WO2019194250A1

WO2019194250A1 - Steel sheet and method for producing steel sheet

Info

Publication number: WO2019194250A1
Application number: PCT/JP2019/014877
Authority: WO
Inventors: 力岡本; 林　宏太郎; 武豊田
Original assignee: 日本製鉄株式会社
Priority date: 2018-04-03
Filing date: 2019-04-03
Publication date: 2019-10-10
Also published as: TW201942385A; JPWO2019194250A1; JP6683292B2; TWI688666B

Abstract

Provided is a steel sheet that has excellent production properties, a high yield point, excellent elongation characteristics, a small yield elongation, high strength, and a high concentration of Mn. The steel sheet contains, in mass%, more than 0.10% and less than 0.55% of C, 0.001% or more and less than 3.50% of Si, more than 4.00% and less than 9.00% of Mn, and 0.001% or more and less than 3.00% of soluble Al. The steel sheet is characterized in that the metal composition at a position at 1/8^th of the thickness from the surface in an L cross-section contains, by area ratio, 10% or more of an austenite phase and 10% or more of a ferrite phase, the area ratio of unrecrystallized ferrite within the ferrite phase is 30-70%, the ratio CMnγ/CMnα of the average Mn concentration CMnγ in the austentite phase and the average Mn concentration CMnα in the ferrite phase is 1.2 or more, and the average dislocation density of the ferrite phase is 4×10¹²/m² or more.

Description

Steel plate and method for manufacturing steel plate

The present disclosure relates to a steel sheet having a high content of Mn and a manufacturing method thereof.

In order to achieve both weight reduction and collision safety of automobile bodies and parts, etc., the strength of steel plates, which are these materials, is being increased. In general, when the strength of a steel plate is increased, the elongation is reduced and the formability of the steel plate is impaired. Therefore, in order to use a high-strength steel sheet as a member for automobiles, it is necessary to increase both strength and formability, which are contradictory properties.

In order to improve elongation, so-called TRIP (Transformation Induced Plasticity) steel using transformation-induced plasticity of retained austenite (residual γ) has been proposed so far (for example, Patent Document 1).

Residual austenite is obtained by concentrating C in austenite so that austenite does not transform into another structure even at room temperature. As a technique for stabilizing austenite, it has been proposed to contain carbide precipitation-inhibiting elements such as Si and Al in the steel sheet, and to enrich C in the austenite during the bainite transformation that occurs in the steel sheet during the manufacturing stage of the steel sheet. Yes. In this technique, if the C content contained in the steel plate is large, austenite can be further stabilized and the amount of retained austenite can be increased. As a result, a steel plate having excellent strength and elongation properties can be produced. However, when a steel plate is used for a structural member, welding is often performed on the steel plate. However, if the C content in the steel plate is large, the weldability is deteriorated, so that use as a structural member is limited. Therefore, it is desired to improve both the formability and strength of the steel sheet with a smaller C content.

Further, as a steel sheet having a retained austenite amount larger than that of the TRIP steel and having a ductility exceeding that of the TRIP steel, steel added with 3.5% or more of Mn (Patent Document 2), or addition of Mn exceeding 4.0% Steel (Non-patent Document 1) has been proposed. Since the steel contains a large amount of Mn, the effect of reducing the weight of the member used is also remarkable. However, the requirement for suppressing the yield elongation (YP-El) was not clear while enhancing the elongation characteristics and improving the yield point that most affected the impact characteristics.

Japanese Patent Laid-Open No. 5-59429 JP 2013-76162 A

Therefore, a steel sheet with a high Mn concentration having a high yield point, excellent elongation characteristics, small yield elongation, and high strength is desired.

In order to ensure a high yield point, excellent elongation characteristics, small yield elongation, and high strength in a steel sheet having a high Mn concentration, the inventors have made the austenite phase 10% or more in the steel sheet by area ratio. 10% or more of the ferrite phase, and the area ratio of non-recrystallized ferrite in the ferrite phase is 30% or more and 70% or less, and the ratio between the average Mn concentration CMnγ in the austenite phase and the average Mn concentration CMnα in the ferrite phase It was found that it is effective to set CMnγ / CMnα of 1.2 or more and the average dislocation density of the ferrite phase to 4.0 × 10 ¹² / m ² or more.

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

The gist of the present disclosure is as follows.
(1) In mass%,
C: more than 0.10% and less than 0.55%,
Si: 0.001% or more and less than 3.50%,
Mn: more than 4.00% and less than 9.00%,
sol. Al: 0.001% or more and less than 3.00%,
P: 0.100% or less,
S: 0.010% or less,
N: less than 0.050%,
O: less than 0.020%,
Cr: 0% or more and less than 2.00%,
Mo: 0% or more and 2.00% or less,
W: 0% to 2.00%,
Cu: 0% or more and 2.00% or less,
Ni: 0% or more and 2.00% or less,
Ti: 0% or more and 0.300% or less,
Nb: 0% or more and 0.300% or less,
V: 0% or more and 0.300% or less,
B: 0% or more and 0.010% or less,
Ca: 0% or more and 0.010% or less,
Mg: 0% or more and 0.010% or less,
Zr: 0% or more and 0.010% or less,
REM: 0% or more and 0.010% or less,
Sb: 0% or more and 0.050% or less,
Sn: 0% or more and 0.050% or less and Bi: 0% or more and 0.050% or less, with the balance being iron and impurities,
In the L cross section, the metal structure at 1/8 position of the thickness from the surface contains an austenite phase of 10% or more and a ferrite phase of 10% or more in area ratio,
Among the ferrite phases, the area ratio of non-recrystallized ferrite is 30% or more and 70% or less,
CMnγ / CMnα, which is the ratio of the average Mn concentration CMnγ in the austenite phase and the average Mn concentration CMnα in the ferrite phase, is 1.2 or more,
The average dislocation density of the ferrite phase is 4 × 10 ¹² / m ² or more.
(2) In mass%,
Cr: 0.01% or more and less than 2.00%,
Mo: 0.01% or more and 2.00% or less,
W: 0.01% or more and 2.00% or less,
Cu: 0.01% or more and 2.00% or less, and Ni: 0.01% or more and 2.00% or less, containing one or more selected from the group consisting of: The steel plate as described in 1).
(3) In mass%,
Ti: 0.005% or more and 0.300% or less,
Nb: 0.005% or more and 0.300% or less, and V: 0.005% or more and 0.300% or less, containing one or more selected from the group consisting of The steel plate according to 1) or (2).
(4) In mass%,
B: 0.0001% or more and 0.010% or less,
Ca: 0.0001% or more and 0.010% or less,
Mg: 0.0001% or more and 0.010% or less,
Zr: 0.0001% or more and 0.010% or less, and REM: 0.0001% or more and 0.010% or less, comprising one or more selected from the group consisting of The steel sheet according to any one of 1) to (3).
(5) In mass%,
Sb: 0.0005% or more and 0.050% or less,
Sn: 0.0005% or more and 0.050% or less, and Bi: 0.0005% or more and 0.050% or less, containing one or more selected from the group consisting of: The steel plate according to any one of 1) to (4).
(6) The metal structure further includes a tempered martensite phase of 5% or more by area ratio, and the martensite phase is limited to less than 15%, according to any one of the above (1) to (5) The described steel sheet.
(7) The steel sheet according to any one of (1) to (6) above, wherein a hot-dip galvanized layer is provided on the surface of the steel sheet.
(8) The steel sheet according to any one of (1) to (6) above, which has an alloyed hot-dip galvanized layer on the surface of the steel sheet.
(9) Hot rolling the steel having the component according to any one of (1) to (5) above to obtain a hot rolled steel sheet,
The hot-rolled steel sheet is subjected to heat treatment for 1 hour or more in a temperature range where the austenite phase fraction is 20% to 50%, and then subjected to pickling and cold rolling to form a cold-rolled steel sheet,
The cold rolling rate in the cold rolling is 30% or more and 70% or less,
Annealing the cold-rolled steel sheet in a temperature range where the austenite phase fraction is 20% to 50% and holding for 30 seconds to less than 15 minutes; and after the annealing, the rolling reduction is 5.0% or more Applying skin pass rolling, and maintaining the annealing temperature, cooling at an average cooling rate of 2 ° C./second to 2000 ° C./second and holding at a temperature range of 100 ° C. to 500 ° C. for 10 seconds to 1000 seconds. A method for producing a steel sheet characterized by the above.
(10) The method for producing a steel sheet according to (9) above, wherein the difference between the heat treatment temperature and the annealing temperature is equivalent to 15% or less in terms of a difference in austenite phase fraction. .
(11) The method for producing a steel sheet according to (9) or (10), wherein the hot rolling includes finish rolling at a temperature of 750 ° C. or more and 1000 ° C. or less and winding at a temperature of less than 300 ° C. .
(12) The method for producing a steel sheet according to any one of (9) to (11) above, wherein after the annealing, a hot dip galvanizing treatment is performed, and then the skin pass rolling is performed.
(13) The above (12), wherein after the hot dip galvanizing treatment is performed, the hot dip galvanizing is alloyed in a temperature range of 450 ° C. or higher and 620 ° C. or lower, and then the skin pass rolling is performed. The manufacturing method of the steel plate as described in 2.

According to the present disclosure, it is possible to provide a steel sheet having a high content Mn concentration and having a high yield point, excellent elongation characteristics, small yield elongation, and high strength.

FIG. 1 is a stress-strain curve of a steel plate. FIG. 2 is a mapping result showing a distribution state of Mn in a steel sheet heat-treated in a ferrite single-phase region and a ferrite / austenite two-phase region.

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

1. Chemical Composition The reason why the chemical composition of the steel sheet of the present disclosure is specified as described above will be described. In the following description, “%” representing the content of each element means mass% unless otherwise specified.

(C: more than 0.10% and less than 0.55%)
C is an extremely important element for increasing the strength of steel and securing austenite. In order to obtain a sufficient amount of austenite, a C content of more than 0.10% is required. On the other hand, if C is contained excessively, the weldability of the steel sheet is impaired, so the upper limit of the C content is less than 0.55%.

The lower limit of the C content is preferably 0.15% or more, more preferably 0.20% or more. When the lower limit of the C content is 0.15% or more, the strength of martensite and tempered martensite can be effectively increased when the metal structure includes martensite and tempered martensite. The upper limit value of the C content is preferably 0.40% or less, more preferably 0.35% or less. By setting the upper limit value of the C content within the above range, the toughness of the steel sheet can be further increased. .

(Si: 0.001% or more and less than 3.50%)
Si has the effect of suppressing the precipitation of cementite and promoting the austenite residue. Si is an element effective for strengthening tempered martensite when the metal structure contains tempered martensite, homogenizing the structure, and improving workability. In order to acquire the said effect, 0.001% or more of Si content is required. On the other hand, if Si is contained excessively, the plating properties and chemical conversion properties of the steel sheet are impaired, so the upper limit of the Si content is set to less than 3.50%.

The lower limit of the Si content is preferably 0.005% or more, more preferably 0.010% or more. By setting the lower limit of the Si content in the above range, the elongation characteristics of the steel sheet can be further improved. The upper limit of the Si content is preferably 2.00% or less, more preferably 1.00% or less.

(Mn: more than 4.00% and less than 9.00%)
Mn is an element that stabilizes austenite and improves hardenability. Moreover, in the steel plate of this indication, Mn is concentrated in austenite and austenite is stabilized more. In order to stabilize austenite at room temperature, more than 4.00% Mn is required. On the other hand, if the steel sheet contains Mn excessively, the weldability, hole expandability and ductility are impaired, so the upper limit of the Mn content is set to less than 9.00%.

The lower limit of the Mn content is preferably more than 4.20%, more preferably 4.50% or more, and further preferably 4.80% or more. The upper limit of the Mn content is preferably 8.50% or less, more preferably 8.00% or less. By making the lower limit value of Mn content in the above range, the fraction of stable austenite phase can be increased, and by making the upper limit value of Mn content in the above range, deterioration of toughness can be further suppressed. .

(Sol.Al: 0.001% or more and less than 3.00%)
Al is a deoxidizer and should be contained by 0.001% or more. In addition, Al has an effect of improving material stability in order to widen the two-phase temperature range during annealing. The effect increases as the Al content increases. However, excessive addition of Al leads to deterioration of surface properties, paintability, weldability, and the like. The upper limit of Al was made less than 3.00%.

Sol. The lower limit of the Al content is preferably 0.005% or more, more preferably 0.01% or more, and further preferably 0.02% or more. sol. The upper limit of the Al content is preferably 2.00% or less, more preferably 1.00% or less. sol. By making the lower limit value and the upper limit value of the Al content within the above ranges, the balance between the deoxidation effect and the material stability improvement effect, the surface properties, the paintability, and the weldability becomes better. As used herein, “sol.Al” means “acid-soluble Al”.

(P: 0.100% or less)
P is an impurity, and if the steel sheet contains P excessively, the toughness and weldability are impaired. Therefore, the upper limit of the P content is 0.100% or less. The upper limit of the P content is preferably 0.050% or less, more preferably 0.030% or less, and still more preferably 0.020% or less. Since the steel plate according to this embodiment does not require P, P may not be substantially contained, and the lower limit value 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 lower the P content, the better.

(S: 0.010% or less)
S is an impurity, and if the steel sheet contains excessive S, MnS stretched by hot rolling is generated, which causes deterioration of formability such as bendability and hole expansibility. Therefore, the upper limit of the S content is 0.010% or less. The upper limit of the S content is preferably 0.007% or less, more preferably 0.003% or less. Since the steel plate according to the present embodiment does not require S, S may not be substantially contained, and the lower limit value of the S content is 0%. The lower limit of the S content may be more than 0% or 0.001% or more, but the lower the S content, the better.

(N: less than 0.050%)
N is an impurity, and if the steel sheet contains 0.050% or more of N, the toughness is deteriorated. Therefore, the upper limit of the N content is 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 plate according to the present embodiment does not require N, N may not be substantially contained, and the lower limit value of the N content is 0%. The lower limit of the N content may be more than 0% or 0.005% or more, but the lower the N content, the better.

(O: less than 0.020%)
O is an impurity, and if the steel sheet contains 0.020% or more of O, ductility is deteriorated. Therefore, the upper limit of the O content is less than 0.020%. The upper limit of the O content is preferably 0.010% or less, more preferably 0.005% or less, and still more preferably 0.003% or less. Since the steel plate according to the present embodiment does not require O, it may not substantially contain O, and the lower limit value 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 lower the O content, the better.

The steel plate 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 You may contain 2 or more types. However, the steel sheet according to the present embodiment may not contain Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, Sn, and Bi. The lower limit of the content may be 0%.

(Cr: 0% or more and less than 2.00%)
(Mo: 0% to 2.00%)
(W: 0% to 2.00%)
(Cu: 0% to 2.00%)
(Ni: 0% to 2.00%)
Each of Cr, Mo, W, Cu, and Ni is not an essential element for the steel sheet according to the present embodiment, and thus may not be contained, and each content is 0% or more. However, since Cr, Mo, W, Cu, and Ni are elements that improve the strength of the steel sheet, they may be contained. In order to obtain the effect of improving the strength of the steel plate, the steel plate may contain 0.01% or more of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni. . However, if the steel sheet contains these elements in excess, surface flaws during hot rolling are likely to be generated, and further, the strength of the hot rolled steel sheet becomes too high and cold rolling properties may be deteriorated. Therefore, among the contents of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni, the upper limit value of the Cr content is less than 2.00%, and Mo , W, Cu, and Ni, the upper limit of each content shall be 2.00% or less.

(Ti: 0% to 0.300%)
(Nb: 0% to 0.300%)
(V: 0% to 0.300%)
Ti, Nb, and V do not have to be contained because they are not essential elements for the steel sheet according to the present embodiment, and each content is 0% or more. However, since Ti, Nb, and V are elements that generate fine carbides, nitrides, or carbonitrides, they are 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, the lower limit value of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V is preferably 0.005% or more. . On the other hand, when these elements are contained excessively, the strength of the hot-rolled steel sheet is excessively increased, and the cold rollability may be decreased. As for Nb, when the Nb content is 0.300% or less, a delay in recrystallization of the ferrite phase can be suppressed, and a desired structure can be obtained more stably. Therefore, it is preferable to set the upper limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V to 0.300% or less.

(B: 0% to 0.010%)
(Ca: 0% or more and 0.010% or less)
(Mg: 0% to 0.010%)
(Zr: 0% to 0.010%)
(REM: 0% to 0.010%)
B, Ca, Mg, Zr, and REM (rare earth metal) may not be contained because they are not essential elements for the steel sheet of the present disclosure, and each content is 0% or more. However, B, Ca, Mg, Zr, and REM improve the local elongation 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. 0.001% or more. However, an excessive amount of these elements deteriorates the workability of the steel sheet, so the upper limit of the content of each of these elements is 0.010% or less, and 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. As used herein, REM refers to a total of 17 elements of Sc, Y, and lanthanoid, and the REM content refers to the content when there is one REM, and the total content when there are two or more. Point to. REM is also supplied as misch metal, which is generally an alloy of a plurality of types of REM. For this reason, one or more individual elements may be added so that the REM content falls within the above range. For example, the REM content may be added in the form of misch metal. You may make it contain so that it may become this range.

(Sb: 0% to 0.050%)
(Sn: 0% to 0.050%)
(Bi: 0% to 0.050%)
Sb, Sn, and Bi do not need to be contained because they are not essential elements for the steel sheet of the present disclosure, and each content is 0% or more. However, Sb, Sn, and Bi suppress oxidizable elements such as Mn, Si, and / or Al in the steel sheet from diffusing into the steel sheet surface to form oxides, and improve the surface properties and plating properties 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, so the upper limit of the content of each of these elements is preferably 0.050% or less.

The balance is iron and impurities. Impurities are inevitably mixed from steel raw materials or scraps and / or from the steel making process, and elements allowed within a range not impairing the characteristics of the steel sheet according to the present embodiment are exemplified.

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

The metal structure at the 1/8 position (also referred to as 1/8 t portion) of the thickness from the surface of the steel sheet according to the present embodiment includes an austenite phase of 10% or more and a ferrite phase of 10% or more in terms of area ratio. The fraction of each structure varies depending on the heat treatment conditions, and affects the material such as yield point, strength, and elongation characteristics. Since the required material varies depending on, for example, parts for automobiles, heat treatment conditions may be selected as necessary to control the tissue fraction.

The area ratio of each structure can be measured by observing the microstructure at 1/8 position of the thickness from the surface of the steel sheet. The L cross section refers to a surface cut so as to pass through the central axis of the steel plate in parallel with the plate thickness direction and the rolling direction.

(Area ratio of austenite in metal structure of 1 / 8t part of steel plate: 10% or more)
In the steel sheet according to the present embodiment, it is important that the amount of austenite phase in the metal structure is in a predetermined range. Austenite is a structure that increases the ductility of a steel sheet by transformation-induced plasticity. Since austenite can be transformed into martensite by stretching, drawing, stretch flange processing, or bending with tensile deformation, it contributes to improving the strength of the steel sheet. In order to obtain these effects, the steel plate according to the present embodiment needs to contain an austenite phase of 10% or more in area ratio in the metal structure.

The area ratio of the austenite phase is preferably 15% or more, more preferably 20% or more, and further preferably 25% or more. When the area ratio of the austenite phase is 15% or more, the elongation characteristic is maintained to a higher strength.

The higher the area ratio of the austenite phase, the better the moldability. The upper limit of the area ratio of the austenite phase is not particularly defined, but is substantially 40%. The area ratio of the austenite phase can be measured by backscattered electron diffraction (EBSP: Electron Back Back Scattering Pattern). It is possible to measure the area ratio of the austenite phase by measuring at least 8 visual fields in a range of at least 100 μm × 100 μm at a pitch of 0.1 μm and averaging the measured values.

(Area ratio of ferrite in metal structure of 1 / 8t part of steel sheet: 10% or more)
Ferrite is an essential structure for ensuring ductility. The area ratio of the ferrite phase in the metal structure is 10% or more, preferably 15% or more. The upper limit of the area ratio of ferrite is not particularly specified, but is substantially less than 85%.

In the steel sheet according to the present embodiment, preferably, the metal structure at 1/8 position (also referred to as 1 / 8t part) of the thickness from the surface further contains a tempered martensite phase of 5% or more in area ratio, The martensite phase is limited to less than 15%. With such a metal structure, hole expandability can be obtained while ensuring strength.

(Area ratio of tempered martensite phase in metal structure of 1 / 8t part of steel sheet: 5% or more)
The tempered martensite phase is a hard phase and contributes to the improvement of the hole expansion property while ensuring the strength of the steel sheet. In order to ensure the strength while improving the hole expansion property, the area ratio in the metal structure of the tempered martensite phase is preferably 5% or more. When emphasizing the strength of the steel sheet, the area ratio of the tempered martensite phase is preferably 10% or more, more preferably 15% or more, and further preferably 20% or more. The upper limit of the area ratio of the tempered martensite phase is not specified, but is substantially less than 80%. Although the bainite phase may be included in the metal structure, the bainite phase has the same characteristics as the tempered martensite phase, so when the bainite phase is included in the metal structure, the area ratio of the tempered martensite phase is tempered. It is measured including the bainite phase in addition to the martensite phase.

(Area ratio of martensite phase in metal structure of 1 / 8t part of steel sheet: less than 15%)
The martensite (also called fresh martensite) phase is a hard phase that contains a lot of dislocations in its structure, but it is a different structure from the tempered martensite phase, and the hole expandability can be degraded. In order to ensure the property, the area ratio in the metal structure of the martensite phase is preferably less than 15%. Moreover, local elongation can be improved more by making the area ratio in the metal structure of a martensite phase less than 15%. The martensite phase may not be contained in the metal structure. That is, the area ratio in the metal structure of the martensite phase may be 0%. When especially hole expansibility and local elongation are required, the area ratio of the martensite phase is more preferably 10% or less, and further preferably 5% or less.

In the metal structure, the remainder other than the austenite phase, martensite phase, tempered martensite phase (including bainite phase), and ferrite phase can be a structure such as pearlite or cementite.

The area ratios of the ferrite phase, the martensite phase, and the tempered martensite phase are calculated from the structure observation with a scanning electron microscope (SEM). After the L section of the steel plate is mirror-polished, it is corroded with 3% nital (3% nitric acid-ethanol solution), and the metal structure at 1/8 position from the surface is observed with a scanning electron microscope with a magnification of 5000 times. The ferrite phase (including unrecrystallized ferrite) is discriminated as a gray background structure, and martensite is discriminated as a white structure. Tempered martensite appears white like martensite, but the one in which the substructure is confirmed in the crystal grains is determined to be tempered martensite.

Among the ferrite phases, the area ratio of non-recrystallized ferrite is 30% or more, preferably 40% or more. When the area ratio of non-recrystallized ferrite is within the above range, a steel plate having a high yield point can be obtained. If there is too much unrecrystallized ferrite, it will lead to a decrease in ductility, so the upper limit of the area ratio is 70%. The upper limit of the area ratio of non-recrystallized ferrite is more preferably 60%.

As for the area ratio of unrecrystallized ferrite, after discriminating ferrite phase grains as described above, EBSP measurement is performed on this area, and an area with a KAM (Kernel Average Misorientation) value of 1 ° or more is It is calculated by measuring as a crystalline ferrite structure.

CMnγ / CMnα, which is the ratio of the average Mn concentration CMnγ in the austenite phase and the average Mn concentration CMnα in the ferrite phase (all ferrite phases including unrecrystallized ferrite phase), is 1.2 or more, preferably 1.5 or more. . When CMnγ / CMnα is within the above range, sufficient Mn distribution to concentrate Mn is obtained at the place where it was an austenite phase during heat treatment, and a stable austenite phase can be obtained even after short-time annealing. Is obtained. On the other hand, if CMnγ / CMnα is less than 1.2, Mn distribution is not sufficient, and it becomes difficult to obtain an austenite phase by short-time annealing. Moreover, CMnγ / CMnα is preferably less than 2.0. By setting CMnγ / CMnα to less than 2.0, it is possible to suppress the austenite phase from becoming excessively stable and suppress the reduction in ductility improving effect.

CMnγ / CMnα can be measured by EBSP, SEM, and electron beam microanalyzer (EMPA). CMnγ / CMnα can be calculated by measuring the austenite phase and ferrite phase by EBSP and SEM, and measuring CMnγ and CMnα by EMPA.

The average dislocation density in the ferrite phase is 4.0 × 10 ¹² / m ² or more. When the average dislocation density in the ferrite phase is within the above range, the yield strength can be increased while the yield elongation is reduced, and a steel sheet having sufficient elongation can be obtained.

The average dislocation density in the ferrite phase is preferably 5 × 10 ¹³ / m ² or less. By setting the upper limit of the average dislocation density in the ferrite phase within the above range, deterioration of ductility can be suppressed and formability can be maintained.

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

The tensile strength (TS) of the steel sheet according to this embodiment is preferably 780 MPa or more, more preferably 980 MPa or more. This is because when a steel plate is used as an automobile material, the plate thickness is reduced by increasing the strength, thereby contributing to weight reduction. Moreover, in order to use the steel plate which concerns on this embodiment for press forming, it is desirable that elongation (El) is excellent. In that case, TS × El is preferably 25000 MPa ·% or more, more preferably 28000 MPa ·% or more.

The steel sheet according to this embodiment is also excellent in the yield point, and shows a stress-strain curve (SS curve) as shown as “A” in FIG. On the other hand, the heat treatment of the hot-rolled steel sheet in the method of the present disclosure, that is, only annealing for a short time on the cold-rolled steel sheet without performing heat treatment for 1 hour or more in the temperature range where the austenite phase fraction is 20% to 50%. The steel sheet obtained by applying the stress exhibits a stress-strain curve as indicated by “C” having a small elongation.

The steel sheet according to the present embodiment exhibits a stress-strain curve as shown as “A” in FIG. 1 by performing skin pass rolling with a rolling reduction of 5.0% or more. For reference, the stress-strain curve for a steel sheet produced under the same conditions except that skin pass rolling with a rolling reduction of 0.5% is shown as “B”. As will be described in detail later, a steel plate showing a stress-strain curve as shown in “A” has a high tensile strength although its yield point is slightly smaller than that of a steel plate showing a stress-strain curve as shown in “B”. Further, the yield elongation (YP-El) can be suppressed while maintaining the elongation. A steel sheet exhibiting a stress-strain curve as shown as “A” can be dispersed without localizing the strain than “B”. Thereby, local deformation can be suppressed.

The yield ratio YR (the yield point YP is gradually lowered by the tensile strength TS) of the steel sheet according to the present embodiment is preferably 0.68 or more, more preferably 0.70 or more, and further preferably 0.75 or more. Therefore, when using the steel plate which concerns on this embodiment as a raw material of a motor vehicle, it contributes to the improvement of a collision characteristic. Further, the yield elongation (YP-El) of the steel sheet according to this embodiment is preferably less than 2.5%, more preferably less than 1.5%. Moreover, the steel plate according to the present embodiment preferably has excellent hole expansibility (λ), and preferably exhibits a λ of 18% or more, more preferably 22% or more, and further preferably 24% or more. Further, the steel sheet according to the present embodiment preferably has a local elongation of 1.5% or more, more preferably 1.7% or more, and further preferably 2.0% or more, and exhibits good formability.

As described above, the steel sheet of the present disclosure has a sufficiently high yield point, high tensile strength, sufficient elongation characteristics, excellent formability, and low yield elongation. Ideal for parts applications. Furthermore, since the steel sheet of the present disclosure has a high Mn concentration, it contributes to the weight reduction of automobiles, and thus the industrial contribution is extremely remarkable.

3. Manufacturing method Next, the manufacturing method of the steel plate concerning this embodiment is explained.

The steel plate according to the present embodiment is obtained by melting a steel having the above-described chemical composition by a conventional method, casting it to produce a slab or a steel ingot, heating this, and subjecting it to hot rolling. The steel sheet is heat-treated in the ferrite / austenite two-phase region, pickled, cold-rolled at a cold rolling rate of 30% to 70%, and then annealed in the ferrite / austenite two-phase region for a short time. After the annealing, it is manufactured by performing skin pass rolling with a rolling reduction of 5.0% or more.

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 be performed by a normal continuous cold rolling line. In the method of the present disclosure, the annealing can be performed using a continuous annealing line, and thus the productivity is very excellent.

In order to obtain the metal structure of the steel sheet of the present disclosure, the following conditions, in particular, hot rolling conditions, heat treatment conditions for hot rolled steel sheets after hot rolling, cold rolling conditions, annealing conditions, and skin pass rolling 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-described chemical composition, the molten steel may be melted by a normal blast furnace method, and the raw material contains a large amount of scrap like steel produced by the electric furnace method. May be included. The slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting.

¡The above-mentioned slab or steel ingot is heated and hot-rolled to obtain a hot-rolled steel sheet. The temperature of the steel material used for hot rolling is preferably 1100 ° C. or higher and 1300 ° C. or lower. By setting the temperature of the steel material used for hot rolling to 1100 ° C. or higher, the deformation resistance during hot rolling can be further reduced. On the other hand, by reducing the temperature of the steel material used for hot rolling to 1300 ° C. or less, it is possible to suppress a decrease in yield due to an increase in scale loss. In the present specification, the temperature refers to the surface temperature of the central portion of the main surface of the steel material.

The time for holding in the temperature range of 1100 ° C. or higher and 1300 ° C. or lower, which is the preferable temperature range before hot rolling, is not particularly specified, but is preferably 30 minutes or longer in order to improve bendability. More preferably, the above is used. Moreover, in order to suppress an excessive scale loss, it is preferable to set it as 10 hours or less, and it is more preferable to set it as 5 hours or less. In addition, it is a case where direct feed rolling or direct rolling is performed, and it may be subjected to hot rolling as it is without performing heat treatment.

(Finish rolling and winding: finish rolling at 750 ° C. or more and 1000 ° C. or less and winding at less than 300 ° C.)
Finish rolling is performed in hot rolling. The finish rolling start temperature is preferably 750 ° C. or higher and 1000 ° C. or lower. By setting the finish rolling start temperature to 750 ° C. or higher, the deformation resistance during rolling can be reduced, and the structure can be easily controlled. On the other hand, by setting the finishing rolling start temperature to 1000 ° C. or lower, it is possible to prevent the coarsening of the structure in the hot-rolled state, and in addition to the subsequent structure control, the deterioration of the surface properties of the steel sheet due to grain boundary oxidation. Can be suppressed. After finishing rolling, cooling and winding are performed. The winding temperature is preferably less than 300 ° C. By winding at less than 300 ° C., the hot-rolled sheet structure can be made into a full martensite structure. 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 are efficiently performed, respectively. It is possible to wake up. That is, by performing finish rolling at the above temperature and winding at the above temperature, the tempered martensite phase having an area ratio of 5% or more is obtained while limiting the area ratio of the martensite phase to less than 15%. It is possible to obtain a steel sheet excellent in strength and hole expansibility.

(Heat treatment of hot-rolled steel sheet: Hold for 1 hour or more in a temperature range where the austenite phase fraction is 20% to 50%)
The obtained hot-rolled steel sheet is heat-treated for 1 hour or longer in a temperature range where the austenite phase fraction is 20% to 50%. By performing heat treatment in the temperature range where the austenite phase fraction is 20% to 50% within the temperature range of the steel sheet over Ac1 and less than Ac3, Mn is distributed to the austenite to stabilize the austenite. And high ductility can be obtained. By performing heat treatment at a temperature at which the austenite phase fraction is 20% to 50%, the metal structure at the 1/8 position of the thickness from the surface in the L cross section of the steel sheet after annealing has an area ratio of 10% or more austenite. Phases can be included. The temperature range in which the area ratio of the austenite phase is 20% to 50% is determined from the volume change during heating by heating at a heating rate of 0.5 ° C / sec from room temperature in an offline preliminary experiment, depending on the components of the steel sheet. It can be obtained by measuring the austenite phase fraction. The temperature of the heat treatment is preferably a temperature included in a temperature range where the austenite phase fraction is 25% to 40%. The lower limit of the heat treatment holding time is preferably 2 hours or more, more preferably 3 hours or more. The upper limit of the heat treatment holding time is preferably within 10 hours, more preferably within 8 hours.

FIG. 2 shows that after hot rolling, when heat treatment is performed at 650 ° C. for 6 hours in a temperature range in which the austenite phase fraction is 20% to 50% in the two-phase region, and for 15 minutes at 500 ° C. which is the single-phase region. The example of the mapping result showing the distribution state of Mn in the position of 1/8 of the thickness from the surface of the L cross section of the steel sheet when the heat treatment is performed is shown. In the steel plate heat-treated at 650 ° C., Mn is discharged from ferrite and Mn is concentrated in austenite (light-colored region). The Mn concentration in the portion that was ferrite at 650 ° C. becomes low.

¡After heat treatment in a temperature range where the austenite area ratio is 20% to 50%, cooling is performed. Thereby, 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 rolling reduction of 30% to 70% to obtain a cold-rolled steel sheet. If the rolling reduction of cold rolling is less than 30%, the grain size remains large and the austenite reverse transformation is delayed, so that a sufficient austenite phase cannot be obtained. On the other hand, if the rolling reduction is more than 70%, sufficient unrecrystallized ferrite cannot be obtained. The lower limit value of the cold rolling reduction is preferably 40% or more. The upper limit of the cold rolling reduction is preferably 60% or less.

It is preferable to modify the shape by performing mild rolling of more than 0% to about 5% before or after pickling before cold rolling because it is advantageous in terms of ensuring flatness. Moreover, pickling is improved by performing mild rolling before pickling, and removal of the surface concentrating element is promoted, and there is an effect of improving chemical conversion treatment and plating treatment.

(Annealing of cold-rolled steel sheet: Hold for 30 seconds or more and less than 15 minutes in a temperature range where the austenite phase fraction is 20% to 50%)
The obtained cold-rolled steel sheet is annealed while being held in a temperature range where the austenite phase fraction is 20% to 50% for 30 seconds to less than 15 minutes, preferably 1 minute to 5 minutes. The Mn distribution has already been completed by the heat treatment of the hot-rolled steel sheet, and Mn is concentrated in the austenite phase during the heat treatment, so this part immediately becomes an austenite phase even after short-time annealing. Easy and stable austenite can be obtained, and excellent ductility can be obtained by a short annealing treatment. On the other hand, if heat treatment is performed at a temperature at which the austenite phase fraction is less than 20% in the annealing, sufficient austenite is not obtained, and if heat treatment is performed at a temperature exceeding 50%, the austenite phase is easily transformed into a martensite phase. Become. Also, if the annealing time is less than 30 seconds, austenite cannot be obtained sufficiently. Preferably, annealing is performed in a temperature range where the austenite phase fraction is 25% to 40%.

Although not limited by theory, when the cold-rolled steel sheet is heat-treated at the low temperature and in a short time, recovery annealing mainly occurs and recrystallization hardly occurs. For this reason, it is considered that non-recrystallized ferrite remains and the dislocations remain without disappearing, and the yield point before skin pass rolling is increased. According to the method of the present disclosure, in addition to improvement of tensile strength (TS) × elongation (El) due to Mn distribution before skin pass rolling, a steel sheet having a high yield point (YP) due to non-recrystallized ferrite is obtained. Can do.

The difference between the temperature in the heat treatment before cold rolling and the temperature in the annealing after cold rolling is preferably equivalent to 15% or less, more preferably equivalent to 10% or less in terms of the difference in austenite phase fraction. Either the temperature in the heat treatment before cold rolling or the temperature in the annealing after cold rolling may be higher. By making the difference between the temperature in the heat treatment before cold rolling and the temperature in the annealing after cold rolling within the above range, the austenite phase fraction in the heat treatment before cold rolling and the austenite phase in the annealing after cold rolling Since the fraction can be made closer, austenite can be generated only at the location where Mn is concentrated in the annealing after cold rolling. The temperature in the heat treatment before cold rolling and the temperature in the annealing after cold rolling are substantially the maximum temperatures in the heat treatment profile.

(Cooling conditions after annealing: Cool at an average cooling rate of 2 ° C./second or more and 2000 ° C./second or less and hold at a temperature range of 100 ° C. or more and 500 ° C. or less for 10 seconds or more and 1000 seconds or less)
After the annealing temperature is maintained, cooling is performed to an average cooling rate of 2 ° C./second to 2000 ° C./second to a temperature range of 100 ° C. to 500 ° C. By setting the average cooling rate after annealing to 2 ° C./second or more, grain boundary segregation can be suppressed and bendability can be improved. On the other hand, by setting the average cooling rate to 2000 ° C./second or less, the steel plate temperature distribution after the cooling is stopped becomes uniform, so that the flatness of the steel plate can be improved.

冷却 By setting the cooling stop temperature to 500 ° C or lower, it is possible to suppress segregation at the grain boundaries and improve bendability. On the other hand, by setting the cooling stop temperature to 100 ° C. or higher, the generation of strain associated with martensitic transformation can be suppressed, and the flatness of the steel sheet can be improved.

After the cooling, the temperature is maintained at 100 ° C. to 500 ° C. for 10 seconds to 1000 seconds. By cooling to a temperature range of 100 ° C. or more and 500 ° C. or less, martensite is generated, and the subsequent holding causes martensite self-tempering. By setting the holding time in the temperature range of 100 ° C. or more and 500 ° C. or less to 10 seconds or more, C distribution to austenite is sufficiently advanced, and austenite can be stably generated in the structure before the final heat treatment, As a result, it is possible to suppress the formation of massive austenite in the structure after the final heat treatment, and to suppress fluctuations in strength characteristics. On the other hand, even if the holding time is longer than 1000 seconds, the effect of the above action is saturated and the productivity is lowered, so the holding time in the temperature range of 100 ° C. to 500 ° C. is 1000 seconds or less. , Preferably 300 seconds or less, more preferably 180 seconds or less.

The efficiency of the continuous annealing line can be improved by setting the holding temperature to 100 ° C. or higher. On the other hand, by setting the holding temperature to 500 ° C. or lower, grain boundary segregation can be suppressed and bendability can be improved.

After the cooling, it is preferable to cool the steel plate to room temperature.

Skin pass rolling with a rolling reduction of 5.0% or more is performed on the annealed steel sheet. When hot dip galvanizing or alloying hot dip galvanizing is performed on the surface of the steel plate, skin pass rolling with a rolling reduction of 5.0% or more is performed on the steel plate after plating. From the viewpoint of lowering ductility or increasing roll load, skin pass rolling is generally less than 0.5% in the production of conventional steel sheets, but in this embodiment, the rolling reduction is 5.0% or more. By performing skin pass rolling, many dislocations are introduced and work hardening is performed, so that the yield elongation of the steel sheet is increased while reducing the yield elongation of the steel sheet compared to the case of performing normal skin pass (less than 5.0% reduction). Can be adjusted. In the present embodiment, by performing skin pass rolling at a rolling reduction of 5.0% or more, local elongation can be improved while realizing a decrease in yield elongation due to an increase in fresh martensite.

By performing skin pass rolling with a rolling reduction of 5.0% or more, the elongation and yield points are slightly lower than “B” shown as a reference, as shown by “A” in FIG. It is possible to obtain a steel sheet that exhibits a stress-strain curve with high strength and low yield elongation. A steel plate having this characteristic has a large initial absorption energy at the time of collision and can absorb more energy.

By performing skin pass rolling with a rolling reduction of 5.0% or more, the average dislocation density of the ferrite phase can be made 4.0 × 10 ¹² / m ² or more. The average dislocation density is the total dislocation density of fixed dislocations and movable dislocations in the ferrite phase (all ferrite phases including non-recrystallized ferrite phases).

By setting the average dislocation density to 4.0 × 10 ¹² / m ² or more, the elongation and yield point are slightly lower than “B” as shown in FIG. 1, but high tensile strength is maintained. In addition, a steel sheet exhibiting a stress-strain curve with a low yield point elongation can be obtained. A steel plate having such characteristics has a large initial absorption energy and absorbs more energy because it can suppress the localization of deformation at the time of collision while keeping the yield point (YP) at a high level. Can do.

Preferably, the reduction rate of skin pass rolling is set to 10.0% or less. Thereby, sufficient moldability can be secured. By making the rolling reduction of skin pass rolling 10.0% or less, the average dislocation density of the ferrite phase can be made 5 × 10 ¹³ / m ² or less.

The average dislocation density of the ferrite phase can be measured by conventional measurement using a TEM (transmission electron microscope).

The cooling after the annealing may be performed to room temperature as it is when the steel plate is not plated. Moreover, when plating on a steel plate, it can manufacture as follows.

When producing hot dip galvanized steel sheets by hot dip galvanizing on the surface of the steel sheet, the cooling after the annealing is stopped in a temperature range of 430 to 500 ° C., and then the cold rolled steel sheet is immersed in a hot dip galvanizing bath. Then, hot dip galvanizing is performed. The conditions for the plating bath may be within the normal range. What is necessary is just to cool to room temperature after a plating process.

In the case of producing an alloyed hot dip galvanized steel sheet by subjecting the surface of the steel sheet to galvannealed steel sheet, after the hot dip galvanizing treatment is performed on the steel sheet, before the steel sheet is cooled to room temperature, the temperature is changed to 450 to 620 ° C. An alloying treatment of hot dip galvanizing is performed at a temperature. The alloying treatment conditions may be within a normal range.

By manufacturing a steel plate as described above, the steel plate according to this 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 the manufacturing method thereof, and the steel sheet of the present disclosure and the manufacturing method thereof are not limited to the following examples.

1. Production of Steel Plate for Evaluation Steel having chemical components shown in Table 1 was melted in a converter and a slab having a thickness of 245 mm was obtained by continuous casting.

The obtained slab was hot-rolled at a finishing temperature and a winding temperature shown in Table 2 to produce a 2.6 mm thick hot-rolled steel sheet. The obtained hot-rolled steel sheet is subjected to a heat treatment at a temperature and a holding time at an austenite phase fraction shown in Table 2, then pickled, and further subjected to cold rolling at a cold rolling rate shown in Table 2, A cold rolled steel sheet having a thickness of 1.2 mm was made. The heat treatment of the hot-rolled steel sheet was performed in a reducing atmosphere of 98% nitrogen and 2% hydrogen. The alphabetical characters in the “Steel” column shown in Tables 2 to 7 below correspond to the steel type symbols shown in the “Steel” column of Table 1, respectively.

The obtained cold-rolled steel sheet was annealed at a temperature and holding time at which the austenite phase fraction shown in Table 2 was obtained. The cold rolled steel sheet was annealed 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 a temperature difference corresponding to the austenite phase fraction shown in Table 2.

After holding the annealing temperature, the steel sheet was cooled under the conditions of average cooling rate, cooling stop temperature, and holding time shown in Table 2. The example which does not describe the numerical values of the cooling stop temperature and the holding time means an example of cooling after annealing, cooling is not performed in the temperature range of 100 ° C. or higher and 500 ° C. or lower, and is cooled to room temperature as it is after annealing. To do.

For some annealed cold-rolled steel sheets, after annealing, cooling after annealing was stopped at 400 ° C., and the cold-rolled steel sheets were immersed in a 400 ° C. hot-dip galvanizing bath for 2 seconds to perform hot dip galvanizing treatment Went. The conditions of the plating bath are the same as the conventional one. In the case where the alloying process described later was not performed, the sample was cooled to room temperature at an average cooling rate of 10 ° C./second after maintaining at 400 ° C.

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

The annealed cold-rolled steel sheet thus obtained was subjected to skin pass rolling with a rolling reduction of 6.0% to produce a steel sheet.

Separately from the steel sheets produced under the conditions shown in Table 2, as shown in Table 3, skin pass rolling was performed at 3.0%, 9.0% and 12.0% reduction ratios (SPM) to produce steel sheets. did. However, numbers 102, 105, 108, 111, and 115 in Table 3 are for reference and are the same as

numbers

2, 5, 10, 13, and 15 in Table 2, respectively. The same applies to Tables 5 and 6 below.

Separately from the steel sheets produced under the conditions shown in Tables 2 and 3, steel sheets (invention examples and comparative examples) were produced under the conditions shown in Table 4.

2. Evaluation method The steel sheet obtained in each example of Table 2 and Table 3 was subjected to microstructure observation, tensile test, elongation test, and hole expansion test, and ferrite phase (α), austenite phase (γ), and tempered. Martensite phase (TM), martensite phase (FM), unrecrystallized ferrite (non-crystalline α) area ratio, CMnγ / CMnα, yield point (YP), tensile strength (TS), elongation ( El), hole expansibility (λ), yield elongation (YP-El), yield ratio (YR), and TS × El were evaluated. The method of each evaluation is as follows. About the steel plate obtained by each example of Table 4, in addition to the test and evaluation which were performed about the steel plate obtained by each example of Table 2 and Table 3, the local elongation test was implemented.

The area ratio of the austenite phase was measured using backscattered electron diffraction (EBSP: Electron Back Scattering pattern). The L cross-section obtained by cutting the steel plate in parallel with the plate thickness direction and the rolling direction was subjected to mirror polishing by diamond buffing and alumina polishing, and then the microstructure was revealed with 3% nital, and 100 μm at 1/8 position from the surface. The x100 μm range was measured with 8 visual fields at a pitch of 0.1 μm, and the measured values were averaged.

The area ratios of the ferrite phase, the tempered martensite phase, and the martensite phase were calculated from structural observation with a scanning electron microscope (SEM). About the microstructure which carried out the said mirror surface polishing and the nital processing, the range of 0.2 mm x 0.3 mm in the 1/8 position from the surface of the center of the width direction of a steel plate with a scanning electron microscope of 5000 times is carried out at intervals of 0.5 mm. Two visual fields were observed. The area ratio was calculated by measuring 400 to 500 points using the JIS-G0555 point calculation method.

The ferrite phase (including non-recrystallized ferrite) was identified as a gray background structure, and martensite was identified as a white structure. Tempered martensite appears white like martensite, but the one in which the substructure was confirmed in the crystal grains was judged to be tempered martensite.

The area ratio of non-recrystallized ferrite is determined as follows. 100 to 150 ferrite phase grains are discriminated, and EBSP measurement is performed on the discriminated crystal grains to calculate the KAM value of each crystal grain. And a region of 1 ° or more was calculated as an unrecrystallized ferrite structure.

CMnγ / CMnα was measured by EBSP, SEM, and electron beam microanalyzer (EMPA). For the microstructure subjected to the mirror polishing and the nital treatment, 10 points each of austenite phase and ferrite phase were selected using EBSP and SEM, and the average values of 10 points were measured as CMnγ and CMnα, respectively, using EMPA with an acceleration voltage of 15 kV. CMnγ / CMnα was calculated.

(Test method for mechanical properties)
The yield point (YP) and the yield elongation (YP-El) were measured by the methods specified in JIS-Z2241. The yield point means a falling yield point when there is a yield phenomenon, and 0.2% proof stress when there is no yield phenomenon.

A JIS No. 5 tensile test piece was taken from the direction perpendicular to the rolling direction of the steel sheet, the tensile strength (TS) and the elongation (El) were measured, and TS × uEL was calculated. The tensile test was performed by the method prescribed | regulated to JISZ2241: 2011 using the JIS5 tension test piece. The elongation was measured by a method defined in JIS Z2241: 2011 using a JIS No. 5 test piece having a parallel part length of 60 mm and a standard distance of 50 mm as a reference for measuring strain. Uniform elongation is the elongation (strain measured between gauge points) obtained until the maximum test strength (TS) is reached. The measurement of local elongation was calculated by subtracting the value of elongation at the maximum load point (uniform elongation) from the value of elongation (total elongation) when the fractured specimens were butted.

The hole expansibility (λ) was evaluated by the following method. A test piece for hole expansion of 100 mm × 100 mm was cut out from the direction perpendicular to the rolling direction of the steel sheet, and a hole with a diameter of 10 mm was punched in the center with a clearance of 12.5%. The clearance is defined as Clearance (%) = (Die hole diameter−Die diameter) / (Steel plate thickness) / 2 × 100. The test piece with a hole was extruded with a conical punch, the hole was widened, and the test was stopped when a crack progressed inside the hole edge, and the hole diameter d (unit: mm) was measured. The hole expansion ratio λ (%) was calculated from the equation: λ = 100 × (d−10) / 10.

3. Evaluation Results Table 5 shows the evaluation results for the steel plates produced under the conditions shown in Table 2. A steel plate exhibiting TS × El of 25000 MPa ·% or more, a hole expansion ratio (λ) of 18% or more, and a yield ratio (YR) of 0.68 or more, a high yield point, excellent elongation characteristics, a small yield elongation, and Evaluation was made as a steel sheet having high strength.

Table 6 shows the evaluation results for the steel sheets produced under the conditions shown in Table 3. A steel plate (invention example) subjected to skin pass rolling at a reduction rate (SPM) of 9.0% and 12.0% is a steel plate (invention example) subjected to skin pass rolling at a reduction rate (SPM) of 6.0%. Similarly, very low yield elongation (YP-El) was exhibited while maintaining high yield point, good elongation characteristics, and high strength. A steel plate subjected to skin pass rolling at a reduction rate (SPM) of 3.0% (comparative example) is yielded compared with a steel plate subjected to skin pass rolling at a reduction rate (SPM) of 6.0% (invention example). The point (YP) and yield ratio (YR) were low, indicating a large yield elongation (YP-El).

Table 7 shows the evaluation results for the steel sheets produced under the conditions shown in Table 4. By producing a steel sheet under the conditions shown in Table 4, while exhibiting a local elongation of 1.5% or more, TS × El of 25000 MPa ·% or more, a hole expansion rate (λ) of 18% or more, and 0.68 or more A steel sheet having a yield ratio (YR) of 1 was obtained.

Claims

% By mass
C: more than 0.10% and less than 0.55%,
Si: 0.001% or more and less than 3.50%,
Mn: more than 4.00% and less than 9.00%,
sol. Al: 0.001% or more and less than 3.00%,
P: 0.100% or less,
S: 0.010% or less,
N: less than 0.050%,
O: less than 0.020%,
Cr: 0% or more and less than 2.00%,
Mo: 0% or more and 2.00% or less,
W: 0% to 2.00%,
Cu: 0% or more and 2.00% or less,
Ni: 0% or more and 2.00% or less,
Ti: 0% or more and 0.300% or less,
Nb: 0% or more and 0.300% or less,
V: 0% or more and 0.300% or less,
B: 0% or more and 0.010% or less,
Ca: 0% or more and 0.010% or less,
Mg: 0% or more and 0.010% or less,
Zr: 0% or more and 0.010% or less,
REM: 0% or more and 0.010% or less,
Sb: 0% or more and 0.050% or less,
Sn: 0% or more and 0.050% or less and Bi: 0% or more and 0.050% or less, with the balance being iron and impurities,
In the L cross section, the metal structure at 1/8 position of the thickness from the surface contains an austenite phase of 10% or more and a ferrite phase of 10% or more in area ratio,
Among the ferrite phases, the area ratio of non-recrystallized ferrite is 30% or more and 70% or less,
CMnγ / CMnα, which is the ratio of the average Mn concentration CMnγ in the austenite phase and the average Mn concentration CMnα in the ferrite phase, is 1.2 or more,
The average dislocation density of the ferrite phase is 4 × 10 12 / m 2 or more.
% By mass
Cr: 0.01% or more and less than 2.00%,
Mo: 0.01% or more and 2.00% or less,
W: 0.01% or more and 2.00% or less,
It contains one or more selected from the group consisting of Cu: 0.01% or more and 2.00% or less, and Ni: 0.01% or more and 2.00% or less. The steel plate according to 1.
% By mass
Ti: 0.005% or more and 0.300% or less,
It contains one or more selected from the group consisting of Nb: 0.005% or more and 0.300% or less, and V: 0.005% or more and 0.300% or less. The steel plate according to 1 or 2.
% By mass
B: 0.0001% or more and 0.010% or less,
Ca: 0.0001% or more and 0.010% or less,
Mg: 0.0001% or more and 0.010% or less,
Zr: 0.0001% or more and 0.010% or less, and REM: 0.0001% or more and 0.010% or less, comprising one or more selected from the group consisting of: The steel sheet according to any one of 1 to 3.
% By mass
Sb: 0.0005% or more and 0.050% or less,
It contains one or more selected from the group consisting of Sn: 0.0005% or more and 0.050% or less, and Bi: 0.0005% or more and 0.050% or less. The steel sheet according to any one of 1 to 4.
The steel sheet according to any one of claims 1 to 5, wherein the metal structure further contains a tempered martensite phase of 5% or more by area ratio, and the martensite phase is limited to less than 15%.
The steel plate according to any one of claims 1 to 6, further comprising a hot-dip galvanized layer on the surface of the steel plate.
The steel sheet according to any one of claims 1 to 6, wherein the steel sheet has an alloyed hot-dip galvanized layer on the surface thereof.
Applying hot rolling to the steel having the component according to any one of claims 1 to 6 to obtain a hot-rolled steel sheet,
The hot-rolled steel sheet is subjected to heat treatment for 1 hour or more in a temperature range where the austenite phase fraction is 20% to 50%, and then subjected to pickling and cold rolling to form a cold-rolled steel sheet,
The cold rolling rate in the cold rolling is 30% or more and 70% or less,
Annealing the cold-rolled steel sheet in a temperature range where the austenite phase fraction is 20% to 50% and holding for 30 seconds to less than 15 minutes; and after the annealing, the rolling reduction is 5.0% or more Applying skin pass rolling, and maintaining the annealing temperature, cooling at an average cooling rate of 2 ° C./second to 2000 ° C./second and holding at a temperature range of 100 ° C. to 500 ° C. for 10 seconds to 1000 seconds. A method for producing a steel sheet characterized by the above.
The method for producing a steel sheet according to claim 9, wherein a difference between the temperature of the heat treatment and the temperature of the annealing is equivalent to 15% or less in terms of a difference in austenite phase fraction.
The method for producing a steel sheet according to claim 9 or 10, wherein the hot rolling includes finish rolling at a temperature of 750 ° C or higher and 1000 ° C or lower and winding at a temperature of less than 300 ° C.
The method for producing a steel sheet according to any one of claims 9 to 11, wherein after the annealing, a hot dip galvanizing treatment is performed, and then the skin pass rolling is performed.
The steel sheet according to claim 12, wherein after the hot dip galvanizing treatment is performed, the hot dip galvanizing is alloyed in a temperature range of 450 ° C or higher and 620 ° C or lower, and then the skin pass rolling is performed. Manufacturing method.