The Influence of Temperature on the Microstructure and Properties of Nb-V-Ti-Mo Complex Microalloyed High-Strength Fire-Resistant Steel

Xin Wang,Junchang Shen,Chengjia Shang,Shitong Zhou,Qilong Yong,Runnong Chen,Qingyou Liu,Guangjie Da,Zhaodong Li,Zhongmin Yang

doi:10.3390/met11111670

Abstract

Nb-V-Ti-Mo complex microalloyed high-strength fire-resistant steel was obtained through two-stage hot rolling and laminar cooling. The results showed that the microstructure of the steel included bainite ferrite and martensite-austenite constituent (i.e., MA) islands. The experimental steel displayed high strength at room temperature, with a yield strength (YS) of 617 MPa and tensile strength of 813 MPa (elongation = 18.5%). As the temperature increased to 700 °C, the high-temperature yield strength gradually decreased. Electron backscatter diffraction (EBSD) was used to analyze the experimental steels at different temperatures. The grain sizes did not grow significantly. A small number of nanoprecipitates with an average diameter of 29.2 nm were distributed in the matrix of the as-rolled specimen. Upon increasing the temperature, the number of fine nanoprecipitates gradually increased, resulting in a gradual decrease in their average diameter, reaching a minimum of 19.4 nm at 600 °C. The Orowan equation explained well the precipitation strengthening effect of the nanoprecipitates that formed at a high temperature. At both room temperature and 300 °C, the Ashby work hardening theoretical curves were consistent with the experimental true stress-strain curves. Dynamic recovery and recrystallization occurred at 600 °C, which caused the experimental true stress-strain curve to deviate from the calculated curve.

Highlights

In recent years, due to rapid socioeconomic development, many large landmark buildings with multiple floors have been constructed [1,2,3,4]
The yield strength of conventional mild steel at 600 ◦ C is less than 1/2 its room-temperature value, causing it to lose its bearing capacity; fire-resistant steel must meet the criterion: YS (600 ◦ C) ≥ 2/3 YS [3,8]
The yield strength (YS) of steels is determined as a summation of the intrinsic friction stress (YS0 ), solid solution strengthening (YSs ), grain boundary strengthening (YSg ), dislocation strengthening (YSd ), and precipitation strengthening (YSp ) in the following equation

Summary

Introduction

Due to rapid socioeconomic development, many large landmark buildings with multiple floors have been constructed [1,2,3,4]. Compared with conventional mild steel, high-strength low-alloy steel (HSLA) can greatly reduce construction costs (material costs, transportation costs, energy consumption, and carbon emissions) because of its high strength (yield and tensile strength), low weight, green properties, and good safety, and can reduce materials’ usage (including steel, welding materials, and coatings) [5,6]. Due to improved living standards and an increase in population density, buildings have become taller, and decoration is increasingly high-grade, which uses many combustible materials. This increases the fire load density (equivalent combustible mass per unit area), which increases the risk of fire; it has become imperative to study the fire resistance of structural steel [7]. The yield strength of conventional mild steel at 600 ◦ C is less than 1/2 its room-temperature value, causing it to lose its bearing capacity; fire-resistant steel must meet the criterion: YS (600 ◦ C) ≥ 2/3 YS (room-temperature design yield strength) [3,8]

Results

Discussion

Conclusion