Method for Fabricating Steel Sheet for Press Hardening, and Parts Obtained by This Method

This is a divisional of U.S. patent application Ser. No. 17/217,770, filed on Mar. 30, 2021, published as US 2021/0214816 A1, which is a divisional of U.S. patent application Ser. No. 15/500,090, filed Jan. 30, 2017, published as US 2017/0253941 A1, which is a National Stage of International Application PCT/IB2015/001273, filed Jul. 29, 2015 which claims priority of International Application PCT/IB2014/001428, filed Jul. 30, 2014. All of the above applications are hereby incorporated by reference herein.

The invention relates to a fabrication method for steel sheets intended to yield very high strength mechanical parts after press hardening.

As known, hardening by quenching in press (or press hardening) consists of heating steel blanks at a sufficiently high temperature to obtain an austenitic transformation, and then hot stamping the blanks by keeping them within the press tool so as to obtain quenched microstructures. According to a variant of the method, a cold pre-stamping can be done on the blanks in advance before heating and press hardening. These blanks can be precoated, for example with aluminum or zinc alloy. In this case, during heating in a furnace, the precoating alloys with the steel substrate by diffusion so as to create a compound providing surface protection of the part against decarburizing and formation of scale. This compound is suited for hot forming.

The resulting parts are in particular used as structural elements in automotive vehicles for providing anti-intrusion or energy absorption functions. Thus, the following can be cited as implementation examples: bumper crossbeams, door or center pillar reinforcements or frame rails. Such press hardened parts can also be used for example for fabricating tools or parts for agricultural machines.

Depending on the composition of the steel and the cooling speed obtained in the press, the mechanical strength can reach a higher or lower level. Thus, the publication EP 2 137 327 discloses a steel composition containing: 0.040%<C<0.100%, 0.80%<Mn<2.00%, Si<0.30%, S<0.005%, P<0.030%, 0.010%≤Al≤0.070%, 0.015%<Nb<0.100%, 0.030%≤Ti≤0.080%, N<0.009%, Cu, Ni, Mo<0.100%, Ca<0.006%, with which a tensile mechanical strength Rm of over 500 MPa can be obtained after press hardening.

The publication FR 2,780,984 discloses a greater strength level being obtained: a steel sheet containing 0.15%<C<0.5%, 0.5%<Mn<3%, 0.1%<Si<0.5%, 0.01%<Cr<1%, Ti<0.2%, Al and P<0.1%, S<0.05%, 0.0005%<B<0.08% enables a strength Rm over 1000 even over 1500 MPa to be obtained.

Such strengths are satisfactory for many applications. However, demands for reducing the energy consumption of vehicles drives the search for even lighter weight vehicles through the use of parts whose mechanical strength would be even higher, meaning whose strength Rm would be over 1800 MPa. Since some parts are painted and undergo a paint baking cycle, this value is to be reached with or without thermal treatment by baking.

Now, such a level of strength is generally associated with a completely or very predominantly martensitic microstructure. It is known that this type of microstructure has a lower resistance to delayed cracking: after press hardening, the fabricated parts can in fact be susceptible to cracking or breaking after some time, under the conjunction of three factors:

In order to resolve the problem of delayed cracking, rigorously controlling the atmosphere of the reheating furnaces and the conditions of cutting blanks was proposed in order to minimize the level of stresses. Performing thermal post treatments on hot stamped parts was also proposed in order to allow hydrogen degassing. These operations are however constraining for the industry which wants a material that enables avoidance of this risk and overcomes these additional constraints and costs.

Depositing specific coatings on the surface of the steel sheet which reduces hydrogen adsorption was also proposed. However, a simpler process is sought which offers equivalent delayed cracking resistance.

Therefore, one is looking for a fabrication method for parts which would offer simultaneously a very high mechanical strength Rm, and a high resistance to delayed cracking after press hardening; these objectives being a priori difficult to reconcile.

Further, it is known that steel compositions richer in quench-promoting and/or hardening elements (C, Mn, Cr, Mo, etc.) lead to obtaining hot rolled sheets with a higher hardness. Thus, this increased hardness is detrimental for obtaining cold rolled sheets over a large range of thicknesses, considering the limited rolling capacity of some cold rolling mills. A too-high level of strength at the hot rolled sheet stage therefore does not allow very thin cold rolled sheets to be obtained. A method which provides a large range of cold rolled sheet thicknesses is therefore sought.

Additionally, the presence of quench-promoting and/or hardening elements in larger quantities can have consequences during thermomechanical treatment for fabrication because a variation of some parameters (end of rolling temperature, coiling temperature, variation of cooling speed over the width of the rolled strip) may lead to a variation of the mechanical properties within the sheet. A steel composition less sensitive to a variation of certain fabrication parameters is therefore sought so as to fabricate a sheet having good mechanical property homogeneity.

A steel composition is also sought which can be easily coated, in particular through hot-dip, such that the sheet can be available in different forms: uncoated, or coated with aluminum alloy or zinc alloy depending on end-user specifications.

A process is also sought that provides a sheet having good suitability for the mechanical cutting step in order to obtain blanks intended for press hardening, i.e., whose mechanical strength would not be too high at that stage in order to avoid breakdown of the cutting or punching tools.

A goal of the present invention is to resolve all of the problems discussed above by means of an economical fabrication method.

Surprisingly, the inventors have shown that these problems were resolved by supplying a sheet with the composition detailed below, where this sheet furthermore had the feature of having a specific enrichment with nickel in the area of the surface thereof.

For this purpose, present invention provides a rolled steel sheet, for press hardening, for which the chemical composition comprises, with contents expressed by weight: 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%, 0.015%≤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%, with it being understood that the titanium and nitrogen content satisfy: Ti/N>3.42, and that the carbon, manganese, chromium and silicon content satisfy:

with the chemical composition optionally comprising one or more of the following elements: 0.05%≤Mo≤0.65%, 0.001%≤W≤0.30%, 0.0005%≤Ca≤0.005%, with the remainder made up of iron and inevitable impurities coming from preparation, the sheet containing a nickel content Niat any point of the steel near the surface of said sheet over a depth Δ, such that Ni>Ni, where Nidesignates the nominal nickel content of the steel, and such that Nidesignates the maximum nickel content within

and such that

with the depth Δ expressed in microns and the Niand Nicontents expressed in percentages by weight.

According to a first embodiment, the composition of the sheet comprises, by weight: 0.32%≤C≤0.36%, 0.40%≤Mn≤0.80%, 0.05%≤Cr≤1.20%.

According to a second embodiment, the composition of the sheet comprises, by weight: 0.24%≤C≤0.28%, 1.50%≤Mn≤3%.

The silicon content of the sheet is preferably such that: 0.50%≤Si≤0.60%.

According to a further embodiment, the composition comprises, by weight: 0.30%≤Cr≤0.50%.

Preferably, the composition of the sheet comprises, by weight: 0.30%≤Ni≤1.20%, and very preferably: 0.30%≤Ni≤0.50%.

The titanium content is preferably such that: 0.020%≤Ti.

The composition of the sheet advantageously comprises: 0.020%≤Ti≤0.040%.

According to a preferred embodiment, the composition comprises, by weight: 0.15%≤Mo≤0.25%.

The composition preferably comprises by weight: 0.010%≤Nb≤0.060%, and very preferably: 0.030%≤Nb≤0.050%.

According to a further embodiment, the composition comprises, by weight: 0.50%≤Mn≤0.70%.

Advantageously, the microstructure of the steel sheet is ferritic-pearlitic.

According to a preferred embodiment, the steel sheet is a hot rolled sheet.

Preferably, the sheet is a hot rolled and annealed sheet.

According to a further embodiment, the steel sheet is precoated with a metal layer of aluminum or aluminum alloy or aluminum-based alloy.

According to a another embodiment, the steel sheet is precoated with a metal layer of zinc or zinc alloy or zinc-based alloy.

According to another embodiment, the steel sheet is precoated with one coat or several coats of inter-metallic alloys containing aluminum and iron and possibly silicon, where the precoating does not contain free aluminum, of phase τof type FeSiAl, and τof type FeSiAl.

The present invention also provides a part obtained by press hardening of a steel sheet of composition according to any one of the modes above with martensitic or martensitic-bainitic structure.

Preferably, the press hardened part contains a nominal nickel content Ni, in which the nickel content Niin the steel near the surface is greater than Niover a depth Δ, and in that, Nidesignating the maximum nickel content within Δ

and in that:

where the depth Δ is expressed in microns and the contents Niand Niare expressed in percentage by weight.

Advantageously, the press hardened part has a mechanical strength Rm greater than or equal to 1800 MPa.

According to a preferred embodiment, the press hardened part is coated with an aluminum or aluminum-based alloy, or a zinc or zinc-based alloy, resulting from the diffusion between the steel substrate and the precoating during the thermal treatment of press hardening.

The present invention also provides a fabrication method for a hot rolled steel sheet comprising the successive steps according to which an intermediate product with chemical composition according to the one of the embodiments presented above is cast, and then reheated to a temperature between 1250° C. and 1300° C. for a hold time at this temperature between 20 and 45 minutes. The intermediate product is hot rolled until an end of rolling temperature, ERT, between 825° C. and 950° C. in order to obtain a heat rolled sheet, and then the hot rolled sheet is coiled at a temperature between 500° C. and 750° C. in order to obtain a hot rolled and coiled, and then the oxide layer formed during the preceding steps is removed by pickling.

The present invention further provides a fabrication method for cold rolled and annealed sheet, characterized in that it comprises the successive steps according to which a hot rolled sheet is supplied, coiled and pickled, fabricated by the method described above and then this hot rolled, coiled and pickled sheet is cold rolled in order to obtain a cold rolled sheet. This cold rolled sheet is annealed at a temperature between 740° C. and 820° C. in order to obtain a cold rolled and annealed sheet.