Coated steel strips, coated stamped products and methods
A pre-coated steel strip is provided. The steel strip includes a strip of base steel having a length, a width, a first side, and a second side. The length of the strip is at least 100 m and the width is at least 600 mm. An aluminum or an aluminum alloy pre-coating is on at least part of at least one of the first or second sides of the strip of base steel. A thickness t p of the pre-coating is from 20 to 33 micrometers at every location on at least one of the first or second sides. Processes, coated stamped products and land motor vehicles are also provided.
1. A process for making a hot stamped coated steel sheet product, comprising:
(A) heating an aluminum- or aluminum alloy pre-coated steel sheet in a furnace preheated to a temperature and during a time defined by trapezoid ABCD having heating time, temperature coordinates of about A (3 minutes, 930° C.), B (6 minutes, 930° C.), C (13 minutes, 880° C.) and D (4.5 minutes, 880° C.) if thickness of said sheet is greater than or equal to 0.7 mm and less than or equal to 1.5 mm, and by trapezoid EFGH having heating time, temperature coordinates of about E (4 minutes, 940° C.), F (8 minutes, 940° C.), G (13 minutes, 900° C.), and H (6.5 minutes, 900° C.) if thickness of said sheet is greater than 1.5 mm and less than or equal to 3 mm, at a heating rate V c between 20 and 700° C. comprised between 4 and 12° C./s, to obtain a heated blank;
(B) transferring said heated blank to a die; and
(C) stamping said heated blank in said die, to thereby obtain a hot stamped steel sheet product,
wherein the said heated product is cooled at a rate of at least 50° C./s during an interval between when said product exits said furnace and when said product reaches 400° C.
2. A process according to claim 1 , wherein said heated blank is deformed by a quantity higher than 10% during said stamping.
3. A process for making a hot stamped coated steel sheet product, comprising:
hot dip pre-coating a steel strip having a first side and a second side with aluminum or aluminum alloy to obtain a pre-coated steel strip, the pre-coating thickness on at least one of said first side and said second side of said strip having a thickness of from 20 to 33 μm at every location on at least one of said first and second sides of said strip;
cutting said pre-coated steel strip to obtain a sheet;
heating the sheet in a pre-heated furnace to obtain a heated blank in which the steel is austenitic;
transferring said heated blank to a die while maintaining said steel in the austenitic phase; and
stamping said heated blank in a die to obtain a hot stamped steel sheet product in which the steel microstructure is martensitic.
4. A land motor vehicle comprising the heat treated coated steel product produced according to claim 1 .