Seamless carbon steel pipe cross rolling process

The cross-rolling process for seamless carbon steel pipes is a technique where the material rotates, deforms along its own axis, and moves forward between two or three rollers, which rotate in the same direction but have intersecting or inclined longitudinal axes. This method, positioned between longitudinal and cross rolling, is crucial in the production of seamless pipes and is also used for rolling periodic cross-sections such as steel balls.

There are three primary types of inclined roll forming in cross-rolling:

- Cross-rolling for seamless carbon steel pipe production: This includes several stages such as cross-rolling piercing, extension, straightening, and sizing.

- Cross-pass rolling: Characterized by rolling grooves with variable height and pitch on the roll surface, this type is used to produce rotary products with variable lengths and cross-sections, such as steel balls and screws.

- Profile cross-rolling: In this process, three rotating tapered rollers are controlled hydraulically or mechanically to move radially relative to the center of the rolled piece, enabling the rolling of variable-section shafts. This technique is mainly used for producing elongated shafts with variable cross-sections, such as textile spindles, knives, scissors, and surgical instrument blanks.

The cross-rolling method is extensively applied in the thermal expansion of seamless carbon steel pipes. It plays a significant role not only in major thermal expansion processes like piercing but also in basic processes such as pipe rolling, leveling, sizing, stretching, diameter expansion, and spinning. The key difference between cross-rolling, longitudinal rolling, and cross rolling lies in the direction of metal flow. In longitudinal rolling, the metal flows in the same direction as the roll surface, whereas in cross-rolling, the metal flow aligns with the roll surface's movement. Cross-rolling occupies a middle ground, where the deformed metal’s flow direction forms an angle with the deforming tool’s movement, leading to a spiral forward motion as the metal rotates around its axis.

In production, two main types of cross-rolling mills are used: two-roller systems and three-roller systems.

With advancements in technology, the perforation process in thermal expansion has become more refined and automated. The cross-rolling and piercing process can be broken down into three distinct stages:

- Unstable Initial Stage: The metal at the tube blank’s front gradually fills the deformation zone, initiating contact with the rollers. During this stage, primary and secondary occlusions occur.

- Stable Stage: This is the main phase of piercing, where the metal at the front enters the deformation zone, and the process continues until the metal at the tail begins to leave the zone.

- Unstable Final Stage: The metal at the tail end of the tube blank gradually exits the deformation zone, concluding the process as the metal completely leaves the rollers.

These stages show clear differences between stable and unstable processes, particularly visible in the size variations along the capillary. Typically, the front end of the capillary has a larger diameter, the tail end a smaller one, and the middle section remains consistent. This variation is a hallmark of the unstable process, driven by differences in the metal’s interaction with the deformation zone and the axial resistance provided by the plug during the piercing process. As a result, the front end of the tube blank experiences less axial extension and more lateral deformation, leading to a larger diameter. Conversely, the tail end stretches more easily with reduced lateral deformation, resulting in a smaller diameter.