Computer-aided design of transformation toughened blast resistant naval hull steels: Part I

A systematic approach to computer-aidedmaterials design has formulated a new class of ultratough, weldable secondary hardened plate steels combining new levels of strength and toughness while meeting processability requirements. A theoretical design concept integrated the mechanism of precipitated nickel-stabilized dispersed austenite for transformation toughening in an alloy strengthened by combined precipitation of M2C carbides and BCC copper both at an optimal ∼3 nm particle size for efficient strengthening. This concept was adapted to plate steel design by employing amixed bainitic/martensitic matrixmicrostructure produced by air-cooling after solution-treatment and constraining the composition to low carbon content for weldability. With optimized levels of copper and M2C carbide formers based on a quantitative strength model, a required alloy nickel content of 6.5wt% was predicted for optimal austenite stability for transformation toughening at the desired strength level of 160 ksi (1,100MPa) yield strength. A relatively high Cu level of 3.65wt% was employed to allow a carbon limit of 0.05wt% for good weldability, without causing excessive solidification microsegregation.