In the present paper, transformation behavior during continuous cooling in non-deformed and hot deformed Nb-Ti micro-alloyed steels was investigated by using the thermal dilation method. Ti content in Nb-Ti bearing steels varied from 0 to 0.031 mass% with Nb content being kept to be constant. Thermal dilation curves were measured at different cooling rates, from which continuous cooling transformation (CCT) curves were built up. For non-deformed Nb-Ti steels, it was observed that ferrite transformation start temperatures (Ar3) decreased with increasing Ti content up to 0.015 mass%, leveled off in the range of 0.015 to 0.027 mass% Ti, and drastically decreased thereafter. For hot deformed Nb-Ti steels, Ar3 temperature did not exhibit significant difference with Ti addition lower than 0.027 mass%, and decreased drastically by further increasing Ti content. Austenite grain size (Dg), Nb-Ti precipitates and residual strain were taken into account to explain the variation of Ar3 temperatures. Based on the experimental results, mathematical models for the calculation of Ar3 for non-deformed and hot deformed Nb-Ti micro-alloyed steels were developed.

In the present paper, continuous cooling transformation behaviors in Nb micro-alloyed steels were systematically investigated by the thermal dilatation method, during which the effects of Nb contents and hot deformation of austenite on phase transformation behaviors, especially the transformation start temperatures, were studied in detail. The tests were carried out with the samples having been reheated hot deformed or reheated non-deformed prior to the dilation measurements. It was found that the Ar3 temperatures measured from the dilatometric curves changed with increasing Nb content in parabolic ways under both hot deformed and non-deformed conditions. It is believed that dissolved Nb in austenite may have had the Nb solute drag effect that could delay austenite to ferrite transformation. On the other hand, Nb precipitates in austenite could retard the growth of austenite grains and act as potential nucleation sites, both of which could enhance the transformation kinetics. Also, the interaction of strain, precipitation and temperature was estimated by using the Sellars model, which predicted that the strain induced precipitation had occurred in hot deformed Nb steels before phase transformation started, and in non-deformed steels with Nb content greater than 0.023 mass%, precipitation was also likely to have occurred under slow cooling rate before phase transformation started, which could have played an important role in determining Ar3. These factors worked together to make Ar3 changing with Nb content in the parabolic way. Based on the experimental results, a mathematical model for the Ar3 calculation for Nb and C-Mn steels were developed, which exhibited a good accuracy in predicting the Ar3 of the steels with and without hot deformation.

By using isothermal double hit compression tests and applying the 2% offset method, a new model was develoed to predict the microstructural evolution of Nb-bearing steels at temperatures above and below the start temperature of strain-induced precipitation (TP). The TP was developed as a function of ture strain, initial austenite grain size and the Nb content. The activation energy of static recrystallization (Qrex) was expressed as a function of the content of different alloy elements. It was found that Nb played the most important role in increasing the value of Qnex The microstructural observations and measurements confirmed the validity of the model developed in the present investiogation.

On the basis of pilot rolling data on the microstructure and mechanical properties of C-Mn steels, computer models of the relationships between the microstructure and mechanical properties have been established using Artificial Neural Networks (ANNs). These models show good learning precision and generalization, and therefore can be used for predicting the mechanical properties of hot-rolled steel strips.

Ni3AlC5% Cr and Ni3AlC5% Cr+-0.3% Y (wt.%) microcrystalline coatings were produced using a close-field, unbalanced magnetron-sputter deposition (CFUMSD) technique. Isothermal and cyclic-oxidation tests were carried out to assess the oxidation resistance of the coatings. The results showed that Al2O3 formed on the coatings as the main oxidation products, with the formation of θ - and α-Al2O3 scales at 900 and 1200℃, respectiûely. The spallation resistance of the Al2O3 scales formed on the coatings was superior to the oxide scales formed on cast Ni3 Al. After oxidation, interfacial ûoids were obserûed on the oxide-metal interface of the cast alloy while no ûoids were found on the coating surfaces. On the basis of the enhancement of Al diffusion, because of the high density of grain boundaries in the coatings, oxidation mechanisms were proposed.