High strength gradient steels

A gradient steel comprising:

• a central high-retained austenite layer having a first surface and a second surface parallel with the first surface, the central high-retained austenite layer comprising a first amount of carbon;
• a first low-retained austenite layer disposed adjacent to the central high-retained austenite surface of the central high-retained austenite layer;
• a second low-retained austenite layer disposed adjacent to the second surface of the central high-retained austenite layer, wherein the first and second low-retained austenite layers each comprises a second amount of carbon, the second amount of carbon being less than the first amount of carbon;
• a first lean-chemistry, low-martensite layer disposed adjacent to a surface of the first low-retained austenite layer that faces away from the central high-retained austenite layer;
• a second lean-chemistry, low-martensite layer disposed adjacent to a surface of the second low-retained austenite layer that faces away from the central high-retained austenite layer, wherein the first lean-chemistry, low-martensite layer and the second lean-chemistry, low-martensite layer each comprises a third amount of carbon, the third amount of carbon being less than the second amount of carbon;
• a first ferritic layer disposed adjacent to a surface of the first lean-chemistry, low-martensite layer that faces away from the central high-retained austenite layer;
• and
• a second ferritic layer disposed adjacent to a surface of the second lean-chemistry, low-martensite layer that faces away from the central high-retained austenite layer, wherein the first ferritic layer and the second ferritic layer each comprises a fourth amount of carbon, the fourth amount of carbon being less than the third amount of carbon.

A gradient steel comprising:

• a central high-retained austenite layer having a first surface and a second surface parallel with the first surface, the central high-retained austenite layer comprising a greater than or equal to about 50 vol. % to less than or equal to about 70 vol. % of a bainite/tempered martensite phase volume and greater than or equal to about 15 vol. % to less than or equal to about 30 vol. % of a retained austenite phase volume;
• a first low-retained austenite layer disposed adjacent to the first surface of the central high-retained austenite layer;
• a second low-retained austenite layer disposed adjacent to the second surface of the central high-retained austenite layer, wherein the first and second low-retained austenite layers each comprises greater than or equal to about 30 vol. % to less than or equal to about 40 vol. % of a first ferrite phase volume and greater than or equal to about 30 vol. % to less than or equal to about 40 vol. % of a first martensite phase volume;
• a first lean-chemistry, low-martensite layer disposed adjacent to a surface of the first low-retained austenite layer that faces away from the central high-retained austenite layer;
• a second lean-chemistry, low-martensite layer disposed adjacent to a surface of the second low-retained austenite layer that faces away from the central high-retained austenite layer, wherein the first lean-chemistry, low-martensite layer and the second lean-chemistry, low-martensite layer each comprises greater than or equal to about 70 vol. % to less than or equal to about 90 vol. % of a second ferrite phase volume and greater than or equal to about 90 vol. % of a second martensite phase volume;
• a first ferritic layer disposed adjacent to a surface of the first lean-chemistry, low-martensite layer that faces away from the central high-retained austenite layer;
• and
• a second ferritic layer disposed adjacent to a surface of the second lean-chemistry, low-martensite layer that faces away from the central high-retained austenite layer, wherein the first ferritic layer and the second ferritic layer each comprises about 100 vol. % of a third ferrite phase volume.