A martensitic steel and a method of martensitic steel thereof

Abstract

A martensitic steel sheet including of the following elements 0.08%≤C≤0.14%; 1.95%≤Mn≤2.6%; 0.1%≤Si≤0.8%; 0.01%≤Al≤0.1%; 0.001%≤Ti≤0.1%; 0.0001%≤B≤0.05%; 0%≤S≤0.09%; 0%≤P≤0.09%; 0%≤N≤0.09%; 0.1%≤Cr≤1%; 0%≤Ni≤1%; 0%≤Cu≤1%; 0%≤Mo≤0.4%; 0%≤Nb≤0.1%; 0%≤V≤0.1%; 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, the microstructure of said steel including, by area percentage, at least 92% of martensite, a cumulated amount of ferrite and bainite from 1% to 8%, and an optional amount of residual austenite from 0% to 2%.

1 - 15 . (canceled) 
     
     
         16 : A martensitic steel sheet comprising the following composition, expressed in percentage by weight:
 0.08%≤C≤0.14%;   1.95%≤Mn≤2.6%;   0.1%≤Si≤0.8%;   0.01%≤Al≤0.1%;   0.001%≤Ti≤0.1%;   0.0001%≤B≤0.05%;   0%≤S≤0.09%;   0%≤P≤0.09%;   0%≤N≤0.09%;   
       and can contain one or more of the following optional elements
 0.1%≤Cr≤1%; 
 0%≤Ni≤1%; 
 0%≤Cu≤1%; 
 0%≤Mo≤0.4%; 
 0%≤Nb≤0.1%; 
 0%≤V≤0.1%; 
 0%≤Sn≤0.1%; 
 0%≤Pb≤0.1%; 
 0%≤Sb≤0.1%; 
 0.001%≤Ca≤0.01%; 
 a remainder of the composition being composed of iron and unavoidable impurities caused by processing, 
 a microstructure of the steel comprising, by area percentage, at least 92% of martensite, a cumulated amount of ferrite and bainite from 1% to 8%, and an optional amount of residual austenite from 0% to 2%. 
 
     
     
         17 : The martensitic steel sheet according to  claim 16  wherein the composition includes 0.15% to 0.7% of Silicon. 
     
     
         18 : The martensitic steel sheet according to  claim 16  wherein the composition includes 0.09% to 0.13% of Carbon. 
     
     
         19 : The martensitic steel sheet according to  claim 16  wherein the composition includes 2% to 2.5% of Manganese. 
     
     
         20 : The martensitic steel sheet according to  claim 16  wherein the amount of martensite is from 93% to 99%. 
     
     
         21 : The martensitic steel sheet according to  claim 16  wherein the sheet has a hole expansion ration of more than 40%. 
     
     
         22 : A method of production of a martensitic steel sheet comprising the following successive steps:
 providing a semi-finished product with a composition, expressed in percentage by weight:
 0.08%≤C≤0.14%; 
 1.95%≤Mn≤2.6%; 
 0.1%≤Si≤0.8%; 
 0.01%≤Al≤0.1%; 
 0.001%≤Ti≤0.1%; 
 0.0001%≤B≤0.05%; 
 0%≤S≤0.09%; 
 0%≤P≤0.09%; 
 0%≤N≤0.09%; 
   and can contain one or more of the following optional elements
 0.1%≤Cr≤1%; 
 0%≤Ni≤1%; 
 0%≤Cu≤1%; 
 0%≤Mo≤0.4%; 
 0%≤Nb≤0.1%; 
 0%≤V≤0.1%; 
 0%≤Sn≤0.1%; 
 0%≤Pb≤0.1%; 
 0%≤Sb≤0.1%; 
 0.001%≤Ca≤0.01%; 
   a remainder of the composition being composed of iron and unavoidable impurities caused by processing;   reheating the semi-finished product to a temperature from 1000° C. to 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 hot rolled sheet;   cooling the hot rolled sheet to room temperature;   optionally performing scale removal process on the hot rolled steel sheet;   optionally annealing may be performed on hot rolled steel sheet;   optionally performing scale removal process on the hot rolled steel sheet;   then cold rolling the hot rolled steel sheet with a reduction rate CR from 35 to 90% to obtain a cold rolled steel sheet;   then heating the cold rolled steel sheet with a first heating step and a second heating step wherein:
 the first heating step starts from room temperature to a temperature HT1 from 410° C. to 750° C., with a heating rate HR1 of at least 10° C./s; 
 the second heating step starts from HT1 to a temperature Tsoak from Ac3 to Ac3+100° C., with a heating rate HR2 from 0.5° C./s to 50° C./s, and holding for 10 to 500 seconds; 
   then cooling the cold rolled steel sheet with a first cooling step and a second cooling step wherein:
 the first cooling step starts from Tsoak down to a temperature T1 from 620° C. to 750° C., with a cooling rate CR1 from 15° C./s to 150° C./s; 
 the second cooling step starts from T1 down to a temperature T2 from Ms-10° C. to 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 from 150° C. to 300° C. and holding for 100 to 650 seconds;   then cooling to room temperature with a cooling rate of at least 1° C./s to obtain a martensitic steel sheet.   
     
     
         23 : The method according to  claim 22  wherein the coiling temperature is from 475° C. to 625° C. 
     
     
         24 : The method according to  claim 22  wherein Tsoak is from Ac3+10° C. to Ac3+100° C. 
     
     
         25 : The method according to  claim 22  wherein CR1 is from 20° C./s to 120° C./s. 
     
     
         26 : The method according to  claim 22  wherein T1 is from 630° C. to 725° C. 
     
     
         27 : The method according to  claim 22  wherein CR2 is greater than 100° C./s. 
     
     
         28 : The method according to  claim 22  wherein T2 is from Ms—50° C. and 20° C. 
     
     
         29 : The method according to  claim 22  wherein Ttemper is from 200° C. to 300° C. 
     
     
         30 : A method for manufacturing a structural part of a vehicle comprising employing the steel sheet as recited in  claim 16 . 
     
     
         31 : A method for manufacturing a structural part of a vehicle comprising employing the steel sheet manufactured according to the method as recited in  claim 22 .