JP6760543B1 - Steel plate and steel plate manufacturing method - Google Patents

Abstract

C, Si and sol. It contains Al, and further contains Mn: more than 2.50 to less than 4.20% by mass, the balance is iron and impurities, and the metallographic structure at the position 1/8 of the thickness from the surface is the austenite phase in area ratio. : 10% or more, total of tempered martensite phase and austenite phase: 5% or more, ferrite phase: 35% or more, and fresh martensite phase: less than 15%, area of unrecrystallized ferrite phase relative to ferrite phase The ratio is 10 to 50%, CMnγ / CMnα, which is the ratio of the average Mn concentration CMnγ in the austenite phase to the average Mn concentration CMnα in the ferrite phase, is 1.20 or more, and Vickers at the position 1/8 of the thickness from the surface. A steel plate having a hardness variation of 40 Hv or less.

Description

The present disclosure relates to steel sheets and methods for manufacturing them.

In general, when the strength of a steel sheet is increased, the elongation is reduced and the formability of the steel sheet can be reduced. Therefore, in order to use a high-strength steel sheet as a part for an automobile body, it is necessary to improve both strength and formability, which are contradictory characteristics. In addition, since many automobile body parts made of steel sheets require bending, high-strength steel sheets used as body parts are required to have excellent bendability. Therefore, as the mechanical properties of the steel sheet, it is required to have high strength and excellent moldability, and further have excellent bendability.

So far, in order to improve elongation, that is, formability, Mn has been positively added and about 5% by mass of Mn has been contained in the steel sheet to generate retained austenite (residual γ) in the steel. A so-called medium Mn steel utilizing the transformation-induced plasticity has been proposed (for example, Non-Patent Document 1).

Further, a steel sheet to which Mn of 2.60% or more and 4.20% or less is added (Patent Document 1) has been proposed. Since the above-mentioned steel sheet also contains more Mn than general high-strength steel, retained austenite is easily generated, the elongation is high, and excellent formability is exhibited.

International Publication No. 2016/067622

Takashi Furukawa, Osamu Matsumura, Heat Treatment, Japan, Japan Heat Treatment Association, 1997, Vol. 37, No. 4, p. 204

However, since the steel sheet disclosed in Non-Patent Document 1 has a high Mn content, weldability may become a problem when it is used for parts for automobile bodies. Therefore, considering the utility as automobile parts and the like, it is desired to improve both the strength and formability of the steel sheet with a smaller Mn content. Further, since the steel sheet disclosed in Patent Document 1 forms a remarkable band structure, the bendability exhibits remarkable anisotropy. In particular, when the bending ridge line is in the rolling direction, the bendability deteriorates. As described above, when the anisotropy of bendability becomes large, it becomes difficult to produce a square tubular part, and not only the degree of freedom in part design is lowered, but also the yield at the time of forming the part is lowered. Hereinafter, the bendability when the bending ridge line is in the rolling direction is simply referred to as bendability.

Therefore, a steel sheet having excellent elongation characteristics, excellent bendability, and high strength is desired.

In order to ensure excellent elongation characteristics, excellent bendability, and high strength, the present inventors have tempered martensite phase in a steel plate containing a predetermined component in an area% of 10% or more of an austenite phase. The total of the austenite phase and the bainite phase is 5% or more, the ferrite phase is 35% or more, the fresh martensite phase is limited to less than 15%, and the area ratio of the unrecrystallized ferrite phase to the ferrite phase is 10 to 50%. CMnγ / CMnα, which is the ratio of the average Mn concentration CMnγ in the austenite phase to the average Mn concentration CMnα in the ferrite phase, shall be 1.20 or more, and the variation in Vickers hardness at the position 1/8 of the thickness from the surface shall be 40 Hv or less. Was found to be effective.

The steel sheet of the present disclosure and the manufacturing method thereof have been made based on the above findings, and the gist thereof is as follows.

The gist of this disclosure is as follows.
(1)
The chemical composition is mass%,
C: More than 0.15 to less than 0.40%,
Si: 0.001 to less than 2.00%,
Mn: Over 2.50 to less than 4.20%,
sol. Al: 0.001 to less than 1.500%,
P: 0.030% or less,
S: 0.0050% or less,
N: Less than 0.050%,
O: Less than 0.020%,
Cr: 0 to 0.50%,
Mo: 0-2.00%,
W: 0-2.00%,
Cu: 0-2.00%,
Ni: 0-2.00%,
Ti: 0 to 0.300%,
Nb: 0 to 0.300%,
V: 0 to 0.300%,
B: 0 to 0.010%,
Ca: 0-0.010%,
Mg: 0-0.010%,
Zr: 0-0.010%,
REM: 0-0.010%,
Sb: 0 to 0.050%,
Sn: 0 to 0.050%,
Bi: 0 to 0.050%, and the balance: iron and impurities,
The metallographic structure at 1/8 of the thickness from the surface is the area ratio of austenite phase: 10% or more, total tempered martensite phase and bainite phase: 5% or more, ferrite phase: 35% or more, and fresh martensite. Phase: less than 15%
The area ratio of the unrecrystallized ferrite phase to the ferrite phase is 10 to 50%.
The CMnγ / CMnα, which is the ratio of the average Mn concentration CMnγ in the austenite phase to the average Mn concentration CMnα in the ferrite phase, is 1.20 or more, and the variation in Vickers hardness at the position 1/8 of the thickness from the surface. Steel plate of 40 Hv or less.
(2)
When the chemical composition is mass%,
Cr: 0.01-0.50%,
Ti: 0.005 to 0.300%,
Nb: 0.005 to 0.300%,
V: 0.005 to 0.300%, and B: 0.0001 to 0.010%
The steel sheet according to (1), which contains one or more selected from the group consisting of.
(3)
The steel sheet according to (1) or (2), which has a hot-dip galvanized layer on the surface of the steel sheet.
(4)
The steel sheet according to (1) or (2), which has an alloyed hot-dip galvanized layer on the surface of the steel sheet.
(5)
The chemical composition is mass%,
C: More than 0.15 to less than 0.40%,
Si: 0.001 to less than 2.00%,
Mn: Over 2.50 to less than 4.20%,
sol. Al: 0.001 to less than 1.500%,
P: 0.030% or less,
S: 0.0050% or less,
N: Less than 0.050%,
O: Less than 0.020%,
Cr: 0 to 0.50%,
Mo: 0-2.00%,
W: 0-2.00%,
Cu: 0-2.00%,
Ni: 0-2.00%,
Ti: 0 to 0.300%,
Nb: 0 to 0.300%,
V: 0 to 0.300%,
B: 0 to 0.010%,
Ca: 0-0.010%,
Mg: 0-0.010%,
Zr: 0-0.010%,
REM: 0-0.010%,
Sb: 0 to 0.050%,
Sn: 0 to 0.050%,
Bi: 0 to 0.050%, and the balance: iron and steel as impurities, the finish rolling temperature is 1000 ° C or less, the cooling time after the finish rolling is 0.8 seconds or more, and the average cooling after the cooling. Hot rolling at a speed of 30 ° C./sec or higher and a winding temperature of less than 300 ° C. to obtain a hot-rolled steel sheet.
The hot-rolled steel sheet is heat-treated for 1 hour or more in a temperature range where the austenite phase fraction is 20 to 50%, and then pickled and cold-rolled to obtain a cold-rolled steel sheet.
The cold rolling ratio in the cold rolling is 30 to 70%.
The cold-rolled steel sheet is annealed by holding it for 30 seconds or more in a temperature range where the austenite phase fraction is 20 to 65%, and after the annealing temperature is maintained, it is cooled to a temperature range of 100 to 530 ° C. , Holding in a temperature range of 100 to 530 ° C. for 10 to 1000 seconds,
A method for manufacturing a steel sheet, including.
(6)
When the chemical composition is mass%,
Cr: 0.01-0.50%,
Ti: 0.005 to 0.300%,
Nb: 0.005 to 0.300%,
V: 0.005 to 0.300%, and B: 0.0001 to 0.010%
The method for producing a steel sheet according to (5), which contains one or more selected from the group consisting of.
(7)
The method for producing a steel sheet according to (5) or (6), wherein the cooling time after the finish rolling is 1.2 to 4.0 seconds.
(8)
The method for producing a steel sheet according to any one of (5) to (7), wherein hot-dip galvanizing treatment is performed after the annealing.
(9)
The method for producing a steel sheet according to (8), wherein after performing the hot-dip galvanizing treatment, the hot-dip galvanizing treatment is performed in a temperature range of 450 to 620 ° C.

According to the present disclosure, it is possible to provide a steel sheet having excellent elongation characteristics, excellent bendability, and high strength.

Hereinafter, an example of one embodiment of the steel sheet of the present disclosure will be described.

1. 1. Chemical Composition The reason why the chemical composition of the steel sheet of the present disclosure is defined as described above will be described. In the following description, "%" representing the content of each element means mass% unless otherwise specified. In the chemical composition of steel sheets, the numerical range represented by "~" is the lower limit and upper limit of the numerical values before and after "~" unless "super" or "less than" is used. Means the range to be included.

(C: More than 0.15 to less than 0.40%)
C is an extremely important element for increasing the strength of steel and ensuring the austenite phase. A C content of greater than 0.15% is required to obtain a sufficient austenite phase. On the other hand, if C is excessively contained, it becomes difficult to maintain the weldability of the steel sheet, so the upper limit of the C content is set to less than 0.40%. The lower limit of the C content is preferably 0.20% or more, more preferably 0.25% or more. When the C content is 0.20% or more, the formation of the austenite phase can be further promoted. The upper limit of the C content is preferably 0.36% or less, more preferably 0.32% or less, and by setting the upper limit of the C content in the above range, the toughness of the steel sheet can be further enhanced. ..

(Si: 0.001 to less than 2.00%)
Si is an element effective for strengthening the tempered martensite phase, homogenizing the structure, and improving moldability. Si also has the effect of suppressing the precipitation of cementite and promoting the retention of the austenite phase. In order to obtain the above effect, a Si content of 0.001% or more is required. On the other hand, if Si is excessively contained, it becomes difficult to maintain the plating property and chemical conversion treatment property of the steel sheet, so the upper limit of the Si content is set to less than 2.00%. The lower limit of the Si content is preferably 0.005% or more, more preferably 0.010% or more, and further preferably 0.10% or more. By setting the lower limit of the Si content to the above range, the elongation characteristics of the steel sheet can be further improved. The upper limit of the Si content is preferably 1.90% or less, more preferably 1.80% or less.

(Mn: more than 2.50 to less than 4.20%)
Mn is an element that stabilizes the austenite phase and enhances hardenability. Further, in the steel sheet of the present disclosure, Mn is concentrated in the austenite phase to further stabilize the austenite phase. More than 2.50% Mn is required to stabilize the austenite phase at room temperature. On the other hand, the upper limit of the Mn content was set to less than 4.20% in order not to reduce the bendability when the bending ridge line is in the rolling direction while ensuring the weldability. The lower limit of the Mn content is preferably more than 3.00%, more preferably 3.50% or more. The upper limit of the Mn content is preferably 4.10% or less, more preferably 4.00% or less. By setting the lower limit of the Mn content to the above range, the fraction of the stable austenite phase can be increased, and by setting the upper limit of the Mn content to the above range, the bendability can be sufficiently exhibited. it can.

(Sol.Al: less than 0.001-1.500%)
Al is a deoxidizing agent and needs to be contained in an amount of 0.001% or more. In addition, Al also has an action of improving material stability because it widens the temperature range of the two-phase region at the time of annealing. The larger the Al content, the greater the effect. However, if the Al content is excessive, it becomes difficult to maintain the surface texture, paintability, and weldability. The upper limit of Al was set to less than 1.500%. sol. The lower limit of the Al content is preferably 0.005% or more, more preferably 0.010% or more, and further preferably 0.020% or more. sol. The upper limit of the Al content is preferably 1.200% or less, more preferably 1.000% or less. sol. By setting the lower limit value and the upper limit value of the Al content in the above range, the balance between the deoxidizing effect and the material stability improving effect and the surface texture, paintability, and weldability becomes better.

(P: 0.030% or less)
P is an impurity, and if the steel sheet contains P in excess, the bendability is impaired. Therefore, the upper limit of the P content is set to 0.030% or less. The upper limit of the P content is preferably 0.025% or less, more preferably 0.020% or less, still more preferably 0.015% or less. Since the steel sheet according to this embodiment does not require P, the lower limit of the P content is 0%. The lower limit of the P content may be more than 0% or 0.001% or more, but the smaller the P content, the more preferable.

(S: 0.0050% or less)
S is an impurity, and if the steel sheet contains S in excess, the weldability is impaired. Therefore, the upper limit of the S content is set to 0.0050% or less. The upper limit of the S content is preferably 0.0030% or less, more preferably 0.0020% or less. Since the steel sheet according to this embodiment does not require S, the lower limit of the S content is 0%. The lower limit of the S content may be more than 0% or 0.0003% or more, but the smaller the S content, the more preferable.

(N: less than 0.050%)
N is an impurity, and the toughness decreases when the steel sheet contains 0.050% or more of N. Therefore, the upper limit of the N content is set to less than 0.050%. The upper limit of the N content is preferably 0.010% or less, more preferably 0.006% or less. Since the steel sheet according to this embodiment does not require N, the lower limit of the N content is 0%. The lower limit of the N content may be more than 0% or 0.001% or more, but the smaller the N content, the more preferable.

(O: less than 0.020%)
O is an impurity, and ductility is reduced when the steel sheet contains 0.020% or more of O. Therefore, the upper limit of the O content is set to less than 0.020%. The upper limit of the O content is preferably 0.010% or less, more preferably 0.005% or less, still more preferably 0.003% or less. Since the steel sheet according to this embodiment does not require O, the lower limit of the O content is 0%. The lower limit of the O content may be more than 0% or 0.001% or more, but the smaller the O content, the more preferable.

The steel sheet of the present embodiment is further selected from the group consisting of Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, Sn and Bi, or one of them. Two or more kinds may be contained. However, the steel sheet according to the present embodiment does not have to contain Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, Sn and Bi, that is, The lower limit of the content may be 0%.

(Cr: 0 to 0.50%)
(Mo: 0-2.00%)
(W: 0-2.00%)
(Cu: 0-2.00%)
(Ni: 0-2.00%)
Since Cr, Mo, W, Cu, and Ni are not essential elements for the steel sheet according to the present embodiment, their respective contents are 0% or more. However, Cr, Mo, W, Cu, and Ni are elements that improve the strength of the steel sheet and may be contained. In order to obtain the effect of improving the strength of the steel sheet, the steel sheet contains 0.01% or more and 0.04% or more of one or more elements selected from the group consisting of Cr, Mo, W, Cu and Ni, respectively. It may be contained in excess or 0.10% or more. However, if the steel sheet contains these elements in excess, surface scratches are likely to occur during hot rolling, and the strength of the hot rolled steel sheet becomes too high, which may reduce the cold rollability. Therefore, of the content of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni, the upper limit of the Cr content is set to 0.50% or less, and Mo. , W, Cu, and Ni, respectively, shall have an upper limit of 2.00% or less. The upper limit of the Cr content may be 0.45% or less, 0.40% or less, or 0.35% or less, and the upper limit of the respective contents of Mo, W, Cu, and Ni is It may be 1.80% or less, 1.50% or less, 1.20% or less, or 1.00% or less.

(Ti: 0 to 0.300%)
(Nb: 0 to 0.300%)
(V: 0 to 0.300%)
Since Ti, Nb, and V are not essential elements for the steel sheet according to the present embodiment, their respective contents are 0% or more. However, Ti, Nb, and V are elements that generate fine carbides, nitrides, or carbonitrides, and are therefore effective in improving the strength of the steel sheet. Therefore, the steel sheet may contain one or more elements selected from the group consisting of Ti, Nb, and V. In order to obtain the effect of improving the strength of the steel sheet, it is preferable that the lower limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V is 0.005% or more. , 0.010% or more is more preferable. On the other hand, if these elements are excessively contained, the strength of the hot-rolled steel sheet may increase too much and the cold rollability may decrease. Regarding Nb, when the Nb content is 0.300% or less, the delay in recrystallization of the ferrite phase can be suppressed, and a desired structure can be obtained more stably. Therefore, the upper limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V is set to 0.300% or less, preferably 0.250% or less, and more preferably 0. .200% or less.

(B: 0 to 0.010%)
(Ca: 0 to 0.010%)
(Mg: 0 to 0.010%)
(Zr: 0 to 0.010%)
(REM: 0 to 0.010%)
Since B, Ca, Mg, Zr, and REM (rare earth metal) are not essential elements for the steel sheet of the present disclosure, their respective contents are 0% or more. However, B, Ca, Mg, Zr, and REM improve the local ductility and hole expandability of the steel sheet. In order to obtain this effect, the lower limit of each of one or more elements selected from the group consisting of B, Ca, Mg, Zr, and REM is preferably 0.0001% or more, more preferably 0. .001% or more. However, since an excess amount of these elements deteriorates the moldability of the steel sheet, the upper limit of the content of each of these elements is set to 0.010% or less, and the element is selected from the group consisting of B, Ca, Mg, Zr, and REM. The total content of one or more elements is preferably 0.030% or less.

(Sb: 0 to 0.050%)
(Sn: 0 to 0.050%)
(Bi: 0 to 0.050%)
Since Sb, Sn, and Bi are not essential elements for the steel sheet of the present disclosure, their respective contents are 0% or more. However, Sb, Sn, and Bi suppress that easily oxidizing elements such as Mn, Si, and / or Al in the steel sheet are diffused on the surface of the steel sheet to form an oxide, and improve the surface texture and plating property of the steel sheet. Increase. In order to obtain this effect, the lower limit of the content of each of one or more elements selected from the group consisting of Sb, Sn, and Bi is preferably 0.0005% or more, more preferably 0.001. % Or more. On the other hand, if the content of each of these elements exceeds 0.050%, the effect is saturated. Therefore, the upper limit of the content of each of these elements is set to 0.050% or less, preferably 0.040% or less, more preferably. Is 0.030% or less.

The steel sheet of the present embodiment has Cr: 0.01 to 0.50%, Ti: 0.005 to 0.300%, Nb: 0.005 to 0.300%, V: 0 among the above optional components. It may contain one or more of the elements selected from the group consisting of .005 to 0.300% and B: 0.0001 to 0.010%.

The rest of the chemical composition of the steel sheet according to this embodiment is iron and impurities. Examples of impurities include elements that are inevitably mixed from the steel raw material, scrap, and / or the steelmaking process, and are allowed as long as they do not impair the characteristics of the steel sheet according to the present embodiment. In addition, the impurities include elements other than the components described above, which are contained in the steel sheet at a level at which the action and effect peculiar to the element do not affect the characteristics of the steel sheet according to the embodiment of the present invention. Is what you do.

2. 2. Metal structure Next, the metal structure of the steel sheet according to the present embodiment will be described.

The metallographic structure at the 1/8 position (also referred to as 1 / 8t portion) of the thickness from the surface of the steel sheet according to the present embodiment has an area ratio of austenite phase: 10% or more, total of tempered martensite phase and bainite phase: 5% or more, ferrite phase: 35% or more, and fresh martensite phase: less than 15%. The fraction of each structure changes depending on the heat treatment conditions and affects the material of the steel sheet such as strength, elongation characteristics, and bendability.

(Area% of austenite phase in the metal structure of 1 / 8t part of the steel sheet: 10% or more)
In the steel sheet according to the present embodiment, it is important that the amount of the austenite phase in the metal structure is within a predetermined range. The austenite phase is a structure that enhances the elongation characteristics of the steel sheet by transformation-induced plasticity. Since the austenite phase can be transformed into the martensite phase by overhanging, drawing, stretch flangeing, or bending accompanied by tensile deformation, it also contributes to the improvement of the strength of the steel sheet. In order to obtain these effects, the steel sheet according to the present embodiment needs to contain an austenite phase having an area ratio of 10% or more in the metal structure. The area ratio of the austenite phase is preferably 15% or more, more preferably 18% or more. When the area ratio of the austenite phase is 15% or more, further 18% or more, both strength and elongation are compatible, and TS × EL described later becomes high. The upper limit of the area ratio of the austenite phase is not particularly specified, but is substantially 30% or less. The area ratio of the austenite phase is measured by an X-ray diffraction method.

(Area% of fresh martensite phase in the metal structure of 1 / 8t part of the steel sheet: less than 15%)
The fresh martensite phase is a hard phase containing many dislocations in its structure, and is an effective phase for obtaining the strength of a steel sheet. However, in order to significantly deteriorate the bendability, the area ratio of the fresh martensite phase in the metal structure is set to less than 15%. When bendability is particularly required, the area ratio of the fresh martensite phase is preferably 10% or less, more preferably 5% or less, and even more preferably substantially 0%.

(Total area% of tempered martensite phase and bainite phase in the metal structure of 1 / 8t part of steel sheet: 5% or more)
The tempered martensite phase and the bainite phase are also hard phases, but they have a structure different from that of the fresh martensite phase, and contribute to the improvement of bendability while ensuring the strength of the steel sheet. In order to achieve both strength and bendability, the total area ratio of the tempered martensite phase and the bainite phase in the metal structure needs to be 5% or more. When the strength of the steel sheet is emphasized, the total area ratio of the tempered martensite phase and the bainite phase is preferably 10% or more, more preferably 15% or more, still more preferably 20% or more. The upper limit of the total area ratio of the tempered martensite phase and the bainite phase is not particularly limited, but is substantially 50% or less. In the metallographic structure of the steel sheet according to the present embodiment, all of the total area ratios of the tempered martensite phase and the bainite phase are often the area ratios of the tempered martensite. On the other hand, the metal structure may contain a bainite phase, but since the bainite phase has the same characteristics as the tempered martensite phase, even when the metal structure contains a bainite phase, the area ratio of the tempered martensite phase And the area ratio of the bainite phase is also measured.

(Area ratio of ferrite phase in the metal structure of 1 / 8t part of steel sheet: 35% or more)
The ferrite phase is an essential structure for ensuring ductility. In order to secure the required ductility, the area ratio of the ferrite phase in the metal structure is 35% or more. The area ratio of the ferrite phase in the metal structure is preferably 40% or more, more preferably 45% or more. The upper limit of the area ratio of the ferrite phase in the metal structure is not particularly limited, but is substantially 75% or less. Further, the area ratio of the unrecrystallized ferrite phase to the ferrite phase is 10% or more, preferably 20% or more. When the area ratio of the unrecrystallized ferrite phase is within the above range, a steel sheet having a high yield point can be obtained. On the other hand, if there are too many unrecrystallized ferrite phases, ductility will decrease. Therefore, the upper limit of the area ratio of the unrecrystallized ferrite phases is set to 50% or less. The upper limit of the area ratio of the unrecrystallized ferrite phase is more preferably 40% or less.

In the metal structure, the balance other than the austenite phase, the fresh martensite phase, the tempered martensite phase (including the bainite phase), and the ferrite phase can be a structure such as pearlite or cementite. Alternatively, the steel sheet according to the present embodiment may consist only of an austenite phase, a fresh martensite phase, a tempered martensite phase, a bainite phase, and a ferrite phase.

(CMnγ / CMnα ≧ 1.20)
The ratio of the average Mn concentration CMnγ in the austenite phase to the average Mn concentration CMnα in the ferrite phase (all ferrite phases including the unrecrystallized ferrite phase) is 1.20 or more, preferably 1.35 or more. is there. When CMnγ / CMnα is within the above range, Mn distribution that concentrates Mn in the portion that was the austenite phase during heat treatment can be sufficiently obtained, and a stable austenite phase can be obtained even after short-time annealing, resulting in excellent ductility. Is obtained. On the other hand, if CMnγ / CMnα is less than 1.20, the Mn distribution is insufficient and it becomes difficult to obtain the austenite phase by annealing for a short time. The upper limit of CMnγ / CMnα is not particularly specified, but is substantially 1.60 or less.

(Vickers hardness variation at 1 / 8t part of steel sheet is 40Hv or less)
The variation in Vickers hardness at the position of 1/8 of the thickness from the surface of the steel sheet is 40 Hv or less, preferably 30 Hv or less. By optimizing the cooling conditions after finish rolling and the heat treatment conditions of the hot-rolled steel sheet to suppress the variation in Vickers hardness, the structure of the steel sheet according to the present embodiment becomes uniform, and the formation of band structure is suppressed. Flexibility is improved when the bending ridge is in the rolling direction.

Each measurement method will be described below.

(Measurement method of area ratio of austenite phase)
The area ratio of the austenite phase is calculated as follows. From the center of the surface of the steel sheet (midpoint in the width direction of the steel sheet) to 25 mm in the rolling direction, 25 mm in the width direction of the steel sheet (direction perpendicular to the rolling direction), and the thickness of the test piece to the thickness of the annealed sample. break the ice. Then, the test piece is chemically polished to reduce the plate thickness by 1/8 to obtain a test piece having a chemically polished surface. The "direction perpendicular to the rolling direction" is a direction parallel to the surface of the steel sheet and perpendicular to the rolling direction. Next, X-ray diffraction analysis with a measurement range of 2θ of 45 to 105 degrees is performed three times on the surface of the test piece using a Co tube. Then, by analyzing the profile of the obtained austenite phase and averaging each of them, the area ratio of the austenite phase can be obtained.

(Measurement method of ferrite phase, total of tempered martensite phase and bainite phase, and area ratio of fresh martensite phase)
The total of the ferrite phase, the tempered martensite phase and the bainite phase, and the area ratio of the fresh martensite phase are calculated from the microstructure observation by a scanning electron microscope (SEM). The L cross section of the steel sheet is mirror-polished, and then the microstructure is exposed with 3% nital. Using a scanning electron microscope with a magnification of 5000 times, the length is 0.1 mm (length in the plate thickness direction) at a position 1/8 from the surface. Observe the microstructure in the range of (s) x width 0.3 mm (length in the rolling direction). The ferrite phase (including the unrecrystallized ferrite phase) is discriminated as a gray underlying structure, and the austenite phase and the fresh martensite phase are discriminated as a white structure. The area ratio of the fresh martensite phase is calculated by subtracting the area ratio of the austenite phase measured by the X-ray diffraction method from the total area ratio of the austenite phase and the fresh martensite phase. The tempered martensite phase (including the bainite phase) looks white like the fresh martensite phase, but the one in which the substructure is confirmed in the crystal grains is judged to be the tempered martensite phase (including the bainite phase). Calculate by doing. The L cross section refers to a surface obtained by cutting a steel sheet parallel to the rolling direction and perpendicular to the surface of the steel sheet. The L cross section in the present embodiment is a surface cut so as to pass through the center in the width direction of the steel sheet.

(Measuring method of area ratio of unrecrystallized ferrite phase)
The area ratio of the unrecrystallized ferrite phase is determined by performing backscattered electron diffraction (EBSP) measurement on this region, that is, on the ferrite phase region after discriminating the crystal grains of the ferrite phase as described above. It is calculated by judging the region of 1 ° or more in KAM value as an unrecrystallized ferrite phase structure. Therefore, the "area ratio of the unrecrystallized ferrite phase" refers to the ratio of the unrecrystallized ferrite phase to the ferrite phase.

(Measuring method of CMnγ / CMnα)
CMnγ / CMnα is measured by EBSP, SEM, and electron probe microanalyzer (EPMA). As described above, the regions of the austenite phase and the ferrite phase were identified using EBSP and SEM, and CMnγ (Mn concentration of the austenite phase) and CMnα (Mn concentration of the ferrite phase) were measured in the regions by EPMA. Calculate CMnγ / CMnα. More specifically, any austenite phase (γ phase) and ferrite identified for a region of 50 μm in length (length in the plate thickness direction) and 50 μm in width (length in the rolling direction) at a position 1/8 from the surface. For the phase (α phase), the average value of the Mn concentration measured for each of the 10 grains in each phase is calculated, and the values are taken as CMnγ and CMnα to calculate CMnγ / CMnα.

(Measuring method of variation in Vickers hardness)
The L cross section of the steel sheet is mirror-polished, and then the microstructure is exposed by 3% nital, and the variation in Vickers hardness at the position 1/8 of the thickness from the center of the steel sheet surface is measured. As for the variation in Vickers hardness, the load is 100 g, the Vickers hardness at 8 points at a position 1/8 of the thickness from the surface is measured, and the difference between the maximum and the minimum is defined as the variation in Vickers hardness. However, for the eight Vickers indentations, the distance between adjacent indentations shall be 50 to 70 μm. The Vickers hardness is measured in accordance with JIS2244: 2009.

Next, the mechanical properties of the steel sheet according to this embodiment will be described.

(Tensile strength, elongation)
The tensile strength (TS) of the steel sheet according to the present embodiment is preferably 980 MPa or more, more preferably 1180 MPa or more. This is because when a steel sheet is used as a material for automobiles, the thickness is reduced by increasing the strength, which contributes to weight reduction. Further, in order to use the steel sheet according to the present embodiment for press forming, the elongation (EL) is preferably excellent, for example, the elongation (EL) is 20% or more, preferably 22% or more. The TS × EL of the steel sheet according to the present embodiment is preferably 24,000 MPa ·% or more, more preferably 26,000 MPa ·% or more, still more preferably 28,000 MPa ·% or more. Further, the steel sheet according to the present embodiment is also excellent in bendability when the bending ridge line is in the rolling direction.

The steel sheet of the present disclosure can be used for various purposes, and is particularly suitable for structural parts of automobiles such as side sills and contributes to weight reduction of automobiles, so that its industrial contribution is extremely remarkable.

3. 3. Manufacturing Method A method for manufacturing a steel sheet according to the present embodiment will be described. In the following, the numerical range represented by using "~" includes the numerical values before and after "~" as the lower limit value and the upper limit value, except when "super" or "less than" is used. Means a range.

For the steel sheet according to the present embodiment, steel having the above-mentioned chemical composition is melted and cast by a conventional method to produce a slab or an ingot, which is heated to a finish rolling temperature of 1000 ° C. or less, after finish rolling. The hot-rolled steel sheet obtained by hot rolling with an annealing time of 0.8 seconds or more, an average cooling rate after cooling of 30 ° C./sec or more, and a winding temperature of less than 300 ° C. was subjected to ferrite / After heat treatment in the two-phase region of austenite, pickling, cold rolling at a cold rolling rate of 30 to 70%, and then short-time annealing in the two-phase region of ferrite / austenite to 100 to 530 ° C. It is cooled and held at this temperature for manufacturing.

Hot rolling may be performed on a normal continuous hot rolling line. The heat treatment of the hot-rolled steel sheet after hot rolling can be performed in a batch furnace such as a box annealing furnace (BAF) or a tunnel furnace such as a continuous annealing furnace. Cold rolling may also be performed on a normal continuous cold rolling line. In the method of the present disclosure, annealing can be performed using a continuous annealing line, so that the productivity is very excellent.

In order to obtain the metallographic structure of the steel sheet of the present disclosure, the following conditions, particularly hot-rolled conditions, heat treatment conditions for hot-rolled steel sheets after hot rolling, cold rolling conditions, annealing conditions, and cooling conditions are as follows. It is preferable to carry out within the range shown in.

As long as the steel sheet according to the present embodiment has the above-mentioned chemical composition, the molten steel may be melted by a normal blast furnace method, and the raw material is a large amount of scrap like the steel produced by the electric furnace method. It may be included in. The slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting.

The above-mentioned slab or ingot is heated and hot-rolled to obtain a hot-rolled steel sheet. The temperature of the steel material to be subjected to hot rolling is preferably 1100 to 1300 ° C. By setting the temperature of the steel material to be subjected to hot rolling to 1100 ° C. or higher, the deformation resistance during hot rolling can be further reduced. On the other hand, by setting the temperature of the steel material to be subjected to hot rolling to 1300 ° C. or lower, it is possible to suppress a decrease in yield due to an increase in scale loss. As used herein, the temperature refers to the surface temperature at the center of the slab or steel surface.

The time for holding in the above-mentioned preferable temperature range of 1100 to 1300 ° C. before hot rolling is not particularly specified, but in order to improve toughness, it is preferably 30 minutes or more, preferably 1 hour or more. Is even more preferable. Further, in order to suppress excessive scale loss, it is preferably 10 hours or less, and more preferably 5 hours or less. In the case of direct rolling or direct rolling, hot rolling may be performed as it is without heat treatment.

(Finish rolling and winding: Finish rolling at 1000 ° C or less, allowing 0.8 seconds or more to cool after finishing rolling, cooling at an average cooling rate of 30 ° C / sec or more after cooling, and winding at less than 300 ° C)
Finish rolling is performed in hot rolling. The finish rolling start temperature is 1000 ° C or lower, and the finish rolling is performed at 1000 ° C or lower. If the finish rolling start temperature exceeds 1000 ° C, the coarsening of the structure in the hot-rolled state cannot be prevented, the subsequent structure control becomes difficult, and the surface texture of the steel sheet deteriorates due to intergranular oxidation. It also becomes difficult to suppress. The finish rolling start temperature is preferably 750 ° C. or higher. When the finish rolling start temperature is 750 ° C. or higher, the deformation resistance during rolling can be reduced and the structure control can be easily performed.

After finish rolling, allow to cool for 0.8 seconds or longer. Generally, from the viewpoint of microstructure miniaturization, it is preferable to quench immediately after finish rolling. However, in a steel sheet containing more than 2.50% of Mn, such as the steel sheet according to the present embodiment, the austenite particle size immediately after finish rolling becomes non-uniform due to the delay in recrystallization due to Mn segregation. Alternatively, ferrite is formed non-uniformly, and a band structure is likely to be formed during heat treatment of the hot-rolled steel sheet and further during annealing of the cold-rolled steel sheet. Therefore, by allowing to cool for 0.8 seconds or more after finish rolling, the formation of a band structure is suppressed, and the variation in Vickers hardness becomes 40 Hv or less. The upper limit of the cooling time is preferably less than 6.0 seconds. When the cooling time is 6.0 seconds or more, the effect of suppressing band tissue formation is saturated.

Preferably, after the finish rolling, it is allowed to cool for 1.2 to 4.0 seconds. By setting the cooling time within the above range, the austenite grains become more uniform, the structure of the steel sheet after annealing becomes more uniform, and the variation in Vickers hardness becomes 30 Hv or less. The cooling time may be 1.5 seconds or longer, 1.8 seconds or longer, 2.0 seconds or longer, 2.2 seconds or longer, or 2.5 seconds or longer. The cooling time may be 3.8 seconds or less, 3.5 seconds or less, 3.2 seconds or less, or 3.0 seconds or less.

After the above-mentioned cooling, cooling is performed at an average cooling rate of 30 ° C./sec or more. When the average cooling rate is less than 30 ° C./sec, cementite is generated non-uniformly at the block boundary of the hot-rolled plate and the former austenite grain boundary during martensitic transformation, so that the Vickers hardness variation is more than 40 Hv. Become. The upper limit of the average cooling rate is preferably 500 ° C./sec or less. The higher the average cooling rate, the more preferable, and if it is 500 ° C./sec or less, uneven cooling is less likely to occur and cold rollability is less likely to be lowered.

After finish rolling, it is cooled and wound at a temperature of less than 300 ° C. When wound at a temperature of 300 ° C. or higher, the structure of the hot-rolled steel sheet cannot be made into a full martensite structure, and in the heat treatment of the hot-rolled steel sheet and the annealing process of the cold-rolled steel sheet, Mn distribution and austenite reverse transformation, respectively. It becomes difficult to cause the problem efficiently.

(Heat treatment of hot-rolled steel sheet: Hold for 1 hour or more in the temperature range where the austenite phase fraction is 20 to 50%)
The obtained hot-rolled steel sheet is heat-treated for 1 hour or more in a temperature range in which the austenite phase fraction is 20 to 50% in the two-phase region of ferrite / austenite. Mn is distributed to the austenite phase by performing heat treatment in the temperature range where the austenite phase fraction is 20 to 50% in the temperature range of the two-phase region of the steel sheet having more than Ac1 and less than Ac3, and the austenite phase is formed. It can be stabilized to obtain high ductility. On the contrary, if the heat treatment is performed at a temperature where the austenite phase fraction is less than 20% or more than 50%, it becomes difficult to stabilize the austenite phase. Even when the heat treatment is performed in less than 1 hour, it becomes difficult to stabilize the austenite phase. By performing the heat treatment at a temperature at which the austenite phase fraction is 20 to 50% for 1 hour or more, the annealed steel sheet contains an austenite phase of 10% or more in area ratio, and CMnγ / CMnα can be increased. The temperature range in which the austenite phase fraction is 20 to 50% is determined by heating at a heating rate of 0.5 ° C./sec from room temperature in an offline preliminary experiment, depending on the composition of the steel sheet, and from the volume change during heating, austenite It can be obtained by measuring the phase fraction. In order to promote the stabilization of the austenite phase, the lower limit of the heat treatment holding time is preferably 3 hours or more, more preferably 4 hours or more. In order to further promote the stabilization of the austenite phase, the austenite phase fraction during the heat treatment may be 25% or more or 30% or more, or 45% or less or 40% or less. From the viewpoint of productivity, the upper limit of the heat treatment holding time is preferably 10 hours or less, more preferably 8 hours or less. The atmosphere of the heat treatment is not particularly limited, and may be, for example, an atmospheric atmosphere, an inert atmosphere, or a reducing atmosphere containing H _{2 and the} like.

After heat treatment is performed in a temperature range in which the austenite phase fraction is 20 to 50%, cooling is performed. As a result, the Mn distribution state obtained by the heat treatment can be maintained.

The hot-rolled steel sheet is pickled by a conventional method and then cold-rolled at a reduction rate of 30 to 70% (cold rolling rate) to obtain a cold-rolled steel sheet. When the reduction ratio of cold rolling is less than 30%, recrystallization becomes non-uniform, austenite phase is non-uniformly formed, and the variation in Vickers hardness of the annealed steel sheet becomes large. Further, when the rolling reduction ratio is more than 70%, fracture is likely to occur during cold rolling. The lower limit of the rolling reduction rate for cold rolling is preferably 40% or more. The upper limit of the rolling reduction rate for cold rolling is preferably 60% or less.

It is preferable to perform light rolling of about 0 to 5% before or after cold rolling before or after pickling to correct the shape because it is advantageous in terms of ensuring flatness. In addition, light rolling before pickling improves pickling properties, promotes removal of surface-concentrating elements, and has the effect of improving chemical conversion treatment properties and plating treatment properties.

(Annealing of cold-rolled steel sheet: Holds for 30 seconds or more in a temperature range where the austenite phase fraction is 20 to 65%)
The obtained cold-rolled steel sheet is held in a two-phase region of ferrite / austenite in a temperature range where the austenite phase fraction is 20 to 65% for 30 seconds or longer, preferably 1 minute or longer for annealing. Since Mn distribution has already been completed by the heat treatment of the hot-rolled steel sheet and Mn is concentrated in the portion that was the austenite phase during the heat treatment, this portion immediately becomes the austenite phase even if it is annealed for a short time. Easy and stable austenite phase can be obtained, and excellent ductility can be obtained by short-time annealing treatment. On the other hand, if the austenite phase is heat-treated at a temperature of less than 20% in the annealing, a sufficient austenite phase cannot be obtained, and if the heat treatment is performed at a temperature of more than 65%, a ferrite phase is not sufficiently generated, and further. , It becomes easy to transform from the austenite phase to the martensite phase. Further, if the annealing time is less than 30 seconds, recrystallization does not proceed sufficiently. The upper limit of the annealing time is not particularly specified, but from the viewpoint of productivity, it is preferably less than 15 minutes, and more preferably 5 minutes or less. In order to obtain a desired metallographic structure, the austenite phase fraction at the time of annealing may be 25% or more or 30% or more, or 60% or less, 55% or less, 50% or less or 40% or less. Preferably, it is annealed in a temperature range where the austenite phase fraction is 25-40%. Further, the annealing atmosphere may be any of an atmospheric atmosphere, an inert atmosphere, and a reducing atmosphere including H _{2 and the} like.

The difference between the temperature in the heat treatment before the cold rolling and the temperature in the annealing after the cold rolling is preferably equivalent to 15% or less, more preferably 10% or less in terms of the difference in the austenite phase fraction. Whichever is higher, the temperature in the heat treatment before the cold rolling and the temperature in the annealing after the cold rolling. By keeping the difference between the temperature in the heat treatment before cold rolling and the temperature in annealing after cold rolling within the above range, the austenite phase fraction in the heat treatment before cold rolling and the austenite phase in annealing after cold rolling Since the fraction can be brought close to each other, the austenite phase can be generated only in the portion where Mn is concentrated in the annealing after cold rolling. The temperature in the heat treatment before cold rolling and the temperature in annealing after cold rolling are the substantially maximum temperatures in the heat treatment profile.

(Cooling conditions after annealing: Cool to a temperature range of 100 to 530 ° C and hold for 10 to 1000 seconds in a temperature range of 100 to 530 ° C)
After maintaining the annealing temperature, the cold-rolled steel sheet is cooled to a temperature range of 100 to 530 ° C.

When the cooling stop temperature exceeds 530 ° C, it becomes difficult to temper the martensite phase or generate a bainite phase. As a result, a fresh martensite phase is likely to be formed in the metal structure, and after annealing. The bendability of the steel sheet is reduced. Further, when the cooling stop temperature is less than 100 ° C., strain due to martensitic transformation is likely to occur, it becomes difficult to maintain the flatness of the steel sheet, and the improvement of the efficiency of the continuous annealing line is hindered.

The average cooling rate after annealing is preferably 2 to 2000 ° C./sec. When the average cooling rate after annealing is 2 ° C./sec or more, excessive coarsening of the ferrite phase can be further suppressed. Further, when the average cooling rate is 2000 ° C./sec or less, the temperature distribution of the steel sheet after the cooling is stopped tends to be more uniform, and the flatness of the steel sheet can be sufficiently maintained.

After cooling to a temperature range of 100 to 530 ° C., the mixture is held in a temperature range of 100 to 530 ° C. for 10 to 1000 seconds. When the holding time in the temperature range of 100 to 530 ° C. is less than 10 seconds, C distribution to the austenite phase becomes difficult to proceed, and it becomes difficult to stably generate the austenite phase in the metal structure. In addition, it becomes difficult to temper the martensite phase or generate a bainite phase, and as a result, a fresh martensite phase is likely to occur in the metal structure, and the elongation and bendability of the steel sheet after annealing are reduced. It will be easier to do. The holding time in the temperature range of 100 to 530 ° C. is preferably 30 seconds or more. On the other hand, when the holding time exceeds 1000 seconds, the effect of the above action is saturated and the productivity of the continuous annealing line is lowered. Therefore, the holding time in the temperature range of 100 to 530 ° C. is 1000 seconds or less, preferably 300 seconds or less. The holding temperature may be different from the cooling stop temperature as long as it is within the range of 100 to 530 ° C., but the temperature difference between the holding temperature and the cooling stop temperature is preferably within 50 ° C., more preferably substantially. It is 0 ° C.

After holding in the temperature range of 100 to 530 ° C., the steel sheet is cooled to preferably 80 ° C. or lower, more preferably room temperature.

When the steel sheet is not plated, the cooling after annealing may be carried out as it is, preferably at 80 ° C. or lower, more preferably at room temperature. Further, when plating a steel sheet, it can be manufactured as follows.

When hot-dip galvanized steel sheet is manufactured by hot-dip galvanizing the surface of the steel sheet, the cold-rolled steel sheet is cooled to a temperature range of 100 to 530 ° C. after the annealing, and 10 to 1000 in a temperature range of 100 to 530 ° C. After holding for 2 seconds, the temperature is raised to 430 to 500 ° C., and then the cold-dip galvanized steel sheet is immersed in a hot-dip galvanizing bath to perform hot-dip galvanizing treatment. The conditions of the plating bath may be within the normal range. After the plating treatment, it may be cooled to room temperature.

When the surface of a steel sheet is hot-dip galvanized to produce an alloyed hot-dip galvanized steel sheet, the temperature is 450 to 620 ° C. after the steel sheet is hot-dip galvanized and before the steel sheet is cooled to room temperature. The hot dip galvanizing process is alloyed at temperature. The alloying treatment conditions may be within the usual range.

Skin pass rolling may be performed on the annealed steel sheet or the plated steel sheet. The rolling reduction of such skin pass rolling is preferably less than 0 to 5.0% (that is, including the case where skin pass rolling is not performed). The rolling reduction in the case of skin pass rolling is more than 0 to less than 5.0%.

By manufacturing the steel sheet as described above, the steel sheet according to the present embodiment can be obtained.

The steel sheet of the present disclosure will be described more specifically with reference to an example. However, the following examples are examples of the steel sheet of the present disclosure and its manufacturing method, and the steel sheet of the present disclosure and its manufacturing method are not limited to the aspects of the following examples.

1. 1. Production of Steel Sheet for Evaluation The steel having the chemical components shown in Table 1 was melted to obtain a slab having a thickness of 30 mm.

The obtained slab was hot-rolled at the finish rolling start temperature, cooling time, average cooling rate, and winding temperature shown in Table 2 to produce a 2.6 mm thick hot-rolled steel sheet. The obtained hot-rolled steel sheet is heat-treated at a temperature and holding time at which the austenite phase fraction is shown in Table 2, then pickled, and further cold-rolled at the cold rolling ratio shown in Table 2. A cold-rolled steel plate was made. The heat treatment of the hot-rolled steel sheet was carried out in a reducing atmosphere of 98% nitrogen and 2% hydrogen. The example in which the austenite phase fraction and the holding time of the heat treatment are not described means an example in which the heat treatment is not performed after the hot rolling and the cold rolling is performed as it is after the winding.

The obtained cold-rolled steel sheet was annealed at a temperature and holding time at which the austenite phase fraction was shown in Table 3. Annealing of the cold-rolled steel sheet was carried out in a reducing atmosphere of 98% nitrogen and 2% hydrogen. The heat treatment temperature of the hot-rolled steel sheet and the annealing temperature of the cold-rolled steel sheet were temperature differences corresponding to the austenite phase fraction difference shown in Table 3. After maintaining the annealing temperature, the steel sheet was cooled under the conditions of the cooling stop temperature shown in Table 3, the holding temperature after the cooling stop, and the holding time. The average cooling rate after maintaining the annealing temperature was 100 ° C./sec. An example in which the values of the cooling stop temperature, the holding temperature after the cooling stop, and the holding time are not described is the cooling after annealing, without stopping and holding the cooling in the temperature range of 100 to 530 ° C., and at room temperature as it is after annealing. It means an example of cooling to.

For some hot-dip cold-rolled steel sheets, after annealing, the cold-dip steel sheets are cooled and held at the cooling stop temperature, holding temperature and holding time shown in Table 3, and then heated to 460 ° C. It was held at that temperature for 10 seconds and immersed in a hot-dip galvanizing bath at 460 ° C. for 2 seconds to perform a hot-dip galvanizing treatment. The conditions of the plating bath are the same as those of the conventional one. When the alloying treatment described later was not performed, the product was cooled to room temperature at an average cooling rate of 10 ° C./sec after the hot-dip galvanizing treatment.

Some annealed cold-rolled steel sheets are subjected to hot-dip galvanizing treatment, then heated to 500 ° C. at 10 ° C./sec without cooling to room temperature, and held at 500 ° C. for 5 seconds for alloying treatment. After that, the mixture was cooled to room temperature at an average cooling rate of 10 ° C./sec.

The annealed cold-rolled steel sheet thus obtained was subjected to skin pass rolling with a reduction ratio of 0.5% to prepare the steel sheets of each example.

2. 2. Evaluation method X-ray diffraction measurement, microstructure observation, tensile test, and bending test were carried out on the annealed cold-rolled steel sheets of each example produced under the conditions shown in Tables 2 and 3, and the ferrite phase (α) and austenite phase were carried out. Area of (γ), annealed martensite phase (TM) (including bainite phase), fresh martensite phase (FM), and unrecrystallized ferrite phase (unrecrystallized α) relative to the ferrite phase. The rate, CMnγ / CMnα, and Vickers hardness variation ΔHv were evaluated. Each evaluation method is as described in the above embodiment. Further, the tensile strength (TS), elongation (EL), TS × EL, and bendability (Rmin) were evaluated for the annealed cold-rolled steel sheets of each example as described below.

(Test method for mechanical properties)
A JIS No. 5 tensile test piece was taken from a direction perpendicular to the rolling direction of the steel sheet, and a tensile test and an elongation test were performed to measure the tensile strength (TS) and the elongation (EL). The tensile test was carried out by the method specified in JIS-Z2241: 2011 using a JIS No. 5 tensile test piece. The elongation test was carried out by the method specified in JIS-Z2241: 2011 using a JIS No. 5 test piece having a parallel portion length of 50 mm.

Flexibility (Rmin) was evaluated by performing a bending test. In the bending test, the test piece having a width of 15 mm (direction of bending ridge), a length of 50 mm (direction of right angle to rolling), and the thickness of the annealed sample (direction of plate thickness) is set so that the bending ridge is in the rolling direction. The test piece was sampled from the central portion of the surface of the steel plate, and the test piece was pushed into the V block with a V-shaped punch having a tip angle of 90 degrees and a tip R of 2.5 times the plate thickness. After that, the bending ridgeline was observed, and when the ridgeline was not cracked, the bending property was regarded as "good". When the ridgeline was cracked, the bendability was regarded as "poor". Furthermore, for a steel plate that did not crack when pushed into the V block with a 2.5 times V type punch, another test piece was V block with a V type punch with a tip R 1.5 times the plate thickness. I pushed it into. After that, the bending ridgeline was observed, and when the ridgeline was not cracked, the bending property was regarded as "better".

3. 3. Evaluation Results Table 4 shows the evaluation results of the steel sheets manufactured under the conditions shown in Tables 2 and 3. In the example of the present invention, a steel sheet showing TS of 980 MPa or more, TS × EL of 24000 MPa ·% or more, and “good” Rmin was obtained.

Example No. Since 1-3, 6, 7, 9, 10, 13-15, 17, 20-22, 27, 30, 32 and 33 were produced according to a predetermined production method, a desired metal structure was obtained, which was excellent. It had characteristics (strength and elongation characteristics (TS × EL value), and bendability).

Example No. In No. 4, the austenite phase fraction of the heat treatment after hot rolling was low, and the desired metal structure could not be obtained. Therefore, sufficient strength and elongation characteristics (TS × EL value) could not be obtained. Example No. No. 5 contained an excessive amount of P, so that sufficient bendability could not be obtained. Example No. In No. 8, the austenite phase fraction at the time of annealing was low, and a desired metal structure could not be obtained. Therefore, sufficient strength and elongation characteristics (TS × EL value) could not be obtained. Example No. No. 11 had a low cold rolling ratio, could not suppress variations in Vickers hardness, and could not obtain sufficient bendability. Example No. In No. 12, the Mn content was insufficient and the desired metal structure could not be obtained, so that sufficient strength and elongation characteristics (TS × EL value) could not be obtained. Example No. In No. 16, the winding temperature was high, the values of CMnγ / CMnα were insufficient, and sufficient strength and elongation characteristics (values of TS × EL) could not be obtained. Example No. In No. 18, since cooling was not performed after annealing, a desired metal structure could not be obtained, and sufficient bendability could not be obtained. Example No. No. 19 had a high austenite phase fraction at the time of annealing, a desired metal structure could not be obtained, sufficient strength and elongation characteristics (TS × EL value), and sufficient bendability could not be obtained. Example No. In No. 23, the average cooling rate after allowing to cool after hot rolling was low, variation in Vickers hardness could not be suppressed, and sufficient bendability could not be obtained. Example No. No. 24 had a short holding time in cooling after annealing, a desired metal structure could not be obtained, sufficient strength and elongation characteristics (TS × EL value), and sufficient bendability could not be obtained. Example No. In No. 25, the cooling stop temperature and the holding temperature after annealing were high, a desired metal structure could not be obtained, and sufficient bendability could not be obtained. Example No. In No. 26, the C content was insufficient, the desired metal structure could not be obtained, and sufficient strength and elongation characteristics (TS × EL value) could not be obtained. Example No. In No. 28, the austenite phase fraction of the heat treatment after hot rolling was high, the desired metallographic structure could not be obtained, the values of CMnγ / CMnα were insufficient, and sufficient strength and elongation characteristics (values of TS × EL) were obtained. I couldn't get it. Example No. In No. 29, the holding time in the heat treatment after hot rolling was short, the desired metal structure could not be obtained, the values of CMnγ / CMnα were insufficient, and sufficient strength and elongation characteristics (values of TS × EL) were obtained. I couldn't. Example No. No. 31 had a short holding time in annealing, a desired metal structure could not be obtained, and sufficient strength and elongation characteristics (TS × EL value) could not be obtained. Example No. In No. 34, the cooling time after hot rolling was short, the variation in Vickers hardness could not be suppressed, and sufficient bendability could not be obtained. Example No. In No. 35, Mn was excessively contained, so that sufficient bendability could not be obtained. Example No. No. 36 did not undergo heat treatment after hot rolling, so that sufficient strength and elongation characteristics (TS × EL value) could not be obtained.

Claims (9)

The chemical composition is mass%,
C: More than 0.15 to less than 0.40%,
Si: 0.001 to less than 2.00%,
Mn: Over 2.50 to less than 4.20%,
sol. Al: 0.001 to less than 1.500%,
P: 0.030% or less,
S: 0.0050% or less,
N: Less than 0.050%,
O: Less than 0.020%,
Cr: 0 to 0.50%,
Mo: 0-2.00%,
W: 0-2.00%,
Cu: 0-2.00%,
Ni: 0-2.00%,
Ti: 0 to 0.300%,
Nb: 0 to 0.300%,
V: 0 to 0.300%,
B: 0 to 0.010%,
Ca: 0-0.010%,
Mg: 0-0.010%,
Zr: 0-0.010%,
REM: 0-0.010%,
Sb: 0 to 0.050%,
Sn: 0 to 0.050%,
Bi: 0 to 0.050%, and the balance: iron and impurities,
The metallographic structure at 1/8 of the thickness from the surface is the area ratio of austenite phase: 10% or more, total tempered martensite phase and bainite phase: 5% or more, ferrite phase: 35% or more, and fresh martensite. Phase: less than 15%
The area ratio of the unrecrystallized ferrite phase to the ferrite phase is 10 to 50%.
The CMnγ / CMnα, which is the ratio of the average Mn concentration CMnγ in the austenite phase to the average Mn concentration CMnα in the ferrite phase, is 1.20 or more, and the variation in Vickers hardness at the position 1/8 of the thickness from the surface. Steel plate of 40 Hv or less. When the chemical composition is mass%,
Cr: 0.01-0.50%,
Ti: 0.005 to 0.300%,
Nb: 0.005 to 0.300%,
V: 0.005 to 0.300%, and B: 0.0001 to 0.010%
The steel sheet according to claim 1, which contains one kind or two or more kinds selected from the group consisting of. The steel sheet according to claim 1 or 2, which has a hot-dip galvanized layer on the surface of the steel sheet. The steel sheet according to claim 1 or 2, which has an alloyed hot-dip galvanized layer on the surface of the steel sheet. The chemical composition is mass%,
C: More than 0.15 to less than 0.40%,
Si: 0.001 to less than 2.00%,
Mn: Over 2.50 to less than 4.20%,
sol. Al: 0.001 to less than 1.500%,
P: 0.030% or less,
S: 0.0050% or less,
N: Less than 0.050%,
O: Less than 0.020%,
Cr: 0 to 0.50%,
Mo: 0-2.00%,
W: 0-2.00%,
Cu: 0-2.00%,
Ni: 0-2.00%,
Ti: 0 to 0.300%,
Nb: 0 to 0.300%,
V: 0 to 0.300%,
B: 0 to 0.010%,
Ca: 0-0.010%,
Mg: 0-0.010%,
Zr: 0-0.010%,
REM: 0-0.010%,
Sb: 0 to 0.050%,
Sn: 0 to 0.050%,
Bi: 0 to 0.050%, and the balance: iron and steel as impurities, the finish rolling temperature is 1000 ° C or less, the cooling time after the finish rolling is 0.8 seconds or more, and the average cooling after the cooling. Hot rolling at a speed of 30 ° C./sec or higher and a winding temperature of less than 300 ° C. to obtain a hot-rolled steel sheet.
The hot-rolled steel sheet is heat-treated for 1 hour or more in a temperature range where the austenite phase fraction is 20 to 50%, and then pickled and cold-rolled to obtain a cold-rolled steel sheet.
The cold rolling ratio in the cold rolling is 30 to 70%.
The cold-rolled steel sheet is annealed by holding it for 30 seconds or more in a temperature range where the austenite phase fraction is 20 to 65%, and after the annealing temperature is maintained, it is cooled to a temperature range of 100 to 530 ° C. , Holding in a temperature range of 100 to 530 ° C. for 10 to 1000 seconds,
The method for manufacturing a steel sheet according to claim 1 . When the chemical composition is mass%,
Cr: 0.01-0.50%,
Ti: 0.005 to 0.300%,
Nb: 0.005 to 0.300%,
V: 0.005 to 0.300%, and B: 0.0001 to 0.010%
The method for producing a steel sheet according to claim 5, which contains one or more selected from the group consisting of. The method for producing a steel sheet according to claim 5 or 6, wherein the cooling time after the finish rolling is 1.2 to 4.0 seconds. The method for producing a steel sheet according to any one of claims 5 to 7, wherein hot-dip galvanizing treatment is performed after the annealing. The method for producing a steel sheet according to claim 8, wherein after performing the hot-dip galvanizing treatment, the hot-dip galvanizing treatment is performed in a temperature range of 450 to 620 ° C.