ABSTRACT

The effect of low-alloy additions on hardenability and the consequent response to tempering of high strength low-C steels is presented. Various as-quenched materials with microstructures consisting of low-C (granular) bainitic, mixed bainitic/martensitic and fully martensitic microstructures were produced and their response to tempering in terms of evolution of microstructure and precipitation, and changes in strength-toughness combination were studied. Results show that:

• Austenite grain size (AGS) has an important role in determining steel hardenability and as-quenched microstructure, for a given chemical composition and cooling rate. For a given cooling rate, an increase of Mo content decreases the transformation temperatures and promotes martensite formation.
• Microstructure after tempering consists of complex aggregates of high and lowangle units and carbides.
• Toughness (50%FATT) is related to the inverse square root of the packet size.
• For a given AGS, the increase of martensite volume fraction formed after quenching leads to a finer packet, enhancing toughness. In bainitic microstructures, coarsening of AGS leads to larger packets with possible detrimental effects on toughness. In microstructures predominantly constituted of martensite, the packets appear always fine (< 5 m), independently from AGS.
• The average cell size is the key factor in defining the yield strength according to a “Hall-Petch” law.

Results from such investigation were used to identify target microstructures for high strength Q&T seamless linepipe steels with minimum specified yield strength