MODELING OF MICROSTRUCTURAL EVOLUTION IN MICROALLOYED STEEL DURING HOT FORGING PROCESS

The microstractural evolution of microalloyed steel during hot forging process was investigated using physical simulation experiments. The dynamic recrystallized fraction was described by modifying Avrami＇s equation, the parameters of which were determined by single hit compression tests. Double hit compression tests were performed to model the equation describing the static recrystallized fraction, and the obtained predicted values were in good agreement with the measured values. Austenitic grain growth was modeled as： Dinc^5 = D0^5 ＋ 1.6 × 10^32t·exp （ -716870/RT ） using isothermal tests. Furthermore, an equation describing the dynamic recrystallized grain size was given as Ddyn=3771·Z^-0.2. The models of microstructural evolution could be applied to the numerical simulation of hot forging.

New Type Austenite Dynamic Recrystallization of Microalloyed Forging Steels 38MnVS During Forging Process

Hot compression tests of microalloyed forging steels 38MnVS were carried out on the Gleeble-3800 thermo-mechanical simulator at the deformation temperatures from 950 to 1 150 ℃ with the strain rates ranging from 0.1 to 10 s^（-1）. The effects of the deformation temperature and strain rate on the austenite dynamic recrystallization and microstructural changes were researched. The experimental results show that the dynamic recrystallization accelerated with the increase of the deformation temperatures and the decrease of the strain rate. The activation energy of dynamic recrystallization was calculated, which was about 275.453 kJ/mol. The relation between the dynamic recrystallization and the Z-parameter was investigated, and the state chart of the dynamic recrystallization of the microalloyed forging steel 38MnVS was made according to the experimental data and the deformation parameters.

Constitutive Model for F45V Microalloyed Forging Steel at High Temperature

Isothermal hot compression experiments of F45V, a microalloyed steel, were performed on a Gleeble-1500 thermo-mechanical simulator at temperatures of 950--1200 ;C and strain rate of 0.01--10 s -1. Based on the experi- mental flow stress curves, a constitutive model that was expressed by the hyperbolic laws in an Arrhenius-type equa- tion was established, and the material parameters of the model were expressed as 6th order polynomial form of strain. Standard statistical parameters such as correlation coefficient and average absolute relative error were em- ployed to quantify the predictability of the model. They were found to be 0. 995 and 4.34% respectively. The results show that the established constitutive mode[ can predict the magnitude and tendency of flow stress with the increase of deformation accurately, and can be used for the numerical simulation of hot forging process of the F45V steel.

Detection and analysis of magnetic particle testing defects on heavy truck crankshaft manufactured by microalloyed medium-carbon forging steel

An unqualifed six-cylinder heavy truck crankshaft has been studied to investigate the cause of magnetic particle testing defects on the rod journals.Large-sized long-striped MnS inclusions are regarded as the major cause for the magnetic particle testing failure because they have been detected in situ under the magnetic particle indications.Through the observation of macroscopic structures of the rod journals and corresponding counterweight blocks,it is found that for the 1#and 3#rod journals,the center metal of the original hot-rolled bar has been extruded to the inboard edge of the rod journals and large-sized long-striped MnS inclusions are exposed on the surface after fash removal,leading to the failure of magnetic particle testing.As for the 2#rod journal,the center metal of the original bar has not been extruded to the surface and MnS inclusions on the rod journal surface are small in size,few in number,resulting in passing the magnetic particle testing.If the quality of the hot-rolled bars fuctuates,it is more recommended to apply magnetic particle testing on samples at the center of bars before forging to evaluate the severity of defects caused by the long-striped MnS inclusions for fear of the scrap of the fnal crankshafts.

Dynamic Recrystallization Behavior of Microalloyed Forged Steel

The dynamic recrystallization behavior of microalloyed forged steel was investigated with a compression test in the temperature range of 1 223-1 473 K and a strain rate of 0. 01-5 s^-1. Activation energy was calculated to be 305.9 kJ/mol by regression analysis. Modeling equations were developed to represent the dynamic recrystallization volume fraction and grain size. Parameters of the modeling equations were determined as a function of the Zener-Hollomon parameter. The developed modeling equation will be combined with finite element modeling to predict microstructural change during the hot forging processing.