Formation mechanism of surface scale defects in hot rolling process

Abstract

Thick oxide scale on steel may result in serious surface defects on hot-worked products. Yield efficiency and productivity of processes are considerably deteriorated by formation of defects. It is highly demanded to establish a way to produce hot-worked steels free from surface defects by controlling oxide scale. The oxide scale shows various behaviors in hot rolling; (a) uniform deformation with matrix steel, (b) cracking, (c) fragmentation, (d) indentation to matrix steel, etc. Through observations using glass coating, it is found that the behavior strongly depends on the rolling temperature as well as the scale thickness before rolling. Temperature drop due to contact with cold rolls is found to cause the cracking and a major reason for the thickness dependence. It is found that the scale cracking is predictable using the estimated scale temperature and the ductile-brittle transition temperature of Wustite (FeO). Then, methodology to produce hot rolled steels without surface defects in industrial processes is presented.

Introduction

In industrial hot working of steels and other alloys, it is always important to improve productivity and yield efficiency of processes. Better dimensional accuracy, surface quality, micro-structure and properties of products are also demanded. As the oxide scale formed on the workpiece could be a cause of these problems, it is highly preferable to establish a way to control the scale behavior [1]. Mechanical and water-jet descalers are often used in steel plants to make the scale subjected to hot working sufficiently thin and to improve surface quality. However, frequent descaling increases scale loss and decreases yield efficiency. It takes more machines and electricity. It is not easy to design the specifications and position of descalers in steel plants. In this paper, the authors propose a method to predict formation of scale defects in steel hot rolling. As an application, a technique to produce sheets free from scale defects in industrial processes with consideration of scale behavior is also presented.

Although high-temperature oxidation phenomenon has been studied widely by chemists and metallurgists, the behavior of the scale during hot-working has not been made clear sufficiently. This is because in situ observation is difficult due to high-temperature and high-speed nature of hot working. In addition, the scale starts to grow in atmosphere even after the hot working. In order to avoid the secondary oxidation, Okada [2] and Sun et al. [3] used cooling chambers filled with nitrogen to observe the as hot-rolled scale on the steel sheet. On the other hand, the authors proposed to coat the steel surface immediately after hot rolling using oxide glass [4]. The deformation behavior of oxide scale on low carbon steel and the influence on rolling characteristics were studied under several hot-rolling conditions. In addition, it was found that thick scale decreases friction coefficient by non-uniform scale deformation and decreases the heat transfer from the hot steel to the cold rolls [5].

Through these experiments, it was made clear that the scale shows various complicated behaviors during hot rolling; (a) uniform deformation with matrix steel, (b) cracking, (c) fragmentation, (d) indentation to matrix steel, etc. It is notable that the scale is not always brittle but shows some ductility depending on hot rolling conditions. Lenard reported that the behavior depends on the temperature as well as the scale thickness [6]. As the ductility of the oxide scale increases remarkably with increasing temperature in tensile test [7], the deformation of scale in hot rolling may be affected by the real surface temperature in the roll bite.

In this study, hot rolling experiments were conducted to investigate the scale deformation including the thickness dependence in detail. Through the observations, the complicated scale deformations were classified based on morphologies after the rolling. Then, the scale deformation was predicted with the scale temperature estimated by simple thermal analysis. Using reported properties of Wustite (FeO) at high temperature, fracture conditions of the scale which can account for the experimental behaviors are proposed. Using the conditions, methodology to avoid scale defects in industrial hot rolling processes is derived.