JP7657372B2 - Steel plate and its manufacturing method - Google Patents

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No. 63/236,426, entitled "Steel Sheet and Method of Producing Same," filed August 24, 2021, the entire disclosure of which is incorporated herein by reference.

The present invention relates to a steel plate and its manufacturing method.

In order to reduce the weight of automobile bodies, reduce _CO2 emissions, and improve crashworthiness, the strength of steel sheets for automobiles is being increased, and new regulations are being introduced one after another. As a result, the number of cases in which high-strength steel sheets with a tensile strength (TS) of 1470 MPa or more are used in the main structural parts of automobiles is increasing.

High-strength steel sheets used in automobiles are required to have excellent yield strength (YS) and tensile strength (TS). For example, automobile bumpers and other structural components are required to have excellent shock absorption properties in the event of a collision, so it is preferable to use steel sheets with excellent yield strength (YS) and tensile strength (TS), which are correlated with shock absorption properties.

Furthermore, automotive steel sheets are painted before use, and are subjected to chemical conversion treatments such as phosphate treatment as a pretreatment for painting. Since there is concern about delayed fracture due to hydrogen that penetrates from chemicals during the chemical conversion treatment of steel sheets, automotive steel sheets are required to have excellent delayed fracture resistance. In order to increase the proportion of high-strength steel sheets used in automotive parts, it is necessary to comprehensively satisfy these characteristics.

In response to these requirements, various high-strength steel sheets have been proposed. For example, Patent Document 1 describes a composition of a steel sheet containing, in mass %, C: 0.10% or more and 0.6% or less, Si: 1.0% or more and 3.0% or less, Mn: more than 2.5% and 10.0% or less, P: 0.05% or less, S: 0.02% or less, Al: 0.01% or more and 1.5% or less, N: 0.005% or less, Cu: 0.05% or more and 0.50% or less, with the balance being iron and unavoidable impurities, the coverage of the steel sheet surface by oxides mainly composed of Si is 1% or less, the coverage of the steel sheet surface by iron-based oxides is 40% or less, and _CuS / _CuB is 4.0 or less ( _CuS is the Cu concentration in the steel sheet surface layer, Cu The document describes a high-strength cold-rolled steel sheet that satisfies the above conditions ( _B is the Cu concentration in the base material) and has a tensile strength of 1180 MPa or more (Claim 1)," and in this steel sheet, "the steel structure has a total volume fraction of tempered martensite and/or bainite of 40% or more and 100% or less, a volume fraction of ferrite of 0% or more and 60% or less, and a volume fraction of retained austenite of 2% or more and 30% or less (Claim 2)."

Patent Document 2 also describes a composition containing, by mass%, C: 0.06 to 0.25%, Si: 0.6 to 2.5%, Mn: 2.3 to 3.5%, P: 0.02% or less, S: 0.01% or less, sol.Al: less than 0.50%, N: less than 0.015%, with the balance being iron and unavoidable impurities, and a composition containing, by area ratio, ferrite: 6 to 80%, a structure consisting of one or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ: 20 to 94%, and a volume fraction of retained γ: 7 to 20%, with a grain width of 0.18 to 0.60 μm, a grain length of 1.7 to 7.0 _μm , and an aspect ratio of 5 to 15. The steel sheet has _{a γUB} of 0.2 to 5%, and a total area ratio of fresh martensite having a circle-equivalent grain diameter of 1.5 to 15 μm and an aspect ratio of 3 or less and/or a circle-equivalent grain diameter of 1.5 to 15 μm and an aspect ratio of 3 or less: S _γBlock is 3% or less (including 0%) (Claim 1)."

International Publication No. WO 2017/141953 (US2019/040490A1) International Publication No. WO 2018/190416 (US2020/157647A1)

The purpose of Patent Document 1 is to provide a high-strength steel sheet that has a tensile strength of 1180 MPa or more and is excellent in delayed fracture resistance and phosphate treatment properties. However, the description in Patent Document 1 does not take into account the yield strength (YS), which correlates with the impact absorption properties during a collision. In addition, the evaluation of delayed fracture resistance is performed using test pieces that have been ground, and the change in delayed fracture resistance due to shear conditions is not taken into account.

The aim of Patent Document 2 is to provide a steel sheet having a tensile strength of 780 to 1470 MPa, high ductility, and excellent stretch flange formability. However, Patent Document 2 does not take into consideration the yield strength (YS) and delayed fracture resistance properties that are correlated with the impact absorption properties during a collision.

In view of the above problems, the present invention aims to provide a steel sheet having high yield strength YS, high tensile strength TS, and excellent delayed fracture resistance, and a manufacturing method thereof.

The inventors conducted extensive research to solve the above problems and discovered the following:

(1) By increasing the amount of tempered martensite to 90% or more, a TS of 1,470 MPa or more can be achieved.

(2) By setting the amount of tempered martensite to 90% or more and the carbon concentration in the retained austenite to 0.35% or more, a YS of 1,100 MPa or more can be achieved.

(3) By keeping the amount of retained austenite to 7% or less and the total amount of bainitic ferrite and fresh martensite to 9% or less, excellent resistance to delayed fracture can be achieved.

The gist of the present invention, which was completed based on the above findings, is as follows:

[1] In mass%,
C: 0.24-0.28%,
Si: 0.40-0.80%,
Mn: 2.30-2.70%,
Cu: 0.010-1.000%,
P: 0.001-0.100%,
S: 0.0001-0.0200%,
Al: 0.010 to 0.050%, and N: 0.0010 to 0.0100%
Contains
Optionally,
Ti: 0.1000% or less,
B: 0.01000% or less,
Nb: 0.1000% or less,
Cr: 1.00% or less,
V: 0.100% or less,
Mo: 0.500% or less,
Ni: 0.500% or less,
As: 0.500% or less,
Sb: 0.200% or less,
Sn: 0.200% or less,
Ta: 0.100% or less,
Ca: 0.0200% or less,
Mg: 0.0200% or less,
Zn: 0.0200% or less,
Co: 0.0200% or less,
A composition further containing one or more selected from the group consisting of Zr: 0.0200% or less, and REM: 0.0200% or less, with the balance being Fe and unavoidable impurities;
In terms of volume ratio,
Tempered martensite: 90% or more,
Retained austenite: 1 to 7%
Bainitic ferrite and/or fresh martensite: 3-9% in total, and Ferrite: 0-5%
a structure in which the carbon concentration in the retained austenite is 0.35% or more;
A tensile strength TS of 1470 to 1650 MPa;
A yield strength YS of 1100 MPa or more;
A steel plate having

[2] The steel plate described in [1] above, in which the yield strength YS is 1200 MPa or more.

[3] Prepare a non-plated steel sheet having the composition described in [1] above,
The steel plate is heated to a heating temperature T1 of 850° C. or higher,
The steel sheet is held at the heating temperature T1 for 10 to 1000 seconds,
The steel sheet is cooled from the heating temperature T1 to a cooling stop temperature T2 of 130 to 170 ° C.
(i) an average cooling rate from the heating temperature T1 to 550° C.: 16° C./s or more; and (ii) an average cooling rate from 550° C. to the cooling stop temperature T2: 150° C./s or less;
The steel sheet is held at the cooling stop temperature T2 for 1 to 200 seconds,
The steel plate is heated from the cooling stop temperature T2 to a tempering temperature T3 of 280 to 350 ° C. at an average heating rate of 10 ° C./s or more,
The steel plate is held at the tempering temperature T3 for 10 to 1000 seconds;
A method for producing a steel plate, comprising: cooling the steel plate to 50° C. or less;

[4] The method for producing the steel sheet described in [3] above, further comprising a step of subjecting the steel sheet to temper rolling with an elongation rate of 0.1 to 1.0% after the cooling to 50°C or less.

The steel plate of the present invention has high yield strength YS, high tensile strength TS, and excellent resistance to delayed fracture. According to the manufacturing method of the steel plate of the present invention, it is possible to manufacture a steel plate having high yield strength YS, high tensile strength TS, and excellent resistance to delayed fracture.

The steel plate (high-strength steel plate) according to one embodiment of the present invention has a specified component composition, a specified structure, and specified mechanical properties.

First, the composition of the steel plate according to this embodiment will be described. In the following description, "%" representing the content of the component elements of the steel plate means "mass %" unless otherwise specified.

C: 0.24% or more and 0.28% or less C is one of the important basic components of steel, and in the present invention, it is an important element that affects the carbon concentration and TS in the retained austenite. If the C content is less than 0.24%, (i) the carbon concentration in the retained austenite decreases, causing a decrease in YS, and (ii) it becomes difficult to achieve a TS of 1470 MPa or more. Therefore, the C content is set to 0.24% or more, preferably 0.25% or more. On the other hand, if the C content exceeds 0.28%, the strength of the steel plate increases too much, making it difficult to achieve a TS of 1650 MPa or less. Therefore, the C content is set to 0.28% or less, preferably 0.27% or less.

Si: 0.40% or more and 0.80% or less Si is one of the important basic components of steel, and in the present invention, it is an important element that affects the amount of retained austenite and the carbon concentration in the retained austenite. If the Si content is less than 0.40%, the carbon concentration in the retained austenite decreases, and the YS decreases. Therefore, the Si content is set to 0.40% or more, preferably 0.50% or more. On the other hand, if the Si content exceeds 0.80%, the phase fraction of the retained austenite increases, and the delayed fracture resistance property decreases. It is also known that the phosphating property decreases with an increase in the Si content. Therefore, the Si content is set to 0.80% or less, preferably 0.70% or less.

Mn: 2.30% or more and 2.70% or less Mn is one of the important basic components of steel, and in the present invention, it is an important element that affects the phase fraction of tempered martensite, the phase fraction of ferrite, and delayed fracture resistance. If the Mn content is less than 2.30%, the phase fraction of ferrite increases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the Mn content is set to 2.30% or more, preferably 2.40% or more. On the other hand, if the Mn content exceeds 2.70%, the delayed fracture resistance decreases. It is also known that the phosphating property decreases with an increase in the Mn content. Therefore, the Mn content is set to 2.70% or less, preferably 2.60% or less.

Cu: 0.010% to 1.000% Cu is one of the important basic components of steel, and is an important element that affects delayed fracture resistance, particularly in the present invention. If the Cu content is less than 0.010%, the delayed fracture resistance is reduced. Therefore, the Cu content is set to 0.010% or more, preferably 0.050% or more. On the other hand, if the Cu content exceeds 1.000%, the slab becomes embrittled and easily cracked in the casting process, and productivity is significantly reduced. It is also known that the phosphating property decreases with an increase in the Cu content. Therefore, the Cu content is set to 1.000% or less, preferably 0.900% or less.

P: 0.001% or more and 0.100% or less If the P content exceeds 0.100%, P segregates at the prior austenite grain boundary, embrittling the grain boundary, and the delayed fracture resistance property is reduced. Therefore, the P content is set to 0.100% or less, preferably 0.070% or less, and more preferably 0.050% or less. In addition, due to the constraints of production technology, the P content is typically set to 0.001% or more.

S: 0.0001% or more and 0.0200% or less If the S content exceeds 0.0200%, S exists as sulfides and may become the starting point of delayed fracture. Therefore, the S content is set to 0.0200% or less, preferably 0.0100% or less, and more preferably 0.0050% or less. In addition, due to restrictions on production technology, the S content is typically set to 0.0001% or more.

Al: 0.010% or more and 0.050% or less Al increases the strength of the steel sheet and makes it easier to achieve a TS of 1470 MPa or more. Therefore, the Al content is set to 0.010% or more. However, if the Al content exceeds 0.050%, the amount of ferrite increases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the Al content is set to 0.050% or less, preferably 0.040% or less, and more preferably 0.020% or less.

N: 0.0010% or more and 0.0100% or less If the N content exceeds 0.0100%, the slab becomes brittle and easily cracks during the casting process, and productivity drops significantly. Therefore, the N content is set to 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less. In addition, due to restrictions on production technology, the N content is set to 0.0010% or more.

In various embodiments, the composition of the steel sheet contains one or more elements selected from Ti, B, and Nb in the following content ranges:

Ti: 0.1000% or less Ti increases the strength of the steel plate and facilitates the realization of a TS of 1470 MPa or more. Therefore, the Ti content is preferably 0.0010% or more, more preferably 0.0050% or more. On the other hand, if the Ti content exceeds 0.1000%, the slab becomes brittle and easily cracks during the casting process, and productivity is significantly reduced. Therefore, when Ti is added, the Ti content is 0.1000% or less, preferably 0.0600% or less.

B: 0.01000% or less B suppresses the formation of ferrite during cooling, making it easier to achieve a TS of 1470 MPa or more. Therefore, the B content is preferably 0.00010% or more, more preferably 0.00100% or more. On the other hand, if the B content exceeds 0.01000%, the slab becomes brittle and easily cracks during the casting process, and productivity is significantly reduced. Therefore, when B is added, the B content is 0.01000% or less, preferably 0.00500% or less.

Nb: 0.1000% or less Nb increases the strength of the steel plate, facilitates the realization of a TS of 1470 MPa or more, and improves the delayed fracture resistance by combining with C to form Nb-based carbides and serving as a hydrogen trapping site. Therefore, the Nb content is preferably 0.0010% or more, more preferably 0.0050% or more. On the other hand, if the Nb content exceeds 0.1000%, the slab becomes brittle and easily cracks during the casting process, significantly reducing productivity. Therefore, when Nb is added, the Nb content is 0.1000% or less, preferably 0.0600% or less.

[Cu] + 10 × [Nb]: 0.15 or more and 2.00 or less (preferred conditions)
As a result of the research by the present inventors, it was found that when [Cu] + 10 × [Nb] is 0.15 or more, the delayed fracture resistance property is improved. Therefore, it is preferable that [Cu] + 10 × [Nb] is 0.15 or more. [Cu] and [Nb] respectively mean the amount of Cu and the amount of Nb (mass%) in the composition. On the other hand, it is preferable that [Cu] + 10 × [Nb] is 2.00 or less from the upper limit of the amount of Cu and the amount of Nb (mass%).

In various embodiments, the composition of the steel sheet includes one or more elements selected from Cr, V, Mo, Ni, As, Sb, Sn, Ta, Ca, Mg, Zn, Co, Zr, and REM in the following content ranges:

Cr: 1.00% or less Cr not only plays a role as a solid solution strengthening element, but also stabilizes austenite during the cooling process during continuous annealing and suppresses the formation of ferrite, thereby increasing the strength of the steel sheet. In order to obtain such effects, the Cr content is preferably 0.01% or more, more preferably 0.02% or more. On the other hand, if the Cr content exceeds 1.00%, a large amount of coarse precipitates and inclusions are formed, which reduces the ultimate deformability of the steel. Therefore, when Cr is added, the Cr content is 1.00% or less, preferably 0.70% or less.

V: 0.100% or less V increases the strength of the steel sheet. To obtain this effect, the V content is preferably 0.001% or more, more preferably 0.005% or more. On the other hand, if the V content exceeds 0.100%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformability of the steel. Therefore, when V is added, the V content is 0.100% or less, preferably 0.060% or less.

Mo: 0.500% or less Mo not only plays a role as a solid solution strengthening element, but also stabilizes austenite during the cooling process during continuous annealing and suppresses the formation of ferrite, thereby increasing the strength of the steel sheet. In order to obtain such effects, the Mo content is preferably 0.010% or more, more preferably 0.020% or more. On the other hand, if the Mo content exceeds 0.500%, a large amount of coarse precipitates and inclusions are formed, which reduces the ultimate deformability of the steel. Therefore, when Mo is added, the Mo content is 0.500% or less, preferably 0.450% or less.

Ni: 0.500% or less Ni stabilizes austenite during the cooling process during continuous annealing and suppresses the formation of ferrite, thereby increasing the strength of the steel sheet. In order to obtain such an effect, the Ni content is preferably 0.010% or more, more preferably 0.020% or more. On the other hand, if the Ni content exceeds 0.500%, the slab becomes embrittled and easily cracked during the casting process, and productivity is significantly reduced. Therefore, when Ni is added, the Ni content is 0.500% or less, preferably 0.450% or less.

As: 0.500% or less As increases the strength of steel plate. To obtain this effect, the As content is preferably 0.001% or more, more preferably 0.005% or more. On the other hand, if the As content exceeds 0.500%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformability of the steel. Therefore, when As is added, the As content is 0.500% or less, preferably 0.060% or less.

Sb: 0.200% or less Sb suppresses surface softening and increases the strength of the steel plate. To obtain such an effect, the Sb content is preferably 0.001% or more, more preferably 0.005% or more. On the other hand, if the Sb content exceeds 0.200%, the slab becomes brittle and easily cracks during the casting process, and productivity is significantly reduced. Therefore, when Sb is added, the Sb content is 0.200% or less, preferably 0.100% or less.

Sn: 0.200% or less Sn suppresses surface softening and increases the strength of the steel sheet. To obtain such an effect, the Sn content is preferably 0.001% or more, more preferably 0.005% or more. On the other hand, if the Sn content exceeds 0.200%, the slab becomes brittle and easily cracks during the casting process, and productivity is significantly reduced. Therefore, when Sn is added, the Sn content is 0.200% or less, preferably 0.100% or less.

Ta: 0.100% or less Ta increases the strength of the steel plate. To obtain this effect, the Ta content is preferably 0.001% or more, more preferably 0.005% or more. On the other hand, if the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformability of the steel. Therefore, when Ta is added, the Ta content is 0.100% or less, preferably 0.050% or less.

Ca: 0.0200% or less Ca is an element used for deoxidization, and is also an element that is effective in making the shape of sulfides spherical, improving the ultimate deformability of the steel sheet, and improving delayed fracture resistance. In order to obtain such effects, the Ca content is preferably 0.0001% or more. On the other hand, if the Ca content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformability of the steel. Therefore, when Ca is added, the Ca content is set to 0.0200% or less.

Mg: 0.0200% or less Mg is an element used for deoxidization, and is also an element that is effective in making the shape of sulfides spherical, improving the ultimate deformability of the steel sheet, and improving delayed fracture resistance. In order to obtain such effects, the Mg content is preferably 0.0001% or more. On the other hand, if the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are generated, reducing the ultimate deformability of the steel. Therefore, when Mg is added, the Mg content is set to 0.0200% or less.

Zn: 0.0200% or less Zn is an element that is effective in making the shape of inclusions spherical, improving the ultimate deformability of the steel sheet, and improving delayed fracture resistance. In order to obtain such effects, the Zn content is preferably 0.0010% or more. On the other hand, if the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformability of the steel. Therefore, when Zn is added, the Zn content is set to 0.0200% or less.

Co: 0.0200% or less Co is an element that is effective in making the shape of inclusions spherical, improving the ultimate deformability of the steel sheet, and improving delayed fracture resistance. In order to obtain such effects, the Co content is preferably 0.0010% or more. On the other hand, if the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are generated, which reduces the ultimate deformability of the steel. Therefore, when Co is added, the Co content is set to 0.0200% or less.

Zr: 0.0200% or less Zr is an element that is effective in making the shape of inclusions spherical, improving the ultimate deformability of the steel sheet, and improving delayed fracture resistance. In order to obtain such effects, the Zr content is preferably 0.0010% or more. On the other hand, if the Zr content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are generated, lowering the ultimate deformability of the steel. Therefore, when Zr is added, the Zr content is set to 0.0200% or less.

REM: 0.0200% or less REM (Rare Earth Metal) is an element that is effective in making the shape of inclusions spherical, improving the ultimate deformability of the steel plate, and improving delayed fracture resistance. To obtain such effects, the REM content is preferably 0.0010% or more. On the other hand, if the REM content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are generated, reducing the ultimate deformability of the steel. Therefore, when REM is added, the REM content is set to 0.0200% or less.

In the composition of the steel sheet, the balance other than the above elements consists of Fe and inevitable impurities. Note that, when the content of the above optional elements is less than the preferable lower limit, the above optional elements are included as inevitable impurities so as not to impair the effects of the present invention.

Next, the steel structure of the steel plate according to this embodiment will be described. The steel structure includes tempered martensite as the main phase, a predetermined amount of retained austenite, a predetermined amount of one or both of bainitic ferrite and fresh martensite, and optional ferrite.

Tempered martensite: 90% or more by volume Having tempered martensite as the main phase is useful for achieving a TS of 1470 MPa or more. From the viewpoint of increasing the TS, the amount of tempered martensite needs to be 90% or more, preferably 92% or more, and more preferably 94% or more.

The volume fraction of tempered martensite is measured as follows. The L-section of the steel plate is polished and then corroded with 3% by volume of nital. The L-section is observed at 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) in 10 fields of view at a magnification of 2000 times using an SEM to obtain a structural image. In the above structural image, the tempered martensite has a structure with fine internal irregularities and carbides inside. The area fraction of tempered martensite is determined in the 10 fields of view, and the average value is calculated. Since the area fraction of tempered martensite is almost constant in the direction perpendicular to the L-section, this average value is regarded as the "volume fraction of tempered martensite."

Retained austenite: 1% to 7% by volume If the amount of retained austenite exceeds 7%, the delayed fracture resistance property is reduced. The reason for the reduction in delayed fracture resistance property due to retained austenite is that the retained austenite is transformed into processing-induced martensite by processing, and becomes a harder structure than the tempered martensite, which is the main phase. Therefore, the amount of retained austenite is set to 7% or less, preferably 6% or less. On the other hand, the amount of retained austenite depends on the cooling stop temperature T2, and it is difficult to set the cooling stop temperature T2 to less than 130°C due to production technology constraints. Therefore, the amount of retained austenite is set to 1% or more, preferably 2% or more.

Here, the method for measuring the volume fraction of retained austenite is as follows. The steel sheet is polished from the surface to expose the surface at 1/4 of the sheet thickness. In the first step of such polishing, mechanical polishing is performed from the surface of the steel sheet to a surface that is 0.1 mm closer to the surface than the 1/4 of the sheet thickness position. Then, in the second step, chemical polishing is performed to reduce the thickness of the steel sheet by 0.1 mm to expose the surface at the 1/4 of the sheet thickness position. For the exposed surface at the 1/4 of the sheet thickness position, the integrated intensity of the diffraction peaks of the {200}, {220}, and {311} planes of fcc iron and the {200}, {211}, and {220} planes of bcc iron is measured using CoKα radiation in an X-ray diffraction device. The integrated intensity ratio (fcc/(fcc+bcc)) is calculated for all combinations (total of 9) of the three faces of fcc iron and the three faces of bcc iron. The average of the nine integrated intensity ratios obtained is calculated and used as the "volume fraction of retained austenite."

One or both of bainitic ferrite and fresh martensite: total volume fraction of 3% to 9% If the total amount of bainitic ferrite and fresh martensite exceeds 9%, the delayed fracture resistance property is reduced. The reason for the reduction in delayed fracture resistance due to bainitic ferrite and fresh martensite is that both of them form structures with different hardness from the main phase, tempered martensite. Therefore, the total amount is set to 9% or less, preferably 8% or less, and more preferably 5% or less. On the other hand, due to constraints on production technology, the total amount is set to 3% or more.

Here, the method for measuring the total volume ratio of bainitic ferrite and fresh martensite is as follows. After polishing the L-section of the steel plate, it is etched with 3 volume % nital. The L-section is observed at 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) at 10 fields of view at a magnification of 2000 times using an SEM to obtain a structural image. In the above structural image, bainitic ferrite, fresh martensite, and retained austenite have fine internal irregularities and are structures that do not have carbides inside. The total area ratios of these three phases are obtained in the 10 fields of view, and their average value is calculated. Since the total area ratios of these three phases are almost constant in the direction perpendicular to the L-section, this average value is regarded as the "total volume ratio" of these three phases. The total volume ratio of bainitic ferrite and fresh martensite can be obtained by subtracting the volume ratio of retained austenite measured by the above-mentioned method from the total volume ratio of these three phases.

Ferrite: Volume fraction 0% to 5% Since ferrite is a soft structure, if the ferrite content exceeds 5%, it becomes difficult to achieve a TS of 1470 MPa or more. Therefore, the ferrite content is set to 5% or less, preferably 3% or less, and more preferably 2% or less.

The method for measuring the volume fraction of ferrite is as follows. After polishing the L-section of the steel plate, it is corroded with 3 volume % nital. The L-section is observed at 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) in 10 fields of view at a magnification of 2000 times using an SEM to obtain a structural image. In the above structural image, the ferrite is a structure with concave portions and flat inside. The area fraction of ferrite is obtained in the 10 fields of view, and the average value is calculated. Since the area fraction of ferrite is almost constant in the direction perpendicular to the L-section, this average value is regarded as the "volume fraction of ferrite".

Carbon concentration in retained austenite: 0.35% or more If the carbon concentration in the retained austenite is less than 0.35%, the main cause of yielding of the steel sheet changes from tempered martensite to retained austenite, making it difficult to achieve a YS of 1100 MPa or more. Therefore, the carbon concentration in the retained austenite is set to 0.35% or more, and preferably 0.40% or more. On the other hand, due to constraints in production technology, the carbon concentration in the retained austenite is preferably set to 1.00% or less.

The method for measuring the carbon concentration in the retained austenite using CoKα radiation in an X-ray diffractometer is as follows: First, the lattice constant a of the retained austenite was calculated from the diffraction peak shift amount of the (220) plane of the austenite using formula (1), and the carbon concentration in the retained austenite was calculated by substituting the obtained lattice constant a of the retained austenite into formula (2).
a=1.79021√2/sinθ...(1)
a=3.578+0.00095[Mn]+0.022[N]+0.0006[Cr]+0.0031[Mo]+0.0051[Nb]+0.0039[Ti]+0.0056[Al]+0.033[C]...(2)
In addition,
a: Lattice constant of retained austenite (Å)
θ: value (rad) obtained by dividing the diffraction peak angle of the (220) plane by 2
[M]: Content (mass%) of element M in retained austenite
That is, [C] in formula (2) is the carbon concentration in the retained austenite. However, in the present disclosure, the content (mass%) of element M other than C in the retained austenite is defined as the content (mass%) in the entire steel.

Tensile strength TS: 1470 MPa or more and 1650 MPa or less The steel plate of this embodiment has a tensile strength TS of 1470 MPa or more and 1650 MPa or less.

Yield strength YS: 1100 MPa or more The steel plate of this embodiment has a YS of 1100 MPa or more, preferably a YS of 1150 MPa or more, and more preferably a YS of 1200 MPa or more. The steel plate of this embodiment preferably has a YS of 1470 MPa or less.

Yield ratio YR: 0.75 or more (optimal condition)
The steel sheet of the present embodiment preferably has a YR of 0.75 or more, more preferably 0.80 or more, and preferably 1.0 or less, where YR=YS/TS.

In one embodiment of the present invention, a method for manufacturing a steel sheet (high-strength steel sheet) includes preparing an unplated steel sheet having the above-mentioned composition, and annealing the steel sheet under predetermined conditions. Specifically, the annealing includes heating the steel sheet to a predetermined heating temperature T1, holding the steel sheet at T1 for a predetermined time t1, then continuously cooling the steel sheet to a predetermined cooling stop temperature T2, holding the steel sheet at T2 for a predetermined time t2, then heating the steel sheet to a predetermined tempering temperature T3, holding the steel sheet at T3 for a predetermined time t3, and then cooling the steel sheet to 50°C or less. This method makes it possible to suitably manufacture a steel sheet having the above-mentioned composition, structure, and mechanical properties.

In this embodiment, the unplated steel sheet subjected to annealing is preferably a cold-rolled steel sheet. A suitable manufacturing process for the cold-rolled steel sheet is described below.

First, a steel slab having the above-mentioned composition is manufactured. There are no particular limitations on the method for manufacturing the steel slab, and any known melting method using a converter, electric furnace, etc. can be used. In order to prevent macrosegregation, it is preferable to manufacture the steel slab by a continuous casting method.

The steel slab is then hot-rolled to obtain a hot-rolled steel sheet. Examples of methods for hot-rolling the steel slab include a method of rolling the steel slab after heating it, a method of directly rolling the steel slab after continuous casting without heating it, and a method of heating the steel slab for a short time after continuous casting and rolling it. The slab heating temperature, slab soaking time, finish rolling temperature, and coiling temperature in hot rolling are not particularly limited, but the slab heating temperature is preferably 1100°C or more and 1300°C or less, the slab soaking time is preferably 30 minutes or more and 250 minutes or less, the finish rolling temperature is preferably the _Ar3 transformation point or more, and the coiling temperature is preferably 350°C or more and 650°C or less.

Next, the hot-rolled steel sheet is pickled. Pickling removes oxides from the steel sheet surface, which contributes to ensuring good phosphating properties and plating quality in the final high-strength steel sheet product. Pickling may be performed once or multiple times.

Then, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. Cold rolling may be performed as is after pickling, or cold rolling may be performed after pickling and heat treatment. The reduction ratio in cold rolling is not particularly limited, but is preferably 30% to 80%. The effects of the present invention can be obtained without any particular limitations on the number of rolling passes and the reduction ratio of each pass. The thickness of the cold-rolled steel sheet is not particularly limited, but is preferably 0.6 mm to 2.0 mm.

Heating temperature T1: 850°C or higher If the heating temperature T1 is less than 850°C, the annealing treatment will be performed in the two-phase region of ferrite and austenite, so the amount of ferrite will exceed 5% after annealing, making it difficult to achieve a TS of 1470 MPa or higher. Therefore, the heating temperature T1 is set to 850°C or higher (austenitization temperature region), and preferably 860°C or higher. Note that there is no particular upper limit to the heating temperature T1, but due to production technology constraints, the heating temperature T1 is preferably set to 1000°C or lower.

Holding time t1 at heating temperature T1: 10 seconds or more and 1000 seconds or less If the holding time t1 is less than 10 seconds, austenitization is insufficient, the amount of ferrite after annealing exceeds 5%, and it becomes difficult to achieve a TS of 1470 MPa or more. Therefore, the holding time t1 is set to 10 seconds or more, preferably 50 seconds or more, and more preferably 100 seconds or more. On the other hand, if the holding time t1 exceeds 1000 seconds, the prior austenite grain size increases excessively, and the delayed fracture resistance property decreases. Therefore, the holding time t1 is set to 1000 seconds or less, preferably 500 seconds or less, and more preferably 400 seconds or less.

Average cooling rate θ1 from heating temperature T1 to 550°C: 16°C/s or more If the average cooling rate θ1 is less than 16°C/s, bainite transformation occurs in the temperature range from heating temperature T1 to 550°C, the total amount of bainitic ferrite and fresh martensite becomes 9% or more, and the delayed fracture resistance property decreases. Therefore, the average cooling rate θ1 is set to 16°C/s or more, and preferably 20°C/s or more. There is no particular upper limit to the average cooling rate θ1, but due to constraints on production technology, the average cooling rate θ1 is preferably set to 300°C/s or less.

Average cooling rate θ2 from 550 ° C. to cooling stop temperature T2: 150 ° C./s or less If the average cooling rate θ2 exceeds 150 ° C./s, carbon distribution from martensite to retained austenite during cooling is hindered, and the carbon concentration in the retained austenite becomes less than 0.35%, and as a result, the main cause of yielding of the steel sheet changes from tempered martensite to retained austenite, making it difficult to achieve a YS of 1100 MPa or more. Therefore, the average cooling rate θ2 is set to 150 ° C./s or less, preferably 120 ° C./s or less, and more preferably 100 ° C./s or less. There is no particular limit to the lower limit of the average cooling rate θ2, but due to constraints on production technology, the average cooling rate θ2 is preferably set to 5 ° C./s or more.

Continuous cooling from heating temperature T1 to cooling stop temperature T2 In one embodiment, it has been found that it is necessary for the steel sheet to be continuously cooled from the heating temperature T1 to the cooling stop temperature T2, i.e., the steel sheet temperature is gradually decreased, in order to realize a YS of 1100 MPa or more. For example, if isothermal holding is performed for 1 second or more in the temperature range from the heating temperature T1 to the cooling stop temperature T2, it becomes difficult to realize a YS of 1100 MPa or more. Therefore, isothermal holding is not performed for 1 second or more in the temperature range from the heating temperature T1 to the cooling stop temperature T2. Similarly, reheating is not performed in the temperature range from the heating temperature T1 to the cooling stop temperature T2.

Cooling stop temperature T2: 130°C or higher and 170°C or lower If the cooling stop temperature T2 exceeds 170°C, the amount of retained austenite exceeds 7%, and the delayed fracture resistance property is deteriorated. Therefore, the cooling stop temperature T2 is set to 170°C or lower, and preferably 160°C or lower. On the other hand, due to constraints on production technology, the cooling stop temperature T2 is set to 130°C or higher, and preferably 140°C or higher.

Holding time t2 at cooling stop temperature T2: 1.0 seconds or more and 200.0 seconds or less If the holding time t2 is less than 1.0 seconds, the martensitic transformation is insufficient and the amount of retained austenite exceeds 7%, so that the delayed fracture resistance property is deteriorated. Therefore, the holding time t2 is set to 1.0 seconds or more, preferably 5.0 seconds or more. On the other hand, if the holding time t2 exceeds 200.0 seconds, the amount of carbide precipitation increases, so that the carbon concentration in the retained austenite becomes less than 0.35%, and as a result, the main cause of the yield of the steel sheet changes from tempered martensite to retained austenite, making it difficult to achieve a YS of 1100 MPa or more. Therefore, the holding time t2 is set to 200.0 seconds or less, preferably 150.0 seconds or less.

Average heating rate θ3 from cooling stop temperature T2 to tempering temperature T3: 10° C./s or more When the average heating rate θ3 is less than 10° C./s, the amount of carbide precipitation increases, so the carbon concentration in the retained austenite becomes less than 0.35%, and as a result, the main cause of the yield of the steel sheet changes from tempered martensite to retained austenite, making it difficult to achieve a YS of 1100 MPa or more. In addition, when the average heating rate θ3 is less than 10° C./s, bainite transformation occurs, the total amount of bainitic ferrite and fresh martensite becomes 9% or more, and the delayed fracture resistance property decreases. Therefore, the average heating rate θ3 is 10° C./s or more, preferably 15° C./s or more. There is no particular upper limit to the average heating rate θ3, but due to constraints on production technology, the average cooling rate θ3 is preferably 200° C./s or less.

Tempering temperature T3: 280°C or more and 350°C or less When the tempering temperature T3 exceeds 350°C, the tempering of martensite proceeds excessively, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the tempering temperature T3 is set to 350°C or less, preferably 340°C or less. On the other hand, when the tempering temperature T3 is less than 280°C, carbon distribution from martensite to austenite becomes insufficient, and the carbon concentration in the retained austenite becomes less than 0.35%, and as a result, the main cause of the yield of the steel sheet changes from tempered martensite to retained austenite, making it difficult to achieve a YS of 1100 MPa or more. Therefore, the tempering temperature T3 is set to 280°C or more, preferably 290°C or more.

Holding time t3 at tempering temperature T3: 10 seconds or more and 1000 seconds or less If the holding time t3 is less than 10 seconds, the bainite transformation at the tempering temperature T3 does not progress, and the amount of retained austenite exceeds 7%, so that the delayed fracture resistance property is deteriorated. Therefore, the holding time t3 is set to 10 seconds or more, preferably 50 seconds or more, and more preferably 100 seconds or more. On the other hand, if the holding time t3 exceeds 1000 seconds, the tempering of martensite progresses excessively, and it becomes difficult to realize a TS of 1470 MPa or more. Therefore, the holding time t3 is set to 1000 seconds or less, preferably 800 seconds or less, and more preferably 600 seconds or less.

Cooling the steel sheet to 50° C. or less After the steel sheet has been held at the tempering temperature T3, the steel sheet is cooled to 50° C. or less, preferably to about room temperature. The method and conditions for this cooling are not particularly limited.

In various embodiments, the steel sheet is then subjected to temper rolling with an elongation rate of 0.1% or more. As a result, the residual austenite with a low carbon concentration undergoes processing-induced martensite transformation, increasing the carbon concentration in the residual austenite and improving the YS. Therefore, when temper rolling is performed, it is preferable to set the elongation rate to 0.1% or more. There is no particular upper limit to the elongation rate, but if the elongation rate is too high, the effect of improving the YS will saturate. In addition, from the viewpoint of constraints on manufacturing equipment, it is preferable to set the elongation rate to 1.0% or less.

Steel having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and made into slabs using the continuous casting method.

Then, the obtained slab was heated, hot-rolled, pickled, and then cold-rolled to obtain a cold-rolled steel sheet. The cold-rolled steel sheet was then annealed under the conditions shown in Table 2, and then cooled to room temperature to obtain a high-strength steel sheet. Note that in some comparative examples, the steel sheet was held at the intermediate holding temperature and for the intermediate holding time shown in Table 2 between the heating temperature T1 and the cooling stop temperature T2. In some invention examples, the steel sheet was cooled to room temperature, and then temper rolled at the elongation rate shown in the "SKP" column in Table 2.

For each of the high-strength steel plates obtained in the above manner, the volume fraction of tempered martensite, the volume fraction of retained austenite, the total volume fraction of bainitic ferrite and fresh martensite, the volume fraction of ferrite, and the carbon concentration in the retained austenite were determined according to the methods described above. The results are shown in Table 3.

In addition, the high-strength steel plates in each example were subjected to the following tensile tests and evaluation of delayed fracture resistance properties.

[Tensile test]
A JIS No. 5 test piece (gauge length 50 mm, parallel part width 25 mm) was taken from each high-strength steel plate so that the direction perpendicular to the rolling direction was the longitudinal direction of the test piece, and a tensile test was carried out according to JIS Z 2241. The tensile test was carried out under the condition of a crosshead speed of 1.67×10 ⁻¹ mm/s, and YS and TS were measured. In the present invention, TS of 1470 MPa or more and 1650 MPa or less was judged to be acceptable. YS of less than 1100 MPa was evaluated as "x", 1100 MPa or more and less than 1200 MPa was evaluated as "○", and 1200 MPa or more was evaluated as "◎", and YS of 1100 MPa or more was judged to be excellent. In addition, the yield ratio YS was calculated from YS and TS and shown in Table 3.

[Evaluation of delayed fracture resistance]
The delayed fracture resistance evaluation was performed by an immersion test. The high-strength steel plate of each example was sheared to 30 m x 110 mm with the direction perpendicular to the rolling direction as the longitudinal direction, and a hole for passing a bolt was drilled to prepare a test piece. The rake angle during shearing was unified to 0°, and the shear clearance was changed to 5, 10, 15, 20, 25, 30, and 35%. After bending the test piece with a 90° V-bending punch and a die with a curvature radius of 10 mm at the tip, a stress of 1000 MPa was applied to the apex of the test piece by the bolt. The test piece in the stressed state was immersed in hydrochloric acid at 25°C and pH 3 for 100 hours. The shear clearance range where no cracks occurred was evaluated as "x", the shear clearance range where no cracks occurred was 10% or more and less than 15% was evaluated as "○", and the shear clearance range where no cracks occurred was 15% or more was evaluated as "◎". A specimen having a shear clearance range of 10% or more where no cracks occur was deemed to have excellent delayed fracture resistance.

As is clear from Table 3, the examples of the present invention have a TS of 1470 MPa or more and 1650 MPa or less, a YS of 1100 MPa or more, and excellent delayed fracture resistance. On the other hand, the comparative examples are inferior in one or more of TS, YS, and delayed fracture resistance.

The high-strength steel plate of the present invention can be suitably used as a structural member for automobile parts and the like, and contributes to improving fuel efficiency by reducing the weight of the vehicle body.

Claims (4)

In mass percent,
C: 0.24-0.28%,
Si: 0.40-0.80%,
Mn: 2.30-2.70%,
Cu: 0.010-1.000%,
P: 0.001-0.100%,
S: 0.0001-0.0200%,
Al: 0.010 to 0.050%, and N: 0.0010 to 0.0100%
Contains
Optionally,
Ti: 0.1000% or less,
B: 0.01000% or less,
Nb: 0.1000% or less,
Cr: 1.00% or less,
V: 0.100% or less,
Mo: 0.500% or less,
Ni: 0.500% or less,
As: 0.500% or less,
Sb: 0.200% or less,
Sn: 0.200% or less,
Ta: 0.100% or less,
Ca: 0.0200% or less,
Mg: 0.0200% or less,
Zn: 0.0200% or less,
Co: 0.0200% or less,
A composition further containing one or more selected from the group consisting of Zr: 0.0200% or less, and REM: 0.0200% or less, with the balance being Fe and unavoidable impurities;
In terms of volume ratio,
Tempered martensite: 90% or more,
Retained austenite: 1 to 7%
Bainitic ferrite and/or fresh martensite: 3-9% in total, and Ferrite: 0-5%
a structure in which the carbon concentration in the retained austenite is 0.35% or more;
A tensile strength TS of 1470 to 1650 MPa;
A yield strength YS of 1100 MPa or more;
A steel plate having The steel plate according to claim 1, wherein the yield strength YS is 1200 MPa or more. An unplated steel sheet having the composition according to claim 1 is prepared,
The steel plate is heated to a heating temperature T1 of 850° C. or higher,
The steel sheet is held at the heating temperature T1 for 10 to 1000 seconds,
The steel sheet is cooled from the heating temperature T1 to a cooling stop temperature T2 of 130 to 170 ° C.
(i) an average cooling rate from the heating temperature T1 to 550° C.: 16° C./s or more; and (ii) an average cooling rate from 550° C. to the cooling stop temperature T2: 150° C./s or less;
The steel sheet is held at the cooling stop temperature T2 for 1.0 to 200.0 seconds,
The steel plate is heated from the cooling stop temperature T2 to a tempering temperature T3 of 280 to 350 ° C. at an average heating rate of 10 ° C./s or more,
The steel plate is held at the tempering temperature T3 for 10 to 1000 seconds;
A method for producing the steel plate according to claim 1, comprising the step of cooling the steel plate to 50°C or less. The method for manufacturing the steel plate according to claim 3, further comprising a step of subjecting the steel plate to temper rolling with an elongation rate of 0.1 to 1.0% after the cooling to 50°C or less.