| Because of its good weldability, Microalloyed steel is widely applied. The mechanical properties of weld metal, which are largely determined by microstructure, are the most important service properties in microalloyed steel. The best combination of straight and toughness of weld metal is obtained when it contains more than 65% of AF with an average size of about 1μm. Thus, study on the formation mechanism of AF in weld metal of microalloyed steel has both theoretical and practical significance. In this paper, the transformation dynamics and thermodynamics of AF in weld metal in microalloyed steel were thoroughly investigated. The influence of heat input on microstructure and properties of weld metal and inclusions in weld metal were also studied in depth.The T-tc-f dynamics curves of AF transformation in weld metal in microalloyed steel have been established using the method of thermal simulating The following laws of AF transformation occurred on analyzing these dynamics curves: ①The temperature range of AF transformation is between 670℃ and 540℃;② AF transformation is a typical diffusion transformation, including nucleation and growth with incubation period. With increasing heat input, the incubation period becomes longer;③ The dynamics curves of AF transformation are incomplete "C" curve without obvious "nose tip";④ With increasing heat input, the AF content initially increased and then decreased;⑤ With the increasing of transformation temperature, both the f - T and f - tc curves show "S" shape, that is, the volume fraction of AF transformation initially increased slowly, then rapidly and then slowly again;⑥ With increasing heat input, AF transformation shifts to higher temperature.In this paper, the thermodynamics driving forces of AF transformation were calculated in theory using the models of KRC and regular solution for the first time. The calculated curves, using the models of KRC and regular solution, coincide well and its error is between 10kJ/mol and -10kJ/mol. Hence, both KRC and regular solution model can be used to calculate the thermodynamics of AF transformation ΔGγ→α+γ1, Which is the theoretical value of the driving force of AF transformation in weld metal, is less than -570J/mol. The driving force of AF nucleation and growth increased with the decreasing of AF transformation temperature and the carbon content in weld metal. The total driving force for AF transformation is smaller than that for AF nucleation and growth.Statistical analyses on the size and number of inclusions shows that 60% of theinclusions have an average size of less than 0.6 u m, and only no more than 10% of the inclusions are larger than 1.0 U m. More than 94% of the inclusions, which are AF nucleation sites, are in the range of 0.2 u m and 0.6 u m. Analyses on the mechanisms for AF nucleation at inclusions found that the appearance of multiple nucleation of AF grains around one inclusion can interpreted exactly with AF nucleation at a high energy inert interface and AF nucleation in a region of high strain energy.The number of inclusions changed little with the increasing of heat input, but larger inclusions increase while smaller ones decrease. Under different location of the weld cross-section, the number of inclusions are quite different, for example, at the center of the weld there are the most inclusions but in the fusion zone there are the least Inclusions phases and compositions have been analyzed by electron microanalysis These analyses have shown that the inclusions contained many different compounds, the following inclusion phases have been identified: MnO2, 3MnO ? A12O3 *SiO2, TiN, CaO ? 2A12O3, SiO2, CaO ? 2TiO2 and 0 -A12O3.Heat input has obvious effect on microstructure of weld metal. When increasing heat input, the primary ferrite content increased in which grain boundary ferrite decreased and polygonal ferrite increased, while ferrite side plate decreased and AF initially increased and then decreased. In addition, with increasing heat input, the grain size of the primary ferrite, ferrite side plate and AF also increased and the microstructures of weld metal are coarser. With given heat input, the change of welding parameters has some effect on microstructures. When welding current increased and welding speed decreased, the AF content and grain size in the weld metal decreased a little.The results of mechanical property testing have shown that the weld metal in microalloyed steel has good low-temperature toughness. Under given temperature, with increasing heat input, the low-temperature toughness of weld metal initially increased and then decreased when it reached the maximum value. This coincided well with the variation of AF content. With increasing AF content, the low-temperature toughness of weld metal increased.
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