A METHOD FOR THE MANUFACTURE OF A COATED STEEL SHEET

Abstract

Method for the manufacture of a coated steel sheet including the following successive steps A. the provision of an annealed steel sheet having the following chemical composition in weight: 0.10<C<0.40%, 1.5<Mn<3.0%, 0.7<Si<2.0%, 0.05<Al<1.0%, 0.75<(Si+Al)<3.0%, and on a purely optional basis, one or more elements such as Nb0.5%, B0.005%, Cr1.0%, Mo0.50%, Ni1.0%, Ti0.5%, the remainder of the composition making up of iron and inevitable impurities resulting from the elaboration, such steel sheet being annealed at a temperature between 600 to 1200 C., B. the coating of the steel sheet obtained in step A) with a first coating including nickel, such first coating not including iron, and having a thickness equal or above 0.5 m and C. the coating of the steel sheet obtained in step B) with a second coating based on zinc. A steel sheet, a spot welded joint and further uses of the steel sheet are also provided.

Claims

1-27. (canceled)

28. A method for the manufacture of a coated steel sheet comprising the following successive steps: providing an annealed steel substrate having the following chemical composition in weight: 0.10<C<0.40%, 1.5<Mn<3.0%, 0.7<Si<2.0%, 0.05<Al<1.0%, 0.75<(Si+Al)<3.0%, and on a purely optional basis, one or more elements such as Nb0.5%, B0.005%, Cr1.0%, Mo0.50%, Ni1.0%, Ti0.5%, remainder of the composition making up of iron and inevitable impurities resulting from processing, the steel sheet being annealed at a temperature between 600 to 1200 C.; coating of the annealed steel substrate with a first coating including nickel but not including iron, and having a thickness equal or above 0.5 m; and coating, with a second coating based on zinc, the annealed steel substrate coated with the first coating.

29. The method as recited in claim 28 wherein the steel substrate is annealed in a continuous annealing.

30. The method as recited in claim 28 wherein the annealing is performed in an atmosphere comprising from 1 to 30% of H.sub.2 at a dew point between 10 and 60 C.

31. The method as recited in claim 28 wherein the first coating includes above 80% by weight of nickel.

32. The method as recited in claim 31 wherein the first coating includes above 90% by weight of nickel.

33. The method as recited in claim 32 wherein the first coating consists of nickel.

34. The method as recited in claim 28 wherein the first coating does not include phosphorus, nickel hydroxide or sulfur compounds.

35. The method as recited in claim 28 wherein the first coating has a thickness equal or above 1.0 m.

36. The method as recited in claim 35 wherein the first coating has a thickness equal or above 1.6 m.

37. The method as recited in claim 36 wherein the first coating has a thickness between 1.8 to 7.0 m.

38. The method as recited in claim 28 wherein the second coating includes above 50% of zinc.

39. The method as recited in claim 38 wherein the second coating includes above 75% of zinc.

40. The method as recited in claim 39 wherein the second coating includes above 90% of zinc.

41. The method as recited in claim 28 wherein the second coating does not comprise nickel.

42. The method as recited in claim 40 wherein the second coating consists of zinc.

43. A steel sheet obtainable from the method as recited in claim 28, the steel sheet comprising a steel substrate coated with a first coating comprising nickel and having a thickness equal or above 0.5 m, the first coating being directly topped by a zinc based layer.

44. The steel sheet as recited in claim 43 wherein a steel microstructure of the steel substrate includes from 1 to 50% of residual austenite, from 1 to 60% of martensite and optionally at least one element chosen from: bainite, ferrite, cementite and pearlite.

45. The steel sheet as recited in claim 44 wherein the microstructure comprises from 5 to 25% of residual austenite.

46. The steel sheet as recited in claim 44 wherein the microstructure comprises from 1 to 60% of tempered martensite.

47. The steel sheet as recited in claim 44 wherein the microstructure comprises from 10 to 40% of bainite.

48. The steel sheet as recited in claim 44 wherein the microstructure comprises from 1 to 25% of ferrite.

49. The steel sheet as recited in claim 44 wherein the microstructure comprises from 1 to 15% of untempered martensite.

50. A spot welded joint comprising: at least two metal sheets including a first metal sheet being the steel sheet as recited in claim 43, the joint containing less than 3 cracks having a size above 100 m and wherein the longest crack has a length below 300 m.

51. The spot welded joint as recited in claim 50 wherein a second metal sheet of the metal sheets is a steel sheet or an aluminum sheet.

52. The spot welded joint as recited in claim 50 wherein a second metal sheet of the metal sheets is a second steel sheet including an annealed second steel substrate with a first coating including nickel but not including iron, and having a thickness equal or above 0.5 m; and with a second coating based on zinc.

53. The spot welded joint as recited in claim 50 wherein the at least two metal sheets include a third metal sheet being a steel sheet or an aluminum sheet.

54. A spot welded joint comprising: at least two metal sheets including a first metal sheet being a steel sheet obtained by the method as recited in claim 28, the joint containing less than 3 cracks having a size above 100 m and wherein the longest crack has a length below 300 m.

55. An automotive vehicle part comprising the steel sheet as recited in claim 43.

56. An automotive vehicle part comprising a steel sheet obtained by the method as recited in claim 28.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0030] FIG. 1 shows schematically a non-limiting example of a spot welded joint with three steel sheets made according to the present invention.

DETAILED DESCRIPTION

[0031] Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

[0032] The designation steel or steel sheet means a steel sheet, a coil, a plate having a composition allowing the part to achieve a tensile strength up to 2500 MPa and more preferably up to 2000 MPa. For example, the tensile strength is above or equal to 500 MPa, preferably above or equal to 980 MPa, advantageously above or equal to 1180 MPa and even above or equal 1470 MPa.

[0033] The invention relates to method for the manufacture of a coated steel sheet comprising the following successive steps: [0034] A. The provision of an annealed steel sheet having the following chemical composition in weight: [0035] 0.10<C<0.40%, [0036] 1.5<Mn<3.0%, [0037] 0.7<Si<2.0%, [0038] 0.05<Al<1.0%, [0039] 0.75<(Si+Al)<3.0%, [0040] and on a purely optional basis, one or more elements such as [0041] Nb0.5%, [0042] B0.005%, [0043] Cr1.0%, [0044] Mo0.50%, [0045] Ni1.0%, [0046] Ti0.5%, [0047] the remainder of the composition making up of iron and inevitable impurities resulting from the elaboration, such steel sheet being annealed at a temperature between 600 to 1200 C., [0048] B. the coating of the steel sheet obtained in step A) with a first coating comprising nickel, such first coating not comprising iron, having a thickness equal or above 0.5 m and [0049] C. the coating of the steel sheet obtained in step B) with a second coating based on zinc.

[0050] Without willing to be bound by any theory, it seems that Ni, being present at the interface between the steel having the above specific chemical composition and the overlying zinc coating, prevents liquid zinc penetration into steel during any heating steps being for example a welding. Thus, by applying the method according to the present invention, it is possible to obtain a barrier layer to LME.

[0051] Preferably, in step A), the steel sheet is annealed in a continuous annealing. For example, the continuous annealing comprises a heating, a soaking and a cooling step. It can further comprise a pre-heating step.

[0052] Advantageously, the thermal treatment is performed in an atmosphere comprising from 1 to 30% of H.sub.2 at a dew point between 10 and 60 C. For example, the atmosphere comprises from 1 to 10% of H.sub.2 at a dew point between 10 C. and 60 C.

[0053] In step B), the first coating comprising nickel is deposited by any deposition method known by one skilled in the art. It can be deposited by vacuum deposition or electro-plating method. Preferably, it is deposited by electro-plating method.

[0054] Preferably, in step B), the first coating comprises above 80%, more preferably above 90% by weight of nickel. Preferably, in step B), the first coating does not comprise phosphorus, nickel hydroxide or sulfur compounds such as sulfate salt.

[0055] In a preferred embodiment, the first coating consists of nickel. In this embodiment, the amount of nickel is >99% by weight and preferably is of 100%.

[0056] Preferably, in step A), the first coating has a thickness equal or above 1.0 m and advantageously equal or above 1.6 m. More preferably, the first coating has a thickness between 1.8 to 7.0 m.

[0057] Advantageously, in step C), the second layer comprises above 50%, more preferably above 75% of zinc and advantageously above 90% of zinc. Preferably, the second layer does not comprise nickel. The second layer can be deposited by any deposition method known by one skilled in the art. It can be by hot-dip coating, by vacuum deposition or by electro-galvanizing.

[0058] For example, the coating based on zinc comprises 0.01-8.0% Al, optionally 0.2-8.0% Mg, the remainder being Zn.

[0059] Preferably, the coating based on zinc is deposited by hot-dip galvanizing. In this embodiment, the molten bath can also comprise unavoidable impurities and residuals elements from feeding ingots or from the passage of the steel sheet in the molten bath. For example, the optionally impurities are chosen from Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, the content by weight of each additional element being inferior to 0.3% by weight. The residual elements from feeding ingots or from the passage of the steel sheet in the molten bath can be iron with a content up to 5.0%, preferably 3.0%, by weight.

[0060] In a preferred embodiment, the second layer consists of zinc. When the coating is deposited by hot-dip galvanizing, the percentage of Aluminum is comprised between 0.15 and 0.40% in the bath.

[0061] With the method according to the present invention, a steel sheet coated with a first coating comprising nickel and having a thickness equal or above 0.5 m, such coating being directly topped by a zinc based layer, is obtained. It is believed that the first coating acts like a barrier layer to LME and prevent liquid metal to penetrate inside into the steel.

[0062] Preferably, the steel sheet has a microstructure comprising from 1 to 50% of residual austenite, from 1 to 60% of martensite and optionally at least one element chosen from: bainite, ferrite, cementite and pearlite. In this case, the martensite can be tempered or untempered.

[0063] In a preferred embodiment, the steel sheet has a microstructure comprising from 5 to 25% of residual austenite.

[0064] Preferably, the steel sheet has a microstructure comprising from 1 to 60% and more preferably between 10 to 60% of tempered martensite.

[0065] Advantageously, the steel sheet has a microstructure comprising from 10 to 40% of bainite, such bainite comprising from 10 to 20% of lower bainite, from 0 to 15% of upper bainite and from 0 to 5% of carbide free bainite.

[0066] Preferably, the steel sheet has a microstructure comprising from 1 to 25% of ferrite.

[0067] Preferably, the steel sheet has a microstructure comprising from 1 to 15% untempered martensite.

[0068] After the manufacture of a steel sheet, in order to produce some parts of a vehicle, it is known to assembly by welding two metal sheets. Thus, a spot welded joint is formed during the welding of at least two metal sheets, said spot being the link between the at least two metal sheets.

[0069] To produce a spot welded joint according to the invention, the welding is performed with an effective intensity is between 3 kA and 15 kA and the force applied on the electrodes is between 150 and 850 daN with said electrode active face diameter being between 4 and 10 mm.

[0070] Thus, a spot welded joint of at least two metal sheets, comprising the coated steel sheet according to the present invention, is obtained, such said joint containing less than 3 cracks having a size above 100 m and wherein the longest crack has a length below 300 m. Preferably, the second metal sheet is a steel sheet or an aluminum sheet. More preferably, the second metal sheet is a steel sheet according to the present invention.

[0071] In another embodiment, the spot welded joint comprises a third metal sheet being a steel sheet or an aluminum sheet. For example, the third metal sheet is a steel sheet according to the present invention. FIG. 1 thus shows schematically a spot weld 40 for joining three metal sheets each with a steel substrate 10, 20, 30, first coating 12, 22, 32 and second coating 14, 24, 34, respectively.

[0072] The steel sheet or the spot welded joint according to the present invention can be used for the manufacture of parts for automotive vehicle.

[0073] The invention will now be explained in trials carried out for information only. They are not limiting.

Example

[0074] For all samples, steel sheets used have the following composition in weight percent: C=0.37 wt. %, Mn=1.9 wt. %, Si=1.9 wt. %, Cr=0.35 wt. %, Al=0.05 wt. % and Mo=0.1 wt. %.

[0075] Trials 1 to 4 were prepared by performing an annealing in a continuous annealing in an atmosphere comprising 5% of H.sub.2 and 95% of N.sub.2 at a dew point of 60 C. The steel sheets were heated at a temperature of 900 C. Then, Trials 1 to 4 were coated with a different nickel coating thicknesses deposited by electro-galvanizing method. Finally, a zinc coating was deposited by electro-galvanizing method.

[0076] Trial 5 was prepared by deposited a zinc coating by electro-galvanizing method after the continuous annealing of the steel sheet under similar atmosphere.

[0077] The resistance to LME of above Trial samples were evaluated by resistance spot welding method. To this end, for each Trial, two coated steel sheets were welded together by resistance spot welding. The type of the electrode was ISO Type B with a diameter of 16 mm; the force of the electrode was of 5 kN and the flow rate of water of was 1.5 g/min. The details welding cycle has been reported in Table 1

TABLE-US-00001 TABLE 1 Welding schedule Weld time Pulses Pulse (cy) Cool time (cy) Hold time (cy) Cycle 2 12 2 10

[0078] The number of cracks above 100 m was then evaluated using an optical microscope as well as SEM (Scanning Electron Microscopy) as shown in Table 2.

TABLE-US-00002 TABLE 2 LME crack details after spot welding (2 layer stack-up condition) Number of cracks Thick- Thick- (>100 m) Maximum 1.sup.st ness 2.sup.nd ness per spot crack Trials coating (m) coating (m) weld length Trial 1* Ni 1.5 Zn (EG) 7 1 250 Trial 2* Ni 2.0 Zn (EG) 7 0 150 Trial 3* Ni 3.5 Zn (EG) 7 0 0 Trial 4* Ni 6.4 Zn (EG) 7 0 0 Trial 5 Zn (EG) 7 3 760 *according to the present invention.

[0079] Trials according to the present invention show an excellent resistance to LME as compared to Trial 5. Indeed, the number of cracks of Trials according to the present invention is very low, even nonexistent, compared to Trial 5.

[0080] For each Trial, three coated steel sheets were also welded together by resistance spot welding. The number of cracks above 100 m was then evaluated using an optical microscope as well as SEM (Scanning Electron Microscopy) as shown in Table 3.

TABLE-US-00003 TABLE 3 LME crack details after spot welding (3 layer stack-up condition) Number of Maximum cracks (>100 m) crack length Trials per spot weld (m) Trial 1* 2 250 Trial 2* 2 300 Trial 3* 0 250 Trial 4* 0 150 Trial 5 7 850 *: according to the present invention.

[0081] Trials according to the present invention show an excellent resistance to LME as compared to Trial 5.