Microalloying and a new post forging treatment of medium carbon steels

In the last years, the automotive industry has been demanding a continuous reduction of weight in order to compensate for the introduction of new constructive components according to higher requirements on passenger safety and driving comfort, as well as to accomplish with new strict environmental regulations. The adoption of new materials with improved mechanical properties and/or lower density, together with modern cost effective processing technologies have contributed greatly to reach the above objectives [1], Particularly microalloyed steels, which may provide a higher strength with small alloying additions and low heat treatment expenses, have evolved into an important category of engineering materials suitable for the manufacturing of hull, frame and motor components. As an example, the classical quench and tempering (QT) of alloyed steels followed by straightening operations and stress relieving cycles has been progressively substituted by the continuous cooling (CC) of more economical microalloyed steels from the forging temperature. An accurate control of the cooling rate adjusts the properties of the ferritic-pearlitic microstructure obtained. This technology results in lower processing costs and eliminates product rejectances due to quench-cracking. However there are limitations in the mechanical properties achievable by this processing route if compared with the QT-operation [2], and hardly any cooling rate may be encountered which would lead to a balance of strength and ductility comparable to that after QT. In this context a new post forging treatment (two-step cooling, TSC) has been presented elsewhere [3] as a promising alternative to the CC of medium carbon microalloyed steels. This paper is focused on the particular contributions of different microalloying additions with optimized deformation schedules to improve the mechanical properties achievable through this new technology.