Process Development and Application of Small Diameter GCr15 Bearing Steel Tubes

Literature Overview

This paper by Wang Zenghai, Wen Bo, Gao Heping, and Lin Zhen, published in Steel Pipe (2021, Vol. 50, No. 4, pp. 56-58), documents the development and trial production of small diameter GCr15 bearing steel tubes using the hot continuous rolling (hot finishing mill) process. The research was conducted by the Steel Pipe Company of Inner Mongolia Baogang Steel Union Co., Ltd., a major Chinese steel producer. The study addresses a specific manufacturing challenge: the production of small diameter bearing steel tubes, which have traditionally been produced by cold drawing or cold rolling processes, using the more cost-effective hot continuous rolling route.

Core Technical Findings

The paper reports that the hot continuous rolling process is technically feasible for producing small diameter GCr15 bearing steel tubes, but identifies internal folding (internal fold) as the predominant quality issue encountered during trial production. The authors attribute the internal fold defect to three primary factors: center segregation or cracking in the continuous casting billet, unreasonable reheating furnace temperature profiles, and improper setting of the piercing mill process parameters.

Process Parameters and Trial Production Results

Technical Analysis of Internal Fold Defect

Internal folding in hot-rolled steel tubes is a severe surface and subsurface defect that occurs when the inner surface of the tube wall is folded inward during the forming process. This defect is particularly problematic in bearing steel tubes because it can propagate during subsequent cold working operations and ultimately lead to premature fatigue failure in service.

Root Cause Analysis Using FMEA Methodology

Metallurgical Considerations for GCr15 Steel

GCr15 is a through-hardening bearing steel with a high carbon content (approximately 1.0%) and chromium content (approximately 1.5%). The high carbon content makes this steel particularly susceptible to:

• Center segregation: The high carbon content promotes center segregation during continuous casting, where carbon and other alloying elements accumulate in the center of the billet. This creates a chemically inhomogeneous microstructure that is prone to cracking and folding during hot working.
• Thermal cracking: The high carbon content reduces the hot ductility of the steel, making it more susceptible to thermal cracking during reheating and hot working.
• Decarburization: The high carbon content increases the tendency for surface decarburization during reheating, which affects the final hardness and wear resistance of the bearing tube.

Process Development and Optimization

Continuous Casting Billet Quality Control

The quality of the continuous casting (CC) billet is the foundation of the entire production process. For small diameter bearing steel tubes, the following billet quality criteria should be established:

1. Center segregation control: The billet should be cast with a low center segregation ratio, achieved through optimized casting parameters including low casting speed, high superheat control, and electromagnetic stirring.
2. Center porosity and cracking: The billet should be free from center porosity and center cracks, which can be verified by ultrasonic testing (UT) and/or hydrostatic testing.
3. Chemical homogeneity: The chemical composition should be uniform throughout the billet cross-section, with no significant center segregation of carbon, chromium, or other alloying elements.

Reheating Furnace Optimization

The reheating furnace temperature profile is critical for achieving proper deformation behavior during piercing:

• Soaking temperature: Should be controlled to avoid excessive grain growth while ensuring uniform temperature throughout the billet cross-section. For GCr15 steel, the soaking temperature should typically be in the range of 1150-1200°C, depending on the billet diameter.
• Heating rate: Should be controlled to prevent thermal stresses that could cause surface or internal cracking. The heating rate should be reduced for larger diameter billets.
• Atmosphere control: The furnace atmosphere should be controlled to minimize decarburization and oxidation. A neutral or slightly reducing atmosphere is preferred.

Piercing Mill Parameter Optimization

The piercing mill is the critical process step for producing the initial hollow shell from the solid billet:

• Piercing plug geometry: The plug nose angle and diameter should be optimized to achieve uniform deformation and complete elimination of center defects. A smaller plug nose angle promotes more uniform deformation but increases the required piercing force.
• Piercing reduction: The reduction should be sufficient to completely eliminate the center of the billet, which contains the center segregation and porosity. A piercing reduction of 30-40% is typically required for bearing steel.
• Roll gap and plug alignment: Precise alignment of the roll gap and plug is essential to avoid eccentric piercing and non-uniform wall thickness.

Engineering Practice and Quality Control

The trial production results demonstrate that the hot continuous rolling process is technically feasible for small diameter GCr15 bearing steel tubes, but the quality issues encountered highlight the need for comprehensive process control and quality assurance. The following quality control measures should be implemented:

1. Incoming billet inspection: 100% ultrasonic testing of CC billets to detect center porosity and internal cracks.
2. Process monitoring: Real-time monitoring of reheating furnace temperatures, piercing mill parameters, and rolling mill parameters.
3. In-process inspection: Visual inspection and eddy current testing of the hollow shell after piercing to detect internal folds.
4. Final product testing: Comprehensive mechanical testing, chemical analysis, and non-destructive testing of the finished tubes.

Key Questions and Reflections

The trial production results raise several important questions for future research and development:

• What are the optimal continuous casting parameters for GCr15 steel to produce billets with minimal center segregation?
• How does the reheating temperature profile affect the microstructure and deformation behavior of GCr15 steel during piercing?
• What is the minimum piercing reduction required to completely eliminate center defects in GCr15 steel billets?
• Can the internal fold defect be completely eliminated through process optimization, or is it inherent to the hot continuous rolling of high-carbon bearing steels?

From a manufacturing economics perspective, the hot continuous rolling process offers significant cost advantages over cold drawing and cold rolling for small diameter tubes. The cold drawing process requires multiple drawing passes, intermediate annealing, and extensive surface finishing, resulting in high energy consumption and long production cycles. The hot continuous rolling process can produce tubes in a single continuous operation, with significantly lower energy consumption and shorter production cycles. However, the quality challenges associated with hot rolling of bearing steels must be overcome to realize these economic benefits.

Study Insights and Implications

This paper documents an important step in the development of cost-effective production methods for small diameter bearing steel tubes. The identification of internal fold as the predominant quality issue provides a clear focus for future process optimization efforts. The three root causes identified (billet quality, reheating furnace profile, and piercing mill parameters) represent a comprehensive framework for addressing the quality challenges.

For steel pipe manufacturers considering the adoption of hot continuous rolling for bearing steel tubes, the key lessons from this study are:

• Invest in high-quality continuous casting technology and billet quality control.
• Develop and validate reheating furnace temperature profiles through systematic trials.
• Optimize piercing mill parameters through parametric studies and process simulation.
• Implement comprehensive quality control at every stage of the production process.

The study also highlights the importance of process development and validation in steel pipe manufacturing. The trial production approach, while time-consuming and costly, provides invaluable process knowledge that cannot be obtained through theoretical analysis alone. Manufacturers should invest in systematic process development programs to optimize production processes and achieve consistent quality.

The ultimate goal of this research is to enable the production of small diameter GCr15 bearing steel tubes with quality equivalent to cold-drawn tubes, but at significantly lower cost. Achieving this goal will require continued research and development, but the trial production results provide a solid foundation for future progress.