Manufacturing Method for Hot-Stamped Parts

This application is a continuation of International Application No. PCT/KR2023/003608 filed on Mar. 17, 2023, which claims under 35 U.S.C. § 119(a) the benefit of Korean Patent Application No. 10-2022-0191109 filed on Dec. 30, 2022, the entire contents of which applications are incorporated by reference herein.

The present disclosure relates to a method of manufacturing hot-stamped parts.

As environmental and fuel efficiency regulations have been strengthened worldwide, the need for lighter vehicular materials is increasing. Accordingly, ultra-high-strength steel and hot-stamping steel have been actively researched and developed.

Hot stamping includes a process in which steel sheets are heated to a high temperature in a heating furnace and then rapidly cooled while being formed in a press to manufacture high-strength parts. In addition, a piercing process may be additionally performed to form holes by cutting/processing the high-strength parts.

The piercing process may be performed using laser equipment or press dies. However, when laser equipment is used, a processing time may increase, and when press dies are used, the quality of sheared surfaces may deteriorate.

In one aspect, embodiments of the present disclosure are characterized by deriving a maximum delay time for hot piercing by considering factors such as the thickness of a blank, the start temperature of forming, etc.

An embodiment of the present disclosure provides a method of manufacturing hot-stamped parts, the method including: a heating operation of heating a blank; a transferring operation of transferring the heated blank to a press die including a punch; and a forming and piercing operation of hot-forming the transferred blank into a shape of a hot-stamped part and hot-piercing the transferred blank to form a pierced portion in the hot-stamped part, wherein a hot piercing delay time in the forming and piercing operation is within a range of 0 second to λfrom a time point at which the press die reaches a bottom dead point, and λis derived by Equation below:

(In the Equation above, λrefers to a maximum delay time (s) of hot piercing, arefers to a correction coefficient considering a forming start temperature and hot piercing conditions, T refers to the forming start temperature (° C.), brefers to a correction coefficient considering a pressing force of the press die, crefers to a correction coefficient considering thickness sensitivity of a material, and t refers to a thickness (mm) of the material).

According to the embodiment, in the Equation above, amay may have a value greater than 0 and less than or equal to 0.001, bmay may have a value greater than 0 and less than or equal to 0.65, and cmay may have a value ranging from about 1 to about 1.2.

According to the embodiment, in the transferring operation of transferring the heated blank, the heated blank suitably may be air cooled at room temperature.

According to the embodiment, the heating operation of heating the blank may include: a multi-stage heating operation in which the blank may be heated step by step; and a soaking operation in which the blank may be heated to a temperature of Ac3 to 1000° C. Heating step by step can include applying a first temperature for at least 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100 seconds or more and then applying a second heating temperature (e.g. a second temperature may be 3, 5, 10 degrees centigrade or more higher than the first temperature) and that second temperature for at least 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100 seconds or more, and then optionally applying subsequent higher or lower temperatures for such time periods.

According to the embodiment, the heating operation of heating the blank suitably may be performed in a heating furnace, and the heating furnace suitably may include a plurality of zones having different temperature ranges.

According to the embodiment, in the forming and piercing operation, the transferred blank suitably may be cooled in the press die.

According to the embodiment, the pierced portion suitably may include at least two pierced portions.

Other aspects, features, and advantages, in addition to those described above, will become apparent from the following detailed description for carrying out the disclosure, the claims, and the accompanying drawings.

As described above, according to an embodiment of the present disclosure, a maximum delay time for hot piercing is derived by considering factors such as the thickness of a blank or the start temperature of forming, thereby enabling flexible process design and facilitating the quality control of manufactured hot-stamped parts. However, the scope of the present disclosure is not limited by these effects.

The present disclosure may have various different forms and various embodiments, and specific embodiments are illustrated in the accompanying drawings and are described herein in detail. Effects and features of the present disclosure, and methods of achieving the effects and features will become apparent with reference to the accompanying drawings and the embodiments described below in detail. However, the present disclosure is not limited to the embodiments described below and may be implemented in various forms

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

In the following embodiments, the terms of a singular form may include plural forms unless referred to the contrary.

In the following embodiments, terms such as “comprise,” “include,” and “have” specify the presence of features or elements stated in the specification, but do not preclude the presence or addition of one or more other features or elements.

In the drawings, the sizes of elements may be exaggerated or reduced for ease of illustration. For example, in the drawings, the size or thickness of each element may be arbitrarily shown for illustrative purposes, and thus the present disclosure should not be construed as being limited thereto.

The order of processes explained in one embodiment may be changed in a modification of the embodiment or another embodiment. For example, two processes sequentially explained may be performed substantially at the same time or in the reverse of the explained order.

In the present specification, the expression “A and/or B” indicates A, B, or A and B. In addition, the expression “at least one of A and B” indicates A, B, or A and B.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the drawings, like reference numerals denote like elements, and overlapping descriptions thereof will be omitted.

is a perspective view schematically illustrating a hot-stamped part according to an embodiment of the present disclosure.

Referring to, according to an embodiment of the present disclosure, a hot-stamped partmay include a pierced portion. In an embodiment, the hot-stamped partmay include two pierced portions. The pierced portionmay include a first pierced portionand a second pierced portion. However, the present disclosure is not limited thereto. The hot-stamped partmay include one pierced portionor three or more pierced portions.

Although not illustrated, the hot-stamped partmay include an additional pierced portion in addition to the pierced portion. In this case, the pierced portionmay be formed by hot piercing, and the additional pierced portion may be formed by cold piercing or laser piercing. The hot-stamped partmay include one or more additional pierced portions.

In one embodiment, an additional pierced portion may be formed using the pierced portion. This will be described in more detail below.

In addition, although not illustrated, the hot-stamped partmay include an edge portion. In this case, the edge portion of the hot-stamped partmay refer to a side extending along the length of the hot-stamped part.

is a flowchart schematically illustrating a method of manufacturing hot-stamped parts according to an embodiment of the present disclosure.

Referring to, according to the embodiment, the method of manufacturing hot-stamped parts may include a preparing operation S, a heating operation S, a transferring operation S, and a forming/piercing operation S.

is a flowchart schematically illustrating the preparing operation Sin the method of manufacturing hot-stamped parts, according to an embodiment of the present disclosure, andis a plan view schematically illustrating a blankaccording to an embodiment of the present disclosure.

Referring to, the preparing operation Smay be an operation of preparing a blankfor hot stamping. In one embodiment, the preparing operation Smay include a hot rolling operation S, a cooling/coiling operation S, a cold rolling operation S, an annealing operation S, a plating operation S, and a cutting operation S.

First, an operation of reheating a steel slab may be performed. In the slab reheating operation, a steel slab obtained through a continuous casting process is reheated to a predetermined temperature, and thus, components that have segregated during casting may be redissolved. In one embodiment, a slab reheating temperature (SRT) may range from about 1,200° C. to about 1,400° C. When the slab reheating temperature (SRT) is lower than about 1,200° C., components that have segregated during casting may not be sufficiently redissolved, making it difficult to achieve significant homogenization of alloy elements and a significant solid solution effect of titanium (Ti). Although a higher slab reheating temperature (SRT) is advantageous for homogenization, a slab reheating temperature (SRT) exceeding about 1,400° C. may lead to an increase in the grain size of austenite, which may reduce strength, and may also increase manufacturing costs due to excessive heating.

In the hot rolling operation S, a reheated steel sheet may be hot rolled at a predetermined finishing delivery temperature. A hot-rolled steel sheet may be produced through the hot rolling operation S. In one embodiment, the finishing delivery temperature (FDT) may range from about 880° C. to about 950° C. Here, when the finishing delivery temperature (FDT) is lower than about 880° C., it may be difficult to secure workability of the steel sheet due to the occurrence of mixed microstructures caused by rolling in an abnormal region, and the workability of the steel sheet may deteriorate due to microstructural non-uniformity. Furthermore, during hot rolling, it may be difficult to transfer the steel sheet due to abrupt phase transitions. When the finishing delivery temperature (FDT) exceeds about 950° C., austenite grain coarsening may occur, and TiC precipitate coarsening may also occur, leading to deterioration in the quality of hot-stamped parts.

In the cooling/coiling operation S, the hot-rolled steel sheet may be cooled to a predetermined coiling temperature (CT) and then coiled. In one embodiment, the coiling temperature of the cooling/coiling operation Smay range from about 550° C. to about 800° C. The coiling temperature affects the redistribution of carbon. When the coiling temperature is lower than about 550° C., the fraction of low-temperature phases may increase due to overcooling, which may lead to an increase in strength, an increase in rolling load during cold rolling, and a significant decrease in ductility. Conversely, when the coiling temperature exceeds about 800° C., abnormal grain growth or excessive grain growth may occur, potentially lowering formability and strength.

In the cold rolling operation S, the coiled hot-rolled steel sheet may be uncoiled, pickled, and then cold rolled. Here, the pickling may be performed to remove scale from the coiled hot-rolled steel sheet, that is, to remove scale from a hot-rolled coil produced by hot rolling. A cold-rolled steel sheet may be produced through the cold rolling operation S.

In the annealing operation S, the cold-rolled steel sheet may be annealed at a temperature of about 700° C. or higher. For example, the annealing operation Smay include heating the cold-rolled steel sheet and cooling the heated cold-rolled steel sheet at a predetermined cooling rate. In the annealing operation S, the cold-rolled steel sheet may be annealed. The annealing operation Smay be performed in an annealing furnace.

In one embodiment, the annealing temperature of the cold-rolled steel sheet may range from about 750° C. to about 900° C. When the annealing temperature of the cold-rolled steel sheet is lower than about 750° C., a desired microstructure may not be obtained, and recrystallization may not be sufficiently completed. On the other hand, when the annealing temperature of the cold-rolled steel sheet exceeds about 900° C., manufacturing process efficiency may decrease because the annealing temperature is excessively high. Therefore, when the annealing temperature of the cold-rolled steel sheet is in the range of about 750° C. to about 900° C., a desired microstructure, sufficient recrystallization, and improved manufacturing process efficiency may be achieved.

The plating operation Smay be an operation of forming a plating layer on the annealed cold-rolled steel sheet. In one embodiment, a plating layer may be formed on the annealed cold-rolled steel sheet through the plating operation S. In this case, the plating layer may include a zinc (Zn)-based plating layer or an aluminum (Al)-based plating layer.

Specifically, in the plating operation S, the annealed cold-rolled steel sheet may be immersed in a plating bath. At this time, the plating bath may be maintained at a temperature of about 400° C. to about 700° C. The density of the plating layer may range from about 40 g/mto about 200 g/m, based on both surfaces of a base material of the cold-rolled steel sheet. After the plating operation S, the cold-rolled steel sheet on which the plating layer is formed may be wound into a coil form.

Althoughshows that the cold rolling operation S, the annealing operation S, and the plating operation Sare performed after the cooling/coiling operation S, the present disclosure is not limited thereto. At least one of the cold rolling operation S, the annealing operation S, and the plating operation Smay be omitted. For example, the cold rolling operation Sand the annealing operation Smay be omitted. In this case, the hot-rolled steel sheet on which the plating layer is formed after the plating operation Smay be wound into a coil form.

Thereafter, in the cutting operation S, the steel sheet (for example, a cold-rolled steel sheet or a hot-rolled steel sheet) wound into the shape of a coil may be uncoiled and may then be cut to form a blankusing a laser or a cold press die. At this time, the blankmay include an outer portion (or edge) of the coil. For example, the blankmay include an outer portion (or edge) of the steel sheet.

Referring to, after the operation Sof preparing a blank, the heating operation Smay be performed to heat the blank. The type of heating in the heating operation Smay include direct heating and indirect heating. The type of heating in the heating operation Smay be one of direct heating and indirect heating, or a combination thereof.

In one embodiment, the heating operation Smay be performed by heating the blankin a heating furnace. The heating furnace may include a single zone with a single temperature range (or a single temperature), or a plurality of zones with different temperature ranges. When the heating furnace includes a single zone with one temperature range, the blankmay be heated to a target temperature within the temperature range. At this time, the target temperature may range from Ac3 to about 1,000° C. That is, the blankmay be heated in the heating furnace having a temperature range of Ac3 to about 1,000° C. until the temperature of the blankreaches a temperature ranging from Ac3 to about 1,000° C.

On the other hand, when the heating furnace includes a plurality of zones having different temperature ranges, the blankmay be heated in the heating furnace to the target temperature through the different temperature ranges.

is a flowchart schematically illustrating the heating operation Sin the method of manufacturing hot-stamped parts, according to an embodiment of the present disclosure.is a view illustrating a heating furnace having a plurality of zones for the heating operation Sin the method of manufacturing hot-stamped parts, according to an embodiment of the present disclosure.

Referring to, in the heating operation S, the blank(refer to) may be heated in the heating furnace having a plurality of zones with different temperature ranges. As shown in, the heating operation Smay include a multi-stage heating operation Sand a soaking operation S. In the multi-stage heating operation Sand the soaking operation S, the blankmay be heated while being transferred through the plurality of zones provided in the heating furnace.

In one embodiment, the overall temperature of the heating furnace may range from about 680° C. to about 1,000° C. Specifically, the overall temperature of the heating furnace in which the multi-stage heating operation Sand the soaking operation Sare performed may range from about 680° C. to about 1,000° C. In this case, the temperature of the heating furnace in which the multi-stage heating operation Sis performed may range from about 680° C. to about Ac3, and the temperature of the heating furnace in which the soaking operation Sis performed may range from about Ac3 to about 1,000° C.