CN116601320A - High-strength steel sheet excellent in workability and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a steel sheet that can be used for automobile parts and the like, and relates to a steel sheet excellent in balance of strength and ductility, balance of strength and hole expansibility, and yield ratio evaluation index, and a method for producing the same.

Description

High-strength steel sheet excellent in workability and manufacturing method thereof

technical field

The present invention relates to a steel sheet that can be used for automobile parts and the like, and to a steel sheet having high strength characteristics and excellent workability, and a method for producing the same.

Background technique

In recent years, in order to protect the global environment, the automobile industry is paying attention to methods that can reduce the weight of materials and ensure the stability of passengers. To meet such demands for stability and light weight, the application of high-strength steel sheets is increasing dramatically. Generally, it is known that the workability of a steel sheet decreases as the strength of the steel sheet increases. Therefore, among steel sheets for automobile parts, there is a need for a steel sheet having high strength characteristics and excellent workability represented by ductility, hole expandability, and the like.

It is known that Transformation Induced Plasticity (TRIP) steel utilizing transformation induced plasticity of retained austenite has a complex microstructure composed of ferrite, bainite, martensite and retained austenite, etc. Therefore, it has high strength characteristics and has a certain level of workability.

As techniques for further improving the workability of steel sheets, Patent Document 1 and Patent Document 2 disclose a method utilizing tempered martensite. Tempered martensite made by tempering hard martensite is softened martensite, so tempered martensite is different from existing untempered martensite (newborn martensite) Martensite) there is a difference in strength. Therefore, when young martensite is suppressed and tempered martensite is formed, workability can be increased.

However, in the techniques disclosed in Patent Document 1 and Patent Document 2, the balance of tensile strength and elongation (TS ² *EL ^1/2 ) cannot satisfy 3.0*10 ⁶ to 6.2*10 ⁶ (MPa ² % ^{1/ 2} ), which means that it is difficult to secure a steel sheet excellent in both strength and ductility.

In addition, 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, the ferrite-pearlite transformation is suppressed and the formation of bainite is induced, so that both strength and workability can be achieved.

However, in the technique disclosed in Patent Document 3, it is not possible to simultaneously ensure the balance of tensile strength and elongation (B _TE ) of 3.0*10 ⁶ to 6.2*10 ⁶ (MPa ² % ^1/2 ), 6.0*10 ⁶ Balance of tensile strength and hole expansion ratio (B _TH ) to 11.5*10 ⁶ (MPa ² % ^1/2 ) and yield strength ratio evaluation index (I _YR ) of 0.15 to 0.42, so it means that it is difficult to secure strength, A steel plate with excellent hole expandability, ductility, and yield ratio.

That is, the demand for a steel sheet excellent in balance of tensile strength and elongation (B _TE ), balance of tensile strength and hole expansion ratio (B _TH ), and yield ratio evaluation index (I _YR ) cannot be met at present.

(Prior art literature)

(Patent Document 1) Korean Laid-Open Patent Publication No. 10-2006-0118602

(Patent Document 2) Japanese Laid-Open Patent Publication No. 2009-019258

(Patent Document 3) Japanese Laid-Open Patent Publication No. 2016-216808

Contents of the invention

technical problem to be solved

According to one aspect of the present invention, it is possible to provide a steel plate having an excellent balance of tensile strength and elongation, a balance of tensile strength and hole expansion rate, and a yield ratio evaluation index by optimizing the composition and fine structure of the steel plate and its method of manufacture.

The technical problem to be solved by the present invention is not limited to the above content. The additional technical problems to be solved by the present invention are described in the entire description, and those skilled in the art can easily understand the additional technical problems to be solved by the present invention from the content recorded in the description of the present invention.

Technical solutions

In the high-strength steel sheet excellent in workability according to one aspect of the present invention, the steel sheet may contain: C: 0.1-0.25%, Si: 0.01-1.5%, 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%, the balance of Fe and unavoidable impurities, the fine structure can contain bainite, tempering Martensite, young martensite, retained austenite, and other unavoidable structures, the steel sheet may satisfy the following [Relational Expression 1].

[relational expression 1]

0.03≤[B] _{FM /[B]TM≤0.55}

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

In % by weight, the steel sheet may further contain any one or more of the following (1) to (8).

(1) More than one of Ti: 0-0.5%, Nb: 0-0.5% and V: 0-0.5%,

(2) More than one of Cr: 0-3.0% and Mo: 0-3.0%,

(3) More than one of Cu: 0-4.0% and Ni: 0-4.0%,

(4) Ca: 0-0.05%, REM except Y: 0-0.05% and Mg: 0-0.05% or more,

(5) More than one of W: 0-0.5% and Zr: 0-0.5%,

(6) More than one of Sb: 0-0.5% and Sn: 0-0.5%,

(7) More than one of Y: 0-0.2% and Hf: 0-0.2%,

(8) Co: 0-1.5%.

In terms of volume fraction, the microstructure of the steel plate may contain 10-30% bainite, 50-70% tempered martensite, 10-30% new martensite, 2-10% retained austenite Tensile, 5% or less (including 0%) ferrite.

_[ ^{_ _ _ _} The balance (B _TH ) of the tensile strength and hole expansion rate expressed by relational expression 3] can satisfy 6.0*10 ⁶ to 11.5*10 ⁶ (MPa ² % ^1/2 ), the yield strength represented by the following [relational expression 4] The specific evaluation index (I _YR ) can satisfy 0.15 to 0.42.

[relational expression 2]

B _TE = [tensile strength (TS, MPa)] ² * [elongation (El, %)] ^1/2

[relational expression 3]

BTH = [tensile strength (TS, MPa)] ² * [hole expansion rate (HER, %)] ^1/2

[relational expression 4]

I _YR ＝1-[Yield strength ratio (YR)]

A method for manufacturing a high-strength steel sheet with excellent workability according to one aspect of the present invention may include the following steps: providing a cold-rolled steel sheet, in % by weight, the steel sheet comprising: C: 0.1-0.25%, Si: 0.01-1.5% %, 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%, the balance of Fe and unavoidable impurities ; 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 (secondary heating ), and then kept for 50-1200 seconds (one-time keeping); the steel plate kept once was cooled to a temperature range of 200-400°C at an average cooling rate above 1° C./second (one-time cooling); The steel plate is heated to a temperature range of 350-550°C at an average heating rate of 5°C/s or more (three times of heating), and then kept for more than 50 seconds (secondary hold); The average cooling rate is cooled to room temperature (secondary cooling).

The steel slab may further contain any one of the following (1) to (8).

(1) More than one of Ti: 0-0.5%, Nb: 0-0.5% and V: 0-0.5%,

(2) More than one of Cr: 0-3.0% and Mo: 0-3.0%,

(3) More than one of Cu: 0-4.0% and Ni: 0-4.0%,

(4) Ca: 0-0.05%, REM except Y: 0-0.05% and Mg: 0-0.05% or more,

(5) More than one of W: 0-0.5% and Zr: 0-0.5%,

(6) More than one of Sb: 0-0.5% and Sn: 0-0.5%,

(7) More than one of Y: 0-0.2% and Hf: 0-0.2%,

(8) Co: 0-1.5%.

The cold-rolled steel plate can be provided by the following steps: heating the billet to 1000-1350°C; performing hot finish rolling in the temperature range of 800-1000°C; rolling the steel sheet; pickling the rolled steel sheet; and cold rolling the pickled steel sheet at a reduction rate of 30-90%.

Beneficial effect

According to a preferred aspect of the present invention, it is possible to provide a material that can be suitably used for automobile parts and the like because it has an excellent balance of tensile strength and ductility, a balance of tensile strength and hole expandability, and a yield ratio evaluation index. Steel plate and its manufacturing method.

best practice

The present invention relates to a high-strength steel sheet excellent in workability and a method for producing the same. Preferred specific examples of the present invention will be described below. The specific embodiments of the present invention can be modified into various forms, and it should not be construed that the scope of the present invention is limited to the specific embodiments described below. This specific embodiment is provided to explain the present invention in more detail to those skilled in the art.

The inventors of the present invention realized that in a boron (B) type transformation-induced plasticity (TRIP) steel comprising bainite, tempered martensite, young martensite and retained austenite, the tempered martensite The structure fractions of tempered martensite, young martensite and retained austenite are controlled within a certain range, and the content of boron (B) contained in tempered martensite and young martensite is controlled within a certain range, and the retained austenite When the shape and size of the tenite are controlled within a certain range, the balance of excellent tensile strength and ductility, the balance of excellent tensile strength and hole expandability, and the excellent yield ratio evaluation index can be ensured at the same time. Recognizing this, they devised a method that can effectively balance excellent strength, yield ratio, ductility, and hole expandability, thereby completing the present invention.

Hereinafter, a high-strength steel sheet excellent in workability according to one aspect of the present invention will be described in detail.

In the high-strength steel sheet excellent in workability according to one aspect of the present invention, by weight %, C: 0.1-0.25%, Si: 0.01-1.5%, 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%, the balance of Fe and unavoidable impurities, the fine structure can contain bainite, tempered martensite body, young martensite, retained austenite and other unavoidable structures, the steel plate may satisfy the following [relational expression 1].

[relational expression 1]

0.03≤[B] _{FM /[B]TM≤0.55}

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

Hereinafter, the steel composition of the present invention will be described in more detail. Hereinafter, % indicating the content of each element is based on weight unless otherwise specified.

In the high-strength steel sheet excellent in workability according to one aspect of the present invention, in % by weight, C: 0.1-0.25%, Si: 0.01-1.5%, 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%, the balance of Fe and unavoidable impurities. In addition, it may further include: 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 except 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: up to 0.5% (including 0%), Y: up to 0.2% (including 0%), Hf: up to 0.2% (including 0%), and Co: up to 1.5% (including 0%) .

Carbon (C): 0.1-0.25%

Carbon (C) is an element essential for securing the strength of the steel sheet, and is an element that stabilizes retained austenite that contributes to improving the ductility of the steel sheet. Therefore, in order to achieve the effects as described above, 0.1% or more of carbon (C) may be contained in the present invention. A preferable carbon (C) content may exceed 0.1%, and may be 0.11% or more, 0.12% or more. On the other hand, when the carbon (C) content exceeds a certain level, ductility decreases due to an excessive increase in strength, and weldability may deteriorate. Therefore, the upper limit of the carbon (C) content may be limited to 0.25% in the present invention. The carbon (C) content may be 0.24% or less, more preferably the carbon (C) content may be 0.23% or less.

Silicon (Si): 0.01-1.5% or less

Silicon (Si) is an element that contributes to an increase in strength by solid solution strengthening, and is also an element that improves workability by homogenizing the structure. In addition, silicon (Si) is an element that contributes to the formation of retained austenite by suppressing the precipitation of cementite. Therefore, in order to achieve the effects described above, silicon (Si) may be added in an amount of 0.01% or more in the present invention. 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, when the silicon (Si) content exceeds a certain level, the problem of plating defects such as non-plating phenomenon is caused in the plating process, and the weldability of the steel sheet may be reduced, so the silicon (Si) content can be adjusted in the present invention. The upper limit is 1.5%. A preferable upper limit of the silicon (Si) content may be 1.48%, and a more preferable upper limit of the silicon (Si) content may be 1.46%.

Manganese (Mn): 1.0-4.0%

Manganese (Mn) is an element useful for improving both strength and ductility. Therefore, in order to achieve the above-mentioned effects, in the present invention, manganese (Mn) may be added in an amount of 1.0% or more. 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 too much manganese (Mn) is added, since the bainite transformation time increases, the enrichment of carbon (C) in austenite is insufficient, so there is a problem that the desired austenite fraction cannot be ensured. The problem. Therefore, the upper limit of the manganese (Mn) content may be limited to 4.0% in the present invention. A preferable upper limit of manganese (Mn) content may be 3.9%.

Aluminum (Al): 0.01-1.5%

Aluminum (Al) is an element that exerts a deoxidizing effect by combining with oxygen in steel. In addition, like silicon (Si), aluminum (Al) is an element that stabilizes retained austenite by suppressing the precipitation of cementite. Therefore, in order to achieve the effects as described above, aluminum (Al) may be added in an amount of 0.01% or more in the present invention. 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, if too much aluminum (Al) is added, the inclusions of the steel sheet may increase, and the workability of the steel sheet may be reduced, so the upper limit of the aluminum (Al) content may be limited to 1.5% in the present invention. A preferable upper limit of the aluminum (Al) content may be 1.48%.

Phosphorus (P): 0.15% or less (including 0%)

Phosphorus (P) is an element contained as an impurity that deteriorates impact toughness. Therefore, the content of phosphorus (P) is preferably controlled below 0.15%.

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 contained as an impurity and forms nitrides during continuous casting to cause 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 increases strength by increasing hardenability, and is also an element that suppresses nucleation of grain boundaries. Furthermore, the object of the present invention is to simultaneously secure an excellent balance of tensile strength and elongation, an excellent balance of tensile strength and hole expandability, and an excellent Yield ratio evaluation index, so boron (B) must be added in the present invention. Therefore, in the present invention, 0.0005% or more of boron (B) may be added for the above-mentioned effects. However, when added boron (B) exceeds a certain level, not only the characteristic effect is excessive, but also the production cost increases, so the upper limit of the boron (B) content can be limited to 0.005% in the present invention.

In addition, in the steel sheet of the present invention, there is an alloy composition that may be further included in addition to the above-mentioned alloy components, and this will be described in detail below.

Titanium (Ti): 0-0.5%, niobium (Nb): 0-0.5% and vanadium (V): 0-0.5% or more

Titanium (Ti), niobium (Nb), and vanadium (V) are elements that refine crystal grains by forming precipitates, and are also elements that contribute to the improvement of the strength and impact toughness of the steel sheet. Therefore, in order to As an effect, in the present invention, one or more of titanium (Ti), niobium (Nb) and vanadium (V) may be added. However, when the content of each of titanium (Ti), niobium (Nb) and vanadium (V) exceeds a certain level, excessive precipitates are formed, thereby reducing impact toughness, and causing an increase in manufacturing cost. Therefore, in this In the invention, the content of titanium (Ti), niobium (Nb) and vanadium (V) can be limited to 0.5% or less.

More than one of chromium (Cr): 0-3.0% and molybdenum (Mo): 0-3.0%

Chromium (Cr) and molybdenum (Mo) suppress austenite decomposition at the time of alloying treatment, and like manganese (Mn), chromium (Cr) and molybdenum (Mo) are elements that stabilize austenite, therefore, in order to In the present invention, one or more of chromium (Cr) and molybdenum (Mo) may be added. However, when the content of chromium (Cr) and molybdenum (Mo) exceeds a certain level, the enrichment of carbon (C) in austenite is insufficient due to the increase of the bainite transformation time, thus failing to ensure the desired residual Austenitic fraction. Therefore, in the present invention, the contents of chromium (Cr) and molybdenum (Mo) can be limited to 3.0% or less.

One or more of copper (Cu): 0-4.0% and nickel (Ni): 0-4.0%

Copper (Cu) and nickel (Ni) are elements that stabilize austenite and inhibit corrosion. In addition, copper (Cu) and nickel (Ni) are elements that are enriched on the surface of the steel sheet and prevent intrusion of hydrogen migrating into the steel sheet to suppress hydrogen-induced delayed fracture. Therefore, in the present invention, one or more of copper (Cu) and nickel (Ni) may be added for the above-mentioned effects. However, when the content of copper (Cu) and nickel (Ni) exceeds a certain level, excessive characteristic effects are caused, and it becomes a cause of increased manufacturing cost, so copper (Cu) and nickel (Ni) can be combined in the present invention. The content is limited to 4.0% or less, respectively.

One or more of calcium (Ca): 0-0.05%, magnesium (Mg): 0-0.05%, and rare earth elements (REM) except yttrium (Y): 0-0.05%

Wherein, the rare earth element (REM) refers to scandium (Sc), yttrium (Y) and lanthanoid elements. Rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) are elements that contribute to the improvement of the ductility of steel sheets by spheroidizing sulfides. In the invention, one or more rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) may be added. However, when the content of rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) exceeds a certain level, excessive characteristic effects are caused, and it becomes a cause of increased manufacturing costs. Therefore, in the present invention, The contents of calcium (Ca), magnesium (Mg), and rare earth elements (REM) other than yttrium (Y) can be limited to 0.05% or less, respectively.

Tungsten (W): 0-0.5% and Zirconium (Zr): 0-0.5% or more

Tungsten (W) and zirconium (Zr) are elements that increase the strength of the steel sheet by increasing the hardenability. Therefore, in order to achieve the above-mentioned effects, one of tungsten (W) and zirconium (Zr) can be added in the present invention. above. However, when the content of tungsten (W) and zirconium (Zr) exceeds a certain level, excessive characteristic effects are caused, and it becomes a cause of increased manufacturing cost, so in the present invention, tungsten (W) and zirconium (Zr) can be combined The content is limited to 0.5% or less, respectively.

One or more of antimony (Sb): 0-0.5% and tin (Sn): 0-0.5%

Antimony (Sb) and tin (Sn) are elements that improve the plating wettability and plating adhesion of steel sheets, therefore, antimony (Sb) and tin (Sn) may be added in the present invention for the above-mentioned effects more than one of them. However, when the content of antimony (Sb) and tin (Sn) exceeds a certain level, the brittleness of the steel plate increases, and cracks may occur during hot working or cold working, so antimony (Sb) and tin (Sn) can be combined in the present invention ) content is limited to 0.5% or less.

More than one of yttrium (Y): 0-0.2% and hafnium (Hf): 0-0.2%

Yttrium (Y) and hafnium (Hf) are elements that improve the corrosion resistance of steel sheets. Therefore, in the present invention, one or more of yttrium (Y) and hafnium (Hf) may be added for the above effects. However, when 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) can be limited to 0.2 %the following.

Cobalt (Co): 0-1.5%

Cobalt (Co) is an element that increases the effect of TRIP by promoting bainitic transformation, and therefore, cobalt (Co) may be added in the present invention for the above-mentioned effect. However, when the content of cobalt (Co) exceeds a certain level, the weldability and ductility of the steel sheet may be deteriorated, so the content of cobalt (Co) may be limited to 1.5% or less in the present invention.

A high-strength steel sheet excellent in workability according to an aspect of the present invention may contain a balance of Fe and other unavoidable impurities in addition to the above-mentioned components. However, unwanted impurities are unavoidably mixed in from raw materials or the surrounding environment during a normal manufacturing process, and therefore these impurities cannot be completely eliminated. These impurities are well known to those skilled in the art, and therefore not all of them are specifically mentioned in this description. In addition, further addition of active ingredients other than the above-mentioned ingredients is not completely excluded. .

In the high-strength steel sheet excellent in workability according to one aspect of the present invention, the fine structure may contain bainite, tempered martensite, fresh martensite, retained austenite and other inevitable organization.

Both untempered martensite (fresh martensite, FM) and tempered martensite (tempered martensite, TM) are fine structures that increase the strength of the steel sheet. However, compared with tempered martensite, young martensite has a characteristic of lowering the ductility and rim workability of the steel sheet. In addition, young martensite tends to lower the yield ratio of the steel sheet compared with tempered martensite. This is because the fine structure of the tempered martensite is softened by the tempering heat treatment. Therefore, in order to ensure the balance of tensile strength and elongation (TS ² *EL ^1/2 ), the balance of tensile strength and hole expansion ratio (TS ² *HER ^1/2 ) and yield ratio evaluation expected by the present invention The index (1-YR), preferably controls the structure fraction of tempered martensite and young martensite. In order to meet the balance of tensile strength and elongation (TS ² *EL ^1/2 ) of 3.0*10 ⁶ or more, and the 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, the fraction of tempered martensite is preferably limited to 50% by volume or more, and the fraction of young martensite is preferably limited to 10% by volume or more. A more preferable fraction of tempered martensite may be 52 volume % or more or 54 volume % or more, and a more preferable fraction of young martensite may be 12 volume % or more. On the other hand, when tempered martensite or young martensite is excessively formed, the ductility and rim workability are lowered, and eventually the balance of tensile strength and elongation of 3.0*10 ⁶ or more cannot be satisfied at the same time (TS ² *EL ^1/2 ), 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 may be limited to 70% by volume or less, and the fraction of young martensite may be limited to 30% by volume or less. A more preferred fraction of tempered martensite may be less than 68% by volume or less than 65% by volume, and a more preferred fraction of young martensite may be less than 25% by volume.

In order to ensure the balance of tensile strength and elongation (TS ² *EL ^1/2 ), the balance of tensile strength and hole expansion ratio (TS ² *HER ^1/2 ) and yield ratio evaluation at the desired level of the present invention Index (1-YR), the fraction of bainite that needs to be optimized. In order to ensure the balance of tensile strength and elongation (TS ² *EL ^1/2 ) of 3.0*10 ⁶ or more, and the 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 fraction of bainite to 10 volume % or more. A more preferable fraction of bainite may be 12 volume % or more or 14 volume % or more. On the other hand, when too much bainite is formed, the fraction of tempered martensite will eventually be reduced, so in order to ensure the desired balance of tensile strength and elongation (TS ² *EL ^1/2 ) , the balance of tensile strength and hole expansion rate (TS ² *HER ^1/2 ) and the yield strength 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, or 28 volume % or less or 26 volume % or less.

Steel sheets containing retained austenite have excellent ductility and workability due to transformation-induced plasticity that occurs when transforming from austenite to martensite during processing. When the fraction of retained austenite is less than a certain level, the balance of tensile strength and elongation (TS ² *EL ^1/2 ) is less than 3.0*10 ⁶ (MPa ² % ^1/2 ), which is not preferable. In addition, when the fraction of retained austenite exceeds a certain level, local elongation (Local Elongation) may decrease, or spot weldability may decrease. Therefore, in order to obtain a steel sheet excellent in balance of tensile strength and elongation (TS ² *EL ^1/2 ), the fraction of retained austenite may be limited to a range of 2-10% in the present invention. A preferred fraction of retained austenite may be 3 vol% or more or 8 vol% or less.

In the steel sheet of the present invention, ferrite, pearlite, island martensite (martensite-austenite composition (Martensite Austenite Constituent, M-A) etc. may be contained as unavoidable structure. When too much iron is formed During ferrite, the intensity of steel plate may reduce, so the fraction of ferrite can be limited to below 5 volume % (including 0%) in the present invention.And, when forming too much pearlite, the workability of steel plate will be reduced Reduced, or the fraction of retained austenite may be reduced, so the present invention aims to limit the formation of pearlite as much as possible.

A high-strength steel sheet excellent in workability according to an aspect of the present invention may satisfy the following [Relational Expression 1].

[relational expression 1]

0.03≤[B] _{FM /[B]TM≤0.55}

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

In order to ensure the desired balance of tensile strength and elongation (TS ² *EL ^1/2 ), balance of tensile strength and hole expansion ratio (TS ² *HER ^1/2 ) and yield ratio evaluation index (1- YR), the structure fraction of tempered martensite, young martensite and retained austenite can be controlled in a certain range in the present invention, and the boron (B) contained in tempered martensite and young martensite The proportion of the content is controlled within a certain range, and the proportion of the retained austenite relative to the specific size, shape and type of the retained austenite is controlled within a certain range.

In the present invention, as shown in [Relational Equation 1], the content of boron (B) contained in young martensite ([B] _FM , % by weight) is compared with the content of boron (B) contained in tempered martensite The ratio of content ([B] _TM , weight %) is controlled in the range of 0.03 to 0.55, so the tensile strength and elongation of 3.0*10 ⁶ to 6.2*10 ⁶ (MPa ² % ^1/2 ) can be ensured at the same time Balance (B _TE ), tensile strength and hole expansion ratio (B _TH ) of 6.0*10 ⁶ to 11.5*10 ⁶ (MPa ² % ^1/2 ), and yield ratio evaluation index (I _YR ) of 0.15 to 0.42 ).

As a result of the inventors of the present invention's intensive research on a method for securing the physical properties of boron (B)-added TRIP steel, although the theoretical basis has not yet been clearly elucidated, it has been noted that only when boron (B) contained in the nascent martensite When the ratio of the content to the content of boron (B) contained in the tempered martensite satisfies a certain range, desired physical properties of the present invention can be ensured. In particular, it was confirmed that the yield ratio of the steel sheet showed a certain tendency depending on the content ratio of boron (B) contained in the tempered martensite and the young martensite. Therefore, in the present invention, as shown in [Relational Expression 1], the ratio of the boron (B) content contained in the young martensite to the boron (B) content contained in the tempered martensite is limited to 0.03 to 0.55 Therefore, the desired balance of tensile strength and elongation (TS ² *EL ^1/2 ), balance of tensile strength and hole expansion ratio (TS ² *HER ^1/2 ) and yield ratio evaluation can be ensured Index (1-YR).

In the high-strength steel sheet excellent in workability according to an aspect of the present invention, the balance (B _TE ) of tensile strength and elongation represented by the following [Relational Expression 2] may satisfy 3.0*10 ⁶ to 6.2*10 ⁶ ( MPa ² % ^1/2 ), and the balance (B _TH ) of tensile strength and hole expansion rate expressed by the following [Relational Expression 3] can satisfy 6.0*10 ⁶ to 11.5*10 ⁶ (MPa ² % ^1/2 ) , and the yield ratio evaluation index (I _YR ) represented by the following [Relational Expression 4] may satisfy 0.15 to 0.42.

[relational expression 2]

B _TE = [tensile strength (TS, MPa)] ² * [elongation (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 strength ratio (YR)]

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

A method of manufacturing a high-strength steel sheet according to an aspect of the present invention may include the steps of: heating a cold-rolled steel sheet having a predetermined alloy composition to 700° C. at an average heating rate of 5° C./second or more (primary heating), Heating to the temperature range of Ac3 to 920°C at an average heating rate of °C/sec or less (secondary heating), and then holding for 50-1200 seconds (primary hold); cooling the steel plate held for the first time at an average rate of 1°C/sec or more Speed cooling to a temperature range of 200-400°C (primary cooling); heating the steel plate once cooled to a temperature range of 350-550°C at an average heating rate of 5°C/s or more (three heatings), and then holding for 50 seconds The above (secondary holding); the steel sheet held for the second time is cooled to normal temperature at an average cooling rate of 1° C./second or more (secondary cooling).

The cold-rolled steel sheet can be provided by the following steps: heating a billet with a predetermined alloy composition to 1000-1350°C; performing hot finish rolling in the temperature range of 800-1000°C; , coiling the hot-rolled steel plate; pickling the coiled steel plate; and cold-rolling the pickled steel plate at a reduction rate of 30-90%.

Billet preparation and heating

A billet 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 above-mentioned steel sheet, the description of the alloy composition of the steel sheet is substituted for the description of the alloy composition of the steel sheet.

The prepared steel slab can be heated to a certain temperature range, and the heating temperature of the steel slab at this time can be in the range of 1000-1350°C. When the heating temperature of the slab is lower than 1000°C, hot rolling may be performed in a temperature range below the desired hot finish rolling temperature range, and when the heating temperature of the slab exceeds 1350°C, it may melt due to reaching the melting point of steel.

Hot rolled and coiled

The heated steel slab can be hot-rolled to provide a hot-rolled steel sheet. The hot finish rolling temperature during hot rolling is preferably in the range of 800-1000°C. Excessive rolling load may become a problem when the hot finish rolling temperature is lower than 800°C, and when the hot finish rolling temperature exceeds 1000°C, coarse grains of the hot rolled steel sheet are formed, thus possibly causing the physical properties of the final steel sheet decrease.

The hot-rolled steel sheet that has been hot-rolled can be cooled at an average cooling rate above 10°C/s, and can be coiled at a temperature range of 350-650°C. This is because, when the coiling temperature is lower than 350°C, coiling is not easy, and when the coiling temperature exceeds 650°C, surface scale is formed inside the hot-rolled steel sheet, so pickling may be difficult.

pickling and cold rolling

After the coiled hot-rolled coil is uncoiled, it may be pickled and cold-rolled in order to remove scale formed on the surface of the steel sheet. The conditions of pickling and cold rolling are not particularly limited in the present invention, but cold rolling is preferably performed at a total reduction ratio of 30-90%. When the total reduction ratio 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 made into an uncoated cold-rolled steel sheet through an annealing heat treatment process, or can be made into a coated steel sheet through a plating process in order to impart corrosion resistance. Plating methods such as hot-dip galvanizing, electro-galvanizing, and hot-dip aluminizing can be used, and the methods and types thereof are not particularly limited.

Annealing heat treatment

In the present invention, an annealing heat treatment process is performed in order to ensure both the strength and workability of the steel sheet.

The cold-rolled steel sheet is heated to 700°C at an average heating rate of 5°C/s or more (primary heating), and heated to a temperature range from Ac3 to 920°C at an average heating rate of 5°C/s or less (secondary heating), and then Hold for 50-1200 seconds (one hold).

When the average heating rate of one heating to 700°C is less than 5°C/sec, the massive austenite is formed from the ferrite and cementite formed during the heating process, and as a result, the final structure cannot form a fine refractory structure. fire martensite and retained austenite. Therefore, the desired balance of tensile strength and elongation (TS ² *EL ^1/2 ) and the balance of tensile strength and hole expansion ratio (TS ² *HER ^1/2 ) cannot be achieved. In addition, when the secondary heating rate until the primary holding temperature exceeds 5°C/s, the cementite formed during the heating process is accelerated to transform into austenite, forming a large amount of massive austenite, and finally the structure is coarsened , and boron (B) cannot be sufficiently enriched in tempered martensite. Therefore, [B] _FM /[B] _TM exceeds 0.55, and the balance of tensile strength and elongation (TS ² *EL ^1/2 ), balance of tensile strength and hole expansion ( TS ² *HER ^1/2 ) and yield ratio evaluation index (I _YR ).

When the primary holding temperature is less than Ac3 (two-phase region), more than 5% by volume of ferrite is formed, so the balance of tensile strength and elongation (TS ² *EL ^1/2 ) and the balance of tensile strength and porosity Balance (TS ² *HER ^1/2 ) may decrease. In addition, when the holding time at one time is less than 50 seconds, the structure cannot be sufficiently homogenized, so the physical properties of the steel sheet may be lowered. The upper limits of the primary holding temperature and primary holding time are not particularly limited, but in order to prevent the decrease in toughness due to grain coarsening, the primary holding temperature is preferably limited to 920°C or less, and the primary holding time is preferably limited to 1200 seconds or less.

After the primary holding, it can be cooled to a primary cooling termination temperature of 200-400° C. (primary cooling) at an average cooling rate of 1° C./second or more. When the average cooling rate of primary cooling is less than 1°C/sec, the fraction of retained austenite becomes insufficient due to slow cooling, so the balance of tensile strength and elongation (TS ² *EL ^1/2 ) of the steel sheet may be reduce. The upper limit of the average cooling rate in primary cooling is not particularly specified, but is preferably 100°C/sec or less. When the primary cooling termination temperature is lower than 200 °C, too much tempered martensite is formed, and the retained austenite is insufficient, so the balance of tensile strength and elongation (TS ² *EL ^1/2 ) and tensile strength of the steel plate The balance of tensile strength and hole expansion ratio (TS ² *HER ^1/2 ) may decrease. On the other hand, when the primary cooling termination temperature exceeds 400°C, too much bainite is formed, and tempered martensite is insufficient, so the balance of tensile strength and elongation of the steel plate (TS ² *EL ^1/2 ) And the balance of tensile strength and porosity (TS ² *HER ^1/2 ) may decrease.

After secondary cooling, it may be heated to a temperature range of 350-550° C. at an average heating rate of 5° C./sec or more (tertiary heating), and then held for 50 seconds or more (secondary hold). The upper limit of the average heating rate for three times of heating is not particularly specified, but is preferably 100° C./sec or less. When the secondary holding temperature is lower than 350° C. or the secondary holding time is less than 50 seconds, too much tempered martensite is formed, so it is difficult to ensure the fraction of retained austenite. As a result, the balance between tensile strength and elongation (TS ² *EL ^1/2 ) and the balance between tensile strength and hole expansion ratio (TS ² *HER ^1/2 ) may decrease. When the secondary holding temperature exceeds 550°C or the secondary holding time exceeds 155000 seconds, the fraction of retained austenite is insufficient, so the balance of tensile strength and elongation (TS ² *EL ^1/2 ) of the steel sheet may decrease.

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

In the high-strength steel sheet excellent in workability produced by the above-mentioned production method, the fine structure may contain bainite, tempered martensite, young martensite, retained austenite and other unavoidable structures, as a preferable one Examples, by volume fraction, may contain 10-30% bainite, 50-70% tempered martensite, 10-30% young martensite, 2-10% retained austenite, 5 % below (including 0%) of ferrite.

In the steel sheet manufactured by the above manufacturing method, the balance of tensile strength and elongation (B _TE ) represented by the following [Relational Expression 2] may satisfy 3.0*10 ⁶ to 6.2*10 ⁶ (MPa ² % ^1/2 ) , the balance (B _TH ) of tensile strength and porosity expressed by the following [Relational Expression 3] can satisfy 6.0*10 ⁶ to 11.5*10 ⁶ (MPa ² % ^1/2 ), given by the following [Relational Expression 4] The expressed yield ratio evaluation index (I _YR ) can satisfy 0.15 to 0.42.

[relational expression 2]

B _TE = [tensile strength (TS, MPa)] ² * [elongation (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 strength ratio (YR)]

Detailed ways

Hereinafter, the high-strength steel sheet excellent in workability and its manufacturing method 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 only for understanding the present invention, and are not intended to limit the scope of rights of the present invention. The scope of rights of the present invention is determined by the content described in the claims and the content reasonably derived therefrom.

(Example)

A steel billet with a thickness of 100mm having an alloy composition (the balance being Fe and unavoidable impurities) described in Table 1 below was produced, heated at 1200°C, and then hot finish rolled at 900°C. Thereafter, cooling was performed at an average cooling rate of 30° C./sec, and coiling was performed at the coiling temperatures shown in Table 2 and Table 3 to manufacture hot-rolled steel sheets with a thickness of 3 mm. After that, it is pickled to remove surface scale, and then cold rolled to a thickness of 1.5mm.

Thereafter, heat treatment was performed under the annealing heat treatment conditions described in Table 2 to Table 5 below, thereby manufacturing steel sheets. In Table 2 and Table 3 below, the single-phase region represents the temperature range from Ac3 to 920°C, and the two-phase region represents the temperature range below Ac3°C.

The microstructures of the steel sheets produced as described above were observed, and the results are shown in Tables 6 and 7. The section of the polished test piece was etched with nitric acid alcohol solution, and then the ferrite (F), bainite (B), tempered martensite (TM), and new martensite (FM) in the microstructure were observed by SEM. ) and pearlite (P). After the nital solution etching, the structure having no unevenness on the surface of the test piece was classified as ferrite, and the structure having a layered structure of cementite and ferrite was classified as pearlite. Both bainite (B) and tempered martensite (TM) are observed in lath-like and massive shapes, making it difficult to distinguish, so bainite and tempered martensite are calculated using expansion curves after expansion evaluation Fraction. That is, a value obtained by subtracting the fraction of tempered martensite calculated from the expansion curve from the fraction of bainite and tempered martensite measured by SEM observation was determined as the fraction of bainite. Additionally, fresh martensite (FM) and retained austenite (retained gamma) are also difficult to distinguish, so the fractions of martensite and retained austenite observed by the SEM will be subtracted by X-ray diffraction The value of the calculated fraction of retained austenite was determined as the fraction of young martensite.

In addition, [B] _FM /[B] _TM of the steel plate, the balance of tensile strength and elongation (TS ² *EL ^1/2 ), the balance of tensile strength and hole expansion ratio (TS ² *HER ^1/2 ) and Yield Ratio Evaluation Index (I _YR ) were measured and evaluated, and the results are shown in Table 8 and Table 9.

The boron (B) content in young martensite ([B] _FM ) and the boron (B) content in tempered martensite ([B] _TM ) were determined using an Electron Probe MicroAnalyser (Electron Probe MicroAnalyser, EPMA) Concentration of boron (B) measured in young and tempered martensite.

Tensile strength (TS) and elongation (El) are evaluated by a tensile test, based on the direction of 90° with respect to the rolling direction of the rolled plate, and the test piece is taken and evaluated according to JIS No. 5 standard, so that Tensile strength (TS) and elongation (El) were measured. The hole expansion rate (HER) is evaluated by the hole expansion test. After forming a punching hole of 10mmΨ (the inner diameter of the mold is 10.3mm, and the gap is 12.5%), the conical punch with a vertex angle of 60° is placed along the burr of the punching hole. (burr) is inserted into the punched hole in the direction of the outside, and the peripheral part of the punched hole is squeezed and expanded at a moving speed of 20 mm/min, and then calculated using the following [Relational Expression 5].

[relational formula 5]

Hole expansion rate (HER,%)={(DD ₀ )/D ₀ }×100

In the relational expression 5, D represents the pore diameter (mm) when the crack penetrates the steel plate along the thickness direction, and D ₀ represents the initial pore diameter (mm).

[Table 1]

[Table 2]

[table 3]

[Table 4]

[table 5]

[Table 6]

[Table 7]

[Table 8]

[Table 9]

As shown in Tables 1 to 9, it can be seen that in the case of the test piece satisfying the conditions proposed in the present invention, [relational expression 1] is satisfied, and the balance between tensile strength and elongation (B _TE ) satisfies 3.0*10 ⁶ To 6.2*10 ⁶ (MPa ² % ^1/2 ), the balance of tensile strength and hole expansion ratio (B _TH ) meets 6.0*10 ⁶ to 11.5*10 ⁶ (MPa ² % ^1/2 ), yield ratio evaluation The index (I _YR ) satisfies 0.15 to 0.42.

In the test piece 2, the primary average heating rate was less than 5°C/sec, so tempered martensite and retained austenite were insufficient. As a result, the balance of tensile strength and elongation (B _TE ) of the test piece 2 was less than 3.0*10 ⁶ , and the balance of tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

In the test piece 3, the secondary average heating rate exceeded 5°C/sec, so massive austenite was formed, and boron (B) was not enriched in the tempered martensite. As a result, [B] _FM /[B] _TM of test piece 3 exceeded 0.55, the yield ratio evaluation index (I _YR ) exceeded 0.42, the balance between tensile strength and elongation (B _TE ) was less than 3.0*10 ⁶ , and the tensile The balance (B _TH ) of tensile strength and hole expansion rate is less than 6.0*10 ⁶ .

In test piece 4, it was carried out in the two-phase region where the primary temperature was kept lower than Ac3, so the fraction of ferrite was exceeded. As a result, the balance of tensile strength and elongation (B _TE ) of test piece 4 was less than 3.0*10 ⁶ , and the balance of tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

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

In test piece 6, the primary cooling termination temperature was lower than 200° C., so the fraction of tempered martensite was excessive, and the fraction of retained austenite was insufficient. As a result, the balance of tensile strength and elongation (B _TE ) of test piece 6 was less than 3.0*10 ⁶ , and the balance of tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

In test piece 7, the primary cooling termination temperature exceeded 400° C., so the fraction of bainite was excessive, and the fraction of tempered martensite was insufficient. As a result, the balance of tensile strength and elongation (B _TE ) of test piece 7 was less than 3.0*10 ⁶ , and the balance of tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

In test piece 8, the secondary holding temperature was lower than 350° C., so the fraction of tempered martensite was excessive, and the fraction of retained austenite was insufficient. As a result, the balance of tensile strength and elongation (B _TE ) of test piece 8 was less than 3.0*10 ⁶ , and the balance of tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

In test piece 9, the secondary holding temperature exceeded 550° C., so the fraction of retained austenite was insufficient. As a result, the balance (B _TE ) of the tensile strength and elongation of the test piece 9 was less than 3.0*10 ^{6 .}

In the test piece 10, the secondary holding time was less than 50 seconds, so the fraction of tempered martensite was excessive, and the fraction of retained austenite was insufficient. As a result, the balance of tensile strength and elongation (B _TE ) of the test piece 10 was less than 3.0*10 ⁶ , and the balance of tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ^{6 .}

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

In test piece 33, the content of carbon (C) is low, so the balance of tensile strength and elongation (B _TE ) is less than 3.0*10 ⁶ , and the balance of tensile strength and hole expansion rate (B _TH ) is less than 6.0*10 ⁶ .

In the test piece 34, the carbon (C) content was high, so the fraction of tempered martensite was insufficient, and the fraction of young martensite was excessive, and the fraction of retained austenite was excessive. As a result, the balance of tensile strength and elongation (B _TE ) of the test piece 34 was less than 3.0*10 ⁶ , and the balance of tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

In the test piece 35, the silicon (Si) content was low, so the fraction of retained austenite was insufficient. As a result, the balance between tensile strength and elongation (B _TE ) of the test piece 35 was less than 3.0*10 ⁶ .

In the test piece 36, the content of silicon (Si) was high, so the fraction of young martensite exceeded. As a result, the balance between tensile strength and elongation (B _TE ) of the test piece 36 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 .

In the test piece 37, the content of aluminum (Al) was high, so the fraction of young martensite exceeded. As a result, the balance between tensile strength and elongation (B _TE ) of the test piece 37 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 .

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

In test piece 39, the content of manganese (Mn) was high, so the fraction of young martensite exceeded. As a result, the balance of tensile strength and elongation (B _TE ) of test piece 39 was less than 3.0*10 ⁶ , and the balance of tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

In the test piece 40, the chromium (Cr) content is high, so the fraction of young martensite exceeds. As a result, the balance between tensile strength and elongation (B _TE ) of the 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 .

In the test piece 41, the molybdenum (Mo) content was high, so the fraction of young martensite exceeded. As a result, the balance of tensile strength and elongation (B _TE ) of the test piece 41 was less than 3.0*10 ⁶ , and the balance of tensile strength and hole expansion rate (B _TH ) was less than 6.0*10 ⁶ .

In the test piece 42, the boron (B) content was low, so boron (B) could not be enriched in the tempered martensite. As a result, [B] _FM /[B] _TM of the test piece 42 exceeded 0.55, and the yield ratio evaluation index (I _YR ) exceeded 0.42.

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

As mentioned above, the present invention has been described in detail through the examples, but other forms of examples are also possible. Therefore, the technical idea and scope of the claims are not limited to the embodiments.

Claims (7)

1. A high-strength steel plate with excellent workability, in % by weight, the steel plate comprises: C: 0.1-0.25%, Si: 0.01-1.5%, 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%, the balance of Fe and unavoidable impurities, the microstructure includes bainite, tempered martensite, newborn martensite Tentenite, retained austenite and other unavoidable structures, the steel plate satisfies the following [relational formula 1], [relational expression 1] 0.03≤[B] _{FM /[B]TM≤0.55} In the relational expression 1, [B] _FM is the content of boron (B) contained in the young martensite, wherein the unit of the content is % by weight; [B] _TM is the content of boron (B) contained in the tempered martensite (B) content, wherein, the unit of content is % by weight. 2. The high-strength steel plate with excellent workability according to claim 1, wherein, in weight %, the steel plate further comprises any one or more of the following (1) to (8): (1) More than one of Ti: 0-0.5%, Nb: 0-0.5% and V: 0-0.5%, (2) More than one of Cr: 0-3.0% and Mo: 0-3.0%, (3) More than one of Cu: 0-4.0% and Ni: 0-4.0%, (4) Ca: 0-0.05%, REM except Y: 0-0.05% and Mg: 0-0.05% or more, (5) More than one of W: 0-0.5% and Zr: 0-0.5%, (6) More than one of Sb: 0-0.5% and Sn: 0-0.5%, (7) More than one of Y: 0-0.2% and Hf: 0-0.2%, (8) Co: 0-1.5%. 3. The high-strength steel plate with excellent workability according to claim 1, wherein, in terms of volume fraction, the microstructure of the steel plate comprises: 10-30% bainite, 50-70% tempered martensite body, 10-30% new martensite, 2-10% retained austenite, 5% or less and including 0% ferrite. 4. The high-strength steel sheet excellent in workability according to claim 1, wherein, in the steel sheet, the balance B _TE of tensile strength and elongation represented by the following [Relational Expression 2] satisfies 3.0* ¹⁰⁶ to 6.2*10 ⁶ (MPa ² % ^1/2 ), the balance B _TH of the tensile strength and hole expansion rate expressed by the following [Relational Expression 3] satisfies 6.0*10 ⁶ to 11.5*10 ⁶ (MPa ² % ^1/2 ), the yield strength ratio evaluation index I _YR expressed by the following [relational formula 4] satisfies 0.15 to 0.42, [relational expression 2] B _TE = [tensile strength (TS, MPa)] ² * [elongation (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). 5. A method for manufacturing a high-strength steel plate with excellent workability, comprising the steps of: A cold-rolled steel sheet is provided. In weight percent, the steel sheet includes: C: 0.1-0.25%, Si: 0.01-1.5%, Mn: 1.0-4.0%, Al: 0.01-1.5%, P: less than 0.15%, S: less than 0.03%, N: less than 0.03%, B: 0.0005-0.005%, the balance of Fe and unavoidable impurities; The cold-rolled steel sheet is heated to 700°C at an average heating rate of 5°C/sec or more (primary heating), and heated to a temperature range of Ac3 to 920°C at an average heating rate of 5°C/sec or less (secondary heating) , and then hold for 50-1200 seconds (one hold); cooling the steel plate held once to a temperature range of 200-400° C. at an average cooling rate of 1° C./second or more (primary cooling); heating the steel plate cooled once to a temperature range of 350-550° C. at an average heating rate of 5° C./second or more (three heatings), and then holding it for more than 50 seconds (secondary holding); and The steel sheet held for the second time is cooled to normal temperature at an average cooling rate of 1° C./second or more (secondary cooling). 6. The method for manufacturing a high-strength steel plate with excellent workability according to claim 5, wherein the steel slab further comprises any one or more of the following (1) to (8): (1) More than one of Ti: 0-0.5%, Nb: 0-0.5% and V: 0-0.5%, (2) More than one of Cr: 0-3.0% and Mo: 0-3.0%, (3) More than one of Cu: 0-4.0% and Ni: 0-4.0%, (4) Ca: 0-0.05%, REM except Y: 0-0.05% and Mg: 0-0.05% or more, (5) More than one of W: 0-0.5% and Zr: 0-0.5%, (6) More than one of Sb: 0-0.5% and Sn: 0-0.5%, (7) More than one of Y: 0-0.2% and Hf: 0-0.2%, (8) Co: 0-1.5%. 7. The method for manufacturing a high-strength steel sheet excellent in workability according to claim 5, wherein the cold-rolled steel sheet is provided by the following steps: Heating the billet to 1000-1350°C; Hot finish rolling in the temperature range of 800-1000°C; Coiling the hot-rolled steel plate within a temperature range of 350-650°C; pickling the rolled steel plate; and The pickled steel sheet is cold-rolled at a reduction ratio of 30-90%.