Investigation On Microstructure And Properties Of High Silicon Bainitic Steels And Effect Of Vanadium Microalloyng

The microstructure of the carbide-free bainite is a mixed microstructure composed of lathlike bainitic ferrite and film-like retained austenite which is distributed among the lath of bainite and inside the bainite. Compared with the conventional microstructures of bainite and martensite, the carbide-free bainite has become a research focus recently because high strength and excellent ductility can be achieved in this type steel. In the recent years, increasing researches in which the treatment processes of bainitic steels are investigated and developed to obtain the microstructure of carbide-free bainite or the mixed microstructure of carbide-free bainite and martensite were done. However, because the processes to obtain the microstructure of carbide-free bainite are comparatively complicated, the production and application of this type steel fall under a definite restriction. Therefore, it has a very important significance to do basic investigation on bainitic steels in order to afford theoretic reference for their production and practical application and their development on high performance, low cost and good process capacity.Based on the above background, two kinds of high-silicon bainite steels0.35C-1.6Si-1.6Mn-0.8Cr and0.33C-1.6Si-1.6Mn-0.8Cr-0.13V is selected as the study object in this paper. The austempering and continuous cooling processes (air-cooling or wind-cooling) are used to treat the high-silicon bainite steels. In order to obtain the carbide-free bainite or carbide-free bainite/martensite dual-phase structure, the microstructure evolution process and the rule of two kinds of high-silicon bainite steels during austempering and continuous coolling are systematically investigated by by optical microscopy (OM), scanning electron microscopy (SEM), laser scanning confocal microscopy (LSCM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). And the evolution process and the rule of the various properties during the above treatments are also systematically investigated by. Impact test, tensile test, micro-hardness test and abrasive wear test, etc. The effects of austempering temperature and time on the carbide-free bainitic microstructure, the morphology and the distribution of the retained austenite as well as their effects on the mechanical properties and wear resistance, the effects of V microalloying and different cooling methods on the microstructure and the properties and the effects of tempering after continuous cooling on the structures and the performances of high-silicon bainitic steels are discussed. The Research Results show that:1. The microstructure of0.35C-1.6Si-1.6Mn-0.8Cr steel subjected to austempering at different temperature is fundamentally composed of bainite and martensite among which retained austenites are distributed.The bainite content and the content of retained austenite and bainite beam size increase with austempering temperature increase, but the carbon content in retained austenite do not significantly change with the austempering temperature. The microstructure when the austempering temperature is lower (280℃) is a mixed microstructure composed of bainite and martensite. After austempered at310℃for120min, the ε-carbide precipitation similar to that during self-tempering of the martensite takes place within bainite. The complete bainite structure can be obtained after austempered if austempering temperature is higher than310Κ.2. With increasing the austempering temperature, all of the hardness, strength and toughness of0.35C-1.6Si-1.6Mn-0.8Cr steel subjected to austempering treatment decrease, but the elongation increases significantly, With prolonging the austempering time, the hardness and strength do not obviously change, but the elongation and toughness significantly increase when the austempering time is over30min; the wear rate firstly reduces and then increases, and is the lowest after austempered for30min. The austemper treatment process of the0.35C-1.6Si-1.6Mn-0.8Cr steel for obtaining the optimal coordination of strength and toughness and the wear resistance is:austempering at310"C for30min. The hardness is41HRC, Rm=1580MPa, Ro.2=1140MPa, A=15.6%, aKV=47J/cm2and the wear rate is10.5mg/M when the steel is subjected to this treatment.3. The microstructure of0.33C-1.6Si-1.6Mn-0.8Cr-0.13V steel subjected to austempering at different temperature is a mixed microstructure composed of bainite and martensite. With increasing the austempering temperature, the bainite beam size and the volume fraction of retained austenite increase, but the content of carbon in retained austenite do’not be obviously changed. Self-tempering phenomenon occurs in martensite and the e-carbide precipitates during air cooling after austempered at280℃. The microstructure is composed of complete bainite when the austempering temperature is310℃or a temperature higher than310℃. The austempering time has little effect on microstructure. However, the volume fraction of retained austenite is firstly increased and then reduced with prolonging austempering time, and has a maximum fraction after austempered at310℃for30min.4. With increasing the austempering temperature, all of the hardness、 strength and toughness of0.33C-1.6Si-1.6Mn-0.8Cr-0.13V steel subjected to austempering treatment decrease, but only the elongation increases. With prolonging the austempering time, both of elongation and toughness are improved but the hardness and strength are rarely changed. The wear rate firstly decreases and then increases with the austempering time, and has the lowest valume, i.e., the wear resistance is the best, when austempered at310℃for30min. The technological conditions for the steel to obtain the optimal coordination of strength and toughness and wear resistance is also that to austemper at310℃for30min. After treated by this condition, the hardness of the steel is45HRC, Rm=1610MPa, Ro.2=1100MPa, A=15.4%, aKv=53J/cm2and the wear rate is10.15mg/M.5. Both of the0.35C-1.6Si-1.6Mn-0.8Cr and0.33C-1.6Si-1.6Mn-0.8Cr-0.13V steels obtain the optimal properties after austempered at310℃for30min, and the V alloying steel shows more better one. This is because the microstructure of the V-free steel is one composed mainly of bainite while the microstructure of the V alloying steel is a mixed microstructure of bainite and martensite and a small quantity of carbide precitates even after subjected to the same treatment. And it is also because V micro-alloying significantly refine the bainitic ferrite and increase the volume fraction of the stable retained austenite.6. The microstructure of the0.35C-1.6Si-1.6Mn-0.8Cr steel air-cooled after austentized is a mixed microstructure composed of ferrite, pearlite, bainite in which there is no the precipitation of the carbide. The microstructure do not obviusly be changed after tempered at low and medium temperature. The volume fraction of retained austenite is higher in the microstructure of steel untempered after air-cooled, and decrease in the microstructure of the tempered steel because their decomposition occurs during tempering. The hardness and strength of the specimens untempered after air-cooled are not high and their elongation and toughness are lower. After tempered at low or medium temperatures, all of the hardness, strength and toughness decrease and only the elongation obviously increases. Increasing the tempering temperature has no large effect on properties. The specimens untempered after air-cooled shows the lowest wear rate, namely have the best wear resistance. The tempering process decreases the wear resistance. The wear mechanism is micro-cutting.7. The microstructure of0.33C-1.6Si-1.6Mn-0.8Cr-0.13V steel air-cooled after austentized is a mixed microstructure composed of ferrite, bainite and martensite in which there are a small volume of retained austenite and no carbide precipitates. The specimens with such microstructure show higher hardness, strength and elongation but lower toughness. The bainite ferrite continues to grow up during tempering at low or medium temperatures. After tempered, the hardness and toughness are increased in varying degrees and the tensile strength and elongation are significantly decreased. On the other hand, the yield strength is firstly decreased and then increased. As a whole, the tempering temperature has little effect on properties. The specimens untempered after air-cooled show the highest wear rate namely the worst wear resistance. After tempered, the hardness and toughness are increased and the wear rate is decreased, namely the wear resistance is improved.8. For the air cooling, the addition of V to high-silicon bainite steel improves the hardenability of the steel and refines the original austenite grains in the steel. Because V microalloyed steel directly enters into the bainite transformation zone and no ferrite and pearlite transformation occurs during heat-treating by the same techonological parameters, it is easier to form the bainite microstructure. All of the hardness, strength and toughness of the V micoalloyed steel are higher than those of the V-free steel and the elongation is similar between the two kinds of steels. The wear rate of the V microalloyed steel is lower than that of the V-free steel.9. The microstructure of0.35C-1.6Si-1.6Mn-0.8Cr steel blow-cooled after austentized is a mixed microstructure composed of ferrite, bainite and twinned martensite in which there is a small volume fraction of retained austenite. The specimens with such microstructure have hardness≥44HRC, strength≥1500MPa, elongation≥12%, toughness≥35J/cm2, namely the coordination of the strength and the toughness is excellent. After tempered at low temperature the hardness and yield strength are further increased the variation in the microstructure is not clear, but the tensile strength is slightly decreased, plasticity (elongation) and toughness do not be significantly changed, authough the variation in the microstructure. After tempered at medium temperature, the boundary of bainite becomes unclear, and the precipitations of the ε-carbides similar to the martensite self-tempering and the alloy carbide. The wear rates of the specimens subjected to blow cooling and tempering after blow-cooled is almost the same and the wear resistance is similar. The wear mechanism is micro-cutting. This steel has excellent coordination of strength and toughness and better wear resistanc after blow-cooled and tempered at250℃after austentized at920℃.10. The microstructure of0.33C-1.6Si-1.6Mn-0.8Cr-0.13V steel blow-cooled after austentized is a mixed microstructure composed of lower bainite and martensite in which a small volume fraction of the retained austenite. The specimens with such microstructure show an excellent coordination of strength and toughness, namely, hardness>50HRC, strength≥1500MPa, elongation≥15%, toughness≥50J/cm2. The microstructure is rarely changed after tempered at low temperature. After tempered at medium temperature, the boundaries of bainite become unclear, and the ε-carbides precipitate; the hardness and strength are further increased, the toughness is not clearly changed and the elongation is significantly decreased. Because the hardnesses and toughnesses of this steel subjected to polytechnic treatments are similar each other and the strength and ductility has no effect on the wear rate, the wear rate and hence the wear resistance of this steel subjected to different treatments is also similar each other, the wear mechanism is micro-cutting without exception.11. Because V micro-alloying improves the hardenability of the high-silicon bainite steel and refines the original austenite grain, the V alloyed steel has the higher hardness, strength, plasticity and toughness, and has lower wear rate and better wear resistance than the V-free steel. The specimens of V-free steel subjected to blow-cooling after austentized at920℃for30min and tempering at250℃for1hour after blow-cooled show a good coordination of strength and toughness, and their properties can satisfy the request from the conditions using wear-resistant steel. However, the specimens of V microallyed steel subjected to blow cooling and no tempering after austentized at920℃for30min already show a good coordination of strength and toughness and their properties can satisfy the request from the conditions using wear-resistant steel. It is obvious that V microalloying can simplify the heat treatment process and optimize the service properties of steel.