Heat treated cold rolled steel sheet and a method of manufacturing thereof

Abstract

A heat treated cold rolled steel sheet with the following elements, 0.1%≤C≤0.2%; 1.2%≤Mn≤2.2%; 0.05%≤Si≤0.6%; 0.001%≤Al≤0.1%; 0.01%≤Cr≤0.5 %; 0%≤S≤0.09%; 0%≤P≤0.09%; 0%≤N≤0.09%; 0%≤Mo≤0.5%; 0%≤Ti≤0.1%; 0%≤Nb≤0.1%; 0%≤V≤0.1%; 0%≤Ni≤1%; 0%≤Cu≤1%; 0%≤Ca≤0.005%; 0%≤B≤0.05%; the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel having, by area percentage, 60% to 85% of tempered martensite, a cumulated amount of ferrite and bainite of 15% to 38%, an optional amount of residual austenite of 0% to 5% and an optional amount of fresh martensite of 0 to 5%.

What is claimed is: 
     
         1 - 18 . (canceled) 
     
     
         19 . A heat treated cold rolled steel sheet comprising a composition, expressed in percentage by weight, of the following elements:
 0.1%≤C≤0.2%;   1.2%≤Mn≤2.2%;   0.05%≤Si≤0.6%;   0.001%≤Al≤0.1%;   0.01%≤Cr≤0.5%;   0%≤S≤0.09%;   0%≤P≤0.09%;   0%≤N≤0.09%;   
       and optionally one or more of the following optional elements
 0%≤Mo≤0.5%; 
 0%≤Ti≤0.1%; 
 0%≤Nb≤0.1%; 
 0%≤V≤0.1%; 
 0%≤Ni≤1%; 
 0%≤Cu≤1%; 
 0%≤Ca≤0.005%; 
 0%≤B≤0.05%; 
 a remainder of the composition being composed of iron and unavoidable impurities caused by processing, 
 the microstructure of the steel including, by area percentage, 60% to 85% of tempered martensite, a cumulated amount of ferrite and bainite of 15% to 38%, an optional amount of residual austenite of 0% to 5% and an optional amount of fresh martensite of 0 to 5%. 
 
     
     
         20 . The heat treated cold rolled steel sheet as recited in  claim 19  wherein the composition includes 0.12% to 0.19% of carbon. 
     
     
         21 . The heat treated cold rolled steel sheet as recited in  claim 19  wherein the composition includes 0.1% to 0.5% of Silicon. 
     
     
         22 . The heat treated cold rolled steel sheet as recited in  claim 19  wherein the composition includes 0.001% to 0.09% of Aluminum. 
     
     
         23 . The heat treated cold rolled steel sheet as recited in  claim 19  wherein the composition includes 1.3% to 2.1% of Manganese. 
     
     
         24 . The heat treated cold rolled steel sheet as recited in  claim 19  wherein the composition includes 0.1% to 0.4% of Chromium. 
     
     
         25 . The heat treated cold rolled steel sheet as recited in  claim 19  wherein the amount of tempered martensite is between 62% and 80%. 
     
     
         26 . The heat treated cold rolled steel sheet as recited in  claim 19  wherein the cumulated amount of ferrite and bainite is between 20% and 37%. 
     
     
         27 . The heat treated cold rolled steel sheet as recited in  claim 19  wherein the sheet has an ultimate tensile strength of 980 MPa to 1150 MPa in the transverse direction, and an ultimate tensile strength of 980 MPa to 1150 MPa in the rolling direction. 
     
     
         28 . A method of production of heat treated cold rolled steel sheet comprising the following successive steps:
 providing a semi-finished product having a composition, expressed in percentage by weight, of the following elements:   0.1%≤C≤0.2%;   1.2%≤Mn≤2.2%;   0.05%≤Si≤0.6%;   0.001%≤Al≤0.1%;   0.01%≤Cr≤0.5%;   0%≤S≤0.09%;   0%≤P≤0.09%;   0%≤N≤0.09%;   
       and optionally one or more of the following optional elements
 0%≤Mo≤0.5%; 
 0%≤Ti≤0.1%; 
 0%≤Nb≤0.1%; 
 0%≤V≤0.1%; 
 0%≤Ni≤1%; 
 0%≤Cu≤1%; 
 0%≤Ca≤0.005%; 
 0%≤B≤0.05%; 
 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 from Ac3 to 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 said hot rolled sheet; 
 cooling the said hot rolled sheet to room temperature; 
 optionally performing scale removal process on said hot rolled steel sheet; 
 optionally performing annealing on hot rolled steel sheet; 
 optionally performing scale removal process on said 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 in a two step heating wherein:
 the first step of heating the cold rolled steel sheet starts from room temperature to a temperature HT1 from 550° C. to 750° C., with a heating rate HR1 of at least 10° C./s; 
 the second step of heating starts from HT1 to a temperature Tsoak from Ac3 to Ac3+100° C., with a heating rate HR2 from 1° C./s to 15° C./s, where it 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 from 630° C. to 685° C., with a cooling rate CR1 from 1° C./s to 15° C./s; 
 the second step of cooling starts from Ti down to a temperature T2 from Ms−10° C. to 15° C., with a cooling rate CR2 of at least 100° C./s; 
 
 then reheating the cold rolled steel sheet at a rate of at least 5° C./s to a tempering temperature Ttemper between 150° C. and 300° C. where the cold rolled steel sheet is held during 100 to 600 seconds wherein ΔT=(T1−Ttemper) must be from 415° C. to 455° C.; and 
 then cooling to room temperature with a cooling rate of at least 1° C./s to obtain a heat treated cold rolled steel sheet. 
 
     
     
         29 . The method as recited in  claim 28  wherein the coiling temperature is from 475° C. to 625° C. 
     
     
         30 . The method as recited in  claim 28  wherein Tsoak is from Ac3+10° C. to Ac3+100° C. 
     
     
         31 . The method as recited in  claim 28  wherein CR1 is from 1° C./s to 10° C./s. 
     
     
         32 . The method as recited in  claim 28  wherein T1 is from 640° C. to 685° C. 
     
     
         33 . The method as recited in  claim 28  wherein CR2 is at least 200° C./s. 
     
     
         34 . The method as recited in  claim 28  wherein T2 is from Ms−20° C. to 20° C. 
     
     
         35 . The method as recited in  claim 28  wherein Ttemper is from 175° C. to 280° C. 
     
     
         36 . A method for manufacturing a structural part of a vehicle comprising performing the method as recited in  claim 28 . 
     
     
         37 . A method for manufacturing a structural part of a vehicle comprising employing the steel sheet as recited in  claim 19 .