Cold rolled heat treated steel sheet and a method of manufacturing thereof
A cold rolled and heat treated steel sheet having a composition with the following elements, expressed in percentage by weight:0.10%≤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 can contain one or more of the following optional 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%, 0.0001%≤Calcium≤0.005%, 0%≤Vanadium≤0.1%, 0%≤Boron≤0.003%, 0%≤Cerium≤0.1%, 0%≤Magnesium≤0.010%, 0%≤Zirconium≤0.010% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel sheet having in area fraction, 10 to 30% Residual Austenite, 10 to 40% Bainite, 5% to 50% Annealed Martensite, 1% to 20% Quenched Martensite and less than 30% Tempered Martensite, wherein the cumulated amounts of Bainite and Residual Austenite is more than or equal to 25%.
1. A method of production of a cold rolled heat treated steel sheet comprising the following successive steps:
providing a semi-finished product with a steel composition comprising the following elements, expressed in percentage by weight:
0.10%≤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 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%
0.001%≤Copper≤2%
0.01%≤Nickel≤3%
0.0001%≤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 the 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 sheet to room temperature;
cold rolling the hot rolled steel sheet with a reduction rate between 35 and 90% to obtain a cold rolled steel sheet;
performing a first annealing by heating the said 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 sheet for a time of 10 to 500 seconds;
cooling the cold rolled sheet at a rate greater than 20° C./s to a temperature below 500° C. to define an annealed steel sheet;
performing a second annealing by heating the annealed steel sheet at a rate greater than 3° C./s to a soaking temperature between T soaking and Ac3 and holding the annealed steel sheet for a time of 10 to 500 seconds;
cooling the annealed steel sheet at a rate greater than 20° C./s to a temperature range between Tc max and Tc min :
TC max =565-601*(1−Exp(−0.868*C))−34*Mn−13*Si−10*Cr+13*Al−361*Nb
TC min =565-601*(1−Exp(−1.736*C))−34*Mn−13*Si−10*Cr+13*Al−361*Nb wherein C, Mn, Si, Cr, Al and Nb are in wt. % of the elements in the steel composition; and
bringing the annealed steel sheet to a temperature range between 350° C. and 550° C., holding the annealed steel sheet for a time of 5 to 500 seconds and cooling the annealed steel sheet down to room temperature with a cooling rate of at least 1° C./s to obtain the cold rolled heat treated steel sheet;
wherein Tc max , Tc min Ac3, and T soaking , are in ° C.
2. The method as recited in claim 1 wherein the coiling temperature is below 570° C.
3. The method as recited in claim 1 wherein the hot rolling finishing temperature is between Ac3 and Ac3+100° C.
4. The method as recited in claim 1 wherein the cooling of the hot rolled steel sheet at the cooling rate above 30° C./s to the coiling temperature below 600° C. includes cooling at the rate greater than 30° C./s to a temperature below 500° C.
5. The method as recited in claim 1 wherein the annealed steel sheet is continuously annealed between T soaking and Ac3 for the 10 to 500 seconds to have an Austenite to Annealed Martensite ratio between 50:50 to 90:10.
6. The method as recited in claim 1 further comprising performing a scale removal process on the hot rolled steel sheet.
7. The method as recited in claim 1 further comprising performing tempering the annealed steel sheet between 120° C. and 250° C.
8. The method as recited in claim 1 , wherein a microstructure of the cold rolled heat treated steel sheet is free of pearlite, ferrite and cementite.
9. The method as recited in claim 6 further comprising annealing the hot rolled steel sheet at a temperature between 400° C. and 750° C.
10. The method as recited in claim 9 further comprising performing a further scale removal process on the hot rolled steel sheet.
11. The method as recited in claim 10 further comprising performing tempering the annealed steel sheet between 120° C. and 250° C.