The present invention relates to steel parts that are hot formed and press hardened through a cooling step achieved by holding the parts in the press tool. These parts are used as structural elements in automotive vehicles for anti-intrusion or energy absorption functions. Such parts can also be used for example for the fabrication of tools or parts for agricultural machinery.

BACKGROUND

In such type of applications, it is desirable to have steel parts that combine high mechanical strength, high impact resistance, good corrosion resistance and dimensional accuracy. This combination is particularly desirable in the automobile industry. Automotive parts such as front or rear rails, roof rails, B-pillars, and chassis parts such as lower control arms, engine cradles, need more particularly these properties.

The press hardening process has been disclosed in the publication GB1490 535. A hardened steel part is obtained by heating a steel blank to a temperature at which the steel is transformed into austenite and then hot formed in a press. The blank is simultaneously rapidly cooled in the press tool and held so to prevent distortion thus obtaining a martensitic and/or bainitic microstructure. The steel used may have the following composition: C<0.4%, 0.5-2.0% Mn, S and P<0.05, 0.1-0.5% Cr, 0.05-0.5% Mo, <0.1% Ti, 0.005-0.01% B, <0.1% Al. However, this publication does not provide a solution for obtaining simultaneously high mechanical resistance and elongation, good bendability and weldability.

The fabrication of parts with good corrosion resistance and tensile strength higher than 1500 MPa is disclosed by the publication FR2780984: an aluminized steel sheet with 0.15-0.5% C, 0.5-3% Mn, 0.1-0.5% Si, 0.01-1% Cr, <0.2% Ti, 0.1% Al and P, <0.05% S, 0.0005-0.08% B, is heated, formed and rapidly cooled. However, due to the high tensile strength level, the total elongation in tensile test is lower than 6%.

The publication EP2137327 discloses the press hardening of a steel blank with a composition containing: 0.040-0.100% C, 0.80-2.00% Mn, <0.30% Si, <0.005% S, <0.030% P, 0.01-0.070% Al, 0.015-0.100% Al, 0.030-0.080% Ti, <0.009% N, <0.100% Cu, Ni, Mo, <0.006% Ca. After press hardening, a tensile strength higher than 500 MPa can be obtained. However, due to the nature of the microstructure, which is equiaxed ferrite, it is not possible to achieve very high tensile strength level.

The document EP1865086 discloses a steel composition comprising 0.1-0.2% C, 0.05-0.3% Si, 0.8-1.8% Mn, 0.5-1.8% Ni, ≤0.015% P, ≤0.003% S, 0.0002-0.008% B, optionally 0.01-0.1% Ti, optionally 0.01-0.05% Al, optionally 0.002-0.005% N. This composition makes it possible to manufacture a press hardened part with a tensile strength higher than 1000 MPa and with elongation higher than 10%. However, due to its high nickel content, this steel is costly to manufacture.

The document EP1881083 discloses a press hardened part made from a steel alloy containing 0.11-0.18% C, 0.10-0.30% Si, 1.60-2.20% Mn, <0.0015% P, <0.010% S, 1.00-2.00% Cr, <0.020% N, 0.020-0.060% Nb, 0.001-0.004% B, 0.001-0.050% Ti. The part has a tensile strength higher than 1200 MPa and a total elongation more than 12%. However, due to its high chromium content, this steel is costly to manufacture.

SUMMARY OF THE INVENTION

Thus, it is desired to have a press hardened part and a manufacturing process that would not have the previous limitations. It is more particularly desired to have a press hardened steel part with a thickness comprised between 0.8 and 4 mm and a yield stress YS comprised between 700 and 950 MPa, a tensile stress TS comprised between 950 and 1200 MPa, and a high ductility characterized by a bending angle higher to 75°.

It is also desired to have a press hardened part with a fracture strain under plane strain condition, higher than 0.60.

As heavily deformed areas in the press hardened parts, such as for example the radii zones, are subjected to high stress concentration during further service conditions or during vehicles collisions, it is also desirable to have press hardened parts which would display higher ductility in these deformed zones.

It is also desirable to have press hardened parts which would be easily weldable, and press hardened welded joints with high ductility and free from significant softening in the Heat Affected Zones.

It is also desirable to have steel sheets that would be suitable for Laser welding: this process is very sensitive to misalignment defects that can be due to insufficient flatness: thus, sheets with very good flatness properties are required for Laser welding.

It is also desirable to have a steel sheet that could be easily weldable either in a homogeneous process (i.e. welding of two sheets with the same composition) or in heterogeneous process (welding of two sheets with different steel compositions) and further press hardened, and that these press hardened welds have high mechanical properties.

It is also desired to have a steel composition for press hardening that could be available either in uncoated state or with a metallic coating providing to the steel substrate a corrosion resistance after press hardening.

The present invention provides a press hardened steel part with a steel chemical composition comprising, in weight: 0.062%≤C≤0.095%, 1.4%≤Mn≤1.9%, 0.2%≤Si≤0.5%, 0.020%≤Al≤0.070%, 0.02%≤Cr≤0.1%, wherein 1.5%≤(C+Mn+Si+Cr)≤2.7%, 0.040%≤Nb≤0.060%, 3.4×N≤Ti≤8×N, wherein: 0.044%≤(Nb+Ti)≤0.090%, 0.0005: B≤0.004%, 0.001%≤N≤0.009%, 0.0005%≤S≤0.003%, 0.001%≤P≤0.020% optionally: 0.0001%≤Ca≤0.003%, the remainder being Fe and unavoidable impurities, and wherein the microstructure comprises, in the majority of the part, in surface fractions: less than 40% of bainite, less than 5% of austenite, less than 5% of ferrite, the remainder being martensite, said martensite consisting of fresh martensite and of self-tempered martensite.

Preferably, the composition is such that: 1.7%≤(C+Mn+Si+Cr)≤2.3%.

In a preferred embodiment, the C content of the steel part is such that: 0.065%≤C≤0.095%

Preferably, the microstructure comprises at least 5% in surface fraction of self-tempered martensite.

The sum of fresh martensite and of self-tempered martensite surface fractions is preferably comprised between 65 and 100%.

According to a preferred embodiment, the average size of titanium nitrides is less than 2 micrometers in the outer zones comprised between one quarter thickness of the part, and the closest surface of the part.

Preferably, the average length of sulfides is less than 120 micrometers in the outer zones comprised between one quarter thickness of the part, and the closest surface of the part.

According to a preferred embodiment, the press hardened steel part comprises at least one hot deformed zone (A) with a deformation quantity higher than 0.15, and at least one zone (B) having experienced the same cooling cycle in press hardening than zone (A), wherein the deformation quantity is less than 0.05.

The difference in hardness between the zone (B) and the hot deformed zone (A) is preferably more than 20 HV.

Preferably, the average lath width of the martensitic-bainitic structure in the hot deformed zone (A) is reduced by more than 50% as compared to the lath width of the martensitic-bainitic structure in the zone (B).

In a preferred embodiment, the average lath width of the martensitic-bainitic structure in the hot deformed zone (A) is less than 1 μm.

The average lath width of the martensitic-bainitic structure in the zone (B) is preferably comprised between 1 and 2.5 μm.

According to one embodiment of the invention, the press hardened steel part is coated with a metallic coating.

The metallic coating is preferably zinc-based alloy or zinc alloy.

Preferably, the metallic coating is aluminum-based alloy or aluminum alloy.

In a preferred embodiment, the press hardened part has a yield stress comprised between 700 and 950 MPa, a tensile stress TS comprised between 950 and 1200 MPa, and a bending angle higher than 75°.

According to a preferred embodiment, the press hardened steel part has a variable thickness.

Very preferably, the variable thickness is produced by a continuous flexible rolling process.

The present invention also provides a press hardened laser welded steel part, wherein at least one first steel part of the weld is an Al coated part as described above, welded with at least at least one second steel part, the composition of which contains from 0.065 to 0.38% of carbon in weight, and wherein the weld metal between the first steel part and the second steel part has an aluminum content less than 0.3% in weight, and wherein the first steel part, the second steel part, and the weld metal, are press hardened in the same operation.

The present invention further provides a process for manufacturing a press hardened steel part comprising the following and successive steps:providing a steel semi-product with the composition mentioned above,hot rolling the semi-product to obtain a hot rolled steel sheet,coiling the hot rolled steel sheet at a coiling temperature Tc comprised between 550° C. and Ms, Ms being the martensitic transformation start temperature of the steel sheet, so as to obtain a coiled steel sheet,optionally cold rolling the coiled steel sheet,annealing the steel sheet at an annealing temperature Ta in order to obtain less than 10% of unrecrystallized area fraction, so to obtain an annealed steel sheet,cutting the annealed steel sheet to a predetermined shape so to obtain a blank,heating the blank and holding it at a temperature Tm comprised between 890 and 950° C., the holding duration Dm at the temperature Tm being comprised between 1 and 10 minutes, so as to obtain a heated blank,transferring the heated blank within a forming press, the transfer duration Dt being less than 10 s,hot forming the heated blank in the forming press so as to obtain a formed part,cooling said formed part at a cooling rate CR1 comprised between 40 and 360° C./s in a temperature range between 750 and 450° C., and at an cooling rate CR2 between 15 to 150° C./s in a temperature range comprised 450° C. and 250° C., wherein CR2<CR1.

Preferably, the cold rolling ratio is comprised between 50 and 80%.

The annealing temperature Ta is preferably comprised between 800 and 850° C., and very preferably between 800 and 835° C.

In a particular embodiment, the blank is cold formed before heating said blank at said temperature Tm.

Preferably, the hot forming is performed with a deformation quantity higher than 0.15 in at least one hot deformed zone of the part,

In a preferred embodiment, the annealed steel sheet is precoated with metallic precoating, before cutting the annealed steel blank to a predetermined shape.

The metallic precoating is preferably zinc, or zinc-based alloy, or zinc alloy.

Preferably, the metallic precoating is aluminum, or aluminum-based alloy, or aluminum alloy.

According to a preferred embodiment, the sheet is precoated with at least one intermetallic layer containing Al and iron, and optionally silicon, and the precoating contains neither free Al, nor τ5phase of Fe3Si2Al12 type, nor τ6phase of Fe2Si2Al9 type.

In another preferred embodiment, the metallic precoating comprises a layer of aluminum or an aluminum-based alloy or an aluminum alloy, topped by a layer of zinc or zinc-based alloy or a zinc alloy.

The invention even further provides a process for manufacturing a press hardened Laser welded steel part, comprising the successive following steps of:providing at least one first steel sheet with a composition as mentioned, precoated with a metallic precoating of aluminum, or aluminum-based alloy, or aluminum alloy,providing at least one second steel sheet with a composition containing, from 0.065 to 0.38% of carbon in weight, precoated with a metallic precoating of aluminum, or aluminum-based alloy, or aluminum alloy,removing a portion of the thickness of the aluminum precoating at the upper and lower sides along one side of the periphery of the first steel sheet and the second steel sheet, thencreating a welded blank by Laser welding the first steel sheet and the second steel sheet, such that the aluminum content in the weld metal is lower than 0.3% in weight, the Laser welding being performed along the periphery wherein a portion of the aluminum precoating has been removed,heating the welded blank and holding it at a temperature Tm comprised between 890 and 950° C., the holding duration Dm at said temperature being comprised between 1 and 10 minutes, so as to obtain a heated welded blank,transferring the heated welded blank within a forming press, the transfer duration Dt being less than 10 s,hot forming the heated welded blank in the forming press so as to obtain a welded formed part,cooling the welded formed part at a cooling rate CR1 comprised between 40 and 360° C./s in a temperature range between 750 and 450° C., and at an cooling rate CR2 between 15 to 150° C./s in a temperature range comprised 450° C. and 250° C., wherein CR2<CR1.

Preferably, the holding duration Dm is comprised between 1 and 6 minutes

The invention also provides use of a part as described above, or manufactured according to a process as described above, for the manufacturing of structural or safety parts of vehicles.

