High-strength steel plate with excellent workability and manufacturing method thereof

Abstract

The present invention relates to a steel sheet used for automobile parts, etc., and provides a steel sheet that has an excellent balance between strength and ductility, a good balance between strength and hole expandability, and an excellent yield ratio evaluation index, and a method for manufacturing the same. [Solution] Provided is a high-strength steel sheet with excellent workability, which contains, by weight, C: 0.1-0.25%, Si: 0.01-1.5%, Mn: 1-4%, Al: 0.01-1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005-0.005%, with the remainder being Fe and unavoidable impurities, and which contains, as a microstructure, bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures, and which satisfies the following [Relational Formula 1] and [Relational Formula 2]. [Equation 1] 0.03≦[B]FM / [B]TM≦0.55 [Equation 2] V(1.2μm,γ) / V(γ)≧0.12

Description

[Technical Field]

[0001] The present invention relates to a steel sheet that can be used for automobile parts, etc., and has high strength The present invention relates to a steel sheet that has excellent workability while retaining the required properties, and a method for manufacturing the same. [Background technology]

[0002] Recently, the automobile industry has been trying to reduce the weight of materials to protect the global environment, while at the same time providing passenger stability. We are focusing on methods that can ensure stability. Therefore, the use of high-strength steel sheets is rapidly increasing. It is known that the workability of steel sheets decreases as the temperature increases. This steel plate has high strength characteristics while also having excellent workability, such as ductility and hole expandability. This is the reality that is required.

[0003] TRIP (Transformation Induced Plasticity) using retained austenite Induced Plasticity (IP) steel is a composite of ferrite, bainite, and martensitic It has a complex microstructure consisting of ferrite, ferrite, and retained austenite, and therefore has high strength properties. However, it is known to have a certain level of workability.

[0004] As a technology to further improve the workability of steel sheets, a method utilizing tempered martensite is particularly This is disclosed in Patent Documents 1 and 2. Since tempered martensite produced by the annealing process is softened martensite, Tempered martensite is a type of martensite that is not tempered as conventionally (fresh) Therefore, there is a difference in strength between fresh martensite and the martensite. If the temperature is controlled to form tempered martensite, the workability can be improved.

[0005] However, in the techniques disclosed in Patent Documents 1 and 2, the balance between tensile strength and elongation is poor. (TS 2 *EL 1 / 2 ) is 3.0*10 6 ~6.2*10 6 (MPa 2 % 1 / 2 ) range This means that it is difficult to obtain a steel plate that is excellent in both strength and ductility. do.

[0006] On the other hand, another technique for improving the formability of steel sheets is the addition of boron (B). A method for inducing the formation of nitrite is disclosed in Patent Document 3. When boron (B) is added, In this case, the ferrite-pearlite transformation is suppressed and the formation of bainite is induced, resulting in improved strength and It is possible to achieve both good workability and good processability.

[0007] However, in the technology disclosed in Patent Document 3, 3.0*10 6 ~6.2*10 6 ( MPa 2 % 1 / 2 ) balance of tensile strength and elongation (B TE ), 6.0*10 6 ~11. 5*10 6 (MPa 2 % 1 / 2 ) balance of tensile strength and hole expansion rate (B TH ) and 0.1 Yield ratio index (I) of 0.5 to 0.42 YR ) cannot be secured at the same time, so this is This means that it is difficult to obtain a steel sheet that is excellent in all of strength, hole expandability, ductility, and yield ratio.

[0008] In other words, the balance between tensile strength and elongation (B TE ), balance of tensile strength and hole expansion rate ( B TH ) and yield ratio evaluation index (I YR ) do not all meet the requirements for superior steel plates. This is the reality. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Korean Patent Publication No. 10-2006-0118602 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-019258 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-216808 Summary of the Invention [Problem to be solved by the invention]

[0010] According to one aspect of the present invention, the composition and microstructure of a steel sheet are optimized to achieve a balance between tensile strength and elongation. The steel plate manufactured from this has excellent balance between tensile strength and hole expansion rate, and yield ratio evaluation index. A method for making the same can be provided.

[0011] The object of the present invention is not limited to the above. Further object of the present invention is to The present invention is described in the contents of the above, and a person having ordinary skill in the art to which the present invention pertains can easily understand the invention. For example, it is not difficult to understand the further object of the present invention from the contents described in the specification of the present invention. There is no. [Means for solving the problem]

[0012] The high strength steel sheet having excellent formability according to one aspect of the present invention comprises, in weight percent, C: 0.1 to 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 0.005%, the remainder being Fe and unavoidable impurities, with the microstructure being bainite, tempered Martensite, fresh martensite, retained austenite and other unavoidable The following [Relationship 1] and [Relationship 2] can be satisfied. [Equation 1] 0.03≦[B] FM / [B] TM ≦0.55 In the above relational expression 1, [B] FM is the boron (B ) content (wt%), [B] TM is boron (B) contained in tempered martensite The content (wt%). [Equation 2] V(1.2μm,γ) / V(γ)≧0.12 In the above relational expression 2, V(1.2 μm, γ) is the residual grain size of 1.2 μm or more. is the fraction of austenite (volume%), and V(γ) is the fraction of retained austenite in the steel plate ( % by volume.

[0013] The steel plate may further contain, by weight %, one or more of the following (1) to (8): can. (1) One or more of Ti: 0-0.5%, Nb: 0-0.5%, and V: 0-0.5% above (2) At least one 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) Ca: 0-0.05%, REM excluding Y: 0-0.05%, and Mg: 0-0.0 1 or more of 5% (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%

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

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

[0016] According to one aspect of the present invention, there is provided a method for producing a high strength steel sheet having excellent formability, the method comprising the steps of: ~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: 0.0 To provide a cold-rolled steel sheet containing 0.005-0.005% of Cu, the remainder being Fe and unavoidable impurities. The cold-rolled steel sheet is heated to 700°C at an average heating rate of 5°C / s or more (1 Heat to a temperature range of Ac3 to 920°C at an average heating rate of 5°C / s or less (secondary heating). After heating, the steel sheet is maintained for 50 to 1200 seconds (first maintenance). Cool to a temperature range of 350-550°C at an average cooling rate of 2-100°C / s (primary cooling) After that, the steel plate is maintained for 5 to 600 seconds (secondary maintenance); Cooling (secondary cooling) to a temperature range of 200-400°C at an average cooling rate of 0°C / s; The secondarily cooled steel plate was heated to a temperature range of 350 to 550°C at an average heating rate of 5 to 100°C / s. After heating to the desired temperature (third heating), maintain the temperature for 50 seconds or more (third maintenance). The process includes a step of cooling the cooled steel plate to room temperature at an average cooling rate of 1°C / s or more (tertiary cooling). can be done.

[0017] The steel slab may further include 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% above (2) At least one 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) Ca: 0-0.05%, REM excluding Y: 0-0.05%, and Mg: 0-0.0 1 or more of 5% (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%

[0018] The cold-rolled steel sheet is then subjected to a step of heating the steel slab to 1000 to 1350°C; Finish hot rolling in the temperature range of 350~650℃; A step of winding the hot-rolled steel sheet; a step of pickling the wound steel sheet; and a step of pickling the pickled steel sheet. The washed steel sheet is cold-rolled at a reduction rate of 30 to 90%. Cut. [Effects of the Invention]

[0019] According to a preferred embodiment of the present invention, the balance between tensile strength and ductility, the balance between tensile strength and hole expandability, Steel with excellent balance and yield ratio evaluation index, suitable for use in automobile parts A plate and a method for manufacturing the same can be provided. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention relates to a high-strength steel sheet having excellent workability and a method for manufacturing the same. The preferred embodiments of the present invention will be described below. The embodiments of the present invention may be modified in various ways. The scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiment is not intended to be limiting unless it is specifically stated that the present invention is not limited to the above embodiment. is provided to further explain the

[0021] The inventors of the present invention have found that bainite, tempered martensite, fresh martensite and boron (B)-added transformation induced plasticity (TIP) containing retained austenite In tion Induced Plasticity (TRIP) steel, tempering mar The fraction of martensite, fresh martensite and retained austenite is within a certain range. Controlled boron (B) content in tempered martensite and fresh martensite The amount of residual austenite is controlled within a certain range, and the shape and size of the residual austenite are controlled within a certain range. When controlling the temperature, it is necessary to achieve a balance of excellent tensile strength and ductility, and a balance of excellent tensile strength and hole expansion. It has become clear that it is possible to simultaneously secure an excellent yield ratio evaluation index. We have devised a method that can effectively achieve excellent strength, yield ratio, ductility, and hole expandability at the same time. As a result, the present invention was achieved.

[0022] Hereinafter, a high-strength steel sheet with excellent workability according to one embodiment of the present invention will be described in more detail.

[0023] The high strength steel sheet having excellent formability according to one aspect of the present invention comprises, in weight percent, C: 0.1 to 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 0.005%, the remainder being Fe and unavoidable impurities, with the microstructure being bainite, tempered Martensite, fresh martensite, retained austenite and other unavoidable The following [Relationship 1] and [Relationship 2] can be satisfied. [Equation 1] 0.03≦[B] FM / [B] TM ≦0.55 In the above relational expression 1, [B] FM is the boron (B ) content (wt%), [B] TM is boron (B) contained in tempered martensite The content (wt%). [Equation 2] V(1.2μm,γ) / V(γ)≧0.12 In the above relational expression 2, V(1.2 μm, γ) is the residual grain size of 1.2 μm or more. is the fraction of austenite (volume%), and V(γ) is the fraction of retained austenite in the steel plate ( % by volume.

[0024] The steel composition of the present invention will be described in more detail below. The percentages indicating the content of ingredients are based on weight.

[0025] The high strength steel sheet having excellent formability according to one aspect of the present invention comprises, in weight percent, C: 0.1 to 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 0.005%, the remainder contains Fe and unavoidable impurities. In addition, Ti: 0.5% or less ( 0% inclusive), Nb: 0.5% or less (0% inclusive), V: 0.5% or less (0% inclusive), Cr: 3.0% or less (including 0%), Mo: 3.0% or less (including 0%), Cu: 4.0% or less (0 % inclusive), Ni: 4.0% or less (0% inclusive), Ca: 0.05% or less (0% inclusive), Y Excluding REM: 0.05% or less (including 0%), Mg: 0.05% or less (including 0%), W: 0 0.5% or less (0% included), Zr: 0.5% or less (0% included), Sb: 0.5% or less (0% (including), 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%). This can be done.

[0026] Carbon (C): 0.1~0.25% Carbon (C) is an essential element for ensuring the strength of steel plates, and also contributes to improving the ductility of steel plates. It is also an element that stabilizes the retained austenite. In order to achieve the desired effect, the carbon (C) content can be 0.1% or more. C) The content may be more than 0.1%, may be 0.11% or more, and may be less than 0.1 2% or more. On the other hand, if the carbon (C) content exceeds a certain level, The increase in strength at this stage may result in a decrease in ductility and a deterioration in weldability. can limit the carbon (C) content to 0.25%. The carbon (C) content is The carbon (C) content can be 0.24% or less, and more preferably 0.23% or less. It is possible.

[0027] Silicon (Si): 0.01 to 1.5% or less Silicon (Si) is an element that contributes to improving strength through solid solution strengthening, and it homogenizes the structure. Silicon (Si) is also an element that improves workability by increasing the precipitation of cementite. Therefore, the present invention is an element that suppresses the deposition of iron and contributes to the formation of retained austenite. To achieve this effect, 0.01% or more of silicon (Si) can be added. The preferred silicon (Si) content is 0.02% or more, and more preferably The silicon (Si) content can be 0.04% or more, provided that silicon (S i) If the content exceeds a certain level, it may cause plating defects such as non-plating during the plating process. This invention not only induces but also reduces the weldability of steel plates. i) The upper limit of the silicon (Si) content can be limited to 1.5%. The upper limit of the silicon (Si) content can be 1.48%, and a more preferred upper limit of the silicon (Si) content is 1. It could be .46%.

[0028] Manganese (Mn): 1.0-4.0% Manganese (Mn) is a useful element for increasing both strength and ductility. In order to achieve this effect, the present invention adds 1.0% or more of manganese (Mn). The lower limit of the manganese (Mn) content can be preferably 1.2%. A more preferable lower limit of the manganese (Mn) content may be 1.4%. When excessive manganese (Mn) is added, the bainite transformation time increases and the amount of manganese in the austenite increases. Since the carbon (C) concentration becomes insufficient, it is necessary to ensure the desired austenite fraction. Therefore, the present invention provides a method for reducing the manganese (Mn) content. The upper limit of manganese (Mn) content can be limited to 4.0%. The preferred upper limit of manganese (Mn) content is 3. It can be 9%.

[0029] Aluminum (Al): 0.01 to 1.5% Aluminum (Al) is an element that combines with oxygen in steel to deoxidize it. Aluminum (Al) suppresses cementite precipitation in the same way as silicon (Si) and reduces the amount of residual austenite. It is also an element that stabilizes stagnite. Therefore, the present invention achieves such an effect. In order to improve the corrosion resistance, 0.01% or more of aluminum (Al) can be added. The aluminum (Al) content can be 0.03% or more, and more preferably aluminum The (Al) content can be 0.05% or more. On the other hand, if aluminum (Al) is too high, If added too frequently, it not only increases the number of inclusions in the steel sheet but also reduces the workability of the steel sheet. Therefore, the present invention limits the upper limit of the aluminum (Al) content to 1.5%. The preferred upper limit of the aluminum (Al) content may be 1.48%. .

[0030] Phosphorus (P): 0.15% or less (including 0%) Phosphorus (P) is an element that is contained as an impurity and deteriorates impact toughness. The phosphorus (P) content is preferably controlled to 0.15% or less.

[0031] Sulfur (S): 0.03% or less (0% included) Sulfur (S) is contained as an impurity and forms MnS in steel sheets, which causes deterioration of ductility. Therefore, the sulfur (S) content is preferably 0.03% or less.

[0032] Nitrogen (N): 0.03% or less (including 0%) Nitrogen (N) is contained 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. stomach.

[0033] Boron (B): 0.0005 to 0.005% Boron (B) is an element that improves hardenability and increases strength, and also promotes nucleation of grain boundaries. Furthermore, the present invention is also an element that suppresses the concentration of boron (B) in tempered martensite. This provides an excellent balance of tensile strength and elongation, an excellent balance of tensile strength and hole expandability, and In order to simultaneously secure an excellent yield ratio evaluation index, boron (B) is used in the present invention. Therefore, the present invention provides a method for producing such an effect by adding 0.00 0.5% or more of boron (B) can be added. However, if the boron (B) exceeds a certain level, If it is added in excess, it will not only have excessive properties but also cause an increase in manufacturing costs. In the present invention, the upper limit of the boron (B) content can be limited to 0.005%.

[0034] On the other hand, the steel sheet of the present invention does not have any alloy composition that can be additionally contained in addition to the above-mentioned alloy components. , which will be explained in more detail below.

[0035] Titanium (Ti): 0-0.5%, Niobium (Nb): 0-0.5%, and Vanadium (V ): One or more of 0 to 0.5% Titanium (Ti), niobium (Nb) and vanadium (V) form precipitates to refine the grains. It is an element that strengthens the strength and impact toughness of steel sheets. Recently, titanium (Ti), niobium (Nb) and vanadium (V) have been used for this purpose. However, titanium (Ti), niobium (Nb) and vanadium can be added in one or more of these. When the content of each element exceeds a certain level, excessive precipitates are formed, resulting in poor impact toughness. This not only reduces the efficiency but also causes an increase in manufacturing costs. The content of niobium (Nb) and vanadium (V) can be limited to 0.5% or less. Cut.

[0036] At least one of chromium (Cr): 0-3.0% and molybdenum (Mo): 0-3.0% Chromium (Cr) and molybdenum (Mo) suppress the decomposition of austenite during alloying treatment. It is also an element that stabilizes austenite, just like manganese (Mn). Therefore, the present invention uses one of chromium (Cr) and molybdenum (Mo) for this effect. However, the content of chromium (Cr) and molybdenum (Mo) When exceeds a certain level, the bainite transformation time increases and the carbon ( C) The amount of enrichment is insufficient, making it difficult to secure the desired fraction of retained austenite. Therefore, the present invention aims to limit the content of chromium (Cr) and molybdenum (Mo) to Each of these can be limited to 3.0% or less.

[0037] Copper (Cu): 0-4.0% and Nickel (Ni): 0-4.0% or more Copper (Cu) and nickel (Ni) are elements that stabilize austenite and inhibit corrosion. In addition, copper (Cu) and nickel (Ni) concentrate on the surface of the steel sheet and migrate into the steel sheet. It is also an element that prevents hydrogen penetration and suppresses delayed hydrogen fracture. To achieve this effect, it is recommended to add one or more of copper (Cu) and nickel (Ni). However, if the copper (Cu) and nickel (Ni) content exceeds a certain level, This not only has an excessive effect on the properties but also causes an increase in manufacturing costs, so the present invention uses copper (Cu) and The nickel (Ni) content can be limited to 4.0% or less.

[0038] Calcium (Ca): 0-0.05%, Magnesium (Mg): 0-0.05%, and Rare earth elements (REM) excluding yttrium (Y): 0-0.05% of one or more Here, rare earth elements (REM) are scandium (Sc), yttrium (Y) and la The elements of the calcium group are calcium (Ca), magnesium (Mg), and yttrium. Rare earth elements (REM) other than Y (Y) improve the ductility of steel sheets by making sulfides spherical. The present invention uses calcium (Ca), magnesium (Mg) and other elements that contribute to this effect. Addition of one or more rare earth elements (REM) excluding magnesium (Mg) and yttrium (Y) However, calcium (Ca), magnesium (Mg), and yttrium If the content of rare earth elements (REM) other than (Y) exceeds a certain level, excessive properties may occur. This not only increases the production cost but also the effectiveness of calcium (Ca), magnesium (Mg), The content of rare earth elements (REM) excluding magnesium (Mg) and yttrium (Y) is It can be limited to 0.05% or less.

[0039] Tungsten (W): 0-0.5% and Zirconium (Zr): 0-0.5% more than species Tungsten (W) and zirconium (Zr) improve the hardenability and increase the strength of the steel plate. Therefore, the present invention uses tungsten (W) and diamine (Diamine) for this effect. One or more of tungsten (W) and zinc (Zr) can be added. And if the content of zirconium (Zr) exceeds a certain level, it will not only have excessive property effects. This is a cause of increased manufacturing costs, so the present invention uses tungsten (W) and zirconium The content of (Zr) can be limited to 0.5% or less.

[0040] One or more of antimony (Sb): 0-0.5% and tin (Sn): 0-0.5% Antimony (Sb) and tin (Sn) improve the plating wettability and plating adhesion of steel sheets. Therefore, the present invention uses antimony (Sb) and tin (S n), but antimony (Sb) and tin (Sn ) content exceeds a certain level, the brittleness of the steel sheet increases and the steel sheet becomes brittle during hot or cold working. Therefore, the present invention does not include antimony (Sb) and tin (Sn). The content of each can be limited to 0.5% or less.

[0041] One of yttrium (Y): 0-0.2% and hafnium (Hf): 0-0.2% End Yttrium (Y) and hafnium (Hf) are elements that improve the corrosion resistance of steel sheets. Therefore, the present invention uses yttrium (Y) and hafnium (Hf) to achieve this effect. One or more of these can be added. However, yttrium (Y) and hafnium (Hf) If the content exceeds a certain level, the ductility of the steel sheet may deteriorate. The content of yttrium (Y) and hafnium (Hf) is limited to 0.2% or less. It is possible.

[0042] Cobalt (Co): 0-1.5% Cobalt (Co) is an element that promotes bainite transformation and increases the TRIP effect. Therefore, in the present invention, cobalt (Co) can be added for this effect. If the content of cobalt (Co) exceeds a certain level, the weldability and ductility of the steel plate will deteriorate. Therefore, the present invention limits the content of cobalt (Co) to 1.5% or less. can be done.

[0043] The high strength steel sheet excellent in formability according to one embodiment of the present invention contains the remaining Fe and However, in the normal manufacturing process, raw materials or surrounding environment may contain impurities such as chlorine, methyl methacrylate ... Since unintended impurities can inevitably be mixed in from the boundary, this must be completely eliminated. These impurities can be easily identified by anyone skilled in the art. Since it is well known, not all of the contents thereof will be specifically mentioned in this specification. This does not mean that the addition of additional effective ingredients other than the ingredients listed above is completely excluded.

[0044] The high strength steel sheet excellent in formability according to one aspect of the present invention is a steel sheet having bainite, tempered martensite, and Tempered Martensite, Fresh Martensite h Martensite), retained austenite and other unavoidable structures are microstructured It can be included as:

[0045] Untempered martensite (fresh martensite, FM) and tempered All of the tempered martensite (TM) improves the strength of the steel plate. However, compared to tempered martensite, fresh martensite has a fine structure that The martensite has the characteristic of reducing the ductility and burring ability of the steel sheet. Compared to martensite, fresh martensite tends to reduce the yield ratio of steel plates. This is because the tempering heat treatment softens the microstructure of tempered martensite. Therefore, the balance between tensile strength and elongation (TS 2 *EL 1 / 2 ), tensile Balance between strength and hole expansion rate (TS 2 *HER 1 / 2 ) and yield ratio evaluation index (1-YR) To ensure the optimum ratio of tempered martensite to fresh martensite, It is preferable. 3.0*10 6The balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), 6.0*10 6 The balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and and a yield ratio rating index (1-YR) of 0.42 or less. The fraction of fresh martensite is limited to 50% by volume or more, and the fraction of fresh martensite is limited to 10% by volume or more. It is more preferable that the tempered martensite fraction is 52% by volume or more. The fraction of fresh martensite can be 54% by volume or more, and more preferably, the fraction of fresh martensite is 1 On the other hand, the content of tempered martensite or fresh martensite can be 2% by volume or more. If excessive tensile stress is formed, the ductility and burring property will decrease, resulting in a 3.0*1 0 6 The balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), 6.0*10 6 The above Balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and a yield ratio rating of 0.42 or less Therefore, the present invention is not able to simultaneously satisfy the index (1-YR). The fraction of martensite is limited to 70% by volume or less, and the fraction of fresh martensite is limited to 30% by volume or less. The more preferable tempered martensite fraction is 68% by volume. % or less by volume or 65% or less by volume, more preferably fresh martensite. The fraction of cellulose can be 25% by volume or less.

[0046] The balance of tensile strength and elongation (TS) 2 *EL 1 / 2 ), pull Balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and yield ratio evaluation index (1-YR) To ensure this, it is necessary to optimize the bainite fraction. 6 The above quotes Balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), 6.0*10 6 More than tensile strength and holes Expansion Rate Balance (TS 2 *HER 1 / 2 ) and a yield ratio evaluation index (1-Y) of 0.42 or less In order to ensure the above R), it is preferable to control the bainite fraction to 10% by volume or more. More preferably, the bainite fraction is 12% by volume or more, or 14% by volume or more. On the other hand, if excessive bainite is formed, the resulting separation of tempered martensite will be This causes a decrease in the tensile strength and elongation ratio, so the desired balance between tensile strength and elongation ratio (TS 2 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ), and yield ratio evaluation index (1 To ensure the desired annealing temperature (YR), the bainite fraction can be limited to 30% by volume or less. The preferred bainite fraction is 12% by volume or more, or 14% by volume or more, or 28% by volume or more. % or less or 26% by volume or less.

[0047] Steel sheets containing retained austenite undergo austenite transformation into martensite during processing. It has excellent ductility and workability due to the transformation-induced plasticity that occurs during the transformation of the retained austenite. When the fraction of nitrite is below a certain level, the balance between tensile strength and elongation (TS 2 *EL 1 / 2 ) is 3.0*10 6 (MPa 2 %1 / 2 ), which is not desirable. When the fraction of retained austenite exceeds a certain level, the local elongation (Local El The welding resistance may decrease or the spot weldability may decrease. The present invention provides a balance between tensile strength and elongation (TS 2 *EL 1 / 2 ) to obtain excellent steel sheets. The fraction of retained austenite can be limited to the range of 2 to 10%. The austenite fraction can be greater than or equal to 3% by volume or less than or equal to 8% by volume.

[0048] The steel sheet of the present invention has an inevitable structure of ferrite, pearlite, and island martensite. (Martensite Austenite Constituent, MA), etc. If ferrite is formed excessively, the strength of the steel sheet may decrease. Therefore, the present invention can limit the ferrite fraction to 5% by volume or less (including 0%). In addition, if pearlite is formed excessively, the workability of the steel sheet will decrease or residual austenite will form. Since the fraction of stenite may be reduced, the present invention aims to minimize the formation of pearlite. Restrict.

[0049] The high-strength steel sheet excellent in formability according to one aspect of the present invention has the following [Relational Formula 1] and [Relational Formula 2]: 2] can be satisfied. [Equation 1] 0.03≦[B] FM / [B] TM ≦0.55 In the above relational expression 1, [B] FM is the boron (B ) content (wt%), [B] TM is boron (B) contained in tempered martensite The content (wt%). [Equation 2] V(1.2μm,γ) / V(γ)≧0.12 In the above relational expression 2, V(1.2 μm, γ) is the residual grain size of 1.2 μm or more. is the fraction of austenite (volume%), and V(γ) is the fraction of retained austenite in the steel plate ( % by volume.

[0050] The present invention achieves the desired balance between tensile strength and elongation (TS 2 *EL 1 / 2 ), tensile strength and hole expansion rate balance (TS 2 *HER 1 / 2 ) and yield ratio evaluation index (1-YR) To achieve this, tempered martensite, fresh martensite and retained austenite are used. In addition to controlling the fraction of tempered martensite within a certain range, The percentage of boron (B) contained in the austenite is controlled within a certain range, and the overall residual austenite is The ratio of the specific size, shape and type of retained austenite to the austenite is controlled within a certain range. do.

[0051] The present invention is based on the boron (B) content of tempered martensite as shown in [Relationship 1]. ([B] TM , wt. % of boron (B) content in fresh martensite Amount([B] FM , weight %) ratio is controlled to the range of 0.03 to 0.55, 0 6 ~6.2*10 6 (MPa 2 % 1 / 2 ) balance of tensile strength and elongation (B TE ), 6.0*10 6 ~11.5*10 6 (MPa 2 % 1 / 2) balance between tensile strength and hole expansion rate S (B TH ) and yield ratio evaluation index (I YR ) at the same time can be done.

[0052] The inventors of the present invention have conducted in-depth research into methods for ensuring the physical properties of boron (B)-added TRIP steel. Although the theoretical basis has not been clearly clarified as a result of the research, tempered martensite The boron (B) content in fresh martensite relative to the B content in the site The properties aimed at by the present invention can be ensured only when the content of (B) satisfies a certain range. In particular, we focused on the fact that tempered martensite and fresh martensite can be The yield ratio of steel sheets tends to be constant depending on the percentage of boron (B) contained in the tensite. Therefore, the present invention has been confirmed to be based on the tempered martensitic steel as shown in [Relationship 1]. The boron (B) content in fresh martensite relative to the boron (B) content in martensite (B) The content ratio is limited to the range of 0.03 to 0.55, so that the desired tensile strength and Elongation balance (TS 2 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *H ER 1 / 2 ) and yield ratio evaluation index (1-YR).

[0053] Furthermore, the inventors of the present invention have investigated not only the fraction of retained austenite but also the total retained austenite. The ratio of retained austenite of a specific size to the austenite is important for ensuring strength and workability. It was found to be an element.

[0054] The percentage of retained austenite with an average grain size of 1.2 μm or more The more the average grain size is increased, the more effective it is in improving the workability of the steel sheet. The retained austenite above is the residual austenite that has been heat treated at the bainite forming temperature and has an increased average size. The average grain size of the retained austenite is 1.2 μm or less. This is a structure in which the driving force for transformation to martensite is relatively suppressed. Retained austenite with a grain size of 1.2 μm or more inhibits transformation into martensite. Therefore, the proportion of retained austenite with an average grain size of 1.2 μm or more is above a certain level. In this case, the workability of the steel sheet can be further effectively improved.

[0055] In the present invention, the fraction of the total retained austenite contained in the steel sheet (V( γ), the fraction of retained austenite with an average grain size of 1.2 μm or more (V (1.2μm, γ), volume % ratio is controlled to 0.12 or more, and the target tensile strength is obtained. Balance of strength and elongation (TS 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) can be effectively ensured.

[0056] The high strength steel sheet excellent in formability according to one aspect of the present invention has a drawing strength represented by the following [Relational Formula 3]. Balance of tensile strength and elongation (B TE ) is 3.0*10 6 ~6.2*10 6 (MPa 2 % 1 / 2 ) and the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 ~11.5*10 6 (MPa 2% 1 / 2 ) and satisfy the following [relationship The yield ratio evaluation index (I YR ) can satisfy 0.15 to 0.42. [Equation 3] B TE = [tensile strength (TS, MPa)] 2 *[Elongation rate (El, %)] 1 / 2 [Equation 4] B TH = [tensile strength (TS, MPa)] 2 *[Hole expansion rate (HER, %)] 1 / 2 [Equation 5] I YR =1-[yield ratio (YR)]

[0057] An example of a method for producing the steel sheet of the present invention will be described in detail below.

[0058] A method for producing a high-strength steel sheet according to one aspect of the present invention comprises: The steel plate is heated to 700°C at an average heating rate of 5°C / s or more (primary heating), and then heated at an average heating rate of 5°C / s or less. After heating to a temperature range of Ac3 to 920°C at an average heating rate of 50 to 120°C (secondary heating), 00 seconds (first maintenance); the steel sheet is maintained at an average temperature of 2 to 100°C / s. After cooling to a temperature range of 350 to 550°C at a cooling rate (primary cooling), maintain the temperature for 5 to 600 seconds. The second-holding step is to cool the steel plate at an average cooling rate of 2 to 100°C / s. The step of cooling (secondary cooling) the steel sheet to a temperature range of 200~400℃; Heat to a temperature range of 350 to 550°C at an average heating rate of 5 to 100°C / s (third heating). After that, maintain the temperature for 50 seconds or more (third maintenance); after the third maintenance, the steel plate is heated at a rate of 1°C / s or more. The step of cooling to room temperature at an average cooling rate (tertiary cooling) may be included.

[0059] The cold-rolled steel sheet is produced by rolling a steel slab having a predetermined alloy composition at a temperature of 1000 to 1350°C. Finish hot rolling in the temperature range of 800 to 1000°C; a step of winding the hot-rolled steel sheet at a temperature in the range of 50°C; and cold rolling the pickled steel sheet at a reduction ratio of 30 to 90%. can be provided as follows.

[0060] Steel slab preparation and heating A steel slab having a predetermined alloy composition is prepared. The steel slab of the present invention is the alloy of the steel plate described above. Therefore, the alloy composition of the steel slab is explained in accordance with the above-mentioned steel This replaces the explanation of the alloy composition of the plate.

[0061] The prepared steel slab can be heated within a certain temperature range, and the heating of the steel slab The temperature can be in the range of 1000-1350°C. In this case, there is a risk that hot rolling will occur in a temperature range below the temperature range of the intended finish hot rolling. If the heating temperature of the steel slab exceeds 1350℃, the melting point of the steel may be reached and the steel may melt. There it is.

[0062] Hot rolling and coiling The heated steel slab can be hot-rolled to provide a hot-rolled steel sheet. The temperature of the finish hot rolling at this time is preferably in the range of 800 to 1000°C. If the temperature is less than 800°C, excessive rolling load may become a problem. If the temperature exceeds 1000℃, the grain size of the hot-rolled steel sheet will become coarse, resulting in poor physical properties of the final steel sheet. may cause a decline.

[0063] After hot rolling, the hot-rolled steel sheet is cooled at an average cooling rate of 10°C / s or more. The coiling temperature is 350 to 650°C. If the winding temperature is below 650℃, it is difficult to wind the film. If the winding temperature is above 650℃, the surface Scale may form deep inside the hot-rolled steel sheet, making pickling difficult. .

[0064] Pickling and cold rolling After the hot-rolled coil is uncoiled, the scale formed on the surface of the steel sheet is removed. In the present invention, pickling and cold rolling are carried out to remove the oxidative stress. Although the rolling conditions are not particularly limited, the cold rolling can be performed at a cumulative reduction rate of 30 to 90%. It is preferable that the cumulative reduction rate of cold rolling exceeds 90% due to the high strength of the steel sheet. It may be difficult to extend the period in a short time.

[0065] Cold-rolled steel sheets are either uncoated or made into corrosion-resistant cold-rolled steel sheets after undergoing an annealing heat treatment process. To give it strength, it can be made of plated steel sheet through a plating process. Applying plating methods such as hot-dip galvanizing, electrogalvanizing, and hot-dip aluminum plating The method and type of the method are not particularly limited.

[0066] Annealing heat treatment In the present invention, an annealing heat treatment step is carried out in order to simultaneously ensure the strength and workability of the steel sheet.

[0067] The cold-rolled steel plate is heated to 700°C (primary heating) at an average heating rate of 5°C / s or more. The sample was heated to a temperature range of Ac3 to 920°C at an average heating rate of 5°C / s or less (secondary heating). Then maintain for 50 to 1200 seconds (primary maintenance).

[0068] If the average heating rate of the first heating to 700°C is less than 5°C / s, Massive austenite is formed from the ferrite and cementite that are formed, resulting in the final structure. As a result, it becomes impossible to form fine tempered martensite and retained austenite as a structure. This allows the desired balance between tensile strength and elongation (TS 2 *EL 1 / 2 ) and quotes Balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) cannot be realized. In addition, if the secondary heating rate to the primary maintenance temperature exceeds 5℃ / s, the The transformation of the cementite into austenite is accelerated, and a large amount of blocky austenite is formed. The final structure becomes coarse and boron (B) is not sufficiently concentrated in the tempered martensite. This may cause [B] FM / [B] TM exceeds 0.55 This allows for the desired balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), tensile strength and hole expansion rate balance (TS 2 *HER 1 / 2 ) and yield ratio evaluation index (I YR ) to be realized You will no longer be able to do this.

[0069] If the primary temperature is below Ac3 (two-phase region), 5% or more by volume of ferrite is formed. Therefore, the balance between tensile strength and elongation (TS 2 *EL 1 / 2 ) and tensile strength and hole expansion Rate Balance (TS 2 *HER 1 / 2 ) may decrease. Also, the primary maintenance time If the time is less than 50 seconds, the structure cannot be sufficiently homogenized, and the physical properties of the steel sheet will deteriorate. There are no particular restrictions on the upper limits of the primary maintenance temperature and primary maintenance time, but the temperature and time may be affected by the coarsening of crystal grains. To prevent a decrease in toughness, the first maintenance temperature is 920℃ or less, and the first maintenance time is 1200 seconds. It is preferable to limit it to the following:

[0070] After the first cooling period, the temperature range is 350 to 550°C with an average cooling rate of 2°C / s or more. After cooling to (primary cooling), it can be maintained at that temperature range for 5 seconds or more (secondary maintenance). If the average cooling rate of the primary cooling is less than 2°C / s, the retained austenite will be lost due to slow cooling. The tensile strength and elongation balance of the steel sheet (TS 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease There is no need to specify an upper limit for the average cooling rate of the first cooling, but it should be 100°C or less. If the second maintenance temperature is less than 350°C, the V(1 .2μm, γ) / V(γ) and balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) On the other hand, if the secondary maintenance temperature exceeds 550℃, the residual austenite The balance between tensile strength and elongation of the steel sheet (TS 2 *EL 1 / 2 ) and tensile strength and hole expansion rate balance (TS 2 *HER 1 / 2 ) may decrease. If the next maintenance time is less than 5 seconds, the heat treatment time is insufficient and the V(1.2μm, γ) / V( γ) and the balance between tensile strength and hole expansion rate (TS2 *HER 1 / 2 ) may decrease There is no need to specify an upper limit for the secondary maintenance time, but it is preferable to limit it to 600 seconds or less. Desirable.

[0071] After the second hold, cool to the first cooling stop temperature of 200-400°C at an average cooling rate of 2°C / s or more. If the average cooling rate of the secondary cooling is less than 2°C / s, the cooling rate will be slow. Slow cooling results in a shortage of retained austenite, which reduces the tensile strength and elongation of the steel plate. Rate Balance (TS 2 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 There is no need to specify an upper limit for the average cooling rate of the secondary cooling. However, it is preferable to set the rate to 100°C / s or less. If the primary cooling stop temperature is less than 200°C, In this case, excessive tempered martensite is formed and the residual austenite is insufficient, resulting in the V( 1.2μm, γ) / V(γ), balance of tensile strength and elongation (TS 2 *EL 1 / 2 ) and Balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease. If the first cooling stop temperature exceeds 400°C, excessive bainite is formed, and the tempering matrix The balance between tensile strength and elongation of the steel sheet is poor due to the lack of ruthenium. 2 *EL 1 / 2 ) and Balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease.

[0072] After the secondary cooling, the temperature is increased to 350-550°C at an average heating rate of 5°C / s or more. After heating (third heating), it can be maintained for 50 seconds or more (third maintenance). There is no need to specify an upper limit for the uniform heating rate, but it is preferable to set it to 100°C / s or less. If the third holding temperature is less than 350°C or the third holding time is less than 50 seconds, the tempered martensite will Excessive sites are formed, making it difficult to maintain the fraction of retained austenite. As a result, V(1. 2μm, γ) / V(γ), balance of tensile strength and elongation (TS 2 *EL 1 / 2 ) and tensile Balance between strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease. If the temperature exceeds 550°C or the third maintenance time exceeds 155,000 seconds, residual The proportion of oxidized steel sheet is insufficient, and the V(1.2μm, γ) / V(γ), tensile strength and elongation Rate Balance (TS 2 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease.

[0073] After the third maintenance, it can be cooled to room temperature (third cooling) at an average cooling rate of 1℃ / s or more. do.

[0074] The high-strength steel plate with excellent workability manufactured by the above-mentioned manufacturing method has the following microstructure: Bainite, tempered martensite, fresh martensite, retained austenite and and other unavoidable structures. A preferred example is a volume fraction of 10 to 3 0% bainite, 50-70% tempered martensite, 10-30% fresh martensite Ruthenium, 2-10% retained austenite, and 5% or less (including 0%) ferrite It can include.

[0075] The steel sheet manufactured by the above-mentioned manufacturing method has a tensile strength expressed by the following [Relationship 3]. and growth rate balance (B TE ) is 3.0*10 6 ~6.2*10 6 (MPa 2 % 1 / 2 ) The balance between tensile strength and hole expansion rate (B TH ) is 6 .0*10 6 ~11.5*10 6 (MPa 2 % 1 / 2 ) and the following [Relationship 5] Yield ratio evaluation index (I YR ) can satisfy 0.15 to 0.42. [Equation 3] B TE = [tensile strength (TS, MPa)] 2 *[Elongation rate (El, %)] 1 / 2 [Equation 4] B TH = [tensile strength (TS, MPa)] 2 *[Hole expansion rate (HER, %)] 1 / 2 [Equation 5] I YR =1-[yield ratio (YR)] [Example]

[0076] Hereinafter, a high-strength steel plate having excellent workability according to one embodiment of the present invention and its The manufacturing method will be described in more detail. The following examples are provided to aid in the understanding of the present invention. It should be noted that this is not intended to limit the scope of the present invention. The scope of the invention is determined by the matters described in the claims and by the facts that can be reasonably inferred from them. It is determined by the term.

[0077] (Example) A 100 mm thick alloy having the alloy composition shown in Table 1 below (the remainder is Fe and unavoidable impurities) was used. A steel slab of 100 mm was produced and heated at 1200°C, and then finish hot rolled at 900°C. After this, the sheet was cooled at an average cooling rate of 30°C / s, and wound at the winding temperatures shown in Tables 2 and 3. After that, the hot-rolled steel sheets were pickled to remove surface scale and then cut into 1.5 mm thick strips. The sheet was cold rolled to a thickness of 1 mm.

[0078] Thereafter, heat treatment was carried out under the annealing heat treatment conditions shown in Tables 2 to 5 below to produce steel sheets. In Tables 2 and 3, the single-phase region refers to the temperature range of Ac3 to 920°C, and the two-phase region refers to the temperature range of Ac This refers to a temperature range of less than 3°C.

[0079] The microstructure of the steel sheets thus produced was observed, and the results are shown in Tables 6 and 7. The microstructure consists of ferrite (F), bainite (B), tempered martensite (TM), Fresh martensite (FM) and pearlite (P) were determined by navigating the cross section of the polished specimen. After ital etching, the specimen was observed via SEM. The smooth structure on the surface of the piece is divided into ferrite and the lamellar structure of cementite and ferrite is The microstructures were divided into pearlite and bainite (B) and tempered martensite (T). M) are observed in lath and block forms and are difficult to distinguish, so they are considered to be bainite and tempered masonry. The fraction of ruthenium was calculated using the expansion curve after dilatation evaluation. The fraction of bainite and tempered martensite measured by SEM observation through the dilatation curve The calculated tempered martensite fraction was subtracted from the bainite fraction to determine the bainite fraction. It is also not easy to distinguish between fresh martensite (FM) and retained austenite (retained γ). Therefore, the fractions of martensite and retained austenite observed by SEM were used for X-ray diffraction. The fraction of fresh martensite is calculated by subtracting the fraction of retained austenite calculated by the diffraction method. was determined by the rate.

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

[0081] Boron (B) content in fresh martensite ([B] FM ) and tempered martensite Boron (B) content in the site ([B] TM ) is an EPMA (Electron Probe MicroAnalyzer) was used to measure fresh martensite and tempered martensite. It was determined by the boron (B) concentration measured in the tensite. The average grain size is 1.2 μm or more. The retained austenite (V(1.2μm, γ)) was measured by EBSD (Electron Ba Using the phase map of backscatter diffraction (CKD), and measured.

[0082] Tensile strength (TS) and elongation (El) are evaluated by tensile testing, and the rolling direction of the rolled plate material The test piece was taken based on JIS No. 5 standard and evaluated at a 90° angle to the Tensile strength (TS) and elongation (El) were measured. Hole expansion ratio (HER) was measured by the hole expansion test. It was evaluated by punching a 10mmΨ hole (die inner diameter 10.3mm, clearance 12.5 After forming the hole, a conical punch with a 60° apex angle is used to remove the burrs from the hole. Insert the workpiece into the punched hole in the direction indicated by the arrow and move it at a speed of 20 mm / min. After compressing and expanding the area, the following [relationship 6] was used to calculate. [Equation 6] Hole expansion rate (HER, %) = {(D-D0) / D0} × 100 In the above equation 6, D is the hole diameter (m m), and D0 means the initial hole diameter (mm).

[0083] [Table 1]

[0084] [Table 2]

[0085] [Table 3]

[0086] [Table 4]

[0087] [Table 5]

[0088] [Table 6]

[0089] [Table 7]

[0090] [Table 8]

[0091] [Table 9]

[0092] As shown in Tables 1 to 9 above, in the case of a test piece that satisfies the conditions presented in the present invention, [relationship 1] and [Relationship 2], and the balance between tensile strength and elongation (B TE ) is 3.0 *10 6 ~6.2*10 6 (MPa 2 % 1 / 2 ) and balance between tensile strength and hole expansion rate. S (B TH ) is 6.0*10 6 ~11.5*10 6 (MPa 2 % 1 / 2 ) and surrender Relative Evaluation Index (I YR ) satisfies 0.15 to 0.42.

[0093] Specimen 2 was subjected to a primary average heating rate of less than 5°C / s, and the tempered martensite and residual As a result, specimen 2 had a poor balance between tensile strength and elongation (B T E ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0094] Specimen 3 was subjected to a secondary average heating rate of more than 5°C / s, and blocky austenite was formed. As a result, boron (B) could not be concentrated in the tempered martensite. is [B] FM / [B] TM exceeds 0.55, and the yield ratio evaluation index (I YR ) exceeds 0.42, Balance of tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate Lance (B TH ) is 6.0*10 6 It was less than.

[0095] Test piece 4 was performed in the two-phase region where the primary temperature was less than Ac3, and the ferrite fraction exceeded As a result, test piece 4 had a good balance between tensile strength and elongation (B TE ) is 3.0*10 6 less than , the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0096] Test piece 5 was subjected to a primary cooling rate of less than 1°C / s, and the fraction of retained austenite was As a result, the balance between tensile strength and elongation (B TE ) is 3.0*1 0 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0097] For test piece 6, the second maintenance temperature was less than 350°C, and the heat treatment temperature was insufficient. As a result, for specimen 6, V(1.2μm, γ) / V(γ) was less than 0.12, and the tensile strength and hole expansion ratio Balance (B TH ) is 6.0*10 6 It was less than.

[0098] Test piece 7 was tested at a secondary holding temperature of over 550°C, and the fraction of retained austenite was insufficient. As a result, specimen 7 had a good balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0099] For test piece 8, the second maintenance time was less than 5 seconds, which meant that the heat treatment time was insufficient. Test piece 8 has a V(1.2μm, γ) / V(γ) of less than 0.12, and the tensile strength and hole expansion ratio are not uniform. ance (B TH ) is 6.0*10 6 It was less than.

[0100] Specimen 9 was subjected to secondary cooling at a rate of less than 2°C / s, and the fraction of retained austenite was As a result, the balance between tensile strength and elongation (B TE ) is 3.0*1 0 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0101] The test piece 10 was subjected to a primary cooling stop temperature of less than 200°C, and was a tempered martensite. As a result, specimen 10 was found to have a V(1 .2μm, γ) / V(γ) is less than 0.12, and the balance of tensile strength and elongation (B TE ) is 3 .0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 Less than It was.

[0102] Test piece 11 was performed at a primary cooling stop temperature of over 400°C, and the bainite fraction exceeded As a result, the tensile strength and elongation of specimen 11 were low. Rate Balance (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0103] Test piece 12 was tested at a third temperature of less than 350°C, and the fraction of tempered martensite As a result, the fraction of retained austenite was insufficient. μm, γ) / V(γ) is less than 0.12, and the balance between tensile strength and elongation (B TE ) is 3.0 *10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 was less than .

[0104] Test piece 13 was tested at a third temperature of over 550°C, and the fraction of retained austenite was As a result, the V(1.2μm, γ) / V(γ) of test piece 16 was less than 0.12, and Balance of tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% tensile strength and hole expansion rate ance (B TH ) is 6.0*10 6 It was less than.

[0105] Test piece 14 was tested for a third holding time of less than 50 seconds, and the fraction of tempered martensite was As a result, specimen 14 was found to have a V (1.2μ m, γ) / V(γ) is less than 0.12, and the balance between tensile strength and elongation (B TE ) is 3.0* 10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0106] Test piece 15 was tested for a third holding time of over 155,000 seconds, and the amount of retained austenite was As a result, the V(1.2μm, γ) / V(γ) ratio of specimen 15 was less than 0.12. The balance of tensile strength and elongation (B TE ) is 3.0*10 6 Less than tensile strength and hole expansion rate Balance (B TH ) is 6.0*10 6 It was less than.

[0107] Test piece 37 has a low carbon (C) content and exhibits a good balance between tensile strength and elongation (B TE )but 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 Less than there were.

[0108] Specimen 38 has a high carbon (C) content and a low tempered martensite fraction. The fraction of Rösch martensite was excessive, and the fraction of retained austenite was excessive. As a result, specimen 38 had a good balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than tensile strength Balance of degree and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0109] Specimen 39 has a low silicon (Si) content and a low fraction of retained austenite. As a result, test piece 39 had a good balance between tensile strength and elongation (B TE ) is 3.0*10 6 Not yet The balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0110] Specimen 40 has a high silicon (Si) content and a high fraction of fresh martensite. As a result, the balance between tensile strength and elongation (B TE ) is 3.0*1 0 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0111] Specimen 41 has a high aluminum (Al) content and is a distribution of fresh martensite. As a result, the balance between tensile strength and elongation (B TE ) is 3.0 *10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 was less than .

[0112] Specimen 42 has a low manganese (Mn) content, and pearlite formation results in a loss of retained austenite. As a result, the balance between tensile strength and elongation (B TE )but 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 Less than there were.

[0113] Specimen 43 has a high manganese (Mn) content and a high fraction of fresh martensite. As a result, the balance between tensile strength and elongation (B TE ) is 3.0*1 0 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0114] Specimen 44 has a high chromium (Cr) content and the fraction of fresh martensite is very high. As a result, the test piece 44 had a good balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0115] Specimen 45 has a high molybdenum (Mo) content and a low fraction of fresh martensite. As a result, the balance between tensile strength and elongation (B TE ) is 3.0* 10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0116] Specimen 46 has a low boron (B) content and contains no boron (B) in the tempered martensite. As a result, test piece 51 was [B] FM / [B] TM exceeds 0.55 The yield ratio evaluation index (I YR ) exceeded 0.42.

[0117] Specimen 47 has a high boron (B) content and contains boron (B) in the tempered martensite. As a result, test piece 52 was found to be [B] FM / [B] TM is less than 0.03 The yield ratio evaluation index (I YR ) was less than 0.15.

[0118] Although the present invention has been described in detail above by way of examples, other embodiments are also possible. Therefore, the spirit and scope of the following claims are not limited to the examples. Not determined.

Claims

1. In weight percent, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4. 0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N:

0. 0.03% or less, B: 0.0005 to 0.005%, the remainder including Fe and inevitable impurities, The microstructures include bainite, tempered martensite, fresh martensite, and residual martensite. Contains retained austenite and other unavoidable structures. A high-strength steel plate with excellent formability that satisfies the following [Relational Formula 1] and [Relational Formula 2]. [Relationship 1] 0.03≦[B] ＦＭ / [B] ＴＭ ≦0.55 In the above-mentioned relational formula 1, [B] ＦＭ is the boron (B) contained in fresh martensite. ) content (wt%), [B] ＴＭ is boron (B) contained in tempered martensite The content (wt%) is [Relationship 2] V(1.2 μm, γ) / V(γ)≧0.12 In the above-mentioned relational expression 2, V(1.2 μm, γ) is a residual material having an average crystal grain size of 1.2 μm or more. is the fraction of austenite (volume %), and V(γ) is the fraction of retained austenite in the steel plate ( % by volume.

2. The steel plate contains, in weight percent, at least one selected from the group consisting of the following (1) to (8): The high-strength steel plate with excellent workability according to claim 1 , further comprising a seed. (1) One or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5% above (2) One or more of Cr: 0 to 3.0% and Mo: 0 to 3.0% (3) One or more of Cu: 0 to 4.0% and Ni: 0 to 4.0% (4) Ca: 0 to 0.05%, REM excluding Y: 0 to 0.05%, and Mg: 0 to 0.0 One or more of the 5% (5) One or more of W: 0 to 0.5% and Zr: 0 to 0.5% (6) One or more of Sb: 0 to 0.5% and Sn: 0 to 0.5% (7) One or more of Y: 0 to 0.2% and Hf: 0 to 0.2% (8) Co: 0-1.5%

3. The microstructure of the steel plate is, by volume fraction, 10 to 30% bainite and 50 to 70% tempered. Martensite, 10-30% fresh martensite, 2-10% retained austenite 10. The method of claim 1, further comprising the step of: High strength steel plate.

4. The steel sheet has a balance between tensile strength and elongation (B ＴＥ )but 3.0*10 ６ ~6.2*10 ６ (MPa ２ % １／２ ) and the following [Relationship 4] is satisfied. The balance between tensile strength and hole expansion rate (B ＴＨ ) is 6.0*10 ６ ~11.5*10 ６ (MPa ２ % １／２ ) and the yield ratio evaluation index (I) is expressed by the following [Relationship 5]. ＹＲ 2. The high-strength steel plate having excellent formability according to claim 1, wherein σ is 0.15 to 0.

42. [Relationship 3] B ＴＥ = [tensile strength (TS, MPa)] ２ * [Elongation rate (El,%)] １／２ [Relationship 4] B ＴＨ = [tensile strength (TS, MPa)] ２ * [Hole expansion rate (HER, %)] １／２ [Relationship 5] I ＹＲ =1 - [Submission Ratio (YR)]

5. In weight percent, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4. 0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N:

0. 0.03% or less, B: 0.0005 to 0.005%, the remainder being Fe and unavoidable impurities. providing a rolled steel sheet; The cold-rolled steel sheet is heated to 700°C at an average heating rate of 5°C / s or more (first heating ) and then heated to a temperature range of Ac3 to 920°C at an average heating rate of 5°C / s or less (secondary heating). and then maintaining the temperature for 50 to 1200 seconds (first maintenance); The steel sheet is cooled to a temperature of 350 to 550°C at an average cooling rate of 2 to 100°C / s. cooling to the desired range (first cooling) and then maintaining the temperature for 5 to 600 seconds (second maintaining); The second-maintained steel sheet is cooled to a temperature of 200 to 400°C at an average cooling rate of 2 to 100°C / s. cooling to a desired temperature range (secondary cooling); The secondary cooled steel plate is heated to a temperature of 350 to 550°C at an average heating rate of 5 to 100°C / s. and then maintaining the temperature for 50 seconds or more (third maintenance); The steel plate maintained in the third cooling step is cooled to room temperature at an average cooling rate of 1°C / s or more (third cooling). A method for manufacturing a high-strength steel plate with excellent formability, including the step of:

6. The steel slab further includes any one or more of the following (1) to (8): A method for manufacturing a high-strength steel plate having excellent workability as described above. (1) One or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5% above (2) One or more of Cr: 0 to 3.0% and Mo: 0 to 3.0% (3) One or more of Cu: 0 to 4.0% and Ni: 0 to 4.0% (4) Ca: 0 to 0.05%, REM excluding Y: 0 to 0.05%, and Mg: 0 to 0.0 One or more of the 5% (5) One or more of W: 0 to 0.5% and Zr: 0 to 0.5% (6) One or more of Sb: 0 to 0.5% and Sn: 0 to 0.5% (7) One or more of Y: 0 to 0.2% and Hf: 0 to 0.2% (8) Co: 0-1.5%

7. The cold-rolled steel sheet is heating the steel slab to 1000-1350°C; Finish hot rolling in the temperature range of 800 to 1000°C; Coiling the hot-rolled steel sheet at a temperature in the range of 350 to 650°C; pickling the coiled steel sheet; and cold rolling the pickled steel sheet at a reduction of 30 to 90%; The method for producing a high-strength steel plate having excellent workability according to claim 5.

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