The difference between hot rolling and cold rolling

Cold-rolled seamless steel pipes are usually of small diameter, while hot-rolled seamless steel pipes are usually of large diameter. Cold-rolled seamless steel tubes have higher precision than hot-rolled seamless steel tubes, and their price is also higher. Depending on the manufacturing process, seamless steel pipes are divided into hot-rolled (extruded) seamless steel pipes and cold-drawn (rolled) seamless steel pipes. Cold-drawn (rolled) tubes are further divided into round tubes and shaped tubes. a. Process flow overview: Hot rolling (extrusion of seamless steel pipes): Round tube billet → Heating → Piercing → Three-roll skew rolling, continuous rolling or extrusion → Tube removal → Dimensioning (or reduction in diameter) → Cooling → Billet tube → Straightening → Hydrostatic testing (or flaw detection) → Marking → Storage. Cold-drawn (rolled) seamless steel tubes: round tube billet → heating → piercing → end forming → annealing → pickling → oiling (copper plating) → multiple passes of cold drawing (cold rolling) → tube blank → heat treatment → straightening → hydrostatic testing (flaw detection) → marking → storage.

Cold-rolled products are typically thin-walled tubes with a glossy surface and high dimensional accuracy. Hot-rolled pipes are mostly thick-walled and large in size; they have an oxide layer on their surface, making them dark in color, and they rust easily

Things from the internet: 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-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 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 leading to 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. 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 the 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 plate, the requirements for dimensional accuracy are relatively low, and it is not easy to encounter shape-related problems; thus, 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, shape defects 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 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 extrusion to force deformation. 　　Difference: Simply put, 1. Cold-rolled steel sheets have a certain level of gloss on their surface and feel 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 unrusted areas are purplish-black (due to iron oxide scale). 　　The performance advantages of cold-rolled and hot-rolled sheets are as follows: 1. Higher precision, with the thickness variation of cold-rolled steel 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 steel 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 high hardness. Hot rolling is the process of shaping steel at high temperatures. 1. Hot Rolling: Using continuously cast slabs or initially rolled slabs as raw materials, these are heated in a step-type furnace; after descaling with high-pressure water, they go into the roughing mill. The material from the roughing mill has its ends trimmed before it enters the finishing mill, where rolling is carried out under computer control. After final rolling, the steel undergoes laminar cooling (with the cooling rate controlled by a computer) and is then coiled by a coiler to form straight coils. The ends of straightened curled hair often have a tongue-shaped or fish-tail shape, with poor accuracy in terms of thickness and width; defects such as wavy edges, folded edges, and tower shapes are commonly present at the edges. Its coil weight is high, and the inner diameter of the steel coil is 760 mm. (It is commonly used in the pipe manufacturing industry. ) After the straight steel coils undergo processes such as end trimming, tail trimming, edge trimming, as well as multiple rounds of straightening and leveling in the finishing line, they are then cut into sheets or re-rolled to produce products such as hot-rolled steel sheets, leveled hot-rolled steel coils, and slitted strips. If the hot-rolled finish coil is pickled to remove scale and then coated with oil, it becomes hot-rolled pickled sheet coil. This product tends to partially replace cold-rolled steel sheets; it has a moderate price and is very popular among users. 2. Cold rolling: Using hot-rolled steel coils as raw material, acid washing is carried out to remove the oxide scale, followed by cold continuous rolling. The resulting product is a hard-rolled coil. Due to the cold work hardening resulting from continuous cold deformation, the strength and hardness of this coil increase while its toughness and ductility decrease; as a result, its formability worsens, and it can only be used for parts that require simple deformation. Hard-rolled coils can be used as raw material for hot-dip galvanizing plants, as all hot-dip galvanizing lines are equipped with annealing lines. The weight of the hardened coil is generally between 6 and 13.5 tons, with an inner diameter of 610 mm for the steel coil. Generally, cold-rolled sheets and coils should undergo continuous annealing (using CAPL lines) or bell-type furnace annealing to eliminate work hardening and rolling stresses, so as to meet the mechanical property requirements specified in the relevant standards. 　　Cold-rolled steel sheets have superior surface quality, appearance, and dimensional accuracy compared to hot-rolled sheets, and their thickness can be reduced to around 0.18 mm, which is why they are highly favored by many users. Deep processing of products using cold-rolled steel coils as a base material results in high-value-added products. Such as electro-galvanizing, hot-dip galvanizing, fingerprint-resistant electro-galvanizing, colored coated steel coils, vibration-damping composite steel sheets, and PVC-coated steel sheets

Both hot rolling and cold rolling are processes for shaping steel sections or steel plates; they have a significant impact on the structure and properties of the steel. Hot rolling is the primary method used for rolling steel, while cold rolling is employed only for producing small-sized steel sections and thin sheets. I. Hot rolling: Advantages: It can disrupt the casting structure of the steel ingot, 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, thereby making the steel less isotropic to a certain extent ； Bubbles, cracks, and porosity 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 (interlayer formation). 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 strain, which is much larger than the strain caused by loads ； 2. Residual stresses caused by uneven cooling. Residual stress is the stress that exists in a state of internal self-equilibrium in the absence of external forces. All hot-rolled steel sections have such residual stress, and 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 may have adverse effects on deformation, stability, fatigue resistance, and other aspects. II. Cold rolling refers to the process of processing steel plates or strips into various shapes through cold working methods such as cold drawing, cold bending, and cold extrusion at room temperature. Advantages: fast forming speed, high output, and no damage to the coating; it can be made into a variety of cross-sectional shapes to meet the requirements of different usage conditions ； Cold rolling can cause significant plastic deformation in steel, thereby increasing its yield strength. Disadvantages: 1. Although it is not subjected to hot plastic compression during the forming process, residual stresses still exist within the cross-section, which inevitably affects the overall and local buckling properties of the steel material ； 2. Cold-rolled section shapes generally have an open cross-section, resulting in a low free torsional stiffness of the cross-section. It is prone to torsion under bending, and to bending-torsional buckling under compression; its torsional resistance is poor ； 3. Cold-rolled formed steel has a relatively thin wall thickness, and no thickening is applied at the corners where the plates meet, resulting in a weak capacity to withstand localized concentrated loads. III. The main differences between hot rolling and cold rolling are as follows: 1. Cold-rolled shaped steel allows for local buckling of the cross-section, thereby enabling full utilization of the load-bearing capacity of the member after buckling ； On the other hand, hot-rolled section steel does not allow local buckling of its cross-section. 2. The causes of residual stress in hot-rolled and cold-rolled sections are different, so there are also significant differences in their distribution across the cross-section. The residual stress distribution on the cross-section of cold-formed thin-walled steel sections is of a bending type, whereas that on the cross-sections of hot-rolled or welded steel sections is of a thin-film type. 3. The free torsional stiffness of hot-rolled sections is higher than that of cold-rolled sections, so the torsional resistance of hot-rolled sections is superior to that of cold-rolled sections.