THE CONTROLLED COOLING SIMULATION OF A CASE - HARDENING STEEL ROLLED INTO COILS

Transcription

1 SIMULACE ŘÍZENÉHO OCHLAZOVÁNÍ CEMENTAČNÍ OCELI VÁLCOVANÉ DO SVITKŮ THE CONTROLLED COOLING SIMULATION OF A CASE - HARDENING STEEL ROLLED INTO COILS Tomáš Gajdzica a, Bogdan Kotas a, Milan Kotas a, Karel Milan Čmiel b, Ivo Schindler a, Jiří Kliber a, Viktor Tittel c a VŠB - TU Ostrava, Fakulta metalurgie a materiálového inženýrství, 17. listopadu 15, Ostrava - Poruba, ČR, tomas.gajdzica@vsb.cz, milan.kotas@vsb.cz, bogdan.kotas@centrum.cz, ivo.schindler@vsb.cz, jiri.kliber@vsb.cz b TŘINECKÉ ŽELEZÁRNY, a. s., Průmyslová 1000, Třinec, ČR, karel.cmiel@trz.cz c STU Bratislava, MTF Trnava, Bottová 25, Trnava, SR, viktor.tittel@stuba.sk Abstract Presently the demand for rolled-out material in coils is augmented by the majority of steel manufacturers who specialize themselves in the drawing of SBQ steels. Generally, the main reason for that is conditioned and supported with a simpler insitu manipulation and pulling in the drawing-line. Within the scope of revamping the Light-Section Rolling Mill of TŽ, a.s. and expanding contemporary assortment of steels, a brand new Garret-type winding line has been installed. This facility is fully capable of processing the material of the required parameters. Overall contributions of the new investment lie not only in the production possibilities of 2-tonne coils or in an enhanced system of binding and pressing, but mainly in new controlled rolling options. However, there s a significant difference between cooling of coils and bars determined by natural convection hence it s nowadays important to optimize thoroughly the technology of controlled cooling of SBQ steels with respect to different steel qualities. A controlled cooling simulation incorporating the laboratory analysis of specific steel samples belong among progressive methods to effectuate such optimization without any time and financial stress imposed upon the rolling facility due to additional trials and sampling. The simulation has been accomplished at VŠB - TU of Ostrava under the conditions of laboratory rolling train TANDEM. A few low-carbon steels have been used with aim to determine the influence of final rolling temperature and the way of cooling in Garret winding machines on the final microstructure and mechanical properties of a casehardening steel used in this article. The objective was based on optimizing the temperature conditions for controlled rolling and cooling at the Light-Section Rolling Mill the samples taken from rolled-out products were analysed both metallographically and mechanically (tensile tests) to determine final mechanical properties of thus processed material. Keywords: continuous light mild, coiling line, SBQ steel, thermomechanical rolling, case hardening steel, controlled cooling, rolled coil, laboratory rolling, temperature, microstructure analysis, mechanical properties. 1

2 1. INTRODUCTION Development of rolling at controlled temperatures, i.e. finish rolling at a defined temperature and temperature control before transfer of products to the cooling bed, took place in bar rolling plants at the end of 1980 s. However, general studies of controlled rolling and related experiments leading to improvement in mechanical properties date back to 1950 s and 1960 s [1, 2]. The ever increasing customer requirements comprise a new trend beside the traditional demand for high-quality rolled material with specific mechanical and microstructural properties achievable through controlled rolling or cooling. It is a demand for delivery of bars or coils, which is the form of product preferred by a number of customers due to further processing requirements. In view of this fact, new Garret coiling line was installed and research into its use has been initiated at the university. The investigation was focused on wide-scale utilization of capabilities of the line in controlled rolling of SBQ-type steel for coiling and on optimization of controlled cooling processes which has major impact on final mechanical properties of the processed material. This includes optimization of finishrolling temperatures in relation to coiling temperatures and their influence on resulting properties of the material. At this initial stage of investigation, tests and experiments on site would be very ineffective due to required stoppages and resulting financial losses. For this reason, it was necessary to rely on laboratory experiments. Within certain boundaries, it is possible to accurately simulate the real-world conditions of rolling of steel bars. Simulations of controlled cooling of specified case-hardening steel grade were conducted in the TANDEM rolling mill installed at VŠB - TU of Ostrava. Case hardening is a thermochemical treatment process, whereby the surface of, typically, low-carbon steel becomes enriched with carbon from a solid, liquid or gaseous medium. The resulting carbon concentration in the case is either eutectoid or slightly hyper-eutectoid; normally between 0.7 and 0.9%. The purpose is to achieve high hardness and wear resistance of the surface of the part combined with tough core of the part [3]. The primary objective of the laboratory simulation was to determine the influence of the finish-rolling temperature and cooling in Garret coilers on microstructural and mechanical properties of the material. Microstructure of rolled products was examined by metallographic observation and tensile testing of the material at room temperature was performed. 2. EXPERIMENTAL Low-alloyed case-hardening 16MnCrS5 steel with carbon content of 0.182% was used for the experiment. The as-delivered material consisted in croppings from continuously-cast 150 x 150 mm sections. Specimens with h:16 mm, w:40 mm and l:130 mm were prepared for laboratory rolling. All specimens were heated to 1,100 C in an electric resistance furnace. Each specimen was rolled in reversing fashion in both two-high rolling stands of the TANDEM mill [4]. A workpiece with a thickness of 10 mm was produced by two roughing passes. The initial as-cast microstructure of the piece became refined by repeated static recrystallization. The following processing stage consisted in air-cooling of the product to a defined finish-rolling temperature and two consecutive passes. The average thickness of the final product was 6.4 mm. Total height reduction achieved in final rolling operations was therefore about 36%. During rolling and cooling, the surface temperature of specimens was measured by 2 pyrometers and 2 high-speed scan-thermometers. 2

3 Finished rolled products were cooled by 3 different methods: air-cooling at an average rate of 3.3 C/s slow cooling in a furnace at 600 C for 10 to 20 minutes (depending on the finishrolling temperature) immediately upon the final rolling operation a similar furnace cooling process applied upon cooling of the product down to coiling temperature by repeated passages through water sprays. An electric resistance furnace heated to 600 C was used for simulation of slow cooling of material in the Garret coiler. Holding times in furnace were calculated by simplified computer simulation of cooling of a steel workpiece of given size. After reaching 600 C, the specimen was removed from the furnace and air-cooled down to room temperature. Where needed, the specimen was carefully straightened in a press at about 500 C. Table 1. contains parameters of the rolling and cooling processes for specimens. The chart in Fig. 1. shows an example of rolling force F [kn] and surface temperature T [ C] values as recorded by a computer during water spray cooling and air cooling of rolled products. Table 1. Specimen processing parameters 16MnCrS5 Temperature Temperature Specimen Finishrollinrolling Cooling Specimen Finish- Cooling Coiling Coiling M M M M Air M M Air M M Rolling forces F [kn] Temperatures T [ C] Fig. 1. Temperatures and rolling forces measured in processing of M7 specimen (finish rolling temperature of 870 C / water spray / coiling temperature of 780 C) 3

4 2.1 Microstructure analysis On cooling, the rolled pieces were sectioned to obtain specimens for metallographic observation and for tensile testing. The microstructure was observed in longitudinal direction - on a vertical cross-section in the centre (half-width) of the product. Microstructure of air-cooled products consists of phases formed by hardening, ferrite grains and pearlite islands. The number and size of the latter increased with decreasing finish-rolling temperature, see Fig. 2. Microstructure formed during slow cooling in a furnace consisted of ferrite and pearlite. Lower finish-rolling temperature led to formation of finer grain but also to more pronounced banding, as shown in Fig. 3. and 4. The use of a water spray slightly reduced the banding and resulted in formation of finer grain. When coiling was preceded by application of a water spray, the impact of the coiling temperature on microstructural characteristics was indistinct. a) M8 finish-rolling at 980 C / air b) M9 finish-rolling at 870 C / air Fig. 2. Microstructure of air-cooled 16MnCrS5 steel specimens a) M3 finish-rolling / furnace b) M5 water spray cooling to 910 C / furnace 4

5 c) M6 water spray cooling to 870 C / furnace d) M10 water spray cooling to 840 C / furnace Fig. 3. Microstructure of furnace-cooled 16MnCrS5 steel specimens upon finish-rolling at 980 C a) M4 finish-rolling / furnace b) M7 water spray cooling to 780 C / furnace Fig. 4. Microstructure of furnace-cooled 16MnCrS5 steel specimens upon finish-rolling at 870 C 2.2 Mechanical properties Two specimens for room temperature tensile test according to standard ČSN EN were cut from every examined rolled piece in longitudinal direction. Measured values included yield strength R p [MPa], ultimate tensile strength R m [MPa] and elongation A 5 [%]. The results are shown in Table 2. The coiling temperature was selected as the independent variable for plotting the chart, as the influence of the finish-rolling temperature had not been identified due to the data scatter. The graph in Fig. 5. clearly shows that neither finish-rolling nor coiling temperatures within the ranges used had any notable impact on the strength of the material. However, this only applies to specimens cooled slowly in the furnace. Air-cooled products showed markedly higher yield and ultimate strengths and lower elongation in all cases. Plasticity properties of furnace-cooled specimens show slight dependence on the coiling temperature Tn. 5

6 Table 2. Mechanical properties of laboratory-rolled products of 16MnCrS5 steel Specimen Temperature Finish-rolling T d [ C] Coiling T n [ C] M MnCrS5 Cooling M M M M M Air M Air M Yield strength [MPa] Strength [MPa] Ductility [%] Fig. 5. Dependence of mechanical properties of rolled products of 16MnCrS5 steel on the coiling temperature 3. SUMMARY Physical modelling using TANDEM laboratory rolling mill at VŠB Technical University of Ostrava was employed for investigation of the effect of finish-rolling temperature and cooling in Garret coilers on microstructural and mechanical properties of selected 16MnCrS5 case-hardening steel grade. 6

7 Examined variables included the temperature of entry in the ASC (Automatic System Control) stand in the KJT (light section rolling mill), the coiling temperature and, additionally, the type of cooling of the rolled products (3 types of cooling). Heating of the rolled product in an electric resistance furnace served as simulation of slow cooling of the material in a Garret coiler. Holding times in a furnace were calculated by simplified computer simulation of cooling of a steel workpiece of given size. The microstructure of 16MnCrS5 steel upon slow cooling in a furnace consisted of ferrite and pearlite. Lower finish-rolling temperature led to formation of finer grain but also to more pronounced banding. When coiling was preceded by application of a water spray, the impact of the coiling temperature on microstructural characteristics was indistinct. Microstructure of air-cooled products consists of phases formed by hardening, ferrite grains and pearlite islands, the number and size of which increased with decreasing finish-rolling temperature. Mechanical properties measured by room temperature tensile tests are in line with the results of metallographic analysis of individual steels. An important finding was the fact that the achieved strength and plasticity can be correlated with sufficient reliability only with the coiling temperature ( C), as the effect of coiling temperature completely prevails over the effect of finish-rolling temperatures ( C). The strength of the steel upon simulation of slow cooling in the coiler was virtually independent of the coiling temperature. Elongation values show mild dependence on the coiling temperature with indistinct local maxima at about 870 C. Yield stress and ultimate tensile strength of the steel, however, considerably increased upon faster air-cooling, which was accompanied by a decrease in elongation in all cases. The results suggest that incorporation of slow cooling of rolled products in Garret coiler has substantial impact on microstructural and mechanical properties of the steels in question: causing an increase in plasticity and decrease in strength. The experiments were performed with the aid of technical equipment developed under the research plan no. MSM (Ministry of Education, Youth and Sports of the Czech Republic). REFERENCES [1] EHL, R., et al. Temperature - controlled rolling of long products - current state of the art. Stahl und Eisen, 2006, 126, č. 5, s. S13-S18. [2] TAMURA, I., et al. Thermomechanical Processing of High Strength Low Alloy Steels. London: Butterworth & Co. Ltd, [3] HLUCHÝ, M., MODRÁČEK, O., PAŇÁK, R. Strojírenská technologie 1-2. díl. Metalografie a tepelné zpracování Praha: Scientia, [4] 7