Cold rolled and coated steel sheet and a method of manufacturing thereof

Abstract

A cold rolled and coated steel sheet, the steel including 0.30%≤carbon 0.45%, 1≤manganese≤2.5%, 0.9%≤silicon≤2.2%, 0%≤aluminum≤0.09%, 0.001≤niobium≤0.09%, 0%≤phosphorus≤0.02%, 0%≤sulfur≤0.03%, 0%≤nitrogen≤0.09%, and optionally one or more of the following elements 0%≤molybdenum≤0.5%, 0%≤chromium≤0.6%, 0%≤titanium≤0.06%, 0%≤vanadium≤0.1%, 0%≤calcium≤0.005%, 0%≤boron≤0.010%, 0%≤Magnesium≤0.05%, 0%≤Zirconium≤0.05%, 0%≤Cerium≤0.1%, and the balance including iron and unavoidable impurities, the steel sheet having a microstructure comprising 35% to 65% Partitioned Martensite, 15% to 40% of Bainite, 14% to 30% of residual austenite, 4% to 15% of ferrite and 0% to 10% fresh martensite in area fractions, the balance being partitioned martensite.

What is claimed is: 
     
         1 - 15 . (canceled) 
     
     
         16 . A cold rolled and coated steel sheet, the steel comprising a composition, in weight percentage of:
 0.30% ≤carbon≤0.45%,   1% ≤manganese≤2.5%,   0.9% ≤silicon≤2.2%,   0% ≤aluminum≤0.09%,   0.001%≤niobium≤0.09%,   0%≤phosphorus≤0.02%,   0%≤sulfur≤0.03%,   0% ≤nitrogen≤0.09%, and   optionally one or more of the following elements:   0%≤molybdenum≤0.5%,   0%≤chromium≤0.6%,   0%≤titanium≤0.06%,   0%≤vanadium≤0.1%,   0%≤calcium≤0.005%,   0%≤boron≤0.010%,   0%≤Magnesium≤0.05%,   a balance including iron and unavoidable impurities,
 the steel sheet having a microstructure comprising 35% to 65% Partitioned Martensite, 15% to 40% of Bainite, 14% to 30% of residual austenite, 4% to 15% of ferrite and 0% to 10% fresh martensite in area fractions, a microstructure balance being partitioned martensite. 
   
     
     
         17 . The cold rolled and coated steel sheet as recited in  claim 16  wherein the composition includes 1.2% to 2.5% of manganese. 
     
     
         18 . The cold rolled and coated steel sheet as recited in  claim 16  wherein the composition includes 0.32% to 0.45% of Carbon. 
     
     
         19 . The cold rolled and coated steel sheet as recited in  claim 16  wherein the composition includes 1% to 2.1% of Silicon. 
     
     
         20 . The cold rolled and coated steel sheet as recited in  claim 16  wherein the composition includes 0.001% to 0.08% of Niobium. 
     
     
         21 . The cold rolled and coated steel sheet as recited in  claim 16  wherein the microstructure contains 35% to 63% of partition martensite. 
     
     
         22 . The cold rolled and coated steel sheet as recited in  claim 16  wherein the microstructure contains residual 14% to 28% of residual austenite. 
     
     
         23 . The cold rolled and coated steel sheet as recited in  claim 16  wherein the microstructure contains 15% to 35% of bainite. 
     
     
         24 . The cold rolled and coated steel sheet as recited in  claim 16  wherein the steel sheet has a tensile strength greater than or equal to 1170 MPa, and a total elongation of 18% of more. 
     
     
         25 . The cold rolled and coated steel sheet as recited in  claim 16  wherein the steel sheet has a yield strength greater than or equal to 730 MPa. 
     
     
         26 . A method of manufacturing of a cold rolled and coated steel sheet comprising the following successive steps:
 providing a steel composition, in weight percentage of:
 0.30% ≤carbon≤0.45%, 
 1% ≤manganese≤2.5%, 
 0.9% ≤silicon≤2.2%, 
 0% ≤aluminum≤0.09%, 
 0.001%≤niobium≤0.09%, 
 0%≤phosphorus≤0.02%, 
 0%≤sulfur≤0.03%, 
 0% ≤nitrogen≤0.09%, and 
   optionally one or more of the following elements:
 0%≤molybdenum≤0.5%, 
 0%≤chromium≤0.6%, 
 0%≤titanium≤0.06%, 
 0%≤vanadium≤0.1%, 
 0%≤calcium≤0.005%, 
 0%≤boron≤0.010%, 
 0%≤Magnesium≤0.05%, 
 0%≤Zirconium≤0.05%, 
 0%≤Cerium≤0.1%, 
   a balance including iron and unavoidable impurities, to obtain a semi-finished product;
 reheating the semi-finished product to a temperature above 1000° C.; 
 rolling the the semi-finished product completely in the austenitic range wherein the hot rolling finishing temperature is greater than or equal to 850° C. to obtain a hot rolled steel sheet; 
 cooling the sheet at a cooling rate above 3° C./s to a temperature below or equal to 650° C.; and coiling the hot rolled sheet at temperature of coiling below 650° C. cooling the hot rolled sheet; 
 performing an optional scale removal process on the hot rolled steel sheet; 
 subjecting the hot rolled steel sheet to an optional annealing at a temperature from 350° C. to 750° C. for 1 h to 96 h; 
 performing a further optional scale removal process on the hot rolled annealed steel sheet; 
 cold rolling the said hot rolled steel sheet with a reduction rate from 35 to 70% to obtain a cold rolled steel sheet; 
 annealing the cold rolled steel sheet by heating the steel sheet from room temperature to a temperature a soaking temperature TA from Ac3 -10° C. to Ac3 -50° C., with a heating rate HR1 from 2° C./s and 70° C./s; then 
 performing an annealing at TA during 10 to 1000 seconds, time being selected to obtain a minimum percentage of 90% austenite at the end of the soaking; then 
 cooling the cold rolled steel sheet from TA to cooling stop temperature CS1 from Ms-40° C. to Ms-130° C. with a cooling rate CR1 from 1° C./s to 1000° C./s, and holding the cold rolled steel sheet at CS1 during 1 to 200 seconds; then 
 heating the cold rolled steel sheet from CS1 temperature to an overaging temperature TOA from 350° C. to 450° C. at an average heating rate HR3 from 1° C./s to 100° C./s; then 
 overaging the cold rolled steel sheet at TOA during 5 to 500 seconds; then 
 bringing the cold rolled steel sheet to a coating temperature between 420° C. to 680° C. for coating the cold rolled steel sheet; and thereafter 
 cooling the coated cold rolled steel sheet to room temperature to obtain a cold rolled and coated steel sheet. 
   
     
     
         27 . The method as recited in  claim 26  wherein the TA temperature is from 760° C. to 840° C. 
     
     
         28 . The method as recited in  claim 26  wherein the CS1 temperature is from 190° C. to 250° C. 
     
     
         29 . The method as recited in  claim 26  wherein the TOA temperature is from 360° C. to 440° C. 
     
     
         30 . A method comprising:
 manufacturing structural or safety parts of a vehicle by employing the cold rolled and coated steel sheet as recited in  claim 16 .   
     
     
         31 . A method comprising:
 manufacturing structural or safety parts of a vehicle by employing the cold rolled and coated steel sheet manufactured according to the method as recited in  claim 26 .