JP2024500723A - 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 manufacturing the same. be.

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 focused on ways to reduce the weight of materials in order to protect the global 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 and are also excellent in workability, typified by ductility and hole expandability.

TRIP (Transformation Induced Plasticity) steel, which uses the transformation-induced plasticity of retained austenite, has a complex microstructure consisting of ferrite, bainite, martensite, retained austenite, etc. It is known to have good workability.

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 made by tempering hard martensite, is softened martensite, so there is a difference in strength between tempered martensite and conventional untempered martensite (fresh martensite). exist. Therefore, when fresh martensite is suppressed and tempered martensite is formed, workability can be increased.

However, in the techniques disclosed in Patent Documents 1 and 2, the balance between tensile strength and elongation (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 with excellent 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). When boron (B) is added, ferrite-pearlite transformation is suppressed and bainite formation 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. This means that it is difficult to secure a steel plate with excellent strength, hole expandability, ductility, and yield ratio.

That is, the actual situation is that the balance between tensile strength and elongation rate (B _TE ), the balance between tensile strength and hole expansion rate (B _TH ), and the yield ratio evaluation index (I _YR ) all do not meet the requirements for a steel plate that is excellent.

Korean Published Patent No. 10-2006-0118602 JP2009-019258A JP2016-216808A

According to one aspect of the present invention, 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 by optimizing the composition and microstructure of the steel plate, and a method for manufacturing the same can be provided.

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

A high-strength steel plate with excellent workability according to one embodiment of the present invention has C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 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%, remainder contains Fe and unavoidable impurities, and contains bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures as fine structures, and satisfies [Relational Expression 1] and [Relational Expression 2] below. can.
[Relational expression 1]
0.03≦[B] _FM /[B] _TM ≦0.55
In the above relational expression 1, [B] _FM is the boron (B) content (% by weight) contained in fresh martensite, and [B] _TM is the boron (B) content (% by weight) contained in tempered martensite. weight%).
[Relational expression 2]
V(1.2 μm, γ)/V(γ)≧0.12
In the above relational expression 2, V (1.2 μm, γ) is the fraction (volume %) of retained austenite with an average grain size of 1.2 μm or more, and V (γ) is the fraction (volume %) of retained austenite in the steel sheet. volume%).

The steel plate may further contain any 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%, excluding Y One or more of REM: 0 to 0.05% and Mg: 0 to 0.05% (5) One or more of W: 0 to 0.5% and Zr: 0 to 0.5% (6) One or more of Sb: 0 to 0.5% and Sn: 0 to 0.5% (7) One or more of 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 (0 %) of ferrite.

The above steel plate has a balance between tensile strength and elongation (B _TE ) expressed by [Relational Expression 3] below, which 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 4] 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 5] can satisfy 0.15 to 0.42.
[Relational expression 3]
B _TE = [Tensile strength (TS, MPa)] ² * [Elongation rate (El, %)] ^1/2
[Relational expression 4]
B _TH = [Tensile strength (TS, MPa)] ² * [Hole expansion ratio (HER, %)] ^1/2
[Relational expression 5]
I _YR = 1 - [yield ratio (YR)]

A method for manufacturing a high-strength steel sheet with excellent workability according to one embodiment 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; heating the cold rolled steel plate to 700°C at an average heating rate of 5°C/s or more (primary heating); After heating to a temperature range of Ac3 to 920°C at an average heating rate of 5°C/s or less (secondary heating), maintaining the temperature for 50 to 1200 seconds (primary maintenance); After cooling to a temperature range of 350 to 550 °C at an average cooling rate of 100 °C / s (primary cooling), maintaining for 5 to 600 seconds (secondary maintenance); Step of cooling (secondary cooling) to a temperature range of 200 to 400 °C at an average cooling rate of 5 to 100 °C/s; cooling the above-mentioned secondary cooled steel plate to a temperature of 350 to 550 °C at an average heating rate of 5 to 100 °C/s After heating to a certain temperature range (tertiary heating), maintaining the steel plate for 50 seconds or more (tertiary maintenance); cooling the tertiary-maintained steel plate to room temperature at an average cooling rate of 1°C/s or more (tertiary cooling) It can include stages.

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%, excluding Y One or more of REM: 0 to 0.05% and Mg: 0 to 0.05% (5) One or more of W: 0 to 0.5% and Zr: 0 to 0.5% (6) One or more of Sb: 0 to 0.5% and Sn: 0 to 0.5% (7) One or more of Y: 0 to 0.2% and Hf: 0 to 0.2% (8) Co: 0-1.5%

The cold-rolled steel plate is obtained by heating the steel slab to 1,000 to 1,350°C; final hot rolling in a temperature range of 800 to 1,000°C; and hot rolling in a temperature range of 350 to 650°C. The method may be provided through the following steps: winding up the 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 embodiment 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 used suitably for automobile parts, etc., and a method for manufacturing the same. can do.

The present invention relates to a high-strength steel plate with excellent workability and a method for manufacturing the same, and preferred embodiments of the present invention will be described below. The embodiments of the invention may be modified in various ways and the scope of the invention should not be construed as being limited to the embodiments described below. These embodiments are provided so that this invention will be more fully understood to those skilled in the art to which the invention pertains.

The inventors of the present invention have discovered that in a boron (B)-added Transformation Induced Plasticity (TRIP) steel containing bainite, tempered martensite, fresh martensite, and retained austenite, tempered martensite, fresh martensite, and retained austenite When controlling the microstructure fraction of austenite within a certain range, controlling the boron (B) content contained in tempered martensite and fresh martensite within a certain range, and controlling the shape and size of retained austenite within a certain range. It has been recognized that 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 embodiment of the present invention will be described in more detail.

The high-strength steel sheet with excellent workability according to one embodiment of the present invention has 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 satisfies [Relational Expression 1] and [Relational Expression 2] below. can.
[Relational expression 1]
0.03≦[B] _FM /[B] _TM ≦0.55
In the above relational expression 1, [B] _FM is the boron (B) content (wt%) contained in fresh martensite, and [B] _TM is the boron (B) content (% by weight) contained in tempered martensite. weight%).
[Relational expression 2]
V(1.2 μm, γ)/V(γ)≧0.12
In the above relational expression 2, V (1.2 μm, γ) is the fraction (volume %) of retained austenite with an average grain size of 1.2 μm or more, and V (γ) is the fraction (volume %) of retained austenite in the steel sheet. volume%).

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

The high-strength steel sheet with excellent workability according to one embodiment of the present invention has 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% Below (including 0%), Mo: 3.0% or below (including 0%), Cu: below 4.0% (including 0%), Ni: below 4.0% (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 (0% included), Sb: 0.5% or less (0% included), Sn: 0.5% or less (0% included), Y: 0.2% or less (0% ), Hf: 0.2% or less (0% included), and Co: 1.5% or less (0% included).

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, the present invention can include 0.1% or more of carbon (C) to achieve such effects. Preferred carbon (C) content can be greater than 0.1%, can 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, the present invention can limit the upper limit of carbon (C) content to 0.25%. The carbon (C) content can be 0.24% or less, and more preferably the carbon (C) content can 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 this 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 plating defects such as unplated areas in the plating process, but also reduces the weldability of the steel sheet. The upper limit of 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 this effect. 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 excessively, the bainite transformation time increases and the carbon (C) concentration in austenite becomes insufficient, making it difficult to secure the desired austenite fraction. There is a problem that it cannot be done. Therefore, the present invention can limit the upper limit of the manganese (Mn) content to 4.0%. A preferable upper limit of the manganese (Mn) content can be 3.9%.

Aluminum (Al): 0.01-1.5%
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, 0.01% or more of aluminum (Al) can be added to achieve this effect. 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 excessively, not only the number of inclusions in the steel sheet increases, but also the workability of the steel sheet can be reduced. It can be limited to .5%. A preferable upper limit of the aluminum (Al) content can be 1.48%.

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 content of nitrogen (N) 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. Furthermore, the present invention simultaneously ensures 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. Therefore, boron (B) must be 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 not only causes excessive property effects but also increases manufacturing costs, so the present invention sets the upper limit of the boron (B) content 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) Vanadium (V) is an element that forms 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 increases manufacturing costs. Therefore, in the present invention, the content 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. 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. Unable to secure austenite fraction. Therefore, the present invention can limit the contents of chromium (Cr) and molybdenum (Mo) 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. It is. Copper (Cu) and nickel (Ni) are also elements that concentrate 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 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 Ni) can be limited to 4.0% or less.

One or more of the following: Calcium (Ca): 0 to 0.05%, Magnesium (Mg): 0 to 0.05%, and Rare earth elements (REM) excluding yttrium (Y): 0 to 0.05% Click here The 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 spherical. One or more of rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) may be added for effectiveness. However, if the content of rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) exceeds a certain level, it will cause not only excessive characteristic effects but also an increase in manufacturing costs. Therefore, 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. Therefore, 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 characteristic effects but also increases manufacturing costs. The content of Zr) 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 the plating wettability and plating adhesion of the steel plate. Since it is an element, one or more of antimony (Sb) and tin (Sn) can be added in the present invention 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. 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 content of yttrium (Y) and hafnium (Hf) exceeds a certain level, the ductility of the steel sheet may deteriorate. Each can be limited to 0.2% 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 this 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. can do.

The high-strength steel sheet with excellent workability according to one embodiment of the present invention can 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 having ordinary skill in the art, and therefore their full contents are not specifically mentioned herein. Furthermore, addition of effective ingredients other than those mentioned above is not completely excluded.

A high-strength steel sheet with excellent workability according to one embodiment of the present invention can contain bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures as a microstructure. .

Untempered martensite (fresh martensite, FM) and tempered martensite (tempered martensite, TM) are all 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 the steel sheet. Furthermore, fresh martensite tends to lower the yield ratio of the steel sheet compared to tempered martensite. This is because the fine structure of the 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- It is preferable to control the texture fraction of tempered martensite and fresh martensite in order to ensure YR). Balance of 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 ) In order to satisfy the yield ratio evaluation index (1-YR) of 0.42 or less, limit the fraction of tempered martensite to 50% by volume or more, and limit the fraction of fresh martensite to 10% by volume or more. is preferred. A more preferred fraction of tempered martensite can be 52 volume % or more or 54 volume % or more, and a more preferred fraction of fresh martensite can be 12 volume % or more. On the other hand, if tempered martensite or fresh martensite is formed excessively, the ductility and burring property are reduced, resulting in a balance between tensile strength and elongation of 3.0 * ¹⁰⁶ or more (TS ² * EL ^{1/ 2} ), it becomes impossible to simultaneously satisfy the balance between tensile strength and hole expansion ratio (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. . 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 can be 68 vol.% or less or 65 vol.% or less, and a more preferred fraction of fresh martensite can be 25 vol.% 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- In order to ensure YR), it is necessary to optimize the bainite fraction. Balance of 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 ) In order to ensure 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 can be 12 volume % or more or 14 volume % or more. On the other hand, if bainite is formed excessively, it will result in a decrease in the fraction of tempered martensite, so the desired balance between tensile strength and elongation (TS ² * EL ^1/2 ), tensile strength and hole In order to ensure the balance of expansion ratio (TS ² *HER ^1/2 ) and yield ratio evaluation index (1-YR), the bainite fraction can be limited to 30% by volume or less. A preferred fraction of bainite can be 12 vol.% or more, 14 vol.% or more, or 28 vol.% or less, or 26 vol.% 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 ), so Undesirable. 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 ). The preferred fraction of retained austenite can be 3% by volume or more and 8% by volume or less.

The steel sheet of the present invention may contain ferrite, pearlite, island-like martensite (Martensite Austenite Constituent, MA), etc. as an unavoidable structure. If excessive ferrite is formed, the strength of the steel sheet may be reduced, 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 limits the formation of pearlite as much as possible.

A high-strength steel plate with excellent workability according to one embodiment of the present invention can satisfy the following [Relational Expression 1] and [Relational Expression 2].
[Relational expression 1]
0.03≦[B] _FM /[B] _TM ≦0.55
In the above relational expression 1, [B] _FM is the boron (B) content (wt%) contained in fresh martensite, and [B] _TM is the boron (B) content (% by weight) contained in tempered martensite. weight%).
[Relational expression 2]
V(1.2 μm, γ)/V(γ)≧0.12
In the above relational expression 2, V (1.2 μm, γ) is the fraction (volume %) of retained austenite with an average grain size of 1.2 μm or more, and V (γ) is the fraction (volume %) of retained austenite in the steel sheet. volume%).

The present invention provides 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 ) In order to ensure the The ratio of retained austenite of a specific size, shape, and type to the total retained austenite is controlled within a certain range.

In the present invention, as shown in [Relational Expression 1], the boron (B) content ([B] _TM , weight %) contained in fresh martensite is calculated by B] Tensile strength and elongation 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 hole expansion ratio (B _TH ) of 6.0*10 ⁶ to 11.5*10 ⁶ (MPa ² % ^1/2 ) and 0.15 to 0.42 It is possible to simultaneously secure a yield ratio evaluation index (I _YR ) of .

The inventor of the present invention has conducted in-depth research into a method for securing the physical properties of boron (B)-added TRIP steel, and although the theoretical basis has not been clearly clarified, the inventor has discovered that boron contained in tempered martensite We focused on the point 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 the (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 boron (B) content ratio contained in the tempered martensite and fresh martensite. Therefore, according to the present invention, the ratio of the boron (B) content contained in fresh martensite to the boron (B) content contained in tempered martensite is 0.03 to 0.55 as shown in [Relational Expression 1]. In order ^to limit ^{the range of} 1-YR) can be secured.

Furthermore, the inventor of the present invention found that not only the fraction of retained austenite but also the ratio of retained austenite of a specific size to the total retained austenite is an important factor in ensuring strength and workability.

As the proportion of retained austenite having an average crystal grain size of 1.2 μm or more among retained austenite increases, the workability of the steel sheet can be improved. Retained austenite with an average grain size of 1.2 μm or more is retained austenite whose average size has been increased by heat treatment at a bainite-forming temperature, and is more likely to change to martensite than retained austenite with an average grain size of 1.2 μm or less. This is a tissue in which the driving force for metamorphosis is relatively suppressed. Therefore, retained austenite with an average grain size of 1.2 μm or more is inhibited from transforming into martensite, so if the proportion of retained austenite with an average grain size of 1.2 μm or more is above a certain level, It is possible to further effectively improve the workability of.

The present invention provides the ratio of retained austenite having an average crystal grain size of 1.2 μm or more (V( In order to control the ratio of 1.2 μm, γ), volume %) to 0.12 or more, the desired 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 ) can be effectively secured.

The high-strength steel plate with excellent workability according to one embodiment of the present invention has a balance between tensile strength and elongation (B _TE ) expressed by the following [Relational Expression 3] 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 [Relational expression 4] below 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 5] can satisfy 0.15 to 0.42.
[Relational expression 3]
B _TE = [Tensile strength (TS, MPa)] ² * [Elongation rate (El, %)] ^1/2
[Relational expression 4]
B _TH = [Tensile strength (TS, MPa)] ² * [Hole expansion ratio (HER, %)] ^1/2
[Relational expression 5]
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 Ac3 to 920°C at an average heating rate of ℃/s or less, maintaining the temperature for 50 to 1200 seconds (primary maintenance); After cooling to a temperature range of 350 to 550 °C at an average cooling rate of °C/s (primary cooling), maintaining for 5 to 600 seconds (secondary maintenance); Cooling (secondary cooling) to a temperature range of 200 to 400 °C at an average cooling rate of 5 to 100 °C/s; (tertiary heating) and then maintaining the steel plate for 50 seconds or more (tertiary maintenance); cooling the tertiary-maintained steel plate to room temperature at an average cooling rate of 1°C/s or more (tertiary cooling) can include.

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 completing hot rolling in a temperature range of 350 to 650°C. Winding the hot-rolled steel plate; pickling the rolled-up steel plate; and cold-rolling the pickled steel plate at a reduction rate of 30 to 90%. can be done.

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 replaces the description of the alloy composition of the steel plate described above.

The prepared steel slab can be heated in a certain temperature range, and the heating temperature of the steel slab can be in the range of 1000-1350°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 finishing 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 temperature of finish hot rolling 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 become coarse. This may cause deterioration of the physical properties of the final steel sheet.

The 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. If the winding temperature is less than 350°C, winding is not easy, and if the winding temperature exceeds 650°C, surface scale may form inside the hot rolled steel sheet, making pickling difficult. There is.

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. Although pickling and cold rolling conditions are not particularly limited in the present invention, cold rolling is preferably performed at a cumulative reduction rate of 30 to 90%. When the cumulative reduction rate of cold rolling exceeds 90%, it may be difficult to perform cold rolling in a short time due to the high strength of the steel sheet.

The cold-rolled steel sheet can be manufactured as an unplated cold-rolled steel sheet through an annealing heat treatment process, or it can be manufactured as 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 In the present invention, an annealing heat treatment process is carried out in order to simultaneously ensure the strength and workability of the steel sheet.

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), it is maintained for 50 to 1200 seconds (primary maintenance).

If the average heating rate of the 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 as a result, the final structure is fine tempered martensite. It becomes impossible to form retained austenite. This makes it impossible to achieve the desired balance between tensile strength and elongation rate (TS ² *EL ^1/2 ) and between tensile strength and hole expansion rate (TS ² *HER ^1/2 ). Furthermore, when the secondary heating rate to the primary maintenance temperature exceeds 5°C/s, the transformation from cementite to austenite generated during heating is accelerated, a large amount of massive austenite is formed, and the final structure is may become coarse and boron (B) may not be sufficiently concentrated in the tempered martensite. As a result, [B] _FM / [B] _TM exceeds 0.55, achieving the desired level of balance between tensile strength and elongation (TS ² * EL ^1/2 ), tensile strength and hole expansion. ratio balance (TS ² *HER ^1/2 ) and yield ratio evaluation index (I _YR ) become impossible to achieve.

When the primary maintenance temperature is less than Ac3 (two-phase region), 5% by volume or more of ferrite is formed, and the balance between tensile strength and elongation rate (TS ² * EL ^1/2 ) and tensile strength and hole expansion change accordingly. The rate balance (TS ² *HER ^1/2 ) may decrease. Furthermore, 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. The upper limits of the primary maintenance temperature and primary maintenance time are not particularly limited, but in order to prevent a decrease in toughness due to grain coarsening, the primary maintenance temperature is limited to 920°C or less and the primary maintenance time is limited to 1200 seconds or less. It is preferable to do so.

After primary maintenance, cool to a temperature range of 350 to 550°C at an average cooling rate of 2°C/s or more (primary cooling), and then maintain the temperature range for 5 seconds or more (secondary maintenance). be able to. When the average cooling rate of the primary cooling is less than 2°C/s, the fraction of retained austenite is insufficient due to slow cooling, and the balance between tensile strength and elongation rate (TS ² *EL ^1/2 ) and tensile The balance between strength and hole expansion rate (TS ² *HER ^1/2 ) may deteriorate. The upper limit of the average cooling rate of primary cooling does not need to be particularly specified, but it is preferably 100° C. or less. When the secondary maintenance temperature is less than 350°C, the low heat treatment temperature reduces the steel plate's V (1.2 μm, γ)/V (γ) and the balance between tensile strength and hole expansion rate (TS ² * HER ^1/2 ). There are things to do. On the other hand, if the secondary maintenance temperature exceeds 550°C, the residual austenite is insufficient and 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 decrease. In addition, if the secondary holding time is less than 5 seconds, the heat treatment time is insufficient and the balance between V (1.2 μm, γ)/V (γ) and tensile strength and hole expansion rate of the steel plate (TS ² * HER ^{1 /2} ) may decrease. The upper limit of the secondary maintenance time does not need to be specifically defined, but it is preferably limited to 600 seconds or less.

After the secondary maintenance, it can be cooled (secondary cooling) to the primary cooling stop temperature of 200 to 400°C at an average cooling rate of 2°C/s or more. When the average cooling rate of secondary cooling is less than 2°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 ), tensile The balance between strength and hole expansion rate (TS ² *HER ^1/2 ) may deteriorate. Although the upper limit of the average cooling rate of secondary 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 poor balance between V (1.2 μm, γ)/V (γ), tensile strength and elongation of the steel plate. (TS ² *EL ^1/2 ) and the balance between tensile strength and hole expansion rate (TS ² *HER ^1/2 ) may decrease. 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 maintained for 50 seconds or more (tertiary maintenance). The upper limit of the average heating rate of tertiary heating does not need to be particularly specified, but it is preferably 100° C./s or less. When the tertiary holding temperature is less than 350° C. or the tertiary holding time is less than 50 seconds, tempered martensite is excessively formed and it is difficult to ensure a sufficient proportion of retained austenite. As a result, V(1.2 μm, γ)/V(γ), 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 decrease. If the tertiary maintenance temperature exceeds 550°C or the tertiary maintenance time exceeds 155,000 seconds, the fraction of retained austenite is insufficient and the steel plate's V(1.2μm, γ)/V(γ) , 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 decrease.

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

The high-strength steel plate 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, Contains, by volume fraction, 10-30% bainite, 50-70% tempered martensite, 10-30% fresh martensite, 2-10% retained austenite, and 5% or less (including 0%) ferrite. be able to.

The steel plate manufactured by the above 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 4] 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 5] can satisfy 0.15 to 0.42.
[Relational expression 3]
B _TE = [Tensile strength (TS, MPa)] ² * [Elongation rate (El, %)] ^1/2
[Relational expression 4]
B _TH = [Tensile strength (TS, MPa)] ² * [Hole expansion ratio (HER, %)] ^1/2
[Relational expression 5]
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 embodiment 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 to help understand the present invention, and are not intended to limit the scope 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. 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 sheet manufactured in this way was observed and the results are shown in Tables 6 and 7. Among the microstructures, ferrite (F), bainite (B), tempered martensite (TM), fresh martensite (FM), and pearlite (P) were analyzed using SEM after nital etching the cross section of the polished specimen. I observed it. After nital etching, the structure with no irregularities on the surface of the test piece was sectioned with ferrite, and the structure with a lamellar structure of cementite and ferrite was sectioned with pearlite. Since bainite (B) and tempered martensite (TM) are all observed in lath and block forms and are difficult to distinguish, the fractions of bainite and tempered martensite were calculated using expansion curves after dilation evaluation. That is, the bainite fraction was determined by subtracting the tempered martensite fraction calculated through the expansion curve from the bainite and tempered martensite fraction measured by SEM observation. 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 using the fraction of martensite and retained austenite observed by the above SEM. The value obtained by subtracting the ratio was determined as the fraction of fresh martensite.

On the other hand, the [B] _FM / [B] _TM of the steel plate, V (1.2 μm, γ) / V (γ), balance between tensile strength and elongation (TS ² * EL ^1/2 ), tensile strength and hole expansion The ratio balance (TS ² *HER ^1/2 ) and yield ratio evaluation index (I _YR ) were measured and evaluated, and the results are shown in Tables 8 and 9.

The boron (B) content ([B] _FM ) in fresh martensite and the boron (B) content ([B] _TM ) in tempered martensite were measured using EPMA (Electron Probe MicroAnalyser). It was determined by the boron (B) concentration measured within the tempered martensite. The retained austenite (V (1.2 μm, γ)) having an average crystal grain size of 1.2 μm or more was measured using a phase map of EBSD (Electron Backscatter Diffraction).

Tensile strength (TS) and elongation rate (El) are evaluated by a tensile test, and the tensile strength is evaluated using a test piece taken based on the JIS No. 5 standard with a direction of 90 degrees to the rolling direction of the rolled plate material. (TS) and elongation rate (El) were measured. The hole expansion ratio (HER) is evaluated by a hole expansion test. After forming a punched hole of 10 mmΨ (die inner diameter 10.3 mm, clearance 12.5%), a conical punch with an apex angle of 60° is used to burr the punched hole. (burr) was inserted into the punched hole in the direction toward the outside, and the surrounding area of the punched 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 9 above, in the case of a test piece that satisfies the conditions presented in the present invention, both [Relational Expression 1] and [Relational Expression 2] are satisfied, and the balance between tensile strength and elongation rate (B _TE ) satisfies 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 ) is 6.0*10 ⁶ to 11.5* 10 ⁶ (MPa ² % ^1/2 ), and the yield ratio evaluation index (I _YR ) is 0.15 to 0.42.

Test piece 2 was tested at a primary average heating rate of less than 5° C./s, resulting in insufficient tempered martensite and retained austenite. As a result, the balance between tensile strength and elongation rate (B _TE ) of test piece 2 was less than 3.0*10 ⁶ , and the balance between tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

Test piece 3 was tested at a secondary average heating rate of over 5° C./s, so that massive austenite was formed and boron (B) could not be concentrated 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 maintenance temperature was less than Ac3, and the ferrite fraction exceeded. As a result, the balance between tensile strength and elongation rate (B _TE ) of test piece 4 was less than 3.0*10 ⁶ , and the balance between tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

Test piece 5 was tested at a primary average cooling rate of less than 1° C./s, and the residual austenite fraction was insufficient. As a result, the balance between tensile strength and elongation rate (B _TE ) of test piece 5 was less than 3.0*10 ⁶ , and the balance between tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

For test piece 6, the secondary maintenance temperature was less than 350° C., and the heat treatment temperature was insufficient. As a result, test piece 6 had a V (1.2 μm, γ)/V (γ) of less than 0.12, and a balance between tensile strength and hole expansion rate (B _TH ) of less than 6.0*10 ⁶ .

Test piece 7 was tested at a secondary maintenance temperature of over 550° C., and the fraction of retained austenite was insufficient. As a result, the balance between tensile strength and elongation rate (B _TE ) of test piece 7 was less than 3.0*10 ⁶ , and the balance between tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

For test piece 8, the secondary holding time was less than 5 seconds, and the heat treatment time was insufficient. As a result, test piece 8 had a V (1.2 μm, γ)/V (γ) of less than 0.12, and a balance between tensile strength and hole expansion rate (B _TH ) of less than 6.0*10 ⁶ .

Test piece 9 was tested at a secondary average cooling rate of less than 2° C./s, and the residual austenite fraction was insufficient. As a result, test piece 9 had a balance between tensile strength and elongation rate ( _BTE ) of less than 3.0* ¹⁰⁶ , and a balance between tensile strength and hole expansion rate ( _BTH ) of less than 6.0* ¹⁰⁶ . .

Test piece 10 was tested at a primary cooling stop temperature of less than 200° C., and the fraction of tempered martensite exceeded the fraction of retained austenite. As a result, test piece 10 had a V (1.2 μm, γ)/V (γ) of less than 0.12, a balance between tensile strength and elongation (B _TE ) of less than 3.0*10 ⁶ , and tensile strength and hole The balance of expansion ratio (B _TH ) was less than 6.0*10 ⁶ .

Test piece 11 was tested at a primary cooling stop temperature of over 400° C., resulting in an excess of bainite and an insufficient tempered martensite fraction. As a result, the balance between tensile strength and elongation rate (B _TE ) of test piece 11 was less than 3.0*10 ⁶ , and the balance between tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

Test piece 12 was carried out at a tertiary maintenance temperature of less than 350° C., and the fraction of tempered martensite was exceeded and the fraction of retained austenite was insufficient. As a result, test piece 12 had a V (1.2 μm, γ)/V (γ) of less than 0.12, a balance between tensile strength and elongation (B _TE ) of less than 3.0*10 ⁶ , and tensile strength and hole The balance of expansion ratio (B _TH ) was less than 6.0*10 ⁶ .

Test piece 13 was tested at a tertiary maintenance temperature of over 550° C., and the residual austenite fraction was insufficient. As a result, test piece 16 had a V (1.2 μm, γ)/V (γ) of less than 0.12, a balance between tensile strength and elongation (B _TE ) of less than 3.0*10 ⁶ , and a tensile strength and hole The balance of expansion ratio (B _TH ) was less than 6.0*10 ⁶ .

For test piece 14, the tertiary holding time was less than 50 seconds, and the fraction of tempered martensite exceeded and the fraction of retained austenite was insufficient. As a result, test piece 14 had a V (1.2 μm, γ)/V (γ) of less than 0.12, a balance between tensile strength and elongation (B _TE ) of less than 3.0*10 ⁶ , and tensile strength and hole The balance of expansion ratio (B _TH ) was less than 6.0*10 ⁶ .

Test piece 15 was tested with a tertiary holding time exceeding 155,000 seconds, resulting in an insufficient fraction of retained austenite. As a result, test piece 15 had a V (1.2 μm, γ)/V (γ) of less than 0.12, a balance between tensile strength and elongation (B _TE ) of less than 3.0*10 ⁶ , and tensile strength and hole The balance of expansion ratio (B _TH ) was less than 6.0*10 ⁶ .

Test piece 37 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. It was less than 0* ¹⁰⁶ .

Test piece 38 had a high carbon (C) content, the fraction of tempered martensite was insufficient, the fraction of fresh martensite exceeded, and the fraction of retained austenite exceeded. As a result, the balance between tensile strength and elongation rate (B _TE ) of test piece 38 was less than 3.0*10 ⁶ , and the balance between tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

Test piece 39 had a low silicon (Si) content, and the fraction of retained austenite was insufficient. As a result, the balance between tensile strength and elongation rate (B _TE ) of test piece 39 was less than 3.0*10 ⁶ , and the balance between tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

Test piece 40 had a high silicon (Si) content, and the fraction of fresh martensite exceeded. As a result, the balance between tensile strength and elongation rate (B _TE ) of test piece 40 was less than 3.0*10 ⁶ , and the balance between tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

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

Test piece 42 had a low manganese (Mn) content, and the fraction of retained austenite was insufficient due to pearlite formation. As a result, the balance between tensile strength and elongation rate (B _TE ) of test piece 42 was less than 3.0*10 ⁶ , and the balance between tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

Test piece 43 had a high manganese (Mn) content and exceeded the fresh martensite fraction. As a result, the balance between tensile strength and elongation rate (B _TE ) of test piece 43 was less than 3.0*10 ⁶ , and the balance between tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

Test piece 44 had a high chromium (Cr) content and an excessive fresh martensite fraction. As a result, the balance between tensile strength and elongation rate (B _TE ) of test piece 44 was less than 3.0*10 ⁶ , and the balance between tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

Test piece 45 had a high molybdenum (Mo) content and an excessive fresh martensite fraction. As a result, the balance between tensile strength and elongation rate (B _TE ) of test piece 45 was less than 3.0*10 ⁶ , and the balance between tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

Test piece 46 had a low boron (B) content, and boron (B) could not be concentrated in the tempered martensite. As a result, the test piece 51 had a [B] _FM /[B] _TM of over 0.55 and a yield ratio evaluation index (I _YR ) of over 0.42.

Test piece 47 had a high boron (B) content, and boron (B) was excessively concentrated in the tempered martensite. As a result, the test piece 52 had a [B] _FM /[B] _TM of less than 0.03 and a yield ratio evaluation index (I _YR ) of less than 0.15.

Although the present invention has been described above in detail by way of examples, embodiments with different forms 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] and [Relational Expression 2] below.
[Relational expression 1]
0.03≦[B] _FM /[B] _TM ≦0.55
In the above relational expression 1, [B] _FM is the boron (B) content (% by weight) contained in fresh martensite, and [B] _TM is the boron (B) content (% by weight) contained in tempered martensite. weight%).
[Relational expression 2]
V(1.2 μm, γ)/V(γ)≧0.12
In the above relational expression 2, V (1.2 μm, γ) is the fraction (volume %) of retained austenite with an average grain size of 1.2 μm or more, and V (γ) is the fraction (volume %) of retained austenite in the steel sheet. volume%). The high-strength steel plate with excellent workability according to claim 1, wherein the steel plate further contains at least one selected from the group consisting 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%, excluding Y One or more of REM: 0 to 0.05% and Mg: 0 to 0.05% (5) One or more of W: 0 to 0.5% and Zr: 0 to 0.5% (6) One or more of Sb: 0 to 0.5% and Sn: 0 to 0.5% (7) One or more of 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 (0 The high-strength steel plate with excellent workability according to claim 1, which contains ferrite of %). The steel plate has a balance between tensile strength and elongation (B _TE ) expressed by the following [Relational Expression 3] 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 4] 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 5] satisfies 0.15 to 0.42.
[Relational expression 3]
B _TE = [Tensile strength (TS, MPa)] ² * [Elongation rate (El, %)] ^1/2
[Relational expression 4]
B _TH = [Tensile strength (TS, MPa)] ² * [Hole expansion ratio (HER, %)] ^1/2
[Relational expression 5]
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. step;
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), maintaining for 50 to 1200 seconds (first maintaining);
Cooling the primarily maintained steel plate to a temperature range of 350 to 550°C at an average cooling rate of 2 to 100°C/s (primary cooling), and then maintaining it for 5 to 600 seconds (secondary maintenance);
cooling the secondary maintained steel plate to a temperature range of 200 to 400°C at an average cooling rate of 2 to 100°C/s (secondary cooling);
heating the secondarily cooled steel plate to a temperature range of 350 to 550°C at an average heating rate of 5 to 100°C/s (tertiary heating), and then maintaining it for 50 seconds or more (tertiary maintenance); and A method for producing a high-strength steel plate with excellent workability, including a step of cooling a tertiary-maintained steel plate to room temperature at an average cooling rate of 1° C./s or more (tertiary 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%, excluding Y One or more of REM: 0 to 0.05% and Mg: 0 to 0.05% (5) One or more of W: 0 to 0.5% and Zr: 0 to 0.5% (6) One or more of Sb: 0 to 0.5% and Sn: 0 to 0.5% (7) One or more of 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;
Finish hot rolling at a temperature range of 800-1000°C;
coiling the hot rolled steel sheet at a temperature range of 350 to 650°C;
The excellent workability according to claim 5, which is provided by: pickling the rolled-up steel plate; and cold rolling the pickled steel plate at a rolling reduction of 30 to 90%. A method for manufacturing high-strength steel plates.