JP7678370B2 - hot rolled steel plate - Google Patents

Description

The present invention relates to a hot-rolled steel sheet. Specifically, the present invention relates to a hot-rolled steel sheet that is used by being formed into various shapes by press working or the like, and in particular to a hot-rolled steel sheet that has high strength and excellent shear workability.
This application claims priority based on Japanese Patent Application No. 2021-146231, filed on September 8, 2021, the contents of which are incorporated herein by reference.

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

In order to achieve both lightweight vehicle bodies and crashworthiness, the use of high-strength steel plates to make components thinner is being considered. For this reason, there is a strong demand for steel plates that combine high strength with excellent formability, and several technologies have been proposed to meet these demands. Automotive components are formed by press molding, and blank plates for press molding are often manufactured by shearing, which has high productivity. Blank plates manufactured by shearing must have excellent end surface precision after shearing. For example, if the surface roughness of the fracture surface after shearing increases, fatigue properties and bendability decrease.

Regarding shear workability, for example, Patent Document 1 discloses a technology for controlling the burr height after punching by controlling the ratio ds/db of the surface ferrite grain size ds to the internal ferrite crystal grains db to 0.95 or less.
Patent Document 2 discloses a technique for improving peeling and curling at the sheet end surface by reducing the P content.

Japanese Patent Application Publication No. 10-168544 Japanese Patent Application Publication No. 2005-298924

However, Patent Document 1 targets IF steel, and it may be difficult to apply it to high-strength components of 980 MPa or more. Patent Document 2 also does not achieve a strength of 980 MPa or more, and does not consider the roughness of the fracture surface at the sheared end surface after shear processing.

The present invention has been made in consideration of the above problems of the conventional technology, and has an object to provide a hot-rolled steel sheet having high strength and excellent shear workability.
In the present invention, "having excellent shear workability" means that the surface roughness Rz of the fracture surface at the sheared end surface after shear work is 30.0 μm or less, and "having high strength" means that the tensile strength is 980 MPa or more.

In view of the above-mentioned problems, the inventors have conducted extensive research into the relationship between the chemical composition and metal structure of hot-rolled steel sheets and their mechanical properties, and have obtained the following findings (a) to (d) and completed the present invention.

(a) In order to reduce the surface roughness of the fracture surface at the shear end surface, it is necessary to limit the area fraction of ferrite, which has a high work-hardening capacity, and pearlite, which causes the formation of coarse voids.

(b) In order to reduce the surface roughness of the fracture surface at the sheared end surface, it is also important to have a metal structure in which a certain amount of coarse Ti-based carbides are present and the crystal grain size is small. The reason for this is that the coarse Ti-based carbides are thought to be carbides that precipitated in the austenite structure during hot rolling. These coarse Ti-based carbides are incompatible with the parent phase after transformation at low temperatures. Therefore, it is presumed that the dispersed precipitation of these coarse and incompatible Ti-based carbides disperses the voids that occur during shear deformation, which, together with the refinement of the crystal grains, reduces the surface roughness of the fracture surface.

(c) In order to maintain a certain amount of incoherent and coarse Ti-based carbides, a hot rolling process in the high temperature range is important. For example, it is effective to perform hot rolling with a total reduction rate of 70% or more in the temperature range of 1100°C to SRT (°C).

(d) In order to reduce the crystal grain size of the metal structure, in addition to increasing the total reduction in the high temperature range described above, it is also important to increase the reduction in hot rolling in the low temperature range. For example, it is effective to perform hot rolling with a total reduction of 80% or more in the temperature range from below 1100°C to finishing rolling. The pinning effect of the coarse Ti-based carbides precipitated in the high temperature range refines the austenite structure, and in combination with the subsequent cooling conditions, it is possible to create a structure with a small crystal grain size.

The gist of the present invention made based on the above findings is as follows.
[1] A hot-rolled steel sheet according to one embodiment of the present invention has, in mass%, C: 0.050 to 0.200%, Si: 0.005 to 2.000%, Mn: 0.50 to 4.00%, P: 0.100% or less, S: 0.0100% or less, sol. Al: 0.001-1.00%, Ti: 0.150-0.400%, N: 0.0010-0.0200%, Nb: 0-0.200%, V: 0-1.000%, Mo: 0-1.000%, Cu: 0-1.00%, Ni: 0-1.00%, Cr: 0-2.00%, W: 0-1.00%, B: 0-0.0040%, Ca: 0-0.0100%, Mg: 0-0.0100%, REM: 0-0.0100%, Bi: 0-0.0200%, balance: Fe and impurities. and when a region from the surface to 1/8 to 3/8 of the plate thickness in the plate thickness direction is defined as a 1/4 depth position, the metal structure at the 1/4 depth position contains, in area fractions, retained austenite: less than 3.0%, ferrite: less than 30.0%, and pearlite: less than 5.0%, with the remaining structure being composed of one or more of bainite, martensite, and tempered martensite, and at the 1/4 depth position, the average number density of Ti-based carbides having a major axis of 15 nm or more is 1.0 x ¹⁰⁴ pieces/ ^mm2 or more, the average crystal grain size dq is 15.0 μm or less, and the tensile strength is 980 MPa or more.
[2] In the hot-rolled steel sheet described in [1], when a region from the surface to 50 μm from the surface in the sheet thickness direction is defined as a surface layer portion, a ratio ds/dq of an average grain size ds of the surface layer portion to the average grain size dq at the 1/4 depth position may be 0.95 or less.
[3] In the hot-rolled steel sheet according to [1] or [2], the chemical composition is, in mass%, Nb: 0.001 to 0.200%, V: 0.005 to 1.000%, Mo: 0.001 to 1.000%, Cu: 0.02 to 1.00%, Ni: 0.02 to 1.00%, Cr: 0.02 to 2.00%, W: 0.020 to 1.00%, B: 0.0001 to 0.0040%, Ca: 0.0002 to 0.0100%, Mg: 0.0002 to 0.0100%, REM: 0.0002 to 0.0100%, Bi: 0.0002 to 0.0200%. It may contain one or more selected from the group consisting of.

According to the above-mentioned aspect of the present invention, a hot-rolled steel sheet having high strength and excellent shear workability can be obtained.
The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for automobile parts, machine structural parts, and further building parts.

FIG. 13 is a diagram for explaining a sheared end surface after shearing.

A hot-rolled steel sheet according to one embodiment of the present invention (a hot-rolled steel sheet according to this embodiment) has a predetermined chemical composition, and when a region from the surface to ⅛ of the sheet thickness in the sheet thickness direction is defined as a ¼ depth position, the metal structure at the ¼ depth position contains, in terms of area fraction, retained austenite: less than 3.0%, ferrite: less than 30.0%, and pearlite: less than 5.0%, and at the ¼ depth position, the average number density of Ti-based carbides having a major axis of 15 nm or more is 1×10 ⁴ pieces/mm ² or more, the average crystal grain size dq is 15.0 μm or less, and the tensile strength of the hot-rolled steel sheet is 980 MPa or more.
The characteristics of the hot-rolled steel sheet according to this embodiment (hereinafter, sometimes simply referred to as steel sheet) will be described in more detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible within the scope of the present invention. The numerical ranges described below with "to" include the lower limit and the upper limit. The numerical values indicated as "less than" or "more than" are not included in the numerical range.

[Chemical composition]
First, the chemical composition of the hot-rolled steel sheet according to the present embodiment will be described. In the following description, percentages relating to the chemical composition of the steel sheet are mass percentages unless otherwise specified.

(C: 0.050-0.200%)
C is an element that increases the fraction of the hard phase and increases the strength of ferrite by combining with precipitation strengthening elements such as Ti, Nb, and V. If the C content is less than 0.050%, it is difficult to obtain the desired strength. Therefore, the C content is set to 0.050% or more. The C content is preferably 0.060% or more, more preferably 0.070% or more, and even more preferably 0.080% or more.
On the other hand, if the C content exceeds 0.200%, the weldability of the hot-rolled steel sheet is reduced. Therefore, the C content is set to 0.200% or less, and preferably 0.150% or less.

(Si: 0.005-2.000%)
Si is an element that acts to increase the strength of hot-rolled steel sheet by solid solution strengthening. In addition, Si acts to improve the soundness of steel by deoxidization (suppressing the occurrence of defects such as blowholes in steel). If the Si content is less than 0.005%, the above-mentioned effects cannot be obtained. Therefore, the Si content is set to 0.005% or more. The Si content is preferably 0.010% or more.
On the other hand, Si is an element that deteriorates the surface quality and chemical conversion treatability of the hot-rolled steel sheet, and also has the effect of promoting the formation of residual austenite by suppressing the precipitation of cementite from austenite. If the Si content exceeds 2.000%, residual austenite is formed and the surface roughness of the fracture surface deteriorates. Therefore, the Si content is set to 2.000% or less. The Si content is preferably 1.500% or less, and more preferably 1.300% or less.

(Mn: 0.50-4.00%)
Mn is an element that has the effect of suppressing ferrite transformation and increasing the strength of the hot-rolled steel sheet. If the Mn content is less than 0.50%, a tensile strength of 980 MPa or more cannot be obtained. Therefore, the Mn content is set to 0.50% or more. The Mn content is preferably 0.80% or more, and more preferably 1.00% or more. In addition, when the area fraction of ferrite is reduced, the Mn content is even more preferably 1.40% or more, and further preferably 1.50% or more.
On the other hand, if the Mn content exceeds 4.00%, cracks occur near the center of the sheet thickness due to central segregation of Mn, and the surface roughness of the sheared end surface after shearing deteriorates. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.50% or less, and more preferably 3.00% or less.

(P: 0.100% or less)
P is an element that is generally contained as an impurity. P is an element that easily segregates, and if the P content exceeds 0.100%, the bending workability decreases due to grain boundary segregation. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.030% or less.
Although it is not necessary to specify a lower limit for the P content, it is preferable to set the P content to 0.001% or more from the viewpoint of refining costs. In addition, P is also an element that has the effect of increasing the strength of the hot-rolled steel sheet by solid solution strengthening. Therefore, P may be intentionally contained. In that case, the P content may be set to 0.002% or more.

(S: 0.0100% or less)
S is an element contained as an impurity, and forms sulfide-based inclusions in steel, thereby reducing the bending workability of the hot-rolled steel sheet. If the S content exceeds 0.0100%, the bending workability of the hot-rolled steel sheet is significantly reduced. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0050% or less. There is no need to specify a lower limit for the S content, but from the viewpoint of refining costs, it is preferable to set the S content to 0.0001% or more.

(sol. Al: 0.001 to 1.00%)
Al, like Si, is an element that has the effect of deoxidizing steel and improving the soundness of the steel. If the sol. Al content is less than 0.001%, the above-mentioned effect cannot be obtained. Therefore, the sol. Al content is set to 0.001% or more. The sol. Al content is preferably 0.01% or more.
On the other hand, if the sol. Al content exceeds 1.00%, the above effects are saturated and it is economically undesirable. Therefore, the sol. Al content is set to 1.00% or less. The sol. Al content is preferably 0.80% or less, more preferably 0.60% or less.
Sol. Al means acid-soluble Al, which indicates Al present in the steel in a solid solution state.

(Ti: 0.150-0.400%)
Ti is an element that reduces the surface roughness of the fracture surface at the sheared end surface by precipitating as coarse Ti-based carbides in the high temperature region of hot rolling. It is also an element that suppresses the recovery, recrystallization and grain growth of the austenite structure and refines the metal structure after transformation. Furthermore, Ti is an element that precipitates as fine Ti-based carbides even during cooling after hot rolling (after completion of finish rolling) and improves the strength of the steel by precipitation strengthening. If the Ti content is less than 0.150%, the driving force for precipitating Ti-based carbides in the high temperature region of hot rolling is small, and the desired number density of Ti-based carbides cannot be obtained. Therefore, the Ti content is set to 0.150% or more. The Ti content is preferably 0.170% or more, more preferably 0.190% or more, and even more preferably 0.210% or more.
On the other hand, if the Ti content exceeds 0.400%, rectangular, coarse nitrides with a major axis of several μm are formed, which reduces bending workability. Therefore, the Ti content is set to 0.400% or less. The Ti content is preferably 0.350% or less, and more preferably 0.300% or less.
The Ti-based carbide refers to a carbide having a NaCl-type crystal structure containing Ti. If such a carbide contains Ti, it also includes those containing small amounts of other carbide-forming alloy elements, such as Mo, Nb, V, Cr, and W, within the range of the chemical composition specified in this embodiment. It also includes carbonitrides in which part of the carbon is replaced with nitrogen.

(N: 0.0010-0.0200%)
N is an element that forms nitrides or carbonitrides with Ti, Nb, V, etc., and suppresses the coarsening of austenite during slab heating, thereby refining the metal structure. If the N content is less than 0.0010%, it becomes difficult to exert the above-mentioned effect. Therefore, the N content is set to 0.0010% or more. The N content is preferably 0.0015% or more.
On the other hand, if the N content exceeds 0.0200%, coarse Ti nitrides are formed, and bending workability is deteriorated. Therefore, the N content is set to 0.0200% or less. The N content is preferably 0.0150% or less, more preferably 0.0100% or less, and even more preferably 0.0060% or less.

The balance of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and impurities. In the present embodiment, the term "impurities" refers to substances mixed in from raw materials such as ore, scrap, or the manufacturing environment, and/or substances that are allowed to be present within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment.
On the other hand, the hot-rolled steel sheet according to the present embodiment may contain one or more of Nb, V, Mo, Cu, Ni, Cr, W, B, Ca, Mg, REM, and Bi as optional elements instead of a part of Fe. Since it is not essential to contain the optional elements, the lower limit of the content is 0%. Hereinafter, the optional elements will be described in detail.

(Nb: 0-0.200%)
Nb is an optional element. Nb is an element that precipitates in steel as carbides, nitrides, carbonitrides, etc., and has the effect of increasing the tensile strength of the steel sheet. In order to obtain these effects, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more.
On the other hand, if the Nb content exceeds 0.200%, the above effect saturates and the rolling load during finish rolling increases, making rolling difficult. Therefore, when Nb is contained, the Nb content is set to 0.200% or less. The Nb content is preferably 0.170% or less, more preferably 0.140% or less, and even more preferably 0.110% or less.

(V: 0-1.000%)
V is an optional element. V precipitates in steel as carbides, nitrides, carbonitrides, or the like, and has the effect of improving the tensile strength of the steel sheet. In order to obtain these effects, the V content is preferably 0.005% or more. The V content is more preferably 0.010% or more.
On the other hand, if the V content exceeds 1.000%, the workability of the hot-rolled steel sheet decreases. Therefore, when V is contained, the V content is set to 1.000% or less. The V content is more preferably 0.800% or less, and even more preferably 0.600% or less.

(Mo: 0-1.000%)
Mo is an optional element. Mo is an element that has the effect of increasing the hardenability of steel and forming carbides and carbonitrides to increase the strength of the steel sheet. In order to obtain these effects, the Mo content is preferably 0.001% or more. The Mo content is more preferably 0.005% or more.
On the other hand, if the Mo content exceeds 1.000%, the cracking sensitivity of the slab may increase. Therefore, when Mo is contained, the Mo content is set to 1.000% or less. The Mo content is more preferably 0.800% or less, and even more preferably 0.600% or less.

(Cu: 0-1.00%)
Cu is an optional element. Cu is an element that has the effect of improving the toughness of steel and the effect of increasing the tensile strength. In order to obtain these effects, the Cu content is preferably 0.02% or more. The Cu content is more preferably 0.08% or more.
On the other hand, excessive Cu content may deteriorate the weldability of the steel sheet. Therefore, when Cu is contained, the Cu content is set to 1.00% or less. The Cu content is more preferably 0.50% or less, and even more preferably 0.30% or less.

(Ni: 0-1.00%)
Ni is an optional element. Ni is an element that has the effect of improving the toughness of steel and the effect of increasing the tensile strength. In order to obtain these effects, the Ni content is preferably 0.02% or more. The Ni content is more preferably 0.10% or more.
On the other hand, excessive Ni content increases the alloy cost and may deteriorate the toughness of the welded heat affected zone of the steel plate. Therefore, when Ni is contained, the Ni content is set to 1.00% or less. The Ni content is more preferably 0.50% or less, and even more preferably 0.30% or less.

(Cr: 0-2.00%)
Cr is an optional element. Cr is an element that has the effect of increasing the hardenability of steel and forming carbides and carbonitrides to increase the strength of the steel plate. To obtain this effect, the Cr content is preferably 0.02% or more. The Cr content is more preferably 0.05% or more.
On the other hand, if Cr is contained in excess, the chemical conversion treatability deteriorates. Therefore, when Cr is contained, the Cr content is set to 2.00% or less. The Cr content is more preferably 1.50% or less, even more preferably 1.00% or less, and particularly preferably 0.50% or less.

(W: 0-1.00%)
W is an optional element. W is an element that has the effect of increasing tensile strength by forming carbides and carbonitrides. In order to obtain this effect, the W content is preferably 0.02% or more.
On the other hand, even if W is contained in a certain amount or more, the above-mentioned effect is saturated and the alloy cost increases. Therefore, when W is contained, the W content is set to 1.00% or less. The W content is preferably 0.80% or less.

(B: 0-0.0040%)
B is an optional element. B is an element that has the effect of increasing the tensile strength of the steel sheet by grain boundary strengthening and solid solution strengthening. To obtain this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more.
On the other hand, if the B content exceeds 0.0040%, the above effects are saturated and the alloy cost increases. Therefore, when B is contained, the B content is set to 0.0040% or less. The B content is more preferably 0.0030% or less, and even more preferably 0.0020% or less.

(Ca: 0-0.0100%)
Ca is an optional element. Ca is an element that has the effect of dispersing a large number of fine oxides in molten steel and refining the metal structure of the steel sheet. Ca is also an element that has the effect of fixing S in the molten steel as spherical CaS and suppressing the generation of elongation inclusions such as MnS, thereby improving the stretch flangeability of the steel sheet. In order to obtain these effects, the Ca content is preferably 0.0002% or more. The Ca content is more preferably 0.0005% or more.
On the other hand, if the Ca content exceeds 0.0100%, the amount of CaO in the steel increases, which may adversely affect the toughness of the steel plate. Therefore, when Ca is contained, the Ca content is set to 0.0100% or less. The Ca content is more preferably 0.0050% or less, and even more preferably 0.0030% or less.

(Mg: 0-0.0100%)
Mg is an optional element. Mg, like Ca, forms oxides and sulfides in molten steel, suppresses the formation of coarse MnS, disperses a large number of fine oxides, and has the effect of refining the metal structure of the steel sheet. In order to obtain these effects, the Mg content is preferably 0.0002% or more. The Mg content is more preferably 0.0005% or more.
On the other hand, if the Mg content exceeds 0.0100%, the amount of oxides in the steel increases, adversely affecting the toughness of the steel plate. Therefore, when Mg is contained, the Mg content is set to 0.0100% or less. The Mg content is more preferably 0.0050% or less, and even more preferably 0.0030% or less.

(REM: 0-0.0100%)
REM is an optional element. Like Ca, REM also forms oxides and sulfides in molten steel, suppresses the formation of coarse MnS, disperses a large number of fine oxides, and has the effect of refining the metal structure of the steel sheet. To obtain these effects, the REM content is preferably 0.0002% or more. The REM content is more preferably 0.0005% or more.
On the other hand, if the REM content exceeds 0.0100%, the amount of oxides in the steel increases, which may adversely affect the toughness of the steel plate. Therefore, when REM is contained, the REM content is preferably 0.0100% or less. The REM content is more preferably 0.0050% or less, and even more preferably 0.0030% or less.
Here, REM (rare earth) refers to a total of 17 elements consisting of Sc, Y and lanthanoids. In this embodiment, the content of REM refers to the total content of these elements.

(Bi: 0-0.0200%)
Bi is an optional element. B is an element that has the effect of refining the solidification structure and improving the formability of the steel sheet. In order to obtain this effect, the Bi content is preferably 0.0001% or more. The Bi content is more preferably 0.0005% or more.
On the other hand, if the Bi content exceeds 0.0200%, the above effects are saturated and the alloy cost increases. Therefore, when Bi is contained, the Bi content is set to 0.0200% or less. The Bi content is more preferably 0.0100% or less, and even more preferably 0.0070% or less.

The chemical composition of the above-mentioned hot-rolled steel sheet may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Sol. Al may be measured by ICP-AES using the filtrate after thermally decomposing the sample with acid. C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method.

[Metal structure]
Next, the metal structure of the hot-rolled steel sheet according to this embodiment will be described.
In the hot-rolled steel sheet according to the present embodiment, when the region from the surface to 1/8 to 3/8 of the sheet thickness is defined as the 1/4 depth position, the metal structure at the 1/4 depth position contains, in terms of area fraction, less than 30.0% ferrite, less than 3.0% retained austenite, and less than 5.0% pearlite. In addition, in the metal structure at the 1/4 depth position of the hot-rolled steel sheet according to the present embodiment, the average crystal grain size is 15.0 μm or less, and the average number density of Ti-based carbides having a major axis of 15 nm or more is 1.0 × 10 ⁴ pieces / mm ² or more (10,000 pieces / mm ² or more) (Ti-based carbides having a diameter of 15 nm or more are precipitated at an average number density of 1.0 × 10 ⁴ pieces / mm ² or more). The reason for specifying the metal structure in the region from the surface of the steel sheet to 1/8 to 3/8 of the sheet thickness (1/4 depth position) is that the metal structure at this position shows a typical metal structure of the steel sheet.

(Ferrite area fraction: less than 30.0%)
Ferrite is a structure that is generated when fcc transforms to bcc at a relatively high temperature. Since ferrite has a high work hardening ability, if the area fraction of ferrite is too high, the deformation amount of the fracture surface at the sheared edge increases, and the roughness of the fracture surface increases. Therefore, the area fraction of ferrite is less than 30.0%. The area fraction of ferrite is preferably 20.0% or less, more preferably 10.0% or less, and even more preferably 8.0% or less. The smaller the area fraction of ferrite, the more preferable it is, and it may be 0%, but considering productivity, the area fraction of ferrite may be 1.0% or more, 2.0% or more, or 3.0% or more.

(Area fraction of retained austenite: less than 3.0%)
In the hot-rolled steel sheet according to the present embodiment, if the area fraction of the retained austenite is too high, the surface roughness of the fracture surface at the sheared end surface may increase. This is presumably because the retained austenite forms coarse voids. In particular, if the area fraction of the retained austenite is 3.0% or more, the shear workability of the hot-rolled steel sheet deteriorates and the surface roughness of the fracture surface increases. Therefore, the area fraction of the retained austenite is less than 3.0%. The area fraction of the retained austenite is preferably less than 1.5%, more preferably less than 1.0%. Since the smaller the amount of retained austenite, the area fraction of the retained austenite may be 0%.

(Pearlite area fraction: less than 5.0%)
Pearlite is a lamellar metal structure in which cementite is precipitated in layers between ferrite. Pearlite is also a soft metal structure compared to bainite and martensite. If the area fraction of pearlite is 5.0% or more, carbon is consumed by the cementite contained in pearlite, and the strength of the remaining structures, martensite, tempered martensite, and bainite, decreases, making it impossible to obtain a tensile strength of 980 MPa or more. Therefore, the area fraction of pearlite is set to less than 5.0%. The area fraction of pearlite is preferably 3.0% or less, more preferably 2.0%, and even more preferably 1.0% or less, or may be 0%.

In order to ensure a tensile strength of 980 MPa or more, the hot-rolled steel sheet according to this embodiment preferably has a residual structure other than the retained austenite, ferrite, and pearlite, which is a hard structure, consisting of one or more of bainite, martensite, and tempered martensite. The area fraction of one or more of bainite, martensite, and tempered martensite is preferably 70.0% or more, more preferably 80.0% or more, and even more preferably 90.0% or more.

The area fraction of each structure constituting the metal structure is measured by the following method. A plate thickness cross section parallel to the rolling direction is mirror-finished and polished for 8 minutes at room temperature using colloidal silica that does not contain an alkaline solution to remove strain introduced into the surface layer of the sample. At any position in the longitudinal direction of the sample cross section, a region with a length of 50 μm and a depth of 1/8 of the plate thickness from the surface to a depth of 3/8 of the plate thickness from the surface is measured by electron backscatter diffraction at measurement intervals of 0.1 μm to obtain crystal orientation information. For the measurement, an EBSD device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the EBSD device is 9.6×10 ⁻⁵ Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62. Furthermore, a backscattered electron image is taken in the same field of view. First, the crystal grains in which ferrite and cementite are precipitated in layers are identified from the backscattered electron image, and the area fraction of the crystal grains is calculated to obtain the area fraction of pearlite. Then, for the crystal grains other than those determined to be pearlite, the obtained crystal orientation information is used to determine the regions with a Grain Average Misorientation value of 1.0° or less as ferrite using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. The area fraction of the regions determined to be ferrite is obtained to obtain the area fraction of ferrite.

Next, under the condition that the boundary of 5° or more in the remaining region (region with a grain average misorientation value of more than 1.0°) is defined as a grain boundary, when the maximum value of the "Grain Average IQ" of the ferrite region is Iα, the region exceeding Iα/2 is extracted as bainite, and the region below Iα/2 is extracted as "pearlite, martensite, and tempered martensite". The area fraction of bainite is obtained by calculating the area fraction of the extracted bainite. In addition, the area fraction of the extracted "pearlite, martensite, and tempered martensite" is calculated, and the total area fraction of martensite and tempered martensite is obtained by subtracting the area fraction of pearlite obtained by the above-mentioned EBSD analysis.

Methods for measuring the area fraction of retained austenite include X-ray diffraction, EBSD (Electron Backscatter Diffraction Pattern) analysis, magnetic measurement, etc., and the measured value may differ depending on the measurement method. In this embodiment, the area fraction of retained austenite is measured by X-ray diffraction.
In the measurement of the area fraction of retained austenite by X-ray diffraction in this embodiment, the area fraction of retained austenite is obtained by determining the integrated intensities of a total of six peaks, namely, α(110), α(200), α(211), γ(111), γ(200), and γ(220), using Co-Kα radiation at the 1/4 depth position of the hot-rolled steel sheet (the region from 1/8 depth of the sheet thickness from the surface to 3/8 depth of the sheet thickness from the surface), and calculating using the intensity averaging method.

(The average density of Ti-based carbides with a major axis of 15 nm or more is 1.0 x ¹⁰⁴ / ^mm2 or more)
In the hot-rolled steel sheet according to the present embodiment, coarse Ti-based carbides having a major axis of 15 nm or more are precipitated. Due to the presence of these coarse Ti-based carbides and the refinement of the average crystal grain size described below, voids at the time of fracture in shear processing are dispersed, and the surface roughness of the fracture surface at the sheared end surface is reduced. For this purpose, it is necessary that the average number density of Ti-based carbides having a major axis of 15 nm or more is 1.0×10 ⁴ (10,000) pieces/mm ² or more. The average number density of Ti-based carbides having a major axis of 15 nm or more is preferably 2.0×10 ⁴ pieces/mm ² or more, more preferably 4.0×10 ⁴ pieces/mm ² or more. The average number density of Ti-based carbides having a major axis of 15 nm or more is preferably as high as possible, but may be 5.0×10 ⁶ pieces/mm ² or less because the amount of solute C required for strengthening fine Ti-based carbides and bainite and martensite precipitated during cooling after completion of hot rolling may decrease, causing a decrease in strength. Here, the Ti-based carbide refers to a carbide having a NaCl-type crystal structure containing Ti. If such a carbide contains Ti, it may contain a small amount of other carbide-forming alloy elements. Within the range of the chemical composition specified in this embodiment, the Ti-based carbide may contain other carbide-forming alloy elements, such as Mo, Nb, V, Cr, and W. Furthermore, the Ti-based carbide may be a carbonitride in which part of the carbon is replaced with nitrogen.

The average number density of Ti-based carbides is determined by photographing 20 fields of view, each field being a 2.0 μm×2.0 μm area, at a magnification of 50,000 times with a TEM at a 1/4 depth position, analyzing precipitates observed within the fields of view with energy dispersive X-ray spectroscopy (EDS), determining that precipitates in which Ti and C are detected are Ti-based carbides, and measuring the major axis (longest diameter) of each precipitate (Ti-based carbide).The number of Ti-based carbides with a major axis of 15 nm or more per ^mm2 is then counted to determine the number density.

(Average grain size: 15.0 μm or less)
In the metal structure, if the average grain size is coarse, the roughness of the fracture surface of the sheared end surface increases. Therefore, the average grain size (dq) at the 1/4 depth position is set to 15.0 μm or less. The average grain size is preferably 12.0 μm or less, and more preferably 10.0 μm or less. The smaller the average grain size, the better, so there is no particular lower limit. However, in normal hot rolling, it is technically difficult to refine the average grain size to below 1.0 μm. Therefore, the average grain size may be 1.0 μm or more, or 4.0 μm or more.

(Preferably, the ratio of the average crystal grain size ds of the surface layer to the average crystal grain size dq at the 1/4 depth position, ds/dq, is 0.95 or less.)
The higher the strength of the hot-rolled steel sheet, the more likely it is that cracks will occur from the inside of the bend during bending (hereinafter referred to as "inner bending cracks"). In particular, in the case of the hot-rolled steel sheet according to the present embodiment, which has a tensile strength of 980 MPa or more, inner bending cracks are likely to occur.
The mechanism of cracking on the inside of the bend is believed to be as follows. That is, during bending, compressive stress is generated on the inside of the bend. At first, the entire inside of the bend deforms uniformly as bending progresses, but as the amount of deformation increases, the uniform deformation alone is no longer able to support the deformation, and deformation progresses as strain concentrates locally (generation of shear deformation bands). As these shear deformation bands continue to grow, cracks initiate and grow from the inside surface of the bend along the shear bands.
The reason why internal cracks become more likely to occur as strength increases is believed to be that the decrease in work hardening ability associated with high strength causes uneven deformation, resulting in the formation of shear deformation bands early in processing (or under loose processing conditions).

The present inventors have studied a method for suppressing internal cracks in high-strength steel sheets. As a result, they have found that the finer the crystal grain size in the surface layer of the hot-rolled steel sheet, the more localized strain concentration is suppressed, and the less likely internal cracks are to occur. More specifically, when the surface layer is the region from the surface to 50 μm in the sheet thickness direction from the surface (the region from the surface to 50 μm in the sheet thickness direction from the surface), they have found that internal cracks in bending can be suppressed by setting ds/dq, which is the ratio of the average crystal grain size ds in the surface layer of the hot-rolled steel sheet to the average crystal grain size dq at the 1/4 depth position, to 0.95 or less. Therefore, when obtaining a hot-rolled steel sheet that has high strength, excellent shear workability, and excellent bendability (suppressing internal cracks in bending during bending), it is preferable to set ds/dq to 0.95 or less. ds/dq is more preferably 0.90 or less, and even more preferably 0.85 or less. The lower limit of ds/dq is not particularly specified, but may be 0.50 or more.

The average grain size at the surface layer and at the 1/4 depth position are measured at the surface layer (the area from the surface to a depth of 50 μm from the surface) and at the 1/4 depth position (the area from 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface) of the hot-rolled steel plate in the thickness cross section parallel to the rolling direction of the hot-rolled steel plate, by analyzing using EBSD at a magnification of 1200 times, with an area of 40 μm x 30 μm as one field of view, at least five fields of view. Regarding the measurement, the area where the angle difference between adjacent measurement points is 15° or more and the circle equivalent diameter is 0.3 μm or more is defined as a grain boundary, and the area average grain size is calculated. The area average grain size obtained at each measurement position is the average grain size of the surface layer and the average grain size at the 1/4 depth position.

<Mechanical properties>
(Tensile strength: 980 MPa or more)
The hot-rolled steel sheet according to this embodiment has a tensile (maximum) strength of 980 MPa or more. If the tensile strength is less than 980 MPa, the applicable parts are limited, and the contribution to reducing the weight of the vehicle body is small. The tensile strength is preferably 1000 MPa or more, more preferably 1080 MPa or more, and even more preferably 1180 MPa or more. There is no need to particularly limit the upper limit, but from the viewpoint of suppressing die wear, the tensile strength may be 1780 MPa or less.

The tensile strength of hot-rolled steel sheets is evaluated in accordance with JIS Z 2241:2011. The test specimens are JIS Z 2241:2011 No. 5 test specimens, and the test direction is perpendicular to the rolling direction.

The thickness of the hot-rolled steel sheet according to this embodiment is not particularly limited, but may be 1.2 to 10.0 mm. If the thickness of the hot-rolled steel sheet is less than 1.2 mm, it may be difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the hot-rolled steel sheet according to this embodiment may be 1.2 mm or more. More preferably, it is 1.4 mm or more. On the other hand, if the thickness exceeds 10.0 mm, it may be difficult to refine the metal structure, and it may be difficult to obtain the above-mentioned metal structure. Therefore, the thickness may be 10.0 mm or less. More preferably, it is 8.0 mm or less. Even more preferably, it is 6.0 mm or less.

<Manufacturing conditions>
The manufacturing method of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but the hot-rolled steel sheet can be obtained by a manufacturing method including the following steps.
(I) A heating step in which a slab or piece of steel having a predetermined chemical composition is heated.
(II) A hot rolling step in which the slab or the steel billet after the heating step is subjected to multi-pass hot rolling using a plurality of rolling stands to obtain a hot-rolled steel sheet.
(III) A winding step of winding the hot-rolled steel sheet.
Preferred conditions for each step will be described below.

[Heating process]
(Heating temperature: over 1300°C and SRT (°C) or higher)
The heating temperature of the slab or steel slab to be subjected to hot rolling is set to be more than 1300°C and equal to or higher than the temperature SRT (°C) represented by the following formula (1). In normal hot rolling, it is considered undesirable to heat to a temperature of more than 1300°C because of the yield reduction caused by the mass loss of iron due to oxides in the heating furnace and the generation of surface defects due to melting of scale. However, in order to obtain the coarse Ti-based carbides with the desired number density possessed by the hot-rolled steel sheet according to this embodiment, it is necessary to sufficiently solutionize the Ti-based carbides in the heating process. Therefore, the temperature of the slab or steel slab to be subjected to hot rolling is heated to be more than 1300°C and equal to or higher than SRT (°C). Here, "the temperature of the slab or steel piece is higher than 1300°C and equal to or higher than SRT (°C)" means that the temperature of the slab or steel piece is higher than the higher of 1300°C and SRT (°C), or when SRT (°C) is higher than 1300°C, the temperature of the SRT and the temperature of the slab or steel piece are the same.
On the other hand, if the heating temperature exceeds 1400° C., thick scale may be generated, resulting in a decrease in yield and significant damage to the heating furnace. Therefore, the heating temperature is preferably 1400° C. or lower.
SRT (°C) = 1630 + 90 × ln ([C] × [Ti])…(1)
In the above formula (1), the [element symbol] indicates the content of each element in the slab or steel piece in mass%, and ln is the natural logarithm.
The slab or billet to be heated may be one obtained by continuous casting or casting and blooming, or may be one that has been subjected to hot or cold working. The chemical composition of the slab or billet does not change substantially during the manufacturing process, so it may be the same as the chemical composition of the hot-rolled steel sheet to be obtained.

[Hot rolling process]
In the hot rolling process, a heated slab or steel billet is subjected to multi-pass hot rolling using multiple rolling stands to produce a hot-rolled steel sheet. Multi-pass hot rolling can be performed using a reverse mill or a tandem mill, but from the viewpoint of industrial productivity, it is preferable to use a tandem mill for at least the final several stages.

(Total reduction rate in the temperature range of 1100°C or higher and SRT (°C) or lower: 70% or more)
In the manufacturing method of the hot-rolled steel sheet according to the present embodiment, the total reduction in the hot rolling in the temperature range of 1100°C or more and SRT (°C) or less is increased to refine the recrystallized austenite and to cause processing-induced precipitation of coarse Ti-based carbides having a major axis of 15 nm or more in a short time during the limited time during rolling. If the total reduction in this temperature range is low, it becomes difficult to obtain a fine structure or desired Ti-based carbides. Specifically, in order to refine the recrystallized austenite and obtain the desired Ti-based carbides, the total reduction in the temperature range of 1100°C or more and SRT (°C) or less is set to 70% or more. If the total reduction in the above temperature range is less than 70%, the desired coarse Ti-based carbides cannot be obtained. The total reduction is preferably 75% or more, more preferably 80% or more. The higher the total reduction in the temperature range of 1100°C or more and SRT (°C) or less, the more preferable.

(Total rolling reduction in the temperature range above the hot rolling completion temperature FT (°C) below 1100°C: 80% or more)
In the manufacturing method of the hot-rolled steel sheet according to this embodiment, the rolling reduction in the temperature range of 1100°C or higher is controlled as described above, and then the total rolling reduction in the temperature range of less than 1100°C (FT (°C)) or higher is increased, and further cooling after hot rolling is performed under the conditions described below, thereby refining the average crystal grain size.
If the total rolling reduction in the temperature range of FT (°C) or more and less than 1100°C is less than 80%, the average crystal grain size after transformation becomes coarse. Therefore, the total rolling reduction in the temperature range of FT (°C) or more and less than 1100°C is set to 80% or more. The total rolling reduction is preferably 85% or more, more preferably 90% or more. The higher the total rolling reduction in the temperature range of FT (°C) or more and less than 1100°C, the better, but since the industrial limit is about 99%, it may be 99% or less.

In order to refine the average crystal grain size, it is important to repeat processing and recrystallization to refine the austenite structure, whether rolling in the temperature range of 1100°C or higher and SRT (°C) or lower, or rolling in the temperature range of less than 1100°C and FT°C or higher. For this reason, rolling is performed two or more times in each temperature range.

In the hot rolling process, the total reduction rate in each temperature range is the percentage of the total reduction in this temperature range (the difference between the entrance thickness before the first pass in rolling in this temperature range and the exit thickness after the final pass in rolling in this temperature range) based on the entrance thickness before the first pass in a given temperature range.

(Hot rolling completion temperature FT (°C): Ar3 (°C) or higher calculated by the following formula (2))
If FT is less than Ar3 (°C), ferrite transformation proceeds during finish rolling, and coarse ferrite grains are generated in part or most of the steel, resulting in poor shear workability. Therefore, FT is set to Ar3 (°C) or more.
On the other hand, if the FT exceeds 1,050° C., the structure becomes coarse, and the shear workability decreases. Therefore, the FT is set to 1,050° C. or less. The FT is preferably 1,030° C. or less, and more preferably 1,010° C. or less.
The temperature during hot rolling refers to the surface temperature of the steel material, and can be measured using a radiation thermometer or the like.
Ar3(°C)=901-325×[C]+33×[Si]-92×[Mn]+287×[P]+40×[sol. Al] (2)
In the above formula (2), the [element symbol] indicates the content of each element in mass %, and 0 is substituted when the element is not contained.

(After completion of hot rolling, average cooling rate to 600°C or less: 50°C/sec or more)
(Preferably, after completion of hot rolling, the temperature is cooled to a temperature range of FT-50°C or less within 1.0 second.)
After completion of the hot rolling, accelerated cooling is performed to a temperature range of 600° C. or lower at an average cooling rate of 50° C./sec or higher in order to suppress the formation of ferrite and pearlite.
The average cooling rate referred to here means a 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.
Although there is no particular upper limit to the average cooling rate, a high cooling rate requires large-scale cooling equipment, which increases the equipment costs. Therefore, in consideration of the equipment costs, a cooling rate of 300° C./sec or less is preferable.
In addition, in the cooling after hot rolling, in order to suppress the growth of austenite grains refined by hot rolling, it is more preferable to cool to 50°C or more within 1.0 second after the completion of hot rolling (the temperature drop is set to 50°C or more). In order to cool to a temperature range of the hot rolling completion temperature FT-50°C or less within 1.0 second after the completion of hot rolling, cooling is performed at a high average cooling rate immediately after the completion of hot rolling. For example, cooling water may be sprayed onto the steel sheet surface. By cooling to a temperature range of FT-50°C or less within 1.0 second after the completion of hot rolling, the crystal grain size of the surface layer can be refined and the resistance to internal bending cracks can be improved (the occurrence of internal bending cracks during bending can be suppressed).

(Dwell time in the temperature range of 600 to 750°C: 5.0 seconds or less)
In order to suppress the generation of ferrite and pearlite and harden the matrix structure to obtain a tensile strength of 980 MPa or more, it is preferable that the residence time in the ferrite transformation temperature range of 600 to 750°C is 5.0 seconds or less. The residence time in the temperature range of 600 to 750°C is more preferably 2.0 seconds or less.

[Winding process]
(Winding temperature: less than 600°C)
The hot-rolled steel sheet after cooling under the above conditions is coiled. The coiling temperature (almost equal to the cooling stop temperature) is set to a temperature range of less than 600°C. By setting the coiling temperature in this temperature range, a bainite or martensite structure is obtained. In addition, by suppressing grain growth after coiling, a high-strength and fine structure can be obtained, and as a result, excellent shear workability can be obtained.

In the manufacturing method of the hot-rolled steel sheet according to this embodiment, the structure and carbide precipitation state are controlled in the processes up to the coiling process. Therefore, it is preferable not to carry out any processes that affect the structure or carbide state after the coiling process.

Next, the effect of one aspect of the present invention will be explained in more detail using an example. However, the conditions in the example are merely an example of conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to this example of conditions. The present invention may adopt various conditions as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

Steel having the chemical composition shown in Table 1 was melted and slabs with a thickness of 240 to 300 mm were produced by continuous casting. The resulting slabs were used to obtain the hot-rolled steel sheets shown in Tables 3A and 3B under the manufacturing conditions shown in Tables 2A and 2B. Two or more passes of reduction were performed in both rolling in the temperature range of 1100°C or higher and SRT (°C) or lower, and rolling in the temperature range of less than 1100°C and FT°C or higher.

For the obtained hot-rolled steel sheet, the area fraction of the metal structure at the 1/4 depth position, the average number density of Ti-based carbides with a major axis of 15 nm or more, the average grain size, and the ratio ds/dq of the average grain size dq at the 1/4 depth position from the surface to the average grain size ds of the surface layer were determined by the above-mentioned method. The obtained measurement results are shown in Tables 3A and 3B.

In addition, the tensile strength TS, shear workability, and resistance to internal bending cracking of the obtained hot-rolled steel sheets were evaluated as follows.

[Tensile properties]
The tensile strength of the hot-rolled steel sheet was evaluated in accordance with JIS Z 2241: 2011. The test specimen was a No. 5 test specimen of JIS Z 2241: 2011, and the test direction was perpendicular to the rolling direction.
When the tensile strength TS was 980 MPa or more, the hot-rolled steel sheet was judged to be high-strength and passed the test, whereas when the tensile strength TS was less than 980 MPa, the hot-rolled steel sheet was judged to be inferior in strength and failed the test.

[Shear processability]
The shear workability of the hot-rolled steel sheets was evaluated by determining the surface roughness Rz (μm) of the fracture surface at the end face after punching in a punching test.
A punched hole with a diameter of 10 mm and a clearance of 20% was made at a punching speed of 3 m/s. Next, using a laser microscope, the surface roughness Rz (μm) of the fracture surface of the end face at four locations in total in the rolling direction and the direction perpendicular to the rolling direction of the punched hole was measured, and the maximum value among the measured values was used for evaluation.
When Rz was 30.0 μm or less, the hot-rolled steel sheet was judged to have excellent shear workability. The fracture surface is a punched end surface separated by a crack generated near the cutting edge after the completion of shear deformation, as shown in FIG.

[Resistance to internal cracking during bending]
The resistance to internal cracking due to bending was evaluated by the following bending test.
A test piece having a rectangular shape of 100 mm x 30 mm was cut out from the hot-rolled steel sheet to obtain a bending test piece. For both bending (L-axis bending) in which the bending ridgeline is parallel to the rolling direction (L direction) and bending (C-axis bending) in which the bending ridgeline is parallel to the direction perpendicular to the rolling direction (C direction), the bending internal cracking resistance was investigated in accordance with JIS Z 2248:2014 (V-block 90° bending test), and the minimum bending radius at which no cracks were generated was obtained. The value obtained by dividing the average value of the minimum bending radius of the L axis and the C axis by the plate thickness was taken as the limit bending R/t, which was used as an index value of bending internal cracking resistance. When R/t was 2.5 or less, it was determined that the hot-rolled steel sheet had excellent bending internal cracking resistance.
However, to determine the presence or absence of cracks, the test piece after the V-block 90° bending test was cut along a plane parallel to the bending direction and perpendicular to the plate surface, and the cross section was mirror-polished to a polished finish, after which the cracks were observed under an optical microscope. If the length of the crack observed on the inside of the bent test piece exceeded 30 μm, it was determined that a crack was present.
The results are shown in Tables 3A and 3B.

As can be seen from Tables 1 to 3B, the hot-rolled steel sheets according to the present invention (sample numbers 1 to 5, 11 to 27) have excellent strength and shear workability. In addition, among the examples of the present invention, the hot-rolled steel sheets having a ds/dq ratio of 0.95 or less have excellent resistance to internal bending cracking in addition to the above-mentioned properties.
On the other hand, it is found that the hot-rolled steel sheets according to the comparative examples do not have either one or more of excellent strength and shear workability.

In the comparative example, sample No. 6, the heating temperature of the slab was low. Therefore, the Ti-based carbides were not sufficiently dissolved during heating, and the average number density of Ti-based carbides with a major axis of 15 nm or more was low. As a result, the roughness of the fracture surface was high (shear workability was low).
In the comparative example, sample No. 7, the total rolling reduction was low in the temperature range of 1100°C or more and SRT (°C) or less. Therefore, the average number density of Ti-based carbides having a major axis of 15 nm or more was low. As a result, the roughness of the fracture surface became large.
In the comparative example, sample No. 8, the total rolling reduction in the temperature range of FT (°C) or higher was low and the average grain size was large. As a result, the roughness of the fracture surface was large.
In the comparative example, sample No. 9, after completion of hot rolling, the average cooling rate to 600°C or less was small, and the residence time at 600 to 750°C was long. Therefore, the area fraction of ferrite was high, and the average grain size was large. As a result, the tensile strength was low, and the roughness of the fracture surface was large. In addition, the ds/dq was high, and the resistance to internal bending cracking was also low.
In the comparative example, sample No. 10, the coiling temperature was high. Therefore, the surface area fraction of ferrite was high. As a result, the tensile strength was low and the roughness of the fracture surface was large.
Sample No. 28, which is a comparative example, had a low C content. As a result, the tensile strength was low.
The comparative sample, sample No. 29, had a high Si content, which resulted in a large area of retained austenite and roughness of the fracture surface.
The comparative sample No. 30 had a low Ti content. Therefore, the average number density of Ti-based carbides having a major axis of 15 nm or more was low. As a result, the roughness of the fracture surface was large.
The comparative example, sample No. 31, had a low Mn content, and as a result, had a low tensile strength.

According to the present invention, it is possible to obtain a hot-rolled steel sheet having high strength and excellent shear workability. The hot-rolled steel sheet of the present invention is suitable as an industrial material used for automobile parts, machine structural parts, and even building parts, and has high industrial applicability.

Claims (3)

In mass percent,
C: 0.050-0.200%,
Si: 0.005-2.000%,
Mn: 0.50-4.00%,
P: 0.100% or less,
S: 0.0100% or less,
sol. Al: 0.001 to 1.00%,
Ti: 0.150-0.400%,
N: 0.0010-0.0200%,
Nb: 0 to 0.200%,
V: 0-1.000%,
Mo: 0-1.000%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Cr: 0-2.00%,
W: 0-1.00%,
B: 0 to 0.0040%,
Ca: 0-0.0100%,
Mg: 0 to 0.0100%,
REM: 0-0.0100%,
Bi: 0 to 0.0200%,
The balance has a chemical composition consisting of Fe and impurities,
When a region from the surface to 1/8 to 3/8 of the plate thickness in the plate thickness direction is defined as a 1/4 depth position, the metal structure at the 1/4 depth position has an area fraction of:
Retained austenite: less than 3.0%
Ferrite: less than 30.0%
Perlite: less than 5.0%
The remaining structure is composed of one or more of bainite, martensite, and tempered martensite,
At the 1/4 depth position,
The average number density of Ti-based carbides having a major axis of 15 nm or more is 1.0 × 10 ⁴ / mm ² or more,
Average crystal grain size dq: 15.0 μm or less,
A hot-rolled steel sheet having a tensile strength of 980 MPa or more. When the area from the surface to 50 μm in the plate thickness direction from the surface is defined as the surface layer,
The hot-rolled steel sheet according to claim 1, characterized in that a ratio ds/dq of an average grain size ds of the surface layer portion to the average grain size dq at the 1/4 depth position is 0.95 or less. The chemical composition, in mass%,
Nb: 0.001-0.200%,
V: 0.005-1.000%,
Mo: 0.001 to 1.000%,
Cu: 0.02-1.00%,
Ni: 0.02-1.00%,
Cr: 0.02-2.00%,
W: 0.020-1.00%,
B: 0.0001 to 0.0040%,
Ca: 0.0002-0.0100%,
Mg: 0.0002 to 0.0100%,
REM: 0.0002-0.0100%,
Bi: 0.0002-0.0200%,
The hot-rolled steel sheet according to claim 1 or 2, further comprising one or more selected from the group consisting of:

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