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

Abstract

A cold rolled and heat treated steel sheet having a composition having the following elements: 0.13%≤Carbon≤0.18%, 1.1%≤Manganese≤1.8%, 0.5%≤Silicon≤0.9%, 0.6%≤Aluminum≤1%, 0.002%≤Phosphorus≤0.02%, 0%≤Sulfur≤0.003%, 0%≤Nitrogen≤0.007% and can contain one or more of the following optional elements: 0.05%≤Chromium≤1%, 0.001%≤Molybdenum≤0.5%, 0.001%≤Niobium≤0.1%, 0.001%≤Titanium≤0.1%, 0.01%≤Copper≤2%, 0.01%≤Nickel≤3%, 0.0001%≤Calcium≤0.005%, 0%≤Vanadium≤0.1%, 0%≤Boron≤0.003%, 0%≤Cerium≤0.1%, 0%≤Magnesium≤0.010%, 0%≤Zirconium≤0.010%, the remainder composition being iron and unavoidable impurities caused by processing, the microstructure of the steel sheet being in area fraction, 60 to 75% Ferrite, 20 to 30% Bainite, 10 to 15% Residual Austenite, and 0% to 5% Martensite, wherein the cumulated amounts of Residual Austenite and Ferrite is between 70% and 80%.

What is claimed is: 
     
         1 - 20 . (canceled) 
     
     
         21 . A cold rolled steel sheet having a composition comprising the following elements, expressed in percentage by weight:
 0.13%≤Carbon≤0.18%   1.1%≤Manganese≤1.8%   0.5%≤Silicon≤0.9%   0.6%≤Aluminum≤1%   0.002%≤Phosphorus≤0.02%   0%≤Sulfur≤0.003%.   0%≤Nitrogen≤0.007%   and optionally one or more of the following elements:   0.05%≤Chromium≤1%   0.001%≤Molybdenum≤0.5%   0.001%≤Niobium≤0.1%   0.001%≤Titanium≤0.1%   0.01%≤Copper≤2%   0.01%≤Nickel≤3%   0.0001%≤Calcium≤0.005%   0%≤Vanadium≤0.1%   0%≤Boron≤0.003%   0%≤Cerium≤0.1%   0%≤Magnesium≤0.010%   0%≤Zirconium≤0.010%;   a remainder of the composition being composed of iron and unavoidable impurities caused by processing,   a microstructure of the steel sheet comprising in area fraction, 60 to 75% Ferrite, 20 to 30% Bainite, 10 to 15% Residual Austenite, and 0% to 5% Martensite, wherein cumulated amounts of the Residual Austenite and the Ferrite is between 70% and 80%.   
     
     
         22 . The cold rolled steel sheet as recited in  claim 21  wherein the composition includes 0.6% to 0.8% of Silicon. 
     
     
         23 . The cold rolled steel sheet as recited in  claim 21  wherein the composition includes 0.14% to 0.18% of Carbon. 
     
     
         24 . The cold rolled steel sheet as recited in  claim 23  wherein the composition includes 0.6% to 0.8% of Aluminum. 
     
     
         25 . The cold rolled steel sheet as recited in  claim 21  wherein the composition includes 1.2% to 1.8% of Manganese. 
     
     
         26 . The cold rolled steel sheet as recited in  claim 25  wherein the composition includes 1.3% to 1.7% of Manganese. 
     
     
         27 . The cold rolled steel sheet as recited in  claim 21  wherein the cumulated amount of the Ferrite and the Residual Austenite is between 73% and 80% and the percentage of Residual Austenite is less than 13%. 
     
     
         28 . The cold rolled steel sheet as recited in  claim 21  wherein the amount of Martensite is between 0% and 3%. 
     
     
         29 . The cold rolled steel sheet as recited in  claim 21  wherein the Carbon content of Residual Austenite is between 0.9 to 1.1%. 
     
     
         30 . The cold rolled steel sheet as recited in  claim 21  wherein the steel sheet has an ultimate tensile strength of 600 MPa or more, and a total elongation of 31% or more. 
     
     
         31 . The cold rolled steel sheet as recited in  claim 30  wherein said steel sheet has yield strength of 320 MPa or more and a total elongation of 33% or more. 
     
     
         32 . The cold rolled steel sheet as recited in  claim 21  wherein the steel sheet is coated. 
     
     
         33 . A method of production of a cold rolled steel sheet comprising the following successive steps:
 providing a semi-finished product having a steel composition comprising the following elements, expressed in percentage by weight:
 0.13%≤Carbon≤0.18% 
 1.1%≤Manganese≤1.8% 
 0.5%≤Silicon≤0.9% 
 0.6%≤Aluminum≤1% 
 0.002%≤Phosphorus≤0.02% 
 0%≤Sulfur≤0.003%. 
 0%≤Nitrogen≤0.007% 
   and optionally one or more of the following elements:
 0.05%≤Chromium≤1% 
 0.001%≤Molybdenum≤0.5% 
 0.001%≤Niobium≤0.1% 
 0.001%≤Titanium≤0.1% 
 0.01%≤Copper≤2% 
 0.01%≤Nickel≤3% 
 0.0001%≤Calcium≤0.005% 
 0%≤Vanadium≤0.1% 
 0%≤Boron≤0.003% 
 0%≤Cerium≤0.1% 
 0%≤Magnesium≤0.010% 
 0%≤Zirconium≤0.010%; 
   a remainder of the composition being composed of iron and unavoidable impurities caused by processing;   reheating the semi-finished product to a temperature between 1150° C. and 1280° C.;   rolling the semi-finished product in the austenitic range wherein the hot rolling finishing temperature is between Ac1+50° C. and Ac1+250° C. to obtain a hot rolled steel sheet;   cooling the sheet at a cooling rate above 30° C./s to a coiling temperature below 625° C.;   and coiling the hot rolled sheet;   cooling the hot rolled sheet to room temperature;   optionally performing scale removal process on said hot rolled steel sheet;   optionally annealing the hot rolled steel sheet at temperature between 400° C. and 750° C.;   optionally performing 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 performing an annealing at a soaking temperature between Ac1+30° C. and Ac3 for a duration between 10 and 500 seconds by heating the cold rolled steel sheet by a two step heating wherein:
 in step one of the two step heating, the cold rolled steel sheet is heated at a heating rate between 10° C./s and 40° C./s to a temperature range between 550° C. and 650° C.; 
 then in step two of the two step heating, the cold rolled steel sheet is heated at a heating rate between 1° C./s and 5° C./s from a temperature range between 550° C. and 650° C. to the annealing soaking temperature and maintained at the annealing soaking temperature, 
   then cooling the cold rolled steel sheet in a two step cooling wherein:
 in step one of the two step cooling, the cold rolled steel sheet is cooled at a cooling rate of less 5° C./s to temperature range between 600° C. and 720° C.; 
 then in step two of the two step cooling, the sheet is cooled at a cooling rate between 10° C./s to 100° C./s from a temperature range between 600° C. and 720° C. to an overaging temperature; 
   then the cold rolled steel sheet is overaged at a temperature range between 250° C. and 470° C. for 5 to 500 seconds and then cooling to room temperature to obtain the produced cold rolled steel sheet.   
     
     
         34 . The method as recited in  claim 33  wherein the coiling temperature is below 600° C. 
     
     
         35 . The method as recited in  claim 33  wherein the finishing rolling temperature is between Ac1+50° C. and Ac1+200° C. 
     
     
         36 . The method as recited in  claim 33  wherein the cooling rate after annealing is less than 3° C./s in the temperature range between 625° C. and 720° C. 
     
     
         37 . The method as recited in  claim 33  wherein the cold rolled steel sheet is annealed between Ac1+30° C. and Ac3 and the temperature of annealing is selected so as to ensure the presence of at least 30% of austenite at the end of the soaking. 
     
     
         38 . The method as recited in  claim 33  wherein the cold rolled steel sheet can be coated a temperature range between 400° C. and 480° C. 
     
     
         39 . A structural or safety part of a vehicle comprising the steel sheet as recited in  claim 21 . 
     
     
         40 . A vehicle comprising the structural of safety part recited in  claim 39 . 
     
     
         41 . A method for manufacturing a structural or safety part of a vehicle comprising the method as recited in  claim 33 . 
     
     
         42 . A vehicle comprising the structural or safety part obtained according to  claim 41 .