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The present paper deals with the development of low carbon (, 0 . 04%) microalloyed steel plates for high grade (> X80, yield strength > 550 MPa) longitudinally welded large diameter pipes for gas transmission. The interest in increasing the grade level of steels for linepipe is to improve transportation efficiency by an increase in operation pressure. In addition, increasing the grade allows thinner wall pipes to be used, reducing costs and improving the rate of field joint welding. A general requirement for linepipe steels is a simultaneous increase in fracture toughness as the strength increases. However, it is remarkably difficult to achieve a combination of high strength and good low temperature toughness. Bainitic microstructures have shown the capability to attain such requirements.

Two convenient experimental methods are described for ascertaining dispersibility characteristics of carbon black. Both are based on the rate of development of optical density with dispersion or mixing time in a dilute suspension. One procedure uses polybutadiene medium and a roll mill to effect dispersion, the other chloroform and ultrasonic agitation. Correlations with conventional dispersion test results based on photomicrography are good. Advantages are precision and objectivity, not available to the same degree in photomicrograph evaluations.

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