Research On Heat Treatment Process And Alloying Of Steel Bar For Prestressesd Concrete With High Mechanical Property

Pre-stressed concrete steel bars with high strength and high toughness,lowrelaxation, high bond stress, excellent welding capacity and heading capacity havebeen used widely in high strength pre-stressed concrete centrifugal pipe pile, high-risebuilding foundation and elevated bridge pier concrete pipe pile, electric pole, railroadtie and so on. This subject was undertaken by the China Iron and Steel ResearchGroup Limited, and focusing on the common problems for domestic PC steel bar,such as poor delayed fracture resistance, mechanical property instability and highyield ratio.This subject studies carefully the principle laws of the heat treatment process andmicro-alloying elements effecting on mechanical properties and the ability ofresistance to delayed fracture, in order to obtain optimal heat treatment process.Conclusions are as the following:（1）The experiments obtained the CCT graph of20MnSi steel by expansionmethod, and got the conclusion that when the quenching cooling rate is faster than30℃/s, martensite can be obtained. For the20MnSi steel，the optimal quenchingtemperature is940℃and the quenching time should be less than5s. Inductionheating method refined organization in the process of quenching and tempering. Withthe tempering temperature increased, the yield strength and tensile strength decreased,but elongation and percentage of total elongation increased. The optimal temperingtemperature is between400~450℃.（2）Increasing the content of silicon is conducive to improve the ductility andhigh temperature relaxation properties of the steel bars, but the yield ratio alsoincreased. The silicon content should be between0.20%and0.70%to make sure theAgt is above3.5%and yield ratio below95%. The Vanadium elements could promotethe increase of yield ratio, but it is conducive to improve the overall performance ofstrength and ductility. When higher strength is required, it can be added appropriately,but no more than0.08%.（3）With the quenching temperature increasing, the martensite after quenching became coarse and easily gathered hydrogen, so the plastic and impact toughnessdecreased after high temperature quenching, and delayed fracture resistance alsoreduced. The steel with the Vanadium elements have higher strength and resistance todelayed fracture property due to VC leading to grain refining and diffused distributionof carbide.