Effect Of Alloying Elements And High Magnetic Field On Pearlite Transformation And Micro-structure Of High Carbon Steel

With China's"strategic Silk Road Economic Belt"plan and"big bay area and the economic belt along the river"construction plan,more and more bridges across the river and across the sea will be built.The bridge construction is bound to develop in the direction of super large span and lightweight.Therefore,bridge cable steel is required to have excellent comprehensive mechanical properties.Ultra high strength,ultra-fine structure and homogenization have become the"bottleneck"of the development of bridge cable steel.Every 100 MPa increase in the strength grade of the bridge cable steel will reduce the weight of the main cable by about 10%.The bridge cable steel with tensile strength of 2000 MPa has been successfully applied to the Hutong Yangtze River Bridge.The improvement of the strength grade of the bridge cable steel has become a bottleneck problem in the current bridge construction,which needs to be solved.Therefore,it has become an important scientific and technological issue to study the influence of alloying elements and heat treatment process on the microstructure of high carbon pearlite steel for bridge cables.In this paper,the effects of different alloying elements like carbon(C),aluminum(Al),niobium(Nb),and 12-T(Tesla)high magnetic field(as one of the process parameters)on pearlite transformation of high carbon steel for bridge cable are reported,and provides a research idea to change the microstructure of high carbon steel.By increasing the content of C(C>0.77 wt.%),Al or Nb microalloying can be used to make it still hypoeutectoid steel or eutectoid steel,High magnetic field is used to further promote pearlite transformation of high carbon steel to achieve the goal of obtain ing fine and uniform microstructure,low density and lightweight,and the main conclusions are listed below.(1)The effect of C element on pearlite transformation of high carbon steel was evaluated.The results showed that carbon improves the stability of austenite and reduces the start temperature of eutectoid transformation from 756°C to 738°C.The increase in carbon percentage accelerated the diffusion speed of pearlite transformation,increased the end temperature,and narrowed down the transformation region.The continuous furnace cooling led to a compact microstructure,decreased the interlamellar spacing by about 55 nm,and increased the hardness by about 22.8 HV10.(2)The effect of Al element on pearlite transformation of high carbon steel was investigated.The addition of 1.99 wt.%and 3.97 wt.%of Al pushed the eutectoid point of the test steel to a higher temperature and to a higher carbon percentage,1.053 wt.%,and 746°C and 1.361 wt.%and 738°C,respectively.The results showed that the A l element reduces the stability of austenite,pushes up the starting temperature of pearlite transformation,and accelerates the transformation speed.Apart from that,the end temperature also moved up,the percentage of proeutectoid ferrite increased by a bout22.6%,and the interlamellar spacing decreased by about 48 nm with the addition of Al.(3)The effect of Nb element on pearlite transformation of high carbon steel was studied.It was observed that the addition of 0.025 wt.%Nb increases the start temperature of pearlite transformation of high carbon steel when cooled at a lower rate but decreases the start temperature when cooled at a faster rate.The undercooling of pearlite transformation increased by about 13.3°C,and the interlamellar spacing decreased by about 20 nm.(4)The effect of the high magnetic field on pearlite transformation of high carbon steel was evaluated.The eutectoid point of high carbon steel moved toward high temperature and to a high carbon content direction under the 12-T magnetic field.After continuous cooling in the furnace,the pearlite cluster size of high carbon steel increased by about 2.6?m,interlamellar spacing increased by about 61 nm,Vickers hardness value decreased by about 14 HV10,cementite thickness increased by 14.9 nm,and the percentage of small-angle grain boundary(<15°)decreased by 3.35%approximately.(5)The high magnetic field led to the magnetostriction of the ferrite matrix,resulting in the carbon atoms could not enter smoothly into the octahedral gap for diffusion,and the diffusion rate of carbon was reduced.However,high magnetic field increases the driving force and the speed of pearlite transformation.(6)The synergistic effect of the Al element and 12-T magnetic field the driving force of pearlite phase transformation increases by-242.6 J·mol^-1,the eutectoid eutectoid point(pearlite phase transformation)further moves towards high carbon and high temperature,and the eutectoid temperature increases from 738°C increased to785°C,increased by 47°C.The carbon content of eutectoid point is increased from1.053wt.%to 1.478wt%,increased by 0.425wt%.The effect of high magnetic field and Al element on pearlite transformation and micro-structure of high carbon steel is the same function.(7)The driving force of pearlite transformation in high carbon steel was increased due to the synergistic effect of Nb element and high magnetic field,the eutectoid point temperature of pearlite phase transformation increases from 749°C increased to 793°C,increased by 44°C.The Nb refining grain makes the pearlite phase transformation have more nucleation sites,and the high magnetic field makes the driving force of pearlite phase transformation increase.Under the synergistic effect of the Al element and high magnetic field,the pearlite phase transformation accelerates and the transformation region becomes larger.