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

Abstract

A heat treated and cold rolled steel sheet having a composition including of the following elements 0.09%≤Carbon≤0.15%, 1.8%≤Manganese≤2.5%, 0.2%≤Silicon≤0.7%, 0.01%≤Aluminum≤0.1%, 0%≤Phosphorus≤0.09%, 0%≤Sulfur≤0.09%, 0%≤Nitrogen≤0.09%, 0%≤Niobium≤0.1%, 0%≤Titanium≤0.1%, 0%≤Chromium≤1%, 0%≤Molybdenum≤1%, 0%≤Vanadium≤0.1%, 0%≤Calcium≤0.005%, 0%≤Boron≤0.01%, 0%≤Cerium≤0.1%, 0%≤Magnesium≤0.05%, 0%≤Zirconium≤0.05% the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of said steel sheet comprising in area fraction, 65 to 85% Tempered Martensite, 0% to 5% Residual Austenite and a cumulative presence of Ferrite and Bainite between 15 and 35%.

What is claimed is: 
     
         1 - 19 . (canceled) 
     
     
         20 . A heat treated and cold rolled steel sheet having a composition comprising of the following elements, expressed in percentage by weight:
 0.09%≤Carbon≤0.15%   1.8%≤Manganese≤2.5%   0.2%≤Silicon≤0.7%   0.01%≤Aluminum≤0.1%   0%≤Phosphorus≤0.09%   0%≤Sulfur≤0.09%.   0%≤Nitrogen≤0.09%   and optionally one or more of the following elements   0%≤Niobium≤0.1%   0%≤Titanium≤0.1%   0%≤Chromium≤1%   0≤Molybdenum≤1%   0%≤Vanadium≤0.1%   0%≤Calcium≤0.005%   0%≤Boron≤0.01%   0%≤Cerium≤0.1%   0%≤Magnesium≤0.05%   0%≤Zirconium≤0.05%   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, 65 to 85% Tempered Martensite, 0% to 5% Residual Austenite and a cumulative amount of Ferrite and Bainite between 15 and 35%.   
     
     
         21 . The heat treated and cold rolled steel sheet as recited in  claim 20  wherein the composition includes 0.3% to 0.7% of Silicon. 
     
     
         22 . The heat treated and cold rolled steel sheet as recited in  claim 20  wherein the composition includes 0.01% to 0.08% of Aluminum. 
     
     
         23 . The heat treated and cold rolled steel sheet as recited in  claim 20  wherein the composition includes 1.9% to 2.4% of Manganese. 
     
     
         24 . The heat treated and cold rolled steel sheet as recited in  claim 20  wherein the composition includes 0.1% to 0.13% of Carbon. 
     
     
         25 . The heat treated and cold rolled steel sheet as recited in  claim 20  wherein the cumulated amount of Silicon and Aluminum is between 0.3% and 0.8%. 
     
     
         26 . The heat treated and cold rolled steel sheet as recited in  claim 20  wherein the cumulated amount of Ferrite and Bainite is between 22% and 35% wherein the percentage of Ferrite is at least 15% of total area fraction of steel. 
     
     
         27 . The heat treated and cold rolled steel sheet as recited in  claim 20  wherein the carbon content of residual austenite is between 0.7% and 0.9%. 
     
     
         28 . The heat treated and cold rolled steel sheet as recited in  claim 20  wherein the tempered martensite is between 65% and 80%. 
     
     
         29 . The heat treated and cold rolled steel sheet as recited in  claim 20  wherein the Bainite is between 0% and 10%. 
     
     
         30 . The heat treated and cold rolled steel sheet as recited in  claim 20  wherein the steel sheet has an ultimate tensile strength of 950 MPa or more, and a total elongation of 8% or more. 
     
     
         31 . The heat treated and cold rolled steel sheet as recited in  claim 30  wherein the steel sheet has an ultimate tensile strength of 1000 MPa or more and a hole expansion ratio of greater of at least 55%. 
     
     
         32 . A method of production of a heat treated and cold rolled steel sheet comprising the following successive steps:
 providing a semi-finished product having a composition comprising of the following elements, expressed in percentage by weight:   0.09%≤Carbon≤0.15%   1.8%≤Manganese≤2.5%   0.2%≤Silicon≤0.7%   0.01%≤Aluminum≤0.1%   0%≤Phosphorus≤0.09%   0%≤Sulfur≤0.09%.   0%≤Nitrogen≤0.09%   and optionally one or more of the following elements   0%≤Niobium≤0.1%   0%≤Titanium≤0.1%   0%≤Chromium≤1%   0≤Molybdenum≤1%   0%≤Vanadium≤0.1%   0%≤Calcium≤0.005%   0%≤Boron≤0.01%   0%≤Cerium≤0.1%   0%≤Magnesium≤0.05%   0%≤Zirconium≤0.05%   a remainder of the composition being composed of iron and unavoidable impurities caused by processing,   reheating the semi-finished product to a temperature between 1000° C. and 1250° C.;   rolling the semi-finished product in the temperature range between Ac3 and Ac3+100° C. wherein a 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 below 600° C.; and coiling the hot rolled steel;   cooling the hot rolled steel to room temperature;   optionally performing scale removal process on said hot rolled steel sheet;   optionally annealing is performed on the hot rolled steel sheet 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;   annealing the cold rolled steel sheet in two steps heating wherein:
 the first step starts from heating the steel sheet to a temperature HT1 between 600° C. and 650° C., with a heating rate HR1 of at least 10° C./s, 
 the second step starts from heating further the steel sheet from HT1 to an annealing temperature range between Ac3 and Ac3+200° C., with a heating rate HR2 of 1° C./s or more, HR2 being lower than HR1; 
   then perform annealing at annealing temperature for 5 to 1000 seconds;   then cooling the cold rolled steel sheet in a three step cooling wherein:
 the first step starts from cooling the steel sheet from the annealing temperature to a temperature CT1 between 675° C. and 725° C., with a cooling rate CR1 of 10° C./s or less, 
 the second step starts from cooling further the steel sheet from CT1 to CT2 between 450° C. and 550° C., with a cooling rate CR2 of 30° C./s or more, 
 the third step starts from cooling further the steel sheet from CT2 to CT3 between Ms−50° C. and 20° C., with a cooling rate CR2 of 200° C./s or more; 
   then heating the cold rolled steel sheet at a heating rate of at least 10° C./s to a tempering temperature between 300° C. and 380° C. and tempered during 100 s to 1000 seconds;   and then cooling the cold rolled steel sheet to room temperature range to obtain a heat treated and cold rolled steel sheet.   
     
     
         33 . The method as recited in  claim 33  wherein the coiling temperature is between 350° C. and 600° C. 
     
     
         34 . The method as recited in  claim 33  wherein the HT1 temperature is between 600° C. and 630° C. with a BTU heating rate of at least 15° C./s. 
     
     
         35 . The method as recited in  claim 33  wherein annealing soaking temperature is between Ac3+10° C. and Ac3+150° C. 
     
     
         36 . The method as recited in  claim 33  wherein the tempering temperature range is between 320° C. and 360° C. 
     
     
         37 . A method for the manufacture of a structural or safety part of a vehicle comprising performing the method as recited in  claim 33 . 
     
     
         38 . A method for the manufacture of a structural or safety part of a vehicle comprising employing the steel sheet as recited in  claim 20 . 
     
     
         39 . A vehicle comprising the structural or safety part obtained by performing the method as recited in  claim 37 . 
     
     
         40 . A vehicle comprising the structural or safety part obtained by performing the method as recited in  claim 38 .