JP7779298B2 - High-strength steel plate, steel strip, method for manufacturing high-strength steel plate, method for manufacturing steel strip and component - Google Patents

Description

The present invention relates to high-strength steel plates, steel strips, manufacturing methods for high-strength steel plates, manufacturing methods for steel strips, and components. In particular, the present invention relates to high-strength steel plates and manufacturing methods that enable the production of parts with high dimensional accuracy and excellent material stability with little variation in YR, and have a TS of 980 MPa or more and an El of 10% or more, making them suitable for components used in industrial fields such as automotive and electrical machinery that are formed by cold pressing.

In recent years, there has been a growing 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 use of high-strength steel sheets in automotive parts.

Conventionally, 780 MPa and 980 MPa 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 two-phase region. In particular, 980 MPa grade steel sheets have a problem in that slight temperature changes in the two-phase region cause abrupt changes in the ferrite and martensite fractions, which can easily lead to variations in strength even within the same coil.
Furthermore, high-strength steel sheets used in automotive reinforcement and frame structural components are required to be able to manufacture parts with high dimensional accuracy. To manufacture parts with high dimensional accuracy, it is important to control the yield ratio (YR = yield strength YS / tensile strength TS) of the steel sheet within a certain range. Controlling the yield ratio (YR) of the steel sheet within a certain range suppresses springback after steel sheet forming and improves dimensional accuracy during forming. Furthermore, increasing the yield ratio (YR) of the steel sheet can increase the impact absorption energy of the part during a collision. However, variations in mechanical properties within the same coil result in 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 manufacture steel sheets with minimal variation in mechanical properties within the same coil.

For example, Patent Document 1 specifies the area ratios of ferrite, bainitic ferrite, and martensite, the volume ratio and average grain size of retained austenite, and further specifies that the average Mn content (mass %) in the retained austenite is at least 1.2 times the Mn content (mass %) in the steel, and that at least 60% of the total retained austenite is at least 2.1 times the C content (mass %) in the steel. This technology discloses a high-strength steel sheet that has a tensile strength (TS) of at least 780 MPa, excellent ductility, stretch flangeability, and material stability.

Patent Document 2 also discloses a technology that specifies the area ratio of tempered martensite with a hardness of 330 Hv or more and 450 Hv or less, specifies the grain size of the remaining ferrite, and further specifies the ratio between the upper and lower limits of the Mn concentration in the cross section of the steel sheet in the thickness direction, thereby achieving a high-strength steel sheet with a tensile strength of 980 MPa or more, an elongation of 13% or more, and reduced variation in stretch flangeability.

International Publication No. 2016/021193 JP 2010-65307 A

However, the technology described in Patent Document 1 requires that the cold-rolled steel sheet be heated to a temperature of 820°C or higher and 950°C or lower, then 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 likely to result in higher energy costs.

Furthermore, while the technology described in Patent Document 2 reduces variation in stretch flangeability for steel sheets of 980 MPa or higher, it does not take into consideration variation in YR, which is likely to be a problem for 980 MPa-class steel sheets.

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%, all of which are manufactured using a simpler method than conventional techniques and have excellent material stability, a problem encountered with 980 MPa-class steel sheets. The above-mentioned steel sheets refer to hot-rolled steel sheets, cold-rolled steel sheets, and plated steel sheets such as GA and GI, while the above-mentioned steel strips refer to steel strips (also called coils) made from the above-mentioned steel sheets.

YR can be calculated using the following formula: YR = YS/TS x 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 amount of variation in YR in the width direction of the steel sheet is 10% or less.

In order to solve the above problems, the inventors have conducted extensive research into the chemical composition and microstructure of steel sheets, and as a result have found that it is possible to control the Mn distribution in steel and the microstructural structure of steel sheets by controlling the slab heating conditions and the temperature from hot rolling to coiling, and by changing the amount of heat received by the steel sheet during annealing, and that it is possible to obtain steel with high strength, a predetermined 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 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] Further, as a component composition, Ti: 0.500% or less by mass%,
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] The 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] The high-strength steel sheet according to any one of [1] to [3], characterized in that the steel sheet surface has a plating layer.
[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 comprising 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 step of raising the temperature to a heating target temperature _T0 °C that is Ac1°C or higher and satisfies formula 2, then annealing at an annealing temperature _T1 °C that satisfies formula ₃ , and cooling from _T1 °C to 400°C at an average cooling rate of 5.0°C/s or higher, with a cooling start temperature of T1°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,
a heat input parameter Q defined by Equation 4, which is a time t _h seconds from Ac1°C passing through a temperature rise in the heating step to the heating target temperature _{T0 °} C and then to the annealing temperature _T1 °C, an 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 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.
(Formula 2) T ₁ <T ₀
(Formula 3) 720<T ₁ <Ac3+100
(Formula 4) Q=[{(Ac1+T ₁ )/2}×t _h /60] + T ₁ ×t/60
(Formula 5) Q/10≦Toa(logtoa)
[9] A method for producing a steel strip according to [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 step of raising the temperature to a heating target temperature _T0 °C that is Ac1°C or higher and satisfies formula 2, then annealing at an annealing temperature _T1 °C that satisfies formula ₃ , and cooling from _T1 °C to 400°C at an average cooling rate of 5.0°C/s or higher, with a cooling start temperature of T1°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,
a heat input parameter Q defined by Equation 4, which is a time t _h seconds from Ac1°C passing through a temperature rise in the heating step to the heating target temperature _{T0 °} C and then to the annealing temperature _T1 °C, an 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) T ₁ <T ₀
(Formula 3) 720<T ₁ <Ac3+100
(Formula 4) Q=[{(Ac1+T ₁ )/2}×t _h /60] + T ₁ ×t/60
(Formula 5) Q/10≦Toa(logtoa)
[10] The 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] The 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 producing 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 using the high-strength steel plate according to any one of [1] to [5].

The present invention makes it possible to obtain high-strength steel plates, steel strips, and components with TS of 980 MPa or more, El of 10% or more, YR of 50% to 85%, and excellent material stability with little variation in YR. Therefore, the present invention is extremely valuable in industrial fields such as automobiles and electrical equipment, and is particularly useful for reducing the weight of automobile body frame parts.

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.

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, it is set to 0.022% or more, and more preferably, it is set to 0.024% or more. On the other hand, if the C content exceeds 0.200%, the annealing temperature dependence of the strength of the martensite phase becomes too high, impairing the material stability. Therefore, the C content is set to 0.200% or less. Preferably, it is set to 0.180% or less. More preferably, it is set to 0.160% or less.

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. Preferably, it is set to 0.02% or more, and more preferably, it is set to 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, which will easily form unrecrystallized ferrite, particularly during annealing, 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.

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 and fresh martensite, thereby impairing material stability. For this reason, the Mn content is set to 2.50% or more. Preferably, it is set to 2.70% or more, more preferably 2.90% or more, and even more preferably 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 4.50% or less, more preferably 4.00% or less, and even more preferably 3.90% or less.

P: 0.100% or less P may have an adverse effect on workability by segregating at grain boundaries and causing embrittlement, so its amount must be 0.100% or less. Therefore, the P content is set to 0.100% or less. Preferably, it is set to 0.080% or less. More preferably, it is set to 0.050% or less. Although there is no particular restriction on the lower limit, 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.

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

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, it is set to 0.080% or less. More preferably, it is set to 0.070% or less. There is no particular lower limit for the Al content, but it is preferably set to 0.001% or more. More preferably, it is set to 0.005% or more.

N: 0.0100% or less N has an adverse 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 smaller the amount, 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.

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, 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.

(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 Mn content is small relative to the C and Si content, 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, resulting in 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.

In addition to the above components, the alloy may contain one or more of the following elements, by mass: 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.

Ti: 0.500% or less Ti contributes to precipitation strengthening and further refines the prior austenite grain size, resulting in refinement of tempered martensite and bainite, and is therefore effective in improving steel strength. However, if Ti is contained in an amount exceeding 0.500%, Ti may remain in an undissolved state when the steel material is heated before hot rolling, increasing the number of coarse precipitates and reducing ductility. Therefore, when Ti is contained, the Ti content is set to 0.500% or less, preferably 0.100% or less, and more preferably 0.070% or less.

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, when 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.

V: 0.200% or less V contributes to precipitation strengthening and further refines the prior austenite grain size, resulting in refinement of tempered martensite and bainite, thereby effectively improving steel strength. While the lower limit is not particularly limited, a content of 0.001% or more is preferable to obtain 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, when V is contained, the V content is set to 0.200% or less, preferably 0.180% or less.

Ta: 0.10% or less Like Ti, Ta contributes to high strength by forming alloy carbides and alloy carbonitrides. Ta also 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, when Ta is added, the Ta content should be 0.10% or less, preferably 0.08% or less, and more preferably 0.07% or less.

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 the lower limit is not particularly limited, a content of 0.01% or more is preferable to obtain 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, when W is added, the W content is set to 0.10% or less, preferably 0.08% or less, and more preferably 0.05% or less.

B: 0.0100% or less B is an element that can improve hardenability by segregating at austenite grain boundaries, forming a structure mainly composed of tempered martensite and bainite, and improving the strength of the steel sheet, so it can be added as needed. While there is no particular lower limit, to obtain the above-mentioned effect, 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, it is set to 0.0090% or less. More preferably, it is set to 0.0080% or less. Even more preferably, it is set to 0.0070% or less.

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

Mo: 1.00% or less Mo has the effect of improving the balance between strength and ductility, so it can be contained 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 contained 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 contained, the Mo content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

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 it to 0.01% or more. However, if the content exceeds 1.00%, the area ratio of fresh martensite becomes excessively large, resulting in reduced dimensional accuracy and ductility during forming. Therefore, when Co is added, the Co content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

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 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 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.

Cu: 1.00% or less Cu is an element effective in strengthening steel, so it can be contained 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 contained in an amount exceeding 1.00%, the area ratio of tempered martensite, bainite, and fresh martensite becomes excessive, resulting in a decrease in dimensional accuracy and ductility during forming. Therefore, when Cu is contained, the Cu content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

Sn: 0.200% or less, Sb: 0.200% or less Sn and Sb suppress decarburization in a region of several tens of micrometers in the surface layer of the steel sheet caused by nitriding or oxidation of the steel sheet surface, prevent a decrease in the area ratio of tempered martensite on the steel sheet surface, and can be contained as needed to ensure strength and material stability. While there are no particular lower limits, to achieve the above effects, it is preferable that the contents of these elements be 0.001% or more. However, excessive content of either 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 contained, the contents of Sn and Sb are set to 0.200% or less, preferably 0.100% or less, and more preferably 0.050% or less.

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, it is preferable that these contents be 0.0010% or more. However, excessive content exceeding 0.0100% each can cause an increase in inclusions, which can lead 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, they should be 0.0090% or less. More preferably, they should be 0.0080% or less.

Zr: 0.100% or less, Te: 0.100% or less Zr and Te can be added as needed to improve the hardenability of the steel and to improve the strength of the steel by forming fine Zr-containing carbides and refining tempered martensite and bainite. While there are no particular lower limits, the contents of these elements 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 contained, the Zr and Te contents should each be 0.100% or less. Preferably, they should be 0.080% or less. More preferably, they should be 0.050% or less.

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.

Bi: 0.200% or less If Bi is 0.200% or less, coarse precipitates and inclusions do not increase, and the material stability of the steel sheet is not impaired. Therefore, when Bi is contained, the Bi content is set to 0.200% or less. Preferably, it is set to 0.150% or less. More preferably, it is set to 0.100% or less. There is no particular lower limit, but it is preferable that these contents be 0.001% or more.

REM: 0.0100% or less REM is an element that makes sulfides spheroidal. There is no particular lower limit, but the content is preferably 0.0010% or more. However, excessive content exceeding 0.0100% may cause an increase in inclusions, which may lead to surface and internal defects and may result in a decrease in ductility. Therefore, the REM content is set to 0.0100% or less. Preferably, it is set to 0.0090% or less. More preferably, it is set to 0.0080% or less.

Note that the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Co, Ni, Cu, Sn, Sb, Ca, Mg, Zr, Te, Hf, Bi, and REM are considered to be included as unavoidable impurities because the effects of the present invention are not impaired when their respective contents are below the preferred lower limit values.

(2) Next, we will explain the microstructure of high-strength steel plates (steel plates).

Total area ratio of ferrite and bainitic ferrite: 30% or more and 70% or less Ferrite and bainitic ferrite contribute to the ductility of steel sheet. If the total area ratio of ferrite and bainitic ferrite is less than 30%, the ductility will be less than 10%, so the sum of the area ratios of ferrite and bainitic ferrite needs to be at least 30% or more, preferably 32% or more. On the other hand, if the sum of the area ratios 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 ratios of bainite and tempered martensite needs to be 70% or less, preferably 68% or less.

Area fraction of tempered martensite: 10% or more and 60% or less 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%, TS is less than 980 MPa. Therefore, the area fraction of tempered martensite must be at least 10% or more. Preferably, it is 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, it is 58% or less.

Fresh martensite area fraction: 5% or more and 35% or less. Fresh martensite is a very hard phase, and dispersing fresh martensite throughout the structure can produce steel with excellent dimensional accuracy during forming. To fully achieve 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%. Note that, in the present invention, the effects of the present invention are not impaired even if residual structures such as ferrite, bainitic ferrite, tempered martensite, and retained austenite and pearlite other than fresh martensite are present. If a residual structure is present, it is assumed that the residual structure includes at least one of retained austenite and pearlite. Specifically, the area fraction of at least one of retained austenite and pearlite is 0% or more and 20% or less. The area fraction of the residual structure is preferably 15% or less, more preferably 10% or less, and even more preferably 5% or less.

The ratio of the amount of Mn contained in the 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 %).
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 large TS fluctuations relative to the holding temperature, is suppressed, resulting in a stabilized YR, a predetermined 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.

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. Since the strength of martensite is determined by the carbon content, determining the ratio of the carbon content in fresh martensite to the carbon content in the steel sheet components 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 obtained, and a steel sheet with superior material stability can be obtained. To fully achieve these effects, the ratio of the 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.

(3) Next, a manufacturing method and manufacturing conditions for high-strength steel plates (steel plates) 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 exist 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 coarse 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 longer. While there is no particular upper limit on the heating temperature of the steel material, if the heating temperature exceeds 1400°C, the amount of oxidation increases, resulting in increased scale loss and reduced productivity. Therefore, the heating temperature is preferably 1400°C or lower. There is also no upper limit on the heating time, but it is preferably 48.0 hours or shorter from the viewpoint of suppressing decarburization from the slab surface due to heating.

Finishing temperature of hot rolling: 800°C or higher. The heated steel material is hot-rolled to produce a hot-rolled steel sheet. At a finishing rolling temperature below 800°C, the reduction ratio increases while the austenite is in an unrecrystallized state, leading to the development of an abnormal texture and significant in-plane anisotropy in the final product. Furthermore, if rolling is completed in the two-phase region, there will be areas where C and Mn are significantly concentrated and areas where they are not, depending on the phase. Because Mn has a slow diffusion rate, if this uneven distribution of Mn occurs, the austenite produced through the heating and reheating processes will have areas with high and low Mn concentrations. The high-Mn-concentration areas exist as retained austenite, and the Mn concentration in the region that will ultimately become tempered martensite will likely be low. As a result, it becomes difficult to achieve the desired YR, and material stability is impaired. Furthermore, precipitates dissolved during heating of the steel material are reprecipitated, which increases the rolling load and the rolling load, hindering cold rolling. Therefore, the finish rolling delivery temperature of hot rolling, i.e., the finish rolling end temperature, is set to 800°C or higher. It is preferably 850°C or higher. It is 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) produced 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 surface roughness of the pressed product 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.

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 heat treatments of the subsequent 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, which may cause a deterioration in the surface appearance after cold rolling. Therefore, the coiling temperature after hot rolling is set to 700°C or less. Preferably, it is 670°C or less. More preferably, it is 650°C or less.
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.

The obtained hot-rolled steel sheet (hot-rolled coil) is subjected to hot-rolled sheet annealing in which the sheet is held at a holding temperature of Ac1 or higher and Ac3 or lower for 3 seconds or longer and 216,000 seconds or shorter, which can effectively promote the concentration of Mn in martensite in the final structure, and therefore this hot-rolled sheet annealing may be performed as necessary.
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 the formulas (7) and (8), [C], [Si], [Mn], [Cu], [Ni], [Cr], and [Mo] each represent the mass percentage of the element contained in the steel.

The resulting hot-rolled steel sheet (hot-rolled coil) may then be subjected to treatments such as pickling, if necessary. The pickling method for the hot-rolled coil may follow conventional methods. Skin-pass rolling may also be performed to correct the shape of the hot-rolled coil and improve its pickling properties.

After hot rolling and/or pickling, the steel may be heat treated directly, or may be cold rolled and then heat treated. If cold rolling is performed, the cold reduction is preferably 25% or more, and more preferably 30% or more. However, 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.

[Heating process]
Heating temperature reached T ₀ ℃
As defined by formula 2, by setting the ultimate heating temperature T ₀ °C to a temperature higher than the annealing temperature T ₁ °C, it is possible to promote Mn concentration in martensite in the final structure. Therefore, the ultimate heating temperature T ₀ °C is set to a temperature higher than the annealing temperature T ₁ °C so as to satisfy the following formula 2. Furthermore, in order to cause austenite transformation in the heating step 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 T ₀ °C needs to be Ac1 or higher. Furthermore, although there is no particular upper limit set for the ultimate heating temperature T ₀ °C, in consideration of the diffusion of Mn in two-phase annealing, it is preferable that the ultimate heating temperature T ₀ °C be Ac3 or lower.
(Formula 2) T ₁ <T ₀

Annealing temperature T ₁ °C
The annealing temperature T ₁ °C must satisfy Equation 3 for the reasons described below. If the annealing temperature T ₁ °C is 720 °C or lower, the area fraction of ferrite generated during annealing becomes excessive, and the total area fraction of ferrite and bainitic ferrite exceeds 70%, resulting in a TS of less than 980 MPa. On the other hand, if the annealing temperature T ₁ °C is Ac3 + 100 °C or higher, the total area fraction of ferrite and bainitic ferrite becomes less than 30%, resulting in poor ductility. Therefore, the annealing temperature T ₁ °C must satisfy Equation 3.
(Formula 3) 720<T ₁ <Ac3+100

Heating with a heat input parameter Q of 3200 or more and 4800 or less In the heating process, the heat input parameter Q (°C x min) is defined by the time t _h seconds from _Ac1 °C passed during temperature rise to the heating target temperature _T0 °C and then to the annealing temperature _T1 °C, the annealing time t seconds at the annealing temperature _T1 °C, and the annealing temperature T1°C, and is 3200 or more and 4800 or less. This is one of the important constituent features of the present invention.

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. In other words, 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.

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 becomes 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 more 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 transformation to bainite, which has a large TS variation with the holding temperature during the reheating process, resulting in unstable YR and poor material stability. Therefore, the heat input parameter Q needs to be 4800 or less, preferably 3950 or less, and more preferably 3900 or less.
(Formula 4) Q=[{(Ac1+T ₁ )/2}×t _h /60] + T ₁ ×t/60

Cooling is performed under conditions where the average cooling rate from T ₁ °C to 400 °C is 5.0 °C/s or more. If T ₁ °C is the cooling start temperature and the average cooling rate in the temperature range from T ₁ °C to 400 °C is less than 5.0 °C/s, transformation to bainite, which has a large TS variation relative to the holding temperature, will proceed, resulting in fluctuations in YR and impairing material stability. Therefore, the average cooling rate in the temperature range from T ₁ °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 _{T 1} °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.

[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 becomes 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 becomes greater than 35%, the ductility decreases, the YR decreases, and the dimensional accuracy during forming becomes poor. For this reason, the cooling stop temperature Tsq°C is set to 370°C or lower.

Holding temperature Toa°C: A temperature higher than the cooling stop temperature Tsq°C, and 180°C to 430°C. After cooling is stopped, by raising the temperature to a temperature higher than the cooling stop temperature, it is possible to convert the martensite generated at the time of cooling stop into tempered martensite, and an excessive increase in YR can be suppressed. However, if the heating temperature is less than 180°C, the area fraction of tempered martensite will be less than 10%, and 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 YR and resulting in poor dimensional accuracy during forming.

Holding temperature: Toa°C; holding time: 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 of fresh martensite during holding can be controlled and the steel sheet strength can be maintained.

If the value determined by Equation 5 based on 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 the holding time toa seconds must satisfy Equation 5.
(Formula 5) Q/10≦Toa(logtoa)
On the other hand, if the value determined by the holding temperature and holding time exceeds Q/5, C distribution from fresh martensite is promoted, and the ratio of the amount of C contained in fresh martensite to the amount of C contained in the steel sheet may become less than 1.50. In addition, C concentration in untransformed austenite remaining in the steel sheet is suppressed, so that transformation to bainite, which has a large TS variation with the holding temperature during holding, is promoted, and as a result, YR varies and material stability may be deteriorated. Therefore, it is preferable to satisfy formula 6.
(Formula 6) Toa (logtoa)≦Q/5

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

There are no particular limitations on the plating conditions, but the coating weight (coating weight per side) is preferably 20 g/ ^m2 or more from the viewpoints of corrosion resistance and coating weight control, and is preferably 120 g/ ^m2 or less from the viewpoint of adhesion. The coating weight is more preferably 25 g/ ^m2 or more, and even more preferably 30 g/ ^m2 or more. The coating weight is more preferably 100 g/ ^m2 or less, and even more preferably 70 g/ ^m2 or less.

For hot-dip galvanizing, it is preferable to use a zinc plating bath containing 0.08% to 0.30% Al. Furthermore, the effects of the present invention remain the same even if the plating bath contains elements other than Al, Mg, and Si, such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn.

When hot-dip galvanizing is performed, the hot-dip galvanizing treatment is followed by the hot-dip galvanizing treatment at 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, resulting in an area ratio of fresh martensite of less than 5%, which may result in poor dimensional accuracy during forming. Therefore, when performing the hot-dip galvanizing treatment, it is preferable to perform the hot-dip galvanizing treatment at a temperature range of 450°C to 600°C. The Fe concentration of 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.

Note that the steel strips covered by this invention also include coiled steel strips, with a total length of 50m to 3000m.

(4) Members Next, members according to one embodiment of the present invention will be described.

A component according to one embodiment of the present invention is a component made using the high-strength steel plate according to one embodiment of the present invention described above. A component according to one embodiment of the present invention, for example, the component according to one embodiment of the present invention described above, is preferably a component for an automobile frame structural part or an automobile reinforcing part.

The high-strength steel sheet according to one embodiment of the present invention described above 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, a member according to one embodiment of the present invention can contribute to reducing the weight of the vehicle body, and can be particularly suitably used for automotive frame structural parts or general components for automotive reinforcing parts.

Steels having the chemical compositions shown in Table 1, with the balance being Fe and unavoidable impurities, were melted in a converter and formed into slabs by continuous casting. The resulting 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 slabs were hot-rolled without being cold-rolled, to obtain hot-rolled steel sheets (HR).
Further, hot-dip galvanizing treatment was performed to obtain a hot-dip galvanized steel sheet (GI) and a galvannealed steel sheet (GA). For the hot-dip galvanized steel sheet (GI), a zinc bath containing 0.19 mass% Al was used, and for the galvannealed steel sheet (GA), a zinc bath containing 0.14 mass% Al was used, and the bath temperature was 465°C. The coating weight was 45 g/ ^m2 per side (double-sided coating), and for the GA, the Fe concentration in the coating layer was adjusted to be within the range of 9 mass% to 12 mass%.
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.

The area ratios 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 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). Using the resulting structural images, the area ratios of each structure (total of ferrite and bainitic ferrite, tempered martensite, and fresh martensite) were calculated. Furthermore, in the above structural images, ferrite and bainitic ferrite were defined as the black structural regions, fresh martensite as the light gray structural regions, and tempered martensite as the dark gray structural regions in which carbides precipitated.

The area fraction of retained austenite was determined by polishing the steel plate from the 1/4 position down to 0.1 mm, then chemically polishing a further 0.1 mm. The integrated intensity ratios of the diffraction peaks for the {200}, {220}, and {311} planes of fcc iron and the {200}, {211}, and {220} planes of bcc iron were measured using CoKα radiation in an X-ray diffractometer, and the average value of the nine integrated intensity ratios obtained 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%.

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 which a lamellar structure was confirmed in the obtained structure image as pearlite.

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

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), and the obtained quantitative values were divided by the amounts of Mn and C contained in the steel sheet to determine the ratio of the amount of Mn contained in the tempered martensite to the amount of Mn contained in the steel sheet and the ratio of the amount of C contained in the fresh martensite to the amount of C contained in the steel sheet.
The EPMA measurements were carried out in an 18 μm × 18 μm area at a position corresponding to 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). C analysis was carried out at an accelerating voltage of 7 kV, a current of 50 nA, and an analysis time of 20 ms/pixel, and Mn analysis was carried out at an accelerating voltage of 9 kV, a current of 200 nA, and an analysis time of 100 ms/pixel. C quantification was carried out using a calibration curve using XPP quantification results for a pure material and a steel sample with Si and Mn distribution.

The material stability was evaluated by preparing tensile test specimens 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 specimens. 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, 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 indicate favorable material stability, a variation of 7% or less was considered to indicate more favorable material stability, and a variation of 4% or less was considered to indicate even more favorable material stability. Regarding the method for collecting samples from the entire length of the coil, the tip samples were randomly collected within a range from the tip to 1/10 of the entire length of the coil, the center samples were randomly collected from a position halfway along the entire length of the coil to ±1/10 of the entire length of the coil in the longitudinal direction, and the tail samples were randomly collected within a range from the tail to 1/10 of the entire length of the coil. The above-mentioned sample collection positions in the width direction were all 1/4 width.

All of the high-strength steel sheets in the examples of the present invention 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 material stability. On the other hand, the comparative examples are inferior in at least one of the properties of strength and material stability.

Claims (24)

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 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. Further, the composition of the alloy is, in mass%, 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: 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 %. 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 %. A high-strength steel sheet according to any one of claims 1 to 4, characterized in that the steel sheet surface has a plating layer. The high-strength steel plate according to claim 5, characterized in that the plating layer is an alloyed plating layer. A steel strip characterized by being made from the high-strength steel plate described in any one of claims 1 to 4. A steel strip characterized by being made from the high-strength steel plate described in claim 5. A steel strip characterized by being made from the high-strength steel plate described in claim 6. 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 step, a heating step is performed in which the temperature is increased to a heating target temperature T0°C that is _Ac1 °C or higher and satisfies formula 2, and then annealing is performed at an annealing temperature _T1 °C that satisfies formula 3, and the cooling start temperature is set to _T1 °C, and cooling is performed under the condition that the average cooling rate from _T1 °C to 400°C is 5.0°C/s or higher;
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,
a heat input parameter Q defined by Equation 4, which is a time t _h seconds from Ac1°C passing through a temperature rise in the heating step to the heating target temperature _{T0 °} C and then to the annealing temperature _T1 °C, an 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 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.
(Formula 2) T ₁ <T ₀
(Formula 3) 720<T ₁ <Ac3+100
(Formula 4) Q=[{(Ac1+T ₁ )/2}×t _h /60] + T ₁ ×t/60
(Formula 5) Q/10≦Toa(logtoa) 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 step, a heating step is performed in which the temperature is increased to a heating target temperature T0°C that is _Ac1 °C or higher and satisfies formula 2, and then annealing is performed at an annealing temperature _T1 °C that satisfies formula 3, and the cooling start temperature is set to _T1 °C, and cooling is performed under the condition that the average cooling rate from _T1 °C to 400°C is 5.0°C/s or higher;
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,
a heat input parameter Q defined by Equation 4, which is a time t _h seconds from Ac1°C passing through a temperature rise in the heating step to the heating target temperature _{T0 °} C and then to the annealing temperature _T1 °C, an 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) T ₁ <T ₀
(Formula 3) 720<T ₁ <Ac3+100
(Formula 4) Q=[{(Ac1+T ₁ )/2}×t _h /60] + T ₁ ×t/60
(Formula 5) Q/10≦Toa(logtoa) The method for manufacturing high-strength steel plate according to claim 10, characterized in that cold rolling is performed after the hot rolling process and before the heating process. The method for manufacturing high-strength steel sheet according to claim 10, characterized in that after the reheating step, a plating treatment is performed to form a plating layer on the steel sheet surface. The method for manufacturing high-strength steel sheet according to claim 12, characterized in that after the reheating step, a plating process is performed to form a plating layer on the steel sheet surface. The method for manufacturing a steel strip according to claim 11, characterized in that cold rolling is performed after the hot rolling process and before the heating process. The method for manufacturing a steel strip described in claim 11, characterized in that after the reheating process, a plating process is performed to form a plating layer on the steel sheet surface. The method for manufacturing a steel strip described in claim 15, characterized in that after the reheating process, a plating process is performed to form a plating layer on the steel sheet surface. The method for manufacturing high-strength steel sheet described in claim 13, characterized in that the plating treatment is an alloying plating treatment to form an alloying plating layer. The method for manufacturing high-strength steel sheet described in claim 14, characterized in that the plating treatment is an alloying plating treatment to form an alloying plating layer. The method for manufacturing a steel strip described in claim 16, characterized in that the plating process is an alloying plating process to form an alloying plating layer. The method for manufacturing a steel strip described in claim 17, characterized in that the plating process is an alloying plating process to form an alloying plating layer. A component characterized by using the high-strength steel plate described in any one of claims 1 to 4. A component characterized by using the high-strength steel plate described in claim 5. A component characterized by using the high-strength steel plate described in claim 6.