CN118139998A - Hot rolled steel plate - Google Patents

Abstract

The hot-rolled steel sheet has a specified chemical composition and metal structure, and in the texture from the surface to a region at a depth of 1/8 of the plate thickness from the above-mentioned surface, the pole density of the {001}<110>, {111}<110> and {112}<110> orientation groups is 2.0 to 8.0, and in the texture from a depth of 1/8 of the plate thickness from the above-mentioned surface to a depth of 1/2 of the plate thickness from the above-mentioned surface, the pole density of the {110}<112> orientation is 2.0 to 4.0, and the tensile strength of the above-mentioned hot-rolled steel sheet is greater than 980 MPa.

Description

Hot rolled steel plate

Technical Field

The present invention relates to hot rolled steel sheets.

This application claims priority based on Japanese Patent Application No. 2021-168627 filed in Japan on October 14, 2021, and the contents of which are incorporated herein.

Background technique

From the perspective of protecting the global environment, the lightweighting of automobile bodies is being carried out for the purpose of improving the fuel efficiency of automobiles. In order to further lighten the automobile bodies, it is necessary to increase the strength of the steel sheets used for the automobile bodies. However, generally speaking, if the strength of the steel sheets is increased, the formability is reduced.

As a method for improving the formability of a steel plate, there is a method of making the metal structure of the steel plate contain retained austenite. However, if the metal structure of the steel plate contains retained austenite, the ductility is improved, but the isotropy of the ductility may be deteriorated and the hole expansion may be deteriorated. When performing bending, hole expansion and flanging, it is required that the anisotropy of the ductility is reduced, that is, the isotropy of the ductility is excellent. Furthermore, when performing the above-mentioned processing, excellent hole expansion is also required.

Patent document 1 discloses a hot-rolled steel plate whose microstructure is mainly composed of bainite, in which the hard phase composed of martensite and/or austenite accounts for more than 3% and less than 20% in terms of area fraction, the structure with an aspect ratio of more than 3 in the hard phase present in the central part of the plate thickness accounts for more than 60%, the length of the hard phase present in the central part of the plate thickness in the rolling direction is less than 20 μm, the sum of the X-ray random intensity ratios of the <011> orientation and the <111> orientation observed from the rolling direction is more than 3.5, and the X-ray random intensity ratio of the <001> orientation observed from the rolling direction is less than 1.0.

Prior art literature

Patent Literature

Patent Document 1: International Publication No. 2016/010004

Summary of the invention

Problems to be solved by the invention

However, in order to further reduce the weight of the automobile body, it is necessary to further improve the strength in Patent Document 1. In addition, in Patent Document 1, the isotropy of ductility is not considered.

An object of the present invention is to provide a hot-rolled steel sheet having high strength, isotropy of ductility and excellent hole expandability.

Means for solving problems

In view of the above problems, the inventors of the present invention have repeatedly conducted intensive studies on the relationship between the chemical composition and metal structure of hot-rolled steel sheets and mechanical properties, and as a result, they have obtained the following findings and have completed the present invention.

The inventors of the present invention have recognized that it is important to control the texture of the surface region and the inner region of the hot-rolled steel sheet in order to improve the isotropy of the ductility and the hole expansion of the hot-rolled steel sheet. In addition, the inventors of the present invention have recognized that it is effective to control the texture of the surface region and the inner region of the hot-rolled steel sheet, in particular, to control the finishing rolling conditions.

The gist of the present invention made based on the above findings is as follows.

(1) The chemical composition of the hot-rolled steel sheet according to one embodiment of the present invention contains, in terms of mass %, the following:

C: 0.100～0.350％,

Si: 0.010～3.00%,

Mn: 1.00～4.00%,

Sol.Al: 0.001～2.000％,

Si+sol.Al: 1.00% or more,

Ti: 0.010～0.380％,

P: 0.100% or less,

S: 0.0300% or less,

N: 0.1000% or less,

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％,

One or more of Zr, Co, Zn and W: 0 to 1.00% in total, and

Sn: 0～0.050%,

The rest contains Fe and impurities.

The metal structure in the region from 1/8 of the plate thickness to 3/8 of the plate thickness from the surface comprises, in terms of area %, retained austenite: 10-20%, primary martensite: 10% or less, and bainite: 70-90%,

In the texture from the surface to a region having a depth of 1/8 of the plate thickness from the surface, the pole density of the {001}<110>, {111}<110> and {112}<110> orientation groups is 2.0 to 8.0,

In the texture in the region from 1/8 of the plate thickness to 1/2 of the plate thickness from the surface, the pole density of the {110}<112> orientation is 2.0 to 4.0,

The tensile strength of the hot-rolled steel sheet is 980 MPa or more.

(2) The hot-rolled steel sheet according to (1) above, wherein the chemical composition may contain, in terms of mass %, one or more elements selected from the following:

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: 0.0005～0.1000％, and

Bi: 0.0005～0.020%.

Effects of the Invention

According to the above aspects of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, isotropy of ductility, and excellent hole expandability.

Detailed ways

The chemical composition and metal structure of the hot-rolled steel sheet of this embodiment will be described in more detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made within the scope of the present invention.

For the numerical ranges described below with "to", the lower limit and upper limit are included in the range. For the numerical values expressed as "lower than" or "exceeding", the value is not included in the numerical range. In the following description, "%" about chemical composition means "mass %" unless otherwise specified.

chemical components

The chemical composition of the hot-rolled steel sheet according to the present embodiment includes, by 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: 1.00% or more, Ti: 0.010-0.380%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and the remainder: Fe and impurities.

Hereinafter, each element will be described in detail.

C: 0.100～0.350%

C is an element necessary to obtain desired strength. If the C content is less than 0.100%, it becomes difficult to obtain desired strength. Therefore, the C content is set to 0.100% or more. The C content is preferably 0.120% or more, 0.150% or more.

On the other hand, when the C content exceeds 0.350%, the transformation rate slows down and MA (a mixed phase of primary martensite and retained austenite) is easily generated, making it difficult to obtain excellent ductility, isotropy and hole expandability. Therefore, the C content is set to 0.350% or less. The C content is preferably 0.330% or less, 0.310% or less.

Si: 0.010～3.00%

Si has the effect of delaying the precipitation of cementite. Through this effect, the amount of austenite remaining without phase transformation, i.e. the area ratio of retained austenite, can be increased. In addition, by ensuring the amount of solid solution C in the hard phase in large quantities and preventing the coarsening of cementite, the strength can be improved. In addition, Si itself also has the effect of improving the strength of the hot-rolled steel sheet by solid solution strengthening. In addition, Si has the effect of making steel sound (suppressing defects such as pores in steel) by deoxidation. When the Si content is less than 0.010%, the effect brought by the above-mentioned effect cannot be obtained. Therefore, the Si content is set to more than 0.010%. The Si content is preferably more than 0.50%, more than 1.00%, more than 1.20%, and more than 1.50%.

On the other hand, when the Si content exceeds 3.00%, the precipitation of cementite is significantly delayed, and the amount of retained austenite becomes excessive, so it is not preferred. In addition, the surface properties and chemical conversion treatment properties of the hot-rolled steel sheet, and then the ductility and weldability are significantly deteriorated, and the _A3 phase transformation point is significantly increased. As a result, it becomes 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 the 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, 1.80% or more.

On the other hand, when the Mn content exceeds 4.00%, the isotropy of the ductility and the hole expandability 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, 3.50% or less.

Sol.Al: 0.001～2.000%

Sol.Al has the following effects, similar to Si: it improves the steel by deoxidation, and promotes the formation of retained austenite by inhibiting the precipitation of cementite from austenite. When the sol.Al content is less than 0.001%, the effects brought about by the above effects 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 it is not economically preferred. Furthermore, the _A3 transformation point rises significantly, and it becomes difficult to perform hot rolling stably. 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.

It should be noted that in the present embodiment, sol.Al refers to acid-soluble Al, and means solid-solution Al present in the steel in a solid-solution state.

Si+sol.Al: 1.00% or more

Both Si and sol.Al have the effect of delaying the precipitation of cementite, and through 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 brought by the above-mentioned effect cannot be obtained. Therefore, the total content of Si and sol.Al is set to 1.00% or more. Preferably, it is 1.20% or more and 1.50% or more.

In addition, Si in "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 the recrystallization and grain growth of austenite between the stands of hot rolling. By suppressing the recrystallization of austenite between the stands, 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 effect cannot be obtained. Therefore, the Ti content is set to 0.010% or more. Preferably, it is 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 caused by TiN are generated, and the toughness of the hot-rolled steel sheet deteriorates. Therefore, the Ti content is set to 0.380% or less, preferably 0.320% or less or 0.300% or less.

P: 0.100% or less

P is an element generally contained in steel as an impurity, but has the effect of improving the strength of hot-rolled steel sheets by solid solution strengthening. Therefore, P may be actively contained. However, P is an element that is prone to segregation. If the P content exceeds 0.100%, the isotropic degradation of hole expansion and ductility caused by grain boundary segregation becomes significant. 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 particularly specified, 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. It forms sulfide inclusions in steel and deteriorates the isotropy of the hole expansion and ductility of the hot-rolled steel sheet. If the S content exceeds 0.0300%, the isotropy of the hole expansion and ductility of the hot-rolled steel sheet will be 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 does not need to be particularly specified, but is preferably set to 0.0001% from the viewpoint of refining cost.

N: 0.1000% or less

N is an element contained in steel as an impurity, and has the effect of deteriorating the isotropy of the hole expansion and ductility of the hot-rolled steel sheet. When the N content exceeds 0.1000%, the isotropy of the hole expansion and ductility of the hot-rolled steel sheet is significantly deteriorated. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0800% or less, 0.0700% or less.

The lower limit of the N content does not need to be particularly specified, but in order to promote the 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 a large amount of O is contained in steel, coarse oxides are formed which become the starting point of destruction, causing brittle fracture and hydrogen cracking. Therefore, the O content is set to 0.0100% or less. The O content is preferably set to 0.0080% or less, or 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, or 0.0010% or more.

The remainder of the chemical composition of the hot-rolled steel sheet of the present embodiment includes Fe and impurities. In the present embodiment, impurities refer to elements mixed from ore, waste materials or manufacturing environment as raw materials or elements intentionally added in trace amounts, and are elements that are allowed within the range that does not adversely affect the hot-rolled steel sheet of the present embodiment.

The chemical composition of the hot-rolled steel sheet of the present embodiment may contain the following elements as optional elements in addition to the above elements. When the above optional elements are not contained, the lower limit of the content is 0%. Each optional element will be described in detail below.

Nb: 0.005-0.100% and V: 0.005-0.500%

Both Nb and V are elements that inhibit recrystallization and grain growth of austenite between stands of hot rolling, similarly to Ti. Therefore, one or more of these elements may be contained. In order to more reliably obtain the effects brought about by the above-mentioned actions, it is preferred that the Nb content be set to 0.005% or more, or the V content be set to 0.005% or more.

However, even if these elements are contained in excess, the effects due to the above-mentioned actions are saturated, which is 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: 0.01-2.00%, Cr: 0.01-2.00%, Mo: 0.01-1.00%, Ni: 0.02-2.00%, and B: 0.0001-0.0100%

Cu, Cr, Mo, Ni and B all have the effect of improving the hardenability of the hot-rolled steel sheet. In addition, Cr and Ni have the effect of stabilizing the retained austenite, and Cu and Mo have the effect of precipitating carbides in the steel to improve the strength of the hot-rolled steel sheet. Furthermore, when Cu is contained, Ni has the effect of effectively suppressing the grain boundary cracking of the slab caused by Cu. Therefore, one or more of these elements may also be contained.

As described above, Cu has the function of improving the hardenability of the steel sheet and precipitating as carbides in the steel at low temperatures to improve the strength of the hot-rolled steel sheet. In order to more reliably obtain the effects brought about by the above functions, the Cu content is preferably set to 0.01% or more.

However, when 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.

As described above, Cr has the function of improving the hardenability of the steel sheet and the function of stabilizing retained austenite. In order to more reliably obtain the effects of the above functions, 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 reduced. Therefore, the Cr content is set to 2.00% or less.

As described above, Mo has the function of improving the hardenability of the steel sheet and the function of precipitating carbides in the steel to improve the strength. In order to more reliably obtain the effects of the above functions, the Mo content is preferably set to 0.01% or more.

However, even if the Mo content exceeds 1.00%, the effect due to the above-mentioned action is saturated, which is not economically preferable. Therefore, the Mo content is set to 1.00% or less.

As described above, Ni has the effect of improving the hardenability of the steel sheet. In addition, when Cu is contained, Ni has the effect of effectively suppressing the grain boundary cracking of the slab caused by Cu. In order to more reliably obtain the effect brought about by the above-mentioned effect, it is preferred that the Ni content be set to 0.02% or more.

Ni is an expensive element, so containing a large amount is not economically preferable. Therefore, the Ni content is set to 2.00% or less.

As described above, B has the function of improving the hardenability of the steel sheet. In order to more reliably obtain the effect of this function, the B content is preferably set to 0.0001% or more.

However, when the B content exceeds 0.0100%, the hole expandability and isotropy of the ductility of the hot-rolled steel sheet are significantly deteriorated, so the B content is made 0.0100% or less.

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 hot-rolled steel sheets by controlling the shape of inclusions to a preferred shape. In addition, Bi has the effect of improving the formability of hot-rolled steel sheets by refining the solidification structure. Therefore, one or more of these elements may also be contained. In order to more reliably obtain the effects brought about by the above-mentioned effects, it is preferred to set any one or more of Ca, Mg, REM and Bi to 0.0005% or more. However, if the Ca content or the Mg content exceeds 0.0200%, or if the REM content exceeds 0.1000%, inclusions are excessively generated in the steel, which may deteriorate the isotropy of the hole expansion and ductility of the hot-rolled steel sheet. In addition, even if the Bi content is set to more than 0.020%, the effect brought about by the above-mentioned effects is saturated, which is not economically preferred. Therefore, the Ca content and the Mg content are set to less than 0.0200%, the REM content is set to less than 0.1000%, and the Bi content is set to less than 0.020%. The Bi content is preferably 0.010% or less.

Here, REM refers to a total of 17 elements including Sc, Y and lanthanoid elements, and the above-mentioned REM content refers to the total content of these elements. In the case of lanthanoid elements, they are added in the form of mixed rare earth alloys in industry.

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 inventors of the present invention have confirmed that even if these elements are contained in an amount of 1.00% or less in total, the effect of the hot-rolled steel sheet of the present embodiment is not impaired. Therefore, one or more of Zr, Co, Zn and W may be contained in an amount of 1.00% or less in total.

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. However, since there is a possibility of generation of defects during hot rolling, the Sn content is set to 0.050% or less.

The chemical composition of the hot-rolled steel sheet mentioned above can be determined by general analytical methods. For example, it can be determined by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). It should be noted that sol.Al can be determined by using the filtrate after the sample is heated and decomposed with acid and then measured by ICP-AES. C and S can be measured by combustion-infrared absorption method, N can be measured by inert gas melting-thermal conductivity method, and O can be measured by inert gas melting-non-dispersive infrared absorption method.

Metal structure of hot rolled steel plate

Next, the metal structure of the hot-rolled steel sheet according to the present embodiment will be described.

In the hot-rolled steel sheet of the present embodiment, the metal structure in the region from a depth of 1/8 of the plate thickness from the surface to a depth of 3/8 of the plate thickness from the surface includes, in terms of area %, retained austenite: 10 to 20%, primary martensite: less than 10% and bainite: 70 to 90%, and in the texture from the surface to a region from a depth of 1/8 of the plate thickness from the above-mentioned surface, the pole density of the {001}<110>, {111}<110> and {112}<110> orientation groups is 2.0 to 8.0, and in the texture from a depth of 1/8 of the plate thickness from the above-mentioned surface to a depth of 1/2 of the plate thickness from the above-mentioned surface, the pole density of the {110}<112> orientation is 2.0 to 4.0.

It should be noted that in this embodiment, the area ratios of retained austenite, primary martensite, and bainite at a depth position of 1/4 of the plate thickness from the surface (a region from 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface) of the plate thickness cross section parallel to the rolling direction are specified. The reason for this is that the metal structure at this position represents a representative metal structure of the hot-rolled steel sheet.

Retained austenite: 10-20%

Retained austenite is an isotropic structure that improves the hole expansion and ductility of the hot-rolled steel sheet. If the area ratio of retained austenite is less than 10%, the desired isotropy of hole expansion and ductility cannot be obtained. Therefore, the area ratio of retained austenite is set to 10% or more. Preferably, it is 12% or more or 13% or more.

On the other hand, if the area ratio of retained austenite exceeds 20%, the desired strength cannot be obtained. Therefore, the area ratio of retained austenite is set to 20% or less, preferably 18% or less, or 17% or less.

Primary martensite: less than 10%

Fresh martensite is a hard structure, so it helps to improve the strength of hot-rolled steel sheets. However, fresh martensite is also an isotropic structure that lacks hole expansion and ductility. If the area ratio of fresh martensite exceeds 10%, the desired isotropy of hole expansion and ductility cannot be obtained. Therefore, the area ratio of fresh martensite is set to 10% or less. Preferably, it is 8% or less, 6% or less, 4% or less, or 2% or less. The area ratio of fresh martensite can also be 0%.

Bainite: 70-90%

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, it is 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 expandability cannot be obtained. Therefore, the area ratio of bainite is set to 90% or less. Preferably, it is less than 90%, 88% or less, or 85% or less.

The area ratio of the structure other than retained austenite in each of the above-mentioned structures was measured by the following method.

From the hot-rolled steel plate, a test piece is collected in such a way that the metal structure at a depth of 1/4 of the plate thickness from the surface (an area from 1/8 of the plate thickness to 3/8 of the plate thickness from the surface) of the plate thickness section parallel to the rolling direction can be observed. Next, after grinding the plate thickness section, the ground surface is corroded with nitric acid, and an optical microscope and a scanning electron microscope (SEM) are used to observe the structure of an area of 30μm×30μm. The observation area is set to at least 3 areas. The area ratio of bainite is obtained by image analysis of the tissue photograph obtained by the tissue observation. Afterwards, for the same observation position, Lepera corrosion is performed, and the structure is observed using an optical microscope and a scanning electron microscope, and the obtained tissue photograph is image analyzed to obtain the area ratio of primary martensite.

In the above-mentioned tissue observation, each tissue was identified by the following method.

Primary martensite has a high dislocation density and has a lower structure such as lath blocks and lath bundles in the grains. Therefore, it can be distinguished from other metal structures by electron channel contrast imaging using a scanning electron microscope.

The following structures are regarded as bainite: a structure that is a collection of lath-shaped grains, and is not a primary martensite in a structure that does not contain Fe-based carbides with a long diameter of 20 nm or more in the structure, or a structure that contains Fe-based carbides with a long diameter of 20 nm or more in the structure, and the Fe-based carbides have a single variant, that is, a structure of Fe-based carbides extending in the same direction. Here, Fe-based carbides extending in the same direction refer to those whose elongation directions of Fe-based carbides differ by 5° or less.

The area ratio of retained austenite is measured by the following method.

In the present embodiment, the area ratio of retained austenite is measured by X-ray diffraction. First, at a depth of 1/4 of the plate thickness from the surface (a region from 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface) of the plate thickness section parallel to the rolling direction of the hot-rolled steel plate, Co-Kα rays are used to obtain the integrated intensity of a total of 6 peaks of α(110), α(200), α(211), γ(111), γ(200), and γ(220), and the volume ratio of retained austenite is calculated using the intensity average method. The volume ratio of the retained austenite is regarded as the area ratio of the retained austenite.

The pole density of the {001}<110>, {111}<110>, and {112}<110> orientation groups in the texture from the surface to a depth of 1/8 of the plate thickness: 2.0 to 8.0

If the pole density of the {001}<110>, {111}<110> and {112}<110> orientation groups in the texture from the surface to the region 1/8 of the plate thickness from the surface (hereinafter sometimes referred to as the surface region) is less than 2.0, the isotropy of the ductility and the hole expandability of the hot-rolled steel sheet are deteriorated. Therefore, the pole density of the {001}<110>, {111}<110> and {112}<110> orientation groups in the texture of the surface region is set to 2.0 or more. Preferably, it is 2.2 or more, 2.5 or more, or 2.7 or more.

If the pole density of the {001}<110>, {111}<110> and {112}<110> orientation groups in the texture of the surface region exceeds 8.0, the isotropy of the ductility and the hole expandability of the hot-rolled steel sheet are deteriorated. Therefore, the pole density of the {001}<110>, {111}<110> and {112}<110> orientation groups in the texture of the surface region is set to 8.0 or less. Preferably, it is 7.5 or less or 7.0 or less.

The pole density of the {110}<112> orientation in the texture of the region from 1/8 of the plate thickness to 1/2 of the plate thickness from the surface: 2.0 to 4.0

If the pole density of the {110}<112> orientation in the texture of the region from 1/8 of the plate thickness to 1/2 of the plate thickness from the surface (hereinafter sometimes referred to as the internal region) exceeds 4.0, the isotropy of the ductility and the hole expandability of the hot-rolled steel sheet are deteriorated. Therefore, the pole density of the {110}<112> orientation in the texture of the internal region is set to 4.0 or less. Preferably, it is 3.6 or less, 3.2 or less, or 3.0 or less.

From the viewpoint of suppressing strength degradation, the pole density of the {110}<112> orientation in the texture of the inner region is set to 2.0 or more, preferably 2.3 or more or 2.5 or more.

The pole density was determined using a device combining a scanning electron microscope and an EBSD analyzer and OIM Analysis (registered trademark) manufactured by AMETEK. The pole density of the {001}<110>, {111}<110>, and {112}<110> orientation groups in the texture of the surface region and the pole density of {110}<112> in the texture of the internal region were determined using the crystal orientation distribution function (ODF) representing the three-dimensional texture calculated by using the orientation data measured by the EBSD (Electron BackScattering Diffraction) method and the spherical harmonics.

It should be noted that the measurement range is set from the surface to the area with a depth of 1/8 of the plate thickness from the surface for the surface layer area, and from the area with a depth of 1/8 of the plate thickness from the surface to the area with a depth of 1/2 of the plate thickness from the surface for the internal area. The measurement pitch is 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 with {hkl} oriented in the normal direction of the sheet surface and <uvw> oriented in the rolling direction.

It should be noted that the rolling direction of the hot-rolled steel sheet can be determined by the following method.

First, a test piece is collected in such a way that the plate thickness section of the hot-rolled steel plate can be observed. The plate thickness section of the collected test piece is finished by mirror polishing and then observed using an optical microscope. The observation range is set to the entire thickness of the plate, and the area with dark brightness is judged as an inclusion. For inclusions with a long axis length of 40 μm or more in the inclusion, the direction parallel to the direction in which the inclusion extends is judged as the rolling direction.

Mechanical properties

The tensile (maximum) strength of the hot-rolled steel sheet of this embodiment is 980 MPa or more. By setting the tensile strength to 980 MPa or more, it is possible to further contribute to the lightweighting of the vehicle body. More preferably, the tensile strength is 1180 MPa or more. The upper limit does not need to be particularly limited, but may also be set to 1470 MPa, 1300 MPa or less, or 1200 MPa 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 indicator of isotropy of ductility, is preferably ±3.0% or less.

The hole expansion ratio which is an index of hole expansion property is preferably 40% or more.

The tensile strength TS and the total elongation EL were measured using the No. 5 test piece of JIS Z 2241: 2011 in accordance with JIS Z 2241: 2011. The sampling position of the tensile test piece was set to be 1/4 of the distance from the end in the width direction of the plate, and the direction (C direction) perpendicular to the rolling direction was set as the length direction. It should be noted that, regarding the total elongation EL, a tensile test was also performed on a tensile test piece with a direction (L direction) parallel to the rolling direction as the length direction, and the total elongation in the L direction was measured.

The hole expansion ratio λ is measured using a No. 5 test piece of JIS Z 2241: 2011 in accordance with JIS Z 2256: 2010. The hole expansion test piece may be collected at a position that is 1/4 of the distance from the end in the sheet width direction of the hot-rolled steel sheet.

Plate thickness

The plate thickness of the hot-rolled steel plate of the present embodiment is not particularly limited, but can also be set to 1.2 to 8.0 mm. By setting the plate thickness of the hot-rolled steel plate to 1.2 mm or more, it becomes easy to ensure the rolling completion temperature, and the rolling load can be reduced, and hot rolling can be easily performed. Therefore, the plate thickness of the hot-rolled steel plate of the present embodiment can also be set to 1.2 mm or more. Preferably, it is 1.4 mm or more. In addition, by setting the plate thickness to 8.0 mm or less, it is possible that the control of the texture becomes difficult, and it becomes difficult to obtain the above-mentioned texture. Therefore, the plate thickness can also be set to 8.0 mm or less. Preferably, it is 6.0 mm or less.

Plating

The hot-rolled steel sheet of the present embodiment with the above-mentioned chemical composition and metal structure can also be made into surface-treated steel sheet by making the surface equipped with coating for the purpose of improving corrosion resistance. The coating can be an electroplated layer or a hot-dip coated layer. As the electroplated layer, electrogalvanized layer, electroplated Zn-Ni alloy layer, etc. can be exemplified. As the hot-dip coated layer, hot-dip galvanized layer, alloyed hot-dip galvanized layer, hot-dip aluminum layer, hot-dip Zn-Al alloy layer, hot-dip Zn-Al-Mg alloy layer, hot-dip Zn-Al-Mg-Si alloy layer, etc. can be exemplified. The coating adhesion amount is not particularly limited and can be set to be the same as before. In addition, after plating, appropriate chemical conversion treatment (such as coating and drying of the chromium-free chemical conversion treatment solution of silicate system) is implemented to further improve corrosion resistance.

Next, a preferred method for producing a hot-rolled steel sheet according to the present embodiment is described. The preferred method for producing a hot-rolled steel sheet according to the present embodiment includes the following steps (a) to (d). It should be noted that the temperature in the following description refers to the surface temperature of the steel sheet unless otherwise specified.

(a) A heating step of heating the slab having the above-mentioned chemical composition to a temperature range of 1100°C or higher and lower than 1350°C.

(b) A finishing rolling step of finishing the heated slab using a rolling mill having a plurality of stands, and satisfying the following conditions (I) to (V).

(I) The finish rolling start temperature is set to 850°C or higher.

(II) In each of the last four stands among the plurality of stands, rolling is performed so that σ represented by the following formula (1) becomes 40 to 80.

σ＝exp(0.753+3000/T)×ε ^0.21 ×ε' ^0.13 (1)

Where T is the temperature just before entering each rack (°C), ε is the equivalent plastic strain, and ε’ is the strain rate.

(III) Set the time between the last four passes between each stand to 0.1 to 10.0 seconds.

(IV) The cumulative reduction ratio of the last four stands is set to 60% or more.

(V) The finishing temperature of the finishing rolling is set to 850 to 1000°C.

(c) A cooling step of air cooling for 2.0 to 4.0 seconds after the finish rolling is completed, and then cooling so that the average cooling rate to the temperature range of 450 to 550° C. becomes 100° C./second or more.

(d) A coiling step of coiling the steel sheet after cooling.

Hereinafter, each step will be described.

(a) Heating process

In the heating step, the slab having the above chemical composition is preferably heated to a temperature range of 1100° C. or higher and lower than 1350° C. The method for manufacturing the slab does not need to be particularly limited, and the following common method can be applied: molten steel having the above chemical composition is melted in a converter or the like, and the slab is made by a casting method such as continuous casting. It should be noted that an ingot casting-opening method can also be used.

In the slab, most of the carbonitride-forming elements such as Ti exist in the slab as coarse carbonitrides with uneven distribution. The coarse precipitates (carbonitrides) that exist in an uneven distribution will deteriorate the properties of the hot-rolled steel sheet (such as tensile strength, ductility, hole expansion, etc.). Therefore, the slab before hot rolling is heated in the desired temperature range to dissolve the coarse precipitates. In order to fully dissolve the coarse precipitates before hot rolling, it is preferred to set the heating temperature of the slab to above 1100°C. However, if the heating temperature of the slab becomes too high, the yield will be reduced due to the generation of surface defects and the peeling of oxide scale. Therefore, the heating temperature of the steel raw material is preferably set to below 1350°C.

The slab is heated to a temperature range of 1100°C or higher and lower than 1350°C and held for a predetermined time. However, if the holding time exceeds 4800 seconds, the amount of scale generated increases. As a result, scale bites may easily occur in the subsequent finishing rolling process, and the surface quality of the hot-rolled steel sheet may deteriorate. Therefore, the holding time in the temperature range of 1100°C or higher and lower than 1350°C is preferably set to 4800 seconds or less.

Rough rolling process

The slab may be subjected to rough rolling between the heating step and the finish rolling step. The conditions for the rough rolling are not particularly limited as long as a desired thin slab size can be obtained.

(b) Finishing process

In the finish rolling step, the heated slab is finish rolled using a rolling mill having a plurality of stands. At this time, it is preferable that the following conditions (I) to (V) are satisfied.

In addition, it is preferable to perform descaling before finish rolling or during rolling between rolling stands of finish rolling.

(I) Finish rolling starting temperature: 850°C or above

The start temperature of finishing rolling (the entry temperature of the first pass of finishing rolling) is preferably set to 850°C or higher. If the start temperature of finishing rolling is lower than 850°C, the rolling in a part of the plurality of rolling stands (especially the stands in the first half) is performed in the dual-phase region temperature of ferrite + austenite. As a result, there is a possibility that the processed structure remains after finishing rolling, and the strength and ductility of the hot-rolled steel sheet deteriorate. Therefore, the start temperature of finishing rolling is preferably set to 850°C or higher.

In order to suppress the coarsening of austenite, the finish rolling start temperature may be set to 1100° C. or lower.

(II) In the last four racks, σ is expressed by the following equation (1): 40 to 80

σ＝exp(0.753+3000/T)·ε ^0.21 ·ε' ^0.13 (1)

Wherein, T is the temperature (°C) just before entering each rack (i.e., the inlet temperature), ε is the equivalent plastic strain, and ε’ is the strain velocity.

In other words, the fact that σ is 40 to 80 in each of the last four racks means that σ of the fourth to last rack, σ of the third to last rack, σ of the second to last rack, and σ of the final rack are all 40 to 80.

If there is even one frame with σ lower than 40, it is possible that the strain required for the development of the texture in the surface region cannot be appropriately given in each of the last four frames. As a result, in the texture from the surface to the region with a depth of 1/8 of the plate thickness from the surface, the pole density of the {001}<110>, {111}<110>, and {112}<110> orientation groups may not be optimally controlled. In addition, the texture in the internal region may not be optimally controlled. Therefore, it is preferable that σ in each of the last four frames is set to 40 or more.

In addition, if there is even one stand with σ exceeding 80, the above texture may not be developed, dynamic recrystallization may be exhibited, and the structure may be randomized. As a result, the isotropy of ductility and hole expansion of the hot-rolled steel sheet may be deteriorated. Therefore, σ in each of the last four stands is preferably set to 80 or less.

It should be noted that when the entry plate thickness is set to h and the exit plate thickness is set to H, the equivalent plastic strain, i.e., ε, can be obtained by ε = (2/√3) × (h/H). In addition, when the rolling time is set to t (s), the strain rate, i.e., ε', can be obtained by ε' = ε/t.

The rolling time t refers to the time during which the steel sheet is in contact with the rolls and strain is applied to the steel sheet.

(III) The time between the last four passes between the racks: 0.1 to 10.0 seconds

If there is even one pass time between the last four stands that exceeds 10.0 seconds, recovery and recrystallization between passes will progress. As a result, accumulation of strain may become difficult, and the desired texture may not be obtained in the surface layer area and the internal area. Therefore, the pass time between the last four stands is preferably set to 10.0 seconds or less.

The inter-pass time between the last four stands is preferably short, but shortening the inter-pass time is limited by the installation space of each stand and the rolling speed, so it is preferably set to 0.1 second or more.

It should be noted that the inter-pass time between the last four racks is 0.1 to 10.0 seconds. In other words, the inter-pass time between the 4th to last rack and the 3rd to last rack, the inter-pass time between the 3rd to last rack and the 2nd to last rack, and the inter-pass time between the 2nd to last rack and the final rack are all 0.1 to 10.0 seconds.

(IV) Cumulative reduction rate of the last 4 racks: more than 60%

When the cumulative reduction ratio of the last four stands is less than 60%, the density of dislocations introduced into the unrecrystallized austenite may decrease. If the density of dislocations introduced into the unrecrystallized austenite decreases, it may become difficult to obtain the desired texture, and the isotropy of the hole expandability and ductility of the hot-rolled steel sheet may deteriorate. Therefore, the cumulative reduction ratio of the last four stands is preferably set to 60% or more.

It should be noted that if the cumulative reduction ratio of the last four stands exceeds 97%, the shape of the hot-rolled steel sheet may be deteriorated. Therefore, the cumulative reduction ratio of the last four stands may be set to 97% or less.

It should be noted that when the inlet plate thickness of the fourth to last stand is set to t0 and the outlet plate thickness of the final stand is set to t1, the cumulative reduction ratio of the last four stands can be expressed as {1-(t1/t0)}×100(%).

(V) Finishing temperature: 850～1000℃

When the finishing temperature (the exit temperature of the final stand) is lower than 850°C, the rolling is performed in the dual-phase region of ferrite + austenite. As a result, the processed structure may remain after rolling, and the isotropy of the strength and ductility of the hot-rolled steel sheet may deteriorate. Therefore, the finishing temperature is preferably set to 850°C or higher.

In addition, in the slab having the chemical composition of the present embodiment, the non-recrystallized austenite region is generally a temperature region below 1000°C. Therefore, if the finish rolling finish temperature exceeds 1000°C, the austenite grains grow, and the grain length of the martensite of the hot-rolled steel sheet obtained after cooling becomes larger. As a result, it may become difficult to obtain the desired texture, and the isotropy of the strength and ductility of the hot-rolled steel sheet deteriorates. Therefore, the finish rolling finish temperature is preferably set to 1000°C or less.

(c) Cooling process

In the cooling step, it is preferred that air cooling be performed for 2.0 to 4.0 seconds after the finish rolling is completed, and then cooling be performed so that the average cooling rate to the temperature range of 550 to 450° C. becomes 100° C./second or more.

Air cooling time: 2.0~4.0 seconds

After the finish rolling is completed, air cooling is preferably performed for 2.0 to 4.0 seconds. If the air cooling time is less than 2.0 seconds or exceeds 4.0 seconds, the desired amount of bainite may not be obtained. Therefore, air cooling is preferably performed for 2.0 to 4.0 seconds.

It should be noted that in the present embodiment, air cooling refers to cooling at an average cooling rate of less than 10° C./second.

In this embodiment, it is preferable to provide a cooling facility at the rear stage of the finishing rolling facility, and cool the steel sheet after the finishing rolling after passing through the cooling facility. It should be noted that the cooling mentioned here does not include the air cooling mentioned above.

The cooling device is preferably set to a device capable of cooling the steel plate at an average cooling rate of 100° C./sec or more. As the cooling device as described above, for example, a water cooling device using water as a cooling medium can be exemplified.

The average cooling rate in the cooling process is set to the value obtained by dividing the temperature drop of the steel plate 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 so-called start of cooling is set to the time when the steel plate is introduced into the cooling equipment; the so-called end of cooling is set to the time when the steel plate is taken out of the cooling equipment.

In addition, the cooling device includes a device without an air cooling section in the middle and a device with one or more air cooling sections in the middle. In the present embodiment, any cooling device can be used. Even when a cooling device with an air cooling section is used, the average cooling rate from the start of cooling to the end of cooling only needs to be 100°C/second or more.

Average cooling rate from air cooling end temperature to 450-550°C temperature range: 100°C/sec or more

If the average cooling rate from the air cooling end temperature to the temperature range of 450 to 550°C is lower than 100°C/sec, ferrite may be easily formed and the 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°C is preferably set to 100°C/sec or more.

(d) Coil-up process

In the coiling process, the steel sheet cooled to a temperature range of 450 to 550°C is preferably coiled into a coil. Since the steel sheet is coiled immediately after cooling, the coiling temperature is approximately equal to the cooling stop temperature. If the coiling temperature is lower than 450°C, the desired amount of bainite may not be obtained, and the isotropy of hole expansion and ductility may deteriorate. In addition, if the coiling temperature exceeds 550°C, ferrite and pearlite may be generated in large quantities, and the desired strength cannot be obtained. Therefore, the coiling temperature is preferably set to a temperature range of 450 to 550°C.

After coiling, air cooling is performed. It should be noted that after coiling, the hot rolled steel sheet may be subjected to temper rolling according to conventional methods, and may also be subjected to pickling to remove the oxide scale formed on the surface. Alternatively, a coating treatment such as aluminum plating, aluminum-zinc plating, aluminum-silicon plating, hot dip galvanizing, electrogalvanizing, alloyed hot dip galvanizing, or chemical conversion treatment may be further performed.

The hot-rolled steel sheet according to the present embodiment can be stably produced by the preferred production method described above.

Example

Next, the embodiment of the present invention is described, but the conditions in the embodiment are a conditional example adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to this conditional example. As long as it does not deviate from the gist of the present invention and achieves the purpose of the present invention, the present invention can adopt various conditions.

Molten steel having the chemical composition shown in Table 1 was melted in a converter and slabs were obtained by continuous casting. Then, the slabs were heated under the conditions shown in Tables 2A and 2B, rough rolled, and then finished rolled under the conditions shown in Tables 2A and 2B. After the finish rolling, the slabs were cooled and coiled under the conditions shown in Tables 3A and 3B to obtain hot rolled steel sheets having the thickness shown in Tables 3A and 3B.

In addition, in the heating process, the holding time at the heating temperature described in Table 2A and Table 2B was set to 4800 seconds or less.

In addition, cooling after finishing rolling (except air cooling) is set to cooling by water cooling, which is performed by passing the steel plate through a water cooling device without an air cooling section in the middle. The average cooling rate in Table 3A and Table 3B is the value obtained by dividing the temperature drop of the steel plate from the time of introduction to the water cooling device to the time of exiting the water cooling device by the time required for the steel plate to pass through the water cooling device.

Test pieces were collected from the obtained hot-rolled steel sheets, and the area ratio of each structure, the polar density of the 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-mentioned method.

The obtained results are shown in Table 4A and Table 4B.

When the obtained tensile strength is 980 MPa or more, it is judged as having high strength and is passed. On the other hand, when the obtained tensile strength is less than 980 MPa, it is judged as not having high strength and is failed.

When the difference between the obtained total elongation in the C direction and the total elongation in the L direction is ±3.0% or less, it is judged as acceptable as having excellent isotropy of 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%, it is judged as unacceptable as not having excellent isotropy of ductility.

When the obtained hole expansion ratio is 40% or more, it is judged as having excellent hole expansion property and is passed. On the other hand, when the hole expansion ratio is less than 40%, it is judged as not having excellent hole expansion property and is failed.

[Table 1]

[Table 2A]

[Table 2B]

[Table 3A]

The underline indicates that the manufacturing conditions are not preferred.

[Table 3B]

The underline indicates that the manufacturing conditions are not preferred.

[Table 4A]

[Table 4B]

As can be seen from Table 4A and Table 4B, in the examples of the present invention, a hot-rolled steel sheet having high strength, isotropy, and excellent ductility and hole expandability was obtained.

On the other hand, the comparative examples whose chemical composition and/or metal structure are not within the range specified in the present invention are inferior in one or more of the above-mentioned characteristics.

Industrial Applicability

According to the above aspects of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, isotropy of ductility, and excellent hole expandability.

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％。