Rock drill rods, as crucial components connecting the drill bit and rock drilling tools in drilling projects, significantly impact mining efficiency. Hollow steel is the primary raw material for manufacturing rock drill rods. Since rock drill rod is a slender rod working under complex stress conditions, it requires high quality of  hollow steel. Currently, there are two main production processes for hollow steel: hot piercing-hot rolling and hot rolling with core.

1.  Hot Piercing-Hot Rolling Method

The hot piercing-hot rolling method is an effective way to produce hollow steel and is very common in China. The process involves several steps to process round billets into the required hexagonal and round hollow steel.

The general process is as follows:

Initially, round billets are cut according to production needs, preparing them for subsequent heating and piercing processes. A specially designed medium frequency inductor heats the round billets, providing preheating, temperature raising, and insulation, reaching the piercing temperature (1140-1200°C). Subsequently, a two-roll piercing machine performs oblique rolling piercing. The outer diameter and wall thickness of the pierced billet must meet subsequent process requirements. A three-roll tube rolling machine then processes the billet to achieve the required external and internal diameters. After diameter reduction, the billet requires forming rolling on a three-roll or six-roll mill to produce the required hollow steel. Finally, the rolled hollow steel undergoes cooling process to ensure the stability of its structure and properties.

The hot piercing-hot rolling technique uses advanced heating and piercing equipment, as well as precise control techniques, ensuring dimensional accuracy and ensuring the core holes are properly rounded. However, during the piercing and diameter reduction stages, fine spiral cracks and decarburization are easily to occur on the surface of the core hole, resulting in poor performance and stability of the manufactured rock drill rods, making them economically suitable for use in low hardness, uniformly distributed rock drilling environments.

2.  Hot Rolling with Core Method

The hot rolling with core method, originating in Europe, utilizes continuous cast billets and high manganese steel core materials.

The process is roughly as follows:

Continuous cast billets are cut according to production needs. After center drilling, an 80Mn14 core material is inserted, followed by heating in a continuous heating furnace. The heating temperature is controlled between 1050 and 1200°C. After exiting the furnace, the billet enters the rolling stage. Before rolling, high-pressure water descales the billet, removing the oxidized layer. The unit then rolls the billet into required hexagonal or round hollow steel shapes, with rolling temperatures controlled between 950 and 1180°C, and final rolling temperatures controlled between 830 and 950°C. After rolling, the rolled piece is transferred to a cooling bed for temperature and cooling control, then the core is withdrawn.

Due to the different expansion coefficients between the high manganese steel and the hollow steel billets, high manganese steel shrinks uniformly,  and is extendable by 40%-60%, the core is easily withdrawn when cooled to room temperature, resulting in a smooth core holes and good mechanical properties of the hollow steel. The hot rolling with core method produces high-quality, stable-performance hollow steel, well-suited for hard, unevenly distributed, fractured rock layers. This method is also one of the mature hollow steel production techniques currently available worldwide.