WO2024128737A1 - Tôle d'acier à haute résistance ayant un rapport d'élasticité élevé, et son procédé de fabrication - Google Patents

Tôle d'acier à haute résistance ayant un rapport d'élasticité élevé, et son procédé de fabrication Download PDF

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WO2024128737A1
WO2024128737A1 PCT/KR2023/020374 KR2023020374W WO2024128737A1 WO 2024128737 A1 WO2024128737 A1 WO 2024128737A1 KR 2023020374 W KR2023020374 W KR 2023020374W WO 2024128737 A1 WO2024128737 A1 WO 2024128737A1
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steel sheet
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steel
ferrite
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한성호
이재훈
최용훈
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주식회사 포스코
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  • the present invention relates to materials used for automobile interior panels, reinforcements, etc., and to a high-yield ratio high-strength steel plate and a method of manufacturing the same.
  • high-strength steel plates capable of reducing plate thickness is increasing in automobile parts.
  • high-strength steel plates are widely used in automobile bodies to ensure the safety of occupants, and to improve the crash performance of automobile bodies, the yield strength of steel is increased to efficiently absorb collision energy even with low deformation.
  • steel plates with a high yield ratio are required.
  • DP steel dual phase steel
  • DP steel has a problem of low yield ratio because its main phase is soft ferrite and hard structures such as bainite, martensite, and tempered martensite are used as secondary phases. Therefore, there are some limits to the application of DP steel for automobile parts that absorb impact energy while suppressing deformation.
  • Patent Document 1 proposed a steel sheet that prevents recrystallization of ferrite and has a structure composed of unrecrystallized ferrite and a hard second phase. However, if excessive unrecrystallized ferrite exists, strength and yield ratio increase, but there is a problem of insufficient formability due to low elongation.
  • Patent Documents 5 to 7 proposed a steel sheet with improved elongation flangeability by using unrecrystallized ferrite and using unrecrystallized ferrite having an intermediate hardness of soft ferrite and hard secondary phase.
  • the steel sheets proposed in Patent Documents 5 and 6 have a small amount of Nb or Ti added, so the recrystallization inhibition effect is small, so rapid heating is necessary during annealing, and the steel sheets proposed in Patent Document 7 have a small amount of Nb or Ti added. Since the temperature increase rate during annealing is low, below 10°C/s, the time available for recrystallization increases, and the effect of using unrecrystallized ferrite is not sufficient.
  • Patent Document 1 Japanese Patent Laid-open Publication 1978-005018
  • Patent Document 2 Japanese Patent Application Publication 2007-138261
  • Patent Document 3 Japanese Patent Laid-open Publication 2007-107099
  • Patent Document 4 Japanese Patent Laid-open Publication 2001-152288
  • Patent Document 5 Japanese Patent Laid-open Publication 2008-106351
  • Patent Document 6 Japanese Patent Laid-open Publication 2008-106352
  • Patent Document 7 Japanese Patent Laid-open Publication 2008-156680
  • One aspect of the present invention is to provide a steel plate with a high yield ratio and high strength, and excellent formability, and a method for manufacturing the same.
  • One aspect of the present invention is weight percent, carbon (C): 0.05 to 0.12%, manganese (Mn): 1.0 to 1.8%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03. % or less (excluding 0%), Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less (excluding 0%), Aluminum (sol.Al): 0.01 to 0.08%, titanium (Ti): 0.02 ⁇ 0.06%, Niobium (Nb): 0.02 ⁇ 0.06%, Boron (B): 0.005% or less (excluding 0%), including the remaining Fe and inevitable impurities,
  • the microstructure includes 80 to 99% of ferrite in terms of area percent, the remainder includes pearlite and other inevitable structures, and unrecrystallized ferrite is 20 to 50% of the ferrite,
  • a steel plate having an aspect ratio of 5 to 15 of the ferrite is provided.
  • the total amount of Ti and Nb may be 0.1% or less.
  • the cold rolled steel sheet may satisfy the following relational expression 1.
  • the steel sheet may further include a hot-dip galvanized layer on the surface.
  • the steel plate may have a yield strength of 460 MPa or more and a tensile strength of 520 MPa or more.
  • the product of yield strength and elongation may be 8600 or more.
  • Another embodiment of the present invention is by weight percentage, carbon (C): 0.05 to 0.12%, manganese (Mn): 1.0 to 1.8%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%), Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less (excluding 0%), Aluminum (sol.Al): 0.01 to 0.08%, Ti (titanium): 0.02 ⁇ 0.06%, niobium (Nb): 0.02 ⁇ 0.06%, boron (B): 0.005% or less (excluding 0%), steel slabs containing the remaining Fe and inevitable impurities are stored at 1100 ⁇ 1250°C. Heating with;
  • a method of manufacturing a steel sheet including the step of continuously annealing the cold rolled steel sheet at a temperature range of 770 to 820°C.
  • the method of manufacturing the steel plate can satisfy the conditions of [Relational Expression 2] and [Relational Expression 3] below.
  • CR is the cold rolling reduction rate (%)
  • SS is the annealing temperature (°C)
  • LS is the line speed (mpm) during continuous annealing.
  • the method of manufacturing the steel sheet may further include hot-dip galvanizing the continuously annealed steel sheet.
  • the steel plate of the present invention has high strength and high yield ratio, so when used as an inner plate, reinforcement, etc., the resistance (collision resistance characteristics) increases during a collision, which is advantageous in ensuring the safety of passengers. Additionally, according to the present invention, a steel plate with excellent formability can be provided.
  • Figure 1 is a photograph showing the microstructure of invention steel 1 in Examples.
  • Non-recrystallized ferrite is ferrite stretched in the rolling direction by cold rolling, meaning that recrystallization has not been completed and dislocations within the particles have been recovered.
  • the material of the cold rolled steel sheet has large deviations in the width and length directions, and even a slight difference in the unrecrystallized fraction may cause the material to appear very non-uniform. Therefore, it was recognized that the best method was to minimize the unredetermining tissue as much as possible.
  • Ti titanium
  • Nb niobium
  • C carbon
  • the annealing temperature must be managed very high or a special process is required to suppress precipitation of TiC, NbC, etc.
  • the special process is to shorten the time for TiC, NbC, etc. to precipitate through very rapid quenching or very rapid heating.
  • this is a process that cannot be achieved during normal operation, and special equipment is required to achieve it.
  • it is necessary to maintain a temperature of almost 900°C or higher.
  • Such high-temperature annealing can cause problems such as coil meandering and increased manufacturing costs. Even if the annealing temperature is raised to 900°C or higher, the yield strength decreases due to material softening due to the creation of a recrystallization structure, so it is difficult to secure the high yield ratio required in the present invention.
  • the present inventors conducted in-depth research to manufacture a steel plate with a high yield ratio, preferably a yield strength of 460 to 600 MPa, a tensile strength of 520 to 700 MPa, and a yield ratio of 0.8 to 0.9.
  • a method was derived that could secure the above-mentioned yield ratio using unrecrystallized ferrite and at the same time have excellent formability with an elongation of 10% or more, leading to the present invention.
  • the content of the alloy composition is based on weight%.
  • the steel sheet contains carbon (C): 0.05-0.12%, manganese (Mn): 1.0-1.8%, silicon (Si): 0.6% or less (excluding 0%), phosphorus (P): 0.03% or less (excluding 0%). , Sulfur (S): 0.01% or less (excluding 0%), Nitrogen (N): 0.01% or less (excluding 0%), Aluminum (sol.Al): 0.01 to 0.08%, Titanium (Ti): 0.02 to 0.06 %, niobium (Nb): 0.02 ⁇ 0.06%, boron (B): 0.005% or less (excluding 0%).
  • Carbon (C) is an element that contributes to the increase in strength and the creation of pearlite, and is added in an appropriate amount to secure the target strength. In addition, it is an essential element to give strength to steel sheets by forming precipitates with Ti, Nb, etc. in the ferrite phase. If it is less than 0.05%, it is difficult to secure the strength required for the present invention steel, and if it exceeds 0.12%, formability or weldability will be deteriorated, so it is effective that the C content is 0.05 to 0.12%.
  • Manganese (Mn) is an element that lowers the Ac3 transformation temperature, which is the temperature at which Ac1 and ⁇ - ⁇ transformations are completed and austenite becomes a single phase. That is, if the amount of Mn is small, it is necessary to increase the annealing temperature to promote transformation, which makes it difficult to secure the appropriate fraction of unrecrystallized ferrite required by the present invention. Additionally, Mn is an element that contributes to solid solution strengthening along with Si and is also effective in increasing strength. From this point of view, a Mn content of 1.0% or more is effective. On the other hand, if the Mn content exceeds 1.8%, hardenability increases, bainite and martensite are easily formed, and the yield ratio is lowered, so it is effective not to exceed 1.8%.
  • Si is a deoxidizing element and is effective in increasing strength as a solid solution strengthening element.
  • the Si amount exceeds 0.6%, Ac1 becomes too high and it is necessary to increase the annealing temperature, which accelerates transformation and makes it difficult to secure unrecrystallized ferrite. Therefore, it is effective to manage it to 0.6% or less.
  • excessive addition of Si may cause problems such as deterioration of plating adhesion due to oxide during hot dip galvanizing.
  • 0% is excluded.
  • Phosphorus (P) is an impurity and segregates at grain boundaries, causing a decrease in the toughness of the steel sheet and deterioration of weldability.
  • the alloying reaction is very slow during hot dip galvanizing and productivity decreases, it is effective for P to be 0.03% or less.
  • S Sulfur
  • S is an impurity that is inevitably included in steel, and it is desirable to keep its content as low as possible. Therefore, considering that the S content in steel is inevitably included, 0% is excluded. In particular, since S in steel increases the possibility of causing red heat embrittlement, it is effective to control its content to 0.01% or less.
  • N Nitrogen
  • 0% is excluded (i.e., exceeds 0%).
  • the refining cost of the steel increases rapidly, so it is managed to 0.01% or less, which is the range within which operating conditions are possible.
  • Acid-soluble aluminum is an element added for particle size refinement and deoxidation. If the sol.Al content is less than 0.01%, aluminum killed (Al-killed) steel cannot be manufactured in a normal stable state. On the other hand, if the sol.Al content exceeds 0.08%, it is advantageous to increase strength due to the grain refinement effect, but the possibility of surface defects in the plated steel sheet increases due to excessive formation of inclusions during steelmaking operation. In addition, because there is a problem that causes a sharp increase in manufacturing cost, it is desirable to manage the sol.Al content at 0.01 to 0.08%.
  • the Ti and Nb are elements that promote the retention of unrecrystallized ferrite by suppressing recrystallization of ferrite during an annealing process for deformed ferrite generated by cold rolling.
  • the amount of unrecrystallized ferrite increases due to the formation of carbides such as TiC and NbC, the yield strength and yield ratio may increase excessively, and excessive addition of alloy elements may cause an increase in manufacturing costs.
  • Boron (B) is an element that improves hardenability, increases strength, and suppresses the nucleation of grain boundaries. If the content of B exceeds 0.005% by weight, the effect becomes excessive and causes an increase in manufacturing costs, so it is preferable to control the content of B to 0.005% by weight or less.
  • the unavoidable impurities may be included as long as they can be unintentionally introduced during the manufacturing process of ordinary cold rolled steel sheets (and plated steel sheets). Since a person skilled in the art can easily understand the meaning, it is not particularly limited here.
  • the microstructure of the steel sheet is expressed in terms of area percentage and includes 80 to 99% ferrite, and the remainder includes pearlite and other inevitable structures.
  • the inevitable structure is not particularly limited, but may be cementite, carbide, etc. More specifically, ferrite may be included in 80 to 95%.
  • the unrecrystallized ferrite it is effective for the unrecrystallized ferrite to be 20 to 50% by area among the ferrites.
  • the area % of the non-nodulated ferrite refers to the fraction of the entire microstructure.
  • the fraction of unrecrystallized ferrite can be determined by analyzing crystal orientation measurement data from electron back scattering diffraction (EBSD) using the Kernel Average Misorientation method (KAM method). Since the KAM method can quantitatively express the crystal orientation difference with adjacent pixels (measurement points), in the present invention, particles with an average crystal orientation difference with adjacent measurement points of less than 1° are defined as unrecrystallized ferrite. In order to secure sufficient yield strength and yield ratio, it is effective for the area percent of the unrecrystallized ferrite to be 20 to 50%.
  • the unrecrystallized ferrite is less than 20%, sufficient yield strength and yield ratio cannot be obtained, and if it exceeds 50%, the yield strength and yield ratio become excessive due to the high unrecrystallized structure, and the aspect ratio of the grains increases. . Therefore, it is effective to have 20 to 350% of unrecrystallized ferrite.
  • the total ferrite fraction including the unrecrystallized ferrite is 80 to 99%. More specifically, a total ferrite fraction of 80-95% may be more effective.
  • the steel sheet of the present invention contains pearlite and inevitable structures other than ferrite.
  • microstructure of the steel sheet especially the grain aspect ratio (A/R) of ferrite, is 5 to 15.
  • the grain aspect ratio is determined by etching the microstructure with 5% Nital etching solution, observing it at 500x with a scanning electron microscope (SEM), and analyzing the grains using the Image Analyzer program.
  • the major axis length and minor axis length were obtained.
  • the aspect ratio was obtained as the major axis length of the grain/ellipse minor axis length.
  • the average aspect ratio of each ferrite obtained by this technique was defined as the grain aspect ratio.
  • A/R exceeds 15
  • the formation of such excessive stretched grains causes an excessive increase in yield strength, exceeding the yield strength and yield ratio required by the present invention.
  • A/R is less than 5, it means that recrystallization has progressed to a significant extent, which means that due to softness of the steel, the yield strength is insufficient and the yield ratio is lower than the level required for the steel of the present invention.
  • the X-ray diffraction integrated intensity ratio is the relative intensity based on the X-ray diffraction integrated intensity of the non-oriented standard sample.
  • X-ray diffraction can be performed using an X-ray diffraction device widely used in the technical field to which the present invention belongs, such as an energy dispersive type. If the X-ray diffraction integrated intensity ratio calculated in equation 1 above exceeds 2, it means that the (222) texture, that is, the fraction of recrystallized texture increases, which means that the fraction of unrecrystallized ferrite in the steel is not formed in an appropriate range. This means that the aspect ratio cannot be secured.
  • a steel sheet that satisfies the above alloy composition and microstructure may have a yield strength of 460 MPa or more, a tensile strength of 520 MPa or more, and a yield ratio of 0.8 to 0.9. At the same time, the steel sheet may have an elongation of 10% or more than 10%. More specifically, the yield strength of the steel plate may be 460 to 600 MPa. The tensile strength of the steel plate can be 520 to 700 MPa.
  • the product of the yield strength and elongation of the steel plate may be 8500 or more. More specifically, the product of yield strength and elongation may be 8900 or more. Typically, as yield strength increases, elongation may decrease. However, according to one embodiment of the present invention, the product of yield strength and elongation is 8500 or more, so the yield strength and elongation can be improved simultaneously.
  • the steel sheet of the present invention may include a plating layer to improve corrosion resistance.
  • the plating layer is not particularly limited, and any plating type or plating method performed in the technical field to which the present invention belongs is sufficient.
  • a preferred example may be a hot-dip galvanized layer.
  • a steel slab satisfying the above-described composition can be manufactured through the process of reheating, hot rolling, coiling, cold rolling, and annealing. Below, each process is described in detail.
  • the reheated steel slab is hot rolled at a temperature of 880°C or higher to produce a hot rolled steel sheet. If the temperature of the hot rolling is less than 880°C, ferrite transformation occurs during rolling and an elongated structure is created, which may cause problems such as anisotropy deterioration and cold rolling resistance. Therefore, the hot rolling is performed at 880°C or higher. It is effective.
  • Cold rolled steel sheets are manufactured by cold rolling the hot rolled steel sheets after coiling and pickling. It is effective to perform the cold rolling reduction rate (cold rolling reduction rate) at 45 to 70%. If the cold rolling reduction ratio is less than 45%, the recrystallization driving force is very low and excessive unrecrystallized ferrite is formed, making it difficult to secure the strength required in the present invention. On the other hand, when the cold rolling reduction ratio exceeds 70%, the recrystallization driving force becomes too high, making it easy to recrystallize ferrite even at low annealing temperatures, making it difficult to manufacture high-strength steel with a yield ratio of 0.8 to 0.9.
  • CR is the cold rolling reduction rate (%)
  • SS is the annealing temperature (°C)
  • LS is the line speed (mpm) during continuous annealing.
  • the above [Relational Expression 2] and [Relational Expression 3] are operation factors that control the recrystallization driving force and include annealing temperature, cold rolling reduction rate, and line speed during annealing.
  • it is effective for the transfer speed to be 90 to 150 mpm.
  • the transfer speed is controlled differently depending on the thickness of the steel plate. In other words, for thick materials, the line speed is low, and for thin materials, high-speed work is performed. It is desirable to manage the cold rolling reduction rate and annealing temperature together to suit these conditions.
  • plating may be additionally performed after the continuous annealing.
  • the plating may be performed in a manner commonly performed in the technical field to which the present invention pertains, and the type and method of plating are not particularly limited.
  • hot dip galvanizing conditions are not particularly limited, and hot dip galvanizing can be performed under normal conditions that can be applied in the same technical field.
  • the steel sheet according to an embodiment of the present invention may include a hot dip galvanizing layer on the surface.
  • a molten zinc-based galvanized steel sheet is manufactured by immersing the steel sheet in a molten zinc-based plating bath at 440 to 500°C.
  • the steel sheet may be subjected to alloying heat treatment.
  • the hot-dip galvanized steel sheet is subjected to alloying heat treatment at a temperature range of 460 to 530° C. and then cooled to room temperature. You can. Through alloying heat treatment, the steel sheet may include an alloyed hot-dip galvanized layer on the surface.
  • temper rolling can be performed. Temper rolling can also be performed within the normal range of 0.1 to 1.0%. If the temper rolling elongation is less than 0.1%, it is difficult to control the plate shape. On the other hand, if it exceeds 1.0%, material deterioration due to excessive increase in dislocation density in the surface layer may occur, and side effects such as plate fracture may occur due to limitations in facility capacity.
  • CR is the cold rolling reduction rate (%)
  • SS is the continuous annealing temperature (°C)
  • LS is the feed speed (line speed, mpm).
  • a tensile test was performed in the rolling direction using the DIN-L standard to measure the yield strength (YP), tensile strength (TS), and elongation (El.) of the steel sheet. It is shown in Table 3 above.
  • the grain aspect ratio of ferrite and the fractions (area %) of unrecrystallized and recrystallized ferrite were measured using the microstructure measured using the previously described scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). Meanwhile, the X-ray diffraction intensity values for each texture component were measured for the manufactured steel sheet, and the X-ray diffraction intensity ratio was calculated using the equation in Equation 1.
  • invention steels 1 to 8 that meet the alloy composition and manufacturing conditions of the present invention have a yield strength of 469 to 545 MPa, a tensile strength of 545 to 655 MPa, an elongation of 18 to 21%, and a yield ratio (YR) of 0.82 to 0.86, and the product of yield strength and elongation is 8900 or more, satisfying the mechanical properties presented in the present invention steel.
  • the inventive steel has a grain aspect ratio of 5.9 to 10.2, which satisfies the condition of 5 to 15 grain aspect ratio proposed in the present invention, and the unrecrystallized ferrite fraction is 29 to 45%, which is 29 to 45%.
  • the suggested 20 to 50% conditions are met.
  • the X-ray diffraction ratio of these steels is also 0.6 to 1.2, which sufficiently satisfies the conditions presented in the present invention.
  • Figure 1 shows an SEM photograph showing the microstructure of the annealed plate of invention steel 1 among the examples.
  • the microstructure included unrecrystallized ferrite (code 1) and recrystallized ferrite (code 2), and some pearlite.
  • Comparative steels 1 and 3 had cold rolling reduction rates much lower than the conditions suggested in the present invention. This caused a lack of ferrite recrystallization during annealing, so the yield strength was very high and the yield ratio exceeded the standards of the present invention. Additionally, the grain aspect ratio value was very high due to lack of recrystallization.
  • comparative steels 2 and 7 have a very high cold rolling reduction rate of 80%.
  • the cold rolling reduction rate is high, ferrite recrystallization easily occurs even at low annealing temperatures. This is due to a decrease in strength due to an increase in the recrystallization fraction, making it impossible to satisfy the yield strength and yield ratio conditions required for the steel of the present invention.
  • the grain aspect ratio and X-ray integrated intensity exceeded the standards of the present invention.
  • Comparative steel 4 had very low Ti and Nb addition amounts of 0.01% each. Recrystallization was promoted due to the lack of TiC and NbC precipitates, and the fraction of unrecrystallized ferrite after annealing was lowered, and as a result, the yield strength and yield ratio were outside the conditions of the present invention.
  • Comparative steels 5 and 8 are cases where no Ti or Nb is added. Due to the lack of precipitates in the steel, recrystallization easily occurred during annealing, and as a result, the yield strength of the steel sheet was low and the grain aspect ratio and X-ray diffraction intensity ratio did not meet the conditions of the present invention.
  • Comparative steel 6 has the same composition as comparative steel 5 and is a steel with no added Nb, and its cold rolling reduction rate was very low at 35% under conditions of insufficient precipitates, so it did not satisfy the conditions of the present invention even after high-temperature annealing at 860°C.
  • Comparative steel 10 has a Mn content outside the range of the present invention and an annealing temperature of 750°C, which is very low. This low annealing temperature causes excessive lack of ferrite recrystallization, which causes problems such as deterioration of workability when the elongation is below 10% due to excessive increase in yield strength and yield ratio.
  • Comparative steel 11 has a carbon content of 0.14%, which is outside the composition range of the present invention steel. Due to the excessive carbon content, the amount of carbide in the steel increased, which caused problems such as increased yield ratio and deterioration of elongation. Additionally, excessive addition of carbon causes deterioration of weldability.
  • Comparative steel 12 had a Ti content of 0.08%, which was outside the standard for the present invention steel, and the total amount of Ti+Nb was also outside the standard. This increase in carbonitride forming elements causes excessive TiC and NbC precipitation, which causes problems such as increased yield ratio due to delayed recrystallization.

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Abstract

La présente invention concerne un matériau utilisé pour un panneau intérieur d'automobile, un renfort et similaire, et une tôle d'acier à haute résistance ayant un rapport d'élasticité élevé, ainsi que son procédé de fabrication.
PCT/KR2023/020374 2022-12-12 2023-12-12 Tôle d'acier à haute résistance ayant un rapport d'élasticité élevé, et son procédé de fabrication WO2024128737A1 (fr)

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2008190032A (ja) * 2007-01-10 2008-08-21 Nippon Steel Corp 加工性及び耐衝突特性に優れた高強度冷延鋼板及びその製造方法
JP2010285656A (ja) * 2009-06-11 2010-12-24 Nippon Steel Corp 析出強化型冷延鋼板及びその製造方法
JP2011021224A (ja) * 2009-07-15 2011-02-03 Jfe Steel Corp 高強度冷延鋼板およびその製造方法
KR20160097348A (ko) * 2014-01-06 2016-08-17 신닛테츠스미킨 카부시키카이샤 열간 성형 부재 및 그 제조 방법
US20180057907A1 (en) * 2015-03-27 2018-03-01 Jfe Steel Corporation High-strength steel sheet and production method therefor

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2008190032A (ja) * 2007-01-10 2008-08-21 Nippon Steel Corp 加工性及び耐衝突特性に優れた高強度冷延鋼板及びその製造方法
JP2010285656A (ja) * 2009-06-11 2010-12-24 Nippon Steel Corp 析出強化型冷延鋼板及びその製造方法
JP2011021224A (ja) * 2009-07-15 2011-02-03 Jfe Steel Corp 高強度冷延鋼板およびその製造方法
KR20160097348A (ko) * 2014-01-06 2016-08-17 신닛테츠스미킨 카부시키카이샤 열간 성형 부재 및 그 제조 방법
US20180057907A1 (en) * 2015-03-27 2018-03-01 Jfe Steel Corporation High-strength steel sheet and production method therefor

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