KR20210078603A - High strength steel sheet having excellent workability and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a steel sheet that can be used for automobile parts, etc., and relates to a steel sheet having excellent balance between strength and ductility, excellent balance between strength and hole expandability, and excellent bending workability, and a method for manufacturing the same. The steel sheet of the present invention contains 0.25 to 0.75 wt% of C, 4.0 wt% or less of Si, 0.9 to 5.0 wt% of Mn, 5.0 wt% or less of Al, 0.15 wt% or less of P, 0.03 wt% or less of S, 0.03 wt% or less of N, and the balance Fe and unavoidable impurities, and includes, as the microstructure, 30 to 70 vol% of tempered martensite, 10 to 45 vol% of bainite, 10 to 40 vol% of residual austenite, 3 to 20 vol% ferrite, and unavoidable structures.

Description

High strength steel sheet having excellent workability and method for manufacturing the same}

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

Recently, the automobile industry is paying attention to ways to reduce material weight and secure occupant stability in order to protect the global environment. In order to meet these demands for stability and weight reduction, the application of high-strength steel sheets is rapidly increasing. In general, it is known that as the strength of the steel sheet increases, the workability of the steel sheet decreases. Therefore, in the steel sheet for automobile parts, a steel sheet having high strength characteristics and excellent workability typified by ductility, bending workability, and hole expandability is required.

As a technique for improving the workability of a steel sheet, a method of utilizing tempered martensite is disclosed in Patent Documents 1 and 2. Since tempered martensite made by tempering hard martensite is soft martensite, there is a difference in strength between tempered martensite and existing untempered martensite (fresh martensite). Therefore, when fresh martensite is suppressed and tempered martensite is formed, workability may be increased.

However, with the techniques disclosed in Patent Documents 1 and 2, the balance between tensile strength and elongation (TSХEl) does not satisfy 22,000 MPa% or more, which means that it is difficult to secure a steel sheet having excellent strength and ductility.

On the other hand, TRIP (Transformation Induced Plasticity) steel using the transformation-induced plasticity of retained austenite has been developed in order to obtain both high strength and excellent workability for automobile member steel sheets. Patent Document 3 discloses TRIP steel having excellent strength and workability.

In Patent Document 3, including polygonal ferrite, retained austenite and martensite, it was attempted to improve ductility and workability, but bainite is the main phase, so high strength cannot be secured, and the tensile strength and elongation It can be seen that the balance (TSХEl) also does not satisfy 22,000 MPa% or more.

That is, while having high strength, the demand for a steel sheet having excellent workability, such as ductility, bendability, and hole expandability, is not satisfied.

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

According to one aspect of the present invention, a high-strength steel sheet having excellent ductility, bendability and hole expandability by optimizing the composition and microstructure of the steel sheet and a method for manufacturing the same can be provided.

The subject of the present invention is not limited to the above matters. Additional problems of the present invention are described in the overall content of the specification, and those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional problems of the present invention from the contents described in the specification of the present invention.

High-strength steel sheet excellent in workability according to an aspect of the present invention, by weight, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, including the remaining Fe and unavoidable impurities, as a microstructure, 30-70 vol% tempered martensite, 10-45 vol% bainite, 10-40 vol% of retained austenite, 3 to 20 vol% of ferrite and an unavoidable structure, and the following [Relational Expression 1] may be satisfied.

[Relational Expression 1]

1.02 ≤ [Si+Al] _F / [Si+Al] _av ≤ 1.45

In Relation 1, [Si+Al] _F is the average total content of Si and Al contained in the ferrite (weight%), and [Si+Al] _av is the average total content of Si and Al contained in the steel sheet (weight%) )to be.

The steel plate may further include any one or more of the following (1) to (9).

(1) Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5% at least one of

(2) at least one of Cr: 0 to 3.0% and Mo: 0 to 3.0%

(3) at least one of Cu: 0 to 4.5% and Ni: 0 to 4.5%

(4) B: 0~0.005%

(5) Ca: 0 to 0.05%, REM except for Y: 0 to 0.05%, and Mg: at least one of 0 to 0.05%

(6) at least one of W: 0 to 0.5% and Zr: 0 to 0.5%

(7) at least one of Sb: 0 to 0.5% and Sn: 0 to 0.5%

(8) at least one of Y: 0 to 0.2% and Hf: 0 to 0.2%

(9) Co: 0~1.5%

The total content of Si and Al (Si+Al) may be 1.0 to 6.0 wt%.

_{The steel sheet has a balance (B T E} ) of tensile strength and elongation expressed by the following [Relational Expression 2] of 22,000 (MPa%) or more, and the tensile strength and hole expansion rate expressed by [Relational Expression 3] below the balance _{(B T · H) 7 *} 10 6 (MPa 2% 1/2) is over, it can be [expression 4] range of bending rate (B _R) of 0.5 to 3.0 represented by the following.

[Relational Expression 2]

B _{T E} = [Tensile strength (TS, MPa)] * [Elongation (El, %)]

[Relational Expression 3]

B _{T H} = [Tensile strength (TS, MPa)] ² * [Hole expansion rate (HER, %)] ^1/2

[Relational Expression 4]

B _R = R/t

In Relation 4, R means the minimum bending radius (mm) at which cracks do not occur after the 90° bending test, and t means the thickness (mm) of the steel sheet.

High-strength steel sheet excellent in workability according to another aspect of the present invention, by weight, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less , S: 0.03% or less, N: 0.03% or less, the remainder is a step of heating a steel slab containing Fe and unavoidable impurities, and hot rolling; winding the hot-rolled steel sheet; performing hot rolling annealing heat treatment on the wound steel sheet in a temperature range of 650 to 850°C for 600 to 1700 seconds; cold rolling the hot-rolled annealing heat-treated steel sheet; heating (primary heating) the cold-rolled steel sheet to a temperature range of Ac1 or more and less than Ac3 at an average temperature increase rate of 5° C./s or more, and maintaining (primary maintenance) for 50 seconds or more; cooling (primary cooling) to a temperature range of 100 to 300°C at an average cooling rate of 1°C/s or more; heating (secondary heating) the first cooled steel sheet to a temperature range of 300 to 500°C, and maintaining (secondary maintenance) for 50 seconds or more; and cooling to room temperature (secondary cooling).

The steel slab may further include any one or more of the following (1) to (9).

(1) Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5% at least one of

(2) at least one of Cr: 0 to 3.0% and Mo: 0 to 3.0%

(3) at least one of Cu: 0 to 4.5% and Ni: 0 to 4.5%

(4) B: 0~0.005%

(5) Ca: 0 to 0.05%, REM except for Y: 0 to 0.05%, and Mg: at least one of 0 to 0.05%

(6) at least one of W: 0 to 0.5% and Zr: 0 to 0.5%

(7) at least one of Sb: 0 to 0.5% and Sn: 0 to 0.5%

(8) at least one of Y: 0 to 0.2% and Hf: 0 to 0.2%

(9) Co: 0~1.5%

The total content of Si and Al contained in the steel slab (Si+Al) may be 1.0 to 6.0% by weight.

The steel slab is heated to a temperature range of 1000 ~ 1350 ℃, it may be finish hot rolling in a temperature range of 800 ~ 1000 ℃.

The hot-rolled steel sheet may be wound in a temperature range of 300 ~ 600 ℃.

The rolling reduction of the cold rolling may be 30 to 90%.

The cooling rate of the secondary cooling may be 1°C/s or more.

According to a preferred aspect of the present invention, it is possible to provide a steel sheet particularly suitable for automobile parts because it has excellent strength as well as excellent workability such as ductility, bending workability and hole expandability.

The present invention relates to a high-strength steel sheet having excellent workability and a method for manufacturing the same, and preferred embodiments of the present invention will be described below. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided in order to further detail the present invention to those of ordinary skill in the art to which the present invention pertains.

The inventors of the present invention, in transformation induced plasticity (TRIP) steel containing bainite, tempered martensite, retained austenite and ferrite, promotes the stabilization of retained austenite and, at the same time, retained austenite and ferrite When the ratio of a specific component included in the ferrite is controlled within a certain range, it was recognized that it is possible to simultaneously secure the workability and strength of the steel sheet by reducing the difference in hardness between the phases of retained austenite and ferrite. By identifying this, a method for improving the ductility and workability of high-strength steel was devised, leading to the present invention.

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

High-strength steel sheet excellent in workability according to an aspect of the present invention, by weight, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, including the remaining Fe and unavoidable impurities, as a microstructure, 30-70 vol% tempered martensite, 10-45 vol% bainite, 10-40 vol% of retained austenite, 3 to 20 vol% of ferrite and an unavoidable structure, and the following [Relational Expression 1] may be satisfied.

[Relational Expression 1]

1.02 ≤ [Si+Al] _F / [Si+Al] _av ≤ 1.45

In Relation 1, [Si+Al] _F is the average total content of Si and Al contained in the ferrite (weight%), and [Si+Al] _av is the average total content of Si and Al contained in the steel sheet (weight%) )to be.

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

High-strength steel sheet excellent in workability according to an aspect of the present invention, by weight, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, remaining Fe and unavoidable impurities, and additionally Ti: 0.5% or less (including 0%), Nb: 0.5% or less (including 0%), V: 0.5% or less (including 0%), Cr: 3.0% or less (including 0%), Mo: 3.0% or less (including 0%), Cu: 4.5% or less (including 0%), Ni: 4.5% or less (including 0%) , B: 0.005% or less (including 0%), Ca: 0.05% or less (including 0%), REM excluding Y: 0.05% or less (including 0%), Mg: 0.05% or less (including 0%), W : 0.5% or less (including 0%), Zr: 0.5% or less (including 0%), Sb: 0.5% or less (including 0%), Sn: 0.5% or less (including 0%), Y: 0.2% or less (0%) %), Hf: 0.2% or less (including 0%), Co: 1.5% or less (including 0%) may further include one or more. In addition, the total content of Si and Al (Si+Al) may be 1.0 to 6.0%.

Carbon (C): 0.25 to 0.75%

Carbon (C) is an element essential for securing the strength of a steel sheet, and is also an element for stabilizing retained austenite, which contributes to the improvement of ductility of the steel sheet. Therefore, the present invention may contain 0.25% or more of carbon (C) to achieve such an effect. A preferred carbon (C) content may be greater than 0.25%, may be greater than 0.27%, and may be greater than or equal to 0.30%. More preferably, the carbon (C) content may be 0.31% or more. On the other hand, when the carbon (C) content exceeds a certain level, cold rolling may become difficult due to an excessive increase in strength. Therefore, the present invention may limit the upper limit of the carbon (C) content to 0.75%. The carbon (C) content may be 0.70% or less, and a more preferable carbon content (C) may be 0.67% or less.

Silicon (Si): 4.0% or less (excluding 0%)

Silicon (Si) is an element that contributes to strength improvement by solid solution strengthening, and is also an element that improves workability by strengthening ferrite and homogenizing the structure. In addition, silicon (Si) is an element contributing to generation of retained austenite by suppressing precipitation of cementite. Therefore, in the present invention, silicon (Si) may be necessarily added to achieve such an effect. A preferable silicon (Si) content may be 0.02% or more, and a more preferable silicon (Si) content may be 0.05% or more. However, when the silicon (Si) content exceeds a certain level, it not only causes a plating defect problem such as non-plating in the plating process, but also reduces the weldability of the steel sheet. The present invention provides an upper limit of the silicon (Si) content can be limited to 4.0%. A preferable upper limit of the silicon (Si) content may be 3.8%, and a more preferable upper limit of the silicon (Si) content may be 3.5%.

Aluminum (Al): 5.0% or less (excluding 0%)

Aluminum (Al) is an element that deoxidizes by combining with oxygen in steel. In addition, aluminum (Al) is also an element that suppresses cementite precipitation and stabilizes retained austenite, similarly to silicon (Si). Therefore, in the present invention, aluminum (Al) may be necessarily added to achieve such an effect. A preferable aluminum (Al) content may be 0.05% or more, and a more preferable aluminum (Al) content may be 0.1% or more. On the other hand, when aluminum (Al) is added excessively, inclusions of the steel sheet are increased, and the workability of the steel sheet can be reduced, so the present invention can limit the upper limit of the aluminum (Al) content to 5.0%. . The upper limit of the preferable aluminum (Al) content may be 4.75%, and the more preferable upper limit of the aluminum (Al) content may be 4.5%.

Meanwhile, the total content (Si+Al) of silicon (Si) and aluminum (Al) is preferably 1.0 to 6.0%. Since silicon (Si) and aluminum (Al) are components that affect microstructure formation in the present invention, affecting ductility, bendability and hole expandability, the total content of silicon (Si) and aluminum (Al) is 1.0~ It is preferably 6.0%. More preferably, the total content (Si+Al) of silicon (Si) and aluminum (Al) may be 1.5% or more, and may be 4.0% or less.

Manganese (Mn): 0.9~5.0%

Manganese (Mn) is a useful element for increasing both strength and ductility. Therefore, the present invention may limit the lower limit of the manganese (Mn) content to 0.9% in order to achieve such an effect. A preferred lower limit of the manganese (Mn) content may be 1.0%, and a more preferred lower limit of the manganese (Mn) content may be 1.1%. On the other hand, when manganese (Mn) is excessively added, the bainite transformation time increases, so that the carbon (C) concentration in the austenite is not sufficient, there is a problem that the target austenite fraction cannot be secured. Therefore, the present invention may limit the upper limit of the manganese (Mn) content to 5.0%. A preferable upper limit of the manganese (Mn) content may be 4.7%, and a more preferable upper limit of the manganese (Mn) content may be 4.5%.

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

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

Sulfur (S): 0.03% or less (including 0%)

Sulfur (S) is an element that is contained as an impurity to form MnS in the steel sheet and deteriorate 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 causes cracks in the slab by forming nitride during continuous casting as it is contained as an impurity. Therefore, the content of nitrogen (N) is preferably 0.03% or less.

On the other hand, the steel sheet of the present invention has an alloy composition that may be additionally included in addition to the above-described alloy components, which will be described in detail below.

Titanium (Ti): 0 to 0.5%, niobium (Nb): 0 to 0.5%, and vanadium (V): 0 to 0.5% at least one of

Titanium (Ti), niobium (Nb), and vanadium (V) are elements that make precipitates and refine crystal grains, and are elements that also contribute to the improvement of strength and impact toughness of a steel sheet, so the present invention provides titanium (Ti) for this effect. ), at least one of niobium (Nb) and vanadium (V) may be added. However, when the respective contents of titanium (Ti), niobium (Nb) and vanadium (V) exceed a certain level, excessive precipitates are formed to decrease impact toughness and increase manufacturing cost, so the present invention Silver may limit the content of titanium (Ti), niobium (Nb), and vanadium (V) to 0.5% or less, respectively.

Chromium (Cr): 0 to 3.0% and molybdenum (Mo): 0 to 3.0% at least one of

Chromium (Cr) and molybdenum (Mo) are elements that not only suppress austenite decomposition during alloying treatment, but also stabilize austenite in the same way as manganese (Mn), so the present invention provides chromium (Cr) and At least one of molybdenum (Mo) may be added. However, when the content of chromium (Cr) and molybdenum (Mo) exceeds a certain level, the bainite transformation time increases and the carbon (C) concentration in austenite becomes insufficient, so the desired retained austenite fraction is secured. Can not. Accordingly, the present invention may limit the content of chromium (Cr) and molybdenum (Mo) to 3.0% or less, respectively.

Copper (Cu): 0 to 4.5% and Nickel (Ni): 0 to 4.5% at least one of

Copper (Cu) and nickel (Ni) are elements that stabilize austenite and inhibit corrosion. In addition, copper (Cu) and nickel (Ni) are also elements that are concentrated on the surface of the steel sheet to prevent hydrogen intrusion from moving into the steel sheet, thereby suppressing delayed hydrogen destruction. Accordingly, in the present invention, at least one of copper (Cu) and nickel (Ni) may be added for such an effect. However, when the content of copper (Cu) and nickel (Ni) exceeds a certain level, it causes not only excessive characteristic effects, but also an increase in manufacturing cost. Therefore, in the present invention, the content of copper (Cu) and nickel (Ni) is increased, respectively. It can be limited to 4.5% or less.

Boron (B): 0~0.005%

Boron (B) is an element that improves hardenability to increase strength, and is also an element that suppresses nucleation of grain boundaries. Therefore, in the present invention, boron (B) may be added for this effect. However, when the content of boron (B) exceeds a certain level, it causes excessive characteristic effects as well as an increase in manufacturing cost, so the present invention may limit the content of boron (B) to 0.005% or less.

Calcium (Ca): 0 to 0.05%, Magnesium (Mg): 0 to 0.05%, and rare earth elements (REM) excluding yttrium (Y): at least one of 0 to 0.05%

Here, the rare earth element (REM) means scandium (Sc), yttrium (Y), and a lanthanide element. Since rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) are elements that contribute to the improvement of ductility of a steel sheet by spheroidizing sulfides, the present invention provides calcium (Ca), At least one of rare earth elements (REM) other than 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, it causes excessive characteristic effects as well as an increase in manufacturing cost, so the present invention provides calcium ( Ca), magnesium (Mg), and the content of rare earth elements (REM) excluding yttrium (Y) may be limited to 0.05% or less, respectively.

Tungsten (W): 0 to 0.5% and Zirconium (Zr): 0 to 0.5% at least one of

Since tungsten (W) and zirconium (Zr) are elements that increase the strength of a steel sheet by improving hardenability, in the present invention, one or more of tungsten (W) and zirconium (Zr) may be added for this effect. . However, when the content of tungsten (W) and zirconium (Zr) exceeds a certain level, it causes excessive characteristic effects as well as an increase in manufacturing cost, so the present invention sets the content of tungsten (W) and zirconium (Zr) to 0.5 % or less.

Antimony (Sb): 0 to 0.5% and Tin (Sn): 0 to 0.5% at least one of

Since antimony (Sb) and tin (Sn) are elements that improve the plating wettability and plating adhesion of the steel sheet, in the present invention, at least one of antimony (Sb) and tin (Sn) may be added for such an effect. However, when the content of antimony (Sb) and tin (Sn) exceeds a certain level, the brittleness of the steel sheet increases and cracks may occur during hot working or cold working, so the present invention provides antimony (Sb) and tin (Sn) ) may be limited to 0.5% or less, respectively.

At least one of yttrium (Y): 0-0.2% and hafnium (Hf): 0-0.2%

Since yttrium (Y) and hafnium (Hf) are elements that improve the corrosion resistance of the steel sheet, in the present invention, at least one of yttrium (Y) and hafnium (Hf) may be added for this effect. However, when the content of yttrium (Y) and hafnium (Hf) exceeds a certain level, the ductility of the steel sheet may be deteriorated, so the present invention sets the content of yttrium (Y) and hafnium (Hf) to 0.2% or less, respectively. can be limited

Cobalt (Co): 0~1.5%

Since cobalt (Co) is an element that increases the TRIP effect by promoting bainite transformation, cobalt (Co) may be added for this effect in the present invention. However, when the content of cobalt (Co) exceeds a certain level, since weldability and ductility of the steel sheet may be deteriorated, the present invention may limit the content of cobalt (Co) to 1.5% or less.

The high-strength steel sheet having excellent workability according to an aspect of the present invention may include remaining Fe and other unavoidable impurities in addition to the above-described components. However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, it cannot be completely excluded. Since these impurities are known to those of ordinary skill in the art, all contents thereof are not specifically mentioned in the present specification. In addition, additional addition of effective ingredients other than the above-mentioned ingredients is not entirely excluded.

The high-strength steel sheet having excellent workability according to an aspect of the present invention may include tempered martensite, bainite, retained austenite and ferrite as a microstructure. As a preferred example, the high-strength steel sheet having excellent workability according to an aspect of the present invention, by volume fraction, is 30 to 70% tempered martensite, 10 to 45% bainite, 10 to 40% retained austenite, It may contain 3-20% ferrite and unavoidable texture. As an unavoidable structure of the present invention, fresh martensite, perlite, martensite martensite (Martensite Austenite Constituent, M-A) and the like may be included. When fresh martensite or pearlite is excessively formed, the workability of the steel sheet may decrease or the fraction of retained austenite may be reduced.

The high-strength steel sheet having excellent workability according to an aspect of the present invention, as shown in the following [Relational Expression 1], the average total content of silicon (Si) and aluminum (Al) contained in the steel sheet ([Si+Al] _av , wt% ) to the average total content ([Si+Al] _F , wt%) of silicon (Si) and aluminum (Al) contained in ferrite may satisfy the range of 1.02 to 1.45.

[Relational Expression 1]

1.02 ≤ [Si+Al] _F / [Si+Al] _av ≤ 1.45

In addition, the high-strength steel sheet excellent in workability according to one aspect of the present invention has a balance (B _{T E} ) of tensile strength and elongation expressed by the following [Relational Expression 2] of 22,000 (MPa%) or more, and the following [Relational Expression 2] 3] the tensile strength and the hole expansion rate balance (B _{T · H)} 7 × 10 ^6, and (MPa ^{2% 1/2)} or more, the bending rate, expressed in [expression 4] below, in which is represented by (B _R ) satisfies the range of 0.5 to 3.0, so it can have excellent balance between strength and ductility and balance between strength and hole expandability, as well as have excellent bendability.

[Relational Expression 2]

B _{T E} = [Tensile strength (TS, MPa)] * [Elongation (El, %)]

[Relational Expression 3]

B _{T H} = [Tensile strength (TS, MPa)] ² * [Hole expansion rate (HER, %)] ^1/2

[Relational Expression 4]

B _R = R/t

In Relation 4, R means the minimum bending radius (mm) at which cracks do not occur after the 90° bending test, and t means the thickness (mm) of the steel sheet.

In the present invention, it is important to stabilize the retained austenite in the steel sheet because it is intended to simultaneously secure excellent ductility and bendability as well as high strength properties. In order to stabilize retained austenite, it is necessary to enrich carbon (C) and manganese (Mn) in ferrite, bainite, and tempered martensite of the steel sheet into austenite. However, if carbon (C) is concentrated in austenite by utilizing ferrite, the strength of the steel sheet may be insufficient due to the low strength characteristics of ferrite, and excessive interphase hardness difference may occur, thereby reducing the hole expansion rate (HER). Therefore, the present invention intends to enrich carbon (C) and manganese (Mn) into austenite by utilizing bainite and tempered martensite.

When the content of silicon (Si) and aluminum (Al) in retained austenite is limited to a certain range, carbon (C) and manganese (Mn) can be concentrated in large amounts from bainite and tempered martensite into retained austenite, Residual austenite can be effectively stabilized. In addition, by limiting the content of silicon (Si) and aluminum (Al) in austenite to a certain range, it is possible to increase the content of silicon (Si) and aluminum (Al) in ferrite. As the content of silicon (Si) and aluminum (Al) in ferrite increases, the hardness of ferrite increases, and the hardness difference between the phases of ferrite, which is a soft structure, and tempered martensite, bainite, and retained austenite, which is a hard structure, can be effectively reduced. .

Therefore, the present invention provides an average of silicon (Si) and aluminum (Al) contained in ferrite with respect to the average total content ([Si+Al] _av , wt%) of silicon (Si) and aluminum (Al) contained in the steel sheet. Since the ratio of the total content ([Si+Al] _F , wt%) is limited to 1.02 or more, it is possible to effectively reduce the hardness difference between the phases of the soft tissue and the hard tissue. On the other hand, if the content of silicon (Si) and aluminum (Al) in the ferrite is excessive, rather the ferrite is excessively hardened and the workability is deteriorated. Therefore, the desired balance between tensile strength and elongation (TSХEl), tensile strength and hole expansion rate balance (TS ² ХHER ^1/2) and the bending rate is impossible to ensure both (R / t). Therefore, the present invention relates to the average total content of silicon (Si) and aluminum (Al) contained in the steel sheet ([Si+Al] _av , weight %) of silicon (Si) and aluminum (Al) contained in ferrite according to the present invention. ) of the average total content ([Si+Al] _F , wt%) may be limited to 1.45 or less.

The steel sheet containing retained austenite has excellent ductility and bendability 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 (TSХEl) may be less than 22,000 MPa%, or the bending workability (R/t) may exceed 3.0. On the other hand, when the fraction of retained austenite exceeds a certain level, local elongation may be reduced. Therefore, in the present invention, in order to obtain a steel sheet excellent in the balance of tensile strength and elongation (TSХEl) as well as in the bending workability (R/t), the fraction of residual austenite can be limited in the range of 10 to 40% by volume.

On the other hand, both untempered martensite (fresh martensite) and tempered martensite are microstructures that improve the strength of the steel sheet. However, compared with tempered martensite, fresh martensite has a property of greatly reducing the ductility and hole expandability of the steel sheet. This is because the microstructure of tempered martensite is softened by the tempering heat treatment. Therefore, in the present invention, it is preferable to utilize tempered martensite in order to provide a steel sheet having excellent balance between strength and ductility, balance between strength and hole expandability, and bending workability. When the fraction of tempered martensite is less than a certain level, the balance of tensile strength and elongation of 22,000 MPa% or more (TSХEl) or the balance of tensile strength and hole expansion ratio of ^{7*10 6} (MPa ² % ^{1/2 ) or more (TS 2} ХHER) ^1/2 ) is difficult to secure, and when the fraction of tempered martensite exceeds a certain level, ductility and workability decrease, and the balance between tensile strength and elongation (TSХEl) is less than 22,000 MPa%, or the bending workability (R/ t) exceeding 3.0 is undesirable. Accordingly, the present invention is the balance of tensile strength and elongation (TSХEl), balance of tensile strength and the hole expansion rate (TS ² ХHER ^1/2) and a bending ratio (R / t) the tempered martensite in order to obtain a steel sheet excellent The fraction of can be limited in the range of 30 to 70 vol%.

In order to improve the balance of tensile strength and elongation (TSХEl), the balance of tensile strength and hole expansion rate (TS ² ХHER ^1/2 ), and the flexural workability (R/t), bainite is appropriately included as a microstructure. it is preferable Bainite fraction is only when more than a certain level, the balance of tensile strength and elongation than 22,000MPa% ^{(TSХEl), 7 * 10 6} (MPa 2% 1/2) or more of the balance of the tensile strength and the hole expansion rate (TS ² ХHER ^1/2) and 0.5 it is possible to ensure a bending ratio (R / t) of 3.0. On the other hand, if the fraction of bainite is excessive, a decrease in the tempered martensite fraction is necessarily accompanied, so the balance of tensile strength and elongation (TSХEl), the balance of tensile strength and hole expansion rate (TS ²⁾ ХHER ^1/2), and it is impossible to ensure the bending ratio (R / t). Therefore, the present invention can limit the fraction of bainite in the range of 10 to 45 vol%.

Since ferrite is an element contributing to ductility improvement, it is possible to secure the balance (TSХEl) between tensile strength and elongation, which is the purpose of the present invention, only when the fraction of ferrite is above a certain level. However, if the fraction of ferrite is excessive, the difference in hardness between phases increases and the hole expansion rate (HER) may be lowered. Therefore, the balance between tensile strength and hole expansion rate (TS ² ХHER ^1/2 ), which is the purpose of the present invention, is not achieved. can't get it Accordingly, the present invention may limit the fraction of ferrite to a range of 3 to 20 vol%.

Hereinafter, an example of a method for manufacturing a 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 includes the steps of preparing a steel slab having a predetermined component, heating the steel slab, and hot rolling; winding the hot-rolled steel sheet; performing hot rolling annealing heat treatment on the wound steel sheet in a temperature range of 650 to 850°C for 600 to 1700 seconds; cold rolling the hot-rolled annealing heat-treated steel sheet; heating (primary heating) the cold-rolled steel sheet to a temperature range of Ac1 or more and less than Ac3 at an average temperature increase rate of 5°C/s or more, and maintaining (primary maintenance) for 50 seconds or more; cooling (primary cooling) to a temperature range of 100 to 300°C at an average cooling rate of 1°C/s or more; heating (secondary heating) the first cooled steel sheet to a temperature range of 300 to 500°C, and maintaining (secondary maintenance) for 50 seconds or more; and cooling to room temperature (secondary cooling).

Steel slab preparation and heating

A steel slab having a predetermined component is prepared. Since the steel slab of the present invention has an alloy composition corresponding to the alloy composition of the steel plate described above, the description of the alloy composition of the steel slab is replaced with the description of the alloy composition of the steel plate described above.

The prepared steel slab may be heated to a certain temperature range, and the heating temperature of the steel slab at this time may be in the range of 1000 to 1350 °C. If the heating temperature of the steel slab is less than 1000℃, it may be hot rolled in the temperature range below the target finish hot rolling temperature range. If the heating temperature of the steel slab exceeds 1350℃, it will reach the melting point of the steel and melt. because it has potential.

hot rolled and wound

The heated steel slab may be hot rolled to provide a hot rolled steel sheet. The finish hot rolling temperature during hot rolling is preferably in the range of 800 to 1000 °C. If the finish hot rolling temperature is less than 800 °C, excessive rolling load may be a problem, and if the finish hot rolling temperature exceeds 1000 °C, coarse grains of the hot rolled steel sheet are formed, which may cause deterioration of the physical properties of the final steel sheet. Because.

The hot-rolled steel sheet after the hot rolling has been completed may be cooled at an average cooling rate of 10°C/s or more, and may be wound at a temperature of 300 to 600°C. If the coiling temperature is less than 300 ℃, winding is not easy, and when the coiling temperature exceeds 600 ℃, the surface scale (scale) is formed even inside the hot-rolled steel sheet This is because there is a possibility that it may make pickling difficult.

hot annealing heat treatment

It is preferable to perform a hot rolling annealing heat treatment process to facilitate pickling and cold rolling, which are subsequent processes after winding. The hot rolling annealing heat treatment can be performed for 600 to 1700 seconds in a temperature range of 650 to 850 °C. When the hot rolling annealing heat treatment temperature is less than 650 ° C. or less than 600 seconds, which is the hot rolling annealing heat treatment time, the strength of the hot rolling annealing heat treated steel sheet is high, and subsequent cold rolling may not be easy. On the other hand, when the hot-rolling annealing heat treatment temperature exceeds 850° C. or exceeds 1700 seconds, which is the hot-rolling annealing heat treatment time, pickling may not be easy due to a scale formed deep inside the steel sheet.

Pickling and cold rolling

After the hot rolling annealing heat treatment, in order to remove the scale generated on the surface of the steel sheet, pickling may be performed, and cold rolling may 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 ratio of 30 to 90%. When the cumulative 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 may be manufactured as an unplated cold-rolled steel sheet through an annealing heat treatment process, or may be manufactured as a plated steel sheet through a plating process to impart corrosion resistance. For plating, plating methods such as hot-dip galvanizing, electro-galvanizing, and hot-dip aluminum plating may be applied, and the method and type thereof are not particularly limited.

Annealing heat treatment

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

The cold-rolled steel sheet is heated (primary heating) to a temperature range of Ac1 or more and less than Ac3 (ideal range), and maintained (primary maintenance) in the temperature range for 50 seconds or more. If the primary heating or primary maintenance temperature is Ac3 or higher (single-phase region), the desired ferrite structure cannot be realized, so the desired level of [Si+Al] _F / [Si+Al] _av and tensile strength and hole expansion rate The balance of TS ² ХHER ^1/2 cannot be implemented. In addition, when the primary heating or primary maintenance temperature is in a temperature range less than Ac1, sufficient heating is not performed, so there is a fear that the microstructure of the present invention may not be realized even by subsequent heat treatment. The average temperature increase rate of the primary heating may be 5 ℃/s or more.

If the primary holding time is less than 50 seconds, the structure may not be sufficiently homogenized and the physical properties of the steel sheet may be deteriorated. The upper limit of the primary holding time is not particularly limited, but the primary heating time is preferably limited to 1200 seconds or less in order to prevent a decrease in toughness due to grain coarsening.

After the primary maintenance, it can be cooled (primary cooling) to a primary cooling stop temperature of 100 to 300°C at a primary cooling rate of 1°C/s or more at an average cooling rate. The upper limit of the primary cooling rate does not need to be specifically defined, but is preferably set to 100°C/s or less. When the primary cooling stop temperature is less than 100 ° C, tempered martensite is excessively formed and the amount of residual austenite is insufficient, so [Si+Al] _F / [Si+Al] _av , tensile strength and elongation of the steel sheet balance (TSХEl) and bending workability (R/t) may be reduced. On the other hand, when the primary cooling stop temperature exceeds 300°C, bainite is excessively formed and the amount of tempered martensite is insufficient, so the balance of tensile strength and elongation (TSХEl) and tensile strength and hole expansion rate of the steel sheet may lower the balance (TS ² ХHER ^1/2 ).

After the primary cooling, heating (secondary heating) to a secondary heating temperature of 300 to 500 °C at a secondary heating rate of 5 °C/s or more at an average temperature increase rate, and maintaining at the temperature range for more than 50 seconds (secondary maintenance) can do. The upper limit of the secondary temperature increase rate does not need to be particularly specified, but is preferably set to 100°C/s or less. If the secondary heating or secondary holding temperature is less than 300 ° C, or the holding time is less than 50 seconds, tempered martensite is excessively formed, and the silicon (Si) and aluminum (Al) content control in the steel is insufficient, so the purpose It is difficult to secure the retained austenite fraction. As a result, [Si+Al] _F / [Si+Al] _av , the balance between tensile strength and elongation (TSХEl) and bending workability (R/t) of the steel sheet may be reduced. On the other hand, when the secondary heating or holding temperature exceeds 500°C or the secondary holding time exceeds 126,000 seconds, the control of the silicon (Si) and aluminum (Al) content in the steel is insufficient to secure the fraction of retained austenite. hard to do As a result, [Si+Al] _F / [Si+Al] _av and the balance between tensile strength and elongation (TSХEl) of the steel sheet may be deteriorated.

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

The high-strength steel sheet with excellent workability manufactured by the above-described manufacturing method may include tempered martensite, bainite, retained austenite and ferrite as a microstructure, and as a preferred example, 30 to 70% by volume fraction of tempered martensite, 10-45% bainite, 10-40% retained austenite, 3-20% ferrite and unavoidable structure.

In addition, the high-strength steel sheet with excellent workability produced by the above-described manufacturing method, as shown in the following [Relational Expression 1], the average total content of silicon (Si) and aluminum (Al) contained in the steel sheet ([Si + Al] _av , weight %) of the average total content of silicon (Si) and aluminum (Al) contained in ferrite ([Si+Al] _F , wt%) may satisfy the range of 1.02 to 1.45, and the following [Relational Expression] The balance between tensile strength and elongation expressed in 2] (B _{T E} ) is 22,000 (MPa%) or more, and the balance between tensile strength and hole expansion rate (B _{T H} ) expressed in [Relational Expression 3] below is 7*10 ⁶ (MPa ² % ^1/2 ) or more, and the bending workability ( _BR ) expressed by [Relational Expression 4] below may satisfy the range of 0.5 to 3.0.

[Relational Expression 1]

1.02 ≤ [Si+Al] _F / [Si+Al] _av ≤ 1.45

[Relational Expression 2]

B _{T E} = [Tensile strength (TS, MPa)] * [Elongation (EL, %)]

[Relational Expression 3]

B _{T H} = [Tensile strength (TS, MPa)] ² * [Hole expansion rate (HER, %)] ^1/2

[Relational Expression 4]

B _R = R/t

In Relation 4, R means the minimum bending radius (mm) at which cracks do not occur after the 90° bending test, and t means the thickness (mm) of the steel sheet.

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

(Example)

A steel slab having a thickness of 100 mm having the alloy composition shown in Table 1 (the remainder being Fe and unavoidable impurities) was prepared, heated at 1200° C., and then finish hot rolling was performed at 900° C. Then, it was cooled at an average cooling rate of 30° C./s, and wound at the coiling temperature of Tables 2 and 3 to prepare a hot-rolled steel sheet having a thickness of 3 mm. The hot rolled steel sheet was subjected to hot rolling annealing heat treatment under the conditions of Tables 2 and 3. Then, after removing the surface scale by pickling, cold rolling was performed to a thickness of 1.5 mm.

Thereafter, heat treatment was performed under the annealing heat treatment conditions shown in Tables 2 to 5 to prepare steel sheets.

The microstructure of the thus-prepared steel sheet was observed, and the results are shown in Tables 6 and 7. Among the microstructures, ferrite (F), bainite (B), tempered martensite (TM), and perlite (P) were observed through SEM after nital etching the polished specimen cross section. The fractions of bainite and tempered martensite, which are difficult to distinguish among them, were calculated using an expansion curve after evaluation of dilatation. On the other hand, since fresh martensite (FM) and retained austenite (residual γ) are also difficult to distinguish, the fraction of retained austenite calculated by X-ray diffraction method is subtracted from the fraction of martensite and retained austenite observed by the SEM. The value was determined as the fresh martensite fraction.

On the other hand, the steel sheet _{[Si + Al] F / [} Si + Al] av, balance of tensile strength and elongation (TSХEl), balance of tensile strength and the hole expansion rate ^{(TS 2 ХHER 1/2)} , the bending rate (R /t) was observed, and the results are shown in Tables 8 and 9.

_{The average total content ([Si+Al] F} , wt%) of silicon (Si) and aluminum (Al) contained in ferrite was measured using an EPMA (Electron Probe MicroAnalyser), and the silicon (Si) and The average total content of aluminum (Al) ([Si+Al] _av , wt%) was calculated from the alloy composition content of the steel sheet.

Tensile strength (TS) and elongation (El) were evaluated through a tensile test, and tensile strength (TS) and elongation were evaluated using specimens taken according to JIS No. (El) was measured. The bending workability (R/t) was evaluated by the V-bending test, and the minimum bending radius R where cracks do not occur after 90° bending test by taking a specimen based on the 90° direction with respect to the rolling direction of the rolled sheet It was calculated by dividing by the thickness t of . The hole expansion rate (HER) was evaluated through the hole expansion test, and after forming a 10mmØ punching hole (die inner diameter 10.3mm, clearance 12.5%), a conical punch with an apex angle of 60° was used. After inserting into the punching hole in the desired direction, pressing and expanding the periphery of the punching hole at a moving speed of 20 mm/min, it was calculated using the following [Relational Expression 5].

[Relational Expression 5]

Hole expansion rate (HER, %) = {(D - D ₀ ) / D ₀ } x 100

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

steel grade Chemical composition (wt%) C Si Mn P S Al N Cr Mo Etc A 0.36 1.77 1.79 0.009 0.0011 0.48 0.0028 0.64 B 0.34 2.15 1.65 0.011 0.0010 0.51 0.0034 0.31 0.28 C 0.37 1.86 1.96 0.010 0.0008 0.59 0.0025 0.53 D 0.35 1.58 3.87 0.008 0.0011 0.52 0.0033 0.45 E 0.38 1.73 2.25 0.010 0.0010 0.76 0.0034 F 0.43 1.59 2.16 0.012 0.0011 0.64 0.0028 G 0.70 1.62 1.84 0.008 0.0008 0.60 0.0031 H 0.33 1.14 2.03 0.011 0.0010 1.25 0.0030 I 0.37 0.65 1.93 0.008 0.0013 2.47 0.0024 J 0.39 0.03 2.25 0.010 0.0010 4.68 0.0033 Ti: 0.04 K 0.46 1.57 2.57 0.008 0.0011 0.54 0.0034 Nb: 0.05 L 0.49 1.94 1.94 0.009 0.0008 0.39 0.0021 V: 0.04 M 0.34 1.83 2.02 0.010 0.0011 0.28 0.0030 Ni: 0.38 N 0.35 1.59 2.36 0.011 0.0013 0.42 0.0033 Cu: 0.32 O 0.36 1.55 1.64 0.013 0.0007 0.48 0.0027 B: 0.003 P 0.43 1.62 2.11 0.012 0.0008 0.67 0.0022 Ca: 0.001 Q 0.39 1.74 2.25 0.008 0.0009 0.59 0.0032 REM: 0.001 R 0.42 1.69 2.03 0.008 0.0008 0.62 0.0030 Mg: 0.002 S 0.35 1.75 2.14 0.009 0.0011 0.53 0.0037 W: 0.12 T 0.32 2.04 2.20 0.011 0.0011 0.43 0.0041 Zr: 0.10 U 0.34 1.62 2.17 0.009 0.0010 0.58 0.0035 Sb: 0.02 V 0.38 1.80 2.12 0.008 0.0011 0.72 0.0042 Sn: 0.01 W 0.37 1.54 2.04 0.010 0.0011 0.63 0.0028 Y: 0.01 X 0.28 3.26 2.36 0.009 0.0007 0.49 0.0032 Hf: 0.01 Y 0.34 2.18 2.25 0.011 0.0001 0.45 0.0027 Co: 0.37 XA 0.22 1.79 2.14 0.008 0.0008 0.51 0.0023 XB 0.79 1.84 1.88 0.012 0.0008 0.53 0.0031 XC 0.36 0.02 1.96 0.008 0.0010 0.03 0.0035 XD 0.33 4.19 1.85 0.010 0.0011 0.03 0.0030 XE 0.37 0.02 2.02 0.012 0.0008 5.18 0.0023 XF 0.39 1.93 0.74 0.009 0.0011 0.47 0.0022 XG 0.41 1.77 5.32 0.011 0.0009 0.53 0.0030 XH 0.37 1.85 1.83 0.008 0.0011 0.58 0.0034 3.46 XI 0.42 1.82 2.17 0.010 0.0009 0.65 0.0023 3.52

Psalter
number steel grade hot rolled steel
winding temperature
(℃) hot rolled steel
Annealing temperature
(℃) hot rolled steel
Annealing time
(s) 1st average
heating rate
(℃/s) 1st maintenance
temperature range
(℃) 1st maintenance
time
(s) One A 500 800 1400 10 Lee Sang Station 120 2 A 550 900 1200 poor pickling 3 A 550 600 1300 Fracture during cold rolling 4 B 400 750 1800 poor pickling 5 B 450 750 500 Fracture during cold rolling 6 B 450 700 1200 10 single phase station 120 7 B 400 750 1600 10 Lee Sang Station 120 8 B 500 800 1500 10 Lee Sang Station 120 9 B 400 750 1000 10 Lee Sang Station 120 10 B 550 750 1200 10 Lee Sang Station 120 11 C 500 700 900 10 Lee Sang Station 120 12 C 550 700 1200 10 Lee Sang Station 120 13 C 450 750 1000 10 Lee Sang Station 120 14 C 500 700 1400 10 Lee Sang Station 120 15 C 500 750 1400 10 Lee Sang Station 120 16 C 450 650 1300 10 Lee Sang Station 120 17 C 550 650 1500 10 Lee Sang Station 120 18 D 500 750 1200 10 Lee Sang Station 120 19 E 500 650 1700 10 Lee Sang Station 120 20 F 450 700 600 10 Lee Sang Station 120 21 G 500 750 1300 10 Lee Sang Station 120 22 H 450 850 800 10 Lee Sang Station 120 23 I 350 700 1200 10 Lee Sang Station 120 24 J 550 700 1300 10 Lee Sang Station 120 25 K 500 800 1500 10 Lee Sang Station 120

Psalter
number steel grade hot rolled steel
winding temperature
(℃) hot rolled steel
Annealing temperature
(℃) hot rolled steel
Annealing time
(s) 1st average
heating rate
(℃/s) 1st maintenance
temperature range
(℃) 1st maintenance
time
(s) 26 L 550 700 1400 10 Lee Sang Station 120 27 M 450 800 1000 10 Lee Sang Station 120 28 N 400 750 1100 10 Lee Sang Station 120 29 O 500 800 1500 10 Lee Sang Station 120 30 P 550 700 1300 10 Lee Sang Station 120 31 Q 450 800 1200 10 Lee Sang Station 120 32 R 500 700 1400 10 Lee Sang Station 120 33 S 550 750 1200 10 Lee Sang Station 120 34 T 550 750 1100 10 Lee Sang Station 120 35 U 400 800 1000 10 Lee Sang Station 120 36 V 500 700 1200 10 Lee Sang Station 120 37 W 450 800 1400 10 Lee Sang Station 120 38 X 450 700 1500 10 Lee Sang Station 120 39 Y 500 750 1100 10 Lee Sang Station 120 40 XA 500 800 1300 10 Lee Sang Station 120 41 XB 550 700 1400 10 Lee Sang Station 120 42 XC 550 750 1000 10 Lee Sang Station 120 43 XD 500 700 1200 10 Lee Sang Station 120 44 XE 450 800 1400 10 Lee Sang Station 120 45 XF 400 750 1200 10 Lee Sang Station 120 46 XG 550 700 1100 10 Lee Sang Station 120 47 XH 500 800 1300 10 Lee Sang Station 120 48 XI 550 800 1500 10 Lee Sang Station 120

Psalter
number steel grade 1st average
cooling rate
(℃/s) primary cooling
stop temperature
(℃) 2nd average
heating rate
(℃/s) secondary maintenance
Temperature
(℃) secondary maintenance
time
(s) 2nd average
cooling rate
(℃/s) One A 20 220 15 450 300 10 2 A poor pickling 3 A Fracture during cold rolling 4 B poor pickling 5 B Fracture during cold rolling 6 B 20 210 15 400 300 10 7 B 0.5 180 15 450 300 10 8 B 20 190 15 400 300 10 9 B 20 120 15 400 300 10 10 B 20 210 15 400 300 10 11 C 20 200 15 450 600 10 12 C 20 80 15 400 300 10 13 C 20 330 15 400 300 10 14 C 20 220 15 270 300 10 15 C 20 230 15 530 300 10 16 C 20 180 15 400 40 10 17 C 20 180 15 450 144,000 10 18 D 20 210 15 400 300 10 19 E 20 230 15 400 300 10 20 F 20 280 15 400 300 10 21 G 20 220 15 400 300 10 22 H 20 220 15 350 300 10 23 I 20 180 15 400 600 10 24 J 20 200 15 400 300 10 25 K 20 220 15 450 300 10

Psalter
number steel grade 1st average
cooling rate
(℃/s) primary cooling
stop temperature
(℃) 2nd average
heating rate
(℃/s) secondary maintenance
Temperature
(℃) secondary maintenance
time
(s) 2nd average
cooling rate
(℃/s) 26 L 20 210 15 400 300 10 27 M 20 220 15 400 300 10 28 N 20 190 15 400 300 10 29 O 20 180 15 450 300 10 30 P 20 200 15 450 300 10 31 Q 20 220 15 400 300 10 32 R 20 210 15 400 600 10 33 S 20 230 15 400 300 10 34 T 20 200 15 400 300 10 35 U 20 230 15 400 600 10 36 V 20 210 15 350 300 10 37 W 20 190 15 400 300 10 38 X 20 220 15 400 300 10 39 Y 20 200 15 450 300 10 40 XA 20 180 15 400 300 10 41 XB 20 200 15 400 300 10 42 XC 20 230 15 400 300 10 43 XD 20 200 15 400 300 10 44 XE 20 210 15 450 300 10 45 XF 20 190 15 400 300 10 46 XG 20 200 15 400 600 10 47 XH 20 220 15 400 300 10 48 XI 20 180 15 450 300 10

Psalter
number steel grade ferrite
(vol.%) bainite
(vol.%) tempered
marten
site
(vol.%) fresh
marten
site
(vol.%) residual
Auste
Night
(vol.%) perlite
(vol.%) One A 12 17 52 0 19 0 2 A poor pickling 3 A Fracture during cold rolling 4 B poor pickling 5 B Fracture during cold rolling 6 B One 23 61 0 15 0 7 B 24 14 46 0 5 11 8 B 5 17 62 0 16 0 9 B 10 18 54 0 18 0 10 B 12 15 53 One 19 0 11 C 11 21 51 0 17 0 12 C 9 5 79 0 7 0 13 C 12 63 6 0 19 0 14 C 10 14 72 0 4 0 15 C 8 18 67 One 6 0 16 C 9 12 73 0 6 0 17 C 11 19 59 4 7 0 18 D 11 18 51 One 19 0 19 E 10 17 57 0 16 0 20 F 5 21 54 0 20 0 21 G 6 41 39 0 14 0 22 H 18 15 51 0 16 0 23 I 11 16 56 0 17 0 24 J 9 12 61 0 18 0 25 K 7 13 46 0 34 0

Psalter
number steel grade ferrite
(vol.%) bainite
(vol.%) tempered
marten
site
(vol.%) fresh
marten
site
(vol.%) residual
Auste
Night
(vol.%) perlite
(vol.%) 26 L 11 21 52 One 15 0 27 M 8 16 58 One 17 0 28 N 6 15 60 One 18 0 29 O 7 19 57 One 16 0 30 P 10 21 52 0 17 0 31 Q 8 17 59 One 15 0 32 R 7 16 42 0 35 0 33 S 11 18 49 One 21 0 34 T 10 19 53 0 18 0 35 U 9 20 54 One 16 0 36 V 8 17 55 0 20 0 37 W 11 18 54 0 17 0 38 X 10 22 47 0 21 0 39 Y 8 19 55 0 18 0 40 XA 10 17 59 0 14 0 41 XB 5 14 21 16 44 0 42 XC 11 22 62 0 5 0 43 XD 6 13 43 22 16 0 44 XE 5 17 40 19 19 0 45 XF 6 16 64 0 6 8 46 XG 4 15 51 15 15 0 47 XH 7 16 41 23 13 0 48 XI 6 20 44 18 12 0

Psalter
number steel grade [Si+Al] _F /[Si+Al] _av B _{T E}
(MPa%) B _{T H}
(MPa ^{2% 1/2)} R/t One A 1.17 31,542 10,819,415 1.58 2 A poor pickling 3 A Fracture during cold rolling 4 B poor pickling 5 B Fracture during cold rolling 6 B 0.97 28,215 6,485,296 2.03 7 B 1.53 16,328 7,229,178 2.16 8 B 1.26 29,631 8,992,265 1.62 9 B 1.14 30,027 12,855,180 2.25 10 B 1.22 32,592 10,502,114 1.88 11 C 1.20 30,894 9,899,273 2.14 12 C 1.48 16,442 7,778,447 4.15 13 C 1.18 20,611 6,655,107 2.37 14 C 1.54 13,607 7,272,268 4.38 15 C 1.49 20,851 8,568,314 2.03 16 C 1.57 16,874 8,352,875 5.19 17 C 1.52 19,866 7,780,616 2.40 18 D 1.29 30,894 9,635,245 1.52 19 E 1.37 32,250 10,772,531 2.72 20 F 1.43 29,236 10,505,903 1.94 21 G 1.15 31,333 11,214,821 1.27 22 H 1.10 30,017 9,804,695 2.75 23 I 1.28 29,537 10,028,681 2.52 24 J 1.21 31,044 9,925,290 1.78 25 K 1.11 30,314 10,070,923 1.97

Psalter
number steel grade [Si+Al] _F /[Si+Al] _av B _{T E}
(MPa%) B _{T H}
(MPa ^{2% 1/2)} R/t 26 L 1.23 30,502 10,152,414 1.72 27 M 1.27 28,899 11,180,290 1.85 28 N 1.20 32,692 9,987,755 1.79 29 O 1.16 35,909 10,225,208 2.23 30 P 1.12 29,106 10,502,607 1.43 31 Q 1.18 30,735 9,820,075 2.26 32 R 1.20 32,486 10,988,836 1.47 33 S 1.19 29,596 11,032,009 1.60 34 T 1.15 31,181 9,951,238 1.51 35 U 1.13 30,592 8,816,156 2.25 36 V 1.24 29,172 10,333,893 1.63 37 W 1.27 30,050 9,659,478 1.76 38 X 1.29 31,290 10,756,274 2.08 39 Y 1.14 30,294 11,088,165 2.04 40 XA 1.18 17,128 5,397,276 2.11 41 XB 1.22 20,732 5,914,019 5.53 42 XC 1.53 13,082 7,778,447 4.62 43 XD 1.15 24,944 8,709,941 4.81 44 XE 1.17 28,030 8,590,607 6.57 45 XF 1.49 15,485 7,857,409 2.26 46 XG 1.20 24,866 8,028,337 5.04 47 XH 1.27 23,463 7,721,541 5.38 48 XI 1.16 27,487 7,502,405 6.13

As shown in Tables 1 to 9, in the case of the specimens satisfying the conditions presented in the present invention, the values of [Si+Al] _F / [Si+Al] _av satisfy the range of 1.02 to 1.45, and the tensile strength and the balance (TSХEl) of the elongation is not less than 22,000MPa%, and the balance (TS ² ХHER ^1/2) is ^{7 * 10 6 (MPa 2%} 1/2) or more of tensile strength and the hole expansion rate, bending rate ( R/t) satisfies the range of 0.5 to 3.0, and it can be seen that excellent strength and workability are simultaneously provided.

Specimens 2 to 5 overlap the alloy composition range of the present invention, but since the hot rolling annealing temperature and time are out of the scope of the present invention, it can be confirmed that pickling failure occurs or fracture occurs during cold rolling.

In Specimen 6, the amount of ferrite formed was insufficient because the primary heating or maintaining temperature in the annealing heat treatment process after cold rolling exceeded the range limited by the present invention. As a result, in specimen 6, [Si+Al] _F / [Si+Al] _av was less than 1.02, and the balance of tensile strength and hole expansion rate (TS ² ХHER ^1/2 ) was 7*10 ⁶ (MPa ² % ^{1). /2} ) can be confirmed.

In Specimen 7, in the annealing heat treatment process after cold rolling, the primary cooling rate did not reach the limit of the present invention, so ferrite was excessively formed and retained austenite was small. As a result, it can be seen that in Specimen 7, [Si+Al] _F / [Si+Al] _av exceeds 1.45, and the balance of tensile strength and elongation (TSХEl) is less than 22,000 MPa%.

Specimen 12 had a low primary cooling stop temperature, so tempered martensite was excessively formed, and retained austenite was low. As a result, in Specimen 12, [Si+Al] _F / [Si+Al] _av exceeded 1.45, the balance of tensile strength and elongation (TSХEl) was less than 22,000 MPa%, and the bending workability (R/t) was 3.0 can be seen to exceed .

Specimen 13 had a high primary cooling stop temperature, so bainite was excessively formed, and tempered martensite was less formed. As a result, the specimen 13 is less than the 22,000MPa% balance (TSХEl) of the tensile strength elongation and the balance (TS ² ХHER ^1/2) of the tensile strength and the hole expansion rate of ^{7 * 10 6 (MPa 2%} 1/2) It can be seen that less than

Specimen 14 had a low secondary heating or holding temperature, so tempered martensite was excessively formed and retained austenite was low. As a result, in Specimen 14, [Si+Al] _F / [Si+Al] _av exceeded 1.45, the balance of tensile strength and elongation (TSХEl) was less than 22,000 MPa%, and the bending workability (R/t) was 3.0 can be seen to exceed .

Specimen 15 had insufficient residual austenite formation due to high secondary heating or holding temperature, [Si+Al] _F / [Si+Al] _av exceeding 1.45, and the balance of tensile strength and elongation (TSХEl) was 22,000 MPa. It can be seen that less than %

Specimen 16 had insufficient secondary holding time, so tempered martensite was excessively formed, and retained austenite was low. As a result, in Specimen 16, [Si+Al] _F / [Si+Al] _av exceeded 1.45, the balance of tensile strength and elongation (TSХEl) was less than 22,000 MPa%, and the bending workability (R/t) was 3.0 can be seen to exceed .

In specimen 17, the amount of retained austenite formation was insufficient due to excessive secondary holding time, [Si+Al] _F / [Si+Al] _av exceeded 1.45, and the balance of tensile strength and elongation (TSХEl) was 22,000 MPa% It can be seen that less than

Specimens 40 to 48 meet the manufacturing conditions presented in the present invention, but are outside the alloy composition range. In these cases, [Si+Al] _F / [Si+Al] _av of the present invention, the balance of tensile strength and elongation (TSХEl), and the balance of tensile strength and hole expansion (TS ² ХHER ^1/2 ) are 7*10 ^{It can be seen that 6} (MPa ² % ^1/2 ) and bending workability (R/t) conditions are not simultaneously satisfied. On the other hand, in specimen 42, the total content of aluminum (Al) and silicon (Si) is less than 1.0%, [Si+Al] _F / [Si+Al] _av , the balance of tensile strength and elongation (TSХEl) and bending workability ( R/t) condition is not satisfied.

Although the present invention has been described in detail through examples above, other types of embodiments are also possible. Therefore, the spirit and scope of the claims set forth below are not limited to the embodiments.

Claims (11)

By weight%, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, remainder Fe and unavoidable impurities;
As a microstructure, it contains 30-70 vol % of tempered martensite, 10-45 vol % of bainite, 10-40 vol % of retained austenite, 3-20 vol % of ferrite and an unavoidable structure,
A high-strength steel sheet with excellent workability that satisfies the following [Relational Expression 1].
[Relational Expression 1]
1.02 ≤ [Si+Al] _F / [Si+Al] _av ≤ 1.45
In Relation 1, [Si+Al] _F is the average total content of Si and Al contained in the ferrite (weight%), and [Si+Al] _av is the average total content of Si and Al contained in the steel sheet (weight%) )to be.
According to claim 1,
The steel sheet, further comprising any one or more of the following (1) to (9), high-strength steel sheet excellent workability.
(1) Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5% at least one of
(2) at least one of Cr: 0 to 3.0% and Mo: 0 to 3.0%
(3) at least one of Cu: 0 to 4.5% and Ni: 0 to 4.5%
(4) B: 0~0.005%
(5) Ca: 0 to 0.05%, REM except for Y: 0 to 0.05%, and Mg: at least one of 0 to 0.05%
(6) at least one of W: 0 to 0.5% and Zr: 0 to 0.5%
(7) at least one of Sb: 0 to 0.5% and Sn: 0 to 0.5%
(8) at least one of Y: 0 to 0.2% and Hf: 0 to 0.2%
(9) Co: 0~1.5%
According to claim 1,
The total content of Si and Al (Si + Al) is 1.0 to 6.0 wt%, a high strength steel sheet with excellent workability.
According to claim 1,
_{The steel sheet has a balance (B T E} ) of tensile strength and elongation expressed by the following [Relational Expression 2] of 22,000 (MPa%) or more, and the tensile strength and hole expansion rate expressed by [Relational Expression 3] below balance (B _{T · H)} is ^{7 * 10 6 (MPa 2%} 1/2) or more, the bending rate, expressed in [expression 4] below (B _R) of 0.5 to 3.0, and processability is excellent high-strength steel sheet .
[Relational Expression 2]
B _{T E} = [Tensile strength (TS, MPa)] * [Elongation (El, %)]
[Relational Expression 3]
B _{T H} = [Tensile strength (TS, MPa)] ² * [Hole expansion rate (HER, %)] ^1/2
[Relational Expression 4]
B _R = R/t
In Relation 4, R means the minimum bending radius (mm) at which cracks do not occur after the 90° bending test, and t means the thickness (mm) of the steel sheet.
In weight %, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, the rest is heating and hot rolling a steel slab containing Fe and unavoidable impurities;
winding the hot-rolled steel sheet;
hot-rolling and annealing the wound steel sheet in a temperature range of 650 to 850° C. for 600 to 1700 seconds;
cold rolling the hot-rolled annealing heat-treated steel sheet;
heating (primary heating) the cold-rolled steel sheet to a temperature range of Ac1 or more and less than Ac3 at an average temperature increase rate of 5° C./s or more, and maintaining (primary maintenance) for 50 seconds or more;
cooling (primary cooling) to a temperature range of 100 to 300°C at an average cooling rate of 1°C/s or more;
heating (secondary heating) the first cooled steel sheet to a temperature range of 300 to 500°C, and maintaining (secondary maintenance) for 50 seconds or more; and
A method of manufacturing a high-strength steel sheet having excellent workability, including; cooling to room temperature (secondary cooling).
6. The method of claim 5,
The steel slab further comprises any one or more of the following (1) to (9), a method of manufacturing a high strength steel sheet excellent in workability.
(1) Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5% at least one of
(2) at least one of Cr: 0 to 3.0% and Mo: 0 to 3.0%
(3) at least one of Cu: 0 to 4.5% and Ni: 0 to 4.5%
(4) B: 0~0.005%
(5) Ca: 0 to 0.05%, REM except for Y: 0 to 0.05%, and Mg: at least one of 0 to 0.05%
(6) at least one of W: 0 to 0.5% and Zr: 0 to 0.5%
(7) at least one of Sb: 0 to 0.5% and Sn: 0 to 0.5%
(8) at least one of Y: 0 to 0.2% and Hf: 0 to 0.2%
(9) Co: 0~1.5%
6. The method of claim 5,
The total content of Si and Al contained in the steel slab (Si + Al) is 1.0 to 6.0 wt%, a method of manufacturing a high strength steel sheet having excellent workability.
6. The method of claim 5,
The steel slab is heated to a temperature range of 1000 ~ 1350 ℃, finish hot rolling in a temperature range of 800 ~ 1000 ℃, a method of manufacturing a high strength steel sheet with excellent workability.
6. The method of claim 5,
The hot-rolled steel sheet is wound in a temperature range of 300 ~ 600 ℃, a method of manufacturing a high strength steel sheet excellent workability.
6. The method of claim 5,
The reduction ratio of the cold rolling is 30 to 90%, a method of manufacturing a high strength steel sheet excellent workability.
6. The method of claim 5,
The cooling rate of the secondary cooling is 1 ℃ / s or more, a method of manufacturing a high strength steel sheet excellent workability.