Steel plate and its manufacturing method

Abstract

This steel sheet has a prescribed chemical composition and a tensile strength of at least 780 MPa and exhibits a ferrite area ratio of at least 5%, a ferrite + bainite total area ratio of 10-90%, a martensite + tempered martensite total area ratio of 10-90%, and a pearlite + retained austenite total area ratio of 0-10%. With reference to the total number of the ferrite and bainite crystal grains, the percentage for the number of ferrite and bainite crystal grains having an area of not more than 6 µm2 is at least 40% for this steel sheet and the percentage for the number of ferrite and bainite crystal grains having an area of at least 50 µm2 is not more than 5%. The maximum Mn content in the region from the boundary between the ferrite and martensite or tempered martensite to 0.5 µm toward the inside of the ferrite grain in the direction perpendicular to this boundary is at least 0.30 mass% less than the average Mn content in the steel sheet.

Description

[Technical Field]

[0001] This invention relates to steel plates and methods for manufacturing them. This application claims priority based on Japanese Patent Application No. 2021-122923, filed in Japan on July 28, 2021, and the contents of that application are incorporated herein by reference. [Background technology]

[0002] In recent years, automobile manufacturers have been actively developing technologies to lighten vehicle bodies in order to improve fuel efficiency. Lightening the weight of steel materials used, such as by reducing the thickness of steel plates, can easily lighten a vehicle. However, in the case of automobiles, emphasis is also placed on improving collision resistance to ensure occupant safety. Therefore, simply reducing the weight of steel materials is not a viable option, and vehicle weight reduction is not easy. To achieve both vehicle weight reduction and collision resistance, thinning components using high-strength steel plates is being considered. On the other hand, steel plates used in automobile parts are formed into the shape of the part, but as the strength of the steel plate increases, its formability usually deteriorates. Therefore, there is a strong demand for steel plates used in automobile parts that possess both high strength and excellent formability.

[0003] As such a high-strength steel sheet, for example, Patent Document 1 describes a steel sheet with excellent elongation, hole-expanding properties, bendability, and delayed fracture resistance, satisfying the following mass% requirements: C: 0.15~0.25%, Si: 1.00~2.20%, Mn: 2.00~3.50%, P: 0.05% or less, S: 0.005% or less, Al: 0.01~0.50%, N: 0.010% or less, B: 0.0003~0.0050%, and also containing one or two selected from Ti: 0.005~0.05%, Cu: 0.003~0.50%, Ni: 0.003~0.50%, Sn: 0.003~0.50%, Co: 0.003~0.05%, and Mo: 0.003~0.50%. A high-strength TRIP steel sheet is disclosed, characterized in that it contains more than one species, with the remainder being Fe and unavoidable impurities, and the microstructure consists of ferrite with an average grain size of 2 μm or less in volume fraction of 15% or less (including 0%), retained austenite with an average grain size of 2 μm or less in volume fraction of 2 to 15%, martensite with an average grain size of 3 μm or less in volume fraction of 10% or less (including 0%), and the remainder being bainite and tempered martensite with an average grain size of 6 μm or less, and on average contains 10 or more cementite particles with a particle size of 0.04 μm or more within the bainite and tempered martensite grains.

[0004] Patent Document 2 discloses a high-strength cold-rolled steel sheet that combines high strength (tensile strength (TS): 980 MPa or more) and excellent bendability, having a specific component composition and a specific steel structure in which the area ratio of the ferrite phase is 30% to 70%, the area ratio of the martensite phase is 30% to 70%, the average grain size of the ferrite grains is 3.5 μm or less, the standard deviation of the grain size of the ferrite grains is 1.5 μm or less, the average aspect ratio of the ferrite grains is 1.8 or less, the average grain size of the martensite grains is 3.0 μm or less, and the average aspect ratio of the martensite grains is 2.5 or less, and the tensile strength is 980 MPa or more.

[0005] Patent Document 3 discloses a high-strength steel sheet with a yield strength (YS) of 780 MPa or more, a tensile strength (TS) of 1180 MPa or more, excellent spot weldability, ductility, and bending workability. The C content is 0.15% or less, the ferrite area ratio is 8 - 45%, the martensite area ratio is 55 - 85%, and the ratio of martensite adjacent only to ferrite in the entire structure is 15% or less. The average crystal grain size of ferrite and martensite is 10 μm or less, and the area ratio of ferrite with a crystal grain size of 10 μm or more among the ferrite present in the range from a depth of 20 μm from the steel sheet surface to a depth of 100 μm from the steel sheet surface is less than 5%.

[0006] Patent Document 4 discloses a steel sheet with little variation in mechanical properties (especially strength and ductility). In mass%, it contains C: 0.10 - 0.25%, Si: 0.5 - 2.0%, Mn: 1.0 - 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 - 0.05%, N: 0.01% or less, respectively, and the balance consists of iron and inevitable impurities. It has a constituent composition. It contains 20 - 50% of ferrite, which is a soft first phase, by area ratio, and the balance consists of tempered martensite and / or tempered bainite, which are hard second phases. Among all the particles of the ferrite, the total area of particles with an average particle size of 10 - 25 μm occupies 80% or more of the total area of all the particles of the ferrite. And the dispersion state of cementite particles with a circle equivalent diameter of 0.3 μm or more present in all the particles of the ferrite is such that it is more than 0.15 and 1.0 or less per 1 μm of the ferrite, and the tensile strength is 980 MPa or more. A high-strength cold-rolled steel sheet is disclosed. 2

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

[0008] By the way, steel sheets used in automobile parts and the like are often formed into parts by methods such as pressing and punching. In recent years, press forming conditions have become more stringent, and strains exceeding uniform elongation are often introduced into the formed parts. While Patent Documents 1-4 mention increasing strength and combining good ductility and bendability, they do not mention suppressing crack occurrence when press forming is performed under such stringent conditions, leaving room for improvement. Therefore, the object of the present invention is to provide a steel sheet and a method for manufacturing the same that can suppress the occurrence of cracks during press forming (excellent fracture resistance) even when a strain exceeding uniform elongation is introduced. [Means for solving the problem]

[0009] The inventors investigated a method to suppress crack formation (obtain excellent fracture resistance) when introducing a strain exceeding uniform elongation into a steel sheet containing ferrite and / or bainite and martensite and / or tempered martensite. As a result, they found that even with a strain exceeding uniform elongation, crack formation during press forming can be suppressed if a high true stress state can be maintained.

[0010] This invention was made in view of the above-mentioned problems. The gist of this invention is as follows. [1] A steel sheet according to one aspect of the present invention has the following composition by mass%, C: 0.07~0.15%, Si: 0.01~2.00%, Mn: 1.5~3.0%, P: 0~0.020%, S: 0~0.0200%, Al: 0.001~1.000%, N: 0~0.0200%, O: 0~0.0200%, Co: 0~0.500%, Ni :0~1.000%, Cu:0~0.500%, Mo:0~1.000%, Cr:0~2.000%, Ti:0~0.5000%, Nb:0~0.50% , V:0~0.500%, W:0~0.100%, Ta:0~0.100%, B:0~0.0100%, Mg:0~0.050%, Ca:0~0.050% The chemical composition consists of Zr: 0-0.050%, REM: 0-0.100%, Sn: 0-0.050%, Sb: 0-0.050%, As: 0-0.050%, and the remainder being Fe and impurities; the tensile strength is 780 MPa or higher; in the microstructure, the area ratio of ferrite is 5% or more, the combined area ratio of ferrite and bainite is 10% or more and 90% or less, the combined area ratio of martensite and tempered martensite is 10% or more and 90% or less, and the combined area ratio of pearlite and retained austenite is 0% or more and 10% or less; and the area is 6 μm² relative to the total number of crystal grains of ferrite and bainite. 2 The following conditions must be met: the number of ferrite and bainite grains must be 40% or more, and the area must be 50 μm². 2 The above conditions are met, the number ratio of ferrite and bainite crystal grains is 5% or less, and the maximum Mn content in the region from the interface between the ferrite and the martensite or tempered martensite to 0.5 μm perpendicular to the interface and toward the inside of the ferrite grains is 0.30 mass% or more lower than the average Mn content of the steel sheet. In the steel plate described in [2][1], the area is 6 μm 2 The average aspect ratio of the crystal grains of the ferrite and bainite may be 1.0 or more and 2.0 or less. The steel sheet described in [3][1] or [2] may have a coating layer on its surface containing zinc, aluminum, magnesium, or an alloy thereof. [4] A method for manufacturing a steel sheet according to another aspect of the present invention is:A method for manufacturing steel plates as described in [1] or [2], In mass%, C: 0.07~0.15%, Si: 0.01~2.00%, Mn: 1.5~3.0%, P: 0~0.020%, S: 0~0.0200%, Al: 0.001~1.000%, N: 0~0.0200%, O: 0~0.02 00%, Co:0~0.500%, Ni:0~1.000%, Cu:0~0.500%, Mo:0~1.000%, Cr:0~2.000%, Ti:0~0.5000%, Nb:0~0.50%, V:0~0.500%, W:0~0 A hot rolling process to obtain a hot-rolled steel sheet by hot rolling a slab having a chemical composition consisting of 0.100%, Ta:0~0.100%, B:0~0.0100%, Mg:0~0.050%, Ca:0~0.050%, Zr:0~0.050%, REM:0~0.100%, Sn:0~0.050%, Sb:0~0.050%, As:0~0.050%, and the remainder: Fe and impurities; and a winding process to obtain a hot-rolled steel sheet by hot rolling the hot-rolled steel sheet at an average cooling rate of 30°C / second or more and a temperature of 650°C or less and 450°C or more. A winding step in which the hot-rolled steel sheet is cooled to a temperature and wound at the winding temperature; a holding step in which the hot-rolled steel sheet after the winding step is held for 2 to 8 hours in a temperature range from the winding temperature to the winding temperature - 50°C; a cooling step in which the hot-rolled steel sheet after the holding step is cooled to a temperature of 300°C or less at an average cooling rate of 0.1°C / second or more; a cold rolling step in which the hot-rolled steel sheet after the cooling step is cold-rolled with a thickness reduction rate of 20 to 80% to obtain a cold-rolled steel sheet; and the cold-rolled steel The hot rolling process includes an annealing step in which the plate is heated to an annealing temperature of 740 to 900°C at an average heating rate of 5°C / second or more, and held at the annealing temperature for 60 to 300 seconds, wherein the hot rolling process is performed using a rolling mill having four or more stands, with the first stand being the first stand and the final stand being the nth stand, the plate thickness reduction rate at each stand from the nth-3rd stand to the nth stand being 30% or more, and the rolling temperature at the nth stand being 900°C or less. In the steel sheet manufacturing method described in [5][4], a coating layer containing zinc, aluminum, magnesium, or an alloy thereof may be formed on the surface of the steel sheet during the annealing step. [Effects of the Invention]

[0011] According to the above aspects of the present invention, it is possible to provide a steel plate with excellent fracture resistance and a method for manufacturing the same. [Brief explanation of the drawing]

[0012] [Figure 1] This is a diagram explaining how to find Δσ. [Modes for carrying out the invention]

[0013] The following describes a steel sheet (a steel sheet according to this embodiment) and a method for manufacturing the same according to one embodiment of the present invention.

[0014] <Steel plate> The steel sheet according to this embodiment has a predetermined chemical composition described later, a tensile strength of 780 MPa or more, and in its microstructure, the area ratio of ferrite is 5% or more, the combined area ratio of ferrite and bainite is 10% or more and 90% or less, the combined area ratio of martensite and tempered martensite is 10% or more and 90% or less, and the combined area ratio of pearlite and retained austenite is 0% or more and 10% or less, and the area is 6 μm² relative to the total number of crystal grains of ferrite and bainite. 2 The following conditions apply: the number of ferrite and bainite grains is 40% or more, and the area is 50 μm². 2 The number ratio of ferrite and bainite crystal grains is 5% or less, and the maximum Mn content in the region from the interface between the ferrite and the martensite or tempered martensite to 0.5 μm perpendicular to the interface and toward the inside of the ferrite grains is 0.30 mass% or more lower than the average Mn content of the steel sheet.

[0015] [Chemical composition] This section explains the content of each element constituting the chemical composition and the reasons for its limitation. In this embodiment, the percentage (%) for each element's content refers to mass percentage. Furthermore, numbers enclosed by a "~" indicate that the values ​​at both ends of the range are included as the lower or upper limit. For example, 0.07~0.15% indicates that the content is between 0.07% and 0.15%.

[0016] C: 0.07~0.15% Carbon (C) is an element necessary to secure a predetermined amount of martensite and improve the strength of the steel sheet. If the C content is less than 0.07%, it is difficult to obtain the predetermined amount of martensite, and a tensile strength of 780 MPa or higher cannot be secured. Therefore, the C content should be 0.07% or higher. Preferably, the C content is 0.09% or higher. On the other hand, if the carbon content exceeds 0.15%, ferrite formation is suppressed, leading to a decrease in elongation and deterioration of the ductility of the punched-out end face. Therefore, the carbon content should be 0.15% or less. Preferably, the carbon content is 0.13% or less.

[0017] Si: 0.01~2.00% Si acts as a solid solution strengthening element, increasing the strength of steel sheets. Furthermore, it is an effective element for obtaining structures containing martensite, bainite, and residual gamma. To achieve this effect, the Si content should be 0.01% or higher. A Si content of 0.10% or higher is also acceptable. On the other hand, if the Si content exceeds 2.00%, press formability deteriorates, chemical conversion treatment performance decreases, and the ductility of the punched end face deteriorates. For this reason, the Si content should be 2.00% or less. When applying hot-dip galvanizing, problems such as reduced plating adhesion and decreased productivity due to delayed alloying reaction occur, so it is preferable to keep the Si content at 1.20% or less.

[0018] Mn: 1.5~3.0% Mn is an element that contributes to improving the strength of steel sheets. Furthermore, Mn has the effect of suppressing ferrite transformation that occurs during heat treatment in continuous annealing equipment or continuous hot-dip galvanizing equipment. If the Mn content is less than 1.5%, these effects are not fully realized, and ferrite exceeding the required area ratio is generated, making it impossible to obtain a tensile strength of 780 MPa or higher. Therefore, the Mn content should be 1.5% or higher. Preferably, the Mn content is 1.7% or higher, and more preferably 1.9% or higher. On the other hand, if the Mn content exceeds 3.0%, the ferrite transformation is excessively suppressed, making it impossible to secure the required amount of ferrite, and resulting in reduced elongation. Therefore, the Mn content should be 3.0% or less. Preferably, the Mn content is 2.7% or less.

[0019] P: 0~0.020% P is an impurity element that segregates in the center of the steel sheet thickness, reducing its toughness. Furthermore, when steel sheets are welded, P embrittles the welded joint. If the P content exceeds 0.020%, the weld strength, hole expansion properties, and ductility of the punched end face are significantly reduced. Therefore, the P content should be 0.020% or less. Preferably, the P content is 0.010% or less. A low P content is preferable, and it may even be 0%. However, reducing the P content to less than 0.0001% in practical steel sheets significantly increases manufacturing costs, making it economically unfavorable. Therefore, the P content may be 0.0001% or more.

[0020] S: 0~0.0200% S is an impurity element that reduces weldability and manufacturability during casting and hot rolling. S also forms coarse MnS, reducing hole-expandability. When the S content exceeds 0.0200%, the decrease in weldability, hole-expandability, and ductility of punched edges becomes significant. Therefore, the S content should be 0.0200% or less. Preferably, the S content is 0.0050% or less. A lower S content is preferable, and it may even be 0%, but reducing the S content to less than 0.0001% in practical steel sheets significantly increases manufacturing costs, making it economically disadvantageous. Therefore, the S content may be 0.0001% or more.

[0021] Al: 0.001~1.000% Al acts as a deoxidizing agent for steel and is also an element that stabilizes ferrite. To obtain these effects, the Al content should be 0.001% or higher. On the other hand, if the Al content exceeds 1.000%, coarse Al oxides are formed, reducing ductility. Therefore, the Al content should be 1.000% or less. Preferably, the Al content is 0.500% or less.

[0022] N: 0~0.0200% Nitrogen (N) is an element that forms coarse nitrides, reducing bendability and hole-expandability. Furthermore, N is a cause of blowhole formation during welding. When the N content exceeds 0.0200%, coarse nitrides are formed, leading to a significant decrease in formability and increased blowhole formation. Therefore, the N content should be 0.0200% or less. A lower N content is preferable, and it may even be 0%, but reducing the N content to less than 0.0005% in practical steel sheets significantly increases manufacturing costs, making it economically undesirable. Therefore, the N content may be 0.0005% or more.

[0023] O: 0~0.0200% O is an element that forms coarse oxides, degrading formability and fracture resistance. Furthermore, O is a cause of blowhole formation during welding. When the O content exceeds 0.0200%, the presence of coarse oxides significantly degrades formability and ductility of punched edges, and blowhole formation becomes pronounced. Therefore, the O content should be 0.0200% or less. A lower O content is preferable, and it may even be 0%, but reducing the O content to less than 0.0001% in practical steel sheets significantly increases manufacturing costs, making it economically undesirable. Therefore, the O content may be 0.0001% or more.

[0024] Co: 0~0.500% Co is an effective element for increasing the strength of steel plates. While the Co content may be 0%, to obtain the above effects, the Co content is preferably 0.001% or more, and more preferably 0.010% or more. On the other hand, if the Co content is too high, the ductility of the steel sheet may decrease, potentially reducing its formability. For this reason, the Co content is kept below 0.500%.

[0025] Ni: 0~1.000% Ni, like Co, is an effective element for increasing the strength of steel sheets. While the Ni content can be 0%, to obtain the above effects, the Ni content is preferably 0.001% or more, and more preferably 0.010% or more. On the other hand, if the nickel content is too high, the ductility of the steel sheet may decrease, potentially reducing its formability. For this reason, the nickel content is kept below 1.000%.

[0026] Cu: 0~0.500% Cu is an element that contributes to improving the strength of steel plates. While the Cu content can be 0%, it is preferable that the Cu content be 0.001% or more to obtain the above effects. On the other hand, if the copper content is too high, there is a risk that productivity in hot rolling will decrease due to red-hot brittleness. Therefore, the copper content is kept below 0.500%.

[0027] Mo: 0~1.000% Mo, like Mn, is an element that contributes to increasing the strength of steel sheets. While the Mo content can be 0%, it is preferable that the Mo content be 0.010% or more to obtain the above effects. On the other hand, if the Mo content exceeds 1.000%, coarse Mo carbides may form, potentially reducing the cold formability of the steel sheet. For this reason, the Mo content is kept below 1.000%.

[0028] Cr: 0~2.000% Cr, like Mn and Mo, is an element that contributes to increasing the strength of steel sheets. While the Cr content can be 0%, to obtain the above effects, the Cr content is preferably 0.001% or more, and more preferably 0.100% or more. On the other hand, if the Cr content exceeds 2,000%, coarse Cr nitrides may form, potentially reducing the cold formability of the steel sheet. For this reason, the Cr content is kept below 2,000%.

[0029] Ti: 0~0.5000% Ti is an effective element for strengthening ferrite, as well as for controlling the morphology of carbides, and is also effective in refining the microstructure and improving the toughness of steel sheets. The Ti content may be 0%, but to obtain the above effects, the Ti content is preferably 0.0001% or more, and more preferably 0.0010% or more. On the other hand, if the Ti content is excessive, coarse Ti oxides or TiN may be formed, potentially reducing the formability of the steel sheet. Therefore, from the viewpoint of ensuring the formability of the steel sheet, the Ti content should be 0.5000% or less.

[0030] Nb: 0~0.50% Nb, like Ti, is an effective element for controlling the morphology of carbides and is also effective in refining the microstructure and improving the toughness of steel sheets. The Nb content may be 0%, but to obtain the above effects, the Nb content is preferably 0.001% or more, and more preferably 0.01% or more. On the other hand, if the Nb content is excessive, many fine, hard Nb carbides will precipitate, and as the strength of the steel sheet increases, its ductility will deteriorate significantly, potentially reducing the formability of the steel sheet. For this reason, the Nb content is kept below 0.50%.

[0031] V: 0~0.500% Like Ti and Nb, V is an effective element for controlling the morphology of carbides and is also effective in refining the microstructure and improving the toughness of steel sheets. The V content can be 0%, but to obtain the above effects, it is preferable that the V content be 0.001% or more. On the other hand, if the V content is excessive, a large number of fine V carbides may precipitate, increasing the strength of the steel while reducing its ductility and potentially decreasing the formability of the steel sheet. For this reason, the V content should be 0.500% or less.

[0032] W: 0~0.100% Like Nb and V, W is an effective element for controlling the morphology of carbides. Furthermore, W is also an effective element for improving the strength of steel sheets. While a W content of 0% is acceptable, a W content of 0.001% or more is preferable to achieve the above effects. On the other hand, if the W content is too high, many fine W carbides will precipitate, increasing the strength of the steel sheet, which in turn may decrease its ductility and reduce its cold workability. For this reason, the W content should be 0.100% or less.

[0033] Ta: 0~0.100% Like W, Ta is an effective element for controlling the morphology of carbides and improving the strength of steel sheets. While the Ta content can be 0%, it is preferable that the Ta content be 0.001% or higher to obtain the above effects. On the other hand, if the Ta content is excessive, a large number of fine Ta carbides will precipitate, increasing the strength of the steel sheet, which in turn may decrease its ductility and thus its cold workability. For this reason, the Ta content should be 0.100% or less. Preferably, the Ta content should be 0.020% or less, and more preferably 0.010% or less.

[0034] B: 0~0.0100% B is an element that suppresses the formation of ferrite and pearlite during the cooling process from the austenite temperature range, and promotes the formation of low-temperature transformation structures such as bainite or martensite. Furthermore, B is an element that is effective in increasing the strength of steel. The B content may be 0%, but to obtain the above effects, the B content is preferably 0.0001% or more. On the other hand, if the B content is excessive, coarse B oxides are generated, and these B oxides can become the starting point for void formation during press forming, potentially reducing the formability of the steel sheet. Therefore, the B content is 0.0100% or less.

[0035] Mg: 0~0.050% Mg is an element that controls the morphology of sulfides and oxides, contributing to improved bendability of steel sheets. While the Mg content may be 0%, to obtain the above effects, the Mg content is preferably 0.0001% or more, and more preferably 0.001% or more. On the other hand, if the Mg content is excessive, coarse inclusions may form, potentially reducing cold formability. Therefore, the Mg content should be 0.050% or less. Preferably, the Mg content should be 0.040% or less.

[0036] Ca: 0~0.050% Like Mg, Ca is an element that can control the form of sulfides even in trace amounts. While the Ca content can be 0%, it is preferable that the Ca content be 0.001% or higher in order to obtain the above effect. On the other hand, if the Ca content is excessive, coarse Ca oxides are generated. These coarse Ca oxides can become the starting point for crack formation during cold forming. Therefore, the Ca content should be 0.050% or less. Preferably, the Ca content should be 0.030% or less.

[0037] Zr: 0~0.050% Zr, like Mg and Ca, is an element that can control the form of sulfides even in trace amounts. While the Zr content can be 0%, it is preferable that the Zr content be 0.001% or higher to obtain the above effects. On the other hand, if the Zr content is excessive, coarse Zr oxides may be generated, potentially reducing cold formability. Therefore, the Zr content should be 0.050% or less. Preferably, the Zr content should be 0.040% or less.

[0038] REM: 0~0.100% REM is an element that is effective in controlling the morphology of sulfides even in trace amounts. While the REM content may be 0%, it is preferable that the REM content be 0.001% or higher in order to obtain the above effects. On the other hand, if the REM content is too high, coarse REM oxides may be generated, potentially reducing processability and fracture resistance. Therefore, the REM content should be 0.100% or less. Preferably, the REM content is 0.050% or less. Here, REM stands for Rare Earth Metal, and refers to the collective term for 17 elements: scandium (Sc) and yttrium (Y), and 15 elements from lanthanum (La) to lutetium (Lu) (lanthanoids). In this embodiment, "REM" consists of one or more elements selected from these rare earth elements, and "REM content" is the total amount of rare earth elements. REM is often added as mischmetal and may contain a combination of the above-mentioned lanthanoid series elements in addition to La and Ce. The effects of this embodiment are still achieved even if lanthanoid series elements other than La and Ce are included as impurities. The effects of this embodiment are also achieved even if metallic La or Ce is added.

[0039] Sn: 0~0.050% Sn (Tin) is an element that can be present in steel sheets when scrap is used as a raw material. Furthermore, Sn can cause a decrease in the cold formability of steel sheets due to ferrite embrittlement. Since the adverse effects are significant when the Sn content exceeds 0.050%, the Sn content should be 0.050% or less. A Sn content of 0.040% or less is preferable. A lower Sn content is preferable, so it may be 0%, but reducing the Sn content to less than 0.001% would lead to an excessive increase in refining costs. Therefore, the Sn content may be 0.001% or more.

[0040] Sb: 0~0.050% Sb, like sn, is an element that can be present in steel sheets when scrap is used as a raw material. Sb is an element that can strongly segregate at grain boundaries, potentially leading to embrittlement of grain boundaries, a decrease in ductility, and a decrease in cold formability. Since the adverse effects are significant when the Sb content exceeds 0.050%, the Sb content should be 0.050% or less. Preferably, the Sb content is 0.040% or less. A lower Sb content is preferable, so it could be 0%, but reducing the Sb content to less than 0.001% would lead to an excessive increase in refining costs. Therefore, the Sb content may be 0.001% or higher.

[0041] As: 0~0.050% As, like Sn and Sb, is an element that can be present in steel sheets when scrap is used as a raw material. As is an element that strongly segregates at grain boundaries and may lead to a decrease in cold formability. If the As content exceeds 0.050%, the adverse effects are significant, so the As content should be 0.050% or less. Preferably, the As content should be 0.040% or less. A lower As content is preferable, and it may be 0%, but reducing the As content to less than 0.001% would lead to an excessive increase in refining costs. Therefore, the As content may be 0.001% or more.

[0042] In the chemical composition of the steel sheet according to this embodiment, the remainder of the elements other than those mentioned above consists of Fe and impurities. The impurities are elements that are introduced from the steel raw materials and / or during the steelmaking process, and whose presence is permissible to the extent that they do not impair the properties of the steel sheet according to this embodiment, and also mean elements that are not components that were intentionally added to the steel sheet.

[0043] The above chemical composition can be measured, for example, by using spark emission spectrometry (Spark-OES, commonly known as cant-vac) or ICP emission spectrometry / mass spectrometry (ICP-OES / ICP-MS). The average content measured by this method is that of the steel sheet.

[0044] <Microorganism> The microstructure (metallic structure) of the steel sheet according to this embodiment will be described below. Hereinafter, the microstructure fraction will be expressed as an area fraction, so the unit "%" of the microstructure fraction means area %. Furthermore, in the steel sheet according to this embodiment, the microstructure in the range of 1 / 8 to 3 / 8 of the sheet thickness, centered at a point 1 / 4 of the sheet thickness from the surface of the steel sheet, is defined as follows. This range of microstructure is defined because it is a typical structure of the steel sheet and has a high correlation with its properties.

[0045] (Total area percentage of ferrite and bainite: 10% or more, 90% or less) (Ferrite area ratio: 5% or more) Ferrite and bainite have a soft structure and are therefore easily deformable, contributing to improved fracture resistance. When the total area ratio of ferrite and bainite is 10% or more, sufficient elongation can be obtained and moldability is improved. For this reason, the total area ratio of ferrite and bainite is set to 10% or more. Preferably, the total area ratio of ferrite and bainite is 20% or more, more preferably 25% or more. On the other hand, in order to ensure a predetermined tensile strength, the total area ratio of ferrite and bainite is set to 90% or less. Preferably, the total area ratio of ferrite and bainite is 70% or less, and more preferably 50% or less. Furthermore, compared to bainite, ferrite is softer and has better ductility, thus contributing to improved fracture resistance. For this reason, the area ratio of ferrite should be 5% or more. Preferably, the area ratio of ferrite is more than 5%, more preferably 7% or more, and even more preferably 10% or more.

[0046] (Total area ratio of martensite and tempered martensite: 10% or more, 90% or less) Martensite (so-called fresh martensite) and tempered martensite are hard structures and therefore contribute to improving tensile strength. By making the total area of ​​martensite and tempered martensite 10% or more, high strength can be achieved, making it easier to secure a tensile strength of 780 MPa or more. To secure even higher tensile strength, it is preferable to increase the total area ratio of martensite and tempered martensite. For example, to secure a tensile strength of 900 MPa or more, the total area ratio of martensite and tempered martensite is preferably 45% or more, more preferably 50% or more, and even more preferably 55% or more. Also, to secure a tensile strength of 1100 MPa or more, the total area ratio of martensite and tempered martensite is preferably 70% or more, and more preferably 80% or more. On the other hand, if the combined area ratio of martensite and tempered martensite exceeds 90%, sufficient elongation cannot be obtained, and the formability deteriorates. Therefore, the combined area ratio should be 90% or less. From the viewpoint of formability, the combined area ratio of martensite and tempered martensite is preferably 85% or less, more preferably 80% or less.

[0047] (Total area percentage of perlite and retained austenite: 0% or more, 10% or less) Perlite is a structure containing hard cementite, which can become the starting point for void formation during press molding, degrading its fracture resistance. Retained austenite is a microstructure that contributes to improved elongation through transformation-induced plasticity (TRIP). However, the martensite formed by the transformation of retained austenite is extremely hard, becoming a starting point for void formation and degrading fracture resistance. Therefore, the total area ratio of pearlite and retained austenite should be 10% or less. Preferably, the total area ratio is 5% or less. The area ratio of retained austenite is preferably 5% or less, more preferably less than 3%. In the steel sheet according to this embodiment, pearlite and retained austenite may not be included. That is, the total area ratio may be 0%.

[0048] This document describes the identification of ferrite, bainite, martensite, tempered martensite, pearlite, and retained austenite, as well as the calculation of their area and area ratio. The identification of each metallographic structure and the calculation of its area and area fraction can be performed by observing a 100 μm × 100 μm area of ​​the steel sheet cross-section, parallel to the rolling direction and perpendicular to the sheet surface, using a scanning electron microscope at a magnification of 1,000 to 50,000 times, depending on the structure being examined. This can be done using EBSD (Electron Back Scattering Diffraction), X-ray diffraction, or Nital reagent or Repera reagent. For measuring the area fraction of any structure, measurements are taken at three locations, and the average value is calculated.

[0049] The area and area fraction of ferrite grains can be measured by the following method. Specifically, using an EBSD attached to a scanning electron microscope, measurements are taken at intervals (pitch) of 0.2 μm from the surface of the steel plate, centered at a point 1 / 4 of the plate thickness from the surface, in the range of 1 / 8 to 3 / 8 of the plate thickness. The value of the local average misorientation (GAM) is calculated from the measurement data. The region where the grain average misorientation value is less than 0.5° is then identified as ferrite, and its area and area fraction are measured. Here, the local average misorientation is calculated by determining the orientation difference between adjacent measurement points in a region surrounded by grain boundaries where the crystal orientation difference is 5° or more, and averaging this value over all measurement points within the crystal grain.

[0050] To determine the area and area ratio of bainite crystal grains, a sample is taken from the cross-section of the steel plate parallel to the rolling direction, the observation surface is polished, and the surface is etched with nital reagent. The area from the surface to 1 / 8 to 3 / 8 of the plate thickness, centered at a point 1 / 4 of the plate thickness from the surface, is observed using a field emission scanning electron microscope (FE-SEM), and the area and area ratio are calculated using known image analysis software. For example, "ImageJ" can be used to calculate the area ratio. "ImageJ" is open-source, public-domain image processing software that is widely used among those skilled in the art. In FE-SEM observation, for example, the microstructure on an observation surface defined as a square with sides of 100 μm is distinguished as follows: Bainite is an aggregate of lath-like crystal grains that does not contain iron-based carbides with a major axis of 20 nm or more, or contains iron-based carbides with a major axis of 20 nm or more, and these carbides belong to a single variant, i.e., a group of iron-based carbides elongated in the same direction. Here, a group of iron-based carbides elongated in the same direction means that the difference in the elongation direction of the iron-based carbide group is within 5°. Bainite is counted as one bainite crystal grain when surrounded by grain boundaries with an orientation difference of 15° or more.

[0051] The area ratio of martensite and tempered martensite can be calculated by taking a sample with the plate thickness cross-section parallel to the rolling direction of the steel plate as the observation surface, polishing the observation surface, etching it with a Lepera reagent, observing and photographing the range from 1 / 8 to 3 / 8 of the plate thickness centered at the position of 1 / 4 of the plate thickness from the surface by FE-SEM, and subtracting the area ratio of the retained austenite measured using X-rays described later (details will be described later) from the area ratio of the uncorroded region. The observation range on the observation surface is, for example, a range of a square with a side length of 100 μm.

[0052] The area ratio of the retained austenite is thinned from the surface to the position of the thickness from 1 / 8 to 3 / 8 of the plate thickness by electrolytic polishing or chemical polishing. The polished surface is subjected to X-ray diffraction using MoKα line as the characteristic X-ray, and the area ratio of the retained austenite is calculated from the integrated intensity ratio of the diffraction peaks of (200), (211) of the bcc phase and (200), (220), (311) of the fcc phase, and the value at the position of 1 / 4 of the plate thickness is taken.

[0053] The area ratio of pearlite is determined by taking a sample with the plate thickness cross-section parallel to the rolling direction of the steel plate as the observation surface, polishing the observation surface, etching it with a nital reagent, and observing the range of 1 / 8 to 3 / 8 thickness centered at the position of 1 / 4 of the plate thickness from the surface of the steel plate using a secondary electron image obtained by a scanning electron microscope. The region photographed with a bright contrast in the secondary electron image is regarded as pearlite, and the area ratio is calculated using the above-mentioned image analysis software "ImageJ". The observation range on the observation surface is, for example, a range of a square with a side length of 100 μm.

[0054] (The ratio of the number of ferrite and bainite crystal grains with an area of 6 μm 2 or less to the total number of ferrite and bainite crystal grains is 40% or more) The ratio (N6 / N T ) of the number (N6) of ferrite and bainite crystal grains with an area of 6 μm 2 or less to the total number (N TThis is one of the indicators that greatly affects the fracture resistance characteristics of the steel plate according to this embodiment. The area is 6 μm relative to the total number of ferrite and bainite crystal grains. 2 The ratio of the number of crystal grains (fine grains) below (N6 / N T When the area increases, voids are less likely to form near the molded area during molding, and the voids that do form are less likely to connect with each other, improving fracture resistance. Specifically, in ferrite and bainite, when the area is 6 μm 2 The ratio of the number of crystal grains (N6 / N) T By setting the ratio to 40% or more, the fracture resistance is higher than that of automotive parts with the same level of strength. Therefore, the area is 6 μm 2 The ratio of the number of crystal grains (N6 / N) T ) should be 40% or more. (N6 / N T ) is preferably 50% or more, more preferably 55% or more. Area 6 μm in ferrite and bainite 2 The proportion of the following crystal grains may be 90% or less from the viewpoint of suppressing yield strength elongation.

[0055] (Area of ​​50 μm relative to the total number of ferrite and bainite crystal grains) 2 (The proportion of ferrite and bainite crystal grains is 5% or less.) Total number of ferrite and bainite crystal grains (N T ) with an area of ​​50 μm 2 The number of supercrystalline ferrite and bainite grains (N 50 ) proportion (N 50 / N T This is one of the indicators that greatly affects the fracture resistance characteristics of the steel plate according to this embodiment. Area of ​​50 μm in ferrite and bainite 2 When the proportion of supercrystalline grains increases, voids are more likely to form near the molded area during molding, and these voids are more likely to connect with each other, resulting in a decrease in fracture resistance. For this reason, area 50 μm 2 The ratio of the number of supercrystalline grains (N 50 / NT ) shall be 5% or less. (N 50 / N T ) is preferably 3% or less. (N 50 / N T Since a lower value is preferable, no lower limit is set, but it may be set to 1% or more from the perspective of suppressing the increase in manufacturing costs associated with precise control.

[0056] The grain size and number ratio of ferrite and bainite crystal grains are calculated from the image analysis results using "EBSD" and "ImageJ" performed within the same field of view.

[0057] (The maximum Mn content in the region from the interface between ferrite and martensite or tempered martensite up to 0.5 μm perpendicular to the interface and toward the inside of the ferrite grains is 0.30 mass% or more lower than the average Mn content of the steel sheet.) The inventors have found that in the case of a steel sheet containing soft ferrite and hard martensite and tempered martensite, if the Mn content in the ferrite near the interface between ferrite and martensite or tempered martensite (the interface between ferrite and martensite, and the interface between ferrite and tempered martensite) is high, the fracture resistance decreases. Therefore, in the steel sheet according to this embodiment, the maximum Mn content in the region from the interface between ferrite and martensite or tempered martensite to 0.5 μm inward from the interface (i.e., within the ferrite, in the range from the interface to 0.5 μm) is reduced by 0.30 mass% or more compared to the average Mn content of the steel sheet. If the maximum Mn content in the above region is greater than (average Mn content of the steel plate - 0.30 mass%), a sufficient improvement in fracture resistance cannot be obtained.

[0058] Whether the maximum Mn content in the region from the interface between ferrite and martensite or tempered martensite to 0.5 μm perpendicular to the interface and toward the inside of the ferrite grains is 0.30 mass% or more lower than the average Mn content of the steel sheet is determined by the following method. Specifically, after identifying ferrite, martensite, and tempered martensite using the method described above, line analysis is performed using EPMA in a range of 0.5 μm or more from the interface between two opposing ferrite and martensite or between ferrite and tempered martensite that are 3.0 μm or less apart, perpendicular to the interface and toward the inside of the ferrite grains. The difference between the maximum Mn content in the ferrite in the range of 0.5 μm from the interface obtained by line analysis and the average Mn content of the steel sheet (average Mn content of the steel sheet - maximum Mn content in the range of 0.5 μm from the interface) is defined as ΔMn. Line analysis is performed at 10 locations, and if the average of the 10 ΔMn values ​​is 0.30 (mass%) or more, it is determined that the Mn content in the region from the interface between ferrite and martensite or tempered martensite to 0.5 μm perpendicular to the interface and toward the inside of the ferrite grains is 0.30 mass% or more lower than the average Mn content of the steel sheet (ΔMn≧0.30).

[0059] (Preferably, an area of ​​6 μm 2 The following are examples of ferrite and bainite crystal grains with an average aspect ratio between 1.0 and 2.0. Area is 6 μm 2 The average aspect ratio of the ferrite and bainite grains, as described below, is also one of the indicators that affects fracture resistance. Generally, the smaller the aspect ratio and the more equiaxed the grains, the less likely stress concentration is to occur at the interface. Therefore, to obtain better fracture resistance, it is preferable to set the average aspect ratio of the ferrite and bainite grains to 1.0 or more and 2.0 or less. This effect is difficult to obtain when the average aspect ratio exceeds 2.0. The average aspect ratio is more preferably 1.0 or more and 1.5 or less. In this embodiment, the aspect ratio refers to the ratio of the longest diameter (major axis) of a ferrite crystal grain to the longest diameter (minor axis) of the ferrite grain perpendicular to it. The same applies to the aspect ratio of bainite crystal grains. Area is 6 μm 2 While there are no particular limitations on the supergrain size, as its contribution to void connectivity is relatively small, elongated grains are preferable from the viewpoint of reducing stress concentration at the interface, so the average aspect ratio may be greater than 2.0 and less than or equal to 5.0.

[0060] <Mechanical properties> [Tensile strength: 780 MPa or higher] In this embodiment, the steel sheet has a tensile strength of 780 MPa or higher, taking into consideration its contribution to reducing the weight of automobiles through application to automotive parts. Considering its contribution to reducing the weight of automobiles, the tensile strength is preferably 980 MPa or higher, and more preferably 1180 MPa or higher. On the other hand, while there is no need to limit the upper limit of tensile strength, as tensile strength increases, properties such as elongation and hole-expanding ability may decrease, so the tensile strength may be limited to 1500 MPa or less.

[0061] [The difference between stress σ2 and tensile strength σ1 at a uniform elongation of +1.0% (Δσ, which is σ1-σ2) is 50 MPa or less.] To suppress crack formation during press forming, it is important to maintain a high true stress at strain levels exceeding uniform elongation. In particular, in the case of a stress-strain curve as shown in Figure 1, if σ1 is the stress at the position where the stress is maximum, the elongation at that point is the uniform elongation (u-El), and σ2 is the stress at uniform elongation + 1.0% (the stress at the strain level (horizontal axis) when the elongation is uniform elongation + 1.0%), then the fracture resistance is improved if Δσ (σ1-σ2) is 50 MPa or less. This is thought to be because Δσ is an indicator that represents the amount of voids generated that reduce true stress, and it shows a good correlation with crack initiation during press forming. Therefore, in the steel plate according to this embodiment, the goal is for Δσ to be 50 MPa or less. More preferably, Δσ is 40 MPa or less.

[0062] The tensile strength σ1 and the stress σ2 at a uniform elongation of +1.0% of the steel plate are determined by performing a tensile test in accordance with JIS Z 2241:2011 using a No. 5 test specimen.

[0063] The steel sheet according to this embodiment described above may have a coating layer on its surface containing zinc, aluminum, magnesium, or an alloy thereof. The presence of a coating layer on the surface of the steel sheet improves corrosion resistance. The coating layer may be a known coating layer. For example, when steel plates are used in corrosive environments, concerns about perforation and other issues may prevent them from being thinned below a certain thickness, even if their strength is increased. One of the purposes of increasing the strength of steel plates is to reduce weight by making them thinner, so even if high-strength steel plates are developed, their application is limited if their corrosion resistance is poor. Having a coating layer containing zinc, aluminum, magnesium, or alloys thereof on the surface is preferable because it improves corrosion resistance and broadens the range of applications. When a steel sheet has a coating layer (e.g., a plating layer) on its surface, the term "surface" in the context of "a range of 1 / 8 to 3 / 8 of the sheet thickness centered at a point 1 / 4 of the sheet thickness from the surface of the steel sheet" refers to the base metal surface excluding the coating layer.

[0064] The thickness of the steel plate according to this embodiment is not limited to a specific range, but considering strength, versatility, and manufacturability, 0.3 to 6.0 mm is preferred.

[0065] <Manufacturing method> The steel sheet according to this embodiment can be obtained by a manufacturing method comprising the following steps, although the manufacturing method is not particularly limited. (I) A hot rolling process in which a slab having a predetermined chemical composition is hot-rolled to obtain a hot-rolled steel sheet, (II) A winding process in which the hot-rolled steel sheet is cooled to a winding temperature of 650°C or less and 450°C or more at an average cooling rate of 30°C / second or more, and then wound at the winding temperature. (III) A holding step in which the hot-rolled steel sheet after the winding step is held for 2 to 8 hours in a temperature range from the winding temperature to the winding temperature - 50°C. (IV) A cooling step in which the hot-rolled steel sheet after the holding step is cooled to a temperature of 300°C or less at an average cooling rate of 0.1°C / second or more. (V) A cold rolling process in which the hot-rolled steel sheet after the cooling process is cold-rolled with a thickness reduction rate of 20 to 80% to obtain a cold-rolled steel sheet. (VI) An annealing process in which the cold-rolled steel sheet is heated to an annealing temperature of 740 to 900°C at an average heating rate of 5°C / second or more, and held at the annealing temperature for 60 to 300 seconds. The following describes the preferred conditions for each process.

[0066] [Hot rolling process] In the hot rolling process, a slab having a predetermined chemical composition (in the case of obtaining a steel sheet according to this embodiment, the same chemical composition as the steel sheet according to this embodiment) is heated and hot-rolled to obtain a hot-rolled steel sheet. The slabs to be heated may be obtained by continuous casting or casting and bract rolling, but they may also be those that have undergone hot working or cold working. While the heating temperature is not limited, if it is below 1100°C, the carbides and sulfides generated during casting may not dissolve and may become coarser, potentially degrading the press-formability. Therefore, the heating temperature is preferably 1100°C or higher, and more preferably 1150°C or higher.

[0067] Furthermore, in the hot rolling process, the finish rolling is carried out using a rolling mill with four or more stands. If the first stand is designated as the first stand and the final stand as the nth stand, the thickness reduction rate at each stand from the (n-3)th stand to the nth stand is set to 30% or more, and the rolling temperature at the final stand (nth stand) is set to 900°C or less. That is, for example, if the rolling mill has seven stands, the thickness reduction rates at the fourth, fifth, sixth, and seventh stands are set to 30% or more, and the rolling temperature at the seventh stand is set to 900°C or less. In this finish rolling, the austenite grain size is refined by recrystallization during rolling, and a large amount of strain is introduced into the austenite to increase the number of sites where ferrite nuclei are generated, thereby refining the crystal grains of the hot-rolled steel sheet. If the thickness reduction rate at any of the stands is less than 30%, or if the rolling temperature at the nth stand exceeds 900°C, the hot-rolled structure will become coarse and mixed, and the structure after the annealing process described later will also become coarse. If the completion temperature of hot rolling is below 830°C, the rolling reaction force increases, making it difficult to stably obtain the target thickness. For this reason, it is preferable that the rolling temperature at the final stand be 830°C or higher. Furthermore, even if the reduction ratio is greater than 50%, the effect of grain refinement saturates, and the equipment load increases excessively due to the increase in rolling load. For this reason, it is preferable that the thickness reduction rates at the n-3rd to nth stands be 50% or less. Furthermore, finish rolling is performed using a rolling mill with four or more stands, in order to ensure continuous rolling with short inter-pass times between the final four passes. This is because if the inter-pass time is long, even if a large reduction in plate thickness is performed, the strain will recover between passes, and sufficient strain will not accumulate. Conventionally, it was sometimes performed to reduce grain size by 20% or more in three or more passes solely for the purpose of grain refinement. However, in this embodiment, in order to concentrate Mn into the cementite, which was not the objective in conventional methods, the final four passes (at stands n-3 to n) are reduced in thickness by 30% or more, as described above. Even when performing three or more passes with a reduction ratio of 20% or more, reducing the final four passes in thickness by 30% or more is not usually done because it not only saturates the grain refinement effect but also concentrates the rolling load on the subsequent four stands, leading to decreased productivity and the risk of equipment trouble.

[0068] [Winding process] In the winding process, the hot-rolled steel sheet after the hot-rolling process is cooled to a winding temperature of 650°C or lower and 450°C or higher at an average cooling rate of 30°C / second or more, and then wound at the aforementioned winding temperature. By rapidly cooling after hot rolling, the transformation to ferrite and pearlite at high temperatures is suppressed, and the ferrite transformation occurs at low temperatures where the driving force for transformation is high, thereby obtaining a structure having fine ferrite and fine cementite formed at the grain boundaries of that ferrite. If the average cooling rate is less than 30°C / second, or the cooling stop temperature (winding temperature) exceeds 650°C, coarse ferrite and pearlite containing coarse carbides will be formed unevenly. Since coarse carbides are difficult to dissolve in the annealing process, the resulting structure after annealing will be coarse and mixed. On the other hand, if the winding temperature falls below 450°C, the strength of the hot-rolled steel sheet becomes excessive, increasing the cold-rolling load and degrading productivity. The average cooling rate is preferably 100°C / second or less in order to stably obtain the target cooling stop temperature.

[0069] [Holding process] In the holding process, the steel sheet after the winding process is held for 2 to 8 hours in a temperature range from the winding temperature to the winding temperature minus 50°C. In this retention process, Mn primarily diffuses along ferrite grain boundaries and becomes concentrated in the cementite. As mentioned above, making the ferrite grains finer increases the number of diffusion pathways, promoting the concentration of Mn in the cementite (for example, the Mn content in the cementite exceeds 3.0%). Furthermore, when Mn is concentrated in the cementite, a Mn-deficient layer with a low Mn content is formed nearby. However, if the holding time in the temperature range from the winding temperature to the winding temperature - 50°C exceeds 8 hours, the cementite will coarseen. Since coarse carbides are difficult to dissolve in the annealing process, if the carbides coarseen, the structure after annealing will be coarse and mixed. For this reason, the holding time should be kept within 8 hours. In order to sufficiently concentrate Mn in the cementite, the holding time in the temperature range from the winding temperature to the winding temperature - 50°C should be 2 hours or more.

[0070] [Cooling process] In the cooling process, the hot-rolled steel sheet after the holding process is cooled to a temperature of 300°C or lower at an average cooling rate of 0.1°C / second or higher. If the average cooling rate after the holding process, down to the cooling stop temperature of 300°C or less, is less than 0.1°C / second, there is a risk of cementite coarsening. If the average cooling rate is high, hard martensite is more likely to form if untransformed γ remains. In this case, there is a concern that the hot-rolled sheet will become stronger, leading to an increase in the cold-rolling load. Therefore, an average cooling rate of 12.0°C / second or less is preferable.

[0071] [Cold rolling process] In the cold rolling process, the hot-rolled steel sheet, after the cooling process, is cold-rolled to obtain a cold-rolled steel sheet with a thickness reduction rate of 20-80%. If the plate thickness reduction rate is less than 20%, strain accumulation in the steel plate is insufficient, and the austenite nucleation sites become non-uniform. In this case, during the subsequent annealing process, the grain size becomes coarser or mixed, resulting in an area of ​​6 μm². 2 The following are the number ratios and area of ​​ferrite and bainite crystal grains: 2As a result, the ratio of ferrite and bainite crystal grains does not fall within the desired range. Furthermore, the aspect ratio of the crystal grains also increases. Consequently, the fracture resistance deteriorates. On the other hand, if the plate thickness reduction rate exceeds 80%, the cold rolling load becomes excessive, leading to a decrease in productivity. Therefore, the plate thickness reduction rate should be between 20% and 80%. Preferably, the plate thickness reduction rate is between 30% and 80%. There are no restrictions on the cold rolling method; the number of rolling passes and the reduction rate for each pass can be set as appropriate. Pickling may be performed under known conditions before cold rolling.

[0072] [Annealing process] In the annealing process, the cold-rolled steel sheet is heated to an annealing temperature of 740-900°C at an average heating rate of 5°C / second or more, and then held at that annealing temperature (740-900°C) for 60 seconds or more. If the average heating rate is less than 5°C / second, the Mn concentrated in cementite (θ) in the hot-rolled steel sheet may diffuse into the Mn-deficient layer with a low Mn content, potentially causing the Mn-deficient layer to disappear. Therefore, the average heating rate to the annealing temperature should be 5°C / second or higher. In particular, since Mn diffusion is likely to occur above 550°C, it is preferable to maintain an average heating rate of 5°C / second or higher in the temperature range above 550°C. Controlling the average heating rate to exceed 50°C / second requires excessive capital investment. Therefore, from an economic standpoint, an average heating rate of 50°C / second or lower is preferable. Furthermore, if the annealing temperature is below 740°C, the austenite content is low, the area ratio of martensite after annealing and tempered martensite becomes less than 10%, and the tensile strength becomes less than 780 MPa. On the other hand, at annealing temperatures above 900°C, the metal structure becomes coarser, and the fracture resistance deteriorates. Therefore, the annealing temperature should be between 740°C and 900°C. Preferably, the annealing temperature should be between 780°C and 850°C. If the holding time (residence time) at the annealing temperature is less than 60 seconds, sufficient austenite will not form, the area ratio of martensite after annealing and tempered martensite will be less than 10%, and the tensile strength will be less than 780 MPa. Therefore, the holding time at the annealing temperature should be 60 seconds or more. Preferably, the holding time is 70 seconds or more, more preferably 80 seconds or more. On the other hand, if the annealing time exceeds 300 seconds, the crystal grains will coarse. Therefore, the annealing time should be 300 seconds or less.

[0073] The cooling rate after heating in the annealing process is not limited, but after slow cooling to achieve the desired ferrite fraction, rapid cooling is performed to generate martensite. Holding and reheating steps to temper the martensite may also be included.

[0074] It is believed that by cold-rolling a hot-rolled steel sheet having fine cementite and a Mn-deficient layer at the ferrite grain boundary and then annealing it under the above conditions, the Mn-deficient layer with a low Mn content will become ferrite after annealing, and the fine cementite will dissolve to become martensite (or retained austenite) with a high Mn content, thereby obtaining the microstructure of the steel sheet according to this embodiment.

[0075] In the annealing process, a coating layer, such as a plating layer containing zinc, aluminum, magnesium, or alloys thereof, may be formed on the surface of the steel sheet to improve its corrosion resistance. For example, the steel sheet may be immersed in a plating bath during the cooling process after holding to form a hot-dip plating. Alternatively, this hot-dip plating may be heated to a predetermined temperature to alloy it, resulting in alloyed hot-dip plating. Furthermore, the plating layer may also contain Fe, Al, Mg, Mn, Si, Cr, Ni, Cu, etc. Any of the above methods may be used to form a plating layer for the purpose of improving corrosion resistance. The plating conditions and alloying conditions should be known conditions applied according to the composition of the plating. [Examples]

[0076] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to these examples.

[0077] Various slabs with the chemical compositions listed in Tables 1-1 and 1-2 (continuation of 1-1) were prepared by casting. In Tables 1-1 and 1-2, blank spaces indicate that a substance was not intentionally added. The units for the components of each slab are in mass percent, with the remainder being Fe and impurities. This slab was heated to a temperature range of 1150°C to 1250°C, and hot-rolled according to the conditions described in Tables 2-1 and 2-2 to obtain hot-rolled steel sheets. These hot-rolled steel sheets were then cooled to the winding temperature according to the conditions described in Tables 2-3 and 2-4 and wound up. In the hot-rolling process, a rolling mill with four or more stands was used for the finish rolling. The rolled hot-rolled steel sheet was held in the temperature range from the roll temperature to the roll temperature - 50°C for the durations shown in Tables 2-3 and 2-4, and then cooled to a temperature range of 300°C or less. After cooling, the hot-rolled steel sheet was pickled and then cold-rolled at the thickness reduction rates shown in Tables 2-3 and 2-4 to obtain a cold-rolled steel sheet with a thickness of 1.4 mm. The obtained cold-rolled steel sheets were annealed under the conditions described in Tables 2-5 and 2-6. Some of the steel sheets were immersed in a zinc plating bath during the cooling process of annealing to form a hot-dip galvanized layer on the surface. Furthermore, some of the plated steel sheets underwent an alloying treatment to convert the hot-dip galvanized layer into an alloyed hot-dip galvanized layer. In Tables 2-5 and 2-6, "Presence or absence of plating" indicates whether or not hot-dip galvanizing was performed during the continuous annealing process, and "Presence or absence of alloying" indicates whether or not an alloying treatment was performed after hot-dip galvanizing. Furthermore, after annealing, the cooling rate was adjusted to achieve a desirable area ratio between ferrite, martensite, and tempered martensite. Based on the above, cold-rolled steel sheets for tests No. 1-51 and 53-58 were obtained. Test No. 52 was discontinued because the cold rolling load was excessive and cold rolling could not be performed.

[0078] [Table 1-1]

[0079] [Table 1-2]

[0080] [Table 2-1]

[0081] [Table 2-2]

[0082] [Table 2-3]

[0083] [Table 2-4]

[0084] [Table 2-5]

[0085] [Table 2-6]

[0086] The area ratio of the metallographic structure (ferrite, bainite, martensite, tempered martensite, pearlite, and retained austenite (retained γ)) in the range of 1 / 8 to 3 / 8 of the thickness of these cold-rolled steel sheets, centered at a point 1 / 4 of the thickness from the surface, and the total number of ferrite and bainite grains in the 1 / 4 thickness portion. T For a given area of ​​6 μm 2 The ratio of the number of crystal grains N6 for ferrite and bainite is as follows (N6 / N T ), the total number of ferrite and bainite crystal grains N T In contrast, the area is 50 μm 2The number of crystal grains N of supercrystalline ferrite and bainite. 50 The proportion (N 50 / N T ), the difference ΔMn between the maximum Mn concentration in the region from the interface between ferrite and martensite to 0.5 μm perpendicular to the interface and toward the inside of the ferrite grains, and the area of ​​6 μm at 1 / 4 of the plate thickness. 2 The average aspect ratios of ferrite and bainite were evaluated as follows. The results are shown in Table 3. These evaluations were carried out according to the method described above.

[0087] Furthermore, the tensile strength (TS), uniform elongation (uEl), and σΔ of these cold-rolled steel sheets were evaluated using the method described below. The results are shown in Tables 3-1 to 3-4. In Tables 1-1 to 3-4, underlined values ​​indicate that the results are outside the scope of the present invention or that desirable properties were not obtained.

[0088] [Table 3-1]

[0089] [Table 3-2]

[0090] [Table 3-3]

[0091] [Table 3-4]

[0092] The tensile strength (TS) (σ1), uniform elongation (u-El), and Δσ (the difference between stress σ2 and tensile strength σ1 at uniform elongation +1.0%) of the steel sheet were evaluated by taking a JIS No. 5 test specimen from the steel sheet so that its longitudinal direction was perpendicular to the rolling direction of the steel sheet, and performing a tensile test in accordance with JIS Z 2241:2011. Steel plates with a tensile strength (TS) of 780 MPa or higher were deemed to meet the requirements for tensile strength. Furthermore, a uniform elongation (u-El) of 5.5% or higher was considered to indicate excellent moldability. Furthermore, if Δσ is 50 MPa or less, it was determined that the material has excellent fracture resistance.

[0093] As can be seen from Tables 1-1 to 3-4, the inventive examples (Tests No. 1 to No. 37, No. 58) that fall within the scope of the present invention in terms of both chemical composition and manufacturing conditions, have a microstructure fraction and microstructure characteristics ((N6 / N T ), (N 50 / N T ), ΔMn, area is 6 μm 2 The average aspect ratio of ferrite and bainite, as well as their properties, all fell within the scope of the invention, exhibiting excellent strength, moldability, and fracture resistance. On the other hand, comparative examples (Tests No. 38 to No. 57) that did not meet the scope of the invention in either chemical composition or manufacturing conditions had inferior strength, moldability, and fracture resistance because their chemical composition, microstructure fraction, and microstructure characteristics fell outside the scope of the invention.

[0094] Tests No. 38 to No. 47 were comparative examples where the manufacturing conditions were within the scope of the present invention, but the chemical composition was outside the scope of the invention, resulting in inferiority in at least one of the properties: strength, moldability, and fracture resistance.

[0095] Tests No. 48 to No. 57 are comparative examples in which the chemical composition was within the scope of the present invention, but one or more of the conditions in the manufacturing method fell outside the scope of the present invention. Test No. 48 was N6 / N because the temperature of the final stand in the hot rolling process was too high. T and N 50 / N T Insufficient resources were secured, and the formation of a Mn-deficient layer was inadequate (ΔMn was less than 0.30). As a result, Δσ, an indicator of fracture resistance, did not meet the target. In test No. 49, the thickness reduction rate from the n-3rd stand to the n-1st stand was too small, and in test No. 50, the cooling rate after finish rolling was too slow. As a result, the cementite in the hot-rolled steel sheet could not be uniformly dispersed in large quantities, leading to insufficient refinement of the post-annealing structure and insufficient formation of a Mn-deficient layer. T and N 50 / N T This fell outside the scope of the present invention. As a result, Δσ did not satisfy the target. In test No. 52, the coiling temperature was too low, which significantly increased the strength of the hot-rolled steel sheet, leading to an increase in the cold rolling load and making cold rolling impossible. In test No. 53, the holding time during the holding process was too short, resulting in insufficient enrichment of elements such as Mn into the carbide, and consequently, insufficient formation of a Mn-deficient layer after annealing. As a result, Δσ did not meet the target. In test No. 54, the rate of thickness reduction during the cold rolling process was too large, resulting in insufficient strain accumulation in the steel sheet, which led to non-uniform austenite nucleation sites during annealing, and N 50 / N T This fell outside the scope of the present invention. As a result, Δσ did not satisfy the target. In test No. 55, the heating rate during the annealing process was too fast, causing the Mn-deficient layer to disappear during annealing. As a result, ΔMn was less than 0.30, and Δσ did not meet the target. In test No. 56, the heating temperature during the annealing process was too high, causing the Mn-deficient layer to disappear during annealing, resulting in a ΔMn value of less than 0.30. Also, N 50 / N T This fell outside the scope of the present invention. As a result, Δσ did not satisfy the target. In test No. 57, the holding time during heating in the annealing process was too long, causing the Mn-deficient layer to disappear during annealing, resulting in a ΔMn value of less than 0.30. Also, N 50 / N T This fell outside the scope of the present invention. As a result, Δσ did not satisfy the target. [Industrial applicability]

[0096] According to the present invention, it is possible to provide a steel plate with excellent fracture resistance and a method for manufacturing the same. Therefore, it has high potential for industrial application.

Claims

1. In mass percent, C: 0.07-0.15%, Si: 0.01-2.00%, Mn: 1.5-3.0%, P: 0-0.020%, S: 0-0.0200%, Al: 0.001-1.000%, N: 0 to 0.0200%, O: 0 to 0.0200%, Co: 0 to 0.500%, Ni: 0-1.000%, Cu: 0-0.500%, Mo: 0-1.000%, Cr: 0-2.000%, Ti: 0 to 0.5000%, Nb: 0 to 0.50%, V: 0 to 0.500%, W: 0-0.100%, Ta: 0-0.100%, B: 0 to 0.0100%, Mg: 0 to 0.050%, Ca: 0-0.050%, Zr: 0 to 0.050%, REM: 0-0.100%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, As: 0-0.050%, and Remainder: Fe and impurities It has a chemical composition consisting of, The tensile strength is 780 MPa or more. In microorganisms, The area ratio of ferrite is 5% or more. The area ratio of the aforementioned ferrite and bainite is 10% or more and 90% or less in total. The total area ratio of martensite and tempered martensite is 10% or more and 90% or less. The total area ratio of perlite and retained austenite is 0% or more and 10% or less. And, With respect to the total number of crystal grains of the ferrite and bainite, Area is 6 μm ２ The following conditions apply: the number of ferrite and bainite crystal grains is 40% or more. Area 50 μm ２ The above conditions are met, and the number of ferrite and bainite crystal grains is 5% or less. The maximum Mn content in the region from the interface between the ferrite and the martensite or tempered martensite to 0.5 μm in the direction perpendicular to the interface and toward the inside of the ferrite grains is 0.30 mass% or more lower than the average Mn content of the steel sheet. A steel plate characterized by the following features.

2. Area is 6 μm ２ The average aspect ratio of the crystal grains of the ferrite and bainite is 1.0 or more and 2.0 or less, The steel plate according to claim 1, characterized in that

3. The surface has a coating layer containing zinc, aluminum, magnesium, or an alloy thereof. A steel plate according to claim 1 or 2, characterized in that...

4. A method for manufacturing a steel sheet according to claim 1 or 2, In mass%, C: 0.07 to 0.15%, Si: 0.01 to 2.00%, Mn: 1.5 to 3.0%, P: 0 to 0.020%, S: 0 to 0.0200%, Al: 0.001 to 1.000%, N: 0 to 0.0200. %, O: 0-0.0200%, Co: 0-0.500%, Ni: 0-1.000%, Cu: 0-0.500%, Mo: 0-1.000%, Cr: 0-2.000%, Ti: 0-0.5000%, Nb: 0-0. A hot rolling process to obtain a hot-rolled steel sheet by hot rolling a slab having a chemical composition consisting of 50%, V: 0-0.500%, W: 0-0.100%, Ta: 0-0.100%, B: 0-0.0100%, Mg: 0-0.050%, Ca: 0-0.050%, Zr: 0-0.050%, REM: 0-0.100%, Sn: 0-0.050%, Sb: 0-0.050%, As: 0-0.050%, and the remainder: Fe and impurities, The hot-rolled steel sheet is cooled to a winding temperature of 650°C or lower and 450°C or higher at an average cooling rate of 30°C / second or more, and then wound at the winding temperature in a winding process. A holding step is performed to hold the hot-rolled steel sheet after the winding step in a temperature range from the winding temperature to the winding temperature minus 50°C for a holding time of 2 to 8 hours. A cooling step is performed to cool the hot-rolled steel sheet after the holding step to a temperature of 300°C or lower at an average cooling rate of 0.1°C / second or more. A cold rolling step is performed to obtain a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet after the cooling step with a thickness reduction rate of 20 to 80%, The cold-rolled steel sheet is heated to an annealing temperature of 740 to 900°C at an average heating rate of 5°C / second or more, and held at the annealing temperature for 60 to 300 seconds in an annealing process, It has, The hot rolling process is as follows: When finish rolling is performed using a rolling mill having four or more stands, with the first stand designated as the first stand and the final stand as the nth stand, the thickness reduction rate at each stand from the nth-3rd stand to the nth stand is set to 30% or more, and the rolling temperature at the nth stand is set to 900°C or less. A method for manufacturing steel plates, characterized by the following:

5. In the annealing process, a coating layer containing zinc, aluminum, magnesium, or an alloy thereof is formed on the surface of the steel sheet. A method for manufacturing a steel plate according to claim 4, characterized in that

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