Microalloying And Strengthening Mechanism Of Ultra-high Strength Bridge Cable Steel

With the implementation of the strategic plan of“Made in China 2025”manufacturing power,China’s wire rope industry is facing new challenges.Steel strands prepared from steel wire ropes are often used in the cables of suspension bridges,and are also key components related to life and production safety.The suspension bridge consists of four parts:main cable,tower,stiffener beam and anchor.The bridge load is transmitted to the tower through the main cable.The main cable is the main structural component of the suspension bridge,which bears all the constant load and dynamic load of the superstructure of the bridge,and its production quality will directly affect the construction quality and construction period of the suspension bridge.Due to the long production process and complex production process,the quality control factors of steel wire for bridge cables are affected in many aspects.In order to meet the demand of national key projects,further improve the quality of steel wire rope and clarify its strengthening mechanism,this paper has carried out investigation on the development process of steel wire for new bridge cables at the level of 2000 MPa.In this paper,the influence of temperature and microalloying on the austenite grain size of bridge cable steel was first established from mechanisms,and the recrystallization law under the condition of Nb microalloying was mainly studied to provide parameter guidance for industrial production.Then,the controlled cooling process and microalloying of the raw material wire rod produced by steel wire were compared,and the main and secondary influencing factors of strengthening mechanism were clarified.Finally,a comprehensive and systematic explanation model for microstructure evolution and cementite dissolution mechanism of steel wire during drawing is proposed.The main conclusions are as follows:（1）Nb microalloying can significantly refine the austenite grain size above 1200°C,and the composition of Mn instead of Si has little effect on the growth of austenite grain.Si can increase the growth rate of austenite by increasing the diffusion activation of carbon in austenite,and refine the original austenite grain size to a certain extent.（2）After Nb microalloying,the static recrystallization fraction of bridge cable steel increases with the increase of deformation temperature and intermediate residence time,and has a certain inhibitory effect on the whole recrystallization process of experimental steel.When the strain rate is 0.01 s^-1,Nb-free steel has typical dynamic recrystallization characteristics at 850～900°C,and has dynamic recovery characteristics at 750～850°C.Nb-bearing steel shows dynamic recovery characteristics and work hardening characteristics at the same temperature.Based on the experimental data of thermal compression,the thermal deformation activation energies of Nb-free steel and Nb-bearing steel were determined as36.36 k J/mol and 76.52 k J/mol,respectively.（3）The addition of Nb delayed the pearlite transformation at the cooling rate of1.00°C/s,and promoted the pearlite transformation at the cooling rate of 0.05°C/s.At1.00°C/s,the temperature at the beginning of the pearlite transition was increased from745°C to 751°C,and the temperature at the end of the pearlite transition was increased from 719°C to 722°C;at 0.05°C/s,the temperature at the beginning of the pearlite transition was reduced from 699°C to 689°C,and the temperature at the end of the pearlite transition was reduced from 649°C to 637°C.The effect of Nb microalloying on the growth of pearlite depends more on the driving force of undercooling reduction,and its effect on carbon diffusion during pearlite transformation is relatively weak.（4）Nb microalloying refines pearlite colony size and interlamellar spacing,so that the dislocation density of the test steel increases and the residual stress decreases.Due to its uniform grain size and more random texture,the microstructure uniformity of the new wire is improved.（5）Compared with the Stelmor controlled cooling process,smaller pearlite interlamellar spacing has been obtained for the 600°C water bath isothermal cooling process.The composition of high Mn and low Si has changed the phase transition point of the wire,and the microstructure of the secondary cementite is obtained.However,the interlamellar spacing and colony size of pearlite are also refined,which improves the mechanical properties of wire.High Si content significantly reduces the stability of cementite,which is not conducive to the work hardening of steel wire drawing.（6）In the drawing process,the cementite in the steel wire is partially dissolved mainly through the dislocation mechanism and the Gibbs-Thomson effect.The dissolution caused by thermodynamic diffusion may play a secondary role in the dissolution of cementite.Drawing makes the pearlite lamellar direction gradually uniform from uniform random distribution to parallel to drawing direction;the interlamellar spacing between ferrite and cementite decreases with the increase of drawing strain.Due to the dissolution of cementite,the actual measured cementite thickness is significantly less than the calculated thickness.The synergistic effect of cementite and ferrite nano-layer composite structure ensures that the yield strength of steel wire is improved through interfacial strengthening,which effectively mitigates the inverse Hall-Petch effect.With the increase of drawing passes,the preferential accumulated strain on ferrite gradually diffuses to the cementite.