Study On TMCP And Welding Processes Of X90 Pipeline Steel

In China,the delivery capacity of single natural gas pipeline for newly-built mainline construction will be increased from 30 to 45 billion cubic meters per year and above,namely,is 1.5 times more than previous one.Hence,it is vital to further raise delivery pressure and applicate higher-grade steel pipeline with larger diameter and thicker wall.In order to achieve such a high long-distance delivery capacity for new generation of natural gas pipeline,the optimum solution is to employ a delivery pressure of 12 MPa grade and X90 steel pipeline with 1422 mm in diameter and 16.0~27.4 mm in thickness.The X80 grade steel has been thought as the first choice for fabricating long-distance natural gas pipeline since it was well developled.However,the X90 grade steel has rarely been reported,requires better combination of ultrahigh strength,excellent toughness,good ductility and adequate weldability,and meet more challenges than X80 grade steel.As such,in this thesis,the effects of TMCP parameters on microstructure and mechanical properties of X90 pipeline steel were investigated systematically,and the mechaims for governing the yield strength were discussed.Simultaneously,the effect of welding heat input parameter on microstructure and impact properties of sumulated coarsed-grained heat affected zone for girth welding of X90 steel pipeline was also investigated,for revealling the role of one specific structure in controlling the impact toughness.The results can be obtained as the following.The processes of X90 steel by billet reheating at 1100~1250 °C rough rooling with deformation amount of 25%~50% at 1050 °C and 0.1 s-1,and dwelting for 1~20 s was reproduced using a Gleeble-3500 machine,and the evolution of recrystallized austenite grain(RAG)was estimated.It is shown that the complete recrystallization of strained austenite took placed when the deformation amount for tough rolling is greater than 30%.The average size of RAG decreased with the decreasing reheating temperature due to the gradual resolution of Nb into the matrix,or with the increasing deformation amount due to an impoved nucleation ratio.However,the growth of RAG took place during the following process of dwelling,which is controlled by element diffustion,and the varization of RAG size with the dwelling time follows the classical Johnson-Mehl-Avrami(JMA)model.A homogenous distribution of refined RAG can be obtained in X90 steel by reheating at 1200 °C and rough rolling at 1050 °C for a deformation amount of greater than 30%.The Gleeble-3500 testing machine was used to simulate tow-hit deformation for rough and finish rolling of X90 steel,and the effects of deformation temperature and interval time between tow neighbouring hits on the RAG were estimated.It is indicated that the volume fraction of RAG increased with the increasing deformation temperature and interval time,and hence,both the dynamic softening and the uniformity of RAG were improved.The static recrystallization(SRC)was refrained by the Nb solute dragging after deformation at 1000 °C.When the deformation temperature was lowered to 950 °C,the SRC was further refrained by the Nb-contained particle pinning,resulting an even lower fraction of RAG.The optimized temperature window for tow-stage controlled rolling was therefore determined as rough rolling at 1050 °C or above,and finish rolling at 950 °C or low.The Gleeble-3500 testing machine was used to simulate the processes of two-stage controlled rolling and controlled colling,and the effects of finish rolling temperature(FRT),starting cooling temperature(SCT)and finish cooling temperature(FCT)on microstructure and mechanical properties were systmetically investigated.It is shown that the microstructure was consisted of quasi-polygonal ferrite(QPF),granular bainitic ferrite,lath bainitic ferrite and martensite/austenite(M-A)constituent.With the decreasing FRT,the QPF decreased while the BF increased.The refinement of matrix microstructure was achieved,as a rerult of an enhanced nucleation ratio by an increasing density of dislocations.The diffusion of C atoms into γ phase was also enhanced,which lead to the formation of more M-A islands in smaller size.Besides,the precipitation of carbonitrides was promoted and hence,the fraction of large/small precipites increased/decreased.As a whole,the yield strength(YS),impact energy(IE)and yield ratio(YR)were improved,while the uniform elongation(UEL)were weakened by lowering FRT.With the decreasing SCT,the OPF and GBF increased at an expense of LBF,and accordingly the dislocation density decreased.The matrix microstructure was refined slightly at 780 °C and then was coarsen by further lowering SCT.At high temperature stage,the enhanced diffusion of C lead to an increasing amount and size of M-A islands,while the improved precipitation also brough about an increasing/decreasing fraction of large/small particles.Totally,the YS was dereased,and the YR and UEL was increased slightly at 780 °C and then dereased by further lowering SCT.The IE exhibited an opposite tendency to the YR.With the decreasing FCT,the OPF and GBF decreased while the LBF increased,and accordingly the dislocation density increased.Besides,the microstructure was refined,and correspondingly,the M-A islands and precipitated particles decreased.As results,the YS,YR and IE increased at an expense of UEL.Based on the above-described rsults,the target papameters for TMCP of X90 steel were optimized as finish rolling at 810 °C,initial cooling at 780 °C and final cooing at 400 °C.The Gleeble-3500 machine was used to simulate the thermal cycle of girth welding coarse-grained heat-affected zone(CGHAZ),and explored the effect of welding heat imput on the microstructure and impact toughness in the CGHAZ of X90 pipeline steel.The results show that the mixed microstructure consists of GBF,LBF and M-A constituent.With the decreasing heat input,GB and M-A constituent decreased,LB increased,prior austenite grain(PAG)refined,the bainite packet and block size decreased.Hence,the impact energy at-20 °C increased,the average cleavage facet size decreased and the impact toughness improved significantly with the decreasing heat input.Moreover,the average bainite packet size is nearly equal to the average cleavage facet size,and the bainite packet boundary can strongly impede the crack propagation,indicating that the bainitic packet is the most effective unit in control of impact toughness in the simulated CGHAZ of X90 pipeline steel.Finally,the optimized parameters of TMCP and girth welding were employed to conduct the trial production of X90 steel coil,plate,spiral welded pipe,and girth welding procedure qualification test.The mechanical properties and welding performance of X90 steel can fully meet the technical requirements.The results can provide references for industrial production and application of X90 pipeline steel.