JP2019014933A - Steel sheet and method of producing the same - Google Patents

Abstract

To provide a steel sheet having excellent strength and ductility as well as suppressed work hardening, and a method of producing the steel sheet.SOLUTION: A steel sheet contains 0.05-0.25 mass% C, more than 0 mass% and 3.0 mass% or less Si, 5.0-10.0 mass% Mn, more than 0 mass% and 0.100 mass% or less P, more than 0 mass% and 0.010 mass% or less S, 0.001-3.0 mass% Al, more than 0 mass% and 0.0100 mass% or less N, and the balance iron with inevitable impurities. In the steel sheet, the area ratio of ferrite is 40% or more and less than 80%, the area ratio of martensite is less than 25%, and the area ratio of retained austenite is 20% or more. The total area ratio of structures other than the ferrite, the martensite and the retained austenite is less than 10%. The average crystal grain size of the retained austenite is 1.0 μm or less, and the coupling rate of the retained austenite is 5.0 or more.SELECTED DRAWING: None

Description

  The present invention relates to a steel plate and a method for manufacturing the steel plate. In particular, for example, the present invention relates to a steel plate that can be used for automotive parts and the like and a method for manufacturing the same.

  Steel sheets used for automotive parts are required to be thin in order to improve fuel efficiency through weight reduction, and high strength is required in order to achieve both thinning and securing component strength. Yes. In addition, steel sheets used for automobile parts are required to have high energy absorption capability in the event of a collision in consideration of collision safety, and in addition to high strength, high ductility is required.

  In order to realize high strength and high ductility, it is necessary to increase ductility by improving TS × EL (elongation) in addition to increasing strength by improving tensile strength (TS). For this reason, it is known that it is effective to introduce a large amount of retained austenite into the steel sheet structure while increasing the strength of the steel sheet, and to utilize the transformation induced plasticity (TRIP) effect caused by the work-induced martensitic transformation of the retained austenite. It has been. That is, in order to increase energy absorption at the time of collision, it is effective to increase the retained austenite that undergoes work-induced martensite transformation during collision deformation. In addition, by suppressing the work-induced martensitic transformation of retained austenite during component forming, work hardening during forming can be suppressed, and even a high strength steel plate has the same low press load and high dimensions as a low strength steel plate. Accuracy can be achieved.

  For example, in Patent Document 1, the area ratio of martensite to the entire steel sheet structure is 15% or more and 90% or less, the amount of retained austenite is 10% or more and 50% or less, and 50% or more of the martensite is tempered martensite. A steel sheet is disclosed in which the area ratio of the tempered martensite to the entire steel sheet structure is 10% or more and the area ratio of the polygonal ferrite to the entire steel sheet structure is 10% or less (including 0%). In the steel sheet, TS is 1470 MPa or more and TS × EL is 29000 MPa% or more.

  Patent Document 2 includes ferrite having an area ratio of 30% to 80%, martensite of 0% to 17%, and retained austenite of 8% or more in volume ratio. A steel sheet having an average crystal grain size of 2 μm or less is disclosed. The steel sheet has TS of 780 MPa or more and TS × EL of 22000 MPa ·% or more.

JP 2011-184756 A JP 2012-237054 A

  However, in spite of extensive studies including the above-mentioned technology, in the production of actual automotive parts, large work hardening occurs when forming a steel sheet into a part shape, which increases the press load. However, problems such as deterioration in dimensional accuracy have occurred. Therefore, it is difficult to achieve high strength and high ductility and to suppress work hardening.

  This invention is made | formed in view of such a condition, One of the objectives is providing the steel plate which was excellent in intensity | strength and ductility, and the work hardening was suppressed, and another one objective. Is to provide its manufacturing method.

Aspect 1 of the present invention
C: 0.05-0.25 mass%,
Si: more than 0% by mass, 3.0% by mass or less,
Mn: 5.0 to 10.0% by mass,
P: more than 0% by mass, 0.100% by mass or less,
S: more than 0% by mass, 0.010% by mass or less,
Al: 0.001 to 3.0% by mass, and N: more than 0% by mass, 0.0100% by mass or less, with the balance consisting of iron and inevitable impurities,
The area ratio of ferrite is 40% or more and less than 80%,
The area ratio of martensite is less than 25%,
The area ratio of retained austenite is 20% or more,
The total area ratio of the structure other than ferrite, martensite and retained austenite is less than 10%,
The average crystal grain size of retained austenite is 1.0 μm or less,
It is a steel plate in which the connection ratio of retained austenite is 5.0 or more.

  Aspect 2 of the present invention is Cr: 0.01-0.20% by mass, Mo: 0.01-0.20% by mass, Cu: 0.01-0.20% by mass, Ni: 0.01-0 The steel plate according to aspect 1, further containing one or more selected from the group consisting of 20 mass% and B: 0.0001 to 0.02 mass%.

  Aspect 3 of the present invention is selected from the group consisting of Ca: 0.0005 to 0.01 mass%, Mg: 0.0005 to 0.01 mass%, and REM: 0.0001 to 0.01 mass%. It is a steel plate of the aspect 1 or 2 which further contains 1 or more types.

Aspect 4 of the present invention
Hot-rolling a steel slab having the chemical composition according to any one of aspects 1 to 3 and then cooling to room temperature to obtain a hot-rolled sheet; and
A softening annealing step of holding the hot-rolled sheet at a softening annealing temperature of (Ac1 + 30 ° C.) to (Ac1 + 100 ° C.) for 1.0 to 72 hours;
A cold rolling step of cold rolling the hot rolled sheet after the softening annealing at a cold rolling rate of 25 to 65% to obtain a cold rolled sheet;
The cold-rolled sheet is heated to a soaking temperature of [(Ac1 + Ac3) / 2-50 ° C.] to [(Ac1 + Ac3) / 2 + 10 ° C.] at an average heating rate of 3.0 ° C./second or more. It is a manufacturing method of a steel plate including a soaking step for 10 to 1800 seconds at a heat temperature.

  In one embodiment of the present invention, it is possible to provide a steel sheet that is excellent in strength and ductility and has suppressed work hardening, and in another embodiment, it is possible to provide a manufacturing method thereof. is there.

The present inventors have intensively studied to solve the above problems. As a result, the microstructure of the steel sheet is controlled so as to have a large anisotropy between the C direction (direction perpendicular to the rolling direction) and the L direction (rolling direction) regarding the processing-induced martensitic transformation behavior of retained austenite. Thus, it has been found that high strength and high ductility can be achieved and work hardening can be suppressed. By using such a steel plate, even when the strength of the steel plate is high, when forming a part, the remaining austenite is processed in a direction in which it is unlikely to undergo processing-induced transformation, thereby suppressing work hardening and ensuring formability. In addition, the absorbed energy of the component can be improved by deforming in a direction in which the machining-induced transformation is likely to proceed in the event of a collision. In such a steel sheet, as described above, the processing-induced martensitic transformation behavior is different between the C direction and the L direction, so that the TS is different between the C direction and the L direction, that is, there is TS anisotropy. .
As a result of further studies based on the above findings, the present inventors have limited the introduction amount of hard martensite in order to make the steel sheet have the desired TS, TS × EL and TS anisotropy. The present inventors have found that it is effective to use ferrite having high ductility as a parent phase, to increase the area ratio of retained austenite, and to control the crystal grain size and the coupling ratio.
Hereinafter, the details of the steel sheet and the manufacturing method thereof according to the embodiment of the present invention will be described.

1. Steel structure The steel sheet according to the embodiment of the present invention has an area ratio of ferrite of 40% or more and less than 80%, an area ratio of martensite of less than 25%, and an area ratio of retained austenite of 20% or more. The total area ratio of the structure other than ferrite, martensite and retained austenite is less than 10%, the average crystal grain size of retained austenite is 1.0 μm or less, and the connection ratio of retained austenite is 5.0 or more. .
Hereinafter, each configuration will be described in detail. The “area ratio” means the area ratio with respect to the entire tissue.

(1) Area ratio of ferrite: 40% or more and less than 80% By using ferrite as a main phase, desired TS and TS × EL can be obtained together with transformation-induced plasticity of retained austenite. When the area ratio of the ferrite is less than 40%, TS × EL decreases because the ductility of the parent phase is insufficient. On the other hand, when the area ratio of ferrite is 80% or more, TS cannot be secured. Therefore, the area ratio of ferrite is 40% or more and less than 80%. The area ratio of ferrite is preferably 45% or more, and preferably 75% or less.

(2) Area ratio of martensite: less than 25% When martensite is contained in the steel structure in an area ratio of 25% or more, the elongation decreases, and TS × EL decreases. Therefore, the area ratio of martensite is less than 25%. The area ratio of martensite is preferably 22% or less, more preferably 20% or less. The lower limit of the area ratio of martensite is not particularly limited, and may be 0% from the viewpoint of obtaining good elongation and TS × EL.
In the embodiment of the present invention, “martensite” means both “as-quenched martensite” and “tempered martensite”.

(3) Area ratio of retained austenite: 20% or more In addition to ferrite as a parent phase, retained austenite is introduced as a second phase. Residual austenite has the effect of increasing TS × EL by transformation-induced martensite transformation. In order to obtain good mechanical properties, the area ratio of retained austenite is 20% or more. The area ratio of retained austenite is preferably 25%, more preferably 30%. The upper limit of the area ratio of retained austenite is not particularly limited as long as the area ratio of ferrite and the area ratio of martensite are within the above ranges.

(4) Total area ratio of structures other than ferrite, martensite and retained austenite: less than 10% Examples of structures other than ferrite, martensite and retained austenite include pearlite, bainite and cementite. When the total area ratio of the structure other than ferrite, martensite and retained austenite is 10% or more, TS × EL is lowered. Therefore, the total area ratio of the structure other than ferrite, martensite and retained austenite is set to less than 10%. The area ratio is preferably 5% or less. The lower limit of the area ratio is not particularly limited, and may be 0% from the viewpoint of obtaining good TS × EL.
Hereinafter, structures other than ferrite, martensite, and retained austenite may be referred to as “other structures”.

(5) Average crystal grain size of retained austenite: 1.0 μm or less Cracks originating from work-induced martensite by setting the average crystal grain size of residual austenite to 1.0 μm or less and finely dispersing individual grains. Generation | occurrence | production can be suppressed and the fall of TSxEL can be prevented. When the average crystal grain size of the retained austenite exceeds 1.0 μm, the retained austenite is transformed into coarse martensite, which causes early breakage or cracks. The average crystal grain size of retained austenite is preferably 0.94 μm or less, more preferably 0.8 μm or less.
In this specification, the cross section of steel is measured with an EBSP analyzer, and from the analysis data of EBSP, the boundary where the crystal orientation difference (slant angle) exceeds 15 °, that is, the residual austenite grains are defined with the large-angle grain boundaries as crystal grain boundaries. Define.

(6) Retention ratio of retained austenite: 5.0 or more Remarkable between work hardening behavior when deforming in the L direction and work hardening behavior when deforming in the C direction by connecting individual retained austenite grains. Differences appear. Although the detailed mechanism is unknown, the deformation-induced martensite transformation of retained austenite is suppressed during deformation in the L direction, while the deformation-induced martensite transformation of retained austenite is not suppressed during deformation in the C direction. Since the work hardening is promoted by the TRIP effect, the anisotropy of TS can be increased, so that the difference is considered to be expressed.
“Residual austenite is connected” means that two or more retained austenite grains are in contact with a large-angle grain boundary as a boundary. A group of retained austenite grains in which the retained austenite grains are continuously connected may be referred to as “retained austenite connection product”.
The “retained austenite connection ratio” means a value obtained by dividing the total number of residual austenite grains by the total number of unretained residual austenite grains and residual austenite.
If the residual austenite connectivity is less than 5.0, the desired anisotropy cannot be obtained. Therefore, the residual austenite connection rate is 5.0 or more, preferably 6.0 or more, more preferably 7.0 or more.

  Hereinafter, an example of an evaluation method of the area ratio of each steel structure, the average crystal grain size of retained austenite, and the connection ratio will be described.

[Measurement of area ratio of steel structure]
After polishing the plate thickness cross section perpendicular to the rolling direction of the steel plate and corroding it with a Picral solution to reveal the structure, FE-SEM (Field-Emission Scanning Electron Microscope, electric field) Using an emission scanning electron microscope), 10 fields of 10 μm × 12 μm are randomly photographed at a magnification of 10,000 times to obtain an SEM image. The obtained SEM image was subjected to tissue classification, and the area ratio of each tissue was calculated for each visual field using image analysis software, for example, MEDIA CYBERNETICS image analysis software “ImagePro Plus ver. 7.0”. Let the average value of 10 visual fields be the area ratio of each structure | tissue.

The area ratio of ferrite and martensite may be measured as follows. That is, since it is difficult to distinguish between ferritic and as-quenched martensite in the steel as-annealed structure, the temperature range in which only cementite precipitation occurs in the as-quenched martensite without change in the structure fraction (for example, 300 Tempering is performed at 30 ° C. for 30 minutes). Using tempered steel, the structure is observed in the same way as above, and the ratio of the area ratio of ferrite to the total area ratio of ferrite and martensite (region where carbides are precipitated) is calculated for each field of view. Then, the average value A of 10 fields of the ratio is obtained, and the area ratio of ferrite and the area ratio of martensite are obtained using the following formulas (1) and (2).

Ferrite area ratio (%)
= [100− (area ratio of retained austenite + total area ratio of structure other than ferrite, martensite and retained austenite)] × A (1)

Martensite area ratio (%)
= [100- (area ratio of retained austenite + total area ratio of structure other than ferrite, martensite and retained austenite)] × (1-A) (2)

[Measurement of average grain size of retained austenite]
The thickness cross section perpendicular to the rolling direction of the steel plate is polished, and the region of plate thickness / 4 is selected at random using the EBSD (Electron Backscatter Diffraction (Electronic Backscatter Diffraction)) analyzer attached to the FE-SEM. Measurement is performed at a step interval of 0.05 μm for the 5 fields of 20 μm × 20 μm. Using an analysis software, for example, analysis software “OIM Analysis 7” manufactured by TSL Solutions, the average crystal grain size is calculated for each field while limiting to the region of residual austenite, and the average value of five fields of view is the average crystal of residual austenite. The particle size. In the above measurement, the retained austenite grains are defined with the boundary where the crystal orientation difference (oblique angle) exceeds 15 °, that is, the large-angle grain boundary as the crystal grain boundary.

[Measurement of residual austenite connectivity]
In the measurement by EBSD, since the retained austenite grains are defined with the boundary where the crystal orientation difference (oblique angle) exceeds 15 °, that is, the large angle grain boundary as the crystal grain boundary, the individual residuals constituting the connected structure of the retained austenite. Each austenite is counted individually. Therefore, the total number of retained austenite grains can be measured by EBSD.
On the other hand, in the measurement by FE-SEM, the individual retained austenite grains constituting the connected structure of retained austenite cannot be counted individually, but the connected structure of retained austenite can be counted as one. Therefore, the total number of residual austenite grains that are not connected and the connected product of residual austenite can be measured by FE-SEM.
Utilizing the above characteristics of the EBSD measurement and the FE-SEM measurement, the connection ratio of retained austenite can be measured as follows.
The total number of residual austenite grains is calculated for each of the five visual fields observed at the time of measuring the average crystal grain size of residual austenite by EBSD, and the average value B of the five visual fields of the total number is obtained. For the measurement of the number, analysis software, for example, analysis software “OIM Analysis 7” manufactured by TSL Solutions may be used. In the case of the above measurement, when the retained austenite grains are not in contact with each other at the position of the thickness / 4, the retained austenite grains are connected even if the retained austenite grains are in contact with each other at other positions. It may be treated as not.
Further, with respect to the SEM image of the 10 fields of view obtained in the measurement of the area ratio of retained austenite by FE-SEM, image analysis software, for example, image analysis software “ImagePro Plus ver. 7.0” manufactured by MEDIA CYBERNETICS is used. The total number of residual austenite grains that are not connected and the connected structure of residual austenite is calculated for each field of view, and the average value C of the 10 fields of the number is obtained.
The connection ratio of retained austenite is obtained using the following equation (3).

Connection ratio of retained austenite = B / C (3)

2. Chemical Component Composition The steel sheet according to the embodiment of the present invention has C: 0.05 to 0.25 mass%, Si: more than 0 mass%, 3.0 mass% or less, Mn: 5.0 to 10.0 mass%. , P: more than 0% by mass, 0.100% by mass or less, S: more than 0% by mass, 0.010% by mass or less, Al: 0.001 to 3.0% by mass, and N: more than 0% by mass, 0 0.0100% by mass or less, with the balance being iron and inevitable impurities.
Hereinafter, each element will be described in detail.

(1) C: 0.05 to 0.25% by mass
C, together with Mn, contributes to an increase in the retained austenite fraction and an improvement in the stability of the retained austenite with respect to processing as an austenite stabilizing element. In order to effectively exhibit such an action, the C content needs to be 0.05% by mass or more, and preferably 0.10% by mass or more. However, if the C content exceeds 0.25% by mass, hard martensite is excessively generated in the final annealing, and the weldability is deteriorated. Therefore, the C content is 0.25% by mass or less, preferably 0.20% by mass or less.

(2) Si: more than 0% by mass and 3.0% by mass or less Si is useful as a solid solution strengthening element for ferrite, and contributes to a higher TS while minimizing the decrease in EL. However, when it adds excessively, local ductility will fall and TSxEL will be reduced. Therefore, the Si content is set to more than 0% by mass and 3.0% by mass or less. Si content becomes like this. Preferably it is 0.1 mass% or more, More preferably, it is 0.2 mass% or more, Preferably it is 1.5 mass% or less, More preferably, it is 0.5 mass% or less.

(3) Mn: 5.0 to 10.0% by mass
Mn as an austenite stabilizing element contributes to an increase in the retained austenite fraction and an improvement in stability of the retained austenite. In order to effectively exhibit such an action, the Mn content needs to be 5.0% by mass or more, and preferably 6.0% by mass or more. However, if the Mn content exceeds 10.0% by mass, the retained austenite becomes coarse and TS × EL decreases. Therefore, Mn content is 10.0 mass% or less, Preferably it is 9.0 mass% or less.

(4) P: more than 0% by mass and 0.100% by mass or less P is inevitably present as an impurity element, and if it exceeds 0.100% by mass, EL deteriorates. Therefore, the P content is 0.100% by mass or less. The P content is preferably 0.03% by mass or less. The P content is preferably as low as possible and is most preferably 0% by mass, but it may remain above 0% by mass, for example, about 0.001% by mass due to restrictions on the production process.

(5) S: more than 0% by mass, 0.010% by mass or less S is an element which inevitably exists as an impurity element, forms sulfide inclusions such as MnS, and lowers EL as a starting point of cracking. It is. For this reason, S content shall be 0.010 mass% or less. The S content is preferably 0.005% by mass or less. The S content is preferably as low as possible, and most preferably 0% by mass. However, it may exceed 0% by mass, for example, about 0.001% by mass due to restrictions on the production process.

(6) Al: 0.001 to 3.0 mass%,
Al is used as a deoxidizing material, but if its content is less than 0.001% by mass, a sufficient cleaning effect of the steel cannot be obtained, whereas if the Al content exceeds 3.0% by mass, steel is used. Embrittles and causes cracks in the steel during casting. Therefore, Al content shall be 0.001-3.0 mass%. Al content becomes like this. Preferably it is 0.5 mass% or more, More preferably, it is 0.8 mass% or more, Preferably it is 2.8 mass% or less, More preferably, it is 2.5 mass% or less.

(7) N: more than 0% by mass, 0.0100% by mass or less N is unavoidably present as an impurity element, reduces elongation by strain aging, and precipitates as coarse nitrides by combining with Al. To lower TS × El. Therefore, the N content is desirably as low as possible, and the N content is set to 0.0100% by mass or less. The N content is preferably 0.006% by mass or less. The N content is preferably as low as possible, and most preferably 0% by mass. However, it may remain above 0% by mass, for example, about 0.001% by mass due to restrictions on the production process.

(8) Balance The basic components are as described above, and the balance is iron and inevitable impurities (for example, Sb). Inevitable impurities are elements that are brought in depending on the situation of raw materials, materials, manufacturing equipment and the like.
For example, as in P, S and N, the smaller the content, the better. Therefore, it is an unavoidable impurity, but there are elements whose composition range is separately defined as described above. For this reason, in this specification, the term “inevitable impurities” constituting the balance is a concept that excludes elements whose composition range is separately defined.

  Furthermore, the steel plate according to the embodiment of the present invention may contain the following optional elements as necessary, and the properties of the steel plate are further improved according to the contained components.

(9) Cr: 0.01 to 0.20 mass%, Mo: 0.01 to 0.20 mass%, Cu: 0.01 to 0.20 mass%, Ni: 0.01 to 0.20 mass% And B: one or more selected from the group consisting of 0.0001 to 0.02 mass% Cr, Mo, Cu, Ni and B are elements useful as steel strengthening elements. In order to effectively exhibit such an action, the contents of Cr, Mo, Cu and Ni are each preferably 0.01% by mass or more, more preferably 0.05% by mass or more, and the B content Is preferably 0.0001% by mass or more, more preferably 0.0002% by mass or more. However, even if these elements are contained excessively, the above effects are saturated and economically useless, so the contents of Cr, Mo, Cu and Ni are preferably 0.20% by mass or less, More preferably, it is 0.15 mass% or less, B content becomes like this. Preferably it is 0.02 mass% or less, More preferably, it is 0.006 mass% or less.

(10) One or more kinds selected from the group consisting of Ca: 0.0005 to 0.01 mass%, Mg: 0.0005 to 0.01 mass%, and REM: 0.0001 to 0.01 mass% Ca Mg and REM are effective elements for controlling the form of sulfide in steel and improving workability. Here, examples of the REM (rare earth element) used in the embodiment of the present invention include Sc, Y, and lanthanoid. In order to effectively exhibit the above action, the Ca and Mg contents are each preferably 0.0005 mass% or more, more preferably 0.001 mass% or more, and the REM content is preferably 0.00. It is 0001% by mass or more, more preferably 0.0002% by mass or more. However, even if these elements are contained excessively, the above effects are saturated and economically useless, so the Ca and Mg contents are each preferably 0.01% by mass or less, more preferably 0%. 0.003 mass% or less, and the REM content is preferably 0.01 mass% or less, more preferably 0.006 mass% or less.

4). Manufacturing method The manufacturing method of the steel plate which concerns on embodiment of this invention is the hot rolling process of (1) hot-rolling the steel slab which has the above-mentioned chemical component composition, and cooling to room temperature after that, and ( 2) a softening annealing step of holding the hot-rolled sheet at a softening annealing temperature of (Ac1 + 30 ° C.) to (Ac1 + 100 ° C.) for 1.0 to 72 hours; and (3) a hot-rolled plate after the softening annealing. A cold rolling step in which a cold rolled sheet is obtained by cold rolling at a cold rolling rate of 25 to 65%, and (4) the cold rolled sheet at an average heating rate of 3.0 ° C./second or more, [ And (Ac1 + Ac3) / 2-50 [deg.] C.] to [(Ac1 + Ac3) / 2 + 10 [deg.] C.], and a soaking step is held for 10 to 1800 seconds at the soaking temperature.
Hereinafter, each process is explained in full detail.

(1) Hot-rolling step A steel slab having the above chemical composition is hot-rolled and then cooled to room temperature to obtain a hot-rolled sheet. Hot rolling conditions are not particularly limited. For example, cast steel slabs are charged directly into the heating furnace, or the cast steel slabs are once cooled to room temperature, then charged into the heating furnace, soaked, and after hot rolling, wound up and cooled. A hot-rolled coil (hot-rolled plate) may be used.

(2) Softening annealing step The obtained hot-rolled sheet is held at a softening annealing temperature of (Ac1 + 30 ° C.) to (Ac1 + 100 ° C.) for 1.0 to 72 hours. By subjecting the hot-rolled sheet to softening annealing under the above conditions before cold rolling, the steel structure is a two-phase structure of tempered martensite and fine austenite existing between the laths of the tempered martensite. can do. When softening annealing is not performed, not only the desired final annealed structure cannot be obtained, but the strength of the hot-rolled sheet is too high, so that cold rolling is substantially impossible.
When the softening annealing temperature is less than Ac1 + 30 ° C., or when the holding time is less than 1.0 hour, fine austenite existing between the laths of tempered martensite cannot be obtained sufficiently, so that the final structure As a result, residual austenite having a high connection rate cannot be obtained, and the anisotropy of TS is lowered. On the other hand, when the softening annealing temperature is more than Ac1 + 100 ° C. or when the holding time is more than 72 hours, austenite is coarsened, so fine residual austenite grains cannot be obtained in the final structure, and TS × EL descend.

  The means for softening annealing is not particularly limited, but it is preferable to use a batch furnace because soaking is required for a long time. Moreover, you may perform pickling before softening annealing.

(3) Cold-rolling process The hot-rolled sheet after softening annealing is cold-rolled at a cold rolling rate of 25 to 65% to obtain a cold-rolled sheet. By cold rolling, a part of retained austenite between laths of tempered martensite generated by softening annealing is transformed into work-induced martensite. When the cold rolling rate is less than 25%, the formation of work-induced martensite is insufficient, so that the residual austenite connection rate decreases in the final structure, and TS anisotropy decreases. On the other hand, when the cold rolling rate exceeds 65%, cracks starting from the work-induced martensite are generated, so that cold rolling becomes impossible.

(4) Soaking step The obtained cold-rolled sheet is soaked from [(Ac1 + Ac3) / 2-50 ° C.] to [(Ac1 + Ac3) / 2 + 10 ° C.] at an average heating rate of 3.0 ° C./second or more. The temperature is raised to a temperature and held at the soaking temperature for 10 to 1800 seconds.

  In order to refine the retained austenite, the temperature is raised at 3.0 ° C./second or more to the soaking temperature. When the average heating rate is less than 3.0 ° C./second, the retained austenite becomes coarse and TS × EL decreases. The upper limit of the average heating rate is not particularly limited.

  The temperature is maintained for 10 to 1800 seconds at a soaking temperature within a predetermined temperature range of the ferrite-austenite two-phase region, that is, [(Ac1 + Ac3) / 2-50 ° C.] to [(Ac1 + Ac3) / 2 + 10 ° C.]. By performing soaking within the temperature range, the area ratio of ferrite and the area ratio of retained austenite are controlled. When the soaking temperature is less than [(Ac1 + Ac3) / 2-50 ° C.], or when the holding time is less than 10 seconds, the amount of austenite to be generated is insufficient, so the area ratio of residual austenite is reduced in the final structure. In addition, the anisotropy of TS, TS × EL, and TS decreases. On the other hand, when the soaking temperature is higher than [(Ac1 + Ac3) / 2 + 10 ° C.], or when the holding time is longer than 1800 seconds, the austenite to be generated becomes excessive and coarsens at the same time. The area ratio becomes excessive, the area ratio of ferrite and / or retained austenite decreases, and the retained austenite coarsens, and the anisotropy of TS × EL and TS decreases. During soaking, the temperature may vary as long as it is within the range of the soaking temperature.

The cooling conditions after the soaking step are not particularly limited. It may be rapidly cooled to room temperature by gas jet or water cooling, and may be gradually cooled by air cooling. Moreover, you may hold | maintain at predetermined temperature in the middle of cooling.
After the soaking step, it may be cooled to a predetermined temperature and immersed in a plating bath to obtain a plated steel plate, or after the soaking step, it may be supercooled and then reheated and immersed in a plating bath to obtain a plated steel plate. . After forming a plated steel plate, the alloying may be performed by heating in an alloying process. Further, skin pass rolling may be applied at a rolling reduction in a normal process range.

  As described above, the manufacturing method of the steel sheet according to the embodiment of the present invention has been described. However, a person skilled in the art who understands the desired characteristics of the steel sheet according to the embodiment of the present invention performs trial and error, and relates to the embodiment of the present invention. There is a possibility of finding a method other than the above-described production method, which is a method for producing a steel plate having desired characteristics.

  Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can be implemented with appropriate modifications within the scope that can meet the above-mentioned and later-described gist, and they are all within the technical scope of the present invention. Is included.

In order to confirm the applicability of the present invention, a laboratory test was conducted as follows. First, steel shown in Table 1 below was melted. Ac1 and Ac3 were experimentally determined by performing a temperature increase test using a cold-rolled sheet described later at a temperature increase rate of 3.0 ° C./s and measuring the shrinkage associated with austenite formation. The molten steel is processed into a steel slab with a thickness of 50 mm by hot forging, soaked at 1200 ° C. for 30 minutes, roughly rolled to 12 mm, soaked again at 1200 ° C. for 30 minutes, and then hot rolled. After finishing to a plate thickness of 2.3 mm, cooling to 500 ° C. by water cooling, charging in an atmospheric furnace heated to 500 ° C., holding for 30 minutes, cooling the furnace, and simulating coil cooling by winding. Thereafter, softening annealing was performed in an atmospheric furnace under the conditions shown in Table 2, and after air cooling, the scale was removed by pickling and cold rolling to produce a cold-rolled sheet having a thickness of 1.4 mm. However, production No. For No. 5, the front and back surfaces of the hot-rolled sheet finished to 2.3 mm were equally reduced in thickness to 1.75 mm, and a cold-rolled sheet of 1.4 mm was produced by cold rolling (cold rolling rate 20%). The soaking process was simulated by an atmosphere control heat treatment simulator. After soaking in the soaking process, it was cooled to 200 ° C. with a gas jet (abbreviated as “GJ” in Table 2), and then air-cooled.
In Tables 1 to 3, the underlined numerical values indicate that they are out of the scope of the present invention.

  For each steel plate obtained as described above, the area ratio of the steel structure, the average crystal grain size of retained austenite, and the connection ratio of retained austenite were measured in the following manners (1) to (3).

[Measurement of area ratio of steel structure]
After polishing the plate thickness section perpendicular to the rolling direction of the steel plate and corroding it with the Picral solution to reveal the structure, the Schottky field emission scanning electron microscope manufactured by JEOL Ltd. was applied to the area of plate thickness / 4. Then, 10 fields of view of a region of 10 μm × 12 μm were randomly photographed at a magnification of 10,000 times to obtain an SEM image. Regarding the obtained SEM image, a region that is particularly corroded and observed with a black contrast is determined to be retained austenite. Further, a structure other than ferrite, martensite, and retained austenite is classified, and image analysis software “MEDIA CYBERNETICS” ImagePro Plus ver. 7.0 "was used to calculate the area ratio of each tissue for each field of view, and the average value of 10 fields was taken as the area ratio of each tissue.

The area ratio of ferrite and martensite was measured as follows.
The steel plate is kept at 300 ° C. for 30 minutes and tempered, and the tempered steel is used to observe the structure in the same manner as described above, and the total of ferrite and martensite (region where carbide is precipitated) is obtained. The ratio of the area ratio of ferrite to the area ratio is calculated for each field of view, the average value A of 10 fields of the ratio is obtained, and the area ratio of ferrite and martensite are calculated using the following formulas (1) and (2). The area ratio was determined.

Ferrite area ratio (%)
= [100− (area ratio of retained austenite + total area ratio of structure other than ferrite, martensite and retained austenite)] × A (1)

Martensite area ratio (%)
= [100- (area ratio of retained austenite + total area ratio of structure other than ferrite, martensite and retained austenite)] × (1-A) (2)

[Measurement of average grain size of retained austenite]
The thickness cross section perpendicular to the rolling direction of the steel plate is polished, and the region of plate thickness / 4 is selected at random using the EBSD (Electron Backscatter Diffraction (Electronic Backscatter Diffraction)) analyzer attached to the FE-SEM. The measurement was performed at a step interval of 0.05 μm for the 5 fields of 20 μm × 20 μm. Using the analysis software “OIM Analysis 7” manufactured by TSL Solutions, the average crystal grain size is calculated for each visual field limited to the residual austenite region, and the average value of the five visual fields is defined as the average crystal grain size of the residual austenite. At the time of the above measurement, residual austenite grains were defined with the boundary where the crystal orientation difference (oblique angle) exceeded 15 °, that is, the large-angle grain boundary as the crystal grain boundary.

[Measurement of residual austenite connectivity]
For the five visual fields observed when measuring the average crystal grain size of residual austenite by EBSD, the total number of residual austenite grains was calculated for each visual field, and the average value B of the five visual fields of the total number was obtained. For measurement of the number, analysis software “OIM Analysis 7” manufactured by TSL Solutions was used. In the case of the above measurement, when the retained austenite grains are not in contact with each other at the position of the thickness / 4, the retained austenite grains are connected even if the retained austenite grains are in contact with each other at other positions. Treated as not.
Further, with respect to the SEM image of the 10 fields of view obtained in the measurement of the area ratio of retained austenite by FE-SEM, image analysis software, for example, image analysis software “ImagePro Plus ver. 7.0” manufactured by MEDIA CYBERNETICS is used. The total number of residual austenite grains not connected and the connected structure of residual austenite was calculated for each field of view, and the average value C of the 10 fields of the number was calculated.
Using the following equation (3), the connection ratio of retained austenite was determined.

Connection ratio of retained austenite = B / C (3)

[Measurement of TS and EL]
About each steel plate obtained as mentioned above, the mechanical characteristic was measured by the tension test. The tensile test was performed by collecting JIS No. 5 test pieces from a direction perpendicular to the rolling direction (C direction) and a direction parallel to the rolling direction (L direction), and TS (TS _C ) and EL ( EL _C ) and TS (TS _L ) in the L direction were measured. Then, TS _C × EL _C and ΔTS = TS _C −TS _L were calculated.

Table 3 shows the measurement results. Regarding mechanical properties of the steel sheet, TS _C : 1180 MPa or higher, TS _C × EL _C : 27000 MPa% or higher, and ΔTS: 100 MPa or higher are indicated as “O” as acceptable, and the others are rejected. Indicated by “x”.
In Table 3, “α” represents ferrite, “M” represents martensite, and “γ _R ” represents retained austenite.

As shown in Table 3, steel No. which is the steel of the invention (evaluation is ○). 1, 12, 14, 17, 18, and 20 to 25 are examples that satisfy all the requirements defined in the embodiments of the present invention, and TS _C , TS _C × EL _C, and ΔTS are all acceptable standards. Thus, it was confirmed that a high-strength steel ductile steel plate having anisotropy in tensile strength was obtained.

On the other hand, steel No. which is a comparative steel (evaluation of x). 2 to 11, 13, 15, 16, and 19 are comparative examples that do not satisfy the requirements defined in the embodiment of the present invention, and at least one of TS _C , TS _C × EL _C, and ΔTS is inferior.

  Steel No. In No. 2, the softening annealing temperature was too low, the connection ratio of residual austenite was insufficient, and ΔTS was inferior.

Steel No. In No. 3, the softening annealing temperature was too high, the retained austenite was coarsened, and TS _C × EL _C was inferior.

  Steel No. In No. 4, the retention time of softening annealing was too short, the connection ratio of residual austenite was insufficient, and ΔTS was inferior.

  Steel No. In No. 5, the cold rolling rate was too low, the residual austenite connection rate was insufficient, and ΔTS was inferior.

Steel No. In No. 6, the average heating rate to the soaking temperature after cold rolling was too low, the retained austenite was coarsened, and TS _C × EL _C was inferior.

Steel No. In No. 7, the soaking temperature was too low and the ferrite was excessive, while the retained austenite was insufficient, and TS _C , TS _C × EL _C and ΔTS were inferior.

Steel No. In No. 8, the soaking temperature was too high and the residual austenite was insufficient, while the martensite was excessive, the residual austenite was coarsened, and TS _C × EL _C and ΔTS were inferior.

Steel No. In No. 9, the retention time in soaking was too short, and the ferrite was excessive, while the retained austenite was insufficient, and TS _C , TS _C × EL _C and ΔTS were inferior.

Steel No. In No. 10, the retention time was too long in soaking, and the retained austenite was insufficient. On the other hand, martensite was excessive, the retained austenite was coarsened, and TS _C × EL _C and ΔTS were inferior.

Steel No. 11 (steel type B) had a C content that was too low and an excess of ferrite, while residual austenite was insufficient and TS _C , TS _C × EL _C and ΔTS were inferior.

Steel No. No. 13 (steel type D) had a too high C content and insufficient ferrite, while martensite was excessive and TS _C × EL _C was inferior.

Steel No. 15 (steel type F) had an excessively high Si content and was inferior in TS _C × EL _C.

Steel No. No. 16 (steel type G) had a too low Mn content, a shortage of retained austenite, and inferior TS _C , TS _C × EL _C and ΔTS.

Steel No. 19 (steel type J) had an excessively high Mn content, coarsened retained austenite, and inferior TS _C × EL _C.

  As a result, the applicability of the present invention was confirmed.

Claims (4)

C: 0.05-0.25 mass%,
Si: more than 0% by mass, 3.0% by mass or less,
Mn: 5.0 to 10.0% by mass,
P: more than 0% by mass, 0.100% by mass or less,
S: more than 0% by mass, 0.010% by mass or less,
Al: 0.001 to 3.0% by mass, and N: more than 0% by mass, 0.0100% by mass or less, with the balance consisting of iron and inevitable impurities,
The area ratio of ferrite is 40% or more and less than 80%,
The area ratio of martensite is less than 25%,
The area ratio of retained austenite is 20% or more,
The total area ratio of the structure other than ferrite, martensite and retained austenite is less than 10%,
The average crystal grain size of retained austenite is 1.0 μm or less,
A steel plate having a connection ratio of retained austenite of 5.0 or more. Cr: 0.01-0.20 mass%,
Mo: 0.01-0.20 mass%,
Cu: 0.01-0.20 mass%,
The steel plate according to claim 1, further comprising at least one selected from the group consisting of Ni: 0.01 to 0.20 mass% and B: 0.0001 to 0.02 mass%. Ca: 0.0005 to 0.01% by mass,
The steel plate according to claim 1 or 2, further comprising at least one selected from the group consisting of Mg: 0.0005 to 0.01 mass% and REM: 0.0001 to 0.01 mass%. Hot-rolling a steel slab having the chemical composition according to any one of claims 1 to 3 and then cooling to room temperature to obtain a hot-rolled sheet;
A softening annealing step of holding the hot-rolled sheet at a softening annealing temperature of (Ac1 + 30 ° C.) to (Ac1 + 100 ° C.) for 1.0 to 72 hours;
A cold rolling step of cold rolling the hot rolled sheet after the softening annealing at a cold rolling rate of 25 to 65% to obtain a cold rolled sheet;
The cold-rolled sheet is heated to a soaking temperature of [(Ac1 + Ac3) / 2-50 ° C.] to [(Ac1 + Ac3) / 2 + 10 ° C.] at an average heating rate of 3.0 ° C./second or more. A method for producing a steel sheet, comprising a soaking step of holding at a heat temperature for 10 to 1800 seconds.