Hot-rolled steel plate

Abstract

This hot-rolled steel sheet has a desired chemical composition; the metal structure of this hot-rolled steel sheet at a position where the depth from the surface is 1 / 4 in the sheet thickness direction comprises, in area%, less than 3.0% of residual austenite, not less than 15.0% but less than 60.0% of ferrite and less than 5.0% of pearlite; alloy carbides in the ferrite has an average sphere equivalent radius of not less than 0.5 nm but less than 10.0 nm, and an average number density of not less than 0.10 × 1016 per cm3 but less than 1.45 × 1016 per cm3; the E value that indicates the periodicity of the metal structure is 10.7 or more; the I value that indicates the uniformity of the metal structure is 1.020 or more; and the standard deviation of the Mn concentration is 0.60% by mass or less.

Description

[Technical Field]

[0001] This invention relates to hot-rolled steel sheets. More specifically, it relates to hot-rolled steel sheets that are formed into various shapes by press working or the like and used for various purposes, and in particular to hot-rolled steel sheets that are high in strength and have excellent ductility, fatigue properties, and shear workability. This application claims priority based on Japanese Patent Application No. 2022-135960, filed in Japan on August 29, 2022, and the contents of that application 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 lighten vehicle bodies in order to improve fuel efficiency. However, since emphasis is also placed on improving collision resistance in order to ensure the safety of occupants, reducing vehicle body weight is not easy.

[0003] To achieve both vehicle weight reduction and collision resistance, the use of high-strength steel plates to thin components is being considered. Therefore, there is a strong demand for steel plates that possess both high strength and excellent formability. Several technologies have been proposed to meet these requirements. While the required formability varies depending on the component due to the various processing methods used for automotive components, ductility is considered a crucial indicator of formability.

[0004] Furthermore, while automotive components are formed by press forming, the blank sheets used for this press forming are often manufactured by shearing, which offers high productivity. Blank sheets manufactured by shearing require excellent edge face accuracy after shearing. For example, if a secondary shear surface occurs on the edge face after shearing (sheared edge face), where the surface consists of a shear surface, a fracture surface, and another shear surface, the accuracy of the sheared edge face deteriorates significantly.

[0005] For example, Patent Document 1 discloses a high-strength steel sheet with a tensile strength of 980 MPa or more, excellent ductility and flange elongation, in which a second phase composed of retained austenite and / or martensite is finely dispersed in crystal grains.

[0006] Patent Document 2 discloses a technique for controlling the burr height after punching by controlling the ratio d s of the ferrite grain size d b on the surface to the ferrite crystal grain d s inside to 0.95 or less. b

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0008]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

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

[0010] In addition, higher-strength steel sheets may be required to have more excellent fatigue characteristics.

[0011] The present invention has been made in view of the above problems of the prior art, and an object thereof is to provide a hot-rolled steel sheet having high strength, excellent ductility, fatigue characteristics, and shearing properties.

Means for Solving the Problems

[0012] The gist of the present invention is as follows. (1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition in 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: a total of 0.060 to 0.500%, sol.Al: 0.001 to 2.oo0%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Cu: 0 to 2.00%, Cr: 0 to 2.00%, Mo: 0 to 1.00%, Ni: 0 to 2.00%, B: 0 to 0.0100%, Ca: 0 to 0.0200%, Mg: 0 to 0.0200%, REM: 0 to 0.1000%, Bi: 0~0.0200%, As: 0~0.100%, It contains one or more of the following elements: Zr, Co, Zn, and W, totaling 0-1.00%, and Sn: 0-0.05%. The remainder consists of Fe and impurities. The metallic structure, In area percentage, The residual austenite content is less than 3.0%. The ferrite content is 15.0% or more and less than 60.0%. The perlite content is less than 5.0%. The average equivalent spherical radius of the alloy carbides in the ferrite is 0.5 nm or more and less than 10.0 nm, and the average number density is 0.10 × 10⁻⁶. 16 pieces / cm 3 The above is 1.45 × 10 16 pieces / cm 3 It is less than, The E value indicating the periodicity of the aforementioned metal structure is 10.7 or higher. The I value indicating the uniformity of the metal structure is 1.020 or higher. The standard deviation of the Mn concentration is 0.60 mass% or less. (2) The hot-rolled steel sheet described in (1) above has the chemical composition in mass %, Cu: 0.01~2.00%, Cr: 0.01~2.00%, Mo: 0.01~1.00%, Ni: 0.02~2.00%, B: 0.0001~0.0100%, Ca: 0.0005~0.0200%, Mg: 0.0005~0.0200%, REM: 0.0005~0.1000%, Bi: 0.0005~0.0200%, and As: 0.001~0.100% It may contain one or more selected from the group consisting of the following. [Effects of the Invention]

[0013] According to the above embodiment of the present invention, a hot-rolled steel sheet having high strength, as well as excellent ductility, fatigue properties, and shear workability can be obtained. The hot-rolled steel sheet according to the above embodiment of the present invention is suitable as an industrial material used in automobile components, machine structural components, and building components. [Brief explanation of the drawing]

[0014] [Figure 1] This is an example of a sheared end face of a hot-rolled steel sheet according to the present invention. [Figure 2] This is an example of a sheared end face of a hot-rolled steel sheet in a comparative example. [Modes for carrying out the invention]

[0015] The chemical composition and microstructure of the hot-rolled steel sheet according to this embodiment will be described in detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the invention.

[0016] The numerical ranges indicated below, separated by a "~", include both a lower and upper limit. Numbers indicated as "less than" or "greater than" are not included in the numerical range. In the following explanation, percentages for the chemical composition of steel plates refer to mass percentages unless otherwise specified.

[0017] chemical composition The hot-rolled steel sheet according to this embodiment contains, by mass%, C: 0.050-0.250%, Si: 0.05-3.00%, Mn: 1.00-4.00%, one or more of Ti, Nb, and V: 0.060-0.500% 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, and the remainder: Fe and impurities. Each element is described in detail below.

[0018] C: 0.050~0.250% Carbon (C) increases the area ratio of the hard phase and, by bonding with precipitation-strengthening elements such as Ti, Nb, and V, increases the strength of ferrite. If the C content is less than 0.050%, the desired strength cannot be obtained. Therefore, the C content should be 0.050% or more. Preferably, the C content is 0.060% or more, more preferably 0.070% or more, and even more preferably greater than 0.070%, 0.075% or more, or 0.080% or more. On the other hand, if the carbon content exceeds 0.250%, the area ratio of ferrite decreases, which reduces the ductility of the hot-rolled steel sheet. Therefore, the carbon content should be 0.250% or less. Preferably, the carbon content is 0.200% or less, 0.180% or less, or 0.150% or less.

[0019] Si: 0.05~3.00% Si has the effect of promoting ferrite formation to improve the ductility of hot-rolled steel sheets and the effect of solid-solution strengthening of ferrite to increase the strength of hot-rolled steel sheets. In addition, Si has the effect of soundening steel through 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 should be 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 properties, chemical treatment properties, ductility, and weldability of the steel sheet deteriorate significantly, and the A3 transformation point rises significantly. This makes it difficult to perform stable hot rolling. Therefore, the Si content should be 3.00% or less. Preferably, the Si content is 2.50% or less, and more preferably 2.00% or less or 1.50% or less.

[0020] Mn: 1.00~4.00% Mn has the effect of suppressing ferrite transformation and increasing the strength of hot-rolled steel sheets. If the Mn content is less than 1.00%, the desired tensile strength cannot be obtained. Therefore, the Mn content should be 1.00% or more. Preferably, the Mn content is 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 becomes a periodic band-like structure due to Mn segregation, making it difficult to obtain the desired shear workability. Therefore, the Mn content should be 4.00% or less. Preferably, the Mn content is 3.50% or less, and more preferably 3.00% or less or 2.50% or less.

[0021] Ti, Nb, and V: One or more of these: 0.060-0.500% in total Ti, Nb, and V precipitate as carbides and nitrides in steel, improving its strength through precipitation strengthening. Furthermore, they are essential elements for obtaining desired fatigue properties. If the total content of Ti, Nb, and V is less than 0.060%, these effects cannot be obtained. Therefore, the total content of Ti, Nb, and V should be 0.060% or more. Note that it is not necessary for all three elements to be present; it is sufficient if any one of them is present, as long as 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, and even more preferably 0.120% 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 should be 0.500% or less. Preferably, it should be 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% Al, like Si, has the effect of sounding down steel through deoxidation, and also promotes ferrite formation, thereby increasing the ductility of hot-rolled steel sheets. The effects of the above effects cannot be obtained if the sol.Al content is less than 0.001%. Therefore, the sol.Al content should be 0.001% or more. Preferably, the sol.Al content is 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 effect becomes saturated and it is not economically desirable; therefore, the sol.Al content should be 2.000% or less. Preferably, the sol.Al content is 0.400% or less, more preferably 0.300% or less, and even more preferably 0.250% or less. Note that sol.Al refers to acid-soluble Al, specifically solid-solution Al present in steel.

[0023] P:0.100% or less P is an element that enhances the strength of hot-rolled steel sheets through solid solution strengthening. Therefore, P may be actively included. 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 should be 0.100% or less. Preferably, the P content is 0.030% or less. There is no need to specifically define the lower limit of the P content, but from the viewpoint of refining costs, it is preferable to set it at 0.001%.

[0024] S: 0.0300% or less S forms sulfide inclusions in the steel, reducing the ductility of hot-rolled steel sheets. If the S content exceeds 0.0300%, the ductility of the hot-rolled steel sheet decreases significantly. Therefore, the S content should be 0.0300% or less. Preferably, the S content is 0.0050% or less. There is no need to specifically define the lower limit of the S content, but from the viewpoint of refining costs, it is preferable to set it at 0.0001%.

[0025] N: 0.1000% or less N 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 sheet decreases significantly. Therefore, the N content should be 0.1000% or less. Preferably, the N content is 0.0800% or less, more preferably 0.0700% or less, and even more preferably 0.0050% or less. There is no need to specifically define the lower limit of the N content, but when one or more of Ti, Nb, and V are included to further refine the metal structure, it is preferable that the N content be 0.0010% or more, and more preferably 0.0020% or more, in order to promote the precipitation of carbonitrides.

[0026] O: 0.0100% or less When oxygen (O) is present in large quantities in steel, it forms coarse oxides that act as fracture initiation points, leading to brittle fracture and hydrogen-induced cracking. Therefore, the O content should be 0.0100% or less. Preferably, the O content is 0.0080% or less, more preferably 0.0055% or less, and even more preferably 0.0050% or less. In order to disperse a large number of fine oxides during the deoxidation of molten steel, the O content may be 0.0005% or more, or 0.0010% or more.

[0027] The remainder of the chemical composition of the hot-rolled steel sheet according to this embodiment may be Fe and impurities. In this embodiment, impurities are those introduced from raw materials such as ore, scrap, or the manufacturing environment. things It means.

[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. The lower limit of the content when no optional elements are included is 0%. The optional elements will be described in detail below.

[0029] Cu: 0.01~2.00% Cr: 0.01~2.00% Mo: 0.01~1.00% Ni: 0.02~2.00% B: 0.0001~0.0100% Cu, Cr, Mo, Ni, and B all have the effect of increasing the hardenability of hot-rolled steel sheets. In addition, Cu and Mo precipitate as carbides in the steel and have the effect of increasing the strength of hot-rolled steel sheets. Furthermore, when Cu is included, Ni has the effect of effectively suppressing grain boundary cracking of the slab caused by Cu. Therefore, one or more of these elements may be included.

[0030] As mentioned above, Cu has the effect of increasing the hardenability of hot-rolled steel sheets and increasing the strength of hot-rolled steel sheets by precipitating as carbides in the steel at low temperatures. To more reliably obtain the effects of 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 should be 2.00% or less. The Cu content is preferably 1.50% or less, and more preferably 1.00% or less.

[0031] As mentioned above, chromium (Cr) has the effect of improving the hardenability of hot-rolled steel sheets. To more reliably obtain the effects of the above effect, it is preferable to have a Cr content of 0.01% or more, and more preferably 0.05% or more. However, if the Cr content exceeds 2.00%, the chemical treatment properties of the hot-rolled steel sheet are significantly reduced. Therefore, the Cr content should be 2.00% or less.

[0032] As described above, Mo has the effect of increasing the hardenability of hot-rolled steel sheets and increasing the strength of hot-rolled steel sheets by precipitating as carbides in the steel. To more reliably obtain the effects of the above effects, it is preferable to have a Mo content of 0.01% or more, and more preferably 0.02% or more. However, if the Mo content exceeds 1.00%, the effects of the above effects saturate, which is not economically desirable. Therefore, the Mo content should be 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 enhances the hardenability of hot-rolled steel sheets. Furthermore, when Cu is included, Ni effectively suppresses grain boundary cracking in the slab caused by Cu. To more reliably obtain the effects of the above, it is preferable that the Ni content be 0.02% or higher. Since Ni is an expensive element, including large amounts is not economically desirable. Therefore, the Ni content should be 2.00% or less.

[0034] As mentioned above, B has the effect of improving the hardenability of hot-rolled steel sheets. To more reliably obtain the effect of this action, it is preferable to have a B content of 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 decreases significantly, so the B content should be 0.0100% or less. It is preferable that the B content be 0.0050% or less.

[0035] Ca: 0.0005~0.0200% Mg: 0.0005~0.0200% REM: 0.0005~0.1000% Bi: 0.0005~0.0200% Ca, Mg, and REM all have the effect of improving the ductility of hot-rolled steel sheets by adjusting the shape of inclusions in the steel to a desirable shape. Bi also has the effect of improving the ductility of hot-rolled steel sheets by refining the solidification structure. Therefore, one or more of these elements may be included. To more reliably obtain the effects of the above actions, it is preferable to have one or more of Ca, Mg, REM, and Bi at a concentration of 0.0005% or more. However, if the Ca 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 decrease the ductility of the hot-rolled steel sheet. Furthermore, even if the Bi content exceeds 0.0200%, the effects of the above actions become saturated, which is economically undesirable. Therefore, the Ca and Mg content should be 0.0200% or less, the REM content 0.1000% or less, and the Bi content 0.0200% or less. The Bi content is preferably 0.0100% 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 added industrially in the form of mischmetal.

[0036] As: 0.001~0.100% As contributes to improving the ductility of hot-rolled steel sheets by lowering the austenite single-phase formation temperature, thereby refining the prior austenite grains. To reliably obtain this effect, it is preferable to have an As content of 0.001% or more. On the other hand, even if a large amount of As is included, the above effect will saturate, so the As content should be 0.100% or less.

[0037] One or more of the following elements: Zr, Co, Zn, and W: Total 0-1.00% Sn: 0~0.05% Regarding Zr, Co, Zn, and W, the inventors have confirmed that even if these elements are contained in a total amount of 1.00% or less, the effects of the hot-rolled steel sheet according to this embodiment are not impaired. Therefore, one or more of Zr, Co, Zn, and W may be contained in a total amount of 1.00% or less. In addition, the inventors have confirmed that even if a small amount of Sn is contained, the effects of the hot-rolled steel sheet according to this embodiment are not impaired. However, if a large amount of Sn is contained, defects may occur during hot rolling, so the Sn content is set to 0.05% or less.

[0038] The chemical composition of the hot-rolled steel sheet described above may be measured by a general analysis method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Incidentally, sol.Al may be measured by ICP-AES using the filtrate after the sample is thermally decomposed with an acid. C and S may be measured using combustion-infrared absorption method, N may be measured using inert gas fusion-thermal conductivity method, and O may be measured using inert gas fusion-non-dispersive infrared absorption method. When the hot-rolled steel sheet has a plating layer, a coating film, etc. on its surface, if necessary, the plating layer, the coating film, etc. are removed by mechanical grinding or the like, and then the chemical composition is analyzed.

[0039] Metallographic structure of the hot-rolled steel sheet Next, the metallographic 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 metallographic structure in which, in area%, the retained austenite is less than 3.0%, the ferrite is 15.0% or more and less than 60.0%, the pearlite is less than 5.0%, the average spherical equivalent radius of the alloy carbide in the ferrite is 0.5 nm or more and less than 10.0 nm, the average number density is 0.10×10 16 pieces / cm 3 or more and 1.45×10 16 pieces / cm 3 less than, 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.

[0040] Because the hot-rolled steel sheet according to this embodiment has the above-described metal structure, it can be obtained to have high strength, as well as excellent ductility, fatigue properties, and shear workability. In this embodiment, the microstructure fraction, average equivalent sphere radius and average number density of alloy carbides, E value, I value, and standard deviation of Mn concentration are defined for the hot-rolled steel sheet at a position 1 / 4 of the way from the surface in the thickness direction and at a position 1 / 4 of the way from the end face in the direction perpendicular to the rolling direction and the thickness direction (width direction) (at any position in the rolling direction). This is because the microstructure at this position represents the typical microstructure of the steel sheet. Furthermore, the term "surface" here refers to the interface between the plating layer and the steel sheet when the hot-rolled steel sheet has a plating layer or coating.

[0041] Area percentage of retained austenite: less than 3.0% Retained austenite is a metallic structure that exists as a face-centered cubic lattice even at room temperature. Retained austenite enhances 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, inhibiting stable crack initiation and causing the formation of secondary shear surfaces. When the area ratio of retained austenite exceeds 3.0%, the above effect becomes apparent, and the shear workability of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of retained austenite should be less than 3.0%. Preferably, the area ratio of retained austenite is less than 1.5%, and more preferably less than 1.0%. Since less retained austenite is preferable, the area ratio of retained austenite may be 0%.

[0042] Methods for measuring the area fraction of retained austenite include X-ray diffraction, EBSP (Electron Back Scattering Diffraction Pattern) analysis, and magnetic measurement. In this embodiment, the area fraction of retained austenite is measured by X-ray diffraction. In this embodiment, the measurement of the retained austenite area fraction by X-ray diffraction is performed. First, a sample is taken from a cross-section of the hot-rolled steel sheet at a position 1 / 4 of the way from the surface in the thickness direction, so that the metal structure can be observed in an area of ​​1 mm or more at any position in the rolling direction and 1 mm or more centered on the position 1 / 4 of the way from the end face in the width direction. The integrated intensities of a total of six peaks, α(110), α(200), α(211), γ(111), γ(200), and γ(220), are determined from the above sample using Co-Kα radiation. Next, the volume fraction of retained austenite is obtained from the integrated intensities using the intensity averaging method, and this is considered to be the area fraction of retained austenite.

[0043] Ferrite area ratio: 15.0% or more, less than 60.0% Ferrite is a microstructure that forms when fcc transforms into bcc at relatively high temperatures. Because ferrite has a high work hardening rate, it has the effect of improving the strength-ductility balance of hot-rolled steel sheets. To obtain the above effect, the area ratio of ferrite should be 15.0% or more. Preferably, the area ratio of ferrite is 20.0% or more, more preferably 25.0% or more, and even more preferably 30.0% or more. On the other hand, because ferrite has low strength, if the area ratio is excessive, the desired strength cannot be obtained. For this reason, the ferrite area ratio should be less than 60.0%. Preferably it is 50.0% or less, and more preferably 45.0% or less.

[0044] Perlite area ratio: Less than 5.0% Pearlite is a lamellar metallic structure in which cementite is precipitated in layers between ferrites, and it is a softer metallic structure compared to bainite and martensite. If the area ratio of pearlite is 5.0% or more, carbon is consumed by the cementite contained in the pearlite, reducing the strength of the remaining martensite and bainite, and the desired strength cannot be obtained. Furthermore, microvoids that degrade ductility are formed early at the interface between the ferrite and cementite contained in the pearlite, so if the area ratio of pearlite is 5.0% or more, the desired ductility and fatigue properties cannot be obtained. Therefore, the area ratio of pearlite should be less than 5.0%. Preferably, the area ratio of pearlite is 3.0% or less. In order to improve the elongation flangeability of the hot-rolled steel sheet, it is preferable to reduce the area ratio of pearlite as much as possible, and it is even more preferable that the area ratio of pearlite is 0%.

[0045] Furthermore, the steel sheet according to this embodiment includes a hard structure consisting of one or more types of bainite, martensite, and tempered martensite, with a total area ratio of 32.0% or more and less than 85.0%, as the remaining structure other than retained austenite, ferrite, and pearlite.

[0046] The area fraction of metallic structures other than retained austenite is measured by the following method. First, a sample is taken from the hot-rolled steel sheet at a position 1 / 4 of the way from the end face in the width direction, with a cross section parallel to the rolling direction, so that the metallic structure in the region at a position 1 / 4 of the way from the surface in the thickness direction can be observed. Depending on the measuring device, the sample should be large enough to allow observation of about 10 mm in the rolling direction. Next, the observation cross section of the sample is polished to a mirror finish, and then polished for 8 minutes at room temperature with colloidal silica that does not contain alkaline solutions to remove the strain introduced into the surface layer of the sample. Crystal orientation information is obtained by measuring at measurement intervals of 0.1 μm using electron backscatter diffraction in a region of 200 μm or more at any position in the rolling direction of the observation cross section, and a region of 200 μm or more centered on the position 1 / 4 of the way from the surface in the thickness direction. For the above 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. In this case, the vacuum level inside the EBSD analyzer is 9.6 × 10⁻⁶ -5 The parameters are set to Pa or less, with an accelerating voltage of 15kV, an irradiation current level of 13, and an electron beam irradiation level of 62.

[0047] Furthermore, backscattered electron images are captured within the same field of view. First, crystal grains in which ferrite and cementite are precipitated in layers are identified from the backscattered electron images, and the area ratio of these crystal grains is calculated to obtain the area ratio of pearlite. Then, for the crystal grains excluding those identified as pearlite, the obtained crystal orientation information is used with the "Grain Average Misorientation" function installed in the "OIM Analysis®" software attached to the EBSD analyzer to determine that regions with a Grain Average Misorientation value of 1.0° or less are ferrite. At this time, the Grain Tolerance Angle is set to 15°, and the area ratio of ferrite is obtained by calculating the area of ​​the regions identified as ferrite.

[0048] The area ratio of the remaining microstructure is obtained by subtracting the area ratios of retained austenite, pearlite, and ferrite from 100%. Furthermore, based on the chemical composition of the hot-rolled steel sheet and the manufacturing conditions, it can be estimated that the remaining microstructure is a hard microstructure consisting of one or more types of bainite, martensite, and tempered martensite. The rolling direction of hot-rolled steel sheets is determined by the following method. First, a test piece is taken from the hot-rolled steel sheet so that the thickness cross-section can be observed. The thickness cross-section of the taken test piece is mirror-polished, etched with a saturated picric acid solution, and then observed using an optical microscope. The observation range is the entire thickness of the sheet, and the direction of grain stretching is determined. Here, the angle difference between the stretching direction and the thickness direction is denoted as θ. Furthermore, the surface at the 1 / 4 thickness position, perpendicular to the thickness direction and parallel to the stretching direction as described above, is polished in the same way as the thickness cross-section, and the direction of grain stretching is determined. Here, the angle difference between the thickness cross-section and the stretching direction is denoted as φ. The thickness direction is defined as Z according to ISO 3785. In a spherical coordinate system with the axis as the axis, the direction with angles θ and φ obtained from the grain stretching directions in the two cross-sections described above is identified as the rolling direction. The grain stretching direction θ and φ can be determined by using the "Analyze Particles" function of the image analysis software "ImageJ" with a circularity of 0.7 or less to obtain the grain stretching direction.

[0049] Average equivalent sphere radius of alloy carbides in ferrite: 0.5 nm or more, less than 10.0 nm In this embodiment, the hot-rolled steel sheet has preferably controlled average spherical equivalent radius and average number density of alloy carbides in the ferrite. If the average spherical equivalent radius of the alloy carbides in the ferrite is less than 0.5 nm, the strength of the ferrite against repeated deformation cannot be sufficiently increased, and the desired fatigue strength cannot be obtained. Therefore, the average spherical equivalent radius of the alloy carbides in the ferrite is set to 0.5 nm or more. Preferably, the average spherical equivalent radius of the alloy carbides in the ferrite is 1.0 nm or more. On the other hand, if the average equivalent sphere radius of the alloy carbides in the ferrite is 10.0 nm or more, the strength of the ferrite cannot be sufficiently increased, and due to the difference in hardness between crystal grains, a crack is generated 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, a secondary shear surface is easily formed, and the desired shear workability cannot be obtained in the hot-rolled steel sheet. For this reason, the average equivalent sphere radius of the alloy carbides in the ferrite should be less than 10.0 nm. Preferably, the average equivalent sphere radius of the alloy carbides in the ferrite is 8.0 nm or less, 6.0 nm or less, or 4.0 nm or less, and more preferably less than 2.0 nm.

[0050] Average number density of alloy carbides in ferrite: 0.10 × 10 16 pieces / cm 3 The above is 1.45 × 10 16 pieces / cm 3 less than The average number density of alloy carbides in ferrite is 0.10 × 10⁻⁶. 16 pieces / cm 3 Less than or 1.45 × 10 16 pieces / cm 3 If the values ​​are above this level, the strength of the ferrite against repeated deformation cannot be sufficiently increased, and the desired fatigue strength cannot be obtained. Therefore, the average number density of alloy carbides in the ferrite should be 0.10 × 10⁶. 16 pieces / cm 3 The above is 1.45 × 10 16 pieces / cm 3 The average number density of alloy carbides in ferrite is preferably 0.50 × 10⁻⁶. 16 pieces / cm 3 The above is more 1.00 × 10 16 pieces / cm 3 That concludes the explanation. Furthermore, the average number density of alloy carbides in the ferrite is preferably 1.40 × 10⁻⁶. 16 pieces / cm 3 The following, more preferably 1.20 × 10 16 pieces / cm 3 The following, and more preferably 1.10 × 10 16 pieces / cm 3 The following applies:

[0051] In this embodiment, the term "alloy carbide" refers to a carbide containing one or more of the elements Ti, Nb, Mo, and V.

[0052] The equivalent spherical radius and number density of alloy carbides in ferrite are measured using a three-dimensional atom probe. For the three-dimensional atom probe measurement, the laser wavelength (λ) is set to 355 nm, the laser power to 30 pJ, and the temperature of the needle-shaped specimen to 50 K. The apparatus used for the three-dimensional atom probe measurement is not particularly limited. For example, the LEAP4000XHR, manufactured by AMETEK Corporation, is used as a three-dimensional atom probe measuring apparatus.

[0053] For ferrite grains within the observation field obtained by the aforementioned EBSD, where the area ratio of each tissue was measured, samples were collected using a FIB (Focused Ion Beam) apparatus. The collected samples were processed into a needle shape using a well-known method, and by using a three-dimensional atom probe, the equivalent spherical radius and number density of fine precipitates ranging from less than 1 nm to several tens of nm can be accurately measured. The number density of precipitates can be obtained by dividing the number of precipitates contained in the region measured by the three-dimensional atom probe by the volume of the measurement region, for precipitates identified as alloy carbides by the method described later.

[0054] 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 a single precipitate is obtained by dividing the total volume of precipitates by the number of precipitates. From the obtained volume of precipitates, the equivalent radius of a sphere is calculated, assuming that the precipitates are spherical.

[0055] The method described above is applied to 30,000 nm 3By performing the measurement on five or more measurement data points with the volume of the above measurement area, the average number density and average sphere equivalent radius are obtained. The area where the amount of Ga introduced during FIB processing is less than 0.025 at% is designated as the observation area, and the area where Ga is present at 0.025 at% or more is excluded from the measurement area. To confirm the amount of Ga, the amount of Ga in the longitudinal direction of the needle sample can be confirmed using the 1D Concentration Profile function of the data analysis software IVAS 3.6.14 (CAMECA Instruments Inc.).

[0056] Furthermore, whether the observed precipitates are alloy carbides or not is determined by using the Cluster Analysis function of the analysis software IVAS 3.6.14 to identify the data acquired by the three-dimensional atom probe. max =1.2nm, Order=10, N min Using the analytical parameters =10, L=0.5nm, and de erosion=0.5nm, precipitates recognized as clusters are identified as alloy carbides.

[0057] E-value: 10.7 or higher I value: 1.020 or higher To suppress the formation of secondary shear surfaces, it is important to allow the fracture surface to form only after a sufficient shear surface has been formed, and it is necessary to prevent premature crack formation from the cutting edge of the tool during shearing. To achieve this, it is important that the periodicity of the metal structure is low and the uniformity of the metal structure is high. In this embodiment, the formation of secondary shear surfaces is suppressed 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.

[0058] The E value represents the periodicity of the metal structure. When the brightness is periodically arranged due to the formation of a band-like structure, i.e., when the periodicity of the metal structure is high, the E value decreases. In this embodiment, it is necessary to have a metal structure with low periodicity, so it is necessary to increase the E value. If the E value is less than 10.7, secondary shear surfaces are likely to occur. Starting from the periodically arranged structure, a crack is generated from the cutting edge of the shearing tool very early in the shearing process, forming a fracture surface, and then another shear surface is formed. This is presumed to make secondary shear surfaces more likely to occur. Therefore, the E value should be 10.7 or higher. Preferably it is 10.8 or higher, and more preferably 11.0 or higher. A higher E value is preferable, and there is no particular upper limit, but it may be 13.0 or lower, 12.5 or lower, or 12.0 or lower.

[0059] The I value represents the uniformity of the metal structure, and increases as the area of ​​regions with a constant brightness increases. A high I value means high uniformity of the metal structure. In this embodiment, a highly uniform metal structure is required, so the I value needs to be increased. If the I value is less than 1.020, due to the influence of precipitates within the crystal grains and the hardness distribution caused by elemental concentration differences, cracks will occur from the cutting edge of the shearing tool very early in the shearing process, forming a fracture surface, and then a shear surface will be formed again. This is presumed to make secondary shear surfaces more likely to occur. Therefore, the I value should be 1.020 or higher. Preferably it is 1.025 or higher, and more preferably 1.030 or higher. A higher I value is preferable, and there is no particular upper limit, but it may be 1.200 or less, 1.150 or less, or 1.100 or less.

[0060] The E-value and I-value can be obtained by the following method. In this embodiment, the imaging area of ​​the SEM image taken to calculate the E and I values ​​is 200 μm × 200 μm, centered at a position 1 / 4 of the way from the end face in the width direction, parallel to the rolling direction, and with a position 1 / 4 of the way from the surface in the thickness direction, and the number of observation fields is 5. A Hitachi High-Technologies Corporation SU-6600 Schottky electron gun is used to acquire the SEM image, 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 in 256-level grayscale.

[0061] Next, the obtained SEM image is cropped to an 880 x 880 pixel region, and a smoothing process is applied to it using the contrast enhancement limit ratio of 2.0 and a tile grid size of 8 x 8, as described in Non-Patent Literature 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 frequency values ​​of luminance between adjacent pixels are collected in matrix form using the gray-level co-occurrence matrix method (GLCM method) described in Non-Patent Literature 1.

[0062] The matrix of 179 frequency values ​​obtained by the above method is divided into p, where k is the rotation angle from the original image. k This is expressed as (k=0···89, 91,···179). For each image, the generated p k After summing the values ​​for all k (k=0···89, 91···179), a 256×256 matrix P is calculated, normalized so that the sum of each component is 1. Furthermore, the E value and I value are calculated using equations (1) and (2) described in Non-Patent Document 2. The average value obtained from measurements across the entire field of view is also calculated. In equations (1) and (2) below, P(i,j) is the gray-level co-occurrence matrix, and the value in the i-th row and j-th column of matrix P is denoted as P(i,j). As mentioned above, this is calculated using a 256×256 matrix P, so if you want to emphasize this point, you can modify equation (1) to equation (1') and equation (2) to equation (2'). In equations (1') and (2') below, the value in the i-th row and j-th column of matrix P is denoted as P(i,j). ij It is written as follows.

[0063]

number

[0064]

number

[0065]

number

[0066]

number

[0067] Standard deviation of Mn concentration: 0.60% by mass or less The standard deviation of the Mn concentration in the hot-rolled steel sheet according to this embodiment is 0.60 mass% or less. This allows for uniform dispersion of the hard phase and prevents 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 surfaces can be suppressed. The standard deviation of the Mn concentration is preferably 0.50 mass% or less, and more preferably 0.47 mass% or less. From the viewpoint of suppressing excessive burrs, a smaller lower limit for the standard deviation of the Mn concentration is desirable, but due to constraints of the manufacturing process, the practical lower limit is 0.10 mass%.

[0068] The standard deviation of Mn concentration can be obtained by the following method. First, a sample is taken at a position 1 / 4 of the way from the end face in the width direction of the plate, with a cross-section parallel to the rolling direction, so that 1 / 4 of the surface in the thickness direction can be observed. Depending on the measuring device, the sample should be large enough to allow observation of approximately 10 mm in the rolling direction. Next, after the sample is mirror-polished, the standard deviation of the Mn concentration is measured using an electron probe microanalyzer (EPMA). The measurement conditions are an acceleration voltage of 15 kV and a magnification of 5000x, and distribution images are measured at more than 40,000 points in a region of 20 μm in the rolling direction and 20 μm in the thickness direction at measurement intervals of 0.1 μm. Then, the standard deviation of the Mn concentration is obtained by calculating the standard deviation based on the Mn concentrations obtained from all measurement points.

[0069] Tensile properties The tensile strength characteristics (tensile strength and total elongation) of the mechanical properties of hot-rolled steel sheets shall be evaluated in accordance with JIS Z 2241:2011. The test specimen shall be a No. 5 test specimen as specified in JIS Z 2241:2011. The test specimen shall be taken from a position 1 / 4 of the way from the end face in the width direction of the hot-rolled steel sheet, with the width direction being the longitudinal direction of the test specimen.

[0070] The hot-rolled steel sheet according to this embodiment preferably has a tensile (maximum) strength of 980 MPa or more. More preferably, the tensile strength is 1000 MPa or more. By setting the tensile strength to 980 MPa or more, it can contribute significantly to vehicle body weight reduction without limiting the applicable parts. There is no particular need to limit the upper limit, but from the viewpoint of suppressing mold wear, it may be set to 1780 MPa.

[0071] 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. Furthermore, the product of tensile strength and total elongation is more preferably 14,000 MPa·% or more, and even more preferably 15,000 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, it is possible to significantly contribute to vehicle weight reduction without limiting the applicable parts.

[0072] Fatigue characteristics When material hardening occurs during repeated deformation, the amount of plastic deformation during repeated deformation decreases, which tends to extend the fatigue life. Therefore, it is preferable for material hardening to occur during repeated deformation. Whether or not hardening occurs during repeated deformation can be determined by the following method. A test specimen designated "1-15" is taken from the end face in the width direction of the hot-rolled steel sheet at a position 1 / 4 of the way from the end face in the width direction of the sheet, in accordance with JIS Z 2275-1978, so that the width direction is the longitudinal direction of the test specimen. Using this test specimen, a plane bending fatigue test is performed with a repeated stress that results in a fracture cycle of 3 million to less than 10 million cycles, in accordance with JIS Z 2275-1978. The torque during the fatigue test or the value of the strain gauge attached to the test specimen is measured to evaluate the change in repeated stress. The repeated stress at 100 cycles is used as the reference stress, and if the repeated stress is 5% or more higher than the above reference stress in the range of 100,000 to 1 million cycles, it can be determined that repeated hardening has occurred and the hot-rolled steel sheet has excellent fatigue properties.

[0073] 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 secure 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, when the plate thickness exceeds 8.0 mm, it becomes difficult to refine the metal structure, and it may be difficult to obtain the metal structure described above. Therefore, the plate thickness may be 8.0 mm or less. Preferably, it is 6.0 mm or less.

[0074] Plating layer The hot-rolled steel sheet according to this embodiment, having the chemical composition and metal structure described above, may be surface-treated by providing a plating layer on its surface for the purpose of improving corrosion resistance, etc. The plating layer may be an electroplating layer or a hot-dip galvanizing layer. Examples of electroplating layers include electro-zinc plating and electro-Zn-Ni alloy plating. Examples of hot-dip galvanizing layers include hot-dip galvanizing, alloyed hot-dip galvanizing, 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 amount of plating is not particularly limited and may be the same as in the conventional method. Furthermore, it is possible to further improve corrosion resistance by applying an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment solution) after plating.

[0075] Manufacturing conditions A preferred method for manufacturing the hot-rolled steel sheet according to this embodiment having the above-described chemical composition and metal structure is as follows.

[0076] To obtain the hot-rolled steel sheet according to this embodiment, it is effective to heat the slab under predetermined conditions, then perform hot rolling, accelerate cooling to a predetermined temperature range, then slow cooling, and control the cooling history until winding.

[0077] In the preferred manufacturing method for hot-rolled steel sheets according to this embodiment, the following steps (1) to (10) are performed sequentially. In this embodiment, the slab temperature and steel sheet temperature refer to the surface temperature of the slab and the surface temperature of the steel sheet. Furthermore, stress refers to the tension applied to the steel sheet in the rolling direction. Stress can be controlled by adjusting the rotational speed of the rolling stand and the winding device, and can be determined by dividing the measured load in the rolling direction by the cross-sectional area of ​​the sheet passing through. (1) The slab is heated in a temperature range of 700°C or higher and 850°C or lower for 900 seconds or more, then heated further and held in a temperature range of 1100°C or higher for 6000 seconds or more. (2) Hot rolling is performed in a temperature range of 850°C or higher and 1100°C or lower, such that the total thickness of the sheet is reduced by 90% or more. (3) The rolling process one stage before the final stage is carried out at a temperature of 900°C or higher and less than 1010°C, and a stress of 170 kPa or more is applied to the steel plate after the rolling process one stage before the final stage of hot rolling, and before the final stage of rolling. (4) The reduction ratio in the final stage of hot rolling shall be 8% or more, and the hot rolling shall be completed such that the rolling completion temperature Tf is 900°C or higher and less than 1010°C. (5) Perform a light reduction in the temperature range of 840°C or higher and less than 900°C, such that the total reduction in plate thickness is 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 rolling under light reduction, as well as the stress applied to the steel sheet after the final stage of light reduction and until the steel sheet cools to 800°C, shall be less than 200 kPa. (7) After light compression is complete, accelerate cooling is performed to a temperature range of 600°C or higher but less than 680°C at an average cooling rate of 50°C / second or higher. (8) Perform slow cooling for 2.0 seconds or more in the temperature range of 600°C or higher and below 680°C, with an average cooling rate of less than 5°C / s. (9) Cool to a temperature range of 350°C or lower at an average cooling rate of 50°C / s or higher. (10) Wind up at a temperature of 350℃ or less.

[0078] By employing the above manufacturing method, hot-rolled steel sheets with high strength, excellent ductility, fatigue properties, and shear workability can be reliably produced. Specifically, by appropriately controlling the slab heating conditions and hot-rolling conditions, Mn segregation is reduced and pre-transformation austenite is made equiaxed. Combined with the cooling conditions after hot rolling, described later, hot-rolled steel sheets with the desired microstructure can be reliably produced.

[0079] (1) Slab temperature and holding time when subjecting to hot rolling Slabs to be subjected to hot rolling can be those obtained by continuous casting or by casting and ingot division, and if necessary, those that have been subjected to hot working or cold working can be used. When heating slabs to be subjected to hot rolling, it is preferable to hold them in a temperature range of 700°C to 850°C for 900 seconds or more, and then heat them further and hold them in a temperature range of 1100°C or higher for 6000 seconds or more.

[0080] Furthermore, when holding the material in a temperature range of 700°C or higher and 850°C or lower, the steel plate temperature may be varied within this temperature range or kept constant. Also, when holding the material at 1100°C or higher, the steel plate temperature may be varied within a temperature range of 1100°C or higher or kept constant.

[0081] During austenite transformation in the temperature range of 700°C to 850°C, Mn is distributed between ferrite and austenite, and by extending the transformation time, Mn can diffuse within the ferrite region. This eliminates microsegregation of Mn that is unevenly distributed in the slab and significantly reduces the standard deviation of the Mn concentration. Therefore, it is preferable to hold the material in the temperature range of 700°C to 850°C for 900 seconds or more. Furthermore, holding the material in the temperature range of 1100°C or higher for 6000 seconds or more can significantly reduce the standard deviation of the Mn concentration.

[0082] Hot rolling is preferably performed using a reverse mill or tandem mill as a multi-pass rolling process. Particularly from the viewpoint of industrial productivity and stress loading on the steel sheet during rolling, it is more preferable to perform hot rolling using a tandem mill for at least the last two stages.

[0083] (2) Reduction ratio in hot rolling: Total thickness reduction of 90% or more in the temperature range of 850°C or above and 1100°C or below. Hot rolling in a temperature range of 850°C to 1100°C that results in a total thickness reduction of 90% or more is preferable. This primarily refines the recrystallized austenite grains and promotes the accumulation of strain energy within the unrecrystallized austenite grains. As a result, austenite recrystallization is promoted, as is the atomic diffusion of Mn, which can reduce the standard deviation of the Mn concentration. Therefore, it is preferable to perform hot rolling in a temperature range of 850°C to 1100°C that results in a total thickness reduction of 90% or more.

[0084] Furthermore, the total thickness reduction in the temperature range between 850°C and 1100°C can be expressed as {(t0-t1) / t0} × 100(%), where t0 is the thickness of the sheet metal at the entrance before the first rolling stage in this temperature range, and t1 is the thickness of the sheet metal at the exit stage after the final rolling stage in this temperature range.

[0085] (3) Rolling temperature of the stage before the final stage: 900°C or higher, less than 1010°C; stress after rolling the stage before the final stage of hot rolling, and before rolling the final stage: 170kPa or higher It is preferable to perform the rolling one stage before the final stage at 900°C or higher and less than 1010°C, and to apply a stress of 170 kPa or higher to the steel sheet after the rolling one stage before the final stage of hot rolling, and before the final stage of rolling. This ensures that the {110} recrystallized austenite after the rolling one stage before the final stage is formed. <001> The number of crystal grains with this crystal orientation can be reduced. {110} <001> Since this crystal orientation is difficult to recrystallize, suppressing the formation of this crystal orientation effectively promotes recrystallization by the final stage of reduction. As a result, the band-like 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 higher. The stress applied to the steel sheet refers to the tension applied in the longitudinal direction of the steel sheet, and can be controlled by adjusting the roll rotation speed during tandem rolling. There is no particular upper limit on the stress applied to the steel plate, but it can be kept below 350 kPa.

[0086] (4) Reduction ratio in the final stage of hot rolling: 8% or more, hot rolling completion temperature Tf: 900℃ or higher, less than 1010℃ It is preferable that the reduction ratio in the final stage of hot rolling be 8% or more, and the hot rolling completion temperature Tf be 900°C or higher. By setting the reduction ratio in the final stage of hot rolling to 8% or more, recrystallization due to the final stage of reduction can be promoted. As a result, the band-like structure of the hot-rolled steel sheet is improved, the periodicity of the metal structure is reduced, and the E value increases. By setting the hot rolling completion temperature Tf to 900°C or higher, an excessive increase in the number of ferrite nucleation sites in austenite can be suppressed. As a result, the formation of ferrite in the final structure (metal structure of the hot-rolled steel sheet after manufacturing) can be suppressed, and the desired strength can be obtained. Furthermore, by setting Tf to less than 1010°C, the coarsening of austenite grain size can be suppressed, the periodicity of the metal structure can be reduced, and the desired E value can be obtained. There is no particular upper limit to the reduction ratio in the final stage of hot rolling, but it can be 30% or less, and it is preferable to set it to 20% or less, and even more preferably to 15% or less.

[0087] (5) Perform a light reduction in the temperature range of 840°C or higher and less than 900°C, such that the total reduction in plate thickness is 5% or more and less than 8%. After the final stage of hot rolling, it is preferable to perform a light reduction in a temperature range of 840°C or higher and less than 900°C, resulting in a total thickness reduction of 5% or more and less than 8%. This allows the average equivalent spherical radius and average number density of alloy carbides in the ferrite to be controlled to desired values. Light reduction may be performed, for example, at the final stage of the finishing rolling mill, or by introducing new reduction equipment between the finishing rolling mill and the cooling bed.

[0088] Furthermore, the total thickness reduction under light rolling can be expressed as {(t0-t1) / t0} × 100(%), where t0 is the inlet thickness before the first rolling under light rolling, and t1 is the outlet thickness after the final rolling stage under light rolling.

[0089] (6) Stress applied to the steel sheet after the final stage of hot rolling and before the first stage of light rolling, and stress applied to the steel sheet after the final stage of light rolling and before the steel sheet cools to 800°C: Less than 200kPa The stress applied to the steel sheet after the final stage of hot rolling and before the first stage of light rolling, and the stress applied to the steel sheet after the final stage of light rolling and until the steel sheet cools to 800°C, are preferably less than 200 kPa. By keeping the stress applied to the steel sheet at these points below 200 kPa, austenite recrystallization preferentially proceeds in the rolling direction, suppressing an increase in the periodicity of the metal structure. As a result, a desired E value can be obtained. The stress applied to the steel sheet at these points is more preferably 180 kPa or less.

[0090] (7) After light compression is complete, accelerate cooling to a temperature range of 600°C or higher but less than 680°C at an average cooling rate of 50°C / second or higher. To suppress the growth of austenite grains that have been refined by hot rolling, it is preferable to accelerate cooling to a temperature range below 680°C at an average cooling rate of 50°C / second or more after light reduction is completed. By accelerating cooling to a temperature range below 680°C, the average number density of alloy carbides in the ferrite is reduced to 1.45 × 10⁻⁶. 16 pieces / cm 3 It can be set to less than 50°C / second. Furthermore, by setting the average cooling rate of accelerated cooling up to a temperature range of 600°C or higher and below 680°C to 50°C / second or higher, the excessive formation of perlite can be suppressed.

[0091] In this context, the average cooling rate refers to 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 completion 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 completion of accelerated cooling.

[0092] While there is no specific upper limit for the cooling rate, increasing the cooling rate will require larger cooling equipment and thus higher equipment costs. Therefore, considering equipment costs, an average cooling rate of 300°C / second or less is preferable. Furthermore, the ferrite area ratio should be 15.0% or more, and the average number density of alloy carbides in the ferrite should be 0.10 × 10⁻⁶. 16 pieces / cm 3 In addition, to ensure that the average equivalent spherical radius of the alloy carbides in the ferrite is 0.5 nm or more, it is preferable that the cooling stop temperature for accelerated cooling be 600°C or higher. To achieve the average cooling rate described above, after light reduction is complete, a cooling method with a high average cooling rate can be performed, for example, by spraying cooling water onto the surface of the steel plate.

[0093] (8) Perform slow cooling for 2.0 seconds or more in the temperature range of 600°C or higher and below 680°C, with an average cooling rate of less than 5°C / s. By performing slow cooling for 2.0 seconds or more at a temperature range of 600°C or higher and below 680°C, with an average cooling rate of less than 5°C / s, the desired amount of ferrite can be obtained. Furthermore, the number density of alloy carbides in the ferrite can be controlled to the desired amount. In this context, the average cooling rate refers to the value obtained by dividing the temperature drop of the steel plate from the cooling stop temperature of accelerated cooling to the cooling stop temperature of slow cooling by the time required from the stop of accelerated cooling to the stop of slow cooling.

[0094] The duration of slow cooling is preferably 3.0 seconds or longer. The upper limit of the slow cooling duration is determined by the equipment layout, but it should generally be less than 10.0 seconds. There is no particular lower limit for the average cooling rate of slow cooling, but since raising the temperature without cooling would involve significant investment in equipment, it may be set to 0°C / s or higher.

[0095] (9) Average cooling rate to winding temperature: 50°C / second or more To suppress the area ratio of pearlite and obtain the desired strength, it is preferable to set the average cooling rate from the slow cooling stop temperature to the winding temperature to 50°C / second or higher. This makes the matrix structure harder and allows for the control of the average equivalent spherical radius and average number density of alloy carbides in the ferrite to the desired amounts. In this context, the average cooling rate refers to the value obtained by dividing the temperature drop of the steel plate from the cooling stop temperature (where the average cooling rate is less than 5°C / s) to the winding temperature by the time required from the stopping of slow cooling (where the average cooling rate is less than 5°C / s) to winding.

[0096] (10) Winding temperature: 350℃ or less The winding temperature is preferably 350°C or lower. By keeping the winding temperature below 350°C, the amount of iron carbide precipitation 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. [Examples]

[0097] Next, the effects of one aspect of the present invention will be described in more detail with reference to examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0098] Steel having the chemical compositions shown in Tables 1 and 2 was melted, and slabs with a thickness of 240 to 300 mm were produced by continuous casting. Using the obtained slabs, hot-rolled steel sheets shown in Tables 5A to 6B were obtained under the manufacturing conditions shown in Tables 3A to 4B. The average cooling rate for slow cooling was set to less than 5°C / s. Furthermore, since the lower limit of measurement for the winding temperature listed in Tables 4A and 4B is 50°C, the actual winding temperature in the examples where 50°C is listed is 50°C or lower. Additionally, the rolling stage immediately preceding the final stage of hot rolling was performed at a temperature between 900°C and 1010°C.

[0099] For the obtained hot-rolled steel sheets, the area ratio of the metal structure, E value, I value, standard deviation of Mn concentration, average equivalent sphere radius and average number density of alloy carbides in ferrite, tensile strength TS, and total elongation El were determined using the method described above. Furthermore, the fatigue properties were evaluated by performing a plane bending fatigue test using the method described above. The obtained measurement results are shown in Tables 5A to 6B.

[0100] Method for evaluating the properties of hot-rolled steel sheets Tensile properties A hot-rolled steel sheet was deemed acceptable if its tensile strength (TS) was 980 MPa or higher, its total elongation (El) was 10.0% or higher, and its tensile strength (TS) × total elongation (El) was 13000 MPa·% or higher, indicating that it possessed high strength and excellent ductility. If any of these conditions were not met, the sheet was deemed unacceptable, indicating that it did not possess high strength and excellent ductility.

[0101] Fatigue characteristics Using the method described above, a planar bending fatigue test was conducted. If the repeated stress was 5% or more higher than the reference stress in the range of several hundred thousand to one million cycles, repeated hardening occurred, and the hot-rolled steel sheet was judged to be acceptable as having excellent fatigue properties. On the other hand, if the repeated stress did not exceed 5% above the standard stress in the range of 100,000 to 1,000,000 cycles, repeated hardening did not occur, and the hot-rolled steel sheet was deemed unsuitable as it did not possess excellent fatigue properties. Examples judged as passing are marked "Good" in the fatigue characteristics column of the table, while examples judged as failing are marked "NG" in the table.

[0102] Shear workability (evaluation of secondary shear surface) The shear workability of hot-rolled steel sheets was evaluated by punching tests. Three punched holes were created for each embodiment with a hole diameter of 10 mm, a clearance of 10%, and a punching speed of 3 m / s. Next, the plate thickness cross section perpendicular to the rolling direction and the plate thickness cross section parallel to the rolling direction of the punched holes were embedded in resin, and the cross-sectional shape was photographed using a scanning electron microscope. In the obtained observation photographs, the shear end surface as shown in Figure 1 or Figure 2 can be observed. Figure 1 is an example of the shear end surface of a hot-rolled steel sheet according to the present invention, and Figure 2 is an example of the shear end surface of a hot-rolled steel sheet according to a comparative example. In Figure 1, the shear end surface consists of a dart - shear surface - fracture surface - shear surface - fracture surface - burr. On the other hand, in Figure 2, the shear end surface consists of a dart - shear surface - fracture surface - shear surface - fracture surface - burr. Here, "burr" refers to a smooth, rounded surface area, "shear surface" refers to the area of ​​the punched end separated by shear deformation, "fracture surface" refers to the area of ​​the punched end separated by a crack originating near the cutting edge, and "burr" refers to a surface with a projection that protrudes from the lower surface of the hot-rolled steel sheet.

[0103] Of the obtained shear end faces, if a shear surface-fracture surface-shear surface pattern, as shown in Figure 2, was observed on two faces perpendicular to the rolling direction and two faces parallel to the rolling direction, it was determined that a secondary shear surface had been formed. Four faces were observed for each punched hole, for a total of 12 faces. If no secondary shear surfaces were observed, the hot-rolled steel sheet was judged to have excellent shear workability and was deemed acceptable. On the other hand, if even one secondary shear surface was formed, the hot-rolled steel sheet was judged to have poor shear workability and was deemed unacceptable. Examples judged as acceptable were marked "Good" in the shear workability column of the table, and examples judged as unacceptable were marked "NG" in the table.

[0104] [Table 1]

[0105] [Table 2]

[0106] [Table 3A]

[0107] [Table 3B]

[0108] [Table 4A]

[0109] [Table 4B]

[0110] [Table 5A]

[0111] [Table 5B]

[0112] [Table 6A]

[0113] [Table 6B]

[0114] Tables 5A to 6B show that the hot-rolled steel sheet according to the present invention has high strength while also possessing excellent ductility, fatigue properties, and shear workability. On the other hand, it can be seen that the hot-rolled steel sheet in the comparative example does not possess one or more of the above characteristics.

Claims

1. The chemical composition is expressed in mass percent. C: 0.050-0.250%, Si: 0.05-3.00%, Mn: 1.00-4.00%, One or more of Ti, Nb, and V: total of 0.060–0.500% 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 to 0.0100%, Ca: 0-0.0200%, Mg: 0 to 0.0200%, REM: 0-0.1000%, Bi: 0 to 0.0200%, As: 0 to 0.100%, It contains one or more of the following: Zr, Co, Zn, and W, totaling 0-1.00%, and Sn: 0-0.05%. The remainder consists of Fe and impurities. The metallic structure, In area percentage, The residual austenite content is less than 3.0%. The ferrite content is 15.0% or more and less than 60.0%. The perlite content is less than 5.0%. The average equivalent spherical radius of the alloy carbides in the ferrite is 0.5 nm or more and less than 10.0 nm, and the average number density is 0.10 × 10⁻⁶. １６ pieces / cm ３ The above is 1.45 x 10 １６ pieces / cm ３ It is less than, The E value indicating the periodicity of the metal structure is 10.7 or higher. The I value indicating the uniformity of the metal structure is 1.020 or higher. A hot-rolled steel sheet characterized by having a standard deviation of Mn concentration of 0.60% by mass or less.

2. The aforementioned 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: 0.0005-0.1000%, Bi: 0.0005 to 0.0200%, and The hot-rolled steel sheet according to claim 1, characterized in that it contains one or more types selected from the group consisting of As: 0.001 to 0.100%.

Images