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Dong-Han Seo and Jang-Yong Yoo Technical Research Laboratories, POSCO, Pohang 790-785, Korea

Jae-il Janga) Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Korea (Received 2 August 2008; accepted 9 October 2008)

An attempt was made to predict the macroscopic plastic flow of a high-performance pipeline steel, consisting of dual constituent phases (soft ferrite and hard bainite), by performing nanoindentation experiments on each microphase with two spherical indenters that have different radii (550 nm and 3.3 mm). The procedure is based on the well known concepts of indentation stress-strain and constraint factor, which make it possible to relate indentation hardness to the plastic flow of the phases. Additional consideration of the indentation size effect for sphere and application of a simple “ruleof-mixture” led us to a reasonably successful estimation of the macroscopic plastic flow of the steel from the microphases properties, which was verified by comparing the predicted stress-strain curve with that directly measured from the conventional tensile test of a bulky sample.

I. INTRODUCTION

In constructing pipelines for transporting natural gas and crucial oil over a long distance, application of higher-strength linepipes has many economical advantages1 such as the increase in transportation efficiency (which can be achieved by increasing operating pressure) and the reduction in materials costs (by decreasing wall thickness and thus total tonnage of pipeline steel and welding consumables). Accordingly, many efforts have been competitively made to develop and apply highergrade pipeline steels beyond conventional API X65 steel that have a yield strength of 65 ksi (
450 MPa). Recently, the application of a high-strength linepipe such as API X80 grade have been increased, and API X100 and even X120 grade steels have been considered for practical use in the field.1–5 One of the most recent challenges in this research area of pipeline engineering is developing advanced steels for new design-concept pipelines (referred to as “strain-based design pipeline”) that are applicable to the seismic and permafrost regions where large plastic deformation can be introduced to buried linepipes.6,7 In addition to high strength, an important requirement for this strain-based design pipeline steel is excellent deformability, namely high work-hardening ability (and thus low yield-to-tensile a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0101

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J. Mater. Res., Vol. 24, No. 3, Mar 2009

strength ratio). Because dual-phase microstructures consisting of hard and soft phases are known to have a higher hardenability than single-phase structures,3,8,9 two types of microstructures have been extensively considered for the high-deformability pipeline steels; ferrite-bainite and bainite-martensite. For such a dual-phase steel, to optimize the volume fraction of each phase is essential to obtain proper target properties. In this regard, some pioneering work w