KR20220096961A - Hot stamping component, and method for manufacturing the same - Google Patents

Abstract

A hot stamping component according to one embodiment of the present invention comprises: a steel plate; a plating layer including an intermixed layer, a first layer (Al-rich Fe_xAl_ySi_z), a second layer (Fe-rich Fe_xAl_ySi_z), and a third layer (Al-rich Fe_xAl_ySi_z) located on the steel plate and sequentially laminated; and an oxide layer located on the plating layer and having a thickness of 30 nm to 100 nm. The hot stamping component has improved weldability.

Description

HOT STAMPING COMPONENT, AND METHOD FOR MANUFACTURING THE SAME

FIELD OF THE INVENTION The present invention relates to hot stamping parts and methods of manufacturing the same.

Joining and welding are essential in automobile assembly. The resistance spot welding process with various advantages is the most widely applied welding method because it is suitable for car body welding. Resistance spot weld quality is the most important management point for car body quality (rigidity and durability characteristics, etc.), and securing welding quality is the most important task.

In recent years, in a situation in which the application rate of hot stamping steel sheet is increasing in order to reduce vehicle body weight and secure stability, welding quality must also be secured along with this, but in reality, the welding characteristics of hot stamping steel for hot forming are poor. In the case of high-strength steel, the contact resistance and volume resistance are high due to high rigidity and alloy content, so the heat generation rate is high when the welding current is applied. In particular, in the case of aluminum (Al)-based plated hot stamping steel sheet, an alloying reaction occurs due to diffusion of aluminum (Al) and silicon (Si) in the plating layer and iron (Fe) in the base material in the heating furnace heat treatment process for the hot stamping method. Due to this, an interdiffusion layer and various intermetallic compound (IMC) layers are formed, which increases the surface resistance, so that expulsion occurs during welding, and the weldability is more vulnerable because the sensitivity is very high. Therefore, it is sensitive to occurrence of intermediate expulsion and internal defects such as limiting nugget growth due to occurrence of intermediate expulsion occur, making it difficult to secure quality.

As a related technology, there is Korean Patent Laid-Open Publication No. 10-2018-0095757 (Title of the Invention: Method of Manufacturing Hot Stamping Part).

No. 10-2018-0095757

SUMMARY Embodiments of the present invention provide a hot stamping part with improved weldability, and a method of manufacturing the same.

A hot stamping part according to an embodiment of the present invention includes a steel plate; An intermixed layer, a first layer (Al-rich Fe _x Al _y Si _z ), a second layer (Fe-rich Fe _x Al _y Si _z ), and a third layer (Al) disposed on the steel plate and sequentially stacked -rich Fe _x Al _y Si _z ) including a plating layer; and an oxide layer positioned on the plating layer and having a thickness of 30 nm to 100 nm.

The intermixed layer may include a first intermixed layer adjacent to the steel sheet and a second intermixed layer adjacent to the first intermixed layer, wherein the first intermixed layer and the second intermixed layer may have different phases.

The Si content of the second intermixed layer may be less than the Si content of the first intermixed layer.

The first intermixed layer (Fe-rich Fe _x Al _y Si _z ) is x: 80wt% to 90wt%, y: 5wt% to 20wt%, z: more than 0wt% and 5wt% or less, and the second intermixed layer (Fe -rich Fe _x Al _y Si _z ) is x: 40wt% to 60wt%, y: 30wt% to 50wt%, z: It may be more than 0wt% and 20wt% or less.

A ratio of the thickness of the second intermixed layer to the total thickness of the intermixed layer may be less than 40%.

The intermixed layer may have a thickness of 4 μm to 10 μm.

The second intermixed layer may have the same phase as the second layer (Fe-rich Fe _x Al _y Si _z ).

An area fraction of the first layer and the third layer with respect to the plating layer may be 40% or more.

The first layer (Al-rich Fe _x Al _y Si _z ) is x: 30wt% to 50wt%, y: 50wt% to 70wt%, z: more than 0wt% and 10wt% or less, and the second layer (Fe-rich) Fe _x Al _y Si _z ) is x: 40wt% to 60wt%, y: 30wt% to 50wt%, z: more than 0wt% and 20wt% or less, and the third layer (Al-rich Fe _x Al _y Si _z ) is x: 30wt% to 50wt%, y: 50wt% to 70wt%, z: may be more than 0wt% and 10wt% or less.

A method of manufacturing a hot stamping part according to an embodiment of the present invention includes: introducing a blank into a heating furnace having a plurality of sections having different temperature ranges; a multi-stage heating step of heating the blank step by step; and a crack heating step of crack heating the blank, wherein in the multi-stage heating step and the crack heating step, an intermixed layer sequentially stacked, a first layer (Al-rich Fe _x Al _y Si _z ), a second A plating layer including a layer (Fe-rich Fe _x Al _y Si _z ) and a third layer (Al-rich Fe _x Al _y Si _z ) is formed, wherein the intermixed layer includes a first intermixed layer adjacent to the steel sheet and the formed to include a second intermixed layer adjacent to the first intermixed layer.

A ratio of a length of a section in which the blank is heated in multiple stages in the plurality of sections and a length of a section in which the blank is heated by cracking may satisfy 1:1 to 4:1.

The temperature of the plurality of sections may increase in a direction from the entrance of the heating furnace to the exit direction of the heating furnace.

The temperature difference between two adjacent sections of the section heating the blank in multiple stages may be greater than 0°C and less than or equal to 100°C.

A temperature of a section in which the blank is heated by cracking among the plurality of sections may be higher than a temperature of sections in which the blank is heated in multiple stages.

The blank may stay in the furnace for 180 seconds to 360 seconds.

In the multi-stage heating step and the crack heating step, an oxide layer having a thickness of 30 nm to 100 nm may be formed on the plating layer.

In the hot stamping part, the first intermixed layer and the second intermixed layer may have different phases.

The Si content of the first intermixed layer may be less than the Si content of the second intermixed layer.

The first intermixed layer (Fe-rich Fe _x Al _y Si _z ) is x: 80wt% to 90wt%, y: 5wt% to 20wt%, z: more than 0wt% and 5wt% or less, and the second intermixed layer (Fe -rich Fe _x Al _y Si _z ) is x: 40wt% to 60wt%, y: 30wt% to 50wt%, z: It may be more than 0wt% and 20wt% or less.

A ratio of the thickness of the second intermixed layer to the total thickness of the intermixed layer may be less than 40%.

The intermixed layer may have a thickness of 4 μm to 10 μm.

The second intermixed layer may have the same phase as the second layer (Fe-rich Fe _x Al _y Si _z ).

after the crack heating step, transferring the crack-heated blank from the heating furnace to a press mold; forming a molded body by hot stamping the transferred blank; and cooling the formed body.

According to the embodiments of the present invention, a multi-layer plating layer including a layer having excellent weldability is formed by multi-stage heating and crack heating of a blank for hot stamping, and the thickness of the oxide film is reduced by controlling the temperature of the heating step. Weldability can be maximized.

1 is a photograph showing traces of scattering of the melt in the hot stamping homogeneous welding site.
FIG. 2 is a photograph showing an example in which pores are formed due to scattering of the melt, resulting in a deviation in welding quality.
3A and 3B are cross-sectional views illustrating a cross-section of a component for hot stamping according to an embodiment.
4 is a flowchart schematically illustrating a method of manufacturing a hot stamping part according to an embodiment.
5 is a diagram illustrating a heating furnace having a plurality of sections in a multi-stage heating step and a crack heating step of a method for manufacturing a hot stamping part according to an embodiment.
6 is a view comparing the properties of the hot stamping parts according to the crack heating temperature according to the embodiment.
7 is a graph comparing surface contact resistance according to a crack heating temperature of a hot stamping part according to an embodiment.
8 is a cross-sectional view illustrating a cross-section of a hot stamping part according to different embodiments.

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 methods 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.

Hereinafter, resistance spot welding is a welding method in which an object to be joined is basically pressed with two upper and lower electrodes, and then a current is passed between the electrodes while maintaining the pressure to melt using the electrical resistance heat of the joined object. At this time, heat generation starts at the bonding surface due to the contact resistance. In the case of aluminum (Al)-silicon (Si) plating material, an alloying reaction occurs due to mutual diffusion of aluminum (Al) and silicon (Si) in the plating layer and iron (Fe) in the base material in the heating furnace heat treatment process for the hot stamping method. , thereby forming an interdiffusion layer and various intermetallic compounds (IMC), and even an oxide film may be formed on the surface layer. Therefore, the surface contact resistance of the welded part is very increased, and the heat generation rate due to electrical resistance is very high at the interface of the joint where the plating layer is in contact, so molten nuggets are first generated, and the rapid growth of these nuggets causes expulsion. deepening and the welding quality deteriorates.

1 is a photograph showing traces of scattering of the melt in the same type of hot stamping welding, and FIG. 2 is a photograph showing an example in which pores (yellow arrows) are formed due to the scattering of the melt, resulting in a deviation in welding quality due to blowing. As such, there is a great difficulty in securing and managing the quality of hot stamping parts due to scattering of the melt.

In the present invention, an oxide film may be formed on the surface of the component during the hot stamping process, and in the case of the Al-Si plating material described above, Al ₂ O ₃ An aluminum-based oxide film may be formed. Aluminum-based oxide is a material that reduces weldability in resistance spot welding, and has a problem in that the resistance is high at the welding site, the heat generation rate is very high, and the weldability is deteriorated. Accordingly, an object of the present invention is to provide a method for manufacturing hot stamping with improved weldability by controlling the heating temperature and thus the thickness of the oxide film, and a hot stamping part manufactured according to the method.

3A and 3B are cross-sectional views illustrating a cross-section of a component for hot stamping according to an embodiment.

3A and 3B, the hot stamping component 10 includes a steel plate 100, a plating layer 200 positioned on the steel plate 100, and an oxide layer 300 positioned on the plating layer 200. can do. Another layer may be further included between the plating layer 200 and the oxide layer 300 or on the oxide layer 300 .

The 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 alloying element in a predetermined content. For example, the steel sheet 100 is carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), titanium (Ti), boron (B), the balance iron (Fe), and other unavoidable impurities. In addition, the steel sheet 100 may further include one or more of niobium (Nb), molybdenum (Mo), and aluminum (Al).

The plating layer 200 is provided in multiple layers on at least one surface of the steel sheet 100 , and the interdiffusion layer 210 , the first layer 220 , the second layer 230 and the third sequentially stacked from the steel sheet 100 . layer 240 . Hereinafter, the first layer 220 is Al-rich Fe _x Al _y Si _z , the second layer 230 is Fe-rich Fe _x Al _y Si _z , and the third layer 240 is Al-rich Fe _x Al _y It may be expressed as Si _z . That is, the first layer 220 and the third layer 240 may be an Al-rich layer, and the second layer 230 may be an Fe-rich layer.

The intermixed layer 210 includes a first intermixed layer 211 adjacent to the steel sheet 100 and a second intermixed layer 212 adjacent to the first intermixed layer 211 , and the plating layer 200 according to an embodiment. ) may be provided with at least five layers or more. The two intermixed layers 211 and 212 may have different phases. Each of the first intermixed layer 211 and the second intermixed layer 212 may be denoted as Fe-rich Fe _x Al _y Si _z , and the Si content of the first intermixed layer 211 is the second intermixed layer 21 . may be smaller than the Si content of More specifically, the first intermixed layer 211 has x: 80 wt% to 90 wt%, y: 5 wt% to 20 wt%, z: more than 0 wt% and 5 wt% or less, and the second intermixed layer 212 is x: 40 wt% to 60 wt%, y: 30 wt% to 50 wt%, z: may be more than 0 wt% and 20 wt% or less.

The first intermixed layer 211 is a layer adjacent to the steel sheet 100 and has a relatively high resistance compared to the second intermixed layer 212 , and has a relatively low Al content and a relatively low Si content. Conversely, the second intermixed layer 212 is a layer adjacent to the first layer 220, which is an Al-rich layer, and has a relatively high resistance compared to the first layer 220, which is an Al-rich layer, and has a relatively low Al content, and Si The content is relatively high.

The total thickness of the intermixed layer 210 may be, for example, about 4 μm to about 10 μm.

The first layer 220 is an Al-rich layer (Al-rich Fe _x Al _y Si _z ), for example, having a composition of x=30-50wt%, y=50-70wt.%, z=0-10wt%. can

The second layer 230 is a Fe-rich layer (Fe-rich Fe _x Al _y Si _z ), for example, having a composition of x = 40 to 60 wt%, y = 30 to 50 wt.%, z = 0 to 20 wt%. can

The third layer 240 is an Al-rich layer (Al-rich Fe _x Al _y Si _z ), for example, having a composition of x=30-50wt%, y=50-70wt.%, z=0-10wt%. can

In this case, the second intermixed layer 212 may have the same phase as the second layer (Fe-rich Fe _x Al _y Si _z ) 230 .

The intermixed layer 210 positioned on the surface of the steel sheet 100 and the second layer 230 (Fe-rich) positioned in the middle are layers having a high resistance phase, and due to the high resistance during welding, the high A higher fraction of a layer with a resistive phase is disadvantageous for welding. The first layer 220 and the third layer 240, which are Al-rich Fe _x Al _y Si _z layers, are layers having a low-resistance phase, and the higher the fraction of the layer having the low-resistance phase, the more advantageous it is for welding. do. The electrical resistance of the plating layer 200, that is, the contact resistance, is closely related to the electrical properties of the component and the fraction between the layers. Accordingly, the high-resistance layers 210 and 230 and the low-resistance layers 220 and 240 can maximize the weldability of the hot stamping part 10 by optimizing the fraction.

For example, in order to maximize weldability, the area fraction of the first layer 220 and the third layer 240, which are low-resistance layers, is about 40% or more with respect to the entire plating layer 200, and the intermixed layer 210, which is a high-resistance layer, is ) and the area fraction of the second layer 230 may be controlled to be about 60% or less. Factors constituting the plating layer 200 provided in the multi-layer are expressed by equations as follows.

40% < V _{Al-rich FexAlySiz} , 60%> V _{intermixed layer} + V _{Fe-rich FexAlySiz} = 1- V _{Al-rich FexAlySiz}

40% < V _{Al-rich FexAlySiz} (50wt% < y < 70wt%, 30wt% < x+z < 50wt% in Al-rich FexAlySiz)

60% > V _{intermixed layer} + V _{Fe-rich FexAlySiz} (30wt% < y < 50wt%, 50wt% < x+z < 70wt% in Fe-rich FexAlySiz)

On the other hand, the electrical resistance of the plating layer 200 is closely related to the silicon (Si) content in the plating layer 200, and in general, as the Si content decreases, spatter can be reduced in the spot welding process and welding strength can be secured. That is, in the present invention, the lower the fraction (area or thickness) of the second intermixed layer 212 having a relatively high Si content in the intermixed layer 210 is advantageous in securing weldability. Accordingly, for example, the ratio of the thickness of the second intermixed layer 212 to the total thickness of the intermixed layer 210 may be less than about 40%, and the thickness of the first intermixed layer 211 with respect to the total thickness of the intermixed layer 210 may be less than about 40%. The thickness ratio may be greater than about 60%. For example, the first intermixed layer 211 may have a fraction of about 62.3% and the second intermixed layer 212 may have a fraction of about 37.7% with respect to the total thickness of the intermixed layer 210 .

The oxide layer 300 is an oxide film of the hot stamping part positioned on the plating layer 200 and may include an aluminum-based oxide, for example, Al ₂ O ₃ . The thickness of the oxide layer 300 may be about 30 nm to about 100 nm. In the hot stamping process, the thickness of the oxide layer 300 may be formed differently depending on the heating temperature. The oxide layer 300 increases resistance during resistance point welding and increases the exothermic temperature, which is disadvantageous for welding. It is necessary to minimize This will be described in more detail with reference to FIG. 8 to be described later.

4 is a flowchart schematically illustrating a method of manufacturing a hot stamping part according to an embodiment.

Referring to FIG. 4 , the method for manufacturing a hot stamping part according to an embodiment includes a blank forming step (S310), a multi-stage heating step (S320), a crack heating step (S330), a transferring step (S340), and a forming step (S350) , and a cooling step (S360).

The blank forming step ( S310 ) may be a step of preparing and cutting the steel sheet 100 on which the plating layer 200 is formed to form a blank. In the blank forming step ( S310 ), a blank may be formed by cutting a steel sheet into a desired shape according to the purpose. In the step of preparing the steel sheet 100 on which the plating layer 200 is formed, a steel slab having a known composition applied for hot stamping is prepared, and the steel slab is subjected to at least one of known hot rolling and cold rolling. By annealing heat treatment, it may proceed to the process of manufacturing the steel sheet material. After the annealing heat treatment, an Al-Si-based plating layer or a Zn plating layer may be formed on the steel sheet material by a known method.

The multi-stage heating step (S320) may be a step of heating the blank step by step, and the crack heating step (S330) may be a step of heating the multi-step heated blank to a uniform temperature. In the multi-stage heating step (S320), the blank may pass through a plurality of sections provided in the heating furnace and the temperature may be raised step by step. Among the plurality of sections provided in the heating furnace, a plurality of sections in which the multi-stage heating step (S320) is performed may exist. In addition, the temperature is set for each section so as to increase from the inlet of the heating furnace into which the blank is input to the exit direction of the furnace in which the blank is taken out, so that the temperature of the blank can be increased step by step. (S330) may be made.In the crack heating step (S330), the multi-stage heated blank may be heat-treated while passing through the section of the heating furnace set at a temperature of Ac3 to 1,000° C. Preferably, in the crack heating step (S330) The multi-stage heated blank may be crack-heated at a temperature of 900° C. to 1,000° C. More preferably, in the crack heating step S330, the multi-stage heated blank may be crack-heated at a temperature of 900° C. to 970° C. Also , there may be at least one section in which the crack heating step (S330) is performed among a plurality of sections provided in the heating furnace.Specific features of the multi-stage heating step (S320) and the crack heating step (S330) and the remaining steps (S350) ~ S370) will be further described with reference to FIG. 5 to be described later.

5 is a diagram illustrating a heating furnace having a plurality of sections in a multi-stage heating step and a crack heating step of a method for manufacturing a hot stamping part according to an embodiment.

Referring to FIG. 5 , the heating furnace according to an embodiment may include a plurality of sections having different temperature ranges. More specifically, the heating furnace has a first section P1 having a first temperature range T1 , a second section P2 having a second temperature range T2 , and a third section having a third temperature range T3 . A section P3, a fourth section P4 having a fourth temperature range T4, a fifth section P5 having a fifth temperature range T5, a sixth section having a sixth temperature range T6 ( P6), and a seventh section P7 having a seventh temperature range T7 may be provided.

For example, in the multi-stage heating step (S320), the blank may be heated in stages while passing through a plurality of sections (eg, the first section (P1) to the fourth section (P4)) defined in the heating furnace. In addition, in the crack heating step ( S330 ), the blank heated in multiple stages in the first section ( P1 ) to the fourth section ( P4 ) may be heated by cracking in the fifth section ( P5 ) to the seventh section ( P7 ).

The first section P1 to the seventh section P7 may be sequentially disposed in the heating furnace. The first section P1 having the first temperature range T1 is adjacent to the inlet of the furnace into which the blank is put, and the seventh section P7 having the seventh temperature range T7 is the furnace through which the blank is discharged. may be adjacent to the exit of Accordingly, the first section P1 having the first temperature range T1 may be the first section of the heating furnace, and the seventh section P7 having the seventh temperature range T7 is the last section of the furnace can Among the plurality of sections of the heating furnace, the fifth section (P5), the sixth section (P6), and the seventh section (P7) may be sections in which crack heating is performed, not a section in which multi-stage heating is performed.

The temperature of a plurality of sections provided in the heating furnace, for example, the temperature of the first section (P1) to the seventh section (P7) can increase in the direction of the outlet of the furnace from which the blank is taken out from the inlet of the furnace into which the blank is put. have. However, the temperatures of the fifth section P5 , the sixth section P6 , and the seventh section P7 may be the same. In addition, a temperature difference between two adjacent sections among a plurality of sections provided in the heating furnace may be greater than 0°C and less than or equal to 100°C. For example, the temperature difference between the first section P1 and the second section P2 may be greater than 0°C and less than or equal to 100°C.

For example, the first temperature range T1 of the first section P1 may be 840°C to 860°C, and may be 835°C to 865°C. The second temperature range T2 of the second section P2 may be 870°C to 890°C, or 865°C to 895°C. The third temperature range T3 of the third section P3 may be 900°C to 920°C, and 895°C to 925°C. The fourth temperature range T4 of the fourth section P4 may be 920°C to 940°C, and may be 915°C to 945°C. The fifth temperature range T5 of the fifth section P5 may be Ac3 to 1,000°C. Preferably, the fifth temperature range T5 of the fifth section P5 may be 900°C or more and 1,000°C or less. More preferably, the fifth temperature range T5 of the fifth section P5 may be 900°C or more and 1000°C or less. The sixth temperature range T6 of the sixth section P6 and the seventh temperature range T7 of the seventh section P7 may be the same as the fifth temperature range T5 of the fifth section P5. .

5, the heating furnace according to an embodiment is illustrated as having seven sections having different temperature ranges, but the present invention is not limited thereto. Five, six, or eight sections having different temperature ranges may be provided in the furnace.

The crack heating step ( S330 ) may be performed in the last part of the plurality of sections of the heating furnace. For example, the crack heating step may be performed in the fifth section (P5), the sixth section (P6), and the seventh section (P7) of the heating furnace. When a plurality of sections are provided in the heating furnace, if the length of one section is long, there may be problems such as temperature change in the section. Accordingly, the section in which the crack heating step is performed is divided into a fifth section (P5), a sixth section (P6), and a seventh section (P7), the fifth section (P5), the sixth section (P6), And the seventh section P7 may have the same temperature range in the heating furnace.

In the crack heating step (S330), the multi-stage heated blank may be crack-heated at a temperature of Ac3 to 1,000°C. Preferably, in the crack heating step (S330), the multi-stage heated blank may be crack-heated at a temperature of 900 °C to 1,000 °C. When the temperature for crack heating is less than 900°C, it is difficult to secure the material of the hot stamping part, and when it exceeds 1000°C, there is a problem in that the surface contact resistance is excessively high. Furthermore, even within the range of about 900 ° C. to about 1000 ° C., while minimizing the thickness of the surface oxide layer 300 , which is a cause of increasing the exothermic temperature during welding, at the same time increasing the fraction of the intermixed layer having a high relative Si content advantageous for securing weldability. You need to set the range. To this end, for example, in the crack heating step ( S330 ), the multi-stage heated blank may be crack-heated at a temperature of about 900°C or higher and less than about 970°C.

For example, the ratio of the length (D1) of the section in which the blank is heated in multiple stages to the length (D2) of the section in which the blank is heated by cracking may be 1:1 to 4:1. More specifically, the sum of the lengths of the first section (P1) to the fourth section (P4), which is a section in which the blank is heated in multiple stages, and the fifth section (P5), to the section where the blank is heated by cracking (P7) The ratio of the sum of the lengths of may satisfy 1:1 to 4:1. When the ratio of the length (D1) of the section in which the blank is heated in multiple stages (D1) to the length (D2) of the section in which the blank is heated by cracking (D2) exceeds 1:1 due to the increase in the length of the section where the blank is heated by cracking, austenite in the section where the crack is heated (FCC) tissue may be generated, which may increase the amount of hydrogen permeation into the blank, resulting in increased delayed rupture. In addition, when the ratio of the length (D1) of the section in which the blank is heated in multiple stages (D1) to the length (D2) of the section in which the blank is heated by cracking is less than 4:1 because the length of the section in which the blank is heated by cracking is reduced, the section is heated by cracking (time) Since this is not sufficiently secured, the strength of the parts manufactured by the manufacturing process of the hot stamping parts may be non-uniform.

For example, the length of a section in which the uniform heating step S330 is performed among a plurality of sections provided in the heating furnace may be 20% to 50% of the total length of the heating furnace. In addition, in the multi-stage heating step ( S320 ) and the crack heating step ( S330 ), at least two blanks having different thicknesses may be simultaneously transferred into the heating furnace.

For example, the blank may stay in the furnace for 180 seconds to 360 seconds. That is, the time during which the blank is heated in multiple stages and cracked may be 180 seconds to 360 seconds. If the residence time of the blank in the furnace is less than 180 seconds, it may be difficult to sufficiently crack at the desired cracking temperature. In addition, when the residence time of the blank in the furnace exceeds 360 seconds, the amount of hydrogen penetrating into the blank increases, thereby increasing the risk of delayed fracture, and corrosion resistance after hot stamping may decrease.

As the heating steps S320 and S330 are performed, an alloying reaction may be performed due to internal diffusion of Al and Si of the plating layer 200 and Fe contained in the steel sheet 100, and thereby the plating layer 200 is As described with reference to FIG. 3A , it may be formed in a multi-layer structure of five or more layers.

The transfer step (S340) is a step of transferring the heated blank from the heating furnace to the press mold. The blank heated in the transfer step (S340) may be air-cooled for 10 to 15 seconds.

The forming step ( S350 ) is a step of hot stamping the transferred blank to form a molded body. The cooling step (S360) is a step of cooling the formed body.

A final product may be formed by cooling the molded body at the same time as molding into a final part shape in a press mold. A cooling channel through which a refrigerant circulates may be provided in the press mold. It is possible to rapidly cool the heated blank by circulating the refrigerant supplied through the cooling channel provided in the press mold. At this time, in order to prevent a spring back phenomenon of the plate material and maintain a desired shape, rapid cooling may be performed while pressurizing 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 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, the cooling of the material is not sufficiently performed, and the temperature deviation of each part due to the residual heat may affect the water intake quality. In addition, mechanical properties may not be secured because a sufficient amount of martensite is not generated. On the other hand, when the holding time in the press mold exceeds 20 seconds, the holding time in the press mold becomes longer and productivity may decrease.

The plating layer 200 of the hot stamping part controlled through the manufacturing method according to an embodiment of the present invention as described above may be formed in multiple layers, and according to an embodiment, five layers that are sequentially stacked and can be distinguished by component composition may include

In addition, the oxide layer 300 on the plating layer 200 formed by controlling the heating temperature in the manufacturing method may be formed to a thickness of about 30 nm to about 100 nm.

6 is a view comparing the properties of the hot stamping parts according to the crack heating temperature according to the embodiment.

Referring to FIG. 6 , the part characteristics of the hot stamping part formed according to the left crack heating temperature, in this figure, a welding current (kA), a spatter current (kA), and a surface contact resistance (mΩ) are shown. It is assumed that the crack heating time is about 300 s. The welding current (kA) is a current value in which the starting value of one section is shown in the section. For example, a section in which the welding current is shown as 6.0 kA may have a range of about 6.0 kA to about 6.5 kA.

According to the hatching shown, no spatter (A), low spatter content (B) when spatter is measured to be about 50% or less, and heavy spatter content (C) when spatter is about 50% or more and less than 100% , the case where the spatter is about 100% corresponds to the high spatter content (D), and is illustrated by hatching corresponding to each example in each column. In this case, A may be a case in which the spatter is not visually confirmed. Hereinafter, for convenience of description, they may be referred to as A, B, C, and D. In this case, it may be defined as the spatter upper limit current (kA) including the period from the non-spatter generation (A) section to the section having the small amount of spatter (B). It can be explained that as the spatter upper limit current increases, the spatter sensitivity decreases and the weldability improves.

Specifically, the examples of the drawings are the results of experiments based on welding conditions as shown in Table 1 below, but these are only examples and the conditions of the embodiments of the present invention are not limited thereto.

First, when the crack heating temperature is about 880 ℃, spatter appears as 100% in all welding current sections shown in FIG. 4 and corresponds to D, the spatter upper limit current is about 8.0 kA, and the surface contact resistance is measured as 0.51 mΩ became

When the crack heating temperature is about 900 °C, the welding current corresponds to A in the range of 5.5 kA to 6.7 kA, and the welding current corresponds to B in the section of about 6.9 kA, so the spatter upper limit current is about 7.1 kA and the surface contact resistance was measured to be 0.55 mΩ.

When the crack heating temperature is about 930 ℃, the welding current corresponds to A in the range of 5.5 kA to 6.5 kA, and the welding current corresponds to B in the section of about 6.7 kA, so the spatter upper limit current is about 6.9 kA and the surface contact resistance was measured to be 0.67 mΩ.

When the crack heating temperature is about 950℃, the welding current corresponds to A in the range of 5.5kA to 6.5kA, and corresponds to C in the section where the welding current is about 6.7kA. Therefore, the upper limit of the spatter current is about 6.7 kA. The surface contact resistance was measured to be 0.75 mΩ.

When the crack heating temperature is about 970 °C, the welding current corresponds to A in the section of about 5.5 kA, and the adjacent welding current corresponds to C in the section of about 6.0 kA, so the spatter upper limit current is about 6.0 kA, and the surface contact resistance is It was measured to be 0.81 mΩ.

First, when the crack heating temperature is about 1000 ℃, since it is a part with a heavy spatter content (C) and a high spatter content (D) in all the welding current sections shown in FIG. 4, the spatter upper limit current is about 5.0 kA and the surface The contact resistance was measured to be 0.92 mΩ.

Here, it will be described with reference to FIG. 7 together. 7 is a graph comparing surface contact resistance according to a crack heating temperature of a hot stamping part according to an embodiment.

Referring to FIG. 7 , the surface contact resistance (mΩ) for each crack heating temperature according to the embodiment of FIG. 6 is shown. When the heating temperature is about 1200°C, the spatter upper limit current is about 5.0 kA, but the surface contact resistance is too high, about 0.9 mΩ or more, so the weldability is inferior. Therefore, it is preferable to control the crack heating temperature to about 1000° C. or less. On the other hand, when the heating temperature is less than about 880 ℃, there is a problem in that it is difficult to secure the desired properties of the hot stamping steel sheet. Accordingly, in the manufacturing process of the hot stamping part according to an embodiment of the present invention, the temperature at which the blank is cracked and heated may be controlled to be about 900° C. or more and about 1000° C. or less. In particular, in order to optimize weldability, the heating temperature for cracking may be about 900° C. or more and about 1000° C. or less.

As described above, in the method of manufacturing a hot stamping part according to an embodiment, it can be seen that the lower the crack heating temperature, the higher the spatter upper limit current, and the lower the surface contact resistance during resistance point welding of the part. Accordingly, the heating temperature during welding may be lowered and weldability may be improved. In addition, it was confirmed that the thickness of the oxide layer 300 (refer to FIG. 3b ) of the hot stamping part decreases as the crack heating temperature decreases, which will be described later in FIG. 8 .

8 is a cross-sectional view illustrating a cross-section of a hot stamping part according to different embodiments. 8(a), 8(b) and 8(c) are cross-sectional views showing cross-sections of hot stamping parts manufactured at crack heating temperatures of 970°C, 930°C and 900°C, respectively. In each of FIGS. 8(a), 8(b) and 8(c), an oxide layer 300 disposed on the plating layer 200 (specifically, it may be the third layer 240) is illustrated. Embodiments according to the present figure may further include a first passivation layer 310 and a second passivation layer 320 on the oxide layer 300 . The first protective layer 310 may include Pt, the second protective layer 320 may include carbon, and the protective layers 310 and 320 protect the plating layer 200 from additional oxidation, nitridation, etc. can play a protective role.

The oxide layer 300 may include Al ₂ O ₃ as shown in this figure. The oxide layer thickness d1 of FIG. 8(a) is about 78 nm, the oxide layer thickness d2 of FIG. 8(b) is about 56 nm, and the oxide layer thickness d3 of FIG. 8(c) is about 38 nm. became On the other hand, as described in FIGS. 6 and 7 , it was confirmed that the lower the heating temperature within the range of about 900°C to about 1000°C, the higher the spatter upper limit current and the lower the surface contact resistance, the better the weldability. In this figure, since the thickness of the aluminum-based oxide layer 300 becomes thinner as the heating temperature decreases within the heating temperature range, it can be seen that the thinner the thickness of the oxide layer 300, the better the weldability. The thickness of the oxide layer 300 for ensuring the weldability of the hot stamping part may be about 30 nm to about 100 nm, and preferably about 30 nm to 80 nm.

As described above, according to the embodiments of the present invention, a multi-layer plating layer including a layer having excellent weldability is formed by multi-stage heating and crack heating of a blank for hot stamping, and the thickness of the oxide film is decreased by controlling the temperature of the heating step. reduced to maximize weldability.

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 (23)

grater;
An intermixed layer, a first layer (Al-rich Fe _x Al _y Si _z ), a second layer (Fe-rich Fe _x Al _y Si _z ), and a third layer (Al) disposed on the steel plate and sequentially stacked -rich Fe _x Al _y Si _z ) including a plating layer; and
an oxide layer positioned on the plating layer and having a thickness of 30 nm to 100 nm;
Including, hot stamping parts. According to claim 1,
the intermixed layer comprises a first intermixed layer adjacent to the steel sheet and a second intermixed layer adjacent to the first intermixed layer;
wherein the first intermixed layer and the second intermixed layer have different phases. 3. The method of claim 2,
wherein the Si content of the second intermixed layer is less than the Si content of the first intermixed layer. 4. The method of claim 3,
The first intermixed layer (Fe-rich Fe _x Al _y Si _z ) is x: 80wt% to 90wt%, y: 5wt% to 20wt%, z: more than 0wt% 5wt% or less,
The second intermixed layer (Fe-rich Fe _x Al _y Si _z ) is x: 40wt% to 60wt%, y: 30wt% to 50wt%, z: more than 0wt% and 20wt% or less, hot stamping part. 4. The method of claim 3,
and a ratio of the thickness of the second intermixed layer to the total thickness of the intermixed layer is less than 40%. According to claim 1,
wherein the intermixed layer has a thickness of 4 μm to 10 μm. 3. The method of claim 2,
and the second intermixed layer has the same phase as the second layer (Fe-rich Fe _x Al _y Si _z ). According to claim 1,
and an area fraction of the first layer and the third layer with respect to the plating layer is 40% or more. 9. The method of claim 8,
The first layer (Al-rich Fe _x Al _y Si _z ) is x: 30wt% to 50wt%, y: 50wt% to 70wt%, z: more than 0wt% and 10wt% or less,
The second layer (Fe-rich Fe _x Al _y Si _z ) is x: 40wt% to 60wt%, y: 30wt% to 50wt%, z: more than 0wt% 20wt% or less,
The third layer (Al-rich Fe _x Al _y Si _z ) is x: 30wt% to 50wt%, y: 50wt% to 70wt%, z: more than 0wt% and 10wt% or less, hot stamping part. Putting the blank into a heating furnace having a plurality of sections having different temperature ranges;
a multi-stage heating step of heating the blank step by step; and
a crack heating step of crack heating the blank;
including,
In the multi-stage heating step, and the crack heating step,
Sequentially stacked intermixed layers, a first layer (Al-rich Fe _x Al _y Si _z ), a second layer (Fe-rich Fe _x Al _y Si _z ), and a third layer (Al-rich Fe _x Al _y Si ) _z ) is formed with a plating layer comprising,
wherein the intermixed layer is formed to include a first intermixed layer adjacent the steel sheet and a second intermixed layer adjacent the first intermixed layer. 11. The method of claim 10,
The ratio of the length of the section for heating the blank in multiple stages in the plurality of sections to the length of the section for crack heating the blank satisfies 1:1 to 4:1, the method of manufacturing a hot stamping part. 12. The method of claim 11,
The temperature of the plurality of sections increases from the inlet of the furnace to the outlet of the furnace, the method of manufacturing a hot stamping part. 13. The method of claim 12,
The temperature difference between two adjacent sections of the section heating the blank in multiple stages is greater than 0° C. and less than or equal to 100° C., a method of manufacturing a hot stamping part. 12. The method of claim 11,
A method of manufacturing a hot stamping part, wherein a temperature of a section heating the blank by cracking among the plurality of sections is higher than a temperature of sections heating the blank in multiple stages. 11. The method of claim 10,
wherein the blank stays in the furnace for 180 seconds to 360 seconds. 14. The method of claim 13,
In the multi-stage heating step, and the crack heating step,
A method of manufacturing a hot stamping part, located on the plating layer, wherein an oxide layer having a thickness of 30 nm to 100 nm is formed. 17. The method of claim 16,
The hot stamping part comprises:
wherein the first intermixed layer and the second intermixed layer have different phases. 18. The method of claim 17,
wherein the Si content of the first intermixed layer is less than the Si content of the second intermixed layer. 19. The method of claim 18,
The first intermixed layer (Fe-rich Fe _x Al _y Si _z ) is x: 80wt% to 90wt%, y: 5wt% to 20wt%, z: more than 0wt% 5wt% or less,
The second intermixed layer (Fe-rich Fe _x Al _y Si _z ) is x: 40wt% to 60wt%, y: 30wt% to 50wt%, z: more than 0wt% 20wt% or less
A method of manufacturing hot stamping parts. 19. The method of claim 18,
and a ratio of the thickness of the second intermixed layer to the total thickness of the intermixed layer is less than 40%. 17. The method of claim 16,
wherein the intermixed layer has a thickness of 4 μm to 10 μm. 18. The method of claim 17,
and the second intermixed layer has the same phase as the second layer (Fe-rich Fe _x Al _y Si _z ). 11. The method of claim 10,
After the crack heating step,
transferring the crack-heated blank from the furnace to a press mold;
forming a molded body by hot stamping the transferred blank; and
cooling the formed body;
Further comprising, a method of manufacturing a hot stamping part.

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