The Influence of Niobium Addition on the Mechanical Properties and Microstructure of High-Strength Low-Alloy Steel Processed Through a Conventional Rolling Mill

Abstract

The high-strength low-alloy (HSLA) steels present a challenge in balancing mechanical performance, cost and formability. The HSLA steel design involves adjusting the carbon (C) content and incorporating microalloying elements such as niobium (Nb), vanadium (V), titanium (Ti), which contribute through precipitation hardening, solid solution strengthening, and grain refinement. Nb is the primary microalloying element; it enhances tensile properties by refining the austenitic grain size during hot rolling, increasing hardness, and providing strength, usually, via Nb(C,N) or NbC precipitation. The morphology, size, and distribution of these precipitates critically impact the steel’s behavior. Thermo-Mechanical Control Processing (TMCP) further enhances material properties through Nb addition. However, the effectiveness of Nb in the continuous rolling of long products requires thorough evaluation. This study addresses this issue by quantifying and correlating the effect of Nb under conditions different from those of TMCP, evaluating its effect on the grain size, tensile properties, impact energy absorption, and hardness of AISI 4320 steel following conventional rolling. The results indicate that Nb addition between 200 and 500 ppm improves the yield strength by approximately 10%, while the ultimate tensile strength increases by about 6%. The particle size distribution of the precipitates reveals unusual findings regarding the grain refinement capability of Nb.