Int J Fract (2011) 168:259–266 DOI 10.1007/s10704-010-9573-7
© Springer Science+Business Media B.V. 2010
LETTERS IN FRACTURE AND MICROMECHANICS
THE INFLUENCE OF VANADIUM MICROALLOYING ON VOIDS OCCURENCE IN LOW-ALLOYED Cr-Mo STEELS AFTER CONTINUOUS CASTING Aleš Hendrych1, Miroslav Kvíþala2, Vladimír Matolin3, OndĜej Životský1 and Petr Jandaþka1 1 Department of Physics, VŠB-Technical University of Ostrava, 17. listopadu 15/2172, 70800 Ostrava, Czech Republic e-mail:
[email protected] 2 Department of Materials Engineering, VŠB-Technical Univerzity of Ostrava, 17. listopadu 15/2172, 70800 Ostrava, Czech Republic 3 Department of Surface and Plasma Science, Charles University in Prague, V Holešoviþkách 2, 180 00, Prague 8, Czech Republic Abstract. We discuss the correlation between segregation of carbide forming elements (vanadium) and void initiation and propagation in low-alloyed Cr-Mo steels. The internal defects are created during blooms straightening in radial type of casting machine due to strain deformation field in the temperature range characteristic for vanadium nitride, carbide or carbonitride precipitation. Based on the statistical analysis (1097 continuously cast blooms) of three low-alloyed Cr-Mo steel variants with different vanadium content, we conclude that the void occurence is strongly associated with the level of vanadium content. The experiments performed by means of microstructural, fractographic investigations and microchemical point analysis proved that preffered areas for void initiation are interdendritic segregations in the core of bloom. We observed vanadium carbide precipitates that are situated close to the cracks, whereas the content of vanadium was 7,5 times more then in an average smelt composition. The further development of defects is connected with fast heating up to the austenization temperature in soaking pit. Our results confirm that marked decrease of defects can be achieved by optimization of production process, i. e. optimized casting speed, steel overheat above the liquidus temperature and/or application of M-EMS. Keywords: crack, carbide precipitation, interdendritic segregation, low-alloyed steels. 1. Introduction. It is generally known that at niobium microalloyed steels high abundance of surface fracture is usually detected (Patrick and Ludlow (1994)). Nowadays, to suppress this problems, the niobium is often substituted by vanadium, therefore the vanadium microalloying is of great interest. This process (Parsons and Edmonds (1987)) improves the fatigue properties, enhances abrasion resistance and hardening capacity in steels and also suppresses abnormal austenitic grain growth after hot rolling (Epicier et al. (1987)). Moreover, vanadium microalloying decreases the steel vulnerability to hydrogen embrittlement. On the other hand the negative influence of vanadium can be expressed only during bloom`s straightening or bloom`s cooling after rolling. The process of vanadium precipitation is realized in form of vanadium nitride VN, vanadium carbide VC or carbonitride V(C,N) (Fourlaris at al. (1995), Dunlop et al. (1987)). In fact, VN are rarely observed, in most cases complex V(C,N) are taking place due to the nitrogen accessibility. The temperature interval of 1100 - 1180°C is known to be crucial for
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VN precipitation. At temperature that is below 1100°C, the ratio of carbon in V(C,N) precipitates is slightly increasing. The entire content of nitrogen is segregated at temperature of 920°C. The precipitation of vanadium exclusively in form of VC is realized below the temperature of 920°C. The process of vanadium precipitation ends aproximately at the temperature of 700°C (Baker et al. (2009)). Due to the fact that hot rolling temperature is too high for VN, V(C,N) precipitation along the austenitic grain boundaries (e. g. dissolving of VC in low-alloyed steels is known to be approximatelly at 900°C), it is more probable that negative effect of precipitation is realized during continuous casting ("critical" temperature interval of bloom`s straightening at 800 - 1000°C) (Rezaeian et al. (2008)). The vanadium microalloying of Cr-Mo-based steels and its infuence on internal defect is still questionable and was not successfully explained. Therefore it is necessary to extend the knowledge of the vanadium microalloying and its relation to the internal defect occurance as well as the heating strategy during the production procedure. In this paper we discuss the initiation and propagation of voids in CrMo steels. 2. Experimental methods. Three types (25CrMo4, 34CrMo4, 42CrMo4) of low-alloyed CrMo steels were investigated. The chemical composition of the steels is given in Table 1. The ensemble of 1097 continuously cast blooms was statistically analysed using the Statgraphics Centurion programme. The round blooms (550 mm in diameter) were hot rolled into the square billets (260 x 260 mm). Only billets with central voids (see Fig. 1) detected during the ultrasonic inspection (UI) were chosen for fracture investigation, microchemical and microstructure analysis. The samples were cooled in liquid nitrogen (77 K) and broken by Charpy impact tester. After etching in Nital, we performed the microstructural analysis by means of optical microscopy (Olympus PM63). The microchemical studies of the interdendritic segregations were carried out using the scanning electron microscopy SEM (Hitachi S3500N with Thermonoran 445a EDX microprobe). The fractographic investigations and the specification of the vanadium content in fracture areas were utilised by SEM (TESCAN Mira with Bruker Quantax 200 EDX microprobe). Table 1. Chemical composition of low-alloyed Cr-Mo steels (wt. %)
Steel
C
Mn
Si
P
S
Cr
Ni
Mo
V
25CrMo4
0,25
0,74
0,25
0,013
0,004
1,18
0,25
0,20
0,044
34CrMo4
0,34
0,74
0,25
0,015
0,005
1,17
0,25
0,20
0,088
42CrMo4
0,42
0,74
0,25
0,015
0,005
1,18
0,25
0,20
0,005
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The influence of vanadium microalloying on voids occurence in low-alloyed
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Figure 1. Cross-section of the 25CrMo4 square billet (260 x 260 mm) with detected fracture after hot rolling.
3. Results and discussion 3.1 Fractography and EDX interpretation
Average number of UI findings
0,8 0,7 0,6
42CrMo4
0,5
25CrMo4
0,4
34CrMo4
0,3 0,2 0,1 0
Figure 2. The graph describing the correlation between the average number of UI findings and corresponding Cr--Mo steel type. For instance, number 0,2 denotes that for every ten blooms, there are 2 imperfect samples.
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Figure 2 shows average number of UI findings at three inspected Cr-Mo steels with different amount of vanadium. It is seen that due to the corresponding vanadium content in order of 0,088 wt. %, the steel 34CrMo4 exhibits the highest fracture failure, in comparison to the 42CrMo4 steel showing only one imperfect sample for every hundred blooms. Due to the fact, that 25CrMo4 steel variant is the most industrially used, we aim our concern at this type of Cr-Mo steel. By means of SEM and EDX analysis we studied fracture surfaces of blooms centre. From Fig. 3 (upper set of images), one can see smooth fracture with occasional failure. The fatigue failure is not characterized by elevated content of vanadium and carbon. On the other hand, the relation is seen between vanadium precipitates and crack initiation in the central part of the blooms (see Fig. 3 - lower set of images). The stressed places, where the crack is initiating and propagating are fringed by VC particles. The content of vanadium and carbon is markedly higher than in smelt composition (7,5 times for vanadium), see Table 1. We have to point out that due to the integral character of the EDX probe, we could not analyse the carbidic particle itself, therefore the result is partially affected by the nearest neighbourhood. However, vanadium precipitates act as prefferential regions where the crack nucleates whereas coupled interdendritic segregations create the motive force for crack propagation.
cps/eV
0,9 0,8 0,7 0,6 0,5
Mo C
V Cr
Mn Fe
Ni
0,4 0,3 0,2 0,1 0,0
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0,20
0,40
0,60
0,80
1,00
The influence of vanadium microalloying on voids occurence in low-alloyed
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cps/eV
0,30 0,25 0,20 Mo C
V Cr
Mn Fe
Ni
0,15 0,10 0,05 0,0 0,20
0,40
0,60
0,80
1,00
Figure 3. The SEM of fracture surfaces of 25CrMo4 steel continuously cast bloom centre (550 mm in diameter) with correcponding element abundance that were collected during EDX analysis (upper and lower set of images). See vanadium and carbon content taken from highlighted area (VC particle, that fringes the crack).
3.2 Microstructure and EDX interpretation Microstructural point analysis with using the EDX probe that was realized on bloom crosssection of 25CrMo4 steel type without M-EMS showed accentuated molybdenum, manganese and chromium volume mainly close to the interdendritic segregation (see Fig. 4 upper set of images). The voids in case of suitable conditions connect to each other and create the potentiality of the fracture. Fig. 4 - (lower set of images) aparently demonstrates that the chromium, molybdenum and manganese content in interdendritic segregation is higher than in blooms which were cast without M-EMS. The key role of the void initiation in the steel plays the rate of interdendritic segregations. One could expect that two negative processess may compete. First one takes place in case that M-EMS was not utilised. The steel structure is composed of numerous interdendritic segregations coupled together. It simplifies the crack initiation. On the other hand markedly higher content of chromium, molybdenum in interdendritic segregations while using the M-EMS implies that these regions are characterized by higher resistance to deformation during bloom's straightening. Based on our results crack initiation and propagation processess which are accelerated by interdendritic segregations while M--EMS is not applied are more important and affect the steel structure more often.
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Chemical composition (wt. %)
264
Void
Segregation Bainite
2,5 2 1,5
Mn Cr
1
Si Ni
0,5
Mo
0 Void
Segregation
Bainite
500 Pm
Void Segregation
Chemical composition (wt. %)
2,5 2 Mn Cr
1,5
Si
1
Ni Mo
0,5 0 Void
Segregation
600 Pm
Figure 4. Microstructures of 25CrMo4 steel continuously cast bloom centre (550 mm in diameter). Included subplots denote the chemical composition of elements that was taken from EDX analysis (M-EMS not applied--upper set of images, M-EMS applied--lower set of images).
3.3 Statistical analysis The statistical analysis of three Cr-Mo steel variants with different vanadium content has been made to interpret the relation between the crack occurence in blooms and the chemical composition and/or continuous casting conditions. We have found out that vanadium microalloyed steels are markedly vulnerable to internal voids during casting than those that are free of vanadium. To lower the crack occurance in vanadium microalloyed Cr-Mo steels, it is necessary to optimize the casting speed and steel overheating temperature in tundish. Furthermore M-EMS application contributes to improvement of continuously cast bloom´s quality especially in case when casting speed and overheating temperature are out of optimum. The influence of M-EMS application in dependence on the casting speed and steel overheating temperature is depicted on Fig. 5. The negative effect of slow casting speed and steel overheating temperature can be suppressed by M-EMS application.
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Average number of UI findings
The influence of vanadium microalloying on voids occurence in low-alloyed
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0,8 0,6
without M-EMS with M-EMS
0,4 0,2 0 20
30
40
50
60
Average number of UI findings
dT(°C) 0,8 0,6 without M-EMS 0,4
with M-EMS
0,2 0 0,28
0,29
0,3
0,31
0,32
0,33
Casting speed (m/min.)
Figure 5. The influence of M-EMS application (with or without) on casting speed and steel overheating temperature due to the average number of UI findings.
4. Conclusion Statistical analysis, microstructural and microchemical observations of three vanadium microalloyed Cr-Mo steels have shown, that initiation and propagation of internal defects are strongly associated with vanadium content. Originating microcracs are propagating along the interdendritic segregations and expand through the material. Modification and optimization of casting conditions such as high casting speed, low steel overheating
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temperature and M-EMS applications cause marked decrease of crack occurence and in this way contribute to steel quality improvement. Acknowledgements This work was partially supported by the grants SP/2010150, CZ.1.05/2.1.00/01.0040, 106/09/1587. References Baker, T. N. (2009). Processes, microstructure and properties of vanadium microalloyed steels, Materials Science and Technology, 25, 1083-1107. Dunlop, G. L., Carlsson, C-J., Frimodig, G. (1978). Precipitation of VC in ferrite and pearlite during direct transformation of a medium carbon microalloyed steel, Metallurgical and Materials Transactions A, 9, 261-266. Epicier, T., Acevedo, D., Perez, M. (2008). Crystallographic structure of vanadium carbide precipitates in a model Fe-C-V steel, Philosophical Magazine, 88, 31-45. Fourlaris, G., Baker, A. J., Papadimitriou G. D. (1995). A microscopic investigation of the precipitation phenomena observed during the pearlite reaction in vanadium alloyed carbon steels, Acta Metallurgica et Materialia, 43, 37333742. Parsons, S. A., Edmonds, D. V. (1987). Microstructure and mechanical properties of medium-carbon ferrite-pearlite steel microalloyed with vanadium, Materials Science and Technology, 3, 894-904. Patrick, B., Ludlow, V. (1994). Development of casting practices to minimise transverse cracking in microalloyed steels, Revue de Metallurgie. Cahiers D'Informations Techniques, 91, 1081--1089. Rezaeian, A., Zarandi, F., Yue, S. (2008) Mechanism of hot ductility improvement of a peritectic steel containing vanadium using very-high-temperature compression. Metallurgical and Materials Transactions A, 39A, 2644.
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