Effects of cooling process on microstructure, mechanical properties and precipitation behaviors of niobium-titanium micro-alloyed steel

Abstract

The tested steels were cooled to room temperature using different cooling paths after two-stage rolling, and effects of cooling paths on microstructure, mechanical properties and precipitation behaviors of Nb-Ti micro-alloyed steels were investigated. The results show the hot rolled plates with fine grain were produced at the cooling path of ultra fast cooling + air cooling, and the average grain size, lower yield strength and ultimate tensile strength are about 7.76 μm, 425 MPa and 500 MPa, respectively. The fine precipitation particles ranging from 2 nm to 7 nm were observed in the samples cooling with ultra fast cooling + furnace cooling, but are only a few globular precipitates in the samples cooling with ultra fast cooling + air cooling. The inter-phase precipitation was observed in samples cooling with air cooling after finish rolling. These plates with different cooling paths were annealed at 700°C for 300 s. The precipitation particles were obviously coarsened during annealing. It can be found that the average grain size of the samples with cooling path of ultra fast cooling + furnace cooling is 6.47 μm and the increments of lower yield strength and ultimate tensile strength are about 50 and 30 MPa, respectively. The strength increment mainly depends on fine grain strengthening. For niobium-titanium micro-alloyed steels containing 0.03%Nb (mass fraction), because the volume fraction of precipitates is limited, grain boundaries strengthening is higher than precipitation hardening, making changes of strength be in good agreement with that of grain size. In addition, the strain hardening exponent is mainly related to average grain size, and strain hardening exponent increases with average grain size increasing.