JP7442645B2 - High-strength steel plate with excellent workability and its manufacturing method - Google Patents

Description

The present invention relates to a steel plate that can be used for automobile parts, etc., and relates to a steel plate that has high strength characteristics and excellent workability, and a method for manufacturing the same.

Recently, the automotive industry has been looking at ways to reduce the weight of materials in order to protect the environment, while at the same time ensuring occupant stability. In order to meet such demands for stability and weight reduction, the application of high-strength steel plates is rapidly increasing. It is generally known that the higher the strength of a steel plate, the lower the workability of the steel plate. Therefore, there is a current demand for steel sheets for automobile parts that have high strength characteristics but also have excellent workability, typified by ductility, bending workability, hole expandability, and the like.

As a technique for improving the workability of steel sheets, methods of utilizing tempered martensite are disclosed in Patent Documents 1 and 2. Tempered martensite, which is produced by tempering hard martensite, is softened martensite, so tempered martensite has a higher strength than existing untempered martensite (fresh martensite). There is a difference between Therefore, by suppressing fresh martensite and forming tempered martensite, workability can be increased.

However, the techniques disclosed in Patent Documents 1 and 2 cannot satisfy the balance between tensile strength and elongation (TS x El) of 22,000 MPa% or more. This means that it is difficult to secure.

On the other hand, TRIP (Transformation Induced Plasticity) steel, which uses the transformation-induced plasticity of retained austenite, has been developed for steel sheets for automobile parts in order to obtain all the characteristics of high strength and excellent workability. Patent Document 3 discloses TRIP steel that has excellent strength and workability.

In Patent Document 3, an attempt was made to improve ductility and workability by containing polygonal ferrite, retained austenite, and martensite, but since the main phase is bainite, high strength could not be secured, and tensile strength It can be seen that the balance between strength and elongation (TS x El) also does not satisfy 22,000 MPa% or more.

In other words, the actual situation is that, although it has high strength, it does not meet the requirements for a steel plate that has excellent workability, typified by ductility, bending workability, hole expandability, and the like.

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

According to one aspect of the present invention, it is possible to provide a high-strength steel plate having excellent ductility, bending workability, and hole expandability by optimizing the composition and microstructure of a steel plate, and a method for manufacturing the same.

The object of the present invention is not limited to the matters described above. Further problems of the present invention are described in the general content of the specification, and a person having ordinary knowledge in the technical field to which the present invention pertains will be able to understand from the content described in the specification of the present invention. There is no difficulty in understanding the further subject of the invention.

The high-strength steel sheet with excellent workability according to one aspect of the present invention has, in weight percent, C: 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, including remaining Fe and unavoidable impurities, and as a microstructure, tempered martensite, It contains bainite, retained austenite, ferrite, and unavoidable structure, and can satisfy the following [Relational Expression 1] and [Relational Expression 2].

[Relational expression 1]
1.1≦[Si+Al] _F /[Si+Al] _γ ≦3.0
In the above relational expression 1, [Si+Al] _F is the average total content (weight%) of Si and Al contained in the ferrite, and [Si+Al] _γ is the average total content (weight %) of Si and Al contained in the retained austenite. %).

[Relational expression 2]
T(γ)/V(γ)≧0.1
In the above relational expression 2, T(γ) is the fraction (volume %) of tempered retained austenite in the steel plate, and V(γ) is the fraction (volume %) of retained austenite in the steel plate.

The steel plate may further include one or more of the following (1) to (9).
(1) One or more of Ti: 0-0.5%, Nb: 0-0.5% and V: 0-0.5% (2) Cr: 0-3.0% and Mo: 0- One or more types among 3.0% (3) One or more types among Cu: 0 to 4.5% and Ni: 0 to 4.5% (4) B: 0 to 0.005%
(5) One or more of Ca: 0 to 0.05%, REM excluding Y: 0 to 0.05%, and Mg: 0 to 0.05% (6) W: 0 to 0.5% and Zr : One or more types among 0 to 0.5% (7) One or more types among Sb: 0 to 0.5% and Sn: 0 to 0.5% (8) Y: 0 to 0.2% and Hf : One or more of 0 to 0.2% (9) Co: 0 to 1.5%
The total content of Si and Al (Si+Al) may be 1.0 to 6.0% by weight.

The microstructure of the steel sheet may include 30-70% by volume of tempered martensite, 10-45% by volume of bainite, 10-40% by volume of retained austenite, and 3-20% by volume of ferrite. The above steel plate has a balance between tensile strength and elongation (B _{T E} ) expressed by the following [Relational Expression 3] of 22,000 (MPa%) or more, and is expressed by the following [Relational Expression 4]. The balance between tensile strength and hole expansion rate (B _T・H ) is 7×10 ⁶ (MPa ² % ^1/2 ) or more, and the bending rate (B _R ) expressed by the following [Relational expression 5] A high-strength steel plate with excellent workability that has a value of 0.5 to 3.0.

[Relational expression 3]
B _T・E = [Tensile strength (TS, MPa)] × [Elongation rate (El, %)]
[Relational expression 4]
B _T・H = [Tensile strength (TS, MPa)] ² × [Hole expansion ratio (HER, %)] ^1/2
[Relational expression 5]
B _R =R/t
In the above relational expression 5, R means the minimum bending radius (mm) at which no crack occurs after a 90° bending test, and t means the thickness (mm) of the steel plate.

According to another aspect of the present invention, a method for producing a high-strength steel sheet with excellent workability includes, in weight percent, C: 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 is cold rolled containing Fe and inevitable impurities. a step of providing a steel plate; a step of heating the cold rolled steel plate to a temperature range of Ac1 or more and less than Ac3 (primary heating) and holding it for 50 seconds or more (primary holding); and an average cooling rate of 1°C. /s or more to a temperature range of 600 to 850°C (first cooling stop temperature) (primary cooling), and an average cooling rate of 2°C/s or more to a temperature range of 300 to 500°C (primary cooling). (secondary cooling) and holding in this temperature range for 5 seconds or more (secondary holding), and cooling (secondary cooling) to a temperature range of 100 to 300°C (secondary cooling stop temperature) at an average cooling rate of 2°C/s or more. tertiary cooling), heating to a temperature range of 300 to 500°C (secondary heating) and holding in this temperature range for 50 seconds or more (tertiary holding), and cooling to room temperature (fourth cooling). It can include steps.

The cold rolled steel plate may further include one or more of the following (1) to (9).
(1) One or more of Ti: 0-0.5%, Nb: 0-0.5% and V: 0-0.5% (2) Cr: 0-3.0% and Mo: 0- One or more types among 3.0% (3) One or more types among Cu: 0 to 4.5% and Ni: 0 to 4.5% (4) B: 0 to 0.005%
(5) One or more of Ca: 0 to 0.05%, REM excluding Y: 0 to 0.05%, and Mg: 0 to 0.05% (6) W: 0 to 0.5% and Zr : One or more types among 0 to 0.5% (7) One or more types among Sb: 0 to 0.5% and Sn: 0 to 0.5% (8) Y: 0 to 0.2% and Hf : One or more of 0 to 0.2% (9) Co: 0 to 1.5%
The total content of Si and Al (Si+Al) contained in the cold rolled steel sheet may be 1.0 to 6.0% by weight.

The above-mentioned cold rolled steel plate is produced by heating the steel slab to 1000 to 1350°C, finishing hot rolling in a temperature range of 800 to 1000°C, and performing the above hot rolling in a temperature range of 300 to 600°C. a step of winding up the rolled steel sheet; a step of hot-rolling the rolled-up steel sheet for 600-1700 seconds at a temperature range of 650-850°C; and cold rolling at a reduction rate. The cooling rate of the primary cooling (Vc1) and the cooling rate of the secondary cooling (Vc2) can satisfy the relationship of Vc1<Vc2.

According to a preferred aspect of the present invention, it is possible to provide a steel plate that not only has excellent strength but also has excellent workability such as ductility, bending workability, and hole expandability, and is particularly suitable for automobile parts.

The present invention relates to a high-strength steel plate with excellent workability and a method for manufacturing the same, and preferred implementation examples of the present invention will be described below. The implementations of the invention may be modified in various forms, and the scope of the invention should not be construed as limited to the implementations described below. This implementation is provided to fully convey the scope of the invention to those skilled in the art to which the invention pertains.

The inventors of the present invention have attempted to stabilize retained austenite in TRIP steel containing bainite, tempered martensite, retained austenite, and ferrite, and to stabilize retained austenite contained in retained austenite and ferrite. It has come to be recognized that when controlling the ratio of specific components within a certain range, it is possible to simultaneously ensure the workability and strength of steel sheets by reducing the hardness difference between the retained austenite and ferrite phases. . After investigating this problem, they discovered a method for improving the ductility and workability of high-strength steel, leading to the present invention. Hereinafter, a high-strength steel plate with excellent workability according to one aspect of the present invention will be described in more detail.

The high-strength steel sheet with excellent workability according to one aspect of the present invention has, in weight percent, C: 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, including remaining Fe and unavoidable impurities, and as a microstructure, tempered martensite, It contains bainite, retained austenite, ferrite, and unavoidable structure, and can satisfy the following [Relational Expression 1] and [Relational Expression 2].

[Relational expression 1]
1.1≦[Si+Al] _F /[Si+Al] _γ ≦3.0
In the above relational expression 1, [Si+Al] _F is the average total content (weight%) of Si and Al contained in the ferrite, and [Si+Al] _γ is the average total content (weight %) of Si and Al contained in the retained austenite. %).

[Relational expression 2]
T(γ)/V(γ)≧0.1
In the above relational expression 2, T(γ) is the fraction (volume %) of tempered retained austenite in the steel plate, and V(γ) is the fraction (volume %) of retained austenite in the steel plate.

Hereinafter, the steel composition of the present invention will be explained in more detail. In the following, unless otherwise specified, percentages indicating the content of each element are based on weight.

The high-strength steel sheet with excellent workability according to one aspect of the present invention has, in weight percent, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Contains Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, remaining Fe and unavoidable impurities. Additionally, Ti: 0.5% or less (including 0%), Nb: 0.5% or less (including 0%), V: 0.5% or less (including 0%), Cr: 3 .0% or less (including 0%), Mo: 3.0% or less (including 0%), Cu: 4.5% or less (including 0%), Ni: 4.5% or less (including 0%) ), B: 0.005% or less (including 0%), Ca: 0.05% or less (including 0%), REM excluding Y: 0.05% or less (including 0%), Mg: 0.05% or less (including 0%), W: 0.5% or less (including 0%), Zr: 0.5% or less (including 0%), Sb: 0.5% or less (0% ), Sn: 0.5% or less (including 0%), Y: 0.2% or less (including 0%), Hf: 0.2% or less (including 0%), Co: 1. It can further contain one or more of 5% or less (including 0%). Note that the total content of Si and Al (Si+Al) may be 1.0 to 6.0%.

Carbon (C): 0.25-0.75%
Carbon (C) is an element essential to ensuring the strength of a steel plate, and is also an element that stabilizes retained austenite, which contributes to improving the ductility of a steel plate. Therefore, the present invention may include 0.25% or more carbon (C) to achieve such effects. A preferred carbon (C) content may be greater than 0.25%, may be greater than or equal to 0.27%, and may be greater than or equal to 0.30%. A more preferable carbon (C) content may be 0.31% or more. On the other hand, if the carbon (C) content exceeds a certain level, cooling rolling may become difficult due to an excessive increase in strength. Therefore, the present invention can limit the upper limit of carbon (C) content to 0.75%. The carbon (C) content may be 0.70% or less, and the more preferable carbon (C) content may be 0.67% or less.

Silicon (Si): 4.0% or less (excluding 0%)
Silicon (Si) is an element that contributes to improving strength through solid solution strengthening, and is also an element that improves workability by strengthening ferrite and making the structure uniform. Furthermore, silicon (Si) is an element that suppresses the precipitation of cementite and contributes to the formation of retained austenite. Therefore, in the present invention, silicon (Si) can be essentially added to achieve this effect. A preferable silicon (Si) content may be 0.02% or more, and a more preferable silicon (Si) content may be 0.05% or more. However, if the silicon (Si) content exceeds a certain level, it may not only cause problems such as unplated plating defects during the plating process, but also reduce the weldability of the steel sheet. The upper limit of the (Si) content can be limited to 4.0%. A preferable upper limit of silicon (Si) content may be 3.8%, and a more preferable upper limit of silicon (Si) content may be 3.5%.

Aluminum (Al): 5.0% or less (excluding 0%)
Aluminum (Al) is an element that combines with oxygen in steel to deoxidize it. Further, like silicon (Si), aluminum (Al) is also an element that suppresses cementite precipitation and stabilizes retained austenite. Therefore, in the present invention, aluminum (Al) can be essentially added to achieve this effect. A preferable aluminum (Al) content may be 0.05% or more, and a more preferable aluminum (Al) content may be 0.1% or more. On the other hand, if aluminum (Al) is added in excess, not only the number of inclusions in the steel sheet increases, but also the workability of the steel sheet may be reduced. It can be limited to .0%. A preferable upper limit of the aluminum (Al) content may be 4.75%, and a more preferable upper limit of the aluminum (Al) content may be 4.5%.

Meanwhile, the total content of silicon (Si) and aluminum (Al) (Si+Al) is preferably 1.0 to 6.0%. In the present invention, silicon (Si) and aluminum (Al) are components that affect the formation of a microstructure and affect ductility, bending workability, and hole expandability. The total content of is preferably 1.0 to 6.0%. More preferably, the total content of silicon (Si) and aluminum (Al) (Si+Al) may be 1.5% or more and 4.0% or less.

Manganese (Mn): 0.9-5.0%
Manganese (Mn) is a useful element for increasing both strength and ductility. Therefore, the present invention can limit the lower limit of the manganese (Mn) content to 0.9% to achieve this effect. A preferable lower limit of the manganese (Mn) content may be 1.0%, and a more preferable lower limit of the manganese (Mn) content may be 1.1%. On the other hand, when manganese (Mn) is added in excess, the bainite transformation time increases and the concentration of carbon (C) in austenite becomes insufficient, resulting in the problem that the desired austenite fraction cannot be secured. A point exists. Therefore, the present invention can limit the upper limit of the manganese (Mn) content to 5.0%. A preferable upper limit of the manganese (Mn) content may be 4.7%, and a more preferable upper limit of the manganese (Mn) content may be 4.5%.

Phosphorus (P): 0.15% or less (including 0%)
Phosphorus (P) is an element that is contained as an impurity and deteriorates impact toughness. Therefore, it is preferable to control the phosphorus (P) content to 0.15% or less.

Sulfur (S): 0.03% or less (including 0%)
Sulfur (S) is an element that is contained as an impurity, forms MnS in the steel sheet, and deteriorates ductility. Therefore, the content of sulfur (S) is preferably 0.03% or less.

Nitrogen (N): 0.03% or less (including 0%)
Nitrogen (N) is an element that is contained as an impurity and forms nitrides during continuous casting, causing cracks in the slab. Therefore, the nitrogen (N) content is preferably 0.03% or less.

Meanwhile, the steel sheet of the present invention may include an alloy composition other than the above-mentioned alloy components, which will be described in detail below.

One or more of titanium (Ti): 0 to 0.5%, niobium (Nb): 0 to 0.5%, and vanadium (V): 0 to 0.5% Titanium (Ti), niobium (Nb), and Vanadium (V) is an element that creates precipitates and refines crystal grains, and also contributes to improving the strength and impact toughness of steel sheets. One or more of (Ti), niobium (Nb), and vanadium (V) can be added. However, if the content of titanium (Ti), niobium (Nb), and vanadium (V) exceeds a certain level, excessive precipitates are formed, which not only reduces impact toughness but also causes an increase in manufacturing costs. Therefore, the present invention can limit the contents of titanium (Ti), niobium (Nb), and vanadium (V) to 0.5% or less, respectively.

One or more of chromium (Cr): 0 to 3.0% and molybdenum (Mo): 0 to 3.0% Chromium (Cr) and molybdenum (Mo) only suppress austenite decomposition during alloying treatment. However, like manganese (Mn), it is an element that stabilizes austenite. Therefore, in the present invention, it is possible to add one or more of chromium (Cr) and molybdenum (Mo) for this effect. can. However, if the content of chromium (Cr) and molybdenum (Mo) exceeds a certain level, the bainite transformation time increases and the amount of carbon (C) enriched in austenite becomes insufficient, so that the desired residual austenite cannot be obtained. Unable to secure percentage. Therefore, the present invention can limit the contents of chromium (Cr) and molybdenum (Mo) to 3.0% or less.

One or more of copper (Cu): 0 to 4.5% and nickel (Ni): 0 to 4.5% Copper (Cu) and nickel (Ni) are elements that stabilize austenite and suppress corrosion. be. Copper (Cu) and nickel (Ni) are also elements that concentrate on the surface of the steel sheet to prevent hydrogen from entering the steel sheet and suppress hydrogen-delayed fracture. Therefore, in the present invention, one or more of copper (Cu) and nickel (Ni) can be added for such effects. However, if the content of copper (Cu) and nickel (Ni) exceeds a certain level, it not only causes excessive property effects but also increases manufacturing costs. The content of each can be limited to 4.5% or less.

Boron (B): 0-0.005%
Boron (B) is an element that improves hardenability and increases strength, and is also an element that suppresses nucleation at grain boundaries. Therefore, in the present invention, boron (B) can be added for such an effect. However, if the content of boron (B) exceeds a certain level, it not only causes excessive property effects but also increases production costs, so the present invention limits the content of boron (B) to 0.005% or less. can do.

One or more of calcium (Ca): 0 to 0.05%, magnesium (Mg): 0 to 0.05%, and rare earth elements (REM) excluding yttrium (Y): 0 to 0.05%, where, Rare earth elements (REM) mean scandium (Sc), yttrium (Y), and lanthanum group elements. Rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) are elements that contribute to improving the ductility of steel sheets by making sulfides spheroidal. For this purpose, one or more of rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) can be added. However, if the content of rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) exceeds a certain level, this will not only cause excessive property effects but also increase manufacturing costs. In the present invention, the content of rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) can be limited to 0.05% or less.

One or more of tungsten (W): 0 to 0.5% and zirconium (Zr): 0 to 0.5% Tungsten (W) and zirconium (Zr) improve the hardenability and increase the strength of the steel plate. Since it is an element, in the present invention, one or more of tungsten (W) and zirconium (Zr) can be added for such an effect. However, if the content of tungsten (W) and zirconium (Zr) exceeds a certain level, it not only causes excessive property effects but also increases manufacturing costs. The content of each can be limited to 0.5% or less.

One or more of antimony (Sb): 0 to 0.5% and tin (Sn): 0 to 0.5% Antimony (Sb) and tin (Sn) improve plating wettability and plating adhesion of steel sheets. Therefore, in the present invention, one or more of antimony (Sb) and tin (Sn) can be added for such an effect. However, if the content of antimony (Sb) and tin (Sn) exceeds a certain level, the brittleness of the steel sheet may increase and cracks may occur during hot working or cold working. (Sb) and tin (Sn) contents can be limited to 0.5% or less, respectively.

One or more of yttrium (Y): 0 to 0.2% and hafnium (Hf): 0 to 0.2% Yttrium (Y) and hafnium (Hf) are elements that improve the corrosion resistance of steel sheets. In the present invention, one or more of yttrium (Y) and hafnium (Hf) can be added to achieve this effect. However, if the content of yttrium (Y) and hafnium (Hf) exceeds a certain level, the ductility of the steel sheet may deteriorate. .2% or less.

Cobalt (Co): 0-1.5%
Since cobalt (Co) is an element that promotes bainite transformation and increases the TRIP effect, the present invention can add cobalt (Co) for such an effect. However, if the cobalt (Co) content exceeds a certain level, the weldability and ductility of the steel plate may deteriorate, so the present invention limits the cobalt (Co) content to 1.5% or less. I can do it.

The high-strength steel sheet with excellent workability according to one aspect of the present invention may contain residual Fe and other unavoidable impurities in addition to the above-mentioned components. However, in normal manufacturing processes, unintended impurities may inevitably be mixed in from raw materials or the surrounding environment, so this cannot be completely eliminated. These impurities are known to anyone with ordinary knowledge in this technical field, and therefore, all contents thereof are not specifically mentioned in this specification. Furthermore, the addition of further active ingredients in addition to those mentioned above is not entirely excluded.

A high-strength steel sheet with excellent workability according to one aspect of the present invention can contain tempered martensite, bainite, retained austenite, and ferrite as a microstructure. As a preferred example, the high-strength steel sheet with excellent workability according to one aspect of the present invention has a volume fraction of 30 to 70% tempered martensite, 10 to 45% bainite, 10 to 40% retained austenite, It can contain 3-20% ferrite and unavoidable structure. The inevitable structure of the present invention may include fresh martensite, pearlite, island martensite (Martensite Austenite Constitution, MA), and the like. Excessive formation of fresh martensite or pearlite may reduce the workability of the steel sheet or reduce the fraction of retained austenite.

A high-strength steel sheet with excellent workability according to one aspect of the present invention has an average total content of silicon (Si) and aluminum (Al) contained in retained austenite ([Si+Al] The ratio of the average total content of silicon (Si) and aluminum (Al) contained in the ferrite ([Si+Al] _F , weight %) to _γ , weight %) satisfies the range of 1.1 to 3.0, and the following As shown in [Relational Expression 2], the ratio of the fraction of tempered retained austenite in the steel plate (T(γ), volume %) to the fraction of retained austenite in the steel plate (V(γ), volume %) is 0.1 or more It may be.

[Relational expression 1]
1.1≦[Si+Al] _F /[Si+Al] _γ ≦3.0

[Relational expression 2]
T(γ)/V(γ)≧0.1

A high-strength steel plate with excellent workability according to one aspect of the present invention has a balance between tensile strength and elongation (B _{T E} ) of 22,000 (MPa%) or more as expressed by [Relational Expression 3] below. The balance ^between tensile strength and ^hole expansion rate ⁽ _B ] Since the bending rate (B _R ) expressed by It can have processability.

[Relational expression 3]
B _T・E = [Tensile strength (TS, MPa)] × [Elongation rate (El, %)]

[Relational expression 4]
B _T・H = [Tensile strength (TS, MPa)] ² × [Hole expansion ratio (HER, %)] ^1/2

[Relational expression 5]
B _R =R/t
In the above relational expression 5, R means the minimum bending radius (mm) at which no crack occurs after a 90° bending test, and t means the thickness (mm) of the steel plate.

Since the present invention aims to simultaneously ensure not only high strength properties but also excellent ductility and bending workability, it is important to stabilize retained austenite in the steel sheet. In order to stabilize retained austenite, it is necessary to enrich carbon (C) and manganese (Mn) in ferrite, bainite, and tempered martensite of the steel sheet to austenite. However, if ferrite is used to enrich carbon (C) in austenite, the strength of the steel sheet may be insufficient due to the low strength properties of ferrite, and an excessive difference in hardness between the phases may occur, leading to hole enlargement. There is a risk that the rate (HER) may decrease. Therefore, the present invention utilizes bainite and tempered martensite to enrich carbon (C) and manganese (Mn) in austenite.

When limiting the content of silicon (Si) and aluminum (Al) in retained austenite to a certain range, carbon (C) and manganese (Mn) must be enriched in large amounts in retained austenite from bainite and tempered martensite. Therefore, retained austenite can be effectively stabilized. Furthermore, by limiting the contents of silicon (Si) and aluminum (Al) in austenite to a certain range, the contents of silicon (Si) and aluminum (Al) in ferrite can be increased. As the content of silicon (Si) and aluminum (Al) in ferrite increases, the hardness of ferrite increases, and the hardness difference between the phases of ferrite, which is a soft structure, and tempered martensite, bainite, and retained austenite, which are hard structures, increases. can be effectively reduced.

Therefore, the present invention aims at increasing the amount of silicon (Si) and aluminum (Al) contained in ferrite relative to the average total content ([Si+Al] _γ , weight %) of silicon (Si) and aluminum (Al) contained in retained austenite. Since the ratio of the average total content ([Si+Al] _F , weight %) is limited to 1.1 or more, the hardness difference between the soft tissue and hard tissue phases can be effectively reduced. On the other hand, if the content of silicon (Si) and aluminum (Al) in ferrite is excessive, the ferrite becomes excessively hard and its workability decreases, resulting in the desired balance between tensile strength and elongation (TS x El). ), the balance between tensile strength and hole expansion rate (TS ² ×HER ^1/2 ), and bending rate (R/t) cannot be ensured. Therefore, the present invention aims at increasing the amount of silicon (Si) and aluminum (Al) contained in ferrite relative to the average total content ([Si+Al] _γ , weight %) of silicon (Si) and aluminum (Al) contained in retained austenite. The ratio of the average total content ([Si+Al] _F , weight %) can be limited to 3.0 or less.

On the other hand, tempered retained austenite in retained austenite is heat-treated at the formation temperature of bainite to increase its average size, and can suppress the transformation from austenite to martensite and improve the workability of steel sheets. can. That is, in order to improve the ductility and workability of the steel sheet, it is preferable to increase the fraction of tempered retained austenite in the retained austenite.

Therefore, the high-strength steel sheet with excellent workability according to one aspect of the present invention has a ratio of tempered retained austenite (T(γ), volume %) to the retained austenite fraction (V(γ), volume %) of the steel sheet. %) can be limited to 0.1 or more. If the ratio of the fraction of tempered retained austenite in the steel plate (T(γ), volume %) to the fraction of retained austenite in the steel plate (V(γ), volume %) is less than 0.1, the bending rate (R/ There is a problem that t) no longer satisfies 0.5 to 3.0, and the desired workability cannot be secured.

A steel plate containing retained austenite has excellent ductility and bending workability due to transformation-induced plasticity that occurs during transformation from austenite to martensite during processing. If the fraction of retained austenite is less than a certain level, the balance between tensile strength and elongation (TS x El) is less than 22,000 MPa%, or the bending rate (R/t) is less than 3.0. can be exceeded. On the other hand, if the fraction of retained austenite exceeds a certain level, local elongation may decrease. Therefore, the present invention aims to increase the fraction of retained austenite to 10 to 40% by volume in order to obtain a steel plate that not only has a good balance between tensile strength and elongation rate (TS x El) but also has an excellent bending rate (R/t). Can be limited to a range.

On the other hand, both untempered martensite (fresh martensite) and tempered martensite are microstructures that improve the strength of the steel sheet. However, compared to tempered martensite, fresh martensite has characteristics that greatly reduce the ductility and hole expandability of the steel sheet. This is because the microstructure of tempered martensite is softened by the tempering heat treatment. Therefore, in the present invention, it is preferable to utilize tempered martensite in order to provide a steel plate with excellent balance between strength and ductility, balance between strength and hole expandability, and excellent bending workability. If the fraction of tempered martensite is below a certain level, the balance between tensile strength and elongation (TS x El) of 22,000 MPa% or more or the tensile strength and hole of 7 x 10 ⁶ (MPa ² % ^1/2 ) or more It is difficult to maintain the balance of expansion ratio (TS ² × HER ^1/2 ), and when the fraction of tempered martensite exceeds a certain level, ductility and workability decrease, resulting in a balance of tensile strength and elongation (TS × HER 1/2). El) is less than 22,000 MPa% or the bending rate (R/t) exceeds 3.0, which is not preferable. Therefore, the present invention provides a steel plate with excellent balance between tensile strength and elongation rate (TS x El), balance between tensile strength and hole expansion rate (TS ² x HER ^1/2 ), and bending rate (R/t). In order to achieve this, the fraction of tempered martensite can be limited to a range of 30-70% by volume.

In order to improve the balance between tensile strength and elongation rate (TS x El), the balance between tensile strength and hole expansion rate (TS ² x HER ^1/2 ), and the bending rate (R/t), bainite as a microstructure is required. is preferably included appropriately. Only when the bainite fraction is above a certain level, the balance between tensile strength and elongation (TS x El) of 22,000 MPa% or more, tensile strength and hole of 7 x 10 ⁶ (MPa ² % ^1/2 ) or more A balance in expansion ratio (TS ² ×HER ^1/2 ) and a bending ratio (R/t) of 0.5 to 3.0 can be ensured. On the other hand, if the bainite fraction is excessive, the tempered martensite fraction inevitably decreases, so that the balance between tensile strength and elongation (TS x El), which is the objective of the present invention, and the tensile strength The balance between the hole expansion rate (TS ² ×HER ^1/2 ) and the bending rate (R/t) cannot be ensured. Therefore, the present invention can limit the fraction of bainite to a range of 10 to 45% by volume.

Since ferrite is an element that contributes to improving ductility, the balance between tensile strength and elongation (TS x El) that is the objective of the present invention can be ensured only when the ferrite fraction is above a certain level. . However, if the ferrite fraction is excessive, the difference in hardness between phases may increase and the hole expansion ratio (HER) may decrease. (TS ² ×HER ^1/2 ) cannot be secured. Therefore, the present invention can limit the ferrite fraction to a range of 3 to 20% by volume.

Below, an example of the method for manufacturing the steel plate of the present invention will be explained in detail.
A method for manufacturing a high-strength steel plate with excellent workability according to one aspect of the present invention includes the steps of providing a cold-rolled steel plate having a predetermined composition, and heating the cold-rolled steel plate at a temperature of Ac1 or more and less than Ac3. heating to a temperature range of 600 to 850°C (primary cooling stop temperature) at an average cooling rate of 1°C/s or more and holding for 50 seconds or more (primary holding). (primary cooling) and cooling to a temperature range of 300 to 500°C at an average cooling rate of 2°C/s or more (secondary cooling) and holding this temperature range for 5 seconds or more (secondary holding). , a stage of cooling (tertiary cooling) to a temperature range of 100 to 300 °C (secondary cooling stop temperature) at an average cooling rate of 2 °C/s or more, and a stage of heating to a temperature range of 300 to 500 °C (secondary heating). ), and may include the steps of holding in this temperature range for 50 seconds or more (tertiary holding) and cooling to room temperature (fourth cooling).

In addition, the cold rolled steel plate of the present invention can be obtained by heating the steel slab to 1000 to 1350°C, finishing hot rolling in a temperature range of 800 to 1000°C, and performing hot rolling in a temperature range of 300 to 600°C. a step of winding the hot-rolled steel plate; a step of hot-rolling and annealing the rolled-up steel plate at a temperature range of 650-850°C for 600-1700 seconds; and cold rolling at a rolling reduction of ~90%.

Preparation and heating of steel slab A steel slab having predetermined components is prepared. Since the steel slab of the present invention has an alloy composition corresponding to the alloy composition of the steel plate described above, the description of the alloy composition of the steel slab is replaced by the description of the alloy composition of the steel plate described above.

The prepared steel slab can be heated to a certain temperature range. The heating temperature of the steel slab at this time may be in the range of 1000 to 1350°C. This means that if the heating temperature of the steel slab is less than 1000°C, there is a risk that it will be hot rolled in a temperature range below the target finishing hot rolling temperature range, and if the heating temperature of the steel slab exceeds 1350°C, This is because there is a risk that it will reach the melting point of steel and melt.

Hot Rolling and Coiling The heated steel slab can be hot rolled to provide hot rolled steel sheet. The finish hot rolling temperature during hot rolling is preferably in the range of 800 to 1000°C. This is because if the finish hot rolling temperature is less than 800℃, excessive rolling load may become a problem, and if the finish hot rolling temperature exceeds 1000℃, the crystal grains of the hot rolled steel sheet may become coarse. This is because there is a possibility that the physical properties of the final steel sheet will deteriorate.

A hot-rolled steel sheet that has been hot-rolled can be cooled at an average cooling rate of 10°C/s or more, and can be rolled up at a temperature of 300 to 600°C. This is because when the coiling temperature is less than 300℃, it is not easy to coil, and when the coiling temperature exceeds 600℃, surface scale is formed to the inside of the hot rolled steel sheet, making pickling difficult. This is because there is a risk that

Hot Rolling Annealing Heat Treatment In order to easily perform pickling and cold rolling, which are subsequent steps after coiling, it is preferable to perform a hot rolling annealing heat treatment step. The hot rolling annealing heat treatment can be performed at a temperature range of 650 to 850° C. for 600 to 1700 seconds. If the hot rolling annealing heat treatment temperature is less than 650°C or the hot rolling annealing heat treatment time is less than 600 seconds, the strength of the hot rolling annealing heat treated steel plate may be high and subsequent cold rolling may not be easy. . On the other hand, if the hot rolling annealing heat treatment temperature exceeds 850°C or the hot rolling annealing heat treatment time exceeds 1700 seconds, pickling may not be easy due to scale formed deep inside the steel plate. There is.

Pickling and Cold Rolling After hot rolling annealing heat treatment, pickling can be performed to remove scale generated on the surface of the steel sheet, and then cold rolling can be performed. In the present invention, the conditions for pickling and cold rolling are not particularly limited, but cold rolling is preferably performed at a cumulative reduction rate of 30 to 90%. If the cumulative reduction ratio in cold rolling exceeds 90%, it may be difficult to cold roll in a short time due to the high strength of the steel sheet.

Cold rolled steel sheets can be produced as unplated cold rolled steel sheets through an annealing heat treatment process, or can be produced as plated steel sheets through a plating process to impart corrosion resistance. For plating, plating methods such as hot-dip galvanizing, electrolytic galvanizing, and hot-dip aluminum plating can be applied, and the method and type thereof are not particularly limited.

Annealing Heat Treatment The present invention performs an annealing heat treatment process in order to simultaneously ensure the strength and workability of the steel plate.

A cold rolled steel plate is heated (primary heating) to a temperature range of Ac1 or more and less than Ac3 (two-phase region), and held in the temperature range for 50 seconds or more (primary holding). If the primary heating or primary holding temperature is Ac3 or higher (single phase range), the desired ferrite structure cannot be achieved, so the desired level of [Si+Al] _F /[Si+Al] _γ and tensile strength It becomes impossible to achieve the balance between the hole expansion rate and the hole expansion rate (TS ² ×HER ^1/2 ). Further, when the primary heating or primary holding temperature is in a temperature range below Ac1, sufficient heating is not performed, so there is a possibility that the microstructure aimed at by the present invention cannot be achieved even by the subsequent heat treatment. The average temperature increase rate of the primary heating may be 5° C./s or more.

If the primary holding time is less than 50 seconds, the structure cannot be made sufficiently uniform, and the physical properties of the steel sheet may deteriorate. Although the upper limit of the primary holding time is not particularly limited, it is preferable to limit the primary heating time to 1200 seconds or less in order to prevent a decrease in toughness due to coarsening of crystal grains.

After the primary holding, it is preferable to cool (primary cooling) to a temperature range of 600 to 850°C (primary cooling stop temperature) at an average cooling rate of 1°C/s or more. The upper limit of the average cooling rate of primary cooling does not need to be particularly defined, but it is preferably limited to 100° C./s or less. When the primary cooling stop temperature is less than 600°C, ferrite is formed excessively and residual austenite is insufficient, resulting in poor balance between [Si+Al] _F /[Si+Al] _γ and tensile strength and elongation (TS x El). may decrease. Moreover, since it is preferable that the upper limit of the primary cooling stop temperature is 30° C. or lower than the above-mentioned primary holding temperature, the upper limit of the primary cooling stopping temperature can be limited to 850° C.

After primary cooling, it is preferable to cool to a temperature range of 300 to 500° C. at an average cooling rate of 2° C./s or more (secondary cooling) and hold the temperature range for 5 seconds or more (secondary holding). When the average cooling rate of secondary cooling is less than 2°C/s, ferrite is formed excessively and residual austenite is insufficient, resulting in poor [Si+Al] _F /[Si+Al] _γ and the balance between tensile strength and elongation (TS ×El) may decrease. The upper limit of the average cooling rate of secondary cooling does not need to be specifically defined, but it is preferably limited to 100° C./s or less. On the other hand, when the secondary holding temperature exceeds 500°C, the residual austenite is insufficient and the balance between [Si + Al] _F / [Si + Al] _γ , T (γ) / V (γ), tensile strength and elongation rate (TS × El ) and bending rate (R/t) may decrease. Moreover, when the secondary holding temperature is less than 300° C., T(γ)/V(γ) and bending rate (R/t) may decrease due to the low heat treatment temperature. When the secondary holding time is less than 5 seconds, there is a possibility that the heat treatment time is insufficient and T(γ)/V(γ) and bending rate (R/t) decrease. On the other hand, the upper limit of the secondary holding time does not need to be particularly specified, but it is preferably 600 seconds or less.

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

After the secondary holding, it is preferable to cool (tertiary cooling) to a temperature range of 100 to 300°C (secondary cooling stop temperature) at an average cooling rate of 2°C/s or more. When the average cooling rate of the tertiary cooling is less than 2°C/s, the [Si+Al] _F /[Si+Al] _γ , T(γ)/V(γ) and bending rate (R/t) of the steel plate decrease due to slow cooling. There is a possibility that it will decrease. Although the upper limit of the average cooling rate of tertiary cooling does not need to be particularly specified, it is preferably limited to 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, resulting in T(γ)/V(γ) and the balance between tensile strength and elongation (TS x El ) may decrease. On the other hand, when the secondary cooling stop temperature is less than 100°C, tempered martensite is excessively formed and residual austenite is insufficient, resulting in [Si+Al] _F /[Si+Al] _γ , T(γ)/V (γ), the balance between tensile strength and elongation rate (TS×El), and the bending rate (R/t) may decrease.

After the tertiary cooling, it is preferable to heat it to a temperature range of 300 to 500° C. (secondary heating) and hold it in the temperature range for 10 seconds or more (tertiary holding). When the tertiary holding temperature exceeds 550°C, the retained austenite is insufficient and [Si+Al] _F / [Si+Al] _γ , T (γ) / V (γ), balance between tensile strength and elongation (TS × El), and There is a possibility that the bending rate (R/t) will decrease. On the other hand, when the tertiary holding temperature is less than 350°C, the holding temperature is low and residual austenite is insufficient, resulting in a decrease in [Si+Al] _F /[Si+Al] _γ , T(γ)/V(γ), and tensile strength. The elongation rate balance (TS×El) and bending rate (R/t) may decrease. If the tertiary holding time is less than 50 seconds, tempered martensite is excessively formed and residual austenite is insufficient, resulting in poor [Si+Al] _F /[Si+Al] _γ , T(γ)/V(γ), and tensile strength. The elongation rate balance (TS×El) and bending rate (R/t) may decrease. Although the upper limit of the tertiary holding time is not particularly limited, a preferable tertiary holding time may be 1800 seconds or less.

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

The high-strength steel sheet with excellent workability produced by the above production method can contain tempered martensite, bainite, retained austenite, and ferrite as a microstructure, and as a preferable example, the volume fraction is 30 to 30. It can contain 70% tempered martensite, 10-45% bainite, 10-40% retained austenite, 3-20% ferrite and unavoidable structure.

In addition, the high-strength steel sheet with excellent workability manufactured by the above-mentioned manufacturing method has an average total content of silicon (Si) and aluminum (Al) contained in retained austenite, as shown in [Relational Expression 1] below. The ratio of the average total content ([Si+Al] _F , weight %) of silicon (Si) and aluminum (Al) contained in the ferrite to ([Si+Al]] _γ , weight %) is in the range of 1.1 to 3.0. can be fulfilled. In addition, as shown in [Relational Expression 2] below, the ratio of the fraction of tempered retained austenite (T (γ), volume %) of the steel plate to the fraction of retained austenite (V (γ), volume %) of the steel plate is It may be 0.1 or more.

[Relational expression 1]
1.1≦[Si+Al] _F /[Si+Al] _γ ≦3.0

[Relational expression 2]
T(γ)/V(γ)≧0.1

The high-strength steel plate with excellent workability manufactured by the above-mentioned manufacturing method has a balance between tensile strength and elongation rate (B _{T E} ) of 22,000 (MPa%), which is expressed by the following [Relational Expression 3]. Therefore ^, the balance ^between tensile strength and hole expansion rate ⁽ _B The bending rate (B _R ) expressed by Equation 5] can satisfy the range of 0.5 to 3.0.

[Relational expression 3]
B _T・E = [Tensile strength (TS, MPa)] × [Elongation rate (EL, %)]

[Relational expression 4]
B _T・H = [Tensile strength (TS, MPa)] ² × [Hole expansion ratio (HER, %)] ^1/2

[Relational expression 5]
B _R =R/t
In the above relational expression 5, R means the minimum bending radius (mm) at which no crack occurs after a 90° bending test, and t means the thickness (mm) of the steel plate.

Hereinafter, a high-strength steel plate with excellent workability and a method for manufacturing the same according to one aspect of the present invention will be described in detail through specific examples. It should be noted that the following examples are provided to aid understanding of the present invention, and are not intended to specify the scope of rights of the present invention. The scope of rights in the present invention is determined by the matters stated in the claims and matters reasonably inferred therefrom.

(Example)
A 100 mm thick steel slab having the alloy composition shown in Table 1 below (the remainder being Fe and unavoidable impurities) was manufactured, heated at 1200°C, and then finished hot rolled at 900°C. Thereafter, it was cooled at an average cooling rate of 30° C./s and rolled up at the winding temperatures shown in Tables 2 and 3 to produce a hot rolled steel sheet with a thickness of 3 mm. The above hot rolled steel sheets were subjected to hot rolling annealing heat treatment under the conditions shown in Tables 2 and 3. Thereafter, after pickling to remove surface scale, cold rolling was performed to a thickness of 1.5 mm. Thereafter, heat treatment was performed under the annealing heat treatment conditions shown in Tables 2 to 7 to produce steel plates.

The microstructure of the steel sheet manufactured in this manner was observed, and the results are shown in Tables 8 and 9. Among the microstructures, ferrite (F), bainite (B), tempered martensite (TM), and pearlite (P) were observed through SEM after nital etching the cross section of the polished sample. Among these, the fraction of bainite and tempered martensite, which are difficult to distinguish, was calculated using an expansion curve after dilation evaluation. On the other hand, fresh martensite (FM) and retained austenite (retained γ) are also not easily distinguishable, so the retained austenite fraction calculated by X-ray diffraction method from the fraction of martensite and retained austenite observed by the above SEM The value obtained by subtracting the fraction was defined as the fresh martensite fraction.

On the other hand, [Si+Al] _F / [Si+Al] _γ , T (γ) / V (γ), balance between tensile strength and elongation rate (TS × El), balance between tensile strength and hole expansion rate (TS ² × HER ^1/2 ) and the bending rate (R/t) were observed, and the results are shown in Tables 10 and 11.

Average total content of silicon (Si) and aluminum (Al) contained in retained austenite ([Si+Al] _γ , weight%) and average total content of silicon (Si) and aluminum (Al) contained in ferrite ([Si+Al] ] _F , weight %) was measured using EPMA (Electron Probe MicroAnalyser). Further, the fraction of retained austenite (V(γ)) of the steel plate was determined by the area measured within the retained austenite using an EPMA phase map.

Tensile strength (TS) and elongation rate (El) were evaluated by a tensile test, using specimens taken in accordance with JIS No. TS) and elongation rate (El) were measured. The bending rate (R/t) is evaluated by a V-bending test, where specimens are taken at 90° to the rolling direction of the rolled plate material and the minimum bending radius at which no cracks occur after the 90° bending test. It was determined and calculated by dividing R by the thickness t of the plate material. The hole expansion ratio (HER) was evaluated through a hole expansion test. After forming a 10mmΦ punch hole (die inner diameter 10.3mm, clearance 12.5%), a conical punch with a 60° apex angle was used to burr the punched hole (HER). burr) was inserted into the punch hole in a direction facing outward, and the surrounding area of the punch hole was compressed and expanded at a moving speed of 20 mm/min, and then calculated using the following [Relational Expression 6].

[Relational expression 6]
Hole expansion rate (HER, %) = {(DD ₀ )/D ₀ }×100
In the above relational expression 6, D means the hole diameter (mm) when a crack penetrates the steel plate along the thickness direction, and D ₀ means the initial hole diameter (mm).

As shown in Tables 1 to 11 above, in the case of specimens satisfying the conditions presented in the present invention, the value of [Si+Al] _F /[Si+Al] _γ satisfies the range of 1.1 to 3.0, and T( γ)/V(γ) is 0.1 or more, the balance between tensile strength and elongation rate (TS × El) is 22,000 MPa% or more, and the balance between tensile strength and hole expansion rate (TS ² × HER ^{1/ 2} ) is 7×10 ⁶ (MPa ² % ^1/2 ) or more, the bending rate (R/t) satisfies the range of 0.5 to 3.0, and has excellent strength and workability at the same time. I understand.

Test specimens 2 to 5 have the same alloy composition range as the present invention, but the hot rolling annealing temperature and time are outside the range of the present invention, resulting in poor pickling and breakage during cold rolling. It can be confirmed that

In sample 6, the amount of ferrite formed was insufficient because the primary heating or holding temperature in the annealing heat treatment process after cold rolling exceeded the range limited by the present invention. As a result, the value of [Si+Al] _F /[Si+Al] _γ of specimen 6 was less than 1.1, and the balance between tensile strength and hole expansion rate (TS ² × HER ^1/2 ) was 7 × 10 ⁶ (MPa It can be confirmed that it is less than ² % ^1/2 ).

In specimen 8, the primary cooling stop temperature was low, so ferrite was formed excessively, and residual austenite was formed in small amounts. As a result, it can be confirmed that specimen 8 has a value of [Si+Al] _F /[Si+Al] _γ exceeding 3.0 and a balance between tensile strength and elongation rate (TS x El) of less than 22,000 MPa%. .

In specimen 9, the average cooling rate of the secondary cooling was low, so ferrite was formed excessively, and retained austenite was formed in small amounts. As a result, it can be confirmed that specimen 9 has a value of [Si+Al] _F /[Si+Al] _γ exceeding 3.0 and a balance between tensile strength and elongation rate (TS x El) of less than 22,000 MPa%. .

Specimen 12 had a high secondary holding temperature and was formed with little retained austenite. As a result, specimen 12 had a [Si+Al] _F /[Si+Al] _γ value of more than 3.0, T(γ)/V(γ) of less than 0.5, and a good balance between tensile strength and elongation. It can be confirmed that (TS×El) is less than 22,000 MPa% and the bending rate (R/t) is more than 3.0.

Specimen 13 has a low secondary holding temperature and T(γ)/V(γ) of less than 0.1, and a bending rate (R/t) of more than 3.0. It can be confirmed that the subsequent holding time is short, T(γ)/V(γ) is less than 0.1, and the bending rate (R/t) exceeds 3.0.

Specimen 15 has a low average cooling rate in tertiary cooling, a value of [Si+Al] _F /[Si+Al] _γ exceeding 3.0, and a value of T(γ)/V(γ) of less than 0.5. It can be confirmed that the bending rate (R/t) exceeds 3.0.

In specimen 16, the secondary cooling stop temperature was high, and excessive bainite was formed, while less tempered martensite was formed. As a result, it can be confirmed that the specimen 16 has a T(γ)/V(γ) of less than 0.1 and a balance between tensile strength and elongation (TS×El) of less than 22,000 MPa%.

In sample 17, the secondary cooling stop temperature was low, and tempered martensite was formed excessively, and retained austenite was formed in small amounts. As a result, specimen 17 had a [Si+Al] _F /[Si+Al] _γ value of more than 3.0, T(γ)/V(γ) of less than 0.1, and a good balance between tensile strength and elongation. It can be confirmed that (TS×El) is less than 22,000 MPa% and the bending rate (R/t) is more than 3.0.

Sample 18 had a high tertiary holding temperature and was formed with little retained austenite. As a result, specimen 18 had a [Si+Al] _F /[Si+Al] _γ value of over 3.0, T(γ)/V(γ) of less than 0.1, and a good balance between tensile strength and elongation. It can be confirmed that (TS×El) is less than 22,000 MPa% and the bending rate (R/t) is more than 3.0.

Specimen 19 has a low tertiary holding temperature, a value of [Si+Al] _F / [Si+Al] _γ of more than 3.0, a value of T(γ)/V(γ) of less than 0.1, and a low tensile strength. It can be confirmed that the elongation rate balance (TS×El) is less than 22,000 MPa% and the bending rate (R/t) is more than 3.0.

In specimen 20, the tertiary holding time was short, and tempered martensite was formed excessively, and retained austenite was formed too little. As a result, specimen 20 had a [Si+Al] _F /[Si+Al] _γ value of more than 3.0, T(γ)/V(γ) of less than 0.1, and a good balance between tensile strength and elongation. It can be confirmed that (TS×El) is less than 22,000 MPa% and the bending rate (R/t) is more than 3.0.

Test specimens 43 to 51 satisfy the manufacturing conditions presented in the present invention, but are outside the alloy composition range. In these cases, the present invention's [Si+Al] _F /[Si+Al] _γ , T(γ)/V(γ), balance between tensile strength and elongation rate (TS x El), balance between tensile strength and hole expansion rate It can be confirmed that (TS ² ×HER ^1/2 ) does not simultaneously satisfy the conditions of 7×10 ⁶ (MPa ² % ^1/2 ) and bending rate (R/t). On the other hand, specimen 45 has a total content of aluminum (Al) and silicon (Si) of less than 1.0%, and the balance between [Si+Al] _F /[Si+Al] _γ and tensile strength and elongation rate (TS It can be confirmed that the conditions of xEl) and bending rate (R/t) are not satisfied.

As mentioned above, the present invention has been explained in detail through examples, but embodiments with different forms are also possible. Therefore, the spirit and scope of the claims described below are not limited to the embodiments.

Claims (10)

In mass %, C: 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, and N: 0.03% or less , the remainder consisting of Fe and inevitable impurities,
The microstructure contains 30-70% by volume of tempered martensite, 10-45% by volume of bainite, 10-40% by volume of retained austenite, and ferrite , and the rest consists of an unavoidable structure.
A high-strength steel plate with excellent workability that satisfies [Relational Expression 1] and [Relational Expression 2] below.
[Relational expression 1]
1.1≦[Si+Al] _F /[Si+Al] _γ ≦3.0
In the above relational expression 1, [Si+Al] _F is the average total content (mass%) of Si and Al contained in the ferrite, and [Si+Al] _γ is the average total content ( mass %) of Si and Al contained in the retained austenite. mass %).
[Relational expression 2]
T(γ)/V(γ)≧0.1
In the above relational expression 2, T(γ) is the fraction (volume %) of tempered retained austenite in the steel plate, and V(γ) is the fraction (volume %) of retained austenite in the steel plate. The high-strength steel plate with excellent workability according to claim 1, wherein the steel plate further contains one or more of the following (1) to (9) (excluding a total amount of 0%) .
(1) One or more of Ti: 0-0.5%, Nb: 0-0.5% and V: 0-0.5% (2) Cr: 0-3.0% and Mo: 0- One or more types among 3.0% (3) One or more types among Cu: 0 to 4.5% and Ni: 0 to 4.5% (4) B: 0 to 0.005%
(5) One or more of Ca: 0 to 0.05%, REM excluding Y: 0 to 0.05%, and Mg: 0 to 0.05% (6) W: 0 to 0.5% and Zr : One or more types among 0 to 0.5% (7) One or more types among Sb: 0 to 0.5% and Sn: 0 to 0.5% (8) Y: 0 to 0.2% and Hf : One or more of 0 to 0.2% (9) Co: 0 to 1.5% The high-strength steel sheet with excellent workability according to claim 1, wherein the total content of Si and Al (Si+Al) is 1.0 to 6.0% by mass . The high-strength steel plate with excellent workability according to claim 1 , wherein the microstructure of the steel plate contains 3 to 20% by volume of ferrite. The steel plate has a balance between tensile strength and elongation (B _{T E} ) expressed by the following [Relational Expression 3] of 22,000 (MPa%) or more, and is expressed by the following [Relational Expression 4]. The balance between tensile strength and hole expansion rate (B _T・H ) is 7×10 ⁶ (MPa ² % ^1/2 ) or more, and the bending rate (B _R ) expressed by the following [Relational expression 5] 2. The high-strength steel plate with excellent workability according to claim 1, wherein the steel sheet has a value of 0.5 to 3.0.
[Relational expression 3]
B _T・E = [Tensile strength (TS, MPa)] × [Elongation rate (El, %)]
[Relational expression 4]
B _T・H = [Tensile strength (TS, MPa)] ² × [Hole expansion ratio (HER, %)] ^1/2
[Relational expression 5]
B _R =R/t
In the above relational expression 5, R means the minimum bending radius (mm) at which no crack occurs after a 90° bending test, and t means the thickness (mm) of the steel plate. In mass %, C: 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, and N: 0.03% or less, with the remainder consisting of Fe and unavoidable impurities;
heating the cold rolled steel plate to a temperature range of Ac1 or more and less than Ac3 (primary heating) and holding it for 50 seconds or more (primary holding);
A stage of cooling (primary cooling) to a temperature range of 600 to 850 °C (primary cooling stop temperature) at an average cooling rate of 1 °C / s or more,
Cooling to a temperature range of 300 to 500°C at an average cooling rate of 2°C/s or more (secondary cooling) and holding in this temperature range for 5 seconds or more (secondary holding);
A step of cooling (tertiary cooling) to a temperature range of 100 to 300 °C (secondary cooling stop temperature) at an average cooling rate of 2 °C / s or more,
heating to a temperature range of 300 to 500°C (secondary heating) and holding in this temperature range for 50 seconds or more (tertiary holding);
The method for producing a high-strength steel plate with excellent workability according to any one of claims 1 to 5, comprising a step of cooling to room temperature (quaternary cooling). The cold-rolled steel plate further contains one or more of the following (1) to (9) (excluding 0% in total amount): Method of manufacturing strength steel plate.
(1) One or more of Ti: 0-0.5%, Nb: 0-0.5% and V: 0-0.5% (2) Cr: 0-3.0% and Mo: 0- One or more types among 3.0% (3) One or more types among Cu: 0 to 4.5% and Ni: 0 to 4.5% (4) B: 0 to 0.005%
(5) One or more of Ca: 0 to 0.05%, REM excluding Y: 0 to 0.05%, and Mg: 0 to 0.05% (6) W: 0 to 0.5% and Zr : One or more types among 0 to 0.5% (7) One or more types among Sb: 0 to 0.5% and Sn: 0 to 0.5% (8) Y: 0 to 0.2% and Hf : One or more of 0 to 0.2% (9) Co: 0 to 1.5% Production of a high-strength steel plate with excellent workability according to claim 6, wherein the total content of Si and Al (Si + Al) contained in the cold-rolled steel plate is 1.0 to 6.0% by mass . Method. The cold rolled steel plate is
heating the steel slab to 1000-1350°C;
finishing hot rolling in a temperature range of 800 to 1000°C;
Winding the hot rolled steel sheet at a temperature range of 300 to 600°C;
Hot rolling annealing the rolled steel plate at a temperature range of 650 to 850°C for 600 to 1700 seconds;
7. The method for producing a high-strength steel plate with excellent workability according to claim 6, which is provided by the step of cold rolling the hot-rolled annealed and heat-treated steel plate at a reduction ratio of 30 to 90%. 7. The method for manufacturing a high-strength steel plate with excellent workability according to claim 6, wherein the cooling rate (Vc1) of the primary cooling and the cooling rate (Vc2) of the secondary cooling satisfy the relationship of Vc1<Vc2.