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

Abstract

A cold rolled martensitic steel sheet including the following elements, 0.3%≤C≤0.4%; 0.5% ≤Mn≤1%; 0.2%≤Si≤0.6%; 0.1%≤Cr≤1%; 0.01%≤Al≤1%; 0.01%≤Mo≤0.5%; 0.001%≤Ti≤0.1%; 0%≤S≤0.09%; 0%≤P≤0.09%; 0%≤N≤0.09%; 0%≤Nb≤0.1%; 0%≤V≤0.1%; 0%≤Ni≤1%; 0%≤Cu≤1%; 0%≤B≤0.05%; 0.001%≤Ca≤0.01%; 0%≤Sn≤0.1%; 0%≤Pb≤0.1%; 0%≤Sb≤0.1%; the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of steel, 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 having the following elements, expressed in percentage by weight:
 0.3%≤C≤0.4%;   0.5%≤Mn≤1%;   0.2%≤Si≤0.6%;   0.1%≤Cr≤1%;   0.01%≤Al≤1%;   0.01%≤Mo≤0.5%;   0.001%≤Ti≤0.1%;   0%≤S≤0.09%;   0%≤P≤0.09%;   0%≤N≤0.09%;   
       and optionally one or more of the following elements:
 0%≤Nb≤0.1%; 
 0%≤V≤0.1%; 
 0%≤Ni≤1%; 
 0%≤Cu≤1%; 
 0%≤B≤0.05%; 
 0.001%≤Cad 0.01%; 
 0%≤Sn≤0.1%; 
 0%≤Pb≤0.1%; 
 0% ≤Sb≤0.1%; 
 a remainder of the composition being composed of iron and unavoidable impurities caused by processing,
 a microstructure 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 0.3% to 0.36% of carbon. 
     
     
         21 . The cold rolled martensitic steel sheet as recited in  claim 19  wherein wherein the composition includes 0.3% to 0.38% of Carbon. 
     
     
         22 . The cold rolled martensitic steel sheet as recited in  claim 19  wherein the composition includes 0.01% to 0.5% of Aluminum. 
     
     
         23 . The cold rolled martensitic steel sheet as recited in  claim 19  wherein the composition includes 0.5% to 0.9% of Manganese. 
     
     
         24 . The cold rolled martensitic steel sheet as recited in  claim 19  wherein the composition includes 0.3% to 0.9% 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 steel sheet has an ultimate tensile strength of 1700 MPa or more, and a yield strength of 1500 MPa or more. 
     
     
         28 . A method of production of a cold rolled martensitic steel sheet comprising the following successive steps:
 providing semi-finished product with a composition having the following elements, expressed in percentage by weight:   0.3%≤C≤0.4%;   0.5%≤Mn≤1%;   0.2%≤Si≤0.6%;   0.1%≤Cr≤1%;   0.01%≤Al≤1%;   0.01%≤Mo≤0.5%;   0.001%≤Ti≤0.1%;   0%≤S≤0.09%;   0%≤P≤0.09%;   0%≤N≤0.09%;   
       and optionally one or more of the following elements:
 0%≤Nb≤0.1%; 
 0%≤V≤0.1%; 
 0% 23  Ni≤1%; 
 0%≤Cu≤1%; 
 0%≤B≤0.05%; 
 0.001%≤Ca≤0.01%; 
 0%≤Sn≤0.1%; 
 0%≤Pb≤0.1%; 
 0%≤Sb≤0.1%; 
 a remainder of the composition being composed of iron and unavoidable impurities caused by processing; 
 reheating the semi-finished product to a temperature between 1000° C. and 1280° C.; 
 rolling the 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 sheet at a cooling rate of at least 20° C./s to a coiling temperature below 650° C.; and coiling the hot rolled sheet; 
 cooling the hot rolled sheet to room temperature; 
 optionally performing a scale removal process on the hot rolled steel sheet; 
 optionally annealing the hot rolled steel sheet; 
 optionally performing a further 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; 
 heating the the cold rolled steel sheet in a two step heating wherein:
 the first step of heating the cold rolled steel sheet starts from room temperature to a temperature HT1 between 550° C. and 750° C., with a cooling rate HR1 of at least 10° C./s; 
 the second step of heating starts from HT1 to a temperature Tsoak between Ac3 and Ac3+100° C., with a heating rate HR2 between 1° C./s and 50° C./s, and holding for 10 to 500 seconds; 
 
 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 630° C. and 750° C., with a cooling rate CR1 between 30° C./s and 150° C./s; 
 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; 
 
 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. and holding for 100 to 600 seconds; and 
 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 30° C./s and 120° C./s. 
     
     
         32 . The method as recited in  claim 28  wherein T1 is between 640° C. and 725° C. 
     
     
         33 . The method as recited in  claim 28  wherein CR2 is at least 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 the manufacture of a structural part of a vehicle comprising performing the method as recited in  claim 28 . 
     
     
         37 . A method for the manufacture of a structural part of a vehicle comprising employing cold rolled martensitic steel sheet as recited in  claim 19 .