JP2019039037A - Steel sheet and method for manufacturing the same - Google Patents

Abstract

To provide a steel sheet excellent in strength, ductility and hole expandability and to provide a method for manufacturing the steel sheet.SOLUTION: A steel sheet contains C:0.05 to 0.25 mass%, Si:1.0 to 3.0 mass%, Mn:5.0 to 10.0 mass%, P:more than 0 mass% to 0.100 mass%, S:more than 0 mass% to 0.010 mass%, Al:0.001 to 3.0 mass% and N:more than 0 mass% to 0.0100 mass% and the balance iron with inevitable impurities and has a structure in which an area ratio of ferrite is 40% to less than 80%; an area ratio of martensite is less than 20%; an area ratio of residual austenite is 20% or more; an area ratio of the total of the structure other than ferrite, martensite and residual austenite is less than 10%; an average crystal grain size of residual austenite is 1.0 μm or less; and an average value of a ratio dW/dT of the length dW of residual austenite grain in a sheet width direction and a length dT thereof in a sheet thickness direction is 1.4 or more.SELECTED DRAWING: None

Description

  The present invention relates to a steel plate and a method for producing the same, and more particularly, to a steel plate that can be used for, for example, automobile parts and the method for producing the same.

  Steel sheets used in the manufacture of automotive parts are required to be thin in order to improve fuel efficiency by reducing weight, and high strength is required to achieve both thinning and ensuring the strength of parts. ing. In addition, steel sheets used in the manufacture of automotive parts are required to have high energy absorption capacity 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.

  Furthermore, steel sheets used in the manufacture of automotive parts are required to have excellent formability in order to be processed into parts with complex shapes, and in particular, have excellent hole expansibility (λ), which is an index of local deformability. Is required.

  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, TS × EL of 22000 MPa% or more, and is excellent in λ.

JP 2011-184756 A JP 2012-237054 A

  However, in spite of extensive studies including the above-described technology, it is difficult to produce a steel sheet that achieves high strength and high ductility and is excellent in hole expansibility.

  The present invention has been made in view of such circumstances, and one of its purposes is to provide a steel sheet excellent in strength, ductility and hole expansibility, and another purpose is its production. Is to provide a method.

Aspect 1 of the present invention
C: 0.05-0.25 mass%,
Si: 1.0-3.0 mass%,
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 20%,
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 average value of the ratio dW / dT of the length dW in the plate width direction and the length dT in the plate thickness direction of the retained austenite grains is 1.4 or more.

  Aspect 2 of the present invention is Cr: 0.01-0.40 mass%, Mo: 0.01-0.40 mass%, Cu: 0.01-0.40 mass%, Ni: 0.01-0 The steel sheet according to aspect 1, further containing one or more selected from the group consisting of .40% by mass and B: 0.0001 to 0.01% by 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
After heating the steel slab having the chemical composition according to any one of the aspects 1 to 3 to a heating temperature of 1050 to 1150 ° C., a finishing pressure of 20% or more at a finishing temperature of 800 ° C. or more and less than 860 ° C. A hot rolling step of hot rolling at a rate 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 500 ° C. to (Ac1 + 30 ° C.) for 0.5 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 75% to obtain a cold rolled sheet;
The cold-rolled sheet is heated to a soaking temperature of [(Ac1 + Ac3) / 2-90 ° C.] to [(Ac1 + Ac3) / 2-20 ° C.] at an average heating rate of 3.0 ° C./second or more, And a soaking step in which the soaking temperature is maintained for 10 to 1800 seconds.

  In one embodiment of the present invention, a steel plate excellent in strength, ductility and hole expansibility can be provided, and in another embodiment, a manufacturing method thereof can be provided.

The present inventors have intensively studied to solve the above problems. As a result, in order to combine high-strength steel sheets with the desired TS, TS × EL, and λ, while restricting the amount of hard martensite introduced, the high-strength ferrite is used as the parent phase, and the area of residual austenite It has been found that it is effective to increase the rate and control the crystal grain size and grain shape.
Regarding the shape of the retained austenite grains, specifically, the inventors of the present invention have a shape that extends in the direction parallel to the plate surface (that is, the plate width direction and the rolling direction) and is compressed in the plate thickness direction (for example, It has been found that, when punching is performed in a hole expanding process, the martensitic transformation of retained austenite can be suppressed, and as a result, λ can be improved.
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 20%, 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 length dW of the retained austenite grain in the plate width direction is The average value of the ratio dW / dT to the length dT in the plate thickness direction is 1.4 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 making ferrite rich in ductility 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 20% When a large amount of martensite is contained in the structure of the steel sheet before deformation by processing or the like, that is, when martensite is included in the steel structure at an area ratio of 20% or more, elongation occurs. As TS × EL decreases, λ also decreases because martensite acts as a starting point for destruction. Therefore, the area ratio of martensite is less than 20%. The area ratio of martensite is preferably 15% or less, more preferably 10% or less. The lower limit of the martensite area ratio 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% or more, and more preferably 30% or more. 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 retained 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 retained austenite exceeds 1.0 μm, when the steel sheet is deformed by processing or the like, the retained austenite is transformed into coarse martensite, and TS × EL and / or λ decreases. The average crystal grain size of the 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 by an EBSD (Electron Back Scatter Diffraction (Electron Back Scattering Diffraction)) analyzer, and from the analysis data of EBSP, the crystal orientation difference (oblique angle) exceeds 15 °, that is, a large angle grain. Residual austenite grains are defined with the boundaries as grain boundaries.

(6) Ratio dW / dT ratio of length dW in the plate width direction and length dT in the plate thickness direction of the retained austenite grains: 1.4 or more The shape of the retained austenite grains is elongated in the plate width direction. In addition, by controlling the shape to be compressed in the thickness direction (for example, approximately elliptical), the transformation-induced plasticity effect is obtained during tensile processing, while the transformation of residual austenite to hard martensite is suppressed during punching of hole expansion. Λ can be improved. Therefore, the average value of the ratio dW / dT of the length dW in the plate width direction and the length dT in the plate thickness direction of the retained austenite grains is set to 1.4 or more. The average value of dW / dT is preferably 1.5 or more, more preferably 1.6 or more.
Of the directions parallel to the plate surface, it is also assumed that the degree of elongation of the retained austenite grains differs between the plate width direction and the rolling direction, but in the embodiment of the present invention, the degree of extension in the plate width direction is It is selected as an index representing the degree of expansion in the direction parallel to the surface.

  Hereinafter, methods for evaluating the area ratio of each steel structure, the average grain size of retained austenite, and dW / dT will be exemplified.

[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]
Polishing the plate thickness section perpendicular to the rolling direction of the steel plate, and targeting the region of plate thickness / 4, 5 regions of 20 μm × 20 μm region randomly selected by the EBSD analyzer attached to the FE-SEM, Measurement is performed at a step interval of 0.05 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 ratio dW / dT of length dW in the plate width direction and length dT in the plate thickness direction of residual austenite grains]
DW / dT is obtained as follows using the SEM images of the above 10 fields of view (area of 10,000 times, 10 μm × 12 μm) observed at the time of measuring the area ratio of the steel structure.
For the SEM image, 10 straight lines are drawn at equal intervals (approximately 0.5 μm) in each of the plate width direction and the plate thickness direction. And measure the length of all the line segments where the 10 straight lines in the plate width direction overlap with the retained austenite region, divide the total length of all the line segments by the number of the line segments, An average value of the lengths of the line segments is obtained, and the average value is defined as a length dW of the retained austenite grains in the plate width direction. Similarly in the thickness direction, the lengths of all the line segments where the 10 straight lines in the thickness direction overlap with the retained austenite region are measured, and the total length of all the line segments is calculated. By dividing by the number, an average value of the lengths of the line segments is obtained, and the average value is set as a length dT of the retained austenite grains in the thickness direction. DW / dT is calculated from the obtained dW and dT.
The SEM images for all 10 fields are measured as described above, dW / dT is calculated for each field, and the average value of dW / dT for all 10 fields is obtained.

2. Chemical Component Composition The steel sheet according to the embodiment of the present invention has C: 0.05 to 0.25% by mass, Si: 1.0 to 3.0% by mass, 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% It contains less than mass%, and the balance consists of 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 increasing the retained austenite fraction and improving the stability of the retained austenite in processing as an austenite stabilizing element, and is useful for securing the strength of the steel sheet. 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: 1.0 to 3.0% by mass
Si is useful as a solid solution strengthening element for ferrite, and contributes to higher TS while suppressing the decrease in EL. Further, by suppressing recovery and recrystallization of austenite in hot rolling, dW / dT that defines the shape of austenite grains in the final structure can be controlled within a desired range. In order to effectively exhibit these actions, the Si content is set to 1.0% by mass or more. However, when Si is added excessively, the local ductility is lowered and TS × EL is lowered, so the Si content is 3.0 mass% or less. Si content becomes like this. Preferably it is 1.1 mass% or more, More preferably, it is 1.3 mass% or more, Preferably it is 2.8 mass% or less, More preferably, it is 2.6 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 and / or λ 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. Accordingly, the N content is desirably as low as possible, and the N content is 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: 0.01-0.40 mass%, Mo: 0.01-0.40 mass%, Cu: 0.01-0.40 mass%, Ni: 0.01-0.40 mass% And B: one or more selected from the group consisting of 0.0001 to 0.01% by 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 0.40% by mass or less, More preferably, it is 0.30 mass% or less, B content becomes like this. Preferably it is 0.01 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.

3. Manufacturing method The manufacturing method of the steel plate which concerns on embodiment of this invention is (800) 800 degreeC or more and less than 860 degreeC, after heating up the steel slab which has the above-mentioned chemical component composition to the heating temperature of 1050-1150 degreeC. A hot rolling step of hot rolling at a finishing temperature of 20% or more at a finishing temperature and then cooling to room temperature to obtain a hot rolled sheet; (2) the hot rolled sheet at a temperature of 500 ° C. to (Ac1 + 30 ° C.) A softening annealing step of holding at a softening annealing temperature for 0.5 to 72 hours; and (3) cold rolling the hot rolled sheet after the softening annealing at a cold rolling rate of 25 to 75%. A cold rolling step for obtaining a plate, and (4) the cold rolled plate at an average temperature increase rate of 3.0 ° C./second or more, [(Ac1 + Ac3) / 2-90 ° C.] to [(Ac1 + Ac3) / 2-20. C]], and the temperature is maintained for 10 to 1800 seconds at the soaking temperature. including.
Hereinafter, each process is explained in full detail.

(1) Hot rolling process After heating up the steel slab which has the above-mentioned chemical component composition to the heating temperature of 1050-1150 degreeC, it hot-rolls. Finish rolling is performed at a finishing reduction rate of 20% or more at a finishing temperature of 800 ° C. or more and less than 860 ° C., and then cooled to room temperature to obtain a hot-rolled sheet.

(1-1) Steel slab heating temperature: 1050 to 1150 ° C.
By regulating the heating temperature of the steel slab to 1150 ° C. or less, it is possible to suppress the coarsening of the austenite grains, effectively introduce strain into the austenite by hot rolling, and obtain a processed austenite in which recrystallization is suppressed. When the heating temperature is higher than 1150 ° C., austenite becomes coarse, an appropriate processed austenite cannot be obtained by hot rolling, and an average value of dW / dT that defines the shape of retained austenite grains in the final structure is 1.4 or more. It can not be. Moreover, in order to suppress an excessive increase in deformation resistance and edge cracking during rolling, the heating temperature is set to 1050 ° C. or higher. Further, from the viewpoint of productivity, the heating time when heating the steel slab is preferably 24 hours or less. The cast steel slab may be directly charged into the heating furnace, or the cast steel slab may be once cooled to room temperature and then charged into the heating furnace and heated.

(1-2) Finishing temperature: 800 ° C. or more, less than 860 ° C., Finishing reduction ratio: 20% or more Work austenite can be obtained by defining the conditions of finishing rolling, which is the final pass of hot rolling. When the finishing temperature is 860 ° C. or higher, or when the finishing reduction ratio is less than 20%, processed austenite is not obtained, and the average value of dW / dT that defines the shape of residual austenite grains in the final structure is 1 .4 or higher. However, if the finishing temperature is too low, the load on the rolling mill increases rapidly, so the finishing temperature is 800 ° C. or higher. The upper limit of the finishing reduction ratio is not particularly limited. After hot rolling, it may be wound and cooled to form a hot rolled coil (hot rolled plate).

(2) Softening annealing step The obtained hot-rolled sheet is held at a softening annealing temperature of 500 ° C to (Ac1 + 30 ° C) for 0.5 to 72 hours. By tempering the martensite structure obtained in the hot rolling process, the structure is softened to a strength capable of cold rolling.
When the softening annealing temperature is less than 500 ° C. or when the holding time is less than 0.5 hour, the high-strength martensite generated from the processed austenite cannot be sufficiently softened, and cold rolling is performed. It becomes difficult to implement. On the other hand, when the softening annealing temperature is higher than (Ac1 + 30 ° C.) or when the holding time is longer than 72 hours, a large amount of austenite is newly generated and coarsened. Grains are not obtained and TS × EL and / or λ decreases.
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 step The hot-rolled sheet after softening annealing is cold-rolled at a cold rolling rate of 25 to 75% to obtain a cold-rolled sheet. In cold rolling, in addition to contributing to the introduction of processed austenite by introducing a large amount of dislocations into tempered martensite generated by softening annealing, it introduces residual austenite that extends in a direction parallel to the plate surface. Also contributes. When the cold rolling rate is less than 25%, the average value of dW / dT that defines the shape of retained austenite grains decreases, so λ decreases. On the other hand, when the cold rolling rate exceeds 75%, cracks starting from the work-induced martensite occur, and thus cold rolling becomes impossible.

(4) Soaking step The obtained cold-rolled sheet is [(Ac1 + Ac3) / 2-90 ° C] to [(Ac1 + Ac3) / 2-20 ° C] at an average temperature increase rate of 3.0 ° C / second or more. The temperature is raised to a soaking temperature and held at the soaking temperature for 10 to 1800 seconds.

(4-1) Average heating rate: 3.0 ° C./second or more In order to refine the retained austenite, the temperature is raised to 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 and / or λ decreases. The upper limit of the average heating rate is not particularly limited.

(4-2) Soaking temperature: [(Ac1 + Ac3) / 2-90 ° C.] to [(Ac1 + Ac3) / 2-20 ° C.], holding time: 10 to 1800 seconds By soaking in the two-phase region of ferrite-austenite. Further, Mn added in a large amount can be concentrated in the austenite to increase the stability of the austenite. Thereby, even if it cools to room temperature, 20% or more of retained austenite can be obtained in area ratio. Moreover, the fraction of ferrite and martensite in the final structure can be controlled by soaking in a predetermined temperature range in the ferrite-austenite two-phase region. When the soaking temperature is less than [(Ac1 + Ac3) / 2-90 ° C.], or when the holding time is less than 10 seconds, the amount of austenite to be produced is insufficient, so the area ratio of residual austenite is reduced in the final structure. To do. On the other hand, when the soaking temperature is higher than [(Ac1 + Ac3) / 2−20 ° C.], or when the holding time is longer than 1800 seconds, austenite is excessively generated, so that the area ratio of martensite in the final structure is As a result, the area ratio of ferrite and / or the area ratio of retained austenite decreases, and at the same time, retained austenite becomes coarse. The holding time is preferably 20 seconds or longer, and preferably 900 seconds or shorter.
Note that during soaking, there may be an increase (decrease) in temperature 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 reduction rate in the range of a normal process.

  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./second and measuring the shrinkage associated with austenite formation. The melted steel was processed into a slab having a thickness of 50 mm by hot forging, soaked for 120 minutes at the heating temperature shown in Table 2 below, and hot rolled. Finish with a finishing temperature and finishing reduction ratio shown in Table 2 to a plate thickness of 4.0 mm, cool to 500 ° C. with water cooling, insert into an atmospheric furnace heated to 500 ° C., hold for 30 minutes, cool down by furnace, and wind up Coil cooling was simulated. Then, softening annealing is performed in an atmospheric furnace under the conditions shown in Table 2, and after air cooling, the scale is removed by pickling, and cold rolling is performed at a cold rolling rate shown in Table 2 to obtain a sheet thickness of 1.4 mm. A rolled sheet was produced. However, production No. For No. 7, 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 in an atmosphere-controlled heat treatment simulator, soaked under the conditions shown in Table 2, cooled to 200 ° C. with a gas jet, and then air-cooled.
In Tables 1 and 2, 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 grain size of retained austenite, dW / dT, TS, EL, and λ were measured in the following manner.

[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]
Polishing the plate thickness section perpendicular to the rolling direction of the steel plate, and targeting the region of plate thickness / 4, 5 regions of 20 μm × 20 μm region randomly selected by the EBSD analyzer attached to the FE-SEM, Measurement was performed at a step interval of 0.05 μm. Using the analysis software “OIM Analysis 7” manufactured by TSL Solutions, the average crystal grain size was calculated for each visual field while limiting to the residual austenite region, and the average value of the five visual fields was 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 ratio dW / dT of length dW in the plate width direction and length dT in the plate thickness direction of residual austenite grains]
DW / dT was determined as follows using the SEM images of the above 10 fields of view (area of 10,000 times, 10 μm × 12 μm) observed when measuring the area ratio of the steel structure.
For the SEM image, 10 straight lines were drawn at equal intervals (approximately 0.5 μm) in each of the plate width direction and the plate thickness direction. And measure the length of all the line segments where the 10 straight lines in the plate width direction overlap with the retained austenite region, divide the total length of all the line segments by the number of the line segments, The average value of the lengths of the line segments was determined, and the average value was defined as the length dW of the retained austenite grains in the plate width direction. Similarly in the thickness direction, the lengths of all the line segments where the 10 straight lines in the thickness direction overlap with the retained austenite region are measured, and the total length of all the line segments is calculated. By dividing by the number, the average value of the length of the line segment was obtained, and the average value was defined as the length dT of the retained austenite grain in the plate thickness direction. DW / dT was calculated from the obtained dW and dT.
The SEM images of all 10 fields were measured as described above, dW / dT was calculated for each field, and the average value of dW / dT for all 10 fields was determined.

[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 carried out by collecting JIS No. 5 test pieces from the direction perpendicular to the rolling direction (C direction), measuring TS and EL, and calculating TS × EL.

[Measurement of hole expansion ratio (λ)]
The hole expansion ratio λ was measured as follows in accordance with Japanese Industrial Standard (JIS Z 2256: 2010).
About each steel plate obtained as mentioned above, the test piece of 70 mm x 70 mm size was extract | collected from the plate | board surface direction center part. A punched hole having a diameter d ₀ (d ₀ = 10 mm) is formed in the test piece, a punch having a tip angle of 60 ° is pushed into the punched hole, and the diameter of the punched hole when the generated crack penetrates the plate thickness of the test piece. d was measured, and λ was obtained using the following equation (4).

λ (%) = {(d−d ₀ ) / d ₀ } × 100 (4)

Table 3 shows the measurement results. Regarding the mechanical properties of the steel sheet, those satisfying all of TS: 980 MPa or more, TS × EL: 29000 MPa% or more, and λ: 20% or more are indicated as “◯” as acceptable, and the others satisfying “×” ".
In Table 3, “α” indicates ferrite, “M” indicates martensite, “γ _R ” indicates retained austenite, and the underlined numerical values indicate that they are out of the scope of the present invention.

  As shown in Table 3, steel No. which is the steel of the invention (evaluation is ○). 1, 14, 17, 20, 21, and 23 to 27 are examples that satisfy all the requirements defined in the embodiments of the present invention, and TS, TS × EL, and λ all satisfy the acceptance criteria. Thus, it was confirmed that a steel sheet excellent in strength, ductility and hole expansibility was obtained.

  On the other hand, steel No. which is a comparative steel (evaluation of x). 2 to 13, 15, 16, 18, 19, and 22 are comparative examples that do not satisfy the requirements defined in the embodiment of the present invention, and at least one of TS, TS × EL, and λ was inferior.

  Steel No. In No. 2, the heating temperature in the hot rolling process was too high, the degree of elongation of retained austenite grains in the sheet width direction was insufficient, dW / dT was reduced, and λ was inferior.

  Steel No. In No. 3, the finishing temperature in the hot rolling process was too high, the degree of elongation of retained austenite grains in the sheet width direction was insufficient, dW / dT was reduced, and λ was inferior.

  Steel No. In No. 4, the finish reduction ratio in the hot rolling process was too low, the degree of elongation of the retained austenite grains in the sheet width direction was insufficient, dW / dT was reduced, and λ was inferior.

  Steel No. In No. 5, the softening annealing temperature in the softening annealing process was too high, the retained austenite was coarsened, and λ was inferior.

  Steel No. In No. 6, the retention time in the softening annealing process was too long, the retained austenite was coarsened, and λ was inferior.

  Steel No. In No. 7, the cold rolling rate was too low, the degree of elongation of retained austenite grains in the sheet width direction was insufficient, dW / dT was reduced, and λ was inferior.

  Steel No. In No. 8, the average heating rate in the soaking process was too slow, the retained austenite was coarsened, and λ was inferior.

  Steel No. In No. 9, the soaking temperature in the soaking step was too low, the ferrite was excessive, but the retained austenite was insufficient, and TS × EL was inferior.

  Steel No. In No. 10, the soaking temperature in the soaking step was too high, and the retained austenite was insufficient, while the martensite was excessive, the remaining austenite was coarsened, and TS × EL and λ were inferior.

  Steel No. In No. 11, the holding time in the soaking process was too short, the ferrite was excessive, but the retained austenite was insufficient, and TS × EL was inferior.

  Steel No. In No. 12, the retention time in the soaking process was too long, and the retained austenite was insufficient while martensite was excessive, the retained austenite was coarsened, and TS × EL and λ were inferior.

  Steel No. 13, 15, 16, 18, 19, and 22 were inferior in at least one of TS, TS × EL, and λ because the chemical composition did not meet the requirements defined in the embodiments of the present invention.

  Steel No. No. 13 (steel type B) had an excessively low C content and an excess of ferrite, but lacked retained austenite, and was inferior in TS and TS × EL.

  Steel No. 15 (steel type D) had an excessively high C content, excessive martensite, and inferior TS × EL and λ.

  Steel No. In No. 16 (steel type E), the Si content was too low, the degree of elongation of retained austenite grains in the sheet width direction was insufficient, dW / dT was reduced, and λ was inferior.

  Steel No. 18 (steel type G) had an excessively high Si content and an inferior TS × EL.

  Steel No. 19 (steel type H) had an excessively low Mn content, lack of retained austenite, and inferior TS × EL.

  Steel No. No. 22 (steel type K) had an excessively high Mn content, coarsened retained austenite, and inferior TS × EL and λ.

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

Claims (4)

C: 0.05-0.25 mass%,
Si: 1.0-3.0 mass%,
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 20%,
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 in which the average value of the ratio dW / dT of the length dW in the plate width direction and the length dT in the plate thickness direction of the retained austenite grains is 1.4 or more. Cr: 0.01-0.40 mass%,
Mo: 0.01-0.40 mass%,
Cu: 0.01-0.40 mass%,
The steel plate according to claim 1, further comprising one or more selected from the group consisting of Ni: 0.01-0.40 mass% and B: 0.0001-0.01 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%. After heating the steel slab having the chemical composition according to any one of claims 1 to 3 to a heating temperature of 1050 to 1150 ° C, at a finishing temperature of 800 ° C or more and less than 860 ° C, 20% or more. A hot rolling step of hot rolling at a finishing reduction ratio of, 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 500 ° C. to (Ac1 + 30 ° C.) for 0.5 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 75% to obtain a cold rolled sheet;
The cold-rolled sheet is heated to a soaking temperature of [(Ac1 + Ac3) / 2-90 ° C.] to [(Ac1 + Ac3) / 2-20 ° C.] at an average heating rate of 3.0 ° C./second or more, And a soaking step of maintaining the soaking temperature for 10 to 1800 seconds.