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•102s-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•102s-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.U članku se analizira utjecaj brzine deformacije na ponašanje nelegiranog čelika s Re (granica razvlačenja) 210 do 550 MPa tijekom deformacije. Motre se rezultati utjecaja brzine deformacije u rasponu 10-3 do 2,5•102s-1 na granicu razvlačenja, vlačnu čvrstoću (Rm) istezanja (A) i kontrakciju (Z). Dodatno se prikazuje utjecaj brzine deformacije na vrijednost Erichsenovog broja IE. Povećanjem brzine deformacije u intervalu 10-3 do 2,5•102s-1 povećava se odnos Re/Rm i to intenzivnije za čelik s nižom vrijednošću Re. Povećanjem brzine deformacije do cca 1 s-1 povećava se IE ispitivanog čelika pri čemu Re/Rm = 0,82. Iznad te brzine odnos Re/Rm se povećava, a IE izrazito se smanjuje

J. Michel’

M. Buršák

English

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M. BUR[ÁK, J. MICHEL’INFLUENCE OF THE STRAIN RATE ON THE MECHANICALAND TECHNOLOGICAL PROPERTIES OF STEEL SHEETSReceived – Prispjelo: 2009-09-18Accepted – Prihva}eno: 2010-05-10Preliminary Note – Prethodno priop}enjeINTRODUCTIONIncreasing of strain rate during forming of semiproducts or products is one of the ways of production in-tensification. Therefore an attention is paid to the studyof the influence of the strain rate on the material behav-iour in the deformation process, but also on the method-ology of evaluation of formability at increased strainrates (including impact) 1,2. In general, it applies thatincreased strain rates result in increased strength char-acteristics of materials, while the yield stress is rising upmore intensively than the tensile strength 3. As a re-sult, increased strain rate results in an increased Re/Rm ra-tio, and for certain materials is ratio Re/Rm > 1. This factsignificantly influences the formability (especially com-pressibility) of materials due to of localization of plasticdeformation to “suitable” areas.Plastic deformation is characterized by the fact thatits development is markedly non-homogeneous. The de-gree of non-homogeneity is a function of internal andexternal factors. The internal factors are internalmikrostructure of material, as follows: number andstructure of phases, grain size and structure type. By in-creasing of grain size and number of phases, non-homo-geneity of plastic deformation significantly increased.The temperature, strain rate and the stress state are cru-cial external factors 4,5.The sensitivity of materials on strain rate during theforming process is, as it was mentioned earlier, a func-tion of material, and therefore it is beneficial to analyzethis sensitivity, especially for new developed materialsintended for the cold forming. This is necessary in orderto determine the limit state, as well as the properties ofthe final product.The main aim of the paper is to extend the knowl-edge and to mention on certain problems occurring dur-ing forming at increased rates (up to impact loadings).EXPERIMENTAL MATERIAL AND METHODSExperimental programme was realized on samplestaken from stripes produced of un-alloyed high-gradeand micro-alloyed cold rolled steels with yield stressranged from 210 to 550 MPa. Cut-outs from the steelsheet and flat test specimens oriented in the rolling di-rection were machined out for the tensile test. The sameprocedure was applied for the samples used in Erichsendeep-drawing tests.METALURGIJA 49 (2010) 4, 317-320 317The paper analyses the influence of strain rate on the behaviour of un-alloyed steels with Re (yield strength) inthe range of 210 … 550 MPa in the deformation process. It analyses the results of the influence of strain rateranging from 10–3 to 2,5·102 s–1 on the yield strength, the ultimate tensile strength (Rm), the elongation (A) andthe reduction of area (Z). Achieved results of strain rate in relationship on values of Erichsen number IE are alsogiven. By increasing of strain rate ranging from 10–3 to 2,5·102 s–1 the ratio Re/Rm is increased, whereas it was ob-served more intensively for steels with the lower value of Re. By increasing of strain rate up to 1 s–1 are IE valuesof tested steels increased, whereas the ratio Re/Rm was equal 0,82. After exceeding of this strain rate was the ra-tio Re/Rm increased and IE value is remarkable decreased.Key words: Plastic deformation, Erichsen number IE, tensile testsUtjecaj brzine deformacije na mehani~ka i tehnolo{ka svojstva ~eli~nih traka. U ~lanku se analizira ut-jecaj brzine deformacije na pona{anje nelegiranog ~elika s Re (granica razvla~enja) 210 do 550 MPa tijekom de-formacije. Motre se rezultati utjecaja brzine deformacije u rasponu 10–3 do 2,5·102 s–1 na granicu razvla~enja,vla~nu ~vrsto}u (Rm) istezanja (A) i kontrakciju (Z). Dodatno se prikazuje utjecaj brzine deformacije na vrijednostErichsenovog broja IE. Pove}anjem brzine deformacije u intervalu 10–3 do 2,5·102 s–1 pove}ava se odnos Re/Rmi to intenzivnije za ~elik s ni`om vrijedno{}u Re. Pove}anjem brzine deformacije do cca 1 s–1 pove}ava se IE ispiti-vanog ~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 pokusISSN 0543-5846METABK 49(4) 317-320 (2010)UDC – UDK 669.14-418:539.37:620.17=111M. Bur{ák, J. Michel’ - Faculty of Metallurgy, Technical University ofKo{ice, SlovakiaFor static tensile tests a universal test machineINSTRON 1185 was used and this machine was used re-tooled with an Erichsen test fixture for the deep-drawingtests in the press tool velocity interval from 3,3·10–3m·s–1 to 1,10–1 m·s–1. Deep-drawing tests with speedpunch up to 2,5 m·s–1 were done by drop tower.EXPERIMENTAL RESULTS AND DISCUSSIONThe influence of strain rate on the basic mechanicalproperties of tested steel C4 is shown in Figure 1, whichindicates that increased strain rates result in increasedstrength properties, whereas the intensity of growth of Reis higher than of Rm. The dependence of the strength prop-erties on the strain rate for un-alloyed high-grade steels inthe range from 10–3 to 103 s–1 was described using para-metric equations, mostly in the form presented in 4,6,8,R R ke e  ln( /  )     0 0R R km m  ln( /  )     0 0where Re, Rm are the yield strength and the ultimate ten-sile strength at the given strain rate , Re0 and Rm0 , arethe yield strength and the tensile strength at the staticstrain rate (10–3 s–1).When the material is more homogeneous and there isthe lower amount of obstacles for dislocations move-ment, the more sensitive to the strain rate is. Figure 2shows the influence of the strain rate on the increment ofthe yield strength Re of C33 steel after various heattreatments.The as-quenched steel has the lowest sensitivity to ,because the martensitic makrostructure has the highestnumber of obstacles to dislocations movement, and theas-normalized steel has the highest sensitivity.Similarly, Figure 3 documents the influence of  onRe and Rm valuesof C4 and E500TS steels with differ-ent grain size.The grain boundaries are insuperable obstacles todislocation movement, therefore the finer grain causedthe more obstructions, and the steel is less sensitive tothe strain rate . In terms of assessment of formability,the Re/Rm ratio is the most important criterion.Test results realized on different steel grades con-firmed fact that by increasing of strain rate the ratioRe/Rm is also increased and the intensity of that increaseis a function of material. The lower amount of obstruc-tions for dislocations movement in material causes themore remarkable influence of . Figure 4 shows an ex-ample of influence of grain size to ratio Re/Rm for differ-ent strain rates.Analysis of achieved results of strength properties(Re, Rm) showed that un-alloyed high-grade steels dedi-cated for cold forming up to critical strain rate kr allo-cated the ratio Re/Rm < 1 and from the view of macro vol-ume up to that strain rate should be keeping the plasticstability till deformation responded to stress Rm.318 METALURGIJA 49 (2010) 4, 317-320M. BUR[ÁK et al.: INFLUENCE OF THE STRAIN RATE ON THE MECHANICAL AND TECHNOLOGICAL PROPERTIES...Figure 1 Influence of the strain rate  on mechanical pro-perties of steel C4.Figure 2 Influence of the strain rate on the increment ofstrength properties Re or Rm, after heat treat-ment, compared with the initial state ( =10–3s–1) of steel C33, after various heat treatments.Figure 3 Influence of the strain rate on the increment ofstrength properties ÄRe or ÄRm, compared withthe initial state at  =10-3 s-1 for various steelgrades.Figure 4 Influence of the strain rate on Re/Rm for varioussteel grades.The critical value kr is affected by internal structureof material and commonly should be stated that by in-creasing Re value also the value kr increase. Table 1shows values of Re, Rm, Re/Rm of tested steels at charac-teristic strain rates as well as the characteristics of itsstate (carbon content, content of micro-alloying ele-ments and grain size).Based on the Re/Rm ratio obtained from the static ten-sile tests, the tested steels can be divided into threegroups: steels with Re < 300 MPa and Re/Rm < 0,7, steelswith Re >300 MPa and Re/Rm > 0,7, and the third groupconsists by steels with Re > 500 MPa and Re/Rm > 0,8.Figure 5 shows the graphic relationships vs. A and vs.Z of tested steels. A decrease of the elongation with anincrease of value is only shown for steels with Re < 300MPa after exceeding of ratio Re/Rm > 0,82. Steels with ahigher yield stress maintain or even increase their elon-gation at ratio Re/Rm > 0,82.Results of Erichsen deep-drawing tests showed thatthe indentation depth up to the fracture of the indentedcup (IE) is actually the same for both used steel grades,although there are big differences in elongation A80. Thiscan be related to the changes in the stress distributionduring the Erichsen deep-drawing test on the one sideand during tensile test on the other.Basic information about the formability of the steelsheet can be obtained by tensile test. However, the com-pressibility of the sheet is affected by a number of fac-tors arising from the applied production technology1,2,6. Important characteristics of compressibility canbe obtained by the technological tests. In Table 2 aresummarized the results of Erichsen deep-drawing test(IE - is the Erichsen number) for different movement rateof punch knife.The influence of strain rate on the IE values is plottedin Figure 6. As can be seen that by increasing of theloading rate up to about v = 0,2 m·s–1 is a slight growth ofthe IE, then there is a slight decrease of the IE value.For the highest loading rate of 2,5 m·s–1 is that de-crease more remarkable. The loading rate of 0,2 m·s–1which marked the start of IE decrease, roughly corre-METALURGIJA 49 (2010) 4, 317-320 319M. BUR[ÁK et al.: INFLUENCE OF THE STRAIN RATE ON THE MECHANICAL AND TECHNOLOGICAL PROPERTIES...Table 1 Mechanical properties of tested steels at characteristic strain ratesTested steelsMaterialcharacteristics/s–110–3 2,5·10–1 1 2,5–102C4C < 0,04%d = 0,031 mmRe /MPaRe/RmA5 /%2100,65472750,8462900,83444000,9335E280GC < 0,04 %d = 0,009 mmRe /MPaRe/RmA5 /%2950,70303400,79273500,80254590,922H340LADNb, V < 0,1 %d = 0,008 mmRe /MPaRe/RmA5 /%3500,81263800,79264300,82265400,9424C33C < 0,33 %d = 0.012 mmRe /MPaRe/RmA5 /%4400,62274510,62264700,63266300,8027C33quenchedRe /MPaRe/Rm19100,8119100,8119100,8319350,81C33 quenched + tempered 300°CRe /MPaRe/Rm14800,9215250,9515400,9516001,0C33 quenched + tempered 550°CRe /MPaRe/Rm8200,908400,908450,909000,90S460 MCC < 0,07 % Nbd = 0,006 mmRe /MPaRe/RmA5 /%5370,86305500,86305920,86307000,8729E500TS C < 0,08 %, Nb,Tid = 0,0056 mmRe /MPaRe/RmA5 /%5400,84395400,84395500,8539,56400,9440Figure 5 Influence of the strain rate  on the elongationA5 and the reduction of area Z of various testedsteel grades.sponded to the strain rate 1 s–1. The experiments are ingood agreement with data reported in references1,6,7,9, declaring that up to the strain rate of about 1 s–1there is no significant decrease of steel sheet formabilityby the increase of the loading rate, and therefore in thiszone the traditional deep-drawing criteria can be applied.At this strain rate is ratio Re/Rm for H340LAD steel equal0,82. At strain rate of 2·102 s–1 is ratio Re/Rm = 0,9.During testing, but also during production by form-ing is deformation rate due to inhomogeneous process ofplastic deformation changing and instantaneous defor-mation speed of particular position is almost higher thanthe mean deformation strain rates.The outstanding decrease of IE value at impact load-ing (v = 2,5 m·s–1, 102 s–1) is caused due to differentfactors. We assume that the decisive factor is the in-crease of macro heterogeneity of plastic deformation ofthe cup by deformation rate increasing and also defor-mation localization into the critical parts of the cup. Thiscan results in a decrease of the total value of plasticity.Changes of the friction between the tool and sheetshould have an influence, too 9,10.CONCLUSIONSThe aim of the paper was to judge the influence ofstrain rate in the range from 10-3 to 2,5·102 s–1 on the me-chanical properties, with regards to the plasticity of un-al-loyed high-grade steels with the yield strength rangedfrom 210 to 550 MPa. Based on the analysis of experi-mental results obtained from a long time period and liter-ature sources, the following conclusions can be stated:– The resistance of material to plastic deformation isincreases with strain rate increasing, herewith thestrength properties of tested steels as well as ratioRe/Rm are increased.– The intensity of increase of ratio Re/Rm with an in-creasing strain rate is a function of the internalstructure of material. The intensity of increase inratio Re/Rm with an increasing strain rate is thehighest for steels with Re < 300 MPa, lower forsteels with Re < 500 MPa, and slightly for steelswith Re > 500 MPa.– The influence of the strain rate on the plasticitycharacteristics (elongation and reduction of area)is related to the ratio Re/Rm. Only steels with Re <300 MPa exhibit a decreasing elongation in thestudied strain rate interval, and it from the strainrate where ratio Re/Rm > 0,82. Steels with thehigher values of yield stress are keeping or in-creasing its elongation, respectively.– Movement rate of punch knife during the stampingprocess (Erichsen deep-drawing test) up to rateabout 0,2 m·s–1 what responds to average strain rateabout 0,1 m·s–1 causes the slight increase ofdeep-drawing. Exceeding of this rate leads to thedecrease of deep-drawing, whereas it was more in-tensive for steel sheet with the lower yield stress.– The decrease of Erichsen number was observed atstrain rate for ratio Re/Rm > 0,82.REFERENCES1 M. Bur{ák, I. Mamuzi~, Metalurgija, 46 (2007), 1, 37-402 J. Janovec, J. Ziegelheim, Rùst ú`itných vlastností u tenkýchautomobilových plechù, In.: Technológie ´99, STU Bratisla-va, 8.-9.9.1999, 3193 P. Veles, Mechanické vlastnosti a skú{anie kovov, AlfaBratislava, 19894 J. Michel’ Materiálové in`inierstvo, 3, 1996, 225 J. Elfmark, Plasticita kovù, V[B Ostrava, 19846 J. Michel’, E. ^i`márová, S. Oru`inská, Kovové materiály,37, (1999) 3, 191-1957 E ^i`márová, J. Miche¾, Acta Metallurgica Slovaca, 9,(2003) 90-968 E. ^i`márová, et al., Metalurgija, 43, (2004) 3, 211-2149 E. Spi{ák,E, et al., Výrobné in`inierstvo, Ko{ice, 200310 O. Hrivòák, E. Evin, Lisovate¾nos plechov, Elfa, Ko{ice,2004Acknowledgement: This work has been supported byAPVV Agency under No. APVV-0326-07.Note: The responsible translator for English language is PeterHornak, Slovakia320 METALURGIJA 49 (2010) 4, 317-320M. BUR[ÁK et al.: INFLUENCE OF THE STRAIN RATE ON THE MECHANICAL AND TECHNOLOGICAL PROPERTIES...Table 2 Results of Erichsen deep-drawing test for different movement rate of punch knifeMark of steelsv/m·s-13,3·10–3 8,34·10–3 1,6·10–2 2,0·10–1 2,5H220YDIE/mm12,5 12,6 13,0 12,7 11,2H340LAD 12,4 12,5 12,8 12,6 11,3H380LAD 12,4 12,5 12,6 12,5 11,5Figure 6 Influence of pressing tool velocity on Erichsennumber IE for investigated steel.

Utjecaj brzine deformacije na mehanička i tehnološka svojstva čeličnih traka

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