Low density cold rolled and annealed steel sheet, method of production thereof and use of such steel to produce vehicle parts

Abstract

A low density cold rolled and annealed steel sheet having 0.12%≤carbon≤0.25%, 3%≤manganese≤10%, 3.5%≤aluminum≤6.5%, 0%≤phosphorus≤0.1%, 0%≤sulfur≤0.03%, 0%≤nitrogen≤0.1%, 0%≤silicon≤2%, 0.01%≤niobium≤0.03%, 0.01%≤titanium≤0.2%, 0%≤molybdenum≤0.5%, 0%≤chromium≤0.6%, 0.01%≤copper≤2.0%, 0.01%≤nickel≤3.0%, 0%≤calcium≤0.005%, 0%≤boron≤0.01%, 0%≤Magnesium≤0.005%, 0%≤Zirconium≤0.005%, 0%≤Cerium≤0.1%, and the balance including iron, the steel sheet having a microstructure comprising 60% to 90% Delta ferrite, 8% to 30% of residual austenite having average grain size between 0.6 and 2 microns, 1.0% to 10% of alpha-ferrite having average grain size between 0.6 and 2 microns and 0% to 2% of kappa precipitates (Fe,Mn)3AlCx, where x is strictly lower than 1.

What is claimed is: 
     
         1 - 14 . (canceled) 
     
     
         15 . A low density cold rolled and annealed steel sheet comprising by weight:
 0.12%≤carbon≤0.25%,   3%≤manganese≤10%,   3.5%≤aluminum≤6.5%,   0%≤phosphorus≤0.1%,   0%≤sulfur≤0.03%,   0%≤nitrogen≤0.1%,   
       and optionally one or more of the following elements
 0%≤silicon≤2%, 
 0.01%≤niobium≤0.03%, 
 0.01%≤titanium≤0.2%, 
 0%≤molybdenum≤0.5%, 
 0%≤chromium≤0.6%, 
 0.01%≤copper≤2.0%, 
 0.01%≤nickel≤3.0%, 
 0%≤calcium≤0.005%, 
 0%≤boron≤0.01%, 
 0%≤Magnesium≤0.005%, 
 0%≤Zirconium≤0.005%, 
 0%≤Cerium≤0.1%, 
 
       and a balance including iron and unavoidable impurities;
 the steel sheet having a microstructure comprising, in area fraction, 60% to 90% Delta ferrite, 8% to 30% of residual austenite having an average grain size between 0.6 and 2 microns, 1.0% to 10% of alpha-ferrite having an average grain size between 0.6 and 1.85 microns and 0% to 2% of kappa precipitates (Fe,Mn)3AICx, where x is strictly lower than 1. 
 
     
     
         16 . The steel sheet as recited in  claim 15  wherein the carbon content is between 0.13% and 0.2%. 
     
     
         17 . The steel sheet as recited in  claim 15  wherein the manganese content is between 4% and 9%. 
     
     
         18 . The steel sheet as recited in  claim 15  wherein the residual austenite content is between 9% and 29%. 
     
     
         19 . The steel sheet as recited in  claim 15  wherein the kappa precipitates content is between 0% and 1%. 
     
     
         20 . The steel sheet as recited in  claim 15  wherein the alpha-ferrite content is between 2% and 10% having an average grain size between 0.6 and 1.2 microns. 
     
     
         21 . The steel sheet as recited in  claim 15  wherein the steel sheet is covered by a metallic coating. 
     
     
         22 . A method for producing a steel sheet comprising the following steps:
 feeding a slab having a composition by weight of:
 0.12%≤carbon≤0.25%, 
 3%≤manganese≤10%, 
 3.5%≤aluminum≤6.5%, 
 0%≤phosphorus≤0.1%, 
 0%≤sulfur≤0.03%, 
 0%≤nitrogen≤0.1%, 
   and optionally one or more of the following elements   0%≤silicon≤2%,   0.01%≤niobium≤0.03%,   0.01%≤titanium≤0.2%,   0%≤molybdenum≤0.5%,   0%≤chromium≤0.6%,   0.01%≤copper≤2.0%,   0.01%≤nickel≤3.0%,   0%≤calcium≤0.005%,   0%≤boron≤0.01%,   0%≤Magnesium≤0.005%,   0%≤Zirconium≤0.005%,   0%≤Cerium≤0.1%,   and a balance including iron and unavoidable impurities;   reheating the slab at a temperature above 1000° C. and hot rolling the slab with a final rolling temperature of at least 750° C. to define a hot rolled steel sheet;   coiling the hot rolled steel sheet at a temperature below 720° C.;   cooling the hot rolled steel sheet;   performing pickling on said hot rolled steel sheet;   cold rolling the hot rolled steel sheet with a reduction rate from 30 to 90% to obtain a cold rolled steel sheet;   annealing the cold rolled steel sheet by heating the steel sheet from room temperature to an annealing temperature from 840° C. to 1000° C., with a heating rate of at least 1° C./s;   then perform annealing for less than 1000 seconds;   then cooling the cold rolled steel sheet to a cooling stop temperature from 480° C. to room temperature with a cooling rate of at least 3° C./s, and optionally holding the cold rolled steel sheet between 100° C. to 480° C. for 1 to 200 seconds; and   thereafter cooling the cold rolled steel sheet to room temperature to obtain a low density cold rolled and annealed steel sheet.   
     
     
         23 . The method as recited in  claim 22  wherein the annealing temperature is between 850° C. and 975° C. 
     
     
         24 . The method as recited in  claim 22  wherein the coiling temperature is between 350° C. and 720° C. 
     
     
         25 . The method as recited in  claim 22  wherein the holding time of the annealing is less than 600 seconds. 
     
     
         26 . The method as recited in  claim 22  wherein the heating rate for the annealing is more than 3° C./s. 
     
     
         27 . The method as recited in  claim 22  further comprising a final coating step. 
     
     
         28 . A method for manufacturing a structural or safety part of a vehicle comprising the method as recited in  claim 22 . 
     
     
         29 . A method for manufacturing a structural or safety part of a vehicle comprising employing the steel sheet as recited in  claim 15 .