TW201829791A - Steel plate having a predetermined chemical composition and excellent elongation and hole expandability - Google Patents

Abstract

A steel plate, which has a predetermined chemical composition and a metal structure represented by the following: in terms of an area fraction, ferrite iron: 30% to 50%, granular bainite: 5% to 20%, martensite: 30 %~55%, bainite: 35% or less, and the residual austenite and pearlite: less than 10% in total. Preferably, the tensile strength of the steel plate is 1180 MPa or more, the stretchability is 10% or more, and the hole expansion value is 20% or more. More preferably, the VDA bending angle when the thickness is t(mm) is "7.69t 2 - 38.4t + 109" or more.

Description

Steel plate

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

In order to suppress the amount of carbon dioxide exhaust gas derived from automobiles, the automobile body using high-strength steel sheets continues to be lighter. In addition, in order to ensure the safety of the rider, most of the high-strength steel sheets are used in the vehicle body. In order to make the car body more lightweight, further strength improvement is important. On the one hand, depending on the parts of the vehicle body, it is required to have excellent formability. For example, high-strength steel sheets for parts for skeletons are required to have excellent elongation and hole expandability.

However, both strength improvement and formability improvement are both difficult.

For example, Patent Document 1 describes controlling the nano hardness distribution of a steel sheet composed of ferrite iron and 麻田散铁 for both strength and workability. In Patent Document 1, a lateral bending test simulating the formation of an elongated flange is also described for the evaluation of workability. However, there is no record of bending.

The steel sheet described in Patent Document 2 is a main phase of the granulated iron structure, and is excellent in strength and flexibility, but lacks ductility, and may be cracked during press forming.

For example, the TRIP (Transformation Induced Plasticity) steel containing the residual Worth iron is described in Patent Document 3. In the TRIP steel, the residual Worstian iron present in the steel is metamorphosed into the granulated iron during the forming, whereby excellent ductility is obtained. However, at the time of molding, the vestibular iron is metamorphosed by the residual Worthite iron, which is hard, and is likely to be a starting point of cracking, and is also a cause of deterioration of hole expandability or bendability. [Prior Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Publication No. Hei 7-74412 (Patent Document 3): JP-A-H10-130776

[Problem to be Solved by the Invention] An object of the present invention is to provide a steel sheet which has high strength and which can obtain excellent elongation and hole expandability. [Means to solve the problem]

In order to solve the above problems, the inventors of the present invention conducted intensive studies. As a result, it is judged that the following is important: the area ratio of fertilized iron is 30% to 50%, the area of Matian iron is 30% to 55%, and the area of granular toughened iron is 5 %~20%, the area ratio of toughened iron is less than 35%, and the area ratio of residual Worthite and Borne iron is less than 10%. The tempered Ma Tian loose iron is included in the Ma Tian loose iron. Granular toughening iron is mainly composed of tough ferrite iron with low difference density and hardly contains hard ferritic carbon iron. Therefore, it is harder than fertilized iron, and is more tough than iron and granulated iron. It is soft. Accordingly, the granular toughened iron exhibits superiority to the toughened iron and the granitic iron. Furthermore, the granular toughened iron is harder than the ferrite, but softer than the toughened iron and the granulated iron. Therefore, at the time of reaming, the interface between the self-fertilized iron and the granulated iron or the toughened iron can be inhibited. There are gaps.

The inventors of the present invention conducted in-depth research based on these knowledge findings, and thus thought of various aspects of the invention shown below.

(1) A steel sheet characterized by having the chemical composition represented by the following: C: 0.09% to 0.15%, Si: 0.2% to 2.5%, Al: 0.01% to 1.00%, Mn: 1.0% ~3.0%, P: 0.02% or less, S: 0.01% or less, N: 0.007% or less, O: 0.006% or less, Cr: 0.00% to 1.00%, Mo: 0.00% to 1.00%, B: 0.0000% to 0.010 %, Nb: 0.000%~0.30%, Ti: 0.000%~0.30%, Ni: 0.00%~1.00%, Cu: 0.00%~1.00%, V: 0.000%~0.50%, Mg: 0.0000%~0.04%, REM: 0.0000%~0.04%, and the remainder: Fe and impurities; and the steel plate has the following metal structure in terms of area fraction: ferrite iron: 30%~50%, granular toughening iron: 5%~ 20%, Ma Tian loose iron: 30% ~ 55%, toughened iron: less than 35%, and residual Worthfield iron and Borne iron: a total of less than 10%.

(2) The steel sheet according to (1), which has a tensile strength of 1180 MPa or more, an elongation of 10% or more, and a hole expansion value of 20% or more.

(3) The steel sheet according to (1) or 2, wherein the VDA angle when the thickness is t ( mm ) is "7.69t ^{2 -} 38.4t + 109" or more.

(4) The steel sheet according to any one of (1) to (3) which has a hot-dip galvanized layer on the surface.

(5) The steel sheet according to any one of (1) to (3) which has an alloyed hot-dip galvanized layer on the surface.

[Effect of the Invention] According to the present invention, since a granular toughened iron or the like having an appropriate area fraction is contained in a metal structure, high strength, excellent elongation, and hole expandability can be obtained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described.

First, the metal structure of the steel sheet according to the embodiment will be described. The details are as described later, but the steel sheet according to the embodiment of the present invention is produced by hot rolling, cold rolling, annealing, tempering, or the like of steel. Therefore, the metal structure of the steel sheet not only considers the characteristics of the steel sheet, but also considers the phase transition state in the treatment. The steel sheet of the present embodiment has the following metal structure: area fraction meter, ferrite iron: 30% to 50%, granular toughened iron: 5% to 20%, 麻田散铁: 30% to 55%, toughening Iron: less than 35%, and residual Worthite and Borne: total below 10%.

(Ferrous iron: 30%~50%) Because of its soft texture, ferrite is easy to deform and contribute to increased elongation. Fertilizer iron also contributes to the phase change of the ferrite iron into a grainy tough iron. If the area fraction of the ferrite iron is less than 30%, sufficient stretchability is not obtained. Furthermore, an appropriate granular tough iron area fraction is not available. Accordingly, the area fraction of the ferrite iron is 30% or more, preferably 35% or more. On the one hand, if the ferrite iron area fraction exceeds 50%, it is difficult to obtain a tensile strength of 1180 MPa or more. Accordingly, the area fraction of the ferrite iron is below 50%, preferably below 45%.

(granular toughening iron: 5%~20%) Granular toughening iron is mainly composed of tough ferrite iron with a low difference in density of about 10 ¹³ m/m ³ and hardly contains hard snow Carbon iron, therefore, is harder than fat iron and softer than toughened iron. Therefore, the granular toughened iron exhibits superior elongation than the toughened iron. Granular toughening iron is harder than fat iron, but softer than toughened iron and granulated iron. Therefore, when reaming is processed, it can suppress the gap between the self-fertilizer iron and the granulated iron and the self-fertilizer iron and There is a gap in the interface of the toughened iron. If the area fraction of granular toughened iron is less than 5%, such effects cannot be fully obtained. Accordingly, the area fraction of the granular toughened iron is 5% or more, preferably 10% or more. On the other hand, if the area fraction of the granular toughened iron exceeds 20%, the granulated iron cannot be sufficiently obtained, and it is difficult to obtain a tensile strength of 1180 MPa or more. Accordingly, the area fraction of the granular toughened iron is 20% or less, preferably 15% or less.

(Ma Tian loose iron: 30% ~ 55%) Ma Tian loose iron is high in density and is hard tissue, so it helps to increase tensile strength. If the area fraction of the granulated iron is less than 30%, the tensile strength of 1180 MPa or more cannot be obtained. Accordingly, the area fraction of the granulated iron is more than 30%, preferably more than 35%. On the one hand, if the area fraction of the Ma Tian loose iron exceeds 55%, it will not be able to obtain sufficient elongation. Accordingly, the area fraction of the granulated iron is below 55%, preferably below 50%. The quarrying iron and the tempered granulated iron in the quenching state belong to the granulated iron. That is, the area fraction of the granulated iron is the sum of the area fraction of the quenched granulated iron and the area fraction of the tempered granulated iron. The method of obtaining the tempered granulated iron is not limited, and the tempered granulated iron may be obtained by automatic tempering during cooling, or may be obtained by tempering heat treatment after continuous annealing.

(toughened iron: less than 35%) The toughened iron is mainly composed of toughened ferrite iron and hard swarf carbon iron with a high density of about 1.0×10 ¹⁴ m/m ³ , and helps to improve Tensile Strength. However, if the area fraction of the toughened iron is 35% or more, the area fraction of the granulated iron which is more effective than the toughened iron to improve the tensile strength is insufficient, so it is difficult to obtain a tensile strength of 1180 MPa or more. Therefore, the area fraction of the toughened iron is set to be less than 35%.

(Residual Worthite iron and Wolla iron: a total of 10% or less) Residual Worth iron is deformed to cause metamorphosis and metamorphosis toward the granulated iron, and thus has excellent work hardening and high uniform elongation. However, the processing-induced granulated iron can significantly deteriorate the hole expandability. Since the Bora iron contains hard ferritic carbon iron, it becomes a starting point of void generation at the time of reaming processing, and the hole expandability is deteriorated. In particular, if the total area fraction of the remaining Worthite iron and the Borne iron exceeds 10%, the hole expandability is remarkably deteriorated. Therefore, the area ratio of the remaining Worthfield iron and the Bora iron is set below 10%.

Identification of ferrite iron, granular toughening iron, granulated iron, toughened iron, residual Worth iron and Borne iron, and specific area fractions, for example, electron back scattering Diffraction: EBSD) method, X-ray measurement, or scanning electron microscope (SEM). For SEM observation, for example, a sample is etched using a Nital reagent or a LePera solution, and a cross section parallel to the rolling direction and the thickness direction and/or perpendicular to the sample is observed at a magnification of 1000 to 50,000 times. The section of the rolling direction. The metal structure of the steel sheet can be represented by a metal structure in a region where the depth of the steel material is about 1/4 of the thickness of the steel sheet. For example, if the thickness of the steel sheet is 1.2 mm, a metal structure in a region having a depth of about 0.3 mm from the surface of the steel material can be used as a representative.

The area fraction of the ferrite iron can be specified, for example, using an electronically channelized contrast image obtained by SEM observation. The electron channel contrast image is represented by the difference in the crystal orientation difference in the crystal grain as the contrast difference. The comparative uniform part in the electron channel contrast image is the ferrite iron. In this method, for example, a region from the surface of the steel material having a depth of 1/8 to 3/8 of the thickness of the steel sheet is taken as an observation object.

The area fraction of the residual Worthite iron can be specified, for example, by X-ray measurement. In this method, for example, a portion from the surface of the steel material up to 1/4 of the thickness of the steel sheet is removed by mechanical polishing and chemical polishing, and a MoKα line is used as the characteristic X-ray. Therefore, using the following equation, 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 integral intensity ratio is used to calculate the area fraction of the residual Worthfield iron. Sγ=(I _200f +I _220f +I _311f )/(I _200b +I _211b )×100 (Sγ is the area fraction of residual Worth iron: I _200f , I _220f , I _311f respectively represent fcc phase (200 The diffraction peak intensities of (220) and (311); I _200b and I _211b represent the diffraction peak intensities of (b) and (211) of the bcc phase, respectively.

The area fraction of the quenched granulated iron can be determined by, for example, field emission-scanning electron microscope (FE-SEM) observation and X-ray measurement. In this method, for example, a region from the surface of the steel material having a depth of 1/8 to 3/8 of the thickness of the steel sheet is used as an observation object, and a lining solution for corrosion is used. Since the structure which cannot be corroded by the Riedera liquid is the quenched granulated iron and the residual Worthite iron, the residual Worth determined by the X-ray is subtracted from the area fraction of the area which cannot be corroded by the Ribeira liquid. Tian Iron's area fraction Sγ can specify the area fraction of the quenched Ma Tian loose iron. The area fraction of the quenched granulated iron can be specified, for example, using an electron-channel contrast image obtained by SEM observation. In the electron channel contrast image, the difference in density is high, and the area of the lower structure such as a block or a packet in the grain is a quenched granulated iron. The area fraction of the tempered granulated iron is ok, for example, by FE-SEM observation. In this method, for example, a region from the surface of the steel material having a depth of 1/8 to 3/8 of the thickness of the steel sheet is used as an observation object, and a nitrate reagent for corrosion is used. Therefore, based on the location and variability of Xueming carbon iron, the tempered granulated iron was identified. The tempered granulated iron contains ferritic carbon iron in the interior of the lath of the granulated iron. There are two or more kinds of crystal orientations between the Ma Tian loose iron needle and the Xueming carbon iron. Therefore, the Xueming carbon iron contained in the tempered Ma Tian loose iron has a plurality of variations. Based on the location and variation of the ferritic carbon iron, the tempered granulated iron can be identified and the area fraction is specified.

Toughened iron is possible, for example, by FE-SEM observation. In this method, for example, a region from the surface of the steel material having a depth of 1/8 to 3/8 of the thickness of the steel sheet is used as an observation object, and a nitrate reagent for corrosion is used. In addition, based on the location of the sulphur carbon iron and its variability, the toughened iron was identified. Toughened iron contains upper toughened iron and lower toughened iron. The upper toughened iron contains swarf carbon iron or residual Worth iron at the interface of the tough, fermented iron of the lath. The lower toughened iron contains swarf carbon iron located inside the long strip of tough ferrite. The relationship between the crystal orientation of the toughened ferrite iron and the swarf carbon iron is one, and therefore the smectite carbon iron contained in the lower toughened iron has the same variation. Based on the location and variation of the ferritic carbon iron, the toughened iron can be identified and the area fraction can be specified.

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

Granular toughening iron is a secondary electron image observation using a conventional etching method using a scanning electron microscope, which is difficult to distinguish from ferrite iron. The inventors of the present invention have intensively reviewed and found that the granular toughened iron has a slight crystal orientation difference in the grains. Therefore, it can be distinguished from the ferrite iron by detecting a slight crystal orientation difference in the crystal grains. Here, a specific specific method of the area fraction of the granular toughened iron will be described. In this method, for example, a region from the surface of the steel material having a depth of 1/8 to 3/8 of the thickness of the steel sheet is used as an observation object, and the crystal orientation of a plurality of spaces (pixels) in the region is determined by the EBSD method. The interval of 0.2 μm was measured, and the result of GAM (Grain Average Misorientation) was calculated from the result. In this calculation, when the crystal orientation difference between adjacent pixels is 5° or more, there is a grain boundary between the pixels, and the crystal orientation between adjacent pixels in the region surrounded by the grain boundaries is calculated. Poor and find the average of the difference. This average is the GAM value. In doing so, the toughened ferrite iron was detected to have a slight crystal orientation difference. The region where the GAM value is 0.5° or more is any one of granular toughened iron, toughened iron, tempered granulated iron, ferritic iron or 麻田散铁. Therefore, the value obtained by subtracting the total area fraction of the toughened iron, the tempered granulated iron, the ferritic iron, and the granulated iron from the area ratio of the GAM value in the region above 0.5° is the granular toughened iron. Area fraction.

Next, the steel sheet according to the embodiment of the present invention and the chemical composition used in 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, or the like. Therefore, the chemical composition of the steel sheet and the steel embryo is not only taken into consideration by the characteristics of the steel sheet, but also by the treatment thereof. In the following description, the unit "%" of the content of each element contained in the steel sheet and the steel embryo means "% by mass" unless otherwise specified. In the present embodiment, the steel sheet has the chemical composition shown below: C: 0.09% to 0.15%, Si: 0.2% to 2.5%, Al: 0.01% to 1.00%, Mn: 1.0% to 3.0%, P: 0.02% or less, S: 0.01% or less, N: 0.007% or less, O: 0.006% or less, Cr: 0.00% to 1.00%, Mo: 0.00% to 1.00%, B: 0.0000% to 0.010%, Nb: 0.000% to 0.30%, Ti: 0.000% to 0.30%, Ni: 0.00% to 1.00%, Cu: 0.00% to 1.00%, V: 0.000% to 0.50%, Mg: 0.0000% to 0.04%, REM: 0.0000% to 0.04%, and remaining Part: Fe and impurities. The impurities are exemplified by those contained in raw materials such as ore and scrap, or those included in the production steps.

(C: 0.09% to 0.15%) C is an increase in tensile strength. When the C content is less than 0.09%, it is difficult to obtain a tensile strength of 1180 MPa or more. Therefore, the C content is set to 0.09% or more, preferably 0.10% or more. On the other hand, when the C content exceeds 0.15%, sufficient elongation can not be obtained because the formation of ferrite iron is inhibited. Therefore, the C content is 0.15% or less, preferably 0.13% or less.

(Si: 0.2%~2.5%) Si inhibits ferritic carbon iron and contributes to the formation of granular toughened iron. Granular toughening iron is a structure in which a plurality of toughened ferrite irons are returned at their interfaces to form a piece of tissue. Therefore, if there is ferritic carbon iron at the interface of the tough ferrite, no granular toughened iron will be formed there. When the Si content is less than 0.2%, ferritic carbon iron is excessively formed, and sufficient granular toughened iron cannot be obtained. Therefore, the Si content is set to be 0.2% or more. On the other hand, if the Si content exceeds 2.5%, cracks in the steel are likely to occur during hot rolling. Therefore, the Si content should be set to 2.5% or less.

(Al: 0.01%~1.00%) Al inhibits ferritic carbon iron and contributes to the formation of granular toughened iron. Granular toughening iron is a structure in which a plurality of toughened ferrite irons are returned at their interfaces to form a piece of tissue. Therefore, if there is ferritic carbon iron at the interface of the tough ferrite, no granular toughened iron will be formed there. Al is also an element that can be used as a deoxidizer. When the total content of Al is less than 0.01%, too much ferritic carbon iron is formed, and sufficient granular toughened iron cannot be obtained. Therefore, the Al content should be set to 0.01% or more. On the other hand, if the Al content exceeds 1.00%, cracks in the steel are likely to occur during hot rolling. Further, the number density of the coarse inclusions of the Al system is increased to cause deterioration of the hole expandability. Therefore, the Al content is set to be 1.00% or less.

(Mn: 1.0% to 3.0%) In the continuous annealing after cold rolling or the heat treatment on the plating line, Mn suppresses the deformation of the ferrite and iron and contributes to the improvement of strength. When the total Mn content is less than 1.0%, the area fraction of the ferrite iron is too high, and it is difficult to obtain a tensile strength of 1180 MPa or more. Therefore, the Mn content is set to 1.0% or more. When the Mn content exceeds 3.0%, the area fraction of the ferrite iron becomes too small, and sufficient tensile strength cannot be obtained. Therefore, the Mn content is set to 3.0% or less.

(P: 0.04% or less) P is not an essential element and is contained, for example, as an impurity in steel. P lowers the hole expandability and segregates at the center of 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 exceeds 0.04%, the hole expandability is remarkably lowered. Therefore, the P content is 0.04% or less, preferably 0.01% or less. Furthermore, the cost is required to reduce the P content, and if it is to be lowered to less than 0.0001%, the cost is remarkably increased.

(S: 0.01% or less) S is not an essential element and is contained, for example, as an impurity in steel. S lowers the weldability, lowers the manufacturability at the time of casting and hot rolling, or forms 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, and the decrease in manufacturability and the decrease in hole expandability are remarkable. Therefore, the S content is 0.01% or less, preferably 0.005% or less. Furthermore, the cost is required to reduce the S content, and if it is to be reduced to less than 0.0001%, the cost is remarkably increased.

(N: 0.01% or less) N is not an essential element and is contained, for example, as an impurity in steel. N forms a coarse nitride. The coarse nitride causes a decrease in flexibility and hole expandability, and further, pores are generated during welding. Therefore, the lower the N content, the better. In particular, when the N content exceeds 0.01%, the hole expandability is remarkably lowered, and the occurrence of pores is remarkable. Therefore, the N content is 0.01% or less, preferably 0.008% or less. Cost is required for the reduction of the N content, and if it is to be lowered to less than 0.0005%, the cost is significantly increased.

(O: 0.006% or less) O is not an essential element and is contained, for example, as an impurity in steel. O forms a coarse oxide. The coarse oxide causes a decrease in flexibility and hole expandability, and further, pores are generated during welding. Therefore, the lower the O content, the better. In particular, when the O content exceeds 0.006%, the hole expandability is remarkably lowered, and the occurrence of pores is remarkable. Therefore, the O content is 0.006% or less, preferably 0.005% or less. The cost is required to reduce the O content, and if it is to be reduced to less than 0.0005%, the cost is significantly increased.

Cr, Mo, Ni, Cu, Nb, Ti, V, B, Ca, Mg, and REM are unnecessary elements, and the like is a limited amount of any element that is appropriately adjusted in a steel sheet and steel.

(Cr: 0.00% to 1.00%, Mo: 0.00% to 1.00%, Ni: 0.00% to 1.00%, Cu: 0.00% to 1.00%) Annealing or plating after cold rolling of Cr, Mo, Ni and Cu It will inhibit the fat and iron metamorphosis and contribute to the increase of strength. Therefore, Cr, Mo, Ni or Cu or any combination thereof may be contained. In order to obtain sufficient effect, it is preferable that the Cr content is set to 0.10% or more, 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 set to 0.05% or more. However, when the Cr content exceeds 1.00%, or the Mo content is more than 1.00%, or 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 elongation cannot be obtained. Sex. Therefore, any of the Cr content, the Mo content, the Ni content, and the Cu content is set to 1.00% or less. That is, it is preferably in accordance with Cr: 0.10% to 1.00%, Mo: 0.01% to 1.00%, Ni: 0.05% to 1.00%, or Cu: 0.05% to 1.00%, or any combination thereof.

(Nb: 0.000% to 0.30%, Ti: 0.000% to 0.30%, V: 0.000% to 0.50%) Nb, Ti and V are finely granulated by Worthite iron in an annealing step after cold rolling or the like. Increase the area of the grain boundary of the Worthfield Iron and promote the metamorphosis of the ferrite. Therefore, it is also possible to contain Ni, Ti or V or any combination thereof. In order to obtain sufficient effect, it is preferable that the Nb content is set to 0.005% or more, the Ti content is set to 0.005% or more, and the V content is set to 0.005% or more. However, 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%, and the area fraction of the ferrite iron becomes excessive, and sufficient tensile strength cannot be 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 preferably in accordance with Nb: 0.005% to 0.30%, Ti: 0.005% to 0.30%, or V: 0.005% to 0.50%, or any combination thereof.

(B: 0.0000% to 0.010%) B is segregated on the grain boundary of the Worthfield iron during annealing after cold rolling, and suppresses the deformation of the ferrite. Therefore, B can also be included in it. In order to obtain sufficient effect, it is preferred that the B content be set to 0.0001% or more. However, when the B content exceeds 0.010%, the area fraction of the ferrite iron is set too low, and sufficient elongation cannot be obtained. Therefore, the B content is made 0.010% or less. That is, it is preferably composed of B: 0.0001% to 0.010%.

(Ca: 0.0000% to 0.04%, Mg: 0.0000% to 0.04%, REM: 0.0000% to 0.04%) Ca, Mg, and REM inhibit the form of oxides and sulfides, and contribute to the increase in hole expandability. Therefore, it is also possible to contain Ca, Mg or REM or any combination thereof. In order to obtain sufficient effect, it is preferred that any of the Ca content, the Mg content, and the REM content is 0.0005% or more. However, if the Ca content exceeds 0.04%, or the Mg content exceeds 0.04%, or the REM content exceeds 0.04%, a coarse oxide is formed and sufficient hole expandability cannot be obtained. Therefore, any of the Ca content, the Mg content, and the REM content is set to 0.04% or less, preferably 0.01% or less. That is, it is preferably in accordance with Ca: 0.0005% to 0.04%, Mg: 0.0005% to 0.04%, or REM: 0.0005% to 0.04%, or any combination thereof.

REM is a generic term for a total of 17 elements belonging to the elements of Sc, Y and lanthanide, and the REM content is the total content of these elements. The REM is, for example, contained in a mixed rare earth metal (mischmetal), and the addition of the REM may be, for example, addition of a mixed rare earth metal, or addition of a metal RE such as a metal La or a metal Ce.

According to the present embodiment, the following bending properties can be obtained: for example, a tensile strength of 1180 MPa or more, an elongation of 10% or more, a hole expandability of 20% or more, and a VDA angle when the thickness is t (mm) is "7.69". Flexibility above t ² -38.4t+109". That is, it can obtain high strength, excellent stretchability, hole expandability, and flexibility. This steel plate is easily formed into a skeleton part such as an automobile, and can secure safety in the event 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 the embodiment of the present invention, the steel preform having the chemical composition described above is sequentially subjected to hot rolling, pickling, cold rolling, and annealing.

There is no particular limitation on the manufacturing method previously performed on the casting. That is, after being melted by a blast furnace or an electric furnace or the like, various secondary refinings can be carried out. Also, metal scrap can be used as a raw material.

The cast steel obtained is temporarily cooled to a low temperature, and then may be heated again to be supplied to hot rolling, or the cast steel may be continuously supplied to hot rolling.

Hot rolling starts at a temperature above 1100 ° C and ends at a temperature above Ar ₃ . In cold rolling, the rolling reduction ratio is set to 30% or more and 80% or less. During annealing, maintained at above Ac Ac _{1:00 3:00} maximum heating temperature of less than 10 seconds, in the subsequent cooling, since the Ar ₃ point of the cooling rate is provided until at least 650 ℃ 0.5 ℃ / 50 sec The cooling rate from 650 ° C to 450 ° C is set to be 0.5 ° C / sec or more and 5 ° C / sec or less.

When the temperature at which the hot rolling is started is less than 1,100 ° C, the elements other than Fe may not be sufficiently solid-solved in Fe, and coarse alloy carbides may remain therein, which may cause embrittlement during hot rolling. Therefore, hot rolling starts at a temperature of 1100 ° C or higher. The hot rolling temperature is started, for example, at a steel embryo heating temperature. As the steel slab, for example, a steel slab obtained by continuous casting and a steel slab made of a thin steel slab can be used. The steel blank may also be supplied to the hot rolling equipment after being cast at a temperature of 1100 ° C or higher, or may be cooled to less than 1100 ° C and then heated to be supplied to the hot rolling equipment.

When the temperature at the end of hot rolling is lower than the Ar ₃ point, the Worthite iron and the ferrite iron are contained in the metal structure of the hot rolled steel sheet. Due to the difference in mechanical properties between the Worthite iron and the ferrite iron, It will become difficult to perform hot rolling after cold rolling. Therefore, hot rolling is terminated at a temperature above Ar ₃ point. When the hot rolling is terminated at a temperature above Ar ₃ , the rolling load in hot rolling can be relatively reduced.

The hot rolling includes rough rolling and final rolling, and finally, rolling may be performed by joining a plurality of steel sheets obtained by rough rolling and continuously rolling. Further, it is also possible to perform the final rolling after the rough rolling of the rough rolling plate is temporarily taken up. The coiling temperature is set to be 500 ° C or more and 650 ° C or less. If the coiling temperature exceeds 650 ° C, the productivity is deteriorated. Therefore, the coiling temperature is set to be 650 ° C or less. On the other hand, if the coiling temperature is lower than 500 ° C, the hardness of the hot-rolled steel sheet is too high, and thereafter it is difficult to perform cold rolling. Therefore, the coiling temperature is set at 500 ° C or higher.

The hot-rolled steel sheet obtained in this manner is pickled in order to remove surface oxides. Pickling is carried out once or twice. The oxide on the surface of the hot-rolled steel sheet is removed by pickling to increase the processability and plating properties.

In the case where the cold rolling reduction ratio is less than 30%, it is difficult to keep the shape of the cold rolled steel sheet flat, and sufficient ductility cannot be obtained. Therefore, the rolling reduction ratio of cold rolling is set to 30% or more. On the other hand, if the rolling reduction ratio of cold rolling exceeds 80%, the rolling load becomes too large, and cold rolling becomes difficult. Therefore, the rolling reduction ratio of cold rolling is set to 80% or more.

The heating rate of the cold-rolled steel sheet when passing through the continuous annealing line or the plating line is not particularly limited.

Annealed, by the maximum heating temperature for ₃ point or less of the Ac Ac _1:00 for 10 seconds, to generate austenite. Worthite iron, after cooling, metamorphosed into fermented iron, granular tough iron or 麻田散铁. If the temperature is kept below the Ac ₁ point, or the holding time is less than 10 seconds, the Worthite iron cannot be sufficiently formed. On the other hand, if the maximum heating temperature exceeds Ac ₃ point, the ferrite iron cannot be obtained, and the ductility is insufficient. Therefore, the maximum heating temperature is below Ac ₃ point above Ac ₁ point, and the holding time is 10 seconds or more.

In the cooling from the highest heating temperature, the average cooling rate (first average cooling rate) in the temperature range from Ar _{3 to} 650 ° C is set to 0.5 ° C / sec to 50 ° C / sec. If the average cooling rate is less than 0.5 ° C / sec, the ferrite iron or the Borne iron is excessively generated by the Worthite iron during the cooling process. As a result, it has become difficult to ensure sufficient area fraction of the granulated iron in the field, and it is difficult to obtain a tensile strength of 1180 MPa or more. Even if the average cooling rate is increased, there is no material problem, and the average cooling rate is excessively increased, resulting in an increase in manufacturing cost, so the average cooling rate is set to 50 ° C / sec or less. In the cooling method, it may be either roll cooling, air cooling or water cooling, or any of these.

The average cooling rate (second average cooling rate) from 650 ° C to 450 ° C is set at 0.5 ° C / sec to 5 ° C / sec, and a granular toughened iron having an appropriate area fraction can be produced. As described above, the granular, toughened iron is a structure in which a plurality of toughened ferrite irons are returned at their interfaces to form a piece of tissue. Such a differential recovery can occur in a temperature range below 650 °C. However, when the cooling rate in this temperature range exceeds 5 ° C / sec, the difference row cannot be sufficiently recovered, and the area fraction of the granular toughened iron is insufficient. Therefore, it is preferable to set the average cooling rate in this temperature range to 5 ° C / sec or less. On the one hand, if the cooling rate in this temperature range is less than 0.5 ° C / sec, the area fraction of the granular toughened iron and the toughened iron becomes too large, and it is necessary to obtain a tensile strength of 1180 MPa or more. It has become difficult. Therefore, it is preferable to set the average cooling rate in this temperature range to 0.5 ° C / sec or more. The cooling method may also be continuous cooling, slant cooling or isothermal holding or the like.

In this way, 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 an electric plating treatment or a vapor deposition treatment, and more preferably, it may be alloyed after the plating treatment. The steel sheet may be subjected to surface treatment such as organic film formation, film lamination, organic salt/inorganic salt treatment, or chromium-free treatment.

In the case of performing a hot-dip galvanizing treatment as a plating treatment on a steel sheet, for example, heating or cooling the temperature of the steel sheet to a temperature at which the steel sheet is passed through a zinc plating bath, the temperature is a ratio of zinc plating The temperature of the bath is lower than the temperature above 40 ° C and is higher than the temperature of the zinc plating bath by a temperature of 50 ° C or less. 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 remainder: 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. If the temperature is lower than 460 ° C, the alloying is insufficient. If the temperature exceeds 600 ° C, the alloying is excessive and the corrosion resistance is deteriorated. By alloying, 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.

It is to be understood that the above-described embodiments are merely illustrative of the specific embodiments of the invention, and are not intended to limit the technical scope of the invention. That is, the present invention can be implemented in various forms as long as it does not deviate from the technical idea or its main features.

[Embodiment] Next, an embodiment of the present invention will be described. The conditions of the examples are a conditional example used to confirm the practicability and effects of the present invention, and the present invention is not limited by such a condition. The present invention can be formed by various conditions as long as the object of the present invention can be achieved without departing from the gist of the present invention.

(First Test) In the first test, steel slabs having chemical compositions shown in Tables 1 to 2 were produced, and the steel slabs were hot rolled to obtain hot rolled steel sheets. The blank fields in Tables 1 to 2 indicate that the content of the element is below the detection limit, and the remainder is Fe and impurities. The bottom line in Table 2 indicates that the value is outside the scope of the present invention.

[Table 1]

[Table 2]

Thereafter, pickling, cold rolling, and annealing of the hot rolled steel sheet are performed to obtain a steel sheet. The conditions of hot rolling, cold rolling and annealing are shown in Tables 3 to 7. The bottom line in Tables 3 to 7 indicates that the value falls outside the scope of the present invention.

[table 3]

[Table 4]

[table 5]

[Table 6]

[Table 7]

Thereafter, the area fraction fF of the ferrite iron in each steel sheet, the area fraction fM of the granulated iron, the area fraction fGB of the granular toughened iron, the area fraction fB of the toughened iron, and the area of the ferritic iron were measured. The fractional rate fP and the area fraction of residual Worthite iron are fR-γ. These results are shown in Tables 8 to 12. The bottom line in Tables 8 to 12 associated with these results indicates that the value is outside the scope of the present invention.

Therefore, the tensile test, the hole expansion test, and the bending test of each steel plate were performed. In the tensile test, a Japanese Industrial Standard JIS No. 5 test piece was taken from the steel sheet at a right angle to the rolling direction, and the tensile strength TS and the total elongation EL were measured in accordance with JIS Z2242. In the hole expansion test, the hole expansion ratio λ was measured in accordance with the description of JIS Z2256. In the bending test, the test was carried out in accordance with the specifications of 238-100 of the Verbin der Automobilindustrie (VDA), and the VDA angle α was measured. These results are shown in Tables 8 to 12. The bottom line in Tables 8 to 12 associated with these results indicates that the value is outside the desired range. The range expected here is that TS is 1180 MPa or more, EL is 10% or more, λ is 20% or more, and VDA bending angle α is a reference value α ₀ or more (when thickness is t (mm), α ₀ = 7.69 t ² -38.4t+109).

[Table 8]

[Table 9]

[Table 10]

[Table 11]

[Table 12]

As shown in Tables 8 to 10, the samples within the scope of the present invention can obtain high strength, excellent elongation, and hole expandability.

Sample No. 71 has a low tensile strength because the C content is too low. In sample No. 72, since the C content was too high, the hole expansion ratio was low. In sample No. 73, since Si was too low in content, the tensile strength was low. In sample No. 74, since the Si content was too high, the tensile strength was low. In sample No. 75, since the Mn content was too low, the tensile strength and the hole expansion ratio were low. In sample No. 76, since the Mn content was too high, the hole expansion ratio and the VDA angle were low. In sample No. 77, since the P content was too high, tensile strength, elongation, hole expansion ratio, and VDA angle were low. In sample No. 78, since the S content was too high, the tensile strength and the hole expansion ratio were low. In sample No. 79, since the Al content was too low, tensile strength, elongation, and hole expansion ratio were low. In sample No. 80, since the Al content was too high, tensile strength, elongation, hole expansion ratio, and VDA angle were low. In sample No. 81, since the N content was too high, the hole expansion ratio was low. In sample No. 82, since the O content was too high, the hole expansion ratio and the VDA angle were low.

In sample No. 83, since the Cr content was too high, the hole expansion ratio was low. In sample No. 84, since the Mo content was too high, the hole expansion ratio was low. In sample No. 85, since the Ni content was too high, the hole expansion ratio was low. In sample No. 86, since the Cu content was too high, the tensile strength and the hole expansion ratio were low. In sample No. 87, since the Nb content was too high, the tensile strength and the hole expansion ratio were low. In sample No. 88, since the Ti content was too high, tensile strength, elongation, hole expansion ratio, and VDA angle were low. In sample No. 89, since the V content was too high, the hole expansion ratio and the VDA angle were low. In sample No. 90, since the B content was too high, tensile strength, elongation, hole expansion ratio, and VDA angle were low. In sample No. 91, since the Ca content was too high, the elongation, the hole expansion ratio, and the VDA angle were low. In sample No. 92, since the Mg content was too high, the tensile strength, the hole expansion ratio, and the びVDA angle were low. In sample No. 93, since the REM content was too high, the elongation, the hole expansion ratio, and the VDA angle were low.

In sample No. 94, the steel embryo heating temperature was too low, and undesired cracking occurred during hot rolling, and the through-plate could not be performed thereafter. In sample No. 95, since the finishing temperature of the finishing rolling was too low and the shape was deteriorated during the hot rolling, the through-plate could not be performed thereafter. In sample No. 96, since the coiling temperature was excessive, the hot-rolled steel sheet became too hard, and thereafter cold rolling could not be performed. In sample No. 97, since the coiling temperature was too high and the area fraction of the granulated iron was insufficient, the elongation, the hole expansion ratio, and the VDA angle were low. In sample No. 98, since the cold rolling reduction ratio was too low, the shape was deteriorated during cold rolling, and thereafter cold rolling could not be performed. In sample No. 99, since the cold rolling reduction ratio was too high, the rolling load was excessively increased, and thereafter cold rolling could not be performed. In sample No. 100, since the maximum heating temperature of the annealing was too high, the area fraction of the ferrite iron was insufficient, and the area fraction of the toughened iron was too large, so the elongation was low. In sample No. 101, since the maximum heating temperature of annealing is too low, the area fraction of ferrite iron and ferritic iron is too much, and the area fraction of granulated iron and granular toughened iron is insufficient, so tensile strength and expansion The porosity is low. In sample No. 102, since the holding time of the highest heating temperature is too short, the area fraction of the ferrite iron and the ferrite is too high, and the area fraction of the granulated iron and the granular toughened iron is insufficient, so the tensile strength and expansion The porosity and VDA angle are low. In sample No. 103, since the holding time of the highest heating temperature was too short, the area fraction of the ferrite was excessive, the area fraction of the granular toughened iron was insufficient, and the tensile strength was low. In sample No. 104, the first average cooling rate was too low, the area fraction of the ferrite iron was too high, and the area fraction of the granulated iron was insufficient, so the tensile strength was low. In sample No. 105, since the first average cooling rate is too high, the area fraction of the ferrite iron is insufficient, and the area fraction of the granular toughened iron and the ferrite is too high, the tensile strength and the VDA angle are low. In sample No. 106, since the second average cooling rate was excessive, the area fraction of the granulated iron was insufficient, so the tensile strength, the hole expansion ratio, and the VDA angle were low. In sample No. 107, since the second average cooling rate is too low, the area fraction of the granulated iron and the granular toughened iron is insufficient, and the area fraction of the toughened iron is too high, the tensile strength and the VDA angle are low. In sample No. 108, since the second average cooling rate is too high, the area fraction of the granulated iron and the granular toughened iron is insufficient, and the area fraction of the toughened iron is too high, so the tensile strength and the reaming are performed. The rate and VDA corner are low.

Industrial Applicability The present invention can be utilized, for example, in an industry associated with steel sheets suitable for automobile parts.

Claims (5)

A steel sheet characterized by having the following chemical composition: C: 0.09% to 0.15%, Si: 0.2% to 2.5%, Al: 0.01% to 1.00%, Mn: 1.0% to 3.0% , P: 0.02% or less, S: 0.01% or less, N: 0.007% or less, O: 0.006% or less, Cr: 0.00% to 1.00%, Mo: 0.00% to 1.00%, B: 0.0000% to 0.010%, Nb : 0.000% to 0.30%, Ti: 0.000% to 0.30%, Ni: 0.00% to 1.00%, Cu: 0.00% to 1.00%, V: 0.000% to 0.50%, Mg: 0.0000% to 0.04%, REM: 0.0000 %~0.04%, and the remainder: Fe and impurities; The steel plate has the following metal structure in terms of area fraction: ferrite iron: 30%~50%, granular toughening iron: 5%~20%, Ma Tian Dispersion iron: 30%~55%, toughened iron: less than 35%, and residual Worthite iron and Borne iron: total less than 10%. The steel sheet according to claim 1 has a tensile strength of 1180 MPa or more, a tensile property of 10% or more, and a pore expansion value of 20% or more. For the steel sheet of claim 1, the VDA angle when the thickness is t (mm) is "7.69t ^{2 -} 38.4t + 109" or more. A steel sheet according to any one of claims 1 to 3, which has a hot-dip galvanized layer on the surface. A steel sheet according to any one of claims 1 to 3, which has an alloyed hot-dip galvanized layer on the surface.