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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 an intermediate product made from high-quality hot-rolled thin sheets. The oxide layer is removed using a pickling unit, the edges are trimmed, and further finishing is carried out; its surface quality and performance requirements (mainly with regard to cold bending or stamping) lie between those of hot-rolled sheets and cold-rolled sheets. Hot-rolled sheets have relatively lower strength and poorer surface quality (due to oxidation and low surface finish), but they possess good plasticity and are generally used for medium-thick plates. 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, resulting in improved surface quality, 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, leading to 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 performed; 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 is difficult to control deformation, the welds are hard to grind, deformation is substantial, and reshaping is challenging ； It will increase the workload; the yield strength under the same welding conditions needs to be determined through welding experiments. 4. 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 developing 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 increases; the effectiveness of pickling needs to be confirmed through experiments. 2. Electrostatic powder spraying: Poor surface smoothness 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 work? 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 are red, while the un-rusted 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 ； Currently, the minimum thickness achievable through hot rolling is 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 iron oxide 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 finish), but they exhibit 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 can 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 early warning of potential hazards during use, and accidents can easily occur when the load exceeds the allowable limit. 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 easily forms 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 called cold rolling, while rolling above the recrystallization temperature is called hot rolling. The recrystallization temperature of steel is 450–600°C. Hot rolling, as the name implies, involves workpieces at high temperatures; therefore, their resistance to deformation is low, allowing for large amounts of deformation. Taking steel plate rolling as an example, the thickness of the continuous cast slab is generally around 230 mm, while after rough rolling and finish rolling, the final thickness is 1–20 mm. Meanwhile, due to the small width-to-thickness ratio of the steel plate and relatively low requirements for dimensional accuracy, plate shape problems are unlikely to occur; thus, controlling the crown is the main objective. 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 precision rolling in order to regulate the microstructure and mechanical properties of the steel strip. In cold rolling, there is generally no heating process prior to rolling. However, due to the small thickness of the strip steel, shape problems are likely to 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 thin sheets, cold-rolled products and hot-rolled products represent the 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 transforming 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. Differences 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 (they are oxidized and have a rough surface). However, they possess good plasticity; they are generally medium- to thick-gauge sheets. Cold-rolled sheets, on the other hand, have high strength and hardness, as well as excellent 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 can 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 early warning of potential hazards during use, and accidents can easily occur when the load exceeds the allowable limit. 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 film covering 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 storage areas or under cover. Compared to the hot-rolled state, metal materials in the cold-rolled state exhibit 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. How to distinguish between Q235 and Q345 steel plate materials? It is generally impossible to tell the difference between Q235 and Q345 by looking at them. Color differences are not related to the material of the steel, but rather result from different cooling methods after the steel is rolled. Generally, the surface is red after natural cooling. If the quenching method is used and a dense oxide layer forms on the surface, it will appear black. For general strength design, Q345 is used because it has higher strength than Q235 steel, which saves material – 15% to 20% less compared to Q235. For stability control design, it’s better to use Q235. The price difference is 3%—8%. As for identification, there are several opinions: A: 1. In factories, trial welding can be used to roughly distinguish between the two materials. For example, a small round steel bar is welded to each of two steel plates using E43 welding rods, and then a shear force is applied; the material of the two steel plates can be roughly determined based on the pattern of failure. 2. In the factory, grinding with a grinding wheel can also be used to roughly distinguish between the two materials. When grinding Q235 steel with a grinder, the sparks that fly off are round particles with a dark color. The spark of Q345 is branched and bright in color. 3. Additionally, the two types of steel can be roughly distinguished based on the color difference in their shear surfaces. Generally, the cut surface of Q345 steel appears white. B: 1. The color of the steel plate can be used to distinguish between Q235 and Q345 materials: Q235 has a bluish color, while Q345 has a slightly reddish tint (this is only true for steel that has just arrived; over time, it becomes difficult to tell the difference). 2. The most reliable method for identifying the material is chemical analysis, as the carbon content, as well as other chemical components, differ between Q235 and Q345. (This is a foolproof method.) 3. To distinguish between Q235 and Q345 materials, use welding: join two steel plates of unknown material together and weld them using ordinary welding rods; if cracks appear on one of the plates, it indicates that the material is Q345. (This is practical experience.) Why is there a distinction between hot-rolled steel and cold-rolled steel, and what are the differences? Both hot rolling and cold rolling are processes for shaping steel plates or profiles, and they have a significant impact on the structure and properties of the steel. The rolling of steel is mainly done by hot rolling; cold rolling is typically used only for producing precision steel products such as small-sized steel profiles and thin sheets. Common hot- and cold-rolling scenarios for steel: Wire rods: Diameter ranges from 5.5 to 40 mm; they are supplied in coils and are all hot-rolled products. After cold drawing, it becomes cold-drawn material. Round steel: Apart from the precision-sized bright bars, it is generally hot-rolled; there are also forged bars (with forging marks on their surface). Coiled steel: available in both hot-rolled and cold-rolled forms, with cold-rolled products generally being thinner. Steel plates: Cold-rolled plates are generally thin, such as those used in automobiles ； There are many hot-rolled medium and thick plates; some have a thickness similar to that of cold-rolled plates, but their appearance is significantly different. Angle steel: all hot-rolled. Steel pipes: available in both welded hot-rolled and cold-drawn types. Channel steel and H-beam: hot-rolled. Reinforcing bars: hot-rolled. By definition, ingots or billets of steel are difficult to deform at room temperature and not easy to process; therefore, they are usually heated to 1100–1250°C before rolling, and this rolling process is known as hot rolling. 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. Most steel is rolled using the hot rolling method. Steel delivered in the hot-rolled state develops an oxide scale on its surface due to the high temperatures, which confers a certain degree of corrosion resistance, allowing it to be stored outdoors. However, this layer of iron oxide also makes the surface of hot-rolled steel rough and results in large size variations; therefore, for steel that requires a smooth surface, precise dimensions, and good mechanical properties, it is necessary to use hot-rolled semi-finished or finished products as raw materials and then proceed with cold rolling. Advantages: It features a fast forming speed and high productivity; it does not damage the coating. It can be shaped into various cross-sectional forms to meet different application requirements. Cold rolling enables significant plastic deformation of steel, thereby increasing its yield point. Disadvantages: 1. Although no hot plastic compression occurs during the forming process, residual stresses remain within the cross-section, which inevitably affects the overall and local buckling properties of the steel material; 2. Cold-rolled sections generally have an open cross-section, resulting in a low free torsional stiffness for such sections. It is prone to torsion under bending stress, and to bending-torsional buckling under compressive stress, resulting in poor torsional resistance; 3. The wall thickness of cold-rolled formed steel is relatively small, and there is no thickening at the corners where the plates meet, leading to a weak capacity to withstand localized concentrated loads. Cold rolling is a rolling process in which steel is compressed under the pressure of rollers at room temperature to change its shape. Although the processing process also raises the temperature of the steel sheet, it is still called cold rolling. More specifically, cold-rolling uses hot-rolled steel coils as raw material; after pickling to remove the oxide scale, pressure processing is carried out, and the resulting product is a hard-rolled coil. Commonly used cold-rolled steels such as galvanized and colored steel sheets require annealing, which gives them good plasticity and elongation rates; they are therefore widely used in industries such as automobiles, home appliances, and hardware. Cold-rolled sheets have a certain level of smoothness on their surface; they feel smooth to the touch, and this is mainly due to the pickling process. The surface finish of hot-rolled sheets generally does not meet the required standards; therefore, hot-rolled steel strips need to be cold-rolled. Additionally, the thinnest thickness of hot-rolled steel strips is usually 1.0 mm, while cold-rolling can achieve a thickness of 0.1 mm. Hot rolling is rolling above the crystallization temperature, while cold rolling is rolling below the crystallization temperature. The change in the shape of steel due to cold rolling is a form of continuous cold deformation; the cold working hardening resulting from this process increases the strength and hardness of the rolled coil, while reducing its toughness and ductility. For end-use purposes, cold rolling deteriorates the stamping properties, making the product suitable for parts that require only simple deformation. Advantages: It can disrupt the casting structure of steel ingots, refine the grain structure of the steel, and eliminate defects in the microstructure, thereby making the steel structure denser and improving its mechanical properties. This improvement is mainly evident in the direction of rolling, which allows the steel to become less isotropic to a certain extent; the bubbles, cracks, and porosities formed during casting can also be welded together under high temperature and pressure. Disadvantages: 1. After hot rolling, the non-metallic inclusions within the steel (mainly sulfides and oxides, as well as silicates) are compressed into thin layers, resulting in delamination. Stratification **degrades** the tensile properties of steel in the thickness direction, and interlayer tearing may occur during weld contraction. The local strain induced by weld shrinkage often reaches several times the yield stress, which is much greater than the strain caused by loads; 2. Residual stresses resulting from uneven cooling. Residual stress is a type of stress that is self-balanced internally in the absence of external forces. All hot-rolled steel sections exhibit such residual stresses; generally, the larger the cross-sectional size of the steel section, the greater the residual stress. Although residual stress is in self-equilibrium, it still has a certain impact on the performance of steel components under external forces. It can have adverse effects on deformation, stability, fatigue resistance, and other aspects. Summary: The main difference between cold rolling and hot rolling lies in the temperature during the rolling process. ““Cold” refers to normal temperature, while “hot” refers to high temperature. From a metallurgical perspective, the boundary between cold rolling and hot rolling should be defined by the recrystallization temperature. That is, rolling below the recrystallization temperature is called cold rolling, while rolling above the recrystallization temperature is called hot rolling. The recrystallization temperature of steel is 450–600°C.