The difference between hot rolled and cold rolled coils

2022-04-09
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I. Definition of both

1. Hot rolling

With continuous casting plate also or primary south plate also as raw materials, heated by the step dynamic heating furnace, high-pressure water descaling into the rough rolling mill, rough rolling material by cutting the head, tail, and then into the finishing mill, the implementation of computer-controlled rolling, the final rolling that is after laminar cooling (computer-controlled cooling rate) and winding machine rolled, become straight hair volume. Straight hair volume of the head, tail is often tongue and fish tail shape, thickness, width accuracy is poor, the side often exists wave-shaped, folded edge, tower-shaped defects. Its volume weight is heavier, the inner diameter of the steel coil is 760. (General pipe making industry enjoys the use of.) The straight hair volume by cutting the head, cutting the tail, cutting the edge and multiple passes of straightening, leveling and other finishing line processing, and then cut the plate or re-roll, that is, become: hot-rolled steel sheet, flat hot-rolled steel coil, longitudinal strip and other products. If the hot-rolled finishing coil is pickled to remove oxidation and oiled, it becomes hot-rolled pickled plate coil. The product has a partial replacement of cold-rolled plate tendency, the price is moderate, loved by the majority of users.

2. Cold rolling

Hot rolled steel coil as raw material, after pickling to remove oxide skin cold rolling, the finished product is rolled hard coil, due to continuous cold deformation caused by cold hardening rolled hard coil strength, hardness, toughness indicators decline, so the stamping performance will deteriorate, can only be used for simple deformation of the parts. Rolled hard volume can be used as raw material for hot dip galvanizing plant, because the hot dip galvanizing unit are set up with tongue annealing line. Rolled hard rolls generally weigh 6 to 13.5 tons, and the inner diameter of the steel coil is 610mm.

General cold rolling plate, volume should be continuous annealing (CAPL unit) or hooded furnace annealing to eliminate the hardening and rolling stress, to achieve the mechanical properties of the corresponding standard indicators.

The surface quality, appearance and dimensional accuracy of cold-rolled steel sheets are better than that of hot-rolled sheets, and the thickness of its products are right rolled thin to about 0.18mm, so it is favored by the majority of users. Deep processing of products with cold rolled steel coils as substrate becomes high value-added products. Such as electro-galvanized, hot-dip galvanized, fingerprint-resistant electro-galvanized, color-coated steel coil and vibration-damping composite steel sheet, PVC laminated steel sheet.

II. Briefly describe the difference between the two

The surface of cold-rolled sheet has a certain glossy hand touching the mold is relatively smooth, similar to the kind of water used to drink the very common steel cup. The surface of hot-rolled sheet is similar to that of many ordinary steel sheets in the market if it is not pickled, with the rusted surface being red and the unrusted surface being purple-black (iron oxide). Cold rolling and hot rolling are two different rolling techniques. As the name implies, cold rolling is the tying of steel at room temperature, and this steel is hard. Hot rolling is the rolling of steel at high temperatures.

The performance advantages of cold rolled sheet versus hot turned sheet are:

(1) Higher precision, cold-rolled strip thickness difference does not exceed 0.01 to 0.03mm.

(2) Thinner size, the thinnest cold-rolled can roll 0.001mm steel strip; hot-rolled can now be as thin as 0.78mm.

(3) Better surface quality, cold-rolled steel can even produce a mirror surface: while the surface side of the hot-rolled plate has iron oxide, pockmarks and other defects.

(4) Cold-rolled sheet can be adjusted according to user requirements for its office properties such as tensile strength and process desires such as stamping performance.

III. Detailing the difference between the two 

Hot-rolled sheet has low hardness, easy processing and good ductility. Cold-rolled plate hardness is high, relatively difficult to process, but not easy to deformation, higher strength. Hot-rolled plate strength is relatively low, the surface quality is poor (oxidation, low finish), but good plasticity, generally for the thick plate, cold-rolled plate strength, high hardness and high surface finish, generally for Jiang plate, can be used as a stamping plate. Hot-rolled steel plate, mechanical properties far less than cold processing, but also second to forging processing, but has better toughness and ductility. Cold-rolled steel plate due to a certain degree of work hardening, low toughness, but can achieve a better yield-to-strong ratio, used to cold bending spring and other parts, while the yield point is closer to the tensile strength, so the use of the process of danger without foresight, in the load exceeds the allowable load is prone to accidents.

By definition, ingots or steel is also difficult to deform at room temperature, not easy to process, generally heated to 1100 ~ 1250C for rolling, this rolling process is called hot rolling. Most steel is rolled by hot method. But because the surface of the steel at high temperatures is easy to generate iron oxide, so that the hot-rolled steel surface roughness, size fluctuations, so the requirements of the surface finish, accurate size, good mechanical properties of steel, hot-rolled semi-finished or finished products as raw materials and then cold-rolled method of production. Rolling at room temperature, a stream of understanding for cold rolling, from a metallurgical point of view, the boundary between cold milk and hot rolling should be distinguished by the recrystallization temperature. That is, below the recrystallization temperature of rolling for cold rolling, above the recrystallization temperature of rolling for hot rolling. The recrystallization temperature of steel is 450-600℃.

Hot rolling, as the name implies, the temperature of the rolled parts is high, so the deformation resistance is small and a large deformation can be achieved. Take the rolling of steel plate as an example, the general continuous casting also thickness of about 230mm, and after roughing and finishing turning rolling, contraindicated final thickness of 1 ~ 20mm. at the same time, due to the small width-thickness ratio of the steel plate, the dimensional accuracy requirements are relatively low, not easy to plate shape problems, to control convexity. For the organization requirements, a stream through the control of rolling control cooling to achieve, that is, to control the fine single opening rolling temperature, final rolling temperature and crimping temperature to control the microstructure of the strip and machine branch performance. Cold rolling, generally before the opening of the rolling is no heating process. However, due to the small thickness of the strip, it is easy to have plate shape problems. Moreover, the cold rolling is finished product, so a lot of tedious processes are used to control the dimensional accuracy and surface quality of the strip. The production line of cold rolling is long, with many equipments and complicated processes. As the users' requirements for dimensional accuracy, plate shape and surface quality of strip steel increase, the control models, L1 and L2 systems, and plate shape control means of cold turning rolling units are relatively hot to be more. Moreover, the temperature of rolls and strip steel is one of the more important control indexes.

Cold-rolled products and hot-rolled products thin plate line, is the difference between the upper process and the lower process, hot-rolled products is the raw material for cold-rolled products, cold rolling will be treated by pickling the hot rolled steel coil on the machine using a roll mill, rolling, are cold processing molding, mainly thick specifications of the hot-rolled plate rolled into thin specifications of the cold-rolled plate, usually such as 3.0m hot-rolled plate on the machine rolling can produce 0.3-0.7mm cold rolled coil The main principle is to use the principle of extrusion to force deformation.

IV.The difference between cold-rolled and hot-rolled stainless steel sheets

Stainless steel sheet by rolling, can be divided into cold rolling and hot rolling, to recrystallization temperature as a point of distinction. Hot-rolled is relative to cold-rolled, cold-rolled is the south system below the crystallization temperature, while hot south is the rolling above the crystallization temperature.

Here we analyze the difference between cold rolled sheet and hot sheet in terms of characteristics.

Cold rolled sheet:

1. good stretching, toughness, but the price is more expensive;

2. cold-rolled processing surface without oxidation, good quality;

3. Cold rolling is generally used to produce strips, and its rolling speed is higher;

4. The thickness accuracy of cold-rolled is higher than that of hot-rolled. Mechanical properties, surface, etc. are above the hot-rolled;

5. cold-rolled steel sheet due to a certain degree of work hardening, low toughness, but to achieve a better yield to strength ratio, used to cold bend spring and other parts;

6. cold plate because the yield point is closer to the tensile strength, so the use of the process of danger without foresight, in the load exceeds the permissible load is prone to accidents;

7. The surface of cold-formed plate without electroplating is gray;

8. The smoothness of the surface of cold-tied plate can be higher than that of hot-tied plate after plating.  

Hot rolled sheet:

1. Poor toughness and surface flatness, lower price;

2. the temperature of hot rolling is similar to the temperature of forging;

3. hot plate using hot tying processing surface oxidation, plate thickness has a lower difference;

4. the price of hot-rolled products as well as the use of relative cold-rolled to a much wider range of;

5. hot-rolled steel plate, mechanical properties far less than cold processing, also second to forging processing, but has better toughness and ductility;

6. The surface of the hot plate without electroplating becomes dark brown;

7. from the smoothness of the surface after plating can be distinguished from the smoothness of the hot-rolled plate is lower.

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