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A high strength steel with a combination of ∼930 MPa yield strength and excellent low temperature toughness with an upper shelf energy of above 200 J and ductile brittle transition temperature (DBTT) of lower than −90 °C is developed. The strengthening and toughening mechanisms are investigated systematically based on the detailed characterization on microstructures including the matrix and precipitates. The results indicate that the steel is composed of a fine lath martensite with rod-like Cu precipitates. The high strength is achieved by a combination of solid-solution strengthening, dislocation strengthening, grain boundary strengthening and precipitation strengthening of Cu-precipitates. The instrumented Charpy impact results further indicate that the crack propagation is the main factor affecting DBTT while the dislocation density has an obvious effect on both crack initiation and propagation. The fine lath structure of the low carbon martensite enhances the crack resistance and delays the rapid unstable crack propagation at low temperatures. Both the strengthening and toughening are thoroughly discussed in details. • The instrumented Charpy impact results indicate the crack propagation is the main factor affecting DBTT. • The dislocation density has an obvious effect on both crack initiation and propagation. • The combination of Cu-rich precipitates and lath martensite achieves high strength and high toughness. [ABSTRACT FROM AUTHOR]

A high strength steel with a combination of ∼930 MPa yield strength and excellent low temperature toughness with an upper shelf energy of above 200 J and ductile brittle transition temperature (DBTT) of lower than −90 °C is developed. The strengthening and toughening mechanisms are investigated systematically based on the detailed characterization on microstructures including the matrix and precipitates. The results indicate that the steel is composed of a fine lath martensite with rod-like Cu precipitates. The high strength is achieved by a combination of solid-solution strengthening, dislocation strengthening, grain boundary strengthening and precipitation strengthening of Cu-precipitates. The instrumented Charpy impact results further indicate that the crack propagation is the main factor affecting DBTT while the dislocation density has an obvious effect on both crack initiation and propagation. The fine lath structure of the low carbon martensite enhances the crack resistance and delays the rapid unstable crack propagation at low temperatures. Both the strengthening and toughening are thoroughly discussed in details. • The instrumented Charpy impact results indicate the crack propagation is the main factor affecting DBTT. • The dislocation density has an obvious effect on both crack initiation and propagation. • The combination of Cu-rich precipitates and lath martensite achieves high strength and high toughness. [ABSTRACT FROM AUTHOR]