Hot-rolled steel plate

Abstract

This hot-rolled steel sheet has a specific 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, while having an E value of 10.7 or more, an I value of 1.020 or more, a CS value of -8.0 × 105 to 8.0 × 105, and a standard deviation of the Mn concentration of 0.60% by mass or less; and, in the surface, the number density of Si-Al-Cr oxides having a sphere equivalent radius of 0.500 µm or more is 2.0 × 103 per cm2 or less, and the number density of Si-Al-Cr oxides having a sphere equivalent radius of 0.005 µm to 0.050 µm is 1.0 × 105 per cm2 or more.

Description

[Technical Field]

[0001] This invention relates to hot-rolled steel sheets. This application claims priority based on Japanese Patent Application No. 2022-174630, filed in Japan on October 31, 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, and several technologies have been proposed to meet these requirements. Because automotive components are processed in various ways, the required formability differs depending on the component. Among these, the rate of reduction in plate thickness at critical fracture and ductility are considered important indicators of formability.

[0004] The critical fracture thickness reduction rate is a value determined from the minimum thickness of the tensile test specimen after fracture, compared to the thickness of the tensile test specimen before fracture. A low critical fracture thickness reduction rate is undesirable because it makes the specimen more prone to premature fracture when tensile strain is applied during press forming.

[0005] Automotive components are formed by press forming, but the blank sheets used for press forming are often manufactured by shearing, which offers high productivity. Blank sheets manufactured by shearing require excellent edge face accuracy after shearing.

[0006] For example, if a secondary shear surface occurs on the end face after shearing (sheared end face), where the surface consists of a shear surface, a fracture surface, and another shear surface, the accuracy of the sheared end face deteriorates significantly.

[0007] Furthermore, steel sheets applied to automotive components are required to have better corrosion resistance.

[0008] For example, Patent Document 1 discloses a hot-rolled steel sheet that controls the Mn segregation degree and P segregation degree in the central part of the sheet thickness and serves as a material for a cold-rolled steel sheet with excellent surface properties after press working. However, Patent Document 1 does not consider the limiting fracture thickness reduction rate, shearing workability, and corrosion resistance of the hot-rolled steel sheet.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Non-Patent Documents

[0010]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0011] However, Patent Document 1 does not consider the rate of reduction in thickness at the limit fracture, shear workability, and corrosion resistance of hot-rolled steel sheets.

[0012] This invention has been made in view of the above-mentioned circumstances, and aims to provide a hot-rolled steel sheet having high strength and a low rate of reduction in plate thickness at critical fracture, as well as excellent ductility, shear workability, and corrosion resistance. [Means for solving the problem]

[0013] The gist of this invention is as follows: (1) A hot-rolled steel sheet according to one aspect of the present invention has a chemical composition in mass% of: C: 0.050~0.250%, Si: 0.40~1.00%, Mn: 1.00~1.50%, sol.Al: 0.100~0.500%, Cr: 0.50~2.00%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Ti: 0~0.500%, Nb: 0~0.500%, V: 0~0.500%, Cu: 0~2.00%, Mo: 0~1.00%, Ni: 0~2.00%, B: 0~0.0100%, Ca: 0~0.0200%, Mg: 0~0.0200%, REM: 0~0.1000%, Bi: 0~0.0200%, As: 0~0.100%, Zr: 0~1.00%, Co: 0~1.00%, Zn: 0~1.00%, W: 0~1.00%, and It contains Sn: 0~0.05%, The remainder consists of Fe and impurities. The following equations (A) and (B) are satisfied, The microstructure at a position 1 / 4 of the way from the surface in the thickness direction is 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 entropy value obtained by analyzing the SEM image of the metal structure using the gray-level co-occurrence matrix method is 10.7 or greater, as shown in the following formula (1). The inverse difference normalized value shown in formula (2) below is 1.020 or greater, The Cluster Shade value shown in formula (3) below is -8.0 × 10 5 ~8.0×10 5 And, The standard deviation of the Mn concentration is 0.60 mass% or less. On the aforementioned surface, the number density of Si-Al-Cr oxide particles with an equivalent spherical radius of 0.500 μm or more is 2.0 × 10⁻¹⁴ 3 pieces / cm 2 The following conditions apply, and the number density of Si-Al-Cr oxides with an equivalent spherical radius of 0.005 to 0.050 μm is 1.0 × 10⁻¹⁰ 5 pieces / cm 2 That's all. 0.060%≦Ti+Nb+V≦0.500% …(A) Zr + Co + Zn + W ≤ 1.00% …(B) However, in formulas (A) and (B) above, each element symbol indicates the mass percentage content of that element, and if the element is not contained, 0% is substituted. Here, P(i,j) in equations (1) to (5) below is the gray level co-occurrence matrix, L in equation (2) below is the number of grayscale levels that the SEM image can take, i and j in equations (2) and (3) below are natural numbers from 1 to L, and μ in equation (3) below x and μ yThese are represented by equations (4) and (5) below, respectively.

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[0014] According to the above embodiment of the present invention, a hot-rolled steel sheet can be obtained that has high strength and a low reduction in thickness at critical fracture, as well as excellent ductility, shear workability, and corrosion resistance. 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 even building components. [Brief explanation of the drawing]

[0015] [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]

[0016] 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.

[0017] The numerical ranges indicated below, separated by a "~", include both a lower and upper limit. Numbers marked "less than" or "greater than" are not included in the numerical range. In the following explanation, percentages related to chemical composition refer to mass percentages unless otherwise specified.

[0018] chemical composition The chemical composition of the hot-rolled steel sheet according to this embodiment is, in mass%, C: 0.050~0.250%, Si: 0.40~1.00%, Mn: 1.00~1.50%, sol.Al: 0.100~0.500%, Cr: 0.50~2.00%, 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, satisfying formula (A) (0.060% ≤ Ti + Nb + V ≤ 0.500%). The following provides a detailed explanation of each element.

[0019] 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 carbon content is less than 0.050%, the desired strength cannot be obtained. Therefore, the carbon content should be 0.050% or more. Preferably, the carbon content is 0.055% or more, more preferably 0.060% or more, and even more preferably 0.065% 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 0.150% or less.

[0020] Si: 0.40~1.00% 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 0.40%, the above effects cannot be obtained. Therefore, the Si content should be 0.40% or more. The Si content is preferably 0.50% or more, and more preferably 0.60% or more. However, if the Si content exceeds 1.00%, the number density of oxides on the steel sheet surface increases, degrading the chemical conversion treatment properties. In addition, ferrite tends to form excessively, reducing the strength. Therefore, the Si content should be 1.00% or less. Preferably, the Si content is 0.90% or less, and more preferably 0.80% or less.

[0021] Mn: 1.00~1.50% 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 strength cannot be obtained. Therefore, the Mn content should be 1.00% or more. Preferably, the Mn content is 1.10% or more, and more preferably 1.20% or more. On the other hand, if the Mn content exceeds 1.50%, the ferrite transformation becomes difficult due to the combined effect of Mn and Cr, which is included as a quenching element. Therefore, the Mn content should be 1.50% or less. Preferably, the Mn content is 1.45% or less, and more preferably 1.40% or less.

[0022] Ti: 0~0.500% Nb: 0~0.500% V: 0~0.500% 0.060%≦Ti+Nb+V≦0.500% …(A) However, each element symbol in formula (A) above indicates the mass percentage content of that element, and if the element is not contained, 0% is substituted. Ti, Nb, and V are elements that precipitate as carbides and nitrides in steel, improving the strength of the steel through precipitation strengthening. 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. That is, the value of the middle side of formula (A) above should be 0.060% or more. Note that it is not necessary for all of Ti, Nb, and V 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. The content of Ti, Nb, and V individually is preferably 0.001% 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. That is, the value of the middle side of formula (A) 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.

[0023] sol.Al: 0.100~0.500% Al, like Si, has the effect of deoxidizing steel and making it sounder, as well as promoting ferrite formation and increasing the ductility of hot-rolled steel sheets. If the sol.Al content is less than 0.100%, the above effects cannot be obtained. Therefore, the sol.Al content should be 0.100% or more. Preferably, the sol.Al content is 0.200% or more. On the other hand, if the sol.Al content exceeds 0.500%, the above effect saturates and it becomes economically undesirable. Therefore, the sol.Al content should be 0.500% or less. Preferably, the sol.Al content is 0.450% or less, more preferably 0.400% or less, and even more preferably 0.350% or less. Note that sol.Al refers to acid-soluble Al, specifically solid-solution Al present in steel.

[0024] Cr: 0.50~2.00% Cr has the effect of improving the hardenability of hot-rolled steel sheets. In addition, Cr forms oxides with Si and Al. If the Cr content is less than 0.50%, the effect of improving hardenability cannot be obtained. Furthermore, the Si-Al-Cr oxide cannot be preferably formed on the surface. For this reason, the Cr content should be 0.50% or more. Preferably, the Cr content is 0.60% or more, more preferably 0.70% or more, and even more preferably 0.80% or more. On the other hand, if the Cr content exceeds 2.00%, the chemical treatment properties of the hot-rolled steel sheet deteriorate significantly. Therefore, the Cr content should be 2.00% or less. Preferably, the Cr content is 1.80% or less, and more preferably 1.60% or less.

[0025] 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 and the rate of reduction in limit fracture thickness of the hot-rolled steel sheet 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 particular need to specify a lower limit for the P content, and it may be 0%, but from the viewpoint of refining costs, it is preferable to have a P content of 0.001% or more.

[0026] S: 0.0300% or less S forms sulfide inclusions in the steel, reducing the ductility and the rate of reduction in thickness at critical fracture of hot-rolled steel sheets. When the S content exceeds 0.0300%, the ductility and the rate of reduction in thickness at critical fracture of hot-rolled steel sheets decrease significantly. Therefore, the S content should be 0.0300% or less. Preferably, the S content is 0.0050% or less. There is no particular need to specify a lower limit for the S content, and it may be 0%, but from the viewpoint of refining costs, it is preferable to have a S content of 0.0001% or more.

[0027] N: 0.1000% or less N has the effect of reducing the ductility and the rate of reduction in thickness at critical fracture of hot-rolled steel sheets. When the N content exceeds 0.1000%, the ductility and the rate of reduction in thickness at critical fracture of hot-rolled steel sheets decrease 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 0.0100% or less. There is no particular need to specify a lower limit for the N content, and it may be 0%, 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.

[0028] 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, and more preferably 0.0050% or less. The O content may be 0%, but 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.

[0029] 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 mean substances introduced from the raw materials such as ore, scrap, or the manufacturing environment, and / or substances that are permissible to the extent that they do not adversely affect the hot-rolled steel sheet according to this embodiment.

[0030] 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 these optional elements are not included is 0%. The optional elements will be described in detail below.

[0031] Cu: 0.01~2.00% Mo: 0.01~1.00% Ni: 0.02~2.00% B: 0.0001~0.0100% Cu, 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 the hot-rolled steel sheet. 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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 a content of 0.0005% or more of one or more of Ca, Mg, REM, and Bi. 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.

[0037] 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.

[0038] Zr: 0.01~1.00% Co: 0.01~1.00% Zn: 0.01~1.00% W: 0.01~1.00% Zr + Co + Zn + W ≤ 1.00% …(B) Sn: 0.01~0.05% However, each element symbol in formula (B) above indicates the mass percentage content of that element, and 0 is substituted if the element is not contained. The inventors have confirmed that the effects of the hot-rolled steel sheet according to this embodiment are not impaired even if the total amount of these elements is 1.00% or less. Therefore, one or more of Zr, Co, Zn, and W may be included in total at a concentration of 1.00% or less. That is, the value on the left side of formula (B) may be 1.00% or less. Since Zr, Co, Zn, and W do not need to be included, their respective contents may be 0%. In order to improve the strength of the steel sheet by solid solution strengthening, the contents of Zr, Co, Zn, and W may each be 0.01% or more. Furthermore, the inventors have 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 included. However, since defects may occur during hot rolling if a large amount of Sn is included, the Sn content should be 0.05% or less. Sn may not be included at all, so the Sn content may be 0%. In order to improve the corrosion resistance of the hot-rolled steel sheet, the Sn content may be 0.01% or more.

[0039] The chemical composition of the hot-rolled steel sheet described above can be measured using general analytical methods. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Sol.Al can be measured using ICP-AES with the filtrate obtained after heating and decomposing the sample with acid.C and S can be measured using combustion-infrared absorption spectroscopy, N can be measured using inert gas fusion-thermal conductivity spectroscopy, and O can be measured using inert gas fusion-nondispersive infrared absorption spectroscopy. If the hot-rolled steel sheet has a plating layer on its surface, the plating layer may be removed by mechanical grinding or other means as needed before the chemical composition analysis is performed.

[0040] Metallographic structure of hot-rolled steel sheet Next, the metallographic structure of the hot-rolled steel sheet according to the present embodiment will be described. In the hot-rolled steel sheet according to the present embodiment, the metallographic structure at the 1 / 4 position from the surface in the plate thickness direction has, in terms of area%, less than 3.0% retained austenite, 15.0% or more and less than 60.0% ferrite, less than 5.0% pearlite, and the Entropy value represented by the following formula (1) obtained by analyzing the SEM image of the metallographic structure by the gray level co-occurrence matrix method is 10.7 or more, the Inverse difference normalized value represented by the following formula (2) is 1.020 or more, and the Cluster Shade value represented by the following formula (3) is -8.0×10 5 ~8.0×10 5 and the standard deviation of the Mn concentration is 0.60 mass% or less, and on the surface, the number density of Si-Al-Cr oxides with a spherical equivalent radius of 0.500 μm or more is 2.0×10 3 0 2 per cm 5 >or less, and the number density of Si-Al-Cr oxides with a spherical equivalent radius of 0.005 to 0.050 μm is 1.0×10 2 or more.

[0041] Therefore, the hot-rolled steel sheet according to the present embodiment can obtain high strength, limiting fracture thickness reduction rate, and excellent ductility, shearing workability, and corrosion resistance. In the present embodiment, the volume fraction, Entropy value, Inverse difference normalized value, Cluster Shade value, and standard deviation of the Mn concentration of the metallographic structure in the region at the 1 / 4 position from the surface in the plate thickness direction are defined. The reason is that the metallographic structure at this position shows a typical metallographic structure of the steel sheet. Also, the "surface" referred to here means the interface between the plating layer and the steel sheet when the hot-rolled steel sheet has a plating layer, and the "1 / 4 depth position from the surface" means the position at a depth of 1 / 4 of the plate thickness in the plate thickness direction from the surface of the hot-rolled steel sheet.

[0042] 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 processing, becoming a crack initiation point during deformation and causing a decrease in the rate of reduction in plate thickness at critical fracture. The above effect becomes apparent when the area ratio of retained austenite is 3.0% or more. 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 a lower amount of retained austenite is preferable, the area ratio of retained austenite may be 0%.

[0043] 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.

[0044] 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 a 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 a region of 1 mm or more at any position in the rolling direction and in a region of 1 mm or more centered on the center in a direction perpendicular to the rolling direction and the thickness direction. The sheet is then thinned by mechanical polishing and chemical polishing so that the surface is at a position 1 / 4 of the way from the surface in the thickness 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 using an X-ray diffractometer (e.g., Rigaku RINT-2500, Co-Kα rays). Next, the volume fraction of retained austenite is calculated from the integrated intensities using the intensity averaging method. The obtained volume fraction of retained austenite is considered to be the area fraction of retained austenite.

[0045] 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 it 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.

[0046] Perlite area ratio: Less than 5.0% Pearlite is a lamellar metallic structure in which cementite is precipitated in layers between ferrite crystals, and it is a softer metallic structure compared to bainite and martensite. If the area ratio of pearlite is 5.0% or more, the 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. Therefore, the area ratio of pearlite should be less than 5.0%. Preferably, the area ratio of pearlite is 3.0% or less. To improve the elongation flange properties of hot-rolled steel sheets, it is preferable to reduce the area ratio of perlite as much as possible, and it is even more preferable that the area ratio of perlite be 0%.

[0047] Furthermore, the hot-rolled steel sheet according to this embodiment may include a hard structure consisting of one or more types of bainite, martensite, and tempered martensite, with a total area ratio of more than 32.0% and 85.0% or less, as the remaining structure other than retained austenite, ferrite, and pearlite.

[0048] The area fractions of ferrite and pearlite are measured by the following method. First, a sample is taken at the center in the direction perpendicular to the rolling direction and the thickness direction, with a thickness cross section parallel to the rolling direction, so that the metal structure in the region at a position 1 / 4 of the way from the surface in the thickness direction can be observed. The sample should be large enough to allow observation of approximately 10 mm in the rolling direction. Next, the sample cross section is polished to a mirror finish, and then polished for 8 minutes at room temperature with colloidal silica with a particle size of 0.25 μm that does not contain alkaline solutions to remove the strain introduced into the surface of the sample. Crystal orientation information is obtained by electron backscatter diffraction measurement at measurement intervals of 0.1 μm in the rolling direction and thickness direction in regions of 200 μm or more at any position in the rolling direction of the sample cross section, and around a position 1 / 4 of the way from the surface in the thickness direction, with a measurement interval of 200 μm or more. The above measurements utilize an EBSD analysis system consisting of a thermal field emission scanning electron microscope (e.g., JEOL JSM-7001F) and an EBSD detector (e.g., TSL DVC5 detector). The vacuum level inside the EBSD analysis system is 9.6 × 10⁻⁶. -5 The parameters are set to Pa or less, the acceleration voltage to 15kV, the irradiation current level to 13, and the electron beam irradiation level to 62. The number of observation fields will be 5.

[0049] Furthermore, backscattered electron images are taken in the same field of view from which the aforementioned crystal orientation information was obtained. For this image acquisition, the acceleration voltage is 15kV, the irradiation current level is 12-13, and the electron beam irradiation level is 62. The focal length (WD: Working Distance) is 5mm. Ferrite and pearlite are identified from the backscattered electron images and the aforementioned crystal orientation information. First, in the backscattered electron image, crystal grains in which cementite has precipitated in a lamellar pattern are identified. In the backscattered electron image, cementite is observed as a white contrast. The cementite in this pearlite has a lamellar morphology, and crystal grains in which the white contrast of the lamellar morphology is observed at intervals of 1.0 μm or less are identified as pearlite crystal grains. The area ratio of pearlite is obtained by calculating the area ratio of these crystal grains. Subsequently, for the grains excluding those identified as pearlite, the obtained crystal orientation information is used with the "Grain Average Misorientation" function included 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 the regions identified as ferrite is calculated to obtain the area ratio of ferrite.

[0050] The area percentage of the remaining tissue is obtained by subtracting the area percentages of retained austenite, ferrite, and pearlite from 100%. Furthermore, in this embodiment, the rolling direction of the hot-rolled steel sheet is determined by the following method. First, a test specimen is taken so that a cross-section parallel to the surface of the hot-rolled steel sheet can be observed. After finishing the cross-section of the test specimen taken in the thickness direction with mirror polishing, it is observed using an optical microscope. The observation surface is a plane parallel to the sheet surface at an arbitrary depth in the range of 1 / 4 to 1 / 2 in the thickness direction, and the direction parallel to the grain elongation direction on this observation surface is identified as the rolling direction.

[0051] Entropy value: 10.7 or higher Inverse difference normalized 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 entropy value (E value), which indicates the periodicity of the metal structure, and the inverse difference normalized value (I value), which indicates the uniformity of the metal structure.

[0052] 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.

[0053] 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 lower, 1.150 or lower, or 1.100 or lower.

[0054] Cluster Shade value: -8.0 × 10 5 ~8.0×10 5 The Cluster Shade (CS) value indicates the degree of distortion in the metal structure. The CS value is positive if there are many points with brightness above the average value in the image obtained from photographing the metal structure, and negative if there are many points with brightness below the average value.

[0055] In secondary electron images from an electron microscope, brightness increases in areas with large surface irregularities and decreases in areas with small irregularities. Surface irregularities of an object are greatly influenced by the grain size and intensity distribution within the metal structure. In this embodiment, the CS value increases when the intensity variation of the metal structure is large or the microstructure units are small, and decreases when the intensity variation is small or the microstructure units are large.

[0056] In this embodiment, it is important to keep the CS value within a desired range close to 0. The CS value is -8.0 × 10⁻⁶. 5 If the value is less than -8.0 × 10, the rate of reduction in thickness at critical fracture of hot-rolled steel sheet decreases. This is presumed to be because larger grain sizes are present in the metal structure, and these grains preferentially fracture during extreme deformation. Therefore, the CS value should be -8.0 × 10. 5 The above is sufficient. Preferably -7.5 × 105 The above is preferable, and more preferably -7.0 × 10 5 That's all. On the other hand, the CS value is 8.0 × 10 5 When the value is excessive, the rate of reduction in thickness at the limit fracture of hot-rolled steel sheets decreases. This is presumably because there is a large variation in microscopic strength within the metal structure, causing strain to concentrate locally during extreme deformation and making fracture more likely. Therefore, the CS value is 8.0 × 10⁻⁶. 5 The following applies. Preferably 7.5 × 10 5 The following, and more preferably 7.0 × 10 5 The following applies:

[0057] The E, I, and CS values ​​can be obtained by the following methods. In this embodiment, the imaging area of ​​the SEM image taken to calculate the E value, I value, and CS value is 160 μm × 160 μm, centered at the 1 / 4 position from the surface in the thickness direction, with a cross-section parallel to the rolling 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.

[0058] Next, the obtained SEM image is cropped into an 880×880 pixel area (the observation area is 160μm×160μm in actual size), and the image is subjected to smoothing processing with a contrast enhancement limit of 2.0 and a tile grid size of 8×8, 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 luminance frequency values ​​between adjacent pixels are collected in matrix form using the GLCM method described in Non-Patent Literature 1.

[0059] 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. kThis 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, I value, and CS value are calculated using the following equations (1) to (5) described in Non-Patent Literature 2. The average value obtained from measurements across the entire field of view is also calculated. In equations (1) to (5) 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, it is calculated using a 256×256 matrix P, so if you want to emphasize this point, you can modify equations (1) to (5) below to equations (1') to (5'). Here, in equation (2) below, L is the number of grayscale levels that the SEM image can take (quantization levels of grayscale), and in this embodiment, as mentioned above, the SEM image is output in 256 grayscale levels, so L is 256. In equations (2) and (3) below, i and j are natural numbers from 1 to L, and in equation (3) below, μ x and μ y These are represented by equations (4) and (5) below, respectively. In the following equations (1') to (5'), the value at the i-th row and j-th column of matrix P is given by P. ij It is written as follows.

[0060]

number

[0061]

number

[0062]

number

[0063]

number

[0064]

number

[0065]

number

[0066]

number

[0067]

number

[0068]

number

[0069]

number

[0070] 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%.

[0071] The standard deviation of Mn concentration can be obtained by the following method. First, a sample is taken at the center in the direction perpendicular to the rolling direction and the thickness direction, with a cross section parallel to the rolling direction, so that 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 be observed at a size of approximately 10 mm in the rolling direction. Next, after the above sample is mirror-polished, the standard deviation of 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 a distribution image is measured in a range of 20 μm in the rolling direction of the sample and 1 / 4 of the way from the surface in the thickness direction, with the distribution image centered at a position 20 μm in the thickness direction of the sample. More specifically, the measurement interval is set to 0.1 μm, and the Mn concentration is measured at more than 40,000 locations. Then, the standard deviation of Mn concentration is obtained by calculating the standard deviation based on the Mn concentrations obtained from all measurement points.

[0072] Number density of Si-Al-Cr oxides on the surface with an equivalent spherical radius of 0.500 μm or more: 2.0 × 10⁻¹⁴ 3 pieces / cm 2 below Number density of Si-Al-Cr oxides on the surface with an equivalent spherical radius of 0.005 to 0.050 μm: 1.0 × 10⁻¹⁶ 5 pieces / cm 2 That's all. The inventors have discovered that the corrosion resistance of hot-rolled steel sheets can be improved by controlling the number density of Si-Al-Cr oxide particles formed on the surface, specifically those with an equivalent spherical radius of 0.500 μm or more and those with an equivalent spherical radius of 0.005 to 0.050 μm, within a desired range.

[0073] On the surface, the number density of Si-Al-Cr oxide particles with an equivalent spherical radius of 0.500 μm or more is 2.0 × 10⁻¹⁶ 3 pieces / cm 2 If the number density is excessive, the electrochemical reaction between the solution and the steel sheet is locally hindered during chemical conversion treatment, preventing the zinc phosphate film from adhering to the steel sheet and degrading the corrosion resistance of the hot-rolled steel sheet. Therefore, the number density of Si-Al-Cr oxides with an equivalent spherical radius of 0.500 μm or more on the surface should be 2.0 × 10⁻⁶. 3 pieces / cm 2The following applies: Preferably 1.8 × 10 3 pieces / cm 2 The following, more preferably 1.5 × 10 3 pieces / cm 2 The following applies: The number density of Si-Al-Cr oxides with an equivalent spherical radius of 0.500 μm or more is 0.1 × 10⁻¹⁶ 3 pieces / cm 2 You may leave it at that.

[0074] On the surface, the number density of Si-Al-Cr oxide particles with an equivalent spherical radius of 0.005 to 0.050 μm is 1.0 × 10⁻¹⁰ 5 pieces / cm 2 If the number density is less than 10, the corrosion resistance of the hot-rolled steel sheet deteriorates. The mechanism is not entirely clear, but it is thought that in the case of minute Si-Al-Cr oxides, the zinc phosphate film adheres more easily due to the anchoring effect caused by the increase in surface area, rather than due to the effect of hindering the electrochemical reaction. Therefore, the number density of Si-Al-Cr oxides on the surface with an equivalent spherical radius of 0.005 to 0.050 μm is 1.0 × 10⁻⁶. 5 pieces / cm 2 The above is required. Preferably 3.0 × 10 5 pieces / cm 2 That is all, more preferably 5.0 × 10 5 pieces / cm 2 That's all. The number density of Si-Al-Cr oxides with an equivalent spherical radius of 0.005 to 0.050 μm is 100.0 × 10⁻⁶ 5 pieces / cm 2 The following is also acceptable.

[0075] In this embodiment, however, there is no need to limit the number density of Si-Al-Cr oxides with an equivalent spherical radius greater than 0.050 μm and less than 0.500 μm, or the number density of Si-Al-Cr oxides with an equivalent spherical radius less than 0.005 μm, on the surface. This is because, in the hot-rolled steel sheet produced by the chemical composition of this embodiment and the manufacturing method described later, amounts of Si-Al-Cr oxides with an equivalent spherical radius greater than 0.050 μm and less than 0.500 μm, and Si-Al-Cr oxides with an equivalent spherical radius less than 0.005 μm, that would adversely affect the properties of the hot-rolled steel sheet according to this embodiment, are not formed.

[0076] The number density of Si-Al-Cr oxide on the surface is measured by the following method. A sample is cut from a hot-rolled steel sheet so that the surface in the thickness direction becomes the observation surface. The observation surface is degreased at 60°C for 60 seconds using an FC-E6403 manufactured by Nippon Parkerizing Co., Ltd., and then ultrasonically cleaned by immersion in acetone for 90 seconds. After that, 10 or more fields of view are observed at a magnification of 3000x. The composition of the precipitates can be measured by EDS (energy-dispersive X-ray spectrometer). From the precipitates containing Si, Al, Cr, and O, regions with an equivalent spherical radius of 0.500 μm or more and regions with an equivalent spherical radius of 0.005 to 0.050 μm are selected, and the number of these in each field of view is counted and divided by the measurement area to obtain the number density of Si-Al-Cr oxides with an equivalent spherical radius of 0.500 μm or more, and the number density of Si-Al-Cr oxides with an equivalent spherical radius of 0.005 to 0.050 μm.

[0077] Furthermore, if EDS analysis of the precipitate detects at least 10 atomic percent each of Si, Cr, and O, and at least 5 atomic percent of Al, the precipitate is considered to be a Si-Al-Cr oxide.

[0078] Furthermore, if the hot-rolled steel sheet has scale on its surface, the sample should be pickled under the following conditions before being subjected to the surface treatment described above. The pickling treatment can be carried out by conventional methods, for example, by immersing the sheet in hydrochloric acid with a concentration of 3-10% by volume at a temperature of 85-98°C for 20-300 seconds. The pickling may be performed once or in multiple stages as needed. The above pickling time (20-300 seconds) refers to the time of the single pickling if pickling is performed only once, and the total time of the multiple picklings if pickling is performed multiple times. It is preferable to set the pickling temperature to 85°C or higher because it can sufficiently remove oxides from the surface. There is no particular upper limit to the pickling temperature, but in reality it is around 98°C. If the pickling time exceeds 300 seconds, the surface roughness becomes excessively rough, the surface properties deteriorate, and furthermore, the irregularities remaining after cold rolling may produce a notch-like effect, which may worsen the bendability of the hot-rolled steel sheet. The upper limit of the pickling time is preferably 200 seconds.

[0079] If the hot-rolled steel sheet has a surface treatment film such as a plating layer and paint on its surface, the surface treatment film is removed, and the resulting base metal surface is subjected to the pickling treatment described above before being subjected to the surface treatment described above. The method for removing the surface treatment film can be appropriately selected according to the type of surface treatment film, within a range that does not affect the surface roughness of the base metal. For example, if the surface treatment film is a zinc plating layer such as electroplated zinc, electroplated Zn-Ni alloy, hot-dip galvanized zinc, alloyed hot-dip galvanized zinc, hot-dip Zn-Al alloy, hot-dip Zn-Al-Mg alloy, or hot-dip Zn-Al-Mg-Si alloy, the zinc plating layer can be dissolved using dilute hydrochloric acid with an inhibitor added. This allows only the zinc plating layer to be peeled off from the steel sheet. An inhibitor is an additive used to suppress changes in roughness due to the prevention of over-dissolution of the base metal. For example, hydrochloric acid diluted to 5 volume% can be used with "Ibit No. 700BK," a corrosion inhibitor for hydrochloric acid pickling manufactured by Asahi Chemical Industry Co., Ltd., added to achieve a concentration of 0.6 g / L. Furthermore, if the surface treatment film is an aluminum plating layer such as molten aluminum plating, the aluminum plating is dissolved by sequentially immersing it in a dilute hydrochloric acid aqueous solution to which sodium hydroxide aqueous solution and hexamethylenetetramine have been added, in accordance with the description in JIS G 3314:2019, and continuing to immerse it until the foaming caused by the dissolution of the plating subsides. If the surface treatment film is electrodeposited coating, the electrodeposited coating is removed using a stripping agent (Neoriver SP-751: manufactured by Sansai Chemical Co., Ltd.).

[0080] Tensile strength characteristics The tensile strength characteristics (tensile strength and total elongation) of hot-rolled steel sheets, among other mechanical properties, shall be evaluated in accordance with JIS Z 2241:2011. The test specimen shall be a No. 5 specimen as specified in JIS Z 2241:2011. The test specimen shall be taken at a position 1 / 4 of the way from the end face in a direction perpendicular to the rolling direction and the thickness direction, with the width direction being the longitudinal direction of the test specimen.

[0081] The hot-rolled steel sheet according to this embodiment has a tensile strength of 980 MPa or more. Preferably, it is 1000 MPa or more. If the tensile strength is less than 980 MPa, the applicable parts are limited and the contribution to vehicle body weight reduction is small. 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.

[0082] Furthermore, the total elongation of the hot-rolled steel sheet according to this embodiment is preferably 10.0% or more, and the product of tensile strength and total elongation (TS × El) is preferably 13,000 MPa·% or more. A total elongation of 11.0% or more is more preferable, and 13.0% or more is even more preferable. Furthermore, the product of tensile strength and total elongation is more preferably 14,000 MPa·% or more, and 15,000 MPa·% MPa or more is even more preferable. By setting the total elongation to 10.0% or more and the product of tensile strength and total elongation to 13,000 MPa·% or more, it is possible to significantly contribute to vehicle body weight reduction without limiting the applicable parts.

[0083] 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, if the thickness exceeds 8.0 mm, it may be difficult to refine the metal structure and obtain the metal structure described above. Therefore, the thickness may be 8.0 mm or less. Preferably, it is 6.0 mm or less.

[0084] 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.

[0085] 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.

[0086] In the preferred manufacturing method for hot-rolled steel sheets according to this embodiment, the following steps (1) to (11) 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.

[0087] (1) After holding the slab at a temperature of 700-850°C for 900 seconds or more, further heat it and hold it at a temperature of 1100°C or higher for 6000 seconds or more. (2) Descaling shall be performed at least once in a temperature range of 1200°C or higher before rough rolling, and descaling shall be performed at least twice in a temperature range of 1150°C or higher during rough rolling, and the maximum total reduction ratio between descalings in the temperature range of 1150°C or higher shall be 40% or higher. (3) The maximum temperature from the final stage of rough rolling, when the reduction ratio exceeds 20%, until descaling after the completion of rough rolling shall be 1130°C or lower. (4) Hot rolling is performed so that the total reduction ratio in the temperature range of 850 to 1100°C is 90% or more. (5) A stress of 170 kPa or more is applied to the steel plate between the completion of the second-to-last stage of hot rolling and the start of the final stage of rolling. (6) 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. (7) After the completion of the final stage of hot rolling, a stress of less than 200 kPa is applied to the steel plate until it cools to 800°C. (8) After hot rolling is complete, the material is cooled to a temperature range of 600-780°C at an average cooling rate of 50°C / s or more. (9) Perform slow cooling for 2.0 seconds or more in the temperature range of 600-780°C with an average cooling rate of less than 5°C / s. (10) After the slow cooling is complete, the temperature range of 450-600°C is cooled so that the average cooling rate is 30°C / s or more and less than 50°C / s. (11) Cool the coils so that the average cooling rate in the temperature range of ~450°C is 50°C / s or more. (12) Wind up at a temperature of 350℃ or less.

[0088] By adopting the above manufacturing method, it is possible to stably produce hot-rolled steel sheets with high strength, a low reduction in thickness at critical fracture, and excellent ductility, shear workability, and corrosion resistance.

[0089] (1) Slab temperature and holding time when subjecting to hot rolling Slabs used for hot rolling can be those obtained by continuous casting or by casting and splitting. Furthermore, if necessary, slabs that have undergone hot or cold working can be used.

[0090] For slabs to be subjected to hot rolling, it is preferable to hold the slab at a temperature of 700-850°C for 900 seconds or more during slab heating, and then further heat it to a temperature of 1100°C or higher and hold it for 6000 seconds or more. During the holding period in the 700-850°C temperature range, the steel plate temperature may be varied within this range or kept constant. Similarly, during the holding period at 1100°C or higher, the steel plate temperature may be varied within the 1100°C or higher temperature range or kept constant.

[0091] During austenite transformation in the temperature range of 700-850°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 Mn concentration. Furthermore, by maintaining the temperature range above 1100°C for 6000 seconds or more, the standard deviation of Mn concentration can be significantly reduced.

[0092] 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. Hot rolling is a process that includes rough rolling and finish rolling, each of which involves multiple rolling passes (stages). Rough rolling is a process in which the slab is rolled to a minimum of 25 mm, and finish rolling is a process in which the sheet after rough rolling is rolled to the target sheet thickness.

[0093] (2) Descaling conditions before rough rolling, descaling during rough rolling, and rolling conditions It is preferable to perform descaling at a temperature of 1200°C or higher at least once before rough rolling. Performing descaling at a temperature of 1200°C or higher at least once before rough rolling removes the primary scale formed in the heating furnace and suppresses the occurrence of subsequent descaling defects. As a result, the number density of Si-Al-Cr oxides with an equivalent spherical radius of 0.500 μm or more can be favorably controlled on the surface of the hot-rolled steel sheet. There is no particular upper limit to the number of descaling cycles at a temperature of 1200°C or higher, but it may be 5 cycles or less.

[0094] In rough rolling, multiple rolling and descaling processes are performed. During rough rolling, descaling is performed between rolling processes or after multiple rolling processes. In this embodiment, it is preferable to perform two or more descaling processes in a temperature range of 1150°C or higher during rough rolling, and to set the maximum total reduction ratio between descaling processes in the temperature range of 1150°C or higher to 40% or higher. By performing two or more descaling processes in the temperature range of 1150°C or higher, the scale thickness formed in the preliminary stage of rough rolling is reduced or the scale is removed, thereby allowing for favorable control of the number density of Si-Al-Cr oxides with an equivalent spherical radius of 0.500 μm or higher on the surface of the hot-rolled steel sheet. Furthermore, by setting the maximum total reduction ratio between descaling processes in the temperature range of 1150°C or higher to 40% or higher, the effect of descaling can be enhanced, and the number density of Si-Al-Cr oxides with an equivalent spherical radius of 0.500 μm or higher on the surface of the hot-rolled steel sheet can be favorably controlled.

[0095] To achieve a maximum total reduction ratio of 40% or more during descaling in the temperature range above 1150°C, for example, one rolling operation should be performed during a certain descaling operation in the temperature range above 1150°C that results in a reduction ratio of 40% or more, or multiple rolling operations should be performed to achieve a total reduction ratio of 40% or more.

[0096] The total reduction ratio between each descaling step in the temperature range above 1150°C can be expressed as {(t0-t1) / t0} × 100(%), where t0 is the plate thickness before the nth descaling step in the temperature range above 1150°C, and t1 is the exit plate thickness after the (n+1)th descaling step in the temperature range above 1150°C. Between the nth descaling step and the (n+1)th descaling step, only one rolling step may be performed, or multiple rolling steps may be performed. Descaling can be performed by water jetting.

[0097] (3) Temperature conditions during rough rolling and after completion of rough rolling In rough rolling, it is desirable to keep the maximum temperature from the final stage of rolling, where the reduction ratio exceeds 20%, until descaling after rough rolling is complete, below 1130°C. After rough rolling, descaling is performed to remove the scale generated during rough rolling. Also, after the final stage of rough rolling, the temperature of the steel sheet rises due to reheating from within the steel sheet. In this embodiment, it is preferable to control the maximum temperature of the steel sheet that rises due to reheating from the final stage of rolling, where the reduction ratio exceeds 20%, until descaling after rough rolling is complete, to below 1130°C. By keeping the maximum temperature from the final stage of rolling, where the reduction ratio exceeds 20%, until descaling after rough rolling is complete, below 1130°C, the number density of Si-Al-Cr oxide with an equivalent spherical radius of 0.005 to 0.050 μm can be favorably controlled.

[0098] Furthermore, even when heating is performed to raise the steel sheet temperature from the final stage of rough rolling, when the reduction ratio exceeds 20%, until descaling is carried out after the rough rolling is completed, in order to achieve uniform heating of the steel sheet, it is preferable to control the maximum temperature to 1130°C or lower. Furthermore, the "final stage of rolling where the reduction ratio exceeds 20%" as used here does not necessarily refer to the final rolling step of the finish rolling process. For example, if the reduction ratio of the final rolling step of the finish rolling process is 20% or less, and the reduction ratio of the rolling step immediately preceding the final stage exceeds 20%, then this preceding rolling step is considered the "final stage of rolling where the reduction ratio exceeds 20%."

[0099] (4) Total reduction ratio of hot rolling: 90% or more in the temperature range of 850~1100℃ Hot rolling in the 850-1100°C temperature range to achieve a total reduction ratio of 90% or more primarily refines the recrystallized austenite grains and promotes the accumulation of strain energy within the unrecrystallized austenite grains. This promotes the recrystallization of austenite and the atomic diffusion of Mn, thereby reducing the standard deviation of the Mn concentration. Therefore, it is preferable to perform hot rolling in the 850-1100°C temperature range to achieve a total reduction ratio of 90% or more. Note that the term "hot rolling" as used here includes rough rolling and finish rolling.

[0100] Furthermore, the reduction ratio in the temperature range of 850 to 1100°C can be expressed as {(t0-t1) / t0} × 100(%), where t0 is the thickness of the plate at the entrance before the first rolling stage in this temperature range, and t1 is the thickness of the plate at the exit stage after the final rolling stage in this temperature range.

[0101] (5) Stress to be applied to the steel plate from after the completion of the second-to-last stage of hot rolling until before the start of the final stage of rolling: 170 kPa or more It is preferable to apply a stress of 170 kPa or more to the steel sheet from the completion of the rolling stage before the final stage of hot rolling until the start of the final stage of rolling. This ensures that the {110} recrystallized austenite after the completion of the rolling 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 reduction stage. 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.

[0102] Note that the rolling process immediately preceding the final stage of hot rolling, as used here, refers to the rolling process immediately preceding the final stage of finish rolling. For example, if finish rolling is performed in seven passes (F1, F2...F6, F7), then it refers to the sixth pass (F6).

[0103] If the stress applied to the steel plate is less than 170 kPa, it may not be possible to achieve the desired E value. More preferably, the stress applied to the steel plate is 190 kPa or higher. The stress applied to a steel plate refers to the tension applied in the longitudinal direction of the steel plate. This can be controlled by adjusting the roll rotation speed during tandem rolling, and can be determined by dividing the load in the rolling direction, measured at the rolling stand, by the cross-sectional area of ​​the steel plate passing through it.

[0104] (6) 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 a high-strength hot-rolled steel sheet 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 E value can be set to a desired value.

[0105] (7) Stress applied to the steel sheet from the completion of the final stage of hot rolling until the steel sheet cools to 800°C: Less than 200kPa It is preferable to apply a stress of less than 200 kPa to the steel sheet from the completion of the final stage of hot rolling until the steel sheet cools to 800°C. By applying a stress of less than 200 kPa to the steel sheet, austenite recrystallization preferentially proceeds in the rolling direction, and the increase in the periodicity of the metal structure can be suppressed. As a result, the E value can be set to a desired value. The stress applied to the steel sheet is more preferably 180 kPa or less.

[0106] (8) Accelerated cooling to a temperature range of 600-780°C at an average cooling rate of 50°C / s or higher. By performing accelerated cooling to a temperature range of 780°C or lower at an average cooling rate of 50°C / s or higher after hot rolling is complete, the formation of ferrite and pearlite, which have low precipitation strengthening properties, can be suppressed. This increases the strength of the hot-rolled steel sheet. 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.

[0107] 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, a rate of 300°C / s or less is preferable. Furthermore, the cooling stop temperature for accelerated cooling should be set to 600°C or higher in order to perform the slow cooling described later.

[0108] (9) Perform slow cooling for 2.0 seconds or more in the temperature range of 600-780°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 600-780°C with an average cooling rate of less than 5°C / s, sufficient precipitation-strengthened ferrite can be deposited. This makes it possible to achieve both strength and ductility in hot-rolled steel sheets.

[0109] 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.

[0110] 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.

[0111] (10) After slow cooling is complete, cool the temperature in the 450-600°C range so that the average cooling rate is 30°C / s or more and less than 50°C / s. After the above slow cooling is complete, it is preferable to cool the material so that the average cooling rate in the 450-600°C temperature range is 30°C / s or more and less than 50°C / s. By setting the average cooling rate in the above temperature range to 30°C / s or more and less than 50°C / s, the CS value can be set to the desired value. If the average cooling rate is 50°C / s or more, a flat, lath-like structure with low brightness is likely to be formed, and the CS value will be -8.0 × 10⁻⁶. 5 The CS value will be less than 8.0 × 10⁻⁶. When the average cooling rate is less than 30°C / s, carbon concentration in the untransformed portion is promoted, increasing the strength of the hard tissue and widening the strength difference between it and the soft tissue, so the CS value will be less than 8.0 × 10⁻⁶. 5 It becomes super.

[0112] 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 for slow cooling (average cooling rate less than 5°C / s) to the cooling stop temperature for cooling (average cooling rate of 30°C / s or more but less than 50°C / s) by the time required from the time of cessation of slow cooling (average cooling rate less than 5°C / s) to the time of cessation of cooling (average cooling rate of 30°C / s or more but less than 50°C / s).

[0113] (11) Average cooling rate in the temperature range of winding temperature ~450℃: 50℃ / s or higher To suppress the area ratio of pearlite and retained austenite and obtain the desired strength and moldability, it is preferable to set the average cooling rate in the winding temperature range of ~450°C to 50°C / s or higher. This makes the matrix structure harder.

[0114] In this context, the average cooling rate refers to the value obtained by dividing the temperature drop of the steel sheet from the cooling stop temperature to the winding temperature for cooling with an average cooling rate of 30°C / s or more and less than 50°C / s by the time required from the cooling stop to winding for cooling with an average cooling rate of 30°C / s or more and less than 50°C / s.

[0115] (12) Winding temperature: 350℃ or less The winding temperature should be 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 I value can be increased, and the occurrence of secondary shear surfaces can be suppressed. [Examples]

[0116] 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.

[0117] 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 and 6B were obtained under the manufacturing conditions shown in Tables 3A to 4B. Furthermore, the average cooling rate for slow cooling was set to less than 5°C / s. Also, 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.

[0118] For the obtained hot-rolled steel sheets, the area ratio of the metal structure, E value, I value, CS value, standard deviation of Mn concentration, number density of Si-Al-Cr oxides with an equivalent spherical radius of 0.500 μm or more and number density of Si-Al-Cr oxides with an equivalent spherical radius of 0.005 to 0.050 μm on the surface, tensile strength TS, and total elongation El were determined using the method described above. The obtained measurement results are shown in Tables 5A and 6B.

[0119] 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.

[0120] Reduction in plate thickness at critical fracture The rate of thickness reduction at the limit fracture of hot-rolled steel sheets was evaluated by tensile testing. Tensile tests were performed using the same method as when evaluating the tensile properties. The limit fracture thickness reduction rate was obtained by calculating (t1-t2)×100 / t1, where t1 was the thickness of the plate before the tensile test, and t2 was the minimum thickness of the plate at the center of the width direction (short side direction) of the tensile test specimen after fracture. The tensile test was performed five times, and the limit fracture thickness reduction rate was obtained by calculating the average of the three values ​​excluding the maximum and minimum values.

[0121] If the reduction in thickness at critical fracture was 60.0% or higher, the hot-rolled steel sheet was judged to be acceptable as having a high reduction in thickness at critical fracture. On the other hand, if the reduction in thickness at critical fracture was less than 60.0%, the hot-rolled steel sheet was judged to be unacceptable as not having a high reduction in thickness at critical fracture.

[0122] 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 cross-sections perpendicular to the rolling direction and parallel to the rolling direction of the punched holes were embedded in resin, and the cross-sectional shapes were photographed using a scanning electron microscope. In the obtained observation photographs, the shear end faces shown in Figure 1 or Figure 2 can be observed. Figure 1 is an example of the shear end face of a hot-rolled steel sheet according to the present invention, and Figure 2 is an example of the shear end face of a hot-rolled steel sheet according to a comparative example. In Figure 1, the shear end face consists of a burr, shear surface, fracture surface, and burr. On the other hand, in Figure 2, the shear end face consists of a burr, shear surface, fracture surface, shear surface, fracture surface, and burr. Here, a burr is a smooth, R-shaped surface area, a shear surface is a region of the punched end face separated by shear deformation, a fracture surface is a region of the punched end face separated by a crack originating near the cutting edge, and a burr is a surface with a projection that protrudes from the lower surface of the hot-rolled steel sheet.

[0123] Of the obtained shear end faces, if a shear surface-fracture surface-shear surface pattern, such 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 found, the hot-rolled steel sheet was judged to have excellent shear workability and was deemed acceptable, and "None" was written in the table. On the other hand, if even one secondary shear surface was formed, the hot-rolled steel sheet was deemed not to have excellent shear workability and was deemed unacceptable, and "Present" was written in the table.

[0124] Corrosion resistance A 70mm x 40mm sample was taken from a hot-rolled steel sheet after pickling. After treating it with zinc phosphate (SD5350 system: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.), three locations along the length of the specimen (center and both ends) were observed at 1000x magnification using a scanning electron microscope (SEM). The pickling conditions were the same as those for the pickling treatment described above. If the coverage rate of the chemical conversion treated crystals was 98% or more of the surface area, the hot-rolled steel sheet was judged to have excellent corrosion resistance and was deemed acceptable. On the other hand, if the coverage rate of the chemical conversion treated crystals was less than 98% of the surface area, the hot-rolled steel sheet was judged not to have excellent corrosion resistance and was deemed unacceptable.

[0125] [Table 1]

[0126] [Table 2]

[0127] [Table 3A]

[0128] [Table 3B]

[0129] [Table 4A]

[0130] [Table 4B]

[0131] [Table 5A]

[0132] [Table 5B]

[0133] [Table 6A]

[0134] [Table 6B]

[0135] Tables 5A to 6B show that the hot-rolled steel sheets according to the present invention have high strength and a low rate of reduction in thickness at critical fracture, as well as excellent ductility, shear workability, and corrosion resistance. On the other hand, it can be seen that the hot-rolled steel sheet in the comparative example has deteriorated in one or more properties. [Industrial applicability]

[0136] According to the above embodiment of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and a low reduction in thickness at critical fracture, as well as excellent ductility, shear workability, and corrosion resistance. The hot-rolled steel sheet according to the present invention is suitable as an industrial material used in automobile components, machine structural components, and even building components.

Claims

1. The chemical composition is expressed in mass percent. C: 0.050-0.250%, Si: 0.40-1.00%, Mn: 1.00-1.50%, Sol. Al: 0.100–0.500%, Cr: 0.50-2.00%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Ti: 0 to 0.500%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 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%, Zr: 0 to 1.00%, Co: 0-1.00%, Zn: 0 to 1.00%, W: 0-1.00%, and Sn: Contains 0-0.05%, The remainder consists of Fe and impurities. The following equations (A) and (B) are satisfied, The microstructure at a position 1 / 4 of the way from the surface in the thickness direction is 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 Entropy value, expressed by the following formula (1), obtained by analyzing the SEM image of the metal structure using the gray-level co-occurrence matrix method, is 10.7 or greater. The Inverse difference normalized value shown in the following formula (2) is 1.020 or greater, The Cluster Shade value shown in the following formula (3) is -8.0 × 10 ５ ~8.0 x 10 ５ And, The standard deviation of the Mn concentration is 0.60 mass% or less. On the aforementioned surface, the number density of Si-Al-Cr oxide particles with an equivalent spherical radius of 0.500 μm or more is 2.0 × 10⁻¹⁴ ３ pieces / cm ２ The following conditions apply, and the number density of Si-Al-Cr oxides with an equivalent spherical radius of 0.005 to 0.050 μm is 1.0 × 10⁻¹⁰ ５ pieces / cm ２ A hot-rolled steel sheet characterized by the above. 0.060%≦Ti+Nb+V≦0.500%…(A) Zr+Co+Zn+W≦1.00%…(B) However, in formulas (A) and (B) above, each element symbol indicates the mass percentage content of that element, and if the element is not contained, 0% is substituted. Here, P(i,j) in equations (1) to (5) below is the gray level co-occurrence matrix, L in equation (2) below is the number of grayscale levels that the SEM image can take, i and j in equations (2) and (3) below are natural numbers from 1 to L, and μ in equation (3) below ｘ and μ ｙ These are represented by equations (4) and (5) below, respectively. 【Number 1】 [Math 2] [Math 3] [Math 4] [Math 5]

2. The aforementioned chemical composition, in mass%, Ti: 0.001 to 0.500%, Nb: 0.001-0.500%, V: 0.001-0.500%, Cu: 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: 0.0005-0.0200%, As: 0.001 to 0.100%, Zr: 0.01-1.00%, Co: 0.01 to 1.00%, Zn: 0.01-1.00%, W: 0.01 to 1.00%, and Sn: 0.01~0.05% It contains one or more selected from the group consisting of the following: The hot-rolled steel sheet according to feature 1.

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