WO2019194251A1

WO2019194251A1 - Steel sheet and method for producing steel sheet

Info

Publication number: WO2019194251A1
Application number: PCT/JP2019/014878
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: JP6683291B2; TWI688664B; JPWO2019194251A1; TW201942383A

Abstract

Provided is a steel sheet that has good collision characteristics, excellent elongation characteristics, excellent hole expandability, 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 4.20% 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, 5% or more of a tempered martensite phase, and 10% or more of a ferrite phase, the martensite phase is limited to less than 15%, the area ratio of unrecrystallized ferrite within the ferrite phase is 30-70%, and 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.

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 addition, since many automotive body parts made of steel plates are formed by press working, high-tensile steel sheets used for body parts are required to have excellent press formability. Therefore, as a mechanical characteristic of a steel plate, it is required to have a high hole expanding property (stretch flange formability) while having high strength.

In order to improve workability and formability, so-called TRIP (Transformation Induced Plasticity) steel using transformation-induced plasticity of retained austenite (residual γ) has been proposed (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), and 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 requirements for improving the impact characteristics while enhancing the elongation characteristics were not clear.

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

Therefore, a steel sheet having a high content of Mn having good impact characteristics, excellent elongation characteristics, excellent hole expansibility, and high strength is desired.

In order to ensure excellent elongation characteristics, good collision characteristics, excellent hole expansibility, and high strength in a steel sheet having a high Mn concentration, the inventors have provided an austenitic phase in the steel sheet at an area ratio. 10% or more, tempered martensite phase 5% or more, ferrite phase 10% or more, martensite phase is limited to less than 15%, and the area ratio of unrecrystallized ferrite in ferrite phase is 30% or more It was found that it is effective to set 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, to 1.2 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 4.20% 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 of the thickness from the surface contains an austenite phase of 10% or more, an tempered martensite phase of 5% or more, and a ferrite phase of 10% or more in terms of area ratio, The site phase is limited to less than 15%,
Among the ferrite phases, the area ratio of non-recrystallized ferrite is 30% or more and 70% or less,
CMnγ / CMnα, which is a 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.
(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 steel sheet according to any one of (1) to (5), wherein an average dislocation density of the ferrite phase is 2 × 10 ¹² / m ² or more and less than 4 × 10 ¹² / m ² .
(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) A steel having the component according to any one of (1) to (5) above is subjected to hot rolling at a finishing temperature of 1000 ° C. or less and a winding temperature of less than 300 ° C. To do,
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 or more and 15 minutes or less, and after holding the temperature of the annealing, an average cooling rate of 2 ° C. / A method for producing a steel sheet, wherein the steel sheet is cooled at a temperature in a range from 100 ° C. to 2000 ° C./second and held in a temperature range from 100 ° C. to 500 ° C. for 10 seconds to 1000 seconds.
(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 hot dip galvanizing is performed after the annealing.
(12) The method for producing a steel sheet according to (11) above, 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. .
(13) The method for producing a steel sheet according to (9) or (10) above, wherein after the annealing, skin pass rolling is performed at a rolling reduction of 1.0% or more and less than 5.0%.
(14) The method for producing a steel sheet according to (11) above, wherein after the hot dip galvanizing treatment, skin pass rolling with a rolling reduction of 1.0% or more and less than 5.0% is performed.
(15) The method for producing a steel sheet according to (12) above, wherein after the alloying treatment, skin pass rolling with a rolling reduction of 1.0% or more and less than 5.0% is performed.

According to the present disclosure, it is possible to provide a steel sheet having a high content of Mn having good impact characteristics, excellent elongation characteristics, excellent hole expanding properties, 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 value of the C content is 0.15% or more, the strength of martensite and tempered martensite can be effectively increased. 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 is an element effective for strengthening tempered martensite, homogenizing the structure, and improving workability. Si also has the effect of suppressing the precipitation of cementite and promoting the retention of austenite. 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 4.20% 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. To stabilize austenite at room temperature, more than 4.20% 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 value of the Mn content is preferably more than 4.50%, more 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 the effect of increasing the material stability in order to widen the temperature range of the two-phase region 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 / 8t part) of the thickness from the surface of the steel sheet according to the present embodiment is an area ratio of 10% or more austenite phase, 5% or more tempered martensite phase, and It contains 10% or more of the ferrite phase, and the martensite phase is limited to less than 15%. The fraction of each structure varies depending on the heat treatment conditions and affects materials such as yield point, strength, elongation characteristics, and hole expandability. 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 martensite phase in metal structure of 1 / 8t part of steel sheet: less than 15%)
The martensite (also referred to as fresh martensite) phase is a hard phase containing many dislocations in its structure, and is an effective phase for obtaining the strength of the steel sheet. However, the area ratio in the metal structure of the martensite phase is set to less than 15% in order to remarkably deteriorate the hole expandability. Moreover, local elongation can be improved 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 preferably 10% or less, and more preferably 5% or less.

(Area ratio of tempered martensite phase in metal structure of 1 / 8t part of steel sheet: 5% or more)
The tempered martensite phase is also a hard phase, but has a different structure from the martensite phase, and contributes to the improvement of the hole expanding property while ensuring the strength of the steel sheet. In order to achieve both strength and hole expandability, the area ratio in the metal structure of the tempered martensite phase needs to be 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 ferrite phase in metal structure of 1 / 8t part of steel sheet: 10% or more)
The ferrite phase 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 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.

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 28000 MPa ·% or more, more preferably 30000 MPa ·% or more, and further preferably 32000 MPa ·% or more. Further, the steel sheet according to the present embodiment is further excellent in hole expansibility (λ), preferably 20% or more, more preferably 25% or more, and further preferably 30% or more. Further, the steel sheet according to the present embodiment preferably has a yield ratio YR of 0.65 or more (obtained by gradually increasing the yield point YP with the tensile strength TS) and exhibits good collision characteristics. Further, the steel sheet according to the present embodiment preferably has a local elongation of 1.5% or more, more preferably 1.8% or more, and further preferably 2.0%, and exhibits good formability.

Furthermore, an example of the steel sheet according to the present embodiment is excellent in yield point YP and elongation, and shows a stress-strain curve (SS curve) as shown by “A” in FIG. On the other hand, heat treatment of the hot-rolled steel sheet in the method of the present disclosure, that is, annealing the cold-rolled steel sheet for a short time without performing heat treatment for 1 hour or more in a temperature range where the austenite phase fraction is 20% to 50%. The steel sheet obtained by applying only this exhibits a stress-strain curve as indicated by “C” having a small elongation. Thus, it is shown that an example of the steel sheet according to the present embodiment can obtain a higher yield point YP and elongation than a steel sheet obtained by performing only a short time annealing shown in “C”. The yield ratio YR of the steel sheet “A” shown in FIG. 1 is preferably 0.80 or more, more preferably 0.90 or more.

Another example of the steel sheet according to the present embodiment is “A” as shown as “B” in FIG. 1 by performing a relatively large skin pass rolling with a rolling reduction of 1.0% or more and less than 5.0%. It can also show a stress-strain curve that is slightly lower but has a good yield point and has a lower yield elongation than “A”. A steel sheet showing a stress-strain curve as shown in “A” preferably exhibits a yield ratio YR of 0.80 or more, while a steel sheet showing a stress-strain curve as shown in “B” is preferably 0.8. It exhibits a yield ratio YR of 65 or more, and preferably exhibits a yield elongation (YP-El) of less than 10%. A steel sheet showing a stress-strain curve as indicated by “B” has a smaller yield elongation, and therefore, can be dispersed without localizing the strain than “A”. Thereby, local deformation can be suppressed. The metal structures of the “A” and “B” steel plates will be described later.

As described above, the steel sheet of the present disclosure has high strength, good elongation characteristics, good hole expandability, and excellent formability. In addition, since the steel sheet “A” has a higher yield point, the initial absorbed energy at the time of collision can be improved, and the steel sheet “B” has a good yield point and has a small yield elongation. Energy can be suppressed and more energy can be absorbed. Therefore, both the steel sheets “A” and “B” are excellent in collision characteristics. These steel plates are optimal for structural parts of automobiles such as pillars, and any one of the steel plates can be selected according to the usage. Furthermore, the steel sheet of the present disclosure has a high content of Mn and contributes to the weight reduction of automobiles, so that the industrial contribution is extremely remarkable.

Next, a method of manufacturing a steel sheet showing a stress-strain curve of “A” will be described, and then a method of manufacturing a steel sheet showing a stress-strain curve of “B” will be described.

3. Manufacturing method The manufacturing method of the steel plate which concerns on this embodiment is demonstrated.

The steel plate according to the present embodiment is prepared by melting steel having the above-described chemical composition in a conventional manner, casting it to produce a slab or a steel ingot, and heating this to a finishing temperature of 1000 ° C. or less and a winding temperature of After hot rolling at less than 300 ° C., the obtained hot-rolled steel sheet is heat-treated in a ferrite / austenite two-phase region, pickled, and then cold-rolled at a cold rolling rate of 30% to 70%. Then, it is manufactured by annealing for a short time in a ferrite / austenite two-phase region.

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, and annealing conditions are as follows. It is preferable to carry out within.

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.

(Finishing rolling and winding: Finishing rolling below 1000 ° C and winding below 300 ° C)
Finish rolling is performed in hot rolling. Finish rolling start temperature shall be 1000 degrees C or less. If the finishing rolling start temperature exceeds 1000 ° C., the coarsening of the structure in the hot-rolled state cannot be prevented and the subsequent structure control becomes difficult, and the surface properties of the steel sheet are deteriorated due to grain boundary oxidation. It is also difficult to suppress. After finishing rolling, it cools and winds at less than 300 degreeC. When rolled up at 300 ° C or higher, the hot-rolled sheet structure cannot be made into a full martensite structure, and Mn distribution and austenite reverse transformation are efficiently performed in the heat treatment of the hot-rolled steel sheet and the annealing process of the cold-rolled steel sheet, respectively. It becomes difficult to wake up. By setting the finish rolling conditions within the above range, it is possible to secure a tempered martensite phase having an area ratio of 5% or more while limiting the area ratio of the martensite phase to less than 15%. 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 can be easily controlled.

(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. Conversely, if the heat treatment is performed at a temperature where the austenite phase fraction is less than 20% or more than 50% in the heat treatment, it becomes difficult to stabilize the austenite. Even when the heat treatment is performed in less than 1 hour, it becomes difficult to stabilize the austenite. By performing heat treatment for 1 hour or more at a temperature at which the austenite phase fraction is 20% to 50%, the metal structure at 1/8 of the thickness from the surface in the L cross section of the annealed steel sheet is 10% in area ratio. The above austenite phase 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 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, from the viewpoint of productivity.

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 held for 30 seconds to less than 15 minutes, preferably 1 minute to 5 minutes or less, in a temperature range where the austenite phase fraction is 20% to 50% in the two-phase region and is annealed. Do. The Mn distribution has already been completed in the heat treatment of the hot-rolled steel sheet, and Mn is concentrated in the place that was austenite during the heat treatment. 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 where the austenite phase fraction is less than 20% in the annealing, sufficient austenite cannot be obtained, and if heat treatment is performed at a temperature higher than 50%, the austenite phase tends to be transformed into a martensite phase. 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 unrecrystallized ferrite remains and the dislocations remain without disappearing, and the yield point increases. According to the method of the present disclosure, in addition to improvement of tensile strength (TS) × elongation (El) due to Mn distribution, a steel plate having a high yield point (YP) due to non-recrystallized ferrite and having excellent hole expandability is obtained. Obtainable.

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.

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.

Skin pass rolling may be performed on the steel plate after annealing or the steel plate after plating. The rolling reduction of such skin pass rolling is preferably 0% or more and less than 5.0% (that is, including the case where skin pass rolling is not performed), and more preferably 4.0% or less. In addition, when performing hot dip galvanization or alloying hot dip galvanization on the surface of a steel plate, skin pass rolling is performed to the steel plate after plating. The rolling reduction of the skin pass rolling can be appropriately selected according to the desired characteristics to be expressed in the steel plate. For example, when skin pass rolling with a rolling reduction of 0% or more and less than 1.0% is performed, finally a steel sheet having a higher yield point and a higher yield elongation is obtained as shown as “A” in FIG. Can do. Those having this characteristic can increase the initial absorption energy at the time of collision.

On the other hand, by performing skin pass rolling with a rolling reduction of 1.0% or more and less than 5.0%, it has a good yield point although it is slightly lower than “A”, as shown as “B” in FIG. In addition, a steel sheet exhibiting a stress-strain curve having a yield elongation smaller than that of “A” can be obtained.

Specifically, by reduction ratio perform skin pass rolling of less than 1.0% to 5.0% of the average dislocation density of the ferrite phase, 2 × 10 ¹² / m ² or more 4 × 10 ¹² / m less than ² Can be. In addition, when skin pass rolling with a rolling reduction of 0% or more and less than 1.0% is performed, the average dislocation density of the ferrite phase is less than 2 × 10 ¹² / m ² . 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). Although the average dislocation density of the ferrite phase is in the range of 2 × 10 ¹² / m ² or more and less than 4 × 10 ¹² / m ² , it is slightly lower than “A” as shown as “B” in FIG. A steel sheet having a good yield point and exhibiting a stress-strain curve with a yield elongation smaller than “A” can be obtained. A steel plate having such characteristics can suppress the localized deformation at the time of a collision, so that the absorbed energy can be increased. In this way, characteristics suitable for the member can be created depending on how the skin pass is applied.

When the average dislocation density is 2 × 10 ¹² / m ² or more, a steel sheet exhibiting a stress-strain curve having a good yield point and a small yield elongation can be obtained although it is slightly low. If the average dislocation density is 4 × 10 ¹² / m ² or more, that is, the rolling reduction is 5.0% or more, it is difficult to ensure sufficient elongation. In order to further suppress the variation in dislocation density in the steel sheet, the rolling reduction of skin pass rolling is preferably more than 2.0%.

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

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 thus obtained annealed cold-rolled steel sheet was subjected to skin pass rolling with a rolling reduction of 0.5% to produce steel sheets (invention examples and comparative examples).

Separately from the steel sheets produced under the conditions shown in Table 2, the annealed cold-rolled steel sheets obtained under the conditions shown in Table 3 were subjected to skin pass rolling at the rolling reduction (SPM) shown in Table 3 to obtain the steel sheets (invention examples). Produced. However, numbers 101, 104, 108, 111, and 114 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), and unrecrystallized ferrite (non-crystalline α) area ratio, CMnγ / CMnα, yield point (YP), yield elongation (YP-El), Tensile strength (TS), elongation (El), hole expansibility (λ), 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 sheet exhibiting TS × El of 28000 MPa ·% or more, a hole expansion ratio (λ) of 20% or more, and a yield ratio (YR) of 0.65 or more, has good impact characteristics, excellent elongation characteristics, and excellent hole expansion. The steel sheet was evaluated as a steel sheet having high properties and 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 2.0% and 4.0% is a steel plate (invention example) subjected to skin pass rolling at a reduction rate (SPM) of 0.5%. In comparison, although the yield point (YP) and the yield ratio (YR) are slightly decreased, the yield ratio is 0.65 or more, and the yield elongation (YP-El) is suppressed to less than 10%, while 20% or more. A steel plate exhibiting a high TS × El of 28,000 MPa ·% or more was obtained.

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 28000 MPa ·% or more, a hole expansion rate (λ) of 20% or more, and 0.65 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 4.20% 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 of the thickness from the surface contains an austenite phase of 10% or more, an tempered martensite phase of 5% or more, and a ferrite phase of 10% or more in terms of area ratio, The site phase is limited to less than 15%,
Among the ferrite phases, the area ratio of non-recrystallized ferrite is 30% or more and 70% or less,
CMnγ / CMnα, which is a 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.
% 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 an average dislocation density of the ferrite phase is 2 x 10 12 / m 2 or more and less than 4 x 10 12 / m 2 .
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.
A steel having the component according to any one of claims 1 to 5 is subjected to hot rolling with a finishing temperature of 1000 ° C or lower and a winding temperature of less than 300 ° C to form 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 or more and less than 15 minutes, and after holding the temperature of the annealing, an average cooling rate of 2 ° C. / A method for producing a steel sheet, wherein the steel sheet is cooled at a temperature in a range from 100 ° C. to 2000 ° C./second and held in a temperature range from 100 ° C. to 500 ° C. for 10 seconds to 1000 seconds.
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 hot-dip galvanizing is performed after the annealing.
The method for producing a steel sheet according to claim 11, 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.
The method for producing a steel sheet according to claim 9 or 10, wherein after the annealing, skin pass rolling with a rolling reduction of 1.0% or more and less than 5.0% is performed.
The method for producing a steel sheet according to claim 11, wherein skin pass rolling with a rolling reduction of 1.0% or more and less than 5.0% is performed after the hot dip galvanizing treatment.
13. The method for producing a steel sheet according to claim 12, wherein after the alloying treatment, skin pass rolling with a rolling reduction of 1.0% or more and less than 5.0% is performed.