The Thermomechanical Controlled Processing of High-Strength Steel Plate: A New View of Toughness Based on Modern Metallography

Abstract:

Modern thermomechanical controlled processing (TMCP) of advanced steels is now an important processing route in the production of engineering structures and products that are of value to society. The principles of TMCP are now practiced in the hot mill, cold mill and press forming shops around the world. Successful TMCP means that the proper metallurgical microstructure has been obtained in the required areas of the steel. The ideal microstructure is often defined by the correct phase balance and dimensions either of the parent austenite or final ferritic phase. Technological and economic demands have led to ever increasing levels of strength, especially for applications such as large diameter linepipe. The operative yield strengths in 18mm hot rolled plate have increased from X52(ferrite pearlite) in 1970 to X80(ferrite-bainite) today. The next frontier is the X100-X120 strength range, where bainitic or martensitic microstructures are required. It is clear that achieving a high-strength bainitic microstructure in heavy plate requires a high Carbon Equivalent value (C. E. II or Pcm), a rapid cooling rate, and a low water-end temperature. The requirement of high toughness and good weldability also means a low carbon content. This paper will describe the results of a research program where a steel of C. E. 0.56 and Pcm 0.23 was reheated, rough rolled for grain refinement, finish rolled for austenite pancaking, and direct quenched to below the Bs temperature. It was found that the strength and especially the toughness of the fully processed plates could not be explained using conventional metallographic techniques in conjunction with known structure-property relationships. However, the application of modern metallographic techniques based on FEG-SEM incorporating OIM led to microstructural characterization that more fully explained the observed mechanical properties. Of particular importance were the amount of MA micro-constituent, the crystallographic packet size of the bainite, and the high angle boundary character, especially the CSL boundaries, found in the microstructure. In the future, improved modeling of microstructural evolution and attendant mechanical properties will incorporate these important features.

• Periodical: Materials Science Forum (Volume 762)
• Pages: 38-46
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.762.38
• Online since: July 2013
• Authors: Xiao Jun Liang, Ming Jian Hua, C. Isaac Garcia, Anthony J. DeArdo
• Keywords: Bainitic Steels, DBTT, Direct Quenching, EBSD-OIM, Linepipe Steels, M-A Microconstituent, Microalloyed Steel, Plate Steels, Sigma Boundaries, Strength, TMCP, Toughness

References

[1] Terada, Y., et al., Development of API X100 UOE line pipe. Nippon Steel Technical Report(Japan), 1997. 72: pp.47-52.

[2] Koo, J.Y., et al.,. Int. J. Offshore Polar Eng. Vol. 14 (2004) p.2.

[3] Okaguchi, S., et al.,. Int. J. Offshore Polar Eng. Vol. 14 (2004) p.29.

[4] Hillenbrand, H.G., et al. Development of high strength material and pipe production technology for grade X120 line pipe. 2004.

DOI: 10.1115/ipc2004-0224

[5] Takeuchi, I., et al. Crack arrestability of high-pressure gas pipelines by X100 or X120. 2006.

[6] Dearden, J., HO'Niell: Trans. Inst. Weld Vol. 3 (1940) p.203.

[7] Ito, Y. and K. Bessho, J Jpn Weld. Soc. Vol. 37 (1968) p.98.

[8] Suikkanen, P., Development and processing of low carbon bainitic steels, Doctoral thesis, 2009, University of Oulu.

[9] Suikkanen, P.P., C. Cayron, A.J. DeArdo, L.P. Karjalainen, Crystallographic Analysis of Isothermally Transformed Bainite in 0.2C-2.0Mn-1.5Si-0.6Cr Steel Using EBSD, J. Materials Sci. Techn. (China), 2013 (in press).

DOI: 10.1016/j.jmst.2013.01.015

[10] Dáz-Fuentes, M., A. Iza-Mendia, and I. Gutierrez,. Metall. Mater. Trans. A Vol. 34 (2003), p.2505.

[11] Liang, X., C.I. Garcia, and A.J. DeArdo. Effect of bainite transformation on grain boundary character distribution and DBTT in high-strength linepipe steel. in AIST Steel Properties and Applications Conference Proceedings - Combined with MS and T'12, Materials Science and Technology 2012. 2012. Pittsburgh, PA.

[12] Dieter, G.E. and D. Bacon, Mechanical metallurgy. Vol. 2. 1986: McGraw-Hill New York.

[13] Lepera, F.S., J. Met. Vol. 32 (1980), p.38.