Cold-rolled steel sheet for hot forming having excellent surface quality, hot-formed member, and method for producing same

Abstract

The present invention relates to a cold-rolled steel sheet for hot forming, a hot-formed member, and a method for manufacturing the same, and more particularly, to a cold-rolled steel sheet for hot forming, a hot-formed member, and a method for manufacturing the same, 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 component, and a method for manufacturing the same. More specifically, the present invention relates to a cold-rolled steel sheet for hot forming and a hot-formed component having excellent surface quality, and a method for manufacturing the same. Background Art

[0002] In recent years, a lot of research has been conducted on automobile structural parts in order to improve fuel efficiency by reducing the weight of automobiles and to improve collision resistance by increasing strength. In particular, parts manufactured by hot forming are widely used as automobile structural parts.

[0003] Patent Document 1 proposes that after heating the Al-Si plated steel sheet to 850°C or higher, the structure of the component is formed into martensite by hot forming and rapid cooling using a press, thereby ensuring an ultra-high strength of 1600 MPa or higher tensile strength. In addition, it is disclosed that the alloy layer and diffusion layer formed by the diffusion of Fe from the base material to the plated layer during the heat treatment process can ensure corrosion resistance and spot weldability without shot blasting.

[0004] However, the Al-Si plated steel sheet needs to form an Al-Si plating layer, and thus requires a separate plating process, which causes a problem of reduced economy and productivity.

[0005] In the case of non-plated materials, spot weldability cannot be ensured due to the oxide layer generated during heat treatment, and a shot peening process is required to remove the oxide layer. In addition, there is a 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 component, and a method for producing the same that can ensure excellent corrosion resistance and spot weldability without a plating process and a shot peening process.

[0007] [Prior art literature]

[0008] (Patent Document 1) U.S. Patent No. 6296805 (published on October 2, 2001) Summary of the invention

[0009] 1. Technical issues to be resolved

[0010] According to one aspect of the present invention, an object is to provide a cold-rolled steel sheet for hot forming, a hot-formed component, and a method for producing the same having excellent surface quality.

[0011] The technical problems of the present invention are not limited to the above contents. It is not difficult for those skilled in the art to understand the additional technical problems of the present invention from the overall content of this specification.

[0012] (II) Technical solution

[0013] One aspect of the present invention can provide a cold-rolled steel sheet, comprising a base steel sheet and a first oxide layer on the base steel sheet, wherein the first oxide layer contains two or more of Fe, Mn, Cr, and Si, and has a thickness of 5-500 nm.

[0014] The base steel plate may include, by weight%, C: 0.05-0.4%, Si: 0.5-3.0%, Cr: 0.3-5.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, N: 0.001-0.02%, the balance of Fe and other inevitable impurities.

[0015] The base steel plate may contain one or more of Ti: 0.001-0.4%, Nb: 0.001-0.4%, Zr: 0.001-0.4%, V: 0.001-0.4%, B: 0.0001-0.01%, Mo: 0.001-1.0%, W: 0.001-1.0%, Cu: 0.005-2.0%, Ni: 0.005-2.0%, Sb: 0.001-1.0%, Sn: 0.001-1.0%, and rare earth elements (REM): 0.0001-0.02%.

[0016] In terms of weight %, the sum of the contents of Si, Mn and Cr in the first oxide layer may be greater than 30%.

[0017] The cold-rolled steel sheet may contain 5 area % or more of ferrite and cementite as a fine structure.

[0018] Another aspect of the present invention can provide a component, which includes a base steel plate, a first oxide layer on the base steel plate and a second oxide layer on the first oxide layer, wherein the first oxide layer contains two or more of Fe, Mn, Cr, and Si, and the second oxide layer is composed of an Fe-based oxide, and the second oxide layer has a thickness of 0.1-10μm.

[0019] In terms of weight %, the base steel plate may include C: 0.05-0.4%, Si: 0.5-3.0%, Cr: 0.3% or more and less than 5.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, N: 0.001-0.02%, the balance Fe and other inevitable impurities.

[0020] The base steel plate may contain one or more of Ti: 0.001-0.4%, Nb: 0.001-0.4%, Zr: 0.001-0.4%, V: 0.001-0.4%, B: 0.0001-0.01%, Mo: 0.001-1.0%, W: 0.001-1.0%, Cu: 0.005-2.0%, Ni: 0.005-2.0%, Sb: 0.001-1.0%, Sn: 0.001-1.0%, and rare earth elements (REM): 0.0001-0.02%.

[0021] In terms of weight %, the sum of the contents of Si, Mn and Cr in the second oxide layer may be less than 30%.

[0022] The microstructure of the component may contain martensite or bainite as a main phase.

[0023] The tensile strength of the component may be 500 MPa or more.

[0024] Another aspect of the present invention can provide a method for manufacturing a cold-rolled steel sheet, the manufacturing method comprising the following steps: reheating a steel billet; hot rolling the reheated steel billet; cooling and coiling the hot-rolled steel sheet; cold rolling the coiled steel sheet; and continuously annealing the cold-rolled steel sheet, wherein during the continuous annealing, the heating rate at an atmosphere temperature from room temperature to 700°C is 3.0-20.0°C / second, the heating rate at an atmosphere temperature of 700-800°C is 0.08-1.5°C / second, the heating rate at an atmosphere temperature of 800-900°C is 0.01-1.5°C / second, and the heating rate at an atmosphere temperature of 900-1000°C is 0.01-1.0°C / second.

[0025] The steel billet may include, by weight%, C: 0.05-0.4%, Si: 0.5-3.0%, Cr: 0.3-5.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, N: 0.001-0.02%, the balance of Fe and other inevitable impurities.

[0026] The steel billet may contain one or more of Ti: 0.001-0.4%, Nb: 0.001-0.4%, Zr: 0.001-0.4%, V: 0.001-0.4%, B: 0.0001-0.01%, Mo: 0.001-1.0%, W: 0.001-1.0%, Cu: 0.005-2.0%, Ni: 0.005-2.0%, Sb: 0.001-1.0%, Sn: 0.001-1.0%, and rare earth elements (REM): 0.0001-0.02%.

[0027] The reheating may be performed in a temperature range of 1000-1300°C, the hot rolling may be performed at a finishing temperature of Ar3 to 1000°C, the coiling may be performed in a temperature range of Ms to 750°C, and the cold rolling may be performed at a reduction rate of 30-80%.

[0028] The continuous annealing can be performed at a temperature of 700-900° C. for the steel plate, and the continuous annealing time can be 1-1000 seconds.

[0029] Another aspect of the present invention may provide a method for manufacturing a component, the manufacturing method comprising the following steps: heat treating a cold-rolled steel sheet; and hot forming the heat-treated steel sheet and then cooling it, wherein during the heat treatment, the A value defined in the following relationship 1 is 0.6 to 1.0.

[0030] [Equation 1]

[0031] A=(T+0.2t) / 1210

[0032] (In the formula, T represents the heating temperature in °C, and t represents the total heating time in seconds.)

[0033] During the heat treatment, the material may be heated to the heating temperature at an atmosphere heating rate of 1-1000° C. / sec.

[0034] During the heat treatment, the heating temperature may be 700-1000° C. and the heating time may be 150-1000 seconds.

[0035] During the cooling after the hot forming, the cooling may be performed at a cooling rate of 10-1000° C. / sec to a temperature below the Mf temperature.

[0036] (III) Beneficial effects

[0037] According to one aspect of the present invention, a cold-rolled steel sheet for hot forming, a hot formed component, and a method for producing the same having excellent surface quality can be provided.

[0038] According to one aspect of the present invention, a cold-rolled steel sheet for hot forming, a hot-formed component and a method for manufacturing the same can be provided, which can be applied to automobile structural protection materials or reinforcement materials, etc. and have excellent surface quality and collision resistance characteristics without plating and shot peening processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 The surface qualities of Inventive Example 2 and Comparative Example 10 after hot forming are shown.

[0040] Figure 2 The phosphate coverage after hot forming of Inventive Example 1 and Comparative Example 2 is shown.

[0041] Figure 3 Shown is the change in surface quality according to thermoforming conditions.

[0042] Figure 4 These are photographs showing the surfaces of Inventive Example 11 and Comparative Example 24 after thermoforming. Best Mode for Carrying Out the Invention

[0043] Below, the preferred specific embodiments of the present invention are described. The specific embodiments of the present invention can be modified into various forms, and it should not be interpreted that the scope of the present invention is limited to the specific embodiments described below. This specific embodiment is provided to explain the present invention in more detail to those skilled in the art.

[0044] The present inventors have recognized that, in the case of non-plated cold-rolled steel sheets for hot forming, excellent surface quality of the parts cannot be ensured due to the oxide layer generated during the heat treatment performed when manufacturing the hot-formed parts, and a shot peening process is required to remove the oxide layer. In order to solve the above-mentioned problems while ensuring excellent surface quality, the present inventors have conducted intensive research.

[0045] As a result, it was confirmed that by precisely controlling the alloy composition and manufacturing conditions, in particular, by precisely controlling the contents of Cr, Si, and Mn and the rate of change of the atmosphere temperature during annealing, a composite oxide layer of the target thickness can be formed during annealing, and by controlling the hot forming process conditions, excellent surface quality can be ensured without the need for a shot peening process, thereby completing the present invention.

[0046] Hereinafter, the present invention will be described in detail.

[0047] Hereinafter, the cold-rolled steel sheet of the present invention will be described in detail.

[0048] 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.

[0049] 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.

[0050] In the present invention, unless otherwise specified, % indicating the content of each element is based on weight.

[0051] The base steel plate of the cold rolled steel plate according to one aspect of the present invention may contain, in weight %, C: 0.05-0.4%, Si: 0.5-3.0%, Cr: 0.3-5.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, N: 0.001-0.02%, the balance of Fe and other inevitable impurities.

[0052] Carbon (C): 0.05-0.4%

[0053] Carbon (C) is an essential element for improving the strength of heat-treated parts and needs to be added in an appropriate amount. When the content of carbon (C) is less than 0.05%, there may be a problem of difficulty in ensuring sufficient strength. On the other hand, when the content of carbon (C) exceeds 0.4%, the strength of the hot-rolled material is too high when the hot-rolled material is cold-rolled, the cold rolling property is significantly deteriorated, and the spot weldability may be greatly reduced. According to another aspect of the present invention, the lower limit of the carbon (C) content can be 0.06%. According to another aspect of the present invention, the upper limit of the carbon (C) content can be 0.38%, and according to another aspect, the upper limit of the carbon (C) content can be 0.36%.

[0054] Silicon (Si): 0.5-3.0%

[0055] Silicon (Si) plays an important role in enriching the surface of the cold-rolled steel sheet when annealing it on a continuous annealing line to form a silicon (Si)-based oxide layer, and inhibits the formation of Fe, Mn, and Cr oxide layers in the hot forming process, thereby ensuring the spot weldability of the parts. When the content of silicon (Si) is less than 0.5%, the above effect may be insufficient. According to another aspect of the present invention, the lower limit of the silicon (Si) content may be 0.8%. On the other hand, when the content of silicon (Si) exceeds 3.0%, an overly thick Si-based amorphous oxide layer is formed, so there may be a problem of reduced spot weldability. According to another aspect of the present invention, the upper limit of the silicon (Si) content may be 2.8%.

[0056] Chromium (Cr): 0.3-5.0%

[0057] Chromium (Cr) not only improves the hardenability of the steel sheet, but also can play a role in stably helping to form a surface Si-based amorphous oxide layer through appropriate reaction with Si. When the content of chromium (Cr) is less than 0.3%, the above effect may be insufficient. According to another aspect of the present invention, the lower limit of the chromium (Cr) content can be 0.5%. On the other hand, when the content of chromium (Cr) exceeds 5.0%, its effect is saturated, and there may be a problem of increased manufacturing cost. According to another aspect of the present invention, the upper limit of the chromium (Cr) content can be 4.5%.

[0058] Manganese (Mn): 0.01-4.0%

[0059] Manganese (Mn) can not only ensure the solid solution strengthening effect, but also in hot forming parts, in order to reduce the critical cooling rate for ensuring martensite, it is necessary to add manganese (Mn). When the content of manganese (Mn) is less than 0.01%, the above effect may be insufficient. According to another aspect of the present invention, the lower limit of manganese (Mn) content can be 0.05%. On the other hand, when the content of manganese (Mn) exceeds 4.0%, the strength of the steel sheet before the hot forming process is excessively increased, so the blanking operation may become difficult, and due to the addition of too much alloy iron, there may be the disadvantages of increased cost and deterioration of spot weldability. According to another aspect of the present invention, the upper limit of manganese (Mn) content can be 3.9%, and according to another aspect, the upper limit of manganese (Mn) content can be 3.8%.

[0060] Aluminum (Al): 0.001-0.4%

[0061] Aluminum (Al) can play a deoxidation role together with Si in steelmaking, thereby improving the cleanliness of steel. When the content of aluminum (Al) is less than 0.001%, the above effect may be insufficient. According to another aspect of the present invention, the lower limit of the aluminum (Al) content may be 0.005%. On the other hand, when the content of aluminum (Al) exceeds 0.4%, the Ac3 temperature rises excessively, and there may be a problem of needing to increase the heating temperature. According to another aspect of the present invention, the upper limit of the aluminum (Al) content may be 0.3%, and according to another aspect, the upper limit of the aluminum (Al) content may be 0.2%.

[0062] Phosphorus (P): 0.001-0.05%

[0063] Phosphorus (P) is an impurity. In order to control the content of phosphorus (P) to less than 0.001%, a large amount of manufacturing cost may be required, so the lower limit of the phosphorus (P) content may be limited to 0.001%. On the other hand, when the content of phosphorus (P) exceeds 0.05%, the weldability of the hot-formed parts may be greatly reduced. According to another aspect of the present invention, the upper limit of the phosphorus (P) content may be 0.03%.

[0064] Sulfur (S): 0.0001-0.02%

[0065] Sulfur (S) is an impurity. In order to control the content of sulfur (S) to less than 0.0001%, a large amount of manufacturing cost may be required, so the lower limit of the sulfur (S) content may be limited to 0.0001%. On the other hand, when the content of sulfur (S) exceeds 0.02%, the ductility, impact properties and weldability of the parts may be hindered. According to another aspect of the present invention, the upper limit of the sulfur (S) content may be 0.01%.

[0066] Nitrogen (N): 0.001-0.02%,

[0067] Nitrogen (N) is an impurity. In order to control the content of nitrogen (N) to less than 0.001%, a large amount of manufacturing cost may be required, so the lower limit of the nitrogen (N) content may be limited to 0.001%. On the other hand, when the nitrogen (N) content exceeds 0.02%, not only cracks are easily generated during slab continuous casting, but also the impact properties may be reduced. According to one aspect of the present invention, the upper limit of the nitrogen (N) content may be 0.01%.

[0068] The steel material of the present invention may contain a balance of iron (Fe) and inevitable impurities in addition to the above composition. Inevitable impurities may be accidentally mixed in the usual manufacturing process, and therefore these impurities cannot be excluded. Such impurities are well known to those skilled in the art of ordinary steel manufacturing, and therefore all of them are not particularly mentioned in this specification.

[0069] The base steel plate of the cold-rolled steel plate according to one aspect of the present invention may contain one or more of Ti: 0.001-0.4%, Nb: 0.001-0.4%, Zr: 0.001-0.4%, V: 0.001-0.4%, B: 0.0001-0.01%, Mo: 0.001-1.0%, W: 0.001-1.0%, Cu: 0.005-2.0%, Ni: 0.005-2.0%, Sb: 0.001-1.0%, Sn: 0.001-1.0%, and rare earth elements (REM): 0.0001-0.02%.

[0070] Titanium (Ti), niobium (Nb), zirconium (Zr), vanadium (V): 0.001-0.4%

[0071] Titanium (Ti), niobium (Nb), zirconium (Zr), and vanadium (V) are effective in improving the strength of heat-treated parts and stabilizing retained austenite and improving impact toughness by forming fine precipitates. When the content of titanium (Ti), niobium (Nb), zirconium (Zr), and vanadium (V) (when two or more are added, it refers to their sum) is less than 0.001%, the above effects may be insufficient. When the content of titanium (Ti), niobium (Nb), zirconium (Zr), and vanadium (V) exceeds 0.4%, not only the effects are saturated, but also the cost may increase due to the addition of too much alloy iron.

[0072] Boron (B): 0.0001-0.01%

[0073] Boron (B) is an element that can improve hardenability even when added in a small amount, and segregates at the original austenite grain boundaries to suppress the brittleness of hot-formed parts caused by grain boundary segregation of P and / or S. When the content of boron (B) is less than 0.001%, the above effect may be insufficient, and when the content of boron (B) exceeds 0.01%, not only is its effect saturated, but hot brittleness may be caused during hot rolling. According to one aspect of the present invention, the upper limit of the boron (B) content may be 0.005%.

[0074] Molybdenum (Mo), tungsten (W): 0.001-1.0%

[0075] Molybdenum (Mo) and tungsten (W) are added to improve hardenability and strength and grain refinement through precipitation strengthening effect. When the content of molybdenum (Mo) and tungsten (W) is less than 0.001%, the above effects are insufficient, and when the content of molybdenum (Mo) and tungsten (W) exceeds 1.0%, not only the effects are saturated, but also there may be a problem of increased cost.

[0076] Copper (Cu), Nickel (Ni): 0.005-2.0%

[0077] Copper (Cu) can be added as an element to improve strength by forming fine precipitates. In addition, when copper (Cu) is added alone, hot brittleness may be caused, so nickel (Ni) may be added as needed. When the content of copper (Cu) and nickel (Ni) is less than 0.005%, the above effect may be insufficient, and when the content of copper (Cu) and nickel (Ni) exceeds 2.0%, it may lead to excessive increase in cost.

[0078] Antimony (Sb), Tin (Sn): 0.001-1.0%

[0079] Antimony (Sb) and tin (Sn) have the effect of inhibiting the formation of oxides that may be generated at the surface grain boundaries of hot-rolled steel materials to which Si is added, thereby inhibiting the dent defects caused by the detachment of the surface grain boundaries during the annealing of the cold-rolled material. In order to obtain the above-mentioned effects, antimony (Sb) and tin (Sn) of 0.001% or more may be added. On the other hand, when the content of antimony (Sb) and tin (Sn) exceeds 1.0%, not only the cost is excessively increased, but also the solid solution at the grain boundaries of the slab may cause edge cracks of the coil during hot rolling.

[0080] Rare earth elements (REM): 0.0001-0.02%

[0081] Rare earth elements (REM) can control the activity of Fe in steel, thereby controlling the thickness of the Fe oxide scale formed on the surface during hot forming. In order to obtain this effect, it is preferred to add 0.0001% or more of rare earth elements (REM). On the other hand, when the content of rare earth elements (REM) exceeds 0.02%, the ability to control the activity of Fe is lost, and thus the surface quality may deteriorate. According to one aspect of the present invention, the content of rare earth elements (REM) can be controlled to be less than 0.01%.

[0082] The first oxide layer according to one aspect of the present invention may include two or more of Fe, Mn, Cr, and Si, and may have a thickness of 5-500 nm.

[0083] 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. In order to control the thickness of the second oxide layer of the component after hot forming, it is necessary to accurately control the thickness by controlling the heating rate of the atmosphere temperature when manufacturing the cold-rolled steel sheet. According to one aspect of the present invention, the sum of the contents of Si, Mn, and Cr can be 30% or more in terms of weight % relative to the first oxide layer itself, and according to another aspect, the sum of the contents of Si, Mn, and Cr can be 90% or less.

[0084] When the thickness of the first oxide layer is less than 5 nm, an excessively thick second oxide layer is formed after hot forming, so that the effect of improving the surface quality is minimal, and it may be difficult to ensure excellent surface quality. On the other hand, when the thickness of the first oxide layer exceeds 500 nm, it is not easy to form a second oxide layer after hot forming, so that the phosphate treatment property becomes poor, and it is difficult to ensure excellent surface quality, and it may be difficult to ensure sufficient coating corrosion resistance. The upper limit of the thickness of the first oxide layer according to one aspect of the present invention may be 490 nm, and the lower limit of the thickness of the first oxide layer according to another aspect may be 5.5 nm. The first oxide layer of the present invention may be formed continuously or discontinuously.

[0085] According to one aspect of the present invention, the first oxide layer may include two or more of Fe, Mn, Cr, and Si, and may have a thickness of 5.0-500.0 nm.

[0086] Hereinafter, the microstructure of the cold-rolled steel sheet of the present invention will be described in detail.

[0087] In the present invention, unless otherwise specified, % indicating the fraction of fine structures is based on area.

[0088] The cold-rolled steel sheet according to one aspect of the present invention may contain ferrite and cementite. In the present invention, the area fractions thereof are not particularly limited, but more preferably, the ferrite and cementite may be 5 area % or more.

[0089] When a blank is manufactured in order to form a cold-rolled steel sheet into a hot-formed part, die wear may easily occur when the strength of the blank is too high. More specifically, the feature of the microstructure may refer to the microstructure of the base steel sheet of the cold-rolled steel sheet. When this is not considered, bainite, martensite, etc. may be included, but bainite, martensite, etc. are not excluded.

[0090] Hereinafter, the components of the present invention will be described in detail.

[0091] A component according to an aspect of the present invention may include a base steel plate, a first oxide layer formed on the base steel plate, and a second oxide layer formed on the first oxide layer.

[0092] The composition of the base steel sheet of the component according to one aspect of the present invention is the same as that of the above-mentioned cold-rolled steel sheet, and therefore will not be described separately.

[0093] The second oxide layer according to one aspect of the present invention may be composed of an Fe-based oxide, and may have a thickness of 0.1-10 μm.

[0094] When the thickness of the second oxide layer exceeds 10 μm, there is a problem that it is difficult to ensure excellent surface quality due to excessive oxide formation, for example, peeling of surface oxide after hot forming, etc. On the other hand, when the thickness of the second oxide layer is less than 0.1 μm, phosphate treatment property becomes poor and it may be difficult to ensure excellent surface quality. According to one aspect of the present invention, the sum of the contents of Si, Mn and Cr may be less than 30% in terms of weight % relative to the second oxide layer itself, and preferably exceeds 0%.

[0095] Hereinafter, the microstructure of the component of the present invention will be described in detail.

[0096] In the present invention, unless otherwise specified, % indicating the fraction of fine structures is based on area.

[0097] The microstructure of the component according to one aspect of the present invention may contain martensite or bainite as a main phase.

[0098] The hot-formed part of the present invention may contain martensite or bainite as a main phase to ensure high strength. More specifically, the microstructure feature may refer to the microstructure of the base steel plate of the part. In the present invention, the main phase may refer to the phase with the largest area fraction among the multiple phases constituting the microstructure. In particular, although the area fraction is not particularly limited, according to one aspect of the present invention, it may be 5 area % or more.

[0099] Hereinafter, the method for producing the cold-rolled steel sheet and the component according to the present invention will be described in detail.

[0100] The cold-rolled steel sheet according to one aspect of the present invention can be manufactured by reheating, hot rolling, coiling, cold rolling, and continuously annealing a steel billet satisfying the above-mentioned alloy composition.

[0101] Reheating

[0102] The steel billet satisfying the alloy composition of the present invention may be reheated to a temperature range of 1000-1300°C.

[0103] When the reheating temperature is lower than 1000° C., there is a problem that it is difficult to homogenize the slab structure, and when the reheating temperature exceeds 1300° C., excessive oxides may be formed and the manufacturing cost may increase.

[0104] Hot Rolling

[0105] The reheated steel slab may be hot rolled at a finishing rolling temperature of Ar3 to 1000°C.

[0106] When the finishing temperature is lower than Ar3, due to the ease of two-phase rolling, a mixed grain structure may be generated in the surface layer, and it may be difficult to control the shape of the hot-rolled steel sheet. On the other hand, when the finishing temperature exceeds 1000°C, the grains of the hot-rolled steel sheet may become coarse.

[0107] Cooling and winding

[0108] The hot rolled steel sheet may be cooled and coiled at a temperature ranging from Ms to 750°C.

[0109] When the coiling temperature is lower than Ms (martensitic transformation start temperature), the strength of the hot-rolled steel sheet increases excessively, which may reduce the cold rolling property. On the other hand, when the coiling temperature exceeds 750°C, the thickness of the oxide layer increases, causing surface grain boundary oxidation, so the pickling property deteriorates, and the problem of causing surface grain boundary detachment may occur during annealing in a continuous annealing furnace. During cooling, the cooling rate is not particularly limited, and air cooling may be performed.

[0110] Cold Rolling

[0111] The coiled steel sheet may be cold rolled.

[0112] In the present invention, the reduction ratio of cold rolling is not particularly limited, but in order to ensure the target thickness, the rolling can be performed at a reduction ratio of 30 to 80%.

[0113] In the present invention, in order to more accurately control the thickness of the steel plate, cold rolling may be performed, and pickling may be performed before cold rolling.

[0114] Continuous annealing

[0115] The cold-rolled steel sheet can be subjected to continuous annealing. During the continuous annealing, the heating rate at an atmosphere temperature from room temperature to 700°C can be 3.0-20.0°C / second, the heating rate at an atmosphere temperature of 700-800°C can be 0.08-1.5°C / second, the heating rate at an atmosphere temperature of 800-900°C can be 0.01-1.5°C / second, and the heating rate at an atmosphere temperature of 900-1000°C can be 0.01-1.0°C / second.

[0116] In order to accurately control the thickness of the first oxide layer on the surface during continuous annealing, in the present invention, the heating rate of the atmosphere temperature is more strictly controlled. When the heating rate of the atmosphere temperature does not reach the lower limit specified, too much first oxide layer is formed and exceeds 500 nm, and a second oxide layer of sufficient thickness cannot be ensured after hot forming, so the phosphate treatment property is deteriorated, and thus excellent surface quality may not be ensured. On the other hand, when the heating rate of the atmosphere temperature exceeds the upper limit specified, the formation of the first oxide layer is not obvious, and too much second oxide layer is formed after hot forming, so that the surface quality may be deteriorated, such as surface oxide scale peeling, etc.

[0117] According to one aspect of the present invention, the continuous annealing can be performed in a temperature range of 700-900° C. for the steel plate. When the annealing temperature is lower than 700° C., the rolling structure generated by cold rolling may be difficult to recover and recrystallize. On the other hand, when the annealing temperature exceeds 900° C., the annealing equipment may be deteriorated, which may become a factor in increasing process costs due to frequent equipment replacement, etc.

[0118] According to one aspect of the present invention, the continuous annealing time may be 1-1000 seconds. When the annealing time is less than 1 second, it is difficult to obtain the annealing effect, while when the annealing time exceeds 1000 seconds, productivity may be reduced.

[0119] The component according to one aspect of the present invention may be manufactured by subjecting the cold-rolled steel sheet manufactured by the above method to heat treatment, hot forming, and cooling.

[0120] Heat Treatment

[0121] The cold rolled steel sheet according to one aspect of the present invention is heat treated, wherein the value of A defined in the following relational expression 1 may be 0.6 to 1.0, and the heat treatment may be performed after being heated to a heating temperature at a heating rate of 1-1000° C. / sec.

[0122] In the present invention, in order to accurately control the thickness of the second oxide layer, the heating temperature and the heating time can be controlled by the relational expression 1. When the A value defined in the relational expression 1 is less than 0.6, the thickness of the second oxide layer formed during the heat treatment is not obvious, and it may be difficult to ensure excellent surface quality, such as deterioration of phosphate treatment properties. On the other hand, when the A value defined in the relational expression 1 exceeds 1.0, the thickness of the second oxide layer is too thick, and there is a problem of difficulty in ensuring excellent surface quality due to the peeling of the oxide layer.

[0123] The heating rate in the heat treatment step may refer to the heating rate of the atmosphere of the continuous annealing furnace. When the heating rate is less than 1°C / second, it may be difficult to fully ensure productivity, and when the heating rate exceeds 1000°C / second, there may be a problem of requiring high-cost equipment. According to one aspect of the present invention, the heating temperature may be above 700°C, and the heating temperature may be below 1000°C. According to one aspect of the present invention, the heating time may be above 150 seconds, and the heating time may be below 1000 seconds.

[0124] [Equation 1]

[0125] A=(T+0.2t) / 1210

[0126] (In the formula, T represents the heating temperature in °C, and t represents the total heating time in seconds.)

[0127] Thermoforming and cooling

[0128] The heat-treated steel sheet may be hot-formed and then cooled to a temperature below the Mf temperature at a cooling rate of 10-1000° C. / sec.

[0129] When the cooling rate is less than 10°C / sec, unnecessary ferrite and pearlite are formed, making it difficult to ensure the desired level of tensile strength. On the other hand, when the cooling rate exceeds 1000°C / sec, expensive special cooling equipment is required to control the cooling rate, so productivity may be reduced.

[0130] When the cooling end temperature exceeds Mf (martensitic transformation end temperature) and the cooling is terminated and then cooled to room temperature again, it may be difficult to ensure the shape freezeability of the hot formed part.

[0131] The thermoformed component according to one aspect of the present invention manufactured as described above can have a tensile strength of 500 MPa or more, and thus can ensure characteristics excellent in strength and collision resistance.

[0132] The present invention is described in more detail below by way of examples. However, it should be noted that the following examples are only used to illustrate the present invention and are described in more detail, and are not intended to limit the scope of the present invention. DETAILED DESCRIPTION

[0133] The present invention is described in more detail below by way of examples. However, it should be noted that the following examples are only used to illustrate the present invention and are described in more detail, and are not intended to limit the scope of the present invention.

[0134] (Example)

[0135] A slab having a thickness of 40 mm and 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 finishing temperature of 900° C. to produce a hot rolled steel sheet having 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 rolling reduction rate of 50% to produce a cold rolled steel sheet.

[0136] [Table 1]

[0137]

[0138] The cold rolled steel sheet manufactured as described above was continuously annealed under the conditions of Table 2 below. At this time, the continuous annealing was performed at a steel sheet temperature of 780°C. In addition, heat treatment was performed under the conditions of Table 2 below, and cooling was performed after hot forming. During the heat treatment, the heating rate was 5°C / second, and after hot forming, the steel sheet was cooled to room temperature at a cooling rate of 30°C / second.

[0139] [Table 2]

[0140]

[0141]

[0142] [Equation 1] A=(T+0.2t) / 1210

[0143] (In the formula, T represents the heating temperature in °C, and t represents the total heating time in seconds.)

[0144] Table 3 below shows the thickness of the first oxide layer of the cold-rolled steel sheet measured after continuous annealing, and also shows the thickness of the second oxide layer of the component measured after subsequent heat treatment, hot forming and cooling. The thickness of the first oxide layer is measured using a transmission electron microscope (TEM), and the thickness of the second oxide layer is the average value of the results of measurement at three locations using a transmission electron microscope (TEM) and an electron beam microanalysis method (EPMA). In addition, for items of poor quality, whether the oxide scale is peeled off is visually observed and shown. When visually observed, when the area is 5mm 2 When more than 10 peelings of the above oxide scale occur, it is judged to be poor. The phosphate coverage is to measure the area of ​​the part where phosphate crystals are not formed after observing the structure through a scanning electron microscope (SEM), and judge whether the quality is poor. At this time, when the phosphate crystal formation part exceeds 70%, it is judged that the phosphate characteristics are good. In addition, for the manufactured parts, the yield strength, tensile strength and elongation are measured and shown. The yield strength, tensile strength and elongation are based on the ISO6892 standard, using a JIS-5 test piece for room temperature tensile test.

[0145] [Table 3]

[0146]

[0147]

[0148]

[0149] The cold-rolled steel sheets and parts satisfying the alloy composition and manufacturing conditions of the present invention form the first oxide layer and the second oxide layer within the thickness ranges proposed in the present invention, and also ensure excellent surface quality after hot forming.

[0150] Figure 1 The photographs are shown to show the surface quality after hot forming of Inventive Example 2 and Comparative Example 10. Compared with Inventive Example 2, Comparative Example 10 formed thick oxide, and thus it was confirmed that the quality was inferior.

[0151] Figure 2 The phosphate coverage after hot forming is shown for Inventive Example 1 and Comparative Example 2. In Comparative Example 2, it was confirmed that less than 70% of phosphate crystals were formed.

[0152] Figure 3The change in surface quality according to the heat treatment temperature and time conditions is shown. The surface state is observed by adjusting the hot forming temperature and time. When the heat treatment conditions are not within the range limited by Relational Formula 1, it can be confirmed that too much second oxide layer is formed on the surface after heat treatment, so the surface oxide scale peels off, or the surface quality deteriorates due to the formation of too much first oxide layer, such as poor phosphate treatment performance.

[0153] Figure 4 These are photographs of the surfaces after hot forming of Inventive Example 11 and Comparative Example 24. Comparative Example 24 was heat treated under heat treatment conditions exceeding those limited by Relational Expression 1, and compared with Inventive Example 11, oxide scale was confirmed by visual observation of the surface.

[0154] In Comparative Examples 1, 4, 5, 7 and 10, the contents of the elements did not meet the required ranges of the present invention, and thus the first oxide layer could not be properly formed. Figure 1 As shown, the thickness of the second oxide layer after hot forming exceeds 10 μm, and too much surface oxide scale is formed after hot forming, so excellent surface quality cannot be ensured.

[0155] In Comparative Examples 2, 3, 6, 8, 9 and 11, the contents of the components exceeded the required ranges of the present invention, and excessive first oxide layers were formed after annealing. Therefore, a sufficient second oxide layer was not formed during hot forming, and less than 70% of phosphate crystals were formed, resulting in poor surface quality.

[0156] In addition, in the cases of Comparative Examples 12, 15, 17, 18 and 20, in which at least one of the heating rates in each interval was out of the range proposed in the present invention during continuous annealing, the first oxide layer was not sufficiently formed, and an excessive amount of the second oxide layer was formed after hot forming, so that excellent surface quality, such as peeling of the surface oxide scale, could not be ensured.

[0157] In the case of Comparative Examples 13, 14, 16 and 19, the heating rate of the atmosphere temperature in each interval did not reach the lower limit of the heating rate specified in the present invention, resulting in excessive first oxide layer formation and poor phosphate treatability, thus failing to ensure excellent surface quality.

[0158] In addition, in the case of Comparative Examples 21 and 22 where the atmosphere temperature in each interval did not reach part of the lower limit or exceeded the upper limit, too much first oxide layer was formed, making it difficult to form a second oxide layer after hot forming, so the phosphate treatability deteriorated and excellent surface quality could not be ensured.

[0159] In the case of Comparative Examples 23, 25, 27 and 29, the lower limit of the conditions restricted in Relationship 1 was not reached, so the second oxide layer could not be fully formed, and thus poor phosphate treatability was shown. In the case of Comparative Examples 24, 26, 28 and 30 that exceeded the upper limit of the proposed range, the thickness of the second oxide layer formed became thicker, so a large amount of Fe oxide scale was generated, resulting in poor surface quality.

[0160] The present invention has been described in detail above by using the embodiments, but embodiments different from these are also possible. Therefore, the technical concept and scope of the claims are not limited to the embodiments.

Claims

1. A cold-rolled steel sheet comprising a base steel sheet and a first oxide layer on the base steel sheet, wherein the first oxide layer comprises two or more of Fe, Mn, Cr, and Si, and has a thickness of 5-500 nm.

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

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

4. The cold-rolled steel sheet according to any one of claims 1 to 3, wherein: In terms of weight %, the sum of the contents of Si, Mn and Cr in the first oxide layer is greater than 30%.

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 fine structure.

6. A component comprising a base steel plate, a first oxide layer on the base steel plate, and a second oxide layer on the first oxide layer, wherein the first oxide layer contains two or more of Fe, Mn, Cr, and Si, and the second oxide layer is composed of an Fe-based oxide, and the second oxide layer has a thickness of 0.1-10 μm.

7. The component according to claim 6, wherein The base steel plate comprises, in weight%, C: 0.05-0.4%, Si: 0.5-3.0%, Cr: 0.3% or more and less than 5.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, N: 0.001-0.02%, the balance of Fe and other inevitable impurities.

8. The component according to claim 7, wherein The base steel plate contains one or more of Ti: 0.001-0.4%, Nb: 0.001-0.4%, Zr: 0.001-0.4%, V: 0.001-0.4%, B: 0.0001-0.01%, Mo: 0.001-1.0%, W: 0.001-1.0%, Cu: 0.005-2.0%, Ni: 0.005-2.0%, Sb: 0.001-1.0%, Sn: 0.001-1.0%, and rare earth elements (REM): 0.0001-0.02%.

9. A component according to any one of claims 6 to 8, wherein In terms of weight %, the sum of the contents of Si, Mn and Cr in the second oxide layer is less than 30%.

10. A component according to any one of claims 6 to 9, wherein The microstructure of the component contains martensite or bainite as a main phase.

11. A component according to any one of claims 6 to 10, wherein The tensile strength of the component is 500 MPa or more.

12. A method for manufacturing a cold-rolled steel sheet, comprising the following steps: reheating the steel billet; hot rolling the reheated steel billet; Cooling and coiling the hot-rolled steel plate; cold rolling the coiled steel plate; as well as The cold-rolled steel sheet is continuously annealed. During the continuous annealing, the heating rate from room temperature to 700°C is 3.0-20.0°C / second, the heating rate from 700-800°C is 0.08-1.5°C / second, the heating rate from 800-900°C is 0.01-1.5°C / second, and the heating rate from 900-1000°C is 0.01-1.0°C / second.

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

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

15. The method for manufacturing a cold-rolled steel sheet according to any one of claims 12 to 14, wherein: The reheating is performed at a temperature range of 1000-1300° C., the hot rolling is performed at a finishing temperature of Ar3 to 1000° C., the coiling is performed at a temperature range of Ms to 750° C., and the cold rolling is performed at a reduction ratio of 30-80%.

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

17. A method for manufacturing a component, comprising the following steps: heat-treating the cold-rolled steel sheet according to any one of claims 12 to 16; and The heat-treated steel sheet is hot-formed and then cooled. Wherein, during the heat treatment, the value of A defined in the following relational formula 1 is 0.6 to 1.0, [Equation 1] A=(T+0.2t) / 1210 Wherein, T represents the heating temperature in °C, and t represents the total heating time in seconds.

18. The method for manufacturing a component according to claim 17, wherein: During the heat treatment, the material is heated to a heating temperature at an atmosphere heating rate of 1-1000° C. / sec.

19. The method for manufacturing a component according to claim 17 or 18, wherein: During the heat treatment, the heating temperature is 700-1000° C. and the heating time is 150-1000 seconds.

20. A method for manufacturing a component according to any one of claims 17 to 19, wherein: During the cooling after the hot forming, the cooling is performed at a cooling rate of 10-1000° C. / s to a temperature below the martensitic transformation finish (Mf) temperature.

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