Fundamentals of Rail Steel: Historical Evolution of Steel Rail Rolling Methods (Part 6)
1. Pass Rolling
The rolling technology for steel rails evolved through multiple stages. Initially, rails were produced via blooming mills in a single-heat process, directly rolling ingots into finished rails. Later, the U.S. adopted steam-driven three-high reversible mills, typically configured with three stands, significantly improving production efficiency. In early practices, ingots were first bloomed into rectangular billets using cogging mills and then transported via roller tables for final shaping. However, prolonged rolling time led to temperature loss, and mismatched capacities between cogging and finishing mills prompted a shift to a two-stage process: ingots were pre-bloomed into billets and reheated at larger mills before final rolling.
Given the complex rail profile, specialized cap-shaped passes (2–4 grooves) were designed on two-high reversible blooming mills to gradually form rail-like sections with high-leg geometries. Roughing utilized closed rail passes, while intermediate and finishing were performed on two-high or three-high mills. Typical mill layouts included three-stand cross-country mills or a "1+3" hybrid configuration (1 blooming mill + 3 three-high mills). Nevertheless, pass rolling faced limitations such as inconsistent head geometry, asymmetric cross-sections, and dimensional tolerances.
In 1900, American engineer J.S. Seaman revolutionized the industry with his universal rolling method patent (U.S. Patent 647,821) for rail finishing, catalyzing global adoption of universal mills and ushering in a new era of rail production.
2. Gary Rail Finishing Method
From 1901 to 1930, U-Steel's Gary Plant (U.S.) pioneered a vertical-roll-based finishing process. Pre-finishing and final passes using vertical rolls targeted the rail head and base respectively, achieving improved dimensional accuracy in the head and enhanced work hardening effects across both regions.
3. H.Hahn's Universal Rolling Experiment
In 1928, H.Hahn enhanced universal rolling by integrating backup rolls with edging functions before and after the universal mill. This configuration provided supplemental compression to the head and base areas that lacked direct reduction in conventional universal stands, achieving more uniform work hardening.
4. Wendel-Sidelor Industrial Universal Rolling
Developed by R.Stambach at France's Wendel-Sidelor Hayange Plant (1967), this patented system combined a reversible two-high blooming mill, universal mill, and edger. The process first shaped blooms into rail profiles, then precisely finished them through coordinated universal/edging passes. The intense deformation at the head and base delivered superior dimensional tolerance (e.g., ±0.3mm in height) and mechanical properties, establishing it as the global standard.
5.Technological Advancements and Innovations in Universal Rail Rolling
Following Wendel-Sidelor's pioneering universal rail rolling technology, global steelmakers have continuously refined the process for enhanced precision and performance.
Nippon Steel introduced an asymmetric vertical roll system – deploying smaller-diameter vertical rolls (φ450mm) for rail head compression and larger rolls (φ650mm) for the base. This innovation addressed profile asymmetry-induced bending, achieving uniform deformation and reducing dimensional tolerance to ±0.2mm.
In the 1990s, Germany's SMS Group pioneered the "2+3" tandem rolling process at Hyundai Steel Korea. Two universal mills operate sequentially: the first stand shapes the web and one head side, while the second processes the head's upper/lower surfaces. Alternating deformation eliminates head-web misalignment, achieving ±0.15mm height tolerance. This configuration has become the global benchmark.
6.Key Features of Universal Rail Rolling
(1) Asymmetric Vertical Roll Design:
The universal mill employs left-right asymmetric vertical rolls – smaller-diameter rolls (e.g., φ500mm) for the rail head (higher reduction) and larger rolls (e.g., φ700mm) for the base (lower reduction). This ensures synchronized roll contact and balanced rolling forces (±5% deviation), preventing rail bending or lateral shifting during entry.
(2) Hydraulic AGC System:
A hydraulic Automatic Gauge Control (AGC) system is implemented for all four rolls, enabling rapid (≤0.5s) and precise (±0.05mm) roll gap adjustments to maintain consistent profile geometry.
(3) Quick-Shifting Edger:
The two-high edger features multiple grooves and lateral shifting capability (response time <10s). It controls head/base widths without web deformation, coordinating with universal mill pass schedules.
(4) Adjustable Rolling Line Alignment:
Rolling line height is dynamically adjusted via entry guides and table elevation (±15mm range), ensuring symmetrical compression by aligning the rail axis with the mill centerline.
7.Advantages of Universal Rail Rolling
(1) Enhanced Metallurgical Properties:
Vertical rolls refine grain structure (ASTM 8-10), improving tensile strength (≥880MPa) and fatigue crack resistance at the base.
(2) Reduced Residual Stresses:
Bidirectional compression (horizontal + vertical) minimizes residual stress (<50MPa) compared to pass rolling (>120MPa).
(3) Superior Dimensional Accuracy:
Achieves ±0.15mm head radius tolerance and ±0.3mm rail height, exceeding EN 13674-1 Class A specifications.
(4) Lower Roll Consumption:
Simplified roll profiles reduce wear rates to 0.5-1.2kg/t vs. 1.2-7.2kg/t in groove rolling.
(5) High Mill Efficiency:
AGC-enabled roll gap control increases uptime to 92%, with pass adjustment completed in ≤3 minutes.
(6) Uniform Deformation:
Balanced elongation (horizontal: 1.15-1.25; vertical: 1.10-1.18) eliminates surface scratches from guides.
(7) Higher Reduction Ratios:
Per-pass elongation coefficients reach 1.25-1.4, surpassing groove rolling's 1.2-1.23 limits.
(8) Improved Surface Quality:
Open-pass design prevents scale entrapment, yielding surface roughness Ra≤12.5μm.
