KR102412116B1 - Hot stamping component and manufacturing method thereof - Google Patents

Abstract

In an embodiment of the present invention, heating a blank in which an Al-Si-based pre-coating layer is formed on a steel plate and at least one surface of the steel plate; forming the heated blank using a press mold; and cooling the molded blank in the press mold; in the step of heating the blank, the precoat layer is alloyed to form a plating layer, and heating the blank includes: A first heating step of heating to a first temperature and a second heating step of heating from the first temperature to a second temperature, wherein in the first heating step, the steel sheet is directly heated by a resistance heating method, Disclosed is a method for manufacturing a hot stamping part in which the temperature increase rate of the first heating step is 8° C./s to 16° C./s.

Description

Hot stamping component and manufacturing method thereof

Embodiments of the present invention relate to hot stamping parts and methods of manufacturing the same.

Recently, as environmental regulations and safety standards in the automobile industry are strengthened, the application of high-strength steel for weight reduction and stability of automobiles is increasing. On the other hand, high-strength steel can secure high-strength characteristics compared to weight, but fracture of the material occurs during processing or a spring-back phenomenon, so that it is difficult to form a product having a complex and precise shape. Therefore, the application of hot stamping molding as a method for solving these problems is expanding.

In hot stamping forming, the steel sheet is pressed at a high temperature, so it is easy to form the steel. As a technology related thereto, there is Korean Patent Application Laid-Open No. 10-2018-0095757 (Title of the Invention: Method of Manufacturing Hot Stamping Parts). However, in hot stamping forming, since the steel sheet is heated to a high temperature, there is a problem in that the surface of the steel sheet is oxidized. In order to solve this problem, the invention of US Patent No. 6,296,805 proposes a method of hot stamping forming a steel sheet subjected to aluminum plating. According to the invention of US Patent No. 6,296,805, since the aluminum plating layer is present on the surface of the steel sheet, oxidation of the surface of the steel sheet by heating the steel sheet can be prevented.

However, when the steel sheet is heated, Fe from the steel sheet is diffused into the aluminum plating layer to alloy the aluminum plating layer, and when such an aluminum plating steel sheet is hot stamped, cracks may occur in the plating layer having brittleness due to alloying. On the other hand, since the aluminum plating layer does not have sacrificial corrosion resistance, when cracks occur in the plating layer, the surface of the steel sheet is exposed and the corrosion resistance of the hot stamping part may be rapidly reduced.

US 6,296,805 No. 10-2018-0095757

SUMMARY Embodiments of the present invention provide a hot stamping part that prevents cracks from occurring in a plating layer of the hot stamping part, and has improved weldability and paintability, and a method of manufacturing the same.

In an embodiment of the present invention, heating a blank in which an Al-Si-based pre-coating layer is formed on a steel plate and at least one surface of the steel plate; forming the heated blank using a press mold; and cooling the molded blank in the press mold; in the step of heating the blank, the precoat layer is alloyed to form a plating layer, and heating the blank includes: A first heating step of heating to a first temperature and a second heating step of heating from the first temperature to a second temperature, wherein in the first heating step, the steel sheet is directly heated by a resistance heating method, Disclosed is a method for manufacturing a hot stamping part in which the temperature increase rate of the first heating step is 8° C./s to 16° C./s.

In this embodiment, a region containing 90% to 100% of Al may be formed in the plating layer.

In this embodiment, the region containing 90% to 100% of Al may be formed on the surface layer of the plating layer.

In this embodiment, the thickness of the region containing 90% to 100% of Al may be 0.01 μm to 10 μm.

In this embodiment, the first temperature may be 650 °C, and the second temperature may be Ac3 or higher.

In this embodiment, the method may further include surface-treating the surface of the plating layer with a phosphate.

In this embodiment, in the first heating step, at least a portion of the Al of the precoat layer maintains a solid state, and in the second heating step, the phase change of the steel sheet is completed, and the All Al can be melted.

In this embodiment, the first heating step has a first heating time, the second heating step has a second heating time, the first heating time for the sum of the first heating time and the second heating time The ratio of the heating time may be less than 20%.

In this embodiment, before the cooling step, the blank may be held in the press mold for 3 seconds to 20 seconds.

In this embodiment, the steel sheet, carbon (C) 0.19 wt% to 0.25 wt%, silicon (Si) 0.1 wt% to 0.6 wt%, manganese (Mn) 0.5 wt% to 2.0 wt%, based on the total weight of the steel sheet %, phosphorus (P) 0.03 wt% or less, sulfur (S) 0.03 wt% or less, chromium (Cr) 0.05 wt% to 0.6 wt%, boron (B) 0.0005 wt% to 0.005 wt%, balance iron and unavoidable impurities may include

In this embodiment, the plating layer may include at least two or more intermetallic compounds selected from the group consisting of Fe ₂ Al ₅ , FeAl ₃ , FeAl and FeAlSi compounds.

In this embodiment, the plating layer is a first layer containing the Fe ₂ Al ₅ or FeAl ₃ as a main component, a second layer containing the FeAl or FeAlSi as a main component, and the Fe ₂ Al ₅ or FeAl ₃ as a main component In the third layer, a region containing 90% to 100% of Al may be formed.

Another embodiment of the present invention also discloses a hot stamping part manufactured by the above manufacturing method.

According to the embodiments of the present invention, a region having an Al content of 90% to 100% is formed in the plating layer of the hot stamping component, thereby preventing cracks in the plating layer or the plating layer from falling off, thereby improving corrosion resistance, hot stamping The weldability and paintability of parts may be improved.

1 is a flowchart schematically illustrating a method of manufacturing a hot stamping part according to an embodiment of the present invention.
FIG. 2 is a flowchart schematically illustrating an example of a method of manufacturing the blank of FIG. 1 .
FIG. 3 is a diagram schematically illustrating a temperature profile during manufacturing of the hot stamping part of FIG. 1 .
4 is a perspective view schematically illustrating an induction heating method in the blank heating step of FIG. 1 .
5 is a perspective view schematically illustrating a conductive heating method in the blank heating step of FIG. 1 .
6 is a cross-sectional view schematically illustrating an example of a cross-section of a hot stamping part according to an embodiment of the present invention.
7 and 8 are views showing a cross section of a plating layer and a distribution of Fe and Al in the plating layer of Comparative Examples and Examples, respectively.
9 and 10 are diagrams illustrating surface cracks in the plating layers of Comparative Examples and Examples, respectively.
11 is a view showing welding resistance values of Comparative Examples and Examples.
12 and 13 are views showing the surface treatment results of the plating layers of Comparative Examples and Examples, respectively.
14 and 15 are views showing corrosion resistance test results of Comparative Examples and Examples, respectively.

Since the present invention can apply various transformations and can have various embodiments, specific embodiments are illustrated in the drawings and described in detail in the detailed description. Effects and features of the present invention, and a method for achieving them will become apparent with reference to the embodiments described below in detail in conjunction 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, second, etc. are used for the purpose of distinguishing one component from another, not in a limiting sense.

In the following examples, the singular expression includes the plural expression unless the context clearly dictates otherwise.

In the following embodiments, terms such as include or have means that the features or components described in the specification are present, and the possibility that one or more other features or components will be added is not excluded in advance.

In the following embodiments, when it is said that a part such as a film, region, or component is on or on another part, not only when it is directly on the other part, but also another film, region, component, etc. is interposed therebetween. Including cases where there is

In the drawings, the size of the components may be exaggerated or reduced for convenience of description. For example, since the size and thickness of each component shown in the drawings are arbitrarily indicated for convenience of description, the present invention is not necessarily limited to the illustrated bar.

In cases where certain embodiments may be implemented otherwise, a specific process sequence may be performed different from the described sequence. For example, two processes described in succession may be performed substantially simultaneously, or may be performed in an order opposite to the order described.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings, and the same or corresponding components are given the same reference numerals when described with reference to the drawings.

1 is a flowchart schematically illustrating a method of manufacturing a hot stamping part according to an embodiment of the present invention.

The method of manufacturing a hot stamping part according to an embodiment of the present invention may include a blank preparation step (S100), a blank heating step (S200), a transferring step (S300), a forming step (S400) and a cooling step (S500). and may further include a surface treatment and painting step (S600).

The blank preparation step S100 is a step of preparing a blank for heating the blank, and the blank may be formed by cutting a steel plate (or a base material) for forming a hot stamping part. The steel sheet may be manufactured by performing hot rolling or cold rolling on a steel slab, followed by annealing heat treatment. In addition, after the annealing heat treatment, an Al-based precoat layer may be formed on at least one surface of the annealed steel sheet. This will be described later with reference to FIG. 2 .

Meanwhile, the steel sheet contains 0.19 wt% to 0.25 wt% of carbon (C), 0.1 wt% to 0.6 wt% of silicon (Si), 0.5 wt% to 2.0 wt% of manganese (Mn), and 0.03 wt% of phosphorus (P), based on the total weight of the steel sheet. wt% or less, sulfur (S) 0.03 wt% or less, chromium (Cr) 0.05 wt% to 0.6 wt%, boron (B) 0.0005 wt% to 0.005 wt%, the balance may include iron and unavoidable impurities. In addition, the steel sheet may further include a predetermined amount of calcium (Ca).

Carbon (C) acts as an austenite stabilizing element in the steel sheet. Carbon is a major element that determines the strength and hardness of the steel sheet, and after the hot stamping process, it is added for the purpose of securing the tensile strength of the steel sheet (eg, tensile strength of 1,350 MPa or more) and securing the hardenability. Such carbon may be included in an amount of 0.19 wt% to 0.25 wt% based on the total weight of the steel sheet. When the carbon content is less than 0.19 wt%, it is difficult to secure a hard phase (martensite, etc.), so it is difficult to satisfy the mechanical strength of the steel sheet. Conversely, when the carbon content exceeds 0.25 wt%, brittleness of the steel sheet or a problem of reducing bending performance may occur.

Silicon (Si) acts as a ferrite stabilizing element in the steel sheet. Silicon (Si) as a solid solution strengthening element improves the ductility of the steel sheet and improves the carbon concentration in austenite by suppressing the formation of carbides in the low-temperature region. In addition, silicon is a key element in hot rolling, cold rolling, hot pressing, homogenizing the structure (perlite, manganese segregation zone control), and fine dispersion of ferrite. Silicon serves as a martensitic strength heterogeneity control element to improve collision performance. Such silicon may be included in an amount of 0.1 wt% to 0.6 wt% based on the total weight of the steel sheet. If the content of silicon is less than 0.1wt%, it is difficult to obtain the above-mentioned effects, and cementite formation and coarsening may occur in the final hot stamping martensite structure, and the uniforming effect of the steel sheet is insignificant and the V-bending angle cannot be secured. . Conversely, when the content of silicon exceeds 0.6wt%, hot-rolling and cold-rolling loads increase, hot-rolled red scale becomes excessive, and plating properties of the steel sheet may be deteriorated.

Manganese (Mn) acts as an austenite stabilizing element in the steel sheet. Manganese is added to increase hardenability and strength during heat treatment. Such manganese may be included in 0.5wt% to 2.0wt% based on the total weight of the steel sheet. When the manganese content is less than 0.5 wt%, the hardenability effect is not sufficient, and the hard phase fraction in the molded article after hot stamping may be insufficient due to insufficient hardenability. On the other hand, when the content of manganese exceeds 2.0 wt%, ductility and toughness due to manganese segregation or pearlite bands may be reduced, which may cause deterioration in bending performance and may cause a heterogeneous microstructure.

Phosphorus (P) may be included in an amount greater than 0 and 0.03 wt% or less based on the total weight of the steel sheet in order to prevent deterioration of the toughness of the steel sheet. When the phosphorus content exceeds 0.03 wt%, iron phosphide compounds are formed to deteriorate toughness and weldability, and cracks may be induced in the steel sheet during the manufacturing process.

Sulfur (S) may be included in an amount greater than 0 and 0.003 wt% or less based on the total weight of the steel sheet. If the sulfur content exceeds 0.003 wt%, hot workability, weldability, and impact properties are deteriorated, and surface defects such as cracks may occur due to the formation of large inclusions.

Chromium (Cr) is added for the purpose of improving the hardenability and strength of the steel sheet. Chromium makes it possible to refine grains and secure strength through precipitation hardening. Such chromium may be included in an amount of 0.05 wt% to 0.6 wt% based on the total weight of the steel sheet. When the content of chromium is less than 0.05wt%, the precipitation hardening effect is low, and on the contrary, when the content of chromium exceeds 0.6wt%, the Cr-based precipitates and the matrix solid solution increase to decrease toughness, and production cost increases due to cost increase may increase.

Boron (B) is added for the purpose of securing the hardenability and strength of the steel sheet by suppressing the transformation of ferrite, pearlite and bainite to secure a martensitic structure. In addition, boron segregates at grain boundaries to increase hardenability by lowering grain boundary energy, and has an effect of grain refinement due to an increase in austenite grain growth temperature. Such boron may be included in an amount of 0.0005 wt% to 0.005 wt% based on the total weight of the steel sheet. When boron is included in the above range, it is possible to prevent the occurrence of brittleness at the hard phase grain boundary, and secure high toughness and bendability. When the content of boron is less than 0.0005wt%, the hardenability effect is insufficient. Conversely, when the content of boron exceeds 0.005wt%, the solid solubility is low, so that the hardenability is deteriorated or the hardenability is deteriorated or It may cause embrittlement at high temperatures, and toughness and bendability may be reduced due to the occurrence of hard phase intergranular embrittlement.

Meanwhile, other unavoidable impurities may include nitrogen (N) and the like. When nitrogen (N) is added in a large amount, the amount of dissolved nitrogen may increase, thereby reducing the impact properties and elongation of the steel sheet. Nitrogen may be included in an amount greater than 0 and 0.001 wt% or less based on the total weight of the steel sheet. When the nitrogen content exceeds 0.001 wt%, impact properties and elongation of the steel sheet may be deteriorated.

Calcium (Ca) may be added to control the inclusion shape. Such calcium may be included in an amount of 0.003 wt% or less based on the total weight of the steel sheet.

Meanwhile, the steel sheet may further include an additive. The additive is a carbide generating element that contributes to the formation of precipitates in the steel sheet 10 . Specifically, the additive may include at least one of titanium (Ti), niobium (Nb), and vanadium (V).

Titanium (Ti) forms precipitates such as TiC and/or TiN at a high temperature, thereby effectively contributing to austenite grain refinement. Such titanium may be included in an amount of 0.001 wt% to 0.050 wt% based on the total weight of the steel plate 10 . When titanium is included in the content range, poor performance and coarsening of precipitates can be prevented, and physical properties of the steel can be easily secured, and defects such as cracks can be prevented on the steel surface. On the other hand, when the content of titanium exceeds 0.050 wt%, the precipitates are coarsened, and elongation and bendability may decrease.

Niobium (Nb) and vanadium (V) may increase strength and toughness according to a decrease in martensite packet size. Each of niobium and vanadium may be included in an amount of 0.01 wt% to 0.1 wt% based on the total weight of the steel sheet 10 . When niobium and vanadium are included in the above range, the crystal grain refinement effect of the steel sheet 10 is excellent in the hot rolling and cold rolling process, and it prevents cracks in the slab and brittle fracture of the product during steelmaking/playing, It is possible to minimize the formation of precipitates.

The blank heating step (S200) is a step of heating the blank by putting the blank into a heating furnace. In the blank heating step (S200), the blank is heated to a temperature of Ac3 or higher so that the transformation from ferrite to full austenite can be completed for the steel sheet of the blank. On the other hand, in the blank heating step (S200), the precoat layer is alloyed by the mutual diffusion of the steel sheet and the precoat layer of the blank to form a plating layer.

Meanwhile, the blank heating step ( S200 ) may include a first heating step of heating from room temperature to a first temperature, and a second heating step of heating to a second temperature that is a final heating temperature thereafter. The first temperature may be 650 ℃, the second temperature may be more than Ac3.

On the other hand, from room temperature to 650 °C, the average temperature increase rate is controlled to 8 °C/s to 16 °C/s, and when the steel sheet of the blank is directly heated by the resistance heating method, the area in which the aluminum content is 90% to 100% is the plating layer. can be included in Specifically, a region having an aluminum content of 90% to 100% may be formed in the surface layer of the plating layer.

As such, when a region having an aluminum content of 90% to 100% is formed in the plating layer, cracks are prevented from occurring in the plating layer having a brittleness due to alloying, and as the resistance of the plating layer decreases, the weldability of hot stamping parts is improved. can

The transfer step (S300) is a step of transferring the heated blank from the heating furnace to the press mold. In the step of transferring the heated blank from the heating furnace to the press mold, the heated blank may be air-cooled for 7 seconds to 15 seconds, preferably, it may be air-cooled for 10 seconds to 15 seconds.

The forming step (S400) is a step of hot stamping the blank transferred to the press mold to form a molded body, and the cooling step (S500) is a step of cooling the molded body in the press mold.

After hot stamping, before the cooling step (S500), the blank may be held in the press mold for 3 seconds to 20 seconds. If the holding time in the press mold is less than 3 seconds, a sufficient amount of martensite may not be generated and mechanical properties may not be secured. In addition, when the holding time in the press mold exceeds 20 seconds, the holding time in the press mold becomes long and productivity may decrease.

Meanwhile, a cooling channel through which a refrigerant circulates may be provided inside the press mold. The molded body may be rapidly cooled by circulation of a refrigerant supplied through a cooling channel provided in the press mold. At this time, in order to prevent a spring back phenomenon of the steel sheet and to maintain a desired shape, rapid cooling may be performed while pressing the press die in a closed state. In forming and cooling the heated blank, the average cooling rate can be cooled to at least 10° C./s or more to the martensite end temperature.

The surface treatment and painting step ( S600 ) is a step of phosphorylating the surface of the molded body formed by hot stamping and then painting. According to the present invention, since a region having an aluminum content of 90% to 100% is formed in the surface layer of the plating layer, phosphate is uniformly formed on the surface of the plating layer during surface treatment by phosphorylation, and as a result, the coating film is densely formed during coating Thereby, the corrosion resistance of the hot stamping part can be improved.

FIG. 2 is a flowchart schematically illustrating an example of a method of manufacturing the blank of FIG. 1 .

As shown in Figure 2, the blank manufacturing method, after reheating of the steel slab, hot rolling step (S110), cooling / winding step (S120), cold rolling step (S130), annealing heat treatment step (S140) and plating step (S150) may be included. Meanwhile, although steps S110 to S150 are illustrated as independent steps in FIG. 2 , some of steps S110 to S150 may be performed in one process, and some may be omitted if necessary.

First, a steel slab in a semi-finished state to be subjected to a process of forming a steel sheet of a blank is prepared. The steel slab is, based on the total weight, carbon (C) 0.19 wt% to 0.25 wt%, silicon (Si) 0.1 wt% to 0.6 wt%, manganese (Mn) 0.5 wt% to 2.0 wt%, phosphorus (P) 0.03 wt% % or less, sulfur (S) 0.03 wt% or less, chromium (Cr) 0.05 wt% to 0.6 wt%, boron (B) 0.0005 wt% to 0.005 wt%, the balance may include iron and unavoidable impurities. In addition, the slab may further include a predetermined amount of calcium (Ca).

Such slabs may be reheated for hot rolling. By reheating the slab to a predetermined temperature by reheating, the segregated components are re-dissolved during casting. In one embodiment, the slab reheating temperature (Slab Reheating Temperature: SRT) may be 1200 ℃ to 1300 ℃. If the slab reheating temperature (SRT) is lower than 1200 ℃, the segregated components during casting are not sufficiently re-dissolved, so it is difficult to see the effect of homogenizing the alloy elements greatly, and there is a problem in that it is difficult to see the effect of solid solution of titanium (Ti) significantly. The higher the slab reheating temperature (SRT), the more favorable for homogenization, but when it exceeds 1300° C., the austenite grain size increases, making it difficult to secure strength, and only the manufacturing cost of the steel sheet may increase due to the excessive heating process.

The hot rolling step (S110) is a step of manufacturing a steel sheet by hot rolling the reheated slab at a predetermined finishing delivery temperature (FDT) range. In one embodiment, the finish rolling temperature (FDT) range may be controlled to 840 ℃ to 920 ℃. When the finish rolling temperature (FDT) is less than 840 ° C, it is difficult to secure the workability of the steel sheet due to the occurrence of a mixed structure due to rolling in an abnormal region, and there is a problem in that workability is deteriorated due to microstructure non-uniformity, as well as hot rolling due to rapid phase change There may be a problem of marketability in the middle. Conversely, when the finish rolling temperature (FDT) exceeds 920° C., the austenite grains are coarsened. In addition, there is a risk that the TiC precipitates are coarsened to deteriorate the final member performance.

In the cooling/winding step (S120), the hot-rolled steel sheet is cooled and wound in a predetermined coiling temperature (CT) range. In one embodiment, the winding temperature may be 550 °C to 800 °C. The coiling temperature affects the redistribution of carbon (C), and when the coiling temperature is less than 550 ° C, the fraction of the low-temperature phase due to overcooling increases, so that the strength increases and the rolling load during cold rolling may intensify, and the ductility decreases rapidly There is a problem that Conversely, when the coiling temperature exceeds 800° C., there is a problem in that formability and strength deteriorate due to abnormal crystal grain growth or excessive crystal grain growth.

Cold rolling step (S130) is a step of cold rolling after uncoiling the wound steel sheet, pickling treatment. At this time, pickling is performed for the purpose of removing the scale of the wound steel sheet, that is, the hot rolled coil manufactured through the above hot rolling process. Meanwhile, in one embodiment, the rolling reduction during cold rolling may be controlled to 30% to 70%, but is not limited thereto.

The annealing heat treatment step (S140) is a step of annealing the cold rolled steel sheet at a temperature of 700 ° C. or higher. In one embodiment, the annealing heat treatment includes heating the cold rolled steel sheet and cooling the heated cold rolled steel sheet at a predetermined cooling rate.

The plating step (S150) is a step of forming a precoat layer on the annealed heat-treated steel sheet. In one embodiment, in the plating step ( S600 ), an Al-Si pre-coating layer may be formed on the annealed heat-treated steel sheet.

Specifically, the plating step (S600) is a step of immersing the steel sheet in a plating bath having a temperature of 650 ° C. to 700 ° C. to form a hot-dip plated layer on the surface of the steel sheet, and cooling the steel sheet on which the hot-dip plated layer is formed to form a pre-coating layer. A cooling step may be included. In this case, the plating bath may include Si, Fe, Al, Mn, Cr, Mg, Ti, Zn, Sb, Sn, Cu, Ni, Co, In, Bi, etc. as an additive element, but is not limited thereto.

The precoat layer is formed on the surface of the steel sheet and is formed between a surface layer containing 80 wt% or more of aluminum (Al) and between the surface layer and the steel sheet, and includes aluminum-iron (Al-Fe) and aluminum-iron-silicon (Al-Fe-). Si) may include an alloying layer including an intermetallic compound. The alloying layer may contain 20 wt% to 70 wt% of iron (Fe). For example, the surface layer may contain 80 to 100% by weight of aluminum, and may have an average thickness of 10 μm to 40 μm.

As such, by performing the hot stamping process described in FIG. 1 on the blank manufactured through steps S110 to S150 as described above, a hot stamping part satisfying the required strength and bendability can be manufactured. On the other hand, when the blank is heated in a heating furnace to perform the hot stamping process, mutual diffusion occurs between the steel sheet and the precoat layer in the heating process, and the precoat layer is alloyed to form a plating layer. Since such a plating layer is very brittle, cracks may occur after the hot stamping process, and the steel sheet may be exposed by the generated cracks, thereby causing corrosion of the steel sheet.

However, in the present invention, when the blank is heated to perform the hot stamping process, the average temperature increase rate is controlled to 8° C./s to 16° C./s while directly heating the steel sheet of the blank by resistance heating from room temperature to 650° C. Accordingly, by forming a region having an aluminum content of 90% to 100% in the plating layer, it is possible to prevent or minimize cracks in the plating layer.

3 is a view schematically showing a temperature profile during the manufacturing of the hot stamping part of FIG. 1 , FIG. 4 is a perspective view schematically illustrating an induction heating method in the blank heating step of FIG. 1 , and FIG. 5 is a blank heating step of FIG. 1 A perspective view schematically showing a conduction heating method in, and FIG. 6 is a cross-sectional view schematically illustrating an example of a cross-section of a hot stamping part according to an embodiment of the present invention.

First, referring to FIG. 3 , the blank is heated to a temperature of Ac3 or higher so that the transformation of the steel sheet to full austenite can be completed. For example, the blank may be heated to 850°C to 970°C. Meanwhile, the total heating rate up to the final heating temperature of the blank may be 4° C./s or more. By heating the blank, the plating layer is converted into a Fe-Al-based plating layer in which Si is dissolved.

6 shows an example of a cross-section of a plating layer of a hot stamping part. 6 illustrates a cross-section of a plating layer formed on one surface of a steel sheet except for the steel sheet for convenience of explanation. The plating layer may include at least two or more intermetallic compounds selected from the group consisting of Fe ₂ Al ₅ , FeAl ₃ , FeAl and FeAlSi compounds. For example, the plating layer includes a first layer 11 containing Fe ₂ Al ₅ or FeAl ₃ as a main component, a second layer 12 containing FeAl or FeAlSi as a main component, and a third containing Fe ₂ Al ₅ or FeAl ₃ as a main component. It may have a laminated structure of layers 13 . In addition, an interface between the plating layer and the steel sheet may include an interdiffusion layer formed of Fe, Al, and Si.

On the other hand, in the process of heating the blank, when the average temperature increase rate up to 650 °C is controlled to 8 °C/s to 16 °C/s, the region 14 in which the content of aluminum in the third layer 13 is 90% to 100% is formed can be formed.

Since the region 14 having an aluminum content of 90% to 100% has greater ductility compared to the third layer 13, when the region 14 having an aluminum content of 90% to 100% is formed, during the hot pressing process It is possible to prevent cracks from occurring in the plating layer, and it is possible to improve weldability by lowering the contact resistance value of the plating layer. In addition, during the surface treatment of the plating layer with phosphate, the phosphate is uniformly formed on the surface of the plating layer, so that the coating film is densely formed, so that corrosion resistance can be improved.

In order to form the region 14 having such an aluminum content of 90% to 100%, in the process of heating the blank, heating to 650° C. may be performed by directly heating the steel sheet of the blank. For example, the blank heating up to 650° C. may be performed by a resistance heating method such as an induction heating method or a conductive heating method.

As an example, in the induction heating method, as shown in FIG. 4 , a high-frequency current is passed through a coil around the blank (B) so that an induced current flows through the blank (B), and the steel plate of the blank is made by the induced current. It can be heated directly. The induction heating method includes a longitudinal field, a cross field, a face field, etc. depending on the type of the induction coil, but the present invention is not limited thereto.

As another example, in the conduction heating method, as shown in FIG. 5 , the blank (B) is fixed with a pair of electrodes (E), and current flows through the pair of electrodes (E). Heating by electric resistance heat is also called energization heating method. On the other hand, the blank (B) includes a steel sheet and a precoat layer formed on the surface of the steel sheet. Since the resistance of the steel sheet is greater than that of the precoat layer, the steel sheet can be directly heated by the conduction heating method.

As such, in the step of heating the blank, when the steel sheet is directly heated to 650° C., where the pre-plating layer starts to melt, heat is generated from the inside of the blank, so that at least a portion of the aluminum of the pre-coating layer is sufficiently melted by the heat conducted from the steel sheet. Although it maintains an undissolved solid state, an intermetallic compound layer may be formed in the steel sheet by diffusion of Fe and Al. When the blank is heated to the final temperature in this state, even if the phase transformation of the steel sheet is completed and the Al of the pre-plating layer is completely melted, sufficient time for the Fe of the steel sheet to diffuse to the surface layer of the pre-plating layer is insufficient, and in the formed intermetallic compound layer By blocking the diffusion of Al, the region 14 having an aluminum content of 90% to 100% may be finally formed in the plating layer.

Meanwhile, the heating method from 650° C. to the final temperature is not limited to the resistance heating method, and may be radiant heating using a roller table furnace or a walking beam furnace. Upon heating to the final temperature, all of Al in the precoat layer is melted.

In addition, in order to form the region 14 in which the content of aluminum is 90% to 100% in the plating layer, it is necessary to have a temperature increase rate of 8°C/s or more up to 650°C. If the temperature increase rate up to 650° C. is less than 8° C./s, it is difficult to finally form the region 14 having an aluminum content of 90% to 100% in the plating layer because the time for Fe diffusion is sufficient. On the other hand, if the temperature increase rate up to 650° C. is greater than 16° C./s, the depth and width of the region 14 in which the aluminum content is 90% to 100% becomes too large, so that during the hot pressing process, the molten Al is transferred to the press mold. There may be a problem of sticking.

Therefore, in the heating step of the blank, the blank is heated by a method of directly heating the steel sheet up to 650° C., and the temperature increase rate at this time is 8° C./s to 16° C./s, whereby the third layer 13 with high brittleness A region 14 having an aluminum content of 90% to 100% may be formed therein.

The region 14 having an aluminum content of 90% to 100% formed in this way may have a discontinuous shape, and may have a depth of 0.01 μm to 10 μm. Here, the depth of the region 14 in which the aluminum content is 90% to 100% means the maximum depth from the surface layer of the plating layer. If the depth of the region 14 in which the aluminum content is 90% to 100% is less than 0.01 μm, it is difficult to prevent cracks in the plating layer and improve weldability according to the decrease in the contact resistance value of the plating layer, and the aluminum content is 90 % to 100%, if the depth of the region 14 is greater than 10 μm, there may be a problem in that molten Al is sintered in the press mold during the hot pressing process.

Meanwhile, the heating time (t1) up to 650°C may be less than 20% of the total heating time (t1+t2). If the heating time (t1) to 650 °C is 20% or more of the total heating time (t1 + t2), as the heating time to 650 °C increases, the time for Fe diffusion can be sufficiently secured, so that the plating layer It is difficult to form the region 14 in which the aluminum content is 90% to 100%.

Hereinafter, the present invention will be described in more detail through examples. However, the following examples are provided to explain 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 and changed by those skilled in the art within the scope of the present invention.

<Blank manufacturing and hot pressing process>

A steel slab of the following components was subjected to hot rolling, cooling/winding, cold rolling, and annealing heat treatment to form a steel sheet (thickness of 1.2 mm), and then hot-dip plating was performed on the surface of the steel sheet to form a pre-coating layer to prepare a blank.

Ingredients (wt%) C Si Mn P S Cr B 0.2 0.32 1.18 0.015 0.004 0.03 0.0005

Hot-dip Al plating was performed by using a non-oxidizing furnace-reducing furnace type line and adjusting the amount of deposition of the hot-dip plated layer to 80 g/m ² to 180 g/m ² on one side by gas wiping after plating, and then cooling. At this time, the plating bath was set to contain 7 wt% of Si, 2.5 wt% of Fe, and the remainder of Al in a temperature range of 600 °C to 700 °C. In addition, the steel sheet was passed through a plating bath at a speed of 100mpm to 200mpm, and then cooled to room temperature at an average cooling rate of 25°C/s and then cut to prepare a blank.

The manufactured blank was heated to a temperature of Ac3 or higher at an average temperature increase rate of 3 ° C./s or higher, and then rapidly cooled to 300 ° C. or lower at an average speed of 10 ° C./s or higher while applying an external force with a press to manufacture a hot pressing part.

On the other hand, the comparative examples and examples below have only a difference in which the heating rate of the blank is set differently up to 650° C. in the heating step of the blank during the hot press process.

Specifically, the comparative example is a result of the temperature increase rate up to 650 ℃ set to 7 ℃ / s, the Example is the result of setting the temperature increase rate up to 650 ℃ 8 ℃ / s.

7 shows the distribution of Fe and Al in the plating layer and the cross section of the plating layer as a comparative example, and FIG. 8 shows the distribution of Fe and Al in the cross section of the plating layer and the plating layer as an example. In addition, Table 2 below is the result of analyzing the composition at three points (①, ②, ③) in FIGS. 7 and 8, respectively.

division Comparative Example (FIG. 7) Example (Fig. 8) Al (%) Si (%) Fe (%) Al (%) Si (%) Fe (%) ① 50.92 6.88 42.2 100 0 0 ② 17.88 13.97 43.08 100 0 0 ③ 45.94 3.09 38.67 80 18 0

7 and 8, in the case of the comparative example, the plating layer is formed of the first layer 11, the second layer 12, and the third layer 13, whereas in the case of the embodiment, the plating layer is Consists of the first layer 11, the second layer 12, and the third layer 13, but an additional region 14 having an aluminum content of 90% to 100% is further formed in the third layer 13 can be known

More specifically, looking at the distribution of Al and Fe of FIG. 7 and the results of Table 2 together, in the case of FIG. 7, it can be seen that Fe is distributed in all regions of the plating layer. This is a result of the overall alloying of the plating layer because Fe can be sufficiently diffused into the plating layer by setting the temperature increase rate to 650° C. less than 8° C./s in the heating step of the blank.

On the other hand, looking at the distribution of Al and Fe of FIG. 8 and the results of Table 2 together, it can be seen that Fe does not diffuse to the surface layer of the plating layer. That is, in the case of the embodiment, as a result of setting the temperature increase rate to 650° C. at least 8° C./s in the heating step of the blank, the region 14 in which the aluminum content is additionally 90% to 100% in the third layer 13 is It can be seen that more is formed.

<Crack inspection of coating layer>

Cracks generated in the plating layer in the Comparative Examples and Examples were visually inspected. 9 and 10 show cracks occurring in the plating layer in Comparative Examples and Examples, respectively.

As can be clearly seen from FIGS. 9 and 10 , in the case of FIG. 9 , the number of cracks C generated on the surface of the plating layer is larger than in FIG. 10 , and the width and depth of each crack C are larger than in FIG. 10 . it can be seen that That is, in FIG. 10 , it can be seen that the occurrence of cracks (C) in the plating layer can be prevented or minimized by additionally forming a region having a content of 90% to 100% of aluminum having high ductility in the surface layer of the plating layer.

<Evaluation of welding resistance characteristics>

11 is a graph showing results of resistance measurement during spot welding for Comparative Examples and Examples. For resistance measurement, a welding tip of 6 mm at a pressure of 350 kgf for 4 minutes at 930° C. was applied to the welding part, and a current was applied to measure the contact resistance.

As can be seen from FIG. 11 , it can be seen that, compared to the comparative example, the overall contact resistance during welding as well as the contact resistance at the start of welding was reduced. That is, it can be confirmed that the Example has excellent weldability compared to the Comparative Example. This is because, in the case of the embodiment, since a region having an aluminum content of 90% to 100% is additionally formed in the plating layer, compounds having high specific resistance such as Fe ₂ Al ₅ , FeAl ₃ , FeAl and FeAlSi are reduced.

<Evaluation of corrosion resistance>

12 and 13 are views showing the results of surface treatment of the plating layers of Comparative Examples and Examples, respectively, and FIGS. 14 and 15 are views showing results of corrosion resistance tests of Comparative Examples and Examples, respectively.

12 and 13 show the results of phosphorylation treatment on the surface of the plating layer in Comparative Examples and Examples, respectively, and FIGS. 14 and 15 are respectively after phosphorylation treatment and forming scratches on the plating layer, corrosion resistance evaluation A result is shown.

Phosphating was performed by immersing the hot stamping parts of Comparative Examples and Examples in a phosphate treatment solution at 40° C. for 2 minutes. As a result of the phosphate treatment, as can be seen in FIGS. 12 and 13 , it can be seen that the Example has more uniform phosphate crystals than the Comparative Example. This is because, in the case of the embodiment, a geometric structure to which a phosphate can be attached can be formed by forming an oxide layer in a region having an aluminum content of 90% to 100% in the plating layer.

On the other hand, the corrosion resistance evaluation of FIGS. 14 and 15 is the result of forming a scratch (S) on the coating film, spraying 5% NaCl salt water, and observing the occurrence of the red rust phenomenon after 120 hours have elapsed. As a result of FIGS. 14 and 15 , it can be seen that the red rust phenomenon is significantly reduced in the case of the example compared to the comparative example. This means that not only the coating is densely formed, but the plating layer has better resistance to the red rust phenomenon.

As described above, the present invention has been described with reference to one embodiment shown in the drawings, but it will be understood that various modifications and variations of the embodiments are possible therefrom by those of ordinary skill in the art. Accordingly, the true technical protection scope of the present invention should be determined by the technical spirit of the appended claims.

Claims (13)

heating a steel sheet and a blank in which an Al-Si-based pre-coating layer is formed on at least one surface of the steel sheet;
forming the heated blank using a press mold; and
In the press mold, cooling the molded blank;
In the step of heating the blank, the precoat layer is alloyed to form a plating layer,
The step of heating the blank,
a first heating step of heating the blank to a first temperature and a second heating step of heating the blank from the first temperature to a second temperature;
In the first heating step, the steel sheet is directly heated by a resistance heating method,
The temperature increase rate of the first heating step is 8 ℃ / s to 16 ℃ / s,
A region containing 90% to 100% of Al is formed in the plating layer, and the region containing 90% to 100% of Al is formed on a surface layer of the plating layer. delete delete According to claim 1,
The thickness of the region containing 90% to 100% of Al is 0.01㎛ to 10㎛ method of manufacturing a hot stamping part. According to claim 1,
The first temperature is 650°C, and the second temperature is Ac3 or more. According to claim 1,
The method of manufacturing a hot stamping part further comprising the step of surface-treating the surface of the plating layer with a phosphate. According to claim 1,
In the first heating step, at least a portion of the Al of the precoat layer maintains a solid state,
In the second heating step, the phase change of the steel sheet is completed, and the Al of the precoat layer is entirely melted. According to claim 1,
The first heating step has a first heating time, the second heating step has a second heating time,
and a ratio of the first heating time to the sum of the first heating time and the second heating time is less than 20%. According to claim 1,
Before the cooling step, the blank is held in the press mold for 3 to 20 seconds. According to claim 1,
The steel sheet is, based on the total weight of the steel sheet, carbon (C) 0.19 wt% to 0.25 wt%, silicon (Si) 0.1 wt% to 0.6 wt%, manganese (Mn) 0.5 wt% to 2.0 wt%, phosphorus (P) Hot stamping parts containing 0.03 wt% or less, sulfur (S) 0.03 wt% or less, chromium (Cr) 0.05 wt% to 0.6 wt%, boron (B) 0.0005 wt% to 0.005 wt%, balance iron and unavoidable impurities manufacturing method. According to claim 1,
The plating layer is Fe ₂ Al ₅ , FeAl ₃ , a method of manufacturing a hot stamping part comprising at least two or more intermetallic compounds selected from the group consisting of FeAl and FeAlSi compounds. 12. The method of claim 11,
The plating layer is a first layer containing the Fe ₂ Al ₅ or FeAl ₃ as a main component, a second layer containing the FeAl or FeAlSi as a main component, and a third layer containing the Fe ₂ Al ₅ or FeAl ₃ as a main component. have a layered structure,
A method of manufacturing a hot stamping part, wherein a region containing 90% to 100% of Al is formed in the third layer. A hot stamping part manufactured by the manufacturing method of any one of claims 1 to 12.

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