EP2987568B1

EP2987568B1 - Hot press forming device for coated steel and hot press forming method using same

Info

Publication number: EP2987568B1
Application number: EP13882172.3A
Authority: EP
Inventors: Hong-Gee Kim; Jong-Won Choi; Kyung-Seok Oh; Dong-Jin Kim
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2013-04-19
Filing date: 2013-12-24
Publication date: 2018-08-01
Anticipated expiration: 2033-12-24
Also published as: US20160082496A1; JP6106804B2; CN105307793A; EP2987568A4; EP2987568A1; KR101482395B1; WO2014171610A1; KR20140125562A; JP2016518256A

Description

[Technical Field]

The present disclosure relates to a hot press forming (HPF) device and an HPF method using the HPF device, and more particularly, to an HPF device and method for forming coated steel according to the preambles of claims 1 and 4 respectively.

[Background Art]

Recently, automobile manufacturers have increased the use of high-strength materials in order to manufacture eco-friendly, fuel-saving, light automotive parts satisfying social needs. However, high-strength materials are difficult to form into desired shapes because of problems such as spring back and difficulty in maintaining dimensions, and thus the use of high-strength materials is limited.
These problems related with formability may be solved by manufacturing high-strength parts in a way of forming high-strength materials into desired shapes at high temperatures guaranteeing good formability, and rapidly cooling the formed high-strength materials in dies. This method is called "hot press forming (HPF)." Parts having a degree of strength equal to or greater than 1500 MPa may be manufactured by the HPF method.
In a HPF process of the related art, steel blanks are heated to 900°C or higher and are then pressed. However, when steel blanks are heated, scale may form on the surfaces of the steel blanks due to oxidation. Therefore, after the HPF process, additional processes such as a shot blasting process may be performed to remove scale from formed products. In addition, the corrosion resistance of products manufactured by the HPF method is inferior to that of coated products.
To address these problems, Patent Document 1 has proposed a method of forming an aluminous coating layer on a steel sheet, the aluminous coating layer withstanding severe environments of a heating furnace, suppressing the oxidation of the steel sheet, and forming a corrosion resistant aluminum (Al) passive film on the steel sheet.
However, although such Al-coated materials have a high degree of resistance to high temperatures, the corrosion resistance of the Al-coated materials is inferior to the corrosion resistance of materials coated with zinc (Zn) by a sacrificial anode method, and the manufacturing costs of the Al-coated materials are high. Therefore, there has been increasing interest in methods of using Zn-coated materials.
However, if Zn-coated materials are heated to a high temperature and are then formed into parts, micro cracks having a size of about 10 µm to 30 µm may be formed in walls of the parts, thereby deteriorating the properties of the parts such as bendability. Therefore, the application of Zn-coated materials is limited.
(Patent Document 1) US Patent No.: 6,296,805
US 2012/067098 A1 discloses a method performed by a power press machine composed of a concave die, a convex die with two shoulders and two depressors separately beside the convex die. The method includes the steps of: a) placing a high-strength steel (HSS) on the convex die and under the concave die; b) moving the depressors to have two opposite side portions of the HSS clamped between the concave die and the depressors; c) moving the convex die to press the HSS and keeping the side portions immovable; d) forming two steps adjacent to the side portions by the shoulders; and e) cutting off the steps with the side portions.
JP 2009 101421 discloses a device for forming a pressed article comprising: a lower die having a female type, an upper die having a male type, a cushion pin which holds the material to be formed, a plate hold-down device, a cam driver which moves in response to the upper die, a cam slider which has a notch formed at its inside end to grip an unformed part of the article to be formed and presses the article in the direction of the center of pressure with the cam driver.

[Disclosure]

[Technical Problem]

Aspects of the present disclosure may provide a hot press forming (HPF) device for performing a HPF process on coated steel, particularly zinc (Zn)-coated steel while reducing the formation of microcracks in a formed product and imparting uniform properties to the formed product, and a HPF method using the HPF device.

[Technical Solution]

According to an aspect of the present disclosure, a hot press forming (HPF) device for forming coated steel according to claim 1 is provided. According to another aspect of the present disclosure, a HPF method for forming coated steel according to claim 4 is provided.

[Advantageous Effects]

According to the present disclosure, when coated steel such as zinc (Zn)-coated steel is processed through a hot press forming (HPF) process, the formation of micro cracks in formed products may be reduced, and the formed products may have a high degree of formability such as bendability. In addition, formed products having high quality may be produced, and particularly, shaped portions of the formed products may have uniform properties.

[Description of Drawings]

FIG. 1 is a schematic view illustrating a hot press forming (HPF) device and method of the related art.
FIG. 2 is a view illustrating a shaped portion of a formed product manufactured by a HPF method of the related art.
FIG. 3 is a schematic view illustrating plastic strain in the formed product manufactured by the HPF method of the related art.
FIG. 4 is a schematic view illustrating an exemplary HPF device and method according to an exemplary embodiment of the present disclosure.
FIG. 5 is a schematic view illustrating plastic strain in a formed product manufactured according to the exemplary embodiment of the present disclosure.
FIG. 6 is a schematic view illustrating an exemplary HPF device and method according to another exemplary embodiment of the present disclosure.
FIG. 7(a) is a schematic view illustrating plastic strain in a formed product manufactured by a method of the related art, and FIG. 7(b) is a schematic view illustrating plastic strain in a formed product manufactured according to the other exemplary embodiment of the present disclosure.
FIG. 8(a) is an image of a shaped portion of a formed product manufactured by a method of the related art, and FIG. 8(b) is an image of a shaped portion of the formed product manufactured according to the other exemplary embodiment of the present disclosure.

[Best Mode]

The inventors have found that if coated steel, particularly zinc (Zn)-coated steel, is subjected to a hot press forming (HPF) process, formed parts (formed products) have micro cracks (very small cracks or micro cracks), and the properties of the formed products are not uniform because of non-uniform cooling at shaped portions of the formed products. Thus, the inventors have conducted research to solve these problems.
Exemplary embodiments of the present disclosure will now be described in detail with reference to the accompanying drawings. However, the accompanying drawings are for illustrative purposes only and are not intended to limit the scope of the present invention.
FIG. 1 is a schematic view illustrating a HPF process of the related art. As illustrated in FIG. 1, in a HPF process, a heated blank 10 is placed between an upper die 11 and a lower die 12 and is then pressed using the upper and lower dies to produce a formed product 13.
FIG. 2 illustrates a surface of a shaped portion of coated steel after the coated steel is processed by a HPF method of the related art as illustrated in FIG. 1. As illustrated in FIG. 2, formed products 13 made of coated steel through a HPF process of the related art had micro cracks in shaped portions of the formed products 13.
To analyze reasons for this, plastic strain in a formed product illustrated in FIG. 2 was analyzed, and results of the analysis are illustrated in FIG. 3. As illustrated in FIG. 3, the formed product 13 produced using a HPF device of the related art had an excessive amount of plastic deformation at the shaped portion of the formed product 13, and micro cracks were formed in the shaped portion.
In detail, micro cracks were formed in a wall of the shaped portion, especially at a lower end portion of the wall of the shaped portion because of concentrated deformation on the lower end portion. The design shape of the formed product 13 may be modified to reduce deformation. However, it may not be easy to modify the design shape of the formed product 13 because of limitations to the change of design. Therefore, the inventors have invented a method of using a cam for reducing deformation and micro cracks.
In addition, when a blank is pressed between upper and lower dies, the thickness of the blank is reduced at a shaped portion of the blank, and a narrow gap is formed between the blank and the upper and lower dies. As a result, when the blank is cooled in the dies after the blank is pressed, the blank is not uniformly cooled, and thus properties of the shaped portion of the blank are deteriorated.
Therefore, the inventors have invented a HPF device configured to prevent the generation of micro cracks in formed products and impart uniform properties to formed products, and a HPF method using the HPF device.
First, the HPF device of the present disclosure will be described in detail.
The HPF device of the present disclosure includes: an upper die and a lower die configured to constrain a portion of a blank; and a cam configured to deform a non-constrained portion of the blank to form a shaped portion.
The cam forms a shaped portion while moving in a direction different from directions in which the upper and lower dies move.
FIG. 4 is a schematic view illustrating an exemplary HPF device 20 according to an exemplary embodiment of the present disclosure. As shown in FIG. 4, the HPF device 20 of the exemplary embodiment of the present disclosure includes upper 21 and lower 22 dies, and cams 23 between the upper 21 and lower 22 dies. An HPF device of the related art such as that illustrated in FIG. 1 includes no cam, and when a blank is pressed using the HPF device of the related art, upper and lower dies of the HPF device are used to constrain the blank.
However, when a blank 24 is pressed using the HPF device 20 of the exemplary embodiment of the present disclosure, the upper 21 and lower 22 dies constrain a portion of the blank 24, and a non-constrained portion of the blank 24 is formed using the cams 23 to form a shaped portion. In the HPF device 20 illustrated in FIG. 4, the cams 23 move in horizontal directions independent of the upper 21 and lower 22 dies moving in vertical directions, in order to form a shaped portion.
When the shaped portion is formed, the plastic deformation of the shaped portion is distributed by the cams 23. That is, as illustrated in FIG. 4, when a blank 24 is pressed into a desired shape using the HPF device 20 of the exemplary embodiment of the present disclosure, the upper 21 and lower 22 dies constrain and shape a portion of the blank 24, and the cams 23 move to shape another portion of the blank 24 not constrained by the upper 21 and lower 22 dies.
In the HPF device 20 illustrated in FIG. 4, the cams 23 are provided in addition to the upper 21 and lower 22 dies.
FIG. 5 illustrates plastic strain in a formed product 25 manufactured using the HPF device 20 illustrated in FIG. 4, the plastic strain being measured by analysis on forming. When the results shown in FIG. 5 are compared with the results shown in FIG. 3, the plastic deformation of a shaped portion of the formed product 25 produced using the HPF device 20 of the exemplary embodiments of the present disclosure is markedly reduced. Therefore, the formation of micro cracks may be markedly reduced in products manufactured using the HPF device 20 of the exemplary embodiment of the present disclosure.
Another exemplary HPF device 30 is illustrated in FIG. 6 according to another exemplary embodiment of the present disclosure. In the HPF device 30 illustrated in FIG. 6, cams 33 are provided separate from upper 31 and lower 32 dies.
In the HPF device 30 illustrated in FIG. 6, the upper 31 and lower 32 dies constrain a blank 34 to fix the blank 34, and forming of the blank 34 is performed substantially by the cams 33. That is, the upper 31 and lower 32 dies fix the blank 34, and the cams 33 form the blank 34 while moving at predetermined angles.
FIG. 7(b) illustrates plastic strain in a formed product 35 manufactured using the HPF device 30 illustrated in FIG. 6, the plastic strain being measured by analysis on forming. FIG. 7(a) illustrates plastic strain in a formed product produced by a method of the related art. Referring to FIGS. 7(a) and 7(b), the plastic strain in the formed product 35 (FIG. 7(b)) produced by the HPF device 30 of the other exemplary embodiment of the present disclosure is much lower than the plastic strain in the formed product (FIG. 7(a)) produced by the related-art method.
In addition, FIG. 8(a) illustrates a surface of a shaped portion of the formed product 35 manufactured using the HPF device 30 illustrated in FIG. 6, and FIG. 8(b) illustrates a surface of a shaped portion of a formed product 50 manufactured using a HPF device of the related art. Referring to FIG. 8(a), the formed product 35 manufactured using the HPF device 30 of the other exemplary embodiment of the present disclosure does not have a large micro crack developed to base steel. However, referring to FIG. 8(a), a large micro crack 51 is formed in the base steel of the formed product 50.
In addition, an exemplary embodiment of the present disclosure provides a HPF method for forming coated steel. Hereinafter, the HPF method will be described in detail.
According to the HPF method of the exemplary embodiment of the present disclosure, a prepared blank is heated and formed in a HPF device.
As illustrated in FIG. 4 and FIG. 6, upper 21, 31 and lower 22, 32 dies of the HPF device 20, 30 are used to constrain a portion of the blank 24, 34, and cams 23, 33 of the HPF device 20, 30 are used to form a non-constrained portion of the blank 24, 34 to form a shaped portion.
In the example illustrated in FIG. 4, the upper 21and lower 22 dies of the HPF device 20 are used to constrain and form a portion of the blank 24, and the cams 23 of the HPF device 20 are used to form a non-constrained portion of the blank 24 while moving to the non-constrained portion of the blank 24 to complete forming. Unlike this, in the example illustrated in FIG. 6, although the upper die 31 constrains the lower die 32, the upper 31 and lower 32 dies are not involved in forming, and the cams 33 form a portion of the blank 34 while being moved.
According to a HPF method of the related art as shown in FIG. 1, when a portion of a blank 10 is formed, the portion of the blank 10 continuously undergoes plastic deformation due to friction. Therefore, the portion has a large amount of plastic deformation after the forming, and thus micro cracks may be formed in the shaped portion. As a result, formed products having poor bendability and formability may be manufactured. Moreover, a shaped portion having undergone continuous deformation may have a more reduced thickness than the other portion. In this case, when the blank 10 is cooled, since the shaped portion is not in uniform contact with the dies, the shaped portion may not be uniformly cooled, and thus may have non-uniform properties.
However, according to the HPF method of the exemplary embodiment of the present disclosure, as illustrated in FIGS. 4 and 6, when a portion of the blank 24, 34 is formed, the portion does not continuously undergo plastic deformation, thereby preventing the formation of micro cracks in the portion and a decrease in the thickness of the portion. In addition, since the cams 23, 33 push the portion against the dies 21, 22, 31, 32, the portion and the dies 21, 22, 31, 32 may be reliably brought into contact with each other, and after the blank 24, 34 is cooled, the portion may have uniform properties.
Meanwhile, the blank 24, 34 may be uniformly heated to have the same temperature, or may be heated to a relatively high temperature in some region and a relatively low temperature in the other region in order to produce a multi-strength formed product.
In detail, the entire region of the blank 24, 34 may be heated to a temperature equal to or higher than an A3 temperature of the blank 24, 34, or the blank 24, 34 may be heated to a temperature equal to or higher than the A3 temperature in a predetermined region and to a temperature equal to or lower than an A1 temperature of the blank 24, 34 in another region.
In the former case, the entire region of a product formed by the HPF method may have a high degree of strength, and in the latter case, a multi-strength product may be formed by the HPF method. The multi-strength product may have a relatively high degree of strength in a region heated to a relatively high temperature and a relatively low degree of strength in a region heated to a relatively low temperature.
In the above, any heating method may be used. That is, any method used in the related art to heat steel may be used. For example, the blank 24, 34 may be heated in the atmosphere of a heating furnace or using an induction heating device.
After the blank is completely formed, the blank 24, 34 is cooled. For example, the blank 24, 34 may be indirectly cooled by cooling the dies of the HPF device 20, 30. However, cooling of the blank 24, 34 is not limited thereto. In addition, cooling conditions generally used in an HPF method of the related art may be used.

Claims

A hot press forming (HPF) device (20) for forming coated steel, the HPF device (20) comprising an upper die (21) and a lower die (22),
characterised in that the upper (21) and lower dies (22) constrain a portion of a blank (24), and the HPF device (20) further comprises a cam (23) configured to form another portion of the blank (24) not constrained by the upper (21) and lower dies (22) in order to form a shaped portion, wherein the cam (23) in use is movable in a direction different from directions in which the upper (21) and lower dies (22) are capable of being moved.
The HPF device (20) of claim 1, wherein the cam (23) is disposed between the upper (21) and lower dies (22).
The HPF device (20) of claim 1, wherein the cam (23) is separate from a portion of the upper die (21) or the lower die (22) .
A hot press forming (HPF) method for forming coated steel, the HPF method comprising:
heating a blank (24);

forming the heated blank (24) using a HPF device (20); and

cooling the formed blank (24),

characterised in that in the forming of the heated blank (24), a portion of the heated blank (24) is constrained by upper (21) and lower dies (22) of the HPF device (20), and another portion of the heated blank (24) not constrained by the upper (21) and lower dies (22) is formed by a cam (23) in order to form a shaped portion, wherein in the forming of the heated blank (24), the cam (23) is moved in a direction different from directions in which the upper (21) and lower dies (22) are moved.
The HPF method of claim 4, wherein the cam (23) is disposed between the upper (21) and lower dies (22).
The HPF method of claim 4, wherein the cam (23) is separate from a portion of the upper die (21) or the lower die (22) .
The HPF method of claim 4, wherein in the heating of the blank (24), the blank (24) is entirely heated to a temperature equal to or higher than an A3 temperature of the blank (24), or the blank (24) is heated to a temperature equal to or higher than the A3 temperature of the blank (24) in a predetermined region and to a temperature equal to or lower than an A1 temperature of the blank (24) in another region.