High-strength steel plate with excellent workability and method for manufacturing the same

Abstract

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

Description

[Technical Field]

[0001] This invention relates to a steel sheet that can be used in automobile parts and the like, and more particularly to a steel sheet that possesses high strength properties while also having excellent workability, and a method for manufacturing the same. [Background technology]

[0002] Recently, the automotive industry has been focusing on methods to reduce material weight in order to protect the global environment while simultaneously ensuring passenger stability. To meet these demands for stability and weight reduction, the application of high-strength steel sheets has increased dramatically. Generally, it is 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 possess high strength characteristics while also exhibiting excellent workability, such as ductility and hole expandability.

[0003] TRIP (Transformation-Induced Plasticity) steel, which utilizes transformation-induced plasticity of retained austenite, is known to possess high strength properties while also having a certain level of workability due to its complex microstructure consisting of ferrite, bainite, martensite, and retained austenite.

[0004] Patent documents 1 and 2 disclose a method for utilizing tempered martensite as a technique to further improve the workability of steel sheets. Tempered martensite, produced by tempering hard martensite, is softened martensite, and therefore has a difference in strength compared to conventional untempered martensite (fresh martensite). Consequently, workability can be increased by suppressing the formation of fresh martensite and creating tempered martensite.

[0005] However, the technologies disclosed in Patent Documents 1 and 2 do not balance tensile strength and elongation (TS 2 *EL 1 / 2) is 3.0*10 6 ~6.2*10 6 (MPa 2 % 1 / 2 ) is not satisfied, which means it is difficult to ensure a steel plate with excellent strength and ductility.

[0006] On the other hand, as another technique for improving the workability of the steel plate, a method of inducing the formation of bainite by adding boron (B) is disclosed in Patent Document 3. When adding boron (B), in order to suppress the ferrite-pearlite transformation and induce the formation of bainite, it is possible to achieve both strength and workability.

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

[0008] That is, the situation is that the requirements for a steel plate with excellent balance of tensile strength and elongation (B TE ), balance of tensile strength and hole expansion rate (B TH ) and 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, a steel sheet and a method for manufacturing the same can be provided, which optimize the composition and microstructure of the steel sheet to achieve an excellent balance between tensile strength and elongation, a balance between tensile strength and porosity expansion, and a yield ratio evaluation index.

[0011] The problems that the present invention will address are not limited to those described above. Further problems that the present invention will address are described throughout the specification, and any person with ordinary skill in the art to which the present invention belongs will have no difficulty understanding these further problems from the contents of the specification. [Means for solving the problem]

[0012] A high-strength steel sheet with excellent workability according to one aspect of the present invention contains, by weight %, C: 0.1~0.25%, Si: 0.01~1.5%, Mn: 1.0~4.0%, Al: 0.01~1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005~0.005%, with the remainder being Fe and unavoidable impurities, and its microstructure includes bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures, and can satisfy the following [Relationship Formula 1] and [Relationship Formula 2]. [Relationship 1] 0.03 ≤ [B] FM / [B] TM ≤0.55 In the above relational expression 1, [B] FM This is the boron (B) content (weight %) contained in fresh martensite, and [B] TM This represents the boron (B) content (weight %) contained in the tempered martensite. [Relationship 2] V(1.2μm, γ) / V(γ)≧0.12 In the above relational equation 2, V(1.2μm, γ) is the fraction (volume %) of retained austenite with an average grain size of 1.2μm or more, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet.

[0013] The above steel plate may further contain, by weight %, one or more of the following (1) to (8): (1) One or more of the following: Ti: 0-0.5%, Nb: 0-0.5%, and V: 0-0.5% (2) One or more of the following: Cr: 0-3.0% and Mo: 0-3.0% (3) One or more of the following: Cu: 0-4.0% and Ni: 0-4.0% (4) Ca: 0-0.05%, REM (excluding Y): 0-0.05%, and Mg: 0-0.05% (one or more of these). (5) One or more of the following: W: 0-0.5% and Zr: 0-0.5% (6) One or more of the following: Sb: 0-0.5% and Sn: 0-0.5% (7) One or more of the following: Y: 0-0.2% and Hf: 0-0.2% (8) Co: 0~1.5%

[0014] The microstructure of the above steel sheet may contain, by volume fraction, 10-30% bainite, 50-70% tempered martensite, 10-30% fresh martensite, 2-10% retained austenite, and 5% or less (including 0%) ferrite.

[0015] The above steel plate has a balance between tensile strength and elongation, as shown in [Relationship Equation 3] below (B TE ) is 3.0*10 6 ~6.2*10 6 ( MPa 2 % 1 / 2 ) satisfies the following conditions and the balance between tensile strength and hole expansion ratio (B TH ) is 6.0*10 6 ~11.5*10 6 ( MPa 2 % 1 / 2) satisfies the following and is expressed by the yield ratio evaluation index (I YR ) can satisfy 0.15 to 0.42. [Relationship 3] B TE =[Tensile Strength (TS, MPa)] 2 *[Growth rate (El, %)] 1 / 2 [Relationship Equation 4] B TH =[Tensile Strength (TS, MPa)] 2 *[Hole expansion rate (HER, %)] 1 / 2 [Relationship 5] I YR =1-[yield ratio (YR)]

[0016] A method for manufacturing a high-strength steel sheet with excellent workability according to one aspect of the present invention comprises the steps of: providing a cold-rolled steel sheet containing, by weight %, C: 0.1~0.25%, Si: 0.01~1.5%, Mn: 1.0~4.0%, Al: 0.01~1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005~0.005%, with the remainder being 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 to a temperature range of Ac3~920°C at an average heating rate of 5°C / s or less (secondary heating), and then maintaining for 50~1200 seconds (primary maintenance). The process may include the following steps: ) a step of first maintaining the steel plate; a step of first maintaining the steel plate at an average cooling rate of 2 to 100°C / s to a temperature range of 350 to 550°C (first cooling), and then maintaining it for 5 to 600 seconds (second maintenance); a step of second maintaining the steel plate at an average cooling rate of 2 to 100°C / s to a temperature range of 200 to 400°C (second cooling); a step of second heating the steel plate at an average heating rate of 5 to 100°C / s to a temperature range of 350 to 550°C (tertiary heating), and then maintaining it for 50 seconds or more (tertiary maintenance); and a step of third maintaining the steel plate at an average cooling rate of 1°C / s or more to room temperature (tertiary cooling).

[0017] The above steel slab may further include one or more of the following (1) to (8): (1) One or more of the following: Ti: 0-0.5%, Nb: 0-0.5%, and V: 0-0.5% (2) One or more of the following: Cr: 0-3.0% and Mo: 0-3.0% (3) One or more of the following: Cu: 0-4.0% and Ni: 0-4.0% (4) Ca: 0-0.05%, REM (excluding Y): 0-0.05%, and Mg: 0-0.05% (one or more of these). (5) One or more of the following: W: 0-0.5% and Zr: 0-0.5% (6) One or more of the following: Sb: 0-0.5% and Sn: 0-0.5% (7) One or more of the following: Y: 0-0.2% and Hf: 0-0.2% (8) Co: 0~1.5%

[0018] The cold-rolled steel sheet described above can be provided through the following steps: heating a steel slab to 1000-1350°C; finishing hot-rolling in a temperature range of 800-1000°C; winding the hot-rolled steel sheet in a temperature range of 350-650°C; pickling the wound steel sheet; and cold-rolling the pickled steel sheet with a reduction ratio of 30-90%. [Effects of the Invention]

[0019] According to a preferred embodiment of the present invention, a steel sheet and a method for manufacturing the same can be provided, which have excellent balance between tensile strength and ductility, balance between tensile strength and hole expandability, and yield ratio evaluation index, and can be used for applications such as automobile parts. [Modes for carrying out the invention]

[0020] The present invention relates to a high-strength steel sheet with excellent workability and a method for manufacturing the same, and preferred embodiments of the present invention will be described below. Embodiments of the present invention can be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to further illustrate the present invention to persons who are ordinary skill in the art to which the invention pertains.

[0021] The inventors of the present invention have come to realize that in boron (B)-added transformation-induced plasticity (TRIP) steel containing bainite, tempered martensite, fresh martensite, and retained austenite, it is possible to simultaneously secure an excellent balance between tensile strength and ductility, an excellent balance between tensile strength and hole expansion, and an excellent yield ratio evaluation index by controlling the microstructural fractions of tempered martensite, fresh martensite, and retained austenite within a certain range, controlling the boron (B) content in tempered martensite and fresh martensite within a certain range, and controlling the shape and size of retained austenite within a certain range. By investigating this and devising a method that can effectively achieve excellent strength, yield ratio, ductility, and hole expansion, the inventors have arrived at the present invention.

[0022] The following describes in more detail a high-strength steel sheet with excellent workability according to one aspect of the present invention.

[0023] A high-strength steel sheet with excellent workability according to one aspect of the present invention contains, by weight %, C: 0.1~0.25%, Si: 0.01~1.5%, Mn: 1.0~4.0%, Al: 0.01~1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005~0.005%, with the remainder being Fe and unavoidable impurities, and its microstructure includes bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures, and can satisfy the following [Relationship Formula 1] and [Relationship Formula 2]. [Relationship 1] 0.03 ≤ [B] FM / [B] TM ≤0.55 In the above relational expression 1, [B] FM This is the boron (B) content (weight %) contained in fresh martensite, and [B] TM This represents the boron (B) content (weight %) contained in the tempered martensite. [Relationship 2] V(1.2μm, γ) / V(γ)≧0.12 In the above relational equation 2, V(1.2μm, γ) is the fraction (volume %) of retained austenite with an average grain size of 1.2μm or more, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet.

[0024] 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.

[0025] A high-strength steel sheet with excellent workability according to one aspect of the present invention contains, by weight percent, C: 0.1-0.25%, Si: 0.01-1.5%, Mn: 1.0-4.0%, Al: 0.01-1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005-0.005%, with the remainder being Fe and unavoidable impurities. In addition, it may further contain one or more of the following: 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%), and Co: 1.5% or less (including 0%).

[0026] Carbon (C): 0.1~0.25% Carbon (C) is an essential element for ensuring the strength of steel sheets, and it also stabilizes retained austenite, which contributes to improving the ductility of steel sheets. Therefore, the present invention may contain 0.1% or more of carbon (C) to achieve these effects. A preferred carbon (C) content can be more than 0.1%, 0.11% or more, and 0.12% or more. On the other hand, if the carbon (C) content exceeds a certain level, excessive strength increase may reduce ductility and deteriorate weldability. Therefore, the present invention may limit the upper limit of the carbon (C) content to 0.25%. The carbon (C) content can be 0.24% or less, and a more preferred carbon (C) content can be 0.23% or less.

[0027] Silicon (Si): 0.01-1.5% or less Silicon (Si) is an element that contributes to strength improvement through solid solution strengthening and also improves workability by homogenizing the structure. Furthermore, silicon (Si) is an element that suppresses cementite precipitation and contributes to the formation of retained austenite. Therefore, the present invention can add 0.01% or more of silicon (Si) to achieve these effects. A preferred silicon (Si) content can be 0.02% or more, and a more preferred silicon (Si) content can be 0.04% or more. However, if the silicon (Si) content exceeds a certain level, it may not only induce plating defects such as unplated areas in the plating process but also reduce the weldability of the steel sheet. Therefore, the present invention can limit the upper limit of the silicon (Si) content to 1.5%. A preferred upper limit of silicon (Si) content can be 1.48%, and a more preferred upper limit of silicon (Si) content can be 1.46%.

[0028] Manganese (Mn): 1.0~4.0% Manganese (Mn) is a useful element for increasing both strength and ductility. Therefore, the present invention allows for the addition of 1.0% or more of manganese (Mn) to achieve such effects. The lower limit of the preferred manganese (Mn) content can be 1.2%, and the lower limit of the more preferred manganese (Mn) content can be 1.4%. On the other hand, if manganese (Mn) is added in excess, the bainite transformation time increases, resulting in insufficient carbon (C) concentration in the austenite, which presents the problem of not being able to secure the desired fraction of austenite. Therefore, the present invention can limit the upper limit of the manganese (Mn) content to 4.0%. The upper limit of the preferred manganese (Mn) content can be 3.9%.

[0029] Aluminum (Al): 0.01~1.5% Aluminum (Al) is an element that combines with oxygen in steel to deoxidize it. Furthermore, like silicon (Si), aluminum (Al) is an element that suppresses cementite precipitation and stabilizes retained austenite. Therefore, the present invention allows for the addition of 0.01% or more of aluminum (Al) to achieve these effects. A preferred aluminum (Al) content can be 0.03% or more, and a more preferred aluminum (Al) content can be 0.05% or more. On the other hand, if aluminum (Al) is added excessively, not only will the inclusions in the steel sheet increase, but the workability of the steel sheet may also decrease. Therefore, the present invention limits the upper limit of the aluminum (Al) content to 1.5%. A preferred upper limit of aluminum (Al) content can be 1.48%.

[0030] Phosphorus (P): 0.15% or less (including 0%) Phosphorus (P) is an element that, when present as an impurity, degrades impact toughness. Therefore, it is preferable to control the phosphorus (P) content to 0.15% or less.

[0031] Sulfur (S): 0.03% or less (including 0%) Sulfur (S) is an element that, when present as an impurity, forms MnS in the steel sheet, degrading its ductility. Therefore, it is preferable that the sulfur (S) content be 0.03% or less.

[0032] Nitrogen (N): 0.03% or less (including 0%) Nitrogen (N) is an element that, when present as an impurity, forms nitrides during continuous casting, causing cracks in the slab. Therefore, it is preferable that the nitrogen (N) content be 0.03% or less.

[0033] Boron (B): 0.0005~0.005% Boron (B) is an element that improves hardenability and increases strength, and also suppresses nucleation at grain boundaries. Furthermore, the present invention aims to simultaneously secure an excellent balance between tensile strength and elongation, an excellent balance between tensile strength and hole expansion, and an excellent yield ratio evaluation index by enriching the tempered martensite with boron (B). Therefore, boron (B) must be added in the present invention. Accordingly, the present invention may add 0.0005% or more of boron (B) for these effects. However, if boron (B) is added in excess of a certain level, it will not only cause excessive property effects but also increase manufacturing costs. Therefore, the present invention may limit the upper limit of the boron (B) content to 0.005%.

[0034] 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 explained in detail below.

[0035] Titanium (Ti): 0-0.5%, Niobium (Nb): 0-0.5%, and Vanadium (V): 0-0.5% (one or more of these) Titanium (Ti), niobium (Nb), and vanadium (V) are elements that form precipitates and refine the crystal grains, and also contribute to improving the strength and impact toughness of steel sheets. Therefore, in this invention, one or more of titanium (Ti), niobium (Nb), and vanadium (V) can be added for these effects. However, if the content of each of titanium (Ti), niobium (Nb), and vanadium (V) exceeds a certain level, excessive precipitates will be formed, which will not only reduce impact toughness but also increase manufacturing costs. Therefore, in this invention, the content of each of titanium (Ti), niobium (Nb), and vanadium (V) can be limited to 0.5% or less.

[0036] Chromium (Cr): 0-3.0% and molybdenum (Mo): 0-3.0% (one or more of these) Chromium (Cr) and molybdenum (Mo) are elements that not only suppress austenite decomposition during alloying but also stabilize austenite, similar to manganese (Mn). Therefore, in this invention, one or more of chromium (Cr) and molybdenum (Mo) can be added for these effects. 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 the austenite becomes insufficient, making it impossible to secure the desired fraction of retained austenite. Accordingly, in this invention, the content of chromium (Cr) and molybdenum (Mo) can be limited to 3.0% or less each.

[0037] Copper (Cu): 0-4.0% and Nickel (Ni): 0-4.0% (one or more of these) Copper (Cu) and nickel (Ni) are elements that stabilize austenite and suppress corrosion. Furthermore, copper (Cu) and nickel (Ni) concentrate on the surface of the steel sheet, preventing hydrogen intrusion into the steel sheet and suppressing hydrogen-delayed breakdown. Therefore, the present invention allows for the addition of one or more of copper (Cu) and nickel (Ni) for these effects. However, if the content of copper (Cu) and nickel (Ni) exceeds a certain level, it can lead to not only excessive property effects but also an increase in manufacturing costs. Therefore, the present invention limits the content of copper (Cu) and nickel (Ni) to 4.0% or less, respectively.

[0038] One or more of the following: 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 the lanthanum group elements. Rare earth elements (REM), excluding calcium (Ca), magnesium (Mg), and yttrium (Y), contribute to improving the ductility of steel sheets by spheroidizing sulfides. Therefore, the present invention allows for the addition of one or more of these rare earth elements (REM), excluding calcium (Ca), magnesium (Mg), and yttrium (Y), for this effect. However, if the content of rare earth elements (REM), excluding calcium (Ca), magnesium (Mg), and yttrium (Y), exceeds a certain level, it can lead to excessive property effects as well as increased manufacturing costs. Therefore, the present invention can limit the content of each rare earth element (REM), excluding calcium (Ca), magnesium (Mg), and yttrium (Y), to 0.05% or less.

[0039] One or more of the following: 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 sheets. Therefore, the present invention may add one or more of tungsten (W) and zirconium (Zr) for this effect. However, if the content of tungsten (W) and zirconium (Zr) exceeds a certain level, it will not only cause excessive property effects but also increase manufacturing costs. Therefore, the present invention may limit the content of tungsten (W) and zirconium (Zr) to 0.5% or less, respectively.

[0040] One or more of the following: antimony (Sb): 0-0.5% and tin (Sn): 0-0.5% Since antimony (Sb) and tin (Sn) are elements that improve the wettability and adhesion of plating on steel sheets, the present invention may add one or more of antimony (Sb) and tin (Sn) for these effects. However, if the content of antimony (Sb) and tin (Sn) exceeds a certain level, the brittleness of the steel sheet may increase and cracks may occur during hot working or cold working. Therefore, the present invention may limit the content of antimony (Sb) and tin (Sn) to 0.5% or less, respectively.

[0041] Yttrium (Y): 0-0.2% and Hafnium (Hf): 0-0.2% (at least one of these) Since yttrium (Y) and hafnium (Hf) are elements that improve the corrosion resistance of steel sheets, the present invention may add one or more of yttrium (Y) and hafnium (Hf) 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 the present invention may limit the content of yttrium (Y) and hafnium (Hf) to 0.2% or less each.

[0042] Cobalt (Co): 0-1.5% Since cobalt (Co) is an element that promotes bainite transformation and increases the TRIP effect, the present invention may incorporate cobalt (Co) for this effect. However, if the cobalt (Co) content exceeds a certain level, the weldability and ductility of the steel sheet may deteriorate, so the present invention may limit the cobalt (Co) content to 1.5% or less.

[0043] A high-strength steel sheet with excellent workability according to one aspect of the present invention may contain the remaining Fe and other unavoidable impurities in addition to the components described above. However, since unintended impurities are inevitably introduced from raw materials or the surrounding environment during the normal manufacturing process, it is not possible to completely eliminate them. These impurities are recognizable to anyone with ordinary skill in the art, and therefore, their full details are not specifically mentioned herein. Furthermore, the addition of other effective components in addition to the components described above is not entirely excluded.

[0044] A 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 unavoidable structures as part of its microstructure.

[0045] Both untempered martensite (fresh martensite, FM) and tempered martensite (tempered martensite, TM) are microstructures that improve the strength of steel sheets. However, fresh martensite has the characteristic of reducing the ductility and burring properties of steel sheets compared to tempered martensite. Furthermore, fresh martensite tends to lower the yield ratio of steel sheets compared to tempered martensite. This is because the microstructure of tempered martensite softens due to the tempering heat treatment. Therefore, the balance between tensile strength and elongation (TS) that this invention aims for is not achieved. 2 *EL 1 / 2 ), balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2It is preferable to control the tissue fraction of tempered martensite and fresh martensite in order to ensure the yield ratio evaluation index (1-YR). 3.0*10 6 The balance between tensile strength and elongation (TS) 2 *EL 1 / 2 ), 6.0*10 6 The balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 In order to satisfy the yield ratio evaluation index (1-YR) of 0.42 or less, it is preferable to limit the fraction of tempered martensite to 50 volume% or more and the fraction of fresh martensite to 10 volume% or more. A more preferable fraction of tempered martensite can be 52 volume% or more or 54 volume% or more, and a more preferable fraction of fresh martensite can be 12 volume% or more. On the other hand, if tempered martensite or fresh martensite is formed excessively, the ductility and burring properties will decrease, and ultimately, 3.0*10 6 The balance between tensile strength and elongation (TS) 2 *EL 1 / 2 ), 6.0*10 6 The balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and a yield ratio evaluation index (1-YR) of 0.42 or less cannot be satisfied simultaneously. Therefore, the present invention can limit the fraction of tempered martensite to 70 volume% or less and the fraction of fresh martensite to 30 volume% or less. A more preferable fraction of tempered martensite can be 68 volume% or less or 65 volume% or less, and a more preferable fraction of fresh martensite can be 25 volume% or less.

[0046] The balance between tensile strength and elongation at the level targeted by this invention (TS 2 *EL 1 / 2 ), balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 To ensure the yield ratio evaluation index (1-YR), optimization of the bainite fraction is necessary. 3.0*10 6The balance between tensile strength and elongation (TS) 2 *EL 1 / 2 ), 6.0*10 6 The balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 In order to ensure a yield ratio evaluation index (1-YR) of 0.42 or less, it is preferable to control the bainite fraction to 10 volume% or more. A more preferable bainite fraction can be 12 volume% or more or 14 volume% or more. On the other hand, if bainite is formed excessively, it will result in a decrease in the tempered martensite fraction, thus making it difficult to achieve the desired balance between tensile strength and elongation (TS 2 *EL 1 / 2 ), balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ), and in order to ensure the yield ratio evaluation index (1-YR), the bainite fraction can be limited to 30 volume% or less. Preferred bainite fractions may be 12 volume% or more, 14 volume% or more, 28 volume% or less, or 26 volume% or less.

[0047] Steel sheets containing retained austenite exhibit 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 below a certain level, the balance between tensile strength and elongation (TS) is important. 2 *EL 1 / 2 ) is 3.0*10 6 ( MPa 2 % 1 / 2 Since it is less than ), it is undesirable. On the other hand, if the fraction of retained austenite exceeds a certain level, the local elongation may decrease or the spot weldability may decrease. Therefore, the present invention aims to balance tensile strength and elongation (TS 2 *EL 1 / 2 To obtain steel sheets with excellent properties, the fraction of retained austenite can be limited to a range of 2-10%. A preferred fraction of retained austenite can be 3 volume% or more, or 8 volume% or less.

[0048] The steel sheet of the present invention can include, 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, the present invention can limit the fraction of ferrite to 5% by volume (including 0%). Also, when pearlite is excessively formed, the workability of the steel sheet may decrease or the fraction of retained austenite may be reduced. Therefore, the present invention limits the formation of pearlite as much as possible.

[0049] The high-strength steel sheet excellent in workability according to one aspect of the present invention can satisfy the following [Relational Expression 1] and [Relational Expression 2]. [Relational Expression 1] 0.03 ≦ [B] FM / [B] TM ≦ 0.55 In the above Relational Expression 1, [B] FM is the boron (B) content (wt%) contained in fresh martensite, and [B] TM is the boron (B) content (wt%) contained in tempered martensite. [Relational Expression 2] V(1.2μm, γ) / V(γ) ≧ 0.12 In the above Relational Expression 2, V(1.2μm, γ) is the fraction (volume%) of retained austenite having an average crystal grain size of 1.2 μm or more, and V(γ) is the fraction (volume%) of retained austenite in the steel sheet.

[0050] The present invention aims at the balance between the target tensile strength and elongation (TS 2 *EL 1 / 2 ), the balance between the tensile strength and hole expansion ratio (TS 2 *HER 1 / 2In order to ensure the yield ratio evaluation index (1 - YR), not only the tissue fractions of tempered martensite, fresh martensite and retained austenite are controlled within a certain range, but also the ratio of the boron (B) content in tempered martensite to the boron (B) content in fresh martensite is controlled within a certain range, and the ratio of retained austenite of a specific size, shape and type to the total retained austenite is controlled within a certain range.

[0051] The present invention controls the ratio of the boron (B) content ([B] TM , wt%) contained in fresh martensite to the boron (B) content ([B] FM , wt%) contained in tempered martensite within the range of 0.03 to 0.55, so as to balance the tensile strength and elongation rate (B) of 3.0*10 6 ~6.2*10 6 (MPa 2 %, 1 / 2 TE ), the balance of tensile strength and hole expansion rate (B) of 6.0*10 6 ~11.5*10 6 (MPa 2 %, 1 / 2 TH ) and the yield ratio evaluation index (I YR ) of 0.15 to 0.42 can be ensured simultaneously.

[0052] The inventors of this invention conducted in-depth research on methods for ensuring the physical properties of boron(B)-added TRIP steel. While the theoretical basis has not been clearly established, they focused on the fact that the desired physical properties can be ensured only when the ratio of boron(B) content in fresh martensite to that in tempered martensite meets a certain range. In particular, it was confirmed that the yield ratio of the steel sheet shows a certain tendency depending on the ratio of boron(B) content in tempered martensite and fresh martensite. Therefore, this invention limits the ratio of boron(B) content in fresh martensite to that in tempered martensite to a range of 0.03 to 0.55, as shown in [Relationship Formula 1], thereby achieving the desired balance between tensile strength and elongation (TS 2 *EL 1 / 2 ), balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and the yield ratio evaluation index (1-YR) can be secured.

[0053] Furthermore, the inventors of this invention have found that not only the fraction of retained austenite, but also the ratio of retained austenite of a specific size to the total retained austenite is an important factor in ensuring strength and workability.

[0054] The higher the proportion of retained austenite with an average grain size of 1.2 μm or more, the more it contributes to improving the workability of steel sheets. Retained austenite with an average grain size of 1.2 μm or more is retained austenite whose average size has increased due to heat treatment at the bainite formation temperature, and it is a structure in which the driving force for transformation to martensite is relatively suppressed compared to retained austenite with an average grain size of 1.2 μm or less. Therefore, since the transformation to martensite is suppressed for retained austenite with an average grain size of 1.2 μm or more, the workability of steel sheets can be further effectively improved when the proportion of retained austenite with an average grain size of 1.2 μm or more is above a certain level.

[0055] This invention aims to control the ratio of the fraction of retained austenite with an average grain size of 1.2 μm or larger (V(1.2 μm, γ), volume%) to the total fraction of retained austenite contained in the steel sheet (V(γ), volume%) as shown in [Relationship Formula 2], to 0.12 or more, thereby achieving the desired balance between tensile strength and elongation (TS 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) can be effectively secured.

[0056] A high-strength steel sheet with excellent workability according to one aspect of the present invention has a balance between tensile strength and elongation (B) represented by the following [Relational Equation 3]. TE ) is 3.0*10 6 ~6.2*10 6 ( MPa 2 % 1 / 2 ) satisfies the following conditions and the balance between tensile strength and hole expansion ratio (B TH ) is 6.0*10 6 ~11.5*10 6 ( MPa 2 % 1 / 2 ) satisfies the following and is expressed by the yield ratio evaluation index (I YR ) can satisfy 0.15 to 0.42. [Relationship 3] B TE =[Tensile Strength (TS, MPa)] 2 *[Growth rate (El, %)] 1 / 2 [Relationship Equation 4] B TH =[Tensile Strength (TS, MPa)] 2 *[Hole expansion rate (HER, %)] 1 / 2 [Relationship 5] I YR =1-[yield ratio (YR)]

[0057] The following describes in detail an example of a method for manufacturing the steel sheet of the present invention.

[0058] A method for manufacturing a high-strength steel sheet according to one aspect of the present invention involves heating a cold-rolled steel sheet 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 maintaining it for 50 to 1200 seconds (primary maintenance); and then cooling the steel sheet that has been primary maintained to a temperature range of 350 to 550°C at an average cooling rate of 2 to 100°C / s (primary cooling), and then for 5 to 600 seconds The procedure may include: a step of maintaining the temperature for a few seconds (secondary maintenance); a step of cooling the secondary-maintained steel plate to a temperature range of 200-400°C at an average cooling rate of 2-100°C / s (secondary cooling); a step of heating the secondary-cooled steel plate to a temperature range of 350-550°C at an average heating rate of 5-100°C / s (tertiary heating), and then maintaining the temperature for 50 seconds or more (tertiary maintenance); and a step of cooling the tertiary-maintained steel plate to room temperature at an average cooling rate of 1°C / s or more (tertiary cooling).

[0059] The cold-rolled steel sheet described above can be provided through the following steps: heating a steel slab having a predetermined alloy composition to 1000-1350°C; finishing hot-rolling in a temperature range of 800-1000°C; winding the hot-rolled steel sheet in a temperature range of 350-650°C; pickling the wound steel sheet; and cold-rolling the pickled steel sheet with a reduction ratio of 30-90%.

[0060] Preparation and heating of steel slabs A steel slab having a predetermined alloy composition is prepared. Since the steel slab of the present invention has an alloy composition corresponding to the alloy composition of the steel plate described above, the explanation of the alloy composition of the steel slab is equivalent to the explanation of the alloy composition of the steel plate described above.

[0061] The prepared steel slab can be heated within a certain temperature range, which may be between 1000 and 1350°C. If the heating temperature of the steel slab is below 1000°C, there is a risk that it will be hot-rolled at a temperature below the target temperature range for finish hot rolling. 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.

[0062] Hot rolling and winding Heated steel slabs can be hot-rolled to produce hot-rolled steel sheets. The preferred temperature for the finish hot-rolling is in the range of 800 to 1000°C. If the finish hot-rolling temperature is below 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 form coarsely, potentially leading to a decrease in the physical properties of the final steel sheet.

[0063] Hot-rolled steel sheets, once hot-rolled, can be cooled at an average cooling rate of 10°C / s or higher and wound up in a temperature range of 350 to 650°C. If the winding temperature is below 350°C, winding is not easy, and if the winding temperature exceeds 650°C, surface scale may form inside the hot-rolled steel sheet, making pickling difficult.

[0064] Pickling and cold rolling After uncoiling the wound hot-rolled coil, the steel sheet can be pickled to remove scale formed on its surface, and then cold-rolled. While the pickling and cold-rolling conditions are not particularly limited in this invention, cold rolling is preferably performed with a cumulative reduction ratio of 30-90%. If the cumulative reduction ratio of cold rolling exceeds 90%, it may be difficult to perform the cold rolling in a short time due to the high strength of the steel sheet.

[0065] Cold-rolled steel sheets can be produced as unplated cold-rolled steel sheets through an annealing heat treatment process, or as plated steel sheets through a plating process to impart corrosion resistance. Plating methods such as hot-dip galvanizing, electro-galvanizing, and hot-dip aluminum plating can be applied, and there are no particular limitations on the method or type.

[0066] Annealing heat treatment This invention involves performing an annealing heat treatment process to simultaneously ensure the strength and workability of the steel plate.

[0067] Cold-rolled steel sheets are heated to 700°C at an average heating rate of 5°C / s or higher (primary heating), then heated to a temperature range of Ac3 to 920°C at an average heating rate of 5°C / s or lower (secondary heating), and maintained at that temperature for 50 to 1200 seconds (primary maintenance).

[0068] If the average heating rate of the primary heating to 700°C is less than 5°C / s, massive austenite is formed from the ferrite and cementite generated during heating, and as a result, it becomes impossible to form the final structure of fine tempered martensite and retained austenite. This makes it difficult to achieve the desired balance between tensile strength and elongation (TS). 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) becomes impossible to achieve. Also, if the secondary heating rate to the primary maintenance temperature exceeds 5°C / s, the transformation from cementite to austenite generated during heating is accelerated, a large amount of bulky austenite is formed, the final structure becomes coarser, and boron (B) may not be sufficiently concentrated in the tempered martensite. As a result, [B] FM / [B] TM When this value exceeds 0.55, the balance between tensile strength and elongation (TS) becomes less than the desired level. 2 *EL 1 / 2 ), balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and yield ratio evaluation index (I YR This will make it impossible to achieve.

[0069] When the primary maintenance temperature is below Ac3 (two-phase region), 5 volume% or more of ferrite is formed, and the balance between tensile strength and elongation (TS) is adjusted accordingly. 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2) may decrease. Also, if the primary maintenance time is less than 50 seconds, the microstructure may not be sufficiently homogenized, and the physical properties of the steel sheet may deteriorate. There are no particular upper limits on the primary maintenance temperature and primary maintenance time, but in order to prevent a decrease in toughness due to grain coarsening, it is preferable to limit the primary maintenance temperature to 920°C or less and the primary maintenance time to 1200 seconds or less.

[0070] After primary maintenance, the material can be cooled to a temperature range of 350-550°C at an average cooling rate of 2°C / s or higher (primary cooling), and then maintained at that temperature range for 5 seconds or more (secondary maintenance). If the average cooling rate of primary cooling is less than 2°C / s, the slow cooling will result in an insufficient fraction of retained austenite, thereby compromising the balance between the tensile strength and elongation of the steel sheet (TS). 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease. There is no need to specifically specify an upper limit for the average cooling rate of primary cooling, but it is preferable to keep it below 100°C. If the secondary maintenance temperature is below 350°C, the balance between V(1.2μm,γ) / V(γ) and tensile strength and hole expansion rate (TS) of the steel plate may decrease due to the low heat treatment temperature. 2 *HER 1 / 2 ) may decrease. On the other hand, if the secondary maintenance temperature exceeds 550°C, the retained austenite may become insufficient, and the balance between the tensile strength and elongation of the steel sheet (TS 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease. Also, if the above secondary holding time is less than 5 seconds, the heat treatment time will be insufficient, and the balance between the V(1.2μm,γ) / V(γ) and tensile strength and hole expansion rate (TS) of the steel sheet may decrease. 2 *HER 1 / 2 ) may decrease. There is no need to specifically specify an upper limit for the secondary maintenance time, but it is preferable to limit it to 600 seconds or less.

[0071] After secondary maintenance, the steel can be cooled to the primary cooling stop temperature of 200-400°C at an average cooling rate of 2°C / s or higher (secondary cooling). If the average cooling rate of secondary cooling is less than 2°C / s, the slow cooling will result in a deficiency in the fraction of retained austenite, thereby compromising the balance between the tensile strength and elongation of the steel sheet (TS). 2 *EL 1 / 2 ), balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease. There is no need to specifically specify an upper limit for the average cooling rate of secondary cooling, but it is preferable to keep it below 100°C / s. If the primary cooling stop temperature is below 200°C, excessive tempering martensite will be formed, and retained austenite will be insufficient, resulting in a decrease in the V(1.2μm,γ) / V(γ) ratio of the steel sheet, and a balance between tensile strength and elongation (TS). 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease. On the other hand, if the primary cooling stop temperature exceeds 400°C, excessive bainite formation occurs, and tempered martensite becomes insufficient, resulting in a balance between the tensile strength and elongation (TS) of the steel sheet. 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease.

[0072] After secondary cooling, the material can be heated to a temperature range of 350-550°C at an average heating rate of 5°C / s or higher (tertiary heating), and then maintained for 50 seconds or more (tertiary maintenance). While there is no specific upper limit to the average heating rate for tertiary heating, it is preferable to keep it below 100°C / s. If the tertiary maintenance temperature is below 350°C or the tertiary maintenance time is less than 50 seconds, excessive tempered martensite formation occurs, making it difficult to secure a sufficient fraction of retained austenite. As a result, the V(1.2μm,γ) / V(γ), the balance between tensile strength and elongation (TS) may be affected. 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2) may decrease. If the tertiary maintenance temperature exceeds 550°C or the tertiary maintenance time exceeds 155,000 seconds, the fraction of retained austenite will be insufficient, and the balance between the V(1.2μm,γ) / V(γ) and tensile strength and elongation (TS) of the steel sheet may decrease. 2 *EL 1 / 2 ), balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease.

[0073] After tertiary maintenance, the temperature can be cooled to room temperature at an average cooling rate of 1°C / s or higher (tertiary cooling).

[0074] The high-strength steel sheet with excellent workability produced by the manufacturing method described above may contain bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable microstructures. A preferred example may contain, by volume fraction, 10-30% bainite, 50-70% tempered martensite, 10-30% fresh martensite, 2-10% retained austenite, and 5% or less (including 0%) ferrite.

[0075] The steel plate manufactured by the above-described manufacturing method has a balance between tensile strength and elongation (B) expressed in the following [Relationship Equation 3]. TE ) is 3.0*10 6 ~6.2*10 6 ( MPa 2 % 1 / 2 ) satisfies the following conditions and the balance between tensile strength and hole expansion ratio (B TH ) is 6.0*10 6 ~11.5*10 6 ( MPa 2 % 1 / 2 ) satisfies the following and is expressed by the yield ratio evaluation index (I YR ) can satisfy 0.15 to 0.42. [Relationship 3] B TE =[Tensile Strength (TS, MPa)] 2 *[Growth rate (El, %)] 1 / 2 [Relationship Equation 4] B TH =[Tensile Strength (TS, MPa)] 2 *[Hole expansion rate (HER, %)] 1 / 2 [Relationship 5] I YR =1-[yield ratio (YR)] [Examples]

[0076] The following describes in more detail, with reference to specific examples, a high-strength steel sheet with excellent workability and a method for manufacturing the same according to one aspect of the present invention. 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. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0077] (Examples) A 100 mm thick steel slab with the alloy composition listed in Table 1 (the remainder being Fe and unavoidable impurities) was manufactured, heated to 1200°C, and then finished hot-rolled at 900°C. After this, it was cooled at an average cooling rate of 30°C / s and wound at the winding temperatures shown in Tables 2 and 3 to produce a 3 mm thick hot-rolled steel sheet. After pickling to remove surface scale, it was cold-rolled to a thickness of 1.5 mm.

[0078] Subsequently, steel plates were manufactured by heat treatment under the annealing heat treatment conditions described in Tables 2 to 5 below. In Tables 2 and 3 below, the single-phase region refers to the temperature range of Ac3 to 920°C, and the two-phase region refers to the temperature range of Ac3°C or less.

[0079] 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), fresh martensite (FM), and pearlite (P) were observed via SEM after nital etching of the cross-section of polished specimens. After nital etching, the smooth structure on the surface of the specimen was separated by ferrite, and the structure with a lamellar structure of cementite and ferrite was separated by pearlite. Since bainite (B) and tempered martensite (TM) were all observed in lath and block forms and were difficult to distinguish, the fractions of bainite and tempered martensite were calculated using expansion curves after dilatation evaluation. In other words, the fraction of bainite was determined by subtracting the fraction of tempered martensite calculated via the expansion curve from the fractions of bainite and tempered martensite measured by SEM observation. On the other hand, since fresh martensite (FM) and retained austenite (retained γ) are not easily distinguishable, the fraction of fresh martensite was determined by subtracting the fraction of retained austenite calculated by X-ray diffraction from the fraction of martensite and retained austenite observed by the above SEM.

[0080] On the other hand, steel plate [B] FM / [B] TM V(1.2μm,γ) / V(γ), balance between tensile strength and elongation (TS 2 *EL 1 / 2 ), balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and yield ratio evaluation index (I YR The following parameters were measured and evaluated, and the results are shown in Tables 8 and 9.

[0081] Boron (B) content in fresh martensite ([B] FM ) and the boron (B) content in tempered martensite ([B] TMThe boron (B) concentration was determined by measuring it in fresh and tempered martensite using EPMA (Electron Probe MicroAnalyser). Retained austenite (V(1.2 μm, γ)) with an average grain size of 1.2 μm or larger was measured using an EBSD (Electron Backscatter Diffraction) phase map.

[0082] Tensile strength (TS) and elongation (El) were evaluated by tensile testing. Test specimens were taken according to JIS No. 5 standard, with the reference direction being 90° to the rolling direction of the rolled sheet material, and the tensile strength (TS) and elongation (El) were measured. Hole expansion rate (HER) was evaluated by hole expansion testing. After forming a 10 mmΨ punched hole (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 the direction where the burr of the punched hole was facing outwards. The peripheral area of ​​the punched hole was compressed and expanded at a moving speed of 20 mm / min, and then calculated using the following [Relationship Formula 6]. [Relationship 6] Hole expansion rate (HER, %) = {(D-D0) / D0} × 100 In the above relational equation 6, D represents the diameter of the hole (mm) when the crack penetrates the steel plate along the thickness direction, and D0 represents the initial diameter of the hole (mm).

[0083] [Table 1]

[0084] [Table 2]

[0085] [Table 3]

[0086] [Table 4]

[0087] [Table 5]

[0088] [Table 6]

[0089] [Table 7]

[0090] [Table 8]

[0091] [Table 9]

[0092] As shown in Tables 1-9 above, in the case of a test specimen that satisfies the conditions presented in this invention, both [Relationship Formula 1] and [Relationship Formula 2] are satisfied, and the balance between tensile strength and elongation (B TE ) is 3.0*10 6 ~6.2*10 6 ( MPa 2 % 1 / 2 ) satisfies the requirements and balances tensile strength and hole expansion ratio (B TH ) is 6.0*10 6 ~11.5*10 6 ( MPa 2 % 1 / 2 ) satisfies the yield ratio evaluation index (I YR It can be seen that ) satisfies 0.15 to 0.42.

[0093] Test specimen 2 was conducted at a primary mean heating rate of less than 5°C / s, resulting in insufficient tempered martensite and retained austenite. As a result, the balance between tensile strength and elongation of test specimen 2 was (B TE ) is 3.0*10 6Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0094] Test specimen 3 was subjected to a secondary mean heating rate exceeding 5°C / s, resulting in the formation of massive austenite and failure to concentrate boron (B) in the tempered martensite. As a result, test specimen 3 was [B] FM / [B] TM The yield ratio evaluation index (I) is above 0.55. YR ) exceeds 0.42, balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0095] Test specimen 4 was tested in a two-phase region where the primary maintenance temperature was below Ac3, and the ferrite fraction exceeded the limit. As a result, the balance between tensile strength and elongation of test specimen 4 was (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0096] Test specimen 5 was subjected to a primary mean cooling rate of less than 1°C / s, resulting in an insufficient fraction of retained austenite. As a result, the balance between tensile strength and elongation of test specimen 5 was (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0097] Test specimen 6 was subjected to a secondary maintenance temperature of less than 350°C, resulting in insufficient heat treatment temperature. As a result, test specimen 6 had a V(1.2μm,γ) / V(γ) of less than 0.12, and the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than [amount missing].

[0098] Test specimen 7 was subjected to a secondary maintenance temperature exceeding 550°C, resulting in an insufficient fraction of retained austenite. As a result, the balance between tensile strength and elongation of test specimen 7 was (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0099] Test specimen 8 was subjected to a secondary holding time of less than 5 seconds, resulting in insufficient heat treatment time. As a result, test specimen 8 had a V(1.2μm,γ) / V(γ) of less than 0.12, and the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than [amount missing].

[0100] Test specimen 9 was subjected to a secondary mean cooling rate of less than 2°C / s, resulting in an insufficient fraction of retained austenite. As a result, the balance between tensile strength and elongation of test specimen 9 was (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0101] Test specimen 10 was subjected to a primary cooling stop temperature of less than 200°C, resulting in an excess of tempered martensite fraction and a deficiency of retained austenite fraction. As a result, test specimen 10 had a V(1.2μm,γ) / V(γ) ratio of less than 0.12, and the balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0102] Test specimen 11 was subjected to a primary cooling stop temperature exceeding 400°C, resulting in an excess of bainite fraction and a deficiency of tempered martensite fraction. As a result, the balance between tensile strength and elongation of test specimen 11 was (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0103] Test specimen 12 was subjected to a tertiary maintenance temperature of less than 350°C, resulting in an excess of tempered martensite fraction and a deficiency of retained austenite fraction. As a result, test specimen 12 had a V(1.2μm,γ) / V(γ) ratio of less than 0.12, and the balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0104] Test specimen 13 was subjected to a tertiary maintenance temperature exceeding 550°C, resulting in an insufficient fraction of retained austenite. As a result, test specimen 16 had a V(1.2μm,γ) / V(γ) of less than 0.12, and the balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0105] Test specimen 14 was subjected to a tertiary maintenance time of less than 50 s, resulting in an excess of tempered martensite fraction and a deficiency of retained austenite fraction. As a result, test specimen 14 had a V(1.2 μm, γ) / V(γ) of less than 0.12, and a balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0106] Test specimen 15 was subjected to a tertiary maintenance time exceeding 155,000 s, resulting in an insufficient fraction of retained austenite. As a result, test specimen 15 had a V(1.2 μm, γ) / V(γ) of less than 0.12, and the balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0107] Test specimen 37 has a low carbon (C) content and a good balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0108] Test specimen 38 had a high carbon (C) content, insufficient tempered martensite fraction, excessive fresh martensite fraction, and excessive retained austenite fraction. As a result, test specimen 38 had an imbalance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0109] Test specimen 39 had a low silicon (Si) content and an insufficient fraction of retained austenite. As a result, the balance between tensile strength and elongation of test specimen 39 was (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0110] Test specimen 40 had a high silicon (Si) content and an excess of fresh martensite. As a result, the balance between tensile strength and elongation of test specimen 40 was (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0111] Test specimen 41 had a high aluminum (Al) content and an excess of fresh martensite. As a result, the balance between tensile strength and elongation of test specimen 41 was (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0112] Test specimen 42 had a low manganese (Mn) content, resulting in insufficient residual austenite fraction during pearlite formation. As a result, the balance between tensile strength and elongation of test specimen 42 was (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0113] Test specimen 43 had a high manganese (Mn) content and an excess of fresh martensite. As a result, the balance between tensile strength and elongation of test specimen 43 was (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0114] Test specimen 44 had a high chromium (Cr) content and an excess of fresh martensite. As a result, the balance between tensile strength and elongation of test specimen 44 was (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0115] Test specimen 45 had a high molybdenum (Mo) content and an excess of fresh martensite. As a result, the balance between tensile strength and elongation of test specimen 45 was (B TE ) is 3.0*10 6 Less than (B) the balance between tensile strength and hole expansion rate TH ) is 6.0*10 6 It was less than [amount missing].

[0116] Test specimen 46 had a low boron (B) content, and boron (B) could not be concentrated in the tempered martensite. As a result, test specimen 51 was [B] FM / [B] TM The yield ratio evaluation index (I) exceeded 0.55. YR ) exceeded 0.42.

[0117] Test specimen 47 had a high boron (B) content, resulting in excessive concentration of boron (B) in the tempered martensite. As a result, test specimen 52 was [B] FM / [B] TM The yield ratio evaluation index (I) is less than 0.03. YR The value was less than 0.15.

[0118] Although the present invention has been described in detail above with reference to examples, other forms of embodiments are also possible. Therefore, the technical idea and scope of the claims described below are not limited to the examples.

Claims

1. In weight percent, it contains C: 0.1-0.25%, Si: 0.01-1.5%, Mn: 1.0-4.0%, Al: 0.01-1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, and B: 0.0005-0.005%, with the remainder being Fe and unavoidable impurities. The microstructure includes bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures. The microstructure comprises, by volume fraction, 10-30% bainite, 50-70% tempered martensite, 5% or less (including 0%) ferrite, 10-30% fresh martensite, and 2-10% retained austenite. The following [Relationship 1] and [Relationship 2] are satisfied, A high-strength steel sheet with excellent workability, having a yield ratio evaluation index (I YR) of 0.15 to 0.42, as expressed by the following [Relationship Formula 5]. [Relationship 1] 0.03≦[B] ＦＭ / [B] ＴＭ ≦0.55 In the aforementioned relational expression 1, [B] ＦＭ This is the boron (B) content (weight %) contained in fresh martensite, and [B] ＴＭ This represents the boron (B) content (weight %) in the tempered martensite. [Relationship Equation 2] V(1.2 μm, γ) / V(γ)≧0.12 In the above relational equation 2, V(1.2 μm, γ) is the fraction (volume %) of retained austenite with an average grain size of 1.2 μm or more, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet. [Relationship Equation 5] I YR = 1 - [yield ratio (YR)]

2. The steel plate further comprises, by weight %, at least one selected from the group consisting of (1) to (8) below, as described in claim 1, a high-strength steel plate with excellent workability. (1) One or more of the following: Ti: 0-0.5%, Nb: 0-0.5%, and V: 0-0.5% (2) One or more of the following: Cr: 0-3.0% and Mo: 0-3.0% (3) One or more of the following: Cu: 0-4.0% and Ni: 0-4.0% (4) Ca: 0-0.05%, REM (excluding Y): 0-0.05%, and Mg: 0-0.05% (one or more of these). (5) One or more of the following: W: 0-0.5% and Zr: 0-0.5% (6) One or more of the following: Sb: 0-0.5% and Sn: 0-0.5% (7) One or more of the following: Y: 0-0.2% and Hf: 0-0.2% (8) Co: 0-1.5%

3. The steel sheet satisfies the balance (B) of tensile strength and elongation rate represented by the following [Formula 3] within the range of 3.0×10 ＴＥ to 6.2×10 ６ (MPa ６ % ２ ), and satisfies the balance (B) of tensile strength and hole expansion rate represented by the following [Formula 4] within the range of 6.0×10 １／２ to 11.5×10 ＴＨ (MPa ６ % ６ ). The high-strength steel sheet with excellent workability according to Claim 1.​​​​ [Relationship Equation 3] B ＴＥ = [Tensile strength (TS, MPa)] ２ * [Growth rate (El, %)] １／２ [Relational Equation 4] B ＴＨ = [Tensile strength (TS, MPa)] ２ * [Hole expansion rate (HER, %)] １／２

4. A step of cold rolling a steel slab containing, by weight percent, C: 0.1-0.25%, Si: 0.01-1.5%, Mn: 1.0-4.0%, Al: 0.01-1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, and B: 0.0005-0.005%, with the remainder being Fe and unavoidable impurities; This step involves heating a cold-rolled steel sheet to 700°C at an average heating rate of 5°C / s or more (primary heating), then 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 finally maintaining the temperature for 50 to 1200 seconds (primary maintenance); The steel plate that has been maintained in the primary stage is cooled to a temperature range of 350 to 550°C at an average cooling rate of 2 to 100°C / s (primary cooling), and then maintained for 5 to 600 seconds (secondary maintenance); The second stage involves cooling the secondarily maintained steel plate to a temperature range of 200 to 400°C at an average cooling rate of 2 to 100°C / s (secondary cooling); The second-cooled steel plate is heated to a temperature range of 350 to 550°C at an average heating rate of 5 to 100°C / s (tertiary heating), and then maintained for 50 seconds or more (tertiary maintenance); and A method for manufacturing a high-strength steel sheet with excellent workability according to any one of claims 1 to 3, comprising the step of cooling the tertiarily maintained steel sheet to room temperature at an average cooling rate of 1°C / s or more (tertiary cooling).

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

6. The step of cold rolling the steel slab is as follows: The step of heating the steel slab to 1000 to 1350°C; The stage of finish hot rolling at a temperature range of 800 to 1000°C; A step of winding up the hot-rolled steel sheet at a temperature range of 350 to 650°C; The step of pickling the rolled steel sheet; and A method for manufacturing a high-strength steel sheet with excellent workability according to claim 4, comprising the step of cold-rolling the pickled steel sheet at a reduction ratio of 30 to 90%.

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