Evaluation of friction in the hot rolling of steel bars by means of on line forward slip measurements

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In spite of the importance of friction it is still a weak point in the analysis of rolling. Most studies assume that a coefficient of friction (μ) is representative for the roll gap. Forward slip measurements under normal production conditions in a bar mill have been used to evaluate the coefficient of friction, completed by laboratory measurements in a high-temperature test rig, and stability measurements in oval–square and flat–edge oval pass sequences. Coefficients of friction in the range 0.35–0.40 have been evaluated from the forward slip measurements. The rolling speed did not exceed 0.2 m/s and measurements were made in the temperature range 1000–1150°C. No clear influence of temperature on friction could be observed. The stability measurements show similar results. However, no tests have been made at temperatures other than 1000°C, and no influence of temperature on friction can be evaluated. In the high-temperature test rig the influence of temperature on friction is strong at temperatures exceeding 900°C, and probably the mechanism of friction changes in the test rig at higher temperatures, due to the lack of plastic deformation in the contact zone. Further investigations are necessary to formulate a relevant model for friction, valid for the entire range of parameters in a bar or wire rod mill.

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