The development of ultrafine-grained hot rolling products using advanced thermomechanical processing

Abstract

The aim of the work is development of industry guidance concerning production of ultrafine-grained (UFG) High Strength Low Alloy (HSLA) steels using strain-induced dynamic phase transformations during advanced thermomechanical processing. In the first part of the work, the effect of processing parameters on the grain refinement was studied. Based on the obtained results, a multiscale computer model was developed in the second part of the work that was subsequently used to predict the mechanical response of studied structures. As an overall outcome, a process window was established for the production of UFG steels that can be adopted in existing hot rolling mills.

Introduction

For metal forming it is generally assumed that the shape, properties and dimensions of the final product are directly controlled by the mechanical state of the process and material mechanical response represented by the flow stress, but in practice the microstructure evolution can change the situation drastically. The evolution of the microstructure is connected with physical processes taking place in the deformed steels, such as recrystallization, precipitation and phase transformation, depending on the chemical composition and particular parameters of the metal forming process, i.e. the strain, strain rate and temperature. One example of a modern structural material in which several microstructural phenomena are utilized is ultrafine-grained (UFG) microalloyed steel [1], [2]. At present, several different branches of industry are interested in UFG steels [3]. The superior properties of these steels result from microstructural design that utilizes the effects of both processing and work hardening. The development of UFG microalloyed steels will require unique second phase elements/microstructure combinations to achieve the desired properties. A large volume fraction and fine dispersion of a second phase effectively increase the work hardening rate by promoting the accumulation of dislocation around interphase boundaries [4]. Interpreting these microstructures can be difficult with traditional light, transmission or scanning electron microscopy. Electron backscattered diffraction (EBSD) analysis appears to offer a means to quantify such complex microstructures more effectively and will be used in the present study.

Advanced UFG steels are currently the fastest growing group of structural materials. However, progress in their further development requires the use of modeling processes to understand both microstructure development and the mechanisms responsible for the final mechanical properties [5], [6]. These models must be capable of predicting deformation conditions at critical locations in the process and over a wide range of steel grades. The real challenge for state-of-the-art modeling of material behavior is the process of UFG microalloyed steel manufacturing. There is a growing market for UFG microalloyed steels, and the factors that encourage this expansion are numerous and complex. In the hot working of niobium-micro-alloyed steels, the initial microstructural inhomogeneity of austenite has a significant influence on the metallurgical state of the finished product. This situation may strongly affect the damage behavior of materials [7]. Monitoring the impact of hot deformation conditions on the microstructure development, precipitation process, and resulting defects (i.e., sub-grains, dislocation structure, and deformation bands) under industrial conditions is difficult and expensive. However, using well constructed and experimentally verified models of microstructure development and strengthening mechanisms, it is possible to control the inhomogeneities in such a way that optimal mechanical properties can be expected in a precise location [8]. These possibilities seem to be very attractive in hot rolled or forged shape products, as well as in near shape casting or compact strip production.

In recent years modeling of the phenomena discussed above started to be employed directly in such advanced thermomechanical processing (ATP) as intercritical rolling (i.e., deformation in the austenite-ferrite phase region) or processing based on strain-induced dynamic transformations (SIDT) and in particular strain-induced dynamic ferrite transformation (SIDFT) [9]. In literature, these two terms are used to talk about Strain Induced Dynamic Transformations. It may be used to describe both the austenite to ferrite and reverse dynamic transformation of ferrite to austenite. The term SIDFT is used specifically when one talks about dynamic transformation of austenite to ferrite upon cooling. Here, the idea is to increase the as-hot rolled strength by increasing austenite pancaking, thereby refining and work hardening the ferrite. Deformation controlled austenite morphology or processing in the two-phase region seems to produce higher quality products, which has drawn the attention of scientists. However, more work is needed to maximize the effects of the physically based multiscale modeling in real industrial conditions of metal forming of UFG microalloyed steels. In addition, it is still not well understood how the grain size of austenite and how the history of deformation affect the phase transformation of products. Particular interest should be put on the critical stored energy for triggering the strain-induced ferrite transformation.

Accordingly, the goal of the present study is to investigate the intricate relationship between thermomechanical processing – particularly SIDT – and microstructure refinement of UFG microalloyed steels. This paper will discuss also the direct benefits of computer modeling that can be obtained when multiscale models that have been verified in laboratory conditions are implemented in the real industrial conditions of ATP.