Hot rolled steel sheet and a method of manufacturing thereof

20:

• A hot rolled steel sheet having a composition comprising the following elements, expressed in percentage by weight:
• 0.11%≤Carbon≤0.16%
• 1%≤Manganese≤2%
• 0.1%≤Silicon≤0.7%
• 0 02%≤Aluminum≤0.1%
• 0.15%≤Molybdenum≤0.4%
• 0.15%≤Vanadium≤0.4%
• 0.002%≤Phosphorus≤0.02%
• 0%≤Sulfur≤0.005%.
• 0%≤Nitrogen≤0.01%,
• and optionally one or more of the following elements:
• 0%≤Chromium≤0.5%
• 0%≤Niobium≤0.05%
• 0.0001%≤Calcium≤0.005%
• 0%≤Boron≤0.001%
• 0%≤Magnesium≤0.0010%
• 0%≤Titanium≤0.01%,
• with 0.3%≤Mo+V+Nb≤0.6%
• 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, 70% to 90% Bainite, 10% to 25% Ferrite, wherein a cumulated amount of Bainite and Ferrite is at least 90% and a cumulated amount of Residual Austenite and Martensite is between 0% and 10%.

30:

• A method of production of a hot rolled heat treated 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:
• 0.11%≤Carbon≤0.16%
• 1%≤Manganese≤2%
• 0.1%≤Silicon≤0.7%
• 0 02%≤Aluminum≤0.1%
• 0.15%≤Molybdenum≤0.4%
• 0.15%≤Vanadium≤0.4%
• 0.002%≤Phosphorus≤0.02%
• 0%≤Sulfur≤0.005%.
• 0%≤Nitrogen≤0.01%,
• and optionally one or more of the following elements:
• 0%≤Chromium≤0.5%
• 0%≤Niobium≤0.05%
• 0.0001%≤Calcium≤0.005%
• 0%≤Boron≤0.001%
• 0%≤Magnesium≤0.0010%
• 0%≤Titanium≤0.01%,
• with 0.3%≤Mo+V+Nb≤0.6%
• a remainder of the composition being composed of iron and unavoidable impurities caused by processing;
• reheating the semi-finished product to a temperature between 1200° C. and 1300° C.;
• rolling the semi-finished product in the austenitic range wherein the hot rolling finishing temperature is between 850° C. and 975° C. to obtain a hot rolled steel strip;
• then
• cooling the hot rolled strip in three step cooling wherein:
• in step one of the cooling, the hot rolled steel sheet starts from a temperature range between 850° C. and 975° C. to a temperature range between 650° C. and 725° C., with a cooling rate between 40° C./s and 150° C./s;
• the step two of the cooling the hot rolled steel sheet starts from a temperature range between 650° C. and 725° C. to a temperature range between 620° C. and 690° C., said step two having a duration of 1 s to 10 s and being an air cooling;
• and
• in the step three of the cooling the hot rolled steel sheet starts from a temperature range between 620° C. and 690° C. to a temperature range between 450° C. and 550° C. with a cooling rate greater than 20° C./s;
• thereafter
• coiling the hot rolled steel strip at a temperature range between 450° C. and 550° C.;
• cooling the coiled hot rolled steel strip to room temperature.