Optimization of Hot Rolling Process for API-X60 Grade Line Pipe Steel With Lower Yield/Tensile Ratio for ERW Pipe Application

The real challenge of achieving yield to tensile ratio (YR) lower than 0.85 along with YS & TS as per specification especially for ERW pipes was performed because the major concern with higher yield ratio is the resultant lower uniform elongation, lower work hardening exponent and also possible deformation under load (safety) when pipelines are used underground. A study was performed by controlling the HSM process parameters. To ensure tensile properties as per X60 grade with lower yield ratio, and good impact toughness, Finish Rolling Temperature (FRT), Coiling Temperature (CT) and Cooling Rate (CR) were varied at Hot Strip Mill in production scale. Generally, micro-alloyed steels show good strength as desired for X60 grade, along with high yield ratio up to 0.90 especially for lower thickness like 6.4mm due to thermomechanical rolling technology. The HSM process parameters for reducing yield ratio was focused on finding the effect of grain size and uniform microstructure. The results of coil form material showed that higher deformation temperature with normal CT and consistent water-cooling pattern could give optimum YS, TS with lower. YR. Comparatively, there was relatively high amount of change in YS, TS & YR with different combination of FRT, CT and CR. At customer end, after pipe making, it was found that yield ratio was lower than 0.85 with minimized YS drop due to Bauschinger effect. Interestingly, the tensile properties & yield ratio values were differing with respect to ERW pipe manufacturing methods (Cage forming & Linear forming processes). These were attributed to the differences in straining during pipe forming. The study indicated that API-X60 grade steel can be supplied with optimization of HSM process parameter to get desired yield and tensile properties as per specification with yield ratio of 0.84∼0.88 and also achieve desired yield and tensile results after ERW pipe making with yield ratio of 0.85 max.