Hot-rolled steel plate

Abstract

With respect to a hot-rolled steel sheet according to the present invention, the metal structure at 1 / 4 the sheet thickness from the surface comprises, in area%, 2.0% to 30.0% of ferrite, 60.0% to 93.0% of bainite and 5.0% to 20.0% of martensite, while being in contact with a 30° grain boundary; if Iα is the maximum value of the GAIQ of the ferrite, the relative GAIQ is Iα / 3 or less; the area ratio of the martensite having a crystal grain size of 2.0 µm or more is 5.0% or more; and the tensile strength is 980 MPa or more.

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet. This application claims priority based on Japanese Patent Application No. 2021-191745, filed on November 26, 2021, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, efforts to reduce carbon dioxide emissions have been made in many fields from the perspective of protecting the global environment. Automobile manufacturers are also actively developing technologies to reduce the weight of vehicles in order to improve fuel efficiency. However, reducing the weight of vehicles is not easy, as emphasis is also placed on improving crashworthiness to ensure the safety of passengers.

[0003] In order to achieve both lightweight vehicle bodies and crashworthiness, the use of high-strength steel sheets to reduce the thickness of components has been considered. Therefore, there is a strong demand for steel sheets that combine high strength with excellent formability. To meet these requirements, several technologies have been proposed.

[0004] For example, Patent Document 1 discloses a high-strength hot-rolled steel sheet with a low yield ratio and high burring property, characterized in that the amount of insolubility Ti in the steel sheet measured by an extraction residue method is 30% to 70% of the total Ti content. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2009-263774 Summary of the Invention [Problem to be solved by the invention]

[0006] Automotive components are formed by press forming, and the blank sheets used for press forming are often produced by punching. In particular, for high-strength steel sheets with strengths of 980 MPa or more, it is desirable to be able to control the edge of the punched steel sheet with high precision, i.e., to have excellent punching workability.

[0007] As a result of investigations, the present inventors have found that it is necessary to further increase the strength of the material disclosed in Patent Document 1, and that when the strength is increased, there is room for improvement in the yield ratio and punching workability.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and yield ratio, as well as excellent punching workability. [Means for solving the problem]

[0009] The gist of the present invention is as follows. (1) A hot-rolled steel sheet according to one aspect of the present invention has a chemical composition, in mass%, C: 0.045~0.150%, Si: 0.10 to 2.50% Mn: 1.50~3.50% sol.Al: 0.010~1.000%, Ti: 0.050~0.200%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Nb: 0 to 0.050%, V: 0~0.050%, B: 0~0.0100%, Cu: 0-2.00% Cr: 0~2.00%, Mo: 0 to 1.000%, Ni: 0-2.00% Ca: 0 to 0.0200%, Mg: 0 to 0.0200%, REM: 0 to 0.1000%, Bi: 0 to 0.0200%, One or more of Zr, Co, Zn and W: 0 or more in total 0.0400 %,and Sn: 0 to 0.050% the balance being Fe and impurities, The metal structure at 1 / 4 of the plate thickness from the surface is Area % Ferrite: 2.0 to 30.0%, Bainite: 60.0~93.0%, Martensite: 5.0 to 20.0% The area ratio of the martensite which is in contact with the 30° grain boundary and has a relative GAIQ value of Iα / 3 or less when the maximum value of the GAIQ value of the ferrite is Iα and has a grain size of 2.0 μm or more is 5.0% or more, The tensile strength is 980 MPa or more. (2) The hot-rolled steel sheet according to (1) above has the chemical composition, in mass%, Nb: 0.001 to 0.050%, V: 0.001~0.050%, B: 0.0001 to 0.0100%, Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.001 to 1.000%, Ni: 0.01 to 2.00% Ca: 0.0005 to 0.0200%, Mg: 0.0005 to 0.0200%, REM: 0.0005 to 0.1000%, and Bi: 0.0005 to 0.0200%, The compound may contain one or more of the group consisting of: [Effects of the Invention]

[0010] According to the above-described aspects of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and yield ratio, as well as excellent punching workability. DETAILED DESCRIPTION OF THE INVENTION

[0011] The chemical composition and metal structure of the hot-rolled steel sheet according to this embodiment will be described in more detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible within the scope of the present invention.

[0012] Below, numerical ranges indicated with "to" include the lower and upper limits. Numerical values ​​indicated as "less than" or "greater than" are not included in the numerical range. In the following description, percentages regarding the chemical composition of the hot-rolled steel sheet are mass% unless otherwise specified.

[0013] chemical composition The hot-rolled steel sheet according to this embodiment contains, in mass%, C: 0.045 to 0.150%, Si: 0.10 to 2.50%, Mn: 1.50 to 3.50%, sol. Al: 0.010 to 1.000%, Ti: 0.050 to 0.200%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and the balance: Fe and impurities. Each element will be described in detail below.

[0014] C: 0.045 to 0.150% C is an element necessary to obtain the desired strength. If the C content is less than 0.045%, the desired strength cannot be obtained. Therefore, the C content is set to 0.045% or more. The C content is preferably 0.050% or more, 0.055% or more, or 0.060% or more. On the other hand, if the C content exceeds 0.150%, the weldability of the hot-rolled steel sheet decreases. Therefore, the C content is set to 0.150% or less. The C content is preferably 0.120% or less, 0.100% or less, or 0.080% or less.

[0015] Si: 0.10 to 2.50% Si has the effect of promoting the formation of ferrite to improve the ductility of the hot-rolled steel sheet, and the effect of solid-solution strengthening ferrite to increase the strength of the hot-rolled steel sheet. Si also has the effect of sounding the steel by deoxidation (suppressing the occurrence of defects such as blowholes in the steel). If the Si content is less than 0.10%, the above effects cannot be obtained. Therefore, the Si content is set to 0.10% or more. The Si content is preferably 0.50% or more or 0.70% or more. On the other hand, if the Si content exceeds 2.50%, the weldability of the hot-rolled steel sheet decreases. Therefore, the Si content is set to 2.50% or less. The Si content is preferably 2.00% or less, 1.80% or less, or 1.50% or less.

[0016] Mn: 1.50 to 3.50% Mn is an element that improves hardenability and increases the strength of hot-rolled steel sheets. If the Mn content is less than 1.50%, the desired strength cannot be obtained. Therefore, the Mn content is set to 1.50% or more. The Mn content is preferably 1.80% or more, 2.00% or more, or 2.30% or more. On the other hand, if the Mn content exceeds 3.50%, the hardenability becomes excessive and the yield ratio of the hot-rolled steel sheet decreases. Therefore, the Mn content is set to 3.50% or less. The Mn content is preferably 3.30% or less, 3.00% or less, or 2.80% or less.

[0017] sol.Al:0.010~1.000% Al has the effect of improving the quality of steel by deoxidizing it and also has the effect of controlling the ferrite transformation. If the sol. Al content is less than 0.010%, the above effects cannot be obtained. Therefore, the sol. Al content is set to 0.010% or more. The sol. Al content is preferably 0.030% or more, 0.050% or more, 0.080% or more, or 0.100% or more. On the other hand, if the sol.Al content exceeds 1.000%, cluster-like precipitates of alumina are formed, resulting in a decrease in the yield ratio of the hot-rolled steel sheet. Therefore, the sol.Al content is set to 1.000% or less. The sol.Al content is preferably 0.800% or less, 0.600% or less, 0.400% or less, or 0.200% or less. Here, sol. Al means acid-soluble Al, which indicates solute Al that exists in the steel in a solid solution state.

[0018] Ti: 0.050 to 0.200% Ti precipitates in steel as carbides or nitrides, refining the metal structure through a pinning effect, and increasing the strength and yield ratio of the hot-rolled steel sheet through precipitation strengthening. If the Ti content is less than 0.050%, the above effects cannot be obtained. Therefore, the Ti content is set to 0.050% or more. The Ti content is preferably 0.080% or more, 0.100% or more, or 0.130% or more. On the other hand, if the Ti content exceeds 0.200%, excessive precipitation of TiC will occur, deteriorating the punching workability of the hot-rolled steel sheet. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.180% or less or 0.150% or less.

[0019] P:0.100% or less P is an element generally contained in steel as an impurity, and the lower its content, the better. In particular, if the P content exceeds 0.100%, the workability and weldability of the hot-rolled steel sheet deteriorate significantly, and punching workability also deteriorates. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.080% or less, 0.060% or less, or 0.040% or less. The P content is preferably 0%, but may be 0.001% or more from the viewpoint of refining costs.

[0020] S: 0.0300% or less S is an element generally contained in steel as an impurity, and the lower its content, the better. If the S content exceeds 0.0300%, the yield ratio of the hot-rolled steel sheet will decrease significantly. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0200% or less or 0.0100% or less. The S content is preferably 0%, but may be 0.0001% or more from the viewpoint of refining costs.

[0021] N: 0.1000% or less N is an element generally contained in steel as an impurity, and the lower its content, the better. If the N content exceeds 0.1000%, the yield ratio of the hot-rolled steel sheet will decrease significantly. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0800% or less, 0.0600% or less, or 0.0400% or less. The N content is preferably 0%, but may be 0.0010% or more from the viewpoint of refining costs.

[0022] O: 0.0100% or less O is an element that, when contained in large amounts in steel, forms coarse oxides that become the starting point of fracture, causing brittle fracture and hydrogen-induced cracking. If the O content exceeds 0.0100%, brittle fracture and hydrogen-induced cracking are more likely to occur. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, 0.0060% or less, or 0.0040% or less. In order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be set to 0.0005% or more, or 0.0010% or more.

[0023] The balance of the chemical composition of the heat-rolled steel sheet according to this embodiment may be Fe and impurities. In this embodiment, the impurities refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment, and / or substances that are allowed to the extent that they do not adversely affect the heat-rolled steel sheet according to this embodiment.

[0024] The hot-rolled steel sheet according to this embodiment may contain the following elements as optional elements in place of a portion of Fe. When these optional elements are not contained, the lower limit of the content is 0%. The optional elements will be described in detail below.

[0025] Nb: 0.001 to 0.050% Nb is an element that precipitates finely in steel as carbides and nitrides, improving the strength of steel through precipitation strengthening. To reliably obtain this effect, the Nb content is preferably 0.001% or more. However, if the Nb content exceeds 0.050%, the yield ratio of the hot-rolled steel sheet deteriorates, so the Nb content is set to 0.050% or less.

[0026] V: 0.001 to 0.050% Like Nb, V is an element that precipitates in steel as fine carbides and nitrides, improving the strength of steel through precipitation strengthening. To reliably obtain this effect, the V content is preferably 0.001% or more. However, if the V content exceeds 0.050%, the yield ratio of the hot-rolled steel sheet deteriorates, so the V content is set to 0.050% or less.

[0027] B: 0.0001 to 0.0100% B has the effect of improving the hardenability of the hot-rolled steel sheet. To reliably obtain this effect, the B content is preferably 0.0001% or more. However, if the B content exceeds 0.0100%, the yield ratio of the hot-rolled steel sheet decreases significantly, so the B content is set to 0.0100% or less.

[0028] Cu: 0.01 to 2.00% Cu has the effect of improving the hardenability of the hot-rolled steel sheet and the effect of precipitating as carbides in the steel at low temperatures to increase the strength of the hot-rolled steel sheet. To reliably obtain these effects, the Cu content is preferably 0.01% or more. However, if the Cu content exceeds 2.00%, intergranular cracking of the slab may occur, and therefore the Cu content is set to 2.00% or less.

[0029] Cr: 0.01 to 2.00% Cr has the effect of improving the hardenability of the hot-rolled steel sheet, and in order to reliably obtain this effect, the Cr content is preferably 0.01% or more. However, if the Cr content exceeds 2.00%, the chemical conversion treatability of the hot-rolled steel sheet is significantly reduced, and therefore the Cr content is set to 2.00% or less.

[0030] Mo: 0.001 to 1.000% Mo has the effect of improving the hardenability of the hot-rolled steel sheet and the effect of precipitating in the steel as carbides to increase the strength of the hot-rolled steel sheet. To ensure these effects, the Mo content is preferably 0.001% or more. However, even if the Mo content exceeds 1.000%, the above effects saturate and it is not economically preferable, so the Mo content is set to 1.000% or less.

[0031] Ni: 0.01 to 2.00% Ni has the effect of improving the hardenability of the hot-rolled steel sheet, and in order to reliably obtain this effect, the Ni content is preferably 0.01% or more. However, since Ni is an expensive element, it is not economically preferable to add a large amount of Ni, so the Ni content is set to 2.00% or less.

[0032] Ca: 0.0005 to 0.0200% Ca has the effect of adjusting the shape of inclusions in steel to a preferred shape, thereby increasing the yield ratio of the hot-rolled steel sheet. To reliably obtain this effect, the Ca content is preferably 0.0005% or more. However, if the Ca content exceeds 0.0200%, excessive inclusions are formed in the steel, resulting in a decrease in the yield ratio of the hot-rolled steel sheet, so the Ca content is set to 0.0200% or less.

[0033] Mg: 0.0005 to 0.0200% Mg has the effect of adjusting the shape of inclusions in steel to a preferred shape, thereby increasing the yield ratio of the hot-rolled steel sheet. To reliably obtain this effect, the Mg content is preferably 0.0005% or more. However, if the Mg content exceeds 0.0200%, excessive inclusions are formed in the steel, resulting in a decrease in the yield ratio of the hot-rolled steel sheet, so the Mg content is set to 0.0200% or less.

[0034] REM: 0.0005 to 0.1000% REM has the effect of increasing the yield ratio of a hot-rolled steel sheet by adjusting the shape of inclusions in the steel to a preferred shape. To reliably obtain this effect, the REM content is preferably 0.0005% or more. However, if the REM content exceeds 0.1000%, excessive inclusions are formed in the steel, reducing the yield ratio of the hot-rolled steel sheet, so the REM content is set to 0.1000% or less. Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements. In the case of lanthanides, they are industrially added in the form of misch metal.

[0035] Bi: 0.0005 to 0.0200% Furthermore, Bi has the effect of increasing the yield ratio of the hot-rolled steel sheet by refining the solidification structure. To more reliably obtain this effect, the Bi content is preferably 0.0005% or more. However, if the Bi content exceeds 0.0200%, the above effects will saturate, which is not economically preferable. Therefore, the Bi content is set to 0.0200% or less.

[0036] One or more of Zr, Co, Zn and W: 0 to 1.0000% in total Sn: 0 to 0.050% The inventors have confirmed that the effects of the hot-rolled steel sheet according to this embodiment are not impaired even if the total content of Zr, Co, Zn, and W is 1.0000% or less. Therefore, one or more of Zr, Co, Zn, and W may be contained in a total content of 1.0000% or less. The inventors have also confirmed that the effects of the hot-rolled steel sheet according to this embodiment are not impaired even if a small amount of Sn is added. However, if a large amount of Sn is added, defects may occur during hot rolling, so the Sn content is set to 0.050% or less.

[0037] The chemical composition of the above-mentioned hot-rolled steel sheet may be measured by a common analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Sol-Al may be measured by ICP-AES using the filtrate obtained by thermally decomposing a sample with acid. C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method. If the hot-rolled steel sheet is a surface-treated steel sheet having a plating layer, the analysis of the chemical composition is carried out after mechanically grinding off 150 μm or more of the front and back surfaces of the steel sheet including the surface plating layer.

[0038] Metal structure of hot-rolled steel sheet Next, the metal structure of the hot-rolled steel sheet according to this embodiment will be described. In the hot-rolled steel sheet according to this embodiment, the metal structure at the 1 / 4 position of the sheet thickness from the surface is Area % Ferrite: 2.0 to 30.0%, Bainite: 60.0~93.0%, Martensite: Contains 5.0 to 20.0% The area fraction of the martensite, which is in contact with the 30° grain boundary, has a relative GAIQ value of Iα / 3 or less when the maximum GAIQ value of the ferrite is Iα, and has a grain size of 2.0 μm or more, is 5.0% or more.

[0039] In this embodiment, the position 1 / 4 of the plate thickness from the surface refers to the region from 1 / 8 of the plate thickness depth from the surface to 3 / 8 of the plate thickness depth from the surface of the hot-rolled steel plate. The reason for specifying the metallographic structure at this position is that the metallographic structure at this position shows a typical metallographic structure of the hot-rolled steel plate. Each provision will be explained below.

[0040] Ferrite area ratio: 2.0 to 30.0% Ferrite is a structure that is formed when fcc transforms to bcc at a relatively high temperature. If the area fraction of ferrite is less than 2.0%, the desired yield ratio cannot be obtained. Therefore, the area fraction of ferrite is set to 2.0% or more. Preferably, it is 5.0% or more, 8.0% or more, or 10.0% or more. On the other hand, if the area ratio of ferrite exceeds 30.0%, the desired strength cannot be obtained. Therefore, the area ratio of ferrite is set to 30.0% or less, preferably 27.0% or less, 25.0% or less, or 20.0% or less.

[0041] Bainite: 60.0~93.0% Bainite is a structure consisting of fine crystal grains and carbides. If the area fraction of bainite is less than 60.0%, the desired strength and yield ratio cannot be obtained. Therefore, the area fraction of bainite is set to 60.0% or more. Preferably, it is 65.0% or more, 70.0% or more, 75.0% or more, or 80.0% or more. On the other hand, if the area fraction of bainite exceeds 93.0%, the desired yield ratio cannot be obtained. Therefore, the area fraction of bainite is set to 93.0% or less, preferably 90.0% or less, 87.0% or less, or 80.0% or less.

[0042] Martensite: 5.0 to 20.0% Martensite is a structure that increases the strength of a hot-rolled steel sheet. If the area fraction of martensite is less than 5.0%, the desired strength cannot be obtained. Therefore, the area fraction of martensite is set to 5.0% or more, preferably 8.0% or more or 10.0% or more. On the other hand, if the area fraction of martensite exceeds 20.0%, the desired yield ratio cannot be obtained. Therefore, the area fraction of martensite is set to 20.0% or less, preferably 18.0% or less or 15.0% or less.

[0043] The heat-rolled steel sheet according to this embodiment may contain less than 5.0% in total of retained austenite and pearlite as the remaining structure.

[0044] The area ratio of each structure is measured by the following method. First, a test piece is taken from the hot-rolled steel sheet in a thickness cross section parallel to the rolling direction so that the metal structure can be observed at a position 1 / 4 of the thickness from the surface (a region from 1 / 8 depth to 3 / 8 depth from the surface) and at the center position in the sheet width direction.

[0045] The cross section of the specimen was polished using #600 to #1500 silicon carbide paper, then polished to a mirror finish using a liquid in which diamond powder with a grain size of 1 to 6 μm was dispersed in alcohol or pure water. Next, the specimen was polished using colloidal silica without alkaline solution at room temperature to remove the strain introduced into the surface of the specimen. At any position along the longitudinal direction of the specimen, a 50 μm long region extending from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness was measured using electron backscatter diffraction at 0.1 μm measurement intervals to obtain crystal orientation information.

[0046] For the measurements, an EBSD analyzer consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) was used. The vacuum level inside the EBSD analyzer was 9.6 × 10 -5 Pa or less, acceleration voltage is 15 kV, probe current level is 13, and electron beam irradiation level is 62.

[0047] From the obtained crystal orientation information, the "Grain Orientation Spread" function installed in the "OIM Analysis (registered trademark)" software attached to the EBSD analyzer is used to extract regions where the "Grain Orientation Spread" is 1° or less as ferrite, under the condition that boundaries with a crystal orientation difference of 15° or more are considered to be grain boundaries. The area ratio of the extracted ferrite is obtained by calculating the area ratio of ferrite.

[0048] Next, a Grain Average Image Quality map (GAIQ map) is obtained using the "Grain Average Image Quality" function. In the obtained GAIQ map, areas surrounded by grain boundaries with a crystal orientation misorientation of 15° or more are defined as crystal grains. If the maximum "Grain Average Image Quality value (GAIQ value)" of the areas extracted as ferrite is defined as Iα, areas with a relative GAIQ value exceeding Iα / 2 are extracted as bainite, and areas with a relative GAIQ value of Iα / 2 or less are extracted as martensite. The area fractions of the extracted bainite and martensite areas are calculated to obtain the area fractions of each. The area ratio of the remaining tissue is obtained by subtracting the area ratio of the above tissue from 100%.

[0049] Contamination on the surface of the observation surface can be removed by buffing using alumina particles with a particle size of 0.1 μm or less, or by Ar ion sputtering.

[0050] Area ratio of martensite that is in contact with the 30° grain boundary, has a relative GAIQ value of Iα / 3 or less when the maximum GAIQ value of ferrite is Iα, and has a grain size of 2.0 μm or more: 5.0% or more The martensite can be expressed in other words as martensite that satisfies the following conditions (I) to (III). (I) 30° contact with grain boundary. (II) When the maximum value of the GAIQ value of ferrite is Iα, the relative GAIQ value is Iα / 3 or less. (III) The crystal grain size is 2.0 μm or more. If the area fraction of martensite satisfying the above conditions (I) to (III) is less than 5.0%, the punching workability of the hot-rolled steel sheet will deteriorate. Therefore, the area fraction of martensite is set to 5.0% or more, preferably 8.0% or more or 10.0% or more. Although there is no particular upper limit, the area ratio of martensite may be set to 20.0% or less or 15.0% or less.

[0051] A higher GAIQ value indicates a lower dislocation density, and a lower GAIQ value indicates a higher dislocation density. Therefore, the GAIQ value is a parameter that can reflect the dislocation density of crystal grains. When the maximum GAIQ value of ferrite is Iα, martensite with a relative GAIQ value of Iα / 3 or less is harder than other martensites. In this embodiment, the area ratio of hard martensite in contact with the 30° grain boundary is controlled to improve the punching workability of the hot-rolled steel sheet. Note that martensite with a grain size of less than 2.0 μm does not need to be particularly controlled because it does not affect the punching workability of the hot-rolled steel sheet.

[0052] The area ratio of martensite is measured by the following method. First, a test piece is collected and processed using the same method as when measuring the area fraction of the microstructure. The measurement position is 1 / 4 of the sheet thickness from the surface (the region from 1 / 8 depth to 3 / 8 depth from the surface) and at the center in the sheet width direction. Next, the 30° grain boundary is identified using the "Grain Orientation Spread" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. Next, a GAIQ map is obtained using the same method as when measuring the area fraction of the microstructure, and ferrite and martensite are extracted. The maximum GAIQ value Iα of the ferrite is obtained, and martensite with a relative GAIQ value of Iα / 3 or less is identified. This identifies martensite that is in contact with the 30° grain boundary and has a relative GAIQ value of Iα / 3 or less (conditions (I) and (II)). Note that martensite in contact with the 30° grain boundary also includes martensite present on the 30° grain boundary.

[0053] The grain size of the martensite is obtained by calculating the circle-equivalent diameter of the martensite satisfying the conditions (I) and (II) in the GAIQ map, and thereby the martensite having a grain size of 2.0 μm or more is identified (condition (III)).

[0054] By calculating the area fraction of martensite that satisfies conditions (I) to (III) in the GAIQ map, the area fraction of martensite that is in contact with the 30° grain boundary, has a relative GAIQ value of Iα / 3 or less, and has a grain size of 2.0 μm or more is obtained.

[0055] strength The hot-rolled steel sheet according to this embodiment has a tensile strength of 980 MPa or more. A tensile strength of 980 MPa or more can contribute to reducing the weight of the vehicle body. There is no particular upper limit to the tensile strength, but it may be 1400 MPa or less.

[0056] The tensile strength is measured by a tensile test in accordance with JIS Z 2241:2011. The test piece used is a No. 5 test piece of JIS Z 2241:2011. The tensile test piece is taken from a quarter section from the end in the plate width direction, with the direction perpendicular to the rolling direction as the longitudinal direction.

[0057] yield ratio The hot-rolled steel sheet according to this embodiment has a yield ratio of 0.75 or more. The yield ratio can be determined by dividing the yield stress by the tensile strength (yield stress / tensile strength). The yield stress is obtained by conducting a tensile test using the method described above. If the hot-rolled steel sheet undergoes discontinuous yielding, the upper yield point is considered to be the yield stress, and if it undergoes continuous yielding, the 0.2% proof stress is considered to be the yield stress.

[0058] Punching processability The hot-rolled steel sheet according to this embodiment preferably has excellent punched end surface quality when subjected to a punching test. The punched end surface quality is evaluated by the following method. Test specimens are taken from the hot-rolled steel sheet. Punched holes are made in the test specimen with a hole diameter of 20 mm, a shear angle of 5°, a clearance of 15.0%, and a punching speed of 80 mm / s. The test specimens are taken so that four cross sections of the punched holes can be seen at approximately 90° intervals, and are embedded in resin. The punched end surface is photographed using a scanning electron microscope. The obtained photographs are observed, and if no large cracks exceeding 100 μm are observed in the thickness direction or perpendicular to the sheet, the hot-rolled steel sheet can be judged to have excellent punching workability. Furthermore, if no cracks of 50 μm or more are observed in the thickness direction or perpendicular to the sheet, the hot-rolled steel sheet can be judged to have particularly excellent punching workability.

[0059] Plate thickness The thickness of the hot-rolled steel sheet according to this embodiment is not particularly limited, but may be 0.6 to 8.0 mm. By making the thickness of the hot-rolled steel sheet 0.6 mm or more, it is possible to prevent the rolling load from becoming excessive, and hot rolling can be easily performed. Furthermore, by making the thickness 8.0 mm or less, it is possible to easily obtain the above-mentioned metal structure.

[0060] plating layer The hot-rolled steel sheet may be provided with a plating layer on the surface to improve corrosion resistance or the like, thereby forming a surface-treated steel sheet. The plating layer may be an electroplated layer or a hot-dip plated layer. Examples of electroplated layers include electrogalvanized plating and electrolytic Zn-Ni alloy plating. Examples of hot-dip plated layers include hot-dip galvanized plating, alloyed hot-dip galvanized plating, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. The coating weight is not particularly limited and may be the same as conventional coating weights. Furthermore, corrosion resistance can be further improved by performing an appropriate chemical conversion treatment after plating (for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying it).

[0061] Manufacturing conditions In a preferred method for manufacturing a hot-rolled steel sheet according to this embodiment, the following steps (1) to (7) are carried out in order. Note that the temperatures of the slab and the steel sheet in this embodiment refer to the surface temperatures of the slab and the steel sheet. (1) After casting a slab having the above-mentioned chemical composition, it is heated before being cooled to a temperature range of 500°C or less, and is held at a temperature range of 1220°C or more for 30 minutes or more. (2) The slab width is reduced at a reduction rate of 10% or more in a temperature range of 1200°C or more. (3) In rough rolling, the rough rolling completion temperature is set to a temperature range of 1100°C or higher, the cumulative reduction rate of all stages is set to 70% or higher, the reduction rate of each of the final three stages is set to less than 20%, and the reduction rate of each of all stages is set to less than 40%. (4) After the completion of rough rolling and before the start of finish rolling, the material is held in a temperature range of 1000°C or higher for 30 seconds or more. (5) The finish rolling completion temperature FT is set in a temperature range of T1 (°C) - 100°C or higher, the cumulative reduction rate of the finish rolling is set to 75% or higher, and the cumulative reduction rate of the final two stages is set to less than 30%. T1 (°C) can be obtained by the following formula (A): The element symbols in the formula indicate the content of each element in mass %, and 0 is substituted when the element is not contained. T1=937+168×Ti+3545×Nb+4500×B...(A) (6) The average cooling rate in the temperature range of the finish rolling completion temperature FT to 700°C is 30°C / s or more. (7) Coiling in the temperature range of 450 to 650°C.

[0062] The hot-rolled steel sheet according to this embodiment can be stably produced by a manufacturing method in which the above steps are closely and inseparably controlled. Each step will be described below.

[0063] (1) Slab heating The slab to be subjected to hot rolling is preferably heated after casting and before being cooled to a temperature range of 500°C or less, and held in a temperature range of 1220°C or more for 30 minutes or more. During holding in a temperature range of 1220°C or more, the steel sheet temperature may be varied or may be constant. By not cooling the slab to temperatures below 500°C, the precipitation of coarse Ti can be suppressed and Ti can be fully dissolved. This enhances the precipitation strengthening ability of ferrite and bainite, thereby increasing the strength and yield ratio of the hot-rolled steel sheet. Furthermore, by holding the steel at temperatures above 1220°C for 30 minutes or longer, sufficient solutionization can be achieved and prior austenite grains can be coarsened, maximizing the effectiveness of the slab width reduction process in the subsequent process. As a result, the area fraction of martensite adjacent to the 30° grain boundary, with a relative GAIQ value of Iα / 3 or less (where Iα is the maximum GAIQ value of ferrite), and with a grain size of 2.0 μm or more (hereinafter sometimes referred to as the martensite area fraction at the 30° grain boundary) can be increased.

[0064] The manufacturing process prior to hot rolling is not particularly limited. After melting in a blast furnace or electric furnace, various secondary smelting processes may be performed, and then a slab may be cast by a conventional continuous casting method. Scrap may also be used as the raw material.

[0065] (2) Slab width reduction After the heating and holding, it is preferable to carry out width reduction of the slab at a temperature of 1200°C or higher with a reduction ratio of 10% or more. By carrying out width reduction of the slab under these conditions, the coarsened prior austenite grains can be elongated in the thickness direction. As a result, the martensite area fraction of the 30° grain boundary can be increased.

[0066] The reduction ratio for slab width reduction can be expressed as (1-w1 / w0) x 100 (%), where w0 is the width direction length of the slab before reduction and w1 is the width direction length of the slab after reduction. Examples of methods for reducing the slab width include a method in which the slab is rolled using rolls installed so that the rotation axis is perpendicular to the plate surface of the slab.

[0067] (3)Rough rolling In rough rolling, it is preferable that the rough rolling completion temperature be in a temperature range of 1100°C or higher, the cumulative reduction rate for all stages be 70% or higher, and each of the final three stages be less than 20%, with each reduction rate for all stages being less than 40%. By setting the rough rolling completion temperature to a temperature range of 1100°C or higher, the cumulative reduction rate for all stages be 70% or higher, and each of the final three stages be less than 20%, the prior austenite grains elongated in the thickness direction can be made equiaxed, and recrystallization in the intragranular deformation bands can be promoted to obtain uniform prior austenite grains. As a result, the martensite area fraction of the 30° grain boundary can be increased. Furthermore, by setting the reduction ratios in all stages to less than 40%, it is possible to suppress the elongation of prior austenite grains in the rolling direction, thereby increasing the martensite area ratio at the 30° grain boundaries.

[0068] It is not desirable to perform reverse rolling during rough rolling, because if reverse rolling is performed during rough rolling, the shape of the prior austenite grains cannot be controlled favorably, and as a result, the martensite area ratio of the 30° grain boundary cannot be controlled favorably.

[0069] (4) Holding after rough rolling is completed and before finish rolling begins After the completion of rough rolling and before the start of finish rolling, it is preferable to hold the steel sheet at a temperature of 1000°C or higher for 30 seconds or longer. Holding the steel sheet under these conditions promotes recrystallization in the intragranular deformation bands, thereby obtaining uniform prior austenite grains. As a result, the martensite area fraction of the 30° grain boundary can be increased.

[0070] Methods for maintaining the steel sheet in the above temperature range include, for example, heating in a heating furnace after completion of rough rolling, or using a heat-retaining cover. In the above-mentioned maintenance, the steel sheet temperature may be constant or may be varied in a temperature range of 1000°C or higher.

[0071] (5) Finish rolling It is preferable that the finish rolling completion temperature FT is set in a temperature range of T1 (°C) - 100°C or higher, the cumulative reduction rate of the finish rolling is set to 75% or higher, and the cumulative reduction rate of the final two stages is set to less than 30%. By performing the finish rolling under these conditions, it is possible to promote recrystallization while controlling the prior austenite grains to be equiaxed. As a result, it is possible to increase the martensite area fraction of the 30° grain boundary.

[0072] (6) Cooling after finishing rolling After the finish rolling is completed, the average cooling rate is preferably 30° C. / s or more in the temperature range of the finish rolling completion temperature FT to 700° C. By cooling under these conditions, the desired amounts of ferrite and bainite can be obtained.

[0073] In this embodiment, the average cooling rate refers to the value obtained by dividing the temperature drop of the steel sheet from the start of cooling to the completion of cooling by the time required from the start of cooling to the completion of cooling.

[0074] (7) Winding Coiling is preferably carried out in a temperature range of 450 to 650°C. By setting the coiling temperature in the temperature range of 450°C or higher, it is possible to obtain the desired amounts of ferrite and martensite. Furthermore, by setting the coiling temperature in the temperature range of 650°C or lower, it is possible to obtain the desired amounts of ferrite and bainite.

[0075] After coiling, the steel sheet may be cooled to room temperature. Thereafter, if necessary, pickling and cold rolling may be performed by a conventional method. In cold rolling, the cumulative reduction ratio may be set to 50% or more. To flatten the hot-rolled steel sheet and adjust the surface roughness, temper rolling may be performed if necessary. [Example]

[0076] Next, the effects of one embodiment of the present invention will be explained in more detail using examples, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0077] Steels having the chemical compositions shown in Table 1 were melted and continuously cast into slabs with thicknesses of 240 to 300 mm. The resulting slabs were used to obtain hot-rolled steel sheets shown in Tables 4A and 4B under the manufacturing conditions shown in Tables 2A to 3B.

[0078] The obtained hot-rolled steel sheets were subjected to metallographic observation, tensile testing, and punching testing by the methods described above. The measurement results are shown in Tables 4A and 4B.

[0079] If the tensile strength was 980 MPa or more, the hot-rolled steel sheet was judged to be high-strength and passed the test, whereas if the tensile strength was less than 980 MPa, the hot-rolled steel sheet was judged not to be high-strength and passed the test.

[0080] If the yield ratio (yield stress / tensile strength) was 0.75 or more, the hot-rolled steel sheet was judged to have a high yield ratio and to have passed the test. On the other hand, if the yield ratio was less than 0.75, the hot-rolled steel sheet was judged to have an unsatisfactory yield ratio and to have failed the test.

[0081] In the microstructure photograph obtained from the punching test, if no cracks of 50 μm or more were observed in the thickness direction or perpendicular direction, the punching workability was deemed particularly good and was recorded as "E" (Excellent) in the table. If cracks of more than 50 μm and less than 100 μm were observed in the thickness direction or perpendicular direction, the punching workability was deemed good and was recorded as "G" (Good) in the table. If cracks of more than 100 μm were observed in the thickness direction or perpendicular direction, the punching workability was deemed poor and was recorded as "B" (Bad) in the table. When the evaluation was "E" or "G", the hot-rolled steel sheet was judged to have excellent punching workability and to have passed the test. On the other hand, when the evaluation was "B", the hot-rolled steel sheet was judged to have poor punching workability and to have failed the test.

[0082] [Table 1]

[0083] [Table 2A]

[0084] [Table 2B]

[0085] [Table 3A]

[0086] [Table 3B]

[0087] [Table 4A]

[0088] [Table 4B]

[0089] It can be seen from Tables 4A and 4B that the hot-rolled steel sheets according to the examples of the present invention have high strength and yield ratio, as well as excellent punching workability. On the other hand, it is clear that the hot-rolled steel sheets according to the comparative examples do not have one or more of the above properties. [Industrial Applicability]

[0090] According to the above-described aspects of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and yield ratio, as well as excellent punching workability.

Claims

1. The chemical composition, in mass%, is C: 0.045-0.150%, Si: 0.10-2.50%, Mn: 1.50-3.50%, sol. Al: 0.010 to 1.000%, Ti: 0.050-0.200%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Nb: 0 to 0.050%, V: 0 to 0.050%, B: 0 to 0.0100%, Cu: 0-2.00%, Cr: 0-2.00%, Mo: 0-1.000%, Ni: 0-2.00%, Ca: 0-0.0200%, Mg: 0 to 0.0200%, REM: 0-0.1000%, Bi: 0 to 0.0200%, One or more of Zr, Co, Zn, and W: 0 to 0.0400% in total; and Sn: 0 to 0.050%; the balance being Fe and impurities; The metal structure at 1 / 4 of the plate thickness from the surface is In area %, Ferrite: 2.0 to 30.0%, Bainite: 60.0 to 93.0%, Martensite: 5.0 to 20.0% The area ratio of the martensite which is in contact with the 30° grain boundary, has a relative GAIQ value of Iα / 3 or less when the maximum value of the GAIQ value of the ferrite is Iα, and has a grain size of 2.0 μm or more is 5.0% or more, A hot-rolled steel sheet having a tensile strength of 980 MPa or more.

2. The chemical composition is, in mass %, Nb: 0.001 to 0.050%, V: 0.001 to 0.050%, B: 0.0001 to 0.0100%, Cu: 0.01-2.00%, Cr: 0.01-2.00%, Mo: 0.001 to 1.000%, Ni: 0.01-2.00%, Ca: 0.0005-0.0200%, Mg: 0.0005-0.0200%, REM: 0.0005 to 0.1000%, and Bi: 0.0005-0.0200%, The hot-rolled steel sheet according to claim 1, characterized in that it contains one or more of the group consisting of

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