Continuous Cooling Transformation Behavior and Microstructure Control of Nb Microalloyed Reinforcing Bars

In this paper,CCT(continuous cooling transformation)diagrams are determined for Nb-containing reinforcing bars with different Nb,Mn additions,and initial austenite grain sizes by simulating industrial conditions via dilatometry tests.It was found that coarse austenite grain size,high Mn content and Nb remaining in solution all increase hardenability of Nb-containing rebars,namely lower Ar3 for acicular phase transformation products,which leads to continuous yielding during tensile deformation when the volume fraction of acicular ferrite or bainite microstructure reaches a certain volume fraction.By coupling with actual cooling rates for different size rebars,it can be explained why bainitic structure is prone to form in the center of rebars,especially for small size rebars.In order to achieve required ferrite-pearlite microstructure,cooling strategy is optimized for industrial production.

Strengthening Effects of Niobium on High Strength Rebars

It is well known that the strengthening mechanisms of Nb microalloying on low carbon flat product have been widely studied and recognized since microalloying 75, but the application research and strengthening effects of niobium on middle carbon steel like rebars have been incomplete and under argument due to some factors like limited solubility of Nb（CN） in reheating stage, dominant application of vanadium in rebars, as well as uncertain strengthening effect resulted from high rolling temperature and fast rolling speed in finishing rolling stage. Combining with new version of GB1499.2-2018： Steel for the reinforcement of concrete-Part 2： Hot rolled ribbed bars, it is very necessary to clarify the strengthening effects of niobium on high strength rebars for cost saving. In this paper, some new phenomena have been presented and discussed based on industrial trial results of Nb-bearing HRB400E, HRB500E and HRB600（E）, and then strengthening effects of niobium on high strength rebars were discussed.

Tensile deformation behavior of high strength anti-seismic steel with multi-phase microstructure

To investigate the tensile deformation behavior of high strength anti-seismic steel with multi-phase microstructure,tensile tests with strains of 0.05,0.12 and 0.22 were performed at room temperature.Microstructure of tested steels was observed by means of optical microscopy（OM）,transmission electron microscopy（TEM）and scanning electron microscopy（SEM）.Tensile mechanical properties of tested steels were obtained,and the influence of bainite content on deformation behavior was also discussed.Meanwhile,the deformation mechanism of steel with three kinds of microstructures of bainite,pearlite and ferrite was analyzed.Results show that tested steel with high volume fraction of bainite exhibits a continuous deformation behavior,and this may be attributed to a higher bainite volume fraction and a lower mobile dislocation density.The morphology of microstructure will influence the mechanical properties of tested steels.An increasing content of bainite can improve the tensile strength,but reduce the plasticity and toughness of the tested steels.In the deformation process of 0.039 Nb steel,the ferrite and bainite have priorities to deform,and the deformation exhibits co-deformation of all microstructures in the later stage of deformation.In the deformation process of 0.024Nb-0.032 Vsteel,the ferrite and pearlite have priorities to deform,and the deformation exhibits co-deformation of all microstructures in the later stage of deformation.