Rolling of round bar made of steel 32CrB4 was simulated physically. The interrupted torsion test led to the final microstructure with coarser grains, however, thanks to comparatively large dimensions of the sample this type of experiment was not so sensitive to the cooling rate of the final cooling as compression test. Too high temperature of the end of controlled cooling of the samples after compression test led to partial self-quenching of the investigated material. Decelerated cooling below the temperature of the start of martensitic transformation led in both applied types of compression tests to the parameters of the resulting microstructure and hardness closest to the parameters obtained after laboratory rolling

I. Schindler

J. Kliber

K. M. Čmiel

P. Kawulok

P. Unucka

R. Kawulok

S. Rusz

Rolling of round bar made of steel 32CrB4 was simulated physically. The interrupted torsion test led to the fi nal
microstructure with coarser grains, however, thanks to comparatively large dimensions of the sample this type of
experiment was not so sensitive to the cooling rate of the fi nal cooling as compression test. Too high temperature
of the end of controlled cooling of the samples after compression test led to partial self-quenching of the investigated
material. Decelerated cooling below the temperature of the start of martensitic transformation led in both
applied types of compression tests to the parameters of the resulting microstructure and hardness closest to the
parameters obtained after laboratory rolling.Web of Science53330229

Kawulok, Petr

Kawulok, Rostislav

Schindler, Ivo

Rusz, Stanislav

Kliber, Jiří

Unucka, Petr

Čmiel, Karel Milan

DSpace at VSB Technical University of Ostrava

299METALURGIJA 53 (2014) 3, 299-302P. KAWULOK, R. KAWULOK, I. SCHINDLER, S. RUSZ, J. KLIBER, P. UNUCKA, K. M. ČMIELCREDIBILITY OF VARIOUS PLASTOMETRIC METHODS IN SIMULATION OF HOT ROLLING OF THE STEEL ROUND BARReceived - Prispjelo: 2013-06-21Accepted - Prihvaćeno: 2013-12-15Original Scientifi c Paper – Izvorni znanstveni radISSN 0543-5846METABK 53(3) 299-302 (2014)UDC – UDK 621.77: 620.18: 620.178.15=111P. Kawulok, R. Kawulok, I. Schindler, S. Rusz, J. Kliber - VŠB – Tech-nical University of Ostrava, Faculty of Metallurgy and Materials Engi-neering, Czech RepublicP. Unucka - Material and Metallurgical Research Ltd., Czech RepublicK. M. Čmiel - Třinecké železárny a.s., Czech RepublicRolling of round bar made of steel 32CrB4 was simulated physically. The interrupted torsion test led to the fi nal microstructure with coarser grains, however, thanks to comparatively large dimensions of the sample this type of experiment was not so sensitive to the cooling rate of the fi nal cooling as compression test. Too high temperature of the end of controlled cooling of the samples after compression test led to partial self-quenching of the investi-gated material. Decelerated cooling below the temperature of the start of martensitic transformation led in both applied types of compression tests to the parameters of the resulting microstructure and hardness closest to the parameters obtained after laboratory rolling.Key words: laboratory hot rolling, metallographic analysis, torsion test, compression testsINTRODUCTIONSimulations, be it mathematical (based most often on the Finite elements method (FEM) [1,2]), or physical simulations are an efﬁ cient method for optimization of rolling technologies. It is possible to use for physical simulations the laboratory rolling mills or specialized plastometers [3-7]. Advantage of plastometers consists namely in their greater universality at selection of experi-mental conditions [8-11]. On the other hand advantage of the laboratory rolling mills consists in the possibility of use of larger samples, which entails possibilities of easier evaluation not only of structural, but also of mechanical properties of the investigated material [12,13].In December 2012 at the VŠB-TU Ostrava two de-vices were put into operation, which were unique within the Czech Republic: semi-continuous laboratory mill for rolling of round bars and hot deformation simulator HDS-20 made by the company DSI, equipped with the simulation module Hydrawedge II [14,15]. Both these devices complement suitably the torsion plastometer working in the MMR [16].The object of our works was comparison of results of physical simulations performed on both plastometers with the results obtained at laboratory rolling of the steel 32CrB4 on the reversible rougher of the semi-con-tinuous mill.EXPERIMENTAL PROCEDUREChemical composition of the steel 32CrB4, which is designated for manufacture of fasteners, was 0,334 C – 0,90 Mn – 0,193 Si – 0,014 P – 0,003 S – 1,21 Cr – 0,0295 Ti – 0,0035 B (in wt. %).The initial blank of the investigated steel with diam-eter of 52 mm was austenitized at the temperature of 1 443 K and then rolled by 8 passes to the ﬁ nal diameter of 20 mm. Rolling parameters are summarized in Table 1.Table 1 Rolling parameters No. of pass Equivalent strainStrain  rateDeformation temperatureInterpass time/ - / s-1 / K / s1. 0,14 1,8 1 4332. 0,24 2,3 1 373 163. 0,24 3,0 1 323 124. 0,27 3,3 1 273 135. 0,26 4,2 1 223 126. 0,25 4,3 1 173 147. 0,26 5,2 1 128 148. 0,26 5,4 1 083 18The ﬁ nal rolled product was cooled at the average cooling rate of 2,5 K·s-1 to the temperature of 873 K, and then cooled at the average cooling rate of 0,7 K·s-1 to the temperature of 773 K, which was determined by high-speed temperature scanners LAND.Torsion simulations of the described rolling process were performed on two types of cylindrical samples. The samples with the deformed part with dimensions Ø 6 × 50 mm are more suitable from the viewpoint of uni-formity of strain, however, in their case we achieved 5 times lower strain rate than at laboratory rolling. Shorter samples of the type Ø 6 × 10 mm made it possible to apply identical strain rates as at rolling.We performed on the simulator HDS-20 the plain strain compression tests (PSCT) on prismatic samples 300  METALURGIJA 53 (2014) 3, 299-302P. KAWULOK et al.: CREDIBILITY OF VARIOUS PLASTOMETRIC METHODS IN SIMULATION OF HOT ROLLING...Microstructure of the laboratory rolled product was formed by a mixture of ferrite and pearlite with slightly band arrangement. Islands of bainite occurred sporadi-cally. Average size of pearlitic blocks was approx. 14,5 µm. Laboratory rolled product had an average hardness HV1 = 244 ± 5.Due to the fact that torsion test is characterized by principal irregularity of strain across the sample cross-section, metallographic analyses were performed at the point of its representative radius, i.e. at the distance of 1 mm below the surface [19]. Microstructure of both twisted samples was formed by a mixture of ferrite and pearlite with minority occurrence of bainite – see Fig-ure 2a), 2b).We can see on the longer sample greater banding in the direction of twisting. Inﬂ uence of the applied strain rate on the resulting structure was not signiﬁ cantly mani-fested, since all reductions were due to their small mag-nitude connected with the static and not dynamic recrys-tallization (which can be derived from the shape of the stress-stain curves). This fact was conﬁ rmed also by the with the height of 10 mm, width of 15 mm and length of 20 mm (which are preferentially designated for multi-reduction simulations of forming processes [14,17]), as well as uniaxial compression tests (UACT) with use of cylindrical samples with dimensions Ø 10 × 15 mm (suitable namely for determination of material ﬂ ow stress [18]). In the ﬁ rst stage of plastometric tests the controlled cooling of the samples was terminated at achievement of the temperature of 773 K.The laboratory rolled product, as well as plastomet-rically formed samples were subjected to metallograph-ic analyses. At the testing of HV1 hardness we made always 5 indentures, from the results of which we cal-culated the average and standard deviation.DISCUSSION OF RESULTSMetallographic analyses were performed in the centre of vertical sections of the samples. Microstructure of the laboratory rolled product is documented in Figure 1.Figure 1  Microstructure of the laboratory rolled product (longitudinal section)a) length of the deformed part of 10 mmb) length of the deformed part of 50 mmFigure 2 Microstructure of samples after torsion testingdetermined average values of hardness, which were in the case of short sample HV1 = 295 ± 10, and in the case of long sample HV1 = 289 ± 19. Increase in hardness in comparison with the rolled product should be attributed to the occurrence of bainite. On the other hand greater coarseness of the microstructure (mean size of pearlitic blocks was 35 µm in the long sample, and 40 µm in the short sample) reduced relatively the measured hardness.The samples obtained by compression tests showed microstructure formed predominantly by the mixture of bainite and martensite with minority share of ferrite and pearlite – see Figure 3. In the case of PSCT the harden-ing components showed banding character. Noticeable occurrence of hardening components inﬂ uenced funda-mentally the hardness values (HV1 = 413 ± 80 after PSCT, or HV1 = 369 ± 60 after UACT), which did not correspond to the hardness of the laboratory rolling product.The reason of disagreement of structural and me-chanical properties of individual samples was the ge-ometry of the samples and cooling conditions after forming. After completion of the torsion test the sample is for certain time still surrounded by a quartz tube and also due to its larger dimensions it cools down relatively slowly, which spontaneously prevented an excessive 301METALURGIJA 53 (2014) 3, 299-302P. KAWULOK et al.: CREDIBILITY OF VARIOUS PLASTOMETRIC METHODS IN SIMULATION OF HOT ROLLING...creation of hardening components. In the case of com-pression tests we obtained samples with the thickness of 1,9 mm (PSCT), or 3,1 mm (UACT), which after com-pletion of the controlled cooling cooled down signiﬁ -cantly more quickly. Since it is possible to estimate the martensitic transformation start temperature by calcula-tion in the software QTSteel [20] to be for the given steel 628 K, it is evident that free cooling of the samples below the temperature of 773 K led to partial quenching of material.For this reason we performed the compression tests again under analogical conditions of forming, but with additional controlled cooling of the samples from the temperature of 773 K down to the temperature of 473 K at the cooling rate of 0,5 K·s-1 (determined from the evolution of cooling of the laboratory rolling product), in order to suppress the spontaneous hardening of mate-rial. Thus obtained sample after PSCT showed ﬁ ne-grained microstructure formed by the mixture of ferrite and pearlite with a very speciﬁ c (non-lamellar) mor-phology, which was completed by minority occurrence of bainitic bands – see Figure 4a. The resulting hardness of the majority share of microstructure of the sample was HV1 = 282 ± 12, and the mean size of pearlitic blocks was approx. 17 µm.In the case of the sample after UACT we obtained microstructure of similar composition, but with more dis-tinctly distinguishable pearlitic blocks with the mean size of 23 µm – see Figure 4b. Average hardness of this sam-ple was HV1 = 297 ± 21. Microstructure and hardness of the samples obtained by compression tests with compre-hensive mode of cooling approached already very close the properties of the laboratory rolling product.SUMMARYWith use of the selected plastometric methods we simulated hot rolling of round bars from the steel 32CrB4 on reversing stand of the laboratory rolling mill. It was veriﬁ ed that torsion test was for simple sim-ulations of high-temperature forming processes well us-able and that it provided resulting mechanical proper-ties of material quite similar to the properties of the relevant rolled product. The resulting microstructure is, however, markedly coarser. Inﬂ uence of the strain rate (in the model scale of 1:1 or 1:5) on results of the tor-sion tests was in this speciﬁ c case negligible, since the controlling structure-forming mechanism during form-ing was static, and not dynamic recrystallization inﬂ u-enced fundamentally by the strain rate.It was established that too high temperature of the end of controlled cooling of the samples after compres-sion tests  led to partial self-hardening of the investi-gated material, when very small ﬁ nal thickness of the sample, as well as its too rapid and to some extent ir-regular cooling played negative role. Decelerated cool-ing below the martensitic transformation start tempera-ture yielded in both applied types of compression tests to the parameters of resulting microstructure and hard-ness that were the closest to the given parameters after laboratory rolling.It was manifested, in the case of the investigated steel 32CrB4, that the cooling mode in its low-tempera-ture stage had fundamental inﬂ uence on the ﬁ nal prop-erties of the plastometrically formed sample. Larg-er samples have from this perspective relative advan-Figure 3  Microstructure of samples after UACT – controlled cooling to the temperature of 773 Ka) after PSCTb) after UACTFigure 4  Microstructure after compression testing – controlled cooling to the temperature of 473 K302  METALURGIJA 53 (2014) 3, 299-302P. KAWULOK et al.: CREDIBILITY OF VARIOUS PLASTOMETRIC METHODS IN SIMULATION OF HOT ROLLING...tage at control of the instant cooling rate. It means that for physical simulations and cooling of more bulky long rolled products the test by uniaxial compression is suf-ﬁ cient. Plain strain compression test appears to be more suitable for simulations of the hot rolled ﬂ at products and sections with smaller thickness [21], when only this test can generate a band arranged structure analogically to the longitudinal rolling.AcknowledgementsThis paper was created within the project CZ.1.05/2.1.00/01.0040 “Regional Materials Science and Technology Centre” within the frame of the opera-tion programme “Research and Development for Inno-vations” ﬁ nanced by the Structural Funds and from the state budget of the Czech Republic; and within the stu-dents’ grant project SP2013/111 supported at VŠB – TU Ostrava by the Ministry of Education of the Czech Re-public.REFERENCES[1] R. Fabík et al. Metalurgija, 51 (2012) 3, 341-344.[2] D.S. Qian et al. Transactions of Nonferrous Metals Society of China, 22 (2012) 2, s247-s253.[3] I. Schindler et al. High Temperature Materials and Proces-ses, 32 (2013) 2, 133-138.[4] J.-Q. Sun et al. Materials & Design, 32 (2011) 3, 1612-1616.[5] M. Knapinski et al. Metalurgija, 52 (2013) 2, 239-242.[6] J. Bořuta et al. Hutnické listy, 61 (2008) 1, 80-87.[7] S. Grosseiber et al. Steel Research International, 83 (2012) 5, 445-455.[8] M. Opiela, A. Grajcar. Archives of Civil and Mechanical Engineering, 12 (2012), 327-333.[9] B. Mirzakhani et al. Journal of Iron and Steel Research, International, 17 (2010) 3, 34-39.[10] R. Pernis, J. Kasala, J. Bořuta. Acta Metallurgica Slovaca, 15 (2009) 1, 5-14.[11] K.M. Banks, A .Tuling, B. Mintz. Materials Science and Technology, 28 (2012) 5, 536-542.[12] M. Adamczyk, D. Kuc, E. Hadasik. Archives of Civil and Mechanical Engineering, 8 (2008) 3, 5-13.[13] S. Chai et al. Materials Science and Engineering A, 588 (2013), 208-213.[14] I. Schindler et al. Hutnické listy, 66 (2013) 4, 13-17.[15] I. Schindler, P. Kawulok. Hutnické listy, 66 (2013) 4, 85-90.[16] P. Unucka, J. Bořuta, A. Bořuta. 20th International Confe-rence on Metallurgy and Materials, „Metal 2011”, Ostrava: Tanger Ltd, 2011, CD ROM, paper no 781.[17] S.T. Mandziej. Materials and technology, 44 (2010) 3, 105-119.[18] W.-X. Wu et al. Transactions of Nonferrous Metals Society of China, 22 (2012) 5, 1169-1175.[19] I. Schindler, J. Bořuta. Utilization Potentialities of the Tor-sion Plastometer, Silesian Technical University, Katowice, 1998, 104 pp.[20] Information on http://www.ita-tech.cz/.[21] I. Schindler et al. Hutnické listy, 66 (2013) 4, 61-66.Note:  The responsible translator for English language is B. Škandera, Dobrá, Czech Republic

Credibility of various plastometric methods in simulation of hot rolling of the steel round bar

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Credibility of various plastometric methods in simulation of hot rolling of the steel round bar

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