Microstructural Characteristics and Mechanical Properties of YSt-355 Structural Steel

The introduction of YSt-355 plain carbon steel, with its higher nominal yield strength, is poised to replace YSt-310 in the steel construction industry. Despite this potential, comprehensive studies on the microstructural and mechanical performance of YSt-355 plain carbon steel are lacking. This paper presents an extensive investigation into the microstructural characteristics and material properties of YSt-355 structural steel tubes (SSTs) at both room temperature (RT) and elevated temperatures (ETs). The study’s primary objectives were (1) to determine the material properties of YSt-355 at RT and ET, and (2) to conduct a microstructural analysis of the tested steel specimens. Material properties were assessed through tension tests conducted at RT and ET, with temperatures varying from 100°C to 800°C. Additionally, a detailed study on material inhomogeneity, supported by robust reliability analysis, was conducted. At the microstructural level, scanning electron microscope (SEM) analysis, X-ray diffraction (XRD) analysis, and hardness tests were performed on steel specimens tested at ET. For YSt-355, an insignificant depreciation in the yield strength (14%) is observed up to 400°C; however, the rate of depreciation of yield strength is 18% per 100°C beyond 400°C. Beyond 400°C, the yield strength reduction factor of YSt-355 plain carbon steel was higher than that of YSt-310 by 10%, whereas it possesses 10% less strength-retaining ability compared to fire-resistant steels. The results indicate that YSt-355 exhibits superior performance compared to YSt-310, S355, S355J2H, S355JO, and Q355 at the material level; however, its performance is lower when compared to the same grade of fire-resistant steels. The variance in strength and deformability properties of structural steels was found to be significantly influenced by the regions within the SST. Microstructural studies reveal a phase transformation in YSt-355 was observed beyond 700°C; moreover, a huge drop in hardness (24%) was observed at 700°C.