Precipitation of NbC and effect of Mn on the strength properties of hot strip HSLA low carbon steel

Two low-carbon microalloyed steels were investigated: 0.07C-0.02Nb-0.33Mn and 0.07C-0.028Nb-1.1Mn. The two steels were hot rolled in a similar way, followed by either air cooling to room temperature (ACRT) or accelerated cooling (AC) to 680 degrees C and held isothermally at 650 degrees C for various times prior to ACRT. The mechanical properties of the steels in their fully-processed condition were evaluated. As expected, the steel with the higher Mn content showed the highest tensile (YS and UTS) properties for all processing conditions. The results of this study also indicated that the tensile properties of both steels remained unchanged, prior to and after the isothermal treatments. The central goal of this study was to conduct a detailed microstructural analysis which could help in the understanding of the observed mechanical properties. The results from the ACRT samples after hot rolling revealed two distinct precipitation sequences: (i) precipitation of fine NbC (5 nm in average size) in austenite, and (ii) interphase and multivariant precipitation of NbC in ferrite, The samples which were AC and then held isothermally also exhibited fine NbC precipitates. All the precipitates observed in these samples had formed in austenite. The absence of the Baker-Nutting (B-N) orientation relationship clearly confirmed that precipitation in ferrite did not take place. Atom Probe analysis of the precipitates revealed that the stoichiometry of the precipitates was close to NbC,,. From the detailed TEM microstructural analysis, all the hardening increments contributing to the yield strength of the steels were computed the results showed a good agreement with the measured tensile properties.