Yong Lin Kang

Effect of Partitioning Temperature on Work Hardening Behavior of Q&P Steels

Study on the instantaneous n-value of 0.3C-1.5Mn-1.5Si steel subjected to 1-step quenching and partitioning process (Q&P) is presented. The result indicated that the curves of instantaneous n-value vs. true strain could be divided into three stages. First, the instantaneous n-value shows a rapid decrease, then a comparative stable stage was observed due to the TRIP phenomenon of retained austenite, at last the instantaneous n-value decreases sharply to zero when necking appearance. The relationship of instantaneous n-value and partitioning temperature (PT) was analyzed; n-value decreased along with the rise of PT due to a higher carbon concentration and lower volume fraction of retained austenite was obtained at higher PT which could influence the uniform elongation and the stability of retained austenite.

Study on Microstructure-Processing Relationship of a Semisolid Rheocasting A357 Aluminum Alloy

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.

3D Thermal Mechanical Coupled Elasto-Plastic Finite Element Analysis in the Whole Rolling Process of H-Beam

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.

Simulation of Intercritical Austenization of a C-Mn Cold Rolled Dual Phase Steel

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.

Microstructure and Property of Rheo-Diecasting Magnesium-Alloy with Forced Convection Mixing Process

Taking AZ91D magnesium alloy as experimental material, the rheo-diecasting process was implemented by combining the self-developed forced convection rheomoulding device with high pressure die casting machine. Microstructural characteristics of the RDC tensile samples were discussed. Microstructure of semisolid slurry at different rotational speed prepared by FCR device was investigated. Tensile properties and fracture morphology of rheo-diecasting process in different conditions were studied as well. The results indicate that the excellent semisolid microstructure of AZ91D tensile samples can be obtained by forced convection rheo-diecasting process. With the increase of rotational speed, the mean size of the primary particles diminishes and the shape factor increases. The tensile properties of as-cast RDC samples are obviously better than conventional diecasting samples. T6 heat treatment can increase tensile strength and elongation of RDC tensile samples to 286MPa and 6.1%, respectively. The yield strength and elongation of T4 heat treated RDC samples are 100MPa and 12.6%.

Forced Convection Rheomoulding Process for Semisolid Slurry Preparation and Microstructure Evolution of 7075 Aluminum Alloy

A self-developed forced convection rheomoulding (FCR) machine for light alloy semisolid slurry preparation is introduced, which is high efficient, reliable and easy to maintain. Its core component contains a shearing system, a central temperature processing system and a discharge system. Flow characteristics and the parameter of FCR process affecting semisolid slurry organization are analysed. Taking 7075 aluminium alloy as experimental material, experimental results show that the FCR process could obtain semisolid microstructure with the small primary α-Al particles of nearly spherical and uniformly distributed. The melt of aluminium alloy involves complex convection mixing characteristics when flowing in FCR device. As the pouring temperature decreases, the primary particles become fine and homogeneous with the mean size diminished and the shape factor increased.

Effect of Partitioning Time on the Microstructure and Mechanical Properties of Q&P Steels

The microstructure and properties of Q&P steel were studied by means of tensile test, OM and SEM after simulating heat treatment process in salt bath furnace. The results showed that the main microstructure of Q&P steel was lath martensite and retained film austenite. With the increase of partitioning time, the morphology of the parallel martensite lath became clear and ordered. With the trivialness and disorder with massive martensite appearing, the yield strengths and tensile strength decreased initially and then increased. On the other hand, the elongation increased initially and then decreased. This was because of that the retained austenite is unstable at the beginning for low carbon content, and the carbide precipitated after a long partitioning time. Therefore, there was an optimum partitioning time to obtain the best properties combination. Under 250 quenching temperature and 350 partitioning temperature, partitioning time was 60s, the tensile strength and elongation were 1027MPa and 27%, respectively. The product of strength and elongation was up to 27729MPa·%.

Prediction and Simulation of Microstructure-Property and Residual Stress for Large Size Hot Rolled H-Beam

Based on the full process 3D elastic-plastic thermal mechanical coupled simulation of large size H-Beam, the relevant issues about the residual stress distribution of H-Beam has been systematically analyzed, which include the formation and control of residual stress. In addition, based on the conclusion of the large size H-Beam residual stress analysis, the method of cooling for the control of residual stress has been provided and experiments about the outside cooling of the flange has been implemented. The result of the experiments proved that the coercive water cooling for the outside surface of flange largely reduced the residual stress of the web, which can avoid the web wave after cooling and crack phenomenon during utility. At the same time, on the basis of the whole rolling process finite element simulations, with the help of model of austenite evolution, phase transition and microstructure and property prediction, the simulation of 3D microstructure evolution and property prediction has been fulfilled. By means of microstructure research, it has been proved that the simulation result meet the genuine microstructure very well, which has revealed the feasibility of this method.

Research of Three Dimensional Finite Element Simulation of Rail during Cooling and Pre-Bending Process

In this paper, with the thermal mechanical coupled method and the whole rolling process simulation result using as the initial thermal field, the thermal field and deformation of rail during cooling have been simulated, then rail pre-bending process is proposed according rail bending deformation curve during cooling The results show that the bending curve formula fitting by rail cooling deformation curve is accurate comparing with the actual situation. Moreover, pre-bending process is set by fitting formula, and the rail recovers straightly after cooling.

Micromechanical Modelling for Bauschinger Effect of Dual Phase Steel

The aim of this study is to simulate the micromechanics of dual phase (DP) 600 steel during tension-compression deformation and apply the calculated stress-strain curves to identify material parameters of phenomenological constitutive models with focus on Bauschinger effects. Image-based and statistical representative volume element (RVE) models were built to perform virtual tension-compression tests. The effects of mesh size, dimension, martensite volume fraction and martensitic particle size on stress-strain response were also studied. The results showed that the RVE models gave good overall prediction compared with experiment data. Material parameters of Chaboche nonlinear kinematic model were identified by fitting the resulted cyclic strain-stress curve from virtual tests using optimization software LS-OPT.