Hot-rolled steel plate

Abstract

This hot-rolled steel sheet has a desired chemical composition and a metal structure, wherein: the average spherical equivalent radius of alloy carbide in ferrite is 0.5-5.0 nm (exclusive of 5.0); the average number density is at least 3.5 × 1016 / cm3; the E value indicating the periodicity of the metal structure is at least 10.7; the I value indicating the uniformity of the metal structure is at least 1.020; the standard deviation of the Mn concentration is at most 0.60 mass%; and the tensile strength is at least 980 MPa.

Description

[Technical Field]

[0001] The present invention relates to a hot-rolled steel sheet, specifically to a hot-rolled steel sheet that is used after being formed into various shapes by press working or the like, and particularly to a hot-rolled steel sheet that has high strength and excellent ductility, fatigue properties, and shear workability. This application claims priority based on Japanese Patent Application No. 2022-015116, filed on February 2, 2022, 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. For this reason, steel sheets that combine high strength with excellent formability are highly desired. To meet these requirements, several technologies have been proposed. Since there are various processing methods for automotive components, the required formability varies depending on the component to be used, but ductility is considered an important indicator of formability.

[0004] Furthermore, automotive components are formed by press molding, and the blank sheets used for this press molding are often produced by shearing, which has high productivity. Blank sheets produced by shearing require excellent edge precision after shearing. For example, if a secondary shear plane occurs on the edge (sheared edge) after shearing, where the surface configuration is a sheared surface-fracture surface-sheared surface, the precision of the sheared edge will deteriorate significantly.

[0005] For example, Patent Document 1 discloses a high-strength steel sheet having a tensile strength of 980 MPa or more, excellent in ductility and stretch flangeability, and in which a second phase consisting of retained austenite and / or martensite is finely dispersed within the crystal grains.

[0006] In Patent Document 2, the ferrite grain size d s and the internal ferrite grains d b Relative to d s / d b A technique is disclosed in which the burr height after punching is controlled by controlling the value of the square root of the square root to 0.95 or less. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2005-179703 [Patent Document 2] Japanese Patent Application Publication No. 10-168544 [Non-patent literature]

[0008] [Non-Patent Document 1] J. Webel, J. Gola, D. Britz, F. Mucklich, Materials Characterization 144 (2018) 584-596 [Non-patent document 2] DL Naik, HU Sajid, R. Kiran, Metals 2019, 9, 546 [Non-patent document 3] K. Zuiderveld, Contrast Limited Adaptive Histogram Equalization, Chapter VIII.5, Graphics Gems IV. PS Heckbert (Eds.), Cambridge, MA, Academic Press, 1994, pp. 474-485 Summary of the Invention [Problem to be solved by the invention]

[0009] The techniques disclosed in Patent Documents 1 and 2 are techniques for improving either ductility or the end face properties after shearing, but Patent Documents 1 and 2 do not mention a technique for achieving both of these properties.

[0010] Furthermore, there are cases where high strength steel sheets are required to have better fatigue properties.

[0011] The present invention has been made in view of the above-mentioned problems of the prior art, and has an object to provide a hot-rolled steel sheet having high strength, as well as excellent ductility, fatigue properties and shear workability. [Means for solving the problem]

[0012] 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: 0.050~0.250%, Si: 0.05 to 3.00%, Mn: 1.00~4.00%, One or more of Ti, Nb, and V: 0.060 to 0.500% in total, sol.Al:0.001~2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Cu: 0-2.00% Cr: 0~2.00%, Mo: 0-1.00%, Ni: 0-2.00% B: 0~0.0100%, Ca: 0 to 0.0200%, Mg: 0 to 0.0200%, REM: 0 to 0.1000%, Bi: 0 to 0.020% One or more of Zr, Co, Zn, and W: 0 to 1.00% in total, and Sn: 0 to 0.05% the balance being Fe and impurities, The metal structure is Area % The retained austenite is less than 3.0%. Ferrite is 15.0% or more and less than 60.0%; pearlite is less than 5.0%; The alloy carbide in the ferrite has an average equivalent sphere radius of 0.5 nm or more and less than 5.0 nm, and an average number density of 3.5 × 10 16 pieces / cm 3 That's all, The E value indicating the periodicity of the metal structure is 10.7 or more, The I value indicating the uniformity of the metal structure is 1.020 or more, The standard deviation of the Mn concentration is 0.60 mass% or less, The tensile strength is 980 MPa or more. (2) The hot-rolled steel sheet according to (1) above has the chemical composition, in mass%, Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 1.00%, Ni: 0.02 to 2.00% B: 0.0001 to 0.0100%, Ca: 0.0005 to 0.0200%, Mg: 0.0005 to 0.0200%, REM: 0.0005 to 0.1000%, and Bi: 0.0005 to 0.020% The compound may contain one or more selected from the group consisting of: [Effects of the Invention]

[0013] According to the above-described aspects of the present invention, it is possible to obtain a hot-rolled steel sheet having high strength, as well as excellent ductility, fatigue properties, and shear workability. The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for automobile parts, machine structural parts, and even building parts. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is an example of a sheared end surface of a hot-rolled steel sheet according to an example of the present invention. [Figure 2] 1 is an example of a sheared end surface of a hot-rolled steel sheet according to a comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0015] 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.

[0016] 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.

[0017] chemical composition The hot-rolled steel sheet according to this embodiment contains, by mass%, C: 0.050 to 0.250%, Si: 0.05 to 3.00%, Mn: 1.00 to 4.00%, one or more of Ti, Nb, and V: 0.060 to 0.500% in total, sol. Al: 0.001 to 2.000%, 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.

[0018] C: 0.050 to 0.250% C increases the area ratio of the hard phase and also increases the strength of ferrite by combining with precipitation strengthening elements such as Ti, Nb, and V. If the C content is less than 0.050%, the desired strength cannot be obtained. Therefore, the C content is set to 0.050% or more. The C content is preferably 0.060% or more, more preferably 0.070% or more, and even more preferably 0.080% or more. On the other hand, if the C content exceeds 0.250%, the area ratio of ferrite decreases, resulting in a decrease in the ductility of the hot-rolled steel sheet. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.200% or less, 0.180% or less, or 0.150% or less.

[0019] Si: 0.05 to 3.00% 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 0.05%, the above effects cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.50% or more, and more preferably 0.80% or more. On the other hand, if the Si content exceeds 3.00%, the surface quality and chemical conversion treatability of the steel sheet, as well as its ductility and weldability, are significantly deteriorated, and the A3 transformation point is significantly increased. This makes it difficult to perform stable hot rolling. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less, and more preferably 2.00% or less or 1.50% or less.

[0020] Mn: 1.00 to 4.00% Mn has the effect of suppressing ferrite transformation and increasing the strength of the hot-rolled steel sheet. If the Mn content is less than 1.00%, the desired tensile strength cannot be obtained. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.30% or more, and more preferably 1.50% or more. On the other hand, if the Mn content exceeds 4.00%, the hard phase will have a periodic band shape due to Mn segregation, making it difficult to obtain the desired shear workability. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.50% or less, and more preferably 3.00% or less or 2.50% or less.

[0021] One or more of Ti, Nb, and V: 0.060 to 0.500% in total Ti, Nb, and V precipitate finely in steel as carbides and nitrides, improving the strength of the steel through precipitation strengthening. Furthermore, they are essential elements for achieving desired fatigue properties. If the total content of Ti, Nb, and V is less than 0.060%, these effects cannot be achieved. Therefore, the total content of Ti, Nb, and V is set to 0.060% or more. It is not necessary to contain all of Ti, Nb, and V; any one of them may be present, provided that its content is 0.060% or more. The total content of Ti, Nb, and V is preferably 0.080% or more, more preferably 0.100% or more. On the other hand, if the total content of Ti, Nb, and V exceeds 0.500%, the workability of the hot-rolled steel sheet deteriorates. Therefore, the total content of Ti, Nb, and V is set to 0.500% or less, preferably 0.300% or less, more preferably 0.250% or less, and even more preferably 0.200% or less.

[0022] sol.Al:0.001~2.000% Like Si, Al has the effect of improving the soundness of steel by deoxidizing, and also promotes the formation of ferrite, thereby improving the ductility of hot-rolled steel sheets. If the sol. Al content is less than 0.001%, the above effects cannot be obtained. Therefore, the sol. Al content is set to 0.001% or more. The sol. Al content is preferably 0.010% or more, and more preferably 0.020% or more or 0.030% or more. On the other hand, if the sol.Al content exceeds 2.000%, the above effects saturate and it is economically undesirable, so the sol.Al content is set to 2.000% or less. The sol.Al content is preferably 1.500% or less, more preferably 1.000% or less, and even more preferably 0.500% or less. Here, sol. Al means acid-soluble Al, which indicates solute Al that exists in the steel in a solid solution state.

[0023] P:0.100% or less P is an element that is generally contained as an impurity, but it also has the effect of increasing the strength of hot-rolled steel sheets through solid-solution strengthening. Therefore, P may be intentionally contained. However, P is an element that easily segregates, and if the P content exceeds 0.100%, the decrease in ductility due to grain boundary segregation becomes significant. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.030% or less. There is no particular need to specify a lower limit for the P content, but it may be set to 0%. From the viewpoint of refining costs, the P content is preferably set to 0.001%.

[0024] S: 0.0300% or less S is an element contained as an impurity, and forms sulfide-based inclusions in steel, reducing the ductility of the hot-rolled steel sheet. If the S content exceeds 0.0300%, the ductility of the hot-rolled steel sheet will be significantly reduced. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0050% or less. There is no particular need to specify a lower limit for the S content, but it may be set to 0%. From the viewpoint of refining costs, the S content is preferably set to 0.0001%.

[0025] N: 0.1000% or less N is an element contained in steel as an impurity and has the effect of reducing the ductility of hot-rolled steel sheets. If the N content exceeds 0.1000%, the ductility of the hot-rolled steel sheets will be significantly reduced. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0800% or less, more preferably 0.0700% or less, and even more preferably 0.0100% or less or 0.0050% or less. There is no particular need to specify a lower limit for the N content, but it may be 0%. When one or more of Ti, Nb, and V are added to further refine the metal structure, the N content is preferably 0.0010% or more, more preferably 0.0020% or more, in order to promote the precipitation of carbonitrides.

[0026] O: 0.0100% or less If the O content in steel is too high, it forms coarse oxides that act as fracture initiation sites, causing brittle fracture and hydrogen-induced cracking. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, more preferably 0.0055% or less or 0.0050% 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.

[0027] 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.

[0028] 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 no optional elements are contained, the lower limit of the content is 0%. The optional elements will be described in detail below.

[0029] Cu: 0.01 to 2.00% Cr: 0.01 to 2.00% Mo: 0.01 to 1.00% Ni: 0.02 to 2.00% B: 0.0001 to 0.0100% Cu, Cr, Mo, Ni, and B all have the effect of improving the hardenability of hot-rolled steel sheets. Cu and Mo also have the effect of precipitating as carbides in the steel to increase the strength of the hot-rolled steel sheets. Furthermore, when Cu is contained, Ni has the effect of effectively suppressing intergranular cracking of the slab caused by Cu. Therefore, one or more of these elements may be contained.

[0030] As mentioned above, Cu has the effect of improving the hardenability of hot-rolled steel sheets and precipitating as carbides in the steel at low temperatures to increase the strength of the hot-rolled steel sheets. To ensure the above effects, the Cu content is preferably 0.01% or more, and more preferably 0.05% or more. However, if the Cu content exceeds 2.00%, grain boundary cracking of the slab may occur. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.50% or less, and more preferably 1.00% or less.

[0031] As mentioned above, Cr has the effect of improving the hardenability of hot-rolled steel sheets. To ensure the above effect, the Cr content is preferably 0.01% or more, and more preferably 0.05% or more. However, if the Cr content exceeds 2.00%, the chemical conversion treatability of the hot-rolled steel sheet is significantly reduced. Therefore, the Cr content is set to 2.00% or less.

[0032] As mentioned above, Mo has the effect of improving the hardenability of hot-rolled steel sheets and of precipitating as carbides in the steel to increase the strength of the hot-rolled steel sheets. To ensure the above effects, the Mo content is preferably 0.01% or more, and more preferably 0.02% or more. However, even if the Mo content exceeds 1.00%, the above effects are saturated and it is not economically preferable. Therefore, the Mo content is set to 1.00% or less. The Mo content is preferably 0.50% or less, and more preferably 0.20% or less.

[0033] As mentioned above, Ni has the effect of improving the hardenability of hot-rolled steel sheets. Furthermore, when Cu is added, Ni has the effect of effectively suppressing grain boundary cracking of slabs caused by Cu. To more reliably obtain the above-mentioned effects, the Ni content is preferably 0.02% or more. Since Ni is an expensive element, it is economically undesirable to add a large amount. Therefore, the Ni content is set to 2.00% or less.

[0034] As mentioned above, B has the effect of improving the hardenability of hot-rolled steel sheets. To ensure this effect, the B content is preferably 0.0001% or more, and more preferably 0.0002% or more. However, if the B content exceeds 0.0100%, the formability of the hot-rolled steel sheet is significantly reduced, so the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less.

[0035] Ca: 0.0005 to 0.0200% Mg: 0.0005 to 0.0200% REM: 0.0005 to 0.1000% Bi: 0.0005 to 0.020% Ca, Mg, and REM all have the effect of adjusting the shape of inclusions in steel to a preferred shape, thereby improving the ductility of hot-rolled steel sheets. Bi also has the effect of refining the solidification structure, thereby improving the ductility of hot-rolled steel sheets. Therefore, one or more of these elements may be added. To ensure the above-mentioned effects, it is preferable to set the content of any one or more of Ca, Mg, REM, and Bi to 0.0005% or more. However, if the Ca content or Mg content exceeds 0.0200%, or if the REM content exceeds 0.1000%, excessive inclusions may be formed in the steel, which may actually reduce the ductility of the hot-rolled steel sheets. Furthermore, even if the Bi content exceeds 0.020%, the effects of the above-mentioned effects saturate, making it economically undesirable. Therefore, the Ca content and Mg content are set to 0.0200% or less, the REM content to 0.1000% or less, and the Bi content to 0.020% or less. The Bi content is preferably 0.010% 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.

[0036] One or more of Zr, Co, Zn and W: 0 to 1.00% in total Sn: 0 to 0.05% 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 1.00% or less. Therefore, one or more of Zr, Co, Zn, and W may be contained in a total content of 1.00% 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 0.05% 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.

[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. The hot-rolled steel sheet according to this embodiment has a metal structure in which, in area %, retained austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is less than 5.0%, and the average equivalent sphere radius of alloy carbides in the ferrite is 0.5 nm or more and less than 5.0 nm, and the average number density is 3.5 × 10 16 pieces / cm 3 or more, the E value indicating the periodicity of the metallographic structure is 10.7 or more, the I value indicating the uniformity of the metallographic structure is 1.020 or more, and the standard deviation of the Mn concentration is 0.60 mass % or less.

[0039] The hot-rolled steel sheet according to this embodiment has the above-described metal structure, and therefore can obtain high strength, excellent ductility, fatigue properties, and shear workability. In this embodiment, the structure fraction, the average spherical equivalent radius and average number density of alloy carbides, the E value, the I value, and the standard deviation of the Mn concentration in the metal structure at a depth of 1 / 4 of the sheet thickness from the surface (a region from 1 / 8 of the sheet thickness from the surface to 3 / 8 of the sheet thickness from the surface) and at the center position in the sheet width direction in a cross section parallel to the rolling direction are specified because the metal structure at this position represents a typical metal structure of the steel sheet.

[0040] Area ratio of retained austenite: less than 3.0% Retained austenite is a metal structure that exists as a face-centered cubic lattice even at room temperature. Retained austenite has the effect of increasing the ductility of hot-rolled steel sheets through transformation-induced plasticity (TRIP). On the other hand, retained austenite transforms into high-carbon martensite during shearing, hindering stable crack initiation and causing the formation of secondary shear planes. If the area fraction of retained austenite is 3.0% or more, the above effects become apparent, deteriorating the shear workability of the hot-rolled steel sheets. Therefore, the area fraction of retained austenite is set to less than 3.0%. The area fraction of retained austenite is preferably less than 1.5%, more preferably less than 1.0%. Since the smaller the amount of retained austenite, the more preferable it is, the area fraction of retained austenite may be 0%.

[0041] Methods for measuring the area fraction of retained austenite include X-ray diffraction, EBSP (Electron Back Scattering Diffraction Pattern) analysis, magnetic measurement, etc. In this embodiment, the area fraction of retained austenite is measured by X-ray diffraction. In the measurement of the area fraction of retained austenite by X-ray diffraction in this embodiment, first, in a cross section parallel to the rolling direction at a depth of 1 / 4 of the sheet thickness of the hot-rolled steel sheet (a region from 1 / 8 of the sheet thickness depth from the surface to 3 / 8 of the sheet thickness depth from the surface) and at the center position in the sheet width direction, Co-Kα radiation is used to determine the integrated intensities of a total of six peaks: α(110), α(200), α(211), γ(111), γ(200), and γ(220), and the volume fraction of retained austenite is calculated using the intensity averaging method. The obtained volume fraction of retained austenite is regarded as the area fraction of retained austenite.

[0042] Ferrite area ratio: 15.0% or more, less than 60.0% Ferrite is a structure that is formed when fcc transforms to bcc at relatively high temperatures. Ferrite has a high work hardening rate, which improves the strength-ductility balance of hot-rolled steel sheets. To achieve this effect, the area fraction of ferrite is set to 15.0% or more. It is preferably 20.0% or more, more preferably 25.0% or more, and even more preferably 30.0% or more. On the other hand, since ferrite has low strength, if the area ratio is excessive, the desired strength cannot be obtained. Therefore, the ferrite area ratio is set to less than 60.0%, preferably 50.0% or less, and more preferably 45.0% or less.

[0043] Perlite area ratio: less than 5.0% Pearlite is a lamellar metal structure in which cementite is precipitated in layers between ferrite, and is softer than bainite or martensite. If the area fraction of pearlite is 5.0% or more, carbon is consumed by the cementite contained in pearlite, reducing the strength of the remaining martensite and bainite, making it impossible to obtain the desired strength. Therefore, the area fraction of pearlite is set to less than 5.0%. The area fraction of pearlite is preferably 3.0% or less. In order to improve the stretch flangeability of the hot-rolled steel sheet, it is preferable to reduce the area fraction of pearlite as much as possible, and it is even more preferable that the area fraction of pearlite is 0%.

[0044] The steel sheet according to this embodiment includes, as the remaining structure other than the retained austenite, ferrite, and pearlite, a hard structure consisting of one or more of bainite, martensite, and tempered martensite, with a total area ratio of 32.0% or more and less than 85.0%.

[0045] The area ratio of the metallographic structure is measured using the following method. A thickness cross section parallel to the rolling direction is polished to a mirror finish and then polished for 8 minutes at room temperature using colloidal silica that does not contain alkaline solution to remove strain introduced into the surface layer of the sample. At any position along the longitudinal direction of the sample cross section, a length of 50 μm, a position 1 / 4 of the thickness from the surface (the region from 1 / 8 of the thickness from the surface to 3 / 8 of the thickness from the surface), and a region at the center in the width direction of the plate are measured using electron backscatter diffraction at measurement intervals of 0.1 μm to obtain crystal orientation information. For the measurement, an EBSD analyzer consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) is used. The vacuum level inside the EBSD analyzer is 9.6 × 10 -5 Pa or less, acceleration voltage is 15 kV, probe current level is 13, and electron beam irradiation level is 62.

[0046] Furthermore, a backscattered electron image is taken in the same field of view. First, crystal grains in which ferrite and cementite are precipitated in layers are identified from the backscattered electron image, and the area ratio of these crystal grains is calculated to obtain the area ratio of pearlite. Then, for the crystal grains other than those identified as pearlite, the obtained crystal orientation information is used to determine the area where the grain average misorientation value is 1.0° or less as ferrite using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. In this case, the grain tolerance angle is set to 15°, and the area ratio of ferrite is obtained by calculating the area of ​​the area identified as ferrite.

[0047] Average spherical equivalent radius of alloy carbides in ferrite: 0.5 nm or more, less than 5.0 nm The hot-rolled steel sheet according to this embodiment has excellent fatigue properties because the mean spherical equivalent radius and mean number density of alloy carbides in ferrite are preferably controlled. If the mean spherical equivalent radius of alloy carbides in ferrite is less than 0.5 nm, the strength of ferrite against repeated deformation cannot be sufficiently increased, and the desired fatigue strength cannot be obtained. Therefore, the mean spherical equivalent radius of alloy carbides in ferrite is set to 0.5 nm or more. The mean spherical equivalent radius of alloy carbides in ferrite is preferably set to 1.0 nm or more. On the other hand, if the mean spherical equivalent radius of alloy carbides in ferrite is set to 5.0 nm or more, the strength of ferrite cannot be sufficiently increased, and due to the difference in hardness between crystal grains, cracks initiate from the cutting edge of the shearing tool very early in the shearing process, forming a fracture surface, and then a shear surface is formed again. As a result, secondary shear surfaces are likely to be formed, and the desired shearing workability cannot be obtained in the hot-rolled steel sheet. Therefore, the mean spherical equivalent radius of alloy carbides in ferrite is set to less than 5.0 nm. The average equivalent-sphere radius of the alloy carbides in the ferrite is preferably 4.0 nm or less, 3.0 nm or less, or 2.0 nm or less, and more preferably less than 1.5 nm.

[0048] Average number density of alloy carbides in ferrite: 3.5×10 16 pieces / cm 3 End The average number density of alloy carbides in ferrite is 3.5 × 10 16 pieces / cm 3 If the average number density of alloy carbides in ferrite is less than 3.5 × 10, the strength of the ferrite against repeated deformation cannot be sufficiently increased, and the desired fatigue strength cannot be obtained. 16 pieces / cm 3 The average number density of alloy carbides in the ferrite is preferably 5.0 × 10 16 pieces / cm 3 That's it, 10.0 x 10 16 pieces / cm 3 or more than 20.0 x 10 16 pieces / cm 3 That's all. Although the upper limit of the average number density of alloy carbides in ferrite is not particularly specified, the higher the density, the better. However, in the chemical composition and metal structure of the hot-rolled steel sheet according to this embodiment, the average number density of alloy carbides in ferrite is set to 1.0 × 10 19 pieces / cm 3 Therefore, the average number density of alloy carbides in ferrite is 1.0 × 10 19 pieces / cm 3 The following may also be used.

[0049] In this embodiment, the alloy carbide refers to a carbide containing one or more of Ti, Nb, Mo, and V.

[0050] The sphere-equivalent radius and number density of alloy carbides in ferrite are measured using a three-dimensional atom probe. In the three-dimensional atom probe measurement, the laser wavelength (λ) is 355 nm, the laser power is 30 pJ, and the temperature of the needle-shaped test piece is 50 K. The device used for the three-dimensional atom probe measurement is not particularly limited. For example, the three-dimensional atom probe measurement device is a product name LEAP4000XHR manufactured by Ametec Co., Ltd.

[0051] After measuring the area fraction of each structure using EBSD, samples are taken from the ferrite grains within the observation field using a focused ion beam (FIB) device. The sample is then processed into a needle shape using a well-known method, and a three-dimensional atom probe is used to accurately measure the spherical equivalent radius and number density of fine precipitates, with spherical equivalent radii ranging from less than 1 nm to several tens of nm. The number density of precipitates can be obtained by dividing the number of precipitates in the area measured with the three-dimensional atom probe by the volume of the measurement area, for precipitates identified as alloy carbides using the method described below.

[0052] The total volume of precipitates within the measurement area is obtained by dividing the total number of atoms of alloying elements (Ti, Nb, Mo, V, C) contained in all precipitates within the measurement area by the atomic density of the alloy carbide. The volume of the precipitates is obtained by dividing the total volume of the precipitates by the number of precipitates. From the obtained volume of the precipitates, the sphere-equivalent radius is calculated, assuming that the precipitates are spherical.

[0053] The above method was applied at 30,000 nm. 3 The average number density and average spherical equivalent radius are obtained by performing measurements on five or more samples with a volume of the measurement area above. The observation area is the area where the Ga introduced during FIB processing is less than 0.025 at%; areas with Ga contamination of 0.025 at% or more are excluded from the measurement area. The Ga amount can be confirmed in the longitudinal direction of the needle sample using the 1D Concentration Profile function in the data analysis software IVAS 3.6.14 (CAMECA Instruments Inc.).

[0054] Whether or not the observed precipitates are alloy carbides is determined by using the Cluster Analysis function of the analysis software IVAS 3.6.14, which uses data acquired by a three-dimensional atom probe. The analysis parameters used are dmax = 1.2 nm, Order = 10, Nmin = 10, L = 0.5 nm, and d erosion = 0.5 nm, and precipitates recognized as clusters are identified as alloy carbides.

[0055] E value: 10.7 or higher I value: 1.020 or more To prevent the secondary shear plane from occurring, it is important to form a fracture surface after sufficient shear planes have been formed, and it is necessary to prevent early cracks from occurring from the cutting edge of the tool during shearing. To achieve this, it is important that the metal structure has low periodicity and high uniformity. In this embodiment, the secondary shear plane is prevented from occurring by controlling the E (Entropy) value, which indicates the periodicity of the metal structure, and the I (Inverse Difference Normalized) value, which indicates the uniformity of the metal structure.

[0056] The E value represents the periodicity of the metal structure. When brightness is periodically arranged due to the influence of the formation of a band-shaped structure, i.e., when the periodicity of the metal structure is high, the E value decreases. In this embodiment, a metal structure with low periodicity is required, so the E value needs to be increased. If the E value is less than 10.7, secondary shear planes are likely to occur. Starting from the periodically arranged structure, cracks are initiated from the cutting edge of the shearing tool very early in the shearing process, forming a fracture surface, and then a shear surface is formed again. This is presumably what makes secondary shear planes more likely to occur. Therefore, the E value is set to 10.7 or more. It is preferably 10.8 or more, and more preferably 11.0 or more. The higher the E value, the better. There is no particular upper limit specified, but it may be 13.0 or less, 12.5 or less, or 12.0 or less.

[0057] The I value represents the uniformity of the metal structure, and increases as the area of ​​the region with a constant brightness increases. A high I value indicates a high uniformity of the metal structure. In this embodiment, a highly uniform metal structure is required, so the I value must be increased. If the I value is less than 1.020, cracks will initiate from the cutting edge of the shearing tool very early in the shearing process due to the influence of precipitates within the crystal grains and the hardness distribution caused by differences in element concentration, forming a fracture surface, and then a shear surface will form again. This is presumably likely to make secondary shear surfaces more likely to occur. Therefore, the I value is set to 1.020 or more. It is preferably 1.025 or more, and more preferably 1.030 or more. The higher the I value, the better. There is no particular upper limit, but it may be 1.200 or less, 1.150 or less, or 1.100 or less.

[0058] The E value and I value can be obtained by the following method. In this embodiment, the region of the SEM image taken to calculate the E value and I value is a position at a depth of 1 / 4 of the sheet thickness from the surface of the steel sheet (a region from a depth of 1 / 8 of the sheet thickness from the surface to a depth of 3 / 8 of the sheet thickness from the surface) and at the center position in the sheet width direction in the sheet thickness cross section parallel to the rolling direction. To take the SEM image, a SU-6600 Schottky electron gun manufactured by Hitachi High-Technologies Corporation is used, with a tungsten emitter and an acceleration voltage of 1.5 kV. Under these settings, the SEM image is output at a magnification of 1000x and a grayscale of 256 gradations.

[0059] Next, the obtained SEM image is cut into an 880 x 880 pixel region and subjected to smoothing processing with a tile grid size of 8 x 8 and a contrast enhancement limiting factor of 2.0, as described in Non-Patent Document 3. The smoothed SEM image is rotated counterclockwise in 1-degree increments from 0 to 179 degrees, excluding 90 degrees, and an image is created for each degree, resulting in a total of 179 images. Next, for each of these 179 images, the GLCM method described in Non-Patent Document 1 is used to extract the frequency values ​​of brightness between adjacent pixels in the form of a matrix.

[0060] The matrix of 179 frequency values ​​collected by the above method is expressed as p k (k=0···89, 91,···179). For each image, the generated p k are summed for all k (k=0...89, 91...179), and then a 256 x 256 matrix P is calculated that is normalized so that the sum of each component is 1. Furthermore, the E value and I value are calculated using the following formulas (1) and (2) described in Non-Patent Document 2. In the following formulas (1) and (2), the value in the ith row and jth column of the matrix P is taken as P ij It is written as follows.

[0061]

number

[0062]

number

[0063] Standard deviation of Mn concentration: 0.60 mass% or less The standard deviation of the Mn concentration at a depth of 1 / 4 of the sheet thickness from the surface (a region from 1 / 8 of the sheet thickness from the surface to 3 / 8 of the sheet thickness from the surface) and at the center position in the sheet width direction of the hot-rolled steel sheet according to this embodiment is 0.60% by mass or less. This allows the hard phase to be uniformly dispersed, preventing cracks from occurring from the cutting edge of the shearing tool very early in the shearing process. As a result, the occurrence of secondary shear planes can be suppressed. The standard deviation of the Mn concentration is preferably 0.50% by mass or less, and more preferably 0.47% by mass or less. From the viewpoint of suppressing excessive burrs, the lower limit of the standard deviation of the Mn concentration is preferably as small as possible, but due to constraints of the manufacturing process, the practical lower limit is 0.10% by mass.

[0064] After mirror polishing a cross section of the hot-rolled steel sheet parallel to the rolling direction, the standard deviation of the Mn concentration is measured using an electron probe microanalyzer (EPMA) at a depth of 1 / 4 of the sheet thickness from the surface (a region from 1 / 8 of the sheet thickness from the surface to 3 / 8 of the sheet thickness from the surface) and at the center position in the sheet width direction. The measurement conditions are an acceleration voltage of 15 kV and a magnification of 5000x, and a distribution image is measured over a range of 20 μm in the rolling direction and 20 μm in the thickness direction of the sample. More specifically, the measurement interval is 0.1 μm, and the Mn concentration is measured at 40,000 or more locations. The standard deviation is then calculated based on the Mn concentrations obtained from all measurement points to obtain the standard deviation of the Mn concentration.

[0065] Tensile properties Among the mechanical properties of hot-rolled steel sheets, the tensile strength characteristics (tensile strength, total elongation) are evaluated in accordance with JIS Z 2241:2011. The test specimen is a No. 5 test specimen of JIS Z 2241:2011. The tensile test specimen is taken from a quarter section from the end in the sheet width direction, with the direction perpendicular to the rolling direction as the longitudinal direction.

[0066] The hot-rolled steel sheet according to this embodiment has a tensile (maximum) strength of 980 MPa or more. The tensile strength is preferably 1000 MPa or more. By making the tensile strength 980 MPa or more, the applicable parts are not limited and it can greatly contribute to reducing the weight of the vehicle body. There is no particular need to set an upper limit, but it may be 1780 MPa from the viewpoint of suppressing die wear.

[0067] The total elongation is preferably 10.0% or more, and the product of tensile strength and total elongation (TS × El) is preferably 13,000 MPa·% or more. The total elongation is more preferably 11.0% or more, and even more preferably 13.0% or more. The product of tensile strength and total elongation is more preferably 14,000 MPa·% or more, and even more preferably 15,000 MPa·% MPa or more. By achieving a total elongation of 10.0% or more and a product of tensile strength and total elongation of 13,000 MPa·% or more, there are no restrictions on the parts to which the steel can be applied, and it can make a significant contribution to reducing the weight of vehicle bodies.

[0068] Fatigue properties If softening occurs during repeated deformation, the fatigue life may be significantly reduced. Therefore, it is preferable that softening does not occur during repeated deformation. Whether softening occurs during repeated deformation can be determined by the following method. Test pieces are taken from the 1 / 4 position in the width direction of the hot-rolled steel sheet in accordance with JIS Z 2275-1978 so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction. Using these test pieces, a plane bending fatigue test is performed in accordance with JIS Z 2275-1978. The number of repetitions to fracture is 1 × 10 5 More than 3 times 10 5 Repeated stress less than 3 × 10 5 More than 3 times 10 6 Repeated stress less than 3 × 10 6 More than 1×10 7A plane bending fatigue test is performed at a cyclic stress of less than 100 cycles. The torque during the fatigue test or the value of a strain gauge attached to the test piece is measured to evaluate the change in cyclic stress. The cyclic hardening rate is calculated in the plane bending fatigue test at each cyclic stress. The cyclic hardening rate is defined as (minimum value of cyclic stress at more than 100 cycles / cyclic stress at 100 cycles). If the minimum value of the cyclic hardening rate at each cyclic stress is 1.00 or more, it can be determined that cyclic softening does not occur and that the hot-rolled steel sheet has excellent fatigue properties.

[0069] Plate thickness The thickness of the hot-rolled steel sheet according to this embodiment is not particularly limited, but may be 0.5 to 8.0 mm. If the thickness of the hot-rolled steel sheet is less than 0.5 mm, it may be difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the hot-rolled steel sheet according to this embodiment may be 0.5 mm or more. Preferably, it is 1.2 mm or more or 1.4 mm or more. On the other hand, if the plate thickness exceeds 8.0 mm, it may be difficult to refine the metal structure, and it may be difficult to obtain the above-mentioned metal structure. Therefore, the plate thickness may be 8.0 mm or less, and preferably 6.0 mm or less.

[0070] plating layer The hot-rolled steel sheet according to this embodiment, having the above-described chemical composition and metallographic structure, may be provided with a plating layer on its surface to provide a surface-treated steel sheet for the purpose of improving corrosion resistance, etc. The plating layer may be an electroplated layer or a hot-dip plated layer. Examples of electroplated layers include electrogalvanized coating and electrolytic Zn-Ni alloy coating. Examples of hot-dip plated layers include hot-dip galvanized coating, alloyed hot-dip galvanized coating, hot-dip aluminum coating, hot-dip Zn-Al alloy coating, hot-dip Zn-Al-Mg alloy coating, and hot-dip Zn-Al-Mg-Si alloy coating. The coating weight is not particularly limited and may be the same as conventional coatings. 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).

[0071] Manufacturing conditions A suitable method for producing the hot-rolled steel sheet according to this embodiment having the above-described chemical composition and metallographic structure is as follows.

[0072] In order to obtain the hot-rolled steel sheet according to this embodiment, it is effective to heat a slab under predetermined conditions, then hot-roll it, accelerate cooling it to a predetermined temperature range, then slow cooling it, and control the cooling history until coiling.

[0073] In a preferred method for manufacturing a hot-rolled steel sheet according to this embodiment, the following steps (1) to (10) are performed in sequence. Note that the slab temperature and the steel sheet temperature in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet. Also, the stress refers to the stress applied in the rolling direction of the steel sheet. (1) The slab is held in a temperature range of 700 to 850°C for 900 seconds or more, and then further heated and held in a temperature range of 1100°C or higher for 6000 seconds or more. (2) Hot rolling is performed in the temperature range of 850 to 1100°C so that the plate thickness is reduced by a total of 90% or more. (3) The next to last stage of rolling is carried out at a temperature of 900°C or higher and lower than 1010°C, and a stress of 170 kPa or higher is applied to the steel sheet after the next to last stage of hot rolling and before the final stage of rolling. (4) The reduction rate in the final stage of hot rolling is set to 8% or more, and hot rolling is completed so that the rolling completion temperature Tf is 900°C or more and less than 1010°C. (5) In the temperature range of 840°C or more and less than 900°C, soft reduction is performed so that the plate thickness is reduced by a total of 5% or more and less than 8%. (6) The stress applied to the steel sheet after the final stage of hot rolling and before the first soft reduction rolling, and the stress applied to the steel sheet after the final stage of soft reduction rolling until the steel sheet is cooled to 800°C, shall be less than 200 kPa. (7) After completion of soft reduction, accelerate cooling is performed to a temperature range of 680 to 730°C at an average cooling rate of 50°C / second or more. (8) Slow cooling is carried out for 2.0 seconds or more in the temperature range of 680 to 730°C at an average cooling rate of less than 5°C / s. (9) Cool to a temperature range of 350°C or less at an average cooling rate of 50°C / s or more. (10) Coiling is performed at a temperature of 350°C or less.

[0074] By adopting the above-mentioned manufacturing method, it is possible to stably manufacture a hot-rolled steel sheet having high strength, as well as excellent ductility, fatigue properties, and shear workability. That is, by appropriately controlling the slab heating conditions and hot rolling conditions, it is possible to reduce Mn segregation and make pre-transformation austenite equiaxed, and in combination with the cooling conditions after hot rolling described below, it is possible to stably manufacture a hot-rolled steel sheet having a desired metallographic structure.

[0075] (1) Slab temperature and holding time when subjected to hot rolling The slabs used for hot rolling can be slabs obtained by continuous casting or by casting and blooming, and if necessary, can be hot-worked or cold-worked. The slabs used for hot rolling are preferably heated at a temperature of 700 to 850°C for 900 seconds or more, and then further heated and held at a temperature of 1100°C or higher for 6000 seconds or more. The upper limit of the heating temperature during slab heating is not particularly limited, but may be 1350°C or lower from the viewpoint of thermal efficiency.

[0076] When the steel sheet is held in the temperature range of 700 to 850°C, the temperature may be varied within this temperature range or may be kept constant. When the steel sheet is held at 1100°C or higher, the temperature may be varied within the temperature range of 1100°C or higher or may be kept constant.

[0077] During austenite transformation in the temperature range of 700 to 850°C, Mn distributes between ferrite and austenite, and by extending the transformation time, Mn can diffuse within the ferrite region. This eliminates Mn microsegregation unevenly distributed in the slab and significantly reduces the standard deviation of the Mn concentration. Therefore, it is preferable to hold the slab in the temperature range of 700 to 850°C for 900 seconds or more. Furthermore, by holding the slab in the temperature range of 1100°C or higher for 6000 seconds or more, the standard deviation of the Mn concentration can be significantly reduced.

[0078] The hot rolling is preferably performed using a reverse mill or a tandem mill as a multi-pass rolling. In particular, from the viewpoints of industrial productivity and stress load on the steel sheet during rolling, it is more preferable to perform hot rolling using a tandem mill for at least the final two stages.

[0079] (2) Hot rolling reduction: A total thickness reduction of 90% or more in the temperature range of 850 to 1100°C Hot rolling in a temperature range of 850 to 1100°C to reduce the thickness by 90% or more in total mainly results in the refinement of recrystallized austenite grains and promotes the accumulation of strain energy in unrecrystallized austenite grains. This promotes austenite recrystallization and Mn atomic diffusion, thereby reducing the standard deviation of the Mn concentration. Therefore, it is preferable to perform hot rolling in a temperature range of 850 to 1100°C to reduce the thickness by 90% or more in total.

[0080] The total thickness reduction in the temperature range of 850 to 1100°C can be expressed as {(t0-t1) / t0} × 100(%), where t0 is the entrance thickness before the first rolling in this temperature range and t1 is the exit thickness after the final rolling in this temperature range.

[0081] (3) Rolling temperature before the last stage: 900°C or higher, but lower than 1010°C; stress after rolling before the last stage of hot rolling and before rolling in the last stage: 170 kPa or higher It is preferable that the next to last stage of hot rolling is performed at 900°C or higher and lower than 1010°C, and the stress applied to the steel sheet after the next to last stage of hot rolling and before the final stage of rolling is 170 kPa or higher. As a result, the {110} <001> The number of grains with a crystal orientation of {110} can be reduced. <001> Since this is a crystal orientation that is difficult to recrystallize, suppressing the formation of this crystal orientation can effectively promote recrystallization during the final reduction stage. As a result, the band-shaped structure of the hot-rolled steel sheet is improved, the periodicity of the metal structure is reduced, and the E value increases. If the stress applied to the steel sheet is less than 170 kPa, the desired E value may not be obtained. The stress applied to the steel sheet is more preferably 190 kPa or more. There is no particular upper limit to the stress applied to the steel sheet, but it can be 350 kPa or less. The stress applied to the steel sheet is tension in the rolling direction and can be controlled by adjusting the roll rotation speed during tandem rolling.

[0082] (4) Reduction ratio in the final stage of hot rolling: 8% or more, hot rolling completion temperature Tf: 900°C or more, less than 1010°C It is preferable that the reduction rate in the final stage of hot rolling is 8% or more, and the hot rolling completion temperature Tf is 900°C or more. By setting the reduction rate in the final stage of hot rolling to 8% or more, recrystallization due to the reduction in the final stage can be promoted. As a result, the band-shaped structure of the hot-rolled steel sheet is improved, the periodicity of the metallographic structure is reduced, and the E value is increased. By setting the hot-rolling completion temperature Tf to 900°C or more, an excessive increase in the number of ferrite nucleation sites in austenite can be suppressed. As a result, the generation of ferrite in the final structure (the metallographic structure of the hot-rolled steel sheet after production) can be suppressed, and the desired strength can be obtained. Note that the upper limit of the reduction rate in the final stage of hot rolling is not particularly limited, but can be set to 40% or less. Furthermore, by setting Tf to less than 1010°C, coarsening of the austenite grain size can be suppressed, the periodicity of the metallographic structure can be reduced, and the desired E value can be obtained.

[0083] (5) In the temperature range of 840°C or more and less than 900°C, soft reduction is performed so that the plate thickness is reduced by a total of 5% or more and less than 8%. After the final stage of hot rolling, it is preferable to perform soft reduction so as to reduce the sheet thickness by a total of 5% to less than 8% in a temperature range of 840° C. to less than 900° C. This makes it possible to control the average equivalent sphere radius and average number density of alloy carbides in ferrite to desired values. The soft reduction may be carried out, for example, in the final stage of a finishing mill, or by installing new reduction equipment between the finishing mill and the cooling bed. The soft reduction may also be carried out in multiple stages using multiple rolls.

[0084] The total thickness reduction during soft reduction can be expressed as {(t0-t1) / t0}×100(%), where t0 is the entrance thickness before the first soft reduction and t1 is the exit thickness after the final soft reduction.

[0085] (6) The stress applied to the steel plate after the final stage of hot rolling and before the first soft reduction, and the stress applied to the steel plate after the final stage of soft reduction and until the steel plate is cooled to 800°C: less than 200 kPa The stress applied to the steel sheet after the final stage of hot rolling and before the first soft reduction rolling, and the stress applied to the steel sheet after the final stage of soft reduction rolling until the steel sheet is cooled to 800°C, are each preferably less than 200 kPa. By applying a stress of less than 200 kPa to the steel sheet at the above locations, austenite recrystallization proceeds preferentially in the rolling direction, and an increase in the periodicity of the metal structure can be suppressed. As a result, the desired E value can be obtained. The stress applied to the steel sheet at the above locations is more preferably 180 kPa or less.

[0086] (7) After soft reduction is completed, accelerated cooling is performed at an average cooling rate of 50°C / sec or more to a temperature range of 680-730°C. In order to suppress the growth of austenite grains refined by hot rolling, it is preferable to perform accelerated cooling at an average cooling rate of 50°C / sec or more to a temperature range of 730°C or less after completion of soft reduction. By performing accelerated cooling to a temperature range of 730°C or less, the formation of ferrite or pearlite, which has a small amount of precipitation strengthening, can be suppressed, thereby improving the strength of the hot-rolled steel sheet.

[0087] The average cooling rate referred to here means the value obtained by dividing the temperature drop of the steel plate from the start of accelerated cooling (when the steel plate is introduced into the cooling equipment) to the end of accelerated cooling (when the steel plate is removed from the cooling equipment) by the time required from the start of accelerated cooling to the end of accelerated cooling.

[0088] Although there is no particular upper limit for the cooling rate, increasing the cooling rate requires larger cooling equipment and higher equipment costs. Therefore, considering equipment costs, the average cooling rate is preferably 300°C / sec or less. In addition, to obtain the desired amount of ferrite, the cooling stop temperature of the accelerated cooling should be 680°C or higher. In order to realize the above-mentioned average cooling rate, cooling at a high average cooling rate may be carried out after completion of soft reduction, for example by spraying cooling water onto the surface of the steel sheet.

[0089] (8) In the temperature range of 680 to 730°C, slow cooling is performed at an average cooling rate of less than 5°C / s for 2.0 seconds or more. By performing slow cooling at an average cooling rate of less than 5°C / s for 2.0 seconds or longer in the temperature range of 680 to 730°C, precipitation-strengthened ferrite can be sufficiently precipitated. This allows the hot-rolled steel sheet to achieve both strength and ductility. The average cooling rate here refers to the value obtained by dividing the temperature drop of the steel sheet from the cooling stop temperature of accelerated cooling to the slow cooling stop temperature by the time required from the stop of accelerated cooling to the stop of slow cooling.

[0090] The time for slow cooling is preferably 3.0 seconds or more. The upper limit of the time for slow cooling is determined by the equipment layout, but it is generally sufficient to set it to less than 10.0 seconds. There is no particular lower limit for the average cooling rate for slow cooling, but since raising the temperature without cooling requires a large investment in equipment, it may be set to 0°C / s or more.

[0091] (9) Average cooling rate to coiling temperature: 50°C / sec or more In order to suppress the area ratio of pearlite and obtain the desired strength, it is preferable to set the average cooling rate from the cooling stop temperature of the slow cooling to the coiling temperature to 50°C / sec or more, which makes it possible to harden the matrix structure. The average cooling rate referred to here means the value obtained by dividing the temperature drop of the steel sheet from the cooling stop temperature of slow cooling, where the average cooling rate is less than 5°C / s, to the coiling temperature, by the time required from the stop of slow cooling, where the average cooling rate is less than 5°C / s, to coiling.

[0092] (10) Winding temperature: 350°C or less The coiling temperature is preferably 350°C or less. By setting the coiling temperature to 350°C or less, the amount of iron carbide precipitated can be reduced, and the variation in hardness distribution within the hard phase can be reduced. As a result, the desired I value can be obtained. The lower limit of the coiling temperature is not particularly limited, but can be room temperature. [Example]

[0093] 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.

[0094] Steels having the chemical compositions shown in Tables 1 and 2 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 5-1 to 6-2 under the manufacturing conditions shown in Tables 3-1 to 4-2. The average cooling rate of the slow cooling was less than 5°C / s. In addition, since 50°C is the lower limit of measurement for the coiling temperatures listed in Tables 4-1 and 4-2, the actual coiling temperature in examples listed as 50°C is 50°C or less. In addition, except for Production No. 7, the rolling one stage before the final stage of hot rolling was carried out at 900°C or higher and less than 1010°C. The hot-rolled steel sheet of Production No. 46 was produced under the conditions where the above-mentioned soft reduction was not performed. In Production No. 46, the load stress was 170 kPa from the time of rolling in the final stage of hot rolling until the steel sheet was cooled to 800°C.

[0095] The area fraction of the metallographic structure, E value, I value, standard deviation of Mn concentration, average spherical equivalent radius and average number density of alloy carbides in ferrite, tensile strength TS, and total elongation El were determined for the obtained hot-rolled steel sheets using the methods described above. Furthermore, fatigue properties were evaluated by performing a plane bending fatigue test using the methods described above. The measurement results are shown in Tables 5-1 to 6-2. The remaining structure was determined to be a hard structure, based on the chemical composition and manufacturing method of the steel, and to be one or more of bainite, martensite, and tempered martensite.

[0096] Evaluation method for properties of hot-rolled steel sheets Tensile properties If the tensile strength TS was 980 MPa or more, the total elongation El was 10.0% or more, and the tensile strength TS × total elongation El was 13,000 MPa·% or more, the hot-rolled steel sheet was deemed to have high strength and excellent ductility and passed the test. If any one of these conditions was not met, the hot-rolled steel sheet was deemed to have neither high strength nor excellent ductility and was therefore deemed to have failed the test.

[0097] Fatigue properties The cyclic hardening rate was determined for each cyclic stress by performing a plane bending fatigue test using the method described above. If the minimum value of the cyclic hardening rate for each cyclic stress was 1.00 or more, the hot-rolled steel sheet was judged to have passed the test, since no cyclic softening occurred and the sheet had excellent fatigue properties. On the other hand, if the minimum value of the cyclic hardening rate for each cyclic stress was less than 1.00, the hot-rolled steel sheet was judged to have poor fatigue properties and failed the test.

[0098] Shear workability (secondary shear surface evaluation) The shear workability of the hot-rolled steel sheets was evaluated by a punching test. Three punched holes were created for each example with a hole diameter of 10 mm, a clearance of 10%, and a punching speed of 3 m / s. Next, the thickness cross sections of the punched holes perpendicular to the rolling direction and parallel to the rolling direction were embedded in resin, and the cross-sectional shapes were photographed using a scanning electron microscope. The sheared edge surfaces shown in Figure 1 and Figure 2 can be observed in the obtained observation photographs. Note that Figure 1 shows an example of the sheared edge surface of a hot-rolled steel sheet according to an example of the present invention, and Figure 2 shows an example of the sheared edge surface of a hot-rolled steel sheet according to a comparative example. Figure 1 shows the sheared edge surface with sag, shear surface, fracture surface, and burr. Meanwhile, Figure 2 shows the sheared edge surface with sag, shear surface, fracture surface, shear surface, fracture surface, and burr. Here, sag refers to the smooth, rounded surface area, shear surface refers to the area of ​​the punched edge that has been separated by shear deformation, fracture surface refers to the area of ​​the punched edge that has been separated by a crack that has initiated near the cutting edge, and burr refers to the surface with a protrusion that protrudes from the underside of the hot-rolled steel plate.

[0099] If a sheared surface-fracture surface-sheared surface pattern, such as that shown in Figure 2, was observed on two surfaces perpendicular to the rolling direction and two surfaces parallel to the rolling direction among the obtained sheared edges, it was determined that a secondary sheared surface had been formed. Four surfaces per punched hole, for a total of 12 surfaces, were observed, and if no secondary sheared surfaces were found, the hot-rolled steel sheet was deemed to have excellent shear workability and passed the test, and this was recorded as "absent" in the table. On the other hand, if even one secondary sheared surface was found, the hot-rolled steel sheet was deemed to have poor shear workability and failed the test, and this was recorded as "present" in the table.

[0100]

Table 1

[0101]

Table 2

[0102]

Table 3-1

[0103]

Table 3-2

[0104]

Table 4-1

[0105]

Table 4-2

[0106]

Table 5-1

[0107]

Table 5-2

[0108]

Table 6-1

[0109]

Table 6-2

[0110] It can be seen from Tables 5-1 to 6-2 that the hot-rolled steel sheets according to the examples of the present invention have high strength, as well as excellent ductility, fatigue properties and shear 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]

[0111] According to the above-described aspects of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, as well as excellent ductility, fatigue properties, and shear workability. The hot-rolled steel sheet according to the present invention is suitable as an industrial material used for automobile parts, machine structural parts, and even building parts.

Claims

1. The chemical composition, in mass%, is C: 0.050-0.250%, Si: 0.05-3.00%, Mn: 1.00-4.00%, one or more of Ti, Nb, and V: 0.060 to 0.500% in total; sol. Al: 0.001 to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Cu: 0-2.00%, Cr: 0-2.00%, Mo: 0-1.00%, Ni: 0-2.00%, B: 0 to 0.0100%, Ca: 0-0.0200%, Mg: 0 to 0.0200%, REM: 0-0.1000%, Bi: 0 to 0.020%, One or more of Zr, Co, Zn, and W: 0 to 1.00% in total; and Sn: 0 to 0.05%; the balance being Fe and impurities; The metal structure is In area %, The retained austenite is less than 3.0%; Ferrite is 15.0% or more and less than 60.0%; Pearlite is less than 5.0%; The alloy carbide in the ferrite has an average equivalent-sphere radius of 0.5 nm or more and less than 5.0 nm, and an average number density of 3.5 × 10 １６ pieces / cm ３ That's all, the E value indicating the periodicity of the metal structure is 10.7 or more, The I value indicating the uniformity of the metal structure is 1.020 or more, The standard deviation of the Mn concentration is 0.60 mass% or less, A hot-rolled steel sheet having a tensile strength of 980 MPa or more.

2. The chemical composition is, in mass %, Cu: 0.01-2.00%, Cr: 0.01-2.00%, Mo: 0.01-1.00%, Ni: 0.02-2.00%, B: 0.0001 to 0.0100%, Ca: 0.0005-0.0200%, Mg: 0.0005-0.0200%, REM: 0.0005 to 0.1000%, and Bi: 0.0005-0.020% The hot-rolled steel sheet according to claim 1, further comprising one or more selected from the group consisting of:

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