During the production process of hot-rolled and cold-rolled steel strips, edge cracks are a typical and frequently occurring quality defect. Even if the cracks are minor, they can lead to increased cutting edge loss and unqualified appearance of the products. In severe cases, they can cause the steel strip to break, leading to machine downtime and production halt, and subsequently result in failure of stamping and bending processes, directly causing batch degradation, customer complaints and economic losses. 

Many production teams discovered cracks and merely adopted temporary remedies by increasing the cutting edge size, but failed to address the root cause, resulting in the recurrence of defects. This article, based on the metal deformation mechanism and practical on-site experience, thoroughly analyzes the causes and evolution patterns of the edge cracks in the steel strip, and provides corresponding full-process prevention and control measures to help the front-line personnel completely solve this problem. 

 

 

I. Common Types and Appearance Characteristics of Edge Cracks on Steel Sheets

Based on the on-site production conditions and according to their morphology and distribution, edge cracks can be mainly classified into four categories, which can be quickly distinguished by comparison: 

The longitudinal edge cracks extend along the rolling direction, varying in length. They are mostly concentrated on the edges of the strip steel and appear in a continuous or intermittent pattern. These cracks are the most common type, mainly caused by edge stress concentration and the extension of original defects in the billet. 

The transverse edge cracks are perpendicular to the rolling direction. They are mostly in the form of short strips or fine lines, often appearing in patches at the head and tail areas of the strip steel, and are closely related to the fluctuations in rolling tension and sudden temperature changes. 

The jagged edge has an irregular saw-tooth shape. It is accompanied by local peeling and chipping. This is usually caused by improper pressing of the stand roll or disorderly metal flow at the edge. 

The deep-through cracks extend from the surface into the matrix of the base material, with a large depth, and are highly likely to cause interruption bands during rolling. They are mostly caused by deterioration of the original defects such as inclusions and shrinkage cavities within the billet. 

 

 

II. Core Mechanism of Edge Cracks in Strip Steel

From the perspective of metal plastic deformation and mechanics, the edges of the strip steel are inherently stress concentration areas, which is the fundamental condition for the generation of cracks. 

During the rolling process of the strip steel, the base metal undergoes plastic elongation under the pressure of the rolls. The metal in the middle of the plate surface flows smoothly; while the edges of the strip steel have no lateral constraints, and the metal has a large degree of freedom for lateral flow. The internal tensile stress and compressive stress alternate and superimpose, resulting in stress values that are much higher than those in the central area of the plate surface. 

When additional adverse external conditions are added, the stress exceeds the breaking strength of the steel, and cracks will gradually form: 

If there are minor inclusions, pores, or grain boundary brittleness defects on the edges of the steel billet / raw material, the location of these defects will become a stress concentration point. Under the action of the rolling force, these defects will continuously expand and eventually manifest as surface cracks. 

Under high-temperature rolling conditions, the edges of the strip steel show overheating and overburning. The metal grains become coarse, the strength of the grain boundaries drops significantly, the plasticity deteriorates, and even slight external force can cause cracking. 

Abnormalities in rolling tension, stand roll pressure, and cooling intensity will further amplify the stress difference at the edge, accelerating the initiation and expansion of microcracks. 

In simple terms: Edge stress concentration is the inherent condition, while factors such as raw materials, temperature, process, and equipment defects are the triggering causes. When these two factors combine, visible edge cracks are formed. 

 

 

III. Multi-dimensional Analysis of the Causes of Edge Cracks

The cracks are not a problem caused by a single process; rather, they are the cumulative result of various chain issues including the billet, heating, descaling, rolling, equipment, and cooling. Let's go through them one by one as follows: 

(1) Primary defects of raw material blanks (root causes)

This is the primary reason for the repeated occurrence of cracks. The defects will be continuously elongated and enlarged during the rolling process. 

The corner areas of the continuous casting billets are prone to contain shrinkage cavities, porosity, bubbles, and non-metallic inclusions, especially sulfide and oxide inclusions, which can disrupt the continuity of the metal. 

At the edges of the cast billet, there are corner cracks, subsurface cracks, and overly deep vibration marks. The original microcracks continue to expand after multiple rolling processes. 

The cross-sectional dimensions of the billet deviate significantly, there are burrs at the edges, and the corners are sharp. During the rolling process, there is a sudden change in local force, which causes cracking. 

(2) Inadequate control of the heating process (the main cause of high-temperature embrittlement)

The furnace temperature was too high, and some areas were overheated. The grain size at the edge of the steel strip became coarse, and the grain boundaries oxidized. The plasticity of the steel sharply decreased, which is commonly known as "overheating", and it is highly prone to generating hot cracks. 

The billet was heated unevenly, with a significant temperature difference between the edges and the center. The metal's plasticity was inconsistent, and the deformation was not synchronized, resulting in additional stress. 

The furnace chamber has excessive air leakage and the edges are severely oxidized, resulting in a thick layer of brittle oxide scale. During the rolling process, the oxide scale falls off, causing the base metal to crack along with it. 

(III) Surface Deformation and Quality Issues

The high-pressure water descaling process is not thorough, leaving thick iron oxide scales at the edges. During the rolling process, the hard oxide scales are pressed into the base material, creating tiny cracks, which may subsequently develop into fissures. At the same time, the increased oxide scales increase the local friction resistance, further exacerbating the uneven stress at the edges. 

(4) Inappropriate rolling process parameters (the main inducing factor)

Abnormal parameters of the vertical rolls: The opening of the vertical rolls is too small, and the side pressure is too high. This causes forced extrusion of the steel strip edges, resulting in blocked metal flow and the formation of extrusion cracks; wear and rough surfaces of the vertical rolls can also scratch the edges, leading to additional cracks. 

Improper reduction system: The reduction amount in a single pass is too large, causing the local deformation rate to exceed the plasticity limit of the steel, resulting in premature cracking at the edges; the reduction amounts allocated among each machine frame are uneven, and stress accumulates step by step. 

Tension control imbalance: The tension between the frames is too high, causing the steel strip to be forcibly stretched as a whole. The weak edges are the first to develop stretching cracks; in the head and tail sections, the absence of tension results in sudden stress changes, which is also prone to causing short cracks. 

The use of bent rolls and misaligned rolls is improper, causing the force distribution on the plate surface to be uneven, and significantly increasing the load on one side of the edge. 

(5) Equipment and tooling malfunctions

The guide rails and guard plates are installed with deviations and suffer severe wear, continuously scraping the edge of the steel strip, resulting in mechanical scratches. The scratched areas evolve into cracks. 

The surface of the rolls is unevenly worn, the gap between the rolls is misaligned, the pressing amounts on both sides of the strip steel are inconsistent, and the stress on one side exceeds the standard. 

The frame and bearing clearance are too large, resulting in vibration and movement during operation, which leads to a decrease in the stability of the rolling process and causes cracks to be induced by the impact stress. 

(6) Effects of the cooling and winding processes

Excessive water supply for edge cooling and concentrated spraying result in a sudden drop in the temperature of the steel edge, generating huge thermal stress and causing cold cracks; uneven cooling also leads to inconsistent contraction of the plate surface, causing the edges to crack under tension. 

The setting of the winding tension is unreasonable. During the winding process, the steel strip is repeatedly subjected to force, causing the existing microcracks to further expand. 

 

 

IV. Implementation of Comprehensive Prevention Measures (Can be directly executed on-site)

Based on the above causes, following the principle of prioritizing source control, supplemented by process management, and with equipment guarantee as the ultimate safeguard, a graded prevention plan has been formulated. 

(1) Control of raw material and billet: Cut off inherent defects at the source

Strengthen the inspection of continuous casting billets upon entry, conduct batch-by-batch inspections of the corners and edges of the billets. Billets with corner cracks, shrinkage cavities, subsurface inclusions, or deep indentations should be sorted out separately and strictly prohibited from entering the rolling process; 

Pre-treatment of sharp edges and burrs on the raw material by chamfering can reduce stress concentration during rolling. 

Stable continuous casting process, reducing the occurrence of defects on the edge of the cast billet, and improving the quality of raw materials from the upstream. 

(2) Optimize heating process to avoid high-temperature embrittlement

Strictly follow the temperature standards corresponding to the steel type, prevent overheating and local overheating, and focus on controlling the temperature at the corners of the billets to avoid overburning; 

Regularly inspect and maintain the furnace body and burners, seal off the leakage points, ensure uniform heating of the billet as a whole, and reduce the temperature difference between the edges and the middle part. 

Shorten the waiting time for the billet before rolling to avoid prolonged oxidation and deterioration of grain boundaries under high-temperature conditions. 

(3) Strengthen the descaling system to ensure a clean surface

Maintain the pressure and flow rate of the descaling water within the process standard range. Conduct daily inspections and unclog the nozzles to ensure that the descaling covers the entire edge of the steel strip without any oxide scale remaining. 

Optimize the water quality and filtration system, reduce nozzle blockage, and prevent local descaling failure. 

(4) Optimize the rolling process and control the stress reasonably

Standardize the use of the vertical rolls: Set a reasonable side pressure based on the steel strip specifications, and prohibit excessive compression at the edges; Regularly grind and replace the vertical rolls to ensure smooth roll surfaces. 

Scientifically allocate the reduction amount: Adhere to the principle of "uniform deformation per pass", do not blindly increase the single-pass reduction rate, and appropriately reduce the reduction amount of the first few passes for steel types prone to cracking. 

Stable micro-tension rolling: Fix the tension parameters of each stand to prevent significant fluctuations in tension; Optimize the speed-up and speed-down logic to reduce sudden stress changes at the beginning and end of the strip. 

Use the bending roll and skew roll functions as needed to ensure uniform force distribution across the entire sheet surface and avoid single-sided overloading. 

(5) Daily maintenance of equipment and tooling, to prevent mechanical damage

Before each shift starts, check the position of the guides and side guard plates to ensure accurate alignment, without looseness or uneven wear. If any scratches on the steel strip are observed, immediately adjust and replace them. 

Regularly calibrate the frame, bearings and roll gap of the rolling mill, eliminate equipment vibration and displacement problems, and ensure smooth rolling operation. 

The rolls are periodically ground to ensure the surface is intact and to prevent any defects on the roll from damaging the steel strip. 

(6) Precise control of cooling and winding

Adjust the layout of the cooling spray, distribute the cooling water at the edges to avoid concentrated rapid cooling in certain areas; ensure uniform cooling throughout the line and reduce thermal stress; 

According to the specifications and steel type, the winding tension is matched. The tension is gradually increased to avoid sudden force applied to the edges. 

(7) Online inspection and edge trimming control

Assign dedicated personnel to monitor the edge condition of the steel strip online. In case of any minor cracks or peeling, immediately report and adjust the parameters. 

Reasonably set the trimming allowance, without wasting the raw material, completely remove the edge areas with micro cracks to prevent the cracks from extending. 

 

 

V. Emergency Disposal Plan for On-site Cracks

If a batch of edge cracks are detected during production, the following steps should be taken for rapid handling to minimize downtime and losses: 

First, observe the crack patterns: For longitudinal long cracks, focus on inspecting the billet, the vertical rolls, and the tension; for transverse fine cracks, pay special attention to checking the heating temperature and the cooling system. 

Temporarily slightly reduce the lateral pressure on the stand rolls, decrease the single-pass reduction amount, and alleviate the stress at the edges. 

Fine-tune the cooling pipeline, weaken the concentrated cooling at the edges, and reduce thermal stress. 

Increase the cutting margin appropriately to isolate the defective area and prevent crack propagation from causing strip breakage. 

Conduct offline random inspections of the raw materials, analyze the fracture microstructure, identify the root causes of the defects, and thoroughly rectify them before resuming normal production. 

 

 

VI. Correction of Common Operational Mistakes at the Site

❌ Mistake 1: When cracks appear at the edges, simply increase the cutting edge size without investigating the root cause 

✅ Correct answer: Increasing the cutting edge is only a temporary solution. It cannot eliminate the internal problems and will only increase the loss of finished products. It is necessary to simultaneously trace back and rectify the process and raw materials. 

❌ Misconception 2: To increase production, blindly increase the reduction ratio and the lateral pressure of the stand roll 

✅ Correct: If the deformation exceeds the plastic limit of the steel, it will directly cause large-scale edge cracks and strip breakage, resulting in a loss that outweighs the gains. 

❌ Misconception 3: Ignoring the inspection of raw materials, believing that minor defects will naturally disappear after rolling. 

✅ Correct translation: Micro-defects at the edge of the billet will continuously expand during the rolling process, which is the main cause of stubborn edge cracking. Source inspection must not be omitted. 

❌ Misconception 4: The cooling system only focuses on the surface temperature of the board, without considering the spray conditions at the edges. 

✅ Correct translation: The thermal stress generated by the rapid cooling at the edge is the main cause of cold zone cracking. The uniformity of cooling needs to be strictly controlled throughout the process. 

 

 

VII. Summary

The root cause of edge cracks in the steel strip is stress concentration, and multiple process defects are the driving force. To solve the problem permanently, we cannot rely solely on post-event edge cutting and remediation. Instead, a full-process control system covering "raw material inspection → heating temperature control → process optimization → equipment maintenance → online inspection" must be established. 

The frontline teams should learn to trace back the causes of cracks based on their shapes, distinguish whether it is a defect in the raw materials, a temperature issue, or a problem with the process equipment, and then take appropriate measures. By implementing various standards at each shift and each process, it is possible to significantly reduce the defect rate of edge cracks, minimize the loss of cut edges, effectively avoid the risk of belt breakage and shutdown, and achieve stable production, quality improvement, and cost reduction.