JP4901346B2 - High-strength steel sheet with excellent stretch flangeability and fatigue characteristics - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue characteristics, which is suitable as a material for an automobile underbody member.

  There is an increasing demand for high strength and light weight hot-rolled steel sheets for automobiles from the viewpoint of improving the safety of automobiles and improving fuel efficiency leading to environmental conservation. Among the parts for automobiles, the weight of frames and arms, especially called undercarriage systems, occupies a high proportion of the weight of the entire vehicle body, so by increasing the strength of the materials used for these parts, It becomes possible to realize the weight reduction. Moreover, the material used for this suspension system is required to have high fatigue characteristics from the viewpoint of durability against vibration during running.

  However, as the strength and fatigue resistance increase, the hole expansibility tends to decrease in the same way as the ductility, and in the application of high-strength steel sheets to the undercarriage system of automobiles with complex shapes, Hole expandability is an important consideration.

  For this reason, several steel plates have been proposed for the purpose of achieving both mechanical strength characteristics, fatigue characteristics and hole expansibility (workability). For example, Patent Document 1 proposes a steel plate (generally referred to as a DP steel plate) in which fine Cu precipitates or solid solutions are dispersed in a ferrite-structure and martensite-phase composite structure steel sheet. In the disclosed technique shown in Patent Document 1, Cu precipitates having a particle size of 2 nm or less composed of solid solution of Cu or Cu alone are very effective for improving fatigue characteristics, and workability is not impaired. As a result, the composition ratio of various components is limited.

  Although such DP steel sheet is excellent in the balance between strength and ductility and fatigue characteristics, it is known that the stretch flangeability evaluated by the hole expansion test is still inferior. One reason for this is that DP steel is a composite of a soft ferrite phase and a hard martensite phase, so the boundary between both phases cannot follow deformation during hole expansion and tends to be the starting point of fracture. It is believed that there is.

On the other hand, a high-strength hot-rolled steel sheet that satisfies not only fatigue characteristics but also severe stretch flangeability requirements for recent materials for wheels and suspension members has been proposed (see, for example, Patent Document 2). ). In the disclosed technique of Patent Document 2, the main phase is made a bainite structure by reducing C as much as possible, and a ferrite structure that is solid solution strengthened or precipitation strengthened is contained in an appropriate volume ratio, and the hardness of these ferrite and bainite. The gist is to reduce the difference and avoid the formation of coarse carbides.
Japanese Patent Laid-Open No. 11-199973 Japanese Patent Laid-Open No. 2001-200331

  By the way, the high-strength hot-rolled steel sheet having a steel sheet structure mainly composed of a bainite phase and suppressing the formation of coarse carbides as disclosed in the above-mentioned Patent Document 2 certainly exhibits excellent stretch flangeability, but Cu It cannot be said that the fatigue properties are necessarily superior to those of the contained DP steel sheet. In addition, the generation of cracks cannot be prevented when severe hole enlargement processing is performed only by suppressing the formation of coarse carbides. According to the study by the present inventors, it has been found that these causes are the presence of elongated sulfide inclusions mainly composed of MnS in the steel sheet. When subjected to repeated deformation, internal defects are generated around the extended coarse MnS inclusions on or near the surface layer, and the fatigue properties are deteriorated by propagating as cracks, and also extended coarse MnS inclusions. This is because the object tends to become a starting point of crack generation during the hole expanding process.

  For this reason, it is desirable to make the MnS inclusions in steel as fine as possible without stretching as much as possible.

  However, Mn, together with C and Si, is an element that contributes effectively to increasing the strength of materials. In high-strength steel sheets, it is common to set the Mn concentration high to ensure strength, and secondary refining. If the process of removing S is not carried out in the process, the S concentration will be 50 ppm or more. For this reason, MnS is usually present in the slab. When the slab is hot-rolled and cold-rolled, MnS is easily deformed, so that it becomes a stretched MnS-based inclusion, which causes a decrease in fatigue characteristics and stretch flangeability (hole expansion workability). However, no example of suggesting a hot-rolled steel sheet excellent in stretch flangeability and fatigue properties from the viewpoint of MnS precipitation / deformation control has been found.

  Therefore, the present invention has been devised in view of the above-mentioned problems, and its object is to precipitate as fine MnS in the slab, and further, without being deformed during rolling, causing cracking. An object of the present invention is to provide a high-strength steel sheet excellent in stretch flangeability and fatigue characteristics, which is improved in stretch flangeability and fatigue characteristics, by being dispersed in the steel sheet as fine spherical inclusions which are difficult to start.

  In order to solve the above-mentioned problems, the present inventor precipitated as fine MnS in the slab and further dispersed in the steel sheet as fine spherical inclusions that are not deformed during rolling and are unlikely to start cracking. Research focused on elucidating additive elements that do not degrade fatigue properties and methods that cause fatigue. As a result, MnS was deposited on fine and hard Ce oxide, La oxide, cerium oxysulfide and lanthanum oxysulfide produced by deoxidation by addition of Ce and La, and deformation of this deposited MnS during rolling Therefore, the stretched coarse MnS is remarkably reduced in the steel sheet, and it becomes difficult for these MnS inclusions to become the starting point of crack generation and the path of crack propagation during repetitive deformation and hole expansion processing. It has been clarified that it leads to improvement in fatigue resistance as described above.

  The summary of the high-strength steel sheet excellent in stretch flangeability and fatigue characteristics according to the present invention is as follows.

  (1) By mass%, C: 0.03-0.10%, Si: 0.08-1.5%, Mn: 1.0-3.0%, P: 0.05% or less, S: 0.002% or more, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, acid-soluble Ti: less than 0.008%, total of one or two of Ce or La : A steel plate containing 0.0005 to 0.04%, the balance being iron and inevitable impurities, inclusions existing in the steel plate and having an equivalent circle diameter of 1 μm or more, and a major axis / minor axis of 5 The number ratio of the above stretched inclusions is 20% or less.

  (2) By mass%, C: 0.03-0.10%, Si: 0.08-1.5%, Mn: 1.0-3.0%, P: 0.05% or less, S: 0.002% or more, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, acid-soluble Ti: less than 0.008%, total of one or two of Ce or La : 0.0005-0.04% steel, the balance being iron and unavoidable impurities, and the steel plate contains one or two of Ce or La oxide or oxysulfide with MnS It is characterized by containing 10% or more of precipitated inclusions in the number ratio.

(3) By mass%, C: 0.03-0.10%, Si: 0.08-1.5%, Mn: 1.0-3.0%, P: 0.05% or less, S: 0.002% or more, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, acid-soluble Ti: less than 0.008%, total of one or two of Ce or La : A steel plate containing 0.0005 to 0.04%, the balance being iron and inevitable impurities, inclusions existing in the steel plate and having an equivalent circle diameter of 1 μm or more, and a major axis / minor axis of 5 A volume number density of the above-described stretched inclusions is 1.0 × 10 ⁴ pieces / mm ³ or less.

(4) By mass%, C: 0.03-0.10%, Si: 0.08-1.5%, Mn: 1.0-3.0%, P: 0.05% or less, S: 0.002% or more, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, acid-soluble Ti: less than 0.008%, total of one or two of Ce or La : 0.0005-0.04% steel, the balance being iron and unavoidable impurities, and the steel plate contains one or two of Ce or La oxide or oxysulfide with MnS The volume number density of the inclusions deposited is 1.0 × 10 ³ pieces / mm ³ or more.

  (5) By mass%, C: 0.03-0.10%, Si: 0.08-1.5%, Mn: 1.0-3.0%, P: 0.05% or less, S: 0.002% or more, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, acid-soluble Ti: less than 0.008%, total of one or two of Ce or La : A steel plate containing 0.0005 to 0.04%, the balance being iron and inevitable impurities, an inclusion with an equivalent circle diameter of 1 μm or more present in the steel plate, and a major axis / minor axis of 5 or more A high-strength steel sheet excellent in stretch flangeability and fatigue characteristics, characterized in that the average equivalent circle diameter of the stretched inclusions is 10 μm or less.

  (6) By mass%, C: 0.03-0.10%, Si: 0.08-1.5%, Mn: 1.0-3.0%, P: 0.05% or less, S: 0.002% or more, N: 0.0005 to 0.01%, acid-soluble Al: 0.01% or less, acid-soluble Ti: less than 0.008%, total of one or two of Ce or La : 0.0005-0.04% steel, the balance being iron and unavoidable impurities, and the steel plate contains one or two of Ce or La oxide or oxysulfide with MnS Precipitated inclusions are present, and the inclusion contains 0.5 to 50% by mass of one or two of Ce or La in an average composition, and is excellent in stretch flangeability and fatigue characteristics High strength steel plate.

  According to the method of the present invention, it is precipitated as fine MnS in a slab, and is not deformed during rolling, and is dispersed in a steel plate as fine spherical inclusions that are unlikely to start cracking. A high-strength hot-rolled steel sheet having excellent fatigue characteristics can be obtained.

  Hereinafter, as the best mode for carrying out the present invention, a high-strength steel sheet excellent in stretch flangeability and fatigue characteristics will be described in detail. Hereinafter, the mass% in the composition is simply described as%.

  First, the experiment that led to the completion of the present invention will be described.

  The inventor contains C: 0.07%, Si: 0.2%, Mn: 1.2%, P: 0.01% or less, S: 0.005%, N: 0.003%. The molten steel whose balance is Fe was deoxidized using various elements to produce a steel ingot. The obtained steel ingot was hot-rolled to obtain a 3 mm hot-rolled steel sheet. These manufactured hot-rolled steel sheets were subjected to a hole expansion test and a fatigue test, and the inclusion number density, morphology and average composition in the steel sheets were investigated.

  As a result, it was found that a steel sheet which was deoxidized by adding at least Ce and La after adding Si with almost no deoxidation with Al was most excellent in stretch flangeability and fatigue characteristics. The reason is that MnS is deposited on fine and hard Ce oxide, La oxide, cerium oxysulfide and lanthanum oxysulfide produced by deoxidation by addition of Ce and La, and this MnS is also deposited during rolling. Since deformation hardly occurs, the stretched coarse MnS is remarkably reduced in the steel sheet. As a result, at the time of repeated deformation or hole expansion processing, these MnS inclusions are unlikely to become a starting point of crack generation or a path of crack propagation, which leads to improvement in fatigue resistance as described above.

The reason why Ce oxide, La oxide, cerium oxysulfide and lanthanum oxysulfide are refined is that Ce and La, to which SiO ₂ inclusions initially generated by Si deoxidation are added later, are reduced and decomposed. Forming fine Ce oxide, La oxide, cerium oxysulfide and lanthanum oxysulfide, and interfacial energy between the generated Ce oxide, La oxide, cerium oxysulfide and lanthanum oxysulfide itself and molten steel This is because the aggregation and coalescence after the generation is also suppressed because of the low N.

  Based on the knowledge obtained from these experimental studies, the present inventor has studied the chemical composition conditions of the steel sheet and completed the present invention as will be described below.

  Hereinafter, the reason why the chemical component is limited in the present invention will be described.

C: 0.03-0.10%
C is the most basic element for controlling the hardenability and strength of steel, and contributes to the improvement of fatigue strength by increasing the hardness and depth of the hardened hardened layer. That is, this C is an essential element for securing the strength of the steel sheet, and at least 0.03% is required to obtain a high-strength steel sheet. However, if this C is excessively contained, a cementite phase will be generated even if C is fixed by Ti carbide generation as in the prior art or even if cooling conditions are used. This cementite phase induces work hardening of the steel sheet and is not preferable for improving stretch flange characteristics. For this reason, in this invention, from a viewpoint of improving workability, the density | concentration of C shall be 0.10% or less.

Si: 0.08 to 1.5%
Since Si is a major deoxidizing element in molten steel to which Al or Ti is not added as much as in the present invention, it is extremely important in the present invention. Si also has a function of increasing the number of nucleation sites of austenite during quenching heating, suppressing austenite grain growth, and refining the grain size of the quenched hardened layer. This Si suppresses carbide formation and suppresses a decrease in grain boundary strength due to the carbide. Furthermore, this Si is effective for the formation of a bainite structure and plays an important role in terms of ensuring the strength of the entire material. Reducing the dissolved oxygen concentration in the molten steel, once to generate the SiO ₂ inclusions (Ce added later the SiO ₂ inclusions, in order to fine inclusions by reduction with La) It is necessary to add 0.08% or more of Si. Therefore, in the present invention, the lower limit of Si is set to 0.08%. On the other hand, if the Si concentration is too high, the SiO ₂ concentration in the inclusions becomes high and large inclusions are likely to be formed, and the toughness becomes extremely poor, resulting in increased surface decarburization and surface flaws. Therefore, the fatigue characteristics are worsened. In addition, excessive addition of Si adversely affects weldability and ductility. Therefore, in the present invention, the upper limit of Si is set to 1.5%.

Mn: 1.0-3.0%
Mn is an element useful for deoxidation in the steelmaking stage, and is an element effective for increasing the strength of the steel sheet together with C and Si. In order to obtain such an effect, it is necessary to contain 1.0% or more of this Mn. However, when Mn is contained in an amount exceeding 3.0%, ductility is lowered due to segregation of Mn and increase in solid solution strengthening. Further, since the weldability and the base metal toughness are also deteriorated, the upper limit of Mn is set to 3.0%.

P: 0.05% or less P is effective in that it acts as a substitutional solid solution strengthening element smaller than Fe atoms, but it segregates at the austenite grain boundaries and lowers the grain boundary strength, thereby torsional fatigue strength. And the processability is liable to be deteriorated, so 0.05% or less. Further, if solid solution strengthening is not necessary, it is not necessary to add P, and the lower limit value of P includes 0%.

S: 0.002% or more S is segregated as an impurity, and since S forms a coarse stretch inclusion of MnS and deteriorates stretch flangeability, it is desirable that the concentration be as low as possible. Conventionally, in order to secure stretch flangeability, it has been necessary to extremely low sulfurize the S concentration to less than 0.002%. However, in the present invention, MnS is precipitated on the fine and hard Ce oxide, La oxide, cerium oxysulfide, and lanthanum oxysulfide, and deformation hardly occurs during rolling, thereby preventing inclusions from being stretched. The upper limit value of the S concentration is not particularly specified.

  Further, in order to reduce the S concentration to less than 0.002%, which is conventional, it is necessary to considerably strengthen the desulfurization treatment by secondary refining, and the desulfurization treatment cost for realizing the concentration becomes too high. In addition, since the effect of controlling the form of MnS is hardly exhibited, the lower limit value of the S concentration is set to 0.002%.

N: 0.0005 to 0.01%
N is an element that is inevitably mixed in steel because nitrogen in the air is taken in during the treatment of molten steel. N forms nitrides with Al, Ti, etc., and promotes refinement of the base material structure. However, if this N is added excessively, coarse precipitates are generated even if a small amount of Al or a small amount of Ti is used, and the stretch flangeability is deteriorated. For this reason, in the present invention, the upper limit of the concentration of N is set to 0.01%. On the other hand, if the N concentration is less than 0.0005%, the cost increases, so 0.0005% is set as the lower limit.

Acid-soluble Al: 0.01% or less Acid-soluble Al is desirable to be suppressed as much as possible because its oxides are likely to be clustered and become coarse and deteriorate stretch flangeability and fatigue characteristics. However, up to 0.01% is permitted as a preliminary deoxidizer. This is because when the acid-soluble Al concentration exceeds 0.01%, the Al ₂ O ₃ content in the inclusions exceeds 50%, and inclusions cluster. From the viewpoint of preventing clustering, the acid-soluble Al concentration should be low, and the lower limit includes 0%. The acid-soluble Al concentration is an analytical method using the measurement of the concentration of Al dissolved in an acid. The dissolved Al dissolves in an acid and Al ₂ O ₃ does not dissolve in an acid. Here, examples of the acid include a mixed acid mixed at a ratio (mass ratio) of hydrochloric acid 1, nitric acid 1, and water 2. By using such an acid, it can be separated into Al soluble in acid and Al ₂ O ₃ not soluble in acid, and the acid soluble Al concentration can be measured.

Acid-soluble Ti: less than 0.008% Acid-soluble Ti also has an oxide that can easily be coarsened because its oxides are likely to cluster and form coarse TiN inclusions by combining with N in the steel. Molten Ti is less than 0.008%, and the lower limit includes 0%. The acid-soluble Ti concentration is an analytical method that measures the concentration of Ti dissolved in an acid, and uses that dissolved Ti dissolves in an acid and Ti oxide does not dissolve in an acid. Here, examples of the acid include a mixed acid mixed at a ratio (mass ratio) of hydrochloric acid 1, nitric acid 1, and water 2. By using such an acid, it can be separated into Ti soluble in acid and Ti oxide not soluble in acid, and the acid-soluble Ti concentration can be measured.

Total of one or two of Ce or La: 0.0005 to 0.04%
Ce and La reduce SiO ₂ produced by Si deoxidation, become a MnS precipitation site, and are hard, fine and difficult to deform during rolling (eg, Ce ₂ O ₃ , CeO ₂ ), cerium. Oxysulfide (eg, Ce ₂ O ₂ S), La oxide (eg, La ₂ O ₃ , LaO ₂ ), lanthanum oxysulfide (eg, La ₂ O ₂ S), Ce oxide-La oxide, or Ce It has the effect of forming inclusions with rhodium oxysulfide-lanthanum oxysulfide as the main phase (50% or more as a guide).

Here, the inclusions may contain a part of MnO, SiO ₂ , or Al ₂ O ₃ depending on deoxidation conditions. However, if the main phase is the above oxide, it sufficiently functions as a MnS precipitation site. However, the effect of making the inclusion fine and hard is not impaired. In order to obtain such inclusions, the total concentration of one or two of Ce or La needs to be 0.0005% or more and 0.04% or less. If the total concentration of one or two of Ce or La is less than 0.0005%, SiO ₂ inclusions cannot be reduced, and if it exceeds 0.04%, cerium oxysulfide and lanthanum oxysulfide are produced in large amounts, and are coarse. It becomes an inclusion and deteriorates stretch flangeability and fatigue characteristics.

  Next, the presence conditions of inclusions in the steel sheet of the present invention will be described. The steel sheet means a rolled sheet obtained through hot rolling or further cold rolling.

In order to obtain a steel sheet excellent in stretch flangeability and fatigue characteristics, it is important to reduce as much as possible the stretched and coarse MnS inclusions in the steel sheet, which are likely to be the starting point of crack generation and the path of crack propagation. The present inventor has experimentally found that MnS inclusions having an equivalent circle diameter of less than 1 μm are harmless as crack initiation points and do not deteriorate stretch flangeability and fatigue characteristics, and the equivalent circle diameter of 1 μm. Since the above inclusions can be easily observed with a scanning electron microscope (SEM) or the like, the shape and composition of the inclusions having a circle equivalent diameter of 1 μm or more in the steel sheet are investigated, and the MnS type inclusions are investigated. Distribution state was evaluated. Here, the equivalent circle diameter is defined as (major axis × minor axis) ^0.5 obtained from the length and minor axis of the inclusion observed in the cross section.
The upper limit of the equivalent circle diameter of the MnS-based inclusion is not particularly specified, but in reality, an MnS-based inclusion of about 1 mm may be observed.

  The ratio of the number of stretched inclusions was determined by analyzing the composition of a plurality of inclusions (for example, about 50) having a circle-equivalent diameter of 1 μm or more selected at random using SEM, and determining the major axis and minor axis of the inclusions from the SEM image. taking measurement. Here, when the elongated inclusion is an inclusion having a major axis / minor axis (ratio of stretching) of 5 or more, the number of the detected elongated inclusions is the total number of inclusions examined (50 in the above example). The ratio of the number of the stretched inclusions can be obtained by dividing by the degree.

  The reason why the inclusion stretching ratio was set to 5 or more is that inclusions with a stretching ratio of 5 or more in the comparative steel sheet to which Ce and La were not added were mostly MnS inclusions. In addition, although the upper limit of the extending | stretching ratio of MnS type inclusions is not prescribed | regulated in particular, MnS type inclusions of about 50 extending | stretching ratio may be observed actually.

As a result, it has been found that stretch flangeability and fatigue characteristics are improved in a steel sheet whose form is controlled to have a number ratio of stretched inclusions having a stretching ratio of 5 or more to 20% or less. That is, when the number ratio of stretch inclusions having a stretch ratio of 5 or more exceeds 20%, the number ratio of MnS-based stretch inclusions that are likely to be the starting point of cracking increases, and the stretch flangeability and fatigue characteristics decrease. In the present invention, the ratio of the number of stretched inclusions having a stretching ratio of 5 or more is 20% or less. Moreover, since the stretch flangeability and fatigue characteristics are so good that there are few stretched MnS inclusions, the lower limit of the number ratio of stretch inclusions having a stretch ratio of 5 or more includes 0%.
Here, an inclusion having a circle equivalent diameter of 1 μm or more and a lower limit value of the number ratio of the drawing inclusions having a drawing ratio of 5 or more means 0% is an inclusion having a circle equivalent diameter of 1 μm or more. This is the case where there are no stretch ratios of 5 or more, or even when the stretch inclusions have a stretch ratio of 5 or more, the equivalent circle diameters are all less than 1 μm.

  In addition, in a steel sheet in which the number ratio of stretched inclusions having a stretching ratio of 5 or more is controlled to 20% or less, MnS is contained in an oxide or oxysulfide of one or two of Ce or La correspondingly. It is in a deposited form. The form of the inclusion is not particularly limited as long as MnS is precipitated in an oxide or oxysulfide composed of one or two kinds of Ce or La, and is not particularly limited. In many cases, MnS is precipitated around a seed oxide or oxysulfide as a nucleus.

  In addition, inclusions in which MnS is precipitated on one or two kinds of oxides or oxysulfides of Ce or La are not easily deformed even during rolling. It has become.

  Here, the spherical inclusions judged not to be stretched are not particularly defined, but are inclusions having a stretching ratio of 3 or less in the steel sheet, preferably inclusions of 2 or less. This is because in the slab stage before rolling, the stretching ratio in the inclusions in which MnS was precipitated on the oxide or oxysulfide of one or two of Ce or La was 3 or less. In addition, since the spherical inclusion that is determined not to be stretched is completely spherical, the stretch ratio is 1, so the lower limit of the stretch ratio is 1.

  Such investigation of the number ratio of inclusions was performed in the same manner as the number ratio investigation of stretched inclusions. As a result, in the steel sheet in which the number ratio of inclusions in the form of MnS precipitated in one or two kinds of oxides or oxysulfides of Ce or La is controlled to be 10% or more, stretch flangeability and fatigue characteristics are improved. It turned out to improve. When the number ratio of inclusions in the form of MnS precipitated on one or two kinds of oxides or oxysulfides of Ce or La is less than 10%, the number ratio of MnS-based stretched inclusions corresponding to this. Too much increases, stretch flangeability and fatigue characteristics deteriorate. For this reason, the number ratio of inclusions in the form in which MnS is precipitated in one or two kinds of oxides or oxysulfides of Ce or La is set to 10% or more. In addition, stretch flangeability and fatigue characteristics are better when a large amount of MnS is precipitated in one or two kinds of oxides or oxysulfides of Ce or La. Therefore, the upper limit of the number ratio is 100%. Including.

  Incidentally, inclusions in the form of MnS precipitated on oxide or oxysulfide consisting of one or two of Ce or La are not easily deformed even during rolling, and the equivalent circle diameter is not particularly specified, It may be 1 μm or more. However, since it is feared that if it is too large, it becomes a starting point of cracking, the upper limit is preferably about 50 μm.

  On the other hand, this inclusion is not easily deformed even during rolling, and when the equivalent circle diameter is less than 1 μm, it does not become a starting point of cracking, so the lower limit of the equivalent circle diameter is not particularly specified.

  Next, the existence condition of inclusions in the steel sheet of the present invention described above is defined by the number density of inclusions per unit volume.

The particle size distribution of the inclusion was carried out by SEM evaluation of the electrolytic surface by the speed method. The SEM evaluation of the electrolytic surface by the speed method is to evaluate the size and number density of inclusions by polishing the surface of the sample piece, performing electrolysis by the speed method, and directly observing the sample surface by SEM. The speed method is a method of electrolyzing the sample surface using 10% acetylacetone-1% tetramethylammonium chloride-methanol to extract inclusions, and the amount of electrolysis is 1 C per 1 cm ² area of the sample surface. Was electrolyzed. The SEM image of the surface electrolyzed in this manner was subjected to image processing, and the frequency (number) distribution with respect to the equivalent circle diameter was obtained. The average equivalent circle diameter was calculated from the frequency distribution of the particle diameters, and the number density of inclusions per volume was also calculated by dividing the frequency by the area of the observed visual field and the depth determined from the amount of electrolysis.

As a result of evaluating the volume number density of inclusions having an equivalent circle diameter of 1 μm or more and an elongation ratio of 5 or more, which becomes the starting point of crack generation and deteriorates stretch flangeability and fatigue characteristics, it is 1.0 × 10 ⁴ pieces / mm ³ or less. It has been found that stretch flangeability and fatigue properties are improved. When the volume number density of stretched inclusions having an equivalent circle diameter of 1 μm or more and a stretching ratio of 5 or more exceeds 1.0 × 10 ⁴ pieces / mm ³ , the number density of MnS-based stretched inclusions that are likely to be the starting point of cracking is increased. Too much and the stretch flangeability and fatigue characteristics deteriorate. Therefore, the volume number density of stretched inclusions having an equivalent circle diameter of 1 μm or more and a stretch ratio of 5 or more is set to 1.0 × 10 ⁴ pieces / mm ³ or less. Further, since the stretched flangeability and fatigue characteristics are better as the number of MnS-based inclusions stretched is smaller, the lower limit value of the volume number density of stretched inclusions having an equivalent circle diameter of 1 μm or more and a stretching ratio of 5 or more is 0%. Including.

  Here, the lower limit value of the volume number density of stretched inclusions having an equivalent circle diameter of 1 μm or more and a stretch ratio of 5 or more means 0%, which is the same as described above.

Further, in a steel sheet whose shape is controlled to have a volume number density of a stretched inclusion having a diameter of 1 μm or more and a draw ratio of 5 or more of 1.0 × 10 ⁴ pieces / mm ³ or less, it is not stretched correspondingly. The MnS inclusions were in the form of MnS deposited on an oxide or oxysulfide of one or two of Ce or La, and the shape was almost spherical inclusions.

  As the form of the inclusion, as described above, MnS may be precipitated in the oxide or oxysulfide of one or two of Ce or La, and is not particularly limited. In many cases, MnS is precipitated around one or two oxides or oxysulfide as a nucleus.

  The spherical inclusion is not particularly specified, but is an inclusion having a drawing ratio of 3 or less in the steel plate, preferably an inclusion having 2 or less. Here, since the stretching ratio is 1 if it is completely spherical, the lower limit of the stretching ratio is 1.

As a result of investigating the volume number density of such inclusions, the volume number density of inclusions in the form of MnS deposited around an oxide or oxysulfide of one or two kinds of Ce or La as a nucleus is 1 It was found that a steel sheet whose precipitation was controlled to 0.0 × 10 ³ pieces / mm ³ or more improved stretch flangeability and fatigue characteristics. When the volume number density of inclusions in the form of MnS precipitated on oxide or oxysulfide of one or two of Ce or La becomes less than 1.0 × 10 ³ / mm ³ , correspondingly, Inclusions in the form of MnS precipitated on oxide or oxysulfide of one or two of Ce or La, because the number ratio of MnS-based stretched inclusions becomes too large, and the stretch flangeability and fatigue characteristics deteriorate. The volume number density is defined as 1.0 × 10 ³ pieces / mm ³ or more. In addition, stretch flangeability and fatigue strength are better when a large amount of MnS is precipitated with one or two oxides of Ce or La or oxysulfide as the core, so the upper limit of the volume number density is It is not specified.

  In addition, the equivalent circle diameter of the inclusion in which MnS is precipitated on the oxide or oxysulfide of one or two kinds of Ce or La is not particularly defined as described above, and may be 1 μm or more. However, if this equivalent circle diameter is too large, there is a concern that cracking will start, so the upper limit is preferably about 50 μm.

  On the other hand, when the circle equivalent diameter of the inclusion is less than 1 μm, there is no problem at all, and therefore the lower limit is not particularly specified.

  Next, the existence condition of the drawn inclusions in the steel sheet of the present invention described above was defined by the upper limit of the equivalent circle diameter. Specifically, as a result of evaluating the average equivalent circle diameter of inclusions having a circle equivalent diameter of 1 μm or more and an elongation ratio of 5 or more, which becomes the starting point of crack generation and deteriorates stretch flangeability and fatigue characteristics, the average of the extension inclusions was evaluated. It has been found that stretch flangeability and fatigue characteristics are improved when the equivalent circle diameter is 10 μm or less. This is because the average circle equivalent diameter of the stretched inclusions increases as the number ratio of stretched inclusions with a circle equivalent diameter of 1 μm or more and a stretch ratio of 5 or more increases. The equivalent diameter is defined as an index. This is presumed that as the amount of Mn and S in the molten steel increases, the number of MnS produced increases and the size of the produced MnS also coarsens.

  Therefore, if the number of stretched inclusions with an equivalent circle diameter of 1 μm or more and a stretch ratio of 5 or more exceeds 10 μm, the number ratio of the stretch inclusions exceeds 20% accordingly, which is the starting point for cracking. Since the number ratio of easy and coarse MnS-based stretch inclusions increases too much, and the stretch flangeability and fatigue characteristics deteriorate, the average equivalent circle diameter of stretch inclusions with a circle equivalent diameter of 1 μm or more and a stretch ratio of 5 or more is 10 μm. The following.

  The definition that the average equivalent circle diameter of stretched inclusions having a circle equivalent diameter of 1 μm or more and a stretching ratio of 5 or more is 10 μm or less is that the inclusion having a circle equivalent diameter of 1 μm or more exists in the steel sheet. This means that the lower limit of the equivalent circle diameter is 1 μm.

  On the other hand, in the steel sheet of the present invention described above, inclusions in which MnS is precipitated as an existence condition of the form in which MnS is precipitated in oxides or oxysulfides of one or two of Ce or La are included. It was defined by the content of the average composition of Ce or La.

  Specifically, as described above, in order to improve stretch flangeability and fatigue characteristics, MnS is precipitated in an oxide or oxysulfide composed of one or two kinds of Ce or La to prevent the extension of MnS. This is very important.

  As the form of the inclusion, as described above, MnS may be precipitated in the oxide or oxysulfide of one or two of Ce or La, and is not particularly limited. In many cases, MnS is precipitated around one or two oxides or oxysulfide as a nucleus.

  The spherical inclusion is not particularly specified, but is an inclusion having a drawing ratio of 3 or less in the steel plate, preferably an inclusion having 2 or less. Here, if it is perfectly spherical, the stretching ratio is 1, so the lower limit of the stretching ratio is 1.

  Therefore, in order to clarify the effective composition for suppressing the stretching of MnS inclusions, a composition analysis of inclusions in the form of MnS precipitated on one or two oxides of Ce or La or oxysulfide was performed. .

  However, if the inclusion has a circle equivalent diameter of 1 μm or more, the observation is easy, and therefore, for convenience, a circle equivalent diameter of 1 μm or more was targeted. However, if the observation is possible, inclusions having an equivalent circle diameter of less than 1 μm may be included.

  In addition, inclusions in which MnS is deposited on oxides or oxysulfides of one or two types of Ce or La are not stretched, so that the stretching ratio is all 3 or less. confirmed. Therefore, a composition analysis was performed on inclusions having an equivalent circle diameter of 1 μm or more and a stretching ratio of 3 or less.

  As a result, stretch flangeability and fatigue when inclusion of 0.5 to 50% of the average composition of one or two of Ce or La in an inclusion having an equivalent circle diameter of 1 μm or more and an elongation ratio of 3 or less is included. It was found that the characteristics were improved. When the average content of one or two of Ce or La in an inclusion having an equivalent circle diameter of 1 μm or more and a stretching ratio of 3 or less is less than 0.5% by mass, one or two of Ce or La Since the number ratio of inclusions in the form of MnS precipitated in the oxide or oxysulfide consisting of seeds is greatly reduced, the number ratio of MnS-based stretch inclusions that are likely to be the starting point of cracking is excessively increased. Stretch flangeability and fatigue characteristics decrease.

  On the other hand, when the average content of one or two of Ce or La in inclusions having an equivalent circle diameter of 1 μm or more and a stretching ratio of 3 or less exceeds 50%, cerium oxysulfide and lanthanum oxysulfide are obtained. Since it is produced in a large amount and becomes a coarse inclusion having an equivalent circle diameter of about 50 μm or more, stretch flangeability and fatigue characteristics are deteriorated.

  Next, the structure of the steel plate will be described.

  In the present invention, stretch flangeability and fatigue properties are improved by controlling MnS inclusions, and the microstructure of the steel sheet is not particularly limited, but in order to obtain excellent stretch flangeability, bainitic It is preferable to have a structure having ferrite as a main phase. The area ratio of the bainitic ferrite phase in the steel sheet is preferably 50% or more, more preferably 80% or more, and still more preferably 100%. Further, the balance can contain 20% or more of bainite phase or polygonal ferrite phase, and it is desirable to avoid the martensite phase as much as possible.

  Next, manufacturing conditions will be described.

  In the present invention, Si, Mn, P, etc. in molten steel blown in a converter and decarburized, or further decarburized using a vacuum degasser, and the C concentration is 0.03 to 0.1%. In this case, deoxidation and component adjustment are performed, and Al or Ti is not added, or when oxygen adjustment is required, acid-soluble Al or acid-soluble Ti is slightly left so that it remains slightly. The components are adjusted by adding Al or Ti, and then adding one or two of Ce or La. The molten steel thus produced is continuously cast to produce a slab.

  For continuous casting, not only is it applied to normal slab continuous casting of about 250 mm thickness, but it is also used for thin slab continuous casting where the mold thickness of blooms and billets and slab continuous casting machines is thinner than usual, for example 150 mm or less. And is fully applicable.

  The hot rolling conditions for producing a high strength hot rolled steel sheet will be described.

  The heating temperature of the slab before hot rolling is preferably 1150 ° C. or higher in order to dissolve carbonitrides in the steel. By solid-dissolving them, the formation of polygonal ferrite is suppressed in the cooling process after rolling, and a structure mainly composed of bainitic ferrite phase which is preferable for stretch flangeability can be obtained. On the other hand, when the heating temperature of the slab before hot rolling exceeds 1250 ° C., oxidation of the slab surface becomes remarkable, and in particular, wedge-shaped surface defects due to selective oxidation of grain boundaries remain after descaling, Since the surface quality after rolling is impaired, the upper limit is preferably set to 1250 ° C.

After being heated to the above temperature range, normal hot rolling is performed. In the process, the finish rolling completion temperature is important when the structure of the steel sheet is controlled. If the finish rolling completion temperature is less than Ar ₃ point + 30 ° C., the crystal grain size of the surface layer portion tends to be coarse, which is not preferable in terms of fatigue characteristics. On the other hand, since easily generated undesirably polygonal ferrite phase for stretch flange formability is Ar ₃ point + 200 ° C. greater, so the upper limit is preferably made Ar ₃ point + 200 ° C..

  Moreover, the average cooling rate of the steel sheet after finish rolling is set to 40 ° C./second or more, and cooling to the range of 300 to 500 ° C. suppresses the formation of polygonal ferrite phase, and mainly bainitic ferrite phase. It is effective to obtain an organization.

  When the average cooling rate is less than 40 ° C./second, a polygonal ferrite phase is easily generated, which is not preferable. On the other hand, there is no need to set an upper limit on the cooling rate in terms of structure control, but too high a cooling rate may cause uneven cooling of the steel sheet, and it is expensive to manufacture equipment that enables such cooling. It is thought that this will lead to an increase in the price of the steel sheet. From such a viewpoint, the upper limit of the cooling rate is preferably set to 100 ° C./second.

  In addition, when the cooling stop temperature is lower than 300 ° C., a martensite phase which is not preferable for stretch flangeability is generated, so the lower limit is set to 300 ° C. Therefore, the winding temperature of the hot-rolled coil is preferably set to 300 ° C. or higher in order to suppress the formation of a martensite phase that extremely deteriorates the stretch flangeability.

  On the other hand, when the temperature exceeds 500 ° C., the formation of polygonal ferrite phase cannot be suppressed, and in steel containing Cu, Cu may precipitate locally in the ferrite phase, which may reduce the effect of improving fatigue properties. The winding temperature is preferably 500 ° C. or lower. Therefore, by winding at 500 ° C. or lower, carbonitride precipitates in the subsequent cooling process, and the amount of solid solution C and N in the ferrite phase is reduced, resulting in improved stretch flangeability.

  Examples of the present invention will be described below together with comparative examples.

The slab which shows a chemical component in Table 1 was hot-rolled on the conditions shown in Table 2, and the hot rolled sheet of thickness 3.2mm was obtained.

  In Table 1, steel numbers (hereinafter referred to as steel numbers) 1, 3, 5, and 7 are composed of compositions within the range of the high-strength steel sheets according to the present invention, and steel numbers 2, 4, 6, No. 8 is a comparative steel deviating from the range of the high-strength steel sheet according to the present invention. Steel Nos. 2, 4, and 6 are slabs containing acid-soluble Al in excess of 0.01%. In Steel No. 8, the total of one or two of Ce or La is less than 0.0005. It is configured as a slab reduced to a maximum.

By the way, in Table 1, steel numbers 1 and 2, steel plates 3 and 4, steel numbers 5 and 6, and steel numbers 7 and 8 can be compared. In addition, the acid-soluble Al and the like are made different from each other after having the same composition.

  In Table 2, as Condition A, the heating temperature is 1250 ° C., the finish rolling completion temperature is 845 ° C., the cooling rate after finish rolling is 75 ° C./second, the winding temperature is 450 ° C., and Condition B is The heating temperature is 1200 ° C., the finish rolling completion temperature is 825 ° C., the cooling rate after finish rolling is 45 ° C./second, and the winding temperature is 450 ° C.

  For Steel No. 1 and Steel No. 2, Condition A, for Steel No. 3 and Steel No. 4, Condition B, for Steel No. 5 and Steel No. 6, Condition A, Furthermore, by applying the condition B to the steel numbers 7 and 8, the influence of the chemical composition can be compared under the same manufacturing conditions.

  As basic properties of the steel sheet thus obtained, strength, ductility, stretch flangeability, and fatigue limit ratio were examined.

  In addition, as the state of the presence of the stretched inclusions in the steel sheet, the number ratio, volume number density, and average equivalent circle diameter of the inclusions with a stretching ratio of 5 or more were examined for all inclusions of 1 μm or more.

  Furthermore, as the state of the presence of unstretched inclusions in the steel sheet, inclusions in which MnS is precipitated on oxides or oxysulfides of one or two types of Ce or La, targeting inclusions of 1 μm or more in all. The average value of the number ratio and the volume number density and the total content of one or two of Ce or La in inclusions having a stretching ratio of 3 or less were examined.

  The reason why inclusions of 1 μm or more are used is that, in addition to easy observation, inclusions of less than 1 μm do not affect stretch flangeability and deterioration of fatigue characteristics.

The results are shown in Table 3 for each combination of steel and rolling conditions.

The strength and ductility were obtained by a tensile test of a JIS No. 5 specimen taken in parallel with the rolling direction. Stretch flangeability is measured by measuring the hole diameter D (mm) at the time when a through-thickness crack is generated by expanding a punched hole with a diameter of 10 mm with a 60 ° conical punch in the center of a 150 mm × 150 mm steel plate. The hole expansion value λ = (D−10) / 10. Further, the fatigue limit ratio used as an index representing the fatigue characteristics is a value obtained by dividing the 2 × 10 ⁶ time strength (σW) obtained by a method according to JIS Z 2275 by the strength (σB) of the steel sheet (σW / σB) was evaluated.

  Note that the test piece is a No. 1 test piece defined in the same standard, with a parallel part of 25 mm, a radius of curvature R of 100 mm, and a thickness of 3.0 mm obtained by equally grinding both surfaces of the original plate (hot rolled plate). .

  Further, the inclusions were observed by SEM, and the major axis and the minor axis were measured for 50 inclusions having a circle-equivalent diameter of 1 μm or more selected at random. Further, using the quantitative analysis function of SEM, composition analysis was performed on 50 inclusions with a diameter of 1 μm or more selected at random. Using these results, the number ratio of inclusions having a stretching ratio of 5 or more, the average equivalent circle diameter of inclusions having a stretching ratio of 5 or more, and oxides or oxysulfides of one or two types of Ce or La were found to contain MnS. The average value of the total number of one kind or two kinds of Ce or La in the number ratio of the inclusions deposited and the inclusion ratio of 3 or less in the stretching ratio was obtained. Further, the volume number density of inclusions by shape was calculated by SEM evaluation of the electrolytic surface by the speed method.

As apparent from Table 3, in steel Nos. 1, 3, 5, and 7 to which the method of the present invention was applied, by depositing MnS on an oxide or oxysulfide of one or two of Ce or La, Stretched MnS inclusions could be reduced in the steel sheet. That is, the ratio of the number of inclusions in which MnS is precipitated in one or two oxides or oxysulfides of Ce or La in the steel sheet is 10% or more, and the volume number density of the inclusions is 1.0 × 10 ^By making the average content of one or two of Ce or La in the inclusions of ³ pieces / mm ³ or more and in the steel sheet with a stretching ratio of 3 or less 0.5% to 50%, The number ratio of stretched inclusions with a circle equivalent diameter of 1 μm or more and a stretch ratio of 5 or more is 20% or less, the volume number density of the inclusions is 1.0 × 10 ⁴ pieces / mm ³ or less, and the average equivalent circle diameter of the inclusions Can be made 10 μm or less. As a result, compared with the comparative steel, steel Nos. 1, 3, 5, and 7 as the steels of the present invention were able to obtain steel plates that were excellent in stretch flangeability and fatigue characteristics. However, in the comparative steels (steel Nos. 2, 4, 6, and 8), the distribution of the elongated MnS inclusions and the inclusions in which MnS is precipitated in the oxide or oxysulfide of one or two of Ce or La. Since the state is different from the distribution state defined in the present invention, the MnS inclusions stretched during the processing of the steel sheet became the starting point of cracking, and the stretch flangeability and fatigue characteristics were reduced.

Claims (6)

% By mass
C: 0.03-0.10%,
Si: 0.08 to 1.5%,
Mn: 1.0 to 3.0%
P: 0.05% or less,
S: 0.002% or more,
N: 0.0005 to 0.01%,
Acid-soluble Al: 0.01% or less,
Acid soluble Ti: less than 0.008%,
Total of one or two of Ce or La: 0.0005 to 0.04%,
The balance is a steel plate made of iron and inevitable impurities,
Stretch flangeability and fatigue characteristics, characterized in that inclusions with an equivalent circle diameter of 1 μm or more present in the steel sheet and the ratio of the number of drawn inclusions having a major axis / minor axis of 5 or more are 20% or less Excellent high strength steel plate. % By mass
C: 0.03-0.10%,
Si: 0.08 to 1.5%,
Mn: 1.0 to 3.0%
P: 0.05% or less,
S: 0.002% or more,
N: 0.0005 to 0.01%,
Acid-soluble Al: 0.01% or less,
Acid soluble Ti: less than 0.008%,
Total of one or two of Ce or La: 0.0005 to 0.04%,
The balance is a steel plate made of iron and inevitable impurities,
The steel sheet contains 10% or more of inclusions in which MnS is precipitated in one or two kinds of oxides or oxysulfides of Ce or La, and is excellent in stretch flangeability and fatigue characteristics. High strength steel plate. % By mass
C: 0.03-0.10%,
Si: 0.08 to 1.5%,
Mn: 1.0 to 3.0%
P: 0.05% or less,
S: 0.002% or more,
N: 0.0005 to 0.01%,
Acid-soluble Al: 0.01% or less,
Acid soluble Ti: less than 0.008%,
Total of one or two of Ce or La: 0.0005 to 0.04%,
The balance is a steel plate made of iron and inevitable impurities,
The volume number density of the inclusions having an equivalent circle diameter of 1 μm or more present in the steel sheet and having a major axis / minor axis of 5 or more is 1.0 × 10 ⁴ pieces / mm ³ or less. High-strength steel sheet with excellent stretch flangeability and fatigue characteristics. % By mass
C: 0.03-0.10%,
Si: 0.08 to 1.5%,
Mn: 1.0 to 3.0%
P: 0.05% or less,
S: 0.002% or more,
N: 0.0005 to 0.01%,
Acid-soluble Al: 0.01% or less,
Acid soluble Ti: less than 0.008%,
Total of one or two of Ce or La: 0.0005 to 0.04%,
The balance is a steel plate made of iron and inevitable impurities,
The steel sheet is characterized in that the volume number density of inclusions in which MnS is precipitated on one or two kinds of oxides or oxysulfides of Ce or La is 1.0 × 10 ³ pieces / mm ³ or more. High strength steel sheet with excellent stretch flangeability and fatigue characteristics. % By mass
C: 0.03-0.10%,
Si: 0.08 to 1.5%,
Mn: 1.0 to 3.0%
P: 0.05% or less,
S: 0.002% or more,
N: 0.0005 to 0.01%,
Acid-soluble Al: 0.01% or less,
Acid soluble Ti: less than 0.008%,
Total of one or two of Ce or La: 0.0005 to 0.04%,
The balance is a steel plate made of iron and inevitable impurities,
Stretch flangeability and fatigue characteristics characterized by inclusions having an equivalent circle diameter of 1 μm or more present in the steel sheet, and an average equivalent circle diameter of elongated inclusions having a major axis / minor axis of 5 or more being 10 μm or less. Excellent high strength steel plate. % By mass
C: 0.03-0.10%,
Si: 0.08 to 1.5%,
Mn: 1.0 to 3.0%
P: 0.05% or less,
S: 0.002% or more,
N: 0.0005 to 0.01%,
Acid-soluble Al: 0.01% or less,
Acid soluble Ti: less than 0.008%,
Total of one or two of Ce or La: 0.0005 to 0.04%,
The balance is a steel plate made of iron and inevitable impurities,
In the steel sheet there are oxides consisting of one or two of Ce or La or inclusions in which MnS is precipitated in oxysulfide, and the inclusions contain one or two kinds of Ce or La in average composition. A high-strength steel sheet excellent in stretch flangeability and fatigue characteristics, characterized by containing a total of 0.5 to 50% by mass.