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O. HÍRE[ et al.: MECHANICAL PROPERTIES OF FORGINGS DEPENDING ON THE CHANGES IN SHAPE... Figure 6 Typical folds of ESR forgings Figure 7 Chemical microanalysis of folds in ESR forgings CONCLUSIONS The presented results show that the electroslag remelting forgings have significantly higher values of mechanical characteristics than the values of requirements recommended. Therefore steel produced in arc furnace and then refined by ESR technology follows to its conditions suitable for the designer to design products with better utility properties. An important benefit of ESR technology lies in plastic properties distribution homogenously through volume of forging. The plastic properties of ESR forgings are also significantly higher at high yield point level than the properties of forgings without ESR. This positive effect of ESR to steel forgings is due to distribution of non-ferrous inclusions in the steel. Forgings without ESR has inclusions segregated in cluster but this effect did not appear in ESR forgings. The refinement effect causes that some inclusions got stuck in the slug (a third of them according to analysis) and the rest, breaking off the slug, are segregated individually in the liquid. Individual segregation of inclusions does not affect mechanical properties of steel in a negative way if compared to inclusion segregated in clusters. REFERENCES 1 Híre{, O., Mimopecná rafinácia ocelí. In: Zborník Akademická Dubnica, 1999, pp. 219 2 Pernis, R., Teória tvárnenia kovov. TnU AD v Tren~íne, 2007, Tren~ín, pp. 104-110 3 Híre{, O., Mäsiar, H., Barényi, I.: Vplyv elektrotroskového pretavovania dlhých výkovkov z CrMoNi ocele na jej plastické vlastnosti. In: Zborník predná{ok z medzinárodnej vedeckej konferencie FORMING 2002, Luha~ovice 2002, I, pp. 113-115 4 Barényi, I. Dizerta~ná práca. TnU AD v Tren~íne, 2008, pp. 57-67 5 Ptá~ek, L. et al., Náuka o materiálu II. Akademické nakladatelství CERM, Brno, 2002 6 [tepánek A., Vydarený V, Electroslag remelting Cr-Ni-Mo alloy steels, In: Acta Metalurgica Slovaca, 3(1997), 23-28 7 Zábavník, V., Bur{ák, M., Materiál, tepelné spracovanie, kontrola kvality. EMILENA Ko{ice, 2004 8 Li~ková, M., Drsnost’povrchu plazmovo striekaných povlakov, In: Strojárstvo, MediaST s.r.o., 4(2008), 106-108 Note: English translation corrected by: Mária Igazova, TnU, Tren~ín, Slovakia 316 METALURGIJA 49 (2010) 4, 313-316
M. BUR[ÁK, J. MICHEL’ INFLUENCE OF THE STRAIN RATE ON THE MECHANICAL AND TECHNOLOGICAL PROPERTIES OF STEEL SHEETS INTRODUCTION Increasing of strain rate during forming of semi products or products is one of the ways of production intensification. Therefore an attention is paid to the study of the influence of the strain rate on the material behaviour in the deformation process, but also on the methodology of evaluation of formability at increased strain rates (including impact) 1,2. In general, it applies that increased strain rates result in increased strength characteristics of materials, while the yield stress is rising up more intensively than the tensile strength 3. Asaresult, increased strain rate results in an increased Re/Rm ratio, and for certain materials is ratio Re/Rm > 1. This fact significantly influences the formability (especially compressibility) of materials due to of localization of plastic deformation to “suitable” areas. Plastic deformation is characterized by the fact that its development is markedly non-homogeneous. The degree of non-homogeneity is a function of internal and external factors. The internal factors are internal mikrostructure of material, as follows: number and structure of phases, grain size and structure type. By in- Received – Prispjelo: 2009-09-18 Accepted – Prihva}eno: 2010-05-10 Preliminary Note – Prethodno priop}enje The paper analyses the influence of strain rate on the behaviour of un-alloyed steels with Re (yield strength) in the range of 210 … 550 MPa in the deformation process. It analyses the results of the influence of strain rate ranging from 10 –3 to 2,5·10 2 s –1 on the yield strength, the ultimate tensile strength (Rm), the elongation (A) and the reduction of area (Z). Achieved results of strain rate in relationship on values of Erichsen number IE are also given. By increasing of strain rate ranging from 10 –3 to 2,5·10 2 s –1 the ratio Re/Rm is increased, whereas it was observed more intensively for steels with the lower value of Re. By increasing of strain rate up to 1 s –1 are IE values of tested steels increased, whereas the ratio Re/Rm was equal 0,82. After exceeding of this strain rate was the ratio Re/Rm increased and IE value is remarkable decreased. Key words: Plastic deformation, Erichsen number IE, tensile tests Utjecaj brzine deformacije na mehani~ka i tehnolo{ka svojstva ~eli~nih traka. U ~lanku se analizira utjecaj brzine deformacije na pona{anje nelegiranog ~elika s Re (granica razvla~enja) 210 do 550 MPa tijekom deformacije. Motre se rezultati utjecaja brzine deformacije u rasponu 10 –3 do 2,5·10 2 s –1 na granicu razvla~enja, vla~nu ~vrsto}u (Rm) istezanja (A) i kontrakciju (Z). Dodatno se prikazuje utjecaj brzine deformacije na vrijednost Erichsenovog broja IE. Pove}anjem brzine deformacije u intervalu 10 –3 do 2,5·10 2 s –1 pove}ava se odnos Re/Rm i to intenzivnije za ~elik s niòm vrijedno{}u Re. Pove}anjem brzine deformacije do cca 1 s –1 pove}ava se IE ispitivanog ~elika pri ~emu Re/Rm = 0,82. Iznad te brzine odnos Re/Rm se pove}ava, a IE izrazito se smanjuje. Klju~ne rije~i: plasti~na deformacije, Erichsen broj IE, vla~ni pokus M. Bur{ák, J. Michel’ - Faculty of Metallurgy, Technical University of Ko{ice, Slovakia ISSN 0543-5846 METABK 49(4) 317-320 (2010) UDC – UDK 669.14-418:539.37:620.17=111 creasing of grain size and number of phases, non-homogeneity of plastic deformation significantly increased. The temperature, strain rate and the stress state are crucial external factors 4,5. The sensitivity of materials on strain rate during the forming process is, as it was mentioned earlier, a function of material, and therefore it is beneficial to analyze this sensitivity, especially for new developed materials intended for the cold forming. This is necessary in order to determine the limit state, as well as the properties of the final product. The main aim of the paper is to extend the knowledge and to mention on certain problems occurring during forming at increased rates (up to impact loadings). EXPERIMENTAL MATERIAL AND METHODS Experimental programme was realized on samples taken from stripes produced of un-alloyed high-grade and micro-alloyed cold rolled steels with yield stress ranged from 210 to 550 MPa. Cut-outs from the steel sheet and flat test specimens oriented in the rolling direction were machined out for the tensile test. The same procedure was applied for the samples used in Erichsen deep-drawing tests. METALURGIJA 49 (2010) 4, 317-320 317
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