Analysis of common surface defects of hot rolled steel sheet
Hot-rolled steel sheets and strip products are one of the important steel products. With the application and development of hot-rolled sheets in all aspects, users’ requirements for products are becoming more and more stringent. While users pay more attention to the quality of steel plates, they also pay more attention to the appearance quality of products.
To a large extent, the appearance quality is one of the important indicators for evaluating the quality of hot-rolled steel sheets. If the appearance quality of the product does not meet the requirements, the user will not recognize and accept it. Therefore, in the production process, both must ensure The performance of the product, but also 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 products produced may not fully meet the requirements of customers, and various defects on the surface of the steel plate It is inevitable, and the occurrence of these defects runs through the entire production process of continuous hot rolling. It not only affects the surface appearance of the subsequently formed device, but also is closely related to the effect of subsequent processing, processing cost and so on.
In order to further improve the surface quality of hot-rolled steel sheets, domestic and foreign production plants have taken many measures to correct and improve, such as the combination of chemical phosphorus removal and mechanical phosphorus removal, continuous annealing, and steel purification, etc.
Classification of common surface defects of hot rolled steel sheet
Defect classification should be based on the relevant definitions and descriptions in the relevant technical standards and technical conditions of hot-rolled steel sheets. At the same time, it should be formulated and described corresponding to the increase in the number of steel types in recent years, the diversification of production methods, and the actual forms and characteristics of defects. . Common defects of hot-rolled steel sheets can be divided into five categories: surface defects, plate shape defects, composition properties, appearance defects of the entire coil, and geometric dimensions.
There has been a unified international judging standard for the classification of surface defects of hot-rolled steel plates. According to the different factors such as their causes and shapes, the surface defects of medium and thick plates are divided into 33 types.
However, the various types may be related to each other, and many defects are also very similar in appearance, which is easier to cause confusion. Therefore, to summarize and organize these defects, they can be divided into five major categories:
- (1) Pockmarks;
- (2) Folding and scratching;
- (3) Inclusion;
- (4) Scarring;
- (5) Cracks.
The shape defects can generally be divided into 3 categories:
- Medium waves: Wave-like undulations along the longitudinal direction, mostly in thin gauge or low carbon steel plates;
- Edge waves: mostly appear in thin-gauge, low-carbon steel products, with undulating edges;
- The base plate of the pattern plate is uneven: the base plate of the pattern has obvious protrusions and depressions, which are fish-scale, distributed along the longitudinal band, and some are distributed along the surface of the steel plate.
The appearance defect of the whole roll
In terms of appearance, the appearance defects of the entire roll can generally be divided into 5 categories:
- Tower shape: a shape like a tower, mostly in rolled thick gauge products;
- Loose coil: There is a large gap between the coil layer and the layer, which mostly appears in the rolled thick gauge products;
- Staggered layer: the coil layer is uneven, or between multiple layers and multiple layers, and it mostly appears in rolled products of thick specifications;
- Hanging damage: the two ends of the coil are damaged; there are gaps between the layers of the coil;
- Collapse: The appearance of the steel coil is oval or irregularly round.
From the performance of ingredients, it can be divided into 3 categories:
- Excessive composition: a certain composition or multiple compositions are beyond the scope required by the planned steel number;
- The tensile strength exceeds the standard M: the tensile strength exceeds the range required by the planned steel number;
- Excessive elongation: The elongation exceeds the required range of the planned steel grade.
In terms of geometric dimensions, it can be divided into 3 categories:
- Thickness discrepancy: all or part of the thickness of the steel coil exceeds the tolerance range of the planned thickness;
- Width incompatibility: the total or partial width of the steel coil exceeds the tolerance range of the planned width;
- The height of the tread plate is not enough: the height of the pattern is less than the contract stipulation. It is easy to occur when rolling thickness of 5.8mm or more.
Analysis of common surface defects of hot rolled steel sheet
Hot-rolled steel sheets are affected by various factors in the production process:
It mainly includes three aspects: the heating and dephosphorization of the slab, the hot charging process and the surface condition of the roll. The reasons for these defects are also various, and the reasons for some of the common defects are analyzed.
Pockmarks are rough surfaces with unevenness on the surface of the steel, also known as pockmarked surfaces. They are generally continuous in slices, and some are distributed locally or periodically. The reason is the poor quality of the rolling and the different surface hardness. Or lose the chilled layer, uneven wear, the finished hole or the front hole of the finished product is worn, rusted or adhered to broken iron oxide. During the heating process, the slab is seriously oxidized, and the iron sheet is pressed into the surface during rolling and formed after falling off small pits.
The cause is poor processing or severe wear of the guide plate.
The edges are not smooth, the guide and guard device is installed and adjusted improperly, the pressure on the rolling piece is too large, causing scratches, the guide plate or the hole pattern adheres to the iron oxide scale to cause scratches, and the floor, spokes, and cold beds in the hot rolling area are moved. The steel turning equipment has sharp corners, the rolling piece is scratched when passing, or the sulfur content in the big gas is high, the slider is easy to nodulate, causing the surface of the slab to fold and scratch.
Inclusions refer to non-metallic inclusions with a certain depth on the surface of the steel, generally distributed in dots, strips, or blocks, and the color is dark red, light yellow, etc.
The main reasons are that the non-metallic inclusions originally brought on the surface of the ingot and billet were not cleaned up and rolled into the surface of the steel; during the heating process, the refractory materials, coal ash and cinder on the furnace roof or furnace end fell on the surface of the rigid billet. It is cleaned up and rolled into the surface of the rolled piece; in addition, the environment around the rolling mill is not clean, and the surface of the rolled piece is stuck with non-metallic inclusions may also produce inclusions.
Scars are scar-like metal flakes on the surface of steel, which are irregularly distributed. The scars are of different sizes and depths. There are often inclusions under the scars.
The cause of scarring is improper operation when casting the steel ingot, which makes the scattered or splashing molten steel stick to the mold wall, and after being oxidized, it sticks to the surface of the steel ingot. After rolling, it forms scars on the surface, or due to the sticky mold and convexity on the surface of the steel ingot. Defects such as bag, net pattern, heavy skin or turning skin will form scars during rolling. During the rolling process, the surface of the finished hole is scratched at a certain pass. The fins are attached to the surface of the rolled piece, or the rolling groove The surface is severely abraded, and then rolled into scars, some of which are caused by improper cleaning of the slab “beard”.
There are mainly longitudinal cracks, transverse cracks and star-shaped cracks. From the cross section of steel, the cracks have sharp roots, have a certain depth and are perpendicular to the surface, and there are serious decarburization phenomena and non-metallic inclusions around them.
The reason is that the thickness of the primary shell is uneven, the stress is concentrated in the thin part of the shell, or the thermal stress caused by the temperature difference between the inside and outside of the shell, the hydrostatic pressure of the steel resists the stress caused by the solidification and shrinkage of the shell in the thickness direction. Cracks occur when the stress exceeds the tensile strength of the blank shell.
- The lateral thickness of the cast slab is uneven, which causes the elongation of each point in the lateral direction to be different during rolling;
- The cvc spoke is not matched well, which leads to inconsistent relative reduction rates in the lateral direction of the rolled piece.
During smelting and refining, the composition of molten steel was not strictly controlled in strict accordance with the technical standards and agreement conditions required by the contract, or the composition of molten steel changed due to excessive pouring time. This has a lot to do with the pouring process, operation level, and equipment condition.
- The computer control system fails, and the rolling parameter control fluctuates;
- The looper or coiling tension is inappropriate or large fluctuations; the slab size deviation is large;
- The vertical spoke side pressure is inappropriate; the rolling process is inappropriate.
The reasons for other defects will not be analyzed one by one here.
Measures to prevent surface defects of hot-rolled steel sheets
In the whole process of hot rolling production, the occurrence of steel plate defects is an unavoidable problem. After finding the cause of the defect, how to avoid the defect is very important.
Prevent pitting on the surface of hot rolled steel sheet
Since the causes of pitting are various, the measures to prevent and eliminate pitting are also different:
- When changing the spokes, check the rolls carefully and do not use severely rusted rolls;
- It is necessary to promptly replace worn spokes or hole shapes;
- Improve the material of the rolling web, increase the wear resistance, keep the rolling groove cool, and use the hot rolling process lubricant to reduce wear and improve the wear resistance of the rolling groove;
- Control the heating temperature of the material to keep positive pressure in the furnace and reduce oxidizing gas. For some alloy steels that are easy to oxidize but the oxide scale is not easy to fall off, the surface of the rigid material should be covered with iron sheet to protect heating;
- Use high-pressure water and compressed air to remove the oxide scale on the surface of the rolling piece before or during the rolling process.
Prevent the surface of hot-rolled steel sheet from folding and scratching
Measures to prevent folding and scratching are:
- The processed guide board must meet the requirements, with smooth edges;
- Correctly adjust the guide and guard device;
- Eliminate sharp corners on the spokes, the upper cover of the ring, and the steel moving and turning equipment;
- It is necessary to check the operation of the transportation stop and troubleshoot in time.
Prevent scarring on the surface of hot rolled steel sheet
Measures to prevent and eliminate scarring are:
- Improve ingot casting and whole mold operations or continuous casting operations to improve the quality of steel ingots;
- Strengthen the quality inspection of steel ingots and billets, and the rigid billets with scarring defects must be put into production after removal;
- Strictly perform the scoring operation of the rolling spoke, and timely replace the severely worn guide plate and rolling groove to prevent the rolling piece from being scratched.
Prevent surface cracks of hot rolled steel sheet
Measures to prevent cracks are:
- Improve the metallurgical quality of steelmaking and continuous casting, improve the surface quality of continuous casting slabs, and strictly inspect, and the unqualified slabs will not be put into production;
- Strictly control the heating temperature;
- Improve the rolling system, prevent uneven cooling, adopt uniform reduction and reasonable steel turning pass;
- Reasonable cooling after rolling is controlled to make the rolled piece cool evenly.
Prevent the surface of the hot-rolled steel plate from towering
- Need to regularly check and replace the side guides;
- Optimize the winding process operation.
Prevent surface collapse of hot rolled steel sheet
- Reduce the length of the straight section as much as possible;
- Appropriately reduce the coiling temperature; reasonably store the steel coil.
Prevent insufficient height of checkered plate on the surface of hot rolled steel plate
Strictly engrave the pattern spokes, increase the rolling pressure, and change the spokes in time.
The production process of hot-rolled steel and strip products is more complicated. In this process, it is very difficult to control product defects. Therefore, it needs better technology and more experience to gradually improve.
The classification of steel plate defects is a meticulous and rigorous work, which not only respects the traditional classification methods, but also conforms to the existing definitions of relevant standards.
This article classifies the surface defects of hot-rolled steel sheets, analyzes the causes of some of them, and introduces methods to prevent some defects. However, with the gradual increase in the number of steel grades and the diversification of production methods, new defects and morphological changes will continue to occur. This problem needs further research and discussion.
Source: China Hot Rolled Steel Sheet Manufacturer – Yaang Pipe Industry (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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