The shape of hot-rolled coils must satisfy certain dimensional tolerances required for downstream processes and handling. Some steel grades are more susceptible to the so-called “coil sagging” — coil shape distortion when a coil is not perfectly round. An important factor responsible for coil sag is phase transformation of austenite during cooling after hot rolling, when a significant portion of austenite remains in the material during strip coiling. Aiming at keeping the coil shape at ac-ceptable levels with less impact on hot strip mill productivity, ArcelorMittal utilizes several techniques of process optimiza-tion, including a new technology developed and patented by its Research Center in East Chicago. This article summarizes ArcelorMittal knowledge and industrial experience in this field.

The interaction of Nb content and coiling temperature is important to properties such as strength and low-temperature toughness of heavy-gauge (≥22 mm) X80 line pipe steels. In this study, two levels of Nb are examined as well as two dif-ferent coiling temperatures for the alloy with a lower level of Nb through the use of pilot-scale vacuum induction melting, casting, rolling and coiling thermal simulations. Tensile testing is completed and also drop weight tear testing, which was performed at two temperatures and orientations. Microstructural characterization provides insight into the differences ob-served from tensile and fracture testing.

Knowledge from the contribution to ductility performance from the through-thickness as-rolled austenite grain size/homogeneity and niobium precipitate evolution in normalized/quench-and-tempered steels has been applied to the heat-affected zone (HAZ) ductility performance of API and structural steel welds. The through-thickness austenite grain size/homogeneity and Nb precipitation evolution developed during hot rolling has been observed to be beneficial to influ-encing the HAZ grain size and microstructure to peak welding temperature. This has resulted in stable and enhanced duc-tility performance of the HAZ in various applications such as API pipeline steels for natural gas or H2 transmission.

In the hot strip mill, camber is an important quality characteristic of a transfer bar coming out of a roughing mill. Excessive camber negatively impacts subsequent process in a finishing mill. Therefore, it is desirable to measure and control the bar camber during rolling in a roughing mill. Centerline deviation measurements cannot distinguish bar movements from bar shape changes. A new camber gauge system for roughing mill applications allows for characterization of the true bar shape, providing camber/curvature information that can potentially be used by operator/control system to correct the bar shape. Camber sensing and its current application are described.