EFFECT OF COPPER ON PROPERTIES OF FINE-GRAINED LOW-CARBON BORON STEEL

Low carbon steels are used in variety of structural states. One of the most used is fine grained dual or multiphase structure with prevailing ferrite. Martensite/austenite islands forms rest of the structure. At least two of the phases - pearlite, bainite, martensite and rest austenite - occur in multi-phase steels. Main strengthening mechanisms are grain refinement and the hard phase content. The mostly ferritic soft matrix ensures sufficient plasticity. The constraining phenomena is cohesion of soft ferrite and the hard phase for the steel performance. Voids open upon deformation at the boundary between these constituents and lead to fracture. The difference in strength can be reduced by softening of the hard phase or by strengthening the ferrite. Copper alloying has possibility to do ferrite strengthening by precipitation. The experimental steel was subjected to controlled rolling to achieve dual- or multi-phase structure. Deformation in intercritical region resulted in fine ferritic matrix. Samples were subsequently quenched in water to transform remaining austenite into hard martensite or bainite. The delay between rolling and quenching gave opportunity for Cu to precipitate in freshly-formed ferrite and strengthen it. This approach leaves opportunity to gain precipitation strengthening in soft phase with no loss in hard phase strength. This is different from conventional quenching and tempering approach. The tempering acts also as precipitation hardening, but the tempering decreases strength of martensite in far higher rate than Cu precipitation can strengthen the ferritic matrix.