Laser welding method for producing a semi-finished sheet metal product made of hardenable steel and comprising a coating based on aluminium or aluminium-silicon

Abstract

A method for producing a tailor-made semi-finished sheet metal product where two steel sheets of different material grades and/or thicknesses are joined by laser welding. At least one of the sheets is press-hardenable steel having a metallic coating of aluminium or aluminium-silicon. Filler wire is fed into the weld melt. The filler wire is substantially free of aluminium and contains at least one alloy element which promotes the formation of austenite in a content that is at least 0.1 wt. % greater than that in the press-hardenable steel. The filler wire is heated before being fed into the weld melt. The steel sheets have a gap delimited by the edges of the sheets having an average width of at least 0.15 mm. The ratio of the volume of filler wire inserted into the gap to the volume of the steel sheet material melted by the laser beam is at least 20%.

Claims

1. A method for producing a tailor-made semi-finished sheet metal product that can be hot formed, in which at least two steel sheets of different material grades and/or thicknesses are joined by means of laser welding, at least one of the steel sheets being produced from press-hardenable steel and at least one of the steel sheets having a metallic coating based on aluminium or aluminium-silicon on at least one outer surface, the laser welding being carried out while filler wire is fed into a weld melt produced solely by means of at least one laser beam, the filler wire being substantially free of aluminium and containing at least one alloy element which promotes the formation of austenite and is present in the filler wire in a content by weight that is at least 0.1 wt. % greater than the content in the press-hardenable steel, and the filler wire is heated by a heating device before being fed into the weld melt, wherein, before welding, the edges of the steel sheets that are to be welded to one another are spaced apart from one another such that there is a gap between the edges of the steel sheets which is delimited by the edges of the steel sheets and has an average width extending between the edges of the steel sheets of at least 0.15 mm, and at the time of welding, the metallic coating on the at least one outer surface of the at least one steel sheet extends to the edge of the at least one steel sheet, and the amount of material of the filler wire which is inserted into the gap is such that the ratio of the volume of filler wire inserted into the gap to the volume of the steel sheet material melted by the at least one laser beam is 30 to 60%.

2. The method according to claim 1, wherein the steel sheets are positioned such that the gap delimited by the edges of said sheets that are to be welded to one another has an average width in the range of from 0.15 to 0.5 mm.

3. The method according to claim 1, wherein the heating device heats the filler wire inductively, electrically, conductively or by heat radiation before said wire is fed into the weld melt.

4. The method according to claim 1, wherein the filler wire is heated by means of the heating device to a temperature of at least 100 C. before being fed into the weld melt.

5. The method according to claim 1, wherein the press-hardenable steel is a manganese-boron steel.

6. The method according to claim 1, wherein the press-hardenable steel has the following composition: 0.10 to 0.50 wt. % C, a maximum of 0.40 wt. % Si, 0.50 to 2.0 wt. % Mn, a maximum of 0.025 wt. % P, a maximum of 0.010 wt. % S, a maximum of 0.60 wt. % Cr, a maximum of 0.50 wt. % Mo, a maximum of 0.050 wt. % Ti, 0.0008 to 0.0070 wt. % B, a minimum of 0.010 wt. % Al, and the balance Fe and unavoidable impurities.

7. The method according to claim 1, wherein at least one of the steel sheets is produced from microalloyed steel.

8. The method according to claim 7, wherein the microalloyed steel has the following composition: 0.05 to 0.15 wt. % C, a maximum of 0.35 wt. % Si, 0.40 to 1.20 wt. % Mn, a maximum of 0.030 wt. % P, a maximum of 0.025 wt. % S, 0.01 to 0.12 wt. % Nb, 0.02 to 0.18 wt. % Ti, 0.0008 to 0.0070 wt. % B, a minimum of 0.010 wt. % Al, and the balance Fe and unavoidable impurities.

9. The method according to claim 1, wherein the at least one laser beam in the form of a line focus beam is directed towards the edges of the steel sheets that are to be welded to one another such that a longitudinal axis of the line focus beam impinging on the edges extends substantially in parallel with said edges.

10. The method according to claim 1, wherein the filler wire contains at least one of manganese and nickel as the alloy elements that promote the formation of austenite.

11. The method according to claim 1, wherein the filler wire has the following composition: 0.05 to 0.15 wt. % C, 0.5 to 2.0 wt. % Si, 1.0 to 3.0 wt. % Mn, 0.5 to 2.0 wt. % Cr+Mo, 1.0 to 4.0 wt. % Ni, and the balance Fe and unavoidable impurities.

12. The method according to claim 1, wherein the filler wire has a carbon content by weight that is at least 0.1 wt. % lower than the carbon content of the press-hardenable steel.

13. The method according to claim 1, wherein the speed in terms of m/min. at which the filler wire is inserted into the gap is 70 to 100% of the laser welding speed in terms of m/min.

14. The method according to claim 1, wherein protective gas is supplied to the weld melt during the laser welding.

15. The method according to claim 14, wherein pure argon or a mixture of argon and carbon dioxide is used as the protective gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective view of parts of a device for carrying out the laser welding method according to the invention, in which two steel blanks, which are of substantially the same thickness and differ from one another by the material grades thereof, are welded to one another in a butt joint;

(2) FIG. 2 is a cross section through a portion of the steel blanks from FIG. 1 that are welded to one another;

(3) FIG. 3 is a perspective view of parts of a device for carrying out the laser welding method according to the invention, in which two steel blanks of different thickness and different material grades are welded to one another in a butt joint; and

(4) FIG. 4 is a cross section through a portion of the steel blanks from FIG. 3 that are welded to one another.

DESCRIPTION OF THE INVENTION

(5) FIG. 1 schematically shows parts of a laser welding device by means of which the laser welding method according to the invention can be carried out. The device comprises a base or movable support plate (not shown) on which two blanks or sheets 1, 2 made of steel of different material grades are arranged. One of the sheets 1, 2 is produced from press-hardenable steel, preferably manganese-boron steel, whereas the other sheet 2 or 1 is produced from a steel of a relatively low deep-drawing grade, preferably a microalloyed steel.

(6) The press-hardenable steel can have the following chemical composition for example:

(7) a maximum of 0.4 wt. % C,

(8) a maximum of 0.4 wt. % Si,

(9) a maximum of 2.0 wt. % Mn,

(10) a maximum of 0.025 wt. % P,

(11) a maximum of 0.010 wt. % S,

(12) a maximum of 0.8 wt. % Cr+Mo,

(13) a maximum of 0.05 wt. % Ti,

(14) a maximum of 0.007 wt. % B,

(15) a minimum of 0.015 wt. % Al, and

(16) the balance Fe and unavoidable impurities.

(17) In the delivery state, i.e. before heat treatment and rapid cooling, the yield point Re of the press-hardenable steel 1 or 2 is preferably at least 300 MPa, the tensile strength Rm thereof is at least 480 MPa, and the elongation at fracture A.sub.80 thereof is in the range of from 10 to 15%. After being hot formed (press hardened), i.e. after being heated to an austentising temperature of approximately 900 to 920 C. and subsequently rapidly cooled, said steel sheet 1 or 2 has a yield point Re of approximately 1,100 MPa, a tensile strength Rm of from approximately 1,500 to 2,000 MPa, and an elongation at fracture A.sub.80 of approximately 5%.

(18) The steel of the sheet 2 or 1 having a relatively low deep-drawing grade or the microalloyed steel has, by contrast, the following chemical composition for example:

(19) a maximum of 0.1 wt. % C,

(20) a maximum of 0.35 wt. % Si,

(21) a maximum of 1.0 wt. % Mn,

(22) a maximum of 0.030 wt. % P,

(23) a maximum of 0.025 wt. % S,

(24) a maximum of 0.10 wt. % Nb,

(25) a maximum of 0.15 wt. % Ti,

(26) a maximum of 0.007 wt. % B,

(27) a minimum of 0.015 wt. % Al, and

(28) the balance Fe and unavoidable impurities.

(29) At least one of the sheets 1, 2 comprises a metallic coating 1.1, 2.1 based on aluminium or aluminium-silicon. In the example shown in FIG. 1, both sheets 1, 2 are provided with a coating 1.1, 2.1 of this kind. The coating 1.1, 2.1 can typically be applied to a steel strip by means of a continuous hot-dip coating process, from which strip the sheets 1, 2 are subsequently obtained by being cut to size.

(30) The sheets 1, 2 shown in FIG. 1 are of substantially the same thickness. The thickness of the sheets 1, 2, including the coating 1.1, 2.1, is in the range of from 0.6 to 3.0 mm, for example. The thickness of the coating 1.1, 2.1 on the relevant upper or lower face of the sheet 1, 2 is, for example, in the range of from approximately 10 to 120 m, and is preferably less than or equal to 50 m.

(31) Shown above the sheets 1, 2 is a portion of a laser welding head 3 which is provided with optics (not shown) for supplying a laser beam 4, and a focusing apparatus for concentrating the laser beam 4. Furthermore, a line 5 for supplying protective gas is arranged on the laser welding head 3. The protective gas line 5 opens substantially into the focal region of the laser beam 4 or the weld melt 6 produced by means of the laser beam 4. Pure argon or a mixture of argon and helium and/or carbon dioxide is preferably used as the protective gas.

(32) Additionally, the laser welding head 3 is assigned a wire feed apparatus 7 by means of which the weld melt 6 is supplied with a specific filler material in the form of a wire 8, which is also melted by means of the laser beam 4. The weld seam is provided with reference numeral 9. The filler wire 8 is substantially free of aluminium and contains at least one alloy element, preferably manganese and/or nickel, which promotes the formation of austenite or stabilises the austenite.

(33) The blanks or sheets 1, 2 are joined in a butt joint such that there is a gap G, the width of which is at least 0.15 mm, preferably at least 0.2 mm. The average width b of the gap G which is delimited by the sheet edges that are to be welded to one another is in the range of from 0.15 to 0.5 mm. In the coated steel sheet 1 and/or 2, the aluminium or aluminium-silicon coating 1.1, 2.1 extends as far as the sheet edge to be welded in the butt joint. The sheets 1, 2 are thus welded without the coating being (previously) removed from the rim of the sheet edge to be welded.

(34) The focusing apparatus concentrates the laser beam 4 so as to form a substantially punctiform or circular focal point or preferably so as to form a focal line. The diameter or the width of the laser beam 4 at the point at which it impinges on the sheets 1, 2 is in the range of from approximately 0.7 to 0.9 mm. The relatively wide gap G, the width b of which is at least 0.15 mm and can be in the range of from 0.25 to 0.5 mm for example, ensures that less material of the sheets 1, 2 and therefore also less volume of the aluminium-containing coating 1.1, 2.1, is melted and flows into the melt 6. The gap G is filled with the melted material of the filler wire 8 which has a diameter in the range of from approximately 0.8 to 1.2 mm in the solid state. Insertion of the filler material into the gap G results in significant thinning and a homogenous distribution of the aluminium flowing into the melt 6 from the melted rim of the coating 1.1, 2.1. The ratio of the volume of filler wire inserted into the gap G to the volume of the steel sheet material melted by means of the laser beam 4 is at least 20%, and is preferably approximately in the range of from 30 to 60%.

(35) The filler wire 8 has the following chemical composition for example:

(36) 0.1 wt. % C,

(37) 0.9 wt. % Si,

(38) 2.2 wt. % Mn,

(39) 0.4 wt. % Cr,

(40) 0.6 wt. % Mo,

(41) 2.2 wt. % Ni, and

(42) the balance Fe and unavoidable impurities.

(43) In this case, the manganese content of the filler wire 8 is greater than the manganese content of the press-hardenable steel sheet. The manganese content of the filler wire 8 is preferably at least 0.2 wt. % greater than the manganese content of the press-hardenable steel sheet. It is also advantageous for the chromium and molybdenum content of the filler wire 8 to also be greater than that of the press-hardenable steel sheet 1 or 2. The combined chromium-molybdenum content of the filler wire 8 is preferably at least 0.1 wt. % greater than the combined chromium-molybdenum content of the press-hardenable steel sheet 1 or 2. The nickel content of the filler wire 8 is preferably in the range of from 1.0 to 4.0 wt. %, in particular in the range of from 2.0 to 2.5 wt. %.

(44) Furthermore, the filler wire 8 preferably has a lower carbon content than the press-hardenable steel sheet 1 or 2. The carbon content of the filler wire 8 is preferably in the range of from 0.05 to 0.15 wt. %.

(45) The filler wire 8 is fed in a heated state to the melt 6 produced by means of the laser beam 4. For this purpose, the wire feed apparatus 7 is equipped with a heating device (not shown) which heats the filler wire 8 preferably inductively, electrically, conductive or by means of heat radiation. The portion of the filler wire 8 heated in this way has a temperature of at least 60 C. for example, preferably at least 150 C., particularly preferably at least 180 C.

(46) With regard to the laser source of the laser welding device, the laser type thereof is for example a CO.sub.2 laser, diode laser or fibre laser. During the welding process, the laser source provides an energy input per unit length of at least 0.3 kJ/cm with a laser output of at least 7 kW. The welding speed is for example in the range of from 3 to 9 m/min or preferably above 8 m/min. In this case, the filler wire 8 is fed in at a speed in the range of from 70 to 100% of the laser welding speed.

(47) The embodiment shown in FIGS. 3 and 4 differs from the embodiment in FIGS. 1 and 2 in that the steel sheets 1, 2 are of different thicknesses and therefore there is a discontinuity d in thickness of at least 0.2 mm at the butt joint. For example, the press-hardenable steel sheet 1 has a sheet thickness in the range of from 0.5 mm to 1.2 mm, whereas the sheet 2 made of microalloyed steel or relatively ductile steel has a sheet thickness in the range of from 1.4 mm to 3.0 mm.