Turning of Chilled Cast Iron Rolls
Chilled cast iron rolls are core components of steel rolling equipment. Their surface layer is made of highly wear-resistant white cast iron (HRC 50-65 hardness), while their core is made of tough gray cast iron (HB 180-220 hardness). These material properties present significant challenges for turning. Turning results in severe tool wear, cracking of the machined surface, and high cutting vibration, which severely impacts the roll’s cylindricity and surface quality. For example, a φ800×2000mm cold roll has a machining allowance of 10-15mm. Improper processing can increase tool consumption by over 50% and potentially render the roll scrapped due to surface cracking. Therefore, turning chilled cast iron rolls requires targeted solutions to address issues such as tool wear resistance, vibration control, and surface integrity.
The choice of tool material and geometry must accommodate the high hardness of chilled cast iron. The surface layer of white cast iron reaches a hardness exceeding HRC60 and contains a large amount of cementite and network carbides, placing extremely high demands on tool wear resistance. Ordinary carbide tools (such as YG8) wear 5-8 times faster than those used on gray cast iron. Therefore, ultra-hard tool materials are essential. Cubic boron nitride (CBN) tools, with a hardness of HV3000-4000 and wear resistance 10-20 times that of carbide, are suitable for finish turning. Ceramic tools (such as Al2O3-based ceramics) are less expensive than CBN and can be used for rough and semi-finish turning, but they offer lower impact resistance. Tool geometry requires a negative rake angle (-5° to -10°) to enhance cutting edge strength; a clearance angle of 4°-6° to reduce flank wear; and a leading rake angle of 90° to reduce radial cutting forces and vibration. A nose radius of 1-2mm prevents stress concentration that can lead to edge chipping. After a rolling mill adopted CBN tools, the single-tool processing length increased from 50m to 500m and the tool cost was reduced by 60%.
Determining cutting parameters requires balancing efficiency and tool life. During rough turning, to quickly remove the surface hardened layer, the cutting speed is controlled at 60-80 m/min, the feed rate at 0.3-0.5 mm/r, and the back cut depth at 2-3 mm. Although tool wear is faster at this speed, the stock removal per unit time is maximized. During semi-finishing turning, the speed is increased to 80-100 m/min, the feed rate at 0.2-0.3 mm/r, and the back cut depth at 1-2 mm, laying the foundation for finish turning. During finish turning, the speed is reduced to 50-70 m/min, the feed rate at 0.1-0.2 mm/r, and the back cut depth at 0.5-1 mm, ensuring a surface roughness Ra ≤ 1.6 μm. Excessively high cutting speeds can intensify thermochemical wear of CBN tools. When the speed exceeds 120 m/min, tool life is shortened by 50%. Excessive feed rate will increase the cutting force and cause chatter marks on the roller surface. Experiments show that when the feed rate is reduced from 0.4mm/r to 0.2mm/r, the surface waviness drops from 5μm to below 1μm.
Vibration control and cooling lubrication technologies guarantee machining quality. The cutting process of chilled cast iron rolls is intermittent (alternating surface and core material), which is prone to high-frequency vibration (1000-3000Hz), leading to surface chatter marks and tool chipping. Solutions include: using a rigid flange to connect the machine tool spindle to the roll, combined with an end-drive center, to minimize the connection gap (≤0.01mm); installing a damper on the toolholder to absorb vibration energy through viscous damping, reducing the amplitude by over 60%; and selecting heavy-duty machine tools (such as the CW61125), which have a bed rigidity 30% higher than that of conventional lathes and a natural frequency that avoids cutting vibrations. The cooling system utilizes a combination of oil mist lubrication and high-pressure cooling: oil mist (compressed air + extreme pressure cutting oil) is precisely sprayed onto the cutting edge through a nozzle, forming a lubricating film. Simultaneously, an emulsion at a flow rate of 30-50L/min is used to flush the chips and reduce the temperature in the cutting zone. For CBN tools, insufficient cooling will cause thermal cracks on the cutting edge, so it is necessary to ensure that the cooling system pressure is ≥0.5MPa and the flow rate is ≥40L/min.
The process and quality inspection must balance surface protection. The machining process for chilled cast iron rolls is as follows: rough turning (removing scale and defective layers) → flaw detection (magnetic particle inspection for surface cracks) → semi-finishing turning (machining to the finished size with a 1-2mm margin) → stress relief (holding at 200-250°C for 6 hours) → finish turning (to the finished size). During rough turning, excessive cutting forces that could cause surface flaking should be avoided. Backcut depth should not exceed 3mm, and the first cut should be light (0.5-1mm) to remove loose surface structures. After finish turning, the rolls are tested for cylindricity (≤0.03mm/m), surface roughness (Ra ≤0.8μm), and hardness (HRC 55±2). Penetrant testing is also used to inspect the surface for microcracks. Superfinish rolls are also ground and polished to reduce the surface roughness to Ra 0.02μm, improving the surface quality of the rolled strip. A steel company optimized its process to reduce the grinding amount of the rolls from 2mm to 0.5mm, extending their service life by 30% and reducing the surface defect rate of the rolled strip by 80%.
