Development and Application of Continuous Casting Weld-Overlay Composite Rolls at Pangang

Literature Overview

This 1999 study from Pangang (Panzhihua Iron and Steel Group) documents the development and industrial application of weld-overlay composite rolls for continuous casting operations. Published in Iron and Steel Vanadium Titanium, the research represents a significant advancement in roll technology for the steel industry, addressing the critical need for durable rolls in the demanding environment of continuous casting.

Core Technical Content

Continuous casting rolls experience extreme conditions: contact with molten steel at 1400–1500 °C, intense thermal cycling, mechanical pressure from the solidifying shell, and erosion from steel flow. Traditional solid rolls made of cast iron or steel wear rapidly, requiring frequent replacement and causing production interruptions. The weld-overlay composite roll concept combines a tough steel core with a wear-resistant overlay layer to achieve both structural integrity and surface durability.

Composite Roll Design

Component	Material	Function
Core	Low-carbon steel (20 steel or 45 steel)	Structural support, impact resistance
Bond layer	Medium-carbon alloy steel (e.g., 40Cr)	Transition between core and overlay
Overlay layer	High-carbon high-chromium cast iron or alloy steel	Wear resistance, thermal stability
Total thickness	600–800 mm diameter	Standard for slab casting
Overlay thickness	15–25 mm	Wear-resistant working surface

Manufacturing Process

The composite roll manufacturing process involves several critical steps:

Core preparation: The steel core is machined to precise dimensions and inspected for internal defects.
Surface preparation: The core surface is roughened to 50–100 μm Ra to ensure mechanical interlock with the overlay.
Preheating: The core is preheated to 600–700 °C to reduce thermal shock during overlay application.
Overlay application: Multiple layers of overlay material are applied using submerged arc welding (SAW) or electroslag welding (ESW).
Post-weld treatment: Stress relief annealing at 600–650 °C to reduce residual stresses.
Machining: The overlay surface is ground to final dimensions and surface finish requirements.

Technical Analysis

Metallurgical Bonding

The bond between the core and overlay layers is critical to roll performance. The researchers investigated several factors affecting bond quality:

Dilution rate: Controlled at 10–20% to ensure adequate alloying without excessive softening of the overlay.
Microstructure: The bond zone exhibits a mixed microstructure of martensite and bainite, providing good toughness and strength.
Bond strength: Tested through ring tensile tests, achieving bond strengths of 200–300 MPa.
Thermal cycling resistance: The overlay must withstand 50,000+ thermal cycles without cracking or delamination.

Performance Comparison

Performance Metric	Solid Cast Iron Roll	Weld-Overlay Composite Roll	Improvement
Service life	1–2 months	4–6 months	300–500%
Surface hardness	45–50 HRC	58–65 HRC	15–30%
Thermal cracking resistance	Poor	Good	Significant
Impact resistance	Poor	Excellent	Dramatic
Cost per ton of steel	High	Low	40–60% reduction

Engineering Practice Integration

The Pangang study documents successful industrial application of composite rolls in slab continuous casting operations. Key lessons learned include:

Installation and alignment: Precise roll alignment is critical to prevent edge loading and premature wear. Laser alignment systems are recommended for new installations.
Burn-in procedure: New rolls require a controlled burn-in period of 24–48 hours at reduced casting speed to allow thermal expansion and seating.
Monitoring: Regular inspection of roll surface condition, including measurement of overlay thickness and identification of wear patterns.
Repair capability: Worn overlay layers can be removed and renewed through re-welding, extending roll life further.

Defect Analysis

Common defects in weld-overlay composite rolls and their prevention:

Defect	Cause	Prevention
Overlay delamination	Insufficient bond strength	Optimize preheat; ensure clean surface; control dilution
Overlay cracking	Excessive residual stress	Post-weld stress relief; controlled cooling
Uneven wear	Roll misalignment or eccentricity	Precision machining; laser alignment
Overlay spalling	Thermal fatigue	Proper overlay thickness; adequate toughness

Key Insights and Reflections

The Pangang research demonstrates the economic and technical advantages of weld-overlay composite rolls for continuous casting applications. The 300–500% improvement in service life translates directly to reduced production costs and improved operational reliability. The study also highlights the importance of process control and quality assurance in composite roll manufacturing.

From a broader perspective, this work contributed to the widespread adoption of composite roll technology in the Chinese steel industry. Subsequent developments have expanded the application to other rolling mills, including hot strip mills and cold rolling mills, with similar benefits in terms of life extension and cost reduction.

The research also underscores the value of industry-academia collaboration in solving practical engineering problems. The involvement of Pangang's research institute, mechanical manufacturing company, and production units ensured that the developed technology was both technically sound and practically implementable.