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Über dieses Buch

In this book, advanced steel technologies mainly developed at the National Institute for Materials Science (NIMS), Japan, for structure control, mechanical properties, and the related mechanisms are introduced and discussed. NIMS has long worked on developing advanced steel techniques, namely, producing advanced steels by using only simple alloying elements such as carbon, manganese, and silicon, and also by utilizing steel scrap. The hope is that this approach will lead to a technology of a so-called steel-to-steel recycling process, with the ultimate goal of a recycling process such as an automotive-steel-to-automotive-steel recycling process to take the place of the current cascade-type recycling system. The main idea is to utilize ultra-grain refining structures and hetero structures as well as martensite structures. In particular, the focus of this book is on tensile strength and toughness of advanced steels from both the fundamental and engineering points of view. Fundamentally, a unique approach to analysis is taken, based on fracture surface energy as effective grain size is employed to better understand the mechanism of property improvement. From the engineering point of view, in fracture toughness such factors as crack tip opening displacement (CTOD) of advanced steels are evaluated in comparison with those of conventional steels.

Inhaltsverzeichnis

Frontmatter

Chapter 1. Introduction: Issues Concerning Environmental Problems and Related Advanced Steel Techniques

Abstract
It is important to establish fundamental concepts for ultra-grain refining of structural steels in order to find their future industrial applications. This book presents and discusses in detail some advanced steel techniques developed at NIMS, Japan, with regard to structural control, improvement in mechanical properties, and mechanisms involved therein. Here, the focus is specifically on the tensile strength and toughness of advanced steels from research and engineering points of view.
Toshihiro Hanamura, Hai Qiu

Chapter 2. Ultra-Fine Grained Steel: Relationship Between Grain Size and Tensile Properties

Abstract
Characteristic ferrite grain growth occurs in parallel with the Ostwald ripening of cementite particles during annealing of submicron-grained ferrite/cementite steel with a heterogeneous and dense distribution of cementite particles. The applicability of the Hall-Petch relation to the hardness and average ferrite grain size is demonstrated as a predictive means to show a significant potential for hardening by grain refining. The lower yield stress, upper yield stress, and ultimate tensile stress tend to have monotonic relationships with the carbon content. True stress increases with increase in the carbon content. However, the strain-hardening rate increases when the carbon content is increased to 0.3 wt% C, after which the strain-hardening rate remains almost constant even with further increase in the carbon content. This strain-hardening is reflected in a similar change in terms of uniform elongation.
Toshihiro Hanamura, Hai Qiu

Chapter 3. Ultra-Fine-Grained Steel: Relationship Between Grain Size and Impact Properties

Abstract
The study on the relationship between the effective grain size, d EFF, and ductile-to-brittle transition temperature in impact tests indicated that the microstructure of ultra-fine-grained ferrite/cementite (F/C) belongs to the same group composed of quenched (Q) and quench-and-tempered (QT) microstructures, while the microstructures of ferrite/pearlite (F/P) belongs to a different group. According to the estimated fracture stress, ultra-fine- grained ferrite/cementite (UGF/C) exhibited the highest fracture stress among the four microstructures. The UGF/C steel has excellent fracture toughness because of its characteristic small d EFF and high surface energy of fracture in comparison to other steel structures. The low absorbed energy with a ductile dimple fracture in the lower shelf region was found to be a characteristic feature of the UGF/C steel. In ultra-fine-grained steel, a transition from an energy-absorbent ductile mode to an energy-absorbent brittle mode existed in impact tests and some dense and small-sized dimples were observed in the lower shelf energy region.
Toshihiro Hanamura, Hai Qiu

Chapter 4. Ultra-Fine-Grained Steel: Fracture Toughness (Crack-Tip-Opening Displacement)

Abstract
Fracture toughness is directly dependent on the stress and strain distribution ahead of a crack and the critical values of stress and strain for crack initiation. The ferrite grain size strongly affects these parameters. The variation in these parameters caused by the ferrite grain refinement determines the fracture toughness and fracture mode. This chapter discusses in detail the effect of ferrite grain size on these parameters and evaluates the fracture toughness (crack-tip-opening displacement) of the ultra-fine-grained steel.
Toshihiro Hanamura, Hai Qiu

Chapter 5. Summary

Abstract
This chapter focuses on the tensile strength and toughness of advanced steels from both the fundamental and engineering points of view. From the fundamental point of view, a unique approach of analysis based on the fracture surface energy by means of the effective grain size is employed to better understand the mechanisms of property improvement. From the engineering point of view, fracture toughness such as crack-tip-opening displacement in advanced steels is evaluated experimentally and compared to the values exhibited by already established conventional steels.
Toshihiro Hanamura, Hai Qiu
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