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 having high strength and excellent ductility and shear workability.
This application claims priority based on Japanese Patent Application No. 2019-040857 filed in Japan on March 6, 2019, and the contents thereof are incorporated herein by reference.
Background technology
[0002]
In recent years, from the viewpoint of global environmental protection, efforts have been made to reduce carbon dioxide emissions in many fields. Automakers 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 considered 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 to meet these demands. Among them, steel sheets containing retained austenite show excellent ductility due to transformation-induced plasticity (TRIP), and therefore many studies have been made so far.
[0004]
For example, in Patent Document 1, high strength for automobiles having excellent 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. Steel plates are disclosed. In a steel sheet containing retained austenite in its metal structure, austenite undergoes martensitic 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 miniaturizing 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 elongation 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]
Patent Documents 3 and 4 disclose a high-strength hot-rolled steel sheet having excellent ductility and stretch flangeability and a method for manufacturing the same. According to Patent Document 3, the product is cooled to a temperature range of 720 ° C. or lower within 1 second after the completion of hot rolling, and is allowed to stay in a temperature range of more than 500 ° C. and 720 ° C. or lower for a residence time of 1 to 20 seconds, and then 350 to. A method for producing a high-strength hot-rolled steel sheet, which is wound in a temperature range of 500 ° C. and has good ductility and stretch flangeability, is disclosed. Further, in Patent Document 4, bainite is mainly used, and an appropriate amount of polygonal ferrite and retained austenite are contained, and the average of grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in the steel structure excluding 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.
Prior art literature
Patent documents
[0007]
Patent Document 1: Japanese Patent Application Laid-Open No. 11-61326
Patent Document 2: Japanese Patent No. 4109619
Patent Document 3: Japanese Patent No. 565571
Patent Document 4: Japanese Patent No. 6241273
Outline of the invention
Problems to be solved by the invention
[0008]
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-molded blank plates are often manufactured by high-productivity shearing. In particular, in a high-strength steel plate of 980 MPa or more, the load required for post-treatment such as coining after shearing is large, so it is desired to control the height difference of the end face after shearing with particularly high accuracy.
[0009]
The techniques disclosed in Patent Documents 1 to 4 are all techniques for improving the press formability of ductility and expansion of holes, but there is no reference to the technique for improving the shear workability, and the member is press-molded. Post-treatment is required at the stage, and it is estimated that the manufacturing cost will increase.
[0010]
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 shear workability.
Means to solve problems
[0011]
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 sheet and the relationship between the metallographic structure and the mechanical properties. Completed the invention. In addition, having excellent shearing workability means that the height difference of the end face after shearing work is small. Further, having high strength or having excellent strength means that the tensile (maximum) strength is 980 MPa or more.
[0012]
(A) In order to obtain excellent tensile (maximum) strength, it is preferable that the matrix structure of the metal structure is hard. That is, it is preferable that the soft structure fraction such as ferrite and bainite is as small as possible.
[0013]
(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.
[0014]
(C) In order to impart excellent ductility to a high-strength hot-rolled steel sheet, it is effective to contain an appropriate amount of retained austenite that can enhance ductility by transformation-induced plasticity (TRIP).
[0015]
(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 minimum retention time after the transformation of bainite and tempered martensite is stopped. However, if it is held for too long, austenite is decomposed and the amount of retained austenite is reduced, so it is effective to set an appropriate holding time.
[0016]
(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 52 ° with respect to the <110> direction are 7 A large number of grain boundaries are formed.
[0017]
(F) 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, the grain boundary density is high and the crystal orientation difference is 7 ° with respect to the <110> direction, that is, the metal structure is uniformly dispersed, that is, the crystal orientation difference is 7 ° with respect to the <110> direction. In a metal structure having a large total grain boundary length, dislocations can be easily introduced into the metal structure during shearing, and deformation of the material during shearing is promoted. As a result, the height difference of the end face after shearing is suppressed.
[0018]
(G) In order to uniformly disperse the grain boundaries having a crystal orientation difference of 7 ° and the grain boundaries having a crystal orientation difference of 52 ° about the <110> direction, the standard deviation of the Mn concentration should be a certain value or less. There is a need. In order to keep the standard deviation of the Mn concentration below a certain value, the slab is heated in a temperature range of 700 ° C to 850 ° C for 900 seconds or longer, held at 1100 ° C or higher for 6000 seconds or longer, and 850 ° C to 1100. It is effective to perform hot rolling so that the total plate thickness is reduced by 90% or more in the temperature range of ° C. By preferably controlling the residence time in the temperature range of 700 ° C to 850 ° C and the reduction in plate thickness in the temperature range of 850 ° C to 1100 ° C, the microsegregation of Mn becomes smaller, so the standard deviation of the Mn concentration is set to a certain value or less. can do. As a result, the grain boundaries having a crystal orientation difference of 7 ° and the grain boundaries having a crystal orientation difference of 52 ° can be uniformly distributed with the <110> direction as the axis, and the height difference of the end face after shearing can be reduced. can do.
[0019]
(H) To increase the length of the grain boundaries having a crystal orientation difference of 7 ° and decrease the length of the grain boundaries having a crystal orientation difference of 52 ° about the <110> direction, the winding temperature It is effective to set the temperature above a predetermined temperature.
[0020]
The gist of the present invention made based on the above findings is as follows.
[0021]
(1) The hot-rolled steel sheet according to one aspect of the present invention has a chemical composition of mass%.
C: 0.100 to 0.250%,
Si: 0.05 to 3.00%,
Mn: 1.00 to 4.00%,
sol. Al: 0.001 to 2.000%,
P: 0.100% or less,
S: 0.0300% or less,
N: 0.1000% or less,
O: 0.0100% or less,
Ti: 0 to 0.300%,
Nb: 0 to 0.100%,
V: 0 to 0.500%,
Cu: 0 to 2.00%,
Cr: 0 to 2.00%,
Mo: 0 to 1.000%,
Ni: 0 to 2.00%,
B: 0 to 0.0100%,
Ca: 0-0.0200%,
Mg: 0-0.0200%,
REM: 0 to 0.1000%,
Bi: 0 to 0.020%,
One or more of Zr, Co, Zn and W: 0-1.00% in total, and
Sn: 0 to 0.050%
Containing, the balance 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.
Residual austenite is 3.0% or more in area%,
With the <110> direction as the axis, L 52 / L 7, which is the ratio between the grain boundary length L 52 having a crystal orientation difference of 52 ° and the grain boundary length L 7 having a crystal orientation difference of 7 °, is 0.10 or more and 0.18 or less,
The standard deviation of Mn concentration is 0.60% by mass or less,
It is characterized by having a tensile strength of 980 MPa or more.
(2) The hot-rolled steel sheet according to (1) above has a chemical composition of% by mass.
Ti: 0.005 to 0.300%,
Nb: 0.005 to 0.100%,
V: 0.005 to 0.500%,
Cu: 0.01-2.00%,
Cr: 0.01-2.00%,
Mo: 0.010 to 1.000%,
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.
The invention's effect
[0022]
According to the above aspect according to the present invention, a hot-rolled steel sheet having excellent strength, ductility and shear workability can be obtained. The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for automobile members, mechanical structural members, and building members.
A brief description of the drawing
[0023]
[Fig. 1] Fig. 1 is a diagram for explaining a method of measuring a height difference of an end face after shearing.
Embodiment for carrying out the invention
[0024]
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 limit range described below includes the lower limit value and the upper limit value. 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 hot-rolled steel sheet is mass% unless otherwise specified.
[0025]
1. 1. Chemical composition
The hot-rolled steel sheet according to the present embodiment has C: 0.100 to 0.250%, Si: 0.05 to 3.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and the balance: Fe and impurities including. Each element will be described in detail below.
[0026]
(1-1) C: 0.100 to 0.250%
C has the effect of stabilizing retained austenite. If the C content is less than 0.100%, it becomes difficult to obtain the desired surface integral of retained austenite. Therefore, the C content is set to 0.100% or more. The C content is preferably 0.120% or more, more preferably 0.150% or more. On the other hand, when the C content exceeds 0.250%, pearlite Is preferentially produced, and the production of retained austenite becomes insufficient, making it difficult to obtain a desired surface integral of retained austenite. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.220% or less.
[0027]
(1-2) Si: 0.05 to 3.00%
Si has the effect 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 deoxidizing the steel to make it sound (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, when the Si content exceeds 3.00%, the surface texture and chemical conversion treatment property of the steel sheet, 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 and 2.50% or less.
[0028]
(1-3) Mn: 1.00 to 4.00%
Mn has the effect of suppressing ferrite transformation and increasing the strength of the steel sheet. 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 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 obtained. It becomes difficult to obtain. 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.
[0029]
(1-4) sol. Al: 0.001 to 2.000%
Like Si, Al has the effect of deoxidizing the steel to make the steel sheet sound, and also has the effect of promoting the formation of retained austenite by suppressing the precipitation of cementite from austenite. 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.
[0030]
(1-5) P: 0.100% or less
P is an element that is generally contained as an impurity, but it is also an element that has the effect 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 formability and toughness are significantly deteriorated due to the 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.
[0031]
(1-6) S: 0.0300% or less
S is an element contained as an impurity and forms sulfide-based inclusions in the steel to reduce the formability of the hot-rolled steel sheet. When the S content exceeds 0.0300%, the formability 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.
[0032]
(1-7) 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 steel sheet. When the N content exceeds 0.1000%, the formability 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, 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, Nb and V are contained to make the metal structure finer, the precipitation of carbonitride The N content is preferably 0.0010% or more, and more preferably 0.0020% or more in order to promote the above.
[0033]
(1-8) 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 during deoxidation of the molten steel.
[0034]
The balance of the chemical composition of the hot-rolled steel sheet according to this embodiment is composed of Fe and impurities. In the present embodiment, the impurity means an ore as a raw material, scrap, or a substance mixed from a manufacturing environment, etc., which is permitted as long as it does not adversely affect the hot-rolled steel sheet according to the present embodiment. do.
[0035]
In addition to the above elements, the hot-rolled steel sheet according to the present embodiment may optionally contain Ti, Nb, V, Cu, Cr, Mo, Ni, B, Ca, Mg, REM, Bi, Zr, Co, Zn, W and Sn. It may be contained as an element. 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]
(1-9) Ti: 0.005 to 0.300%, Nb: 0.005 to 0.100% and V: 0.005 to 0.500%
Since Ti, Nb and V all precipitate as carbides or nitrides in steel and have an action of refining the metal structure by the pinning effect, one or more of these elements are contained. May be good. In order to obtain the effect of the above action more reliably, the Ti content should be 0.005% or more, the Nb content should be 0.005% or more, or the V content should be 0.005% or more. It is preferable to do so. 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, the Nb content is 0.100% or less, and the V content is 0.500% or less.
[0037]
(1-10) 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 the 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, when Ni contains 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.
[0038]
Cu has the effect of enhancing the hardenability of the steel sheet and the effect of precipitating it 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 cracking of the slab may occur. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.50% or less and 1.00% or less.
[0039]
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 deteriorated. Therefore, the Cr content is set to 2.00% or less.
[0040]
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 1.000% or less. The Mo content is preferably 0.500% or less and 0.200% or less.
[0041]
As mentioned above, Ni has the 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.
[0042]
As described above, B has the 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 formability 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.
[0043]
(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 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, 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 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 lanthanoids, and the content of the above REM refers to the total content of these elements. In the case of lanthanoids, they are 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 sheet 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 sheet according to the present embodiment is not impaired even if a small amount of Sn is contained, but defects may occur during hot rolling. The Sn content is 0.050% or less.

The scope of the claims
[Claim 1]
The chemical composition is mass%,
C: 0.100 to 0.250%,
Si: 0.05 to 3.00%,
Mn: 1.00 to 4.00%,
sol. Al: 0.001 to 2.000%,
P: 0.100% or less,
S: 0.0300% or less,
N: 0.1000% or less,
O: 0.0100% or less,
Ti: 0 to 0.300%,
Nb: 0 to 0.100%,
V: 0 to 0.500%,
Cu: 0 to 2.00%,
Cr: 0 to 2.00%,
Mo: 0 to 1.000%,
Ni: 0 to 2.00%,
B: 0 to 0.0100%,
Ca: 0-0.0200%,
Mg: 0-0.0200%,
REM: 0 to 0.1000%,
Bi: 0 to 0.020%,
One or more of Zr, Co, Zn and W: 0-1.00% in total, and
Sn: 0 to 0.050%
Containing, the balance 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.
Residual austenite is 3.0% or more in area%,
With the <110> direction as the axis, L 52 / L 7, which is the ratio between the grain boundary length L 52 having a crystal orientation difference of 52 ° and the grain boundary length L 7 having a crystal orientation difference of 7 °, is 0.10 or more and 0.18 or less,
The standard deviation of Mn concentration is 0.60% by mass or less,
A hot-rolled steel sheet characterized by a tensile strength of 980 MPa or more.
[Claim 2]
The chemical composition is by mass%
Ti: 0.005 to 0.300%,
Nb: 0.005 to 0.100%,
V: 0.005 to 0.500%,
Cu: 0.01-2.00%,
Cr: 0.01-2.00%,
Mo: 0.010 to 1.000%,
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%
The hot-rolled steel sheet according to claim 1, wherein the hot-rolled steel sheet contains one kind or two or more kinds selected from the group consisting of.