The wear-resistant steel plate used for forging is the most widely used class of wear-resistant steel plate at present, and its forging process not only basically determines the shape of the parts, but also basically determines the process performance and service performance of the parts. 37MnSiVS steel is a new type of medium carbon wear-resistant steel plate for expansion connecting rod developed in recent years, which has excellent comprehensive mechanical properties and expansion performance, and can be used to manufacture high-performance automotive engine connecting rod by expansion process.
Compared with the traditional medium carbon wear-resistant steel such as 38MnVS steel, the mass fraction of microalloyed element vanadium in the steel is increased from the usual 0.08% ~ 0.15% to 0.25% ~ 0.35%. The significant increase of vanadium content will inevitably affect the thermal deformation behavior of steel.
37MnSiVS steel with high vanadium content was selected for the test, smelted in a 200kg vacuum induction furnace in the laboratory, and poured into 110kg ingot, whose chemical composition (mass fraction, %) is as follows: C0.38%, Si0.77, Mn1.07, P0.032, S0.085, Cr0.18, V0.28, Al0.017. The ingot is then forged into a round rod with a diameter of 50mm by two fires, and air cooled after forging. A thermal simulated cylindrical specimen with a diameter of 8mm and a height of 15mm was extracted from the 1/2 radius of the round rod.
A single pass compression thermal simulation test was carried out on the Gleeble 3800 thermal simulation machine. The sample was heated to 1200℃ at the rate of 10℃/s, held for 2min, and then cooled to the deformation temperatures of 1150, 1100, 1000 and 900℃ at the rate of 10℃/s, respectively. After being held for 10s (eliminating the temperature gradient within the sample), a single pass compression deformation was carried out at the deformation rates of 0.1, 0.5, 1 and 10s-1, respectively. The deformation is 60% (true strain is 0.92). A layer of graphite sheet is inserted at the contact between the indenter of the thermal simulator and the two ends of the sample for lubrication, so that the sample is deformed as uniformly as possible. After deformation, the sample was immediately water-cooled to room temperature to retain the austenitic grain morphology.
After quenching, the sample was cut along the axis with wire cutting, and after grinding and polishing, it was etched with saturated picric acid + detergent solution to show the original austenite grains. The austenite grain was observed by metallographic image analyzer and photographed. The grain size of austenite was measured by transsectional method. In order to confirm whether there is vanadium carbonitride precipitation before and after deformation, the samples cooled to 900℃ were water quenched before and after deformation. Transmission electron microscope (TEM) samples were prepared by electrolysis with 10% perchloric acid alcohol solution and the microstructure was observed under HITACHI H-800 TEM.
The results show that the thermal deformation characteristics of the test material are basically the same as that of the traditional medium carbon microalloy wear-resistant steel plate, and the higher temperature and lower strain rate are conducive to dynamic recrystallization. When the deformation temperature of the test material is lower than 1000℃, the time of recrystallization is further extended. The results of transmission electron microscopy showed that the vanadium in the test material mainly existed in the form of solid solution in austenite, which affected the dynamic recrystallization behavior of austenite. The thermal deformation activation energy of the experimental material is 364.9kJ/mol, and the thermal deformation equation and the relationship between the dynamic recrystallization grain size and Zener-Hollomon parameters are obtained. This study provides theoretical and experimental basis for the optimization of steel grade composition and the formulation of rational forging process.
