JP5291568B2 - Evaluation method of delayed fracture resistance of steel sheet molded products - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an evaluation method that can properly and accurately determine the resistance to delayed fracture of various processed locations in a high-strength steel plate molding for which complicate press working is performed. <P>SOLUTION: The method for evaluating the resistance to delayed fracture of the steel plate molding includes the delayed fracture hydrogen amount estimation step, wherein the corresponding relationship between the amount of hydrogen contained in the steel in the event of generation of the delayed fracture and the strain of the crystal grain in a structure of the steel, in the event of generation of the delayed fracture is used to determine the amount of hydrogen, corresponding to the strain of the crystal grain in a structure of the locations to be evaluated in the steel plate molding, thereby estimating the amount of hydrogen to generate the delayed fracture at the locations to be evaluated. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a method for evaluating delayed fracture resistance of a steel sheet formed by pressing a high-strength steel sheet.

  In recent years, from the viewpoint of protecting the global environment, there has been a demand for lower fuel consumption of automobiles, and as one of the methods for realizing this, a reduction in the weight of the vehicle body is required. At the same time, improvement in automobile crash safety is also demanded, but there is a concern that the weight of the vehicle body may increase when conventional steel plates are used. Therefore, in order to solve these two conflicting problems, a thin steel plate that can be thinned and has high strength has been developed. In recent years, in the field of automotive parts steel plate, an ultra-high strength such that the tensile strength exceeds 1180 MPa. Strength steel plates are starting to be applied.

  Such a high-strength steel sheet is subjected to complicated press work such as bending and drawing in the field of automobile parts, and is used as a part that constitutes bumpers and impact beams for automobiles. In recent years, the demand for improved formability has also increased. That is, there is an urgent need to provide a high-strength steel sheet having high strength and suitable formability according to each application.

  On the other hand, with the increase in strength of steel sheets, there is concern about the problem of delayed fracture of steel sheet molded products. Delayed fracture refers to a phenomenon of sudden brittle fracture without plastic deformation after a certain period of time has passed since the start of use due to hydrogen entering the interior of the steel sheet molded product in the usage environment. Therefore, there has been a demand for a method for evaluating that delayed fracture is less likely to occur in a steel sheet molded article in an actual use environment (delayed fracture resistance).

  Therefore, in the field of components such as bolts, PC steel wires, and line pipes that have been increased in strength from the past, hydrogen has been artificially introduced into steel materials as described in Patent Document 1 and Non-Patent Document 1. A delayed fracture resistance evaluation method for embrittlement of the steel material has been used. For example, the evaluation method according to Patent Document 1 discloses a representative method used for evaluating delayed fracture resistance of bolts and PC steel wire steel. These methods apply tension to a tensile test piece in an aqueous solution environment, artificially embrittle the tensile test piece by introducing hydrogen into the steel, and stress applied to the tensile test piece. The hydrogen concentration was analyzed and evaluated as a measure of the conditions under which delayed fracture occurs.

  However, the method for evaluating delayed fracture resistance in the field of steel plates (particularly thin steel plates) is different from the method for evaluating parts such as bolts. That is, since components such as bolts are used as finished products, it is not necessary to consider the influence of the degree of processing, additional stress, residual stress, end face condition, etc. in the evaluation. On the other hand, since a steel plate is used as a part after being subjected to press working or the like, an accurate evaluation is not possible unless effects such as degree of work, additional stress, residual stress, and end face state are taken into consideration.

  Therefore, Patent Document 2 describes a method in which a steel sheet is cut out and bent into a U-shape, and then stressed by tightening both ends with bolts and nuts to evaluate as a test piece. The test piece was subjected to electrolytic emulsification in an acidic solution to cause hydrogen embrittlement, and the amount of hydrogen in the test piece at that time was measured to evaluate as a measure of conditions under which delayed fracture occurred. By adopting such a configuration, the evaluation apparatus and the evaluation method according to Patent Document 2 are resistant to the influence of the environment in which the steel sheet molded product is used, the degree of processing, the additional stress, the residual stress, the end face state, the bending radius, and the like. The delayed fracture property could be evaluated.

  Patent Document 3 describes a method in which a steel sheet is cut into a strip shape, holes are formed near both ends in the longitudinal direction, bent into a U shape, and evaluated as a test piece. Then, a strain gauge is attached to the surface of the processed part of the test piece, a bolt is passed through the hole, the desired stress is applied by tightening the bolt while observing the strain with the strain gauge, and the test piece is immersed in dilute sulfuric acid. I let you. Then, a negative voltage is applied to the test piece to generate hydrogen by electrolyzing dilute sulfuric acid, and the time until the change of the strain gauge value appears (time until delayed fracture occurs) is measured. It was a thing.

  Further, in Non-Patent Document 2, a delayed fracture resistance evaluation map using bending radius, residual stress, and delayed fracture hydrogen amount in the processed part of the actual part of the 1180 MPa class steel plate that is a strength level at which delayed fracture is a concern. Has been proposed. This delayed fracture resistance evaluation map is a condition in which delayed fracture occurs by plotting and mapping the bending radius, residual stress, and hydrogen amount when delayed fracture occurs in a three-dimensional graph. Is a visual representation.

JP 2004-309197 A JP-A-2005-134152 JP-A-7-146225

Matsuyama Atsushi "Delayed Destruction" Nikkan Kogyo Shimbun, August 31, 1989 P159-161 Automotive Technical Papers Vol. 39, no. 5 “Development of integrated door impact beam by cold pressing of 1180 MPa class steel plate” September 2008 P133-138

  Due to the recent rapid increase in the weight of vehicles and the improvement in collision safety, ultra high strength steel plates of 1180 MPa or more are beginning to be applied to steel plate molded products subjected to complicated press working and bending. These steel plate products are processed by deep drawing, overhanging, stretch flange processing, etc. by pressing, in addition to the conventional processing of simple round bar shapes and simple bending. There is a change in the crystal level itself. That is, in the steel material structure, distortion of crystal grains, dislocations, increase of defects, collapse of crystal grains and structure form, and the like are caused with processing. Therefore, in the vertical wall part where material inflow (deep drawing process) and material outflow (stretch flange process, overhang process) occur, in the vicinity of the punched hole subjected to punching, and in the part subjected to multiple times of bending-bending. There is concern over the occurrence of delayed fracture over conventional simple bending.

  However, in the conventional evaluation method described above, since the portion that can be evaluated is only a portion subjected to simple bending, the vertical wall portion, which is likely to cause delayed fracture in a pressed steel sheet product, It was not possible to evaluate the vicinity of the hole and the part subjected to a plurality of bending-bending processes. In addition, as in the evaluation methods described in Patent Document 2 and Non-Patent Document 2, if the evaluation of delayed fracture resistance is performed in the relationship of “bending radius−residual stress−hydrogen amount”, the evaluation result is temporarily good. Even in the case of a steel sheet molded product determined as “delayed”, delayed fracture might actually occur and the accuracy of evaluation was lacking.

  In addition, in the evaluation method according to Patent Document 3, only a steel sheet formed by bending with a bending radius of 15 mm and a load stress of 1000 MPa can be evaluated, and the delay resistance of the steel sheet formed by processing under other conditions is not possible. It was difficult to evaluate destructibility. Furthermore, since the evaluation method according to Patent Document 3 is only a portion where the evaluable portion is subjected to a simple bending process, the vertical wall portion that is likely to cause delayed fracture in the actual steel sheet formed by pressing, It was not possible to evaluate the vicinity of the punched hole and the part subjected to a plurality of bending-bending processes.

  The present invention has been made in view of such problems, and in high-strength steel sheet molded products subjected to complicated press work, it is possible to appropriately and accurately determine delayed fracture resistance of various processed parts. The purpose is to provide an evaluation method.

  In order to solve the above-described problems, the present inventor has intensively studied a method for evaluating delayed fracture resistance in steel sheet molded products using high-strength steel sheets. In the field of high-strength steel sheets, a method for evaluating delayed fracture resistance has not yet been established, and a method of applying tension to the bolt as described above, or a simple bending process that simulates a press-formed product. A method in which stress is applied to the test piece is used. However, for press-molded products, there are, for example, deep drawing, overhanging, stretch flange processing, etc. in addition to bending, and conventional methods cannot cope with these processing methods. It was. That is, conventionally, only the “bending radius” has been used as an index of the degree of processing of a steel sheet molded product that affects delayed fracture.

  On the other hand, in the present invention, by focusing on the change in the steel material structure caused by the press work, and using “crystal grain distortion” as an index of the degree of work of the steel sheet molded product, complicated press work that could not be evaluated conventionally is performed. It was also found that delayed fracture resistance can be accurately evaluated for the formed steel sheet product. In other words, as an index indicating the degree of processing of a steel sheet molded article, not “bending radius” focusing only on the shape of the steel sheet molded article but “crystal grain distortion” which more essentially indicates the degree of processing of the steel sheet molded article. Thus, it has been found that it is possible to evaluate all processing methods other than simple bending processing, that is, a vertical wall portion of a steel sheet molded product, the vicinity of a punched hole, a portion subjected to a plurality of bending-bending processes, etc. It was.

  That is, the evaluation method of delayed fracture resistance of the steel sheet molded article according to the present invention is an evaluation method of delayed fracture resistance of the steel sheet molded article, and the amount of hydrogen contained in the steel material when delayed fracture occurs, The amount of hydrogen corresponding to the distortion of the crystal grains in the structure of the evaluation part of the steel sheet molded product is obtained using the relationship in which the distortion of the crystal grains in the structure of the steel material when the delayed fracture occurs is correlated. Thus, a delayed fracture hydrogen amount estimation step for estimating a hydrogen amount causing the delayed fracture at the evaluation site is performed.

  The method for evaluating delayed fracture resistance having such a configuration is a processing part (evaluation part) of a steel sheet molded article in a relationship in which the amount of hydrogen in a test piece and the distortion of crystal grains are associated with each other when delayed fracture occurs. The amount of hydrogen corresponding to the distortion of the crystal grain can be obtained quickly and easily by comparing the distortion of the crystal grain in, and the amount of delayed fracture hydrogen can be accurately estimated. The delayed fracture hydrogen amount refers to the amount of hydrogen in the steel sheet molded product when delayed fracture occurs in the steel sheet molded product.

  Furthermore, the evaluation method of delayed fracture resistance of the steel sheet molded article according to the present invention is an evaluation method of delayed fracture resistance of the steel sheet molded article, and the amount of hydrogen contained in the steel material when delayed fracture occurs, Using the relationship that correlates the crystal grain distortion in the structure of the steel material when the delayed fracture occurs, the crystal grain distortion corresponding to the amount of hydrogen contained in the evaluation part of the steel sheet molded product is obtained. Thus, a delayed fracture crystal grain strain estimation step for estimating the strain of crystal grains causing the delayed fracture at the evaluation site is performed.

  The method for evaluating delayed fracture resistance having such a configuration is based on the conditions in which the amount of hydrogen in the test piece and the distortion of the crystal grains when delayed fracture occurs are correlated with the amount of hydrogen in the processed part of the steel sheet molded product. By comparing, the distortion of the crystal grain corresponding to the amount of hydrogen can be obtained quickly and easily, and the delayed fracture crystal grain distortion can be accurately estimated. The delayed fracture crystal grain distortion refers to the distortion of crystal grains in the structure of the steel sheet molded article when delayed fracture occurs in the steel sheet molded article.

  Further, in the method for evaluating delayed fracture resistance of a steel sheet molded article according to the present invention, in the delayed fracture hydrogen amount estimation step, the amount of hydrogen contained in the steel material when the delayed fracture occurs, and the delayed fracture occurs. Crystal grains in the structure of the evaluation part of the steel sheet molded article using a relationship in which the distortion of the crystal grains in the structure of the steel material when performing the residual stress of the steel material when the delayed fracture occurs It is preferable to obtain the amount of hydrogen corresponding to the strain of and the residual stress of the evaluation site.

  The delayed fracture resistance evaluation method with such a configuration is that delayed fracture occurs in the test piece by adding residual stress data to the conditions for obtaining the hydrogen content or crystal grain distortion of the steel sheet molded product. A more accurate relationship can be derived. And the amount of delayed fracture hydrogen corresponding to these can be calculated | required more correctly by collating the distortion and residual stress of the crystal grain in the process site | part of a steel plate molded product with this relationship.

  Further, the method for evaluating delayed fracture resistance of a steel sheet molded article according to the present invention includes the amount of hydrogen contained in the steel material when the delayed fracture occurs in the delayed fracture crystal strain estimation step, and the delayed fracture. Hydrogen contained in the evaluation part of the steel sheet molded article using a relationship in which the distortion of crystal grains in the structure of the steel material when generated and the residual stress of the steel material when the delayed fracture occurs are associated with each other It is preferable to obtain the distortion of the crystal grains corresponding to the amount and the residual stress of the evaluation site.

  The delayed fracture resistance evaluation method with such a configuration is that delayed fracture occurs in the test piece by adding residual stress data to the conditions for obtaining the hydrogen content or crystal grain distortion of the steel sheet molded product. A more accurate relationship can be derived. And the distortion of the crystal grain corresponding to these can be calculated | required more correctly by comparing the hydrogen content and residual stress contained in the process site | part of a steel plate molded product with this relationship.

  Further, in the method for evaluating delayed fracture resistance of a steel sheet molded article according to the present invention, before the delayed fracture hydrogen amount estimation step or the delayed fracture crystal grain strain estimation step, create a test piece subjected to forming, A test piece hydrogen introduction step for introducing hydrogen into the test piece until delayed fracture occurs, and a test piece measurement for measuring the hydrogen content of the test piece and the distortion of crystal grains when the delayed fracture occurs. It is preferable to perform a step and a delayed fracture condition deriving step of deriving the relationship by associating the hydrogen amount of the test piece measured in the test piece measuring step and the distortion of crystal grains.

  The evaluation method for delayed fracture resistance having such a configuration is to measure the amount of hydrogen and strain of crystal grains at the time of delayed fracture occurrence using test pieces subjected to various forming processes, and obtain these data. The associated relationship is derived. Then, by measuring the distortion of crystal grains or the amount of hydrogen in the processed part of the steel sheet molded article and comparing this with the relationship, the amount of delayed fracture hydrogen or the distortion of crystal grains corresponding thereto can be obtained more accurately.

  Further, in the method for evaluating delayed fracture resistance of a steel sheet molded article according to the present invention, before the delayed fracture hydrogen amount estimation step or the delayed fracture crystal grain strain estimation step, create a test piece subjected to forming, A test piece hydrogen introduction step for applying hydrogen into the test piece until residual stress is applied to the test piece and delayed fracture occurs in the test piece, and the amount of hydrogen in the test piece when the delayed fracture occurs And the test piece measurement step for measuring the strain of the crystal grains, the hydrogen amount of the test piece measured in the test piece measurement step, the strain of the crystal grains, and the residual stress applied to the test piece, and the relationship It is preferable to perform a delayed fracture condition deriving step for deriving.

  The evaluation method of delayed fracture resistance having such a configuration uses a test piece subjected to various molding processes to measure the amount of hydrogen and the distortion of crystal grains when delayed fracture occurs in advance. A relationship in which the residual stress is associated is derived. Then, by measuring the distortion of crystal grains or the amount of hydrogen in the processed part of the steel sheet molded article and comparing this with the relationship, the amount of delayed fracture hydrogen or the distortion of crystal grains corresponding thereto can be obtained more accurately.

  And the evaluation method of the delayed fracture resistance of the steel sheet molded product according to the present invention is such that the processing strain is an area ratio at which the CI (Confidence Index) for azimuth determination in EBSP is 0.1 or less, and IQ for azimuth determination in EBSP. (Image Quality), KAM (Kernel Average Misorientation) for azimuth determination in EBSP, or distortion measured by XRD (X-ray diffraction) is preferable.

  The evaluation method of delayed fracture resistance having such a configuration can quickly and accurately measure the crystal structure of the steel structure in the steel sheet molded article, that is, the size of the crystal grain distortion, by using EBSP or XRD. it can.

  According to the method for evaluating delayed fracture resistance of a steel sheet molded article according to the present invention, by using “crystal strain” as an evaluation index instead of “bending radius” which is a conventional evaluation index of delayed fracture resistance. The delayed fracture resistance of the steel sheet molded product can be more accurately evaluated in the relationship of “crystal grain distortion−residual stress−hydrogen content”. In other words, by measuring the distortion of the crystal grains of the steel sheet molded product, the evaluation of dense delayed fracture resistance taking into account the distortion of the crystal grains, dislocations, increase in defects, collapse of the crystal grains and structure morphology, etc. accompanying the processing. Can be performed. Therefore, in addition to simple round bar shape processing and simple bending processing, vertical wall parts that cause material inflow (deep drawing processing) and material outflow (stretch flange processing, overhang processing), and punching with punching processing It is possible to accurately evaluate the delayed fracture resistance of the portion in the vicinity of the hole and subjected to a plurality of bending-bending processes. Further, by measuring and analyzing the crystal orientation by EBSP / XRD, the delayed fracture resistance can be evaluated more quickly than the conventional evaluation method using “bending radius”.

It is a flowchart which shows the evaluation method which concerns on one Embodiment of this invention. It is a figure which shows the typical processing method of press work, (a) is a figure which shows deep drawing, (b) is a figure which shows overhanging, (c) is a figure which shows stretch flange processing, d) is a diagram showing bending. It is a figure which shows the distortion of the crystal grain in the structure | tissue of the steel materials by processing, (a) is a figure which shows the state of the crystal grain before a process, (b) is a figure which shows the state of the crystal grain after a process, is there. It is a figure which shows the measurement site | part of EBSP, (a) is a figure which shows the measurement site | part of EBSP in a flat part, (b) is a figure which shows the measurement site | part of EBSP in a bending process part. It is a figure which shows the creation process of delayed fracture resistance, (a) is a figure which shows the process of creating a delayed fracture occurrence boundary area, (b) is a figure which shows the process of creating a delayed fracture estimated boundary area, c) is a figure which shows the process of plotting the measured value of an actual steel plate molded article. It is a figure which shows the press-molded article used for the evaluation method which concerns on this invention, (a) is a figure which shows the press-formed article of the shape i using A steel, (b) is the figure ro which used B steel. (C) is a figure which shows the press-formed product of the shape C using C steel. In the conventional evaluation method, it is a figure which shows the delayed fracture resistance evaluation map created based on the evaluation result which used "bending radius" as the parameter | index of delayed fracture resistance. In the evaluation method which concerns on this invention, it is a figure which shows the delayed fracture resistance evaluation map created based on the evaluation result which used "the area ratio which becomes CI <= 0.1" as the parameter | index of delayed fracture resistance. In the evaluation method which concerns on this invention, it is a figure which shows the delayed fracture resistance evaluation map produced based on the evaluation result which used "IQ" as the parameter | index of delayed fracture resistance. In the evaluation method which concerns on this invention, it is a figure which shows the delayed fracture resistance evaluation map produced based on the evaluation result which used "KAM" as the parameter | index of delayed fracture resistance. In the evaluation method which concerns on this invention, it is a figure which shows the delayed fracture resistance evaluation map created based on the evaluation result which used "strain measurement by XRD" as the parameter | index of delayed fracture resistance.

<First Embodiment>
An evaluation method according to the first embodiment of the present invention will be described. In the evaluation method according to the present embodiment, as shown in FIG. 1, a test piece hydrogen introduction step (step S1) for introducing hydrogen into the test piece in order to cause delayed fracture, and the amount of hydrogen in the test piece are measured. The test piece hydrogen amount measurement step (step S2), the test piece crystal strain measurement step (step S3) for measuring the strain of crystal grains of the test piece, and the correspondence relationship between the hydrogen amount in the test piece and the strain of crystal grains. A delayed fracture condition derivation step (step S4) for extracting the conditions for the occurrence of delayed fracture and the molded product crystal grain strain measurement step (step S5) for measuring the crystal grain strain of the steel plate molded product, and delayed fracture occurs. A delayed fracture hydrogen amount estimation step (step S6) for estimating the amount of hydrogen at which delayed fracture occurs based on the conditions to be performed.

  Here, either the test piece hydrogen amount measurement step (step S2) or the test piece crystal grain strain measurement step (step S3) may be performed first. Steps S1 to S4 are steps for measuring sample / reference data when delayed fracture occurs as will be described later. Once the data is measured, only step S5 and subsequent steps are performed. May be. Hereinafter, each step will be described in detail.

(1) Test piece hydrogen introduction process This process is a process in which hydrogen is introduced into a plurality of test pieces that have been subjected to various forming processes, and the test piece is artificially delayed. In this way, by investigating delayed fracture properties using a plurality of test pieces in advance, sample and reference data such as the amount of hydrogen in steel when delayed fracture occurs, the distortion of crystal grains in the steel structure, etc. Can be obtained. This process will be described in detail as follows.

(1-1) Preparation of test piece As mentioned above, the test piece is for obtaining sample and reference data for evaluating delayed fracture resistance. It is preferable to do. In the evaluation method according to the present embodiment, as an example, a specimen having three types of compositions and four types of representative processing sections in press working is created, but it is of course possible to use more composition / processing sections. Is possible.

  The evaluation method according to the present embodiment can be evaluated with any steel sheet having any tensile strength. Therefore, a specimen can be cut out from a high-strength steel sheet having a tensile strength of 1180 MPa or more to simulate a high-strength steel sheet molded product, and can be used as a sample / reference for evaluation.

  In this embodiment, the composition of the test piece is the same as that of the steel sheet molded product to be evaluated. Here, the reason why the compositions of the test piece and the steel sheet molded product are matched is that the composition of the steel material also affects the amount of delayed fracture hydrogen, and therefore it is preferable to match these for accurate evaluation. However, the present invention is not limited to this, and when it is known in advance that the composition of the test piece exhibits the same delayed fracture characteristics as the composition of the steel sheet molded product, the composition of the test piece is different from the composition of the steel plate molded product. There is no problem. Further, the composition and delayed fracture characteristics of the steel sheet molded product and the test piece do not need to be completely matched, and a difference that does not adversely affect the evaluation result is allowed. In addition, about the manufacturing method of a test piece, a conventionally well-known method is used, and manufacturing conditions, such as a workability, should be set with reference to the part of the steel sheet product to be evaluated and the deformation limit curve of the base steel sheet. Can do.

  For example, as shown in FIG. 2, the test piece can be prepared in various processing sections. In FIG. 2, (a) is a deep drawing test piece with a different drawing ratio for simulating a vertical wall portion (deep drawing) in an actual steel sheet molded product. Moreover, (b) is an Erichsen test piece for simulating a drawn portion (overhang processing) in an actual steel sheet molded product. Moreover, (c) is a stretch flange test piece for simulating the vicinity of a punched hole (stretch flange processing) in an actual steel sheet molded product. Further, (d) is a U-bending specimen in which the bending stress and the applied stress to the bending portion are changed in order to simulate bending in an actual steel sheet molded product. In the evaluation method according to the present embodiment, since the delayed fracture characteristics are evaluated based on the distortion of the crystal grains, it is not necessary to match the processing category of the test piece and the processing category of the evaluation site in the steel sheet molded product. Accordingly, it is of course possible to evaluate the delayed fracture characteristics of the bent portion of the steel sheet molded product based on the measurement data obtained from the test piece subjected to deep drawing.

  By using such a test piece, the evaluation method according to the present embodiment can obtain in advance measurement data that takes into account the strain that occurs in the steel structure due to deep drawability, overhangability, stretch flangeability, and bendability. That is, as described later, by correlating these measurement data and deriving the relationship, it is possible to evaluate the delayed fracture resistance of an actual steel sheet molded product having the same characteristics.

(1-2) Introduction of hydrogen into the test piece Next, hydrogen is artificially introduced into the steel material constituting each test piece by the hydrogen introduction means. At this time, since the delayed fracture characteristics are also affected by the residual stress of the steel material, a plurality of test pieces having different residual stresses are used. In addition, it is not necessary to make the residual stress given to a test piece and the residual stress of the evaluation site | part of a steel plate molded product correspond, and evaluation of delayed fracture resistance is possible.

  As a means for introducing hydrogen, an acid dipping method in which a test piece is immersed in an acid solution such as hydrochloric acid, an electrolytic solution is electrolyzed by passing a current between the cathode and the anode, using the test piece as a cathode and platinum as an anode in an electrolytic solution. However, the cathodic charging method in which hydrogen generated at that time is introduced into the steel material, the corrosion of the steel material is generated by repeating the environment in which the neutral chloride aqueous solution is sprayed and the temperature and humidity are controlled. Combined cycle method that introduces the generated hydrogen into the steel material, air exposure method that installs the test piece in the actual atmospheric environment and introduces the hydrogen generated due to the corrosion reaction occurring in the natural environment into the steel material, etc. The following means can be used.

  However, the combined cycle method and the atmospheric exposure method require a long time for introduction, and the amount of hydrogen that penetrates into the steel material due to the corrosion reaction is 0.1. Since the amount is as small as several ppm, the test piece may not crack (delayed fracture). Therefore, as a method for introducing hydrogen into the test piece, it is preferable to use an acid immersion method or a cathodic charge from the viewpoint of shortening the test time and controlling the amount of hydrogen in the steel material.

  Examples of the solution used in the acid dipping method include an aqueous hydrochloric acid solution. Here, the concentration of the aqueous hydrochloric acid solution is preferably in the range of 0.01 to 10%. If the concentration of the hydrochloric acid aqueous solution exceeds 10%, the dissolution reaction of the test piece and the hydrogen generation reaction in the hydrochloric acid aqueous solution become too active, and hydrogen is introduced rapidly and in large quantities into the steel material. On the other hand, when the concentration of the aqueous hydrochloric acid solution is less than 0.01%, the dissolution reaction of the test piece is suppressed, but at the same time, the hydrogen generation reaction is also suppressed, so that almost no hydrogen intrusion into the steel material occurs. The piece may not crack.

  In the cathode charging method, the amount of hydrogen introduced into the steel material can be controlled by controlling the concentration of the solution containing the electrolyte and controlling the negative current density and time to be applied. Therefore, it is simple as a method for evaluating the delayed fracture resistance having a correlation with the amount of hydrogen in the steel material, and has the advantage that the amount of hydrogen in the steel material can be arbitrarily controlled as described above. In addition, what is necessary is just to use current control apparatuses, such as the existing potentiostat, for control of the electric current to apply.

  In the cathodic charging method, the pH of the solution containing the electrolyte is preferably 6 or less in order to efficiently introduce hydrogen into the steel material. If the pH exceeds 6, hydrogen may not be efficiently introduced into the steel material. Further, as the solution containing the electrolyte, a weakly acidic to acidic aqueous solution containing thiocyanate (potassium thiocyanate, sodium thiocyanate, etc.) known to have a catalytic action for introducing hydrogen into the steel material is used. It is preferable.

(2) Test piece measuring step This step is a step of measuring the amount of hydrogen contained in the test piece when delayed fracture occurs and the distortion (working strain) of crystal grains. This step can be divided into a test piece hydrogen amount measurement step, a test piece crystal grain strain measurement step, and a test piece residual stress measurement step, and either step may be performed first. Also, in these steps, as a test piece measuring means, a hydrogen amount measuring means in the test piece hydrogen amount measuring step, a processing strain measuring means in the test piece crystal grain strain measuring step, and a residual stress measurement in the test piece residual stress measuring step. Use means. Hereinafter, each step will be described.

(2-1) Test piece hydrogen amount measuring step This step is a step of measuring the hydrogen amount in the test piece by the hydrogen amount measuring means. That is, hydrogen is introduced by any of the methods described above until cracks (delayed fracture) occur in the test piece, and the introduction of hydrogen is stopped when it is confirmed that the cracks have occurred. In addition, the judgment of the presence or absence of a crack is made by whether or not the crack of the test piece can be visually confirmed as a line. In addition, a strain gauge is attached in the vicinity of the cracked part, and a crack is judged by a change in its value. Alternatively, a method of determining a crack based on a potential change that occurs when a crack occurs may be used.

  And the test piece for hydrogen amount analysis is rapidly cut out from a test piece so that the introduced hydrogen may not scatter. When cutting out the test piece for hydrogen amount analysis, it is preferable to use a method of cutting out with a rotating grindstone while spraying cooling water so that the hydrogen introduced into the steel material is not scattered by the heat generated at the time of cutting.

  Of the hydrogen contained in the test piece cut out by such a method, the delayed fracture is affected by diffusible hydrogen that can move freely in steel even at relatively low temperatures. It is called. In this step, the amount of diffusible hydrogen measured from the test piece for analyzing the amount of hydrogen in which cracking has occurred (hereinafter simply referred to as the amount of hydrogen) is the object of measurement.

  Examples of the hydrogen amount measuring means include measurement using an atmospheric pressure ionization mass spectrometer (API-MS). As measurement conditions, it is preferable that the temperature increase rate during the measurement is 1 ° C./min to 20 ° C./min. This is not preferable if the rate of temperature increase is less than 1 ° C./min, because it takes a very long time to measure diffusible hydrogen and the efficiency deteriorates. When the rate of temperature increase exceeds 20 ° C./min, This is because the hydrogen release efficiency is lowered and the temperature control becomes difficult, and the diffusible hydrogen cannot be accurately measured. In addition, from the viewpoint of measurement efficiency, it is more preferable that the rate of temperature rise is 5 ° C./min or more and 15 ° C./min or less. Moreover, it is preferable to determine the amount of diffusible hydrogen to be the total amount (integrated value) of hydrogen released in the temperature range from room temperature to 300 ° C.

(2-2) Test Piece Crystal Strain Measurement Step This step is a step of measuring the processing strain of crystal grains in the structure of the test piece by means of processing strain measurement means. In this way, by measuring the strain of the crystal grains in the structure of the test piece, it is possible to obtain a more precise resistance that takes into account the distortion of the crystal grains, dislocations, an increase in defects, the collapse of the crystal grains and the structure morphology, etc. Evaluation of delayed fracture is possible. Therefore, in addition to simple round bar shape processing and simple bending processing, vertical wall parts that cause material inflow (deep drawing processing) and material outflow (stretch flange processing, overhang processing), and punching with punching processing It is possible to accurately evaluate the delayed fracture resistance of the portion in the vicinity of the hole and subjected to a plurality of bending-bending processes.

  With reference to FIG. 3, the distortion of the crystal grains of the steel material constituting the test piece will be described. As shown to Fig.3 (a), the crystal grain of the steel material which has not been pressed is the state in which the orientation of the crystal grain is aligned in one direction. Therefore, there are few factors for hydrogen to enter the crystal grains, and delayed fracture is less likely to occur.

  On the other hand, as shown in FIG. 3B, the crystal grains of the steel material subjected to press working or the like are in a state in which the orientation of the crystal grains is disturbed. In addition, distortion A occurs in the crystal grains, and voids (voids) B occur in the crystal grains. Furthermore, cell walls C are also formed at the crystal grain boundaries due to the dislocation of crystal grains. Therefore, there is a high possibility that hydrogen enters the strain A and the void B, and delayed fracture is likely to occur. As described above, the evaluation method according to the present embodiment is characterized in that the distortion of the crystal grains accompanying the processing of the steel material affects the delayed fracture and uses the distortion of the crystal grains as an index of the delayed fracture resistance. It is said.

  As processing distortion measuring means for measuring the distortion of crystal grains of each test piece, evaluation by EBSP (Electron Back Scatter Diffraction Pattern) or measurement by XRD (X-ray diffraction) is possible. It is preferable to measure the distortion of crystal grains using any of these methods. Hereinafter, each measuring means will be described in detail.

(2-2-1) Measurement of processing strain of crystal grains by EBSP EBSP is a Kikuchi pattern (Kikuchi line) obtained from reflected electrons generated when an electron beam is incident on the surface of a test piece. By analyzing this pattern, the crystal orientation of the electron beam incident position can be determined. The Kikuchi pattern refers to a pattern that appears behind the electron diffraction image in a pair of black and white parallel lines, strips or arrays when the electron beam hitting the crystal is scattered and diffracted.

  The determination of crystal orientation by EBSP is a TEM (Transmission Electron Microscope) that can identify steel structures in the thickness direction, which are determined to be the same in ordinary microscopic observations but have different crystal orientation differences, by color difference. There are advantages such as the ability to measure bulk (bulk) samples, which is difficult with Microscope), the need to create a thin film sample for observation, and a dramatic reduction in measurement and analysis time. .

  The measurement of crystal grain distortion by EBSP is preferably performed by structural evaluation using an FE-SEM (Field Emission-Scanning Electron Microscope) equipped with an EBSP detector. It is possible to analyze the crystal orientation distribution (crystal grain distortion) on the surface of the test piece by scanning the surface of the test piece in two dimensions with the FE-SEM and measuring the crystal orientation at every predetermined pitch. it can. In addition, as FE-SEM provided with the EBSP detector used for a measurement, "JEOL Co., Ltd. field emission scanning electron microscope JSM-6500F" can be used, for example.

  As shown in FIG. 4, the measurement site is a measurement area (about 150 μm × 150 μm, measurement interval is 0.1 μm) centering on the position of 1/4 of the thickness of the test piece 100, up to the measurement site. A sample for EBSP measurement is prepared by a polishing method for polishing. However, when polishing to the measurement site, it is preferable to perform electrolytic polishing in order to prevent the influence of the structure change due to polishing. In addition, when measuring the flat part of a test piece, as shown to Fig.4 (a), the arbitrary areas in the position advanced 1/4 in the plate | board thickness direction from the surface are measured. Moreover, when measuring the process site | part (for example, bending process part) of a test piece, as shown in FIG.4 (b), the process site | part in the position advanced 1/4 in the plate | board thickness direction from the surface is measured.

  Next, a test piece is set in the lens barrel of the FE-SEM, the measurement site is irradiated with an electron beam, and EBSP is projected on the screen. This is photographed with a high-sensitivity camera and captured as image data into a computer. Then, EBSP image analysis is performed, and the orientation of the recognized crystal system is determined by comparison with a simulation pattern using a known crystal system (FCC: face-centered cubic lattice, BCC: body-centered cubic lattice). An electronic computer is used for the analysis, and as software used for the analysis, for example, “OIM (Orientation Imaging Microscooy) Analysis 5.2 manufactured by EDAX-TSL” can be used.

  In the measurement of distortion of crystal grains by EBSP, CI (Confidence Index), IQ (Image Quality), KAM are used as indexes (parameters) for quantitatively evaluating changes in steel structure due to processing applied to a steel plate. It is preferable to use (Kernel Average Misorientation). For analysis other than IQ, it is preferable to exclude data with CI = 0.1 or less, which has extremely low reliability in determining the azimuth angle including grain boundaries and the like. Hereinafter, each index will be described in detail.

[EBSP index: evaluation of area ratio with CI ≦ 0.1]
CI is a parameter called a reliability evaluation index, and is a numerical value indicating the crystal orientation determination accuracy in the analysis region. In determining the orientation of a crystal, the orientation in the case of the best match is calculated by comparison with the diffraction pattern of a given crystal system. Therefore, the reliability of data can be statistically evaluated by quantifying (1 to 0) the accuracy of azimuth determination as a reliability evaluation index (CI). The CI value is closer to 1 and the crystal orientation determination accuracy is higher, and the CI value is closer to 0 and the crystal orientation determination accuracy is lower.

  In the measurement of crystal grain distortion by EBSP, the crystal orientation may differ from the actual one depending on the beam state or test piece state. Therefore, in the case of limited diffraction pattern, superimposed diffraction pattern, or unclear pattern However, the most likely orientation is automatically performed. For example, when processing such as bending or drawing is applied to a high-strength steel plate, dislocations and distortion of crystal grains increase due to the processing, and the orientation of crystal grains may collapse. In these parts, it is difficult to determine the crystal orientation by OIM, the orientation is not determined, and the CI value becomes low (in relation to the above-mentioned IQ, it is generally difficult to determine the orientation when the IQ value is low) Often). Therefore, in the measurement site, the area ratio of the location where the orientation could not be determined (CI value is 0.1 or less) is calculated to be “area ratio where CI ≦ 0.1”, and the steel material is processed by processing. It was used as an indicator of changes that occurred in the organization.

[EBSP index: IQ evaluation]
IQ is a value related to the intensity of the Kikuchi pattern after EBSP image processing (Hough transform), and is a numerical value obtained by parameterizing the completeness of the crystal at the measurement site. The IQ value is high when the integrity of the crystal is high, and the IQ value is low when the integrity is low. The deterioration of the IQ value is affected by factors due to processing such as slip lines in the analysis region, defects at the boundary of dislocation cells, and a decrease in crystal integrity due to an elastic strain field. IQ is a numerical value of the definition of the diffraction pattern in EBSP, and the map showing the result is displayed in a gradation of white (high) to black (low) corresponding to this numerical value. The IQ distribution shows the distribution state of this numerical value, and the IQ average value of the entire measurement visual field is used as an index of the change caused in the tissue by processing.

[EBSP index: KAM evaluation]
KAM indicates a strain distribution based on local azimuth change, and is represented by an average value of azimuth differences between six adjacent pixels in a map showing the results. In the OIM of the analysis software described above, whether or not characteristics related to the processing state of the measurement specimen, the internal residual strain, and the like can be expressed based on the orientation difference between pixels. When there is a minute angle change between the pixels, the map is drawn based on the assumption that the crystal is deformed by processing in that portion and is related to the residual strain. Therefore, the average of the azimuth differences between the pixels in this map is calculated, and the average value is used as the KAM value to represent the strain distribution based on the local azimuth change.

  Here, when there is an orientation difference of 5 ° or more between adjacent measurement points, the orientation difference is considered as a subgrain boundary or cell wall and excluded from the calculation of KAM. Therefore, the analysis was performed considering only the orientation difference of 5 ° or less as the orientation fluctuation in the crystal grains, and the KAM average value of the entire measurement visual field was used as an index of the change caused in the steel structure by the processing.

(2-2-2) Measurement of crystal grain distortion by XRD In the evaluation method according to the present embodiment, in addition to the above-described EBSP, the crystal grain distortion in the steel material is directly measured by XRD (X-ray diffraction). You can also. XRD is a technique for analyzing a crystal structure of a substance by utilizing a phenomenon in which X-rays are diffracted by a crystal lattice. As a measurement site of strain in each test piece, a deep drawing test piece (FIG. 2A) is a steel plate surface layer portion 10 in a portion 10 mm below the tip of the vertical wall portion, an Erichsen test piece (FIG. 2B). Is a spherical top 20, an elongated flange test piece (FIG. (C)) is a punched hole vicinity 30, and a U-bend test piece (FIG. (D)) is a bent top 40.

  As an analyzer used for measurement of crystal grain distortion by XRD, for example, “X-ray diffractometer RAD-RU330 manufactured by Rigaku Denki” can be used. Moreover, as measurement conditions, for example, θ / 2θ scanning continuous measurement is performed, Co is used as a target, a monochromator (Kα line) is used for monochromatization, scanning speed: 2 ° / min, sampling width: 0.02 °, Measurement angle (2θ): 30 to 130 ° is preferable.

  Then, the half-value width is calculated by peak separation (fitting) from the measurement data obtained under these conditions, and the Hall-Williamson method (ρ = (14.4 × ε2) / b2, ρ: dislocation density, b: Burgers The distortion of the processed part of the steel sheet can be calculated from the vector). Since crystal grain strain measurement by XRD is a simple and rapid method, it is a suitable measurement method for evaluating a large number of samples.

(2-3) Specimen Residual Stress Measurement Step In the evaluation method according to this embodiment, the test piece residual stress measurement step can be further performed. The test piece residual stress measurement step is a step of measuring the residual stress of the test piece in which the delayed fracture has occurred by the residual stress measurement means. Here, the residual stress means the stress remaining inside the metal after processing. XRD or a strain gauge can be used as a means for measuring the residual stress of the test piece.

  As described above, in the evaluation method according to the present embodiment, the residual stress of the steel sheet molded product used for the evaluation can be any value by measuring the residual stress of the test pieces subjected to various forming processes. The delayed fracture resistance can be accurately evaluated. That is, the delayed fracture resistance of the steel sheet molded product can be more accurately evaluated in the relationship of “crystal grain strain−residual stress−hydrogen content”.

(3) Delayed fracture condition derivation process This process consists of the amount of hydrogen in the test piece measured in the previous steps, the distortion of crystal grains in the structure of the test piece, and the residual stress applied to the test piece. This is a step of deriving a condition estimated that delayed fracture occurs in the test piece using the associated relationship. Here, measurement data collected from test pieces that have been subjected to various forming processes in the same way as steel sheet molded products that are subject to evaluation of delayed fracture characteristics, that is, the amount of hydrogen when delayed fracture occurs, distortion of crystal grains, and residual Corresponding relationships are derived by converting stress relationships into functions using known calculation means. By this step, the correspondence relationship between the amount of hydrogen when the delayed fracture occurs, the strain of the crystal grains, and the residual stress can be obtained as a function including the three factors of the hydrogen amount, the strain of the crystal grains, and the residual stress as variables. it can.

(4) Molded product measuring step (4-1) Molded product crystal grain strain measuring step This step is a step of measuring the strain of crystal grains in the structure of the evaluation site in the steel sheet molded product to be evaluated for delayed fracture characteristics. It is. The measurement of the distortion of the crystal grain of the steel sheet molded product is performed using the same processing strain measuring means (EBSP, XRD) as the test piece.

(4-2) Molded Product Residual Stress Measurement Step This step is a step of measuring the residual stress in the structure of the evaluation site in the steel plate molded product that is the object of evaluation of delayed fracture characteristics. The measurement of the residual stress of the steel sheet molded product is performed using the same processing strain measuring means (XRD, strain gauge) as that of the test piece.

(5) Delayed fracture hydrogen amount estimation step This step uses the relationship derived in the delayed fracture condition derivation step, and in addition to the processing strain of the crystal grain of the steel sheet molded product measured in the molded product crystal grain strain measurement step. The process of obtaining the hydrogen amount corresponding to the residual stress of the steel sheet molded product measured in the molded product residual stress measurement process and estimating the hydrogen amount that causes delayed fracture (the hydrogen amount expected to accumulate in the steel sheet molded product) It is.

  Estimated amount of delayed fracture hydrogen is calculated by the relationship derived in the delayed fracture condition derivation process, that is, a function (relation) with three factors as variables: hydrogen content of specimen, crystal grain distortion and residual stress. It can be calculated by putting the values of residual stress and crystal grain distortion at the evaluation site of the product. The amount of hydrogen at which delayed fracture occurs can be more easily estimated visually by using a delayed fracture evaluation map described later. The estimation method using the delayed fracture property evaluation map will be described later.

  As described above, the evaluation method according to the present embodiment first measures the hydrogen amount, crystal grain distortion, and residual stress when delayed fracture occurs using test pieces subjected to various forming processes. The relationship which matched is derived | led-out. Then, by measuring the distortion and residual stress of the crystal grains of the steel sheet molded product to be evaluated and comparing with the previously determined relationship, the amount of delayed fracture hydrogen of the steel sheet molded product (expected to accumulate in the steel sheet molded product) The amount of hydrogen produced) can be easily estimated. Here, it can be seen that the larger the amount of delayed fracture hydrogen, the better the delayed fracture resistance of the steel material.

  As described above, the delayed fracture hydrogen amount is affected by the composition, residual stress, and processing conditions (working degree) of the steel sheet molded product. Therefore, if the amount of hydrogen expected to accumulate in the steel sheet molded product to be evaluated reaches (exceeds) the amount of delayed fracture hydrogen estimated by the above method, the composition and processing conditions of the steel sheet, Consider reviewing the shape. In addition, for a plurality of steel materials with different compositions, the corresponding relationship between the amount of hydrogen, crystal grain distortion and residual stress when delayed fracture occurs in the delayed fracture condition derivation process, and comparing them, it is possible to It is possible to evaluate which composition is preferable as the composition to be used.

  In the evaluation method according to the present embodiment, when it is difficult to predict the amount of hydrogen accumulated in the steel sheet molded product, instead of the above-described molded product crystal grain strain measurement step and delayed fracture hydrogen amount estimation step, forming is performed. The product hydrogen amount measuring step and the delayed fracture crystal grain strain estimating step can be performed. Here, the molded product hydrogen amount measuring step is a step of actually measuring the amount of hydrogen contained in the actual steel sheet molded product by the same hydrogen amount measuring means as that of the test piece. The delayed fracture crystal grain distortion estimation process uses the relationship derived in the delayed fracture condition derivation process to measure the hydrogen content of the steel sheet molded product measured in the molded product hydrogen content measurement process, or in addition to the molded product residual stress measurement. In this step, the distortion of the crystal grains corresponding to the residual stress of the steel sheet molded article measured in the process is obtained, and the distortion of the crystal grains causing delayed fracture is estimated.

(Creation of delayed fracture resistance evaluation map)
In the evaluation method according to the present embodiment, a delayed fracture resistance evaluation map can be created based on values and relationships obtained in each step. By creating a delayed fracture resistance evaluation map, delayed fracture resistance can be more visually evaluated.

  The delayed fracture resistance evaluation map is created using a three-dimensional graph in which the crystal grain distortion, residual stress, and hydrogen amount are X-axis, Y-axis, and Z-axis, respectively. Hereinafter, a delayed fracture resistance evaluation map using a three-dimensional graph will be briefly described.

[Plotting and mapping of hydrogen content, crystal grain strain, residual stress in the specimen]
First, as shown in FIG. 5 (a), the amount of hydrogen in the test piece when the delayed fracture measured in the test piece measurement step occurs, the distortion of the crystal grains and the residual stress are plotted on a three-dimensional graph with ▲. Each point is connected to create a delayed fracture occurrence boundary region 8 indicated by a dark color in the drawing. Here, the delayed fracture occurrence boundary region 8 is a visual indication of the conditions under which delayed fracture occurs, and the amount of hydrogen in the steel sheet molded product is within this region, or from this region the Z axis If it is above the direction (if the amount of hydrogen is large), it can be considered that delayed fracture occurs. In FIG. 5A, nine test pieces are measured and nine points are plotted with ▲, but more measurement values may be plotted.

  Next, as shown in FIG. 5B, a delayed fracture estimation boundary region 9 shown in a light color in the drawing is created outside the delayed fracture occurrence boundary region 8. Here, the delayed fracture estimated boundary region 9 is estimated that delayed fracture occurs based on a function obtained from the amount of hydrogen in the test piece, crystal grain distortion, and residual stress measured in the second and third steps. If the amount of hydrogen in the steel sheet molded product is within this region, or if it is above this region in the Z-axis direction (if the amount of hydrogen is large), it will be delayed. It can be estimated that destruction occurs.

  Thus, the delayed fracture estimated boundary region 9 is formed by extending the delayed fracture occurrence boundary region 8 based on the same function. Preferably, a sample having the same crystal grain distortion as the plot point used for creating the delayed fracture occurrence boundary region 8 and changing the residual stress is prepared to measure the delayed fracture hydrogen amount. Then, after plotting similarly, fitting by the least square method is performed to create the delayed fracture estimated boundary region 9.

[Plots of hydrogen content, crystal grain distortion, and residual stress in steel sheet molded products]
Next, as shown in FIG. 5 (c), crystal grain distortion and residual stress at the evaluation site in the steel sheet molded product measured in the molded product measurement step, and hydrogen expected to accumulate in the steel sheet molded product. The points corresponding to the quantities are plotted on a three-dimensional graph. Here, the plot points of ◯ are not included in the delayed fracture occurrence boundary region 8 and the delayed fracture estimated boundary region 9 and the measured values of the steel sheet molded product are located below these regions in the Z-axis direction. It is shown that. That is, the measured value of the steel sheet molded product in which delayed fracture does not occur is shown. Note that the plot points marked with ○ are actually located below these regions even if they appear to be included in the delayed fracture occurrence boundary region 8 or the delayed fracture estimated boundary region 9 in the figure.

  In addition, the plot point of x indicates that the measured value of the steel sheet molded product is included in the delayed fracture occurrence boundary region 8 or the delayed fracture estimated boundary region 9 or located above these regions in the Z-axis direction. Show. That is, the measured value of the steel sheet molded product estimated to cause delayed fracture is shown. In addition, even if it is a case where it appears that it is not included in the delayed fracture occurrence boundary region 8 or the delayed fracture estimated boundary region 9 in the figure, the x plotted points are actually located inside these regions. .

  In FIG. 5C, the measured values a and b are plotted below the delayed fracture occurrence boundary region 8 and the delayed fracture estimated boundary region 9 in the Z-axis direction. Therefore, it is presumed that delayed fracture does not occur in the processed part of the steel sheet molded product corresponding to the measured values a and b. The measured value c is plotted in the delayed fracture occurrence boundary region 8, the measured value d is plotted in the delayed fracture estimated boundary region 9, and the measured values e and f are the delayed fracture occurrence boundary region 8 and the delayed fracture occurrence. Plotted above the estimated boundary region 9 in the Z-axis direction. Therefore, it can be estimated that delayed fracture occurs in the processed portion of the steel sheet molded product corresponding to the measured values c, d, e, and f.

  Since the delayed fracture resistance evaluation map is a visual representation of the conditions under which delayed fracture occurs, the strain of crystal grains in the actual processed part of the steel sheet molded article is measured, and the delayed fracture occurrence boundary region 8 In addition, the amount of delayed fracture hydrogen for causing delayed fracture at the machining site can be estimated by comparing with the estimated fracture region 9. Alternatively, by measuring the amount of hydrogen in the processing site of the actual steel sheet molded product and comparing it with the delayed fracture occurrence boundary region 8 and the delayed fracture estimation boundary region 9, the crystal grains for causing delayed fracture in the processing site Distortion can be estimated.

  In the present embodiment, as a preferred embodiment, in the delayed fracture condition derivation step, the correspondence between the three factors of the amount of hydrogen causing delayed fracture, crystal grain distortion, and residual stress is obtained. The invention is not limited to this. For example, in the test piece measurement process, the same residual stress as that applied to the evaluation part of the steel sheet molded article is applied to each test piece, and the amount of hydrogen in the test piece and crystal grain distortion at the time when delayed fracture occurs. Measure. Then, in the delayed fracture condition deriving step, it is possible to obtain the correspondence between the two factors of the amount of hydrogen that causes delayed fracture and the distortion of crystal grains.

  In the case of this modification, the function obtained by the delayed fracture condition deriving step has the amount of hydrogen and the distortion of crystal grains as variables. Further, the delayed fracture resistance evaluation map is a two-dimensional map with the amount of hydrogen and the distortion of crystal grains as axes, and the boundary conditions are defined by lines rather than planes. In the delayed fracture hydrogen amount estimation step, the relationship derived in the delayed fracture condition derivation step, i.e., the function having the hydrogen amount and the crystal grain strain as variables, is used as a function of the distortion of the crystal grain in the structure of the evaluation part of the steel sheet molded product. By entering a value, an estimated value of the delayed fracture hydrogen amount is calculated. It should be noted that the residual stress applied to the test piece does not necessarily need to be completely matched with the residual stress applied to the evaluation part of the steel sheet molded product, and a difference that does not adversely affect the evaluation result is allowed.

  Further, in the present embodiment, by actually performing the test piece hydrogen introduction step, the test piece measurement step, and the delayed fracture condition derivation step, the correspondence between the hydrogen amount when the delayed fracture occurs, the distortion of the crystal grains, and the residual stress However, the present invention is not limited to this, and the above correspondence may be obtained from various documents. Furthermore, in this embodiment, the delayed fracture resistance evaluation map was created and the delayed fracture resistance evaluation was performed. The delayed fracture resistance evaluation map is used to visually grasp the presence or absence of delayed fracture. Therefore, the delayed fracture resistance can be evaluated without using the delayed fracture resistance evaluation map. Note that these modifications can be similarly applied to the second embodiment described below.

<Second Embodiment>
An evaluation method according to the second embodiment of the present invention will be described. The evaluation method according to the present embodiment is the same as that of the first embodiment described above with respect to steps S1 to S4 in FIG. Therefore, step S4 and subsequent steps will be described.

  In this embodiment, after obtaining the correspondence relationship between the amount of hydrogen, the distortion of crystal grains, and the residual stress when delayed fracture occurs in the delayed fracture condition deriving step of step S4, hydrogen that is allowed to accumulate in the steel sheet molded product is allowed. Based on the amount, a critical machining strain is obtained (step S5 ′ (not shown)). Specifically, the distortion of the crystal grain is calculated by substituting the value of the hydrogen quantity that is allowed to accumulate in the evaluation part of the steel sheet molded product into the function having the hydrogen amount, the distortion of the crystal grain, and the residual stress as variables. As a result, an estimated value of the critical machining strain can be obtained.

  Here, when the index indicating the distortion of crystal grains is a property that increases according to the processing amount, the larger the estimated value of the critical processing strain, the better the delayed fracture resistance of the steel material. I understand. Further, the composition and residual stress of the steel sheet molded product affect the critical working strain. Therefore, when the distortion of the crystal grains corresponding to the amount of processing applied to the actual steel sheet molded product reaches (exceeds) the limit processing strain estimated by the above method, the composition and processing conditions of the steel plate, the steel plate molded product Consider review of the shape.

  Hereinafter, the present invention will be described more specifically with reference to examples. It should be noted that the present invention is not limited by the following examples, and can be implemented with appropriate modifications within a range that can be adapted to the spirit of the present invention, all of which are included in the technical scope of the present invention. .

(Sample material)
Three types of steel plates (A to C steels) having a strength level of 1180 to 1470 MPa class shown in Table 1 were used as the steel plates constituting the test pieces and steel plate molded products used in this example. The three types of steel plates were produced using a conventionally known method.

(Creation of specimen)
2A, 2B, and 2D described above in order to simulate the change in the material structure that the steel sheet undergoes by machining in a steel sheet molded product obtained by pressing A to C steel (hereinafter referred to as a press molded product). The test piece shown in was produced. That is, three types of (a) deep drawing test piece, (b) Erichsen test piece, and (d) U-bending test piece were prepared for each of steels A to C. Here, in Comparative Example 1 described later, only the test piece of (d) was used for comparison with the conventional evaluation method (an evaluation method focusing on the bending radius). On the other hand, in Examples 1 to 4 described later, three types of test pieces (a), (b), and (d) were used to show the effects of the evaluation method according to the present invention. The detailed preparation method of each test piece is as follows.

  (A) About the deep drawing processed test piece, a test piece of φ90 is cut out from the steel plates A to C shown in Table 1, and the test piece is restrained with dies and wrinkle restraint (wrinkle restraining pressure: 10 kN). Two types of test pieces were prepared so that the drawing ratio was 1.5 and 2 at R10, punch diameter: φ60 / φ45, molding speed: 5 mm / min (total 6 types). Note that press oil was used as the lubricant.

  (B) For the Erichsen test piece, a 90 mm × 90 mm test piece was cut out from the steel plates A to C shown in Table 1, and the test piece was restrained with dies and wrinkle suppression (wrinkle suppression pressure: 10 kN). A punch having a sphere diameter of 20 mm was pushed into a 27 mm die hole by 5 mm at a speed of 5 mm / min (total 3 types). Note that press oil was used as the lubricant.

  (D) The U-bending test pieces are three types of bending radii of 5 mm, 10 mm, and 15 mm from the investigation results of several types of press-formed products using high-strength steel plates exceeding 1180 MPa, and the applied stress to each bending part. (Residual stress in a press-formed product was simulated) was set to three types of 500 MPa, 1000 MPa, and 1500 MPa. Then, three types of U-bending specimens were prepared using the steel plates A to C shown in Table 1 under the conditions of a bending radius of 5 mm-addition stress of 1500 MPa, a bending radius of 10 mm-addition stress of 1000 MPa, and a bending radius of 15 mm-addition stress of 500 MPa. (9 types in total).

  (D) About U bending test piece, the test piece of 30 mm x 150 mm was cut out from the steel plates AC of Table 1 by making the rolling direction into the length. Then, a φ12 hole is machined at a position of 45 mm from both ends in the longitudinal direction of the cut-out test piece, and after bending with the above-described three bending radii, a bolt is passed through the φ12 hole, and a nut is used. The above-mentioned three kinds of stress were applied to the bent portion by tightening. Here, the applied stress value to the bent portion was a value calculated from the strain amount of the strain gauge and the Young's modulus.

(Introduction of hydrogen into the specimen)
As for the method of introducing hydrogen into each test piece, cathodic charging was used from the viewpoint that the test time and the amount of hydrogen in the steel material can be easily controlled. Regarding the conditions for cathodic charging, the test solution was 0.5 M-H ₂ SO ₄ +0.01 M-KSCN, and current was applied until cracking (delayed fracture) occurred in the test piece at a current density of 0.1 mA / mm ^2. did.

(Measurement of the amount of hydrogen in the specimen)
After cutting out a test piece so that a crack part might be included, the hydrogen amount in steel materials was measured with the atmospheric pressure ionization mass spectrometer (API-MS). The heating rate of the atmospheric pressure ionization mass spectrometer was 12 ° C./min.

(Measurement of residual stress and distortion of crystal grains)
For each of the test pieces (a), (b), and (d), the residual stress was measured by XRD. In addition, for each test piece in which cracking (delayed fracture) occurred, the microstructure analysis of crystal grain distortion by EBSP (evaluation of area ratio satisfying CI ≦ 0.1, IQ, KAM) and the crystal grain analysis by XRD Strain measurements were taken. Note that (d) U-bending specimens are three types of analysis examples of post-processed structures: bending radius 5 mm—additional stress 1500 MPa, bending radius 10 mm—additional stress 1000 MPa, bending radius 15 mm—additional stress 500 MPa, as described above. The analysis was conducted.

  The measurement of the distortion of the crystal grains by EBSP was performed with “JEM-6500F, a field emission scanning electron microscope manufactured by JEOL Ltd.”, and “OIM manufactured by EDAX-TSL” was used as analysis software. In the measurement, polishing by electropolishing was performed up to the measurement sites shown in FIGS. 4 (a) and 4 (b).

  Measurement of crystal grain distortion by XRD was performed using “X-ray diffractometer RAD-RU330 manufactured by Rigaku Corporation”. The measurement is θ / 2θ scanning continuous measurement, using Co as a target and a monochromator (Kα ray) for monochromatization, scanning speed: 2 ° / min, sampling width 0.02 °, measurement angle (2θ): 30 Performed at ~ 130 °. As the measurement site, the deep-drawn test piece is as shown in FIG. 2 (a), and the steel sheet surface layer portion 10 and the Erichsen test piece are 10mm below the tip of the vertical wall as shown in FIG. 2 (b). The spherical top 20, the stretch flange test piece is a punched hole vicinity 30 as shown in FIG. 4C, and the U-bend test piece is a bent top 40 as shown in FIG. .

  Table 2 shows the amount of hydrogen, residual stress, crystal grain strain (area ratio where CI ≦ 0.1, IQ, KAM), and XRD strain, respectively.

(Creation of press-molded products)
Next, actual press-formed products 101, 102, and 103 as shown in FIG. 6 were prepared from the steel plates A to C shown in Table 1 using a conventionally known manufacturing method. A press-formed product 101 in FIG. 6A is obtained by pressing the steel A in Table 1 into a shape A. Moreover, the press-formed product 102 in FIG. 6B is obtained by pressing the B steel in Table 1 into a shape B. Moreover, the press-formed product 103 in FIG. 6C is obtained by pressing the C steel in Table 1 into a shape C.

  For the press-formed products 101, 102, and 103, seven parts (parts 1 to 7 surrounded by a broken line ○) subjected to typical press processing are selected, and the amount of hydrogen, distortion of crystal grains, and residual stress described later are selected. The measurement site was used. Here, the parts 1 and 2 shown in FIG. 6 are bending parts, the parts 3 and 4 are drawing parts, the part 5 is a punching part, the part 6 is a vertical wall part, and the part 7 is a flat part.

(Introduction of hydrogen into press-molded products, measurement of hydrogen content, and confirmation of cracks)
Since it was difficult to introduce hydrogen by cathodic charging, the press-formed products 101, 102, and 103 were immersed in a 3% HCl solution for 100 hours to introduce hydrogen into the steel structure. And the presence or absence of a crack is confirmed visually, and when the crack is confirmed, the press-molded products 101, 102, 103 are taken out from the 3% HCl solution, and the sites 1 to 1 are detected with an atmospheric pressure ionization mass spectrometer (API-MS). The amount of hydrogen in 7 was measured. The temperature increase rate of the atmospheric pressure ionization mass spectrometer at that time was set to 12 ° C./min.

(Measurement of residual stress and bending radius of press-formed products)
Residual stress was measured by XRD for portions 1 to 7 of the press-formed products 101, 102, and 103. Further, for comparison between the prior art and the present invention, the bending radii of the parts 1 to 7 of the press-formed products 101, 102, 103 were measured by a conventionally known method. The column of “Measurement results” in Table 3 shows actual measurement values obtained by measuring the hydrogen amount, presence / absence of cracks, residual stress, and bending radius of the parts 1 to 7 of the press-formed products 101, 102, and 103. In the item of “presence / absence of crack”, “◯” indicates that no crack is generated in the part, and “X” indicates that the part is cracked.

  As shown in Table 3, the amount of hydrogen contained in the portions 1 to 7 of the press-formed products 101, 102, and 103 was in the range of 0.49 to 0.66 ppm. Further, in the press-formed product 101 (steel material A-shape A), a crack occurred in the portion 6 (vertical wall portion). In the press-formed product 102 (steel material B—shape B), cracks occurred in all the parts other than the part 7 (flat portion). In the press-formed product 103 (steel material C-shape C), cracks occurred in the part 3 (drawing part) and the part 6 (vertical wall part).

  The residual stresses of the parts 1 to 7 of the press-formed products 101, 102, and 103 were in the range of 300 to 1200 MPa. Moreover, the bending radii of the parts 1 to 7 of the press-formed products 101, 102, and 103 were in the range of 7 to 20 mm. However, for the parts 4 to 7 other than the parts 1 and 2 where the bending process was performed and the part 3 where the drawing process was performed, the bending radius could not be measured due to the shape of the press-formed product.

(Measurement of crystal grain distortion of press-molded products)
For the parts 1 to 7 of the press-formed products 101, 102, 103, the structure analysis of crystal grain distortion by EBSP (evaluation of area ratio satisfying CI ≦ 0.1, IQ, KAM) and the distortion of crystal grains by XRD Measurements were made. Here, the measurement equipment, the measurement method, and the measurement conditions are the same as in the case of the test piece described above. Table 3 shows the measurement results.

(Creation of delayed fracture resistance evaluation map)
Based on the measurement results, a delayed fracture resistance evaluation map was prepared. Specifically, the amount of hydrogen, the bending radius or crystal grain distortion, and the residual stress in each test piece were plotted on a three-dimensional graph, and each point was connected in a matrix to create a delayed fracture occurrence boundary region 8. Further, a delayed fracture estimated boundary region 9 was created outside the delayed fracture occurrence boundary region 8 based on a function obtained from the amount of hydrogen in the test piece, bending radius or crystal grain distortion, and residual stress. Then, the amount of hydrogen, bending radius, crystal grain distortion, and residual stress in the steel sheet molded product were plotted on a three-dimensional graph.

  The results of creating a delayed fracture resistance evaluation map are shown in FIGS. Here, the delayed fracture resistance evaluation map in this example is Comparative Example 1 (an evaluation method using “bending radius” as an index of delayed fracture resistance) and Example 1 (an area where “CI ≦ 0.1”). Rate "as an index for delayed fracture resistance), Example 2 (an evaluation method using" IQ "as an index for delayed fracture resistance), and Example 3 (" KAM "for delayed fracture resistance) Evaluation method used as index) Five methods were prepared in Example 4 (an evaluation method used as an index of "XRD strain" delayed fracture resistance). Comparative Example 1 is an evaluation method using a conventional “bending radius” as an index, and Examples 1 to 4 are evaluation methods according to the present invention.

  7 to 11, those plotted with ▲ are measured values of each test piece, and those plotted with ◯ or × are measured values of the parts 1 to 7 of the press-formed products 101, 102, and 103. And ◯ indicates a measured value of a site where cracks are estimated not to occur, and x indicates a measured value of sites where cracks are estimated to occur.

  Even when the plot points of ○ appear to be included in the delayed fracture occurrence boundary region 8 or the delayed fracture estimated boundary region 9 in the figure, they are actually located below these regions. Even if the plot points do not appear to be included in the delayed fracture occurrence boundary region 8 or the delayed fracture estimated boundary region 9 in the figure, they are actually located inside these regions. Although not shown in FIGS. 7 to 11, the same axes as those in FIG. 5 are the X axis, the Y axis, and the Z axis. Hereinafter, individual evaluation results will be described.

(Comparative Example 1)
FIG. 7 is a delayed fracture resistance evaluation map created based on an evaluation result using “bending radius” as an index of delayed fracture resistance in the conventional evaluation method. 7A, 7 </ b> B, and 7 </ b> C, the X axis is a bending radius, the Y axis is a residual stress, and the Z axis is a hydrogen amount (diffusible hydrogen amount). Here, referring to FIGS. 7A, 7 </ b> B, and 7 </ b> C, it can be seen that the sum of the measurement points of ◯ and X is 6 or less in any map. Here, the measurement points of ◯ and X in the map correspond to the parts 1 to 7 of the press-formed products 101, 102, 103. Therefore, it can be seen that when the “bending radius” is used as an index of delayed fracture resistance, the delayed fracture resistance cannot be properly evaluated for some of the parts 1 to 7 of the press-formed products 101, 102, 103.

  In the column of “Comparative Example 1” in the “Evaluation Result” column of Table 3, the evaluation results of cracks of the press-formed products 101, 102, 103 estimated by the evaluation method according to Comparative Example 1 are shown. Here, the “presence / absence of cracks” shown in the column “Measurement results” in Table 3 indicates whether or not the press-formed products 101, 102, 103 are actually cracked. Therefore, if the delayed fracture resistance evaluation is accurate, the “Measurement Result” column and the “Evaluation Result” column in Table 3 must be completely the same. In Table 3, the portions where the measurement results and the evaluation results do not match are indicated by thick frames.

  Referring to Table 3, in any of the press-formed products 101, 102, 103, it is possible to evaluate delayed fracture resistance of the part 5 (punched part), the part 6 (vertical wall part), and the part 7 (flat part). could not. That is, the part 5 and the part 6 have a very large bending radius due to the processing, and have a shape determined that the bending processing is not applied, and thus cannot be plotted on the map of FIG. Moreover, since the bending process itself was not added, the site | part 7 was not able to be plotted on the above-mentioned map of FIG. Further, the portion 3 of the C steel-shape C (press-formed product 103) is erroneously evaluated as “O” without cracking although cracking actually occurs. Therefore, it can be seen that when the “bending radius” is used as an index of delayed fracture resistance, the delayed fracture resistance cannot be evaluated for some of the portions 1 to 7 of the press-formed products 101, 102, 103.

Example 1
FIG. 8 is a delayed fracture resistance evaluation map created based on an evaluation result using “area ratio satisfying CI ≦ 0.1” as an index of delayed fracture resistance in the evaluation method according to the present invention. 8 (a), (b), and (c), the X-axis is the area ratio satisfying CI ≦ 0.1, the Y-axis is the residual stress, and the Z-axis is the hydrogen amount (diffusible hydrogen amount). Here, with reference to FIGS. 8A, 8B, and 8C, it can be seen that the total of the measurement points of ◯ and X is 7 in any map. Therefore, it can be seen that by using “area ratio satisfying CI ≦ 0.1” as an index of delayed fracture resistance, the delayed fracture resistance of all parts can be evaluated. Further, referring to the column of “Example 1” in the column of “Evaluation result” in Table 3, it can be seen that any of the evaluation results is the same as the “Measurement result” in Table 3. Therefore, it can be seen that the evaluation method using “area ratio satisfying CI ≦ 0.1” as an index of delayed fracture resistance is accurate.

(Example 2)
FIG. 9 is a delayed fracture resistance evaluation map created based on an evaluation result using “IQ” as an index of delayed fracture resistance in the evaluation method according to the present invention. 9A, 9B, and 9C, the X-axis is IQ, the Y-axis is residual stress, and the Z-axis is the amount of hydrogen (diffusible hydrogen amount). Here, referring to FIGS. 9A, 9 </ b> B, and 9 </ b> C, it can be seen that the total of the measurement points of ◯ and X is 7 in any map. Therefore, it can be seen that the delayed fracture resistance of all the parts can be evaluated by using “IQ” as an index of delayed fracture resistance. Further, referring to the column of “Example 2” in the column of “Evaluation result” in Table 3, it can be seen that any of the evaluation results is the same as the “Measurement result” in Table 3. Therefore, it can be seen that the evaluation method using “IQ” as an index of delayed fracture resistance is accurate.

(Example 3)
FIG. 10 is a delayed fracture resistance evaluation map created based on an evaluation result using “KAM” as an index of delayed fracture resistance in the evaluation method according to the present invention. 10A, 10B, and 10C, the X axis is KAM, the Y axis is the residual stress, and the Z axis is the amount of hydrogen (diffusible hydrogen amount). Here, referring to FIGS. 10A, 10 </ b> B, and 10 </ b> C, it can be seen that the total of the measurement points of ◯ and X is 7 in any map. Therefore, it can be seen that the delayed fracture resistance of all the parts can be evaluated by using “KAM” as an index of delayed fracture resistance. Further, referring to the column of “Example 3” in the column of “Evaluation result” in Table 3, it can be seen that any of the evaluation results is the same as the “Measurement result” in Table 3. Therefore, it can be seen that the evaluation method using “KAM” as an index of delayed fracture resistance is accurate.

Example 4
FIG. 11 is a delayed fracture resistance evaluation map created based on an evaluation result using “XRD strain (crystal grain strain due to X-ray diffraction)” as an index of delayed fracture resistance in the evaluation method according to the present invention. is there. In FIGS. 11A, 11B, and 11C, the X-axis is XRD strain, the Y-axis is residual stress, and the Z-axis is the amount of hydrogen (diffusible hydrogen amount). Here, referring to FIGS. 11A, 11 </ b> B, and 11 </ b> C, it can be seen that the total of the measurement points of ○ and X is 7 in any map. Therefore, it can be understood that the delayed fracture resistance of all the parts can be evaluated by using “XRD strain” as an index of delayed fracture resistance. Further, referring to the column of “Example 4” in the column of “Evaluation result” in Table 3, it can be seen that any of the evaluation results is the same as the “Measurement result” in Table 3. Therefore, it can be seen that the evaluation method using “XRD strain” as an index of delayed fracture resistance is accurate.

  Thus, according to the method for evaluating delayed fracture resistance of a steel sheet molded article according to the present invention, instead of the “bending radius” that is a conventional evaluation index of delayed fracture resistance, a “crystal grain distortion” evaluation index and By doing so, the delayed fracture resistance of the steel sheet molded product can be more accurately evaluated in the relationship of “crystal grain strain−residual stress−hydrogen content”. In other words, by measuring the distortion of crystal grains in steel sheet molded products, a more precise evaluation of delayed fracture resistance considering the distortion of grains, dislocations, increase of defects, collapse of crystal grains / structure morphology, etc. Can be performed.

1 part 2 part 3 part 4 part 5 part 6 part 7 part 8 Delayed fracture occurrence boundary area 9 Delayed fracture estimated boundary area 10 Steel plate surface layer part 20 Spherical head part 30 Near punched hole 40 Head part 100 Test piece 101 Press-formed product (Steel A-Shape A)
102 Press-formed products (Steel B-Shape B)
103 Press-formed product (C steel-shape C)
A Strain B Void C Cell wall a Measured value b Measured value c Measured value d Measured value e Measured value f Measured value S1 Specimen hydrogen introduction process S2 Specimen hydrogen content measurement process S3 Specimen crystal grain strain measurement process S4 Delayed fracture condition Derivation process S5 Molded crystal grain strain measurement process S6 Delayed fracture hydrogen quantity estimation process

Claims (10)

A method for evaluating delayed fracture resistance of steel sheet molded products,
Evaluation of the steel sheet molded product using a relationship in which the amount of hydrogen contained in the steel material when delayed fracture occurs and the distortion of crystal grains in the structure of the steel material when delayed fracture occurs are associated with each other A steel sheet molded article characterized by performing a delayed fracture hydrogen amount estimation step for estimating the amount of hydrogen that causes the delayed fracture in the evaluation site by obtaining a hydrogen amount corresponding to the distortion of crystal grains in the structure of the site. Evaluation method of delayed fracture resistance. A method for evaluating delayed fracture resistance of steel sheet molded products,
Evaluation of the steel sheet molded product using a relationship in which the amount of hydrogen contained in the steel material when delayed fracture occurs and the distortion of crystal grains in the structure of the steel material when delayed fracture occurs are associated with each other Performing a delayed fracture crystal strain estimation step of estimating a strain of a crystal grain that causes the delayed fracture in the evaluation site by obtaining a strain of the crystal grain corresponding to the amount of hydrogen contained in the site. Evaluation method of delayed fracture resistance of steel sheet molded products. In the delayed fracture hydrogen amount estimation step,
The amount of hydrogen contained in the steel material when the delayed fracture occurs, the distortion of crystal grains in the structure of the steel material when the delayed fracture occurs, and the residual stress of the steel material when the delayed fracture occurs, 2. The steel sheet according to claim 1, wherein the amount of hydrogen corresponding to the distortion of the crystal grains in the structure of the evaluation part of the steel sheet molded article and the residual stress of the evaluation part is obtained using the relationship in which Evaluation method of delayed fracture resistance of molded products. In the delayed fracture crystal grain strain estimation step,
The amount of hydrogen contained in the steel material when the delayed fracture occurs, the distortion of crystal grains in the structure of the steel material when the delayed fracture occurs, and the residual stress of the steel material when the delayed fracture occurs, 3. The steel sheet according to claim 2, wherein the amount of hydrogen contained in the evaluation part of the steel sheet molded product and the distortion of the crystal grains corresponding to the residual stress of the evaluation part are obtained using the relationship in which Evaluation method of delayed fracture resistance of molded products. Before the delayed fracture hydrogen amount estimation step or the delayed fracture crystal grain strain estimation step,
A test piece hydrogen introduction step of creating a test piece subjected to molding and introducing hydrogen into the test piece until delayed fracture occurs in the test piece,
A test piece measurement step for measuring the hydrogen amount and crystal grain distortion of the test piece when the delayed fracture occurs,
Associating the amount of hydrogen of the test piece measured in the test piece measurement step and the distortion of the crystal grains, the delayed fracture condition derivation step for deriving the relationship,
The method for evaluating delayed fracture resistance of a steel sheet molded article according to claim 1 or 2, wherein: Before the delayed fracture hydrogen amount estimation step or the delayed fracture crystal grain strain estimation step,
A test piece hydrogen introduction step of creating a test piece subjected to molding processing, applying residual stress to the test piece, and introducing hydrogen into the test piece until delayed fracture occurs,
A test piece measurement step for measuring the hydrogen amount and crystal grain distortion of the test piece when the delayed fracture occurs,
A delayed fracture condition deriving step of deriving the relationship by associating the hydrogen amount of the test piece measured in the test piece measuring step, the distortion of crystal grains, and the residual stress applied to the test piece. The method for evaluating delayed fracture resistance of a steel sheet molded article according to claim 3 or 5.   The distortion of the crystal grains is represented by an area ratio at which a CI (Confidence Index) for determining an orientation in EBSP is 0.1 or less. The evaluation method of the delayed fracture resistance of the steel plate molded article described in 1.   The delay of the steel sheet molded article according to any one of claims 1 to 6, wherein the distortion of the crystal grains is expressed by IQ (Image Quality) of orientation determination in EBSP. Destructive evaluation method.   The distortion of the crystal grain is represented by KAM (Kernel Average Misorientation) of orientation determination in EBSP, and the steel plate molded product according to any one of claims 1 to 6, wherein Evaluation method of delayed fracture property.   The distortion of the crystal grain is represented by the distortion measured by XRD (X-ray diffraction), The steel sheet molded article according to any one of claims 1 to 6 characterized by things. Evaluation method of delayed fracture resistance.