High temperature oxide scale characteristics of low carbon steel in hot rolling

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 100 experimental rolling mill at various speeds and reductions. The rolling temperatures were between 1000 and 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. © 2004 Elsevier B.V. All rights reserved. Keywords: Hot rolling; Oxide scales; Surface roughness

1. Introduction A scale layer is always formed on the strip surface during 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 0.6 s [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 0.1 and 1.0 s−1 strain 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: [email protected] (W. Sun).

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

range of 830–1150 ◦ C. Yu and Lenard [9] estimated the resistance to deformation of the scale layer during hot rolling of carbon steel strips. However, little is known how the surface texture is transferred 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 100 experimental rolling mill with rolls of 225 mm diameter and 254 mm barrel roll, which is made of high speed steel (HSS) with a hardness of HRC55 and Ra of 0.4 ␮m in

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Table 1 Chemical composition of the steel Element

Chemical composition (wt.%)

C Si Mn P S Cr Ni Cu Mo Al–T Ti

0.18 0.18 0.95 0.026 0.027 0.10 0.067 0.13 0.19 0.004 <0.003

surface finishing. This rolling mill can operate at a rolling speed up to 1 m s−1 , a load up to 150 t and a torque up to 13000 kN m. It is equipped with two load cells to determine the compensations for the mill modulus and bearing clearance, and two torque gauges on the shaft to measure the individual roll torque. Two position transducers are used to control the roll gap. Two DMC-450 radiation thickness gauges at entry and exit and a hydraulic AGC are available on the Hille 100 rolling mill. A Pentium III computer was used in the experiment. Its maximum sampling rate is 250 k s−1 . In addition to the two optical pyrometers for controlling the surface temperature, a Flir PM390 Thermo-camera by Thermoteknix Systems Limited was also used to measure the temperature field of the strip surface. Nitrogen was fed to the hearth of the reheating furnace in order to control the scale thickness and the steel bars were cooled in a box connected with nitrogen gas straight after rolling. A Nanoscope IIIA AFM from Digital Instruments and Leico E440 SEM were used to analyze the topography of the strip surface. A Leico optical microscope and a Philips PW1730 X-ray diffraction meter were also used characterize the morphologies and kinetics of the oxide scale. 2.2. Materials The test specimens were commercial low-carbon steel strip of dimension 10 mm×100 mm in thickness and width. They were machined to 9.20 mm thickness, 100 mm width and 450 mm length. The surfaces were carefully prepared to 0.5 to 0.7 ␮m in surface finishing, so that any influence of the original surface defects occurred from the industrial manufacturing could be prevented. Table 1 shows the chemical composition of the steel. 2.3. Experiment procedure The temperature of the furnace was first raised to 1100 ◦ C and the nitrogen gas was then connected. Five minutes later, the test sample was put into the furnace hearth, waiting for the furnace temperature to reach 1100 ◦ C, the sample was soaked for 10 min to ensure uniform temperature. This pro-

cedure took about 23–25 min. The sample was quickly rolled at 1000 ◦ C with a reduction from 7.5 to 41% and rolling speeds between 0.12 and 0.72 m s−1 . After rolling, the sample were transferred into the cooling box which was connected to N2 flow and a flow rate of 20 l min−1 was selected. This rolling test was carried out one sample after another. The other series of tests were carried out in a similar procedure, but the furnace was not connected with N2 and the steel samples were manually tapped to remove the oxide scale which was developed while soaking in the furnace. This is to remove the surface primary scale before rolling and to simulate the descaling effect. There were five or four samples heated each time for 55–70 or 145–160 min, respectively, to examine the effect of heating time on the behavior of the secondary scale under different deformation conditions. Samples for AFM, SEM and microscopic analysis were prepared from the rolled plates.

3. Results and discussion 3.1. Topography of the oxide scale before and after rolling As mentioned before, the steel surface is covered with a layer of oxide scale during hot rolling. Thus, the topology of the rolled steel product surface here is actually the topology of oxide scale. 3D images of the steel surface after rolling can be seen from Fig. 1(a) and (c). The samples were heated for 60 and 65 min of which the soaking period is for 10 and 15 min, respectively. After they were extracted from the reheating furnace, manual descaling operation was carried out mechanically on the samples at the entry of the roll bite. About 4–6 s later, the samples were rolled. Then the samples were picked up at the exit of roll bite and stored in a cooling box, which was connected to N2 flow. The total time taken from descaling to the cooling box were 10.5–13 s. Section analysis (Fig. 1(b) and (d)) was carried out along the rolling direction with Digital Nanoscope software version 5.12b. As for the sample deformed at 25% at the rolling speed of 0.12 m s−1 , the maximum vertical distance between the peak and the valley, indicated with reversed triangles, was 1.912 ␮m over a horizontal distance of 6.45 ␮m (see Fig. 1(b)). When the sample was deformed at 33.7% at the rolling speed of 0.12 m s−1 , the value of the maximum vertical distance was 1.391 ␮m over a horizontal distance of 6.04 ␮m. These values seem to be much smaller than those in [12], which were obtained from simulated oxidizing tests on a Gleeble 3500 Thermo-Mechanical Simulator. As it will be discussed later, the total thickness of the scale on the descaled steel surface after rolling is <26 ␮m. Deformation effects could have an influence on the surface topology. A further examination of the surface morphology was conducted by scanning on the sample surface with SEM. Fig. 2 is the micrographs taken on the secondary oxide scale surfaces that were non-deformed, micrograph (a), and after deformed 7.6% (Fig. 2(b)) and 25% (Fig. 2(c)), respectively,

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Fig. 1. 3D image and section analysis of the rolled steel product surface: (a) and (b) after 25% deformation; (c) and (d) after 33.7% deformation. Rolling speed was 0.12 m s−1 .

at the rolling speed of 0.12 m s−1 . On the non-deformed oxide scale surface, there are occasionally irregular long thin cracks, which are probably attributed by thermal-stress during cooling, see photo (a) in Fig. 2. After deformation, short and coarse cracks appear transversely to the rolling direction (Fig. 2(b) and (c)). These SEM micrographs reveal that visible plastic deformation the scale and the substrate have occurred. This is similar and consistent to the wear behavior which was observed by Vergne et al. [13]. Fig. 2(b) and (c) also show that the plastically deformed areas exhibit the extruded “under-cells” where the surfaces are oxidized. 3.2. Deformation behavior of the oxide scale and morphological features of the secondary scale Morphologies of the primary oxide scale were discussed in previous research [1–3,8,9]. However, due to their differ-

ent mechanical properties, the scale may display a different behavior when deformed together with hot steel substrate by compression and tension. Fig. 3 illustrates the microstructures of the primary and secondary oxide scales before and after deformation. The samples for the test of primary oxide scale in Fig. 3 were heated one by one for about 23–25 min each in the furnace. Results of the rolling test show that the primary scale exhibits its significant plastic deformation. Before deformation, the thickness of the scale turns out to be 297 ␮m, which is much larger than the values in [9,14]. The reason could be that the heating time in [9,14] was much shorter than the present test. Cracks and large pores can be found in Fig. 3(a). After it was deformed 15.8% at rolling speed of 0.12 m s−1 , the primary scale became 220 ␮m (Fig. 3(b)). At the deformation of 33.7% when it was rolled at the speed of 0.48 m s−1 , the thickness was 127.7 ␮m (Fig. 3(c)). This means that the scale

Fig. 2. Secondary oxide scale surface micrograph: (a) non-deformed; (b) and (c) deformed.

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Fig. 3. Microstructures of the primary and secondary oxide scales before and after deformation: (a) and (d) non-deformed primary and secondary scale; (b) primary scale deformed 15.8% at rolling speed of 0.12 m s−1 ; (c) primary scale deformed 33.7% at rolling speed of 0.48 m s−1 ; (e) and (f) secondary scale deformed 15.8% at rolling speed of 0.24 and 0.72 m s−1 , respectively.

deformed 25.9% when the sample was subjected a 15.8% deformation and that it would deform 57% when the sample deformed 33.7%. There is a significant trend that the scale thickness decreases quickly with the bulk deformation. The rolling speeds also play a similar role on the thickness of the oxide scale (see Fig. 4). Photos in Fig. 3(d)–(f) are the microstructures for the secondary oxide scales before and after deformation. The samples were heated for 145–160 min before it was descaled. The initial secondary scale thickness was 29.5 ␮m in this experiment. X-ray diffraction results show that there are three

components in the secondary layer, which are hematite, magnetite and wustite. Magnetite shows its strongest presence at 2θ ◦ of 30.1, 35.5, 57.1 and 62.6◦ . However, hematite reveals its significant presence at 33.1, 35.7, 49.5 and 54.2◦ even though it is shadowed by magnetite (Fig. 5). From Fig. 3(d), the interface between the scale and the substrate became rougher. Under a reduction of 15.8%, the scale thickness became 24 and 24.2 ␮m, respectively, at rolling speeds of 0.24 and 0.72 m s−1 . Although a 15.8% bulk deformation could reduce the thickness of the secondary scale about 13.5%, the rolling speed does not seem

Fig. 4. Effects of reduction and rolling speed on the thickness of primary and secondary oxide scales. (a) Reductions from 7.6 to 41.3%; (b) rolling speeds from 0.12 to 0.72 m s−1 .

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Fig. 5. X-ray diffraction result of the secondary oxide scale.

to have an effect on the secondary scale thickness (see Fig. 4(b)). On the other hand, the bulk reduction does not have remarkable effects on the thickness of secondary scale (see Fig. 4(a)). The samples were heated for 55–70 min totally. The approximate thickness of the secondary scales were 10.9–11.4 ␮m after the samples were deformed 7.6, 25 and 33.7% at rolling speed of 0.12 m s−1 . This could be the reason that the total scale thickness was so small that the difference in scale thickness, which was caused by the deformation, could be compensated by oxidation while the sample was delivered to the cooling box. However, the results of the test show another interesting outcome that the holding time of the samples will affect the secondary scale thickness significantly. The magnitude of the scale thickness value for 55 to 70 min heating time (Fig. 4(a)) is less than half of the value of the scales which were produced in 145–160 min (Fig. 4(b)). This implies that the thermal history of the steel in the furnace plays an important role on the oxide scale thickness even after descaling.

3.3. Surface roughness After rolling, surface roughness was measured to examine the effects of rolling parameters on the surface roughness. The results are shown in Fig. 6. Bulk deformation affects significantly the surface finish of the sample surface. When the bulk deformation increases, the surface roughness decreases remarkably when oxide scale is thick (see Fig. 6(a)). For the steel surface with primary oxide scale, the surface roughness increases with reduction at first. According to the present test, after reduction reached 15.8%, surface roughness decreases significantly with reduction. As for the descaled surfaces, the roughness decreases very quickly when reduction increases from 7.6 to 25%. But when the bulk deformation increases further, the roughness reduction slows down. However, when there is a thin scale layer on the steel surface, the ability of deformation to improve the surface roughness is impaired. On the other hand, the effect of rolling speeds on the surface roughness is complex in this experiment. In the case

Fig. 6. Effects of reductions and rolling speeds on the surface roughness.

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of 55–70 min of heating, the roughness of both the descaled and non-descaled increases with rolling speed. On the other hand, the roughness decreases with increasing rolling speed when the sample heating time is 145–160 min. However, heavy reduction has a significant influence on the surface finish.

4. Conclusion In this paper, hot rolling tests were carried out on a Hille 100 mill. Topology and SEM analysis indicate that cracks occurred in the secondary oxide scale when subjected to bulk deformation and new “under-cell” extruded. Morphological examination shows that the primary oxide scale deforms when the bulk reduction increases or when the rolling speed increases at a certain reduction. However, an increase of bulk deformation or increase of rolling speed has limited influence in reducing the secondary scale thickness. The secondary oxide scale is composed of three components similar to those for the primary scale. The thickness is greatly influenced by the heating history. More importantly, the bulk deformation has a significant effect on the surface finish of the hot rolled steel products.

Acknowledgements This project is supported by an ARC-linkage grant. The first author would like to thank Australian Government for scholarship support (IPRS and UPA) to undertake this research. The authors are greatly thankful for the assistance from Dr. Hongtao Zhu while the rolling tests were carrying out.

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