Hot Rolling Process Optimization for 20CrMoNbVTiB Continuous Casting Round Billet Seamless Steel Tubes for 110S Grade Oil Country Pipe

Literature Overview

This paper, published in the journal Steel Pipe (Vol. 52, Issue 1, 2023, pp. 52-56), addresses a critical production challenge faced by Shandong Molong Petroleum Machinery Co., Ltd. The authors—Liang Hongxing, Sun Jing, and Ma Jishun—investigate the hot rolling process for Φ180 mm continuous rolling tube mills when using 20CrMoNbVTiB continuous casting round billets to produce 110S grade oil country seamless steel tubes. The core problem identified is that this microalloyed steel grade exhibits poor plasticity during the hot rolling stage, high deformation resistance, and frequent process failures, resulting in an original yield rate of only 81.6%. The paper proposes and validates a comprehensive set of process optimization measures that improved the yield rate to 91.5% and significantly extended the service life of the piercing mandrel.

Core Technical Challenges of 20CrMoNbVTiB Microalloyed Steel

The 20CrMoNbVTiB steel grade is a low-alloy microalloyed steel designed for oil country applications where high strength, good toughness, and resistance to sour service are required. The addition of niobium (Nb), vanadium (V), titanium (Ti), and boron (B) provides precipitation strengthening and grain refinement benefits. However, these microalloying elements create significant challenges during hot rolling:

• Precipitation behavior: NbC, NbVC, and TiC precipitates formed during solidification and slow cooling in continuous casting reduce austenite grain growth during reheating, but they also increase deformation resistance at rolling temperatures.
• Deformation resistance: The combined effect of alloying elements raises the flow stress substantially compared to plain carbon or low-carbon steels, demanding higher rolling forces and potentially causing equipment overload or defect formation.
• Plasticity limitations: The microalloyed steel has a narrower hot working window, meaning the temperature range in which acceptable deformation can occur without cracking is relatively restricted.
• Mandrel wear: The high deformation resistance and elevated rolling temperatures accelerate the wear of the piercing mandrel, which is the most critical and expensive consumable in the continuous rolling tube mill.

Process Optimization Measures and Technical Rationale

The authors propose five key optimization measures, each of which I will analyze for its metallurgical and mechanical rationale.

Ring Furnace Reheating Temperature Increase

The first measure involves raising the reheating temperature in the ring furnace. For 20CrMoNbVTiB continuous casting billets, the target reheating temperature was increased to ensure sufficient austenite homogeneity and adequate softening of the microalloyed precipitates. In continuous casting billets, the presence of non-metallic inclusions and segregation patterns can create localized weak spots. Higher reheating temperatures promote:

• Complete dissolution of coarse NbC and TiC precipitates that formed during the slow cooling of the casting.
• Uniform austenite grain size across the cross-section, reducing the risk of surface cracking during piercing.
• Lower deformation resistance at the piercing stage by increasing the degree of austenite softening.

The recommended reheating temperature range for this grade is approximately 1150-1200°C, which is higher than the typical 1080-1150°C used for plain carbon steels. However, excessive temperatures must be avoided to prevent excessive grain coarsening and surface oxidation.

Piercing Machine Parameters: Low Speed with Large Feed Angle

The piercing operation is the most critical and defect-prone stage in seamless tube production. The authors recommend using a lower piercing mill speed combined with a larger feed angle. The technical rationale is as follows:

• Lower piercing speed: Reduces the instantaneous deformation rate, allowing more time for dynamic recrystallization and stress relaxation. This is particularly important for microalloyed steels where the deformation resistance is high and the material is more susceptible to cracking.
• Larger feed angle: Increases the axial compression component of the deformation, which promotes more uniform deformation distribution along the billet length. A larger feed angle also reduces the radial tensile stress on the outer surface of the piercing mandrel, thereby reducing the risk of surface cracks and extending mandrel life.

The combination of these two parameters effectively reduces the peak stress on both the material and the mandrel, which directly addresses the two main failure modes observed in the original process.

Use of Broken-In Rolling Mills

The recommendation to use properly broken-in (磨合) rolling mills is a practical but often overlooked measure. New rolling mills have a hard, smooth surface that can cause galling and surface tearing on the hot steel, especially for alloyed grades with high deformation resistance. The breaking-in process gradually smooths the roll surface, removes residual machining marks, and creates a surface finish that reduces friction between the roll and the steel. For microalloyed steels, this reduces the risk of surface defects such as scratches, laps, and inclusion pull-out.

Continuous Rolling Mill Roll Speed Adjustment

The adjustment of roll speeds in the continuous rolling mill is aimed at optimizing the deformation distribution and reducing the risk of over-rolling or under-rolling. The specific adjustments depend on the desired final wall thickness and the deformation resistance of the material. For 20CrMoNbVTiB, the roll speeds were adjusted to ensure that the total reduction is distributed evenly across the stands, avoiding excessive reduction in any single stand that could cause wall thinning defects or roll damage.

Intermediate Reheating in Medium Frequency Induction Furnace Before Reduction

Perhaps the most innovative measure is the use of a medium frequency induction furnace to reheat the blank tube before the reduction stand. This addresses a specific problem: by the time the tube reaches the reduction stand, the temperature has dropped significantly due to the long rolling time and air cooling between stands. For microalloyed steels, the reduced temperature increases deformation resistance and can cause cracking during reduction. The intermediate reheating restores the tube temperature to an optimal range (approximately 950-1050°C), ensuring sufficient plasticity for the reduction operation without requiring excessive rolling force.

Results and Engineering Significance

The optimized process achieved remarkable results:

The improvement of nearly 10 percentage points in yield rate represents a substantial economic benefit. For a production line processing thousands of tonnes per month, this translates to hundreds of tonnes of additional product and significant savings in raw material costs. The improvement in mandrel life reduces both consumable costs and production downtime associated with mandrel replacement.

Study Insights and Engineering Practice Implications

This paper provides valuable lessons for engineers working with microalloyed seamless steel tubes. The key insight is that the process optimization must be holistic rather than focusing on a single parameter. Each measure addresses a specific aspect of the process chain, and their combined effect produces a synergistic improvement. The use of intermediate reheating is particularly noteworthy, as it represents a process innovation that decouples the temperature requirements of different rolling stages.

From a quality control perspective, the process optimization also has implications for the final product properties. By ensuring more uniform deformation and appropriate temperature control, the resulting tube is more likely to meet the mechanical property requirements of the 110S grade specification, including yield strength, tensile strength, elongation, and impact toughness at the required test temperatures.

The methodology employed—identifying the root cause of process failures, proposing targeted solutions, and validating through production trials—is a classic PDCA (Plan-Do-Check-Act) approach that is applicable to similar problems in other steel grades and product configurations. Engineers dealing with microalloyed steel hot rolling should consider similar systematic approaches to process optimization, particularly when facing yield rate challenges with new or difficult-to-roll grades.