15. - 17. 5. 2013, Brno, Czech Republic, EU OPTIMIZATION OF THE TUBE ROLLING PROCESS FOR 25CRMO4 STEEL GRADE Ľudovít PARILÁK a, c, Milan MOJŽIŠ a , Tomáš KVAČKAJ b , Lucia DOMOVCOVÁ a a ŽP Research and Development Centre, Podbrezova, Slovak Republic, EU, parilak@zelpo.sk b Železiarne Podbrezová, Podbrezova, Slovak Republic, EU, kvackaj.tomas@zelpo.sk c Technical University in Kosice, Presov, Slovak Republic, EU Abstract In the process of tube rolling in Železiarne Podbrezová it is crucial to properly maintain the selected thermomechanical process parameters, namely: heating of the tube stock in a walking beam furnace, maintaining the uniform plastic deformation in all rolling stands of a stretch-reducing mill and, finally, keeping the finishing rolling temperature of the tube over the A r3 temperature. In this paper, optimization results for 25CrMo4 tube rolling are being presented, considering the final tube with a 2,6 mm wall thickness. For this, the stretch-reduction parameter in all rolling stands as well as the finishing rolling temperature has been optimized. Keywords: tube rolling, 25CrMo4 steel grade, plastic deformation, stretch-reducing mill 1. INTRODUCTION In Železiarne Podbrezová, steel grade 25CrMo4 is being produced and subsequently hot-rolled in seamless tube rolling mill, producing the tubes with diameter from 31.8 mm up to 76.1 mm and wall thickness from 2.6 mm up to 5.6 mm. These tubes are frequently used as a semi-finished product for pressure vessels and also for applications in power industry, thanks to their favorable creep-resistant properties up to 400 °C [1]. The tube rolling represents a sophisticated technology, consisting of several technological sub-steps: billet heating in a rotary hearth furnace (with zone #1 - 1230°C, zone #2 - 1269°C, zone #3 - 1297°C, zone #4. - 1285°C), billet piercing, elongation, rolling on a push-bench and mandrel removal on a detaching mill (reeler). After this, the tube blank is reheated in a walking-beam furnace (temperatures from 1035 °C to 1050 °C), thus ready for final rolling operation on a 28-stand stretch-reducing mill. For 25CrMo4 steel grade and wall thickness of 2.6 mm, material failure (rupture) between adjacent rolling stands occurred frequently. This problem plagued the rolling mill economically, rendering this problem a high priority one to be solved [1]. 2. EXPERIMENT In the first iteration we focused on all parameters of technological sub-steps, going from rotary hearth furnace up to the walking beam furnace. However, we have encountered no flaws leading to material failure in the stretch-reducing mill. In the second iteration we focused on: thermal-deformation processes during rolling, walking-beam furnace temperature and stretch-reducing mill inlet/outlet temperature. We analyzed the strain rates given by stretch ratio in adjacent stretch-reducing rolling stands. Results presented were obtained in a plant experiment, using heat No. 13520 (see Tab. 1). Dilatometric analysis, performed on TU Košice, reveals threshold temperatures A r1 = 687 °C, A r3 = 764 °C, A c1 = 758 °C and A c3 = 814 °C, respectively [2]. After tempering, 25CrMo4 grade should exhibit the hypoeutectoid, ferritic-pearlitic structure (with approx. 30 wt% of ferrite), Fig. 1, 2.