Influence of microalloying and billet heating temperature on microstructure and properties of new type of high-nitrogen rebar steel
Thermo-Calc thermodynamic software was employed to investigate the effects of nitrogen,vanadium,and niobium on the precipitation phases in rebar steel,leading to the design of a high-nitrogen rebar composition.A trial production of the new high-nitrogen rebar steel was conducted using the BOF→LF→CC process,and the effects of billet heating temperature(1 050,1 100 and 1 150℃)on the microstructure and mechanical properties of the rebar steel were studied.Results indicated with the increase of nitrogen mass fraction in steel,the temperature at which the precipitated phase begins to precipitate and the number of precipitations increase.Enhanced nitrogen content effectively improves precipitation strengthening while reducing production costs.As niobium content increased,the temperature at which Nb(C,N)begins to precipitate rose along with the precipitate volume,concurrently the precipitation of V(C,N)is inhibited.The optimal mass fraction for nitrogen,vanadium,and niobium in the steel were determined to be 0.020%,0.020%,and 0.005 0%,respectively.With higher billet heating temperatures,the grain size of the steel enlarged,and the proportion of pearlite initially decreased before increasing.When the billet heating temperature was lower than the precipitation temperature of the precipitated phase,the precipitates could not be fully redissolved.The optimal billet heating temperature for the new high-nitrogen threaded steel was determined to be 1 100℃.
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