Title of invention : Hot-rolled steel sheet
Technical field
[0001]
　The present invention relates to hot-rolled steel sheets. 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 particularly to a hot-rolled steel sheet that is high in strength and excellent in hole expansibility and shear workability.
　This application claims priority based on Japanese Patent Application No. 2020-010945 filed in Japan on January 27, 2020, the content of which is incorporated herein.
Background technology
[0002]
　In recent years, efforts have been made to reduce carbon dioxide emissions in many fields from the viewpoint of protecting the global environment. Automobile manufacturers are actively developing technologies to reduce the weight of automobile bodies for the purpose of reducing fuel consumption. However, it is not easy to reduce the weight of the car body because the emphasis is placed on improving crashworthiness in order to ensure the safety of passengers.
[0003]
　In order to achieve both weight reduction of the vehicle body and collision resistance, thinning of members using high-strength steel sheets has been studied. Therefore, there is a strong demand for a steel sheet having both high strength and excellent formability, and several techniques have been conventionally proposed to meet these demands.
[0004]
　Since there are various processing methods for automotive parts, the required formability varies depending on the applied parts, but among them, hole expandability is positioned as an important index of formability. Automobile members are formed by press molding, and the press-molded blank plates are often manufactured by shearing, which is highly productive.
[0005]
　For example, Patent Document 1 discloses a high-strength automobile excellent in collision safety and formability, in which retained austenite having an average crystal grain size of 5 μm or less is dispersed in ferrite having an average crystal grain size of 10 μm or less. A steel plate is disclosed. In a steel sheet containing retained austenite in the metal structure, the austenite transforms into martensite during working, and although it exhibits a large elongation due to transformation-induced plasticity, the formation of hard martensite impairs the hole expansibility. Patent Document 1 discloses that refinement of ferrite and retained austenite improves not only ductility but also hole expansibility.
[0006]
　Patent Document 2 discloses a high-strength steel sheet having a tensile strength of 980 MPa or more with excellent elongation and hole-expanding properties, in which a second phase composed of retained austenite and/or martensite is finely dispersed in grains. there is
[0007]
　Patent Documents 3 and 4 disclose a high-strength hot-rolled steel sheet having excellent ductility and hole expansibility and a method for producing the same. In Patent Document 3, after cooling to a temperature range of 720 ° C. or less within 1 second after the completion of hot rolling, and staying in a temperature range of 500 ° C. to 720 ° C. for a residence time of 1 to 20 seconds, 350 to 350 ° C. A method for producing a high-strength hot-rolled steel sheet having good ductility and stretch-flangeability, which is wound in a temperature range of 500°C, is disclosed.
[0008]
　Further, in Patent Document 4, there is disclosed an average of grains surrounded by grain boundaries having a crystal orientation difference of 15° or more in a steel structure that is mainly composed of bainite, has an appropriate amount of polygonal ferrite and retained austenite, and excludes retained austenite. A high-strength hot-rolled steel sheet having a grain size of 15 μm or less and having good ductility and stretch-flangeability is disclosed.
prior art documents
patent literature
[0009]
Patent Document 1: Japanese Patent Laid-Open No. 11-61326
Patent Document 2: Japanese Patent Document
2005-179703 Patent Document 3: Japanese Patent Document 2012-251200
Patent Document 4: Japanese Patent Document 2015-124410 publication
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0010]
　As described above, automobile parts are formed by press molding, and the press-molded blank plates are often manufactured by shearing, which is highly productive. In particular, with high-strength steel sheets of 980 MPa or more, the load required for post-treatment such as coining after shearing increases, so it is desired to control the unevenness of the fracture surface on the end face after shearing with particularly high accuracy. there is
[0011]
　The techniques disclosed in Patent Documents 1 to 4 are all techniques for improving strength and press formability during hole expansion, but there is no mention of techniques for improving shear workability, and parts are press-formed. It is presumed that post-processing will be required at the stage of manufacturing, and the manufacturing cost will increase.
[0012]
　The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent hole expansibility and shear workability.
Means to solve problems
[0013]
　In view of the problems described above, the present inventors have extensively studied the relationship between the chemical composition and metallographic structure of hot-rolled steel sheets and their mechanical properties. As a result, the following findings (a) to (f) were obtained, and the present invention was completed.
　In addition, having excellent shearing workability means that the unevenness of the fracture surface on the end face after shearing is small. Moreover, having excellent strength or high strength indicates that the tensile strength is 980 MPa or more.
[0014]
(a) In order to obtain excellent tensile (maximum) strength and hole expansibility, it is preferable that the matrix structure of the metal structure is hard. That is, it is preferable that the fraction of soft structures such as ferrite and retained austenite be as small as possible.
[0015]
(b) In order to form a large amount of martensite and tempered martensite, it is effective to rapidly cool austenite to a predetermined temperature range. Therefore, it is effective to cool to a predetermined temperature range without performing intermediate air cooling during the hot rolling process.
[0016]
(c) A hard structure is generally formed in a phase transformation at 600° C. or less. A large amount of grain boundaries with an angle of 7° are formed.
[0017]
(d) When a grain boundary is generated with a crystal orientation difference of 60° with the <110> direction as the axis, dislocations are significantly accumulated in the structure and the elastic strain increases. Therefore, in a metal structure in which such grain boundaries are dense and uniformly distributed (i.e., the length of the grain boundaries with a crystal misorientation of 60° with the <110> direction as the axis is large), In addition, the strength of the material is increased, plastic deformation during shearing is suppressed, and the formation of irregularities on the fracture surface on the end face after shearing is remarkably suppressed.
[0018]
(e) In order to uniformly disperse the grain boundaries having a crystal orientation difference of 60° with the <110> direction as the axis, the standard deviation of the Mn concentration must be set to a certain value or less. In order to keep the standard deviation of the Mn concentration below a certain value, when heating the slab, it is held in the temperature range of 700 to 850° C. for 900 seconds or longer, then further heated, and then heated in the temperature range of 1100° C. or higher for 6000 seconds or longer. It is effective to maintain the temperature and perform hot rolling in a temperature range of 850 to 1100° C. so as to reduce the total thickness of the sheet by 90% or more.
[0019]
(f) increasing the density of the length of the grain boundary with a crystal misorientation of 60° around the <110> direction, and the length of the grain boundary with a crystal misorientation of 7° around the <110> direction; In order to reduce the density of the coil, it is effective to set the coiling temperature below a predetermined temperature. When the coiling temperature is equal to or higher than the predetermined temperature, the density of the grain boundary with a crystal orientation difference of 60° with the <110> direction as an axis decreases, and the crystal orientation difference with the <110> direction as an axis is 7°. The grain boundary length density increases.
[0020]
　The gist of the present invention made based on the above knowledge is as follows.
(1) A hot-rolled steel sheet according to an aspect of the present invention has a chemical composition in mass% of
C: 0.040 to 0.250%,
Si: 0.05 to 3.00%,
Mn: 0.50 . ~4.00%,
sol. Al: 0.001 to 2.000%,
P: 0.100% or less,
S: 0.0300% or less,
N: 0.1000% or less,
O: 0.0100% or less,
Ti: 0 to 0.300 %,
Nb: 0 to 0.100%,
V: 0 to 0.500%,
Cu: 0 to 2.00%,
Cr: 0 to 2.00%,
Mo: 0 to 1.00%,
Ni: 0 ~2.00%,
B: 0-0.0100%,
Ca: 0-0.0200%,
Mg: 0-0.0200%,
REM: 0-0.1000%,
Bi: 0-0.020% ,
one or more of Zr, Co, Zn and W: 0 to 1.00% in total, and
Sn: 0 to 0.050%,
　The balance is composed of Fe and impurities, and the
　metal structure, in terms of area %, contains
　　a total of more than 92.0% and 100.0% or less of martensite and tempered martensite, and
　　less than 3.0% of retained austenite. , the
　　ferrite is less than 5.0%, the density S 60
　　of the length of the grain boundary with a crystal misorientation of 60° with the <110> direction as the axis, and the length of the grain boundary with a crystal misorientation of 7° S60 / S7 , which is the ratio of the thickness density S7, is more than 0.34 and less than 0.60, 　　the standard deviation of the Mn concentration is 0.60% by mass or less, and the 　tensile strength is 980 MPa or more. be. (2) The hot-rolled steel sheet described in (1) above may have an average crystal grain size of less than 3.0 μm in the surface layer. (3) The hot-rolled steel sheet according to (1) or (2) above has the chemical composition, in mass%, of Ti: 0.005 to 0.300%, Nb: 0.005 to 0.100%, V: 0.005-0.500%, Cu: 0.01-2.00%, Cr: 0.01-2.00%,

Mo: 0.01-1.00%,
Ni: 0.02-2.00%,
B: 0.0001-0.0100%,
Ca: 0.0005-0.0200%,
Mg: 0.0005- It may contain one or more selected from the group consisting of 0.0200%,
REM: 0.0005 to 0.1000%, and
Bi: 0.0005 to 0.020% .
Effect of the invention
[0021]
　According to the aspect of the present invention, it is possible to obtain a hot-rolled steel sheet having excellent strength, hole expansibility and shear workability. In addition, according to the preferred embodiment of the present invention, it is possible to obtain a hot-rolled steel sheet that has the above properties and further suppresses the occurrence of internal bending cracks, that is, has excellent resistance to internal bending cracks. can.
[0022]
　The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for automobile members, mechanical structural members, and building members.
Brief description of the drawing
[0023]
FIG. 1 is a diagram for explaining a method for measuring the size of unevenness of a fractured surface on an end face after shearing; FIG.
MODE FOR CARRYING OUT THE INVENTION
[0024]
　The chemical composition and metallographic structure of the hot-rolled steel sheet (hereinafter sometimes simply referred to as steel sheet) according to the present embodiment will be described more specifically below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the scope of the present invention.
[0025]
　The numerical limits described below with "-" in between include the lower limit and the upper limit. Any numerical value indicated as "less than" or "greater than" excludes that value from the numerical range. In the following description, % regarding the chemical composition of the hot-rolled steel sheet is % by mass unless otherwise specified.
[0026]
1. Chemical composition
　The hot-rolled steel sheet according to the present embodiment is, in mass%, C: 0.040 to 0.250%, Si: 0.05 to 3.00%, Mn: 0.50 to 4.00%, sol . Al: 0.001 to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and the balance: Fe and impurities including. Each element will be described in detail below.
[0027]
(1-1) C: 0.040 to 0.250%
　C increases the area fraction of the hard phase. Also, C increases the strength of martensite by combining with precipitation strengthening elements such as Ti, Nb, and V. If the C content is less than 0.040%, it becomes difficult to obtain the desired strength. Therefore, the C content should be 0.040% or more. The C content is preferably 0.060% or more, more preferably 0.070% or more.
　On the other hand, if the C content exceeds 0.250%, the formation of low-strength pearlite is promoted, and the area fractions of martensite and tempered martensite decrease, thereby decreasing the strength of the hot-rolled steel sheet. Therefore, the C content should be 0.250% or less. The C content is preferably 0.150% or less.
[0028]
(1-2) Si: 0.05 to 3.00%
　Si has the effect of delaying the precipitation of cementite. By this action, the area fraction of martensite and tempered martensite can be increased, and the strength of the hot-rolled steel sheet can be increased by solid-solution strengthening. In addition, Si has the effect of making steel sound by deoxidizing (suppressing the occurrence of defects such as blowholes in steel). If the Si content is less than 0.05%, the above effects cannot be obtained. Therefore, the Si content should be 0.05% or more. The Si content is preferably 0.50% or more and 1.00% or more.
　However, if the Si content exceeds 3.00%, the surface properties and chemical conversion treatability of the hot-rolled steel sheet, as well as the hole expandability and weldability, are significantly deteriorated, and the A3 transformation point is significantly increased. This makes it difficult to stably perform hot rolling. Therefore, the Si content should be 3.00% or less. The Si content is preferably 2.70% or less, more preferably 2.50% or less.
[0029]
(1-3) Mn: 0.50 to 4.00%
　Mn 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 should be 0.50% or more. The Mn content is preferably 1.00% or more, 1.50% or more, or 1.80% or more.
　On the other hand, if the Mn content exceeds 4.00%, the crystal orientation difference of the crystal grains in the hard phase becomes uneven due to the segregation of Mn, and the unevenness of the fracture surface on the end face after shearing increases. Therefore, the Mn content should be 4.00% or less. The Mn content is preferably 3.70% or less and 3.50% or less.
[0030]
(1-4) sol. Al: 0.001 to 2.000%
　Al, like Si, has the effect of deoxidizing steel to make it sound, and by suppressing the precipitation of cementite from austenite, martensite and tempered martensite has the effect of increasing the area fraction of sol. If the Al content is less than 0.001%, the above effects cannot be obtained. Therefore, sol. Al content shall be 0.001% or more. sol. The Al content is preferably 0.010% or more.
　On the other hand, sol. If the Al content exceeds 2.000%, the above effect saturates and is economically unfavorable. Al content is 2.000% or less. sol. The Al content is preferably 1.500% or less and 1.300% or less.
　In addition, in this embodiment, sol. Al means acid-soluble Al, and indicates solid-solution Al present in steel in a solid-solution state.
[0031]
(1-5) P: 0.100% or less
　P is an element that is generally contained as an impurity, but it is also an element that increases strength by solid solution strengthening. Therefore, P may be positively contained, but P is an element that easily segregates, and if the P content exceeds 0.100%, the hole expansibility due to grain boundary segregation will decrease significantly. . Therefore, the P content should be 0.100% or less. The P content is preferably 0.030% or less.
　Although the lower limit of the P content does not have to be specified, it is preferably 0.001% from the viewpoint of refining cost.
[0032]
(1-6) S: 0.0300% or less
　S is an element contained as an impurity, and forms sulfide-based inclusions in the steel to reduce the expansibility of the hot-rolled steel sheet. If the S content exceeds 0.0300%, the hole expansibility of the hot-rolled steel sheet is remarkably lowered. Therefore, the S content should be 0.0300% or less. The S content is preferably 0.0050% or less.
　Although the lower limit of the S content does not have to be specified, it is preferably 0.0001% from the viewpoint of refining cost.
[0033]
(1-7) N: 0.1000% or less
　N is an element contained in steel as an impurity, and has the effect of reducing the hole expansibility of the hot rolled steel sheet. If the N content exceeds 0.1000%, the hole expansibility of the hot-rolled steel sheet is remarkably lowered. Therefore, the N content should be 0.1000% or less. The N content is preferably 0.0800% or less, more preferably 0.0700% or less.
　The lower limit of the N content does not need to be specified in particular, but when one or more of Ti, Nb and V are contained as described later to refine the metal structure, precipitation of carbonitrides The N content is preferably 0.0010% or more, more preferably 0.0020% or more, in order to promote the
[0034]
(1-8) O: 0.0100% or less
　When contained in a large amount of O in steel, it forms coarse oxides that act as fracture starting points, causing brittle fracture and hydrogen-induced cracking. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less and 0.0050% or less.
　The O content may be 0.0005% or more and 0.0010% or more in order to disperse a large number of fine oxides when deoxidizing molten steel.
[0035]
　The rest 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 ores used as raw materials, scraps, or impurities that are mixed in from the manufacturing environment, etc., and are permissible within a range that does not adversely affect the hot-rolled steel sheet according to the present embodiment. do.
[0036]
　The hot-rolled steel sheet according to the present embodiment contains Ti, Nb, V, Cu, Cr, Mo, Ni, B, Ca, Mg, REM, Bi, Zr, Co, Zn, W and Sn in addition to the above elements. It may be contained as an element. The lower limit of the content when the optional element is not included is 0%. The optional elements will be described in detail below.
[0037]
(1-9) Ti: 0.005 to 0.300%, Nb: 0.005 to 0.100% and V: 0.005 to 0.500%
　Ti, Nb and V are all contained in steel One or two or more of these elements may be contained because they precipitate as carbides or nitrides and have the effect of refining the metal structure due to the pinning effect. In order to more reliably obtain the effect of the above action, the Ti content should be 0.005% or more, the Nb content should be 0.005% or more, or the V content should be 0.005% or more. preferably. That is, the content of at least one of Ti, Nb and V is preferably 0.005% or more.
　However, even if these elements are excessively contained, the effect of the above action is saturated, which is economically unfavorable. Therefore, the Ti content is 0.300% or less, the Nb content is 0.100% or less, and the V content is 0.500% or less. The Ti content is preferably 0.200% or less, 0.150% or less, 0.120% or less, 0.110% or less, or 0.100% or less.
[0038]
(1-10) Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.01 to 1.00%, Ni: 0.02 to 2.00% and B : 0.0001 to 0.0100%
　Cu, Cr, Mo, Ni and B all have the effect of increasing the hardenability of hot-rolled steel sheets. Further, Cr and Ni have the effect of stabilizing austenite, and Cu and Mo have the effect of precipitating carbides in steel at low temperatures to increase strength. Furthermore, when Cu is contained, Ni has the effect of effectively suppressing intergranular cracking of the slab caused by Cu. Therefore, one or more of these elements may be contained.
[0039]
　As described above, Cu has the effect of increasing the hardenability of the hot-rolled steel sheet and the effect of increasing the strength of the hot-rolled steel sheet by being precipitated as carbides in the steel at low temperatures. In order to more reliably obtain the effects of the above action, the Cu content is preferably 0.01% or more, more preferably 0.05% or more. However, if the Cu content exceeds 2.00%, intergranular cracking of the slab may occur. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.50% or less and 1.00% or less.
[0040]
　As described above, Cr has the effect of increasing the hardenability of the hot-rolled steel sheet and the effect of increasing the strength by precipitating carbides in the steel at low temperatures. In order to more reliably obtain the effects of the above action, the Cr content is preferably 0.01% or more and 0.05% or more. However, if the Cr content exceeds 2.00%, the chemical conversion treatability of the steel sheet is remarkably lowered. Therefore, the Cr content should be 2.00% or less.
[0041]
　As described above, Mo has the effect of increasing the hardenability of the hot-rolled steel sheet and the effect of precipitating carbides in the steel to increase the strength. In order to more reliably obtain the effects of the above action, the Mo content is preferably 0.01% or more and 0.02% or more. However, even if the Mo content exceeds 1.00%, the effect of the above action is saturated, which is economically unfavorable. Therefore, the Mo content should be 1.00% or less. The Mo content is preferably 0.50% or less and 0.20% or less.
[0042]
　As described above, Ni has the effect of enhancing the hardenability of the hot-rolled steel sheet. In addition, when Cu is contained, Ni has the effect of effectively suppressing intergranular cracking of the slab caused by Cu. In order to more reliably obtain the effects of the above action, the Ni content is preferably 0.02% or more. Since Ni is an expensive element, it is economically unfavorable to contain a large amount of Ni. Therefore, the Ni content is set to 2.00% or less.
[0043]
　As described above, B has the effect of enhancing the hardenability of the hot rolled steel sheet. In order to more reliably obtain the effect of this action, the B content is preferably 0.0001% or more and 0.0002% or more. However, if the B content exceeds 0.0100%, the hole expansibility of the steel sheet is remarkably lowered, so the B content is made 0.0100% or less. The B content is preferably 0.0050% or less.
[0044]
(1-11) Ca: 0.0005 to 0.0200%, Mg: 0.0005 to 0.0200%, REM: 0.0005 to 0.1000% and Bi: 0.0005 to 0.020%
　Ca, Both Mg and REM have the effect of improving the formability of the hot-rolled steel sheet by adjusting the shape of inclusions to a preferred shape. Bi also has the effect of increasing the formability of the hot-rolled steel sheet by refining the solidified structure. Therefore, one or more of these elements may be contained.
[0045]
　In order to more reliably obtain the effects of the above actions, it is preferable that at least one of Ca, Mg, REM and Bi is 0.0005% or more. However, when the Ca content or Mg content exceeds 0.0200%, or the REM content exceeds 0.1000%, inclusions are excessively formed in the steel, and the hole expandability of the hot rolled steel sheet is rather reduced. may reduce Moreover, even if the Bi content exceeds 0.020%, the above effect is saturated, which is economically unfavorable. Therefore, the Ca content and Mg content are set to 0.0200% or less, the REM content to 0.1000% or less, and the Bi content to 0.020% or less. The Bi content is preferably 0.010% or less.
[0046]
　Here, REM refers to a total of 17 elements consisting of Sc, Y and lanthanides, and the REM content refers to the total content of these elements. In the case of lanthanides, they are industrially added in the form of mischmetals.
[0047]
(1-12) One or more of Zr, Co, Zn and W: 0 to 1.00% in total and Sn: 0 to 0.050
　% confirmed that the effect of the hot-rolled steel sheet according to the present embodiment is not impaired even if the total content of these elements is 1.00% or less. Therefore, one or more of Zr, Co, Zn and W may be contained in a total amount of 1.00% or less.
[0048]
　In addition, the inventors have confirmed that even if a small amount of Sn is contained, the effect of the hot-rolled steel sheet according to the present embodiment is not impaired. However, if a large amount of Sn is contained, flaws may occur during hot rolling, so the Sn content is made 0.050% or less.
[0049]
　The chemical composition of the hot-rolled steel sheet described above may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). In addition, sol. Al can be measured by ICP-AES using the filtrate obtained by 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.
[0050]
2. Metal Structure of Hot-Rolled Steel Sheet Next, the metal structure of the hot-rolled steel sheet according to
　the present embodiment will be described.
　In the hot-rolled steel sheet according to the present embodiment, the metal structure has a total of more than 92.0% and 100.0% or less of martensite and tempered martensite, less than 3.0% of retained austenite, and ferrite is less than 5.0%, the density S 60 of the length of the grain boundary with a crystal misorientation of 60° with the <110> direction as the axis, and the length of the grain boundary with a crystal misorientation of 7° S60 / S7 , which is a ratio to density S7 , is more than 0.34 and less than 0.60, and the standard deviation of Mn concentration is 0.60% by mass or less. Therefore, the hot-rolled steel sheet according to this embodiment can obtain excellent strength, ductility and shear workability.
[0051]
　In addition, in this embodiment, the metallographic structure of the section parallel to the rolling direction is defined at a depth of 1/4 of the sheet thickness from the surface and at the central position in the sheet width direction. The reason is that the metallographic structure at this position shows the typical metallographic structure of the steel plate.
　The position at a depth of 1/4 of the plate thickness from the surface means a region from a depth of 1/8 of the plate thickness to a depth of 3/8 of the plate thickness from the surface.
[0052]
(2-1) Area fraction of retained austenite: less than 3.0%
　Retained austenite is a structure that exists as a face-centered cubic lattice even at room temperature. Retained austenite has the effect of increasing the ductility of hot-rolled steel sheets by transformation-induced plasticity (TRIP). On the other hand, retained austenite transforms into high-carbon martensite during shearing, which inhibits stable crack initiation and causes increased unevenness of the fracture surface on the end face after shearing. When the area fraction of retained austenite is 3.0% or more, the above effect becomes apparent, and not only the shear workability of the hot-rolled steel sheet deteriorates (the unevenness of the fracture surface at the end face increases), but also the hole expansibility decreases. . Therefore, the area fraction of retained austenite should be less than 3.0%. The area fraction of retained austenite is preferably less than 1.0%. The area fraction of retained austenite may be 0% because the smaller the retained austenite, the better.
[0053]
(2-2) Area fraction of ferrite: less than 5.0%
　Ferrite is generally a soft structure. If the ferrite content exceeds a predetermined amount, not only the desired strength cannot be obtained, but also the area of ​​the sheared surface on the end surface after shearing increases. An increase in the area of ​​the sheared surface on the end surface after shearing is not preferable because the unevenness of the fractured surface increases. When the area fraction of ferrite is 5.0% or more, the above effect becomes apparent, and the shear workability of the hot-rolled steel sheet deteriorates. Therefore, the area fraction of ferrite should be less than 5.0%. The area fraction of ferrite is preferably less than 1.0%. The area fraction of ferrite may be 0% because the smaller the ferrite content, the better.
[0054]
　Methods for measuring the area fraction of retained austenite include X-ray diffraction, EBSP (Electron Back Scattering Diffraction Pattern) analysis, magnetic measurement, and the like, and the measured value may vary depending on the measurement method. . In this embodiment, the area fraction of retained austenite is measured by X-ray diffraction.
[0055]
　In the measurement of the retained austenite area fraction by X-ray diffraction in this embodiment, first, 1/4 depth of the plate thickness of the steel plate (1/8 depth of the plate thickness from the surface to 3/8 depth of the plate thickness from the surface In a cross section parallel to the rolling direction at the center position in the plate width direction, α(110), α(200), α(211), γ(111), γ( 200) and γ(220) are obtained, and the area fraction of retained austenite is obtained by calculating using the intensity average method.
[0056]
　The area fraction of ferrite is measured by the following method. A cross-section perpendicular to the rolling direction is mirror-finished and polished with colloidal silica containing no alkaline solution at room temperature for 8 minutes to remove the strain introduced to the surface layer of the sample. At an arbitrary position in the longitudinal direction of the sample cross section, electron backscattering at a measurement interval of 0.1 μm in a region of 50 μm in length, 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface Crystal orientation information is obtained by measurement using a diffraction method.
[0057]
　For the measurement, an EBSD analyzer composed 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 analysis apparatus 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. The obtained crystal orientation information was analyzed using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" (manufactured by AMETEK) attached to the EBSD analysis device, and the Grain Average Misorientation value was 1.0°. The following regions are judged to be ferrite. The area fraction of ferrite is obtained by calculating the area fraction of the region determined to be ferrite.
[0058]
(2-3) Total area fraction of martensite and tempered martensite: more than 92.0% and 100.0% or less
　The total area fraction of martensite and tempered martensite is 92.0% or less Otherwise, the desired strength cannot be obtained. Therefore, the sum of the area fractions of martensite and tempered martensite should be greater than 92.0%. In addition, it is not necessary to contain both martensite and tempered martensite, and when either martensite or tempered martensite is contained, the area fraction thereof should be more than 92.0%. When both martensite and tempered martensite are included, the sum of the area fractions of martensite and tempered martensite should be greater than 92.0%. The total area fraction of martensite and tempered martensite is preferably 95.0% or more, 97.0% or more, 99.0% or more.
　Since the total area fraction of martensite and tempered martensite is preferably as large as possible, it may be 100.0%.
[0059]
　Methods for measuring the area fractions of martensite and tempered martensite are described below.
　First, in order to observe the same region as the EBSD measurement region in which the area fraction of ferrite was measured with an SEM, a Vickers indentation is stamped in the vicinity of the observation position. After that, leaving the structure of the observation surface, contamination on the surface layer is removed by polishing, and nital etching is performed. Next, the same field of view as the EBSD observation surface is observed with a SEM at a magnification of 3000 times.
[0060]
　In the EBSD measurement, among the regions determined to be residual structures, those regions having substructures within grains and having multiple variants of cementite precipitated are determined to be tempered martensite. A region with high brightness and in which the substructure is not revealed by etching is judged as "martensite and retained austenite". By calculating the respective area fractions, the area fractions of tempered martensite and "martensite and retained austenite" are obtained. The area fraction of martensite can be obtained by subtracting the area fraction of retained austenite obtained by the above-described X-ray diffraction from the area fraction of "martensite and retained austenite" obtained.
[0061]
　For removing contaminants from the surface layer of the observation surface, buffing using alumina particles having a particle size of 0.1 μm or less, Ar ion sputtering, or the like may be used.
[0062]
(2-4) With the <110> direction as the axis, the grain boundary length density S60 with a crystal misorientation of 60 ° and the grain boundary length density S7 with a crystal misorientation of 7 ° 　In order to obtain a hot-rolled steel sheet having a tensile strength of 980 MPa or more , the ratio S60 / S7 is more than 0.34 and less than 0.60 .
A hard structure is generally formed in a phase transformation at 600° C. or less. Certain grain boundaries are formed abundantly.
[0063]
　At the time of formation of grain boundaries with a crystal orientation difference of 60° with the <110> direction as the axis, dislocations are significantly accumulated in the structure and elastic strain increases. Therefore, the density of grain boundaries with a crystal orientation difference of 60° with the <110> direction as an axis is high and is uniformly dispersed (that is, the grain boundaries with a crystal orientation difference of 60° with the <110> direction as an axis) In a metal structure with a large length density, the strength of the material is increased, plastic deformation during shearing is suppressed, and unevenness of the fracture surface on the end face after shearing is suppressed.
[0064]
　On the other hand, at the grain boundary where the crystal orientation difference is 7° with the <110> direction as the axis, the dislocation density inside the structure is low and the elastic strain is also small, so the fracture surface on the end face after shearing is significantly uneven. growing. Therefore, with the <110> direction as the axis, the density of the length of the grain boundary with a crystal misorientation of 60° is S60, and the density of the grain boundary with a crystal misorientation of 7° is S7 . Then, the size of unevenness of the fracture surface on the end face after shearing is governed by S 60 /S 7 .
[0065]
　When S60 / S7 is 0.34 or less, not only is the tensile strength of the hot-rolled steel sheet unable to be 980 MPa or more, but also the unevenness of the fracture surface on the end face after shearing becomes large. Therefore, S60 / S7 should be greater than 0.34. Preferably, it is 0.40 or more and 0.45 or more. In order to suppress unevenness of the fractured surface on the end surface after shearing, it is desirable that S60 /S7 is as large as possible, but the practical upper limit is 0.60. Therefore, S60 / S7 should be less than 0.60.
[0066]
　A grain boundary having a crystal orientation difference of X° with the <110> direction as an axis is defined as two crystal grains A and B that are adjacent to each other at a certain grain boundary. It refers to a grain boundary having a crystallographic relationship in which the crystal orientations of the crystal grain A and the crystal grain B match when rotated by X degrees about the 110> axis. However, considering the measurement accuracy of the crystal orientation, a misorientation of ±4° from the matching orientation relationship is allowed.
[0067]
　In this embodiment, with the <110> direction as an axis, the density S60 of the length of the grain boundary where the crystal misorientation is 60° and the density S7 of the length of the grain boundary where the crystal misorientation is 7° are determined by EBSP. - Measured using the OIM (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy) method. In the EBSP-OIM method, an electron beam is irradiated to a highly tilted sample in a scanning electron microscope (SEM), the Kikuchi pattern formed by backscattering is photographed with a high-sensitivity camera, and the photographed photograph is image-processed by a computer. By doing so, the crystal orientation of the irradiation point can be measured in a short time.
[0068]
　The EBSP-OIM method is performed using an EBSD analysis apparatus composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector, and OIM Analysis (registered trademark) manufactured by AMETEK. Since the EBSP-OIM method can analyze the microstructure and crystal orientation of the sample surface, it is possible to quantitatively determine the length of the grain boundary having a specific crystal orientation difference. Also, the analyzable area of ​​the EBSP-OIM method is the area that can be observed with the SEM. Although it depends on the resolution of the SEM, the EBSP-OIM method enables analysis with a minimum resolution of 20 nm.
[0069]
　1/4 depth of the plate thickness from the surface of the steel plate in the cross section parallel to the rolling direction (area from 1/8 depth of the plate thickness to 3/8 depth of the plate thickness from the surface) and the center position in the width direction In the measurement of the length of the specific grain boundary in the metal structure, analysis is performed in at least 5 fields of view at a magnification of 1200 times in an area of ​​40 µm x 30 µm. Then, S60 is obtained by dividing the average value of the length of grain boundaries with a crystal misorientation of 60° around the <110> direction by the area of ​​the measurement region . Similarly, S7 is obtained by dividing the average length of grain boundaries with a crystal misorientation of 7° around the <110> direction by the area of ​​the measurement region . In addition, as described above, a misorientation of ±4° is allowed.
[0070]
　Note that retained austenite is not a structure generated by phase transformation at 600° C. or less and has no effect of accumulating dislocations, so retained austenite is not analyzed in this measurement method. That is, in the present embodiment, with the <110> direction as an axis, the grain boundary length density S 60 with a crystal misorientation of 60° and the grain boundary length density S 7 with a crystal misorientation of 7° are those of martensite, tempered martensite and ferrite. In the EBSP-OIM method, retained austenite having a crystal structure of fcc can be excluded from analysis.
[0071]
(2-5) Standard deviation of Mn concentration: 0.60 mass% or less
　1/4 depth of plate thickness from surface of hot-rolled steel sheet according to the present embodiment (1/8 depth of plate thickness from surface to surface The standard deviation of the Mn concentration is 0.60% by mass or less in the area of ​​3/8 depth of the sheet thickness) and at the central position in the sheet width direction. This makes it possible to uniformly disperse grain boundaries having a crystal orientation difference of 60° around the <110> direction. As a result, the unevenness of the fracture surface on the end face after shearing can be reduced. The standard deviation of the Mn concentration is preferably 0.55% by mass or less, 0.50% by mass or less, or 0.40% by mass or less.
[0072]
　Since the lower limit of the standard deviation of the Mn concentration suppresses unevenness of the fracture surface on the end face after shearing, the smaller the value, the better, but due to the restrictions of the manufacturing process, the practical lower limit is 0.10% by mass. .
[0073]
　The standard deviation of Mn concentration is measured by the following method.
　After the L cross section of the hot-rolled steel sheet is mirror-polished, the depth of 1/4 of the plate thickness from the surface (area from 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface) and the plate width The directional center position is measured with an electron probe microanalyzer (EPMA) to measure the standard deviation of the Mn concentration. The measurement conditions are an acceleration voltage of 15 kV, a magnification of 5000, and a distribution image in a range of 20 μm in the rolling direction of the sample and 20 μm in the thickness direction of the sample. More specifically, the measurement interval is set to 0.1 μm, and the Mn concentration is measured at 40,000 or more locations. Then, the standard deviation of the Mn concentration is obtained by calculating the standard deviation based on the Mn concentrations obtained from all measurement points.
[0074]
(2-6) Average crystal grain size of
　surface layer: less than 3.0 μm If the crystal grain size of the surface layer is fine, it is possible to suppress bending cracks in the hot-rolled steel sheet. The higher the strength of the steel sheet, the more likely it is that cracks will occur from the inside of the bend during bending (hereinafter referred to as internal bending cracks).
[0075]
　The mechanism of internal bending cracks is presumed as follows. During bending, compressive stress is generated inside the bend. At first, the entire inner side of the bend deforms uniformly as the work progresses, but as the amount of work increases, the uniform deformation becomes unable to bear the deformation, and the deformation progresses as the strain concentrates locally (the occurrence of shear deformation bands). . As this shear deformation band grows further, a crack occurs and grows along the shear band from the inner surface of the bend. The reason why inner bending cracks are more likely to occur as the strength increases is that due to the decrease in work hardening ability that accompanies the increase in strength, it becomes difficult for uniform deformation to proceed, and uneven deformation tends to occur, which can lead to early ( Or under loose processing conditions) is presumed to be due to the occurrence of shear deformation bands.
[0076]
　The present inventors' research has revealed that the internal bending cracks become conspicuous in steel sheets having a tensile strength of 980 MPa or more. In addition, the present inventors have found that the finer the crystal grain size of the surface layer of the hot-rolled steel sheet, the more the local strain concentration is suppressed and the bending inner cracks are less likely to occur. In order to obtain the above effects, the average grain size of the surface layer of the hot-rolled steel sheet is preferably less than 3.0 μm. More preferably, the thickness is 2.5 μm or less. Although the lower limit is not particularly limited, it may be 1.0 μm or more, 1.5 μm or more, or 2.0 μm or more.
　In this embodiment, the surface layer is a region from the surface of the hot-rolled steel sheet to a depth of 50 μm from the surface.
[0077]
　The crystal grain size of the surface layer is measured using the aforementioned EBSP-OIM method. In the cross section parallel to the rolling direction, in the region from the surface of the hot-rolled steel sheet to the depth of 50 μm from the surface and the center position in the width direction, the magnification is 1200 times, and the analysis is performed in at least 5 fields of view in an area of ​​40 μm × 30 μm, A place where the angle difference between adjacent measurement points is 5° or more is defined as a grain boundary, and the area-average grain size is calculated. The obtained area-average crystal grain size is taken as the average crystal grain size of the surface layer.
[0078]
　Note that retained austenite is not a structure generated by phase transformation at 600° C. or less and has no effect of accumulating dislocations, so retained austenite is not analyzed in this measurement method. That is, in this embodiment, the average crystal grain size of the surface layer is that of martensite, tempered martensite, and ferrite. In the EBSP-OIM method, retained austenite having a crystal structure of fcc can be excluded from analysis.
[0079]
3. Tensile Strength Characteristics
　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 vehicle weight reduction is small. Although the upper limit is not particularly limited, it may be 1780 MPa from the viewpoint of mold wear suppression.
[0080]
　Tensile strength is measured according to JIS Z 2241:2011 using a No. 5 test piece of JIS Z 2241:2011. A tensile test piece is taken from a quarter portion from the edge in the width direction of the sheet, and the direction perpendicular to the rolling direction is taken as the longitudinal direction.
[0081]
4. Hole-expansion property
　The hot-rolled steel sheet according to the present embodiment preferably has a hole-expansion ratio λ of 62% or more. When the hole expansion ratio λ is 62% or more, it is possible to obtain a hot-rolled steel sheet that greatly contributes to weight reduction of the vehicle body without limiting applicable parts. There is no need to specifically limit the upper limit.
[0082]
　The hole expansion ratio λ is measured according to JIS Z 2256:2010 using a No. 5 test piece of JIS Z 2241:2011. The hole-expanding test piece may be sampled from the 1/4 part from the edge in the width direction of the sheet.
　Further, the product of tensile strength and hole expansion (TS×λ), which is an index of hole expandability, is preferably 60000 MPa·% or more. When the product of tensile strength and hole expansion is 60000 MPa·% or more, it is possible to obtain a hot-rolled steel sheet that greatly contributes to vehicle weight reduction without limiting applicable parts.
[0083]
5. Thickness The thickness
　of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be 0.5 to 8.0 mm. By setting the thickness of the hot-rolled steel sheet to 0.5 mm or more, it becomes easy to secure the rolling completion temperature, the rolling load can be reduced, and hot rolling can be easily performed. Therefore, the thickness of the hot-rolled steel sheet according to this embodiment may be 0.5 mm or more. It is preferably 1.2 mm or more and 1.4 mm or more. Further, by setting the plate thickness to 8.0 mm or less, the metal structure can be easily refined, and the metal structure described above can be easily secured. Therefore, the plate thickness may be 8.0 mm or less. Preferably, it is 6.0 mm or less.
[0084]
6. Others
(6-1) Coated Layer
　The hot-rolled steel sheet according to the present embodiment having the chemical composition and metallographic structure described above may be provided with a coated layer on the surface thereof for the purpose of improving corrosion resistance, etc., to form a surface-treated steel sheet. The plating layer may be an electroplating layer or a hot dipping layer. Examples of the electroplating layer include electrogalvanizing and electroplating of Zn—Ni alloy. Examples of hot-dip coating layers include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn--Al alloy plating, hot-dip Zn--Al--Mg alloy plating, and hot-dip Zn--Al--Mg--Si alloy plating. be.
[0085]
　The amount of plating deposited is not particularly limited, and may be the same as the conventional one. Further, it is possible to further improve the corrosion resistance by applying an appropriate chemical conversion treatment (for example, applying a silicate-based chromium-free chemical conversion treatment solution and drying) after plating.
[0086]
7. Manufacturing Conditions
　A preferred method for manufacturing the hot-rolled steel sheet according to the present embodiment having the chemical composition and metallographic structure described above is as follows.
[0087]
　In order to obtain the hot-rolled steel sheet according to the present embodiment, the slab is heated under predetermined conditions, then hot-rolled, acceleratedly cooled to a predetermined temperature range, and the cooling history after winding is controlled. is effective.
[0088]
　In a preferred method for manufacturing a hot-rolled steel sheet according to this embodiment, the following steps (1) to (7) are sequentially performed. The temperature of the slab and the temperature of the steel plate in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel plate.
(1) The slab is held in the temperature range of 700 to 850°C for 900 seconds or longer, then further heated and held in the temperature range of 1100°C or higher for 6000 seconds or longer.
(2) Hot rolling is performed in a temperature range of 850 to 1100° C. so that the total thickness reduction is 90% or more.
(3) Hot rolling is completed so that the hot rolling completion temperature Tf becomes equal to or higher than the temperature T1 (° C.) represented by the following formula <1>.
(4) Accelerated cooling is started within 1.5 seconds after the completion of hot rolling, and the average cooling rate to the temperature range below the temperature T2 (° C.) represented by the following formula <2> is 30 ° C./s or more. and
　More preferably, the steel sheet is cooled to a temperature range equal to or lower than the hot rolling completion temperature Tf-50°C within 1.0 second after the completion of hot rolling.
(5) Cool from T2 (°C) to the winding temperature at an average cooling rate of 30°C/s or more.
(6) Set the winding temperature to a temperature range of 300° C. or less.
[0089]
T1 (°C) = 868 - 396 x [C] - 68.1 x [Mn] + 24.6 x [Si] - 36.1 x [Ni] - 24.8 x [Cr] - 20.7 x [Cu ]+250×[sol. Al] ... <1>
T2 (° C.) = 770 - 270 x [C] - 90 x [Mn] - 37 x [Ni] - 70 x [Cr] - 83 x [Mo] ... <2>
　However, each formula [Element symbol] in the figure indicates the content (% by mass) of each element in the steel. If the element is not contained, 0 is substituted.
[0090]
(7-1) Slab, slab temperature and holding time when subjected to hot rolling As the slab subjected to
　hot rolling, a slab obtained by continuous casting, a slab obtained by casting or blooming, etc. can be used. If necessary, those obtained by adding hot working or cold working to them can be used.
[0091]
　The slab subjected to hot rolling is preferably held in the temperature range of 700 to 850° C. during heating for 900 seconds or longer, then further heated and held in the temperature range of 1100° C. or higher for 6000 seconds or longer. In addition, when the steel sheet is held in the temperature range of 700 to 850° C., the temperature of the steel sheet may be varied within this temperature range, or may be kept constant. Further, when the steel sheet is held in the temperature range of 1100° C. or higher, the steel sheet temperature may be varied in the temperature range of 1100° C. or higher, or may be constant.
[0092]
　In the austenite transformation at 700-850° C., Mn is distributed between ferrite and austenite, and by prolonging the transformation time, Mn can diffuse into the ferrite region. As a result, the Mn microsegregation unevenly distributed in the slab can be eliminated, and the standard deviation of the Mn concentration can be significantly reduced. By reducing the standard deviation of the Mn concentration, in the final metal structure, the grain boundaries with a crystal orientation difference of 60 ° with the <110> direction as the axis can be uniformly dispersed. The unevenness of the fracture surface can be reduced.
　Moreover, in order to make the austenite grains uniform when heating the slab, it is preferable to heat the slab in a temperature range of 1100° C. or higher for 6000 seconds or longer.
[0093]
　For hot rolling, it is preferable to use a reverse mill or a tandem mill as multi-pass rolling. In particular, from the viewpoint of industrial productivity, it is more preferable to perform hot rolling using a tandem mill for at least the final several stages.
[0094]
(7-2) Reduction ratio of hot rolling: A total thickness reduction of 90% or more in
　the temperature range of 850 to 1100°C. Rolling mainly refines recrystallized austenite grains, promotes the accumulation of strain energy in non-recrystallized austenite grains, promotes recrystallization of austenite, and promotes atomic diffusion of Mn. can be accelerated and the standard deviation of the Mn concentration can be reduced.
[0095]
　By reducing the standard deviation of the Mn concentration, in the final metal structure, the grain boundaries with a crystal orientation difference of 60 ° with the <110> direction as the axis can be uniformly dispersed. The unevenness of the fracture surface can be reduced. Therefore, hot rolling is carried out in the temperature range of 850 to 1100° C. so that the total thickness reduction is 90% or more.
[0096]
　The thickness reduction in the temperature range of 850 to 1100 ° C. is defined as the entrance thickness t0 before the first pass in rolling in this temperature range, and the exit thickness after the final pass in rolling in this temperature range as t1. , it can be expressed as (t 0 −t 1 )/t 0 ×100(%).
[0097]
(7-3) Hot rolling completion temperature Tf: T1 (°C) or higher
　The hot rolling completion temperature Tf is preferably T1 (°C) or higher. By setting the hot rolling completion temperature Tf to T1 (° C.) or higher, an excessive increase in the number of ferrite nucleation sites in austenite can be suppressed, and the final structure (metal structure of the hot-rolled steel sheet after production) It is possible to suppress the formation of ferrite and obtain a high-strength hot-rolled steel sheet.
[0098]
(7-4) Accelerated cooling after completion of hot rolling: Accelerated cooling is started within 1.5 seconds, and thinning is performed by
　hot rolling at an average cooling rate of 30 ° C./s or more to T2 (° C.) or lower. In order to suppress the growth of granulated austenite grains, it is preferable to perform accelerated cooling to T2 (°C) or less at an average cooling rate of 30°C/s or more within 1.5 seconds after the completion of hot rolling.
[0099]
　Formation of ferrite and pearlite can be suppressed by performing accelerated cooling to T2 (° C.) or less at an average cooling rate of 30° C./s or more within 1.5 seconds after the completion of hot rolling. This improves the strength of the hot-rolled steel sheet. The average cooling rate here means the temperature drop range of the steel plate from the start of accelerated cooling (when the steel plate is introduced into the cooling equipment) to T2 (°C), and the steel plate temperature from the start of accelerated cooling to T2 (°C). ) is the value divided by the time required to reach
[0100]
　In the accelerated cooling after the completion of hot rolling, the time until the start of cooling is set to within 1.5 seconds, and the average cooling rate to T2 (° C.) or lower is set to 30 ° C./s or higher, so that ferrite transformation inside the steel plate and/or bainite transformation and/or pearlite transformation can be suppressed, and TS≧980 MPa can be obtained. Therefore, within 1.5 seconds after the completion of hot rolling, accelerated cooling is performed so that the average cooling rate to T2 (° C.) or lower is 30° C./s or higher.
[0101]
　Although the upper limit of the average cooling rate is not specified, if the cooling rate is increased, the cooling equipment becomes large-scaled and the equipment cost increases. Therefore, considering the facility cost, the average cooling rate of accelerated cooling is preferably 300° C./s or less. Also, the cooling stop temperature of accelerated cooling is preferably 350° C. or less.
[0102]
　In the cooling after completion of hot rolling, it is more preferable to cool to a temperature range of hot rolling completion temperature Tf−50° C. within 1.0 second after completion of hot rolling. This is because the growth of austenite crystal grains refined by hot rolling can be suppressed. In order to cool to a temperature range of the hot rolling completion temperature Tf-50 ° C. or less within 1.0 second after the completion of hot rolling, cooling with a high average cooling rate is performed immediately after the completion of hot rolling, such as cooling water should be sprayed onto the steel plate surface. By cooling to a temperature range of Tf-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 of the hot-rolled steel sheet can be enhanced.
[0103]
　Within 1.0 second after the completion of hot rolling, after cooling to the temperature range of the hot rolling completion temperature Tf-50 ° C., as described above, the average cooling rate to T2 (° C.) or less is 30 ° C./s. Accelerated cooling may be performed as described above.
[0104]
(7-5) The average cooling rate from T2 (°C) to the coiling temperature is 30°C/s or more
　. It is preferable that the average cooling rate from the film to the winding temperature is 30° C./s or more. Thereby, the matrix structure can be made hard. The average cooling rate here is a value obtained by dividing the temperature drop range of the steel sheet from T2 (°C) to the coiling temperature by the time required from when the steel plate temperature reaches T2 (°C) to coiling. That's what I mean.
[0105]
　By setting the average cooling rate to 30° C./s or higher, the area fractions of ferrite, bainite, and pearlite can be suppressed, and strength and hole expansibility can be ensured. Therefore, the average cooling rate from T2 (°C) to the winding temperature should be 30°C/s or more.
[0106]
(7-6) Winding temperature: 300°C or less The
　winding temperature is preferably 300°C or less. By setting the coiling temperature to 300° C. or lower, the driving force for transformation from austenite to bcc can be increased, and the deformation strength of austenite can be increased. Therefore, when transforming from austenite to bainite and martensite, the density S 60 of the length of the grain boundary having a crystal orientation difference of 60° with the <110> direction as the axis can be suppressed, and S 60 /S 7 is 0.60. can be less than As a result, it is possible to reduce unevenness of the fracture surface on the end surface after shearing. In addition, it is possible to suppress deterioration of the hole expansibility due to the influence of retained austenite. Therefore, the winding temperature is preferably 300° C. or lower. The winding temperature is more preferably 50° C. or lower.
Example
[0107]
　Next, the effects of one aspect of the present invention will be described in more detail with reference to examples. The present invention is not limited to this one conditional example. Various conditions can be adopted in the present invention as long as the objects of the present invention are achieved without departing from the gist of the present invention.
[0108]
　Steel No. in Tables 1 and 2. Steels having chemical compositions shown in A to S were melted, and slabs with a thickness of 240 to 300 mm were produced by continuous casting. Using the obtained slabs, hot-rolled steel sheets shown in Tables 4A and 4B were obtained under the manufacturing conditions shown in Tables 3A and 3B.
[0109]
　The slabs were held in the temperature range of 700 to 850° C. for the holding times shown in Tables 3A and 3B, then further heated to the heating temperatures shown in Tables 3A and 3B and held. Also, accelerated cooling was started within 1.5 seconds after the completion of hot rolling.
[0110]
　The area fraction of each structure, the S60/S7, the standard deviation of the Mn concentration, and the average grain size of the surface layer were determined for the obtained hot-rolled steel sheets by the methods described above . The measurement results obtained are shown in Tables 4A and 4B.
[0111]
　Evaluation method of properties of hot-rolled steel sheet
　(1) Tensile strength properties and hole expansion
　ratio Z 2256:2010 was evaluated. The test piece was JIS Z 2241:2011 No. 5 test piece. The tensile test piece was sampled from the 1/4 part from the edge in the width direction, and the direction perpendicular to the rolling direction was taken as the longitudinal direction.
[0112]
　When the tensile strength TS≧980 MPa was satisfied, the strength was judged to be excellent and passed. On the other hand, when the tensile strength TS was less than 980 MPa, it was judged to be unacceptable because the strength was inferior.
　Moreover, when the tensile strength TS×hole expansion ratio λ≧60000 (MPa·%) was satisfied, the hole expandability was judged to be excellent and it was judged as acceptable. On the other hand, when tensile strength TS×hole expanding ratio λ<60000 (MPa·%), the hole expanding property was judged to be inferior and it was determined to be unacceptable.
[0113]
(2) Shear workability
　The shear workability of the hot-rolled steel sheet was evaluated by measuring the unevenness of the fracture surface on the end face after shearing by a punching test. Five punched holes were made with a hole diameter of 10 mm, a clearance of 10%, and a punching speed of 3 m/s. Next, ten cross-sections of five punched holes parallel to the rolling direction were embedded in resin, and the cross-sectional shape was photographed with a scanning electron microscope. In the observation photograph obtained, it was possible to observe a machined cross section composed of sag, sheared surface, fractured surface, and burrs as shown in FIG.
[0114]
　A sag is an R-shaped smooth surface region. The burr is the area of ​​the end surface, and the burr is the surface having protrusions protruding from the lower surface of the hot-rolled steel sheet.
[0115]
　In the observed photograph, a straight line (straight line 1 in FIG. 1) was drawn parallel to the sheared surface of the hot-rolled steel sheet and passing through the starting point A of the burr. Furthermore, a straight line 2-1 that is parallel to the straight line 1 and passes through the point B where the distance from the straight line 1 is the maximum in the recess of the fracture surface, and parallel to the straight line 1 and in the convex portion of the fracture surface , a straight line 2-1 passing through the point C, which is the greatest distance from the straight line 1. The half value of the distance between the straight line 2-1 and the straight line 2-2 (half the value of d in FIG. 1) was defined as the size of unevenness of the fractured surface. The size of unevenness of the fractured surface is measured for 10 end faces obtained from five punched holes, and if the maximum value of the unevenness of the fractured surface is 3.0 μm or less, the shear workability is excellent. It was determined that it passed as On the other hand, if the maximum value of the unevenness of the fractured surface was more than 3.0 μm, it was judged to be unsatisfactory due to poor shear workability.
[0116]
(3) Resistance to internal bending cracks
　A bending test piece is a strip-shaped test piece of 100 mm × 30 mm cut from the 1/2 position in the width direction of the hot-rolled steel sheet, and evaluated for resistance to internal bending cracks by the following bending test. did.
[0117]
　For both bending (L-axis bending) in which the bending ridge is parallel to the rolling direction (L direction) and bending (C-axis bending) in which the bending ridge is parallel to the direction (C direction) perpendicular to the rolling direction, JIS Z 2248: 2014 (V-block 90° bending test), the resistance to internal bending cracks was investigated, the minimum bending radius that does not cause cracking was determined, and the average value R of the minimum bending radii of the L-axis and C-axis was taken as the plate thickness. The value obtained by dividing by t was defined as the limit bending R/t and used as an index value of bendability. When R/t≦3.0, the hot-rolled steel sheet was judged to be excellent in resistance to internal bending cracks.
[0118]
　However, for the presence or absence of cracks, the cross section of the test piece after the V-block 90° bending test is cut in a plane parallel to the bending direction and perpendicular to the plate surface. When the crack length observed inside the bending exceeded 30 µm, it was determined that there was a crack.
　The measurement results obtained are shown in Tables 4A and 4B.
[0119]
[table 1]

[0120]
[Table 2]

[0121]
[Table 3A]

[0122]
[Table 3B]

[0123]
[Table 4A]

[0124]
[Table 4B]

[0125]
　As can be seen from Tables 4A and 4B, production no. In Nos. 1, 2, 6 and 11-23, hot-rolled steel sheets having excellent strength, hole expandability and shear workability were obtained. Furthermore, the production No. 1 having an average grain size of less than 3.0 μm in the surface layer. In Nos. 1, 2, 12 to 19 and 21 to 23, hot-rolled steel sheets having the above properties and further excellent resistance to internal bending cracks were obtained.
[0126]
　On the other hand, production Nos. 1, 2, 3, 4, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 14, 14, 14, 14, 14, 14, 14, 18, 18, 19, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20 Samples 3 to 5, 7 to 10 and 24 to 27 were inferior in at least one of the properties (tensile strength TS, hole expansion ratio λ, shear workability).
Industrial applicability
[0127]
　According to the aspect of the present invention, it is possible to provide a hot-rolled steel sheet having excellent strength, hole expansibility and shear workability. In addition, according to the preferred embodiment of the present invention, it is possible to obtain a hot-rolled steel sheet that has the above properties and further suppresses the occurrence of internal bending cracks, that is, has excellent resistance to internal bending cracks. can.
[0128]
　INDUSTRIAL APPLICABILITY The hot-rolled steel sheet according to the present invention is suitable as an industrial material used for automobile members, mechanical structural members, and building members.
The scope of the claims
[Claim 1]
　The chemical composition is mass %,
C: 0.040 to 0.250%,
Si: 0.05 to 3.00%,
Mn: 0.50 to 4.00%,
sol. Al: 0.001 to 2.000%,
P: 0.100% or less,
S: 0.0300% or less,
N: 0.1000% or less,
O: 0.0100% or less,
Ti: 0 to 0.300 %,
Nb: 0 to 0.100%,
V: 0 to 0.500%,
Cu: 0 to 2.00%,
Cr: 0 to 2.00%,
Mo: 0 to 1.00%,
Ni: 0 ~2.00%,
B: 0-0.0100%,
Ca: 0-0.0200%,
Mg: 0-0.0200%,
REM: 0-0.1000%,
Bi: 0-0.020% ,
One or more of Zr, Co, Zn and W: 0 to 1.00% in total, and
Sn: 0 to 0.050%, the
　balance being Fe and impurities, and the
　metal structure is , in area %,
　　martensite and tempered martensite totaling more than 92.0% but not more than 100.0%,
　　retained austenite less than 3.0%,
　　ferrite less than 5.0%, and
　　<110> direction As the axis, S60 / S7 , which is the ratio of the grain boundary length density S60 with a crystal misorientation of 60 ° to the grain boundary length density S7 with a crystal misorientation of 7° , is A hot-rolled steel sheet having a Mn concentration of more than 0.34 and less than 0.60, 　　a standard deviation of Mn concentration of 0.60% by mass or less, and a 　tensile strength of 980 MPa or more.

[Claim 2]
　2. The hot-rolled steel sheet according to claim 1, wherein the surface layer has an average grain size of less than 3.0 [mu]m.
[Claim 3]
　The chemical composition is, in mass %,
Ti: 0.005 to 0.300%,
Nb: 0.005 to 0.100%,
V: 0.005 to 0.500%,
Cu: 0.01 to 2.0%. 00%,
Cr: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
Ni: 0.02 to 2.00%,
B: 0.0001 to 0.0100%,
Ca: 0 .0005 to 0.0200%,
Mg: 0.0005 to 0.0200%,
REM: 0.0005 to 0.1000%, and
Bi: 0.0005 to 0.020
% 3. The hot-rolled steel sheet according to claim 1 or 2, wherein the hot-rolled steel sheet contains two or more kinds.