Toshihiro Hanamura

Phosphorus-induced dislocation structure variation in the warm-rolled ultrafine-grained low-carbon steels

Abstract Dislocation structures in warm-rolled low-carbon steels with ultrafine ferrite grains (∼1 μm) are analyzed by both transmission electron microscopy (TEM) observation and the modified Warren–Averbach procedures for X-ray diffraction profiles. In good accordance with the TEM observation results, the dislocation density is derived by the X-ray diffraction profile analysis to be 1.06×10 14 and 1.98×10 14 m −2 , in the as-rolled 0.15C and 0.15C–0.1P steel, respectively. In contrast to the straight dislocation segments in the 0.15C steel, tangled and curved dislocations are observed in the phosphorus steel at both the as-rolled and annealed conditions. The larger q value and smaller dislocation arrangement parameter, M=R e ρ , are observed in the phosphorus steel. Phosphorus in the low-carbon steels tends to prompt the ratio of screw dislocation component and the dislocation arrangement with a stronger interaction during plastic deformation. Not only the dislocation density, but also the dislocation interaction behaviors control the changes of yield strength during annealing the as-rolled steels at 723 K. The modified Warren–Averbach X-ray diffraction profile analysis may be useful in characterizing the dislocation interaction behavior quantitatively.

Low absorbed energy ductile dimple fracture in lower shelf region in an ultrafine grained ferrite/cementite steel

The curve of absorbed energyvs test temperature for an ultrafine grained ferrite/cementite steel showed a transition from an upper shelf energy to a lower shelf energy,i.e., a transition from an energy-absorbent ductile mode to an energy-absorbent brittle mode. Some dense and small-sized dimples were observed in the lower shelf region. A significant and consistent difference existed between ductile-to-brittle transition temperature and impact energy transition temperature, that is, low absorbed energy but undergoing a ductile dimple fracture at a certain low temperature.

Introduction: Issues Concerning Environmental Problems and Related Advanced Steel Techniques

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.

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

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.

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

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.

Effect of Austenite Grain Size on the Mechanical Properties in Air-Cooled 0.1C-5Mn Martensitic Steel

In 0.1C-5Mn steels, 5%Mn addition increases hardening ability and makes 100% martensitic transformation even in air cooling without water quenching. Their Ms and Mf temperatures are in the range of 350-250°C, and subzero treatment is not needed. This makes it possible to measure Ms and Mf temperatures accurately by dilatometry. Utilizing a newly developed experimental technique that makes it possible to examine phase transformation behavior and conduct tensile testing with the same specimen, we examined these relationships with identical specimens and obtained the following results. Ms temperature decreases as much as 40 K with a decrease in austenite grain size from 254 to 30 m. Regarding martensite structure, the packet size and the block length decrease, while the lath width does not change, with the refinement of austenite grain size by about one tenth. True stress - true strain curves obtained up to fracture elucidates that the austenite refinement substantially improves true fracture strength and greatly increases true fracture strain of martensite, potentially invalidating the conventional concept of a trade-off balance between strength and ductility.

Dependence of Yield Strength on Ferrite Grain Size in Ferrite/Cementite and Ferrite/Pearlite Structures of Medium Carbon Steel

Medium carbon steels have low yield ratio (YR). In order to obtain high yield strength (YS) for them, different microstructures including ferrite(F)/cementite(C) and ferrite(F)/pearlite(P) are prepared and the microstructure for high YR as well as its mechanism is clarified. The purpose of this study is, therefore, to elucidate the effect of ferrite grain size on YS in the relationship between F/P and F/C structures having the same tensile strength (TS) level. In case of F/C, YS can be significantly improved through grain refinement with a slight decrease in ductile properties. This is particularly clear by comparison of F/C with a ferrite grain size of 0.6m and F/P, where both structures show the same TS level while the YS level is significantly different; F/P representing much smaller YS than that of F/C. It is also to be noted that F/P microstructure shows low YS compared to its high TS with a generation of dislocations at the interface between ferrite and cementite in its pearlite phase. In conclusion it is necessary to consider the difference in the YS-TS relationship between F/C and F/P in order to make the most of its forging processing.

Microstructural Evolution Deformed Heavily by High Strain Rate at High Temperature Range in Ultra Low Carbon Steel

The microstructural change was observed during large strain high Z deformation with high strain rate in high temperature range using ultra low carbon steel. The finer grains were obtained as decreasing the deformation temperature and increasing the strain rate. And the fraction of high angle grain boundaries became higher in low deformation temperature and strong texture of ferrite recrystallized dynamically was measured such as ND//<100>,<111> and RD//<110>. The change of grain size could be analyzed by Zener-Hollomon parameter, whereas the duration has large effect on the deviation of expected grain size in deformation with high strain rate.

Ductility Improvement of Direct-Cast Gamma TiAl-Based Alloy Sheet

For improving the room temperature tensile ductility of direct-cast gamma TiAl sheets without affecting their high-temperature strength, direct sheet casting with TiB{sub 2} particle dispersion is employed and conducted. The TiB{sub 2} addition and rapid cooling results in the formation of a fine equiaxed grain microstructure with an average grain size of {approximately}10{micro}m, contributing to the increase in the room temperature ductility to 2.1% with the high-temperature tensile strength kept at about 200 MPa. This improvement of room-temperature ductility is attributable to the following fact. The high oxygen content of this material, about 2,500 wt.ppm, is not harmful to the tensile ductility when the oxygen is in the solid solution of the {alpha}{sub 2} lamellar phase or in oxide particles, which are fine enough not to cause brittleness to the matrix. From these findings, a principle is proposed that oxygen is not harmful to the ductility of gamma TiAl when its microstructure containing oxygen is fine enough.