High-strength plain carbon steels having improved formability

Abstract

Fully killed high-strength plain carbon steels consisting essentially of 0.06% to .20% carbon, .4% to 1.2% manganese, .005% to .3% silicon, .04% maximum sulfur, .04% maximum phosphorus, an inclusion shape control agent comprising .05% to .20% zirconium, or .01% to .10% of rare earths or .01% to .10% mischmetal, balance iron are characterized in a hot-rolled finished condition by a yield strength in excess of 35,000 p.s.i., an ultimate tensile strength in excess of 55,000 p.s.i., ductility as measured by percent elongation (2 inches) in excess of 30%, good toughness and superior formability. The steels are hot-rolled finished in the temperature range 1550° F. to 1650° F., cooled at a rate within the temperature range 20° F. to 135° F. per second and collected by coiling or piling within a temperature range of 900° F. to 1200° F., preferably between 1025° F. to 1175° F.

We claim: 
     
       1. A killed high-strength plain carbon 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 preferred temperature range of 1025° F. to 1175° F., the steel being characterized in a hot rolled condition by a yield strength in excess of 35,000 p.s.i., an ultimate tensile strength in excess of 55,000 p.s.i., ductility as measured by percent elongation (2 inches) in excess of 30%, good toughness and formability, said steel consisting essentially of .06% to .20% carbon, .4% to 1.2% manganese, .005% to .3% silicon, sulfur in an amount up to .04%, .04% maximum phosphorus, a sulfide inclusion shape-control agent selected from the group consisting of .05% 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. 
     
     
       2. The steel of claim 1 wherein the sulfide inclusion shape-control agent comprises .05% to .20% zirconium. 
     
     
       3. The steel of claim 1 wherein the sulfide inclusion shape-control agent comprises .01% to .10% of rare earths. 
     
     
       4. A killed high-strength plain carbon 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 preferred temperature range of 1025° F. to 1175° F., the steel being characterized in a hot rolled condition by a yield strength in excess of 35,000 p.s.i., an ultimate tensile strength in excess of 55,000 p.s.i., ductility as measured by percent elongation (2 inches) in excess of 30%, good toughness and formability, said steel consisting essentially of .12% to .[..15%.]. .Iadd..16% .Iaddend.carbon, .5% to .7% manganese, .[..3%.]. .Iadd..1% .Iaddend.maximum silicon, sulfur in an amount up to .02%, .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. 
     
     
       5. The steel of claim 4 wherein the sulfide inclusion shape-control agent comprises .[..05% to .20%.]. .Iadd..08% to .12% .Iaddend.zirconium. 
     
     
       6. The steel of claim 4 wherein the sulfide inclusion shape-control agent comprises .01% to .10% of rare earths.