Low-carbon steels of superior mechanical and corrosion properties and process of making thereof
AssigneeMMFX Steel Corporation of America
InventorGareth Thomas
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Abstract
Alloy steels that combine high strength and toughness with high corrosion resistance are achieved by a dislocated lath microstructure, in which dislocated martensite laths that are substantially free of twinning alternate with thin films of retained austenite, with an absence of autotempered carbides, nitrides and carbonitrides in both the dislocated martensite laths and the retained austenite films. This microstructure is achieved by selecting an alloy composition whose martensite start temperature is 350° C. or greater, and by selecting a cooling regime from the austenite phase through the martensite transition region that avoids regions in which autotempering occurs.
Claims27
A process for manufacturing a high-strength, corrosion-resistant, tough alloy carbon steel, comprising:
- (a) forming an alloy composition consisting of iron and at least one alloying element comprising carbon in proportions selected to provide said alloy composition with a martensite transition range having a martensite start temperature Ms of at least about 350° C., said proportions further selected to permit air-cooling of said alloy composition through said martensite transition range without forming carbides;
- (b) heating said alloy composition to a temperature sufficiently high to cause austenitization thereof, under conditions causing said alloy composition to assume a homogeneous austenite phase with all alloying elements in solution;
- and
- (c) cooling said homogeneous austenite phase through said martensite transition range at a cooling rate sufficiently fast to avoid the occurrence of autotempering, to achieve a microstructure containing laths of martensite alternating with films of retained austenite and containing substantially no carbides.
Dependencies are inferred from the claim text.