Fig. 11. Comparison of the effect of the addition of (0.5-2 %) Mn, (0-1.5 %) Al, (0-1.5 %) Si to the steel presented in Table 2 on austenite volume fraction, austenite carbon content, carbon equivalent calculated with Eq. (1) and ideal diameter (DI) at 800°C; (a) effect of Mn-Al combinations with 0 % Si, (b) effect of Mn-Si combinations with 0 % Al, (c) effect of Si-Al combinations with 1.5 % Mn.

The effects of variations in free nitrogen content on the microstructure and room-temperature tensile properties of laboratory processed dual-phase (DP) sheet steels with a base composition (wt pct) of 0.1C-2.0Mn-0.2Mo-0.2Cr were evaluated. Systematic variations in aluminum and nitrogen content along with coiling temperature were used to create a range of interstitial nitrogen contents and microstructure variations after hot rolling. Two cold rolling reductions of 40 and 70 pct were applied before intercritical annealing. Tensile properties show increasing strength and decreasing total elongation with increasing nitrogen content. Increasing the coiling temperature and adding Al diminished the N effects, while increased cold reduction increased tensile strength in some instances. Microstructural observations indicate an increase in martensite fraction with increased prior cold reduction, while increased coiling temperature decreased the fraction of martensite.

No abstract

The isothermal transformation of bainite in an advanced high-strength steel (AHSS) in the temperature range of 422°C to 482°C for transformation times of 10–6000 s has been examined using light optical microscopy (LOM), and orientation imaging microscopy (OIM). Thermodynamic calculations were done using equilibrium thermodynamic simulation software to produce a theoretical time-temperature-transformation (TTT) diagram, to which the experimental results of this study were compared. Advanced microstructural techniques, such as orientation-imaging-microscopy–electron-backscatter-diffraction (OIM-EBSD), image quality (IQ) phase identification, and polarized light microscopy, were used to observe and quantify the progress of the bainitic transformation during isothermal transformation. Orientation imaging microscopy using electron backscatter diffraction image quality (EBSD-IQ) was used with each sample to quantify the amount of bainite transformed. Results showed that the isothermal transformation at 422°C had 80.1 % bainite, whereas the isothermal transformation at 482°C had 85.5 % bainite transformed after 1200 s; while, after 6000 s, the amount of bainite increased to 90.1 % and 93.4 % respectively. As a result the chemical composition of the steel investigated and the microstructural characteristics of the bainite observed, it was not possible to discern the type of bainite formed at higher and lower temperatures because of the absence of carbides. The kinetics of transformation from the experimental results clearly indicated that the isothermal transformation is slower than that predicted by the theoretical thermodynamic program. The results of this study are presented and discussed.