Study On The Fundamental Rules Of Microstructure And Mechanical Properties Control In Microalloyed Steels Processed By TMCP

Thermomechanical control process (TMCP) plays a significant role in microstructure and mechanical properties control of steel materials. The conventional TMCP technology depending on microalloying elements addition and heavy reduction at low temperature was used for a long time. However, there are some limitations in conventional TMCP technology. Since the beginning of the 21st century, the new generation TMCP with the key technology of ultra fast cooling has been widely used to tap the potential of steel materials and overcome limitations of conventional TMCP technology. And more work has been done in cooling path control, but the transformation rules, microstructural characteristics and mechanism of strength and toughness need to be further investigated and the austenite structures evolution and strain-induced precipitation behaviors in austenite have not been paid sufficient attention. So there is theoretical significance and application value in the study on fundamental rules of microstructure and mechanical properties control in microalloyed steels processed by TMCP.In the present paper, on the basis of TMCP, the key issues, i.e., the evolution of austenite structures and strain-induced precipitation during rolling and cooling, bainite transformation and the physical metallurgy rules of microstructure and mechanical properties control at ultra fast cooling were further investigated. The following main innovative work has been drawn in this paper:1. The effects of rolling and cooling parameters on the austenite structures refinement were systematically investigated for the first time. The refinement mechanism of austenite structures for different controlled rolling processes was determined.The austenite structures control is of great importance in thermomechanical control process. Although the hot deformation behaviors of steels have been further investigated, the systematical work about the effects of thermomechanical control process parameters on the austenite structures evolution has not been reported. Based on hot rolling practice, the effects of temperature, strain, strain rate and cooling rate on the austenite grain size were investigated using single-pass compression tests. The results show that the dynamic recrystallization and metadynamic recrystallization are the main refinement mechanism for the deformation temperatures of 1150 and 1100℃, strains of 0.5 and 0.8 and strain rates ranging from 0.1 to 10s-1, and the static recrystallization is the main refinement mechanism for the deformation temperatures of 1050 and 1000℃, strains of 0.5 and 0.8 and strain rates ranging from 1 to 10s-1. It was found that increasing the strain from 0.0 to 0.5 can significantly refine austenite structures. However, by increasing the strain from 0.5 to 0.8 can not continue to refine austenite structures for the higher deformation temperatures of 1150 and 1100℃, and the austenite structures were coarsened, while the austenite structures can be further refined for the lower deformation temperature of 1000℃. In addition, we found that the austenite grain size is proportional to ε-p and v-q throughout the range of strain rates and cooling rates studied, and p equals ～0.139 and ～0.036 for the higher deformation temperatures of 1150 and 1100℃ and the lower deformation temperatures of 1050 and 1000℃, respectively, indicating that the effect of strain rate on austenite grain refinement is vigorous at higher deformation temperatures. Furthermore, the mathematical model to calculate austenite grain size was established, and the calculated values are in better agreement with measured ones.2. The effects of cooling parameters on CCP (Continuous Cooling Precipitation, CCP) curves were systematically investigated, and the role of ultra fast cooling in controlling strain-induced precipitation was explained.The effects of chemical composition and deformation parameters on PTT (Precipitation Temperature Time, PTT) curves have been reported in many published papers, however, the study on continuous cooling precipitation behaviors is less. For hot rolling practice, the precipitation takes place during continuous cooling, so the further study on continuous cooling precipitation behaviors is of great importance. The PTT curves were determined using two-stage interrupted compression tests at first and then the CCP curves were calculated using additivity rule. The results show that the precipitation start temperature is lowered as the cooling rate is increased, indicating that the degree of supercooling increases with the increase of cooling rate. The CCP curves shift to the left at first and then shift to the right with the decrease of cooling start temperature, and the critical cooling rate for precipitation start is～ 10.2,～8.9 and～1.9℃/s for cooling start temperatures of 1050,950 and 850℃, respectively. Although the strain-induced precipitation of Nb can not be avoided at rolling temperatures ranging from 950 to 850℃, the amount of precipitation in austenite can be significantly lowered using ultra fast cooling.3. The influence of Mo content on bainite transformation behavior was systematically investigated.The bainite transformation field was refined in the present paper, and the effect of Mo content on GF (Granular Ferrite, GF) and BF (Bainite Ferrite, BF) transformation were investigated. The empirical equation was established to calculate GF transformation start temperature.The results show that molybdenum addition of 0.17 wt% does not noticeably alter the transformation behaviors, whereas 0.38 wt% significantly. In addition, the molybdenum addition of 0.38 wt% can significantly lower the GF transformation start temperature, reduce GF transformation field and expand BF transformation field throughout the range of cooling rates studied. By comparisons of continuous cooling transformation curves between Mo-free steel and Mo-0.38 wt% steel, it can be deduced that using ultra fast cooling can also increase the volume fraction of BF for Mo-free steel and the higher transformation strengthening can be realized.4. It was found that the balance of strength and toughness can be realized using ultra fast cooling during bainite transformation. The mechanism that using ultra fast cooling can improve strength and toughness of hot rolled steel was clarified.Using ultra fast cooling can effectively refine the size of M/A islands, promote the formation of lath bainite with high misorientation between laths, suppress the re-partition of carbon, and enhance the relative frequency of high-angle grain boundaries. However, for the higher ultra fast cooling finish temperature of 560℃, the bainite transformation mainly takes place during air cooling. The sufficient re-partition of carbon results in the formation of block-form austenite with carbon-rich, which can be transformed to twin martensite with zone axis of B=[113] and twin plan of (pqr)=(21-1) below the martensite transformation start temperature, indicating that these larger block-form M/A islands are mainly twin martensite islands. The balance of high strength with yield strength of 876MPa and better toughness with ductile brittle transition temperature of lower than -60℃ was realized using the cooling path of ultra fast cooling â†'400℃â†'air cooling. For the cooling path of ultra fast coolingâ†'560 ℃â†'air cooling, the microcracks can easily nucleate at larger block-form brittle twin martensite or twin martensite-matrix interface and easily propagate through twin martensite or along twin-martensite-matrix interface, furthermore, low-angle grain boundaries, even high-angle grain boundaries, can not effectively arrest cracks propagation, resulting in the higher ductile brittle transition temperature. However, for the cooling path of ultra fast coolingâ†'400℃â†'air cooling, the microvoids can hardly nucleate at fine M/A islands or carbides and their growth is not along lath boundaries, but through bainite laths with high misorientation between laths. Moreover, the larger plastic deformation is observed at turning sites or coalescence sites, resulting in the lower ductile brittle transition temperature.5. The study on the effect of complete recrystallization controlled rolling (RCR)+ultra fast cooling (UFC) on the strength and toughness of the tested steel has been done.The thermomechanical control process of RCR+UFC was proposed. The austenite grain size can be refined to～16.5μm by recrystallization controlled rolling. The recrystallized austenite is transformed to granular bainite, upper bainite and lath bainite and the fine M/A islands can be also attained after ultra fast cooling. The higher strength can be also gained. And its ductile brittle transition temperature is～-49℃, showing higher impact toughness.