Cold rolled steel sheet and manufacturing method therefor

Abstract

The present invention relates to a cold rolled steel sheet and a manufacturing method therefor, and more specifically to a high-strength cold rolled steel sheet having an excellent elongation rate and hole expansion ratio, and a manufacturing method therefor.

Description

Cold rolled steel sheet and its manufacturing method

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more particularly, to a high-strength cold-rolled steel sheet having excellent elongation and hole expandability and a method for manufacturing the same.

[0002] The automotive industry has recently been focusing on ways to reduce material weight while simultaneously ensuring passenger safety to protect the environment. To meet these demands for safety and weight reduction, the use of high-strength steel sheets is rapidly increasing. Generally, as steel sheet strength increases, ductility and workability decrease. Therefore, steel sheets for automotive components require superior strength, ductility, and workability.

[0003] As a technology for improving the ductility of steel plates, methods utilizing tempered martensite are disclosed in Patent Documents 1 and 2. Tempered martensite, created by tempering hard martensite, is a softened form of martensite and exhibits a difference in strength from conventional untempered martensite (fresh martensite). Suppressing fresh martensite and forming tempered martensite increases ductility and workability.

[0004] However, the technology disclosed in Patent Documents 1 and 2 does not satisfy the product of tensile strength and elongation (TS×El) of 22,000 MPa% or more, making it difficult to secure both excellent strength and ductility.

[0005] Meanwhile, TRIP (Transformation Induced Plasticity) steel, which utilizes the transformation-induced plasticity of retained austenite, has been developed to achieve high strength, ductility, and excellent workability for automotive steel sheets. Patent Documents 3 and 4 disclose TRIP steel with excellent ductility and workability.

[0006] Patent Document 3 attempted to improve ductility and workability by including polygonal ferrite, retained austenite, and martensite, but it was found that bainite was the main phase, so high strength could not be secured, and TS×El did not satisfy 22,000 MPa% or more.

[0007] Patent Document 4 improves ductility and workability by forming a composite structure including ferrite formation, refinement of retained austenite, and tempered martensite, but there is a problem that it is difficult to secure high strength because a large amount of soft ferrite is included.

[0008] Even now, the demand for steel plates with high strength and excellent ductility has not been met.

[0009] [Prior Art Literature]

[0010] [Patent Document]

[0011] (Patent Document 1) Korean Patent Publication No. 10-2006-0118602

[0012] (Patent Document 2) Japanese Patent Publication No. 2009-019258

[0013] (Patent Document 3) Korean Patent Publication No. 10-2014-0012167

[0014] (Patent Document 4) Korean Patent Publication No. 10-2010-0092503

[0015] According to one embodiment of the present invention, a cold rolled steel sheet and a method for manufacturing the same are provided.

[0016] According to one embodiment of the present invention, it is an object to provide a high-strength cold-rolled steel sheet having excellent elongation and hole expandability and a method for manufacturing the same.

[0017] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding additional objectives of the present invention from the overall content of this specification.

[0018] According to one embodiment of the present invention, it contains, in wt%, C: 0.25 to 0.75%, Si: 4.00% or less, Mn: 0.90 to 5.00%, Al: 5.00% or less, P: 0.150% or less, S: 0.0300% or less, N: 0.0300% or less, B: 0.0001 to 0.0050%, the remainder being Fe and unavoidable impurities.

[0019] The microstructure contains, by area %, 30-75% tempered martensite and 10-40% retained austenite.

[0020] Unit area 175 μm in the area from the surface to the center of thickness 35 μm 2 It may be a cold rolled steel sheet having one or more micro-voids with a diameter of 0.3 μm or more.

[0021] The above unit area may be 35 μm in depth and 5 μm in width from the surface toward the center of thickness.

[0022] The above cold rolled steel sheet may further include one or more of the following (1) to (8).

[0023] (1) At least one of the following: Ti: 0.5% or less, Nb: 0.5% or less, and V: 0.5% or less

[0024] (2) Cr: 3.0% or less and Mo: 3.0% or less, at least one of these

[0025] (3) Cu: 4.5% or less and Ni: 4.5% or less, at least one of these

[0026] (4) Ca: 0.05% or less, REM excluding Y: 0.05% or less, and Mg: 0.05% or less, at least one of these

[0027] (5) W: 0.5% or less and Zr: 0.5% or less, at least one of these

[0028] (6) Sb: 0.5% or less and Sn: 0.5% or less, at least one of these

[0029] (7) Y: 0.2% or less and Hf: 0.2% or less, at least one of these

[0030] (8) Co: 1.5% or less

[0031] The above cold rolled steel sheet may have a total Si and Al content of 1.00 to 6.00%.

[0032] The above microstructure may include, in area %, 30 to 75% tempered martensite, 10 to 50% bainite, 10 to 40% retained austenite, 5% or less ferrite, and unavoidable structures.

[0033] The above cold-rolled steel sheet may have a product of tensile strength and elongation (TS×El) of 22,000 MPa% or more, and R / t (R is the minimum bending radius (mm) at which no crack occurs after a 90° bending test, and t is the thickness (mm) of the steel sheet) of 0.50 to 3.00.

[0034] According to one embodiment of the present invention, there is provided a step of reheating a steel slab containing, in wt%, C: 0.25 to 0.75%, Si: 4.00% or less, Mn: 0.90 to 5.00%, Al: 5.00% or less, P: 0.150% or less, S: 0.0300% or less, N: 0.0300% or less, B: 0.0001 to 0.0050%, the remainder being Fe and unavoidable impurities;

[0035] A step of finishing hot rolling the above reheated steel slab;

[0036] A step of coiling the above-mentioned hot-rolled steel sheet after cooling it;

[0037] The above rolled steel plate is subjected to a hot rolling annealing heat treatment at a temperature range of 500 to 750°C for 600 to 1700 seconds;

[0038] A step of cold rolling the above hot-rolled and annealed steel sheet;

[0039] A first heating and maintaining step of heating the above cold-rolled steel sheet to a temperature range of Ac1 or higher in an atmosphere having a dew point temperature of -15 to 30°C and then maintaining it for 120 seconds or longer;

[0040] A step of first cooling the above-mentioned first heated and maintained steel plate to a temperature range of 100 to 300°C at an average cooling rate of 20.0°C / s or more;

[0041] A second heating and maintaining step of heating the first cooled steel plate to a temperature range of 300 to 500°C and then maintaining it for 300 seconds or more; and

[0042] It may be a method for manufacturing a cold-rolled steel sheet, including a step of secondarily cooling the second heated and maintained steel sheet to room temperature.

[0043] The above steel slab may further include one or more of the following (1) to (8).

[0044] (1) At least one of the following: Ti: 0.5% or less, Nb: 0.5% or less, and V: 0.5% or less

[0045] (2) Cr: 3.0% or less and Mo: 3.0% or less, at least one of these

[0046] (3) Cu: 4.5% or less and Ni: 4.5% or less, at least one of these

[0047] (4) Ca: 0.05% or less, REM excluding Y: 0.05% or less, and Mg: 0.05% or less, at least one of these

[0048] (5) W: 0.5% or less and Zr: 0.5% or less, at least one of these

[0049] (6) Sb: 0.5% or less and Sn: 0.5% or less, at least one of these

[0050] (7) Y: 0.2% or less and Hf: 0.2% or less, at least one of these

[0051] (8) Co: 1.5% or less

[0052] The above steel slab may have a sum of Si and Al contents of 1.00 to 6.00%.

[0053] The above reheating step is performed at a temperature range of 1000 to 1350°C,

[0054] The above finishing hot rolling step is performed at a temperature range of 800 to 1000°C;

[0055] The above-mentioned winding step cools the above-mentioned hot-rolled steel sheet to a temperature range of 300 to 600°C at an average cooling rate of 10°C / s or more,

[0056] The above cold rolling step can be performed at a reduction ratio of 30 to 90%.

[0057] The above first heating and maintenance step heats at an average heating rate of 10°C / s or more,

[0058] The above second heating and maintenance step heats at an average heating rate of 15°C / s or more,

[0059] The above secondary cooling step can cool at an average cooling rate of 1°C / s or more.

[0060] According to one embodiment of the present invention, a cold rolled steel sheet and a method for manufacturing the same can be provided.

[0061] According to one embodiment of the present invention, a high-strength cold-rolled steel sheet having excellent elongation and hole expandability and a method for manufacturing the same can be provided.

[0062] According to one embodiment of the present invention, a cold-rolled steel sheet and a manufacturing method thereof can be provided that can be used as a steel sheet for automobile structures requiring both weight reduction and stability by securing excellent strength, ductility, and processing characteristics.

[0063] Although not essential, it should be noted that the technical solutions according to each aspect of the present invention can also be usefully applied to other aspects of the present invention. Furthermore, the compositions and various useful parameters according to each aspect of the present invention can be appropriately combined with other aspects to achieve beneficial effects.

[0064] Preferred embodiments of the present invention are described below. These embodiments may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to provide a more detailed explanation of the present invention to those skilled in the art.

[0065] Hereinafter, the present invention will be described in detail.

[0066] Below, the steel composition of the present invention is described in detail.

[0067] Unless otherwise specifically stated in the present invention, the percentage indicating the content of each element is based on weight.

[0068] A steel sheet according to one embodiment of the present invention may include, in wt%, C: 0.25 to 0.75%, Si: 4.00% or less, Mn: 0.90 to 5.00%, Al: 5.00% or less, P: 0.150% or less, S: 0.0300% or less, N: 0.0300% or less, B: 0.0001 to 0.0050%, the remainder being Fe and unavoidable impurities.

[0069] Carbon (C): 0.25~0.75%

[0070] Carbon (C) is an essential element for imparting strength to steel sheets, and is also a stabilizing element for retained austenite, which increases the ductility of the steel sheet. If the carbon (C) content is less than 0.25%, it may be difficult to secure the desired tensile strength. According to one embodiment of the present invention, the lower limit of the carbon (C) content may be 0.31%. On the other hand, if the content exceeds 0.75%, cold rolling may be difficult, making it difficult to manufacture the steel sheet. According to one embodiment of the present invention, the upper limit may be 0.55%.

[0071] Silicon (Si): 4.00% or less

[0072] Silicon (Si) is an element that enhances strength through solid solution strengthening, strengthens ferrite, homogenizes the structure, and improves workability. Furthermore, it is an element that suppresses cementite precipitation and contributes to the formation of retained austenite. If the silicon (Si) content exceeds 4.00%, plating defects such as underplating may occur during the plating process, and the weldability of the steel sheet may be reduced. According to one embodiment of the present invention, the upper limit may be 3.00%.

[0073] Manganese (Mn): 0.90~5.00%

[0074] Manganese (Mn) is a useful element for increasing both strength and ductility. To achieve the aforementioned effects, manganese (Mn) may be added in an amount of 0.90% or more. According to one embodiment of the present invention, the lower limit may be 1.00%. On the other hand, if the content exceeds 5.0%, the bainite transformation time increases, resulting in insufficient carbon enrichment in austenite, and thus the desired level of retained austenite fraction may not be secured. According to one embodiment of the present invention, the upper limit may be 3.50%.

[0075] Aluminum (Al): 5.00% or less

[0076] Aluminum (Al) is an element that combines with oxygen in steel to act as a deoxidizer. Furthermore, like silicon (Si), it is an element that stabilizes retained austenite by suppressing cementite precipitation. If the aluminum (Al) content exceeds 5.00%, the workability of the steel sheet deteriorates and inclusions may increase. According to one embodiment of the present invention, the upper limit may be 3.00%.

[0077] Phosphorus (P): 0.150% or less

[0078] Phosphorus (P) is an element that deteriorates impact toughness when contained as an impurity. Therefore, in the present invention, the phosphorus (P) content can be controlled to 0.150% or less.

[0079] Sulfur (S): 0.0300% or less

[0080] Sulfur (S) is an element that is contained as an impurity and forms MnS in steel plates, thereby deteriorating ductility. Therefore, in the present invention, the content of sulfur (S) can be controlled to 0.0300% or less.

[0081] Nitrogen (N): 0.0300% or less

[0082] Nitrogen (N) is an element that is contained as an impurity and forms nitrides during continuous casting, causing cracks in slabs. Therefore, in the present invention, the nitrogen (N) content can be controlled to 0.0300% or less.

[0083] Boron (B): 0.0001~0.0050%

[0084] Boron (B) not only strengthens grain boundaries, thereby providing resistance to LME and hydrogen embrittlement cracking, but also suppresses ferrite transformation during cooling after annealing. To achieve these effects, boron (B) may be added in amounts of 0.0001% or more. However, excessive amounts may deteriorate hot-rollability, and excessive accumulation of boron (B) on the surface may impair plating properties. Therefore, according to one embodiment of the present invention, the upper limit of the boron (B) content may be limited to 0.0050%.

[0085] In addition to the composition described above, the steel of the present invention may contain remaining iron (Fe) and unavoidable impurities. Unavoidable impurities can be unintentionally incorporated during the typical manufacturing process, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the art of steel manufacturing, their full content is not specifically discussed in this specification.

[0086] A steel plate according to one embodiment of the present invention may further include one or more of the following (1) to (8).

[0087] (1) At least one of the following: Ti: 0.5% or less, Nb: 0.5% or less, and V: 0.5% or less

[0088] (2) Cr: 3.0% or less and Mo: 3.0% or less, at least one of these

[0089] (3) Cu: 4.5% or less and Ni: 4.5% or less, at least one of these

[0090] (4) Ca: 0.05% or less, REM excluding Y: 0.05% or less, and Mg: 0.05% or less, at least one of these

[0091] (5) W: 0.5% or less and Zr: 0.5% or less, at least one of these

[0092] (6) Sb: 0.5% or less and Sn: 0.5% or less, at least one of these

[0093] (7) Y: 0.2% or less and Hf: 0.2% or less, at least one of these

[0094] (8) Co: 1.5% or less

[0095] Titanium (Ti): 0.5% or less, niobium (Nb): 0.5% or less, and vanadium (V): 0.5% or less, at least one of these

[0096] The above Ti, Nb, and V are elements that form precipitates and refine grains. They can be added to improve the strength and impact toughness of steel sheets. If the content of each of the above Ti, Nb, and V exceeds 0.5%, excessive precipitate formation not only reduces impact toughness but also causes an increase in manufacturing costs, so the content may be 0.5% or less.

[0097] Chromium (Cr): 3.0% or less and molybdenum (Mo): 3.0% or less, at least one of these

[0098] The above Cr and Mo are elements that suppress austenite decomposition during alloying treatment and, like Mn, stabilize austenite. If the content of each of Cr and Mo exceeds 3.0%, the bainite transformation time increases, and the C enrichment in austenite is insufficient, making it impossible to secure the desired retained austenite fraction. Therefore, the content of each of Cr and Mo may be 3.0% or less.

[0099] Copper (Cu): 4.5% or less and Nickel (Ni): 4.5% or less, at least one of these

[0100] The Cu and Ni above are elements that stabilize austenite and inhibit corrosion. The Cu and Ni are concentrated on the surface of the steel plate, preventing hydrogen from penetrating into the steel plate, thereby suppressing hydrogen-delayed fracture. If the Cu and Ni content exceeds 4.5%, it not only causes excessive characteristic effects but also increases manufacturing costs. Therefore, the Cu and Ni content may be 4.5% or less.

[0101] Calcium (Ca): 0.05% or less, rare earth elements (REM) excluding yttrium (Y): 0.05% or less, and magnesium (Mg): 0.05% or less, at least one of these

[0102] The above REM refers to a total of 17 elements including Sc, Y, and lanthanides. REM, excluding Ca, Mg, and Y, can improve the ductility of steel sheets by spheroidizing sulfides. If the content of each REM, excluding Ca, Mg, and Y, exceeds 0.05%, it will cause excessive characteristic effects as well as an increase in manufacturing costs. Therefore, the content of each REM, excluding Ca, Mg, and Y, may be 0.05% or less.

[0103] Tungsten (W): 0.5% or less and zirconium (Zr): 0.5% or less, at least one of these

[0104] The above W and Zr are elements that improve hardenability and increase the strength of the steel sheet. If the content of each of W and Zr exceeds 0.5%, it not only causes excessive characteristic effects but also increases manufacturing costs. Therefore, the content of each of W and Zr may be 0.5% or less.

[0105] Antimony (Sb): 0.5% or less and tin (Sn): 0.5% or less, at least one of these

[0106] The above Sb and Sn are elements that improve the plating wettability and plating adhesion of the steel sheet. If the content of each of the above Sb and Sn exceeds 0.5%, the brittleness of the steel sheet increases, and cracks may occur during hot or cold working. Therefore, the content of each of the above Sb and Sn may be 0.5% or less.

[0107] Yttrium (Y): 0.2% or less and Hafnium (Hf): 0.2% or less, at least one of these

[0108] The above Y and Hf are elements that improve the corrosion resistance of the steel plate. If the content of each of Y and Hf exceeds 0.2%, the ductility of the steel plate may deteriorate. Therefore, the content of each of Y and Hf may be 0.2% or less.

[0109] Cobalt (Co): 1.5% or less

[0110] The above Co is an element that promotes bainite transformation and increases the TRIP effect. If the Co content exceeds 1.5%, the weldability and ductility of the steel plate may deteriorate. Therefore, the Co content may be 1.5% or less.

[0111] In one embodiment of the present invention, the steel sheet may have a sum of Si and Al contents of 1.00 to 6.00%.

[0112] Silicon (Si) + Aluminum (Al): 1.00~6.00%

[0113] The above Si and Al are components that affect the formation of microstructure in the present invention, thereby affecting ductility and bending workability. Therefore, in order to have excellent ductility and bending workability, the sum of the Si and Al contents may be limited to 1.00 to 6.00%. If the sum of the Si and Al contents is less than 1.00%, the above-described effect may not be secured. According to one embodiment of the present invention, the lower limit may be 1.50%. On the other hand, if the content exceeds 6.00%, there may be a problem that excessive oxides are formed during hot rolling, which deteriorates operability and also deteriorates playability. According to one embodiment of the present invention, the upper limit may be 4.00%.

[0114] Below, the steel microstructure of the present invention is described in detail.

[0115] Unless otherwise specifically stated in the present invention, the percentage indicating the fraction of microstructure is based on area.

[0116] The microstructure of the steel sheet according to one embodiment of the present invention may include, in terms of area %, 30 to 75% tempered martensite and 10 to 40% retained austenite. Furthermore, the microstructure of the steel sheet according to one embodiment of the present invention may include, in terms of area %, 30 to 75% tempered martensite, 10 to 50% bainite, 10 to 40% retained austenite, 5% or less ferrite, and unavoidable structures.

[0117] In the present invention, the inevitable structure may include fresh martensite, pearlite, and martensite austenite constituent (MA). If the inevitable structure is excessively formed, the ductility and workability of the steel sheet may be deteriorated or the fraction of retained austenite may be reduced. In the present invention, the upper limit may be limited to 20%.

[0118] Meanwhile, both untempered martensite (fresh martensite) and tempered martensite have microstructures that enhance the strength of steel sheets. However, compared to tempered martensite, fresh martensite has the characteristic of significantly reducing the ductility of the steel sheet. This is because the microstructure of tempered martensite is softened by the tempering heat treatment. Therefore, in order to provide the steel sheet of the present invention with excellent strength-ductility balance and workability, it is preferable to utilize tempered martensite.

[0119] If the fraction of the tempered martensite is less than 30%, it may be difficult to secure the desired product of tensile strength and elongation. According to one embodiment of the present invention, it may be 40% or more. On the other hand, if the fraction exceeds 75%, it may reduce ductility and workability, making it difficult to secure the product of tensile strength and elongation or may reduce bendability. According to one embodiment of the present invention, it may be 70% or less.

[0120] According to one embodiment of the present invention, bainite may be appropriately included to improve the balance of strength and ductility and workability of the steel plate.

[0121] When the above-mentioned bainite fraction is 10% or more, the product of tensile strength and elongation, as well as bendability, can be secured simultaneously. According to one embodiment of the present invention, it may be 15% or more. On the other hand, when the bainite fraction exceeds 50%, it may be difficult to secure the desired tensile strength and elongation due to the relatively reduced tempered martensite fraction. According to one embodiment of the present invention, it may be 45% or less.

[0122] Steel plates containing retained austenite can have excellent ductility and workability due to transformation-induced plasticity that occurs during transformation from austenite to martensite during processing.

[0123] If the retained austenite content is less than 10%, it may be difficult to properly secure the desired properties. According to one embodiment of the present invention, it may be 15% or more. On the other hand, if the retained austenite content exceeds 40%, local elongation may decrease. Therefore, in order to obtain a steel sheet with excellent balance of strength and ductility as well as workability, the retained austenite content may be limited to 10 to 40%. According to one embodiment of the present invention, it may be 35% or less.

[0124] Ferrite can increase the hardness difference between phases by introducing soft phases, which can have a negative effect on securing hole expandability, so its upper limit can be limited to 5%.

[0125] According to one embodiment of the present invention, a unit area of ​​175 μm in a region from the surface of the steel plate to the center of the thickness of 35 μm 2 There may be one or more micro-voids with a diameter of 0.3 μm or more. According to one embodiment of the present invention, the unit area may have a depth of 35 μm and a width of 5 μm from the surface toward the center of thickness.

[0126] However, the above width does not mean that it is limited to the y-axis when defining the thickness direction of the steel plate as the z-axis, the length direction as the x-axis, and the width direction as the y-axis. That is, the unit area may mean a virtual area that selects one direction based on one axis in the thickness direction. In one embodiment, the unit area may have a depth of 35 μm and a width of 5 μm, and in another embodiment, the unit area may have a depth of 35 μm and a length of 5 μm. In addition, the width and length directions may each mean a 45 degree direction.

[0127] According to one embodiment of the present invention, when applying an internal annealing oxidation method that oxidizes alloying elements such as Mn and Si inside the steel by controlling the heating temperature, oxygen partial pressure in the annealing furnace, or dew point during the first heating, Mn and Si oxides are preferentially formed on the surface of the steel, and then Mn and Si are oxidized by oxygen diffused into the steel, thereby suppressing surface diffusion. At this time, the Mn and Si-based oxides diffused into the extreme surface layer create fine voids in the Fe site existing in the base material, and are effective not only in improving R / t by forming extreme surface layer ferrite but also in securing hydrogen embrittlement resistance by the hydrogen trapping effect caused by the voids.

[0128] Therefore, according to one embodiment of the present invention, a unit area of ​​175 μm is provided in an area from the surface of the steel plate to 35 μm in the direction of the center of thickness. 2 When the number of pores having a diameter of 0.3 μm or more is at least one, it may be advantageous for securing hydrogen embrittlement resistance. According to one embodiment of the present invention, the number of pores may be at least three.

[0129] According to one embodiment of the present invention, a steel plate may have a product of tensile strength and elongation (TS×El) of 22,000 MPa% or more, and R / t (R is a minimum bending radius (mm) at which no crack occurs after a 90° bending test, and t is a thickness (mm) of the steel plate) of 0.50 to 3.00.

[0130] Below, the steel manufacturing method of the present invention is described in detail.

[0131] A steel plate according to one embodiment of the present invention can be manufactured by reheating a steel slab satisfying the above-described alloy composition, finishing hot rolling, cooling, coiling, hot annealing heat treatment, cold rolling, first heating and holding, first cooling, second heating and holding, and second cooling.

[0132] Reheating

[0133] A steel slab satisfying the alloy composition of the present invention can be reheated in a temperature range of 1000 to 1350°C.

[0134] If the reheating temperature is below 1000℃, there is a possibility that hot rolling will occur below the final hot rolling temperature range. In addition, if the heating temperature exceeds 1350℃, there is a possibility that the steel will reach its melting point and melt.

[0135] Finishing hot rolling

[0136] The above reheated steel slab can be finished by hot rolling at a temperature range of 800 to 1000°C.

[0137] If the above finishing hot rolling temperature is below 800℃, the high strength of the steel may place a great burden on the rolling mill. In addition, if the finishing hot rolling temperature exceeds 1000℃, the grain size of the steel sheet after hot rolling may become coarse, which may deteriorate the physical properties of the high-strength steel sheet.

[0138] Cooling and winding

[0139] The above hot-rolled steel sheet can be cooled to a temperature range of 300 to 600°C at an average cooling rate of 10°C / s or more and then coiled.

[0140] If the coiling temperature is lower than 300°C, coiling may not be easy. On the other hand, if the coiling temperature exceeds 600°C, scale generated on the surface of the hot-rolled steel sheet may form inside the steel sheet, making pickling difficult.

[0141] If the average cooling rate is less than 10°C / s, hot rolling productivity may be reduced, and during production, there may be a disadvantage in that a cooling medium with low cooling capacity must be deliberately adopted. Meanwhile, the upper limit of the average cooling rate is not particularly limited, but according to one embodiment of the present invention, it may be 100°C / s or less.

[0142] Hot rolling annealing heat treatment

[0143] The above-mentioned rolled steel plate can be heat-treated by hot rolling annealing at a temperature range of 500 to 750°C for 600 to 1700 seconds.

[0144] After the above winding, a hot rolling annealing heat treatment process can be performed to facilitate acid washing and cold rolling.

[0145] If the hot-rolling annealing heat treatment temperature is less than 500°C or is performed for less than 600 seconds, the strength of the hot-rolling annealing heat treatment steel sheet may be high, making cold rolling difficult. According to one embodiment of the present invention, it may be 530°C or higher. On the other hand, if the hot-rolling annealing heat treatment temperature exceeds 750°C or is performed for more than 1700 seconds, pickling may not be easy due to scales that are deeply formed inside the steel sheet. According to one embodiment of the present invention, it may be 730°C or lower. According to one embodiment of the present invention, it may be 700°C or lower.

[0146] According to one embodiment of the present invention, before cold rolling is performed after hot rolling annealing, pickling may be performed to remove scale formed on the surface of the steel sheet.

[0147] cold rolling

[0148] The above hot-rolled and annealed steel plate can be cold-rolled at a cumulative reduction ratio of 30 to 90%.

[0149] The present invention does not specifically limit the cold rolling conditions, and the cold rolling can be performed at a cumulative reduction ratio of 30 to 90%. When cold rolling, if the cumulative reduction ratio exceeds 90%, it may be difficult to perform cold rolling in a short period of time due to the high strength of the steel sheet. On the other hand, if the cumulative reduction ratio is less than 30%, it may be difficult to secure the desired thickness.

[0150] Primary heating and maintenance

[0151] The above cold-rolled steel sheet can be heated to a temperature range of Ac1 or higher at an average heating rate of 10°C / s or higher in an atmosphere of a dew point temperature of -15 to 30°C and maintained for 120 seconds or longer.

[0152] In order to secure high strength and excellent ductility and processability according to the present invention, an annealing heat treatment process can be performed.

[0153] If the above first heating and holding temperature is less than Ac1, ferrite may be formed, and bainite, retained austenite, and tempered martensite may not be sufficiently formed, making it impossible to secure the desired microstructure and properties.

[0154] Additionally, if the holding time is less than 120 seconds, the tissue may not be sufficiently homogenized, which may result in a deterioration in the physical properties of the steel plate.

[0155] The upper limit of the first heating temperature and the upper limit of the first holding time are not particularly limited, but in order to suppress the decrease in toughness due to grain coarsening, the heating temperature may be limited to 950°C or less and the holding time may be limited to 1200 seconds or less according to one embodiment of the present invention.

[0156] According to one embodiment of the present invention, during primary heating and maintenance, the dew point temperature can be limited to -15 to 30°C.

[0157] Internal oxidation of steel sheets can be achieved by controlling the dew point temperature (Dew Point). This internal oxidation can secure plating properties and light-mechanical etching (LME) resistance, and can be beneficial for improving the R / t ratio by introducing a soft ferrite phase in the polar surface layer. Furthermore, the micro-voids formed in the polar surface layer through internal oxidation serve as hydrogen trap sites, effectively improving hydrogen embrittlement resistance.

[0158] If the dew point temperature is below -15℃, the amount of oxygen flowing into the steel decreases, which only aggravates surface oxidation and does not cause internal oxidation, so there is a concern that a large amount of oxides will exist on the surface and the above-mentioned beneficial effects cannot be obtained. According to one embodiment of the present invention, the dew point temperature may be -10℃ or higher. On the other hand, if the temperature exceeds 30℃, internal oxidation increases, which suppresses the diffusion of alloying elements and increases the effect of suppressing surface oxidation, but the amount of water vapor supplied increases rapidly, so the capacity of the humidifying equipment must be unnecessarily large, and the cooled water vapor may condense and be applied for a long period of time in continuous annealing, which may cause equipment problems. According to one embodiment of the present invention, the dew point temperature may be 20℃ or lower.

[0159] In addition, according to one embodiment of the present invention, in order to obtain a significant effect during internal oxidation, the hydrogen concentration in the atmosphere gas may be included at 1% or more in volume% during annealing. If the hydrogen concentration is less than 1%, the trace amount of oxygen inevitably included in the H2 and N2 gases cannot be effectively oxidized and removed, which may cause the oxygen partial pressure to increase and cause surface oxidation of the steel sheet. On the other hand, if the hydrogen concentration exceeds 70%, the risk of explosion and the cost of high-hydrogen work increase in the event of a gas leak, and therefore the hydrogen concentration may be limited to 70% or less. Except for the hydrogen, the hydrogen may be substantially nitrogen, excluding impurity gases inevitably included.

[0160] Primary cooling

[0161] The above-mentioned primary heated and maintained steel plate can be primary cooled to a temperature range of 100 to 300°C at an average cooling rate of 20.0°C / s or more.

[0162] In the present invention, for productivity, the average cooling rate during the primary cooling may be limited to 20.0°C / s or more. The upper limit of the primary cooling rate need not be specifically defined, and may be limited to 100.0°C / s or less.

[0163] When the primary cooling is performed at a final temperature below 100°C, excessive tempered martensite may be formed and insufficient retained austenite may result in failure to satisfy the product of tensile strength and elongation, which may result in reduced bending workability. On the other hand, when the primary cooling is performed at a final temperature above 300°C, excessive bainite may be formed and insufficient tempered martensite may be formed, which may reduce the product of tensile strength and elongation of the steel sheet.

[0164] Secondary heating and maintenance

[0165] The above-mentioned primary cooled steel plate can be heated to a temperature range of 300 to 500°C at an average heating rate of 15°C / s or more and maintained for 300 seconds or more.

[0166] In the present invention, heating can be performed at an average heating rate of 15°C / s or higher for productivity. The upper limit of the heating rate need not be specifically defined, and can be limited to 100°C / s or lower.

[0167] During the secondary heating and holding, if the temperature is less than 300°C or the holding time is less than 300 seconds, the tempered martensite fraction may be excessive, and it is difficult to secure the retained austenite fraction. As a result, the TS×El and bending workability of the steel sheet may be reduced. On the other hand, if the temperature exceeds 500°C or the holding time is excessive, it may be difficult to secure a sufficient retained austenite fraction. As a result, the TS×El of the steel sheet may be reduced. According to one embodiment of the present invention, the upper limit of the secondary holding time may be limited to 17200 seconds.

[0168] Secondary cooling

[0169] The above-mentioned second heating and maintenance steel plate can be secondarily cooled to room temperature at an average cooling rate of 1°C / s or more.

[0170] In the present invention, for productivity, the average cooling rate during secondary cooling may be limited to 1°C / s or more. The upper limit of the primary cooling rate need not be specifically defined, and may be limited to 100°C / s or less.

[0171] Cold-rolled steel sheets can be manufactured into uncoated cold-rolled steel sheets through an annealing heat treatment process, or can be manufactured into coated steel sheets through a plating process to impart corrosion resistance. According to one embodiment of the present invention, plating can be applied using a plating method such as hot-dip galvanizing, electrogalvanizing, or hot-dip aluminum plating, and the method and type thereof are not particularly limited.

[0172] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and explain the present invention in more detail and are not intended to limit the scope of the present invention.

[0173] (Example)

[0174] A steel slab having a thickness of 100 mm having the alloy composition described in Table 1 below was manufactured, heated at 1200°C, then final hot-rolled at 900°C, cooled at an average cooling rate of 30°C / s, and coiled at 450 to 550°C to manufacture a hot-rolled steel sheet having a thickness of 3 mm. Subsequently, the hot-rolled steel sheet was subjected to hot-rolling annealing heat treatment under the conditions described in Table 2. Thereafter, the surface scale was removed by pickling, and then cold-rolled to a thickness of 1.5 mm. In addition, first heating and holding, first cooling, second heating and holding, and second cooling were performed under the conditions described in Table 2 below. At this time, during the first heating that was not initiated, the average heating rate was applied equally at 10°C / s. In addition, during the second heating, the average heating rate was 15°C / s, and during the second cooling, the average cooling rate was applied equally at 10°C / s.

[0175] Steel alloy composition (weight %) CSiMnPSAlNBCrMoOther A0.391.982.130.0110.00080.020.00320.00180.51--B0.382.032.210.010.00130.020.00280.00160.230.18-C0.371.951.880.010.00100.020.00290.0020-0.47-D0.332.313.950.0090.00120.030.00300.0022-0.49-E0.411.852.060.0080.00090.030.00310. 0017---F0.521.682.330.0090.00080.020.00270.0021---G0.721.642.4 10.0120.00110.020.00340.0015---H0.380.872.110.0110.0011.930.003 30.0020---I0.361.082.070.0110.00132.350.00310.0020---J0.350.02 1.950.010.00104.670.00300.0019--Ti:0.05K0.431.741.930.0080.0011 0.020.00350.0022--Nb:0.05L0.411.891.880.0090.00110.020.00280.0 023--V:0.05M0.391.751.920.0110.00120.020.00270.0016--Ni:0.36N0. 381.892.180.0120.00130.030.00240.0017--Cu:0.35O0.381.682.220.0 130.00070.030.00280.0030--B:0.003P0.361.882.260.0120.00080.020. 00260.0023--Ca:0.002Q0.371.842.370.0080.00090.020.00310.0020-- REM:0.001R0.441.732.450.0090.00090.020.00310.0015--Mg:0.001S0.4 21.772.380.010.0010.020.00340.0020--W:0.11T0.311.952.190.010.0 0110.020.00330.0019--Zr:0.1U0.321.982.030.0090.00130.030.00320.0016--Sb:0.02V0.391.822.410.0080.00120.020.00300.0016--Sn:0.02W0.361.782.260.0090.00120.020.00270.0019--Y:0.01X0.373.642.140.0090.00070.030.00290.0015--Hf:0.01Y0.372.272.180.0110.00070.030.00280.0023--Co:0.35XA0.211.922.050.0110.00080.030.00240.0020---XB0.781.942.110.0080.00110.020.00310.0015---XC0.390.022.160.0120.00120.030.00270.0017---XD0.384.262.070.0120.00090.020.00320.0019---XE0.400.032.310.0080.00105.310.00260.0022---XF0.411.840.750.0090.00100.020.00330.0017---XG0.381.885.640.0110.00120.020.00310.0019---.

[0176] Specimen number Steel grade Coiling heat treatment 1st heating and holding 1st cooling 2nd heating and holding Temperature (℃) Temperature (℃) Time (s) Dew point temperature (℃) Temperature (℃) Time (s) Average cooling rate (℃ / s) End temperature (℃) Temperature (℃) Time (s) 1A500590120015.388012020.01804003002A5009001000 Poor pickling 3A5004801300 Fracture occurred during cold rolling 4A4507501800 Poor pickling 5A500750500 During cold rolling Fracture occurrence 6A500750150015.373012020.02204003007A5507501200-44.6880120.02004003008A500750120014.38801200.52004003009B5006101300-45.288012020.025040030010B5006101000-43.788012020.013035060011B55061080015 .188012020.027045030012C500610100014.488012020.022040030013C5007501200-42.688012020.07040030014C4 50750110015.488012020.033040030015C5007001100-44.888012020.021027030016C550750100015.688012020.021 053030017C5007501300-44.888012020.01804004018C500750150015.588012020.018040017019D500590160015.78 8012020.018040030020E50059090014.988012020.018040030021F550590100015.288012020.020040030022G450620 1700-45.288012020.020035030023H500620120015.788012020.020040060024I45060060015.188012020.02004003 0025J500630140014.888012020.022040030026K50063010001588012020.022040030027L500600120015.688012020.022045030028M550610150014.988012020.022040060029N500610110014.788012020.022040030030O5006301 50015.588012020.018040030031P450620130015.388012020.018040030032Q450590120014.688012020.01803 5030033R500590120015.488012020.018040030034S500650140015.688012020.018040060035T500610120015 .888012020.020040030036U550630160014.688012020.020040030037V500620110015.488012020.0200450300 38W450610120015.888012020.020040030039X500590120015.688012020.020040060040Y45059090014.98801 2020.022040030041XA500750150014.188012020.022040030042XB5007501300-43.288012020.022040030043X C500700110014.688012020.022040030044XD5507501400-44.788012020.022040030045XE5007501200-45.488 012020.020040030046XF500700160014.788012020.020040030047XG4507501700-41.588012020.0200400300.

[0177] The microstructure of the steel plate manufactured in this way was observed, the physical properties were measured, and the results are shown in Table 3 below.

[0178] Among the microstructures, ferrite (F), bainite (B), tempered martensite (TM), and pearlite (P) were observed through SEM after etching the polished specimen cross-section with nital. Among them, bainite and tempered martensite, which are difficult to distinguish, were calculated in fraction using the expansion curve after dilatation evaluation. Meanwhile, since fresh martensite (FM) and retained austenite (RA) are also difficult to distinguish, the fraction of fresh martensite was determined by subtracting the fraction of retained austenite calculated by X-ray diffraction from the fraction of martensite and retained austenite observed by SEM.

[0179] In the case of micro-voids, a cross-section was observed in Compo mode within a certain range with a depth of 35 μm and a width of 5 μm from the surface of the steel plate toward the center of the thickness using a scanning electron microscope (SEM), and the number of voids with a diameter of 0.3 μm or more was measured.

[0180] In addition, the product of tensile strength and elongation was calculated and presented, and R / t and hydrogen embrittlement characteristics were measured and presented.

[0181] The product of the above tensile strength and elongation (TS×El) and R / t were evaluated by tensile tests and V-bending tests. The tensile test was performed using specimens taken at 90° to the rolling direction of the rolled plate in accordance with JIS No. 5, and TS×El was determined. R / t was determined by dividing the minimum bending radius R at which no cracks occur after a 90° bending test using specimens taken at 90° to the rolling direction of the rolled plate by the thickness t of the plate.

[0182] Hydrogen embrittlement resistance was judged to be excellent if no cracks or fractures occurred within 96 hours when a load was applied according to ASTM G39 standard and immersed in a 0.1 N HCl solution.

[0183] Specimen number Steel grade Microstructure Property Classification Fraction (area %) Microvoid number (pieces) TS × El (MPa %) R / t Hydrogen embrittlement FBTMFMRAP1A02156122017302561.69◎ Invention example 12A Pickling defect Comparative example 13A Fracture occurrence during cold rolling Comparative example 24A Pickling defect Comparative example 35A During cold rolling Comparative Example of Fracture Occurrence 46A3341016117135381.75◎Comparative Example 57A21857950-281044.82-Comparative Example 68A141158131322214622.51◎Comparative Example 79B021610180-298101.85-Comparative Example 810B016630210-325531.92-Comparative Example 911B0255511907271271.85◎Invention Example 212C02951218020315412.14◎Invention Example 313C0293050-179436.47 -Comparative Example 1014C0764119015216832.75◎Comparative Example 1115C01578250-116708.66-Comparative Example 1216C0246718021200422.51◎Comparative Example 1317C01477270-182608.24-Comparative Example 1418C029624508217102.87-Comparative Example 1519D0225402407247562.38◎Invention Example 420E01468018022323131.82◎Invention Example 521F02553121029309 301.76◎Invention Example 622G041352220-277592.83-Comparative Example 1623H0235112509248482.05◎Invention Example 724I01956124010287982.34◎Invention Example 825J02158021024256931.78◎Invention Example 926K0245901703310681.92◎Invention Example 1027L01566118013286882.74◎Invention Example 1128M01763020011243002.31◎Invention Example 1229N0196 1119024270922.06◎Invention Example 1330O0295411605278871.88◎Invention Example 1431P02555119026280811.96◎Invention Example 1532Q02157220014269512.05◎Invention Example 1633R01553032012320382.81◎Invention Example 1734S0265212108291572.55◎Invention Example 1835T02656018013313432.53◎Invention Example 1936U02455219018248272.68◎Invention Example 2037V0215702202285972.07◎Invention Example 2138W02059021017254302.46◎Invention Example 2239X0255502007302642.15◎Invention Example 2340Y02358118019315441.68◎Invention Example 2441XA01871011022196942.41◎Comparative Example 1742XB016241446 0-208718.47-Comparative Example 1843XC0296911010105224.28◎Comparative Example 1944XD0154123210-280057.25-Comparative Example 2045XE0224318170-275136.86-Comparative Example 2146XF02463067-155322.83-Comparative Example 2247XG0125015230-231646.37-Comparative Example 23.

[0184] * F: Ferrite, B: Bainite, TM: Tempered Martensite, FM: Fresh Martensite, RA: Retained Austenite, P: Pearlite As shown in Table 3 above, in the case of the invention examples satisfying the conditions of the present invention, the microstructure characteristics proposed in the present invention were satisfied, and the physical properties targeted by the present invention were also secured.

[0185] On the other hand, Comparative Examples 1 to 4 are examples in which the alloy composition range proposed by the present invention is satisfied, but the temperature and time during hot-rolling annealing heat treatment are outside the scope of the present invention. As a result, pickling defects occurred, and fractures occurred during cold rolling.

[0186] In Comparative Example 5, the temperature was low during the first heating and holding, resulting in excessive ferrite formation and insufficient bainite and tempered martensite fractions. In addition, the target tensile strength and elongation product range was not achieved.

[0187] In Comparative Example 6, the structure became uneven due to insufficient holding time during the first heating and holding, resulting in excessive ferrite formation and insufficient bainite and retained austenite fractions. As a result, the desired level of physical properties could not be achieved.

[0188] In Comparative Example 7, the average cooling rate was low during the first cooling, resulting in excessive ferrite formation and insufficient retained austenite fraction. As a result, the product of tensile strength and elongation was inferior.

[0189] Comparative Examples 8, 9, and 16 are examples in which the dew point temperature during primary heating and maintenance did not satisfy the conditions of the present invention. As a result, micro-voids were not formed, and hydrogen embrittlement characteristics were poor.

[0190] Comparative Examples 10 and 11 are examples that fall outside the end temperature range during the first cooling proposed by the present invention. In Comparative Example 10, the end temperature was low during the first cooling, resulting in excessive formation of tempered martensite and insufficient fraction of retained austenite. As a result, the product of tensile strength and elongation was not achieved. In Comparative Example 11, the end temperature was high during the first cooling, resulting in excessive formation of bainite and insufficient formation of tempered martensite. As a result, the product of tensile strength and elongation was not achieved at the proposed level.

[0191] Comparative Examples 12 and 13 are examples where the temperature during secondary heating and holding was outside the scope of the present invention. As a result, retained austenite was not formed within the desired range, and thus the desired tensile strength-elongation product could not be achieved. In particular, in the case of Comparative Example 12, tempered martensite was excessively formed, resulting in an R / t exceeding 3.0.

[0192] Comparative Examples 14 and 15 are examples of insufficient or excessive holding times during secondary heating and holding. In Comparative Example 14, excessive tempered martensite was formed and the retained austenite fraction was insufficient, failing to secure the desired physical properties. Comparative Example 15 also failed to secure the desired level of physical properties due to the insufficient retained austenite fraction.

[0193] Comparative Examples 17 to 23 satisfy the manufacturing conditions proposed in the present invention, but the alloy composition deviates from these conditions. In these cases, none of the properties proposed in the present invention are satisfied. In particular, in Comparative Example 19, the combined Si and Al contents are less than 1.0%, and it can be confirmed that neither the retained austenite fraction nor the physical properties are satisfied.

[0194] While the present invention has been described in detail through examples above, other embodiments are possible. Therefore, the technical spirit and scope of the claims set forth below are not limited to the examples.

Claims

1. Contains, in weight%, C: 0.25 to 0.75%, Si: 4.00% or less, Mn: 0.90 to 5.00%, Al: 5.00% or less, P: 0.150% or less, S: 0.0300% or less, N: 0.0300% or less, B: 0.0001 to 0.0050%, the remainder being Fe and unavoidable impurities. The microstructure contains, by area %, 30–75% tempered martensite and 10–40% retained austenite. Unit area 175 μm in the area from the surface to the center of thickness 35 μm 2 Cold rolled steel sheet having at least one micro-void with a diameter of 0.3 μm or more.

2. In claim 1, The above unit area is a cold rolled steel plate having a depth of 35 μm and a width of 5 μm from the surface to the center of thickness.

3. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet further comprising at least one of the following (1) to (8). (1) At least one of Ti: 0.5% or less, Nb: 0.5% or less, and V: 0.5% or less (2) Cr: 3.0% or less and Mo: 3.0% or less, at least one of these (3) Cu: 4.5% or less and Ni: 4.5% or less, at least one of these (4) Ca: 0.05% or less, REM excluding Y: 0.05% or less, and Mg: 0.05% or less, at least one of these (5) W: 0.5% or less and Zr: 0.5% or less, at least one of these (6) Sb: 0.5% or less and Sn: 0.5% or less, at least one of these (7) Y: 0.2% or less and Hf: 0.2% or less, at least one of these (8) Co: 1.5% or less 4. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a total Si and Al content of 1.00 to 6.00%.

5. In claim 1, The above microstructure is a cold rolled steel sheet containing, in area %, 30 to 75% tempered martensite, 10 to 50% bainite, 10 to 40% retained austenite, 5% or less ferrite, and unavoidable structures.

6. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a product of tensile strength and elongation (TS × El) of 22,000 MPa% or more, and R / t (R is the minimum bending radius (mm) at which no crack occurs after a 90° bending test, and t is the thickness of the steel sheet (mm)) of 0.50 to 3.

00.

7. A step of reheating a steel slab containing, by weight%, C: 0.25 to 0.75%, Si: 4.00% or less, Mn: 0.90 to 5.00%, Al: 5.00% or less, P: 0.150% or less, S: 0.0300% or less, N: 0.0300% or less, B: 0.0001 to 0.0050%, the remainder being Fe and unavoidable impurities; A step of final hot rolling the above reheated steel slab; A step of coiling the above-mentioned hot-rolled steel sheet after cooling it; The above rolled steel plate is subjected to a hot-rolled annealing heat treatment step at a temperature range of 500 to 750°C for 600 to 1700 seconds; A step of cold rolling the above hot-rolled and annealed steel sheet; A first heating and maintaining step of heating the cold-rolled steel sheet to a temperature range of Ac1 or higher in an atmosphere having a dew point temperature of -15 to 30°C and then maintaining it for 120 seconds or longer; A step of first cooling the above-mentioned first heated and maintained steel plate to a temperature range of 100 to 300°C at an average cooling rate of 20.0°C / s or more; A second heating and maintaining step of heating the first cooled steel plate to a temperature range of 300 to 500°C and then maintaining it for 300 seconds or longer; and A method for manufacturing a cold rolled steel sheet, comprising: a step of secondarily cooling the second heated and maintained steel sheet to room temperature.

8. In claim 7, A method for manufacturing a cold rolled steel sheet, wherein the steel slab further comprises one or more of the following (1) to (8). (1) At least one of Ti: 0.5% or less, Nb: 0.5% or less, and V: 0.5% or less (2) Cr: 3.0% or less and Mo: 3.0% or less, at least one of these (3) Cu: 4.5% or less and Ni: 4.5% or less, at least one of these (4) Ca: 0.05% or less, REM excluding Y: 0.05% or less, and Mg: 0.05% or less, at least one of these (5) W: 0.5% or less and Zr: 0.5% or less, at least one of these (6) Sb: 0.5% or less and Sn: 0.5% or less, at least one of these (7) Y: 0.2% or less and Hf: 0.2% or less, at least one of these (8) Co: 1.5% or less 9. In claim 7, The above steel slab is a cold rolled steel sheet manufacturing method having a total Si and Al content of 1.00 to 6.00%.

10. In claim 7, The above reheating step is performed at a temperature range of 1000 to 1350℃. The above finishing hot rolling step is performed at a temperature range of 800 to 1000°C; The above-mentioned coiling step cools the above-mentioned finished hot-rolled steel sheet to a temperature range of 300 to 600°C at an average cooling rate of 10°C / s or more, A method for manufacturing cold rolled steel sheets, wherein the above cold rolling step is performed at a reduction ratio of 30 to 90%.

11. In claim 7, The above first heating and maintenance step heats at an average heating rate of 10℃ / s or more, The above second heating and maintenance step heats at an average heating rate of 15℃ / s or more, A method for manufacturing a cold rolled steel sheet, wherein the second cooling step is performed at an average cooling rate of 1℃ / s or more.