Papers by Author: Yong Lin Kang

Abstract: The microstructure and properties of aging at room temperature of dual phase steel produced by continuous annealing were investigated by using mechanical property test, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). In order to know the mechanisms of aging of dual phase steel produced by this process, the relative theories of dissolution and diffusion of interstitials, dislocation and precipitation are proposed and discussed. From analysis, the results showed that dual phase steel produced by continuous annealing had high strain aging resistance after overaging.

Abstract: Semi-solid metal forming (SSM) has been recognized as a new forming technology, which is different from the present metal forming methods. Basic research on SSM has been put into operation and a number of SSM techniques have been widely applied in industry. In the application of SSM technique, at present, it is mainly used to produce the low melting point alloys such as Al-base, Zn-base and Mg-base alloys, but the high melting point alloys, for example steels as the most widely useful metal are not extensively studied and applied. In the present work, the electromagnetic stirring method was used to prepare semi-solid slurry of spring steel–60Si2Mn and stainless steel–1Cr18Ni9Ti. At the same time, spring steel–60Si2Mn and stainless steel–1Cr18Ni9Ti were directly rolled into thick strips in the semi-solid state (Rheo-rolling). It is aimed at studying the microstructure and properties of the strips to establish the feasibility of rheo-rolling for the production of the steel strips. According to the present research work, it has been shown that rheo-rolling process combines the casting and hot rolling into a single step for near net-shape production, compared with the conventional hot-rolled metallurgical process. Besides being such a cost-effective process, rheo-rolling process possesses irregular crystal grains such as rosette-type primary crystals in the microstructures because of sufficient agitation during solidification. The overall homogenization of the macrostructures in the whole part of steel ingot can be achieved.

Abstract: An improved and self-developed semisolid preparing and rheomoulding device — rotating barrel rheomoulding machine (RBRM) for light alloys in laboratory is introduced in this paper. It mainly consists of a melting furnace, a shearing system with two relative-rotating conical barrels, a central temperature control unit, gas protection system and a die-casting system. Microstructure-processing relationship of A357 aluminum alloy obtained by the RBRM process is investigated by different intensity of turbulence and different shear rate. The experimental results show that the improved RBRM is capable of eliminating coarse dendrites, and producing small and spherical solid particles uniformly distributed in a eutectic matrix. In addition, the process can eliminate entrapped gas and reduce fine shrinkage pores in the specimens as well. Compared with the original self-developed device in our laboratory, the improved equipment has the following advantages: accurate control of stirring temperature; small volume and convenient manipulation; fine and spherical solid particles, chemical and microstructural uniformity throughout the specimens and so on.

Abstract: Formation of austenite strongly influences the microstructures and mechanical properties of dual phase steels. In present work, austenization process during intercritical annealing was studied in a Fe-C-Mn steel using Gleeble-1500 thermal simulator and quantitative microscopy. The experimental results show that austenite formation is separated into three different stages: (i) growth of high carbon austenite into pearlite rapidly until pearlite dissolution is completed; (ii) slower growth of austenite into ferrite; (iii) very slow equilibration between ferrite and austenite. The thermodynamic and kinetic analyses show that growth of austenite into ferrite is controlled by carbon diffusion in austenite in the primary stage and manganese diffusion in ferrite in the subsequent stage because diffusion coefficient of Mn in ferrite is several orders of magnitude smaller than that of C in austenite. The slow final equilibration between ferrite and austenite is obtained by manganese diffusion through the austenite. Based on the analysis, one dimensional diffusion model of intercritical austenization was developed and solved using finite volume method on the assumption that solute flux was local balance at interface, and the kinetics calculated was compared with experimental results. Simulated results indicate that growth of austenite reaches paraequilibrium in about one second, but remains thousands of seconds to reach final equilibrium. Simulated concentration profiles show that manganese atoms transferred from ferrite congregate in austenite near phase interface, which is consistent with the experimental phenomenon.

Abstract: Different cooling parameters, including the initial cooling temperature, finishing cooling temperature and cooling rate, have a significant effect on the final microstructures and properties for pipeline steels. In present work, Gleeble-1500 thermal-mechanical simulator was used to investigate the microstructural evolution of X70 pipeline steels under different cooling conditions, and the microstructures obtained were analyzed using optical microscope and transmission electron microscope. The experimental results showed that when the initial and finishing cooling temperatures are controlled in the range of 740~760°C and 500~520°C respectively, the microstructure of X70 pipeline steels reveals a proper content (about 12%) of pre-eutectoid ferrite besides acicular ferrite and M/A (Martensite/Austenite) island, which can guarantee an optimum combination of strength and ductility. In contrast to the conventional way using CCT curve, this methodology shows a higher accuracy and operability, and used in industrial production to achieve good effect.

Abstract: In the H-beam rolling process, the deformations and temperature field of workpiece significantly influence the mechanical properties due to the change of product microstructure. Prediction of them is important for the groove design and passes sequences. To get the deformation behavior and the temperature field of workpiece, commercial FEM program LS-DYNA has been used to analyze the whole process of H-beam rolling. The approach is based on 3D thermal mechanical coupled finite element method. The rolling process is divided into several units for calculation. The mesh of workpiece is rebuilt in the simulation for reducing the influence of element distortion. The result shows that, the temperature at the wed to flange position maintains the highest during the whole rolling process, while area of the web the lowest. After the rolling, temperature difference is above 150K between the web’s surface and flange’s inside surface, approximate 130K on the flange’s outside surface and 200K in the cross section. The simulation results show good agreement with the measured temperature data.

Abstract: In this paper, with a newly self-developed rotating barrel rheomoulding machine(RBRM), microstructures and mechanical properties of rheo-die casting A356 alloy were studied. In order to clearly show the characteristic of rheo-die casting, liquid die casting and semi-solid casting were done too. The experimental results showed that microstructures of rheo-die casting were composed of solid grains, which were finer and rounder, and had fewer pores. In the three technologies, integrated mechanical properties of semi-solid rheo-die casting were the best.

Abstract: In this paper the microstructure evolution of AZ91HP magnesium alloy casts produced under different disturbing treatment conditions such as non-disturbing, argon blowing and mechanical stirring in semi-solid state after holding a short time was studied. The results show that the grains of AZ91HP alloy casts refined, the secondary dendritic arms grown and the dendrites tend to rosette shape with the decrease of holding temperature. External disturbance can accelerate the evolution process, and the spherical primary α phase formed easily under mechanical stirring treatment condition. The formation of non-dendritic structure is owing to ripening of the dendritic arms, refinement of the grains and movement of the primary formed solid phases. Disturbing treatments in semi-solid state induce more equilibrium solidification and decrease the amount of brittle β-Mg17Al12 phase. Blowing argon into the refined and modified magnesium alloy in semi-solid state can obtain homogeneous non-dendritic structure and the net shaped β phase distributed on α-Mg phase boundaries become fine and thin, this may be have a good effect on the mechanical properties of the magnesium alloy casts. Due to the low disturbing strength, argon blowing can maintain the metallurgy quality of the semi-solid slurry well, and also have high efficiency to make it. This technology need not new complex equipments and can be practiced in conventional casting conditions, so it may be used in industrialize manufacture.

Abstract: In this paper, SiC nanoparticles reinforced AM60 alloys were fabricated by semisolid process with pretreatment of SiC nanoparticles. The microstructure, hardness and mechanical properties of the alloys have been investigated. The X-ray photoelectron spectroscopy (XPS) was also used for analyzing elements chemical status of the nanoparticles and the alloys. The microstructure, hardness and mechanical properties of SiC nanoparticles reinforced magnesium alloys are better than those of AM60 alloys without SiC nanoparticles addition. The results show that semisolid process was suitable for fabricating SiC nanoparticles reinforced magnesium alloys.