KR102660481B1 - Coated steel sheet, method for manufacturing the same, and method for manufacturing hot stamping component - Google Patents

Abstract

One embodiment of the present invention includes a base steel sheet, a coating layer disposed on the base steel sheet, and an oil layer disposed on the coating layer and containing oil, wherein the oil has a kinematic viscosity at 40°C of 20 mm ² / s to 65 mm ² /s, a coated steel sheet and a manufacturing method thereof, and a manufacturing method of a hot stamping part using the coated steel sheet are disclosed.

Description

Coated steel sheet, method of manufacturing the same, and method of manufacturing hot stamping parts {COATED STEEL SHEET, METHOD FOR MANUFACTURING THE SAME, AND METHOD FOR MANUFACTURING HOT STAMPING COMPONENT}

Embodiments of the present invention relate to coated steel sheets, methods of manufacturing same, and methods of manufacturing hot stamping parts.

As environmental regulations and safety standards are strengthened in the automobile industry, the need for high-strength yet lightweight vehicle materials is increasing. Hot stamping technology is attracting attention as a method for manufacturing such high-strength and lightweight vehicle materials, and research and development on hot stamping materials is being actively conducted. The hot stamping process generally consists of heating/forming/cooling/trimming and utilizes the phase transformation and microstructure of the material during the process.

With the introduction of the hot stamping process, it became possible to manufacture parts that were difficult to form, which were impossible to manufacture with existing high-strength steel sheets. In addition, aluminum-silicon (Al-Si) hot-dip plating, which has excellent heat resistance, was applied to prevent surface oxidation of the base steel sheet due to high-temperature heating before forming. Related technologies include US Patent No. 6,296,805.

However, when a hot-dip galvanized steel sheet is heated, the iron inside the steel sheet diffuses into the aluminum plating layer, and the plating layer becomes alloyed and becomes brittle. Accordingly, there is a risk that defects such as cracks may occur in the plating layer during forming, exposing the steel sheet below the plating layer to the outside, and reducing corrosion resistance. Additionally, when plating a steel sheet, surface problems such as dross and ash may occur due to hot dip plating, and workability may be greatly reduced.

U.S. Patent Publication No. 6,296,805

Embodiments of the present invention provide a coated steel sheet that can prevent or minimize the problem of peeling off the coating layer on the surface of the steel sheet after hot stamping heat treatment, a method of manufacturing the same, and a method of manufacturing hot stamping parts using the coated steel sheet. However, these tasks are illustrative and do not limit the scope of the present invention.

In one aspect of the present invention, a base steel plate; and a coating layer disposed on the base steel sheet; and an oil layer disposed on the coating layer and containing oil, wherein the oil has a kinematic viscosity of 20 mm ² /s to 65 mm ² /s at 40°C.

In one embodiment, the adhesion amount of the oil layer may be 0.05 g/m ² to 1.0 g/m ² .

In one embodiment, the coating layer is formed of a coating composition, and the coating composition includes 40% to 75% by weight of metal particles, 5% to 20% by weight of binder, and 20% by weight of solvent based on the total weight of the coating composition. It may include % to 50% by weight.

In one embodiment, the metal particles include zinc (Zn), aluminum (Al), magnesium (Mg), silicon (Si), nickel (Ni), titanium (Ti), zirconium (Zr), or alloys thereof. can do.

In one embodiment, the thickness of the coating layer may be 5 ㎛ to 20 ㎛.

In one embodiment, the base steel sheet includes carbon (C): 0.01 wt% to 0.5 wt%, silicon (Si): 0.01 wt% to 3.0 wt%, manganese (Mn): 0.3 wt% to 3.0 wt%, phosphorus ( P): 0.1% by weight or less, sulfur (S): 0.1% by weight or less, and the remainder may include iron (Fe) and other unavoidable impurities.

In one embodiment, the base steel sheet contains boron (B): 0.0001% by weight to 0.005% by weight, titanium (Ti): 0.01% by weight to 0.1% by weight, niobium (Nb): 0.01% by weight to 0.1% by weight, chromium ( Cr): 0.01% by weight to 0.5% by weight, molybdenum (Mo): 0.01% by weight to 0.5% by weight, and nickel (Ni): 0.01% by weight to 1.0% by weight.

In another aspect of the present invention, applying a coating composition to a base steel plate; Forming a coating layer by drying the applied coating composition; and applying oil on the coating layer to form an oil layer, wherein the oil has a kinematic viscosity of 20 mm ² /s to 65 mm ² /s at 40 ° C. .

In one embodiment, the oil may be applied in an amount of 0.05 g/m ² to 1.0 g/m ² .

In one embodiment, the coating composition may include 40% to 75% by weight of metal particles, 5% to 20% by weight of binder, and 20% to 50% by weight of solvent based on the total weight of the coating composition. there is.

In one embodiment, the thickness of the coating layer after drying may be 5 ㎛ to 20 ㎛.

In another aspect of the present invention, forming a blank by cutting a coated steel sheet including a base steel sheet, a coating layer, and an oil layer; Injecting the blank into a heating furnace; and heating the blank. A method of manufacturing a hot stamping part is disclosed.

In one embodiment, in the step of heating the blank, the blank may have a surface temperature increase rate of 10°C/s to 40°C/s at a surface temperature of room temperature to 200°C.

In one embodiment, in the step of heating the blank, the blank may have a surface temperature increase rate of 10°C/s to 40°C/s at a surface temperature of 200°C to 450°C.

In one embodiment, in the step of heating the blank, the blank may have a surface temperature increase rate of 10°C/s to 30°C/s at a surface temperature of 450°C to 700°C.

In one embodiment, after heating the blank, transferring the heated blank from the heating furnace to a press mold; Forming a molded body by hot stamping the transferred blank; It may further include cooling the formed molded body.

According to embodiments of the present invention, the coated steel sheet is disposed on the coating layer and includes an oil layer containing an oil component, thereby improving the adhesion of the coating layer to the steel sheet after hot stamping heat treatment.

Accordingly, mold contamination can be reduced, workability can be improved, and maintenance costs can be reduced when manufacturing hot stamping parts.

Additionally, the corrosion resistance of the hot stamping parts being manufactured can be improved. Of course, the scope of the present invention is not limited by this effect.

1 is a cross-sectional view showing a cross-section of a coated steel sheet according to an embodiment of the present invention.
Figure 2 is a flowchart showing a method of manufacturing a hot stamping part according to an embodiment of the present invention.
Figure 3 is a flowchart showing a method of manufacturing a coated steel sheet according to an embodiment of the present invention.
Figure 4 is a schematic diagram showing a coating device for manufacturing a coated steel sheet according to an embodiment of the present invention.
Figure 5 is a graph showing the change in surface temperature of a blank over time according to an embodiment and a comparative example of the present invention.
Figure 6 is a view showing a cross-section of a blank after heat treatment according to an embodiment of the present invention and a comparative example observed using a scanning electron microscope (SEM).

Since the present invention can be modified in various ways and can have various embodiments, specific embodiments will be illustrated in the drawings and described in detail in the detailed description. The effects and features of the present invention and methods for achieving them will become clear by referring to the embodiments described in detail below along with the drawings. However, the present invention is not limited to the embodiments disclosed below and may be implemented in various forms.

In the following embodiments, terms such as first and second are used not in a limiting sense but for the purpose of distinguishing one component from another component.

In the following examples, singular terms include plural terms unless the context clearly dictates otherwise.

In the following embodiments, terms such as include or have mean that the features or components described in the specification exist, and do not exclude in advance the possibility of adding one or more other features or components.

In the following embodiments, when a part of a film, region, component, etc. is said to be on or on another part, it is not only the case where it is directly on top of the other part, but also when another film, region, component, etc. is interposed between them. Also includes cases where there are.

In the drawings, the sizes of components may be exaggerated or reduced for convenience of explanation. For example, the size and thickness of each component shown in the drawings are shown arbitrarily for convenience of explanation, so the present invention is not necessarily limited to what is shown.

In cases where an embodiment can be implemented differently, a specific process sequence may be performed differently from the described sequence. For example, two processes described in succession may be performed substantially at the same time, or may be performed in an order opposite to that in which they are described.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, and when described with reference to the drawings, identical or corresponding components will be assigned the same reference numerals.

1 is a cross-sectional view showing a cross-section of a coated steel sheet according to an embodiment of the present invention.

Referring to Figure 1, the coated steel sheet 10 according to an embodiment of the present invention includes a base steel sheet 100, a coating layer 200 located on the base steel sheet 100, and an oil layer located on the coating layer 200 ( 300).

The base steel sheet 100 may be a steel sheet manufactured by performing a hot rolling process and/or a cold rolling process on a steel slab cast to contain a predetermined content of a predetermined element.

As an example, the base steel sheet 100 may contain carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), the remainder of iron (Fe), and other unavoidable impurities. . Additionally, the steel sheet 100 may further include one or more of boron (B), titanium (Ti), niobium (Nb), chromium (Cr), molybdenum (Mo), and nickel (Ni).

In one embodiment, the base steel sheet 100 contains carbon (C): 0.01 wt% to 0.5 wt%, silicon (Si): 0.01 wt% to 3.0 wt%, manganese (Mn): 0.3 wt% to 3.0 wt%, Phosphorus (P): 0.1% by weight or less, sulfur (S): 0.1% by weight or less, and the remainder may contain iron (Fe) and other unavoidable impurities. In addition, the base steel sheet 100 contains boron (B): 0.0001% by weight to 0.005% by weight, titanium (Ti): 0.01% by weight to 0.1% by weight, niobium (Nb): 0.01% by weight to 0.1% by weight, and chromium (Cr). ): 0.01% by weight to 0.5% by weight, molybdenum (Mo): 0.01% by weight to 0.5% by weight, and nickel (Ni): 0.01% by weight to 1.0% by weight.

Carbon (C) is a major element that determines the strength and hardness of the base steel sheet 100, and is added for the purpose of securing the tensile strength and hardenability characteristics of the base steel sheet 100 after the hot stamping process. As an example, carbon may be included in an amount of 0.01% by weight to 0.5% by weight based on the total weight of the base steel plate 100. If the carbon content is less than 0.01% by weight, it may be difficult to secure the mechanical strength of the base steel sheet 100. On the other hand, if the carbon content exceeds 0.5% by weight, the toughness of the base steel sheet 100 may decrease or problems with brittleness control may occur.

Silicon (Si) is a solid solution strengthening element and can improve the strength and ductility of the base steel sheet 100. Additionally, silicon can play a role in suppressing the formation of cementite, which is the starting point of cracks caused by hydrogen embrittlement. This silicon may be included in an amount of 0.01% by weight to 3.0% by weight based on the total weight of the base steel plate 100. If the silicon content is less than 0.1% by weight, it is difficult to obtain the above-mentioned effect, and conversely, if the silicon content exceeds 3.0% by weight, the coating characteristics of the base steel sheet 100 may deteriorate.

Manganese (Mn) is added to increase hardenability and strength during heat treatment. Manganese may be included in an amount of 0.3% by weight to 3.0% by weight based on the total weight of the base steel plate 100. If the manganese content is less than 0.3% by weight, the grain refining effect may not be sufficient, and the hard phase fraction of the hot stamping part may be insufficient. Conversely, if the manganese content exceeds 3.0% by weight, ductility and toughness may be reduced due to manganese segregation or pearlite bands, which may cause deterioration in bending performance and generate a heterogeneous microstructure.

Phosphorus (P) is added to prevent the toughness of the base steel sheet 100 from decreasing. Phosphorus may be included in an amount greater than 0 and less than or equal to 0.1% by weight based on the total weight of the base steel sheet (100). If the phosphorus content exceeds 0.1% by weight, iron phosphide compounds are formed, which reduces toughness and may cause cracks in the base steel sheet 100 during the manufacturing process.

Sulfur (S) may be included in an amount of more than 0 and less than or equal to 0.1% by weight based on the total weight of the base steel plate (100). If the sulfur content exceeds 0.1% by weight, hot workability deteriorates and surface defects such as cracks may occur due to the formation of large inclusions.

Boron (B) is added for the purpose of securing the hardenability and strength of the base steel sheet 100 by securing the martensite structure, and has the effect of grain refinement by increasing the austenite grain growth temperature. Boron may be included in an amount of 0.0001% by weight to 0.005% by weight based on the total weight of the base steel sheet (100). If the boron content is less than 0.0001% by weight, the effect of improving hardenability may not be sufficient. On the other hand, if the boron content exceeds 0.005% by weight, the risk of brittleness and poor elongation may increase.

Titanium (Ti) is added to strengthen hardenability and improve material quality by forming precipitates after hot stamping heat treatment. In addition, titanium forms precipitated phases such as Ti(C,N) at high temperatures, which can effectively contribute to austenite grain refinement. Titanium may be included in an amount of 0.01% by weight to 0.1% by weight based on the total weight of the base steel plate 100. If the titanium content is less than 0.01% by weight, the formation of precipitates is minimal and the grain refining effect may not be sufficient. On the other hand, if titanium is more than 0.1% by weight, a decrease in elongation and toughness may occur.

Niobium (Nb) is added to increase strength and toughness as the packet size of martensite decreases. Niobium may be included in an amount of 0.01% by weight to 0.1% by weight based on the total weight of the base steel plate 100. When niobium is included in the above range, the grain refining effect of steel materials is excellent in the hot rolling and cold rolling processes, the occurrence of cracks in slabs and brittle fractures of products during steelmaking/rolling are prevented, and the generation of steelmaking coarse precipitates is minimized. You can.

Chromium (Cr) is added for the purpose of improving the hardenability and strength of the base steel sheet 100. Chromium may be included in an amount of 0.01% by weight to 0.5% by weight based on the total weight of the base steel sheet 100. If the chromium content is less than 0.01% by weight, the effects of improving hardenability and strength may not be sufficient. On the other hand, if the chromium content exceeds 0.5% by weight, production costs may increase and the toughness of the base steel sheet 100 may decrease.

Molybdenum (Mo) is added for the purpose of improving the strength of the base steel sheet 100 by suppressing coarsening of precipitates and increasing hardenability during hot rolling and hot pressing. Molybdenum may be included in an amount of 0.01% by weight to 0.5% by weight based on the total weight of the base steel plate 100.

Nickel (Ni) is added for the purpose of improving the hardenability and toughness of the base steel sheet 100. Nickel may be included in an amount of 0.01% by weight to 1.0% by weight based on the total weight of the base steel plate 100. If the nickel content is less than 0.01% by weight, the effect of its addition is insignificant. On the other hand, if the nickel content exceeds 1.0% by weight, the processability of the base steel sheet 100 may decrease and the manufacturing cost may increase.

The coating layer 200 may be formed using a coating composition, and the coating composition may include metal particles, a binder, and a solvent. The coating composition may include about 40% to about 75% by weight of metal particles, about 5% to about 20% by weight of binder, and about 20% to about 50% by weight of solvent, based on the total weight of the coating composition. .

The metal particles may include zinc (Zn), aluminum (Al), magnesium (Mg), silicon (Si), nickel (Ni), titanium (Ti), zirconium (Zr), or alloys thereof. The coating composition may include, for example, at least one of zinc (Zn), aluminum (Al), and magnesium (Mg).

Metal particles may be included in an amount of about 40% by weight to about 75% by weight based on the total weight of the coating composition. If the content of metal particles is less than about 40% by weight, the oxidation resistance and heat resistance of the coating layer 200 may be reduced. On the other hand, if the content of metal particles exceeds about 75% by weight, the adhesion of the coating layer 200 may decrease and a powdering phenomenon may occur.

The metal particles may be spherical, oval, or flake-shaped, but are not limited thereto. However, the flake type may be preferable to ensure oxidation resistance and heat resistance.

The binder can be a conventional binder. Binders are, for example, organic compounds such as polyurethanes, isocyanates, polyesters, epoxy resins, alkyd resins, phenolic resins, melamine resins, acrylic and methacrylic resins, hydrolyzed alkylalkoxy silanes, alkoxy silanes, amino silanes or mixtures. And it may be at least one selected from the group consisting of condensed oligosiloxane, polysiloxane, silicone, and silicone resin.

The binder may be included in an amount of about 5% by weight to about 20% by weight, preferably about 10% by weight to about 20% by weight, based on the total weight of the coating composition. If the binder content is less than about 5% by weight, the adhesion of the coating layer 200 may decrease before hot stamping heat treatment, resulting in peeling and powdering. On the other hand, if the binder content exceeds about 20% by weight, the binder remains after hot stamping heat treatment, so the adhesion of the coating layer 200 may decrease and a powdering phenomenon may occur.

The solvent can be a conventional solvent. Solvents include, for example, alcohol-based solvents such as ethanol, methanol isopropanol, and normal butanol, ketone-based solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone, ether-based solvents such as ethyl acetate and butyl acetate, and toluene. , may include one or more selected from the group consisting of aromatic hydrocarbon-based solvents such as xylene, but are not limited thereto.

The solvent may be included in an amount of about 20% by weight to about 50% by weight based on the total weight of the coating composition. If the solvent content is less than about 20% by weight, the drying performance of the coating composition may be reduced when forming the coating layer 200. On the other hand, if the solvent content exceeds about 50% by weight, changes in the properties of the coating composition may occur due to the volatility of the solvent.

The coating layer 200 may be formed on at least one surface of the base steel sheet 100. The thickness of the coating layer 200 may be about 5 ㎛ to about 20 ㎛, preferably about 8 ㎛ to about 20 ㎛. The thickness of the coating layer 200 is based on the dried thickness and may mean the average thickness over the entire area of the coating layer 200. If the thickness of the coating layer 200 is less than about 5 ㎛, the oxidation resistance and heat resistance of the coating layer 200 are reduced, and thickness control may be difficult when forming the coating layer 200. If the thickness of the coating layer 200 exceeds about 20 ㎛, the total amount of binder in the coating layer 200 increases, resulting in residual binder after hot stamping heat treatment, which may cause peeling of the coating layer 200.

The oil layer 300 may be formed on the surface of the coating layer 200. The oil layer 300 may include an oil component. The oil layer 300 formed on the surface of the coating layer 200 is used to determine the surface temperature of the blank (i.e., the cut coated steel sheet 10), the reaction within the coating layer, and the resulting adhesion of the coating layer 200 in the blank heating step described later. It can have an impact.

The oil may have a kinematic viscosity at 40°C of about 20 mm ² /s to about 65 mm ² /s, preferably about 25 mm ² /s to about 65 mm ² /s. If the kinematic viscosity (40°C) of the oil is less than about 20 mm ² /s, the effect of improving the adhesion of the coating layer 200 due to the formation of the oil layer 300 may be minimal. On the other hand, if the kinematic viscosity (40°C) of the oil exceeds about 65 mm ² /s, the surface of the hot stamping part manufactured after hot stamping heat treatment may be stained. In one embodiment, the oil may be "ANTICORIT RP 4107 S" or "ANTICORIT PL 3802 39 S", a commercial product from FUCHS. In this specification, kinematic viscosity (40°C) means kinematic viscosity measured at 40°C, and may mean kinematic viscosity measured by JIS K 2283:2000 “Crude oil and petroleum products - Kinematic viscosity test method and viscosity index calculation method.

The adhesion amount of the oil layer 300 may be about 0.05 g/m ² to about 1.0 g/m ² , and preferably about 0.05 g/m ² to about 0.2 g/m ² . If the adhesion amount of the oil layer 300 is less than about 0.05 g/m ² , the oil layer 300 may not be formed uniformly on the coating layer 200, and the coating layer 200 may be damaged due to the formation of the oil layer 300. The effect of improving adhesion may be minimal. On the other hand, if the adhesion amount of the oil layer 300 exceeds about 1.0 g/m ² , the surface of the hot stamping part manufactured after hot stamping heat treatment may be stained.

The thickness of the oil layer 300 may be about 1 ㎛ or less. The thickness of the oil layer 300 is based on the thickness of the oil applied and attached to the coating layer 200, and may be the average thickness over the entire area of the oil layer 300. If the thickness of the oil layer 300 exceeds about 1 μm, a residual oil layer may exist after hot stamping heat treatment, or the surface of the hot stamping part manufactured after hot stamping heat treatment may be stained.

FIG. 2 is a flowchart showing a method of manufacturing a hot stamping part according to an embodiment of the present invention, and FIG. 3 is a flowchart showing a method of manufacturing a coated steel sheet according to an embodiment of the present invention. Hereinafter, a method of manufacturing hot stamping parts will be described with reference to FIGS. 2 and 3.

Referring to FIG. 2, the manufacturing method of a hot stamping part according to an embodiment includes a coated steel plate manufacturing step (S110), a blank forming step (S120), a blank input step (S130), a blank heating step (S140), and a blank transferring step. (S150), a molded body forming step (S160), and a molded body cooling step (S170).

As shown in Figure 3, the coated steel sheet manufacturing step (S110) includes a hot rolling step (S210), cooling/winding step (S220), cold rolling step (S230), annealing heat treatment step (S240), and coating layer formation of the steel slab. It may include a step (S250) and an oil layer forming step (S260).

The base steel plate 100 (FIG. 1) may be formed through hot rolling and/or cold rolling on a steel slab and then annealing heat treatment. First, prepare a semi-finished steel slab that is the subject of the process of forming a coated steel sheet. At this time, the steel slab may contain carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), the remainder of iron (Fe), and other unavoidable impurities. Additionally, the steel slab may further include one or more of boron (B), titanium (Ti), niobium (Nb), chromium (Cr), molybdenum (Mo), and nickel (Ni).

A reheating step of the steel slab for hot rolling is carried out. In the reheating step of the steel slab, the steel slab obtained through a continuous casting process is reheated to a predetermined temperature to re-employ the components segregated during casting. As an example, the slab reheating temperature (SRT) may be 1,200°C to 1,400°C. If the slab reheating temperature (SRT) is lower than 1,200°C, the components segregated during casting are not sufficiently re-dissolved, making it difficult to significantly homogenize the alloy elements and the solid solution effect of titanium (Ti). The higher the slab reheating temperature (SRT), the more advantageous it is for homogenization. However, if the slab reheating temperature (SRT) exceeds 1,400°C, the austenite crystal grain size increases, making it difficult to secure strength and increasing the manufacturing cost of the steel sheet due to excessive heating process. This may rise.

In the hot rolling step (S210), the reheated steel slab may be hot rolled at a predetermined finish rolling temperature. As an example, the finishing delivery temperature (FDT) may be 880°C to 950°C. At this time, if the finish rolling temperature (FDT) is lower than 880°C, it is difficult to secure the machinability of the steel sheet due to the occurrence of a mixed structure due to abnormal region rolling, and there is a problem of deterioration of machinability due to microstructure unevenness and rapid phase change. This may cause problems with sheetability during hot rolling. If the finish rolling temperature (FDT) exceeds 950°C, austenite grains may become coarse. Additionally, TiC precipitates may become coarse, which may deteriorate the performance of hot stamping parts.

In the cooling/winding step (S220), the hot rolled steel sheet can be cooled to a predetermined coiling temperature (CT) and then wound. As an example, the coiling temperature may be 550°C to 800°C. The coiling temperature affects the redistribution of carbon (C). If the coiling temperature is less than 550℃, the low-temperature phase fraction increases due to overcooling, which may increase strength, and there is a risk that the rolling load during cold rolling will intensify, and ductility This can deteriorate rapidly. Conversely, if the coiling temperature exceeds 800°C, deterioration of formability and strength may occur due to abnormal or excessive crystal grain growth.

In the cold rolling step (S230), the coiled steel sheet may be uncoiled, pickled, and then cold rolled. At this time, pickling is performed for the purpose of removing scale from the wound steel sheet, that is, the hot-rolled coil manufactured through the hot-rolling process.

The annealing heat treatment step (S240) may be a step of annealing the cold rolled steel sheet at a temperature of 700°C or higher. As an example, annealing heat treatment includes heating a cold-rolled sheet and cooling the heated cold-rolled sheet at a predetermined cooling rate.

The coating layer forming step (S250) is a step of forming a coating layer to improve corrosion resistance and heat resistance of the annealed heat-treated steel sheet. For example, in the coating layer forming step (S250), the coating layer 200 (FIG. 1) may be formed on the annealed heat-treated steel sheet, that is, the base steel sheet 100 (FIG. 1). The coating layer 200 is formed of a coating composition, which contains about 40% to about 75% by weight of metal particles, about 5% to about 20% by weight of binder, and about 20% by weight of solvent, based on the total weight of the coating composition. % to about 50% by weight.

The coating layer forming step (S250) may include applying a coating composition on the base steel plate 100 (FIG. 1) and drying the applied coating composition.

The application step may be performed through various processes of applying the coating composition to the surface of the steel plate. For example, it may be performed by one or more methods selected from the group consisting of coil coating, curtain coating, spray coating, dip coating, and dip coating using a roll coater.

In one embodiment, the application step may be performed using a coil coating method using a roll coater. In this regard, Figure 4 shows a roll-to-roll coating device 20. The coating device 20 includes a receiving portion 21 containing the coating composition, a pickup roller (PUR) 22 that picks up the coating composition, and an applicator roller that rotates in conjunction with the pickup roller 22 and coats the coating composition on the steel sheet. (APR)(23). The rotation direction of the pickup roller 22 and the rotation direction of the applicator roller 23 may be opposite to each other, and through this movement, the coating composition can be transferred from the pickup roller 22 through the applicator roller 23 to the steel plate. .

The movement direction of the applicator roller 23 and the steel plate may be forward/reverse, but the forward direction is preferable for controlling the thickness of the coating layer. The pickup roller 22 and the applicator roller 23 may be provided as one or as a pair. This coating method can be economical and efficient because it does not require a process of melting the metal compared to a plating method that melts the metal.

The drying step is a step of drying the coating composition applied on the base steel sheet 100 (Figure 1) to form a coating layer 200 (Figure 1) and strengthening the adhesion between the coating layer 200 and the base steel sheet 100. It can be. The drying step may be performed by one or more of a hot air method, an induction heating method, and a near-infrared heating method. In one embodiment, drying may be performed at Peal Metal Temperature (PMT), for example, at 80°C to 140°C for 3 to 60 seconds. The thickness of the coating layer 200 after drying may be 5 ㎛ to 20 ㎛.

The oil layer forming step (S260) may include applying oil on the coating layer 200 (FIG. 1). The oil may have a kinematic viscosity at 40°C of about 20 mm ² /s to about 65 mm ² /s. The oil may be applied on the coating layer 200 in an amount of about 0.05 g/m ² to about 1.0 g/m ² , preferably about 0.05 g/m ² to about 0.2 g/m ² . The thickness of the applied oil layer 300 may be about 1 μm or less.

The blank forming step (S120) is a step of forming a blank by cutting the coated steel plate 10 (FIG. 1) according to the purpose. In one embodiment, a blank for hot stamping can be provided by cutting the coated steel sheet 10.

The blank input step (S130) may be a step of inputting the blank, that is, the cut coated steel sheet 10 (FIG. 1) into the heating furnace. After the blank input step (S130), a blank heating step (S140) may be performed. The blank heating step (S140) may be a step of heat treating the blank in a heating furnace.

The blank heating step (S140) may include a temperature increase step and an isothermal step. The temperature raising step may be a step in which the blank passes through a plurality of sections of different temperatures provided in the heating furnace and is heated in multiple stages. The temperature in the heating furnace is set to increase in the direction from the entrance of the heating furnace where the blank is input to the outlet of the heating furnace where the blank is taken out, so that the temperature of the coated steel sheet can be gradually raised. The heating furnace is provided with a plurality of sections, and the plurality of sections may be provided with a uniform temperature. The isothermal step after the temperature raising step may be a step of crack heating the blank to a constant temperature in the last part of the heating furnace. The temperature of the section where the blank is cracked and heated may be higher or the same as the temperature of the sections where the blank is heated in multiple stages. However, the present invention is not limited to this. In another embodiment, the blank heating step (S140) may be a step of heat treating the blank in a heating furnace at a constant temperature.

In one embodiment, the blank heating step (S140) may be performed in a heating furnace maintained at about 800° C. to about 1000° C., preferably about 850° C. to about 950° C., on the blank, that is, the cut coated steel sheet 10. there is.

The blank heating step (S140) may be performed for 2 to 10 minutes. As an example, the blank may remain in the heating furnace for 2 to 10 minutes. In one embodiment, the time for multi-stage heating and crack heating of the blank in the heating furnace may be 2 to 10 minutes. If the blank stays in the heating furnace for less than 2 minutes, it may be difficult to sufficiently crack at the desired cracking temperature. In addition, when the blank stays in the heating furnace for more than 10 minutes, the amount of hydrogen penetrating into the blank increases, increasing the risk of delayed rupture and reducing corrosion resistance after hot stamping.

In the blank heating step (S140), the oil component constituting the oil layer 300 (FIG. 2) of the blank may absorb heat and be sublimated. Accordingly, compared to a blank that does not include an oil layer, the rate of increase in surface temperature of the blank may be small under the same conditions. The oil layer can be completely removed in the step of heating the blank. Meanwhile, during heat treatment, the binder in the coating layer 200 of the blank (Figure 2) may be decomposed and discharged to the outside. The metal particles in the coating layer may be alloyed with the iron in the steel sheet.

After the blank heating step (S140), a blank transfer step (S150), a molded body forming step (S160), and a molded body cooling step (S170) may be further performed.

The blank transfer step (S150) may be a step of transferring the heat-treated blank from the heating furnace to the press mold. In the step of transferring the heat-treated blank from the heating furnace to the press mold, the heat-treated blank may be air-cooled for 5 to 15 seconds.

The molded body forming step (S160) may be a step of forming a molded body by hot stamping the transferred blank. The molded body cooling step (S170) may be a step of cooling the formed molded body.

After being molded into the final part shape in a press mold, the molded body may be cooled to form the final product. The press mold may be provided with a cooling channel in which refrigerant circulates. It is possible to rapidly cool the heat-treated blank by circulating refrigerant supplied through a cooling channel provided in the press mold. At this time, in order to prevent the spring back phenomenon of the plate and maintain the desired shape, rapid cooling can be performed while pressing with the press mold closed. When forming and cooling the heat-treated blank, the average cooling rate can be at least 10°C/s to the martensite end temperature. The blank may be held in the press mold for 3 to 20 seconds. If the holding time in the press mold is less than 3 seconds, sufficient cooling of the material may not occur, resulting in thermal deformation due to residual heat of the product and temperature differences in each area, which may deteriorate dimensional quality. Additionally, if the holding time in the press mold exceeds 20 seconds, the holding time in the press mold becomes longer and productivity may decrease.

Figure 5 is a graph showing the change in surface temperature of a blank over time according to an embodiment and a comparative example of the present invention. Specifically, temperature change over time of a blank (A) including a base steel sheet, a coating layer, and an oil layer as an example, and a blank (B) including a base steel sheet and a coating layer but not an oil layer as a comparative example. This is a graph showing . As an example, the coating layer may include metal particles such as magnesium (Mg), aluminum (Al), and zinc (Zn) particles and a binder. Figure 5 may be the result of heat treatment performed on the blanks of one example and one comparative example under the same conditions, for example, at 950°C for 5 minutes.

Referring to FIG. 5, the blank (A) including the oil layer of one embodiment has a surface temperature of room temperature to about 700° C. compared to the blank (B) that does not include the oil layer of a comparative example. It can be seen that the temperature increase rate is low. Specifically, the blank (A) formed with an oil layer containing an oil with a kinematic viscosity (40°C) of about 20 mm ² /s to about 65 mm ² /s is stored at room temperature compared to the blank (B) without an oil layer. It can be seen that the rate of increase in surface temperature is small at a surface temperature of about 700°C. Meanwhile, at a surface temperature of about 700°C to about 950°C, the temperature increase rate of the surface temperature of the blank (A) including the oil layer is slightly greater than the temperature increase rate of the surface temperature of the blank (B) without the oil layer. You can see that they appear almost identical.

In the step of heating the blank, the oil layer of the blank may sublimate at a relatively low temperature (e.g., a temperature of about 150°C to about 250°C) while blocking oxygen from flowing into the coating layer and absorbing some of the heat. Accordingly, the rate of increase in the surface temperature of the blank may be lowered. In one embodiment, the blank (A) may have a surface temperature increase rate of 10°C/s to 40°C/s at a surface temperature of room temperature to 200°C. Additionally, the blank (A) may have a surface temperature increase rate of 10°C/s to 40°C/s at a surface temperature of 200°C to 450°C. The blank (A) may have a surface temperature increase rate of 10°C/s to 30°C/s at a surface temperature of 450°C to 700°C. Additionally, the blank (A) may have a surface temperature increase rate of 10°C/s to 40°C/s at a surface temperature of room temperature to 450°C. Here, the surface temperature increase rate means the average temperature increase rate in a predetermined surface temperature section.

Figure 6 shows an image (left) of a cross-section of a blank according to an embodiment of the present invention and a comparative example observed with a scanning electron microscope (SEM), and an energy dispersive X-ray detector (EDX) of the scanning electron microscope (SEM). Shows an image (right) in which the composition analysis of the cross section of the blank was performed using . Specifically, the image on the right shows the distribution of zinc (Zn) in the cross section of the blank.

In the step of heating the blank, if the surface temperature of the blank rises above the melting point of the zinc contained in the coating layer (about 400°C to about 450°C), zinc moves to the surface of the coating layer and is oxidized, forming zinc-based oxide. . At this time, zinc-based oxide is presumed to be a factor that reduces the adhesion of the coating layer. However, as explained with reference to FIG. 5, when the surface temperature increase rate of the blank (A) appears to be slow due to the inclusion of the oil layer, iron (Fe) diffused from the base steel sheet at a temperature lower than the melting point of zinc and within the coating layer Alloying of zinc may occur and the melting point of zinc may increase. Therefore, zinc-based oxide formed on the surface of the coating layer may be reduced as zinc is alloyed with iron and the like inside the coating layer before diffusing to the surface of the coating layer.

Referring to FIG. 6, in the case of the blank (B) that does not include the oil layer of one comparative example after heat treatment, zinc is distributed on the surface of the coating layer 200'' and forms a zinc-based oxide, whereas in one embodiment In the case of the blank (A) including the oil layer, it can be confirmed that zinc is diffusely distributed inside the coating layer (200'). According to one embodiment of the present invention, by forming an oil layer on the coating layer, the adhesion of the coating layer 200' to the base steel sheet 100' after heat treatment can be improved.

Below, the present invention will be described in more detail through examples. However, the following examples are intended to illustrate the present invention in more detail, and the scope of the present invention is not limited by the following examples. The following examples can be appropriately modified by those skilled in the art within the scope of the present invention.

oil properties Surface temperature increase rate (℃/s) oil Kinematic viscosity (40℃, mm ² /s = cst) Flash point (℃) Room temperature ~ 200℃ 200℃ ~ 450℃ Room temperature ~ 450℃ 450℃ ~ 700℃ Example 1 O 22 200-220 33.3 33.7 33.5 22.8 Example 2 O 36 200-220 32.2 32.7 32.5 23.8 Example 3 O 60 210-250 31.2 31.9 31.7 26.4 Comparative Example 1 X - - 90.9 61.1 70.6 25.8 Comparative example 2 O 16 130-150 88.9 62.1 71.6 24.8 Comparative example 3 O 18 140-160 87.5 63.1 72.3 23.2 Comparative example 4 O 90 210-250 28.8 32.9 29.2 25.1

Table 1 is a table showing the oil properties of the oil layer for manufacturing hot stamping parts according to an experimental example of the present invention, and the rate of increase in surface temperature of the blank during heat treatment.

The base steel sheets of Examples 1 to 3 and Comparative Examples 1 to 4 contained 0.01% to 0.5% by weight of carbon (C), 0.01% to 3.0% by weight of silicon (Si), and 0.3% by weight of manganese (Mn). % to 3.0% by weight, phosphorus (P) exceeding 0 but not more than 0.1% by weight, sulfur (S) exceeding 0 and not exceeding 0.1% by weight, boron (B), titanium (Ti), niobium (Nb), chromium (Cr), molybdenum ( It contains one or more of Mo) and nickel (Ni), the balance of iron, and other inevitable impurities. Hot rolling, cooling/winding, and annealing heat treatment were performed on the base steel sheets of Examples 1 to 3 and Comparative Examples 1 to 4 under the same conditions, and a coating layer was applied to improve oxidation resistance and heat resistance on the base steel sheets. was formed.

In Examples 1 to 3 and Comparative Examples 2 to 4, 0.1 g/m ² of oil was applied to the surface of the coating layer to form an oil layer. In Comparative Example 1, no oil layer was formed on the coating layer. The oil in Examples 1 to 3 used was commercial product "ANTICORIT RP 4107 S" or "ANTICORIT PL 3802 39 S" from FUCHS. Comparative Examples 2 and 3 used Beomwoo's commercial products "BW80HG", "RP955", or "BW70PS(1)". Comparative Example 4 used “ANTICORIT PLS 100T,” a commercial product from FUCHS. Referring to Table 1, Examples 1 to 3 include oils with a kinematic viscosity (40°C) of 25 mm ² /s to 60 mm ² /s, while Comparative Examples 2 and 3 have a kinematic viscosity (40°C). It includes oil with a kinematic viscosity (40°C) of less than 25 mm ² /s, and Comparative Example 4 includes oil with a kinematic viscosity (40°C) of more than 60 mm ² /s.

The coated steel sheets of Examples 1 to 3 and Comparative Examples 1 to 4 were cut to form blanks, and the blanks were put into a heating furnace and heat treated at 950°C for 5 minutes. Afterwards, hot stamping parts were manufactured using the heat-treated blank.

Referring to Table 1, the blanks of Examples 1 to 3 and Comparative Example 4 had a surface temperature increase rate of 10°C/s to 40°C/s at a surface temperature of room temperature to 200°C during heat treatment, and a temperature increase rate of 200°C/s. It has a surface temperature increase rate of 10°C/s to 40°C/s at a surface temperature of ℃ to 450°C, and a surface temperature increase rate of 10°C/s to 30°C/s at a surface temperature of 450°C to 700°C. It has a surface temperature increase rate of 10°C/s to 40°C/s at a surface temperature of room temperature to 450°C. On the other hand, the blanks of Comparative Example 1 and Comparative Example 2 had a surface temperature of room temperature to 200°C, a surface temperature of 200°C to 450°C, and a surface temperature exceeding 40°C/s in the surface temperature range of room temperature to 450°C during heat treatment. It has a temperature increase rate that exceeds 30°C/s at a surface temperature of 450°C to 700°C.

<Observation of surface appearance>

The surface appearance of the hot stamping parts of Examples 1 to 3 and Comparative Examples 1 to 4 was visually observed. If the surface appearance of the part is good, it is marked as "O", and if the surface appearance of the part is unsatisfactory, such as stains on the surface of the part, it is marked as "X". The results are shown in Table 2 below.

<Evaluation of adhesion of coating layer>

The adhesion of the coating layer was evaluated for the hot stamping parts of Examples 1 to 3 and Comparative Examples 1 to 4. A 5R bending test was performed on each part using a press machine. Afterwards, the bent specimen was reprocessed into a flat plate. An experiment was conducted by attaching and removing tape (made by Ichiban, Japan) to the outer surface where there was a lot of deformation in the bent area. Afterwards, the amount of coating layer attached to the tape was visually confirmed.

The adhesion of the coating layer of the Examples and Comparative Examples was evaluated based on the adhesion of the plating layer of the hot stamping part to which aluminum-silicon (Al-Si) hot-dip plating was applied to the base steel sheet. If the adhesion of the coating layer was excellent, it was marked as "O", and if the adhesion of the coating layer was poor, it was marked as "X". The results are shown in Table 2 below.

Surface appearance observation Adhesion evaluation Example 1 O O Example 2 O O Example 3 O O Comparative Example 1 O X Comparative example 2 O X Comparative Example 3 O X Comparative example 4 X O

Referring to Table 2, Examples 1 to 3 showed good surface appearance and excellent adhesion. On the other hand, in Comparative Examples 1 to 3, peeling of the coating layer was observed and adhesion was found to be insufficient, and in Comparative Example 4, stains existed on the surface of the part and the surface appearance was found to be poor.

Referring to Tables 1 and 2, Examples 1 to 3 include oils in which the oil layer has a kinematic viscosity (40°C) of about 20 mm ² /s to about 65 mm ² /s, and the oil layer During heat treatment, the surface temperature increase rate of the blank decreases, so that the surface temperature increase rate is 10℃/s to 40℃/s at a surface temperature of room temperature to 200℃, and 10℃/s at a surface temperature of 200℃ to 450℃. It has a surface temperature increase rate of 40°C to 40°C, has a surface temperature increase rate of 10°C/s to 30°C/s at a surface temperature of 450°C to 700°C, and has a surface temperature increase rate of 10°C/s to 30°C/s at a surface temperature of 450°C to 450°C. It can be seen that the adhesion of the coating layer is improved by having a surface temperature increase rate of ℃/s to 40℃/s.

However, Comparative Examples 1 to 3 do not include an oil layer or the oil layer includes oil with a kinematic viscosity (40°C) of less than about 20 mm ² /s, and the effect of reducing the surface temperature increase rate of the blank due to the oil layer Since it is insignificant, it can be seen that the adhesion of the coating layer is not improved.

In addition, in the case of Comparative Example 4, the surface temperature increase rate of the blank decreased during heat treatment, improving the adhesion of the coating layer, but the kinematic viscosity (40°C) of the oil contained in the oil layer exceeded 65 mm ² /s, so after heat treatment It can be seen that defects occur in the surface appearance of hot stamping parts.

Therefore, according to an experimental example of the present invention, the coated steel sheet includes an oil layer containing oil with a kinematic viscosity (40°C) of about 20 mm ² /s to about 65 mm ² /s, and the surface temperature of the blank during heat treatment By having the temperature increase rate within the above-described temperature increase rate range for each surface temperature section, the surface appearance of the hot stamping part manufactured by the hot stamping manufacturing method of the present invention can be improved while the adhesion of the oxidation-resistant coating layer can be improved.

Accordingly, mold contamination can be reduced, workability can be increased, and maintenance costs can be reduced when manufacturing hot stamping parts. Additionally, the corrosion resistance and weldability of the manufactured hot stamping parts can be improved.

The present invention has been described with reference to an embodiment shown in the drawings, but this is merely an example, and those skilled in the art will understand that various modifications and variations of the embodiment are possible therefrom. Therefore, the true scope of technical protection of the present invention should be determined by the technical spirit of the attached patent claims.

10: Coated steel plate
100, 100': Base steel plate
200, 200', 200'': Coating layer
300: Oil layer

Claims (16)

body steel plate; and
A coating layer disposed on the base steel sheet; and
An oil layer disposed on the coating layer and containing oil,
The oil has a kinematic viscosity at 40°C of 20 mm ² /s to 65 mm ² /s, a coated steel sheet. According to paragraph 1,
The coated steel sheet has an adhesion amount of the oil layer of 0.05 g/m ² to 1.0 g/m ² . According to paragraph 1,
The coating layer is formed of a coating composition,
The coating composition includes 40% to 75% by weight of metal particles, 5% to 20% by weight of binder, and 20% to 50% by weight of solvent based on the total weight of the coating composition. According to paragraph 3,
The metal particles include zinc (Zn), aluminum (Al), magnesium (Mg), silicon (Si), nickel (Ni), titanium (Ti), zirconium (Zr), or alloys thereof. According to paragraph 1,
A coated steel sheet where the thickness of the coating layer is 5 ㎛ to 20 ㎛. According to paragraph 1,
The base steel sheet includes carbon (C): 0.01% to 0.5% by weight, silicon (Si): 0.01% to 3.0% by weight, manganese (Mn): 0.3% to 3.0% by weight, and phosphorus (P): 0.1% by weight. % or less, sulfur (S): 0.1% by weight or less, and the balance containing iron (Fe) and other inevitable impurities. Coated steel sheet. According to clause 6,
The base steel sheet contains boron (B): 0.0001% by weight to 0.005% by weight, titanium (Ti): 0.01% by weight to 0.1% by weight, niobium (Nb): 0.01% by weight to 0.1% by weight, and chromium (Cr): 0.01% by weight. % to 0.5% by weight, molybdenum (Mo): 0.01% to 0.5% by weight, and nickel (Ni): 0.01% to 1.0% by weight, a coated steel sheet. Applying the coating composition onto the base steel sheet;
Forming a coating layer by drying the applied coating composition; and
It includes forming an oil layer by applying oil on the coating layer,
The oil has a kinematic viscosity at 40°C of 20 mm ² /s to 65 mm ² /s. According to clause 8,
The oil is applied in an amount of 0.05 g/m ² to 1.0 g/m ² . According to clause 8,
The coating composition includes 40% to 75% by weight of metal particles, 5% to 20% by weight of binder, and 20% to 50% by weight of solvent based on the total weight of the coating composition. . According to clause 8,
A method of manufacturing a coated steel sheet, wherein the thickness of the coating layer after drying is 5 ㎛ to 20 ㎛. Forming a blank by cutting a coated steel sheet including a base steel sheet, a coating layer, and an oil layer;
Injecting the blank into a heating furnace; and
A method of manufacturing a hot stamping part, comprising: heating the blank. According to clause 12,
In the step of heating the blank,
The blank has a surface temperature increase rate of 10°C/s to 40°C/s at a surface temperature of room temperature to 200°C. According to clause 12,
In the step of heating the blank,
The method of manufacturing a hot stamping part, wherein the blank has a surface temperature increase rate of 10°C/s to 40°C/s at a surface temperature of 200°C to 450°C. According to clause 12,
In the step of heating the blank,
The blank has a surface temperature increase rate of 10°C/s to 30°C/s at a surface temperature of 450°C to 700°C. According to clause 12,
After heating the blank,
transferring the heated blank from the heating furnace to a press mold;
Forming a molded body by hot stamping the transferred blank;
A method of manufacturing a hot stamping part, further comprising: cooling the formed molded body.

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