Recrystallization Behavior And Microstructure Evolution In ERW Process Of Low Carbon Ferritic Stainless Steel

Low-carbon ferritic stainless steel is widely used because of their outstanding hightemperature strength,thermal fatigue resistance,high temperature oxidation resistance,and low production cost.ERW(Electrical Resistance Weld)pipes of low-carbon ferritic stainless steels have broad prospects for development,due to the excellent service performance,high dimensional accuracy,high production efficiency and low production cost.In this paper,T4003 low-carbon ferritic stainless steel coils produced by TISCO as the raw material,and the newly built ERW355 and ERW660high-frequency straight seam resistance welded steel pipe production line of Tianjin Welded Pipe Co.,Ltd.were used to develop the processing technology of large-diameter stainless steel welded pipe to meet the belt conveyor and the mine lightweight,corrosion resistant operating requirements of gas pipe.Based on the analysis of the static and dynamic recrystallization behaviors,microstructure evolution during the high temperature thermal cycling,post-welding heat treatment and galvanic corrosion mechanism of the low-carbon ferrite stainless steel,the mass production technology was explored.The main research results are as follows:(1)Analysis of static and dynamic recrystallization behaviors of the low-carbon ferritic stainless steel.The increase of static recrystallization annealing temperature results in the increase of martensite fraction,coarsening and dissolution of M₂₃C₆ precipitate,but has little effect on the nano-scale MX phase that possesses high thermal stability.The effects of microstructure evolution on mechanical properties during annealing process were evaluated quantitatively.In view of solid solution strengthening,grain boundary strengthening,dislocation strengthening and secondary phase strengthening mechanisms,the predicted yield strength of the samples at different annealing temperatures was calculated,which is in good agreement with the measured values.The increase of deformation temperature leads to the increase of fine equiaxed deformation-induced ferrite content,the decrease of martensite content and the obvious softening characteristics of stress-strain behavior;the increase of deformation amount can effectively promote deformation-induced ferrite transformation,and reduce the martensite transformation rate by enhancing metastable austenite strengthening mechanism;the slower the strain rate,the more obvious the dynamic softening phenomenon in the stress-strain curve.As the strain rate increases,the dislocations in the metastable austenite that are deformed cannot be recovered,which can provide more nucleation sites for subsequent martensite transformation.(2)High temperature thermal cycling behavior of low carbon ferritic stainless steel.The increase of thermal cycling temperature and the extension of holding time both leads to the growth of the prior austenite grains.When the holding time is longer,the changing regularity of ferrite phase fraction is consistent with the equilibrium phase diagram calculated by JMat Pro.On the contrary,when the holding time is shorter,the ferrite content is far away from the calculated value for the condition of thermodynamic equilibrium.The driving force of grain growth is the decrease of interfacial energy per unit volume.Based on this,a kinetic model of grain growth was established,which can describe the grain growth process under different thermal cycle temperatures and holding time conditions.The results predicted by the proposed model show that the longer the holding time,the lower the grain growth rate,and the higher the thermal cycle temperature,the higher the grain growth rate.Both the increase of thermal cycle temperature and the extension of holding time would increase the starting temperature of martensite transformation(M_s).According to the developed kinetic model of M_s point and original austenite grain size(D_?),it is found that the M_s point is proportional to ln(D_?).(3)Study on post-weld annealing process of low-carbon ferritic stainless steel ERW joint.The ERW joints of low-carbon ferritic stainless steel have the large residual stress after air cooling.At room temperature,the microstructrure of weld zone is composed of martensite and ferrite,and the toughness is extremely poor.When annealing temperature rises to 950°C,the martensite structure in the weld zone of the joint is decomposed completely and transformed to ferrite and bainite.At the same time,the residual stress in the weld zone is fully released,so that the toughness of carbon ferrite ERW joint is improved.With the extension of annealing time,the martensite in the weld zone is decomposed completely,the content of bainite formed after air cooling increases,and the residual stress is gradually eliminated,which improves the toughness of the weld zone.Both annealing at 950°C for 3 minutes and for 30 minutes at 850°C can improve the toughness of low-carbon ferritic stainless steel ERW joints.(4)Corrosion behavior of low-carbon ferritic stainless steel ERW welded joints in the simulated seawater.The electrochemical noise characteristic analysis of the joint shows that the corrosion type of the base metal and joint of the low carbon ferritic stainless steel is the typical local corrosion.As the corrosion time prolongs,the potential noise and current noise often change abruptly,due to the combination of local corrosion and self-repair of the passivation film.The galvanic current signal analysis of the joint shows that the order of corrosion resistance from strong to weak is:weld zone>base metal>low temperature heat affected zone>high temperature heat affected zone.The polarization curves of the joint show that the corrosion mechanism in different regions of the joints is basically the same.According to the results of the self-corrosion potential and corrosion current density fitted by Tafel extrapolation method,the corrosion resistance in the high temperature heat affected zone is the worst and that in the weld zone is the best.The results of constant current-constant potential accelerated corrosion test show that,after constant current-constant potential accelerated corrosion,the joints present a typical groove corrosion pattern.The groove initiating area is located in the heat-affected zone,and the corrosion degree in the weld zone is relatively light,which is consistent with the electrochemical test results.(5)Industrialization practice of large diameter low-carbon ferritic stainless steel ERW welded pipe.Through trial production and mass-scale production,it can be confirmed that ERW welded pipe unit can produce large-diameter low-carbon ferritic stainless steel welded pipe through process optimization without large-scale remoulding,which is suitable for advanced structural pipe fields.The production speed of ERW welded stainless steel pipes is several times faster than that of other production methods for the welded stainless steel pipes.The former has high efficiency and large-scale production.The output per unit line can reach 100,000 tons per year,which is beneficial for the promotion and mass application of the developed products.