Steel alloy for a low-alloy high-strength steel

Abstract

A low alloy high-strength carbide-free bainitic steel for producing strips, sheets and pipes is disclosed with the following chemical composition (in weight %) 0.10-0.70 C, 0.25-4.00 Si, 0.05-3.00 Al, 1.00-3.00 Mn, 0.10-2.00 Cr, 0.001-0.50 Nb, 0.001-0.025 N, max 0.15 P, max 0.05 S, remainder iron and steel tramp elements with optional addition of one or more elements of Mo, Ni, Co, W, Nb, Ti, or V and Zr and rare earths with the proviso the for avoiding primary precipitations of AlN the condition Al×N<5×10 −3 (weight %) and for suppressing the cementite formation the condition Si+Al>4×C (weight %) are satisfied.

What is claimed is: 
     
         1 . A steel alloy for a low alloy high-strength carbide-free bainitic steel for producing strips, sheets and pipes with the following chemical composition (in weight %):
 0.10-0.70 C   0.25-4.00 Si   0.05-3.00 Al   1.00-3.00 Mn   0.10-2.00 Cr   0.001-0.50 Nb   0.001-0.025 N   max 0.15 P   max 0.05 S   remainder iron and steel tramp elements with optional addition of one or more elements of Mo, Ni, Co, W, Nb, Ti, or V and Zr and rare earths with the proviso the for avoiding primary precipitations of AlN the condition Al×N<5×10 −3  (weight %) and for suppressing the cementite formation the condition Si+Al>4×C (weight %) is satisfied.   
     
     
         2 . The steel alloy according to  claim 1 , having the following contents in weight %:
 0.15-0.60 C   0.50-1.75 Si   0.07-1.50 Al   1.50-2.50 Mn   0.10-1.75 Cr   0.001-0.10 Nb   0.001-0.015 N   
     
     
         3 . The steel alloy according to  claim 2 , having the following contents in weight %:
 0.18-0.50 C   0.75-1.5 Si   0.09-0.75 Al   1.70-2.50 Mn   0.10-1.5% Cr   0.001-0.05 Nb   0.002-0.010 N   
     
     
         4 . The steel alloy according to  claim 2 , having the following contents in weight percent:
 max. 5.00 Ni   max. 1.00 Mo   max. 2.00 Co   max. 1.50 W   max. 0.10 Ti   max. 0.20 V   wherein the total content of Ti, V is maximally 0.20% and the total content of Ni, Mo, Co, W is maximally 5.50 weight %.   
     
     
         5 . The steel alloy according to  claim 1 , wherein the microstructure consists of carbide free bainite and residual austenite with a proportion of at least 75% bainite at least 10% residual austenite and up to maximally 5% martensite. 
     
     
         6 . The steel alloy according to  claim 1 , wherein for achieving demanded material properties the following conditions for transformation kinetic martensite start temperature and microstructure formation are adhered to:
 ferritic transformation kinetic with C, Mn, Si and Al corresponding to the element contents in weight % and T to a cooling rate in C/s:
   (35×C)+(10×Mn)−Si−(5×Al)+Cr>13 /{dot over (T)}+ 10
 
   bainitic transformation kinetic with C, Mn and Al corresponding to the element contents in weight % and T to the cooling rate in C/s:
   400×exp[(−7×C)−(4×Mn)+8Al+3 ]/{dot over (T)}> 1   
 
   martensite start temperature with, C, Mn, Si Al and Mo correspond to the element contents in weight %:
   525−(350×C)−(45×Mn)−(16×Mo)−(5×Al)<<400
 
   stabilization of residual austenite with C, Si and Al corresponding to the element contents in weight %):
   Si+Al>4×C
 
   avoiding primary AlN precipitations with Al and N corresponding to the element contents in weight %:
   Al×N<5×10 −3  
 
   satisfying the demanded combination of the mechanical properties:
   C+Si/6+Mn/4+Cr+Mo)/3>1 
   
     
     
         7 . The steel alloy according to  claim 1 , characterized in that an average distance of residual austenite lamellas is less than 750 nm. 
     
     
         8 . The steel alloy according to  claim 7 , wherein the average distance of the residual austenite lamellas is less than 500 nm. 
     
     
         9 . The steel alloy according to  claim 1  for use in production of hot or cold rolled strips sheet metals pipes profiles or for forged parts for the automobile industry, construction industry and machine construction and rods and wires. 
     
     
         10 . The steel alloy according to  claim 1  for use in wear parts and parts for armors.