Hot-Formed Previously Welded Steel Part with very High Mechanical Resistance and Production Method

Abstract

A welded steel part with a very high mechanical strength is provided. The welded steel part is obtained by heating followed by hot forming, then cooling of at least one welded blank obtained by butt welding of at least one first and one second sheet. The at least one first and second sheets including, at least in part, a steel substrate and a pre-coating which includes an intermetallic alloy layer in contact with the steel substrate, topped by a metal alloy layer of aluminum or aluminum-based alloy. A method for the fabrication of a welded steel part and the fabrication of structural or safety parts for automotive vehicles are also provided.

Claims

1. A welded steel part obtained by heating in the austenitic range followed by hot forming, then cooling, of at least one welded blank obtained by butt welding of at least a first and a second sheet, the first and second sheet comprised at least in part of a steel substrate and a pre-coating which is constituted by an intermetallic alloy layer in contact with the steel substrate, topped by a metal alloy layer of an aluminum or aluminum-base alloy, wherein over at least a portion of a the metal alloy layer is removed from the edges in direct proximity to the weld metal zone resulting from the welding operation and constituting the bond between the first and second sheets, while the intermetallic alloy layer is retained, and in that, over at least a portion of the weld metal zone, the ratio between the carbon content of the at least a portion of the weld zone and the carbon content of the substrate of the first or second sheet, whichever has the highest carbon content Cmax, is between 1.27 and 1.59.

2. The steel part of claim 1, wherein a composition of the steel substrate of at least the first or the second sheet, comprises the following elements, expressed in percent by weight: 0.10%≦C≦0.5%; 0.5%≦Mn≦3%; 0.1%≦Si≦1%; 0.01%≦Cr≦1%; Ti≦0.2%; Al≦0.1%; S≦0.05%; P≦0.1%; and 0.0002%≦B≦0.010%, the balance being iron and unavoidable impurities from processing.

3. The steel part as recited in claim 1, wherein the ratio between the hardness of the welded metal zone and the hardness of the substrate of one of the first or second sheets having a higher carbon content is greater than 1.029+(0.36 Cmax), whereby Cmax is expressed in percent by weight.

4. The steel part as recited in claim 1, wherein the composition of the steel substrate of at least the first or second sheet includes the following elements, expressed in percent by weight: 0.15%≦C≦0.4%; 0.8%≦Mn≦2.3%; 0.1%≦Si≦0.35%; 0.01%≦Cr≦1%; Ti≦0.1%; Al≦0.1%; S≦0.03%; P≦0.05%; and 0.0005%≦B≦0.010%, the balance being iron and unavoidable impurities from processing.

5. The steel part as recited in claim 2, wherein the composition of the steel substrate of at least the first or second sheet includes the following elements, expressed in percent by weight: 0.15%≦C≦0.25%; 0.8%≦Mn≦1.8%; 0.1%≦Si≦0.35%; 0.01%≦Cr≦0.5%; Ti≦0.1%; Al≦0.1%; S≦0.05%; P≦0.1%; and 0.0002%≦B≦0.005%, the balance being iron and unavoidable impurities from processing.

6. The steel part as recited in claim 1, wherein the carbon content of the weld metal zone is less than or equal to 0.35% by weight.

7. The steel part as recited in claim 1, wherein the metal alloy layer of the pre-coating includes, expressed in percent by weight, between 8 and 11% silicon and between 2 and 4% iron, the remainder of the metal alloy layer composition consisting of aluminum and unavoidable impurities.

8. The steel part as recited claim 1, wherein a microstructure of the weld metal zone includes no ferrite.

9. The steel part as recited in claim 1, wherein a microstructure of the weld metal zone is martensitic.

10. The steel part as recited in claim 1, wherein hot forming of the welded blank is performed by a hot stamping operation.

11. The steel part as recited in claim 1, wherein the aluminum or aluminum alloy of the metal alloy layer is removed from respective cut edges of peripheral edges of the first and second sheets destined to undergo the welding operation.

12. A method for the fabrication of a welded steel part as recited in claim 1, comprising the steps of: providing at least the first and the second steel sheet; removing the metal alloy layer from at least one surface of a portion of the peripheral edges of each of the first and second steel sheets; butt welding the first and the second steel sheets at a level of the respective edges of the first and second steel sheets from which the metal alloy layer has been removed with a laser source and using a filler metal wire over at least a portion of the welded metal zone, the filler metal wire having a carbon content higher than that of the steel substrates of at least one of the first or second sheets to obtain a welded blank; heating the welded blank to give the weld metal zone an austenitic structure; hot forming and heating the welded blank to obtain a steel part; and cooling the steel part at a controlled rate to obtain specified mechanical strength characteristics.

13. The method of claim 12, wherein a composition of the steel substrate of at least the first or the second sheet, comprises the following elements, expressed in percent by weight: 0.10%≦C≦0.5%; 0.5%≦Mn≦3%; 0.1%≦Si≦1%; 0.01%≦Cr≦1%; Ti≦0.2%; Al≦0.1%; S≦0.05%; P≦0.1%; and 0.0002%≦B≦0.010%, the balance being iron and unavoidable impurities from processing.

14. The method as recited in claim 12, wherein the metal alloy layer has been removed from each of the facing surfaces of the respective peripheral edges of each of the first and second steel sheets leaving the intermetallic alloy layer in place.

15. The method as recited in claim 12, wherein a width of a zone from which the metal alloy layer has been removed at the level of the peripheral edge of the first and second sheets destined to undergo the welding operation is between 0.2 and 2.2 mm.

16. The method as recited in claim 12, wherein a composition of the steel substrate of at least the first or second sheet includes the following elements, expressed in percent by weight 0.15%≦C≦0.4%; 0.8%≦Mn≦2.3%; 0.1%≦Si≦0.35%; 0.01%≦Cr≦1%; Ti≦0.1%; Al≦0.1%; S≦0.03%; P≦0.05%; and 0.0005%≦B≦0.010%, the balance being iron and unavoidable impurities from processing.

17. The method as recited in claim 12, wherein the composition of the steel substrate of at least the first or the second sheet includes the following elements, expressed in percent by weight: 0.15%≦C≦0.25%; 0.8%≦Mn≦1.8%; 0.1%≦Si≦0.35%; 0.01%≦Cr≦0.5%; Ti≦0.1%; Al≦0.1%; S≦0.05%; P≦0.1%; and 0.0002%≦B≦0.005%, the balance being iron and unavoidable impurities from processing.

18. The method as recited in claim 12, wherein during the step of butt welding, the peripheral edges of the first and second steel sheets to be welded are located at a distance of 0.1 mm or less from each other.

19. The method as recited in claim 12, wherein a linear welding energy of the laser source during the welding operation is greater than 0.3 kJ/cm.

20. The method as recited in claim 19, wherein the laser source is a CO.sub.2 gas laser type, which delivers a linear welding energy greater than 1.4 kJ/cm, or a solid-state laser type which delivers a linear welding energy greater than 0.3 kJ/cm.

21. The method as recited in claim 20, wherein a welding speed is between 3 meters/minute and 8 meters/minute, and a power of the CO.sub.2 gas laser is greater than or equal to 7 kW and a power of the solid state laser is greater than or equal to 4 kW.

22. The method as recited in claim 12, wherein the butt welding step is performed under a helium or argon cover gas.

23. The method as recited in claim 22, wherein a flow rate of helium or argon during the butt welding step is greater than or equal to 15 liters per minute.

24. The method as recited in claim 12, wherein the filler metal wire comprises the following elements, expressed in percent by weight: 0.6%≦C≦1.5%; 1%≦Mn≦4%; 0.1%≦Si≦0.6%; Cr≦2%; and Ti≦0.2% the balance being iron and unavoidable impurities from processing.

25. The method as recited in claim 24, wherein the filler metal wire includes the following elements, expressed in percent by weight: 0.65%≦C≦0.75%; 1.95%≦Mn≦2.05%; 0.35%≦Si≦0.45%; 0.95%≦Cr≦1.05%; and 0.15% Ti≦0.25% the balance being iron and unavoidable impurities from processing.

26. The method as recited in claim 25, wherein a proportion of filler metal relative to a volume of the weld metal zone is between 12% and 26% and the welding speed is between 3 and 7 meters/minute.

27. The method as recited in claim 26, wherein filler metal relative to the volume of the weld metal zone and the welding speed are within a range (50) illustrated in FIG. 8.

28. The method as recited in claim 27, wherein a proportion of filler metal relative to the volume of the weld metal zone is between 12% and 26%, the welding speed is between 3 and 7 meters per minute and when the welding speed is greater than 3.5 meters per minute, the proportion of filler metal relative to the volume of the weld metal zone and the welding speed is such that Y≦−3.86X+39.5, whereby Y designates the proportion of filler metal expressed as a volume percent and X designates the welding speed expressed in meters per minute.

29. The method as recited in claim 26, wherein the proportion of filler metal relative to the volume of the weld metal zone is between 14 and 16%, a helium or argon flow rate is between 13 and 17 liters per minute, a diameter at a point of impact on the sheet of the laser beam is between 500 and 700 μm, and an extremity of the filler wire is at a distance from the point of impact of the laser beam on the sheet of between 2 and 3 mm.

30. The method as recited in claim 12, wherein a cooling rate of the weld metal zone during the hot forming step is greater than or equal to a critical martensitic hardening rate of the weld metal zone.

31. A structural or safety part for an automotive vehicle comprising: a welded steel part as recited in claim 1.

32. The method as recited in claim 12, wherein the steps are performed successively.

33. The method as recited in claim 12, wherein the structure of the weld metal zone is completely austenitic.

34. The method as recited in claim 12, wherein on respective cut edges of the peripheral edges of the first and second sheets destined to be welded, the metal alloy layer is removed during a previous cutting operation of each of the first and second sheets.

35. The steel part as recited in claim 1, wherein the welded steel part includes a mechanical strength greater than 1230 MPa.

36. A method of fabricating a welded steel part comprising: providing at least a first and a second steel sheet each having a steel substrate and a pre-coating which includes an AlSiFe intermetallic alloy layer in contact with the steel substrate, the first and second steel sheets having different compositions or thicknesses, a composition of the steel substrate of the first and the second steel sheets comprising the following elements, expressed in percent by weight: 0.10%≦C≦0.5%; 0.5%≦Mn≦3%; 0.1%≦Si≦1%; 0.01%≦Cr≦1%; Ti≦0.2%; Al≦0.1%; S≦0.05%; P≦0.1%; and 0.0002%≦B≦0.010%, the balance being iron and unavoidable impurities from processing; laser butt welding the first and the second steel sheets at respective peripheral edges of the first and second steel sheets by using a filler wire over at least a portion of the length of the welded zone; then heating the welded blank to give the weld metal zone a microstructure that contains no ferrite; wherein the filler wire has a filler wire composition compensating for presence of aluminum in the AlSiFe intermetallic alloy layer in the weld metal zone.

37. The method of claim 36, wherein a critical hardening rate is not increased in the weld metal zone compared to a critical hardening rate of the first and second steel sheets.

38. The method of claim 36, wherein a fracture that occurs in the base metal and no the weld metal zone when the weld joint is subjected to a uniaxial tensile stress perpendicular to the joint.

39. The method of claim 36, wherein a mechanical strength of the weld metal zone is greater than that of the first and second steel sheets.

40. The method of claim 36, wherein the weld metal zone has an increased carbon content with respect to a carbon content of the substrate of the first or second steel sheet having the higher carbon content.

41. The method of claim 40, wherein the increased carbon content of the weld metal zone resulting from the welding operation and constituting the bond between the first and second sheets is at least 1.27 times a carbon content of the substrate of the first or second steel sheet having the higher carbon content.

42. The method of claim 36, wherein the welded steel part that has a mechanical strength greater than 1230 MPa.

43. The method of claim 36, wherein an Mn content of the filler wire is 4% by weight, and an Mn content of the steel substrate of the first or second sheet is 2.3% by weight.

44. The method of claim 36, wherein a carbon content of the filler wire is 0.6%≦C≦1.5% by weight, and wherein the substrate of the first and second steel sheets have a carbon content between 0.1%≦C≦0.5% by weight.

45. The method of claim 36, wherein the filler wire comprises the following elements, expressed in percent by weight: 0.6%≦C≦1.5%; 1%≦Mn≦4%; 0.1%≦Si≦0.6%; Cr≦2%; and Ti≦0.2%; the balance being iron and unavoidable impurities from processing.

46. The method of claim 36, further comprising, prior to the providing step, removing the metal alloy layer from a portion of the first and second steel sheets, leaving in place the intermetallic alloy layer.

47. The method of claim 46, further comprising, after the heating step, hot forming and heating the welded blank to obtain a steel part, then cooling the steel part at a controlled rate.

48. The method of claim 36, wherein, on respective cut edges of the peripheral edges of the first and second sheets destined to be subjected to the welding operation, aluminum or aluminum alloy is removed.

49. The method of claim 48, wherein the aluminum or aluminum alloy on the respective cut edges results from a previous cutting operation of each of the first and second sheets.

50. The method as in claim 36, wherein an Mn content of the filler wire is 1%≦Mn≦4% by weight and an Mn content of the substrate of the first and second steel sheet is 0.8%≦Mn≦2.3% by weight.

51. The method of claim 36, wherein an Mn content of the filler wire is 1%≦Mn≦4% by weight and an Mn content of the substrate of the first or second steel sheet is 0.8%≦Mn≦1.8% by weight.

52. The method of claim 36, wherein said heating comprises heating to a temperature of 900 degrees C.

53. A method for the fabrication of a welded steel part comprising providing at least a first steel sheet with a pre-coating including an intermetallic alloy layer; providing at least a second steel sheet with a pre-coating including an intermetallic alloy layer, the first and second steel sheets having different compositions or thicknesses; forming a welded blank by butt laser welding the at least first and the at least second steel sheets to form a weld metal zone and form a bond between the at least first and the at least second sheets, wherein a filler metal wire is used over at least a portion of the welded metal zone; heating the welded blank in the an austenitic range followed by hot forming and then cooling to form the welded steel part; wherein a carbon content over at least a portion of the weld metal zone is between 1.27 and 1.59 times the carbon content of whichever of the first or second steel sheet has a higher carbon content and wherein a composition of the steel substrate of the at least first or second sheet having a higher carbon content comprises carbon expressed in percent by weight of 0.1%≦C≦0.5% by weight, and wherein a carbon content of the filler wire is 0.6%≦C≦1.5%, by weight.

54. The method of claim 53, wherein the intermetallic alloy layer is an AlSiFe intermetallic alloy layer.

55. The method of claim 53, wherein the C carbon content of the first or second steel sheet having the higher carbon content is 0.15%≦C≦0.25% by weight.

56. The method of claim 55, wherein the C carbon content of the steel sheet having the higher carbon content is 0.2%≦C≦0.25% by weight.

57. The method of claim 53, wherein the C carbon content of the filler wire is approximately 0.7% by weight and the C carbon content of the first or second steel sheet having the higher carbon content is 0.2%≦C≦0.25% by weight.

58. The method of claim 53, wherein the C carbon content of the filler wire is 0.65%≦C≦0.75% by weight.

59. The method of claim 53, wherein an Mn content of the filler wire is 1%≦Mn≦4% by weight and an Mn content of the first or second steel sheet having the higher carbon content is 0.8%%≦Mn≦2.3% by weight.

60. The method of claim 53, wherein an Mn content of the filler wire is 1%≦Mn≦4% by weight and an Mn content of the first or second steel sheet having the higher carbon content is 0.8%%≦Mn≦1.8% by weight.

61. The method of claim 53, wherein an Mn content of the filler wire is 1%≦Mn≦4% by weight and an Mn content of the first or second steel sheet having the higher carbon content is 0.8%%≦Mn≦2.3% by weight.

62. The method of claim 53, wherein an Mn content of the filler wire is 1.95%≦Mn≦2.05% by weight.

63. The method of claim 53, wherein an Mn content of the filler wire is 1%≦Mn≦4% by weight and an Mn content of the first or second steel sheet having the higher carbon content is 0.5%≦Mn≦0.2.3% by weight.

64. The method of claim 53, wherein the a ratio between a hardness of the weld metal zone and a hardness of a substrate of the first or second steel sheet that has the higher carbon content Cmax is greater than 1.029+(0.36 Cmax), where Cmax is expressed in percent by weight.

65. The method of claim 53, wherein the carbon content of the weld metal zone is less than or equal to 0.35% by weight.

66. The method of claim 53, wherein a microstructure of the weld metal zone contains no ferrite.

67. The method of claim 53, wherein a microstructure of the weld metal zone is martensitic.

68. The method of claim 53, wherein the heating comprises heating to a temperature of 900 C.

69. A method of manufacturing a tailor welded blank, comprising: laser-welding blank plates elements made of coated steel having different strengths compositions or thicknesses using a filler wire over at least a portion of a length of an area to be welded to define a weld metal zone, wherein the plates comprise a steel substrate and a coating layer in the weld metal zone which contacts the steel substrate, and wherein the filler wire has a higher amount of an austenite-stabilizing element carbon than the composition of the coated steel plate base material, such that the welded weld metal zone can have a full austenite structure without forming a ferrite structure has a microstructure containing no ferrite even upon intrusion of a the coating layer into the welded zone.

70. The method of claim 69, wherein the coating is an AlSiFe intermetallic alloy layer.

71. The method of claim 69, wherein the carbon content of the steel substrate having the higher carbon content is 0.15%≦C≦0.25% by weight.

72. The method of claim 71, wherein the carbon content of the sheet substrate having the higher carbon content is 0.2%≦C≦0.25% by weight.

73. The method of claim 69, wherein the carbon content of the filler wire is approximately 0.7% by weight and the carbon content of the steel substrate having the higher carbon content is 0.2%≦C≦0.25%. by weight.

74. The method of claim 69, wherein the carbon content of the filler wire is 0.65%≦C≦0.75% by weight.

75. The method of claim 69, wherein a n content of the filler wire is 1%≦Mn≦4% by weight and an Mn content of the steel substrate having the higher carbon content is 0.8%%≦Mn≦2.3% by weight.

76. The method of claim 69, wherein an Mn content of the filler wire is 1%≦Mn≦4% by weight and an Mn content of the steel substrate having the higher carbon content is 0.8%%≦Mn≦1.8% by weight.

77. The method of claim 69, wherein an Mn content of the filler wire is 1%≦Mn≦4% by weight and an Mn content of the steel substrate having the higher carbon content is 0.8%%≦Mn≦2.3% by weight.

78. The method of claim 69, wherein an Mn content of the filler wire is 1.95%≦Mn≦2.05% by weight.

79. The method of claim 69, wherein an Mn content of the filler wire is 1%≦Mn≦4% by weight and an Mn content of the steel substrate having the higher carbon content is 0.5%≦Mn≦0.2.3% by weight.

80. The method of claim 69, wherein the carbon content of the weld metal zone is less than or equal to 0.35% by weight.

81. The method of claim 69, comprising heating the laser welded blanks to a temperature of 900 C so that the weld metal zone has the microstructure containing no ferrite.

82. The method of clam 69, wherein a ratio between a carbon content of the weld metal zone and a carbon content of the steel substrate having a higher carbon content is between 1.27 and 1.59

Description

BRIEF DESCRIPTION OF THE FIGURES

[0134] Other characteristics and advantages of the invention are portrayed in detail in the following description, which is presented exclusively by way of example and is in no way intended to be restrictive, with reference to the accompanying figures in which:

[0135] FIG. 1 presented above represents the comparative profile of the hardness of the base metal and of the weld metal zone as a function of the cooling rate during the hot stamping, for a welded steel part of the prior art,

[0136] FIG. 2 is a schematic illustration of the sheet used in the implementation of the method claimed by the invention,

[0137] FIG. 3 is a schematic illustration of the beginning of the welding operation of the method claimed by the invention,

[0138] FIG. 4 is a schematic illustration of the end of the welding operation of the method claimed by the invention,

[0139] FIG. 5 illustrates the profile of the mechanical tensile fracture strength of the weld metal zone, whereby the stress is exerted perpendicularly in relation to the welded joint, as a function of the percentage of filler metal in the weld metal zone during the method claimed by the invention, and for two different cooling rates during the hot stamping,

[0140] FIG. 6 illustrates the location of the fracture, either in the base metal or in the weld metal zone, as a function of the ratio between the carbon content of the weld metal zone and the carbon content of the base metal,

[0141] FIG. 7 is a graph that illustrates one example of a micro-hardness profile (hardness under a load of 200 g) of a welded steel part fabricated from two sheets of different thickness and stamped according to the invention and of the zone adjacent to the weld metal zone,

[0142] FIG. 8 is a graph illustrating the optimum operating limit conditions of the method claimed by the invention in terms of percentage of filler metal and welding speed, and

[0143] FIG. 9 illustrates the variation of toughness in the weld metal zone as a function of the temperature for different carbon contents.

DETAILED DESCRIPTION

[0144] In the method claimed by the invention, two sheets coated by immersion in a bath of molten aluminum according to a method called continuous “dip coating” as described in publication EP971044 are provided. The term sheet is used in a broad sense as any strip or object obtained by cutting from a strip, coil or sheet.

[0145] The aluminum bath which is the object of the dipping operation can also include from 9 to 10% silicon and from 2 to 3.5% iron.

[0146] The steel constituting the steel substrate of the sheets exhibits the following composition, expressed in per cent by weight: [0147] 0.10%≦C≦0.5% [0148] 0.5%≦Mn≦3% [0149] 0.1%≦Si≦1% [0150] 0.01%≦Cr≦1% [0151] Ti≦0.2% [0152] Al≦0.1% [0153] S≦0.05% [0154] P≦0.1% [0155] 0.0002%≦B≦0.010%,
the balance being iron and unavoidable impurities from processing.

[0156] Preferably, the composition of the steel will be as follows: [0157] 0.15%≦C≦0.4% [0158] 0.8%>Mn≦2.3% [0159] 0.1% >Si≦0.35% [0160] 0.01%>Cr≦1% [0161] Ti>0.1% [0162] Al≦0.1% [0163] S≦0.03% [0164] P≦0.05% [0165] 0.0005%≦B≦0.010%,
the balance being iron and unavoidable impurities from processing.

[0166] Even more preferably, and in accordance with the following description, the composition of the steel will be as follows: [0167] 0.15%≦C≦0.25% [0168] 0.8%≦Mn≦1.8% [0169] 0.1%≦Si≦0.35% [0170] 0.01%≦Cr≦0.5% [0171] Ti≦0.1% [0172] Al≦0.1% [0173] S≦0.05% [0174] P≦0.1% [0175] 0.0002%≦B≦0.005%,
the balance being iron and unavoidable impurities from processing.

[0176] The sheets to be welded to one another can be of identical or different compositions.

[0177] The coating, which will be called the “pre-coating” at this stage in the following description, exhibits the following characteristics resulting from the immersion of the sheet in the aluminum bath: with reference to FIG. 2, the pre-coating 3 of the sheet 4 has two layers 5, 7 of different types.

[0178] First, an intermetallic alloy layer 5 of the AlSiFe type is in contact with the surface of the steel substrate 6 of the sheet 4. This intermetallic alloy layer 5 results from the reaction between the steel substrate 6 and the aluminum bath.

[0179] In addition, this intermetallic alloy layer 5 is topped by a metal alloy layer 7 which forms a surface layer of the pre-coating 3.

[0180] The pre-coating 3 is present on the two opposite faces 8a, 8b of the sheet 4.

[0181] In the method claimed by the invention, the metal alloy layer 7 is removed at the level of the periphery 9 of the sheet 4 which is destined to undergo the subsequent welding operation.

[0182] In FIG. 2, only the upper surface 8a is the object of this removal, but the metal alloy layer 7 will advantageously be removed peripherally at the level of the two opposite faces 8a, 8b of the sheet 4.

[0183] The intermetallic alloy layer 5 therefore remains at the level of the periphery 9 of the sheet 4 which is destined to undergo the welding operation.

[0184] The ablation of the metal layer 7 can be performed by a brushing operation because the metal layer 7 which is removed has a hardness which is less than the hardness of the intermetallic alloy layer 5 which remains.

[0185] A technician skilled in the art will understand how to adapt the parameters relating to the brushing to make possible the removal of the metal layer 7 on the periphery 9 of the sheet 4.

[0186] It is also possible to remove the metal alloy layer using a laser beam directed toward the periphery 9 of the sheet 4.

[0187] The interaction between the laser beam and the pre-coating 3 causes a vaporization and an expulsion of the metal alloy layer 7.

[0188] The width over which the metal alloy layer 7 at the level of the periphery 9 of the sheet 4 is removed is between 0.2 and 2.2 millimeters.

[0189] In addition, the intermetallic alloy layer 5 that remains at the level of the periphery 9 of the sheet 4 is on the order of 5 μm thick.

[0190] These two modes of ablation (brushing and laser) of the metal alloy layer are the subject of publication EP2007545.

[0191] The previous cutting operations of the sheet 4, as well as the operation of removing the metal alloy layer 7 as described above can involve a portion of the pre-coating 3 at the level of the cut edge 10 of the periphery 9 of the sheet 4 destined to be the object of the welding operation. Therefore there are traces of aluminum or aluminum alloy at the level of this cut edge 10.

[0192] According to the method claimed by the invention, these traces of aluminum or aluminum alloy at the level of the cut edge 10 of the sheet 4 are also removed by brushing prior to the welding operation.

[0193] With reference to FIG. 3, a first sheet 11 and the second sheet 12, each having a respective substrate 25, 26, and each having on their respective opposite faces 13a, 13b; 14a, 14b a pre-coating 15, 16 consisting of an intermetallic alloy layer 17, 18 topped by a metal alloy layer 19, 20, are placed end to end according to conventional laser welding techniques by contact between their respective peripheries 21, 22, on which on one hand the metal alloy layer 19, 20 has been removed at the level of their opposite faces 13a, 13b; 14a, 14b, and on the cut edges 23, 24 from which the pre-coating 15, 16 deposited during the shearing operation has also been removed.

[0194] The maximum distance between the respective cut edges 23, 24 of the two sheets 11, 12 is 0.1 mm, whereby the placement of this clearance between the cut edges 23, 24 of the two sheets 11, 12 promotes the deposit of the filler metal during the welding operation.

[0195] As illustrated in FIG. 3, the welding operation according to the method claimed by the invention consists of a laser beam 30 directed at the level of the junction between the two sheets 11, 12, combined with a filler wire 32 that melts at the point of impact 31 of the laser beam. The welding method in question is therefore laser welding with filler metal.

[0196] The laser source used must be high-powered and can be selected from among a laser CO.sub.2 gas type laser source with a wavelength of 10 micrometers or a solid state laser source with a wavelength of 1 micrometer.

[0197] On account of the thickness of the two sheets 11, 12 which is less than 3 mm, the power of the CO.sub.2 gas laser must be greater than or equal to 7 kW while the power of the solid state laser must be greater than or equal to 4 kW.

[0198] The diameter of the laser beam at the point of its impact on the sheets must be approximately 600 μm for both types of laser source.

[0199] Finally, the extremity 32a of the filler wire 32 must be located approximately 3 mm from the point of impact P of the laser beam 30 on the junction between the sheets 11 and 12 for a solid state laser source and approximately 2 mm from the laser beam 30 for a CO.sub.2 gas laser type laser source.

[0200] These conditions make it possible to obtain a complete melting of the filler wire 32 as well as a satisfactory mixing with the steel substrate at the level of the weld.

[0201] In addition, these powers will make it possible to use a welding speed sufficient to prevent the precipitation of boron nitrides and/or other segregation problems.

[0202] The filler wire must meet two requirements: [0203] first, the quantity of metal added by this filler wire 32 must be such that the laser source is able to melt it in its entirety and to produce a relatively homogeneous mixture at the level of the weld. In addition, the quantity of metal added must not result in an overthickness of the weld by more than 10% in relation to the lowest thickness of the two sheets if the latter are not the same thickness, in accordance with the quality standards in force in the automobile industry. [0204] the composition of the filler wire must also make it possible, in combination with the other parameters of the welding process, to obtain a weld, the mechanical strength characteristics of which are comparable, after hot forming and cooling, with the mechanical strength characteristics of the first 11 and second 12 welded sheets.

[0205] Finally, during the welding step, cover gas protection must be provided to prevent the oxidation and decarburization of the zone which is being welded, to prevent the formation of boron nitride in the weld metal zone and potential cold cracking phenomena caused by the absorption of hydrogen.

[0206] This cover gas protection is achieved by using helium and/or argon.

[0207] With reference to FIG. 4, the welding operation leads to the formation of a weld metal zone 35 at the junction between the two sheets 11, 12 which subsequently solidifies, thereby forming the weld. The term “weld metal zone” is used to identify this weld even after solidification of this weld metal zone 35.

[0208] Measures can be taken for the parts that undergo a less rapid local cooling during the hot forming to add a filler wire only in certain portions of the length of the weld metal zone and not to add the filler metal wire in the remaining joints.

[0209] The welded blank 37 resulting from the welding operation therefore has a weld metal zone 35 that does not contain intermetallic alloy because of the prior removal of the metal alloy layer 19, 20 as explained above.

[0210] In addition, as illustrated in FIG. 4, the edges 36 in direct proximity to the weld metal zone 35 are free of the metal alloy layer 19, 20 on account of the fact that the width of the weld metal zone 35 is less than the width of the welding zone which does not include a metal alloy layer 19, 20.

[0211] Although FIG. 4 illustrates the simple case of a welded blank fabricated from a first sheet 11 and the second sheet 12, it is possible in the method claimed by the invention to use a larger number of sheets which are welded to one another.

[0212] The welded blank 37 thereby obtained is then subjected to a heating process to obtain an austenitic transformation in all of the parts of this blank. This blank is then hot formed, preferably by hot stamping. This step is followed by a cooling conducted by contact in the stamping die at a cooling rate which is discussed below, and results in a welded steel part.

[0213] In the following description, the reference to a welded steel part refers to the finished piece following the hot stamping of the welded blank, the fabrication of which is described above.

[0214] For a type 22MnB5 steel (C=0.20-0.25%, Mn=1.1-1.35%, Si=0.15-0.35%, Al=0.020-0.060%, Ti=0.020-0.050%, Cr=0.15-0.30%, B=0.002-0.004%, the contents being expressed in per cent by weight and the balance consisting of iron and the impurities resulting from processing), table 1 below presents the conditions of the welding method used to fabricate a welded steel part for which the hardness of the weld metal and hot-stamped zone is at least equal to the hardness of one or the other of the two sheets 11, 12.

[0215] These conditions are indicated in terms of welding speed, volume percentage of the filler metal in relation to the weld metal zone and the chemical composition of the filler wire expressed in per cent by weight. The tests that were conducted to determine these boundary conditions were performed with a CO.sub.2 gas laser source with a power greater than 7 kilowatts and a solid state laser source with a power greater than 4 kilowatts under a helium and/or argon cover gas at a flow rate greater than 15 liters/minute.

TABLE-US-00001 TABLE 1 Welding Proportion Composition of the speed of filler filler wire-% by weight (m/min) metal (%) C Mn Si Cr Ti Minimum 3 10 0.6 1 0.1 0 0 Maximum 8 26 1.5 4 0.6 2 0.2

[0216] In the framework of another example, tests were conducted with a filler wire having the composition indicated below, in per cent by weight: C=0.7%, Si=0.4%, Mn=2%, Cr=1% and Ti=0.2, the remainder consisting of iron and impurities resulting from processing.

[0217] The tests that were conducted to determine these boundary conditions were performed with a CO.sub.2 gas laser source with a power greater than 7 kilowatts and a solid state laser source with a power greater than 4 kilowatts under a helium and/or argon cover gas at a flow rate greater than 15 liters/minute. All the results obtained and presented below are similar, regardless of the laser source used.

[0218] With reference to FIG. 8, the appearance of the weld metal zone and the quality of the mixing of the filler wire and the molten metal are examined for different percentages of filler metal and welding speeds.

[0219] For the experimental points identified as references 40 and 41, the results in terms of dilution and surface appearance of the weld metal zone are satisfactory, while for the experimental points identified as 42, the results are not satisfactory.

[0220] FIG. 5 illustrates the tensile fracture strength of the hot-stamped welded steel part as a function of the percentage of filler metal in the weld metal zone for two cooling rates of 30 and 50° C. per second.

[0221] The experimental points identified as reference 43 correspond to a cooling rate of 30° C. per second and the experimental points identified as reference 44 correspond to a cooling rate of 50° C. per second. These two rates correspond respectively to an efficient extraction of heat thanks to tight contact between the part and the press die (50° C. per second) and to a less tight contact on account of a lower closing pressure and/or a difference in thickness between the sheets to be welded (30° C. per second).

[0222] When the hot stamped welded blanks are cooled at a rate of 50° C. per second, the tensile strength is between 1470 and 1545 MPa and the fracture occurs in the base metal.

[0223] When the hot stamped welded blanks are cooled at a rate of 30° C. per second, and when the volume proportion of the filler metal is between 4.3 and 11.5%, the fracture occurs in the weld metal zone and the mechanical tensile strength is between 1230 and 1270 MPa.

[0224] On the other hand, when the hot stamped welded blanks are cooled at a rate of 30° C. per second, and when the volume proportion of filler metal is 14.7%, the fracture occurs in the base metal with a mechanical strength of 1410 MPa.

[0225] Therefore, a proportion of filler metal greater than 12% makes it possible to systematically obtain a fracture outside the welded joint, both in the efficiently cooled zones in the hot stamped part and in the less efficiently cooled zones.

[0226] FIG. 6 illustrates the location of the fracture, either in the base metal as indicated on step 45, or in the weld metal zone indicated on step 46, when the welded joints are subjected to a uniaxial tensile force perpendicular to the seam, as a function of the ratio between the carbon content of the weld metal zone and the carbon content of the base metal, starting from the experimental points 43, 44 presented with reference to FIG. 5 and identified respectively as 43a and 44b in FIG. 6.

[0227] It has been shown that when this ratio is greater than 1.27 (line D1), the fracture occurs systematically in the base metal, in spite of the modifications of hardenability due to the presence of aluminum in the weld metal zone, and in spite of the slower cooling rate resulting from incomplete contact between the part and the die. FIG. 6 also shows that beyond a ratio of 1.59 (line D2), a particular brittleness occurs.

[0228] This maximum ratio of 1.59 between the carbon content of the weld metal zone and the carbon content of the base metal is also obtained by determining the critical conditions that lead to the sudden fracture of a martensitic structure weld comprising a surface defect, when stress is applied perpendicular to the welding direction.

[0229] For this purpose, consideration is given to the case of two sheets 11, 12, the thickness w of which is 3 mm, and a slot type defect in the weld metal zone, the depth of which is 10% of the thickness of the sheets 11, 22, i.e. a depth of 0.3 mm.

[0230] The expression of the stress intensity factor K.sub.1 expressed in MPa√m is as follows:

K.sub.1=kσ√πa

in which [0231] k is the shape factor, and determined in particular on the basis of the ratio a/w [0232] σ is the stress applied to the weld, expressed in MPa, and [0233] a is the depth of the defect in question, expressed in meters.

[0234] To evaluate the stress intensity factor, a case of severe stress is considered, in which the applied stress σ is equal to the elastic limit Re.

[0235] Table 2 below expresses the elastic limit Re and the stress intensity factor K.sub.1 for four levels of carbon in the weld metal zone varying between 0.2% and 0.4% for a martensitic microstructure.

TABLE-US-00002 TABLE 2 0.2% C 0.3% C 0.35% C 0.4% C Re (MPa) 1200 1350 1425 1500 K.sub.I(MPa {square root over (m)}) 41.3 46.4 49.0 51.6

[0236] Reference is made to FIG. 9, which shows the variation of the critical stress intensity factor K.sub.IC as a function of the temperature for carbon contents varying between 0.2 and 0.4% and martensitic microstructures. The curve 60 relates to a carbon content of 0.2% C, curve 61 to a carbon content of 0.3% C, curve 62 to a carbon content of 0.35% C and curve 63 to a carbon content of 0.4% C.

[0237] This FIG. 9 presents the values of the stress intensity factor K.sub.1 expressed in table 2 for each of the levels of carbon content, identified respectively as 64 for a carbon content of 0.2% C, 65 for a carbon content of 0.3%, 66 for a carbon content of 0.35% and 67 for a carbon content of 0.4%.

[0238] The risk of sudden fracture of the weld at −50° C. is therefore eliminated when the toughness K.sub.IC at this temperature is greater than the stress intensity factor K.sub.1.

[0239] FIG. 9 shows that this condition is satisfied provided that the carbon content does not exceed 0.35%.

[0240] The result is a maximum carbon content in the weld metal zone of 0.35%. Considering a welded joint fabricated from two sheets of type 22MnB5 steel, i.e. containing 0.22% carbon, the limit value of the ratio between the carbon content of the weld metal zone and the carbon content of the steel sheet beyond which there is a risk of sudden fracture in the weld metal zone is therefore 1.59.

[0241] In addition, the fact that the fracture always occurs in the base metal beyond this value of 1.27 is unexpected, because the toughness of the molten metal decreases as the carbon content increases. Coupled with the effect of stress concentrations which is unavoidable in the welded joint, the fracture should rather have occurred in the molten metal on account of a lack of toughness for the highest carbon levels.

[0242] For this purpose, the risk of sudden fracture in a weld at −50° C., as determined under the conditions specified above, was compared with the risk of sudden fracture at this same temperature in the base metal, where the base metal contained a defect in the thickness of its metal coating.

[0243] The defect in question is a micro-defect 30 μm deep corresponding to the thickness of the metal alloy coating. For a type 22MnB5 steel with the carbon content of 0.22%, the elastic limit Re is 1250 MPa. If this steel is stressed at a stress level equal to its elastic limit, the stress intensity factor K.sub.1is 13.6 MPa. √m.

[0244] By referring to this letter value in FIG. 9 under reference number 68, it can be determined that the sudden fracture should theoretically occur in the weld metal zone and not in the base metal. However, contrary to what was expected, the inventors found that when the ratio between the carbon content of the weld metal zone and the carbon content of the base metal is between 1.27 and 1.59, the fracture systematically occurs in the base metal and not in the weld metal zone. In summary, the inventors have found that the increase in the carbon content in this specific range makes it possible to increase the strength characteristics of the weld metal zone of the hot stamped part, and without any increase in the risk of sudden fracture in this zone, an altogether unexpected effect.

[0245] In addition, the inventors have sought to define a simple method to define the zone claimed by the invention on the basis of the hardness characteristics of the weld metal zone and of the neighboring base metal in the hot stamped part. The significant hardness of the weld metal zone is linked to its martensitic microstructure, which does not contain any ferrite. It is known that the hardness of a steel with a martensitic structure is principally a function of its carbon content. Consequently, it is possible to define, on the basis of the above results, the ratio Z between the hardness of the weld metal zone and the hardness of the neighboring base metal which must be respected.

[0246] In the case of the welding of sheets of different compositions, Cmax designates the carbon content of the sheet that has the highest carbon content. In the case of welding of identical sheets, Cmax designates their carbon content. A fracture in the base metal during the application of tensile stress to a welded joint occurs when the ratio Z is greater than a critical value which is a function of Cmax, i.e. 1.029+(0.36 Cmax).

[0247] For the welding of identical sheets containing 0.22% carbon, a fracture in the base metal is therefore observed when the ratio Z is greater than 1.108, i.e. when the hardness of the weld metal zone exceeds the hardness of the base metal by approximately 11%.

[0248] With reference to FIG. 7, the curves 47 and 48 represent the evolution of the microhardness in the weld metal zone and in the neighboring zones of the welded zone represented on the respective micrographs M1 and M2, for a volume percentage of filler metal of 15% and for different thicknesses of welded sheets.

[0249] For the curve 47, relative to a cooling rate of 30° C. per second, the micro hardness measurements were conducted at the level of the lateral edge of the weld metal zone at one-half the thickness of the thinnest sheet as illustrated in the micrograph M1 by the dotted line X1.

[0250] For the curve 48, relative to a cooling rate of 50° C. per second, the micro hardness measurements were conducted at the level of the bottom of the weld metal zone at one-half the thickness of the thinnest sheet as illustrated in the micrograph M2 by the dotted line X2.

[0251] With reference to FIG. 8, the preferred limit conditions in terms of percentage of filler metal and welding speed for the specific composition of filler wire defined above and containing 0.7% carbon are defined by the hatched zone 50.

[0252] This zone 50 is delimited by four boundaries 51, 52, 53, 54.

[0253] The first boundary 51 defines the lower limit of the percentage of filler metal. The percentage of filler metal must therefore be greater than 12% to keep the welded zone from exhibiting mechanical strength characteristics that are too weak.

[0254] The second boundary 52 defines the upper limit of the percentage of filler metal. The percentage of filler metal must therefore be less than 26%, because above this limit, the welded zone exhibits a brittleness which is incompatible with the required properties.

[0255] The third boundary 53 defines the lower limit of the welding speed. The welding speed must therefore be greater than 3 meters per minute to obtain a satisfactory geometry of the weld bead and to prevent oxidation phenomena.

[0256] Finally, the fourth boundary 54 defines the upper limit of the welding speed and is in the shape of a curve.

[0257] This fourth boundary 54 is defined on the basis of the experimental points 40, 41, 42 discussed above and for which the experimental points 42 correspond to specimens for which the mixing between the filler metal and the base metal is insufficient and/or the weld does not penetrate to a sufficient depth. In addition, the curved shape of this fourth boundary 54 is estimated with reference to requirements specific to the welding operation.

[0258] In fact, the capacity of the laser source to melt the filler wire and to cause a relatively homogeneous mixing has an influence on the maximum percentage of filler metal and on the welding speed.

[0259] For this purpose, for a welding speed of 4 meters per minute, for example, the percentage of filler metal must not be greater than approximately 25%.

[0260] For a higher welding speed, the proportion of filler metal must be limited.

[0261] In approximation of this fourth boundary 54, the equation of the straight line 55 that passes through a first point 56 located at the junction between the upper part of the fourth boundary 54 and the second boundary 52, and through a second point 57 located at the junction between the lower part of the fourth boundary 54 and the first boundary 51 was estimated.

[0262] The equation of this straight line 55 is Y=3.86X+39.5 where Y is the percentage of filler metal and X is the welding speed expressed in meters per minute.

[0263] It can therefore be assumed approximately that the fourth boundary defining the maximum limit of the welding speed is defined by the straight line 55 for a welding speed greater than 3.5 m/m.

[0264] Therefore, the invention makes it possible to economically fabricate structural and safety parts for the automobile industry.