- Email: [email protected]
Effect of micro-defects on the surface brightness of cold-rolled stainless-steel strip
~m'mllof
Materials Process g Technology ELSEVIER
Journal of Materials Processing Technology 69 (1997) 106-111
Effect of micro-defects on the surface brightness of cold-rolled stainless-steel strip Kazuhito Kenmochi a,*, Ikuo Yarita a, Hideo Abe a, Akihiko Fukuhara b Tomio Komatu c, Hiroyuki Kaito b a Mechanical Processing, Instrumentation and Control Research Center. Technical Research Laboratories, Kawasakz Steel Corporation. l Kawasaki-cho. Chuo-ku, Chiba 260, Japan b Chiba I~.'ks, Kawasaki Steel Corporation, Chiba, Japan c Mizusima Works, Kawasaki Steel Corporation, Okayama, Japan
Received 16 February 1996
Abstract Cold-rolled stainless-steel strip can be produced efficiently by high-speed rolling in a cluster-type rolling mill with small-diameter work rolls, or in a tandem cold-rolling mill with large-diameter work rolls. The surface brightness of the strip is deteriorated
by high-speed rolling, and the surface brightness of a strip rolled by large-diameter work rolls becomes worse than that of a strip rolled by small-diameter work rolls. In this study, the effect of micro-defects on surface brightness is examined with laboratory-scale and actual rolling mills, and the mechanisms for the occurrence of micro-pits in cold rolling are discussed. The surface brightness was affected strongly by micro-defects on the surface, the brightness being improved for decreasing surface-area ratio of the micro-defects. Micro-defects can be reduced to four types: micro-pits originating from the surface roughness of the mother sheet; oil-pits formed during cold rolling; grooves formed by inter-granular corrosion during pickling; and scratches due to the surface roughness of the rolls. The micro-pits remaining on the surface of cold-rolled sheet were affected by the diameter of the work rolls, the surface roughness of the mother sheet, and the rolling reduction. The effects of rolling speed and of the viscosity of the rolling oil were minor under the experimental conditions investigated. © 1997 Published by Elsevier Science S.A. Keywords: Micro-defects; Cold rolling; Stainless steel; Surface brightness
1. Introduction Cold-rolled stainless-steel strip is generally rolled by a cluster-type reversible rolling mill such as a 12-high or a 20-high mill which uses small-diameter work rolls. Producing the stainless-steel strip more efficiently has been attempted recently by high-speed rolling in the cluster mill, or in a tandem cold mill which uses largediameter work rolls [1]. The advantage of tandem coldrolling over reversing rolling is greater production efficiency. Surface brightness is an important quality of stainless-steel strip. However, it is reduced by high-speed rolling [2], the surface brightness of stainless-steel strip rolled by large-diameter work rolls being worse than that rolled by small-diameter work rolls [3]. It is thus * Corresponding author. Fax: + 81 43 2622854.
necessary to improve surface brightness to efficiently produce cold-rolled stainless-steel strip. It is well known that the surface condition of the strip after cold rolling affects the surface brightness of a cold rolled product [4]. Rolling conditions such as the roll diameter and the viscosity of the rolling oil affect the surface condition of the strip [2-5]. The oil film thickness at the entrance to the roll bite, which is determined by the rolling conditions, has been investigated, and conditions to effect a decrease in the film thickness have been proposed [6,7]. The surface brightness is evaluated by visual evaluation and by a surface gloss meter. How surface brightness depends on surface roughness has been studied [8] but no quantitative relationship has been advanced in previous studies. The present authors found that an important factor which affects the surface condition of cold-rolled strip is micro-defects existing on the surface [l]. In this study,
0924-0136/97/$17.00 © 1997 Published by Elsevier Science S.A. All rights reserved. PII S0924-0136(97)00003 -4
K. Kemnochi et al. "Journal of Materials Processing Techm~logy 69 (1997) #06-] ##
107
Table 1 Rolling conditions in each cold rolling mill Rolling mill Work roll diameter (ram) Maximum rolling speed (m min-~) Rolling oil (viscosity, mm s-~ at 50°C) Coolant Steel Thickness of mother/finished strip (ram) Number of stand or number of pass Surface roughness on mother strip (tam Ra)
Tandem mill
Sendzimir mitl
530
50 300
A|St430 4.0/1.0 3.2
Mineral oil (35) 3% emulsion, 6~°C AtS1304 3.0/1.0 5 s~.and 3.5
the micro-defects on the surface of cold-rolled strips of ferritic AISI430 and austenitic AISI304 have been observed, the effect of the micro-defects on the surface brightness being evaluated quantitatively. The mechanisms by which micro-pits due to the surface roughness of the mother strip are flattened by cold rolling are explained.
2. Experimental method To study micro-defects occurring on the surface of cold-rolled strip, an experiment was conducted by rolling the strip under the conditions shown in Table 1. In the tandem cold mill employed, the work-roll diameter was 530 ram, and in the 20-high Sendzimir mill, the work-roll diameter was 50 mm. The work rolls were polished to give a roll surface average roughness of 0.3 gm Ra at the middle pass, and 0.1 gm Ra at the final pass. The micro-defects were observed by optical microscope and scanning electron microscopy (SEM), and then classified according to their shapes. The surfacearea ratio of micro-defects was measured using an image processor [9] and the surface brightness of the strip was measured using a surface gloss meter [10]. To study the micro-pits remaining on the surface of cold-rolled strip, an experiment was also conducted by rolling the strip under the conditions shown in Table 2. In the 4-high reversing mill the work-roll diameter was 530 mm, and in the 12-high mill the work-roll diameter was 290 mm. The surface of the strip after each rolling pass was observed, the micro-defects classified, and the surface area ratio of the micro-pits measured by an image processor. Sheet was also rolled using a laboratory-scale mill under the conditions shown in Table 3, the work-roll diameter of the mill being 200 mm. The sheet was stopped during cold rolling, and the surface of the sheet located in the roll bite observed by microscope. The surface roughness was also measured by a contact-type three-dimensional surface roughness meter. AISI430 and AISI304 sheets, after annealing and pickling of the hot rolled strip, were used for the experiments.
Alg1430 8 pass 3.0
Mineral oil I15t Neat, 50°C AIS1304 4.0/1.0 9 pass 3.3
3. Experimental results
3. I. Observation of the toM-rolled strip sulface The surface of strips rolled by the tandem cold mill and Sendzimir mill are shown in Fig. 1. Micro-defects of black grains, defects in the width direction, and defects in the rolling direction can be observed on the surface of the AISI430 strip rolled by the tandem cold mill, whilst micro-defects of black grains, defects in the form of a mesh, and defects in the rolling direction can be observed on the surface of the AISI304 strip rolled by the tandem cold mill.
3.2. Relationship between micro-defects and surface brightness AIS1430 strip was rolled under the conditions shown in Tables 1 and 2. and was then annealed, pickled and temper rolled. The surface-area ratio of micro-defects in the strip was measured, and the effect of the ratio on the surface brightness was examined, the results being show in Fig. 2. Surface brightness is seen to be related to the surface-area ratio of the micro-defects and is improved with reduction of the micro-defects. Table 2 Rolling conditions in the actual reversing mill Work roll diameter (ram) Surface roughness of work roll (gin Ra)
1st pass 2nd and 3rd passes 4th pass
Steel Thickness of mother strip {ram) Surface roughness on mother strip (tam Ra) Delivery speed at final pass (m rain-i) Total reduction in thickness (%) Rolling oil Viscosity of rolling oil (ram s-t at 50°C) Cootant
Standard conditions.
290", 530 0.3, 0.5% 0.8 0.3 0.1 AIS1430 3.0 2.0- 3.2" 300 60, 73, 76~, 80 Synthet;c oil 8, 11, 13~ 5% emulsion, 50°C
108
K. Kenmochi et aL ~Journal of Materials Processing Technology 69 (1997) !06-111
Table 3 Rolling conditions in the laboratory mill Rolling mill Work roll diameter (mm) Surface roughness of work roll (lam Ra) Steel Thickness of mother sheet (ram) Surface roughness on mother sheet (p.m Ra) Reduction in thickness (%) Rolling oil (Applied to strip) Viscosity of rolling oil (mm s-l at 50"C)
~'!000 t',1
2-high mill 200 0.03 AIS1304 3.0 3.2 20 Synthetic oil 13
~
-
-
,oo Boo
k
400
200 [ •
0
I
I
5
10
=
II
15
_.=
20
Surface-area ratio of micro-defects(%)
Fig. 2. Effect of the surface-area ratio of micro-defects on the surface brightness after finished skinpass-rolling.
4. Discussion
4.1. Mechanisms for the occurrence o f micro-defects To clarify the mechanisms for the occurrence of micro-defects, the surfaces of AIS1430 and AISI304 strips were observed by SEM, the results being shown in Fig. 3. The grain boundary on the surface of the mother strip can be recognized at the bottom of a micro-defect of black grain. The micro-defects are micro-pits caused by the surface roughness of the mother strip, and remain on the surface of the cold-rolled strip. Micro-defects in the rolling direction seem to be a trace printed by the surface roughness of the roll on the surface of the strip (hereafter called scratches). Microdefects in the width direction arise from the area of the scratch as shown in Fig. 3, and take the form of wrinkles. Therefore, the defects are described as oil-pits, as explained by Onuki et al. [11]. Micro-defects in the form of a mesh are characteristic in AIS1304 strip. By comparing the surface of the cold-rolled strip with that of the mother strip, it is found that the defect of the cold-rolled strip surface is a groove formed by intergranular corrosion on the mother strip.
Rolling direction
4.2. Effect o f rolling conditions on the remahthtg micro-pits AISI430 steel strip was rolled under the conditions shown in Table 2. The surface-area ratio of the micropits remaining on the cold-rolled strip was measured. The relationships between the area ratio and roll diameter, the surface roughness of the mother strip, the surface roughness of the roll, the rolling reduction, the rolling speed and the viscosity of the rolling oil were studied, the results being shown in Figs. 4 and 5. With decreasing roll diameter and decreasing surface roughness of the mother strip, the surface area ratio of the micro-pits decreases, whilst with increasing surface roughness of the roll and increasing rolling reduction, the area ratio of the micro-pits decreases. In addition, with decreasing rolling speed, the area ratio of the micro-pits decreases slightly but with increasing viscosity of the rolling oil, the area ratio virtually does nat decrease under the present experimental conditions. The effect of the rolling conditions on the surface brightness has been discussed, relating the oil-film thickness resulting from hydro-dynamic lubrication at
200~
i
-.-•
,
.;r •
"
,
(d)
Fig. 1. Micrographs of the surface of stainless steel strips after cold rolling. (a, b) AISI430 strips, (c, d) AIS1304 strips; {a, c) were rolled in the tandem cold mill; and (b, d) were rolled in the Sendzimir mill.
Fig. 3. SEM micrographs of the surface of the mother strip and the cold-rolling strip. (a, b, c) AISI430 strips, (d, e, f) AISI304 strips; (a, d) surface of the mother strip; (b, e) micro-defects of black grain (micro-pits arising from the surface roughness of the mother strip); (c) micro-defect in the width direction (oil-pit); (f) micro-defect in the form of mesh (groove formed of intergranular corrosion).
K. Kemnochi et al. / Journal of Materials" Processing Technology 69 (1997) 106-111
.•
3
'"
(b) 0
200
•o
~
300
400
SO0
J.S
2.0
+~'
2.5
I ,c,
0.0
0.2
0.4
0+6
0.8
8O 60 i.0
G
Roll roughness az I st pass6~rn Ra) Surface roughness of mother strip0am Ra)
Roll diamelcrfram)
109
Fig. 4. Effect of, (a~ roll diameter, (b) surface roughness of the mother strip, and (c) roll roughness at 1st pass, on the surface-mea ratio of micro-pits.
E
40 20
o
I
AISI304, Non-lubricated: []:Floli diameter 200mm II:Roli diameter 50ram
80"
+
6O
u,)
4O
the entrance to the roll bite and the occurrence of oil-pits in the roll bite. It is known that rolling speed and the viscosity of the rolling oil strongly affect the occurrence of oil-pits and the surface brightness [2,6,7]. However, it is found that in these experiments, the rolling speed and the viscosity of the rolling oil have less effect on the micro-pits remaining than the effect of the other rolling conditions. Therefore, the mechanism for the retention of micro-pits on the surface of coldrolled strip cannot be simply explained by the oil-film thickness induced by hydro-dynamic lubrication.
4.3. Mechanism for the retention of micro-pits 4.3.1. Behavior in the roll bite The three-dimensional profile of the surface roughness and the surface area of the micro-pits of a sheet in the roll-bite were measured by stopping the laboratoryscale mill, the results being shown in Fig. 6. It is found that the micro-pits decrease remarkably at the entrance to the roll bite, but with little further decrease from the center to the exit of the roll bite. Therefore, this decrease in micro-pits at the entrance to the roll bite is due to the behavior of the rolling oil, to the roll diameter, to the surface roughness of the sheet, and so on.
20" 0 0
I
Z
3
4
5
10
Distance from entranceof roll bite(mm) Fig. 6. Changes in the surface-arcaratio of micro-pits in the roll bite.
4.3.2. Effect of enclosing rolling oil in micro-pits at the entrance to the roll bite 4.3.2.1. Effect of the roll diameter. Fig. 7 shows the effect of the roll diameter on the occurrence of micropits. In this figure, ~1 is the angle of the roll bite using a large-diameter roll, and ~ is the angle of the roll bite using a small-diameter roll. The amount of rolling oil enclosed in a micro-pit decreases at the entrance to the roll bite with increase in the angle of the roll bite, therefore the volume of micro-pits decreases. The relationship between angle v. and the roll diameter is represented by Eq. (1) [12]: cos ~ = 1 - Ah/(2R')
(1)
where R' is the flattened roll radius, and Ah is the rolling draft. From Fig. 4(a), the amount of micro-pits decreases with decreasing roll diameter.
4.3.2.2. Effect of the surface roughness profile of the mother sheet. It is supposed from Fig. 7 that the
+!Ill\
,.+
(a)
%0-& ~; ~; R ~ u o ~ o n a 1st pa~(%)
100 200 300 4 0 0
Rolling s13eedaa final ~ fro/mira
6
8
10
12
14
16
Viscwsitvof nfilino~oil ¢mmX/seew , 5 ~
Fig. 5. Effect of, (a) reduction at 1st pass, (b) rolling speed at final pass, and (c) viscosity of the rolling oil, on the surface-area ratio of
micro-pits.
amount of rolling oil enclosed in the micro-pits decreases with decreasing surface-area ratio of the micropits at the entrance to the roll bite. It can be seen in Fig. 4(b) that the surface-area ratio of the micro-pits decreases with decreasing surface roughness of the mother sheet. The surface average roughness (Ra) of the mother sheet was 2 - 4 gm Ra, and the surface mean roughness spacing (2a) was 50-200 .um 2a. The oil film thickness enclosed by hydro-dynamic lubrication during the cold rolling of stainless-steel strip is below 0.1 ~tm [13], this being calculated by Azushima's method [14] under the conditions shown in Table 1.
110
K. genmochi et al. ~Journal of Materials Processing Technology 69 (1997) 106-111
Sheet
'
~\ R~olliig~1 X ' ~ % , = ("-Largediameterwork m l , ~ ~ ' ~
Rolll~ Entr--anceofroll bite =. ~ _ . N' • ~, X~, ~ - . . ~ ~ ~:~lueezlng rollingoil from micro-pits ....... ~ . . ~ . . 7,~_~-~.~a,2~K2~t_,~ \ throughgroovesformed of the surface '~ ~, ~, X ' ~ ' ~ 1 -- 1 - ' ~ ,,~.roughnessof the roll Surfaceroughnesson mothersheet
Sheet
~ , . , ~ , . .
Endsure o?",,,=
rollingoil
Fig. 7. Schematic diagram of the conditions for enclosing rolling oil in a micro-pit and for squeezing rolling oil from a micro-pit through grooves formed by the surface roughness of the roll at the entrance to the roll bite.
The amount of rolling oil introduced into the roll bite is affected strongly by mechanical enclosure in micro-pits, because the surface roughness of the mother sheet is greater than the oil-film thickness at the entrance to the role bite. Moreover, the surface roughness of the mother sheet affects the surface-area ratio of micro-pits more strongly than does the rolling speed, as shown by comparison of Fig. 4(b) and Fig. 5(b).
4.3.2.3. Effect of rolling oil. The retention of the micropits is affected by whether or not rolling oil is supplied, as shown in Fig. 6. The amount of micro-pits in the case of non-lubricated rolling decreases more strongly than it does when rolling oil is supplied. The compressibility of a gas as such air is about 7 x 10-6 Pa-1 at room temperature [15], for instance, whilst the compressibility of the lubricant using in the rolling oil is 5 ~ 10x 10 - l ° Pa -1 [16], so that the lubricant is harder to compress than air. Therefore, when rolling oil is enclosed in the micro-pit, the shape of the micro-pit is easily retained in the roll bite. 4.3.2.4. Effect of the surface roughness of the roll. The surface roughness of the roll affects the micro-pits, as shown in Fig. 4(c). Fig. 7 shows the schematic diagram and the conditions for rolling the mother sheet at the entrance to the roll bite when rolling with rolls of large surface roughness. With increasing surface roughness of the roll, rolling oil is squeezed out from the micro-pits through the grooves formed by the surface roughness of the roll at the entrance to the roll bite. From this result, it is supposed that the micro-pits in cold-rolled sheet decrease with decrease in the amount of rolling oil enclosed in the micro-pits.
4.3.3. Effect of squeezing rolling oil from the micro-pits in the roll bite As shown in Fig. 6, the micro-pits decrease in passing through the roll bite. It is supposed that the pressure arising in the rolling oil in the micro-pits increases with increasing rolling force from the entrance up to the neutral point in roll bite during rolling. It is also supposed that the micropits decrease by squeezing out of the rolling oil from the pits when the pressure of the rolling oil in the micro-pits is equal to that around the micro-pits [17]. As shown in Fig. 5(a), it is found that the amount of micro-pits decreases with increasing rolling reduction. The oil-film thickness increases by hydrodynamic lubrication at the entrance to the roll bite, and increasing oil-film thickness results in little decrease in the micropits. In the roll bite, it is supposed that the pressure at the edge of a micro-pit increases due to elasto-hydrodynamic phenomena [3]. It can be explained that rolling oil is squeezed out and the micro-pits are easily decreased when the pressure at the edge becomes greater than the pressure around the micro-pit [18]. The behavior of squeezing out of the rolling oil at the entrance to the roll bite is contrary to the behavior at the edge of a micro-pit. For example, when the rolling speed or viscosity of the rolling oil are increased, the oil-film thickness at the entrance to the roll bite increases and the amount of micro-pits decreases little. In contrast, the pressure at the edge of a micit)-pit ioc~edses and rolling oil in the micro-pits is squeezed out, so that the amount of micro-pits decreases easily. Therefore, it can be explained that the effects of rolling speed and the viscosity of the rolling oil on decreasing the amount of micro-pits become smaller than the effect of reduction, as shown in Fig. 5, because of cancelling of the behavior of hydro-dynamic lubrication.
K. Kemnochi et al. / Journal of Materials Processing Technology 69 (t997) 106-111
~ ~1
5. Condusions
Re~erences
The effect of micro-defects on the surface brightness of cold-rolled AISI430 and AISt304 strips have been studied, and the mechamsms for the origination of micro-pits from the surface roughness of the mother sheet after cold rolling have been made clear. The conclusions drawn are as follows: (1) Surface brightness is affected strongly by microdefects, the brightness being improved with decreasing surface-area ratio of micro-defects. (2) Micro-defects can be identified being of four types: micro-pits originating from the surface roughness of the mother sheet; oil-pits formed during cold rolling; grooves formed by inter-granular corrosion during pickling; and scratches due to the surface roughness of the rolls. (3) The micro-pits decrease remarkably at the entrance to the roll bite, but further decrease only slightly from the center to the exit of the roll bite. (4) Micro-pRs remaining on the surface of the coldrolle~: sheet were affected by the work roll diameter, the surface roughness of the mother sheet, and the rolling reduction. Under the experimental conditions investigated, the effect of the rolling speed and of the viscosity of the rolling oil were slight. (5) Micro-pits on the surface of the sheet were flattened in the roll bite in proportion to the amount of rolling oil squeezed from the pits.
Ill K. Kenmochi, I. Yafita, H. Abe, A. Fukuhara, T. Komatu, H. Ka[to, A. Kishida, CAMP-ISIJ 4 (1991} 492. [2] A. Azushima, K. Noro, Y. lyanagi, H. Degawa, Tetsu-toHagane 76 (1990) 576. [3] M. Wino~S. Shido, Y. Tomari, K. Onodera and H. Milazaki, Proc. Jpn. Spring Conf. Technol. Plast., 1986, pp. 159. [4] S. Sawatani, S. Minamino, H. Nishimura, T. Mizunuma, Seitetu Kenkyu 292 (1977) 100. [5] A. Azushima, J. Kihara, I. Gokyu, J. Jpn. Soc. Technol. Plast. 18 (1977) 337. [6] K. Mizuno, J. Jpn. Soc. Technol. Plast. 12 (1971) 369. [7] ~. Azushima, Tetsu-to-Hagane 64 (1978) 317. [8] H. Hasunuma, J. Nara, J. Phys. Soc. Jpn. 12 (1957) 1117. [9] A. Miyazima, M. YanagJsawa, O. Furukimi, F. Saito, Kawasaki Steel Giho 21 (1989) 362. [10] JIS-Z8741, Method of Measurement for Specular Glossiness. [11] T. Onuki, S. Yasutomi, S. Sotoyama Y. Hasiguti, Y. Tomari, S. Sonoda, S. Hironaka, J. Jpn. Soe " ~ibol. 35 (1990) 845. [12] L.R. Underwood, The Rolling of Metals, Sheet Met. Ind. 22 (1945) 1719. [13] K. Kenmochi, I. Yarita, H. Abe, A. Fukuhara, T. Komatu, H. Kaito, A. Kishida, CAMP-ISIJ 4 (1991) 494. [14] A. Azushima, K. Kihara, M. Miyagawa, Proc. Jpn. Spring Conf. Technol. Plast., 1977, pp. 1. [15] JSME Mechanical Engineer's Handbook. A: Fundamentals, Maruzen, 1990, pp. A5-5. [16] Landolt-Bornstein Zahlenwerte und Funktionen aus Phisik, Chemic, Astronomic, Geophisik und Teehnik, 6 Aufl., II Band, 1 Tell, Springer-Verlag, Berlin, 1971, pp. $437, $438. [17] K. Kenmochi, I. Yarita, H. Abe, E. Kawazumi, Y. Seino, M. Kobayashi, CAMP-ISIJ 4 (1991) 1585. [18] A. Azushima, H. Kudou, Proc. Jpn. Spring Conf. Technol. Piast., 1938, pp. 131.
Materials Process g Technology ELSEVIER
Journal of Materials Processing Technology 69 (1997) 106-111
Effect of micro-defects on the surface brightness of cold-rolled stainless-steel strip Kazuhito Kenmochi a,*, Ikuo Yarita a, Hideo Abe a, Akihiko Fukuhara b Tomio Komatu c, Hiroyuki Kaito b a Mechanical Processing, Instrumentation and Control Research Center. Technical Research Laboratories, Kawasakz Steel Corporation. l Kawasaki-cho. Chuo-ku, Chiba 260, Japan b Chiba I~.'ks, Kawasaki Steel Corporation, Chiba, Japan c Mizusima Works, Kawasaki Steel Corporation, Okayama, Japan
Received 16 February 1996
Abstract Cold-rolled stainless-steel strip can be produced efficiently by high-speed rolling in a cluster-type rolling mill with small-diameter work rolls, or in a tandem cold-rolling mill with large-diameter work rolls. The surface brightness of the strip is deteriorated
by high-speed rolling, and the surface brightness of a strip rolled by large-diameter work rolls becomes worse than that of a strip rolled by small-diameter work rolls. In this study, the effect of micro-defects on surface brightness is examined with laboratory-scale and actual rolling mills, and the mechanisms for the occurrence of micro-pits in cold rolling are discussed. The surface brightness was affected strongly by micro-defects on the surface, the brightness being improved for decreasing surface-area ratio of the micro-defects. Micro-defects can be reduced to four types: micro-pits originating from the surface roughness of the mother sheet; oil-pits formed during cold rolling; grooves formed by inter-granular corrosion during pickling; and scratches due to the surface roughness of the rolls. The micro-pits remaining on the surface of cold-rolled sheet were affected by the diameter of the work rolls, the surface roughness of the mother sheet, and the rolling reduction. The effects of rolling speed and of the viscosity of the rolling oil were minor under the experimental conditions investigated. © 1997 Published by Elsevier Science S.A. Keywords: Micro-defects; Cold rolling; Stainless steel; Surface brightness
1. Introduction Cold-rolled stainless-steel strip is generally rolled by a cluster-type reversible rolling mill such as a 12-high or a 20-high mill which uses small-diameter work rolls. Producing the stainless-steel strip more efficiently has been attempted recently by high-speed rolling in the cluster mill, or in a tandem cold mill which uses largediameter work rolls [1]. The advantage of tandem coldrolling over reversing rolling is greater production efficiency. Surface brightness is an important quality of stainless-steel strip. However, it is reduced by high-speed rolling [2], the surface brightness of stainless-steel strip rolled by large-diameter work rolls being worse than that rolled by small-diameter work rolls [3]. It is thus * Corresponding author. Fax: + 81 43 2622854.
necessary to improve surface brightness to efficiently produce cold-rolled stainless-steel strip. It is well known that the surface condition of the strip after cold rolling affects the surface brightness of a cold rolled product [4]. Rolling conditions such as the roll diameter and the viscosity of the rolling oil affect the surface condition of the strip [2-5]. The oil film thickness at the entrance to the roll bite, which is determined by the rolling conditions, has been investigated, and conditions to effect a decrease in the film thickness have been proposed [6,7]. The surface brightness is evaluated by visual evaluation and by a surface gloss meter. How surface brightness depends on surface roughness has been studied [8] but no quantitative relationship has been advanced in previous studies. The present authors found that an important factor which affects the surface condition of cold-rolled strip is micro-defects existing on the surface [l]. In this study,
0924-0136/97/$17.00 © 1997 Published by Elsevier Science S.A. All rights reserved. PII S0924-0136(97)00003 -4
K. Kemnochi et al. "Journal of Materials Processing Techm~logy 69 (1997) #06-] ##
107
Table 1 Rolling conditions in each cold rolling mill Rolling mill Work roll diameter (ram) Maximum rolling speed (m min-~) Rolling oil (viscosity, mm s-~ at 50°C) Coolant Steel Thickness of mother/finished strip (ram) Number of stand or number of pass Surface roughness on mother strip (tam Ra)
Tandem mill
Sendzimir mitl
530
50 300
A|St430 4.0/1.0 3.2
Mineral oil (35) 3% emulsion, 6~°C AtS1304 3.0/1.0 5 s~.and 3.5
the micro-defects on the surface of cold-rolled strips of ferritic AISI430 and austenitic AISI304 have been observed, the effect of the micro-defects on the surface brightness being evaluated quantitatively. The mechanisms by which micro-pits due to the surface roughness of the mother strip are flattened by cold rolling are explained.
2. Experimental method To study micro-defects occurring on the surface of cold-rolled strip, an experiment was conducted by rolling the strip under the conditions shown in Table 1. In the tandem cold mill employed, the work-roll diameter was 530 ram, and in the 20-high Sendzimir mill, the work-roll diameter was 50 mm. The work rolls were polished to give a roll surface average roughness of 0.3 gm Ra at the middle pass, and 0.1 gm Ra at the final pass. The micro-defects were observed by optical microscope and scanning electron microscopy (SEM), and then classified according to their shapes. The surfacearea ratio of micro-defects was measured using an image processor [9] and the surface brightness of the strip was measured using a surface gloss meter [10]. To study the micro-pits remaining on the surface of cold-rolled strip, an experiment was also conducted by rolling the strip under the conditions shown in Table 2. In the 4-high reversing mill the work-roll diameter was 530 mm, and in the 12-high mill the work-roll diameter was 290 mm. The surface of the strip after each rolling pass was observed, the micro-defects classified, and the surface area ratio of the micro-pits measured by an image processor. Sheet was also rolled using a laboratory-scale mill under the conditions shown in Table 3, the work-roll diameter of the mill being 200 mm. The sheet was stopped during cold rolling, and the surface of the sheet located in the roll bite observed by microscope. The surface roughness was also measured by a contact-type three-dimensional surface roughness meter. AISI430 and AISI304 sheets, after annealing and pickling of the hot rolled strip, were used for the experiments.
Alg1430 8 pass 3.0
Mineral oil I15t Neat, 50°C AIS1304 4.0/1.0 9 pass 3.3
3. Experimental results
3. I. Observation of the toM-rolled strip sulface The surface of strips rolled by the tandem cold mill and Sendzimir mill are shown in Fig. 1. Micro-defects of black grains, defects in the width direction, and defects in the rolling direction can be observed on the surface of the AISI430 strip rolled by the tandem cold mill, whilst micro-defects of black grains, defects in the form of a mesh, and defects in the rolling direction can be observed on the surface of the AISI304 strip rolled by the tandem cold mill.
3.2. Relationship between micro-defects and surface brightness AIS1430 strip was rolled under the conditions shown in Tables 1 and 2. and was then annealed, pickled and temper rolled. The surface-area ratio of micro-defects in the strip was measured, and the effect of the ratio on the surface brightness was examined, the results being show in Fig. 2. Surface brightness is seen to be related to the surface-area ratio of the micro-defects and is improved with reduction of the micro-defects. Table 2 Rolling conditions in the actual reversing mill Work roll diameter (ram) Surface roughness of work roll (gin Ra)
1st pass 2nd and 3rd passes 4th pass
Steel Thickness of mother strip {ram) Surface roughness on mother strip (tam Ra) Delivery speed at final pass (m rain-i) Total reduction in thickness (%) Rolling oil Viscosity of rolling oil (ram s-t at 50°C) Cootant
Standard conditions.
290", 530 0.3, 0.5% 0.8 0.3 0.1 AIS1430 3.0 2.0- 3.2" 300 60, 73, 76~, 80 Synthet;c oil 8, 11, 13~ 5% emulsion, 50°C
108
K. Kenmochi et aL ~Journal of Materials Processing Technology 69 (1997) !06-111
Table 3 Rolling conditions in the laboratory mill Rolling mill Work roll diameter (mm) Surface roughness of work roll (lam Ra) Steel Thickness of mother sheet (ram) Surface roughness on mother sheet (p.m Ra) Reduction in thickness (%) Rolling oil (Applied to strip) Viscosity of rolling oil (mm s-l at 50"C)
~'!000 t',1
2-high mill 200 0.03 AIS1304 3.0 3.2 20 Synthetic oil 13
~
-
-
,oo Boo
k
400
200 [ •
0
I
I
5
10
=
II
15
_.=
20
Surface-area ratio of micro-defects(%)
Fig. 2. Effect of the surface-area ratio of micro-defects on the surface brightness after finished skinpass-rolling.
4. Discussion
4.1. Mechanisms for the occurrence o f micro-defects To clarify the mechanisms for the occurrence of micro-defects, the surfaces of AIS1430 and AISI304 strips were observed by SEM, the results being shown in Fig. 3. The grain boundary on the surface of the mother strip can be recognized at the bottom of a micro-defect of black grain. The micro-defects are micro-pits caused by the surface roughness of the mother strip, and remain on the surface of the cold-rolled strip. Micro-defects in the rolling direction seem to be a trace printed by the surface roughness of the roll on the surface of the strip (hereafter called scratches). Microdefects in the width direction arise from the area of the scratch as shown in Fig. 3, and take the form of wrinkles. Therefore, the defects are described as oil-pits, as explained by Onuki et al. [11]. Micro-defects in the form of a mesh are characteristic in AIS1304 strip. By comparing the surface of the cold-rolled strip with that of the mother strip, it is found that the defect of the cold-rolled strip surface is a groove formed by intergranular corrosion on the mother strip.
Rolling direction
4.2. Effect o f rolling conditions on the remahthtg micro-pits AISI430 steel strip was rolled under the conditions shown in Table 2. The surface-area ratio of the micropits remaining on the cold-rolled strip was measured. The relationships between the area ratio and roll diameter, the surface roughness of the mother strip, the surface roughness of the roll, the rolling reduction, the rolling speed and the viscosity of the rolling oil were studied, the results being shown in Figs. 4 and 5. With decreasing roll diameter and decreasing surface roughness of the mother strip, the surface area ratio of the micro-pits decreases, whilst with increasing surface roughness of the roll and increasing rolling reduction, the area ratio of the micro-pits decreases. In addition, with decreasing rolling speed, the area ratio of the micro-pits decreases slightly but with increasing viscosity of the rolling oil, the area ratio virtually does nat decrease under the present experimental conditions. The effect of the rolling conditions on the surface brightness has been discussed, relating the oil-film thickness resulting from hydro-dynamic lubrication at
200~
i
-.-•
,
.;r •
"
,
(d)
Fig. 1. Micrographs of the surface of stainless steel strips after cold rolling. (a, b) AISI430 strips, (c, d) AIS1304 strips; {a, c) were rolled in the tandem cold mill; and (b, d) were rolled in the Sendzimir mill.
Fig. 3. SEM micrographs of the surface of the mother strip and the cold-rolling strip. (a, b, c) AISI430 strips, (d, e, f) AISI304 strips; (a, d) surface of the mother strip; (b, e) micro-defects of black grain (micro-pits arising from the surface roughness of the mother strip); (c) micro-defect in the width direction (oil-pit); (f) micro-defect in the form of mesh (groove formed of intergranular corrosion).
K. Kemnochi et al. / Journal of Materials" Processing Technology 69 (1997) 106-111
.•
3
'"
(b) 0
200
•o
~
300
400
SO0
J.S
2.0
+~'
2.5
I ,c,
0.0
0.2
0.4
0+6
0.8
8O 60 i.0
G
Roll roughness az I st pass6~rn Ra) Surface roughness of mother strip0am Ra)
Roll diamelcrfram)
109
Fig. 4. Effect of, (a~ roll diameter, (b) surface roughness of the mother strip, and (c) roll roughness at 1st pass, on the surface-mea ratio of micro-pits.
E
40 20
o
I
AISI304, Non-lubricated: []:Floli diameter 200mm II:Roli diameter 50ram
80"
+
6O
u,)
4O
the entrance to the roll bite and the occurrence of oil-pits in the roll bite. It is known that rolling speed and the viscosity of the rolling oil strongly affect the occurrence of oil-pits and the surface brightness [2,6,7]. However, it is found that in these experiments, the rolling speed and the viscosity of the rolling oil have less effect on the micro-pits remaining than the effect of the other rolling conditions. Therefore, the mechanism for the retention of micro-pits on the surface of coldrolled strip cannot be simply explained by the oil-film thickness induced by hydro-dynamic lubrication.
4.3. Mechanism for the retention of micro-pits 4.3.1. Behavior in the roll bite The three-dimensional profile of the surface roughness and the surface area of the micro-pits of a sheet in the roll-bite were measured by stopping the laboratoryscale mill, the results being shown in Fig. 6. It is found that the micro-pits decrease remarkably at the entrance to the roll bite, but with little further decrease from the center to the exit of the roll bite. Therefore, this decrease in micro-pits at the entrance to the roll bite is due to the behavior of the rolling oil, to the roll diameter, to the surface roughness of the sheet, and so on.
20" 0 0
I
Z
3
4
5
10
Distance from entranceof roll bite(mm) Fig. 6. Changes in the surface-arcaratio of micro-pits in the roll bite.
4.3.2. Effect of enclosing rolling oil in micro-pits at the entrance to the roll bite 4.3.2.1. Effect of the roll diameter. Fig. 7 shows the effect of the roll diameter on the occurrence of micropits. In this figure, ~1 is the angle of the roll bite using a large-diameter roll, and ~ is the angle of the roll bite using a small-diameter roll. The amount of rolling oil enclosed in a micro-pit decreases at the entrance to the roll bite with increase in the angle of the roll bite, therefore the volume of micro-pits decreases. The relationship between angle v. and the roll diameter is represented by Eq. (1) [12]: cos ~ = 1 - Ah/(2R')
(1)
where R' is the flattened roll radius, and Ah is the rolling draft. From Fig. 4(a), the amount of micro-pits decreases with decreasing roll diameter.
4.3.2.2. Effect of the surface roughness profile of the mother sheet. It is supposed from Fig. 7 that the
+!Ill\
,.+
(a)
%0-& ~; ~; R ~ u o ~ o n a 1st pa~(%)
100 200 300 4 0 0
Rolling s13eedaa final ~ fro/mira
6
8
10
12
14
16
Viscwsitvof nfilino~oil ¢mmX/seew , 5 ~
Fig. 5. Effect of, (a) reduction at 1st pass, (b) rolling speed at final pass, and (c) viscosity of the rolling oil, on the surface-area ratio of
micro-pits.
amount of rolling oil enclosed in the micro-pits decreases with decreasing surface-area ratio of the micropits at the entrance to the roll bite. It can be seen in Fig. 4(b) that the surface-area ratio of the micro-pits decreases with decreasing surface roughness of the mother sheet. The surface average roughness (Ra) of the mother sheet was 2 - 4 gm Ra, and the surface mean roughness spacing (2a) was 50-200 .um 2a. The oil film thickness enclosed by hydro-dynamic lubrication during the cold rolling of stainless-steel strip is below 0.1 ~tm [13], this being calculated by Azushima's method [14] under the conditions shown in Table 1.
110
K. genmochi et al. ~Journal of Materials Processing Technology 69 (1997) 106-111
Sheet
'
~\ R~olliig~1 X ' ~ % , = ("-Largediameterwork m l , ~ ~ ' ~
Rolll~ Entr--anceofroll bite =. ~ _ . N' • ~, X~, ~ - . . ~ ~ ~:~lueezlng rollingoil from micro-pits ....... ~ . . ~ . . 7,~_~-~.~a,2~K2~t_,~ \ throughgroovesformed of the surface '~ ~, ~, X ' ~ ' ~ 1 -- 1 - ' ~ ,,~.roughnessof the roll Surfaceroughnesson mothersheet
Sheet
~ , . , ~ , . .
Endsure o?",,,=
rollingoil
Fig. 7. Schematic diagram of the conditions for enclosing rolling oil in a micro-pit and for squeezing rolling oil from a micro-pit through grooves formed by the surface roughness of the roll at the entrance to the roll bite.
The amount of rolling oil introduced into the roll bite is affected strongly by mechanical enclosure in micro-pits, because the surface roughness of the mother sheet is greater than the oil-film thickness at the entrance to the role bite. Moreover, the surface roughness of the mother sheet affects the surface-area ratio of micro-pits more strongly than does the rolling speed, as shown by comparison of Fig. 4(b) and Fig. 5(b).
4.3.2.3. Effect of rolling oil. The retention of the micropits is affected by whether or not rolling oil is supplied, as shown in Fig. 6. The amount of micro-pits in the case of non-lubricated rolling decreases more strongly than it does when rolling oil is supplied. The compressibility of a gas as such air is about 7 x 10-6 Pa-1 at room temperature [15], for instance, whilst the compressibility of the lubricant using in the rolling oil is 5 ~ 10x 10 - l ° Pa -1 [16], so that the lubricant is harder to compress than air. Therefore, when rolling oil is enclosed in the micro-pit, the shape of the micro-pit is easily retained in the roll bite. 4.3.2.4. Effect of the surface roughness of the roll. The surface roughness of the roll affects the micro-pits, as shown in Fig. 4(c). Fig. 7 shows the schematic diagram and the conditions for rolling the mother sheet at the entrance to the roll bite when rolling with rolls of large surface roughness. With increasing surface roughness of the roll, rolling oil is squeezed out from the micro-pits through the grooves formed by the surface roughness of the roll at the entrance to the roll bite. From this result, it is supposed that the micro-pits in cold-rolled sheet decrease with decrease in the amount of rolling oil enclosed in the micro-pits.
4.3.3. Effect of squeezing rolling oil from the micro-pits in the roll bite As shown in Fig. 6, the micro-pits decrease in passing through the roll bite. It is supposed that the pressure arising in the rolling oil in the micro-pits increases with increasing rolling force from the entrance up to the neutral point in roll bite during rolling. It is also supposed that the micropits decrease by squeezing out of the rolling oil from the pits when the pressure of the rolling oil in the micro-pits is equal to that around the micro-pits [17]. As shown in Fig. 5(a), it is found that the amount of micro-pits decreases with increasing rolling reduction. The oil-film thickness increases by hydrodynamic lubrication at the entrance to the roll bite, and increasing oil-film thickness results in little decrease in the micropits. In the roll bite, it is supposed that the pressure at the edge of a micro-pit increases due to elasto-hydrodynamic phenomena [3]. It can be explained that rolling oil is squeezed out and the micro-pits are easily decreased when the pressure at the edge becomes greater than the pressure around the micro-pit [18]. The behavior of squeezing out of the rolling oil at the entrance to the roll bite is contrary to the behavior at the edge of a micro-pit. For example, when the rolling speed or viscosity of the rolling oil are increased, the oil-film thickness at the entrance to the roll bite increases and the amount of micro-pits decreases little. In contrast, the pressure at the edge of a micit)-pit ioc~edses and rolling oil in the micro-pits is squeezed out, so that the amount of micro-pits decreases easily. Therefore, it can be explained that the effects of rolling speed and the viscosity of the rolling oil on decreasing the amount of micro-pits become smaller than the effect of reduction, as shown in Fig. 5, because of cancelling of the behavior of hydro-dynamic lubrication.
K. Kemnochi et al. / Journal of Materials Processing Technology 69 (t997) 106-111
~ ~1
5. Condusions
Re~erences
The effect of micro-defects on the surface brightness of cold-rolled AISI430 and AISt304 strips have been studied, and the mechamsms for the origination of micro-pits from the surface roughness of the mother sheet after cold rolling have been made clear. The conclusions drawn are as follows: (1) Surface brightness is affected strongly by microdefects, the brightness being improved with decreasing surface-area ratio of micro-defects. (2) Micro-defects can be identified being of four types: micro-pits originating from the surface roughness of the mother sheet; oil-pits formed during cold rolling; grooves formed by inter-granular corrosion during pickling; and scratches due to the surface roughness of the rolls. (3) The micro-pits decrease remarkably at the entrance to the roll bite, but further decrease only slightly from the center to the exit of the roll bite. (4) Micro-pRs remaining on the surface of the coldrolle~: sheet were affected by the work roll diameter, the surface roughness of the mother sheet, and the rolling reduction. Under the experimental conditions investigated, the effect of the rolling speed and of the viscosity of the rolling oil were slight. (5) Micro-pits on the surface of the sheet were flattened in the roll bite in proportion to the amount of rolling oil squeezed from the pits.
Ill K. Kenmochi, I. Yafita, H. Abe, A. Fukuhara, T. Komatu, H. Ka[to, A. Kishida, CAMP-ISIJ 4 (1991} 492. [2] A. Azushima, K. Noro, Y. lyanagi, H. Degawa, Tetsu-toHagane 76 (1990) 576. [3] M. Wino~S. Shido, Y. Tomari, K. Onodera and H. Milazaki, Proc. Jpn. Spring Conf. Technol. Plast., 1986, pp. 159. [4] S. Sawatani, S. Minamino, H. Nishimura, T. Mizunuma, Seitetu Kenkyu 292 (1977) 100. [5] A. Azushima, J. Kihara, I. Gokyu, J. Jpn. Soc. Technol. Plast. 18 (1977) 337. [6] K. Mizuno, J. Jpn. Soc. Technol. Plast. 12 (1971) 369. [7] ~. Azushima, Tetsu-to-Hagane 64 (1978) 317. [8] H. Hasunuma, J. Nara, J. Phys. Soc. Jpn. 12 (1957) 1117. [9] A. Miyazima, M. YanagJsawa, O. Furukimi, F. Saito, Kawasaki Steel Giho 21 (1989) 362. [10] JIS-Z8741, Method of Measurement for Specular Glossiness. [11] T. Onuki, S. Yasutomi, S. Sotoyama Y. Hasiguti, Y. Tomari, S. Sonoda, S. Hironaka, J. Jpn. Soe " ~ibol. 35 (1990) 845. [12] L.R. Underwood, The Rolling of Metals, Sheet Met. Ind. 22 (1945) 1719. [13] K. Kenmochi, I. Yarita, H. Abe, A. Fukuhara, T. Komatu, H. Kaito, A. Kishida, CAMP-ISIJ 4 (1991) 494. [14] A. Azushima, K. Kihara, M. Miyagawa, Proc. Jpn. Spring Conf. Technol. Plast., 1977, pp. 1. [15] JSME Mechanical Engineer's Handbook. A: Fundamentals, Maruzen, 1990, pp. A5-5. [16] Landolt-Bornstein Zahlenwerte und Funktionen aus Phisik, Chemic, Astronomic, Geophisik und Teehnik, 6 Aufl., II Band, 1 Tell, Springer-Verlag, Berlin, 1971, pp. $437, $438. [17] K. Kenmochi, I. Yarita, H. Abe, E. Kawazumi, Y. Seino, M. Kobayashi, CAMP-ISIJ 4 (1991) 1585. [18] A. Azushima, H. Kudou, Proc. Jpn. Spring Conf. Technol. Piast., 1938, pp. 131.