The present invention relates to a hot-rolled steel sheet. Specifically, the present invention relates to a hot-rolled steel sheet that is formed into various shapes by press working or the like and is used, and in particular, a hot-rolled steel sheet that has high strength and is excellent in ductility and stretch flangeability.
　The present application claims priority based on Japanese Patent Application No. 2018-197937 filed in Japan on October 19, 2018, the contents of which are incorporated herein by reference.
Background technology
[0002]
　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 technologies for reducing the weight of vehicle bodies with the aim of reducing fuel consumption. However, it is not easy to reduce the weight of the vehicle body because the emphasis is on improving the collision resistance to ensure the safety of the occupants.
[0003]
　Therefore, in order to achieve both weight reduction of the vehicle body and collision resistance, it is being studied to thin the member by using a high-strength steel plate. Therefore, a steel sheet having both high strength and excellent formability is strongly desired, and some techniques have been conventionally proposed in order to meet these demands. Among them, steel sheets containing retained austenite have been studied many times because they exhibit excellent ductility due to transformation-induced plasticity (TRIP).
[0004]
　For example, Patent Document 1 describes high strength for automobiles having excellent collision resistance and moldability, in which retained austenite having an average crystal particle size of 5 μm or less is dispersed in ferrite having an average crystal particle size of 10 μm or less. Steel plates are disclosed. In a steel sheet containing retained austenite in its metal structure, austenite undergoes martensitic transformation during processing and exhibits a large elongation due to transformation-induced plasticity, but the formation of hard martensite impairs hole expansion. Patent Document 1 discloses that not only ductility but also hole expansion property is improved by miniaturizing ferrite and retained austenite.
[0005]
　Patent Document 2 discloses a high-strength steel plate having excellent elongation and stretch flangeability and a tensile strength of 980 MPa or more, in which a second phase composed of retained austenite and / or martensite is finely dispersed in crystal grains. There is.
[0006]
　Patent Documents 3 and 4 disclose a high-strength hot-rolled steel sheet having excellent ductility and stretch flangeability, and a method for producing the same. According to Patent Document 3, after cooling to a temperature range of 720 ° C. or lower within 1 second after the completion of hot rolling, and staying in a temperature range of more than 500 ° C. and 720 ° C. or lower for a residence time of 1 to 20 seconds, 350 to 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. Further, Patent Document 4 describes the average of grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in a steel structure excluding retained austenite, which is mainly composed of bainite and has an appropriate amount of polygonal ferrite and retained austenite. A high-strength hot-rolled steel sheet having a particle size of 15 μm or less and having good ductility and stretch flangeability is disclosed.
[0007]
　Patent Document 5 contains 90% or more of grain-shaped tempered martensite in terms of body integration rate, or 90% or more of both grain-shaped tempered martensite and lower bainite in total body integration rate. The average aspect ratio of the effective crystal grains of the tempered martensite and the lower bainite is 2 or less, the effective crystal grain size of the tempered martensite and the lower bainite is 10 μm or less, and the tempered martensite and the lower part are For strength and low temperature toughness, characterized by having a structure in which 1 × 10 6 (pieces / mm 2 ) or more of iron-based carbides are present in bainite, and having a martensite layer or an alloyed martensite layer on the surface. Excellent hot-rolled steel sheets are disclosed.
Prior art literature
Patent documents
[0008]
Patent Document 1: Japanese Patent Application Laid-Open No. 11-61326
Patent Document 2: Japanese Patent
Application Laid-Open No. 2005-179703 Patent Document 3: Japanese Patent Application Laid-Open No. 2012-251200
Patent Document 4: Japanese Patent Application Laid-Open No. 2015-124410 No.
Patent Document 5: Japanese Patent No. 613017
Outline of the invention
Problems to be solved by the invention
[0009]
　Since there are various processing styles for automobile parts, the required moldability differs depending on the member to which it is applied, but among them, ductility and stretch flangeability are positioned as important indicators of moldability. It is desired that automobile parts have both ductility and stretch flangeability at a high level. Further, it is desired that a steel sheet containing retained austenite also has a high level of ductility and stretch flangeability, but precise temperature control is required in the manufacturing process, and when actually manufactured, the material varies in the plate width direction. Has the drawback of being large.
[0010]
　The high-strength steel sheet for automobiles disclosed in Patent Document 1 is said to have improved ductility and hole expansion due to the miniaturization of ferrite and retained austenite, but the maximum hole expansion ratio is 1.5, which is sufficient. It is hard to say that it has press moldability. Further, in order to increase the work hardening index and improve the collision resistance, it is necessary to use a soft ferrite phase as the main phase, and high tensile strength may not be obtained.
[0011]
　The high-strength steel sheet disclosed in Patent Document 2 contains a large amount of expensive elements such as Cu and Ni in order to refine the second phase to nano size and disperse it in the crystal grains, or it is long at high temperature. It is necessary to carry out a solution treatment for a period of time, which may significantly increase the manufacturing cost or decrease the productivity.
[0012]
　In the method for manufacturing a high-strength hot-rolled steel sheet disclosed in Patent Document 3, rapid cooling at a cooling rate of several hundred ° C./s or more is continued up to a temperature of around 700 ° C., so that the plate temperature cannot be easily controlled in the mass production process. In some cases.
[0013]
　Although the high-strength hot-rolled steel sheet disclosed in Patent Document 4 has high strength and good ductility and stretch flangeability, it is necessary to control the structural non-uniformity in the plate thickness direction, and it is necessary to control the structure non-uniformity in the plate thickness direction. It is presumed that the yield may decrease significantly.
[0014]
　The hot-rolled steel sheet disclosed in Patent Document 5 is manufactured under a condition that the winding temperature is 100 ° C. or higher and lower than 400 ° C. and the residence time in the temperature range where retained austenite is generated is not sufficiently secured. And may not be excellent in ductility (TS-EL balance).
[0015]
　The present invention has been made in view of the above 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 ductility and stretch flangeability. More preferably, it is an object of the present invention to provide a hot-rolled steel sheet having the above-mentioned characteristics and having a small material variation in the plate width direction.
　In the present invention, hot rolling is excellent in the above-mentioned various properties (strength, ductility and stretch flangeability) while satisfying low temperature toughness, which is a general property required for steel sheets applied to automobile parts and the like. An object of the present invention is to provide a steel plate.
Means to solve problems
[0016]
　In view of the above-mentioned problems, the present inventors have obtained the following findings (a) to (g) as a result of intensive studies on the chemical composition of the hot-rolled steel sheet and the relationship between the metallographic structure and the mechanical properties. The invention was completed.
[0017]
(A) In order to obtain excellent maximum tensile strength (hereinafter, may be referred to as strength or tensile strength), the metal structure is preferably hard, and in order to obtain excellent stretch flangeability, the metal structure is preferable. Is preferably homogeneous. Therefore, bainite and tempered martensite, which are hard and homogeneous structures, are suitable for the hot-rolled steel sheet to have both high strength and excellent elongation and flangeability. Bainite and tempered martensite are mainly used. It is important to have a metal structure with a small area fraction of ferrite, pearlite and martensite.
[0018]
(B) However, since bainite and tempered martensite are structures having poor ductility, excellent ductility cannot be ensured simply by using a metal structure mainly composed of these.
[0019]
(C) In order to make the hot-rolled steel sheet also have excellent ductility, it is effective to contain an appropriate amount of retained austenite that can enhance the ductility by transformation-induced plasticity (TRIP).
[0020]
(D) In ​​order to stabilize retained austenite at room temperature, it is effective to concentrate C diffused from bainite and tempered martensite during winding into austenite. Therefore, it is effective to secure the residence time in a specific temperature range after the transformation of bainite and tempered martensite is stopped. However, if this residence time becomes too long, austenite is decomposed and the amount of retained austenite decreases, so it is important to set an appropriate residence time.
[0021]
(E) When the coil is wound, the cooling rate differs greatly between the central portion in the plate width direction and the position on the end face side in the plate width direction, and there is a difference in the residence time after the martensitic transformation is stopped. The area fraction changes, which causes material variation in the plate width direction. The material variation in the plate width direction is the balance between tensile strength and ductility (TS × EL) in the central portion in the plate width direction and the position on the end face side in the plate width direction (a predetermined distance from the center portion to the end face side). It means the difference between the balance between tensile strength and ductility (TS × EL) in position).
[0022]
(F) By containing Nb, the time from the retention of martensitic transformation to the start of decomposition of austenite (transformation retention time) is significantly lengthened, so that the hot-rolled steel sheet when the coil is wound is extended. If the cooling rate between the central portion in the plate width direction and the end portion in the plate width direction of the hot-rolled steel sheet is controlled within a certain range, the material variation between the central position in the plate width direction and the position on the end face side in the plate width direction can be reduced. be able to.
[0023]
(G) Residual austenite can increase ductility by transformation-induced plasticity (TRIP), but transforms into hard martensite by transformation-induced plasticity (TRIP) to reduce toughness. In the case of martensite as the mother phase, the minimum low temperature toughness required for steel sheets for undercarriage parts of automobiles cannot be obtained. However, low-temperature toughness is ensured by refining the average crystal grain size of the metal structure and by precipitating an appropriate amount of iron-based carbides to reduce the amount of solid-dissolved C in the matrix as bainite or tempered martensite. Can be done.
[0024]
　The gist of the present invention made based on the above findings is as follows.
[0025]
(1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition of% by mass,
C: 0.100 to 0.250%,
Si: 0.05 to 3.00%,
Mn: 1.00. ~ 4.00%,
Nb: 0.005 ~ 0.050%,
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 %,
V: 0 to 0.500%,
Cu: 0 to 2.00%,
Cr: 0 to 2.00%,
Mo: 0 to 1.000%,
Ni: 0 to 2.00%,
B: 0 ~ 0.0100%,
Ca: 0 ~ 0.0200%,
Mg: 0 ~ 0.0200%,
REM: 0 ~ 0.1000%,
Bi: 0 ~ 0.020%,
Zr, Co, Zn and W One or more of them: 0 to 1.00% in total and
Sn: 0 to 0.050%, the
　balance consisting of Fe and impurities.
　A total of 77.0 to 97. Bainite and tempered martensite in a plate width cross section parallel to the rolling direction, with a metal structure at a depth of 1/4 of the plate thickness from the surface and at the center position in the plate width direction in% area. The metal containing 0%, 0 to 5.0% ferrite, 0 to 5.0% pearlite, 3.0% or more retained austenite, 0 to 10.0% martensite, and excluding the retained austenite. The average crystal grain size of the structure is 7.0 μm or less, the C concentration in the retained austenite is 0.5% by mass or more, and the number density of iron-based carbides having a diameter of 20 nm or more is 1.0 × 10 6 pieces / mm 2 or more.
(2) The hot-rolled steel plate according to (1) above has a plate width cross section parallel to the rolling direction, a depth of 1/4 of the plate thickness from the surface and a central position in the plate width direction, and the plate from the surface. 1/4 depth of thickness and 300 mm from the center position in the plate width direction to one end side in the plate width direction, 1/4 depth of the plate thickness from the surface and from the center position in the plate width direction to the plate width direction 600 mm position on one end side, 1/4 depth from the surface to the plate thickness and 300 mm position from the center position in the plate width direction to the other end side in the plate width direction, and 1/4 of the plate thickness from the surface. When the residual austenite in the metal structure at the depth and 600 mm from the center position in the plate width direction to the other end side in the plate width direction is γ, γ D1 , γ D2 , γ W1 and γ W2 in area%, respectively . γ / γ D1 , γ / γ D2, Γ / γ W1 and γ / γ W2 are 0.8 or more and less than 1.2, respectively, at a
　depth of 1/4 of the plate thickness from the surface and at the center position in the plate width direction, and from the surface to the plate thickness. 1/4 depth and 300 mm from the center position in the plate width direction to the one end side in the plate width direction, 1/4 depth from the surface to the plate thickness and from the center position in the plate width direction to the plate width direction. A position of 600 mm on one end side, a position of 1/4 of the plate thickness from the surface and a position of 300 mm on the other end side of the plate width direction from the center position in the plate width direction, and 1 / of the plate thickness from the surface. C γC , C γD1 , C γD2 , C γW1 in mass% of the C concentration in the retained austenite in the metal structure at a depth of 4 and 600 mm from the center position in the plate width direction to the other end side in the plate width direction. And C γW2 , C γC / C γD1 , C γC / C γD2 , C γC / C γW1 and C γC / C γW2 may be 0.8 or more and less than 1.2, respectively.
(3) The hot-rolled steel sheet according to (1) or (2) above has a chemical composition of% by mass,
Ti: 0.005 to 0.300%,
V: 0.005 to 0.500%, and so on.
Cu: 0.01 to 2.00%,
Cr: 0.01 to 2.00%,
Mo: 0.010 to 1.000%,
Ni: 0.02 to 2.00%,
B: 0.0001 to
% 0.0100,
Ca: 0.0005 ~
0.0200%, Mg: 0.0005 ~ 0.0200%, REM: 0.0005 ~ 0.1000%, and
Bi: 0.0005 ~ 0.020%
from It may contain one or more selected from the group.
The invention's effect
[0026]
　According to the above aspect according to the present invention, it is possible to provide a hot-rolled steel sheet having excellent strength, ductility, stretch flangeability and low temperature toughness. Further, according to a preferred embodiment of the present invention, it is possible to provide a hot-rolled steel sheet having the above-mentioned various characteristics and having a small material variation in the plate width direction.
　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.
Mode for carrying out the invention
[0027]
　The chemical composition and metallographic structure of the hot-rolled steel sheet (hereinafter, may be simply referred to as a steel sheet) according to the present embodiment will be specifically described below. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications can be made without departing from the spirit of the present invention.
　The numerical limitation range described below includes the lower limit value and the upper limit value. Numerical values ​​that indicate "less than" or "greater than" do not fall within the numerical range. In the following description,% regarding the chemical composition of the steel sheet is mass% unless otherwise specified.
[0028]
1. 1. Chemical composition
　The hot-rolled steel sheet according to this embodiment has C: 0.100 to 0.250%, Si: 0.05 to 3.00%, Mn: 1.00 to 4.00%, Nb in mass%. : 0.005 to 0.050%, sol. Al: 0.001 to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less and O: 0.0100% or less, and the balance is Fe and Consists of impurities. Each element will be described in detail below.
[0029]
(1-1) C: 0.100 to 0.250%
　C has an action of promoting the formation of bainite and an action of stabilizing retained austenite. If the C content is less than 0.100%, it becomes difficult to obtain the desired bainite surface integral and retained austenite surface integral. Failure to obtain the desired bainite surface integral may make it difficult to obtain the desired bainite and tempered martensite surface integral. Therefore, the C content is set to 0.100% or more. The C content is preferably 0.120% or more and 0.150% or more. On the other hand, when the C content exceeds 0.250%, pearlite is preferentially produced and the formation of bainite and retained austenite becomes insufficient, and a desired surface integral of bainite and an area fraction of retained austenite can be obtained. It becomes difficult. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.220% or less.
[0030]
(1-2) Si: 0.05 to 3.00%
　Si has an action of delaying the precipitation of cementite. By this action, the amount of austenite remaining untransformed, that is, the surface integral of the retained austenite can be increased, and the strength of the steel sheet can be increased by solid solution strengthening. Further, Si has an action of making the steel sound by deoxidation (suppressing the occurrence of defects such as blow holes in the steel). If the Si content is less than 0.05%, the effect of the above action cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.50% or more and 1.00% or more. However, the Si content is 3.00% greater than the surface texture and chemical conversion treatability of the steel sheet, and further with ductility and weldability is significantly degraded, A 3 transformation point increases significantly. This makes it difficult to perform hot rolling in a stable manner. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.70% or less and 2.50% or less.
[0031]
(1-3) Mn: 1.00 to 4.00%
　Mn has an action of suppressing ferrite transformation and promoting the formation of bainite. If the Mn content is less than 1.00%, the desired surface integral of bainite cannot be obtained. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.50% or more, more preferably 1.80% or more. On the other hand, when the Mn content exceeds 4.00%, the completion of the bainite transformation is delayed, so that carbon concentration to austenite is not promoted, the formation of retained austenite becomes insufficient, and the desired surface integral of retained austenite is used. It becomes difficult to obtain the rate. Furthermore, it becomes difficult to increase the C concentration in the retained austenite. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less and 3.50% or less.
[0032]
(1-4) Nb: 0.005 to 0.050% In
　this embodiment, Nb is an important element. Nb is usually contained in steel for the purpose of precipitating and strengthening ferrite with carbides and for the purpose of refining the austenite grain size by controlled rolling. In addition to these effects, the present inventors have newly added that Nb has an effect of significantly increasing the time from the transformation retention of bainite and tempered martensite to the start of decomposition of austenite (transformation retention time). Found in. Due to the prolonged transformation retention time, it becomes difficult for austenite to decompose into cementite and martensite after the winding process, and even if the difference in cooling rate in the plate width direction of the hot-rolled steel sheet is large, the area of ​​retained austenite The rate can be kept constant. That is, when the coil is wound, the average cooling rate at the central portion of the hot-rolled steel sheet having a relatively slow cooling rate in the plate width direction and the end portion of the hot-rolled steel sheet having a relatively high cooling rate in the plate width direction is determined. If the range is controlled, material variation can be reduced.
[0033]
　The mechanism of lengthening the transformation retention time by Nb is not clear, but it is considered that when retained austenite is decomposed to form ferrite, Nb carbides are precipitated and the further growth of ferrite is delayed. Since the above effect is exhibited when the Nb content is 0.005% or more, the Nb content is set to 0.005% or more. The Nb content is preferably 0.010% or more and 0.015% or more. On the other hand, when the Nb content exceeds 0.050%, the effect of prolonging the transformation retention time is saturated, recrystallization of austenite during rolling is suppressed, and bainite or tempered martensite and retained austenite are layered. Since it is generated, the stretch flangeability of the steel sheet is reduced. Therefore, the Nb content is set to 0.050% or less. The Nb content is preferably 0.040% or less and 0.030% or less.
[0034]
(1-5) sol. Al: 0.001 to 2.000%
　Al, like Si, has the effect of deoxidizing the steel to make the steel sheet sound, and suppresses the precipitation of cementite from austenite to form retained austenite. Has the effect of promoting. sol. If the Al content is less than 0.001%, the effect of the above action cannot be obtained. Therefore, sol. The Al content is 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 effects are saturated and economically unfavorable. The Al content is 2.000% or less. sol. The Al content is preferably 1.500% or less and 1.300% or less. In addition, sol. Al is an abbreviation for soluble Al.
[0035]
(1-6) P: 0.100% or less
　P is an element generally contained as an impurity, but it is also an element having an action of increasing the strength by strengthening the solid solution. Therefore, P may be positively contained, but P is an element that is easily segregated, and when the P content exceeds 0.100%, the moldability and toughness are significantly reduced due to grain boundary segregation. Become. Therefore, the P content is limited to 0.100% or less. The P content is preferably 0.030% or less. The lower limit of the P content does not need to be specified, but is preferably 0.001% from the viewpoint of refining cost.
[0036]
(1-7) S: 0.0300% or less
　S is an element contained as an impurity and forms sulfide-based inclusions in the steel to reduce the formability of the hot-rolled steel sheet. When the S content exceeds 0.0300%, the moldability of the steel sheet is significantly lowered. Therefore, the S content is limited to 0.0300% or less. The S content is preferably 0.0050% or less. The lower limit of the S content does not need to be specified, but is preferably 0.0001% from the viewpoint of refining cost.
[0037]
(1-8) N: 0.1000% or less
　N is an element contained in steel as an impurity and has an action of lowering the moldability of the steel sheet. When the N content exceeds 0.1000%, the moldability of the steel sheet is significantly lowered. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0800% or less, and more preferably 0.0700% or less. The lower limit of the N content does not need to be specified in particular, but as will be described later, when one or more of Ti and V are contained to refine the metal structure, precipitation of carbonitride is promoted. The N content is preferably 0.0010% or more, and more preferably 0.0020% or more.
[0038]
(1-9) O: 0.0100% or less
　O forms a coarse oxide that becomes a starting point of fracture when it is contained in a large amount in steel, and causes brittle fracture and hydrogen-induced cracking. Therefore, the O content is limited 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 the molten steel is deoxidized.
[0039]
　The balance of the chemical composition of the hot-rolled steel sheet according to the present embodiment consists of Fe and impurities. In the present embodiment, the impurities mean those mixed from ore as a raw material, scrap, manufacturing environment, etc., and are allowed as long as they do not adversely affect the hot-rolled steel sheet according to the present embodiment. do.
[0040]
　In addition to the above elements, the hot-rolled steel sheet according to the present embodiment contains Ti, V, Cu, Cr, Mo, Ni, B, Ca, Mg, REM, Bi, Zr, Co, Zn, W and Sn as optional elements. It may be contained. When the above optional element is not contained, the lower limit of the content is 0%. Hereinafter, the above optional elements will be described in detail.
[0041]
(1-10) Ti: 0.005 to 0.300% and V: 0.005 to 0.500%
　Both Ti and V are precipitated as carbides or nitrides in steel and are metal due to the pinning effect. Since it has an action of making the structure finer, these elements may be contained as needed. In order to obtain the effect of the above action more reliably, it is preferable that the Ti content is 0.005% or more, or the V content is 0.005% or more. However, even if these elements are excessively contained, the effect of the above action is saturated and it is economically unfavorable. Therefore, the Ti content is 0.300% or less, and the V content is 0.500% or less.
[0042]
(1-11) Cu: 0.01 to 2.00%, Cr: 0.01 to 2.00%, Mo: 0.010 to 1.000%, Ni: 0.02 to 2.00% and B : 0.0001 to 0.0100%
　Cu, Cr, Mo, Ni and B all have an effect of enhancing the hardenability of the steel sheet. Further, Cr and Ni have an action of stabilizing retained austenite, and Cu and Mo have an action of precipitating carbides in steel to increase the strength. Further, Ni has an effect of effectively suppressing the grain boundary cracking of the slab caused by Cu when Cu is contained. Therefore, these elements may be contained as needed.
[0043]
　Cu has an action of enhancing the hardenability of the steel sheet and an action of precipitating as carbide in the steel at a low temperature to increase the strength of the steel sheet. In order to obtain the effect of the above action more reliably, the Cu content is preferably 0.01% or more, and more preferably 0.05% or more. However, if the Cu content exceeds 2.00%, grain boundary cracks in 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.
[0044]
　As described above, Cr has an action of enhancing the hardenability of the steel sheet and an action of stabilizing retained austenite. In order to obtain the effect of the above action more reliably, the Cr content is preferably 0.01% or more and 0.05% or more. However, when the Cr content exceeds 2.00%, the chemical conversion treatment property of the steel sheet is significantly lowered. Therefore, the Cr content is set to 2.00% or less.
[0045]
　As described above, Mo has an action of enhancing the hardenability of the steel sheet and an action of precipitating carbides in the steel to increase the strength. In order to obtain the effect of the above action more reliably, the Mo content is preferably 0.010% or more and 0.020% or more. However, even if the Mo content exceeds 1.000%, the effect of the above action is saturated and economically unfavorable. Therefore, the Mo content is set to 1.000% or less. The Mo content is preferably 0.500% or less and 0.200% or less.
[0046]
　As described above, Ni has an effect of enhancing the hardenability of the steel sheet. Further, when Ni contains Cu, it has an effect of effectively suppressing the grain boundary cracking of the slab caused by Cu. In order to obtain the effect of the above action more reliably, the Ni content is preferably 0.02% or more. Since Ni is an expensive element, it is economically unfavorable to contain it in a large amount. Therefore, the Ni content is set to 2.00% or less.
[0047]
　As described above, B has an effect of enhancing the hardenability of the steel sheet. In order to obtain the effect of this action more reliably, the B content is preferably 0.0001% or more and 0.0002% or more. However, if the B content exceeds 0.0100%, the moldability of the steel sheet is significantly lowered, so the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less.
[0048]
(1-12) 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 an effect of improving the formability of the steel sheet by adjusting the shape of the inclusions to a preferable shape. In addition, Bi has an effect of improving the formability of the steel sheet by miniaturizing the solidified structure. Therefore, these elements may be contained as needed. In order to obtain the effect of the above action more reliably, it is preferable that any one or more of Ca, Mg, REM and Bi is 0.0005% or more. However, when the Ca content or Mg content exceeds 0.0200%, or when the REM content exceeds 0.1000%, inclusions are excessively generated in the steel, which in turn lowers the formability of the steel sheet. In some cases. Further, even if the Bi content exceeds 0.020%, the effect of the above action is saturated, which is economically unfavorable. Therefore, the Ca content and Mg content are 0.0200% or less, the REM content is 0.1000% or less, and the Bi content is 0.020% or less. The Bi content is preferably 0.010% or less.
　Here, REM refers to a total of 17 elements composed of Sc, Y and lanthanoid, and the content of REM refers to the total content of these elements. In the case of lanthanoids, they are industrially added in the form of misch metal.
[0049]
(1-13) One or more of Zr, Co, Zn and W: 0 to 1.00% in total and Sn: 0 to 0.050% For
　Zr, Co, Zn and W, the present inventors. Et al. Have 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 of 1.00% or less.
　Further, the present inventors have confirmed that the effect of the hot-rolled steel sheet according to the present embodiment is not impaired even if a small amount of Sn is contained, but flaws may occur during hot rolling. The Sn content is 0.050% or less.
[0050]
2. Metallic 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, in the plate width cross section parallel to the rolling direction, the metal structure at a depth of 1/4 of the plate thickness from the surface and at the center position in the plate width direction is an area fraction (area%). Bainite and tempered martensite total 77.0-97.0%, ferrite 0-5.0%, pearlite 0-5.0%, retained austenite 3.0% or more, martensite 0- By containing 10.0%, a maximum tensile strength of 980 MPa or more and high press formability (ductility and stretch flangeability) can be obtained. In the present embodiment, the reason for defining the metal structure at the depth of 1/4 of the plate thickness from the surface and the center position in the plate width direction of the plate width cross section parallel to the rolling direction is that the metal structure at this position is a steel plate. This is because it shows a typical metal structure of. Here, the plate width cross section parallel to the rolling direction refers to a cross section parallel to the rolling direction, parallel to the plate thickness direction, and perpendicular to the plate width direction (so-called L cross section).
[0051]
(2-1) Total area fraction of
　bainite and tempered martensite : 77.0-97.0% Bainite and tempered martensite are the most important metallographic structures in this embodiment.
　Bainite is a collection of lath-shaped crystal grains. The bainite includes an upper bainite which is an aggregate of laths containing carbides between laths and a lower bainite containing iron-based carbides having a major axis of 5 nm or more inside. The iron-based carbides precipitated in the lower bainite belong to a single variant, i.e., a group of iron-based carbides extending in the same direction. Tempering martensite is a collection of lath-shaped crystal grains, and contains iron-based carbides having a major axis of 5 nm or more inside. The iron carbides in tempered martensite belong to a plurality of variants, i.e., a group of iron carbides extending in different directions.
[0052]
　As described above, bainite and tempered martensite are hard and homogeneous metal structures, which are suitable for making steel sheets have both high strength and excellent stretch flangeability. If the total surface integral of bainite and tempered martensite is less than 77.0%, the steel sheet cannot have both high strength and excellent stretch flangeability. Therefore, the total surface integral of bainite and tempered martensite shall be 77.0% or more. The total surface integral of bainite and tempered martensite is preferably 85.0% or more, more preferably 90.0% or more. Since the hot-rolled steel sheet according to the present embodiment contains 3.0% or more of retained austenite, the total area fraction of bainite and tempered martensite is 97.0% or less.
[0053]
(2-2) Surface integral of ferrite: 0 to 5.0%
　Ferrite is a massive crystal grain and has a metal structure that does not contain a substructure such as lath inside. When the area fraction of soft ferrite exceeds 5.0%, the interface between ferrite and bainite or tempered martensite, which tends to be the starting point of void generation, and the interface between ferrite and retained austenite increase, especially in steel plates. Stretchable flangeability is reduced. Therefore, the surface integral of ferrite shall be 5.0% or less. The surface integral of ferrite is preferably 4.0% or less, 3.0% or less, and less than 2.0%. In order to improve the stretch flangeability of the steel sheet, the surface integral of ferrite is preferably reduced as much as possible, and the lower limit thereof is 0%.
[0054]
(2-3) Area fraction of pearlite: 0 to 5.0%
　Pearlite is a lamellar metal structure in which cementite is deposited in layers between ferrites, and is a soft metal structure as compared with bainite. When the area fraction of pearlite exceeds 5.0%, the interface between pearlite and bainite or tempered martensite, which tends to be the starting point of voids, and the interface between pearlite and retained austenite increase, resulting in elongation of the steel plate in particular. Flangeability is reduced. Therefore, the surface integral of pearlite is set to 5.0% or less. The surface integral of pearlite is preferably 4.0% or less, 3.0% or less, and 2.0% or less. In order to improve the stretch flangeability of the steel sheet, the surface integral of the pearlite is preferably reduced as much as possible, and the lower limit thereof is 0%.
[0055]
(2-4) Area fraction of martensite: 0 to 10.0% In the
　present embodiment, martensite is defined as a metal structure in which carbides having a diameter of 5 nm or more are not precipitated between laths and in laths. Martensite is a very hard structure and greatly contributes to increasing the strength of steel sheets. On the other hand, when martensite is contained in the metal structure, the interface between martensite and bainite and tempered martensite, which are the parent phases, becomes the starting point of void generation, and the stretch flangeability of the steel sheet is particularly deteriorated. Further, since martensite has a hard structure, it deteriorates the low temperature toughness of the steel sheet. Therefore, the surface integral ratio of martensite shall be 10.0% or less. Preferably, it is 8% or less, 6% or less, and 3% or less. Since the hot-rolled steel sheet according to the present embodiment contains a predetermined amount of bainite and tempered martensite, a desired strength can be ensured even when martensite is not contained. In order to obtain the desired stretch flangeability, the surface integral of martensite is preferably reduced as much as possible, and the lower limit thereof is 0%.
[0056]
　The bainite, tempered martensite, ferrite, pearlite, and martensite constituting the metal structure of the hot-rolled steel sheet according to the present embodiment as described above can be identified by the following methods, confirmed of their existence positions, and have an area. Measure the fraction.
[0057]
　First, a Nital reagent and a reagent disclosed in JP-A-59-219473 are used to corrode a plate width cross section parallel to the rolling direction. Regarding the corrosion of the plate width cross section, specifically, a solution prepared by dissolving 1 to 5 g of picric acid in 100 ml of ethanol was used as liquid A, and 1 to 25 g of sodium thiosulfate and 1 to 5 g of citric acid were added to 100 ml of water. The dissolved solution is referred to as solution B, and solution A and solution B are mixed at a ratio of 1: 1 to form a mixed solution, and 1.5 to 4% of nitric acid is further added to the total amount of this mixed solution. The mixed solution is used as the pretreatment solution. Further, a liquid obtained by adding and mixing the above-mentioned pretreatment liquid in a ratio of 10% with respect to the total amount of the 2% nital liquid to the 2% nital liquid is used as a post-treatment liquid. The cross section of the plate width parallel to the rolling direction is immersed in the pretreatment liquid for 3 to 15 seconds, washed with alcohol and dried, then immersed in the posttreatment liquid for 3 to 20 seconds, washed with water and dried. , The above-mentioned plate width cross section is corroded. In addition,% about the reagent is all volume%, and the ratio is a volume ratio.
[0058]
　Next, by observing at least three regions of 40 μm × 30 μm at a magnification of 1000 to 100,000 times using a scanning electron microscope at a depth of 1/4 of the plate thickness and at the center position in the plate width direction from the surface of the steel plate. The metallographic structure is identified, the position of the metal structure is confirmed, and the area fraction is measured. Since it is difficult to distinguish between lower bainite and tempered martensite by the above-mentioned measurement method, it is not necessary to distinguish between the two in this embodiment. That is, the total area fraction of "bainite and tempered martensite" is obtained by measuring the area fraction of "upper bainite" and "lower bainite or tempered martensite". As described above, the upper bainite is an aggregate of laths and has a structure containing carbides between the laths, and the lower bainite is a structure containing iron-based carbides having a major axis of 5 nm or more and extending in the same direction inside. Tempered martensite is a collection of lath-shaped crystal grains, and is a structure containing iron-based carbides having a major axis of 5 nm or more and extending in different directions.
[0059]
(2-5)
　Surface integral of retained austenite : 3.0% or more Retained austenite is a metal structure that exists as a face-centered cubic lattice even at room temperature. Residual austenite has the effect of increasing the ductility of the steel sheet due to transformation-induced plasticity (TRIP). If the surface integral of the retained austenite is less than 3.0%, the effect of the above action cannot be obtained and the ductility of the steel sheet deteriorates. Therefore, the surface integral of retained austenite is set to 3.0% or more. The surface integral of the retained austenite is preferably 5.0% or more, more preferably 7.0% or more, still more preferably 8.0% or more. The upper limit of the area fraction of retained austenite does not need to be specified in particular, but since the area fraction of retained austenite that can be secured in the chemical composition of the hot-rolled steel sheet according to the present embodiment is approximately 20.0%, retained austenite. The upper limit of the area fraction of is 20.0%.
[0060]
　Methods for measuring the area fraction of retained austenite include X-ray diffraction, EBSP (Electron Backscattering Diffraction) analysis, and magnetic measurement methods, and the measured values ​​may differ depending on the measurement method. .. In this embodiment, the surface integral of retained austenite is measured by X-ray diffraction.
　In the measurement of the residual austenite surface integral by X-ray diffraction in the present embodiment, first, a Co-Kα ray is used in a plate width cross section parallel to the rolling direction at a depth position of 1/4 of the plate thickness of the steel plate. 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 and calculated using the intensity averaging method to remain. Obtain the surface integral of austenite.
[0061]
　In this embodiment, in order to measure the area fraction of bainite, tempered martensite, ferrite, pearlite and martensite (area fraction other than retained austenite) and the area fraction of retained austenite by different measuring methods. , The total of the above two area fractions may not be 100.0%. If the total of the surface integrals other than the retained austenite and the surface integrals of the retained austenite does not reach 100.0%, the above two surface integrals are adjusted so that the total becomes 100.0%. For example, when the total of the area fraction other than retained austenite and the area fraction of retained austenite is 101.0%, in order to make the total of both 100.0%, other than the retained austenite obtained by measurement. The value obtained by multiplying the area fraction of the above by 100.0 / 101.0 is defined as the area fraction other than retained austenite, and the area fraction of the retained austenite obtained by the measurement is multiplied by 100.0 / 101.0. The value is defined as the area fraction of retained austenite.
　If the sum of the surface integrals other than the retained austenite and the area fractions of the retained austenite is less than 95.0% or more than 105.0%, the area fractions are measured again.
[0062]
(2-6) Average crystal grain size of
　metal structure excluding retained austenite : 7.0 μm or less Average of metal structure excluding retained austenite (main phase bainite and tempered martensite, ferrite, pearlite and martensite) By making the crystal grain size (hereinafter, sometimes simply referred to as the average crystal grain size) finer, the low temperature toughness of the steel sheet is improved. If the average crystal grain size exceeds 7.0 μm, vTrs ≦ −50 ° C., which is an index of low temperature toughness required for steel sheets for undercarriage parts of automobiles, cannot be satisfied. Therefore, the average crystal grain size is set to 7.0 μm or less. It is not necessary to limit the lower limit of the average crystal grain size. The smaller the average crystal grain size is, the more preferable it is. However, since it may be practically difficult to set the average crystal grain size to less than 1.0 μm from the viewpoint of manufacturing equipment, the average crystal grain size may be 1.0 μm or more. ..
[0063]
　In the present embodiment, the crystal grains are defined by using the EBSP-OIM (Electron Backscatter Diffraction Pattern Microscopic) method. In the EBSP-OIM method, a highly inclined sample is irradiated with an electron beam 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 waiting time. The EBSP-OIM method is performed using a device that combines a scanning electron microscope and an EBSP analyzer and an OIM Analysis (registered trademark) manufactured by AMETEK. In the EBSP-OIM method, the fine structure and crystal orientation of the sample surface can be quantitatively analyzed. The analyzable area of ​​the EBSP-OIM method is an area that can be observed by SEM. Although it depends on the resolution of the SEM, according to the EBSP-OIM method, analysis can be performed with a minimum resolution of 20 nm. Since the threshold value of the large-angle grain boundary, which is generally recognized as a crystal grain boundary, is 15 °, in the present embodiment, a crystal grain having an orientation difference of 15 ° or more between adjacent crystal grains is defined as one crystal grain. The crystal grains are visualized from the mapped image, and the average crystal grain size of the area average calculated by OIM Analysis is obtained.
[0064]
　In measuring the average crystal grain size of the metal structure at a depth of 1/4 of the plate thickness from the surface of the steel plate and at the center position in the plate width direction in the plate width cross section parallel to the rolling direction, a magnification of 1200 times, a region of 40 μm × 30 μm. Then, the effective grain size of the crystal grains in at least 10 visual fields is measured, and the average of the effective crystal grain sizes is taken as the average crystal grain size. In this measurement method, since the area fraction is small for structures other than the main phase, it is judged that the effect is small, and the average grain boundaries of bainite and tempered martensite, which are the main phases, and ferrite, pearlite, and martensite Does not distinguish from average grain size. That is, the average crystal grain size measured by the above-mentioned measuring method is the average crystal grain size of bainite, tempered martensite, ferrite, pearlite and martensite. In the measurement of the effective crystal grain size of pearlite, the effective crystal grain size of ferrite in pearlite is measured instead of the effective crystal grain size of the pearlite block.
[0065]
(2-7) C concentration in retained austenite: 0.5% by mass or more By
　setting the C concentration (carbon concentration) in retained austenite to 0.5% by mass or more, the retained austenite is moderately stabilized and the late deformation stage. Since a large amount of deformation-induced plasticity (TRIP) occurs in the high strain region of the steel plate, the ductility and elongation flangeability of the steel plate can be improved. Therefore, the C concentration in the retained austenite is set to 0.5% by mass or more. The C concentration in the retained austenite is more preferably 0.7% by mass or more. Further, by setting the C concentration in the retained austenite to 2.0% by mass or less, excessive stabilization of the retained austenite can be suppressed, and transformation-induced plasticity (TRIP) can be more reliably expressed. Therefore, the C concentration in the retained austenite is preferably 2.0% by mass or less.
[0066]
　The C concentration in the retained austenite is determined by X-ray diffraction. Specifically, X-ray analysis with Cu—Kα rays was performed on the metal structure at a depth of 1/4 of the plate thickness from the steel plate surface and the center position in the plate width direction in the plate width cross section parallel to the rolling direction, and retained austenite. The lattice constant a (unit: angstrom) is obtained from the reflection angles of the (200) plane, (220) plane, and (311) plane, and the C concentration (Cγ) in the retained austenite is calculated according to the following equation (1).
[0067]
　Cγ = (a-3.572) /0.033 ... (1)
[0068]
(2-8) Number of iron-based carbides with a diameter of 20 nm or more Density: 1.0 × 10 6 pieces / mm 2 or
　more Iron-based carbides with a diameter of 20 nm or more are contained in steel 1.0 × 10 6 pieces / mm 2 or more The reason for this is to increase the low temperature toughness of the matrix and to obtain a balance between excellent strength and low temperature toughness.
　When the matrix of the steel sheet is martensite as hardened, the strength is excellent but the low temperature toughness is poor, so that the low temperature toughness is desired to be improved. Therefore, by precipitating a predetermined number or more of iron-based carbides in the steel, the low-temperature toughness of the main phase is improved, and the low-temperature toughness (vTrs ≦ −50 ° C.) required for steel sheets for undercarriage parts of automobiles is achieved. The iron-based carbide in the present embodiment means that the length of the major axis is less than 1 μm. That is, the coarse carbides precipitated between cementite and bainite lath in pearlite having a major axis length of 1 μm or more are not included in the iron-based carbides.
[0069]
　When the present inventors investigated the relationship between the low temperature toughness of hot-rolled steel sheets and the number density of iron-based carbides, it was excellent to set the number density of iron-based carbides to 1.0 × 10 6 pieces / mm 2 or more. It was clarified that low temperature toughness can be obtained. Therefore, in the present embodiment, the number density of iron-based carbides is 1.0 × in the metal structure at a depth of 1/4 of the plate thickness from the steel plate surface and at the center position in the plate width direction in the plate width cross section parallel to the rolling direction. 10 6 pieces / mm 2 or more. The number density of iron-based carbides is preferably 5.0 × 10 6 pieces / mm 2 or more, and more preferably 1.0 × 10 7 pieces / mm 2 or more. The number density of iron-based carbides may be 1.0 × 10 10 pieces / mm 2 or less. This is because if the number density of iron-based carbides exceeds 1.0 × 10 10 pieces / mm 2 , carbon concentration in the retained austenite does not occur, and the carbon concentration in the retained austenite may decrease.
　Further, the size of the iron-based carbides precipitated on the hot-rolled steel sheet according to the present embodiment is as small as 300 nm or less, and most of them are deposited in the lath of martensite and bainite, so it is estimated that the low temperature toughness of the steel sheet is not deteriorated. Will be done.
[0070]
　To measure the number density of iron-based carbides, a sample is taken with the plate width cross section parallel to the rolling direction as the observation surface, the observation surface is polished, night-tar etching is performed, and the depth is 1/4 of the plate thickness from the steel plate surface. This is performed by observing a plate thickness in the range of 1/8 to 3/8 centered on the center position in the plate width direction with a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope). The number density of iron-based carbides is obtained by observing with a magnification of 20000 times and 10 or more fields of view, measuring the number density of iron-based carbides, and calculating the average thereof.
[0071]
(2-9) γ / γ D1 , γ / γ D2 , γ / γ W1 and γ / γ W2 : 0.8 or more and less than 1.2, and C γC / C γD1 , C γC / C γD2 , C γC / C γW1 and C γC / C γW2 : 0.8 or more and less than 1.2
　Plate width cross section parallel to the rolling direction, 1/4 depth from the surface to the plate width and center position in the plate width direction, surface to plate thickness 300 mm from the center position in the plate width direction to one end side in the plate width direction, 1/4 depth from the surface and 600 mm from the center position in the plate width direction to one end side in the plate width direction. Position, 1/4 depth from the surface to the plate thickness and 300 mm from the center position in the plate width direction to the other end side in the plate width direction, 1/4 depth from the surface to the plate thickness and the plate width from the center position in the plate width direction The area fractions of retained austenite in the metallographic structure 600 mm on the other end side of the direction are γ, γ D1 , γ D2 , and γ W1 , respectively. And γ W2 , γ / γ D1 , γ / γ D2 , γ / γ W1 and γ / γ W2 are 0.8 or more and less than 1.2, respectively, and the depth from the surface to 1/4 of the plate thickness. And the center position in the plate width direction, the depth of 1/4 of the plate thickness from the surface and the position of 300 mm from the center position in the plate width direction to one end side in the plate width direction, the depth of 1/4 of the plate thickness from the surface and the center in the plate width direction. 600 mm from the position to one end in the plate width direction, 1/4 depth from the surface to the plate thickness and 300 mm from the center position in the plate width direction to the other end in the plate width direction, and 1/4 of the plate thickness from the surface. depth and sheet width direction center position respectively the C concentration of the retained austenite in the metallic structure of the position of 600mm on the other end side in the plate width direction in mass% from C rC , C Ganmadi1 , C Ganmadi2 , C Ganmadaburyu1 and C Ganmadaburyu2 and C γC / C γD1 , C γC / C γD2 , C γC / C γW1When C γC / C γW2 is 0.8 or more and less than 1.2, respectively, the material variation between the center position in the plate width direction and the position on the end face side in the plate width direction can be further reduced. When the values ​​of γ / γ D1 , γ / γ D2 , γ / γ W1 and γ / γ W2 do not satisfy the above conditions, the frequency of occurrence of the transformation-induced plasticity (TRIP) phenomenon differs depending on the plate width direction. The product of strength and ductility varies widely, which may cause a decrease in yield. Further, when the values ​​of C γC / C γD1 , C γC / C γD2 , C γC / C γW1 and C γC / C γW2 do not satisfy the above conditions, the stability of retained austenite differs depending on the plate width direction. , The product of strength and ductility varies widely, which may cause a decrease in yield. In the present embodiment, the other end side in the plate width direction refers to the opposite side of the one end side in the plate width direction.
[0072]
　In the plate width cross section parallel to the rolling direction, 1/4 depth from the surface and the center position in the plate width direction, 1/4 depth from the surface and the center position in the plate width direction to one end side in the plate width direction. 300 mm from the surface, 1/4 depth from the surface and 600 mm from the center position in the width direction to one end in the width direction, 1/4 depth from the surface and the center position in the width direction. Area fraction of retained austenite in the metal structure at a position of 300 mm on the other end side in the plate width direction and a depth of 1/4 of the plate thickness from the surface and a position 600 mm on the other end side in the plate width direction from the center position in the plate width direction. (Γ, γ D1 , γ D2 , γ W1 and γ W2 ), and C concentration in% by mass in retained austenite in the metallographic structure at each of the above positions (C γC , C γD1 , C γD2 , C γW1 and C γW2). ) Is measured at each position by the above-mentioned method for measuring the area fraction of retained austenite and the method for measuring the C concentration in retained austenite.
[0073]
3. 3.
　Plate thickness The plate thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be 1.2 to 8.0 mm. If the thickness of the hot-rolled steel sheet is less than 1.2 mm, it may be difficult to secure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the hot-rolled steel sheet according to the present invention may be 1.2 mm or more. It is preferably 1.4 mm or more. On the other hand, if the plate thickness exceeds 8.0 mm, it may be difficult to miniaturize the metal structure, and it may be difficult to secure the above-mentioned metal structure. Therefore, the plate thickness may be 8.0 mm or less. It is preferably 6.0 mm or less.
[0074]
4. Other
(4-1) Plating Layer The
　hot-rolled steel sheet according to the present embodiment having the above-mentioned chemical composition and metal structure may be provided with a plating layer on the surface for the purpose of improving corrosion resistance or the like to be a surface-treated steel sheet. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include electrogalvanization and electroZn—Ni alloy plating. Examples of the hot-dip plating layer include hot-dip zinc plating, alloyed hot-dip zinc plating, 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. NS. The amount of plating adhered is not particularly limited and may be the same as the conventional one. Further, it is also possible to further enhance the corrosion resistance by subjecting an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment liquid) after plating.
[0075]
5. Manufacturing Conditions
　A suitable manufacturing method for the hot-rolled steel sheet according to the present embodiment having the above-mentioned chemical composition and metal structure is as follows.
[0076]
　In order to obtain the hot-rolled steel sheet according to the present embodiment, the hot-rolled steel sheet is hot-rolled under predetermined conditions, cooled to a predetermined temperature range, wound, and then the end portion of the hot-rolled steel sheet in the plate width direction and heat. It is important to control the cooling history of the central part of the rolled steel sheet in the plate width direction.
[0077]
　In a preferred method for producing a hot-rolled steel sheet according to the present 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 heated to a temperature T1 (° C.) or higher represented by the following formula (2).
(2) Hot rolling is performed in a temperature range of 850 to 1100 ° C. so that the total plate thickness is reduced by 90% or more.
(3) Hot rolling is completed at a temperature T2 (° C.) or higher represented by the following formula (3).
(4) Cooling is started within 1.5 seconds after the completion of hot rolling, and the temperature is cooled to the temperature T3 (° C.) or less represented by the following formula (4) at an average cooling rate of 50 ° C./sec or more.
(5) Cooling from the cooling stop temperature to the winding temperature at an average cooling rate of 10 ° C./sec or more.
(6) Winding is performed at (T4-100) ° C. to (T4 + 50) ° C. with respect to the temperature T4 (° C.) represented by the following formula (5).
(7) In cooling after winding, the lower limit of the residence time is condition I (80 at 450 ° C. or higher) in a predetermined temperature range at the end of the hot-rolled steel sheet in the plate width direction and the central portion of the hot-rolled steel sheet in the plate width direction. Satisfying any one or more of seconds or more, 200 seconds or more at 400 ° C or higher, and 1000 seconds or longer at 350 ° C or higher, and the upper limit of the residence time is condition II (2000 seconds or less at 450 ° C or higher and 8000 at 400 ° C or higher). Cool to satisfy (all within seconds and within 30,000 seconds at 350 ° C. or higher).
[0078]
T1 (° C.) =-273.15 + 6770 / (2.25-log ([Nb] × [C])) ... (2)
T2 (° C.) = 868-396 × [C] -68.1 × [Mn] + 24.6 x [Si] -36.1 x [Ni] -24.8 x [Cr] -20.7 x [Cu] + 250 x [Al] ... (3)
T3 (° C.) = 770-270 x [ C] -90 x [Mn] -37 x [Ni] -70 x [Cr] -83 x [Mo] ... (4)
T4 (° C.) = 591-474 x [C] -33 x [Mn] -17 × [Ni] -17 × [Cr] -21 × [Mo] ... (5)
　However, the [element symbol] in each formula indicates the content (mass%) of each element in steel, and contains the element. If not, substitute 0. Further, the log in the above formula (2) indicates a common logarithm having a base of 10.
[0079]
(5-1) Slab, slab temperature when subjected to hot rolling, mode of
　hot rolling As the slab to be subjected to hot rolling, a slab obtained by continuous casting, a slab obtained by casting / slab, or the like is used. It can be used, and if necessary, hot-rolled or cold-rolled products can be used.
[0080]
　The temperature of the slab to be subjected to hot rolling may be a temperature at which NbC precipitated during casting can be dissolved, and is set to T1 (° C.) or higher represented by the above formula (2). From the viewpoint of suppressing scale loss, the slab heating temperature is preferably 1350 ° C. or lower. If the slab to be subjected to hot rolling is a slab obtained by continuous casting or a slab obtained by bulk rolling and is in a high temperature state (T1 (° C.) or higher), it is hot as it is without heating. It may be used for rolling.
[0081]
　For hot rolling, it is preferable to use a lever mill or a tandem mill for multi-pass rolling. In particular, from the viewpoint of industrial productivity, it is more preferable that at least the final several steps are hot-rolled using a tandem mill.
[0082]
(5-2) Tempering rate of hot rolling: Total plate thickness reduction of 90% or more in
　the temperature range of 850 to 1100 ° C. Hotness such that total plate thickness reduction is 90% or more in the temperature range of 850 to 1100 ° C. By rolling, the recrystallized austenite grains are mainly refined, and the accumulation of strain energy in the unrecrystallized austenite grains is promoted, and the average crystal grains of bainite and tempered martensite, which are the main phases, are promoted. The diameter becomes finer. Therefore, hot rolling is performed so that the total plate thickness is reduced by 90% or more in the temperature range of 850 to 1100 ° C. The plate thickness reduction in the temperature range of 850 to 1100 ° C. means the inlet plate thickness t 0 before the first pass in rolling in this temperature range, and the outlet plate thickness after the final pass in rolling in this temperature range is t 1 When, it can be expressed as (t 0 − t 1 ) / t 0 × 100 (%).
[0083]
(5-3) Hot rolling completion temperature: T2 (° C.) or higher The
　hot rolling completion temperature is T2 (° C.) or higher. By setting the completion temperature of hot rolling to T2 (° C.) or higher, it is possible to suppress an excessive increase in the number of ferrite nucleation sites in austenite, and in the final structure (metal structure of hot-rolled steel sheet after production). The area fraction of ferrite can be suppressed to 5.0% or less.
[0084]
(5-4) Cooling after completion of hot rolling: Cooling was started within 1.5 seconds and
　finely divided by cooling hot rolling to T3 (° C.) or lower at an average cooling rate of 50 ° C./sec or higher . In order to suppress the growth of austenite crystal grains, cooling is performed to T3 (° C.) or lower at an average cooling rate of 50 ° C./sec or higher within 1.5 seconds after the completion of hot rolling.
　By cooling to T3 (° C) or lower at an average cooling rate of 50 ° C./sec or higher within 1.5 seconds after the completion of hot rolling, the formation of ferrite and pearlite is suppressed, and bainite and tempered martensite The area fraction can be increased. As a result, the uniformity in the metal structure is improved, and the strength and stretch flangeability of the steel sheet are improved. The average cooling rate here is the temperature drop width of the steel sheet from the start of cooling (when the steel sheet is introduced into the cooling equipment) to the completion of cooling (when the steel sheet is taken out from the cooling equipment) from the start of cooling. The value divided by the time required to complete cooling. In cooling after the completion of hot rolling, if the time to start cooling is more than 1.5 seconds, the average cooling rate is less than 50 ° C / sec, or the cooling stop temperature is more than T3 (° C). , Ferrite transformation and / or pearlite transformation inside the steel sheet becomes remarkable, and it becomes difficult to obtain a metal structure mainly composed of bainite and tempered martensite. Therefore, within 1.5 seconds after the completion of hot rolling, cooling is performed to T3 (° C.) or lower at an average cooling rate of 50 ° C./sec or higher. The upper limit of the cooling rate is not specified, but if the cooling rate is increased, the cooling equipment becomes large and the equipment cost increases. Therefore, considering the equipment cost, 300 ° C./sec or less is preferable. The cooling shutdown temperature is preferably (T4-100) ° C. or higher.
[0085]
(5-5) Average cooling rate from the cooling stop temperature of cooling to the take-up temperature: 10 ° C./sec or more In
　order to suppress the area fraction of pearlite to 5.0% or less, the take-up temperature from the cooling stop temperature of cooling The average cooling rate up to is 10 ° C./sec or more. As a result, the surface integral of bainite and tempered martensite is increased, and the balance between the strength and stretch flangeability of the steel sheet can be improved. The average cooling rate here means a value obtained by dividing the temperature drop width of the steel sheet from the cooling stop temperature of cooling to the take-up temperature by the time required from the stop of cooling to the take-up. If the average cooling rate is less than 10 ° C./sec, the surface integral of pearlite increases, the strength decreases, and the ductility decreases. Therefore, the average cooling rate from the cooling stop temperature of cooling to the winding temperature is set to 10 ° C./sec or more. The upper limit of the cooling rate is not specified, but if the cooling rate is increased, the cooling equipment becomes large and the equipment cost increases. Therefore, considering the equipment cost, 300 ° C./sec or less is preferable.
[0086]
(5-6) Winding temperature: (T4-100) ° C. to (T4 + 50) ° C. The
　winding temperature is (T4-100) ° C. to (T4 + 50) ° C. If the take-up temperature is less than (T4-100) ° C, carbon diffusion from bainite and tempered martensite into austenite does not proceed and austenite is not stabilized, so residual austenite with an area fraction of 3.0% or more Is difficult to obtain, and the ductility of the steel plate is reduced. In addition, the density of iron-based carbides also decreases, so the low-temperature toughness of the steel sheet also deteriorates. Further, when the winding temperature exceeds (T4 + 50) ° C., carbon diffused from bainite and tempered martensite is excessively precipitated in steel as iron-based carbides, so that carbon is sufficiently concentrated in austenite. Without doing so, it becomes difficult to increase the C concentration in the retained austenite to 0.5% by mass or more. Therefore, the winding temperature is (T4-100) ° C. to (T4 + 50) ° C.
[0087]
(5-7) Cooling after winding: The lower limit of the residence time satisfies the following condition I in a predetermined temperature range at the end of the hot-rolled steel sheet in the plate width direction and the central portion of the hot-rolled steel sheet in the plate width direction. Cool so that the upper limit of the residence time satisfies the following condition II.
Condition I: 80 seconds or more at 450 ° C or higher, 200 seconds or longer at 400 ° C or higher, and 1000 seconds or longer at 350 ° C or higher
Condition II: 2000 seconds or longer at 450 ° C or higher and 8000 seconds or shorter at 400 ° C or higher In addition
　, in cooling after all winding at 350 ° C. or higher and within 30,000 seconds , the lower limit of the residence time in a predetermined temperature range of the end portion of the hot-rolled steel plate in the plate width direction and the central portion in the plate width direction of the hot-rolled steel plate is a condition I. By cooling to satisfy the above, that is, by ensuring a residence time that satisfies any one or more of 80 seconds or more at 450 ° C. or higher, 200 seconds or longer at 400 ° C. or higher, and 1000 seconds or longer at 350 ° C. or higher. It promotes the diffusion of carbon from bainite and tempered martensite to austenite, increases the area fraction of retained austenite, and facilitates the suppression of decomposition of retained austenite. In this embodiment, the temperature at the end of the hot-rolled steel sheet in the plate width direction is measured with a contact-type or non-contact-type thermometer. The temperature at the center of the hot-rolled steel sheet in the width direction is measured by a thermocouple or calculated by heat transfer analysis. If the lower limit of the residence time does not satisfy the condition 1, that is, the residence time does not satisfy all of 40 ° C. or higher for 80 seconds or longer, 400 ° C. or higher for 200 seconds or longer, and 350 ° C. or higher for 1000 seconds or longer, bainite and tempering. Carbon is not sufficiently diffused from the tempered martensite into austenite, making it difficult to increase the area fraction of retained austenite to 3.0% or more and the C concentration in retained austenite to 0.5% by mass or more. As a result, the ductility of the steel plate decreases.
[0088]
　On the other hand, in cooling after winding, if the upper limit of the residence time in the predetermined temperature range of the end portion of the hot-rolled steel sheet in the plate width direction and the central portion in the plate width direction of the hot-rolled steel sheet does not satisfy the condition II, that is, the residence time. If any one of the above is applicable to more than 2000 seconds at 450 ° C or higher, more than 8000 seconds at 400 ° C or higher, or more than 30,000 seconds at 350 ° C or higher, austenite decomposes into iron-based charcoal and tempered martensite. Therefore, the ductility of the steel sheet is reduced. Therefore, the cooling is performed so that the upper limit of the residence time satisfies the condition II, that is, all of the conditions are satisfied within 2000 seconds at 450 ° C. or higher, within 8000 seconds at 400 ° C. or higher, and within 30,000 seconds at 350 ° C. or higher. Based on the above, cooling after winding is performed in a predetermined temperature range at the end of the hot-rolled steel sheet in the plate width direction and the central portion of the hot-rolled steel sheet in the plate width direction, and the lower limit of the residence time is condition I (80 at 450 ° C. or higher). Satisfying any one or more of seconds or more, 200 seconds or more at 400 ° C or higher, and 1000 seconds or longer at 350 ° C or higher, and the upper limit of the residence time is condition II (2000 seconds or less at 450 ° C or higher and 8000 at 400 ° C or higher). Cool to satisfy (all within seconds and within 30,000 seconds at 350 ° C. or higher). Cooling of the end portion of the hot-rolled steel sheet in the plate width direction and the central portion of the hot-rolled steel plate in the plate width direction after winding may be controlled by a heat insulating cover, an edge mask, mist cooling, or the like.
Example
[0089]
　Next, the effect of one aspect of the present invention will be described more specifically by way of examples. The conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention. The present invention is not limited to this one-condition example. The present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
[0090]
　Steel Nos. In Tables 1 and 2. Steels having the chemical compositions shown in A to Z were melted and continuously cast to produce slabs having a thickness of 240 to 300 mm. Using the obtained slab, a hot-rolled steel sheet was obtained under the production conditions shown in Tables 3 to 6.
　The production No. After winding, No. 35 was cold-rolled at the cold rolling ratio shown in Table 6 and annealed at the annealing holding temperature and annealing holding time shown in Table 6. After that, the mixture was cooled to the cooling shutdown temperature at the primary cooling rate shown in Table 6, and then held for the post-cooling holding time shown in Table 6. In Table 5, the manufacturing No. For No. 35, the residence time after hot rolling and winding and before annealing in Table 6 is described.
　In addition, the production No. In the cooling after hot rolling, 36 and 37 were temporarily stopped at the residence temperature shown in Table 4, allowed to stay at the residence temperature for the residence time shown in Table 4, and then cooled again.

The scope of the claims
[Claim 1]
　The chemical composition is
C: 0.100 to 0.250%,
Si: 0.05 to 3.00%,
Mn: 1.00 to 4.00%,
Nb: 0.005 to 0.050 in mass%. %,
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 %,
V: 0 to 0.500%,
Cu: 0 to 2.00%,
Cr: 0 to 2.00%,
Mo: 0 to 1.000%,
Ni: 0 to 2.00%,
B: 0 ~ 0.0100%,
Ca: 0 ~ 0.0200%,
Mg: 0 ~ 0.0200%,
REM: 0 ~ 0.1000%,
Bi: 0 ~ 0.020%,
Zr, Co, Zn and W One or more of them: 0 to 1.00% in total and
Sn: 0 to 0.050%, the
　balance consisting of Fe and impurities.
　A
　total of 77.0 to 97. Bainite and tempered martensite in a plate width cross section parallel to the rolling direction, with a metal structure at a depth of 1/4 of the plate thickness from the surface and at the center position in the plate width direction in% area . The 　metal 　containing 0%,
　0 to 5.0% ferrite, 0 to 5.0%
　pearlite,
　3.0% or more retained austenite
, 0 to 10.0%
martensite, and excluding the retained austenite. The average crystal grain size of the structure is 7.0 μm or less,
　the C concentration in the retained austenite is 0.5% by mass or more, and
　the number density of iron-based carbides having a diameter of 20 nm or more is 1.0 × 10 6 pieces / A hot-rolled steel plate having a thickness of 2 mm or more.
[Claim 2]
　In the plate width cross section parallel to the rolling direction,
　from the surface to the 1/4 depth of the plate thickness and the center position in the
　plate width direction, and from the surface to the 1/4 depth of the plate thickness and the center position in the plate width direction. A position of 300 mm on one end side in the plate width direction, a position
　1/4 of the plate thickness from the surface and a position of 600 mm on the one end side in the plate width direction from the center position in the plate width direction, and the
　plate thickness from the surface. 1/4 depth and 300 mm from the center position in the plate width direction to the other end side in the plate width direction, and
　1/4 depth of the plate thickness from the surface and from the center position in the plate width direction to the plate width direction. When the
retained austenite in the metal structure at the position of 600 mm on the other end side is γ, γ D1 , γ D2 , γ W1 and γ W2 in area%, respectively , γ / γ D1 , γ / γ D2 and γ / γ W1 And γ / γ W2 are 0.8 or more and less than 1.2, respectively, at a
　depth of 1/4 of the plate thickness from the surface and at the center position in the plate width direction.
　A position
　of 1/4 depth of the plate thickness from the surface and 300 mm from the center position in the plate width direction to the one end side in the plate width direction , 1/4 depth of the plate thickness from the surface and the center in the plate width direction. A position of 600 mm from the position to the one end side in the plate width direction, a position
　of 1/4 of the plate thickness from the
　surface and a position of 300 mm from the center position in the plate width direction to the other end side in the plate width direction, and the surface.
C γC and C in mass% of the C concentration in the retained austenite in the metal structure at a depth of 1/4 of the plate thickness and 600 mm from the center position in the plate width direction to the other end side in the plate width direction , respectively. When γD1 , C γD2 , C γW1 and C γW2 , C γC / C γD1 , C γC / C γD2 , C γC / C γW1 and C γC / C γW2The hot-rolled steel sheet according to claim 1, wherein is 0.8 or more and less than 1.2, respectively.
[Claim 3]
　The chemical composition is, in mass%,
Ti: 0.005 to 0.300%,
V: 0.005 to 0.500%,
Cu: 0.01 to 2.00%,
Cr: 0.01 to 2. 00%,
Mo: 0.010 to 1.000%,
Ni: 0.02 to 2.00%,
B: 0.0001 to 0.0100%,
Ca: 0.0005 to 0.0200%,
Mg: 0 It is characterized by containing one or more selected from the group consisting of 0005 to 0.0200%,
REM: 0.0005 to 0.1000%, and
Bi: 0.0005 to 0.020%.
The hot-rolled steel plate according to claim 1 or 2.