JP7554827B2 - High-strength steel plate with excellent workability and manufacturing method thereof - Google Patents

Description

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

In recent years, the automotive industry has been focusing on ways to reduce the weight of materials in order to protect the global environment while at the same time ensuring stability for passengers. To meet this demand for stability and weight reduction, the use of high-strength steel sheets has rapidly increased. It is generally known that the higher the strength of a steel sheet, the lower its workability becomes. Therefore, there is a demand for steel sheets for automotive parts that have high strength properties while also having excellent workability, such as ductility, bending workability, and hole expansion property.

Patent Documents 1 and 2 disclose a method of utilizing tempered martensite as a technology for improving the workability of steel sheets. Tempered martensite, which is made by tempering hard martensite, is softened martensite, so there is a difference in strength between tempered martensite and existing untempered martensite (fresh martensite). Therefore, by suppressing fresh martensite to form tempered martensite, workability can be improved.

However, the technologies disclosed in Patent Documents 1 and 2 do not achieve a balance between tensile strength and elongation (TS x El) of 22,000 MPa% or more, which means that it is difficult to obtain steel sheets with excellent strength and ductility.

On the other hand, in order to obtain both high strength and excellent workability in steel sheets for automotive components, TRIP (Transformation Induced Plasticity) steel was developed using organic sintering to transform retained austenite. Patent Document 3 discloses TRIP steel with excellent strength and workability.

In Patent Document 3, polygonal ferrite, retained austenite, and martensite are included to improve ductility and workability, but because bainite is the main phase, high strength cannot be ensured, and the balance between tensile strength and elongation (TS x El) does not meet the requirement of 22,000 MPa% or more.

In other words, the current situation is that the demands for steel sheets that have high strength but also excellent workability, such as ductility, bending workability, and hole expansion ability, are not met.

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

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

The object of the present invention is not limited to the above. Further object of the present invention is described in the entire contents of the specification, and a person having ordinary knowledge in the technical field to which the present invention pertains will have no difficulty in understanding the further object of the present invention from the contents described in the specification of the present invention.

A high-strength steel plate with excellent workability according to one aspect of the present invention contains, by weight, 0.25 to 0.75% C, 4.0% or less Si, 0.9 to 5.0% Mn, 5.0% or less Al, 0.15% or less P, 0.03% or less S, 0.03% or less N, with the remainder being Fe and unavoidable impurities, and contains, as a microstructure, 30 to 70% by volume of tempered martensite, 10 to 45% by volume of bainite, 10 to 40% by volume of retained austenite, 3 to 20% by volume of ferrite, and unavoidable structures, and can satisfy the following [Relational Formula 1].
[Relationship 1]
1.1≦[Si+Al] _F /[Si+Al] _γ ≦3.0
In the above Relational Formula 1, [Si+Al] _F is the average total content (wt%) of Si and Al contained in ferrite, and [Si+Al] _γ is the average total content (wt%) of Si and Al contained in retained austenite.

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

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

The steel sheet may have a balance between tensile strength and elongation (B _T·E ) represented by the following [Relationship 2] of 22,000 (MPa%) or more, a balance between tensile strength and hole expansion ratio (B _T·H ) represented by the following [Relationship 3] of 7×10 ⁶ (MPa ² % ^1/2 ) or more, and a bending ratio (B _R ) represented by the following [Relationship 4] that satisfies the range of 0.5 to 3.0.
[Relationship 2]
_BTE = [tensile strength (TS, MPa)] × [elongation rate (El, %)]
[Relationship 3]
_BTH = [tensile strength (TS, MPa)] ² × [hole expansion ratio (HER,%)] ^1/2
[Relationship 4]
_BR = R/t
In the above Relational Formula 4, R means the minimum bending radius (mm) at which no cracks occur after a 90° bending test, and t means the thickness (mm) of the steel plate.

According to another aspect of the present invention, a method for producing a high-strength steel sheet having excellent workability includes the steps of heating and hot-rolling a steel slab containing, 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, and the remainder being Fe and inevitable impurities; coiling the hot-rolled steel sheet; and heat-treating the coiled steel sheet by hot-rolling annealing at a temperature range of 650 to 850°C for 600 to 1700 seconds. ; the step of cold rolling the steel sheet heat-treated by the hot rolling annealing; the step of heating the cold rolled steel sheet to a temperature range of Ac1 or more and less than Ac3 (primary heating) and maintaining it for 50 seconds or more (primary maintenance); the step of cooling to a temperature range of 100 to 300°C at an average cooling rate of 1°C/s or more (primary cooling); the step of heating the primarily cooled steel sheet to a temperature range of 300 to 500°C (secondary heating) and maintaining it for 50 seconds or more (secondary maintenance); and the step of cooling to room temperature (secondary cooling).

The steel slab may further include any one or more of the following (1) to (9):
(1) One or more of Ti: 0-0.5%, Nb: 0-0.5%, and V: 0-0.5%; (2) One or more of Cr: 0-3.0% and Mo: 0-3.0%; (3) One or more of Cu: 0-4.5% and Ni: 0-4.5%; (4) B: 0-0.005%.
(5) One or more of Ca: 0-0.05%, REM excluding Y: 0-0.05%, and Mg: 0-0.05%. (6) One or more of W: 0-0.5%, and Zr: 0-0.5%. (7) One or more of Sb: 0-0.5%, and Sn: 0-0.5%. (8) One or more of Y: 0-0.2%, and Hf: 0-0.2%. (9) Co: 0-1.5%.

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

The steel slab may be heated to a temperature range of 1000-1350°C and finish hot rolled to a temperature range of 800-1000°C.

The hot-rolled steel sheet may be coiled at a temperature range of 300 to 600°C.

The reduction ratio 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 that is not only excellent in strength but also in workability such as ductility, bending workability, and hole expansion property, and is particularly suitable for use in automotive parts.

The present invention relates to a high-strength steel plate with excellent workability and a manufacturing method thereof. Preferred embodiments of the present invention are described below. The embodiments of the present invention may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments are provided to explain the present invention in more detail to those having ordinary skill in the art to which the invention pertains.

The inventors of the present invention have come to realize that in transformation induced plasticity (TRIP) steel containing bainite, tempered martensite, retained austenite, and ferrite, when the retained austenite is stabilized and the ratio of specific components contained in the retained austenite and ferrite is controlled within a certain range, the interphase hardness difference between the retained austenite and ferrite is reduced, thereby simultaneously ensuring the workability and strength of the steel sheet. Having investigated this, the inventors have devised a method for improving the ductility and workability of high-strength steel, leading to the present invention.

The high-strength steel sheet with excellent workability according to one aspect of the present invention will be described in more detail below.

A high-strength steel plate with excellent workability according to one aspect of the present invention contains, by weight, 0.25 to 0.75% C, 4.0% or less Si, 0.9 to 5.0% Mn, 5.0% or less Al, 0.15% or less P, 0.03% or less S, 0.03% or less N, with the remainder being Fe and unavoidable impurities, and contains, as a microstructure, 30 to 70% by volume of tempered martensite, 10 to 45% by volume of bainite, 10 to 40% by volume of retained austenite, 3 to 20% by volume of ferrite, and unavoidable structures, and can satisfy the following [Relational Formula 1].
[Relationship 1]
1.1≦[Si+Al] _F /[Si+Al] _γ ≦3.0
In the above Relational Formula 1, [Si+Al] _F is the average total content (wt%) of Si and Al contained in ferrite, and [Si+Al] _γ is the average total content (wt%) of Si and Al contained in retained austenite.

The steel composition of the present invention will be described in more detail below. Unless otherwise specified, the percentages representing the content of each element are based on weight.

A high-strength steel sheet with excellent workability according to one aspect of the present invention contains, by weight, C: 0.25-0.75%, Si: 4.0% or less, Mn: 0.9-5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, the remainder being Fe and inevitable impurities. In addition, 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: It may further contain one or more of the following: 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 (including 0%), Hf: 0.2% or less (including 0%), Co: 1.5% or less (including 0%). The total content of Si and Al (Si+Al) may be 1.0 to 6.0%.

Carbon (C): 0.25-0.75%
Carbon (C) is an essential element for ensuring the strength of the steel plate, and also an element for stabilizing the retained austenite that contributes to improving the ductility of the steel plate. Therefore, the present invention can contain 0.25% or more of carbon (C) to achieve this effect. A preferred carbon (C) content may be more than 0.25%, 0.27% or more, or 0.30% or more. A more preferred carbon (C) content may be 0.31% or more. On the other hand, if the carbon (C) content exceeds a certain level, cold rolling may become difficult due to an excessive increase in strength. Therefore, the present invention can 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 preferred 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 improving strength through solid solution strengthening, and is also an element that strengthens ferrite and homogenizes the structure to improve workability. Silicon (Si) is also an element that inhibits the precipitation of cementite and contributes to the generation of retained austenite. Therefore, in the present invention, silicon (Si) can be added as an essential component to achieve such effects. A preferred silicon (Si) content may be 0.02% or more, and a more preferred silicon (Si) content may be 0.05% or more. However, if the silicon (Si) content exceeds a certain level, it may not only cause a plating defect problem like unplated in the plating process, but also may reduce the weldability of the steel sheet, so the present invention may limit the upper limit of the silicon (Si) content to 4.0%. A preferred upper limit of the silicon (Si) content may be 3.8%, and a more preferred 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 combines with oxygen in steel to perform deoxidation. Aluminum (Al) is also an element that inhibits cementite precipitation and stabilizes retained austenite, similar to silicon (Si). Therefore, in the present invention, aluminum (Al) can be added as an essential component to achieve such effects. A preferred aluminum (Al) content may be 0.05% or more, and a more preferred aluminum (Al) content may be 0.1% or more. On the other hand, if aluminum (Al) is added excessively, not only the inclusions in the steel sheet may be increased, but also the workability of the steel sheet may be reduced, so the present invention can limit the upper limit of the aluminum (Al) content to 5.0%. A preferred upper limit of the aluminum (Al) content may be 4.75%, and a more preferred upper limit of the aluminum (Al) content may be 4.5%.

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

Manganese (Mn): 0.9-5.0%
Manganese (Mn) is an element useful for increasing both strength and ductility. Therefore, in order to achieve this effect, the present invention may limit the lower limit of the manganese (Mn) content to 0.9%. 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 added in excess, the bainite transformation time increases and the carbon (C) concentration in the austenite becomes insufficient, so that the desired austenite fraction cannot be secured. Therefore, the present invention may limit the upper limit of the manganese (Mn) content to 5.0%. A preferred upper limit of the manganese (Mn) content may be 4.7%, and a more preferred 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 reduces impact toughness, and therefore the phosphorus (P) content is preferably controlled to 0.15% or less.

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

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

On the other hand, the steel sheet of the present invention may contain additional alloy compositions in addition to the alloy components described above, which will be described in detail below.

At least one of titanium (Ti): 0-0.5%, niobium (Nb): 0-0.5%, and vanadium (V): 0-0.5% Titanium (Ti), niobium (Nb), and vanadium (V) are elements that form precipitates to refine crystal grains and also contribute to improving the strength and impact toughness of steel sheets, so the present invention may add at least one of titanium (Ti), niobium (Nb), and vanadium (V) for this effect. However, if the content of each of titanium (Ti), niobium (Nb), and vanadium (V) exceeds a certain level, excessive precipitates are formed, which not only reduces impact toughness but also causes an increase in manufacturing costs, so the present invention may limit the content of titanium (Ti), niobium (Nb), and vanadium (V) to 0.5% or less.

At least one of chromium (Cr): 0-3.0% and molybdenum (Mo): 0-3.0% Chromium (Cr) and molybdenum (Mo) are elements that not only suppress austenite decomposition during alloying treatment but also stabilize austenite like manganese (Mn), so the present invention can add at least one of chromium (Cr) and molybdenum (Mo) for this effect. However, if the content of chromium (Cr) and molybdenum (Mo) exceeds a certain level, the bainite transformation time increases and the amount of carbon (C) enriched in austenite becomes insufficient, so that the desired fraction of retained austenite cannot be secured. Therefore, the content of chromium (Cr) and molybdenum (Mo) can be limited to 3.0% or less, respectively, in the present invention.

At least one of copper (Cu): 0-4.5% and nickel (Ni): 0-4.5% Copper (Cu) and nickel (Ni) are elements that stabilize austenite and inhibit corrosion. Copper (Cu) and nickel (Ni) are also elements that concentrate on the surface of the steel sheet, prevent hydrogen penetration that moves into the steel sheet, and inhibit delayed hydrogen fracture. Therefore, in the present invention, at least one of copper (Cu) and nickel (Ni) can be added for such effects. However, if the content of copper (Cu) and nickel (Ni) exceeds a certain level, it not only causes excessive characteristic effects but also causes an increase in manufacturing costs, so the content of copper (Cu) and nickel (Ni) can be limited to 4.5% or less, respectively, in the present invention.

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

At least one of calcium (Ca): 0-0.05%, magnesium (Mg): 0-0.05%, and rare earth elements (REM) excluding yttrium (Y): 0-0.05%. Here, rare earth elements (REM) refer to scandium (Sc), yttrium (Y), and lanthanum group elements. Since rare earth elements (REM) excluding calcium (Ca), magnesium (Mg), and yttrium (Y) are elements that contribute to improving the ductility of steel sheets by making sulfides spherical, the present invention can add at least one of rare earth elements (REM) excluding calcium (Ca), magnesium (Mg), and yttrium (Y) for such an effect. However, if the content of rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) exceeds a certain level, it may cause excessive property effects as well as an increase in manufacturing costs, so the present invention may limit the content of rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) to 0.05% or less.

At least one of tungsten (W): 0-0.5% and zirconium (Zr): 0-0.5% Tungsten (W) and zirconium (Zr) are elements that improve hardenability and increase the strength of steel sheet, so the present invention may add at least one of tungsten (W) and zirconium (Zr) for this effect. However, if the content of tungsten (W) and zirconium (Zr) exceeds a certain level, it may cause excessive property effects as well as an increase in manufacturing costs, so the present invention may limit the content of tungsten (W) and zirconium (Zr) to 0.5% or less, respectively.

At least one 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, so the present invention may add at least one of antimony (Sb) and tin (Sn) for this effect. However, if 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 may limit the content of antimony (Sb) and tin (Sn) to 0.5% or less, respectively.

At least 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, so in the present invention, at least one of yttrium (Y) and hafnium (Hf) may be added for this effect. However, if the content of yttrium (Y) and hafnium (Hf) exceeds a certain level, the ductility of the steel sheet may deteriorate, so in the present invention, the content of yttrium (Y) and hafnium (Hf) may be limited to 0.2% or less, respectively.

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

The high-strength steel plate with excellent workability according to one aspect of the present invention may contain the remaining Fe and other inevitable impurities in addition to the above-mentioned components. However, since unintended impurities may be inevitably mixed in from raw materials or the surrounding environment during normal manufacturing processes, these cannot be completely excluded. Since these impurities are known to anyone with ordinary knowledge in this technical field, the contents of all of them will not be specifically mentioned in this specification. Furthermore, the addition of additional effective components in addition to the above-mentioned components is not completely excluded.

The high-strength steel sheet with excellent workability according to one aspect of the present invention may include tempered martensite, bainite, retained austenite, and ferrite as microstructures. As a preferred example, the high-strength steel sheet with excellent workability according to one aspect of the present invention may include, by volume fraction, 30 to 70% tempered martensite, 10 to 45% bainite, 10 to 40% retained austenite, 3 to 20% ferrite, and unavoidable structures. The unavoidable structures of the present invention may include fresh martensite, pearlite, island martensite (martensite austenite constituent, M-A), and the like. If fresh martensite or pearlite is formed excessively, the workability of the steel sheet may decrease or the fraction of retained austenite may be reduced.

A high-strength steel plate with excellent workability according to one aspect of the present invention can satisfy the ratio of the average total content of silicon (Si) and aluminum (Al) contained in ferrite ([Si+Al] _F , wt %) to the average total content of silicon (Si) and aluminum (Al) contained in retained austenite ([Si+Al] _γ , wt %) in the range of 1.1 to 3.0, as shown in the following [Relational Formula 1].
[Relationship 1]
1.1≦[Si+Al] _F /[Si+Al] _γ ≦3.0

In addition, the high-strength steel sheet having excellent workability according to one aspect of the present invention has a balance between tensile strength and elongation rate (B _T·E ) represented by the following [Relationship 2] of 22,000 (MPa%) or more, a balance between tensile strength and hole expansion rate (B _T·H ) represented by the following [Relationship 3] of 7 × 10 ⁶ (MPa ² % ^1/2 ) or more, and a bending work ratio (B _R ) represented by the following [Relationship 4] satisfies the range of 0.5 to 3.0. Therefore, not only does it have an excellent balance between strength and ductility and a balance between strength and hole expandability, but it also has excellent bending workability.
[Relationship 2]
_BTE = [tensile strength (TS, MPa)] × [elongation rate (El, %)]
[Relationship 3]
_BTH = [tensile strength (TS, MPa)] ² × [hole expansion ratio (HER,%)] ^1/2
[Relationship 4]
_BR = R/t
In the above Relational Formula 4, R means the minimum bending radius (mm) at which no cracks occur after a 90° bending test, and t means the thickness (mm) of the steel plate.

In the present invention, it is important to stabilize the retained austenite in the steel sheet in order to simultaneously secure not only high strength properties but also excellent ductility and bending workability. In order to stabilize the retained austenite, it is necessary to concentrate carbon (C) and manganese (Mn) in the ferrite, bainite, and tempered martensite of the steel sheet in the austenite. However, if carbon (C) is concentrated in the austenite using ferrite, the strength of the steel sheet may be insufficient due to the low strength properties of ferrite, and excessive interphase hardness differences may occur, resulting in a decrease in the hole expansion ratio (HER). Therefore, the present invention concentrates carbon (C) and manganese (Mn) in the austenite using bainite and tempered martensite.

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

Therefore, in the present invention, the ratio of the average total content ([Si+Al] F , wt%) of silicon (Si) and aluminum (Al) contained in ferrite to the average total content ([Si+Al] _γ , wt%) of _silicon (Si) and aluminum (Al) contained in the retained austenite is limited to 1.1 or more, so that the interphase hardness difference between the soft structure and the hard structure can be effectively reduced. On the other hand, if the content of silicon (Si) and aluminum (Al) in ferrite is excessive, the ferrite becomes excessively hard and the workability decreases, so that the desired balance between tensile strength and elongation rate (TS×El), the balance between tensile strength and hole expansion rate (TS ² ×HER ^1/2 ), and the bending rate (R/t) cannot all be secured. Therefore, the present invention can limit the ratio of the average total content of silicon (Si) and aluminum (Al) contained in ferrite ([Si+Al] _F , wt %) to the average total content of silicon (Si) and aluminum (Al) contained in retained austenite ([Si+Al] _γ , wt %) to 3.0 or less.

Steel sheets containing retained austenite have excellent ductility and bending workability due to transformation organic sintering that occurs when austenite transforms into martensite during processing. If the fraction of retained austenite is less than a certain level, the balance between tensile strength and elongation (TS x El) may be less than 22,000 MPa% or the bending workability (R/t) may exceed 3.0. On the other hand, if the fraction of retained austenite exceeds a certain level, the local elongation may decrease. Therefore, in the present invention, the fraction of retained austenite can be limited to a range of 10 to 40 volume % in order to obtain a steel sheet with an excellent bending workability (R/t) as well as a good balance between tensile strength and elongation (TS x El).

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

In order to improve the balance between tensile strength and elongation (TS×El), the balance between tensile strength and hole expansion ratio (TS ² ×HER ^1/2 ), and the bending ratio (R/t), it is preferable that bainite is appropriately contained as a microstructure. Only when the fraction of bainite is at a certain level or higher, the balance between tensile strength and elongation ratio (TS×El) of 22,000 MPa% or more, the balance between tensile strength and hole expansion ratio (TS ² ×HER ^1/2 ) of 7×10 ⁶ (MPa ² % ^1/2 ) or more, and the bending ratio (R/t) of 0.5 to 3.0 can be ensured. On the other hand, when the fraction of bainite is excessive, a decrease in the fraction of tempered martensite is inevitably accompanied, and as a result, the balance between tensile strength and elongation ratio (TS×El), the balance between tensile strength and hole expansion ratio (TS ² ×HER ^1/2 ), and the bending ratio (R/t) targeted by the present invention cannot be ensured. Therefore, the present invention can limit the bainite fraction to the range of 10 to 45 volume percent.

Since ferrite is an element that contributes to improving ductility, the balance between tensile strength and elongation (TS×El) targeted by the present invention can be ensured only when the fraction of ferrite is at a certain level or higher. However, if the fraction of ferrite is excessive, the interphase hardness difference increases and the hole expansion ratio (HER) may decrease, making it impossible to ensure the balance between tensile strength and hole expansion ratio (TS ² ×HER ^1/2 ) targeted by the present invention. Therefore, the present invention can limit the fraction of ferrite to a range of 3 to 20 volume percent.

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

A method for manufacturing a high-strength steel sheet according to one aspect of the present invention includes the steps of preparing a steel slab having a predetermined composition, heating the steel slab, and hot rolling the steel slab; winding the hot-rolled steel sheet; heat-treating the wound steel sheet by hot-rolling annealing at a temperature range of 650 to 850°C for 600 to 1700 seconds; cold-rolling the steel sheet heat-treated by hot-rolling annealing; heating the cold-rolled steel sheet to a temperature range of Ac1 to Ac3 at an average heating rate of 5°C/s or more (primary heating) and maintaining the temperature for 50 seconds or more (primary maintenance); cooling to a temperature range of 100 to 300°C at an average cooling rate of 1°C/s or more (primary cooling); heating the primarily cooled steel sheet to a temperature range of 300 to 500°C (secondary heating) and maintaining the temperature for 50 seconds or more (secondary maintenance); and cooling to room temperature (secondary cooling).

[0033] A steel slab having a predetermined composition is prepared. Since the steel slab of the present invention has an alloy composition corresponding to the alloy composition of the steel plate described above, the description of the alloy composition of the steel slab replaces the description of the alloy composition of the steel plate described above.

The prepared steel slab can be heated 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°C, there is a risk that it will be hot rolled in a temperature range below the temperature range of the intended finish hot rolling, and if the heating temperature of the steel slab exceeds 1350°C, there is a risk that it will reach the melting point of the steel and melt.

Hot Rolling and Coiling 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 become a problem, and if the finish hot rolling temperature exceeds 1000°C, the crystal grains of the hot rolled steel sheet may be formed coarsely, which may cause a decrease in the physical properties of the final steel sheet.

After hot rolling, the hot-rolled steel sheet may be cooled at an average cooling rate of 10°C/s or more and coiled at a temperature of 300 to 600°C. If the coiling temperature is less than 300°C, coiling is not easy, and if the coiling temperature exceeds 600°C, surface scale may form inside the hot-rolled steel sheet, making pickling difficult.

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

Pickling and cold rolling In order to remove scale formed on the steel sheet surface after the heat treatment by hot rolling annealing, pickling and cold rolling can be performed. In the present invention, the conditions of pickling and cold rolling are not particularly limited, but it is preferable that cold rolling is performed at a cumulative reduction of 30 to 90%. If the cumulative reduction 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 produced as an uncoated cold-rolled steel sheet after an annealing heat treatment process, or as a coated steel sheet after a coating process to impart corrosion resistance. Coating methods such as hot-dip galvanizing, electrolytic galvanizing, and hot-dip aluminum plating can be applied, and the method and type are not particularly limited.

Annealing Heat Treatment In the present invention, an annealing heat treatment step is carried out in order to simultaneously ensure the strength and workability of the steel sheet.

The cold-rolled steel sheet is heated (primary heating) in a temperature range of Ac1 or more and less than Ac3 (two-phase region) and maintained at that temperature range for 50 seconds or more (primary maintenance). If the primary heating or primary maintenance temperature is Ac3 or more (single-phase region), the desired ferrite structure cannot be realized, and the desired level of [Si+Al] _F /[Si+Al] _γ and the balance between tensile strength and hole expansion ratio ( ^TS2 ×HER1 ^/2 ) cannot be realized. In addition, if the primary heating or primary maintenance temperature is in a temperature range less than Ac1, sufficient heating is not performed, and the microstructure desired by the present invention may not be realized even by the subsequent heat treatment. The average heating rate of the primary heating may be 5°C/s or more.

If the primary maintenance time is less than 50 seconds, the structure may not be sufficiently homogenized, and the physical properties of the steel sheet may deteriorate. There is no particular upper limit to the primary maintenance time, but it is preferable to limit the primary heating time to 1200 seconds or less to prevent a decrease in toughness due to grain coarsening.

After the first maintenance, the steel sheet can be cooled (primary cooling) to a primary cooling stop temperature of 100 to 300°C at a primary cooling rate of an average cooling rate of 1°C/s or more. The upper limit of the primary cooling rate does not need to be specified, but it is preferable to set it to 100°C or less. When the primary cooling stop temperature is less than 100°C, tempered martensite is excessively formed, and the amount of retained austenite formed is insufficient, which can reduce the [Si+Al] _F /[Si+Al] _γ , the balance between tensile strength and elongation rate (TS×El), and the bending work rate (R/t) of the steel sheet. On the other hand, when the primary cooling stop temperature exceeds 300°C, bainite is excessively formed, and the amount of tempered martensite formed is insufficient, which can reduce the balance between tensile strength and elongation rate (TS×El) and the balance between tensile strength and hole expansion rate ( ^TS2 ×HER1 ^/2 ) of the steel sheet.

After the primary cooling, the steel sheet is heated to a secondary heating temperature of 300 to 500°C at an average heating rate of 5°C/s or more (secondary heating) and maintained at that temperature range for 50 seconds or more (secondary maintenance). The upper limit of the secondary heating rate does not need to be specified, but it is preferably 100°C or less. If the secondary heating or secondary maintenance temperature is less than 300°C or the maintenance time is less than 50 seconds, tempered martensite is excessively formed, the content of silicon (Si) and aluminum (Al) in the steel is insufficiently controlled, and it is difficult to ensure the desired fraction of retained austenite. As a result, the [Si+Al] _F /[Si+Al] _γ , the balance between tensile strength and elongation rate (TS×El), and the bending rate (R/t) of the steel sheet may decrease. On the other hand, when the secondary heating or holding temperature exceeds 500° C. or the secondary holding time is 144,000 seconds or longer, the content of silicon (Si) and aluminum (Al) in the steel is insufficiently controlled, making it difficult to ensure the fraction of retained austenite, and as a result, the [Si+Al] _F /[Si+Al] _γ and the balance between tensile strength and elongation (TS×El) of the steel sheet may decrease.

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 plate with excellent workability manufactured by the above-mentioned manufacturing method can contain tempered martensite, bainite, retained austenite and ferrite as microstructures, and as a preferred example, can contain, by volume fraction, 30-70% tempered martensite, 10-45% bainite, 10-40% retained austenite, 3-20% ferrite and unavoidable structures.

In addition, the high-strength steel plate having excellent workability manufactured by the above-mentioned manufacturing method can satisfy the ratio of the average total content of silicon (Si) and aluminum (Al) contained in ferrite ([Si+Al] _F , weight%) to the average total content of silicon (Si) and aluminum (Al) contained in retained austenite ([Si+Al] _γ , weight%) in the range of 1.1 to 3.0 as shown in the following [Relational Formula 1], the balance between tensile strength and elongation rate (B _T·E ) represented by the following [Relational Formula 2] is 22,000 (MPa%) or more, the balance between tensile strength and hole expansion rate (B _T·H ) represented by the following [Relational Formula 3] is 7×10 ⁶ (MPa ² % ^1/2 ) or more, and the bending work rate (B _R ) represented by the following [Relational Formula 4] can satisfy the range of 0.5 to 3.0.
[Relationship 1]
1.1≦[Si+Al] _F /[Si+Al] _γ ≦3.0
[Relationship 2]
_BTE = [tensile strength (TS, MPa)] × [elongation rate (EL, %)]
[Relationship 3]
_BTH = [tensile strength (TS, MPa)] ² × [hole expansion ratio (HER,%)] ^1/2
[Relationship 4]
_BR = R/t
In the above Relational Formula 4, R means the minimum bending radius (mm) at which no cracks occur after a 90° bending test, and t means the thickness (mm) of the steel plate.

The high-strength steel sheet with excellent workability and the manufacturing method thereof according to one aspect of the present invention will be described in more detail below with reference to specific examples. It should be noted that the following examples are intended to aid in understanding the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

(Example)
A steel slab having a thickness of 100 mm and having the alloy composition shown in Table 1 below (the remainder being Fe and unavoidable impurities) was manufactured, heated at 1200°C, and then subjected to finish hot rolling at 900°C. Thereafter, the steel slab was cooled at an average cooling rate of 30°C/s and coiled at the coiling temperatures shown in Tables 2 and 3 to manufacture a hot-rolled steel sheet having a thickness of 3 mm. The hot-rolled steel sheet was heat treated by hot rolling annealing under the conditions shown in Tables 2 and 3. Thereafter, the steel slab was pickled to remove surface scale, and then cold rolled to a thickness of 1.5 mm.

Then, heat treatment was performed under the annealing heat treatment conditions in Tables 2 to 5 to produce steel sheets.

The microstructure of the steel sheets manufactured in this manner was observed, and the results are shown in Tables 6 and 7. Among the microstructures, ferrite (F), bainite (B), tempered martensite (TM), and pearlite (P) were observed through a SEM after nital etching of the polished cross section of the test specimen. Of these, the fractions of bainite and tempered martensite, which are difficult to distinguish, were calculated using dilatation curves after dilation evaluation. Meanwhile, since it is also difficult to distinguish between fresh martensite (FM) and retained austenite (residual γ), the fraction of retained austenite calculated by X-ray diffraction method was subtracted from the fractions of martensite and retained austenite observed with the SEM to determine the fraction of fresh martensite.

On the other hand, the [Si+Al] _/ [Si+Al] _γ , the balance between tensile strength and elongation rate (TS×El), the balance between tensile strength and hole expansion rate ( ^TS2 ×HER1 ^/2 ), and the bending rate (R/t) of the steel sheet were observed. The results are shown in Tables 8 and 9.

The average total content of silicon (Si) and aluminum (Al) contained in the retained austenite ([Si+Al] _γ , % by weight) and the average total content of silicon (Si) and aluminum (Al) contained in the ferrite ([Si+Al] _F , % by weight) were measured using an EPMA (Electron Probe MicroAnalyser).

The tensile strength (TS) and elongation (El) were evaluated by a tensile test, and the tensile strength (TS) and elongation (El) were measured using test pieces taken based on the 90° direction relative to the rolling direction of the rolled plate material according to JIS No. 5. The bending rate (R/t) was evaluated by a V-bending test, and the test pieces were taken based on the 90° direction relative to the rolling direction of the rolled plate material, and the minimum bending radius R at which no cracks were generated after the 90° bending test was divided by the thickness t of the plate material. The hole expansion ratio (HER) was evaluated by a hole expansion test. After forming a punched hole of 10 mmφ (die inner diameter 10.3 mm, clearance 12.5%), a conical punch with an apex angle of 60° was inserted into the punched hole in a direction such that the burr of the punched hole was on the outside, and the periphery of the punched hole was compressed and expanded at a moving speed of 20 mm/min. Then, the HER ratio was calculated using the following [Relationship 5].
[Relationship 5]
Hole expansion ratio (HER,%)={(D−D ₀ )/D ₀ }×100
In the above Relational Formula 5, D means the hole diameter (mm) when the crack penetrates the steel plate along the thickness direction, and _D0 means the initial hole diameter (mm).

As shown in Tables 1 to 9 above, in the case of test pieces satisfying the conditions presented in the present invention, the value of [Si+Al] _F /[Si+Al] _γ is in the range of 1.1 to 3.0, the balance between tensile strength and elongation rate (TS×El) is 22,000 MPa% or more, the balance between tensile strength and hole expansion rate (TS ² ×HER ^1/2 ) is 7×10 ⁶ (MPa ² % ^1/2 ) or more, and the bending rate (R/t) is in the range of 0.5 to 3.0, which shows that the test pieces have excellent strength and workability at the same time.

Test pieces 2 to 5 overlap with the alloy composition range of the present invention, but the hot rolling annealing temperature and time are outside the range of the present invention, so it can be confirmed that poor pickling occurred and fracture occurred during cold rolling.

In the case of test piece 6, the amount of ferrite formed was insufficient because the primary heating or maintenance temperature in the annealing heat treatment process after cold rolling exceeded the range limited by the present invention. As a result, it can be confirmed that the value of [Si+Al] _F /[Si+Al] _γ in test piece 6 is less than 1.1, and the balance between tensile strength and hole expansion ratio ( ^TS2 ×HER1 ^/2 ) is less than 7× ¹⁰⁶ ( ^MPa2 % ^1/2 ).

In the case of test piece 7, the primary cooling rate during the annealing heat treatment after cold rolling was not within the range limited by the present invention, so that ferrite was formed excessively and retained austenite was formed in small amounts. As a result, it can be confirmed that the value of [Si+Al] _F /[Si+Al] _γ of test piece 7 exceeds 3.0, and the balance between tensile strength and elongation (TS×El) is less than 22,000 MPa%.

In the test piece 12, the primary cooling stop temperature was low, so that tempered martensite was excessively formed and the amount of retained austenite was small. As a result, it can be confirmed that the value of [Si+Al] _F /[Si+Al] _γ of the test piece 12 exceeds 3.0, the balance between the tensile strength and the elongation rate (TS×El) is less than 22,000 MPa%, and the bending work ratio (R/t) exceeds 3.0.

In test piece 13, the primary cooling stop temperature was high, so that bainite was excessively formed and tempered martensite was not formed much. As a result, it can be confirmed that the balance between tensile strength and elongation rate (TS×El) of test piece 13 is less than 22,000 MPa%, and the balance between tensile strength and hole expansion rate ( ^TS2 ×HER1 ^/2 ) is less than 7× ¹⁰⁶ ( ^MPa2 % ^1/2 ).

In the test piece 14, the secondary heating or holding temperature was low, so that tempered martensite was excessively formed and retained austenite was little formed. As a result, it was confirmed that the value of [Si+Al] _F /[Si+Al] _γ of the test piece 14 exceeded 3.0, the balance between tensile strength and elongation (TS×El) was less than 22,000 MPa%, and the bending ratio (R/t) exceeded 3.0.

In the case of the test piece 15, the secondary heating or maintenance temperature was high, so that the amount of the retained austenite formed was insufficient, and the value of [Si+Al] _F /[Si+Al] _γ exceeded 3.0, and the balance between the tensile strength and the elongation rate (TS×El) was less than 22,000 MPa%.

In the case of the test piece 16, the second holding time was insufficient, so that the tempered martensite was excessively formed and the retained austenite was little formed. As a result, it can be confirmed that the value of [Si+Al] _F /[Si+Al] _γ of the test piece 16 exceeds 3.0, the balance between the tensile strength and the elongation rate (TS×El) is less than 22,000 MPa%, and the bending work ratio (R/t) exceeds 3.0.

It can be seen that in test piece 17, the secondary maintenance time was excessive, resulting in an insufficient amount of retained austenite formation, the value of [Si+Al] _F /[Si+Al] _γ exceeded 3.0, and the balance between tensile strength and elongation (TS×El) was less than 22,000 MPa%.

The test pieces 40 to 48 satisfy the manufacturing conditions presented in the present invention, but are outside the range of the alloy composition. In these cases, it can be confirmed that the [Si+Al] _F /[Si+Al] _γ of the present invention, the balance between tensile strength and elongation rate (TS×El), the balance between tensile strength and hole expansion rate (TS ² ×HER ^1/2 ) of 7×10 ⁶ (MPa ² % ^1/2 ), and the bending rate (R/t) conditions cannot be simultaneously satisfied. On the other hand, the test piece 42, in which the total content of aluminum (Al) and silicon (Si) is less than 1.0%, does not satisfy the conditions of [Si+Al] _F /[Si+Al] _γ , the balance between tensile strength and elongation rate (TS×El), and the bending rate (R/t).

The present invention has been described in detail above using examples, but other embodiments are also possible. Therefore, the technical ideas and scope of the claims described below are not limited to the examples.

Claims (9)

In weight percent, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, the remainder being Fe and unavoidable impurities;
The total content of Si and Al (Si+Al) is 1.0 to 6.0 wt.%;
The microstructure includes 30 to 70 volume % tempered martensite, 10 to 45 volume % bainite, 10 to 40 volume % retained austenite, 3 to 20 volume % ferrite, and unavoidable structures;
A steel plate satisfying the following [Relationship 1].
[Relationship 1]
1.1≦[Si+Al] _F /[Si+Al] _γ ≦3.0
In the above-mentioned Relational Formula 1, [Si+Al] _F is the average total content (wt%) of Si and Al contained in ferrite, and [Si+Al] _γ is the average total content (wt%) of Si and Al contained in retained austenite. The steel plate according to claim 1, further comprising any one or more of the following (1) to (9):
(1) One or more of Ti: 0-0.5%, Nb: 0-0.5%, and V: 0-0.5%; (2) One or more of Cr: 0-3.0% and Mo: 0-3.0%; (3) One or more of Cu: 0-4.5% and Ni: 0-4.5%; (4) B: 0-0.005%.
(5) One or more of Ca: 0-0.05%, REM excluding Y: 0-0.05%, and Mg: 0-0.05%. (6) One or more of W: 0-0.5%, and Zr: 0-0.5%. (7) One or more of Sb: 0-0.5%, and Sn: 0-0.5%. (8) One or more of Y: 0-0.2%, and Hf: 0-0.2%. (9) Co: 0-1.5%. The steel sheet according to claim 1, wherein the balance (B _T·E ) between tensile strength and elongation rate represented by the following [Relationship 2] is 22,000 (MPa%) or more, the balance (B _T·H ) between tensile strength and hole expansion rate represented by the following [Relationship 3] is 7×10 ⁶ (MPa ² % ^1/2 ) or more, and the bending ratio (B _R ) represented by the following [Relationship 4] is 0.5 to 3.0.
[Relationship 2]
_BTE = [tensile strength (TS, MPa)] × [elongation rate (El, %)]
[Relationship 3]
_BTH = [tensile strength (TS, MPa)] ² × [hole expansion ratio (HER,%)] ^1/2
[Relationship 4]
_BR = R/t
In the above Relational Formula 4, R means the minimum bending radius (mm) at which no cracks occur after a 90° bending test, and t means the thickness (mm) of the steel plate. heating and hot rolling a steel slab having, by weight percent, C: 0.25-0.75%, Si: 4.0% or less, Mn: 0.9-5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, and the remainder being Fe and unavoidable impurities , and the total content of Si and Al (Si+Al) being 1.0-6.0% by weight;
coiling the hot rolled steel sheet;
heat treating the coiled steel sheet by hot rolling annealing at a temperature range of 650 to 850° C. for 600 to 1700 seconds;
cold rolling the steel sheet heat-treated by the hot rolling annealing;
heating the cold-rolled steel sheet to a temperature range of Ac1 or more and less than Ac3 (primary heating) and maintaining the temperature for 50 seconds or more (primary maintenance);
a step of cooling (primary cooling) to a temperature range of 100 to 300° C. at an average cooling rate of 1.0° C./s or more;
The method for manufacturing a steel sheet according to any one of claims 1 to 3, comprising: heating the primarily cooled steel sheet to a temperature range of 300 to 500°C (secondary heating) and maintaining the temperature for 50 seconds or more and 144,000 seconds or less (secondary maintenance); and cooling the steel sheet to room temperature (secondary cooling). The method for producing a steel plate according to claim 4, wherein the steel slab further comprises any one or more of the following (1) to (9):
(1) One or more of Ti: 0-0.5%, Nb: 0-0.5%, and V: 0-0.5%; (2) One or more of Cr: 0-3.0% and Mo: 0-3.0%; (3) One or more of Cu: 0-4.5% and Ni: 0-4.5%; (4) B: 0-0.005%.
(5) One or more of Ca: 0-0.05%, REM excluding Y: 0-0.05%, and Mg: 0-0.05%. (6) One or more of W: 0-0.5%, and Zr: 0-0.5%. (7) One or more of Sb: 0-0.5%, and Sn: 0-0.5%. (8) One or more of Y: 0-0.2%, and Hf: 0-0.2%. (9) Co: 0-1.5%. The method for manufacturing steel plate according to claim 4, wherein the steel slab is heated to a temperature range of 1000 to 1350°C and finish hot rolled to a temperature range of 800 to 1000°C. The method for manufacturing steel sheet according to claim 4, wherein the hot-rolled steel sheet is coiled at a temperature range of 300 to 600°C. The method for manufacturing steel sheet according to claim 4, wherein the cold rolling reduction is 30 to 90%. The method for manufacturing steel sheet according to claim 4, wherein the cooling rate of the secondary cooling is 1°C/s or more.