What are the details of the hot rolling piercing process for seamless carbon steel pipes

Seamless carbon steel pipes are core components in machinery manufacturing, hydraulic transmission, and engineering machinery. Their production processes are mainly divided into two categories: hot rolling and cold drawing. Among them, the hot rolling piercing process, with its advantages of high forming efficiency, large capacity, low cost, and suitability for large-specification casing production, has become the mainstream forming method for medium and large-sized seamless carbon steel pipes (outer diameter 80~200mm), and is widely used in mass production scenarios. The hot rolling piercing process uses carbon steel round ingots as raw materials. After being heated to a plastic state in a furnace, the solid round ingots are processed into hollow tubes through the coordinated action of components such as the piercing mill, guide plate, and mandrel, laying the foundation for subsequent finishing and heat treatment processes. The hot rolling piercing process is the "shaping" stage in the forming of seamless carbon steel pipes. The matching degree of its key process parameters directly determines the wall thickness uniformity, roundness, surface quality, and internal microstructure of the hollow tube.

First, what is the principle of the hot rolling piercing process for seamless carbon steel pipes?

The core principle of hot rolling piercing technology for seamless carbon steel pipes is to utilize the plastic deformation characteristics of metal at high temperatures. Through the coordinated action of the piercing mill's rolls, guide plates, and mandrel, a solid round steel ingot is gradually rolled into a hollow tube. The entire process is mainly divided into three stages: the biting stage, the piercing stage, and the leveling stage.

1. Biting Stage: The biting stage is the initial stage of hot rolling piercing. The carbon steel round ingot, heated to a plastic state, is fed into the piercing mill. Through the rotational friction of the rolls, the ingot is bitten between the rolls and the mandrel. At this point, the ingot begins to bear radial pressure and axial tension, and the surface metal undergoes initial plastic deformation, laying the foundation for subsequent piercing. The core requirement of this stage is to ensure a smooth biting of the ingot, avoiding slippage or deviation; otherwise, it will lead to eccentric piercing in the subsequent stages.

2. Piercing Stage: The piercing stage is the core of the entire process. After the round steel ingot is bitten in, under the pressure of the rolls, the push of the mandrel, and the guidance of the guide plate, the internal metal is gradually pierced, forming a hollow tube. In this stage, the plastic deformation of the metal is most intense, with radial and axial deformation occurring simultaneously. The wall thickness is gradually reduced to the preset size, and the uniformity of this deformation directly determines the wall thickness uniformity and the roundness of the hollow tube.

3. Sizing Stage: The sizing stage is the final stage of the piercing process. After the piercing stage, the hollow tube enters the sizing section. Through further pressure and trimming by the sizing rolls, the wall thickness and roundness deviations of the hollow tube are corrected, the metal grains are refined, and the surface quality is improved, ensuring that the dimensional accuracy and surface quality of the hollow tube meet the requirements of subsequent processing.

Secondly, what are the impacts of key parameters in the hot rolling piercing process of carbon steel seamless pipes on product quality?

Taking 20# and 45# carbon steel seamless pipes as research objects, and combining production practice, this paper analyzes the influence mechanism of the four key parameters—heating temperature, piercing speed, mandrel extension, and guide plate spacing—on product quality, clarifies the reasonable control range of each parameter, and provides a theoretical basis for subsequent parameter optimization.

1. Influence of Heating Temperature: Heating temperature is the primary parameter in the hot rolling piercing process, directly determining the plasticity, deformation resistance, and metal microstructure of carbon steel, and has the most significant impact on product quality. The plasticity of carbon steel increases with increasing heating temperature, while its deformation resistance decreases. However, excessively high or low heating temperatures can lead to various quality defects. When the heating temperature is too low, the carbon steel lacks sufficient plasticity and has high deformation resistance, making metal deformation difficult during piercing. This can result in problems such as slippage during insertion and excessive piercing force, leading to defects in the hollow tube, including uneven wall thickness, eccentricity, and surface scratches. Simultaneously, stress concentration is significant during deformation, easily causing surface cracks, especially in 45# medium carbon steel, which has a higher carbon content and relatively poorer plasticity; the incidence of crack defects increases significantly during low-temperature piercing. When the heating temperature is too high, carbon steel will overheat and burn, causing the metal grains to coarsen rapidly, leading to a decrease in the mechanical properties of the hollow tube (reduced strength and hardness, and poor toughness). Simultaneously, excessively high temperatures will accelerate oxidation on the metal surface, forming a thick oxide scale. The peeling off of this scale will cause defects such as pitting and cratering on the surface of the hollow tube, and in severe cases, surface cracks. Furthermore, overheating will also lead to a loose internal metal structure and delamination defects, affecting the load-bearing capacity of the tube.

2. The Influence of Piercing Speed: Piercing speed refers to the linear speed of the rolls, which directly determines the deformation speed and piercing efficiency of the metal, affecting the uniformity of metal deformation and the stress state. Piercing speeds that are too fast or too slow will lead to uneven metal deformation and cause quality defects. When the piercing speed is too slow, the metal remains in the piercing area for too long, undergoing excessive compression cycles, leading to localized over-deformation and defects such as thinner wall thickness and surface scratches. Simultaneously, prolonged residence time exacerbates surface oxidation, causing oxide scale to adhere to the hollow tube surface and affecting surface quality. Furthermore, excessively slow piercing speeds reduce production efficiency and increase production costs. When the piercing speed is too fast, the metal deforms too quickly, preventing sufficient plastic deformation and causing internal stress concentration, which can easily lead to surface cracks and internal folds. Additionally, excessive speeds can cause unstable biting, resulting in deviation and eccentricity, affecting the wall thickness uniformity and roundness of the hollow tube. Moreover, excessively fast piercing speeds also accelerate mandrel wear, shortening mandrel lifespan and indirectly affecting product quality.

3. The Influence of Mandrel Forward Extension: Mandrel forward extension refers to the offset between the mandrel head and the roll centerline. It directly determines the penetration position and stress state of the metal during piercing, and has the most significant impact on the wall thickness uniformity and eccentricity defects of the hollow tube. Excessive or insufficient mandrel extension will lead to uneven stress on the metal, causing defects such as eccentricity and wall thickness deviation. If the mandrel extension is too small, the pushing force on the metal is insufficient, resulting in the metal penetrating too far back. During piercing, the metal undergoes excessive radial deformation and insufficient axial deformation, leading to a thicker hollow capillary tube with a rough inner wall and a tendency for internal folding defects. Simultaneously, insufficient pushing force can cause unstable biting, resulting in deviation and exacerbating eccentricity. If the mandrel extension is too large, the pushing force on the metal is excessive, resulting in the metal penetrating too far forward. During piercing, the metal undergoes excessive axial deformation and insufficient radial deformation, leading to a thinner hollow capillary tube with poor wall thickness uniformity and a tendency for external folding and surface cracks. Furthermore, excessive pushing force will accelerate mandrel wear and may even damage the mandrel, interrupting the production process.

4. The Influence of Guide Plate Spacing: Guide plate spacing refers to the distance between the guide plate and the rolls. Its function is to guide and position the round steel ingot and hollow tube, ensuring stability during metal deformation and preventing problems such as deviation and eccentricity. It directly affects the roundness and wall thickness uniformity of the hollow tube. Excessive or insufficient guide plate spacing will affect the guiding effect and cause quality defects. If the guide plate spacing is too small, the excessive extrusion force on the metal will lead to defects such as scratches and abrasions on the surface of the hollow tube. Simultaneously, excessive extrusion force will also cause uneven metal deformation, resulting in wall thickness deviations and out-of-tolerance roundness. Furthermore, an excessively small guide plate spacing will increase the resistance to metal movement, accelerate the wear of the rolls and guide plates, and affect production stability. When the spacing between the guide plates is too large, their guiding and positioning functions are weakened. This can lead to problems such as misalignment and wobbling of the round steel ingot and hollow tube during the piercing process, resulting in defects like eccentricity and uneven wall thickness in the hollow tube. Furthermore, an excessively large spacing leads to a lack of constraint during metal deformation, causing surface defects such as localized protrusions and depressions, thus affecting surface quality.

Third, safeguard measures for the optimized hot rolling piercing process of carbon steel seamless pipes:

(1) Strengthen heating temperature control: Adopt an intelligent temperature control system to monitor the furnace temperature in real time, ensuring temperature fluctuations are ≤±10℃. Regularly calibrate the temperature control equipment to avoid temperature deviations.

(2) Optimize piercing speed control: Adopt a digital control system to accurately set the roll linear speed, maintaining speed stability and avoiding fluctuations.

(3) Strengthen the maintenance of the mandrel and guide plate: Regularly check the mandrel extension and guide plate spacing, adjust deviations promptly, and regularly replace worn mandrels and guide plates to ensure accurate guidance and positioning.

(4) Establish a parameter ledger to record the optimal parameter combinations for different steel grades and specifications. Regularly analyze production data and fine-tune parameters as needed based on changes in raw material quality and equipment status.

(5) Strengthen operator training: Improve the professional competence of operators and ensure they operate strictly according to the optimized parameters to avoid human error.

Fourth, Conclusion.

The four key parameters of the hot rolling piercing process—heating temperature, piercing speed, mandrel extension, and guide plate spacing—significantly affect the quality of seamless carbon steel pipes. These parameters are interconnected and mutually restrictive; therefore, setting a reasonable combination of parameters is crucial for improving product quality and reducing defect rates. The optimized hot rolling piercing process is highly targeted and feasible, suitable for the mass production of medium to large-sized seamless carbon steel pipes, and can provide theoretical support and practical reference for enterprises. In the future, by combining digital simulation technology and intelligent detection technology, process parameters can be further optimized to simulate the deformation patterns of metal during piercing and achieve intelligent parameter control. Simultaneously, by incorporating surface treatment technology to reduce metal surface oxidation, product quality can be further improved, driving the carbon steel seamless pipe industry towards refinement and efficiency.