Research On Metallurgical Fundamental And Solidification Of Low Carbon TWIP Steels

Twinning induced plasticity steel has both excellent strength and plasticity properties, as well as good strain hardening property. It has been regarded as the ideal next generation steel for modern vehicle. At present, much work has been done on the mechanism of twinning induced plasticity and mechanical properties, but few work has been reported on the smelt and solidification of TWIP steel. As the development of automobile industry, TWIP steel is on the eve of industrial mass production, the research on the smelt, solidification and inclusion of TWIP steel which effect the production seriously has very important significance. According to this, systematic researches have been carried out on these four important aspects:(1)Key smelting technology of AOD-ESR process for TWIP steel.(2) Determination liquidus temperature precisely and identification of the solidification model of TWIP steel under high temperature.(3) macro-segregation and micro-segregation of TWIP steel.(4)Non-metallic inclusions in TWIP steel. According to the above research, following conclusions can be given.(1)In order to produce the TWIP steel by AOD-ESR process, a series of key technologies based on characteristics of steel grade and theoretical analysis were carried out to ensure the quality of steel, a mathematical model for the AOD oxidation refining process has been proposed to optimized production technology. It included deeply decarbonizing in AOD, proper arrangement of alloying order and the temperature control. For ESR process, reducing the oxidation burning loss and adopting lower average rate of melt, as well as stepped feeding, are important to produce qualified production.(2) High manganese TWIP steel has a serious problem under high temperature that manganese evaporation will damage the instrument, a improved DTA method has been used to solve this problem. Meanwhile, a method to determination liquidus temperature precisely was implemented, and on the basis of DTA experimental results of fifth alloys, a equation to predict liquidus temperature of high manganese has been proposed, it is in good agreement with experiments and Thermocalc software results, error of the results can be in the range of10℃(3) High temperature solidification mode of TWIP steel has been analyzed by differential thermal analysis(DTA) and thermodynamic software, and it also studied the solidification mode of Fe-Mn-C-Al TWIP steel to make a comparision. It is found that the solidification mode of low carbon Fe-Mn-Si-Al TWIP steel was γâ†'+γâ†'L, it has low crack initiation probability. But for Fe-Mn-C-Al TWIP steel, the solidification section may be between CA and CB (γâ†'■γ+δâ†'L+γ+δâ†'L+δâ†'L) because of carbon content, it belonged to hypo-peritectic steels and has a significant probability of occurrence of cracks and flaws.(4) Macro-segregation was analyzed by drill hole sampling method, the results showed that carbon segregation degree was relatively serious from the edge of ingot cross-section to the center, but it had minor hazard due to low carbon content. Moreover, the manganese element which had the highest content in TWIP steel had lower segregation degree than other elements. Micro-segregation was analyzed by adopting SEM and EDS, electron probe microanalyzer, and the microsegregation model to analyze element segregation in TWIP steel, it showed that the segregation index of manganese and silicon increased with rising solid fraction, but aluminum element had inverse relationship that segregation index decreased when solid fraction grew, the experimental result from electron probe microanalyzer confirmed this law.(5) Non-metallic inclusions from laboratorial ingot, AOD and ESR processes were detected by SEM and EDS. It is found that eight types of inclusions existed in TWIP steel, the dominant types of inclusions were AIN, Al2O3, MnS and its composite inclusions. Thermodynamics analysis and kinetics model indicated that AlN can precipitate in the liquid phase in TWIP steel, which was different from the conclusion from the literature that AIN can only precipitate in solid or solidification front. High-temperature laser scanning confocal microscopy was applied to observe inclusion directly in liquid state, it clearly observed AIN inclusion precipitating in liquid phase, moving together and colliding to grew up, the whole process confirmed the conclusion that AlN inclusions can precipitate in the liquid phase in TWIP steel.