The article presents results of investigation of structure and properties of low-carbon steel grade P2-04BCH after application of ECAE at temperature of approx. 290C. The ECAE method leads to significant improvement of strength of investigated material. Investigation of structure was made by combination of TEM and FEG SEM together with EBSD. It was proven the ECAE method enables obtaining of ultra fine-grained ferritic structure formed by recrystallised grains with very low density of dislocations and with a small portion of spheroidised carbides, which occurred usually on the boundaries of ferritic grains.Članak prikazuje rezultate istraživanja strukture i svojstava niskougljičnog čelika kvalitete P2-04BCH nakon kanalno kutne ekstruzije (KKE) pri temperaturi od oko 290°C. Zabilježen je značajan porast čvrstoće materijala. Struktura je ispitana kombinacijom transmisijskog elektronskog mikroskopa (TEM), skenirajućeg elektronskog mikroskopa s emisijom polja (FEG SEM) te elektronskom difrakcijom (EBSD). Dokazano je da se jednakokanalnom kutnom ekstruzijom postiže izuzetno sitnozrnata feritna struktura rekristalizacijom zrna, uz vrlo nisku gustoću dislokacija i mali udio sferičnih karbida s rasporedom, obično na granicama feritnih zrna

L. Kander

M. Greger

M. Černý

V. Vodárek

English

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M. GREGER, V. VODÁREK, L. KANDER, M. ^ERNÝWORKING STEEL P2-04BCHBY EQUAL CHANNEL ANGULAR EXTRUSION (ECAE)Received – Prispjelo: 2008 -09 – 09Received – Prihva}eno: 2009 - 01 – 20Original Scientific Paper – Izvorni znanstveni radINTRODUCTIONEnhancement of strength properties of polycrysta-lline metallic materials with preservation of sufficienttoughness can be achieved by refining of grains [1, 2].Dependence between the grain size and the level of yieldstrength is described by the Petch–Hall relation y k d  01 2 (1)where y is flow stress, 0 and k are constants, d is grainsize.This relation can be used in extensive interval ofgrain sizes, up to several tens of nanometres 3. Thesearching of possibilities of efficient grain-refinementof structure of technical materials led to important modi-fications of technology of thermo-mechanical treat-ment, which enable obtaining grain size at the level ofseveral micrometres.The most efficient processes are the following: defor-mation induced ferritic transformation, dynamic re-crys-tallisation of austenite during hot deformation with sub-sequent 	 transformation, hot rolling at inter-criticalinterval of temperatures and dynamic recrystallisation offerrite after large hot deformation 3.Further refining of grain size requires, however, ap-plication of extreme value of plastic deformation of ma-terial. During last two decennia many methods were de-veloped that enable achievement of severe plastic defor-mation. Important position among them holds the equalchannel angular extrusion (ECAE) 4-6. Principle ofthis method consists of severe deformations of massivesamples realised by shear without change of cross sec-tion. The sample is pressed through a die, in which twochannels intersect, forming an angle usually of 90°.Pressing is made either at room or at increased tempera-ture. Equivalent deformation can achieve the value of 10or even higher.The most critical for development of microstructureand resulting properties of samples is above all numberof passes and selection of deformation route (manner ofturning of the sample after each pass). It was establishedfrom the analysis of shear characteristics at various de-formation routes that turning of the sample by 90° wasoptimal. Many works, dealing with optimisation of thelaboratory ECAE equipment, were published. Prom-ising modifications for production of ultra fine-grainedmassive semi-products in industrial practice have ap-peared 3, 4, 6, 7.The ECAE method makes it possible to obtain thegrain size of several hundreds of nanometres 8-11.Materials with sub-micron size of sub-grains/grains (d =METALURGIJA 48 (2009) 4, 263-266 263The article presents results of investigation of structure and properties of low-carbon steel grade P2-04BCH af-ter application of ECAE at temperature of approx. 290 °C. The ECAE method leads to significant improvementof strength of investigated material. Investigation of structure was made by combination of TEM and FEG SEMtogether with EBSD. It was proven the ECAE method enables obtaining of ultra fine-grained ferritic structureformed by recrystallised grains with very low density of dislocations and with a small portion of spheroidisedcarbides, which occurred usually on the boundaries of ferritic grains.Key words:nanomaterials, low-carbon steel, ECAE deformation, EBSDObrada ~elika P2-04BCH kutno kanalnom ekstruzijom (KKE). ^lanak prikazuje rezultate istra`ivanjastrukture i svojstava niskouglji~nog ~elika kvalitete P2-04BCH nakon kanalno kutne ekstruzije (KKE) pri tempe-raturi od oko 290 °C. Zabilje`en je zna~ajan porast ~vrsto}e materijala. Struktura je ispitana kombinacijom tran-smisijskog elektronskog mikroskopa (TEM), skeniraju}eg elektronskog mikroskopa s emisijom polja (FEG SEM)te elektronskom difrakcijom (EBSD). Dokazano je da se jednakokanalnom kutnom ekstruzijom posti`e izuzetnositnozrnata feritna struktura rekristalizacijom zrna, uz vrlo nisku gusto}u dislokacija i mali udio sferi~nih karbi-da s rasporedom, obi~no na granicama feritnih zrna.Klju~ne rije~i: nanomaterijali, niskouglji~ni ~elik, deformiranje jednakokanalnom kutnom ekstruzijom, povrat-no rasipaju}a elektronska difrakcijaISSN 0543-5846METABK 48(4) 263-266 (2009)UDC-UDK 669.14:539.377:620.17=111M. Greger, V. Vodárek, M. ^erný, Faculty of Metallurgy and MaterialsEngineering, V[B-TU Ostrava, Czech RepublicL. Kander, Material & Metallurgical Research Ltd, Ostrava, Czech Re-public0,1-1 m) are usually classified as ultra fine-grainedmaterials 3.The ECAE method was so far unsuccessful at at-tempts of obtaining nanometric materials, i.e. materialswith grain size under 0,1 m. Characterisation of thefraction of sub-grains formed by recovery and grainsseparated by high-angle boundaries, which are formedby re-crystallisation 11, is very important for under-standing the mechanisms of materials structure evolu-tion at application of methods of extreme plastic defor-mation. Definition of difference between sub-grains andgrains is not rigid. The values of 10-15° [1, 2 are usu-ally given as a critical misorientation angle. It is knownthat grains separated by high-angle boundaries havegenerally much more important influence on the level ofmechanical properties than sub-grains divided bylow-angle boundaries 2. In the area of grain size underapprox. 0,3 m the classical mechanism of plastic defor-mation by dislocation sliding is replaced by other mech-anisms. The most important causes of this phenomenoncomprise increasing surface of grain boundaries per unitof volume of material, decrease of dislocation densityinside the grains with grain size under 0,1 m, and local-isation of deformation into shear bands. Important prob-lems connected with development of ultra fine-grainedmaterials include in the first place lower level of plastic-ity non-homogeneity of structure across cross section ofthe pressed blanks and thermal stability of ultra fine-gra-ined structure at higher temperatures 3.Majority of works published until now dealt with ap-plication of the ECAE method on pure metals, whilemuch less attention was given to investigation of com-mercial steels 10. Our article summarises the resultsobtained at investigation of severe plastic deformationby the ECAE method on strength characteristics andstructure of a low-carbon steel P2-04BCH. Detailed in-vestigation of structure evolution was made not onlywith use of Transmission Electron Microscopy (TEM),but also by Scanning Electron Microscopy (SEM) incombination with Electron Back Scattered Diffraction(EBSD) 12, which enables characterisation of miso-rientation angles of individual crystallites on the surfaceof metallographic sections. At present if the Field Emis-sion Gun (FEG) is used, it is possible to obtain the spa-tial resolution of approx. 0,1m.Experimental material and techniqueInvestigation was made with the use of a commerciallow-carbon steel grade P2-04BCH. Table 1 gives itschemical composition.Table 1. Chemical composition of the steel P2-04BCH/ mas. %C Mn Si Cr Mo Ti B0,034 0,67 0,23 0,10 0,017 0,001 0,002The material supplied was in the state after free aircooling from the rolling temperature. Cylindrical samplesof dimensions 12×60 mm were manufactured from thisinitial material. The angle between the channels of the usedECAE die was 105°. This design made possible to reducedeformation resistance and it ensured good filling of dieedges 13. The samples were before the pressing re-heatedin the furnace to temperature of approx. 320 °C, tempera-ture of the ECAE die was approx. 290 °C. Deformationroute Bc was applied (turning of the sample after each passby 90° in the same direction), moreover the front end of thesample was replaced by the rear end of the sample. Thisdeformation route is considered generally as the quickestmanner of achievement of homogenous structure formedby equiaxed grains 14. The maximum number of realisedpasses through the ECAE die was 16.Samples for tensile test were prepared from individualdeformed samples. This test was made at room tempera-ture. For the purposes of structural analysis sections wereprepared perpendicularly to the longitudinal axis of thesamples after 4 (equivalent deformation  = 3,5) and 8 ( =7,1) passes through the ECAE die. Final polishing of thesamples for the SEM analysis was made with use of colloi-dal solution of SiO2 with granularity 0,05 m. Crystal ori-entation maps (COM), study on misorientation angles ofindividual sub-grains/grains and statistic evaluation ofgrain size was made by the apparatus Sirion 200 FEG SEMequipped with the HKL Technology Channel 5 EBSD sys-tem. Thin foils for the TEM were prepared perpendicularlyto the longitudinal axis of the samples from the approxi-mately ¼ of the diameter of initial samples. The foils wereprepared by electrolytic polishing in the solution contain-ing 5 % of HClO4 and 95 % of CH3COOH at the roomtemperature and voltage of 60 V. The TEM investigationwas performed on the microscope JEOL JEM 2100equipped with the PGT EDX analyser.EXPERIMENTALRESULTS AND DISCUSSIONMicrostructure and mechanicalproperties of steel in the initial stateResults of the tensile test of the supplied material atthe room temperature are given in Table 2.Table 2. Results of tensile test of the steel P2-04BCHRp0,2 / MPa Rm / MPa A / % Z / %281 355 31,5 72,5Microstructure of steel was formed by equiaxedgrains of ferrite, which were discontinuously decoratedby carbidic particles, see Figure 1. Small islands of de-composed ferritic-carbidic component were present in avery small quantity at the boundaries of ferritic grains.Small precipitates were observed also inside ferriticgrains. Average size of ferritic grains was approx. 35 m.264 METALURGIJA 48 (2009) 4, 263-266M. GREGER et al.: WORKING STEEL P2-04BCH BY EQUAL CHANNEL ANGULAR EXTRUSION (ECAE)Microstructure and mechanical propertiesof steel after application of ECAESevere plastic deformation of the investigated steel inthe ECAE die lead to significant enhancement of strengthproperties. Obtained results are shown in Figure 2.The biggest increase in strength properties wasfound after the first two passes. Next passes resultedonly in very gradual enhancement of strength parame-ters. After 16 passes even slight decrease of strengthproperties was already observed.Microstructure of the steel investigated after 4 ECAEpasses was non-homogeneous, original ferritic grainswere largely deformed. Deformed ferritic grains formedin elongated bands are clearly visible in Figure 3.TEM analysis proved the original equiaxed ferriticgrains were replaced by stretched sub-grains/grains ofvariable size. Sub-grains/grains formed usually elon-gated parallel bands, see Figure 4.EBSD results obtained on the sample after 8 ECAEpasses were processed in the form of crystal orientationmaps (COM), where the areas of various orientations onthe sample surface are discriminated by differentcolouring. The obtained results were further processedby computer as follows:– in the areas, where the misorientation angle of ad-jacent pixels was greater than 2°, the boundarieswere plotted. In this manner boundaries ofsub-grains were visualised, as well as boundariesof grains separated by high-angle boundaries.– in order to differentiate between the sub-grainsand grains the boundaries of grains were plottedonly in the areas, where misorientation of adjacentpixels exceeded 10°.– grain boundaries were plotted in the areas, wheremisorientation of adjacent pixels exceeded 20°.Map of crystal orientations (COM), shown in Figure5, documents a large quantity of differently orientedsub-grains/grains in the investigated area.Results of statistic processing of misorientation an-gles of sub-grains and grains in the sample after 8 ECAEpasses are shown in Figure 6. It is obvious thatsub-grains with the misorientation angle under 10°formed only in approx. 15 % of all ferritic grains. Thisconfirms the fact the majority of ferritic grains wasformed by the mechanism of re-crystallisation. No pref-erential occurrence of special boundaries was observedin the area of high-angle boundaries, such as e.g. twinMETALURGIJA 48 (2009) 4, 263-266 265M. GREGER et al.: WORKING STEEL P2-04BCH BY EQUAL CHANNEL ANGULAR EXTRUSION (ECAE)Figure 1. Microstructure of steel in the initial stateFigure 2. Results of tensile tests of material deformed inthe ECAE dieFigure 3. Microstructure of the sample after 4 ECAE passesFigure 4. Substructure of the sample after 4 ECAE passesboundaries 15-17. The biggest share of sub-grains cor-responded to the misorientation angles up to 4°.CONCLUSIONSThe results obtained from analysis of influence of se-vere plastic deformation by the ECAE method on struc-ture and properties of the low-carbon steel gradeP2-04BCH can be summarised as follows:– Deformation of investigated steel by the ECAEmethod at the temperature of approx. 290 °C led tosignificant improvement of strength properties.The decisive increase in strength was found afterthe first two passes through the ECAE die.– Deformation applied in 8 ECAE passes led to for-mation of ultra fine-grained ferritic structure with asmall fraction of globular carbidic particles, whichwere usually present at the boundaries of ferriticgrains. Density of dislocations inside ferritic grainswas very low. Majority of ferritic grains wasformed by the mechanism of re-crystallisation ofdeformed metallic matrix.– Sub-grains with an angle of misorientation under10° were formed after 8 ECAE passes only in ap-proximately 15 % of all ferritic grains.– Average size of ferritic grains with high-angleboundaries after 8 ECAE passes was 0,32±0,20m. However, the analysis could not include thegrains, the size of which was smaller than 0,1 m.In comparison with the as received state the grainsize was refined by two orders.This work was supported by the Ministry of Educa-tion, Youth and Sports of the Czech Republic in theframe of Research Program, Contract No. MSM6198910013.REFERENCES1 H. Gleiter: Deformation of Polycrystals: Mechanisms andMicrostructures, ed. N. Hansen et al. (Roslide, Denmark :Riso national Laboratory, 1981, 15.2 P. Fajfar, Metalurgija 45 (2006) 3, 239-241.3 J. Zrník, I. Mamuzi}, S. V. Dobatkin, Metalurgija 45 (2006)4, 323-331.4 R. Z. Valiev, Mat. Sci. 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[kandera, Dobrá, Czech Republic.266 METALURGIJA 48 (2009) 4, 263-266M. GREGER et al.: WORKING STEEL P2-04BCH BY EQUAL CHANNEL ANGULAR EXTRUSION (ECAE)Figure 5. Crystal orientation map, sample after 8 ECAEpassesFigure 6. Distribution of misorientation angles ofsub-grain and grain boundaries, sample after 8ECAE passes

Obrada čelika P2-04BCH kutno kanalnom ekstruzijom (KKE)

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