Materials Science Forum ISSN: 1662-9752, Vol. 788, pp 384-389 doi:10.4028/www.scientific.net/MSF.788.384 © 2014 Trans Tech Publications, Switzerland
Online: 2014-04-22
Effects of Processing Parameters on the Evolution of Microstructure and Hole Expansion Property of F/B Dual Phase Steels Xinjun Shen1, a, Xiangjun Zhang1, b, Fengqin Ji1, c, Jun Chen1, d, Shuai Tang1, e* and Guodong Wang1, f 1
The State Key Laboratory of Rolling and Automation, Northeastern University P. O. Box 105, No 11, Lane 3, Wenhua Road, HePing District, Shenyang 110819, China. a
[email protected],
[email protected],
[email protected],
[email protected],
[email protected],
[email protected]
d
Keywords: F/B dual phase steels, finishing rolling temperature, start cooling temperature, hole expansion property.
Abstract. With the development of auto industry, the F/B dual phase steels were widely used in wheel and underpan due to their lower cost and higher hole expansion property. In the present investigation, the effects of the finishing rolling temperature and initial cooling temperature of ultra fast cooling (UFC) on microstructure and hole expansion property of a low carbon C-Mn steel were investigated using optical microscope (OM) and scanning electron microscope (SEM). It has been found that lowering the finishing rolling temperature can refine the microstructure and the volume fraction of ferrite is increased as the initial cooling temperature of UFC is lowered. Furthermore, both lowering the finishing rolling temperature and the initial cooling temperature of UFC can improve hole expansion property. Introduction Automobile wheel steels demand the combination of strength and formability, especially hole expansion property due to its manufacturing processes, such as drawing process and hole expanding process which determine the high hole expansion ratio of the used steels [1,2]. Dual phase (DP) steels and steels with ferrite and pearlite microstructure can be used to fabricate automobile wheels. But, DP steels with ferrite and martensite microstructure have low hole expansion ratio results from high hardness rate between martensite and ferrite, meanwhile, martensite will be tempered in heat affected zone (HAZ) after flash welding which will lead to softness and decrease of fatigue strength. So, the DP steels with ferrite and martensite microstructure aren’t the desired materials [3-5]. For ferrite and pearlite microstructure steels, its strength is low. So, if the second phase can be replaced by bainite whose strength is between martensite and pearlite and have suitable formability will realize the combination of strength and formability. Ferrite and bainite dual phase steels have high hole expansion ratio which is a desired material to manufacture automobile wheels [6,7]. Some researchers have investigated the relationships between tensile properties and hole expansion property of C-Mn steels [8], influence of bainite/martensite-content on the tensile properties of low carbon dual steels [5] and influence of Ti and Nb on the strength-ductility-hole expansion ratio balance of hot-rolled low carbon high-strength steel sheets[9]. In this paper, the influence of process parameters on microstructure and hole expansion ratio of a low carbon C-Mn steel was investigated. The steels were produced by thermo-mechanical control process (TMCP) with ultra-fast cooling (UFC), and investigated the influence of finishing rolling temperature and start cooling temperature on the evolution of microstructure and hole expansion ratio of a low carbon C-Mn steel. Experimental Procedure Material and hot-rolling experiment. The chemical composition of the low carbon C-Mn steel is shown in Table 1. Slabs of 45mm thickness were reheated at 1200oC for 2h for solution, and All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 137.226.128.175, RWTH Aachen, Aachen, Germany-03/02/16,09:06:31)
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subsequently hot rolled according to a thickness change of 45→32→19→12→7.9→5.6→4.5→4mm. After hot rolling, the steel sheets were air cooled to a special temperature during this period the overcooling austenite transforms to ferrite followed by UFC cooled to a special temperature and then held in an asbestos case to ensure the remained austenite transforms to bainite. To investigate the effects of finishing rolling temperature, the start cooling temperature was same, and the finishing rolling temperature was same when investigates the effects of start cooling temperature. To simulate coiling process the sheets were held in an asbestos case after UFC. The thermal history of the hot-rolling process is shown in Fig. 1. The process parameters are listed in Table 2. Table 1 Chemical composition of the low carbon C-Mn steel in the experiment(wt. %) C Si Mn P S 0.058 0.51 1.31 0.0046 0.003
Fig. 1 Thermal history of the hot-rolling process Microstructure observation. The microstructure was characterized using optical microscopy (OM) and scanning electron microscopy (SEM). Samples of approximate dimension of 10mm×6mm×4mm (l × b × h) were prepared using electric spark cutting and then were ground and polished up to 0.25μm finish along surface perpendicular to transverse direction using standard metallographic practice. The samples were then etched with 4% nital for 10-12s. The samples were first observed by LAICA 2500M optical microscopy and then observed by FEI Quanta 600 scanning electron microscopy. The grain size of ferrite was measured by intercept method and the fraction of bainite was measured using Image Pro-Plus (IPP) soft both using five images. Table 2 Process parameters of the tested steels Finishing rolling Start cooling Process parameters temperature (oC) temperature (oC) 870 745 Effects of finishing 840 755 rolling temperature 830 749 831 777 Effects of starting 835 725 cooling temperature 845 690
Coiling temperature (oC) 470 520 520 447 460 471
Hole expanding test. The hole expanding test was carried out according to GB/T 15825.4-2008. The specimen was fabricated of 90mm×90mm×2.5mm (l× b× h) followed by pierced by a 16.5mm-diameter punch at center of the specimen. A BCS-30D experimental equipment was used at a speed of 3mm/min with a blank holder force of 50KN.
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The hole expansion ratio was calculated by following Eq. 1. Here, do is the initial diameter of the hole and d is the diameter of the hole expanded as far as the first through-thickness crack is just observed. 𝑑−𝑑 λ = 𝑑 0 × 100% (1) 0
Results and Discussion Finishing rolling temperature. Fig. 2 shows the typical optical microstructure of specimens of different finishing rolling temperature. The typical SEM micrographs of the harder phase are shown in Fig. 3 to take a deep insight of the harder phase. It is found that the tested steels are all F/B dual phase steels: the light-colored phase is polygonal ferrite and the saturated phase is banite. The polygonal ferrite is transformed during air cooling period after hot-rolling due the slow cooling rate, and the harder phase (bainite) is transformed during simulating coiling period after UFC. Fig.4 shows the ferritic volume and grain size of the tested steels varying with finishing rolling temperature. Lowering the finishing rolling temperature, the fraction of ferrite increases and the bainite is refined while the ferritic grain size changes slightly. Fig.5 shows the holes expansion ratio of the tested steels varying with the finishing rolling temperature. The holes expansion ratio increases with the reducing of the finishing rolling temperature. C A B
Fig. 2 Typical OM micrographs of the low carbon B/F dual steels varying with the finishing rolling temperature, A: 870oC. B: 840oC. C: 830oC. A
B
C
Fig. 3 Typical SEM micrographs of the harder phase varying with finishing rolling temperature A: 870oC. B: 840oC. C: 830oC. The grain size of austenite will be refined or the austenite will be more “pancaked” after hot-rolling as finishing rolling temperature is reduced [11, 12]. So, the Ar3 temperature and nucleation rate will be elevated, and in turn the ferritic volume will increase. Thus, the above ferritic volume increases with the reducing of finishing rolling temperature. In terms of the slight change of ferritic grain size, it may be due to the insensitivity of the ferritic grain size to finishing rolling temperature. Some researchers have also obtained the same conclusion [13, 14]. The bainite phase is refined as finishing rolling temperature is reduced as shown in Fig. 2 and Fig. 3. So, we can get the conclusion that lowering finishing rolling temperature associates with finer microstructure.
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10
Ferritic volume,%
82
180 9
80 8 78 76
7
74 6
Hole expansion ratio,%
Ferritic volume Ferritic grain size
Ferritic grain suze,μm
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72 70
5 830
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o
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Finishing temperature, C
Finishing temperature,
Fig. 4 Ferritic volume and grain size of the tested steels varying with finishing rolling temperature.
Fig. 5 Hole expansion ratio of tested steels varying with finishing rolling temperature
℃
Thus, the hole expansion property of tested steels is improved as finishing rolling temperature is reduced results from the increase of ferritic volume and the refinement of bainite phase, which favor to mechanical compatibilities between ferrite and bainite during hole expanding tests [15], as shown in Fig. 5. Start cooling temperature. Fig. 6 shows the typical optical microstructure of different start cooling temperature of the tested steels. The typical SEM micrographs of the harder phase are shown in Fig.7 to take a deep insight of the harder phase. It is found that the tested steels are also all F/B dual phase steels. Fig.8 shows the ferritic volume and grain size varying with start cooling temperature. Lowering start cooling temperature, the ferritic volume increases while ferritic grain size changes slightly. Meanwhile, the harder phase (bainite) is refined with the reducing of start cooling temperature, as shown in Fig.8. Fig.9 shows the holes expansion ratio of the tested steels varying with the start cooling temperature. The holes expansion ratio increases with the reducing of the start cooling temperature. A
B
C
Fig. 6 Typical OM micrographs of the tested steels varying with the initial cooling temperature A: 777 oC. B: 725 oC. C: 690 oC. A
B
C
Fig. 7 Typical SEM micrographs of the harder phase varying with the initial cooling temperature A: 770oC. B: 725o. C: 690oC.
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8
78 7 76 74 6 72
Ferritic grain size,μm
80
Ferritic volume,%
120
Ferritic volume Ferritic grain size
70
Hole expansion ratio,%
82
115 110 105 100 95 90 85
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5 680
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780
o
Start cooling temperature, C
Fig. 8 Ferritic volume and grain size of tested steels varying with initial cooling temperature.
680
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o
Start colling temperature, C
Fig. 9 Hole expansion ratio of tested steels varying with initial cooling temperature.
Transformation of polygonal ferrite is occurred during air cooling period after hot-rolling. The transformation of ferritic belongs to diffusional transformation [16, 17], which is a process of nucleation and growth. This process needs time to complete. Lower the start cooling temperature is, longer the time for ferritic transformation is. Thus, the ferritic volume is increased when start cooling temperature is reduced, as shown in Fig. 8. In terms of the slight change of ferritic grain size, it may be due to the insensitiveness of the ferritic grain size to start cooling temperature in this low carbon C-Mn dual phase steels, as shown in Fig. 9. The increase of ferritic volume will in turn refine the harder bainite phase. Polygonal ferritic phase is a soft phase with high plastic deformation ability [18]. Additionally, as start cooling temperature is reduced, the bainite is also refined, so the holes expansion property will increase due to the higher ferritic volume and the increased mechanical compatibilities between ferrite and bainite during hole expanding tests, as shown in Fig. 9. Conclusion In this study, the effects of the finishing rolling temperature and the initail cooling temperature on the evolution of microstructure and holes property of a low carbon C-Mn steel have been examined and the following results have been obtained. (1) Reducing the finishing rolling temperature, the microstructure is refined. The ferritic volume increases but the grain size of ferrite changes slightly, and the bainite phase is refined. The hole expansion property of the tested steels is improved due to the high compatibility of deformation process between ferrite and bainite during hole expansion test. (2) Reducing the initial cooling temperature of UFC, the ferritic volume increases and bainite is refined while the ferritic grain size changes slightly. The holes expansion property of the tested steels is improved due to the changes of the microstructure. Acknowledgements The authors express their gratitude for the financial support by both the National Natural Science Foundation of China (Nos.51204049 and 51101033) and Fundamental Research Funds for the Central Universities (N110307002).
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Reference [1] Yanqiu Wang, Development of automobile wheel steels, North Vanadium Titanium. 2(2011) 15-17. [2] T. Shimizu, Y. Funakawa, S. Kaneko, High strength steel sheets for automobile suspension and chassis use-high strength hot-rolled steel sheets with excellent press formability and durability for critical safety parts, JFE Technical Report. 4(2004) 25-30. [3] M. Sudo, T. Iwai, Deformation behavior and mechanical properties of ferrite-bainite-martensite (triphase) steel, Transactions ISIJ. 23(1983) 294-302. [4] M. Sudo, S. I. Hashimoto, S. Kambe, Niobium bearing ferrite-bainite high strength hot-rolled sheet steel with improved formability, Transactions ISIJ. 23(1983) 303-311. [5] A. Kumar, S. B. Singh, K. K. Ray, Influence of bainite/mastensite-content on the tensile properties of low carbon dual-phase steels, Materials Science and Engineering A. 474(2008) 270-282. [6] Masayuki Kinoshita, Hot-rolled high strength steels with excellent stretch-flangeable property, Wuhan Iron and Steel Corporation Technology. 10(1995) 44-50. [7] Yonglin Kang, Control of Quality and Formability of Modern Car Sheet, Beijing, 1999. [8] X. Fang, Z. Fan, B. Ralph, The relationships between tensile properties and hole expansion property of C-Mn steels, Journal of Materials Science. 38(2003) 3877-3882. [9] K. Kamibayashi, Y. Tanabe, Y. Takemoto, I. Shimizu, T. Senuma, Influence of Ti and Nb on the strength-ductility-hole expansion ratio balance of hot-rolled low carbon high-strength steel sheets, ISIJ International. 52(2012) 151-157. [10] D. H. Hanlon, J. Sietsma, S. V. Zwaag, The effect of plastic deformation of austenite on the kinetics of subsequent ferrite formation, ISIJ International. 41(2001) 1028-1036. [11] P. Cizek, B. P. Wynne, C. H. J. Davies, B. C. Muddle, P. D. Hodgson, Effect of composition and austenite deformation on the transformation characteristics of low-carbon and ultra-carbon microalloyed steels, Metallurgical and Materials Transactions A. 33A(2002) 1331-1349. [12] Yi Dong, Xiaoguang Shi, Bin Han, Effect of finishing temperature on microstructure and properties of fine grained hot-rolled dual-phase steel, Heat Treatment of Metals. 36(2011) 64-67. [13] Yi Dong, Xiaoguang Shi, Bin Han, Rendong Liu, Effect of finishing temperature on the microstructure and mechanical properties of Si-Mn hot-rolled dual-phase steel, Journal of Plasticity Engineering. 18(2011) 81-84. [14] Wenjin Nie, Chengjia Shang, Hailong Guan, Xiaobing Zhang, Shaohui Chen, Control of microstructures of ferrite/bainite (F/B) dual-phase steels and analysis of their resistance to deformation behavior, Acta metallurgical sinica. 48(2012) 298-306. [15] E. A. Wilson, The γ→α transformation in low carbon irons, ISIJ International. 34 (1994) 615-630. [16] Y. V. Leeuwen, Sietsma , S. V. Zwaag, The influence of carbon diffusion on the character of γ→α the phase transformation in steel, ISIJ International. 43(2003) 767-773. [17] W. C. Jeong, Strength and formability of ultra-low-carbon Ti-IF steels, Metallurgical and Materials Transactions A. 31A(2000) 1305-1306.
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Effects of Processing Parameters on the Evolution of Microstructure and Hole Expansion Property of F/B Dual Phase Steels 10.4028/www.scientific.net/MSF.788.384 DOI References [15] E. A. Wilson, The γ→α transformation in low carbon irons, ISIJ International. 34 (1994) 615-630. 10.2355/isijinternational.34.615
