温度模拟

Journal of Materials Processing Technology 155–156(2004)

1307–1312

High temperature oxide scale characteristics of low

carbon steel in hot rolling

Weihua Sun ∗,A.K.Tieu,Zhengyi Jiang,Cheng Lu

Faculty of Engineering,University of Wollongong,North-fields Avenue,Wollongong,NSW 2522,Australia

Abstract

In this paper,hot rolling tests of low-carbon steel were carried out on a 2-high Hille 100experimental rolling mill at various speeds and reductions.The rolling temperatures were between 1000and 1030◦C.Nitrogen protection was used to control the scale thickness when the test samples were heated in the furnace and cooled in a cooling box.The rolling forces,rolling torques and scale thicknesses before and after rolling were measured.Surface characteristics of the steel samples were analyzed with SEM and Atomic Force Microscope (AFM).X-ray diffraction was performed to characterize the phase composition of the scale layers.The effects of scale thickness on rolling force and torque were also investigated.

©2004Elsevier B.V .All rights reserved.

Keywords:Hot rolling;Oxide scales;Surface roughness

1.Introduction

A scale layer is always formed on the strip surface dur-ing hot rolling process on a hot strip mill.Primary scales formed on the surface of slabs in the reheating furnace are removed by descaler.Secondary scale builds up again when the slab is waiting for further processing before each pass and before entering the finishing stands.Furthermore,tertiary oxidation scale layer develops after the slab is repeatedly descaled.Thus,the roll/strip interface in hot rolling always includes layers of scale.When the scale is subjected to rolling in the roll bite,it can act as a lubricant if it is thick and ductile,or as abrasives in the three-body wear mechanism if it is hard and brittle.

Previous researches show that the oxide scale on the steel surface contains three layers:hematite,magnetite and wustite [1–3],even after any oxidation period longer than 0.6s [4].Each scale element has different morphology and mechanical properties [5,6].Tiley et al.[7]carried out hot compression tests on industrial reheating furnace scale of mild steel,indicating considerable plastic deformation when it was deformed at 30%strain with 0.1and 1.0s −1strain rate over the temperature range of 650–850◦C.Krzyzanowski and Beynon [8]conducted hot tension tests after producing scales of 10–300␮m in thickness and also found noticeable plastic deformation in the temperature

∗

Corresponding author.

E-mail address:ws42@.au (W.Sun).

range of 830–1150◦C.Yu and Lenard [9]estimated the re-sistance to deformation of the scale layer during hot rolling of carbon steel strips.

However,little is known how the surface texture is trans-ferred from one pass to the next.It has been suggested that the texture is established by the work roll surface roughness during the rolling process [10].In the case of hot rolling,the actual surface texture is generated by a combination of the asperity contact and the lubricant contact between the strip and the roll.It was assumed that the scale is compressed and begins to be elongated when it enters the roll bite [11].When the oxide scale fractures and undertakes extreme pressure during rolling,the hot metal is extruded partially up into the fine cracks and hence modifying the strip surface roughness.The objective of the present work is to examine the effect of hot rolling parameters on the characteristics of the oxide scale of low carbon steel.The morphology of the scale and surface features of the strip before and after rolling was analyzed.Deformation behavior of the scale and its effects on the rolling force and torques were also examined.2.Experiment equipment,materials and procedures 2.1.Equipment

Hot rolling experiments were carried out on a 2-high Hille 100experimental rolling mill with rolls of 225mm di-ameter and 254mm barrel roll,which is made of high speed steel (HSS)with a hardness of HRC55and R a of 0.4␮m in

0924-0136/$–see front matter ©2004Elsevier B.V .All rights reserved.doi:10.1016/j.jmatprotec.2004.04.167