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

Description

The present invention relates to a method for the manufacture of a coated steel sheet. The invention is particularly well suited for the manufacture of automotive vehicles.

BACKGROUND

Zinc based coatings are generally used because they allows for a protection against corrosion, thanks to barrier protection and cathodic protection. The barrier effect is obtained by the application of a metallic coating on steel surface. Thus, the metallic coating prevents the contact between steel and corrosive atmosphere. The barrier effect is independent from the nature of coating and substrate. On the contrary, sacrificial cathodic protection is based on the fact that zinc is a metal less noble that steel. Thus, if corrosion occurs, zinc is consumed preferentially to steel. Cathodic protection is essential in areas where steel is directly exposed to corrosive atmosphere, like cut edges where the surrounding zinc will be consumed before steel.

However, when heating steps are performed on such zinc coated steel sheets, for example hot press hardening or welding, cracks are observed in steel which spread from the steel/coating interface. Indeed, occasionally, there is a reduction of metal mechanical properties due to the presence of cracks in coated steel sheet after the above operation. These cracks appear with the following conditions: high temperature; contact with a liquid metal having a low melting point (such as zinc) in addition to presence of tensile stress; heterogeneous diffusion of molten metal with substrate grain and grain boundaries. The designation for such phenomenon known as liquid metal embrittlement (LME), and also called liquid metal assisted cracking (LMAC).

US2012/0100391 discloses a method for manufacturing a hot-dip galvanized steel sheet having good plating qualities, plating adhesion and spot weldability, the method comprising:

coating a base steel sheet with Ni in a coating amount (C_Ni) of 0.1-1.0 g/m²;

heating the Ni-coated steel sheet in a reducing atmosphere;

cooling the heated steel sheet to the temperature (X_S) at which the steel sheet is fed into a galvanizing bath; and

feeding and immersing the cooled steel sheet in the galvanizing bath having an effective Al concentration (C_Al) of 0.11-0.14 wt % and a temperature (T_p) of 440460° C., wherein the temperature (X_S) at which the steel sheet is fed into the galvanizing bath satisfies the following relationship: C_Ni·(X_S−T_P)/2C_Al=5-100.

It also discloses a hot-dip galvanized steel sheet wherein the alloy phase is a Fe—Zn alloy phase accounting for 1-20% of the cross-sectional area of the galvanized layer.

SUMMARY OF THE INVENTION

However, in the above method, galvanizing was carried out in a bath comprising from 0.11 to 0.14 wt. % Al bath and thus inhibition layer was very week and Fe—Zn intermetallic phases formed. In industrial scale, this method is difficult to apply since the spot weldability depends on controlling parameters, including the amount of Ni coated, the Al concentration of the galvanizing bath, and the difference between the temperature of the galvanizing bath and the temperature at which the steel sheet is fed into the galvanizing bath. Moreover, the spot weldability performed is evaluated based on the electrode life, i.e. the number of continuous welding spots at the time when the nugget diameter reached 4√t (t: steel sheet thickness) was measured. There is no mention of a reduction of the presence of cracks in coated steel sheet after the spot welding.

It is an object of the present invention to provide a steel sheet coated with a metallic coating which does not have LME issues. It aims to make available, in particular, an easy to implement method in order to obtain a part which does not have LME issues after the forming and/or the welding.

The present invention provides a method for the manufacture of a coated steel sheet comprising 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
    • Nb≤0.5%,
    • B≤0.005%,
    • Cr≤1.0%,
    • Mo≤0.50%,
    • Ni≤1.0%,
    • Ti≤0.5%,
  • 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 comprising nickel, such first coating not comprising iron, having a thickness equal or above 0.5 μm 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.

BRIEF DESCRIPTION OF THE DRAWING

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

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

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.

The invention relates to method for the manufacture of a coated steel sheet comprising 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
    • Nb≤0.5%,
    • B≤0.005%,
    • Cr≤1.0%,
    • Mo≤0.50%,
    • Ni≤1.0%,
    • Ti≤0.5%,
  • 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 comprising nickel, such first coating not comprising iron, having a thickness equal or above 0.5 μm and
  • C. the coating of the steel sheet obtained in step B) with a second coating based on zinc.

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.

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.

Advantageously, the thermal treatment is performed in an atmosphere comprising from 1 to 30% of H₂ at a dew point between −10 and −60° C. For example, the atmosphere comprises from 1 to 10% of H₂ at a dew point between −10° C. and −60° C.

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.

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.

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%.

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

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.

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

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.

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.

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 μm, 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.

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.

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

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

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.

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

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

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.

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.

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 μm and wherein the longest crack has a length below 300 μm. 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.

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.

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

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

Example

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. %.

Trials 1 to 4 were prepared by performing an annealing in a continuous annealing in an atmosphere comprising 5% of H₂ and 95% of N₂ 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.

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

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

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

TABLE 2
LME crack details after spot welding (2 layer stack-up condition)
					Number
					of cracks
		Thick-		Thick-	(>100 μm)	Maximum
	1st	ness	2nd	ness	per spot	crack
Trials	coating	(μm)	coating	(μm)	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.

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.

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

TABLE 3
LME crack details after spot welding (3 layer stack-up condition)
	Number of	Maximum
	cracks (>100 μm)	crack length
Trials	per spot weld	(μm)
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.

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

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 Nb≤0.5%,B≤0.005%,Cr≤1.0%,Mo≤0.50%,Ni≤1.0%,Ti≤0.5%,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 μm; andcoating, 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 H2 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 μm.

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

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

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 μm, 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 μm and wherein the longest crack has a length below 300 μm.

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 μm; 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 μm and wherein the longest crack has a length below 300 μm.

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.