KR20200075957A - Steel sheet having excellent workability and balance of strength and ductility, and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a steel plate that can be used in automobile parts, etc., and to a steel plate having excellent balance of strength and ductility and excellent workability, and a manufacturing method of the same. The present invention includes more than 0.25 to 0.75 wt% of C, 4.0 wt% or less of Si, 0.9 to 5.0 wt% of Mn, 5.0 wt% or less of Al, 0.15 wt% or less of P, 0.03 wt% or less of S, 0.03 wt% or less of N, and the rest containing Fe and unavoidable impurities. A microstructure includes tempered martensite, bainite and retained austenite.

Description

Steel sheet with excellent balance of strength and ductility and processability and its manufacturing method{STEEL SHEET HAVING EXCELLENT WORKABILITY AND BALANCE OF STRENGTH AND DUCTILITY, AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a steel sheet that can be used in automobile parts, and the like, and relates to a steel sheet having excellent balance of strength and ductility, and excellent workability, and a method for manufacturing the same.

Recently, the automobile industry is paying attention to measures to reduce the weight of materials and secure passenger safety at the same time to protect the global environment. In order to meet this stability and light weight, the application of high-strength steel sheet is rapidly increasing. In general, the higher the strength of the steel sheet is, the lower the ductility and the workability are. In the steel sheet for automobile members, a balance between strength and ductility and excellent workability are required.

As a technique for improving the ductility of a steel sheet, a method of utilizing tempered martensite is disclosed in Patent Documents 1 and 2. Tempered martensite, made by tempering hard martensite, is a softened martensite, and shows a difference in strength from the existing non-tempered martensite (fresh martensite). Inhibiting fresh martensite and forming tempered martensite increases ductility and processability.

However, with the technology disclosed in Patent Documents 1 and 2, the product of tensile strength and elongation (TS×EL) does not satisfy more than 22,000 MPa%, making it difficult to secure a balance of excellent strength and ductility.

On the other hand, in order to obtain all of the characteristics of high strength, ductility, and processability, the steel plate for automobile members has developed TRIP (Transformation Induced Plasticity) steel using the residual organic plasticity of austenite. Patent documents 3 and 4 disclose TRIP steels having excellent ductility and workability.

In Patent Document 3, including ferrite of polygon, residual austenite and martensite, it was intended to improve ductility and processability, but bainite was used as the main phase, so high strength was not secured, and TS×EL was 22,000. It can be seen that MPa or more is not satisfied.

Patent Document 4 improves ductility and processability by forming a ferrite formation, refinement of residual austenite, and forming a composite structure containing tempered martensite, but it is difficult to secure high strength because it contains a large amount of soft ferrite. There is.

Even now, the balance between strength and ductility is excellent, but the situation has not been met for the demand for a steel sheet having excellent workability.

Korean Patent Publication No. 10-2006-0118602 Japanese Patent Application Publication No. 2009-019258 Korean Patent Publication No. 10-2014-0012167 Korean Patent Publication No. 10-2010-0092503

One aspect of the present invention is to provide a method for manufacturing a steel sheet having excellent workability and a method of manufacturing the same while securing a balance of excellent strength and ductility by optimizing the composition and microstructure of the steel sheet.

The subject of this invention is not limited to the above-mentioned matter. Additional objects of the present invention are described in the overall contents of the specification, and those skilled in the art to which the present invention pertains will have no difficulty in understanding additional objects of the present invention from the contents described in the specification of the present invention.

One aspect of the present invention, by weight, C: greater than 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N : 0.03% or less, the rest contains Fe and unavoidable impurities,

Microstructures include tumper martensite, bainite and residual austenite,

It relates to a steel sheet excellent in the balance and workability of strength and ductility satisfying the following [Relational Formula 1] and [Relational Formula 2].

[Relationship 1]

0.55 ≤ [Si+Al]γ / [Si+Al]av ≤ 0.85

(However, [Si+Al]γ is the content of Si and Al contained in the retained austenite (% by weight), and [Si+Al]av is the content of Si and Al contained in the steel sheet (% by weight))

[Relationship 2]

V(1.2㎛, γ) / V(γ) ≥ 0.1

(However, V (1.2㎛, γ) is the fraction of the retained austenite having an average grain size of 1.2㎛ or more, V(γ) is the residual austenite fraction of the steel sheet)

Another aspect of the present invention in weight%, C: more than 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less , N: 0.03% or less, the rest providing a cold rolled steel sheet containing Fe and unavoidable impurities;

Heating the cold rolled steel sheet to Ar3 or higher (primary heating), and maintaining it for at least 50 seconds (primary holding);

Cooling (primary cooling) to an average cooling rate of 1°C/s or higher, to a temperature range of 600-850°C (primary cooling stop temperature);

Cooling to an average cooling rate of 2°C/s or more, to a temperature range of 350 to 550°C (secondary cooling), and maintaining for 5 seconds or more (secondary maintenance) in this temperature range;

Cooling to an average cooling rate of 1°C/s or more, to a temperature range of 250 to 450°C (3rd cooling), and maintaining for 5 seconds or more in this temperature range (3rd holding);

Cooling to an average cooling rate of 2°C/s or more, to a temperature range of 100 to 300°C (secondary cooling stop temperature) (fourth cooling);

Heating to a temperature range of 300 to 500°C (secondary heating), and maintaining at least 50 seconds (fourth maintenance) in this temperature range; And

It relates to a method of manufacturing a steel sheet having excellent balance between strength and ductility and processability, including cooling to room temperature (5th cooling).

ADVANTAGE OF THE INVENTION According to this invention, the steel plate which has excellent strength and ductility balance and workability can be provided for automotive structural use which simultaneously requires weight reduction and stability.

The inventors of the present invention seek to stabilize the retained austenite in the transformation induced plasticity (TRIP) steel including bainite, tempered martensite, residual austenite, and the like, to retain residual austenite size and shape. Through this, it was recognized that the influence on the balance of strength, ductility and bending workability was obtained. By investigating this, a steel plate excellent in balance and workability of strength and ductility and a method for manufacturing the same were devised.

Hereinafter, the present invention will be described in detail. First, the alloy composition of the steel sheet of the present invention will be described in detail.

The steel sheet of the present invention by weight (hereinafter, %), C: more than 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03 % Or less, N: 0.03% or less, the rest include Fe and unavoidable impurities. Additionally, Ti: 0-0.5%, Nb: 0-0.5%, V: 0-0.5%, Cr: 0-3.0%, Mo: 0-3.0%, Cu: 0-4.5%, Ni: 0-4.5% , B: 0 to 0.005%, Ca: 0 to 0.05%, REM excluding Y: 0 to 0.05%, Mg: 0 to 0.05%, W: 0 to 0.5%, Zr: 0 to 0.5%, Sb: 0 ~0.5%, Sn: 0~0.5%, Y: 0~0.2%, Hf: 0~0.2% and Co: 0~1.5%. Hereinafter, each alloy composition will be described in detail.

Carbon (C): more than 0.25 to 0.75%

C is an indispensable element for imparting the strength of the steel sheet, and is a stabilizing element of retained austenite that increases the ductility of the steel sheet. If the C content is 0.25% or less, it is difficult to secure the required tensile strength, and if it exceeds 0.75%, cold rolling is difficult to produce a steel sheet. Therefore, the content of C is preferably greater than 0.25 to less than 0.75%. The content of C is more preferably 0.31 to 0.75%.

Silicon (Si): 4.0% or less (excluding 0)

The Si is an element having an effect of improving strength by solid solution strengthening, and is an element that strengthens ferrite, homogenizes the structure, and improves processability. In addition, it is an element that suppresses precipitation of cementite and contributes to the formation of residual austenite. When the Si exceeds 4.0%, the plating defect problem such as unplating in the plating process and the weldability of the steel sheet are lowered, so the Si content is preferably 4.0% or less.

Aluminum (Al): 5.0% or less (excluding 0)

The Al is an element that deoxidizes by bonding with oxygen in the steel. In addition, it is an element that stabilizes the retained austenite by suppressing cementite precipitation similar to Si. When the Al content exceeds 5.0%, the workability of the steel sheet is deteriorated and inclusions are increased. Therefore, the Al content is preferably 5.0% or less.

On the other hand, the sum of Si and Al (Si+Al) is preferably 1.0 to 6.0%. The Si and Al are components that affect the formation of microstructures in the present invention and affect the ductility and bending workability. Therefore, in order to have excellent ductility and bending workability, it is preferable that the total amount of Si and Al is 1.0 to 6.0%. More preferably, it contains 1.5 to 4.0%.

Manganese (Mn): 0.9~5.0%

The Mn is an element useful for increasing strength and ductility together. Although the above effect can be obtained at 0.9% or more, when it exceeds 5.0%, the weldability and impact toughness of the steel sheet is deteriorated. In addition, when Mn is included in excess of 5.0%, the bainite transformation time increases, and C concentration in austenite is insufficient, so that the required fraction of retained austenite cannot be obtained. Therefore, the content of Mn is preferably 0.9 to 5.0%.

Phosphorus (P): 0.15% or less

The P is an element that is contained as impurities and deteriorates impact toughness. Therefore, it is preferable to manage the content of P to 0.15% or less.

Sulfur (S): 0.03% or less

The S is an element that is contained as an impurity to make MnS in the steel sheet, and deteriorates ductility. Therefore, the content of S is preferably 0.03% or less.

Nitrogen (N): 0.03% or less

The N is an element that is contained as an impurity to produce nitride during continuous casting to cause cracking of the slab. Therefore, the content of N is preferably 0.03% or less.

The rest contains Fe and impurities that are inevitably included. On the other hand, the steel sheet of the present invention has an alloy composition that can be additionally included in addition to the above-described alloy components, which will be described in detail below.

Titanium (Ti): 0 to 0.5%, Niobium (Nb): 0 to 0.5%, Vanadium (V): 0 to 0.5%

The Ti, Nb and V are elements that make precipitates to refine crystal grains. It may be contained in order to improve the strength and impact toughness of the steel sheet. When each content of Ti, Nb, and V exceeds 0.5%, not only the impact toughness is lowered due to excessive precipitate formation, but also the cause of increase in manufacturing cost, the content is preferably 0.5% or less.

Chromium (Cr): 0 to 3.0% and molybdenum (Mo): 0 to 3.0%

Cr and Mo are elements that suppress austenite decomposition during alloying treatment and stabilize austenite in the same manner as Mn. When each content of Cr and Mo exceeds 3.0%, the bainite transformation time increases, and the concentration of C in austenite is insufficient, so that a required fraction of austenite cannot be obtained. Therefore, it is preferable that each content of Cr and Mo is 3.0% or less.

Copper (Cu): 0 to 4.5% and nickel (Ni): 0 to 4.5%

Cu and Ni are elements that stabilize austenite and inhibit corrosion. The Cu and Ni are concentrated to the surface of the steel sheet to prevent hydrogen invasion that moves into the steel sheet, thereby suppressing hydrogen delay destruction. When each content of Cu and Ni exceeds 4.5%, it causes not only excessive characteristic effects, but also an increase in manufacturing cost. Therefore, it is preferable that each content of Cu and Ni is 4.5% or less.

Boron (B): 0~0.005%

The B is an element that improves hardenability to increase strength and suppress nucleation of grain boundaries. When the content of B exceeds 0.005%, it causes not only excessive characteristic effects but also an increase in manufacturing cost. Therefore, the content of B is preferably 0.005% or less.

Calcium (Ca): 0 to 0.05%, Magnesium (Mg): 0 to 0.05%, and rare earth elements (REM) excluding yttrium (Y): 0 to 0.05%

The REM refers to a total of 17 elements of Sc, Y and lanthanoid. REM except Ca, Mg and Y can improve the ductility of the steel sheet by spheroidizing the sulfide. When each content of REM excluding Ca, Mg and Y exceeds 0.05%, it causes not only excessive characteristic effects but also manufacturing cost increase. Therefore, each content of REM excluding Ca, Mg, and Y is preferably 0.05% or less.

Tungsten (W): 0 to 0.5% and zirconium (Zr): 0 to 0.5%

The W and Zr are elements that improve the hardenability and increase the strength of the steel sheet. When each content of the W and Zr exceeds 0.5%, it causes not only excessive characteristic effects but also an increase in manufacturing cost. Therefore, it is preferable that each content of W and Zr is 0.5% or less.

Antimony (Sb): 0~0.5% and Tin (Sn): 0~0.5%

The Sb and Sn are elements that improve the plating wettability and plating adhesion of the steel sheet. When each content of the Sb and Sn exceeds 0.5%, the brittleness of the steel sheet increases, and cracking may occur during hot working or cold working. Therefore, each content of the Sb and Sn is preferably 0.5% or less.

Yttrium (Y): 0 to 0.2% and Hafnium (Hf): 0 to 0.2%

Y and Hf are elements that improve the corrosion resistance of the steel sheet. When each content of Y and Hf exceeds 0.2%, ductility of the steel sheet may be deteriorated. Therefore, each content of Y and Hf is preferably 0.2% or less.

Cobalt (Co): 0~1.5%

The Co is an element that promotes bainite transformation and increases the TRIP effect. When the Co content exceeds 1.5%, weldability and ductility of the steel sheet may deteriorate. Therefore, the content of Co is preferably 1.5% or less.

The microstructure of the steel sheet of the present invention includes tempered martensite, bainite and residual austenite. As a preferred example, in a volume fraction, containing 30 to 75% of tempered martensite, 10 to 50% of bainite, and 10 to 40% of retained austenite, consisting of 5% or less of ferrite and remainder inevitable tissue . Examples of the inevitable structure include fresh martensite, pearlite, and martensite Austenite Constituent (M-A). When the fresh martensite or pearlite is excessively formed, the ductility and workability of the steel sheet may be deteriorated or the fraction of retained austenite may be reduced.

As shown in the following relational expression 1, Si and Al contents ([Si+Al]γ, weight%) contained in the retained austenite are included in the steel sheet, and Si and Al contents ([Si+Al]av, weight%) It is preferable that the value divided by is 0.55 to 0.85.

[Relationship 1]

0.55 ≤ [Si+Al]γ / [Si+Al]av ≤ 0.85

The steel sheet of the present invention has a product of tensile strength and elongation (TS×EL) of 22,000 MPa% or more, and R/t (R is a minimum bending radius (mm) that does not crack after a 90° bending test, and t is a steel sheet Thickness (mm)) of 0.5 to 3.0, excellent balance of strength and ductility, and excellent workability.

In order to secure this, it is important to stabilize the retained austenite of the steel sheet. In order to stabilize the retained austenite, it is necessary to concentrate C and Mn into austenite in ferrite, bainite and tempered martensite of the steel sheet. However, when C is concentrated into austenite by using ferrite, the strength of the steel sheet may be insufficient due to the low strength property of ferrite. Therefore, it is preferable to concentrate C and Mn into austenite by utilizing bainite and tempered martensite. In addition, by controlling the Si and Al content ([Si+Al]γ) in the retained austenite, a large amount of C and Mn can be concentrated in the retained austenite from bainite and tempered martensite. Therefore, it is possible to stabilize the retained austenite by controlling Si and Al in the retained austenite. Thus, in the present invention, [Si+Al]γ / [Si+Al]av is set to 0.55 or more to stabilize the retained austenite. However, when [Si+Al]γ / [Si+Al]av exceeds 0.85, C and Mn thickening in residual austenite is insufficient, and thus residual austenite becomes unstable in tensile strain, leading to deterioration in ductility and workability. TS x EL is less than 22,000 MPa%, or R/t exceeds 3.0, which is not preferable.

The steel sheet containing the retained austenite has excellent ductility and workability due to the transformation organic plasticity generated during transformation from austenite to martensite during processing. When the retained austenite of the steel sheet is less than 10%, TS×EL may be less than 22,000 MPa% or R/t may exceed 3.0. On the other hand, if the residual austenite fraction exceeds 40%, local elongation may be deteriorated. Therefore, in order to obtain a steel sheet having both excellent balance of strength and ductility and workability, the fraction of the retained austenite is preferably 10-40%.

On the other hand, residual austenite having an average grain size of 1.2 µm or more among the retained austenite is heat-treated at a bainite formation temperature to increase the average size to suppress transformation of austenite to martensite, thereby improving the workability of the steel sheet. .

In addition, the residual austenite in the form of lath among the retained austenite affects the workability of the steel sheet. The retained austenite is divided into a retained austenite in the form of a wreath formed between bainite phases and a retained austenite in a block form formed in a portion without bainite phases. As the residual austenite in the block form is further transformed into bainite in the heat treatment process, the residual austenite in the form of wrest is increased, and eventually the workability of the steel sheet can be improved.

Therefore, in order to improve the ductility and workability of the steel sheet, it is preferable to increase the residual austenite fraction having an average grain size of 1.2 µm or more among residual austenite and the residual austenite fraction in a lath form. Therefore, in the present invention, it is preferable to satisfy the following relational expression 2 and relational expression 3. When the value of the following relational expression 2 is less than 0.1 or the value of the relational expression 3 is less than 0.5, R/t does not satisfy 0.5 to 3.0, so it is difficult to secure excellent workability.

[Relationship 2]

V(1.2㎛, γ) / V(γ) ≥ 0.1

(However, V (1.2㎛, γ) is the fraction of the retained austenite having an average grain size of 1.2㎛ or more, V(γ) is the residual austenite fraction of the steel sheet)

[Relationship 3]

V(lath, γ) / V(γ) ≥ 0.5

(However, V(lath, γ) is the fraction of retained austenite in the form of lath, and V(γ) is the fraction of retained austenite in the steel sheet)

On the other hand, both non-tempered martensite (fresh martensite) and tempered martensite are microstructures that improve the strength of the steel sheet. However, when compared with tempered martensite, fresh martensite has a property of significantly reducing the ductility of the steel sheet. This is because the microstructure of tempered martensite is softened by tempering heat treatment. Therefore, it is preferable to utilize tempered martensite in order to provide a steel sheet excellent in balance and workability of strength and ductility of the present invention. If the fraction (volume fraction) of the tempered martensite is less than 30%, it is difficult to secure TS×EL of 22,000 MPa or more, and when it exceeds 75%, ductility and workability are reduced, and TS×EL is less than 22,000 MPa. Or R/t is more than 3.0 is undesirable.

In order to improve the balance and workability of the strength and ductility of the steel sheet, it is preferable to contain bainite appropriately. When the bainite fraction (volume fraction) is 10% or more, TS×EL is 22,000 MPa% or more and R/t is 0.5 to 3.0. However, more than 50% of bainite decreases the fraction of tempered martensite relatively, resulting in TS×EL of less than 22,000 MPa%, which is undesirable.

Hereinafter, an example of a method for manufacturing the steel sheet of the present invention will be described in detail. In the steel sheet manufacturing method of the present invention, first, a steel ingot or steel slab having the above-described alloy composition is prepared, and the steel ingot or steel slab is heated to hot roll, and then wound, pickled and cold rolled to prepare a cold rolled steel sheet.

As an example, it is preferable to heat the steel ingot or steel slab to a temperature of 1000 to 1350°C and finish hot roll to a temperature of 800 to 1000°C. When the heating temperature is less than 1000°C, there is a possibility that hot rolling will be performed below the finish hot rolling temperature range. Further, when the heating temperature exceeds 1350°C, there is a possibility that the melting point of the steel is reached and melted. On the other hand, when the finish hot rolling temperature is less than 800°C, the high strength of the steel may place a heavy burden on the rolling mill. In addition, when the finish hot-rolling temperature exceeds 1000°C, after hot rolling, the grains of the steel sheet are coarse to deteriorate the physical properties of the high-strength steel sheet. In order to refine the crystal grains of the hot-rolled steel sheet, it is preferable to cool at a cooling rate of 10° C./s or higher after finishing hot rolling, and wind up at a temperature of 350 to 700° C. When the coiling temperature is less than 350°C, coiling is not easy, and when it exceeds 700°C, a scale generated on the surface of the hot-rolled steel sheet is formed up to the inside of the steel sheet, thereby making pickling difficult. On the other hand, after the winding, to remove the scale formed on the surface of the steel sheet is pickled and cold rolled. The pickling and cold rolling conditions are not particularly limited, and the cold rolling is preferably set to a cumulative rolling reduction of 30 to 90%. When the cumulative reduction ratio of cold rolling exceeds 90%, there is a possibility that it is difficult to perform cold rolling in a short time due to the high strength of the steel sheet.

The cold rolled steel sheet may be made of an unplated cold rolled steel sheet through an annealing heat treatment process, or may be made of a plated steel sheet through a plating process to impart corrosion resistance. For plating, plating methods such as hot-dip galvanizing, electro-galvanizing, and hot-dip aluminum plating can be applied, and the method and type are not particularly limited.

In order to secure the balance and workability of strength and ductility according to the present invention, an annealing heat treatment step is performed. Hereinafter, an example will be described in detail.

The cold rolled steel sheet is heated to Ac3 or higher (primary heating), and maintained for 50 seconds or longer (primary holding).

When the primary heating or primary holding temperature is less than Ac3, ferrite may be formed and residual austenite is insufficient, so that [Si+Al]γ / [Si+Al]av, TS×EL and bending workability of the steel sheet It can lower. In addition, when the primary holding time is less than 50 seconds, the structure cannot be sufficiently homogenized, thereby deteriorating the physical properties of the steel sheet. The upper limit of the primary heating to the holding temperature and the upper limit of the primary holding time are not particularly required, but in order to suppress toughness reduction due to grain coarsening, the primary heating to holding temperature is 950°C or lower, primary The holding time is preferably 1200 seconds or less.

After the primary maintenance, it is preferable to cool (primary cooling) to a temperature range of 600 to 850 °C of primary cooling at an average cooling rate of 1 °C/s or higher. The upper limit of the primary average cooling rate is not particularly required, and the primary average cooling rate is preferably 100° C./s or less. When the primary cooling stop temperature is less than 600°C, ferrite is excessively formed and residual austenite is insufficient, which may degrade [Si+Al]γ / [Si+Al]av, TS×EL and bending workability. . In addition, the upper limit of the primary cooling stop temperature is preferably 30°C or less than the primary holding temperature, and is preferably 850°C.

After the primary cooling, it is preferable to cool (secondary cooling) to a temperature range of 350°C to 550°C at an average cooling rate of 2°C/s or higher, and maintain it for 5 seconds or longer (secondary maintenance) in this temperature range. When the secondary average cooling rate is less than 2°C/s, ferrite is excessively formed, and residual austenite is insufficient to improve [Si+Al]γ / [Si+Al]av, TS×EL and bending workability of the steel sheet. It can decrease. The upper limit of the secondary average cooling rate is not particularly required, and the secondary cooling rate is preferably 100°C/s or less. On the other hand, when the secondary holding temperature exceeds 550°C, the residual austenite is insufficient, and the [Si+Al]γ / [Si+Al]av, V(lath, γ) / V(γ), TS× of the steel sheet EL and bending workability can be reduced. In addition, if it is less than 350°C, V (1.2 μm, γ) / V (γ) and bending workability of the steel sheet may be reduced to a low heat treatment temperature. If the second holding time is less than 5 seconds, the heat treatment time is insufficient, and thus V(1.2㎛, γ) / V(γ), V(lath, γ) / V(γ) and bending workability of the steel sheet may be deteriorated. The upper limit of the second holding time is not particularly required, and the second holding time is preferably 600 seconds or less.

Meanwhile, the primary cooling rate Vc1 is preferably smaller than the secondary cooling rate Vc2 (Vc1 <Vc2).

After the second holding, it is preferable to cool (3rd cooling) to a temperature range of 250 to 450°C at an average cooling rate of 1°C/s or more, and to hold for 5 seconds or more (3rd holding) in this temperature range. The upper limit of the third average cooling rate is not particularly required, and the third average cooling rate is preferably 100° C./s or less. When the third holding temperature exceeds 450°C, V(lath, γ) / V(γ) and bending workability may be deteriorated due to a high heat treatment temperature. On the other hand, if it is less than 250° C., V (1.2 μm, γ) / V (γ), V (lath, γ) / V (γ) and bending workability of the steel sheet may be reduced to a low heat treatment temperature. When the third holding time is less than 5 seconds, the heat treatment time is insufficient, and thus V(1.2㎛, γ) / V(γ), V(lath, γ) / V(γ) and bending workability of the steel sheet may be deteriorated. The upper limit of the third holding time is not particularly required, and the third holding time is preferably 600 seconds or less.

After the third maintenance, it is preferable to cool (quaternary cooling) to a temperature range of 100 to 300°C, which is the secondary cooling stop temperature, at an average cooling rate of 2°C/s or higher. When the fourth cooling rate is less than 2°C/s, bending workability of the steel sheet may be reduced due to slow cooling. The upper limit of the fourth average cooling rate is not particularly required, and the fourth average cooling rate is preferably 100° C./s or less. On the other hand, when the secondary cooling stop temperature exceeds 300°C, bainite is excessively formed and the tempered martensite is insufficient, which may degrade TS×EL of the steel sheet. On the other hand, below 100°C, tempered martensite is excessively formed and there is insufficient residual austenite, so [Si+Al]γ / [Si+Al]av, V(1.2㎛,γ)/V(γ), TSХEL And bending workability.

After the fourth cooling, it is preferable to heat (secondary heating) to a temperature range of 300 to 500° C., and to maintain at least 50 seconds (fourth maintenance) in this temperature range. When the fourth holding temperature exceeds 500°C, the residual austenite fraction becomes insufficient, and the [Si+Al]γ / [Si+Al]av, V(1.2㎛, γ) / V(γ) of the steel sheet, TSХEL and bending workability can be deteriorated. On the other hand, if it is less than 300°C, control of the Si content in the retained austenite is insufficient, resulting in insufficient residual austenite fraction, and eventually [Si+Al]γ / [Si+Al]av, V(1.2㎛, γ of the steel sheet) ) / V(γ), TSХEL and bending workability can be reduced. In addition, when the fourth holding time is less than 50 seconds, tempered martensite is excessive and residual austenite is insufficient, so that [Si+Al]γ / [Si+Al]av, V(1.2㎛, γ) / V of the steel sheet (γ), TSХEL and bending workability can be reduced. In addition, when the fourth holding time exceeds 172,000 seconds, it is difficult to secure the fraction of the retained austenite due to insufficient control of Si and Al content in the retained austenite. As a result, [Si+Al]γ/[Si+Al]av, TSХEL and bending workability of the steel sheet are lowered.

After the fourth maintenance, it is preferable to cool (5th cooling) to room temperature at an average cooling rate of 1°C/s or more.

Hereinafter, embodiments of the present invention will be described in detail. It is necessary to note that the following examples are only for understanding the present invention and are not intended to specify the scope of the present invention. The scope of rights of the present invention is determined by matters described in the claims and reasonably inferred therefrom.

(Example)

A steel slab having a thickness of 30 mm having an alloy composition (the rest is Fe and an unavoidable impurity) according to Table 1 was prepared, heated at 1200° C., and then hot rolled at 900° C. and averaged at 30° C./s. It was cooled at a cooling rate and wound at 600°C to produce a hot rolled steel sheet with a thickness of 3 mm. After removing the surface scale by pickling the hot rolled steel sheet, cold rolling was performed to a thickness of 1.5 mm.

Thereafter, heat treatment was performed under the annealing heat treatment conditions disclosed in Tables 2 to 5 to prepare steel sheets.

The microstructure of the steel sheet thus manufactured was observed, and the results are shown in Tables 6 and 7. Among the microstructures, ferrite (F), bainite (B), tempered martensite (TM), and pearlite (P) were observed through SEM after the etched surface of the polished specimen. Of these, bainite and tempered martensite, which are difficult to distinguish, calculated fractions using an expansion curve after the evaluation of the orientation. On the other hand, since fresh martensite (FM) and residual austenite (residual γ) are also difficult to distinguish, the fraction of martensite and residual austenite observed by the SEM is subtracted from the fraction of residual austenite calculated by X-ray diffraction. The value was determined as the fresh martensite fraction.

On the other hand, [Si+Al]γ / [Si+Al]av, V(1.2㎛, γ) / V(γ), V(lath, γ) / V(γ), TS×EL, R/t was observed, and the results are shown in Tables 8 and 9.

The Si and Al content ([Si+Al]γ) contained in the retained austenite was determined using the EPMA (Electron Probe MicroAnalyser) to determine the Si+Al content measured in the retained austenite phase. The [Si+Al]av means the average Si+Al content of the entire steel sheet.

Residual austenite (V (1.2 µm, γ)) having an average grain size of 1.2 µm or more and residual austenite (V(lath, γ)) having a wth shape are retained using a phase map of EPMA. It was determined by the area measured in the austenite phase. The measured area of the retained austenite phase refers to the volume of the retained austenite in terms of the entire steel sheet.

The TS×EL and R/t were evaluated by tensile test and V-bending test. In the tensile test, TS×EL was determined by evaluating the test pieces collected according to JIS 5 standards based on the 90° direction of the rolling direction of the rolled sheet. R/t was determined by dividing the minimum bending radius R, which does not cause cracks after the 90° bending test, by dividing the thickness t of the sheet by taking a specimen based on the 90° direction with respect to the rolling direction of the rolled sheet.

Steel Chemical composition (% by weight) C Si Mn P S Al N Cr Mo Etc A 0.39 1.98 2.13 0.011 0.0008 0.02 0.0032 0.51 B 0.38 2.03 2.21 0.010 0.0013 0.02 0.0028 0.23 0.18 C 0.37 1.95 1.88 0.010 0.0010 0.02 0.0029 0.47 D 0.33 2.31 3.95 0.009 0.0012 0.03 0.0030 0.49 E 0.41 1.85 2.06 0.008 0.0009 0.03 0.0031 F 0.52 1.68 2.33 0.009 0.0008 0.02 0.0027 G 0.72 1.64 2.41 0.012 0.0011 0.02 0.0034 H 0.38 0.87 2.11 0.011 0.0010 1.93 0.0033 I 0.36 1.08 2.07 0.011 0.0013 2.35 0.0031 J 0.35 0.02 1.95 0.010 0.0010 4.67 0.0030 Ti: 0.05 K 0.43 1.74 1.93 0.008 0.0011 0.02 0.0035 Nb: 0.05 L 0.41 1.89 1.88 0.009 0.0011 0.02 0.0028 V: 0.05 M 0.39 1.75 1.92 0.011 0.0012 0.02 0.0027 Ni: 0.36 N 0.38 1.89 2.18 0.012 0.0013 0.03 0.0024 Cu: 0.35 O 0.38 1.68 2.22 0.013 0.0007 0.03 0.0028 B: 0.003 P 0.36 1.88 2.26 0.012 0.0008 0.02 0.0026 Ca: 0.002 Q 0.37 1.84 2.37 0.008 0.0009 0.02 0.0031 REM: 0.001 R 0.44 1.73 2.45 0.009 0.0009 0.02 0.0031 Mg: 0.001 S 0.42 1.77 2.38 0.010 0.0010 0.02 0.0034 W: 0.11 T 0.31 1.95 2.19 0.010 0.0011 0.02 0.0033 Zr: 0.10 U 0.32 1.98 2.03 0.009 0.0013 0.03 0.0032 Sb: 0.02 V 0.39 1.82 2.41 0.008 0.0012 0.02 0.0030 Sn: 0.02 W 0.36 1.78 2.26 0.009 0.0012 0.02 0.0027 Y: 0.01 X 0.37 3.64 2.14 0.009 0.0007 0.03 0.0029 Hf: 0.01 Y 0.37 2.27 2.18 0.011 0.0007 0.03 0.0028 Co: 0.35 XA 0.21 1.92 2.05 0.011 0.0008 0.03 0.0024 XB 0.78 1.94 2.11 0.008 0.0011 0.02 0.0031 XC 0.39 0.02 2.16 0.012 0.0012 0.03 0.0027 XD 0.38 4.26 2.07 0.012 0.0009 0.02 0.0032 XE 0.40 0.03 2.31 0.008 0.0010 5.31 0.0026 XF 0.41 1.84 0.75 0.009 0.0010 0.02 0.0033 XG 0.38 1.88 5.64 0.011 0.0012 0.02 0.0031 XH 0.38 1.96 2.20 0.010 0.0011 0.02 0.0030 3.38 XI 0.36 1.89 2.08 0.009 0.0010 0.02 0.0027 3.41

division number Steel Primary
Average
heating
speed
(℃/s) Primary
maintain
Temperature
(℃) Primary
maintain
time
(s) Primary
Average
Cooling
speed
(℃/s) Primary
Cooling
stop
Temperature
(℃) Secondary
Average
Cooling
speed
(℃/s) Secondary
maintain
Temperature
(℃) Secondary
maintain
time
(s) 3rd
Average
Cooling
speed
(℃/s) Inventive Example One A 10 880 120 10 700 20 425 40 10 Comparative example 2 A 10 730 120 10 700 20 425 40 10 Comparative example 3 A 10 880 One 10 700 20 425 40 10 Comparative example 4 A 10 880 120 10 580 20 425 40 10 Inventive Example 5 A 10 880 120 10 820 20 425 40 10 Comparative example 6 A 10 880 120 10 700 0.5 425 40 10 Comparative example 7 A 10 880 120 10 700 20 580 40 10 Comparative example 8 A 10 880 120 10 700 20 425 2 10 Comparative example 9 A 10 880 120 10 700 20 320 40 10 Inventive Example 10 B 10 880 120 10 700 20 425 40 10 Comparative example 11 B 10 880 120 10 700 20 520 40 10 Comparative example 12 B 10 880 120 10 700 20 425 40 10 Comparative example 13 B 10 880 120 10 700 20 425 40 10 Comparative example 14 B 10 880 120 10 700 20 425 40 10 Comparative example 15 B 10 880 120 10 700 20 425 40 10 Comparative example 16 B 10 880 120 10 700 20 425 40 10 Comparative example 17 B 10 880 120 10 700 20 425 40 10 Comparative example 18 B 10 880 120 10 700 20 425 40 10 Comparative example 19 B 10 880 120 10 700 20 425 40 10 Comparative example 20 B 10 880 120 10 700 20 425 40 10 Inventive Example 21 C 10 880 120 10 700 20 425 40 10 Inventive Example 22 D 10 880 120 10 700 20 425 40 10 Inventive Example 23 E 10 880 120 10 700 20 425 40 10 Inventive Example 24 F 10 880 120 10 700 20 425 40 10 Inventive Example 25 G 10 880 120 10 700 20 500 40 10 Inventive Example 26 H 10 880 120 10 700 20 400 40 10 Inventive Example 27 I 10 880 120 10 700 20 425 40 10 Inventive Example 28 J 10 880 120 10 700 20 425 40 10 Inventive Example 29 K 10 880 120 10 700 20 425 40 10 Inventive Example 30 L 10 880 120 10 700 20 425 40 10

division number Steel Primary
Average
heating
speed
(℃/s) Primary
maintain
Temperature
(℃) Primary
maintain
time
(s) Primary
Average
Cooling
speed
(℃/s) Primary
Cooling
stop
Temperature
(℃) Secondary
Average
Cooling
speed
(℃/s) Secondary
maintain
Temperature
(℃) Secondary
maintain
time
(s) 3rd
Average
Cooling
speed
(℃/s) Inventive Example 31 M 10 880 120 10 700 20 425 40 10 Inventive Example 32 N 10 880 120 10 700 20 425 40 10 Inventive Example 33 O 10 880 120 10 700 20 425 40 10 Inventive Example 34 P 10 880 120 10 700 20 425 40 10 Inventive Example 35 Q 10 880 120 10 700 20 425 40 10 Inventive Example 36 R 10 880 120 10 700 20 425 40 10 Inventive Example 37 S 10 880 120 10 700 20 425 40 10 Inventive Example 38 T 10 880 120 10 700 20 425 40 10 Inventive Example 39 U 10 880 120 10 700 20 425 40 10 Inventive Example 40 V 10 880 120 10 700 20 425 40 10 Inventive Example 41 W 10 880 120 10 700 20 425 40 10 Inventive Example 42 X 10 880 120 10 700 20 425 40 10 Inventive Example 43 Y 10 880 120 10 700 20 425 40 10 Comparative example 44 XA 10 880 120 10 700 20 425 40 10 Comparative example 45 XB 10 880 120 10 700 20 425 40 10 Comparative example 46 XC 10 880 120 10 700 20 425 40 10 Comparative example 47 XD 10 880 120 10 700 20 425 40 10 Comparative example 48 XE 10 880 120 10 700 20 425 40 10 Comparative example 49 XF 10 880 120 10 700 20 425 40 10 Comparative example 50 XG 10 880 120 10 700 20 425 40 10 Comparative example 51 XH 10 880 120 10 700 20 425 40 10 Comparative example 52 XI 10 880 120 10 700 20 425 40 10

division number Steel 3rd
maintain
Temperature
(℃) 3rd
maintain
time
(s) 4th
Average
Cooling
speed
(℃/s) Secondary
Cooling
stop
Temperature
(℃) Secondary
Average
heating
speed
(℃/s) 4th
maintain
Temperature
(℃) 4th
maintain
time
(s) 5th
Average
Cooling
speed
(℃/s) Inventive Example One A 375 40 20 200 15 400 300 10 Comparative example 2 A 375 40 20 200 15 400 300 10 Comparative example 3 A 375 40 20 190 15 400 300 10 Comparative example 4 A 375 40 20 200 15 400 300 10 Inventive Example 5 A 375 40 20 210 15 400 300 10 Comparative example 6 A 375 40 20 200 15 400 300 10 Comparative example 7 A 375 40 20 200 15 400 300 10 Comparative example 8 A 375 40 20 200 15 400 300 10 Comparative example 9 A 270 40 20 220 15 400 300 10 Inventive Example 10 B 375 40 20 200 15 400 300 10 Comparative example 11 B 480 40 20 200 15 400 300 10 Comparative example 12 B 220 40 20 180 15 400 300 10 Comparative example 13 B 375 2 20 200 15 400 300 10 Comparative example 14 B 375 40 One 200 15 400 300 10 Comparative example 15 B 375 40 20 70 15 400 300 10 Comparative example 16 B 375 40 20 330 15 400 300 10 Comparative example 17 B 375 40 20 200 15 270 300 10 Comparative example 18 B 375 40 20 200 15 530 300 10 Comparative example 19 B 375 40 20 200 15 400 40 10 Comparative example 20 B 375 40 20 200 15 400 172,800 10 Inventive Example 21 C 375 40 20 220 15 400 300 10 Inventive Example 22 D 375 40 20 130 15 400 300 10 Inventive Example 23 E 375 40 20 270 15 400 300 10 Inventive Example 24 F 375 40 20 180 15 400 300 10 Inventive Example 25 G 400 40 20 200 15 400 300 10 Inventive Example 26 H 300 40 20 200 15 400 300 10 Inventive Example 27 I 375 40 20 190 15 400 300 10 Inventive Example 28 J 375 40 20 180 15 350 300 10 Inventive Example 29 K 375 40 20 200 15 450 300 10 Inventive Example 30 L 375 40 20 210 15 400 300 10

division number Steel 3rd
maintain
Temperature
(℃) 3rd
maintain
time
(s) 4th
Average
Cooling
speed
(℃/s) Secondary
Cooling
stop
Temperature
(℃) Secondary
Average
heating
speed
(℃/s) 4th
maintain
Temperature
(℃) 4th
maintain
time
(s) 5th
Average
Cooling
speed
(℃/s) Inventive Example 31 M 375 40 20 200 15 400 300 10 Inventive Example 32 N 375 40 20 220 15 400 600 10 Inventive Example 33 O 375 40 20 200 15 400 300 10 Inventive Example 34 P 375 40 20 150 15 400 300 10 Inventive Example 35 Q 375 40 20 250 15 400 300 10 Inventive Example 36 R 375 40 20 200 15 400 600 10 Inventive Example 37 S 375 40 20 200 15 400 300 10 Inventive Example 38 T 375 40 20 180 15 400 300 10 Inventive Example 39 U 375 40 20 200 15 400 300 10 Inventive Example 40 V 375 40 20 200 15 400 300 10 Inventive Example 41 W 375 40 20 190 15 400 300 10 Inventive Example 42 X 375 40 20 210 15 400 300 10 Inventive Example 43 Y 375 40 20 200 15 400 300 10 Comparative example 44 XA 375 40 20 200 15 400 300 10 Comparative example 45 XB 375 40 20 200 15 400 300 10 Comparative example 46 XC 375 40 20 190 15 400 300 10 Comparative example 47 XD 375 40 20 200 15 400 300 10 Comparative example 48 XE 375 40 20 180 15 400 300 10 Comparative example 49 XF 375 40 20 220 15 400 300 10 Comparative example 50 XG 375 40 20 200 15 400 300 10 Comparative example 51 XH 375 40 20 210 15 400 300 10 Comparative example 52 XI 375 40 20 200 15 400 300 10

division number Steel F
(vol.%) B
(vol.%) TM
(vol.%) FM
(vol.%) P
(vol.%) Residual γ
(vol.%) Inventive Example One A 0 25 53 One 0 21 Comparative example 2 A 31 8 7 0 48 6 Comparative example 3 A 22 13 47 10 0 8 Comparative example 4 A 19 22 51 One 0 7 Inventive Example 5 A 0 21 62 0 0 17 Comparative example 6 A 18 17 59 0 0 6 Comparative example 7 A 0 25 66 One 0 8 Comparative example 8 A 0 16 65 2 0 17 Comparative example 9 A 0 17 63 One 0 19 Inventive Example 10 B 0 28 50 0 0 22 Comparative example 11 B 0 23 56 0 0 21 Comparative example 12 B 0 15 67 0 0 18 Comparative example 13 B 0 24 56 2 0 18 Comparative example 14 B 0 17 67 0 0 16 Comparative example 15 B 0 11 84 0 0 5 Comparative example 16 B 0 78 5 0 0 17 Comparative example 17 B 0 16 56 23 0 5 Comparative example 18 B 0 27 63 2 0 8 Comparative example 19 B 0 16 77 One 0 6 Comparative example 20 B 0 34 56 3 0 7 Inventive Example 21 C 0 21 58 0 0 21 Inventive Example 22 D 0 24 59 0 0 17 Inventive Example 23 E 0 15 66 One 0 18 Inventive Example 24 F 0 17 63 0 0 20 Inventive Example 25 G 0 19 61 One 0 19 Inventive Example 26 H 0 44 35 0 0 21 Inventive Example 27 I 0 23 55 0 0 22 Inventive Example 28 J 0 21 60 One 0 18 Inventive Example 29 K 0 22 61 0 0 17 Inventive Example 30 L 0 19 59 2 0 20

division number Steel F
(vol.%) B
(vol.%) TM
(vol.%) FM
(vol.%) P
(vol.%) Residual γ
(vol.%) Inventive Example 31 M 0 20 58 One 0 21 Inventive Example 32 N 0 23 55 0 0 22 Inventive Example 33 O 0 32 51 0 0 17 Inventive Example 34 P 0 28 53 0 0 19 Inventive Example 35 Q 0 24 56 0 0 20 Inventive Example 36 R 0 19 48 0 0 33 Inventive Example 37 S 0 29 52 One 0 18 Inventive Example 38 T 0 28 50 One 0 21 Inventive Example 39 U 0 25 59 0 0 16 Inventive Example 40 V 0 24 58 One 0 17 Inventive Example 41 W 0 22 56 0 0 22 Inventive Example 42 X 0 27 55 0 0 18 Inventive Example 43 Y 0 26 53 0 0 21 Comparative example 44 XA 0 23 64 One 0 12 Comparative example 45 XB 0 21 18 16 0 45 Comparative example 46 XC 0 32 66 0 0 2 Comparative example 47 XD 0 18 40 25 0 17 Comparative example 48 XE 0 26 38 20 0 16 Comparative example 49 XF 0 27 57 One 9 6 Comparative example 50 XG 0 19 46 17 0 18 Comparative example 51 XH 0 18 48 20 0 14 Comparative example 52 XI 0 21 45 18 0 16

division number Steel [Si+Al]γ
/[Si+Al]av V(1.2㎛,γ)
/V(γ) V(lath,γ)
/V(γ) TSХEL (MPa%) R/t Inventive Example One A 0.75 0.21 0.67 31684 1.42 Comparative example 2 A 0.96 0.12 0.58 18741 4.38 Comparative example 3 A 0.95 0.13 0.56 27358 5.05 Comparative example 4 A 0.92 0.16 0.53 20578 3.73 Inventive Example 5 A 0.71 0.19 0.61 30251 1.85 Comparative example 6 A 0.89 0.14 0.58 21627 3.66 Comparative example 7 A 0.90 0.15 0.41 20084 4.41 Comparative example 8 A 0.73 0.08 0.36 25383 3.48 Comparative example 9 A 0.78 0.06 0.52 23629 3.27 Inventive Example 10 B 0.67 0.18 0.71 30371 2.04 Comparative example 11 B 0.75 0.14 0.45 28597 3.36 Comparative example 12 B 0.79 0.07 0.34 29478 3.78 Comparative example 13 B 0.74 0.09 0.28 28146 4.73 Comparative example 14 B 0.77 0.13 0.54 27482 3.52 Comparative example 15 B 0.91 0.08 0.55 19025 8.47 Comparative example 16 B 0.79 0.15 0.59 20319 2.45 Comparative example 17 B 0.98 0.05 0.52 12739 8.16 Comparative example 18 B 0.99 0.07 0.54 19582 3.87 Comparative example 19 B 0.96 0.06 0.58 18217 6.89 Comparative example 20 B 0.94 0.13 0.56 20108 4.58 Inventive Example 21 C 0.72 0.24 0.65 31820 2.04 Inventive Example 22 D 0.74 0.19 0.61 30265 2.17 Inventive Example 23 E 0.65 0.22 0.82 28951 2.05 Inventive Example 24 F 0.73 0.15 0.67 25816 1.02 Inventive Example 25 G 0.58 0.18 0.61 27105 0.93 Inventive Example 26 H 0.78 0.16 0.58 30617 2.36 Inventive Example 27 I 0.74 0.23 0.75 28453 2.57 Inventive Example 28 J 0.82 0.21 0.59 25806 0.55 Inventive Example 29 K 0.78 0.45 0.63 27008 2.29 Inventive Example 30 L 0.72 0.15 0.56 29327 2.16

division number Steel [Si+Al]γ
/[Si+Al]av V(1.2㎛,γ)
/V(γ) V(lath,γ)
/V(γ) TSХEL (MPa%) R/t Inventive Example 31 M 0.68 0.13 0.59 27058 2.24 Inventive Example 32 N 0.72 0.16 0.54 24839 2.31 Inventive Example 33 O 0.73 0.19 0.63 28136 1.82 Inventive Example 34 P 0.75 0.22 0.60 28865 1.55 Inventive Example 35 Q 0.80 0.18 0.65 32152 2.09 Inventive Example 36 R 0.71 0.12 0.62 27863 2.17 Inventive Example 37 S 0.74 0.20 0.57 31354 2.28 Inventive Example 38 T 0.67 0.17 0.52 30702 1.93 Inventive Example 39 U 0.65 0.23 0.58 25135 2.38 Inventive Example 40 V 0.78 0.16 0.60 27637 1.86 Inventive Example 41 W 0.77 0.13 0.63 25528 1.61 Inventive Example 42 X 0.69 0.16 0.57 29831 2.64 Inventive Example 43 Y 0.64 0.14 0.59 30812 2.53 Comparative example 44 XA 0.73 0.15 0.54 18139 2.73 Comparative example 45 XB 0.70 0.18 0.58 19054 5.36 Comparative example 46 XC 0.95 0.21 0.55 11362 5.71 Comparative example 47 XD 0.74 0.12 0.56 25964 4.29 Comparative example 48 XE 0.71 0.13 0.57 24183 3.95 Comparative example 49 XF 0.93 0.18 0.53 15861 3.62 Comparative example 50 XG 0.77 0.14 0.61 23427 4.53 Comparative example 51 XH 0.75 0.17 0.59 22896 5.22 Comparative example 52 XI 0.73 0.19 0.58 23269 4.85

As can be seen from the results of Tables 1 to 9, in the example of the invention that satisfies the conditions presented in the present invention, TS×EL is 22,000 MPa% or more, and R/t satisfies 0.5 to 3.0, so that the balance of strength and ductility It can be confirmed that it is excellent and secures excellent workability.

On the other hand, in the comparative example, it was confirmed that the conditions suggested in the present invention were not satisfied, and the balance and workability of strength and ductility were poor.

Specifically, in Comparative Examples No. 44 to 50, the contents of C, Si, Mn, and Al among the essential alloy components of the present invention were out of the scope of the present invention, and the strength and ductility of the present invention were poor. You can see that In particular, No. The comparative example of 46 is a case where the sum of Si and Al (Si+Al) is very low, [Si+Al]γ/[Si+Al]av exceeds 0.85, and TS×EL is less than 22,000 MPa%, R/t exceeded 3.0 and was shown as a comparative example.

Nos. 51 and 52 were Cr and Mo, respectively, exceeding the ranges presented in the present invention, and fresh martensite (FM) increased, so that R/t exceeded 3.0.

On the other hand, in the comparative example of No. 2, since the primary holding temperature is low, ferrite is excessively formed, and there is a lack of bainite, retained austenite, and tempered martensite, [Si+Al]γ / [Si+Al]av It exceeded 0.85, TS×EL was less than 22,000 MPa%, and R/t exceeded 3.0. In the comparative example of No. 3, the primary holding time was short, and the tissues were uniform, resulting in excessive formation of ferrite, insufficient residual austenite, and [Si+Al]γ / [Si+Al]av exceeding 0.85. And R/t exceeded 3.0.

The comparative examples of Nos. 4 and 6 are cases in which the primary cooling stop temperature is low or the secondary average cooling rate is low, respectively, and ferrite is excessively formed, and there is insufficient residual austenite, [Si+Al]γ / [Si+Al]av exceeds 0.85, TS x EL is less than 22,000 MPa%, and R/t exceeds 3.0.

In the comparative example of No.7, the secondary holding temperature was low and the residual austenite was insufficient, so that [Si+Al]γ / [Si+Al]av exceeded 0.85, and V(lath, γ) / V(γ) Is less than 0.5, TS×EL is less than 22,000 MPa%, and R/t exceeds 3.0.

In the comparative example of No. 8, the second holding time is short, so that V(1.2 μm, γ) / V(γ) is less than 0.1, V(lath, γ) / V(γ) is less than 0.5, and R/t is 3.0. Exceeded. In the comparative example of No. 9, the secondary holding temperature was high, so that V (1.2 μm, γ) / V (γ) was less than 0.1, and R/t exceeded 3.0. In the comparative example of No. 11, the tertiary holding temperature was high, V(lath, γ) / V(γ) was less than 0.5, and t exceeded 3.0. In Comparative Examples No. 12 and 13, V (1.2 μm, γ) / V (γ) was less than 0.1 because the third holding temperature was low or the third holding time was short, respectively, and V(lath, γ) / V( γ) was less than 0.5 and t exceeded 3.0.

In Comparative Example No. 14, the fourth average cooling rate was low, so R/t exceeded 3.0. No. In Comparative Example 15, the secondary cooling stop temperature was low, and excessive tempered martensite was formed, and residual austenite was insufficiently formed, so that [Si+Al]γ/[Si+Al]av exceeded 0.85, V(1.2 μm, γ) / V(γ) was less than 0.1, TS×EL was less than 22,000 MPa%, and R/t was greater than 3.0. On the other hand, in the comparative example of No. 16, the secondary cooling stop temperature was high, excessively formed of bainite, and insufficiently formed of tempered martensite, and TS×EL was less than 22,000 MPa%.

The comparative examples of Nos. 17 and 18, respectively, are cases where the fourth holding temperature is low or high, the residual austenite is insufficiently formed, and [Si+Al]γ / [Si+Al]av exceeds 0.85, V (1.2 μm, γ)/V(γ) was less than 0.1, TS×EL was less than 22,000 MPa%, and R/t exceeded 3.0.

In the comparative example of No. 19, the fourth holding time is short, excessively formed of tempered martensite, and lack of residual austenite, so that [Si+Al]γ/[Si+Al]av exceeds 0.85, V (1.2 μm, γ)/V(γ) was less than 0.1, TS×EL was less than 22,000 MPa%, and R/t exceeded 3.0. On the other hand, the comparative example of No. 20 has a long 4th holding time and lacks residual austenite, so that [Si+Al]γ/[Si+Al]av exceeds 0.85 and TS×EL is less than 22,000 MPa%. And R/t exceeded 3.0.

Claims (10)

In weight percent, C: more than 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, remainder Contains Fe and unavoidable impurities,
The microstructure includes tumper martensite, bainite and residual austenite,
A steel sheet excellent in balance and workability of strength and ductility satisfying the following [Relational Formula 1] and [Relational Formula 2].
[Relational Formula 1]
0.55 ≤ [Si+Al]γ / [Si+Al]av ≤ 0.85
(However, [Si+Al]γ is the content of Si and Al contained in the retained austenite (% by weight), and [Si+Al]av is the content of Si and Al contained in the steel sheet (% by weight))
[Relationship 2]
V(1.2㎛, γ) / V(γ) ≥ 0.1
(However, V (1.2㎛, γ) is the fraction of the retained austenite having an average grain size of 1.2㎛ or more, V(γ) is the residual austenite fraction of the steel sheet)
The method according to claim 1,
The steel sheet is a steel sheet excellent in balance and workability of strength and ductility further comprising any one or more of the following (1) to (9).
(1) Ti: 0~0.5%, Nb: 0~0.5% and V: 0~0.5%
(2) Cr: 0~3.0% and Mo: 0~3.0%
(3) Cu: 0 to 4.5% and Ni: 0 to 4.5%
(4) B: 0~0.005%
(5) Ca: 0 to 0.05%, excluding REM: 0 to 0.05% and Mg: 0 to 0.05% or more
(6) W: 0~0.5% and Zr: 0~0.5%
(7) Sb: 0~0.5% and Sn: 0~0.5% or more
(8) Y: 0~0.2% and Hf: 0~0.2% or more
(9) Co: 0~1.5%
The method according to claim 1,
The microstructure of the steel sheet is a volume fraction, strength of 30 to 75% of tempered martensite, 10 to 50% of bainite, 10 to 40% of retained austenite, 5% or less of ferrite and inevitable structure. Steel sheet with excellent ductility balance and processability.
The method according to claim 1,
The Si and Al is a steel sheet excellent in the balance and workability of the strength and ductility of the sum (Si + Al) of 1.0 to 6.0%.
The method according to claim 1,
The steel sheet has a product of tensile strength and elongation (TS×EL) of 22,000 MPa% or more, and R/t (R is a minimum bending radius (mm) that does not crack after a 90° bending test, and t is the thickness of the steel sheet (Mm) is a steel plate with excellent strength and ductility balance and workability of 0.5 to 3.0.
The method according to claim 1,
The steel sheet is a steel sheet excellent in balance and workability of strength and ductility satisfying the following [Relational Formula 3].
[Relationship 3]
V(lath, γ) / V(γ) ≥ 0.5
(However, V(lath, γ) is the fraction of retained austenite in the form of lath, and V(γ) is the fraction of retained austenite in the steel sheet)
In weight percent, C: more than 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, remainder Providing a cold rolled steel sheet containing Fe and unavoidable impurities;
Heating the cold rolled steel sheet to Ar3 or higher (primary heating), and maintaining it for at least 50 seconds (primary maintenance);
Cooling (primary cooling) to an average cooling rate of 1°C/s or higher, to a temperature range of 600-850°C (primary cooling stop temperature);
Cooling to an average cooling rate of 2°C/s or more, to a temperature range of 350 to 550°C (secondary cooling), and maintaining for 5 seconds or more (secondary maintenance) in this temperature range;
Cooling to an average cooling rate of 1°C/s or more, to a temperature range of 250 to 450°C (3rd cooling), and maintaining for 5 seconds or more in this temperature range (3rd holding);
Cooling to an average cooling rate of 2°C/s or more, to a temperature range of 100 to 300°C (secondary cooling stop temperature) (fourth cooling);
Heating to a temperature range of 300 to 500°C (secondary heating), and maintaining at least 50 seconds (fourth maintenance) in this temperature range; And
Cooling to room temperature (5th cooling)
Method for producing a steel sheet excellent in balance and workability of strength and ductility comprising a.
The method according to claim 7,
The cold-rolled steel sheet is a method of manufacturing a steel sheet excellent in balance and workability of strength and ductility further comprising any one or more of the following (1) to (9).
(1) Ti: 0~0.5%, Nb: 0~0.5% and V: 0~0.5%
(2) Cr: 0~3.0% and Mo: 0~3.0%
(3) Cu: 0 to 4.5% and Ni: 0 to 4.5%
(4) B: 0~0.005%
(5) Ca: 0 to 0.05%, excluding REM: 0 to 0.05% and Mg: 0 to 0.05% or more
(6) W: 0~0.5% and Zr: 0~0.5%
(7) Sb: 0~0.5% and Sn: 0~0.5% or more
(8) Y: 0~0.2% and Hf: 0~0.2% or more
(9) Co: 0~1.5%
The method according to claim 7,
The preparation of the cold rolled steel sheet comprises heating the steel slab to 1000 to 1350°C;
Hot finishing rolling in a temperature range of 800 to 1000°C;
Winding in a temperature range of 350 ~ 700 ℃; And
Cold rolling at a reduction rate of 30% or more
Method for producing a steel sheet excellent in balance and workability, including strength and ductility.
The method according to claim 7,
The primary cooling rate (Vc1) and the secondary cooling rate (Vc2) is a method of manufacturing a steel sheet excellent in balance and workability of strength and ductility satisfying the relationship of Vc1 <Vc2.