Hot-rolled steel sheet and method for producing the same

Abstract

To provide a high-strength hot-rolled steel sheet, with a yield strength of at least 500 MPa and with excellent ductility and shearability, and a method for producing the same.SOLUTION: A hot-rolled steel sheet has a chemical composition comprising C, Si, Mn, P, S, Al, N, Nb, and optionally added elements and includes carbides containing Nb with (area fraction of KAM value of 1.0 or more and 4.0 or less) / (area fraction of KAM value less than 1.0) of 0.05 or more, a coefficient of variation of grain size of 0.55 or more, a solid-solution Nb content in the steel of 0.03% or less, and an average particle size of 10 nm or less, the hot-rolled steel sheet exhibiting a yield strength of 500 MPa or more and a uniform elongation of 10% or more. Further, a method for producing the hot-rolled steel sheet includes: a rough rolling step in which a steel material having the above chemical composition is heated, or, after casting, rough-rolled without heating, and finished at a rough rolling completion temperature of 1000°C or more and 1100°C or less; a finish rolling step in which a rolling start temperature is 950°C or more and a total draft ratio of a first pass and a second pass is 70% or less; a cooling step; a coiling step; and a post-coiling coil cooling 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 ductility and shear property, and to a method for producing the same.The hot-rolled steel sheet of the present invention is suitable as a material for a wide range of applications, including automotive parts and non-automotive parts. [Background technology]

[0002] In recent years, from the perspective of protecting the global environment, industries across the globe have been moving toward increasing the strength of hot-rolled steel sheets in order to limit CO2 emissions. However, formability is often 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, so a balance between ductility and shear strength is desired. Generally, as the strength of steel sheets increases, their formability and shear strength tend to deteriorate. Therefore, improvements in formability and shear strength are essential to further expand the use of high-strength hot-rolled steel sheets. Furthermore, from the viewpoint of extending the life of components and improving their appearance, steel sheets with good galvanizability are also desired.

[0003] To solve these problems, various techniques for improving the formability of steel sheets have been proposed.

[0004] For example, Patent Document 1 describes that a hot-rolled steel sheet having a tensile strength of 780 MPa or more and excellent stretch flangeability can be obtained by precipitating precipitates containing Ti and having a size of less than 20 nm, with a total volume fraction of 95% or more of the ferrite phase and bainite phase, which have a small difference in hardness, and a volume fraction of the ferrite phase being 50 to 90%.

[0005] Furthermore, Patent Document 2 states that a hot-rolled steel sheet having excellent bendability and a tensile strength of 780 MPa or more can be obtained by precipitating 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 in a metal structure consisting of 95% or more ferrite crystal grains.

[0006] Patent Document 3 discloses a method for manufacturing 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 a dislocation density of 1×10 14 ~1×10 16 m -2 By precipitating TiC precipitates with an average diameter of 2.0 nm or less within the crystal grains at a density above a specified level, it is possible to obtain hot-rolled steel sheets with a tensile strength of 780 MPa or more and minimal damage to punched edge surfaces. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-68945 [Patent Document 2] International Publication No. 2013 / 99196 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-179539 Summary of the Invention [Problem to be solved by the invention]

[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 the uniform elongation required in the present invention. 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. In Patent Document 1, even steels with a small amount of Si added are unable to provide the structure and properties required in the present invention due to their composition and manufacturing method.

[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, Patent Documents 1 and 3 require complex control of runout before winding, which causes a problem of poor manufacturing stability.

[0013] The present invention was developed in consideration 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 properties, and a manufacturing method thereof. [Means for solving the problem]

[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 1.0 mm or more and 3.6 mm or less. To obtain good ductility in a hot-rolled steel sheet, it is effective to impart high uniform elongation. High uniform elongation is easily achieved with a structure that is coiled at 600°C or higher, at which point dislocations are easily recovered.

[0015] In conventional hot rolling, the structure of steel sheets coiled at temperatures above 600°C is primarily ferrite. However, in a structure primarily composed of ferrite, the grain size distribution is narrow and uniform, and crystal strain is present throughout the steel sheet. This makes crack propagation unstable during shearing, and the desired shear strength cannot be achieved. Furthermore, many prior art technologies use silicon to harden the inherently soft ferrite, but silicon reduces galvanic properties.

[0016] Therefore, a study was carried out focusing on the transformation of the metal structure of steel sheets under coiling conditions at 600°C or higher, with the Si content reduced as much as possible. As a result, by setting the coiling temperature in hot rolling at 600°C or higher and 700°C or lower, and controlling the rough rolling, finish rolling, and cooling conditions, a structure with unprecedented crystal grain size distribution and strain distribution was obtained.

[0017] The shear strength is improved by dispersing crystal grains with a high KAM value, which stabilizes crack propagation during shear. Furthermore, the crystal grains with a high KAM value obtained by this invention have higher ductility than bainite or martensite with lath structures, making it possible to achieve both excellent shear strength and ductility.

[0018] To broaden the grain size distribution, partial recrystallization of austenite during rough rolling is effective. To achieve this, it is necessary to control the Nb content and rough rolling temperature, which change the recrystallization behavior of austenite. It was found that to obtain a structure with a high KAM value, a large amount of fine NbC must be precipitated at the austenite-ferrite interface during the austenite-ferrite transformation, changing the austenite-ferrite interface movement. To achieve this, it is necessary to add a large amount of Nb and suppress the amount of NbC that precipitates within the austenite grains during finish rolling. It was found that the desired steel sheet structure can be obtained by controlling the Nb content and finish rolling temperature.

[0019] It was found that a microstructure with this grain size distribution and strain distribution promotes stable crack propagation during shear. It was found that hot-rolled steel sheets with this new microstructure have excellent shear properties, with a yield strength of over 500 MPa and a uniform elongation of over 10%.

[0020] 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: 0.02% or more and 0.18% 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: 0.0060% or less, Nb: 0.08% or more and 0.20% or less, optionally further containing at least one component selected from the following groups A to D, Group A: at least one selected from V: 0% or more and 0.2% or less, Ti: 0% or more and 0.1% 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% 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. and the balance Fe and unavoidable impurities, and the metal structure has a ratio of (area fraction of KAM value of 1.0 or more and 4.0 or less) / (area fraction 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, an amount of dissolved Nb present in the steel of 0.03% or less, and Nb-containing carbides with an average particle size of 10 nm or less, and a yield strength of 500 MPa or more and a uniform elongation of 10% or more. [2] In the above [1], the hot-rolled steel sheet has a plating layer on the surface thereof.

[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] In mass%, C: 0.02% or more and 0.18% or less, Si: less than 0.15%, Mn: 0.7% or more 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: 0.0060% or less, Nb: 0.08% or more and 0.20% or less, optionally, Contains at least one component from Groups A to D below, Group A: at least one selected from V: 0% or more and 0.2% or less, Ti: 0% or more and 0.1% 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% 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, A rough rolling process involves heating a steel material having a composition with the balance being Fe and unavoidable impurities to a heating temperature of 1150°C or higher, or without heating after casting, and rough rolling it to form a sheet bar with a rough rolling completion temperature of 1000°C to 1100°C. A finish rolling process is performed on the sheet bar at a starting temperature of 950°C or higher, a total reduction rate of the first and second passes of 70% or less, a temperature at the finish rolling exit of 850°C or higher, and a rolling speed at the finish rolling exit of 500°C. The method for producing a hot-rolled steel sheet includes: a finish rolling process in which the hot-rolled steel sheet is finish-rolled at mpm or more to obtain a hot-rolled steel sheet; a cooling process in which the hot-rolled steel sheet is cooled to a cooling stop temperature of 600°C or more and 700°C or less at an average cooling rate of 40°C / s or more; a coiling process in which the cooled hot-rolled steel sheet is coiled at a coiling temperature of 600°C or more and 700°C or less; and a coil cooling process in which, after the coiling, the coiled coil is cooled to 500°C at an average cooling rate of 50°C / h or more. [4] In the above [3], the method for manufacturing a hot-rolled steel sheet further includes 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 1070°C or higher, and in the finish rolling step, the joined sheet bar is finish-rolled. [5] In the above [3] or [4], 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. [6] The method for producing a hot-rolled steel sheet according to the above [5], further comprising an alloying step of subjecting the plated hot-rolled steel sheet to an alloying treatment at a temperature of 460°C or higher and 600°C or lower. [Effects of the Invention]

[0022] According to the present invention, it is possible to manufacture high-strength hot-rolled steel sheets that have high strength with a yield strength (YS) of 500 MPa or more, excellent ductility with a uniform elongation of 10%, and good shear properties. Use of the hot-rolled steel sheets according to the present invention makes it possible to reduce the thickness compared to conventional materials, thereby contributing to the reduction of CO2 emissions. DETAILED DESCRIPTION OF THE INVENTION

[0023] 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: 0.02% or more and 0.18% 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: 0.0060% or less, and Nb: 0.08% or more and 0.20% or less. Each component will be explained below. In the following explanation, "%" representing the content of a component means "% by mass."

[0024] C: 0.02% or more and 0.18% or less C is an element that combines with Nb to increase the strength of the steel sheet and delay the transformation during coiling. To obtain a desired steel sheet structure with a yield strength of 500 MPa or more, the C content is 0.02% or more. On the other hand, if the C content exceeds 0.18%, coarse NbC formed in the casting stage remains in the hot-rolled steel sheet, making it impossible to obtain a yield strength of 500 MPa or more. Therefore, the C content is 0.02% or more and 0.18% or less. Preferably, the C content is 0.07% or more and 0.15% or less.

[0025] Si: Less than 0.15% In the present invention, 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 the present invention. 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: over 0.7% and up to 2.5% Mn is an element that contributes to delaying transformation during coiling. To obtain the desired steel sheet structure, the Mn content must exceed 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 must be greater than 0.7% and not more than 2.5%, and preferably be 0.8% or more and not more than 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 its content as much as possible. In the present invention, the P content can be tolerated up to 0.05%. The P content is preferably 0.04% or less, but for use in environments where better shear strength is required, it is more preferable to suppress it to 0.03% or less. However, 0.002% may be unavoidably mixed in during manufacturing.

[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 it as much as possible. In the present invention, S content is allowed up to 0.010%, so the upper limit of the S content is set to 0.005%. The S content is preferably 0.003% or less, but for use in environments requiring stricter toughness, it is more preferable to suppress it to 0.002% or less. During manufacturing, 0.0001% or less 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 0.010% or more and 0.070% or less.

[0030] N: 0.0060% or less N is a harmful element that bonds with Nb to form coarse Nb-containing nitrides, reducing strength and shear resistance. Therefore, it is preferable to reduce N content as much as possible, but up to 0.0060% is acceptable. The N content is more preferably 0.005% or less. In manufacturing, 0.0005% may be unavoidably mixed in.

[0031] Nb: 0.08% or more and 0.20% or less Nb combines with C to form fine carbides containing Nb, thereby contributing to the strength of steel sheets. Furthermore, Nb precipitates at the austenite-ferrite interface during the austenite-to-ferrite transformation, pinning the interface and delaying the transformation. The Nb content required to obtain the desired yield strength and microstructure is 0.08% or more. On the other hand, if the Nb content exceeds 0.20%, the steel becomes excessively hard, making it impossible to obtain a uniform elongation of 10% or more. Therefore, the Nb content range is set to 0.08% or more and 0.20% or less. The Nb content is preferably 0.08% or more and 0.20% or less.

[0032] In order to effectively delay the austenite-to-ferrite transformation during coiling, it is preferable to satisfy formula (1), which takes into consideration the elemental enrichment in austenite near the interface that prevents the austenite-to-ferrite transformation from moving at the interface, the driving force for the austenite-to-ferrite transformation, and the pinning effect of carbides containing Nb. 5.3[%C*]+0.4[%Mn]+7.0[%Nb*]≧1.2 ···(1) however, [%Nb*] = [%Nb] - 93 [%N] / 14, [%C*]=[%C]―12[%Nb] / 93. Here, [%C], [%Mn], [%N], and [%Nb] are the C content, Mn content, N content, and Nb content in mass%, respectively.

[0033] 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 D below may further be contained.

[0034] Group A: at least one selected from V: 0% or more and 0.2% or less, Ti: 0% or more and 0.1% 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. V, Ti, 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, Ti, Mo, Zr, Hf, and W is preferably 0% or more. On the other hand, if each element is contained in an amount greater than the specified amount, NbC cannot be dissolved during slab reheating, and sufficient fine NbC cannot be obtained, making it impossible to obtain the desired structure.

[0035] 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. Cu, Ni, Cr, and B are elements that mainly change the austenite-to-ferrite transformation behavior. 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 an amount greater than the specified amount, the austenite-ferrite interface migration speed during the austenite-to-ferrite transformation decreases, making it impossible to obtain a 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 properties. Therefore, it is preferable that the content of one or more elements selected from Ca, Mg, REM, and Co is 0% or more. On the other hand, even if they are added in large amounts, not only will the effect saturate, but various properties such as weldability may also be deteriorated. Therefore, the upper limits of the contents of Ca, Mg, REM, and Co are specified as above.

[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 that are mixed in as impurities. The upper limit of the content of each element is specified as above so that it does not affect the performance of the steel sheet.

[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 is a hot-rolled steel sheet having a yield strength of 500 MPa or more and a uniform elongation of 10% or more, in which (area ratio of KAM value of 1.0 or more and 4.0 or less) / (area ratio of KAM value less than 1.0) is 0.05 or more, a coefficient of variation of crystal grain size is 0.55 or more, the amount of dissolved Nb present in the steel is 0.03% or less, and Nb-containing carbides have an average particle size of 10 nm or less.

[0040] (Area ratio of KAM value 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 has not been conventionally obtained at a coiling temperature of 600°C or higher, coexist with crystals with a small KAM value that are highly ductile. To obtain properties that combine ductility and shear resistance, (area fraction with a KAM value of 1.0 or more and 4.0 or less) / (area fraction with a KAM value of less than 1.0) (hereinafter also referred to as "KAM ratio") is 0.05 or more. Preferably, the KAM ratio is 0.07 or more. Examples of structures with a high KAM value include martensite and retained austenite. These are observed as crystal grains with a white contrast using a scanning electron microscope (SEM), and the amount of retained austenite can be measured using X-ray diffraction. In this embodiment, the amount of these structures produced is small, less than 2%.

[0041] Coefficient of variation of grain size: 0.55 or more The grain size of steel sheets can be determined by EBSD analysis. Mixing grains with different grain sizes improves shear strength. The desired shear strength is achieved when the coefficient of variation of grain size given by equation (2) is 0.55 or greater. The preferred coefficient of variation of grain size is 0.65 or greater. (coefficient of variation of particle size) = (standard deviation of particle size) / (average particle size) ≥ 0.55 (2)

[0042] Amount of dissolved Nb in steel: 0.03% or less The strengthening effect of Nb-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. When the coiling temperature is below 600°C, Nb does not precipitate but remains in a solid solution state, making it impossible to obtain the desired yield strength. Therefore, to obtain a yield strength of 500 MPa or more, the amount of solute Nb is 0.03% or less. Preferably, the amount of solute Nb is 0.02% or less.

[0043] Average particle size of Nb-containing carbides: 10 nm or less In this embodiment, the steel sheet is strengthened by Nb-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 the Nb-containing carbides dispersed in the steel is set to 10 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 Nb-containing carbides is set to 5 nm or less.

[0044] 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 of this embodiment includes 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).

[0045] Next, a method for manufacturing a hot-rolled steel sheet according to this embodiment will be described. <Hot-rolled steel sheet manufacturing method> The method for producing a hot-rolled steel sheet according to this embodiment contains, in mass%, C: 0.02% or more and 0.18% 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: 0.0060% or less, Nb: 0.08% or more and 0.20% or less, and optionally further contains at least one component selected from the following groups A to D: Group A: at least one selected from V: 0% or more and 0.2% or less, Ti: 0% or more and 0.1% 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% to 0.01%, Mg: 0% to 0.01%, REM: 0% to 1.0%, and Co: 0% to 0.01%. Group D: A steel material having a chemical composition consisting of 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, with the balance being Fe and unavoidable impurities, is heated to a heating temperature of 1150°C or more, or is cast without heating, and is subjected to rough rolling to a rolling completion temperature of 1000°C or more and 110°C or more. the sheet bar is finish-rolled to a hot-rolled steel sheet at a finish rolling start temperature of 950°C or higher, a total reduction rate of 70% or lower for the first and second passes, a finish rolling exit temperature of 850°C or higher, and a finishing rolling exit speed of 500mpm or higher; the 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; the coiling process is performed to coil the cooled hot-rolled steel sheet at a coiling temperature of 600°C or higher and 700°C or lower; and the coil cooling process is performed to cool the coiled coil to 500°C at an average cooling rate of 50°C / h or higher after coiling.

[0046] 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.

[0047] Heating temperature: Heat to 1150°C or above, or do not heat after casting If coarse carbides containing Nb are present during finish rolling, ductility and shear strength decrease. When the temperature of the cast slab drops to 1150°C, Nb-containing carbides precipitate in the slab, and the grains grow to form coarse carbides. Therefore, if the slab temperature falls below 1150°C, it is necessary to heat it to 1150°C or higher to dissolve the Nb-containing carbides. The preferred heating temperature is 1180°C or higher. If the slab temperature after casting does not fall below 1200°C, it can be hot-rolled without heating. There is no specific upper limit, but 1300°C is a manufacturing constraint to avoid thermal damage in the annealing furnace.

[0048] Rough rolling completion temperature: 1000℃ or higher and 1100℃ 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 is below 1000°C, the austenite will become an unrecrystallized structure, while if it exceeds 1100°C, it will become a fully recrystallized structure, in which case a structure with a high grain size distribution cannot be obtained. For this reason, the rough rolling completion temperature is set to 1000°C or higher and 1100°C or lower. Preferably, the rough rolling completion temperature is 1010°C or higher and 1080°C or lower.

[0049] Starting temperature of finish rolling: 950°C or higher, total reduction of first and second passes: 70% or less The structure with a high KAM value, which is a feature of this embodiment, is obtained by suppressing the precipitation of coarse carbides containing Nb that precipitate in austenite due to the processing effect of finish rolling, and by precipitating fine carbides containing Nb at the interface between austenite and ferrite during the transformation from austenite to ferrite. Coarse Nb precipitates in austenite are likely to occur when steel is manufactured at low temperatures and low rolling speeds. By suppressing the precipitation of coarse Nb in austenite and precipitating a large amount of fine NbC at the interface between austenite and ferrite, the pinning effect of NbC causes 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 starting 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. The preferred total reduction ratio of the first and second passes is 50% or more and 67% or less.

[0051] Finishing rolling exit temperature: 850°C or higher, finishing rolling exit speed: 500mpm 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 sheet threading speed at the finish rolling exit. The amount of Nb-containing carbides precipitated by rolling varies depending on the processing temperature, processing amount, and holding time after processing. To suppress the amount of Nb-containing carbides precipitated in austenite by processing, it is effective to increase the processing temperature, promote the recovery of processed austenite, and shorten the holding time after processing. To achieve this, the temperature and rolling speed at the finish rolling exit are 850°C or higher and 500 mpm or higher, respectively. Preferably, the temperature and rolling speed at the finish rolling exit are 900°C or higher and 550 mpm or higher, respectively. The thickness of the steel plate after finish rolling is 1.0 mm or more and 3.6 mm or less.

[0052] Cooling process of hot rolled steel sheet after finish rolling: Average cooling rate of 40°C / s or more until the cooling stop temperature is between 600°C and 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 austenite-ferrite interface increases. As a result, the pinning effect at the interface is insufficient, and not only is the desired microstructure not obtained, but the strengthening achieved by the precipitation of Ti-containing carbides is reduced, preventing the steel from achieving a yield strength of 700 MPa or more. 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, 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 started 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 properties such as a yield strength of 700 MPa or more or a uniform elongation of 8.5% or more. For this reason, the cooling stop temperature of the hot-rolled steel sheet after finish rolling is 600°C or higher. The preferred cooling stop temperature is 610°C or higher and 690°C or lower, and the preferred average cooling rate is 50°C / s or higher.

[0054] Winding process: 600℃ or more and 700℃ or less When the coiling temperature of a hot-rolled steel sheet exceeds 700°C, the temperature at which the austenite-to-ferrite transformation begins increases, and the particle size of Nb-containing carbides that precipitate at the austenite-ferrite interface increases. As a result, the pinning effect at the interface is insufficient, and not only is the desired microstructure not obtained, but the amount of strengthening achieved by the precipitation of Nb-containing carbides is reduced, and a yield strength of 500 MPa or more is not achieved. On the other hand, when the coiling temperature is below 600°C, the amount of solute Nb 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. For the above reasons, the coiling temperature is 600° C. or higher and 700° C. or lower. Preferably, the coiling temperature range is 610° C. or higher and 680° C. or lower.

[0055] Coil cooling process: After winding, 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 after coiling at an average cooling rate of 50°C / h or more. Preferably, the average cooling rate is 75°C / h. In the temperature range below 500°C, the change in structure is small, so cooling by normal air cooling may be used.

[0056] Sheet bar joining process Between the rough rolling and finish rolling steps, the rough rolled sheet bar is joined to the preceding sheet bar at 1070°C or higher. If the temperature is below 1070° C., it becomes difficult to perform rolling at the finish rolling start temperature of 950° C. or higher. The preferred heating temperature of the sheet bar during joining is 1100° C. or higher.

[0057] Plating process Alloying process 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.

[0058] 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 can be used as the coating process. The coating bath can 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 can contain one or more of Zn, Si, Al, Ni, and Mg. [Example]

[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 using continuous casting method> Steel material having a thickness of 250 mm and 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 steel sheets for evaluation. The thickness of the steel sheets after finish rolling was 1.0 to 3.6 mm.

[0061] <Manufacturing method using 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, temper-rolled to an elongation of 0.1 to 0.5%, and pickled to produce steel sheets for evaluation.

[0062] <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 galvanized in a continuous hot-dip galvanizing line (CGL) under the conditions shown in Table 3. In this way, continuous hot-dip galvanized steel sheets (GI) and alloyed hot-dip galvanized steel sheets (GA) were produced. 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.

[0063] (i) Metal structure analysis method The KAM value and the coefficient of variation of the grain size were measured using the EBSD method. The area of the field of view analyzed was 2500 μm 2 The data was acquired with a step width of 0.5 μm. The acquired image data was analyzed using OIM Analysis software (TSL). The KAM value analysis was performed under the condition of 1st nearest neighbor. The grain size distribution was acquired in "Grain Size (diameter)" mode, and the coefficient of variation of the grain size was calculated from the average grain size and standard deviation.

[0064] (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.

[0065] (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 Nb contained in this electrolytic solution was considered to be the amount of dissolved Nb, and the amount of dissolved Nb in mass% was calculated from the ratio of the amount of dissolved matrix to the amount of dissolved Nb.

[0066] (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 1 to 4. Tensile tests were conducted 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, samples with a yield strength of 500 MPa or more and a uniform elongation of 10% or more were considered to be inventive examples.

[0067] (v) Punchability From the hot-rolled steel sheets obtained under the conditions shown in Tables 1 to 4, 10 mm diameter punching was performed three times at 5% pitches with a clearance of 5% to 30%, and the lengths of abnormalities such as roughness and cracks that occurred on the punched end faces were investigated. In Table 5, if the total length of the abnormalities caused by the three punching processes was 3% or less of the total punched end face length (= 10π × 3 mm), it was marked with "O" as it was determined that the characteristic (shear resistance) required by the present invention was not met, and if it exceeded 3%, it was marked with "X" as it was determined that the characteristic (shear resistance) required by the present invention was not met.

[0068] All of the inventive examples had a yield strength (YS) of 500 MPa or more, and exhibited good ductility and shear properties. On the other hand, the comparative examples outside the scope of the present invention either failed to obtain the tensile properties or failed to obtain the shear properties required by the present invention.

[0069] [Table 1]

[0070] [Table 2]

[0071] [Table 3]

[0072] [Table 4]

[0073] [Table 5]

Claims

1. In mass%, C: 0.02% or more and 0.18% or less, Si: less than 0.15% Mn: more than 0.7% and not more than 2.5%; 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: 0.08% or more and 0.20% or less, Optionally, it further contains at least one component from Groups A to D below, Group A; At least one selected from V: 0% or more and 0.2% or less, Ti: 0% or more and 0.1% 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 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; The balance has a composition consisting of Fe and unavoidable impurities, The metal structure is (area ratio of KAM value of 1.0 or more and 4.0 or less) / (area ratio of KAM value less than 1.0) is 0.05 or more, The coefficient of variation of the crystal grain size is 0.55 or more, The amount of solute Nb present in the steel is 0.03% or less, and The carbide contains Nb and has an average particle size of 10 nm or less. Hot-rolled steel sheet with a yield strength of 500 MPa or more and a uniform elongation of 10% or more.

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. In mass%, C: 0.02% or more and 0.18% or less, Si: less than 0.15% Mn: more than 0.7% and not more than 2.5%; 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: 0.08% or more and 0.20% or less, Optionally, it further contains at least one component from Groups A to D below, Group A; At least one selected from V: 0% or more and 0.2% or less, Ti: 0% or more and 0.1% 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 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; a rough rolling step in which a steel material having a component composition with the balance being Fe and unavoidable impurities is heated to a heating temperature of 1150°C or higher, or is roughly rolled without heating after casting, to obtain a sheet bar having a rough rolling completion temperature of 1000°C or higher and 1100°C or lower; a finish rolling step in which the sheet bar is finish rolled at a finish rolling start temperature of 950°C or higher, a total reduction rate of the first and second passes of 70% or lower, a finish rolling exit temperature of 850°C or higher, and a finish rolling exit speed of 500 mpm or higher 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; a coil cooling step of cooling the wound coil after the winding at an average cooling rate of 50°C / h or more to 500°C; A method for producing a hot-rolled steel sheet, comprising:

4. A joining step of joining the rough-rolled sheet bar and a preceding sheet bar at 1070°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.

5. Further, 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 producing a hot-rolled steel sheet according to claim 3 or 4, further comprising the steps of:

6. The method for manufacturing 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 of 460°C or higher and 600°C or lower.

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