Tensile behavior of normalized low carbon Nb-microalloyed steel in the presence of rare earth elements

Abstract

In this study, attempts have been made to study the effects of RE on the tensile behavior of a low carbon Nb-microalloyed steel which confronts yield point phenomenon during its tensile test. The results of the tensile tests and microstructural examinations presented in this research showed that the upper and lower yield points increase by the RE addition which was mainly attributed to the indirect effects of RE on the Nb-precipitation manner and refinement of the microstructure. The results also demonstrated that the flow behavior (oscillation in stress level) during the Lüders strain zone is considerably different for the base and RE-added steels. It was found that the RE-added steel undergoes a uniform propagation of the Lüders bands while the base steel showed a distinct non-uninform Lüders strain with rough fluctuations of stress level within this area. This could be due to the uniformity in distribution of nanoprecipitates and solute, e.g. C, atoms in the RE-added steels. Moreover, It was observed that the Lüders strain increases in the presence of RE which could be probably attributed to the finer ferrite grains and the change in nanoprecipitation behavior caused by RE addition. A significant increase in total elongation of the RE-added steel was also observed.

Introduction

Microalloyed ferritic-pearlitic steels are placed in a group of High-Strength Low-Alloy (HSLA) steels which have attained a continuous and ever-increasing development so far [1], [2]. These steels contain very small amounts of strong carbide or carbonitride-forming elements such as niobium for precipitation strengthening and grain refinement purposes. Carbon content of such steels has been reduced to improve both weldability and toughness, because, the strengthening effects of microalloying elements can compensate the strength reduction caused by the reduction in carbon content [1], [3], [4], [5]. From the chemical composition point of view, the unique mechanical properties of such steels mainly result from the mere presence of the microalloying elements. These elements provide a combination of strengthening mechanisms that serve as obstacles for dislocation movement during deformation processes [6], [7]. Among these, niobium is the more effective strengthening element, able to strongly increase the yield strength by precipitation hardening and by promoting the grain refinement of ferrite grains [8], [9], [10], [11], [12], [13].

In addition to the precipitation hardening promoted by these microalloying elements, the fineness of the microstructural components would also affect the tensile behavior. It has been reported that grain refinement of proeutectoid ferrite would favorably contribute to the strength and toughness properties of the steels [14], [15], [16], [17]. The reduction in the pearlite nodule size would also contribute to the enhancement of the steels strength [15], [18]. Besides, in the low carbon steels, where pearlite coexists with ferrite in the microstructure, the contribution of pearlite volume fraction to the strength is considered to obey the law of mixtures [14], [16], [17].

The characteristic tensile behavior of polycrystalline materials consists in a smooth transition from the elastic to the elastic–plastic region with a steadily rising stress–strain curve, prior occurrence of necking. While, low carbon steels display a rheological behavior at the beginning of plastic deformation where an abrupt transition from the elastic to elastic–plastic state with a characteristic drop in the stress–strain curve takes place [19]. This is referred to the yield point phenomenon, and is associated with the occurrence of Lüders bands. However, for some applications, in order to improve the deep drawability and surface quality, continuous yielding (elimination of Lüders bands) is usually preferred [20]. The main reason for appearance of Lüders bands in such steels is the pinning of dislocations by the dissolved C and N atoms which form the so-called Cottrell's atmospheres [20], [21].

In steel industry, Rare Earth elements (RE) are known as strong inclusion modifiers [22], [23], [24], [25]. There are abundant published reports dedicated to the effects of RE on tensile property of steels [26], [27], [28], [29]. Some of these works show contradictory results regarding the effects of RE on the strength which implies somehow that there is no clear relationship between the tensile strength and the amount of RE in steels. However, a consensus regarding the positive effect of RE on elongation and toughness properties exists in the literature [26], [28], [30], [31]. Despite the extensive studies carried out in this field, the effect of RE on the tensile behavior of low carbon Nb-microalloyed steels, which is talented to undergo a yield point phenomenon, has not been studied so far and the conducted experimental reports suffer from a lack of proper discussion and explanation.

In a recent work of the authors, it has been found out that RE addition to such a low carbon microalloyed steel would refine the pearlite nodules and ferrite grains [32]. In a separate report, the authors have also showed and discussed that RE would affect the fraction, roundness factor, and size of the inclusions that exist in low carbon steels [33]. Besides, it has been reported that RE are able to affect the solubility of Nb and change the Nb-precipitation manner in microalloyed steels [33], [34], [35]. Regarding the effect of Nb-precipitation on the tensile behavior, Reskovic and Jandrlic [19] have claimed that the severity of Lüders effect is influenced by the presence and distribution manner of Nb-precipitates in microalloyed steels.

One can deduce that, more attention should be paid to the use of RE in steels aimed at playing roles in transformations taken place in steels rather than their sole influence on inclusion modification. Hereupon, encouraged by an extensive literature research and net results obtained from our works concerned with RE addition to the normalized steels, in this study, a misch metal (containing Ce and La) was added to a low carbon Nb-microalloyed steel to elucidate their influence on the tensile behavior. In fact, this study mainly focuses on the unraveled influences of RE on the Nb precipitation manner and its consequences on the tensile behavior which would be associated with a change in the yield point phenomenon. Detailed microstructural examinations enabled us to discover the link between the addition of RE and the tensile behavior of the steel under investigation; the addition of RE to low carbon microalloyed steels would change the microalloying precipitation manner (by inclusion modification), which in turn will have an impact on their tensile behavior (especially the Lüders effect). Since the Lüders effect/yield point phenomenon is an important factor interfering with deformation process of low carbon steels, the outcomes of this study could shed light on the advantages and drawbacks of RE addition to such steels.