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Introduction to Hot-rolled Steel Plate Products 1. Introduction to hot-rolled steel plate products: Hot-rolled steel plate and strip products are made from slabs (mainly continuous casting slabs); after heating, they are processed into strips using roughing and finishing rolling mills. The hot steel strip emerging from the last rolling mill in the finish rolling stage is cooled by laminar flow to the desired temperature, and then rolled into a steel strip coil by a coiler. The cooled steel strip coil is processed through various finishing lines (leveling, straightening, slitting or cutting, inspection, weighing, packaging, and labeling) according to the specific requirements of the customer, to be turned into steel plates, leveled coils, and slitted steel strip products. Due to their excellent properties such as high strength, good toughness, ease of processing and forming, and superior weldability, hot-rolled steel plate products are widely used in manufacturing industries such as ships, automobiles, bridges, construction, machinery, and pressure vessels. With the increasing maturity of new technologies for controlling hot-rolling dimensions, shape, and surface quality, as well as the continuous introduction of new products, hot-rolled steel plate and strip products are being used more and more widely and gaining greater competitiveness in the market. Generally speaking, hot-rolled steel plate products come in a wide variety of steel grades and specifications, and have a broad range of applications. They are widely used in the manufacture of various structures, as well as in the production of automobiles, bridges, ships, boilers, pressure vessels, and more. Due to their various applications, different requirements are placed on the material properties, surface quality, dimensions, and dimensional accuracy of steel plates. Therefore, it is necessary to understand the types, materials, characteristics, and uses of hot-rolled steel plate products in order to make economical and rational use of them. 2. Key considerations for mechanical properties (1) Mechanical properties: The mechanical properties of steel plates refer to those characteristics related to their elastic or inelastic responses under stress, or those involving the stress-strain relationship. Tensile strength, yield point, elongation, and impact absorption energy are the main indicators that represent the mechanical properties of hot-rolled steel plates. Its size indicates the degree to which the steel can resist various forces; it is a key criterion for assessing the quality of steel plate materials, and it also serves as the main basis for selecting materials and performing strength calculations when designing steel plate components. (2) Mechanical property tests: The experiments to determine the mechanical properties of hot-rolled steel plates mainly include tensile tests and impact tests. (3) Yield strength: The stress at which, during the tensile testing of a specimen, the load either does not increase or begins to decrease while the specimen continues to elongate (deform). The lower the yield strength of the steel, the less force is required to cause permanent deformation, meaning it is easier to shape and process. (4) Tensile strength: The maximum stress that a specimen can withstand before it breaks when being stretched. When the external stress applied to a material exceeds its tensile strength, it will crack. Therefore, the higher the tensile strength of a steel plate material, the better its ability to withstand large external stresses without cracking. (5) Elongation: The percentage of the increase in length of the gauge section of the specimen after it is pulled apart, relative to the original gauge length. The larger the ratio of elongation, the better the material’s ability to undergo permanent deformation (plasticity) before failing under stress ； Conversely, the plasticity is worse. (6) Impact energy (impact absorption energy): During impact testing, it refers to the energy absorbed by a specimen of specified shape and size when it breaks under a single impact force. The magnitude of the impact energy indicates the resistance of the metal material to impact loads. The higher the impact energy, the greater the material’s resistance to sudden brittle fracture. 3. Selection of hot-rolled steel plate products (1) Relationship between mechanical properties, formability, and service properties. To give the steel plate the desired shape, it is necessary to cause permanent deformation in it; the processes used for this purpose can include local or overall bending, deep drawing, tensioning, or a combination of these forming methods. (1) The yield strength of thin steel sheets indicates their formability and strength after shaping. For the shaping of ordinary carbon steel sheets, if the yield point is too high, excessive springback may occur, the sheets are prone to breaking during shaping, tool wear accelerates, and defects arise due to poor plasticity. However, when the yield strength of the material is less than 140 Mpa, it may not be able to withstand the stresses applied during the forming process. For steel plates used in more complex forming or stamping operations, a relatively low yield strength is generally required; moreover, the lower the yield ratio, the better the formability of the steel plate. (2) The cold formability of medium and thick plates is directly related to the yield strength and elongation of the material. The lower the yield strength value, the less stress is required to cause permanent deformation ； The higher the elongation value, the greater the ductility, which allows the material to withstand large amounts of deformation without breaking. (3) For steel plates used in building structures, bridges, and mechanical components, a certain tensile strength is required for the steel material in order to prevent the components from breaking, while a certain yield strength is needed to prevent deformation of those components. Therefore, for steel materials intended for such applications, minimum values or ranges for tensile strength and yield strength must be specified. (4) For steel plates used to withstand deformation under impact loads, such as those in ships, bridges, and oil and gas pipelines, it is necessary to ensure they possess a sufficiently high impact toughness – namely, an adequate impact energy value – in order to prevent brittle fracture during use. (2) Selection of steel plate types. Hot-rolled steel plate products include strips (coils) and steel plates obtained by cutting them. Steel strips (coils) can be further divided into straight coils and finished coils (segmented coils, flat segmented coils, and slitted coil strips). Since the straight-coiled strips are not re-rolled, the portions at the ends where the steel strip thickness changes are not removed, and they are not straightened or leveled, these strips have bulges at the ends and uneven thickness and width. Defects such as wavy edges, folded edges, tower-shaped shapes, and wrinkles (waist folds) after uncoiling can occur. Therefore, for applications that require high surface quality and good sheet shape, hot-rolled straight-coiled strips are not suitable; instead, leveled strips that have been re-rolled and leveled on a finishing line should be used.