Patent ID: 11920207
Assignee: ARCELORMITTAL
Field: Materials, metallurgy (Chemistry)
Classification: CPC C  B | IPC B  C

Claim 12:
13. A method of production of the cold rolled heat treated steel sheet as recited in claim 1 comprising the following successive steps:
providing semi-finished product having a steel composition comprising the following elements, expressed in percentage by weight:
0.1%<Carbon<0.5%
1%<Manganese<3.4%
0.5%<Silicon<2.5%
0.03%<Aluminum<1.5%
0%<Sulfur<0.003%
0.002%<Phosphorus<0.02%
0%<Nitrogen<0.01%
and optionally containing one or more of the following elements
0.05%<Chromium<1%
0.001%<Molybdenum<0.5%
0.001%<Niobium<0.1%
0.001%<Titanium<0.1%
0.01%<Copper<2%
0.01%<Nickel<3%
0.001%<Calcium<0.005%
0%<Vanadium<0.1%
0%<Boron<0.003%
0%<Cerium<0.1%
0%<Magnesium<0.010%
0%<Zirconium<0.010%
a remainder being iron and unavoidable impurities caused by processing;
reheating the semi-finished product to a temperature between 1200° C. and 1280° C.;
rolling the semi-finished product in an austenitic range wherein the hot rolling finishing temperature is above Ac3 to obtain a hot rolled steel sheet;
cooling the hot rolled steel sheet at a cooling rate above 30° C./s to a coiling temperature below 600° C. and coiling the hot rolled steel sheet;
cooling the hot rolled steel sheet to room temperature;
optionally performing a scale removal process on the hot rolled steel sheet;
optionally annealing the hot rolled steel sheet at temperature between 400° C. and 750° C.;
optionally performing a scale removal process on the hot rolled steel sheet;
cold rolling the hot rolled steel sheet with a reduction rate between 35 and 90% to obtain a cold rolled steel sheet;
heating the cold rolled steel sheet at a rate greater than 3° C./s to a soaking temperature between Ac3 and Ac3+100° C. and holding the cold rolled steel sheet for a time of 10 to 500 seconds;
cooling the cold rolled steel sheet at a rate greater than 20° C./s to a temperature below 500° C. to define an annealed cold rolled steel sheet;
cooling the annealed cold rolled steel sheet to room temperature;
optionally tempering the annealed cold rolled steel sheet between 120° C. and 250° C.;
heating the annealed cold rolled steel sheet at a rate greater than 3° C./s to a soaking temperature between Ac3 and Ac3+100° C. and holding the annealed cold rolled steel sheet of 10 to 500 seconds;
cooling the annealed cold rolled steel sheet at a rate greater than 20° C./s to a temperature range between TCmax and TCmin wherein:

TCmax=565−601*(1−Exp(−0.868*C))−34*Mn−13*Si−10*Cr+13*Al−361*Nb

TCmin=535−601*(1−Exp(−0.868*C))−34*Mn−13*Si−10*Cr+13*AI−361*Nb; and

bringing the annealed cold rolled steel sheet to a holding temperature range between 350° C. and 550° C. for a time period between 5 and 500 seconds and then cooling the annealed cold rolled steel sheet to room temperature with a cooling rate of at least 1° C./s to obtain the cold rolled heat treated steel sheet; and
optionally coating the cold rolled heat treated steel sheet;
thereby producing the cold rolled heat treated steel sheet of claim 1.