During the operation of six-high temper mills on continuous annealing lines, periodic transverse roll mark defects frequently appear on the strip surface right after roll change and mill closing. This surface defect is known as temper mill transverse roll mark. The defect interval matches the circumference of the work roll surface. It is a typical surface quality defect in continuous annealing lines. This defect directly leads to product downgrade. It also restricts the stable production of high-grade steel with strict surface requirements.
Traditional industry solutions mostly focus on the roll body. Operators optimize roll plating processes. They improve roll grinding accuracy and adopt rolls with higher hardness. These steps can reduce the probability of roll marks. Однако, the rising output of high-performance steel grades increases requirements for roll material and plating. This pushes up production costs. These methods can only ease the defects. They cannot eliminate temper mill transverse roll mark fundamentally. To solve this problem thoroughly, in-depth analysis is required. We analyze equipment action sequence, hydraulic control logic and collision mechanics.
## 1. Formation Mechanism of Temper Mill Transverse Roll Mark
The roll surface damage that causes temper mill transverse roll mark does not occur during normal rolling. It mainly takes place in the mill closing sequence after roll replacement.
We break down the automatic mill closing sequence. Roll surface damage happens when the roll gap shrinks from 15 мм до 0 мм. Rolling force rises from 0 kN to the minimum rolling force of 450 кН.
In this phase, the equipment runs under closed-loop rolling force control. A fixed-parameter PI controller handles pressure regulation. The original controller uses a relatively large proportional coefficient. It also has a short integral time.
Before the work rolls touch the steel strip, the PI controller output already hits the maximum limit value. When the work rolls make instant contact with the strip, the actual rolling force rises rapidly. It keeps rising until it reaches the 450 kN minimum rolling force. The mill closing action finishes at this point.
At the moment of contact, the controller output is saturated. The hydraulic screwdown mechanism carries large motion inertia. It creates violent impact collision between work rolls and the strip.
The original PI parameters deliver fast response. But they easily cause output overshoot. They may even trigger system oscillation. This further amplifies collision impact at contact. The impact creates micro-damages on the work roll surface. In subsequent rolling, the roll surface defects transfer periodically onto the strip surface. Окончательно, regular temper mill transverse roll marks form.
## 2. Mechanical Mechanism of Collision during Mill Closing
Based on momentum conservation law, collision impact force can be calculated by momentum formula. When work rolls collide with the strip, the speed drops to nearly zero after contact.
The resultant force on the roll surface is proportional to roll weight and instant contact speed. It is inversely proportional to collision duration.
With fixed roll weight, two approaches can reduce impact load on roll surface. The first way is to reduce the work roll speed at the moment of contact. The second way is to extend collision time for shock buffering.
### 2.1 Reduce Instant Contact Speed
The screwdown speed of work rolls is driven by HGC hydraulic cylinders. Cylinder speed is determined by electro-hydraulic servo valve opening. It is also affected by hydraulic pump outlet pressure and roll load inside the mill.
With stable hydraulic pressure and load, servo valve opening controls flow rate. Larger opening brings faster screwdown speed. Smaller opening enables smoother mill closing. Поэтому, limiting servo valve opening at the late stage of mill closing can lower the instant movement speed of work rolls. It reduces collision kinetic energy.
### 2.2 Extend Collision Duration
The collision process refers to the whole period when rolling force increases from zero to the minimum rolling force. This process is affected by PI controller proportional and integral parameters. It is also affected by the minimum rolling force setpoint.
Simply raising the minimum rolling force can extend collision time. Yet it will change the elongation adjustment window of the temper mill. It will disturb the stability of tempering processes. This adjustment is not recommended for field production.
## 3. Control Improvement Solutions for On-site Production
As analyzed above, excessive impact during mill closing is the root cause of temper mill transverse roll mark. Improvements should not be limited to roll modification. Optimizing control logic during mill closing can reduce collision impact. It guarantees equipment safety and stable production.
### 3.1 Optimize Hydraulic Control Strategy for Mill Closing
Use separate control schemes for normal rolling and mill closing. Do not use fixed PI parameters for all conditions.
At the late screwdown stage of mill closing, properly decrease the proportional coefficient of the PI controller. Extend integral time to suppress controller overshoot and oscillation. This achieves a gentler rolling force rise curve.
Set independent output limits for the closing process. This prevents saturated controller output before roll-strip contact. It weakens inertia impact of HGC hydraulic cylinders. Restore original control parameters in normal rolling phase. This maintains response capability for elongation adjustment.
### 3.2 Segmented Speed Control for HGC Hydraulic Cylinder
Adopt segmented speed control for mill closing. Maintain fast screwdown speed under large roll gaps. This improves roll change efficiency.
When work rolls are about to touch the strip, reduce servo valve opening to slow down the hydraulic cylinder. This cuts the instant contact speed of work rolls. It reduces collision momentum.
The solution only requires program modification. It needs no mechanical reconstruction. It brings little disruption to production.
### 3.3 Maintain Standardized Roll Management
Control optimization is an electro-hydraulic improvement. Basic roll management cannot be neglected.
Roll grinding must follow strict process standards. Inspect roll surfaces for dents, scratches and peeling before installation. Protect rolls during storage, transportation and roll change. Avoid mechanical damage during mounting. Even optimized control logic cannot eliminate roll marks if the roll surface already has damages.
### 3.4 Production Tracking and Verification
After parameter adjustment, track strip surface quality for the first several coils after roll change. Monitor the severity and cycle of transverse roll marks. Record rolling force curves. Ensure no obvious pressure spikes or oscillation.
Fine-tune control parameters for different steel grades and strip specifications. Find the balance point between production efficiency and surface quality.
## 4. Заключение
Temper mill transverse roll marks formed during mill closing of six-high temper mills originate from impact load. The load comes from the hydraulic control system at the late closing stage. The impact creates micro-defects on work roll surfaces. These defects are transferred to steel strips.
Simply upgrading roll grade is a passive solution. It cannot solve the issue fundamentally. Prioritize segmented optimization of mill closing sequence, PI control parameters and HGC screwdown speed. These steps mitigate collision impact.
Combined with standardized roll maintenance, this method effectively suppresses temper mill transverse roll mark defects. It reduces product downgrade rates. It cuts roll spare part consumption and improves overall economic benefits of the production line.
