New bainitic steels for forgings

Abstract

Steels with a bainitic microstructure offer great possibilities for highly stressed forged components. The variety of different bainitic morphologies requests for an aligned thermal treatment after forging in order to achieve the maximum performance. In dependence of the alloying concept and heat treatment bainite is composed of different microstructural components like the ferritic primary phase and the secondary phase, which consists of either carbides, martensite and/or austenite. Different combinations of mechanical properties can thereby be adjusted in these steels, dependent on the arrangement of the primary and secondary phase. Three steels have been investigated, which contain approximately 0.22% C, 1.5% Si, 1.5% Mn, 0.08% Mo, 0.003% B and 0.01 Ti. Their chromium contents vary between 0% for grade 1 and 1.3% for grades 2 and 3. The niobium content varies between 0% for grades 1 and 2 and 0.03% for grade 3. The Si addition is utilized to suppress the carbide formation, so a carbide free bainitic microstructure is expected to form, whereas Mo and B are employed in order to promote the bainite formation. The bainitic microstructure of these steels can be formed either after isothermal phase transformation or after continuous cooling. These two process routes lead to different results with regard to the mechanical properties, especially the Y/T-ratio. While after isothermal transformation the low chromium containing grade 1 exhibits a higher Y/T-ratio than grades 2 and 3, this fact is turned around for the case of the bainite formation after continuous cooling. In the latter case the high chromium containing steel exhibits both higher yield and tensile strength. These differences in mechanical properties can be correlated to characteristic features of the primary and secondary phases of the bainitic microstructures. The specific role of chromium is explained by its effect on the phase transformation kinetics.

Introduction

The commonly used forging steels for automotive applications are on the one hand the precipitation hardening ferritic-pearlitic steels (PHFP-steel) and on the other hand the quenched and tempered (Q&T) forging steels. In order to obtain similar strength properties in PHFP-steels as in Q&T steels, the microstructure and the mechanical properties of these steels are controlled by adding microalloying elements (V, Nb and Ti). Those have a significant influence on the precipitation strengthening and the austenite grain size [1]. The advantages of these PHFP steels compared to Q&T steels are the elimination of an additional heat treatment step which includes a hardening, tempering and stress relieving due to a controlled cooling directly after hot forging (Fig. 1) and an improved machinability [2], [3].

However, forging steels with ferritic/pearlitic microstructures show inferior values of yield strength and toughness compared to the Q&T steels [4]. In order to improve the toughness while maintaining high strength values a bainitic microstructure can be employed [5], [6], [7]. Fig. 2 shows the achievable tensile strengths in dependence of the microstructure for PHFP-M and high strength ductile bainitic (HDB) steels. The increase in strength for the PHFP-M steel is achieved by reduction of the ferritic volume fraction, the decrease in the pearlite lamellae spacing λ and the addition of the microalloying elements Nb and Ti which results in additional precipitates besides the vanadium nitrides [8]. The aimed for microstructure in the HDB steel consists mainly of bainitic ferrite and retained austenite instead of carbides [9], [10] as the bainitic second phase [11].

In this paper the focus will be on the characterization of HDB steels with different chemical compositions and after different process routes. Therefore, in the examined steels the chromium content is varied. Chromium is a commonly used alloying element that enhances the hardenability of steel. It is also a strong carbide forming element [12] and a weak austenite stabiliser. In addition, chromium is one of the elements that hinder the diffusion of carbon significantly which results in a solute drag like effect (SDLE). The SDLE was originally proposed by Kinsman and Aaronson in order to explain the sluggish growth kinetics of ferrite in steels containing specific alloying elements such as Ni, Cr and Mo [8]. They assumed that substitutional alloying elements are accumulated in areas of the ferrite/austenite boundaries. These elements can reduce the activity of C in areas besides the boundaries, which results in a concentration peak of C in austenite directly ahead of the growing ferrite and reduces the ferrite growth kinetics. This effect, which is called the incomplete reaction phenomenon, can even disrupt the transformation and leads to a bay area (Fig. 3) without any transformation occurring between the ferrite/pearlite phase field and the bainite phase field [13].