Metallurgical Design, Development and Strengthening Mechanisms for HRB600E Reinforcing Bar

摘要

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.