CN118139998A - Hot rolled steel sheet - Google Patents

Hot rolled steel sheet Download PDF

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Publication number
CN118139998A
CN118139998A CN202280066930.XA CN202280066930A CN118139998A CN 118139998 A CN118139998 A CN 118139998A CN 202280066930 A CN202280066930 A CN 202280066930A CN 118139998 A CN118139998 A CN 118139998A
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steel sheet
less
hot
rolled steel
content
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荻巢靖之
豊田武
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Nippon Steel Corp
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Nippon Steel and Sumitomo Metal Corp
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    • C22C—ALLOYS
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
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    • C21D2211/002—Bainite
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    • C21D2211/00—Microstructure comprising significant phases
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Abstract

The hot-rolled steel sheet has a predetermined chemical composition and a predetermined metal structure, wherein the {001} <110>, {111} <110>, and {112} <110> orientation groups have a polar density of 2.0 to 8.0 in the texture of the region having a depth of 1/8 from the surface to the thickness of the sheet, the {110} <112> orientation group has a polar density of 2.0 to 4.0 in the texture of the region having a depth of 1/8 from the surface to the thickness of the sheet, and the hot-rolled steel sheet has a tensile strength of 980MPa or more.

Description

Hot rolled steel sheet
Technical Field
The present invention relates to a hot rolled steel sheet.
The present application claims priority based on the japanese patent application No. 2021-168727, 10-14 of 2021, and the contents of which are incorporated herein by reference.
Background
From the viewpoint of global environment protection, the weight of automobile bodies has been reduced for the purpose of improving the fuel efficiency of automobiles. In order to reduce the weight of an automobile body, it is necessary to increase the strength of a steel sheet applied to the automobile body. However, in general, if the strength of the steel sheet is increased, the formability is lowered.
As a method for improving formability of a steel sheet, there is a method of containing retained austenite in a metal structure of a steel sheet. However, if the residual austenite is contained in the metal structure of the steel sheet, ductility is improved, but isotropy of ductility may be deteriorated and hole expansibility may be deteriorated. In bending, reaming and flanging, it is required that the anisotropy of ductility be reduced, that is, that the ductility be excellent in isotropy. Further, when the above-described processing is performed, excellent hole expansibility is also required.
Patent document 1 discloses a hot-rolled steel sheet having a microstructure mainly composed of bainite, in which the hard phase composed of martensite and/or austenite is 3% or more and less than 20% in terms of area fraction, the microstructure having an aspect ratio of 3 or more in the hard phase present in the central portion of the sheet thickness is 60% or more, the length in the rolling direction of the hard phase present in the central portion of the sheet thickness is less than 20 μm, the sum of the X-ray random intensity ratios of <011> orientation and <111> orientation seen from the rolling direction is 3.5 or more, and the X-ray random intensity ratio of <001> orientation seen from the rolling direction is 1.0 or less.
Prior art literature
Patent literature
Patent document 1: international publication No. 2016/010004
Disclosure of Invention
Problems to be solved by the invention
However, in patent document 1, in order to further reduce the weight of the automobile body, further improvement in strength is required. In patent document 1, isotropy of ductility is not considered.
The purpose of the present invention is to provide a hot-rolled steel sheet that has high strength and excellent ductility, isotropy and hole expansibility.
Means for solving the problems
In view of the above problems, the inventors of the present invention have repeatedly studied the relationship between the chemical composition and the metal structure of a hot-rolled steel sheet and the mechanical properties, and as a result, have obtained the following findings, and have completed the present invention.
The inventors of the present invention recognized that: in order to improve the isotropy and hole expansibility of the ductility of the hot-rolled steel sheet, it is important to control the texture of the surface layer region and the inner region of the hot-rolled steel sheet. Furthermore, the inventors of the present invention recognized that: in order to control the texture of the surface layer region and the inner region of the hot rolled steel sheet, in particular, it is effective to control the finish rolling conditions.
The gist of the present invention based on the above knowledge is as follows.
(1) The hot rolled steel sheet according to an embodiment of the present invention has a chemical composition comprising, in mass%:
C：0.100～0.350％、
Si：0.010～3.00％、
Mn：1.00～4.00％、
sol.Al：0.001～2.000％、
si+sol.al: more than 1.00%,
Ti：0.010～0.380％、
P:0.100% or less,
S:0.0300% or less,
N: less than 0.1000 percent,
O:0.0100% or less,
Nb：0～0.100％、
V：0～0.500％、
Cu：0～2.00％、
Cr：0～2.00％、
Mo：0～1.00％、
Ni：0～2.00％、
B：0～0.0100％、
Ca：0～0.0200％、
Mg：0～0.0200％、
REM：0～0.1000％、
Bi：0～0.020％、
1 Or more than 2 of Zr, co, zn and W: 0 to 1.00%, sum of
Sn：0～0.050％，
The remainder comprising Fe and impurities,
The metallic structure in the region from 1/8 depth to 3/8 depth from the surface to the plate thickness contains retained austenite in area%: 10-20 percent of primary martensite: less than 10% and bainite: 70 to 90 percent,
In the texture of the region from the surface to a depth of 1/8 of the plate thickness, the polar densities of the {001} <110>, {111} <110> and {112} <110> orientation groups are 2.0 to 8.0,
In the texture of the region from 1/8 depth to 1/2 depth of the plate thickness from the surface, the polar density of the {110} <112> orientation is 2.0 to 4.0,
The hot-rolled steel sheet has a tensile strength of 980MPa or more.
(2) The hot-rolled steel sheet according to the above (1), wherein the chemical composition may contain 1 or 2 or more kinds of elements selected from the following elements in mass%:
Nb：0.005～0.100％、
V：0.005～0.500％、
Cu：0.01～2.00％、
Cr：0.01～2.00％、
Mo：0.01～1.00％、
Ni：0.02～2.00％、
B：0.0001～0.0100％、
Ca：0.0005～0.0200％、
Mg：0.0005～0.0200％、
REM:0.0005 to 0.1000 percent
Bi：0.0005～0.020％。
Effects of the invention
According to the above aspect of the present invention, a hot rolled steel sheet having high strength and excellent ductility and isotropy and hole expansibility can be provided.
Detailed Description
The chemical composition and the microstructure of the hot-rolled steel sheet according to the present embodiment will be described in more detail below. However, the present invention is not limited to the configuration disclosed in the present embodiment, and various modifications may be made without departing from the scope of the present invention.
The lower limit and the upper limit of the numerical value limiting ranges described in the following "to" are included in the ranges. For values expressed as "below" or "above," the value is not included in the range of values. In the following description, "%" concerning chemical composition is "% by mass" unless otherwise specified.
Chemical composition
The chemical composition of the hot-rolled steel sheet according to the present embodiment includes C:0.100 to 0.350 percent of Si:0.010 to 3.00 percent of Mn: 1.00-4.00%, sol.Al:0.001 to 2.000 percent of Si+sol.Al: more than 1.00 percent of Ti:0.010 to 0.380 percent, P:0.100% or less, S:0.0300% or less, N: less than 0.1000%, O: less than 0.0100% and the remainder: fe and impurities.
Hereinafter, each element will be described in detail.
C：0.100～0.350％
C is an element necessary for obtaining a desired strength. When the C content is less than 0.100%, it becomes difficult to obtain a desired strength. Therefore, the C content is set to 0.100% or more. The C content is preferably 0.120% or more and 0.150% or more.
On the other hand, when the C content exceeds 0.350%, MA (mixed phase of primary martensite and retained austenite) is easily generated due to a slow transformation rate, and it becomes difficult to obtain excellent isotropy and hole expansibility of ductility. Therefore, the C content is set to 0.350% or less. The C content is preferably 0.330% or less and 0.310% or less.
Si：0.010～3.00％
Si has a function of delaying precipitation of cementite. This action can increase the area ratio of retained austenite, which is the amount of austenite remaining without transformation. Further, strength can be improved by securing a large amount of solid solution C in the hard phase and preventing coarsening of cementite. Si itself also has an effect of improving the strength of the hot-rolled steel sheet by solid solution strengthening. Si also has a function of strengthening steel (suppressing defects such as occurrence of voids in steel) by deoxidizing. If the Si content is less than 0.010%, the effect due to the above-mentioned action cannot be obtained. Therefore, the Si content is set to 0.010% or more. The Si content is preferably 0.50% or more, 1.00% or more, 1.20% or more, or 1.50% or more.
On the other hand, if the Si content exceeds 3.00%, precipitation of cementite is significantly delayed, and the amount of retained austenite becomes excessive, which is not preferable. Further, the surface properties and chemical convertibility, and further ductility and weldability of the hot-rolled steel sheet are remarkably deteriorated, and the a 3 transformation point is remarkably increased. This makes it difficult to perform hot rolling stably. 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.
Mn：1.00～4.00％
Mn has an effect of suppressing ferrite transformation and increasing the strength of the hot-rolled steel sheet. When the Mn content is less than 1.00%, the desired strength cannot be obtained. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.50% or more and 1.80% or more.
On the other hand, when the Mn content exceeds 4.00%, the isotropy and hole expansibility of the ductility of the hot-rolled steel sheet deteriorate. 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.
sol.Al：0.001～2.000％
Sol.al has the following effects like Si: the steel is healed by deoxidization, and the formation of residual austenite is promoted by suppressing precipitation of cementite from austenite. When the al content is less than 0.001%, the effect due to the above-mentioned action cannot be obtained. Therefore, the sol.Al content is set to 0.001% or more. The sol.Al content is preferably 0.010% or more.
On the other hand, when the sol.al content exceeds 2.000%, the above effect is saturated and is economically unfavorable. Further, the a 3 transformation point significantly increases, and it becomes difficult to stably perform hot rolling. Therefore, the sol.al content is set to 2.000% or less. The sol.Al content is preferably 1.500% or less and 1.300% or less.
In the present embodiment, sol.al means acid-soluble Al, and indicates solid-solution Al existing in steel in a solid-solution state.
Si+sol.al: more than 1.00%
Both Si and sol.al have an effect of delaying precipitation of cementite, and by this effect, the amount of austenite remaining without transformation, that is, the area ratio of retained austenite can be increased. When the total content of Si and sol.al is less than 1.00%, the effect due to the above-described action cannot be obtained. Therefore, the total content of Si and sol.al is set to 1.00% or more. Preferably 1.20% or more and 1.50% or more.
Si of "si+sol.al" represents the content of Si in mass%, and sol.al represents the content of sol.al in mass%.
Ti：0.010～0.380％
Ti is an element effective for suppressing recrystallization of austenite between hot rolled frames and grain growth. By suppressing recrystallization of austenite between frames, strain can be further accumulated. As a result, the texture of the hot rolled steel sheet can be preferably controlled. If the Ti content is less than 0.010%, the above-mentioned effects cannot be obtained. Therefore, the Ti content is set to 0.010% or more. Preferably 0.050% or more, 0.070% or more, or 0.080% or more.
On the other hand, if the Ti content exceeds 0.380%, inclusions due to TiN are generated, and the toughness of the hot-rolled steel sheet is deteriorated. Therefore, the Ti content is set to 0.380% or less. Preferably 0.320% or less or 0.300% or less.
P: less than 0.100%
P is an element that is generally contained in steel as an impurity, but has an effect of improving the strength of a hot-rolled steel sheet by solid solution strengthening. Therefore, P may be positively contained. However, P is an element that is liable to segregate, and if the P content exceeds 0.100%, deterioration of hole expansibility and isotropy of ductility due to grain boundary segregation becomes remarkable. 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 is not particularly limited, but is preferably set to 0.001% from the viewpoint of refining cost.
S:0.0300% or less
S is an element contained in steel as an impurity, and forms sulfide-based inclusions in steel, thereby deteriorating the hole expansibility and the isotropy of the ductility of the hot-rolled steel sheet. If the S content exceeds 0.0300%, the hole expansibility and the isotropy of the ductility of the hot-rolled steel sheet are significantly deteriorated. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0050% or less.
The lower limit of the S content is not particularly limited, but is preferably set to 0.0001% from the viewpoint of refining cost.
N: less than 0.1000%
N is an element contained in steel as an impurity, and has an effect of degrading the hole expansibility and the isotropy of the ductility of the hot-rolled steel sheet. When the N content exceeds 0.1000%, the hole expansibility and the isotropy of the ductility of the hot-rolled steel sheet are significantly deteriorated. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0800% or less and 0.0700% or less.
The lower limit of the N content is not particularly limited, but in order to promote precipitation of carbonitrides, the N content is preferably set to 0.0010% or more, and more preferably set to 0.0020% or more.
O:0.0100% or less
If O is contained in a large amount in steel, coarse oxides are formed as starting points of fracture, causing brittle fracture and hydrogen induced cracking. Therefore, the O content is set to 0.0100% or less. The O content is preferably set to 0.0080% or less and 0.0050% or less.
In order to disperse a large amount of fine oxides during deoxidation of molten steel, the O content may be set to 0.0005% or more and 0.0010% or more.
The remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment contains Fe and impurities. In the present embodiment, the impurities are elements mixed from ores, scraps, manufacturing environments, and the like as raw materials or intentionally added in a trace amount, and are allowed within a range that does not adversely affect the hot-rolled steel sheet of the present embodiment.
The chemical composition of the hot rolled steel sheet according to the present embodiment may contain the following elements as optional elements in addition to the above elements. The lower limit of the content in the case of not containing the above optional elements is 0%. Hereinafter, each optional element will be described in detail.
Nb: 0.005-0.100% and V: 0.005-0.500%
Nb and V are elements that suppress recrystallization of austenite and grain growth between hot-rolled frames, similarly to Ti. Therefore, 1 or 2 or more of these elements may be contained. In order to obtain the effect by the above action more reliably, it is preferable to set the Nb content to 0.005% or more or set the V content to 0.005% or more.
However, even if these elements are contained excessively, the effects due to the above-mentioned actions are saturated, and are not economically preferable. Therefore, the Nb content is set to 0.100% or less, and the V content is set to 0.500% or less.
Cu:0.01 to 2.00 percent of Cr:0.01 to 2.00 percent of Mo:0.01 to 1.00 percent of Ni:0.02 to 2.00 percent and B: 0.0001-0.0100%
Cu, cr, mo, ni and B each have an effect of improving hardenability of a hot-rolled steel sheet. In addition, cr and Ni have an effect of stabilizing retained austenite, and Cu and Mo have an effect of precipitating carbide in steel to improve strength of a hot-rolled steel sheet. Further, when Cu is contained, ni has an effect of effectively suppressing grain boundary cracking of a slab due to Cu. Accordingly, 1 or 2 or more of these elements may be contained.
As described above, cu has an effect of improving hardenability of a steel sheet and an effect of improving strength of a hot rolled steel sheet by precipitating carbide in the steel at a low temperature. In order to obtain the effect by the above action more reliably, the Cu content is preferably set to 0.01% or more.
However, if the Cu content exceeds 2.00%, there is a possibility that grain boundary cracking of the slab may occur. Therefore, the Cu content is set to 2.00% or less.
As described above, cr has an effect of improving hardenability of the steel sheet and an effect of stabilizing retained austenite. In order to obtain the effect by the above action more reliably, the Cr content is preferably set to 0.01% or more.
However, when the Cr content exceeds 2.00%, the chemical conversion treatability of the hot-rolled steel sheet is significantly lowered. Therefore, the Cr content is set to 2.00% or less.
As described above, mo has an effect of improving hardenability of a steel sheet and an effect of precipitating carbide in the steel to improve strength. In order to obtain the effect by the above action more reliably, the Mo content is preferably set to 0.01% or more.
However, even if the Mo content is set to more than 1.00%, the effect due to the above action is saturated, which is not economically preferable. Therefore, the Mo content is set to 1.00% or less.
As described above, ni has an effect of improving hardenability of the steel sheet. In addition, when Cu is contained, ni has an effect of effectively suppressing grain boundary cracking of slabs due to Cu. In order to obtain the effect by the above action more reliably, the Ni content is preferably set to 0.02% or more.
Ni is an expensive element, and thus it is not economically preferable to contain a large amount. Therefore, the Ni content is set to 2.00% or less.
As described above, B has an effect of improving hardenability of the steel sheet. In order to obtain the effect by this action more reliably, the B content is preferably set to 0.0001% or more.
However, when the B content exceeds 0.0100%, the hole expansibility and the isotropy of the ductility of the hot-rolled steel sheet are significantly deteriorated, and therefore the B content is set to 0.0100% or less.
Ca:0.0005 to 0.0200 percent, mg: 0.0005-0.0200%, REM:0.0005 to 0.1000 percent of Bi:0.0005 to 0.020%
Ca. Mg and REM each have an effect of improving the formability of the hot rolled steel sheet by controlling the shape of the inclusions to a preferable shape. In addition, bi has an effect of improving formability of a hot rolled steel sheet by refining a solidification structure. Therefore, 1 or 2 or more of these elements may be contained. In order to obtain the effect by the above action more reliably, it is preferable to set at least 0.0005% of any one of Ca, mg, REM and Bi. However, if the Ca content or the Mg content exceeds 0.0200% or if the REM content exceeds 0.1000%, inclusions are excessively formed in the steel, and on the contrary, there is a possibility that the hole expansibility and the isotropy of the ductility of the hot-rolled steel sheet may deteriorate. In addition, even if the Bi content is set to more than 0.020%, the effect due to the above action is saturated, which is not economically preferable. Therefore, the Ca content and Mg content were set to 0.0200% or less, the REM content was set to 0.1000% or less, and the Bi content was set to 0.020% or less. The Bi content is preferably 0.010% or less.
Here, REM means 17 elements in total including Sc, Y and lanthanoid, and the content of REM means the total content of these elements. In the case of lanthanoids, it is industrially added in the form of misch metal alloys.
1 Or more than 2 of Zr, co, zn and W: 0 to 1.00% by weight of Sn:0 to 0.050 percent
Regarding Zr, co, zn, and W, the inventors of the present invention confirmed that: even if these elements are contained in a total amount of 1.00% or less, the effect of the hot-rolled steel sheet of the present embodiment is not impaired. Accordingly, 1 or 2 or more of Zr, co, zn, and W may be contained in total of 1.00% or less.
The inventors of the present invention have confirmed that even if Sn is contained in a small amount, the effect of the hot-rolled steel sheet of the present embodiment is not impaired, but there is a possibility that defects may occur during hot rolling, and therefore the Sn content is set to 0.050% or less.
The chemical composition of the hot-rolled steel sheet may be measured by a general analytical method. For example, the measurement may be performed by using ICP-AES (inductively coupled plasma-atomic emission Spectrometry; inductively Coupled Plasma-Atomic Emission Spectrometry). The sol.Al may be measured by ICP-AES using a filtrate obtained by thermally decomposing a sample with an acid. The measurement of C and S is performed by a combustion-infrared absorption method, the measurement of N is performed by an inert gas melting-thermal conductivity method, and the measurement of O is performed by an inert gas melting-non-dispersive infrared absorption method.
Metal structure of hot rolled steel sheet
Next, the microstructure of the hot-rolled steel sheet according to the present embodiment will be described.
In the hot-rolled steel sheet according to the present embodiment, the metal structure in the region from 1/8 depth to 3/8 depth from the surface to the plate thickness contains retained austenite in area%: 10-20 percent of primary martensite: less than 10% and bainite: 70 to 90%, wherein the polar densities of the {001} <110>, {111} <110> and {112} <110> orientation groups are 2.0 to 8.0 in the texture of the region having a depth of 1/8 from the surface to the plate thickness, and the polar densities of the {110} <112> orientation groups are 2.0 to 4.0 in the texture of the region having a depth of 1/8 from the surface to the plate thickness.
In the present embodiment, the area ratio of retained austenite, primary martensite, and bainite at a position 1/4 depth from the surface of the plate thickness section parallel to the rolling direction (a region 1/8 depth from the surface to 3/8 depth from the surface) is defined. The reason for this is that the microstructure at this location represents a representative microstructure of the hot rolled steel sheet.
Retained austenite: 10 to 20 percent
The retained austenite is an isotropic structure that improves hole expansibility and ductility of the hot-rolled steel sheet. If the area ratio of the retained austenite is less than 10%, isotropy of desired hole expansibility and ductility cannot be obtained. Therefore, the area ratio of the retained austenite is set to 10% or more. Preferably 12% or more or 13% or more.
On the other hand, if the area ratio of the retained austenite exceeds 20%, the desired strength cannot be obtained. Therefore, the area ratio of the retained austenite is set to 20% or less. Preferably 18% or less and 17% or less.
Primary martensite: less than 10 percent
The primary martensite is a hard structure, and thus contributes to improvement in strength of the hot-rolled steel sheet. However, the primary martensite is also an isotropic structure lacking hole expansibility and ductility. If the area ratio of the primary martensite exceeds 10%, isotropy of desired hole expansibility and ductility cannot be obtained. Therefore, the area ratio of the primary martensite is set to 10% or less. Preferably 8% or less, 6% or less, 4% or less, or 2% or less. The area ratio of the primary martensite may be 0%.
Bainite: 70 to 90 percent
Bainite is an isotropic structure that improves the strength and ductility of hot rolled steel sheets. If the area ratio of bainite is less than 70%, the desired strength cannot be obtained. Therefore, the area ratio of bainite is set to 70% or more. Preferably 73% or more, 75% or more, or 77% or more.
On the other hand, if the area ratio of bainite exceeds 90%, the strength becomes too high, and the desired hole expansibility cannot be obtained. Therefore, the area ratio of bainite is set to 90% or less. Preferably less than 90%, 88% or less than 85%.
The area ratio of the above-described tissues other than the retained austenite was measured by the following method.
From the hot-rolled steel sheet, test pieces were collected so that the metal structure was observed at a depth of 1/4 of the plate thickness from the surface (a region of 1/8 of the plate thickness to 3/8 of the plate thickness from the surface) of the plate thickness cross section parallel to the rolling direction. Subsequently, the plate thickness cross section was polished, and then the polished surface was subjected to nitric acid ethanol etching, and the region of 30 μm×30 μm was observed by using an optical microscope and a Scanning Electron Microscope (SEM). The observation area is set to at least 3 areas. The area ratio of bainite is obtained by image analysis of a photograph of the structure obtained by observation of the structure. Then, after Lepera etching was performed at the same observation position, the structure was observed by using an optical microscope and a scanning electron microscope, and the obtained structure photograph was subjected to image analysis, thereby obtaining the area ratio of the primary martensite.
In the above-described tissue observation, each tissue was identified by the following method.
Since primary martensite is a structure having a high dislocation density and a lower structure such as a slab or a lath within a crystal grain, it can be distinguished from other metal structures by electron channel contrast imaging using a scanning electron microscope.
The following structure is considered as bainite: the structure is composed of a collection of lath-shaped crystal grains, and does not contain Fe-based carbide having a length of 20nm or more in the structure, which is not primary martensite, or contains Fe-based carbide having a length of 20nm or more in the structure, and has a single modification, that is, a structure of Fe-based carbide elongated in the same direction. Here, the Fe-based carbide extending in the same direction means that the difference in the extending direction of the Fe-based carbide is within 5 °.
The area ratio of retained austenite was measured by the following method.
In the present embodiment, the area ratio of the retained austenite is measured by X-ray diffraction. First, the integrated strength of the total 6 peaks of α (110), α (200), α (211), γ (111), γ (200), and γ (220) was obtained by using co—kα rays at a depth of 1/4 of the plate thickness from the surface (a region of 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface) in the plate thickness cross section parallel to the rolling direction of the hot rolled steel plate, and the volume fraction of the retained austenite was calculated by using the strength averaging method. The volume fraction of the retained austenite is regarded as the area fraction of the retained austenite.
The {001} <110>, {111} <110>, and {112} <110> orientation groups in the texture of the region having a depth of 1/8 of the plate thickness from the surface: 2.0 to 8.0
If the polar density of {001} <110>, {111} <110>, and {112} <110> orientation groups in the texture of the region (hereinafter, sometimes referred to as the surface layer region) having a depth of 1/8 of the plate thickness from the surface to the surface is lower than 2.0, the isotropy and hole expansibility of the ductility of the hot-rolled steel sheet deteriorate. Therefore, the polar densities of {001} <110>, {111} <110> and {112} <110> orientation groups in the texture of the surface layer region are set to 2.0 or more. Preferably 2.2 or more, 2.5 or more, or 2.7 or more.
If the polar densities of {001} <110>, {111} <110> and {112} <110> orientation groups in the texture of the surface layer region exceed 8.0, the isotropy and hole expansibility of the ductility of the hot-rolled steel sheet deteriorate. Therefore, the polar densities of {001} <110>, {111} <110> and {112} <110> orientation groups in the texture of the surface layer region are set to 8.0 or less. Preferably 7.5 or less or 7.0 or less.
The {110} <112> orientation in the texture of the region from 1/8 depth to 1/2 depth from the surface to the plate: 2.0 to 4.0
If the polar density of the {110} <112> orientation in the texture of the region (hereinafter, may be referred to as the internal region) having a depth of 1/8 to 1/2 of the plate thickness from the surface exceeds 4.0, the isotropy and hole expansibility of the hot-rolled steel sheet deteriorate. Therefore, the {110} <112> orientation in the texture of the inner region has a polar density of 4.0 or less. Preferably 3.6 or less, 3.2 or less, or 3.0 or less.
From the viewpoint of suppressing deterioration of strength, the polar density of {110} <112> orientation in the texture of the internal region is set to 2.0 or more. Preferably 2.3 or more or 2.5 or more.
As the polar density, an OIM Analysis (registered trademark) manufactured by ameteek corporation, which is a combination of a scanning electron microscope and an EBSD Analysis device, was used. From a crystal orientation distribution function (ODF: orientation Distribution Function) representing a three-dimensional texture calculated by calculation using orientation data and spherical harmonics measured by an EBSD (electron back scattering diffraction; electron Back Scattering Diffraction) method, the polar densities of {001} <110>, {111} <110> and {112} <110> orientation groups in the texture of the surface layer region and {110} <112> in the texture of the inner region are obtained.
The measurement range was set to a region having a depth of 1/8 of the plate thickness from the surface to the surface of the surface layer region, and a region having a depth of 1/8 of the plate thickness from the surface to 1/2 of the plate thickness from the surface of the inner region. The measurement pitch was set to 5 μm/step.
{ Hkl } represents a crystal plane parallel to the rolling surface, and < uvw > represents a crystal direction parallel to the rolling direction. That is, { hkl } < uvw > represents a crystal in which { hkl } is oriented in the plate surface normal direction and < uvw > is oriented in the rolling direction.
The rolling direction of the hot rolled steel sheet can be determined by the following method.
First, test pieces were collected so that the thickness and cross section of the hot-rolled steel sheet could be observed. The plate thickness cross section of the test piece thus collected was finished by mirror polishing, and then observed with an optical microscope. The observation range was set to the entire thickness of the sheet thickness, and the area with dark brightness was determined as an inclusion. Among the inclusions, those having a long axis length of 40 μm or more were identified as rolling directions in parallel with the direction in which the inclusions were stretched.
Mechanical properties
The hot-rolled steel sheet according to the present embodiment has a tensile (maximum) strength of 980MPa or more. By setting the tensile strength to 980MPa or more, weight reduction of the vehicle body can be further facilitated. More preferably, the tensile strength is 1180MPa or more. The upper limit is not particularly limited, but may be 1470MPa, 1300MPa or less, or 1200MPa or less.
The difference between the total elongation in the C direction and the total elongation in the L direction (total elongation in the L direction—total elongation in the C direction)/total elongation in the C direction), which is an index of isotropy of the ductility, is preferably ±3.0% or less.
The index of hole expansibility, that is, the hole expansibility is preferably 40% or more.
Tensile strength TS and total elongation EL were measured using JIS Z2241: 2011, test piece No.5 according to JIS Z2241: 2011. The collecting position of the tensile test piece may be set to be 1/4 of the distance from the end in the width direction of the sheet, and the direction (direction C) perpendicular to the rolling direction may be set to be the longitudinal direction. The total elongation EL was measured by also performing a tensile test on a tensile test piece having a longitudinal direction which is a direction (L direction) parallel to the rolling direction.
The hole expansibility λ was measured using JIS Z2241: 2011, test piece No. 5 according to JIS Z2256: 2010, measurement is performed. The collecting position of the reaming test piece is set to be 1/4 part of the end of the hot rolled steel plate in the width direction.
Plate thickness
The thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be set to 1.2 to 8.0mm. By setting the plate thickness of the hot-rolled steel sheet to 1.2mm or more, it becomes easy to ensure the rolling completion temperature, and the rolling load can be reduced, enabling easy hot rolling. Therefore, the thickness of the hot-rolled steel sheet according to the present embodiment may be 1.2mm or more. Preferably 1.4mm or more. Further, when the plate thickness is 8.0mm or less, control of the texture may be difficult, and it may be difficult to obtain the texture. Therefore, the plate thickness may be set to 8.0mm or less. Preferably 6.0mm or less.
Coating layer
The hot-rolled steel sheet of the present embodiment having the above-described chemical composition and metallic structure may be provided with a plating layer on the surface thereof for the purpose of improving corrosion resistance and the like, to thereby produce a surface-treated steel sheet. The plating layer may be a plating layer or a hot dip plating layer. Examples of the plating layer include a zinc plating layer and a Zn-Ni alloy plating layer. Examples of the hot dip coating layer include a hot dip galvanized layer, an alloyed hot dip galvanized layer, a hot dip aluminized layer, a hot dip zn—al alloy layer, a hot dip zn—al—mg alloy layer, and a hot dip zn—al—mg—si alloy layer. The plating deposition amount is not particularly limited, and may be set as in the conventional case. Further, the corrosion resistance can be further improved by performing an appropriate chemical conversion treatment (for example, coating and drying of a silicate-based chromium-free chemical conversion treatment solution) after plating.
Next, a preferred method for producing the hot-rolled steel sheet according to the present embodiment will be described. The method for producing a hot-rolled steel sheet according to the present embodiment preferably includes the following steps (a) to (d). The temperature in the following description refers to the surface temperature of the steel sheet unless otherwise specified.
(A) And a heating step of heating the slab having the chemical composition to a temperature range of 1100 ℃ or higher and 1350 ℃ or lower.
(B) And a finish rolling step of finish rolling the heated slab using a rolling mill having a plurality of stands, wherein the following conditions (I) to (V) are satisfied.
(I) The finish rolling start temperature is set to 850 ℃ or higher.
(II) in the last 4 frames among the plurality of frames, rolling was performed so that the sigma represented by the following formula (1) becomes 40 to 80.
σ＝exp(0.753+3000/T)×ε 0.21×ε' 0.13 (1)
Wherein T is the temperature (DEG C) immediately before entering each rack, epsilon is the equivalent plastic strain, and epsilon' is the strain rate.
(III) the inter-pass time between the last 4 frames is set to 0.1-10.0 seconds.
(IV) the cumulative rolling reduction of the last 4 frames is set to 60% or more.
(V) the finishing temperature is set to 850 to 1000 ℃.
(C) And a cooling step of performing air cooling for 2.0 to 4.0 seconds after finishing finish rolling, and then performing cooling so that the average cooling rate up to a temperature range of 450 to 550 ℃ becomes 100 ℃/sec or more.
(D) And a coiling step of coiling after cooling.
Hereinafter, each step will be described.
(A) Heating process
In the heating step, the slab having the chemical composition described above is preferably heated to a temperature range of 1100 ℃ or higher and lower than 1350 ℃. The method for producing the slab is not particularly limited, and the following general methods can be applied: molten steel having the chemical composition described above is melted by a converter or the like, and a slab is produced by a casting method such as continuous casting. The ingot-cogging method may be used.
In the slab, most of the carbonitride forming elements such as Ti exist as coarse carbonitrides in the slab in a non-uniform distribution. Coarse precipitates (carbonitrides) present in an uneven distribution deteriorate various properties (for example, tensile strength, ductility, hole expansibility, etc.) of the hot-rolled steel sheet. Therefore, the slab before hot rolling is heated in a desired temperature range to form solid solutions of coarse precipitates. In order to sufficiently dissolve the coarse precipitates before hot rolling, the heating temperature of the slab is preferably set to 1100 ℃ or higher. However, if the heating temperature of the slab becomes too high, the occurrence of surface flaws and scale peeling may cause a decrease in yield. Therefore, the heating temperature of the steel raw material is preferably set to be lower than 1350 ℃.
The slab was heated to a temperature range of 1100 ℃ or more and 1350 ℃ or less and held for a predetermined time, but if the holding time exceeds 4800 seconds, the amount of scale generation increases. As a result, scale biting or the like may easily occur in the subsequent finish rolling step, and the surface quality of the hot-rolled steel sheet may be deteriorated. Therefore, the holding time in the temperature range of 1100 ℃ or more and 1350 ℃ or less is preferably set to 4800 seconds or less.
Rough rolling process
The slab may be rough rolled between the heating step and the finish rolling step. The rough rolling is not particularly limited as long as the desired sheet bar size can be obtained.
(B) Finish rolling process
In the finish rolling step, the heated slab is finish rolled by using a rolling mill having a plurality of stands. In this case, the following conditions (I) to (V) are preferably satisfied.
It is preferable to remove the scale before finish rolling or during rolling between rolling stands of finish rolling.
(I) Finish rolling start temperature: above 850 DEG C
The finish rolling start temperature (inlet side temperature of initial pass of finish rolling) is preferably set to 850 ℃ or higher. If the finish rolling start temperature is lower than 850 ℃, rolling in a part of the plurality of rolling stands (particularly the stands in the first half) becomes performed in a ferrite+austenite two-phase region temperature. As a result, the worked structure may remain after finish rolling, and the strength and ductility of the hot-rolled steel sheet may deteriorate. Accordingly, the finish rolling start temperature is preferably set to 850 ℃ or higher.
In order to suppress coarsening of austenite, the finish rolling start temperature may be set to 1100 ℃ or lower.
(II) Sigma represented by the following formula (1) in the last 4 frames: 40 to 80 percent
σ＝exp(0.753+3000/T)·ε 0.21·ε' 0.13 (1)
Where T is the temperature immediately before entering each rack (C.) (i.e., the inlet side temperature), ε is the equivalent plastic strain, ε' is the strain rate.
Sigma between 40 and 80 in the last 4 frames may be in other words: the sigma of the 4 th last frame, the sigma of the 3 rd last frame, the sigma of the 2 nd last frame and the sigma of the final frame are all 40-80.
If there are even 1 machine frame with σ less than 40, the strain required for development of texture of the surface layer region may not be appropriately imparted in the last 4 machine frames. As a result, in the texture of the region having a depth of 1/8 of the plate thickness from the surface to the surface, the polar densities of the {001} <110>, {111} <110> and {112} <110> orientation groups may not be controlled preferably. Furthermore, it may not be preferable to control the texture of the interior region. Therefore, σ in the last 4 frames is preferably set to 40 or more.
Furthermore, if there are even 1 rack with σ exceeding 80, there is a possibility that the texture is underdeveloped, and dynamic recrystallization is exhibited to randomize the structure. As a result, there is a possibility that the isotropy of the ductility and hole expansibility of the hot-rolled steel sheet deteriorate. Therefore, σ in the last 4 frames is preferably set to 80 or less.
When the thickness of the inlet side plate is set to H and the thickness of the outlet side plate is set to H, the equivalent plastic strain ε can be obtained by ε= (2 /) × (H/H). When the rolling time is set to t(s), the strain rate, that is, ε 'can be obtained by ε' =ε/t.
The rolling time t is a time when the steel sheet is brought into contact with the roll and strain is applied to the steel sheet.
(III) inter-pass time between last 4 frames: 0.1 to 10.0 seconds
In the last 4 frames, if there is more than 10.0 seconds of inter-pass time even 1 inter-pass time, the recovery and recrystallization between passes proceeds. As a result, the accumulation of strain may become difficult, and a desired texture may not be obtained in the surface layer region and the inner region. Therefore, the inter-pass time between the last 4 frames is preferably set to 10.0 seconds or less.
The inter-pass time between the last 4 stands is preferably short, but the reduction of the inter-pass time is limited in terms of installation space of each stand and rolling speed, and is therefore preferably set to 0.1 seconds or more.
The inter-pass time between the last 4 frames of 0.1 to 10.0 seconds may be: the inter-pass time between the 4 th and 3 rd frames, the inter-pass time between the 3 rd and 2 nd frames, and the inter-pass time between the 2 nd and final frames are all 0.1-10.0 seconds.
(IV) cumulative reduction of last 4 frames: more than 60 percent
When the cumulative reduction of the last 4 frames is less than 60%, there is a possibility that the dislocation density introduced into unrecrystallized austenite becomes small. If the dislocation density introduced into unrecrystallized austenite becomes small, it may become difficult to obtain a desired texture, and the hole expansibility and the isotropy of the ductility of the hot-rolled steel sheet may deteriorate. Therefore, the cumulative rolling reduction of the last 4 frames is preferably set to 60% or more.
If the cumulative reduction of the last 4 frames exceeds 97%, there is a possibility that the shape of the hot rolled steel sheet may deteriorate. Therefore, the cumulative rolling reduction of the last 4 frames can also be set to 97% or less.
When the inlet plate thickness of the 4 th last frame is set to t0 and the outlet plate thickness of the final frame is set to t1, the cumulative rolling reduction of the last 4 frames can be expressed as {1- (t 1/t 0) } ×100 (%).
(V) finishing temperature of 850-1000 DEG C
When the finish rolling finish temperature (outlet side temperature of the final stand) is lower than 850 ℃, rolling proceeds at a ferrite+austenite two-phase region temperature. This may deteriorate isotropy in strength and ductility of the hot-rolled steel sheet due to the remaining of the worked structure after rolling. Therefore, the finishing temperature is preferably set to 850 ℃ or higher.
In the slab having the chemical composition of the present embodiment, the unrecrystallized austenite region is substantially a temperature region of 1000 ℃. Therefore, if the finish rolling completion temperature exceeds 1000 ℃, austenite grains grow, and the grain length of martensite of the hot-rolled steel sheet obtained after cooling becomes large. As a result, it becomes difficult to obtain a desired texture, and the isotropy of the strength and ductility of the hot-rolled steel sheet may deteriorate. Therefore, the finishing temperature is preferably set to 1000 ℃ or less.
(C) Cooling process
In the cooling step, it is preferable that air cooling is performed for 2.0 to 4.0 seconds after finishing finish rolling, and then cooling is performed so that the average cooling rate up to a temperature range of 550 to 450 ℃ becomes 100 ℃/sec or more.
Air cooling time: 2.0 to 4.0 seconds
After finishing finish rolling, air cooling is preferably performed for 2.0 to 4.0 seconds. If the air cooling is performed for less than 2.0 seconds or more than 4.0 seconds, a desired amount of bainite may not be obtained. Therefore, the air cooling is preferably performed for 2.0 to 4.0 seconds.
In the present embodiment, air cooling means cooling at an average cooling rate of less than 10 ℃/sec.
In the present embodiment, it is preferable that a cooling device is provided at the rear stage of the finish rolling device, and the air cooling is performed while passing the finish rolled steel sheet through the cooling device. Here, the term cooling does not include the above-described air cooling.
The cooling device is preferably set to be a device capable of cooling the steel sheet at an average cooling rate of 100 ℃/sec or more. As the cooling device, for example, a water cooling device using water as a cooling medium can be exemplified.
The average cooling rate in the cooling step is set to a value obtained by dividing the temperature decrease width of the steel sheet from the start of cooling to the end of cooling by the time required from the start of cooling to the end of cooling. The start of cooling is set when the steel sheet is introduced into the cooling facility; the cooling end is set to be when the steel sheet is led out from the cooling device.
Further, there are apparatuses having no air cooling section in the middle and apparatuses having 1 or more air cooling sections in the middle. In this embodiment, any cooling device may be used. Even when a cooling device having an air cooling section is used, the average cooling rate from the start of cooling to the end of cooling may be 100 ℃/sec or more.
Average cooling rate from the air cooling end temperature to a temperature range of 450 to 550 ℃:100 ℃/s or more
If the average cooling rate from the air cooling end temperature to the temperature range of 450 to 550 ℃ is less than 100 ℃/sec, ferrite may be easily formed, and a desired amount of bainite may not be obtained. Therefore, the average cooling rate from the air cooling end temperature to the temperature range of 450 to 550 ℃ is preferably set to 100 ℃/sec or more.
(D) Winding process
In the coiling step, the steel sheet cooled to a temperature range of 450 to 550 ℃ is preferably coiled into a coil shape. Since coiling of the steel sheet is performed immediately after cooling, the coiling temperature is substantially equal to the cooling stop temperature. If the coiling temperature is less than 450 ℃, a desired amount of bainite may not be obtained, and isotropy of hole expansibility and ductility may deteriorate. If the winding temperature exceeds 550 ℃, ferrite and pearlite may be generated in large amounts, and the desired strength may not be obtained. Therefore, the winding temperature is preferably set to a temperature range of 450 to 550 ℃.
And (5) carrying out air cooling after coiling. After coiling, the hot rolled steel sheet may be temper rolled according to a conventional method, or may be pickled to remove scale formed on the surface. Or further plating treatment such as aluminizing, aluminizing-zincing, aluminizing-silicozing, hot dip galvanizing, electrogalvanizing, alloyed hot dip galvanizing, and chemical conversion treatment may be performed.
The hot-rolled steel sheet according to the present embodiment can be stably produced by the preferred production method described above.
Examples
Next, an embodiment of the present invention will be described, but the conditions in the embodiment are one example of conditions adopted for confirming the operability and effect of the present invention, and the present invention is not limited to this one example of conditions. The present invention can employ various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.
Molten steel having the chemical composition shown in table 1 was melted in a converter, and a slab was obtained by a continuous casting method. Next, these slabs were heated under the conditions shown in table 2A and table 2B, rough rolled, and then finish rolled under the conditions shown in table 2A and table 2B. After completion of finish rolling, hot rolled steel sheets having the sheet thicknesses shown in tables 3A and 3B were obtained by cooling and coiling under the conditions shown in tables 3A and 3B.
In the heating step, the holding time at the heating temperature described in table 2A and table 2B was set to 4800 seconds or less.
The cooling after finish rolling (except for air cooling) is set to cooling by water cooling, and the steel sheet is passed through a water cooling facility having no air cooling section in the middle. The average cooling rates in tables 3A and 3B are values obtained by dividing the temperature decrease width of the steel sheet from the time of introduction into the water cooling facility to the time of discharge from the water cooling facility by the passage time required for the steel sheet to pass through the water cooling facility.
Test pieces were collected from the obtained hot-rolled steel sheet, and the area ratio of each structure, the polar density of texture, the tensile strength, the total elongation in the C-direction and the L-direction, and the hole expansion ratio were measured by the above-described methods.
The results obtained are shown in tables 4A and 4B.
When the tensile strength obtained was 980MPa or more, the test piece was judged to be satisfactory as having high strength. On the other hand, when the obtained tensile strength is lower than 980MPa, the steel sheet is judged to be unacceptable as having no high strength.
When the difference between the total elongation in the C direction and the total elongation in the L direction obtained is ±3.0% or less, the product is judged to be acceptable as having excellent isotropy in ductility. On the other hand, when the difference between the total elongation in the C direction and the total elongation in the L direction exceeds ±3.0%, the sheet is judged as unacceptable as isotropic without excellent ductility.
When the obtained hole expansion ratio is 40% or more, the product is judged to be acceptable as having excellent hole expansion properties. On the other hand, when the hole expansion ratio is less than 40%, the material is judged as not having excellent hole expansion properties and is not acceptable.
TABLE 1
[ Table 2A ]
[ Table 2B ]
[ Table 3A ]
The underline indicates that the manufacturing conditions are not preferable.
TABLE 3B
The underline indicates that the manufacturing conditions are not preferable.
[ Table 4A ]
TABLE 4B
As is clear from tables 4A and 4B, in the examples of the present invention, hot-rolled steel sheets having high strength and excellent ductility were obtained.
On the other hand, the chemical composition and/or the metal structure were not inferior to any one or more of the above-mentioned properties of the comparative examples within the range defined in the present invention.
Industrial applicability
According to the above aspect of the present invention, a hot rolled steel sheet having high strength and excellent ductility and isotropy and hole expansibility can be provided.

Claims (2)

1. A hot-rolled steel sheet characterized by comprising, in mass%, the chemical composition:
C：0.100～0.350％、
Si：0.010～3.00％、
Mn：1.00～4.00％、
sol.Al：0.001～2.000％、
si+sol.al: more than 1.00%,
Ti：0.010～0.380％、
P:0.100% or less,
S:0.0300% or less,
N: less than 0.1000 percent,
O:0.0100% or less,
Nb：0～0.100％、
V：0～0.500％、
Cu：0～2.00％、
Cr：0～2.00％、
Mo：0～1.00％、
Ni：0～2.00％、
B：0～0.0100％、
Ca：0～0.0200％、
Mg：0～0.0200％、
REM：0～0.1000％、
Bi：0～0.020％、
1 Or more than 2 of Zr, co, zn and W: 0 to 1.00%, sum of
Sn：0～0.050％，
The remainder comprising Fe and impurities,
The metallic structure in the region from 1/8 depth to 3/8 depth from the surface to the plate thickness contains retained austenite in area%: 10-20 percent of primary martensite: less than 10% and bainite: 70 to 90 percent,
In the texture of the surface to a region of 1/8 depth from the surface by the plate thickness, {001} <110>, {111} <110>, and {112} <110> orientation groups have a polar density of 2.0 to 8.0,
In the texture of the region from 1/8 depth to 1/2 depth of the plate thickness from the surface, the polar density of the {110} <112> orientation is 2.0 to 4.0,
The tensile strength of the hot-rolled steel sheet is 980MPa or more.
2. The hot-rolled steel sheet according to claim 1, wherein the chemical composition contains 1 or 2 or more elements selected from the following elements in mass%:
Nb：0.005～0.100％、
V：0.005～0.500％、
Cu：0.01～2.00％、
Cr：0.01～2.00％、
Mo：0.01～1.00％、
Ni：0.02～2.00％、
B：0.0001～0.0100％、
Ca：0.0005～0.0200％、
Mg：0.0005～0.0200％、
REM:0.0005 to 0.1000 percent
Bi：0.0005～0.020％。
CN202280066930.XA 2021-10-14 2022-10-12 Hot rolled steel sheet Pending CN118139998A (en)

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