JP7394921B2 - Manufacturing method of coated steel plate - Google Patents

Description

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

Zinc-based coatings are commonly used because they can prevent corrosion through barrier and cathodic protection. A barrier effect can be achieved by applying a metal coating to the surface of the steel. The metal coating therefore prevents contact of the steel with corrosive atmospheres. The barrier effect is independent of the coating and substrate properties. In contrast, sacrificial cathodic protection is based on the fact that zinc is a metal with a higher tendency to ionize than steel. Therefore, when corrosion occurs, the zinc is consumed before the steel. Cathodic protection is essential in areas where the steel is directly exposed to a corrosive atmosphere, such as at cut edges where the surrounding zinc is consumed before the steel.

However, when heating processes such as hot press hardening or welding are performed on such zinc-coated steel sheets, cracks propagating from the steel/coating interface are observed in the steel. In fact, sometimes the mechanical properties of the metal deteriorate due to cracks in the coated steel plate that exist after the above operations. These cracks are observed under the following conditions: high temperature, contact with low melting point liquid metals (such as zinc) in the presence of tensile stress, non-uniformity of the molten metal in the grains and grain boundaries of the substrate. It is a diffusion. Such a phenomenon is known under the name liquid metal embrittlement (LME), and is also called liquid metal assisted cracking (LMAC).

US2012100391 discloses a method for producing hot-dip galvanized steel sheets with good plating quality, plating adhesion and spot weldability, and this method includes:
- coating the base steel plate with Ni (C _Ni ) in a coating amount of 0.1-1.0 g/m ² ,
- a step of heating the Ni-coated steel plate in a reducing atmosphere;
- cooling the heated steel plate to the temperature (X _s ) at which the steel plate is introduced into the galvanizing bath, and - the effective Al concentration (C _Al ) is 0.11 to 0.14 wt%, and 440 to 460 This is a step in which a cooled steel sheet is placed in a galvanizing bath having a temperature (T _p ) of ℃ and immersed in it, and the temperature (X _s ) at which the steel sheet is placed in the galvanizing bath is C _Ni (X _s - The process satisfies the relationship: T _p )/2C _Al =5-100.

It further discloses a hot-dip galvanized steel sheet in which the alloy phase is a Fe--Zn alloy phase occupying 1-20% of the cross-sectional area of the galvanized layer.

US Patent Application Publication No. 2012/0100391

However, in the above method, the zinc plating was carried out in a bath containing 0.11-0.14 wt% Al, so the suppression layer was very weak and an Fe-Zn intermetallic phase was formed. Spot weldability depends on control parameters including the amount of Ni in the coating, the Al concentration in the galvanizing bath, and the difference between the temperature of the galvanizing bath and the temperature at which the steel sheet is introduced into the galvanizing bath, so it is difficult to At scale, this method is difficult to apply. Furthermore, the spot weldability performed is evaluated based on the electrode life, that is, the number of continuous welding spots measured when the nugget diameter reaches 4Vt (t: steel plate thickness). There is no mention of a reduction in the presence of cracks in coated steel sheets after spot welding.

It is therefore an object of the invention to provide a steel plate coated with a metal coating that is free from LME problems. It aims at providing a particularly easy-to-implement method for obtaining LME-free parts after forming and/or welding.

This object is achieved by providing a method according to claim 1. The method may also include the features of any of claims 2-18.

Another object is achieved by providing a steel plate according to claim 19. This steel plate can also include the features of any of claims 20-25.

Another object is achieved by providing a spot weld joint according to claim 26. This spot-welded joint may also include the features of claims 27-29.

Finally, another object is achieved by providing the use of a steel plate or assembly according to claim 30.

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

The designation "steel" or "steel sheet" means a sheet, coil, plate of steel having a composition that allows the part to achieve a tensile strength of up to 2500 MPa, more preferably up to 2000 MPa. For example, the tensile strength is 500 MPa or more, preferably 980 MPa or more, advantageously 1180 MPa or more, even 1470 MPa or more.

The present invention relates to a method for manufacturing a coated steel sheet, which includes the following steps.

A. providing a pre-coated steel sheet coated with a first coating comprising iron and nickel;
B. A step of heat treating such a pre-painted steel plate at a temperature of 600 to 1000°C,
C. Coating the steel plate obtained in step B) with a second coating based on zinc.

Without wishing to be bound by any theory, it is an essential feature of the invention that a first coating of iron and nickel is deposited on the steel plate prior to heat treatment. This is because during the heat treatment, on the one hand, Ni diffuses towards the steel sheet, resulting in a Fe--Ni alloy layer. On the other hand, some amount of Ni is still present at the interface between the steel and the coating interface, preventing liquid zinc from penetrating into the steel during any heating steps, such as welding. Thus, by applying the method according to the invention it is possible to obtain a barrier layer against LME.

The first coating comprising iron and nickel is deposited by any deposition method known to those skilled in the art. It can be deposited by vacuum evaporation or electroplating. Preferably it is deposited by electroplating.

Preferably, in step A), the first coating comprises 10 to 75% by weight of iron, more preferably 25 to 65% by weight, advantageously 40 to 60% by weight.

Preferably, in step A) the first coating comprises 25 to 90% by weight of nickel, preferably 35 to 75% by weight, advantageously 40 to 60% by weight.

In a preferred embodiment, in step A) the first coating consists of iron and nickel.

Preferably, in step A) the first coating has a thickness of 0.5 μm or more. More preferably, the first coating has a thickness of 0.8 to 5.0 μm, advantageously 1.0 to 2.0 μm.

Preferably, in step A), the composition of the steel plate is 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, e.g. Nb≦0.5%,
B≦0.005%,
Cr≦1.0%,
Mo≦0.50%,
Ni≦1.0%,
Ti≦0.5%,
including by weight,
The remaining components consist of iron and unavoidable impurities resulting from processing.

Preferably, in step B), the heat treatment is continuous annealing. For example, continuous annealing includes heating, soaking, and cooling steps. It may further include a preheating step.

Advantageously, the heat treatment is carried out in an atmosphere containing 1 to 30% H ₂ with a dew point of -10 to -60°C. For example, the atmosphere contains 1-10% H ₂ with a dew point of -40°C to -60°C.

Advantageously, in step C) the second layer comprises more than 50% zinc, more preferably more than 75% zinc, advantageously more than 90% zinc. The second layer can be deposited by any deposition method known to those skilled in the art. It is possible by hot-dip coating, vacuum deposition or electrogalvanizing.

For example, a zinc-based coating contains 0.01-8.0% Al, optionally 0.2-8.0% Mg, and the balance is Zn.

Preferably, the zinc-based coating is deposited by hot dip galvanizing. In this embodiment, the molten bath may also contain unavoidable impurities and residual elements from the input of the ingot or from the passage of the steel plate through the molten bath. For example, optionally the impurities are selected from Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, and the weight content of each additional element is less than 0.3% by weight. be. The residual element due to the charging of the ingot or the passage of the steel plate through the molten bath may have a content of up to 5.0% by weight, preferably up to 3.0% by weight of iron.

In a preferred embodiment, the second layer consists of zinc. If the coating is deposited by hot-dip galvanizing, Al is included in the bath at a percentage of 0.15-0.40 wt%. Furthermore, the iron present in the first coating reacts with the aluminum to form a restraining layer Fe ₂ Al ₅ , resulting in a reactive wetting behavior during hot-dip galvanizing.

The method according to the invention provides a steel plate coated with a diffusion alloy layer containing iron and nickel, which layer is directly covered by a zinc-based layer. It is believed that the diffusion alloy layer acts like a barrier layer to LME and improves coating adhesion.

Preferably, the steel sheet has a microstructure comprising 1 to 50% retained austenite, 1 to 60% martensite, and optionally at least one component selected from bainite, ferrite, cementite and pearlite. In this case, the martensite may or may not be tempered.

In a preferred embodiment, the steel sheet has a microstructure comprising 5-25% retained austenite.

Preferably, the steel sheet has a microstructure comprising 1-60%, more preferably 10-60% tempered martensite.

Advantageously, the steel sheet has a microstructure comprising 10-40% bainite, such bainite comprising 10-20% lower bainite, 0-15% upper bainite and 0-5% non-carbide. Contains bainite.

Preferably, the steel sheet has a microstructure containing 1 to 25% ferrite.

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

After manufacturing the steel sheets, it is known to weld and assemble two metal sheets in order to manufacture some parts of a vehicle. Thus, a spot weld joint is formed during welding of at least two metal plates, said spot connecting between the at least two metal plates.

To produce spot-welded joints according to the invention, welding is carried out with an effective current of 3 kA to 15 kA, the pressure applied to the electrodes is 150 to 850 daN, and the diameter of the active surface of said electrodes is 4 to 10 mm.

Thus, a spot welded joint of at least two metal plates comprising a coated steel plate according to the invention is obtained, such a joint contains less than three cracks with a size of more than 100 μm, and the length of the longest crack is less than 500 μm.

Preferably, the second metal plate is a steel plate or an aluminum plate. More preferably, the second metal plate is a steel plate according to the invention.

In another embodiment, the spot weld joint includes a third metal plate that is a steel plate or an aluminum plate. For example, the third metal plate is a steel plate according to the present invention.

The steel plate or spot welded joint of the present invention can be used in the manufacture of parts for motor vehicles.

The present invention will now be described for informational purposes only in tests conducted. They are not limiting.

For all samples, the weight percentages of the steel plate components used were: C=0.37%, Mn=1.9wt%, Si=1.9wt%, Cr=0.35wt%, Al=0.05wt% and Mo=0.1wt%.

Prototypes 1 and 2 were prepared by depositing a first coating containing 45% Fe and the remainder Ni. Continuous annealing was then carried out in an atmosphere containing 5% H ₂ and 95% N ₂ at a dew point of −45° C. The precoated steel plate was heated at a temperature of 900°C. Finally, a zinc coating was deposited by hot dip galvanizing with a zinc bath containing 0.2% Al. The bath temperature was 460°C.

For comparison, Prototype 3 was prepared by depositing a zinc coating by electrogalvanizing after continuous annealing of the above steel sheet.

The resistance of prototypes 1 to 3 to LME was evaluated. For this purpose, two coated steel plates were welded together by resistance spot welding for each prototype. The electrode type was ISO type B with a diameter of 16 mm, the electrode pressure was 5 kN, and the water flow rate was 1.5 g/min. The welding cycles are shown in Table 1.

表２に報告されるように、１００μｍを超えるクラックの数を、光学顕微鏡及びＳＥＭ（走査型電子顕微鏡法）を使用して評価した。

The number of cracks larger than 100 μm was evaluated using optical microscopy and SEM (scanning electron microscopy), as reported in Table 2.

本発明による試作は、試作３と比較してＬＭＥに対する優れた耐性を示す。

The prototype according to the invention shows superior resistance to LME compared to prototype 3.

Next, for each prototype, three coated steel plates were welded in a three-layer configuration by resistance spot welding. The number of cracks larger than 100 μm was then evaluated using optical microscopy and SEM (scanning electron microscopy) as reported in Table 3.

本発明による試作は、試作３と比較して、ＬＭＥに対する優れた耐性を示す。

The prototype according to the invention shows superior resistance to LME compared to prototype 3.

Finally, prototypes 1 and 2 were bent at an angle of 90°. Next, adhesive tape was applied and peeled off to check the adhesion of the coating to the base steel. The coating adhesion of these prototypes was excellent.

Claims (29)

A method for manufacturing a coated steel sheet, comprising the following steps: A. providing a pre-coated steel sheet coated with a first coating comprising iron and nickel;
B. A step of heat treating such a pre-painted steel plate at a temperature of 600 to 1000°C,
C. coating the steel plate obtained in step B) with a second coating based on zinc;
A method comprising:
In step A), the composition of the steel plate is
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 one or more elements on an optional basis,
Nb≦0.5%,
B≦0.005%,
Cr≦1.0%,
Mo≦0.50%,
Ni≦1.0%,
Ti≦0.5%,
including;
A method in which the remaining components consist of iron and unavoidable impurities resulting from processing. The method of claim 1, wherein in step A) the first coating comprises 10% to 75% by weight iron. 3. The method of claim 2, wherein in step A) the first coating comprises 25-65% by weight iron. A method according to any one of claims 1 to 3, wherein in step A) the first coating comprises 40 to 60% by weight iron. A method according to any one of claims 1 to 4, wherein in step A) the first coating comprises from 25 to 90% by weight of nickel , provided that the sum of iron and nickel does not exceed 100% by weight. . 6. The method of claim 5, wherein in step A) the first coating comprises 35 to 75% by weight of nickel , provided that the sum of iron and nickel does not exceed 100% by weight . 7. The method of claim 6 , wherein in step A) the first coating comprises 40-60% by weight of nickel, provided that the sum of iron and nickel does not exceed 100% by weight . Method according to any one of the preceding claims, wherein in step A) the first coating consists of iron and nickel. A method according to any one of claims 1 to 8, wherein in step A) the first coating has a thickness of 0.5 μm or more. 10. The method of claim 9, wherein in step A) the first coating has a thickness of 0.8 to 5.0 μm. 11. The method of claim 10, wherein in step A) the first coating has a thickness of 1.0 to 2.0 μm. A method according to any one of claims 1 to 11, wherein in step C) the second layer comprises more than 50% zinc. 13. The method of claim 12, wherein in step C) the second layer comprises greater than 75% zinc. 14. The method of claim 13, wherein in step C) the second layer comprises greater than 90% zinc. 15. The method of claim 14, wherein in step C) the second layer consists of zinc. The method according to any one of claims 1 to 15, wherein in step B), the heat treatment is continuous annealing. The method according to any one of claims 1 to 16, wherein in step B) the heat treatment is carried out in an atmosphere containing 1 to 30% H ₂ at a dew point of -10 to -60°C. 18. A method according to any one of claims 1 to 17, wherein the coated steel sheet is coated with a diffusion alloy layer comprising iron and nickel, such layer being directly covered by a zinc-based layer. 19. The steel microstructure of the coated steel sheet comprises 1 to 50% retained austenite, 1 to 60% martensite, and optionally at least one component selected from bainite, ferrite, cementite and pearlite. Method described. 20. The method of claim 19, wherein the microstructure comprises 5-25% retained austenite. 21. A method according to claim 19 or 20, wherein the microstructure comprises 1 to 60% tempered martensite. 22. A method according to any one of claims 19 to 21, wherein the microstructure comprises 10 to 40% bainite. 23. A method according to any one of claims 19 to 22, wherein the microstructure comprises 1 to 25% ferrite. 24. A method according to any one of claims 19 to 23, wherein the microstructure comprises 1 to 15% untempered martensite. A method for manufacturing a spot welded joint of at least two metal plates, wherein at least one of the at least two metal plates is manufactured by the method according to any one of claims 1 to 24. , the number of cracks in the joint having a size of more than 100 μm is less than 3, and the length of the longest crack is less than 500 μm. 26. The method according to claim 25, wherein the second metal plate is a steel plate or an aluminum plate. 27. The method according to claim 26, wherein the second metal sheet is a steel sheet obtained from the method according to any one of claims 1 to 24. 28. A method according to any one of claims 25 to 27, wherein the spot weld joint comprises a third metal plate, which is a steel plate or an aluminum plate. Manufacture of automotive parts of coated steel sheets manufactured by the method according to any one of claims 1 to 24 or spot welded joints manufactured by the method according to any one of claims 25 to 28. Use for.