Ultra-high-strength Steel And Optimization Of Phase Change Heat Treatment Process

30Cr4Si2NiMoNb ultrahigh strength steel is a new developed industrial forging bar, which belongs to the category of medium alloy steels. Because of the increases of alloying elements Si、Cr、Mo and Nb,30Cr4Si2NiMoNb ultrahigh strength steel has better hardenability campared to ultrahigh strength low alloy steels. As the collocations of alloy elements are changed, the phase transformation and phase composition of30Cr4Si2NiMoNb ultrahigh strength steel during the heat treatment are different from ultrahigh strength medium alloy steel, such as Eglin, VKS9and G50steels. As a result, it’s difficult to determine the optimal heat treatment. In this paper,30Cr4Si2NiMoNb ultrahigh strength steel is set as the research object to systematically simulate the phase transformation in the forgings during the heat treatment and their effects on the final mechanical properties by experiments. Accordingly we can determine the optimal heat treatment based on the results. There are three aspects investigated as follows:(1) The effects of preliminary treatment on the hardness;(2) The effects of austenizing temperature on the strength-toughness;(3) The effects of cooling rate on the microstructures and mechanical properties. There are some results achieved as follows:The effects of annealing temperature and time on the hardness of30Cr4Si2NiMoNb ultrahigh strength steel were investigated. The results have shown that hardness descended as the amount of carbide precipitation increased with the raised annealing temperature and extended time. The hardness decreased slowly annealed at higher temperature than680℃, at the same time the hardness also declined slightly with the extended time at680℃, and the coarsened matrix structure was disadvantageous to the combination property. Consequently, the treatment of680℃×2h is determined as the optimal heat treatment.The phase transformation and phase composition during the austenization and their effects on the mechanical properties of30Cr4Si2NiMoNb ultrahigh strength steel were investigated. The results have shown that the grains were refined when austenitized below930℃, but the undissolved carbide (Cr,Fe,Mo)7C3in the steel worsened the impact toughness. As a result, the final strength-toughness was relatively poor. The coarser grains with increased austenitizing temperature led to the decreases of final strength and toughness when austenitized above930℃. Accordingly930℃was the optimal austenitizing temperature of30Cr4Si2NiMoNb ultrahigh strength steel.The influences of cooling rate on microstructures and mechanical properties of30Cr4Si2NiMoNb ultrahigh strength steel were investigated. The results have shown that martensitic and bainitic transformation took place in30Cr4Si2NiMoNb ultrahigh strength steel respectively cooled at different rates (oil cooling, air cooling,450℃/h and200℃/h). The microstructures of specimens cooled at oil and air were both lath martensite, and the strength was decreased, meanwhile impact toughness increased with the slower cooling rate. The microstructures of specimen cooled at450℃/h were13vol%lower bainite+martensite, the strength was increased and impact toughness was decreased because of the precipitation of vimineous and lamellar carbide. The final strength-toughness deteriorated while the upper bainite appeared in the specimen cooled at200℃/h, and it couldn’t achieve the mechanical performance index in industrial applications. The microhardness declined with the decrease of cooling rate.