What are Hot Rolled Bars? Steel Foundations
Hot rolled bars are products of a bar mill. Depending on the final application, the bars can be either shipped as hot rolled bars or further processed into cold rolled bars, also known as cold finished steel bars. The cross-sectional shape can be round, square, flat or hexagonal.
Most common types of hot rolled bar is merchant bar quality, or MBQ, which is specified when standard steel quality for non-critical applications is needed. Bars come in sizes ranging from 0.25 to 3.5 inch (6.4 to 90 mm) in diameter or side length. Depending on their chemical composition and properties, MBQ bars can be used in a wide variety of applications, such as general purpose structural, machinery parts, frames, fixtures, automotive and agricultural implements and equipment, brackets, stakes, ornamental works, forgings, base plates. As well as many other miscellaneous, non-critical applications that involve mild cold bending, mild hot forming, punching, machining, and/or welding.
Bars can also be produced as special bar quality steels, or SBQ, for more critical applications. SBQ bar can have special requirements for minimal steel impurities or tighter chemistry, dimensional, or property tolerances. They are mainly used in the production of fast-moving automotive and aircraft parts that require high-demanding performance.
Hot rolled bar begins by heating blooms or billets in a reheat furnace. The temperature of the billet exiting the reheat furnace is dependent on the steel grade being rolled and the design of the rolling mill. However, it must be heated above the material’s recrystallization temperature, due to the extreme amount of reduction done during this process.
The reheated steel is cleaned using a descaler and rolled to its final size and shape through a series of rolling passes (generally referred to as roughing, intermediate and finishing passes). Then the bar is cut to the desired length and allowed to cool to ambient temperature.
Processing parameters, such as reheating temperature, hot rolling specifics of reduction per pass, temperature of each rolling pass, interpass times, strain rates in each pass, total amount of rolling reduction, steel chemistry and final cooling time-temperature profile can all influence the final as rolled steel bar properties.
