Effect Of Nb Micro-alloyed On Phase Transition Properties And Hot Deformation Of High-carbon Steel For Bridge Cable

With the rapid development of long-span bridges in recent years,there existed an improving requirements for the strength grade in the bridge cables considered as the main load-bearing parts of bridges.The bridge cable was manufactured by twisting the steel wire,which was generated by cold drawing the high carbon steel wire,thus,it imposed a requirement for the torsional performance of bridge cable steels.In order to improve the strength accompanied with the torsional properties of bridge cable steels,the method of the grain refinement and the pearlite interlamellar spacing reduction was utilized via the niobium microalloyed combined with the controlled rolling and cooling process.Therefore,the influence of niobium on the microstructural evolution and mechanical properties in high-carbon steel for bridge cable was conducted during the process of phase transformation and hot deformation.In this paper,the 92 A test steels added 0.034%(wt%)Nb compared with the Nbfree steel and as well as the 82 B test steels added 0.03%(wt%)Nb compared with the Nb-free steel.Firstly,the thermodynamic date of these four kinds of materials were obtained by Thermo-Calc calculation.And then,continuous cooling transformation experiments and isothermal transformation experiments were performed using Formast-F II automatic expansion instrument,the influence of niobium on the phase transformation in the test steels was investigated.Afterwards,the thermal simulation experiments of the test steels were carried out via the Gleeble-1500 D thermal simulator,the effect of niobium on the hot deformation behavior and microstructure after isothermal transformation was studied.Moreover,the microstructure was characterized and mechanical properties were compared and analyzed as well after the test steels normalized.The conclusions were as following:The analysis of continuous cooling phase transformation shows that when the austenitizing temperature is 1200?,Nb partly solid dissolved into steels,the CCT curve of Nb-added 92 A steel becomes steeper,when the cooling rate is slow(0.03?/s),the temperature of pearlite transformation is higher than Nb-free 92 A steel,when the cooling rate is fast(>0.06?/s),the temperature of pearlite transformation is lower than Nb-free 92 A steel,the CCT curve of Nb-added 82 B steel moves down;When the austenitizing temperature is 900?,Nb exists in a precipitated particle state,pinning the grain boundary and preventing the growth of austenite grains.The result is to reduce the undercooled austenite stability,increase the phase transition temperature of pearlite,reduces the undercooling,and the CCT curves of the Nb-added 92 A steel and 82 B steel moves up;The analysis of isothermal transformation of 92 A steels shows that when the austenitizing temperature is 1200?,the solid solution of Nb can increase the incubation period of pearlite nucleation and the TTT curve of the Nb-added steel moves to right.When the austenitizing temperature is 900?,Nb exists in a precipitated state,refines the grain and the TTT curve of the Nb-added steel moves to left;Comparing the thermal deformation process of Nb-free and Nb-added 92 A steels,the results shows that the the addition of Nb can effectively retard dynamic recrystallization of test steels.Under the same deformation conditions,in the dynamic recrystallization process,the characteristic values in the flow stress curve of the Nbadded steel were higher than that of the Nb-free steel.Solid solution of Nb retard recrystallization through solute drag effect,precipitation of Nb forms carbides pinning grain boundaries has a strong effect to retard recrystallization;Through the isothermal transformation experiment of 92 A steels after deformation,The results indicate that the Nb-added steel can refine the grain and the hardness improved.When the deformation temperature is different and through the isothermal process at 600?,the hardness of is almost same.The deformation temperature mainly influence the grain size,The smallest grain can be obtained when the deformation temperature is 900?;When the deformation temperature is same(1100?)and through the isothermal process at different temperature,the isothermal temperature decreased,the pearlite colony became small and the lamellar spacing refined.When the isothermal temperature is 600?,the microstructure of 92 A test steels is sorbitic pearlite;By comparing the microstructure of the 92 A and 82 B test steels after normalizing at 900°C,niobium can refine grain size obviously,the pearlite colony/block size is reduced,and the interlamellar spacing increases;In the 82 B test steels,niobium can inhibit the formation of network cementite,promote the formation of proeutectoid ferrite,and reduce decarburization.Due to the effect of high Si content in 92 A test steels,the network cementite was not observed.Compared with the mechanical properties after normalizing,the strength and hardness of Nb-added steels are higher than that of Nb-free steels,but the plasticity is lower.