Effects of V–N Microalloying on Microstructure and Property in the Welding Heat Affected Zone of Constructional Steel

Kaiyu Cui,Haifeng Yang,Guodong Wang,Zhengrong Li,Hongyun Zhao,Xinchen Nan,Shengjie Yao

doi:10.3390/met12030480

Abstract

Shielded metal arc welding and welding thermal simulation experiment were carried out for constructional steel containing 0% V and 0.10% V, and the microstructure, precipitation feature, microhardness HV0.2, and −20 °C impact value in the welding heat affected zone (HAZ) were investigated. The results showed that in the coarse-grained heat affected zone (CGHAZ), V and N were completely dissolved in the matrix of steel containing 0.10% V to promote the growth of prior austenite grains, meanwhile the fraction of high angle grain boundaries (HAGBs) decreased, thereby leading to the mean −20 °C impact value decreases from 87 J to 18 J. In the grain refined heat affected zone (GRHAZ), V(C, N) precipitates experience re-dissolution and re-precipitation at grain boundaries, V–N microalloying changes the microstructure from lath bainite + granular bainite + small amount of polygonal ferrite to polygonal ferrite + pearlite + martensite, thereby leading to the mean microhardness decreases from 335 HV0.2 to 207 HV0.2, and the mean −20 °C impact value decreased from 117 J to 103 J. In the intercritical heat affected zone (ICHAZ), V(C, N) precipitates experience re-dissolution, re-precipitation, and growth, causing the formation of micro-sized V(C, N) precipitates, thereby leading to the mean −20 °C impact value decreases from 93 J to 62 J.

Highlights

Steel structure construction possesses advantages such as light weight, high strength, convenient installation, short construction period, excellent earthquake resistance, and high recycling rate [1–3]
According to the distribution (Figure 12c,f) and frequency (Figure 13c,f) of high angle grain boundaries (HAGBs) and low angle grain boundaries (LAGBs) in the simulated intercritical heat affected zone (ICHAZ), the size of the ferrite grains in the simulated ICHAZ of 10 V steel was smaller, so the frequency of HAGBs in the simulated ICHAZ of 10 V steel was higher, which is beneficial in improving the impact toughness
V and N elements in the coarse-grained heat affected zone (CGHAZ) of steel containing 0.10% V and 0.0155% N mainly dissolved in steel, considering that the V:N was far more than the VN stoichiometric ratio of 3.64, so the content of dissolved N in the CGHAZ was high, which will lead to the Ac3 temperature increasing significantly to facilitate the growth of prior austenite grains (PAGs)

Summary

Introduction

Steel structure construction possesses advantages such as light weight, high strength, convenient installation, short construction period, excellent earthquake resistance, and high recycling rate [1–3]. For the V–N microalloying steel, V carbonitride could not precipitate due to a high cooling rate at the coarse-grained heat affected zone (CGHAZ), so the austenitic grains became coarsened [16]. Hutchinson B. et al [17] compared the microstructure and property in the HAZ of the V–N microalloyed steel and Nb microalloyed steel, and indicated that with the decrease in cooling rate, an increased transformation temperature in the HAZ of the V–N microalloyed steel led to the formation of networks of coarse ferrite grains along the prior austenite grain boundaries, thereby resulting in the increases in the impact transition temperature. Zhang J. et al [20] indicated that high-N (240 ppm) V alloyed steel could achieve good weldability because of the formation of V(C, N) precipitates in the austenite region, which act as heterogeneous nucleation sites of intragranular polygonal ferrite and acicular ferrite, but V(C, N) precipitates coarsen with the increase in cooling time t8/5 to reduce the impact toughness in CGHAZ

Methods

Results

Discussion

Conclusion