Original scientific paper Received: June 16, 2014 Accepted: July 11, 2014 Modelling surface degradation in hot rolling work rolls Modeliranje degradacije površine valjev za vroče valjanje Milan Terčelj1, Matevž Fazarinc2, Primož Mrvar1, Borut Žužek3, Peter Fajfar1'* 1Department of Material Science and Metallurgy, Faculty of Natural Sciences and Engineering, University of Ljubljana, Aškerčeva cesta 12, 1000 Ljubljana, Slovenia 2RCJ, d.o.o., Cesta Franceta Prešerna 61, 4270 Jesenice, Slovenia 3IMT, Lepi pot 11, 1000 Ljubljana, Slovenia Corresponding author: E-mail: peter.fajfar@omm.ntf.uni-lj.si Abstract For better understanding of materials behaviour subjected to thermal fatigue of work rolls used in hot rolling mill, two thermal fatigue tests were developed. Tests were performed using Gleeble 1500D thermo-mechan-ical simulator. The first test was applied for the testing of thermal fatigue resistance of work rolls used in common rolling conditions. The average cracks length and the crack density of the specimens were established at varying test temperatures and the number of thermal fatigue cycles. The second test enabled the simulation of the thermal loading of the work rolls during a rolling mill stall. The growth of stall band firecracks after different cooling condition of the work rolls is presented. Key words: hot rolling, work roll, thermal fatigue, fire cracks Izvleček Za boljše razumevanje vedenja materiala izpostavljenega termičnemu utrujanju delovnih valjev, ki se uporabljajo za vroče valjanje, sta bila razvita dva preizkusa za termično utrujanje valjev. Preizkusi so bili izvedeni z uporabo termomehanskega simulatorja Gleeble 1500D. Prvi preizkus je bil uporabljen za preizkušanje odpornosti proti termičnemu utrujanju delovnih valjev v normalnih razmerah valjanja. Za določitev povprečne dolžine in gostote razpok so bili preizkusi izvedeni pri različnih temperaturah in številu ciklov termičnega utrujanja. Drugi preizkus omogoča simulacijo toplotne obremenitve delovnih valjev med zastojem valjalnega stroja. Prikazana je rast razpok v vročem glede na različne razmere pri ohlajanju delovnih valjev. Ključne besede: vroče valjanje, delovni valj, termično utrujanje, razpoke v vročem Introduction Work rolls represent an important segment in the operating cost of a modern rolling mill. They are expensive and a large number of work rolls must be in stock to assure the continuous operating of the mill. During hot rolling work, rolls are subjected to successive heating and cooling conditions. Their surface is exposed to rapid temperature changes due to the contact with hot rolled material and due to cold water used for work rolls cooling. This cyclic heating/ cooling conditions cause thermal fatigue which is one of the most important factors affecting the rate of surface deterioration as well as mechanical fatigue and abrasion. Thermal fatigue is a low-cycle failure mechanism that occurs due to the cyclic thermal loading of the work rolls surfaces. Cooling process of the work rolls is one of the most important tasks during hot rolling. Their surface is exposed to rapid temperature change due to the contact with hot rolled material and cooling with water sprays. If cooling is not intensive enough, wear of rolls and fire cracks appear. Fire cracks can appear after only a few turns of rolls, starting on the surface and growing perpendicular to the surface of the rolls. The intensity of the growth and the depth of the cracks mostly depends on the temperature gradient during alternating heating and cooling[1-3]. The evolution of work roll surface temperature during one revolution is presented in Figure 1[4]. The work roll is divided into angular sections. In the section 1, a high increase of the roll surface temperature is noticed. The reason is in the heat transfer from the hot slab to the roll. The contact time between the slab and the roll is very short, only the skin of the roll is subject to the very high thermal gradients. In section 2, the work roll slowly cools down due to radiation and convection. In section 3, the surface temperature decreases during water cooling. In sections 4 and 5, the surface temperature rises slightly due to the heat transfer from the roll centre. A drop of the surface temperature due to the contact with the back-up roll occurs. In section 6, the temperature decreases due to the water cooling and in section 7, the surface temperature rises due to the radiation of the slab surface. Angle, cp/° Figure 1: Evolution of work roll surface temperature. In this paper, two different tests for the laboratory assessments of the thermal fatigue resistance of work rolls are presented. The first test enables a simulation of the thermal fatigue of work rolls during hot rolling of flat products at common condition[5-7]. Furthermore, with the second test, the thermal fatigue resistance of work rolls in the case of the rolling mill stalls can be investigated. Both tests were implemented in a thermo-mechanical simulator of metallurgical states, the Gleeble 1500D. The resistance heating and the water cooling of the samples as well as air blowing (water emptying process] were computer-controlled. The samples were freely spanned in the working jaws of the Gleeble loading system, keeping the outer force on the samples at 0 N. Experimental Specimen Specimens were machined from the work rolls. They were cylindrically shaped with a borehole in the longitudinal axis that enabled the cooling of the specimens with a stream of water and air (Figure 2]. The reduction of the diameter in the central part of the specimen intensified the temperature gradient during the heating and the cooling stage of the experiment. The temperature was controlled with the thermocouple that was spot-welded in the middle of the reduced part of the specimen (Figures 2, 3]. The thermal fatigue tests were performed on thermal-mechanical simulator Gleeble 1500D. Figure 3: Test cell with specimen, Gleeble 1500D. Thermal fatigue of work rolls Specimens were tested at similar conditions found on the surface of rolls during hot rolling. Specimens were heated to four different temperatures, 400 °C, 500 °C, 600°C and 700°C and then rapidly cooled with water and air. Two series of experiments with 500 and 1 000 thermal fatigue cycles were carried out. Each cycle was composed of three phases: resistance heating, water cooling and cooling of the specimen with air, all in duration of 4.8 s. Material The specimens were electric-discharge machined from an indefinite chill roll. All these rolls were cast as a double layer. The hard working surface consisted of matrix of dendrite grains from bainite and martenzite, ledeburite and some free graphite. The core of the roll can be made either out of the alloyed grey cast iron or out of the alloyed nodular cast iron. These types of roles are used for roughing and finish rolling mills. The typical chemical composition is listed in Table 1. Table 1: Typical chemical composition of the indefinite chill roll in mass fractions, w/% C Si Mn Cr Ni Mo min. 3.0 0.8 0.2 1.4 4.0 0.2 max. 3.4 1.1 0.5 2.0 4.6 0.5 Stall band firecracks For the investigation of the formation of stall band firecracks on the work roll surface, five tests were performed. For the first test, the specimen was heated up to 600 °C for each thermal fatigue cycle and then cooled down with a stream of water and air. This was a base test condition for all other tests. After 500 cycles, the specimens were held at the testing temperature for 15 s or 60 s and then cooled down to the room temperature with water or air (Figure 4). 700-, 650 iSi 10050- oq-.--T-I-T-1-T-1-T-1-T-1-T-1-T-1 2080 2090 2100 2110 2120 2130 2140 2150 2160 Times, f/s 700-, 650600: r- 550500: 450- ---- и 400 -^ 350-