Modeling of Precipitation Hardening during Coiling of Nb–Mo Steels

Jean-Yves Maetz,Nam Goo,Bian Jian,Soo Kim,Hardy Mohrbacher,Matthias Militzer,Jer-Ren Yang,Yu Chen

doi:10.3390/met8100758

Abstract

Nb–Mo low-alloyed steels are promising advanced high strength steels (AHSS) because of the highly dislocated bainitic ferrite microstructure conferring an excellent combination of strength and toughness. In this study, the potential of precipitation strengthening during coiling for hot-strip Nb–Mo-bearing low-carbon steels has been investigated using hot-torsion and aging tests to simulate the hot-rolling process including coiling. The obtained microstructures were characterized using electron backscatter diffraction (EBSD), highlighting the effects of Nb and Mo additions on formation and tempering of the bainitic ferrite microstructures. Further, the evolution of nanometer-sized precipitates was quantified with high-resolution transmission electron microscopy (HR-TEM). The resulting age hardening kinetics have been modelled by combining a phenomenological precipitation strengthening model with a tempering model. Analysis of the model suggests a narrower coiling temperature window to maximize the precipitation strengthening potential in bainite/ferrite high strength low-alloyed (HSLA) steels than that for conventional HSLA steels with polygonal ferrite/pearlite microstructures.

Highlights

High strength low-alloyed (HSLA) and similar microalloyed low-carbon steels have been developed since the 1960s and their production continuously increases, especially for automotive and pipeline industries, because of their high strength, excellent ductility, and good weldability [1,2,3].Originally the high strength of these steels was obtained by a combination of refining the ferrite-pearlite microstructure and precipitation hardening due to adding microalloying elements such as Nb, V, or Ti [1,4]
In the Nb–Mo steels, on the other hand, transformation hardening is another strength contribution that depends in detail on the coiling condition. This aspect was not quantified by Sarkar et al but the current study suggests that the coiling temperature window to have overall strength levels within
The potential of precipitation strengthening during coiling for hot-strip Nb–Mo-bearing low-carbon steels has been investigated with laboratory studies simulating the hot-rolling process

Summary

Introduction

High strength low-alloyed (HSLA) and similar microalloyed low-carbon steels have been developed since the 1960s and their production continuously increases, especially for automotive and pipeline industries, because of their high strength, excellent ductility, and good weldability [1,2,3]. The high strength of these steels was obtained by a combination of refining the ferrite-pearlite microstructure and precipitation hardening due to adding microalloying elements such as Nb, V, or Ti [1,4]. Models have been developed to predict the precipitation behavior for these conventional HSLA steels [4,5]. Nowadays there is a strong driving force to develop new advanced high strength steels (AHSS). Nb-Mo-bearing HSLA steels offer a tremendous opportunity to develop high-performance steels

Objectives

Results

Conclusion