TWI618799B - Steel sheet - Google Patents

Abstract

The steel sheet of one aspect of the present invention contains a predetermined chemical composition, and the metal structure of the 1/4t portion contains 4 to 70% by volume of residual Worthite iron; in the 1/4t portion, [Mn] _γ /[Mn] _ave >1.5, and f _γs /f _γ ≦0.30 in the 1/4t portion, and [C] × [Mn] ≧ 0.15.

Description

Steel plate

FIELD OF THE INVENTION The present invention relates to a steel sheet having excellent formability, i.e., excellent uniform elongation and excellent weldability, and high strength.

Background of the Invention In order to achieve both weight reduction and safety of automobile bodies and parts, steel sheets as such raw materials are continuing to be intensively developed. In general, when the steel sheet is increased in strength, the uniform elongation and the hole expandability are lowered, and the formability of the steel sheet is impaired. Therefore, in order to use a high-strength steel sheet as a member for an automobile, it is necessary to increase both the strength and the formability of the opposite characteristics.

In order to increase the uniform elongation, a steel sheet using a metamorphism-induced plasticity of residual Worthite (residual γ), a so-called TRIP steel sheet (for example, refer to Patent Documents 1 and 2) has been proposed.

The residual Worthfield iron is produced by increasing the concentrations of C and Mn in the Worthite iron, so that the Worthite iron does not metamorphose into other structures even at room temperature. As a technique for stabilizing the Worthite iron, a method has been proposed in which the steel sheet contains Si and Al and the like to suppress carbide precipitation elements, and during the production stage of the steel sheet, the steel sheet is strengthened during the toughening iron deformation state. The concentration of C in the Vostian iron.

Under this technology, the more C content contained in the steel sheet, the more stable the Worstian iron can increase the amount of residual Worth iron, and as a result, a steel plate excellent in both strength and uniform elongation can be produced. . However, when a steel sheet is used for a structural member, the steel sheet is often welded. However, since the C content in the steel sheet is large, the weldability is deteriorated, so that it is limited as a structural member. Therefore, everyone expects to increase both uniform elongation and strength with less C content.

Further, in order to improve the hole expandability of the TRIP steel sheet, a steel having a toughened iron as a main structure has been proposed (for example, Patent Document 3). However, when the ductile iron having low ductility is used as the main structure, the ductility of the steel sheet is lowered, and it is difficult to manufacture a member for automobiles having a complicated shape.

On the other hand, a steel having a Mn addition of more than 3.0% is proposed as a steel sheet having a residual Worstian iron content more than the above-mentioned TRIP steel and having a ductility superior to that of the above TRIP steel (for example, Non-Patent Document 1). However, in this steel grade, there are few cases applicable to components for automobiles, and there is no sufficient discussion on the improvement of the hole expandability. Steel sheets containing more than 3.0% of Mn and having improved hole expandability and the like have not yet been developed.

Patent Document 4 discloses a high-strength steel sheet which is subjected to annealing before cold rolling to improve mechanical properties. However, the steel sheet described in Patent Document 4 is a DP steel excellent in r value, and is not a steel sheet excellent in strength and ductility. In particular, the technique disclosed in Patent Document 4 is to precipitate ferritic carbon iron in the ferrite-iron and ferrite of the steel sheet during the coiling treatment after the hot rolling, and to carry out the cold rolling. The snowy carbon iron is coarsened and spheroidized before. A steel sheet having such a structure is not a steel sheet which can achieve tensile strength, ductility, and hole expandability which are required for an automobile steel sheet. Prior Technical Literature Patent Literature

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

Non-Patent Document Non-Patent Document 1: Furukawa K., Matsumura, Heat Treatment, Japan, Japan Heat Treatment Association, Heisei 9, Vol. 37, No. 4, p. 204

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

In order to improve the weldability, the C content must be lowered. In order to ensure excellent uniform elongation, hole expandability, and high strength with a low C content, the present inventors have found that a residual Worthite iron of 7 area% or more is dispersed in a steel sheet and the form of the residual Worthite iron is controlled. This is effective. The object of the invention is as follows.

(1) The steel sheet according to one aspect of the present invention contains, in terms of unit mass%, C: 0.03 to 0.40%, Si: 0.01 to 5.00%, Mn: 0.50 to 12.00%, Al: 0.001 to 5.000%, and P: 0.150%. Hereinafter, S: 0.0300% or less, N: 0.0100% or less, O: 0.0100% or less, Cr: 0 to 5.00%, Mo: 0 to 5.00%, Ni: 0 to 5.00%, Cu: 0 to 5.00%, and Nb: 0~0.500%, Ti: 0~0.500%, V: 0~0.500%, W: 0~0.500%, B: 0~0.0030%, Ca: 0~0.0500%, Mg: 0~0.0500%, Zr: 0 ~0.0500%, REM: 0~0.0500%, Sb: 0~0.0500%, Sn: 0~0.0500%, As: 0~0.0500%, and Te: 0~0.0500%, and the remainder is composed of iron and impurities; The metal structure of the 1/4t portion contains 4 to 70% by volume of the residual Worthite iron; in the 1/4t portion, the average Mn concentration [Mn] _ave per unit mass% with respect to the entire 1/4t portion, The average Mn concentration [Mn] _{γ in the} residual Worthite iron in terms of unit mass% satisfies Formula 1; in the 1/4t portion, the volume ratio f _{γs of the} residual Worstian iron having an aspect ratio of 2.0 or less and all the residual austenite of the volume fraction _f γ-based satisfies formula 2; in units of mass%, a C content [C] and the Mn content [Mn] is satisfied based formula 3 [Mn] _γ /[Mn] _ave >1.5 (Formula 1) f _γs /f _γ ≦0.30 (Formula 2) [C] × [Mn] ≧ 0.15 (Expression 3) (2) The above (1) The steel sheet may also contain Mn: 3.50 to 12.00% per unit mass%. (3) The steel sheet of the above (1) or (2) may be one or more selected from the group consisting of: Cr: 0.01% to 5.00%, and Mo: 0.01%~5.00%, Ni: 0.01%~5.00%, Cu: 0.01%~5.00%, Nb: 0.005%~0.500%, Ti: 0.005%~0.500%, V: 0.005%~0.500%, W: 0.005% ~0.500%, B: 0.0001%~0.0030%, Ca: 0.0001%~0.0500%, Mg: 0.0001%~0.0500%, Zr: 0.0005%~0.0500%, REM: 0.0005%~0.0500%, Sb: 0.0050%~0.0500 %, Sn: 0.0050% to 0.0500%, As: 0.0050% to 0.0500%, and Te: 0.0050% to 0.0500%. (4) The steel sheet according to any one of the above (1) to (3), which may have a hot-dip galvanized layer on the surface of the steel sheet. (5) The steel sheet according to any one of the above (1) to (3), which may have an alloyed hot-dip galvanized layer on the surface of the steel sheet. Effect of the invention

The steel sheet according to the above aspect of the present invention has excellent weldability because the C content is 0.40% by mass or less. Further, in the steel sheet of the present invention, the product "TS × uEL" of the tensile strength and the uniform elongation is high. This point means that both the tensile strength and the uniform elongation of the opposite characteristics are excellent. Further, in the steel sheet of the present invention, the product "TS × λ" of tensile strength and hole expandability is high. This point means that both the tensile strength and the hole expandability of the opposite characteristics are excellent.

According to the present invention, it is possible to provide a high-strength steel sheet having a tensile strength, a uniform elongation, and a hole expandability superior to those of the prior art at a C content which is less than that of the prior art. When this steel sheet is used, it is possible to improve both the weight reduction and the safety of the automobile, and also to ensure the ease of application of the welding, and thus the industrial contribution is extremely remarkable.

In order to carry out the invention, the inventors of the present invention have repeatedly studied the results of the following: The average Mn concentration [Mn] per unit mass% of the 1/4t portion of the steel sheet in the 1/4t portion of the steel sheet. _Ave , the average Mn concentration in the residual Worthite iron in terms of unit mass% [Mn] _γ satisfies the following (Formula 1), and the volume ratio f _γs of the residual _Worthite iron with an aspect ratio of 2.0 or less and the total residual Worthfield When the volume fraction f _{γ of} iron satisfies the following (Formula 2), a high-strength steel sheet having excellent uniform elongation and hole expandability can be produced at a low C content. Further, the 1/4t portion of the steel sheet is a region from the rolling surface of the steel sheet to a depth of about 1/4 of the thickness t of the steel sheet. The 1/4t portion of the steel sheet may also be defined as a region between the surface of the depth of 1/8t and the surface of the depth of 3/8t from the rolling surface of the steel sheet. [Mn] _γ /[Mn] _ave >1.5...(Formula 1) f _γs /f _γ ≦0.30...(Formula 2)

Hereinafter, only "average Mn concentration in terms of unit mass%" is referred to as "average Mn concentration", and [Mn] _γ /[Mn] _{ave is} referred to as "Mn richness degree". Further, the metal structure of the steel sheet according to the present embodiment described below is a metal structure in a quarter of the steel sheet (that is, a region from a rolling surface of the steel sheet to a depth of about 1/4 of the thickness t of the steel sheet). Characteristics. The 1/4t portion of the steel sheet existing between the rolling surface of the steel sheet and the center surface of the steel sheet in the thickness direction is considered to have an average metal structure. Therefore, in the technical field of the steel sheet, the 1/4t portion of the steel sheet is usually controlled by the metal structure. When the metal structure of the 1/4t portion of the steel sheet according to the present embodiment is controlled as described below, the metal structure of the region other than the 1/4t portion of the steel sheet is also ideally controlled, so that the steel sheet of the present embodiment is pulled. The tensile strength, uniform elongation and hole expandability will be higher than the target value. Therefore, the configuration of the metal structure in the region other than the 1/4t portion of the steel sheet is not particularly limited as long as the metal structure of the 1/4t portion of the steel sheet is within the predetermined range described in the present embodiment. In the present embodiment, the description of the structure of the metal structure of the steel sheet is a metal structure of the 1/4 t portion of the steel sheet unless otherwise specified.

Hereinafter, the reason why the Mn rich degree and the aspect ratio of the Worstian iron are retained as described above will be described.

([Mn] _γ /[Mn] _ave >1.5) In the TRIP steel, during the annealing process, the C concentration in the Worthite iron is increased by causing the fermented iron metamorphosis and/or the toughening iron metamorphism to occur. Vostian Iron will also be stable at room temperature. As a simple method for obtaining a steel sheet having a higher strength and a high uniform elongation, a means for increasing the amount of residual Worstian iron by increasing the C content is known. However, when the C content is increased, the weldability of the steel sheet is deteriorated, so this method cannot be used for a steel sheet used for a structural member or the like to be welded.

Then, the inventors of the present invention have studied a method for producing a steel sheet which can increase the uniform elongation without increasing the C content. As a result, it has been found that, by appropriately controlling the hot rolling conditions, the hot rolling delay, and the heat treatment conditions before the cold rolling, the Mn concentration in the residual Worthite iron is increased to satisfy the formula (1), and the strength and the C content can be obtained. Increase the uniform elongation while remaining unchanged.

The reason for the above-mentioned phenomenon is unknown, but the present inventors believe that by increasing the concentration of the Mn in the Vostian iron stabilized element in the Vostian iron, a larger amount of residual Worth iron can be secured, and the uniform elongation increases. .

When the Mn concentration is 1.5 or less, the uniform elongation is impaired, and the desired mechanical properties cannot be obtained. Further, it is estimated that the steel sheet obtained by the general production conditions has a Mn concentration of about 1.2 to 1.4. A preferred lower limit for the degree of Mn concentration is 1.52, 1.54 or 1.56.

The Mn concentration ratio in the 1/4t portion is obtained by grinding perpendicular to the rolling direction of the steel sheet and the cross section of the rolling surface, and in the 1/4t portion of the cross section, FE-SEM or the like can be used. A device for local analysis was performed to determine [Mn] _γ and [Mn] _ave .

(f _γs /f _γ ≦0.30) The present inventors have found that the volume ratio f _γs of the residual _Worthite iron having an aspect ratio of 2.0 or less and the volume ratio f _{γ of the} total residual Worth iron satisfy the above formula. 2), the hole expansion rate will increase. When the steel sheet satisfying the formula 2 having the same strength is compared with the steel sheet not satisfying the formula 2, the steel sheet satisfying the formula 2 has a high hole expansion ratio.

When the volume ratio f _γs of the residual Worthite iron having an aspect ratio of 2.0 or less is lowered, the reason why the hole expansion ratio is increased is not clear. The inventors of the present invention presumed that the Worstian iron having an aspect ratio of 2.0 or less is present at the boundary of the old Worthite iron and the boundary of the inclusion body, and the like, and most of them have a particle diameter of more than 1.0 μm. In the hole expanding process, the stress tends to concentrate on the residual Worthite iron having an aspect ratio of 2.0 or less and is easily broken. Therefore, it is necessary to set f _γs /f _γ to 0.30 or less. f _γs /f _{γ is} preferably 0.25 or less, more preferably 0.20 or less.

The method of calculating f _γs /f _γ in the 1/4t portion is as follows. First, the residual Worstian iron fraction f _γs having an aspect ratio of 2.0 or less in the _1/4 t portion is obtained by performing EBSP analysis on the 1/4 t portion. In this EBSP analysis, the fcc phase is separated from the bcc phase, and the portion in which the fcc phase is in a block shape is regarded as a residual Worstian iron crystal grain, and the aspect ratio is the ratio of the longest width to the shortest width of the crystal grain. . The total residual Vostian iron amount f _γ in the 1/4 t portion was obtained by measuring X-ray diffraction at 1/4 t.

Next, the components of the steel sheet of the present embodiment will be described. In addition, in the following description, the % of content means mass %.

(C: 0.03 to 0.40%) C is an element which is important for increasing the strength of steel and ensuring the retention of Worstian iron. In order to obtain a sufficient amount of residual Worth iron, it is necessary to have a C content of 0.03% or more. On the other hand, when the excessive C content is contained, the weldability of the steel sheet is impaired, so the upper limit of the C content is made 0.40% or less. A preferred lower limit of the C content is 0.04%, 0.09% or 0.14%. A preferred upper limit for the C content is 0.36%, 0.32% or 0.25%.

(Mn: 0.50 to 12.00%) Mn is an element which stabilizes the Worthite iron and improves the hardenability. Further, in the steel sheet according to the present embodiment, Mn is distributed in the Vostian iron to further stabilize the Worthite iron. In order to stabilize the Worthite iron at room temperature, it is necessary to have Mn of 0.50% or more. On the other hand, when an excessive amount of Mn is contained, the ductility is impaired, so the upper limit of the Mn content is set to 12.00%. A preferred lower limit of the Mn content is 1.50%, 2.30%, 3.00% or 3.50%. A preferred upper limit of the Mn content is 10.00%, 8.00% or 6.00%.

(Si: 0.01 to 5.00%) (Al: 0.001 to 5.000%) Si is an oxygen scavenger and must be contained in an amount of 0.01% or more. Al is also an oxygen scavenger and must contain 0.001% or more. In addition, Si and Al are elements which stabilize the ferrite iron during annealing of the steel sheet, and suppress the precipitation of ferritic carbon iron when the ductile iron is metamorphosed, thereby increasing the C concentration of the Worth iron and helping to ensure the residual Wo The elements of the stone. In addition, Si and Al increase the hole expandability of the steel sheet. This reason is unclear, but it is presumed that due to the tempering of the mother phase, the granulated iron or the ferrite iron hardens, so during processing, between the granulated iron from the Worthite iron and the tempered granulated iron or The difference in hardness between the Ma Tian loose iron and the ferrite iron from the Worthfield iron is small. Although the effect is increased as the Si and Al contents are increased, when excessive Si and Al are contained, surface properties, coating properties, and weldability are deteriorated, so that the upper limit of Si is 5.00% or less. The upper limit of Al is 5.000% or less. Further, when Al is contained in an amount of more than 5.000%, the δ ferrite iron remains at room temperature. The ferrite iron becomes a stretched ferrite iron during the hot rolling delay, and the stress concentrates on the ferrite iron during the tensile test or press forming, so that the steel sheet becomes easily broken. A preferred lower limit of the Si content is 0.40%, 0.90% or 1.20%. A preferred upper limit of the Si content is 4.00%, 3.50% or 3.00%.

(P: 0.150% or less) P is an impurity, and when it is contained in excess, it may impair ductility or weldability. Therefore, the upper limit of the P content is made 0.150% or less. A preferred upper limit of the P content is 0.060%, 0.030% or 0.025%. Since the steel sheet according to the present embodiment is not necessarily P, the lower limit of the P content is 0%. The lower limit of the P content may be more than 0% or may be 0.001%, but it is preferable to reduce the P content as much as possible.

(S: 0.0300% or less) S is an impurity. When an excessive amount is contained, MnS which is extended by hot rolling is formed, and the formability such as ductility and hole expandability is deteriorated. Therefore, the upper limit of the S content is made 0.0300% or less. A preferred upper limit of the S content is 0.0100%, 0.0070% or 0.0040%. Since the steel sheet according to the present embodiment is not required to be S, the lower limit of the S content is 0%. The lower limit value of the S content may be more than 0% or may be 0.0001%, but it is preferable to reduce the S content as much as possible.

(N: 0.0100% or less) N is an impurity, and when it exceeds 0.0100%, local ductility is deteriorated. Therefore, the upper limit of the N content is made 0.0100% or less. A preferred upper limit for the N content is 0.0080%, 0.0060% or 0.0050%. Since the steel sheet according to the present embodiment is not necessarily N, the lower limit of the N content is 0%. The lower limit of the N content may be more than 0% or may be 0.0003%, but it is preferable to reduce the N content as much as possible.

(O: 0.0100% or less) O is an impurity, and when it exceeds 0.0100%, deterioration of ductility is caused. Therefore, the upper limit of the O content is made 0.0100% or less. A preferred upper limit of the O content is 0.0060%, 0.0040% or 0.0020%. Since the steel sheet according to the present embodiment is not necessarily O, the lower limit of the O content is 0%. The lower limit value of the O content may be more than 0% or may be 0.0001%, but it is preferable to reduce the O content as much as possible.

The steel sheet of the present embodiment may further contain one selected from the group consisting of Cr, Mo, Ni, Cu, Nb, Ti, V, W, and B. Or two or more. However, since the steel sheet of the present embodiment is not required to be Cr, Mo, Ni, Cu, Nb, Ti, V, W, and B, the contents of Cr, Mo, Ni, Cu, Nb, Ti, V, W, and B are required. The lower limit is 0%.

(Cr: 0 to 5.00%) (Mo: 0 to 5.00%) (Ni: 0 to 5.00%) (Cu: 0 to 5.00%) For the steel sheet of the present embodiment, Cr, Mo, Ni, and Cu are not essential elements. . However, since Cr, Mo, Ni, and Cu are elements for improving the strength of the steel sheet, they may be contained in the steel sheet of the present embodiment. In order to obtain this effect, the steel sheet may contain one or more elements selected from the group consisting of Cr, Mo, Ni, and Cu, each of which is 0.01% or more. However, when an excessive amount of these elements is contained, the steel sheet strength may become too high and the ductility of the steel sheet may be impaired. Therefore, the upper limit of each of one or two or more elements selected from the group consisting of Cr, Mo, Ni, and Cu is 5.00%.

(Nb: 0 to 0.500%) (Ti: 0 to 0.500%) (V: 0 to 0.500%) (W: 0 to 0.500%) Nb, Ti, V, and W are not essential elements for the steel sheet of the present embodiment. . However, since Nb, Ti, V, and W are elements which form fine carbides, nitrides, or carbonitrides, they are effective in securing the strength of the steel sheet. Therefore, the steel sheet may contain one or two or more elements selected from the group consisting of Nb, Ti, V, and W. In order to obtain this effect, the lower limit value of each of one or two or more elements selected from the group consisting of Nb, Ti, V, and W is preferably 0.005%. On the other hand, when an excessive amount of these elements is contained, the strength of the steel sheet may excessively rise and the ductility of the steel sheet may decrease. Therefore, the upper limit of each of the one or more elements selected from the group consisting of Nb, Ti, V, and W is 0.500%.

(B: 0 to 0.0030%) Since B can delay the start of the fermented iron metamorphic state and the toughened iron metamorphism, and the strength of the steel is increased, B can be contained in the steel sheet of the present embodiment as needed. In order to obtain this effect, the lower limit of the B content should be 0.0001%. On the other hand, an excessive amount of B excessively increases the hardenability of the steel sheet and excessively delays the start of the ferrite iron metamorphosis and the tough iron deformation state, so that the C concentration is prevented from increasing toward the residual Worthite iron phase. Therefore, the upper limit of the B content is made 0.0030%.

The steel sheet of the present embodiment may further contain one or two or more elements selected from the group consisting of Ca, Mg, Zr, and REM (rare earth element). However, since the steel sheet of the present embodiment is not required to contain Ca, Mg, Zr, and REM, the lower limit of the contents of Ca, Mg, Zr, and REM is 0%.

(Ca: 0 to 0.0500%) (Mg: 0 to 0.0500%) (Zr: 0 to 0.0500%) (REM: 0 to 0.0500%) Ca, Mg, Zr, and REM control the shape of sulfides and oxides, and Improve the local ductility and hole expansion of the steel plate. In order to obtain this effect, one or two of a group consisting of 0.0001% or more of Ca, 0.0001% or more of Mg, 0.0005% or more of Zr, and 0.0005% or more of REM may be contained in the steel sheet. the above. However, since an excessive amount of these elements deteriorates the workability of the steel sheet, the upper limit of each of the elements is made 0.0500%. In addition, the total content of one or two or more elements selected from the group consisting of Ca, Mg, Zr, and REM is preferably 0.050% or less.

The steel may further contain one or more selected from the group consisting of Sb, Sn, As, and Te. However, since the steel sheet of the present embodiment is not required to have Sb, Sn, As, and Te, the lower limit of the contents of Sb, Sn, As, and Te is 0%.

(Sb: 0 to 0.0500%) (Sn: 0 to 0.0500%) (As: 0 to 0.0500%) (Te: 0 to 0.0500%) Sb, Sn, As, and Te suppress Mn, Si, and/or in the steel sheet. An easily oxidizable element such as Al diffuses on the surface of the steel sheet to form an oxide, thereby improving the surface properties or plating properties of the steel sheet. In order to obtain this effect, the lower limit value of each of one or two or more elements selected from the group consisting of Sb, Sn, As, and Te may be 0.0050%. On the other hand, when the content of each of these elements exceeds 0.0500%, the effect is saturated, so that the upper limit of each of these elements is 0.0500%.

([C] × [Mn] ≧ 0.15) In the chemical composition of the steel sheet of the present embodiment, the C content [C] and the Mn content [Mn] in terms of unit mass% must satisfy the following formula 3. [C] × [Mn] ≧ 0.15 (Formula 3) Each of C and Mn has the effect of stabilizing the Worthite iron contained in the steel sheet during heat treatment included in the method for producing a steel sheet, and The amount of residual Worth iron in the steel sheet finally obtained is increased. When the product of the C content and the Mn content is 0.15 or more, the residual Worstian iron content is 4% or more, and the Mn concentration is increased in the residual Worth iron to satisfy the formula 1. A preferred lower limit value of [C] × [Mn] is 0.30 or 0.50. Although the upper limit of [C] × [Mn] is not particularly required, it may be 4.80 which is calculated from the upper limit of the C content and the Mn content.

Next, the metal structure of the steel sheet of the present embodiment will be described.

The metal structure of the steel sheet according to the present embodiment contains the remaining Worthite iron, and the other structures are one or two kinds of ferrite iron, residual Worthite iron, new matita loose iron, toughened iron, and tempered hematite loose iron. The above is constituted. The rate of each organization varies depending on the annealing conditions, and affects the strength, ductility, and hole expandability. Since the material is changed in accordance with, for example, automotive parts, it is only necessary to select annealing conditions and control the tissue fraction as needed.

(Amount of Worstian iron remaining in the metal structure of the 1/4t portion of the steel sheet: 4 to 70% by volume) In the steel sheet of the present embodiment, it is important to control the amount of the remaining Worth iron. Residual Worth Iron is to increase the ductility of the steel sheet by metamorphism-inducing plasticity, especially the uniform elongation of the steel sheet. In addition, the residual Worthfield iron will be transformed into a granulated iron by processing, which will help to increase the strength of the steel. In order to obtain such effects, the 1/4t portion of the steel sheet according to the present embodiment must contain 4% or more of the remaining Worthite iron in terms of volume ratio. The lower limit of the volume fraction of the residual Worthite iron is preferably 5%, 7%, 9% or 12%. Further, as described above, the metal structure of the region other than the 1/4t portion of the steel sheet is not limited as long as the metal structure of the 1/4t portion of the steel sheet is within the predetermined range described in the embodiment. The volume fraction of the remaining Worthite iron in the region other than the 1/4t portion of the steel sheet is not limited. In the present embodiment, the content of the content of the remaining Worthite iron or the like is a content indicating the residual Worthite iron or the like of the metal structure of the 1/4 t portion of the steel sheet, unless otherwise specified.

The higher the volume fraction of the residual Worthite iron, the better. However, it is difficult to contain more than 70% of residual Worthite iron in a steel sheet having the above chemical composition. In order to make the residual Worth iron exceed 70% by volume, it is necessary to contain more than 0.40% of C, and in this case, the weldability of the steel sheet may be impaired. Therefore, the upper limit of the volume ratio of the residual Worthite iron is 70% or less.

The remaining portion of the metal structure of the steel sheet according to the present embodiment is not particularly limited as long as the amount of the residual Worth iron is within the predetermined range, and may be appropriately selected in accordance with the characteristics sought. The structure which may be contained in the remainder of the metal structure may be fertilized iron, new granulated iron, tempered iron, and tempered granulated iron, but may also contain tissues and inclusions other than those. Although the upper and lower limits of the ferrite iron, the new Ma Tian loose iron, the toughened iron, and the tempered Ma Tian loose iron are not particularly limited, the preferred upper and lower limits of the above are also described below.

Fertilizer iron is a structure that enhances the ductility of a steel sheet. Although the ferrite iron content can also be 0 area%, in order to maintain both strength and ductility within the target strength level, the area ratio of the ferrite iron should be 10 to 75%. A lower limit of the ferrite iron content is 15 area%, 20 area% or 25 area%. The upper limit of the ferrite iron content is 50 area%, 65 area% or 70 area%.

The metal structure of the steel sheet according to the present embodiment may also contain a new matita loose iron (that is, a tempered loose iron in the field). The new mat is scattered into a hard tissue and is effective in ensuring the strength of the steel sheet. However, when the content of the new Ma Tian San is small, the ductility of the steel sheet becomes high. Therefore, the content of the new Ma Tian San can also be 0 area%. In order to ensure the ductility, the upper limit of the content of the new matte in the steel sheet of the present embodiment may be 25% in terms of the area ratio. A preferred lower limit of the fresh ground mass content is 0.5 area%, 1 area% or 2 area%. The preferred upper limit of the fresh ground mass content is 20 area%, 15 area% or 12 area%.

The metal structure of the steel sheet according to the present embodiment may also contain tempered granulated iron which is obtained by tempering the new granulated iron which is produced after the final annealing. The content of tempered granulated iron can also be 0 area%, but the tempered granulated iron is the organization that enhances the strength and reaming of the steel. Although the content of the tempered iron in the tempering field can be appropriately selected according to the target strength level, the ductility of the steel sheet is ideally ensured when the upper limit of the ferrous iron content in the tempering field is 25%. A preferred lower limit of the tempered granulated iron content is 3 area%, 5 area% or 7 area%. A better upper limit for the tempered granulated iron content is 22 area%, 20 area% or 18 area%.

The metal structure of the steel sheet according to the embodiment may also contain toughened iron. During the toughening iron metamorphosis, the C concentration will increase in the Worthfield iron and the Worthite iron will stabilize. In other words, if heat treatment is performed to contain the toughened iron in the steel sheet finally obtained, the remaining Worthite iron is stabilized. What's more, toughened iron is also a tissue that can improve both steel strength and hole expansion. The content of the toughening iron may be 0 area%, but in order to obtain these effects, it is preferable to contain the toughening iron of 5 area% or more. Since the steel sheet of the present embodiment contains 0.5% or more of Mn, it is difficult to make the toughened iron 50% by area or more. Therefore, the upper limit of the toughening iron content may be 50 area%. A more preferable lower limit of the toughening iron content is 7 area%, 8 area%, or 10 area%. A more preferable upper limit of the toughening iron content is 45 area%, 42 area% or 40 area%.

The metal structure of the steel sheet according to the embodiment may also contain Boron iron. In the cooling during annealing and the alloying treatment, the Borne iron may be metamorphosed by the Worthite iron. The upper limit of the content of the fine iron is preferably 10 area%. When the upper limit of the content of the Borne iron is 10 area%, the amount of residual Worthite iron can be prevented from becoming 4 area% or less, and strength and ductility can be ensured. A more preferable upper limit of the content of the Bored iron is 3 area%, 2 area% or 1 area%. In addition, the lower the content of the Borne iron, the better, so the lower limit of the iron content of the Borne is 0 area%.

Further, when the steel sheet contains one of the ferrite iron and the tempered granulated iron or both, the average crystal grain size of the ferrite iron and the tempered granulated iron is preferably 10 μm or less. At this time, since the structure becomes fine, the steel sheet is increased in strength. Moreover, since the structure is homogenized, the processing strain will be uniformly introduced into the steel sheet to increase the uniform elongation of the steel sheet. The average crystal grain size of the ferrite iron and the tempered granules is preferably 7 μm or less, more preferably 5 μm or less.

The identification method of the above tissue is shown below. Moreover, the identification methods described below are all performed on the 1/4t portion of the steel sheet. The observation of the ferrite iron was carried out by using an optical microscope using a cross section of a sample which was ground and etched with a nital etchant. Since the ferrite iron is white in a general optical microscope image, the area ratio of the white portion is measured and it is the ferrite iron fraction. The area ratio is measured by analyzing the tissue photograph as an image. The volume fraction of the residual Worth iron is obtained by X-ray diffraction. Further, the volume ratio obtained by this means can be considered to be almost the same as the area ratio. The observation of the new Ma Tian loose iron was carried out using an optical microscope using a cross section of the sample which was ground and etched with LePera liquid. The Borne iron is a cross-section of a sample which is ground and etched with a oxidizing agent by SEM, and the area ratio of the area constituting the layered structure is measured and made to be the area ratio of the ferritic iron.

The toughened iron and the tempered granulated iron can be discriminated by using the SEM and observing the cross-section of the appropriately prepared sample at a magnification of 5000 times. The following organizations can be considered as toughened iron: fermented iron, granulated iron, residual Worthite iron and other materials other than the Borne iron, and the snowy carbon iron precipitates in one direction inside; and The following organizations are considered to be tempered granulated iron: fermented iron, granulated iron, residual Worth iron and other materials other than Borne, and the stellite carbon iron precipitates in two directions inside. On the other hand, in order to omit the above-described discrimination operation, a method of identifying the toughened iron and the tempered granulated loose iron may be appropriately selected in accordance with the method of generating the tempered granulated iron. As an example of the method of generating the tempered granulated iron in the steel sheet according to the present embodiment, the steel sheet temperature may be a temperature of Ms to Mf before the austemper treatment of the Worth iron described later. Further, as another example of the method of generating the tempered granulated iron in the steel sheet of the present embodiment, for example, the steel sheet may be tempered after the annealing is completed. When such tempering treatment is not carried out, the tempered granulated iron does not exist.

In the case where the tempered granulated iron does not exist, the area ratio of the structure other than the ferrite iron, the granulated iron, the residual Worth iron and the Bora iron can be regarded as the area ratio of the toughened iron. When the tempering treatment is performed after the annealing to generate the tempered granulated iron, the amount of granulated iron in the steel plate before the tempering treatment can be measured, and the amount of the granulated iron is regarded as the oxidized treatment. The amount of scattered iron in the fire Ma Tian. In the case where the temperature of the steel sheet is set to a temperature of Ms to Mf before the austempering treatment of the Worthite iron to generate the tempered granulated iron, the amount of tempered granulated iron can be estimated based on the amount of change in the volume expansion of the steel sheet. . This is due to the increase in volume when the Worth iron is metamorphosed into granulated iron or toughened iron. The volume increase of the steel sheet within 0.1 second after the temperature of the steel sheet is at the temperature of Ms to Mf may be regarded as the amount of tempered granulated iron, and the volume increase after that may be regarded as the amount of toughened iron.

Next, the mechanical properties of the steel sheet of the present embodiment will be described. The tensile strength of the steel sheet of the present embodiment is preferably 440 MPa or more. This is to reduce the thickness of the steel sheet by using a steel sheet as a raw material for automobiles, which contributes to weight reduction. Further, in order to supply the steel sheet of the present embodiment to press molding, it is preferable to have excellent uniform elongation (uEL) and hole expandability (λ). The product "TS × uEL" of the tensile strength and the uniform elongation is preferably 20,000 MPa·% or more, and the product "TS × λ" of the tensile strength and the hole expandability is preferably 20,000 MPa·% or more. Such mechanical properties can be obtained by making the composition of the component and the metal structure of the 1/4t portion within the above range.

Further, the steel sheet according to the present embodiment may be a hot-dip galvanized steel sheet having a hot-dip galvanized layer provided on the surface thereof, or may be an alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer provided on the surface thereof. The hot-dip galvanized layer or the alloyed hot-dip galvanized layer has an effect of greatly increasing the corrosion resistance and the appearance of the steel sheet of the present embodiment. On the other hand, the hot-dip galvanized layer or the alloyed hot-dip galvanized layer does not impair the mechanical properties of the steel sheet of the present embodiment.

Next, a method of manufacturing the steel sheet according to the embodiment will be described. The steel sheet according to the present embodiment is produced by melting and casting a steel having the above-described composition in a conventional manner to form a steel blank or a steel block, heating it, and performing hot rolling to perform hot rolled steel sheet obtained. After pickling, and after the first annealing, a second annealing is further performed. A cold rolling may be performed between the first annealing and the second annealing as needed.

The hot rolling pass may be carried out in a general continuous hot rolling extension line. The first annealing and the second annealing may be performed in either the annealing furnace or the continuous annealing line as long as the conditions described later are satisfied. Further, the cold rolled steel sheet may be subjected to temper rolling.

The cooling conditions, the coiling conditions, the annealing conditions of the first annealing, the cold rolling rate, and the annealing conditions of the second annealing are respectively limited to the ranges shown below, whereby the formula 1 and the formula can be obtained. The metal structure specified in 2.

As long as the chemical component is within the range of the steel sheet of the above-described embodiment, the molten steel may be melted by a general blast furnace method, or may be a material containing a large amount of waste material, such as steel produced by an electric furnace method. The steel blank can be manufactured by a general continuous casting process, or it can be made by casting a thin flat steel blank.

The steel or steel block is heated and hot rolled. The heating temperature and the hot rolling temperature at this time are not particularly limited.

The hot rolled steel sheet obtained by the completion rolling is cooled and taken up to form a coil. In order to suppress the wave-induced iron metamorphism and the fine refinement crystal grain size, the cooling rate from the hot rolling to the start of winding is preferably as large as possible. Therefore, after the hot rolling, the average cooling rate in the temperature range from 800 ° C to the coiling temperature is 10 ° C / s or more. Further, in order to refine the particle diameter, the average cooling rate in the temperature range from 800 ° C to the coiling temperature after hot rolling is preferably 30 ° C / s or more. In order to control the coiling temperature with good precision, the upper limit of the average cooling rate in the temperature range from 800 ° C to the coiling temperature after the hot rolling is preferably 100 ° C / s or less. In addition, the "average cooling rate in the temperature range from 800 ° C to the coiling temperature" is the elapsed time required to divide the difference between 800 ° C and the coiling temperature by the steel sheet from 800 ° C to the coiling temperature. The value. When the finishing rolling of the hot rolling is completed at less than 800 ° C, the average cooling rate in the temperature range from the current temperature after the completion of the rolling to the coiling temperature is controlled, instead of starting from 800 ° C. The average cooling rate in the temperature range up to the coiling temperature may be sufficient.

Let the coiling temperature after cooling be 600 ° C or less. When the coiling temperature exceeds 600 ° C, the hot-rolled structure becomes uneven structure due to the coarsening of the ferrite-iron-iron, and coarse ferritic carbon is generated in the subsequent first annealing. This coarse stellite carbon iron may cause the residual Worthite iron after the second annealing to become coarse, or the residual Schönsten iron after the second annealing to reduce the residual Worthite iron. In either case, the mechanical properties of the steel sheet deteriorate. The coiling temperature is preferably 550 ° C or lower, more preferably 500 ° C or lower.

The hot rolled steel sheet obtained in the above manner was cooled to room temperature and then annealed. This hot rolling annealing (first annealing) is an important process in the method for producing a steel sheet according to the present embodiment. In order to satisfy the above (Formula 1) in the final structure, the annealing must be utilized to increase the concentration of Mn in the residual Worth iron. This is because the annealing (second annealing) performed only after the cold rolling is delayed does not satisfy (Formula 1).

In the first annealing, the hot-rolled steel sheet is heated and maintained at a temperature of 550 to 750 ° C for 30 minutes or more, and then cooled to below the Ms point. By performing this annealing, a steel sheet satisfying both of (Formula 1) and (Formula 2) can be obtained. When the first annealing temperature is in a temperature region lower than 550 ° C, the residual Worth iron in the structure of the finally obtained steel sheet may become unsatisfiable (Formula 2). Although the reason is not clear, it is presumed that when the first annealing temperature is lower than 550 ° C, coarse ferritic carbon iron will be produced at the old Worthfield iron grain boundary and the inclusion grain boundary during the first annealing. The carbon iron changes to a coarse Worthite iron having an aspect ratio of 2.0 or less at the time of the second annealing. On the other hand, when the first annealing temperature exceeds 750 ° C, the numb loose iron becomes large at the end of the first annealing and becomes unsatisfactory (Formula 1). Although the reason is not clear, the estimation is not satisfied (Formula 1) because the steel sheet structure substantially becomes a single phase of the Worthite iron at the time of the first annealing, and segregation of Mn does not occur. Therefore, the upper limit of the first annealing temperature is 750 °C.

Further, when the steel sheet is heated to the first annealing temperature, the average heating rate in the temperature range of 300 to 550 ° C must be 1 ° C / s or more. When the above conditions are not satisfied, the metal structure of the steel sheet may become unsatisfactory (Formula 2), and the hole expandability may deteriorate. The reason for this is unknown, but it is presumed that when it is lower than 1 ° C / s, the ferritic carbon iron precipitates out of the old Worthfield iron grain boundary and the inclusion grain boundary during heating, and the swarf carbon iron will become an aspect ratio of 2.0 or less. The reason of the sita iron. The limitation of the average heating rate is not only the first annealing but also the second annealing. The reason is the same as the first annealing. In addition, the "average heating rate in a temperature range of 300 to 550 ° C" is a value obtained by dividing the difference between the upper limit and the lower limit of the temperature region (250 ° C) by the time required for the steel sheet temperature to pass through the temperature region.

When annealing, it is not necessary to keep the temperature of the steel sheet at a constant temperature. During the annealing, as long as the steel sheet temperature is in the temperature range of 550 to 750 ° C for 30 minutes or more, the steel sheet temperature may also vary within this range. When the steel sheet temperature is maintained in the range of 550 to 750 ° C for less than 30 minutes, the metal structure of the steel sheet may not be satisfied (Formula 1). It is presumed that this is because the insufficient holding time causes the diffusion distance of Mn to be insufficient, and the concentration of Mn in the Worthite iron cannot be sufficiently increased.

The cold rolling is not an essential step in the production of the steel sheet of the present embodiment. However, in order to adjust the thickness of the plate and adjust the shape, the steel sheet may be cold rolled. The cold rolling extension refines the metal structure of the annealed steel sheet, thereby having the effect of improving mechanical properties. However, when the rolling reduction ratio of the cold rolling is too large, local ductility may be lowered.

After the first annealing or after any cold rolling, the steel sheet is subjected to a second annealing. In the method for producing a steel sheet according to the present embodiment, the annealing temperature in the second annealing is a temperature at which the ferrite iron and the Worthite iron coexist. In the case where the annealing temperature of the second annealing is lower than 550 ° C, the amount of iron field iron obtained in the second annealing becomes small, and it is impossible to leave sufficient residual Worthite iron in the steel sheet finally obtained. When the second annealing temperature exceeds 800 ° C, the Mn which has been increased in the Vostian iron in the first annealing will be diffused again, so that the finally obtained steel sheet may become unsatisfied (Formula 1). Further, when the steel was a steel of a single phase in the Vostian iron at 800 ° C and the second annealing was performed at an annealing temperature of more than 800 ° C, the structure of the steel sheet of the present embodiment could not be obtained.

Further, the temperature holding time (holding time at 550 ° C to 800 ° C) in the second annealing is made 5 seconds or longer. When the temperature of the second annealing is less than 5 seconds, the carbides are not completely dissolved, and the amount of residual Worstian iron obtained is finally reduced. In addition, the cold rolling delay recrystallization does not proceed, so the final result is obtained. The ductility of the steel sheet is greatly deteriorated. Although it is not necessary to specify the upper limit of the temperature retention time of the second annealing, the upper limit is preferably 1000 seconds in consideration of productivity.

Cooling after the second annealing is important in order to promote the transformation from the Worthfield iron phase to the ferrite iron phase and to increase the concentration of C in the untransformed Worthfield iron phase to stabilize the Worthfield iron. When the average cooling rate in the cooling after the second annealing is lower than 1 ° C / s, the ferritic iron is generated, resulting in a decrease in the amount of residual Worstian iron finally obtained. In the steel sheet according to the present embodiment, since the concentration of Mn in the residual Worthite iron is increased, it is acceptable to delay the average cooling rate during cooling after the second annealing to 1 ° C/s. On the other hand, when the average cooling rate in the cooling after the second annealing exceeds 200 ° C / s, since the ferrite iron metamorphosis cannot be sufficiently performed, the upper limit of the average cooling rate in the cooling after the second annealing is preferably 200. °C/s. When the steel sheet is plated, the average cooling rate in the cooling after the second annealing is the average cooling rate in the temperature range from the second annealing temperature to the cooling stop temperature described later (that is, the first cooling rate) The difference between the second annealing temperature and the cooling stop temperature is divided by the time required to cool the steel sheet from the second annealing temperature to the cooling stop temperature, thereby obtaining the value). In the case where the steel sheet is not plated, the average cooling rate in the cooling after the second annealing is the average cooling rate in the temperature range from the second annealing temperature to 430 ° C (that is, the second annealing temperature and the room temperature) The difference is divided by the time required to cool the steel sheet from the second annealing temperature to room temperature, whereby the value obtained).

In the case where the steel sheet is not plated, the cooling after the second annealing can be directly performed to room temperature. Moreover, when plating a steel plate, it manufactures as follows.

In the case of producing a hot-dip galvanized steel sheet, cooling after the second annealing is stopped in a temperature range of 430 to 500 ° C, and then the cold-rolled steel sheet is immersed in a molten zinc plating bath to perform hot-dip galvanizing treatment. The conditions of the plating bath can be set within a general range. After the plating treatment, it is cooled to room temperature.

In the case of producing an alloyed hot-dip galvanized steel sheet, the steel sheet is alloyed at a temperature of 450 to 600 ° C after the steel sheet is subjected to hot-dip galvanizing treatment and before the steel sheet is cooled to room temperature. The alloying treatment conditions can be set within a general range.

According to the above production method, in the steel sheet of the present embodiment, since the concentration of Mn in the Worthite iron is increased, it is possible to obtain a residual Voss which is stable at room temperature by directly cooling to room temperature after the second annealing. Tian Tie. Further, like the general TRIP steel disclosed in Patent Document 1, etc., there is no problem in stopping the cooling after the second annealing at room temperature or higher and maintaining the temperature region.

The steel sheet, the hot-dip galvanized steel sheet, and the alloyed hot-dip galvanized steel sheet of the present embodiment can be obtained by producing a steel sheet as described above. Example

Hereinafter, the present invention will be further described by way of examples. [Example 1]

First, in order to investigate the influence of the heating rate in the first annealing, the experiment described below was performed.

A steel frit having the following chemical composition was produced using a vacuum melting furnace: C: 0.10%, Si: 1.0%, Mn: 4.3%, P: 0.010%, S: 0.0020%, Al: 0.03%, N, in terms of unit mass% : 0.0020% and O: 0.0012%, and the remainder consists of iron and impurities. The obtained steel slab is heated to 1200 ° C, and after hot rolling at a finishing temperature of 921 ° C, the average cooling rate in the temperature range from 800 ° C to the coiling start temperature is 50 ° C / s, and is cooled. And coiling at 450 °C. After the coiled hot-rolled steel sheet was cooled to room temperature, the first annealing was performed. In the heating before the first annealing, the average heating rate in the temperature range of 300 to 550 ° C is various values in the range of 0.01 to 30 ° C / s. The annealing temperature (maximum heating temperature) in the first annealing was 570 °C. In all the hot rolled steel sheets, the temperature retention time in the annealing temperature was 7200 seconds (120 minutes). After the temperature retention is completed, the hot rolled steel sheet is air cooled to room temperature. Thereafter, the hot-rolled steel sheet was subjected to cold rolling at a rolling reduction ratio of 52%, and the cold-rolled steel sheet thus obtained was subjected to second annealing. In the heating before the second annealing, the average heating rate in the temperature range of 300 to 550 ° C was 3 ° C / s, the annealing temperature in the second annealing was 620 ° C, and the holding time was 600 s. Then, the average cooling rate in the temperature range from the second annealing temperature to 430 ° C was 10 ° C / s, and the cold-rolled steel sheet after the second annealing was cooled. [Mn] _γ / [Mn] _ave and f _γs / f _γ of various steel sheets obtained were obtained by the following means.

The Mn concentration ratio in the 1/4t portion was determined by grinding the cross section perpendicular to the rolling direction and the rolling surface of the steel sheet, and using FE-SEM to determine [Mn] _γ and [ Mn] _ave .

Let f _γs /f _γ in the 1/4t portion be as follows. First, the residual Worthite iron fraction f _γs having an aspect ratio of 2.0 or less in the _1/4 t portion is obtained by performing EBSP analysis on the 1/4 t portion. In this EBSP analysis, the fcc phase is separated from the bcc phase, and the portion in which the fcc phase is in a block shape is regarded as a residual Worstian iron crystal grain, and the aspect ratio is the ratio of the longest width to the shortest width of the crystal grain. . The total residual Vostian iron amount f _γ in the 1/4 t portion is obtained by measuring X-ray diffraction in the 1/4 t portion.

The test results are explained below. As shown in Fig. 1, the sample obtained by the production method in which the average heating rate at the first annealing was lower than 1 ° C / s did not satisfy the formula (2). Compared with the other samples, the TS × λ of the sample which did not satisfy the formula (2) was drastically lowered. [Example 2]

Next, in order to investigate the influence of the annealing temperature in the first annealing, the experiment described below was carried out.

The steel slab used in Example 1 was heated to 1200 ° C, and after hot rolling at a completion temperature of 921 ° C, the average cooling rate in the temperature range from 800 ° C to the coiling start temperature was 50 ° C / s. After cooling, the coiling treatment was carried out at 450 °C. After the coiled hot-rolled steel sheet was cooled to room temperature, the first annealing was performed. The average heating rate in the range of 300 to 550 ° C before the first annealing (in the range of 300 ° C to the annealing temperature in the case where the annealing temperature is lower than 550 ° C) is 3.5 ° C / s. Next, the heating of each sample was terminated at 450 to 850 ° C, and then each sample was kept at each annealing temperature for 7200 s, and then each sample was air-cooled to room temperature. Thereafter, each sample was subjected to cold rolling at a cold rolling ratio of 52%, and a second annealing was performed on each sample. The average heating rate in the range of 300 to 550 ° C in the second annealing was 3.5 ° C / s, the annealing temperature was 620 ° C and the annealing time was 600 s. After the completion of the second annealing, the average cooling rate in the temperature range from the second annealing temperature to 430 ° C was 10 ° C / s, and each sample was cooled.

As shown in FIG. 2, the sample prepared by the manufacturing method in which the heating temperature at the time of the first annealing was lower than 550 ° C did not satisfy the formula (2). As a result, the sample prepared by the production method in which the heating temperature at the first annealing was lower than 550 ° C was damaged by TS × λ. On the other hand, as shown in FIG. 3, the sample prepared by the manufacturing method in which the heating temperature at the time of the first annealing exceeded 750 ° C did not satisfy the formula (1). As a result, the TS x uEL of the sample prepared by the production method in which the heating temperature at the first annealing exceeded 750 ° C was impaired. [Example 3]

Next, in order to investigate the influence of the heating rate in the second annealing, the experiment described below was carried out. The steel slab used in Example 1 was heated to 1200 ° C, and after hot rolling at a finishing temperature of 921 ° C, the average cooling rate in the section from 800 ° C to the coiling start temperature was 50 ° C / s. After cooling, the coiling treatment was carried out at 450 °C. Thereafter, in the first annealing, the average heating rate at 300 to 550 ° C was 2.5 ° C / s and each sample was heated, and each sample was kept at 590 ° C for 7200 s, and then each sample was air-cooled to room temperature. Thereafter, each sample was subjected to cold rolling at a cold rolling ratio of 52%, and then each sample was heated at an average heating rate of from 0.01 to 30 ° C/sec in the range of 300 to 550 °C. Further, after each sample was held at the second annealing temperature of 620 ° C for 600 s, the average cooling rate in the temperature range from the second annealing temperature to 430 ° C was 10 ° C / s, and each sample was cooled. As shown in FIG. 4, when the average heating rate at the time of the second annealing is less than 1 ° C/s, the formula (2) is not satisfied, and as a result, TS × λ is impaired. [Example 4]

The steel A~P is melted and cast to form a steel embryo, and the steel embryos are hot rolled into a hot rolled steel sheet, and then the hot rolled steel sheets are supplied to coiling, pickling, first annealing, cold rolling. The steel sheet 1 to 25 is obtained by stretching and second annealing, and optionally supplying it to a plating treatment and an alloying treatment. The chemical composition of steel A~P is shown in Table 1-1 and Table 1-2. The manufacturing conditions of steel 1~25 are shown in Table 2. The microstructure of steel 1~25 is shown in Table 3, while steel 1~ The mechanical properties of 25 are shown in Table 4. The numerical units of the chemical compositions of steels A to P shown in Table 1-1 and Table 1-2 are mass%, and the remainder of the chemical components are iron and impurities. In the case of performing the plating treatment, the cooling stop temperature before immersion in the plating bath was 460 °C. Further, in the case of alloying treatment, the alloying treatment temperature was 520 °C. In addition to the material to be tested No. 4 and the material to be tested No. 7, the "first annealing time" described in Table 2 means the time in which the temperature of the material to be tested is in the range of 550 to 750 °C. The "first annealing time" of the material to be tested No. 4 and the material to be tested No. 7 means the time at which the temperature of the material to be tested is "the first annealing temperature". In addition to the material to be tested No. 8 and the material to be tested No. 21, the "second annealing time" described in Table 2 means the time in which the temperature of the material to be tested is in the range of 550 to 800 °C. The "second annealing time" of the material to be tested No. 8 and the material to be tested No. 21 means the time at which the temperature of the material to be tested is "second annealing temperature". The "average cooling rate after the second annealing" means that the plating material is plated from the second annealing temperature to 460 ° C (that is, the cooling stop temperature before the immersion in the plating bath). The average cooling rate until the plating of the material to be tested is the average cooling rate from the second annealing temperature to 430 ° C.

The volume fraction of the ferrite in the 1/4t portion is taken by photographing the 1/4t portion of the section of the sample which has been polished and etched with the oxidizing agent by an optical microscope, and the image of the tissue is image-analyzed. And ask for it. Further, although the value obtained by this method is the ferrite grain area ratio, the area ratio and the volume ratio are considered to be substantially the same value. The 1/4t portion of the Wolla volume fraction is a 1/4t portion of the cross-sectional view of the sample that has been polished and etched with the oxidizing agent by SEM, and the image of the tissue is image-resolved. Seek. In image analysis, an area having a layered structure is regarded as a wave iron. When the tempering treatment is performed after the annealing is completed to generate the tempered granulated iron, the volume ratio of the tempered granules in the 1/4 t portion is determined by measuring the 1/4 t portion of the steel sheet before the tempering treatment. The amount of iron in the field is determined as the amount of tempered iron in the 1/4t portion obtained after tempering. In the case where the temperature of the steel sheet is at the temperature of Ms~Mf before the austempering treatment of the Worthfield iron, the tempered iron volume of the tempered granule is determined by measuring the temperature of the steel sheet. The amount of increase in the volume of the steel sheet within 0.1 second after the temperature of Ms to Mf was determined as the amount of tempered granulated iron in the 1/4 t portion obtained after the tempering treatment. In the case where the tempered granulated iron is not present, the 1/4t portion of the toughening iron volume ratio is considered to be toughened iron by the ferrite iron, the granulated iron, the residual Worth iron, and the structure other than the Bora iron. It is calculated based on the volume ratio of the 1/4t portion of the ferrite iron, the granulated iron, the residual Worthite iron, and the Bora iron. When the tempering treatment is performed after the annealing is completed to generate the tempered granulated iron, the volume fraction of the 1/4t portion of the toughening iron is the ferrite iron, the granulated iron, the residual Worthite iron, the Bora iron, and the back. The structure other than the loose iron in the fire Ma Tian is regarded as toughened iron, and is calculated based on the volume ratio of the 1/4t portion of the ferrite iron, the granulated iron, the residual Worth iron, the bun iron, and the tempered granulated iron. In the case where the temperature of the steel sheet is at the temperature of Ms to Mf before the austempering treatment of the Worthfield iron, the tempered iron volume fraction of the 1/4t portion is obtained by setting the steel sheet temperature to Ms~ The increase in the volume of the steel sheet after 0.1 second after the temperature of Mf was determined as the volume fraction of the tough iron of the 1/4 t portion. The volume fraction of the residual Worthfield iron in the 1/4t portion is obtained by the X-ray diffraction method. The volume ratio of the new imatian iron in the 1/4t portion was taken as a photograph of the cross section of the sample which was polished and etched with LePera solution by an optical microscope, and the tissue photograph was image-analyzed.

Tensile strength (TS), uniform elongation (u-EL), and ductility (t-EL) were measured by a steel sheet tensile test based on JIS Z 2241. The hole-expanding λ was measured using a test piece of 80 mm square shape and subjected to a hole expansion test based on the Japan Iron and Steel Federation specification JFST1001-1996. Steel sheets of TS × uEL and TS × λ of 20,000 MPa ‧ % or more are regarded as steel sheets excellent in mechanical properties.

[Table 1-1]

[Table 1-2]

[Table 2]

[table 3]

[Table 4]

Examples 1, 3, 5, 6, 9, 11, 13, 15, 17 to 20, which have appropriate chemical compositions and manufacturing conditions, have a residual volume of Worthite iron, [Mn] _ν /[Mn] _ave, and f _νs /f _ν is properly controlled and has excellent mechanical properties.

On the other hand, in Comparative Example 2 in which the annealing time in the first annealing was insufficient and Comparative Example 4 in which the annealing temperature in the first annealing was excessive, since the concentration of Mn in the residual Worth iron was not sufficiently increased, TS × uEL was insufficient. . In Comparative Example 7 in which the annealing temperature in the first annealing was insufficient, the residual Worstian iron volume fraction f _{γs having} an aspect ratio of 2.0 or less was not sufficiently reduced, so TS × λ was insufficient. In Comparative Example 8 in which the annealing temperature in the second annealing was insufficient, TS × uEL was insufficient because the residual Worthite iron was not contained. In Comparative Example 10 in which the average cooling rate after the second annealing was insufficient, the amount of iron in the remaining Worth was insufficient, so TS × uEL was insufficient. In Comparative Example 12 in which the annealing time in the second annealing was insufficient, the amount of iron in the remaining Worthfield was insufficient, so TS × uEL was insufficient. In Comparative Example 14 in which the average cooling rate after the first annealing was insufficient, the residual Worstian iron volume ratio f _{γs having} an aspect ratio of 2.0 or less was not sufficiently reduced, so that TS × λ was insufficient. In Comparative Example 16 in which the average heating rate before the first annealing was insufficient, the residual Worstian iron volume ratio f _{γs having} an aspect ratio of 2.0 or less was not sufficiently reduced, so TS × λ was insufficient. In Comparative Example 21 in which the annealing temperature in the second annealing was excessive, since the Mn which had been increased in the Vostian iron in the first annealing was diffused again, the concentration of Mn in the residual Worth iron was not sufficiently increased, and TS × uEL was insufficient.

In Comparative Example 22 in which the C content was insufficient and Comparative Example 23 in which the Mn content was insufficient, the concentration of Mn in the residual Worth iron was not sufficiently increased, so TS × uEL was insufficient. In Comparative Example 24 in which C × Mn was insufficient, the concentration of Mn in the residual Worth iron was not sufficiently increased, so TS × uEL was insufficient. In Comparative Example 25 in which the Mn content was excessive, the ductility was impaired, and TS × uEL and TS × λ were insufficient.

Figure 1 is a graph showing the effect of the average heating rate on f _γs /f _γ during the first annealing. Figure 2 is a graph showing the effect of the maximum heating temperature on f _γs /f _γ during the first annealing. FIG 3 based on the display _[Mn] γ / [Mn] FIG _ave influence the maximum heating temperature during the first annealing. Figure 4 is a graph showing the effect of the average heating rate on f _γs /f _γ during the second annealing.

Claims (5)

A steel sheet characterized by containing C: 0.03 to 0.40%, Si: 0.01 to 5.00%, Mn: 0.50 to 12.00%, Al: 0.001 to 5.000%, P: 0.150% or less, and S: 0.0300 in terms of unit mass%. % or less, N: 0.0100% or less, O: 0.0100% or less, Cr: 0 to 5.00%, Mo: 0 to 5.00%, Ni: 0 to 5.00%, Cu: 0 to 5.00%, Nb: 0 to 0.500%, Ti: 0~0.500%, V: 0~0.500%, W: 0~0.500%, B: 0~0.0030%, Ca: 0~0.0500%, Mg: 0~0.0500%, Zr: 0~0.0500%, REM : 0~0.0500%, Sb: 0~0.0500%, Sn: 0~0.0500%, As: 0~0.0500%, and Te: 0~0.0500%, and the rest is composed of iron and impurities; and, 1/4t The metal structure of the part contains 4 to 70% by volume of the residual Worthite iron; in the 1/4t portion, the average Mn concentration [Mn] _ave in terms of unit mass% with respect to the entire 1/4t portion, the aforementioned residual The average Mn concentration in the unit mass% [Mn] _γ system satisfies the formula 1; in the 1/4t portion, the volume ratio f _{γs of the} residual Worstian iron having an aspect ratio of 2.0 or less and all the above residues austenitic iron-based volume fraction _f γ satisfies formula 2; in units of mass%, a C content [C] and the Mn content [Mn] is satisfied based _{3; [Mn] γ / [} Mn] ave> 1.5 ... ( Formula _{1); f γs / f γ} ≦ 0.30 ... ( Formula 2); [C] × [ Mn] ≧ 0.15 ... ( Formula 3). The steel sheet according to claim 1, which contains Mn: 3.50 to 12.00% in terms of unit mass%. The steel sheet according to claim 1 or 2, which is selected from the group consisting of one or more selected from the group consisting of Cr: 0.01% to 5.00%, and Mo: 0.01% to 5.00%, in terms of unit mass% Ni: 0.01%~5.00%, Cu: 0.01%~5.00%, Nb: 0.005%~0.500%, Ti: 0.005%~0.500%, V: 0.005%~0.500%, W: 0.005%~0.500%, B : 0.0001%~0.0030%, Ca: 0.0001%~0.0500%, Mg: 0.0001%~0.0500%, Zr: 0.0005%~0.0500%, REM: 0.0005%~0.0500%, Sb: 0.0050%~0.0500%, Sn: 0.0050 %~0.0500%, As: 0.0050%~0.0500%, and Te: 0.0050%~0.0500%. The steel sheet according to claim 1 or 2, wherein the steel sheet has a hot-dip galvanized layer on the surface thereof. The steel sheet according to claim 1 or 2, wherein the surface of the steel sheet has an alloyed hot-dip galvanized layer.