Measurement of Oil Film Thickness Distribution in Roll Bite during Cold Rolling Using Quantum Dots

Lubrication is critical to achieve stable rolling during the cold rolling of flat steel products. However, the oil film thickness distribution in the roll bite and its effect on the friction between the work roll and strip has not yet been clarified. This study aims to elucidate the relationship between the oil film thickness distribution and friction by focusing on the rolling oil viscosity and steel grades because they significantly affect the friction between the work roll and the strip. Rolling oil was prepared with quantum dots (QDs) as the fluorescent additive and used in rolling experiments to determine its distribution. Furthermore, cold rolling experiments were conducted using two types of oils with different viscosities and three steel grades: low-carbon steel (LCS), high-strength steel (HSS), and advanced high-strength steel (AHSS) with tensile strengths of 270, 590, and 1180 MPa, respectively. Subsequently, the oil film thickness distribution on the steel strip surface was visualized by fluorescence microscopy using QDs. The idea that the higher the tensile strength of the steel or the higher the oil viscosity, the wider the rolling oil distribution on the strip surface was demonstrated. The numerical analyses revealed that the rolling oil distribution on the steel sheet surface was wider for AHSS and HSS than that for LCS. The high surface pressure between the roll and the steel plate may have increased the oil leaching area by increasing the oil viscosity. These findings demonstrate that rolling oil permeation from oil pits reduces the friction between the work rolls and the strip.