Press hardening method

Abstract

A press hardening method including the following steps: A. the provision of a steel sheet for heat treatment being optionally coated with a zinc- or aluminum-based pre-coating, B. the flexible rolling of the steel sheet in the rolling direction so as to obtain a steel sheet having a variable thickness, C. the cutting of the rolled steel sheet to obtain a tailored rolled blank, D. the deposition of a hydrogen barrier pre-coating over a thickness from 10 to 550 nm, E. the heat treatment of the tailored rolled blank to obtain a fully austenitic microstructure in the steel, F. the transfer of the tailored rolled blank into a press tool, G. the hot-forming of the tailored rolled blank to obtain a part having a variable thickness,H. the cooling of the part having a variable thickness obtained at step G).

Claims

1-15. (canceled).

16: A press hardening method comprising the following steps: A. providing a steel sheet for heat treatment, the steel sheet being optionally coated with a zinc- or aluminum-based pre-coating; B. flexible rolling the steel sheet in a rolling direction so as to obtain a steel sheet having a variable thickness; C. cutting the rolled steel sheet to obtain a tailored rolled blank; D. depositing a hydrogen barrier pre-coating with a thickness from 10 to 550 nm; E. heat treating the tailored rolled blank to obtain a fully austenitic microstructure in the steel sheet; F. transferring the tailored rolled blank into a press tool; G. hot-formingthe tailored rolled blank to obtain a part having a variable thickness; H. cooling of the part obtained in step G) to obtain a microstructure in steel being martensitic or martensito-bainitic or made of at least 75% in terms of volume fraction of equiaxed ferrite, from 5 to 20% in volume of martensite and of bainite in amount less than or equal to 10% in volume.

17: The press hardening method as recited in claim 16 wherein in step A), the optional zinc- or aluminum-based pre-coating is present and based on aluminum and comprises less than 15% Si and less than 5.0% Fe, optionally 0.1 to 8.0% Mg and optionally 0.1 to 30.0% Zn, a remainder being Al.

18: The press hardening method as recited in claim 16 wherein in step A), the optional zinc- or aluminum-based pre-coating is present and based on zinc and comprises less than 6.0% Al and less than 6.0% of Mg, a remainder being Zn.

19: The press hardening method as recited in claim 16 wherein in step B) the flexible rolling is a hot-rolling or a cold-rolling step.

20: The press hardening method as recited in claim 16 wherein in step D), the hydrogen barrier pre-coating comprises at least one element chosen from the group consisting of: nickel, chromium, magnesium, aluminum and yttrium.

21: The press hardening method as recited in claim 20 wherein in step D), the hydrogen barrier pre-coating consists of nickel and chromium or nickel and aluminum or chromium or magnesium or nickel, aluminum and yttrium.

22: The press hardening method as recited in claim 16 wherein the hydrogen barrier pre-coating of step D) is deposited by physical vapor deposition, by electro-galvanization or roll-coating.

23: The press hardening method as recited in claim 16 wherein in step E), an atmosphere is inert or has an oxidizing power equal or higher than that of an atmosphere consisting of 1% by volume of oxygen and equal or smaller than that of an atmosphere consisting of 50% by volume of oxygen.

24: The press hardening method as recited in claim 23 wherein in step E), the atmosphere has a dew point from −30 to +30° C.

25: The press hardening method as recited in claim 16 wherein in step E), the thermal treatment is performed at a temperature between 800 and 970° C.

26: The press hardening method as recited in claim 16 wherein in step E), the thermal treatment is performed for a dwell time of 1 to 12 minutes.

27: The press hardening method as recited in claim 16 wherein in step G), the hot-forming of the blank is performed at a temperature between 600 and 830° C.

28. A part obtainable from the method as recited in claim 16 comprising a steel sheet with variable thickness, a hydrogen barrier pre-coating with uniform thickness and being alloyed by diffusion of iron from the steel sheet, and topped by an oxide layer containing oxides of iron from the steel sheet and of other elements from the hydrogen barrier pre-coating, such oxide layer having a thickness of 1 μm or less.

29. A part obtainable from the method as recited in claim 16 comprising a steel sheet precoated with a zinc-based pre-coating, both having a variable thickness, a hydrogen barrier pre-coating with uniform thickness and being alloyed by diffusion of iron from the steel sheet and diffusion of zinc and other elements from the zinc-based pre-coating, and topped by an oxide layer containing oxides of iron from the steel sheet and oxides of zinc and of other elements from the pre-coatings, such oxide layer having a thickness of 1.5 μm or less.

30: A part obtainable from the method as recited in claim 16 comprising a steel sheet pre-coated with an aluminum-based pre-coating, both having a variable thickness, a hydrogen barrier pre-coating with uniform thickness and being alloyed by diffusion of iron from the steel sheet and diffusion of aluminum and of other elements from the aluminum-based pre-coating, and topped by an oxide layer containing oxides of iron from the steel sheet and oxides of aluminum and of other elements from the pre-coatings, such oxide layer having a thickness of 1.5 μm or less.

Description

EXAMPLES

[0057] For all samples, steel sheets used are 22MnB5. The composition of the steel is as follows: C=0.2252% ; Mn=1.1735% ; P=0.0126%, S=0.0009% ; N=0.0037% ; Si=0.2534% ; Cu=0.0187% ; Ni=0.0197% ; Cr=0.180% ; Sn=0.004% ; Al=0.0371% ; Nb=0.008% ; Ti=0.0382% ; B=0.0028% ; Mo=0.0017% ; As=0.0023% et V=0.0284%.

[0058] Some steel sheets are coated with a 1.sup.st pre-coating called hereinafter “AluSi®”. This pre-coating comprises 9% by weight of Silicon, 3% by weight of iron, the balance being aluminum. It is deposited by hot-dip galvanization.

[0059] Some steel sheets are precoated with a 2.sup.nd pre-coating deposited by magnetron sputtering.

Example 1: hydrogen test:

[0060] This test is used to determine the quantity of hydrogen adsorbed during the austenitization heat treatment of a press hardening method.

[0061] Trial 1 is a steel sheet coated with a 1.sup.st pre-coating being AluSi® (25 μm). A flexible rolling was performed on Trial 1. Then, Trial 1 was cut to obtain a tailored rolled blank.

[0062] Trial 2 is a steel sheet precoated with a 1.sup.st coating being AluSi® (25 μm) and a 2.sup.nd pre-coating comprising 80% of Ni and 20% of Cr. Then, a flexible rolling was performed on Trial 2. Trial 2 was cut to obtain a tailored rolled blank. In this case, the hydrogen barrier pre-coating was deposited before the flexible rolling.

[0063] Trial 3 is a steel sheet precoated with a 1.sup.st pre-coating being AluSi® (25 μm) and a 2.sup.nd pre-coating comprising 80% of Ni and 20% of Cr. Trial 3 was firstly coated with the 1.sup.st pre-coating of AluSi®. Then, a flexible rolling with a rolling of 50% was performed followed by the cutting to obtain a tailored rolled blank. After, the 2.sup.nd pre-coating comprising 80% of Ni and 20% of Cr was deposited on Trial 3. In this case, the hydrogen barrier pre-coating was deposited after the flexible rolling.

[0064] After, all Trials were heated at a temperature of 900° C. during a dwell time varying between 5 and 10 minutes. The atmosphere during the heat treatment was air. Blanks were transferred into a press tool and hot-stamped in order to obtain parts having a variable thickness. Then, parts were cooled by dipping trials into warm water to obtain a hardening by martensitic transformation.

[0065] Finally, the hydrogen amount absorbed by the steel sheet during the heat treatment was measured by thermic desorption using a Thermal Desorption Analyzer or TDA. To this end, each trial was placed in a quartz room and heated slowly in an infra-red furnace under a nitrogen flow. The released mixture hydrogen/nitrogen was picked up by a leak detector and the hydrogen concentration was measured by a mass spectrometer.

[0066] Results are shown in the following Table 1:

TABLE-US-00001 H.sub.2 Thickness amount 2.sup.nd Deposition 2.sup.nd pre- (ppm pre- of 2.sup.nd pre- Ratio coating by Trials Atmosphere coating coating Ni/Cr (nm) mass) 1 air — — — 200 1.1  2 air Ni/Cr Before 4 200 1.05 80/20 flexible rolling  3* air Ni/Cr After 4 200 0.15 80/20 flexible rolling *example according to the invention.

[0067] Trial 3 according to the present invention releases a significantly lower amount of hydrogen.

[0068] After heat treatment and hot forming, the surface of trial 3 has been analyzed. It comprises following oxides on the surface: Cr.sub.2O.sub.3, NiO, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4 and Al.sub.2O.sub.3.

[0069] From the steel sheet to the external surface, the part of trial 3 comprises the following layers: [0070] an inter-diffusion layer comprising iron from the steel sheet, aluminum, silicon and other elements, having a thickness from 10 to 15 μm, [0071] an alloyed layer containing aluminum, silicon and iron from the steel sheet in a lesser amount than the layer below and other elements, having a thickness from 20 to 35 μm, [0072] a thin layer containing less iron and more oxides than the layers below, having a thickness from 100 to 300 nm,

[0073] a thinner layer containing the highest amount of oxides compared to the layers below, especially Ni, Cr and Al oxides, and located directly below the surface, having a thickness from 50 to 150 nm.