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ScienceDirect Materials Today: Proceedings 2S (2015) S627 – S630
International Conference on Martensitic Transformations, ICOMAT-2014
The effect of chromium on microstructure development during Q-P process H. Jirková*, L. Kučerová, B. Mašek University of West Bohemia, FORTECH, Univerzitni 22, 30614 Pilsen, Czech Republic
Abstract Martensitic microstructures with a controlled fraction of retained austenite have the potential to reach high tensile strength above 1900-2000 MPa with ductility higher than 10 %. These microstructures can be obtained by using the Q&P process. The Q&P process with integrated incremental deformation was tested and optimized for two high-strength steels 42SiCr and 42SiMn with 0.43 % C. The 42SiCr steel contained an additional 1.3 % of Cr. A suitable austenitization temperature was determined in the first step and then quenching and partitioning temperatures and cooling rate after deformation were optimized. The results suggest that the addition of Cr has a significant influence on the mechanical properties. Tensile strength of 1367 MPa with ductility A5mm = 7 % were obtained in 42SiMn without Cr (sample geometry: 2x1.5 mm, 5 mm active length). However, the Cr content of 1.33 % in the 42SiCr steel was responsible for the increase in its strength up to 1965 MPa and for the very good ductility A5mm =21 %. © 2014 The Authors. Published by © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations (http://creativecommons.org/licenses/by-nc-nd/4.0/). an open access under the license (http://creativecommons.org/licenses/by-nc-nd/3.0/). 2014. This Selection andisPeer-review underarticle responsibility ofCC the BY-NC-ND chairs of the International Conference on Martensitic Transformations 2014. Keywords: Q-P process; AHS steels; retained austenite; chromium; Si-Cr
1. Introduction The effects of alloying elements, such as carbon, manganese, silicon and, where relevant, aluminium and phosphorus, which stabilize retained austenite (RA) and delay pearlitic transformation and carbide precipitation, have been described in numerous studies [1-3]. Another alloying element which can play a substantial role in
* H. Jirková. Tel.: +420-377-63-8050; fax: +420-377-63-8052. E-mail address:
[email protected]
2214-7853 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations 2014. doi:10.1016/j.matpr.2015.07.362
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modern heat treatment methods that rely mainly on the martensitic transformation is chromium. Besides delaying bainitic and pearlitic transformation, chromium also improves hardenability and ultimate strength. It is therefore a suitable alloying element for steels treated by the Q&P process. Q&P processing is a heat treatment method which uses rapid cooling from the austenitizing temperature to a temperature between Ms and Mf followed by reheating and holding below or just above the Ms temperature [3,4]. Dutiny this holding time retained austenite is stabilize by carbon diffusion from the oversaturated martensite [5,6]. The resulting microstructure consists of martensite and film-like retained austenite [7]. 2. Experimental Programme The paper describes a Q&P process optimization involving two high-strength steels with a carbon content of 0.43 %. In order to refine grain and improve mechanical properties of the materials, incremental deformation was integrated into the process. The purpose of the deformation was to simulate forming operations in real-world processes and explore the effect of deformation energy on microstructure evolution and phase transformations. The proposed treatment schedules were carried out in a thermomechanical simulator which allows temperature and deformation processes to be controlled with precision. For exploring the effect of Cr on microstructure evolution and thus on mechanical properties, two experimental steels, 42SiCr and 42SiMn, were used. The only difference between them was their Cr content. The 42SiCr steel contained 1.33 % Cr (Table 1). The feedstock for making modelling specimens was in the form of forged and annealed bars. The microstructure of the 42SiCr steel with a hardness of 290 HV10 consisted of pearlite with a small fraction of ferrite. The 42SiMn steel, which contained no chromium, contained 60 % ferrite and its hardness was 266 HV10. The temperatures of phase transformations were found using the JMatPro software. The calculations showed that the absence of Cr is the cause of a higher Ms: 320 °C, instead of 298 °C in the Cr-containing steel. The progress of transformations during cooling is also substantially different (Fig. 1). Ferritic and pearlitic transformations shift to the left: towards higher cooling rates. The ferrite nose separates from the pearlite nose. This indicates that in order to obtain martensitic structure without Cr, higher cooling rates have to be used and the quenching and partitioning temperatures must be changed. Fig. 1. 42SiMn – CCT diagram constructed using the software program JMatPro.
Table 1. Chemical compositions of experimental steels (wt.%). C
Mn
Si
P
S
Cr
Ni
42SiCr
0.43
0.59
2.03
0.009
42SiMn
0.43
0.59
2.03
0.01
Cu
Al
Nb
Mo
Ms [°C]
Mf [°C]
0.004
1.33
0.011
Min.
0.07
-
0.008
0.03
0.03
298
178
0.07
0.07
0.008
0.03
0.03
320
202
Austenitizing temperature was the first key processing parameter which was sought. The treatment was performed in a furnace. A total of six temperatures from the 885-950 °C window were used for processing 42SiCr steel. In all cases, the resulting microstructure consisted of ferrite-free martensite. The hardness values were in the range of 723-763 HV10. In the 42SiMn steel, the temperature interval was wider: 800-1000 °C. Up to the soaking temperature of 900 °C, the final microstructures contained some proportion of ferrite. Hardness values were between 409 and 565 HV10. After the schedule with soaking at 1000 °C, traces of ferrite were scarce and the hardness reached almost 600 HV10. As part of the optimization of the Q&P process, the quenching and austenitizing temperatures and cooling rates were varied. The first schedule was based on previous results achieved with the 42SiCr steel. It consisted of austenitizing at 900/950 °C for 100 s [8, 9]. The austenitizing was followed by 20-step incremental deformation with the accumulated true strain of =5, which was applied within a 900-820 °C temperature interval. The deformation was followed by cooling down to the quenching temperature of 200 °C, reheating to the partitioning temperature of 250 °C and holding for 600 s (Table 2, Table 3, QP_01). In other schedules, a higher quenching temperature of 250 °C was used together with a higher cooling rate (QP_02). In addition, an increased austenitizing temperature of 1000 °C was combined with various quenching temperatures and cooling rates (Table 2, Table 3, QP_03 – QP_05).
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H. Jirková et al. / Materials Today: Proceedings 2S (2015) S627 – S630 Table 2. Q&P process parameters for 42SiCr steel. TA/tA QP_01 QP_04 QP_05
Def. steps
Def. interval (°C)
QT/Qt (°C/s)
Cooling rate (°C/s)
PT/Pt
Rp0.2
Rm
A5mm
HV10
RA
(°C/s)
(°C/s)
(MPa)
(MPa)
(%)
(-)
(%)
900/100 1000/100 1000/100
20 20 20
900-820 1000-850 1000-850
200/10 200/10 200/10
21 30 50
250/600 250/600 250/600
1220 1195 1230
1993 1965 1986
19 21 16
618 611 604
12 13 8
Table 3. Q&P process parameters for 42SiMn steel.
QP_01 QP_02 QP_03 QP_04 QP_05
TA/tA (°C/s)
Def. steps
Def. interval (°C)
QT/Qt (°C/s)
Cooling rate (°C/s)
PT/Pt (°C/s)
Rp0.2 (MPa)
Rm (MPa)
A5mm (%)
HV10 (-)
RA (%)
900/100 950/100 1000/100 1000/100 1000/100
20 20 20 20 20
900-820 950-720 1000-850 1000-850 1000-850
200/10 250/10 250/10 200/10 200/10
21 30 30 30 50
250/600 300/600 300/600 250/600 250/600
534 563 802 713 840
1204 1089 1200 1367 1300
16 20 10 7 16
368 264 358 443 441
5 5 4 4 10
3. Results and Discussion The first schedule QP_01 was based on the best of the previous results obtained with the 42SiCr steel. It was used for both steels (Table 2, Table 3). In the 42SiCr steel, martensitic structure with 12 % of retained austenite was obtained (Fig. 2). Its ultimate strength was 1993 MPa and the elongation reached A5mm = 19 % (Tab. 2). In the 42SiMn steel, which lacked chromium, the microstructure consisted of a majority of martensite and small fractions of ferrite and bainite. Ferrite was found along prior austenite grain boundaries. (Fig. 3). The retained austenite proportion was a mere 5 %. The ultimate strength was 1204 MPa and elongation reached A5mm = 16 %.
Fig. 2. 42SiCr – 900 °C/100 s – 250 °C/600 s – cooling rate 21 °C/s (QP_01).
Fig. 3. 42SiMn – 900 °C/100 s – 250 °C /600 s – cooling rate 21 °C/s (QP_01).
Fig. 4. 42SiMn – 1000 °C/100 s – 300 °C /600 s – cooling rate 50 °C/s (QP_05).
In order to eliminate ferrite from the microstructure of the 42SiMn steel, the cooling rate was increased to 30 °C/s (QP_02). With regard to the Ms level determined previously, the quenching temperature was raised to 250 °C. The resulting microstructure contained an even higher volume fraction of ferrite than in the previous case. This caused its strength to decrease to 1089 MPa. The elongation level was 10 %. When the austenitizing temperature was increased to 1000 °C and other parameters were retained, the strength became higher again: 1200 MPa (QP_03). Elongation, however, was below 10 %. On both steels (QP_04, QP_05), the combination of the reduced quenching temperature of 200 °C, the austenitizing temperature of 1000 °C and higher cooling rates were tried. In the 42SiCr steel, the altered parameters did not substantially alter the microstructure evolution or mechanical properties of the material. In the 42SiMn steel, both cases resulted in a considerable increase in hardness. The converted strength levels were 1367 and 1300 MPa. The cooling rate was then increased to 50 °C/s. The resulting microstructure was fully martensitic with no free ferrite even in the 42SiMn steel (Fig. 4). In order to exactly map the distribution of retained austenite, the microstructure was examined using JEOL JEM2010 transmission electron microscope. The microscope had a tungsten filament. An acceleration voltage of 200 kV was used. The resulting resolution was approx. 1.5 nm. The QP_1 specimen of 42SiCr was chosen for the
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examination. The retained austenite fraction found was 12 %. Selected area electron diffraction (SAED) was employed to seek the configuration of the austenite phase with the face-centred cubic lattice (FCC) in diffraction patterns. Using bright-field illumination, one can discern individual martensite needles which were growing from austenite grain boundaries and between the needles which formed earlier in the course of quenching. Twins were found in the transformed structure, showing that high-carbon twinned martensite was probably present locally (Fig. 5). Dark-field micrographs show that isolated austenite particles are probably present both along prior austenite grain boundaries and between martensite needles (¡Error! No se encuentra el origen de la referencia.). The width of these particles is on the order of tens of nanometres. An important finding is that in some locations retained austenite was detected in the form of scattered particles of varying size even within the matrix (Fig. 7). This finding can explain the good ductility of this steel after the Q-P process.
Fig. 5: 42SiCr – Twinned martensite structure (bright-field micrograph).
Fig. 6: 42SiCr – Detail of RA between martensite particles (dark-field micrograph).
Fig. 7: 42SiCr – Detail of RA dispersed in the microstructure (dark-field micrograph).
4. Conclusion Q&P process variables were optimized in the experimental programme for two high-strength steels which only differed in their chromium levels. The 42SiCr steel containing 1.33 % Cr was found to be suitable for treatment. It was not sensitive to changes in process variables. Martensite microstructure with a small proportion of bainite, with retained austenite and with a strength above 1900 MPa and A5mm elongation of more than 15 % can be obtained within a wide range of parameters. The 42SiMn steel without chromium addition offered narrow process parameter windows. Consequently, it was difficult to obtain ferrite-free martensite with austenite in the microstructure. When identical schedules as those for the 42SiCr steel were used, the resulting elongation was comparable but the strength was lower by 700 MPa. By increasing the austenitizing temperature, reducing the quenching temperature and accelerating the cooling process, the martensite structure with retained austenite as expected after the Q-P process was achieved, which translated into a strength of 1300 MPa at an elongation of 16 %. Acknowledgements This paper includes results created within the project GAČR P107/12/P960 and the project SGS-2012-040. The projects are funded from specific resources of the state budget for research and development. The TEM analysis has been supported by the SUSEN Project CZ.1.05/2.1.00/03.0108. References [1] S. Zaefferer, J. Ohlert, W. Bleck, Acta Mater. 52 (2004) 2765–2778. [2] B.C. De Cooman, Curr. Opin. Solid State Mater Sci 8 (2004) 285–303. [3] D.V. Edmonds, K. He, B.C. De Cooman, D.K. Matlock, J.G. Speer, Mater Sci Eng. A 438–440 (2006) 25–34. [4] T. Tsuchiyma, Mater Sci Eng. A 532 (2012) 585–592. [5] J. Speer, Acta Mater. 51 (2003) 2611–2622. [6] J. Speer, Mater Res. 8 (4) (2005) 417–423. [7] A.J. Clarke, Acta Mater. 56 (2008) 16–22. [8] V. Pileček, H. Jirková, B. Mašek, Adv. Mater. Res. 887–888 (2014) 257. [9] H. Jirková, J. Alloys Compd. 615 (2014) 163–168.
