Effect Of Nitrogen Content On The Microstructure And Properties Of Non-quenched And Tempered 38MnVS6 Steel

Application of non-quenched and tempered steel using in automobile forgings has been restricted seriously by its low toughness. This paper aimed to improve toughness of ferrite-pearlite type non-quenched and tempered steel 38MnVS6 without damage its strength. And it mainly studied the influence of nitrogen content and forging process on microstructure and mechanical properties in 38MnVS6 steel, which based on optimized its chemical compositions.The study found that the influence of nitrogen content and forging process on 38MnVS6 steel was remarkable. As the nitrogen content increased from 36ppm to 260ppm, the strength in room temperature of 38MnVS6 steel increased firstly but then decreased while toughness showed opposite behavior. After comparison,190ppm is the best nitrogen content to obtain strong property. In steel adding 190ppm nitrogen, when finish forging temperature decreased from 990℃ to 860℃, impact energy increased obviously, but decreased through cooling method changed from air cooling to forced air cooling. So, the best forging process is finishing forging at 860 ℃, then air cooling. Under the condition of finish forging at 860℃ and air cooling after forging in steel with 190ppm nitrogen content, tensile strength was 910MPa and impact energy was 82.7J. The microstructure of experimental steels were observed and counted by means of OM and SEM. The results indicated that pearlite+ reticular ferrite was the based microstructure of experimental steel, and surface layer, quarter and central part showed uniform distribution. Under the condition of finish forging at 915℃ then air cooling, the grade of grain size increased from 5.5 to 7, ferrite content increased from 4.0% to 22.4%, pearlite colony size decreased from 13.9μm to 10.8μm and lamellar spacing decreased after added nitrogen content to steel.(Ti,V)(C,N) undissolved after reheating and precipitated after cooling were observed using HRTEM and physical and chemical phase analysis. The results showed that the increased nitrogen content promoted the precipitation of (Ti,V)(C,N) and reduced its size that behaved strong pinning effect to austenite. The more (Ti,V)(C,N) undissolved after reheating, the more nucleation position to intra-granular ferrite which can increase the IGF content was received. While, the fine austenite, the more grain boundary area which can increase boundary ferrite content was obtained. Moreover, in steel with high nitrogen content showed strong precipitation strengthening. Because after adding nitrogen, precipitation of (Ti,V)(C,N) precipitated in ferrite increased and the size reduced. In summary, fine microstructure and incremental ferrite fraction obtained after increasing nitrogen content which function together significantly improved the toughness of the steel. At the same time, as nitrogen content increased, the strengthening effect enhanced which can offset the damage of strength causing by decreased pearlite content.