Method of manufacturing tailor welded blanks

Abstract

A method of manufacturing tailor welded blanks may be capable of improving a quality of a welded portion and shortening a manufacturing time when a tailor welded blank is manufactured using a coated steel plate. The method of manufacturing tailor welded blanks includes welding one or more pairs of different coated steel plates having different thicknesses or strengths with laser using a filler wire.

Claims

1. A method of manufacturing tailor welded blanks, comprising welding one or more pairs of different coated steel plates having different thicknesses or strengths with laser using a filler wire, wherein each of the coated steel plate comprises a steel plate and a AlSi coating layer, where the steel plate comprises 0.19 to 0.25 wt % of C, 0.20 to 0.40 wt % of Si, 1.10 to 1.60 wt % of Mn, 0.03 wt % or less of P, 0.015 wt % or less of S, 0.10 to 0.60 wt % of Cr, 0.0008 to 0.0050 wt % of B, Fe, and other inevitable impurities, and wherein the filler wire comprises 0.6 to 0.9 wt % of C, 0.3 to 0.9 wt % of Mn, 1.6 to 3.0 wt % of Ni as austenite stabilization elements through which a welded portion is transformed to have a full austenite structure at a temperature of 900 to 950 C., when a coated layer of each of the coated steel plate is mixed into the welded portion.

2. The method of claim 1, wherein the filler wire further comprises Fe and other inevitable impurities.

3. The method of claim 1, wherein each of the coated steel plate comprises an AlSi coated layer.

4. The method of claim 1, further comprising forming a laser-welded tailor welded blank by hot stamping using the filler wire and then quenching the same at a quenching rate of 40 C./s or more, so as to transform a welded portion structure into a martensite structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawing, in which:

(2) FIG. 1 is a graph illustrating a transformation curve according to an aluminum content and temperature in the related art;

(3) FIG. 2 is a graph illustrating movement of a transformation curve according to increase of an austenite stabilization element content;

(4) FIG. 3 is a graph illustrating a transformation curve according to increase of a nickel content; and

(5) One of ordinary skill in the art would appreciate the advantages and applications of the present application.

DESCRIPTION OF SPECIFIC EMBODIMENTS

(6) Exemplary embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. Throughout the disclosure, like reference numerals refer to like parts throughout the various figures and embodiments of the present invention.

(7) An main idea of a method of manufacturing tailor welded blanks according to an embodiment of the present invention is to prevent problems such as strength deterioration caused due to mixing of an ingredient of a coated layer into a welded portion when a tailor welded blank having locally different material properties is manufactured by interconnecting one or more pairs of different coated steel plates having different materials or thicknesses with laser welding.

(8) In general, when the coated steel plates are bonded with the laser welding in a state in which the coated layer of the welded portion is not removed, problems such as strength deterioration of the welded portion are caused when the coated layer is melted and mixed into the welded portion. This is because the welded portion exhibits a different material property from a basic material since the welded portion has a mixed ingredient of the basic material of the coated steel plate and the coated layer. For this reason, the strength deterioration of the welded portion is caused.

(9) A boron steel plate having hardenability is mainly used as a basic material of the tailor welded blank for hot stamping forming, and an AlSi coated layer is mainly used as the coated layer.

(10) FIG. 1 is a graph illustrating a conventional transformation curve.

(11) As shown in FIG. 1, when the coated layer is mixed into the welded portion and an Al content of the welded portion is increased, it is impossible to secure a full austenite structure even though the welded portion is heated at a temperature of 900 to 950 C. and the welded portion has a structure in which ferrite and martensite coexist with each other, instead of having a full martensite structure, even though the welded portion is quenched after hot stamping forming. For this reason, the strength deterioration of the welded portion is caused.

(12) In the tailor welded blank used in the present invention, a basic material including 0.19 to 0.25 wt % of C, 0.20 to 0.40 wt % of Si, 1.10 to 1.60 wt % of Mn, 0.03 wt % or less of P, 0.015 wt % or less of S, 0.10 to 0.60 wt % of Cr, 0.0008 to 0.0050 wt % of B, the balance of Fe, and other inevitable impurities such as Cu, Mo, Ti, Nb, and the like unlike the above mentioned components are used, and an AlSi coated layer is used.

(13) In the method of manufacturing tailor welded blanks according to the embodiment of the present invention, it is preferable that the welded portion has a full austenite structure at a temperature of 900 to 950 C. by adjusting ingredients of the welded portion using a filler wire during laser welding.

(14) Accordingly, the welded portion may have a desired material property by increasing strength thereof in such a manner that the welded portion is transformed to have a full martensite structure by quenching the welded portion after hot stamping forming.

(15) FIG. 2 is a graph illustrating movement of a transformation curve according to increase of an austenite stabilization element content.

(16) As shown in FIG. 2, C and Mn are austenite stabilization elements. The transformation curve is moved to the right since a eutectoid temperature A.sub.C3 is decreased as the contents of the elements are increased, and thus an austenite region is increased.

(17) Accordingly, the filler wire according to the embodiment of the present invention preferably contains C and Mn as the austenite stabilization elements.

(18) Thus, the welded portion has the full austenite structure when the welded portion is heated at the temperature of 900 to 950 C. during the hot stamping forming after welding, and the welded portion is transformed to have the full martensite structure when the welded portion is quenched after the hot stamping forming.

(19) It is preferable that the filler wire according to the embodiment of the present invention further contains Ni as the austenite stabilization element.

(20) FIG. 3 is a graph illustrating a transformation curve according to increase of a nickel content.

(21) As shown in FIG. 3, a eutectoid temperature A.sub.C3 is decreased as the nickel content of the filler wire is increased.

(22) The filler wire according to the embodiment of the present invention preferably includes 0.6 to 0.9 wt % of C, 0.3 to 0.9 wt % of Mn, 1.6 to 3.0 wt % of Ni, and the balance of Fe, and other inevitable impurities such as Cu, Mo, Ti, Nb, and the like unlike the above mentioned components.

(23) This is because a ferrite structure coexists in the welded portion at the temperature of 900 to 950 C. during welding since an increase rate of an austenite region is low when the C content is less than 0.6 wt % and a rupture is generated at the welded portion during impact generation such as collisions since hardness and strength of the welded portion are excessively increased when the C content exceeds 0.9 wt %.

(24) In addition, when Mn and Ni are out of the above composition range, the welded portion does not have the full austenite structure at the temperature of 900 to 950 C.

(25) For this reason, there is a problem in that the material properties of the welded portion of the finished product are changed after the hot stamping forming and defects such as a rupture of the welded portion are generated.

(26) A basic material rupture is generated during a tension test due to increase in strength of the welded portion when the Ni content of the filler wire is 1.6 to 3.0 wt %, whereas a welded portion rupture is generated during the tension test when the Ni content is less than 1.6 wt % or exceeds 3.0 wt %.

(27) As shown in FIGS. 2 to 4, when the filler wire according to the embodiment of the present invention satisfies the above composition range, the welded portion has the full austenite structure at the temperature of 900 to 950 C. Consequently, it may be possible to increase the strength of the welded portion by transforming the welded portion to have the full austenite structure in such a manner that the welded portion is quenched after the hot stamping forming.

(28) That is, the filler wire of the present invention may allow the welded portion to have the full martensite structure after the hot stamping forming by the austenite stabilization elements contained in the filler wire even though Al of the coated layer is mixed into the welded portion during welding. Thus, since the hardness and strength of the welded portion are prevented from deteriorating even though the welding is performed without the removal of the coated layer, the welded portion rupture may be prevented.

(29) In addition, in the method of manufacturing tailor welded blanks according to the embodiment of the present invention, it is preferable that the welded portion is transformed to have the martensite structure by quenching the welded portion at a quenching rate of 40 C./s or more after the hot stamping forming.

(30) In the present invention, it is preferable to immediately initiate quenching (within 0.5 seconds) after the hot stamping forming and to set a mean quenching rate during the quenching as 40 C./s or more.

(31) When the quenching rate is set as 40 C./s or less, it is difficult to refine the structure of the welded portion due to grain growth. As a result, it is difficult to transform the structure of the welded portion into the full martensite structure after the quenching.

(32) In accordance with the exemplary embodiment of the present invention, it may be possible to improve productivity and reduce manufacturing cost by shortening of a production time since coated layer removal and recoating processes are not required when a tailor welded blank is manufactured by laser welding.

(33) In addition, since a welded portion has a full martensite structure, a quality of a welded portion may be improved.

(34) While the present invention has been described with respect to the specific embodiments, it will be apparent to 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.