Materials Science Forum Vols. 654-656

Abstract: Two types of oxide dispersion strengthened (ODS) ferritic steels have been produced by mechanical alloying (MA) either in argon or in hydrogen atmosphere, and vacuum hot pressing (VHP). A drastic reduction in the oxygen and nitrogen contents after VHP was strongly affected by hydrogen gas used as the MA atmosphere. MA in hydrogen was found to be effective for refining the steel matrix and enhancing the tensile ductility of the ODS ferritic steels.

Abstract: Grain boundary serrations were produced in heat-resistant austenitic stainless steel containing Cu by applying after solution heat-treatment a holding time in the range 700-800oC between 30 minutes to 3 hours. These special treatments provide an enhancement of the tensile ductility of about 40 and 113%at room temperature and 750oC, respectively, while no significant change in the yield stress and tensile strength could be observed.

Abstract: Thermal cycles of conventional galvanizing and galvannealing processes were applied to low carbon martensitic steels to examine the mechanical property of martensitic steels after their processing in conventional Continuous Galvanizing Lines (CGL). During the thermal cycle simulation, tempering phenomena occurred resulting in changes of microstructure and mechanical properties. In this study, the tensile deformation behavior of martensitic steels was studied in detail in order to understand the tempering phenomena occurring during their processing. It was found that, after tempering, the strain hardening ability decreased drastically and that the plastic flow became localized. An experimental analysis of this phenomenon will be presented based on TEM microstructural observation and Internal Friction measurements.

Abstract: To understand the reason of high work hardening in high nitrogen steel, a scale-bridging analysis consisting of in situ neutron diffraction, in situ electron backscattering diffraction measurement during tensile deformation and weak beam transmission electron microscopy was performed with particular attention to stress partitioning. This study demonstrates the contribution of stress partitioning, back stress due to dislocation pile-up at grain boundaries and dislocation interaction to work hardening at each strain level.

Abstract: Effect of niobium content on laminar precipitate and high temperature mechanical properties of 21-2N vavle steel was systematically studied, using specimens contain 0.26%, 0.43%, 0.65% 0.82% and 1.06% Nb. After different solid solution treatment and 750 °C aging heat treatment, experimental results showed that laminar precipitate was suppressed by niobium addition, and with the increasing of niobium content, laminar precipitate content decreased and size, distribution and morphology of laminar precipitate was meliorated. Then creep rupture strength and fatigue strength of experimental steels are improved with increasing of niobium.

Abstract: Bake hardening steels have to resist strain aging to prevent the yield strength increment and stretcher strain during press process and to enhance the bake hardenability during baking process after painting. The bake hardening steels need to control the solute carbon and the solute nitrogen to improve the bake hardenability. Ti and/or Nb alloying for nitride and carbide precipitation and low carbon content below 0.003% are used to solve strain aging and formability problem for automotive materials. However, in the present study, the effect of micro-precipitation of copper sulfide on the bake hardenability and fatigue properties of extremely low carbon steel has been investigated. The bake hardenability of Cu-alloyed bake hardening (Cu-BH) steel was slightly higher (5MPa) than that of Nb-alloyed bake hardening (Nb-BH) steel, but the fatigue limit of Cu-BH steel was far higher (45MPa) than that of Nb-BH steel. All samples showed the ductile fracture behavior and some samples revealed distinct fatigue stages, such as crack initiation, stable crack growth and unstable crack growth.

Abstract: The state of the microstructure of ferritic heat resistant steels during creep was evaluate by the system free energy, which composes mainly chemical free energy, surface energy and elastic strain energy, and its stress dependence was expressed quantitatively by using a relaxation time. The steels used in this study were P91 (9Cr-1Mo-C-N-V-Nb) steel and P92 (9Cr-Mo-W-C-N-V-Nb-B) steel. The obtained results are as follows: (1) the relaxation time of elastic strain energy was expressed as a function of stress and temperature, (2) surface energy of P92 scarcely decreased during creep due to the formation of the Laves phase, and (3) the relaxation time of the chemical free energy in P92 was larger than that in P91.

Abstract: Experiments of tensile mechanical properties of steel 0Cr18Ni9 are done on the MTS 810 tensile testing machine, and the temperature range is from 20°C to 1200°C. The stress vs. strain curves are obtained. Results show that the elastic modulus, yield stress and tensile strength decrease with increasing temperature .Based on the experiment results, the functions of the elastic modulus, yield strength and tensile strength versus temperature are represented by polynomial.

Abstract: Recent product development has focussed on developing a range of higher strength structural grades, covering the strip thickness range 0.9-1.5 mm, utilising Nb microalloying. A wide range of Nb levels have been successfully strip cast by the CASTRIP® process. Niobium was found to retard austenite grain refinement and substantially enhanced hardenability, which produced significant microstructural hardening and higher strength hot band. Niobium was retained in solid solution enabling further significant strengthening to be achieved from an age hardening heat treatment. Strength levels ranging from 400 MPa to in excess of 550 MPa, in combination with high ductility, were achieved in the age hardened and hot dip galvanised condition, utilising the annealing furnaces of a conventional hot dip galvanising line. The effect of microalloying content and processing conditions on the microstructure and final mechanical properties of Nb microalloyed UCS products produced by the CASTRIP process are described.

Abstract: In recent years, the double cold reduction material (DR material) was developed rapidly with the increase of industry market of computer, mobile telephone, household electrical appliances and pop canning and so on. The DR material has its characteristics such as very thin specification, strong intensity, high flatness and surface quality. In China, only Baosteel has ability to produce with the double cold reduction (DCR) technology. In this paper,based on the five years research and production Practice since 2004, the experimental study, production commissioning and product performance optimization methods in Baosteel have been discussed, such as the mode choice, roll gap lubrication system as well as the control method, the actual production operation and technology parameter set and so on ,so as to promote the development and progress of DR material production. It offers beneficial reference for the technologist on the spot.