High-strength steel plate, steel strip, method for manufacturing high-strength steel plate, method for manufacturing steel strip and component

Abstract

To provide a high-strength steel sheet excellent in material stability to be a problem in a steel sheet of TS of 980 MPa or more, EL of 10% or more, YR of 50% or more and 85% or less, and 980 MPa class which is produced by a method simpler than the conventional technology, a steel strip, and a method for producing a high-strength steel sheet, a method for manufacturing a steel strip, and a member.SOLUTION: A high-strength steel sheet that has a specific component composition and structure, and is characterized by a ratio of 1.05 or more of an amount of Mn contained in tempered martensite to the amount of Mn contained in the steel sheet. However, the amount of Mn contained in tempered martensite is mass%. (Equation 1) Mn / (C+Si)≥1.2, C, Si, and Mn in Equation 1 all represent mass% of each element.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel sheet, a steel strip, a manufacturing method for the high-strength steel sheet, a manufacturing method for the steel strip, and a component. In particular, the present invention relates to a high-strength steel sheet having a TS of 980 MPa or more and an El of 10% or more, which is suitable for components used in industrial fields such as automotive and electrical machinery formed by cold pressing, and which can be manufactured into parts with high dimensional accuracy and has excellent material stability with little variation in YR, and a manufacturing method thereof. [Background technology]

[0002] In recent years, there has been an increasing need to reduce the weight of automobile bodies in order to improve fuel efficiency and protect the global environment. This has led to a demand for the application of high-strength steel sheets to automobile parts.

[0003] Conventionally, 780MPa- and 980MPa-grade steel sheets have been manufactured using DP steel, which has a dual-phase structure of ferrite and martensite. However, DP steel essentially requires annealing in the dual-phase region. 980MPa-grade steel sheets, in particular, have a problem in that slight temperature changes within the dual-phase region cause abrupt changes in the ferrite and martensite fractions, leading to variations in strength even within the same coil. High-strength steel sheets used in automotive reinforcement and structural components must be able to be manufactured with high dimensional accuracy. To achieve this, it is important to control the steel sheet's yield ratio (YR = yield strength YS / tensile strength TS) within a certain range. Controlling the yield ratio (YR) within a certain range reduces springback after steel sheet forming and improves dimensional accuracy during forming. Furthermore, increasing the steel sheet's yield ratio (YR) can increase the impact energy absorbed by the part during a collision. However, variations in mechanical properties within the same coil can lead to differences in the amount of springback during part forming, leading to dimensional errors in the final formed part and reduced productivity of the final product. Therefore, various studies have been conducted to produce steel sheets with minimal variation in mechanical properties within the same coil.

[0004] For example, Patent Document 1 specifies the area fractions of ferrite, bainitic ferrite, and martensite, the volume fraction of retained austenite, and the average crystal grain size, and further specifies that the average Mn content (mass %) in the retained austenite is 1.2 times or more the Mn content (mass %) in the steel, and that 60% or more of the total retained austenite is 2.1 times or more the C content (mass %) in the steel. This technology discloses a high-strength steel sheet that has a tensile strength (TS) of 780 MPa or more, excellent ductility as well as stretch flangeability, and also excellent material stability.

[0005] Furthermore, Patent Document 2 discloses a technology for realizing a high-strength steel sheet having a tensile strength of 980 MPa or more, an elongation of 13% or more, and reduced variation in stretch flangeability, by specifying the area ratio of tempered martensite having a hardness of 330 Hv or more and 450 Hv or less, specifying the grain size of the remaining ferrite, and further specifying the ratio between the upper and lower limits of the Mn concentration in the cross section of the steel sheet in the thickness direction. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2016 / 021193 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-65307 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the technology described in Patent Document 1 requires a process in which the cold-rolled steel sheet is heated to a temperature of 820°C or higher and 950°C or lower, cooled to the Ms point or lower, and then heated again to a temperature of 740°C or higher and 840°C or lower, and then cooled again, which is thought to result in high energy costs.

[0008] Furthermore, although the technology described in Patent Document 2 reduces variations in stretch flangeability for steel sheets of 980 MPa or more, it does not take into consideration variations in YR, which are likely to be a problem for 980 MPa-class steel sheets.

[0009] Therefore, the present invention aims to provide a high-strength steel sheet, steel strip, method for manufacturing a high-strength steel sheet, method for manufacturing a steel strip, and component that has a TS of 980 MPa or more, an El of 10% or more, and a YR of 50% to 85%, and that is excellent in the material stability that is a problem with 980 MPa-class steel sheets, using a manufacturing method simpler than conventional techniques. The above-mentioned steel sheets refer to hot-rolled steel sheets, cold-rolled steel sheets, and plated steel sheets such as GA and GI, and the above-mentioned steel strips refer to steel strips (also called coils) made from the above-mentioned steel sheets.

[0010] YR can be calculated using the following formula: YR = YS / TS × 100 (%) In the present invention, "it is possible to manufacture parts with high dimensional accuracy (high dimensional accuracy during forming)" means that the YR is 50% or more and 85% or less, and "excellent material stability" means that the variation in YR in the width direction of the steel sheet is 10% or less. [Means for solving the problem]

[0011] In order to solve the above problems, the inventors have conducted extensive research into the chemical composition and microstructure of steel sheets. As a result, they have found that it is possible to control the Mn distribution in the steel and the microstructural composition of the steel sheet by controlling the slab heating conditions and the temperature from hot rolling to coiling, and by changing the amount of heat the steel sheet receives during annealing, and that it is possible to obtain steel with high strength, a specified YR, and excellent material stability by controlling the subsequent cooling stop temperature and holding temperature. The present invention has been made based on the above findings, and the gist of the present invention is as follows. [1] In mass %, C: 0.020% or more and 0.200% or less, Si: 0.01% or more and 2.00% or less, Mn: 2.50% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less and O: 0.0100% or less, C, Si, and Mn satisfy the following formula 1, The balance is Fe and unavoidable impurities, The microstructure of the steel plate is The total area ratio of ferrite and bainitic ferrite is 30% or more and 70% or less, The area ratio of tempered martensite is 10% or more and 60% or less, The area ratio of fresh martensite is 5% or more and 35% or less, The remainder is at least one of retained austenite and pearlite, with an area ratio of 0% to 20%. A high-strength steel sheet characterized in that the ratio of the amount of Mn contained in tempered martensite to the amount of Mn contained in the steel sheet is 1.05 or more. However, the amount of Mn contained in the tempered martensite is expressed in mass %. (Formula 1)Mn / (C+Si)≧1.2 In formula 1, C, Si, and Mn each represent the mass % of each element. [2] Furthermore, the component composition is as follows: Ti: 0.500% or less, Nb: 0.500% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less Co: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, REM: 0.0100% or less, The high-strength steel plate according to [1], characterized in that it contains at least one or two or more elements selected from the following: [3] A high-strength steel sheet according to [1] or [2], characterized in that the ratio of the amount of C contained in the fresh martensite to the amount of C contained in the steel sheet is 1.50 or more. However, the amount of C contained in fresh martensite is expressed in mass %. [4] A high-strength steel sheet according to any one of [1] to [3], characterized in that the steel sheet has a plating layer on its surface. [5] The high-strength steel sheet according to [4], wherein the plating layer is an alloyed plating layer. [6] A steel strip comprising the high-strength steel plate according to any one of [1] to [3]. [7] A steel strip characterized by being made of the high-strength steel plate according to [4] or [5]. [8] A method for producing a high-strength steel plate according to any one of [1] to [3], a hot rolling process in which a steel material having the chemical composition according to [1] or [2] is heated, hot rolling is performed at a finish rolling temperature of 800°C or higher, and then coiling is performed at a coiling temperature of 700°C or lower; After the hot rolling process, A heating process in which the temperature is raised to a heating temperature T0 ° C that is equal to or higher than Ac1 ° C and satisfies formula 2, and then annealed at an annealing temperature T1 ° C that satisfies formula 3, and the cooling start temperature is set to T1 ° C, and the temperature is cooled from T1 ° C to 400 ° C at an average cooling rate of 5.0 ° C / s or more; Next, a reheating step is performed in which cooling is performed to a cooling stop temperature Tsq°C of 150°C to 370°C, and the temperature is raised to a temperature higher than the cooling stop temperature Tsq°C and 180°C to 430°C, and then the temperature is held at a holding temperature Toa°C higher than the cooling stop temperature Tsq°C and 180°C to 430°C for a holding time of toa seconds, In the heating step, the time t from Ac1°C through the heating target temperature T0°C to the annealing temperature T1°C is h The heat input parameter Q defined by Equation 4 using the annealing time t seconds at the annealing temperature T1 ° C. and the annealing temperature T1 ° C. is 3200 or more and 4800 (° C. × min) or less, A method for manufacturing a high-strength steel plate, characterized in that in the reheating step, the holding temperature Toa°C, the holding time toa seconds, and the heat input parameter Q satisfy Equation 5. (Formula 2)T1 <T0 (Formula 3)720 <T1<Ac3+100 (Formula 4)Q=[{(Ac1+T1) / 2}×t h / 60] + T1×t / 60 (Equation 5) Q / 10≦Toa(logtoa) [9] A method for producing a steel strip according to the above [6], a hot rolling process in which a steel material having the chemical composition according to [1] or [2] is heated, hot rolling is performed at a finish rolling temperature of 800°C or higher, and then coiling is performed at a coiling temperature of 700°C or lower; After the hot rolling process, A heating process in which the temperature is raised to a heating temperature T0 ° C that is equal to or higher than Ac1 ° C and satisfies formula 2, and then annealed at an annealing temperature T1 ° C that satisfies formula 3, and the cooling start temperature is set to T1 ° C, and the temperature is cooled from T1 ° C to 400 ° C at an average cooling rate of 5.0 ° C / s or more; Next, a reheating step is performed in which cooling is performed to a cooling stop temperature Tsq°C of 150°C to 370°C, and the temperature is raised to a temperature higher than the cooling stop temperature Tsq°C and 180°C to 430°C, and then the temperature is held at a holding temperature Toa°C higher than the cooling stop temperature Tsq°C and 180°C to 430°C for a holding time of toa seconds, In the heating step, the time t from Ac1°C through the heating target temperature T0°C to the annealing temperature T1°C is h The heat input parameter Q defined by Equation 4 using the annealing time t seconds at the annealing temperature T1 ° C. and the annealing temperature T1 ° C. is 3200 or more and 4800 (° C. × min) or less, A method for manufacturing a steel strip, characterized in that in the reheating step, the holding temperature Toa°C, the holding time toa seconds, and the heat input parameter Q satisfy Equation 5. (Formula 2)T1 <T0 (Formula 3)720 <T1<Ac3+100 (Formula 4)Q=[{(Ac1+T1) / 2}×t h / 60] + T1×t / 60 (Equation 5) Q / 10≦Toa(logtoa)

[10] A method for producing a high-strength steel sheet according to [8], characterized in that cold rolling is performed after the hot rolling step and before the heating step.

[11] The method for producing a high-strength steel sheet according to [8] or

[10] , characterized in that after the reheating step, a plating treatment is carried out to form a plating layer on the surface of the steel sheet.

[12] The method for producing a high-strength steel sheet according to

[11] , wherein the plating treatment is an alloying plating treatment to form an alloying plating layer.

[13] A method for producing a steel strip according to [9], characterized in that cold rolling is performed after the hot rolling step and before the heating step.

[14] A method for producing a steel strip according to [9] or

[13] , characterized in that after the reheating step, a plating treatment is carried out to form a plating layer on the surface of the steel sheet.

[15] The method for manufacturing a steel strip according to

[14] , characterized in that the plating treatment is an alloying plating treatment to form an alloying plating layer.

[16] A member, characterized by being made using the high-strength steel plate according to any one of [1] to [5]. [Effects of the Invention]

[0012] According to the present invention, it is possible to obtain high-strength steel sheets, steel strips, and components having excellent material stability with a TS of 980 MPa or more, an El of 10% or more, and a YR of 50% to 85%, with little variation in YR. Therefore, the present invention is of great value in industrial fields such as automobiles and electrical equipment, and is particularly useful for reducing the weight of automobile body frame parts. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be specifically described below. Note that "%" representing the content of component elements means "% by mass" unless otherwise specified. (1) The reasons for limiting the chemical compositions of the high-strength steel plate (steel plate) and steel strip to the above ranges in the present invention will be explained.

[0014] C: 0.020% or more and 0.200% or less C is an element that affects the fraction and strength of tempered martensite, bainite, and fresh martensite. If the C content is less than 0.020%, the fraction of fresh martensite and tempered martensite decreases, making it difficult to obtain the desired TS. Therefore, the C content is set to 0.020% or more, preferably 0.022% or more, and more preferably 0.024% or more. On the other hand, if the C content exceeds 0.200%, the strength of the martensite phase becomes too dependent on the annealing temperature, impairing the material stability. Therefore, the C content is set to 0.200% or less, preferably 0.180% or less, and more preferably 0.160% or less.

[0015] Si: 0.01% or more and 2.00% or less Si is an element that contributes to solid solution strengthening and improves the strength of steel sheet. If the Si content is less than 0.01%, this effect is poor, so the Si content is set to 0.01% or more. It is preferably 0.02% or more, and more preferably 0.04% or more. On the other hand, since Si is an element that suppresses cementite formation, if the Si content exceeds 2.00%, C that does not precipitate as cementite will combine with elements that form carbides, such as Nb and Ti, making it easier for unrecrystallized ferrite to form, particularly during annealing, and resulting in poor material stability. Therefore, the Si content is set to 2.00% or less, preferably 1.50% or less, and more preferably 1.00% or less.

[0016] Mn: 2.50% or more and 5.00% or less Mn is an element that affects the area fraction of tempered martensite and fresh martensite by improving hardenability. If the Mn content is less than 2.50%, a slight decrease in the annealing temperature significantly increases the fraction of soft phases such as ferrite, making it impossible to stably obtain the desired area fraction of tempered martensite or fresh martensite, thereby impairing material stability. For this reason, the Mn content is set to 2.50% or more. It is preferably set to 2.70% or more, more preferably set to 2.90% or more, and even more preferably set to 3.00% or more. On the other hand, if the Mn content exceeds 5.00%, the area fraction of ferrite decreases and ductility deteriorates. Therefore, the Mn content is set to 5.00% or less, preferably set to 4.50% or less, more preferably set to 4.00% or less, and even more preferably set to 3.90% or less.

[0017] P:0.100% or less P segregates at grain boundaries, causing embrittlement and thus adversely affecting workability, so its amount must be 0.100% or less. Therefore, the P content is set to 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less. There is no particular lower limit, but since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferably set to 0.001% or more.

[0018] S: 0.0200% or less S segregates at grain boundaries, embrittling steel during hot working, and may also adversely affect workability through the formation of sulfides, so its content must be 0.0200% or less. Therefore, the S content is set to 0.0200% or less, preferably 0.0180% or less, and more preferably 0.0100% or less. There is no particular lower limit, but due to production technology constraints, it is preferably set to 0.0001% or more.

[0019] Al: 0.100% or less Al acts as a deoxidizer and is an element effective in reducing inclusions in steel, and is preferably added in the deoxidation process. However, a large content of more than 0.100% increases the risk of steel slab cracking during continuous casting, reducing manufacturability. Therefore, the Al content is set to 0.100% or less, preferably 0.080% or less, and more preferably 0.070% or less. There is no particular lower limit for the Al content, but it is preferably 0.001% or more, and more preferably 0.005% or more.

[0020] N: 0.0100% or less N has a negative effect on material stability by forming coarse nitrides, and if the N content exceeds 0.0100%, a large amount of coarse nitrides is formed, resulting in a significant deterioration in material stability. The lower the N content, the better, so the N content is set to 0.0100% or less. Preferably, it is 0.0090% or less. More preferably, it is 0.0080% or less. There is no particular lower limit, but due to constraints on production technology, it is preferably set to 0.0001% or more.

[0021] O: 0.0100% or less O exists as an oxide and reduces the ultimate deformability of the steel sheet, thereby reducing the material stability. Therefore, the O content must be 0.0100% or less, and preferably 0.0050% or less. There is no particular lower limit for the O content, but due to constraints on production technology, the O content is preferably 0.0001% or more.

[0022] (Formula 1)Mn / (C+Si)≧1.2 In the formula 1, C, Si, and Mn each represent the mass percentage of the element contained in the steel. In addition to the above-mentioned composition, if the amount of Mn is small relative to the amounts of C and Si, the area ratio of the tempered martensite or fresh martensite formed will be small, and the strength of the tempered martensite or fresh martensite will be high, resulting in localized areas of high strength and low strength, and poor material stability. Therefore, it is necessary to satisfy Equation 1. Preferably, the right-hand side of Equation 1 is 1.4. More preferably, the right-hand side of Equation 1 is 1.5. The upper limit is preferably Mn / (C+Si)<8.0.

[0023] In addition to the above components, the alloy may contain, by mass%, one or more selected from Ti: 0.500% or less, Nb: 0.500% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, and REM: 0.0100% or less.

[0024] Ti: 0.500% or less Ti contributes to precipitation strengthening and also refines the prior austenite grain size, which in turn refines tempered martensite and bainite, thereby effectively improving steel strength. However, if Ti is contained in an amount exceeding 0.500%, Ti may remain in an undissolved state during heating of the steel material before hot rolling, increasing the number of coarse precipitates and reducing ductility. Therefore, if Ti is contained, the Ti content should be 0.500% or less, preferably 0.100% or less, and more preferably 0.070% or less.

[0025] Nb: 0.500% or less Nb is an element that improves the strength of steel sheets by precipitation strengthening through the formation of fine precipitates. On the other hand, if the Nb content exceeds 0.500%, precipitates such as Nb carbides and nitrides become coarse, making it difficult to uniformly control the precipitates in the steel sheet, resulting in poor material stability. Therefore, if Nb is contained, the Nb content is set to 0.500% or less, preferably 0.100% or less, and more preferably 0.070% or less.

[0026] V:0.200% or less V contributes to precipitation strengthening and also refines the prior austenite grain size, which in turn refines tempered martensite and bainite, thereby effectively improving steel strength. While there is no particular lower limit, a V content of 0.001% or more is preferred to achieve the above-mentioned effects. However, if V is contained in an amount exceeding 0.200%, V may remain in an undissolved state during heating of the steel material before hot rolling, increasing the number of coarse precipitates and reducing ductility. Therefore, if V is contained, the V content should be 0.200% or less, preferably 0.180% or less.

[0027] Ta: 0.10% or less Ta, like Ti, contributes to high strength by forming alloy carbides and alloy carbonitrides. Furthermore, Ta partially dissolves in Nb carbides and Nb carbonitrides to form composite precipitates such as (Nb, Ta)(C, N). This significantly suppresses coarsening of precipitates and stabilizes the contribution of precipitation strengthening to strength. Ta can be added as needed. While there is no particular lower limit, a content of 0.01% or more is preferred to achieve the above-mentioned effects. However, excessive Ta content saturates the precipitate stabilization effect and increases alloy costs. Therefore, if Ta is added, the Ta content should be 0.10% or less, preferably 0.08% or less, and more preferably 0.07% or less.

[0028] W: 0.10% or less W can be added as needed to improve the hardenability of steel and further improve steel strength by refining tempered martensite and bainite. While there is no particular lower limit, a W content of 0.01% or more is preferred to achieve the above-mentioned effects. However, if W is added in excess of 0.10%, the amount of coarse precipitates such as WN and WS remaining in an undissolved state during slab heating in hot rolling may increase, resulting in reduced ductility. Therefore, if W is added, the W content should be 0.10% or less, preferably 0.08% or less, and more preferably 0.05% or less.

[0029] B: 0.0100% or less B is an element that can improve hardenability by segregating at austenite grain boundaries, and can form a structure mainly composed of tempered martensite and bainite, thereby improving the strength of the steel sheet, and therefore can be added as needed. While there is no particular lower limit, to obtain the above-mentioned effects, it is preferable to set the B content to 0.0003% or more. However, if the B content exceeds 0.0100%, coarse precipitates are formed and ductility decreases. Therefore, when B is contained, the B content is set to 0.0100% or less, preferably 0.0090% or less, more preferably 0.0080% or less, and even more preferably 0.0070% or less.

[0030] Cr:1.00% or less Cr has the effect of improving the balance between strength and ductility, so it can be added as needed. There is no particular lower limit, but to obtain the above effect, it is preferable to set the content to 0.01% or more. However, if it is added in excess of 1.00%, the area ratio of fresh martensite becomes excessive, and dimensional accuracy and ductility during forming decrease. Therefore, when Cr is added, the Cr content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

[0031] Mo: 1.00% or less Mo has the effect of improving the balance between strength and ductility, so it can be added as needed. There is no particular lower limit, but to obtain the above effect, it is preferable to set it to 0.01% or more. However, if it is added in excess of 1.00%, the area ratio of fresh martensite becomes excessive, and dimensional accuracy and ductility during forming decrease. Therefore, when Mo is added, the Mo content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

[0032] Co: 1.00% or less Co is an element effective in improving hardenability and strengthening steel, so it can be added as needed. There is no particular lower limit, but to obtain the above effects, it is preferable to set the content at 0.01% or more. However, if the content exceeds 1.00%, the area ratio of fresh martensite becomes too large, resulting in reduced dimensional accuracy and ductility during forming. Therefore, if Co is added, the Co content should be 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

[0033] Ni: 1.00% or less Ni increases the strength of steel through solid solution strengthening, so it can be added as needed. There is no particular lower limit, but to obtain the above effect, it is preferable to set the content to 0.01% or more. However, if it is added in excess of 1.00%, the area ratio of fresh martensite becomes excessive, and dimensional accuracy and ductility during forming decrease. Therefore, when Ni is added, the Ni content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

[0034] Cu:1.00% or less Cu is an element effective in strengthening steel and can be added as needed. While there is no particular lower limit, a content of 0.01% or more is preferred to achieve the above-mentioned effects. However, if the content exceeds 1.00%, the area ratio of tempered martensite, bainite, and fresh martensite becomes excessive, resulting in reduced dimensional accuracy and ductility during forming. Therefore, if Cu is added, the Cu content should be 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

[0035] Sn: 0.200% or less, Sb: 0.200% or less Sn and Sb can be added as needed to suppress decarburization in a region of several tens of micrometers in the surface layer of the steel sheet, which occurs due to nitriding or oxidation of the steel sheet surface, prevent a decrease in the area ratio of tempered martensite on the steel sheet surface, and ensure strength and material stability. While there are no particular lower limits, to achieve the above effects, it is preferable that the content of these elements be 0.001% or more. However, excessive content of any of these elements exceeding 0.200% may cause the steel sheet to become embrittled, resulting in a decrease in ductility. Therefore, when Sn and Sb are added, the content of Sn and Sb should be 0.200% or less, preferably 0.100% or less, and more preferably 0.050% or less.

[0036] Ca: 0.0100% or less, Mg: 0.0100% or less Ca and Mg are elements that make sulfides spheroidal. Although there are no particular lower limits, their contents are preferably 0.0010% or more. However, excessive contents of each exceeding 0.0100% can cause an increase in inclusions, leading to surface and internal defects and reduced ductility. Therefore, when Ca and Mg are contained, the Ca and Mg contents should each be 0.0100% or less, preferably 0.0090% or less, and more preferably 0.0080% or less.

[0037] Zr: 0.100% or less, Te: 0.100% or less Zr and Te can be added as needed to improve the hardenability of steel and to improve steel strength by forming fine Zr-containing carbides and refining tempered martensite and bainite. While there are no particular lower limits, their contents are preferably 0.001% or more. However, excessive content exceeding 0.100% can increase inclusions, causing surface and internal defects, and reducing ductility. Therefore, when Zr and Te are added, their contents should each be 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less.

[0038] Hf: 0.10% or less Hf is an element that affects the distribution of oxides. Although there is no particular lower limit, the content is preferably 0.001% or more. However, excessive addition of more than 0.10% increases coarse precipitates and inclusions, impairing the stability of the material properties of the steel sheet. Therefore, when Hf is contained, the Hf content is set to 0.10% or less, preferably 0.08% or less, and more preferably 0.05% or less.

[0039] Bi:0.200% or less If Bi is 0.200% or less, the amount of coarse precipitates and inclusions will not increase, and the material stability of the steel sheet will not be impaired. Therefore, when Bi is contained, the Bi content is set to 0.200% or less, preferably 0.150% or less, and more preferably 0.100% or less. There is no particular lower limit, but the Bi content is preferably 0.001% or more.

[0040] REM: 0.0100% or less REM is an element that makes sulfides spheroidal. Although there is no particular lower limit, the content is preferably 0.0010% or more. However, excessive content exceeding 0.0100% may cause an increase in inclusions, resulting in surface and internal defects and a decrease in ductility. Therefore, the REM content is set to 0.0100% or less, preferably 0.0090% or less, and more preferably 0.0080% or less.

[0041] In addition, when the content of each of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Co, Ni, Cu, Sn, Sb, Ca, Mg, Zr, Te, Hf, Bi and REM is less than the preferable lower limit value, the effect of the present invention is not impaired, and therefore, these elements are included as unavoidable impurities.

[0042] (2) Next, the microstructure of high-strength steel plate (steel plate) will be explained.

[0043] Total area ratio of ferrite and bainitic ferrite: 30% to 70% Ferrite and bainitic ferrite contribute to the ductility of steel sheets. If the total area fraction of ferrite and bainitic ferrite is less than 30%, the ductility will be less than 10%, so the sum of the area fractions of ferrite and bainitic ferrite must be at least 30% or more, preferably 32% or more. On the other hand, if the sum of the area fractions of ferrite and bainitic ferrite exceeds 70%, the steel structure will be dominated by relatively soft ferrite and bainitic ferrite, and the TS will be less than 980 MPa. Therefore, the sum of the area fractions of bainite and tempered martensite must be 70% or less, preferably 68% or less.

[0044] Area ratio of tempered martensite: 10% to 60% Tempered martensite contributes to the strength of the steel sheet. In addition, including tempered martensite in the steel sheet structure is effective in maintaining high strength. When the area fraction of tempered martensite is less than 10%, the TS is less than 980 MPa. Therefore, the area fraction of tempered martensite must be at least 10% or more, preferably 12% or more. On the other hand, if the area fraction of tempered martensite exceeds 60%, the YR increases and the dimensional accuracy during forming decreases, so the area fraction of tempered martensite must be 60% or less, preferably 58% or less.

[0045] Area ratio of fresh martensite: 5% to 35% Fresh martensite is a very hard phase, and by dispersing fresh martensite in the structure, it is possible to obtain steel with excellent dimensional accuracy during forming. To fully obtain this effect, the area fraction of fresh martensite must be 5% or more. On the other hand, if the area fraction of fresh martensite exceeds 35%, ductility decreases, so the area fraction of fresh martensite must be 35% or less. Preferably, it is 6% or more and less than 32%. In the present invention, the effects of the present invention are not impaired even if a residual structure such as retained austenite and pearlite, excluding ferrite, bainitic ferrite, tempered martensite, and fresh martensite, is present. When a residual structure is present, the residual structure includes at least one of retained austenite and pearlite, and specifically, the area ratio of at least one of retained austenite and pearlite is set to 0% or more and 20% or less. The area ratio of the residual structure is preferably 15% or less, more preferably 10% or less, and even more preferably 5% or less.

[0046] The ratio of the amount of Mn contained in the tempered martensite to the amount of Mn contained in the steel plate is 1.05 or more (however, the amount of Mn contained in the tempered martensite is expressed in mass %). Determining the ratio of the amount of Mn contained in the tempered martensite to the amount of Mn contained in the steel sheet is one of the important constituent items in the present invention. By controlling the amount of Mn distributed in austenite during annealing, transformation to bainite, which has a large TS variation with the holding temperature, is suppressed, resulting in a stabilized YR, a specified YR, and a steel sheet with excellent material stability. Since most of the regions that were austenite during annealing become tempered martensite in the final structure, in order to fully obtain this effect, the ratio of the amount of Mn contained in the tempered martensite to the amount of Mn contained in the steel sheet needs to be 1.05 or more, preferably 1.07 or more, and more preferably 1.10 or more.

[0047] The ratio of the amount of C contained in the fresh martensite to the amount of C contained in the steel sheet is 1.50 or more (however, the amount of C contained in the fresh martensite is expressed in mass %) (preferred condition). The holding in the reheating process promotes carbon enrichment in the untransformed austenite remaining in the steel sheet. Because the strength of martensite is determined by the carbon content, determining the ratio of carbon content in fresh martensite to the carbon content in the steel sheet is effective in achieving a TS of 980 MPa or higher. Furthermore, promoting carbon enrichment in the untransformed austenite remaining in the steel sheet suppresses transformation to bainite, which exhibits large TS fluctuations relative to the holding temperature. As a result, the YR is stabilized, a predetermined YR can be achieved, and a steel sheet with superior material stability can be obtained. To fully achieve these effects, the ratio of carbon content in fresh martensite to the carbon content in the steel sheet is preferably 1.50 or higher, more preferably 1.60 or higher. Although there is no particular upper limit set for the ratio of the amount of C contained in fresh martensite to the amount of C contained in the steel sheet, it is preferably 7.00 or less, and more preferably 6.50 or less.

[0048] (3) Next, the manufacturing method and manufacturing conditions of the high strength steel plate (steel plate) will be explained. [Hot rolling process] The heating conditions for the steel material are not particularly limited, but it is preferable to carry out the heating under the following conditions. Precipitates present during the heating stage of the steel material remain as coarse precipitates in the final steel sheet and do not contribute to strength. Therefore, it is preferable to remelt the coarse precipitates precipitated during casting as much as possible. Furthermore, if the precipitates are not remelted, the risk of problems occurring during hot rolling due to increased rolling load increases. Furthermore, from the viewpoint of scaling off defects such as bubbles and segregations in the surface layer of the steel material, reducing cracks and irregularities on the steel sheet surface, and achieving a smooth steel sheet surface, it is preferable to heat the steel material to 1100°C or higher for 1.0 hour or more. While there is no particular upper limit to the heating temperature of the steel material, a heating temperature above 1400°C increases the amount of oxidation, resulting in increased scale loss and reduced productivity. Therefore, the heating temperature is preferably 1400°C or lower. There is also no upper limit to the heating time, but it is preferably 48.0 hours or less to suppress decarburization from the slab surface due to heating.

[0049] Hot rolling finish temperature: 800°C or higher After heating, the steel material is hot-rolled to produce hot-rolled steel sheet. If the finish rolling temperature is below 800°C, the reduction ratio increases while the austenite is in an unrecrystallized state, leading to the development of abnormal textures and significant in-plane anisotropy in the final product. Furthermore, if rolling is completed in the two-phase region, there will be areas where carbon and manganese are significantly enriched and areas where they are not, depending on the phase. Because manganese diffusion is slow, this uneven distribution of manganese will result in areas with high and low manganese concentrations in the austenite produced through the heating and reheating processes. The high-manganese concentration areas exist as retained austenite, and the manganese concentration in the region that will eventually become tempered martensite will be low. As a result, it becomes difficult to achieve the desired YR and material stability will be compromised. Furthermore, the reprecipitation of precipitates dissolved during heating of the steel material increases the rolling load, which impedes cold rolling. Therefore, the finish rolling delivery temperature of the hot rolling, that is, the finish rolling end temperature, is set to 800°C or higher, preferably 850°C or higher, and more preferably 870°C or higher. Although there is no particular upper limit for the finish rolling end temperature, if the finish rolling temperature exceeds 1000°C, the amount of oxide (scale) generated increases rapidly, the interface between the base steel and the oxide becomes rough, the surface quality after pickling and cold rolling tends to deteriorate, and the crystal grain size becomes excessively coarse, which may cause the surface of the pressed product to become rough during processing. Therefore, the temperature is preferably 950°C or lower, more preferably 930°C or lower, and even more preferably 900°C or lower.

[0050] Coiling temperature after hot rolling: 700°C or less If the coiling temperature after hot rolling is higher than 700°C, not only will the crystal grain size of the hot-rolled sheet structure increase, but the area ratio of ferrite will also increase, and in the subsequent heat treatments of the heating and reheating steps, the area ratio of ferrite and bainitic ferrite will exceed 70%, resulting in a strength of less than 980 MPa for the final annealed sheet (final product). In addition, an oxide film that is difficult to remove by pickling will form on the surface of the hot-rolled sheet, potentially damaging the surface appearance after cold rolling. Therefore, the coiling temperature after hot rolling is set to 700°C or lower. It is preferably 670°C or lower. It is more preferably 650°C or lower. Although there is no particular lower limit for the coiling temperature, if the temperature is lower than 350°C, the strength of the hot-rolled sheet increases, the rolling load in cold rolling increases, and defects in the sheet shape occur, resulting in reduced productivity. Therefore, the lower limit of the coiling temperature is preferably set to 350°C or higher, more preferably 370°C or higher, and even more preferably 400°C or higher.

[0051] The obtained hot-rolled steel sheet (hot-rolled coil) is subjected to hot-rolled sheet annealing, in which the sheet is held at a temperature of Ac1 to Ac3 for 3 s to 216,000 s, which effectively promotes the enrichment of Mn in martensite in the final structure. Therefore, this hot-rolled sheet annealing may be performed as necessary. Incidentally, Ac1 and Ac3 are defined by the following formulas. (Formula 7) Ac1=751-16.3[C]+34.9[Si]-27.5[Mn]-5.5[Cu]-15.9[Ni]+12.7[Cr]+3.4[Mo] (Formula 8) Ac3=881-205.7[C]+53.1[Si]-15[Mn]-20.1[Cu]-0.7[Cr]+41.1[Mo] In (Equation 7) and (Equation 8), [C], [Si], [Mn], [Cu], [Ni], [Cr], and [Mo] each represent the mass percentage of the element contained in the steel.

[0052] Furthermore, the obtained hot-rolled steel sheet (hot-rolled coil) may be subjected to treatment such as pickling as necessary. The pickling method for the hot-rolled coil may be a conventional method. In addition, the hot-rolled coil may be subjected to skin-pass rolling in order to correct the shape and improve the pickling properties.

[0053] After hot rolling and / or pickling, the steel may be heat-treated directly, or may be cold-rolled and then heat-treated. When cold-rolling is performed, the cold reduction is preferably 25% or more, and more preferably 30% or more. On the other hand, excessive reduction increases the rolling load and increases the load on the cold-rolling mill, so the upper limit is preferably 75%, and more preferably 70% or less.

[0054] [Heating process] Heating temperature reached T0℃ As defined by Equation 2, by setting the ultimate heating temperature T0°C to a temperature higher than the annealing temperature T1°C, it is possible to promote Mn concentration in martensite in the final structure. Therefore, the ultimate heating temperature T0°C is set to a temperature higher than the annealing temperature T1°C so as to satisfy the following Equation 2. Furthermore, in order to cause austenite transformation in the heating process and to achieve a fresh martensite fraction of 5% or more and a tempered martensite fraction of 10% or more in the final structure, the ultimate heating temperature T0°C must be set to Ac1 or higher. Furthermore, although there is no particular upper limit set for the ultimate heating temperature T0°C, it is preferable that the ultimate heating temperature T0°C be Ac3 or lower in consideration of the diffusion of Mn during dual-phase annealing. (Formula 2)T1 <T0

[0055] Annealing temperature T1℃ The annealing temperature T1°C must satisfy Equation 3 for the reasons described below. If the annealing temperature T1°C is 720°C or lower, the area fraction of ferrite generated during annealing will be excessive, and the total area fraction of ferrite and bainitic ferrite will exceed 70%, resulting in a TS of less than 980 MPa. On the other hand, if the annealing temperature T1°C is Ac3 + 100°C or higher, the total area fraction of ferrite and bainitic ferrite will be less than 30%, resulting in poor ductility. Therefore, the annealing temperature T1°C must satisfy Equation 3. (Formula 3)720 <T1<Ac3+100

[0056] Heating with heat input parameter Q between 3200 and 4800 In the heating step, the time t from Ac1°C through the heating target temperature T0°C to the annealing temperature T1°C is h It is one of the important constituent features of the present invention that the heat input parameter Q (°C × min) defined by (Equation 4) using the annealing time t seconds at the annealing temperature T1°C and the annealing temperature T1°C is 3200 or more and 4800 or less.

[0057] The inventors conducted experiments by varying the heat input parameter Q and found that, even at the same holding temperature, increasing the heat input parameter Q reduced the fraction of fresh martensite and increased the fraction of tempered martensite, and also changed the amount of Mn contained in the tempered martensite. That is, as the heat input parameter Q increases, the fraction of austenite during annealing increases, reducing the Mn concentration in austenite and increasing the fraction of martensite formed when cooling is stopped. The fresh martensite formed when cooling is stopped becomes tempered martensite during subsequent holding.

[0058] If the heat input parameter is less than 3200, the amount of Mn in the tempered martensite increases, i.e., the Mn concentration in the austenite increases during annealing. Therefore, although bainite transformation is suppressed and dimensional accuracy during forming is improved, the amount of fresh martensite generated when cooling is stopped decreases, and reheating is performed with a large amount of untransformed austenite remaining. As a result, this untransformed austenite transforms into fresh martensite upon cooling after reheating, and the area ratio of fresh martensite in the final structure exceeds 35%, resulting in poor ductility. For this reason, the heat input parameter Q is set to 3200 or more. It is preferably 3250 or more, and more preferably 3300 or more. On the other hand, if the heat input parameter is greater than 4800, the amount of Mn in the tempered martensite decreases, i.e., the ratio of the amount of Mn contained in the tempered martensite to the amount of Mn contained in the steel sheet becomes less than 1.05. This promotes the transformation to bainite, which has a large TS fluctuation relative to the holding temperature during the reheating process, resulting in unstable YR and poor material stability. Therefore, the heat input parameter Q must be 4800 or less, preferably 3950 or less, and more preferably 3900 or less. (Formula 4)Q=[{(Ac1+T1) / 2}×t h / 60] + T1×t / 60

[0059] Cooling is performed under conditions where the average cooling rate from T1℃ to 400℃ is 5.0℃ / s or more. If the cooling start temperature is T1°C and the average cooling rate in the temperature range from T1°C to 400°C is less than 5.0°C / s, the transformation to bainite, which has a large TS variation relative to the holding temperature, will proceed, resulting in a variation in YR and a loss of material stability. Therefore, the average cooling rate in the temperature range from T1°C to 400°C is set to 5.0°C / s or more. There is no particular upper limit set for the cooling temperature in the temperature range from T1°C to 400°C, but in consideration of industrially feasible cooling rates, it is preferably 30.0°C / s or less, and more preferably 27.0°C / s or less.

[0060] [Reheating process] Cooling stop temperature Tsq℃: 150℃~370℃ In the annealing process, if cooling is stopped during cooling at a temperature lower than 150°C, the fraction of fresh martensite generated at the time of cooling stop increases, the area fraction of tempered martensite exceeds 60.0%, the YR becomes excessively high, and the dimensional accuracy during forming is poor. For this reason, the cooling stop temperature Tsq°C is set to 150°C or higher. On the other hand, if cooling is stopped at a temperature higher than 370°C, a large amount of austenite remains at the time of cooling stop, the area fraction of fresh martensite in the final structure exceeds 35%, the ductility decreases, the YR decreases, and the dimensional accuracy during forming is poor. For this reason, the cooling stop temperature Tsq°C is set to 370°C or lower.

[0061] Holding temperature Toa℃: A temperature higher than the cooling stop temperature Tsq℃ and between 180℃ and 430℃ After cooling is stopped, by raising the temperature to a temperature higher than the cooling stop temperature, it is possible to convert the martensite formed at the time of cooling stop into tempered martensite, thereby suppressing an excessive increase in YR. However, if the heating temperature is less than 180°C, the area fraction of tempered martensite will be less than 10%, and the TS will be less than 980 MPa. On the other hand, if the heating temperature is more than 430°C, the area fraction of tempered martensite will be more than 60%, increasing the YR and resulting in poor dimensional accuracy during forming.

[0062] Hold at a temperature of Toa°C for a time of toa seconds. When the range of values ​​determined by Equation 5 based on the holding temperature and holding time in the reheating process satisfies a predetermined relationship with the heat input parameter Q defined by Equation 4 in the heating process, the C diffusion in fresh martensite during holding can be controlled and the steel sheet strength can be maintained.

[0063] If the value determined by Equation 5 due to the holding temperature and holding time is less than Q / 10, the tempering due to holding will be insufficient, the area ratio of fresh martensite in the final structure will exceed 35%, and ductility will be poor. Therefore, the holding temperature Toa°C and holding time toa seconds must satisfy Equation 5. (Equation 5) Q / 10≦Toa(logtoa) On the other hand, if the value determined by the holding temperature and holding time exceeds Q / 5, the distribution of C from fresh martensite is promoted, and the ratio of the amount of C contained in the fresh martensite to the amount of C contained in the steel sheet may become less than 1.50. Furthermore, since the concentration of C in the untransformed austenite remaining in the steel sheet is suppressed, transformation to bainite, which has a large TS variation with the holding temperature during holding, is promoted, and as a result, YR varies and the material stability may be deteriorated. Therefore, it is preferable to satisfy formula 6. (Equation 6) Toa(logtoa)≦Q / 5

[0064] Plating When hot-dip galvanizing is performed, the steel sheet that has been subjected to the annealing treatment is immersed in a galvanizing bath at a temperature of 440°C or higher and 500°C or lower to perform the hot-dip galvanizing treatment, and then the coating weight is adjusted by gas wiping or the like.

[0065] There are no special restrictions on the plating conditions, but the plating weight (amount of plating per side) is set at 20 g / m from the viewpoint of corrosion resistance and plating weight control. 2 It is preferable that the thickness is 120 g / m or more from the viewpoint of adhesion. 2 The coating weight is preferably 25 g / m or less. 2 More preferably, it is 30 g / m or more. 2 It is even more preferable that the plating coverage is 100 g / m or more. 2 It is more preferable that the density is 70 g / m or less. 2 It is even more preferred that:

[0066] It is preferable that a galvanizing bath containing 0.08% to 0.30% Al is used for hot dip galvanizing. Furthermore, the effects of the present invention remain unchanged even if the galvanizing bath contains elements other than Al, Mg, and Si, such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn.

[0067] When hot-dip galvanizing alloying treatment is performed, the hot-dip galvanizing treatment is followed by the hot-dip galvanizing treatment in a temperature range of 450°C to 600°C. If the alloying treatment is performed at a temperature above 600°C, fresh martensite transforms into tempered martensite, and the area ratio of fresh martensite becomes less than 5%, which may result in poor dimensional accuracy during forming. Therefore, when hot-dip galvanizing alloying treatment is performed, it is preferable to perform the hot-dip galvanizing alloying treatment in a temperature range of 450°C to 600°C. The Fe concentration in the plating layer of the alloyed hot-dip galvanized steel sheet is preferably 8 to 17%. The alloyed plating layer is formed by performing the alloying plating treatment.

[0068] The steel strips that are the subject of the present invention include coiled steel strips, and refer to those with a total coil length of 50 m to 3000 m.

[0069] (4) Materials Next, a member according to one embodiment of the present invention will be described.

[0070] A member according to one embodiment of the present invention is a member made using the high-strength steel plate according to one embodiment of the present invention described above. The member according to one embodiment of the present invention, for example, the member according to one embodiment of the present invention described above, is preferably a member for a frame structural part of an automobile or a reinforcement part of an automobile.

[0071] The high-strength steel sheet according to one embodiment of the present invention has a TS of 980 MPa or more, an El of 10% or more, and a YR of 50% to 85%, and is a steel sheet with excellent material stability. Therefore, the member according to one embodiment of the present invention can contribute to reducing the weight of the vehicle body, and can be suitably used in general as a member for automotive frame structural parts or automotive reinforcing parts. [Example]

[0072] Steel having the chemical composition shown in Table 1, with the remainder being Fe and unavoidable impurities, was melted in a converter and formed into slabs by continuous casting. The obtained slabs were hot rolled under the conditions shown in Table 2, followed by cold rolling, annealing, and reheating to obtain high-strength cold-rolled steel sheets (CR). Some of the steel was hot-rolled without cold rolling to obtain hot-rolled steel sheets (HR). Further, hot-dip galvanizing treatment was carried out to obtain hot-dip galvanized steel sheets (GI) and galvannealed steel sheets (GA). For the hot-dip galvanizing bath, a zinc bath containing 0.19 mass% Al was used for the hot-dip galvanized steel sheets (GI), and a zinc bath containing 0.14 mass% Al was used for the galvannealed steel sheets (GA), with the bath temperature set to 465°C. The coating weight was 45 g / m per side. 2 The GA was adjusted so that the Fe concentration in the plating layer was within the range of 9 mass % or more and 12 mass % or less. The cross-sectional microstructure, Mn and C contents in the microstructure, and tensile properties of the obtained steel sheets were investigated. The results are shown in Table 3.

[0073] [Table 1]

[0074] [Table 2]

[0075] [Table 3]

[0076] The area fractions of ferrite, bainitic ferrite, fresh martensite, and tempered martensite were determined by polishing a thickness cross section (L cross section) parallel to the rolling direction of the steel plate, etching it with nital, and observing 10 fields of view at 2000x magnification using a scanning electron microscope (SEM) at the 1 / 4 thickness position (a position corresponding to 1 / 4 of the thickness in the depth direction from the steel plate surface).The area fractions of each structure (total of ferrite and bainitic ferrite, tempered martensite, and fresh martensite) were calculated using the obtained structural images.In addition, in the above structural images, ferrite and bainitic ferrite were defined as the black structural regions, fresh martensite as the light gray structural region, and tempered martensite as the dark gray structural region with carbide precipitation.

[0077] The area fraction of retained austenite was determined by grinding the steel sheet from the 1 / 4 position to 0.1 mm down, then chemically polishing another 0.1 mm, and measuring the integrated intensity ratios of the diffraction peaks of the {200}, {220}, and {311} planes of fcc iron and the {200}, {211}, and {220} planes of bcc iron using CoKα radiation in an X-ray diffractometer. The average value of the nine integrated intensity ratios was used. When retained austenite was measured, the area fraction of retained austenite was subtracted from the area fraction of fresh martensite to ensure that the total area fraction did not exceed 100%.

[0078] Furthermore, the area ratio of pearlite was determined by polishing a thickness cross section (L cross section) of the steel plate parallel to the rolling direction, etching it with nital, and observing 10 fields of view at 2000x magnification using a scanning electron microscope (SEM) at a position 1 / 4 of the plate thickness (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate).The area ratio of pearlite was calculated by defining the area in the obtained structural image where a lamellar structure was confirmed as pearlite.

[0079] Tensile tests were conducted in accordance with JIS Z 2241 (2011) using JIS No. 5 test pieces, which were prepared so that the tensile direction was perpendicular to the rolling direction of the steel sheet. TS (tensile strength), YS (0.2% yield strength), and EL (total elongation) were measured. YR was calculated by dividing the YS value by TS. In the examples, steel sheets with a TS of 980 MPa or more were judged to have achieved the target strength and passed the test. Steel sheets with an EL of 10% or more were judged to have achieved the target formability and passed the test. Steel sheets with a YR of 50% to 85% were judged to have achieved the target dimensional accuracy and passed the test.

[0080] The amount of Mn in the tempered martensite and the amount of C in the fresh martensite were measured by analysis using an FE-EPMA (field emission electron probe microanalyzer). The quantitative values ​​obtained were divided by the amounts of Mn and C contained in the steel sheet to determine the ratio of the amount of Mn in the tempered martensite to the amount of Mn in the steel sheet and the ratio of the amount of C in the fresh martensite to the amount of C in the steel sheet. The EPMA measurements were performed over an 18 μm x 18 μm area at a position 1 / 4 of the plate thickness (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the steel plate surface). C analysis was performed at an accelerating voltage of 7 kV, a current of 50 nA, and an analysis time of 20 ms / pixel, while Mn analysis was performed at an accelerating voltage of 9 kV, a current of 200 nA, and an analysis time of 100 ms / pixel. C quantification was performed using a calibration curve based on the XPP quantification results for the pure material and a steel sample with Si and Mn distribution.

[0081] The material stability was evaluated by preparing tensile test pieces at three locations in the width direction of the steel plate described in the examples: 1 / 4 width, 2 / 4 width, and 3 / 4 width, and calculating the YR of these. If the variation in YR at the three locations was 4% or less, it was judged as ⊚, if it was more than 4% and less than 7%, it was judged as ◯, if it was more than 7% and less than 10%, it was judged as △, and if it was more than 10%, it was judged as ×. In the examples, steels judged as × were used as comparative examples. When the evaluation object was a coil, the above evaluation was performed on a steel plate taken from the center of the coil. Furthermore, for the coils described in the Examples, tensile test specimens were prepared at three locations along the entire length of the coil: the tip, center, and tail end, as described below, and the YR of these specimens was calculated. The material stability in the longitudinal direction of the coil was evaluated based on the variation in YR at these three locations. A variation in YR of 10% or less was considered to have favorable material stability, a variation of 7% or less was considered to have more favorable material stability, and a variation of 4% or less was considered to have even more favorable material stability. Regarding the method for collecting samples from the entire length of the coil, the tip was randomly collected from the tip to 1 / 10 of the entire length of the coil, the center was randomly collected from 1 / 2 of the longitudinal position along the entire length of the coil at ±1 / 10 of the entire length of the coil, and the tail end was randomly collected from the tail to 1 / 10 of the entire length of the coil. The above-mentioned sampling positions in the width direction were all 1 / 4 width.

[0082] The high-strength steel sheets of the invention examples all have a steel sheet strength of 980 MPa or more, an El of 10% or more, and a YR of 50% to 85%, and are high-strength steel sheets with excellent mechanical stability. On the other hand, the comparative examples are inferior in at least one of the properties of strength and mechanical stability.

Claims

1. In mass%, C: 0.020% or more and 0.200% or less, Si: 0.01% or more and 2.00% or less, Mn: 2.50% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less and O: 0.0100% or less, C, Si, and Mn satisfy the following formula 1: The balance is composed of Fe and unavoidable impurities, The microstructure of the steel plate is The total area ratio of ferrite and bainitic ferrite is 30% or more and 70% or less, The area ratio of tempered martensite is 10% or more and 60% or less, The area ratio of fresh martensite is 5% or more and 35% or less, The remainder is at least one of retained austenite and pearlite in an area ratio of 0% to 20%. A high-strength steel sheet characterized in that the ratio of the amount of Mn contained in tempered martensite to the amount of Mn contained in the steel sheet is 1.05 or more. However, the amount of Mn contained in the tempered martensite is expressed in mass %. (Formula 1) Mn / (C+Si)≧1.2 In formula 1, C, Si, and Mn each represent the mass % of each element.

2. Furthermore, the component composition is as follows: Ti: 0.500% or less, Nb: 0.500% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, Bi: 0.200% or less, REM: 0.0100% or less, 2. The high strength steel plate according to claim 1, further comprising at least one or more elements selected from the group consisting of:

3. 2. The high-strength steel plate according to claim 1, wherein the ratio of the amount of C contained in the fresh martensite to the amount of C contained in the steel plate is 1.50 or more. However, the amount of C contained in fresh martensite is expressed in mass %.

4. 3. The high-strength steel plate according to claim 2, wherein the ratio of the amount of C contained in the fresh martensite to the amount of C contained in the steel plate is 1.50 or more. However, the amount of C contained in fresh martensite is expressed in mass %.

5. 5. The high-strength steel sheet according to claim 1, wherein the steel sheet surface has a plating layer.

6. The high-strength steel sheet according to claim 5, wherein the plating layer is an alloyed plating layer.

7. A steel strip comprising the high strength steel plate according to any one of claims 1 to 4.

8. A steel strip comprising the high strength steel plate according to claim 5.

9. A steel strip comprising the high strength steel plate according to claim 6.

10. The method for producing a high-strength steel plate according to any one of claims 1 to 4, a hot rolling step of heating a steel material having the chemical composition according to claim 1 or 2, performing hot rolling at a finish rolling end temperature of 800°C or higher, and then performing coiling at a coiling temperature of 700°C or lower; After the hot rolling process, the heating temperature T is set to Ac1 ° C. or higher and satisfies Equation 2. ０ ° C., and then annealing at an annealing temperature T １ Annealing is performed at ℃, and the cooling start temperature is T １ °C, and T １ a heating step of cooling the material at an average cooling rate of 5.0°C / s or more from 0°C to 400°C; Next, a reheating step of cooling to a cooling stop temperature Tsq°C of 150°C to 370°C, heating to a temperature higher than the cooling stop temperature Tsq°C and 180°C to 430°C, and then holding the temperature Toa°C at a temperature higher than the cooling stop temperature Tsq°C and 180°C to 430°C for a holding time of toa seconds, In the heating step, the temperature is increased from Ac1°C to the heating temperature T ０ ° C. through the annealing temperature T １ Time t until ° C. is reached ｈ seconds and the annealing temperature T １ Annealing time t seconds and annealing temperature T in ° C. １ The heat input parameter Q defined by Equation 4 in terms of ° C. is 3200 or more and 4800 (° C. × min) or less, A method for producing a high strength steel plate, characterized in that in the reheating step, the holding temperature Toa°C, the holding time Toa seconds, and the heat input parameter Q satisfy Equation 5. (Equation 2) T １ <T ０ (Equation 3) 720 < T １ <Ac3+100 (Equation 4) Q = [ { (Ac1 + T １ ) / 2} × t ｈ / 60 ] + T １ × t / 60 (Formula 5) Q / 10≦Toa(logtoa)

11. 8. A method for producing a steel strip according to claim 7, a hot rolling step of heating a steel material having the chemical composition according to claim 1 or 2, performing hot rolling at a finish rolling end temperature of 800°C or higher, and then performing coiling at a coiling temperature of 700°C or lower; After the hot rolling process, the heating temperature T is set to Ac1 ° C. or higher and satisfies Equation 2. ０ ° C., and then annealing at an annealing temperature T １ Annealing is performed at ℃, and the cooling start temperature is T １ °C, and T １ a heating step of cooling the material at an average cooling rate of 5.0°C / s or more from 0°C to 400°C; Next, a reheating step of cooling to a cooling stop temperature Tsq°C of 150°C to 370°C, heating to a temperature higher than the cooling stop temperature Tsq°C and 180°C to 430°C, and then holding the temperature Toa°C at a temperature higher than the cooling stop temperature Tsq°C and 180°C to 430°C for a holding time of toa seconds, In the heating step, the temperature is increased from Ac1°C to the heating temperature T ０ ° C. through the annealing temperature T １ Time t until ° C. is reached ｈ seconds and the annealing temperature T １ Annealing time t seconds and annealing temperature T in ° C. １ The heat input parameter Q defined by Equation 4 in terms of ° C. is 3200 or more and 4800 (° C. × min) or less, A method for manufacturing a steel strip, characterized in that in the reheating step, the holding temperature Toa °C, the holding time Toa seconds, and the heat input parameter Q satisfy Equation 5. (Equation 2) T １ <T ０ (Equation 3) 720 < T １ <Ac3+100 (Equation 4) Q = [ { (Ac1 + T １ ) / 2} × t ｈ / 60 ] + T １ × t / 60 (Formula 5) Q / 10≦Toa(logtoa)

12. The method for producing a high strength steel plate according to claim 10, wherein cold rolling is performed after the hot rolling step and before the heating step.

13. The method for producing a high-strength steel sheet according to claim 10, wherein after the reheating step, a plating treatment is carried out to form a plating layer on the surface of the steel sheet.

14. The method for producing a high-strength steel sheet according to claim 12, wherein after the reheating step, a plating treatment is carried out to form a plating layer on the surface of the steel sheet.

15. The method for producing a steel strip according to claim 11, characterized in that cold rolling is carried out after the hot rolling step and before the heating step.

16. The method for producing a steel strip according to claim 11, wherein after the reheating step, a plating treatment is carried out to form a plating layer on the surface of the steel sheet.

17. The method for producing a steel strip according to claim 15, wherein after the reheating step, a plating treatment is carried out to form a plating layer on the surface of the steel sheet.

18. The method for producing a high-strength steel sheet according to claim 13, wherein the plating treatment is an alloying plating treatment to form an alloying plating layer.

19. The method for producing a high-strength steel sheet according to claim 14, wherein the plating treatment is an alloying plating treatment to form an alloying plating layer.

20. The method for manufacturing a steel strip according to claim 16, wherein the plating treatment is an alloying plating treatment to form an alloying plating layer.

21. The method for manufacturing a steel strip according to claim 17, wherein the plating treatment is an alloying plating treatment to form an alloying plating layer.

22. A member, characterized by being made using the high-strength steel plate according to any one of claims 1 to 4.

23. A member, characterized in that it is made using the high-strength steel plate according to claim 5.

24. A member, characterized in that it is made using the high-strength steel plate according to claim 6.

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