Hot-rolled steel sheet and manufacturing method thereof

Abstract

To provide a hot rolled steel sheet which has a yield strength (YS) of 500 MPa or more and has excellent bending workability and toughness, and to provide a method of producing the same.SOLUTION: Provided is a hot rolled steel sheet of a composition comprising C, Si, Mn, P, S, Al, N, Nb, and optionally selected components, which has, in terms of areal fraction of the metallic structure, 0 to 85% of ferrite, 3% or less of retained austenite, 5% or less of lath morphology, 15% or more of a structure having a KAM value of 1.0, and which has a yield strength of 500 MPa or more with carbides containing Nb with an average particle size of 8 nm or less. Also provided is method producing a hot rolled steel sheet, including; a rough rolling step of heating or not heating a steel raw material having the above-mentioned component composition; a finish rolling step in which the starting temperature is 950°C or lower, the total draft from the first pass to the fifth pass is 75% or more, and the finishing temperature is 860°C or higher or 910°C or lower; a cooling step; and a coiling step.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet having a yield strength of 500 MPa or more and excellent bending workability and toughness, and a method for producing the same. [Background technology]

[0002] In recent years, from the perspective of protecting the global environment, the entire automotive industry has been working to improve the fuel efficiency of automobiles in order to regulate CO2 emissions. The most effective way to improve fuel efficiency is to reduce the weight of automobiles by making the parts thinner, so in recent years the amount of high-strength steel sheets used as materials for automobile parts has been increasing. Generally, as the strength of steel sheets increases, the formability and toughness tend to deteriorate. Therefore, in order to further expand the use of high-strength steel sheets, it is essential that high strength, formability, and toughness are simultaneously achieved.

[0003] To solve these problems, various techniques have been proposed to increase the strength and improve the formability of steel sheets.

[0004] For example, Patent Document 1 discloses a hot-rolled steel sheet in which Ti carbides with an average grain size of less than 6 nm and TiS with an average grain size of 0.5 μm or less are dispersed in ferrite crystals with an area ratio of 95% or more, and it is said that this results in a high-tensile hot-rolled steel sheet with a tensile strength of 780 MPa to 900 MPa and excellent bending workability.

[0005] Patent Document 2 discloses a technology in which a steel slab containing one or more of Ti and Nb is heated and hot-rough rolled to form a steel plate, which is then joined to the rear end of a preceding rough-rolled steel plate and hot-finish rolled in the temperature range of Ar3 to Ar3+50° C. This method is said to produce a hot-rolled steel plate for processing with good toughness. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2013 / 099196 [Patent Document 2] Japanese Patent Application Publication No. 09-227949 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the prior art disclosed in the above patent document has the following problems.

[0008] The technology described in Patent Document 1 does not allow for the microstructure desired in the present invention to be obtained, as seen in, for example, steel sheet No. 5 in the examples, and therefore does not allow for a yield strength of 500 MPa or more to achieve both good bending workability and toughness.

[0009] Furthermore, the technology described in Patent Document 2 does not consistently produce steel sheets having a yield strength of 500 MPa or more, and Patent Document 2 does not suggest any requirements for obtaining good bendability. Furthermore, controlling the hot rolling temperature within a narrow range to obtain high toughness significantly hinders manufacturability, and in some cases it has been impossible to implement depending on the production size.

[0010] The present invention was developed in consideration of the above-mentioned problems of the conventional technology, and aims to provide a hot-rolled steel sheet having a yield strength (YS) of 500 MPa or more and excellent bending workability and toughness, and a manufacturing method thereof. [Means for solving the problem]

[0011] In order to solve the above-mentioned problems, the inventors have conducted extensive research into the requirements for combining bending workability and toughness in a hot-rolled steel sheet having a yield strength of 500 MPa or more. Considering the recent trend toward higher strength hot-rolled steel sheets for automobiles, it is preferable to set the yield strength to 500 MPa or more, and as described below, it is preferable to add Nb and Ti in combination. Since high ductility is necessary to obtain good workability, a process assuming a high coiling temperature that can obtain high total elongation was investigated. In order to obtain a strength of 500 MPa or more at a coiling temperature of 600°C or more, the hot-rolled steel sheet is strengthened with extremely fine nano-sized carbides containing Nb.

[0012] However, it was previously common knowledge that when the coiling temperature of hot-rolled steel sheets is 600°C or higher, which causes the precipitation of Nb-containing carbides, a ferrite phase containing few dislocations is formed in the steel sheet. The fracture surface unit that has a significant effect on the toughness of this ferrite structure is said to be equivalent to the ferrite grain size. After investigating how to refine the ferrite phase, it was concluded that it would be difficult to consistently achieve the desired toughness.

[0013] Therefore, the possibility of forming a crystalline structure other than ferrite when the coiling temperature of this hot-rolled steel sheet is 600°C or higher was investigated extensively, and as a result, a new structure that cannot be classified as either ferrite or bainite was obtained, and it was found that this structure has good strength, bending workability, and toughness. It was discovered that this new structure is generated from fine recrystallized austenite grains that are recrystallized during the finish rolling process of hot rolling.

[0014] The hot-rolled steel sheet according to the present invention, which was developed based on the above findings, has the following configuration. [1] In mass%, C: 0.030% or more and 0.100% or less, Si: 1.5% or less, Mn: 1.3% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% or more and 0.080% or less, N: 0.0060% or less, Nb: 0.03% or more and 0.20% or less, Optionally, it further contains at least one component from Groups A to C below: Group A; Ti: 0.010% or more and 0.150% or less, Group B; B: 0.0002% or more and 0.0050% or less, Group C: A hot-rolled steel sheet having a chemical composition containing one or more of V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W, Ta, Pb, Cs, Zr, Hf, Te, Bi, and Se in total at 1% or less, with the balance being Fe and unavoidable impurities, in which, in terms of area ratio of the metal structure, ferrite is 0% to 85% or less, retained austenite is 3% or less, a lath-type structure is 5% or less, and a structure with a KAM value of 1.0 or more is 15% or more, and which has carbides containing Nb and having an average particle size of 8 nm or less, and has a yield strength of 500 MPa or more. [2] In the above [1], the hot-rolled steel sheet has a plating layer on the surface thereof.

[0015] The method for producing a hot-rolled steel sheet according to the present invention, which was developed based on the above findings, is configured as follows: [3] A method for producing a hot-rolled steel sheet, comprising: a rough rolling step of heating a steel material having the chemical composition described in [1] above to a heating temperature of 1200°C or higher, or without heating, and rough rolling the steel material to a sheet bar; a finish rolling step of finish rolling the sheet bar to a rolling start temperature of 950°C or higher, a total reduction rate of 75% or higher from the first to fifth passes, and a rolling completion temperature of 860°C to 910°C to produce a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet to a cooling stop temperature of 600°C to 700°C at an average cooling rate of 40°C / s or higher; and a coiling step of coiling the cooled hot-rolled steel sheet at a coiling temperature of 600°C to 700°C. [4] In the above [3], a method for producing a hot-rolled steel sheet includes a casting step of casting a steel material having a thickness of 35 mm or more and 200 mm or less and having the component composition described in [1] before the rough rolling step or the finish rolling step, and the hot-rolled steel sheet is made into a sheet bar with or without applying the rough rolling step. [5] In the above [3], the method for producing a hot-rolled steel sheet further comprises a joining step of joining the rough-rolled sheet bar and a preceding sheet bar at 1050°C or higher between the rough rolling step and the finish rolling step, and in the finish rolling step, the joined sheet bar is finish-rolled. [6] In any one of the above [3] to [5], the method for producing a hot-rolled steel sheet further includes a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet at an annealing temperature of 720°C or less, and a plating step of plating the annealed hot-rolled steel sheet. [7] The method for producing a hot-rolled steel sheet according to the above [6], further comprising an alloying step of subjecting the plated hot-rolled steel sheet to an alloying treatment at 480°C or higher and 600°C or lower. [Effects of the Invention]

[0016] According to the present invention, it is possible to manufacture a hot-rolled steel sheet having high strength with a yield strength (YS) of 500 MPa or more, and excellent bending workability and toughness. If the hot-rolled steel sheet according to the present invention is applied to automobile parts, further weight reduction of the automobile parts can be realized. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, the hot-rolled steel sheet according to this embodiment will be described. <Chemical composition of hot-rolled steel sheets> The chemical composition of the hot-rolled steel sheet is, in mass%, C: 0.030% to 0.100%, Si: 1.5% or less, Mn: 1.3% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% to 0.080% or less, N: 0.0060% or less, and Nb: 0.03% to 0.20% or less. Each component is explained below. In the following explanation, "%" representing the content of a component means "% by mass."

[0018] C: 0.030% or more and 0.100% or less By combining with Ti, C contributes to increasing the strength of steel sheets and forming a highly dislocated structure during isothermal transformation. To obtain steel sheets with a yield strength of 500 MPa or more, the C content is set to 0.030% or more. On the other hand, if the C content exceeds 0.100%, coarse cementite precipitates, increasing the risk of reduced bending workability and toughness. Therefore, the C content is set to 0.030% or more and 0.100% or less. Preferably, it is set to 0.035% or more and 0.090% or less.

[0019] Si: 1.5% or less Si is an effective element for improving workability by increasing the elongation of steel sheets and suppressing cementite precipitation. On the other hand, if the Si content exceeds 1.5%, the effect of improving bending workability is reduced, and surface properties and weldability deteriorate, increasing the adverse effects of adding a large amount of Si. Therefore, the Si content is set to 1.5% or less. Preferably, the Si content is set to 1.2% or less. Note that even if the Si content is 0%, the effect of this embodiment is not impaired, but in order to stably generate a structure without a lath structure and with large crystal strain, the Si content is preferably set to 0.15% or more.

[0020] Mn: 1.3% or less Mn improves hardenability and suppresses the formation of ferrite, which has small crystal strain, during the cooling process after hot rolling. In order to stably produce hot-rolled steel sheets, the Mn content is preferably 0.2% or more. Furthermore, in order for austenite to recrystallize during hot rolling, it is preferable for hot working strain to be present, and it is effective to control the contents of Si and Mn, which are substitutional solid solution elements, within a narrow range. To achieve this, it is preferable to satisfy the following formula (1): 0.9≦0.8[%Si]+[%Mn]≦2.7···(1) Here, [%Si] and [%Mn] are the Si content and Mn content in mass %. On the other hand, if the Mn content exceeds 1.3%, the driving force for the transformation from austenite to ferrite is excessively reduced, and a structure with small crystal strain cannot be obtained. Therefore, the Mn content is set to 1.3% or less. Preferably, the Mn content is 1.2% or less.

[0021] P:0.05% or less P is a harmful element that segregates at grain boundaries and reduces toughness, so its content should be minimized to 0.05% or less. The P content is preferably 0.04% or less, but when used in environments requiring even higher toughness, it is more preferable to keep it 0.02% or less. However, 0.002% of P may be unavoidably mixed in during manufacturing.

[0022] S: 0.010% or less S forms coarse sulfides in steel, which elongate and become wedge-shaped inclusions during hot rolling, adversely affecting toughness. Therefore, since S is also a harmful element, its content must be reduced as much as possible, with the S content set to 0.010% or less. Preferably, the S content is 0.003% or less, but for use in environments requiring even stricter toughness, it is more preferable to set it to 0.001% or less. During manufacturing, 0.0001% of S may be unavoidably mixed in.

[0023] Al: 0.005% or more and 0.080% or less When Al is added as a deoxidizer during steelmaking, the Al content is 0.005% or more. Al forms oxides, which reduces bending workability and toughness. Therefore, the Al content is set to 0.080% or less. Preferably, the Al content is 0.010% or more and 0.070% or less.

[0024] N: 0.0060% or less N is a harmful element that combines with Nb to form coarse Nb-containing nitrides, thereby reducing strength, bending workability, and toughness. Therefore, the N content should be reduced as much as possible to 0.0060% or less. Preferably, the N content is 0.0050% or less. During manufacturing, 0.0005% N may be unavoidably mixed in.

[0025] Nb: 0.03% or more and 0.20% or less Nb combines with C and contributes to increasing the strength of steel sheets. To obtain a yield strength of 500 MPa or more, the Nb content is 0.03% or more. On the other hand, if the Nb content exceeds 0.20%, coarse Nb-containing carbonitrides cannot be dissolved in the heating process before hot rolling, and not only does the effect of increasing strength saturate, but it also has a negative effect on bending workability and toughness. Therefore, the Nb content is set to 0.03% or more and 0.20% or less. Preferably, the Nb content is 0.035% or more and 0.20% or less.

[0026] As mentioned above, C contributes to the formation of a structure with large crystal strain, while also being used to form carbides containing Nb by bonding with Nb. Therefore, in order to stably obtain the desired metal structure in the hot-rolled steel sheet according to this embodiment, it is preferable to satisfy the following formula (2). In particular, if the value of formula (2) is less than 1.5, the concentration of C accumulating at grain boundaries during isothermal transformation decreases, making it impossible to stably obtain a structure with large crystal strain. Therefore, it is preferable that the value of formula (2) is 1.5 or more. On the other hand, to obtain a steel sheet with a yield strength of 500 MPa or more, it is necessary to strengthen it with nano-sized carbides containing Nb. However, if the value of formula (2) exceeds 3.5, the coarse NbC cannot be dissolved during slab reheating, resulting in a decrease in the strength and bending workability of the steel sheet. Therefore, it is preferable to set formula (2) to 3.5 or less. 1.5≦([%C] / 12) / ([%Nb] / 93)≦3.5···(2) Here, [%C] and [%Nb] are the C content and Nb content in mass %.

[0027] The above is the basic composition of the hot-rolled steel sheet according to this embodiment, but optionally, at least one element from Groups A to C below may be further contained. Group A; Ti: 0.010% or more and 0.15% or less Group B; B: 0.0002% or more and 0.0050% or less Group C: V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W, Ta, Pb, Cs, Zr, Hf, Te, Bi, and Se, total of 1% or less

[0028] Ti: 0.010% or more and 0.150% or less Ti combines with C to form fine Ti-containing carbides, thereby contributing to the strength of steel sheets. To obtain a yield strength of 500 MPa or more, the Ti content is 0.010% or more. On the other hand, if the Ti content exceeds 0.150%, the coarse Ti-containing carbides cannot be dissolved in the heating process before hot rolling, and not only does the effect of increasing strength saturate, but bending workability and toughness are adversely affected. Therefore, the Ti content is set to a range of 0.010% or more and 0.150% or less. Preferably, the Ti content is 0.02% or more and 0.14% or less.

[0029] Considering the need to increase the strength of automotive components, there are cases where it is preferable to obtain a yield strength of 500 MPa or more. In such cases, the combined addition of Nb and Ti is effective, and by satisfying the following formula (3), a hot-rolled steel sheet with a yield strength of 500 MPa or more can be obtained. 0.10≦([%Nb] / 2)+[%Ti*]···(3) Here, [%Ti*]=[%Ti]-48[%N] / 14, and [%Nb], [%Ti], and [%N] are the Nb content, Ti content, and N content in mass%. When Nb and Ti are added in combination, not only Nb but also Ti combines with C, and C is utilized. Therefore, it is preferable to satisfy the following formula (4) instead of formula (2). 1.5≦([%C] / 12) / ([%Nb] / 93+[%Ti*] / 48)≦3.5 ···(4)

[0030] B: 0.0002% or more and 0.0050% or less B is an element effective in improving hardenability, and ensuring hardenability is necessary to obtain a structure with large crystal strain. By setting the B content to 0.0002% or more, it is possible to stably obtain a desired structure. On the other hand, if the B content exceeds 0.0050%, the effect on the hardenability of the steel saturates, so the B content is set to 0.0050% or less. More preferably, the B content is 0.0004% or more and 0.0030% or less.

[0031] V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W, Ta, Pb, Cs, Zr, Hf, Te, Bi, and Se, total of 1% or less If the total content of any one or more elements is 1% or less, there is little effect on the properties of the hot-rolled steel sheet according to this embodiment. On the other hand, the content of each element is preferably limited to 0.03% or less.

[0032] The chemical composition of the hot-rolled steel sheet according to this embodiment contains the above elements, with the balance being Fe and unavoidable impurities.

[0033] <Metal structure of hot-rolled steel sheet> Next, the metal structure of the hot-rolled steel sheet will be described. The metal structure of the hot-rolled steel sheet according to this embodiment has an area ratio of ferrite of 0% or more and 85% or less, an area ratio of retained austenite of 3% or less, an area ratio of a structure having a lath morphology of 5% or less, an area ratio of a structure with a KAM value of 1.0 or more of 15% or more, and has carbides containing Nb with an average particle size of 8 nm or less. In the following description, "%" representing the metal structure means "area ratio."

[0034] Ferrite: 0% to 85% Ferrite is a structure with inferior toughness because the fracture surface unit at the time of brittle fracture is larger than that of the new structure with large crystal strain in the hot-rolled steel sheet according to this embodiment. To obtain the desired toughness, the area fraction of ferrite needs to be limited to 85% or less. Preferably, the area fraction of ferrite is 80% or less, and more preferably, 70% or less.

[0035] Retained austenite is 3% or less (including 0%) The bainite and tempered martensite defined in this embodiment are those in which a lath structure is observed within the grains. Martensite is a structure observed as a white contrast on an SEM, but since it may be cementite, it can be separated by crystal structure using electron backscatter diffraction (EBSD) analysis. For example, whether bainite, martensite, and tempered martensite satisfy the Kurdjumov-Sachs relationship with the parent phase can be determined by obtaining a (001)α pole figure of a single prior γ grain region. Retained austenite can be determined by XRD analysis of a sample that has been ground from the surface to one-quarter of the sheet thickness and then chemically polished to 0.1 mm or more. In the hot-rolled steel sheet of this embodiment, these structures reduce strength, workability, and toughness. It is preferable to reduce these structures as much as possible, and retained austenite is set to 3% or less. Preferably, the total of bainite, martensite, tempered martensite, martensite and retained austenite is 5% or less, and more preferably 3% or less.

[0036] The ratio of lath-like structure is 5% or less, and the ratio of structure with a KAM value of 1.0 or more is 15% or more. The greatest technical feature of this embodiment is that a structure with large crystal strain and no lath structure is strengthened with Nb-containing carbides with an average particle size of 8 nm or less. Ferrite has small crystal strain, i.e., a KAM value of less than 1. Low-temperature transformation phases such as bainite, martensite, and tempered martensite have a lath structure. Therefore, a structure with large crystal strain and no lath structure cannot be classified as ferrite or bainite. Lath is a structure observed as a plate-like morphology within grains using a transmission electron microscope (TEM) or EBSD analysis. A structure with this lath structure is hard, but has poor workability and does not achieve the desired bendability. A structure with large crystal strain in the present invention is one with a KAM value of 1.0 or more as determined by EBSD analysis. The KAM value indicates a disorder in the crystal structure, and this crystal disorder refines the effective fracture surface unit, achieving greater toughness than ferritic steel. From the above, this structure enables the production of steel sheets with excellent workability and toughness. Therefore, a structure without a lath structure has an area ratio of lath-form structure of 5% or less, and a structure with large crystal strain has a structure with a KAM value of 1.0 or more of 15% or more, more preferably a structure with a KAM value of 1.0 or more of 20% or more.

[0037] Carbide containing Nb with an average particle size of 8 nm or less In this embodiment, the steel sheet is strengthened by carbides containing Nb. To obtain a high-strength hot-rolled steel sheet having a yield strength of 500 MPa or more, the average particle size of the carbides containing Nb dispersed in the steel must be 8 nm or less. To stably obtain a strength having a yield strength of 500 MPa or more, it is preferable that the average particle size of the carbides containing Nb be 5 nm or less. The carbides containing Nb may be composite carbides with Ti. Furthermore, in the production of the hot-rolled steel sheet of this embodiment, when the coiling temperature is set to 600°C or higher, Nb, even though it is a substitutional element, diffuses sufficiently in the steel. By utilizing this property of Nb to diffuse and precipitate Nb in the steel, a steel sheet having a yield strength of 500 MPa or higher can be obtained even with a small amount of bainite, martensite, and tempered martensite structures, which are often used in high-strength steel sheets. To obtain a steel sheet having a yield strength of 500 MPa or higher, 80% or more of the Nb contained is utilized for precipitation. Preferably, 85% or more of the Nb contained is utilized for precipitation.

[0038] The hot-rolled steel sheet according to this embodiment preferably has a plating layer on its surface. Even if the plating layer is formed, the function of the hot-rolled steel sheet is not impaired. The composition of the plating layer is preferably one or more selected from Zn, Si, Al, Ni, and Mg. The plated steel sheet in this embodiment includes any of those that have been subjected to hot-dip galvanizing treatment (GI), those that have been subjected to hot-dip galvanizing treatment followed by alloying treatment (GA), and those that have been subjected to electrogalvanizing treatment (EG).

[0039] Next, a first embodiment of the method for manufacturing a hot-rolled steel sheet according to this embodiment will be described. <Hot-rolled steel sheet manufacturing method> Generally, hot-rolled steel sheets are manufactured by casting a slab (steel material), then loading the slab (steel material) into a heating furnace after its temperature has been reduced to 1000°C or below, where it is heated for a short period of time, and then reducing its thickness to a predetermined thickness on a hot rolling line before being wound into a coil. Alternatively, the slab (steel material) is cast, then cooled to room temperature, and then heated for a long period of time in a heating furnace, before being reduced to a predetermined thickness on a hot rolling line before being wound into a coil. Another manufacturing method involves directly sending the cast slab (steel material) to a hot rolling line without heating it in a heating furnace, where it is reduced to a predetermined thickness and wound into a coil. The method for manufacturing a hot-rolled steel sheet according to this embodiment can be applied not only to a process in which a steel material is heated after casting, but also to a process in which a steel material is directly sent to a hot rolling line without being heated after casting.

[0040] <First form steel material> The melting method for producing the steel material of this embodiment is not particularly limited, and known melting methods such as converters and electric furnaces can be used. Secondary refining may also be performed in a vacuum degassing furnace. Taking productivity and quality into consideration, the molten steel thus adjusted to the above-described composition is then preferably formed into a slab (steel material) by a continuous casting method. Alternatively, the slab may be formed by an ingot casting-blooming rolling method or other known casting methods.

[0041] <First type of rough rolling process> In this embodiment, the steel material is heated to a heating temperature of 1200° C. or higher, or is not heated, and is then roughly rolled to form a sheet bar. <First type of finish rolling process> Next, hot rolling is performed to obtain a hot-rolled steel sheet, with a finish rolling starting temperature of 950°C or higher, a total reduction rate from the first to fifth passes of 75% or higher, and a finish rolling temperature of 860°C to 910°C. <Cooling step of the first embodiment> Next, the hot-rolled hot-rolled steel sheet is cooled to a cooling stop temperature of 600°C or higher and 700°C or lower at an average cooling rate of 40°C / s or higher. <Winding process of the first embodiment> Thereafter, the cooled hot-rolled steel sheet is coiled at a coiling temperature of 600°C or higher and 700°C or lower.

[0042] Heating of steel material: Heating above 1200℃ or not heating Coarse carbides containing Nb precipitated in a slab (steel material) are dissolved in a heating process before hot rolling, resulting in the precipitation of fine carbides containing Nb after hot rolling. Therefore, to obtain Ti-containing carbides with an average particle size of 8 nm or less, the slab (steel material) is heated to 1200°C or higher. The temperature is preferably 1220°C or higher, and when the Nb content is 0.12% or higher, it is more preferable to heat the slab (steel material) to 1240°C or higher. While there is no particular upper limit, 1300°C is a manufacturing constraint for heating the slab (steel material) to avoid thermal damage in the heating furnace. When the steel material is directly sent to the hot rolling line after being kept at 1200°C or higher after casting, the steel material is not heated after casting.

[0043] Finish rolling start temperature: 950°C or higher, total reduction rate from the first to fifth passes: 75% or higher, finish rolling completion temperature: 860°C or higher, 910°C or lower To produce the large crystal strain structure that is characteristic of the hot-rolled steel sheet according to this embodiment, it is necessary to precisely control the hot-rolling conditions. Specifically, fine austenite is formed by the recrystallization of austenite during finish rolling. To achieve this, the starting temperature of finish rolling is set to 950°C or higher, and the total reduction rate from the first to fifth passes is set to 75% or higher. If the chemical composition of the hot-rolled steel sheet according to this embodiment is within the range, austenite recrystallization occurs in the finish rolling stand after the fifth pass. Therefore, finish rolling is performed in five passes or more. If the finish rolling start temperature is below 950°C, austenite recrystallizes early in the finish rolling, and the recrystallized austenite is rolled again. This results in the formation of ferrite, and a structure with large crystal strain cannot be obtained. If the finish rolling start temperature exceeds 1100°C, there is a high possibility that austenite recrystallization will not occur during finish rolling, so the finish rolling start temperature is preferably 1100°C or lower.

[0044] If the finish rolling completion temperature is below 860°C, there is a high risk of ferrite being generated during rolling. On the other hand, if the finish rolling completion temperature is above 910°C, austenite cannot be recrystallized during finish rolling. Therefore, the finish rolling completion temperature is set to 860°C or higher and 910°C or lower. To stably obtain austenite recrystallization, it is preferable that the finish rolling completion temperature be 890°C or lower.

[0045] As mentioned above, to recrystallize austenite by finish rolling, it is necessary to accumulate strain from the first to fifth passes of finish rolling. Therefore, if the rolling interval from the first to fifth passes is long, the strain imparted by rolling will recover, and austenite will not be recrystallized stably by finish rolling. Therefore, from the viewpoint of avoiding the adverse effects of this austenite recovery, it is preferable to set the rolling interval time from the first to fifth passes to at least 1.5 seconds or less.

[0046] Cooling stop temperature after finish rolling is 600°C to 700°C, with an average cooling rate of 40°C / s or more If the cooling rate to 700°C or below after hot rolling is slow, polygonal ferrite (ferrite), which is coarse and has small crystal strain within the grains, will form at high temperatures. To suppress the formation of this ferrite, the material is cooled at an average cooling rate of 40°C / s or more after hot rolling. Preferably, the material is cooled to 700°C or below at an average cooling rate of 50°C / s or more within 2 seconds after hot rolling. On the other hand, if the cooling stop temperature is below 600°C, it becomes difficult to obtain carbides containing Nb, and a steel sheet having a yield strength of 500 MPa or more cannot be obtained. Therefore, the cooling stop temperature is set to 600°C or higher and 700°C or lower. Preferably, the cooling stop temperature is 610°C or higher and 690°C or lower. Here, the average cooling rate can be calculated by {(cooling start temperature) - (cooling completion temperature)} / (forced cooling time other than natural cooling) after hot rolling, in which forced cooling other than natural cooling is performed. An example of a forced cooling method is water cooling.

[0047] Winding temperature: 600℃ or more and 700℃ or less For the same reason as the cooling stop temperature, the coiling temperature is set to 600° C. or higher and 700° C. or lower, and preferably 610° C. or higher and 690° C. or lower. Coiling within this temperature range can suppress the generation of ferrite, bainite, martensite, and retained austenite.

[0048] Next, a second embodiment of the method for manufacturing a hot-rolled steel sheet according to the present embodiment will be described, and the differences from the first embodiment will be described. <Second type casting process> The hot rolled steel sheet according to this embodiment can also be produced by the thin slab continuous casting method. When produced by the thin slab continuous casting method, a steel material having a thickness of 35 mm or more and 200 mm or less is cast. <Second type of rough rolling process> The cast steel material is heated to a heating temperature of 1200°C or higher, or is not heated, and is then roughly rolled as necessary to form a sheet bar. The process from the finish rolling step onwards is the same as in the first embodiment.

[0049] Here, we will explain the slab (steel material) thickness that is specific to the thin slab continuous casting method.

[0050] Slab thickness (steel material): 35mm to 200mm Unlike continuous casting, thin slab casting produces a thin slab before hot rolling, resulting in a low degree of austenite reduction during hot rolling. If the slab thickness is less than 35 mm, a total reduction rate of 75% or more from the first to fifth passes cannot be achieved. On the other hand, if the slab thickness exceeds 200 mm, the casting speed slows down, and the productivity advantage of thin slab casting compared to continuous casting is lost. Therefore, the slab thickness in thin slab casting is between 35 mm and 200 mm.

[0051] Next, a third embodiment of the method for producing a hot-rolled steel sheet according to the present embodiment will be described. In this embodiment, the differences from the first and second embodiments will be described. In the third embodiment, a continuous hot rolling technique can be applied. <Third type of joining process> The sheet bar obtained by the first or second method is joined to the preceding sheet bar at 1050°C or higher before finish rolling. If the temperature is lower than 1050°C, it becomes difficult to roll at the finish rolling start temperature of 950°C or higher. The preferred heating temperature for the sheet bar during joining is 1070°C or higher. The cooling step and subsequent steps are the same as in the first embodiment.

[0052] The method for producing a hot-rolled steel sheet according to this embodiment can employ an annealing step in which the steel sheet is annealed in a continuous annealing line at an annealing temperature of 720°C or less, and a plating step in which the steel sheet is plated in a continuous plating line. Furthermore, the method may include an alloying step in which the plated hot-rolled steel sheet is heated to 480°C or more and 600°C or less and subjected to an alloying treatment. This annealing treatment or this plating treatment does not affect the properties of the hot-rolled steel sheet according to this embodiment. Therefore, it is possible to further plate the surface of the hot-rolled steel sheet to form a plating layer on the steel sheet surface.

[0053] As described above, the coating process and the composition of the coating bath do not affect the properties of the hot-rolled steel sheet according to this embodiment, and therefore any of hot-dip galvanizing, galvannealing, and electrogalvanizing processes can be used as the coating process. The coating bath may contain one or more of Zn, Al, Mg, Si, and Ni. That is, the coating layer formed on the surface of the hot-rolled steel sheet in the coating process may contain one or more of Zn, Al, Mg, Si, and Ni. [Example]

[0054] The embodiments of the present invention will be further explained by way of examples. Note that the present invention is not limited to the manufacturing conditions and product performance shown in the following examples. The embodiments can achieve the desired performance within the scope of the present invention.

[0055] <First form using continuous casting method> Steel materials having a thickness of 250 mm and having the chemical compositions shown in Tables 1-1 and 1-2 were hot rolled under the rough rolling and finish rolling conditions shown in Table 2, and then temper rolled at an elongation rate of 0.1 to 0.5% and pickled to produce steel sheets for evaluation.

[0056] <Second method using thin slab continuous casting> Steel having the chemical composition shown in Table 1-1 was hot rolled into thin slabs under the conditions shown in Table 3, and the slabs were then temper rolled to an elongation of 0.1 to 0.5% and pickled to produce steel sheets for evaluation.

[0057] <Third form using hot continuous rolling method> Steels having the chemical compositions shown in Table 1-1 were joined into sheet bars under the conditions shown in Table 4, and the joined sheet bars were hot rolled, temper rolled to an elongation rate of 0.1 to 0.5%, and pickled to produce steel sheets for evaluation.

[0058] <Manufacturing method for applying a plating layer to a hot-rolled steel sheet> The hot-rolled coils produced under the conditions shown in Table 2 were pickled, and then the hot-rolled steel sheets were subjected to a coating treatment in a continuous hot-dip galvanizing line (CGL) under the conditions shown in Table 5. In this way, continuous hot-dip galvanized steel sheets (GI) and alloyed hot-dip galvanized steel sheets (GA) were produced.

[0059] [Table 1-1]

[0060] [Table 1-2]

[0061] [Table 2]

[0062] [Table 3]

[0063] [Table 4]

[0064] [Table 5]

[0065] [Table 6]

[0066] The hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5 were evaluated in terms of metal structure, tensile properties, bending workability, and toughness by the following methods. The results are shown in Table 6.

[0067] (i) Area ratio of metal structure Test specimens were cut from the hot-rolled steel sheet so that the cross section parallel to the rolling direction served as the observation surface. The center of the sheet thickness was corroded with 1% nital to reveal the structure, and 10 fields of view were photographed at 1 / 4 of the sheet thickness using a scanning electron microscope (SEM) at 2000x magnification and an accelerating voltage of 15 kV. Ferrite is a crystal grain that does not show any corrosion marks within the grains and is observed with a lower brightness than martensite (gray in SEM). Bainite and tempered martensite are crystal grains in which three or more adjacent lath-shaped corrosion marks with a width of 500 nm or less are observed within the grains. Martensite is a crystal grain that does not show any corrosion marks within the grains, but is observed with a higher brightness than ferrite (white in SEM). The area ratio of the metallographic structure was determined using image analysis software (Photoshop Elements and Image J) for the structures separated in this manner. The retained austenite was measured by grinding the surface of the test piece to 3 / 4 of its total thickness, chemically polishing it to 0.1 mm or more, and then measuring the polished surface using X-ray diffraction. The volume fraction of retained austenite was measured using MoKα radiation as the incident radiation source, from the peaks of (200)α, (211)α, (220)α, (200)γ, (220)γ, and (311)γ. The volume fraction of the retained austenite phase obtained in this way was taken as the area fraction of retained austenite.

[0068] The area fraction of highly distorted structures without lath structure was determined using SEM and EBSD. Before observation, the specimens were marked using a Vickers tester or similar instrument to ensure the same field of view for both SEM and EBSD. When observed with SEM, highly distorted structures without lath structure exhibit intragranular corrosion marks. Depending on the shape of the corrosion marks, some may not actually be lath but may appear to be lath. To distinguish between lath-like structures and actual lath structures, rectangular structures within grains with a width of more than 500 nm on the short side and two or fewer adjacent grains were not considered to be lath structures. A structure with a width of less than 500 nm on the short side and three or more adjacent grains was considered to have a lath structure, as observed in bainite or tempered martensite. This lath structure can be more clearly distinguished by observation with a transmission electron microscope (TEM). EBSD analysis was then performed using OIM Analysis software (TSL). By EBSD analysis, the structure within the grain surrounded by the high-angle grain boundary with an angle difference of 15° or more, in which the required KAM value exceeds 1.0 and which does not have a lath structure, is defined as a structure with large crystal distortion without a lath structure, and is classified as 1mm 2 The area ratio was calculated for the above fields of view. The KAM value analysis was performed under the condition of 1st nearest neighbor.

[0069] (ii) Average particle size of Nb-containing carbides A thin film for observation was taken from a location equivalent to 1 / 4 of the plate thickness of the hot-rolled steel plate, and more than 300 Nb-containing carbides were photographed using a transmission electron microscope at a magnification of 600,000 times or more. The circle-equivalent diameters of the photographed Nb-containing carbides were determined, and the average value was used as the average particle size. Nb-containing carbides can be identified by checking for the presence or absence of peaks derived from Nb using EDX, which is attached to the TEM.

[0070] (iii) Analysis of the amount of carbide precipitates containing Nb The front and back surfaces of the test piece were each ground by 25% of the plate thickness, then dissolved in a 10% AA electrolytic solution, the solution was filtered through a filter with a mesh size of 0.2 μm, and the Nb concentration in the filtered electrolytic solution was analyzed using ICP-MS. The amount of precipitated Nb-containing carbides was determined by subtracting the Nb concentration in the electrolytic solution from the amount of Nb contained.

[0071] (iv) Tensile test JIS No. 5 tensile test pieces were prepared perpendicular to the rolling direction from the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5. Tensile tests were performed five times in accordance with the provisions of JIS Z2241 (2011) to determine the average yield strength (YS) and tensile strength (TS). The crosshead speed of the tensile test was 10 mm / min. In Table 6, samples with a yield strength of 500 MPa or more were considered to be inventive examples.

[0072] (v) Bending test Test pieces with a width of 35 mm and a length of 100 mm were taken from the hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5, and the end faces were ground to perform five bending tests using the V-block method described in JIS Z 2248. For test pieces with a yield strength of 500 MPa or more but less than 680 MPa, an R / t of 0.1 or less, and for test pieces with a yield strength of 680 MPa or more, an R / t of 0.5 or less are properties required in the present invention, and the results in Table 6 are marked with "◯". Test pieces in which cracks were observed on the surface of the test piece at least once under the above conditions out of the five tests were marked with "X" as they do not have the properties required in the present invention.

[0073] (vi) Charpy impact test V-notch test pieces as specified in JIS Z 2242 were taken from hot-rolled steel sheets obtained under the conditions shown in Tables 2 to 5, with the longitudinal direction normal to the rolling direction. When the thickness of the hot-rolled steel sheet was less than 10 mm, multiple test pieces were stacked, holes were drilled at the ends of the test pieces, and the pieces were connected with bolts to adjust the thickness to 10±1 mm. Test pieces with a yield strength of 500 MPa or more and less than 680 MPa were immersed in a bath adjusted to -80°C for 10 minutes or more, and then tested according to the method in accordance with JIS Z 2242. The test results are shown in Table 6.2 The above are the characteristics required in this invention, and are marked with "Good" at 30 J / cm 2 Test specimens with a yield strength of 680 MPa or more were immersed in a bath adjusted to -40°C for 10 minutes or more, and then tested according to the method in accordance with JIS Z2242. 2 The above are the characteristics required in this invention, and are marked with "Good" at 30 J / cm 2 Any level below this is marked with "x" as it is not a characteristic required by the present invention.

[0074] All of the inventive examples had a yield strength (YS) of 500 MPa or more, and exhibited good bending workability and toughness. On the other hand, the comparative examples outside the range of the present invention either did not reach a yield strength of 500 MPa or did not exhibit the bending workability or toughness required by the present invention.

Claims

1. In mass%, C: 0.030% or more and 0.100% or less, Si: 1.5% or less, Mn: 1.3% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% or more and 0.080% or less, N: 0.0060% or less, Nb: 0.03% or more and 0.20% or less; Optionally, it further contains at least one component from Groups A to C below: Note Group A; Ti: 0.010% or more and 0.150% or less, Group B; B: 0.0002% or more and 0.0050% or less, Group C: one or more of V, Mo, Sb, REM, Mg, Ca, Sn, Ni, Cu, Co, As, Cr, W, Ta, Pb, Cs, Zr, Hf, Te, Bi, and Se in a total content of 1% or less; The balance has a composition consisting of Fe and unavoidable impurities, The area ratio of the metal structure is Ferrite is 0% or more and 85% or less, Retained austenite is 3% or less, Lath-type structure is 5% or less, The KAM value is 1.0 or more in 15% or more of the tissue, A hot-rolled steel sheet having a yield strength of 500 MPa or more and containing carbides containing Nb with an average particle size of 8 nm or less.

2. The hot-rolled steel sheet according to claim 1, further comprising a plating layer on the surface of the hot-rolled steel sheet.

3. A rough rolling step in which a steel material having the component composition according to claim 1 is roughly rolled into a sheet bar, with or without heating, at a heating temperature of 1200°C or higher; a finish rolling step of finish rolling the sheet bar at a rolling start temperature of 950°C or higher, a total rolling reduction of 75% or higher from the first to fifth passes, and a rolling completion temperature of 860°C or higher and 910°C or lower to obtain a hot-rolled steel sheet; a cooling step of cooling the hot-rolled steel sheet to a cooling stop temperature of 600°C or higher and 700°C or lower at an average cooling rate of 40°C / s or higher; a coiling step of coiling the cooled hot-rolled steel sheet at a coiling temperature of 600°C or higher and 700°C or lower; Including, The metal structure has an area ratio of ferrite of 0% or more and 85% or less, retained austenite of 3% or less, a lath structure of 5% or less, and a structure with a KAM value of 1.0 or more of 15% or more, A method for producing a hot-rolled steel sheet, characterized in that the hot-rolled steel sheet has carbides containing Nb having an average particle size of 8 nm or less.

4. A casting process is included in which a steel material having a thickness of 35 mm or more and 200 mm or less and having the component composition according to claim 1 is cast before the rough rolling process or the finish rolling process, The method for manufacturing a hot-rolled steel sheet according to claim 3, wherein the sheet bar is produced by applying or not applying the rough rolling step.

5. A joining step of joining the rough-rolled sheet bar and a preceding sheet bar at 1050°C or higher is included between the rough rolling step and the finish rolling step, The method for producing a hot-rolled steel sheet according to claim 3, wherein the joined sheet bar is finish-rolled in the finish rolling step.

6. Furthermore, a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet at an annealing temperature of 720 ° C. or less; a plating step of plating the annealed hot-rolled steel sheet; The method for manufacturing a hot-rolled steel sheet according to any one of claims 3 to 5, comprising:

7. The method for manufacturing a hot-rolled steel sheet according to claim 6, further comprising an alloying step of subjecting the plated hot-rolled steel sheet to an alloying treatment at a temperature of 480°C or higher and 600°C or lower.

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