| Summary: | 博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 102 === In related research, Mo element was shown to effectively enhance precipitation strengthening via nanometer-sized carbides and retard the recovery of dislocations in Nb-containing low-carbon bainitic fire-resistant steel. It is of some interest to apply the concept to another steel product, hot-rolled strips. In addition, the present study investigated the effects of adding Mo on interphase precipitation in ferritic hot-rolled strips to promote precipitation strengthening. After the comparison of the effects of Mo on the two types of hot-rolled strips, it was possible to determine whether the addition of Mo is beneficial to bainitic hot-rolled strips.
The composition was primarily designed to be 0.05C-1.7Mn-0.08Nb (wt% based composition) with 0 and 0.1 wt% Mo, separately labeled Nb and Nb-Mo strips. In the practical hot-rolling process, the residual strain in prior austenite grains inevitably promotes greater transformation of ferrite, so in the present study, it was necessary to reduce the coiling temperature and accelerate the cooling rate so as to prevent the formation of a fully ferrite structure in present study. The coiling temperatures (C.T.) in the hot rolling process in this study were 450oC and 550oC. Observations of the microstructure revealed that C.T. 450oC and Mo both efficiently suppressed the ferrite transformation and promoted the granular bainite structure in bainitic hot-rolled strips. Then during the simulated coiling procedure, tempering treatment at 600oC, the strips with granular bainite demonstrated a significant secondary hardening effect. Elevating the coiling temperature to 650oC produced a fully ferrite structure with interphase precipitation in the hot-rolled strips. The strips achieved the same strength as the strips with the secondary hardening effect. However, the addition of 0.1 wt% Mo did not improve the precipitation strengthening by interphase precipitation and induced more unstable second phases in the coiling procedure in Nb-Mo strips with C.T. 650o C. From the above, it is concluded that Mo alloy has a positive effect on Nb-containing low-carbon bainitic strips.
In order to increase the volume fraction of bainite and enhance the secondary hardening effect, the concentration of Mo in Nb-Mo strip with C.T. 450oC was increased to 0.3wt%, labeled the Nb-3Mo strip. In the microstructure, the morphology of bainite was the same as that of granular bainite, and the amount seems to increase with the greater addition of Mo. In further analysis to quantitate granular bainite, the electron backscattering diffraction technique was used to feature the unique misorientation gradient in granular bainite because scanning electron microscopy was unable to distinguish ferrite from granular bainite due to their similar morphologies. In the phase qualification of three hot-rolled strips with C.T. 450oC, it was surprising to find that only the high addition of Mo, 0.3wt%, effectively increased the volume fraction of granular bainite. Further dilatometry experiments led to the conclusion that the formation of ferrite must have been prohibited, allowing more prior austenite regions and the grain boundaries in the promotion of granular bainite.
During tempering treatment, the high addition of Mo rarely promoted the secondary hardening effect of tempered granular bainite in Nb-3Mo strip. To determine the reason, Electron Energy Loss Spectroscopy (EELS), high resolution TEM (HR TEM), Energy dispersive X-ray spectroscopy (EDX), and atom probe tomography (APT) were used to investigate the effect of Mo solute on nanometer carbides and the dislocation structures in tempered bainite. Measurements of dislocation density revealed that no obvious recovery occurred in the three strips. It is probable that, unlike in the high temperature tensile test, the slipping of dislocation lines was slight in the current tempering treatment lacking external stress. Thus, the Mo solute could not retard the movement of static dislocation lines in granular bainite during tempering. In the evolution of nanometer carbides in tempered bainite with increasing Mo, Mo solute was homogeneously dissolved in (Nb,Mo) carbides in tempered bainite. Furthermore, the Mo solute became a significant carbide-forming element, and the carbides were slightly coarsened by absorption of excess Mo solute.
In the reports on fire-resistant steel, the carbon replica method allowed confusion of the nanometer carbides in tempered bainite with strain-induced Nb carbides in prior austenite. This confusion is likely to have caused misunderstanding of the refinement of nanometer carbides by the addition of Mo in tempered bainite . In the present study, HR TEM images were used to focus on the nanometer carbides and statistically analyze their sizes during tempering because the featured Baker-Nutting orientation relationship between carbides and matrix could be recognized accurately with the technique. In addition, the signal of Mo occurred in strain-induced Nb carbides after tempering. It was concluded that Nb and Mo solute segregated toward the strain-induced Nb carbides and then formed a layer of carbides on the surface. It is likely that in previous research, segregation of Mo solute was believed to occur in the nanometer carbides in tempered bainite because the two types of carbides could not be resolved. Therefore, in the present study, it was necessary to dispel the fallacy that the addition of Mo can promote precipitation strengthening via nanometer carbides in tempered bainite in steels. Mo alloy strengthens the Nb-containing low-carbon bainitic hot-rolled strips by increasing the hardenability, rather than by a synergistic effect of Nb and Mo.
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