High-strength low-alloy steels having improved formability

Abstract

Fully killed high-strength low-alloy steels consisting essentially of .06% to .20% carbon, .50% to 1.4% manganese, .01% to .08% columbium or .04% to .12% vanadium, .05% maximum silicon, .04% maximum sulfur, .04% maximum phosphorus and an inclusion shape-control agent comprising either .06% to .20% zirconium, 0.1% to .10% of a rare earth or .01% to .10% mischmetal are characterized in a hot-rolled finished condition by yield strengths in excess of 45,000 p.s.i., ultimate tensile strengths in excess of 60,000 p.s.i., ductilities as measured by percent elongation (2 inches) in excess of 20%, good toughness, superior formability and reduced directionality. The steels are hot-rolled finished in the temperature range 1550° F. to 1650° F., cooled at a rate within the range 20° F. to 135° F. per second and collected by coiling or piling within a temperature range of 1025° F. to 1175° F.

We claim: 
     
       1. A .[.kill.]. .Iadd.killed .Iaddend.high-strength low-alloy steel which has been hot-rolled finished in the temperature range 1550° F. to 1650° F., cooled at a rate within the range of 20° F. to 135° F. per second, and collected within a temperature range of 1025° F. to 1175° F., the steel being characterized in a hot rolled condition by a yield strength in excess of 45,000 p.s.i., an ultimate tensile strength in excess of 60,000 p.s.i., ductility as measured by percent elongation (2 inches) in excess of 20%, good toughness and formability and reduced directionality, said steel consisting essentially of .06% to .20% carbon, .50% to 1.4% manganese, a strengthening agent selected from the group consisting of .01% to .08% columbium and .04% to .12% vanadium, .5% maximum silicon, suflur in an amount up to .04%, .04% maximum phosphorus, a sulfide inclusion shape-control agent selected from the group consisting of .06% to .20% zirconium, .01% to .10% of a rare earth and .01% to .10% mischmetal, balance iron, the sulfide inclusions in the steel having a substantially spherical shape.Iadd., said zirconium being present in a minimum of 0.02% zirconium +6.5 (wt. percent N).Iaddend.. 
     
     
       2. The steel of claim 1 wherein the strengthening agent comprises .01% to .08% columbium. 
     
     
       3. The steel of claim 1 wherein the strengthening agent comprises .04% to .12% vanadium and the sulfide inclusion shape-control agent comprises .01% to .10% of a rare earth. 
     
     
       4. A killed high-strength low-alloy steel which has been hot-rolled finished in the temperature range 1550° F. to 1650° F., cooled at a rate within the range of 20° F. to 135° F. per second and collected within a temperature range of 1025° F. to 1175° F. the steel being characterized in a hot rolled condition by a yield strength in excess of 45,000 p.s.i., an ultimate tensile strength in excess of 60,000 p.s.i., ductility as measured by percent elongation (2 inches) in excess of 20%, good toughness and formability and reduced directionality, said steel consisting essentially of .10% to .15% carbon, .9% to 1.2% manganese, a strengthening agent selected from the group consisting of .02% to .04% columbium and .04% to .07% vanadium, .05% maximum silicon, sulfur in an amount up to .025%, .03% maximum phosphorus, a sulfide inclusion shape-control agent selected from the group consisting of .08% to .12% zirconium, .01% to .10% of a rare earth and .01% to .10% mischmetal, balance iron, the sulfide inclusions in the steel having a substantially spherical shape.Iadd., said zirconium being present in a minimum of at least 0.02% zirconium +6.5 (wt. percent N).Iaddend.. 
     
     
       5. The steel of claim 4 wherein the strengthening agent comprises .02% to .04% columbium. 
     
     
       6. The steel of claim 5 wherein the sulfide inclusion shape-control agent comprises .08% to .12% zirconium. 
     
     
       7. The steel of claim 5 wherein the sulfide inclusion shape-control agent comprises .01% to .10% of a rare earth. 
     
     
       8. The steel of claim 4 wherein the strengthening agent comprises .04% to .07% vanadium. 
     
     
       9. The steel of claim 8 wherein the sulfide inclusion shape-control agent comprises .01% to .10% of a rare earth.