JP2024500743A - High-strength steel plate with excellent workability and its manufacturing method - Google Patents

Abstract

The present invention relates to a steel plate that can be used for automobile parts, etc., and relates to a steel plate that has an excellent balance between strength and ductility, a balance between strength and hole expandability, and a yield ratio evaluation index, and a method for producing the same.

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 properties and excellent workability, and a method for manufacturing the same.

In recent years, the automobile industry has been focusing on ways to reduce the weight of materials in order to protect the global environment and to ensure the safety of passengers. In order to meet such demands for safety and weight reduction, the application of high-strength steel plates is rapidly increasing. It is generally known that as the strength of a steel plate increases, the workability of the steel plate decreases. Therefore, the current situation is that steel sheets for automobile parts are required to have high strength characteristics while also having excellent workability, typified by ductility and hole expandability.

TRIP (Transformation Induced Plasticity) steel, which utilizes the transformation-induced plasticity of retained austenite, has a complex microstructure consisting of ferrite, bainite, martensite, retained austenite, etc., so while it has high strength characteristics, it cannot exceed a certain level. It is known that the processability is as follows.

As a technique for further 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 lower strength than existing untempered martensite (fresh martensite). There are differences. Therefore, by suppressing fresh martensite and forming tempered martensite, workability can be improved.

However, in the techniques disclosed in Patent Documents 1 and 2, the balance between tensile strength and elongation rate (TS ² ×EL ^1/2 ) is 3.0 × 10 ⁶ to 6.2 × 10 ⁶ (MPa ² % ^{1/2). 2} ), which means that it is difficult to secure a steel plate that is excellent in both strength and ductility.

On the other hand, as another technique for improving the workability of steel sheets, Patent Document 3 discloses a method of inducing the formation of bainite by adding boron (B). By adding boron (B), ferrite-pearlite transformation is suppressed and bainite generation is induced, so that both strength and workability can be achieved.

However, in the technology disclosed in Patent Document 3, the balance between tensile strength and elongation (B _TE ) of 3.0×10 ⁶ to 6.2×10 ⁶ (MPa ² % ^1/2 ), 6.0× A balance between tensile strength and hole expansion rate (B _TH ) of 10 ⁶ to 11.5×10 ⁶ (MPa ² % ^1/2 ) and a yield ratio evaluation index (I _YR ) of 0.15 to 0.42 are simultaneously ensured. Therefore, it has been difficult to secure a steel plate that has excellent strength, hole expandability, ductility, and yield ratio.

In other words, the actual situation is that the steel sheet does not meet the requirements for a steel plate that has excellent balance between tensile strength and elongation rate (B _TE ), balance between tensile strength and hole expansion rate (B _TH ), and yield ratio evaluation index (I _YR ). .

Korean Patent Publication No. 10-2006-0118602 Japanese Patent Publication No. 2009-019258 Japanese Patent Publication No. 2016-216808

According to one aspect of the present invention, the composition and microstructure of the steel plate are optimized to provide a steel plate with excellent balance between tensile strength and elongation rate, balance between tensile strength and hole expansion rate, and yield ratio evaluation index, and the same. A manufacturing method can be provided.

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

A high-strength steel plate with excellent workability according to one aspect of the present invention has a weight percentage of C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4. 0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, remainder contains Fe and unavoidable impurities, and contains bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures as fine structures, and can satisfy the following [Relational Expression 1].

[Relational expression 1]
0.03≦[B] _FM /[B] _TM ≦0.55

In the above relational expression 1, [B] _FM is the content (wt%) of boron (B) contained in fresh martensite, and [B] _TM is the content of boron (B) contained in tempered martensite. (% by weight).

The steel plate may further contain one or more of the following (1) to (8) in weight percent.

(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- 1 or more of 3.0% (3) Cu: 0 to 4.0% and Ni: 1 or more of 0 to 4.0% (4) Ca: 0 to 0.05%, REM excluding Y : 0 to 0.05% and Mg: one or more of 0 to 0.05% (5) W: 0 to 0.5% and Zr: one or more of 0 to 0.5% (6) Sb : 0 to 0.5% and Sn: 0 to 0.5% (7) Y: 0 to 0.2% and Hf: 0 to 0.2% (8) Co :0~1.5%

The microstructure of the above steel plate has a volume fraction of 10 to 30% bainite, 50 to 70% tempered martensite, 10 to 30% fresh martensite, 2 to 10% retained austenite, and 5% or less ( ferrite (including 0%).

The above steel plate has a balance between tensile strength and elongation (B _TE ) expressed by the following [Relational Expression 2] that satisfies 3.0×10 ⁶ to 6.2×10 ⁶ (MPa ² % ^1/2 ). , the balance between tensile strength and hole expansion rate (B _TH ) expressed by [Relational Expression 3] below satisfies 6.0×10 ⁶ to 11.5×10 ⁶ (MPa ² % ^1/2 ), and the following The yield ratio evaluation index (I _YR ) expressed by [Relational Expression 4] can satisfy 0.15 to 0.42.

[Relational expression 2]
B _TE = [Tensile strength (TS, MPa)] ² × [Elongation rate (El, %)] ^1/2

[Relational expression 3]
B _TH = [Tensile strength (TS, MPa)] ² × [Hole expansion rate (HER, %)] ^1/2

[Relational expression 4]
I _YR = 1 - [yield ratio (YR)]

A method for producing a high-strength steel sheet with excellent workability according to one aspect of the present invention includes, in weight percent, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 ~4.0%, Al: 0.01~1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005~0.005 %, remaining Fe and unavoidable impurities, and heating the cold rolled steel plate to 700°C at an average heating rate of 5°C/s or more (primary heating). , heating to a temperature range of Ac 3 to 920°C at an average heating rate of 5°C/s or less (secondary heating), and then holding for 50 to 1200 seconds (primary holding); A stage of cooling (primary cooling) to a temperature range of 200 to 400°C at an average cooling rate of 1°C/s or more, and a step of cooling the above-mentioned primary cooled steel plate to 350 to 550°C at an average heating rate of 5°C/s or more. After heating (tertiary heating) to a temperature range of cooling).

The above steel slab may further include any one or more of the following (1) to (8).

(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- 1 or more of 3.0% (3) Cu: 0 to 4.0% and Ni: 1 or more of 0 to 4.0% (4) Ca: 0 to 0.05%, REM excluding Y : 0 to 0.05% and Mg: one or more of 0 to 0.05% (5) W: 0 to 0.5% and Zr: one or more of 0 to 0.5% (6) Sb : 0 to 0.5% and Sn: 0 to 0.5% (7) Y: 0 to 0.2% and Hf: 0 to 0.2% (8) Co :0~1.5%

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 350 to 650°C. The method may be provided through the steps of: winding up the rolled steel sheet, pickling the rolled steel sheet, and cold rolling the pickled steel sheet at a rolling reduction of 30 to 90%.

According to a preferred aspect of the present invention, there is provided a steel plate that has an excellent balance between tensile strength and ductility, a balance between tensile strength and hole expandability, and a yield ratio evaluation index, and can be suitably used for automobile parts, etc., and a method for manufacturing the same. be able to.

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 varied 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 more fully explain the invention to those skilled in the art to which the invention pertains.

The inventors of the present invention have developed a boron (B)-added Transformation Induced Plasticity (TRIP) steel containing bainite, tempered martensite, fresh martensite, and retained austenite. The structure fraction of site and retained austenite is controlled within a certain range, the content of boron (B) contained in tempered martensite and fresh martensite is controlled within a certain range, and the shape and size of retained austenite are controlled within a certain range. It has been recognized that when controlled, it is possible to simultaneously ensure an excellent balance between tensile strength and ductility, an excellent balance between tensile strength and hole expandability, and an excellent yield ratio evaluation index. We investigated this problem and devised a method that can effectively achieve both excellent strength, yield ratio, ductility, and hole expandability, resulting in 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.

A high-strength steel plate with excellent workability according to one aspect of the present invention has a weight percentage of C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4. 0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, remainder contains Fe and unavoidable impurities, and contains bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures as fine structures, and can satisfy the following [Relational Expression 1].

[Relational expression 1]
0.03≦[B] _FM /[B] _TM ≦0.55

In the above relational expression 1, [B] _FM is the content (wt%) of boron (B) contained in fresh martensite, and [B] _TM is the content of boron (B) contained in tempered martensite. (% by weight).

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

A high-strength steel plate with excellent workability according to one aspect of the present invention has a weight percentage of C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4. 0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, remainder Contains 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: 4.0% or less (including 0%), Ni: 4.0% 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 (including 0%), Sn: 0.5% or less (0% ), Y: 0.2% or less (including 0%), Hf: 0.2% or less (including 0%), Co: 1.5% or less (including 0%). may further include.

Carbon (C): 0.1-0.25%
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, in the present invention, 0.1% or more of carbon (C) can be included in order to achieve such effects. A preferred carbon (C) content may be greater than 0.1%, may be greater than or equal to 0.11%, and may be greater than or equal to 0.12%. On the other hand, if the carbon (C) content exceeds a certain level, ductility may decrease due to an excessive increase in strength, and weldability may deteriorate. Therefore, in the present invention, the upper limit of the carbon (C) content can be limited to 0.25%. The carbon (C) content may be 0.24% or less, and the more preferable carbon (C) content may be 0.23% or less.

Silicon (Si): 0.01 to 1.5% or less Silicon (Si) is an element that contributes to improving strength through solid solution strengthening, and is also an element that improves workability by 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, 0.01% or more of silicon (Si) can be added to achieve such an effect. A preferable silicon (Si) content may be 0.02% or more, and a more preferable silicon (Si) content may be 0.04% or more. However, if the silicon (Si) content exceeds a certain level, it not only causes problems of plating defects such as unplated parts in the plating process, but also reduces the weldability of the steel sheet. The upper limit of the (Si) content can be limited to 1.5%. A preferable upper limit of silicon (Si) content may be 1.48%, and a more preferable upper limit of silicon (Si) content may be 1.46%.

Manganese (Mn): 1.0-4.0%
Manganese (Mn) is a useful element for increasing both strength and ductility. Therefore, in the present invention, 1.0% or more of manganese (Mn) can be added to achieve such effects. A preferable lower limit of the manganese (Mn) content may be 1.2%, and a more preferable lower limit of the manganese (Mn) content may be 1.4%. On the other hand, when manganese (Mn) is added in excess, the bainite transformation time increases and the carbon (C) concentration in austenite becomes insufficient, resulting in the problem that the desired austenite fraction cannot be secured. exists. Therefore, in the present invention, the upper limit of the manganese (Mn) content can be limited to 4.0%. A preferable upper limit of the manganese (Mn) content may be 3.9%.

Aluminum (Al): 0.01-1.5%
Aluminum (Al) is an element that combines with oxygen in steel to perform a deoxidizing action. Further, like silicon (Si), aluminum (Al) is also an element that suppresses cementite precipitation and stabilizes retained austenite. Therefore, in the present invention, 0.01% or more of aluminum (Al) can be added to achieve such effects. A preferable aluminum (Al) content may be 0.03% or more, and a more preferable aluminum (Al) content may be 0.05% or more. On the other hand, when aluminum (Al) is added in excess, it not only increases the number of inclusions in the steel sheet but also reduces the workability of the steel sheet. Therefore, in the present invention, the upper limit of the aluminum (Al) content is set to 1 It can be limited to .5%. A preferred upper limit of aluminum (Al) content may be 1.48%.

Phosphorus (P): 0.15% or less (including 0%)
Phosphorus (P) is an element that is included 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 contained as an impurity that 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 included 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.

Boron (B): 0.0005-0.005%
Boron (B) is an element that improves hardenability and increases strength, and is also an element that suppresses nucleation at grain boundaries. In addition, the present invention achieves an excellent balance between tensile strength and elongation, an excellent balance between tensile strength and hole expandability, and an excellent yield ratio evaluation index by enriching boron (B) in tempered martensite. In order to simultaneously ensure the following, boron (B) should be essential added in the present invention. Therefore, in the present invention, 0.0005% or more of boron (B) can be added for such an effect. However, if boron (B) is added in excess of a certain level, it will not only cause excessive property effects but also increase production costs, so in the present invention, the upper limit of the boron (B) content is set to 0. It can be limited to 0.005%.

Meanwhile, the steel sheet of the present invention has an alloy composition that may be additionally included in addition to 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 titanium (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 increases manufacturing costs. Therefore, in the present invention, the contents of titanium (Ti), niobium (Nb), and vanadium (V) can be limited 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, so in the present invention, it is recommended 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 content cannot be achieved. unable to secure a certain rate. Therefore, in the present invention, the contents of chromium (Cr) and molybdenum (Mo) can be limited to 3.0% or less, respectively.

One or more of copper (Cu): 0 to 4.0% and nickel (Ni): 0 to 4.0% Copper (Cu) and nickel (Ni) are elements that stabilize austenite and suppress corrosion. be. Copper (Cu) and nickel (Ni) are also elements that, by concentrating on the surface of the steel sheet, 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 an effect. 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. ) can be limited to 4.0% or less.

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 effect, 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, it will not only cause excessive characteristic 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. ) 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 or cold working. The contents of antimony (Sb) and tin (Sn) 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 contents of yttrium (Y) and hafnium (Hf) exceed a certain level, the ductility of the steel sheet may deteriorate, so in the present invention, the contents of yttrium (Y) and hafnium (Hf) are set to 0. It can be limited to 2% or less.

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

A high-strength steel sheet with excellent workability according to one aspect of the present invention may contain remaining 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 well 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 completely excluded.

A high-strength steel sheet with excellent workability according to an aspect of the present invention may include bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures as a microstructure. .

Both untempered martensite (fresh martensite, FM) and tempered martensite (tempered martensite, TM) are microstructures that improve the strength of steel sheets. However, compared to tempered martensite, fresh martensite has the characteristic of lowering the ductility and burring property of steel sheets. Furthermore, fresh martensite tends to lower the yield ratio of the steel sheet compared to tempered martensite. This is because the microstructure of tempered martensite is softened by the tempering heat treatment. Therefore, the balance between tensile strength and elongation rate (TS ² × EL ^1/2 ), the balance between tensile strength and hole expansion rate (TS ² × HER ^1/2 ), and the yield ratio evaluation index (1 -YR), it is preferable to control the tissue fraction of tempered martensite and fresh martensite. Balance between tensile strength and elongation rate of 3.0×10 ⁶ or more (TS ² × EL ^1/2 ), balance of tensile strength and hole expansion rate of 6.0×10 ⁶ or more (TS ² × HER ^1/2 ) , and in order to satisfy the yield ratio evaluation index (1-YR) of 0.42 or less, the fraction of tempered martensite is limited to 50% by volume or more, and the fraction of fresh martensite is limited to 10% by volume or more. It is preferable to do so. A more preferred fraction of tempered martensite may be 52 volume % or more or 54 volume % or more, and a more preferred fraction of fresh martensite may be 12 volume % or more. On the other hand, when tempered martensite or fresh martensite is excessively formed, ductility and burring properties decrease, resulting in a balance between tensile strength and elongation of 3.0 × 10 ⁶ or more (TS ² × EL ^1/2 ), the balance between tensile strength and hole expansion rate (TS ² × HER ^1/2 ) of 6.0 × 10 ⁶ or more, and the yield ratio evaluation index (1-YR) of 0.42 or less cannot be satisfied at the same time. . Therefore, in the present invention, the fraction of tempered martensite can be limited to 70% by volume or less, and the fraction of fresh martensite can be limited to 30% by volume or less. A more preferred fraction of tempered martensite may be 68 volume % or less or 65 volume % or less, and a more preferred fraction of fresh martensite may be 25 volume % or less.

Balance between tensile strength and elongation rate (TS ² × EL ^1/2 ), balance between tensile strength and hole expansion rate (TS ² × HER ^1/2 ), and yield ratio evaluation index (1 -YR), it is necessary to optimize the bainite fraction. Balance between tensile strength and elongation rate of 3.0×10 ⁶ or more (TS ² × EL ^1/2 ), balance of tensile strength and hole expansion rate of 6.0×10 ⁶ or more (TS ² × HER ^1/2 ) , and a yield ratio evaluation index (1-YR) of 0.42 or less, it is preferable to control the bainite fraction to 10% by volume or more. A more preferable bainite fraction may be 12 volume % or more or 14 volume % or more. On the other hand, when bainite is formed in excess, it induces a decrease in the fraction of tempered martensite, so the desired balance between tensile strength and elongation rate (TS ² × EL ^1/2 ), tensile strength and hole expansion rate is achieved. In order to ensure the balance of (TS ² ×HER ^1/2 ) and the yield ratio evaluation index (1-YR), the fraction of bainite can be limited to 30% by volume or less. A preferable bainite fraction may be 12 volume% or more or 14 volume% or more, and may be 28 volume% or less or 26 volume% or less.

A steel plate containing retained austenite has excellent ductility and workability due to transformation-induced plasticity that occurs during transformation from austenite to martensite during processing. When the fraction of retained austenite is less than a certain level, the balance between tensile strength and elongation (TS ² × EL ^1/2 ) is less than 3.0 × 10 ⁶ (MPa ² % ^1/2 ), which is preferable. do not have. On the other hand, if the fraction of retained austenite exceeds a certain level, local elongation may decrease or spot weldability may decrease. Therefore, in the present invention, the fraction of retained austenite can be limited to a range of 2 to 10% in order to obtain a steel plate with an excellent balance between tensile strength and elongation (TS ² ×EL ^1/2 ). A preferable retained austenite fraction may be 3% by volume or more and 8% by volume or less.

The steel sheet of the present invention can include ferrite, pearlite, island-like martensite (Martensite Austenite Constituent, MA), etc. as inevitable structures. If excessive ferrite is formed, the strength of the steel sheet may decrease, so in the present invention, the ferrite fraction can be limited to 5% by volume (including 0%) or less. Furthermore, if excessive pearlite is formed, the workability of the steel sheet may be reduced or the fraction of retained austenite may be reduced, so the present invention attempts to limit the formation of pearlite as much as possible.

A high-strength steel plate with excellent workability according to one aspect of the present invention can satisfy [Relational Expression 1] below.

[Relational expression 1]
0.03≦[B] _FM /[B] _TM ≦0.55

In the above relational expression 1, [B] _FM is the content (wt%) of boron (B) contained in fresh martensite, and [B] _TM is the content of boron (B) contained in tempered martensite. (% by weight).

The present invention aims to achieve the desired balance between tensile strength and elongation rate (TS ² × EL ^1/2 ), balance between tensile strength and hole expansion rate (TS ² × HER ^1/2 ), and yield ratio evaluation index (1- In order to ensure YR), it is necessary not only to control the structure fractions of tempered martensite, fresh martensite, and retained austenite within a certain range, but also to control the boron (B) content contained in tempered martensite and fresh martensite. The ratio is controlled within a certain range, and the ratio of retained austenite of a specific size, shape, and type to total retained austenite is controlled within a certain range.

The present invention is based on the relationship between the content of boron (B) contained in fresh martensite ([B] _TM , weight %) with respect to the content of boron (B) contained in tempered martensite ([B] TM , weight %) as shown in [Relational Expression 1]. [B] Tensile strength of 3.0×10 ⁶ to 6.2×10 ⁶ (MPa ² % ^1/2 ) in order to control the ratio of _FM ), weight %) in the range of 0.03 to 0.55. balance between tensile strength and elongation rate (B _TE ), balance between tensile strength and hole expansion rate (B _TH ) of 6.0×10 ⁶ to 11.5×10 ⁶ (MPa ² % ^1/2 ), and 0.15 to A yield ratio evaluation index (I _YR ) of 0.42 can be ensured at the same time.

The inventor of the present invention has conducted extensive research into ways to ensure the physical properties of boron (B)-added TRIP steel, and although the theoretical basis has not been clearly investigated, the boron (B) contained in tempered martensite ( It has been noticed that the physical properties targeted by the present invention can be ensured only when the ratio of the boron (B) content contained in fresh martensite to B) content satisfies a certain range. In particular, it was confirmed that the yield ratio of the steel sheet showed a certain tendency depending on the ratio of boron (B) content contained in tempered martensite and fresh martensite. Therefore, the present invention sets the ratio of the boron (B) content contained in fresh martensite to the boron (B) content contained in tempered martensite to be 0.03 to 0.55 as shown in [Relational Expression 1]. In order to limit the range, the desired balance between tensile strength and elongation rate (TS ² × EL ^1/2 ), balance between tensile strength and hole expansion rate (TS ² × HER ^1/2 ), and yield ratio evaluation index ( 1-YR) can be secured.

The high-strength steel plate with excellent workability according to one aspect of the present invention has a balance between tensile strength and elongation (B _TE ) of 3.0×10 ⁶ to 6.2× expressed by the following [Relational Expression 2]. 10 ⁶ (MPa ² % ^1/2 ), and the balance between tensile strength and hole expansion rate (B _TH ) expressed by the following [Relational Expression 3] is 6.0×10 ⁶ to 11.5×10 ⁶ (MPa ² % ^1/2 ), and the yield ratio evaluation index (I _YR ) expressed by the following [Relational Expression 4] can satisfy 0.15 to 0.42.

[Relational expression 2]
B _TE = [Tensile strength (TS, MPa)] ² × [Elongation rate (El, %)] ^1/2

[Relational expression 3]
B _TH = [Tensile strength (TS, MPa)] ² × [Hole expansion rate (HER, %)] ^1/2

[Relational expression 4]
I _YR = 1 - [yield ratio (YR)]

Hereinafter, an example of the method for manufacturing the steel plate of the present invention will be described in detail.

A method for manufacturing a high-strength steel plate according to one aspect of the present invention includes heating a cold-rolled steel plate having a predetermined alloy composition to 700°C at an average heating rate of 5°C/s or more (primary heating), After heating (secondary heating) to a temperature range of Ac 3 to 920°C at an average heating rate of /s or less, holding the steel plate for 50 to 1200 seconds (primary holding), and heating the above-mentioned primarily held steel plate at 1°C/s. A stage of cooling (primary cooling) to a temperature range of 200 to 400 °C at an average cooling rate of 5 °C/s or more, and a step of cooling the above-mentioned primary cooled steel plate to a temperature range of 350 to 550 °C at an average heating rate of 5 °C/s or more. After heating to (tertiary heating), the steel plate is held for 50 seconds or more (secondary holding), and the secondly held steel plate is cooled to room temperature at an average cooling rate of 1°C/s or more (secondary cooling). It can include steps.

The above-mentioned cold rolled steel plate is produced by heating a steel slab having a predetermined alloy composition to 1000 to 1350°C, finishing hot rolling in a temperature range of 800 to 1000°C, and finishing at a temperature of 350 to 650°C. a step of winding the hot-rolled steel sheet in a range, a step of pickling the rolled-up steel sheet, and a step of cold-rolling the pickled steel sheet at a rolling reduction ratio of 30 to 90%; can be provided via.

Preparation and heating of steel slab A steel slab having a predetermined alloy composition 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 will be 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. At this time, the heating temperature of the steel slab may be in the range of 1000 to 1350°C. If the heating temperature of the steel slab is less than 1000°C, there is a risk that the steel slab will be hot rolled in a temperature range below the intended finish hot rolling temperature range, and if the heating temperature of the steel slab exceeds 1350°C, the steel There is a risk that it will reach the melting point 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. If the finish hot rolling temperature is less than 800°C, excessive rolling load may become a problem, and if the finish hot rolling temperature exceeds 1000°C, the crystal grains of the hot rolled steel sheet may be coarsely formed. , which may cause deterioration of the physical properties of the final steel plate.

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 in the range of 350 to 650°C. This is because if the coiling temperature is less than 350℃, it is not easy to coil, and if the coiling temperature exceeds 650℃, surface scale will be formed to the inside of the hot rolled steel sheet, making pickling difficult. This is because there is a risk of making it difficult.

Pickling and Cold Rolling After uncoiling the hot-rolled coil, pickling is 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%. When the cumulative reduction rate of cold rolling exceeds 90%, there is a possibility that it becomes 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 an unplated cold-rolled steel sheet after an annealing heat treatment process, or may be a plated steel sheet 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 to 700°C at an average heating rate of 5°C/s or more (primary heating), and heated to a temperature range of Ac3 to 920°C at an average heating rate of 5°C/s or less (secondary heating). After heating), hold for 50 to 1200 seconds (primary hold).

If the average heating rate of primary heating up to 700°C is less than 5°C/s, massive austenite is formed from the ferrite and cementite generated during heating, and eventually fine tempered martensite forms as the final structure. and retained austenite cannot be formed. As a result, the desired balance between tensile strength and elongation rate (TS ² ×EL ^1/2 ) and balance between tensile strength and hole expansion rate (TS ² ×HER ^1/2 ) cannot be achieved. In addition, if the secondary heating rate to the primary holding temperature exceeds 5°C/s, the transformation from cementite generated during heating to austenite will be accelerated, a large amount of lumpy austenite will be formed, and the final structure will be coarse. , and boron (B) may not be sufficiently concentrated in the tempered martensite. As a result, [B] _FM / [B] _TM exceeds 0.55, and the desired level of balance between tensile strength and elongation rate (TS ² × EL ^1/2 ), tensile strength and hole expansion rate is achieved. balance (TS ² ×HER ^1/2 ) and yield ratio evaluation index (I _YR ) become impossible to achieve.

When the primary holding temperature is less than Ac3 (two-phase region), 5% by volume or more of ferrite is formed, which improves the balance between tensile strength and elongation (TS ² × EL ^1/2 ), and the balance between tensile strength and hole. The balance of spreading ratios (TS ² ×HER ^1/2 ) may deteriorate. Furthermore, if the primary holding time is less than 50 seconds, the structure may not be made sufficiently uniform, and the physical properties of the steel sheet may deteriorate. The upper limits of the primary holding temperature and primary holding time are not particularly limited, but in order to prevent a decrease in toughness due to coarsening of crystal grains, the primary holding temperature is 920°C or less and the primary holding time is 1200 seconds or less. Preferably limited.

After the primary holding, it can be cooled (primary cooling) to a primary cooling stop temperature of 200 to 400°C at an average cooling rate of 1°C/s or more. When the average cooling rate of primary cooling is less than 1°C/s, the fraction of retained austenite becomes insufficient due to slow cooling, and the balance between tensile strength and elongation rate (TS ² × EL ^1/2 ) of the steel plate is thereby affected. It may decrease. Although the upper limit of the average cooling rate of primary cooling does not need to be particularly specified, it is preferably 100° C./s or less. When the primary cooling stop temperature is less than 200°C, tempered martensite is excessively formed and residual austenite is insufficient, resulting in a loss of balance between tensile strength and elongation rate (TS ² × EL ^1/2 ) and tensile strength of the steel plate. The balance between strength and hole expansion rate (TS ² ×HER ^1/2 ) may deteriorate. On the other hand, when the primary cooling stop temperature exceeds 400°C, bainite is formed excessively and tempered martensite is insufficient, resulting in the balance between tensile strength and elongation rate (TS ² × EL ^1/2 ) and tensile strength of the steel plate. The balance between strength and hole expansion rate (TS ² ×HER ^1/2 ) may deteriorate.

After secondary cooling, it can be heated to a temperature range of 350 to 550°C at an average heating rate of 5°C/s or more (tertiary heating) and then held for 50 seconds or more (secondary holding). Although the upper limit of the average heating rate of tertiary heating does not need to be particularly specified, it is preferably 100° C./s or less. If the secondary holding temperature is less than 350° C. or the secondary holding time is less than 50 seconds, tempered martensite is formed excessively, making it difficult to secure a retained austenite fraction. As a result, the balance between tensile strength and elongation rate (TS ² ×EL ^1/2 ) and the balance between tensile strength and hole expansion rate (TS ² ×HER ^1/2 ) may deteriorate. If the secondary holding temperature exceeds 550°C or the secondary holding time exceeds 155,000 seconds, the fraction of retained austenite is insufficient and the balance between tensile strength and elongation of the steel plate (TS ² × EL ^{1 /2} ) may decrease.

After the secondary holding, it can be cooled to room temperature (secondary cooling) at an average cooling rate of 1° C./s or more.

The high-strength steel sheet with excellent workability produced by the above-mentioned production method can contain bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures as a microstructure, and as a preferable example, By volume fraction, 10-30% bainite, 50-70% tempered martensite, 10-30% fresh martensite, 2-10% retained austenite, 5% or less (including 0%) ferrite. can include.

The steel plate manufactured by the above-mentioned manufacturing method has a balance between tensile strength and elongation (B _TE ) of 3.0×10 ⁶ to 6.2×10 ⁶ (MPa ² % ^1/2 ), and the balance between tensile strength and hole expansion rate (B _TH ) expressed by the following [Relational Expression 3] is 6.0×10 ⁶ to 11.5×10 ⁶ (MPa ² % ^{1 /2} ), and the yield ratio evaluation index (I _YR ) expressed by the following [Relational Expression 4] can satisfy 0.15 to 0.42.

[Relational expression 2]
B _TE = [Tensile strength (TS, MPa)] ² × [Elongation rate (El, %)] ^1/2

[Relational expression 3]
B _TH = [Tensile strength (TS, MPa)] ² × [Hole expansion rate (HER, %)] ^1/2

[Relational expression 4]
I _YR = 1 - [yield ratio (YR)]

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 more detail with reference to specific examples. It should be noted that the following examples are provided for understanding 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 wound at the winding temperatures shown in Tables 2 and 3 to produce a hot rolled steel sheet with a thickness of 3 mm. 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 listed in Tables 2 to 5 below to produce steel plates. In Tables 2 and 3 below, the single-phase region means a temperature range of 3 to 920° C. Ac, and the two-phase region means a temperature range of less than 3° C. Ac.

The microstructure of the steel plate thus produced was observed, and the results are shown in Tables 6 and 7. In the microstructure, ferrite (F), bainite (B), tempered martensite (TM), fresh martensite (FM), and pearlite (P) were observed by SEM after nital etching the cross section of the polished specimen. observed through. After nital etching, a structure with no irregularities on the surface of the test piece was classified as ferrite, and a structure with a lamellar structure of cementite and ferrite was classified as pearlite. Both bainite (B) and tempered martensite (TM) are observed in the form of laths and blocks, making it difficult to distinguish between them. The fraction was calculated. That is, the bainite fraction was determined to be the value obtained by subtracting the tempered martensite fraction calculated using the expansion curve from the bainite and tempered martensite fraction measured by SEM observation. On the other hand, since it is not easy to distinguish between fresh martensite (FM) and retained austenite (retained γ), the retained austenite fraction calculated by X-ray diffraction method from the fraction of martensite and retained austenite observed by the SEM mentioned above. The value obtained by subtracting the percentage was determined to be the fresh martensite fraction.

On the other hand, the [B] _FM / [B] _TM of the steel plate, the balance between tensile strength and elongation rate (TS ² × EL ^1/2 ), the balance between tensile strength and hole expansion rate (TS ² × HER ^1/2 ), and The yield ratio evaluation index (I _YR ) was measured and evaluated, and the results are shown in Tables 8 and 9.

The boron (B) content in fresh martensite ([B] _FM ) and the boron (B) content in tempered martensite ([B] _TM ) were measured using EPMA (Electron Probe MicroAnalyser). and the concentration of boron (B) measured in tempered martensite.

Tensile strength (TS) and elongation rate (El) were evaluated by a tensile test using test pieces taken in accordance with JIS No. TS) and elongation rate (El) were measured. The hole expansion rate (HER) was evaluated by a hole expansion test. After forming a punch hole of 10 mmΨ (die inner diameter 10.3 mm, clearance 12.5%), a conical punch with a 60° apex angle was used to burr the punch hole (HER). burr) was inserted into the punch hole in the direction toward the outside, 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 5].

[Relational expression 5]
Hole expansion rate (HER, %) = {(DD ₀ )/D ₀ }×100

In the above relational expression 5, D means the diameter (mm) of the hole when the crack penetrates the steel plate along the thickness direction, and D ₀ means the initial diameter (mm) of the hole.

As shown in Tables 1 to 9 above, in the case of test pieces that satisfy the conditions presented in the present invention, [Relational Expression 1] is satisfied and the balance between tensile strength and elongation (B _TE ) is 3.0 × 10 ⁶ ~ 6.2×10 ⁶ (MPa ² % ^1/2 ), and the balance between tensile strength and hole expansion rate (B _TH ) is 6.0×10 ⁶ to 11.5×10 ⁶ (MPa ² % ^1/2). ), and the yield ratio evaluation index (I _YR ) satisfies 0.15 to 0.42.

Test piece 2 was tested at a primary average heating rate of less than 5° C./s and lacked tempered martensite and retained austenite. As a result, test piece 2 had a balance between tensile strength and elongation rate ( _BTE ) of less than 3.0 x ¹⁰⁶ , and a balance between tensile strength and hole expansion rate ( _BTH ) of less than 6.0 x ¹⁰⁶ . Ta.

Test piece 3 was tested at a secondary average heating rate of over 5° C./s, resulting in the formation of massive austenite and no concentration of boron (B) in the tempered martensite. As a result, test piece 3 had a [B] _FM / [B] _TM of over 0.55, a yield ratio evaluation index (I _YR ) of over 0.42, and a balance between tensile strength and elongation (B _TE ) of 3. ^The balance between tensile strength and hole expansion rate (B _TH ) was less than 6.0×10 ⁶ .

Test piece 4 was tested in a two-phase region where the primary holding temperature was less than Ac3, and the ferrite fraction was excessive. As a result, test piece 4 had a balance between tensile strength and elongation rate ( _BTE ) of less than 3.0 x ¹⁰⁶ , and a balance between tensile strength and hole expansion rate ( _BTH ) of less than 6.0 x ¹⁰⁶ . Ta.

Test piece 5 was tested at a primary average cooling rate of less than 1° C./s, and the retained austenite fraction was insufficient. As a result, the balance between tensile strength and elongation (B _TE ) of test piece 5 was less than 3.0×10 ⁶ .

Test piece 6 was tested at a primary cooling stop temperature of less than 200° C., had an excessive tempered martensite fraction, and an insufficient retained austenite fraction. As a result, test piece 6 had a balance between tensile strength and elongation rate ( _BTE ) of less than 3.0 x ¹⁰⁶ , and a balance between tensile strength and hole expansion rate ( _BTH ) of less than 6.0 x ¹⁰⁶ . Ta.

Test piece 7 was tested at a primary cooling stop temperature of over 400°C, and had an excessive bainite fraction and an insufficient tempered martensite fraction. As a result, test piece 7 had a balance between tensile strength and elongation rate (B _TE ) of less than 3.0 x 10 ⁶ and a balance between tensile strength and hole expansion rate (B _TH ) of less than 6.0 x 10 ⁶ . Ta.

Test piece 8 was tested at a secondary holding temperature of less than 350°C, had an excessive tempered martensite fraction, and an insufficient retained austenite fraction. As a result, test piece 8 had a balance between tensile strength and elongation rate (B _TE ) of less than 3.0 x 10 ⁶ and a balance between tensile strength and hole expansion rate (B _TH ) of less than 6.0 x 10 ⁶ . Ta.

Test piece 9 was tested at a secondary holding temperature of over 550° C., and the retained austenite fraction was insufficient. As a result, test piece 9 had a balance between tensile strength and elongation (B _TE ) of less than 3.0×10 ⁶ .

Test piece 10 was tested with a secondary holding time of less than 50 seconds, and had an excessive tempered martensite fraction and an insufficient retained austenite fraction. As a result, the test piece 10 had a balance between tensile strength and elongation rate (B _TE ) of less than 3.0 x 10 ⁶ and a balance between tensile strength and hole expansion rate (B _TH ) of less than 6.0 x 10 ⁶ . Ta.

Test piece 11 was tested with a secondary holding time exceeding 155,000 seconds, and the retained austenite fraction was insufficient. As a result, the balance between tensile strength and elongation (B _TE ) of test piece 11 was less than 3.0×10 ⁶ .

Test piece 33 has a low carbon (C) content, a balance between tensile strength and elongation rate (B _TE ) of less than 3.0×10 ⁶ , and a balance between tensile strength and hole expansion rate (B _TH ) of 6.0. It was less than ×10 ⁶ .

Test piece 34 had a high carbon (C) content, an insufficient tempered martensite fraction, an excessive fresh martensite fraction, and an excessive retained austenite fraction. As a result, the test piece 34 had a balance between tensile strength and elongation rate (B _TE ) of less than 3.0 x 10 ⁶ and a balance between tensile strength and hole expansion rate (B _TH ) of less than 6.0 x 10 ⁶ . Ta.

Test piece 35 had a low silicon (Si) content and an insufficient retained austenite fraction. As a result, test piece 35 had a balance between tensile strength and elongation (B _TE ) of less than 3.0×10 ⁶ .

Test piece 36 had a high silicon (Si) content and an excessive fresh martensite fraction. As a result, the test piece 36 had a balance between tensile strength and elongation rate (B _TE ) of less than 3.0 x 10 ⁶ and a balance between tensile strength and hole expansion rate (B _TH ) of less than 6.0 x 10 ⁶ . Ta.

Test piece 37 had a high aluminum (Al) content and an excessive fresh martensite fraction. As a result, test piece 37 had a balance between tensile strength and elongation rate (B _TE ) of less than 3.0 x 10 ⁶ and a balance between tensile strength and hole expansion rate (B _TH ) of less than 6.0 x 10 ⁶ . Ta.

Test piece 38 had a low manganese (Mn) content, pearlite was produced, and the retained austenite fraction was insufficient. As a result, the balance between tensile strength and elongation (B _TE ) of test piece 38 was less than 3.0×10 ⁶ .

Test piece 39 had a high manganese (Mn) content and an excessive fresh martensite fraction. As a result, test piece 39 had a balance between tensile strength and elongation rate (B _TE ) of less than 3.0 x 10 ⁶ and a balance between tensile strength and hole expansion rate (B _TH ) of less than 6.0 x 10 ⁶ . Ta.

Test piece 40 had a high chromium (Cr) content and an excessive fresh martensite fraction. As a result, the test piece 40 had a balance between tensile strength and elongation rate (B _TE ) of less than 3.0 x 10 ⁶ and a balance between tensile strength and hole expansion rate (B _TH ) of less than 6.0 x 10 ⁶ . Ta.

Test piece 41 had a high molybdenum (Mo) content and an excessive fresh martensite fraction. As a result, the test piece 41 had a balance between tensile strength and elongation rate (B _TE ) of less than 3.0 x 10 ⁶ and a balance between tensile strength and hole expansion rate (B _TH ) of less than 6.0 x 10 ⁶ . Ta.

Test piece 42 had a low boron (B) content, and boron (B) did not concentrate in the tempered martensite. As a result, the test piece 42 has a [B] _FM /[B] _TM of over 0.55 and a yield ratio evaluation index (I _YR ) of over 0.42.

Test piece 43 had a high boron (B) content, and boron (B) was excessively concentrated in the tempered martensite. As a result, the test piece 43 had a [B] _FM /[B] _TM of less than 0.03 and a yield ratio evaluation index (I _YR ) of less than 0.15.

As mentioned above, although the present invention has been described in detail with reference to examples, embodiments having different forms from these examples are also possible. Therefore, the spirit and scope of the claims described below are not limited to the embodiments.

Claims (7)

In weight%, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, including remaining Fe and inevitable impurities,
The microstructure includes bainite, tempered martensite, fresh martensite, retained austenite and other unavoidable structures,
A high-strength steel plate with excellent workability that satisfies [Relational Expression 1] below.
[Relational expression 1]
0.03≦[B] _FM /[B] _TM ≦0.55
In the above relational expression 1, [B] _FM is the content (wt%) of boron (B) contained in fresh martensite, and [B] _TM is the content of boron (B) contained in tempered martensite. (% by weight). 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 (8) in weight percent.
(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- 1 or more of 3.0% (3) Cu: 0 to 4.0% and Ni: 1 or more of 0 to 4.0% (4) Ca: 0 to 0.05%, REM excluding Y : 0 to 0.05% and Mg: one or more of 0 to 0.05% (5) W: 0 to 0.5% and Zr: one or more of 0 to 0.5% (6) Sb : 0 to 0.5% and Sn: 0 to 0.5% (7) Y: 0 to 0.2% and Hf: 0 to 0.2% (8) Co :0~1.5% The microstructure of the steel plate has a volume fraction of 10 to 30% bainite, 50 to 70% tempered martensite, 10 to 30% fresh martensite, 2 to 10% retained austenite, and 5% or less ( The high-strength steel sheet with excellent workability according to claim 1, which contains ferrite of 0%). The steel plate has a balance between tensile strength and elongation (B _TE ) expressed by the following [Relational Expression 2] that satisfies 3.0×10 ⁶ to 6.2×10 ⁶ (MPa ² % ^1/2 ). , the balance between tensile strength and hole expansion rate (B _TH ) expressed by [Relational Expression 3] below satisfies 6.0×10 ⁶ to 11.5×10 ⁶ (MPa ² % ^1/2 ), and the following The high-strength steel plate with excellent workability according to claim 1, wherein the yield ratio evaluation index (I _YR ) expressed by [Relational expression 4] satisfies 0.15 to 0.42.
[Relational expression 2]
B _TE = [Tensile strength (TS, MPa)] ² × [Elongation rate (El, %)] ^1/2
[Relational expression 3]
B _TH = [Tensile strength (TS, MPa)] ² × [Hole expansion rate (HER, %)] ^1/2
[Relational expression 4]
I _YR = 1 - [yield ratio (YR)] In weight%, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: Provided is a cold rolled steel plate containing 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, remaining Fe and unavoidable impurities. stages and
The cold rolled steel plate is heated to 700°C at an average heating rate of 5°C/s or more (primary heating), and heated to a temperature range of Ac3 to 920°C at an average heating rate of 5°C/s or less (2 After heating (secondary heating), holding for 50 to 1200 seconds (first holding);
cooling the primarily held steel plate to a temperature range of 200 to 400°C at an average cooling rate of 1°C/s or more (primary cooling);
After heating the primarily cooled steel plate to a temperature range of 350 to 550 ° C. at an average heating rate of 5 ° C. / s or more (tertiary heating), holding it for 50 seconds or more (secondary holding);
A method for producing a high-strength steel plate with excellent workability, comprising the step of cooling the secondarily held steel plate to room temperature at an average cooling rate of 1° C./s or more (secondary cooling). The method for producing a high-strength steel plate with excellent workability according to claim 5, wherein the steel slab further includes any one or more of the following (1) to (8).
(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- 1 or more of 3.0% (3) Cu: 0 to 4.0% and Ni: 1 or more of 0 to 4.0% (4) Ca: 0 to 0.05%, REM excluding Y : 0 to 0.05% and Mg: one or more of 0 to 0.05% (5) W: 0 to 0.5% and Zr: one or more of 0 to 0.5% (6) Sb : 0 to 0.5% and Sn: 0 to 0.5% (7) Y: 0 to 0.2% and Hf: 0 to 0.2% (8) Co :0~1.5% 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 350 to 650°C;
Pickling the rolled steel plate;
The method of manufacturing a high-strength steel sheet with excellent workability according to claim 5, comprising the step of cold rolling the pickled steel sheet at a rolling reduction of 30 to 90%.