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

Abstract

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

Description

Technical Field

[0001] This invention relates to a steel sheet that can be used in automotive parts, etc., and to a steel sheet with high strength and excellent processability, and a method for manufacturing the same. Background Technology

[0002] In recent years, to protect the global environment, the automotive industry has focused on methods that can achieve lightweight materials while ensuring passenger stability. To meet this demand for both stability and lightweighting, the application of high-strength steel sheets is increasing dramatically. It is generally known that as the strength of a steel sheet increases, its machinability decreases. Therefore, steel sheets used in automotive components require high strength while also exhibiting excellent machinability, such as ductility and hole-expanding properties.

[0003] Transformation-induced plasticity (TRIP) steel, which utilizes the transformation of retained austenite to induce plasticity, is known to have a complex microstructure composed of ferrite, bainite, martensite, and retained austenite. Therefore, it has high strength properties while also possessing a certain level of machinability.

[0004] As a technique to further improve the workability of steel sheets, Patent Documents 1 and 2 disclose a method utilizing tempered martensite. Tempered martensite, produced by tempering hard martensite, is softened martensite, thus exhibiting a strength difference between tempered martensite and existing untempered martensite (newly formed martensite). Therefore, by suppressing the formation of new martensite and forming tempered martensite, workability can be increased.

[0005] However, in the technologies disclosed in Patent Documents 1 and 2, the balance between tensile strength and elongation (TS) 2 *EL 1 / 2 Unable to meet 3.0*10 6 Up to 6.2*10 6 (MPa 2 % 1 / 2 The range of ) indicates that it is difficult to ensure steel plates with both excellent strength and ductility.

[0006] In addition, as another technique to improve the workability of steel sheets, Patent Document 3 discloses a method for inducing the formation of bainite by adding boron (B). Adding boron (B) suppresses the ferrite-pearlite phase transformation and induces the formation of bainite, thus achieving a balance between strength and workability.

[0007] However, the technology disclosed in Patent Document 3 cannot simultaneously ensure 3.0*10 6 Up to 6.2*10 6 (MPa2 1 / 2 balance (B TE ) of tensile strength and elongation 6 to 11.5*10 6 (MPa 2 1 / 2 balance (B TH ) of tensile strength and hole expansion ratio and a yield ratio evaluation index (I YR ), and thus it is difficult to ensure a steel sheet having excellent strength, hole expandability, ductility, and yield ratio.

[0008] That is, there is currently no steel sheet having excellent balance (B TE ) of tensile strength and elongation, balance (B TH ) of tensile strength and hole expansion ratio, and yield ratio evaluation index (I YR ) that can satisfy the demand.

[0009] (Prior Art Documents)

[0010] (Patent Document 1) Korean Patent Laid-Open Publication No. 10-2006-0118602

[0011] (Patent Document 2) Japanese Patent Laid-Open Publication No. 2009-019258

[0012] (Patent Document 3) Japanese Patent Laid-Open Publication No. 2016-216808 SUMMARY

[0013] PROBLEMS TO BE SOLVED BY THE INVENTION

[0014] According to one aspect of the present application, it is possible to provide a steel sheet having excellent balance of tensile strength and elongation, balance of tensile strength and hole expansion ratio, and yield ratio evaluation index by optimizing the composition and microstructure of the steel sheet, and a method of manufacturing the same.

[0015] The problems to be solved by the present application are not limited to the above. Additional problems to be solved by the present application are described throughout the specification, and those skilled in the art can easily understand the additional problems to be solved by the present application from the contents described in the specification of the present application.

[0016] TECHNICAL SOLUTION

[0017] ​​The high-strength steel sheet having excellent workability according to one aspect of the present application can include, in terms of mass%, 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%, the balance of Fe and inevitable impurities, and a fine structure including bainite, tempered martensite, fresh martensite, residual austenite and other inevitable structures, and can satisfy the following [relation 1].

[0018] [relation 1]

[0019] 0.03≤[B] FM / [B] TM ≤0.55

[0020] In the relation 1, [B] FM is the content (mass%) of boron (B) included in the fresh martensite, and [B] TM is the content (mass%) of boron (B) included in the tempered martensite.

[0021] The steel sheet can further include any one or more of the following (1) to (8) in terms of mass%.

[0022] (1) one or more of Ti: 0-0.5%, Nb: 0-0.5% and V: 0-0.5%,

[0023] (2) one or more of Cr: 0-3.0% and Mo: 0-3.0%,

[0024] (3) one or more of Cu: 0-4.0% and Ni: 0-4.0%,

[0025] (4) one or more of Ca: 0-0.05%, REM except Y: 0-0.05% and Mg: 0-0.05%,

[0026] (5) one or more of W: 0-0.5% and Zr: 0-0.5%,

[0027] (6) one or more of Sb: 0-0.5% and Sn: 0-0.5%,

[0028] (7) one or more of Y: 0-0.2% and Hf: 0-0.2%,

[0029] (8) Co: 0-1.5%.

[0030] The microstructure of the steel sheet can contain, in volume fraction, 10 to 30% of bainite, 50 to 70% of tempered martensite, 10 to 30% of fresh martensite, 2 to 10% of residual austenite, and 5% or less (including 0%) of ferrite.

[0031] In the steel sheet, a balance (B TE ) of the tensile strength and the elongation represented by the following [Equation 2] can satisfy 3.0*10 6 to 6.2*10 6 (MPa 2 %) 1 / 2 , a balance (B TH ) of the tensile strength and the hole expansion ratio represented by the following [Equation 3] can satisfy 6.0*10 6 to 11.5*10 6 (MPa 2 %) 1 / 2 , and an index (I YR ) of the yield ratio represented by the following [Equation 4] can satisfy 0.15 to 0.42.

[0032] [Equation 2]

[0033] B TE = [Tensile strength (TS, MPa)] 2 * [Elongation (El, %)] 1 / 2

[0034] [Equation 3]

[0035] BTH = [Tensile strength (TS, MPa)] 2 * [Hole expansion ratio (HER, %)] 1 / 2

[0036] [Equation 4]

[0037] I YR = 1 - [Yield ratio (YR)]

[0038] The method of manufacturing a high-strength steel sheet having excellent workability according to one aspect of the present application can include the steps of: providing a cold-rolled steel sheet including, in 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 %, the balance of Fe and inevitable impurities; heating the cold-rolled steel sheet to 700 °C at an average heating rate of 5 °C / sec or more (first heating), heating to a temperature range of Ac3 to 920 °C at an average heating rate of 5 °C / sec or less (second heating), and then maintaining for 50-1200 seconds (first maintaining); cooling the first-maintained steel sheet to a temperature range of 200-400 °C at an average cooling rate of 1 °C / sec or more (first cooling); heating the first-cooled steel sheet to a temperature range of 350-550 °C at an average heating rate of 5 °C / sec or more (third heating), and then maintaining for 50 seconds or more (second maintaining); and cooling the second-maintained steel sheet to room temperature at an average cooling rate of 1 °C / sec or more (second cooling).

[0039] The steel billet can further include any one of the following (1) to (8).

[0040] (1) one or more of Ti: 0-0.5 %, Nb: 0-0.5 %, and V: 0-0.5 %,

[0041] (2) one or more of Cr: 0-3.0 % and Mo: 0-3.0 %,

[0042] (3) one or more of Cu: 0-4.0 % and Ni: 0-4.0 %,

[0043] (4) one or more of Ca: 0-0.05 %, REM excluding Y: 0-0.05 %, and Mg: 0-0.05 %,

[0044] (5) one or more of W: 0-0.5 % and Zr: 0-0.5 %,

[0045] (6) one or more of Sb: 0-0.5 % and Sn: 0-0.5 %,

[0046] (7) one or more of Y: 0-0.2 % and Hf: 0-0.2 %,

[0047] (8) Co: 0-1.5 %.

[0048] The cold-rolled steel sheet can be provided by heating a steel billet to 1000-1350°C, performing hot finish rolling at a temperature range of 800-1000°C, coiling the hot-rolled steel sheet at a temperature range of 350-650°C, pickling the coiled steel sheet, and cold-rolling the pickled steel sheet at a reduction of 30-90%.

[0049] Advantages

[0050] According to a preferred aspect of the present application, a steel sheet which can be suitably used for automobile parts and the like, and a manufacturing method thereof, can be provided, which has an excellent balance of tensile strength and ductility, an excellent balance of tensile strength and hole expandability, and an excellent index of evaluation of yield ratio.

[0051] Best Mode for Carrying Out the Invention

[0052] The present application relates to a high-strength steel sheet having excellent workability and a manufacturing method thereof, and a preferred embodiment of the present application will be described below. The embodiment 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 embodiment described below. The present embodiment is provided to more specifically describe the present application to those skilled in the art.

[0053] The inventors of the present application have recognized that in a boron (B)-added transformation-induced plasticity (TRIP) steel including bainite, tempered martensite, fresh martensite, and residual austenite, when the structure fraction of the tempered martensite, fresh martensite, and residual austenite is controlled within a certain range, and the boron (B) content included in the tempered martensite and fresh martensite is controlled within a certain range while the shape and size of the residual austenite is controlled within a certain range, an excellent balance of tensile strength and ductility, an excellent balance of tensile strength and hole expandability, and an excellent index of evaluation of yield ratio can be simultaneously ensured. Recognizing this, a method which can effectively balance excellent strength, yield ratio, ductility, and hole expandability has been designed, thereby completing the present application.

[0054] Hereinafter, a high-strength steel sheet having excellent workability according to an aspect of the present application will be described in detail.

[0055] In the high-strength steel sheet having excellent workability according to an aspect of the present application, 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%, the balance of Fe and inevitable impurities can be included, and a fine structure can include bainite, tempered martensite, fresh martensite, residual austenite, and other inevitable structures, the steel sheet can satisfy the following [relationship 1].

[0056] [Relationship 1]

[0057] 0.03 ≤ [B] FM / [B] TM ≤ 0.55

[0058] In the Relationship 1, [B] FM is the content of boron (B) contained in the newly born martensite (wt. %), and [B] TM is the content of boron (B) contained in the tempered martensite (wt. %).

[0059] 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.

[0060] In the high-strength steel sheet excellent in workability according to one aspect of the present application, 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%, the balance of Fe and inevitable impurities are contained in terms of weight%. In addition, one or more of Ti: 0.5% or less (including 0%), Nb: 0.5% or less (including 0%), V: 0.5% or less (including 0%), Cr: 3.0% or less (including 0%), Mo: 3.0% or less (including 0%), Cu: 4.0% or less (including 0%), Ni: 4.0% or less (including 0%), Ca: 0.05% or less (including 0%), REM 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%) can be further contained.

[0061] Carbon (C): 0.1-0.25%

[0062] Carbon (C) is an element essential to ensure the strength of the steel sheet, and is an element to stabilize retained austenite which contributes to improvement in ductility of the steel sheet. Therefore, in order to achieve the effects as described above, 0.1% or more of carbon (C) can be contained in the present application. The preferable carbon (C) content can exceed 0.1%, and can be 0.11% or more, 0.12% or more. On the other hand, when the carbon (C) content exceeds a certain level, the ductility is reduced due to excessive increase in strength, and the weldability can be deteriorated. Therefore, the upper limit of the carbon (C) content can be limited to 0.25% in the present application. The carbon (C) content can be 0.24% or less, and the more preferable carbon (C) content can be 0.23% or less.

[0063] Silicon (Si): 0.01 to 1.5% or less

[0064] Silicon (Si) is an element which contributes to improvement in strength through solid solution strengthening, and is also an element which improves workability by homogenizing the structure. In addition, silicon (Si) is an element which contributes to formation of retained austenite by suppressing precipitation of cementite. Therefore, in order to achieve the effects as described above, 0.01% or more of silicon (Si) can 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 0.04% or more. However, when the silicon (Si) content exceeds a certain level, plating defects problems such as non-plating phenomenon are induced in the plating process, and the weldability of the steel sheet can be deteriorated, and therefore the upper limit of the silicon (Si) content can be limited to 1.5% in the present application. The upper limit of the preferable silicon (Si) content can be 1.48%, and the upper limit of the more preferable silicon (Si) content can be 1.46%.

[0065] Manganese (Mn): 1.0 to 4.0%

[0066] Manganese (Mn) is a useful element which improves both strength and ductility. Therefore, in order to achieve the effects as described above, 1.0% or more of manganese (Mn) can be added in the present application. The lower limit of the preferable manganese (Mn) content can be 1.2%, and the lower limit of the more preferable manganese (Mn) content can be 1.4%. On the other hand, when too much manganese (Mn) is added, the bainite transformation time is increased, and the enrichment degree of carbon (C) in austenite is insufficient, and therefore there is a problem that the desired austenite fraction cannot be ensured. Therefore, the upper limit of the manganese (Mn) content can be limited to 4.0% in the present application. The upper limit of the preferable manganese (Mn) content can be 3.9%.

[0067] Aluminum (Al): 0.01 to 1.5%

[0068] Aluminum (Al) is an element that plays a deoxidizing role by combining with oxygen in the steel. Also, like silicon (Si), aluminum (Al) is an element that stabilizes residual austenite by suppressing the precipitation of cementite. Therefore, in order to achieve the effects as described above, 0.01% or more of aluminum (Al) can be added in the present application. The preferable aluminum (Al) content can be 0.03% or more, and the more preferable aluminum (Al) content can be 0.05% 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, so the upper limit of the aluminum (Al) content can be limited to 1.5% in the present application. The preferable upper limit of the aluminum (Al) content can be 1.48%.

[0069] Phosphorus (P): 0.15% or less (including 0%)

[0070] Phosphorus (P) is an element that is contained as an impurity and deteriorates the impact toughness. Therefore, the content of phosphorus (P) is preferably controlled to be 0.15% or less.

[0071] Sulfur (S): 0.03% or less (including 0%)

[0072] Sulfur (S) is an element that is contained as an impurity and forms MnS in the steel sheet and deteriorates the ductility. Therefore, the content of sulfur (S) is preferably 0.03% or less.

[0073] Nitrogen (N): 0.03% or less (including 0%)

[0074] Nitrogen (N) is an element that is contained as an impurity and causes cracks of the slab by forming nitrides in continuous casting. Therefore, the content of nitrogen (N) is preferably 0.03% or less.

[0075] Boron (B): 0.0005-0.005%

[0076] Boron (B) is an element that improves the strength by improving the hardenability, and is also an element that suppresses the nucleation of grain boundaries. Also, the purpose of the present application is to simultaneously secure an excellent balance of tensile strength and elongation, an excellent balance of tensile strength and hole expansibility, and an excellent yield ratio evaluation index by the enrichment of boron (B) in tempered martensite, so boron (B) must be added in the present application. Therefore, in order to achieve the effects as described above, 0.0005% or more of boron (B) can be added in the present application. However, when the added boron (B) exceeds a certain level, not only the characteristic effects are excessive, but also the manufacturing cost increases, so the upper limit of the content of boron (B) can be limited to 0.005% in the present application.

[0077] 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.

[0078] One or more of titanium (Ti): 0 - 0.5%, niobium (Nb): 0 - 0.5%, and vanadium (V): 0 - 0.5%

[0079] Titanium (Ti), niobium (Nb), and vanadium (V) are elements that refine grains by forming precipitates, and are also elements that contribute to an increase in strength and impact toughness of the steel sheet, and thus, one or more of titanium (Ti), niobium (Nb), and vanadium (V) can be added in the present application for the above-mentioned effects. However, when the content of each of titanium (Ti), niobium (Nb), and vanadium (V) exceeds a certain level, too many precipitates are formed, thereby reducing impact toughness, and can also be a cause of an increase in manufacturing cost, and thus, the content of each of titanium (Ti), niobium (Nb), and vanadium (V) can be limited to 0.5% or less in the present application.

[0080] One or more of chromium (Cr): 0 - 3.0% and molybdenum (Mo): 0 - 3.0%

[0081] Chromium (Cr) and molybdenum (Mo) inhibit austenite decomposition at the time of alloying treatment, and, like manganese (Mn), chromium (Cr) and molybdenum (Mo) are elements that stabilize austenite, and thus, one or more of chromium (Cr) and molybdenum (Mo) can be added in the present application for the above-mentioned effects. However, when the content of each of chromium (Cr) and molybdenum (Mo) exceeds a certain level, the bainite transformation time increases, and the amount of carbon (C) enrichment in austenite is insufficient, and thus, the desired residual austenite fraction cannot be ensured. Thus, the content of each of chromium (Cr) and molybdenum (Mo) can be limited to 3.0% or less in the present application.

[0082] One or more of copper (Cu): 0 - 4.0% and nickel (Ni): 0 - 4.0%

[0083] Copper (Cu) and nickel (Ni) are elements that stabilize austenite and inhibit corrosion. In addition, copper (Cu) and nickel (Ni) are elements that enrich on the surface of the steel sheet and prevent the invasion of hydrogen that migrates into the steel sheet, thereby inhibiting hydrogen-induced delayed fracture. Thus, one or more of copper (Cu) and nickel (Ni) can be added in the present application for the above-mentioned effects. However, when the content of each of copper (Cu) and nickel (Ni) exceeds a certain level, excessive characteristic effects are caused, and can also be a cause of an increase in manufacturing cost, and thus, the content of each of copper (Cu) and nickel (Ni) can be limited to 4.0% or less in the present application.

[0084] One or more of calcium (Ca): 0 - 0.05%, magnesium (Mg): 0 - 0.05%, and rare earth elements (REM) other than yttrium (Y): 0 - 0.05%

[0085] Among them, the rare earth element (REM) means scandium (Sc), yttrium (Y) and lanthanide series elements. The rare earth element (REM) except calcium (Ca), magnesium (Mg), yttrium (Y) is an element which contributes to improving the ductility of the steel sheet by spheroidizing sulfides, and thus, in order to the effect as described above, one or more of the rare earth element (REM) except calcium (Ca), magnesium (Mg), yttrium (Y) can be added in the present invention. However, when the content of the rare earth element (REM) except calcium (Ca), magnesium (Mg), yttrium (Y) exceeds a certain level, it results in excessive characteristic effects, and also becomes a cause of increasing manufacturing costs, and thus, in the present invention, the content of the rare earth element (REM) except calcium (Ca), magnesium (Mg), yttrium (Y) can be limited to 0.05% or less, respectively.

[0086] One or more of tungsten (W): 0-0.5% and zirconium (Zr): 0-0.5%

[0087] Tungsten (W) and zirconium (Zr) are elements which increase the strength of the steel sheet by improving the hardenability, and thus, in order to the effect as described above, one or more of tungsten (W) and zirconium (Zr) can be added in the present invention. However, when the content of tungsten (W) and zirconium (Zr) exceeds a certain level, it results in excessive characteristic effects, and also becomes a cause of increasing manufacturing costs, and thus, in the present invention, the content of tungsten (W) and zirconium (Zr) can be limited to 0.5% or less, respectively.

[0088] One or more of antimony (Sb): 0-0.5% and tin (Sn): 0-0.5%

[0089] Antimony (Sb) and tin (Sn) are elements which improve the plating wettability and plating adhesion of the steel sheet, and thus, in order to the effect as described above, one or more of antimony (Sb) and tin (Sn) can be added in the present invention. However, when the content of antimony (Sb) and tin (Sn) exceeds a certain level, the brittleness of the steel sheet increases, and cracks can be generated at the time of hot working or cold working, and thus, in the present invention, the content of antimony (Sb) and tin (Sn) can be limited to 0.5% or less, respectively.

[0090] One or more of yttrium (Y): 0-0.2% and hafnium (Hf): 0-0.2%

[0091] Yttrium (Y) and hafnium (Hf) are elements which improve the corrosion resistance of the steel sheet, and thus, in order to the effect as described above, one or more of yttrium (Y) and hafnium (Hf) can be added in the present invention. However, when the content of yttrium (Y) and hafnium (Hf) exceeds a certain level, the ductility of the steel sheet can be deteriorated, and thus, in the present invention, the content of yttrium (Y) and hafnium (Hf) can be limited to 0.2% or less, respectively.

[0092] Cobalt (Co): 0-1.5%

[0093] Cobalt (Co) is an element that increases the TRIP effect by promoting the bainite transformation, and thus, for the above-mentioned effects, cobalt (Co) can be added in the present application. However, when the content of cobalt (Co) exceeds a certain level, the weldability and ductility of the steel sheet can be deteriorated, and thus, the content of cobalt (Co) can be limited to 1.5% or less in the present application.

[0094] The high-strength steel sheet having excellent workability according to one aspect of the present application can contain, in addition to the above-mentioned components, the balance of Fe and other inevitable impurities. However, in the usual manufacturing process, impurities that are not required can be inevitably mixed from the raw material or the surrounding environment, and thus, these impurities cannot be completely excluded. These impurities are well known to those skilled in the art, and thus, all of them are not particularly mentioned in the present specification. In addition, the further addition of effective components other than the above-mentioned components is not completely excluded.

[0095] In the high-strength steel sheet having excellent workability according to one aspect of the present application, the microstructure can include bainite, tempered martensite, fresh martensite, retained austenite, and other inevitable structures.

[0096] Both untempered martensite (fresh martensite, FM) and tempered martensite (tempered martensite, TM) are microstructures that increase the strength of the steel sheet. However, compared to the tempered martensite, the fresh martensite has a characteristic of reducing the ductility and flange workability of the steel sheet. In addition, compared to the tempered martensite, the fresh martensite has a tendency to reduce the yield ratio of the steel sheet. This is because the microstructure of the tempered martensite is softened due to the tempering heat treatment. Thus, in order to secure the balance of the tensile strength and elongation (TS 2 *EL 1 / 2 ), the balance of the tensile strength and the hole expansion ratio (TS 2 *HER 1 / 2 ), and the yield ratio evaluation index (1-YR) as mentioned above, it is preferable to control the structure fraction of the tempered martensite and the fresh martensite. In order to satisfy 3.0*10 6 the balance of the tensile strength and elongation (TS 2 *EL 1 / 2 ), 6.0*10 6 the balance of the tensile strength and the hole expansion ratio (TS 2 *HER 1 / 2The yield strength ratio (1-YR) is below 0.42, and the fraction of tempered martensite is preferably limited to 50% by volume or more, and the fraction of newly formed martensite is preferably limited to 10% by volume or more. More preferably, the fraction of tempered martensite can be 52% by volume or more or 54% by volume or more, and more preferably, the fraction of newly formed martensite can be 12% by volume or more. On the other hand, when excessive tempered martensite or newly formed martensite is formed, ductility and flange workability decrease, ultimately failing to simultaneously satisfy 3.0*10 6 The above balance between tensile strength and elongation (TS) 2 *EL 1 / 2 ), 6.0*10 6 The above balance between tensile strength and porosity (TS) 2 *HER 1 / 2 The yield strength ratio (1-YR) is less than 0.42. Therefore, in this invention, the fraction of tempered martensite can be limited to 70% by volume or less, and the fraction of newly formed martensite can be limited to 30% by volume or less. More preferably, the fraction of tempered martensite can be 68% by volume or less or 65% by volume or less, and more preferably, the fraction of newly formed martensite can be 25% by volume or less.

[0097] To ensure the desired balance of tensile strength and elongation (TS) as required by this invention 2 *EL 1 / 2 Balance between tensile strength and porosity (TS) 2 *HER 1 / 2 The bainite fraction needs to be optimized to determine the yield strength ratio (1-YR) and the bainite ratio evaluation index. To ensure 3.0*10... 6 The above balance between tensile strength and elongation (TS) 2 *EL 1 / 2 ), 6.0*10 6 The above balance between tensile strength and porosity (TS) 2 *HER 1 / 2 For a yield strength ratio (1-YR) of 0.42 or less, the bainite fraction is preferably controlled to be 10% by volume or more. More preferably, the bainite fraction can be 12% by volume or more, or 14% by volume or more. On the other hand, when too much bainite is formed, it will eventually lead to a reduction in the tempered martensite fraction. Therefore, in order to ensure the desired balance between tensile strength and elongation (TS), 2 *EL 1 / 2 Balance between tensile strength and porosity (TS) 2 *HER 1 / 2The yield strength ratio (1-YR) and the yield strength ratio evaluation index can limit the bainite fraction to 30% or less by volume. Preferred bainite fractions can be 12% or more by volume, or 14% or more by volume, or 28% or less by volume, or 26% or less by volume.

[0098] Steel plates containing retained austenite exhibit excellent ductility and machinability due to the transformation-induced plasticity during the austenite-martensite transition in processing. When the fraction of retained austenite is below a certain level, the balance between tensile strength and elongation (TS) is affected. 2 *EL 1 / 2 Less than 3.0*10 6 (MPa 2 % 1 / 2 Therefore, it is not preferred. Furthermore, when the residual austenite fraction exceeds a certain level, local elongation may decrease, or spot weldability may decrease. Therefore, in order to achieve a balance between tensile strength and elongation (TS...),... 2 *EL 1 / 2 In this invention, the retained austenite fraction can be limited to a range of 2-10% for the superior steel sheet. Preferably, the retained austenite fraction can be 3% or more by volume or less by volume.

[0099] In the steel sheet of the present invention, the microstructure may unavoidably include ferrite, pearlite, island martensite (martensite austenite constituent, MA), etc. When excessive ferrite forms, the strength of the steel sheet may decrease; therefore, in the present invention, the ferrite fraction is limited to 5% by volume (including 0%) or less. Furthermore, when excessive pearlite forms, the workability of the steel sheet decreases, or the fraction of retained austenite may decrease; therefore, the present invention aims to limit the formation of pearlite as much as possible.

[0100] According to one aspect of the present invention, a high-strength steel plate with excellent processability can satisfy the following [relationship 1].

[0101] [Relation 1]

[0102] 0.03≤[B] FM / [B] TM ≤0.55

[0103] In relation 1, [B] FM This refers to the boron (B) content (by weight %) in the newly formed martensite. TM It is the boron (B) content (by weight) contained in tempered martensite.

[0104] To ensure the desired balance between tensile strength and elongation (TS)2 *EL 1 / 2 ), balance of tensile strength and elongation (TS 2 *HER 1 / 2 ), and yield ratio evaluation index (1-YR), in the present invention, the fraction of tempered martensite, newly formed martensite and residual austenite can be controlled within a certain range, and the ratio of boron (B) content contained in the tempered martensite and the newly formed martensite can be controlled within a certain range, and the ratio of residual austenite of a certain size, shape and kind with respect to the entire residual austenite can be controlled within a certain range.

[0105] In the present invention, as shown in [Equation 1], the ratio of the content of boron (B) contained in the newly formed martensite ([B] FM , wt%) to the content of boron (B) contained in the tempered martensite ([B] TM , wt%) is controlled within the range of 0.03 to 0.55, so that the balance of tensile strength and elongation (TS 6 *EL 6 (MPa 2 % of elongation), the balance of tensile strength and hole expansion ratio (TS 1 / 2 *HER TE ), and the yield ratio evaluation index (1-YR) can be ensured at the same time. 6 to 11.5*10 6 (MPa 2 % of elongation), the balance of tensile strength and hole expansion ratio (TS 1 / 2 *HER TH ), and the yield ratio evaluation index (1-YR) can be ensured at the same time. YR

[0106] As a result of the inventors of the present invention conducting in-depth research on a method for ensuring the physical properties of a boron (B) added TRIP steel, although the theoretical basis has not been clearly elucidated, it was noted that only when the ratio of the boron (B) content contained in the newly formed martensite to the boron (B) content contained in the tempered martensite satisfies a certain range, the desired physical properties of the present invention can be ensured. In particular, it was confirmed that depending on the ratio of the boron (B) content contained in the tempered martensite and the newly formed martensite, the yield ratio of the steel sheet showed a certain tendency. Therefore, in the present invention, as shown in [Equation 1], the ratio of the boron (B) content contained in the newly formed martensite to the boron (B) content contained in the tempered martensite is limited to the range of 0.03 to 0.55, so that the desired balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), the balance of tensile strength and hole expansion ratio (TS 2 *HER 1 / 2 ), and the yield ratio evaluation index (1-YR) can be ensured.

[0107] ​In the high-strength steel sheet excellent in workability according to one aspect of the present application, the balance (B TE ) of the tensile strength and the elongation represented by the following [Equation 2] can satisfy 3.0*10 6 to 6.2*10 6 (MPa 2 % 1 / 2 ), and the balance (B TH ) of the tensile strength and the hole expansion ratio represented by the following [Equation 3] can satisfy 6.0*10 6 to 11.5*10 6 (MPa 2 % 1 / 2 ), and the yield ratio evaluation index (I YR ) represented by the following [Equation 4] can satisfy 0.15 to 0.42.

[0108] [Equation 2]

[0109] B TE = [Tensile strength (TS, MPa)] 2 * [Elongation (El, %)] 1 / 2

[0110] [Equation 3]

[0111] B TH = [Tensile strength (TS, MPa)] 2 * [Hole expansion ratio (HER, %)] 1 / 2

[0112] [Equation 4]

[0113] I YR = 1 - [Yield ratio (YR)]

[0114] Hereinafter, one example of a method of manufacturing the steel sheet of the present application will be described in detail.

[0115] The method of manufacturing the high-strength steel sheet according to one aspect of the present application can include the steps of heating a cold-rolled steel sheet having a predetermined alloy composition to 700°C at an average heating rate of 5°C / sec or more (first heating), heating to a temperature range of Ac3 to 920°C at an average heating rate of 5°C / sec or less (second heating), and then maintaining for 50 to 1200 seconds (first maintaining); cooling the first-maintained steel sheet to a temperature range of 200 to 400°C at an average cooling rate of 1°C / sec or more (first cooling); heating the first-cooled steel sheet to a temperature range of 350 to 550°C at an average heating rate of 5°C / sec or more (third heating), and then maintaining for 50 seconds or more (second maintaining); and cooling the second-maintained steel sheet to normal temperature at an average cooling rate of 1°C / sec or more (second cooling).

[0116] The cold-rolled steel sheet can be provided by heating a steel slab having a predetermined alloy composition to 1000-1350°C, performing hot finish rolling at a temperature range of 800-1000°C, coiling the hot-rolled steel sheet at a temperature range of 350-650°C, pickling the coiled steel sheet, and cold-rolling the pickled steel sheet at a reduction of 30-90%.

[0117] Preparation and heating of steel slab

[0118] A steel slab having a predetermined alloy composition is prepared. The steel slab of the present application has an alloy composition corresponding to that of the above-described steel sheet, and thus the description of the alloy composition of the steel slab is replaced by that of the alloy composition of the above-described steel sheet.

[0119] The prepared steel slab can be heated to a certain temperature range, and the heating temperature of the steel slab at this time can be in the range of 1000-1350°C. When the heating temperature of the steel slab is lower than 1000°C, hot rolling can be performed in a temperature range below the desired hot finish rolling temperature range, and when the heating temperature of the steel slab exceeds 1350°C, the steel can be melted due to reaching the melting point of the steel.

[0120] Hot rolling and coiling

[0121] The heated steel slab can be hot-rolled to provide a hot-rolled steel sheet. The hot finish rolling temperature at the time of hot rolling is preferably in the range of 800-1000°C. When the hot finish rolling temperature is lower than 800°C, excessive rolling load can be a problem, and when the hot finish rolling temperature exceeds 1000°C, coarse grains of the hot-rolled steel sheet are formed, and thus can cause a decrease in the physical properties of the final steel sheet.

[0122] The hot-rolled steel sheet completed by hot rolling can be cooled at an average cooling rate of 10°C / sec or more, and can be coiled at a temperature range of 350-650°C. This is because, when the coiling temperature is lower than 350°C, coiling is not easy, and when the coiling temperature exceeds 650°C, surface scales are formed to the inside of the hot-rolled steel sheet, and thus it can be difficult to perform pickling.

[0123] Pickling and cold rolling

[0124] After the coiled hot-rolled coil is uncoiled, in order to remove scales formed on the surface of the steel sheet, pickling can be performed, and cold rolling is performed. The pickling and cold rolling conditions are not particularly limited in the present application, but it is preferable to cold-roll at a total reduction of 30-90%. When the total reduction of cold rolling exceeds 90%, it can be difficult to perform cold rolling in a short time due to the high strength of the steel sheet.

[0125] The cold-rolled steel sheet can be manufactured into an uncoated cold-rolled steel sheet by an annealing heat treatment process, or into a plated steel sheet by a plating process in order to impart corrosion resistance. The plating can be performed by a hot-dip galvanizing, an electro-galvanizing, a hot-dip aluminum plating, etc., and the method and type thereof are not particularly limited.

[0126] Annealing heat treatment

[0127] In the present invention, in order to simultaneously secure the strength and workability of the steel sheet, an annealing heat treatment process is performed.

[0128] The cold-rolled steel sheet is heated to 700°C at an average heating rate of 5°C / sec or more (first heating), heated to a temperature range of Ac3 to 920°C at an average heating rate of 5°C / sec or less (second heating), and then held for 50 to 1200 seconds (first holding).

[0129] When the average heating rate of the first heating to 700°C is less than 5°C / sec, massive austenite is formed from ferrite and cementite formed during the heating, and as a result, fine tempered martensite and residual austenite cannot be formed as a final microstructure. Therefore, the balance of tensile strength and elongation (TS 2 *EL 1 / 2 ) and the balance of tensile strength and hole expansion ratio (TS 2 *HER 1 / 2 ) desired cannot be achieved. In addition, when the second heating rate up to the first holding temperature exceeds 5°C / sec, the cementite phase formed during the heating is transformed into austenite, forming a large amount of massive austenite, the final microstructure is coarsened, and boron (B) cannot be sufficiently enriched in the tempered martensite. Therefore, [B] FM / [B] TM exceeds 0.55, and the balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), the balance of tensile strength and hole expansion ratio (TS 2 *HER 1 / 2 ), and the yield strength ratio evaluation index (I YR ) desired cannot be achieved.

[0130] When the first holding temperature is less than Ac3 (dual phase region), ferrite of 5 vol% or more is formed, and therefore the balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), the balance of tensile strength and hole expansion ratio (TS 2 *HER 1 / 2) can be reduced. In addition, when the first holding time is less than 50 seconds, the structure cannot be sufficiently homogenized, and thus the physical properties of the steel sheet can be reduced. The upper limit of the first holding temperature and the first holding time is not particularly limited, but in order to prevent a reduction in toughness due to coarsening of the grains, the first holding temperature is preferably limited to 920°C or lower, and the first holding time is preferably limited to 1200 seconds or lower.

[0131] After the first holding, the temperature can be cooled to a first cooling end temperature (first cooling) of 200 to 400°C at an average cooling rate of 1°C / sec or more. When the average cooling rate of the first cooling is less than 1°C / sec, the fraction of residual austenite becomes insufficient due to slow cooling, and thus the balance of the tensile strength and the elongation (TS 2 *EL 1 / 2 ) of the steel sheet can be reduced. The upper limit of the average cooling rate of the first cooling is not particularly specified, but is preferably 100°C / sec or less. When the first cooling end temperature is lower than 200°C, too much tempered martensite is formed, and the residual austenite is insufficient, and thus the balance of the tensile strength and the elongation (TS 2 *EL 1 / 2 ) and the balance of the tensile strength and the hole expansion ratio (TS 2 *HER 1 / 2 ) of the steel sheet can be reduced. On the other hand, when the first cooling end temperature exceeds 400°C, too much bainite is formed, and the tempered martensite is insufficient, and thus the balance of the tensile strength and the elongation (TS 2 *EL 1 / 2 ) and the balance of the tensile strength and the hole expansion ratio (TS 2 *HER 1 / 2 ) of the steel sheet can be reduced.

[0132] After the second cooling, the temperature can be heated to a temperature range of 350 to 550°C at an average heating rate of 5°C / sec or more (third heating), and then held for 50 seconds or more (second holding). The upper limit of the average heating rate of the third heating is not particularly specified, but is preferably 100°C / sec or less. When the second holding temperature is lower than 350°C or the second holding time is less than 50 seconds, too much tempered martensite is formed, and thus it is difficult to secure the fraction of the residual austenite. As a result, the balance of the tensile strength and the elongation (TS 2 *EL 1 / 2 ) and the balance of the tensile strength and the hole expansion ratio (TS 2 *HER 1 / 2 ) of the steel sheet can be reduced. When the second holding temperature exceeds 550°C or the second holding time exceeds 155000 seconds, the fraction of the residual austenite is insufficient, and thus the balance of the tensile strength and the elongation (TS 2 *EL 1 / 2 ) of the steel sheet can be reduced.

[0133] After the secondary holding, the temperature can be cooled to room temperature at an average cooling rate of 1°C / sec or more (secondary cooling).

[0134] In the high-strength steel sheet having excellent workability manufactured by the manufacturing method described above, the microstructure can include bainite, tempered martensite, fresh martensite, residual austenite, and other unavoidable structures, and as one preferred example, can include 10-30% of bainite, 50-70% of tempered martensite, 10-30% of fresh martensite, 2-10% of residual austenite, and 5% or less (including 0%) of ferrite, in terms of volume fraction.

[0135] In the steel sheet manufactured by the manufacturing method described above, the balance (B TE ) between the tensile strength and the elongation represented by the following [Equation 2] can satisfy 3.0*10 6 to 6.2*10 6 (MPa 2 %) 1 / 2 , the balance (B TH ) between the tensile strength and the hole expansion ratio represented by the following [Equation 3] can satisfy 6.0*10 6 to 11.5*10 6 (MPa 2 %) 1 / 2 , and the yield strength ratio evaluation index (I YR ) represented by the following [Equation 4] can satisfy 0.15 to 0.42.

[0136] [Equation 2]

[0137] B TE = [Tensile strength (TS, MPa)] 2 * [Elongation (El, %)] 1 / 2

[0138] [Equation 3]

[0139] B TH = [Tensile strength (TS, MPa)] 2 * [Hole expansion ratio (HER, %)] 1 / 2

[0140] [Equation 4]

[0141] I YR = 1 - [Yield strength ratio (YR)] DETAILED DESCRIPTION

[0142] 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.

[0143] (Example)

[0144] 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 to 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. Afterward, it is pickled to remove surface oxide scale and then cold-rolled to a thickness of 1.5 mm.

[0145] The steel sheet is then 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 represents the temperature range from Ac3 to 920°C, and the two-phase region represents the temperature range below Ac3°C.

[0146] 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 using a nitric acid-alcohol solution, and the microstructure of ferrite (F), bainite (B), tempered martensite (TM), nascent martensite (FM), and pearlite (P) was observed by SEM. After etching with the nitric acid-alcohol solution, the microstructure without unevenness on the specimen surface was classified as ferrite, and the microstructure with a layered structure of cementite and ferrite was classified as pearlite. Bainite (B) and tempered martensite (TM) were both observed to have lath and blocky morphologies, making them difficult to distinguish. Therefore, the fractions of bainite and tempered martensite were calculated using expansion curves after expansion evaluation. That is, the bainite fraction was determined by subtracting the fraction of tempered martensite calculated by the expansion curves from the fraction of bainite and tempered martensite measured by SEM observation. Furthermore, it is difficult to distinguish between newly formed martensite (FM) and retained austenite (retained γ). Therefore, the fraction of newly formed martensite is determined by subtracting the fraction of retained austenite calculated by X-ray diffraction from the fraction of martensite and retained austenite observed by the SEM.

[0147] In addition, [B] of the steel plate FM / [B] TM Balance between tensile strength and elongation (TS) 2 *EL 1 / 2 Balance between tensile strength and porosity (TS) 2 *HER1 / 2 ) and the ratio of yield strength to tensile strength (Y / T) YR ) were measured and evaluated, and the results thereof are shown in Tables 8 and 9.

[0148] The boron (B) content ([B FM ) and the boron (B) content ([B TM ) was determined as the concentration of boron (B) measured in the as-formed martensite and the tempered martensite using an Electron Probe MicroAnalyser (EPMA).

[0149] The tensile strength (TS) and the elongation (El) were evaluated by a tensile test, and the tensile strength (TS) and the elongation (El) were measured by taking a test piece according to JIS No. 5 standard and evaluating it with a direction of 90° with respect to the rolling direction of the rolled plate as a reference. The hole expansion ratio (HER) was evaluated by a hole expansion test, and after forming a punched hole of 10 mm in diameter (10.3 mm in inner diameter of a die, and 12.5% in clearance), a conical punch having a top angle of 60° was inserted into the punched hole in a direction in which a burr of the punched hole became an outside, and the peripheral portion of the punched hole was extruded and expanded at a moving speed of 20 mm / minute, and then the hole expansion ratio (HER) was calculated using the following [Equation 5].

[0150] [Equation 5]

[0151] Hole expansion ratio (HER, %) = {(D - D0) / D0} x 100

[0152] In the Equation 5, D represents a hole diameter (mm) when a crack penetrates a steel sheet in a thickness direction, and D0represents an initial hole diameter (mm).

[0153] [Table 1]

[0154]

[0155] [Table 2]

[0156]

[0157] [Table 3]

[0158]

[0159] [Table 4]

[0160]

[0161] [Table 5]

[0162]

[0163] [Table 6]

[0164]

[0165] [Table 7]

[0166]

[0167] [Table 8]

[0168]

[0169] [Table 9]

[0170]

[0171] As shown in Tables 1 to 9, in the case of the test pieces satisfying the conditions set forth in the present application, the balance (B TE ) between the tensile strength and the elongation satisfies 3.0*10 6 to 6.2*10 6 (MPa 2 %) 1 / 2 , the balance (B TH ) between the tensile strength and the hole expansion ratio satisfies 6.0*10 6 to 11.5*10 6 (MPa 2 %) 1 / 2 , and the index (I YR ) of the yield strength ratio satisfies 0.15 to 0.42.

[0172] In the test piece 2, the average heating rate in the first stage is less than 5°C / sec, and therefore the tempered martensite and the residual austenite are insufficient. As a result, the balance (B TE ) between the tensile strength and the elongation of the test piece 2 is less than 3.0*10 6 , and the balance (B TH ) between the tensile strength and the hole expansion ratio is less than 6.0*10 6 .

[0173] In the test piece 3, the average heating rate in the second stage exceeds 5°C / sec, and therefore the blocky austenite is formed, and boron (B) is not enriched in the tempered martensite. As a result, the ratio [B] FM / [B] TM of the test piece 3 exceeds 0.55, the index (I YR ) of the yield strength ratio exceeds 0.42, the balance (B TE ) between the tensile strength and the elongation is less than 3.0*10 6 , and the balance (B TH ) between the tensile strength and the hole expansion ratio is less than 6.0*10 6 .

[0174] In the test piece 4, the fraction of ferrite exceeds because the temperature in the dual phase region is kept below Ac3 once. As a result, the balance (B TE ) of the tensile strength and the elongation of the test piece 4 is less than 3.0*10 6 , and the balance (B TH ) of the tensile strength and the hole expansion ratio is less than 6.0*10 6 .

[0175] In the test piece 5, the fraction of residual austenite is insufficient because the average cooling rate is less than 1°C / sec once. As a result, the balance (B TE ) of the tensile strength and the elongation of the test piece 5 is less than 3.0*10 6 .

[0176] In the test piece 6, the fraction of tempered martensite exceeds and the fraction of residual austenite is insufficient because the cooling termination temperature is below 200°C once. As a result, the balance (B TE ) of the tensile strength and the elongation of the test piece 6 is less than 3.0*10 6 , and the balance (B TH ) of the tensile strength and the hole expansion ratio is less than 6.0*10 6 .

[0177] In the test piece 7, the fraction of bainite exceeds and the fraction of tempered martensite is insufficient because the cooling termination temperature exceeds 400°C once. As a result, the balance (B TE ) of the tensile strength and the elongation of the test piece 7 is less than 3.0*10 6 , and the balance (B TH ) of the tensile strength and the hole expansion ratio is less than 6.0*10 6 .

[0178] In the test piece 8, the fraction of tempered martensite exceeds and the fraction of residual austenite is insufficient because the secondary holding temperature is below 350°C. As a result, the balance (B TE ) of the tensile strength and the elongation of the test piece 8 is less than 3.0*10 6 , and the balance (B TH ) of the tensile strength and the hole expansion ratio is less than 6.0*10 6 .

[0179] In the test piece 9, the fraction of residual austenite is insufficient because the secondary holding temperature exceeds 550°C. As a result, the balance (B TE ) of the tensile strength and the elongation of the test piece 9 is less than 3.0*10 6。

[0180] In the test piece 10, the fraction of tempered martensite exceeds and the fraction of residual austenite is insufficient because the secondary holding time is less than 50 seconds. As a result, the balance (BTE ) less than 3.0*10 6 , the balance (B TH ) less than 6.0*10 6。

[0181] In the test piece 11, the secondary holding time exceeds 155,000 seconds, and thus the fraction of residual austenite is insufficient. As a result, the balance (B TE ) between the tensile strength and the elongation of the test piece 11 is less than 3.0*10 6 .

[0182] In the test piece 33, the carbon (C) content is low, and thus the balance (B TE ) between the tensile strength and the elongation is less than 3.0*10 6 , and the balance (B TH ) between the tensile strength and the hole expansion ratio is less than 6.0*10 6 .

[0183] In the test piece 34, the carbon (C) content is high, and thus the fraction of tempered martensite is insufficient, and the fraction of fresh martensite exceeds, and the fraction of residual austenite exceeds. As a result, the balance (B TE ) between the tensile strength and the elongation of the test piece 34 is less than 3.0*10 6 , and the balance (B TH ) between the tensile strength and the hole expansion ratio is less than 6.0*10 6 .

[0184] In the test piece 35, the silicon (Si) content is low, and thus the fraction of residual austenite is insufficient. As a result, the balance (B TE ) between the tensile strength and the elongation of the test piece 35 is less than 3.0*10 6 .

[0185] In the test piece 36, the silicon (Si) content is high, and thus the fraction of fresh martensite exceeds. As a result, the balance (B TE ) between the tensile strength and the elongation of the test piece 36 is less than 3.0*10 6 , and the balance (B TH ) between the tensile strength and the hole expansion ratio is less than 6.0*10 6 .

[0186] In the test piece 37, the aluminum (Al) content is high, and thus the fraction of fresh martensite exceeds. As a result, the balance (B TE ) between the tensile strength and the elongation of the test piece 37 is less than 3.0*10 6 , and the balance (B TH ) between the tensile strength and the hole expansion ratio is less than 6.0*10 6 .

[0187] In the test piece 38, the manganese (Mn) content is low, and thus pearlite is formed, and the fraction of residual austenite is insufficient. As a result, the balance (B TE ) of the tensile strength and the elongation of the test piece 38 is less than 3.0*10 6 .

[0188] In the test piece 39, the manganese (Mn) content is high, and thus the fraction of newly formed martensite exceeds. As a result, the balance (B TE ) of the tensile strength and the elongation of the test piece 39 is less than 3.0*10 6 , and the balance (B TH ) of the tensile strength and the hole expansion ratio is less than 6.0*10 6 .

[0189] In the test piece 40, the chromium (Cr) content is high, and thus the fraction of newly formed martensite exceeds. As a result, the balance (B TE ) of the tensile strength and the elongation of the test piece 40 is less than 3.0*10 6 , and the balance (B TH ) of the tensile strength and the hole expansion ratio is less than 6.0*10 6 .

[0190] In the test piece 41, the molybdenum (Mo) content is high, and thus the fraction of newly formed martensite exceeds. As a result, the balance (B TE ) of the tensile strength and the elongation of the test piece 41 is less than 3.0*10 6 , and the balance (B TH ) of the tensile strength and the hole expansion ratio is less than 6.0*10 6 .

[0191] In the test piece 42, the boron (B) content is low, and thus boron (B) cannot be enriched in tempered martensite. As a result, the ratio of [B] FM / [B] TM of the test piece 42 exceeds 0.55, and the index (I YR ) of the yield strength ratio evaluation exceeds 0.42.

[0192] In the test piece 43, the boron (B) content is high, and thus boron (B) is excessively enriched in tempered martensite. As a result, the ratio of [B] FM / [B] TM of the test piece 43 is less than 0.03, and the index (I YR ) of the yield strength ratio evaluation is less than 0.15.

[0193] The above has described the present application in detail through examples, but other forms of examples can also be included. Therefore, the technical idea and scope of the claims are not limited to the examples.

Claims

1. A high-strength steel plate with excellent workability, comprising, by weight percent: C: 0.1-0.25%, Si: 0.01-1.5%, Mn: 1.0-4.0%, Al: 0.01-1.5%, P: less than 0.15%, S: less than 0.03%, N: less than 0.03%, B: 0.0005-0.005%, with the balance being Fe and unavoidable impurities. By volume fraction, the fine microstructure of the steel plate comprises: 10-30% bainite, 50-70% tempered martensite, 10-30% newly formed martensite, 2-10% retained austenite, less than 5% and including 0% ferrite, and other unavoidable microstructures, wherein the steel plate satisfies the following [Equation 1]. [Relation 1] 0.03≤[B] FM / [B] TM ≤0.55 In relation 1, [B] FM It refers to the boron (B) content contained in the newly formed martensite, among which, The unit for content is weight %; [B] TM This refers to the boron (B) content in the tempered martensite, where the content is expressed in weight.

2. The high-strength steel plate with excellent workability according to claim 1, wherein, The steel plate further comprises, by weight percent, any one or more of the following (1) to (8): (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: 0-4.0% and Ni: 0-4.0% (4) One or more of the following: Ca: 0-0.05%, REM (excluding Y): 0-0.05%, and Mg: 0-0.05%. (5) One or more of W: 0-0.5% and Zr: 0-0.5% (6) One or more of Sb: 0-0.5% and Sn: 0-0.5% (7) One or more of Y: 0-0.2% and Hf: 0-0.2% (8) Co: 0-1.5%.

3. The high-strength steel plate with excellent workability according to claim 1, wherein, In the steel plate, the balance B of tensile strength and elongation is expressed by the following [Equation 2]. TE Satisfies 3.0*10 6 Up to 6.2*10 6 MPa 2 % 1 / 2 The balance between tensile strength and porosity, expressed by the following [Equation 3], is B. TH Satisfies 6.0*10 6 Up to 11.5*10 6 MPa 2 % 1 / 2 The yield strength ratio evaluation index I is expressed by the following [relationship 4]. YR Satisfying 0.15 to 0.42, [Relationship 2] B TE =[Tensile Strength (TS, MPa)] 2 *[Elongation (E1, %)] 1 / 2 [Relationship 3] B TH =[Tensile Strength (TS, MPa)] 2 *[Pore Expansion Ratio (HER, %)] 1 / 2 [Relationship 4] I YR = 1 - [YR (yield ratio)].

4. A method for manufacturing a high-strength steel plate with excellent processability, comprising the following steps: Cold-rolled steel sheets are provided, comprising, by weight percent: C: 0.1-0.25%, Si: 0.01-1.5%, Mn: 1.0-4.0%, Al: 0.01-1.5%, P: less than 0.15%, S: less than 0.03%, N: less than 0.03%, B: 0.0005-0.005%, with the balance being Fe and unavoidable impurities; The cold-rolled steel sheet is heated once at an average heating rate of 5°C / second or higher to 700°C, and then heated a second time at an average heating rate of less than 5°C / second to a temperature range of Ac3 to 920°C, and then held for 50-1200 seconds at a time. The steel plate held in one step is cooled once at an average cooling rate of more than 1°C / second to a temperature range of 200-400°C. The steel plate, after being cooled once, is heated three times at an average heating rate of 5°C / second or higher, to a temperature range of 350-550°C, and then held for at least 50 seconds in two separate heating cycles; and The steel plate, which has been held for a second time, is cooled to room temperature at an average cooling rate of 1°C / second or higher.

5. The method for manufacturing a high-strength steel plate with excellent processability according to claim 4, wherein, The cold-rolled steel sheet further comprises any one or more of the following (1) to (8): (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: 0-4.0% and Ni: 0-4.0% (4) One or more of the following: Ca: 0-0.05%, REM (excluding Y): 0-0.05%, and Mg: 0-0.05%. (5) One or more of W: 0-0.5% and Zr: 0-0.5% (6) One or more of Sb: 0-0.5% and Sn: 0-0.5% (7) One or more of Y: 0-0.2% and Hf: 0-0.2% (8) Co: 0-1.5%.

6. The method for manufacturing a high-strength steel plate with excellent processability according to claim 4, wherein, The cold-rolled steel sheet is provided by the following steps: Heat the steel billet to 1000-1350℃; Hot finishing rolling is performed within a temperature range of 800-1000℃; The hot-rolled steel sheet is coiled within a temperature range of 350-650℃; The coiled steel plate is then pickled; as well as The pickled steel sheet is cold-rolled at a reduction rate of 30-90%.

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