TWI616540B - Steel plate - Google Patents

Abstract

The steel sheet of the present invention has a predetermined chemical composition and has the following metal structure: in terms of area fraction, ferrite iron: 50% to 95%, granular toughening iron: 5% to 48%, and 麻田散铁: 2%~30%, and the upper toughened iron, the lower toughened iron, the tempered Matian loose iron, the residual Worthite iron and the Bora iron: a total of 5% or less.

Description

Steel plate

The present invention relates to a steel sheet suitable for automotive parts.

In order to suppress the amount of carbon dioxide gas emitted from automobiles, lightweighting of automobile bodies using high-strength steel sheets is being carried out. Moreover, in order to ensure the safety of passengers, the use of high-strength steel sheets in the vehicle body is increasing. In order to make the car body lighter, it is important to increase the strength. On the other hand, excellent formability is required for the body member. For example, high strength steel sheets for skeleton system components are required to have excellent stretching and hole expandability.

However, it is difficult to improve both strength and formability. A technique for improving the strength and improving the formability has been proposed (Patent Documents 1 to 3), but sufficient characteristics have not been obtained by these. Prior Technical Literature Patent Literature

Japanese Patent Laid-Open Publication No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei.

Disclosure of the Invention Problems to be Solved by the Invention An object of the present invention is to provide a steel sheet having high strength and excellent stretch and hole expandability. Means to solve the problem

The inventors of the present invention have made efforts to solve the above problems. As a result, it was found that the metal structure contained, in addition to the ferrite iron and the granulated iron, the granulated toughening iron of the area fraction of 5% or more, and the upper toughened iron, the lower toughened iron, the tempered granulated iron, and the residue It is important that the area ratio of Vostian Iron and Bora Iron is 5% or less in total. Because the upper toughened iron and the lower toughened iron are mainly composed of toughened ferrite iron with high indexing density and hard snow carbon, the stretching is poor. On the other hand, the granular toughening iron is mainly composed of the toughened ferrite iron with low translocation density, and it does not contain hard swarf carbon iron, so it is harder than the ferrite, the upper toughened iron and the lower part. Toughened and soft. Therefore, the granular toughened iron exhibits an excellent stretch compared to the upper toughened iron and the lower toughened iron. The granular toughening iron is harder than the ferrite and harder than the granulated iron, so it inhibits the pores generated from the interface between the ferrite iron and the granulated iron during the reaming process.

The inventors of the present application have conducted research on the basis of the knowledge obtained from such observations, and as a result, have considered the aspects of the invention shown below.

(1) A steel sheet characterized by having the chemical composition shown below: C: 0.05% to 0.1%, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less, O: 0.006, by mass% % or less, Si and Al: 0.20% to 2.50% in total, Mn and Cr: 1.0% to 3.0% in total, Mo: 0.00% to 1.00%, Ni: 0.00% to 1.00%, Cu: 0.00% to 1.00%, Nb : 0.000% to 0.30%, Ti: 0.000% to 0.30%, V: 0.000% to 0.50%, B: 0.0000% to 0.01%, Ca: 0.0000% to 0.04%, Mg: 0.0000% to 0.04%, REM: 0.0000 %~0.04%, and the remainder: Fe and impurities; and have the following metal structure: in terms of area fraction, ferrite iron: 50% to 95%, granular toughened iron: 5% to 48%, Ma Tian loose iron: 2% ~ 30%, and the upper toughening iron, the lower toughening iron, the tempering Ma Tian loose iron, the residual Worth iron and the Bora iron: a total of 5% or less.

(2) The steel sheet according to (1), wherein the chemical composition is such that Mo: 0.01% to 1.00%, Ni: 0.05% to 1.00%, or Cu: 0.05% to 1.00%, or any combination thereof.

(3) The steel sheet according to (1) or (2), wherein the chemical composition includes Nb: 0.005% to 0.30%, Ti: 0.005% to 0.30%, or V: 0.005% to 0.50%, or the like. random combination.

(4) The steel sheet according to any one of (1) to (3) wherein B: 0.0001% to 0.01% is established in the chemical composition.

(5) The steel sheet according to any one of (1) to (4) wherein, in the chemical composition, Ca: 0.0005% to 0.04%, Mg: 0.0005% to 0.04%, or REM: 0.0005% to 0.04%, or Any combination of these.

(6) The steel sheet according to any one of (1) to (5) which has a hot-dip galvanized layer on the surface.

(7) The steel sheet according to any one of (1) to (5) which has an alloyed hot-dip galvanized layer on the surface. Effect of the invention

According to the present invention, since the granular toughened iron or the like is contained in the metal structure at an appropriate area fraction, high strength, excellent stretching and hole expandability can be obtained.

Embodiments for Carrying Out the Invention Hereinafter, embodiments of the present invention will be described.

First, the metal structure of the steel sheet according to the embodiment of the present invention will be described. The details will be described later, and the steel sheet according to the embodiment of the present invention is produced by hot rolling, cold rolling, annealing, or the like of steel. Therefore, the metal structure of the steel plate needs to consider not only the characteristics of the steel sheet but also the phase transformation state of the treatment. The steel sheet according to the present embodiment has the following metal structure: in terms of area fraction, ferrite iron: 50% to 95%, granular toughened iron: 5% to 48%, and Ma Tian loose iron: 2% to 30% And the upper toughened iron, the lower toughened iron, the tempered granulated iron, the residual Worthite iron and the Bora iron: a total of 5% or less.

(Fat iron: 50%~95%) Fertilizer iron is a soft tissue that is easily deformed and helps to increase stretching. Fertilizer iron also contributes to the metamorphosis of the ferrite from iron to granular toughened iron. When the area fraction of the ferrite iron is less than 50%, sufficient granular toughened iron is not obtained. Therefore, it is preferable to set the area fraction of the ferrite iron to 50% or more, and to set it to 60% or more. On the other hand, when the area fraction of the ferrite iron is more than 95%, sufficient tensile strength is not obtained. Therefore, the area fraction of the ferrite iron is set to 95% or less, and it is preferably 90% or less.

(granular toughening iron: 5%~48%) Granular toughening iron is mainly composed of toughened ferrite iron with low indexing density, such as 10 ¹³ m/m ³ or so, because it does not contain hard snow Carbon iron, it is harder than the ferrite, harder than the upper toughened iron and the lower toughened iron. Therefore, the granular toughened iron exhibits an excellent stretch compared to the upper toughened iron and the lower toughened iron. The granular toughening iron is harder than the ferrite and harder than the granulated iron, so it inhibits the pores generated from the interface between the ferrite iron and the granulated iron during the reaming process. When the area fraction of the granular toughened iron is less than 5%, such effects are not sufficiently obtained. Therefore, the area fraction of the granular toughened iron is preferably 5% or more, and more preferably 10% or more. On the other hand, when the area fraction of the granular toughened iron is more than 48%, the area fraction of the ferrite iron and/or the granulated iron is inevitably insufficient. Therefore, the area fraction of the granular toughened iron is preferably 48% or less, and preferably 30% or less.

(Ma Tian loose iron: 2% ~ 30%) Because of the high density and hard tissue of the Ma Tian scattered iron, it helps to improve the tensile strength. When the area fraction of the granulated iron is less than 2%, sufficient tensile strength, such as a tensile strength of 590 MPa or more, is not obtained. Therefore, it is preferable to set the area ratio of the granulated iron to 2% or more, and to set it as 5% or more. On the other hand, when the area fraction of the granulated iron is more than 30%, sufficient stretching and hole expandability are not obtained. Therefore, it is preferable to set the area ratio of the granulated iron to 30% or less, and to set it as 20% or less.

(Upper toughened iron, lower toughened iron, tempered Matian loose iron, residual Worthite iron and Bora iron: less than 5% in total) The upper toughened iron and the lower toughened iron are mainly composed of high translocation density such as 1.0× It is composed of 10 ¹⁴ m/m ³ of toughened ferrite iron and hard snowy carbon iron, and the upper toughened iron has more residual Worthite iron. Tempered Ma Tian loose iron contains hard snow-light carbon iron. The upper metamorphic iron, the lower toughened iron and the tempered granulated iron have a high translocation density. Therefore, the upper toughened iron, the lower toughened iron, and the tempered sesame loose iron will cause the elongation to decrease. In the deformation, the residual Worth iron is metamorphosed into a granulated iron by processing-induced metamorphism, and the hole expandability is remarkably deteriorated. Borne iron contains hard stellite carbon iron, which becomes the starting point for pore formation during reaming. Therefore, the lower the area ratio of the upper toughened iron, the lower toughened iron, the tempered granulated iron, the residual Worth iron and the ferritic iron, the better. In particular, when the area ratio of the upper toughened iron, the lower toughened iron, the tempered granulated iron, the residual Worth iron and the ferritic iron is more than 5%, the stretching or hole expanding property or both will be remarkable. The ground is falling. Therefore, the area fraction of the upper toughened iron, the lower toughened iron, the tempered granulated iron, the residual Worth iron, and the ferritic iron is set to be 5% or less in total. Furthermore, the area fraction of the remaining Worth Iron does not include the area fraction of the residual Worth Iron contained in the upper toughened iron.

The identification of the ferrite iron, the granular toughening iron, the granulated iron, the upper toughened iron, the lower toughened iron, the tempered granulated iron, the residual Worth iron and the Bora iron, and the specificity of the area fraction can be borrowed This is carried out, for example, by an electron back scattering diffraction (EBSD) method, an X-ray measurement, or a scanning electron microscope (SEM) observation. For SEM observation, for example, the sample is etched using a nitrate etchant reagent or a Lepera solution, and a cross section parallel to the rolling direction and the thickness direction and/or rolling is observed at a magnification of 1000 to 50,000 times. A section perpendicular to the vertical direction. The metal structure of the steel sheet can be represented by a metal structure having a depth of about 1/4 of the thickness of the steel sheet from the surface thereof. For example, when the thickness of the steel sheet is 1.2 mm, it can be represented by a metal structure having a depth of about 0.3 mm from the surface.

The area fraction of the ferrite iron can be, for example, specific for the electron channel contrast image observed using SEM. The electron channel contrast image shows the difference in crystal orientation in the crystal grains as the difference between the contrasts. The contrasting part of the electron channel contrast image is the ferrite iron. In this method, for example, a region having a depth from the surface of the steel sheet of 1/8 to 3/8 of the thickness of the steel sheet is used as an observation object.

The area fraction of the residual Worthite iron can be specified by, for example, X-ray measurement. In this method, for example, a portion of the steel sheet surface to a quarter of the thickness of the steel sheet is removed by mechanical polishing and chemical polishing, and a MoKa line is used as the characteristic X-ray. In addition, the integrated intensity of the diffraction peaks of (200), (220), and (311) from the body-centered cubic lattice (bcc) phase (200) and (211), and the face-centered cubic lattice (fcc) phase The area fraction of the residual Worthite iron was calculated by the following formula. Sg=(I200f+I220f+I311f)/(I200b+I211b)×100 (Sg shows the area fraction of residual Worth Tin, I200f, I220f, I311f respectively show fcc phase (200), (220), (311 The diffraction peak intensity of I200b and I211b respectively shows the diffraction peak intensity of (b) (b) and (211).

The area fraction of the granulated iron can be specified by, for example, field emission-scanning electron microscope (FE-SEM) observation and X-ray measurement. In this method, for example, a region having a depth from the surface of the steel sheet of 1/8 to 3/8 of the thickness of the steel sheet is used as an observation object, and is etched using a Riepella liquid. The structure that is not corroded by the Riepella solution is the granulated iron and the residual Worthite iron, so the residual Worthfield specific to the X-ray measurement is subtracted from the area fraction of the area not corroded by the Ribeira solution. The area fraction of iron, Sg, can be used to specify the area fraction of the granulated iron. The area fraction of the granulated iron can also be specified using, for example, an electron channel contrast image obtained by SEM observation. In the electron channel contrast image, the translocation density is high, and the area with the lower tissue such as bundles and blocks in the granule is the granulated iron.

The upper toughened iron, the lower toughened iron, and the tempered granulated iron can be specified by, for example, FE-SEM observation. In this method, for example, a region from the surface of the steel sheet having a depth of 1/8 to 3/8 of the thickness of the steel sheet is used as an observation object, and is etched using a nitrate etchant reagent. In addition, as described below, the upper toughened iron, the lower toughened iron, and the tempered granulated iron are identified based on the position and variation of the smectite carbon iron. The upper toughened iron at the interface of the slab-like toughened ferrite iron contains ferritic carbon iron or residual Worth iron. The lower toughened iron contains ferritic carbon iron inside the slab-shaped toughened ferrite iron. The relationship between the grain orientation of the toughened ferrite iron and the smectite carbon iron is one, so the stellite carbon iron contained in the lower toughened iron has the same variant. The tempered Ma Tian loose iron contains Xueming carbon iron inside the Ma Tian loose iron slats. There are two or more kinds of crystal orientations between the slabs of the slabs and the stellites of the smectites. Therefore, the ferritic carbon iron contained in the tempered granulated iron has a large number of variants. The area fraction can be specified by identifying the upper toughening iron, the lower toughening iron, and the tempering Ma Tian loose iron based on the position and variation of such a snow-capped carbon iron.

Borite can be identified by, for example, optical microscopy, specifying its area fraction. In this method, for example, a region from the surface of the steel sheet having a depth of 1/8 to 3/8 of the thickness of the steel sheet is used as an observation object, and is etched using a nitrate etchant reagent. The area showing dark contrast in optical microscopy is a Borne iron.

Even if it was observed by the conventional etching method or the secondary electron image observation using a scanning electron microscope, the granular toughened iron and the ferrite iron were not distinguished. As a result of intensive studies, the present inventors have found that granular toughened iron has a slight crystal orientation difference in the grains. Therefore, by detecting the slight crystal orientation difference in the grain, it can be distinguished from the ferrite iron. Here, a specific specific method of the area fraction of the granular toughened iron will be described. In this method, a region from the surface of the steel sheet having a depth of 1/8 to 3/8 of the thickness of the steel sheet is used as a measurement object, and a plurality of (pixel) crystals in the region are measured by an EBSD method at intervals of 0.2 mm. Orientation, from which the value of GAM (grain average misorientation) is calculated. In this calculation, when the crystal orientation difference between adjacent pixels is 5° or more, it is considered that there is a grain boundary between the pixels, and the crystal orientation difference between adjacent pixels in the region surrounded by the grain boundary is calculated. The average of the differences. This average value is the value of GAM. In this way, it is possible to detect the slight crystal orientation difference of the tough ferrite iron. The area where the value of GAM is 0.5 or more is any one of granular toughening iron, upper toughened iron, lower toughened iron, tempered granian iron, waved iron or 麻田散铁. Therefore, the area fraction of the area where the value of GAM is 0.5° or more is subtracted from the total area fraction of the upper toughened iron, the lower toughened iron, the tempered granulated iron, the ferritic iron, and the granulated iron. The area fraction of granular toughened iron.

Next, the chemical composition of the steel sheet according to the embodiment of the present invention and the flat steel blank used for the production thereof will be described. As described above, the steel sheet according to the embodiment of the present invention is produced by hot rolling, cold rolling, annealing, and the like of a flat steel blank. Therefore, the chemical composition of the steel plate and the flat steel embryo not only needs to consider the characteristics of the steel plate, but also needs to consider such treatment. In the following description, unless otherwise specified, the unit "%" of the content of each element contained in the steel sheet and the flat steel embryo is "% by mass". The steel sheet according to the present embodiment has the chemical composition shown below: C: 0.05% to 0.1%, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less, and O: 0.006% or less, by mass%. Si and Al: 0.20% to 2.50% in total, Mn and Cr: 1.0% to 3.0% in total, Mo: 0.00% to 1.00%, Ni: 0.00% to 1.00%, Cu: 0.00% to 1.00%, and Nb: 0.000% ~0.30%, Ti: 0.000%~0.30%, V: 0.000%~0.50%, B: 0.0000%~0.01%, Ca: 0.0000%~0.04%, Mg: 0.0000%~0.04%, REM (rare earth metal: rare Earth metal): 0.0000%~0.04%, and the rest: Fe and impurities. The impurities may be exemplified by those contained in raw materials such as ore or scrap, and those included in the production steps.

(C: 0.05%~0.1%) C helps to increase the tensile strength. When the C content is less than 0.05%, sufficient tensile strength, for example, a tensile strength of 590 MPa or more is not obtained. Therefore, the C content is preferably 0.05% or more, and more preferably 0.06% or more. On the other hand, when the C content is more than 0.1%, the formation of ferrite iron is suppressed, so that sufficient stretching cannot be obtained. Therefore, the C content is preferably 0.1% or less, and preferably 0.09% or less.

(P: 0.04% or less) P is not an essential element, and is contained as an impurity in steel, for example. P can reduce the hole expandability, segregate at the center in the thickness direction of the steel sheet to lower the toughness, or embrittle the welded portion. Therefore, the lower the P content, the better. In particular, when the P content is more than 0.04%, the hole expandability is remarkably lowered. Therefore, the P content is preferably 0.04% or less, and preferably 0.01% or less. Reducing the P content will increase the cost, and when it is reduced to less than 0.0001%, the cost will increase significantly. Therefore, the P content may be 0.0001% or more.

(S: 0.01% or less) S is not an essential element, and is contained as an impurity in steel, for example. S can lower the weldability, lower the manufacturability at the time of casting and hot rolling, or form a coarse MnS to lower the hole expandability. Therefore, the lower the S content, the better. In particular, when the S content is more than 0.01%, the weldability is lowered, the manufacturability is lowered, and the hole expandability is markedly lowered. Therefore, the S content is preferably 0.01% or less, and is preferably 0.005% or less. Reducing the S content will increase the cost, and if it is reduced to less than 0.0001%, the cost will increase significantly. Therefore, the S content may be 0.0001% or more.

(N: 0.01% or less) N is not an essential element, and is contained as an impurity in steel, for example. N will form a coarse nitride, and the coarse nitride may lower the bendability and hole expandability or cause the pores to be formed during the fusion. Therefore, the lower the N content, the better. In particular, when the N content is more than 0.01%, the hole expandability will be remarkably lowered and the pores will be significantly erected. Therefore, the N content is preferably 0.01% or less, and is preferably 0.008% or less. Reducing the N content will increase the cost, and if it is reduced to less than 0.0005%, the cost will increase significantly. Therefore, the N content may be 0.0005% or more.

(O: 0.006% or less) O is not an essential element, and is contained as an impurity in steel, for example. O will form a coarse oxide, and the coarse oxide can lower the bendability and the hole expandability, and cause the pores to be formed during the fusion. Therefore, the lower the O content, the better. In particular, when the O content is more than 0.006%, the hole expandability will be remarkably lowered and the pores will be apparently real. Therefore, the O content is preferably 0.006% or less, and is preferably 0.005% or less. Reducing the O content will increase the cost, and if it is reduced to less than 0.0005%, the cost will increase significantly. Therefore, the O content may be 0.0005% or more.

(Si and Al: 0.20% to 2.50% in total) Si and Al contribute to the formation of granular toughened iron. Granular toughening iron is a structure in which a large number of tough fertiliser irons are recovered at the transposition of these interfaces and become a piece. Therefore, when smectite carbon iron exists at the interface of the tough and fertilized iron, no granular toughened iron is formed there. Si and Al will inhibit the formation of ferritic carbon iron. When the total content of Si and Al is less than 0.20%, ferritic carbon iron is excessively formed, and granular toughened iron is not sufficiently obtained. Therefore, the Si and Al contents are preferably 0.20% or more in total, and preferably 0.30% or more. On the other hand, when the total content of Si and Al is more than 2.50%, flat steel cracks are likely to occur in the hot rolling. Therefore, the Si and Al contents are preferably 2.50% or less in total, and preferably 2.00% or less. It may contain only either Si or Al, and may contain both Si and Al.

(Mn and Cr: 1.0% to 3.0% in total) Mn and Cr can suppress the deformation of the ferrite and iron during annealing or plating after cold rolling, and contribute to the improvement of strength. When the total content of Mn and Cr is less than 1.0%, the area fraction of the ferrite iron becomes excessive, and sufficient tensile strength, for example, a tensile strength of 590 MPa or more is not obtained. Therefore, the content of Mn and Cr is preferably 1.0% or more in total, and is preferably 1.5% or more. On the other hand, when the total content of Mn and Cr is more than 3.0%, the area fraction of the ferrite iron is too small, and sufficient stretching cannot be obtained. Therefore, the content of Mn and Cr is preferably 3.0% or less in total, and is preferably 2.8% or less. It may contain only either Mn or Cr, and may contain both Mn and Cr.

Mo, Ni, Cu, Nb, Ti, V, B, Ca, Mg, and REM are not essential elements, and may be limited and appropriately contained in a steel sheet and steel in a predetermined amount.

(Mo: 0.00% to 1.00%, Ni: 0.00% to 1.00%, Cu: 0.00% to 1.00%) Mo, Ni, and Cu can suppress the deformation of the ferrite and iron during annealing or plating after cold rolling, and help For improving strength. Therefore, Mo, Ni or Cu or any combination of these may also be contained. In order to sufficiently obtain this effect, the Mo content is set to 0.01% or more, the Ni content is set to 0.05% or more, and the Cu content is preferably 0.05% or more. However, when the Mo content is more than 1.00%, the Ni content is more than 1.00%, or the Cu content is more than 1.00%, the area fraction of the ferrite iron becomes too small, and sufficient stretching is not obtained. Therefore, the Mo content, the Ni content, and the Cu content are all set to 1.00% or less. That is, it is preferable to satisfy Mo: 0.01% to 1.00%, Ni: 0.05% to 1.00%, or Cu: 0.05% to 1.00%, or any combination of these.

(Nb: 0.000% to 0.30%, Ti: 0.000% to 0.30%, V: 0.000% to 0.50%) In the annealing after cold rolling, the Worthite iron is finely granulated by Nb, Ti, and V. The area of the grain boundary of the stone is increased, which promotes the deformation of the ferrite. Therefore, Ni, Ti or V or any combination of these may also be contained. In order to sufficiently obtain this effect, it is preferable to set the Nb content to 0.005% or more, the Ti content to 0.005% or more, and the V content to 0.005% or more. However, when the Nb content is more than 0.30%, the Ti content is more than 0.30%, or the V content is more than 0.50%, the area fraction of the ferrite iron becomes excessive, and sufficient tensile strength is not obtained. Therefore, the Nb content is set to 0.30% or less, the Ti content is set to 0.30% or less, and the V content is set to 0.50% or less. That is, it is preferable to satisfy Nb: 0.005% to 0.30%, Ti: 0.005% to 0.30%, or V: 0.005% to 0.50%, or any combination of these.

(B: 0.0000% to 0.01%) B is segregated at the grain boundary of the Worthite iron in the annealing of the cold rolling and the like to suppress the deformation of the ferrite. Therefore, it is also possible to contain B. In order to sufficiently obtain this effect, the B content is preferably made 0.0001% or more. However, when the B content is more than 0.01%, the area fraction of the ferrite iron becomes too small, and sufficient stretching is not obtained. Therefore, the B content is made 0.01% or less. That is, it is preferable to establish B: 0.0001% to 0.01%.

(Ca: 0.0000%~0.04%, Mg: 0.0000%~0.04%, REM: 0.0000%~0.04%) Ca, Mg and REM can control the form of oxides and sulfides, which contribute to the improvement of hole expandability. Thus, it is also possible to contain Ca, Mg or REM or any combination of these. In order to sufficiently obtain this effect, it is preferable to set the Ca content, the Mg content, and the REM content to 0.0005% or more. However, when the Ca content is more than 0.04%, the Mg content is more than 0.04%, or the REM content is more than 0.04%, a coarse oxide is formed, and sufficient hole expandability is not obtained. Therefore, the Ca content, the Mg content, and the REM content are all set to 0.04% or less, and preferably 0.01% or less. That is, it is preferable to satisfy Ca: 0.0005% to 0.04%, Mg: 0.0005% to 0.04%, or REM: 0.0005% to 0.04%, or any combination of these.

REM is a general term for a total of 17 elements of Sc, Y and elements belonging to the lanthanide system, and the REM content is intended to mean the total content of these elements. The REM is contained, for example, in a rare earth metal alloy, and the addition of REM is, for example, a rare earth metal alloy or a metal REM such as a metal La or a metal Ce.

According to the present embodiment, for example, tensile strength of 590 MPa or more, TS × EL (tensile strength × full tensile) of 15000 MPa × % or more, TS × 1 of 25000 MPa × % or more (tensile strength × hole expansion ratio) ). That is, high strength, excellent stretching and hole expandability can be obtained. This steel sheet is easy to form, for example, a skeleton component of an automobile, and can also ensure safety at the time of collision.

Next, a method of producing a steel sheet according to an embodiment of the present invention will be described. In the method for producing a steel sheet according to an embodiment of the present invention, hot rolling, pickling, cold rolling, and annealing of a flat steel preform having the above chemical composition are sequentially performed.

The hot rolling is started at a temperature of 1100 ° C or higher and ends at a temperature of Ar ₃ or higher. In the cold rolling, the rolling reduction ratio is set to 30% or more and 80% or less. In the annealing, the holding temperature is set to Ac ₁ point or more, the holding time is set to 10 seconds or more, and in the subsequent cooling, the cooling rate in the temperature range of 700 ° C to the Mf point is set to 0.5 ° C / sec or more and 4 ° C / sec or less.

When the temperature at which the hot rolling is started is less than 1,100 ° C, there is a case where the elements other than Fe are not sufficiently dissolved in Fe. Therefore, the hot rolling is started at a temperature of 1100 ° C or higher. The temperature at which the hot rolling is started is, for example, the temperature at which the flat steel is heated. For the flat steel embryo, for example, a flat steel embryo obtained after continuous casting and a flat steel embryo made by a thin flat steel blank casting machine can be used. The flat steel blank may be supplied to the hot rolling equipment while maintaining a temperature of 1100 ° C or higher after casting, or may be heated and then supplied to the hot rolling equipment after cooling to a temperature of less than 1100 ° C.

When the temperature of the hot rolling extension is less than Ar ₃ point, the metal structure of the hot rolled steel sheet will contain Worthfield iron and ferrite iron. The mechanical properties of the Worcester iron and the ferrite iron are different, so there is cold rolling. The case where the processing after the hot rolling is delayed becomes difficult. Therefore, the hot rolling is terminated at a temperature of Ar ₃ or more. The hot rolling delay is terminated at a temperature above Ar ₃ , which can reduce the rolling load in the hot rolling.

The hot rolling pass includes a rough rolling and a final rolling, and the final rolling may also continuously roll the majority of the steel sheets obtained by joining the rough rolling. The coiling temperature is set to 450 ° C or more and 650 ° C or less.

Pickling is carried out once or twice. By pickling, the oxide on the surface of the hot-rolled steel sheet is removed, and chemical conversion treatability and plating property are improved.

When the rolling reduction ratio of the cold rolling is less than 30%, it may be difficult to keep the shape of the cold rolled steel sheet flat or fail to obtain sufficient ductility. Therefore, it is preferable to set the rolling reduction ratio of the cold rolling to 30% or more, and it is preferable to set it as 50% or more. On the other hand, when the rolling reduction ratio of the cold rolling is more than 80%, there is a case where the rolling load becomes excessively heavy or the recrystallization of the ferrite iron is excessively promoted in the annealing after the cold rolling. Therefore, the rolling reduction ratio of the cold rolling is preferably 80% or less, and preferably 70% or less.

During the annealing, the Vostian iron was formed by maintaining the temperature above Ac ₁ point for 10 seconds. After the cooling, the Worth Iron was metamorphosed into ferrite iron, granular tough iron or 麻田散铁. If the temperature is kept less than Ac ₁ point, or the holding time is less than 10 seconds, the Worthite iron is not sufficiently formed. Therefore, the holding temperature is set to Ac ₁ point or more, and the holding time is set to 10 seconds or more.

In the cooling after annealing, granular toughening iron and granulated iron can be formed in the temperature range of 700 ° C to Mf point. As described above, the granular toughening iron is a structure in which a plurality of tough ferrite irons are recovered at the transposition of the interfaces to become a piece. This index can be restored to a temperature domain below 700 ° C. However, when the cooling rate in the temperature range is more than 4 ° C / sec, the indexing is not sufficiently restored, and the area fraction of the granular toughened iron is insufficient. Therefore, the cooling rate in this temperature range is set to 4 ° C / sec or less. On the other hand, when the cooling rate in this temperature range is less than 0.5 ° C / sec, there is a case where the granulated iron is not sufficiently formed. Therefore, the cooling rate in the temperature range is set to 0.5 ° C / sec or more.

Thus, the steel sheet according to the embodiment of the present invention can be produced.

The steel sheet may be subjected to a plating treatment such as a plating treatment or a vapor deposition treatment, or may be subjected to an alloying treatment after the plating treatment. Surface treatment such as organic film formation, film lamination, organic salt/inorganic salt treatment, and chromium-free treatment may be performed on the steel sheet.

When the steel sheet is subjected to a hot-dip galvanizing treatment as a plating treatment, for example, the temperature of the steel sheet is heated or cooled to a temperature lower than the temperature of the galvanizing bath by 40° C. or higher, and the temperature of the galvanizing bath is 50° C. or higher. Galvanized bath. By hot-dip galvanizing, a steel sheet having a hot-dip galvanized layer on the surface, that is, a hot-dip galvanized steel sheet can be obtained. The hot-dip galvanized layer has, for example, Fe: 7 mass% or more and 15 mass% or less, and the remaining portion: a chemical composition represented by Zn, Al, and impurities.

When the alloying treatment is performed after the hot-dip galvanizing treatment, for example, the hot-dip galvanized steel sheet is heated to a temperature of 460 ° C or higher and 600 ° C or lower. When the temperature is less than 460 ° C, there is a case where the alloying is insufficient. When the temperature is more than 600 ° C, there is a case where the alloying is excessive and the corrosion resistance is deteriorated. By alloying treatment, a steel sheet having an alloyed hot-dip galvanized layer on the surface, that is, an alloyed hot-dip galvanized steel sheet can be obtained.

Further, the above-described embodiments are merely examples for embodying the embodiments of the present invention, and are not intended to limit the technical scope of the present invention. In other words, the present invention can be implemented in various forms without departing from the technical idea or main features of the present invention. [Examples]

Next, an embodiment of the present invention will be described. The conditions of the examples are a conditional example used to confirm the applicability and effects of the present invention, and the present invention is not limited by the conditional example. The present invention can be obtained using various conditions without departing from the gist of the present invention and attaining the object of the invention.

(First Test) In the first test, a flat steel embryo having a chemical composition shown in Tables 1 to 4 was produced, and the flat steel blank was hot rolled to obtain a hot rolled steel sheet. The blank columns in Tables 1 to 4 show that the content of the element is less than the detection limit, and the remaining part is Fe and impurities. The bottom line in Tables 1 to 4 shows that the value is outside the scope of the present invention.

[Table 1]

[Table 2]

[table 3]

[Table 4]

Next, pickling, cold rolling, and annealing of the hot rolled steel sheet are performed to obtain a steel sheet. Tables 5 to 7 show the conditions of hot rolling, cold rolling and annealing. The cooling rate in the annealing conditions is the average cooling rate in the temperature range from 700 ° C to the Mf point. Table 8 to Table 10 show the area fraction f _{F of the} ferrite and iron of each steel plate, the area fraction of the granular toughened iron f _GB , the area fraction f _{M of the} granulated iron, and the upper toughened iron and the lower part. The total area fraction f _{T of the} tough iron, the tempered granulated loose iron, the residual Worthite iron and the Borne iron. The bottom lines in Tables 8 to 10 show that the values are outside the scope of the present invention.

[table 5]

[Table 6]

[Table 7]

[Table 8]

[Table 9]

[Table 10]

In addition, the tensile test and the hole expansion test of each steel plate were performed. In the tensile test, the Japanese Industrial Standard JIS No. 5 test piece was taken at right angles to the rolling direction of the steel sheet, and the tensile strength TS and the full tensile EL were measured in accordance with JIS Z2242. In the hole expanding test, the hole expansion ratio l was measured in accordance with the description of JIS Z2256. The results of these are shown in Tables 11 to 13. The bottom line in Tables 11 through 13 shows that the value is outside the expected range. The term range referred to herein is TS 590 MPa or more, TS × EL is 15000 MPa × % or more, and TS × l is 25000 MPa × % or more.

[Table 11]

[Table 12]

[Table 13]

As shown in Tables 11 to 13, the samples in the range of the present invention were able to obtain high strength, excellent stretching and hole expandability.

In sample No. 1, the C content was too low, so the strength was low. In sample No. 5, since the C content was too high, the stretching and hole expandability were low. In sample No. 6, the total content of Si and Al was too low, so that the hole expandability was low. In sample No. 10, since the total content of Si and Al was too high, flat steel cracks were generated in the hot rolling. In sample No. 11, the total content of Mn and Cr was too low, so the strength was low. In sample No. 15, the total content of Mn and Cr was too high, so the stretching and hole expandability were low. In sample No. 18, since the P content was too high, the hole expandability was low. In sample No. 21, since the S content was too high, the hole expandability was low. In sample No. 23, since the N content was too high, the hole expandability was low. In sample No. 25, since the O content was too high, the hole expandability was low.

In sample No. 28, since the Mo content was too high, the stretching was low. In sample No. 31, since the Ni content was too high, the stretching was low. In sample No. 34, since the Cu content was too high, the stretching and hole expandability were low. In sample No. 37, since the Nb content was too high, the strength was low and the hole expandability was low. In sample No. 40, since the Ti content was too high, the strength was low and the hole expandability was low. In sample No. 43, the V content was too high, so the strength was low. In sample No. 46, since the B content was too high, the stretching and hole expandability were low. In sample No. 49, since the Ca content was too high, the hole expandability was low. In sample No. 52, since the Mg content was too high, the hole expandability was low. In sample No. 55, since the REM content was too high, the hole expandability was low.

In sample No. 59, since the total area fraction f _{T was} too high, the hole expandability was low. In sample No. 62, since the area fraction f _{M was} too low and the total area fraction f _{T was} too high, the hole expandability was low. In sample No. 64, since the area fraction f _{F was} too low and the total area fraction f _{T was} too high, the stretching was low. In sample No. 67, since the area fraction f _{GB was} too low and the total area fraction f _{T was} too high, the hole expandability was low. In sample No. 69, since the area fraction f _{GB was} too low, the hole expandability was low. In sample No. 70, since the area fraction f _{GB was} too low and the total area fraction f _{T was} too high, the hole expandability was low. In sample No. 72, since the area fraction f _{GB was} too low and the total area fraction f _{T was} too high, the hole expandability was low. In sample No. 74, since the area fraction f _{GB was} too low, the hole expandability was low. In sample No. 75, since the area fraction f _{GB was} too low, the hole expandability was low. In sample No. 77, since the area fraction f _{GB was} too low and the total area fraction f _{T was} too high, the hole expandability was low. In sample No. 79, since the area fraction f _{GB was} too low and the total area fraction f _{T was} too high, the hole expandability was low. In sample No. 80, since the area fraction f _{GB was} too low and the total area fraction f _{T was} too high, the hole expandability was low. Industrial availability

The present invention can be utilized in a steel plate-related industry suitable for automotive parts.

Claims (7)

A steel sheet characterized by having the chemical composition shown below: C: 0.05% to 0.1%, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less, O: 0.006% or less, in mass%, Si and Al: 0.20% to 2.50% in total, Mn and Cr: 1.0% to 3.0% in total, Mo: 0.00% to 1.00%, Ni: 0.00% to 1.00%, Cu: 0.00% to 1.00%, and Nb: 0.000% ~0.30%, Ti: 0.000%~0.30%, V: 0.000%~0.50%, B: 0.0000%~0.01%, Ca: 0.0000%~0.04%, Mg: 0.0000%~0.04%, REM: 0.0000%~0.04 %, and the remainder: Fe and impurities; and have the following metal structure: in terms of area fraction, ferrite iron: 50% to 95%, granular toughening iron: 5% to 48%, Ma Tian loose iron : 2%~30%, and the upper toughened iron, the lower toughened iron, the tempered Matian loose iron, the residual Worthite iron and the Bora iron: a total of 5% or less. The steel sheet according to claim 1, wherein Mo: 0.01% to 1.00%, Ni: 0.05% to 1.00%, or Cu: 0.05% to 1.00%, or any combination of the above, is established in the chemical composition. The steel sheet according to claim 1, wherein the chemical composition comprises Nb: 0.005% to 0.30%, Ti: 0.005% to 0.30%, or V: 0.005% to 0.50%, or any combination thereof. The steel sheet of claim 1, wherein B: 0.0001% to 0.01% is established in the aforementioned chemical composition. The steel sheet according to claim 1, wherein the chemical composition comprises Ca: 0.0005% to 0.04%, Mg: 0.0005% to 0.04%, or REM: 0.0005% to 0.04%, or any combination thereof. A steel sheet according to any one of claims 1 to 5, which has a hot-dip galvanized layer on the surface. A steel sheet according to any one of claims 1 to 5, which has an alloyed hot-dip galvanized layer on the surface.