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

Abstract

The present invention relates to a steel sheet which can be used in automotive parts and the like, and to a steel sheet which has excellent bending workability and excellent balance between strength and ductility and between strength and hole expandability, and to a method for manufacturing the same.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 202080087053.5, filed on November 25, 2020, entitled "High-strength Steel Sheet with Excellent Workability and Method for Manufacturing the Same", which claims priority to KR 10-2019-0169611. TECHNICAL FIELD

[0002] The present application relates to a steel sheet which can be used for automobile parts and the like, and to a steel sheet having high-strength characteristics and excellent workability and a method for manufacturing the same. BACKGROUND

[0003] In recent years, in order to protect the earth's environment, the automobile industry is focusing on a method that can achieve the light weight of materials while ensuring the stability of passengers. In order to meet such a demand for stability and light weight, the application of high-strength steel sheets is rapidly increasing. In general, it is known that as the strength of a steel sheet increases, the workability of the steel sheet decreases. Therefore, in steel sheets for automobile parts, there is a demand for a steel sheet having high-strength characteristics while having excellent workability represented by ductility, bend workability, and hole expansion workability.

[0004] As a technique for improving the workability of a steel sheet, a method using tempered martensite is disclosed in Patent Document 1 and Patent Document 2. Tempered martensite, which is made by tempering hard martensite, is a soft martensite, and thus there is a difference in strength between tempered martensite and existing untempered martensite (freshly formed martensite). Therefore, when the freshly formed martensite is suppressed and tempered martensite is formed, the workability can be increased.

[0005] However, in the techniques disclosed in Patent Document 1 and Patent Document 2, the balance of tensile strength and elongation (TS x El) cannot satisfy 22000 MPa% or more, which means that it is difficult to secure a steel sheet having both excellent strength and ductility.

[0006] In addition, in order to obtain high-strength and excellent workability characteristics in a steel sheet for automobile parts, a Transformation Induced Plasticity (TRIP) steel using transformation induced plasticity of residual austenite has been developed. A TRIP steel having excellent strength and workability is disclosed in Patent Document 3.

[0007] In Patent Document 3, an attempt is made to improve ductility and workability by including polygonal ferrite, residual austenite, and martensite, but the main phase is bainite, and thus it is not possible to secure high strength, and it is known that the balance of tensile strength and elongation (TS x El) cannot satisfy 22000 MPa% or more.

[0008] That is, the demand for a steel sheet having high strength while having excellent workability represented by ductility, bend workability, and hole expansion, etc. cannot be satisfied at present.

[0009] (Prior Art Document) (Patent Document 1) Korean Patent Laid-Open No. 10-2006-0118602 (Patent Document 2) Japanese Patent Laid-Open No. 2009-019258 (Patent Document 3) Korean Patent Laid-Open No. 10-2014-0012167 SUMMARY

[0010] Technical Problem to be Solved by the Invention According to one aspect of the present invention, it is possible to provide a high-strength steel sheet having excellent ductility, bend workability, and hole expansion by optimizing the composition and microstructure of the steel sheet, and a method for manufacturing the same.

[0011] The technical problem of the present invention is not limited to the above. Additional technical problems of the present invention are described throughout the specification, and those skilled in the art can easily understand the additional technical problems of the present invention from the contents described in the specification of the present invention.

[0012] Technical Means to Solve the Technical Problem The high-strength steel sheet having excellent workability according to one aspect of the present invention can include, in terms of weight%, C: 0.25-0.75%, Si: 4.0% or less, Mn: 0.9-5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, the balance of Fe and inevitable impurities, the microstructure can include ferrite as soft microstructure and tempered martensite, bainite, and residual austenite as hard microstructure, and the following [relation 1] can be satisfied.

[0013] [Relation 1] 0.4 ≤ [H] F / [H] TM+B+γ ≤ 0.9 In the relation 1, [H] F and [H] TM+B+γ are nano-hardness values measured using a nanoindenter, [H] F is an average nano-hardness value (Hv) of ferrite as soft microstructure, and [H] TM+B+γ is an average nano-hardness value (Hv) of tempered martensite, bainite, and residual austenite as hard microstructure.

[0014] The steel sheet can further include any one or more of the following (1) to (9).

[0015] (1) one or more of Ti: 0-0.5%, Nb: 0-0.5%, and V: 0-0.5% (2) one or more of Cr: 0-3.0%, and Mo: 0-3.0% (3) one or more of Cu: 0-4.5%, and Ni: 0-4.5% (4) B: 0-0.005% (5) one or more of Ca: 0-0.05%, REM other than Y: 0-0.05%, and Mg: 0-0.05% (6) one or more of W: 0-0.5%, and Zr: 0-0.5% (7) one or more of Sb: 0-0.5%, and Sn: 0-0.5% (8) one or more of Y: 0-0.2%, and Hf: 0-0.2% (9) Co: 0-1.5% The total content (Si+Al) of the Si and the Al can be 1.0-6.0% by weight.

[0016] The microstructure of the steel sheet can include, in volume fraction, 30-70% of tempered martensite, 10-45% of bainite, 10-40% of retained austenite, 3-20% of ferrite, and unavoidable structures.

[0017] The balance (B T·E ) of the tensile strength and the elongation of the steel sheet, which is represented by the following [relationship 2], can be 22000 (MPa%) or more. T·H The balance (B R ) of the tensile strength and the hole expansion ratio, which is represented by the following [relationship 3], can be satisfied in the range of 0.5 to 3.0. The bending workability (B R ) can be satisfied in the range of 0.5 to 3.0.

[0018] [Relationship 2] B T·E [Relationship 3] B T·H = [Tensile strength (TS, MPa)] 2 [Relationship 4] B R = R / t In the relationship 4, R represents the minimum bending radius (mm) at which no cracks are generated after a 90° bending test, and t represents the thickness (mm) of the steel sheet.

[0019] A method for manufacturing a high-strength steel sheet with excellent processability according to another aspect of the present invention may include the following steps: heating and hot-rolling a steel billet, wherein the steel billet comprises, by weight %,: C: 0.25-0.75%, Si: less than 4.0%, Mn: 0.9-5.0%, Al: less than 5.0%, P: less than 0.15%, S: less than 0.03%, N: less than 0.03%, the balance being Fe and unavoidable impurities; coiling the hot-rolled steel sheet; and subjecting the coiled steel sheet to hot-rolled annealing heat treatment at a temperature range of 650-850°C. The hot-rolled annealed steel sheet is cold-rolled for 600-1700 seconds; the cold-rolled steel sheet is heated to a temperature range of Ac1 or higher but lower than Ac3 (first heating) and held for at least 50 seconds (first holding); the steel sheet is cooled to a temperature range of 100-300°C at an average cooling rate of 1°C / second or higher (first cooling); the steel sheet that has been cooled once is heated to a temperature range of 300-500°C at an average heating rate of 5°C / second or higher (second heating) and held for at least 50 seconds (second holding); and then cooled to room temperature (second cooling).

[0020] The billet may also include any one or more of the following (1) to (9).

[0021] (1) One or more of Ti: 0-0.5%, Nb: 0-0.5% and V: 0-0.5%. (2) One or more of Cr: 0-3.0% and Mo: 0-3.0% (3) One or more of Cu: 0-4.5% and Ni: 0-4.5% (4) B: 0-0.005% (5) One or more of the following: Ca: 0-0.05%, REM (excluding Y): 0-0.05%, and Mg: 0-0.05%. (6) One or more of W: 0-0.5% and Zr: 0-0.5% (7) One or more of Sb: 0-0.5% and Sn: 0-0.5% (8) One or more of Y: 0-0.2% and Hf: 0-0.2% (9) Co: 0-1.5% The total content (Si+Al) of Si and Al contained in the billet can be 1.0-6.0 by weight.

[0022] The steel billet can be heated to a temperature range of 1000-1350℃ and hot-rolled within a temperature range of 800-1000℃.

[0023] The hot-rolled steel sheet can be coiled at a temperature ranging from 300 to 600°C.

[0024] The cold-rolled reduction ratio can be 30 to 90%.

[0025] The secondary cooling rate can be 1°C / sec or more.

[0026] Inventive Effects According to a preferred aspect of the present application, a steel sheet having excellent strength and excellent ductility, bend workability, and hole expansion workability, etc., can be provided, and thus is particularly suitable as a steel sheet for automobile parts. Best Mode for Carrying Out the Invention

[0027] The present application relates to a high-strength steel sheet having excellent workability and a method of manufacturing the same, and preferred embodiments of the present application will be described below. The embodiments of the present application can be modified in various forms, and should not be construed as limiting the scope of the present application to the embodiments described below. The embodiments are provided to more specifically describe the present application to those skilled in the art.

[0028] The inventors of the present application recognized that in a transformation-induced plasticity (TRIP) steel including bainite, tempered martensite, residual austenite, and ferrite, when the stabilization of the residual austenite is achieved and the ratio of specific components included in the residual austenite and the ferrite is controlled within a certain range, the hardness difference between the phases of the residual austenite and the ferrite is reduced, and thus the workability and the strength of the steel sheet can be simultaneously ensured. Through research on this, a method of improving the ductility and the workability of a high-strength steel was designed, and thus the present application was completed.

[0029] Hereinafter, a high-strength steel sheet having excellent workability according to an aspect of the present application will be described in detail.

[0030] The high-strength steel sheet having excellent workability according to an aspect of the present application can include, in weight %, C: 0.25 to 0.75 %, Si: 4.0 % or less, Mn: 0.9 to 5.0 %, Al: 5.0 % or less, P: 0.15 % or less, S: 0.03 % or less, N: 0.03 % or less, the balance of Fe and inevitable impurities, and a fine structure can include ferrite as soft structure and tempered martensite, bainite, and residual austenite as hard structure, and can satisfy the following [Equation 1].

[0031] [Equation 1] 0.4 ≤ [H] F / [H] TM+B+γ ≤ 0.9 In the Equation 1, [H] F and [H]TM+B+γ is a nano hardness value measured using a nano indenter, [H] F is an average nano hardness value (Hv) of ferrite which is soft structure, [H] TM+B+γ is an average nano hardness value (Hv) of tempered martensite, bainite and residual austenite which are hard structure.

[0032] Hereinafter, the steel composition of the present application is explained in more detail. Hereinafter, unless otherwise specified, the % indicating the content of each element is based on weight.

[0033] The high-strength steel sheet having excellent workability according to one aspect of the present application contains, in weight %, C: 0.25-0.75 %, Si: 4.0 % or less, Mn: 0.9-5.0 %, Al: 5.0 % or less, P: 0.15 % or less, S: 0.03 % or less, N: 0.03 % or less, the balance of Fe and inevitable impurities. Further, the steel sheet 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: 4.5 % or less (including 0 %), Ni: 4.5 % or less (including 0 %), B: 0.005 % or less (including 0 %), Ca: 0.05 % or less (including 0 %), REM except Y: 0.05 % or less (including 0 %), Mg: 0.05 % or less (including 0 %), W: 0.5 % or less (including 0 %), Zr: 0.5 % or less (including 0 %), Sb: 0.5 % or less (including 0 %), Sn: 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 %). Also, the total content of Si and Al (Si+Al) can be 1.0-6.0 %.

[0034] Carbon (C): 0.25-0.75 % Carbon (C) is an element essential to secure the strength of the steel sheet, and is an element to stabilize residual austenite which contributes to improvement of the ductility of the steel sheet. Therefore, in order to achieve the effects as described above, 0.25 % or more of carbon (C) can be contained in the present application. The preferable content of carbon (C) can exceed 0.25 %, and can be 0.27 % or more, 0.30 % or more. The more preferable content of carbon (C) can be 0.31 % or more. On the other hand, when the content of carbon (C) exceeds a certain level, it can be difficult to perform cold rolling due to excessive increase in strength. Therefore, the upper limit of the content of carbon (C) can be limited to 0.75 % in the present application. The content of carbon (C) can be 0.70 % or less, and the more preferable content of carbon (C) can be 0.67 % or less.

[0035] Silicon (Si): 4.0% or less (except 0%) Silicon (Si) is an element that contributes to the improvement of strength by solid solution strengthening, and is also an element that improves workability by strengthening ferrite and homogenizing the structure. In addition, silicon (Si) is an element that contributes to the formation of retained austenite by suppressing the precipitation of cementite. Therefore, in order to achieve the effects as described above, silicon (Si) must be added in the present application. The preferable silicon (Si) content can be 0.02% or more, and the more preferable silicon (Si) content can be 0.05% or more. However, when the silicon (Si) content exceeds a certain level, problems such as plating defects that are not plated are induced in the plating process, and the weldability of the steel sheet can be reduced, and thus the upper limit of the silicon (Si) content can be limited to 4.0% in the present application. The upper limit of the preferable silicon (Si) content can be 3.8%, and the upper limit of the more preferable silicon (Si) content can be 3.5%.

[0036] Aluminum (Al): 5.0% or less (except 0%) Aluminum (Al) is an element that plays a deoxidizing role by combining with oxygen in the steel. In addition, like silicon (Si), aluminum (Al) is an element that stabilizes retained austenite by suppressing the precipitation of cementite. Therefore, in order to achieve the effects as described above, aluminum (Al) must be added in the present application. The preferable aluminum (Al) content can be 0.05% or more, and the more preferable aluminum (Al) content can be 0.1% or more. On the other hand, when too much aluminum (Al) is added, the inclusions of the steel sheet increase, and the workability of the steel sheet can be reduced, and thus the upper limit of the aluminum (Al) content can be limited to 5.0% in the present application. The upper limit of the preferable aluminum (Al) content can be 4.75%, and the upper limit of the more preferable aluminum (Al) content can be 4.5%.

[0037] In addition, the total content of silicon (Si) and aluminum (Al) (Si+Al) is preferably 1.0-6.0%. Silicon (Si) and aluminum (Al) are components that affect the formation of a fine structure, and thus affect ductility, bend workability, and hole expansion in the present application, and thus the total content of silicon (Si) and aluminum (Al) is preferably 1.0-6.0%. The more preferable total content of silicon (Si) and aluminum (Al) (Si+Al) can be 1.5% or more, and can be 4.0% or less.

[0038] Manganese (Mn): 0.9-5.0% Manganese (Mn) is a useful element for simultaneously improving strength and ductility. Therefore, in order to achieve the effects as described above, the lower limit of the content of manganese (Mn) can be limited to 0.9% in the present application. The lower limit of the content of manganese (Mn) can be preferably 1.0%, and the lower limit of the content of manganese (Mn) can be more preferably 1.1%. On the other hand, when too much manganese (Mn) is added, since the time of bainite transformation increases, the enrichment degree of carbon (C) in austenite is insufficient, and thus there is a problem in that the desired austenite fraction cannot be ensured. Therefore, the upper limit of the content of manganese (Mn) can be limited to 5.0% in the present application. The upper limit of the content of manganese (Mn) can be preferably 4.7%, and the upper limit of the content of manganese (Mn) can be more preferably 4.5%.

[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, the content of phosphorus (P) is preferably controlled to be 0.15% or less.

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

[0041] Nitrogen (N): 0.03% or less (including 0%) Nitrogen (N) is an element that is contained as an impurity and causes cracks of a slab by forming nitrides in continuous casting. Therefore, the content of nitrogen (N) is preferably 0.03% or less.

[0042] In addition, in the steel sheet of the present application, in addition to the above-described alloying components, there are alloying components that can be further contained, which will be described in detail below.

[0043] One or more of titanium (Ti): 0-0.5%, niobium (Nb): 0-0.5%, and vanadium (V): 0-0.5% Titanium (Ti), niobium (Nb), and vanadium (V) are elements that refine grains by forming precipitates, and are also elements that contribute to improvement of strength and impact toughness of a steel sheet, and thus, in order to achieve the effects as described above, one or more of titanium (Ti), niobium (Nb), and vanadium (V) can be added in the present application. However, when the content of each of titanium (Ti), niobium (Nb), and vanadium (V) exceeds a certain level, too much precipitates are formed, thereby deteriorating impact toughness, and also, can become a cause of increasing manufacturing costs, and thus, in the present application, the content of each of titanium (Ti), niobium (Nb), and vanadium (V) can be limited to 0.5% or less.

[0044] One or more of chromium (Cr): 0-3.0% and molybdenum (Mo): 0-3.0% Chromium (Cr) and molybdenum (Mo) inhibit austenite decomposition during alloying treatment, and like manganese (Mn), chromium (Cr) and molybdenum (Mo) are austenite-stabilizing elements. Therefore, to achieve the effects described above, one or more of chromium (Cr) and molybdenum (Mo) can be added in this invention. However, when the content of chromium (Cr) and molybdenum (Mo) exceeds a certain level, the enrichment of carbon (C) in austenite is insufficient due to the increased bainitic transformation time, thus failing to ensure the desired fraction of retained austenite. Therefore, in this invention, the content of chromium (Cr) and molybdenum (Mo) can be limited to 3.0% or less respectively.

[0045] Copper (Cu): 0-4.5% and Nickel (Ni): 0-4.5% or more Copper (Cu) and nickel (Ni) are elements that stabilize austenite and inhibit corrosion. Furthermore, copper (Cu) and nickel (Ni) are elements that accumulate on the surface of steel plates and prevent the intrusion of hydrogen migrating into the steel plate, thus inhibiting hydrogen-induced delayed fracture. Therefore, to achieve the effects described above, one or more of copper (Cu) and nickel (Ni) can be added in this invention. However, when the content of copper (Cu) and nickel (Ni) exceeds a certain level, it leads to excessive characteristic effects and increases manufacturing costs; therefore, in this invention, the content of copper (Cu) and nickel (Ni) is limited to 4.5% or less each.

[0046] Boron (B): 0-0.005% Boron (B) is an element that increases strength by improving hardenability and also inhibits grain boundary nucleation. Therefore, boron (B) can be added in this invention to achieve the effects described above. However, when the boron (B) content exceeds a certain level, it leads to excessive property effects and increases manufacturing costs. Therefore, in this invention, the boron (B) content is limited to 0.005% or less.

[0047] 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%. Rare earth elements (REMs) refer to scandium (Sc), yttrium (Y), and lanthanides. Rare earth elements (REMs) other than calcium (Ca), magnesium (Mg), and yttrium (Y) contribute to improving the ductility of steel sheets by spheroidizing sulfides. Therefore, to achieve the effects described above, one or more rare earth elements (REMs) other than calcium (Ca), magnesium (Mg), and yttrium (Y) may be added in this invention. However, when the content of rare earth elements (REMs) other than calcium (Ca), magnesium (Mg), and yttrium (Y) exceeds a certain level, it leads to excessive characteristic effects and increases manufacturing costs. Therefore, in this invention, the content of rare earth elements (REMs) other than calcium (Ca), magnesium (Mg), and yttrium (Y) is limited to 0.05% or less.

[0048] One or more of tungsten (W): 0-0.5% and zirconium (Zr): 0-0.5%. Tungsten (W) and zirconium (Zr) are elements that increase the strength of steel plates by improving hardenability. Therefore, in order to achieve the effects described above, one or more of tungsten (W) and zirconium (Zr) can be added in this invention. However, when the content of tungsten (W) and zirconium (Zr) exceeds a certain level, it leads to excessive property effects and increases manufacturing costs. Therefore, in this invention, the content of tungsten (W) and zirconium (Zr) is limited to 0.5% or less for each.

[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 wettability and adhesion of steel plates during plating. Therefore, to achieve the effects described above, one or more of antimony (Sb) and tin (Sn) can be added in this invention. However, when the content of antimony (Sb) and tin (Sn) exceeds a certain level, the brittleness of the steel plate increases, and cracks may occur during hot or cold working. Therefore, in this invention, the content of antimony (Sb) and tin (Sn) can be limited to 0.5% or less respectively.

[0050] One or more of yttrium (Y): 0-0.2% and hafnium (Hf): 0-0.2%. Yttrium (Y) and hafnium (Hf) are elements that improve the corrosion resistance of steel plates. Therefore, to achieve the effects described above, one or more of yttrium (Y) and hafnium (Hf) can be added in this invention. However, when the content of yttrium (Y) and hafnium (Hf) exceeds a certain level, the ductility of the steel plate may deteriorate. Therefore, in this invention, the content of yttrium (Y) and hafnium (Hf) can be limited to 0.2% or less.

[0051] Cobalt (Co): 0-1.5% Cobalt (Co) is an element that enhances the TRIP effect by promoting bainitic phase transformation; therefore, cobalt (Co) can be added in this invention to achieve the effect described above. However, when the cobalt (Co) content exceeds a certain level, the weldability and ductility of the steel sheet may deteriorate; therefore, the cobalt (Co) content can be limited to 1.5% or less in this invention.

[0052] In the high-strength steel sheet with excellent workability according to one aspect of the invention, in addition to the above-mentioned components, a balance of Fe and other unavoidable impurities may be included. However, unwanted impurities inevitably mix in from raw materials or the surrounding environment during normal manufacturing processes, and therefore these impurities cannot be completely eliminated. These impurities are well known to those skilled in the art, and therefore their contents are not specifically mentioned in this specification. Furthermore, the further addition of effective components other than those described above is not completely excluded.

[0053] According to one aspect of the present invention, the fine microstructure of a high-strength steel sheet with excellent machinability may comprise ferrite as a soft structure and tempered martensite, bainite, and retained austenite as hard structures. The terms "soft structure" and "hard structure" can be interpreted as concepts distinguished by their relative hardness difference.

[0054] As a preferred example, the fine microstructure of the high-strength steel sheet with excellent workability according to one aspect of the present invention may, by volume fraction, contain 30-70% tempered martensite, 10-45% bainite, 10-40% retained austenite, 3-20% ferrite, and unavoidable microstructure. The unavoidable microstructure of the present invention may include fresh martensite, pearlite, island martensite (martensite-austenite constituent, MA), etc. When too much fresh martensite or pearlite is formed, the workability of the steel sheet is reduced, or the fraction of retained austenite may be reduced.

[0055] In a high-strength steel sheet with excellent machinability according to one aspect of the present invention, as shown in the following [Relationship 1], the average nanohardness value ([H]) of the soft microstructure (ferrite) is... F The average nanohardness value ([H]) of the hard microstructure (tempered martensite, bainite, and retained austenite) is compared with that of the hard microstructure ([H]). TM+B+γ The ratio of Hv can satisfy the range of 0.4 to 0.9. [Relation 1] 0.4≤[H] F / [H] TM+B+γ ≤0.9 The nanohardness values ​​of hard and soft microstructures can be measured using a nanoindenter (FISCHERSCOPE HM2000). Specifically, the steel plate surface can be electropolished, and then, under an indentation load of 10000 μN, at least 20 points are randomly measured on both the hard and soft microstructures. The average nanohardness value of the hard and soft microstructures is calculated based on the measured values.

[0056] Furthermore, in a high-strength steel sheet with excellent workability according to one aspect of the present invention, the balance between tensile strength and elongation (B) is expressed by the following [relationship 2]. T·E For tensile strength of 22000 (MPa%) or higher, the balance between tensile strength and porosity is expressed by the following [Equation 3] (B T·H )for The bending processing rate (B) is expressed by the following [relationship 4]. R The steel plate meets the range of 0.5 to 3.0, so the steel plate can have an excellent balance of strength and ductility as well as a balance of strength and hole expansion, and can also have excellent bending workability.

[0057] [Relationship 2] B T·E [Relationship 3] B T·H =[Tensile Strength (TS, MPa)] 2 [Relationship 4] B R =R / t In Equation 4, R represents the minimum bending radius (mm) that does not produce cracks after a 90° bending test, and t represents the thickness of the steel plate (mm).

[0058] The objective of this invention is to simultaneously ensure high strength, excellent ductility, and bending workability; therefore, stabilizing the retained austenite in the steel sheet is crucial. To stabilize the retained austenite, it is necessary to enrich carbon (C) and manganese (Mn) in the ferrite, bainite, and tempered martensite of the steel sheet within the austenite. However, when carbon (C) is enriched in austenite using ferrite, the steel sheet may suffer from insufficient strength due to the low strength characteristics of ferrite, and excessive interphase hardness differences may occur, potentially reducing the porosity (HER). Therefore, carbon (C) and manganese (Mn) are enriched in austenite using bainite and tempered martensite.

[0059] When the silicon (Si) and aluminum (Al) content in retained austenite is limited to a certain range, a large amount of carbon (C) and manganese (Mn) can be enriched in the retained austenite from bainite and tempered martensite, thus effectively stabilizing the retained austenite. Furthermore, by limiting the silicon (Si) and aluminum (Al) content in austenite to a certain range, the silicon (Si) and aluminum (Al) content in ferrite can be increased. With the increase of silicon (Si) and aluminum (Al) content in ferrite, the hardness of ferrite increases, and the hardness difference between ferrite (a soft structure) and tempered martensite, bainite, and retained austenite (hard structures) can be effectively reduced.

[0060] When the average nanohardness value of the soft tissue (ferrite) is ([H]) F The average nanohardness value ([H]) of the hard microstructure (tempered martensite, bainite, and retained austenite) is compared with that of the hard microstructure ([H]). TM+B+γ When the ratio of σH to σV is above a certain level, the hardness difference between the soft structure (ferrite) and the hard structure (tempered martensite, bainite, and retained austenite) decreases, thus ensuring the desired balance between tensile strength and elongation (TS×EL) and between tensile strength and porosity (TS). 2 ×HER 1 / 2 ) and bending rate (R / t). On the other hand, when the average nanohardness value of the soft structure (ferrite) ([H]) F The average nanohardness value ([H]) of the hard microstructure (tempered martensite, bainite, and retained austenite) is compared with that of the hard microstructure ([H]). TM+B+γ When the ratio of tensile strength to elongation (TS×E1) is too large, the ferrite becomes excessively hardened, which reduces machinability. Therefore, it is impossible to simultaneously ensure the desired balance between tensile strength and elongation (TS×E1) and the balance between tensile strength and porosity (TS). 2 ×HER 1 / 2 The average nanohardness value ([H]) of the soft microstructure (ferrite) and the bending rate (R / t) can be used in this invention. F The average nanohardness value ([H]) of the hard microstructure (tempered martensite, bainite, and retained austenite) is compared with that of the hard microstructure ([H]). TM+B+γ The ratio of Hv) is limited to the range of 0.4 to 0.9.

[0061] Due to the transformation-induced plasticity that occurs when austenite transforms into martensite during processing, steel sheets containing retained austenite exhibit excellent ductility and bending workability. When the retained austenite fraction is below a certain level, the balance of tensile strength and elongation (TS×E1) is less than 22000 MPa%, or the bending workability (R / t) may exceed 3.0. Furthermore, when the retained austenite fraction exceeds a certain level, local elongation may decrease. Therefore, in this invention, to obtain steel sheets with excellent balance of tensile strength and elongation (TS×E1) and bending workability (R / t), the retained austenite fraction can be limited to a range of 10-40% by volume.

[0062] Furthermore, both untempered martensite (newly formed martensite) and tempered martensite are fine microstructures that improve the strength of steel sheets. However, compared to tempered martensite, newly formed martensite significantly reduces the ductility and porosity of the steel sheet. This is because the fine microstructure of tempered martensite is softened by tempering heat treatment. Therefore, in this invention, tempered martensite is preferred to provide a steel sheet with a balance between strength and ductility, a balance between strength and porosity, and excellent workability. When the fraction of tempered martensite is below a certain level, it is difficult to ensure tensile strength and elongation of 22000 MPa% or higher. The balance between tensile strength and porosity above (TS) 2 ×HER 1 / 2 When the fraction of tempered martensite exceeds a certain level, ductility and workability decrease, resulting in a tensile strength-elongation ratio (TS×E1) of less than 22000 MPa%, or a bending rate (R / t) exceeding 3.0, which is undesirable. Therefore, in this invention, to obtain a balance between tensile strength and elongation (TS×E1) and a balance between tensile strength and porosity (TS... 2 ×HER 1 / 2 Steel sheets with excellent bending workability (R / t) can limit the tempered martensite fraction to a range of 30-70% by volume.

[0063] To improve the balance between tensile strength and elongation (TS×El), and the balance between tensile strength and porosity (TS) 2 ×HER 1 / 2 The microstructure preferably includes bainite, along with the bending work ratio (R / t). Only when the bainite fraction is above a certain level can a balance between tensile strength and elongation (TS×E1) of over 22000 MPa% be ensured. The balance between tensile strength and porosity above (TS) 2 ×HER 1 / 2The bending rate (R / t) is 0.5 to 3.0. On the other hand, when the bainite fraction is too large, it is inevitably accompanied by a decrease in the tempered martensite fraction, so the balance between tensile strength and elongation (TS×E1) and the balance between tensile strength and porosity (TS) desired by the present invention cannot be ensured. 2 ×HER 1 / 2 The percentage of bainite can be limited to 10-45% by volume, as well as the bending rate (R / t).

[0064] Ferrite is an element that helps improve ductility; therefore, only when the ferrite fraction is above a certain level can the desired balance of tensile strength and elongation (TS×E1) be ensured in this invention. However, when the ferrite fraction is too large, the interphase hardness difference increases, which may reduce the porosity (HER), thus failing to ensure the desired balance of tensile strength and porosity (TS×E1) in this invention. 2 ×HER 1 / 2 Therefore, the fraction of ferrite in this invention can be limited to a range of 3-20% by volume.

[0065] The following is a detailed description of an example of a method for manufacturing the steel plate of the present invention.

[0066] A method for manufacturing a high-strength steel plate according to one aspect of the present invention may include the following steps: preparing a steel billet having a specified composition; heating and hot-rolling the steel billet; coiling the hot-rolled steel plate; subjecting the coiled steel plate to hot-rolled annealing heat treatment for 600-1700 seconds at a temperature range of 650-850°C; cold-rolling the hot-rolled annealed steel plate; heating the cold-rolled steel plate to a temperature range of Ac1 or higher but lower than Ac3 (first heating) and holding it for at least 50 seconds (first holding); cooling it to a temperature range of 100-300°C at an average cooling rate of 1°C / second or higher (first cooling); heating the first-cooled steel plate to a temperature range of 300-500°C at an average heating rate of 5°C / second or higher (second heating) and holding it for at least 50 seconds (second holding); and cooling it to room temperature (second cooling).

[0067] Preparation and heating of steel billets Prepare a steel billet having a specified composition. The steel billet of the present invention has an alloy composition corresponding to the alloy composition of the steel plate described above; therefore, the description of the alloy composition of the steel plate is used instead of the description of the alloy composition of the steel billet.

[0068] The prepared steel billet can be heated to a certain temperature range, which can be between 1000-1350℃. This is because when the heating temperature of the steel billet is below 1000℃, hot rolling may be carried out in a temperature range below the desired hot finishing temperature range, and when the heating temperature of the steel billet exceeds 1350℃, it may reach the melting point of steel and melt.

[0069] Hot rolling and coiling Heated steel billets can be hot-rolled to provide hot-rolled steel sheets. The hot finishing temperature during hot rolling is preferably in the range of 800-1000°C. This is because when the hot finishing temperature is below 800°C, excessive rolling load may become a problem, and when the hot finishing temperature exceeds 1000°C, coarse grains are formed in the hot-rolled steel sheet, which may lead to a reduction in the physical properties of the final steel sheet.

[0070] Hot-rolled steel sheets can be cooled at an average cooling rate of 10°C / second or higher and can be coiled at temperatures of 300-600°C. This is because it is difficult to coil when the coiling temperature is below 300°C, and when the coiling temperature exceeds 600°C, surface oxide scale forms inside the hot-rolled steel sheet, making pickling difficult.

[0071] Hot rolling annealing heat treatment To facilitate subsequent pickling and cold rolling processes after coiling, hot rolling annealing is preferred. Hot rolling annealing can be performed at temperatures between 650-850°C for 600-1700 seconds. When the hot rolling annealing temperature is below 650°C or the annealing time is less than 600 seconds, the steel sheet after hot rolling annealing has high strength, making subsequent cold rolling difficult. On the other hand, when the hot rolling annealing temperature exceeds 850°C or the annealing time exceeds 1700 seconds, pickling may be difficult due to the oxide scale formed deep within the steel sheet.

[0072] Pickling and cold rolling After hot rolling and annealing heat treatment, pickling and cold rolling can be performed to remove the oxide scale formed on the surface of the steel sheet. This invention does not impose particular restrictions on the pickling and cold rolling conditions, but it is preferable to perform cold rolling with a cumulative reduction rate of 30-90%. When the cumulative reduction rate 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.

[0073] Cold-rolled steel sheets can be annealed to produce uncoated cold-rolled steel sheets, or coated to impart corrosion resistance. Coating methods such as hot-dip galvanizing, electro-galvanizing, and hot-dip aluminizing can be used, with no particular restrictions on the methods or types.

[0074] Annealing heat treatment In this invention, an annealing heat treatment process is performed to ensure both the strength and processability of the steel plate.

[0075] Cold-rolled steel sheet is heated to a temperature range above Ac1 but below Ac3 (two-phase region) (single heating) and held within this temperature range for at least 50 seconds (one holding). When the temperature of the single heating or holding is above Ac3 (single-phase region), the desired ferrite structure cannot be achieved, and therefore the desired level of [H] cannot be achieved. F / [H] TM+B+γ And the balance between tensile strength and porosity (TS) 2 ×HER 1 / 2 Furthermore, when the temperature during a single heating or holding cycle is below the temperature range of Ac1, sufficient heating cannot be achieved, and even subsequent heat treatment may not be able to achieve the fine microstructure desired by this invention. The average heating rate for a single heating cycle can be 5°C / second or higher.

[0076] When the holding time is less than 50 seconds, the microstructure cannot be sufficiently homogenized, and the physical properties of the steel sheet may be reduced. There is no particular upper limit to the holding time, but to prevent a decrease in toughness due to grain coarsening, the heating time is preferably limited to 1200 seconds or less.

[0077] After one holding period, cooling can be continued at an average cooling rate of 1°C / second or higher to a primary cooling termination temperature of 100-300°C (primary cooling). No specific upper limit needs to be specified for the primary cooling rate, but it is preferably set to below 100°C / second. When the primary cooling termination temperature is below 100°C, excessive tempered martensite is formed, and the amount of retained austenite is insufficient, which may reduce [H]. F / [H] TM+B+γ The balance between tensile strength and elongation (TS×El) and bending workability (R / t) is considered. On the other hand, when the initial cooling termination temperature exceeds 300°C, excessive bainite is formed, and the amount of tempered martensite formation is insufficient, which may reduce the balance between tensile strength and elongation (TS×El) and the balance between tensile strength and porosity (TS). 2 ×HER 1 / 2 ).

[0078] After the initial cooling, a secondary heating process can be performed at an average heating rate of 5°C / second or higher to a secondary heating temperature of 300-500°C (secondary heating), and held within this temperature range for at least 50 seconds (secondary holding). While no specific upper limit is required for the secondary heating rate, it is preferably set to 100°C / second or lower. When the secondary heating or holding temperature is below 300°C or the holding time is less than 50 seconds, excessive tempered martensite forms, and the Si and Al content in the retained austenite is not adequately controlled, making it difficult to ensure the desired retained austenite fraction. Consequently, [H] may decrease. F / [H] TM+B+γ The balance between tensile strength and elongation (TS×E1) and bending workability (R / t) is considered. On the other hand, when the temperature for secondary heating or holding exceeds 500°C or the holding time exceeds 172,000 seconds, the control of Si and Al content in the retained austenite is insufficient, making it difficult to ensure the fraction of retained austenite. As a result, [H] may decrease. F / [H] TM+B+γ And the balance between tensile strength and elongation (TS×El).

[0079] After the second holding period, the temperature can be cooled to room temperature at an average cooling rate of 1°C / second or higher (secondary cooling).

[0080] In the high-strength steel sheet with excellent workability manufactured by the above manufacturing method, the fine microstructure may include tempered martensite, bainite, retained austenite and ferrite. As a preferred example, by volume fraction, the fine microstructure may include 30-70% tempered martensite, 10-45% bainite, 10-40% retained austenite, 3-20% ferrite and unavoidable microstructure.

[0081] Furthermore, in the high-strength steel sheet with excellent workability manufactured by the above-described manufacturing method, as shown in the following [Equation 1], the average nanohardness value ([H]) of the soft microstructure (ferrite) is... F The average nanohardness value ([H]) of the hard microstructure (tempered martensite, bainite, and retained austenite) is compared with that of the hard microstructure ([H]). TM+B+γ The ratio of tensile strength to elongation (B) can satisfy a range of 0.4 to 0.9, as expressed by the following [Equation 2]: T·E The tensile strength and porosity can be above 22000 (MPa%), and the balance between tensile strength and porosity is expressed by the following [relationship 3] (B T·H ) can be The bending processing rate (B) is expressed by the following [relationship 4]. R It can meet the range of 0.5 to 3.0.

[0082] [Relation 1] 0.4≤[H] F / [H] TM+B+γ ≤0.9 [Relationship 2] B T·E [Relationship 3] B T·H =[Tensile Strength (TS, MPa)] 2 [Relationship 4] B R =R / t In Equation 4, R represents the minimum bending radius (mm) that does not produce cracks after a 90° bending test, and t represents the thickness of the steel plate (mm). Detailed Implementation

[0083] The following detailed description, through specific embodiments, illustrates one aspect of the present invention: a high-strength steel plate with excellent processability and its manufacturing method. It should be noted that the following embodiments are merely for understanding the present invention and are not intended to limit the scope of the invention. The scope of the invention is determined by the contents of the claims and the contents reasonably inferred therefrom.

[0084] (Example) A 100 mm thick steel billet with the alloy composition (balance: Fe and unavoidable impurities) listed in Table 1 is manufactured and heated to 1200 °C, then hot-rolled at 900 °C. It is then cooled at an average cooling rate of 30 °C / s and coiled at the coiling temperatures specified in Tables 2 and 3 to produce a 3 mm thick hot-rolled steel sheet. The hot-rolled steel sheet is then subjected to hot-rolling annealing heat treatment according to the conditions in Tables 2 and 3. Afterwards, it is pickled to remove surface oxide scale and then cold-rolled to a thickness of 1.5 mm.

[0085] Then, heat treatment is carried out according to the annealing heat treatment conditions disclosed in Tables 2 to 5 to manufacture steel plates.

[0086] The microstructure of the steel plates manufactured as described above was observed, and the results are shown in Tables 6 and 7. The cross-sections of the polished specimens were etched with a nitric acid-alcohol solution, and then the ferrite (F), bainite (B), tempered martensite (TM), and pearlite (P) in the microstructure were observed by SEM. Bainite and tempered martensite, which were difficult to distinguish, were fractionated using expansion curves after expansion evaluation. Furthermore, newly formed martensite (FM) and retained austenite (retained γ) were also difficult to distinguish; therefore, the fraction of newly formed 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 SEM.

[0087] In addition, regarding the [H] of the steel plate F / [H] TM+B+γ Balance between tensile strength and elongation (TS×El), and balance between tensile strength and porosity (TS). 2 ×HER 1 / 2 The bending rate (R / t) was observed, and the results are shown in Tables 8 and 9.

[0088] Tensile strength (TS) and elongation (El) are evaluated through tensile testing. A test piece is taken and evaluated according to JIS standard No. 5, with a reference angle of 90° relative to the rolling direction of the rolled sheet. The bending rate (R / t) is evaluated through V-bending testing. A test piece is taken with a reference angle of 90° relative to the rolling direction of the rolled sheet. It is calculated by dividing the minimum bending radius R (mm) that does not produce cracks after the 90° bending test by the sheet thickness t (mm). Hole expansion rate (HER) is evaluated through hole expansion testing. After forming a 10mmØ punch (die inner diameter 10.3mm, clearance 12.5%), a conical punch with a 60° apex angle is inserted into the punch with the burr facing outwards. The punch is then compressed and expanded around the perimeter at a moving speed of 20mm / min, and the result is calculated using the following [Equation 5].

[0089] [Relation 5] Hole expansion ratio (HER, %) = {(D-D0) / D0} × 100 In Equation 5, D represents the aperture (mm) when the crack penetrates the steel plate along the thickness direction, and D0 represents the initial aperture (mm).

[0090] Nanoindentation was used to measure the nanohardness values ​​of hard and soft tissues. Specifically, the surfaces of each specimen were electropolished, and then at least 20 points were randomly measured on both hard and soft tissues using a nanoindentation instrument (FISCHERSCOPE HM2000) under an indentation load of 10000 μN. The average nanohardness value of the hard and soft tissues was calculated based on the measured values.

[0091] [Table 1]

[0092] [Table 2]

[0093] [Table 3]

[0094] [Table 4]

[0095] [Table 5]

[0096] [Table 6]

[0097] [Table 7]

[0098] [Table 8]

[0099] [Table 9]

[0100] As shown in Tables 1 to 9, it can be seen that when the test pieces meet the conditions proposed in this invention, [H] F / [H] TM+B+γ The value satisfies the range of 0.4 to 0.9, the balance of tensile strength and elongation (TS×El) is above 22000 MPa%, and the balance of tensile strength and porosity (TS) 2 ×HER 1 / 2 The bending rate (R / t) is above 0.5 to 3.0, thus exhibiting both excellent strength and machinability.

[0101] Test pieces 2 to 5 overlap with the alloy composition range of the present invention, but the hot rolling annealing temperature and time are not within the range of the present invention. Therefore, it can be confirmed that pickling failure or fracture occurred during cold rolling.

[0102] In specimen 6, the temperature was raised or held beyond the limits of this invention during the annealing heat treatment after cold rolling, resulting in insufficient ferrite formation. Consequently, the [H] content of specimen 6 was confirmed. F / [H] TM+B+γ Less than 0.4, the balance between tensile strength and porosity (TS) 2 ×HER 1 / 2 ).

[0103] In specimen 7, the cooling rate during the annealing heat treatment after cold rolling was lower than the range limited by this invention, resulting in the formation of excessive ferrite and a small amount of retained austenite. As a result, the [H] content of specimen 7 can be confirmed. F / [H] TM+B+γ When the value exceeds 0.9, the balance of tensile strength and elongation (TS×El) is less than 22000 MPa.

[0104] In specimen 12, the initial cooling termination temperature was low, resulting in excessive tempered martensite formation and a small amount of retained austenite. As a result, the [H] content of specimen 12 can be confirmed. F / [H] TM+B+γ The tensile strength and elongation balance (TS×El) is less than 22000 MPa%, and the bending rate (R / t) is greater than 3.0.

[0105] In specimen 13, the initial cooling termination temperature was high, resulting in the formation of excessive bainite and a small amount of tempered martensite. Consequently, it can be confirmed that the balance between tensile strength and elongation (TS×E1) of specimen 13 is less than 22000 MPa%, and the balance between tensile strength and porosity (TS) is also less than 22000 MPa%. 2 ×HER 1 / 2 ).

[0106] In specimen 14, the secondary heating or holding at a low temperature resulted in the formation of excessive tempered martensite and a small amount of retained austenite. As a result, the [H] content of specimen 14 can be confirmed. F / [H] TM+B+γ The tensile strength and elongation balance (TS×El) is less than 22000 MPa%, and the bending rate (R / t) is greater than 3.0.

[0107] In specimen 15, the secondary heating or high holding temperature resulted in insufficient formation of retained austenite, and [H] was confirmed. F / [H] TM+B+γ When the value exceeds 0.9, the balance of tensile strength and elongation (TS×El) is less than 22000 MPa.

[0108] In specimen 16, the secondary holding time was insufficient, resulting in excessive tempered martensite formation and a small amount of retained austenite. As a result, the [H] content of specimen 16 can be confirmed. F / [H] TM+B+γ The tensile strength and elongation balance (TS×El) is less than 22000 MPa%, and the bending rate (R / t) is greater than 3.0.

[0109] In specimen 17, the secondary holding time was too long, resulting in insufficient formation of retained austenite, and [H] can be confirmed. F / [H] TM+B+γ When the value exceeds 0.9, the balance of tensile strength and elongation (TS×El) is less than 22000 MPa.

[0110] Test pieces 40 to 48 represent cases where the manufacturing conditions proposed in this invention are met, but the alloy composition falls outside the range proposed in this invention. In these cases, it can be confirmed that the [H] of this invention cannot be simultaneously satisfied. F / [H] TM+B+γ Balance of conditions, tensile strength and elongation (TS×El) Balance of conditions, tensile strength and porosity (TS) 2 ×HER 1 / 2 The conditions and bending rate (R / t) conditions were also considered. Additionally, sample 42, with a total aluminum (Al) and silicon (Si) content of less than 1.0%, confirmed that the [H] requirement could not be met. F / [H] TM+B+γ The balance between tensile strength and elongation (TS×El) and the bending rate (R / t) conditions.

[0111] The present invention has been described in detail above through embodiments, but other embodiments may also be included. Therefore, the technical concept and scope of the claims are not limited to the embodiments.

Claims

1. A high-strength steel sheet excellent in workability, comprising, in mass%, C: 0.31 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 0.33 to 5.0%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, balance of Fe and unavoidable impurities, a microstructure comprising ferrite as soft structure and tempered martensite, bainite and residual austenite as hard structure, the microstructure comprising, in volume fraction, 30 to 70% of tempered martensite, 10 to 45% of bainite, 10 to 40% of residual austenite, 3 to 20% of ferrite and unavoidable structure, and satisfying the following [relation 1], [relation 1] 0.5 < (TS - 600) / (TS - 200) < 1.5 wherein the unit of the tensile strength TS is MPa, the units of the elongation El and the hole expansion ratio HER are %, R represents the minimum bending radius without cracks after a 90° bending test, the unit is mm, and t represents the thickness of the steel sheet, the unit is mm. [relation 2] 0.5 < (TS - 600) / (TS - 200) < 1.5 wherein the unit of the tensile strength TS is MPa, the units of the elongation El and the hole expansion ratio HER are %, R represents the minimum bending radius without cracks after a 90° bending test, the unit is mm, and t represents the thickness of the steel sheet, the unit is mm. [relation 3] 0.5 < (TS - 600) / (TS - 200) < 1.5 wherein the unit of the tensile strength TS is MPa, the units of the elongation El and the hole expansion ratio HER are %, R represents the minimum bending radius without cracks after a 90° bending test, the unit is mm, and t represents the thickness of the steel sheet, the unit is mm. [relation 4] 0.5 < (TS - 600) / (TS - 200) < 1.5 wherein the unit of the tensile strength TS is MPa, the units of the elongation El and the hole expansion ratio HER are %, R represents the minimum bending radius without cracks after a 90° bending test, the unit is mm, and t represents the thickness of the steel sheet, the unit is mm. The balance B of the tensile strength and the elongation of the steel sheet represented by the following [relation expression 2] T·E is 22000 MPa% or more, the balance B of the tensile strength and the hole expansion ratio represented by the following [relation expression 3] T·H The balance B of the bend processing ratio represented by the following [relation expression 4] R is 0.5 to 3.0, In the relation 4, R represents the minimum bending radius without cracks after a 90° bending test, the unit is mm, and t represents the thickness of the steel sheet, the unit is mm. 0.4 ≤ [H] F / [H] TM+B+γ ≤ 0.9 In the relational expression 1, [H] F and [H] TM+B+γ is a nano hardness value measured using a nano indenter, [H] F is an average nano hardness value of ferrite as soft tissue, [H] TM+B+γ is an average nano hardness value of tempered martensite, bainite, and residual austenite as hard tissue, The steel sheet further comprises any one or more of the following (1) to (9): B T·E (1) one or more of Ti: 0 to 0.5%, Nb: 0 to 0.5% and V: 0 to 0.5%, B T·H =[tensile strength TS] 2 (2) one or more of Cr: 0 to 3.0% and Mo: 0 to 3.0%, B R =R / t (3) one or more of Cu: 0 to 4.5% and Ni: 0 to 4.5%, (5) one or more of Ca: 0 to 0.05%, REM other than Y: 0 to 0.05% and Mg: 0 to 0.05%, 2. The high-strength steel sheet with excellent workability according to claim 1, wherein (6) one or more of W: 0 to 0.5% and Zr: 0 to 0.5%, (7) one or more of Sb: 0 to 0.5% and Sn: 0 to 0.5%, (8) one or more of Y: 0 to 0.2% and Hf: 0 to 0.2%, (9) Co: 0 to 1.5%. (4) B：0-0.005%， The total content of the Si and the Al is 1.0 to 6.0 mass%.

4. A method of manufacturing the high-strength steel sheet excellent in workability according to claim 1, comprising the steps of: heating a steel billet comprising, in mass%, C: 0.31 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 0.33 to 5.0%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, balance of Fe and unavoidable impurities, and performing hot rolling; winding the hot-rolled steel sheet; performing hot-rolling annealing heat treatment of the wound steel sheet at a temperature range of 650 to 850°C for 600 to 1700 seconds; cold-rolling the hot-rolling annealing heat-treated steel sheet; heating the cold-rolled steel sheet to a temperature range of Ac1 or higher and lower than Ac3 once, and performing once holding for 50 seconds or more; performing once cooling at an average cooling rate of 1°C / sec or more to a temperature range of 100 to 300°C; and performing tempering heat treatment of the once-cooled steel sheet at a temperature range of 350 to 650°C for 10 to 300 minutes. ​ ​ ​ 3. The high-strength steel sheet with excellent workability according to claim 1, wherein ​ ​ ​ ​ ​ ​ ​ ​ the steel plate once cooled is heated to a temperature range of 300 to 500°C at an average temperature increase rate of 5°C / sec or more for secondary heating, and held for 50 seconds or more for secondary holding; and secondary cooling to normal temperature is performed.

5. The method of producing a high-strength steel sheet excellent in workability according to claim 4, wherein The steel billet further contains any one or more of the following (1) to (9): (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 4.5% and Ni: 0 to 4.5%, (4) B：0-0.005%， (5) one or more of Ca: 0 to 0.05%, REM other than Y: 0 to 0.05%, and Mg: 0 to 0.05%, (6) one or more of W: 0 to 0.5% and Zr: 0 to 0.5%, (7) one or more of Sb: 0 to 0.5% and Sn: 0 to 0.5%, (8) one or more of Y: 0 to 0.2% and Hf: 0 to 0.2%, (9) Co: 0 to 1.5%.

6. The method of producing a high-strength steel sheet excellent in workability according to claim 4, wherein The total content of the Si and the Al contained in the steel billet is 1.0 to 6.0% by weight.

7. The method of producing a high-strength steel sheet excellent in workability according to claim 4, wherein The steel billet is heated to a temperature range of 1000 to 1350°C, and hot finish rolling is performed in a temperature range of 800 to 1000°C.

8. The method of producing a high-strength steel sheet with excellent workability according to claim 4, wherein The hot-rolled steel plate is coiled in a temperature range of 300 to 600°C.

9. The method of producing a high-strength steel sheet with excellent workability according to claim 4, wherein The cold-rolling reduction rate is 30 to 90%.

10. The method of producing a high-strength steel sheet with excellent workability according to claim 4, wherein The cooling rate of the secondary cooling is 1°C / sec or more.

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