KR102178731B1 - High strength steel sheet having excellent workability property, and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a steel sheet that can be used in automobile parts, and in particular, to a high-strength steel sheet having excellent processing characteristics and a method of manufacturing the same.

Description

High strength steel plate with excellent processing characteristics and its manufacturing method {HIGH STRENGTH STEEL SHEET HAVING EXCELLENT WORKABILITY PROPERTY, AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a steel sheet that can be used in automobile parts, and in particular, to a high-strength steel sheet having excellent processing characteristics and a method of manufacturing the same.

In recent years, the automobile industry is paying attention to measures to reduce material weight and secure occupant stability in order to protect the global environment. In order to meet these requirements for stability and weight reduction, the application of high-strength steel sheets is rapidly increasing. In general, since the ductility and workability decrease as the strength of the steel sheet is increased, a balance between strength and ductility and excellent workability are required in a steel sheet for automobile members.

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, which is made by tempering hard martensite, is a softened martensite, and shows a difference in strength from existing untempered martensite (fresh martensite). When fresh martensite is suppressed and tempered martensite is formed, ductility and workability increase.

However, with the technologies disclosed in Patent Documents 1 and 2, it is difficult to secure a high-strength steel sheet having excellent ductility because the product of tensile strength and elongation (TS×El) does not satisfy 22,000 MPa% or more.

On the other hand, in order to obtain both high strength and excellent ductility and workability, the steel sheet for automobile members has developed TRIP (Transformation Induced Plasticity) steel using the transformation organic plasticity of retained austenite. In Patent Documents 3 and 4, TRIP steel having excellent ductility and workability is disclosed.

Patent Document 3 tried to improve ductility and workability, including polygonal ferrite, retained austenite, and martensite, but it was not able to secure high strength because bainite was used as the main phase, and TS×El was also 22,000. It can be seen that the MPa% or more is not satisfied.

In Patent Document 4, ferrite is formed, residual austenite is refined, and a complex structure including tempered martensite is formed to improve ductility and workability, but it is difficult to secure high strength because a large amount of soft ferrite is included. There is.

Until now, the situation has not been able to meet the demand for a high-strength steel sheet excellent in ductility and workability.

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

An aspect of the present invention is to provide a high-strength steel sheet having excellent ductility and workability by optimizing the composition and microstructure of the steel sheet and a method of manufacturing the same.

The subject of the present invention is not limited to the above. Additional problems of the present invention are described in the general contents of the specification, and those of ordinary skill in the art to which the present invention pertains will not have any difficulty in understanding the additional problems of the present invention from the contents described in the specification of the present invention.

One aspect of the present invention is by 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 contains Fe and inevitable impurities,

The microstructure includes tampered martensite, bainite and retained austenite,

It relates to a high-strength steel sheet excellent in processing characteristics satisfying the following [Relational Formula 1].

[Relationship 1]

V(1.2㎛, γ) / V(γ) ≥ 0.1

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

Another aspect of the present invention is by 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: providing a cold-rolled steel sheet containing less than 0.03%, the balance Fe and inevitable impurities;

Heating the cold-rolled steel sheet to at least Ac3 (primary heating) and maintaining it for at least 50 seconds (primary maintenance);

Cooling (primary cooling) to a temperature range of 600 to 850°C at an average cooling rate of 1°C/s or more;

Cooling (secondary cooling) to a temperature range of 300 to 500°C at an average cooling rate of 2°C/s or more, and maintaining (secondary maintenance) in this temperature range for 5 seconds or more;

Cooling (third cooling) to a temperature range of 100 to 300°C at an average cooling rate of 2°C/s or more;

Heating (secondary heating) to a temperature range of 350 to 550°C at an average heating rate of 5°C/s or more, and maintaining (third holding) in this temperature range for 10 seconds or more;

Cooling (4th cooling) to a temperature range of 250 to 450°C at an average cooling rate of 1°C/s or more, and maintaining (4th maintenance) in this temperature range for 10 seconds or more; And

It relates to a method for manufacturing a high-strength steel sheet having excellent processing characteristics, including cooling to room temperature (5th cooling) at an average cooling rate of 1°C/s or more.

Advantageous Effects of Invention According to the present invention, by securing a high-strength steel sheet having excellent ductility and workability, it can be provided for automotive structures requiring weight reduction and stability at the same time.

In the Transformation Induced Plasticity (TRIP) steel including bainite, tempered martensite, and retained austenite, the inventors of the present invention aim to stabilize retained austenite and determine the size and shape of retained austenite. Through this, it was recognized that strength, ductility and workability were affected. By clarifying this, they devised a steel plate with excellent balance of strength and ductility and workability and a method of manufacturing the same.

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 is in% by weight (hereinafter, %), 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, and the remainder contains 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: 0~0.005%, Ca: 0~0.05%, REM excluding Y: 0~0.05%, Mg: 0~0.05%, W: 0~0.5%, Zr: 0~0.5%, Sb: 0 It may include ~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 ~ 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. When the C content is 0.25% or less, it is difficult to secure the necessary tensile strength, and when it exceeds 0.75%, cold rolling is difficult and the steel sheet cannot be manufactured. Therefore, the content of C is preferably more than 0.25 ~ 0.75% or less. The content of C is more preferably 0.31 to 0.75%.

Silicon (Si): 4.0% (excluding 0)

Si is an element having an effect of improving strength by solid solution strengthening, and is an element that strengthens ferrite, uniformizes the structure, and improves workability. In addition, it is an element that contributes to the formation of retained austenite by suppressing the precipitation of cementite. When the Si content exceeds 4.0%, plating defects such as non-plating in the plating process and weldability of the steel sheet are deteriorated, so the content of Si is preferably 4.0% or less.

Aluminum (Al): 5.0% (excluding 0)

The Al is an element that acts on deoxidation by bonding with oxygen in steel. In addition, it is an element that stabilizes retained austenite by suppressing precipitation of cementite like 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 microstructure formation in the present invention and affect ductility and bending workability. Therefore, in order to have excellent ductility and bending workability, the sum of Si and Al is preferably 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 enhancing both strength and ductility. The above effect can be obtained at 0.9% or more, but when it exceeds 5.0%, the weldability and impact toughness of the steel sheet are deteriorated. In addition, when Mn is included in an amount exceeding 5.0%, the bainite transformation time increases and the concentration of C in the austenite is insufficient, and thus the required residual austenite fraction cannot be secured. Therefore, the content of Mn is preferably 0.9 to 5.0%.

Phosphorus (P): 0.15% or less

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

Sulfur (S): 0.03% or less

S is an element that is contained as an impurity, makes 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 and causes a crack in the slab by making nitride during continuous casting. Accordingly, the content of N is preferably 0.03% or less.

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

Titanium (Ti): 0~0.5%, Niobium (Nb): 0~0.5%, Vanadium (V): At least one of 0~0.5%

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

Chromium (Cr): 0-3.0%, Molybdenum (Mo): 0-3.0%

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

Copper (Cu): 0-4.5%, Nickel (Ni): 0-4.5%

The Cu and Ni are elements that stabilize austenite and inhibit corrosion. The Cu and Ni are concentrated on the surface of the steel plate to prevent hydrogen from moving into the steel plate, thereby suppressing the delayed hydrogen destruction. When each content of Cu and Ni exceeds 4.5%, it causes not only an excessive characteristic effect, 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%

B is an element that improves hardenability to increase strength and suppresses nucleation of grain boundaries. When the content of B exceeds 0.005%, it causes an increase in manufacturing cost as well as an excessive characteristic effect. Therefore, the content of B is preferably 0.005% or less.

Calcium (Ca): 0~0.05%, Magnesium (Mg): 0~0.05%, Rare-earth elements (REM) excluding yttrium (Y): 0~0.05%

The REM refers to a total of 17 elements of Sc, Y and lanthanoids. REM except for Ca, Mg, and Y can improve the ductility of the steel sheet by making the sulfide spherical. When each content of REM excluding Ca, Mg and Y exceeds 0.05%, it causes not only an excessive characteristic effect but also an increase in manufacturing cost. Therefore, it is preferable that each content of REM excluding Ca, Mg and Y is 0.05% or less.

Tungsten (W): 0~0.5%, Zirconium (Zr): 0~0.5%

The W and Zr are elements that increase the strength of the steel sheet by improving the hardenability. When each content of W and Zr exceeds 0.5%, it causes not only an excessive characteristic effect 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%, Tin (Sn): 0-0.5%

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

Yttrium (Y): 0~0.2%, Hafnium (Hf): 0~0.2%

The Y and Hf are elements that improve the corrosion resistance of the steel sheet. When the respective contents of Y and Hf exceed 0.2%, the ductility of the steel sheet may be deteriorated. Therefore, it is preferable that each content of Y and Hf is 0.2% or less.

Cobalt (Co): 0~1.5%

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

Next, the microstructure of the steel sheet of the present invention will be described in detail.

The microstructure of the steel sheet of the present invention includes tempered martensite, bainite and retained austenite. As a preferred example, by volume fraction, it contains 30 to 75% of tempered martensite, 10 to 50% of bainite, and 10 to 40% of retained austenite, and consists of 5% or less of ferrite and the balance inevitable. . The inevitable structure includes fresh martensite, perlite, martensite (Martensite Austenite Constituent, M-A), and the like. If the fresh martensite or pearlite is excessively formed, the ductility and workability of the steel sheet may be inferior, or the fraction of retained austenite may be reduced.

The steel sheet of the present invention has a product of tensile strength and elongation (TS×El) of 22,000 MPa% or more, R/t (R is the minimum bending radius (mm) that does not cause cracks after a 90° bending test, and t is the steel sheet The thickness (mm) of) is 0.5 to 3.0, and not only strength but also ductility is excellent, and excellent processing characteristics can be secured.

For this, it is important to stabilize the retained austenite in the steel sheet. In order to stabilize retained austenite, it is necessary to concentrate C and Mn into austenite in ferrite, bainite, and tempered martensite of the steel sheet. However, if C is concentrated in austenite using ferrite, the strength of the steel sheet may be insufficient due to the low strength characteristics of ferrite. Therefore, it is preferable to use bainite and tempered martensite to concentrate C and Mn in austenite.

The steel sheet containing retained austenite has excellent ductility and workability due to the transformation organic plasticity that occurs during transformation from austenite to martensite during processing. When the residual 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, when the residual austenite fraction exceeds 40%, local elingation may decrease. Therefore, in order to obtain a steel sheet excellent in both the balance of strength and ductility and workability, the fraction of retained austenite is preferably 10 to 40%.

Meanwhile, among the retained austenite, retained austenite having an average grain diameter of 1.2 μm or more is heat-treated at the bainite formation temperature to increase the average size, thereby suppressing the transformation from austenite to martensite, thereby improving the workability of the steel sheet. .

Therefore, in order to secure the ductility and workability of the steel sheet, it is preferable to increase the proportion of retained austenite having an average grain size of 1.2 µm or more among retained austenite. Accordingly, in the present invention, it is preferable to satisfy the following relational expression 1. When the value of the following relational expression 1 is less than 0.1, R/t does not satisfy 0.5 to 3.0, and it is difficult to secure excellent workability.

[Relationship 1]

V(1.2㎛, γ) / V(γ) ≥ 0.1

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

Also, among the retained austenite, retained austenite in lath form may affect the workability of the steel sheet. The retained austenite is classified into a rheth-like retained austenite formed between bainite phases and a block-like retained austenite formed in a portion without bainite phases. As the residual austenite in the block form is further transformed into bainite during the heat treatment process, the residual austenite in the form of leth increases, and thus the workability of the steel sheet may be improved.

Therefore, in order to improve ductility and workability, it is desirable to increase the fraction of retained austenite in lath form. Accordingly, in the present invention, it is preferable to satisfy the following relational formula 2. When the value of the following relational expression 2 is less than 0.5, R/t does not satisfy 0.5 to 3.0, and it is difficult to secure excellent workability.

[Relationship 2]

V(lath, γ) / V(γ) ≥ 0.5

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

On the other hand, untempered martensite (fresh martensite) and tempered martensite are both microstructures that improve the strength of the steel sheet. However, compared with tempered martensite, fresh martensite has a property of greatly reducing the ductility of the steel sheet. This is because the microstructure of the tempered martensite is not softened by the tempering heat treatment. Therefore, in order to provide a steel sheet having excellent strength and ductility balance and workability of the present invention, it is preferable to use tempered martensite. If the fraction (volume fraction) of the tempered martensite is less than 30%, it is difficult to secure more than 22,000 MPa% of TS×El, and if it exceeds 75%, ductility and workability are deteriorated, and TS×El is less than 22,000 MPa%. Or R/t is more than 3.0, which is not preferable.

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

Hereinafter, an example of a method of 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-rolled, 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 rolling at a temperature of 800 to 1000°C. When the heating temperature is less than 1000° C., there is a possibility to be hot-rolled below the finish hot-rolling temperature range. In addition, when the heating temperature exceeds 1350°C, it may reach the melting point of the steel and melt. On the other hand, when the finish hot rolling temperature is less than 800°C, a large burden may be placed on the rolling mill due to the high strength of the steel. In addition, when the finish hot rolling temperature exceeds 1000°C, the crystal grains of the steel sheet are coarse after hot rolling, and thus the physical properties of the high-strength steel sheet may be reduced. 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 more after finishing hot rolling, and wind up at a temperature of 350 to 700°C. When the winding temperature is less than 350°C, winding is not easy, and when the winding temperature 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, making pickling difficult. Meanwhile, in order to remove scale generated on the surface of the steel sheet after the winding, it 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 reduction ratio of 30 to 90%. If the cold rolling cumulative reduction ratio exceeds 90%, it may be 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 produced as an unplated cold-rolled steel sheet through an annealing heat treatment process, or may be produced as a plated steel sheet through a plating process to impart corrosion resistance. For plating, a plating method such as hot-dip galvanizing, electro-zinc plating, or hot-dip aluminum plating can be applied, and the method and type are not particularly limited.

In order to ensure the balance of strength and ductility and workability 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 (primary heating) over Ac3, and maintained for at least 50 seconds (primary holding).

When the primary heating or primary holding temperature is less than Ac3, ferrite may be formed, and residual austenite may not be sufficient, so that TS×El and bending workability of the steel sheet may be deteriorated. In addition, when the primary holding time is less than 50 seconds, the structure cannot be sufficiently uniformed, and the physical properties of the steel sheet are deteriorated. The upper limit of the primary heating or holding temperature and the upper limit of the primary holding time do not need to be specifically defined, but in order to suppress the decrease in toughness due to grain coarsening, the primary heating or holding temperature is 950° C. or less, It is preferable that the holding time be 1200 seconds or less.

After the first maintenance, it is preferable to cool (primary cooling) to a temperature range of 600 to 850°C at an average cooling rate of 1°C/s or more. The upper limit of the first average cooling rate does not need to be specifically defined, and the first 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 becomes insufficient, thereby reducing 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 first cooling, it is preferable to cool (secondary cooling) to a temperature range of 300 to 500°C at an average cooling rate of 2°C/s or more, and maintain for 5 seconds or more (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, which may lower TS×El and bending workability of the steel sheet. The upper limit of the secondary average cooling rate does not need to be specifically defined, and is preferably 100°C/s or less. On the other hand, when the secondary holding temperature exceeds 500° C., residual austenite is insufficient, and thus V(lath, γ) / V(γ), TS×El, and bending workability of the steel sheet may be lowered. In addition, if it is less than 300°C, the V(1.2㎛, γ) / V(γ) and bending workability of the steel sheet may be lowered at a low heat treatment temperature. If the secondary holding time is less than 5 seconds, the heat treatment time is insufficient, and the V(1.2㎛, γ) / V(γ) of the steel sheet is insufficient, thereby reducing the bending workability of the steel sheet. The upper limit of the secondary holding time need not be particularly defined, and the secondary holding time is preferably 600 seconds or less.

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

After the secondary maintenance, it is preferable to cool (third cooling) to a temperature range of 100 to 300°C, which is a secondary cooling stop temperature, at an average cooling rate of 2°C/s or more. When the third average cooling rate is less than 2°C/s, the bending workability of the steel sheet may be deteriorated due to slow cooling. The upper limit of the third average cooling rate need not be particularly defined, and the third 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 tempered martensite is insufficient, which may reduce TS×El of the steel sheet. On the other hand, below 100° C., tempered martensite is excessively formed and residual austenite is insufficient, which may lower the V(1.2 μm,γ)/V(γ), TSХEL, and bending workability of the steel sheet.

After the third cooling, it is preferable to heat (secondary heating) to a temperature range of 350 to 550°C at an average heating rate of 5°C/s or more, and to maintain for 10 seconds or more (third maintenance) in this temperature range. The upper limit of the secondary average heating rate does not need to be particularly defined, and the secondary average heating rate is preferably 100°C/s or less. When the third holding temperature exceeds 550° C., the residual austenite fraction becomes insufficient, and thus V(1.2 μm, γ) / V(γ), TSХEL, and bending workability of the steel sheet may be lowered. On the other hand, if it is less than 350°C, V(1.2㎛, γ) / V(γ), TSХEL, and bending workability may be deteriorated due to the low heat treatment temperature. In addition, if the third holding time is less than 10 seconds, the heat treatment time is insufficient, and thus V(1.2㎛, γ) / V(γ), TSХEL and bending workability may be deteriorated. The upper limit of the third holding time need not be particularly defined, and the third holding time is preferably set to 1800 seconds or less.

After the third maintenance, it is preferable to cool (4th cooling) to a temperature range of 250 to 450°C at an average cooling rate of 1°C/s or more, and maintain for 10 seconds or more (4th maintenance) in this temperature range. The upper limit of the fourth average cooling rate does not need to be specifically defined, and the fourth average cooling rate is preferably 100°C/s or less. When the fourth holding temperature exceeds 450° C., V(lath, γ) / V(γ) and bending workability of the steel sheet may be lowered by a high heat treatment temperature. On the other hand, below 250° C., V(1.2 μm, γ) / V(γ), V(lath, γ) / V(γ) and bending workability of the steel sheet may be lowered at a low heat treatment temperature. On the other hand, if the 4th holding time is less than 10 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 reduced. have. The upper limit of the fourth holding time does not need to be specifically defined, and the fourth holding time is preferably 1800 seconds or less.

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

Hereinafter, embodiments of the present invention will be described in detail. It should be noted 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 the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

A steel slab with a thickness of 30 mm having the alloy composition according to Table 1 (the rest are Fe and inevitable impurities) was prepared, heated at 1200°C, and then finished hot-rolled at 900°C, with an average of 30°C/s. It cooled at a cooling rate and wound at 600°C to prepare a hot-rolled steel sheet having a thickness of 3 mm. After pickling the hot-rolled steel sheet to remove the surface scale, 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 a steel sheet.

The microstructure of the thus prepared steel sheet 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 nital etching of the polished specimen cross section. Among them, the fractions of bainite and tempered martensite, which are difficult to distinguish, were calculated using the expansion curve after dilation evaluation. On the other hand, since fresh martensite (FM) and retained austenite (residual γ) are also difficult to distinguish, the fraction of martensite and retained austenite observed by the SEM was subtracted from the fraction of retained austenite calculated by the X-ray diffraction method. The value was determined as the fresh martensite fraction.

Meanwhile, V(1.2㎛, γ) / V(γ), V(lath, γ) / V(γ), TS×El, R/t of the prepared steel sheet were observed, and the results are shown in Table 8 and Table It is shown in 9.

Retained austenite (V (1.2 µm, γ)) and lath-shaped retained austenite (V (lath, γ)) having an average grain size of 1.2 µm or more are retained using the 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 retained austenite as viewed as a whole of the steel sheet.

The TS×El and R/t were evaluated by a tensile test and a V-bending test. The tensile test was evaluated with a test piece taken according to JIS No. 5 standard based on the direction of 90° to the rolling direction of the rolled sheet, and TS×El was determined. R/t was determined as the value obtained by dividing the minimum bending radius R at which no crack occurs after a 90° bending test by taking a specimen based on the 90° direction with respect to the rolling direction of the rolled plate by the thickness t of the plate.

Steel grade Chemical composition (% by weight) C Si Mn P S Al N Cr Mo Other 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 grade 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) Invention example One A 10 880 120 10 700 20 400 50 20 Comparative example 2 A 10 730 120 10 700 20 400 50 20 Comparative example 3 A 10 880 One 10 700 20 400 50 20 Comparative example 4 A 10 880 120 10 580 20 400 50 20 Invention example 5 A 10 880 120 10 820 20 400 50 20 Comparative example 6 A 10 880 120 10 700 0.5 400 50 20 Comparative example 7 A 10 880 120 10 700 20 530 50 20 Comparative example 8 A 10 880 120 10 700 20 400 2 20 Comparative example 9 A 10 880 120 10 700 20 270 50 20 Invention example 10 B 10 880 120 10 700 20 400 50 20 Comparative example 11 B 10 880 120 10 700 20 400 50 One Comparative example 12 B 10 880 120 10 700 20 400 50 20 Comparative example 13 B 10 880 120 10 700 20 400 50 20 Comparative example 14 B 10 880 120 10 700 20 400 50 20 Comparative example 15 B 10 880 120 10 700 20 400 50 20 Comparative example 16 B 10 880 120 10 700 20 400 50 20 Comparative example 17 B 10 880 120 10 700 20 400 50 20 Comparative example 18 B 10 880 120 10 700 20 400 50 20 Comparative example 19 B 10 880 120 10 700 20 400 50 20 Invention example 20 C 10 880 120 10 700 20 400 50 20 Invention example 21 D 10 880 120 10 700 20 400 50 20 Invention example 22 E 10 880 120 10 700 20 400 50 20 Invention example 23 F 10 880 120 10 700 20 400 50 20 Invention example 24 G 10 880 120 10 700 20 450 50 20 Invention example 25 H 10 880 120 10 700 20 350 50 20 Invention example 26 I 10 880 120 10 700 20 400 50 20 Invention example 27 J 10 880 120 10 700 20 400 50 20 Invention example 28 K 10 880 120 10 700 20 400 50 20 Invention example 29 L 10 880 120 10 700 20 400 50 20 Invention example 30 M 10 880 120 10 700 20 400 50 20

division number Steel grade 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) Invention example 31 N 10 880 120 10 700 20 400 50 20 Invention example 32 O 10 880 120 10 700 20 400 50 20 Invention example 33 P 10 880 120 10 700 20 400 50 20 Invention example 34 Q 10 880 120 10 700 20 400 50 20 Invention example 35 R 10 880 120 10 700 20 400 50 20 Invention example 36 S 10 880 120 10 700 20 400 50 20 Invention example 37 T 10 880 120 10 700 20 400 50 20 Invention example 38 U 10 880 120 10 700 20 400 50 20 Invention example 39 V 10 880 120 10 700 20 400 50 20 Invention example 40 W 10 880 120 10 700 20 400 50 20 Invention example 41 X 10 880 120 10 700 20 400 50 20 Invention example 42 Y 10 880 120 10 700 20 400 50 20 Comparative example 43 XA 10 880 120 10 700 20 400 50 20 Comparative example 44 XB 10 880 120 10 700 20 400 50 20 Comparative example 45 XC 10 880 120 10 700 20 400 50 20 Comparative example 46 XD 10 880 120 10 700 20 400 50 20 Comparative example 47 XE 10 880 120 10 700 20 400 50 20 Comparative example 48 XF 10 880 120 10 700 20 400 50 20 Comparative example 49 XG 10 880 120 10 700 20 400 50 20 Comparative example 50 XH 10 880 120 10 700 20 400 50 20 Comparative example 51 XI 10 880 120 10 700 20 400 50 20

division number Steel grade Secondary
Cooling
stop
Temperature
(℃) Secondary
Average
heating
speed
(℃/s) 3rd
maintain
Temperature
(℃) 3rd
maintain
time
(s) 4th
Average
Cooling
speed
(℃/s) 4th
maintain
Temperature
(℃) 4th maintenance
time
(s) 5th
Average
Cooling
speed
(℃/s) Invention example One A 200 15 425 160 10 375 160 10 Comparative example 2 A 200 15 425 160 10 375 160 10 Comparative example 3 A 190 15 425 160 10 375 160 10 Comparative example 4 A 200 15 425 160 10 375 160 10 Invention example 5 A 210 15 425 160 10 375 160 10 Comparative example 6 A 200 15 425 160 10 375 160 10 Comparative example 7 A 200 15 425 160 10 375 160 10 Comparative example 8 A 200 15 425 160 10 375 160 10 Comparative example 9 A 220 15 425 160 10 375 160 10 Invention example 10 B 200 15 425 160 10 375 160 10 Comparative example 11 B 200 15 425 160 10 375 160 10 Comparative example 12 B 70 15 425 160 10 375 160 10 Comparative example 13 B 330 15 425 160 10 375 160 10 Comparative example 14 B 200 15 320 160 10 270 160 10 Comparative example 15 B 200 15 580 160 10 375 160 10 Comparative example 16 B 200 15 425 3 10 375 160 10 Comparative example 17 B 200 15 425 160 10 220 160 10 Comparative example 18 B 200 15 520 160 10 480 160 10 Comparative example 19 B 200 15 425 160 10 375 3 10 Invention example 20 C 220 15 425 160 10 375 160 10 Invention example 21 D 130 15 425 160 10 375 160 10 Invention example 22 E 270 15 425 160 10 375 160 10 Invention example 23 F 180 15 425 160 10 375 160 10 Invention example 24 G 200 15 425 160 10 375 160 10 Invention example 25 H 200 15 425 160 10 375 160 10 Invention example 26 I 190 15 425 160 10 375 160 10 Invention example 27 J 180 15 425 160 10 375 160 10 Invention example 28 K 200 15 425 160 10 375 160 10 Invention example 29 L 210 15 425 160 10 375 160 10 Invention example 30 M 200 15 425 160 10 375 160 10

division number Steel grade Secondary
Cooling
stop
Temperature
(℃) Secondary
Average
heating
speed
(℃/s) 3rd
maintain
Temperature
(℃) 3rd
maintain
time
(s) 4th
Average
Cooling
speed
(℃/s) 4th
maintain
Temperature
(℃) 4th
maintain
time
(s) 5th
Average
Cooling
speed
(℃/s) Invention example 31 N 220 15 425 160 15 375 160 10 Invention example 32 O 200 15 425 160 15 375 160 10 Invention example 33 P 150 15 425 160 15 375 160 10 Invention example 34 Q 250 15 425 160 15 375 160 10 Invention example 35 R 200 15 425 160 15 375 160 10 Invention example 36 S 200 15 425 160 15 375 160 10 Invention example 37 T 180 15 425 160 15 375 160 10 Invention example 38 U 200 15 425 160 15 375 160 10 Invention example 39 V 200 15 425 160 15 375 160 10 Invention example 40 W 190 15 425 160 15 375 160 10 Invention example 41 X 210 15 425 160 15 375 160 10 Invention example 42 Y 200 15 425 160 15 375 160 10 Comparative example 43 XA 200 15 425 160 15 375 160 10 Comparative example 44 XB 200 15 425 160 15 375 160 10 Comparative example 45 XC 190 15 425 160 15 375 160 10 Comparative example 46 XD 200 15 425 160 15 375 160 10 Comparative example 47 XE 180 15 425 160 15 375 160 10 Comparative example 48 XF 220 15 425 160 15 375 160 10 Comparative example 49 XG 200 15 425 160 15 375 160 10 Comparative example 50 XH 210 15 425 160 15 375 160 10 Comparative example 51 XI 200 15 425 160 15 375 160 10

division number Steel grade F
(vol.%) B
(vol.%) TM
(vol.%) FM
(vol.%) P
(vol.%) Residual γ
(vol.%) Invention example One A 0 27 51 0 0 22 Comparative example 2 A 33 7 8 0 47 5 Comparative example 3 A 20 15 47 12 0 6 Comparative example 4 A 18 24 49 One 0 8 Invention example 5 A 0 23 58 0 0 19 Comparative example 6 A 17 19 57 0 0 7 Comparative example 7 A 0 20 64 One 0 15 Comparative example 8 A 0 19 63 0 0 18 Comparative example 9 A 0 21 61 One 0 17 Invention example 10 B 0 25 55 0 0 20 Comparative example 11 B 0 18 64 One 0 17 Comparative example 12 B 0 12 82 0 0 6 Comparative example 13 B 0 75 6 0 0 19 Comparative example 14 B 0 18 63 0 0 19 Comparative example 15 B 0 23 69 0 0 8 Comparative example 16 B 0 18 64 2 0 16 Comparative example 17 B 0 15 67 0 0 18 Comparative example 18 B 0 24 55 One 0 20 Comparative example 19 B 0 22 60 One 0 17 Invention example 20 C 0 23 59 0 0 18 Invention example 21 D 0 18 66 0 0 16 Invention example 22 E 0 19 59 2 0 20 Invention example 23 F 0 17 61 0 0 22 Invention example 24 G 0 20 62 0 0 18 Invention example 25 H 0 45 36 One 0 18 Invention example 26 I 0 22 59 0 0 19 Invention example 27 J 0 19 64 0 0 17 Invention example 28 K 0 21 56 One 0 22 Invention example 29 L 0 17 62 0 0 21 Invention example 30 M 0 24 57 0 0 19

division number Steel grade F
(vol.%) B
(vol.%) TM
(vol.%) FM
(vol.%) P
(vol.%) Residual γ
(vol.%) Invention example 31 N 0 22 61 0 0 17 Invention example 32 O 0 31 47 0 0 22 Invention example 33 P 0 26 53 One 0 20 Invention example 34 Q 0 21 61 0 0 18 Invention example 35 R 0 18 47 One 0 34 Invention example 36 S 0 27 51 One 0 21 Invention example 37 T 0 25 56 0 0 19 Invention example 38 U 0 23 60 0 0 17 Invention example 39 V 0 17 65 0 0 18 Invention example 40 W 0 19 59 One 0 21 Invention example 41 X 0 23 57 One 0 19 Invention example 42 Y 0 22 60 0 0 18 Comparative example 43 XA 0 25 62 0 0 13 Comparative example 44 XB 0 19 22 15 0 44 Comparative example 45 XC 0 29 67 0 0 4 Comparative example 46 XD 0 20 39 22 0 19 Comparative example 47 XE 0 24 41 20 0 15 Comparative example 48 XF 0 26 60 One 6 7 Comparative example 49 XG 0 18 47 15 0 20 Comparative example 50 XH 0 22 44 18 0 16 Comparative example 51 XI 0 23 43 19 0 15

division number Steel grade V(1.2㎛,γ)/V(γ) V(lath,γ)/V(γ) TS×EL (MPa%) R/t Invention example One A 0.23 0.72 31146 1.63 Comparative example 2 A 0.15 0.55 15358 4.64 Comparative example 3 A 0.12 0.54 25724 5.28 Comparative example 4 A 0.14 0.58 19546 4.17 Invention example 5 A 0.21 0.65 31531 2.04 Comparative example 6 A 0.15 0.56 20157 3.37 Comparative example 7 A 0.08 0.58 24913 4.39 Comparative example 8 A 0.06 0.53 28516 3.85 Comparative example 9 A 0.07 0.57 26157 3.34 Invention example 10 B 0.22 0.73 31358 2.17 Comparative example 11 B 0.13 0.54 28597 3.42 Comparative example 12 B 0.08 0.56 21512 3.74 Comparative example 13 B 0.17 0.58 18941 2.29 Comparative example 14 B 0.05 0.55 26345 3.57 Comparative example 15 B 0.16 0.33 15627 7.53 Comparative example 16 B 0.08 0.39 27443 4.38 Comparative example 17 B 0.06 0.46 24838 8.04 Comparative example 18 B 0.16 0.44 26945 3.67 Comparative example 19 B 0.08 0.37 28578 5.87 Invention example 20 C 0.26 0.62 30257 2.32 Invention example 21 D 0.22 0.63 31752 2.01 Invention example 22 E 0.25 0.80 28146 2.47 Invention example 23 F 0.16 0.65 26247 1.14 Invention example 24 G 0.19 0.63 28324 1.38 Invention example 25 H 0.17 0.55 31972 2.22 Invention example 26 I 0.21 0.71 28126 2.42 Invention example 27 J 0.24 0.56 28315 0.79 Invention example 28 K 0.43 0.60 28168 2.04 Invention example 29 L 0.17 0.57 29020 2.28 Invention example 30 M 0.15 0.62 26418 2.64

division number Steel grade V(1.2㎛,γ)/V(γ) V(lath,γ)/V(γ) TS×EL (MPa%) R/t Invention example 31 N 0.18 0.67 28342 2.68 Invention example 32 O 0.17 0.62 29027 1.75 Invention example 33 P 0.20 0.68 30257 1.91 Invention example 34 Q 0.19 0.65 31139 2.15 Invention example 35 R 0.14 0.63 28661 2.68 Invention example 36 S 0.23 0.55 32187 2.33 Invention example 37 T 0.19 0.59 31455 1.84 Invention example 38 U 0.21 0.56 27635 2.42 Invention example 39 V 0.19 0.63 29328 1.84 Invention example 40 W 0.25 0.61 28479 1.77 Invention example 41 X 0.18 0.58 29216 2.53 Invention example 42 Y 0.19 0.63 31338 2.28 Comparative example 43 XA 0.24 0.55 16398 2.82 Comparative example 44 XB 0.19 0.53 17107 4.84 Comparative example 45 XC 0.18 0.56 13894 4.57 Comparative example 46 XD 0.13 0.57 26988 4.06 Comparative example 47 XE 0.14 0.54 26287 3.58 Comparative example 48 XF 0.17 0.53 15374 3.39 Comparative example 49 XG 0.15 0.62 24068 4.17 Comparative example 50 XH 0.19 0.56 23623 5.61 Comparative example 51 XI 0.16 0.54 25180 4.43

As shown in Tables 1 to 9, in the invention examples satisfying the conditions presented in the present invention, TS×El is 22,000 MPa% or more, and R/t is 0.5 to 3.0, so that a high-strength steel sheet having excellent processing characteristics is obtained. Can be seen

In contrast, in the comparative example, it can be seen that the conditions presented in the present invention are not satisfied, and thus the physical properties required by the present invention are not secured.

Specifically, Comparative Examples of Nos. 43 to 49 are those whose alloy compositions such as C, Si, Mn, and Al among the essential alloy components of the present invention are out of the scope of the present invention, and the strength and processing characteristics in the present invention cannot be secured. Can be seen. In particular, No. Comparative Example 45 is a case in which the total amount of Si and Al (Si + Al) is very low, TS × El is less than 22,000 MPa%, R/t exceeds 3.0, and is shown as a comparative example.

Nos. 50 and 51 are cases in which Cr and Mo exceeded the ranges suggested in the present invention, respectively, and fresh martensite (FM) increased, and R/t exceeded 3.0.

On the other hand, in Comparative Example No. 2, since the primary holding temperature was low, ferrite was excessively formed, bainite, retained austenite, and tempered martensite were insufficient, so that TS×El was less than 22,000 MPa%, and R/t Exceeded 3.0. In Comparative Example No. 3, the primary holding time was short, the structure became uneven, the ferrite fraction exceeded 5%, the residual austenite was insufficient, and the R/t exceeded 3.0.

In Comparative Examples of Nos. 4 and 6, respectively, the primary cooling stop temperature is low or the secondary average cooling rate is low, so ferrite is excessively formed, residual austenite is insufficient, TS x El is less than 22,000 MPa%, R /t exceeded 3.0. On the other hand, in Comparative Examples of Nos. 7 to 9, the secondary holding temperature and time were out of the scope of the present invention, so that V(1.2 μm, γ) / V(γ) was less than 0.1, and R/t exceeded 3.0. In Comparative Example No. 11, the third cooling rate was low, and the R/t exceeded 3.0.

In Comparative Example No.12, the secondary cooling stop temperature was low, resulting in excessive formation of tempered martensite and insufficient residual austenite, so that V(1.2㎛, γ) / V(γ) was less than 0.1, and TS×El It was less than 22,000 MPa%, and R/t exceeded 3.0. In Comparative Example No. 13, the secondary cooling stop temperature was high, so that bainite was excessively formed, and tempered martensite was insufficient, so that TS×El was less than 22,000 MPa%.

No. 14 had a low tertiary holding temperature, so V(1.2㎛, γ) / V(γ) was less than 0.1, and R/t exceeded 3.0. On the other hand, Comparative Example No.15 had a high tertiary holding temperature and lacked retained austenite, so that V(lath, γ) / V(γ) was less than 0.5, TS×El was less than 22,000 MPa%, and R/t Exceeded 3.0. Comparative Example of No. 16 has a short 3rd holding time, so V(1.2㎛, γ) / V(γ) is less than 0.1, V(lath, γ) / V(γ) is less than 0.5, and R/t Exceeded 3.0.

Comparative examples of Nos. 17 and 19, respectively, have a low 4th holding temperature or a short 4th holding time, with V(1.2㎛, γ) / V(γ) less than 0.1, and V(lath, γ) / V(γ) was less than 0.5 and R/t exceeded 3.0. On the other hand, in Comparative Example No. 18, the fourth holding temperature was high, so that V(lath, γ) / V(γ) was less than 0.5, and R/t exceeded 3.0.

Claims (10)

In% by weight, C: more than 0.25 and 0.75% or less, 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,
The microstructure contains 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 by volume fraction,
It satisfies the following [relational equation 1] and the following [relational equation 2],
The product of tensile strength and elongation (TS×El) is 22,000 MPa% or more, R/t (R is the minimum bending radius (mm) that does not cause cracks after 90° bending test, and t is the thickness of the steel plate (mm). This is a high-strength steel sheet with excellent processing characteristics of 0.5 to 3.0.
[Relationship 1]
V(1.2㎛, γ) / V(γ) ≥ 0.1
(However, V(1.2㎛, γ) is the fraction of retained austenite having an average grain size of 1.2㎛ or more, and V(γ) is the retained austenite fraction of the steel sheet)
[Relationship 2]
V(lath, γ) / V(γ) ≥ 0.5
(However, V(lath, γ) is the fraction of retained austenite in lath form, and V(γ) is the retained austenite fraction of the steel sheet)
The method according to claim 1,
The steel sheet is a steel sheet having excellent balance of strength and ductility and workability further comprising any one or more of the following (1) to (9).
(1) At least one of Ti: 0~0.5%, Nb: 0~0.5%, and V: 0~0.5%
(2) Cr: 0 to 3.0% and Mo: at least one of 0 to 3.0%
(3) Cu: 0-4.5% and Ni: 0-4.5% at least one of
(4) B: 0~0.005%
(5) Ca: 0~0.05%, REM excluding Y: 0~0.05% and Mg: at least one of 0~0.05%
(6) One or more of W: 0~0.5% and Zr: 0~0.5%
(7) Sb: 0 to 0.5% and Sn: at least one of 0 to 0.5%
(8) One or more of Y: 0~0.2% and Hf: 0~0.2%
(9) Co: 0~1.5%
The method according to claim 1,
The Si and Al is a high-strength steel sheet having excellent processing characteristics in which the sum (Si+Al) is 1.0 to 6.0%.
delete delete delete In% by weight, C: more than 0.25 and 0.75% or less, 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 inevitable impurities;
Heating the cold-rolled steel sheet to at least Ac3 (primary heating) and maintaining it for at least 50 seconds (primary maintenance);
Cooling (primary cooling) to a temperature range of 600 to 850°C at an average cooling rate of 1°C/s or more;
Cooling (secondary cooling) to a temperature range of 300 to 500°C at an average cooling rate of 2°C/s or more, and maintaining (secondary maintenance) in this temperature range for 5 seconds or more;
Cooling (third cooling) to a temperature range of 100 to 300°C at an average cooling rate of 2°C/s or more;
Heating (secondary heating) to a temperature range of 350 to 550°C at an average heating rate of 5°C/s or more, and maintaining (third holding) in this temperature range for 10 seconds or more;
Cooling (4th cooling) to a temperature range of 250 to 450°C at an average cooling rate of 1°C/s or more, and maintaining (4th maintenance) in this temperature range for 10 seconds or more; And
Cooling to room temperature (5th cooling) at an average cooling rate of 1℃/s or more
A method of manufacturing a high-strength steel sheet having excellent processing properties, including.
The method of claim 7,
The cold-rolled steel sheet is a method of manufacturing a high-strength steel sheet having excellent processing characteristics further comprising any one or more of the following (1) to (9).
(1) At least one of Ti: 0~0.5%, Nb: 0~0.5%, and V: 0~0.5%
(2) Cr: 0 to 3.0% and Mo: at least one of 0 to 3.0%
(3) Cu: 0-4.5% and Ni: 0-4.5% at least one of
(4) B: 0~0.005%
(5) Ca: 0~0.05%, REM excluding Y: 0~0.05% and Mg: at least one of 0~0.05%
(6) One or more of W: 0~0.5% and Zr: 0~0.5%
(7) Sb: 0 to 0.5% and Sn: at least one of 0 to 0.5%
(8) One or more of Y: 0~0.2% and Hf: 0~0.2%
(9) Co: 0~1.5%
The method of claim 7,
The preparation of the cold rolled steel sheet includes heating the steel slab to 1000 ~ 1350 °C;
Hot finishing rolling at a temperature range of 800 to 1000°C;
Winding in a temperature range of 350 ~ 700 ℃; And
Cold rolling with a reduction ratio of 30% or more
A method of manufacturing a high-strength steel sheet having excellent processing properties, including.
The method of claim 7,
The first cooling rate (Vc1) and the second cooling rate (Vc2) is a method of manufacturing a high-strength steel sheet having excellent processing characteristics satisfying the relationship of Vc1 <Vc2.