INFLUENCE OF CHEMICAL COMPOSITION ON SPREADING OF STEEL AT HOT LONGITUDINAL ROLLING

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1 INFLUENCE OF CHEMICAL COMPOSITION ON SPREADING OF STEEL AT HOT LONGITUDINAL ROLLING Petr KAWULOK a, Stanislav CZERNEK a, Ivo SCHINDLER a, Rostislav KAWULOK a, Stanislav RUSZ a, Zdeněk SOLOWSKI b, Karel Milan ČMIEL b a VSB Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, 17. listopadu 15, Ostrava Poruba, Czech Republic, petr.kawulok@vsb.cz b Třinecké železárny, a.s., Průmyslová 1000, Třinec Staré Město, Czech Republic, zdenek.solowski@trz.cz Abstract At formation of pass rolls at section rolling mills the ability to predict with sufficient precision the ratio of elongation and spreading of material at its reduction plays a very important role. Perfect filling of the profile gauge depends on it, and thus also resulting dimensional accuracy of cross section of the final rolled product. The objective of the works was to determine, how the spreading is influenced by chemical composition of steel at otherwise constant conditions of rolling. The research was realised by rolling of prismatic samples on the laboratory rolling mill K350, namely on plain rolls at two temperature levels. At first it was necessary to optimise dimensions of the samples and adjustment of the rolling gap in such a way, that degree of spreading was maximised and measurements of dimensional changes of the samples was more precise. The necessary enhancement of engagement ability of the rolls was ensured by bevelling of the tapered part of the sample, which thus gained a semi-wedge shape. Samples from three dozens of steels, which differed significantly by their chemical composition, were rolled under otherwise identical conditions. The measured results led to a formulation of conclusions concerning the influence of rolling temperature and particularly of the selected elements on the degree of spreading of steels. Key words: Hot longitudinal rolling, spreading of steel, semi-wedge shape samples 1. INTRODUCTION Transverse flow of steel in the rolling gap causes an increase of the width of the rolled material, i.e. its spreading. Three types of spreading exist at rolling in dependence on the type of the roll: open, restricted and forced. The spreading itself helps to fill the gauges at rolling of the steel sections, but it supports also creation of additional tensile stresses and limits thus the formability of steel [1-3]. The course and magnitude of spreading is influenced by many factors comprising deformation, strain rate, height of the rolled product before and after rolling, the initial width of the rolled product, roll diameter, temperature, chemical composition, etc. Spreading of steel is an important characteristic for calculation of the rolling plan. When designing the roll passes at the rolling mills for rolling of sections a very important role is played by the ability to predict with reasonable accuracy the ratio of elongation and spreading of material at reduction. It influences the perfect filling of the gauge and in final result the shape accuracy of the cross section of the finished rolled product [4, 5]. For determination of spreading the distribution of steel and of other metallic materials it is possible to use mathematical modelling in the programs, which use the finite element method [6, 7]. Another option consists in simulation of real technological conditions physically on the laboratory rolling mills [8, 9]. The objective of the performed experiments was to evaluate the influence of chemical composition and temperature on the

2 degree of spreading at hot rolling of various types of low-alloyed steels, which are processed at the rolling mills of Třinecké železárny a.s. The laboratory rolling mill K350 was used for this purpose, which is installed in the Institute of modelling and control of forming processes at the VSB-TU Ostrava [10]. 2. EXPERIMENT DESCRIPTION Altogether 29 different steel grades and qualities were chosen for evaluation of the influence of temperature and chemical composition on the degree of spreading. Their approximate chemical composition and calculated carbon equivalent Ceq. are documented in Table 1. Table 1: Chemical composition of investigated steels Steel No. C Mn Si Pmax. Smax. Cu Cr Ceq At the preparatory phase of the experiment an initial shape of the test samples was optimised with respect to achievement of absolutely maximal possible spreading, which should in turn allow for more accurate evaluation and comparison between the achieved results. For investigation of spreading the deformation temperature of 850 C and 950 C were chosen, and therefore relatively high rolling forces were expected. The rolling mill K350 in a four-high configuration with diameter of working rolls of 64.5 mm was chosen for rolling of the samples. Its reduction capabilities are unfortunately limited and they were therefore first tested on prismatic specimens of different cross-sections. For this purpose the samples made of the steel No. 1 were used (see Table 1), in which one of the highest resistance to deformation was expected. From the perspective The chosen rolling stand from the perspective of energy and force parameters has coped even with high deformations, but it was necessary to improve the grip conditions by skewing the lead-in parts of the samples into a wedge shape. For this reason 4 samples were prepared from each of the studied steels

3 with a chamfer of the lead-in part into the wedge shape, which was followed by a prismatic part of the sample with a cross-section of 15 x 15 mm, as is can be seen in Fig 1. a) dimensions of the sample b) photo of the sample Fig. 1. Shape of initial sample The prepared samples were first individually measured and they were then rolled at a nominal roll gap of 5 mm on smooth steel rolls of the rolling stand K350 at the rate of their rotation of 80 min -1, after the direct reheating of material in electric resistance furnaces to the rolling temperature of 850 C or 950 C. Always 2 samples were rolled for each of the investigated steel and selected temperature and the achieved results were averaged. The rolled samples were again measured individually and these results were used as input data for further mathematical analyses. 3. MATHEMATICAL PROCESSING OF GAINED DATA AND DISCUSSION OF RESULTS Altogether 116 rolled products were rolled and then measured. For each of them the absolute deformation of the height h [mm] or of the width b [mm] was afterwards calculated: h h 0 h 1 (1) b b 1 b 0 (2) where h 0, b 0 [mm] is the height or the width of the prismatic part of the sample prior to rolling, and h 1, b 1 [mm] is the height or the width of the prismatic part of the sample after rolling. The degree of spreading of the rolled products from the given steel rolled at the chosen temperature was quantified by the relation b/ h [-]: b b h h 1(1) 0(1) b h 0(1) 1(1) b h 1(2) 0(2) b h 0(2) 1(2) (3) where b 0(1) and b 0(2) [mm] is the width of the 1 st or of the 2 nd sample from the given steel prior to rolling; b 1(1), and b 1(2) [mm] is the width of the 1 st or of the 2 nd sample after rolling at analogical temperature of deformation; h 0(1) and h 0(2) [mm] is the height of the 1 st or of the 2 nd sample (of its prismatic part) from the given steel prior to rolling; h 1(1) and h 1(2) [mm] is the height of the 1 st or of the 2 nd sample rolled at analogical temperature. Table 2 documents the obtained results of spreading of the rolled products.

4 Table 2: Degree of spreading for individual steels and investigated temperatures of deformation Steel No. b/ h C b/ h C difference [%] It follows from the obtained results (see Table 2) that the difference of spreading at both temperatures (when the values achieved at 950 C were chosen as the basis) is in most cases relatively small. The biggest differences (exceeding 10%) were observed in the steels Nos. 28, 25, 23 and 1. At the same time it cannot be said that spreading on average increases or decreases with the decreasing temperature different steels behaved in this respect differently. Our original plan was to relate mathematically the degree of spreading to the chemical composition of the steel via the simply quantified carbon equivalent, but unfortunately it turned out that the spreading is more or less dependent on all 17 analysed chemical elements. That's why a regression model was created with use of the statistical software UNISTAT 5.6 [G] for each of the investigated temperature level, which was constructed as a multiple linear regression in dependence on the contents of individual elements in wt. %: b 950 C C Mn Si Cr N P h S Cu Ni Al Mo W V Ti B Ca Nb b 850 C C Mn Si Cr N P h S Cu Ni Al Mo W V Ti B Ca Nb (4) (5)

5 Each simplification of the regression models (4) and (5) led to significant decrease of their accuracy, which is surprising namely in the case of the elements, the contents of which is practically constant (particularly W). Table 3 documents the excellent accuracy, with which we were able to describe by the developed mathematical models the degree of spreading for all 29 steel for the temperatures of 950 C and 850 C relative error of prediction based on these models is not greater than 5 %. Table 3: Comparison of experimental and predicted values of the degree of spreading for the temperatures of 950 C and 850 C Steel No. b/ h C experimental b/ h C model (4) relative error of the model (4) [%] b/ h C experimental b/ h C model (5) relative error of the model (5) [%] It follows from Tables 2 and 3 that at the temperature of 950 C the highest degree of spreading was achieved in the steels Nos. 5, 27 and 1, and on the other hand by far the lowest degree of spreading was achieved in the steel No. 23. At the temperature of 850 C it is possible to observe the maximal degree of spreading in the steel No. 25 and the minimal spreading in the steel No. 23.

6 4. CONCLUSIONS Laboratory rolling of specially designed wedge-shaped samples identified and quantified the differences in the degree of spreading of 29 different types of low-alloyed steels at the temperatures of 950 C and 850 C. The difference in spreading at both temperatures was in most cases relatively small. Only in the steels Nos. 28, 25, 23 and 1 the difference of the degree of spreading for the investigated temperatures was higher than 10%. It is impossible to state on the basis of the obtained results whether the spreading on average increases or decreases with the decreasing temperature. With use of the already previously used statistical software UNISTAT 5.6 regression models were developed for both deformation temperatures, enabling a very precise prediction of the degree spreading of the investigated steels in dependence on their chemical composition. The advantage of the model designed for the lower temperature of deformation is its ability to react for selected steels to the change of the phase composition. The obtained results and models cannot be mechanically applied to the operating conditions of any industrial rolling (especially when rolling in roll passes) due to specific geometric parameters of forming, but they provide valuable information of comparative type and can be thus very useful especially in at roll pass design. ACKNOWLEDGEMENTS This paper was created within the project No. CZ.1.05/2.1.00/ "Regional Materials Science and Technology Centre" within the frame of the operation programme "Research and Development for Innovations" financed by the Structural Funds and from the state budget of the Czech Republic; and within the students grant project SP2013/111 supported at VŠB TU Ostrava by the Ministry of Education of the Czech Republic. REFERENCES [1] ABO-ELKHIER, M. A modified method for lateral spread in thin strip rolling. Journal of Materials Processing Technology, 2002, Vol. 124, No. 1 2, pp [2] SASSANI, F., SEPEHRI, N. Prediction of spread in hot flat rolling under variable geometry conditions. Journal of Materials Shaping Technology, 1987, Vol. 5, No. 2, pp [3] LENARD, J. G. Primer on flat rolling. 1 st edition. Oxford: Elsevier, pp. [4] KOLLEROVÁ, M., et al. Valcovanie [Rolling]. 1 st edition. Bratislava: Alfa, pp. [5] ESTEBAN, L., ELIZALDE, M. R., OCAŇA, I. Mechanical characterization and finite element modelling of lateral spread in rolling of low carbon steels. Journal of Materials Processing Technology, 2007, Vol. 183, No. 2 3, pp [6] YEA, Y., et al. Prediction of spread, pressure distribution and roll force in ring rolling process using rigid plastic finite element method. Journal of Materials Processing Technology, 2003, Vol. 140, No. 1 3, pp [7] QIAN, D. S., HUA, L., DENG, J. D. FE analysis for radial spread behavior in three-roll cross rolling with small-hole and deep-groove ring. Transactions of Nonferrous Metals Society of China, 2012, Vol. 22, No. 2, pp. s247-s253. [8] VAŠEK, Z., et al. Determination of spread of low carbon steels by wedge rolling test. In: Forming 2005, Conference Proceedings. Ostrava: VŠB-TU Ostrava, 2005, pp [9] KAWULOK, P., et al. Influence of finish-rolling conditions on microstructure and mechanical properties of low-alloy Mn-Ni-Cr-Mo steel grade. Key Engineering Materials, 2011, No. 465, pp [10] [11] SCHINDLER, I., et al. Phenomenological model of hot deformation resistance of the C-Mn-V-B-type HSLA steel. In: Metal 2012, Conference Proceedings. Ostrava : Tanger Ltd, 2012, pp