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 these demands for both stability and lightweighting, the application of high-strength steel sheets is increasing dramatically. It is generally known that as steel sheets achieve higher strength, their 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, formed by tempering hard martensite, is softened martensite, thus creating 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 The balance between tensile strength and elongation (B) TE ), 6.0*10 6 Up to 11.5*10 6 (MPa 2 % 1 / 2 The balance between tensile strength and porosity (B) TH and a yield strength ratio evaluation index (I) of 0.15 to 0.42. YR This means that it is difficult to ensure that the steel plate has excellent strength, porosity, ductility and yield strength ratio.

[0008] That is, it is currently impossible to achieve a balance between tensile strength and elongation (B). TE Balance between tensile strength and porosity (B) TH ) and yield strength ratio evaluation index (I YR The demand for high-quality steel plates is high.

[0009] (Existing technical literature)

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

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

[0012] (Patent Document 3) Japanese Patent Publication No. 2016-216808 Summary of the Invention

[0013] Technical problems to be solved

[0014] According to one aspect of the present invention, a steel plate having an excellent balance of tensile strength and elongation, an excellent balance of tensile strength and porosity, and an excellent yield strength ratio evaluation index by optimizing the composition and microstructure of the steel plate, and a method thereof for manufacturing the same, can be provided.

[0015] The technical problems of this invention are not limited to those described above. Additional technical problems of this invention are described throughout the specification, and those skilled in the art can readily understand these additional technical problems based on the description provided in this specification.

[0016] Technical solution

[0017] According to one aspect of the invention, a high-strength steel plate with excellent workability may contain, 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%, and the balance Fe and unavoidable impurities. The fine microstructure may contain bainite, tempered martensite, newly formed martensite, retained austenite, and other unavoidable microstructures. The steel plate may satisfy the following [Relationship 1] and [Relationship 2].

[0018] [Relation 1]

[0019] 0.03≤[B] FM / [B] TM ≤0.55

[0020] 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.

[0021] [Relation 2]

[0022] T(γ) / V(γ)≥0.08

[0023] In Equation 2, T(γ) is the fraction of tempered retained austenite in the steel plate (volume %), and V(γ) is the fraction of retained austenite in the steel plate (volume %).

[0024] The steel plate may further comprise any one or more of the following (1) to (8) by weight%.

[0025] (1) One or more of Ti: 0-0.5%, Nb: 0-0.5%, and V: 0-0.5%.

[0026] (2) One or more of Cr: 0-3.0% and Mo: 0-3.0%

[0027] (3) One or more of Cu: 0-4.0% and Ni: 0-4.0%

[0028] (4) One or more of the following: Ca: 0-0.05%, REM (excluding Y): 0-0.05%, and Mg: 0-0.05%.

[0029] (5) One or more of W: 0-0.5% and Zr: 0-0.5%

[0030] (6) One or more of Sb: 0-0.5% and Sn: 0-0.5%

[0031] (7) One or more of Y: 0-0.2% and Hf: 0-0.2%

[0032] (8) Co: 0-1.5%.

[0033] The microstructure of the steel plate, by volume fraction, may contain 10-30% bainite, 50-70% tempered martensite, 10-30% newly formed martensite, 2-10% retained austenite, and less than 5% (inclusive) ferrite.

[0034] In the steel plate, the balance between tensile strength and elongation (B) is expressed by the following [relationship 3]. TE It can satisfy 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 4] (B) TH It can satisfy 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 5]. YR It can satisfy values ​​from 0.15 to 0.42.

[0035] [Relationship 3]

[0036] B TE =[Tensile Strength (TS, MPa)] 2 *[Elongation (E1, %)] 1 / 2

[0037] [Relationship 4]

[0038] B TH =[Tensile Strength (TS, MPa)] 2 *[Pore Expansion Ratio (HER, %)] 1 / 2

[0039] [Relation 5]

[0040] I YR =1 - [YR (Yellow Ratio)]

[0041] A method for manufacturing a high-strength steel sheet with excellent processability according to one aspect of the present invention may include the following steps: providing a cold-rolled steel sheet, wherein, by weight percent, the steel sheet comprises: 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%, and the balance being Fe and unavoidable impurities; heating the cold-rolled steel sheet to 700°C at an average heating rate of 5°C / second or higher (first heating), and then heating it to a temperature range of Ac3 to 920°C at an average heating rate of less than 5°C / second (second heating); The steel plate is held for 50-1200 seconds (one holding); the steel plate held for one holding is cooled to a temperature range of 200-400°C at an average cooling rate of 2-100°C / second (one cooling); the steel plate cooled for one cooling is heated to a temperature range of 400-600°C at an average heating rate of 5-100°C / second (three heatings), and then held for 10-1800 seconds (two holdings); the steel plate held for two holdings is cooled to a temperature range of 300-500°C at an average cooling rate of 1-100°C / second (two coolings), and then held for 10-1800 seconds (three holdings); and the steel plate held for three holdings is cooled to room temperature at an average cooling rate of more than 1°C / second (three coolings).

[0042] The billet may further include any one of the following (1) to (8).

[0043] (1) One or more of Ti: 0-0.5%, Nb: 0-0.5%, and V: 0-0.5%.

[0044] (2) One or more of Cr: 0-3.0% and Mo: 0-3.0%

[0045] (3) One or more of Cu: 0-4.0% and Ni: 0-4.0%

[0046] (4) One or more of the following: Ca: 0-0.05%, REM (excluding Y): 0-0.05%, and Mg: 0-0.05%.

[0047] (5) One or more of W: 0-0.5% and Zr: 0-0.5%

[0048] (6) One or more of Sb: 0-0.5% and Sn: 0-0.5%

[0049] (7) One or more of Y: 0-0.2% and Hf: 0-0.2%

[0050] (8) Co: 0-1.5%.

[0051] The cold-rolled steel sheet can be provided by the following steps: heating a steel billet to 1000-1350°C; hot finishing 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 rate of 30-90%.

[0052] The cooling rate (Vc1) of the primary cooling and the cooling rate (Vc2) of the secondary cooling can satisfy the relationship that Vc1 > Vc2.

[0053] Beneficial effects

[0054] According to a preferred aspect of the present invention, a steel sheet and a method thereof can be provided that have an excellent balance of tensile strength and ductility, an excellent balance of tensile strength and pore-expanding properties, and an excellent yield strength ratio evaluation index, and are suitable for automotive parts, etc.

[0055] Best practice

[0056] This invention relates to a high-strength steel plate with excellent workability and a method for manufacturing the same. Preferred embodiments of the invention are described below. These embodiments can be modified in various ways and should not be construed as limiting the scope of the invention to the specific embodiments described below. These specific embodiments are provided to illustrate the invention in more detail to those skilled in the art.

[0057] The inventors of this invention recognized that in boron (B)-added transformation-induced plasticity (TRIP) steel containing bainite, tempered martensite, newly formed martensite, and retained austenite, by controlling the microstructure fractions of tempered martensite, newly formed martensite, and retained austenite within certain ranges, and by controlling the boron (B) content in the tempered martensite and newly formed martensite within certain ranges, as well as the shape and size of the retained austenite within certain ranges, it is possible to simultaneously ensure a balance between excellent tensile strength and ductility, excellent tensile strength and porosity, and an excellent yield strength ratio evaluation index. Recognizing this, a method was designed that effectively balances excellent strength, yield strength ratio, ductility, and porosity, thus completing this invention.

[0058] The high-strength steel sheet with excellent processability according to one aspect of the present invention will now be described in detail.

[0059] According to one aspect of the invention, a high-strength steel plate with excellent workability may contain, 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%, and the balance Fe and unavoidable impurities. The fine microstructure may include bainite, tempered martensite, newly formed martensite, retained austenite, and other unavoidable microstructures. The steel plate may satisfy the following [Relationship 1] and [Relationship 2].

[0060] [Relation 1]

[0061] 0.03≤[B] FM / [B] TM ≤0.55

[0062] 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.

[0063] [Relationship 2]

[0064] T(γ) / V(γ)≥0.08

[0065] In Equation 2, T(γ) is the fraction of tempered retained austenite in the steel plate (volume %), and V(γ) is the fraction of retained austenite in the steel plate (volume %).

[0066] The steel composition of the present invention will now be described in more detail. Unless otherwise stated, all percentages of the content of each element are by weight.

[0067] The high-strength steel sheet with excellent workability according to one aspect of the present invention comprises, 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%, and the balance Fe and unavoidable impurities. In addition, it may further include one or more of the following: Ti: less than 0.5% (inclusive), Nb: less than 0.5% (inclusive), V: less than 0.5% (inclusive), Cr: less than 3.0% (inclusive), Mo: less than 3.0% (inclusive), Cu: less than 4.0% (inclusive), Ni: less than 4.0% (inclusive), Ca: less than 0.05% (inclusive), REM excluding Y: less than 0.05% (inclusive), Mg: less than 0.05% (inclusive), W: less than 0.5% (inclusive), Zr: less than 0.5% (inclusive), Sb: less than 0.5% (inclusive), Sn: less than 0.5% (inclusive), Y: less than 0.2% (inclusive), Hf: less than 0.2% (inclusive), and Co: less than 1.5% (inclusive).

[0068] Carbon (C): 0.1-0.25%

[0069] Carbon (C) is an essential element for ensuring the strength of steel sheets and also stabilizes residual austenite, which helps improve the ductility of steel sheets. Therefore, to achieve the effects described above, the present invention may contain 0.1% or more carbon (C). Preferably, the carbon (C) content may exceed 0.1%, and may be 0.11% or more, or 0.12% or more. On the other hand, when the carbon (C) content exceeds a certain level, ductility may decrease due to excessive strength increase, and weldability may deteriorate. Therefore, the upper limit of the carbon (C) content in the present invention may be limited to 0.25%. The carbon (C) content may be 0.24% or less, and more preferably 0.23% or less.

[0070] Silicon (Si): 0.01-1.5% or less

[0071] Silicon (Si) is an element that helps improve strength through solid solution strengthening and also improves processability by homogenizing the microstructure. Furthermore, silicon (Si) contributes to the formation of retained austenite by inhibiting the precipitation of cementite. Therefore, to achieve the effects described above, 0.01% or more of silicon (Si) can be added in this invention. A preferred silicon (Si) content is 0.02% or more, and a more preferred silicon (Si) content is 0.04% or more. However, when the silicon (Si) content exceeds a certain level, plating defects such as unplated areas may occur during the plating process, and the weldability of the steel sheet may be reduced. Therefore, the upper limit of the silicon (Si) content in this invention is limited to 1.5%. A preferred upper limit of the silicon (Si) content is 1.48%, and a more preferred upper limit of 1.46%.

[0072] Manganese (Mn): 1.0-4.0%

[0073] Manganese (Mn) is a useful element for simultaneously improving strength and ductility. Therefore, to achieve the effects described above, more than 1.0% manganese (Mn) can be added in this invention. A lower limit for the preferred manganese (Mn) content is 1.2%, and a more preferred lower limit is 1.4%. On the other hand, when too much manganese (Mn) is added, the enrichment of carbon (C) in austenite is insufficient due to the increased bainitic transformation time, thus the desired austenite fraction cannot be guaranteed. Therefore, the upper limit for the manganese (Mn) content in this invention can be limited to 4.0%. A preferred upper limit for the manganese (Mn) content is 3.9%.

[0074] Aluminum (Al): 0.01-1.5%

[0075] Aluminum (Al) is an element that deoxidizes steel by combining with oxygen. Furthermore, like silicon (Si), aluminum (Al) stabilizes retained austenite by inhibiting the precipitation of cementite. Therefore, to achieve the effects described above, 0.01% or more of aluminum (Al) can be added in this invention. A preferred aluminum (Al) content is 0.03% or more, and a more preferred aluminum (Al) content is 0.05% or more. On the other hand, when too much aluminum (Al) is added, the inclusions in the steel sheet increase, and the workability of the steel sheet may decrease. Therefore, the upper limit of the aluminum (Al) content in this invention is limited to 1.5%. A preferred upper limit of the aluminum (Al) content is 1.48%.

[0076] Phosphorus (P): 0.15% or less (including 0%)

[0077] Phosphorus (P) is an element that is included as an impurity and reduces impact toughness. Therefore, the phosphorus (P) content is preferably controlled below 0.15%.

[0078] Sulfur (S): less than 0.03% (including 0%)

[0079] Sulfur (S) is an element that is included as an impurity and forms MnS in the steel sheet, thus reducing its ductility. Therefore, the sulfur (S) content is preferably 0.03% or less.

[0080] Nitrogen (N): 0.03% or less (including 0%)

[0081] Nitrogen (N) is an element that is included as an impurity and forms nitrides during continuous casting, causing cracks in the slab. Therefore, the nitrogen (N) content is preferably 0.03% or less.

[0082] Boron (B): 0.0005-0.005%

[0083] Boron (B) is an element that enhances strength by improving hardenability and also inhibits grain boundary nucleation. Furthermore, the objective of this invention is to simultaneously ensure a balance between excellent tensile strength and elongation, excellent tensile strength and porosity, and an excellent yield strength ratio evaluation index through the enrichment of boron (B) in tempered martensite; therefore, boron (B) must be added in this invention. Thus, to achieve the effects described above, more than 0.0005% boron (B) can be added in this invention. However, when the added boron (B) exceeds a certain level, it not only leads to excessive characteristic effects but also increases manufacturing costs; therefore, the upper limit of the boron (B) content in this invention is limited to 0.005%.

[0084] In addition, the steel plate of the present invention may contain other alloy components besides the alloy components described above, which will be described in detail below.

[0085] One or more of the following: titanium (Ti): 0-0.5%, niobium (Nb): 0-0.5%, and vanadium (V): 0-0.5%.

[0086] Titanium (Ti), niobium (Nb), and vanadium (V) are elements that refine grains by forming precipitates, and they also contribute to improving the strength and impact toughness of steel plates. Therefore, to achieve the effects described above, one or more of titanium (Ti), niobium (Nb), and vanadium (V) can be added in this invention. However, when the content of each of titanium (Ti), niobium (Nb), and vanadium (V) exceeds a certain level, excessive precipitates are formed, which reduces impact toughness and increases manufacturing costs. Therefore, in this invention, the content of each of titanium (Ti), niobium (Nb), and vanadium (V) can be limited to 0.5% or less.

[0087] Chromium (Cr): 0-3.0% and molybdenum (Mo): 0-3.0% or more

[0088] Chromium (Cr) and molybdenum (Mo) are elements that inhibit austenite decomposition during alloying and stabilize austenite like manganese (Mn). 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 becomes insufficient due to the increased bainitic transformation time, thus failing to ensure the desired residual austenite fraction. Therefore, in this invention, the content of chromium (Cr) and molybdenum (Mo) can be limited to 3.0% or less each.

[0089] Copper (Cu): 0-4.0% and Nickel (Ni): 0-4.0% or more

[0090] 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, thereby 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 not only leads to excessive characteristic effects but also increases manufacturing costs. Therefore, in this invention, the content of copper (Cu) and nickel (Ni) is limited to 4.0% or less each.

[0091] 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%.

[0092] 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) can 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 not only leads to excessive characteristic effects but also 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.

[0093] One or more of tungsten (W): 0-0.5% and zirconium (Zr): 0-0.5%.

[0094] 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 will not only lead to excessive property effects, but also increase manufacturing costs. Therefore, in this invention, the content of tungsten (W) and zirconium (Zr) can be limited to 0.5% or less each.

[0095] One or more of antimony (Sb): 0-0.5% and tin (Sn): 0-0.5%.

[0096] 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.

[0097] One or more of yttrium (Y): 0-0.2% and hafnium (Hf): 0-0.2%.

[0098] 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.

[0099] Cobalt (Co): 0-1.5%

[0100] 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.

[0101] In addition to the components described above, the high-strength steel sheet with excellent workability according to one aspect of the invention may contain a balance of Fe and other unavoidable impurities. However, undesirable impurities may inevitably be introduced from raw materials or the surrounding environment during normal manufacturing processes, and therefore 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.

[0102] In a high-strength steel sheet with excellent workability according to one aspect of the invention, the microstructure may include bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable microstructures.

[0103] Both untempered martensite (nascent martensite, FM) and tempered martensite (tempered martensite, TM) are fine microstructures that improve the strength of steel sheets. However, compared to tempered martensite, nascent martensite is characterized by reduced ductility and burrability of the steel sheet. Furthermore, compared to tempered martensite, nascent martensite tends to reduce the yield strength ratio of the steel sheet. This is because the fine microstructure of tempered martensite is softened due to the tempering heat treatment. Therefore, in order 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 yield strength ratio (1-YR) and the optimal control of the microstructure fractions of tempered martensite and newly formed martensite are used. To meet the requirement of 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 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 burr resistance 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 0.42 or less. 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.

[0104] 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 must be optimized to obtain the yield strength ratio evaluation index (1-YR). 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 The yield strength ratio (1-YR) is below 0.42, and the bainite fraction is preferably controlled at 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 / 2 The yield strength ratio (1-YR) and other evaluation indices can limit the bainite fraction to 30% or less by volume. Preferred bainite fractions may be 12% or more by volume, or 14% or more by volume, or 28% or less by volume, or 26% or less by volume.

[0105] 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 9% or less by volume.

[0106] 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.

[0107] According to one aspect of the present invention, a high-strength steel plate with excellent processability can satisfy the following [Relation 1] and [Relation 2].

[0108] [Relation 1]

[0109] 0.03≤[B] FM / [B] TM ≤0.55

[0110] 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.

[0111] [Relation 2]

[0112] T(γ) / V(γ)≥0.08

[0113] In Equation 2, T(γ) is the fraction of tempered retained austenite in the steel plate (volume %), and V(γ) is the fraction of retained austenite in the steel plate (volume %).

[0114] 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 / 2 The present invention controls the fractions of tempered martensite, newly formed martensite, and retained austenite within a certain range, controls the proportion of boron (B) content in tempered martensite and newly formed martensite within a certain range, and controls the proportion of a specific type of retained austenite relative to the whole retained austenite within a certain range.

[0115] In this invention, as shown in [Relationship 1], the content of boron (B) contained in the newly formed martensite ([B]) is... FM (in weight %) and the content of boron (B) contained in tempered martensite ([B]). TMThe ratio (by weight%) is controlled within the range of 0.03 to 0.55, thus ensuring 3.0*10 6 Up to 6.2*10 6 (MPa 2 % 1 / 2 The balance between tensile strength and elongation (B) TE ), 6.0*10 6 Up to 11.5*10 6 (MPa 2 % 1 / 2 The balance between tensile strength and porosity (B) TH and a yield strength ratio evaluation index (I) of 0.15 to 0.42. YR ).

[0116] The inventors of this invention conducted in-depth research on methods for ensuring the physical properties of boron (B)-type TRIP steel. Although the theoretical basis has not yet been clearly elucidated, it has been noted that when the ratio of boron (B) content in newly formed martensite to that in tempered martensite meets a certain range, the desired physical properties of this invention can be ensured. In particular, it has been confirmed that the yield strength ratio of the steel plate shows a certain tendency depending on the ratio of boron (B) content in tempered martensite and newly formed martensite. Therefore, in this invention, as shown in [Equation 1], the ratio of boron (B) content in newly formed martensite to that in tempered martensite is limited to a range of 0.03 to 0.55, thereby ensuring the desired balance of tensile strength and elongation (TS). 2 *EL 1 / 2 Balance between tensile strength and porosity (TS) 2 *HER 1 / 2 ) and the yield strength ratio evaluation index (1-YR).

[0117] Furthermore, the inventors of this invention know that not only the fraction of retained austenite, but also the proportion of a specific type of retained austenite relative to the total retained austenite is an important factor in ensuring strength and workability.

[0118] A higher proportion of tempered retained austenite in the retained austenite generally improves the workability of the steel sheet. Tempered retained austenite is retained austenite in which carbon (C) is introduced and enriched during heat treatment at the bainite formation temperature, and is defined as retained austenite with a carbon (C) content (wt%) at least 1.45 times the average carbon (C) content (wt%) of the steel sheet. The relative enrichment of carbon (C), an austenite stabilizing element, in tempered retained austenite suppresses the phase transformation to martensite, and a certain level of tempered retained austenite proportion further enhances the workability of the steel sheet.

[0119] In this invention, as shown in [Equation 2], the fraction (volume%) of tempered retained austenite relative to the total retained austenite contained in the steel sheet is limited to 0.08 or more, thus effectively ensuring the desired balance between tensile strength and elongation (TS). 2 *EL 1 / 2 The balance between tensile strength and porosity (TS) 2 *HER 1 / 2 ).

[0120] 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 3]. TE It can satisfy 3.0*10 6 Up to 6.2*10 6 (MPa 2 % 1 / 2 ), and the balance between tensile strength and porosity is expressed by the following [relationship 4] (B TH It can satisfy 6.0*10 6 Up to 11.5*10 6 (MPa 2 % 1 / 2 Furthermore, the yield strength ratio evaluation index (I) is expressed by the following [relationship 5]. YR It can satisfy values ​​from 0.15 to 0.42.

[0121] [Relationship 3]

[0122] B TE =[Tensile Strength (TS, MPa)] 2 *[Elongation (E1, %)] 1 / 2

[0123] [Relationship 4]

[0124] B TH =[Tensile Strength (TS, MPa)] 2 *[Pore Expansion Ratio (HER, %)] 1 / 2

[0125] [Relation 5]

[0126] I YR =1 - [YR (Yellow Ratio)]

[0127] The following is a detailed description of an example of a method for manufacturing the steel plate of the present invention.

[0128] A method for manufacturing a high-strength steel sheet according to one aspect of the present invention may include the following steps: heating a cold-rolled steel sheet having a predetermined alloy composition to 700°C at an average heating rate of 5°C / second or higher (first heating), heating it to a temperature range of Ac3 to 920°C at an average heating rate of less than 5°C / second (second heating), and then holding it for 50-1200 seconds (first holding); cooling the steel sheet held for the first time to a temperature range of 200-400°C at an average cooling rate of 2-100°C / second (first cooling). The steel plate, after being cooled once, is heated to a temperature range of 400-600°C at an average heating rate of 5-100°C / second (three heatings), and then held for 10-1800 seconds (two holdings); the steel plate, after being held twice, is cooled to a temperature range of 300-500°C at an average cooling rate of 1-100°C / second (two coolings), and then held for 10-1800 seconds (three holdings); and the steel plate, after being held three times, is cooled to room temperature at an average cooling rate of more than 1°C / second (three coolings).

[0129] The cold-rolled steel sheet can be provided by the following steps: heating a steel billet having a predetermined alloy composition to 1000-1350°C; hot finishing rolling in a temperature range of 800-1000°C; coiling the hot-rolled steel sheet in a temperature range of 350-650°C; pickling the coiled steel sheet; and cold rolling the pickled steel sheet at a reduction rate of 30-90%.

[0130] Preparing and heating steel billets

[0131] A steel billet having a specified alloy composition is prepared. The steel billet of the present invention has an alloy composition corresponding to that of the steel plate described above; therefore, the description of the alloy composition of the steel plate described above is used instead of the description of the alloy composition of the steel billet.

[0132] The prepared steel billet can be heated to a certain temperature range, which can be between 1000-1350℃. 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. When the heating temperature of the steel billet exceeds 1350℃, it may reach the melting point of steel and melt.

[0133] Hot rolling and coiling

[0134] 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. 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.

[0135] Hot-rolled steel sheets can be cooled at an average cooling rate of 10°C / second or higher and can be coiled within a temperature range of 350-650°C. This is because it is difficult to coil when the coiling temperature is below 350°C, and when the coiling temperature exceeds 650°C, surface oxide scale forms inside the hot-rolled steel sheet, making pickling difficult.

[0136] Pickling and cold rolling

[0137] After the hot-rolled coil is unwound, it can be pickled to remove the oxide scale formed on the surface of the steel sheet, and then cold-rolled. This invention does not impose particular restrictions on the pickling and cold-rolling conditions, but it is preferable to cold-roll with a cumulative reduction rate of 30-90%. When the cumulative reduction rate of cold rolling exceeds 90%, it may be difficult to cold-roll within a short time due to the high strength of the steel sheet.

[0138] 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.

[0139] Annealing heat treatment

[0140] In this invention, an annealing heat treatment process is performed to ensure both the strength and processability of the steel plate.

[0141] The cold-rolled steel sheet is heated to 700°C at an average heating rate of 5°C / second or higher (first heating), then heated to a temperature range of Ac3 to 920°C at an average heating rate of less than 5°C / second (second heating), and then held for 50-1200 seconds (first holding).

[0142] When the average heating rate during a single heating to 700℃ is less than 5℃ / second, the ferrite and cementite formed during the heating process transform into blocky austenite, resulting in the inability to form fine tempered martensite and retained austenite as the final microstructure. Therefore, the desired balance of T(γ) / V(γ), tensile strength, and elongation (TS) cannot be achieved. 2 *EL 1 / 2 The balance between tensile strength and porosity (TS) 2 *HER 1 / 2 Furthermore, when the secondary heating rate exceeds 5°C / second until the initial holding temperature, the phase transformation from cementite formed during heating to austenite is accelerated, resulting in the formation of large amounts of blocky austenite, ultimately leading to a coarsened microstructure, and boron (B) cannot be sufficiently enriched in the tempered martensite. Therefore, [B] FM / [B] TMExceeding 0.55, and failing to achieve the desired balance of tensile strength and elongation (TS) 2 *EL 1 / 2 Balance between tensile strength and porosity (TS) 2 *HER 1 / 2 ) and yield strength ratio evaluation index (I YR ).

[0143] When the temperature is kept below Ac3 (two-phase region) for a single period, more than 5% ferrite is formed, thus achieving a balance between tensile strength and elongation (TS). 2 *EL 1 / 2 And the balance between tensile strength and porosity (TS) 2 *HER 1 / 2 The temperature and time of the first holding may decrease. Furthermore, when the holding time is less than 50 seconds, the microstructure is not sufficiently homogenized, and therefore the physical properties of the steel sheet may decrease. No particular upper limit is placed on the holding temperature and holding time, but to prevent a decrease in toughness caused by grain coarsening, the holding temperature is preferably limited to 920°C or less, and the holding time is preferably limited to 1200 seconds or less.

[0144] After one holding period, the temperature can be cooled to a primary cooling termination temperature of 200-400℃ at an average cooling rate of 2℃ / s or higher (primary cooling). When the average cooling rate of primary cooling is less than 2℃ / s, the fraction of retained austenite becomes insufficient due to slow cooling, thus affecting the balance of T(γ) / V(γ), tensile strength, and elongation (TS) of the steel plate. 2 *EL 1 / 2 And the balance between tensile strength and porosity (TS) 2 *HER 1 / 2 The temperature may decrease. There is no specific upper limit to the average cooling rate for a single cooling cycle, but it is preferably below 100°C / s. When the single cooling termination temperature is below 200°C, excessive tempered martensite is formed, and the retained austenite is insufficient, thus affecting the balance of T(γ) / V(γ), tensile strength, and elongation (TS) of the steel sheet. 2 *EL 1 / 2 And the balance between tensile strength and porosity (TS) 2 *HER 1 / 2 This may decrease. On the other hand, when the initial cooling termination temperature exceeds 400°C, excessive bainite is formed, and insufficient tempered martensite is produced, thus affecting the balance between the tensile strength and elongation of the steel sheet (TS). 2 *EL 1 / 2 And the balance between tensile strength and porosity (TS) 2 *HER 1 / 2 () may decrease.

[0145] After one cooling, the temperature can be heated to a range of 400-600°C at an average heating rate of 5°C / second or higher (three heatings), and then held for 10-1800 seconds (two holdings). The upper limit of the average heating rate for the three heatings is not specifically specified, but is preferably below 100°C / second. When the two holding temperature is below 400°C, the balance between the tensile strength and porosity of the steel sheet is compromised due to the low heat treatment temperature. 2 *HER 1 / 2 The percentage of retained austenite may decrease when the secondary holding temperature exceeds 600°C, thus affecting the balance of T(γ) / V(γ), tensile strength, and elongation (TS). 2 *EL 1 / 2 And the balance between tensile strength and porosity (TS) 2 *HER 1 / 2 This may decrease. When the secondary holding time is less than 10 seconds, the heat treatment time is insufficient, thus affecting the balance between the tensile strength and the expansion rate of the steel sheet (TS). 2 *HER 1 / 2 The duration may decrease. There is no specific upper limit for the secondary hold time, but it is preferably below 1800 seconds.

[0146] After the second holding, the temperature can be cooled to a range of 300-500°C at an average cooling rate of 1°C / s or higher (secondary cooling), and then held for 10-1800 seconds (third holding). The upper limit of the average cooling rate for the secondary cooling is not specifically specified, but is preferably below 100°C / s. When the third holding temperature is below 300°C, the balance between tensile strength and porosity (TS) is affected due to the low heat treatment temperature. 2 *HER 1 / 2 The value of γ may decrease. On the other hand, when the holding temperature exceeds 500°C three times, the fraction of retained austenite is insufficient, thus affecting the balance of T(γ) / V(γ), tensile strength, and elongation (TS). 2 *EL 1 / 2 And the balance between tensile strength and porosity (TS) 2 *HER 1 / 2 This may decrease. When the holding time for the three cycles is less than 10 seconds, the heat treatment time is insufficient, thus affecting the balance between the tensile strength and the expansion rate of the steel sheet (TS). 2 *HER 1 / 2 The duration may decrease. There is no specific upper limit for the duration of the three hold events, but it is preferably below 1800 seconds.

[0147] The cooling rate of the primary cooling (Vc1) and the cooling rate of the secondary cooling (Vc2) can satisfy the relationship that Vc1 > Vc2.

[0148] After three holding cycles, it can be cooled to room temperature at an average cooling rate of over 1°C / second (three cooling cycles).

[0149] In the high-strength steel sheet with excellent workability manufactured by the above-described manufacturing method, the microstructure may include bainite, tempered martensite, newly formed martensite, retained austenite, and other unavoidable microstructures. As a preferred example, by volume fraction, it may contain 10-30% bainite, 50-70% tempered martensite, 10-30% newly formed martensite, 2-10% retained austenite, and less than 5% (inclusive) ferrite.

[0150] In the steel sheet manufactured by the above manufacturing method, the balance between tensile strength and elongation (B) is expressed by the following [relationship 3]. TE It can satisfy 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 4] (B) TH It can satisfy 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 5]. YR It can satisfy values ​​from 0.15 to 0.42.

[0151] [Relationship 3]

[0152] B TE =[Tensile Strength (TS, MPa)] 2 *[Elongation (E1, %)] 1 / 2

[0153] [Relationship 4]

[0154] B TH =[Tensile Strength (TS, MPa)] 2 *[Pore Expansion Ratio (HER, %)] 1 / 2

[0155] [Relation 5]

[0156] I YR =1 - [YR (Yellow Ratio)] Detailed Implementation

[0157] 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.

[0158] (Example)

[0159] 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. Afterward, it is pickled to remove surface oxide scale and then cold-rolled to a thickness of 1.5 mm.

[0160] 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.

[0161] 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), nascent martensite (FM), and pearlite (P) in the microstructure were 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. The observed bainite (B) and tempered martensite (TM) were both lath and blocky in shape, making them difficult to distinguish. Therefore, the bainite and tempered martensite fractions 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.

[0162] In addition, [B] of the steel plate FM / [B] TM The balance between T(γ) / V(γ), tensile strength and elongation (TS) 2 *EL 1 / 2 Balance between tensile strength and porosity (TS) 2 *HER1 / 2 ) and yield strength ratio evaluation index (I YR The measurements and evaluations were performed, and the results are shown in Tables 8 and 9.

[0163] Boron (B) content in newly formed martensite ([B]) FM The boron (B) content in tempered martensite and [B] content TM The concentration of boron (B) was determined using an electron probe microanalyzer (EPMA) in both nascent and tempered martensite. Tempered retained austenite was distinguished using EPMA based on the measured carbon (C) content within the retained austenite.

[0164] Tensile strength (TS) and elongation (E1) are evaluated through tensile testing. Specimens are obtained and evaluated according to JIS standard No. 5, with a reference angle of 90° relative to the rolling direction of the rolled sheet. Hole expansion rate (HER) is evaluated through hole expansion testing. After forming a 10 mm Ψ punch (die inner diameter 10.3 mm, clearance 12.5%), a conical punch with a 60° apex angle is inserted into the punch along the direction of the burr facing outwards. The punch is then compressed and expanded around the perimeter at a moving speed of 20 mm / min, and the result is calculated using the following [Equation 6].

[0165] [Relationship 6]

[0166] Hole expansion ratio (HER, %) = {(D-D0) / D0} × 100

[0167] In Equation 6, D represents the aperture (mm) of the hole when the crack penetrates the steel plate along the thickness direction, and D0 represents the initial aperture (mm).

[0168] [Table 1]

[0169]

[0170] [Table 2]

[0171]

[0172] [Table 3]

[0173]

[0174] [Table 4]

[0175]

[0176] [Table 5]

[0177]

[0178] [Table 6]

[0179]

[0180] [Table 7]

[0181]

[0182] [Table 8]

[0183]

[0184] [Table 9]

[0185]

[0186] As shown in Tables 1 to 9, it can be seen that the specimens meeting the conditions proposed in this invention all satisfy [Equation 1] and [Equation 2], achieving a balance between tensile strength and elongation (B). TE Satisfies 3.0*10 6 Up to 6.2*10 6 (MPa 2 % 1 / 2 Balance between tensile strength and porosity (B) TH Satisfies 6.0*10 6 Up to 11.5*10 6 (MPa 2 % 1 / 2 ), yield strength ratio evaluation index (I YR It satisfies 0.15 to 0.42.

[0187] In specimen 2, the average heating rate was less than 5°C / second, resulting in insufficient tempered martensite and retained austenite. Consequently, the T(γ) / V(γ) ratio of specimen 2 was less than 0.08, and the balance between tensile strength and elongation (B) was not achieved. TE Less than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0188] In specimen 3, the secondary average heating rate exceeded 5°C / second, thus forming blocky austenite, and boron (B) was not enriched in the tempered martensite. As a result, the [B] in specimen 3... FM / [B] TM A value exceeding 0.55 indicates a yield strength ratio (I) rating. YR When the value exceeds 0.42, the balance between tensile strength and elongation (B) is affected. TE Less than 3.0*10 6The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0189] In specimen 4, the process was carried out in the two-phase region where the temperature was kept below Ac3, thus the ferrite fraction was excessive. As a result, the balance between tensile strength and elongation of specimen 4 (B) was achieved. TE Less than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0190] In specimen 5, the average cooling rate was less than 2°C / s, resulting in insufficient retained austenite fraction. Consequently, the T(γ) / V(γ) ratio of specimen 5 was less than 0.08, and the balance between tensile strength and elongation (B) was not achieved. TE Less than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0191] In specimen 6, the initial cooling termination temperature was below 200°C, resulting in an excessive amount of tempered martensite and insufficient amount of retained austenite. Consequently, specimen 6 had a T(γ) / V(γ) ratio less than 0.08, and the balance between tensile strength and elongation (B) was not achieved. TE Less than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0192] In specimen 7, the initial cooling termination temperature exceeded 400°C, resulting in an excessive bainite fraction and an insufficient tempered martensite fraction. Consequently, the balance between tensile strength and elongation (B0) of specimen 7 was disrupted. TE Less than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0193] In specimen 8, the secondary holding temperature was below 400℃, therefore the heat treatment temperature was insufficient. As a result, the balance between tensile strength and porosity of specimen 8 was compromised (B...). TH Less than 6.0*10 6 .

[0194] In specimen 9, the temperature was held above 600℃ three times, resulting in insufficient retained austenite fraction. Consequently, the T(γ) / V(γ) ratio of specimen 9 was less than 0.08, and the balance between tensile strength and elongation (B...) was compromised. TELess than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0195] In specimen 10, the secondary holding time was less than 10 seconds (s), therefore the heat treatment time was insufficient. As a result, the balance between the tensile strength and porosity of specimen 10 was compromised (B). TH Less than 6.0*10 6 .

[0196] In specimen 11, the temperature was kept below 300°C three times, therefore the heat treatment temperature was insufficient. As a result, the balance between tensile strength and porosity of specimen 11 was not achieved (B... TH Less than 6.0*10 6 .

[0197] In specimen 12, the temperature was held above 500℃ three times, resulting in insufficient retained austenite fraction. Consequently, the T(γ) / V(γ) ratio of specimen 12 was less than 0.08, and the balance between tensile strength and elongation (B...) was compromised. TE Less than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0198] In specimen 13, the holding time was less than 10 seconds in three separate instances, resulting in insufficient heat treatment time. Consequently, the balance between tensile strength and porosity of specimen 13 was compromised (B...). TH Less than 6.0*10 6 .

[0199] In specimen 35, the carbon (C) content is low, therefore the balance between tensile strength and elongation (B) is relatively low. TE Less than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0200] In specimen 36, the carbon (C) content is high, resulting in insufficient tempered martensite and an excess of nascent martensite. Consequently, the balance between tensile strength and elongation (B) in specimen 36 is disrupted. TE Less than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0201] In specimen 37, the silicon (Si) content is low, resulting in an insufficient fraction of retained austenite. Consequently, the T(γ) / V(γ) ratio of specimen 37 is less than 0.08, and the balance between tensile strength and elongation (B) is not optimal. TE Less than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0202] In specimen 38, the silicon (Si) content is high, resulting in an excessive fraction of newly formed martensite. Consequently, the balance between tensile strength and elongation (B) in specimen 38 is disrupted. TE Less than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0203] In specimen 39, the aluminum (Al) content is high, thus exceeding the fraction of newly formed martensite. As a result, the balance between tensile strength and elongation (B) in specimen 39 is disrupted. TE Less than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0204] In specimen 40, the manganese (Mn) content was low, resulting in insufficient retained austenite fraction due to pearlite formation. Consequently, the T(γ) / V(γ) ratio of specimen 40 was less than 0.08, and the balance between tensile strength and elongation (B) was not achieved. TE Less than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0205] In specimen 41, the manganese (Mn) content is high, resulting in an excessive fraction of newly formed martensite. Consequently, the balance between tensile strength and elongation (B) of specimen 41 is disrupted. TE Less than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0206] In specimen 42, the chromium (Cr) content is high, resulting in an excessive fraction of newly formed martensite. Consequently, the balance between tensile strength and elongation (B) of specimen 42 is disrupted. TE Less than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0207] In specimen 43, the molybdenum (Mo) content is high, resulting in an excessive fraction of newly formed martensite. Consequently, the balance between tensile strength and elongation (B) in specimen 43 is disrupted. TE Less than 3.0*10 6 The balance between tensile strength and porosity (B) TH Less than 6.0*10 6 .

[0208] In sample 44, the boron (B) content is low, and boron (B) cannot be enriched in the tempered martensite. As a result, sample 44 has low [B] content. FM / [B] TM A value exceeding 0.55 indicates a yield strength ratio (I) rating. YR () Exceeds 0.42.

[0209] In sample 45, the boron (B) content is high, and boron (B) is excessively enriched in the tempered martensite. As a result, sample 45 has [B]... FM / [B] TM Less than 0.03, yield strength ratio evaluation index (I YR (less than 0.15)

[0210] The present invention has been described in detail above through embodiments, but other embodiments different from these 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 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%, and the balance Fe and unavoidable impurities, wherein the microstructure comprises bainite, tempered martensite, newly formed martensite, retained austenite, and other unavoidable microstructures. The steel plate satisfies the following [relationship 1] and [relationship 2], The balance between the tensile strength and the porosity of the steel plate, expressed by the following [relationship 4], is B. TH Meets 6.0 10 6 Up to 11.5 10 6 (MPa 2 % 1 / 2 ), [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 of content is weight %; [B] TM This refers to the boron (B) content in the tempered martensite, where the content is expressed as a percentage by weight (%). [Relation 2] T(γ) / V(γ)≥0.08 In Equation 2, T(γ) is the fraction of tempered retained austenite in the steel plate, where the unit of the fraction is volume %, and V(γ) is the fraction of retained austenite in the steel plate, where the unit of the fraction is volume %. [Relationship 4] B TH =[Tensile Strength (TS, MPa)] 2 [Vacuum expansion ratio (HER, %)] 1 / 2 .

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, The fine microstructure of the steel plate, by volume fraction, comprises: 10-30% bainite, 50-70% tempered martensite, 10-30% newly formed martensite, 2-10% retained austenite, and less than 5% and including 0% ferrite.

4. 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 3]. TE Meets 3.0 10 6 Up to 6.2 10 6 (MPa 2 % 1 / 2 The yield strength ratio evaluation index I is expressed by the following [relationship 5]. YR Satisfying 0.15 to 0.42, [Relationship 3] B TE =[Tensile Strength (TS, MPa)] 2 [Elongation (E1, %)] 1 / 2 [Relation 5] I YR =1-[YR (Y)].

5. A method for manufacturing a high-strength steel plate with excellent processability, comprising the following steps: Cold-rolled steel sheets are provided, in weight percent, comprising: 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%, and the balance Fe and unavoidable impurities; The cold-rolled steel sheet is heated to 700°C once at an average heating rate of 5°C / second or higher, and then heated to a temperature range of Ac3 to 920°C a second time at an average heating rate of less than 5°C / second, and then held for 50-1200 seconds. The steel plate, held in one step, is cooled to a temperature range of 200-400°C at an average cooling rate of 2-100°C / second. The steel plate, which has been cooled once, is heated three times to a temperature range of 400-600°C at an average heating rate of 5-100°C / second, and then held for 10-1800 seconds. The steel plate, after being held twice, is cooled a second time to a temperature range of 300-500°C at an average cooling rate of 1-100°C / second, and then held three times for 10-1800 seconds; and The steel plate, which has been held three times, is cooled to room temperature three times at an average cooling rate of more than 1°C / second.

6. The method for manufacturing a high-strength steel plate with excellent processability according to claim 5, 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%.

7. The method for manufacturing a high-strength steel plate with excellent processability according to claim 5, wherein, The cold-rolled steel sheet is provided through 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 then cold-rolled at a reduction rate of 30-90%.

8. The method for manufacturing a high-strength steel plate with excellent processability according to claim 5, wherein, The cooling rate Vc1 of the primary cooling and the cooling rate Vc2 of the secondary cooling satisfy the relationship Vc1>Vc2.

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