Summary of Common Surface Defects in Hot-Rolled Steel Plates
1. Hot-rolled steel plates and strips are important steel products. With the increasing application and development of hot-rolled steel plates in various fields, users' requirements for these products are becoming increasingly stringent. While focusing on the quality of the steel plates, users are also paying more attention to the appearance quality of the products. To a large extent, appearance quality is one of the most 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 accept it. Therefore, in the production process, it is necessary to ensure both product performance and appearance quality.
However, in the entire production process of hot-rolled products, due to the influence of various factors such as high temperature, high pressure, high speed, and hardware equipment, the produced products may not fully meet customer requirements. The occurrence of various surface defects in the steel plates is inevitable. These defects occur throughout the entire hot-rolling production process, affecting not only the surface appearance of subsequently formed components but also the effect and cost of subsequent processing.
To further improve the surface quality of hot-rolled steel sheets, domestic and international manufacturers have adopted numerous measures for correction and improvement, such as combining chemical and mechanical descaling, continuous annealing, and steel purification.
Common Surface Defects in Hot-Rolled Steel Sheets
2. Defect classification should primarily rely on the relevant definitions and descriptions in the technical standards and conditions for hot-rolled steel sheets, while also considering the increasing number of steel grades and the diversification of production methods in recent years, and the corresponding actual forms and characteristics of defects. Common defects in hot-rolled steel sheets can be classified into five categories: surface defects, sheet shape defects, compositional properties, overall coil appearance defects, and geometric dimensions.
2.1 Surface Defects There are already unified international evaluation standards for the classification of surface defects in hot-rolled steel sheets. Based on differences in their causes and shapes, surface defects in medium and heavy plates are divided into 33 types.
However, these categories may be interconnected, and many defects appear very similar, making confusion easy. Therefore, these defects can be summarized and categorized into five main types: (1) pitting; (2) folds and scratches; (3) inclusions; (4) scabs; (5) cracks. 2.2 Plate Shape Defects Plate shape defects can generally be divided into three categories:
2.2.1 Central Waves: Wave-like undulations along the longitudinal direction, mostly appearing in thin-gauge or low-carbon steel plates;
2.2.2 Edge Waves: Mostly appearing in thin-gauge, low-carbon steel products, with wavy undulations at the edges;
2.2.3 Uneven Base Plate of Patterned Plates: The base plate of the pattern has obvious protrusions and depressions, resembling fish scales, distributed in longitudinal bands, and sometimes along the plate surface. 2.3 Overall Coil Appearance Defects Overall coil appearance defects can generally be divided into five categories:
2.3.1 Tower Shape: A tower-like shape, often seen in thick-gauge rolled products;
2.3.2 Loose Coil: Large gaps between coil layers, often seen in thick-gauge rolled products;
2.3.3 Misaligned Layers: Uneven layers or multiple layers of coil, often seen in thick-gauge rolled products;
2.3.4 Broken Coil: Damage at individual points at both ends of the coil; gaps between coil layers;
2.3.5 Collapsed Coil: The steel coil has an elliptical or irregular circular appearance.
2.4 Composition and Performance Defects Composition and performance defects can be divided into three categories:
2.4.1 Composition Exceeds Standard: One or more components exceed the range required by the planned steel grade;
2.4.2 Tensile Strength Exceeds Standard (M): Tensile strength exceeds the range required by the planned steel grade;
2.4.3 Elongation Exceeds Standard: Elongation exceeds the range required by the planned steel grade.
2.5 Geometric Dimensions Geometrically, defects can be categorized into three types:
2.5.1 Thickness Inconsistency: The thickness of the steel coil, either entirely or partially, exceeds the tolerance range of the planned thickness.
2.5.2 Width Inconsistency: The width of the steel coil, either entirely or partially, exceeds the tolerance range of the planned width.
2.5.3 Insufficient Pattern Plate Height: The pattern height does not meet the contract specifications. This is prone to occur when rolling thicknesses of 5.8mm or higher.
Analysis of Common Surface Defects in Hot-Rolled Steel Plates
3. Hot-rolled steel plates are affected by various factors during production:
These mainly include three aspects: slab heating and descaling, hot charging process, and the surface condition of the rolling mill. The causes of these defects are varied, and a preliminary analysis of the causes of some common defects is presented.
3.1 Pitting: Pitting is a rough, uneven surface on the steel, also known as pitting. It is generally continuous in patches, but can sometimes appear locally or periodically. Causes include poor rolling mill quality, inconsistent surface hardness, loss of the chilled hardening layer, uneven wear, wear, corrosion, or adhesion of broken iron oxide in the finishing hole or pre-finishing hole grooves. During heating, severe oxidation of the slab causes scale to be pressed into the surface during rolling, forming small pits after detachment.
3.2 Folding and Scratches: Causes include poor processing or severe wear of the guide plates. Unsmooth edges, improper installation or adjustment of the guide devices, excessive pressure on the workpiece causing scratches, iron oxide scale adhering to the guide plates or die causing scratches, steel shifting on the floor, spokes, or cooling bed in the hot rolling zone, sharp corners in the steel turning equipment causing scratches as the workpiece passes through, or high sulfur content in the gas causing nodules to form on the slide block, resulting in folding and scratches on the slab surface.
3.3 Inclusions: Inclusions refer to non-metallic inclusions of a certain depth on the surface of steel, generally appearing as dots, strips, or blocks, and are dark red or pale yellow in color. The main causes include: non-metallic inclusions originally present on the surface of the ingot or billet were not removed and were rolled into the steel surface; during the heating process, refractory materials, coal ash, and slag from the furnace top or end fell onto the billet surface and were not removed and were rolled into the rolled piece surface; additionally, an unclean environment around the rolling mill, where non-metallic inclusions adhere to the surface of the rolled piece, can also cause inclusions.
3.4 Scales: Scales are thin, scar-like metallic flakes on the surface of steel, irregularly distributed, varying in size and depth, and often containing inclusions beneath them. Scale formation is caused by improper operation during steel ingot casting, causing spilled or splashed molten steel to adhere to the die wall, oxidize, and then stick to the ingot surface, forming scale after rolling. Alternatively, defects such as die sticking, bulges, mesh patterns, overlapping layers, or flaking on the ingot surface can also cause scale formation during rolling. During rolling, surface fins formed by scratches in a pass before the finished product hole can adhere to the surface of the rolled piece, or severe wear on the rolling groove surface can lead to scale formation during re-rolling. Some cases are caused by incomplete cleaning of the slab's "beard."
3.5 Cracks: These mainly include longitudinal cracks, transverse cracks, and star-shaped cracks. Viewed from the cross-section of the steel, cracks have sharp roots, a certain depth, and are perpendicular to the surface. Their periphery shows severe decarburization and non-metallic inclusions. The causes include uneven thickness of the initial billet shell, stress concentration in thinner areas, thermal stress caused by temperature differences between the inside and outside of the shell, and stress generated by the static pressure of the molten steel resisting the solidification shrinkage of the shell along its thickness. When the stress exceeds the tensile strength of the billet shell, cracks occur.
3.6 Edge waviness: Uneven transverse thickness of the cast billet leads to different elongation rates at different points during rolling; poor CVC spoke matching results in inconsistent relative reduction rates in the transverse direction of the rolled piece.
3.7 Composition exceeding standards: During smelting and refining, the composition of the molten steel was not strictly controlled according to the technical standards and agreement conditions required by the contract, or the composition of the molten steel changed due to excessive pouring time. This is closely related to the steel casting process, operator skill, and equipment condition.
3.8 Width discrepancy: Malfunction of the computer control system, fluctuations in rolling parameter control; unsuitable or large fluctuations in looper or coiling tension; large deviations in slab dimensions; unsuitable vertical spoke side pressure; unsuitable rolling process.
