JP4016573B2 - High-tensile steel plate excellent in ductility and impact resistance and method for producing the same, and method for producing structural member having impact resistance - Google Patents

Description

これらの鋼片を加熱炉に装入し、１２００℃で３０分間保持した後、炉から取り出して１０００℃まで自然冷却し、圧延開始温度を１０００℃とする熱間仕上圧延を施した。仕上圧延のパス回数は合計３パスで、仕上圧延後の厚さは３．５ｍｍであり、仕上圧延終了温度は８５０℃であった。熱間仕上圧延終了後、ただちに４秒間水スプレー冷却して７２０℃とし（平均冷却速度３３℃／秒）、次いで８秒間自然放冷して６８０℃とし、さらに２秒間の水スプレー冷却を施して６２０℃とし（平均冷却速度３０℃／秒）、これを６２０℃に加熱した炉に装入して３０分間保持した後、２０℃／時で室温まで冷却した。
【００７１】
得られた熱延板は、塩酸溶液を用いて酸洗してスケールを除去した後、合計圧下率６６％で１．２ｍｍまで冷間圧延した。得られた冷延板を、８２０℃に加熱して４０秒間均熱した後、５℃／秒で７００℃まで徐冷した後、５０℃／秒で４００℃まで冷却し、４００℃で３００秒間保持した後、３０℃／秒で室温まで冷却した。得られた焼鈍板に伸び率１．０％の調質圧延を施した。
【００７２】
これらの鋼板の圧延方向について、ＪＩＳ−Ｚ２２０１に規定された５号試験片を用い、ＪＩＳ−Ｚ２２４１の規定に準拠して引張試験をおこなった。引張試験時の応力−歪み曲線における最大荷重時の歪みを一様伸びとし、全伸びと一様伸びの差を求めて局部伸び値とした。
【００７３】
上記調質圧延済みの鋼板から得た圧延方向を長手方向とするブランクを、図２に示すプレス工具を用いてプレス成形し、幅４０ｍｍ、高さ３０ｍｍ、フランジ幅１０ｍｍ、全長２００ｍｍのハット断面部品１を得た。ポンチとダイの肩半径は、共に３．０ｍｍとした。しわ抑え力は、縦壁部２の板厚歪みが１０％となるように調整した。得られたハット断面部品には１７０℃で２０分間保持する焼付け処理を施した後、縦壁部から小片を切り出し、圧延方向に垂直な断面のビッカース硬さを測定した。
【００７４】
図３は、ビッカース硬さの測定位置を示す配置図である。ビッカース硬さ試験はＪＩＳ−Ｚ２２４４の規定に準拠しておこない、試験荷重は４．９Ｎとした。断面内での測定位置は、板厚中心と両表面（板厚の１／８だけ内側の位置）について、それぞれ、０．５ｍｍ間隔で５点測定し、板厚中心の平均をＨＶｃ、両表面の測定値の平均をＨＶｓとした。なお、予成形前の鋼板についても、圧延方向に垂直な断面の板厚中心において、０．５ｍｍ間隔で５点のビッカース硬さを測定し、その平均をＨＶ₀ とした。これらの値から硬度比Ｘを計算した。
【００７５】
上記プレス成形で得たハット断面部品と、同じ冷間圧延し、焼鈍と調質圧延を施した鋼板から得た幅６０ｍｍ、長さ２００ｍｍの平板部品１２を２０ｍｍ間隔でスポット溶接し、閉断面構造部材を作製し、これに１７０℃で２０分間加熱する焼付け処理を施して試験体を作製した。
【００７６】
この試験体を、その長手方向を鉛直にして試験台に装着し、上方から質量が６０ｋｇの錘体を落下させ、１０ｍ／秒の速度で試験体上端に衝突させる落重式軸圧壊試験をおこなった。試験体下部にはロードセルを設置して試験体に作用する荷重を測定し、別途錘体の位置変化を測定して、これらの荷重−変位関係から、錘体が停止するまでに試験体に作用した荷重の平均値Ｐ（ｋＮ）を求め、この軸圧壊平均荷重により耐衝撃特性を判定した。
【００７７】
表２に、加工前の鋼板の引張特性、予成形し焼付け処理したハット断面部品の縦壁部で測定した硬度比および試験体の軸圧壊平均荷重測定結果を示す。
【００７８】
【表２】

表２において試番３〜１１は鋼の化学組成が本発明の規定する条件を満足するものであり、その硬度比Ｘは０．１２以上で、いずれも本発明例である。試番１はＣ含有量が低い鋼Ａを使用し、試番２、１２および１３はＳｉ＋Ａｌ含有量が低い鋼Ｂ、ＬおよびＭを使用したもので、これらは硬度比が０．１２に満たず、いずれも比較例として評価したものである。試験体の軸圧壊平均荷重は、素材とした鋼板の引張強さ（ＴＳ）レベルにより異なるので、耐衝撃特性は鋼板の引張強さに応じてその良否を判断するのが妥当である。表２に示されているように、本発明例である鋼板を用いた試番３〜１１はいずれも優れた軸圧壊平均荷重を示していた。また、本発明鋼はＴＳ×ＥＬで代表される強度−延性バランスにも優れていた。
【００７９】
図４に、表２の軸圧壊平均荷重と、それぞれの加工前の鋼板の引張強さととの関係を示す。図４からわかるように、硬度比Ｘが０．１２以上のものは、比較例に比べ同じ引張強さでも約１０％高い軸圧壊平均荷重を示した。硬度比Ｘが０．１５以上のものはさらに優れていることが判る。
【００８０】
（実施例２）
表１に示した化学組成を有する鋼Ｄ、ＥおよびＨの鋼片に、巻取温度以外は実施例１と同一条件とする熱間仕上圧延を施して実施例１と同一寸法の熱延板とし、実施例１と同様の酸洗、冷間圧延、焼鈍および調質圧延を施して、種々の冷間圧延鋼板を作製した。これらの鋼板について実施例１に記載したのと同様の方法で試験して、引張特性と局部伸びを調査した。得られた結果を、巻取温度と共に表３に示す。
【００８１】
【表３】

表３に示されているように、熱間圧延後の巻取温度が本発明の製造方法で規定する条件を満たす試番１６、１７、２１および２６は特に優れた局部延性を有していた。また、通常の延性（Ｅｌ）や、ＴＳとＥｌの積（いわゆる延性バランス）も良好なものであった。
【００８２】
【発明の効果】
本発明の高張力鋼板は、局部延性に優れるので自動車に代表される複雑な形状を備えた構造部材への加工が容易であるうえ、プレス加工時の曲げ曲げ戻しを伴うプレス成形と焼付け処理により、構造部材としての耐衝撃特性を大幅に向上させることができる。従って本発明の高張力鋼板は、自動車の構造部材の高強度が容易で鋼板の薄肉化による軽量化に有効であるうえ、衝突安全性向上にも有効であり、これらを同時に達成できるので利用価値が極めて大きい。
【図面の簡単な説明】
【図１】構造部材で多用される閉断面構造部材を模した試験体の斜視図である。
【図２】ハット断面部品のプレス成形状態を説明するための断面図である。
【図３】ハット断面部品の縦壁部でのビッカース硬さ測定点を示す模式図である。
【図４】鋼板の引張強さとこれを用いて作製した閉断面構造部材の軸圧壊平均荷重との関係を示すグラフである。
【符号の説明】
１：ハット断面部品、２：縦壁部、３：ポンチ、４：ポンチ肩、５：ダイ、６：ダイ肩、７：ダイ溝、８：しわ押さえ、１１：底部、１２：平板部品、
図２で符号である。[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to a high-strength steel sheet excellent in ductility and impact resistance, suitable for use as a material for a high-strength structural member formed by pressing or bending, and a manufacturing method thereof, and a structural member having impact resistanceofIt relates to a manufacturing method.
[0002]
[Prior art]
Corresponding to the improvement of collision safety and weight reduction in automobiles, the tension of structural members has been increased. At that time, some problems have been pointed out in applying high-strength steel sheets to automobile structural members.
[0003]
Generally, the strength and formability of a steel plate are in a contradictory relationship, and as the strength of the steel plate increases, press forming becomes difficult, and there is a problem that the members to which the high strength steel plate can be applied are limited. In order to solve this problem, a steel sheet having an excellent balance between strength and ductility using the TRIP (Transformaion Induced Plasticity) effect of retained austenite has been developed.
[0004]
For example, in Japanese Patent Application Laid-Open No. 5-117761, the chemical composition is mass% (hereinafter, “%” of the chemical composition means mass%), C: 0.08 to 0.30%, Mn: 1.0 Retained austenite is obtained by annealing a hot-rolled steel plate or a cold-rolled steel plate containing ~ 2.0%, Si: 0.5-2.5%, Al: 0.5-1.5% under specific conditions. A method for producing a high-strength thin steel sheet having a crystal structure having a phase and having both strength and workability is disclosed.
[0005]
Moreover, there is a problem that the static-to-static ratio decreases as the strength of the steel plate is increased, and the impact resistance characteristics are not improved for the use of the high strength steel plate. Here, the static ratio is a static tensile test (with a strain rate of 10^-FourThe strain rate acting on the structural member when the automobile collides against the strength at around 10 sec.^Three  In the case of steel having a small static ratio, the strength at high speed deformation is small even if the static strength is high. For this problem, a method for increasing the strength or static ratio in a dynamic tensile test is disclosed.
[0006]
For example, in JP-A-11-80879, C: 0.04 to 0.30%, containing one or both of Si and Al in total 0.3 to 3.0%, ferrite and 3% by volume or more This is a second phase containing austenite, and changes in the volume fraction of the austenite phase before and after pre-deformation and the difference between the quasi-static deformation strength and the dynamic deformation strength of the pre-deformed steel An excellent work-induced transformation type high-strength steel sheet is disclosed.
[0007]
In JP-A-7-34186, C: 0.01% or less, Si: 0.01-1.5%, Mn: 0.01-3.0%, Al: 0.02-0. Containing 06%, P: 0.15% or less, the region from the steel sheet surface to 50 μm is a composite structure containing bainite or martensite in the ferrite structure, and the region excluding it is a ferrite single-phase structure. A structure in which the average hardness from the surface of the steel sheet after baking to 1/4 t (t: plate thickness) is 1.5 times or more the average hardness of the center thickness (1/4 t to 3/4 t). A thin steel sheet for forming having excellent impact resistance is disclosed.
[0008]
[Problems to be solved by the invention]
As described above, although various types of steel plates with excellent balance between strength and ductility and steel plates with a high static ratio have been developed, the conventional methods have not yet obtained satisfactory solutions. That is, in the technique disclosed in Japanese Patent Application Laid-Open No. 5-117761, the hot rolling conditions are not mentioned in the production method for obtaining a retained austenitic steel sheet, and a good property improving effect is not necessarily required when the structure before annealing is not preferable. There is a problem that cannot be obtained. In particular, the retained austenitic steel sheet has a problem of poor local ductility, and its performance is not sufficient for use in processing that affects local ductility, such as hole expansion and microbending.
[0009]
In JP-A-11-80879, the influence of molding is evaluated by considerable strain, and the dynamic strength is evaluated by a tensile test. However, since the actual press product undergoes bending and bending back deformation at the die shoulder at the time of forming, the mechanical properties are different in the plate thickness direction, and the deformation mode acting on the steel plate at the time of collision is as described above. For example, there is a problem that a sufficient effect may not be obtained by the above method because it is not a simple tensile deformation. For example, when the steel plate is bent and deformed at the time of collision, the surface strain becomes particularly large, so it is desirable that the strength of the steel plate surface is high, but the tensile test method can only evaluate the average strength in the plate thickness direction, so accurate evaluation It was difficult.
[0010]
In JP-A-7-34186, in order to improve strain rate sensitivity (static ratio), it is described that it is effective to distribute structures having different characteristics with respect to strain rate sensitivity in the structure of the same steel sheet, As a method for obtaining a hard composite structure only on the steel sheet surface, surface carburization during annealing and heating and quenching only the surface layer part to a two-phase region temperature are described. However, these methods are not always easy to realize in a general manufacturing process of steel sheets.
[0011]
  The object of the present invention is to solve these problems, and to provide a high-strength steel sheet that has better formability, excellent impact resistance characteristics, and can be manufactured at low cost, and a manufacturing method thereof, and a structural member having impact resistance characteristics.ofIt is to provide a manufacturing method.
[0012]
[Means for Solving the Problems]
In a structural member related to collision safety, it is important that a large amount of energy is absorbed when the formed member is subjected to high-speed crushing deformation. Many of the structural members have a closed cross-section structure made of a thin steel plate. However, when these members are crushed and deformed at high speed, they collapse into a bellows shape or bend and deform to absorb kinetic energy.
[0013]
Such a deformation mode generated in a member made of a thin steel plate at the time of high-speed crushing deformation is not a simple tensile deformation, but mainly a bending-bending return deformation (a deformation in which a bending return occurs following the bending deformation). Along with the bending back bending deformation, the outer side surface of the bending undergoes a compressive deformation after the tensile deformation, and the inner side surface of the bending undergoes a tensile deformation after the compressive deformation. Anyway, the distortion which a surface part produces is larger than the plate | board thickness center part of a steel plate.
[0014]
According to the research results of the present inventor, when the hardness of the steel plate surface portion and the hardness of the plate thickness center portion satisfy a specific relationship, the impact absorption energy absorption that can be absorbed by the member when impact deformation is applied. I learned that I can dramatically improve Noh.
[0015]
FIG. 1 is a perspective view of a test body simulating a member having a closed cross-sectional structure frequently used for structural members. Reference numeral 1 is a hat cross-sectional part, reference numeral 12 is a flat plate part, and both are joined by spot welding. Yes. Reference numeral 2 denotes a vertical wall portion of the hat cross-section component 1, and reference numeral 11 denotes a bottom portion thereof.
[0016]
FIG. 2 is a cross-sectional view for explaining the press-formed state of the hat cross-section component 1, reference numeral 3 is a punch, reference numeral 4 is a punch shoulder, reference numeral 5 is a die, reference numeral 6 is a die shoulder, reference numeral 7 is a die groove, reference numeral 8 is a wrinkle presser.
[0017]
The steel plate sandwiched between the die 5 and the wrinkle retainer 8 is bent and deformed along the curved surface of the punch shoulder 6 as the punch 3 is lowered, and is drawn into the die groove 7. The bent part passing through the punch shoulder 6 is bent back by the die side wall.
[0018]
The vertical wall 2 is subjected to bending and bending back deformation at the die shoulder 6, so that the amount of strain differs in the thickness direction and a difference in hardness occurs. That is, the hardness of the steel plate surface portion is higher than that of the plate thickness center portion. In addition to the bending and bending back deformation, the vertical wall portion 2 is subjected to a tensile force generated between the punch and the wrinkle retainer, so that a plate thickness distortion is generated and the thickness is reduced.
[0019]
The pressed parts are assembled as structural members by welding or the like, painted, and then subjected to a paint baking process (hereinafter also simply referred to as “baking process”) that is held at 170 ° C. for about 20 minutes. At this stage, solid solution atoms (C, N atoms, etc.) in the steel are precipitated, strain aging occurs, and the hardness of the steel increases.
[0020]
According to the results of the inventor's research, residual austenitic steel produced under specific conditions is harder than conventional steel when subjected to pre-formation and baking treatment with tensile bending deformation as described above. Compared to this, it was found that if this was used as a structural member, the energy absorption capability during high-speed axial crushing deformation was greatly improved, and extremely excellent impact resistance was exhibited.
[0021]
The mechanism is not necessarily clear, but is estimated as follows. In general, bending and bending back causes large strain on the surface part and causes work hardening (hardening due to an increase in dislocation density), but due to the Bauschinger effect, work hardening by bending and work hardening by bending back are additive. It is thought that it does not become.
[0022]
In steel containing residual austenite, hardening occurs due to transformation of residual austenite to hard martensite in addition to work hardening caused by the growth of dislocations as strain increases. This hardening by martensite is irrelevant to the Bauschinger effect, and it is considered that the hardness of the surface portion is remarkably increased as a result of the additional increase in hardness in both bending and unbending.
[0023]
In addition, in order to obtain the desired surface hardening characteristics, it has been found that steel having a residual austenite of 5% by volume or more and the balance having a crystal structure substantially composed of ferrite is preferable. The meaning of “substantially” means that bainite structure inevitably generated may be mixed when obtaining retained austenite in the annealing after cold rolling.
[0024]
The present inventor has further conducted various studies on the formability of retained austenitic steel sheet, in particular, a method for improving the local ductility deficiency, which is a problem in conventional retained austenitic steel sheet. It has been found that a steel having a chemical composition can be greatly improved by producing it under specific conditions.
[0025]
That is, in order to improve the local ductility of the cold-rolled steel sheet having retained austenite, the hot-rolling conditions are optimized in the production of the hot-rolled sheet as a base material of the cold-rolled steel sheet, and the hard second phase in the crystal structure of the hot-rolled sheet is obtained. It is important to reduce the volume fraction of and to make the hard second phase softer pearlite, not bainite or martensite. Furthermore, in the hot-rolled sheet, it is effective to improve the ductility of the cold-rolled steel sheet to reduce the second-phase band-like structure (point-like structure) caused by solidification segregation of Mn and P.
[0026]
The mechanism is not necessarily clear, but is estimated as follows.
If there is a large hardness difference between the ferrite phase and the second phase, plastic deformation does not occur uniformly during cold rolling, and microvoids are generated at the interface with the second phase. If the hardness of the second phase is remarkably high, or if the second phase has a band-like structure that exists in a sequence of dots, a large number of microvoids are generated in a sequence of dots and remain after annealing. These microvoids are connected to each other and easily break in a region that has undergone a large strain during the molding of the product. That is, when there are many such microvoids, the local ductility is remarkably impaired, and the local elongation in the tensile test is reduced. Therefore, it is considered effective to improve the local ductility by reducing the volume fraction and hardness of the second phase and eliminating the band-like structure.
[0027]
  The present invention has been completed based on these findings, and the gist thereof is as follows (1) to(8)is there.
[0028]
(1) By mass%, C: 0.05 to 0.25%, Si: 2.0% or less, Al: 0.005 to 2.0%, Mn: 0.8 to 2.5%, P: 0.05% or less, and (Si + Al): 1.0 to 2.5% is satisfied, and the balance has a chemical composition composed of Fe and inevitable impurities, and has tensile strength (TS) and total elongation. (El) product (TS × El) is 21900 MPa ·% or moreThe hardness of the steel plate surface and the center of the plate thickness after the pre-forming with 10% tensile bending deformation as the plate thickness strain and then the baking treatment held at 170 ° C. for 20 minutes satisfies the following formula YouHigh tensile strength steel sheet with excellent ductility and impact resistance;
(HVs-HVc) / HV ₀ ≧ 0.12,
However, HV ₀ : Vickers hardness at the center of the plate thickness before the above preforming,
HVc: Vickers hardness at the center of the plate thickness after the above preforming and baking process,
HVs: Vickers hardness of the surface portion after the above pre-molding and baking treatment.
(2) By mass%, C: 0.05 to 0.25%, Si: 2.0% or less, Al: 0.005 to 2.0%, Mn: 0.8 to 2.5%, P: 0.05% or less, and (Si + Al): 1.0 to 2.5% is satisfied, and Ti and / or Nb is further changed to Ti: 0.003 to 0.05%, Nb: 0.00. 003 to 0.05%, and (Ti + Nb) ≦ 0.05% in a range that satisfies the chemical composition comprising Fe and inevitable impurities, the tensile strength (TS) and the total elongation (El ) Product (TS × El) is 21900 MPa ·% or moreThe hardness of the steel plate surface and the center of the plate thickness after the pre-forming with 10% tensile bending deformation as the plate thickness strain and then the baking treatment held at 170 ° C. for 20 minutes satisfies the following formula YouHigh tensile strength steel sheet with excellent ductility and impact resistance;
(HVs-HVc) / HV ₀ ≧ 0.12,
However, HV ₀ : Vickers hardness at the center of the plate thickness before the above preforming,
HVc: Vickers hardness at the center of the plate thickness after the above preforming and baking process,
HVs: Vickers hardness of the surface portion after the above pre-molding and baking treatment.
(3) The chemical composition further includes, in mass%, one or more of Cu, Ni, and Co in the group consisting of Cu: 0.2-1.0%, Ni: 0.1 0.5%, Co: 0.0005 to 1.0%, and (Cu + Ni + Co) ≦ 1.5% in a range satisfying the ductility and impact resistance described in the above (1) or (2) Excellent high strength steel plate.
[0029]
(4) Above( 1 ) ~ ( 3 ) In any ofA steel slab having a chemical composition is subjected to hot finish rolling with a hot finish rolling start temperature of 1050 ° C. or lower and an end temperature of 800 ° C. or higher, and then cooled to 750 ° C. at a cooling rate of 20 ° C./second or higher. Then, hot rolling having a step of winding at 700 ° C. or lower and Tc (° C.) or higher calculated by the following formula is performed, the obtained steel sheet is pickled, and then the total rolling reduction is 40% or higher and 80%. Cold rolling is performed in the following range, and then kept for 30 seconds or more and 90 seconds or less in the two-phase temperature range of ferrite + austenite, and then the temperature range of 700 ° C or less and 450 ° C or more is 30 ° C / second or more. Cooling, holding for 200 seconds or more and 400 seconds or less in a temperature range of 450 ° C. or less and 370 ° C. or more, and then performing annealing to cool to room temperature,The product (TS × El) of tensile strength (TS) and total elongation (El) is 21900 MPa ·% or more.Production method of high-tensile steel plate with excellent ductility and impact resistance;
Tc (° C.) = 430 + 70 × Mn (%) + 1000 × P (%).
(5) The method for producing a high-tensile steel sheet having excellent ductility and impact resistance as described in (4) above, wherein the steel slab before hot finish rolling is subjected to auxiliary heating.
(6) By mass%, C: 0.05 to 0.25%, Si: 2.0% or less, Al: 0.005 to 2.0%, Mn: 0.8 to 2.5%, P: 0.05% or less, and (Si + Al): 1.0 to 2.5% is satisfied, and the balance includes a chemical composition composed of Fe and inevitable impurities,The product (TS × El) of tensile strength (TS) and total elongation (El) is 21900 MPa ·% or more,The hardness of the steel plate surface portion and the plate thickness center portion after pre-forming with 10% tensile bending deformation in the plate thickness strain and then subjected to baking treatment held at 170 ° C. for 20 minutes satisfies the following formula: A method for producing a structural member having impact resistance characteristics, characterized by subjecting a tensile steel sheet to forming with bending and bending back deformation, followed by coating baking treatment;
(HVs-HVc) / HV₀≧ 0.12,
However, HV₀: Vickers hardness at the center of the plate thickness before the above preforming,
HVc: Vickers hardness at the center of the plate thickness after the above preforming and baking process,
HVs: Vickers hardness of the surface portion after the above pre-molding and baking treatment.
[0030]
(7) By mass%, C: 0.05-0.25%, Si: 2.0% or less, Al: 0.005-2.0%, Mn: 0.8-2.5%, P: 0.05% or less, and (Si + Al): 1.0 to 2.5% is satisfied, and Ti and / or Nb is further changed to Ti: 0.003 to 0.05%, Nb: 0.00. 003 to 0.05%, and (Ti + Nb) ≦ 0.05% in a range that satisfies, the balance comprising a chemical composition consisting of Fe and inevitable impurities,The product (TS × El) of tensile strength (TS) and total elongation (El) is 21900 MPa ·% or more,The hardness of the steel plate surface portion and the plate thickness center portion after pre-forming with 10% tensile bending deformation in the plate thickness strain and then subjected to the baking treatment held at 170 ° C. for 20 minutes satisfies the following formula: A method for producing a structural member having impact resistance characteristics, characterized by subjecting a tensile steel sheet to forming with bending and bending back deformation, followed by coating baking treatment;
(HVs-HVc) / HV₀≧ 0.12,
However, HV₀: Vickers hardness at the center of the plate thickness before the above preforming,
HVc: Vickers hardness at the center of the plate thickness after the above preforming and baking treatment,
HVs: Vickers hardness of the surface portion after the above pre-molding and baking treatment.
(8) The chemical composition is further mass%, and one or more of the group consisting of Cu, Ni, and Co is added to Cu: 0.2 to 1.0%, Ni: 0.1 to 0.1%. The high-tensile steel plate according to (6) or (7) above, containing 0.5%, Co: 0.0005 to 1.0%, and (Cu + Ni + Co) ≦ 1.5%. A method for producing a structural member having impact resistance characteristics, characterized in that a coating baking process is performed after forming with bending deformation.
[0033]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described in detail.
Chemical composition of steel sheet;
C: The most powerful austenite stabilizing element and one of the essential components of the present invention. In order to obtain stable austenite at room temperature after annealing, the C concentration of austenite at the annealing temperature needs to be increased to about 1% or more. Therefore, the C content of steel is set to 0.05% or more.
[0034]
By increasing the C content, the strength of the steel can be increased. However, if the content exceeds 0.25%, the strength becomes too high to be suitable for plastic working applications, and the weldability is also deteriorated. Therefore, the upper limit of the C content is 0.25%. Preferably it is 0.20% or less.
[0035]
Si and Al: These elements are ferrite stabilizing elements. By containing a proper amount of these, the volume fraction of ferrite is increased at the ferrite + austenite two-phase region temperature during annealing, the C concentration of equilibrium austenite is increased, and the effect of stabilizing austenite is obtained.
[0036]
Although Si is not an essential element, Si has an action of suppressing precipitation of carbides, and an effect of concentrating C in austenite is also obtained during bainite transformation during cooling from two-phase annealing.
[0037]
In order to sufficiently obtain these effects, Si and / or Al, Al is sol. A total of 1.0% or more is contained as Al. In addition, all Al content in this invention is sol. Means Al. The ferrite stabilization effect by these elements is saturated when the total content exceeds 2.5%, and adding more than that only impairs the economy, so the total content of both elements is 2.5% or less.
[0038]
Si has the effect of strengthening ferrite, so it is useful to increase the strength of the steel. However, when the Si content exceeds 2.0%, the surface quality deterioration due to the high Si scale characteristic of Si-added steel sheets is significant. become. In order to avoid this, the Si content is set to 2.0% or less. Since Si has an effect of inhibiting the wettability of molten zinc, when performing hot dip galvanizing or alloying hot dip galvanizing, the Si content is preferably 0.8% or less. More preferably, it is 0.6% or less.
[0039]
Since Al does not hinder plating wettability at the time of hot dip galvanizing, it is preferable to contain Al when hot dip galvanizing is performed. Al is used as a deoxidizing material during steel making, but is contained in an amount of 0.005% or more in order to obtain a sufficient deoxidizing effect. If the Al content exceeds 2.0%, inclusions increase in the steel sheet and the ductility is impaired, so the Al content is set to 2.0% or less.
[0040]
Mn: has an austenite stabilizing effect and is one of the essential elements of the high-tensile steel sheet of the present invention. In the cooling process after the two-phase region annealing, the Mn content is set to 0.8% or more so that austenite remains at room temperature without being transformed into martensite. On the other hand, Mn is an element that easily segregates during solidification, and if excessively contained, it segregates to form a band-like structure, and ductility decreases. In order to avoid this, the Mn content is 2.5% or less. Preferably, it is 2.0% or less.
[0041]
P: Although not an essential element, it has an effect of strengthening steel by dissolving in ferrite. Further, when coexisting with Cu, there is also an effect of improving the corrosion resistance by forming a stable protective film on the surface layer of steel. Therefore, in order to obtain these effects, P may be contained in an amount of 0.01% or more. However, P is easily segregated at the time of solidification, and if it is excessively contained, a band-like structure resulting from segregation is generated, and ductility is impaired, and weldability of steel is also deteriorated. Therefore, even when contained, the content is made 0.05% or less. Preferably it is 0.02% or less.
[0042]
Ti, Nb: These elements are not essential, but they are carbide-generating elements, and have the effect of forming fine precipitates and refining the hot rolled sheet crystal structure to increase the strength of the steel sheet. In order to obtain such an effect, either or both of these elements may be contained in an amount of 0.003% to 0.05%. However, when the total content of the two types exceeds 0.05%, the decrease in ductility becomes more significant than the increase in strength. Therefore, when the two types are included simultaneously, the upper limit of the total amount is 0.05%. To do.
[0043]
Further, Ti is easily bonded to N, and TiN is precipitated in preference to the precipitation of AlN, and has an effect of preventing slab cracking due to AlN. In order to obtain this effect, it is preferable to contain Ti in a range that satisfies 0.003% or more and satisfies (Ti / 48) / (N / 14) ≧ 2.
[0044]
Cu, Ni, Co: These elements are not essential, but all of them are not easily dissolved in iron carbide, and since precipitation of carbide is suppressed at the time of bainite transformation, an effect that retained austenite is easily obtained is obtained. In order to obtain these effects, one or more of the group consisting of Cu, Ni, and Co, Cu is 0.2% or more, Ni is 0.1% or more, and Co is 0.0005% or more. You may make it contain. If any element is excessively contained, the bainite transformation becomes insufficient. Therefore, the upper limit in the case of inclusion is 1.0% for Cu, 0.5% for Ni, 1.0% for Co, 2% or more. When contained, the total amount is 1.5% or less. Further, since Cu improves the corrosion resistance when coexisting with P, it may be added for this purpose.
[0045]
In addition, since Cu causes slab cracking, when Cu is contained, it is preferable that Ni is combined and contained within a range that satisfies Ni ≧ Cu / 2.
The balance is Fe and inevitable impurities. Among unavoidable impurities, S precipitates as MnS and not only inhibits ductility, but also consumes Mn added as an austenite stabilizing element as a precipitate, so its content is 0.01% or less. It is good. Further, if the N content is large, it causes slab cracking in AlN and also reduces the ductility of AlN in the product, so the content is preferably 0.005% or less.
[0046]
Surface hardening properties;
As described above, in the member that has been pre-formed and subjected to the baking treatment, the hardness of the steel plate surface portion has a surface hardening property that is higher than the hardness of the central portion by a certain percentage or more. It is extremely important for tensile steel sheets. There are various shapes of structural members, and there are various forming methods, strain distributions, etc., so this surface hardening property is obtained by applying 10% tensile bending to bending back deformation (hereinafter simply referred to as “tensile”). The thickness of the steel sheet before the pre-forming, after the pre-forming with “bending deformation”) and the difference in hardness between the steel sheet surface and the center of the thickness after baking at 170 ° C. for 20 minutes. It is defined that the ratio to the hardness of the central portion (X represented by the following formula, hereinafter, also simply referred to as “hardness ratio”) satisfies the relationship of 0.12 or more.
X = (HVs−HVc) / HV₀
Where HV₀  Is the Vickers hardness at the center of the thickness of the steel sheet before the preforming, HVc is the Vickers hardness at the center of the thickness after the preforming and baking process, and HVs is the preforming and baking process. It is the Vickers hardness of the later surface portion. Here, the Vickers hardness of the surface portion means a value obtained by averaging the hardness measured at a portion 1/8 of the plate thickness away from the steel plate surface for both surfaces. The bending radius in the pre-forming is 2.5 times (2.5 t) the plate thickness (t).
[0047]
According to the steel sheet having a high hardness ratio of the present invention, a structural member having high impact absorption energy when deformed at high speed can be obtained. When the hardness ratio is less than 0.12, the resulting member has insufficient impact characteristics. To obtain a structural member with good impact characteristics, a steel sheet with a hardness ratio of 0.12 or more is used. It is effective to do. Therefore, the high-tensile steel plate of the present invention has a hardness ratio, that is, X of 0.12 or more. In order to select more excellent impact characteristics, a steel sheet having a hardness ratio of 0.15 or more is preferable.
[0048]
The high-tensile steel sheet of the present invention is formed by a press tool having a die, a punch and a wrinkle presser, and has a bending and bending back deformation at least in the vicinity of the die shoulder in the forming process, and is subjected to a normal paint baking process after forming. If it is used for a part, a structural member having excellent impact resistance can be obtained. When the press tool is provided with a wrinkle holding bead or the like, the bending and bending back that the steel plate receives when passing through the bead portion also contributes to the surface hardening.
[0049]
Even if the portion to be bent and bent back is used only for a part of a part constituting the structural member, an effect of improving the impact resistance characteristics can be obtained. If the closed cross-section structural member shown in FIG. 1 is described as an example, the portion to be bent and bent back is the vertical wall portion 2 of the hat cross-section component. The bottom section 11 of the hat cross-section component and the flat plate component 12 are not bent and bent back and thus have no surface hardening characteristics, but they may be used.
[0050]
The high-tensile steel sheet according to the present invention can obtain a desired effect even if it is a surface-treated steel sheet that has been subjected to a treatment such as electroplating or hot dipping, in addition to a cold-rolled steel sheet.
Production method;
The high-tensile steel sheet having excellent ductility and impact resistance according to the present invention is preferably manufactured by hot rolling, cold rolling, and recrystallization annealing a steel having the above chemical composition by the following method. .
[0051]
Steel having the above chemical composition is cast into a slab by a conventional method. The ingot may be rolled to form a steel slab, which may be a slab. The slab is heated by a conventional method and roughly rolled, and then subjected to finish rolling. However, if the slab temperature after casting is high and the finishing temperature described below can be secured, rough slab heating is omitted. It doesn't matter. Moreover, when a thin slab is obtained by a known method such as strip casting, rough rolling may be omitted.
[0052]
Finish rolling: The hot-rolled sheet used as the base material of the high-strength steel sheet of the present invention is composed of ferrite and soft pearlite, and the pearlite is uniformly dispersed in order to obtain a steel sheet having excellent local ductility in the final product. It shall have a crystal structure.
[0053]
If the finish rolling start temperature in hot rolling is excessively high, recovery and recrystallization of austenite during rolling proceeds rapidly, strain accumulation becomes insufficient, and ferrite transformation in the cooling process after rolling is delayed, The volume fraction of soft ferrite is reduced. In order to avoid this, the finish rolling start temperature is set to 1050 ° C. or lower. The lower limit of the finish rolling start temperature is not particularly limited, and is desirably low as long as the finish rolling exit temperature described below is satisfied.
[0054]
The finish rolling finish temperature is 800 ° C. or higher. When the finish rolling finish temperature is a low temperature of less than 800 ° C., ferrite transformation occurs during rolling, and a processed ferrite structure in which crystal grains are extended is obtained, and a hot-rolled steel sheet structure in which the second phase is uniformly dispersed cannot be obtained. .
[0055]
Auxiliary heating: It is necessary to satisfy the entrance side temperature and the exit side temperature of the finish rolling described above over the entire length of the hot-rolled coil. When the steel slab is long, the steel slab temperature is lowered during rolling, and the finishing temperature may not be ensured in the latter half of hot rolling. Moreover, since the entrance temperature of finish rolling is limited low, the finishing temperature may not be ensured due to a temperature drop at the end of the steel slab width direction. In such a case, it is preferable to perform auxiliary heating on the finishing rolling entry side. The auxiliary heating method is not limited, but an electromagnetic induction heating method in which the control of the amount of heat input according to the temperature distribution of the slab on the finishing rolling entry side is easy is preferable.
[0056]
Cooling after finish rolling: After finishing rolling, in order to promote ferrite transformation, rapid cooling to 750 ° C. is performed at a cooling rate of 20 ° C./second or more. When the rapid cooling end temperature is higher than 750 ° C. or the cooling rate is less than 20 ° C./sec, the austenite recovers during the cooling, the processing strain disappears, and the ferrite transformation is difficult to proceed. not good.
[0057]
In order to promote the ferrite transformation, it is desirable to continue for 2 seconds or more in the temperature range until the start of winding after the rapid cooling. This staying treatment is preferably performed by cooling in the above temperature range by air cooling or slow cooling. If the residence time is less than 2 seconds, the ferrite transformation becomes insufficient, which is not good. A more preferable residence time is 5 seconds or more. If the residence time exceeds 10 seconds, the necessary cooling table becomes longer, so the residence time is preferably 10 seconds or less. After the retention treatment, rapid cooling may be performed at an arbitrary cooling rate up to the coiling temperature.
[0058]
Winding temperature: When the winding temperature becomes high, the scale loss increases, and the winding shape of the wound coil collapses because the steel becomes soft. In order to avoid this, the coiling temperature is 700 ° C. or less. Preferably it is 680 degrees C or less, More preferably, it is 650 degrees C or less.
[0059]
Mn has the effect of lowering the Ar3 point and increasing P. Therefore, when segregation of these elements occurs when the steel solidifies, ferrite transformation is delayed at the part of positive segregation in the case of Mn and negative segregation in the case of P, and pearlite precipitates in a band shape, resulting in a band-like structure. Arise. Such steels are prone to microvoids after cold rolling, which may impair the local ductility of the product steel sheet. In a steel having a high content of Mn and P, the band-like structure of pearlite can be reduced by increasing the coiling temperature and promoting ferrite transformation. For this reason, in this invention, the lower limit (Tc) of coiling temperature is prescribed | regulated in relation to Mn and P content.
[0060]
That is, the lower limit Tc (° C.) of the coiling temperature is not less than the value calculated by 430 + 70 × Mn (%) + 1000 × P (%). If the coiling temperature is too low, bainite and martensite are generated as the second phase, and it is necessary to wind at a temperature equal to or higher than the above-described Tc also from this viewpoint.
[0061]
Cold rolling: A hot-rolled steel sheet obtained by hot rolling by the above method is cold-rolled after removing scales by pickling or the like by a conventional method. Cold rolling may be performed according to a conventional method, but the cold rolling reduction is 40% or more and 80% or less in total. When the cold rolling reduction is less than 40%, the rolling efficiency is lowered, and when it exceeds 80%, microvoids between the ferrite and the second phase are increased, which adversely affects the ductility after recrystallization annealing. A preferable cold rolling reduction is 70% or less in total.
[0062]
Annealing: The annealing temperature is set to a temperature range from the Ac1 transformation point to the Ac3 transformation point in order to concentrate C into austenite in the two phases of ferrite and austenite. If the annealing temperature is too low, it takes too much time for the cementite to re-dissolve, and if it is too high, the volume fraction of austenite becomes too large and the C concentration in the austenite is reduced. Preferably it is the range of 800 degreeC or more and 850 degrees C or less.
[0063]
The soaking time is 30 seconds or more in order to sufficiently dissolve cementite. If the soaking time exceeds 90 seconds, the austenite grains are coarsened, which is not preferable, so the soaking time is 90 seconds or less.
[0064]
After completion of soaking, rapid cooling is performed at a temperature range of 700 to 450 ° C. at 30 ° C./second or more in order to suppress pearlite transformation. The cooling rate between the soaking temperature and 700 ° C. is not limited, but in order to increase the volume fraction of ferrite and concentrate C in austenite, it is preferable to cool to 700 ° C. at 10 ° C./second or less. .
[0065]
Following the rapid cooling, the sample is retained for 200 seconds or more and 400 seconds or less in a temperature range of 450 ° C. or lower and 370 ° C. or higher. This residence method may be a method of maintaining a constant temperature, or a method of gradually lowering the temperature between 450 ° C. or less and 370 ° C. in the range of 200 seconds or more and 400 seconds or less. When the residence temperature exceeds 450 ° C., bainite transformation does not occur, and when it is less than 370 ° C., it becomes lower bainite, C concentration to austenite hardly occurs, and a desired retained austenitic steel sheet cannot be obtained.
[0066]
The cooling after the residence is not limited, but the cooling rate may be increased in order to simplify the equipment. Moreover, in order to manufacture a hot dip galvanized steel sheet, you may perform the said annealing process using a continuous hot dip plating line. An alloying heat treatment may be performed in order to obtain alloyed hot dip galvanizing.
[0067]
Temper rolling: After annealing, temper rolling may be performed by a known method for the purpose of adjusting surface roughness, forcing flatness, and reducing elongation at yield point. In that case, in order not to lower the ductility, the temper rolling elongation is preferably 2.0% or less.
[0068]
What is necessary is just to manufacture by a well-known method except the above.
[0069]
【Example】
Example 1
Steel having the chemical composition shown in Table 1 was melted in a laboratory to form a steel ingot having a thickness of 60 mm, a width of 150 mm, and a mass of 17 kg, which was hot forged to have a thickness of 25 mm and a width of 150 mm. I got a piece.
[0070]
[Table 1]

These steel slabs were charged into a heating furnace and held at 1200 ° C. for 30 minutes, then removed from the furnace, naturally cooled to 1000 ° C., and subjected to hot finish rolling at a rolling start temperature of 1000 ° C. The number of passes of finish rolling was 3 passes in total, the thickness after finish rolling was 3.5 mm, and the finish rolling finish temperature was 850 ° C. Immediately after the hot finish rolling, water spray cooling is performed for 4 seconds to 720 ° C. (average cooling rate 33 ° C./second), followed by natural cooling for 8 seconds to 680 ° C., and further water spray cooling for 2 seconds. The temperature was set to 620 ° C. (average cooling rate of 30 ° C./second), and this was placed in a furnace heated to 620 ° C. and held for 30 minutes, and then cooled to room temperature at 20 ° C./hour.
[0071]
The obtained hot-rolled sheet was pickled using a hydrochloric acid solution to remove the scale, and then cold-rolled to 1.2 mm at a total rolling reduction of 66%. The obtained cold-rolled sheet was heated to 820 ° C. and soaked for 40 seconds, then slowly cooled to 700 ° C. at 5 ° C./second, then cooled to 400 ° C. at 50 ° C./second, and 300 seconds at 400 ° C. After being held, it was cooled to room temperature at 30 ° C./second. The obtained annealed sheet was subjected to temper rolling with an elongation of 1.0%.
[0072]
About the rolling direction of these steel plates, the tension test was done based on the prescription | regulation of JIS-Z2241 using the No. 5 test piece prescribed | regulated to JIS-Z2201. The strain at the maximum load in the stress-strain curve at the time of the tensile test was defined as uniform elongation, and the difference between the total elongation and the uniform elongation was determined to obtain the local elongation value.
[0073]
A blank having a rolling direction obtained from the temper-rolled steel plate as a longitudinal direction is press-molded using a press tool shown in FIG. 1 was obtained. Both the punch and die shoulder radii were 3.0 mm. The wrinkle restraining force was adjusted so that the thickness distortion of the vertical wall portion 2 was 10%. The obtained hat cross-section part was subjected to a baking treatment of holding at 170 ° C. for 20 minutes, and then a small piece was cut out from the vertical wall portion, and the Vickers hardness of the cross section perpendicular to the rolling direction was measured.
[0074]
FIG. 3 is a layout diagram showing measurement positions of Vickers hardness. The Vickers hardness test was conducted in accordance with JIS-Z2244, and the test load was 4.9N. The measurement position in the cross section is the thickness center and both surfaces (positions that are 1/8 of the plate thickness inside), each measuring 5 points at intervals of 0.5 mm, the average of the thickness center is HVc, both surfaces The average of the measured values was HVs. In addition, also about the steel plate before preforming, 5 points | pieces of Vickers hardness were measured at 0.5 mm intervals in the sheet thickness center of a cross section perpendicular | vertical to a rolling direction, and the average was set to HV₀  It was. The hardness ratio X was calculated from these values.
[0075]
Spot-welded plate parts 12 having a width of 60 mm and a length of 200 mm obtained from the same cold-rolled steel plate subjected to the above press forming and the same cold-rolled, annealed and temper-rolled steel plate at intervals of 20 mm to provide a closed cross-sectional structure A member was prepared and subjected to a baking treatment of heating at 170 ° C. for 20 minutes to prepare a test body.
[0076]
The test specimen was mounted on a test stand with its longitudinal direction vertical, and a weight drop type shaft crushing test was performed in which a weight weighing 60 kg was dropped from above and collided with the upper end of the test specimen at a speed of 10 m / sec. It was. A load cell is installed at the bottom of the test body to measure the load acting on the test body, separately measuring the position change of the weight body, and acting on the test body before the weight body stops from these load-displacement relationships The average value P (kN) of the applied load was determined, and the impact resistance characteristics were determined based on the average axial crushing load.
[0077]
Table 2 shows the tensile properties of the steel sheet before processing, the hardness ratio measured at the vertical wall portion of the pre-formed and baked hat cross-section, and the axial crush average load measurement result of the specimen.
[0078]
[Table 2]

In Table 2, trial Nos. 3 to 11 are those in which the chemical composition of the steel satisfies the conditions defined by the present invention, and the hardness ratio X is 0.12 or more, and all are examples of the present invention. Trial No. 1 uses steel A with low C content, Trial Nos. 2, 12 and 13 use steels B, L and M with low Si + Al content, which have a hardness ratio of 0.12. All were evaluated as comparative examples. Since the average axial crush load of the test specimen varies depending on the tensile strength (TS) level of the steel sheet used as the material, it is appropriate to judge whether the impact resistance characteristics are good or bad according to the tensile strength of the steel sheet. As shown in Table 2, all of the test numbers 3 to 11 using the steel plates as examples of the present invention showed excellent axial crush average loads. The steel of the present invention was also excellent in the strength-ductility balance represented by TS × EL.
[0079]
In FIG. 4, the relationship between the axial crush average load of Table 2 and the tensile strength of the steel plate before each process is shown. As can be seen from FIG. 4, those having a hardness ratio X of 0.12 or more showed an axial crush average load that was about 10% higher than that of the comparative example even at the same tensile strength. It can be seen that those having a hardness ratio X of 0.15 or more are more excellent.
[0080]
(Example 2)
The steel D, E, and H steel pieces having the chemical compositions shown in Table 1 were hot-finished and rolled with the same conditions as in Example 1 except for the coiling temperature, and the hot-rolled sheet having the same dimensions as in Example 1 Then, the same pickling, cold rolling, annealing and temper rolling as in Example 1 were performed to prepare various cold rolled steel sheets. These steel plates were tested in the same manner as described in Example 1 to investigate tensile properties and local elongation. The obtained results are shown in Table 3 together with the coiling temperature.
[0081]
[Table 3]

As shown in Table 3, the test numbers 16, 17, 21, and 26 that satisfy the conditions specified by the manufacturing method of the present invention for the coiling temperature after hot rolling had particularly excellent local ductility. . Moreover, normal ductility (El) and the product (so-called ductility balance) of TS and El were also favorable.
[0082]
【The invention's effect】
The high-strength steel sheet of the present invention is excellent in local ductility, so that it can be easily processed into a structural member having a complicated shape typified by automobiles, and by press forming and baking processing involving bending and bending back during press processing. The impact resistance characteristics as a structural member can be greatly improved. Therefore, the high-strength steel sheet of the present invention is easy to increase the strength of automobile structural members and is effective for reducing the weight by reducing the thickness of the steel sheet, and is also effective for improving the collision safety. Is extremely large.
[Brief description of the drawings]
FIG. 1 is a perspective view of a test body simulating a closed cross-section structural member frequently used in a structural member.
FIG. 2 is a cross-sectional view for explaining a press-formed state of a hat cross-sectional component.
FIG. 3 is a schematic diagram showing Vickers hardness measurement points at a vertical wall portion of a hat cross-sectional component.
FIG. 4 is a graph showing the relationship between the tensile strength of a steel sheet and the axial crush average load of a closed cross-section structural member produced using the steel sheet.
[Explanation of symbols]
1: Hat cross-section part, 2: Vertical wall part, 3: Punch, 4: Punch shoulder, 5: Die, 6: Die shoulder, 7: Die groove, 8: Wrinkle retainer, 11: Bottom part, 12: Flat part part
It is a code | symbol in FIG.

Claims (8)

In mass%, C: 0.05 to 0.25%, Si: 2.0% or less, Al: 0.005 to 2.0%, Mn: 0.8 to 2.5%, P: 0.05 %, And (Si + Al): 1.0 to 2.5% is satisfied, and the balance has a chemical composition composed of Fe and inevitable impurities, and has tensile strength (TS) and total elongation (El). the product of the (TS × El) is Ri der 21900MPa ·% or more, subjected to preforming with 10% of the tensile bending deformation in the strain thick plate, then after having been subjected to baking treatment for holding at 170 ° C. 20 min high tensile steel hardness of the steel sheet surface portion and the thickness center portion is excellent in ductility and impact properties, characterized that you satisfy the following equation;
(HVs−HVc) /HV0≧0.12.
However, HV0: Vickers hardness at the center of the plate thickness before the above preforming,
HVc: Vickers hardness at the center of the plate thickness after the above preforming and baking process,
HVs: Vickers hardness of the surface portion after the above pre-molding and baking treatment. In mass%, C: 0.05 to 0.25%, Si: 2.0% or less, Al: 0.005 to 2.0%, Mn: 0.8 to 2.5%, P: 0.05 % And (Si + Al): 1.0 to 2.5% is satisfied, and Ti and / or Nb is Ti: 0.003 to 0.05%, Nb: 0.003 to 0 .05% and (Ti + Nb) ≦ 0.05% in a range that satisfies the chemical composition consisting of Fe and inevitable impurities, the tensile strength (TS) and the total elongation (El) product (TS × El) is Ri der 21900MPa ·% or more, the thickness strain was subjected to preforming with 10% of the tensile bending deformation, and then the surface of the steel sheet was subjected to a baking treatment for holding at 170 ° C. 20 min parts and ductility hardness of the plate thickness center is characterized that you satisfy the following formula and excellent impact resistance High-tensile steel sheet;
(HVs−HVc) / HV ₀ ≧ 0.12.
Where HV ₀ : Vickers hardness at the center of the plate thickness before the preforming,
HVc: Vickers hardness at the center of the plate thickness after the above preforming and baking process,
HVs: Vickers hardness of the surface portion after the above pre-molding and baking treatment. Further, the chemical composition is, by mass%, one or two or more of Cu, Ni, Co in the group consisting of Cu: 0.2 to 1.0%, Ni: 0.1 to 0.5. %, Co: 0.0005 to 1.0%, and (Cu + Ni + Co) ≦ 1.5%. The high-tensile steel sheet having excellent ductility and impact resistance according to claim 1. A steel piece having the chemical composition according to any one of claims 1 to 3 is subjected to hot finish rolling with a hot finish rolling start temperature of 1050 ° C or lower and an end temperature of 800 ° C or higher, and then 20 ° C. Cooling to 750 ° C. at a cooling rate of at least / sec, applying hot rolling having a step of winding up to 700 ° C. or higher and Tc (° C.) or higher calculated by the following formula, pickling the obtained steel sheet, and then Then, cold rolling is performed in a range where the total rolling reduction is 40% or more and 80% or less, and then maintained in the two-phase temperature range of ferrite and austenite for 30 seconds or more and 90 seconds or less, and thereafter 700 ° C. or less, 450 ° C. cooling the above temperature range 30 ° C. / sec or higher, 450 ° C. or less, 370 ° C. or higher temperature range at 200 seconds or more, after holding than 400 seconds, and characterized by applying annealing to cool to room temperature, tensile Between strength (TS) and total elongation (El) (TS × El) is the method of producing a high tensile steel sheet having excellent ductility and impact resistance is 21900MPa ·% or more;
Tc (° C.) = 430 + 70 × Mn (%) + 1000 × P (%). The method for producing a high-strength steel sheet excellent in ductility and impact resistance according to claim 4, wherein the steel slab before hot finish rolling is subjected to auxiliary heating. In mass%, C: 0.05 to 0.25%, Si: 2.0% or less, Al: 0.005 to 2.0%, Mn: 0.8 to 2.5%, P: 0.05 %, And (Si + Al): 1.0 to 2.5% is satisfied, and the balance has a chemical composition composed of Fe and inevitable impurities, and has tensile strength (TS) and total elongation (El). (TS × El) is 21900 MPa ·% or more, pre-molding with a tensile strain of 10% is applied to the plate thickness strain, and then subjected to a baking treatment that is held at 170 ° C. for 20 minutes. A structural member with impact resistance, characterized by subjecting a high-tensile steel sheet with hardness at the surface and thickness center of the steel sheet to satisfy the following formula, followed by forming and baking treatment after bending and bending deformation. Manufacturing method of
(HVs−HVc) / HV ₀ ≧ 0.12.
Where HV ₀ : Vickers hardness at the center of the plate thickness before the preforming,
HVc: Vickers hardness at the center of the plate thickness after the above preforming and baking process,
HVs: Vickers hardness of the surface portion after the above pre-molding and baking treatment. In mass%, C: 0.05 to 0.25%, Si: 2.0% or less, Al: 0.005 to 2.0%, Mn: 0.8 to 2.5%, P: 0.05 % And (Si + Al): 1.0 to 2.5% is satisfied, and Ti and / or Nb is Ti: 0.003 to 0.05%, Nb: 0.003 to 0 .05% and (Ti + Nb) ≦ 0.05% in a range that satisfies the chemical composition consisting of Fe and inevitable impurities, the tensile strength (TS) and the total elongation (El) Steel sheet surface after the product (TS × El) is 21900 MPa ·% or more, the sheet thickness is subjected to preforming with a tensile bending deformation of 10% and then held at 170 ° C. for 20 minutes The bending strength of the steel plate and the center of the plate thickness is changed to a high strength steel plate satisfying the following formula. Method of manufacturing a structural member having impact resistance, characterized in that applying the paint baking the after subjected to molding with;
(HVs−HVc) / HV ₀ ≧ 0.12.
Where HV ₀ : Vickers hardness at the center of the plate thickness before the preforming,
HVc: Vickers hardness at the center of the plate thickness after the above preforming and baking process,
HVs: Vickers hardness of the surface portion after the above pre-molding and baking treatment. Further, the chemical composition is, by mass%, one or two or more of Cu, Ni, Co in the group consisting of Cu: 0.2 to 1.0%, Ni: 0.1 to 0.5. %, Co: 0.0005 to 1.0%, and (Cu + Ni + Co) ≦ 1.5% in a range that satisfies the requirements, forming a high-strength steel sheet according to claim 6 or 7 with bending and bending back deformation. A method for producing a structural member having impact resistance characteristics, characterized in that a coating baking process is performed after the coating.

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