Yisheng Zhang

Modeling of the Austenitization of Ultra-high Strength Steel with Cellular Automation Method

A model for simulating the austenitization of ultra-high strength steel during hot stamping is developed using a cellular automata approach. The microstructure state before quenching can be predicted, including grain size, volume fraction of austenite, and distribution of carbon concentration. In this model, a real initial microstructure is used as an input to simulate austenitization, and the intrinsic chemical difference is utilized to describe the ferrite and pearlite phases. The kinetics of austenitization is simulated by simultaneously considering continuous nucleation, grain growth, and grain coarsening. The UHSS is reduced to a Fe-Mn-C ternary system to calculate the driving force during extent growth in ferrite. The simulation results show that the transformation of ferrite to austenite can be divided into three stages in the condition of a heating rate of 10xa0K (−263xa0°C)/s. The transformation rate is determined by two factors, carbon concentration and temperature. The carbon concentration plays a major role at the early stages, as well as the temperature is the main factor at the later stages. The Ac3 calculated is about 1073xa0K (800xa0°C) close to the measured value [1067.1xa0K (794.1xa0°C)]. Austenite grain coarsening was calculated by a curvature-driven model. The simulated morphology of the microstructure agrees well with the experimental result. Most of the dihedrals of the grain boundaries at the triple junctions are close to 120xa0deg. Finally, tensile tests were implied, as dwelling time increased from 3 to 10xa0minutes, the austenite grain size increased from 6.95 to 9.44xa0μm while the tensile strength decreased from 276.4 to 258.3xa0MPa.

Martensitic Phase Transformation from Non-isothermally Deformed Austenite in High Strength Steel 22SiMn2TiB

Non-isothermal compressive deformation was performed on high strength steel 22SiMn2TiB for the study of martensitic phase transformation from deformed austenite. The transformation start temperature Ms decreased with the increase of deformation from 0 to 50 pct, and the variation of deformation rate (0.1 and 10xa0s−1) and the appearance of deformation-induced ferrite and bainite showed no influence on the change of Ms temperature. The deformation at both the rates increased the volume fraction of retained austenite; however, the carbon content of retained austenite decreased at 10xa0s−1 and remained basically unchanged at 0.1xa0s−1. The yield strength of austenite at Ms temperature and the stored energy in deformed austenite were experimentally obtained, with which the relationships between the change of Ms temperature and the thermodynamic driving force for martensitic phase transformation from deformed austenite were established by the use of the Fisher-ADP–Hsu model. And finally, the transformation kinetics was analyzed by the Magee–Koistinen–Marhurger equation.

Nonlinear numerical model with contact for Stockbridge vibration damper and experimental validation

The Stockbridge vibration damper is widely used in overhead transmission lines to reduce Aeolian vibration. Although a linear analytical model has been developed to interpret characteristics of the vibration damper, a much more detailed model is needed to investigate how the nonlinear factors of the structure affect its vibration characteristics. The paper presents a full-scale finite element model of the Stockbridge vibration damper, in which contact conditions are taken into account using the linear perturbation method. Relations between the contact conditions and mode frequencies were studied. It was proved that contact conditions between each two parts of the damper have significant influence on the stiffness of the whole structure. Results obtained from the numerical model compare well with those from the experiment. Finally, this numerical model was applied to investigate how the bonding material between the counterweight and steel strand cable affects the mode frequencies of the vibration damper.

Coupled Model for Carbon Partitioning from Martensite into Austenite During the Quenching Process in Fe-C Steels

In this paper, a coupled model for carbon partitioning from martensite into austenite during the quenching process in Fe-C steels is constructed where the carbon is permitted to partition while the martensite is continuously forming. A diffusion model of carbon at the ‘martensite/austenite interface’ is created where the interface does not move during the carbon partitioning process, and the driving force for carbon partitioning originates from the chemical potential difference. The results show that the martensitic transformation and carbon partitioning affect each other, and that the cooling rate between the martensite start temperature (Ms) and room temperature has a major effect on the volume fraction of the final retained austenite. The simulation results are shown to be in good agreement with experiments.

Advances in tailored hot stamping – innovations in material and local patchwork topology

Hot stamping is now commonplace in the automotive industry. The continuing need by automotive manufacturers to reduce weight while increasing crashworthiness has driven the industry to seek new hot stamping solutions. Tailored hot stamping can be thought to produce a part that has patchwork of hard and soft regions. In this context, patchwork means that there is a relational organization (topology) to the network of hard and soft regions. The next generation of tailored hot stamping will therefore combine new steel grades together into a single part, and secondly will be able to locally tailor material properties to meet detailed engineering targets. The key to meeting engineering demands will be how the patchwork material properties are organized on the part. This paper will briefly outline our latest research in tailoring parts.

Temperature distribution of boron-manganese sheet metal blank by induction heating in application for hot stamping

In order to speed up the production and save more energy in hot stamping process,the induction heating technology as a new effective heating method is considerable.Finite element(FE)-simulation and a series of experiments are carried out to research the temperature homogenization of induction heating with the face inductor.It is found the edge effect has a notable influence on the temperature distribution.Results concerning the mechanical properties of the stamped part as well as surface characteristics will be presented and discussed.

Application of a Model for Quenching and Partitioning in Hot Stamping of High-Strength Steel

Application of quenching and partitioning process in hot stamping has proven to be an effective method to improve the plasticity of advanced high-strength steels (AHSSs). In this study, the hot stamping and partitioning process of advanced high-strength steel 30CrMnSi2Nb is investigated with a hot stamping mold. Given the specific partitioning time and temperature, the influence of quenching temperature on the volume fraction of microstructure evolution and mechanical properties of the above steel are studied in detail. In addition, a model for quenching and partitioning process is applied to predict the carbon diffusion and interface migration during partitioning, which determines the retained austenite volume fraction and final properties of the part. The predicted trends of the retained austenite volume fraction agree with the experimental results. In both cases, the volume fraction of retained austenite increases first and then decreases with the increasing quenching temperature. The optimal quenching temperature is approximately 290xa0°C for 30CrMnSi2Nb with the partition conditions of 425xa0°C and 20xa0seconds. It is suggested that the model can be used to help determine the process parameters to obtain retained austenite as much as possible.

Experimental Investigation of Tailored Hot Stamping Parts

It is known that tailored hot stamped parts, which have locally graded properties, can improve car crashworthiness. In this experimental study, a heated tool was used to decrease the temperature difference between the hot blank and the tool which led to lower cooling rates and softer properties. First, a flat heated tool was used to investigate the effects of process parameters on metallurgical and mechanical properties. Based on the range of parameters examined, press force and quenching time did not have a significant effect on the post-formed mechanical properties. In the next step, a hatshaped channel tool with heating system was used to produce tailored hot stamping parts. The results show considerable differences between hardness values of the top and side faces in the soft section, while the hardness was almost uniform in the hard section. These experimental results generally compare well with the results of previous numerical parametric studies performed by the authors, which identified less robustness of the tailored hot stamping process compared to conventional hot stamping.

Erratum to: Modeling of the Austenitization of Ultra-high Strength Steel with Cellular Automation Method

THE following are corrections to the original article: Page 1, Abstract, line 10: 10 K (–263 C)/s should be 10 K (10 C)/s. Page 4, Fig. 2 caption: 10 K (794.1 C)/s should be 10 K (10 C)/s. Page 5, IV. Procedure of Simulation, second paragraph: 10 K ( 263 C)/s should be 10 K (10 C)/s. Page 7, last paragraph: 10 K ( 263 C)/s should be 10 K (10 C)/s. Page 10, VI. Impact of the Grain Size on Strain–Stress Curve, first paragraph: 10 K ( 263 C)/s should be 10 K (10 C)/s; and 35 K ( 238 C)/s should be 35 K (35 C)/s. Page 10, VII. Summary and Conclusions, second paragraph: 10 K ( 263 C) /s should be 10 K (10 C) /s.