High-strength steel sheet excellent in workability and method for producing the same

Abstract

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

Description

Technical Field

[0001] The present invention relates to a steel sheet that can be used for automotive parts and the like, and relates to a steel sheet having excellent workability while having high strength characteristics, and a method for manufacturing the same.

Background Art

[0002] In the recent automotive industry, in order to protect the global environment, efforts are being made to reduce the weight of materials, and attention is being paid to solutions that can ensure the safety of passengers. In order to meet such requirements for safety and weight reduction, the application of high-strength steel sheets has been rapidly increasing. Generally, it is known that as the strength of the steel sheet increases, the workability of the steel sheet decreases. Therefore, in the case of steel sheets for automotive parts, there is a demand for steel sheets that have high strength characteristics and excellent workability represented by ductility and hole expansion properties.

[0003] TRIP (Transformation Induced Plasticity) steel that utilizes the transformation-induced plasticity of retained austenite has a complex microstructure composed of ferrite, bainite, martensite, retained austenite, etc., and is known to have high strength characteristics and workability above a certain level.

[0004] As a technique for further improving the workability of the steel sheet, methods of utilizing tempered martensite are disclosed in Patent Documents 1 and 2. Since tempered martensite is produced by tempering hard martensite and is softened martensite, there is a difference in strength between tempered martensite and existing untempered martensite (fresh martensite). Therefore, if fresh martensite is suppressed and tempered martensite is formed, the workability can be improved.

[0005] However, in the techniques disclosed in Patent Documents 1 and 2, the balance between the tensile strength and the elongation rate (TS 2 ×EL 1 / 2 ) is 3.0×10 6~6.2×10 6 (MPa 2 % 1 / 2 ) does not satisfy the range, which means it is difficult to ensure a steel sheet excellent in both strength and ductility.

[0006] On the other hand, as another technique for improving the workability of the steel sheet, a method of inducing the formation of bainite by adding boron (B) is disclosed in Patent Document 3. By adding boron (B), the ferrite - pearlite transformation is suppressed and the formation of bainite is induced, so that both strength and workability can be achieved.

[0007] However, in the technique disclosed in Patent Document 3, the balance (B TE ) of the tensile strength and elongation rate of 3.0×10 6 ~6.2×10 6 (MPa 2 %, 1 / 2 ), the balance (B TH ) of the tensile strength and hole expansion rate of 6.0×10 6 ~11.5×10 6 (MPa 2 %, 1 / 2 ) and the yield ratio evaluation index (I YR ) of 0.15 to 0.42 cannot be ensured simultaneously, and it was difficult to ensure a steel sheet excellent in all of strength, hole expandability, ductility, and yield ratio.

[0008] That is, the actual situation is that the requirements for a steel sheet excellent in all of the balance (B TE ) of the tensile strength and elongation rate, the balance (B TH ) of the tensile strength and hole expansion rate, and the yield ratio evaluation index (I YR ) are not satisfied.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

[0010] According to one aspect of the present invention, it is possible to provide a steel sheet having an optimized composition and microstructure of the steel sheet, excellent in all of the balance between tensile strength and elongation, the balance between tensile strength and hole expansion ratio, and the yield ratio evaluation index, and a method for manufacturing the same.

[0011] The problems of the present invention are not limited to the above-mentioned matters. Further problems of the present invention are described in the overall content of the specification, and those having ordinary knowledge in the technical field to which the present invention pertains will have no difficulty in understanding the further problems of the present invention from the content described in the specification of the present invention. [Means for Solving the Problems]

[0012] The high-strength steel sheet excellent in workability according to one aspect of the present invention contains, by weight%, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, and the balance of Fe and inevitable impurities, and as the microstructure, contains bainite, tempered martensite, fresh martensite, retained austenite, and other inevitable structures, and can satisfy the following [Relational Expression 1].[[]]

[0013] [Relational Expression 1] 0.03 ≤ [B] FM / [B] TM ≤ 0.55

[0014] In the above Relational Expression 1, [B] FM is the content (weight%) of boron (B) contained in fresh martensite, and [B] TMis the content (weight %) of boron (B) contained in tempered martensite.

[0015] The above steel plate can further contain any one or more of the following (1) to (8) by weight %.

[0016] (1) One or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5% (2) One or more of Cr: 0 to 3.0% and Mo: 0 to 3.0% (3) One or more of Cu: 0 to 4.0% and Ni: 0 to 4.0% (4) One or more of Ca: 0 to 0.05%, REM excluding Y: 0 to 0.05%, and Mg: 0 to 0.05% (5) One or more of W: 0 to 0.5% and Zr: 0 to 0.5% (6) One or more of Sb: 0 to 0.5% and Sn: 0 to 0.5% (7) One or more of Y: 0 to 0.2% and Hf: 0 to 0.2% (8) Co: 0 to 1.5%

[0017] The microstructure of the above steel plate can contain, by volume fraction, 10 to 30% bainite, 50 to 70% tempered martensite, 10 to 30% fresh martensite, 2 to 10% retained austenite, and 5% or less (including 0%) ferrite.

[0018] The above steel plate has a balance (B TE ) of tensile strength and elongation rate represented by the following [Relationship 2] of 3.0×10 6 ~6.2×10 6 (MPa 2 %) 1 / 2 and a balance (B TH ) of tensile strength and hole expansion rate represented by the following [Relationship 3] of 6.0×10 6 ~11.5×10 6 (MPa 2 %) 1 / 2 and can satisfy a yield ratio evaluation index (I YR ) represented by the following [Relationship 4] of 0.15 to 0.42.

[0019] [Relational Expression 2] B TE =[Tensile Strength (TS, MPa)] 2 ×[Elongation (El, %)] 1 / 2

[0020] [Relational Expression 3] B TH =[Tensile Strength (TS, MPa)] 2 ×[Hole Expansion Ratio (HER, %)] 1 / 2

[0021] [Relational Expression 4] I YR =1 - [Yield Ratio (YR)]

[0022] The method for manufacturing a high-strength steel sheet excellent in workability according to one aspect of the present invention provides a cold-rolled steel sheet containing, by weight%, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, and the balance Fe and unavoidable impurities; heating the cold-rolled steel sheet to 700°C at an average heating rate of 5°C / s or more (primary heating), heating the cold-rolled steel sheet to a temperature range of Ac3 to 920°C at an average heating rate of 5°C / s or less (secondary heating), and then holding for 50 to 1200 seconds (primary holding); cooling the steel sheet held in the primary holding to a temperature range of 200 to 400°C at an average cooling rate of 1°C / s or more (primary cooling); heating the steel sheet cooled in the primary cooling to a temperature range of 350 to 550°C at an average heating rate of 5°C / s or more (tertiary heating), and then holding for 50 seconds or more (secondary holding); and cooling the steel sheet held in the secondary holding to room temperature at an average cooling rate of 1°C / s or more (secondary cooling).

[0023] The steel slab can further contain any one or more of the following (1) to (8).

[0024] (1) One or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5% (2) One or more of Cr: 0 to 3.0% and Mo: 0 to 3.0% (3) One or more of Cu: 0 to 4.0% and Ni: 0 to 4.0% (4) One or more of Ca: 0 to 0.05%, REM excluding Y: 0 to 0.05%, and Mg: 0 to 0.05% (5) One or more of W: 0 to 0.5% and Zr: 0 to 0.5% (6) One or more of Sb: 0 to 0.5% and Sn: 0 to 0.5% (7) One or more of Y: 0 to 0.2% and Hf: 0 to 0.2% (8) Co: 0 to 1.5%

[0025] The cold-rolled steel sheet can be provided through the steps of heating a steel slab at 1000 to 1350°C, finish hot-rolling in a temperature range of 800 to 1000°C, winding up the hot-rolled steel sheet in a temperature range of 350 to 650°C, pickling the wound-up steel sheet, and cold-rolling the pickled steel sheet at a reduction ratio of 30 to 90%.

Effect of the Invention

[0026] According to a preferred aspect of the present invention, it is possible to provide a steel sheet and a method for manufacturing the same, which are excellent in the balance between tensile strength and ductility, the balance between tensile strength and hole expansion property, and yield ratio evaluation index, and are suitably used for automobile parts and the like.

Embodiment for Carrying Out the Invention

[0027] The present invention relates to a high-strength steel sheet excellent in workability and a method for manufacturing the same. Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention can be deformed into various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. This embodiment is provided to explain the present invention in more detail to those having ordinary knowledge in the technical field to which the invention belongs.

[0028] The inventors of the present invention have found that in boron (B)-added transformation-induced plasticity (TRIP) steel containing bainite, tempered martensite, fresh martensite, and retained austenite, when the volume fractions of tempered martensite, fresh martensite, and retained austenite are controlled within a certain range, the contents of boron (B) contained in the tempered martensite and fresh martensite are controlled within a certain range, and the shape and size of the retained austenite are controlled within a certain range, it is possible to simultaneously ensure an excellent balance between tensile strength and ductility, an excellent balance between tensile strength and hole expansion property, and an excellent yield ratio evaluation index. Based on this discovery, they investigated and devised a method that can effectively achieve both excellent strength, yield ratio, ductility, and hole expansion property, leading to the present invention.

[0029] Hereinafter, the high-strength steel sheet with excellent workability according to one aspect of the present invention will be described in more detail.

[0030] The high-strength steel sheet with excellent workability according to one aspect of the present invention contains, by weight%, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, and the balance of Fe and inevitable impurities. As the microstructure, it contains bainite, tempered martensite, fresh martensite, retained austenite, and other inevitable microstructures, and can satisfy the following [Relationship 1].

[0031] [Relationship 1] 0.03 ≤ [B] FM FM / [B] TM TM ≤ 0.55

[0032] In the above Relationship 1, [B] FM FM is the content (weight%) of boron (B) contained in the fresh martensite, and [B] TM TM is the content (weight%) of boron (B) contained in the tempered martensite.

[0033] Hereinafter, the steel composition of the present invention will be described in more detail. Hereinafter, unless otherwise specified, the % indicating the content of each element is based on weight.

[0034] The high-strength steel sheet excellent in workability according to one aspect of the present invention contains, by weight %, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, and the balance Fe and unavoidable impurities. Additionally, it can further contain one or more of Ti: 0.5% or less (including 0%), Nb: 0.5% or less (including 0%), V: 0.5% or less (including 0%), Cr: 3.0% or less (including 0%), Mo: 3.0% or less (including 0%), Cu: 4.0% or less (including 0%), Ni: 4.0% or less (including 0%), Ca: 0.05% or less (including 0%), REM excluding Y: 0.05% or less (including 0%), Mg: 0.05% or less (including 0%), W: 0.5% or less (including 0%), Zr: 0.5% or less (including 0%), Sb: 0.5% or less (including 0%), Sn: 0.5% or less (including 0%), Y: 0.2% or less (including 0%), Hf: 0.2% or less (including 0%), Co: 1.5% or less (including 0%).

[0035] Carbon (C): 0.1 to 0.25% Carbon (C) is an essential element for ensuring the strength of the steel sheet and is also an element that stabilizes retained austenite contributing to the improvement of the ductility of the steel sheet. Therefore, in the present invention, in order to achieve such an effect, carbon (C) can be contained in an amount of 0.1% or more. The preferable carbon (C) content may exceed 0.1%, may be 0.11% or more, or may be 0.12% or more. On the other hand, when the carbon (C) content exceeds a certain level, the ductility may decrease due to excessive strength increase, and the weldability may deteriorate. Therefore, in the present invention, the upper limit of the carbon (C) content can be limited to 0.25%. The carbon (C) content may be 0.24% or less, and a more preferable carbon (C) content may be 0.23% or less.

[0036] Silicon (Si): 0.01 to 1.5% or less Silicon (Si) is an element that contributes to improving strength through solid solution strengthening and is also an element that improves workability by homogenizing the structure. In addition, silicon (Si) suppresses the precipitation of cementite and contributes to the formation of retained austenite. Therefore, in the present invention, in order to achieve such effects, silicon (Si) of 0.01% or more can be added. The preferred silicon (Si) content may be 0.02% or more, and the more preferred silicon (Si) content may be 0.04% or more. However, when the silicon (Si) content exceeds a certain level, it may not only induce problems such as plating defects like unplated areas in the plating process but also reduce the weldability of the steel sheet. Therefore, in the present invention, the upper limit of the silicon (Si) content can be limited to 1.5%. The preferred upper limit of the silicon (Si) content may be 1.48%, and the more preferred upper limit of the silicon (Si) content may be 1.46%.

[0037] Manganese (Mn): 1.0 to 4.0% Manganese (Mn) is an element useful for increasing both strength and ductility. Therefore, in the present invention, in order to achieve such effects, manganese (Mn) of 1.0% or more can be added. The preferred lower limit of the manganese (Mn) content may be 1.2%, and the more preferred lower limit of the manganese (Mn) content may be 1.4%. On the other hand, when manganese (Mn) is added in excess, the bainite transformation time increases and the carbon (C) concentration in austenite becomes insufficient, resulting in a problem that the target austenite fraction cannot be ensured. Therefore, in the present invention, the upper limit of the manganese (Mn) content can be limited to 4.0%. The preferred upper limit of the manganese (Mn) content may be 3.9%.

[0038] Aluminum (Al): 0.01 to 1.5% Aluminum (Al) is an element that combines with oxygen in steel to perform a deoxidizing function. Also, like silicon (Si), aluminum (Al) is an element that suppresses cementite precipitation and stabilizes retained austenite. Therefore, in the present invention, in order to achieve such an effect, aluminum (Al) of 0.01% or more can be added. The preferable aluminum (Al) content may be 0.03% or more, and the more preferable aluminum (Al) content may be 0.05% or more. On the other hand, when aluminum (Al) is added in excess, not only does the inclusion in the steel sheet increase, but it may also deteriorate the workability of the steel sheet. Therefore, in the present invention, the upper limit of the aluminum (Al) content can be limited to 1.5%. The upper limit of the preferable aluminum (Al) content can be 1.48%.

[0039] Phosphorus (P): 0.15% or less (including 0%) Phosphorus (P) is an element that is contained as an impurity and deteriorates impact toughness. Therefore, it is preferable to control the content of phosphorus (P) to 0.15% or less.

[0040] Sulfur (S): 0.03% or less (including 0%) Sulfur (S) is an element that is contained as an impurity, forms MnS in the steel sheet, and deteriorates ductility. Therefore, it is preferable that the content of sulfur (S) is 0.03% or less.

[0041] Nitrogen (N): 0.03% or less (including 0%) Nitrogen (N) is an element that is contained as an impurity, forms nitrides during continuous casting, and causes slab cracking. Therefore, it is preferable that the content of nitrogen (N) is 0.03% or less.

[0042] Boron (B): 0.0005 - 0.005% Boron (B) is an element that improves hardenability and increases strength, and is also an element that suppresses nucleation at grain boundaries. Further, in the present invention, in order to simultaneously secure an excellent balance between tensile strength and elongation, an excellent balance between tensile strength and hole expansion property, and an excellent yield ratio evaluation index by concentrating boron (B) in tempered martensite, boron (B) should be added essentially in the present invention. Therefore, in the present invention, boron (B) can be added in an amount of 0.0005% or more for such an effect. However, when boron (B) is added in excess of a certain level, not only excessive characteristic effects but also an increase in manufacturing cost are caused. Therefore, in the present invention, the upper limit of the content of boron (B) can be limited to 0.005%.

[0043] On the other hand, the steel sheet of the present invention has an alloy composition that can be additionally contained in addition to the alloy components described above, and this will be described in detail below.

[0044] One or more of titanium (Ti): 0 to 0.5%, niobium (Nb): 0 to 0.5%, and vanadium (V): 0 to 0.5% Titanium (Ti), niobium (Nb), and vanadium (V) are elements that form precipitates and refine crystal grains, and also contribute to the improvement of the strength and impact toughness of the steel sheet. Therefore, in the present invention, one or more of titanium (Ti), niobium (Nb), and vanadium (V) can be added for such an effect. However, when the content of each of titanium (Ti), niobium (Nb), and vanadium (V) exceeds a certain level, excessive precipitates are formed, not only the impact toughness decreases, but also the manufacturing cost increases. Therefore, in the present invention, the content of titanium (Ti), niobium (Nb), and vanadium (V) can be limited to 0.5% or less, respectively.

[0045] One or more of chromium (Cr): 0 to 3.0% and molybdenum (Mo): 0 to 3.0% Chromium (Cr) and molybdenum (Mo) are elements that not only suppress austenite decomposition during alloying treatment but also stabilize austenite, similar to manganese (Mn). Therefore, in the present invention, one or more of chromium (Cr) and molybdenum (Mo) can be added for such an effect. However, when the contents of chromium (Cr) and molybdenum (Mo) exceed a certain level, the bainite transformation time increases and the amount of carbon (C) enrichment in austenite becomes insufficient, so that the target retained austenite fraction cannot be ensured. Therefore, in the present invention, the contents of chromium (Cr) and molybdenum (Mo) can be limited to 3.0% or less, respectively.

[0046] Copper (Cu): 0 to 4.0% and nickel (Ni): one or more of 0 to 4.0% Copper (Cu) and nickel (Ni) are elements that stabilize austenite and suppress corrosion. Also, copper (Cu) and nickel (Ni) are elements that prevent the intrusion of hydrogen moving into the steel sheet by concentrating on the surface of the steel sheet and suppress hydrogen delayed fracture. Therefore, in the present invention, one or more of copper (Cu) and nickel (Ni) can be added for such an effect. However, when the contents of copper (Cu) and nickel (Ni) exceed a certain level, not only excessive characteristic effects but also an increase in manufacturing cost is caused. Therefore, in the present invention, the contents of copper (Cu) and nickel (Ni) can be limited to 4.0% or less, respectively.

[0047] Calcium (Ca): 0 to 0.05%, magnesium (Mg): 0 to 0.05%, and rare earth elements (REM) excluding yttrium (Y): one or more of 0 to 0.05% Here, the rare earth elements (REM) refer to scandium (Sc), yttrium (Y), and lanthanide elements. Rare earth elements (REM) excluding calcium (Ca), magnesium (Mg), and yttrium (Y) are elements that contribute to improving the ductility of steel sheets by spheroidizing sulfides. Therefore, in the present invention, for such an effect, one or more of the rare earth elements (REM) excluding calcium (Ca), magnesium (Mg), and yttrium (Y) can be added. However, when the content of rare earth elements (REM) excluding calcium (Ca), magnesium (Mg), and yttrium (Y) exceeds a certain level, it not only causes excessive characteristic effects but also increases the manufacturing cost. Therefore, in the present invention, the content of rare earth elements (REM) excluding calcium (Ca), magnesium (Mg), and yttrium (Y) can be limited to 0.05% or less respectively.

[0048] One or more of tungsten (W): 0 - 0.5% and zirconium (Zr): 0 - 0.5% Since tungsten (W) and zirconium (Zr) are elements that improve hardenability and increase the strength of steel sheets, in the present invention, for such an effect, one or more of tungsten (W) and zirconium (Zr) can be added. However, when the content of tungsten (W) and zirconium (Zr) exceeds a certain level, it not only causes excessive characteristic effects but also increases the manufacturing cost. Therefore, in the present invention, the content of tungsten (W) and zirconium (Zr) can be limited to 0.5% or less respectively.

[0049] One or more of antimony (Sb): 0 - 0.5% and tin (Sn): 0 - 0.5% Antimony (Sb) and tin (Sn) are elements that improve the plating wettability and plating adhesion of steel sheets. Therefore, in the present invention, for such effects, one or more of antimony (Sb) and tin (Sn) can be added. However, when the contents of antimony (Sb) and tin (Sn) exceed a certain level, the brittleness of the steel sheet increases and cracks may occur during hot working or cold working. Therefore, in the present invention, the contents of antimony (Sb) and tin (Sn) can be limited to 0.5% or less, respectively.

[0050] One or more of yttrium (Y): 0 to 0.2% and hafnium (Hf): 0 to 0.2% Yttrium (Y) and hafnium (Hf) are elements that improve the corrosion resistance of steel sheets. Therefore, in the present invention, for such effects, one or more of yttrium (Y) and hafnium (Hf) can be added. However, when the contents of yttrium (Y) and hafnium (Hf) exceed a certain level, the ductility of the steel sheet may deteriorate. Therefore, in the present invention, the contents of yttrium (Y) and hafnium (Hf) can be limited to 0.2% or less, respectively.

[0051] Cobalt (Co): 0 to 1.5% Cobalt (Co) is an element that promotes bainite transformation and increases the TRIP effect. Therefore, in the present invention, for such effects, cobalt (Co) can be added. However, when the content of cobalt (Co) exceeds a certain level, the weldability and ductility of the steel sheet may deteriorate. Therefore, in the present invention, the content of cobalt (Co) can be limited to 1.5% or less.

[0052] The high-strength steel sheet with excellent workability according to one aspect of the present invention may contain the remaining Fe and other inevitable impurities in addition to the components described above. However, in the normal manufacturing process, since unintended impurities may inevitably be mixed in from the raw materials or the surrounding environment, it is impossible to completely eliminate them. Since these impurities are known to anyone with ordinary knowledge in the technical field, all of their details are not particularly mentioned in this specification. Furthermore, the additional addition of effective components other than the components described above is not completely excluded.

[0053] The high-strength steel sheet with excellent workability according to one aspect of the present invention may contain bainite, tempered martensite, fresh martensite, retained austenite, and other inevitable microstructures as its microstructure.

[0054] Both untempered martensite (fresh martensite, FM) and tempered martensite (TM) are microstructures that improve the strength of the steel sheet. However, fresh martensite has the characteristic of reducing the ductility and flanging property of the steel sheet compared to tempered martensite. Also, fresh martensite tends to reduce the yield ratio of the steel sheet compared to tempered martensite. This is because the microstructure of tempered martensite is softened by tempering heat treatment. Therefore, in order to ensure the balance of tensile strength and elongation rate (TS 2 ×EL 1 / 2 ), the balance of tensile strength and hole expansion rate (TS 2 ×HER 1 / 2 ), and the yield ratio evaluation index (1 - YR), it is preferable to control the volume fraction of the tempered martensite and fresh martensite microstructures. A balance of tensile strength and elongation rate (TS 6 ×EL 2 ×EL 1 / 2 ) of 3.0×10 6 or more, and a balance of tensile strength and hole expansion rate (TS 2 ×HER 1 / 2) And in order to satisfy a yield ratio evaluation index (1 - YR) of 0.42 or less, it is preferable to limit the fraction of tempered martensite to 50% by volume or more and limit the fraction of fresh martensite to 10% by volume or more. A more preferable fraction of tempered martensite may be 52% by volume or more or 54% by volume or more, and a more preferable fraction of fresh martensite may be 12% by volume or more. On the other hand, when tempered martensite or fresh martensite is excessively formed, ductility and burring property decrease, and eventually the balance of tensile strength and elongation rate of 3.0×10 6 or more (TS 2 ×EL 1 / 2 ), the balance of tensile strength and hole expansion rate of 6.0×10 6 or more (TS 2 ×HER 1 / 2 ), and a yield ratio evaluation index (1 - YR) of 0.42 or less cannot be satisfied simultaneously. Therefore, in the present invention, the fraction of tempered martensite can be limited to 70% by volume or less, and the fraction of fresh martensite can be limited to 30% by volume or less. A more preferable fraction of tempered martensite may be 68% by volume or less or 65% by volume or less, and a more preferable fraction of fresh martensite may be 25% by volume or less.

[0055] In order to ensure the balance of tensile strength and elongation rate (TS 2 ×EL 1 / 2 ), the balance of tensile strength and hole expansion rate (TS 2 ×HER 1 / 2 ), and the yield ratio evaluation index (1 - YR) at the level aimed for by the present invention, optimization of the bainite fraction is necessary. The balance of tensile strength and elongation rate of 3.0×10 6 or more (TS 2 ×EL 1 / 2 ), the balance of tensile strength and hole expansion rate of 6.0×10 6 or more (TS 2 ×HER 1 / 2) In order to secure a yield ratio evaluation index (1 - YR) of 0.42 or less, it is preferable to control the fraction of bainite to 10% by volume or more. A more preferable fraction of bainite may be 12% by volume or more, or 14% by volume or more. On the other hand, when excessive bainite is formed, it induces a decrease in the fraction of tempered martensite, so the balance of the target tensile strength and elongation (TS 2 ×EL 1 / 2 ), the balance of tensile strength and hole expansion ratio (TS 2 ×HER 1 / 2 ), and the fraction of bainite can be restricted to 30% by volume or less in order to secure the yield ratio evaluation index (1 - YR). The preferable fraction of bainite may be 12% by volume or more, or 14% by volume or more, and may also be 28% by volume or less, or 26% by volume or less.

[0056] A steel sheet containing retained austenite has excellent ductility and workability due to transformation-induced plasticity that occurs during the transformation from austenite to martensite during processing. When the fraction of retained austenite is less than a certain level, the balance of tensile strength and elongation (TS 2 ×EL 1 / 2 ) becomes less than 3.0×10 6 (MPa 2 %, 1 / 2 ) which is not preferable. On the other hand, when the fraction of retained austenite exceeds a certain level, the local elongation rate may decrease or the spot weldability may deteriorate. Therefore, in the present invention, the fraction of retained austenite can be restricted to the range of 2 - 10% in order to obtain a steel sheet excellent in the balance of tensile strength and elongation (TS 2 ×EL 1 / 2 ). The preferable fraction of retained austenite may be 3% by volume or more and may also be 8% by volume or less.

[0057] The steel sheet of the present invention may contain, as inevitable structures, ferrite, pearlite, island martensite (Martensite Austenite Constituent, M-A), etc. When ferrite is excessively formed, the strength of the steel sheet may decrease. Therefore, in the present invention, the fraction of ferrite can be limited to 5% by volume (including 0%). Further, when pearlite is excessively formed, the workability of the steel sheet may decrease or the fraction of retained austenite may decrease. Therefore, in the present invention, the formation of pearlite is to be limited as much as possible.

[0058] The high-strength steel sheet excellent in workability according to one aspect of the present invention can satisfy the following [Relational Expression 1].

[0059] [Relational Expression 1] 0.03 ≦ [B] FM / [B] TM ≦ 0.55

[0060] In the above Relational Expression 1, [B] FM is the content (% by weight) of boron (B) contained in fresh martensite, and [B] TM is the content (% by weight) of boron (B) contained in tempered martensite.

[0061] The present invention controls the tissue fractions of tempered martensite, fresh martensite, and retained austenite within a certain range, controls the ratio of the boron (B) content contained in tempered martensite and fresh martensite within a certain range, and controls the ratio of specific sizes, shapes, and types of retained austenite to all retained austenite within a certain range in order to ensure the balance of the target tensile strength and elongation (TS 2 × EL 1 / 2 ), the balance of tensile strength and hole expansion ratio (TS 2 × HER 1 / 2 ), and the yield ratio evaluation index (1 - YR).

[0062] The present invention, as in [Relational Expression 1], the content of boron (B) contained in tempered martensite ([B]TM , the ratio of the content of boron (B) contained in fresh martensite to the content of boron (B) contained in tempered martensite (weight %) ([B] FM ) is controlled within the range of 0.03 to 0.55, so 3.0×10 6 ~6.2×10 6 (MPa 2 %) of the balance of tensile strength and elongation (B 1 / 2 ), 6.0×10 TE ~11.5×10 6 ~11.5×10 6 (MPa 2 %) of the balance of tensile strength and hole expansion rate (B 1 / 2 ), and a yield ratio evaluation index (I TH ) of 0.15 to 0.42 can be ensured simultaneously. YR ) can be ensured simultaneously.

[0063] As a result of intensive research on the physical property ensuring scheme of boron (B)-added TRIP steel by the inventor of the present invention, although the theoretical basis has not been clearly investigated, it has been noted that the physical properties aimed at by the present invention can be ensured only when the ratio of the content of boron (B) contained in fresh martensite to the content of boron (B) contained in tempered martensite satisfies a certain range. In particular, it has been confirmed that the yield ratio of the steel sheet shows a certain tendency according to the ratio of the content of boron (B) contained in tempered martensite and fresh martensite. Therefore, in the present invention, in order to limit the ratio of the content of boron (B) contained in tempered martensite to the content of boron (B) contained in fresh martensite within the range of 0.03 to 0.55 as in [Relationship 1], the balance of the target tensile strength and elongation (TS 2 ×EL 1 / 2 ), the balance of tensile strength and hole expansion rate (TS 2 ×HER 1 / 2 ), and the yield ratio evaluation index (1 - YR) can be ensured.

[0064] The high-strength steel sheet excellent in workability according to one aspect of the present invention has a balance of tensile strength and elongation (B TE ) represented by the following [Relationship 2] of 3.0×10 6 ~6.2×106 (MPa 2 % 1 / 2 ) satisfies the balance (B TH ) of the tensile strength and the hole expansion rate represented by the following [Relational Expression 3] of 6.0×10 6 ~11.5×10 6 (MPa 2 % 1 / 2 ) and can satisfy the yield ratio evaluation index (I YR ) of 0.15 to 0.42 represented by the following [Relational Expression 4].

[0065] [Relational Expression 2] B TE = [Tensile strength (TS, MPa)] 2 ×[Elongation rate (El, %)] 1 / 2

[0066] [Relational Expression 3] B TH = [Tensile strength (TS, MPa)] 2 ×[Hole expansion rate (HER, %)] 1 / 2

[0067] [Relational Expression 4] I YR = 1 - [Yield ratio (YR)]

[0068] Hereinafter, an example of a method for manufacturing the steel plate of the present invention will be described in detail.

[0069] The method for manufacturing a high-strength steel plate according to one aspect of the present invention includes heating a cold-rolled steel plate having a predetermined alloy composition to 700°C at an average heating rate of 5°C / s or more (primary heating), heating it to a temperature range of Ac3 to 920°C at an average heating rate of 5°C / s or less (secondary heating), and then holding it for 50 to 1200 seconds (primary holding); cooling the steel plate held in the primary holding to a temperature range of 200 to 400°C at an average cooling rate of 1°C / s or more (primary cooling); heating the steel plate cooled in the primary cooling to a temperature range of 350 to 550°C at an average heating rate of 5°C / s or more (tertiary heating), and then holding it for 50 seconds or more (secondary holding); and cooling the steel plate held in the secondary holding to room temperature at an average cooling rate of 1°C / s or more (secondary cooling).

[0070] The cold-rolled steel sheet can be provided through the steps of heating a steel slab having a predetermined alloy composition to 1000-1350°C, finish hot-rolling in a temperature range of 800-1000°C, winding up the hot-rolled steel sheet in a temperature range of 350-650°C, pickling the wound-up steel sheet, and cold-rolling the pickled steel sheet at a reduction ratio of 30-90%.

[0071] Preparation and Heating of Steel Slab Prepare a steel slab having a predetermined alloy composition. Since the steel slab of the present invention has an alloy composition corresponding to the alloy composition of the steel sheet described above, the description of the alloy composition of the steel slab is replaced by the description of the alloy composition of the steel sheet described above.

[0072] The prepared steel slab can be heated to a certain temperature range. At this time, the heating temperature of the steel slab may be in the range of 1000-1350°C. If the heating temperature of the steel slab is less than 1000°C, there is a possibility of hot-rolling in a temperature range below the target finish hot-rolling temperature range. If the heating temperature of the steel slab exceeds 1350°C, there is a possibility of reaching the melting point of the steel and melting.

[0073] Hot-Rolling and Winding Up The heated steel slab can be hot-rolled to provide a hot-rolled steel sheet. The finish hot-rolling temperature during hot-rolling is preferably in the range of 800-1000°C. If the finish hot-rolling temperature is less than 800°C, excessive rolling load may become a problem. If the finish hot-rolling temperature exceeds 1000°C, the crystal grains of the hot-rolled steel sheet may be coarsely formed, which may cause deterioration of the physical properties of the final steel sheet.

[0074] The hot-rolled steel sheet after hot rolling is completed can be cooled at an average cooling rate of 10 °C / s or more and can be coiled in the temperature range of 350 to 650 °C. This is because when the coiling temperature is less than 350 °C, coiling is not easy, and when the coiling temperature exceeds 650 °C, surface scale may be formed up to the inside of the hot-rolled steel sheet, which may make pickling difficult.

[0075] Pickling and cold rolling After uncoiling the coiled hot-rolled coil, pickling can be performed to remove the scale generated on the steel sheet surface, and cold rolling can be carried out. In the present invention, the conditions of pickling and cold rolling are not particularly limited, but cold rolling is preferably carried out 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.

[0076] The cold-rolled steel sheet may be an unplated cold-rolled steel sheet through an annealing heat treatment process, or may be a plated steel sheet through a plating process to impart corrosion resistance. For plating, plating methods such as hot-dip galvanizing, electro-galvanizing, and hot-dip aluminum plating can be applied, and the method and type are not particularly limited.

[0077] Annealing heat treatment The present invention performs an annealing heat treatment process to ensure the strength and workability of the steel sheet simultaneously.

[0078] The cold-rolled steel sheet is heated to 700 °C at an average heating rate of 5 °C / s or more (primary heating), heated to the temperature range of Ac3 to 920 °C at an average heating rate of 5 °C / s or less (secondary heating), and then held for 50 to 1200 seconds (primary holding).

[0079] When the average heating rate of the first heating to 700 °C is less than 5 °C / s, massive austenite is formed from ferrite and cementite generated during heating, and ultimately, fine tempered martensite and retained austenite cannot be formed as the final microstructure. As a result, the balance of the target tensile strength and elongation rate (TS 2 ×EL 1 / 2 ) and the balance of tensile strength and hole expansion rate (TS 2 ×HER 1 / 2 ) cannot be achieved. Also, when the secondary heating rate to the first holding temperature exceeds 5 °C / s, the transformation from cementite to austenite generated during heating is accelerated, a large amount of massive austenite is formed, the final microstructure coarsens, and boron (B) may not be sufficiently concentrated in the tempered martensite. As a result, [B] FM / [B] TM will exceed 0.55, and the balance of the target levels of tensile strength and elongation rate (TS 2 ×EL 1 / 2 ), the balance of tensile strength and hole expansion rate (TS 2 ×HER 1 / 2 ), and the yield ratio evaluation index (I YR ) cannot be achieved.

[0080] When the first holding temperature is below Ac3 (two-phase region), 5% by volume or more of ferrite is formed, thereby potentially reducing the balance of tensile strength and elongation rate (TS 2 ×EL 1 / 2 ) and the balance of tensile strength and hole expansion rate (TS 2 ×HER 1 / 2 ). Also, when the first holding time is less than 50 seconds, the structure may not be sufficiently homogenized, and the physical properties of the steel plate may deteriorate. The upper limits of the first holding temperature and the first holding time are not particularly limited, but in order to prevent a decrease in toughness due to grain coarsening, it is preferable to limit the first holding temperature to 920 °C or lower and the first holding time to 1200 seconds or less.

[0081] After the first holding, it can be cooled (first cooling) to the first cooling stop temperature of 200 to 400 °C at an average cooling rate of 1 °C / s or more. When the average cooling rate of the first cooling is less than 1 °C / s, the fraction of retained austenite is insufficient due to the slow cooling, thereby 2 ×EL 1 / 2 ) may decrease. There is no particular need to specify the upper limit of the average cooling rate of the first cooling, but it is preferably 100 °C / s or less. When the first cooling stop temperature is less than 200 °C, tempered martensite is excessively formed and the retained austenite is insufficient, resulting in the balance of the tensile strength and elongation rate of the steel sheet (TS 2 ×EL 1 / 2 ), and the balance of the tensile strength and hole expansion rate (TS 2 ×HER 1 / 2 ) may decrease. On the other hand, when the first cooling stop temperature exceeds 400 °C, bainite is excessively formed and the tempered martensite is insufficient, resulting in the balance of the tensile strength and elongation rate of the steel sheet (TS 2 ×EL 1 / 2 ), and the balance of the tensile strength and hole expansion rate (TS 2 ×HER 1 / 2 ) may decrease.

[0082] After the second cooling, it can be heated (third heating) to the temperature range of 350 to 550 °C at a heating rate of an average heating rate of 5 °C / s or more and then held (second holding) for 50 seconds or more. There is no particular need to specify the upper limit of the average heating rate of the third heating, but it is preferably 100 °C / s or less. When the second holding temperature is less than 350 °C or the second holding time is less than 50 seconds, tempered martensite is excessively formed and it becomes difficult to secure the fraction of retained austenite. As a result, the balance of the tensile strength and elongation rate (TS 2 ×EL 1 / 2 ), and the balance of the tensile strength and hole expansion rate (TS 2 ×HER 1 / 2 ) may decrease. When the second holding temperature exceeds 550 °C or the second holding time exceeds 155,000 seconds, the fraction of retained austenite is insufficient, resulting in the balance of the tensile strength and elongation rate of the steel sheet (TS 2 ×EL1 / 2 ) may decrease.

[0083] After the secondary holding, it can be cooled to room temperature (secondary cooling) at an average cooling rate of 1 °C / s or more.

[0084] The high-strength steel sheet excellent in workability manufactured by the above manufacturing method can include, as a microstructure, bainite, tempered martensite, fresh martensite, retained austenite, and other inevitable structures. As a preferable example, in terms of volume fraction, it can include 10 to 30% bainite, 50 to 70% tempered martensite, 10 to 30% fresh martensite, 2 to 10% retained austenite, and 5% or less (including 0%) ferrite.

[0085] The steel sheet manufactured by the above manufacturing method has a balance (B TE ) of tensile strength and elongation represented by the following [Relational Expression 2] 6 ~6.2×10 6 (MPa 2 % 1 / 2 ) that satisfies the following, and a balance (B TH ) of tensile strength and hole expansion rate represented by the following [Relational Expression 3] 6 ~11.5×10 6 (MPa 2 % 1 / 2 ) that satisfies the following, and a yield ratio evaluation index (I YR ) represented by the following [Relational Expression 4] can satisfy 0.15 to 0.42.

[0086] [Relational Expression 2] B TE = [Tensile strength (TS, MPa)] 2 × [Elongation rate (El, %)] 1 / 2

[0087] [Relational Expression 3] B TH = [Tensile strength (TS, MPa)] 2 × [Hole expansion rate (HER, %)] 1 / 2

[0088] [Relational Expression 4] I YR = 1 - [Yield Ratio (YR)]

Example

[0089] Hereinafter, a high-strength steel sheet excellent in workability according to one aspect of the present invention and a method for manufacturing the same will be described in more detail with specific examples. It should be noted that the following examples are for understanding the present invention and are not for specifying the scope of rights of the present invention. The scope of rights of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

[0090] (Example) A steel slab with a thickness of 100 mm having the alloy composition described in Table 1 below (the rest is Fe and inevitable impurities) was manufactured, heated at 1200 °C, and then finish hot-rolled at 900 °C. Thereafter, it was cooled at an average cooling rate of 30 °C / s, wound at the coiling temperatures shown in Tables 2 and 3, and a hot-rolled steel sheet with a thickness of 3 mm was manufactured. Thereafter, after pickling to remove the surface scale, cold rolling was performed to a thickness of 1.5 mm.

[0091] Thereafter, heat treatment was performed under the annealing heat treatment conditions described in Tables 2 to 5 below to manufacture a steel sheet. In Tables 2 and 3 below, the single-phase region means a temperature range of Ac3 to 920 °C, and the two-phase region means a temperature range below Ac3 °C.

[0092] The microstructure of the steel sheet manufactured in this way was observed, and the results are shown in Tables 6 and 7. In the microstructure, ferrite (F), bainite (B), tempered martensite (TM), fresh martensite (FM), and pearlite (P) were observed through SEM after nital etching of the cross-section of the polished test piece. After nital etching, the structure with no unevenness on the surface of the test piece was classified as ferrite, and the structure having a lamellar structure of cementite and ferrite was classified as pearlite. Both bainite (B) and tempered martensite (TM) were observed in the form of lath and block, and since they were difficult to distinguish, the fraction was calculated using the dilatation curve after dilatation evaluation for bainite and tempered martensite. That is, the value obtained by subtracting the fraction of tempered martensite calculated using the dilatation curve from the fractions of bainite and tempered martensite measured by SEM observation was determined as the fraction of bainite. On the other hand, since it was not easy to distinguish fresh martensite (FM) and retained austenite (retained γ) either, the value obtained by subtracting the fraction of retained austenite calculated by X-ray diffraction method from the fractions of martensite and retained austenite observed by the above SEM was determined as the fresh martensite fraction.

[0093] On the other hand, for the steel sheet [B] FM / [B] TM , the balance of tensile strength and elongation (TS 2 ×EL 1 / 2 ), the balance of tensile strength and hole expansion rate (TS 2 ×HER 1 / 2 ), and the yield ratio evaluation index (I YR ) were measured and evaluated, and the results are shown in Tables 8 and 9.

[0094] The boron (B) content ([B] FM ) in fresh martensite and the boron (B) content ([B] TM) was determined by the concentration of boron (B) measured in fresh martensite and tempered martensite using an EPMA (Electron Probe MicroAnalyser).

[0095] The tensile strength (TS) and elongation (El) were evaluated by a tensile test, and evaluated using test pieces taken based on JIS No. 5 standard with respect to the direction 90° to the rolling direction of the rolled sheet, and the tensile strength (TS) and elongation (El) were measured. The hole expansion ratio (HER) was evaluated by a hole expansion test. After forming a 10 mm Ψ punch hole (die inner diameter 10.3 mm, clearance 12.5%), a conical punch with a vertex angle of 60° was inserted into the punch hole in the direction where the burr of the punch hole was on the outside, and after compressing and expanding the peripheral part of the punch hole at a moving speed of 20 mm / min, it was calculated using the following [Relational Expression 5].

[0096] [Relational Expression 5] Hole expansion ratio (HER, %) = {(D - D0) / D0} × 100

[0097] In the above Relational Expression 5, D means the diameter (mm) of the hole when the crack penetrates the steel sheet in the thickness direction, and D0 means the initial diameter (mm) of the hole.

[0098]

Table 1

[0099]

Table 2

[0100]

Table 3

[0101]

Table 4

[0102]

Table 5

[0103]

Table 6

[0104]

Table 7

[0105]

Table 8

[0106]

Table 9

[0107] As shown in the above Tables 1 to 9, in the case of the test piece satisfying the conditions presented in the present invention, [Relational Expression 1] is satisfied, and the balance (B TE ) is 3.0×10 6 ~6.2×10 6 (MPa 2 % 1 / 2 ) is satisfied, the balance (B TH ) between the tensile strength and the hole expansion rate is 6.0×10 6 ~11.5×10 6 (MPa 2 % 1 / 2 ) is satisfied, and it can be seen that the yield ratio evaluation index (I YR ) satisfies 0.15 to 0.42.

[0108] Test piece 2 was carried out with a primary average heating rate of less than 5 °C / s, and tempered martensite and retained austenite were insufficient. As a result, for test piece 2, the balance (B TE ) is less than 3.0×10 6 , and the balance (B TH ) between the tensile strength and the hole expansion rate is 6.0×106 was less than

[0109] Test piece 3 was carried out with a secondary average heating rate exceeding 5 °C / s, massive austenite was formed, and boron (B) did not concentrate in the tempered martensite. As a result, test piece 3 had [B] FM / [B] TM exceeding 0.55, yield ratio evaluation index (I YR ) exceeding 0.42, balance of tensile strength and elongation rate (B TE ) less than 3.0×10 6 , balance of tensile strength and hole expansion rate (B TH ) less than 6.0×10 6 was less than

[0110] Test piece 4 was carried out in a two-phase region where the primary holding temperature was below Ac3, and the ferrite fraction was excessive. As a result, test piece 4 had a balance of tensile strength and elongation rate (B TE ) less than 3.0×10 6 , balance of tensile strength and hole expansion rate (B TH ) less than 6.0×10 6 was less than

[0111] Test piece 5 was carried out with a primary average cooling rate of less than 1 °C / s, and the retained austenite fraction was insufficient. As a result, test piece 5 had a balance of tensile strength and elongation rate (B TE ) less than 3.0×10 6 was less than

[0112] Test piece 6 was carried out with a primary cooling stop temperature of less than 200 °C, the tempered martensite fraction was excessive, and the retained austenite fraction was insufficient. As a result, test piece 6 had a balance of tensile strength and elongation rate (B TE ) less than 3.0×10 6 , balance of tensile strength and hole expansion rate (B TH ) less than 6.0×10 6 was less than

[0113] Test piece 7 was tested with the primary cooling stop temperature exceeding 400 °C, resulting in an excessive bainite fraction and an insufficient tempered martensite fraction. As a result, for test piece 7, the balance (B TE ) between the tensile strength and the elongation rate was less than 3.0×10 6 , and the balance (B TH ) between the tensile strength and the hole expansion rate was less than 6.0×10 6 .

[0114] Test piece 8 was tested with the secondary holding temperature less than 350 °C, resulting in an excessive tempered martensite fraction and an insufficient retained austenite fraction. As a result, for test piece 8, the balance (B TE ) between the tensile strength and the elongation rate was less than 3.0×10 6 , and the balance (B TH ) between the tensile strength and the hole expansion rate was less than 6.0×10 6 .

[0115] Test piece 9 was tested with the secondary holding temperature exceeding 550 °C, resulting in an insufficient retained austenite fraction. As a result, for test piece 9, the balance (B TE ) between the tensile strength and the elongation rate was less than 3.0×10 6 .

[0116] Test piece 10 was tested with the secondary holding time less than 50 s, resulting in an excessive tempered martensite fraction and an insufficient retained austenite fraction. As a result, for test piece 10, the balance (B TE ) between the tensile strength and the elongation rate was less than 3.0×10 6 , and the balance (B TH ) between the tensile strength and the hole expansion rate was less than 6.0×10 6 .

[0117] Test piece 11 was tested with the secondary holding time exceeding 155,000 s, resulting in an insufficient retained austenite fraction. As a result, for test piece 11, the balance (B TE ) between the tensile strength and the elongation rate was less than 3.0×10 6 .

[0118] Test piece 33 had a low carbon (C) content, and the balance (B TE ) of tensile strength and elongation rate was less than 3.0×10 6 , and the balance (B TH ) of tensile strength and hole expansion rate was less than 6.0×10 6 .

[0119] Test piece 34 had a high carbon (C) content, insufficient tempered martensite fraction, excessive fresh martensite fraction, and excessive retained austenite fraction. As a result, for test piece 34, the balance (B TE ) of tensile strength and elongation rate was less than 3.0×10 6 , and the balance (B TH ) of tensile strength and hole expansion rate was less than 6.0×10 6 .

[0120] Test piece 35 had a low silicon (Si) content and insufficient retained austenite fraction. As a result, for test piece 35, the balance (B TE ) of tensile strength and elongation rate was less than 3.0×10 6 .

[0121] Test piece 36 had a high silicon (Si) content and excessive fresh martensite fraction. As a result, for test piece 36, the balance (B TE ) of tensile strength and elongation rate was less than 3.0×10 6 , and the balance (B TH ) of tensile strength and hole expansion rate was less than 6.0×10 6 .

[0122] Test piece 37 had a high aluminum (Al) content and excessive fresh martensite fraction. As a result, for test piece 37, the balance (B TE ) of tensile strength and elongation rate was less than 3.0×10 6 , and the balance (B TH ) of tensile strength and hole expansion rate was less than 6.0×10 6 .

[0123] The test piece 38 had a low manganese (Mn) content, pearlite was formed, and the retained austenite fraction was insufficient. As a result, the test piece 38 had a balance (B TE ) of less than 3.0×10 6 .

[0124] The test piece 39 had a high manganese (Mn) content and an excessive fresh martensite fraction. As a result, the test piece 39 had a balance (B TE ) of less than 3.0×10 6 , and a balance (B TH ) of less than 6.0×10 6 between the tensile strength and the hole expansion rate.

[0125] The test piece 40 had a high chromium (Cr) content and an excessive fresh martensite fraction. As a result, the test piece 40 had a balance (B TE ) of less than 3.0×10 6 , and a balance (B TH ) of less than 6.0×10 6 between the tensile strength and the hole expansion rate.

[0126] The test piece 41 had a high molybdenum (Mo) content and an excessive fresh martensite fraction. As a result, the test piece 41 had a balance (B TE ) of less than 3.0×10 6 , and a balance (B TH ) of less than 6.0×10 6 between the tensile strength and the hole expansion rate.

[0127] The test piece 42 had a low boron (B) content and boron (B) did not concentrate in the tempered martensite. As a result, for the test piece 42, [B] FM / [B] TM exceeded 0.55, and the yield ratio evaluation index (I YR ) exceeded 0.42.

[0128] The test piece 43 had a high boron (B) content, and boron (B) was excessively concentrated in the tempered martensite. As a result, for the test piece 43, [B] FM / [B] TM was less than 0.03, and the yield ratio evaluation index (I YR ) was less than 0.15.

[0129] As described above, the present invention has been described in detail with reference to examples, but embodiments in different forms are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the examples.

Claims

1. By weight, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, consisting of the balance Fe and inevitable impurities, The microstructure contains, by volume fraction, 10 to 30% bainite, 50 to 70% tempered martensite, 10 to 30% fresh martensite, 2 to 10% retained austenite, and 5% or less (including 0%) ferrite, A high-strength steel sheet excellent in workability that satisfies the following [Relational Expression 1]. [Relational Expression 1] 0.03 ≤ [B] ＦＭ / [B] ＴＭ ≤ 0.55 In the relational expression 1, [B] ＦＭ is the content (wt%) of boron (B) contained in fresh martensite, and [B] ＴＭ is the content (wt%) of boron (B) contained in tempered martensite.

2. The steel sheet further contains, by weight, any one or more of the following (1) to (8), and is the high-strength steel sheet excellent in workability according to Claim 1. (1) One or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5% (2) One or more of Cr: 0 to 3.0% and Mo: 0 to 3.0% (3) One or more of Cu: 0 to 4.0% and Ni: 0 to 4.0% (4) One or more of Ca: 0 to 0.05%, REM excluding Y: 0 to 0.05%, and Mg: 0 to 0.05% (5) One or more of W: 0 to 0.5% and Zr: 0 to 0.5% (6) One or more of Sb: 0 to 0.5% and Sn: 0 to 0.5% (7) One or more of Y: 0 to 0.2% and Hf: 0 to 0.2% (8) Co: 0 to 1.5%

3. The steel sheet has a balance between tensile strength and elongation (B ＴＥ ) is 3.0 x 10 ６ ~6.2 x 10 ６ (MPa ２ % １／２ ) and the balance between the tensile strength and the hole expansion ratio (B ＴＨ ) is 6.0 x 10 ６ ~11.5 x 10 ６ (MPa ２ % １／２ ) and the yield ratio evaluation index (I ＹＲ 3. The high strength steel plate having excellent workability according to claim 1 or 2, wherein the ratio of tensile strength to compressibility is 0.15 to 0.

42. [Relational Expression 2] B ＴＥ = [Tensile Strength (TS, MPa)] ２ × [Elongation Ratio (El, %)] １／２ [Relational Expression 3] B ＴＨ = [Tensile Strength (TS, MPa)] ２ × [Hole Expansion Ratio (HER, %)] １／２ [Relational Expression 4] I ＹＲ = 1 - [yield ratio (YR)]

4. Providing a steel sheet cold-rolled using a steel slab containing, by weight, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, consisting of the balance Fe and inevitable impurities; Heating the cold-rolled steel sheet to 700°C at an average heating rate of 5°C / s or more (primary heating), heating it to a temperature range of Ac3 to 920°C at an average heating rate of 5°C / s or less (secondary heating), and then holding it for 50 to 1200 seconds (primary holding); Cooling the steel sheet held in the primary holding to a temperature range of 200 to 400°C at an average cooling rate of 1°C / s or more (primary cooling); Heating the steel sheet cooled in the primary cooling to a temperature range of 350 to 550°C at an average heating rate of 5°C / s or more (tertiary heating), and then holding it for 50 seconds or more (secondary holding); cooling the secondarily held steel sheet to room temperature (secondary cooling) at an average cooling rate of 1 °C / s or more, The microstructure, in terms of volume fraction, contains 10 to 30% bainite, 50 to 70% tempered martensite, 10 to 30% fresh martensite, 2 to 10% retained austenite, and 5% or less (including 0%) ferrite, and is a method for manufacturing a high-strength steel sheet with excellent workability.

5. The method for manufacturing a high-strength steel sheet with excellent workability according to claim 4, wherein the steel slab further contains any one or more of the following (1) to (8). (1) One or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5% (2) One or more of Cr: 0 to 3.0% and Mo: 0 to 3.0% (3) One or more of Cu: 0 to 4.0% and Ni: 0 to 4.0% (4) One or more of Ca: 0 to 0.05%, REM excluding Y: 0 to 0.05%, and Mg: 0 to 0.05% (5) One or more of W: 0 to 0.5% and Zr: 0 to 0.5% (6) One or more of Sb: 0 to 0.5% and Sn: 0 to 0.5% (7) One or more of Y: 0 to 0.2% and Hf: 0 to 0.2% (8) Co: 0 to 1.5%

6. The cold-rolled steel sheet is heating the steel slab to 1000 to 1350 °C, performing finish hot rolling in a temperature range of 800 to 1000 °C, coiling the hot-rolled steel sheet in a temperature range of 350 to 650 °C, pickling the coiled steel sheet, cold-rolling the pickled steel sheet at a reduction ratio of 30 to 90%, and is provided through the steps, and is the method for manufacturing a high-strength steel sheet with excellent workability according to claim 4 or 5.

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