Technical field
[0001]
The present invention relates to a hot-rolled steel plate. Specifically, the present invention relates to a hot-rolled steel plate that is formed into various shapes by press processing or the like and is used, and in particular, a hot-rolled steel plate that has high strength and is excellent in ductility and shear workability.
This application claims priority based on Japanese Patent Application No. 2019-181314 filed in Japan on October 1, 2019, and the contents thereof 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. Automakers are also actively developing technologies to reduce the weight of the vehicle body for the purpose 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]
In order to achieve both weight reduction of the vehicle body and collision resistance, it is being considered to thin the members by using high-strength steel plates. Therefore, a steel plate having both high strength and excellent formability is strongly desired. Several techniques have been conventionally proposed to meet these demands. Since there are various processing styles for automobile members, the required formability differs depending on the member to which it is applied, but among them, ductility is positioned as an important index of formability. Further, although automobile members are formed by press molding, the press-formed blank plates are often manufactured by highly productive shearing. Especially for high-strength steel plates of 980 MPa or more, the load required for post-treatment such as coining after shearing is large, so the burr height after shearing is controlled with high accuracy so that post-treatment is not necessary. Is desired.
[0004]
Regarding the technique for improving ductility, for example, Patent Document 1 describes collision resistance and moldability 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. Excellent automotive high-strength steel plates are disclosed. In steel plates containing retained austenite in the metallographic structure, austenite undergoes martensite transformation during processing and exhibits 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 expandability is improved by refining ferrite and retained austenite.
[0005]
Patent Document 2 discloses a high-strength steel plate having a tensile strength of 980 MPa or more, which is excellent in ductility and elongation and flangeability, in which a second phase composed of retained austenite and / or martensite is finely dispersed in crystal grains. There is.
[0006]
As a technique for improving shear workability, for example, in Patent Document 3, the ratio d s / db of the ferrite grain size d s on the surface layer and the ferrite crystal grain db inside is controlled to 0.95 or less. , A technique for controlling the burr height after punching is disclosed.
Patent Document 4 discloses a technique for improving peeling and creases on the end face of a plate by reducing the content of P.
Prior art literature
Patent documents
[0007]
Patent Document 1: Japanese Patent Application Laid-Open No. 11-61326
Patent Document 2: Japanese Patent Application Laid-Open No. 2005-177903
Patent Document 3: Japanese Patent Application Laid-Open No. 10-168544
Patent Document 4: Japanese Patent Application Laid-Open No. 2005-298924
Outline of the invention
Problems to be solved by the invention
[0008]
The techniques disclosed in Patent Documents 1 to 4 are all techniques for improving either the ductility or the end face property after shearing. However, Patent Documents 1 to 3 do not mention a technique for achieving both of these characteristics. Patent Document 4 refers to both shear workability and press formability. However, since the strength of the steel plate disclosed in Patent Document 4 is less than 850 MPa, it may be difficult to apply the technique disclosed in Patent Document 4 to a member having a high strength of 980 MPa or more.
[0009]
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 plate having high strength and excellent ductility and shear workability.
Means to solve the problem
[0010]
In view of the above-mentioned problems, the present inventors have obtained the following findings (a) to (h) as a result of intensive studies on the chemical composition of the hot-rolled steel plate and the relationship between the metallographic structure and the mechanical properties. Completed the invention. In addition, having excellent shearing workability means that the burr height after shearing is small. Further, having excellent strength or high strength means that the tensile strength is 980 MPa or more.
[0011]
(A) In order to obtain excellent tensile (maximum) strength, it is preferable to utilize a hard structure. That is, it is preferable to include martensite or baynite in the tissue.
[0012]
(B) However, since a hard structure is a structure having poor ductility, it is not possible to secure excellent ductility simply by using a metal structure mainly composed of these.
[0013]
(C) In order to provide a high-strength hot-rolled steel plate with excellent ductility, it is effective to contain an appropriate amount of ferrite having high ductility.
[0014]
(D) Since ferrite is generally soft, it is necessary to utilize Ti, Nb, V and the like as precipitation-enhancing elements in order to obtain desired strength. Therefore, it is necessary to apply intermediate air cooling in the hot spreading process to obtain an appropriate amount of precipitation-reinforced ferrite.
[0015]
(E) A hard structure is generally formed in a phase transformation of 600 ° C. or lower, but in this temperature range, the grain boundary and the crystal orientation difference of which the crystal orientation difference is 60 ° with respect to the <110> direction are 7 A large amount of grain boundaries are formed.
[0016]
(F) When a grain boundary having a crystal orientation difference of 7 ° with respect to the <110> direction is formed, dislocations are likely to be accumulated in the hard structure. In a hard phase, such a metal structure having a high density of grain boundaries and uniformly dispersed (that is, a large total length of grain boundaries as described above) is displaced into the hard structure during shearing. Accumulates, so cracks easily occur from inside the hard structure. As a result, cracks are likely to occur even if shearing is performed under a condition where the clearance is large, and the generation of excessive burrs is suppressed.
[0017]
(G) In order to uniformly disperse the grain boundaries having a crystal orientation difference of 7 ° about the <110> direction in the hard phase, it is necessary to set the standard deviation of the Mn concentration to a certain value or less. In order to keep the standard deviation of Mn concentration below a certain value, hold it in the temperature range of 700 to 850 ° C for 900 seconds or more during slab heating, then heat it further and hold it in the temperature range of 1100 ° C or higher for 6000 seconds or more. In addition, it is necessary to perform hot rolling so that the total plate thickness is reduced by 90% or more in the temperature range of 850 ° C to 1100 ° C. If the residence time is short or the plate thickness reduction is small, the microsegregation of Mn becomes large, so that the standard deviation of Mn concentration cannot be kept below a certain value, and the grain boundary with a crystal orientation difference of 7 ° is uniform. Not distributed in.
[0018]
(H) In order to increase the length of the grain boundary having a crystal orientation difference of 7 ° about the <110> direction, it is necessary to quench to room temperature. When cooling is stopped at a temperature of 250 ° C. or higher, the length of the grain boundaries decreases.
[0019]
The gist of the present invention made based on the above findings is as follows.
[0020]
(1) The hot-rolled steel plate according to one aspect of the present invention has a chemical composition of% by mass.
C: 0.050 to 0.250%,
Si: 0.05 to 3.00%,
Mn: 1.00 to 4.00%,
One or more of Ti, Nb and V: 0.060 to 0.500% in total,
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,
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 to 0.1000%,
Bi: 0-0.020%,
One or more of Zr, Co, Zn and W: 0-1.00% in total, and
Sn: contains 0 to 0.050%,
The rest consists of Fe and impurities
In the metal structure with a cross section parallel to the rolling direction, at a depth of 1/4 of the plate thickness from the surface and at the center position in the plate width direction.
Area%, retained austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, pearlite is less than 5.0%, and the crystal orientation is about the <110> direction. L 60 / L 7, which is the ratio of the grain boundary length L 60 having a difference of 60 ° to the grain boundary length L 7 having a crystal orientation difference of 7 °, is less than 0.60.
The standard deviation of Mn concentration is 0.60% by mass or less,
The tensile strength is 980 MPa or more.
(2) The hot-rolled steel plate according to (1) above may have an average crystal grain size of less than 3.0 μm on the surface layer.
(3) The hot-rolled steel plate according to (1) or (2) above has a chemical composition of% by mass.
Cu: 0.01-2.00%,
Cr: 0.01-2.00%,
Mo: 0.01-1.00%,
Ni: 0.02-2.00%,
B: 0.0001 to 0.0100%,
Ca: 0.0005-0.0200%,
Mg: 0.0005-0.0200%,
REM: 0.0005 to 0.1000%, and
Bi: 0.0005-0.020%
It may contain one or more selected from the group consisting of.
Effect of the invention
[0021]
According to the above aspect according to the present invention, a hot-rolled steel plate having excellent strength, ductility and shear workability can be obtained. Further, according to the above-mentioned preferred embodiment according to the present invention, it is possible to obtain a hot-rolled steel plate having the above-mentioned various characteristics and further suppressing the occurrence of bending internal cracking, that is, having excellent bending internal cracking resistance. can.
The hot-rolled steel plate 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.
A brief description of the drawing
[0022]
[Fig. 1] Fig. 1 is a diagram for explaining the burr height after shearing.
Embodiment for carrying out the invention
[0023]
The chemical composition and metallographic structure of the hot-rolled steel plate (hereinafter, may be simply referred to as a steel plate) 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 gist of the present invention.
The lower limit value and the upper limit value are included in the numerical limitation range described below with "~" in between. Numerical values ​​marked "less than" or "greater than" do not fall within the numerical range. In the following description,% regarding the chemical composition of the steel plate is mass% unless otherwise specified.
[0024]
1. 1. Chemical composition
The hot-rolled steel plate according to the present embodiment has C: 0.050 to 0.250%, Si: 0.05 to 3.00%, Mn: 1.00 to 4.00%, Ti, Nb in mass%. And one or more of V: ​​0.060 to 0.500% in total, 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.
[0025]
(1-1) C: 0.050 to 0.250%
C increases the fraction of the hard phase and increases the strength of ferrite by combining with precipitation-enhancing elements such as Ti, Nb, and V. If the C content is less than 0.050%, it becomes difficult to obtain the desired strength. Therefore, the C content is set to 0.050% or more. The C content is preferably 0.060% or more, more preferably 0.070% or more, and even more preferably 0.080% or more. On the other hand, when the C content exceeds 0.250%, the fraction of ferrite is lowered, so that the ductility of the hot-rolled steel plate is lowered. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.200% or less, more preferably 0.150% or less.
[0026]
(1-2) Si: 0.05 to 3.00%
Si has the effect of promoting the formation of ferrite to improve the ductility of the hot-rolled steel plate and the effect of solid-melting and strengthening the ferrite to increase the strength of the hot-rolled steel plate.Have. 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, more preferably 0.80% or more. However, when the Si content exceeds 3.00%, the surface texture and chemical conversion processability of the hot-rolled steel plate, as well as the ductility and weldability, are significantly deteriorated, and the A3 transformation point is significantly increased. 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, more preferably 2.50% or less.
[0027]
(1-3) Mn: 1.00 to 4.00%
Mn has the effect of suppressing ferrite transformation and increasing the strength of hot-rolled steel sheets. If the Mn content is less than 1.00%, a tensile strength of 980 MPa or more 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 angle difference of the crystal grains in the hard phase becomes non-uniform due to the segregation of Mn, and it becomes difficult to obtain the desired shear workability. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less, more preferably 3.50% or less.
[0028]
(1-4) One or more of Ti, Nb and V: 0.060 to 0.500% in total
Ti, Nb and V are elements that finely precipitate in the steel as carbides and nitrides and improve the strength of the steel by strengthening the precipitation. In addition, these elements are elements that fix C by forming the above-mentioned charcoal and suppress the formation of cementite that is harmful to shearability. In order to obtain these effects, the total content of Ti, Nb and V is set to 0.060% or more. It is not necessary that all of Ti, Nb and V are contained, and any one of them may be contained. When only one of them is contained, the content of the element may be 0.060% or more. If the content of any one of them is 0.060% or more, the above effect can be obtained. The total content of Ti, Nb and V is preferably 0.080% or more, more preferably 0.090% or more, and even more preferably 0.100% or more. On the other hand, if the total content of Ti, Nb and V exceeds 0.500%, the workability deteriorates. Therefore, the total content of Ti, Nb and V is set to 0.500% or less. It is preferably 0.300% or less, more preferably 0.250% or less, and even more preferably 0.120% or less.
[0029]
(1-5) sol. Al: 0.001 to 2.000%
Like Si, Al has the effect of deoxidizing the steel to make it sound, and also has the effect of promoting the formation of ferrite and increasing the ductility of the hot-rolled steel plate. 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 means acid-soluble Al, and indicates solid-dissolved Al existing in steel in a solid-dissolved state.
[0030]
(1-6) P: 0.100% or less
P is an element that is generally contained as an impurity, but it is also an element that has the effect of increasing the strength of the hot-rolled steel plate by strengthening the solid solution. Therefore, P may be positively contained. However, P is an element that is easily segregated, and when the P content exceeds 0.100%, the formability and toughness are significantly reduced due to grain boundary segregation. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.030% or less. The lower limit of the P content does not need to be specified, but is preferably 0.001% from the viewpoint of refining cost.
[0031]
(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 plate. When the S content exceeds 0.0300%, the formability of the hot-rolled steel plate is significantly lowered. Therefore, the S content is 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.
[0032]
(1-8) N: 0.1000% or less
N is an element contained in steel as an impurity and has an action of lowering the formability of the hot-rolled steel plate. When the N content exceeds 0.1000%, the formability of the hot-rolled steel plate 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, but when one or more of Ti, Nb and V are contained to further refine the metal structure, the precipitation of carbon nitride is promoted. The N content is preferably 0.0010% or more, and more preferably 0.0020% or more.
[0033]
(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 0.0100% or less. The O content is preferably 0.0080% or less or 0.0050% or less. The O content may be 0.0005% or more or 0.0010% or more in order to disperse a large number of fine oxides during deoxidation of the molten steel.
[0034]
The balance of the chemical composition of the hot-rolled steel plate according to the present embodiment may be Fe and impurities. In the present embodiment, the impurities mean those mixed from ore as a raw material, scrap, manufacturing environment, etc., or those allowed within a range that does not adversely affect the hot-rolled steel plate according to the present embodiment.
[0035]
The hot-rolled steel plate according to the present embodiment contains Cu, Cr, Mo, Ni, B, Ca, Mg, REM, Bi, Zr, Co, Zn, W and Sn as optional elements instead of a part of Fe. You may. 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.
[0036]
(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 : 0.0001-0.0100%
Cu, Cr, Mo, Ni and B all have the effect of enhancing the hardenability of the hot-rolled steel plate. Further, Cr and Ni have an action of stabilizing retained austenite, and Cu and Mo have an action of precipitating as carbides in the steel to increase the strength of the hot-rolled steel plate. Further, when Ni is contained in Cu, it has an effect of effectively suppressing the grain boundary cracking of the slab caused by Cu. Therefore, one or more of these elements may be contained.
[0037]
As described above, Cu has an effect of enhancing the hardenability of the hot-rolled steel plate and an effect of precipitating as carbide in the steel at a low temperature to increase the strength of the hot-rolled steel plate. 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 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 or 1.00% or less.
[0038]
As described above, Cr has an action of enhancing the hardenability of the hot-rolled steel plate 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 more preferably 0.05% or more. However, when the Cr content exceeds 2.00%, the chemical conversion processability of the hot-rolled steel plate is significantly deteriorated. Therefore, the Cr content is set to 2.00% or less.
[0039]
As described above, Mo has an action of enhancing the hardenability of the hot-rolled steel plate and an action of precipitating as carbide in the steel to increase the strength of the hot-rolled steel plate. In order to obtain the effect of the above action more reliably, the Mo content is preferably 0.01% or more, and more preferably 0.02% or more. However, even if the Mo content exceeds 1.00%, the effect of the above action is saturated and is not economically preferable. Therefore, the Mo content is set to 1.00% or less. The Mo content is preferably 0.50% or less or 0.20% or less.
[0040]
As mentioned above, Ni has the effect of enhancing the hardenability of the hot-rolled steel plate. Further, when Cu is contained, Ni 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.
[0041]
As described above, B has an effect of enhancing the hardenability of the hot-rolled steel plate. In order to obtain the effect of this action more reliably, the B content is preferably 0.0001% or more, and more preferably 0.0002% or more. However, if the B content exceeds 0.0100%, the formability of the hot-rolled steel plate is significantly lowered, so the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less.
[0042]
(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, Mg and REM all have the effect of improving the formability of the hot-rolled steel plate by adjusting the shape of the inclusions in the steel to a preferable shape. In addition, Bi has an effect of improving the formability of the hot-rolled steel plate by making the solidified structure finer. Therefore, one or more of these elements may be contained. In order to obtain the effect of the above action more reliably, it is preferable that the content of 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, and on the contrary, the formability of the hot-rolled steel plate is deteriorated. May reduce. 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.
[0043]
Here, REM refers to a total of 17 elements composed of Sc, Y and lanthanoid, and the content of the above REM refers to the total content of these elements. In the case of lantanoids, it is industrially added in the form of misch metal.
[0044]
(1-12) One or more of Zr, Co, Zn and W: 0 to 1.00% in total and Sn: 0 to 0.050%
Regarding Zr, Co, Zn and W, the present inventors have confirmed that even if the total content of these elements is 1.00% or less, the effect of the hot-rolled steel plate according to the present embodiment is not impaired. There is. 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 plate according to the present embodiment is not impaired even if a small amount of Sn is contained. However, if a large amount of Sn is contained, defects may occur during hot rolling, so Sn is contained. The amount shall be 0.050% or less.
[0045]
2. 2. Metal structure of hot-rolled steel plate
Next, the metal structure of the hot-rolled steel plate according to the present embodiment will be described.
The hot-rolled steel plate according to the present embodiment has a cross section parallel to the rolling direction, a depth of 1/4 of the plate thickness from the surface, and a metal structure at the center position in the plate width direction. Less than, ferrite is 15.0% or more and less than 60.0%, pearlite is less than 5.0%, and the grain boundary length is 60 ° with respect to the <110> direction. L 60 / L 7, which is the ratio of L 60 to the grain boundary length L 7 having a crystal orientation difference of 7 °, is less than 0.60, and the standard deviation of Mn concentration is 0.60 mass% or less. be. Therefore, the hot-rolled steel plate according to the present embodiment can obtain high strength, excellent ductility and shear workability. 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 in the cross section parallel to the rolling direction is that the metal structure at this position is representative of the steel plate. This is because it shows a typical metal structure.
[0046]
(2-1) Area fraction of retained austenite: less than 3.0%
Residual austenite is a metal structure that exists as a face-to-center cubic lattice even at room temperature. Residual 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 hinders stable crack generation and causes coarse burrs. When the area fraction of the retained austenite is 3.0% or more, the above-mentioned action becomes apparent and the shearing workability of the hot-rolled steel plate deteriorates. Therefore, the area fraction of retained austenite is less than 3.0%. The area fraction of retained austenite is preferably less than 1.0%. Since the smaller the retained austenite, the more preferable it is, the area fraction of the retained austenite may be 0%.
[0047]
Methods for measuring the area fraction of retained austenite include X-ray diffraction, EBSP (electron backscattering diffraction image, Electron Back Scattering Diffraction Pattern) analysis, and magnetic measurement methods, and the measured values ​​may differ depending on the measurement method. .. In this embodiment, the area fraction of retained austenite is measured by X-ray diffraction.
[0048]
In the measurement of the residual austenite area fraction by X-ray diffraction in the present embodiment, first, Co-Kα is obtained in a cross section parallel to the rolling direction at a depth of 1/4 of the plate thickness of the hot-rolled steel plate and at the center position in the plate width direction. Using a line, determine the integrated intensity of a total of 6 peaks of α (110), α (200), α (211), γ (111), γ (200), and γ (220), and use the intensity averaging method. By calculation, the area fraction of retained austenite is obtained.
[0049]
(2-2) Ferrite area fraction: 15.0% or more and less than 60.0%
Ferrite is a structure formed when fcc is transformed into bcc at a relatively high temperature. Since ferrite has a high processing cure rate, it has the effect of increasing the strength-durability balance of hot-rolled steel sheets. In order to obtain the above action, the area fraction of ferrite shall be 15.0% or more. It is preferably 16.0% or more. On the other hand, since ferrite has a low strength, it is not possible to obtain a desired tensile strength if the area fraction is excessive. Therefore, the ferrite area fraction is set to less than 60.0%. It is preferably 50.0% or less.
[0050]
The hot-rolled steel plate according to the present embodiment may be one of baynite and martensite having a total area fraction of more than 32.0% and 85.0% or less as a residual structure other than retained austenite, ferrite and pearlite. A hard structure consisting of two types is included.
[0051]
(2-3) Area fraction of pearlite: less than 5.0%
Parlite is a lamella-like metal structure in which cementite is deposited in layers between ferrites, and is a soft metal structure compared to baynite and martensite. When the area fraction of pearlite is 5.0% or more, carbon is consumed in the cementite contained in pearlite, the strength of the residual structure martensite or baynite decreases, and it is not possible to obtain a tensile strength of 980 MPa or more. .. Therefore, the area fraction of pearlite is set to less than 5.0%. The area fraction of pearlite is preferably 3.0% or less. In order to improve the stretch flangeability of the hot-rolled steel plate, it is preferable to reduce the area fraction of pearlite as much as possible, and the lower limit thereof is 0%.
[0052]
The area fraction of ferrite and pearlite is measured by the following method. The cross section parallel to the rolling direction at the center position in the plate width direction is mirror-finished and polished at room temperature with colloidal silica containing no alkaline solution for 8 minutes to remove the strain introduced into the surface layer of the sample. 50 μm in length, 1/8 depth from surface to 3/8 depth from surface so that 1/4 depth of plate thickness can be analyzed at any position in the longitudinal direction of the sample cross section. The depth region is measured by the electron backscattering diffraction method at a measurement interval of 0.1 μm to obtain crystal orientation information. For the measurement, an EBSD analyzer composed of a thermal electric field radiation type 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 analyzer is 9.6 × 10 -5 Pa or less, the acceleration voltage is 15 kv, the irradiation current level is 13, and the irradiation level of the electron beam is 62.
[0053]
Furthermore, the reflected electron image is taken in the same field of view. First, the area fraction of pearlite is obtained by identifying the crystal grains in which ferrite and cementite are deposited in layers from the backscattered electron image and calculating the area fraction of the crystal grains. After that, for the crystal grains excluding the crystal grains determined to be pearlite, the obtained crystal orientation information is used for the "Grain Average Misorition" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. Therefore, a region having a Grain Average Misoriation value of 1.0 ° or less is determined to be ferrite. By obtaining the area fraction of the region determined to be ferrite, the area fraction of ferrite is obtained.
[0054]
The area fraction of the residual structure is obtained by subtracting the area fraction of retained austenite, the area fraction of ferrite, and the area fraction of pearlite from 100%.
[0055]
(2-4) The ratio of the grain boundary length L 60 having a crystal orientation difference of 60 ° and the grain boundary length L 7 having a crystal orientation difference of 7 ° with the <110> direction as the axis L. 60 / L 7: Less than 0.60
In order to obtain high strength of 980 MPa or more, it is necessary to make the matrix a hard structure. A hard structure is generally formed in a phase transformation of 600 ° C. or lower, but in this temperature range, the grain boundary with a crystal orientation difference of 60 ° and the crystal orientation difference of 7 ° with the <110> direction as the axis. A large amount of certain grain boundaries are formed. When a grain boundary having a crystal orientation difference of 7 ° with respect to the <110> direction is formed, dislocations are unlikely to accumulate in the hard structure. Therefore, in the hard phase, the hard phase is deformed in such a metal structure in which the density of the grain boundaries is high and the grain boundaries are uniformly dispersed (that is, the total length of the grain boundaries is large as described above). Strain tends to concentrate inside the hard structure, and cracks easily occur from there. As a result, even if shearing is performed under a condition where the clearance is large, cracks are likely to occur from both the punch side and the die side, and the generation of excessive burrs is suppressed.
[0056]
On the other hand, in the grain boundary where the crystal orientation difference is 60 ° with respect to the <110> direction, dislocations are likely to be accumulated in the hard phase. Therefore, in such a metal structure having a high grain boundary density in the hard phase, the hard phase does not deform, so that it is difficult to introduce dislocations into the hard phase during shearing. As a result, the generation of cracks from the inside of the hard phase is suppressed, so that the formation of burrs is delayed and the generation of excessive burrs is promoted. Therefore, when the length of the grain boundary having a crystal orientation difference of 60 ° is L 60 and the length of the grain boundary having a crystal orientation difference of 7 ° is L 7 with respect to the <110> direction, the clearance is large. The susceptibility to excessive burrs after shearing under the conditions is dominated by L60 / L7. When L 60 / L 7 is 0.60 or more, excessive burrs are likely to occur due to the above action. Therefore, in order to improve the shear workability of the hot-rolled steel plate, it is necessary to set L 60 / L 7 to less than 0.60.
[0057]
In addition, the grain boundary in which the crystal orientation difference is X ° with respect to the <110> direction means that when two crystal grains A and crystal grains B adjacent to each other in a certain grain boundary are specified, one crystal grain B is referred to as < 110> Refers to a grain boundary having a crystalline relationship in which the crystal orientations of the crystal grains A and the crystal grains B are matched by rotating X ° along the axis. However, considering the measurement accuracy of the crystal orientation, an orientation difference of ± 4 ° is allowed from the matching orientation relationship.
[0058]
In the present embodiment, the grain boundary lengths L 7 and L 60 as described above are measured by using the EBSP-OIM (Electron Back Scatter Diffraction Pattern-Orientation Image 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 OIM Analysis (registered trademark) manufactured by AMETEK. In the EBSP-OIM method, since the fine structure of the sample surface and the crystal orientation can be analyzed, the length of the grain boundary having a specific crystal orientation difference can be quantitatively obtained. Further, the analyzable area of ​​the EBSP-OIM method is an area that can be observed by SEM. Although it depends on the resolution of SEM, according to the EBSP-OIM method, analysis can be performed with a minimum resolution of 20 nm.
[0059]
In measuring the length of a specific grain boundary of a 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 a cross section parallel to the rolling direction, a region of 1200 times magnification and 40 μm × 30 μm. Then, analysis is performed in at least 5 visual fields, and L60 is obtained by calculating the average value of the lengths of the grain boundaries having a crystal orientation difference of 60 ° with the <110> direction as the axis. Similarly, L 7 is obtained by calculating the average value of the lengths of the grain boundaries having a crystal orientation difference of 7 ° with the <110> direction as the axis. As described above, an orientation difference of ± 4 ° is allowed.
[0060]
It should be noted that ferrite and pearlite are soft phases and have a small effect on the rearrangement accumulation effect inside the hard phase, and retained austenite is not a structure generated by a phase transformation at 600 ° C. or lower and has no rearrangement accumulation effect. Therefore, in this measurement method, ferrite, pearlite and retained austenite are not included in the analysis. The pearlite and ferrite can be excluded from the analysis target by specifying the pearlite by the same method as the method for measuring the area fraction of pearlite and specifying the ferrite by the same method as the method for measuring the area fraction of ferrite. Also. In the EBSP-OIM method, retained austenite having a crystal structure of fcc can be excluded from the analysis target.
[0061]
(2-5) Standard deviation of Mn concentration: 0.60% by mass or less
The standard deviation of the Mn concentration at a depth of 1/4 of the plate thickness from the surface of the hot-rolled steel plate according to the present embodiment and at the center position in the plate width direction is 0.60% by mass or less. This makes it possible to uniformly disperse the grain boundaries having a crystal orientation difference of 7 ° about the <110> direction. As a result, excellent shear workability can be obtained. The lower limit of the standard deviation of the Mn concentration is preferably as small as the value from the viewpoint of suppressing excessive burrs, but the practical lower limit is 0.10% by mass due to the limitation of the manufacturing process.
[0062]
After mirror-polishing the cross section parallel to the rolling direction of the hot-rolled steel plate, measure the depth of 1/4 of the plate thickness from the surface of the hot-rolled steel plate and the center position in the plate width direction with an electronic probe microanalyzer (EPMA), and Mn. Standard deviation of concentration To measure. The measurement conditions are that the acceleration voltage is 15 kV, the magnification is 5000 times, and the distribution image in the range of 20 μm in the sample rolling direction and 20 μm in the sample plate thickness direction is measured. More specifically, the measurement interval is set to 0.1 μm, and the Mn concentration at 40,000 or more points is measured. Next, the standard deviation of the Mn concentration is obtained by calculating the standard deviation based on the Mn concentration obtained from all the measurement points.
[0063]
(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 internal cracking of the hot-rolled steel plate. The higher the strength of the steel plate, the more likely it is that cracks will occur from the inside of the bend during bending (hereinafter referred to as internal bending cracks). The mechanism of internal bending cracking is presumed as follows. During bending, compressive stress is generated inside the bend. At first, the entire inside of the bend is deformed uniformly while processing proceeds, but when the amount of processing increases, the deformation cannot be carried out only by uniform deformation, and the deformation progresses due to the concentration of strain locally (generation of shear deformation zone). As this shear deformation zone grows further, cracks along the shear zone are generated from the inner surface of the bend and grow. The reason why internal bending cracks are more likely to occur with higher strength is that uniform deformation is less likely to proceed due to the decrease in processing hardening ability due to higher strength, and biased deformation is more likely to occur at an early stage of processing ( It is presumed that a shear deformation zone is generated (or under loose processing conditions).
[0064]
According to the research by the present inventors, it was found that the internal bending crack becomes remarkable in the steel plate having a tensile strength of 980 MPa class or higher. Further, the present inventors have found that the finer the crystal grain size of the surface layer of the hot-rolled steel plate, the more the local strain concentration is suppressed and the less likely it is that internal bending cracks occur. In order to obtain the above effect, the average crystal grain size of the surface layer of the hot-rolled steel plate is preferably less than 3.0 μm. More preferably, it is 2.5 μm or less.
In the present embodiment, the surface layer is a region from the surface of the hot-rolled steel plate to a depth of 50 μm from the surface.
[0065]
The crystal grain size of the surface layer is measured by using the above-mentioned EBSP-OIM method. In the region parallel to the rolling direction from the surface to the depth of 50 μm from the surface of the hot-rolled steel plate and in the center position in the plate width direction, analysis was performed in a region of 1200 times magnification and 40 μm × 30 μm in at least 5 fields. A place where the angle difference between adjacent measurement points is 5 ° or more is defined as a crystal grain boundary, and the crystal grain size of the area average is calculated. The obtained area average crystal grain size is defined as the average crystal grain size of the surface layer.
[0066]
Since retained austenite is not a structure generated by a phase transformation at 600 ° C or lower and has no effect of rearrangement accumulation, retained austenite is not included in the analysis in this measurement method. As described above, in the EBSP-OIM method, retained austenite having a crystal structure of fcc can be excluded from the analysis target.
[0067]
3. 3. Tensile strength characteristics
Among the mechanical properties of the hot-rolled steel plate, the tensile strength characteristics (tensile strength, total elongation) are evaluated in accordance with JIS Z 2241: 2011. The test piece shall be JIS Z 2241: 2011 No. 5 test piece. The sampling position of the tensile test piece may be 1/4 of the end portion in the plate width direction, and the direction perpendicular to the rolling direction may be the longitudinal direction.
[0068]
The hot-rolled steel plate 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 of weight reduction of the vehicle body is small. The upper limit is not particularly limited, but may be 1780 MPa from the viewpoint of suppressing mold wear. Further, the product (TS × El) of the tensile strength, which is an index of ductility, and the total elongation is preferably 15000 MPa ·% or more. If the product of the tensile strength and the total elongation is less than 15,000 MPa ·%, the applicable parts are limited and the contribution of weight reduction of the vehicle body is small.
[0069]
4. Plate thickness
The thickness of the hot-rolled steel plate according to the present embodiment is not particularly limited, but may be 0.6 to 8.0 mm. If the thickness of the hot-rolled steel plate is less than 0.6 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 plate according to the present embodiment may be 0.6 mm or more. It is preferably 1.2 mm or more or 1.4 mm or more. On the other hand, if the plate thickness exceeds 8.0 mm, it may be difficult to make the metal structure finer, and it may be difficult to obtain the above-mentioned metal structure. Therefore, the plate thickness may be 8.0 mm or less. It is preferably 6.0 mm or less.
[0070]
5. others
(5-1) Plating layer
The hot-rolled steel plate 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 plate. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of the electroplating layer include electrozinc plating, electroZn—Ni alloy plating, and the like. 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. To. The amount of plating adhesion is not particularly limited and may be the same as before. Further, it is also possible to further enhance the corrosion resistance by applying an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chrome-free chemical conversion treatment liquid) after plating.
[0071]
6. Manufacturing conditions
A suitable manufacturing method for the hot-rolled steel plate according to the present embodiment having the above-mentioned chemical composition and metal structure is as follows.
[0072]
In order to obtain a hot-rolled steel plate according to the present embodiment, the slab is heated under predetermined conditions, then hot-rolled, accelerated and cooled to a predetermined temperature range, then slowly cooled, and cooled until winding. It is important to control the history.
[0073]
In a suitable manufacturing method for the hot-rolled steel plate 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 the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel plate.
(1) The slab is held in a temperature range of 700 ° C. to 850 ° C. for 900 seconds or longer, then further heated and held in a temperature range of 1100 ° C. or higher for 6000 seconds or longer.
(2) 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.
(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) Within 1 second after the completion of hot rolling, after cooling to a temperature range of hot rolling completion temperature Tf-50 ° C or lower, accelerate to a temperature range of 600 to 730 ° C at an average cooling rate of 50 ° C / sec or higher. Cooling. However, it is a more preferable cooling condition to cool to a temperature range of the hot rolling completion temperature Tf-50 ° C. or less within 1 second after the completion of hot rolling.
(5) In the temperature range of 600 to 730 ° C, slow cooling with an average cooling rate of less than 5 ° C / s is performed for 2.0 seconds or longer.
(6) Cool to a temperature range of 250 ° C. or lower at an average cooling rate of 50 ° C./s or higher.
(7) Wind up in a temperature range of 250 ° C. or lower.
[0074]
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)
However, the [element symbol] in the above formula (1) indicates the content (mass%) of each element in the steel. If it does not contain an element, substitute 0.
[0075]
(6-1) Slab, slab temperature and holding time when subjected to hot rolling
As the slab to be subjected to hot rolling, a slab obtained by continuous casting, a slab obtained by casting / slabbing, or the like can be used, and if necessary, a slab obtained by adding hot working or cold working to them is used. be able to. The slab to be subjected to hot rolling needs to be held in a temperature range of 700 to 850 ° C. during heating for 900 seconds or longer, and then further heated and held in a temperature range of 1100 ° C. or higher for 6000 seconds or longer. When holding in the temperature range of 700 to 850 ° C., the steel plate temperature may be changed in this temperature range or may be constant. Further, when holding at 1100 ° C. or higher, the steel plate temperature may be changed in a temperature range of 1100 ° C. or higher, or may be constant.
[0076]
In the austenite transformation at 700 to 850 ° C., Mn is distributed between ferrite and austenite, and by prolonging the transformation time, Mn can be diffused in 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. Further, in order to make the austenite grains uniform during slab heating, it must be heated at 1100 ° C. or higher for 6000 seconds or longer.
[0077]
For hot rolling, it is preferable to use a levers 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 stages are hot-rolled using a tandem mill.
[0078]
(6-2) Hot rolling reduction rate: Total plate thickness reduction of 90% or more in the temperature range of 850 to 1100 ° C.
By performing hot rolling so that the total plate thickness is reduced by 90% or more in the temperature range of 850 to 1100 ° C., the recrystallized austenite grains are mainly made finer and into the unrecrystallized austenite grains. Accumulation of strain energy is promoted, recrystallization of austenite is promoted, atom diffusion of Mn is promoted, and the standard deviation of Mn concentration can be reduced. 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.
[0079]
The plate thickness reduction in the temperature range of 850 to 1100 ° C. means that the inlet plate thickness before the first pass in rolling in this temperature range is t 0, and the exit plate thickness after the final pass in rolling in this temperature range is t. When it is 1, it can be expressed as (t 0-t 1) / t 0 × 100 (%).
[0080] [0080]
(6-3) Hot rolling completion temperature Tf: T1 (° C.) or higher
It is desirable that the hot rolling completion temperature Tf is T1 (° C.) or higher. By setting the hot rolling completion temperature Tf to T1 (° C.) or higher, it is possible to suppress an excessive increase in the number of ferrite nucleation sites in austenite, and the final structure (metal structure of the hot-rolled steel plate after production) can be suppressed. The formation of ferrite can be suppressed, and a high-strength hot-rolled steel plate can be obtained.
[0081]
(6-4) Within 1 second after the completion of hot rolling, cool to a temperature range of hot rolling completion temperature Tf-50 ° C or lower, and then a temperature of 600 to 730 ° C at an average cooling rate of 50 ° C / sec or higher. Accelerate to cool. However, it is a more preferable cooling condition to cool to a temperature range of the hot rolling completion temperature Tf-50 ° C. or less within 1 second after the completion of hot rolling.
[0082]
In order to suppress the growth of austenite crystal grains finely divided by hot rolling, it is more preferable to cool at 50 ° C. or higher within 1 second after the completion of hot rolling. In order to cool to a temperature range of hot rolling completion temperature Tf-50 ° C or less within 1 second after the completion of hot rolling, cooling with a large average cooling rate is performed immediately after the completion of hot rolling, for example, cooling water is applied to the surface of the steel plate. It should be sprayed on. By cooling to a temperature range of Tf-50 ° C. or lower within 1 second after the completion of hot rolling, the crystal grain size of the surface layer can be made finer and the bending internal crack resistance can be improved.
[0083]
Further, by accelerating cooling to 730 ° C or lower at an average cooling rate of 50 ° C./sec or higher, it is possible to suppress the formation of ferrite and pearlite having a small amount of precipitation strengthening. This improves the strength of the hot-rolled steel plate. The average cooling rate here is a value obtained by dividing the temperature drop width of the steel plate from the start of accelerated cooling to the completion of accelerated cooling by the time required from the start of accelerated cooling to the completion of accelerated cooling. ..
[0084]
In cooling after the completion of hot rolling, if the cooling time to the temperature range of the hot rolling completion temperature Tf-50 ° C or lower is more than 1 second, the bending internal cracking property deteriorates. Further, when the average cooling rate during accelerated cooling is less than 50 ° C./sec or the cooling stop temperature is more than 730 ° C., ferrite transformation and / or pearlite transformation in which the amount of precipitation strengthening inside the steel plate is small becomes remarkable. , It becomes difficult to obtain a tensile strength of 980 MPa or more. Therefore, within 1 second after the completion of hot rolling, it should be cooled to a temperature range of hot rolling completion temperature Tf-50 ° C or lower, and then accelerated cooling to 730 ° C or lower at an average cooling rate of 50 ° C / sec or higher.Is preferable. The upper limit of the cooling rate is not particularly specified, but if the cooling rate is increased, the cooling equipment becomes large and the equipment cost increases. Therefore, considering the equipment cost, it is preferably 300 ° C./sec or less. Further, the cooling stop temperature for accelerated cooling is preferably 600 ° C. or higher.
[0085]
(6-5) In the temperature range of 600 to 730 ° C, slow cooling with an average cooling rate of less than 5 ° C / s is performed for 2.0 seconds or longer.
By performing slow cooling with an average cooling rate of less than 5 ° C./s for 2.0 seconds or longer in a temperature range of 600 to 730 ° C., ferrite that has been strengthened by precipitation can be sufficiently precipitated. This makes it possible to achieve both the strength and the ductility of the hot-rolled steel plate. The average cooling rate here is the temperature drop width of the steel plate from the cooling stop temperature of accelerated cooling to the end temperature of slow cooling divided by the time required from the stop of accelerated cooling to the end of slow cooling. It refers to the value.
[0086]
If the time for slow cooling is less than 2.0 seconds, the area ratio of the precipitate-strengthened ferrite does not reach the desired amount, and it becomes difficult to obtain the above action. Therefore, in the temperature range of 600 to 730 ° C, slow cooling with an average cooling rate of less than 5 ° C / s is performed for 2.0 seconds or longer. The time for slow cooling is preferably 3.0 seconds or longer, more preferably 4.0 seconds or longer. The upper limit of the time for slow cooling is determined by the equipment layout, but it should be generally less than 10.0 seconds. Further, although the lower limit of the average cooling rate for slow cooling is not particularly set, raising the temperature without cooling may require a large investment in equipment, and may be set to 0 ° C./s or higher.
[0087]
(6-6) Average cooling rate to winding temperature: 50 ° C / sec or more
In order to suppress the area fraction of pearlite and obtain a tensile strength of 980 MPa or more, the average cooling rate from the cooling stop temperature of slow cooling to the winding temperature is set to 50 ° C./sec or more. As a result, the matrix structure can be made hard. The average cooling rate here means the temperature drop width of the steel plate from the cooling stop temperature of slow cooling where the average cooling rate is less than 5 ° C / s to the winding temperature, and the average cooling rate is less than 5 ° C / s. It is the value divided by the time required from the stop of slow cooling to winding.
[0088]
If the average cooling rate is less than 50 ° C./sec, the area fraction of pearlite increases, the strength of the hot-rolled steel plate decreases, and the ductility decreases. Therefore, the average cooling rate from the cooling stop temperature of slow cooling where the average cooling rate is less than 5 ° C./s to the winding temperature is 50 ° C./sec or more.
[0089]
(6-7) Winding temperature: 250 ° C or less
The winding temperature should be 250 ° C or less. When the winding temperature exceeds 250 ° C., the transformation driving force from austenite to bcc becomes small, and the deformation strength of austenite becomes small. Therefore, when transforming from austenite to baynite and martensite, the length L 60 of the grain boundary where the crystal orientation difference is 60 ° with respect to the <110> direction increases, and L 60 / L 7 exceeds 0.60. Become. As a result, excellent shear workability cannot be obtained. Therefore, the winding temperature is set to 250 ° C. or lower.
Example
[0090]
Next, the effect of one aspect of the present invention will be described more specifically by way of examples, but 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.
[0091]
The steel Nos. of Tables 1 and 2 Steels having the chemical compositions shown in A to V were melted and continuously cast to produce slabs having a thickness of 240 to 300 mm. Using the obtained slab, a hot-rolled steel plate shown in Table 4 was obtained under the manufacturing conditions shown in Table 3. The average cooling rate for slow cooling was set to less than 5 ° C./s.
[0092]
For the obtained hot-rolled steel plate, the area fraction of the metal structure, L60 / L7, the standard deviation of the Mn concentration, and the average crystal grain size of the surface layer were determined by the above-mentioned method. The obtained measurement results are shown in Table 4.
[0093]
Evaluation method of the characteristics of hot-rolled steel plate
(1) Tensile strength characteristics
Among the mechanical properties of the obtained hot-rolled steel plate, the tensile strength characteristics (tensile strength TS and total elongation EL) were evaluated in accordance with JIS Z 2241: 2011. The test piece was JIS Z 2241: 2011 No. 5 test piece. The sampling position of the tensile test piece was 1/4 from the end in the plate width direction, and the direction perpendicular to the rolling direction was the longitudinal direction.
[0094]
When the tensile strength TS ≧ 980 MPa and the tensile strength TS × total elongation El ≧ 15000 (MPa ·%) were satisfied, it was judged to be acceptable as a hot-rolled steel plate having excellent strength and ductility.
[0095]
(2) Shear workability
The shear workability of the hot-rolled steel plate was measured by a punching test. A punched hole was prepared with a hole diameter of 10 mm, a clearance of 25%, and a punching speed of 3 m / s. Next, a cross section perpendicular to the rolling direction of the punched hole was embedded in the resin, and the cross section shape was photographed with a scanning electron microscope. In the obtained observation photograph, the processed cross section as shown in FIG. 1 can be observed. In the observation photograph, a straight line 1 along the lower surface of the steel plate and a straight line 2 passing through the apex of the burr (the point farthest from the lower surface of the steel plate in the burr portion and the plate thickness direction) and parallel to the lower surface of the steel plate are drawn, and these two straight lines are drawn. The distance (d in FIG. 1) was defined as the burr height. The maximum burr height is measured for 10 punched holes in each clearance, and if the maximum burr height is 15.0 μm or less even with a clearance of 25%, it is judged to be a hot-rolled steel plate with excellent shearability and passed. did.
[0096]
(3) Bending resistance and internal cracking resistance
For the bending test piece, a strip-shaped test piece of 100 mm × 30 mm was cut out from the 1/2 position in the width direction of the hot-rolled steel plate, and the bending internal crack resistance was evaluated by the following bending test.
JIS Z for both bending where the bending ridge is parallel to the rolling direction (L direction) (L-axis bending) and bending where the bending ridge is parallel to the direction perpendicular to the rolling direction (C direction) (C-axis bending). In accordance with 2248: 2014 (V block 90 ° bending test), the bending internal crack resistance was investigated, the minimum bending radius without cracking was obtained, and the average value of the minimum bending radius of the L axis and C axis was calculated by the plate thickness. The divided value was used as the limit bending R / t as an index value of bendability. When R / t ≦ 2.5, it was judged that the hot-rolled steel plate had excellent bending internal crack resistance.
[0097]
However, the presence or absence of cracks is determined by mirror-polishing the cross section of the test piece after the V block 90 ° bending test cut on a surface parallel to the bending direction and perpendicular to the plate surface, and then observing the cracks with an optical microscope to determine the presence or absence of cracks. When the crack length observed inside the bend exceeds 30 μm, it is judged that there is a crack.
[0098]
Table 4 shows the obtained measurement results.
[0099]
[table 1]

[0100]
[Table 2]

[0101]
[Table 3]

[0102]
[Table 4]

[0103]
As can be seen from Table 4, the production No. which is an example of the present invention. In 1, 2, 7, 12 to 24, 30 and 31, hot-rolled steel sheets having excellent strength, ductility and shear workability were obtained. Further, Production No. 1 in which the average particle size of the surface layer is less than 3.0 μm. In 1, 2, 13 to 20, 22 to 24, 30 and 31, hot-rolled steel sheets having excellent bending internal crack resistance were obtained.
[0104]
On the other hand, the production No. which is a comparative example. 3 to 6, 8 to 11 and 25 to 29 were inferior in any one or more of the characteristics (tensile strength TS, total elongation EL, shear workability).
Industrial availability
[0105]
According to the above aspect according to the present invention, it is possible to provide a hot-rolled steel plate having excellent strength, ductility and shear workability. Further, according to the above-mentioned preferable aspect of the present invention, it is possible to obtain a hot-rolled steel plate having the above-mentioned various characteristics and further suppressed in bending internal cracking, that is, excellent in bending internal cracking resistance. can.
The hot-rolled steel plate 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: 0.050 to 0.250%,
Si: 0.05 to 3.00%,
Mn: 1.00 to 4.00%,
One or more of Ti, Nb and V: 0.060 to 0.500% in total,
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,
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 to 0.1000%,
Bi: 0-0.020%,
One or more of Zr, Co, Zn and W: 0-1.00% in total, and
Sn: contains 0 to 0.050%,
The rest consists of Fe and impurities
In the metal structure with a cross section parallel to the rolling direction, at a depth of 1/4 of the plate thickness from the surface and at the center position in the plate width direction.
Area%, retained austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, pearlite is less than 5.0%, and the crystal orientation is about the <110> direction. L 60 / L 7, which is the ratio of the grain boundary length L 60 having a difference of 60 ° to the grain boundary length L 7 having a crystal orientation difference of 7 °, is less than 0.60.
The standard deviation of Mn concentration is 0.60% by mass or less,
Tensile strength is 980 MPa or more
A hot-rolled steel plate characterized by this.
[Claim 2]
The hot-rolled steel plate according to claim 1, wherein the average crystal grain size of the surface layer is less than 3.0 μm.
[Claim 3]
The chemical composition is by mass%
Cu: 0.01-2.00%,
Cr: 0.01-2.00%,
Mo: 0.01-1.00%,
Ni: 0.02-2.00%,
B: 0.0001 to 0.0100%,
Ca: 0.0005-0.0200%,
Mg: 0.0005-0.0200%,
REM: 0.0005 to 0.1000%, and
Bi: 0.0005-0.020%
Contains one or more selected from the group consisting of
The hot-rolled steel plate according to claim 1 or 2, characterized in that.