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

Abstract

A cold rolled martensitic steel sheet including 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%; 0%≤N≤0.09%; and can contain one or more of the following optional elements 0%≤Ni≤1%; 0%≤Cu≤1%; 0%≤Mo≤0.4%; 0%≤Nb≤0.1%; 0%≤V≤0.1%; 0%≤B≤0.05%; 0%≤Sn≤0.1%; 0%≤Pb≤0.1%; 0%≤Sb≤0.1%; 0.001%≤Ca≤0.01%; the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel including, 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%.

What is claimed is: 
     
         1 - 18 . (canceled) 
     
     
         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%. 
 
     
     
         20 : The cold rolled martensitic steel sheet as recited in  claim 19  wherein the composition includes 0.16% to 0.24% of Silicon. 
     
     
         21 : The cold rolled martensitic steel sheet as recited in  claim 19  wherein the composition includes 0.11% to 0.19% of Carbon. 
     
     
         22 : The cold rolled martensitic steel sheet as recited in  claim 19  wherein the composition includes 0.01% to 0.05% of Aluminum. 
     
     
         23 : The cold rolled martensitic steel sheet as recited in  claim 19  wherein the composition includes 1.6% to 2.4% of Manganese. 
     
     
         24 : The cold rolled martensitic steel sheet as recited in  claim 19  wherein the composition includes 0.1% to 0.5% of Chromium. 
     
     
         25 : The cold rolled martensitic steel sheet as recited in  claim 19  wherein the amount of martensite is between 96% and 99%. 
     
     
         26 : The cold rolled martensitic steel sheet as recited in  claim 19  wherein the cumulated amount of ferrite and bainite is between 1% and 4%. 
     
     
         27 : The cold rolled martensitic steel sheet as recited in  claim 19  wherein the sheet has an ultimate tensile strength of 1280 MPa or more, and a yield strength of 1100 MPa or more. 
     
     
         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.   
     
     
         29 : The method as recited in  claim 28  wherein the coiling temperature is between 475° C. and 625° C. 
     
     
         30 : The method as recited in  claim 28  wherein Tsoak is between Ac3+10° C. and Ac3+100° C. 
     
     
         31 : The method as recited in  claim 28  wherein CR1 is between 20° C./s and 120° C./s. 
     
     
         32 : The method as recited in  claim 28  wherein T1 is between 660° C. and 725° C. 
     
     
         33 : The method as recited in  claim 28  wherein CR2 is greater than 100° C./s. 
     
     
         34 : The method as recited in  claim 28  wherein T2 is between Ms−50° C. and 20° C. 
     
     
         35 : The method as recited in  claim 28  wherein Ttemper is between 200° C. and 300° C. 
     
     
         36 : A method for manufacturing a structural part of a vehicle comprising performing the method as recited on  claim 28 . 
     
     
         37 : A method for manufacturing a structural part of a vehicle comprising employing the cold rolled martensitic steel sheet as recited in  claim 19 .