Some knowledge about steel plates

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Do you know the difference between pickled steel sheets and hot-rolled steel sheets used in sheet metal work? Pickled steel sheet: It is made from high-quality hot-rolled thin sheets; the oxide layer is removed using a pickling unit, followed by trimming and finishing. Its surface quality and performance requirements (mainly with regard to cold bending or stamping) fall between those of hot-rolled sheets and cold-rolled sheets. Hot-rolled sheets have relatively lower strength and poorer surface quality (with oxidation and low finish), but they exhibit good plasticity; they are generally medium-thick sheets. Compared to hot-rolled sheets, pickled steel sheets have the following advantages: 1. Better surface quality – compared to ordinary hot-rolled sheets, pickled hot-rolled sheets have their surface oxide layer removed, thereby improving the surface quality of the steel, which facilitates welding, coating, and painting. 2. Higher dimensional accuracy – after flattening, the shape of the sheet can be adjusted to reduce irregularities. 3. Improved surface finish, resulting in a better appearance. Effects of using pickled steel sheets on welding: 1. Spot welding: The oxide layer on the surface must be removed before spot welding can be carried out; the specific yield strength requires testing through welding experiments. 2. Bolt welding: The oxide layer needs to be removed, and the specific yield strength again requires testing via welding experiments. 3. Conventional welding: There is significant spatter, it’s difficult to control deformation, the welds are hard to polish, and deformation is substantial, making it hard to reshape the sheet ； It will increase the workload. The yield strength under the same welding conditions needs to be determined through welding experiments. Laser cutting: Laser cutting results in significant deformation. It is not suitable for parts with high dimensional requirements, as it will have a significant impact on subsequent welding. 5. Bending: It has poor cold bending or stamping properties, is prone to bending cracks, which can reduce the product’s service life and lead to failure. II. Effects of using a hot plate on paint: 1. Pretreatment: Due to the large amount of oxide scale on the surface, the pickling time must be increased; the effectiveness of pickling needs to be confirmed through experiments. 2. Electrostatic powder spraying: Poor surface finish will hinder the spraying process. 3. When hot-rolled sheets are left exposed to air, rust formed on them is difficult to remove. Do you know the difference between cold-rolled steel sheets and hot-rolled steel sheets used in sheet metal? 1. The surface of cold-rolled steel sheets has a certain level of gloss, and it feels smooth to the touch, similar to those common steel water bottles used for drinking water. 2. If hot-rolled sheets are not pickled, their surface is similar to that of many ordinary steel sheets available on the market: the rusted areas appear red, while the unrusted areas are purplish-black (due to iron oxide scale). The performance advantages of cold-rolled and hot-rolled sheets are: (1) higher precision, with the thickness variation of cold-rolled strips not exceeding 0.01~0.03 mm. (2) Thinner dimensions; cold rolling can produce steel strips as thin as 0.001 mm ； The thinnest thickness achievable for hot-rolled products is now 0.78 mm. (3) The surface quality is superior; cold-rolled steel sheets can even have a mirror-like surface ； The surface of hot-rolled sheets has defects such as scale and pitting. (4) Cold-rolled sheets can have their mechanical properties such as tensile strength, as well as their process-related properties such as stamping performance, adjusted according to the customer’s requirements. Cold rolling and hot rolling are two different steel rolling techniques. As the name implies, cold rolling involves rolling steel at room temperature, resulting in steel that has a high hardness. Hot rolling is the process of rolling steel at high temperatures. Hot-rolled sheets have low hardness, are easy to process, and possess good ductility. Cold-rolled sheets have high hardness, making them relatively difficult to process, but they are not prone to deformation and possess high strength. Hot-rolled sheets have relatively low strength and poor surface quality (with oxidation and low surface finish), but they possess good ductility; they are generally medium to thick sheets. Cold-rolled sheets, on the other hand, have high strength and hardness as well as a high surface finish; they are usually thin sheets and can be used for stamping purposes. Hot-rolled steel plates have mechanical properties that are far inferior to those of cold-worked steel, and also lower than those of forged steel; however, they possess good toughness and ductility. Cold-rolled steel sheets have low toughness due to a certain degree of work hardening, but they achieve a good yield-to-tensile strength ratio; they are used for manufacturing parts such as cold-formed spring plates. Since the yield point is close to the tensile strength, there is no anticipation of danger during use, and accidents can easily occur when the load exceeds the allowable value. By definition, ingots or billets of steel are difficult to deform at room temperature and not easy to process; they are generally heated to 1100–1250°C for rolling, and this rolling process is known as hot rolling. Most steel is rolled using the hot rolling method. However, since iron oxide scale tends to form on the surface of steel at high temperatures, causing the surface of hot-rolled steel to become rough and resulting in large size variations, steel with a smooth surface, precise dimensions, and good mechanical properties is required. In such cases, hot-rolled semi-finished products or finished products are used as raw materials and then processed through cold rolling. Rolling at room temperature is generally understood as cold rolling; from a metallurgical perspective, the boundary between cold rolling and hot rolling should be determined by the recrystallization temperature. That is, rolling below the recrystallization temperature is cold rolling, while rolling above the recrystallization temperature is hot rolling. The recrystallization temperature of steel is 450–600°C. As the name implies, in hot rolling, the temperature of the rolled material is high; therefore, the resistance to deformation is low, allowing for large amounts of deformation. Taking steel plate rolling as an example, the thickness of a continuous casting slab is generally around 230 mm, while after rough rolling and finish rolling, the final thickness is 1–20 mm. At the same time, due to the low width-to-thickness ratio of the steel plates, the requirements for dimensional accuracy are relatively low, and it is not easy to encounter shape-related issues; therefore, controlling convexity is the main focus. For organizations with specific requirements, this is generally achieved through controlled rolling and cooling, that is, by controlling the starting temperature, final rolling temperature, and coiling temperature during fine rolling in order to regulate the microstructure and mechanical properties of the steel strip. In cold rolling, there is generally no heating step before starting the rolling process. However, due to the thin thickness of the steel strip, sheet shape problems can easily occur. Moreover, the product is obtained after cold rolling; therefore, many complex processes are employed to control the dimensional accuracy and surface quality of the steel strip. The cold rolling production line is long, has many pieces of equipment, and features a complex process. As users place higher demands on the dimensional accuracy, sheet shape, and surface quality of steel strips, the control models, L1 and L2 systems, as well as sheet shape control methods for cold rolling mills are more numerous compared to those used in hot rolling. Moreover, the temperatures of the rolls and the steel strip are also important control parameters. In terms of cold-rolled and hot-rolled products in the form of thin sheets, there is a difference between preceding and subsequent processing steps. Hot-rolled products serve as raw materials for cold-rolled products. Cold rolling involves using roller mills to process hot-rolled steel coils that have been pickled; it is a form of cold working, aimed at converting thick hot-rolled sheets into thinner cold-rolled sheets. For example, a hot-rolled sheet with a thickness of 3.0 mm can be processed to produce cold-rolled coils with a thickness of 0.3–0.7 mm. The underlying principle is the use of compression to force deformation. What is the difference between Q235 hot-rolled steel plates and cold-rolled steel plates? Hot-rolled plates have lower hardness, are easier to process, and possess better ductility. Cold-rolled sheets have high hardness, making them relatively difficult to process, but they are not prone to deformation and possess high strength. Hot-rolled sheets have relatively low strength and poor surface quality (with oxidation and low finish), but they possess good ductility; they are generally medium-thickness sheets. Cold-rolled sheets, on the other hand, have high strength and hardness as well as a high surface finish; they are usually thin sheets and can be used as material for stamping. Hot-rolled steel plates have mechanical properties that are far inferior to those of cold-worked steel, and also lower than those of forged steel; however, they possess good toughness and ductility. Cold-rolled steel sheets have low toughness due to a certain degree of work hardening, but they achieve a good yield-to-tensile strength ratio; they are used for manufacturing parts such as cold-formed spring plates. Since the yield point is close to the tensile strength, there is no anticipation of danger during use, and accidents can easily occur when the load exceeds the allowable value. Hot rolling involves rolling steel plates at high temperatures to make them relatively thinner ； Cold rolling is the process of rolling steel plates at room temperature. Generally, hot rolling is carried out first, followed by cold rolling. When the steel plate is thick, only hot rolling can be used; after it is rolled into a thinner plate, cold rolling is then applied. Hot-rolled steel plates are divided into thick plates (with a thickness greater than 4 mm) and thin plates (with a thickness of 0.35–4 mm) ； Cold-rolled steel sheets come in only one form: thin sheets (with a thickness of 0.2 to 4 mm). The finishing temperature for hot rolling is generally 800–900°C, after which it is usually cooled in air; thus, the hot-rolled state is equivalent to normalizing treatment. Metal materials delivered in the hot-rolled state possess a certain degree of corrosion resistance due to an oxide layer on their surface; therefore, the requirements for storage and handling are not as strict as those for materials delivered in the cold-rolled state. Large and medium-sized steel sections, as well as medium and thick steel plates, can be stored in open areas or under cover. Compared to the hot-rolled state, metal materials in the cold-rolled state have higher dimensional accuracy, better surface quality, lower surface roughness, and superior mechanical properties. However, it is prone to corrosion or rusting; its packaging, storage, and transportation require strict standards. It must be kept in a warehouse, and attention should be paid to controlling the temperature and humidity there.