Study of Delayed Fracture and Low Temperature Toughness on Advanced High Strength Steel
Date Issued
2015
Date
2015
Author(s)
Cheng, Ta-Chien
Abstract
From industrial revolution to the present, steel is the most popular applying alloy in the world. It has been developed for centuries, which could trace back to 1850s. However, the break-through development of steel just occurred in recent decades. In these decades, the newly developed steels are called advance high strength steels (AHSS). Many causes were involved in the linebreak development of AHSS such as the competition of non-ferrous commercial alloys, global climate, raising cost of production, advanced research technique, etc. Until now, two generation of AHSSs have already been developed. linebreak Furthermore, the scheme of the next generation steel was proposed, and was also placed into practice. However, there are still several issues in the develop-ed AHSSs, which are worth discussing. The first part of this research aims at investigating the delayed fracture(DF) of ultrahigh-strength martensitic steels. The main purposes of this study are to building up the evaluation methods of DF and searching for the effective ways to suppress the DF on 15B30 boron added martenstic (B-MART) steels, including slight addition of niobium and vanadium, heat treatments and precise control of carbides. The results of the first part show that these B-MART steels exhibit the most excellent properties. The tensile strength (TS) of as-quenched 15B30 reaches 1900MPa. With addition of vanadium, the TS of 15B30V reaches about 2100MPa. The effect of niobium addition contributes to the yield strength and ductility. In the meantime, the DF of tempered B-MART steels was examined by hydrogen embrittlement test, which showed that, comparing to high temperature, the low temperature tempered 15B30/M steel exhibits a better hydrogen resistance. Further analysis inferred that, this improvement was contributed by the precipitating of epsilon-carbide. On the other hand, the DF of quenched B-MART steels was evaluated through constrain loading technique, that indicated boron had the negative effect on DF. On the contrary, niobium and vanadium had the positive effect on DF. The purpose of second part is to develop the next generation offshore steel, which meets the Norsok MDS-Y70 standard. This investigation is mainly concentrated on the effect of microstructure on low-temperature impact linebreak toughness of the direct water quenched offshore steel. Martensite dominates the microstructure of quenched surface. In contrast, major bainite, martensite and a few ferrite are observed from the central region of quenched specimen. Ductile to brittle transition temperature of tempered martensite is significantly lower than that of bainite dominated microstructure. The presence of bainite greatly impairs low-temperature impact toughness of the steel due to linebreak the presence of low angle interfaces within the bainite packet. The disappearance of high angle interfaces in the bainite packet results in significantly deteriorated low-temperature impact energy of the offshore steel. Similar results are con-firmed in austempered specimens, which are dominated by bainite.
Subjects
AHSS
delayed fracture
offshore steel
impact toughness
EBSD
SDGs
Type
thesis
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