JP3922805B2 - Manufacturing method of high-tensile steel with excellent low-temperature toughness - Google Patents

Description

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【発明の効果】
本発明は、平均フェライト粒径が１μｍ以下の超細粒フェライト組織を鋼材の全厚にわたって達成させて、 vＴrSが−１５０℃以下の極めて優れた低温靱性と脆性き裂伝播停止特性とを有する、引張強度が約５００MPa から９５０MPa 超級までの高張力鋼材を製造するための方法を提供するものであり、構造物の安全性を飛躍的に高めることが可能な高張力鋼材を経済性、生産性を損なうことなく製造できる手段として、産業上の価値の極めて高い発明であると言える。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a high-tensile steel material used for a structural member requiring low temperature toughness. Steel materials produced by this method can be used for general welded steel structures such as marine structures, pressure vessels, shipbuilding, bridges, buildings, line pipes, etc., especially marine structures that require low temperature toughness, It is useful as a steel plate for structures such as shipbuilding. In addition, the present invention can also be applied to steel pipe materials or shape steels that are used as structural members and require low temperature toughness.
[0002]
[Prior art]
Since high-tensile steel materials typified by thick steel plates are used as structural members, low temperature toughness is required from the viewpoint of ensuring the safety of structures. Various methods for improving the low temperature toughness in high-tensile steel materials have been proposed, but the refinement of crystal grain size is almost the only method for improving the low temperature toughness without causing other characteristic deterioration.
[0003]
Recently, the present inventors have already developed a technique for dramatically improving the low-temperature toughness by reducing the ferrite (also referred to as α) grain size to about 3 μm or less and improving the low-temperature toughness. It is disclosed in No. Gazette. The requirements of this technology are that the transformation α is refined by optimizing the heating and hot rolling conditions in the austenite (also referred to as γ) region and the γ / α two-phase region, and the α is recrystallized by processing. It is to obtain ultrafine grains at least.
This technique is practically suitable for achieving ultrafine graining of about 1 to 3 μm, and has a fracture surface transition temperature (vTrS) of 2 mm V notch Charpy impact test of about −100 ° C. to −120 ° C. Although it is a sufficient technique for obtaining toughness stably, in order to achieve further low temperature toughness, it is necessary to further refine the crystal grain size.
[0004]
At present, ultrafine grain refinement on the order of 1 μm has been achieved by ultrafine graining technology similar to the strong machining to α and the subsequent machining and recrystallization of alpha as already disclosed by the present inventors. It's getting on. For example, the Japan Iron and Steel Institute 135th Spring Lecture Meeting Proceedings (CAMP-ISIJ, vol.11, No.3, 1988), page 566, by a method by other pass warm working with the deformation direction changed. An α particle size of about 1 μm has been achieved. However, both are limited to laboratory studies and have not yet been established as practical and industrial means.
[0005]
[Problems to be solved by the invention]
The present invention further develops the ultrafine graining technology by processing and recrystallization that the present inventors have already disclosed, without requiring special equipment, complicated hot working or heat treatment process, And, without limiting to the chemical composition in a special narrow range, it achieves an ultrafine α structure with an average α particle size of 1 μm or less over the entire thickness of the steel, and has extremely excellent toughness with a vTrS of −150 ° C. or less. The present invention provides a method for producing a high-tensile steel material having a tensile strength of about 500 MPa to 950 MPa.
[0006]
[Means for Solving the Problems]
According to the results of the present inventors' research so far, the dominant influence on the crystal grain size obtained by processing and recrystallization of α in the hot rolling stage is from the γ / α two-phase region to α. It is the cumulative reduction ratio in the region and the fineness of the structure before processing. Conventionally, as long as α is ultrafine-grained, it has been considered that α must be sufficiently generated by transformation before being processed into α. In other words, ultra-fine particles can be achieved in steels with relatively low hardenability, or by controlling the cooling so that agglomerate α is finely produced and processing with a large cumulative rolling reduction is applied to the α main structure. did. However, depending on the method, the α particle size usually obtained is in the range of 1 μm to 3 μm, and a crystal particle size of 1 μm or less can be obtained according to a very extreme heat history or severe processing conditions. It has also been found that it is not easy to stably achieve 1 μm or less.
[0007]
Therefore, as a result of further detailed research, the inventors of the present invention naturally have a large influence on the refinement of the previous structure and hot working on the crystal grain size. In order to stably obtain an ultrafine grain structure of 1 μm or less, a fine bainite transformed from unrecrystallized γ containing a large amount of dislocations controlled and rolled at a lower temperature than a fine massive α-based structure. It has been found that it is more advantageous to recrystallize the structure mainly composed of
[0008]
Furthermore, in addition to the above findings, it is necessary to increase the hardenability of the steel more than a certain amount in order to surely cause the bainite transformation, not the bulk α even from fine unrecrystallized γ, a structure different from α In order to recrystallize and make ultrafine grained bainite, it is necessary to set processing conditions different from those in the case where the previous structure is bulk α, and it is also found that an ultrafine grain structure of 1 μm or less is obtained. It led to a new invention. The summary is as follows.
[0009]
(1)% By mass
C: 0.01 to 0.25%
Si: 0.03-1.0%
Mn: 0.30 to 3.0%
Al: 0.003-0.1%
N: 0.001 to 0.01%
As impurities
P: 0.01% or less
S: 0.005% or less,
Contains
Cr: 0.01 to 2.0%
Ni: 0.01-6.0%
Mo: 0.01 to 2.0%
Cu: 0.01 to 1.5%
Of these, it contains one or more, consists of the remainder Fe and inevitable impurities, and(Formula 1) FormulaThe ideal quench critical diameter (D_I)But,25.4-762mmIn the hot rolling of the steel slab,
  Ac₃After heating to a temperature above the transformation point and below 1150 ° C,
After performing the first rolling with a cumulative rolling reduction of 55 to 80%, a rolling start temperature of 850 ° C. or lower, and an end temperature of 750 ° C. or higher,
Second rolling is performed in which rolling with a rolling reduction rate of 30 to 70% per pass includes one or more passes, the cumulative rolling reduction rate is 50 to 95%, the rolling start temperature is 700 ° C. or lower, and the end temperature is 600 ° C. or higher. It is characterized byLowA method for producing high-tensile steel with excellent thermal toughness.
  D _I (Mm) = 12.7・ (C%) 0.5 ・ (1 + 0.64 ・ Si%)
              ・ (1 + 4.10 ・ Mn%) ・ (1 + 0.27 ・ Cu%)
              ・ (1 + 0.52 ・ Ni%) ・ (1 + 2.33 ・ Cr%)
              ・ (1 + 3.14 ・ Mo%) ・ (1 + 1.60 ・ W%)
                                                      ...(Formula 1)
[0010]
(2) The aboveFirst rollingAs a pretreatment of steel,₃Heating to a temperature not lower than the transformation point and not higher than 1050 ° C., and cooling to 300 ° C. or lower at a cooling rate of 0.2 to 20 ° C./s,As described in (1)A method for producing high-tensile steel with excellent low-temperature toughness.
[0011]
(3) The aboveFirst rollingAs a pretreatment of steel,₃After heating to a temperature not lower than the transformation point and not higher than 1150 ° C., preliminary hot rolling with a cumulative reduction rate of 20 to 50% is performed, and cooling to 300 ° C. or lower is performed at a cooling rate of 0.2 to 20 ° C./s. The low temperature toughness described in (1) aboveExcellent high strength steelManufacturing method.
[0012]
(4)% By mass
C: 0.01 to 0.25%
Si: 0.03-1.0%
Mn: 0.30 to 3.0%
Al: 0.003-0.1%
N: 0.001 to 0.01%
As impurities
P: 0.01% or less
S: 0.005% or less,
Contains
Cr: 0.01 to 2.0%
Ni: 0.01-6.0%
Mo: 0.01 to 2.0%
Cu: 0.01 to 1.5%
Among them, the ideal quenching critical diameter (D) containing one or more of them, consisting of the balance Fe and inevitable impurities, and represented by the following formula (formula 1) _I ) Is prior to hot rolling of steel slabs of 25.4 to 762 mm,
The steel slab is made of Ac ₃ Heat to a temperature not lower than the transformation point and not higher than 1050 ° C. and cool to 300 ° C. or lower at a cooling rate of 0.2 to 20 ° C./s.After pretreatment,
After heating in a two-phase region with an austenite fraction of 50-90%, the rolling reduction rate per pass includes 30-70% or more rolling, the cumulative reduction rate is 50-95%, the rolling start temperature is A method for producing a high-tensile steel material excellent in low-temperature toughness, comprising rolling at 700 ° C. or lower and an end temperature of 600 ° C. or higher.
  D _I (Mm) = 12.7 · (C%) 0.5 · (1 + 0.64 · Si%)
              ・ (1 + 4.10 ・ Mn%) ・ (1 + 0.27 ・ Cu%)
              ・ (1 + 0.52 ・ Ni%) ・ (1 + 2.33 ・ Cr%)
              ・ (1 + 3.14 ・ Mo%) ・ (1 + 1.60 ・ W%)
                                                      ...(Formula 1)
(5) Instead of the pretreatment, the steel piece is replaced with Ac ₃ Pre-treatment of heating to a temperature not lower than the transformation point and not higher than 1150 ° C., followed by preliminary hot rolling with a cumulative reduction rate of 20 to 50% and cooling to 300 ° C. or lower at a cooling rate of 0.2 to 20 ° C./s. The method for producing a high-strength steel material having excellent low-temperature toughness as described in (4) above.
[0013]
(6)The steel material after the end of the last hot rolling is accelerated to 20 to 500 ° C. at a cooling rate of 2 to 40 ° C./s.Before(1) ~(5)Low temperature toughnessExcellent high strength steelManufacturing method.
[0014]
(7)450 ° C or higher, Ac₁Tempering below the transformation point, Described in (6)Low temperature toughnessExcellent high strength steelManufacturing method.
[0015]
(8) By mass%
Ti: 0.003-0.10%
V: 0.005-0.50%
Nb: 0.003-0.10%
Zr: 0.003-0.10%
Ta: 0.005 to 0.20%
W: 0.01 to 2.0%
B: 0.0003 to 0.0020%
It is characterized by further containing one or more ofThe above(1) ~(7)Low temperature toughnessExcellent high strength steelManufacturing method.
[0016]
(9) In mass%,
Mg: 0.0005 to 0.01%
Ca: 0.0005 to 0.01%
REM: 0.005-0.10%
It is characterized by further containing one or more of themThe above(1) ~(8)Low temperature toughnessExcellent high strength steelManufacturing method.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
The present invention is characterized in that processing is applied to a bainite main structure as a new means for surpassing the conventional achievement level in a method of ultrafine graining α by processing and recrystallization. Since the bainite structure has a lath structure, it is much more advantageous than the bulk α structure in terms of ease of recrystallization and the fineness of the structure in terms of controlling the grain size after recrystallization. However, in order to make the final α grain size 1 μm or less, it is not sufficient to simply use a bainite structure, but a bainite transformed from unrecrystallized γ having a fine grain size and high dislocation density. We found that this was necessary through detailed experiments.
[0018]
In order to stably transform bainite from fine-grained non-recrystallized γ, it is necessary to optimize the composition of the steel as well as to limit the individual elements for securing strength and toughness and to be an index of hardenability. Hardening critical diameter (D_IIt is also important to keep a certain value above. That is, an optimal combination of chemical composition and manufacturing conditions is required. Hereinafter, the detailed description of the present invention will be divided into manufacturing conditions and chemical composition.
[0019]
  First, the manufacturing conditions of the present invention will be described.
  The method of the present invention firstly states that “the individual elements have the proper chemical composition, and(Formula 1) FormulaA steel slab with an appropriate DI value₃After heating to a temperature not lower than the transformation point and not higher than 1150 ° C., after performing the first rolling at a cumulative rolling reduction of 55 to 80%, a rolling start temperature of 850 ° C. or lower and an end temperature of 750 ° C. or higher, one pass Perform a second rolling with a rolling reduction rate of 30-70% per pass, one or more passes, a cumulative reduction rate of 50-95%, a rolling start temperature of 700 ° C. or lower, and an end temperature of 600 ° C. or higher. The first wayAndFurthermore, in order to achieve further ultra-fine graining and ensure uniformity,₃Heat to a temperature not lower than the transformation point and not higher than 1050 ° C., and cool to 300 ° C. or lower at a cooling rate of 0.2 to 20 ° C./s.₃After heating to a temperature not lower than the transformation point and not higher than 1150 ° C., rolling is performed at a cumulative reduction rate of 20 to 50% and cooling to 300 ° C. or lower at a cooling rate of 0.2 to 20 ° C./s. The method of performing the first method after performing the processing is the second method.AndFurthermore, on the premise that the pre-structure refinement treatment is performed, “the steel slab is reheated to a two-phase region having an austenite fraction of 50 to 90%, and the rolling reduction per pass is 30 to 70%. A method that includes rolling at least one pass, performs rolling with a cumulative rolling reduction of 50 to 95%, a rolling start temperature of 700 ° C. or lower, and an end temperature of 600 ° C. or higher is referred to as a third method.
[0020]
Note that the steel slab in the present invention is an ingot obtained by solidifying molten steel with a mold, a slab such as a slab, billet, or bloom produced by a continuous casting method, and these slabs, mainly for the purpose of adjusting the shape and reducing segregation. It refers to all slabs, steel slabs, and steel plates before being subjected to hot rolling and heat treatment, which are requirements of the present invention, such as steel slabs obtained by partial rolling and hot rolling.
[0021]
The three methods described above are the basic requirements of the present invention. However, according to the necessity of material adjustment such as strength and toughness, after completion of the hot rolling, “20 to 20 ° C./s at a cooling rate of 2 to 40 ° C./s”. Accelerated cooling from ℃ to 300 ℃ ”or“ 450 ℃ or higher, Ac₁It is also possible to perform tempering below the transformation point.
[0022]
The three methods that are the basic requirements of the present invention will be described in more detail.
First, the first method will be described. The steel slab is heated prior to hot rolling._ThreeIt is necessary to set the temperature within the range from the transformation point to 1150 ° C. This is because the heating temperature is Ac_ThreeIf it is less than the transformation point, the part that is not gamma becomes a coarse alpha structure, and is not ultrafine-grained even by subsequent hot rolling, so that the final structure becomes noticeably mixed grains, conversely When the temperature is higher than 1150 ° C., the heated γ particle size becomes coarse, the γ before the transformation becomes insufficiently refined, and the final ultrafine grain becomes insufficient.
[0023]
Steel pieces are Ac_ThreeAfter heating at the transformation point or higher and 1150 ° C. or lower, the first rolling with a cumulative rolling reduction of 55 to 80%, a rolling start temperature of 850 ° C. or lower and an end temperature of 750 ° C. or higher is performed, and γ grains before transformation Adjust the diameter and form. That is, the cumulative rolling reduction needs to be 55% or more in order to make γ finer and sufficiently introduce processing strain. As the cumulative rolling reduction is larger, it is advantageous for the introduction of finer grains and dislocations. However, if the cumulative rolling reduction in the rolling is excessive, it is difficult to secure the rolling reduction in the subsequent second rolling, The upper limit in the first rolling is 80% at which the rolling effect is saturated. On the other hand, if the cumulative rolling reduction is less than 55%, the accumulation of strain in unrecrystallized γ is insufficient, the subdivision of the bainite main structure after transformation becomes insufficient, and the final crystal grain size is refined. Will be insufficient. The first rolling temperature is limited to a start temperature of 850 ° C. or less and an end temperature of 750 ° C. This is because rolling above 850 ° C. has little effect on the γ grain refinement and the introduction of dislocations. This is because it is advantageous to set the rolling start temperature to 850 ° C. or lower in order to fully exhibit the effect of No. 2, and when the end temperature is less than 750 ° C., the rolling effect is saturated while the second rolling is performed. This is because it is difficult to ensure the temperature.
[0024]
If the rolling temperature is 850 ° C. to 750 ° C. within the range of the present invention, the rolling effect of each pass is almost accumulated, so there is no need to limit the rolling reduction of each pass, and the effect of the first rolling is the cumulative rolling reduction. Is achieved if it satisfies the scope of the present invention.
In addition, in this invention, it does not prevent applying the rolling mainly for shape adjustment etc. at the temperature of more than 850 degreeC before this rolling. That is, as long as the cumulative reduction ratio in the first and second rolling can be ensured and the effect of the present invention is not impaired, processing at a temperature higher than 850 ° C. may be added.
[0025]
After adjusting the grain size and form of γ before transformation by the first rolling to refine the bainite main structure after transformation, rolling with a rolling reduction rate of 30 to 70% per pass is 1 pass. Including the above, the second rolling is performed in which the cumulative rolling reduction is 50 to 95%, the rolling start temperature is 700 ° C. or lower, and the end temperature is 600 ° C. or higher.
By the second rolling, the bainite main structure is processed, and through the introduction / recovery / recrystallization process of dislocations, not only α but also the bainite structure is subdivided into an ultrafine grain structure of α as a whole. For that purpose, first, it is necessary to make bainite structure at least 30% or more before sufficiently transforming γ and entering the second rolling. The remainder may be α, γ, or other tissue. If the first rolling is performed, not only bainite but also other transformation structures are sufficiently refined, so there is no problem in ultrafine graining. Further, even if untransformed γ is present, it is transformed into bainite during rolling, and this is not a problem. However, in order to ensure the microstructure, the rolling start temperature needs to be 700 ° C. or lower. If it exceeds 700 ° C., the above pre-processed morphology cannot be stably guaranteed over the entire chemical composition of the present invention. Moreover, the rolling end temperature needs to be 600 degreeC or more. This is because if it is less than 600 ° C., recrystallization of the bainite structure becomes difficult and there are many cases where ultrafine graining is not sufficient, and deformation resistance becomes excessive and the load on the rolling mill becomes excessive. This is because it is difficult to ensure the shape of the steel material.
[0026]
For the above reason, the temperature of the second rolling needs to be in the range of 700 ° C. to 600 ° C., but it is also necessary to define the cumulative rolling reduction and the rolling amount of each pass. That is, in order to sufficiently recrystallize the bainite main structure in rolling at 700 ° C. to 600 ° C. to make it ultrafine grained, it is first assumed that the cumulative rolling reduction is 50 to 95%. That is, if the cumulative rolling reduction is less than 50%, recrystallization is difficult regardless of the rolling conditions of each pass, and there is a limit to the crystal grain size that can be achieved even if recrystallization is performed, so the cumulative rolling reduction is 50 % Or more is required. The larger the cumulative rolling reduction, the more advantageous for finer graining. However, if the cumulative rolling reduction ratio exceeds 95%, the finer graining effect is saturated, while problems arise in securing the temperature and productivity. Therefore, in the present invention, the upper limit is 95%. It was.
[0027]
In the second rolling, the rolling temperature and the cumulative rolling reduction are limited for the above reasons. However, in order to process the bainite main structure so that the α grain size is 1 μm or less, only the rolling temperature and the cumulative rolling reduction are limited. Is not sufficient, and when performing one-pass or multi-pass rolling with a cumulative rolling reduction of 50 to 95%, it is necessary to include one or more rollings with a rolling reduction of 30 to 70% per pass. In the present invention, the rolling temperature is considerably lower due to the need to process the bainite main structure in this invention, but in that case, recrystallization is more difficult than when processing is performed on the α structure main body. It is necessary to promote recrystallization by applying processing under large pressure in such a pass. Also, the difficulty of recrystallization varies depending on the crystal orientation, and depending on the orientation, simply increasing the cumulative rolling reduction does not recrystallize to the end, so it tends to be a mixed grain structure, but also to eliminate such a mixed grain structure It is preferable to perform rolling under large rolling at any stage of rolling. Therefore, the lower limit rolling reduction is 30%. In this case, when the processing rate is large and the temperature rises due to processing heat generation, the recovery and recrystallization of the structure is promoted, and the structure is uniformly refined.
[0028]
  The larger the rolling reduction, the more preferable for the fine graining,
  (1)Recrystallized α grains grow and become coarse due to excessive processing heat generation.
  (2)If the rolling reduction of one pass is excessive, the rolling mill is overloaded.
  (3)It adversely affects the steel plate shape.
For these three reasons, the upper limit of the rolling reduction per pass is 70%. Rolling with a 1-pass rolling reduction of 30 to 70% is more effective at the initial stage of rolling for the purpose of making the α grain size finer, and the cumulative rolling reduction and rolling temperature range are within the scope of the present invention. Then, the necessary ultrafine grain structure can be obtained at any stage of rolling. Moreover, although it is preferable that there are many passes under 30-70% reduction, if at least one pass is included, there is no problem in ultrafine-graining.
[0029]
  That is, “Each element has an appropriate chemical composition, and(Formula 1) FormulaD_IAc₃After heating to a temperature not lower than the transformation point and not higher than 1150 ° C., after performing the first rolling at a cumulative rolling reduction of 55 to 80%, a rolling start temperature of 850 ° C. or lower and an end temperature of 750 ° C. or higher, one pass Perform a second rolling with a rolling reduction rate of 30-70% per pass, one or more passes, a cumulative reduction rate of 50-95%, a rolling start temperature of 700 ° C. or lower, and an end temperature of 600 ° C. or higher. This is the reason for limiting the first method characterized by the above. Next, the reason for limitation of the third method is shown together with the reason for the pre-structural refinement process. The pre-structure refinement process is also effective in the first method, and does not hinder the effect of the first method. By performing the pre-structure refinement process before rolling by the first method, Effective for obtaining toughness higher than that of the first method by reducing the size of the fine particles and the mixed particle size.WaySecond method.
[0030]
  The third method is “the preprocessing according to claim 2 or 3.Same preprocessing asAfter refining the previous structure byWithout performing the first rolling as in the first method,Reheating to a two-phase region with an austenite fraction of 50-80%, including one or more passes with a rolling reduction of 30-70% per pass, a cumulative reduction of 50-95%, and a rolling start temperature of Rolling is performed at 700 ° C. or lower and an end temperature of 600 ° C. or higher ”. That is, unlike the first method, heating is performed to a two-phase region intermediate between the γ region and the α region before hot rolling.The same pretreatment as described inWhen refined, as in the present invention, as in the present invention, in a hardenable steel that has a bainite main structure with fine and non-recrystallized γ, reheating directly into the two-phase region is more effective during rolling. This is because the bainite transformation is promoted from the recrystallized γ, and a desired structure can be stably formed, and the temperature waiting time can be shortened by the lower heating temperature, leading to an improvement in productivity.
[0031]
When reheating to a two-phase region after refining the previous structure, it is necessary to control the γ fraction within a certain range because of the necessity of using a bainite-based structure in the present invention. That is, when the γ fraction is small, it becomes an α-based structure, and it is difficult to obtain an ultrafine grain structure of 1 μm or less. The lower limit of the γ fraction for making the bainite main structure necessary for setting the α particle size of the final structure to 1 μm or less is determined to be 50% based on experiments. On the other hand, if the γ fraction is too large, it is substantially the same as the γ single phase. Therefore, the γ fraction having the effect of promoting bainite transformation has an upper limit of 90%.
[0032]
  As described above, the preprocessing according to claim 2 or 3Same preprocessing asThe rolling condition after the previous structure is refined by the first method is the first methodSecond rolling ofIs exactly the same.
  Next, in the first method, further ultrafine grainingAlthough it is a process that is adopted as necessary for the purpose,In the third method, a pre-process for pre-structural refinement, which is an essential process for ultrafine graining, will be described.
[0033]
  Claim 2Or 4Pretreatment described inThe steelThe coarse solidified structure of the piece is eliminated, and the structure before the processing in the two-phase region to the α region for ultrafine graining is refined. At that time, the steel piece₃Heating to a temperature not lower than the transformation point and not higher than 1050 ° C.₃If it is less than the transformation point, a coarse structure remains regardless of the subsequent cooling conditions, and if it exceeds 1050 ° C., the γ particle size during heating becomes coarse and the structure after cooling becomes insufficiently refined. It is. Moreover, it is necessary to control the cooling after heating to 300 ° C. or less at a cooling rate of 0.2 to 20 ° C./s. This condition is necessary to refine the transformation structure by subsequent cooling even if the heating temperature is specified as described above and the heating γ grain size is refined.
[0034]
That is, since the steel slab is generally thick, the cooling inside the steel slab is often gradually cooled even if it is allowed to cool, in which case the transformation structure becomes coarse. Therefore, in order to refine the structure after cooling, it is necessary to cool at 0.2 ° C./s or more. A larger cooling rate is preferable for refining the structure, but it is not practical to make the cooling rate extremely large for steel slabs with a large plate thickness. Since the appearance of an extremely coarse structure harmful to the crystal structure can be suppressed, the upper limit is set to 20 ° C./s in the present invention. Moreover, the appearance of such a coarse structure can be suppressed regardless of the subsequent cooling conditions if the controlled cooling is performed up to 300 ° C. If the steel piece thickness is relatively small and cooling is performed at 0.2 ° C./s or more by air cooling, naturally air cooling may be used.
[0035]
  Claim 2 aboveOr in the pre-processing described in 4The structure is refined only by heat treatment.Or in the pretreatment described in 5The steel slab structure is refined by hot rolling. Although not only heat treatment but also processing by rolling is advantageous for refining the structure, it also reduces the thickness of the steel slab, so the thickness of the steel slab is thicker than the finished sheet thickness and the cumulative rolling reduction is small. It can be said that this method is preferable when the degree is high.
[0036]
  In the method, first, the steel slab is made Ac.₃Reheat to a temperature above the transformation point and below 1150 ° C. Ac₃Reheating above the transformation point is claimed in claim 2.Or in case of pre-processing described in 4On the other hand, since the subsequent rolling has the effect of refining γ, the upper limit of the reheating temperature is relaxed compared to the case without rolling, but the reheated γ particle size is excessive. In the present invention, the upper limit is set to 1150 ° C., because the grain refinement becomes insufficient even by subsequent rolling.
[0037]
  Ac₃After reheating to a temperature not lower than the transformation point and not higher than 1150 ° C., hot rolling is performed to refine the γ structure. At this time, the cumulative rolling reduction of the hot rolling is set to 20 to 50%. This is Ac₃In the case of rolling following reheating at a transformation point to 1150 ° C., if the cumulative rolling reduction is less than 20%, the γ grains are not sufficiently refined and the resulting transformation structure is not sufficiently refined even when the corner rolling is performed. On the other hand, if it exceeds 50%, the cumulative rolling reduction at the rolling stage for the final ultrafine graining cannot be sufficiently obtained. Claim 2 for cooling after hot rollingOr the pretreatment described in 4In the same manner as described above, as a condition for sufficiently miniaturizing the transformation structure, cooling is performed to 300 ° C. or lower at a cooling rate of 0.2 to 20 ° C./s.
[0038]
The above is an explanation of the basic requirements in the present invention, but other reasons for limiting the requirements related to the manufacturing method are added below.
That is, for the purpose of adjusting the strength of the steel material and improving the toughness, the steel material after the final rolling can be accelerated and cooled to 20 ° C. to 500 ° C. at a cooling rate of 2 to 40 ° C./s. This is because the internal structure is refined by accelerated cooling and the second phase is changed to a harder one. In order to exhibit this effect, the cooling rate needs to be 2 ° C./s or more. A higher cooling rate is advantageous for microstructure refinement and hard phase formation, but it is practically difficult to increase the cooling rate indefinitely with thick materials, so that the accelerated cooling effect on the structure control is not saturated. In the present invention, the upper limit is 40 ° C./s. Moreover, it is preferable to perform this accelerated cooling to 20 to 500 degreeC. In other words, cooling to below 20 ° C is not effective for controlling the structure, and if the accelerated cooling stop temperature exceeds 500 ° C, the structure is not sufficiently refined and it is difficult to form a hard phase. This is because the toughness may deteriorate. In addition, according to the accelerated cooling condition defined in the present invention, it is also effective for suppressing the grain growth of the ultrafine grain structure of the surface layer once formed.
[0039]
  Further, tempering treatment can be performed for the purpose of strength adjustment, toughness improvement, and shape improvement. In that case, it is an essential requirement not to impair the ultrafine grain structure formed in the surface layer portion. In the present invention, the tempering temperature is set to 450 ° C. to Ac.₁Although it is limited to the transformation point, the effect of tempering is not clear at less than 450 ° C.₁This is because if the transformation point is exceeded, the morphology of the superfine grain structure in the surface layer portion is impaired. However, it is more preferable that the tempering temperature does not exceed 700 ° C. in order to more reliably suppress the grain growth of the ultrafine-grained layer. In addition, within the tempering temperature range of the present invention, the heating and holding time for tempering is arbitrary, but similarly, from the viewpoint of preserving the ultrafine grain structure of the surface layer portion, the holding time isWithin 5 hoursIt is preferable that
[0040]
The above are the requirements for the method for producing a high-strength steel material excellent in low-temperature toughness according to the present invention. In order to exert the effect by the production method, it is necessary to limit each chemical component for the reasons described below. There is.
[0041]
That is, C is contained as an effective component for improving the strength of steel, and if it is less than 0.01%, it is difficult to ensure the strength necessary for structural steel, but an excess content exceeding 0.25%. Lowers the toughness and weld crack resistance of the base metal and the welded portion, so the content was made 0.01 to 0.25%.
[0042]
Next, Si is an element effective as a deoxidizing element and for securing the strength of the base material. However, if it is less than 0.03%, deoxidation is insufficient and it is disadvantageous for securing the strength. On the contrary, an excessive content exceeding 1.0% forms a coarse oxide and causes deterioration of ductility and toughness. Therefore, the range of Si is set to 0.03 to 1.0%.
[0043]
Further, Mn is an element necessary for ensuring the strength and toughness of the base material, and it is necessary to contain at least 0.3% or more, but if it is contained excessively, it may cause hard phase formation or grain boundary embrittlement. In order to deteriorate the toughness of the base metal, the toughness of the welded portion, and the weld cracking property, the upper limit was made 3.0% within the allowable range of the material.
[0044]
Al is an element effective for deoxidation, austenite grain size reduction, etc., but in order to exert the effect, it is necessary to contain 0.003% or more. On the other hand, if it exceeds 0.1% and excessively contained, a coarse oxide is formed and the ductility is extremely deteriorated.
[0045]
N is combined with Al and Ti and effectively works for refining γ grains. Further, it is impossible to remove N in steel completely industrially, and reducing it more than necessary is not preferable because it places an excessive load on the manufacturing process. Therefore, the lower limit is set to 0.001% as a range that can be industrially controlled and the load on the manufacturing process is allowable. If excessively contained, solid solution N increases, which may adversely affect ductility and toughness, so the upper limit is made 0.01% as an acceptable range.
[0046]
In the present invention, in order to reliably transform a fine bainite structure from fine / unrecrystallized γ, a critical quench critical diameter (D_I) Must be made appropriate, but the desired D can be obtained only by C, Si, Mn without affecting other characteristics._ISince it is difficult to adjust to a value, it is necessary to form a bainite structure by adding one or more of Cr, Ni, Mo, and Cu, which greatly contribute to hardenability. However, it is necessary to limit each addition amount as follows.
[0047]
Both Cr and Mo are effective elements for improving the strength of the base material, but in order to produce a clear effect, 0.01% or more is necessary. On the other hand, if added over 2.0%, toughness In addition, since the weldability tends to deteriorate, the range is 0.01 to 2.0%.
[0048]
Ni is a very effective element that can improve the strength and toughness of the base material at the same time. However, in order to exert the effect, Ni needs to be contained in an amount of 0.01% or more. If the content increases, the strength and toughness are improved, but even if added over 6.0%, the effect is saturated, but the weldability deteriorates, so the upper limit is made 6.0%. In addition, if the addition exceeds 6.0%, D_IEven if the value is within the range of the present invention, the bainite transformation is suppressed, and an undesirable effect for ultrafine graining is also manifested.
[0049]
Next, Cu has substantially the same effect as Ni, but if it exceeds 1.5%, there is a problem in hot workability, so it is limited to a range of 0.01 to 1.5%.
[0050]
P and S are impurity elements and elements that degrade ductility and toughness, and it is preferable to reduce them as much as possible. However, material deterioration is not so great that the upper limit of P is 0.01% and the upper limit of S is acceptable. Is limited to 0.005%.
[0051]
The above is the reason for limiting the basic components of the steel material of the present invention. In the present invention, one of Ti, V, Nb, Zr, Ta, W, and B is selected as necessary for the adjustment of strength and toughness. It can contain seeds or two or more.
[0052]
Ti is an element that contributes to improving the strength of the base metal by precipitation strengthening, and is also an element effective for refinement of the heated austenite grain size by the formation of TiN, and is also an element effective for improving toughness. Needs to be contained in an amount of 0.003% or more. On the other hand, if it exceeds 0.10%, coarse precipitates and inclusions are formed to deteriorate toughness and ductility, so the upper limit is made 0.10%.
[0053]
V is also an element effective for improving the strength by forming VN. However, if contained excessively, the toughness deteriorates due to precipitation embrittlement. Accordingly, the range in which the effect can be exhibited without causing significant deterioration in toughness is limited to the range of 0.005 to 0.50%.
[0054]
Nb is an element effective for improving strength and toughness by forming Nb (C, N), but if contained excessively, toughness deteriorates due to precipitation embrittlement. Therefore, the range in which the effect can be exhibited without causing deterioration of toughness is limited to the range of 0.003 to 0.10%.
[0055]
Zr is also an element forming a nitride and has the same effect as Ti, but 0.003% or more is required to exhibit the effect. On the other hand, if it exceeds 0.10%, as in Ti, coarse precipitates and inclusions are formed to deteriorate toughness and ductility, so the content is limited to the range of 0.003 to 0.10%.
[0056]
Ta is also an element effective in improving strength and toughness, but 0.005% or more is necessary to exert the effect. On the other hand, if it exceeds 0.20%, precipitation embrittlement, coarse precipitates, and toughness deterioration due to inclusions occur, so the upper limit is made 0.20%.
[0057]
W is effective in increasing the strength of the base metal by solid solution strengthening and precipitation strengthening, but 0.01% or more is necessary to exert the effect. On the other hand, if it exceeds 2.0% and contains excessively, toughness deterioration becomes remarkable, so the upper limit is made 2.0%.
[0058]
B is an element effective for improving toughness by solid solution N fixation and improving strength and toughness by improving hardenability because it binds to N in a small amount, but 0.0003% or more is necessary to exert the effect. is necessary. On the other hand, if the content exceeds 0.0020%, BN becomes coarse and adversely affects ductility and toughness. Moreover, in order to deteriorate weldability, the upper limit is made 0.0020%.
[0059]
Furthermore, in order to improve the base material toughness, the ductility, and the joint toughness, one or more of Mg, Ca, and REM can be contained as necessary.
Mg, Ca, and REM are all effective in improving ductility by suppressing the extension of sulfide during hot rolling. The oxide is refined to contribute to further ultrafine-graining, which is advantageous for improving the base material toughness, and also effectively improves the joint toughness. The lower limit content for exhibiting the effect is 0.0005% for Mg and Ca, and 0.005% for REM. On the other hand, if it is excessively contained, the sulfides and oxides are coarsened and the ductility and toughness are deteriorated. Therefore, the upper limits are 0.01% for Mg and Ca and 0.10% for REM, respectively.
[0060]
  The above is the reason for the limitation of individual elements. In order to reliably transform the fine bainite structure from fine and non-recrystallized γ, not only the limitation of individual elements but also the optimization of hardenability The following is an index of hardenability(Formula 1) FormulaCritical quench critical diameter (D_I)25.4-762mmIt is necessary to.
  D _I (Mm) = 12.7・ (C%) 0.5 ・ (1 + 0.64 ・ Si%)
              ・ (1 + 4.10 ・ Mn%) ・ (1 + 0.27 ・ Cu%)
              ・ (1 + 0.52 ・ Ni%) ・ (1 + 2.33 ・ Cr%)
              ・ (1 + 3.14 ・ Mo%) ・ (1 + 1.60 ・ W%)
                                                      ...(Formula 1)
[0061]
  (Formula 1) FormulaD indicated by_Ivalue25.4mmIf it is less than the range, the hardenability is insufficient, and the ratio of the bainite structure in the transformed structure becomes insufficient, making it difficult to obtain an ultrafine structure of 1 μm or less. On the other hand, D_Ivalue762mmIf the excess exceeds 1, the pre-structural refinement or two-phase region heating, etc., does not perform bainite transformation from γ, but martensite transformation, so the transformation temperature becomes as low as about 400 or less, transformation It becomes virtually impossible to process the tissue. For the above reason, in the present invention,(Formula 1) FormulaCritical quench critical diameter (D_I)25.4-762mmLimited to.
[0062]
【Example】
  The above is an explanation of the requirements of the present invention. Further, the effects of the present invention are shown based on examples.
  Steel sheets were produced under the production conditions shown in Tables 2 and 3 using steel pieces having chemical compositions shown in Table 1. The characteristics of the ferrite structure (average diameter, mixed grain size, grain shape, etc. at the center of the steel sheet thickness) and mechanical properties (strength, 2 mm V notch Charpy impact characteristics, ESSO characteristics) of the manufactured steel sheet The measurement results are also shown in Tables 2 and 3. In addition, Table 2 is the result which showed this invention steel and the comparative example which relate to claims 1-3, Table 3 isClaims 4 and 5It is the result which showed this invention steel related to 1 and a comparative example.
[0063]
  The observation of the structure and the measurement of the particle diameter are carried out based on the scanning electron microscope (SEM) structure photograph of the magnification 1000 times to 5000 times for the steels of the present invention and comparative examples with relatively fine ferrite particle diameters. The comparative example with a coarse particle size was carried out based on a photomicrograph of an optical microscope having a magnification of 500 times. Tensile properties were obtained by collecting round bar tensile test pieces from the center of the thickness in the direction perpendicular to the rolling direction (C direction). A 2 mm V notch Charpy impact test was also conducted by collecting a standard test piece having a thickness of 10 mm from the center of the thickness in the direction perpendicular to the rolling direction (C direction). The Charpy impact characteristics were evaluated by the fracture surface transition temperature (vTrS). Furthermore, the brittle crack propagation stopping property (arrestability) was investigated by a temperature gradient type ESSO test. For ESSO characteristics, specimens are taken from the C direction, and Kca is 400 kgf / mm.^1.5 (= 3920 N / mm ^1.5 )Was evaluated at a temperature (TKca 400).
[0064]
Test No. in Table 2 Test Nos. A1 to A9 and Table 3 A10 to A16 are steel plates produced by the production method of the present invention using steel numbers 1 to 11 having the chemical composition of the present invention, and all of them have an average ferrite particle size of 1 μm or less, and conventional ferrite processing -An ultrafine grain structure that is one level finer than the level of ultrafine grain steel by recrystallization is obtained, and extremely good toughness and arrestability are obtained. That is, the vTrS of the conventional ultrafine-grained steel having an average ferrite particle size of about 3 μm or less is about −100 to −150 ° C., whereas the vTrS of the ultrafine-grained steel of the present invention having an average particle size of 1 μm or less. Are all below -170 ° C, and most of them show a fracture mode mainly composed of ductile fracture even at liquid nitrogen temperature (-196 ° C). Reflecting such extremely high toughness, ESSO characteristics at full thickness However, it is apparent that TKca 400 is very good at −132 ° C. or lower, and the safety of the steel material is dramatically increased.
[0065]
On the other hand, from the results of Tables 2 and 3, the test No. deviating from the scope of the present invention. The steel plates B1 to B11 are the test Nos. Manufactured according to the present invention. It is clear that the toughness and arrestability are significantly inferior to the steel sheets of A1 to A16.
Test No. B1-B5 and No. B11 is an example in which sufficient toughness and arrestability could not be achieved although the production method satisfied the present invention because the chemical composition did not satisfy the present invention.
[0066]
  That is, test no. B1(Formula 1) FormulaD_ISince the value is too small and the structure before the two-phase region processing does not become a bainite-based structure, the finally obtained ferrite grain size is 1.98 μm, which is not less than 1 μm, which is vTrS compared to the steel of the present invention. And arrestability (TKca 400) is inferior.
[0067]
Test No. Conversely, B2 is D_ISince the value is excessive and the hardenability is excessive, the first rolling is performed by the method according to claim 1 and the rolling is performed at a low temperature, but the majority is untransformed austenite. Therefore, an ultrafine-grained ferrite structure is not obtained, and therefore vTrS and arrestability (TKca 400) are significantly inferior to the steel of the present invention.
[0068]
  In addition, Test No. B11 is Test No. Using the same steel piece as B2,Claim 5The two-phase region heating is applied, but the part that was an austenite phase during the two-phase region heating does not cause transformation before and during processing, and the final structure is elongated. As a result, the bainite-based organization was obtained. Like B2, ultrafine graining has not been achieved, and the toughness and arrestability are inferior to the steel of the present invention.
[0069]
Test No. In B3, since C is excessive, fine toughness is sufficient, but a good toughness level is not achieved.
Test No. B4 is inferior in both vTrS and TKca 400 compared to the steel of the present invention, although both P and S are excessive and ultrafine graining is sufficient.
Test No. In B5, since both Cr and Mo are excessive, a good toughness level is not achieved even though ultrafine graining is sufficient.
[0070]
On the other hand, test no. B6, B7, and B8 to B10 satisfy the present invention in chemical composition, but the manufacturing method deviates from the scope of the present invention. Is an example in which the characteristics are inferior to those of the steel sheet produced according to the present invention.
[0071]
Test No. B6 and B7 are comparative examples related to claim 1. In the second rolling, No. B6 has a rolling reduction of 30% or more in any rolling pass. In B7, since the cumulative rolling reduction ratio of the first rolling does not satisfy the range of the present invention, the final ferrite grain size is coarser than that of the steel sheet according to the present invention, and the toughness and arrestability are also the present invention. Inferior to steel.
[0072]
  Tests B8-B10Claims 4 and 5It is a comparative example related to.
  Test No. Since B8 is not subjected to the pre-structural refinement step before the two-phase region heating / rolling, a coarse structure remains in the two-phase region heating stage, and the final structure is also mixed / coarse, toughness, Arrest is inferior.
  Test No. In B9, since rolling reduction after the two-phase region heating is not 30% or more in any rolling pass, ultrafine graining is not sufficient compared to the steel of the present invention, and both vTrS and TKca 400 It stays at the conventional level.
[0073]
Test No. B10 has a low austenite fraction in the heating stage due to the low temperature in the two-phase region, and as a result, is an essential requirement for achieving a ferrite grain size of 1 μm or less. Since the condition of “processing the microstructure mainly composed of fine bainite transformed from recrystallized γ” is not satisfied, the average ferrite grain size can only be reduced to about 2 μm, and by conventional processing and recrystallization of ferrite Only the toughness level of ultrafine-grained steel is obtained.
[0074]
From the above examples, according to the present invention, an ultrafine ferrite structure having an average ferrite grain size of 1 μm or less can be achieved over the entire thickness of the steel material, and production of a high-strength steel material having extremely good Charpy impact properties and arrestability It is clear that will be possible.
[0075]
[Table 1]

[0076]
[Table 2]

[0077]
[Table 3]

[0078]
[Table 4]

[0079]
[Table 5]

[0080]
[Table 6]

[0081]
[Table 7]

[0082]
[Table 8]

[0083]
【The invention's effect】
The present invention achieves an ultrafine ferrite structure with an average ferrite grain size of 1 μm or less over the entire thickness of the steel material, and has vTrS of −150 ° C. or less and extremely excellent low-temperature toughness and brittle crack propagation stopping properties. It provides a method for producing high-tensile steel materials with a tensile strength of about 500MPa to over 950MPa, and can improve the economics and productivity of high-tensile steel materials that can dramatically improve the safety of structures. It can be said that it is an invention with extremely high industrial value as a means that can be manufactured without loss.

Claims (9)

% By mass
C: 0.01 to 0.25%
Si: 0.03-1.0%
Mn: 0.30 to 3.0%
Al: 0.003-0.1%
N: 0.001 to 0.01%
As impurities
P: 0.01% or less S: 0.005% or less,
And further Cr: 0.01-2.0%
Ni: 0.01-6.0%
Mo: 0.01 to 2.0%
Cu: 0.01 to 1.5%
Among them, the ideal quench critical diameter (D _I ) is 25.4 to 762 mm, containing one or two or more types, the balance being Fe and inevitable impurities, and represented by the following formula (Formula 1): In hot rolling of billets,
After heating to a temperature of Ac ₃ transformation point or higher and 1150 ° C. or lower,
After performing the first rolling with a cumulative rolling reduction of 55 to 80%, a rolling start temperature of 850 ° C. or lower, and an end temperature of 750 ° C. or higher,
Second rolling is performed in which rolling with a rolling reduction rate of 30 to 70% per pass includes one or more passes, the cumulative rolling reduction rate is 50 to 95%, the rolling start temperature is 700 ° C. or lower, and the end temperature is 600 ° C. or higher. it shall be the said method for producing a high tensile steel having excellent low-temperature toughness.
D _I (mm) = 12.7 · (C%) 0.5 · (1 + 0.64 · Si%)
・ (1 + 4.10 ・ Mn%) ・ (1 + 0.27 ・ Cu%)
・ (1 + 0.52 ・ Ni%) ・ (1 + 2.33 ・ Cr%)
・ (1 + 3.14 ・ Mo%) ・ (1 + 1.60 ・ W%)
... (Formula 1) The pre-treatment of the first rolling is characterized in that the steel slab is heated to a temperature not lower than the Ac ₃ transformation point and not higher than 1050 ° C. and cooled to not higher than 300 ° C. at a cooling rate of 0.2 to 20 ° C./s. The manufacturing method of the high strength steel materials excellent in the low temperature toughness of Claim 1. As pretreatment for the first rolling , the steel slab is heated to a temperature not lower than the Ac ₃ transformation point and not higher than 1150 ° C., and then subjected to preliminary hot rolling with a cumulative reduction ratio of 20 to 50%, and 0.2 to The method for producing a high-tensile steel material excellent in low-temperature toughness according to claim 1, wherein cooling is performed to 300 ° C. or less at a cooling rate of 20 ° C./s. % By mass
C: 0.01 to 0.25%
Si: 0.03-1.0%
Mn: 0.30 to 3.0%
Al: 0.003-0.1%
N: 0.001 to 0.01%
As impurities
P: 0.01% or less
S: 0.005% or less,
Contains
Cr: 0.01 to 2.0%
Ni: 0.01-6.0%
Mo: 0.01 to 2.0%
Cu: 0.01 to 1.5%
Among them, the ideal quench critical diameter (D _I ) is 25.4 to 762 mm, which contains one or two or more, consists of the balance Fe and inevitable impurities, and represented by the following formula (Formula 1). Prior to hot rolling of billets,
The steel slab is heated to a temperature not lower than Ac ₃ transformation point and not higher than 1050 ° C., and subjected to a pretreatment for cooling to 300 ° C. or lower at a cooling rate of 0.2 to 20 ° C./s ,
After heating in a two-phase region with an austenite fraction of 50-90%, the rolling reduction rate per pass includes 30-70% or more rolling, the cumulative reduction rate is 50-95%, the rolling start temperature is A method for producing a high-tensile steel material excellent in low-temperature toughness, comprising rolling at 700 ° C. or lower and an end temperature of 600 ° C. or higher.
D _I (mm) = 12.7 · (C%) 0.5 · (1 + 0.64 · Si%)
・ (1 + 4.10 ・ Mn%) ・ (1 + 0.27 ・ Cu%)
・ (1 + 0.52 ・ Ni%) ・ (1 + 2.33 ・ Cr%)
・ (1 + 3.14 ・ Mo%) ・ (1 + 1.60 ・ W%)
... (Formula 1) Instead of the pretreatment, the steel piece is replaced with Ac. ₃ Pre-treatment of heating to a temperature not lower than the transformation point and not higher than 1150 ° C., followed by preliminary hot rolling with a cumulative rolling reduction of 20 to 50% and cooling to 300 ° C. or lower at a cooling rate of 0.2 to 20 ° C./s The method for producing a high-tensile steel material excellent in low-temperature toughness according to claim 4, wherein: The low temperature toughness according to any one of claims 1 to 5, wherein the steel material after the final hot rolling is accelerated to 20 to 500 ° C at a cooling rate of 2 to 40 ° C / s. For producing high-strength steel materials with excellent resistance. The method for producing a high-tensile steel material excellent in low-temperature toughness according to claim 6 , wherein tempering is performed at 450 ° C. or higher and Ac ₁ transformation point or lower. % By mass
Ti: 0.003-0.10%
V: 0.005-0.50%
Nb: 0.003-0.10%
Zr: 0.003-0.10%
Ta: 0.005 to 0.20%
W: 0.01 to 2.0%
B: 0.0003 to 0.0020%
1 or 2 types or more of these are further contained, The manufacturing method of the high strength steel materials excellent in the low temperature toughness of any one of Claims 1-7 characterized by the above-mentioned. % By mass
Mg: 0.0005 to 0.01%
Ca: 0.0005 to 0.01%
REM: 0.005-0.10%
The manufacturing method of the high-tensile steel material excellent in the low temperature toughness of any one of Claims 1-8 characterized by further containing 1 type, or 2 or more types.