Chengwei Shao

Hot Deformation Behavior of Vanadium-microalloyed Medium-carbon Steel for Fracture Splitting Connecting Rod

Single compression tests were carried out with a Gleeble-3800 thermal simulator to investigate hot deformation behavior of two vanadium-microalloyed medium-carbon steels for fracture splitting connecting rod. The tests were performed to a total true strain of 0.92 at true strain rates ranging from 10−2 to 10 s−1 and deformation temperature of 900–1150 °C. The results show that hot deformation behavior of the tested steels is similar to that of conventional medium-carbon microalloyed steels and dynamic recrystallization is easier to occur at higher deformation temperature and lower strain rate. The austenite deformation resistance and activation energy of deformation increase with increasing vanadium content from 0. 15% to 0. 28% and thus the starting time of dynamic recrystallization was delayed. Finer recrystallized austenite grain could be obtained at higher strain rate, lower deformation temperature and higher vanadium content. TEM observation of the specimens quenched just before and after deformation reveals that vanadium is mainly in dissolved solute condition in austenite and thus affects the dynamic recrystallization behavior of the tested steels mainly through solute-drag effect.

Thermal stability of retained austenite and mechanical properties of medium-Mn steel during tempering treatment

The thermal stability of retained austenite (RA) and the mechanical properties of the quenched and intercritical annealed 0.1C-5Mn steel with the starting ultrafine lamellar duplex structure of ferrite and retained austenite during tempering within the range from 200 to 500 °C were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and tensile testing. The results showed that there was a slight decrease in the RA volume fraction with increasing tempering temperature up to 400 °C. This caused a slight increase in the ultimate tensile strength (UTS) and a slight decrease in the total elongation (TE); thus, the product of UTS to TE (UTS×TE) as high as 31 GPa • % was obtained and remained nearly unchanged. However, a portion of the RA began to decompose when tempered at 500 °C and thus caused a ~35% decrease of the RA fraction and a ~16% decrease of the value of UTS×TE. It is concluded that the ultrafine lamellar duplex structure is rather stable and the excellent combination of strength and ductility could be retained with tempering temperature up to 400 °C. Thus, thermal processes such as galvanization are feasible for the tested steel provided that their temperatures are not higher than 400 °C.

Enhancing the Mechanical Properties of Vanadium-Microalloyed Medium-Carbon Steel by Optimizing Post-Forging Cooling Conditions

The effects of different cooling conditions after forging on the microstructural characteristics and mechanical properties of a kind of high-content V-microalloyed medium-carbon steel 37MnSiVS were investigated. The effects were studied by using optical microscopy, transmission electron microscopy and tensile tests. Increasing direct cooling rate after forging is found to increase strength while slightly decrease ductility. A significant increase of strength could be obtained after forced air cooling and then short time isothermal holding at 873 K, while strength decreases gradually with further increasing holding time. The variations of microstructural characteristics especially V(C,N) precipitation strengthening effects with cooling conditions are mainly responsible for these variations of tensile properties. For the investigated high V-containing MA steel, a direct cooling strategy after finish forging is proposed, which includes accelerated cooling with forced air in ferrite range, following by short time isothermal holding or very slow cooling at around 873 K and then air cooling.

Influence of Vanadium on Fracture Splitting Property of Medium Carbon Steel

The fracture splitting property of medium carbon steel 37MnSiS microalloyed with V up to 0.45% was investigated by using simulated fracture splitting test, for the development of new crackable medium carbon steel to manufacture high performance connecting rod. Conventional high carbon steel C70S6 was used for comparison. The results show that the volume fraction of both ferrite and V-rich M(C,N) particles increases, and the pearlite interlamellar spacing decreases with increasing V content, which in turn results in gradual increase of strength and decrease of ductility and impact energy. The fracture splitting property of the tested steel could be improved significantly due to the increase of V content mainly through the precipitation hardening mechanism of fine M(C,N) precipitates. The fraction of brittle cleavage fracture in the crack initiation area increases noticeably with increasing V content and full brittle cleavage fracture surface could be obtained when V content was increased to 0.45%. It is concluded that medium carbon steel with V content higher than about 0.28% possesses not only comparable or even higher mechanical properties with those of conventional steel C70S6, but also excellent fracture splitting property, and therefore, is more suitable to fabricate high performance fracture splitting connecting rod.