Cold-rolled steel sheet and manufacturing method therefor

Abstract

The purpose of one aspect of the present invention is to provide a cold-rolled steel sheet and a manufacturing method therefor. The purpose of one preferred aspect of the present invention is to provide an ultra-high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance and bendability, and a manufacturing method therefor.

Description

Cold-rolled steel sheet and method of manufacturing the same

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same.

[0002] For steel materials primarily used as reinforcement components related to the crash safety of automobile passengers, it is required to have excellent processing characteristics, particularly bending characteristics, and ultra-high strength. Accordingly, ultra-high strength steel with a tensile strength of 1470 MPa or higher, manufactured using a single martensitic phase, is already being mass-produced and applied as an automotive reinforcement material.

[0003] Recently, the Hot Press Forming (HPF) method has been developed, in which a material is formed using a die at a high temperature—an environment conducive to forming—and then water-cooled to secure the required strength. Since the HPF method can secure high strength relative to the same thickness, it is widely used in the manufacture of parts; however, the HPF method has the disadvantage of requiring excessive equipment investment and increased process costs. Therefore, there is a need to develop materials for cold stamping and roll forming. Specifically, there is a need to develop cold-rolled steel sheets that are suitable for use in cold stamping and roll forming, possess ultra-high strength of 1700 MPa or higher and a high yield ratio to secure crash performance for the protection of passengers and electric vehicle batteries, and have excellent bending characteristics and shape for forming parts.

[0004] Meanwhile, the introduction of martensite is essential for manufacturing ultra-high-strength steel with a tensile strength of 14,700 MPa or higher. Such steel is prone to brittle fracture caused by hydrogen remaining within or introduced from the outside; this phenomenon is referred to as hydrogen embrittlement. Hydrogen embrittlement causes material failure at a strength lower than the fracture strength, meaning the material can fracture due to hydrogen embrittlement even when a very small stress is applied compared to the actual fracture strength. In particular, sensitivity to hydrogen embrittlement increases as the strength of the steel increases. Furthermore, since resistance to hydrogen embrittlement improves with superior bending characteristics even with the same initial hydrogen content, it is necessary to improve the bending properties of the steel.

[0005] One aspect of the present invention is to provide a cold-rolled steel sheet and a method for manufacturing the same.

[0006] A preferred aspect of the present invention is to provide an ultra-high strength cold-rolled steel sheet with excellent hydrogen embrittlement resistance and bendability, and a method for manufacturing the same.

[0007] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0008] One embodiment of the present invention comprises, in weight%, C: 0.150~0.30%, Si: 0.50~2.0%, Mn: 2.0~3.50%, and the remainder being Fe and other unavoidable impurities; the microstructure comprises, in area%, tempered martensite: 80% or more, and one or more of ferrite and bainite: 20% or less, and the moisture content of precipitates having an average size of 100 nm or more within the tempered martensite is 20 particles / ㎛ 2 A cold-rolled steel sheet having a yield strength of 1000 MPa or more is provided.

[0009] The above cold-rolled steel sheet may additionally contain one or more of Al: 0.10% or less, Cr: 1.0% or less, Mo: 0.50% or less, Ti: 0.10% or less, Nb: 0.10% or less, B: 0.0050% or less, P: 0.030% or less, S: 0.010% or less, N: 0.010% or less, Cu: 0.0030~0.30%, and Ni: 0.0030~0.30%.

[0010] The following relationship 1 can be satisfied.

[0011] [Equation 1] X = -452 + 3161C + 130Si + 407Mn + 175Cr + 585Mo ≥ 1470

[0012] (However, in the above Equation 1, Cr and Mo are each considered to be 0% if not added.)

[0013] The above cold-rolled steel sheet may have a Si / Mn ratio of 0.180 to 0.60.

[0014] The above microstructure may additionally contain 10% or less of retained austenite.

[0015] Another embodiment of the present invention provides a cold-rolled steel sheet comprising a base material and a decarburization region, wherein the base material comprises, in weight percent, C: 0.150~0.30%, Si: 0.50~2.0%, Mn: 2.0~3.50%, the remainder being Fe and other unavoidable impurities, and the decarburization region comprises a first decarburization region and a second decarburization region, wherein the depth of the first decarburization region is 5~70 relative to the total thickness of the decarburization region, and the yield strength is 1000 MPa or higher.

[0016] The above base material may additionally include one or more of Al: 0.10% or less, Cr: 1.0% or less, Mo: 0.50% or less, Ti: 0.10% or less, Nb: 0.10% or less, B: 0.0050% or less, P: 0.030% or less, S: 0.010% or less, N: 0.010% or less, Cu: 0.0030~0.30%, and Ni: 0.0030~0.30%.

[0017] The above base material can satisfy the following relationship 1.

[0018] [Equation 1] X = -452 + 3161C + 130Si + 407Mn + 175Cr + 585Mo ≥ 1470

[0019] (However, in the above Equation 1, Cr and Mo are each considered to be 0% if not added.)

[0020] The above base material may have a Si / Mn ratio of 0.180 to 0.60.

[0021] The above base material may contain, in terms of area %, tempered martensite: 80% or more, and one or more of ferrite and bainite: 20% or less.

[0022] The above microstructure may additionally contain 10% or less of retained austenite.

[0023] The above first decarburization area may have a total of at least one of ferrite and bainite exceeding 20% ​​of the area, and the above second decarburization area may have a total of at least one of ferrite and bainite less than or equal to 20% of the area.

[0024] The above cold-rolled steel sheet may further include an internal oxidation region above the decarburization region.

[0025] The above internal oxidation region may have a Si / Mn ratio of 0.250 to 0.90.

[0026] The ratio (Ra / Rb) of Si / Mn(Ra) of the internal oxidation region and Si / Mn(Rb) of the base material may be 1.0 or greater.

[0027] The above internal oxidation region includes a coarse structure and a fine structure formed by a plurality of grain boundaries, the coarse structure has an average diameter of 1 μm or more, and the fine structure has an average diameter of less than 1 μm, and the region where the fine structure is formed may be located closer to the surface side of the steel plate than the region where the coarse structure is formed.

[0028] Cold-rolled steel sheets according to one embodiment and another embodiment of the present invention may have a residual H content of 0.20 ppm or less.

[0029] A cold-rolled steel sheet according to one embodiment and another embodiment of the present invention may additionally include a plating layer on at least one surface.

[0030] A cold-rolled steel sheet according to one embodiment and another embodiment of the present invention may have a tensile strength of 1470 MPa or more, an elongation of 4.0% or more, and an R / t of 4.0 or less.

[0031] Another embodiment of the present invention provides a method for manufacturing a cold-rolled steel sheet comprising, in weight percent, C: 0.150~0.30%, Si: 0.50~2.0%, Mn: 2.0~3.50%, the remainder being Fe and other unavoidable impurities; a step of continuously annealing the cold-rolled steel sheet at 800~900℃ with a dew point temperature of -10~25℃; a step of first cooling the annealed cold-rolled steel sheet; a step of second cooling the first-cooled cold-rolled steel sheet to a cooling end temperature of 400~600℃ and maintaining it for 500 seconds or less; a step of third cooling the maintained cold-rolled steel sheet; and a step of post-heat treating the third-cooled cold-rolled steel sheet at 150~500℃.

[0032] The step of preparing the cold-rolled steel sheet may include: a step of heating a slab at 1100~1300℃; a step of finishing hot-rolling the heated slab at 850~950℃ to obtain a hot-rolled steel sheet; a step of coiling the hot-rolled steel sheet at 400~650℃; and a step of cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet.

[0033] After the above-mentioned coiling step and before the step of obtaining the above-mentioned cold steel sheet, the method may additionally include a step of heat-treating the coiled hot-rolled steel sheet at 500 to 650°C for 1,800 to 100,000 seconds.

[0034] The step of obtaining the above cold-rolled steel sheet can be carried out with a cold reduction rate of 20 to 70%.

[0035] The above continuous annealing step can be performed for 30 to 120 seconds.

[0036] The above first cooling step can be carried out at an average cooling rate of 10℃ / s or less until the cooling end temperature of 500~700℃.

[0037] The above secondary cooling step can be performed at an average cooling rate of 5 to 50℃ / s.

[0038] After the above-mentioned maintaining step, the maintained cold-rolled steel sheet may have a residual H content of 0.50 ppm or less.

[0039] After the above-mentioned holding step and before the third cooling step, a step of plating the held cold-rolled steel sheet may be additionally included.

[0040] The above third cooling step can be carried out at an average cooling rate of 1 to 100℃ / s until the cooling end temperature of 10℃ to Mf.

[0041] The above post-heat treatment step can be performed for 5 to 100 seconds.

[0042] The above post-heat treatment step can be performed for 1,800 to 100,000 seconds.

[0043] According to one aspect of the present invention, a cold-rolled steel sheet and a method for manufacturing the same can be provided.

[0044] According to a preferred aspect of the present invention, an ultra-high strength cold-rolled steel sheet with excellent hydrogen embrittlement resistance and bendability and a method for manufacturing the same can be provided.

[0045] Figure 1 is a photograph of Comparative Example 5 observed by SEM.

[0046] Figure 2 is a photograph of Invention Example 2 observed with an SEM.

[0047] Figure 3 is a graph related to the C content according to the depth in the thickness direction from the surface of the steel plate for Invention Example 9.

[0048] Figure 4 is a photograph of the surface layer of Invention Example 9 observed with an SEM.

[0049] Figure 5 is a photograph of the surface layer of Invention Example 9 observed with a high-magnification SEM.

[0050] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0051] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0052] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0053] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0054] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0055] Hereinafter, a cold-rolled steel sheet according to an embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. Unless otherwise specified, the alloy composition described below refers to weight percent.

[0056] C: 0.150~0.30%

[0057] The above C is a solid solution strengthening element for improving the strength of steel. If the content of the above C is less than 0.150%, the solid solution strengthening effect is low, which may result in the disadvantage of being difficult to secure the strength targeted by the present invention. If the content of the above C exceeds 0.30%, there may be the disadvantage of inferior spot weldability for welding automotive parts. Therefore, it is advantageous for the content of the above C to have a range of 0.150 to 0.30%. The lower limit of the above C content is more advantageous at 0.155%, and more advantageous at 0.160%. The upper limit of the above C content is more advantageous at 0.28%, more advantageous at 0.26%, and most advantageous at 0.24%.

[0058] Si: 0.50~2.0%

[0059] The above Si is a solid solution strengthening element for improving the strength of steel and is an element that improves bendability by promoting the decarburization reaction of the surface layer of the steel sheet. If the content of the above Si is less than 0.50%, there is a disadvantage in that the above-described effect cannot be obtained. If the content of the above Si exceeds 2.0%, there is a disadvantage in that the strength is weakened by promoting the formation of ferrite during slow cooling after annealing in a heat treatment furnace that has a slow cooling section. Therefore, it is advantageous for the content of the above Si to have a range of 0.50 to 2.0%. The lower limit of the above Si content is more advantageous at 0.55%, and more advantageous at 0.60%. The upper limit of the above Si content is more advantageous at 1.8%, more advantageous at 1.6%, and most advantageous at 1.4%.

[0060] Mn: 2.0~3.50%

[0061] The above Mn is a solid solution strengthening element for martensitic steel and is an element that secures strength by increasing hardenability and suppressing the formation of ferrite during slow cooling in a continuous annealing furnace. If the content of the above Mn is less than 2.0%, there may be a disadvantage in that it is difficult to secure the effects described above. If the content of the above Mn exceeds 3.50%, there may be a disadvantage in that band structures within the microstructure are promoted, resulting in inferior bendability and increased component costs. Therefore, it is advantageous for the content of the above Mn to have a range of 2.00 to 3.50%. The lower limit of the above Mn content is more advantageous at 2.1%, more advantageous at 2.2%, and most advantageous at 2.3%. The upper limit of the above Mn content is more advantageous at 3.3%, more advantageous at 3.1%, and most advantageous at 3.0%.

[0062] The remaining component is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any skilled person in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0063] The cold-rolled steel sheet of the present invention may additionally include one or more of Al: 0.10% or less, Cr: 1.0% or less, Mo: 0.50% or less, Ti: 0.10% or less, Nb: 0.10% or less, B: 0.0050% or less, P: 0.030% or less, S: 0.010% or less, N: 0.010% or less, Cu: 0.0030~0.30%, and Ni: 0.0030~0.30%.

[0064] Al: 0.10% or less

[0065] The above Al is an element that expands the ferrite region. When using a heat treatment furnace with an slow cooling section, it has the disadvantage of promoting ferrite formation and poses a problem that increases the likelihood of causing a decrease in high-temperature hot rolling performance due to AlN formation. If the content of the above Al exceeds 0.10%, the aforementioned disadvantages may occur. Therefore, it is advantageous for the content of the above Al to be in the range of 0.10% or less. It is more advantageous for the above Al content to be 0.08% or less, more advantageous for it to be 0.06% or less, and most advantageous for it to be 0.05% or less. In the present invention, the lower limit of the above Al content is not specifically limited, but as an example, the lower limit may be 0.01%.

[0066] Cr: 1.0% or less

[0067] The above Cr is a hardenable element that facilitates the formation of a martensite structure, and when using a heat treatment furnace with a slow cooling section, it has the advantage of suppressing ferrite formation. If the content of the above Cr exceeds 1.0%, the manufacturing cost increases excessively, and there are disadvantages such as deterioration in surface quality, including chemical treatment properties. Therefore, it is advantageous for the content of the above Cr to be in the range of 1.0% or less. It is more advantageous for the above Cr content to be 0.8% or less, more advantageous for it to be 0.6% or less, and most advantageous for it to be 0.5% or less. In the present invention, the lower limit of the above Cr content is not specifically limited, but as an example, the lower limit may be 0.05%.

[0068] Mo: 0.50% or less

[0069] The above Mo is a hardenable element that facilitates the formation of a low-temperature transformation structure, and when using a heat treatment furnace with a slow cooling section, it has the advantage of suppressing ferrite formation. In addition, it has the effect of delaying austenite grain growth during annealing by suppressing the coarsening of TiC and NbC precipitates. If the content of the above Mo exceeds 0.50%, there may be a disadvantage of excessively increasing manufacturing costs. Therefore, it is advantageous for the content of the above Mo to be in the range of 0.50% or less. It is more advantageous for the above Mo content to be 0.4% or less, more advantageous for it to be 0.3% or less, and most advantageous for it to be 0.2% or less. In the present invention, the lower limit of the above Mo content is not specifically limited, but as an example, the lower limit may be 0.01%.

[0070] Ti: 0.10% or less

[0071] The above Ti is an element that forms nitrides and plays a role in scavenging by precipitating N in the steel as TiN, thereby suppressing the formation of BN and increasing the solid solution B. If the Ti content exceeds 0.10%, there may be a disadvantage in that additional carbides are precipitated in addition to the removal of solid solution N, which reduces the strength of the martensite. Therefore, it is advantageous for the Ti content to be in the range of 0.10% or less. It is more advantageous for the Ti content to be 0.08% or less, more advantageous for it to be 0.07% or less, and most advantageous for it to be 0.06% or less. In the present invention, the lower limit of the Ti content is not specifically limited, but as an example, the lower limit may be 0.01%.

[0072] Nb: 0.10% or less

[0073] The above Nb is an element that contributes to the fineness of the microstructure of the steel sheet by forming carbides and delaying the growth of crystal grains during annealing. If the content of the above Nb exceeds 0.10%, the manufacturing cost increases excessively, and there may be a disadvantage of reduced solid solution C, which is highly effective in increasing the strength of martensite. Therefore, it is advantageous for the content of the above Nb to be in the range of 0.10% or less. It is more advantageous for the above Nb content to be 0.08% or less, more advantageous for it to be 0.07% or less, and most advantageous for it to be 0.06% or less. In the present invention, the lower limit of the above Nb content is not specifically limited, but as an example, the lower limit may be 0.01%.

[0074] B: 0.0050% or less

[0075] The above B is a hardenable element that facilitates the formation of a low-temperature transformation structure, and when utilizing a heat treatment furnace with a slow cooling section, it has the advantage of suppressing ferrite formation. If the content of the above B exceeds 0.0050%, the aforementioned effect becomes saturated, and rather Fe 23There may be a disadvantage that the formation of ferrite is promoted by the precipitation of (C,B)6. Therefore, it is advantageous for the content of B to be in the range of 0.0050% or less. It is more advantageous for the B content to be 0.0045% or less, more advantageous for 0.0040% or less, and most advantageous for 0.0035% or less. In the present invention, the lower limit of the B content is not specifically limited, but as an example, the lower limit may be 0.0010%.

[0076] P: 0.030% or less

[0077] P is an impurity element contained in steel. If the content of P exceeds 0.030%, weldability deteriorates, and it is prone to segregation at grain boundaries, leading to intergranular embrittlement. Furthermore, the grain boundaries are prone to fracture due to hydrogen in the steel, raising concerns about brittleness in the steel. Meanwhile, while it is advantageous for P to be excluded from the steel as much as possible, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is advantageous for the content of P to be 0.030% or less. It is more advantageous for the content of P to be 0.020% or less, and even more advantageous for it to be 0.010% or less.

[0078] S: 0.010% or less

[0079] S is an impurity element included in steel, similar to P. If the content of S exceeds 0.010%, it can impair ductility and weldability, and a large amount of MnS precipitates may be formed, which can lead to inferior bending properties. Meanwhile, although it is advantageous for S not to be included in steel as much as possible, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is advantageous for the content of S to be 0.010% or less. It is more advantageous for the content of S to be 0.0050% or less, and even more advantageous for it to be 0.0030% or less.

[0080] N: 0.010% or less

[0081] N is an impurity element, and if its content exceeds 0.010%, it significantly increases the risk of cracking during continuous casting due to the formation of AlN, etc. Although it is advantageous for the above N not to be included in the steel as much as possible, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is advantageous for the above N content to be in the range of 0.010% or less. It is more advantageous for the above N content to be 0.0080% or less, and even more advantageous for it to be 0.0060% or less.

[0082] Cu: 0.0030~0.30%

[0083] Cu improves corrosion resistance in the operating environment of automobiles and also has the effect of suppressing hydrogen intrusion into the steel sheet by coating the surface of the steel sheet with corrosion products. Furthermore, as an element incorporated when utilizing scrap as a raw material, allowing the incorporation of Cu enables the use of recycled materials as raw materials, thereby reducing manufacturing costs. From this perspective, it is desirable to contain at least 0.0030% Cu, and from the perspective of improving delayed fracture resistance, it is more desirable to contain at least 0.0050% Cu. However, since an excessively high Cu content leads to the occurrence of surface defects, it is desirable to keep the Cu content at 0.30% or less, more desirable at 0.20% or less, and even more desirable at 0.080% or less.

[0084] Ni: 0.0030~0.30%

[0085] Ni, like Cu, is an element that improves corrosion resistance. For this reason, it is desirable to include at least 0.0030% of Ni. However, if the Ni content becomes excessively high, scale formation within the furnace becomes uneven, which can actually cause surface defects. It also leads to increased costs. Therefore, it is desirable to keep the Ni content at 0.30% or less, more desirable at 0.20% or less, and even more desirable at 0.080% or less.

[0086] The cold-rolled steel sheet of the present invention can satisfy the following Equation 1. The following Equation 1 is a compositional relationship formula for securing the strength targeted in the present invention. As the content of the alloying element constituting X increases, the strength of the martensite increases. If the value of X below is less than 1470, it may be difficult to secure the target tensile strength or yield strength. It is more advantageous for the value of X below to be 1480 or higher, more advantageous for 1490 or higher, and most advantageous for 1500 or higher. In the present invention, the upper limit of the value of X below is not specifically limited, but as an example, the upper limit may be 2200.

[0087] [Equation 1] X = -452 + 3161C + 130Si + 407Mn + 175Cr + 585Mo ≥ 1470

[0088] (However, in the above Equation 1, Cr and Mo are each considered to be 0% if not added.)

[0089] The cold-rolled steel sheet of the present invention may have a Si / Mn ratio of 0.180 to 0.60. If the Si / Mn ratio is less than 0.180, it may be difficult to secure the microstructure desired by the present invention. Additionally, it may be difficult to form a second decarburization region within the decarburization region desired by the present invention. If the Si / Mn ratio exceeds 0.60, the hardenability of the base material decreases, and physical properties such as strength, ductility, and bendability may be degraded. It is more advantageous for the lower limit of Si / Mn to be 0.19, and more advantageous for it to be 0.20. It is more advantageous for the upper limit of Si / Mn to be 0.59, and more advantageous for it to be 0.58.

[0090] The microstructure of the cold-rolled steel sheet of the present invention may comprise, in area %, 80% or more of tempered martensite and 20% or less of one or more of ferrite and bainite. The tempered martensite is a structure advantageous for securing strength and bendability. If the fraction of the tempered martensite is less than 80%, there may be a problem in securing the target tensile strength of 1470 MPa. It is more advantageous for the tempered martensite to be 90% or more. Meanwhile, due to the manufacturing process, one or more of ferrite and bainite, which are soft phases relative to the tempered martensite, may inevitably be formed; if the fraction exceeds 20%, it may be difficult to secure the target strength, and it may be difficult to secure the target bendability due to the difference in hardness between the phases with the tempered martensite. The above microstructure may additionally include 10% or less of retained austenite, and the retained austenite is also a structure that is inevitably formed during the manufacturing process. The retained austenite transforms into fresh martensite, which is the structure most susceptible to hydrogen embrittlement during part processing. Accordingly, in order to ensure hydrogen embrittlement resistance, the upper limit of the fraction of the retained austenite can be controlled to 10% in the present invention.

[0091] The moisture content of precipitates with an average size of 100 nm or more in the above bainite and tempered martensite is 20 particles / ㎛ 2 It may be less than or equal to the following: The moisture content of precipitates with an average size of 100 nm or more in the above bainite and tempered martensite is 20 particles / ㎛ 2 If it is less than this value, hydrogen embrittlement and flexibility may be reduced due to coarse precipitates. The moisture content of precipitates with an average size of 100 nm or more is 15 particles / ㎛. 2 It is more advantageous to be less than or equal to 10 particles / ㎛ 2 It is more advantageous to be less than or equal to 5 pieces / ㎛ 2The following is most advantageous. In the present invention, the types of precipitates described above are not specifically limited, but as an example, the precipitates may be one or more of cementite (Fe3C), TiC, TiN, NbC, NbN, and MoC.

[0092] The cold-rolled steel sheet of the present invention may have a residual H content of 0.20 ppm or less. If the residual H content exceeds 0.20 ppm, hydrogen may diffuse into stress concentration areas of welded parts or cut surfaces after manufacturing automotive parts, etc., and hydrogen embrittlement cracks may occur. It is more advantageous for the residual H content to be 0.18 ppm or less, more advantageous for it to be 0.16 ppm or less, and most advantageous for it to be 0.14 ppm or less. In the present invention, the lower limit of the residual H content is not specifically limited, but as an example, the lower limit may be 0.001 ppm.

[0093] The cold-rolled steel sheet of the present invention may additionally include a plating layer on at least one surface. The present invention does not specifically limit the type of plating layer, but as an example, it may be a Zn-based hot-dip plating, a Zn-based alloyed hot-dip plating, an Al-based hot-dip plating, and an Al-based alloyed hot-dip plating.

[0094] The cold-rolled steel sheet of the present invention may have a yield strength of 1000 MPa or more. In addition, the cold-rolled steel sheet of the present invention may have a tensile strength of 1470 MPa or more, an elongation of 4.0% or more, and an R / t of 4.0 or less. In the present invention, the upper limit of the yield strength is not specifically limited, but as an example, the upper limit may be 1400 MPa. In the present invention, the upper limit of the tensile strength is not specifically limited, but as an example, the upper limit may be 2200 MPa. In the present invention, the upper limit of the elongation is not specifically limited, but as an example, the upper limit may be 20%. In the present invention, the lower limit of R / t is not specifically limited, but as an example, the lower limit may be 0.01.

[0095] Hereinafter, a cold-rolled steel sheet according to another embodiment of the present invention will be described.

[0096] A cold-rolled steel sheet according to another embodiment of the present invention comprises a base material and a decarburized region, wherein the base material may comprise, in weight percent, C: 0.150~0.30%, Si: 0.50~2.0%, Mn: 2.0~3.50%, the remainder being Fe and other unavoidable impurities.

[0097] The above base material may additionally include one or more of Al: 0.10% or less, Cr: 1.0% or less, Mo: 0.50% or less, Ti: 0.10% or less, Nb: 0.10% or less, B: 0.0050% or less, P: 0.030% or less, S: 0.010% or less, and N: 0.010% or less.

[0098] The above base material can satisfy the following relationship 1.

[0099] [Equation 1] X = -452 + 3161C + 130Si + 407Mn + 175Cr + 585Mo ≥ 1470

[0100] (However, in the above Equation 1, Cr and Mo are each considered to be 0% if not added.)

[0101] The above base material may have a Si / Mn ratio of 0.180 to 0.60.

[0102] The above base material may contain, in terms of area %, tempered martensite: 80% or more, and one or more of ferrite and bainite: 20% or less.

[0103] The above microstructure may additionally contain 10% or less of retained austenite.

[0104] The above cold-rolled steel sheet may have a residual H content of 0.20 ppm or less.

[0105] At least one surface of the above cold-rolled steel sheet may additionally include a plating layer.

[0106] The above cold-rolled steel sheet may have a yield strength of 1000 MPa or more. In addition, the above cold-rolled steel sheet may have a tensile strength of 1470 MPa or more, an elongation of 4.0% or more, and an R / t of 4.0 or less.

[0107] The above-mentioned related configuration may have the same technical significance as described in the description of the cold-rolled steel sheet according to one embodiment of the present invention.

[0108] The above decarburization region may include a first decarburization region and a second decarburization region. The depth of the first decarburization region may be 5 to 70% of the total thickness of the decarburization region. If the depth of the first decarburization region is less than 5% of the total thickness of the decarburization region, the introduction of a soft layer is insufficient, resulting in a low effect of improving bendability. If the depth of the first decarburization region exceeds 70% of the total thickness of the decarburization region, a soft layer with low strength within the decarburization layer may be predominantly composed, which may result in a disadvantage of reduced strength of the steel plate. The upper limit of the depth of the first decarburization region is more advantageous at 65% of the total thickness of the decarburization region, more advantageous at 60%, and most advantageous at 55%. The depth of the first decarburization region is more advantageous at 6% of the total thickness of the decarburization region, more advantageous at 8%, and most advantageous at 10%.

[0109] The first decarburization region may be formed closer to the surface of the steel plate than the second decarburization region, as decarburization occurs less as it moves from the surface of the steel plate toward the center of the thickness. Additionally, since the hardenability varies by depth depending on the concentration of residual carbon, the composition of the microstructure may vary depending on the depth of the surface layer. The first decarburization region, which is located closer to the surface of the steel plate than the second decarburization region, has a low concentration of residual carbon and thus has low hardenability; accordingly, the total of one or more of ferrite and bainite may exceed 20 area%. The second decarburization region has a lower carbon concentration relative to the base material but has a low degree of decarburization; accordingly, the total of one or more of ferrite and bainite may be 20 area% or less. Although not specifically limited, the second decarburization region may be a region having a C content of 95 weight% or less relative to the average C content of the base material at 1 / 4t (t: steel plate thickness). Meanwhile, the above C content can be measured using GDS (Glow Discharge Spectrometry), etc.

[0110] The cold-rolled steel sheet of the present invention may further include an internal oxidation region above the decarburization region. That is, the internal oxidation region, the first decarburization region, and the second decarburization region may exist in that order from the surface of the steel sheet toward the center of thickness.

[0111] The above internal oxidation region is formed as Si and Mn are concentrated in the form of oxides at the grain boundaries and within the grains in the extreme surface layer of the steel sheet. The Si / Mn ratio of the above internal oxidation region may be 0.250 to 0.90. If the Si / Mn ratio of the above internal oxidation region is less than 0.250, external oxidation occurs instead of internal oxidation on the surface of the steel sheet, which may result in a disadvantage of inferior plating performance of the steel sheet. If the Si / Mn ratio of the above internal oxidation region exceeds 0.90, excessive Si and Mn oxides are generated at the grain boundaries, which may result in a disadvantage of inferior bendability. The lower limit of the Si / Mn ratio of the above internal oxidation region is more advantageous at 0.27, more advantageous at 0.29, and most advantageous at 0.31. The upper limit of the Si / Mn ratio of the above internal oxidation region is more advantageous at 0.88, more advantageous at 0.85, and most advantageous at 0.80.

[0112] The ratio (Ra / Rb) of Si / Mn(Ra) of the internal oxidation region and Si / Mn(Rb) of the base material may be 1.0 or higher. If the ratio (Ra / Rb) is less than 1.0, it implies that the decarburization reaction on the surface of the steel sheet accompanying internal oxidation is insufficient, which may result in a disadvantage of inferior bendability. It is more advantageous for the ratio (Ra / Rb) to be 1.1 or higher, more advantageous for 1.2 or higher, and most advantageous for 1.3 or higher. In the present invention, the upper limit of the ratio (Ra / Rb) is not specifically limited, but as an example, the upper limit may be 3.0.

[0113] The internal oxidation region comprises a coarse structure and a fine structure formed by a plurality of grain boundaries, wherein the coarse structure has an average diameter of 1 μm or more and the fine structure has an average diameter of less than 1 μm, and the region where the fine structure is formed may be located closer to the surface side of the steel plate, where the cooling rate is faster than the region where the coarse structure is formed. By separating the internal oxidation region into such a coarse structure and a fine structure, the propagation of cracks is hindered, thereby improving bendability.

[0114] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0115] First, a cold-rolled steel sheet satisfying the aforementioned alloy composition is prepared. The above-described cold-rolled steel sheet can satisfy the above-described relationship Equation 1.

[0116] The step of preparing the above cold-rolled steel sheet may include the following (1) to (4) processes.

[0117] (1) Step of heating the slab to 1100~1300℃

[0118] If the heating temperature of the above slab is less than 1100℃, there may be a disadvantage in that the load increases rapidly during subsequent hot rolling. If the heating temperature of the above slab exceeds 1300℃, there is a concern that excessive surface scale may be generated, leading to material loss and an increase in energy costs. The lower limit of the heating temperature of the above slab is more advantageous at 1120℃, more advantageous at 1140℃, and most advantageous at 1160℃. The upper limit of the heating temperature of the above slab is more advantageous at 1280℃, more advantageous at 1260℃, and most advantageous at 1240℃.

[0119] (2) A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the heated slab at 850~1000℃

[0120] If the above finishing hot rolling temperature is less than 850°C, there may be a disadvantage in that the hot rolling load increases significantly. If the above finishing hot rolling temperature exceeds 1000°C, surface defects may occur due to scale, and there is a concern that it may cause a shortening of the rolling roll's lifespan. The lower limit of the above finishing hot rolling temperature is more advantageous at 860°C, more advantageous at 870°C, and most advantageous at 880°C. The upper limit of the above finishing hot rolling temperature is more advantageous at 980°C, more advantageous at 960°C, and most advantageous at 940°C.

[0121] (3) Step of coiling the above hot-rolled steel sheet at 400~650℃

[0122] If the above coiling temperature is less than 400℃, the formation of a low-temperature structure is excessive, which increases the strength of the hot-rolled steel sheet and may cause rolling load during subsequent cold rolling. In addition, cooling is required to lower the temperature of the steel sheet, and since processes such as cooling water spraying are required, this may lead to an increase in unnecessary process costs. If the above coiling temperature exceeds 650℃, excessive scale may form on the surface of the hot-rolled steel sheet, causing defects, which is a cause of impeding plating properties. The lower limit of the above coiling temperature is more advantageous at 420℃, more advantageous at 440℃, and most advantageous at 460℃. The upper limit of the above coiling temperature is more advantageous at 640℃, more advantageous at 630℃, and most advantageous at 620℃.

[0123] After the above-mentioned coiling and before the following cold rolling, the above-mentioned coiled hot-rolled steel sheet may additionally include a step of heat-treating the coiled hot-rolled steel sheet at 500 to 650°C for 1,800 to 100,000 seconds. If the heat treatment temperature is less than 500°C, there may be a disadvantage in that the strength reduction effect due to heat treatment is low, leading to an increase in the cold rolling load. If the above-mentioned heat treatment temperature exceeds 650°C, the surface scale of the steel sheet increases due to heat treatment, resulting in poor plating performance and a disadvantage such as an increase in heat treatment costs. The lower limit of the above-mentioned heat treatment temperature is more advantageous at 510°C, more advantageous at 520°C, and most advantageous at 530°C. The upper limit of the above-mentioned heat treatment temperature is more advantageous at 640°C, more advantageous at 630°C, and most advantageous at 620°C. If the above heat treatment time is less than 1,800 seconds, the effect of strength reduction by heat treatment is low, which may result in a disadvantage of increased cold rolling load. If the above heat treatment time exceeds 100,000 seconds, the scale on the steel sheet increases, leading to inferior plating performance and may result in disadvantages such as increased heat treatment costs. The lower limit of the above heat treatment time is more advantageous at 3,600 seconds, more advantageous at 7,200 seconds, and most advantageous at 10,800 seconds. The upper limit of the above heat treatment time is more advantageous at 90,000 seconds, more advantageous at 80,000 seconds, and most advantageous at 70,000 seconds.

[0124] (4) A step of cold-rolling the above-mentioned hot-rolled steel sheet to obtain a cold-rolled steel sheet

[0125] Cold rolling can be performed with a cold reduction rate of 20 to 70%. If the above cold reduction rate is less than 20%, not only is it difficult to secure the target thickness, but it also becomes difficult to correct the shape of the steel sheet. Furthermore, during annealing, the driving force for nucleation and recrystallization of austenite is low, resulting in the formation of coarse austenite and a decrease in the strength of the steel sheet. On the other hand, if the above cold reduction rate exceeds 70%, the likelihood of cold rolling cracks occurring at the edge of the steel sheet increases, and there is a problem of increased cold rolling load. The lower limit of the above cold reduction rate is more advantageous at 25%, more advantageous at 30%, and most advantageous at 35%. The upper limit of the above cold reduction rate is more advantageous at 67%, more advantageous at 66%, and most advantageous at 63%.

[0126] Subsequently, the cold-rolled steel sheet is continuously annealed at 800 to 900°C at a dew point temperature of -10 to 25°C. If the dew point temperature is below -10°C, the internal oxidation and decarburization reactions are insufficient, making it impossible to sufficiently form a first decarburization zone, and consequently, the bendability of the steel sheet cannot be improved. If the dew point temperature exceeds 25°C, an excessive decarburization layer may be formed. In this case, the ratio of the thickness of the decarburization layer, which contains ferrite as the main structure relative to the total thickness of the steel sheet, increases, which may result in the disadvantage of being unable to secure the target strength. The lower limit of the dew point temperature is more advantageous at -8°C, more advantageous at -6°C, and most advantageous at -2°C. The upper limit of the dew point temperature is more advantageous at 23°C, more advantageous at 20°C, and most advantageous at 18°C. If the above continuous annealing temperature is less than 800°C, the austenite reverse transformation may not occur sufficiently, and an excessive ferrite phase may be formed after annealing; in this case, the target strength and bendability cannot be secured. If the above continuous annealing temperature exceeds 900°C, surface oxides may be excessively formed, which may reduce the surface quality and productivity of the steel sheet. In addition, the size of the prior austenite may increase, which may reduce the elongation and strength of the steel sheet. The lower limit of the above continuous annealing temperature is more advantageous at 805°C, more advantageous at 810°C, and most advantageous at 815°C. The upper limit of the above continuous annealing temperature is more advantageous at 890°C, more advantageous at 880°C, and most advantageous at 870°C. The above continuous annealing may be performed for 30 to 120 seconds. If the above continuous annealing time is less than 30 seconds, the austenite reverse transformation may not occur sufficiently, and an excessive amount of ferrite phase may be formed after annealing, in which case the target strength and bendability cannot be secured.If the above continuous annealing time exceeds 120 seconds, surface oxides may be excessively formed, which may lower the surface quality and productivity of the steel sheet, and there is a problem of reduced strength of the steel sheet due to excessive decarburization reaction. The lower limit of the above continuous annealing time is more advantageous at 35 seconds, more advantageous at 40 seconds, and most advantageous at 45 seconds. The upper limit of the above continuous annealing time is more advantageous at 115 seconds, more advantageous at 110 seconds, and most advantageous at 100 seconds.

[0127] Subsequently, the annealed cold-rolled steel sheet is cooled first. The first cooling may be performed at an average cooling rate of 10°C / s or less to a cooling end temperature of 500 to 700°C. If the first cooling end temperature is less than 500°C, there may be a disadvantage in that the bainite phase is excessively formed, making it impossible to secure strength. If the first cooling end temperature exceeds 700°C, there may be a disadvantage in that the ferrite phase is excessively introduced, reducing strength. The lower limit of the first cooling end temperature is more advantageous at 520°C, more advantageous at 540°C, and most advantageous at 560°C. The upper limit of the first cooling end temperature is more advantageous at 690°C, more advantageous at 680°C, and most advantageous at 670°C. If the above first average cooling rate exceeds 10℃ / s, process costs increase due to unnecessary cooling control in the slow cooling section, and there may be disadvantages such as non-uniform plate shape. It is more advantageous for the above first average cooling rate to be 9℃ / s or less, more advantageous for it to be 8℃ / s or less, and most advantageous for it to be 7℃ / s or less. In the present invention, the lower limit of the above first average cooling rate is not specifically limited, but as an example, the lower limit may be 1℃ / s.

[0128] Subsequently, the first-cooled cold-rolled steel sheet is secondarily cooled to a cooling end temperature of 400 to 600°C and maintained for 500 seconds or less. If the second-cooling end temperature and the holding temperature are below 400°C, there may be a disadvantage in that bainite is excessively formed, resulting in reduced strength. If the second-cooling end temperature and the holding temperature exceed 600°C, pearlite is formed, resulting in reduced strength and bendability. The lower limit of the second-cooling end temperature and the holding temperature is more advantageous at 420°C, more advantageous at 440°C, and most advantageous at 460°C. The upper limit of the second-cooling end temperature and the holding temperature is more advantageous at 590°C, more advantageous at 580°C, and most advantageous at 570°C. If the holding time exceeds 500 seconds, a problem may occur in which bainite is excessively formed, resulting in reduced strength. The upper limit of the holding time is more advantageous for 450 seconds, more advantageous for 400 seconds, and most advantageous for 350 seconds. The lower limit of the holding time is not specifically limited, but as an example, the lower limit may be 10 seconds. The secondary cooling may be performed at an average cooling rate of 5 to 50°C / s. If the secondary average cooling rate is less than 5°C / s, there may be disadvantages such as reduced strength due to an increase in the bainite fraction during cooling and inferior productivity as the length of the cooling section becomes unnecessarily long. If the secondary average cooling rate exceeds 50°C / s, there may be disadvantages such as increased process costs due to unnecessary cooling control and deterioration of the plate shape due to rapid cooling. The lower limit of the secondary average cooling rate is more advantageous for 6°C / s and more advantageous for 8°C / s. The upper limit of the above second average cooling rate is more advantageous at 40℃ / s, and more advantageous at 30℃ / s.

[0129] After the above-mentioned maintaining step, the maintained cold-rolled steel sheet may have a residual H content of 0.50 ppm or less.

[0130] After the above-mentioned holding step and before the following third cooling step, a step of plating the held cold-rolled steel sheet may be additionally included. The present invention does not specifically limit the plating method, but as an example, a hot-dip plating method may be used, and after the hot-dip plating, an alloying heat treatment may be performed.

[0131] Subsequently, the maintained cold-rolled steel sheet is cooled a third time. The third cooling can be performed at an average cooling rate of 1 to 100°C / s until a cooling end temperature of 10°C to Mf. If the third cooling end temperature is less than 10°C, there may be disadvantages such as increased process costs due to excessive cooling and inferiority of the sheet shape. If the third cooling end temperature exceeds Mf, post-heat treatment is performed without sufficient martensite formation, resulting in a decrease in the fraction of tempered martensite and a problem of deterioration in bendability. The lower limit of the third cooling end temperature is more advantageous at 15°C, more advantageous at 20°C, and most advantageous at 25°C. The upper limit of the third cooling end temperature is more advantageous at Mf-10°C, more advantageous at Mf-20°C, and most advantageous at Mf-30°C. If the above third average cooling rate is less than 1℃ / s, there may be disadvantages such as reduced strength due to an increase in the bainite fraction during cooling and inferior productivity as the cooling interval becomes longer. If the above third average cooling rate exceeds 100℃ / s, there may be disadvantages such as increased process costs and inferior plate shape due to unnecessary supercooling. The lower limit of the above third average cooling rate is more advantageous at 2℃ / s, and more advantageous at 3℃ / s. The upper limit of the above third average cooling rate is more advantageous at 80℃ / s, more advantageous at 60℃ / s, and most advantageous at 40℃ / s. Meanwhile, the above Mf refers to the temperature at which the martensite transformation is terminated during cooling and can be calculated through Equation 1 below.

[0132] [Equation 1] Mf(°C) = 371 - 412C - 17.4Si - 47.4Mn - 20.9Cr - 17Mo + 49.2Nb + 95Ti + 202B

[0133] Subsequently, the above third-cooled cold-rolled steel sheet is subjected to post-heat treatment at 150 to 500°C. If the post-heat treatment temperature is below 150°C, it may be difficult to secure sufficient yield strength. If the post-heat treatment temperature exceeds 500°C, the moisture content of precipitates with an average size of 100 nm or more within the tempered martensite increases, which may result in insufficient strength or inferior bendability. The lower limit of the post-heat treatment temperature is more advantageous at 160°C, more advantageous at 170°C, and most advantageous at 180°C. The upper limit of the post-heat treatment temperature is more advantageous at 490°C, more advantageous at 480°C, and most advantageous at 470°C.

[0134] The above post-heat treatment may be performed for 5 to 100 seconds. As an example, the above post-heat treatment time may correspond to the case where an IH (Induction Heater) facility is used. If the above post-heat treatment time is less than 5 seconds, there may be a disadvantage of inferior bendability due to insufficient tempering of the martensite. If the above post-heat treatment time exceeds 100 seconds, there may be disadvantages such as inferior bendability due to coarsening of the cementite, increased process costs due to an increase in the line length required for the process, and inferior productivity. The lower limit of the above post-heat treatment time is more advantageous at 6 seconds, more advantageous at 7 seconds, and most desirable at 8 seconds. The upper limit of the above post-heat treatment time is more advantageous at 90 seconds, more advantageous at 80 seconds, and most desirable at 60 seconds.

[0135] The above post-heat treatment may be performed for 1,800 to 100,000 seconds. As an example, the above post-heat treatment time may correspond to the case where a Batch Annealing Furnace (BAF) is used. If the above post-heat treatment time is less than 1,800 seconds, there may be a disadvantage in that the heat treatment in the width and length directions of the coil is not uniform, resulting in material variation within the coil. If the above post-heat treatment time exceeds 100,000 seconds, there may be a disadvantage in that process costs increase. The lower limit of the above post-heat treatment time is more advantageous at 3,600 seconds, more advantageous at 7,200 seconds, and most desirable at 10,800 seconds. The upper limit of the above post-heat treatment time is more advantageous at 90,000 seconds, more advantageous at 80,000 seconds, and most desirable at 70,000 seconds.

[0136] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0137] (Example 1)

[0138] After preparing slabs satisfying the alloy compositions listed in Tables 1 and 2 below, cold-rolled steel sheets were manufactured by heating the slab, finishing hot rolling, coiling, cold rolling, heat treatment, continuous annealing, primary cooling, holding, secondary cooling, and post-heat treatment under the conditions listed in Tables 3 to 5 below. The microstructure, precipitates, residual H content, and mechanical properties of the cold-rolled steel sheets manufactured in this manner were measured, and the results are shown in Table 6 below.

[0139] The types and fractions of the microstructure were measured by taking specimens from 1 / 2t of the steel plate and observing them at 1000x magnification using a scanning electron microscope (SEM). However, the fraction of retained austenite was measured by XRD.

[0140] The moisture content of precipitates with an average size of 100 nm or more in tempered martensite was measured by taking a specimen from 1 / 2t of the steel plate and observing it at 5000x magnification using a Scanning Electron Microscope (SEM).

[0141] The residual H content was measured by using Thermal Desorption Analysis to determine the amount of diffusible hydrogen released at 300°C or lower while heating the steel plate at a heating rate of 100°C / h or less.

[0142] Yield strength, tensile strength, and elongation were measured by processing cold-rolled steel sheets according to JIS standards perpendicular to the rolling direction and then performing a tensile test under conditions of a test speed of 28 mm / min.

[0143] The bendability (R / t) was measured by performing a 90° bending test on the cold-rolled steel sheet and dividing the minimum bending radius (R) value, at which no cracks occur on the surface of the steel sheet, by the thickness of the steel sheet.

[0144] Steel Grade No. Alloy Composition (Wt%) CSI Mn Al Cr Mo Ti Nb 10.26 0.40 2.5 0.03--0.025 -20.28 0.20 2.40.025--0.02 0.02 30.22 1.5 0.2.40.035--0.03 0.03 40.22 1.20 2.7 0.03 0-0.10.03 0.02 50.22 1.402.70.0400.2-0.020-60.201.302.70.0500.30.050.025-70.181.453.00.0300.20.050.040-80.221.402.50.0250.10.050.0300.0390.201.402.70.0300.10.050.030-

[0145] Steel Grade No. Alloy Composition (Wt%) B P S N C uNiSi / Mn X 10.00 25 0.00 8 0.00 10.00 5 --0.16 14 39 20.00 20 0.00 6 0.00 30.00 6 --0.08 14 77 30.00 90.00 5 0.00 30.00 3 --0.63 14 15 40.00 27 0.00 8 0.00 40.00 40.15 0.15 0.44 15 5 750.00290.0060.0020.005--0.52155960.00240.0100.0010.003--0.48153070.00240.0110.00 20.003--0.48159180.00190.0060.0010.005--0.56149090.00220.0080.0020.006--0.521508X = -452 + 3161C + 130Si + 407Mn + 175Cr + 585Mo

[0146] Classification Steel Grade No. Slab Heating Temperature (°C) Finishing Hot Rolling Temperature (°C) Coiling Temperature (°C) Cold Reduction Rate (%) Heat Treatment Temperature (°C) Heat Treatment Time (sec) Comparative Example 1 1 200 900 600 45 -- Comparative Example 2 2 1 21 09 10 61 050 -- Comparative Example 3 3 1 22 09 00 600 50 -- Inventive Example 1 4 1 21 09 30 62 045 -- Inventive Example 2 5 1 19 09 00 59 050 -- Inventive Example 3 5120092060050--Invention Example 46120090059050--Invention Example 57121091060045--Comparative Example 47121090059050--Invention Example 6811809006005055036000Invention Example 7912108905805059043200Comparative Example 59120090060045--Comparative Example 69121090060050--

[0147] Classification Steel Grade No. Dew Point Temperature (°C) Annealing Temperature (°C) Annealing Time (sec) Primary Cooling End Temperature (°C) Primary Average Cooling Rate (°C / s) Secondary Cooling End Temperature / Holding Temperature (°C) Secondary Average Cooling Rate (°C / s) Holding Time (sec) Comparative Example 1 1 2 8 40 60 650 35 50 12 100 Comparative Example 2 2 1 0 8 60 60 640 35 00 12 100 Comparative Example 3 3 1 1 8 30 60 650 34 90 12 100 Invention Example 1 4 1 7 8 40 60 650 45 00 12 100 Invention Example 2 5 1 0 8 40 70 64 0445015150Invention Example 35984060650451010150Invention Example 46785060650349010150Invention Example 57086060640345010150Comparative Example 471078060640345012100Invention Example 681186060640350012100Invention Example 791986070640555012100Comparative Example 591986060650355012100Comparative Example 691586060650335012100

[0148] Classification Steel Grade No. Mf (°C) Tertiary Cooling End Temperature (°C) Tertiary Average Cooling Rate (°C / s) IHBAF Temperature (°C) Time (Sec) Temperature (°C) Time (Sec) Comparative Example 1 1 4 1 2 5 5 20 0 30 -- Comparative Example 2 2 1 3 7 2 5 5 20 0 50 -- Comparative Example 3 3 1 4 7 3 0 5 30 0 10 -- Inventive Example 1 4 1 3 4 3 0 5 -- 2 5 0 7 20 0 0 Inventive Example 2 5 1 2 6 3 0 5 30 0 1 0--Invention Example 3512625530010--Invention Example 4613430530020--Invention Example 5712930540010--Comparative Example 4712930540010--Invention Example 68139255250100--Invention Example 79137255300100--Comparative Example 59137255550100--Comparative Example 69137255300100--

[0149] Classification Microstructure (Area %)™ Moisture content of precipitates with an average size of 100 nm or larger (pieces / ㎛) 2) Residual H Content (ppm) Yield Strength (MPa) Tensile Strength (MPa) Elongation (%) R / t 1 or more of R / T M RAF and B Comparative Example 1 69 328 60.25 105 314 6 18.35.0 Comparative Example 2 64 333 40.229 78 144 14.75.0 Comparative Example 3 8 121 700.119 29 142 37.74.5 Invention Example 1 98 20 20.07 128 1156 96.13.5 Invention Example 2 93 25 30.06 121 0157 48.13.5 Invention Example 3 98 20 50.04 1228 158 47.93.0 Invention Example 4 96 4000.091 24915488.03.0 Invention Example 58731010.08113915108.33.5 Comparative Example 47432340.0894115898.35.5 Invention Example 6953230.06121415347.43.0 Invention Example 7972130.09122115019.12.5 Comparative Example 59721280.09125014509.15.0 Comparative Example 67622280.09132514509.14.5 B: Bainite, TM: Tempered Martensite, RA: Retained Austenite, F: Ferrite

[0150] As can be seen from Tables 1 to 6 above, in the case of Inventive Examples 1 to 7, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, the residual H content, yield strength, tensile strength, elongation, and R / t are excellent as the microstructure desired by the present invention and the moisture content of precipitates with an average size of 100 nm or more in the tempered martensite are secured. In the case of Comparative Example 1, which does not satisfy the C content, Si / Mn, and X values, the residual H content, tensile strength, and R / t are insufficient as the microstructure desired by the present invention is not secured.

[0151] In the case of Comparative Example 2, which does not satisfy the C content, Si / Mn, and X values, it can be seen that the residual H content, yield strength, tensile strength, and R / t are at an insufficient level as the microstructure intended by the present invention is not secured.

[0152] In the case of Comparative Example 3, which does not satisfy the Si / Mn and X values, it can be seen that the yield strength, tensile strength, and R / t are at an insufficient level.

[0153] In the case of Comparative Example 4, which does not satisfy the annealing temperature, it can be seen that the yield strength and R / t are insufficient as the microstructure intended by the present invention is not secured.

[0154] In the case of Comparative Example 5, which does not satisfy the post-heat treatment temperature, it can be seen that the tensile strength and R / t are insufficient because the moisture content of precipitates with an average size of 100 nm or more in the tempered martensite, which is intended to be obtained by the present invention, is not secured.

[0155] In the case of Comparative Example 6, which does not satisfy the second cooling end temperature / holding temperature, it can be seen that the tensile strength and R / t are insufficient as the microstructure intended by the present invention is not secured.

[0156] Figure 1 is a photograph of Comparative Example 5 observed by SEM. Figure 2 is a photograph of Inventive Example 2 observed by SEM. As can be seen from Figures 1 and 2, in the case of Comparative Example 5, coarse cementite in the shape of elongated white grains is excessively formed inside the tempered martensite, whereas in the case of Inventive Example 2, it can be confirmed that less cementite is formed inside the tempered martensite.

[0157] (Example 2)

[0158] After preparing slabs satisfying the alloy compositions listed in Tables 7 and 8 below, cold-rolled steel sheets were manufactured by heating the slab, finishing hot rolling, coiling, cold rolling, heat treatment, continuous annealing, primary cooling, holding, secondary cooling, and post-heat treatment under the conditions listed in Tables 9 to 11 below. The microstructure, decarburization region, internal oxidation region, residual H content, and mechanical properties of the cold-rolled steel sheets manufactured in this manner were measured, and the results are shown in Tables 12 and 13 below.

[0159] The type and fraction of the microstructure of the base material were measured using the method described in Example 1.

[0160] The Si / Mn(Ra) content of the internal oxidation region and the thickness of the decarburization region were measured using GDOES (Glow Discharge Optical Emission Spectrometry).

[0161] The ferrite and bainite fractions in the decarburized region were measured by observing the cross-section of the surface layer of the steel plate at 1000x magnification using a Scanning Electron Microscope (SEM).

[0162] Residual H content, yield strength, tensile strength, elongation, and R / t were measured using the method described in Example 1.

[0163] Steel Grade No. Alloy Composition (Wt%) CSI Mn Al Cr Mo Ti Nb 10.26 0.40 2.5 0.03--0.025-20.28 0.20 2.4 0.025--0.02 0.02 30.22 1.5 0.2 40.03 5--0.03 0.03 40.22 1.20 2.7 0.03 0-0.10.03 0.02 50.22 1.4 0 2.7 0.04 0.2-0.02 0-60.20 1 .302.70.0500.30.050.025-70.181.453.00.0300.20.050.040-80.221.402.50.0250.10.050.0300.0390.201.402.70.0300.10.050.030-100.211.002.90.035-0.050.025-110.280.902.50.030--0.030-

[0164] Steel Grade No. Alloy Composition (Wt%) B P S N C uNiSi / Mn X 10.00 25 0.00 8 0.00 10.00 5--0.16 14 39 20.00 20 0.00 6 0.00 30.00 6--0.08 14 77 30.00 90.00 5 0.00 30.00 3--0.63 14 15 40.00 27 0.00 8 0.00 40.00 40.15 0.15 0.44 15 5 75 0.00 29 0.00 6 0.00 20.00 5--0.52 15 59 60.00240.0100.0010.003--0.48153070.00240.0110.0020.003--0.48159180.00190.0060.0010.005--0.5614909 0.00220.0080.0020.006--0.521508100.00260.0090.0020.005--0.341551110.00280.0070.0020.004--0.361568X = -452 + 3161C + 130Si + 407Mn + 175Cr + 585Mo

[0165] Classification Steel Grade No. Slab Heating Temperature (°C) Finishing Hot Rolling Temperature (°C) Coiling Temperature (°C) Cold Reduction Rate (%) Heat Treatment Temperature (°C) Heat Treatment Time (Sec) Comparative Example 7 1 1 2 0 9 0 6 0 0 45 -- Comparative Example 8 2 1 2 1 0 9 1 0 6 1 0 50 -- Comparative Example 9 3 1 2 2 0 9 0 6 0 0 50 -- Inventive Example 8 4 1 2 1 0 9 3 0 6 2 0 45 -- Inventive Example 9 5 1 1 9 0 9 0 5 9 0 50 -- Inventive Example 10 5 1 2 0 9 2 0 6 0 50 -- Inventive Example 11 6 1 2 0 9 0 5 9 0 50 -- Inventive Example 12 7 1 2 10 91060045--Comparative Example 107121090059050--Inventive Example 13811809006005055036000Inventive Example 14912108905805059043200Comparative Example 119120089057050--Comparative Example 129121090060050--Inventive Example 1510122090062045--Inventive Example 1611120090060050--Comparative Example 1311122090060050--Comparative Example 1411120090060050--

[0166] Classification Steel Grade No. Dew Point Temperature (°C) Annealing Temperature (°C) Annealing Time (sec) Primary Cooling End Temperature (°C) Primary Average Cooling Rate (°C / s) Secondary Cooling End Temperature / Holding Temperature (°C) Secondary Average Cooling Rate (°C / s) Holding Time (sec) Comparative Example 7 1 1 2 8 40 60 650 35 50 12 100 Comparative Example 8 2 1 0 8 60 60 640 35 00 12 100 Comparative Example 9 3 1 1 8 30 60 650 34 90 12 100 Invention Example 8 4 1 7 8 40 60 650 45 00 12 100 Invention Example 9 5 1 0 8 40 70 640 44 50 15 150 Invention Example 1 0 5 9 8 40 60 650 45 10 10 150 Invention Example 1 1 6 7 8 50 60 650 34 90 101 50 Invention Example 127086060640345010150 Comparative Example 1071078060640345012100 Invention Example 1381186060640350012100 Invention Example 1491986070640555012100 Comparative Example 119-3586060650350012100 Comparative Example 1291586060650335012100 Invention Example 15101483060600350010100 Invention Example 16111085050600249010100 Comparative Example 13111085060600349010100 Comparative Example 14111085060650349010100

[0167] Classification Steel Grade No. Mf (°C) Tertiary Cooling End Temperature (°C) Tertiary Average Cooling Rate (°C / s) IHBAF Temperature (°C) Time (Sec) Temperature (°C) Time (Sec) Comparative Example 7 114 125 5200 30-- Comparative Example 8 214 225 5200 50-- Comparative Example 9 314 530 5300 10-- Invention Example 8 4134 305--250 72000 Invention Example 9 5126 305 300 10-- Invention Example 10 5126 255 300 10-- Invention Example 116 134 305 300 20-- Honorary 12712930540010--Comparative Example 10712930540010--Inventional Example 138139255250100--Inventional Example 149137255300100--Comparative Example 119137255300100--Comparative Example 129137255300100--Inventional Example 151013230535010--Inventional Example 161112530525030--Comparative Example 131112530510080--Comparative Example 1411125305----

[0168] Classification Microstructure (Area %) Internal Oxidation Zone 1st Decarburization Zone 2nd Decarburization Zone TMI One or more of RAF and B Si / Mn(Ra)Ra / Rb One or more of F and B (Area %) Depth of Decarburization Zone relative to Total Thickness (%) One of F and B Comparative Example 76993.1280.191.1931100 - Comparative Example 8642.8330.162.0035100 - Comparative Example 9812.3170.811.29555019 Invention Example 898200.651.4860454 Invention Example 9931.950.751.4457556 Invention Example 10981.900.721.3866505 Invention Example 11964.500.661.3859455 Invention Example 12873.2100.541.13453913 Comparative Example 10743.2230. 711.4835100-Invention Example 13952.920.771.3847505 Invention Example 14971.710.811.5656405 Comparative Example 11971.710.551.0619217 Comparative Example 12761.7220.450.8724100-Invention Example 15963.600.441.2935413 Invention Example 16822.9150.391.08362518 Comparative Example 13822.9150.511.42272116 Comparative Example 14812.9160.330.92331917B: Bainite, TM: Tempered martensite, RA: Retained austenite, F: Ferrite, Ra: Si / Mn of the internal oxidation region, Rb: Si / MnRa / Rb of the base material, Rb is the value calculated with 2 significant figures.

[0169] Classification Residual H Content (ppm) Yield Strength (MPa) Tensile Strength (MPa) Elongation (%) R / t Comparative Example 70.25 105 3146 18.35.0 Comparative Example 80.229 78 144 14.75.0 Comparative Example 90.119 29 142 37.74.5 Inventive Example 80.07 128 1156 96.13.5 Inventive Example 90.06 1210 157 48.13.5 Inventive Example 100.04 1228 158 47.93.0 Inventive Example 110.09 1249 1548 8.03.0 Inventive Example 120.08 1139 15108. 33.5 Comparative Example 100.0894115898.35.5 Inventive Example 130.06121415347.43.0 Inventive Example 140.09122115019.12.5 Comparative Example 110.09122115019.14.5 Comparative Example 120.09132514509.14.5 Inventive Example 150.07129816068.63.0 Inventive Example 160.10108115838.83.0 Comparative Example 130.1098815948.83.0 Comparative Example 140.1095716028.83.0

[0170] As can be seen from Tables 7 to 13 above, in the case of Inventive Examples 8 to 16, which satisfy the alloy composition and manufacturing conditions of the present invention, the residual H content, yield strength, tensile strength, elongation, and R / t are excellent as the base material microstructure, internal oxidation region, and decarburization region intended by the present invention are secured. In the case of Comparative Example 7, which does not satisfy the C content, Si / Mn, and X values, the residual H content, tensile strength, and R / t are insufficient as the base material microstructure and Ra intended by the present invention are not secured and the second decarburization region is not formed.

[0171] In the case of Comparative Example 8, which does not satisfy the C content, Si / Mn, and X values, it can be seen that the residual H content, yield strength, tensile strength, and R / t are insufficient because the microstructure of the base material Ra that the present invention aims to obtain is not secured and the second decarburization region is not formed.

[0172] In the case of Comparative Example 9, which does not satisfy the Si / Mn and X values, it can be seen that the yield strength, tensile strength, and R / t are at an insufficient level.

[0173] In the case of Comparative Example 10, which does not satisfy the annealing temperature, it can be seen that the yield strength and R / t are insufficient because the microstructure of the base material to be obtained by the present invention is not secured and the second decarburization region is not formed.

[0174] In the case of Comparative Example 11, which does not satisfy the dew point temperature, it can be seen that the R / t is insufficient because the total fraction of one or more types of ferrite and bainite in the first decarburization region and the depth of the first decarburization region, which the present invention aims to obtain, are not secured.

[0175] In the case of Comparative Example 12, which does not satisfy the second cooling end temperature / holding temperature, it can be seen that the tensile strength, R / t, is insufficient because the base material microstructure, Ra / Rb, which the present invention aims to obtain, is not secured and the second decarburization region is not formed.

[0176] In the case of Comparative Example 13, which does not satisfy the post-heat treatment temperature, it can be seen that the yield strength is insufficient.

[0177] In the case of Comparative Example 14, which did not undergo post-heat treatment, it can be seen that the yield strength is insufficient as the Ra / Rb desired by the present invention is not secured.

[0178] Figure 3 is a graph showing the C content according to the depth in the thickness direction from the surface of the steel plate for Invention Example 9. Figure 4 is a photograph of the surface layer of Invention Example 9 observed by SEM. As can be seen from Figures 3 and 4, it can be confirmed that a first decarburization region of appropriate thickness is formed to satisfy the depth of the first decarburization region relative to the total thickness of the decarburization region proposed by the present invention. In addition, it can be confirmed that an internal oxide layer is formed on the upper side of the first decarburization region.

[0179] Figure 5 is a photograph of the surface layer of Invention Example 9 observed with a high-magnification SEM. As can be seen from Figure 5, in the case of Invention Example 9, the internal oxidation region includes a coarse structure with an average diameter of 1 μm or more formed by a plurality of grain boundaries and a fine structure with an average diameter of less than 1 μm, and it can be confirmed that the region where the fine structure is formed is located closer to the surface side of the steel plate than the region where the coarse structure is formed.

Claims

1. In wt%, it contains C: 0.150~0.30%, Si: 0.50~2.0%, Mn: 2.0~3.50%, the remainder being Fe and other unavoidable impurities, and The microstructure, in area %, contains tempered martensite: 80% or more, and one or more of ferrite and bainite: 20% or less, and The moisture content of precipitates with an average size of 100 nm or more in the above tempered martensite is 20 particles / ㎛ 2 Below, Cold-rolled steel sheet with a yield strength of 1000 MPa or higher.

2. In Paragraph 1, The above cold-rolled steel sheet further comprises one or more of Al: 0.10% or less, Cr: 1.0% or less, Mo: 0.50% or less, Ti: 0.10% or less, Nb: 0.10% or less, B: 0.0050% or less, P: 0.030% or less, S: 0.010% or less, N: 0.010% or less, Cu: 0.0030~0.30%, and Ni: 0.0030~0.30%.

3. In Paragraph 1, Cold-rolled steel sheet satisfying the following relationship 1. [Equation 1] X = -452 + 3161C + 130Si + 407Mn + 175Cr + 585Mo ≥ 1470 (However, in the above Equation 1, Cr and Mo are each considered to be 0% if not added.) 4. In Paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet having a Si / Mn ratio of 0.180 to 0.

60.

5. In Paragraph 1, The above microstructure is a cold-rolled steel sheet further comprising retained austenite: 10% or less.

6. Includes the base material and the decarburization region, The above base material is, In weight percent, it contains C: 0.150~0.30%, Si: 0.50~2.0%, Mn: 2.0~3.50%, the remainder being Fe and other unavoidable impurities, and The above decarburization area is, The above decarburization area includes a first decarburization area and a second decarburization area, and The depth of the first decarburization region is 5 to 70% of the total thickness of the decarburization region, and Cold-rolled steel sheet with a yield strength of 1000 MPa or higher.

7. In Paragraph 6, The above base material is a cold-rolled steel sheet further comprising one or more of Al: 0.10% or less, Cr: 1.0% or less, Mo: 0.50% or less, Ti: 0.10% or less, Nb: 0.10% or less, B: 0.0050% or less, P: 0.030% or less, S: 0.010% or less, N: 0.010% or less, Cu: 0.0030~0.30%, and Ni: 0.0030~0.30%.

8. In Paragraph 6, The above base material is a cold-rolled steel sheet satisfying the following relationship 1. [Equation 1] X = -452 + 3161C + 130Si + 407Mn + 175Cr + 585Mo ≥ 1470 (However, in the above Equation 1, Cr and Mo are each considered to be 0% if not added.) 9. In Paragraph 6, The above base material is a cold-rolled steel sheet with a Si / Mn ratio of 0.180 to 0.

60.

10. In Paragraph 6, The above base material is a cold-rolled steel sheet having a microstructure in area % containing tempered martensite: 80% or more, and one or more of ferrite and bainite: 20% or less.

11. In Paragraph 10, The above microstructure is a cold-rolled steel sheet further comprising retained austenite: 10% or less.

12. In Paragraph 6, The above cold-rolled steel sheet is a cold-rolled steel sheet that further includes an internal oxidation region above the decarburization region.

13. In Paragraph 6, The above-mentioned first decarburization region has a total of at least one type of ferrite and bainite exceeding 20% ​​of the area, and The above second decarburization region is a cold-rolled steel sheet in which the total of one or more types of ferrite and bainite is 20% or less.

14. In Paragraph 12, The above internal oxidation region is a cold-rolled steel sheet with a Si / Mn ratio of 0.250 to 0.

90.

15. In Paragraph 12, A cold-rolled steel sheet in which the ratio (Ra / Rb) of Si / Mn(Ra) of the internal oxidation region and Si / Mn(Rb) of the base material is 1.0 or greater.

16. In Paragraph 12, The above internal oxidation region is, It includes a coarse-grained structure and a fine-grained structure formed by multiple grain boundaries, and The above granular structure has an average diameter of 1㎛ or more, and the above fine structure has an average diameter of less than 1㎛, and The region where the fine-grained structure is formed is a cold-rolled steel sheet located closer to the surface side of the steel sheet than the region where the coarse-grained structure is formed.

17. In Paragraph 1 or 6, The above cold-rolled steel sheet is a cold-rolled steel sheet having a residual H content of 0.20 ppm or less.

18. In Paragraph 1 or 6, The above cold-rolled steel sheet is a cold-rolled steel sheet that additionally includes a plating layer on at least one surface.

19. In Paragraph 1 or 6, The above cold-rolled steel sheet is a cold-rolled steel sheet having a tensile strength of 1470 MPa or more, an elongation of 4.0% or more, and an R / t of 4.0 or less. A step of preparing a cold-rolled steel sheet comprising, in weight% of 20%, C: 0.150~0.30%, Si: 0.50~2.0%, Mn: 2.0~3.50%, and the remainder being Fe and other unavoidable impurities; A step of continuously annealing the above cold-rolled steel sheet at 800~900℃ with a dew point temperature of -10~25℃; A step of first cooling the annealed cold-rolled steel sheet; A step of secondarily cooling the above first-cooled cold-rolled steel sheet to a cooling termination temperature of 400~600℃ and maintaining it for 500 seconds or less; A step of tertiarily cooling the above-mentioned maintained cold-rolled steel sheet; and A method for manufacturing a cold-rolled steel sheet comprising the step of post-heat treating the above-mentioned third-cooled cold-rolled steel sheet at 150 to 500°C.

21. In Paragraph 20, The step of preparing the above cold-rolled steel sheet is, Step of heating the slab at 1100~1300℃; A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the above heated slab at 850~100℃; A step of coiling the above hot-rolled steel sheet at 400~650℃; and A method for manufacturing a cold-rolled steel sheet comprising the step of cold-rolling the above-mentioned coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet.

22. In Paragraph 21, A method for manufacturing a cold-rolled steel sheet, further comprising a step of heat-treating the coiled hot-rolled steel sheet at 500 to 650°C for 1,800 to 100,000 seconds after the coiling step and before the step of obtaining the cold-rolled steel sheet.

23. In Paragraph 21, The step of obtaining the above cold-rolled steel sheet is a method for manufacturing a cold-rolled steel sheet performed with a cold rolling rate of 20 to 70%.

24. In Paragraph 20, A method for manufacturing cold-rolled steel sheets in which the above continuous annealing step is performed for 30 to 120 seconds.

25. In Paragraph 20, A method for manufacturing a cold-rolled steel sheet in which the above first cooling step is performed at an average cooling rate of 10℃ / s or less until a cooling end temperature of 500~700℃.

26. In Paragraph 20, A method for manufacturing cold-rolled steel sheets in which the above secondary cooling step is performed at an average cooling rate of 5 to 50℃ / s.

27. In Paragraph 20, A method for manufacturing a cold-rolled steel sheet in which, after the above-mentioned maintaining step, the maintained cold-rolled steel sheet has a residual H content of 0.50 ppm or less.

28. In Paragraph 20, A method for manufacturing a cold-rolled steel sheet, further comprising a step of plating the maintained cold-rolled steel sheet before a third cooling step after the above-mentioned maintaining step.

29. In Paragraph 20, A method for manufacturing a cold-rolled steel sheet in which the above third cooling step is performed at an average cooling rate of 1 to 100℃ / s until the cooling end temperature of 10℃ to Mf.

30. In Paragraph 20, A method for manufacturing cold-rolled steel sheets in which the above-mentioned post-heat treatment step is performed for 5 to 100 seconds.

31. In Paragraph 20, A method for manufacturing cold-rolled steel sheets in which the above-mentioned post-heat treatment step is performed for 1,800 to 100,000 seconds.

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