Development of a high strength high toughness steel
A bainitic steel with simultaneous high yield strength and high fracture toughness includes at least 5 volume percent austenite as well as iron, carbon, and silicon. The silicon is present in an amount of at least 1.5 weight percent of total weight of the bainitic steel. A method of forming the steel by austempering is also provided.
1. A method of forming a steel, the method comprising:
a) providing a steel alloy composition comprising iron, carbon in an amount from about 0.2 weight percent to about 2.0 weight percent, silicon in an amount from about 1.8 weight percent to about 5 weight percent, manganese in an amount from about 0.1 to 0.4 weight percent, nickel in an amount from about 0.5 to 2 weight percent, chromium in an amount from about 0.5 to about 1.5 weight percent, molybdenum in an amount from about 0.1 to about 0.6 weight percent, and copper in an amount from about 0.1 to about 0.8 weight percent;
b) heating the steel alloy composition to a temperature of at least 500° F., and
c) maintaining the temperature of the steel alloy composition at a temperature of at least 500° F. for a period of time sufficient to induce transformation of the steel alloy to obtain ferrite and carbon-enriched austenite, the ferrite being present in an amount of at least 80 volume percent and the carbon-enriched austenite being present in an amount of at least 5 volume percent and wherein the steel has a tensile strength of at least 1354 MPa.
2. The method of claim 1 wherein the steel alloy is maintained at a temperature of at least 500° F. for a sufficient time to transform from about 5 volume percent to about 20 volume percent of the steel alloy composition to austenite.
3. The method of claim 1 wherein the steel alloy composition comprises silicon in an amount from about 1.8 weight percent to about 2.5 weight percent.
4. The method of claim 1 wherein the steel alloy composition comprises carbon in an amount from about 0.3 weight percent to about 0.7 weight percent.
5. The method of claim 1 wherein the steel alloy includes sulfur in an amount from about 0.001 to about 0.01 weight percent and phosphorus in an amount from about 0.001 to about 0.05 weight percent.
6. The method of claim 1 wherein the steel alloy composition in step c) is maintained at a temperature between 500 to about 800° F.
7. The method of claim 1 wherein the steel alloy composition in step c) is maintained at a temperature between about 600 to about 750° F.
8. A steel comprising at least 80 volume percent ferrite and at least 5 volume percent carbon-enriched austenite, the steel including iron, carbon in an amount from about 0.2 weight percent to about 2.0 weight percent, silicon in an amount from about 1.8 weight percent to about 5 weight percent, manganese in an amount from about 0.1 to 0.4 weight percent, nickel in an amount from about 0.5 to 2 weight percent, chromium in an amount from about 0.5 to about 1.5 weight percent, molybdenum in an amount from about 0.1 to about 0.6 weight percent, and copper in an amount from about 0.1 to about 0.8 weight percent and wherein the steel has a tensile strength of at least 1354 MPa.
9. The steel of claim 8 wherein the silicon is present in an amount from about 1.8 weight percent to about 2.5 weight percent.
10. The steel of claim 8 wherein the carbon is present in an amount from about 0.3 weight percent to about 0.7 weight percent.
11. The steel of claim 8 further comprising sulfur in an amount from about 0.001 to about 0.01 weight percent and phosphorus in an amount from about 0.001 to about 0.05 weight percent.
12. The steel of claim 8 comprising plates of ferrite separated by carbon enriched regions of austenite.
13. A method of forming a steel, the method comprising:
a) providing a steel alloy composition comprising iron, carbon in an amount from about 0.2 weight percent to about 2.0 weight percent, silicon in an amount from about 1.8 weight percent to about 5 weight percent, manganese in an amount from about 0.1 to 0.4 weight percent, nickel in an amount from about 0.5 to 2 weight percent, chromium in an amount from about 0.5 to about 1.5 weight percent, molybdenum in an amount from about 0.1 to about 0.6 weight percent, and copper in an amount from about 0.1 to about 0.8 weight percent;
b) heating the steel alloy composition to a temperature of at least 500° F.; and
c) maintaining the temperature of the steel alloy composition at a temperature of 600 to about 750° F. for a period of time sufficient to induce transformation of the steel alloy to obtain ferrite and carbon-enriched austenite, the ferrite being present in an amount of at least 80 volume percent and the carbon-enriched austenite being present in an amount of at least 5 volume percent and wherein the steel has a tensile strength of at least 1354 MPa.
14. The method of claim 1 wherein carbon content of the austenite is from about 1.51 to about 1.93 weight percent.
15. The method of claim 1 wherein the steel includes plates of ferrite separated by carbon-enriched regions of austenite.
16. The steel of claim 8 wherein carbon content of the austenite is from about 1.51 to about 1.93 weight percent.
17. The method of claim 1 wherein the steel alloy composition is melted, cast, and hot rolled prior to step b).
18. The steel of claim 8 wherein the steel is formed by a method comprising:
a) melting, casting, and hot rolling a steel alloy composition;
b) heating the steel alloy composition to a temperature of at least 500° F., and
c) maintaining the temperature of the steel alloy composition at a temperature of at least 500° F. for a period of time sufficient to induce transformation of the steel alloy to obtain ferrite and carbon-enriched austenite, the ferrite being present in an amount of at least 80 volume percent and the austenite being present in an amount of at least 5 volume percent.
19. The method of claim 1 wherein the steel alloy is melted, cast, and hot rolled prior to step b).