Rolling Temperatures on Sticking Behavior of Ferritic Stainless Steels

ISIJ International, Vol. 38 (1998), No. 7, pp. 739-743

Effect of Roll and Rolling Temperatures on Sticking Behavior of Ferritic Stainless Steels

WonJIN. Jeom-YongCHOIand Yun-YongLEE

Stainless Steel Research Team, Technical Research Laboratories, Pohanglron & Steel Co,, Ltd.. PohangP.O. Box 36, 1, Koedong-dong, Pohang-shi. Kyungbuk, Korea, E-mail: pc543552@smail,posco.kr (Received on December5. 1997.¥ accepted in final form on February 23. 1998)

The sticking behavior of several austenitic and ferritic stainless steels under the hot roiling conditions wasexaminedin detail using a two disk type hot rolling simulator. Thesticking of bare metal to roll surfaces wasstrong!y dependenton the high temperature tensile strength and the oxidation resistance of the stainless steel, Asteel having higher tensile strength and lower oxidation resistance exhibited better resistance against sticking. The sticking occurred in increasing severity in the order of 430J1 L, 436L, 430 and 409L. It was clarified that a high speedsteel (HSS) rol[ wasmorebeneficial to prevent sticking compared to a Hi-Cr roll. KEYWORDS: ferritic stainless steel; sticking behavior; hot rolling; high speedsteel roll; high chromiumroll.

l. Introduction 2. Experiments

Thesticking phenomenonoccurs frequently during the A sticking simulator wasused to investigate the effect hot rolling of ferritic stainless steels, causing surface of hot rolling conditions on sticking behavior. Figure 1 defects on the mill product andscoring on the roll surface. showsthe schematic diagram of the sticking simulator. The sticking of the bare metal, exposed by scale break- The simulation wascarried out by using a two disk type awayduring hot rolling, to the roll surface is affected by hot rolling simulator which consisted of a high frequen- both hot rolling conditions and the stainless steel prop- cy induction heater, water cooling and temperature mea- erties.1~3) According to previous research on sticking suring system, equipped with a contact loading system behavior under various rolling conditions, the sticking between the roll and the mating material. The experi- increases with the increasing contact stress and slip mental conditions of sticking simulation are given in ratio, and is considered to occur in the backward slip Table 1. In order to simulate hot rolling process a new region in the arc of contact.1,4) In addition to the roll- mating material must be used in every revolution. But, ing condition, there is a significant difference in the in this test, a mating material was changed after every sticking resistance of commercial roll materials for hot 20 revolution test. For example, 5newmating materials rolling. This implies that the sticking occurrence is highly were used to simulate 100 revolution test. The sticking dependenton the wear, scratch resistance andmechanical weight was estimated as the difference between the in- properties of the roll material. Onthe other hand, it is itial roll weight and that obtained after simulation. knownthat the 400 series stainless steel is moresensitive Thechemical compositions of steels used in this study to sticking than the 300 series and also that there is a wide variation in sticking resistance the ferritic among Hi9h frequency jnduction coil steels.5) stainless However, despite numerous studies, ~,~l pyrothermometer sticking prevention is still in rolling of / 10 a problem hot .. ferritic high Cr stainless steels. / Mating material This study employeda sticking simulator in order to investigate the effects of hot rolling temperatures and 7 / high temperature mechanical properties of roll and roll- Ci= =1 :: ~' ing material on the sticking behavior. Changesin stick- - ing weight were investigated with rolling temperatures, numberof roll revolutions and rolling materials. Waterspray Also, in order to verify the relationship between the 4~1 sticking tendency and the high temperature properties of 5 [rl stainless steels, high temperature tensile strength and ~1* oxidation resistance of the 300 and the 400 series stain- -- Roll material less steels were measured. Fig. 1. Schematic diagram of the sticking simulator.

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l. variables conditions. Table Experimental and Hi-Cr roll

Variables Condition O. 14 430J 436L430304409L l Roll material HSS, Hi-Cr IL 12 ~ 2 Matmgmatenal 430JIL, 436L, 430, -~O ~1-¥e 409L, 304 0.1 ~J=- 3 Contact stress 400 MPa .O)_ ,D; Simulation ~~ 0.08 4 temperature 700 - 1000 a) 5 Numberof revolutions 5- 100 rev. C 0.06 6 Backwardsli ratio 3401Q :~ ¥-O 0.04 7 Rollin s eed O3m/sec ~5 8 Roughnessofroll(Ra) l O-2.0 m 0.02

20 40 60 80 Ioo Table 2. Chemical compositions ofsteels used. Numberof revolutions wiolo Fig. 2. Changesof the sticking weight Hi-Cr roll with the s ecimen Cr Ni Mo C rqb on numberof revolutions at 900*C. 430JIL 19,l 0.12 O.OIO 0.30 436L 18.7 0.01 o96 O021 0.26 HSSroll 430 16.3 0,08 OO1 o060 409L 11.4 0,07 0,045 o. 1 430JIL436L430304409L ~1 304 18.2 830 o050 13~ ,e-¥ E0,08 ,: ,c,)_ Table 3. Chemical compositions of roll materials used. 0,06 o; wiolo o) 0,04 C Si Mn Ni Cr Mo V ~ (,) HSS 2,0 l .O lO 10 50 2.5 40 0,02 Hi-Cr 29 07 l .O 10 18.0 l .4 0.2 20 40 60 80 1OO Numberof revolutions are shownin Table 2. The five steels consisted of four ferritic stainless steels with various Cr weight percent Fig. 3. Changesof the sticking weight on HSSroll with the numberof revolutions at 900'C. from II to 19 o/o and one austenitic steel as a reference. Also the chemical compositions of the roll materials is 20th, nucleation and growth of sticking shownin Table 3. High temperature tensile tests were processes particles and the separation of these particles from performed using a Gleeble 1500. In order to measure process the roll surface parallel with each other. high temperature tensile strength, specimenswere heated operate These results also that the sticking at 20'C/sec and held at the desired temperature for demonstrated occurred with increasing severity in the order of 430JIL, 5min, then tensile tested at a strain rate of 0.3/min. The 436L, 430, 304 roll less thickness of scale layer formed at various temperatures and 409L. AHSS was prone to sticking than Hi-Cr roll. Figure shows compari- were measured by optica] microscope. The specimens a 4 a of the sticking weight changes for 430 the Hi-Cr were heated in the sticking simulator with a high fre- son on and rolls from the Ist 30th revolution. It quencyinduction heater, at a heating rate lO'C/sec with HSS to was found that the sticking weight of the higher a holding time of 20 sec at the desired temperature. HSSwas than that of the Hi-Cr roll between the Ist and 7th revolution, after 3. Resu]ts and Discussion but the 10th revolution, the situation was reversed, and the saturation of sticking weight of 3.1. The Effect of Roll Revolution Number HSSroll reached before lOth revolution, but Hi-Cr roll In order to investigate the effect of the numberof roll requires 20th revolution, double of the HSSroll. revolutions and roll materials on the sticking behavior The sticking behavior in the initial stages between of austenitic and ferritic stainless steels, the sticking the Ist and 7th revolutions was examined by scanning simulation was carried out. Figures 2 and 3 show the electron microscopy. Figure 5 shows the scanning change of sticking weight on High Chromium(Hi-Cr) electron micrographs of 430JIL particles stuck on the and High Speed Steel (HSS) rolls with numberof roll Hi-Cr andHSSroll surfaces after the 7th revolution. The revolutions at 900'C. The sticking weight drastically arrows on the SEMimagesindicate the 430Jl L particles increased from the first to the 20th revolution, however, stuck on the roll surface. Thenumberof particles on the subsequently, a siight decrease wasnoted in most steels. Hi-Cr roll surface was greater than on the HSSroll In the case of 409L, the sticking weight increased grad- surface, and the particle size on the Hi-Cr roll surface ually. The sticking tendency in this simulation corre- wassmaller than the HSSroll. Theseresults showclearly spondswell to Kato's results2) where sticking occurs im- that the Hi-Cr roll has morenucleation sites of sticking mediately if the specific conditions of occurrence are particles per unit area than the HSSroll. Although HSS satisfied. Accordingly, it is thought that nucleation and rolls have relatively smaller numberof nucleation sites, growth processes of sticking particles occur in the range the sticking weight of the HSSroll is greater than that between the Ist and the 20th revolution, but after the of the Hi-Cr roll during the initial stage up to the lOth

C 1998 ISIJ 740 ISIJ International, Vol. 38 (1998), No. 7

0,08

~c,, E o.06 a =:~ ah---t~= __~___ o IP o 0,04 ~ ,r o :2c / o 0,02 r!'~TT~"'~'L' CO

o o 5 Io 15 20 25 30 35 Numberof revolutions

Fig. 4. Changesof the sticking weight of 430 on Hi-Cr and HSSi'olls wlth the numberof revolutions at 900*C.

Fig. 6. Optical micrographs showing the surface ofthe Hi-Cr and HSSrolls after hot rolling of 430JIL stainless steels.

Table 4. Mechanical properties and microstructures ofHSS and Hi-Cr roll.

Pro erties HSSroll Hi-Cr roll Carbide(MC+ Carbide(M7 Microstructure M7C3+ M6C, C3, 26 501.)+ 12olo) Martensite + Martensite Total 702 558 hardness Hv Hardnessof 650 450 matrix Hv Tensile strength 960 850 MPa Fig. 5. SEMimages showing the surface of Hi-Cr and HSS Resistance to excellent fair roil after sticki ng simulation of430JILstainless steels. heat- c cled fati ue Resistance to excellent fair revolution because of the relatively larger particle size. abrasion But the sticking weight of the HSSroll becomessmaller than that of the Hi-Cr roll after the lOth revolution of HSSand Hi-Cr commercial roll surfaces after hot because of the separation process of the particles which rolling of 430JIL stainless steel. These photographs is enoughto separate from roll surface. Onthe contrary, showthat manyscratches exlst on the surfaces of both the particles stuck on Hi-Cr roll did not becomelarge Hi-Cr and HSSrolls. TheHi-Cr roll displays broad lath during the initia] stage because the particles generated type scratches, but HSSroll has a long needle type form. at manynucleation sites grow competitiveiy with each White particles transferred from the stain]ess steel strip other. Evenafter the IOth revolution, small particles stuck during hot ro]1ing are always found on the roll surface on Hi-Cr roll gradually grow to a critical size to separate scratches, particularly on the relatively broad scratches. from the roll surface. In consequence,the sticking weight This results indicate that the nucleation site of the sticking of the Hi-Cr roll is larger than that of the HSSroll after particle is a scratch on the roll surface. In addition, the the 10th revolution. significant difference In number of nucleation sites of It is well knownthat the sticking weight is depen- sticking particles between Hi-Cr and HSSroll is at- dent on the roughness of roll surface and on the volume tributed to the disparate mechanicai properties between percent of the carbide.5) Also, from the above results the Hi-Cr and HSSroll. Table 4 gives the mechanical there is a difference in the numberof nucleation sites of properties andthe micro-structures of the HSSandHi-Cr sticking particles betweenHi-Cr and HSSrolls, and it is rolls.6) Thereslstance to abrasion and heat cyc]ed fatigue necessary to underst¥and the nature of nucleatlon sites of the HSSroll are superior to those of the Hi-Cr roll on the roll surface. Figure 6showsoptlcal mlcrographs becausethe hardness of the HSSroll is higher than that

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Hi-Cr roll 300 40 4eOJIL 4eOJIL4eo 4SOSO4004 409L409L 0.14 1'F 430J 436L430304409L ~~250 -~- -{~¥~--~¥ ¥ IL O_ ~ ~ ~1 0.12 :~ Ten5ile$trength 30 a eD 11-¥e 200 ~~2 -E ~ ~"""" Thicknes5 ot scale .... 0.1 8cl' ~!: o=,1)* c,, 150 ~ 'o~ .~Jn ~Oo o 0.08 u) .¥' g2 e)~ o) 100 *.O ,: 0.06 '~ --* ~- = c: 10 J. Q, .__._~1~ ~=o 004 50 ~ CO F h 0.02 O 700 750 800 850 900 950 1,000 O ~C) O 700 750 800 850 900 950 1,000 Test temperature( re(~C Test temperatu ) Fig. 9. Changes of tensile strength and scale thickness of stainless steels with Fig. 7. The effect of temperatures on the changes of the temperatures. sticking weight on Hi-Cr roll after 20th revolution. prevent the local plastic failure in the surface layer of HSSroll the stainless steel. The presence of a substantial layer 0.1 430JIL436L430304409L of scale on the rolling material has the role of lubricant ~1 135 and decreases the opportunity for direct contact between E 0.08 *-¥e the roll and bare metal during hot rolling. Therefore, J: it is g) necessary to estimate the high temperature tensile ',D 0.06 ; strength and the thickness of the scale of stainless steels, O, ~-~1 E; in order to investigate the influence of the scale in the ~i 004 O contact arc as well as the high temperature mechanical (D 0.02 properties on the sticking behavior. Figure 9showsthe changesof tensile strength and the thickness of scale layer O 700 750 800 850 900 950 1OOO of stainless steels with temperature. Thehigh temperature Test temperatu re(~ ) tensile strength rapidly decreased, and the thickness of Fig. 8. The effect of temperatures on the changes of the scale layer increased with increasing temperature as might sticking weight on HSSroll after 20th revolution. be expected. The tensile strength decreased in the order of 304, 430, 430JIL and 409Land the thickness of scale of the Hi-Cr roll. Even though the surface roughness layer decreased in the order of 409L, 430, 304and430JIL was similar in starting condition, more scratches are in all ranges of rolling temperature. generated on the surface of the Hi-Cr roll during the hot Fromthe aboveresults, it is concluded that the sticking rolling. Conclusively, the Hi-Cr roll has a higher number resistance above 900'C is expected to be low because of of nucleatlon sites than the HSSroll. the very low tensile strength of stainless steels, however, thick scale formation above900'C can effectively prevent 3.2. The Effect of Hot Rolling Temperature the sticking despite low tensile strength. Onthe contrary, Figure 7 shows the change in sticking weight after in the range below 900'C, the sticking resistance maybe 20th revolution of 5different grade of stainless steels on high due to high tensile strength despite a thin scale Hi-Cr roll with various hot rolling temperatures. The layer. In consequence, the temperature range near 900'C sticking weight of all steels was increased to 900'C, showedthe lowest sticking resistance becauseof the low then subsequently decreased. A similar tendency was tensile strength and the thin scale layer. Thevariation in found in the case of the HSSroll (Fig. 8). This indicates sticking resistance of the 5steels maybe explained by that the temperature range having weak sticking re- these conclusions. The austenitic stainless steel 304, with sistance wasaround 900'C in all ferritic stainless steels. high tensile strength and thin scale layer, and 409Lwith The austenitic stainless steel 304 also followed this ten- low tensile strength andthick sca]e layer, both showgood dency. The sticking occurrence becamesevere with in- sticking resistance. This indicates that if a specific crease of Cr weight percent similar to previous results stainless steel has any one of two properties, high ten- (Figs. 2and 3). sile strength and thick sca]e layer, that steel would show Previous reports indicate that sticking behavior is at- high sticking resistance. But without either of these two tributable to the growth of flake-like particles on the properties, that steel would showlow sticking resistance. roll surface. Namely, whenplastic deformation on the Therefore, 430JILwith low tensile strength andthin scale surface layer of stainless steel in the contact arc between layer is expected to have the lowest sticking resistance. roll and materlal reaches a critical value, a local plastic Generally, the 400 series stainless steel has a lower failure occurs in the surface layer and then, stainless steel sticking resistance than the 300 series. This is because particles generated by local failure are transferred to the the 400 series do not have a high temperature tensile roll surface.2) It was also reported that the existence of strength adequate to prevent local plastic failure of the lubricants in the contact arc is effective to prevent the stainless steel surface layer as in the case of the sameCr sticking.7) From these reports, it is deduced that high weight percent in the 300 series. Particularly, high Cr tensile strength at high temperature is necessary to ferritic stainless steel with high oxidation resistance is

C 1998 [SIJ 742 ISIJ International, Vol. 38 (1998), No. 7 expected to showmuchlower sticking resistance. (5) Thetensile strength decreases in the order of 304, 430, 430JILand409Lat all ranges of rolling temperature 4. Conclusion and the scale thickness decreases in the order of 409L 430, 304 and 430JIL. Sticking occurred severely (1) The sticking weight in sticking simulation more in the order of 409L, 304, 430, 436L and 430JIL. drastically increased from the first to the 20th revolution, however, after the 20th, a s]ight decrease was observed in most of steels. REFERENCES (2) The sticking weight the H1-Cr roll larger l) O. Kato and T. Kawanami: J. Jpn. Soc. Technol. Plast., 28 on was (1987), 264. than that the roll. It is due to lower resistance on HSS a 2) O. Kato, S. Uchida and T. Kimuma:Steel Res., 335 (1989), 35. abrasion the roll that the to of Hi-Cr comparedto of 3) S. Uchida, H. Yamamoto.M. Akata, K. Watanabeand O, Kato: HSS,that is, the Hi-Cr roll had morenucleation sites of What's Newin Roll Technologies of the World, Report of sticking particle per unit area than the HSSroll. Research Committeeon Rolling Roll, ISIJ, Japan, (1995), 183. (3) The sticking weight of all investigated steels in- 4) O. Kato and T. Kawanami:Jpn. Soc. Techno!. Plast., 30 (1989), l03. creased to the rolling temperature 900'C, then decreas- 5) Y. D. Lee, Y. Y. Lee, O. J. Kwon, G. S. Kim and Y, G, Lee: ed subsequently. The temperature range with weak Developmentof Rolling Technologyof 430 Stainless Steel, Report sticking resistance wasaround 900'C. of POSCOResearch Inst., unpublished, (1993). (4) In the temperature range above900'C, the stick- 6) J. H. Ryu, K. B. Kang, P. J. Lee and S, G. Shi: Effect of Rolling ing resistance high because of the thick scale layer Conditions on Roll Surface Roughness, Report of POSCO was Research Inst., unpublished, (1993). despite low tensile strength. On the contrary, in the 7) T. Nakanishi: J, Soc. Tt'ibo!ogists Lubrican! Eng,, 49 1993), 365. range below 900'C, the sticking resistance wasalso high ( due to high tensile strength despite a thin scale layer.

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