Application Research of Nb Microalloying on Medium and High Carbon Long Products

Abstract

It is well known that the strengthening mechanisms of Nb Microalloying on low carbon flat products have been widely studied and recognized since Microalloying 75 in Washington, DC, but the application research of Nb Microalloying on higher carbon long products in excess of 0.020% carbon has been incomplete and full of debate for a long time due to limited solubility of Nb(CN) in austenite, dominant application of V Microalloying in long products, as well as uncertainty about the effects of Nb. In this paper, some new phenomena have been presented and clarified based on the application cases of Nb Microalloying in long products, including high strength rebars microalloyed with small amount Nb and high-carbon wire rod microalloyed with Nb addition up to 0.02% used for prestressed steel strand. Contrary to the traditional opinions, Nb-bearing rebars manifest higher tensile-to-yield ratio compared with V-bearing rebars, especially for high strength rebars of HRB500 and HRB600, which would be discussed based on production processing of rebar products. In addition, Nb Microalloying for high carbon wire rod used for perstressed steel strand also demonstrates encouraging application prospect by reducing net carbide of entectoid steel with carbon content up to 0.087%, as shown from test results of Nb-bearing 87B product. As for these abnormal phenomena, industrial production and subsequent microstructure analysis have been carried out to further validate and clarify the strengthening mechanisms of Nb Microalloying, which will facilitate the understanding of Nb Microalloying technology.