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

Abstract

A cold rolled steel sheet having a composition including of 0.05 % ≤ Carbon ≤ 0.15 %, 1.8% ≤ Manganese ≤ 2.7%, 0.1% ≤ Silicon ≤ 1%,0.01% ≤ Aluminum ≤ 0.8%,0.1% ≤ Chromium ≤ 0.9%,0% ≤ Phosphorus ≤ 0.09%,0.0001% ≤ Titanium ≤ 0.1%,0.0005% ≤ Boron ≤ 0.003%, 0.01% ≤ Niobium ≤ 0.1%, 0 % ≤ Sulfur ≤ 0.09 %, 0 % ≤ Nitrogen ≤ 0.09%, 0% ≤ Vanadium ≤ 0.2%, 0%≤Molybdenum≤0.2%, 0%≤Nickel≤2%, 0% ≤ Copper ≤ 2%, 0% ≤ Calcium ≤ 0.005%, 0% ≤ Cerium ≤ 0.1%, 0% ≤ Magnesium ≦ 0.05%, 0% ≤ Zirconium ≦ 0.05%, the remainder being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel sheet including in area fraction, 40% to 60% martensite, 15 to 40% of inter-critical ferrite, a cumulated amount of 10 to 35% of transformed ferrite and bainite and 0% to 5% of residual austenite.

What is claimed is: 
     
         1 - 18 . (canceled) 
     
     
         19 . A cold rolled steel sheet having a composition comprising the following elements, expressed in percentage by weight:
 0.05 % ≤ Carbon ≤ 0.15 %   1.8% ≤ Manganese ≤ 2.7%   0.1 % ≤ Silicon ≤ 1%   0.01% ≤ Aluminum ≤ 0.8%   0.1% ≤ Chromium ≤ 0.9 %   0 % ≤ Phosphorus ≤ 0.09 %   0.0001% ≤ Titanium ≤ 0.1%   0.0005% ≤ Boron ≤ 0.003%   0.01% ≤ Niobium ≤ 0.1%   0% ≤ Sulfur ≤ 0.09 %   0 % ≤ Nitrogen ≤ 0.09% 
 and one or more of the following optional elements:
 0% ≤ Vanadium ≤ 0.2% 
 0% ≤ Molybdenum ≤ 0.2% 
 0% ≤ Nickel ≤ 2% 
 0% ≤ Copper ≤ 2% 
 0% ≤ Calcium ≤ 0.005% 
 0% ≤ Cerium ≤ 0.1% 
 0% ≤ Magnesium ≦ 0.05% 
 0% ≤ Zirconium ≦ 0.05%; 
 a remainder being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel sheet including, in area fraction, 40% to 60% martensite, 15 to 40% of inter-critical ferrite, a cumulated amount of 10 to 35% of transformed ferrite and bainite and 0% to 5% of residual austenite. 
     
     
         20 . The cold rolled and coated steel sheet as recited in  claim 19  wherein the composition includes 0.2% to 0.9% of Silicon. 
     
     
         21 . The cold rolled and coated steel sheet as recited in  claim 19  wherein the composition includes 0.07% to 0.12% of Carbon. 
     
     
         22 . The cold rolled and coated steel sheet as recited in  claim 19  wherein the composition includes 0.01% to 0.7% of Aluminum. 
     
     
         23 . The cold rolled and coated steel sheet as recited in  claim 19  wherein the composition includes 1.9% to 2.5% of Manganese. 
     
     
         24 . The cold rolled and coated steel sheet as recited in  claim 19  wherein the composition includes 0.2% to 0.8% of Chromium. 
     
     
         25 . The cold rolled and coated steel sheet as recited in  claim 19  wherein the cumulated amounts of Silicon and Aluminum is more than 0.6%. 
     
     
         26 . The cold rolled and coated steel sheet as recited in  claim 19  wherein the cumulated amount of transformed ferrite and bainite is between 15% and 30%. 
     
     
         27 . The cold rolled and coated steel sheet as recited in  claim 19  wherein the martensite amount is between 42% and 58%. 
     
     
         28 . The cold rolled and coated steel sheet as recited in  claim 19  wherein the steel sheet has an ultimate tensile strength of 950 MPa or more, and a total elongation of 14% or more. 
     
     
         29 . The cold rolled and coated steel sheet as recited in  claim 28  wherein said steel sheet has a yield strength of 540 MPa or more. 
     
     
         30 . 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 expressed in percentage by weight:
 0.05 % ≤ Carbon ≤ 0.15 % 
 1.8% ≤ Manganese ≤ 2.7% 
 0.1 % ≤ Silicon ≤ 1% 
 0.01% ≤ Aluminum ≤ 0.8% 
 0.1% ≤ Chromium ≤ 0.9 % 
 0 % ≤ Phosphorus ≤ 0.09 % 
 0.0001% ≤ Titanium ≤ 0.1% 
 0.0005% ≤ Boron ≤ 0.003% 
 0.01% ≤ Niobium ≤ 0.1% 
 0% ≤ Sulfur ≤ 0.09 % 
 0 % ≤ Nitrogen ≤ 0.09% 
   and one or more of the following optional elements:
 0% ≤ Vanadium ≤ 0.2% 
 0% ≤ Molybdenum ≤ 0.2% 
 0% ≤ Nickel ≤ 2% 
 0% ≤ Copper ≤ 2% 
 0% ≤ Calcium ≤ 0.005% 
 0% ≤ Cerium ≤ 0.1% 
 0% ≤ Magnesium ≦ 0.05% 
 0% ≤ Zirconium ≦ 0.05%; 
   a remainder 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 wherein the hot rolling finishing temperature is above Ac3 to obtain a hot rolled steel;   cooling the hot rolled steel at a cooling rate of at least 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 hot rolled steel sheet;   optionally annealing the 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;   heating the cold rolled steel sheet from room temperature to a soaking temperature between Ac1 +60° C. and Ac3,   then performing annealing at the soaking temperature for 5 to 500 seconds   then cooling the cold rolled steel in a two-step cooling process wherein:
 the first step starts from the soaking temperature with a cooling down to a temperature T1 between 550° C. and 650° C., at a cooling rate CR1 of least 3° C./s, 
 the cold rolled steel is then held at T1 during 1 s to 20 s 
 the second step starts then by cooling further the cold rolled steel sheet down from T1 to an overaging temperature T2 between 400° C. and 480° C., at a cooling rate CR2 of at least 3° C./s, 
   then performing overaging at T2 for 5 to 100 seconds,   then optionally bringing to a temperature range between 420° C. and 680° C. to facilitate coating and optionally coated the cold rolled sheet, and   thereafter cooling the cold rolled steel sheet to room temperature at a cooling rate of at least 5° C./s to obtain a cold rolled coated steel sheet.   
     
     
         31 . The method as recited in  claim 30  wherein the coiling temperature is between 475° C. and 625° C. 
     
     
         32 . The method as recited in  claim 30  wherein the soaking temperature is selected so as to ensure the presence of at least 50% of austenite at the end of the soaking. 
     
     
         33 . The method as recited in  claim 30  wherein the temperature for overaging is between 420° C. and 475° C. 
     
     
         34 . The method as recited in  claim 30  wherein the cooling rate after coating is at least 9° C./s. 
     
     
         35 . A method comprising manufacturing a structural or safety part of a vehicle with the steel sheet as recited in  claim 19 . 
     
     
         36 . A vehicle comprising the part obtained according to the method as recited in  claim 35 . 
     
     
         37 . A method comprising manufacturing a structural or safety part of a vehicle including performing the method as recited in  claim 30 . 
     
     
         38 . A vehicle comprising the part obtained according to the method as recited in  claim 37 .