The present invention relates to a hot stamping method used in sheet metal forming, specifically in production of high-strength steel components for crash-relevant parts in the automotive industry.
Lightweight and high-strength body is a main issue in the automotive industry. The hot stamping technology was proposed by Norrbottens Jarnverk AB in Sweden in the early 1970s. In GB Patent No. 1490535 issued to this company, the hot stamping technology is disclosed in detail.
To obtain a vehicle body part having tensile strength of 1 GPa or more by the hot stamping process, the microstructure of a steel blank has to be transformed from austenite to martensite by the quenching process in a press forming apparatus. For the hot stamping, boron steels are used which contains carbon of about 0.2 wt % and uses manganese (Mn) and boron (B) as elements for improving heat treatment performance.
In the hot stamping process, the blank is heated to an austenitization temperature or more, for example, up to 950° C., and then formed in a press forming apparatus, which provides excellent formability and reduces spring-back or delayed fracture, particularly in high-strength parts.
During the hot stamping process, however, surface oxidation of the blank is occurred, and thus oxide scale on the surface of the hot-pressed body part needs to be removed through a descaling process. In order to remove the descaling process, aluminized steel sheets were proposed by Arcelormittal.
For the hot stamping, the blank may be heated in an electric resistance furnace to a temperature between 880° C. and 950° C. to form austenite. The electric resistance furnace may have heating elements provided in its walls and an electric current is directed through the heating elements where it is dissipated as heat. The thermal energy is transferred to the blank by radiation and convection. It takes between 12 minutes and 17 minutes to austenitize a blank of 1.2 mm thick using the electric resistance furnace or a gas furnace, which causes decrease the operating speed and increases the production cost of the hot stamping. Furthermore, the length of the heating furnace when using the electric resistance furnaces and the gas furnaces needs to be extended for the hot stamping, ranging from 23 m to 30 m. This means that large space-based facilities are needed for the hot stamping.
High frequency induction heating may be used for local strengthening of vehicle body parts. A steel body part may be heated up to 1000° C. or more within several seconds by the high frequency induction heating. If it is possible to use the high frequency induction heating for the hot stamping, the length of the heating furnace and the heating time to austenitize blanks can be reduced. Such a fast temperature increase by the high frequency induction heating, however, may cause deformation of the blank. The high frequency induction heating was merely used just for a heat treatment of thick or bulky steel products rather than thin steel sheets for the hot stamping.
The present invention proposes a hybrid heating system having a high frequency induction heating furnace for the hot stamping. High frequency induction heating for press-forming thin steel sheets having a thickness about 0.7 mm to about 1.2 mm has never been adapted before and has never been considered to be possible. The present invention is to overcome the stereotype view and propose an innovative alternative that is able to replace the electric resistance furnace for the hot stamping.
U.S. Pat. No. 5,922,234 discloses technology for induction heating a slab while transferring the slab by using a roller. However, the slab commonly has a thickness of 50 mm to 300 mm and a very long length. The slab is a bulky metal product having a weight of 10 tons or more, furthermore, 50 tons or more and is obviously different from a blank used in hot stamping according to the present invention.
The blank is obtained by blanking a cold rolled coil or a parent material having a sheet shape so as to have a size and a shape required for hot stamping. The blank may be a thin sheet having a thickness of 2 mm or less, mostly, a thickness of about 0.7 mm to about 1.2 mm.
U.S. Pat. No. 5,487,795 discloses technology for heating an impact beam using high frequency induction devices while transferring the impact beam on transfer rollers. However, the impact beam induction-heated in the U.S. Pat. No. 5,487,795 is a bulky metal product previously formed. The impact beam is obviously different from a blank heated for hot stamping in the present invention.
The high frequency induction heating was not used or proposed for hot stamping at the time of filing U.S. application Ser. No. 12/496,254 and Korean Patent Application No. 10-2008-0096912.
A hot stamping method according to the present invention includes: performing a high frequency induction heating on a blank in a first heating furnace while transferring the blank; heating the blank transferred from the first heating furnace while transferring the blank in a second heating furnace; and forming and cooling the blank transferred from the second heating furnace in a press forming apparatus
According to an embodiment, the first heating furnace is surrounded by a housing which may minimize heat loss. However, the heat loss may occur in a portion not completely sealed, in particular, at an inlet of the first heating furnace through which the blank is introduced. The blank is not intentionally cooled during the high frequency induction heating.
According to an embodiment, the blank is continuously transferred without stopping in the high frequency induction heating in the first heating furnace. The blank is transferred by rollers arranged in a moving direction of the blank. When the blank stops on the rollers in the first heating furnace, a portion of the blank, which contacts the rollers, may be locally cooled. A portion of the blank between the rollers may be sagged.
According to an embodiment, the first heating furnace may have at least two heating zones in a transfer direction of the blank, the at least two heating zones being controlled at different target temperatures and different heating rates. The blank may be pre-heated at a lower power level, desirably, heated to a temperature of 250° C. in a first heating zone and may be rapidly heated at a higher power level, desirably, heated to a temperature less than 550° C. in a second heating zone. According to such a heating pattern, deformation or distortion of the blank may be prevented even when temperature sharply rises due to the high frequency induction heating. An inverter and an inductor coil independently controlled are installed in each of the first and second heating zones.
U.S. application Ser. No. 12/496,254 and Korean Patent Application No. 10-2008-0096912 do not emphasize or exactly specify that upper rollers are not provided so as to transfer a blank. However, it is necessary to pay attention that these Applications discloses a feature that deformation of a steel sheet occurring in induction heating is controllable through a space adjustment between an upper roller and a lower roller.
According to an embodiment, the second heating furnace may be an indirect heating furnace, in particular, an electric resistance furnace or a gas furnace, which transfers heat energy to the blank though at least one of radiation and convection. For example, the electric resistance furnace indirectly heats the blank by applying a current to heating elements installed on a furnace wall. The gas furnace uses radiant tubes. The blank may be heated to an austenitization temperature of the blank or more in the second heating furnace.
According to an embodiment, the press forming apparatus includes an upper die and a lower die, which each include a cooling channel formed therein. A high strength body part having a martensite structure is manufactured by forming and quenching the blank heated to the austenitization temperature or more in the second heating furnace
According to embodiments of the present invention, the length and the space for the heating system can be reduced by 50% or more compared to the related art, and the production cost of hot stamping can be significantly reduced.
According to the hot stamping method of the present invention, it is possible to cost-effectively provide a high strength vehicle body part having excellent quality.
Embodiments of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings in which:
Hereinafter, the present invention will be described in detail with reference to the accompanying drawings Like reference numerals refer to like elements for convenience of description.
A hot stamping process and apparatuses used therein according to an embodiment of the present invention will be described with reference to
Referring to
The heating system includes a feed section 100, first and second heating furnaces 200 and 300, and a transit section 400.
As shown in
As shown in
The target temperature of the first heating zone 200a using a relatively low frequency may be 250° C. and the target temperature of the second heating zone 200b using a relatively high frequency may be 550° C. or less. By heating the blank using the two heating zones, it is possible to prevent or suppress deformation or distortion of the blank caused by a sharp increase in temperature.
As shown in
The blank is transferred by the lower rollers 210b which are rotated. The upper rollers 210a are not provided to transfer the blank. When the blank is not deformed, the upper rollers 210a do not contact the blank.
Referring to
The upper rollers 210a rotate together with the lower rollers 210b, at least while the blink 1 is transferred by the lower rollers 210b. The upper rollers 210a rotate in a direction opposite to a rotation direction of the lower rollers 210b, i.e., a direction in which the transferred blank 1 moves forward. After the blank 1 is blocked by the upper rollers 210a, the rotating upper rollers 210a allow the blank 1 to smoothly move in a transfer direction and allow additional problems not to occur.
When a deformation degree of a blank is properly controlled in the high frequency induction heating, some degree of the deformation in the blank may be alleviated to a negligible degree in a subsequent heating process.
The transfer speed of the blank in the first heating furnace 200 is controlled within a range from 70 mm/sec to 90 mm/sec. Referring to
Bakelite panels 240 are connected with drive units for rotating the upper and lower rollers 210a and 210b. Dampers may be provided with the drive units, particularly in bearings to which the upper rollers 210a are connected to absorb the impact from the blank passing on the lower rollers 210b.
The upper and lower rollers 210a and 210b are made of a hollow ceramic material for insulation and have extensions 250 to connect the upper and lower rollers 201a and 210b to drive units.
The second heating furnace 300 may be an indirect heating furnace. An electric resistance furnace or a gas furnace may be used for the second heating furnace 300. The blank may be heated to a temperature of Ac3 or more of the blank (about 950° C.) in the second heating furnace 300.
As shown in
As shown in
A blank position detection sensor 330 and a temperature detection sensor 340 are positioned in the standby section 300c. The position detection sensor 330 for detecting whether or not the blank enters the standby section 300c and is placed in the standby section 300c throughout the entire length thereof. The temperature detection sensor 340 is for confirming if the blank entered into the standby section 300c is sufficiently heated up to 950° C.
The transfer speed of the blank in the heating section 300a is equal to that in the soaking section 300b. The transfer speed of the blank in the standby section 300c is also equal to those in the heating and soaking sections 300a and 300b before the blank is discharged from the standby section 300c. When it is confirmed that the blank completely enters the standby section 300c and is heated, the transfer speed of the blank in the standby section 300c increases and the blank is discharged to the transit section 400. The discharging timing may be determined on the basis of information from the position and temperature detection sensors 330 and 340. After the blank is discharged from the standby section 300c, the transfer speed thereof is gradually reduced to be equal to those for the heating and soaking sections 300a and 300b.
The temperature of the blank decreases rapidly in several seconds until the blank is formed in the press forming apparatus 600 after being discharged from the standby section 300c.
Referring
As shown in
As shown in
Boron steel having an aluminium alloy coating layer may be used as a material of the blank used in the hot stamping process according to the embodiment. In an example, the material of the blank may include 0.4 wt % or less of carbon (C), 0.5 wt % to 2.0 wt % of manganese (Mn), and 0.0005 wt % to 0.1 wt % of boron (B). Furthermore, the material of the blank may be boron steel including 0.2 wt % to 0.25 wt % of carbon (C), 1.10 wt % to 1.35 w % of manganese (Mn), 0.15 wt % to 0.35 wt % of silicon (Si), 0.15 wt % to 0.30 wt % of chrome (Cr), 0.02 wt % to 0.06 wt % of aluminium (Al), 0.002 wt % to 0.004 wt % of boron (B), 0.02 wt % to 0.05 wt % of titanium (Ti), and 0.008 wt % or less of sulphur (S).
The austenitization temperature may be A3 temperature of the boron steel at which a mixture phase of ferrite and austenite is converted into a single phase. The boron steel sheets may have a mixture phase of pearlite and ferrite at room temperature.
While the present invention has been shown and described in connection with the exemplary embodiment, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.
Number | Date | Country | Kind |
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10-2008-0096912 | Oct 2008 | KR | national |
This application is a continuation-in-part of U.S. application Ser. No. 12/496,254 filed on Jul. 1, 2009, the disclosure of which is incorporated herein in its entirety by reference, which is based on and claims the benefit of Korean Patent Application No. 10-2008-0096912, filed on Oct. 2, 2008, the disclosure of which is incorporated herein in its entirety by reference.
Number | Date | Country | |
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Parent | 12496254 | Jul 2009 | US |
Child | 15460889 | US |