Hot rolled steel sheet with ultra-high strength and improved formability and method for producing the same

Abstract

A hot rolled high strength steel sheet with improved formability, including of the following elements in weight %:0.13-0.35 C; 0.80-3.50 Mn; 0.01-2.50 Si; 0.02-1.00 Al; wherein the sum of Si and Al is 0.52-2.50; and optionally one or more selected from: 0.0002-0.0030 Ca; 0.0004-0.0100 REM; 0.10-1.00 Cu; 0.10-1.50 Cr; 0.10-1.00 Ni; 0.05-0.50 Mo; 0.0005-0.0050 B; 0.010-0.100 Nb; 0.010-0.100 Ti; 0.020-0.200 V; wherein the sum of Nb+Ti+V is 0-0.250 and wherein the sum of Cr+Ni+Cu+Mo is 0-1.50, and at most 0.100 P; at most 0.050 S; at most 0.0100 N; the balance inevitable impurities and Fe, wherein the hot rolled high strength steel sheet has a complex phase microstructure including in vol. %:65% or more of a matrix of a mixture of carbide free bainitic ferrite (BF) and/or tempered martensite (TM), and 5 to 25% of retained austenite (RA).

1 . A hot rolled high strength steel sheet having a tensile strength of at least 950 MPa consisting of the following elements in weight %:
 0.13-0.35 C;   0.80-3.50 Mn;   0.01-2.50 Si;   0.02-1.00 Al;   wherein the sum of Si and Al is 0.52-2.50; and   optionally one or more selected from:   0.0002-0.0030 Ca;   0.0004-0.0100 REM;   0.10-1.00 Cu;   0.10-1.50 Cr;   0.10-1.00 Ni;   0.05-0.50 Mo;   0.0005-0.0050 B;   0.010-0.100 Nb;   0.010-0.100 Ti;   0.020-0.200 V;   wherein the sum of Nb+Ti+V is 0-0.250 and   wherein the sum of Cr+Ni+Cu+Mo is 0-1.50, and   at most 0.100 P;   at most 0.050 S;   at most 0.0100 N;   the balance consisting of inevitable impurities and Fe,   wherein the hot rolled high strength steel sheet has a complex phase microstructure comprising in vol. %:   65% or more of a matrix of a mixture of carbide free bainitic ferrite (BF) and/or tempered martensite (TM), and   5 to 25% of retained austenite (RA),   and wherein the microstructure comprises less than 3% carbides.   
     
     
         2 . The hot rolled high strength steel sheet according to  claim 1 , wherein for one or more elements the following range(s) is/are valid:
 0.15-0.32 C;   1.20-3.00 Mn;   0.50-2.00 Si;   0.02-0.80 Al,   wherein the sum of Si and Al is 0.80-2.00; and   optionally one or more selected from:   0.0005-0.0020 Ca;   0.0005-0.0080 REM;   0.15-0.80 Cu;   0.15-1.00 Cr;   0.10-1.00 Ni;   0.10-0.30 Mo;   0.0007-0.0040 B;   0.030-0.050 Nb;   0.030-0.050 Ti;   0.030-0.100 V;   wherein the sum of Nb+Ti+V is 0-0.100;   at most 0.050 P;   at most 0.010 S;   at most 0.0050 N.   
     
     
         3 . The hot rolled high strength steel sheet according to  claim 1 ,
 wherein for one or more elements the following range(s) is/are valid:   0.17-0.30 C;   1.40-2.60 Mn;   0.50-1.50 Si;   0.02-0.60 Al,   wherein the sum of Si and Al is 1.00-1.60; and   optionally one or more selected from:   0.0005-0.001 Ca;   0.0005-0.008 REM;   wherein Cu belongs to the inevitable impurities, and/or   wherein Cr belongs to the inevitable impurities, and/or   wherein Ni belongs to the inevitable impurities, and/or   wherein Mo belongs to the inevitable impurities, and/or   wherein B belongs to the inevitable impurities, and/or   wherein Nb belongs to the inevitable impurities, and/or   wherein Ti belongs to the inevitable impurities, and/or   wherein V belongs to the inevitable impurities, and or;   at most 0.020 P;   at most 0.005 S;   at most 0.0040 N.   
     
     
         4 . The hot rolled high strength steel sheet according to  claim 1 , wherein the retained austenite has a carbon concentration of 0.90-1.60% 
     
     
         5 . The hot rolled high strength steel sheet according to  claim 1 , wherein the microstructure further comprises acicular ferrite (AF), and wherein the total volume of BF+TM+AF is 70-95%. 
     
     
         6 . The hot rolled high strength steel sheet according to  claim 1 , wherein the microstructure contains less than 5% polygonal ferrite. 
     
     
         7 . The hot rolled high strength steel sheet according to  claim 1 , wherein the microstructure comprises less than 20% acicular ferrite. 
     
     
         8 . The hot rolled high strength steel sheet according to  claim 1 , wherein the microstructure comprises less than 15% fresh martensite. 
     
     
         9 . The hot rolled high strength steel sheet according to  claim 1 , having a total elongation of at least 10%. 
     
     
         10 . The hot rolled high strength steel sheet according to  claim 1 , having a hole expansion capacity (HEC) of at least 20%. 
     
     
         11 . A method for producing a hot rolled high strength steel sheet according to  claim 1 , comprising the steps of
 casting a slab having a chemical composition according to  claim 1 ,   Reheating the slab above the Ac3 temperature,   Hot rolling the slab in a temperature range in which the structure of the steel is entirely austenitic,   Finishing the hot rolling at a finish hot rolling temperature higher than the Ar3 transformation point,   cooling the hot-rolled steel strip at a cooling rate (Vcool) of 20 to 100° C./s to a stop temperature (Tstop) below Bs−25° C. and above Mf+25° C.,   Coiling the hot-rolled steel strip at a temperature (Tcoil) below Bs−80° C. and above Ms.   
     
     
         12 . The method of  claim 11 , wherein the cooling rate (Vcool) is between 25 and 80° C./s. 
     
     
         13 . The method of  claim 11 , wherein Tstop is below Bs−25° C. and above Ms, preferably below Bs−50° C. 
     
     
         14 . The method of  claim 11 , wherein Tstop is below Ms and above Mf+25° C., preferably above Mf+50° C. 
     
     
         15 . The method of  claim 11 , wherein Tcoil is below Bs−100° C. and above Ms. 
     
     
         16 . The hot rolled high strength steel sheet according to  claim 1 , wherein the microstructure contains less than 5% polygonal ferrite, wherein the microstructure comprises less than 20% acicular ferrite, wherein the microstructure comprises less than 15% fresh martensite.