KR101738985B1

KR101738985B1 - Hot formed steel part for vehicles and the method for manufacturing the same

Info

Publication number: KR101738985B1
Application number: KR1020150106952A
Authority: KR
Inventors: 김장수; 차홍석; 양대호
Original assignee: 주식회사 엠에스 오토텍
Priority date: 2015-07-29
Filing date: 2015-07-29
Publication date: 2017-05-25
Also published as: KR20170014110A

Abstract

There is introduced a method for manufacturing a high strength automotive steel part having a region having a different thickness. The method includes: blanking an Al-coated steel sheet; Preheating the surface to produce aluminum oxide; And hot rolling the partial area of the blank so as to have at least two regions having different thicknesses. After this process, hot-stamped steel parts have no joints between regions of different thickness.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a hot-

The present invention relates to a steel part for a vehicle, particularly a high strength steel part for a vehicle having a different thickness area, and a manufacturing method thereof.

One of the biggest buzzwords in the automotive industry is light weight and high strength, and hot stamping technology is at the center of these issues. Hot stamping technology was proposed by Norrbottens Jarnverk A.B., Sweden, in the early 1970s, and the company's UK patent 1490535 describes hot stamping technology in detail.

The hot stamping is characterized in that the steel sheet is heated to a high temperature of Ac3 or higher in a heating furnace, and then subjected to heat treatment at the same time as molding in the press, thereby manufacturing a high strength part of 1500 MPa or more. As the material of hot stamping, so-called boron steel containing 0.2% by weight of carbon and using Mn and B as elements for improving the heat treatment performance is used.

This hot stamping technique improves the formability and dimensional accuracy because the steel sheet is formed at a high temperature as well as being excellent in productivity because the molding and the heat treatment are carried out at the same time, and the springback or delayed breakdown, . On the other hand, since the process is performed at a high temperature, oxidation of the surface of the steel sheet can not be avoided, and thus, the molded product must be descaled.

One of the most prominent and widely used Al-coated steel sheets proposed to omit the uncomfortable descaling process is the steel plate. As to the Al-coated steel sheet, US Pat. No. 6,296,805 to Arcelor Mittal can be referred to.

Tailor Welded Blanks (TWB) is a technique that is applied to the manufacturing of vehicle parts for light weight and improved collision safety. TWB is a technology to cut steel sheets of different thicknesses and materials into appropriate sizes and shapes and then weld them with laser to make products of the desired shape.

Referring to Fig. 1, the respective parts from P1 to P4 in the illustrated steel part are different in thickness from each other. For example, P1 may be 2 mm, P2 may be 1.6 mm, P3 may be 1.2 mm, P4 may be 1.7 mm, and these portions may be laser welded to each other before hot forming. The TWB method is useful for lightweight vehicle because it can use thick and high strength steel plate at the part where the strength is needed, and it also helps to improve the stability because the design of the vehicle part can be freely designed according to the collision characteristic.

However, it is very difficult to apply the above-mentioned TWB method to a hot stamping coated steel sheet, for example, an Al-coated steel sheet. This is because when the laser is welded, the strength of the weld is considerably reduced due to the Al plating layer. In U.S. Patent Publication No. 2015-0030382, a problem was solved by a method in which a part of the Al plating layer of the welded portion was peeled in advance and then laser welding was performed.

U.S. Patent Application Publication No. 2010-0086002 (published on April 8, 2010) U.S. Patent Publication No. 2015-0030382 (published on February 29, 2015)

The present invention is based on the results of the study of "Support Project for the Base Economy Base Base Institution" supported by the Ministry of Commerce, Industry and Energy and the Korea Industrial Technology Development Agency.

The present invention has been proposed on the basis of the above-described recognition of the prior art, and a new manufacturing technique of a steel part for a vehicle having two or more areas having different thicknesses is introduced.

The TWB phase as a method of manufacturing a steel part having two or more regions different in thickness from each other in the past was unique. Therefore, no one has considered other options besides TWB for the manufacture of these steel parts, and has only focused on the question of how to treat the Al-plated layer for the application of TWB.

The present inventors intend to provide a new automotive steel part manufacturing technique which is completely different from TWB and can be substituted.

In order to achieve the above object, the hot-formed vehicle steel member proposed by the present invention has two or more regions having different thicknesses but no joints between regions having different thicknesses.

This lack of joints is due to the fact that steel parts are manufactured in such a way that a part of the blank is hot-rolled prior to hot forming to make a difference in thickness between the rolled area and the non-rolled area.

A method for manufacturing a steel part for a vehicle according to the present invention comprises the steps of: cutting a steel sheet having a coating to obtain a blank having a desired shape; Preheating the surface of the blank to form an oxide of the coating; And hot rolling the partial area of the blank where the coating oxide is formed on the surface.

Further, the method for manufacturing a steel part for a vehicle according to the present invention includes the steps of: heating the hot-rolled blank to austenite temperature range; And subjecting the heated blank to hot forming and then quenching the blank. Of course, the step of cooling may be further included.

When the oxide of the film, for example, aluminum oxide, is formed on the surface of the blank by performing the preliminary heating as described above, melting of Al can be prevented at a high temperature, and the subsequent process becomes very convenient. By way of example, aluminum oxide on the blank surface makes it possible to rapidly heat the blank to the austenite temperature range, preferably 950 DEG C, in the hot forming step. The hot forming speed is accelerated.

In addition, aluminum oxide on the blank surface prevents fine cracks from forming on the surface during the hot rolling process.
A method of manufacturing a steel part for a vehicle according to an embodiment includes the steps of: a) cutting a steel sheet having a coating to obtain a blank having a desired shape; b) preheating the surface of the blank to form an oxide of the coating; c) hot rolling the partial area of the blank where the coating oxide is formed on the surface so as to have at least two regions having different thicknesses; And d) heating the hot-rolled blank to austenite temperature range, followed by hot press forming and cooling, wherein hot rolling in step b) is not performed on the entire area of the blank, In at least two or more different directions.
According to the embodiment, in the step a), the steel sheet is cut so that a hole is formed in the obtained blank.
According to the embodiment, in the step a), the steel sheet is cut into a shape corresponding to the side panel on the door side having a hole therein, and the molded product obtained in the step d) is a side panel.
According to the embodiment, in the step c), the blank is rolled using a rolling roll whose edge is gently inclined.

According to the present invention, a hot-formed automotive steel part having two or more regions having different thicknesses can be manufactured without a welded joint.

Since the present invention does not depend on the TWB, there is no need to precisely weld the two steel plates and precisely weld them after appropriately drawing the Al plating layer along the edge of the steel sheet for TWB application.

In addition, TWB requires the use of filler metal wires in the welding process to prevent carbon depletion at the weld site and to adjust the composition, but this need not be the case in the present invention. It is possible to mass-produce a hot-formed automobile steel part having two or more regions having different thicknesses by using the supplied Al-plated steel sheet as it is.

Further, since the steel part according to the present invention does not have a welded joint, there is no problem in that the strength of the welded part often decreases.

1 shows an example of a steel part manufactured by the conventional TWB method,
2 is a flowchart of a steel part manufacturing process according to an embodiment of the present invention,
3A to 3G are views for sequentially illustrating the processes shown in Fig.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same components or parts are denoted by the same reference numerals as much as possible for convenience of description.

The steel component manufacturing method according to the embodiment will be described in the order described in Fig. 3A-3G will be referred to in the description process.

Blanking S1 )

3A and 3B, the blanking S1 is a step of cutting the hot stamping steel sheet 10 into a blank 20 having a desired shape. The steel sheet 10 is an Al-coated steel sheet, and an Al or Al alloy layer is disposed on the outermost surface, and an intermetallic alloy layer is disposed on the inside thereof. The base material is boron steel.
As shown in FIG. 3B, the blank 20 having the holes S formed therein is obtained by blanking the steel sheet 10. The blank 20 having the hole S shown in Fig. 3B is a blank having a shape corresponding to the side panel on the door side.

free Heating S2 )

Referring to FIG. 3C, the preheating S2 is a preheating process before the main heating S3 so that an oxide film, that is, aluminum oxide is formed on the surface of the blank 20. In the case of an Al-coated steel sheet for hot stamping, about 600 ° C is a target.

When aluminum oxide is formed on the surface by the preliminary heating, melting or damage of Al can be prevented in the subsequent hot rolling or hot forming process, and the progress of the subsequent process becomes very convenient.

As one example, aluminum oxide prevents micro cracks from occurring in the hot rolling process, and also enables rapid heating to 950 占 폚 for austenitizing blank blank in the hot forming step. The whole process is easy and quick.

Hot rolling ( S3 )

Referring to Figures 3D and 3E, hot rolling S3 is a step of rolling a part of the blank 20, or partly, into a roll 40 such that a difference in thickness occurs between one area of rolling and the other area .

Some areas may be taken as meaning not the entire area of the blank 20. Even when hot rolling several regions over several areas, it is not necessarily restricted to rolling over the entire area of the blank 20, although the present invention is not limited thereto.

If the thickness before hot rolling is the same throughout the blank 20, the area that has not yet been rolled after the hot rolling step S3 will be the thickest area of the blank 20. The hot rolling step (S3) is designed with this in mind, and it is not necessary to roll over the entire area of the blank (20) unless otherwise specified.

FIG. 3D shows rolling of the area A corresponding to the door seal in the lateral direction by primary hot rolling. The primary hot rolling may proceed several times so that the A region has a desired thickness. Area A becomes thinner than other areas that are not rolled.

3E shows rolling the B region corresponding to the front filler in the longitudinal direction different from the first order by the secondary hot rolling. Likewise, secondary rolling can proceed several times so that the B region has a desired thickness.

It may be necessary to reheat the blank 20 to compensate for the reduced temperature before secondary rolling after primary rolling. In addition, since the secondary rolling direction is different from the primary rolling direction, corresponding means may be required to rotate the blank 20 or to change the direction in which it is placed on the rolling roll 40.

Referring to FIG. 3F, when the area A and the area B are rolled to the same thickness, the rolled part 22 based on the boundary lines L1 and L2 becomes thinner than the part 21 that is not. Of course, the B region can be rolled to a thickness different from that of the A region, and a third or higher rolling can be performed in addition to the secondary rolling.

An important control object in the hot rolling (S3) process is dispersion of mechanical properties. Particularly, roll spreading in the width direction is problematic. In order to prevent this, it is judged that the rolling width needs to be suppressed to 750 mm or less at present.

A step may be formed at the position where the rolling boundary lines L1 and L2 of the blank 20, that is, the edge 41 of the roller passes. The regions overlapping with each other in the A region and the B region do not become a problem, but in the regions L1 and L2 of the non-overlapping regions, abrupt step differences may occur. It may be necessary to design the shape of the roll 40 40 such that the edge 41 of the rolling roll 40 is subjected to a gentle slanting process.

The hot rolling temperature may be performed in a conventional temperature range. However, since hot rolling is an important process that determines the quality of product, it is necessary to design the rolling process and temperature optimally, and this part will be another patent application.

On the other hand, the drawing of the blank 20 may occur during the hot rolling process. Therefore, the above-described blanking (S1) process needs to be optimally designed in consideration of such stretching.

Main heating ( S4 )

Referring to FIG. 3F, the main heating S4 is a step of heating the hot rolled blank 20 to the austenite temperature range of the base material for hot stamping.

Since aluminum oxide is formed on the surface of the Al plating layer through the preheating (S2), the main heating (S4) can proceed rapidly to 950 ° C. The main heating (S4) or the hot press forming (S5) process described below can be referred to U.S. Patent Publication No. 2010-0086002.

Hot press forming ( S5 )

Referring to FIG. 3G, the hot press forming (S5) refers to a process of pressing the austenitized blank 20 into a product of a desired shape and quenching it. A high-strength steel component having a martensite phase is obtained through quenching.
As shown in Fig. 3G, the molded article obtained by the hot press forming (S5) according to the embodiment corresponds to the side panel.

Trimming S6 )

Trimming (S6) refers to a step of removing excess material existing on the edge of the hot press formed product. The excess can be generated by cutting the blank 20 to a larger size than the product for product protection and by stretching the blank 20 during the hot rolling process.

Currently, lasers are used for trimming. The present inventors are developing cold trimming that is more convenient and applicable to mass production.

Characteristics of a steel part for a vehicle manufactured according to the above-described manufacturing method will be described.

Referring to FIG. 3G, the steel part according to the embodiment has two or more regions 31 and 32 having different thicknesses, and there is no welded joint between regions having different thicknesses.

At least one of the regions 31 and 32 having different thicknesses is rolled before hot forming to have a thickness smaller than that of the other regions. As an example, the rolled portion 32 with respect to the rolling boundary lines L1, L2 in Fig. 3G is thinner than the portion 31 that is not.

The steel part according to the embodiment has rolling boundary lines (L1, L2) formed between regions of different thicknesses. The boundary lines L1 and L2 may be sections in which different rolling zones smoothly run without step differences.

The automotive steel part according to the embodiment has a martensite structure in the base material by hot forming, and an oxide film, for example, an aluminum oxide film is covered on the surface. The manufactured steel parts are welded to other steel parts for vehicle manufacturing.

While the invention has been shown and described with respect to the specific embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims It should be understood that it is obvious to those of ordinary skill in the art.

w: welding line 10: steel plate
20: Blank 40: Rolling roll

Claims

a) cutting a steel sheet having a coating to obtain a blank having a shape necessary for press forming;
b) preheating the surface of the blank to form an oxide of the coating;
c) hot rolling the partial area of the blank where the coating oxide is formed on the surface so as to have at least two regions having different thicknesses; And
d) heating the hot-rolled blank to austenite temperature range, followed by hot press forming and cooling,
wherein the hot rolling in step c) is carried out in at least two or more different directions with respect to the plurality of partial areas, not with respect to the entire area of the blank.

The method of claim 1, wherein in step a)
And cutting the steel sheet so that a hole is formed in the obtained blank.

The method according to claim 2, wherein in step a), the steel sheet is cut into a shape corresponding to the side panel on the side of the door having a hole therein,
wherein the molded article obtained in step (d) is a side panel.

2. The method of claim 1, wherein in step c)
And rolling the blank using a rolling roll whose edge is gently inclined.