A cold rolled martensitic steel and a method of martensitic steel thereof

19:

• A cold rolled martensitic steel sheet comprising a composition of the following elements, expressed in percentage by weight:
• 0.1%≤C≤0.2%;
• 1.5%≤Mn≤2.5%;
• 0.1%≤Si≤0.25%;
• 0.1%≤Cr≤1%;
• 0.01%≤Al≤0.1%;
• 0.001%≤Ti≤0.1%;
• 0%≤S≤0.09%;
• 0%≤P≤0.09%;
• and
• 0%≤N≤0.09%;
• and optionally one or more of the following elements:
• 0%≤Ni≤10%;
• 0%≤Cu≤1%;
• 0%≤Mo≤0.4%;
• 0%≤Nb≤00.1%;
• 0%≤V≤0.1%;
• 0%≤B≤0.05%;
• 0%≤Sn≤0.1%;
• 0%≤Pb≤0.1%;
• 0%≤Sb≤0.1%;
• and
• 0.001%≤Ca≤0.01%;
• a remainder of the composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel comprising, by area percentage, at least 95% of martensite, a cumulated amount of ferrite and bainite between 1% and 5%, and an optional amount of residual austenite between 0% and 2%.

28:

• A method of production of a cold rolled martensitic steel sheet comprising the following successive steps:
• providing a semi-finished product with a steel composition of the following elements, expressed in percentage by weight:
• 0.1%≤C≤0.2%;
• 1.5%≤Mn≤2.5%;
• 0.1%≤Si≤0.25%;
• 0.1%≤Cr≤1%;
• 0.01%≤Al≤0.1%;
• 0.001%≤Ti≤0.1%;
• 0%≤S≤0.09%;
• 0%≤P≤0.09%;
• and
• 0%≤N≤0.09%;
• and optionally one or more of the following elements:
• 0%≤Ni≤10%;
• 0%≤Cu≤1%;
• 0%≤Mo≤0.4%;
• 0%≤Nb≤0.10%;
• 0%≤V≤0.1%;
• 0%≤B≤0.05%;
• 0%≤Sn≤0.1%;
• 0%≤Pb≤0.1%;
• 0%≤Sb≤0.1%;
• and
• 0.001%≤Ca≤0.01%;
• reheating the semi-finished product to a temperature between 1000° C. and 1280° C.;
• rolling the said semi-finished product in the austenitic range wherein the hot rolling finishing temperature is between Ac3 and Ac3+100° C. to obtain a hot rolled steel sheet;
• cooling the hot rolled steel sheet at a cooling rate of at least 20° C./s to a coiling temperature which is below 650° C.;
• and coiling the hot rolled steel sheet;
• cooling the hot rolled steel sheet to room temperature;
• optionally performing scale removal process on the hot rolled steel sheet;
• optionally annealing the hot rolled steel sheet;
• 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;
• then heating the cold rolled steel sheet at a rate of at least 2° C./s to a soaking temperature Tsoak between Ac3 and Ac3+100° C. where the cold rolled steel sheet is held during 10 to 500 seconds;
• then cooling the cold rolled steel sheet in a two step cooling wherein:
• the first step of cooling the cold rolled steel sheet starts from Tsoak down to a temperature T1 between 650° C. and 750° C., with a cooling rate CR1 between 15° C./s and 150° C./s;
• and
• the second step of cooling starts from T1 down to a temperature T2 between Ms−10° C. and 20° C., with a cooling rate CR2 of at least 50° C./s,
• then reheating the cold rolled steel sheet at a rate of at least 1° C./s to a tempering temperature Ttemper between 150° C. and 300° C. where it is held during 100 to 600 seconds;
• then cooling to room temperature with a cooling rate of at least 1° C./s to obtain a cold rolled martensitic steel sheet.