1 State Key Laboratory of Coal Mine Disaster Dynamics and Control, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China 2 Chongqing Key Laboratory of Vanadium-Titanium Metallurgy and New Materials, Chongqing University, Chongqing 400044, China
Effect of coarse TiN inclusions and microstructure on impact toughness fluctuation in Ti micro-alloyed steel
1 State Key Laboratory of Coal Mine Disaster Dynamics and Control, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China 2 Chongqing Key Laboratory of Vanadium-Titanium Metallurgy and New Materials, Chongqing University, Chongqing 400044, China
ժҪ Toughness is an important property for steels used in engineering applications. However, recent toughness testing has shown the existence of a significant fluctuation in toughness in a single rolled plate of titanium microalloyed steel (0.17 wt.% C, > 0.05 wt.% Ti, 13 Ti/N ratio). The underlying causes of this fluctuation were investigated by fractography, analysis of microstructure, and measurement of inclusions. Coarse, distributed TiN inclusions were responsible for the toughness variation, as they tended to act as the potent cleavage initiators, forming microcracks. From a calculation of the local fracture stress, the critical size of coarse TiN inclusions for dominating microcrack propagation was 4.93 ��m, and similarly the critical size of ferrite grains for dominating microcrack propagation was 36.6 ��m. Under current casting and thermo-mechanically controlled processing (TMCP) schedules, the toughness fluctuation of rolled steel plates can be attributed primarily to the fraction of coarse TiN inclusions larger than 5 ��m. A corresponding relationship between impact energy and the proportion of coarse TiN inclusions was established in this study. Finally, a normalizing treatment was applied to refine the ferrite grains of rolled steel plates. Despite the presence of coarse TiN inclusions, this refinement in ferrite grains minimized the toughness fluctuation and improved the uniformity of the impact properties of the steel plates.
Abstract��The influence of coarse TiN inclusions on the impact toughness of low-carbon steels microalloyed with titanium was investigated. The microalloyed steels with the total titanium content of 0.053% were selected as impact samples. Charpy V-notch testing results indicated that a significant fluctuation on impact energy was observed. Combined with fractography analysis, microstructure analysis and inclusion investigation, the results revealed that besides the TiN inclusions morphology and size distribution, toughness deterioration was largely attributed to the proportion of the coarse TiN inclusions with size larger than 5��m. Meanwhile, a prediction model of impact energy related to the proportion of coarse TiN inclusions was established. The effects of Ti, N contents and cooling rates during solidification on the precipitation and growth of coarse TiN inclusions were also analyzed with thermodynamic calculation and diffusion-controlled growth model. It was concluded that TiN inclusions began to precipitate at the end of solidification from the dendrites front. Decreasing nitrogen content could significantly reduce the precipitation temperature and decrease the total mass fraction and the final size of TiN inclusions precipitated in mushy zone. TiN inclusions growth tendency was obviously suppressed with the cooling rate increase during solidification. However, titanium content change had little effect on the total mass fraction and the final size of TiN inclusions. Furthermore, in order to ensure that the final TiN size was less than 5��m, the initial nitrogen content in the current steel should be lower than 8.5ppm, or the cooling rate in mushy zone should be more than 2.21 K/s.
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