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Rolling Temperatures on Sticking Behavior of Ferritic Stainless Steels

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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 tempera- tures, 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. 739 C 1998 ISIJ ISIJ International, Vol. 38 (1 998). No. 7 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 par- ticles 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 oc- curred 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.
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