Metallurgical Design, Development and Strengthening Mechanisms for HRB600E Reinforcing Bar

High-strength,anti-seismic requirements represent the development tendency to hot-rolled ribbed bar used for the reinforcement of concrete.In this paper,metallurgical design,development and strengthening mechanisms for 600 MPa reinforcing bar with anti-seismic requirements have been studied and presented based on the production practice of 335 MPa,400 MPa and 500 MPa grades.Through trial and error,it was found that alloy design with niobium plus vanadium is the best way to achieve the optimum combination of yield strength and anti-seismic properties,especially,small amount niobium addition can improve upon tensile-to-yield ratio.In order to clarify the underlying theoretical principles for the strengthening effects,qualitative and quantitative analysis for microstructure had been conducted with the aid of optical microscopy,scanning electron microscopy,transmission electron microscopy and chemical extraction technique,together with the solution and precipitation behavior of niobium and vanadium during the hot-rolling and cooling processes.Based on analysis results,it was found strengthening effects of both niobium and vanadium demonstrate some additional strengthening effects apart from predominated precipitation strengthening and grain refinement strengthening respectively.Although adding high vanadium contents is indispensable to achieve needed yield strength through precipitation strengthening effects,but fine VN particles precipitated in relatively high temperature also bring grain refinement effects for austenite and ferrite.Equally,grain refinement is not overall to describe the effects of niobium in long products,small amount of niobium contents is marked to improve upon tensile-to-yield ratio due to transformation strengthening resulted from the niobium contents in solution.Through the development of HRB600E,strengthening mechanisms of vanadium and niobium in medium carbon long products had been enriched.

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.

Solution and Precipitation Behavior of Nb and Its Role in Medium Carbon Long Product Steels

Controlling of the solution and precipitation for microalloying additions of Nb,V,Ti and their combination is the basic building block of microalloyed steels.Among three microalloying elements,Nb has been regarded the most effective microalloying ingredient used for low carbon flat products due to marked strengthening effect of grain refinement and relatively weak precipitation strengthening effect by matching proper thermomechanical processing(TMP).However,Nb was not viewed as attractive as V for medium carbon long products in the past because of limited solubility during reheating process.What is more,hot working is usually carried out at high temperatures in the recrystallization regime,so it is difficult to exert conventional controlled rolling to obtain pancaked austenite,which has further affected the research and application of Nb in medium carbon long products.Because of these factors,studies of Nb in medium carbon long products were incomplete,and even some recognitions and conclusions are subjected to debate.In order to clarify the strengthening effects of Nb in medium carbon long products,the reinforcing bars have been chosen as experimental steels to clarify the role of Nb on microstructural changes along the whole hot working,cooling processes.In addition,qualitative and quantitative analyses of Nb’s state and distribution in reheating,rolling and cooling had been carried out to illustrate some singularities.