Hot-rolled steel sheet and method for producing same

Abstract

The present invention provides: a hot-rolled steel sheet which has a yield strength of 500 MPa or more, excellent ductility, and excellent shear properties; and a method for producing the hot-rolled steel sheet. The present invention specifically provides a hot-rolled steel sheet which has C, Si, Mn, P, S, Al, N, B, Ti, and an optional component composition. A metal structure of this hot-rolled steel sheet has a value of (the area ratio having KAM value of 1.0 to 4.0 inclusive) / (the area ratio having KAM value of less than 1.0) of 0.05 or more, a variation coefficient of crystal grain size of 0.55 or more, an average particle diameter of carbides that contain Ti of 10 nm or less, a yield strength of 500 MPa or more, and a uniform elongation of 10% or more. The present invention also specifically provides a method for producing the hot-rolled steel sheet, the method including: a rough rolling step in which a steel material having the above-described component composition is subjected to rough rolling after heating or without heating after casting, and the rough rolling completion temperature is 1,000°C to 1,100°C inclusive; a finish rolling step in which the rolling start temperature is 950°C or higher, and the total draft of the first pass and the second pass is 70% or less; a cooling step; a winding step; and a post-winding coil cooling step.

Description

Hot-rolled steel sheet and manufacturing method thereof

[0001] The present invention relates to a hot-rolled steel sheet having a yield strength of 500 MPa or more and excellent ductility and shear property, and to a method for producing the same.The hot-rolled steel sheet of the present invention is suitable for use as a material for automobile parts and for a wide range of applications other than automobiles.

[0002] In recent years, from the perspective of global environmental conservation, CO ２ There is a trend toward increasing the strength of hot-rolled steel sheets in order to reduce CO2 emissions. However, formability often becomes an issue when increasing the strength of hot-rolled steel sheets. For example, automotive suspension parts are subjected to stretch forming, which requires ductility, and stretch flange processing, which processes sheared edges. Therefore, hot-rolled steel sheets are required to have both ductility and shear strength. Generally, as the strength of steel sheets increases, formability and shear strength tend to deteriorate, so improvements in formability and shear strength are essential to further expand the use of high-strength hot-rolled steel sheets. Furthermore, hot-rolled steel sheets with good galvanizability are also desired from the perspective of extending the life of components and improving their appearance.

[0003] In order to solve these problems, various techniques have been proposed to date for improving the formability of hot-rolled steel sheets.

[0004] For example, in Patent Document 1, the total volume fraction of the ferrite phase and the bainite phase, which have a small difference in hardness, is set to 95% or more, the volume fraction of the ferrite phase is set to 50 to 90%, and a predetermined amount of Ti-containing precipitates having a size of less than 20 nm is precipitated, which is said to result in a hot-rolled steel sheet having a tensile strength of 780 MPa or more and excellent stretch flangeability.

[0005] In addition, in Patent Document 2, Ti carbide 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 and precipitated in a metal structure in which 95% or more of the area ratio is made up of ferrite crystal grains, and it is said that this results in a hot-rolled steel sheet with excellent bending workability and a tensile strength of 780 MPa or more and 900 MPa or less.

[0006] Patent Document 3 discloses a steel sheet having a chemical composition in which the mass ratio of the Ti content to the C content, Ti / C, is 0.625 to 3.000, and the dislocation density is 1×10 １４ ~1 x 10 １６m －２ TiC precipitates having an average diameter of 2.0 nm or less are precipitated within the crystal grains at a predetermined density or more. This results in a hot-rolled steel sheet with small punched edge damage and a tensile strength of 780 MPa or more.

[0007] JP 2011-068945 A International Publication No. 2013 / 099196 JP 2017-179539 A

[0008] However, the conventional techniques relating to the hot-rolled steel sheets disclosed in the above patent documents have the following problems.

[0009] The technology proposed in Patent Document 1 utilizes a bainite structure obtained by low-temperature coiling, and therefore does not provide sufficient uniform elongation. Furthermore, many of the steels disclosed in Patent Document 1 are Si-added steels, which reduces galvanizability, and therefore there is also the problem that hot-rolled steel sheets with good galvanizability cannot be obtained. Even in steels with a low Si content, the metal structure and properties that simultaneously achieve yield strength, excellent ductility, and shear strength cannot be obtained because the composition and manufacturing method are inappropriate.

[0010] In the technique proposed in Patent Document 2, the ferrite area ratio is high, and in such a metal structure, the distribution of crystal grain size is narrow, resulting in poor shear strength.

[0011] The technology proposed in Patent Document 3 contains a large amount of dislocations, so that the desired yield strength and uniform elongation cannot be obtained.

[0012] Furthermore, the manufacturing methods described in Patent Documents 1 and 3 require complex control over runout before the hot-rolled steel sheet is coiled, which causes a problem of poor manufacturing stability.

[0013] The present invention has been developed in view of the above-mentioned problems of the conventional technology, and aims to provide a high-strength hot-rolled steel sheet having a yield strength of 500 MPa or more and excellent ductility and shear property, and a manufacturing method thereof.

[0014] In order to solve the above problems, the inventors have conducted extensive research into the requirements for a hot-rolled steel sheet to have both ductility and shear strength. The thickness of the hot-rolled steel sheet targeted by the present invention is in the range of 1.0 mm to 3.6 mm. To obtain good ductility in a hot-rolled steel sheet, it is effective to impart high uniform elongation. The property of high uniform elongation in a hot-rolled steel sheet is easily obtained in a metal structure that is coiled at 600°C or higher, at which dislocation recovery is facilitated.

[0015] In conventional hot rolling, the metal structure of steel sheets coiled at temperatures above 600°C is primarily ferrite. A metal structure primarily composed of ferrite results in a narrow grain distribution and crystal strain throughout the steel sheet. This results in unstable crack propagation during shearing of the steel sheet, making it difficult to achieve the desired shear strength. Furthermore, many prior art technologies utilize Si to harden ferrite, which is inherently soft. However, the inclusion of Si in steel sheets reduces galvanic properties.

[0016] Therefore, a study was conducted focusing on the transformation of the metal structure of a steel sheet under coiling conditions at 600°C or higher, using a chemical composition in which the Si content was reduced as much as possible. As a result, by setting the coiling temperature in hot rolling to 600°C or higher and 700°C or lower and controlling the rough rolling conditions, finish rolling conditions, and cooling conditions, a structure with a crystal grain size distribution and strain distribution not previously seen was obtained.

[0017] Shear strength is improved by dispersing a large number of crystal grains with a high KAM value, which stabilizes crack propagation during shear. Furthermore, crystal grains with a high KAM value have superior ductility compared to bainite or martensite, which have lath-shaped structures. Therefore, it is possible to provide steel sheets that combine excellent shear strength and ductility. The KAM value is an index that represents the plastic strain gradient in a microscopic region.

[0018] To broaden the grain size distribution, it is effective to partially recrystallize austenite during rough rolling in hot rolling. To achieve this, it is necessary to control the Ti content and rough rolling temperature, which change the recrystallization behavior of austenite. It has been found that to obtain a structure with a high KAM value, it is necessary to precipitate a large amount of fine TiC at the interface between austenite and ferrite during the transformation from austenite to ferrite, thereby changing the austenite-ferrite interface movement. To achieve this, it is necessary to contain a large amount of Ti and then suppress the amount of TiC precipitated within the austenite grains during finish rolling. In other words, it has been found that a desired steel sheet structure can be obtained by controlling the Ti content and the finish rolling temperature.

[0019] It was discovered that a metal structure with this grain size distribution and strain distribution promotes stable crack propagation when the steel sheet is sheared. It was found that hot-rolled steel sheets with this new metal structure have excellent shear properties, with a yield strength of 500 MPa or more and a uniform elongation of 10% or more.

[0020] The hot-rolled steel sheet according to the present invention, developed based on the above findings, is configured as follows: [1] Contains, by mass%, C: 0.02% or more and 0.12% or less, Si: less than 0.15%, Mn: more than 0.7% and 2.5% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% or more and 0.080% or less, N: 0.0080% or less, and Ti: 0.06% or more and 0.15% or less, and optionally contains at least one component selected from the following groups A to D, with the balance being Fe and and unavoidable impurities, and the metal structure has a ratio of the area ratio of the KAM value of 1.0 or more to 4.0 or less to the area ratio of the KAM value of less than 1.0 of 0.05 or more, a coefficient of variation of crystal grain size of 0.55 or more, an average particle size of Ti-containing carbides of 10 nm or less, a yield strength of 500 MPa or more, and a uniform elongation of 10% or more. Group A: at least one selected from V: 0% or more and 0.2% or less, Nb: 0% or more and 0.07% or less, Mo: 0% or more and 0.15% or less, Zr: 0% or more and 0.1% or less, Hf: 0% or more and 0.1% or less, and W: 0% or more and 0.1% or less; Group B: at least one selected from Cu: 0% or more and 1.0% or less, Ni: 0% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, and B: 0% or more and 0.010% or less; Group C: at least one selected from Ca: 0% or more and 0.01% or less, Mg: 0% or more and 0.01% or less, REM: 0% or more and 1.0% or less, and Co: 0% or more and 0.01% or less; and Group D: at least one selected from Sb: 0% or more and 0.01% or less, Sn: 0% or more and 0.01% or less, As: 0% or more and 0.01% or less, Ta: 0% or more and 0.01% or less, Pb: 0% or more and 0.01% or less, Cs: 0% or more and 0.01% or less, Te: 0% or more and 0.01% or less, Bi: 0% or more and 0.01% or less, Zn: 0% or more and 0.01% or less, Ge: 0% or more and 0.01% or less, and Sr: 0% or more and 0.01% or less. [2] In the above [1], the hot-rolled steel sheet has a plating layer on its surface.

[0021] 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 steel sheet containing, by mass%, C: 0.02% or more and 0.12% or less, Si: less than 0.15%, Mn: more than 0.7% and 2.5% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% or more and 0.080% or less, N: 0.0080% or less, and Ti: 0.06% or more and 0.15% or less, and optionally containing at least one component selected from the following groups A to D, with the balance being Fe and impurities: The method includes a rough rolling step of rough rolling a steel material having a component composition consisting of unavoidable impurities to form a sheet bar, a finish rolling step of finish rolling the sheet bar to form a hot rolled steel sheet, a cooling step of cooling the hot rolled steel sheet, a winding step of winding the hot rolled steel sheet to form a winding coil, and a coil cooling step of cooling the winding coil, wherein in the rough rolling step, the steel material is heated to 1150°C or higher. Alternatively, after casting, the steel material is held at 1150°C or higher and subjected to rough rolling, and the temperature of the sheet bar when rough rolling is completed is set to a range of 1000°C or higher and 1100°C or lower; in the finish rolling step, the temperature of the rolled material at the start of finish rolling is set to 950°C or higher, the total rolling reduction of the first and second passes is set to 70% or lower, the temperature of the rolled material on the finish rolling exit side is set to 850°C or higher, and the rolling speed on the finish rolling exit side is set to 500 m / min or higher; in the cooling step, the average cooling rate of the hot-rolled steel sheet is set to 40°C / s or higher to a cooling stop temperature in the range of 600°C or higher and 700°C or lower; in the coiling step, the coiling temperature of the hot-rolled steel sheet is set to a range of 600°C or higher and 700°C or lower; and in the coil cooling step, the average cooling rate of the coiled coil is set to 50°C or higher.Group A: at least one selected from V: 0% or more and 0.2% or less, Nb: 0% or more and 0.07% or less, Mo: 0% or more and 0.15% or less, Zr: 0% or more and 0.1% or less, Hf: 0% or more and 0.1% or less, and W: 0% or more and 0.1% or less; Group B: at least one selected from Cu: 0% or more and 1.0% or less, Ni: 0% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, and B: 0% or more and 0.010% or less; Group C: at least one selected from Ca: 0% or more and 0.01% or less, Mg: 0% or more and 0.01% or less, REM: 0% or more and 1.0% or less, and Co: 0% or more and 0.01% or less; and Group D: At least one selected from Sb: 0% or more and 0.01% or less, Sn: 0% or more and 0.01% or less, As: 0% or more and 0.01% or less, Ta: 0% or more and 0.01% or less, Pb: 0% or more and 0.01% or less, Cs: 0% or more and 0.01% or less, Te: 0% or more and 0.01% or less, Bi: 0% or more and 0.01% or less, Zn: 0% or more and 0.01% or less, Ge: 0% or more and 0.01% or less, and Sr: 0% or more and 0.01% or less. [4] The method for producing a hot-rolled steel sheet according to the above item [3], further comprising, between the rough rolling step and the finish rolling step, a joining step of joining the rough-rolled sheet bar and a preceding sheet bar at 1000°C or more, and in the finish rolling step, finish rolling the joined sheet bar. [5] The method for producing a hot-rolled steel sheet according to the above [3] or [4], further comprising a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet at an annealing temperature of 720°C or less to produce a hot-rolled annealed sheet, and a plating step of plating the hot-rolled annealed sheet. [6] The method for producing a hot-rolled steel sheet according to the above [5], further comprising an alloying step of alloying the plated hot-rolled steel sheet at a temperature of 460°C or more and 600°C or less.

[0022] According to the present invention, it is possible to manufacture a high-strength hot-rolled steel sheet having a yield strength (YS) of 500 MPa or more, a uniform elongation of 10% or more, excellent ductility, and good shear properties. By using the hot-rolled steel sheet according to the present invention, it is possible to make the steel thinner than conventional materials, and ２ Contribute to reducing emissions.

[0023] Hereinafter, a hot-rolled steel sheet according to an embodiment of the present invention will be described. <Chemical Composition of Hot-rolled Steel Sheet> The chemical composition of the hot-rolled steel sheet contains, in mass%, C: 0.02% or more and 0.12% or less, Si: less than 0.15%, Mn: more than 0.7% and 2.5% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% or more and 0.080% or less, N: 0.0080% or less, and Ti: 0.06% or more and 0.15% or less. The reasons for limiting the composition of each element will be explained below. In the following description, "%" representing the content of a chemical element means "mass%" unless otherwise specified.

[0024] C: 0.02% or more and 0.12% or less C is an element that combines with Ti to increase the strength of the steel sheet and contribute to delaying transformation during coiling. To obtain a desired steel sheet structure with a yield strength of 500 MPa or more, a C content of 0.02% or more is necessary. On the other hand, if the C content exceeds 0.12%, the amount of cementite formed increases excessively, making it impossible to obtain the desired ductility. Therefore, the C content is set to a range of 0.02% or more and 0.12% or less. The C content is preferably set to a range of 0.035% or more and 0.10% or less.

[0025] Si: Less than 0.15% Si is a harmful element that reduces platability. Furthermore, since Si increases the driving force for the austenite-to-ferrite transformation and impairs the effect of delaying the austenite-to-ferrite transformation during coiling, it is an element that needs to be reduced as much as possible in this embodiment. Therefore, the Si content is less than 0.15%. The Si content is preferably less than 0.10%. Note that even if the Si content is 0%, the effects of the present invention are not impaired.

[0026] Mn: more than 0.7% and not more than 2.5% Mn is an element that contributes to delaying transformation during coiling. To obtain a desired steel sheet structure, the Mn content is set to more than 0.7%. On the other hand, if the Mn content exceeds 2.5%, the austenite-to-ferrite transformation does not proceed during coiling, making it impossible to obtain the desired structure and properties. For this reason, the Mn content is set to a range of more than 0.7% and not more than 2.5%, and preferably to a range of 0.8% to 2.0%.

[0027] P: 0.05% or less P is a harmful element that segregates at grain boundaries and reduces shear strength, so it is preferable to reduce it as much as possible. In this embodiment, the P content can be tolerated up to 0.05%. The P content is preferably 0.04% or less. For use in environments where better shear strength is required, it is more preferable to suppress the P content to 0.03% or less. Meanwhile, in manufacturing, P may be unavoidably mixed in, with the lower limit set to 0.002%.

[0028] S: 0.010% or less S forms coarse sulfides in steel, which elongate and become wedge-shaped inclusions during hot rolling, adversely affecting shear properties. Therefore, since S is also a harmful element, it is preferable to reduce its content as much as possible. In this embodiment, S is tolerable up to 0.010%, so the upper limit of the S content is set to 0.010%. The S content is preferably 0.003% or less. For use in environments requiring stricter toughness, it is more preferable to suppress the S content to 0.002% or less. In manufacturing, the lower limit of the S content is 0.0001%, and S may be unavoidably mixed in.

[0029] 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 reduces ductility and shear strength by forming oxides. Therefore, the Al content is set to 0.080% or less. Preferably, the Al content is set to a range of 0.010% or more and 0.070% or less.

[0030] N: 0.0080% or less N is a harmful element that combines with Ti to form coarse TiN, reducing strength and shear resistance. Therefore, it is preferable to reduce N as much as possible. The N content can be up to 0.0080%. The N content is more preferably 0.0060% or less. In manufacturing, N may be unavoidably mixed in, with the lower limit being 0.0005%.

[0031] Ti: 0.06% or more and 0.15% or less Ti combines with C to form fine carbides containing Ti, thereby contributing to the strength of the steel sheet. Furthermore, Ti precipitates as TiC at the interface between austenite and ferrite during the transformation from austenite to ferrite, thereby pinning the interface and delaying the transformation. To obtain the desired yield strength and metal structure, the Ti content is set to 0.06% or more. On the other hand, if the Ti content exceeds 0.15%, the steel becomes excessively hard, making it impossible to obtain a uniform elongation of 10% or more. Therefore, the Ti content is set to a range of 0.06% or more and 0.15% or less. The Ti content is preferably set to a range of 0.07% or more and 0.14% or less.

[0032] To effectively delay the transformation from austenite to ferrite during coiling, it is preferable to satisfy the following formula (1). Formula (1) takes into consideration the elemental enrichment in austenite near the interface that prevents the austenite-to-ferrite transformation from moving across the interface, the driving force for the transformation from austenite to ferrite, and the pinning effect of Ti-containing carbides. 6.2[%C*] + 0.5[%Mn] + 3.2[%Ti*] ≥ 1.0 (1), where [%Ti*] = [%Ti] - 48[%N] / 14, and [%C*] = [%C] - 12[%Ti] / 48. Here, [%C], [%Mn], [%N], and [%Ti] are the C content, Mn content, N content, and Ti content, respectively, expressed in mass%.

[0033] The above is the basic composition of the hot-rolled steel sheet according to this embodiment. Optionally, the steel sheet may further contain at least one element from Groups A to D below.

[0034] Group A: at least one element selected from V: 0% to 0.2%, Nb: 0% to 0.07%, Mo: 0% to 0.15%, Zr: 0% to 0.1%, Hf: 0% to 0.1%, and W: 0% to 0.1%. V, Nb, Mo, Zr, Hf, and W are elements that contribute to strengthening the steel sheet by forming precipitates. Therefore, the content of one or more elements selected from V, Nb, Mo, Zr, Hf, and W is preferably 0% or more. On the other hand, if each element is contained in an amount exceeding the upper limit, TiC cannot be dissolved during slab reheating, and sufficient fine TiC cannot be obtained during hot rolling, resulting in the failure to obtain the desired metal structure.

[0035] Group B: at least one selected from Cu: 0% to 1.0%, Ni: 0% to 1.0%, Cr: 0% to 1.0%, and B: 0% to 0.010%. Cu, Ni, Cr, and B are elements that mainly change the transformation behavior from austenite to ferrite. Therefore, the content of one or more elements selected from Cu, Ni, Cr, and B is preferably 0% or more. On the other hand, if each element is contained in excess of the upper limit, the interfacial migration speed between austenite and ferrite during the transformation from austenite to ferrite decreases, making it impossible to obtain a metal structure with a high KAM value.

[0036] Group C: at least one selected from Ca: 0% to 0.01%, Mg: 0% to 0.01%, REM: 0% to 1.0%, and Co: 0% to 0.01%. Ca, Mg, REM, and Co are elements that can be expected to change the morphology of inclusions and improve shear strength. Therefore, the content of one or more elements selected from Ca, Mg, REM, and Co is preferably 0% or more. On the other hand, considering that adding large amounts not only saturates the effect but also may deteriorate various properties such as weldability, the upper limits of the contents of Ca, Mg, REM, and Co are specified as above. REM is a collective term for 17 elements, including Sc, Y, and lanthanides, and is expressed as the total content of each element.

[0037] Group D: at least one selected from Sb: 0% or more and 0.01% or less, Sn: 0% or more and 0.01% or less, As: 0% or more and 0.01% or less, Ta: 0% or more and 0.01% or less, Pb: 0% or more and 0.01% or less, Cs: 0% or more and 0.01% or less, Te: 0% or more and 0.01% or less, Bi: 0% or more and 0.01% or less, Zn: 0% or more and 0.01% or less, Ge: 0% or more and 0.01% or less, and Sr: 0% or more and 0.01% or less. Sb, Sn, As, Ta, Pb, Cs, Te, Bi, Zn, Ge, and Sr are elements mixed in as impurities. The upper limit of the content of each element is specified as above as a range that does not affect the properties of the hot-rolled steel sheet according to this embodiment.

[0038] 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.

[0039] <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 a ratio (area ratio of KAM value of 1.0 or more and 4.0 or less) / (area ratio of KAM value less than 1.0) of 0.05 or more, a coefficient of variation of crystal grain size of 0.55 or more, and an average particle size of Ti-containing carbides of 10 nm or less. The amount of solute Ti present in the steel is preferably 0.03% or less.

[0040] (Area ratio of KAM value of 1.0 or more and 4.0 or less) / (Area ratio of KAM value less than 1.0): 0.05 or more The KAM value of a steel sheet obtained by electron backscatter diffraction (EBSD) analysis represents the strain distribution of crystal grains. A major feature of this embodiment is that it has a structure in which crystals with a large KAM value, which have not previously been obtained at coiling temperatures of 600 ° C. or more, and crystals with a small KAM value and high ductility are mixed. To obtain properties that combine ductility and shear resistance, the (Area ratio of KAM value of 1.0 or more and 4.0 or less) / (Area ratio of KAM value less than 1.0) (hereinafter also referred to as the "KAM ratio") is 0.05 or more. Preferably, the KAM ratio is 0.07 or more. While there is no upper limit, the KAM ratio is preferably 0.20 or less from the viewpoint of ensuring sufficient ductility.

[0041] Coefficient of variation of grain size: 0.55 or more The grain size of a steel sheet can be obtained by EBSD analysis. By mixing grains with different grain sizes, good shear properties are achieved. The desired shear properties are obtained when the coefficient of variation of grain size given by the following formula (2) is 0.55 or more. A preferred coefficient of variation of grain size is 0.65 or more. While there is no upper limit, from the viewpoint of material stability, the coefficient of variation of grain size is preferably 0.75 or less. (Coefficient of variation of grain size) = (Standard deviation of grain size) / (Average grain size) ≥ 0.55 ... (2)

[0042] Average particle size of Ti-containing carbides: 10 nm or less In this embodiment, the steel sheet is strengthened by Ti-containing carbides. To obtain a high-strength hot-rolled steel sheet having a yield strength of 500 MPa or more, the average particle size of Ti-containing carbides dispersed in the steel is set to 10 nm or less. To stably obtain a yield strength of 500 MPa or more, it is preferable that the average particle size of Ti-containing carbides is set to 5 nm or less. Although there is no lower limit, from the viewpoint of strength stability, it is preferable that the average particle size of Ti-containing carbides is 1 nm or more.

[0043] Amount of solute Ti present in steel: 0.03% or less The amount of strengthening due to Ti-containing carbides depends not only on the average particle size but also on the amount of precipitation. The amount of precipitation is greatly affected by the coiling temperature, and if the coiling temperature is below 600°C, Ti does not precipitate but remains in a solid solution state, which may prevent the desired yield strength from being obtained. Therefore, to obtain a yield strength of 500 MPa or more, the amount of solute Ti is preferably limited to 0.03% or less, more preferably 0.02% or less. The amount of solute Ti may be 0.

[0044] The hot-rolled steel sheet according to this embodiment preferably has a coating layer on its surface. Even if the coating layer is formed, the function of the hot-rolled steel sheet is not impaired. The composition of the coating layer is preferably at least one selected from Zn, Si, Al, Ni, and Mg. The coated steel sheet according to this embodiment may be any of those that have been subjected to a hot-dip galvanizing treatment (hereinafter also referred to as GI), those that have been subjected to an alloying treatment after hot-dip galvanizing treatment (hereinafter also referred to as GA), and those that have been subjected to an electrogalvanizing treatment (hereinafter also referred to as EG).

[0045] Next, a method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention will be described. <Method for manufacturing a hot-rolled steel sheet> The method for manufacturing a hot-rolled steel sheet according to this embodiment includes a rough rolling process in which a steel material having the chemical composition specified in the above-mentioned hot-rolled steel sheet is rough-rolled to form a sheet bar, a finish rolling process in which the sheet bar is finish-rolled to form a hot-rolled steel sheet, a cooling process in which the hot-rolled steel sheet is cooled, a coiling process in which the hot-rolled steel sheet is wound into a coil, and a coil cooling process in which the coil is cooled. In the rough rolling process, the steel material is heated to 1150°C or higher, or the steel material is held at 1150°C or higher after casting and then rough-rolled, and the temperature of the sheet bar at the completion of rough rolling is set to a range of 1000°C to 1100°C. In the finish rolling process, the temperature of the rolled material at the start of finish rolling is set to 950°C or higher, the total reduction rate of the first and second passes is set to 70% or lower, the temperature of the rolled material at the finish rolling exit is set to 850°C or higher, and the rolling speed at the finish rolling exit is set to 500 m / min or higher. In the cooling process, the average cooling rate of the hot-rolled steel sheet is set to 40°C / s or higher to a cooling stop temperature in the range of 600°C to 700°C. In the coiling process, the coiling temperature of the hot-rolled steel sheet is set to 600°C to 700°C. In the coil cooling process, the average cooling rate of the coiled coil is set to 50°C or higher.

[0046] Generally, in the production of hot-rolled steel sheets, a slab (steel material) is cast, the temperature of which is reduced to 1000°C or less, is loaded into a heating furnace, and the slab (steel material) is heated for a short time, and then reduced to a predetermined thickness on a hot rolling line and wound into a coil. Alternatively, a slab (steel material) is cast, and the slab (steel material) is cooled to room temperature, and then heated for a long time in a heating furnace, and then reduced to a predetermined thickness on a hot rolling line and wound into a coil. Another production method involves directly transporting a 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 producing hot-rolled steel sheets according to this embodiment can be applied not only to a process of heating a steel material after casting, but also to a process of directly transporting a steel material after casting to a hot rolling line without heating it.

[0047] Steel material temperature: Heat to 1150°C or higher, or maintain at 1150°C or higher after casting. If coarse carbides containing Ti are present in the rolled material during finish rolling, ductility and shear strength decrease. If the slab temperature after casting drops below 1150°C, Ti-containing carbides precipitate in the slab and grow into coarse carbides through grain growth. Therefore, if the slab temperature drops below 1150°C, it is necessary to heat the slab to 1150°C or higher to dissolve the Ti-containing carbides. The preferred heating temperature is 1180°C or higher. Although there is no specific upper limit, a manufacturing constraint of 1300°C is imposed on the heating temperature to avoid thermal damage to the heating furnace. If the slab temperature after casting is maintained at 1150°C or higher, hot rolling can be performed without heating the slab. The preferred slab temperature is maintained at 1180°C or higher. Due to manufacturing constraints during slab casting, the upper limit of the slab temperature is preferably set to about 1300°C.

[0048] Rough rolling completion temperature: 1000°C or higher and 1100°C or lower In order to obtain a structure with a high grain size distribution as required in this embodiment, it is necessary to partially recrystallize austenite during rough rolling. If the rough rolling completion temperature, which is the temperature of the rolled material (sheet bar) at the completion of rough rolling of the steel material, is below 1000°C, the austenite will become an unrecrystallized structure. On the other hand, if the rough rolling completion temperature exceeds 1100°C, a fully recrystallized structure will be obtained, and in this case, a structure with a high grain size distribution will not be obtained. For this reason, the rough rolling completion temperature is set to a range of 1000°C or higher and 1100°C or lower. Preferably, the rough rolling completion temperature is set to a range of 1010°C or higher and 1080°C or lower.

[0049] Starting temperature of finish rolling: 950°C or higher, total reduction ratio of the first and second passes: 70% or less. The metal structure with a high KAM value, characterized in this embodiment, is obtained by suppressing the precipitation of coarse Ti-containing carbides that precipitate in austenite due to the processing effect of finish rolling and precipitating fine Ti-containing carbides at the interface between austenite and ferrite during the transformation from austenite to ferrite. Coarse Ti-containing carbides that precipitate in austenite are likely to occur when manufactured at low temperatures and low rolling speeds. In this embodiment, coarse Ti precipitates that precipitate in austenite are suppressed, and a large amount of fine TiC is precipitated at the interface between austenite and ferrite. This can be expected to have a pinning effect due to TiC, causing unusual grain boundary migration, resulting in crystal grains with a high KAM value.

[0050] To avoid the adverse effects of the above-mentioned finish rolling, the temperature of the sheet bar as the start temperature of the finish rolling is set to 950°C or higher, and the total reduction ratio of the first and second passes, where the rolling speed is slower than that of the finish rolling exit side, is limited to 70% or less. A preferred total reduction ratio of the first and second passes is in the range of 50% to 67%. The start temperature of the finish rolling is set lower than the rough rolling completion temperature.

[0051] Finish rolling exit temperature: 850°C or higher, finish rolling exit rolling speed: 500 m / min or higher. To avoid the adverse effects of the above-mentioned finish rolling, it is necessary to control not only the total reduction ratio of the first and second passes of finish rolling, but also the temperature and rolling speed (sheet passing speed) of the hot-rolled steel sheet at the finish rolling exit. The amount of Ti-containing carbides precipitated by rolling varies depending on the processing temperature, processing amount, and holding time after processing. To suppress the amount of Ti-containing carbides precipitated in austenite by processing, it is effective to increase the processing temperature to promote the recovery of processed austenite and shorten the holding time after processing. To achieve this, the finish rolling exit temperature and rolling speed must be 850°C or higher and 500 m / min or higher, respectively. Preferably, the finish rolling exit temperature and rolling speed are 900°C or higher and 550 m / min or higher, respectively. The thickness of the steel sheet after finish rolling is in the range of 1.0 mm to 3.6 mm. From the viewpoint of suppressing coarsening of TiC, the upper limit of the temperature on the delivery side of finish rolling is preferably set to about 950° C. Considering that the thickness of the hot-rolled steel sheet targeted in this embodiment is in the range of 1.0 mm or more and 3.6 mm or less, the upper limit of the rolling speed on the delivery side of finish rolling is preferably set to about 800 m / min.

[0052] Cooling process of the hot-rolled steel sheet after finish rolling: an average cooling rate of 40°C / s or more to a cooling stop temperature in the range of 600°C to 700°C. If the cooling stop temperature of the hot-rolled steel sheet after finish rolling exceeds 700°C or the average cooling rate is less than 40°C / s, the austenite-to-ferrite transformation starts at a higher temperature, and the particle size of Ti-containing carbides precipitated at the interface between austenite and ferrite increases. As a result, the pinning effect at the interface is not sufficiently obtained, and the desired microstructure is not obtained. Furthermore, the amount of strengthening achieved by the precipitation of Ti-containing carbides is reduced, and the yield strength does not even reach 500 MPa. For this reason, the cooling stop temperature and average cooling rate of the hot-rolled steel sheet after finish rolling are set to 700°C or less and 40°C / s or more, respectively.

[0053] The average cooling rate is forced cooling with a cooling rate faster than air cooling by water cooling or the like, and forced cooling is preferably initiated within 3 seconds after the completion of finish rolling. Therefore, the average cooling rate can be calculated by {(finish rolling completion temperature) - (cooling stop temperature)} / (cooling time by forced cooling). If the cooling stop temperature is below 600°C, the amount of solute Ti and dislocation density increase, making it impossible to obtain a yield strength of 500 MPa or more or a uniform elongation of 10% or more. Therefore, the cooling stop temperature of the hot-rolled steel sheet after finish rolling is set to 600°C or higher. The preferred cooling stop temperature is in the range of 610°C to 690°C, and the preferred average cooling rate is 50°C / s or higher. While there is no upper limit, considering equipment constraints, the average cooling rate is preferably set to an upper limit of 200°C / s.

[0054] Coiling process: coiling temperature in the range of 600°C to 700°C. When the coiling temperature of the hot-rolled steel sheet exceeds 700°C, the transformation from austenite to ferrite begins at a high temperature, and the particle size of Ti-containing carbides precipitated at the interface between austenite and ferrite increases. As a result, the pinning effect at the interface is not sufficiently obtained, and the desired microstructure is not obtained. Furthermore, the strengthening amount obtained by precipitating Ti-containing carbides is small, and the yield strength does not even reach 500 MPa. On the other hand, when the coiling temperature is below 600°C, the amount of solute Ti and the dislocation density increase, making it impossible to obtain a yield strength of 500 MPa or more or a uniform elongation of 10% or more. For these reasons, the coiling temperature of the hot-rolled steel sheet is set to a range of 600°C to 700°C. Preferably, the coiling temperature of the hot-rolled steel sheet is set to a range of 610°C to 680°C.

[0055] Coil cooling process: After coiling, the average cooling rate of the coil is 50°C / h or more until it reaches 500°C. If the cooling rate of the coil after coiling is slow, the KAM ratio and the coefficient of variation of the grain size will decrease during the cooling process after coiling, making it impossible to obtain the structure desired in this embodiment. To avoid this adverse effect, the coil is cooled to 500°C at an average cooling rate of 50°C / h or more after coiling. Preferably, the average cooling rate is 75°C / h or more. In the temperature range below 500°C, the change in structure is small, and cooling by normal air cooling may be used. Although there is no upper limit, considering the temperature variation within the coil, it is preferable that the average cooling rate of the coil after coiling be 150°C / h or less until it reaches 500°C.

[0056] Sheet bar joining process Between the rough rolling process and the finish rolling process, the rough-rolled sheet bar and the preceding sheet bar are joined at 1000°C or higher. If the joining temperature of the sheet bar falls below 1000°C, it becomes difficult to roll at the finish rolling start temperature of 950°C or higher in the subsequent finish rolling process. The preferred joining temperature of the sheet bar during joining is 1100°C or higher. From the viewpoint of optimizing the metal structure, it is preferable that the upper limit of the joining temperature of the sheet bar during joining is 1200°C.

[0057] Plating process and alloying process The method for producing a hot-rolled steel sheet according to this embodiment can employ an annealing process in which the hot-rolled steel sheet is annealed in a continuous annealing line at an annealing temperature of 720°C or less, and a plating process in which the hot-rolled steel sheet is plated in a continuous plating line. Furthermore, an alloying process in which the plated hot-rolled steel sheet is heated to a temperature range of 460°C or more and 600°C or less and subjected to an alloying treatment may be included. This annealing process or this plating process does not affect the material properties of the hot-rolled steel sheet according to this embodiment. Therefore, it is possible to plate the surface of the hot-rolled steel sheet to form a plating layer on the steel sheet surface.

[0058] Furthermore, as described above, the coating process and the composition of the coating bath do not affect the material properties of the hot-rolled steel sheet according to this embodiment, and therefore any of hot-dip galvanizing, galvannealed hot-dip galvanizing, and electrogalvanizing can be applied as the coating process. The composition of the coating bath can include at least one element selected from Zn, Al, Mg, Si, and Ni. That is, the composition of the coating layer formed on the surface of the hot-rolled steel sheet in the coating process can include at least one element selected from Zn, Si, Al, Ni, and Mg.

[0059] 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.

[0060] <Manufacturing method by continuous casting method> A 250 mm thick steel material having the chemical composition shown in Table 1 was hot rolled under the rough rolling and finish rolling conditions shown in Table 2, then temper rolled at an elongation rate of 0.1 to 0.5%, and pickled to produce a steel plate to be evaluated. The thickness of the steel plate after finish rolling was in the range of 1.0 mm to 3.6 mm.

[0061]

[0062]

[0063] <Production method for providing a coating layer on a hot-rolled steel sheet> A hot-rolled coil produced under the rough rolling and finish rolling conditions shown in Table 3 was pickled, and then the hot-rolled steel sheet was subjected to a Zn-plating treatment in a continuous hot-dip galvanizing line (CGL) under the plating conditions shown in Table 3. In this way, a hot-dip galvanized steel sheet (GI) and an alloyed hot-dip galvanized steel sheet (GA) were produced.

[0064]

[0065] <Manufacturing method by hot continuous rolling method> Steels having the chemical compositions shown in Table 1 were joined into sheet bars under the conditions shown in Table 4, and the joined sheet bars were hot rolled, then temper rolled to an elongation rate of 0.1 to 0.5%, and pickled to produce steel sheets to be evaluated.

[0066]

[0067] The hot-rolled steel sheets obtained under the conditions shown in Tables 1 to 4 were evaluated in terms of metal structure, tensile properties, and shear properties by the following methods. The results are shown in Table 5.

[0068] (i) Metal structure analysis method The KAM value and the coefficient of variation of the grain size were measured by the EBSD method. The area of ​​the field of view to be analyzed was 2500 μm ２ The data was acquired with a step width of 0.5 μm. The obtained image data was analyzed using OIM Analysis software (TSL). The KAM value analysis was performed under the condition of 1st nearest neighbor. The KAM (Kernel Average Misorientation) value is the average value of the misorientation between the measurement point of interest and the adjacent part, and is an index representing the plastic strain gradient in a micro-region. The grain size distribution was acquired in the "Grain Size (diameter)" mode, and the coefficient of variation of the grain size was calculated from the average grain size and standard deviation using the above-mentioned formula (2).

[0069] (ii) Average particle size of Ti-containing carbides A thin film for observation was collected from a location corresponding to 25% of the plate thickness from the surface of the hot-rolled steel plate, and 300 or more Ti-containing carbides were photographed at a magnification of 600,000 or more using a transmission electron microscope. The circle-equivalent diameters of the photographed Ti-containing carbides were determined, and the average value was used as the average particle size. Ti-containing carbides can be identified by checking the presence or absence of a peak derived from Ti using EDX attached to the TEM.

[0070] (iii) Analysis of the amount of precipitated carbides containing Ti The front and back surfaces of the test specimen were each ground by 25% of the plate thickness, and then dissolved in a 10% acetylacetone-1% tetramethylammonium chloride-methyl alcohol (10% AA) electrolytic solution. The solution was filtered through a filter with a mesh size of 0.2 μm, and the Ti concentration in the filtered electrolytic solution was analyzed using ICP-MS. The amount of Ti contained in this electrolytic solution was considered to be the amount of dissolved Ti, and the amount of dissolved Ti, expressed in mass%, was obtained from the ratio of the amount of dissolved matrix to the amount of dissolved Ti.

[0071] (iv) Tensile Test JIS No. 5 tensile test specimens were prepared from the hot-rolled steel sheets obtained under the conditions shown in Tables 1 to 4 in a direction perpendicular to the rolling direction, and tensile tests were performed five times in accordance with the provisions of JIS Z 2241 (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 5, hot-rolled steel sheets having a yield strength of 500 MPa or more and a uniform elongation of 10% or more were evaluated as having excellent mechanical properties.

[0072] (v) Punchability (shearability) evaluation For the hot-rolled steel sheets obtained under the conditions shown in Tables 1 to 4, punching was performed three times with a diameter of 10 mm at a 5% pitch with a clearance of 5% to 30%. The lengths of abnormalities such as roughness and cracks that occurred on the punched end faces were then investigated. The shearability evaluation in Table 5 was performed as follows. If the sum of the lengths of the abnormalities relative to the sum of the punched end face lengths (= 10π × 3 mm) in the three punching operations was 3% or less, the shearability evaluation column was marked "good." On the other hand, if the sum of the lengths of the abnormalities relative to the sum of the punched end face lengths exceeded 3%, the shearability evaluation column was marked "bad."

[0073] All of the inventive examples had a yield strength (YS) of 500 MPa or more and a uniform elongation of 10% or more, and exhibited good ductility and shear properties. On the other hand, the comparative examples outside the scope of the present invention either did not exhibit tensile properties or were evaluated as inferior in shear properties.

[0074]

Claims

1. A hot-rolled steel sheet having a chemical composition containing, by mass%, C: 0.02% or more and 0.12% or less, Si: less than 0.15%, Mn: more than 0.7% and 2.5% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% or more and 0.080% or less, N: 0.0080% or less, and Ti: 0.06% or more and 0.15% or less, and optionally containing at least one component selected from the following groups A to D, with the balance consisting of Fe and unavoidable impurities, wherein the metal structure has a ratio of the area fraction of KAM values of 1.0 or more and 4.0 or less to the area fraction of KAM values less than 1.0 of 0.05 or more, the coefficient of variation of crystal grain size is 0.55 or more, and the average particle size of carbides containing Ti is 10 nm or less, A hot-rolled steel sheet having a yield strength of 500 MPa or more and a uniform elongation of 10% or more. Group A: at least one selected from V: 0% to 0.2%, Nb: 0% to 0.07%, Mo: 0% to 0.15%, Zr: 0% to 0.1%, Hf: 0% to 0.1%, and W: 0% to 0.1%. Group B: at least one selected from Cu: 0% to 1.0%, Ni: 0% to 1.0%, Cr: 0% to 1.0%, and B: 0% to 0.010%. Group C: at least one selected from Ca: 0% to 0.01%, Mg: 0% to 0.01%, REM: 0% to 1.0%, and Co: 0% to 0.01%. Group D: At least one selected from Sb: 0% or more and 0.01% or less, Sn: 0% or more and 0.01% or less, As: 0% or more and 0.01% or less, Ta: 0% or more and 0.01% or less, Pb: 0% or more and 0.01% or less, Cs: 0% or more and 0.01% or less, Te: 0% or more and 0.01% or less, Bi: 0% or more and 0.01% or less, Zn: 0% or more and 0.01% or less, Ge: 0% or more and 0.01% or less, and Sr: 0% or more and 0.01% or less.

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

3. A steel material containing, by mass%, C: 0.02% or more and 0.12% or less, Si: less than 0.15%, Mn: more than 0.7% and 2.5% or less, P: 0.05% or less, S: 0.010% or less, Al: 0.005% or more and 0.080% or less, N: 0.0080% or less, and Ti: 0.06% or more and 0.15% or less, and optionally containing at least one component selected from the following groups A to D, with the balance being Fe and unavoidable impurities, comprising: a rough rolling process in which a steel material having a component composition is rough rolled to form a sheet bar; a finish rolling process in which the sheet bar is finish rolled to form a hot rolled steel sheet; a cooling process in which the hot rolled steel sheet is cooled; a coiling process in which the hot rolled steel sheet is wound into a winding coil; and a coil cooling process in which the winding coil is cooled; In the rough rolling step, the steel material is heated to 1150°C or higher, or the steel material is held at 1150°C or higher after casting, and rough rolling is performed, with the temperature of the sheet bar at the completion of rough rolling being in the range of 1000°C or higher and 1100°C or lower; in the finish rolling step, the temperature of the rolled material at the start of finish rolling is 950°C or higher, the total reduction rate of the first and second passes is 70% or lower, the temperature of the rolled material on the finish rolling exit side is 850°C or higher, and the rolling speed on the finish rolling exit side is 500m / min or higher; in the cooling step, the average cooling rate of the hot rolled steel sheet is 40°C / s or higher to a cooling stop temperature in the range of 600°C or higher and 700°C or lower; in the coiling step, the coiling temperature of the hot rolled steel sheet is in the range of 600°C or higher and 700°C or lower; In the coil cooling step, the average cooling rate of the wound coil is 50°C / h or more to 500°C.Group A: at least one selected from V: 0% or more and 0.2% or less, Nb: 0% or more and 0.07% or less, Mo: 0% or more and 0.15% or less, Zr: 0% or more and 0.1% or less, Hf: 0% or more and 0.1% or less, and W: 0% or more and 0.1% or less; Group B: at least one selected from Cu: 0% or more and 1.0% or less, Ni: 0% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, and B: 0% or more and 0.010% or less; Group C: at least one selected from Ca: 0% or more and 0.01% or less, Mg: 0% or more and 0.01% or less, REM: 0% or more and 1.0% or less, and Co: 0% or more and 0.01% or less; and Group D: At least one selected from Sb: 0% or more and 0.01% or less, Sn: 0% or more and 0.01% or less, As: 0% or more and 0.01% or less, Ta: 0% or more and 0.01% or less, Pb: 0% or more and 0.01% or less, Cs: 0% or more and 0.01% or less, Te: 0% or more and 0.01% or less, Bi: 0% or more and 0.01% or less, Zn: 0% or more and 0.01% or less, Ge: 0% or more and 0.01% or less, and Sr: 0% or more and 0.01% or less.

4. A method for producing hot-rolled steel sheet according to claim 3, further comprising a joining step between the rough rolling step and the finish rolling step, in which the roughly rolled sheet bar and a preceding sheet bar are joined at 1000°C or higher, and in the finish rolling step, the joined sheet bar is finish-rolled.

5. The method for producing hot-rolled steel sheet according to claim 3 or 4, further comprising: a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet at an annealing temperature of 720°C or less to produce a hot-rolled annealed sheet; and a plating step of plating the hot-rolled annealed sheet.

6. The method for producing a hot-rolled steel sheet according to claim 5, further comprising an alloying step of subjecting the plated hot-rolled steel sheet to an alloying treatment at a temperature in the range of 460°C to 600°C.

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