Hot-rolled steel plate

Abstract

This hot-rolled steel sheet has a specific chemical composition. The metal structure of the hot-rolled steel sheet in a location at a depth of 1 / 4 from the surface in the sheet thickness direction includes, in area%, a total of more than 92.0% and no more than 100.0% of martensite and tempered martensite, less than 3.0% of residual austenite, and less than 5.0% of ferrite while having an E value of 11.0 or more, an I value of less than 1.020, a CS value of -8.0 × 105 to 8.0 × 105 inclusive, and a standard deviation of 0.60 mass% or less in Mn concentration. In the surface, the number density of Si-Al-Cr oxides having an equivalent circle diameter of 0.500 µm or more is 2.0 × 103 oxides per cm2 or less, and the number density of Si-Al-Cr oxides having an equivalent circle diameter of 0.005-0.050 µm is 1.0 × 105 oxides 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. 2022-208520, filed in Japan on December 26, 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 combine high strength with excellent formability, and several technologies have been proposed to meet these requirements. Because automotive components undergo various processing methods, the required formability differs depending on the component applied. Among these, the rate of reduction in plate thickness at critical fracture and hole expansion properties 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 and the thickness of the tensile test specimen before fracture. A higher critical fracture thickness reduction rate is preferable because it makes the specimen less likely to fracture prematurely 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 the linearity of the boundary between the fracture surface and the shear surface at the shear end face is low, the accuracy of the shear end face will deteriorate 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.

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

[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 hole-expanding properties, 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: 0.040~0.250%, Si: 0.40~1.00%, Mn: 1.00~4.00%, 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: 0~0.300%, Nb: 0~0.100%, 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 equation (A) is satisfied, The microstructure at a position 1 / 4 of the way from the surface in the thickness direction is In area percentage, The total amount of martensite and tempered martensite is over 92.0% and 100.0% or less. The residual austenite content is less than 3.0%. The ferrite 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 11.0 or greater, as shown in the following formula (1). The inverse difference normalized value shown in formula (2) below is less than 1.020, 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 circular radius of 0.500 μm or more is 2.0 × 10 3 pieces / cm 2 The following conditions apply, and the number density of Si-Al-Cr oxides with an equivalent circle radius of 0.005 to 0.050 μm is 1.0 × 10⁻¹⁰ 5 pieces / cm 2 That's all. Zr + Co + Zn + W ≤ 1.00 …(A) However, each element symbol in formula (A) above indicates the mass percentage content of that element, and 0 is substituted if the element is not contained. 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.

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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 rate of reduction in plate thickness at critical fracture, as well as excellent hole-expanding properties, 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 building components. [Brief explanation of the drawing]

[0015] [Figure 1] This diagram illustrates a method for measuring the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing. [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: 0.040~0.250%, Si: 0.40~1.00%, Mn: 1.00~4.00%, 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. The following provides a detailed explanation of each element.

[0019] C: 0.040~0.250% Carbon (C) increases the area ratio of the hard phase. If the C content is less than 0.040%, the area ratio of ferrite and the remaining structure in the hot-rolled steel sheet increases, making it impossible to obtain the desired strength and hole-expanding properties. Therefore, the C content should be 0.040% or more. Preferably, the C content is 0.045% or more, and more preferably 0.050% or more. On the other hand, when the carbon content exceeds 0.250%, the flat cementite structure increases, and the E value decreases due to the formation of carbide regions with small brightness differences. Therefore, the carbon content should be 0.250% or less. Preferably, the carbon content is 0.200% or less, or 0.150% or less.

[0020] Si: 0.40~1.00% Si has the effect of promoting ferrite formation, thereby improving the hole-expanding properties of hot-rolled steel sheets, and the effect of solid-solution strengthening of ferrite, thereby increasing 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. Preferably, the Si content is 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: 1.00~4.00% 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 strength cannot be obtained in the hot-rolled steel sheet. 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 4.00%, it is not possible to reduce the standard deviation of the Mn concentration. Therefore, the Mn content should be 4.00% or less. Preferably, the Mn content is 3.50% or less and 3.00% or less, and more preferably 2.50% or less and 2.00% or less.

[0022] 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 improving the hole-expanding properties of hot-rolled steel sheets. The above effects cannot be obtained if the sol.Al content is less than 0.100%. 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.

[0023] 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.

[0024] 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 hole-expanding properties and the rate of reduction in thickness at critical fracture of the hot-rolled steel sheet due to grain boundary segregation become significantly reduced. 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.

[0025] S: 0.0300% or less S forms sulfide inclusions in the steel, reducing the hole-expanding properties and the rate of thickness reduction at critical fracture of hot-rolled steel sheets. When the S content exceeds 0.0300%, the hole-expanding properties and the rate of thickness reduction 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.

[0026] N: 0.1000% or less N has the effect of reducing the hole-expanding properties and the rate of reduction in thickness at critical fracture of hot-rolled steel sheets. When the N content exceeds 0.1000%, the hole-expanding properties 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. The N content is preferably 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 to have an N content of 0.0010% or more, and more preferably 0.0020% or more, in order to promote the precipitation of carbonitrides.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] Ti: 0.001~0.300% Nb: 0.001~0.100% V: 0.001~0.500% Ti, Nb, and V are elements that precipitate as carbides and nitrides in steel, improving the strength of the steel through precipitation strengthening. To more reliably obtain the effects of the above action, it is preferable that the content of at least one of Ti, Nb, or V be 0.001% or more. The content of Ti, Nb, and V is preferably 0.010% or more for each element. On the other hand, if the Ti content exceeds 0.300%, the Nb content exceeds 0.100%, or the V content exceeds 0.500%, the workability of the hot-rolled steel sheet deteriorates. Furthermore, excessive amounts of these elements are not economically desirable. Therefore, the Ti content should be 0.300% or less, the Nb content 0.100% or less, and the V content 0.500% or less.

[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, increasing the strength of the hot-rolled steel sheet. Furthermore, when Cu is included, Ni effectively suppresses grain boundary cracking in 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 hole-expanding properties 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 hole-expanding properties 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 reduce the hole-expanding properties 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 hole-expanding properties 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% …(A) However, each element symbol in formula (A) 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 (A) 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 hot-rolled steel sheet by solid solution strengthening, the contents of Zr, Co, Zn, and W may each be 0.01% or more.

[0039] Sn: 0~0.05% 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%. To improve the corrosion resistance of the hot-rolled steel sheet, the Sn content may be 0.01% or more.

[0040] 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). Note that sol.Al may be measured by ICP-AES using the filtrate after the sample is decomposed by heating 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-nondispersive infrared absorption method. When the hot-rolled steel sheet has a plating layer on its surface, if necessary, the plating layer may be removed by mechanical grinding or the like, and then the chemical composition may be analyzed.

[0041] Metallographic structure of the hot-rolled steel sheet Next, the metallographic structure of the hot-rolled steel sheet according to the present embodiment will be described. The hot-rolled steel sheet according to the present embodiment has a metallographic structure at the 1 / 4 position from the surface in the sheet thickness direction, in terms of area%, the total of martensite and tempered martensite is more than 92.0% and 100.0% or less, the retained austenite is less than 3.0%, the ferrite is less than 5.0%, and by analyzing the SEM image of the metallographic structure by the gray level co-occurrence matrix method, the Entropy value represented by the following formula (1) is 11.0 or more, the Inverse difference normalized value represented by the following formula (2) is less than 1.020, 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 circle-equivalent radius of 0.500 μm or more is 2.0×10 3 pieces / cm 2 or less, and the number density of Si-Al-Cr oxides with a circle-equivalent radius of 0.005 to 0.050 μm is 1.0×10 5 pieces / cm 2 or more.

[0042] Therefore, the hot-rolled steel sheet according to this embodiment can obtain high strength and a low rate of reduction in plate thickness at critical fracture, as well as excellent hole-expanding properties, shear workability, and corrosion resistance. In this embodiment, the microstructure fraction, entropy value, inverse difference normalized value, cluster shade value, and standard deviation of Mn concentration are defined for the region at a position 1 / 4 of the way from the surface in the thickness direction of the sheet. 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.

[0043] 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 hole-expanding properties of hot-rolled steel sheets through transformation-induced plasticity (TRIP). On the other hand, retained austenite transforms into high-carbon martensite during shearing, which inhibits stable crack initiation and reduces the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing. When the area ratio of retained austenite is 3.0% or more, the above effect becomes apparent, and the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing decreases. Furthermore, the desired strength cannot be obtained in the hot-rolled steel sheet. 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%.

[0044] 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.

[0045] 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 longitudinal 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 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.

[0046] Ferrite area ratio: Less than 5.0% Ferrite generally has a soft metallic structure. If the material contains more than a certain amount of ferrite, the desired strength may not be obtained, and the area of ​​the shear surface at the end face after shearing may increase. An increase in the area of ​​the shear surface at the end face after shearing is undesirable because it reduces the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing. This effect becomes apparent when the area ratio of ferrite is 5.0% or more. Therefore, the area ratio of ferrite should be less than 5.0%. The area ratio of ferrite is preferably 3.0% or less, more preferably 2.0% or less, and even more preferably less than 1.0%. Since a lower amount of ferrite is preferable, the area ratio of ferrite may be 0%.

[0047] Total area ratio of martensite and tempered martensite: over 92.0% and 100.0% or less If the combined area ratio of martensite and tempered martensite is 92.0% or less, the desired strength cannot be obtained in the hot-rolled steel sheet. Therefore, the combined area ratio of martensite and tempered martensite should be greater than 92.0%. Preferably, it should be 93.0% or higher, 95.0% or higher, 97.0% or higher, or 99.0% or higher. A higher combined area ratio of martensite and tempered martensite is preferable, so it may be set to 100.0%.

[0048] The hot-rolled steel sheet according to this embodiment may contain one or two types of bainite and pearlite as the remaining structure, with a total area ratio of 0% or more and less than 8.0%. The upper limit of the area ratio of the remaining structure may be 6.0%, 5.0%, 4.0%, 3.0%, or 1.5%.

[0049] The area ratio of each tissue will be measured using the following method. The thickness cross section parallel to the rolling direction is finished to a mirror surface and polished for 8 minutes at room temperature using colloidal silica with a particle size of 0.25 μm, without alkaline solutions, to remove strain introduced into the surface of the sample. Crystal orientation information is obtained in the sample cross section in a region of 50 mm or more at any position in the longitudinal direction and in a region of 50 mm or more centered on the 1 / 4 position from the end face in the thickness direction. The crystal orientation information is obtained by measuring using electron backscatter diffraction at measurement intervals of 0.1 μm. An EBSD analyzer consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) is used for the measurement. At this time, the vacuum level inside the EBSD analyzer 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 observation area is 40,000 μm². 2 Let's assume that.

[0050] Next, a backscattered electron image is taken in the same field of view. From the backscattered electron image, crystal grains in which ferrite and cementite are precipitated in layers are identified, and the area ratio of these crystal grains is calculated to obtain the area ratio of pearlite.

[0051] Subsequently, among the crystal grains excluding those identified as pearlite, the crystal grains determined to have a body-centered cubic structure are analyzed using the "Grain Average Misorientation" function in the "OIM Analysis®" software included with the EBSD analyzer. Regions with a Grain Average Misorientation value of 1.0° or less are identified as 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.

[0052] Next, the methods for measuring the area ratio of martensite and tempered martensite will be explained below. First, to observe the same area as the EBSD measurement area using SEM, a Vickers indentation is imprinted near the observation position. Then, while preserving the structure of the observation surface, surface contaminants are polished off, and nital etching is performed. Next, the same field of view as the EBSD observation surface is observed using SEM at a magnification of 3000x.

[0053] In EBSD measurements, among the regions identified as structures other than ferrite, regions that have a substructure within the grain and where cementite precipitates with multiple variants are identified as tempered martensite. Regions with high brightness and where the substructure is not revealed by etching are identified as "martensite and retained austenite". By calculating the area ratio of each, the area ratio of tempered martensite and the area ratio of "martensite and retained austenite" are obtained. By subtracting the area ratio of retained austenite obtained by the above-mentioned X-ray diffraction from the obtained area ratio of "martensite and retained austenite", the area ratio of martensite is obtained. By calculating the sum of the area ratios of martensite and tempered martensite, the total area ratio of martensite and tempered martensite is obtained.

[0054] To remove contaminants from the surface of the observation area, buff polishing using alumina particles with a particle size of 0.1 μm or less, or methods such as Ar ion sputtering, can be used.

[0055] In this embodiment, since the area percentage of each tissue is measured by X-ray diffraction, EBSD analysis, and SEM observation, the sum of the area percentages of each tissue obtained may not equal 100.0%. If the sum of the area percentages of each tissue obtained by the above method does not equal 100.0%, the area percentages of each tissue are converted so that the sum equals 100.0%. For example, if the sum of the area percentages of each tissue is 103.0%, the area percentage of each tissue is multiplied by "100.0 / 103.0" to obtain the area percentage of each tissue.

[0056] Entropy value: 11.0 or higher Inverse difference normalized value: less than 1.020 To improve the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing, it is important to reduce the periodicity and uniformity of the metal structure. In this embodiment, the linearity of the boundary between the fracture surface, the shear surface and the end face after shearing is improved 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.

[0057] The E value represents the periodicity of the metal structure. When the brightness is periodically arranged due to the formation of band-like structures, 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 11.0, the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing tends to decrease. In a metal structure with high periodicity, i.e., a low E value, cracks are generated starting from the periodically arranged structure, and the fracture surface is formed by traveling along multiple band-like structures present near the starting point. This is presumed to make the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing tend to decrease. Therefore, the E value should be 11.0 or higher. Preferably it is 11.1 or higher, and more preferably 11.2 or higher. A higher E value is preferable, and there is no particular upper limit, but it may be 13.5 or lower, 13.0 or lower, 12.5 or lower, or 12.0 or lower.

[0058] 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, in a hot-rolled steel sheet having a metal structure in which the sum of the area ratios of martensite and tempered martensite exceeds 92.0%, it is necessary to have a metal structure mainly composed of martensite with low uniformity of brightness. For this reason, in this embodiment, it is necessary to reduce the I value. When the uniformity of the metal structure is high, i.e., when the I value is high, cracks are more likely to occur from the tip of the shearing tool due to the influence of precipitates and elemental concentration differences within the crystal grains, as well as hardness differences caused by the soft ferrite phase. As a result, the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing tends to decrease. That is, it is estimated that if the I value is 1.020 or higher, it is not possible to improve the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing. Therefore, the I value should be less than 1.020. Preferably it is 1.015 or lower, and more preferably 1.010 or lower. There is no specific lower limit for the I value, but it may be set at 0.900 or higher, 0.950 or higher, or 1.000 or higher.

[0059] Cluster Shade value: -8.0 × 105 ~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.

[0060] 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.

[0061] 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 × 10 5 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:

[0062] 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 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.

[0063] 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.

[0064] 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, 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.

[0065] 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. The dot symbol "." in equation (1) below represents the product. In equations (2) and (3) below, i and j 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. 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.

[0066]

number

[0067]

number

[0068]

number

[0069]

number

[0070]

number

[0071]

number

[0072]

number

[0073]

number

[0074]

number

[0075]

number

[0076] Standard deviation of Mn concentration: 0.60% by mass or less In this embodiment, the standard deviation of Mn concentration at a position 1 / 4 of the plate thickness from the surface of the hot-rolled steel sheet (the region from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness from the surface) and at the center in the plate width direction is 0.60 mass% or less. This allows for uniform dispersion of the hard phase and prevents a decrease in the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing. The standard deviation of Mn concentration is preferably 0.55 mass% or less or 0.50 mass% or less, and more preferably 0.47 mass% or less. From the viewpoint of suppressing excessive burrs, a smaller lower limit of the standard deviation of Mn concentration is desirable, but due to constraints of the manufacturing process, the practical lower limit is 0.10 mass%. If necessary, the lower limit may be 0.20 mass% or 0.28 mass%.

[0077] The standard deviation of Mn concentration is obtained by the following method. After mirror polishing the thickness cross section of a hot-rolled steel sheet parallel to the rolling direction, the standard deviation of the Mn concentration is measured using an electron probe microanalyzer (EPMA) at a depth of 1 / 4 of the sheet thickness from the surface (the region from 1 / 8 of the sheet thickness to 3 / 8 of the sheet thickness from the surface) and at the center in the sheet width direction. The measurement conditions are an acceleration voltage of 15kV and a magnification of 5000x, and a distribution image is measured in a range of 20μm in the rolling direction and 20μm in the sheet thickness direction. More specifically, the measurement interval is set to 0.1μm, and the Mn concentration is measured at more than 40,000 locations. 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.

[0078] Number density of Si-Al-Cr oxides on the surface with an equivalent circular radius of 0.500 μm or more: 2.0 × 10 3 pieces / cm 2 below Number density of Si-Al-Cr oxides on the surface with an equivalent circular radius of 0.005 to 0.050 μm: 1.0 × 10⁻¹⁴ 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 radius of 0.500 μm or more and those with an equivalent radius of 0.005 to 0.050 μm, within a desired range.

[0079] On the surface, the number density of Si-Al-Cr oxide particles with an equivalent circular radius of 0.500 μm or more is 2.0 × 10⁻⁶ 3 pieces / cm 2 If the number density is excessive, the electrochemical reaction between the solution and the steel sheet is locally hindered during the 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 circular radius of 0.500 μm or more on the surface should be 2.0 × 10⁻⁶. 3 pieces / cm 2 The following applies: The number density of Si-Al-Cr oxides with an equivalent circular radius of 0.500 μm or more on the surface is preferably 1.8 × 10⁻⁶. 3 pieces / cm 2The following, more preferably 1.5 × 10 3 pieces / cm 2 The following applies: The number density of Si-Al-Cr oxides with an equivalent circle radius of 0.500 μm or more is 0.1 × 10⁻⁶ 3 pieces / cm 2 You may leave it at that.

[0080] On the surface, the number density of Si-Al-Cr oxide particles with an equivalent circular 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 clear, but it is thought that in the case of minute Si-Al-Cr oxides, the anchoring effect due to the increase in surface area makes it easier for the zinc phosphate film to adhere, rather than hindering the electrochemical reaction. Therefore, the number density of Si-Al-Cr oxides on the surface with an equivalent circle radius of 0.005 to 0.050 μm is 1.0 × 10⁻⁶. 5 pieces / cm 2 The above is correct. The number density of Si-Al-Cr oxide with an equivalent circular radius on the surface of 0.005 to 0.050 μm is preferably 3.0 × 10⁻⁶. 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 circular radius on the surface of 0.005 to 0.050 μm is 100.0 × 10⁻⁶ 5 pieces / cm 2 Below, 50.0 × 10 5 pieces / cm 2 Below, 30.0 × 10 5 pieces / cm 2 Below, 15.0 × 10 5 pieces / cm 2 The following is also acceptable.

[0081] In this embodiment, however, there is no need to limit the number density of Si-Al-Cr oxides with an equivalent radius of less than 0.005 μm on the surface, nor the number density of Si-Al-Cr oxides with an equivalent radius of more than 0.050 μm and less than 0.500 μm. 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 radius of less than 0.005 μm and Si-Al-Cr oxides with an equivalent radius of more than 0.050 μm and less than 0.500 μm that would adversely affect the properties of the hot-rolled steel sheet according to this embodiment are not formed.

[0082] 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 (plate face) 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, those containing Si, Al, Cr, and O with an equivalent circle radius of 0.500 μm or more and those with an equivalent circle radius of 0.005 to 0.050 μm are selected, their number is counted, and the number density of Si-Al-Cr oxides with an equivalent circle radius of 0.500 μm or more and Si-Al-Cr oxides with an equivalent circle radius of 0.005 to 0.050 μm is obtained by dividing by the measurement area.

[0083] 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. However, if there are precipitates that are similar in grain shape and contrast based on electron beam intensity to precipitates considered to be Si-Al-Cr oxides by EDS analysis, the EDS analysis can be omitted, and the precipitate can be considered as a Si-Al-Cr oxide.

[0084] 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.

[0085] 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.).

[0086] Tensile strength The tensile strength of hot-rolled steel sheets 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 from a position 1 / 4 of the way from the end face in the width direction of the sheet, with the width direction being the longitudinal direction of the test specimen.

[0087] The hot-rolled steel sheet according to this embodiment preferably has a tensile strength of 980 MPa or higher. More preferably, the tensile strength is 1000 MPa or higher. By setting the tensile strength to 980 MPa or higher, the contribution to vehicle body weight reduction can be greatly increased without limiting the applicable parts. There is no particular need to limit the upper limit of the tensile strength, but from the viewpoint of suppressing mold wear, it may be set to 1780 MPa.

[0088] Hole widening properties In this embodiment, the hot-rolled steel sheet preferably has a hole expansion ratio of 55% or more. A hole expansion ratio of 55% or more allows for the production of hot-rolled steel sheets that contribute significantly to vehicle weight reduction without limiting the applicable parts. While there is no particular upper limit to the hole expansion ratio, it may be 85% or less, or 80% or less. The hole expansion ratio (λ) is measured using a No. 5 test specimen according to JIS Z 2241:2011, in accordance with JIS Z 2256:2010. The hole expansion test specimen should be taken from the 1 / 4 portion from the edge in the width direction of the hot-rolled steel sheet.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] (1) After holding the slab at a temperature of 700-850°C for 900 seconds or more, heat it further 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, accelerated cooling is performed so that the average cooling rate up to 600°C is 50°C / s or more. (9) Cool the temperature range of 450-600°C so that the average cooling rate is 30°C / s or more and less than 50°C / s. (10) Accelerated cooling is performed so that the average cooling rate in the winding temperature range of ~450℃ is 50℃ / s or more. (11) Wind up at a temperature of 350℃ or less.

[0094] By adopting the above manufacturing method, it is possible to stably produce hot-rolled steel sheets with high strength, a low rate of reduction in plate thickness at critical fracture, and excellent hole-expanding properties, shear workability, and corrosion resistance.

[0095] (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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] (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 circle 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 limited to 6 cycles or less.

[0100] 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 radius of 0.500 μm or more 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 radius of 0.500 μm or more on the surface of the hot-rolled steel sheet can be favorably controlled.

[0101] 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.

[0102] 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.

[0103] (3) Temperature conditions during rough rolling and after completion of rough rolling In rough rolling, it is preferable to keep the maximum temperature from the final stage of rolling, where the reduction ratio exceeds 20%, until descaling after the completion of rough rolling, below 1130°C. After rough rolling, descaling is performed to remove the scale generated during rough rolling. Furthermore, 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 between the final stage of rolling, where the reduction ratio exceeds 20%, and descaling performed after the completion of rough rolling, below 1130°C. By keeping the maximum temperature between the final stage of rolling, where the reduction ratio exceeds 20%, and descaling performed after the completion of rough rolling, below 1130°C, the number density of Si-Al-Cr oxides with an equivalent circle radius of 0.005 to 0.050 μm can be favorably controlled.

[0104] 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%."

[0105] (4) Total reduction ratio in hot rolling: 90% or more in the temperature range of 850-1100°C 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.

[0106] 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.

[0107] (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.

[0108] 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 passes F1, F2...F6, F7, then it refers to pass F6.

[0109] 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.

[0110] (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.

[0111] (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.

[0112] (8) Average cooling rate from completion of hot rolling to 600°C: 50°C / s or more By accelerating the cooling process from the completion of hot rolling to 600°C so that the average cooling rate is 50°C / s or higher, ferrite, bainite, and / or pearlite transformations within the steel sheet can be suppressed, and the desired strength can be obtained in the hot-rolled steel sheet. Furthermore, the I value can be set to a desired value. Performing air cooling or other methods during the accelerating cooling to 600°C after the completion of hot rolling is undesirable because it may increase the amount of ferrite.

[0113] While there is no specific upper limit for the average cooling rate, increasing the cooling rate requires more complex cooling equipment, leading to higher equipment costs. Therefore, considering equipment costs, it is preferable that the average cooling rate for accelerated cooling be 300°C / s or less.

[0114] 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 600°C by the time required from the start of accelerated cooling until the steel plate temperature reaches 600°C.

[0115] (9) Average cooling rate in the temperature range of 450-600°C: 30°C / s or more, less than 50°C / s After the above accelerated 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 exceeds 50°C / s, coarse crystal grains are likely to form in the metal structure, 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, the strength of hard tissue increases and the strength difference between hard and soft tissue widens, so the CS value will be less than 8.0 × 10⁻⁶. 5 It becomes super.

[0116] 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 (where the average cooling rate is 50°C / s or higher) to the cooling stop temperature of cooling (where the average cooling rate is 30°C / s or higher and less than 50°C / s) by the time required from the time of cessation of accelerated cooling (where the average cooling rate is 50°C / s or higher) to the time of cessation of cooling (where the average cooling rate is 30°C / s or higher and less than 50°C / s).

[0117] (10) Average cooling rate in the temperature range of winding temperature ~450℃: 50℃ / s or higher To reduce the area ratio of perlite and obtain the desired strength in hot-rolled steel sheets, it is preferable to set the average cooling rate in the temperature range of ~450°C (winding temperature) to 50°C / s or higher. This makes the matrix structure harder.

[0118] 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.

[0119] (11) Winding temperature: 350℃ or less The winding temperature is preferably 350°C or lower. By setting the winding temperature to 350°C or lower, the driving force for the transformation from austenite to bcc can be increased, and the deformation strength of the austenite can be increased. As a result, the hard phase is distributed uniformly during the transformation from austenite to martensitic, and variability can be improved. Consequently, the I value can be reduced, and the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing can be improved. Therefore, it is preferable to set the winding temperature to 350°C or lower. [Examples]

[0120] 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.

[0121] 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. Note that the winding temperatures listed in Tables 4A and 4B have a measurement limit of 50°C; therefore, the actual winding temperature for the examples listed as 50°C is 50°C or lower. Furthermore, production No. 15 was cooled to 721°C after hot rolling completion, followed by 6.0 seconds of air cooling. The average cooling rate during air cooling was less than 5.0°C / s.

[0122] 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 radius of 0.500 μm or more on the surface, and number density of Si-Al-Cr oxides with an equivalent radius of 0.005 to 0.050 μm, tensile strength (TS), and hole expansion ratio (λ) were determined using the method described above. The obtained measurement results are shown in Tables 5A to 6B.

[0123] Method for evaluating the properties of hot-rolled steel sheets Tensile strength If the tensile strength (TS) was 980 MPa or higher, it was judged to have high strength and was deemed acceptable. On the other hand, if the tensile strength (TS) was less than 980 MPa, it was judged to have insufficient strength and was deemed unacceptable.

[0124] Hole widening rate If the hole expansion ratio (λ) was 55% or higher, it was judged to be excellent in hole expansion and was deemed acceptable. On the other hand, if the hole expansion ratio (λ) was less than 55%, it was judged to be poor in hole expansion and was deemed unacceptable.

[0125] 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 critical 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 of the tensile test specimen after fracture. The tensile test was performed five times, and the critical fracture thickness reduction rate was obtained by calculating the average of the three values ​​excluding the maximum and minimum values.

[0126] If the reduction in thickness at limit fracture was 75.0% or higher, the hot-rolled steel sheet was judged to be acceptable as having a high reduction in thickness at limit fracture. On the other hand, if the reduction in thickness at limit fracture was less than 75.0%, the hot-rolled steel sheet was judged to be unacceptable as not having a high reduction in thickness at limit fracture.

[0127] Shear workability (evaluation of the linearity of the boundary between the fracture surface and the shear surface) The shear workability of hot-rolled steel sheets was evaluated by performing punching tests to determine the degree of straightness at the boundary between the fracture surface and the shear surface. Five punched holes were created in the center of the width of a hot-rolled steel sheet with a hole diameter of 10 mm, a clearance of 15%, and a punching speed of 3 m / s. Next, the end faces of the five punched holes, parallel to the rolling direction at 10 locations (two end faces per punched hole), were photographed using an optical microscope. The obtained observation photographs show the end faces as shown in Figure 1(a). As shown in Figures 1(a) and (b), burrs, shear surfaces, fracture surfaces, and burrs are observed on the end faces after punching. Figure 1(a) is a schematic diagram of the end face of the punched hole parallel to the rolling direction, and Figure 1(b) is a schematic diagram of the side view of the punched hole. A burr is a smooth, rounded surface; a shear surface is the punched end face separated by shear deformation; a fracture surface is the punched end face separated by a crack that occurred near the cutting edge after the shear deformation was completed; and a burr is a surface with a projection that protrudes from the bottom surface of the hot-rolled steel sheet.

[0128] In the observation photographs of 10 end faces obtained from 5 end faces, the straightness at the boundary between the fracture surface and the shear surface was measured using the method described later, and the maximum value of the obtained straightness was calculated.

[0129] The straightness at the boundary between the fracture surface and the shear surface was obtained by the following method. As shown in Figure 1(b), the boundary points between the shear surface and the fracture surface (points A and B in Figure 1(b)) were determined relative to the end face. The length x of the straight line connecting points A and B was measured. Next, the length y of the curve along the fracture surface-shear surface boundary was measured. The value obtained by dividing the obtained y by x was defined as the degree of straightness at the boundary between the fracture surface and the shear surface.

[0130] If the maximum value of the straightness obtained in the punching test was less than 1.045, the hot-rolled steel sheet was judged to be acceptable as having excellent shear workability. On the other hand, if the maximum value of the obtained linearity was 1.045 or higher, it was judged to be a hot-rolled steel sheet that did not have good shear workability and was therefore deemed unacceptable.

[0131] 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.

[0132] [Table 1]

[0133] [Table 2]

[0134] [Table 3A]

[0135] [Table 3B]

[0136] [Table 4A]

[0137] [Table 4B]

[0138] [Table 5A]

[0139] [Table 5B]

[0140] [Table 6]

[0141] Tables 5A to 6 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 hole-expanding properties, 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]

[0142] 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 rate of reduction in plate thickness at critical fracture, as well as excellent hole-expanding properties, 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: 0.040-0.250%, Si: 0.40-1.00%, Mn: 1.00-4.00%, 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: 0-0.300%, Nb: 0 to 0.100%, 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 equation (A) is satisfied, The microstructure at a position 1 / 4 of the way from the surface in the thickness direction is In area percentage, The total amount of martensite and tempered martensite is more than 92.0% and less than or equal to 100.0%. The residual austenite content is less than 3.0%. The ferrite content is less than 5.0%. The Entropy value, shown in the following formula (1), obtained by analyzing the SEM image of the metal structure using the gray-level co-occurrence matrix method, is 11.0 or greater. The Inverse difference normalized value shown in the following formula (2) is less than 1.020, 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 circular radius of 0.500 μm or more is 2.0 × 10⁻¹⁶ ３ pieces / cm ２ The following conditions apply, and the number density of Si-Al-Cr oxides with an equivalent circle radius of 0.005 to 0.050 μm is 1.0 × 10⁻¹⁶ ５ pieces / cm ２ A hot-rolled steel sheet characterized by the above. Zr+Co+Zn+W≦1.00…(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. 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. [Math 1] 【Number 2】 [Math 3] [Math 4] [Math 5]

2. The aforementioned chemical composition is, in mass%, Ti: 0.001 to 0.300%, Nb: 0.001 to 0.100%, 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% The hot-rolled steel sheet according to claim 1, characterized by containing one or more selected from the group consisting of the above.

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