Patent ID: 7214278

Claim:
A process for manufacturing a high-strength, ductile, corrosion-resistant carbon steel, said process comprising: (a) heating an alloy composition to a temperature sufficiently high to form a starting microstructure comprising a substantially martensite-free austenite phase, said alloy composition having a martensite start temperature of at least about 330° C. and consisting of iron and alloying elements comprising about 0.03% to about 0.35% carbon, about 1.0% to about 11.0% chromium, and at most about 2.0% manganese; (b) cooling said starting microstructure under conditions causing conversion thereof to an intermediate microstructure of austenite, ferrite, and carbides, said intermediate microstructure comprising contiguous phases of austenite and ferrite with carbide precipitates dispersed in said ferrite phases and substantially no carbide precipitates at phase boundaries; and (c) cooling said intermediate microstructure under conditions causing conversion thereof to a final microstructure of martensite, austenite, ferrite, and carbides, said final microstructure comprising martensite-austenite regions consisting of laths of martensite alternating with thin films of austenite, ferrite regions contiguous with said martensite-austenite regions, and carbide precipitates dispersed in said ferrite regions, with substantially no carbide precipitates at interfaces between said martensite laths and said austenite thin films, or at interfaces between said ferrite regions and said martensite-austenite regions.