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

Abstract

A cold rolled and coated steel sheet having a composition including of the following elements, 0.12%≤Carbon≤0.2%, 1.7%≤Manganese≤2.10%, 0.1%≤Silicon≤0.5%, 0.1%≤Aluminum≤0.8%, 0.1%≤Chromium≤0.5%, 0%≤Phosphorus≤0.09%, 0%≤Sulfur≤0.09%, 0%≤Nitrogen≤0.09%, Nickel≤3%, Niobium≤0.1%, Titanium≤0.1%, Calcium≤0.005%, Copper≤2%, Molybdenum≤0.5%, Vanadium≤0.1%, Boron≤0.003%, Cerium≤0.1%, Magnesium≤0.010%, Zirconium≤0.010% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel sheet including in area fraction, 10 to 60% Bainite, 25 to 55% Ferrite, 5% to 15% Residual Austenite wherein carbon content in residual austenite is between 0.7% and 1% and 5% to 18% Martensite, wherein the cumulated amount of Bainite and Ferrite is at least 70%.

1 - 27 . (canceled) 
     
     
         28 . A cold rolled and coated steel sheet comprising:
 a composition of the following elements, expressed in percentage by weight:
   0.12%≤Carbon≤0.2%
 
   1.7%≤Manganese≤2.10%
 
   0.1%≤Silicon≤0.5%
 
   0.1%≤Aluminum≤0.8%
 
   0.1%≤Chromium≤0.5%
 
   0%≤Phosphorus≤0.09%
 
   0%≤Sulfur≤0.09%.
 
   0%≤Nitrogen≤0.09%
 
 and optionally one or more of the following elements:
   Nickel≤3%
 
   Niobium≤0.1%
 
   Titanium≤0.1%
 
   Calcium≤0.005%
 
   Copper≤2%
 
   Molybdenum≤0.5%
 
   Vanadium≤0.1%
 
   Boron≤0.003%
 
   Cerium≤0.1%
 
   Magnesium≤0.010%
 
   Zirconium≤0.010%
 
 
   
       a remainder of the composition being composed of iron and unavoidable impurities caused by processing, a microstructure of the steel sheet including in area fraction, 10 to 60% Bainite, 25 to 55% Ferrite, 5% to 15% Residual Austenite, a carbon content in the Residual Austenite being between 0.7% and 1%, and 5% to 18% Martensite, wherein a cumulated amount of Bainite and Ferrite is at least 70%. 
     
     
         29 . The cold rolled and coated steel sheet as recited in  claim 28  wherein the composition includes 0.1% to 0.4% of Silicon. 
     
     
         30 . The cold rolled and coated steel sheet as recited in  claim 28  wherein the composition includes 0.12% to 0.19% of Carbon. 
     
     
         31 . The cold rolled and coated steel sheet as recited in  claim 28  wherein the composition includes 0.2% to 0.8% of Aluminum. 
     
     
         32 . The cold rolled and coated steel sheet as recited in  claim 28  wherein the composition includes 1.7% to 2.08% of Manganese. 
     
     
         33 . The cold rolled and coated steel sheet as recited in  claim 28  wherein the composition includes 0.1% to 0.4% of Chromium. 
     
     
         34 . The cold rolled and coated steel sheet as recited in  claim 28  wherein the composition includes 1.8% to 2.08% of Manganese. 
     
     
         35 . The cold rolled and coated steel sheet as recited in  claim 30  wherein the composition includes 0.14% to 0.18% of Carbon. 
     
     
         36 . The cold rolled and coated steel sheet as recited in  claim 28  wherein a cumulated amount of Carbon and Manganese is between 2.1% and 2.25%. 
     
     
         37 . The cold rolled and coated steel sheet as recited in  claim 28  wherein a cumulated amount of Silicon and Aluminum is between 0.5% and 0.9%. 
     
     
         38 . The cold rolled and coated steel sheet as recited in  claim 28  wherein the cumulated amount of Ferrite and Bainite is more than or equal to 74% and the percentage of Ferrite is at least 30%. 
     
     
         39 . The cold rolled and coated steel sheet as recited in  claim 28  wherein the carbon content of Residual Austenite is between 0.7% and 0.9%. 
     
     
         40 . The cold rolled and coated steel sheet as recited in  claim 39  wherein the carbon content of Residual Austenite is between 0.7% and 0.8% 
     
     
         41 . The cold rolled and coated steel sheet as recited in  claim 28  wherein the bainite is between 20% and 60%. 
     
     
         42 . The cold rolled and coated steel sheet as recited in  claim 28  wherein the martensite is between 5% and 15%. 
     
     
         43 . The cold rolled and coated steel sheet as recited in  claim 28  wherein the steel sheet has an ultimate tensile strength of 780 MPa or more, and a total elongation of 18% or more. 
     
     
         44 . The cold rolled and coated steel sheet as recited in  claim 43  whereon the steel sheet has an ultimate tensile strength of 800 MPa or more and a total elongation of greater than equal to 20%. 
     
     
         45 . A method of production of a cold rolled and coated steel sheet comprising the following successive steps:
 providing a semi-finished product with a steel composition of the following elements, expressed in percentage by weight:
   0.12%≤Carbon≤0.2%
 
   1.7%≤Manganese≤2.10%
 
   0.1%≤Silicon≤0.5%
 
   0.1%≤Aluminum≤0.8%
 
   0.1%≤Chromium≤0.5%
 
   0%≤Phosphorus≤0.09%
 
   0%≤Sulfur≤0.09%.
 
   0%≤Nitrogen≤0.09%
 
 and optionally one or more of the following elements:
   Nickel≤3%
 
   Niobium≤0.1%
 
   Titanium≤0.1%
 
   Calcium≤0.005%
 
   Copper≤2%
 
   Molybdenum≤0.5%
 
   Vanadium≤0.1%
 
   Boron≤0.003%
 
   Cerium≤0.1%
 
   Magnesium≤0.010%
 
   Zirconium≤0.010%
 
 
   
       a remainder of the steel composition being composed of iron and unavoidable impurities caused by processing reheating said semi-finished product to a temperature between 1000° C. and 1280° C.;
 rolling the semi-finished product in the temperature range between Ac3 and Ac3+100° C. wherein the hot rolling finishing temperature is above Ac3 to obtain a hot rolled steel; 
 cooling the hot rolled steel at a cooling rate above 30° C./s to a coiling temperature between 475° C. and 650° C.; and coiling the hot rolled steel; 
 cooling the hot rolled steel to room temperature; 
 optionally performing a scale removal process on the hot rolled steel sheet; 
 optionally annealing the hot rolled steel sheet between 400° C. and 750° C.; 
 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; 
 annealing the cold rolled steel sheet in two steps heating wherein:
 the first step starts from heating the steel sheet from room temperature to a temperature T1 between 600° C. and 750° C., with a heating rate HR1 of at least 3° C./s, 
 the second step starts from heating further the steel sheet from T1 to a soaking temperature T2 between Ac1 and Ac3, with a heating rate HR2 of 15° C./s or less, HR2 being lower than HR1, 
 
 annealing at T2 for 10 to 500 seconds; 
 cooling the cold rolled steel sheet from T2 to an over aging temperature T over between 375° C. and 480° C. at an average cooling rate of at least 10° C./s, wherein the cooling optionally includes slow cooling sub-step between T2 and a temperature Tsc between 600° C. and 750° C. with a slow cooling rate of 2° C./s or less; 
 over aging the cold rolled steel sheet at T over for 5 to 500 seconds and bringing to a temperature range between 420° C. and 680° C. to facilitate coating; and 
 coating the cold rolled sheet to obtain a cold rolled coated steel sheet. 
 
     
     
         46 . The method as recited in  claim 45  wherein a coiling temperature is between 475° C. and 625° C. 
     
     
         47 . The method as recited in  claim 45  wherein the finishing rolling temperature is more than 850° C. 
     
     
         48 . The method as recited in  claim 45  wherein the average cooling rate after annealing is more than 15° C./s. 
     
     
         49 . The method as recited in  claim 45  wherein T2 is between Ac1+30° C. and Ac3 and T2 is selected so as to ensure the presence of at least 60% of austenite at the end of the annealing. 
     
     
         50 . The method as recited in  claim 49  wherein T2 is selected so as to ensure the presence of at least 70% of austenite at the end of the annealing. 
     
     
         51 . The method as recited in  claim 45  wherein the temperature for over aging T over is between 380° C. and 460° C. 
     
     
         52 . The method as recited in  claim 45  wherein the first step of heating the cold rolled steel sheet ends at a temperature T1 between 650° C. and 750° C. with a heating rate HR1 of at least 5° C./s. 
     
     
         53 . A method for the manufacture of structural or safety parts of a vehicle comprising performing the method as recited in  claim 45 . 
     
     
         54 . A vehicle comprising a part obtained by the method of  claim 53 . 
     
     
         55 . A method for the manufacture of structural or safety parts of a vehicle comprising employing the cold rolled and coated steel sheet as recited in  claim 28 . 
     
     
         56 . A vehicle comprising a part obtained by the method of  claim 55 .