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Analysis and prevention of sticking occurring during hot rolling of ferritic stainless steel
Materials Science and Engineering A 507 (2009) 66–73
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Analysis and prevention of sticking occurring during hot rolling of ferritic stainless steel Dae Jin Ha a , Hyo Kyung Sung a , Sunghak Lee a,∗ , Jong Seog Lee b , Yong Deuk Lee b a b
Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea Stainless Steel Research Group, Technical Research Lab., POSCO, Pohang 790-785, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 October 2008 Received in revised form 24 November 2008 Accepted 24 November 2008 Keywords: Ferritic stainless steel Sticking High speed steel roll Hot rolling
a b s t r a c t Sticking phenomena occurring during hot rolling of a modified STS 430J1L ferritic stainless steel were investigated in this study by using a pilot-plant-scale rolling machine. As the rolling pass proceeded, the Fe–Cr oxide layer formed in a reheating furnace was destroyed, and the destroyed oxides infiltrated into the rolled steel to form a thin oxide layer in the surface region. The sticking did not occur in the surface region containing oxides, whereas it occurred in the surface region without oxides by the separation of the rolled steel at high temperatures. This indicated that the resistance to sticking increased by the increase in the surface hardness when a considerable amount of oxides were formed in the surface region, and that the sticking could be evaluated by the volume fraction and distribution of oxides formed in the surface region. The lubrication and the increase of the rolling speed and rolling temperature beneficially affected to the resistance to sticking because they accelerated the formation of oxides on the steel surface region. In order to prevent or minimize the sticking, thus, it was suggested to increase the thickness of the oxide layer formed in the reheating furnace and to homogeneously distribute oxides along the surface region by controlling the hot-rolling process. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Sticking phenomenon, in which fragments of a rolled material are stuck to a work roll surface, occurs generally during hot rolling, and deteriorate surfaces of both rolls and rolled materials [1]. Particularly in ferritic stainless steels, the sticking leaves various defects such as dents or scratches on surfaces, and poses serious problems of decrease in roll life, hot-rolling productivity, and deterioration of the surface quality of rolled steel products [2–5]. However, it is hard to come up with fundamental and systematic methods to prevent or minimize the sticking since it depends mainly on variety of rolled plates and rolls, rolling temperature, load, speed, and lubrication in contact planes of rolls and rolling steels. The sticking was serious in stainless steels, especially in ferritic stainless steels [6–8], and was less serious when a high speed steel roll was used instead of a high-chromium cast iron roll [9–12]. These results indicate that it is affected by microstructures and hightemperature properties of the roll and rolled steels as well as rolling conditions such as rolling temperature, speed, load, and lubrication [13–16]. For instance, ferritic stainless steels have excellent hardness and strength at room temperature, but their high-temperature
∗ Corresponding author. Tel.: +82 54 279 2140; fax: +82 54 279 5887. E-mail address: [email protected] (S. Lee). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.11.062
hardness drastically decreases at about 900–1100 ◦ C where actual hot rolling starts, resulting in easy separation of the rolled steel due to reduction by rolls. Also, the high-temperature oxidation behavior which can harden the rolled steel surface as oxide layers or oxide particles are formed in the surface region would favorably affect the sticking. Though the sticking can be minimized or prevented by properly designing rolling conditions, only limited information including oxidation effects is available on the sticking of ferritic stainless steels. In particular, how the thick oxide layer formed during reheating slabs or bars in a furnace before hot rolling influences the sticking during hot rolling and how the oxidation behavior is varied with various rolling conditions have hardly been known. In this study, mechanisms of the sticking, which is a representative problem of surface defects occurring during hot rolling of ferritic stainless steels, were investigated by analyzing their microstructure and oxidation behavior. Modified STS430J1L stainless steels, which are representative ferritic stainless steels, were used for the rolled steels, and hot-rolling tests, which can effectively simulate actual hot-rolling stands, were conducted on these steels under varying rolling conditions such as rolling pass, lubrication, rolling speed, and rolling temperature. Based on the results, sticking mechanisms were investigated by analyzing the volume fraction and distribution of oxides formed in the rolled steel surface region, and methods to prevent or minimize the sticking were suggested.
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Table 1 Chemical composition of the modified STS 430J1L stainless steel (wt.%). C Si Mn P S Cr Ni Cu Nb + Ti + Co N
≤0.02 ≤0.5 ≤0.3 ≤0.04 ≤0.03 20–22 0.2–0.3 0.3–0.8 0.3–0.5 ≤0.02
Fig. 2. Low-magnification optical photograph of the tail part of the modified STS 430J1L steel coil.
The stainless steel was sectioned, polished, etched in a Viellela’s etchant (glycerol 45 ml + nitric acid 15 ml + hydrochloric acid 30 ml) for 1 min, and observed by an optical microscope and a scanning electron microscope (SEM). Compositions of oxides formed in the surface region after the hot-rolling test were analyzed by an energy dispersive spectroscopy (EDS), and volume fractions of oxides were quantitatively analyzed by an image analyzer. 3. Results 3.1. Observation of sticking after hot-rolling test
Fig. 1. Optical micrograph of the modified STS 430J1L steel. The ferrite grain size is about 132 m. Etched by a Viellela’s etchant.
2. Experimental The stainless steel used in this study was a modified STS 430J1L grade steel whose chromium content was increased, and its chemical composition is provided in Table 1. It was obtained from a continuous cast slab. The basic optical micrograph of this steel is shown in Fig. 1. The grain size is about 132 m, and the microstructure is composed of ferrite, together with a small amount of etch pits. Grain sizes were quantitatively measured by an image analyzer. Its yield strength, tensile strength, and elongation are 305, 410 MPa, and 36%, respectively, and these properties are similar to those of the conventional STS 430J1L steels [13]. Tensile specimens with a gauge length of 25.4 mm and a gage diameter of 6.3 mm were machined, and were tested at room temperature and at a cross-head speed of 0.5 mm/min by a universal testing machine. A pilot-plant-scale rolling machine was used for the hot-rolling test. It was made by Hitach Metals Co., Japan, and its major specifications and rolling conditions are shown in Table 2. The hot-rolling test was conducted on slabs or bars heated in a large furnace, in which environmental conditions could be controlled as in an actual hot-rolling stand.
Table 2 Main specifications of the pilot-plant-scale rolling machine and hot-rolling test conditions. Classification
Specification
Roll material Mating material Rolling force Roll diameter Rolling temperature Rolling speed
Cast-iron roll Modified STS 430J1L 279–378 ton (Average: 330 ton) 720 mm 1050–1100 ◦ C 70–130 m/min
Fig. 2 is a low-magnification optical photograph of the tail part of the rolled steel coil after the actual hot rolling. The sticking occurs in the areas of 30–40 mm apart from the both edges of the plate, while it was slightly observed in the central area. Average surface roughness (Ra) of the regions numbered by ‘1’ through ‘3’ in the plate was measured, and the results are 7.8, 3.7, and 10.4 m, respectively. This indicates that the regions containing heavy sticking show rougher surfaces. The regions numbered by ‘1’–’3’ were sectioned, and the cross-sectional areas were observed by an SEM as shown in Fig. 3(a) through (c). In the regions marked by ‘1’ and ‘3’, gray particles are hardly observed beneath the surface (Fig. 3(a) and (c)), whereas a considerable amount of gray particles are observed below the surface in the ‘2’ region (Fig. 3(b)). Fig. 4 and Table 3 provide the EDS analysis data of gray colored particles. According to the quantitative EDS analysis data of areas marked by ‘A’–’C’ in Fig. 4, gray particles are found to be Fe–Cr oxides. The surface roughness of the plate center is low because of the less sticking as a considerable amount of oxides are formed here. On the other hand, the surface areas near both edges of the plate shows higher surface roughness and lower surface hardness as the serious sticking occurs here due to almost no presence of oxides. Consequently, the sticking can be prevented or minimized when the rolling conditions are controlled so that as many oxides as possible can be promoted in the plate surface region during hot rolling of stainless steels. 3.2. Hot-rolling test results Hot-rolling tests were conducted by applying high reduction ratios in the pilot-plant-scale rolling machine in order to confirm the occurrence of the sticking. The bar specimens sized by 120 mm × 230 mm × 27 mm were heated for 30 min at 1050 ◦ C in a heating furnace of argon gas atmosphere, and were rolled at a rolling speed of 10 m/min. The plate surface was observed under Table 3 EDS quantitative analysis data of gray colored or white colored areas marked by ‘A’ through ‘C’ in Fig. 4 (wt.%). Detected area
Fe
Cr
O
Si
Mn
A B C
52.33 37.71 19.23
13.38 22.21 43.91
34.29 33.54 36.85
– 6.53 –
– – –
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Fig. 5. Low-magnification optical photographs of modified STS 430J1L steel plates rolled under rolling reduction ratios of (a) 40%, (b) 70%, and (c) 50%+50%. Dotted circles in (b) and (c) indicate visible sticking areas.
Fig. 3. Back-scattered electron images of the cross-section of the regions marked by (a) ‘1’, (b) ‘2’, and (c) ‘3’ in Fig. 2.
Fig. 4. Back-scattered electron images of the cross-section of the ‘2’ region in Fig. 2. Quantitative EDS analysis data of areas marked by ‘A’, ‘B’, and ‘C’ are shown in Table 3.
rolling reduction ratios of 40%, 70%, and 50%+50% after the hotrolling test, and the results are presented in Fig. 5(a) through (c). In the plates rolled under the reduction ratio of 40%, no visible sticking is observed, while the sticking of 200–300 m in length and 50 m in depth (dotted circles in Fig. 5(b) and (c)) has occurred in the plates rolled under the reduction ratios of 70% and 50%+50%. The areas where the sticking occurred were sectioned and observed by an optical microscope, as shown in Fig. 6(a) through (c). In the steel plates rolled under the reduction ratios of 70% and 50%+50%, grooves of about 50 m in depth are observed (Fig. 6(b) and (c)), while a groove of 10 m depth is observed in the plate rolled under the ratio of 40% (Fig. 6(a)). The backscattered electron image of the area around the groove was observed by an SEM, and the results are shown in Fig. 7(a) through (c). The area containing a number of gray colored oxides maintains a smooth surface without any presence of grooves, i.e., sticking (Fig. 7(a)), whereas oxides are hardly observed in the area below grooves (Fig. 7(b) and (c)). This indicates that the surface region does not show fall-offs of the rolled steels even under a considerably high rolling reduction ratio since oxides formed here contribute to the increased surface hardness. In the surface region where oxides are not formed, however, the surface hardness decreases, thereby causing the sticking to occur because of fragments fallen off from the rolled steels when they are contacted with rolls under heavy reduction ratios. The average hardness values measured at the surface regions with and without oxides are about 350 VHN and 180 VHN, respectively, indicating that the one with oxides is twice as hard as the one without oxides.
D.J. Ha et al. / Materials Science and Engineering A 507 (2009) 66–73
Fig. 6. SEM micrographs of the cross-section of the sticking areas in the steel plates rolled under rolling reduction ratios of (a) 40%, (b) 70%, and (c) 50%+50%. Dotted circles indicate sticking areas.
According to Figs. 3(a)–(c) and 7(a)–(c) of the actual hot-rolling results and the hot-rolling test results, respectively, the resistance to sticking is enhanced with increasing surface hardness when lots of Fe–Cr oxides are formed in the steel surface region. However, it is necessary to understand how these oxides are formed. Since stainless steel slabs or bars are kept for long hours in the heating furnace before hot rolling, a thick oxide layer is generally formed on the surface. It is needed to find out whether this oxide layer is not extinguished, infiltrates into the stainless steel surface, and then is dispersed during hot rolling, or is broken off from the surface and new oxides are formed by high-temperature oxidation during hot rolling. A bar specimen of 100 mm × 230 mm × 27 mm in size was heated at 1050 ◦ C for 30 min in a heating furnace of argon gas atmosphere, and then was rolled at a rolling speed of 70 m/min. The number of rolling passes was five, for each of which rolling con-
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Fig. 7. Back-scattered electron images of the cross-section of the sticking areas in the steel plates rolled under rolling reduction ratios of (a) 40%, (b) 70%, and (c) 50%+50%. Gray colored particles located at the surface region are identified to be Fe–Cr oxides by the EDS analysis.
ditions are summarized in Table 4. The reduction ratio is small at 7.6% during the first pass, and it increases to about 40% in the subsequent four passes. The surface of the steel plate was sectioned before rolling and after each pass, and the sectioned microstruc-
Table 4 Rolling condition of each pass of the steel bar reheated at 1050 ◦ C. Rolling pass number
Initial
Plate thickness (mm) Final
Reduction ratio (%)
1 2 3 4 5
26.2 24.2 14.6 8.2 4.7
24.2 14.6 8.2 4.7 3.0
7.6 39.7 43.8 42.7 36.2
Rolling speed (m/min)
70
70
D.J. Ha et al. / Materials Science and Engineering A 507 (2009) 66–73
Fig. 8. Back-scattered electron images of the cross-section of the steel plates after (a) 0, (b) 1, (c) 2, (d) 3, (e) 4, and (f) 5 rolling passes, as listed in Table 4. Insects in (b) and (c) are higher-magnification images of the Fe–Cr oxide layer.
tures were observed in backscattered electron images as shown in Fig. 8(a) through (f). Right after the heating furnace, the bar surface is covered with an oxide layer of about 40 m in thickness, in which a small number of pores and cracks are present (Fig. 8(a)). The EDS analysis data of this oxide layer shows that the oxide layer is composed of Fe–Cr oxides whose composition is similar to that of ones as observed in Fig. 7. After the one pass, the oxide layer formed in the heating furnace remains in 20–30 m thickness (Fig. 8(b)). However, the oxide layer is broken off into large pieces of 5–10 m and fine debris of 1 m because of the reduction by rolls. Though the thickness of the oxide layer is maintained at 20 m after the second pass, the oxide layer is mostly composed of fine oxide debris, together with few large pieces (Fig. 8(c)). Some oxide fragments are observed to have infiltrated into the steel surface as marked by arrows. After the third pass, most of the oxide layer formed in the furnace has disappeared, and a considerable amount of oxides
has infiltrated into the steel (Fig. 8(d)). After the fourth pass, oxides infiltrated inside the steel change into oxides of a thin layer type along the surface, and the overall volume fraction of oxides significantly decreases (Fig. 8(e)). After the final fifth pass, oxides remain thin only in parts of the surface region (Fig. 8(f)). These results of the stepwise rolling test indicate that the resistance to sticking varies with the distribution and volume fraction of oxides formed in the surface region of the steel after the thick oxide layer formed in the furnace is broken off and infiltrates into the steel. This study evaluated the possibility for the sticking to occur by investigating the distribution and volume fraction of oxides after conducting rolling tests under various rolling conditions. The bar specimen of 100 mm × 230 mm × 27 mm in size was heated in a heating furnace at either 1050 or 1100 ◦ C for 30 min, and then rolled for four passes at a rolling speed of 70 or 100 m/min. Table 5 lists the rolling conditions for each pass. Reduction ratio of
D.J. Ha et al. / Materials Science and Engineering A 507 (2009) 66–73 Table 5 Rolling condition of each pass of the reheated steel bar. Rolling pass number
Initial
Plate thickness (mm) Final
Reduction ratio (%)
Rolling speed (m/min)
1 2 3 4
26.2 22.0 15.0 10.0
22.0 15.0 10.0 3.0
16.7 31.8 33.3 70.0
70 100
Table 6 Rolling condition of each steel plate specimen. Plate specimen number
Volume fraction of lubricant in cooling water (%)
Rolling speed (m/min)
Rolling temperature (◦ C)
1 2 3 4 5 6 7 8 9 10 11
0 0 0 0.5 0.5 0.5 0.5 1.0 1.0 1.0 1.0
70 70 100 70 70 100 100 70 70 100 100
1050 1100 1100 1050 1100 1050 1100 1050 1100 1050 1100
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varying volume fraction of lubricant are shown in Fig. 9. In each micrograph, the specimen number is marked on the bottom right according to the differing rolling conditions of Table 6. These micrographs show a clear trend for the oxide volume fraction present in the surface region to increase with increasing volume fraction of lubricant, irrespective of rolling temperature. This verifies the effect of lubricant addition, and indicates that the sticking can be reduced by the increased volume fraction of lubricant. Fig. 10 shows that the oxide volume fraction in the surface region increases with increasing rolling speed under varying lubrication and rolling temperature conditions. In this case, oxides formed in the surface region have an infiltration type, and a considerable amount of oxides infiltrate into the interior as well as the surface. Faster rolling speed works to increase the oxide volume fraction, just like the effect of lubrication, and thus enhances the resistance to sticking. Fig. 9 shows the effect of rolling temperature on oxide formation under varying lubrication conditions. The oxide volume fraction in the surface region stays about the same, irrespective of rolling temperature under varying lubrication conditions. However, when rolled at 1050 ◦ C, oxides have an infiltration type in the surface region, but oxides tend to disperse along the surface in a layer type when rolled at 1100 ◦ C. Since these layer-typed oxides can homogeneously cover the steel surface, it can improve the resistance to sticking. 4. Discussion
the first pass is small at 16.7%, but gradually increases up to 70% after the final pass. Table 6 provides rolling test conditions of the 11 different kinds of plates rolled for four passes with varying conditions such as volume fraction of lubricant contained in the cooling water, rolling temperature, and rolling speed. Back-scattered electron images of the cross-section of the steel plates tested under
In order to clarify the formation mechanism of sticking, which is the most serious problem among various surface defects occurring during rolling of ferritic stainless steels, it is necessary to first understand the high-temperature rolling behavior and to investigate it in relation with microstructures including the oxide volume
Fig. 9. Back-scattered electron images of the cross-section of the steel plates (specimen number; 1, 2, 4, 5, 8, and 9), showing the effect of lubrication on the formation of Fe–Cr oxides located at the surface region. Rolling speed is 70 m/min. Detailed rolling conditions are shown in Table 6.
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Fig. 10. Back-scattered electron images of the cross-section of the steel plates (specimen number; 2, 3, 4, 6, 8, and 10), showing the effect of rolling speed on the formation of Fe–Cr oxides located at the surface region.
fraction formed on the rolled steel surface [14,17–22]. In the present study, hot-rolling tests were conducted by a pilot-plant-scale rolling machine, which can simulate the actual rolling process, the formation process of oxides was studied by steps, and the degree of sticking was evaluated. According to the hot-rolling test results of Fig. 8(a) through (f), the oxide layer formed in the heating furnace is not extinguished during rolling, but is broken off into pieces or fragments and then infiltrates into the steel with increasing number of rolling passes. The decarburized layer formed in the furnace before hot rolling of conventional plain carbon steels or high-strength low alloy steels is shed off by descaling treatment or mostly extinguished during rough rolling. However, most of the oxide layer formed during heating of stainless steels remains even during hot rolling unless it is artificially peeled off by applying a descaling treatment. Though parts of the remaining oxide layer are broken by rolls, a considerable amount of oxides infiltrates into the steel during rolling, and is distributed in the surface region. The intruded oxides are elongated together with the steel as the rolling proceeds, and leave parts of the surface region uncovered by oxides since the amount of oxides is reduced by the elongation. Total reduction ratio in the rolling test of Fig. 8(a) through (f) is about 90%, and the surface area of the steel after the rolling test increases by about 10 times over that before the rolling test. Thus, parts of the thick oxide layer formed in the steel surface before the rolling test are broken off and gone, and a considerable amount of oxides infiltrates into the steel, but the surface regions with hardly any presence of oxides do exist as the surface area increases. The hardness of the surface region without oxides is significantly lower than that of the surface region with oxides, and parts of the steel surface can be removed due to the reduction by rolls. Fig. 7(a) through (c) are examples showing how the removed
steel in the surface region without oxides can grow into the sticking. The oxides infiltrated into the surface region contribute to increased surface hardness, and work for the enhanced resistance to sticking. Consequently, in order to prevent or minimize the sticking in ferritic stainless steels, it is desirable to promote an oxide layer as thick as possible in the heating furnace and to keep the slab temperature as high as possible by installing a heating system after the rough rolling, while the descaling treatment of the oxide layer should not be applied before the rolling. The sticking phenomenon of the ferritic stainless steel is significantly affected by the distribution and volume fraction of oxides present on the steel surface. As shown in Figs. 8(d) and (e) and 10, the oxide distribution can largely be divided into infiltration type and layer type. Infiltration-type oxides are formed when the oxide layer is broken off and infiltrates into the steel. According to Fig. 8(d) and (e), infiltration-type oxides tend to change to layer-type ones as oxides are elongated together with the elongation of the steel as the rolling proceeds, and get distributed in a thin layer type along the surface. Infiltration-type oxides seem to be formed in the process into the layer-type oxides. Since infiltration-type oxides are distributed pretty deep inside the steel, oxides in the region considerably far away from the surface hardly affect the surface hardness when in contact with rolls, and thus do not contribute to the improved resistance to sticking. If oxides are distributed in a layer type, rather than in an infiltration type, in the surface region under a given volume fraction of oxides, the possibility that the surface region is covered with oxides increases, thereby improving the resistance to sticking. Though the resistance to sticking improves with increasing volume fraction of oxides, promoting more of layertype oxides than infiltration-type ones can be more effective way to reduce the sticking.
D.J. Ha et al. / Materials Science and Engineering A 507 (2009) 66–73
Since the sticking phenomenon can be evaluated overall by investigating the distribution and volume fraction of oxides formed in the surface region, the oxidation behavior of the steel plates fabricated by the rolling test under varying conditions of lubrication, rolling temperature, and rolling speed, as specified in Table 6, is examined. Based on the results, the degree of the sticking occurring in actual rolling stands can be predicted. As the volume fraction of lubricant in the cooling water increases, the volume fraction of oxides in the surface region increases (Fig. 9). The friction coefficient between rolls and rolled steel is reduced by the addition of lubricant, as a considerable amount of oxide layer formed in the heating furnace infiltrates into the rolled steel and the loss of the oxide layer decreases. At the faster rolling speed, the volume fraction of oxides in the surface region increases (Fig. 10). This is because the faster rolling speed constitutes to reducing the breakage and loss of the surface oxide layer and to oxide infiltration into the steel. The oxide volume fraction does not vary much with increasing rolling temperature, but the oxide distribution tends to change from an infiltration type to a layer type (Fig. 9). Considering that layer-type oxides work better for the improved surface hardness than infiltrationtype oxides as explained earlier, the less sticking can be expected at higher rolling temperatures. According to the above results, it can be expected that the resistance to sticking can improve with increasing volume fraction of lubricant, rolling speed, and rolling temperature because the surface hardness increases with increasing oxide volume fraction in the surface region and because the oxide distribution changes to the layer-type. Since the rolling test using a pilot-plant-scale rolling machine is a good way to identify sticking phenomenon occurring from actual hot-rolling stands, it can be useful to understand the sticking mechanism and to suggest possible rolling conditions under which the sticking can be prevented or minimized. When the friction coefficient between rolls and rolled steel is reduced by the addition of lubricant, a number of carbides are formed in the surface region, which leads to the enhanced resistance to sticking. Increased rolling speed and rolling temperature also work favorably for the prevention of sticking as they contribute to the increased oxide volume fraction and to the formation of layer-type oxides. Consequently, the sticking problem occurring during hot rolling of stainless steels can be reduced to a great extent by controlling hot-rolling process parameters, under which a number of oxides are promoted and oxides are distributed in a layer type along the surface, and by designing alloys in which the oxide layer can be formed thick in a reheating furnace.
increasing surface hardness, when a considerable amount of Fe–Cr oxides were formed in the steel surface region. (2) As the rolling pass proceeded, the oxide layer formed in the heating furnace was broken off and infiltrated into the steel, thereby forming oxides in the surface region. The surface region without oxides underwent greater reduction in hardness than the surface region with oxides. These findings indicated that the surface hardness and resistance to sticking varied with the distribution and volume fraction of oxides formed in the surface region. (3) Studies on the oxidation behavior after rolling tests under varying lubrication, rolling temperature, and rolling speed revealed that the resistance to sticking was enhanced in the case of higher volume fraction of lubricant because lots of oxides were formed in the surface region when the friction coefficient between rolls and rolled steels decreased. Faster rolling speed and higher rolling temperature also contributed to the enhanced resistance to sticking since they led to the increased volume fraction of oxides or promoted the formation of layer-type oxides. In order to prevent or minimize the sticking in ferritic stainless steels, it was suggested that the oxide layer should be formed as thick as possible in the heating furnace, and that hot-rolling conditions should be controlled so that oxides could be distributed in a layer type along the surface. Acknowledgments This work was supported by the National Research Laboratory Program (No. M10400000361-06J0000-36110) funded by the Korea Science and Engineering Foundation (KOSEF) and by POSCO under a contract No. PD-06903. The authors would like to thank Dr. Kwang Tae Kim and Mr. Yong Joon Choi of POSCO for their help of the hotrolling test. References [1] [2] [3] [4] [5]
[6]
[7] [8]
5. Conclusions In order to clarify the mechanism of the sticking, which is a representative problem of surface defects occurring during hot rolling of ferritic stainless steels, and to present preventative methods against it, hot-rolling tests were conducted on modified STS430J1L ferritic stainless steels under varying rolling conditions. (1) According to the hot-rolling test results, the sticking did not take place, and the surface remained smooth in the surface region containing lots of Fe–Cr oxides. In the surface region without the presence of oxides, the sticking occurred due to the removed fragments of the rolled steels at the time of contact with rolls. This indicated that the resistance to sticking improved with
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[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
O. Kato, T. Kawanami, J. Jpn. Soc. Technol. Plast. 28 (1987) 264. O. Kato, T. Kawanami, J. Jpn. Soc. Technol. Plast. 30 (1989) 103. O. Kato, S. Uchida, T. Kimuma, Seitetsu Kenkyu 335 (1989) 35. T. Nakanishi, J. Soc. Tribologists Lubricant Eng. 49 (1993) 365. S. Uchida, H. Yamamoto, M. Akata, K. Watanabe, O. Kato, What’s New in Roll Technologies of the World, Report of Research Committee on Rolling Roll, ISIJ, Tokyo, Japan, 1995, p. 183. Y.D. Lee, Y.Y. Lee, O.J. Kwon, G.S. Kim, Y.G. Lee, Development of Rolling Technology of 430 Stainless Steel, Technical Report of POSCO Research Institute, 1993. S. Lee, D. Suh, S. Oh, W. Jin, Metal. Mater. Trans. 29A (1998) 696. S.H. Wang, C.C. Wu, C.Y. Chen, J.R. Yang, P.K. Chiu, J.S. Fang, Met. Mater. Int. 13 (2007) 275. M. Pellizzari, A. Molianari, G. Straffelini, Wear 259 (2005) 1281. J.C. Werquin, J.C. Cailand, in: R.B. Corbett (Ed.), Roll for the Metal Working Industries, Iron and Steel Society, Inc., Warrendale, PA, 1990, p. 55. I.D. Choi, D.M. Kim, S.J. Kim, D.M. Bruce, D.K. Matlock, J.G. Speer, Met. Mater. Int. 12 (2006) 13. C.K. Kim, D.-G. Lee, S. Lee, Mater. Sci. Technol. 23 (2007) 1065. C.-Y. Son, C.K. Kim, D.J. Ha, S. Lee, J.S. Lee, K.T. Kim, Y.D. Lee, Metal. Mater. Trans. 38A (2007) 2776. W. Jin, J.Y. Choi, Y.Y. Lee, ISIJ Int. 40 (2000) 789. ASM Handbook, vol. 18, Friction, Lubrication, and Wear Technology. I.D. Choi, D.M. Bruce, D.K. Matlock, J.G. Speer, Met. Mater. Int. 14 (2008) 139. W. Jin, J.Y. Choi, Y.Y. Lee, ISIJ Int. 38 (1998) 739. L.H.S. Luong, T. Heijkoop, Wear 71 (1981) 93. S.Y. Cho, J.H. Lee, Met. Mater. Int. 14 (2008) 193. Y.H. Moon, J.W. Kim, C.J. Van Tyne, Met. Mater. Int. 2 (2005) 169. A. Bidiville, M. Favero, P. Stadelmann, S. Mischler, Wear 263 (2007) 207. W.C. Doo, D.Y. Kim, S.C. Kang, K.W. Yi, Met. Mater. Int. 13 (2007) 249.
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Analysis and prevention of sticking occurring during hot rolling of ferritic stainless steel Dae Jin Ha a , Hyo Kyung Sung a , Sunghak Lee a,∗ , Jong Seog Lee b , Yong Deuk Lee b a b
Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea Stainless Steel Research Group, Technical Research Lab., POSCO, Pohang 790-785, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 October 2008 Received in revised form 24 November 2008 Accepted 24 November 2008 Keywords: Ferritic stainless steel Sticking High speed steel roll Hot rolling
a b s t r a c t Sticking phenomena occurring during hot rolling of a modified STS 430J1L ferritic stainless steel were investigated in this study by using a pilot-plant-scale rolling machine. As the rolling pass proceeded, the Fe–Cr oxide layer formed in a reheating furnace was destroyed, and the destroyed oxides infiltrated into the rolled steel to form a thin oxide layer in the surface region. The sticking did not occur in the surface region containing oxides, whereas it occurred in the surface region without oxides by the separation of the rolled steel at high temperatures. This indicated that the resistance to sticking increased by the increase in the surface hardness when a considerable amount of oxides were formed in the surface region, and that the sticking could be evaluated by the volume fraction and distribution of oxides formed in the surface region. The lubrication and the increase of the rolling speed and rolling temperature beneficially affected to the resistance to sticking because they accelerated the formation of oxides on the steel surface region. In order to prevent or minimize the sticking, thus, it was suggested to increase the thickness of the oxide layer formed in the reheating furnace and to homogeneously distribute oxides along the surface region by controlling the hot-rolling process. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Sticking phenomenon, in which fragments of a rolled material are stuck to a work roll surface, occurs generally during hot rolling, and deteriorate surfaces of both rolls and rolled materials [1]. Particularly in ferritic stainless steels, the sticking leaves various defects such as dents or scratches on surfaces, and poses serious problems of decrease in roll life, hot-rolling productivity, and deterioration of the surface quality of rolled steel products [2–5]. However, it is hard to come up with fundamental and systematic methods to prevent or minimize the sticking since it depends mainly on variety of rolled plates and rolls, rolling temperature, load, speed, and lubrication in contact planes of rolls and rolling steels. The sticking was serious in stainless steels, especially in ferritic stainless steels [6–8], and was less serious when a high speed steel roll was used instead of a high-chromium cast iron roll [9–12]. These results indicate that it is affected by microstructures and hightemperature properties of the roll and rolled steels as well as rolling conditions such as rolling temperature, speed, load, and lubrication [13–16]. For instance, ferritic stainless steels have excellent hardness and strength at room temperature, but their high-temperature
∗ Corresponding author. Tel.: +82 54 279 2140; fax: +82 54 279 5887. E-mail address: [email protected] (S. Lee). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.11.062
hardness drastically decreases at about 900–1100 ◦ C where actual hot rolling starts, resulting in easy separation of the rolled steel due to reduction by rolls. Also, the high-temperature oxidation behavior which can harden the rolled steel surface as oxide layers or oxide particles are formed in the surface region would favorably affect the sticking. Though the sticking can be minimized or prevented by properly designing rolling conditions, only limited information including oxidation effects is available on the sticking of ferritic stainless steels. In particular, how the thick oxide layer formed during reheating slabs or bars in a furnace before hot rolling influences the sticking during hot rolling and how the oxidation behavior is varied with various rolling conditions have hardly been known. In this study, mechanisms of the sticking, which is a representative problem of surface defects occurring during hot rolling of ferritic stainless steels, were investigated by analyzing their microstructure and oxidation behavior. Modified STS430J1L stainless steels, which are representative ferritic stainless steels, were used for the rolled steels, and hot-rolling tests, which can effectively simulate actual hot-rolling stands, were conducted on these steels under varying rolling conditions such as rolling pass, lubrication, rolling speed, and rolling temperature. Based on the results, sticking mechanisms were investigated by analyzing the volume fraction and distribution of oxides formed in the rolled steel surface region, and methods to prevent or minimize the sticking were suggested.
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Table 1 Chemical composition of the modified STS 430J1L stainless steel (wt.%). C Si Mn P S Cr Ni Cu Nb + Ti + Co N
≤0.02 ≤0.5 ≤0.3 ≤0.04 ≤0.03 20–22 0.2–0.3 0.3–0.8 0.3–0.5 ≤0.02
Fig. 2. Low-magnification optical photograph of the tail part of the modified STS 430J1L steel coil.
The stainless steel was sectioned, polished, etched in a Viellela’s etchant (glycerol 45 ml + nitric acid 15 ml + hydrochloric acid 30 ml) for 1 min, and observed by an optical microscope and a scanning electron microscope (SEM). Compositions of oxides formed in the surface region after the hot-rolling test were analyzed by an energy dispersive spectroscopy (EDS), and volume fractions of oxides were quantitatively analyzed by an image analyzer. 3. Results 3.1. Observation of sticking after hot-rolling test
Fig. 1. Optical micrograph of the modified STS 430J1L steel. The ferrite grain size is about 132 m. Etched by a Viellela’s etchant.
2. Experimental The stainless steel used in this study was a modified STS 430J1L grade steel whose chromium content was increased, and its chemical composition is provided in Table 1. It was obtained from a continuous cast slab. The basic optical micrograph of this steel is shown in Fig. 1. The grain size is about 132 m, and the microstructure is composed of ferrite, together with a small amount of etch pits. Grain sizes were quantitatively measured by an image analyzer. Its yield strength, tensile strength, and elongation are 305, 410 MPa, and 36%, respectively, and these properties are similar to those of the conventional STS 430J1L steels [13]. Tensile specimens with a gauge length of 25.4 mm and a gage diameter of 6.3 mm were machined, and were tested at room temperature and at a cross-head speed of 0.5 mm/min by a universal testing machine. A pilot-plant-scale rolling machine was used for the hot-rolling test. It was made by Hitach Metals Co., Japan, and its major specifications and rolling conditions are shown in Table 2. The hot-rolling test was conducted on slabs or bars heated in a large furnace, in which environmental conditions could be controlled as in an actual hot-rolling stand.
Table 2 Main specifications of the pilot-plant-scale rolling machine and hot-rolling test conditions. Classification
Specification
Roll material Mating material Rolling force Roll diameter Rolling temperature Rolling speed
Cast-iron roll Modified STS 430J1L 279–378 ton (Average: 330 ton) 720 mm 1050–1100 ◦ C 70–130 m/min
Fig. 2 is a low-magnification optical photograph of the tail part of the rolled steel coil after the actual hot rolling. The sticking occurs in the areas of 30–40 mm apart from the both edges of the plate, while it was slightly observed in the central area. Average surface roughness (Ra) of the regions numbered by ‘1’ through ‘3’ in the plate was measured, and the results are 7.8, 3.7, and 10.4 m, respectively. This indicates that the regions containing heavy sticking show rougher surfaces. The regions numbered by ‘1’–’3’ were sectioned, and the cross-sectional areas were observed by an SEM as shown in Fig. 3(a) through (c). In the regions marked by ‘1’ and ‘3’, gray particles are hardly observed beneath the surface (Fig. 3(a) and (c)), whereas a considerable amount of gray particles are observed below the surface in the ‘2’ region (Fig. 3(b)). Fig. 4 and Table 3 provide the EDS analysis data of gray colored particles. According to the quantitative EDS analysis data of areas marked by ‘A’–’C’ in Fig. 4, gray particles are found to be Fe–Cr oxides. The surface roughness of the plate center is low because of the less sticking as a considerable amount of oxides are formed here. On the other hand, the surface areas near both edges of the plate shows higher surface roughness and lower surface hardness as the serious sticking occurs here due to almost no presence of oxides. Consequently, the sticking can be prevented or minimized when the rolling conditions are controlled so that as many oxides as possible can be promoted in the plate surface region during hot rolling of stainless steels. 3.2. Hot-rolling test results Hot-rolling tests were conducted by applying high reduction ratios in the pilot-plant-scale rolling machine in order to confirm the occurrence of the sticking. The bar specimens sized by 120 mm × 230 mm × 27 mm were heated for 30 min at 1050 ◦ C in a heating furnace of argon gas atmosphere, and were rolled at a rolling speed of 10 m/min. The plate surface was observed under Table 3 EDS quantitative analysis data of gray colored or white colored areas marked by ‘A’ through ‘C’ in Fig. 4 (wt.%). Detected area
Fe
Cr
O
Si
Mn
A B C
52.33 37.71 19.23
13.38 22.21 43.91
34.29 33.54 36.85
– 6.53 –
– – –
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Fig. 5. Low-magnification optical photographs of modified STS 430J1L steel plates rolled under rolling reduction ratios of (a) 40%, (b) 70%, and (c) 50%+50%. Dotted circles in (b) and (c) indicate visible sticking areas.
Fig. 3. Back-scattered electron images of the cross-section of the regions marked by (a) ‘1’, (b) ‘2’, and (c) ‘3’ in Fig. 2.
Fig. 4. Back-scattered electron images of the cross-section of the ‘2’ region in Fig. 2. Quantitative EDS analysis data of areas marked by ‘A’, ‘B’, and ‘C’ are shown in Table 3.
rolling reduction ratios of 40%, 70%, and 50%+50% after the hotrolling test, and the results are presented in Fig. 5(a) through (c). In the plates rolled under the reduction ratio of 40%, no visible sticking is observed, while the sticking of 200–300 m in length and 50 m in depth (dotted circles in Fig. 5(b) and (c)) has occurred in the plates rolled under the reduction ratios of 70% and 50%+50%. The areas where the sticking occurred were sectioned and observed by an optical microscope, as shown in Fig. 6(a) through (c). In the steel plates rolled under the reduction ratios of 70% and 50%+50%, grooves of about 50 m in depth are observed (Fig. 6(b) and (c)), while a groove of 10 m depth is observed in the plate rolled under the ratio of 40% (Fig. 6(a)). The backscattered electron image of the area around the groove was observed by an SEM, and the results are shown in Fig. 7(a) through (c). The area containing a number of gray colored oxides maintains a smooth surface without any presence of grooves, i.e., sticking (Fig. 7(a)), whereas oxides are hardly observed in the area below grooves (Fig. 7(b) and (c)). This indicates that the surface region does not show fall-offs of the rolled steels even under a considerably high rolling reduction ratio since oxides formed here contribute to the increased surface hardness. In the surface region where oxides are not formed, however, the surface hardness decreases, thereby causing the sticking to occur because of fragments fallen off from the rolled steels when they are contacted with rolls under heavy reduction ratios. The average hardness values measured at the surface regions with and without oxides are about 350 VHN and 180 VHN, respectively, indicating that the one with oxides is twice as hard as the one without oxides.
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Fig. 6. SEM micrographs of the cross-section of the sticking areas in the steel plates rolled under rolling reduction ratios of (a) 40%, (b) 70%, and (c) 50%+50%. Dotted circles indicate sticking areas.
According to Figs. 3(a)–(c) and 7(a)–(c) of the actual hot-rolling results and the hot-rolling test results, respectively, the resistance to sticking is enhanced with increasing surface hardness when lots of Fe–Cr oxides are formed in the steel surface region. However, it is necessary to understand how these oxides are formed. Since stainless steel slabs or bars are kept for long hours in the heating furnace before hot rolling, a thick oxide layer is generally formed on the surface. It is needed to find out whether this oxide layer is not extinguished, infiltrates into the stainless steel surface, and then is dispersed during hot rolling, or is broken off from the surface and new oxides are formed by high-temperature oxidation during hot rolling. A bar specimen of 100 mm × 230 mm × 27 mm in size was heated at 1050 ◦ C for 30 min in a heating furnace of argon gas atmosphere, and then was rolled at a rolling speed of 70 m/min. The number of rolling passes was five, for each of which rolling con-
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Fig. 7. Back-scattered electron images of the cross-section of the sticking areas in the steel plates rolled under rolling reduction ratios of (a) 40%, (b) 70%, and (c) 50%+50%. Gray colored particles located at the surface region are identified to be Fe–Cr oxides by the EDS analysis.
ditions are summarized in Table 4. The reduction ratio is small at 7.6% during the first pass, and it increases to about 40% in the subsequent four passes. The surface of the steel plate was sectioned before rolling and after each pass, and the sectioned microstruc-
Table 4 Rolling condition of each pass of the steel bar reheated at 1050 ◦ C. Rolling pass number
Initial
Plate thickness (mm) Final
Reduction ratio (%)
1 2 3 4 5
26.2 24.2 14.6 8.2 4.7
24.2 14.6 8.2 4.7 3.0
7.6 39.7 43.8 42.7 36.2
Rolling speed (m/min)
70
70
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Fig. 8. Back-scattered electron images of the cross-section of the steel plates after (a) 0, (b) 1, (c) 2, (d) 3, (e) 4, and (f) 5 rolling passes, as listed in Table 4. Insects in (b) and (c) are higher-magnification images of the Fe–Cr oxide layer.
tures were observed in backscattered electron images as shown in Fig. 8(a) through (f). Right after the heating furnace, the bar surface is covered with an oxide layer of about 40 m in thickness, in which a small number of pores and cracks are present (Fig. 8(a)). The EDS analysis data of this oxide layer shows that the oxide layer is composed of Fe–Cr oxides whose composition is similar to that of ones as observed in Fig. 7. After the one pass, the oxide layer formed in the heating furnace remains in 20–30 m thickness (Fig. 8(b)). However, the oxide layer is broken off into large pieces of 5–10 m and fine debris of 1 m because of the reduction by rolls. Though the thickness of the oxide layer is maintained at 20 m after the second pass, the oxide layer is mostly composed of fine oxide debris, together with few large pieces (Fig. 8(c)). Some oxide fragments are observed to have infiltrated into the steel surface as marked by arrows. After the third pass, most of the oxide layer formed in the furnace has disappeared, and a considerable amount of oxides
has infiltrated into the steel (Fig. 8(d)). After the fourth pass, oxides infiltrated inside the steel change into oxides of a thin layer type along the surface, and the overall volume fraction of oxides significantly decreases (Fig. 8(e)). After the final fifth pass, oxides remain thin only in parts of the surface region (Fig. 8(f)). These results of the stepwise rolling test indicate that the resistance to sticking varies with the distribution and volume fraction of oxides formed in the surface region of the steel after the thick oxide layer formed in the furnace is broken off and infiltrates into the steel. This study evaluated the possibility for the sticking to occur by investigating the distribution and volume fraction of oxides after conducting rolling tests under various rolling conditions. The bar specimen of 100 mm × 230 mm × 27 mm in size was heated in a heating furnace at either 1050 or 1100 ◦ C for 30 min, and then rolled for four passes at a rolling speed of 70 or 100 m/min. Table 5 lists the rolling conditions for each pass. Reduction ratio of
D.J. Ha et al. / Materials Science and Engineering A 507 (2009) 66–73 Table 5 Rolling condition of each pass of the reheated steel bar. Rolling pass number
Initial
Plate thickness (mm) Final
Reduction ratio (%)
Rolling speed (m/min)
1 2 3 4
26.2 22.0 15.0 10.0
22.0 15.0 10.0 3.0
16.7 31.8 33.3 70.0
70 100
Table 6 Rolling condition of each steel plate specimen. Plate specimen number
Volume fraction of lubricant in cooling water (%)
Rolling speed (m/min)
Rolling temperature (◦ C)
1 2 3 4 5 6 7 8 9 10 11
0 0 0 0.5 0.5 0.5 0.5 1.0 1.0 1.0 1.0
70 70 100 70 70 100 100 70 70 100 100
1050 1100 1100 1050 1100 1050 1100 1050 1100 1050 1100
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varying volume fraction of lubricant are shown in Fig. 9. In each micrograph, the specimen number is marked on the bottom right according to the differing rolling conditions of Table 6. These micrographs show a clear trend for the oxide volume fraction present in the surface region to increase with increasing volume fraction of lubricant, irrespective of rolling temperature. This verifies the effect of lubricant addition, and indicates that the sticking can be reduced by the increased volume fraction of lubricant. Fig. 10 shows that the oxide volume fraction in the surface region increases with increasing rolling speed under varying lubrication and rolling temperature conditions. In this case, oxides formed in the surface region have an infiltration type, and a considerable amount of oxides infiltrate into the interior as well as the surface. Faster rolling speed works to increase the oxide volume fraction, just like the effect of lubrication, and thus enhances the resistance to sticking. Fig. 9 shows the effect of rolling temperature on oxide formation under varying lubrication conditions. The oxide volume fraction in the surface region stays about the same, irrespective of rolling temperature under varying lubrication conditions. However, when rolled at 1050 ◦ C, oxides have an infiltration type in the surface region, but oxides tend to disperse along the surface in a layer type when rolled at 1100 ◦ C. Since these layer-typed oxides can homogeneously cover the steel surface, it can improve the resistance to sticking. 4. Discussion
the first pass is small at 16.7%, but gradually increases up to 70% after the final pass. Table 6 provides rolling test conditions of the 11 different kinds of plates rolled for four passes with varying conditions such as volume fraction of lubricant contained in the cooling water, rolling temperature, and rolling speed. Back-scattered electron images of the cross-section of the steel plates tested under
In order to clarify the formation mechanism of sticking, which is the most serious problem among various surface defects occurring during rolling of ferritic stainless steels, it is necessary to first understand the high-temperature rolling behavior and to investigate it in relation with microstructures including the oxide volume
Fig. 9. Back-scattered electron images of the cross-section of the steel plates (specimen number; 1, 2, 4, 5, 8, and 9), showing the effect of lubrication on the formation of Fe–Cr oxides located at the surface region. Rolling speed is 70 m/min. Detailed rolling conditions are shown in Table 6.
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Fig. 10. Back-scattered electron images of the cross-section of the steel plates (specimen number; 2, 3, 4, 6, 8, and 10), showing the effect of rolling speed on the formation of Fe–Cr oxides located at the surface region.
fraction formed on the rolled steel surface [14,17–22]. In the present study, hot-rolling tests were conducted by a pilot-plant-scale rolling machine, which can simulate the actual rolling process, the formation process of oxides was studied by steps, and the degree of sticking was evaluated. According to the hot-rolling test results of Fig. 8(a) through (f), the oxide layer formed in the heating furnace is not extinguished during rolling, but is broken off into pieces or fragments and then infiltrates into the steel with increasing number of rolling passes. The decarburized layer formed in the furnace before hot rolling of conventional plain carbon steels or high-strength low alloy steels is shed off by descaling treatment or mostly extinguished during rough rolling. However, most of the oxide layer formed during heating of stainless steels remains even during hot rolling unless it is artificially peeled off by applying a descaling treatment. Though parts of the remaining oxide layer are broken by rolls, a considerable amount of oxides infiltrates into the steel during rolling, and is distributed in the surface region. The intruded oxides are elongated together with the steel as the rolling proceeds, and leave parts of the surface region uncovered by oxides since the amount of oxides is reduced by the elongation. Total reduction ratio in the rolling test of Fig. 8(a) through (f) is about 90%, and the surface area of the steel after the rolling test increases by about 10 times over that before the rolling test. Thus, parts of the thick oxide layer formed in the steel surface before the rolling test are broken off and gone, and a considerable amount of oxides infiltrates into the steel, but the surface regions with hardly any presence of oxides do exist as the surface area increases. The hardness of the surface region without oxides is significantly lower than that of the surface region with oxides, and parts of the steel surface can be removed due to the reduction by rolls. Fig. 7(a) through (c) are examples showing how the removed
steel in the surface region without oxides can grow into the sticking. The oxides infiltrated into the surface region contribute to increased surface hardness, and work for the enhanced resistance to sticking. Consequently, in order to prevent or minimize the sticking in ferritic stainless steels, it is desirable to promote an oxide layer as thick as possible in the heating furnace and to keep the slab temperature as high as possible by installing a heating system after the rough rolling, while the descaling treatment of the oxide layer should not be applied before the rolling. The sticking phenomenon of the ferritic stainless steel is significantly affected by the distribution and volume fraction of oxides present on the steel surface. As shown in Figs. 8(d) and (e) and 10, the oxide distribution can largely be divided into infiltration type and layer type. Infiltration-type oxides are formed when the oxide layer is broken off and infiltrates into the steel. According to Fig. 8(d) and (e), infiltration-type oxides tend to change to layer-type ones as oxides are elongated together with the elongation of the steel as the rolling proceeds, and get distributed in a thin layer type along the surface. Infiltration-type oxides seem to be formed in the process into the layer-type oxides. Since infiltration-type oxides are distributed pretty deep inside the steel, oxides in the region considerably far away from the surface hardly affect the surface hardness when in contact with rolls, and thus do not contribute to the improved resistance to sticking. If oxides are distributed in a layer type, rather than in an infiltration type, in the surface region under a given volume fraction of oxides, the possibility that the surface region is covered with oxides increases, thereby improving the resistance to sticking. Though the resistance to sticking improves with increasing volume fraction of oxides, promoting more of layertype oxides than infiltration-type ones can be more effective way to reduce the sticking.
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Since the sticking phenomenon can be evaluated overall by investigating the distribution and volume fraction of oxides formed in the surface region, the oxidation behavior of the steel plates fabricated by the rolling test under varying conditions of lubrication, rolling temperature, and rolling speed, as specified in Table 6, is examined. Based on the results, the degree of the sticking occurring in actual rolling stands can be predicted. As the volume fraction of lubricant in the cooling water increases, the volume fraction of oxides in the surface region increases (Fig. 9). The friction coefficient between rolls and rolled steel is reduced by the addition of lubricant, as a considerable amount of oxide layer formed in the heating furnace infiltrates into the rolled steel and the loss of the oxide layer decreases. At the faster rolling speed, the volume fraction of oxides in the surface region increases (Fig. 10). This is because the faster rolling speed constitutes to reducing the breakage and loss of the surface oxide layer and to oxide infiltration into the steel. The oxide volume fraction does not vary much with increasing rolling temperature, but the oxide distribution tends to change from an infiltration type to a layer type (Fig. 9). Considering that layer-type oxides work better for the improved surface hardness than infiltrationtype oxides as explained earlier, the less sticking can be expected at higher rolling temperatures. According to the above results, it can be expected that the resistance to sticking can improve with increasing volume fraction of lubricant, rolling speed, and rolling temperature because the surface hardness increases with increasing oxide volume fraction in the surface region and because the oxide distribution changes to the layer-type. Since the rolling test using a pilot-plant-scale rolling machine is a good way to identify sticking phenomenon occurring from actual hot-rolling stands, it can be useful to understand the sticking mechanism and to suggest possible rolling conditions under which the sticking can be prevented or minimized. When the friction coefficient between rolls and rolled steel is reduced by the addition of lubricant, a number of carbides are formed in the surface region, which leads to the enhanced resistance to sticking. Increased rolling speed and rolling temperature also work favorably for the prevention of sticking as they contribute to the increased oxide volume fraction and to the formation of layer-type oxides. Consequently, the sticking problem occurring during hot rolling of stainless steels can be reduced to a great extent by controlling hot-rolling process parameters, under which a number of oxides are promoted and oxides are distributed in a layer type along the surface, and by designing alloys in which the oxide layer can be formed thick in a reheating furnace.
increasing surface hardness, when a considerable amount of Fe–Cr oxides were formed in the steel surface region. (2) As the rolling pass proceeded, the oxide layer formed in the heating furnace was broken off and infiltrated into the steel, thereby forming oxides in the surface region. The surface region without oxides underwent greater reduction in hardness than the surface region with oxides. These findings indicated that the surface hardness and resistance to sticking varied with the distribution and volume fraction of oxides formed in the surface region. (3) Studies on the oxidation behavior after rolling tests under varying lubrication, rolling temperature, and rolling speed revealed that the resistance to sticking was enhanced in the case of higher volume fraction of lubricant because lots of oxides were formed in the surface region when the friction coefficient between rolls and rolled steels decreased. Faster rolling speed and higher rolling temperature also contributed to the enhanced resistance to sticking since they led to the increased volume fraction of oxides or promoted the formation of layer-type oxides. In order to prevent or minimize the sticking in ferritic stainless steels, it was suggested that the oxide layer should be formed as thick as possible in the heating furnace, and that hot-rolling conditions should be controlled so that oxides could be distributed in a layer type along the surface. Acknowledgments This work was supported by the National Research Laboratory Program (No. M10400000361-06J0000-36110) funded by the Korea Science and Engineering Foundation (KOSEF) and by POSCO under a contract No. PD-06903. The authors would like to thank Dr. Kwang Tae Kim and Mr. Yong Joon Choi of POSCO for their help of the hotrolling test. References [1] [2] [3] [4] [5]
[6]
[7] [8]
5. Conclusions In order to clarify the mechanism of the sticking, which is a representative problem of surface defects occurring during hot rolling of ferritic stainless steels, and to present preventative methods against it, hot-rolling tests were conducted on modified STS430J1L ferritic stainless steels under varying rolling conditions. (1) According to the hot-rolling test results, the sticking did not take place, and the surface remained smooth in the surface region containing lots of Fe–Cr oxides. In the surface region without the presence of oxides, the sticking occurred due to the removed fragments of the rolled steels at the time of contact with rolls. This indicated that the resistance to sticking improved with
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[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
O. Kato, T. Kawanami, J. Jpn. Soc. Technol. Plast. 28 (1987) 264. O. Kato, T. Kawanami, J. Jpn. Soc. Technol. Plast. 30 (1989) 103. O. Kato, S. Uchida, T. Kimuma, Seitetsu Kenkyu 335 (1989) 35. T. Nakanishi, J. Soc. Tribologists Lubricant Eng. 49 (1993) 365. S. Uchida, H. Yamamoto, M. Akata, K. Watanabe, O. Kato, What’s New in Roll Technologies of the World, Report of Research Committee on Rolling Roll, ISIJ, Tokyo, Japan, 1995, p. 183. Y.D. Lee, Y.Y. Lee, O.J. Kwon, G.S. Kim, Y.G. Lee, Development of Rolling Technology of 430 Stainless Steel, Technical Report of POSCO Research Institute, 1993. S. Lee, D. Suh, S. Oh, W. Jin, Metal. Mater. Trans. 29A (1998) 696. S.H. Wang, C.C. Wu, C.Y. Chen, J.R. Yang, P.K. Chiu, J.S. Fang, Met. Mater. Int. 13 (2007) 275. M. Pellizzari, A. Molianari, G. Straffelini, Wear 259 (2005) 1281. J.C. Werquin, J.C. Cailand, in: R.B. Corbett (Ed.), Roll for the Metal Working Industries, Iron and Steel Society, Inc., Warrendale, PA, 1990, p. 55. I.D. Choi, D.M. Kim, S.J. Kim, D.M. Bruce, D.K. Matlock, J.G. Speer, Met. Mater. Int. 12 (2006) 13. C.K. Kim, D.-G. Lee, S. Lee, Mater. Sci. Technol. 23 (2007) 1065. C.-Y. Son, C.K. Kim, D.J. Ha, S. Lee, J.S. Lee, K.T. Kim, Y.D. Lee, Metal. Mater. Trans. 38A (2007) 2776. W. Jin, J.Y. Choi, Y.Y. Lee, ISIJ Int. 40 (2000) 789. ASM Handbook, vol. 18, Friction, Lubrication, and Wear Technology. I.D. Choi, D.M. Bruce, D.K. Matlock, J.G. Speer, Met. Mater. Int. 14 (2008) 139. W. Jin, J.Y. Choi, Y.Y. Lee, ISIJ Int. 38 (1998) 739. L.H.S. Luong, T. Heijkoop, Wear 71 (1981) 93. S.Y. Cho, J.H. Lee, Met. Mater. Int. 14 (2008) 193. Y.H. Moon, J.W. Kim, C.J. Van Tyne, Met. Mater. Int. 2 (2005) 169. A. Bidiville, M. Favero, P. Stadelmann, S. Mischler, Wear 263 (2007) 207. W.C. Doo, D.Y. Kim, S.C. Kang, K.W. Yi, Met. Mater. Int. 13 (2007) 249.