| High-pressure boiler pipe steel(HPBPS)is mainly used to manufacture superheater pipe,reheater pipe and gas-guide pipe for high-pressure and ultra-high-pressure boiler,due to its good oxidation resistance,heat resistance and corrosion resistance.However,the surface defects frequently appear in the production process of HPBPS with hypo-peritectic composition and seriously affect the product quality and production efficiency.In this study,the solidification and solid-state phase transformation characteristics of hypo-peritectic HPBPS were investigated to provide the support for improving the product quality and production efficiency of the steel.The main research work and results are as follows:The solidified structure characteristics of the hypo-peritectic HPBPS were studied through directional solidification experiments.At solidification rates of 15,50,and 80 ?m/s,the ?-dendritic structure was observed to be intermittent and enclosed by ?-austenite,and the ?-dendrites developed preferentially along the crystal plane(110)during solidification.With the increasing solidification cooling rate,both the average first and second dendritic spacing showed decreasing trend.Based on the evolution of directional solidified structure,the volume shrinkage of peritectic transformation was analyzed.The influence of cooling rate on the surface cracks of continuous casting billets was discussed in combination with the continuous casting production.The result suggested that lowering the cooling rate of the initial solidification could delay the ??? massive transformation away from the LIT-ZDT brittle zone,which reduced the possibility of surface cracks in the continuous casting production.This provided a new theoretical support for the control of surface cracks by slow-cooling mold in the continuous casting production of hypo-peritectic steel.The characteristics of solidification cracks of steel were studied by directional solidification experiments.Interdendritic cracks were found in the directionally solidified samples of HPBPS(12Cr1MoVG,15CrMoG and 20CrMoG),and sulfide precipitated at the dendritic boundary,which promoted the formation of cracks.By analyzing the characteristics of directional solidification crack,it was proposed to evaluate the crack sensitivity of steel by measuring the area ratio of interdendritic cracks,and the index values of different steel grades were analyzed.The results showed that the crack sensitivity of hypo-peritectic steel(12Cr1MoVG and 15CrMoG)was higher than that of hyper-peritectic steel(20CrMoG),which verified the reliability of this method.On the basis of the solidification segregation model,the thermal strain caused by peritectic phase transformation in the LIT?ZDT brittle zone was calculated to evaluate the crack sensitivity of steel.It was found that the crack sensitivity of 12Cr1MoVG was the strongest,followed by 15CrMoG,and 20CrMoG steel was the weakest.The results were consistent with the results based on directional solidification experiments.In addition,the influence of alloying elements on the crack sensitivity of 12Cr1MoVG and 15CrMoG was calculated.The results showed that the crack sensitivity increased with the increasing silicon,manganese,phosphorus or sulfur content.However,with the increase of chromium,molybdenum or vanadium content,the crack sensitivity decreased.With the help of high temperature confocal laser scanning microscope,the solidification process of hypo-peritectic HPBPS was observed in situ.The influence mechanism of cooling rate on peritectic phase transformation was studied,and the causes and control mechanism of continuous casting surface cracks were explored.The results explained that with the increase of cooling rate,the peritectic transformation(???)interface presented three different modes:plane modality and cellular modality controlled by solute diffusion,and ??? massive transformation controlled by interface process.Different peritectic transformation(???)modes during solidification would lead to different volume shrinkage rates.The uneven growth of initial shell from the difference of volume shrinkage rates result in surface defects of continuous casting billet.In continuous casting production,uniform cooling of molten steel near the meniscus of mold should be controlled as much as possible to reduce the inhomogeneity of the initial shell.With the help of high temperature confocal laser scanning microscope,the heating process of hypo-peritectic HPBPS was observed in situ,and the solid phase transformation and microstructure evolution during heating process were studied.The results showed that when the temperature rised above Acl temperature,Fe3C+??? transformation occurred.After then,?-ferrite phase disappeared first,and cementite remained in the matrix,which was different from the equilibrium state(cementite disappears preferentially).After Fe3C+??? transformation,static recrystallization of austenite happened,accompanied by dissolution of cementite.As the temperature continued to rise,the ? phase first precipitated from the austenite grain boundary and then inside the austenite grain.The continuous cooling transformation of undercooled austenite of hypo-peritectic HPBPS was studied by thermal dilatometer and Gleeble-3500 thermal simulator.The static and dynamic CCT curves of the experimental steel were established,and the influence mechanism of austenite deformation on continuous cooling transformation was analyzed.The results showed that austenite deformation enlarged the ferrite and pearlite areas in CCT curve,increased the starting temperature of ferrite transformation,but inhibited the transformation of bainite and martensite,which resulted in the CCT curve moving to the left.Upper bainite appeared in the undeformed micro structure with cooling rate of 1?20?/s and deformed microstructure with cooling rate of 3?30?/s.The appearance of upper bainite in the micro structure would reduce the mechanical properties of steel and easily leaded to crack formation and propagation.Cooling should be controlled to reduce the appearance of this microstructure in production.Combined with the above research results and actual production analysis,the reasons of surface defects and process control of hypo-peritectic HPBPT produced by steel mills were studied.The results showed that the cause of bar surface defects was the uneven growth of initial shell during continuous casting.The surface defects of bars occurred at the meniscus of mold and were aggravated in the subsequent process of continuous casting and solidification.The surface defects of bars were effectively controlled and the casting speed was improved by adjusting mold flux to uniformly cool the initial solidification of molten steel near meniscus and optimizing the composition of molten steel to reduce the crack sensitivity of steel.On the basis of continuous casting solidification control,the offline cooling of continuous casting billet and hot rolled bar was adjusted,and the surface defects of bar were further improved. |
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