Press hardening method
A press hardening method including: A. providing a steel sheet for heat treatment, being optionally precoated with an aluminum-based pre-coating, B. deposition of a hydrogen barrier pre-coating comprising chromium and not comprising nickel over a thickness from 10 to 550 nm, C. cutting the precoated steel sheet to obtain a blank, D. heat treating the blank at a furnace temperature from 800 to 970° C., during a dwell time from 1 to 12 minutes, in an atmosphere having 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, and having a dew point between −30 and +30° C., E. transferring the blank into a press tool, F. hot-forming at a temperature from 600 to 830° C. to obtain a part, G. cooling the part.
1 . A press hardening method comprising the following steps:
A. providing a steel sheet for heat treatment, the steel sheet being uncoated or having an aluminum-based pre-coating as a top surface of the steel sheet;
B. depositing, (i) directly onto the uncoated steel sheet of step A or (ii) directly onto the aluminum-based pre-coating of step A, a hydrogen barrier pre-coating comprising at least 50% chromium and not comprising nickel with a thickness from 10 to 550 nm;
C. cutting of the precoated steel sheet to obtain a blank;
D. heat treating the blank at a furnace temperature from 800 to 970° C., for a dwell time from 1 to 12 minutes, in an atmosphere having 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 having a dew point between −30 and +30° C.;
E. transferring the blank into a press tool;
F. hot-forming the blank at a temperature from 600 to 830° C. to obtain a part;
G. cooling of the part obtained in step F) 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 bainite in amount less than or equal to 10% in volume.
2 . The press hardening method as recited in claim 1 wherein in step B), the hydrogen barrier pre-coating does not comprise at least one of the elements chosen from Al, Fe, Si, Zn, and N.
3 . The press hardening method as recited in claim 1 wherein in step B), the hydrogen barrier pre-coating consists of chromium.
4 . The press hardening method as recited in claim 1 wherein no further pre-coating is deposited on top of the hydrogen barrier pre-coating between steps C) and G).
5 . The press hardening method as recited in claim 1 wherein in step A), the 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.
6 . The press hardening method as recited in claim 1 wherein the hydrogen barrier pre-coating of step B) is deposited by physical vapor deposition, by electrogalvanization or roll-coating.
7 . The press hardening method as recited in claim 1 wherein in step D), 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.
8 . The press hardening method as recited in claim 7 wherein in step D) the atmosphere is air.
9 . The press hardening method as recited in claim 1 wherein in step D), the heat treatment is performed at a temperature between 840 and 950° C. to obtain a fully austenitic microstructure in the steel.
10 . The method of claim 1 , wherein the microstructure is martensitic.