Amrinder Singh Gill Grant Aaron Thomas Junya Tobata Shigehiro Takajo Yuki Toji

[0001] The present application claims priority from provisional patent application serial no. 63/236,426, entitled “Steel Sheet and Method of Producing Same,” filed on August 24, 2021. The disclosure of application serial no. 63/236,426 is incorporated herein by reference.

Background

[0002] This disclosure relates to a steel sheet and a method of producing the same.

[0003] With the aim of both reducing CO ₂ emissions and improving crashworthiness by reducing the weight of the vehicle body, strengthening of steel sheets for automobiles is progressing and new laws and regulations are being introduced one after another. Accordingly, the application of high-strength steel sheets with a tensile strength (TS) of 1470 MPa or higher is increasing in the major structural parts that form automobiles.

[0004] High-strength steel sheets used in automobiles are required to have excellent yield strength (YS) and tensile strength (TS). For example, automobile structural parts such as bumpers are required to have excellent impact absorption in a crash, and thus it is suitable to use steel sheets with excellent yield strength (YS) and tensile strength (TS), which are correlated with impact absorption.

[0005] In addition, steel sheets for automobiles are used with painting, and subjected to chemical conversion treatment such as phosphate treatment as a pre-treatment for the painting. Since there is concern about delayed fracture caused by hydrogen entering from chemicals during chemical conversion treatment, steel sheets for automobiles are required to have excellent delayed fracture resistance. In order to increase the percentage of high-strength steel sheets applied to automotive parts, it is requested that the above-described properties be satisfied comprehensively.

[0006] Various types of high-strength steel sheets have been proposed to meet these requirements. For example, WO2017/141953A1 (PTL 1, English counterpart: US2019/040490A1) describes "[a] high- strength cold-rolled steel sheet having a chemical composition containing, by 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, and the balance being Fe and inevitable impurities, wherein a steel sheet surface coverage of oxides mainly containing Si is 1 % or less, a steel sheet surface coverage of iron-based oxides is 40 % or less, CUS/CUB is 4.0 or less, and a tensile strength is 1180 MPa or more, where Cus denotes a Cu concentration in a surface layer of the steel sheet, and CUB denotes a Cu concentration in base steel (claim 1)", and "wherein the steel sheet has a microstructure including, in terms of volume ratio, tempered martensite and/or bainite in a total amount of 40 % or more and 100 % or less, ferrite in an amount of 0 % or more and 60 % or less, and retained austenite in an amount of 2 % or more and 30 % or less (claim 2)".

[0007] Further, W02018/190416A1 (PTL 2, English counterpart: US2020/157647A1) describes "[a] steel sheet having a component composition containing, in 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 %, and N: less than 0.015 %, with the balance being iron and incidental impurities, the steel sheet containing, in area ratio, 6 to 80 % of ferrite and 20 to 94 % of a microstructure composed of one or two or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained y, and containing, in volume ratio, 7 to 20 % of retained y, wherein: an area ratio (Sγ _UB ) of retained yuB having a particle width of 0.18 to 0.60 pm, a particle length of 1.7 to 7.0 pm, and an aspect ratio of 5 to 15 is 0.2 to 5 %; and a total area ratio (Sγ _BLOCK ) of fresh martensite having an equivalent circle diameter of 1.5 to 15 μm and an aspect ratio of 3 or less and/or retained y particles having an equivalent circle diameter of 1.5 to 15 μm and an aspect ratio of 3 or less is 3 % or less (including 0%) (claim 1)".

Citation List

[0008] Patent Literature

PTL 1: WO2017/141953A1 (US2019/040490A1) PTL 2: W02018/190416A1 (US2020/157647A1)

Brief Summary

[0009] An object of PTL 1 is to provide a high-strength steel sheet having a tensile strength of 1180 MPa or higher and excellent delayed fracture resistance and phosphatability. However, PTL 1 does not consider the yield strength (YS), which is correlated to impact absorption in a crash. The evaluation of delayed fracture resistance was carried out using ground specimens, and the change in delayed fracture resistance due to shear conditions was not taken into account.

[0010] An object of PTL 2 is to provide a steel sheet having a tensile strength of 780 MPa to 1470 MPa grade, high ductility, and excellent stretch flange formability. However, PTL 2 does not consider the yield strength (YS) and delayed fracture resistance, which are correlated to impact absorption in a crash.

[0011] It would thus be helpful to provide a steel sheet having a high yield strength YS, high tensile strength TS, and excellent delayed fracture resistance, and a method of producing the same.

[0012] To address the above issues, the present inventors conducted intensive study and found the following:

[0013] (1) by containing tempered martensite in an amount of 90 % or more, a TS of 1470 MPa or more can be achieved; [0014] (2) by containing tempered martensite in an amount of 90 % or more and having a carbon concentration in retained austenite of 0.35 % or more, a YS of 1100 MPa or more can be achieved; and

[0015] (3) by containing retained austenite in an amount of 7 % or less and bainitic ferrite and fresh martensite in an amount of 9 % or less in total, excellent delayed fracture resistance can be achieved.

[0016] This disclosure was completed on the basis of the above findings, and the primary features thereof are as described below.

[0017] [1] A steel sheet comprising: a chemical composition containing, in mass%, C: 0.24 % to 0.28 %, Si: 0.40 % to 0.80 %, Mn: 2.30 % to 2.70 %, Cu: 0.010 % to 1.000 %, P: 0.001 % to 0.100 %, S: 0.0001 % to 0.0200 %, Al: 0.010 % to 0.050 %, and N: 0.0010 % to 0.0100 %, and optionally at least one selected from the group consisting of 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, Zr: 0.0200 % or less, and REM: 0.0200 % or less, with the balance being Fe and inevitable impurities; a microstructure comprising, in volume fraction, tempered martensite: 90 % or more, retained austenite: 1 % to 7 %, one or both of bainitic ferrite and fresh martensite: 3 % to 9 % in total, and ferrite: 0 % to 5 %, where the retained austenite has a carbon concentration of 0.35 % or more; a tensile strength TS of 1470 MPa to 1650 MPa ; and a yield strength YS of 1100 MPa or more.

[0018] [2] The steel sheet according to [1], wherein the yield strength YS is 1200 MPa or more.

[0019] [3] A method of producing a steel sheet, comprising: preparing an uncoated steel sheet having the chemical composition as recited in [1]; heating the steel sheet to a heating temperature T1 of 850 °C or higher; holding the steel sheet at the heating temperature T1 for 10 seconds to 1000 seconds; continuously cooling the steel sheet from the heating temperature T1 to a cooling stop temperature T2 of 130 °C to 170 °C under a set of conditions including: (i) an average cooling rate in a temperature range from the heating temperature T1 to 550 °C being 16 °C/s or higher; and (ii) an average cooling rate in a temperature range from 550 °C to the cooling stop temperature T2 being 150 °C/s or lower; holding the steel sheet at the cooling stop temperature T2 for 1 seconds to 200 seconds; heating the steel sheet from the cooling stop temperature T2 to a tempering temperature T3 of 280 °C to 350 °C at an average heating rate of 10 °C/s or higher; holding the steel sheet at the tempering temperature T3 for 10 seconds to 1000 seconds; and cooling the steel sheet to 50 °C or lower, to thereby produce the steel sheet as recited in [1],

[0020] [4] The method of producing a steel sheet according to [3], further comprising: after the cooling to 50 °C or lower, subjecting the steel sheet to temper rolling with an elongation rate of 0.1 % to 1.0 %. [0021] The steel sheet disclosed herein has a high yield strength YS, high tensile strength TS, and excellent delayed fracture resistance. According to the method of producing a steel sheet disclosed herein, it is possible to produce a steel sheet having a high yield strength YS, high tensile strength TS, and excellent delayed fracture resistance.

Detailed Description

[0022] A steel sheet (high-strength steel sheet) according to one of the embodiments disclosed herein comprises a predetermined chemical composition, a predetermined microstructure, and predetermined mechanical properties.

[0023] First, the chemical composition of the steel sheet according to this embodiment will be described below. The "%" representations below indicating the chemical composition of the steel sheet are in "mass%" unless stated otherwise.

[0024] C: 0.24 % or more and 0.28 % or less

C is one of the important basic components of steel, and especially in the present disclosure, it is an important element that affects the carbon concentration in retained austenite and TS. If the C content is less than 0.24 %, (i) the carbon concentration in retained austenite decreases, resulting in a lower YS, and (ii) it is difficult to obtain a TS of 1470 MPa or more. Therefore, the C content is 0.24 % or more, and preferably 0.25 % or more. On the other hand, if the C content exceeds 0.28 %, the strength of the steel sheet increases too much, making it difficult to achieve a TS of not greater than 1650 MPa. Therefore, the C content is 0.28 % or less, and preferably 0.27 % or less.

[0025] Si: 0.40 % or more and 0.80 % or less

Si is one of the important basic components of steel, and especially in the present disclosure, it is an important element that affects the content 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, resulting in a lower YS. Therefore, the Si content is 0.40 % or more, and preferably 0.50 % or more. On the other hand, if the Si content exceeds 0.80 %, the phase fraction of retained austenite is increased, resulting in lower delayed fracture resistance. It is also known that as the Si content increases, the phosphatability decreases. Therefore, the Si content is 0.80 % or less, and preferably 0.70 % or less.

[0026] Mn: 2.30 % or more and 2.70 % or less

Mn is one of the important basic components of steel, and especially in the present disclosure, 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 is increased, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the Mn content is 2.30 % or more, and preferably 2.40 % or more. On the other hand, if the Mn content exceeds 2.70 %, delayed fracture resistance decreases. It is also known that as the Mn content increases, the phosphatability decreases.

Therefore, the Mn content is 2.70 % or less, and preferably 2.60 % or less.

[0027] Cu: 0.010 % or more and 1.000 % or less

Cu is one of the important basic components of steel, and especially in the present disclosure, it is an important element that affects delayed fracture resistance. If the Cu content is less than 0.010 %, delayed fracture resistance decreases. Therefore, the Cu content is 0.010 % or more, and preferably 0.050 % or more. On the other hand, if the content of Cu exceeds 1.000 %, the slab becomes embrittled and prone to cracking during the casting process, resulting in a significant decrease in productivity. It is also known that as the Cu content increases, the phosphatability decreases. Therefore, the Cu content is 1.000 % or less, and preferably 0.900 % or less.

[0028] P: 0.001 % or more and 0.100 % or less

If the P content exceeds 0.100 %, P segregates in prior austenite grain boundaries and embrittles the grain boundaries, resulting in lower delayed fracture resistance. Therefore, the P content is 0.100 % or less, preferably 0.070 % or less, and more preferably 0.050 % or less. Also, under production constraints, the P content is typically 0.001 % or more.

[0029] S: 0.0001 % or more and 0.0200 % or less If the S content exceeds 0.0200 %, S forms a sulfide, which may contribute to delayed fracture. Therefore, the S content is 0.0200 % or less, preferably 0.0100 % or less, and more preferably 0.0050 % or less. Also, under production constraints, the S content is typically 0.0001 % or more.

[0030] Al: 0.010 % or more and 0.050 % or less

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

[0031] N: 0.0010 % or more and 0.0100 % or less

If the N content is more than 0.0100 %, the slab becomes embrittled and prone to cracking during the casting process, resulting in a significant decrease in productivity. Therefore, the N content is 0.0100 % or less, preferably 0.0070 % or less, and more preferably 0.0050 % or less. Also, under production constraints, the N content is 0.0010 % or more.

[0032] In some embodiments, the chemical composition of the steel sheet contains at least one selected from the group consisting of Ti, B, and Nb within the following ranges.

[0033] Ti: 0.1000 % or less Ti increases the strength of the steel sheet and facilitates the provision of a TS of 1470 MPa or more. Therefore, the Ti content is preferably 0.0010 % or more, and more preferably 0.0050 % or more. On the other hand, if the Ti content exceeds 0.1000 %, the slab becomes embrittled and prone to cracking during the casting process, resulting in a significant decrease in productivity.

Therefore, when Ti is added, the Ti content is 0.1000 % or less, and preferably 0.0600 % or less.

[0034] B: 0.01000 % or less

B inhibits the formation of ferrite on cooling and facilitates the provision of a TS of 1470 MPa or more. Therefore, the B content is preferably 0.00010 % or more, and more preferably 0.00100 % or more. On the other hand, if the B content is more than 0.01000 %, the slab becomes embrittled and prone to cracking during the casting process, resulting in a significant decrease in productivity. Therefore, when B is added, the B content is 0.01000 % or less, and preferably 0.00500 % or less.

[0035] Nb: 0.1000 % or less

Nb increases the strength of the steel sheet and facilitates the provision of a TS of 1470 MPa or more. Nb also combines with C to form Nb-based carbides, which serve as hydrogen trapping sites, and thus improves delayed fracture resistance. Therefore, the Nb content is preferably 0.0010 % or more, and more preferably 0.0050 % or more. On the other hand, if the Nb content exceeds 0.1000 %, the slab becomes embrittled and prone to cracking during the casting process, resulting in a significant decrease in productivity. Therefore, when Nb is added, the Nb content is 0.1000 % or less, and preferably 0.0600 % or less.

[0036] [Cu] + 10 x [Nb] = 0.15 or more and 2.00 or less (preferred condition)

Our investigation revealed that when [Cu] + 10 x [Nb] = 0.15 or more, delayed fracture resistance is improved. Therefore, it is preferable that [Cu] + 10 x [Nb] be 0.15 or more. Note that [Cu] and [Nb] respectively represent Cu content and Nb content (in mass%) in the chemical composition. On the other hand, from the respective upper limits of the Cu and Nb contents (in mass%), it is preferable that [Cu] + 10 x [Nb] be 2.00 or less.

[0037] In some embodiments, the chemical composition of the steel sheet contains at least one selected from the group consisting of Cr, V, Mo, Ni, As, Sb, Sn, Ta, Ca, Mg, Zn, Co, Zr, and REM within the following ranges.

[0038] Cr: 1.00 % or less

Cr increases the strength of the steel sheet because it not only serves as a solid-solution-strengthening element, but also enables stabilization of austenite and suppresses the formation of ferrite during the cooling process in continuous annealing. To obtain this effect, the Cr content is preferably 0.01 % or more, and more preferably 0.02 % or more. On the other hand, if the Cr content exceeds 1.00 %, coarse precipitates and inclusions are formed in large quantities, which reduce the ultimate deformability of the steel. Therefore, when Cr is added, the Cr content is 1.00 % or less, and preferably 0.70 % or less.

[0039] 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, and more preferably 0.005 % or more. On the other hand, if the V content exceeds 0.100 %, coarse precipitates and inclusions are formed in large quantities, which reduce the ultimate deformability of the steel. Therefore, when V is added, the V content is 0.100 % or less, and preferably 0.060 % or less.

[0040] Mo: 0.500 % or less

Mo increases the strength of the steel sheet because it not only serves as a solid-solution-strengthening element, but also enables stabilization of austenite and suppresses the formation of ferrite during the cooling process in continuous annealing. To obtain this effect, the Mo content is preferably 0.010 % or more, and more preferably 0.020 % or more. On the other hand, if the Mo content exceeds 0.500 %, coarse precipitates and inclusions are formed in large quantities, which reduce the ultimate deformability of the steel. Therefore, when Mo is added, the Mo content is 0.500 % or less, and preferably 0.450 % or less.

[0041] Ni: 0.500 % or less

Ni increases the strength of the steel sheet because it enables stabilization of austenite and suppresses the formation of ferrite during the cooling process in continuous annealing. To obtain this effect, the Ni content is preferably 0.010 % or more, and more preferably 0.020 % or more. On the other hand, if the Ni content exceeds 0.500 %, the slab becomes embrittled and prone to cracking during the casting process, resulting in a significant decrease in productivity. Therefore, when Ni is added, the Ni content is 0.500 % or less, and preferably 0.450 % or less.

[0042] As: 0.500 % or less

As increases the strength of the steel sheet. To obtain this effect, the As content is preferably 0.001 % or more, and more preferably 0.005 % or more. On the other hand, if the As content exceeds 0.500 %, coarse precipitates and inclusions are formed in large quantities, which reduce the ultimate deformability of the steel. Therefore, when As is added, the As content is 0.500 % or less, and preferably 0.060 % or less.

[0043] Sb: 0.200 % or less

Sb makes the surface layer less prone to softening and increases the strength of the steel sheet. To obtain this effect, the Sb content is preferably 0.001 % or more, and more preferably 0.005 % or more. On the other hand, if the Sb content exceeds 0.200 %, the slab becomes embrittled and prone to cracking during the casting process, resulting in a significant decrease in productivity.

Therefore, when Sb is added, the Sb content is 0.200 % or less, and preferably 0.100 % or less. [0044] Sn: 0.200 % or less

Sn makes the surface layer less prone to softening and increases the strength of the steel sheet. To obtain this effect, the Sn content is preferably 0.001 % or more, and more preferably 0.005 % or more. On the other hand, if the Sn content exceeds 0.200 %, the slab becomes embrittled and prone to cracking during the casting process, resulting in a significant decrease in productivity.

Therefore, when Sn is added, the Sn content is 0.200 % or less, and preferably 0.100 % or less.

[0045] Ta: 0.100 % or less

Ta increases the strength of the steel sheet. To obtain this effect, the Ta content is preferably 0.001 % or more, and more preferably 0.005 % or more. On the other hand, if the Ta content exceeds 0.100 %, coarse precipitates and inclusions are formed in large quantities, which reduce the ultimate deformability of the steel.

Therefore, when Ta is added, the Ta content is 0.100 % or less, and preferably 0.050 % or less.

[0046] Ca: 0.0200 % or less

Ca is an element used for deoxidization, and is also effective in spheroidizing the shape of sulfides and increasing the ultimate deformability of the steel sheet to improve the delayed fracture resistance. To obtain this effect, the Ca content is preferably 0.0001 % or more. On the other hand, if the Ca content exceeds 0.0200 %, coarse precipitates and inclusions are formed in large quantities, which reduce the ultimate deformability of the steel. Therefore, when Ca is added, the Ca content is 0.0200 % or less.

[0047] Mg: 0.0200 % or less

Mg is an element used for deoxidization, and is also effective in spheroidizing the shape of sulfides and increasing the ultimate deformability of the steel sheet to improve the delayed fracture resistance. To obtain this effect, the Mg content is preferably 0.0001 % or more. On the other hand, if the Mg content exceeds 0.0200 %, coarse precipitates and inclusions are formed in large quantities, which reduce the ultimate deformability of the steel. Therefore, when Mg is added, the Mg content is 0.0200 % or less.

[0048] Zn: 0.0200 % or less

Zn is an element effective in spheroidizing the shape of inclusions and increasing the ultimate deformability of the steel sheet to improve the delayed fracture resistance. To obtain this effect, the Zn content is preferably 0.0010 % or more. On the other hand, if the Zn content exceeds 0.0200 %, coarse precipitates and inclusions are formed in large quantities, which reduce the ultimate deformability of the steel. Therefore, when Zn is added, the Zn content is 0.0200 % or less.

[0049] Co: 0.0200 % or less

Co is an element effective in spheroidizing the shape of inclusions and increasing the ultimate deformability of the steel sheet to improve the delayed fracture resistance. To obtain this effect, the Co content is preferably 0.0010 % or more. On the other hand, if the Co content exceeds 0.0200 %, coarse precipitates and inclusions are formed in large quantities, which reduce the ultimate deformability of the steel. Therefore, when Co is added, the Co content is 0.0200 % or less.

[0050] Zr: 0.0200 % or less

Zr is an element effective in spheroidizing the shape of inclusions and increasing the ultimate deformability of the steel sheet to improve the delayed fracture resistance. To obtain this effect, the Zr content is preferably 0.0010 % or more. On the other hand, if the Zr content exceeds 0.0200 %, coarse precipitates and inclusions are formed in large quantities, which reduce the ultimate deformability of the steel. Therefore, when Zr is added, the Zr content is 0.0200 % or less.

[0051] REM: 0.0200 % or less

Rare earth metals (REM) are a group of elements effective in spheroidizing the shape of inclusions and increasing the ultimate deformability of the steel sheet to improve the delayed fracture resistance. To obtain this effect, the REM content is preferably 0.0010 % or more. On the other hand, if the REM content exceeds 0.0200 %, coarse precipitates and inclusions are formed in large quantities, which reduce the ultimate deformability of the steel. Therefore, when REM is added, the REM content is 0.0200 % or less. [0052] In the chemical composition of the steel sheet, the balance other than the above elements is Fe and inevitable impurities. For the above optional elements, if the content is less than the corresponding preferred lower limits, such optional elements may be included as inevitable impurities, as they do not impair the effect of the present disclosure.

[0053] Next, the steel microstructure of the steel sheet according to this embodiment will be described. The steel microstructure comprises 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 optionally ferrite.

[0054] Tempered martensite: 90 % or more in volume fraction

The fact that the microstructure has tempered martensite as the main phase aids in achieving a TS of 1470 MPa or more. From the viewpoint of increasing TS, the content of tempered martensite needs to be 90 % or more, and is preferably 92 % or more, and more preferably 94 % or more.

[0055] In this case, the volume fraction of tempered martensite is measured as follows. Specifically, an L-cross section of the steel sheet is polished, the L-cross section is etched with 3 vol.% nital. Then, the L-cross section is observed at ten locations under an SEM (x2000 magnification) at a 1/4 thickness position (i.e., at a depth of one-fourth of the sheet thickness from the steel sheet surface) to obtain microstructural images. In these microstructural images, tempered martensite is a microstructure with fine irregularities and carbides in the interior. The area ratio of tempered martensite is determined in the ten locations and the results are averaged to give an average value. Since the area ratio of tempered martensite is almost constant in the direction perpendicular to the L-cross section, this average value is regarded as the "volume fraction of tempered martensite".

[0056] Retained austenite: 1 % or more and 7 % or less in volume fraction

If the content of retained austenite exceeds 7 %, delayed fracture resistance decreases. The reason for the decrease in delayed fracture resistance caused by retained austenite is that retained austenite transforms to deformation-induced martensite due to machining, resulting in a harder microstructure than the main phase, tempered martensite. Therefore, the content of retained austenite is 7 % or less, and preferably 6 % or less. On the other hand, since the content of retained austenite depends on the cooling stop temperature T2, it is difficult to control the cooling stop temperature T2 to be lower than 130 °C under production constraints. Therefore, the content of retained austenite is 1 % or more, and preferably 2 % or more.

[0057] In this case, the volume fraction of retained austenite is measured as follows. The steel sheet is polished from its surface to expose a surface at the 1/4 thickness position. In the first step of such polishing, mechanical polishing is performed from the steel sheet surface to a surface closer to the steel sheet surface by 0.1 mm from the 1/4 thickness position. Then, in a 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 thickness position. For the exposed surface at the 1/4 thickness position, measurement is made to determine the integrated intensities of the diffraction peaks of the {200}, {220}, and {311 } planes of fee iron and the {200}, {211 }, and {220} planes of bcc iron, using CoKα radiation in an X-ray diffractometer. The integral intensity ratio (fee / (fee + bee)) is determined for all (nine in total) combinations of the three planes of fee iron and the three planes of bcc iron. The result of averaging the nine integral intensity ratios obtained is used as the "volume fraction of retained austenite".

[0058] One or both of bainitic ferrite and fresh martensite: a total of 3 % or more and 9 % or less in volume fraction

If the total content of bainitic ferrite and fresh martensite exceeds 9 %, delayed fracture resistance decreases. The reason for the decrease in delayed fracture resistance caused by bainitic ferrite and fresh martensite is that both microstructures differ in hardness from the main phase, tempered martensite. Therefore, the total content is 9 % or less, preferably 8 % or less, and more preferably 5 % or less. On the other hand, under production constraints, the total content is 3 % or more.

[0059] In this case, the total volume fraction of bainitic ferrite and fresh martensite is measured as follows. Specifically, an L-cross section of the steel sheet is polished, the L-cross section is etched with 3 vol.% nital. Then, the L-cross section is observed at ten locations under an SEM (x2000 magnification) at a 1/4 thickness position (i.e., at a depth of one-fourth of the sheet thickness from the steel sheet surface) to obtain microstructural images. In these microstructural images, bainitic ferrite, fresh martensite, and retained austenite are microstructures with fine irregularities and without carbides in the interior. The total area ratio of these three phases is determined in the ten locations and the results are averaged to give an average value. Since the total area ratio of the three phases is almost constant in the direction perpendicular to the L-cross section, this average value is regarded as the "total volume fraction" of the three phases. The total volume fraction of bainitic ferrite and fresh martensite can be obtained by subtracting the volume fraction of retained austenite measured as described above from the total volume fraction of the three phases.

[0060] Ferrite: 0 % or more and 5 % or less in volume fraction

Since ferrite is a soft microstructure, if the content of ferrite exceeds 5 %, it is difficult to achieve a TS of 1470 MPa or more.

Therefore, the ferrite content is 5 % or less, preferably 3 % or less, and more preferably 2 % or less.

[0061] In this case, the volume fraction of ferrite is measured as follows. Specifically, an L-cross section of the steel sheet is polished, the L- cross section is etched with 3 vol.% nital. Then, the L-cross section is observed at ten locations under an SEM (x2000 magnification) at a 1/4 thickness position (i.e., at a depth of one- fourth of the sheet thickness from the steel sheet surface) to obtain microstructural images. In these microstructural images, ferrite is a microstructure with a flat concave interior. The area ratio of ferrite is determined in the ten locations and the results are averaged to give an average value. Since the area ratio of ferrite is almost constant in the direction perpendicular to the L-cross section, this average value is regarded as the "volume fraction of ferrite".

[0062] Carbon concentration in retained austenite: 0.35 % or more

If the carbon concentration in retained austenite is less than 0.35 %, the main factor 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 retained austenite is 0.35 % or more, and preferably 0.40 % or more. On the other hand, under production constraints, the carbon concentration in retained austenite is preferably 1.00 % or less.

[0063] In this case, the carbon concentration in retained austenite is measured as follows, using CoKa radiation in an X-ray diffractometer. First, a lattice constant a of retained austenite was calculated from the amount of diffraction peak shift of the (220) plane of austenite using the following Expression (1) below, and the carbon concentration in retained austenite was calculated by substituting the obtained lattice constant a of retained austenite into the following Expression (2): a = 1.79021 √2/sinθ (1), and 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), where a represents a lattice constant (A) of retained austenite,

0 represents a value (rad) obtained by dividing the diffraction peak angle of the (220) plane by 2, and

[M] represents the content (mass%) of element M in retained austenite.

That is, [C] in Expression (2) represents the carbon concentration in retained austenite. In this disclosure, however, the content (mass%) of each element M other than C in retained austenite is defined as the content (mass%) in the entire steel.

[0064] Tensile strength TS: 1470 MPa or more and 1650 MPa or less

The steel sheet according to this embodiment has a tensile strength TS of 1470 MPa or more and 1650 MPa or less.

[0065] Yield strength YS: 1100 MPa or more

The steel sheet according to this embodiment has a YS of 1100 MPa or more, preferably 1150 MPa or more, and more preferably 1200 MPa or more. The steel sheet according to this embodiment preferably has a YS of 1470 MPa or less.

[0066] Yield ratio YR: 0.75 or more (preferred condition)

The steel sheet according to this embodiment has a YR of preferably 0.75 or more, and more preferably 0.80 or more. The steel sheet according to this embodiment has a YR of preferably 1.0 or less. YR is given by: YR = YS / TS. [0067] A method of producing a steel sheet (a high-strength steel sheet) according to one of the embodiments disclosed herein comprises: preparing an uncoated steel sheet having the above-described chemical composition; and annealing the steel sheet under predetermined conditions. The annealing specifically includes: heating the steel sheet to a predetermined heating temperature T1; then holding the steel sheet at T1 for a predetermined period of time tl; continuously cooling the steel sheet from T1 to a predetermined cooling stop temperature T2; then holding the steel sheet at T2 for a predetermined period of time t2; then heating the steel sheet to a predetermined tempering temperature T3; then holding the steel sheet at T3 for a predetermined period of time t3; and then cooling the steel sheet to 50°C or lower. By this method, a steel sheet with the above-described chemical composition, microstructure, and mechanical properties can be produced in a suitable manner.

[0068] In this embodiment, the uncoated steel sheet to be subjected to the annealing is preferably a cold-rolled steel sheet. The following provides a description of a suitable production process for a cold- rolled steel sheet.

[0069] First, a steel slab with the above-described chemical composition is produced. The production method for the steel slab is not particularly limited, it is possible to adopt a known smelting method using a converter, an electric furnace, or the like. The steel slab is preferably produced using a continuous casting method to prevent macro-segregation. [0070] Then, the steel slab is hot rolled to obtain a hot-rolled steel sheet.

Examples of the method of hot rolling the steel slab include a method in which a steel slab is rolled after being heated, a method in which a steel slab is subjected to direct rolling without being heated after continuous casting, and a method in which a steel slab is rolled after being heated for a short period of time after continuous casting. In the hot rolling, the slab heating temperature, the slab soaking duration, the rolling finish temperature, and the coiling temperature are not particularly limited. However, the slab heating temperature is preferably 1100°C or higher. The slab heating temperature is preferably 1300°C or lower. The slab soaking duration is preferably 30 minutes or more. The slab soaking duration is preferably 250 minutes or less. The rolling finish temperature is preferably at or above Ar ₃ transformation temperature. The coiling temperature is preferably 350°C or higher. The coiling temperature is preferably 650°C or lower.

[0071] Then, the hot-rolled steel sheet is subjected to pickling. The pickling removes oxides from the steel sheet surface, which contributes to ensuring good phosphatability and painting quality in the final high-strength steel sheet. The pickling may be performed in one or more batches.

[0072] Then, the hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet. The cold rolling may be carried out directly after the pickling, or heat treatment may be performed after the pickling before the cold rolling. The rolling reduction ratio in the cold rolling is not particularly limited. However, it is preferably 30 % or more. It is preferably 80 % or less. The effect of the disclosure can be obtained without limiting the number of rolling passes or the rolling reduction ratio for each pass. The thickness of the cold-rolled steel sheet is not particularly limited. However, it is preferably 0.6 mm or more. It is preferably 2.0 mm or less.

[0073] Heating temperature T1 : 850°C or higher

If the heating temperature T1 is lower than 850°C , the annealing is performed in a temperature range of a ferrite and austenite dual phase region, which results in a ferrite content exceeding 5 % after the annealing, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the heating temperature T1 is 850°C or higher (i.e., in an austenitizing temperature range), and preferably 860°C or higher. The upper limit of the heating temperature T1 is not particularly limited. However, under production constraints, T1 is preferably 1000°C or lower.

[0074] 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 and the ferrite content will exceed 5 % after the annealing, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the holding time t1 is 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 decreases. Therefore, the holding time t1 is 1000 seconds or less, preferably 500 seconds or less, and more preferably 400 seconds or less.

[0075] Average cooling rate θ1 in a temperature range from the heating temperature T1 to 550°C : 16°C/s or higher

If the average cooling rate θ1 is lower than 16°C/s, bainitic transformation occurs in the temperature range from the heating temperature T1 to 550°C , and the total content of bainitic ferrite and fresh martensite becomes 9 % or more, resulting in lower delayed fracture resistance. Therefore, the average cooling rate θ1 is 16°C/s or higher, and preferably 20°C /s or higher. The upper limit of the average cooling rate θ1 is not particularly limited.

However, under production constraints, the average cooling rate θ1 is preferably 300°C/s or lower.

[0076] Average cooling rate 02 in a temperature range from 550°C to a cooling stop temperature T2: 150°C/s or lower

If the average cooling rate 02 exceeds 150°C/s, carbon distribution from martensite to retained austenite is inhibited during cooling, and the carbon concentration in retained austenite becomes less than 0.35 %. As a result, the main factor 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 02 is 150°C/s or lower, preferably 120°C/s or lower, and more preferably 100°C/s or lower. The lower limit of the average cooling rate 02 is not particularly limited. However, under production constraints, the average cooling rate 02 is preferably 5 °C/s or higher.

[0077] Continuous Cooling from the Heating Temperature T1 to the Cooling Stop Temperature T2

In one embodiment, the steel sheet is continuously cooled from the heating temperature T1 to the cooling stop temperature T2, i.e., the temperature of the steel sheet is gradually decreased, which achieves a YS of 1100 MPa or more. For example, if the steel sheet is held isothermally for 1 second or more in the temperature range from the heating temperature T1 to the cooling stop temperature T2, it is difficult to achieve a YS of 1100 MPa or more. Therefore, the steel sheet shall not be held isothermally for 1 second or more in the temperature range from the heating temperature T1 to the cooling stop temperature T2. Also, reheating of the steel sheet shall not be performed in the temperature range from the heating temperature T1 to the cooling stop temperature T2.

[0078] Cooling stop temperature T2: 130 °C or higher and 170 °C or lower

If the cooling stop temperature T2 exceeds 170 °C, the content of retained austenite exceeds 7 %, resulting in lower delayed fracture resistance. Therefore, the cooling stop temperature T2 is 170 °C or lower, and preferably 160 °C or lower. On the other hand, under production constraints, the cooling stop temperature T2 is 130 °C or higher, and preferably 140 °C or higher. [0079] Holding Time t2 at the Cooling Stop Temperature T2: 1.0 seconds or more and 200.0 seconds or less

If the holding time t2 is shorter than 1.0 seconds, the martensitic transformation is insufficient and the content of retained austenite exceeds 7 %, resulting in lower delayed fracture resistance. Therefore, the holding time t2 is 1.0 seconds or more, and preferably 5.0 seconds or more. On the other hand, if the holding time t2 exceeds 200.0 seconds, the precipitation of carbides increases, and the carbon concentration in the retained austenite becomes less than 0.35 %. As a result, the main factor 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 holding time t2 is 200.0 seconds or less, and preferably 150.0 seconds or less.

[0080] Average heating rate 03 from the cooling stop temperature T2 to the tempering temperature T3: 10 °C/s or higher

If the average heating rate 03 is lower than 10 °C/s, the precipitation of carbides increases, and the carbon concentration in the retained austenite becomes less than 0.35 %. As a result, the main factor 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. Further, if the average heating rate 03 is lower than 10 °C/s, bainitic transformation occurs, and the total content of bainitic ferrite and fresh martensite becomes 9 % or more, resulting in lower delayed fracture resistance. Therefore, the average heating rate 03 is 10 °C/s or higher, and preferably 15 °C/s or higher. The upper limit of the average heating rate 63 is not particularly limited. However, under production constraints, the average cooling rate 03 is preferably 200 °C/s or lower.

[0081] Tempering temperature T3 : 280 °C or higher and 350 °C or lower

If a tempering temperature T3 exceeds 350 °C, tempering of martensite progresses excessively, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the tempering temperature T3 is 350 °C or lower, and preferably 340 °C or lower. On the other hand, if the tempering temperature T3 is lower than 280 °C, the carbon distribution from martensite to austenite is insufficient, and the carbon concentration in retained austenite becomes less than 0.35 %. As a result, the main factor 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 tempering temperature T3 is 280 °C or higher, and preferably 290 °C or higher.

[0082] Holding time t3 at the tempering temperature T3: 10 seconds or more and 1000 seconds or less

If the holding time t3 is shorter than 10 seconds, the bainite transformation does not progress at the tempering temperature T3, and the content of retained austenite exceeds 7 %, resulting in lower delayed fracture resistance. Therefore, the holding time t3 is 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, tempering of martensite progresses excessively, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the holding time t3 is 1000 seconds or less, preferably 800 seconds or less, and more preferably 600 seconds or less.

[0083] Cooling of the steel sheet to 50 °C or lower

After being held at the tempering temperature T3, the steel sheet is cooled to 50 °C or lower, and preferably to around room temperature. The method and conditions of this cooling are not limited.

[0084] In some embodiments, the steel sheet is then subjected to temper rolling with an elongation rate of 0.1 % or more. This causes retained austenite with a low carbon concentration to transform to deformation-induced martensite, resulting in an increased carbon concentration in retained austenite and an improved YS.

Therefore, when temper rolling is performed, the elongation rate is preferably 0.1 % or more. Although the upper limit of the elongation rate is not limited, an excessively high elongation rate does not increase the YS improving effect. In addition, from the viewpoint of restrictions on production lines, the elongation rate is preferably 1.0 % or less.

[0085] Steel samples having the chemical compositions as listed in

Table 1, with the balance being Fe and inevitable impurities, were prepared by steelmaking in a converter, and formed into slabs through continuous casting.

[0087] Then, each resulting slab was heated and hot rolled, then subjected to pickling treatment, and cold rolled to obtain a cold-rolled steel sheet. Then, each cold-rolled steel sheet was subjected to annealing under the conditions listed in Table 2, and then cooled to room temperature to obtain a high-strength steel sheet. In some of the comparative examples, the steel sheets were held at respective intermediate holding temperatures listed in Table 2 in the range of the heating temperature T1 to the cooling stop temperature T2, for respective intermediate holding times listed in Table 2. In some of our examples, the steel sheets were cooled to room temperature, and then subjected to temper rolling with respective elongation rates listed in the "SKP" column in Table 2.

[0089] For each of the high-strength steel sheets thus obtained, 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 retained austenite were determined with the above-described method. The results are listed in Table 3.

[0091] In addition, each of the high-strength steel sheets thus obtained was subjected to tensile test and evaluated for delayed fracture resistance as described below.

[0092] JIS No. 5 test pieces (marking distance: 50 mm, parallel portion width: 25 mm) were taken from each obtained high-strength steel sheet such that the direction perpendicular to the rolling direction was parallel to the longitudinal axis of the test piece, and tensile tests were conducted according to JIS Z 2241. The tensile tests were conducted at a crosshead speed of 1.67 x 10 ^-1 mm/s to measure YS and TS. In this disclosure, TS of 1470 MPa or more and 1650 MPa or less were judged as passed. YS of less than 1100 MPa were evaluated as "poor", YS of 1100 MPa or more and less than 1200 MPa as "good", and YS of 1200 MPa or more as "excellent". In this disclosure, those test pieces with a YS of not less than 1100 MPa were judged as passed. Yield ratio YR was also calculated from YS and TS, and the results are listed in Table 3.

[0093] Delayed fracture resistance was evaluated by immersion tests. Test pieces were prepared by shearing each high-strength steel sheet into 30 m x 110 mm pieces with the direction perpendicular to the rolling direction parallel to the longitudinal direction, and making holes for bolts. The rake angle during shear was unified as 0°, and the shear clearance was varied to 5 %, 10 %, 15 %, 20 %, 25 %, 30 %, and 35 %. After each test piece was bent using a 90° V-bend punch and die with a tip radius of curvature of 10 mm, stress of 1000 MPa was applied to the bend apex of the test piece using bolts. Each stressed test piece was immersed in hydrochloric acid at 25 °C and pH 3 for 100 hours. Those test pieces with a shear clearance range without cracking of less than 10 % were evaluated as "poor," those with a shear clearance range without cracking of 10 % or more and less than 15 % as "good," and those with a shear clearance range without cracking of 15 % or more as "excellent". Those test pieces with a shear clearance range without cracking of not less than 10 % were judged to have excellent delayed fracture resistance.

[0094] As can be seen from Table 3, our examples each 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. In contrast, in the comparative examples, one or more of TS, YS, and delayed fracture resistance are inferior.

[0095] The high-strength steel sheet disclosed herein is suitably usable as a structural member for automotive parts, etc., and contributes to improved fuel efficiency because of the reduction in the weight of automotive bodies.