Low density high strength steel and method for producing said steel

Abstract

A low density high strength steel sheet including 0.15% to 0.25% C, 2.5% to 4% Mn, 0.02% or less P, 0.015% or less S, 6% to 9% Al and 0.01% or less N, the balance being iron and inevitable impurities, wherein 1.7·(Mn—Al)+52.7·C is at least 3 and at most 4.5. A method of producing the low density and high strength steel sheet.

1 . A low density high strength steel sheet comprising 0.15% to 0.25% C, 2.5% to 4% Mn, 0.02% or less P, 0.015% or less S, 6% to 9% Al and 0.01% or less N, optionally comprising one or two or more elements selected from the group consisting of 0.1% to 2.0% Si, 0.1% to 0.3% Cr, 0.05% to 0.5% Mo, 0.1% to 2.0% Ni, 0.1% to 1.0% Cu, 0.0005% to 0.003% B, 0.01% to 0.2% Ti, 0.005% to 0.2% Zr, 0.05% to 1.0% W, 0.001% to 0.2% Ca, the balance being iron and inevitable impurities, wherein 1.7·(Mn—Al)+52.7·C is at least 3 and at most 4.5. 
     
     
         2 . The steel sheet according to  claim 1 , wherein the amount of retained austenite in the sheet is 5% or more. 
     
     
         3 . The steel sheet according to  claim 1 , wherein the carbon content is at most 0.23%. 
     
     
         4 . The steel sheet according to  claim 1 , wherein the density of the steel sheet is at most 7400 kg/m 3 . 
     
     
         5 . The steel sheet according to  claim 1 , wherein a coating layer is provided on one or both of the major surfaces, wherein the coating layer is selected from Zn, Zn—Fe, Zn—Al, Zn—Mg, Zn—Al—Mg, Al—Si, and Al—Mg—Si, and wherein the coating thickness on the or each major surface is between 10 μm and 200 μm. 
     
     
         6 . The steel sheet according to  claim 1 , wherein the value of (36·C+Mn)/Al is at least 1.3 and at most 2.0. 
     
     
         7 . A process for producing a low density high strength steel sheet comprising 0.15% to 0.25% C, 2.5% to 4% Mn, 0.02% or less P, 0.015% or less S, 6% to 9% Al, and 0.01% or less N, optionally comprising one or two or more elements selected from the group consisting of 0.1% to 2.0% Si, 0.1% to 0.3% Cr, 0.05% to 0.5% Mo, 0.1% to 2.0% Ni, 0.1% to 1.0% Cu, 0.0005% to 0.003% B, 0.01% to 0.2% Ti, 0.005% to 0.2% Zr, 0.1% to 1.0% W, and 0.001% to 0.2% Ca, the balance being iron and inevitable impurities, wherein 1.7·(Mn—Al)+52.7·C is at least 3 and at most 4.5, the process comprising the steps of:
 providing a steel slab, thin slab or strip having the said chemical composition; 
 preparing the said slab or strip for hot rolling by reheating it within a range of from 1000° C. to 1250° C.; 
 subjecting the slab or strip to the last finishing hot-rolling step while the rolled strip has a temperature of between 700° C. and 900° C.; 
 coiling the hot-rolled strip at 600° C. or less. 
 
     
     
         8 . Process according to  claim 7 , the process further comprising the steps of:
 cold-rolling the hot-rolled steel slab or strip at a rolling reduction of 40% to 90% to produce a cold-rolled steel strip;   continuously annealing the cold-rolled steel strip at a maximum temperature of 900° C. at an annealing rate of 1 to 20° C./s for 10 to 180 seconds.   
     
     
         9 . Process for producing a low density and high strength steel sheet according to  claim 8 , wherein the continuous annealing process is a recovery annealing. 
     
     
         10 . Process for producing a low density and high strength steel sheet according to  claim 8 , wherein the continuous annealing process is a recrystallisation annealing. 
     
     
         11 . Process according to  claim 7 , wherein the amount of retained austenite in the steel sheet is 5% or more. 
     
     
         12 . Process according to  claim 7 , wherein the value of (36·C+Mn)/Al is at least 1.3 and at most 2.0. 
     
     
         13 . Process according to  claim 8 , wherein after the annealing process a coating layer is provided on one or both of the major surfaces, wherein the coating layer is selected from Zn, Zn—Fe, Zn—Al, Zn—Mg, Zn—Al—Mg, Al—Si, and Al—Mg—Si. 
     
     
         14 . Process according to  claim 8 , wherein the annealing process includes an overageing step. 
     
     
         15 . Process according to  claim 8 , wherein the combination of annealing temperature and annealing time is sufficient to obtain full recrystallisation of the cold-rolled steel strip. 
     
     
         16 . The steel sheet according to  claim 1 , wherein the density of the steel sheet is at most 7300 kg/m 3 , and wherein the steel sheet has a tensile strength of 600 to 900 MPa. 
     
     
         17 . Process according to  claim 8 , wherein after the annealing process a coating layer is provided on one or both of the major surfaces, wherein the coating thickness on the or each major surface is between 10 μm and 200 μm. 
     
     
         18 . Process according to  claim 14 , wherein the overageing step is at a overageing temperature between 350 and 500° C. for a period of between 30 to 180 seconds. 
     
     
         19 . Process according to  claim 18 , wherein the annealing temperature of the annealing process is at least 825° C. and/or at most 875° C. 
     
     
         20 . Process according to  claim 15 , wherein the annealing temperature is 750° C. or more.