Classification and measures of common surface defects in hot-rolled steel plates

Hot rolled steel plates and strip products are one of the important steel varieties. With the application and development of hot-rolled plates in various aspects, users have increasingly strict requirements for products. Users not only focus on the quality of steel plates, but also pay more attention to the appearance quality of products.

To a large extent, appearance quality is one of the important indicators for evaluating the quality of hot-rolled steel plates. If the appearance quality of the product does not meet the requirements, users will not recognize and accept it. Therefore, in the production process, it is necessary to ensure the performance of the product while also paying attention to the appearance quality of the product.

However, in the entire production process of hot rolling products, due to various factors such as high temperature, high pressure, high speed, and hardware equipment, the produced products may not fully meet customer requirements, and the generation of various defects on the surface of steel plates is inevitable. These defects run through the entire hot rolling production process, not only affecting the surface appearance of subsequent formed devices, but also closely related to the effectiveness of subsequent processing, processing costs, and so on.

In order to further improve the surface quality of hot-rolled steel plates, domestic and foreign production plants have taken many measures to correct and improve them, such as combining chemical and mechanical phosphorus removal, using continuous annealing, purifying steel quality, and so on.

Classification of common surface defects in hot-rolled steel plates

The classification of defects should be based on the relevant definitions and descriptions in the technical standards and conditions of hot-rolled steel plates, and should be formulated and described in accordance with the increasing number of steel grades and diversified production methods in recent years, as well as the actual forms and characteristics of defects that occur. The common defects of hot-rolled steel plates can be divided into five categories: surface defects, plate shape defects, composition and performance defects, overall appearance defects, and geometric dimensions.

2.1 Surface defects

There is a unified international evaluation standard for the classification of surface defects in hot-rolled steel plates. According to the different causes and shape factors, the surface defects of medium and thick plates are divided into 33 types.

However, various classes may be interrelated with each other, and many defects are also very similar in appearance, which can easily lead to confusion. Therefore, these defects can be summarized and organized into five main categories: (1) pitting; (2) Folding and scratching; (3) Inclusion; (4) Scar formation; (5) Cracks.

2.2 Plate shape defects

Plate defects can generally be divided into three categories:

2.2.1 Medium wave: It undulates in a wave like pattern along the longitudinal direction, often appearing in thin or low-carbon steel plates;

2.2 2 edge waves: mostly appearing in thin specifications and low-carbon steel products, with undulating edges;

2.2.3 Uneven pattern board substrate: The pattern board has obvious protrusions and indentations, forming a fish scale shape, distributed along the longitudinal strip, and some distributed along the board surface.

2.3 Appearance defects of the entire roll

The appearance defects of the entire roll can generally be divided into 5 categories based on appearance:

2.3.1 Tower shape: a tower like shape, often found in rolled thick gauge products;

2.3.2 Loosening: There are large gaps between the layers of the rolled plate, which are often found in rolled thick specification products;

2.3.3 Staggered layers: The uneven distribution between layers of rolled plates, or between multiple layers, often occurs in rolled thick gauge products;

2.3.4 Hanging damage: Damage occurs at both ends of the roll board; Gaps appear between the layers of the rolling board;

2.3.5 Collapse: The appearance of the steel coil is elliptical or irregularly circular.

2.4 Component performance

From the perspective of component performance, it can be divided into three categories:

2.4.1 Excessive composition: One or more components exceed the range required by the planned steel grade;

2.4.2 Excessive tensile strength M: The tensile strength exceeds the range required by the planned steel grade;

2.4.3 Excessive elongation: The elongation exceeds the range required by the planned steel grade.

2.5 Geometric dimensions

In terms of geometric dimensions, it can be divided into three categories:

2.5.1 Thickness mismatch: The total or partial thickness of the steel coil exceeds the tolerance range of the planned thickness;

2.5.2 Width mismatch: The total or partial width of the steel coil exceeds the tolerance range of the planned width;

2.5.3 Insufficient height of pattern board: The pattern height does not meet the contract requirements. It is prone to occur when rolling over a thickness of 5.8mm.

Analysis of common surface defects in 3 hot-rolled steel plates

Hot rolled steel plates are affected by various factors during the production process:

This mainly includes three aspects: heating and phosphorus removal of slab, hot charging process, and surface condition of rolling spokes. And the reasons for these defects are also various, and a preliminary analysis has been conducted on the causes of some common defects.

3.1 Pits

Pitting is a rough surface that appears uneven on the surface of steel, also known as pitting. It is generally continuous in flakes, some of which exhibit local or periodic distribution. The reason for its occurrence is poor quality of the rolling spokes, uneven surface hardness, or loss of a cold hard layer, uneven wear, and wear of the finished hole or front hole groove. During the heating process, the slab oxidizes severely, and the iron sheet is pressed into the surface during rolling, forming small pits after falling off.

3.2 Folding and Scratches

The cause is poor machining or severe wear of the guide plate.

The edges are not smooth, the installation and adjustment of the guide device are improper, causing scratches on the rolled piece due to excessive pressure, the guide plate or hole is adhered with oxide scale, causing scratches. The floor, spoke, and cooling bed in the hot rolling area move steel, and the flipping equipment has sharp corners, causing scratches when the rolled piece passes through, or the high sulfur content in the coal gas makes the slider prone to nodules, resulting in surface folding and scratches on the slab.

3.3 Inclusions

Inclusions refer to non-metallic inclusions with a certain depth on the surface of steel, generally distributed in the form of dots, stripes, or blocks, with colors ranging from dark red to light yellow.

The main reason for this is that non-metallic inclusions on the surface of steel ingots and billets were not removed and rolled into the steel surface; During the heating process, refractory materials, coal ash, and slag on the top or end of the furnace fall onto the surface of the billet, which are not cleaned and rolled into the surface of the rolled piece; In addition, the environment around the rolling mill is not clean, and non-metallic inclusions adhering to the surface of the rolled parts may also cause inclusions.

3.4 Scarring

Scars are metal sheets on the surface of steel that appear in a scar like pattern, with irregular distribution. Scars vary in size and depth, and inclusions are often present beneath them.

The reason for the formation of scars is improper operation during the casting of steel ingots, which causes the scattered or splashed molten steel to adhere to the mold wall, be oxidized and adhere to the surface of the steel ingot. After rolling, scars are formed on the surface, or due to defects such as sticking to the mold, convex hull, mesh, double skin or flipping on the surface of the steel ingot, scars are formed during rolling. During the rolling process, surface flying wings are formed on the surface of the rolled piece due to scratches in a certain pass before the finished hole, or the surface of the rolling groove is severely worn, and then rolled to form scars. Some are caused by incomplete cleaning of the "beard" of the slab.

3.5 Cracks

There are mainly longitudinal cracks, transverse cracks, and star shaped cracks. From the cross-section of the steel, the cracks have sharp roots, a certain depth, and are perpendicular to the surface. There are serious decarburization phenomena and non-metallic inclusions around them.

The reason for this is due to the uneven thickness of the initial shell, stress concentration in areas where the shell is thin, or thermal stress caused by the temperature difference between the inside and outside of the shell, as well as the stress generated by the static pressure of the steel resisting the solidification shrinkage of the shell along the thickness direction. When the stress exceeds the tensile strength of the shell, cracks occur.

3.6 Edge waves

The uneven transverse thickness of the billet results in different elongation rates at various points during rolling; Poor matching of CVC spokes leads to inconsistent relative compression rates in the transverse direction of the rolled piece.

3.7 Ingredient exceeding standard

During smelting and refining, the composition of the molten steel is not strictly controlled according to the technical standards and agreement conditions required by the contract, or changes in the composition of the molten steel occur due to prolonged pouring time. This is closely related to the pouring process, operational level, and equipment condition.

3.8 Width mismatch

The computer control system has malfunctioned, causing fluctuations in rolling parameter control; Improper or large fluctuations in the tension of the loop or winding; Large deviation in slab size; Inappropriate lateral pressure of the standing beam; The rolling process is not suitable.

For the causes of other defects, we will not analyze them one by one here.

Measures to prevent surface defects on hot-rolled steel plates

The occurrence of steel plate defects is an unavoidable problem throughout the entire process of hot rolling production. Once the cause of the defects is identified, it becomes very important to avoid them.

4.1 Pits

Due to the diverse causes of pitting, measures to prevent and eliminate it also vary:

4.1 When changing spokes, it is necessary to carefully inspect the rolling mill and avoid using severely rusted rolling mills;

4. 1.2 It is necessary to replace worn rolling discs or hole shapes in a timely manner;

4. 1.3 Improve the material of the rolling disc, enhance wear resistance, maintain good cooling of the rolling groove, use hot rolling process lubricant to reduce wear and improve the wear resistance of the rolling groove;

4. 1.4 Control the heating temperature of the material to maintain positive pressure in the furnace and reduce oxidizing gases. For some alloy steels that are prone to oxidation but have iron scales that are not easily detached, iron sheets should be covered on the surface of the steel to protect heating;

4.1 5. Use high-pressure water and compressed air to remove the oxide scale on the surface of the rolled piece before or during the rolling process.

4.2 Folding and Scratches

The measures to prevent folding and scratching include:

4.2 The processed guide plate must meet the requirements, with smooth edges;

4.2.2 Correctly adjust the guide device;

4.2 3. Eliminate sharp corners on the cover plate of the spoke and ring roads, as well as on the steel moving and flipping equipment;

4.2.4 It is necessary to check the operation of transportation dropouts and promptly eliminate faults.

4.3 Scarring

The measures to prevent and eliminate scar formation include:

4.3.1 Improve ingot and whole mold operations or continuous casting operations to improve the quality of steel ingots;

4.3.2 Strengthen the quality inspection of steel ingots and billets, and for billets with scab defects, they must be removed before production;

4.3.3 Strictly carry out the scoring operation of the rolling disc, replace severely worn guide plates and rolling grooves in a timely manner, and prevent scratches on the rolled parts.

4.4 Cracks

The measures to prevent crack formation include:

4.4 1. Improve the metallurgical quality of steelmaking and continuous casting, improve the surface quality of continuous casting billets, conduct strict inspections, and do not put unqualified billets into production;

4.4.2 Strictly control the heating temperature;

4.4 3 Improve the rolling system, prevent uneven cooling, adopt uniform reduction and reasonable flipping passes;

4.4 Reasonable post rolling control cooling to ensure even cooling of the rolled parts.

4.5 Tower shape

Regular inspection and replacement of side guide plates are required; Optimize the coiling process operation.

4.6 Collapse roll

Minimize the length of the straight section as much as possible; Properly reduce the coiling temperature; Reasonable storage of steel coils.

4.7 Insufficient height of pattern plate

Strict engraving of pattern spokes, increasing rolling pressure, and timely replacement of spokes.

The production process of hot rolled steel and strip products is quite complex, and it is very difficult to control product defects in this process. Therefore, better technology and more experience are needed to gradually improve it.