| Q690steel is a low-carbon bainitic steel and is used widely in structural parts. Because bainite has non-equilibrium microstructure, the microstructural stability and properties of heat-affected zones (HAZ) of low-carbon bainitic steel have been of significant interest to researchers. In this study, the ferrite/pearlitic D36steels with different microalloys were chosen to investigate the precipitation characteristics of microalloys. On this basis, the composition design and rolling technology of Q690steel were optimized through computation and simulated rolling experiment. Using the optimized design and technology, two alloyed steels were produced. By subjecting these steels to actual welding and by simulating their welding (using Gleeble1500thermomechanical simulator), the microstructures and properties of the heat-affected zones of the steel Q690were studied. In particular, we focused on the transformation of the martensite-austenite (M-A) constituent as well as the effect of M-A constituent on the impact toughness of the HAZs. The effects of different heat treatments on the parent metal and the HAZs were compared, and the precipitation of cementite was analysed. Different combinations of heat treatments were used to simulate the heat input mechanism and energy, and large samples with microstructures similar to that of the HAZs could be achieved using the selected heat-treatment. On this basis, the relationship between the microstructures and the properties of HAZs could be elucidated. The following conclusions could be drawn:(1) The deformed austenite in V-Ti steel can completely recrystallize after each single-pass deformation during roughing rolling when the appropriate pass reduction is used; this is favorable for the refinement and homogenization of the grains. Using a large pass reduction in the final stage refines the grains effectively, and by lowering the finishing rolling temperature, the grain sizes can be reduced modestly. Given the limitations with respect to the pass reductions used in this study, complete recrystallization could not be achieved after rolling in Nb-bearing steels, and grain refinement only occurred in the final passes during roughing rolling. A small decrease in the finishing rolling temperature aids grain refinement. Increasing the Nb concentration can increase the residual strain and the recrystallization volume fraction, and reduce the grain sizes in the next pass. Ti precipitates in the form of TiN, grain growth during austenization and after recrystallization can effectively be prevented by TiN. Nb precipitates at various defects in the austenite, this suppresses the recrystallization of the deformed austenite. The precipitation of Nb leads to an increase in the dislocation density in the austenite phase. This high dislocation density is retained in the ferrite phase. Some of the Nb precipitates on the inner dislocation lines and exhibits a precipitation hardening effect. V precipitates homogeneously in the ferrite phase, talso exhibiting a precipitaion hardening effect. Adding0.050%of V in the steel can increase its strength by approximately63MPa through precipitation strengthening. Adding0.029%of Nb can increase its strength by approximately123MPa through precipitation strengthening, grain refinement and dislocation strengthening. On the basis of the aforementioned results, it is preliminarily determined that Nb and Ti are the main microalloying elements in the Q690steel, with their additive volumes being approximately0.05%and0.02%, respectively. During the rolling process, the initial rolling temperature was1100℃, the reduction for the first few passes was10mm, respectively, the finishing rolling temperature was980℃, and the reduction for the final six passes was15mm, respectively.(2) Five steels with different Ni, Mo, and B contents were produced in the laboratory. The results indicated that all the air-cooled steels exhibited relatively low strengths. When the Ni and Mo contents were greater than0.32%and0.21%, respectively, and B was added, the strengths of the oil-cooled steels could reach values corresponding to the national standard. When the finishing cooling temperature was below500℃, the steel strength reached levels similar to the national standard. Short tempering after rolling can lead to a higher degree of plasticity and greater toughness in steels. The rolling process of the Q690steel during industrial production was optimized on the basis of the above results:water cooling was performed after rolling, the initial cooling temperature was set to approximately780℃, the finishing cooling temperature was set to lower than500℃, and a tempering treatment was performed at620℃for approximately40min following rolling. The results of trial industrial production showed that the addition of0.05%Nb,0.05%V,0.12%Mo,0.0015%B led to the steel exhibiting unstable properties, with its strength showing relatively large fluctuations. The addition of0.04%Nb,0.06%V,0.14%Mo,0.14%Ni,0.23%Cr,0.0015%B or0.05%Nb,0.50%Cu,0.23%Mo,0.0015%B resulted in all the mechanical properties reaching values corresponding to the national standards, with there being relatively small fluctuations in the properties.(3) The microstructure in the coarse-grained HAZ (CG HAZ) underwent transformation from lath martensite to tempered martensite. It then transformed to upper bainite and finally to granular bainite, with an increase in the heat input. The impact toughness first showed an increase followed by a decrease with the increase in the heat input; the toughness was the highest when the heat input was20kJ·cm-1. The impact toughness at the welding joints is related to the load direction. The toughness reached the lowest value when the cracks expanded along the CG HAZ, increased slightly when the cracks expanded from CG HAZ into the parent metal, and reached the highest value when the cracks expanded along the thickness direction of the sheet. The reduction of the toughness of the CG HAZ is related to the martensite transformation and the carbon precipitates formed at the boundaries of the lath martensite. However, because grain growth is restrained by the precipitated TiN phase and since self-tempering occurs in the martensite during cooling, the decrease in the toughness is relatively low. Embrittlement was observed in the intercritical HAZ (IC HAZ), and the intercritically reheated coarse-grained HAZ (IC CG HAZ); this is related to the blocky M-A constituent at the grain boundaries. The property and size of the M-A constituent are confirmed to be related directly to the embrittlement of the HAZ. When the size of the M-A constituent is above the threshold value (2.0μm) and its deformability is low, the HAZs are embrittled.(4) The Q690steel exhibited relatively high stability in the temperature range of420-470℃and620-670℃, because its dissolved carbon content was relatively low, the degree of precipitation and coarsening of the carbides was relatively low, the lath growth was limited, owing to the carbide pinning effect at the grain boundaries. The carbon precipitated during tempering at low temperatures (420-470℃) mainly caused the growth of the original cementite in the bainite. On the other hand, during high-temperature tempering (520-570℃), owing to the segregation of the impurity atoms at the grain and lath boundaries, the interfacial energy of ferrite/cementite decreased, leading to the large-scale precipitation of cementite at the grain and lath boundaries. This resulted in a drastic fall in impact toughness. During tempering at even higher temperatures (620-670℃), the precipitated carbon mainly served as a supply source for the growth of the originally precipitated cementite. The precipitating behaivour of carbon in the martensite of CG HAZ was similar to that in the bainite, but the degree of precipitation and coarsening of the carbides was relatively high. The large-scale precipitation of cementite at the grain boundaries in CG HAZ promoted the occurrence of temper embrittlement. Considering the improvement of properties of HAZs, Q690steel should be welded with a heat input no more than30kJ·cm-1, and the post-weld heat-treatment should be in the temperature range of420-470℃.To control the occurrence of cold cracking preheat treatment should be conducted at100℃.Further, the Q690steel should be avoid being applied in the temperature range of520-570℃or at high temperature for long time.(5) The microstructures of the as-hot-rolled experimental steel could be transformed into microstructures similar to those of HAZs using different combinations of heat treatments. By exploiting this heat-treatment scheme, large samples with microstructures similar to those of the CG HAZ with different heat input could be achieved, and large samples with microstructures similar to those of the IC HAZ with different peak temperature could also be achieved. Previous studies have shown that compared to the mechanical properties of the parent metal, the yield strength of the coarse-grain zones with an approximate heat input of30kJ·cm-1is lower by23.8%, the elongation is lower by6.6%, and the ductile-brittle transition temperature is higher by approximately20℃. Further, the yield strength of the coarse-grain zones with an approximate heat input of50kJ·cm-1is lower by35.0%, the elongation is lower by22.5%. The discrepancy in hardness between martensite island and the surrounding matrix is confirmed not to be the decisive factor. The critical factor for the embrittlement of HAZs is the size of martensite island, and the critical size is2.0μm. The conclusion is consistent with the nature of embrittlement caused by M-A constituent. Those above results also give indirect evidence to the feasibility of simulation of HAZs using different combinations of heat treatments. |
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