A press hardening method

Abstract

press hardening method includes the following steps: A. the provision of a steel sheet for heat-treatment, precoated with a zinc- or aluminum-based pre-coating, B. the deposition of a hydrogen barrier pre-coating over a thickness from 10 to 550 nm, and comprising at least one element chosen from among: nickel, chromium, magnesium, aluminum and yttrium, C. batch annealing of the precoated steel sheet to obtain a pre-alloyed steel sheet, the cooling after the batch annealing being performed at a speed of 29.0° C.h.sup.−1 or less, D. the cutting of the pre-alloyed steel sheet to obtain blank, E. thermal treatment of the blank to obtain a fully austenitic microstructure in the steel, F. the transfer of the blank into a press tool, G. the hot-forming of the blank to obtain a part, H. the cooling of the part 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, precoated with a zinc- or aluminum-based pre-coating; B. depositing a hydrogen barrier pre-coating with a thickness from 10 to 550 nm; C. batch annealing the precoated steel sheet to obtain a pre-alloyed steel sheet, cooling after the batch annealing being performed at a speed of 29.0° C.h.sup.−1 or less; D. cutting the pre-alloyed steel sheet to obtain a blank; E. thermally treating the blank to obtain a fully austenitic microstructure in the steel sheet; F. transferring the blank into a press tool; G. hot-forming the blank to obtain a part; H. cooling the part obtained in step G) in order to obtain a microstructure in the steel sheet, the microstructure being martensitic or being martensito-bainitic or being 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 B), the hydrogen barrier pre-coating comprises at least one element chosen from the group consisting of: nickel, chromium, magnesium, aluminum and yttrium.

18. The press hardening method as recited in claim 16 wherein in step B), the hydrogen barrier pre-coating consists of nickel and chromium; or nickel and aluminum; or chromium; or nickel and aluminum and yttrium; or magnesium.

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

20. The press hardening method as recited in claim 16 wherein in step A), the zinc- or aluminum-based pre-coating is 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.

21. The press hardening method as recited in claim 16 wherein in step C), the batch annealing is performed at a temperature between 450 and 750° C.

22. The press hardening method as recited in claim 16 wherein in step C), the atmosphere of the batch annealing is air or inert.

23. The press hardening method as recited in claim 16 wherein in step B), the heating rate of the batch annealing is above or equal to 5000° C.h.sup.−1.

24. The press hardening method as recited in claim 16 wherein in step C), the cooling speed is below or equal to 27° C.h.sup.−1.

25. The press hardening method as recited in claim 16 wherein in step C), the batch annealing is performed for 1 to 100 hours.

26. The press hardening method as recited in claim 16 wherein, in steps C and E) independently from each other, the 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.

27. The press hardening method as recited in claim 16 wherein in step E), the atmosphere has a dew point between −30 and +30° C.

28. 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.

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

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

Description

DETAILED DESCRIPTION

[0027] Indeed, without willing to be bound by any theory, the inventors have surprisingly found that when the steel sheet is precoated with a hydrogen barrier pre-coating and when the cooling after the batch annealing is performed at a speed of 29.0° C.h.sup.−1 or less, the absorption of hydrogen into the steel sheet is reduced. Indeed, it is believed that thanks to the hydrogen barrier pre-coating, thermodynamically stable oxides are formed on the surface of the hydrogen barrier pre-coating with a low kinetic of hydrogen diffusion. These thermodynamically stable oxides reduce H.sub.2 absorption. Moreover, it seems that since the atmosphere is oxidizing, the zinc- or aluminum-based pre-coating for anti-corrosion purpose forms also external oxides leading to a thicker oxides layer on the surface of the precoated steel sheet. Finally, it seems that a slow cooling of the batch annealing allows release of the small amounts of hydrogen that could anyway be absorbed during the batch annealing.

[0028] In step A), the steel sheet used is made of steel for heat treatment as described in the European Standard EN 10083. It can have a tensile resistance superior to 500 MPa, advantageously between 500 and 2000 MPa before or after heat-treatment.

[0029] The weight composition of steel sheet is preferably as follows: 0.03%≤C≤0.50% 0.3%≤Mn≤3.0% 0.05%≤Si≤0.8% 0.015%—Ti≤0.2% 0.005%≤Al≤0.1% 0%≤Cr≤2.50% 0%≤S≤0.05% 0%≤P≤0.1%; 0%≤B≤0.010%; 0%≤Ni≤2.5% 0%≤Mo≤0.7% 0%≤Nb≤0.15%; 0%—N≤0.015% 0%≤Cu≤0.15% 0%≤Ca≤0.01% 0%≤W≤0.35%, the balance being iron and unavoidable impurities from the manufacture of steel.

[0030] For example, the steel sheet is 22MnB5 with the following composition: 0.20%≤C≤0.25%; 0.15%≤Si≤0.35%; 1.10%≤Mn≤1.40%; 0%≤Cr≤0.30%; 0%≤Mo≤0.35%; 0%≤P≤0.025%; 0%≤S≤0.005%; 0.020%≤Ti≤0.060%; 0.020%≤Al≤0.060%; 0.002%≤B≤0.004%, the balance being iron and unavoidable impurities from the manufacture of steel.

[0031] The steel sheet can be Usibor®2000 with the following composition: 0.24%≤C≤0.38%; 0.40%≤Mn≤3%; 0.10%≤Si≤0.70%; 0.015%≤Al≤0.070%; 0%≤Cr≤2%; 0.25%≤Ni≤2%; 0.020%≤Ti≤0.10%; 0%≤Nb≤0.060%; 0.0005%≤B≤0.0040%; 0.003%≤N≤0.010%; 0.0001%≤S≤0.005%; 0.0001%≤P≤0.025%; it being understood that the contents of titanium and nitrogen satisfy Ti/N>3.42; and that the contents of carbon, manganese, chromium and silicon satisfy:

[00001]$2,6C+\frac{\mathrm{Mn}}{5,3}+\frac{\mathrm{Cr}}{13}+\frac{\mathrm{Si}}{15}\ge 1,1\%$

[0032] the composition optionally comprising one or more of the following: 0.05%≤Mo≤0.65%; 0.001%≤W≤0.30%; 0.0005%≤Ca≤0.005%, the balance being iron and unavoidable impurities from the manufacture of steel.

[0033] For example, the Steel sheet is Ductibor®500 with the following composition: 0.040%≤C≤0.100%; 0.80%≤Mn≤2.00%; 0%≤Si≤0.30%; 0%≤S≤0.005%; 0%≤P≤0.030%; 0.010%≤Al≤0.070%; 0.015%≤Nb≤0.100%; 0.030%≤Ti≤0.080%; 0%≤N≤0.009%; 0%≤Cu≤0.100%; 0%≤Ni≤0.100%; 0%≤Cr≤0.100%; 0%≤Mo≤0.100%; 0%≤Ca≤0.006%, the balance being iron and unavoidable impurities from the manufacture of steel.

[0034] Steel sheet can be obtained by hot rolling and optionally cold rolling depending on the desired thickness, which can be for example between 0.7 and 3.0 mm.

[0035] Optionally, in step A), the hydrogen barrier pre-coating comprises optional elements 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.

[0036] Preferably in step A), the hydrogen barrier pre-coating comprises at least one element chosen from among: nickel, chromium, aluminum, magnesium and yttrium.

[0037] Preferably in step A), the hydrogen barrier pre-coating consists of nickel and chromium, i.e. the hydrogen barrier pre-coating comprises nickel, chromium and unavoidable impurities. Advantageously, the weight ratio Ni/Cr is between 1.5 and 9. Indeed, without willing to be bound by any theory, it is believed that this specific ratio further decreases the hydrogen absorption during the austenitization treatment.

[0038] In another preferred embodiment, the hydrogen barrier pre-coating consists of nickel and aluminum, i.e. the hydrogen barrier pre-coating comprises Ni, Al and unavoidable impurities.

[0039] In another preferred embodiment, the hydrogen barrier pre-coating consists of chromium at 50% or 75% or 90% by weight. More preferably it consists of chromium, i.e. the hydrogen barrier pre-coating comprises only Cr and unavoidable impurities.

[0040] In another preferred embodiment, the hydrogen barrier pre-coating consists of nickel, aluminum and yttrium, i.e. the hydrogen barrier pre-coating comprises Ni, Al and Y and unavoidable impurities.

[0041] In another preferred embodiment, the hydrogen barrier pre-coating consists of magnesium at 50% or 75% or 90% by weight. More preferably it consists of magnesium, i.e. the hydrogen barrier pre-coating comprises only Mg and unavoidable impurities.

[0042] Preferably, in step A), the hydrogen barrier pre-coating has a thickness between 10 and 550 nm and more preferably between 10 and 90 or between 150 and 250 nm. For example, the thickness of the hydrogen barrier pre-coating is of 50, 200 or 400 nm.

[0043] Without willing to be bound by any theory, it seems that when the hydrogen barrier pre-coating is below 10 nm, there is a risk that hydrogen absorbs into steel because the hydrogen barrier pre-coating does not cover enough the steel sheet. When the hydrogen barrier pre-coating is above 550 nm, it seems that there is a risk that the hydrogen barrier pre-coating becomes more brittle and that the hydrogen absorption begins due to the hydrogen barrier pre-coating brittleness.

[0044] In a preferred embodiment, the zinc- or aluminum-based pre-coating for anti-corrosion purpose is based on aluminum and comprises less than 15% Si, less than 5.0% Fe, optionally 0.1 to 8.0% Mg and optionally 0.1 to 30.0% Zn, the remainder being Al. For example, the zinc- or aluminum-based pre-coating is AluSi®.

[0045] In another preferred embodiment, the zinc- or aluminum-based pre-coating for anti-corrosion purpose is based on zinc and comprises less than 6.0% Al, less than 6.0% of Mg, the remainder being Zn. For example, the zinc- or aluminum-based pre-coating is a zinc coating so to obtain the following product: Usibor® GI.

[0046] The zinc- or aluminum-based pre-coating for anti-corrosion purpose can also comprise impurities and residual elements such iron with a content up to 5.0%, preferably 3.0%, by weight.

[0047] Preferably, the pre-coatings of step A) are deposited by physical vapor deposition, by electro-galvanization, hot-dip galvanization or roll-coating. Preferably, the hydrogen barrier pre-coating is deposited by electron beam induced deposition or roll coating. Preferably, the zinc- or aluminum-based pre-coating is deposited by hot-dip galvanization.

[0048] Optionally, after the deposition of the pre-coatings, a skin-pass can be realized and allows work hardening the coated steel sheet and giving it a roughness facilitating the subsequent shaping. A degreasing and a surface treatment can be applied in order to improve for example adhesive bonding or corrosion resistance.

[0049] Preferably, in step C), the batch annealing is performed at a temperature between 450 and 750° C., preferably between 550 and 750° C.

[0050] Preferably, in step C), the atmosphere of the batch annealing is air or inert.

[0051] Advantageously, in step C), the heating rate of the batch annealing is above or equal to 5000° C.h.sup.−1, more preferably between 10000 and 15000° C.h.sup.−1 or between 20000 and 35000° C.h.sup.−1.

[0052] In step C), the cooling speed is below or equal to 29.0° C.h.sup.−1, more preferably below or equal to 27° C.h.sup.−1 and advantageously between 15 and 27° C.h.sup.−1. Indeed, without willing to be bound by any theory, it is believed that above 29.0° C.h.sup.−1, the cooling is not enough slow to let the hydrogen be released.

[0053] Preferably, in step C), the batch annealing is performed during 1 to 100 hours.

[0054] After, the pre-alloyed steel sheet is cut to obtain a blank.

[0055] A thermal treatment is applied to the blank in a furnace. Preferably, in steps C) and/or E), the 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. The atmosphere may in particular be made of N.sub.2 or Ar or mixtures of nitrogen or argon and gas oxidants such as, for example, oxygen, mixtures of CO and CO.sub.2 or mixtures of H.sub.2 and H.sub.2O. It is also possible to use mixtures of CO and CO.sub.2 or mixtures of H.sub.2 and H.sub.2 without addition of inert gas.

[0056] Preferably, in steps C) and/or E), the atmosphere has an oxidizing power equal or higher than that of an atmosphere consisting of 10% by volume of oxygen and equal or smaller than that of an atmosphere consisting of 30% by volume of oxygen. For example, the atmosphere is air, i.e. consisting of about 78% of N.sub.2, about 21% of O.sub.2 and other gas such as rare gases, carbon dioxide and methane.

[0057] Preferably, in steps C) and/or E), the dew point is between −30 and +30° C., more preferably −20 and +20° C. and advantageously between −15° C. and +15° C. Indeed, without willing to be bound by any theory, it is believed that when the dew point is in the above range, the layer of thermodynamically stable oxides reduce even more the H.sub.2 absorption during the thermal treatment.

[0058] Preferably, the thermal treatment is performed at a temperature between 800 and 970° C. More preferably, the thermal treatment is performed at an austenitization temperature Tm usually between 840 and 950° C., preferably 880 to 930° C. Advantageously, said blank is maintained during a dwell time tm between 1 to 12 minutes, preferably between 3 to 9 minutes. During the thermal treatment before the hot-forming, the pre-coatings forms an alloy layer having a high resistance to corrosion, abrasion, wear and fatigue.

[0059] At ambient temperature, the mechanism of absorption of hydrogen into steel is different from high temperature, in particular the austenitization treatment. Indeed, usually at high temperature, the water in the furnace dissociates at the surface of the steel sheet into hydrogen and oxygen. Without willing to be bound by any theory, it is believed that the hydrogen barrier pre-coating and the slow cooling after the batch annealing can prevent water dissociation at the hydrogen barrier pre-coating surface. This can prevent the hydrogen diffusion through the coating and can allow the hydrogen anyway adsorbed to be released.

[0060] After the thermal treatment, the blank is then transferred to a hot-forming tool and hot-formed at a temperature between 600 and 830° C. The hot-forming can be the hot-stamping or the roll-forming. Preferably, the blank is hot-stamped. The part is then cooled in the hot-forming tool or after the transfer to a specific cooling tool.

[0061] The cooling rate is controlled depending on the steel composition, in such a way that the final microstructure after the hot-forming comprises mostly martensite, preferably contains martensite, or martensite and bainite, or is made of at least 75% of equiaxed ferrite, from 5 to 20% of martensite and bainite in amount less than or equal to 10%.

[0062] A hardened part having excellent resistance to delayed fracture according to the invention is thus obtained by hot forming. Preferably, the part comprises a steel sheet having a variable thickness coated with a hydrogen barrier pre-coating and an oxide layer comprising thermodynamically stable oxides, such hydrogen barrier pre-coating being alloyed through diffusion with the steel sheet.

[0063] More preferably, the part comprises the steel sheet having a variable thickness directly topped by a zinc- or aluminum-based pre-coating, this coating layer being directly topped by the hydrogen barrier pre-coating and an oxide layer comprising thermodynamically stable oxides, such hydrogen barrier pre-coating being alloyed through diffusion with the zinc- or aluminum-based pre-coating, this pre-coating being alloyed with the steel sheet.

[0064] Indeed, without willing to be bound by any theory, it seems that iron from steel diffuses to the surface of the barrier pre-coating during the thermal treatment.

[0065] Preferably, the thermodynamically stable oxides can comprise respectively Cr.sub.2O.sub.3; FeO; NiO; Fe.sub.2O.sub.3, Fe.sub.3O.sub.4, MgO, Y.sub.2O.sub.3, or a mixture thereof.

[0066] If a pre-coating based on zinc is present, the oxides can also comprise ZnO. If a pre-coating based on aluminum is present, the oxides can also comprise Al.sub.2O.sub.3 and/or MgAl.sub.2O.sub.4.

[0067] Preferably, the thickness of the oxide layer is between 10 and 550 nm.

[0068] Preferably, the part is a front rail, a seat cross member, a side sill member, a dash panel cross member, a front floor reinforcement, a rear floor cross member, a rear rail, a B-pillar, a door ring or a shotgun.

[0069] For automotive application, after a phosphating step, the part is dipped in an e-coating bath. Usually, the thickness of the phosphate layer is between 1 and 2 pm and the thickness of the e-coating layer is between 15 and 25 μm, preferably inferior or equal to 20 μm. The cataphoresis layer ensures an additional protection against corrosion. After the e-coating step, other paint layers can be deposited, for example, a primer coat of paint, a basecoat layer and a top coat layer.

[0070] Before applying the e-coating on the part, the part is previously degreased and phosphated so as to ensure the adhesion of the cataphoresis layer.

[0071] The invention will now be explained in trials carried out for information only. They are not limiting.

EXAMPLES

[0072] 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%.

[0073] All the steel sheets are coated with a 1.sup.st pre-coating having an anti-corrosion purpose 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.

[0074] Then, two Trials were coated with a 2.sup.nd pre-coating comprising 80% of Ni and 20% of Cr deposited by magnetron sputtering.

Example 1

Hydrogen Test

[0075] This test is used to determine the quantity of hydrogen absorbed during the austenitization thermal treatment of a press hardening method.

[0076] Trial 1 is a steel sheet coated with a 1.sup.st pre-coating being AluSi® (25 μm). Then, a batch annealing at a temperature of 650° C. was performed during 5 hours. The heating rate was of 10800° C.h.sup.−1. The atmosphere of the batch annealing was air. The cooling after the batch annealing was performed at a speed of 3600° C.h.sup.−1.

[0077] Trial 2 is a steel sheet coated 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. Then, a batch annealing at a temperature of 650° C. was performed during 5 hours. The heating rate was of 10800° C.h.sup.−1. The atmosphere of the batch annealing was air. The cooling after the batch annealing was performed at a speed of 3600° C.h.sup.−1.

[0078] Trial 3 is a steel sheet coated with a 1.sup.st pre-coating being AluSi® (25 μm). Then, a batch annealing at a temperature of 650° C. was performed during 5 hours. The heating rate was of 10800° C.h.sup.−1. The atmosphere of the batch annealing was air. The cooling after the batch annealing was performed at a speed of 25° C.h.sup.−1.

[0079] Trial 4 is a steel sheet coated 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. Then, a batch annealing at a temperature of 650° C. was performed during 5 hours. The heating rate was of 10800° C.h.sup.−1. The atmosphere of the batch annealing was air. The cooling after the batch annealing was performed at a speed of 25° C.h.sup.−1.

[0080] After, all Trials were cut and heated at a temperature of 900° C. during a dwell time of 3 minutes. The atmosphere during the thermal 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.

[0081] Finally, the hydrogen amount absorbed by the trials during the heat treatment was measured by thermic desorption using a TDA or Thermal Desorption Analyser. 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. Results are shown in the following Table 1:

TABLE-US-00001 Thickness H.sub.2 Cooling 2.sup.nd pre- amount 2.sup.nd pre- speed Ratio coating (ppm by Trials coating (° C. .Math. h.sup.−1) Ni/Cr (nm) mass) 1 — 3600 — 200 1.15 2 Ni/Cr 3600 4 — 0.75 80/20 3 — 25 — 200 0.9 4* Ni/Cr 25 4 — 0.35 80/20 *examples according to the invention.

[0082] Trial 4 according to the present invention release a significantly low amount of hydrogen compared to comparative examples.