Can't solve the problem by simply removing the oxide scale? A comprehensive solution covering the entire process from the heating furnace to the final rolling stage.
Surface defects such as spots, stripes, and indentations on hot-rolled steel strips are largely caused by the pressing of iron oxide scale into the material. Many teams, when encountering surface quality complaints, only increase the pressure of the descaling water, but the pressure is fully applied and the defects still recur repeatedly, even getting worse with further adjustments.
In fact, the pressing of iron oxide scale is not just an issue in the descaling process; it runs through the entire process from the formation of scale in the heating furnace, descaling during rough rolling, secondary oxidation between stands, and the shedding of oxide films from the rolls. Focusing solely on the high-pressure water impact does not address the root cause. Today, we will analyze the formation mechanisms of three types of iron oxide scale pressing and present a complete prevention and control plan from heating to finishing rolling.
1. First, distinguish: The three types of pressing defects have completely different root causes.
Many people cannot fix the surface quality problem at the very beginning - they fail to identify which type of iron oxide scale it is. Adjusting the direction will naturally go astray.
1.1 Primary iron oxide scale pressing (heating furnace scale)
The thick layer of oxide scale formed during high-temperature oxidation of the billet in the heating furnace, which remains after incomplete descaling and is pressed into the base material during rolling.
Typical features: Large areas of sheet-like or strip-like black spots, distributed along the rolling direction, with more severe damage at the head and edge;
Core cause: Excessive heating temperature, too long time in the furnace, and overly strong oxidizing atmosphere in the furnace, resulting in an overly thick oxide layer and a dense structure, making it difficult for high-pressure water to completely remove it.
1.2 Secondary iron oxide scale pressing
After rough rolling descaling, the steel strip undergoes secondary oxidation during rolling, forming a thin layer of iron oxide, or the descaling is not thorough during the stand-to-stand process, and is pressed into the strip.
Typical features: Fine spots, point-like pressing, evenly distributed throughout the roll, or fixed to appear after certain stands;
Core cause: Slow rolling rhythm, large temperature drop during the middle billet, failure of stand-to-stand descaling, or excessive temperature at the entrance of the finishing mill leading to accelerated secondary oxidation.
1.3 Rolling roll oxide film shedding pressing
The oxide film formed on the surface of the finishing mill working rolls under high temperature and high pressure periodically peels off and is pressed into the surface of the strip.
Typical features: Periodic, evenly spaced pressing spots, distributed along the rolling direction in a band-like pattern, with the defect spacing corresponding to the circumference of the roll;
Core cause: Inconsistent cooling of the rolls, large fluctuations in rolling rhythm, and unreasonable load distribution, causing premature cracking and shedding of the oxide film on the roll surface.
2. Four core causes for persistent problems
1. Uncontrolled heating process, making the "inherent" iron oxide scale difficult to remove.
This is the most fundamental and also the most easily overlooked aspect.
Exceeding the process limit by 10°C or more in the exit temperature, the thickness of the oxide layer increases exponentially, and the FeO layer is too thick and highly adhesive, making it impossible to remove with conventional descaling pressure;
Excessive time in the furnace: The steel billet and the rolled material remain in the high-temperature section for too long, causing the oxidation to continue and the scale to layer up;
Excessive oxidizing atmosphere in the furnace: An excessive air ratio leads to intensified oxidation of the steel billet, especially in the high-temperature section, where a hard and difficult-to-remove Fe?O? layer is formed.
2. Weakening descaling system, insufficient impact force
Descaling is not just "having water is enough"; insufficient impact force makes the pressure useless.
Nozzle wear and blockage: The nozzle, after long-term use, has a larger diameter wear and clogging at the outlet, resulting in a dispersed jet and a 30% reduction in effective impact force; a common misconception in the field is to only look at the pump station pressure and not check the actual state of the nozzle;
Inconsistent height and angle of the manifold: The nozzle is too high from the surface of the strip or the spray angle is offset, resulting in a significant reduction in actual impact force; many production lines do not calibrate the manifold height for years, and the impact effect is half as good when the strip bends.
Poor water quality and filter failure: The water contains sand and rust, which not only wears the nozzle but also clogs the filter, causing fluctuations in water pressure and unstable jet.
3. Fluctuating rolling rhythm, making secondary oxidation unpredictable
Even if the descaling is clean during rough rolling, it does not mean it will remain clean during the entry to the finishing mill.
Slow rolling rhythm and long time of the middle billet on the roller track leads to rapid secondary oxidation of the surface, generating a new oxide layer;
The descaling process between the stands of the finishing mill is insufficient or ineffective, causing the secondary iron sheets generated by the previous stand to be continuously pressed into the subsequent stand.
For thin-gauge and low-carbon steel, the rolling speed is fast, and the iron sheets cannot be shed in time and are pressed onto the surface, forming fine dots.
4. Unreasonable cooling and load of the rolls, frequent peeling of oxide films
The working rolls of the first few stands of the finishing mill have high surface temperatures and large contact stresses, making it easy to form oxide films.
Insufficient cooling water supply and uneven distribution of the rolls leads to local excessively high temperatures, thick oxide films with poor adhesion;
The rolling load is concentrated on a few stands, with excessive unit pressure, accelerating the fragmentation and shedding of the oxide films;
During the initial roll change and insufficient preheating of the rolls, or large jumps in the rolling plan specifications, the thermal bulge changes drastically, making the oxide films prone to large-scale shedding.
III. Process-wide prevention and control plan, implemented item by item and yielding results
1. Heating furnace end: Temperature control, time control, atmosphere control, reducing scale at the source
Strictly control the exit temperature: Set the temperature limit for each steel type, prohibit over-temperature before rolling; for steel types with high Si content (such as Q355 series), appropriately lower the heating temperature to avoid generating iron olivine (Fe?SiO?) that is difficult to remove;
Optimize the time in the furnace: Control the steel loading rhythm and the empty space in the furnace to reduce the residence time in the high-temperature zone; temporarily stop the machine and cool down promptly to avoid long-term high-temperature oxidation of the billet;
Adjust the atmosphere in the furnace: Reduce the air-fuel ratio in the high-temperature zone appropriately to create a weakly oxidizing atmosphere, reducing oxidation loss; at the same time, ensure uniform heating to avoid local over-heating.
2. Descaling system: Increase impact force, maintain stability, make high-pressure water truly useful
Regularly inspect and replace nozzles: Establish a nozzles wear ledger, replace them regularly according to the rolling tonnage; during each maintenance, check the jet shape of the nozzles one by one, replace those with blockage, skew, or dispersion immediately to ensure a complete jet fan surface and uniform overlapping area;
Adjust the height and angle of the manifold: Optimize the spray angle of the nozzles (usually at a 15° angle from the vertical direction), align with the rolling direction; introduce dynamic adjustment of the manifold height, adjust in real time according to the warpage of the slab to ensure a constant hitting distance;
Strictly control water quality and filtration: Regularly clean the water tank, replace the filter element, ensure the descaling water is clean; maintain the high-pressure pump and pipeline, prevent leakage and pressure loss, ensure the actual impact force of the nozzle outlet meets the standard.
3. Rolling process: Control rhythm, strong descaling, suppress secondary oxidation
Optimize the descaling strategy for rough rolling: Break the old experience of "only applying to odd-numbered passes", for difficult-to-descale steel types, use multiple passes of descaling to ensure a clean surface at the rough rolling outlet;
Accelerate the rolling rhythm: Reduce the waiting time for the intermediate billet to warm up, reduce the degree of secondary oxidation; when the production line temporarily slows down, predict the surface quality risks in advance;
Make good use of the descaling of the finishing rolling: Ensure the effect of F1 front descaling, for high surface requirement steel types, increase the descaling between stands, and promptly remove the secondary-generated oxide scale.
4. Roll control: Stabilize the film, prevent detachment, eliminate periodic defects
Optimize roll cooling: Ensure sufficient cooling water supply and uniform distribution, control the roll surface temperature within a reasonable range, promoting the formation of thin and dense, firmly bonded oxide films;
Reasonably distribute the rolling load: Avoid excessive load on a single rolling mill, balance the reduction amounts of each stand, reducing the stress fragmentation of the oxide film;
Optimize the rolling plan: Smoothly transition specifications and steel types, arrange transitional rolling for roll change, wait for the oxide film on the roll surface to stabilize before producing high surface grade products;
Control the rolling rhythm: Avoid frequent start-stop, large fluctuations in speed, reducing sudden temperature rises and drops on the roll surface, preventing the oxide film from cracking and shedding.
IV. On-site rapid troubleshooting ideas
When encountering defects of iron sheets being pressed into the surface, follow this sequence for inspection, avoiding unnecessary detours:
Examine the defect shape: Large-area sheet-like → Check the heating furnace and rough rolling descaling; fine dots → Check secondary oxidation and finishing rolling descaling; periodic spots → Check roll oxide film detachment; Identify the patterns: If the entire batch shows the same issue → Check the heating process and descaling system status; If the problem occurs after changing the rolls → Check the roll cooling and rolling plan; If it is an occasional and random occurrence → Check for nozzle blockage and excessive temperature of individual billets;
Quick verification: First, check the nozzle status and whether the water pressure meets the standard, then verify the heating temperature and the time in the furnace, and finally investigate the roll cooling and load distribution. Summary
The pressing of iron oxide scale seems to be a minor surface issue, but in fact, it reflects the overall control level of the entire process of heating, descaling, rolling, and roller operation. Simply increasing the descaling pressure will never solve the root problem. By reducing the formation of oxide scale in the furnace at the source, enhancing the actual impact force of the descaling system, controlling secondary oxidation during the rolling process, and stabilizing the state of the oxide film on the rollers, and implementing strict control at each of the four stages, it is possible to continuously and stably improve the surface quality, reduce downstream complaints and downgrade or rejudgment.
