KR20240038998A - hot rolled steel plate - Google Patents

Abstract

This hot-rolled steel sheet has a predetermined chemical composition, and when an area of 1/8 to 3/8 of the sheet thickness in the direction from the surface to the sheet thickness is set to the 1/4 depth position, the metal structure at the 1/4 depth position is: In terms of area fraction, it contains retained austenite: less than 3.0%, ferrite: less than 30.0%, pearlite: less than 5.0%, and at the 1/4 depth position, the average number density of Ti-based carbides with a major axis of 15 nm or more is 1.0. ×10 ⁴ pieces/mm2 or more, the average crystal grain size dq: 15.0 μm or less, and the tensile strength is 980 MPa or more.

Description

hot rolled steel plate

The present invention relates to hot rolled steel sheets. Specifically, it relates to hot-rolled steel sheets that are used by forming them into various shapes through press processing, etc., and in particular, hot-rolled steel sheets that have high strength and excellent shearability.

This application claims priority based on Japanese Patent Application No. 2021-146231, filed in Japan on September 8, 2021, and uses the content here.

In recent years, from the viewpoint of protecting the global environment, efforts have been made to reduce carbon dioxide emissions in many fields. Automobile manufacturers are also actively developing technology to reduce vehicle body weight for the purpose of reducing fuel consumption. However, because emphasis is placed on improving crash resistance characteristics to ensure the safety of occupants, it is not easy to reduce the weight of the car body.

In order to achieve both vehicle body weight reduction and crash resistance characteristics, thinning of members using high-strength steel plates is being considered. For this reason, there is a strong demand for steel plates that have both high strength and excellent formability, and several technologies have conventionally been proposed to meet these demands. Automobile members are formed by press molding, but blank plates for the press molding are often manufactured by shear processing, which is highly productive. In a blank plate manufactured by shear processing, the end surface accuracy after shear processing needs to be excellent. For example, as the surface roughness of the fractured surface after shearing increases, fatigue properties and bendability deteriorate.

Regarding shear workability, for example, Patent Document 1 discloses a technique for controlling the burr height after punching by controlling the ratio ds/db between the surface ferrite grain size ds and the internal ferrite crystal grain db to 0.95 or less.

Patent Document 2 discloses a technique for improving peeling and warping of the plate end surface by reducing the P content.

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

However, Patent Document 1 targets IF steel, and it may be difficult to apply it to high-strength members of 980 MPa or more. Even in Patent Document 2, a strength of 980 MPa or more was not obtained, and the fracture surface roughness of the shear end surface after shearing processing was not studied.

The present invention was made in view of the above-described problems of the prior art, and its purpose is to provide a hot-rolled steel sheet that has high strength and excellent shearability.

In the present invention, having excellent shear workability means that the surface roughness Rz of the fractured surface at the shear end surface after shearing is 30.0 μm or less. Additionally, having high strength indicates that the tensile strength is 980 MPa or more.

In consideration of the above-mentioned problems, the present inventors conducted intensive research on the relationship between the chemical composition and metal structure of hot rolled steel sheets and mechanical properties, and as a result obtained the following findings (a) to (d) and completed the present invention. did.

(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 high work hardening ability, and pearlite, which causes the formation of coarse voids.

(b) In order to reduce the surface roughness of the fracture surface at the shear end surface, it is important to have a metal structure in which coarse Ti-based carbides are present at a certain level or more and the crystal grain size is small. The reason is believed to be that the coarse Ti-based carbides are carbides precipitated within the austenite structure during hot rolling. This coarse Ti-based carbide is inconsistent with the parent phase after transformation at low temperature. Therefore, it is assumed that by dispersing and precipitating this coarse and non-conforming Ti-based carbide, the voids generated during shear deformation are dispersed, and the surface roughness of the fractured surface is reduced along with the refinement of the crystal grains.

(c) In order to ensure that non-conforming and coarse Ti-based carbides exist above a certain level, a hot rolling process in a high temperature range is important. For example, it is effective to perform hot rolling with a total reduction ratio of 70% or more in a 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 ratio in the above-mentioned high temperature range, it is also important to increase the reduction ratio in hot rolling in the low temperature range. For example, it is effective to perform hot rolling with a total reduction ratio of 80% or more in a temperature range from less than 1100°C to the end of rolling. The pinning effect of the coarse Ti-based carbides precipitated in the high temperature range promotes refinement of the austenite structure, and in combination with subsequent cooling conditions, a structure with a small crystal grain size can be created.

The gist of the present invention made based on the above knowledge is as follows.

[1] The hot rolled steel sheet according to one aspect 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 to 1.00%, Ti: 0.150 to 0.400%, N: 0.0010 to 0.0200%, Nb: 0 to 0.200%, V: 0 to 1.000%, Mo: 0 to 1.000%, Cu: 0 to 1.00 %, Ni: 0 to 1.00%, Cr: 0 to 2.00%, W: 0 to 1.00%, B: 0 to 0.0040%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, REM: 0 to 0.0100% , Bi: 0 to 0.0200%, balance: Fe and impurities, and when the area of 1/8 to 3/8 of the sheet thickness from the surface in the sheet thickness direction is set to 1/4 depth, the 1/ The metal structure at the 4-depth position includes, as an area fraction, retained austenite: less than 3.0%, ferrite: less than 30.0%, and pearlite: less than 5.0%, and at the 1/4 depth position, Ti has a major axis of 15 nm or more. The average number density of the system carbide is 1.0 × 10 ⁴ pieces/mm2 or more, the average crystal grain size dq: 15.0 μm or less, and the tensile strength is 980 MPa or more.

[2] The hot-rolled steel sheet described in [1] has the average crystal grain size ds of the surface layer portion and the average of the 1/4 depth position when the surface layer or the area of 50 μm in the sheet thickness direction from the surface is taken as the surface layer portion. The ratio of crystal grain sizes dq, ds/dq, may be 0.95 or less.

[3] The hot rolled steel sheet according to [1] or [2] has the above chemical composition, in mass%, of Nb: 0.001 to 0.200%, V: 0.005 to 1.000%, Mo: 0.001 to 1.000%, and Cu: 0.02 to 0.02%. 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%. You may contain one or two or more types selected from the group consisting of.

According to the above aspect of the present invention, a hot rolled steel sheet having high strength and excellent shearability 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 members, machine structural members, and even building members.

1 is a diagram for explaining the shear end surface after shear processing.

The hot-rolled steel sheet according to one embodiment of the present invention (hot-rolled steel sheet according to the present embodiment) has a predetermined chemical composition, and is formed in a region of 1/8 to 3/8 of the sheet thickness from the surface in the sheet thickness direction to a depth of 1/4. When taken as a position, the metal structure at the 1/4 depth position includes, as an area fraction, retained austenite: less than 3.0%, ferrite: less than 30.0%, pearlite: less than 5.0%, and is located at the 1/4 depth position. In this case, the average number density of Ti-based carbides with a major diameter 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 (hereinafter sometimes simply referred to as steel sheet) according to the present embodiment will be described in more detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various changes are possible without departing from the spirit of the present invention. The numerical limitation range described below with "to" in between includes the lower limit and the upper limit. Numerical values indicated as “less than” or “exceeding” are not included in the numerical range.

[Chemical composition]

First, the chemical composition of the hot rolled steel sheet according to this embodiment will be described. In the following description, % regarding the chemical composition of the steel sheet refers to mass % unless otherwise specified.

(C: 0.050 to 0.200%)

C is an element that increases the strength of ferrite by increasing the hard phase fraction and combining with precipitation strengthening elements such as Ti, Nb, and V. If the C content is less than 0.050%, it becomes 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 decreases. Therefore, the C content is set to 0.200% or less. The C content is preferably 0.150% or less.

(Si: 0.005 to 2.000%)

Si is an element that has the effect of increasing the strength of a hot rolled steel sheet by solid solution strengthening. In addition, Si has the effect of making steel sound by deoxidation (suppressing the occurrence of defects such as blowholes in steel). If the Si content is less than 0.005%, the effect due to the above action 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 has the effect of deteriorating the surface properties and chemical treatment properties of hot-rolled steel sheets and suppressing precipitation of cementite from austenite, thus promoting the formation of retained austenite. If the Si content is more than 2.000%, retained austenite is generated and the surface roughness of the fractured surface deteriorates. Therefore, the Si content is set to 2.000% or less. The Si content is preferably 1.500% or less, more preferably 1.300% or less.

(Mn: 0.50 to 4.00%)

Mn is an element that suppresses ferrite transformation and increases the strength of hot rolled steel sheets. 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. Additionally, when the area fraction of ferrite is reduced, the Mn content is more preferably 1.40% or more, and even more preferably 1.50% or more.

On the other hand, if the Mn content is more than 4.00%, cracks occur near the center of the sheet thickness due to central segregation of Mn, and the surface roughness of the shear 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, more preferably 3.00% or less.

(P: 0.100% or less)

P is an element generally contained as an impurity. P is an element that is prone to segregation, and when the P content exceeds 0.100%, 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.

There is no need to specifically specify the lower limit of the P content, but from the viewpoint of refining costs, it is preferable that the P content is 0.001% or more. In addition, P is also an element that has the effect of increasing the strength of the hot rolled steel sheet through solid solution strengthening. Therefore, P may be actively included. In that case, the P content may be 0.002% or more.

(S: 0.0100% or less)

S is an element contained as an impurity, and is an element that forms sulfide-based inclusions in steel and reduces the bending workability of hot-rolled steel sheets. When the S content exceeds 0.0100%, the bending workability of the hot rolled steel sheet significantly deteriorates. 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 specifically specify the lower limit of the S content, but from the viewpoint of refining costs, it is preferable that the S content is 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 making it sound. If the sol.Al content is less than 0.001%, the effect of the above action 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 is more than 1.00%, the above effect is 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, and refers to Al that exists in steel in a solid solution state.

(Ti: 0.150 to 0.400%)

Ti is an element that reduces the surface roughness of the fractured surface at the shear end surface by precipitating as coarse Ti-based carbides in the high temperature range of hot rolling. Additionally, it is an element that suppresses the recovery, recrystallization, and grain growth of the austenite structure and refines the metal structure after transformation. In addition, 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 steel through precipitation strengthening. If the Ti content is less than 0.150%, the driving force for precipitation of Ti-based carbides in the high temperature range 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 Ti exceeds 0.400%, coarse nitrides with a long diameter of several micrometers in a rectangular shape are formed, and bending workability deteriorates. 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.

Ti-based carbide refers to carbide having a NaCl-type crystal structure containing Ti. If this carbide contains Ti, it also includes 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. Additionally, carbonitrides in which part of carbon is replaced with nitrogen are also included.

(N: 0.0010 to 0.0200%)

N is an element that forms nitrides and carbonitrides such as Ti, Nb, and V, and has the effect of suppressing 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 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 is more than 0.0200%, coarse Ti nitride is formed and bending workability deteriorates. 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 remainder of the chemical composition of the hot rolled steel sheet according to this embodiment may be Fe and impurities. In this embodiment, impurities mean those that are mixed from ore as raw materials, scrap, or the manufacturing environment, and/or that are allowed in the range that do not adversely affect the hot rolled steel sheet according to this embodiment.

On the other hand, the hot rolled steel sheet according to the present embodiment contains one or two types of Nb, V, Mo, Cu, Ni, Cr, W, B, Ca, Mg, REM, and Bi as arbitrary elements instead of part of Fe. It may contain more than one species. Since it is not essential to contain the above arbitrary elements, the lower limit of the content is 0%. Hereinafter, the above arbitrary elements will be described in detail.

(Nb: 0 to 0.200%)

Nb is an arbitrary element. Nb is an element that precipitates in steel as carbide, nitride, carbonitride, etc. and has the effect of increasing the tensile strength of the steel sheet. In order to obtain these effects, it is preferable that the Nb content is 0.001% or more. The Nb content is more preferably 0.005% or more.

On the other hand, when the Nb content exceeds 0.200%, the above effect is saturated and the rolling load during finish rolling increases, making rolling difficult in some cases. Therefore, when containing Nb, 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 to 1.000%)

V is a random element. V is an element that precipitates in steel as carbide, nitride, or carbonitride and has the effect of improving the tensile strength of the steel sheet. In order to obtain these effects, it is preferable that the V content is 0.005% or more. The V content is more preferably 0.010% or more.

On the other hand, when the V content exceeds 1.000%, the workability of the hot rolled steel sheet decreases. Therefore, when it contains V, 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 to 1.000%)

Mo is an arbitrary element. Mo is an element that has the effect of increasing the hardenability of steel and increasing the strength of steel sheets by forming carbides and carbonitrides. In order to obtain these effects, it is preferable that the Mo content is 0.001% or more. Mo content is more preferably 0.005% or more.

On the other hand, when the Mo content exceeds 1.000%, the cracking susceptibility 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 to 1.00%)

Cu is an arbitrary element. Cu is an element that has the effect of improving the toughness of steel and increasing the tensile strength. In order to obtain these effects, it is preferable that the Cu content is 0.02% or more. The Cu content is more preferably 0.08% or more.

On the other hand, if Cu is contained excessively, the weldability of the steel sheet may deteriorate. Therefore, when it contains Cu, 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 to 1.00%)

Ni is an arbitrary element. Ni is an element that has the effect of improving the toughness of steel and increasing the tensile strength. In order to obtain these effects, it is preferable that the Ni content is 0.02% or more. The Ni content is more preferably 0.10% or more.

On the other hand, if Ni is contained excessively, the alloy cost increases and the toughness of the welded heat-affected zone of the steel sheet may deteriorate. Therefore, when containing Ni, 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 to 2.00%)

Cr is an arbitrary element. Cr is an element that has the effect of increasing the hardenability of steel and increasing the strength of steel sheets by forming carbides and carbonitrides. In order to obtain this effect, it is preferable that the Cr content is 0.02% or more. Cr content is more preferably 0.05% or more.

On the other hand, if Cr is contained excessively, chemical treatment properties deteriorate. 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, further preferably 1.00% or less, and particularly preferably 0.50% or less.

(W: 0 to 1.00%)

W is a random element. W is an element that has the effect of increasing tensile strength by forming carbides or carbonitrides. In order to obtain this effect, it is preferable that the W content is 0.02% or more.

On the other hand, even if W is contained above a certain level, the effect of the above action 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 to 0.0040%)

B is a random element. B is an element that has the effect of increasing the tensile strength of the steel sheet through grain boundary strengthening or solid solution strengthening. In order to obtain this effect, it is preferable that the B content is 0.0001% or more. The B content is more preferably 0.0002% or more.

On the other hand, when the B content exceeds 0.0040%, the above effect is 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 to 0.0100%)

Ca is an arbitrary element. Ca is an element that has the effect of dispersing many fine oxides in molten steel and refining the metal structure of the steel sheet. In addition, Ca is an element that has the effect of improving the stretch flangeability of the steel sheet by fixing S in the molten steel as spherical CaS and suppressing the generation of stretching inclusions such as MnS. In order to obtain these effects, it is preferable that the Ca content is 0.0002% or more. The Ca content is more preferably 0.0005% or more.

On the other hand, when the Ca content exceeds 0.0100%, the amount of CaO in the steel increases, which may adversely affect the toughness of the steel sheet. Therefore, when it contains Ca, 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 to 0.0100%)

Mg is an arbitrary element. Mg, like Ca, is an element that forms oxides and sulfides in molten steel, suppresses the formation of coarse MnS, and has the effect of refining the metal structure of the steel sheet by dispersing many fine oxides. In order to obtain these effects, it is preferable that the Mg content is 0.0002% or more. The Mg content is more preferably 0.0005% or more.

On the other hand, when the Mg content exceeds 0.0100%, oxides in the steel increase, adversely affecting the toughness of the steel sheet. 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 to 0.0100%)

REM is a random element. Like Ca, REM is an element that forms oxides and sulfides in molten steel, suppresses the formation of coarse MnS, and has the effect of refining the metal structure of the steel sheet by dispersing many fine oxides. In order to obtain these effects, it is preferable that the REM content is 0.0002% or more. The REM content is more preferably 0.0005% or more.

On the other hand, when the REM content exceeds 0.0100%, oxides in the steel increase, which may adversely affect the toughness of the steel sheet. Therefore, when containing REM, it is preferable that the REM content is 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 elements) refers to a total of 17 elements consisting of Sc, Y, and lanthanoid. In this embodiment, the REM content refers to the total content of these elements.

(Bi: 0 to 0.0200%)

Bi is an arbitrary 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, when the Bi content exceeds 0.0200%, the above effect is saturated and the alloy cost increases. Therefore, when it contains Bi, 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-described hot-rolled steel sheet may be measured by a general analysis method. For example, measurement can be done using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). sol.Al can be measured by ICP-AES using the filtrate after heating and decomposing the sample with acid. C and S can be measured using the combustion-infrared absorption method, and N can 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 area 1/8 to 3/8 of the sheet thickness from the surface is set as the 1/4 depth position, the metal structure at the 1/4 depth position is 30.0% as an area fraction. Contains less than ferrite, less than 3.0% retained austenite, and less than 5.0% pearlite. In addition, the hot rolled steel sheet according to the present embodiment has an average number density of Ti-based carbides having an average crystal grain size of 15.0 μm or less and a major diameter of 15 nm or more in the metal structure at this 1/4 depth position of 1.0 × 10 ⁴ pieces. /mm2 or more (10000 pieces/mm2 or more) (with an average number density of 1.0×10 ⁴ pieces/mm2 or more, Ti-based carbides of 15 nm or more are precipitated). The reason for defining the metal structure in the area of 1/8 to 3/8 of the sheet thickness from the surface of the steel sheet (1/4 depth position) is that the metal structure at this position represents the typical metal structure of the steel sheet. Because it's a bet.

(Area fraction of ferrite: less than 30.0%)

Ferrite is a structure created when fcc is transformed into bcc at a relatively high temperature. Since ferrite has a high work hardening ability, if the area fraction of ferrite is too high, the amount of deformation of the fracture surface at the shear end surface increases, and the fracture surface roughness increases. Therefore, the area fraction of ferrite is set to 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 lower the area fraction of ferrite, the more preferable it may be 0%. However, considering productivity, etc., 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 retained austenite is too high, the surface roughness of the fractured surface at the shear end surface may increase. This is presumed to be because retained austenite forms coarse voids. In particular, when the area fraction of retained austenite is 3.0% or more, the shear workability of the hot rolled steel sheet deteriorates, and the surface roughness of the fractured surface increases. Therefore, the area fraction of retained austenite is set to less than 3.0%. The area fraction of retained austenite is preferably less than 1.5%, more preferably less than 1.0%. Since the smaller the retained austenite, the more desirable it is, so the area fraction of retained austenite may be 0%.

(Area fraction of pearlite: less than 5.0%)

Pearlite is a lamellar metal structure in which cementite is precipitated in layers between ferrites. Additionally, pearlite is a soft metal structure compared to bainite and martensite. If the area fraction of pearlite is 5.0% or more, carbon is consumed in cementite contained in pearlite, the strength of the remaining structures such as 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%, even more preferably 1.0% or less, or may be 0%.

In the hot-rolled steel sheet according to the present embodiment, in order to secure a tensile strength of 980 MPa or more, the remaining structure other than retained austenite, ferrite, and pearlite is one or two or more of hard structures bainite, martensite, and tempered martensite. It is desirable to include. The area fraction of one or more types 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. The sheet thickness cross section parallel to the rolling direction is finished to a mirror finish and polished for 8 minutes using colloidal silica containing no alkaline solution at room temperature to remove the strain introduced into the surface layer of the sample. At an arbitrary position in the longitudinal direction of the sample cross-section, an area 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 was subjected to electron backscattering diffraction at a measurement interval of 0.1 μm. Obtain crystal orientation information by measuring according to . 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 vacuum level within 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. Additionally, a reflected electron image is photographed in the same field of view. First, the crystal grains in which ferrite and cementite are precipitated in layers are identified from the reflected electron image, and the area fraction of the crystal grains is calculated to obtain the area fraction of pearlite. Afterwards, for the crystal grains excluding those determined to be pearlite, the obtained crystal orientation information was analyzed for Grain Average Misorientation using the “Grain Average Misorientation” function installed in the software “OIM Analysis (registered trademark)” included with the EBSD analysis device. The area with a value of 1.0° or less is judged to be ferrite. By calculating the area fraction of the area determined to be ferrite, the area fraction of ferrite is obtained.

Next, under the condition that the grain boundary is defined as a boundary of 5° or more in the remaining region (the region where the Grain Average Misorientation value is greater than 1.0°), when the maximum value of “Grain Average IQ” in the ferrite region is Iα, Iα/ The region exceeding 2 is extracted as bainite, and the region falling below Iα/2 is extracted as “pearlite, martensite, and tempered martensite.” By calculating the area fraction of the extracted bainite, the area fraction of bainite is obtained. Additionally, the area fraction of the extracted "pearlite, martensite and tempered martensite" is calculated, and the area fraction of pearlite obtained by the EBSD analysis described above is subtracted to obtain the sum of the area fractions of martensite and tempered martensite.

Methods for measuring the area fraction of retained austenite include X-ray diffraction, EBSD (Electron Back Scattering Diffraction Pattern) analysis, and magnetic measurement. Measured values may vary 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 retained austenite area fraction by In this case, using the Co-Kα line, the integrated intensity of a total of 6 peaks of α (110), α (200), α (211), γ (111), γ (200), and γ (220) is obtained, By calculating using the strength average method, the area fraction of retained austenite is obtained.

(The average number density of Ti-based carbides with a major diameter of 15 nm or more is 1.0 × 10 ⁴ pieces/㎟ or more.)

In the hot-rolled steel sheet according to this embodiment, coarse Ti-based carbides with a major axis of 15 nm or more are precipitated. Due to the presence of this coarse Ti-based carbide and the refinement of the average crystal grain size described later, voids at the time of fracture during shear processing are dispersed, and the surface roughness of the fracture surface at the shear end surface is reduced. For this purpose, the average number density of Ti-based carbides with a major diameter of 15 nm or more must be 1.0 × 10 ⁴ (10000) pieces/mm 2 or more. The average number density of Ti-based carbides with a major axis of 15 nm or more is preferably 2.0×10 ⁴ pieces/mm 2 or more, and more preferably 4.0×10 ⁴ pieces/mm 2 or more. The larger the average number density of Ti-based carbides with a long diameter of 15 nm or more, the better, but in some cases, the solid solution C required to strengthen fine Ti-based carbides and bainite or martensite, which precipitate during cooling after completion of hot rolling, decreases, causing a decrease in strength. Therefore, it may be 5.0×10 ⁶ pieces/mm2 or less. Here, Ti-based carbide refers to carbide having a NaCl-type crystal structure containing Ti. If such carbide contains Ti, a small amount of other carbide-forming alloy elements may be contained. Within the range of 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. Additionally, 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 was photographed at a 1/4 depth position using a TEM at a magnification of 50,000 times, with an area of 2.0 ㎛ It is analyzed by X-ray spectroscopy (EDS), the precipitates in which Ti and C are detected are judged to be Ti-based carbides, and the major axis (longest diameter) of each precipitate (Ti-based carbide) is measured. Then, the number of Ti-based carbides with a major axis of 15 nm or more per 1 mm2 is counted, and the number density is determined.

(Average crystal grain size: 15.0㎛ or less)

In the metal structure, when the average crystal grain size is coarse, the roughness of the fractured surface of the shear end surface increases. Therefore, the average crystal grain size (dq) at the 1/4 depth position is set to 15.0 μm or less. The average crystal grain size is preferably 12.0 μm or less, and more preferably 10.0 μm or less. Since the smaller the average crystal grain size is, the lower limit is not particularly limited. However, it is technically difficult to refine the average crystal grain size to less than 1.0 μm by ordinary hot rolling. Therefore, the average crystal grain size may be 1.0 μm or more or 4.0 μm or more.

(Preferably, ds/dq, which is the ratio of the average crystal grain size ds of the surface layer and the average crystal grain size dq of the 1/4 depth position: 0.95 or less.)

As the strength of the hot rolled steel sheet increases, cracks are more likely to occur from the inside of the bend during bending processing (hereinafter referred to as inside bending cracks). In particular, as in the hot rolled steel sheet according to the present embodiment, when the tensile strength is 980 MPa or more, internal bending cracking is likely to occur.

The mechanism of bending inner splitting is estimated as follows. In other words, during bending processing, compressive stress occurs inside the bend. At first, processing proceeds while the entire inside of the bend is deformed uniformly, but as the amount of processing increases, the deformation cannot be sustained with only uniform deformation, and deformation progresses as strain is concentrated in the local area (generation of a shear deformation zone). As this shear deformation zone grows further, cracks along the shear zone form and grow from the inner surface of the bend.

The reason why internal bending cracks become more likely to occur with increased strength is because the deformation progresses unevenly due to the decrease in work hardening ability accompanying high strength, and a shear deformation zone occurs early in processing (or under loose processing conditions). It is estimated that

The present inventors studied a method for suppressing internal bending cracking in high-strength steel sheets. As a result, it was found that the finer the crystal grain size of the surface layer of the hot-rolled steel sheet, the more suppressed local strain concentration was, and the more difficult it was for internal bending cracks to occur. More specifically, when the surface or the area of 50 ㎛ from the surface to the sheet thickness direction (the area from the surface to 50 ㎛ in the sheet thickness direction starting from the surface) is taken as the surface layer part, the average determination of the surface layer part of the hot rolled steel sheet It was found that bending internal cracking was suppressed by setting ds/dq, which is the ratio of the grain size ds and 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 excellent bendability (suppresses inner bending cracking during bending) in addition to high strength and excellent shear workability, 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 specifically specified, but may be 0.50 or more.

The average grain size of each of the surface layer portion and the 1/4 depth position is the surface layer portion (the area at a depth of 50 μm from the surface) and 1/4 of the hot rolled steel sheet in the sheet thickness cross section parallel to the rolling direction of the hot rolled steel sheet. At each depth position (area from 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), at a magnification of 1200 times, an area of 40㎛ Regarding the field of view, analysis is performed using EBSD and measurement is performed. Regarding the measurement, an area where the angle difference between adjacent measurement points is 15° or more and the equivalent circle diameter is 0.3 μm or more is defined as a grain boundary, and the area average crystal grain size is calculated. The area average crystal grain size obtained at each measurement position is taken as the average crystal grain size of the surface layer and the average crystal grain size of the 1/4 depth position.

<Machine characteristics>

(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 specifically limit the upper limit, but from the viewpoint of suppressing mold wear, the tensile strength may be 1780 MPa or less.

The tensile strength of hot rolled steel sheets is evaluated based on JIS Z 2241:2011. The test piece is No. 5 of JIS Z 2241:2011, and the test direction is perpendicular to the rolling direction.

The plate 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 becomes difficult to secure the rolling completion temperature, and the rolling load may become excessive, making hot rolling difficult. Therefore, the plate 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 plate thickness exceeds 10.0 mm, it may become difficult to refine the metal structure, making it difficult to obtain the above-mentioned metal structure. Therefore, the plate 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 this embodiment is not particularly limited, but can be obtained by a manufacturing method including the following steps.

(I) A heating process for heating a slab or steel piece having a predetermined chemical composition.

(II) A hot rolling process of performing multi-pass hot rolling on the slab or steel piece after the heating process using a plurality of rolling stands to obtain a hot rolled steel sheet.

(III) A winding process for winding the hot rolled steel sheet.

Below, preferred conditions for each process will be described.

[Heating process]

(Heating temperature: over 1300℃ and above SRT(℃))

The heating temperature of the slab or steel piece to be subjected to hot rolling is set to exceed 1300°C and be equal to or higher than the temperature SRT (°C) expressed by the following formula (1). In normal hot rolling, it is considered undesirable to heat to a temperature exceeding 1300°C for the reason that yield decreases due to mass loss of iron due to oxides in the heating furnace and surface defects are generated due to melting of scale. However, in order to obtain the coarse Ti-based carbide of the desired number density contained in the hot-rolled steel sheet according to the present embodiment, it is necessary to sufficiently solutionize the Ti-based carbide in the heating process. Therefore, the temperature of the slab or steel piece to be subjected to hot rolling is heated to exceed 1300°C and above SRT (°C). Here, “the temperature of the slab or steel piece exceeds 1300°C and SRT(°C) or higher” means that the temperature of the slab or steel piece is higher than the temperature of 1300°C or SRT(°C), whichever is higher, or SRT(°C). If is greater than 1300℃, it means that the temperature of SRT and slab or steel piece are the same.

On the other hand, if the heating temperature exceeds 1400°C, thick scale may be generated, which may lower the yield or cause significant damage to the heating furnace. Therefore, the heating temperature is preferably 1400°C or lower.

However, [element symbol] in the above formula (1) represents the content in mass% of each element in the slab or steel piece. ln is natural logarithm.

The slab or steel piece to be heated may be obtained by continuous casting, casting, or crush rolling, or may be obtained by applying hot working or cold working to it. Since the chemical composition of the slab or steel piece does not change substantially during the manufacturing process, it can be made equivalent to 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 piece is subjected to multi-pass hot rolling using a plurality of rolling stands to manufacture a hot rolled steel sheet. Multi-pass hot rolling can be performed using a reverse mill or tandem mill, but from the viewpoint of industrial productivity, it is preferable to use a tandem mill at least as a final step.

(Total reduction ratio in the temperature range above 1100℃ and below SRT(℃): 70% or more)

In the method for manufacturing a hot rolled steel sheet according to the present embodiment, by increasing the total reduction ratio of hot rolling in the temperature range of 1100°C or higher and SRT (°C) or lower, refinement of recrystallized austenite is achieved, and a strip having a major axis of 15 nm or more is achieved. The Ti-based carbide is processed and induced to precipitate in a short period of time within a limited period of time during rolling. If the total reduction ratio in this temperature range is low, it becomes difficult to obtain a fine structure or desired Ti-based carbide. Specifically, in order to refine the recrystallized austenite and obtain the desired Ti-based carbide, the total reduction ratio in the temperature range of 1100°C or higher and SRT (°C) or lower is set to 70% or higher. If the total reduction ratio in the above temperature range is less than 70%, the desired coarse Ti-based carbide cannot be obtained. The total reduction ratio is preferably 75% or more, more preferably 80% or more. The higher the total reduction ratio in the temperature range of 1100°C or higher and SRT (°C) or lower, the more preferable.

(Total reduction ratio in the temperature range above hot rolling completion temperature FT(℃) below 1100℃: 80% or more)

In the method for manufacturing a hot-rolled steel sheet according to the present embodiment, after controlling the reduction ratio in the temperature range of 1100°C or higher as described above, the total reduction ratio in the temperature range FT(°C) or higher below 1100°C is increased, and further under the conditions described later. By performing cooling after hot rolling, the average crystal grain size is refined.

If the total reduction ratio in the temperature range of less than FT(°C) below 1100°C is less than 80%, the average crystal grain size after transformation becomes coarse. Therefore, the total reduction ratio in the temperature range below FT(°C) below 1100°C is set to 80% or more. The total reduction ratio is preferably 85% or more, more preferably 90% or more. The higher the total reduction ratio in the temperature range of less than FT(°C) below 1100°C, the more preferable it is, but industrially, about 99% is the limit, so it may be 99% or less.

In order to refine the average crystal grain size, the austenite structure is refined by repeating processing and recrystallization in either rolling at a temperature range of 1100°C or higher and SRT (°C) or lower, or rolling at a temperature range of FT°C or higher below 1100°C. It is important. Therefore, in each temperature range, two or more passes of rolling are performed.

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

(Hot rolling completion temperature FT (°C): not less than Ar3 (°C) obtained by equation (2) below)

If FT is less than Ar3 (°C), ferrite transformation progresses during finish rolling, some or most of the coarse ferrite particles are generated, and shearability deteriorates. Therefore, FT is set to Ar3 (°C) or higher.

On the other hand, even if FT exceeds 1050°C, shearability decreases due to coarsening of the structure. Therefore, FT is set to 1050°C or lower. FT is preferably 1030°C or lower, more preferably 1010°C or lower.

The temperature during hot rolling refers to the surface temperature of the steel material and can be measured with a radiation thermometer or the like.

However, [element symbol] in the above formula (2) indicates the content in mass% of each element, and 0 is substituted when it is not contained.

(After completion of hot rolling, average cooling rate to 600℃ or less: 50℃/sec or more)

(Preferably, after completion of hot rolling, cooling to a temperature range below hot rolling completion temperature FT-50°C within 1.0 seconds)

After hot rolling is completed, accelerated cooling is performed to a temperature range of 600°C or lower at an average cooling rate of 50°C/sec or more to suppress the formation of ferrite and pearlite.

The average cooling rate here refers to the temperature drop range of the steel sheet from the start of accelerated cooling (when the steel sheet is introduced into the cooling facility) to the completion of accelerated cooling (when the steel sheet is taken out of the cooling facility). This refers to the value divided by the time required to complete accelerated cooling.

The upper limit of the average cooling rate is not specifically specified, but in order to increase the cooling rate, the cooling facility becomes large-scale and the facility cost increases. For this reason, considering equipment costs, 300°C/sec or less is preferable.

In addition, in cooling after hot rolling, in order to suppress the growth of austenite grains refined by hot rolling, it is recommended to cool to 50°C or more (with the allowable temperature drop being 50°C or more) within 1.0 seconds after completion of hot rolling. It is more desirable. In order to cool to a temperature range below the hot rolling completion temperature FT-50°C within 1.0 seconds after completion of hot rolling, cooling with a large average cooling rate is performed immediately after completion of hot rolling. For example, you can spray coolant on the surface of the steel plate. By cooling to a temperature range of FT-50°C or lower within 1.0 seconds after completion of hot rolling, the crystal grain size of the surface layer can be refined and the bending inner cracking resistance can be increased (the occurrence of bending inner cracking during bending processing can be suppressed). You can.

(Residence time in the temperature range of 600 to 750°C: 5.0 seconds or less)

In order to suppress the formation of ferrite and pearlite, make the matrix structure hard, and obtain a tensile strength of 980 MPa or more, it is desirable to set the residence time in the temperature range of 600 to 750°C, which is the ferrite transformation temperature range, to 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℃)

The hot rolled steel sheet after cooling under the above conditions is wound. The coiling temperature (almost equivalent to the cooling stop temperature) is set to a temperature range of less than 600°C. By setting the coiling temperature to this temperature range, a bainite or martensite structure is obtained. Additionally, by suppressing grain growth after winding, a high-strength and fine structure can be achieved, and as a result, excellent shear processability can be obtained.

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

Example

Next, the effect of one aspect of the present invention will be explained in more detail through examples. However, the conditions in the examples are examples of conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is It is not limited to one conditional example. The present invention can adopt various conditions as long as it achieves the purpose of the present invention without departing from the gist 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 manufactured by continuous casting. Using the obtained slab, hot rolled steel sheets shown in Tables 3A and 3B were obtained according to the manufacturing conditions shown in Tables 2A and 2B. Two or more passes of rolling were performed in either rolling in a temperature range of 1100°C or higher and SRT (°C) or lower and rolling in a temperature range of 1100°C or higher and FT°C or higher.

By the method described above 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 crystal grain size, and the plate thickness from the surface to 1 /4 ds/dq, which is the ratio of the average crystal grain size dq at the depth position and the average crystal grain size ds at the surface layer, was obtained. The obtained measurement results are shown in Tables 3A and 3B.

In addition, the obtained hot rolled steel sheet was evaluated for tensile strength TS, shear workability, and bending internal cracking resistance in the following manner.

[Tensile properties]

The tensile strength of the hot rolled steel sheet was evaluated based on JIS Z 2241:2011. The test piece was No. 5 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, it was judged to be a hot rolled steel sheet with high strength and passed. On the other hand, when the tensile strength TS was less than 980 MPa, the hot rolled steel sheet was considered to have low strength and was judged to be rejected.

[Shear processability]

The shear workability of the hot rolled steel sheet was evaluated by determining the surface roughness Rz (μm) of the fractured surface at the end surface after punching through a punching test.

A punching hole with a clearance of 20% was produced with a hole diameter of 10 mm and a punching speed of 3 m/s. Next, using a laser microscope, the surface roughness Rz (μm) of the fractured surface of the end surface at four points in the rolling direction of the punched hole and the rolling direction orthogonal to the rolling direction was measured, and the maximum value among them was evaluated.

When Rz was 30.0 μm or less, it was determined that the hot rolled steel sheet had excellent shearability. As shown in FIG. 1, the fracture surface is the punched end surface separated by a crack occurring near the blade tip after the end of shear deformation.

[Internal bending and internal splitting]

The bending resistance and internal cracking resistance were evaluated through the following bending test.

A 100 mm x 30 mm rectangular test piece was cut from a hot rolled steel sheet to obtain a bending test piece. JIS Z 2248 for both bending in which the bending ridge is parallel to the rolling direction (L direction) (L-axis bending) and bending in which the bending ridge is parallel to the direction perpendicular to the rolling direction (C-direction) (C-axis bending) :2014 (V block 90° bending test), internal cracking resistance against bending was investigated, and the minimum bending radius at which cracks did not occur was determined. The average value of the minimum bending radii of the L-axis and the C-axis divided by the plate thickness was used as the limit bending R/t, which was used as an index value for internal cracking resistance. When R/t was 2.5 or less, it was determined that the hot rolled steel sheet had excellent bending and internal cracking resistance.

However, the presence or absence of cracks can be determined by mirror-polishing a cross-section of a test piece after a V-block 90° bending test cut along a plane parallel to the bending direction and perpendicular to the plate surface, and then observing cracks with an optical microscope, and observing them on the inside of the bend of the test piece. When the crack length exceeded 30㎛, it was judged that there was a crack.

The obtained results are shown in Table 3A and Table 3B.

[Table 1]

[Table 2A]

[Table 2B]

[Table 3A]

[Table 3B]

As can be seen from Tables 1 to 3B, the hot rolled steel sheets (Nos. 1 to 5, 11 to 27) according to the examples of the present invention have excellent strength and shear workability. In addition, among the examples of the present invention, it can be seen that the hot rolled steel sheet with ds/dq of 0.95 or less has the above-mentioned various properties and also has excellent bending resistance and internal cracking resistance.

On the other hand, it can be seen that the hot rolled steel sheet according to the comparative example does not have any one or more of excellent strength and shear workability.

In sample 6, which is a comparative example, the heating temperature of the slab was low. Therefore, the Ti-based carbide was not sufficiently solutionized during heating, and the average number density of Ti-based carbide with a major axis of 15 nm or more decreased. As a result, the roughness of the fracture surface became rough (shearability was low).

In sample No. 7, which is a comparative example, the total reduction ratio in the temperature range of 1100°C or higher and SRT (°C) or lower was low. Therefore, the average number density of Ti-based carbides with a major axis of 15 nm or more decreased. As a result, the roughness of the fracture surface became rough.

C#8, which is a comparative example, had a low total reduction ratio in the temperature range below 1100°C and above FT (°C). For this reason, the average crystal grain size increased. As a result, the roughness of the fracture surface became rough.

In sample 9, which is a comparative example, the average cooling rate up to 600°C or lower after completion of hot rolling was low and the residence time between 600 and 750°C was long. Therefore, the area fraction of ferrite increased, and the average grain size also increased. As a result, the tensile strength was low and the roughness of the fractured surface became rough. In addition, ds/dq was high and inner bending resistance was low.

In sample number 10, which is a comparative example, the coiling temperature was high. Therefore, the area fraction of ferrite was high. As a result, the tensile strength was low and the roughness of the fractured surface became rough.

Siburn 28, a comparative example, had a low C content. As a result, the tensile strength was low.

Si number 29, a comparative example, had a high Si content. As a result, the area of retained austenite increased, and the roughness of the fracture surface became rough.

Siburn 30, a comparative example, had a low Ti content. Therefore, the average number density of Ti-based carbides with a major axis of 15 nm or more decreased. As a result, the roughness of the fracture surface became rough.

Siburn 31, a comparative example, had a low Mn content. As a result, the tensile strength was low.

According to the present invention, a hot rolled steel sheet having high strength and excellent shearability can be obtained. The hot rolled steel sheet of the present invention is suitable as an industrial material used for automobile members, machine structural members, and even building members, and has high industrial applicability.

Claims (3)

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 to 1.00%,
Ti: 0.150 to 0.400%,
N: 0.0010 to 0.0200%,
Nb: 0 to 0.200%,
V: 0 to 1.000%,
Mo: 0 to 1.000%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Cr: 0 to 2.00%,
W: 0 to 1.00%,
B: 0 to 0.0040%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
REM: 0 to 0.0100%,
Bi: 0 to 0.0200%,
Residue: Fe and impurities
It has a chemical composition consisting of,
When an area of 1/8 to 3/8 of the sheet thickness in the direction from the surface to the sheet thickness is set as the 1/4 depth position, the metal structure at the 1/4 depth position is expressed as an area fraction,
Retained austenite: less than 3.0%,
Ferrite: less than 30.0%,
Perlite: less than 5.0%, including
At the 1/4 depth position,
The average number density of Ti-based carbides with a major diameter of 15 nm or more is 1.0 × 10 ⁴ pieces/mm2 or more,
Average crystal grain size dq: 15.0 ㎛ or less,
Tensile strength of 980 MPa or more
A hot rolled steel sheet characterized in that. According to paragraph 1,
When the surface or the area of 50㎛ in the plate thickness direction from the surface is taken as the surface layer,
ds/dq, which is the ratio of the average crystal grain size ds of the surface layer and the average crystal grain size dq of the 1/4 depth position, is 0.95 or less.
A hot rolled steel sheet characterized in that. According to claim 1 or 2,
The chemical composition is expressed 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%
Containing one or two or more types selected from the group consisting of
A hot rolled steel sheet characterized in that.

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