Cold rolled and coated steel sheet and a method of manufacturing thereof

A cold rolled steel sheet having a composition comprising the following elements, expressed in percentage by weight:

• 0.13%≤Carbon≤0.18%
• 1.1%≤Manganese≤1.8%
• 0.5%≤Silicon≤0.9%
• 0.6%≤Aluminum≤1%
• 0.002%≤Phosphorus≤0.02%
• 0%≤Sulfur≤0.003%.
• 0%≤Nitrogen≤0.007%
• 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%
• 0.01%≤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%;
• a remainder of the composition being composed of iron and unavoidable impurities caused by processing,
• a microstructure of the steel sheet comprising in area fraction, 60 to 75% Ferrite, 20 to 30% Bainite, 10 to 15% Residual Austenite, and 0% to 5% Martensite, wherein cumulated amounts of the Residual Austenite and the Ferrite is between 70% and 80%.

A method of production of a cold rolled steel sheet comprising the following successive steps:

• providing a semi-finished product having a steel composition comprising the following elements, expressed in percentage by weight:
• 0.13%≤Carbon≤0.18%
• 1.1%≤Manganese≤1.8%
• 0.5%≤Silicon≤0.9%
• 0.6%≤Aluminum≤1%
• 0.002%≤Phosphorus≤0.02%
• 0%≤Sulfur≤0.003%.
• 0%≤Nitrogen≤0.007%
• 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%
• 0.01%≤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%;
• a remainder of the composition being composed of iron and unavoidable impurities caused by processing;
• reheating the semi-finished product to a temperature between 1150° C. and 1280° C.;
• rolling the semi-finished product in the austenitic range wherein the hot rolling finishing temperature is between Ac1+50° C. and Ac1+250° C. to obtain a hot rolled steel sheet;
• cooling the sheet at a cooling rate above 30° C./s to a coiling temperature below 625° C.;
• and coiling the hot rolled sheet;
• cooling the hot rolled sheet to room temperature;
• optionally performing scale removal process on said hot rolled steel sheet;
• optionally annealing the hot rolled steel sheet at temperature between 400° C. and 750° C.;
• optionally performing 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;
• then performing an annealing at a soaking temperature between Ac1+30° C. and Ac3 for a duration between 10 and 500 seconds by heating the cold rolled steel sheet by a two step heating wherein:
• in step one of the two step heating, the cold rolled steel sheet is heated at a heating rate between 10° C./s and 40° C./s to a temperature range between 550° C. and 650° C.;
• then in step two of the two step heating, the cold rolled steel sheet is heated at a heating rate between 1° C./s and 5° C./s from a temperature range between 550° C. and 650° C. to the annealing soaking temperature and maintained at the annealing soaking temperature,
• then cooling the cold rolled steel sheet in a two step cooling wherein:
• in step one of the two step cooling, the cold rolled steel sheet is cooled at a cooling rate of less 5° C./s to temperature range between 600° C. and 720° C.;
• then in step two of the two step cooling, the sheet is cooled at a cooling rate between 10° C./s to 100° C./s from a temperature range between 600° C. and 720° C. to an overaging temperature;
• then the cold rolled steel sheet is overaged at a temperature range between 250° C. and 470° C. for 5 to 500 seconds and then cooling to room temperature to obtain the produced cold rolled steel sheet.