Strengthening of Low-Alloy Hot-Forging Steel with Nano- sized Precipitation

3 IMR KINKEN Research Highlights 2012 IMR KINKEN Research Highlights 2012 To reduce the fuel consumption by automobiles, medium carbon steels for forging parts, such as shafts and connecting rods, must be further strengthened. When carbide-forming elements such as Ti, Nb, and V, are alloyed with carbon steels, a fine dispersion of alloy carbides and large precipitation hardening can be achieved. Continuous cooling after austenitization has been frequently reported to produce repeated nucleation of alloy carbides at ferrite/austenite interphase boundaries during the growth of ferrite or pearlite, leading to formation of precipitates in regularly spaced rows. This peculiar precipitation phenomenon is known as interphase precipitation [1]. In mediumand high-carbon steels containing vanadium, interphase precipitation of VC is used for strengthening. However, there are few studies on interphase precipitation in mediumand highcarbon steels, and the precipitates formed through interphase precipitation in those steels have not been characterized quantitatively. In this study [2], the variations in hardness and microstructure, including the size and number density of VC with V addition, have been investigated. Fig. 1 shows the variations in peak hardness at various temperatures as a function of V content. Peak hardness increases linearly with V content at all three temperatures, while the slope at 973 K is smaller than those at 873 K and 923 K. As the temperature at which the ferrite/pearlite transformation occurs decreases, greater hardening is observed. Fig. 2 is a dark-field TEM micrograph of the interphase precipitation of VC in the 0.4% C alloy containing 0.5% V. It is clearly seen that fine VC particles (imaged as bright particles) is aligned in rows parallel to the interphase boundary. Such VC precipitation was also observed in low-alloy eutectoid steel containing vanadium. Precipitation/dispersion strengthening can be classified into the following two categories: a cutting mechanism and a by-pass (Orowan) mechanism. A larger particle size leads to higher strength in the former case and vice-versa in the latter case. The Strengthening of Low-Alloy Hot-Forging Steel with Nanosized Precipitation In a recent national research project on high-performance steels, the strengthening of medium-carbon lowalloy steel for automobile parts was extensively studied. The key technology for strengthening is the use of fine dispersions of alloy carbides several nanometers in size. Here, we describe briefly the microstructure–property relationships from our research conducted in this project.