The commercial technology of rolling 5,5 mm-diameter wire rod in Poland’s most modern rolling line has been examined within the study. The material used for the investigation was the 20MnB4 steel intended for subsequent cold working. From the performed analysis of the investigation results it has been found that the technology of rolling wire rod of cold upsetting steel, which is used currently in the Rolling Mill under examination, allows the production of finished products that can be deformed with a relative reduction of about 33 %. At larger plastic deformations,cracks occur in the material, which disqualifies it from further cold working

H. Dyja

K. Laber

M. Kałamorz

English

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415METALURGIJA 54 (2015) 2, 415-418K. LABER, H. DYJA, M. KAŁAMORZANALYSIS OF THE TECHNOLOGY OF ROLLING 5,5 MM-DIAMETER WIRE ROD OF COLD UPSETTING STEEL IN THE MORGAN BLOCK MILL Received – Primljeno: 2014-03-05Accepted – Prihvaćeno: 2014-09-30Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 54(2) 415-418 (2015)UDC – UDK 621.778:672:620.17:62.001.57=111K. Laber, H. Dyja, M. Kałamorz, Czestochowa University of Technol-ogy, Czestochowa, PolandThe commercial technology of rolling 5,5 mm-diameter wire rod in Poland’s most modern rolling line has been ex-amined within the study. The material used for the investigation was the 20MnB4 steel intended for subsequent cold working. From the performed analysis of the investigation results it has been found that the technology of rolling wire rod of cold upsetting steel, which is used currently in the Rolling Mill under examination, allows the production of finished products that can be deformed with a relative reduction of about 33 %. At larger plastic de-formations, cracks occur in the material, which disqualifies it from further cold working. Keywords: wire rod rolling, cold upsetting steel, mechanical properties, numerical modelling, thermovision exami-nationINTRODUCTIONMaintaining the leading position in the market by wire rod manufacturers is only possible by continuous broadening of their product range offer and enhancing the product quality in respect of mechanical and plastic properties and dimensional accuracy [1, 2].In the opinion of the authors of study [3], the exam-ined technological lines and the production technology employed in them do not currently allow the production of wire rod meeting the continually increasing demands of purchasers for the level and stability of its mechani-cal properties, i.e. the wire rod of quality achieved by the world’s leading manufacturers.Wire rod of cold upsetting steel is used for making fasteners, such as screws, nuts, bolts, etc. Rolled prod-ucts of these steels should be characterized by a uniform microstructure and stable mechanical properties, as well as high surface quality, tight dimensional toler-ances and good formability in subsequent cold plastic working. These steels should be easily weldable and should assure cheap manufacturing of products of com-plex shapes [3].The cold deformability of metallurgical products is determined in the upsetting test. Manufactured wire rod should cold deform with a strain not less than 0,5 and a relative specimen height reduction ratio after upsetting of 50 % [4]. In view of the constantly growing require-ments of purchasers, efforts should be made to improve the properties of produced wire rod.TEST PURPOSE AND SCOPE OF THE STUDYThe paper examines the industrial technology of rolling 5,5 mm-diameter wire rod in one of the most modern wire rod rolling lines enabling rolling at high speeds, which, for 5,5 mm-diameter wire rod, is 110 m/s. The material used for the investigations discussed in this paper was 14 m-long 160 mm-square side 20MnB4 [5] steel stock (concast slab), which was heated up to a temperature of 1 150 C ± 20 C in a stepper furnace. The band temperature distribution during the rolling process, as determined numerically and measured in in-dustrial conditions, was examined. For numerical mod-elling of the rolling process, which determined the band temperature values at individual rolling process stages, among other things, FORGE® [6], a software program relying on the finite-element method, was employed. The following initial and boundary conditions were as-sumed in the examination:  the coefficient of heat ex-change between the band and the rolls αroll = 3 000 W/m2K; the coefficient of heat exchange between the band and the air, αair = 100 W/m2K; the coefficient of heat exchange between the band and the water box wa-ter, αwater = 5 000 W/m2K; the friction coefficient, μ = 0,4; and the friction factor, m = 0,8.For the measurement of the rolled band temperature under industrial conditions, a ThermaCAM SC640 ther-movision camera supplied by FLIR Systems was used. For the processing of obtained results, the ThermaCAM Reporter™ software was employed. The following ob-ject parameters were assumed for thermovision exami-nations: emissivity, 0,82; the distance of the camera from the object, 3 m; ambient temperature, 20 °C; and relative humidity, 50 %.416  METALURGIJA 54 (2015) 2, 415-418K. LABER et al.: ANALYSIS OF THE TECHNOLOGY OF ROLLING 5,5 MM-DIAMETER WIRE ROD OF COLD...In addition, metallographic examinations were car-ried out within the study, which determined the average ferrite grain value. At the next stage of the study, selected mechanical properties of the finished product were determined us-ing the static tensile test.In addition, to establish the possibility of further cold working of the investigated material, cold upset-ting tests were carried out.INVESTIGATION RESULTSThe exact values of the average band temperature and surface temperature before and after individual roll-ing passes and the values of error between the values computed numerically and measured with the thermo-vision camera are shown in Table 1.Table 1  Computed and measured band temperatures in particular rolling standsStand No. Average band temperaturecomputed numerically,tavg. / °CLateral band surface temperature computed numerically,Tlat. / °CLateral band surface temperature measured in actual rolling process,Tlat. / °CError / %before standafterstandbefore standafterstandbefore standafterstandbefore standafterstand1Cont. medium-section mill1 131 1 120 1 086 1 028 1 083 - - -2 1 089 1 108 1 063 1 064 - - - -3 1 090 1 097 1 050 988 - - - -4 1 080 1 081 1 033 1 038 - - - -5 1 076 1 078 1 022 979 - - - -6 1 075 1 076 1 010 1 015 - - - -7 1 074 1 075 1 009 984 965 - 4,55 -8 1 068 1 069 1 014 1 019 - - - -9 1 069 1 072 1 015 1 007 - - - -10 1 071 1 076 1 022 1 029 - - - -11 1 080 1 080 1 028 1 028 - - - -12 1 082 1 084 1 042 1 048 - - - -13 1 086 1 088 1 044 1 051 - - - -14 1 084 1 086 1 056 1 062 - - - -15 1 088 1 089 1 057 1 061 1 046 - 1,05 -16 1 092 1 093 1 066 1 072 - - - -17 1 092 1 093 1 068 1 074 - 1 040 - 3,2719No-twist mill 921 925 872 890 840 - 3,81 -28 1 010 1 015 990 995 - 950 - 4,7429Reducing sizing mill 770 782 734 754 780 - 5,9 -32 810 799 781 781 - - - -Figures 1-2 show example band temperature distri-bution examination results, respectively, numerically computed and measured in the actual rolling process.The analysis of the band temperature distribution in the 5,5 mm-diameter 20MnB4 steel wire rod rolling process has found that the numerically computed tem-perature values correspond to the industrially measured values with high accuracy. Considering the fact that (NTM and RSM) rolling stands in the Morgan block mill are encased, Table 1 only shows the data at the inlet and outlet of individual (NTM and RSM) rolling units. It is found from the analysis of the data in Figure 2 that the average surface temperature of the wire rod as it moved through the STELMOR cooling line was about 750 °C.The least error between the numerically computed band temperature and the band temperature measured during the real rolling process was obtained in rolling stand no. 1, which amounted to 1,05 %. The largest er-ror between the numerically computed and the meas-ured band temperatures was obtained for the band up-stream the first stand of the RSM unit (stand no. 29) of the Morgan rolling line, which was 5,9 %. The differ-ence between the computed and the measured band temperature values might be caused by the occurrence of a large amount of water vapour in the measurement locations.Figure 3 shows example photographs of the micro-structure of finished wire rod. a) b)Figure 1  Distribution of band temperature upstream the rolling stand: a) numerically computed, b) measured using a thermovision cameraFigure 2  Thermogram of temperature distribution in the STELMOR cooling line a) b)Figure 3  Microstructure of the 20MnB4 steel wire rod: a) cross-section, b) longitudinal section (banding)417METALURGIJA 54 (2015) 2, 415-418K. LABER et al.: ANALYSIS OF THE TECHNOLOGY OF ROLLING 5,5 MM-DIAMETER WIRE ROD OF COLD...The average ferrite grain size values, calculated based on the microstructure photos taken at particular points on the cross-section of the 5,5 mm-diameter round wire rod (Figure 4), are given in Table 2.Table 2  Ferrite grain size distribution in the 5,5 mm-diameter round wire rodMeas. location Average ferrite grain size in the analyzed areaD / μm1 20,22 19,13 19,64 19,15 17,96 19,87 22,58 22,99 17,010 19,811 21,612 23,813 19,7Average value 20,2The metallographic examinations have shown that the obtained wire rod has a pearlitic-ferritic structure, and the mean ferrite grain size on the finished product cross-section is about 20 μm. The manufactured 20MnB4 steel wire rod has a banded microstructure that is characterized by the fer-rite and pearlite occurring in the form of alternately ly-ing bands. The formation of the banding might be caused by too low a rate of wire rod cooling, creating the possibility for the carbon to diffuse to a greater dis-tance, which is necessary for the formation of wide fer-rite bands. To reduce the banding, the band cooling rate should be increased, thus shortening the time in which the carbon diffusion can occur.  Steels with a banded structure exhibit a large anisotropy of plastic properties, which are poorer in the direction transverse to the roll-ing direction.At the next stage of the study, tests for the mechani-cal properties of the 20MnB4 steel wire rod manufac-tured following the rolling technology being currently in use. An example tensile test graph for the steel under examination is shown in Figure 5, while the average values of selected mechanical properties are summa-rized in Table 3.Table 3  Average values of selected mechanical properties of steel 20MnB4Yield point, Re / MPaTensile strength Rm / MPaElongation, A / %Reduction of area, Z / MPa358,21 527,02 20,1 72,53At the subsequent stage of the study, the ability of the examined steel grade to be further plastically worked was determined. To this end, a cold upsetting test was carried out. In accordance with the applicable stand-ards, cold upsetting wire rod should be characterized by a plastic strain of 0,5 and a relative specimen height re-duction ratio after upsetting of 50 %.A general view low-carbon steel specimens after cold upsetting up to the 2/3 of the initial height (a plas-tic strain of about 33 %) is shown in Figure 6a, while after upsetting up to the 1/3 of the initial height (a plas-tic strain of about 66 %), in Figure 6b.From the cold upsetting test it has been found that the currently used technology of rolling cold upsetting steel wire rod assures the finished product properties in the plastic strain range up to 33 %. At larger plastic strains, cracks occur in the material, which disqualifies it from further cold working. So, the 20MnB4 steel wire Figure 4  Cross-section of the 5,5 mm-diameter round wire rod with measurement points indicatedFigure 5 Example tensile test curve for steel 20MnB4 a) b)Figure 6  View of the specimens after the cold upsetting process: a) upsetting up to the 2/3 of the initial height; b) upsetting up to the 1/3 of the initial height418  METALURGIJA 54 (2015) 2, 415-418K. LABER et al.: ANALYSIS OF THE TECHNOLOGY OF ROLLING 5,5 MM-DIAMETER WIRE ROD OF COLD...rod does not meet the currently applicable standards for the ability to being further cold worked.At the final stage of the study, the examination of the dimensional accuracy of the obtained wire rod in re-spect of the currently applicable standards was carried out. The diameter of the wire rod manufactured accord-ing to the currently applicable technology was 5,6 mm. The achieved dimension conforms to the applicable standards, although it lies in the positive range of the permissible dimensional deviation. In line with the world’s industrial practice, efforts should be made to manufacture rolled products within the negative range of dimensional tolerance, which will make it possible to achieve considerable savings in met-al and a reduction in the mass of finished product [7].SUMMARYFrom the investigation carried out it can be found that:-  the band temperature values obtained from numer-ical modelling correspond with high accuracy to the band temperature values measured during the actual rolling process. This confirms that the initial parameters assumed for numerical modelling were determined and selected correctly; -  the presently manufactured 20MnB4 steel wire rod has a bandy pearlitic-ferritic microstructure, which adversely affects the properties of the finished product;-  the average ferrite grain size in the wire rod pro-duced according to the currently applicable rolling technology was about 20 μm;-  the yield point of the wire rod produced according to the currently applicable rolling technology was about 358 MPa, the tensile strength about 527 MPa, while the elongation about 20 % and the re-duction of area about 73 %;-  the currently used technology of rolling the 20MnB4 steel wire rod assures the plastic working of finished products at a strain of up to 33 %. At a larger plastic strain, cracks occur in the material, which disqualifies it from further working.The available technical literature lacks detailed stud-ies on technologies for rolling cold upsetting steel wire rod. Therefore, it becomes justifiable to develop new technologies for rolling these steel grades, which will assure finished product of the required microstructure and mechanical properties, i.e. wire rod of the quality achieved by the world’s leading manufacturers, to be obtained.AcknowledgementsThis scientific study was financed from the resourc-es of the National Research and Development Centre in the years 2013-2016 as Applied Research Project No. PBS2/A5/0/2013.REFERENCES[1] Dyja H., Sobczak K., Kawałek A., Knapiński M.: The analysis of the influence of varying types of shape grooves on the behaviour of internal material discontinuities during rolling, Metalurgija, 52 (2013) 1, 35-38.[2] Knapiński M., Dyja H., Kawałek A., Frączek T., Laber K.: Analysis of the microstructure evolution during thermo-mechanical treatment of the steel plates in grade X80-X100, Metalurgija, 52 (2013) 2, 239-242.[3] Danchenko V., Dyja H., Lesik L., Mashkin L., Milenin A.: Technologia i modelowanie procesów walcowania w wykrojach, Politechnika Częstochowska, Prace Dydakt-yczne Wydziału Inżynierii Procesowej, Materiałowej i Fizyki Stosowanej, Seria: Metalurgia Nr 28, Wydawnict-wo Wydziału Inżynierii Procesowej, Materiałowej i Fizyki Stosowanej Politechniki Częstochowskiej, Częstochowa 2002 r.[4]  Polska Norma PN-83/H-04411: Próba spęczania metali, Polski Komitet Normalizacji Miar i Jakości, the „Alfa” Standardization Publishers, Warsaw 1983.[5] Polska Norma PN-EN 10263-4:2004: Stal - Walcówka, pręty i drut do spęczania i wyciskania na zimno. Cześć 4: Warunki techniczne dostawy stali do ulepszania cieplnego, ISBN 83-243-3779-2, Polski Komitet Normalizacyjny, Warsaw 2004.[6] FORGE3®, Reference Guide Release 6.2, Sophia-Antipo-lis, November 2005.[7]  Lesik L., Mróz S., Dyja H.: Wytwarzanie prętów okrągłych w zawężonym zakresie tolerancji wymiarowej. The Con-ference: New Technologies and Achievements in Metallur-gy and Materials Engineering, Series: Metallurgy No. 19, Czestochowa, June 2001, pp. 65-68.Note:  The professional translator for the English language is Czeslaw Grochowina, Studio Tekst, Poland

Analysis of the technology of rolling 5,5 mm-diameter wire rod of cold upsetting steel in the morgan block mill

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