Cold-rolled steel sheet for hot forming with excellent surface quality, hot-formed member, and manufacturing method thereof

Abstract

The present invention relates to a cold-rolled steel sheet for hot forming, a hot-formed member, and a manufacturing method thereof, and more particularly to a cold-rolled steel sheet for hot forming, a hot-formed member, and a manufacturing method thereof, which have excellent surface quality.

Description

[Technical Field]

[0001] The present invention relates to a cold-rolled steel sheet for hot forming, a hot-formed member, and a manufacturing method thereof, and more particularly to a cold-rolled steel sheet for hot forming, a hot-formed member, and a manufacturing method thereof, which have excellent surface quality. [Background technology]

[0002] Recently, much research has been conducted on automotive structural components to improve fuel efficiency by reducing the weight of automobiles and to improve crashworthiness by increasing the strength of automobiles. In particular, components manufactured by hot forming are widely used as automotive structural components.

[0003] Patent Document 1 discloses that ultra-high strength of 1600 MPa or more can be achieved by heating an Al-Si plated steel sheet to 850°C or higher, hot forming it in a press, and then rapidly cooling it to form a martensite structure. It also discloses that corrosion resistance and spot weldability can be ensured without shot blasting due to the alloyed and diffused layers formed by Fe diffusion from the base material to the plated layer during heat treatment.

[0004] However, since an Al-Si plated steel sheet must have an Al-Si plated layer formed, a separate plating process is required, which reduces the cost and productivity.

[0005] On the other hand, in the case of non-plated materials, spot weldability cannot be ensured due to the oxide layer formed during heat treatment, and a shot blasting process is necessarily required to remove this. In addition, there is the problem that it is difficult to ensure corrosion resistance.

[0006] Therefore, there is a need to develop a cold-rolled steel sheet for hot forming, a hot-formed member, and a manufacturing method thereof that can ensure excellent corrosion resistance and spot weldability without requiring a plating process or a shot blasting process. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Patent No. 6296805 (Published October 2, 2001) Summary of the Invention [Problem to be solved by the invention]

[0008] According to one aspect of the present invention, there is provided a cold-rolled steel sheet for hot forming, a hot-formed member, and a method for manufacturing the same, which have excellent surface quality.

[0009] The object of the present invention is not limited to the above-mentioned content, and a person skilled in the art will have no problem in understanding the further object of the present invention from the overall content of this specification. [Means for solving the problem]

[0010] One aspect of the present invention includes a base steel sheet and a first oxide layer on the base steel sheet, The first oxide layer contains two or more of Fe, Mn, Cr and Si, and a cold-rolled steel sheet having a thickness of 5 to 500 nm can be provided.

[0011] The above-mentioned base steel sheet may contain, by weight, C: 0.05 to 0.4%, Si: 0.5 to 3.0%, Cr: 0.3 to 5.0%, Mn: 0.01 to 4.0%, Al: 0.001 to 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, N: 0.001 to 0.02%, the balance being Fe and other unavoidable impurities.

[0012] The base steel sheet may contain one or more of Ti: 0.001 to 0.4%, Nb: 0.001 to 0.4%, Zr: 0.001 to 0.4%, V: 0.001 to 0.4%, B: 0.0001 to 0.01%, Mo: 0.001 to 1.0%, W: 0.001 to 1.0%, Cu: 0.005 to 2.0%, Ni: 0.005 to 2.0%, Sb: 0.001 to 1.0%, Sn: 0.001 to 1.0%, and REM: 0.0001 to 0.02%.

[0013] The first oxide layer may have a total content of Si, Mn and Cr of 30% or more by weight.

[0014] The cold-rolled steel sheet may contain 5% or more by area of ​​ferrite and cementite as a microstructure.

[0015] Another aspect of the present invention is a steel sheet comprising: a base steel sheet; a first oxide layer on the base steel sheet; and a second oxide layer on the first oxide layer, the first oxide layer contains two or more of Fe, Mn, Cr, and Si, The second oxide layer is made of an Fe-based oxide, and a member having a thickness of 0.1 to 10 μm can be provided.

[0016] The above-mentioned base steel sheet may contain, by weight, C: 0.05 to 0.4%, Si: 0.5 to 3.0%, Cr: 0.3% or more but less than 5.0%, Mn: 0.01 to 4.0%, Al: 0.001 to 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, N: 0.001 to 0.02%, the balance being Fe and other unavoidable impurities.

[0017] The base steel sheet may contain one or more of Ti: 0.001 to 0.4%, Nb: 0.001 to 0.4%, Zr: 0.001 to 0.4%, V: 0.001 to 0.4%, B: 0.0001 to 0.01%, Mo: 0.001 to 1.0%, W: 0.001 to 1.0%, Cu: 0.005 to 2.0%, Ni: 0.005 to 2.0%, Sb: 0.001 to 1.0%, Sn: 0.001 to 1.0%, and REM: 0.0001 to 0.02%.

[0018] The second oxide layer may have a total content of Si, Mn and Cr of less than 30% by weight.

[0019] The component may have a microstructure containing martensite or bainite as a major phase.

[0020] The member may have a tensile strength of 500 MPa or more.

[0021] Another aspect of the present invention is a method for manufacturing a steel slab by the steps of: reheating the steel slab; hot rolling the reheated steel slab; cooling and coiling the hot-rolled steel sheet; cold rolling the coiled steel sheet; and The cold-rolled steel sheet is continuously annealed, It is possible to provide a method for producing a cold-rolled steel sheet in which, during the continuous annealing, the heating rate is 3.0 to 20.0°C / s at an atmospheric temperature of room temperature to 700°C, 0.08 to 1.5°C / s at an atmospheric temperature of 700 to 800°C, 0.01 to 1.5°C / s at an atmospheric temperature of 800 to 900°C, and 0.01 to 1.0°C / s at an atmospheric temperature of 900 to 1000°C.

[0022] The above steel slab may contain, by weight, C: 0.05 to 0.4%, Si: 0.5 to 3.0%, Cr: 0.3 to 5.0%, Mn: 0.01 to 4.0%, Al: 0.001 to 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, N: 0.001 to 0.02%, the balance being Fe and other unavoidable impurities.

[0023] The steel slab may contain one or more of Ti: 0.001 to 0.4%, Nb: 0.001 to 0.4%, Zr: 0.001 to 0.4%, V: 0.001 to 0.4%, B: 0.0001 to 0.01%, Mo: 0.001 to 1.0%, W: 0.001 to 1.0%, Cu: 0.005 to 2.0%, Ni: 0.005 to 2.0%, Sb: 0.001 to 1.0%, Sn: 0.001 to 1.0%, and REM: 0.0001 to 0.02%.

[0024] The reheating is carried out in the temperature range of 1000 to 1300°C. The hot rolling is performed at a finish rolling temperature of Ar3 to 1000°C, The above coiling is carried out in the temperature range of Ms to 750°C. The cold rolling can be carried out at a reduction ratio of 30 to 80%.

[0025] The continuous annealing is carried out at a steel sheet temperature in the range of 700 to 900°C, and the continuous annealing time can be 1 to 1000 seconds.

[0026] Another aspect of the present invention is a method for producing a cold-rolled steel sheet by heat treating the cold-rolled steel sheet; and and cooling the heat-treated steel sheet after hot forming, It is possible to provide a method for producing a member in which the value A defined by the following relational expression 1 is 0.6 to 1.0 during the heat treatment.

[0027] [Equation 1] A=(T+0.2t) / 1210 (In the formula, T represents the heating temperature in °C, and t represents the total heating time in seconds.)

[0028] During the heat treatment, the atmosphere can be heated to the heating temperature at a temperature increase rate of 1 to 1000° C. / s.

[0029] During the heat treatment, the heating temperature may be 700 to 1000° C. and the heating time may be 150 to 1000 seconds.

[0030] During cooling after the hot forming, the material can be cooled to the Mf temperature or lower at a cooling rate of 10 to 1000°C / s. [Effects of the Invention]

[0031] According to one aspect of the present invention, it is possible to provide a cold-rolled steel sheet for hot forming, a hot-formed member, and methods for manufacturing the same, which have excellent surface quality.

[0032] According to one aspect of the present invention, it is possible to provide a cold-rolled steel sheet for hot forming, a hot-formed member, and a manufacturing method thereof, which can be applied to automobile structural members or reinforcing members, etc., and which has excellent surface quality and crash resistance properties even without a plating process or a shot blasting process. [Brief explanation of the drawings]

[0033] [Figure 1] 1 shows the surface quality after hot forming of Example 2 of the present invention and Comparative Example 10. [Figure 2] 1 shows the phosphate coverage after hot forming for Example 1 and Comparative Example 2. [Figure 3] This shows the change in surface quality due to hot forming conditions. [Figure 4] 1 is a photograph showing the surface of Example 11 and Comparative Example 24 after hot forming. DETAILED DESCRIPTION OF THE INVENTION

[0034] The following describes preferred embodiments of the present invention. The embodiments of the present invention can be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments are provided to further explain the present invention to those skilled in the art to which the present invention pertains.

[0035] The present inventors recognized that in the case of ungalvanized cold-rolled steel sheets for hot forming, an oxide layer formed during heat treatment in manufacturing hot-formed parts makes it impossible to ensure good surface quality of the parts, and a shot blasting process is required to remove this oxide layer. They conducted extensive research to solve this problem while ensuring good surface quality.

[0036] As a result, the inventors have confirmed that by precisely controlling the alloy composition and manufacturing conditions, particularly the contents of Cr, Si, and Mn, and the rate of change in the atmospheric temperature during annealing, a composite oxide layer can be formed to a target thickness during annealing, and by controlling the process conditions for hot forming, excellent surface quality can be ensured even without a shot blasting process, which led to the completion of the present invention.

[0037] The present invention will be described in detail below.

[0038] The cold-rolled steel sheet of the present invention will be described in detail below.

[0039] A cold-rolled steel sheet according to one aspect of the present invention may include a base steel sheet and a first oxide layer formed on the base steel sheet.

[0040] Hereinafter, the composition of the base steel sheet of the cold-rolled steel sheet according to one aspect of the present invention will be described in detail.

[0041] In the present invention, unless otherwise specified, the percentage representing the content of each element is based on weight.

[0042] The base steel sheet of the cold-rolled steel sheet according to one aspect of the present invention may contain, by weight, C: 0.05 to 0.4%, Si: 0.5 to 3.0%, Cr: 0.3 to 5.0%, Mn: 0.01 to 4.0%, Al: 0.001 to 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, N: 0.001 to 0.02%, with the remainder being Fe and other unavoidable impurities.

[0043] Carbon (C): 0.05~0.4% Carbon (C) is an essential element for improving the strength of heat-treated components and must be added appropriately. If the carbon (C) content is less than 0.05%, it may be difficult to ensure sufficient strength. On the other hand, if the carbon (C) content exceeds 0.4%, the strength of the hot-rolled material may be excessively high when cold-rolling the hot-rolled material, which may significantly deteriorate the cold-rollability and significantly reduce the spot weldability. According to another aspect of the present invention, the lower limit may be 0.06%. According to another aspect of the present invention, the upper limit may be 0.38%, and according to another aspect, the upper limit may be 0.36%.

[0044] Silicon (Si): 0.5 to 3.0% Silicon (Si) not only plays an important role in concentrating on the surface and forming a silicon (Si)-based oxide layer when cold-rolled steel sheet is annealed in a continuous annealing line, but also suppresses the formation of Fe, Mn, and Cr oxide layers during hot forming, thereby ensuring the spot weldability of the component. If the silicon (Si) content is less than 0.5%, the above-mentioned effects may be insufficient. According to another aspect of the present invention, the lower limit may be 0.8%. On the other hand, if the silicon (Si) content exceeds 3.0%, an excessively thick Si-based amorphous oxide layer may be formed, which may actually cause a problem of reduced spot weldability. According to another aspect of the present invention, the upper limit may be 2.8%.

[0045] Chromium (Cr): 0.3 to 5.0% Chromium (Cr) not only improves the hardenability of steel sheet, but also plays a role in stably contributing to the formation of a surface Si-based amorphous oxide layer through an appropriate reaction with Si. If the chromium (Cr) content is less than 0.3%, the above-mentioned effects may be insufficient. According to another aspect of the present invention, the lower limit may be 0.5%. On the other hand, if the chromium (Cr) content exceeds 5.0%, the effects may saturate, and problems such as increased manufacturing costs may occur. According to another aspect of the present invention, the upper limit may be 4.5%.

[0046] Manganese (Mn): 0.01 to 4.0% Manganese (Mn) is necessary not only to ensure the solid solution strengthening effect but also to lower the critical cooling rate for securing martensite in hot-formed components. If the manganese (Mn) content is less than 0.01%, the above-mentioned effects may be insufficient. According to another aspect of the present invention, the lower limit may be 0.05%. On the other hand, if the Mn content exceeds 4.0%, the strength of the steel sheet before the hot-forming process increases excessively, making the blanking process difficult. In addition, the addition of excessive ferroalloy may result in disadvantages such as increased costs and poor spot weldability. According to another aspect of the present invention, the upper limit may be 3.9%. According to another aspect, the upper limit may be 3.8%.

[0047] Aluminum (Al): 0.001 to 0.4% Aluminum (Al), together with Si, acts as a deoxidizer during steelmaking, thereby improving the cleanliness of steel. If the aluminum (Al) content is less than 0.001%, the above-mentioned effects may be insufficient. According to another aspect of the present invention, the lower limit may be 0.005%. On the other hand, if the aluminum (Al) content exceeds 0.4%, the Ac3 temperature may rise excessively, which may necessitate an increase in the heating temperature. According to another aspect of the present invention, the upper limit may be 0.3%, and according to another aspect, the upper limit may be 0.2%.

[0048] Phosphorus (P): 0.001 to 0.05% Phosphorus (P) is an impurity, and controlling its content to less than 0.001% may require high production costs, so the lower limit can be set to 0.001%. On the other hand, if the content exceeds 0.05%, the weldability of hot-formed parts may be significantly reduced. According to another aspect of the present invention, the upper limit may be 0.03%.

[0049] Sulfur (S): 0.0001 to 0.02% Sulfur (S) is an impurity, and controlling its content to less than 0.0001% may require high production costs, so the lower limit can be set to 0.0001%. On the other hand, if the content exceeds 0.02%, the ductility, impact properties, and weldability of the member may be impaired. According to another aspect of the present invention, the upper limit may be 0.01%.

[0050] Nitrogen (N): 0.001-0.02%, Nitrogen (N) is an impurity, and controlling its content to less than 0.001% may require high production costs, so the lower limit can be set to 0.001%. On the other hand, if the N content exceeds 0.02%, the slab may become sensitive to cracking during continuous casting, and impact properties may also be reduced. According to one aspect of the present invention, the upper limit may be 0.01%.

[0051] In addition to the above-described composition, the steel material of the present invention may contain the remaining iron (Fe) and inevitable impurities. The inevitable impurities may be unintentionally mixed in during the normal manufacturing process, and therefore cannot be excluded. Since such impurities are known to any engineer in the field of normal steel manufacturing, the present specification does not mention all of the contents thereof.

[0052] The base steel sheet of the cold-rolled steel sheet according to one aspect of the present invention may contain one or more of Ti: 0.001 to 0.4%, Nb: 0.001 to 0.4%, Zr: 0.001 to 0.4%, V: 0.001 to 0.4%, B: 0.0001 to 0.01%, Mo: 0.001 to 1.0%, W: 0.001 to 1.0%, Cu: 0.005 to 2.0%, Ni: 0.005 to 2.0%, Sb: 0.001 to 1.0%, Sn: 0.001 to 1.0%, and REM: 0.0001 to 0.02%.

[0053] Titanium (Ti), niobium (Nb), zirconium (Zr), vanadium (V): 0.001-0.4% Titanium (Ti), niobium (Nb), zirconium (Zr), and vanadium (V) form fine precipitates, which improve the strength of heat-treated components, stabilize retained austenite by refining crystal grains, and improve impact toughness. If their content (meaning the total content when two or more elements are added) is less than 0.001%, the above-mentioned effects may be insufficient, while if their content exceeds 0.4%, not only will the effects saturate, but the addition of excessive ferroalloys may result in increased costs.

[0054] Boron (B): 0.0001 to 0.01% Boron (B) is an element that can improve hardenability even with a small amount of addition, and can segregate to prior austenite grain boundaries to suppress embrittlement of hot-formed parts due to grain boundary segregation of P and / or S. If the boron (B) content is less than 0.001%, the above-mentioned effects may be insufficient, while if the boron (B) content exceeds 0.01%, not only will the effects saturate but hot embrittlement may occur during hot rolling. In one aspect of the present invention, the upper limit may be 0.005%.

[0055] Molybdenum (Mo), tungsten (W): 0.001 to 1.0% Molybdenum (Mo) and tungsten (W) can be added to improve hardenability, increase strength through precipitation strengthening, and refine grains. If the content is less than 0.001%, the above-mentioned effects become insufficient, while if the content exceeds 1.0%, not only will the effects saturate but costs may also increase.

[0056] Copper (Cu), Nickel (Ni): 0.005-2.0% Copper (Cu) can be added as an element that forms fine precipitates to improve strength. Nickel (Ni) can be added as needed because adding copper (Cu) alone can cause hot embrittlement. If the content is less than 0.005%, the above-mentioned effects may be insufficient, and if the content is more than 2.0%, excessive cost increases may occur.

[0057] Antimony (Sb), tin (Sn): 0.001-1.0% Antimony (Sb) and tin (Sn) have the effect of suppressing the formation of oxides that can form at the surface grain boundaries of hot-rolled steel containing Si, and can also suppress dent defects caused by the shedding of surface grain boundaries during annealing of cold-rolled steel. To achieve this effect, 0.001% or more of antimony and tin can be added. However, if the content exceeds 1.0%, not only will costs increase excessively, but they may also dissolve in the slab grain boundaries and induce coil edge cracks during hot rolling.

[0058] Rare earth elements (REM): 0.0001~0.02% Rare earth elements (REM) can control the activity of Fe in steel and thus the thickness of Fe scale formed on the surface during hot forming. To achieve this effect, it is preferable to add 0.0001% or more of REM elements. On the other hand, if the REM content exceeds 0.02%, the ability to control Fe activity is lost, and surface quality may deteriorate. According to one aspect of the present invention, the REM content can be controlled to 0.01% or less.

[0059] The first oxide layer according to one aspect of the present invention may contain two or more of Fe, Mn, Cr, and Si, and may have a thickness of 5 to 500 nm.

[0060] The first oxide layer is a composite oxide layer containing two or more of Fe, Mn, Cr, and Si, and two or more of these elements are formed. When producing a cold-rolled steel sheet after hot forming in order to control the thickness of the second oxide layer of the component, precise thickness control by controlling the heating rate of the atmospheric temperature is required. According to one aspect of the present invention, the total content of Si, Mn, and Cr, expressed as weight % per se relative to the first oxide layer, may be 30% or more, and according to another aspect, the total content may be 90% or less.

[0061] If the thickness of the first oxide layer is less than 5 nm, the second oxide layer will be formed too thick after hot forming, resulting in little improvement in surface quality and making it difficult to ensure excellent surface quality. On the other hand, if the thickness exceeds 500 nm, the second oxide layer will not be easily formed after hot forming, resulting in poor phosphate treatment properties, making it difficult to ensure excellent surface quality and sufficient coating corrosion resistance. In one aspect of the present invention, the upper limit of the thickness may be 490 nm, and in another aspect, the lower limit of the thickness may be 5.5 nm. The first oxide layer of the present invention can be formed continuously or discontinuously.

[0062] The first oxide layer according to one aspect of the present invention may contain two or more of Fe, Mn, Cr, and Si, and may have a thickness of 5.0 to 500.0 nm.

[0063] The microstructure of the cold-rolled steel sheet of the present invention will be described in detail below.

[0064] In the present invention, unless otherwise specified, the percentages indicating the fractions of the microstructures are based on the area.

[0065] A cold-rolled steel sheet according to one aspect of the present invention may contain ferrite and cementite. In the present invention, there are no particular limitations on the area fractions of ferrite and cementite, but more preferably, the area fractions of ferrite and cementite may be 5 area % or more.

[0066] When cold-rolled steel sheets are used to prepare blanks for hot-formed parts, excessive strength can lead to die wear. More specifically, the microstructural characteristics can refer to the microstructure of the base steel sheet of the cold-rolled steel sheet. When this is not taken into consideration, bainite, martensite, etc. can be included, and are not excluded.

[0067] The components of the present invention will be described in detail below.

[0068] A member according to one aspect of the present invention may include a base steel sheet, a first oxide layer formed on the base steel sheet, and a second oxide layer formed on the first oxide layer.

[0069] The composition of the base steel sheet of the member according to one aspect of the present invention is the same as the composition of the cold-rolled steel sheet described above, and therefore will not be described separately.

[0070] The second oxide layer according to one aspect of the present invention is made of an Fe-based oxide and can have a thickness of 0.1 to 10 μm.

[0071] If the thickness of the second oxide layer exceeds 10 μm, there is a problem that it is difficult to ensure excellent surface quality, such as peeling of the surface oxide after hot forming due to excessive oxide formation. On the other hand, if the thickness is less than 0.1 μm, phosphate treatability may deteriorate, making it difficult to ensure excellent surface quality. According to one aspect of the present invention, the total content of Si, Mn, and Cr, as a weight percentage of the second oxide layer itself, may be less than 30%, and preferably exceeds 0%.

[0072] The microstructure of the member of the present invention will now be described in detail.

[0073] In the present invention, unless otherwise specified, the percentages indicating the fractions of the microstructures are based on the area.

[0074] The member according to one aspect of the present invention may have a microstructure containing martensite or bainite as a main phase.

[0075] The hot-formed member of the present invention may contain martensite or bainite as a main phase to ensure high strength. More specifically, the microstructural characteristics may refer to the microstructure of the base steel sheet of the member. In the present invention, the main phase may refer to the phase having the largest area fraction among multiple phases constituting the microstructure. The area fraction is not particularly limited, but may be 5 area % or more according to one aspect of the present invention.

[0076] Hereinafter, the cold-rolled steel sheet and the method for manufacturing a member according to the present invention will be described in detail.

[0077] A cold-rolled steel sheet according to one aspect of the present invention can be produced by reheating a steel slab satisfying the above-mentioned alloy composition, hot rolling, coiling, cold rolling, and continuous annealing.

[0078] reheating A steel slab satisfying the alloy composition of the present invention can be reheated to a temperature range of 1000 to 1300°C.

[0079] If the reheating temperature is less than 1000°C, it is difficult to homogenize the slab structure, and if the temperature exceeds 1300°C, excessive oxides may be formed, which may increase the production cost.

[0080] hot rolling The reheated steel slab can be hot rolled at a finish rolling temperature of Ar3 to 1000°C.

[0081] If the finish rolling temperature is lower than Ar3, the rolling is likely to occur in the two-phase region, which may cause a duplex structure to form in the surface layer, making it difficult to control the shape of the hot-rolled steel sheet. On the other hand, if the temperature exceeds 1000°C, the crystal grains of the hot-rolled steel sheet may become coarse.

[0082] Cooling and winding The hot-rolled steel sheet can be cooled and coiled in a temperature range of Ms to 750°C.

[0083] If the coiling temperature is lower than Ms (martensitic transformation start temperature), the strength of the hot-rolled steel sheet may become excessively high, which may reduce cold rolling properties. On the other hand, if the temperature exceeds 750°C, the thickness of the oxide layer increases and grain boundary oxidation of the surface layer occurs, which not only deteriorates pickling properties but also may cause problems such as grain boundary shedding of the surface layer when annealing in a continuous annealing furnace. During cooling, the cooling rate is not particularly limited, but air cooling may be performed.

[0084] cold rolling The coiled steel sheet can be cold rolled.

[0085] In the present invention, the reduction rate of cold rolling is not particularly limited, but in order to ensure the target thickness, the reduction rate can be set to 30 to 80%.

[0086] In the present invention, cold rolling can be performed to more precisely control the thickness of the steel sheet, and pickling can be performed before cold rolling.

[0087] continuous annealing The cold-rolled steel sheet may be continuously annealed, and during the continuous annealing, the heating rate may be 3.0 to 20.0°C / s at an ambient temperature of room temperature to 700°C, 0.08 to 1.5°C / s at an ambient temperature of 700 to 800°C, 0.01 to 1.5°C / s at an ambient temperature of 800 to 900°C, or 0.01 to 1.0°C / s at an ambient temperature of 900 to 1000°C.

[0088] In order to precisely control the thickness of the first oxide layer on the surface during continuous annealing, the present invention further strictly controls the heating rate of the atmosphere temperature. If the heating rate of the atmosphere temperature does not reach the suggested lower limit, an excessive first oxide layer will be formed, exceeding 500 nm, and a sufficiently thick second oxide layer will not be obtained after hot forming, which may result in poor phosphating properties and poor surface quality. On the other hand, if the heating rate of the atmosphere temperature exceeds the suggested upper limit, only a small first oxide layer will be formed, and an excessive second oxide layer will be formed after hot forming, which may result in poor surface quality, such as surface scale spalling.

[0089] According to one aspect of the present invention, the continuous annealing can be performed at a steel sheet temperature in the range of 700 to 900°C. If the annealing temperature is less than 700°C, recovery and recrystallization may be difficult to occur in the rolled structure produced by cold rolling. On the other hand, if the temperature exceeds 900°C, the annealing equipment may be deteriorated, and frequent replacement of the equipment may be required, which may increase the process cost.

[0090] According to one aspect of the present invention, the continuous annealing time may be 1 to 1000 seconds. If the annealing time is less than 1 second, it is difficult to obtain the annealing effect, whereas if the annealing time exceeds 1000 seconds, productivity may decrease.

[0091] A member according to one aspect of the present invention can be produced by heat treating, hot forming, and cooling the cold-rolled steel sheet produced by the above-described method.

[0092] Heat Treatment The cold-rolled steel sheet according to one aspect of the present invention is heat-treated. The A value defined by the following relational expression 1 may be 0.6 to 1.0, and the heat treatment may be performed after heating to a heating temperature at a heating rate of 1 to 1000°C / s.

[0093] In the present invention, in order to precisely control the thickness of the second oxide layer, the heating temperature and heating time can be controlled according to Relational Formula 1. If the value A defined by Relational Formula 1 is less than 0.6, the thickness of the second oxide layer formed during heat treatment may be small, resulting in poor phosphating properties and making it difficult to ensure good surface quality. On the other hand, if the value exceeds 1.0, the second oxide layer becomes too thick, causing peeling of the oxide layer, making it difficult to ensure good surface quality.

[0094] The temperature rise rate in the heat treatment step may refer to the temperature rise rate of the atmosphere in a continuous annealing furnace. If the temperature rise rate is less than 1°C / s, it may be difficult to ensure sufficient productivity, and if the rate exceeds 1000°C / s, there may be a problem in that excessively costly equipment is required. According to one aspect of the present invention, the heating temperature may be 700°C or higher and 1000°C or lower. The heating time according to one aspect of the present invention may be 150 seconds or longer and 1000 seconds or shorter.

[0095] [Equation 1] A=(T+0.2t) / 1210 (In the formula, T represents the heating temperature in °C, and t represents the total heating time in seconds.)

[0096] Hot forming and cooling After the heat-treated steel sheet is hot-formed, it can be cooled to the Mf temperature or lower at a cooling rate of 10 to 1000°C / s.

[0097] If the cooling rate is less than 10°C / s, undesired ferrite and pearlite are formed, making it difficult to secure the desired level of tensile strength. On the other hand, if the rate exceeds 1000°C / s, expensive special cooling equipment is required to control the rate, which may reduce productivity.

[0098] If cooling is stopped when the cooling end temperature exceeds Mf (martensitic transformation end temperature) and then the material is cooled again to room temperature, it may become difficult to ensure shape fixability of the hot-formed part.

[0099] The hot-formed member according to one aspect of the present invention thus manufactured has a tensile strength of 500 MPa or more, and can ensure excellent strength and crash resistance. [Example]

[0100] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention.

[0101] (Example) A 40 mm thick slab having the composition shown in Table 1 below was vacuum melted, heated in a heating furnace at 1200°C for 1 hour, and then hot rolled at a finish rolling temperature of 900°C to produce a hot-rolled steel sheet with a final thickness of 3 mm. The hot-rolled steel sheet was air-cooled and coiled at 600°C, and then pickled and cold-rolled at a cold reduction of 50% to produce a cold-rolled steel sheet.

[0102] [Table 1]

[0103] The cold-rolled steel sheets manufactured as described above were subjected to continuous annealing under the conditions shown in Table 2 below, at a steel sheet temperature of 780°C. Further, the sheets were heat-treated under the conditions shown in Table 2 below, and cooled after hot forming. During the heat treatment, a heating rate of 5°C / s was applied, and after hot forming, the sheets were cooled to room temperature at a cooling rate of 30°C / s.

[0104] [Table 2]

[0105] [Relational expression 1] A=(T+0.2t) / 1210 (In the formula, T represents the heating temperature in °C, and t represents the total heating time in seconds.)

[0106] Table 3 below shows the thickness of the first oxide layer measured on the cold-rolled steel sheet after continuous annealing, and also shows the thickness of the second oxide layer measured on the component after subsequent heat treatment, hot forming, and cooling. Here, the thickness of the first oxide layer was measured using a transmission electron microscope (TEM), and the thickness of the second oxide layer was measured at three locations using a transmission electron microscope (TEM) and electron beam microanalysis (EPMA), and the average value was shown. In addition, the presence or absence of scale spalling was visually observed as an item of quality degradation, and the presence or absence of scale spalling was shown. When visually observed, the thickness was measured at three locations using a transmission electron microscope (TEM) and electron beam microanalysis (EPMA), and the average value was shown. 2 If 10 or more points of peeling occurred on the above scale, it was judged to have deteriorated. The phosphate coverage was examined using a scanning electron microscope (SEM) to observe the structure, and the area of ​​the area where phosphate crystals had not formed was measured to determine whether or not there was any quality deterioration. If the phosphate crystal formation area exceeded 70%, the phosphate properties were judged to be good. In addition, the yield strength, tensile strength, and elongation of the manufactured parts were measured and shown. Yield strength, tensile strength, and elongation were measured using JIS-5 test pieces and performed a room temperature tensile test based on ISO 6892 standards.

[0107] [Table 3]

[0108] Cold-rolled steel sheets and members that satisfy the alloy composition and manufacturing conditions of the present invention form first and second oxide layers in the thickness ranges proposed in the present invention, and ensure excellent surface quality even after hot forming.

[0109] Figure 1 shows photographs of the surface quality after hot forming of Example 2 and Comparative Example 10. In Comparative Example 10, thicker oxides were formed compared to Example 2, and the deteriorated quality was confirmed.

[0110] 2 shows the phosphate coverage after hot forming for Inventive Example 1 and Comparative Example 2. In Comparative Example 2, it was confirmed that less than 70% of phosphate crystals were formed.

[0111] Figure 3 shows the change in surface quality depending on the heat treatment temperature and time. The surface condition was observed by adjusting the hot forming temperature and time. It was confirmed that when the heat treatment conditions deviate from the range limited by Relation 1, an excessive second oxide layer is formed on the surface after heat treatment, causing surface scale spalling, or the phosphating ability is deteriorated due to the excessive formation of the first oxide layer, resulting in a deterioration in surface quality.

[0112] 4 is a photograph of the surface after hot forming of Example 11 and Comparative Example 24. Comparative Example 24 was heat-treated beyond the heat treatment conditions limited by Relational Formula 1, and when the surface was visually observed, scale was confirmed compared to Example 11.

[0113] In Comparative Examples 1, 4, 5, 7, and 10, the content of each element did not reach the required range proposed by the present invention, and the formation of the first oxide layer was inappropriate. As a result, as shown in FIG. 1, the thickness of the second oxide layer after hot forming exceeded 10 μm, and excessive surface scale was generated after hot forming, making it impossible to ensure good surface quality.

[0114] In Comparative Examples 2, 3, 6, 8, 9, and 11, the content of each component exceeded the range required by the present invention, resulting in the formation of an excessive first oxide layer after annealing. As a result, a sufficient second oxide layer was not formed during hot forming, resulting in the formation of less than 70% phosphate crystals, and the surface quality was deteriorated.

[0115] In addition, in Comparative Examples 12, 15, 17, 18, and 20, in which the heating rate for each section during continuous annealing exceeded the range proposed by the present invention, the first oxide layer was not sufficiently formed, and an excessive second oxide layer was formed after hot forming, making it impossible to ensure excellent surface quality such as surface scale peeling.

[0116] In the case of Comparative Examples 13, 14, 16, and 19, the rate of increase in the ambient temperature in each section did not reach the lower limit of the rate of increase in the ambient temperature limited by the present invention, and the phosphating property deteriorated due to the excessive formation of the first oxide layer, and excellent surface quality could not be ensured.

[0117] In addition, in Comparative Examples 21 and 22, where the ambient temperature for each section did not reach some of the lower limit values ​​or exceeded some of the upper limit values, an excessive first oxide layer was formed, making it difficult to form a second oxide layer after hot forming, resulting in poor phosphating properties and failure to ensure excellent surface quality.

[0118] In the cases of Comparative Examples 23, 25, 27, and 29, the lower limit of the condition limited by Relational Formula 1 could not be reached, and the second oxide layer was not sufficiently formed, resulting in deteriorated phosphating properties. In the cases of Comparative Examples 24, 26, 28, and 30, which exceeded the upper limit of the proposed range, the thickness of the second oxide layer formed was thick, and a large amount of Fe scale was generated, resulting in deteriorated surface quality.

[0119] Although the present invention has been described in detail with reference to the above embodiments, other embodiments are possible, and the spirit and scope of the following claims should not be limited to the embodiments.

Claims

1. a base steel sheet and a first oxide layer on the base steel sheet; The first oxide layer contains two or more of Fe, Mn, Cr, and Si, and has a thickness of 5 to 500 nm.

2. The cold-rolled steel sheet according to claim 1, wherein the base steel sheet contains, by weight, C: 0.05 to 0.4%, Si: 0.5 to 3.0%, Cr: 0.3 to 5.0%, Mn: 0.01 to 4.0%, Al: 0.001 to 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, N: 0.001 to 0.02%, the balance being Fe and other unavoidable impurities.

3. 3. The cold-rolled steel sheet according to claim 2, wherein the base steel sheet contains one or more of Ti: 0.001 to 0.4%, Nb: 0.001 to 0.4%, Zr: 0.001 to 0.4%, V: 0.001 to 0.4%, B: 0.0001 to 0.01%, Mo: 0.001 to 1.0%, W: 0.001 to 1.0%, Cu: 0.005 to 2.0%, Ni: 0.005 to 2.0%, Sb: 0.001 to 1.0%, Sn: 0.001 to 1.0%, and REM: 0.0001 to 0.02%.

4. The cold-rolled steel sheet according to claim 1 , wherein the first oxide layer has a total content of Si, Mn, and Cr of 30% or more by weight.

5. The cold-rolled steel sheet according to any one of claims 1 to 4, wherein the cold-rolled steel sheet contains 5 area% or more of ferrite and cementite as a microstructure.

6. a base steel sheet, a first oxide layer on the base steel sheet, and a second oxide layer on the first oxide layer; the first oxide layer contains two or more of Fe, Mn, Cr, and Si, The second oxide layer is made of an Fe-based oxide and has a thickness of 0.1 to 10 μm.

7. The member according to claim 6, wherein the base steel sheet contains, by weight, C: 0.05 to 0.4%, Si: 0.5 to 3.0%, Cr: 0.3% or more but less than 5.0%, Mn: 0.01 to 4.0%, Al: 0.001 to 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, N: 0.001 to 0.02%, the balance being Fe and other unavoidable impurities.

8. The member according to claim 7, wherein the base steel sheet contains one or more of Ti: 0.001 to 0.4%, Nb: 0.001 to 0.4%, Zr: 0.001 to 0.4%, V: 0.001 to 0.4%, B: 0.0001 to 0.01%, Mo: 0.001 to 1.0%, W: 0.001 to 1.0%, Cu: 0.005 to 2.0%, Ni: 0.005 to 2.0%, Sb: 0.001 to 1.0%, Sn: 0.001 to 1.0%, and REM: 0.0001 to 0.02%.

9. 9. The member according to claim 6, wherein the second oxide layer has a total Si, Mn and Cr content of less than 30% by weight.

10. The component according to claim 6 , wherein the component has a microstructure containing martensite or bainite as a main phase.

11. 11. The member according to claim 6, wherein the member has a tensile strength of 500 MPa or more.

12. reheating the steel slab; hot rolling the reheated steel slab; cooling and coiling the hot-rolled steel sheet; cold rolling the coiled steel sheet; and The cold-rolled steel sheet is continuously annealed, During the continuous annealing, the heating rate is 3.0 to 20.0°C / s at an atmospheric temperature of room temperature to 700°C, 0.08 to 1.5°C / s at an atmospheric temperature of 700 to 800°C, 0.01 to 1.5°C / s at an atmospheric temperature of 800 to 900°C, and 0.01 to 1.0°C / s at an atmospheric temperature of 900 to 1000°C.

13. The steel slab contains, by weight, C: 0.05 to 0.4%, Si: 0.5 to 3.0%, Cr: 0.3 to 5.0%, Mn: 0.01 to 4.0%, Al: 0.001 to 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, N: 0.001 to 0.02%, the balance being Fe and other unavoidable impurities. The method for producing a cold-rolled steel sheet according to claim 12.

14. The steel slab contains one or more of Ti: 0.001 to 0.4%, Nb: 0.001 to 0.4%, Zr: 0.001 to 0.4%, V: 0.001 to 0.4%, B: 0.0001 to 0.01%, Mo: 0.001 to 1.0%, W: 0.001 to 1.0%, Cu: 0.005 to 2.0%, Ni: 0.005 to 2.0%, Sb: 0.001 to 1.0%, Sn: 0.001 to 1.0%, and REM: 0.0001 to 0.02%. The method for producing a cold-rolled steel sheet according to claim 13,

15. The reheating is carried out in a temperature range of 1000 to 1300°C, The hot rolling is carried out at a finish rolling temperature of Ar3 to 1000°C, The winding is performed in a temperature range of Ms to 750°C, The method for producing a cold-rolled steel sheet according to any one of claims 12 to 14, wherein the cold rolling is performed at a reduction ratio of 30 to 80%.

16. The method for producing a cold-rolled steel sheet according to any one of claims 12 to 15, wherein the continuous annealing is performed at a steel sheet temperature in a temperature range of 700 to 900 ° C., and the continuous annealing time is 1 to 1000 seconds.

17. Heat treating the cold rolled steel sheet according to any one of claims 12 to 16; and cooling the heat-treated steel sheet after hot forming; During the heat treatment, the value A defined by the following relational expression 1 is 0.6 to 1.

0. [Relationship 1] A=(T+0.2t) / 1210 (In the formula, T represents the heating temperature in °C, and t represents the total heating time in seconds.)

18. The method for manufacturing a member according to claim 17, wherein the heat treatment is carried out at a temperature increase rate of 1 to 1000° C. / s up to the heating temperature.

19. The method for manufacturing a member according to claim 17 or 18, wherein the heat treatment is carried out at a heating temperature of 700 to 1000° C. for a heating time of 150 to 1000 seconds.

20. The method for manufacturing a member according to any one of claims 17 to 19, wherein cooling after the hot forming is performed at a cooling rate of 10 to 1000 ° C. / s to a temperature equal to or lower than the Mf temperature.

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