Formation of ferritic-bainitic structure with retained austenite in
Transkrypt
Formation of ferritic-bainitic structure with retained austenite in
ADAM GOŁASZEWSKI, JERZY SZAWŁOWSKI, WIESŁAW ŚWIĄTNICKI Formation of ferritic-bainitic structure with retained austenite in hypoeutectoid steel INTRODUCTION EXPERIMENTAL STUDY The very important aim of developing new grades of steel is to obtain a specific structure which guaranties a combination of high strength and high ductility. A newly developed family of Advanced High Strength Steels (AHSS) meets such requirements to a large extent [1÷4]. These include different multiphase steels such as: Dual Phase (DP), Complex Phase (CP), and Transformation Induced Plasticity (TRIP) steels [1], which give the possibility of obtaining good mechanical properties through manipulating with the type of phase, its volume fraction and spatial arrangement. DP steels are ferritic-martensitic steels, whereas CP steels consist of at least three phases amongst: ferrite, bainite, martensite and austenite. Annealing in the intercritical region plays an important role in the process of forming phase structure in CP and DP steels, since it enables to manipulate with the volume fractions of austenite and ferrite. Another interesting subgroup of AHSS family is steels with TRIP effect, in which carbon-enriched austenite is formed as a result of precise control of phase transformations [2]. The aforementioned phase is transformed into martensite during plastic deformation, which prevents necking deformation and premature rupture [3]. The presence of martensite in addition to ferrite (in the DP steels) results in limited formability when plastic forming methods are used [4]. Replacement of martensite with bainite in such grades of steels may increase this property [4]. In this paper, an attempt to produce a complex-phase steel with ferritic-bainitic structure with retained austenite was described. To form such a structure the process of annealing in intercritical region was used in order to obtain defined fractions of austenite and ferrite, followed by quenching with an isothermal holding in the bottom range of temperatures of bainitic transformation. It was assumed that the presence of retained austenite would increase ductility of steel due to the TRIP effect, whereas low-temperature bainite would provide a high strength due to the significant grain size reduction and limited carbides precipitation during transformation [5]. Such a type of bainite is obtained from austenite containing the adequate amount of carbon and silicon [5], during the process of annealing in the temperatures just above the MS temperature. The research was performed on 35CrSiMn5-5-4 steel which contained adequate amount of silicon, whereas the desired concentration of carbon was expected to be obtained through the intercritical annealing [6]. Steel was subjected to the appropriate heat treatment process, which would allow obtaining desired microstructure. After heat treatment the microstructure was carefully examined by means of light microscopy and transmission electron microscopy (TEM). Moreover, various mechanical tests were performed in order to characterise hardness, strength, ductility and impact strength. Chemical composition of the tested steel is shown in Table 1. The JMatPro [7] computer program was used to select the conditions for intercritical annealing in order to obtain specific phase composition and to achieve the desired carbon concentration in the austenitic phase. The studies of phase transformations occurring in steel and the preheat treatments were conducted with the use of Baehr DIL805L dilatometer on samples with the diameter of 3 mm and the length of 10 mm. The annealing time in the two-phase region was constant at all temperatures and was of 1 hour. The cooling was performed at the rate of 50°C/s with the use of compressed helium. Ac1 and Ac3 temperatures of 35CrSiMn5-5-4 steel were revealed during the heating process at 2°C/min (up to the standard) [8] and 0.28°C/min. The obtained microstructure was examined with the use of Nikon Eclipse MA200 light microscope and transmission electron microscopy (TEM – JEOL JEM1200). The stereological analysis of metallographic specimen was performed by the line secant method [9] in order to characterize the microstructure and the volume fraction of phases. The volume fraction of Vv() austenite was estimated on the basis of the measurement of martensite volume fraction in steel samples after thermal treatment. The samples used in mechanical tests (static tensile test and impact strength test) were annealed in the critical region in a gastight furnace in the nitrogen atmosphere, whereas their isothermal quenching was conducted in the Sn bath. The hardness was examined by Vickers method, with load of 2 kg. Tensile tests on the fivefold samples of 6 mm in diameter were carried out on the Zwick/Roell Z250 testing machine, using an extensometer with 25 mm base. Impact tests were performed on the samples with a "V" notch with the use of the Charpy method. Mgr Adam Gołaszewski ([email protected]), dr hab. inż. Jerzy Szawłowski, dr hab. inż. Wiesław A. Świątnicki – Wydział Inżynierii Materiałowej, Politechnika Warszawska MODELLING OF PHASE COMPOSITION IN THE INTERCRITICAL REGION BY COMPUTER SIMULATIONS In order to obtain a ferritic structure with a low-temperature bainite and retained austenite, a ferritic-austenitic structure with an expected carbon concentration in austenite must be formed at the initial stage of the heat treatment. According to JMatPro (Fig. 1a), the range of temperature for the intercritical region of 35CrSiMn5-5-4 steel is 738÷805°C. Changes in volume fraction of austenite and ferrite as a function of temperature are presented in Figure 1a and the changes in volume fraction of the phases occurring in steel are shown in Figure 1b. Table 1. Chemical composition of 35CrSiMn5-5-4 steel, wt % Tabela 1. Skład chemiczny stali 35CrSiMn5-5-4. % mas. C Mn Si Cr Ni Mo 0.35 0.95 1.3 1.31 0.14 0.018 Al Cu P S Ti V 0.04 0.15 0.01 0.01 0 0.006 Nr 2/2014 ___________________ I N Ż Y N I E R I A M A T E R I A Ł O W A ________________________ 121 Temperature in intercritical region, °C The changes in carbon concentration in austenite in the intercritical region are presented in Figure 1c. At the temperature of 756°C the volume fraction of ferrite is equal to volume fraction of austenite (~49.5%), the rest are carbides. Cementite present in steel dissolves entirely at 757°C, whereas MC and M7C3 carbides dissolve at 755°C and 773°C respectively (Fig. 1b). The only non-metallic inclusions that remain in the whole intercritical region are: manganese sulfide (MnS) and a small amount of titanium carbon sulfide Ti4C2S2. Their presence results from the occurrence of impurities. Complete dissolution of carbides and carbon saturation of austenite occurs therefore at 773°C. The carbon concentration reaches a maximum value of 0.51 wt. % (Fig. 1c) at the temperature of 757°C, in which cementite dissolves completely and the austenite volume fraction is equal to 50.4%. At the temperature of 773°C, in which all the carbides are dissolved, austenite volume fraction equals 76% and carbon concentration is still high (0.46%) as compared to the average carbon concentration in steel (0.35%). Thus, both temperatures: 757ºC – in which the cementite is completely dissolved and 773°C at which all the carbides are dissolved, may be considered to be used in order to obtain a mixture of ferrite and low-temperature bainite in 35CrSiMn5-5-4 steel. At these temperature a high amount of austenite with high carbon concentration may be obtained (Fig. 1c). EXPERIMENTAL DETERMINATION OF Ac1, Ac3 AND MS TEMPERATURES Temperature in intercritical region, °C Temperature in intercritical region, °C Fig. 1. JMatPro simulations for 35CrSiMn5-5-4 steel in the α + γ temperature range: a) phase composition, b) carbide volume fraction, c) carbon concentration in the austenite Rys. 1. Symulacje z programu JMatPro dla stali 35CrSiMn5-5-4 w zakresie temperatury współistnienia α + γ: a) skład fazowy, b) udział węglików, c) stężenie węgla w austenicie Temperature of annealing in the intercritical region is the vital parameter affecting austenite volume fraction in the structure of multi-phase steels. The intercritical region occurs between Ac1 and Ac3 temperatures. Therefore, it is crucial to determine such temperatures adequately. The graph (Fig. 2) presents the values of Ac1 and Ac3 temperatures for 35CrSiMn5-5-4 steel taken from different sources: literature data [10], dilatometric tests and computer simulations. According to the literature and standard dilatometric studies (with the heating rate equal to 2°C/min), Ac1 temperature is between 770 and 780°C. However, the dilatometric tests conducted at lower rate (0.28°C/min) than the standard rate and JMatPro simulations show that the Ac1 temperature is lower. Such decrease in Ac1 temperature may signify that the standard heating rate is too high and does not guarantee equilibrium conditions. The effect of heating rate on Ac1 and Ac3 temperatures was noticed also by Pawłowski [11]. Fig. 2. Comparison of Ac1 and Ac3 temperatures obtained from various sources. Dashed line indicates the annealing temperature before isothermal quenching process, selected for structural and mechanical testing Rys. 2. Zestawienie temperatur Ac1 i Ac3 uzyskanych z różnych źródeł. Przerywaną linią zaznaczono temperaturę wygrzewania przed procesem hartowania izotermicznego wybraną do badań strukturalnych i mechanicznych 122 ________________________ I N Ż Y N I E R I A M A T E R I A Ł O W A __________________ ROK XXXV Figure 2 also reveals the differences between the values of Ac3 temperature. The results obtained from dilatometric tests are higher than these presented in the literature or obtained from computer simulations. The increase in Ac3 temperature at low heating rates observed during dilatometric tests was surprising, since the drop was rather expected as in the case of Ac1 temperature. Such an increase may result from the fact that some subtle changes in length of the sample were observed in the dilatometric curve for the lower heating rate and they might have been unregistered at higher speed. The dashed line presented in Figure 2 indicates the annealing temperature (770°C) in the intercritical region, in which 56% of austenite was obtained, after one hour of annealing. According to the literature data and a standard procedure for determining the characteristic temperatures, that temperature is below or exactly at the same level as Ac1. It means that it would be impossible to obtain such amount of austenite. On the basis of dilatometric tests, the temperatures of the beginnings of martensitic transformation (MS) were selected. As expected, MS temperature decreases with the diminishing of annealing temperature in the intercritical region. Such drop results from an increase in the carbon concentration in austenite, and that is confirmed by the simulations (Fig. 1c and Fig. 3). The carbon concentration in austenite is the result of two parallel processes: carbides dissolution and austenite formation. Carbides dissolution for 35CrSiMn5-5-4 steel initiates at the beginning of austenite formation, so that a small amount of newly formed austenite is enriched in carbon from dissolved carbides (Fig. 1c). At higher temperature, in which the carbides are no longer present, the carbon concentration in austenite begins to decrease steadily until reaching the average concentration level for the investigated steel. This is only the consequence of an increase in austenite volume fraction in the intercritical region. This result confirms the validity of the adopted assumptions and the correctness of the simulations, which showed that the austenite was enriched in carbon to a level higher than the average one during the annealing in the intercritical region. Fig. 3. A change in MS temperature and carbon concentration in the austenite in the intercritical region, in relation to annealing temperature according to the JMatPro simulations Rys. 3. Zmiana temperatury MS i stężenia węgla w austenicie według JMatPro w funkcji temperatury wyżarzania w zakresie międzykrytycznym AUSTENITE VOLUME FRACTION IN STEEL In order to verify experimentally the austenite volume fraction in steel as a function of temperature, the measurements of the phase composition in the samples after the heat treatment were conducted, with the use of a light microscope. It was assumed that the austenite volume fraction is equal to the fraction of martensite areas visible in steel structure after cooling to a room temperature. The austenite volume fraction values obtained from the stereological analysis differ from the results obtained from the simulation, but their changes as a function of temperature exhibit the same tendency as in the case of simulations (Fig. 4). Only at the temperature of 760°C the values of austenite volume fraction are significantly different from that obtained by simulation. Such differences in the results obtained from the measurements and simulations may be caused by the fact that during the onehour annealing in the intercritical region the equilibrium state was not achieved. At the temperature of 760°C, only 5% of austenite in steel structure was obtained after annealing in the intercritical region. This temperature is close to Ac1 temperature, in which a large amount of carbides is present. However, at 770°C the amount of obtained austenite (56%) was similar to the one determined by the simulation for 760°C. Therefore, the temperature of 770°C was selected as the optimal one for the further studies. It can be remarked that even a slight change in the temperature such as five degrees may result in a large change in the austenite volume fraction in steel. This may cause some difficulties in control of the phase composition during the heat treatment. HEAT TREATMENT DESIGN The next step in the study was to design and perform a complete heat treatment, which would consist of the annealing in the intercritical region and of austempering. On the basis of the dilatometric tests, the temperature of isothermal annealing after quenching was chosen to be equal to 300°C which was higher of about 35 degrees than the MS temperature. In order to reduce the time of the whole process, the isothermal holding lasted 1 hour, which would be long enough to enable the transformation in 95%. Dilatometric tests confirmed that during cooling to a room temperature after isothermal quenching, martensitic transformation did not occur, which signified that the retained austenite enriched with carbon during the bainitic transformation was stable. Fig. 4. The amount of austenite in 35CrSiMn5-5-4 steel in the selected temperatures of intercritical annealing determined on the basis of the JMatPro measurements and simulations Rys. 4. Ilość austenitu w stali 35CrSiMn5-5-4 w wybranych temperaturach wygrzewania międzykrytycznego wyznaczona na podstawie pomiarów i symulacji z programu JMatPro Nr 2/2014 ___________________ I N Ż Y N I E R I A M A T E R I A Ł O W A ________________________ 123 MICROSTRUCTURE OF STEEL AFTER ISOTHERMAL TREATMENT In order to verify whether the designed heat treatment would lead to the desired phase structure, the microstructure observations were carried out with the use of light microscope and TEM. 35CrSiMn5-5-4 steel after the incomplete austenitization and isothermal quenching is homogenous and consists of: bainitic grains embedded in a ferritic matrix containing carbides (Fig. 5a). Carbides were observed only in ferrite, which means that they precipitate during the soft annealing process, and that they did not dissolve during the annealing in the intercritical region. It might have resulted from too low temperature of intercritical annealing for the carbides dissolution or from too short time of the annealing. The TEM observations showed that bainite grains were composed of the ferrite laths separated by thin austenite layers and are carbide-free (Fig. 5b). In fact, the diffraction patterns of bainite grains contained reflections coming only from ferrite and austenite. The carbide reflections were not present in diffraction patterns of bainite. These results confirm that in steel submitted to intercritical annealing followed by low temperature austempering it is possible to obtain a carbide free bainite containing retained austenite. The images of the microstructures obtained by light microscope (Fig. 6a and b) are alike. The differences may result from the etching process of such a complex structure. Fig. 6. Microstructure of steel after incomplete austenitization at 770°C for 1 hour and after isothermal quenching at 300°C for 1 h: a) microstructure of a dilatometric sample, b) microstructure of a sample for mechanical testing; LM Rys. 6. Mikrostruktura stali po niepełnej austenityzacji w 770°C przez 1 h i hartowaniu izotermicznym w 300°C przez 1 h: a) mikrostruktura z próbki dylatometrycznej, b) mikrostruktura próbki do badań mechanicznych; LM MECHANICAL PROPERTIES Fig. 5. Microstructure of steel after incomplete austenitization at 770°C for 1 hour and isothermal quenching at 300°C for 1 h: a) microstructure of a dilatometric sample, b) bainitic microstructure; TEM Rys. 5. Mikrostruktura stali po niepełnej austenityzacji w 770°C przez 1 h i hartowaniu izotermicznym w 300°C przez 1 h: a) mikrostruktura z próbki dylatometrycznej, b) mikrostruktura bainitu; TEM The mechanical properties of the examined steel after the intercritical annealing and quenching with isothermal holding were compared to the properties of steel samples obtained after two kinds of heat treatment: (A) full austenitization at 900°C for 1 h followed by quenching with isothermal holding at 310°C for 2 h, (B) conventional quenching and tempering, which was carried out in two stages: (1st) stage-quenching from 900°C in the oil followed by tempering at 700°C, (2nd) stage-quenching from 890oC in the oil and then tempering at 230°C [9]. The results of mechanical tests (Fig. 7) conducted on steel samples after the full austenitization and after austempering (treatment A) are similar to those obtained after quenching and tempering (treatment B). These results are significantly different from those obtained for steel after the treatment presented in this study with the annealing in the intercritical region (incomplete austenitization) and isothermal quenching at 300ºC for 1 h. The greatest differences are visible in terms of hardness (HV) and the yield strength (R0.2). Hardness and yield strength decrease about twice in comparison to the sample isothermally quenched from full austenitization and 124 ________________________ I N Ż Y N I E R I A M A T E R I A Ł O W A __________________ ROK XXXV Fig. 7. Comparison of the mechanical properties after isothermal quenching from incomplete and complete austenitization (A) and after quenching and tempering (B) Rys. 7. Zestawienie właściwości mechanicznych po hartowaniu izotermicznym z niepełnej i pełnej austenityzacji (A) oraz po ulepszaniu cieplnym (B) after quenching and tempering. Low yield ratio (R0.2/Rm – 0.56), as compared to the corresponding value of that parameter (0.79) in steels after the reference treatments (A and B), shows that the applied treatment should improve the formability. After such a treatment, the mechanical strength (Rm) is lower than the one for the isothermal quenching after the full austenitization and lower than the one for quenching and tempering. Nevertheless, it still remains at a high level (1200 MPa). Ultimate elongation (A5) for all three treatments is similar and equal to about 10%. The stress-strain curves are shown in the diagram (Fig. 8) for the samples after isothermal quenching with the full (a) and the incomplete (b) austenitization. The curves overlap for each of the treatments, which means that every one of them would lead to a homogeneous structure and to the similar properties. SUMMARY Fig. 8. Stress-strain curve after incomplete austenitization (a) and after full austenitization (b) Rys. 8. Krzywe rozciągania po niepełnej austenityzacji (a) oraz po pełnej austenityzacji (b) The ability to determine the phase composition is vital in terms of designing the heat treatments for the multiphase steels. The JMatPro computer simulations are an effective and useful tool for modeling the phase composition. By means of such simulations, it was possible to determine the conditions of the heat treatment for 35CrSiMn5-5-4 steel, what allowed to obtain the phase composition as it was planned. Further experimental studies confirmed the results of computer simulations to a large extent. The critical temperatures of the phase transformations as well as the thermal stability of carbides in steel are necessary for designing the heat treatments of complex phase steel. In order to design the annealing treatments in the intercritical region, the initial and final temperatures of austenite formation should be precisely determined. The results presented in this study proved that the standard measurements of Ac1 and Ac3 temperatures are insufficient. The results obtained from the JMatPro simulations and from the dilatometric tests carried out with a low heating rate (lower than recommended) turned out more realistic. These temperatures are only slightly different from the values obtained under standard conditions, which are sufficient for designing classic heat treatments after full austenitization, where the austenitization is always performed at the temperature at least several degrees above the Ac3 temperature. However, selection of the annealing temperature in the intercritical region requires much more precision. Nr 2/2014 ___________________ I N Ż Y N I E R I A M A T E R I A Ł O W A ________________________ 125 The investigations have shown that in 35CrSiMn5-5-4 steel a microstructure composed of bainitic grains, embedded in ferritic matrix, is formed during a heat treatment consisting of an intercritical annealing followed by the low temperature austempering. The bainite formed is carbide-free and is composed of thin ferritic lath separated by the retained austenite layers. The mechanical properties of investigated steel significantly vary in relation to the applied heat treatment. The steel after the isothermal quenching from the incomplete austenitization has low hardness and low yield ratio with the high mechanical strength in comparison to steel after isothermal quenching from the full austenitization and to steel after quenching and tempering. The application of newly-developed heat treatment leads to the interesting properties of steel different from the properties of steel after conventional thermal treatments. ACKNOWLEDGMENT The study was accomplished within the Structural Project “Nanocrystalline structure formation in steels using phase transformation” No. POIG.01.01.02-14-100/09 co-financed by EU, within the funds of the Operational Programme Innovative Economy, 2007÷2013 and supported by Warsaw University of Technology. Special thanks to Prof. Adam Grajcar for his valuable comments and to MA Dorota Pietrzyk for her help with translation. REFERENCES [1] Gronostajski Z.: Metalurgiczne, technologiczne i funkcjonalne podstawy zaawansowanych wysokowytrzymałych stali dla przemysłu motoryzacyjnego. Prace IMŻ 1 (2010) 22÷26. [2] Grajcar A., Opiela M., Griner S.: Rozwój struktury wielofazowej stali typu C-Mn-Si-Al-Nb-Ti ze wzrostem odkształcenia plastycznego na zimno. Inżynieria Materiałowa 1 (2011) 55÷61. 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Journal of Achievements in Materials and Manufacturing Engineering 54 (2) October 2012 185÷193. 126 ________________________ I N Ż Y N I E R I A M A T E R I A Ł O W A __________________ ROK XXXV