Hot rolling
What is hot rolling?
Hot rolling is a metalworking process in which a metal billet is heated to high temperatures and then shaped into thinner pieces between rollers. During heating, the billet reaches its recrystallization temperature, causing its crystalline structure to change and making it easier to shape. The process reduces the thickness of the billet and improves its mechanical properties by increasing its strength and toughness.
Importance and process of hot rolling
The purpose of hot rolling is to shape large pieces of metal, such as steel billets or castings, into thinner and desired forms. The process begins by heating the billet, which can be done in gas or oil-fired furnaces, or for smaller pieces, by induction heating. Heating the metal above its recrystallization temperature—typically 800−1,200 degrees Celsius for steel—allows it to be shaped between the rollers.
The rolling itself usually takes place in multiple stages. First, the billet is roughly shaped in the roughing mill, and then it is further thinned and finished in the finishing stands to its final form. In typical hot rolling lines, the billet is passed several times back and forth through the roller stands until the desired shape and thickness are achieved.
Technical details of hot rolling
Hot rolling results in a significant plastic deformation and improves the internal structure of the metal. During forming, the coarse structure of the metal becomes refined and fine-grained, which in turn enhances its mechanical properties such as strength and toughness. In particular, steel transforms into fine-grained austenite during rolling, and phase changes occurring during cooling affect the steel's properties.
The effect of hot rolling on metal
Hot rolling has a significant impact on the microstructure of the metal, which improves its mechanical properties. During rolling, the metal's crystal lattice is dynamically recrystallized, forming a uniform microstructure that prevents work hardening. The phase changes that take place during cooling alter the character of the metal, making it stronger and tougher.
Advantages and challenges of hot rolling technology
Hot rolling has many advantages. It allows for the efficient and rapid processing of large quantities of metal, making production cost-effective. Hot rolling enables significant plastic deformation and can notably enhance the mechanical properties of the metal.
The challenges in hot rolling include the high temperatures and the demands they place on equipment durability. Controlling the process requires precision, as temperatures and deformations must be carefully managed to ensure a high-quality end product.
Applications of hot rolling
Hot rolling is commonly used in the manufacture of steel plates and profiles. For example, steel beams, sheets, and pipes are often produced through the hot rolling process. Railroad tracks, in particular, are made by hot rolling, as the process allows for the manufacturing of complex cross-sections.
Appearance and further processing of hot-rolled products
Hot-rolled metal surfaces are generally rough and display an oxide layer known as mill scale. This layer can be removed by acid pickling or by using the SCS process, which reveals a smoother surface. The dimensional accuracy of hot-rolled products can be controlled and is typically within 2–5% of the nominal measurements.
Summary
Hot rolling is a key process in the metal industry, resulting in significant forming and improving the microstructure of the metal. The process is especially important for the efficient processing of large metal quantities and for improving strength and toughness. Hot rolling enables the production of widely used steel profiles, sheets, and pipes, enhancing their durability for a range of industrial applications.
