Pinning effect of strain induced Nb(C,N) on case hardening steel under warm forging conditions

Abstract

The effectiveness of Nb in retarding dynamic recrystallization (DRX) during warm forging process (700∘C–900 ∘C) and its dependence on the process parameters(ε,ε˙,T), which are the definers of formation conditions and morphology of the strain induced Nb(C,N), were studied. Due to its capacity of interaction with the austenite phase through solute drag and precipitate pinning effects, Nb is an important variable in the production of high-strength low-alloy steel (HSLA). However, the effectiveness of Nb as austenite conditioner is strictly dependent on its correct manipulation within each manufacturing step of finished or semi-finished products. Computer simulation of the precipitation kinetics of a forging steel 0.03wt. pct. Nb was performed in order to design a thermomechanical processing route based on the time-temperature-precipitation diagram (TTP), thereby ensuring high precipitate phase fraction in short processing time. Characterization of the steel mechanical behavior and its dependence with the formation of Nb(C,N) was done through flow stress curves analysis, the relaxation rate for the microalloyed steel is remarkably lower than for the Nb free steel being a typical stress plateau (carbonitride formation) and a peak stress (recrystallization) respectively observed. Transmission electron microscopy (TEM) was performed in order to characterize the distribution, geometry and dimension of Nb(C,N) generated by different processing routes at different process stages. The solubilization capacity of the adopted and computational designed austenitizing process was also experimentally analyzed. The energy-dispersive X-ray spectroscopy (EDX) method was used in the characterization of the precipitates chemical composition, allowing the identification of Ti-rich precipitation remnants of the austenitizing process and the absence of the element in the Nb(C,N) formed during deformation (strain induced precipitates). The strain, strain rate, temperature and isothermal holding time contribution upon the formation of precipitates were evaluated separately which reveals that it is so important as the processing temperature, the strain rate regime play decisive role in the formation of the particles. In addition, the strain rate dependency of strain induced Nb-precipitates formation at optimal processing temperature was indirectly confirmed by the capacity of the formed precipitates in hinder the static recrystallization (SRX) progress under relaxation tests.The strain induced precipitation formed at optimal processing conditions have circular geometry and size below 10nm, causing high precipitate pinning effect observed in the conditioning of austenite phase by hindering the DRX under warm forging condition.