Microalloying Additions to Commodity C-Mn Structural Steels: Fundamental Strengthening Mechanisms Leading to Improvements in Mechanical Properties, Alloy Optimization, Reduced Alloy Costs and Robustness of Hot Rolling Processing

Abstract

Niobium in steels can be used as substitutional solid solute or as precipitates. In solution, Nb exerts a solute drag effect delaying but usually not interrupting static recrystallization during hot rolling and increasing hardenability during post rolling cooling. Fine precipitates generated during rolling/cooling can interrupt recrystallization in finishing and precipitate in the ferrite matrix increasing strength. As a relatively fine precipitate Nb can also inhibit austenite grain growth during reheating.This paper highlights the idea that micro-additions of Nb, up to 0.02%, to ordinary commodity C-Mn structural steels can improve their strength. Industry trial results are presented giving evidence that mechanical properties can be improved, and a leaner/optimized chemistry may be used by adding these micro-quantities of Nb to otherwise ordinary commodity C-Mn steels.Microstructural analysis of a C-Mn vs. a leaner/optimized C-Mn-micro Nb steel along with austenite evolution modeling using MicroSim-PM© helped identifying which type of metallurgical mechanisms are in-play resulting in higher strengths. This alternative composition has led to lower costs, lower CE, improved microstructure and a more stable process.