Speakers
Description
Heavy‑gauge wide API X70 plates (25–28.2 mm) were developed at JSW Plate Mill through a combination of optimized Nb‑based microalloying, enhanced thermo‑mechanical controlled processing (TMCP), and accelerated Multi-Purpose Interrupted Cooling (MULPIC) cooling to achieve stable strength and low‑temperature ductility. Drop Weight Tear Test (DWTT) performance exceeded 85% shear at 0 °C for all heavy‑gauge plates. Process trials demonstrated that increasing the total metallurgical reduction ratio from conventional values of 5.2-5.6 to 7.3-8.0 enabled stable DWTT performance at 0 °C in heavy‑gauge plates. MicroSim-PM v10.0 optimization indicated that further increasing the reduction ratio to approximately 9.7–10.7, combined with lower reheating temperatures (1200–1210 °C), increased finishing mill reductions (≈70%), and lower cooling stop temperatures, is expected to improve austenite grain size homogeneity (ZD reduced from ~6.7 to ~3.0) and extend stable DWTT performance to −10 °C. Transmission electron microscopy (TEM) based precipitation analysis on representative plates of thickness 25.4 and 28.22 mm identified multiple Nb–Ti–rich precipitate populations, including non‑dissolved precipitates (>100 nm), strain induced precipitates (>10 – ≤100 nm) that started as fine strain induced precipitates at the end of the roughing/beginning of finishing and then coarsened during the remaining finishing passes, and fine strain induced precipitates (≤10 nm) that formed at towards the end of finishing and did not have sufficient time or processing parameters to coarsen. The mean size of fine precipitates (≤10 nm) was similar in both the thickness.
Comparison of the JSW results with published data from other steel producers worldwide suggests that the appropriate NbC precipitate size distribution can play an important role in controlling pipe seam/girth weld HAZ austenite grain growth behavior and corresponding HAZ ductility and hardness.