低碳鋼中橫向裂紋仍然為連續鑄造中常見的問題，過去有文獻提出藉由改善鑄胚表面微觀結構來抑制橫向裂紋形成的問題，然而於次表面(約2至15 ℃mm)仍然發現有橫向裂紋的存在，故了解高溫時鑄胚微觀結構並搭配機械性質測試可幫助我們了解橫向裂紋形成之機理。
本研究首先取樣含有橫向裂紋之SM570鑄胚，觀察裂紋附近之金相組織與裂紋斷裂面之表面結構，待觀察完鑄胚橫向裂紋後再針對不同錳含量(0.67、1.39與1.98 wt%)做沃斯田鐵化，再藉由冷卻速率(0.01、0.05、0.09、3 ℃/s)冷卻至Ar3溫度下水淬以保持高溫時肥粒鐵之微觀結構，並使用三點彎曲搭配數位影像相關係數法來分析應變分布，再比對拉伸表面形成之裂紋與其原始金相結構做比較。
結果顯示橫向裂紋沿沃斯田鐵晶界上之初析肥粒鐵生長，斷裂面可觀察到脆性斷裂與延性凹坑面共存之斷裂面。錳含量與冷卻速率影響肥粒鐵厚度，並藉由擴散控制成長來解釋該現象。三點彎曲結果顯示裂紋形成於肥粒鐵上，同時統計其原始肥粒鐵厚度介於5至15µm之間。

Transverse crack, which perpendicular to the direction of continuous casting, is a main problem in straightening. Although the problem of the transverse crack formation is suppressed by tailoring the microstructure on the slab surface, the transverse cracks having the length of 2 to 15 mm are still found on the subsurface. Thus, this study was separated into two parts. Firstly, the transverse cracks were characterized in low carbon steel of SM570 in order to understand the transverse cracks. Secondly, three steels having Mn content of 0.67, 1.39 and 1.98 wt% which had different chemical composition from SM570 were employed to know the effect of Mn addition on the formation of transverse cracks. In addition, the cooling rates of 0.01, 0.05, 0.09 and 3 ℃/s were investigated after heat treatment to understand the effect of the cooling rate on the formation of transverse cracks. Then, OM and SEM were used to analyze the morphology and microstructure. Dilatometer and three-point bending together with digital image correlation (DIC) technique were applied to measure the Ar3 temperature and the mechanical property, respectively.
In the first part, it was observed that transverse cracks occur and grow along ferrite formed on the boundaries of austenite, and the fracture mode was composed of brittle and ductile fracture. In the second part, increase in manganese content from 0.67 to 1.98 wt% leads to increasing the fraction of ferrite film forming on the boundaries of austenite from 6.4 to 24.3 % and to reducing the thickness of ferrite from 22.7 to 13.0 µm at a cooling rate of 0.05 ℃/s. At manganese content of 1.39wt% the increasing cooling rate from 0.01 to 3 ℃/s results in increasing the fraction of ferrite film forming on the boundaries of austenite from 14.4 to 36.1 % and to reducing the thickness of ferrite from 18.9 to 11.4 µm.

[1] S. Junghans, Apparatus for Continuous Casting Processes, 1938, United States Patent Office, Serial No.184752.
[2] Y. F. Li, G. H. Wen, T. A. Ping, J. Q. Li, and C.L. Xiang, Effect of slab subsurface microstructure evolution on transverse cracking of microalloyed steel during continuous casting, 2014, Journal of Iron and Steel Research, International, 21(8), 737-744.
[3] Y. Maehara, K. Yasumoto, H. Tomono, T. Nagamichi, and Y. Ohmori, Surface cracking mechanism of continuously cast low carbon low alloy steel slabs, 1990, Materials Science and Technology, 6(9), 793-806.
[4] B. Mintz, and J. M. Arrowsmith, Hot-ductility behaviour of C–Mn–Nb–Al steels and its relationship to crack propagation during the straightening of continuously cast strand, 1979, Metals technology, 6(1), 24-32.
[5] H. G. Suzuki, S. Nishimura, J. Imamura, and Y. Nakamura, Embrittlement of steels occurring in the temperature range from 1000 to 600℃, 1984, Transactions of the Iron and Steel Institute of Japan, 24(3), 169-177.
[6] E. Takeuchi, and J. K. Brimacombe, Effect of oscillation-mark formation on the surface quality of continuously cast steel slabs, 1985, Metallurgical Transactions B, 16(3), 605-625.
[7] B. Mintz, S. Yue, and J. J. Jonas, Hot ductility of steels and its relationship to the problem of transverse cracking during continuous casting, 1991, International Materials Reviews, 36(1), 187-220.
[8] N. Pradhan, N. Banerjee, B. B. Reddy, S. K. Sahay, C. S. Viswanathan, P. K. Bhor, and S. Mazumdar, Control of transverse cracking in special quality slabs, 2001, Ironmaking & steelmaking, 28(4), 305-311.
[9] B. Mintz, and D. N. Crowther, Hot ductility of steels and its relationship to the problem of transverse cracking in continuous casting, 2010, International Materials Reviews, 55(3), 168-196.
[10] Y. Ohmori, and Y. Maehara, Precipitation of NbC and hot ductility of austenitic stainless steels, 1984, Transactions of the Japan institute of metals, 25(3), 160-167.
[11] R. Abushosha, S. Ayyad, and B. Mintz, Influence of cooling rate on hot ductility of C-Mn-AI and C-Mn-Nb-Al steels, 1998, Materials science and technology, 14(4), 346-351.
[12] T. Kato, Y. Ito, M. Kawamoto, A. Yamanaka, and T. Watanabe, Prevention of slab surface transverse cracking by microstructure control, 2003, ISIJ international, 43(11), 1742-1750.
[13] N. Baba, K. Ohta, Y. Ito, and T. Kato, Prevention of slab surface transverse cracking at Kashima n° 2 caster with Surface Structure Control (SSC) cooling, 2006, Revue de Métallurgie–International Journal of Metallurgy, 103(4), 174-179.
[14] L. J. Xu, S. L. Zhang, C. G. Qiu, S. T. Qiu, and X. Z. Zhang, Surface microstructure control of microalloyed steel during slab casting, 2017, Journal of Iron and Steel Research International, 24(8), 803-810.
[15] J. K. Brimacombe, and K. Sorimachi, Crack formation in the continuous casting of steel, 1977, Metallurgical transactions B, 8(2), 489-505.
[16] W. D. Callister and D. G. Rethwisch, Materials Science and Engineering(8th ed), 2011, John Wiley & Sons, USA: New York, 319-330.
[17] T. Kasugai, and M. Inagaki, Effect of Mn on transformation behaviors of synthetic weld heat-affected zone, 1980, Transactions of National Research Institute for Metals, 22(4), 258-269.
[18] H. K. Bhadeshia, Diffusional formation of ferrite in iron and its alloys, 1985, Progress in Materials Science, 29(4), 321-386.
[19] J. W. Christian, Theory of Transformations in Metals and Alloys, 1975, Part1, 2nd Edition, PergamonPress, Oxford, 126-148.
[20] R. E. Reed-Hill, R. Abbaschian, and R. Abbaschian, Physical metallurgy principles(4th ed), 2010, Cengage Learning, 481-488.
[21] C. Atkinson, Concentration dependence of D in the growth or dissolution of precipitates, 1967, Acta Metallurgica, 15(7), 1207-1211.
[22] J. R. Bradley, and H. I. Aaronson, Growth kinetics of grain boundary ferrite allotriomorphs in Fe-C-X alloys, 1981, Metallurgical transactions A, 12(10), 1729-1741.
[23] K. R. Kinsman, and H. I. Aaronson, Influence of Al, Co, and Si upon the kinetics of the proeutectoid ferrite reaction, 1973, Metallurgical transactions, 4(4), 959-967.
[24] K. R. Kinsman, E. Eichen, and H. I. Aaronson, Thickening kinetics of proeutectoid ferrite plates in Fe-C alloys, 1975, Metallurgical Transactions A, 6(2), 303-317.
[25] J. R. Bradley, J. M. Rigsbee, and H. I. Aaronson, Growth kinetics of grain boundary ferrite allotriomorphs in Fe−C alloys, 1977, Metallurgical Transactions A, 8(2), 323-333.
[26] M. Enomoto, and H. I. Aaronson, Partition of Mn during the growth of proeutectoid ferrite allotriomorphs in an Fe-1.6 at. pct C-2.8 at. pct Mn alloy, 1987, Metallurgical Transactions A, 18(9), 1547-1557.
[27] P. D. Hodgson, M. R. Hickson, and R. K. Gibbs, The production and mechanical properties of ultrafine ferrite, 1998, Materials Science Forum, 284, 63-72.
[28] B. Hutchinson, D. Lindell, and M. Barnett, Yielding behaviour of martensite in steel, 2015, ISIJ International, 55(5), 1114-1122.
[29] W. H. Peters, and W. F. Ranson, Digital imaging techniques in experimental stress analysis, 1982, Optical engineering, 21(3), 427-431.
[30] J. C. Kuo, D. Chen, S. H. Tung, and M. H. Shih, Prediction of the orientation spread in an aluminum bicrystal during plane strain compression using a DIC-based Taylor model, 2008, Computational Materials Science, 42(4), 564-569.
[31] S. H. Tung, M. H. Shih, and J. C. Kuo, Application of digital image correlation for anisotropic plastic deformation during tension testing, 2010, Optics and Lasers in Engineering, 48(5), 636-641.
[32] J. C. Kuo, S. H. Tung, M. H. Shih, and Y. Y. Lu, Characterisation of Indentation‐Induced Pattern Using Full‐Field Strain Measurement, 2010 Strain, 46(3), 277-282.
[33] J .C. Kuo, D. Chen, S. H. Tung, and M. H. Shih, Misorientation behavior of an aluminum bicrystal with 15.7 symmetric tilt boundary using simple shear, 2007, Journal of materials science, 42(18), 7673-7677.
[34] T. C. Chu, W. F. Ranson, and M. A. Sutton, Applications of digital-image-correlation techniques to experimental mechanics, 1985, Experimental mechanics, 25(3), 232-244.
[35] G. T. Eldis, A Critical Review of Data Sources for Isothermal Transformation and Continuous Cooling Transformation Diagrams, 1977, Hardenability Concepts with Applications to Steel, 126-157.
[36] B. Mintz, J. R. Banerjee, and K. M. Banks, Regression equation for Ar3 temperature for coarse grained as cast steels. 2011, Ironmaking & Steelmaking, 38(3), 197-203.
[37] S. H. Tung, J. C Kuo, and M. H. Shih, Strain distribution analysis using Digital-Image-Correlation techniques, 2005, Proceedings the Eighteenth KKCNN Symposium on Civil Engineering, Taiwan.
[38] K. Lücke, K. Detert A quantitative theory of grain-boundary motion and recrystallization in metals in the presence of impurities, 1957, Acta Metallurgica, 5(11), 628-637.
[39] M. Enomoto, Influence of solute drag on the growth of proeutectoid ferrite in Fe–C–Mn alloy, 1999, Acta materialia, 47(13), 3533-3540.
[40] C. Atkinson, H.B. Aaron, K.R. Kinsman, K. R., and H. I. Aaronson, On the growth kinetics of grain boundary ferrite allotriomorphs, 1973, Metallurgical Transactions, 4(3), 783-792.
[41] C. Poletti, J. Six, M. Hochegger, H. P. Degischer, and S. Ilie, Hot deformation behaviour of low alloy steel, 2011, Steel research international, 82(6), 710-718.
[42] H. Groβheim, K. Schotten, and W. Bleck, Physical simulation of hot rolling in the ferrite range of steels, 1996, Journal of Materials Processing Technology, 60(1-4), 609-614.