Per: peTER geLTEN (tATA sTEEL EUROPE), GEORGE scHIPPER (tATA sTEEL eUROPE), hugo uijtdebroeks (CRm group), pepijn adriaen (crm group)
Abstract:
High-Turbulence Roll Cooling in Tata Steel IJmuiden Hot Strip Mill 2
Tata Steel IJmuiden Hot Strip Mill 2 (HSM2) is currently being upgraded, resulting in an increased rolling force and motor power capacity. With the current conventional spray cooling, a deterioration in work roll & strip surface quality performance is expected as a result of the increased mill capacities. In order to improve the strip surface quality, HSM2, in close collaboration with CRM Group (Belgium), decided to do a full-scale production trial with a prototype High-Turbulence Roll Cooling (HTRC) at the Finishing Mill Stand 2 (FM2) top work roll.
HTRC is a relatively novel way of roll cooling, developed by CRM, using a high turbulent flow regime, operated at a high flow rate (up to 700 m3hr per HTRC unit) and at a low pressure (2-3 bar). HTRC is a cooling unit attached to the stand exit guide with a similar curvature as the work roll, which is positioned very close to the roll surface (5-15 mm). The water flow is going upwards, in the same direction as the circumferential roll speed, which is beneficial to prevent wiper water leakage and also to cool as efficiently and as closely as possible to the gap exit (minimal “wet angle”). Minimizing the wet angle is reducing the repetitive heat penetration into the roll as much as possible, which is believed to be beneficial for roll degradation and thus strip surface quality.
Fluid mechanics tell us that a high turbulent flow regime and thus a high heat transfer coefficient, are achieved by high flow rates (to increase the Reynolds number (Re) values) in combination with perturbation of the flow regime. Therefore, the HTRC unit also has a specifically designed pattern of nozzles, which are creating the perturbation of the flow regime.
Implementing the HTRC prototype was quite a challenge, but rewarding! The HTRC had to be integrated in an existing (spare) exit guide. High pressure pumps needed to be by-passed. Implementation must be possible within an 8 hours stop and fully reversible in case needed. Operationally, key aspect of the HTRC is close positioning to the roll, creating a narrow 5-15 mm gap in-between the HTRC and the roll surface. To prevent scratching the roll, the wiper nose at the bottom side and the blocks at the top side of the HTRC unit are used to maintain the gap distance. Pushing too hard to the roll, leads to (undesired) high wear rate of the wiper nose and the blocks. If the pressure is not high enough, the water will push the HTRC unit away from the roll, possibly leading to water leakage on the strip and a decreased cooling efficiency. In HSM2, lifetime of the wiper and blocks is currently excellent with no water leakage on the strip.
HTRC has been implemented at the FM2 top work roll in August 2018. At the same time, the entry cooling has been blinded, providing opportunities to apply work roll lubrication (which otherwise would possibly be washed away by the entry cooling water). The percentage of rolls with a poor roll surface has dropped more than a factor of 10. In 2020 HTRC will be implemented in stands FM1-FM4, leading to savings on improving roll and strip surface quality. Shutting down redundant high pressure pumps adds significant savings on electricity costs.
