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VÝVOJ ULTRAJEMNÍ STRUKTURY V OCELÍCH V PODMÍNKÁCH EXTRÉMNÍ DEFORMACE ECAP

DEVELOPMENT OF ULTRAFINE STRUCTURE IN STEELS IN CON DITIONS OF SEVERE PLASTIC DEFORMATION ECAP

Libor KRAUS a, Jozef ZRNÍK a,b, Sergey V. DOBATKIN c, George I. RAAB d

a COMTES FHT Inc., Prumyslova 995, 33441 Dobrany, Czech Republic, lkraus@comtesfht.cz

b West Bohemian University, Universitní 22, 30614 Pilsen, Czech Republic, jzrnik@comtesfht.cz

c Baikov Institute of MMS, RAS, Leninsky pr. 49, 105005 Moscow, Russia, dobatkin@ultra.imet.ac.ru

d IPAM, Ufa State Aviation Technical University, 12 K. Marx St., 450000 Ufa, Russia, giraab@mail.ru

Abstract

The article focuses on the results from recent experimental of severe plastic deformation (SPD) of low carbon steel AISI 1010 and medium carbon steel AISI 1045 performed at increased temperatures. The grain refinement of ferrite respectively ferrite-pearlite structure is monitored and described. While LC steel was deformed by ECAP die with a channel angle φ = 90° and by strain εef = 3. The ECAP severe deformation of MC steel was conducted with die channel angle of 120° and εef = 2.6 - 4) and TECAP - 400°C. The high straining in LC steel resulted in extensively elongated ferrite grains with dense dislocation network, and randomly recovered and polygonized structure was observed although. The small period of work hardening appeared at tensile deformation. On the other side, the warm ECAP deformation of MC steel in dependence of increased effective strain resulted in more progressive recovery process in deformed structure. In interior of the elongated ferrite grains the subgrain structure with dislocation network prevailed. As straining increases dynamic polygonization and recrystallization became active to form mixture of polygonized subgrain and submicrocrystalline grain structure. The straining and moderate ECAP temperature caused the cementite lamellae fragmentation and spheroidzation as number of passes increased. The tensile behaviour of the both steels was characterized by strength increase however the absence of strain hardening was found at low carbon steel. The favourable effect of ferrite-pearlite structure modification due straining was reason for extended work hardening period observed at MC steel.

Abstrakt

V příspěvku jsou presentovány nejnovější výsledky z oblasti aplikace extrémní deformace nízko uhlíkové a středně uhlíkové při zvýšené teplotě. Zjemnění zrna je monitorováno a progres ve vývoji feritické a feriticko-perlitické struktuře je charakterizován v příspěvku. Nízkouhlíková ocel (ČSN 12014) byla deformována s využitím ECAP úhlové kanálové metody s úhlem kanálů φ -90° p ři různých teplotách 250 a 300°C a 400°C a počtem přechodů N-3 a N4 (εef -3 a εef -4). Středně uhlíková ocel (ČSN 12045) byla deformována s využitím ECAP deformační metody s úhlem kanálů φ -120°, celkový po čet průchodů N- 4, 5, 6 (εef -2,6 -4) a teplota deformace TECAP - 400°C. Aplikování rozdílné hodnoty nap ětí resultovalo ve vytvořeni smíšené deformační struktury s různím podílem ultrajemných zrn. Použité nižší deformační napětí u nízkouhlíkové oceli způsobilo výrazně prodloužené zrna feritu (polygonizace), které při vyšších napětích byly fragmento- vány a výsledkem bylo vytvoření ultrajemných zrn feritu s výraznou dislokační hustotou v objemu zrn. Výsledkem byl jenom nevýrazný úsek efektu zpevnění na deformačních záznamech. V případe středně¨uhlíkové oceli, v závislosti od velikosti aplikovaného efektivního napětí εef a vyšší teploty deformace se výrazněji uplatnili při formování deformační UJ struktury procesy zotavení a rekrystalizace. Zvyšování εef přispělo také k intenzivnějšímu uplatnění dynamických procesů zotavení a rekrystalizace při vytváření subzrnné a UJ struktury feritu. V důsledku extrémní deformace a zvýšené teploty deformace byla také modifikována lamelární struktura cementitu, u které byly pozorovány změny s ohledem na fragmentaci a

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sferoidizaci lamel cementitu. Deformační chování ocelí, po aplikaci Intenzivní deformace, se projevilo zvýšením pevnostních charakteristik, které zvýšení resultovalo v absenci úseku rovnoměrného zpevnění na deformačních křivkách ve všech případech deformačních výsledků.

Keyword: Carbon steels, warm ECAP, structure analysis, recovery, mechanical properties

1. INTRODUCTION

The fabrication of bulk materials with ultrafine grains has attracted a great deal of attention over the past two decades because of the materials’ enhanced properties [1, 2]. In recent years a worldwide effort employing severe plastic deformation to obtain ultrafine grain structures in steels continues. In the past decade, a number of the various sever plastic deformation (SPD) techniques have been used to refine structure of metals and alloys. To introduce large plastic strain into bulk material, different deformation techniques were used such as ECAP high pressure torsion (HTP), accumulative roll bonding (ARB) and others. It is especially the ECAP generating interest among investigators since it is one of the methods of SPD used for metallic materials to produce nowadays already massive billets with ultrafine grained structure.

Very recently, significant interest has shifted to the use of warm and/or even hot severe deformation in order to produce more stable UFG microstructure [3]. With cold ECAP, low and medium carbon steels can only be pressed by two or three passes with channel intersection of 90° before initiation a failure of sample. The two to four passes successfully conducted with cold ECAP are insufficient and the achievable strain is insufficient to produce a completely refined grain structure [4]. To form stable ultrafine grain structure in metals (alloys), ECAP should be carried out at the temperature corresponding to the temperature of cold working [5].

The purpose of this work is to study the formation of submicrocrystalline structure in low and medium carbon steels subjected to large strain during warm ECAP pressing in dependence on varying temperature and effective strain εef.

2. EXPERIMENTAL PROCEDURES

In this work, two grades of commercial carbon steels with different carbon content were used for experimental. The chemical composition of both steels is as follows: low carbon steel AISI 1010, Fe- 0.1C-0.08Si-0.42Mn (in wt pct) and medium carbon steel AISI 1045, Fe-0.45C-0.23Si-0.63Mn-0.18Cr-0.043Al. Prior to ECAP pressing, AISI 1010 billets were soaked at temperature of 920°C for 1 hour, and billet o f AISI 1045 were soaked at the temperature of 960°C for 2 hours, followed by air cooling. The initial structures of steels experienced thermal treatment, as SEM micrographs, are presented in Fig. 1 and Fig.2. From the treated plates the cylindrical billets with initial diameter of 9 mm and length of 50 mm were cut off for the ECAP deformation The warm ECAP pressing of AISI 1010 steel was performed at two temperatures of 250°C and 300°C respectively, and each billet was pressed up to a total of N = 3 passes. The angle of intersections of the two channels φ was 90°. ECA pressing yielded the effective strain of ε ~3. The AISI 1045 steel billets were subjected to warm ECA pressing at T= 400°C and to higher number of passes, N = 4, 5 , 6 respectively. In this case the intersection angle of channels was of φ = 120°. Effective strain corresponding to one pass was εef ~ 0.67. In both ECAP experimental the route Bc was chosen. The sample was rotated 90° around its longitudinal axis between each pass, in the same direction. The heating of sample prior to pressing was done inside the pre-heated die until sample reached the pressing temperature of 300 and/or 400°C, The microstructural examination of thermally treated initial and also ECAP deformed samples was carried out by utilizing scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Thin foils for TEM observation were sliced normal to the longitudinal axis of ECAP pressed billets. The SEM and TEM micrographs were obtained by using JEOL JSM 6380 SEM operating at 10 kV and JEOL JEM 200FX TEM operating at 200 kV. ¨

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Tensile test were carried out using Zwick universal testing machine equipped with Multisens extensometer. Tensile specimens with gauge length of lo= 20 mm were tested at a constant cross-head speed of 0.016 mm/s until failure. The engineering stress-strain curves were constructed and evaluated with respect to structure changes.

3. EXPERIMENTAL RESULTS AND DISCUSSION

3.1 Microstructure of steel AISI 1010

The substructure of samples

subjected to warm ECAP at

temperature of 250°C and 300°C

investigated by TEM showed the

details of structure transformation.

Comparing effect of different ECAP

temperatures when exposed to N = 3

passes no substantial difference in

microstructure samples was

observed. The substructure analysis

provided the evidence on structure

formation, when both the grain

fragmentation and in-situ recovery,

due to increased ECAP temperature,

contributed to development of UFG structure. For the most parts, in deformed bars, the structure of parallel bands of elongated, formerly equiaxed, ferrite grains is present, Fig. 3. The substructure modification, as concerned the deformation heterogeneity, in bar was observed. High dislocation density and dislocation cells formed inside elongated grains are apparent.

At both ECAP temperatures in some elongated ferrite grains, dislocation activities can be related to progress in polygonization and preliminary nucleation of new subgrains. As temperature of ECAP increased, the tendency for development of submicrocrystalline structure becomes stronger, which can be attributed to local

Obr. 1 . Výchozí struktura nízkouhlíkové oceli ČSN 12010 po žíhání. Fig. 1 . Initial ferrite microstructure of low carbon steel AISI 1010 after solutioning.

Obr. 2. Výchozí microstruktura ferit -perlit oceli ČSN 12045 po tepelném žíhání.. Fig. 2 . Initial ferrite-pearlite structure of AISI 1045 steel after heat treatment.

10 µm

Obr. 3 . TEM mikrostruktura oceli ČSN 12010 po ECAPe při 250°C. Fig. 3 . TEM microstructure after ECAP at 250°C, N-3 passes.

Obr. 4 . TEM mikrostruktura oceli ČSN 12045 po ECAPe při 300°C. Fig. 4 . TEM microstructure after ECAP at 300°C, N-3 passes.

30 µm

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and/or in-situ dynamic polygonization and recrystalization. The more grown up and already equiaxed grains having high angle boundaries and lower dislocations density in grains are documented in Fig. 4. This polygonized structure may provide evidence that formation of well-defined grain boundaries would be attributed to a recovery and recrystallization process. The presence of net pattern in SAED confirms the presence of reasonable portion of boundaries having high angles of misorientation.

3.2 Microstructure of steel AISI 1045

The TEM microstructure of medium carbon steel after ECAP at 400°C are presented in Fig. 5, Fig. 6 and Fig. 7. Substructure analysis proved, when ECAP performed at channel angle of 120°, that deformation process was quite heterogeneous, what resulted in heterogeneity of structure across the bars as well. The areas of severe deformation where cementite lamellae fragmentation and dislocation network in ferrite is evident were observed next to polygonized structure in deformed ferrite grains.

Investigating the cementite deformation the lamellae were observed fractured and spheroidized, as it is shown in Fig. 5, however, locally in some areas the lamellar morphology of cementite was preserved. In deformed ferrite grains, which experienced less straining (εef ~ 2.7) dense dislocation network was often observed together with dislocation cell structure, which dominates. At higher straining εef ~ 3.5 (N-5 passes) and εef ~ 4 (N-6 passes) the subgrains appears more frequently due to polygonization processes and higher stored straining. Applying higher straining the formation of fine submicro grains, were already observed in deformed ferrite grains, as documented in Figs. 6 and 7. As ECAP straining increases the progress in dynamic polygonization proceeded and formation of submicron size grains can be observed in ferrite and also between cementite plates. On the other side straining is insufficient yet, as regards the structure transformation, because lamellar morphology of cementite was still observed, as documented in Fig. 7.

Evaluating structural changes in individual steels, resulting from different straining, the ultrafine grain structure formation in experimental steels, appears to be more effective at higher temperature, regardless the steel and steel constituents. From the observed structural characteristics The driving mechanism for ultrafine grain structure formation appears to be the dynamic polygonization in areas of severely deformed ferrite grains. With strain increasing dislocation substructure in ferrite grains easier transforms due stored

Obr. 5 . Vysoká hustota dislo- lokací v subzrnech feritu po ECAP, N-4. Fig. 5 . High density of disloca- tions in subgrains formed in ferrite after ECAP, N-4.

Obr. 6 . Vytváření zrn v de-formovaném zrnu feritu po aplikaci ECAP, N-5. Fig. 6 . Nucleation of grains in deformed ferrite after ECAP, N-5.

Obr. 7 . Nové jemné zrna v de-formovaném feritu po aplikaci ECAP, N-6. Fig. 7 . New ultrafine grains in former ferrite after ECAP, N-6.

500 nm 500 nm 200 nm

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deformation energy to polygonized subgrains and these to small grains having high angle boundaries. Considering the fact, that progress in structure refining is more evident at higher temperature and at higher strain, it at the same time indicates that to obtain more homogeneous submicron grained structure in both steels the higher straining is needed to be introduced. This limitation follows from the fact that still evident structure heterogeneity was found and especially in order to disintegrate the pearlite structure. On the other side the values of ductility and presence of work hardening period prove (see. Fig. 10) that improved combination of mechanical properties can be obtained.

3.3 Tensile properties of AISI 1010 steel.

The results of tensile testing at room temperature are shown in Fig. 8 for fully annealed condition and for ECAP deformed low carbon steel in Fig. 9. In case of the fully annealed condition, there is an extensive period of strain hardening and a high elongation to failure. The deformation behaviour of ECA deformed specimens is very similar for both specimens and the tensile strength is decreasing as ECAP temperature increases. As similar to other UFG materials, the tensile deformation behaviour of the UFG low carbon steel is characteristic by strength increase and absence of strain hardening. The region of strain hardening prior the softening period is quiet short but detectable and the period of uniform elongation is moderately increasing with increasing temperature of ECAP deformation. The decrease of the UTS for N-3 can be attributed to the more effective dynamic recrystallization process and to partial formation of submicrocrystalline microstructure.

3.4 Tensile properties of AISI 1O45 steel

As concerns the deformation behaviour of the medium carbon steel subjected to ECAP the tensile tests

Obr. 8. Deformační tahové závislosti oceli ČSN 12010 po normalizačním žíhání. Fig. 8 . Engineering stress-strain dependences for AISI 1010 steel after annealing.

Obr. 9 . Deformační tahové závislosti ocele ČSN 12010 po deformaci ECAP, N-3. Fig. 9 . Engineering stress-strain curves for AISI 1010 steel experienced N-3 ECAP passes

Obr. 10. Deformační tahové závislosti oceli ČSN 12045 po ECAP deformaci pro N 4, 5 a 6 přechodů. Fig. 10 . Engineering stress-strain curves of AISI 1045 steel experienced ECAP N-4, 5 and 6 passes

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records are shown in Fig. 10. For all specimens there is quiet extensive region of work hardening, after yielding, and a large elongation to failure as the introduced strain was increased in dependence of ECAP passes introduced. The deformation behaviour of this steel is noticeable different from that of the low carbon steel. Work hardening period for all specimens is evident and is contributing to uniform deformation of steel. At this time such deformation behaviour can be attributed to existence of subgrains and to larger volume fraction of submicrostalline grains in the structure. The more advanced dynamic recovery and dynamic recrystalization contributed as the main mechanisms, which actually contributed by good deal to transformation of deformed structure.

CONCLUSIONS

Microstructural evolution during warm ECAP was studied in low carbon steel AISI 1010 and medium carbon steel AISI 1045. The major results can be summarised as follows: 1. Warm ECAP of low carbon steel leads to formation of heavily deformed substructure consisting of

dislocation cells and subgrains. In the substructure, due to local recovery and polygonization process,

areas with submicrocrystalline structure are formed. Partly recovered plastic ability caused the

decrease of strength properties.

2. Microstructural observation of ECAP AISI 1045 medium carbon steel revealed the appearance of polygonized submicrocrystalline structure of high angle boundaries in large extent, which were formed due to effective activation of dynamical recovery and polygonization process in time of warm ECAP deformation. Formation of ultrafined polygonized structure recovered partially the plastic ability of ECAP steel without strength reduction.

ACKNOWLEDGEMENT

The present work was supported by the Ministry of Education, Youth and Sports of the Czech

Republic in frame of Research proposal the contract No. MSM 2631691901.

REFERENCES

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[3] De Hodgson, P., Hickson, M.R., R.K. Gibbs, R.K. Mater. Sci.Forum, (1998) roč. 63 p. 284-289.

[4] Dobatkin, S.V., Odesski, P.D., Pippan, R., Raab, G.I., Krasilnikov, N.A. Arsenkin, A.M., Russian Metallurgy (Metally), 1 (2004) p. 94-100.

[5] Dobatkin, S.V. “Severe Plastic Deformation of Steels: Structure, Properties and Techniques”, in Investigations and Applications of Severe Plastic Deformation, ed. by T.C. Love and R.Z. Valiev (Kluwer, Netherlands, 2000), vol. 3/80, s. 13-22.