UDK 669.018.254 Professional article/Strokovni članek ISSN 1580-2949 MTAEC9, 47(5)665(2013) INFLUENCE OF THERMOMECHANICAL PROCESSING ON THE COLD DEFORMABILITY OF LOW-CARBON STEEL VPLIV TERMOMEHANSKE PREDELAVE NA HLADNO PREOBLIKOVALNOST MALOOGLJIČNEGA JEKLA Besim Barucija1, Mirsada Oruc1, Omer Beganovic1, Milenko Rimac1, Sulejman Muhamedagic2 1Institute of Metallurgy "Kemal Kapetanovic" Zenica, University of Zenica, Travnička cesta 7, Zenica, Bosnia and Herzegovina 2Faculty of Metallurgy and Materials, University of Zenica, Travnička cesta 7, Zenica, Bosnia and Herzegovina miz@miz.ba Prejem rokopisa — received: 2012-08-31; sprejem za objavo - accepted for publication: 2013-01-29 The group of steels intended for the production of bolts and nuts from the production programme of the Zenica Steel Plant, Zenica, nowadays called Arcelor Mittal Zenica, includes, among others, the steel C8C according to EN 10263-2. In this paper, the results of testing the above-mentioned steel, with the nitrogen content of more than 0.007 %, manufactured within a specifically defined technological programme, are presented. The testing of the rolled bars with a diameter of 12 mm is focused, among the mechanical and metallographic tests, especially on the technological compression test in the cold state that is normally performed as an additional control test in the production. The cold compression of the test samples has been performed in four degrees of deformation and at two different speeds of compressing tools. We determined the dependence of the resistance to deformation of the material on the degree and speed of deformation. The surface inspection of the tested samples was performed with visual control with the aim to determine deformability of the bars produced in this way. Keywords: low-carbon steel, deformation testing, compression, visual control Skupina jekel, namenjena za izdelavo vijakov in matic iz proizvodnega programa Železarne Zenica iz Zenice, danes Arcelor Mittal Zenica, vsebuje med drugim tudi jeklo C8C, skladno s standardom EN 10263-2. V članku so predstavljeni rezultati preizkušanja omenjenega jekla z vsebnostjo dušika več kot 0,007 %, izdelanega po posebni tehnologiji. Preizkušanje valjanih palic premera 12 mm je poleg metalografskih in mehanskih preizkusov usmerjeno predvsem na tehnološki tlačni preizkus v hladnem, kar se navadno izvaja kot dodatni kontrolni preizkus v proizvodnji. Preizkus hladnega stiskanja vzorcev je bil izvršen s štirimi stopnjami deformacije in pri dveh hitrostih stiskanja. Določena je bila odvisnost med odpornostjo proti deformaciji materiala, stopnji deformacije in hitrosti deformacije. Ocena površine preizkušancev je bila izvršena z vizualnim nadzorom z namenom, da se ugotovi deformabilnost palic. Ključne besede: maloogljično jeklo, preizkus deformacije, stiskanje, vizualni nadzor 1 INTRODUCTION Aluminum is used as a deoxidizing agent during the production of low-carbon steels included in the standard EN 10263-2. This type of deoxidation creates favorable conditions for obtaining the steels with a high capability of being shaped in the cold state. Parallel to the process of binding oxygen to aluminum, aluminum also binds nitrogen in aluminum nitride (AlN) precipitates that, according to the known Zener mechanism of the grain boundary pinning,1 block the growth of the austenite grains in the process of static and dynamic recrystalliza-tion and during the thermomechanical rolling process. In the usual process of thermomechanical rolling, according to the original MORGAN technology for the wire rolling mill, we obtain the steel with a fine-grained microstructure (ASTM8-ASTM10). The microstructure of this type of steel is characterized with the increased values of the yield strength and with a high resistance to plastic deformation and decreased plasticity.2 Insufficient plasticity of this steel type is the result of strengthening through a reduction of the ferrite-grain size, the presence of aluminum in the crystal lattice and the presence of aluminum nitride precipitates. Sensitivity of this steel to cold deformation depends on the manner of nitrogen binding in the steel. Namely, if nitrogen is interstitially dissolved in the a-Fe crystal lattice during the process of natural aging, it will bind to iron in Fe4N. This is directly connected to an increased sensitivity of the steel to cold deformation. This sensitivity increases with an increase in the nitrogen content in the steel. But, if nitrogen is in the form of aluminium nitride then the sensitivity of the steel to cold deformation decreases. Particle precipitates and grain boundaries are obstacles for dislocation motion. With an increased number of these obstacles, the resistance of the steel to deformation increases and the plasticity of the steel decreases. Noncoherent precipitates are weaker obstacles for dislocation motion than coherent precipitates because dislocations, during their motion round the non-coherent precipitates, form Orowan loops around such precipitates.3 The formation of Orowan loops represents the initial reaction of the dislocation with the precipitates and a continuation of the plastic deformation, which is accompanied by a deformation strengthening, a further reaction. The external load to be applied to cause the initial reaction and to reach the yield stress represents a measure of precipitation hardening, while an additional increase in the load, allowing a continuation of the reaction represents a measure of deformation strengthening. 2 EXPERIMENTAL WORK Sixteen experimental heats with a weight of 120 t were produced in the LD converters. The chemical compositions of all the heats are within the permissible limits for the steel C8C according to the standard EN 10263-2 (Table 1). Table 1: Chemical composition according to standard EN 10263-2 Tabela 1: Kemijska sestava, skladna s standardom EN 10263-2 Steel Content of elements in mass fractions (w/%) C Simax Mn Pmax Smax Al C8C 0.060.10 0.10 0.250.45 0.020 0.025 0.0200.060 All the produced heats were hot rolled into the semi-products with the dimensions of 115 mm x 115 mm x 12000 mm, and then into the wires with the diameters from 6 mm to 12 mm using a MORGAN -USA wire rolling mill. Eight heats were processed according to the original technological prescriptions defined by MORGAN and the remaining eight heats were processed according to the corrected prescriptions related to the heating of semi-products and the corrected prescription related to the thermomechanical treatment (rolling and cooling). In the phase of the semi-product heating a longer stay of a semi-product in the walking-beam furnace was provided for by increasing the rolling rhythm from 13 s to 16 s, depending on the diameter of the rolled wires. A longer stay of the semi-products in the walking-beam furnace enables the coarsening of the aluminium nitride precipitates. By reducing the semiproduct temperature in the walking-beam furnace by 50 °C we reduced the possibility of a complete dissolution of the aluminium nitride precipitates in the matrix after achieving Ostwald ripening4 at a temperature above 1150 °C. Through an equable rolling rhythm it was provided that the final rolling temperature was between 950-980 °C with the aim of creating the conditions for the static and dynamic recrystallization of the austenite so as to ensure coarse grains in the structure after the phase austenite-ferrite transformation, which is more suitable for further cold plastic deformation. The reduction of the cooling speed after the final rolling pass by stopping the forced cooling and the reduction of the speed of the STELMOR conveyor from 25 m/min to 19 m/min had similar effects. A slow cooling of the rolled wire provides better conditions for the static recrystalli-zation and a growth of the recrystallized austenitic grains. Also, according to Beeghly,5,6 a lower cooling rate creates favourable conditions for a precipitation of aluminium nitride particles in the temperature interval of 950-750 °C in the steel matrix. In this way we prevented dissolution of the nitrogen in the ferrite crystal lattice that is a result of the natural aging process in the steel and of the intense deformation strengthening of the steel during the cold plastic deformation. The size of the secondary grain of the wire produced with the new manufacturing technology is between 6 and 8 per ASTM scale, which is 1 to 2 grades less than the size of the secondary grain of the wire produced with the original MORGAN technology. A microstructural analysis indicates that for both production technologies the ferrite content in the material is between 93 % and 95 % and perlite is between 5 % and 7 %, depending on the carbon content of each heat, in other words, if the carbon content is high the content of perlite is high too. The tensile properties of the hot rolled wires produced with both technologies (old and new) are in accordance with the prescribed values according to the standard EN 10263-2 for the steel C8C. The tensile-strength and yield-strength values for the wires produced with the new technology are lower by 12 MPa to 21 MPa in comparison with the wires produced with the old technology. The values for the elongation and reduction of the area of the wires produced with the new technology are higher by 2.8 % to 4.8 % in comparison with the wires produced with the old technology.7 8 3 RESULTS AND DISCUSION The testing of the steel plasticity in industrial conditions was carried out with a compression test in the cold state on the samples with a height of ho = 1.5do (do - diameter of a rolled wire). The final height was one-third of ho. The estimation of the sample surfaces after cold compression was conducted by using a rating of 0 to 3, where the ratings of 0 and 1 indicated a satisfactory surface and the ratings of 2 and 3 an unsatisfactory one. The shallow grooves that occur as rolling scars as well as the typical fiber surface cracks that mostly appeared on the test samples with a surface estimation of 0 and 1 are not considered as surface defects. The results of the estimation of the tested samples' surface condition after the cold compression for the old and new manufacturing technologies are given in Table 2. The test results clearly indicate that the ability for cold forming of the wires produced with the new technology increased much more than the ability of the wires produced with the old manufacturing technology. The compression test at room temperature in the laboratory conditions was carried out on a SINTECH 10/D (USA) tester at the Polytechnic University of Turin. We tested the rolled wires with a diameter of 12 mm made from the heats 350871 and 350866. The chemical compositions of these heats are given in Table 3. Table 2: Summary review of the estimation of the sample surfaces after the cold compression test of the rolled wires with the diameters of 6 mm to 12 mm, produced with the old and new technologies Tabela 2: Pregled ocene površine po hladnem stiskanju valjane žice s premerom od 6 mm do 12 mm, izdelane po stari in novi tehnologiji Technology Number of rolled coils for plasticity testing Estimation of the sample surface after the cold compression test (no, ni, «2, «3)/% Sum of the coils with the plasticity estimation of 0 and 1 Sum of the coils with the plasticity estimation of 2 and 3 0 1 2 3 no % «1 % n2 % «3 % n0 + n1 % n2 + n3 % Old 509 71 13.9 113 22.2 127 24.9 198 38.9 184 36.2 325 63.8 New 581 306 53.2 251 47.7 14 2.4 10 1.7 557 95.9 24 4.1 Note: no, ni, «2, «3 are numbers of defect classification Table 3: Chemical compositions of the tested heats Tabela 3: Kemijska sestava preizkušanih talin Heat C Si Mn P S Cu Cr Ni Al N w/% 350866 0.07 0.05 0.39 0.015 0.013 0.06 - 0.02 0.047 0.0114 350871 0.08 0.05 0.38 0.010 0.012 0.06 0.02 0.02 0.047 0.0112 The original height of the tested samples was h0 = 24 mm. It means that the ratio between the original height and the diameter (ho = 2do) was not standardized. The cold compression was carried out in four compression degrees and at two deformation speeds (0.5 mm/s and 1.0 mm/s). All the relevant parameters of the process that were measured or calculated are shown in Tables 4 and 5. These tables give the estimations of the tested sample surfaces according to the accepted criterions for industrial conditions with the aim of achieving estimation coherence.7 The degree of deformation is calculated according to the following equation: < = ln h„ h (1) F • h f A • h n N mm (2) where ho is the original height of a sample before the compression, h is the height of a sample after the compression. The resistance of steel to deformation is calculated according to the following equation: where F is the compression force (N), Ao is the original cross-section of a tested sample before the compression. The results of the plasticity testing conducted on the nonstandard samples (ho = 2do) at room temperature and with different degrees and rates of deformation indicate that the low-carbon steel, which deoxidized due to aluminum and was produced with the new thermomecha-nical process on the MORGAN wire rolling mill, is very plastic (Tables 4 and 5). We observed and recorded the defects with the ratings of o or 1, but the o rating is the dominant surface-condition estimation. At the compression-tool speed of 0.5 mm/s, which is the standard requirement, the 0 rating was recorded for the third and fourth deformation degrees, and at the tool speed of 1.0 mm/s, which is a nonstandard requirement, it was determined that the surface condition rating of 0 related to the second and third deformation degrees, while rating 1 related to the fourth degree of deformation. The ratings Table 4: Results of the compression testing at room temperature of the wires with a diameter of 12 mm from the heat 350866 for the testing speeds of 0.5 mm/s and 1.0 mm/s Tabela 4: Rezultati stiskanja pri sobni temperaturi za žico premera 12 mm iz taline 350866 pri hitrostih preizkusa 0,5 mm/s in 1,0 mm/s Sample number Average compression force Reduction of height Compression speed Time of compression Deformation rate Degree of deformation Average resistance to deformation Surface estimation after compression [Exist (+), Nonexist (-)] F/N Ah/mm v/(mm/s) t/s 70 ppm, Ph. D. Thesis, Univezitet u Zenici, 2010 8B. Baručija, M. Oruč, O. Beganovic, M. Rimac, Uticaj aluminijum-nitrida na hladnu deformabilnost niskougljeničnog čelika za izradu vijaka, IX Naučni/stručni simpozij sa melunarodnim učešcem "Metalni i nemetalni materijali", Zenica, BiH, 2012