Hot-rolled steel sheets and strip products are one of the important steel products. With the application and development of hot-rolled sheets in various aspects, users' requirements for products are becoming more and more stringent. While users pay attention to the quality of steel plates, they also pay more attention to and pay more attention to the appearance quality of products.
To a large extent, the appearance quality is one of the most important indicators for evaluating the quality of hot-rolled steel sheets. 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 that The performance of the product, but also pay attention to the appearance quality of the product.
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However, in the whole production process of hot rolling products, due to the influence of 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 may occur. It is inevitable, and the occurrence of these defects runs through the entire hot rolling production process. It not only affects the surface appearance of the subsequent formed device, but also is closely related to the effect of subsequent processing, processing costs, and so on.
In order to further improve the surface quality of hot-rolled steel sheets, domestic and foreign manufacturers have taken many measures to correct and improve, such as combining chemical and mechanical phosphorus removal, continuous annealing, and steel purification. Wait.
2 Classification of common surface defects of hot-rolled steel sheets
The classification of defects should be based on the relevant definitions and descriptions in the relevant technical standards and technical conditions of hot-rolled steel plates, and at the same time, the actual form and characteristics of defects should be formulated and described in combination with the increase in the number of steel types and the diversification of production methods in recent years. . The common defects of hot-rolled steel sheets can be divided into five categories: surface defects, plate shape defects, composition properties, overall coil appearance defects and geometric dimensions.
2.1 Surface defects
There is a unified international evaluation standard for the classification of surface defects of hot-rolled steel sheets. According to the different factors such as their causes and shapes, the defects on the surface of medium-thick plates are divided into 33 types.
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However, various types may be related to each other, and many defects are also very similar in appearance, which is relatively easy to cause confusion. Therefore, these defects can be summarized and organized into five categories: (1) pockmarks; (2) folds and scratches; (3) inclusions; (4) scarring; (5) cracks.
2.2 Shape defects
Shape defects can generally be divided into 3 categories:
2.2.1 Middle waves: Wave undulations along the longitudinal direction, mostly appearing in thin gauge or low-carbon steel plates; 2.2.2 Side waves: Mostly appear in thin gauge, low carbon steel products, with wavy undulations at the edges;
2.2.3 The substrate of the checkered plate is uneven: the base plate of the checkered plate has obvious protrusions and depressions, which are in the shape of fish scales, distributed along the longitudinal band, and some are distributed along the surface of the board.
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2.3 Defects in appearance of the whole roll
The appearance defects of the whole volume can generally be divided into 5 categories in terms of appearance:
2.3.1 Tower shape: tower-like shape, mostly appearing in rolled thick-gauge products;
2.3.2 Unwinding: There are large gaps between the coil layers and layers, which mostly appear in rolled thick-gauge products;
2.3.3 Staggered layers: the unevenness between coil layers or layers, or between layers, mostly occurs in rolled thick-gauge products;
2.3.4 Hanging damage: some parts at both ends of the coil are damaged; there are gaps between the coil layers;
2.3.5 Collapsing coil: The appearance of the steel coil is oval or irregular.
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2.4 Composition performance
In terms of composition performance, it can be divided into 3 categories:
2.4.1 Composition exceeding the standard: one or more components exceed the range required by the planned steel grade;
2.4.2 The tensile strength exceeds the standard 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 Geometry
In terms of geometric size, it can be divided into 3 categories:
2.5.1 Inconsistent thickness: the total or partial thickness of the steel coil exceeds the tolerance range of the planned thickness;
2.5.2 Unsuitable width: the entire or partial width of the steel coil exceeds the tolerance range of the planned width;
2.5.3 The height of the pattern plate is not enough: the height of the pattern is not as specified in the contract. It is easy to occur when rolling thickness above 5.8mm.
3 Analysis of common surface defects of hot-rolled steel sheets
Hot-rolled steel sheets are affected by various factors during the production process:
It mainly includes three aspects: the heating and dephosphorization of the slab, the hot charging process and the surface state of the rolled spoke. The causes of these defects are also various, and a preliminary analysis is made on the causes of some common defects.
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3.1 Pitting
Pockmarks are uneven rough surfaces on the steel surface, also known as pitted surfaces, which are generally continuous sheets, and some are partially or periodically distributed. The reason for this is that the quality of the rolled spokes is poor and the surface hardness is different. Or loss of chilled hard layer, uneven wear, finished hole or front hole rolling groove wear, corrosion or sticking broken iron oxide, during the heating process, the slab is severely oxidized, and the iron sheet is pressed into the surface during rolling, and forms after falling off Tiny pits.
3.2 Folding and scratching
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The reason is that the guide plate is poorly processed or severely worn.
The edge is not smooth, the installation and adjustment of the guide device are improper, the excessive pressure on the rolled piece causes scratches, the guide plate or the hole pattern adheres to the oxide scale and causes scratches, the floor, radial track, and cooling bed of the hot rolling area move steel, The steel turning equipment has sharp corners, and the rolled piece is scratched when it passes through, or the sulfur content in the large gas is high, and the slider is easy to nodulate, causing the surface of the slab to fold and scratch.
3.3 Inclusions
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 colors are dark red, light yellow, etc.
The main reasons are that the non-metallic inclusions originally brought on the surface of the steel ingot and the billet were not cleaned and rolled into the steel surface; during the heating process, the refractory materials, coal ash and cinder on the furnace top or furnace end fell on the surface of the billet, and did not It is 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 on the surface of the rolled piece may also produce inclusions.
3.4 scarring
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Scars are scar-shaped metal flakes on the surface of steel, which are distributed irregularly. The scars are of different sizes and depths. There are often inclusions under the scars.
The reason for scarring is that improper operation when casting steel ingots causes scattered or splashed molten steel to stick to the mold wall, and after being oxidized, sticks to the surface of the steel ingot, forming scars on the surface after rolling, or due to the presence of mold sticking, convexity on the surface of the steel ingot. Defects such as bales, textures, heavy skins or flipped skins will form scars during rolling. During the rolling process, surface fins attached to the surface of the rolled piece due to scratches in a certain pass before the finished hole, or rolling grooves The surface is severely worn, and scarring is caused by re-rolling, some of which are caused by the unclean cleaning of the "beard" of the slab.
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3.5 crack
There are mainly longitudinal cracks, transverse cracks and star cracks. From the steel cross section, the cracks have sharp roots, have a certain depth and are perpendicular to the surface, and there are serious decarburization and non-metallic inclusions around them.
The reason is that the thickness of the primary shell is not uniform, the stress concentration is concentrated in the place where the shell is thin, or the thermal stress caused by the temperature difference between the inside and outside of the shell, the hydrostatic pressure resists the stress of the solidification and shrinkage of the shell along the thickness direction, etc., when Cracks occur when the stress exceeds the tensile strength of the shell.
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3.6 side waves
The uneven lateral thickness of the slab leads to different elongation at each point in the lateral direction during rolling; the poor matching of cvc spokes leads to inconsistent relative reduction rates in the lateral direction of the rolled piece.
3.7 Composition exceeding the standard
During smelting and refining, the composition of molten steel was not strictly controlled in accordance with the technical standards and agreement conditions required by the contract, or the composition of molten steel changed due to excessively long pouring time. This has a lot to do with the pouring process, operation level and equipment status.
3. 8 The width does not match
The computer control system fails, and the rolling parameter control fluctuates; the looper or coiling tension is inappropriate or fluctuates greatly; the slab size deviation is large; the vertical spoke side pressure is inappropriate; the rolling process is inappropriate.
The causes of other defects are not analyzed one by one here.
4 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 defects, how to avoid the occurrence of defects is very important.
4. 1 Pitting
Due to the various causes of pitting, the measures to prevent and eliminate pitting are also different:
4.1.1 When changing the spokes, carefully check the rollers, and do not use severely corroded rollers;
4. 1.2 Replace the worn spokes or hole shapes in time;
4. 1.3 Improve the material of the rolling spokes, improve the wear resistance, keep the rolling groove well cooled, use hot rolling process lubricants, 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 gas. For some alloy steels that are easy to oxidize and the oxide scale is not easy to fall off, an iron sheet should be covered on the surface of the rigid material to protect the heating;
4.1.5 Use high-pressure water and compressed air to remove the iron oxide scale on the surface of the rolled piece before rolling or during the rolling process.
4.2 Folding and scratching
Measures to prevent folding and scratching are:
4.2.1 The processed guide plate must meet the requirements and have smooth edges;
4.2.2 Correctly adjust the guide device;
4.2.3 Eliminate the sharp corners on the radial road, the upper cover plate of the ring road, and the steel moving and turning equipment;
4.2.4 It is necessary to check the operation of the transport drop-off road and troubleshoot in time.
4.3 Scarring
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Measures to prevent and eliminate scarring include:
4.3.1 Improve ingot casting and whole mold operation or continuous casting operation to improve the quality of steel ingot;
4.3.2 Strengthen the quality inspection of steel ingots and blanks, and the steel blanks with scab defects must be put into production after cleaning;
4.3.3 Strictly carry out the scoring operation of the rolled spokes, and replace the seriously worn guide plates and rolling grooves in time to prevent scratches on the rolled pieces.
4.4 Crack
Measures to prevent cracks include:
4.4.1 Improve the metallurgical quality of steelmaking and continuous casting, improve the surface quality of continuous casting slabs, strictly inspect, and unqualified slabs will not be put into production;
4.4.2 Strictly control the heating temperature;
4.4 3 Improve the rolling system, prevent uneven cooling, and adopt uniform reduction and reasonable turning times;
4, 4.4 Reasonably control cooling after rolling to make the rolled piece cool evenly.
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4.5 tower shape
It is necessary to regularly check and replace the side guide plate; optimize the coiling process operation.
4.6 Collapse
Reduce the length of the straight section as much as possible; appropriately reduce the coiling temperature; store the steel coil reasonably.
4.7 Checkered plate height is not enough
Strictly engrave pattern spokes, increase rolling pressure, and replace spokes in time.
The production process of hot-rolled steel and strip products is relatively complicated, and it is very difficult to control product defects in this process, so it needs better technology and more experience to gradually improve.
5 Conclusion
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The classification of steel plate defects is a meticulous and rigorous work, which must not only respect the traditional classification methods, but also comply with the existing definitions of relevant standards.
This paper classifies the surface defects of hot-rolled steel sheets, analyzes the causes of some defects, and introduces the methods to prevent some defects. However, with the gradual increase in the number of steel types and the gradual diversification of production methods, new defects and morphological changes will continue to occur. This issue still needs further research and discussion.
