The difference between hot rolling and cold rolling

By definition, steel ingots or billets are difficult to deform and process at room temperature, and are generally heated to 1100-1250 ℃ for rolling. This rolling process is called hot rolling.

Most steel is rolled by hot rolling method. However, due to the easy formation of oxide scale on the surface of steel at high temperatures, the surface of hot-rolled steel is rough and the size fluctuates greatly. Therefore, it is required to use hot-rolled semi-finished or finished products as raw materials and use cold rolling methods to produce steel with smooth surface, precise size, and good mechanical properties.

Let's take a look!

hot-rolling

Using continuous casting billets or preheated billets as raw materials, they are heated by a step heating furnace, descaling by high-pressure water, and sent to a roughing mill. The roughing mill cuts the head and tail, and then sends it to a precision rolling mill for computer-controlled rolling. Finally, after rolling, it is cooled by laminar flow (computer-controlled cooling rate) and coiled by a coiling machine to become straight coils.

The head and tail of straight curls are often tongue shaped or fish tail shaped, with poor accuracy in thickness and width, and often have defects such as wavy, folded, and tower shaped edges.

The coil weight is relatively large, and the inner diameter of the steel coil is 760mm.

After passing through multiple precision processing production lines such as head cutting, tail cutting, edge cutting, straightening, and leveling, straight coils are cut or re rolled into products such as hot-rolled steel plates, flat hot-rolled steel coils, and longitudinal cut steel strips.

Hot rolled precision processed coils are made by acid pickling and oil coating. This product has a trend of partially replacing cold-rolled sheets, with a moderate price, and is deeply loved by a large number of users.

cold rolling

Using hot-rolled steel coils as raw materials, after acid washing to remove oxide scale, cold rolling is carried out to produce rolled hard coils as finished products.

Due to continuous cold deformation resulting in cold work hardening, the strength and hardness of the rolled hard coil increase, while the toughness and plasticity indicators decrease. Therefore, the stamping performance will deteriorate and can only be used for parts with simple deformation.

Hot dip galvanizing plants can use rolled hard coils as raw materials, because hot-dip galvanizing units are equipped with annealing wires, and the coil weight is generally 6-13.5 tons, with a steel coil inner diameter of 610mm. Cold rolling mill

Plates and coils generally need to undergo continuous annealing (CAPL unit) or hood furnace annealing to eliminate cold work hardening and rolling stress, and achieve the mechanical properties specified in the corresponding standards.

The surface quality, appearance quality, and dimensional accuracy of cold-rolled steel plates are superior to those of hot-rolled plates, and their product thickness can be directly rolled to about 0.18mm, thus being favored by a large number of users. Cold rolled steel coils, as substrates for deep processed products, have become high value-added products.

Such as galvanizing, hot-dip galvanizing, fingerprint resistant galvanizing, color coated steel coils, vibration reducing composite steel plates, PVC coated steel plates, etc.

Briefly describe the difference between the two:

The surface of cold-rolled sheet has a certain glossiness, and it feels relatively smooth to the touch, similar to the common steel cup used for drinking water.

If hot-rolled plates are not pickled, their surface is similar to many ordinary steel plates on the market, with rusted surfaces being red and some surfaces being purple black (iron oxide scale).

The performance advantages of cold-rolled and hot-rolled plates are:

High precision, the thickness difference of cold-rolled strip does not exceed 0.01~0.03mm.

Thinner size, the thinnest cold-rolled steel strip can be rolled to 0.001mm; Hot rolling can now be as thin as 0.78mm.

The surface quality is good, and cold-rolled sheets can even produce mirror surfaces; There are defects such as iron oxide scale and pitting on the surface of hot-rolled plates.

Cold rolled sheets can adjust their tensile strength and stamping performance according to user requirements.

Cold rolling and hot rolling are two different steel rolling processes. As the name suggests, cold rolling is the process of rolling steel at room temperature, which has a higher hardness. Hot rolling is the process of rolling steel at high temperatures.

Specifications

Hot rolled plates have low hardness, easy processing, and good ductility.

Cold rolled plates have high hardness and are relatively difficult to process, but they are not easily deformed and have high strength.

Hot rolled plates have low strength and slightly poor surface quality (oxidation, low smoothness), but good plasticity, generally medium thick plates. Cold rolled plates have high strength, hardness, and surface smoothness, generally thin plates, and can be used as stamping plates. Cold rolled and hot-rolled steel plate

The mechanical properties are far inferior to cold processing and inferior to forging processing, but it has good toughness and ductility.

Cold rolled steel plates have a certain degree of work hardening and low toughness, but can achieve a good yield to strength ratio, and are mostly used for cold bending springs and other parts. At the same time, due to the yield point being closer to the tensile strength, there is no predictability of the dangers during use, and accidents are prone to occur when the load exceeds the allowable load.

According to the definition

By definition, steel ingots or billets are difficult to deform and process at room temperature, and are generally heated to 1100-1250 ℃ for rolling. This rolling process is called hot rolling.

Most steel is rolled by hot rolling method.

However, due to the easy formation of oxide scale on the surface of steel at high temperatures, the surface of hot-rolled steel is rough and the size fluctuates greatly. Therefore, it is required to use hot-rolled semi-finished or finished products as raw materials and use cold rolling methods to produce steel with smooth surface, precise size, and good mechanical properties.

Rolling at room temperature

Rolling at room temperature is generally understood as cold rolling, and from a metallurgical perspective, the boundary between cold rolling and hot rolling should be distinguished by the recrystallization temperature.

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 ℃.

hot-rolling

Hot rolling, as the name suggests, has a high rolling temperature, resulting in low deformation resistance and the ability to achieve significant deformation.

Taking the rolling of steel plates as an example, the thickness of continuous casting billets is generally around 230mm. After rough rolling and precision rolling, the final thickness is 1-20mm.

At the same time, due to the small width to thickness ratio of the steel plate, the dimensional accuracy requirements are relatively low, and it is not easy to have plate shape problems, mainly controlling the convexity.

For organizational requirements, it is generally achieved by controlling rolling and cooling, that is, controlling the opening temperature, final rolling temperature, and edge temperature of precision rolling to control the microstructure and mechanical properties of the strip steel. cold rolling

Generally, there is no pre rolling heating process, but due to the small thickness of the strip steel, it is prone to plate shape problems,

And after cold rolling, it is the finished product, so in order to control the dimensional accuracy and surface quality of the strip steel, many very complicated processes need to be used.

The cold rolling production line is long, with multiple equipment and complex processes. With the increasing demand of users for the dimensional accuracy, plate shape, and surface quality of strip steel, the control models, L1, L2 systems, and plate shape control methods of cold rolling mills are more diverse than those of hot rolling mills.

Moreover, roller temperature and belt temperature are also important control indicators.

cold rolling

The production line of cold-rolled products and hot-rolled products is the difference between the previous process and the next process. Hot rolled products are the raw materials for cold-rolled products. Cold rolling is the process of rolling hot-rolled steel coils on an acid pickling machine using a rolling mill. Rolling is a cold forming process, mainly to roll thick hot-rolled plates into thin cold-rolled plates. Typically, 3.0mm hot-rolled plates can be rolled on machines to produce 0.3-0.7mm cold-rolled coils. The main principle is to use the principle of extrusion for forced deformation.