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Summary of some knowledge about steel plate

Do you know the difference between pickling plate and hot plate used on sheet metal?

Pickling plate: it is an intermediate product with high quality hot rolled sheet as raw material, after the oxidation layer is removed, cut and finished by the pickling machine, the surface quality and application requirements (mainly cold bending forming or stamping performance) are between the hot-rolled plate and the cold rolled plate.
Hot rolled plate: the strength is relatively low, the surface quality is almost (oxidation and finish is low), but the plasticity is good. Generally, it is the characteristics of pickling plate of medium and thick plate compared with hot rolled plate.

  • 1. The surface quality is good. Compared with the ordinary hot-rolled plate, the hot-rolled pickling plate has removed the surface oxide sheet, improved the surface quality of the steel, and is easy to weld, oil and paint.
  • 2. The size accuracy is high, after flatness, the board shape can be changed to a certain extent, thus reducing the deviation of the unevenness.
  • 3Improving the surface finish and enhancing the appearance effect.

The effect of hot plate on welding:

  • 1. Spot welding: because there is oxide skin on the surface, it is necessary to remove it before spot welding. The specific yield strength needs to be welded
  • 2. Bolt welding: oxide scale shall be removed, and welding test shall be conducted for specific yield strength
  • 3. Common welding: large splash, difficult to control deformation, difficult to polish, deformation and shaping difficulties; The yield strength under the same welding conditions will increase the workload, and the welding experiment is needed to determine the yield strength
  • 4. Laser cutting: laser cutting has a large deformation. The parts with high dimensional requirements should not be used, which will have a great impact on the welding at the back.
  • 5. Bending: cold bending forming or stamping performance is low, which is easy to produce bending crack, easy to reduce product service life and cause failure.

Effect of hot plate on paint:

  • 1. Pretreatment: due to the presence of a large number of oxide scales on the surface, the pickling time is increased, and the pickling effect needs to be confirmed by experiments
  • 2. Electrostatic powder spray: the surface finish is not good, which will not be conducive to spraying.
  • 3. Hot rolled plate is placed in air, and once rust is formed, it will be difficult to remove.

Do you know the difference between cold rolled steel plate and hot rolled steel plate for sheet metal?

1. The surface of cold-rolled sheet has a certain glossiness. It feels smooth by hand, similar to the very common steel cup used for drinking water.
2. If the hot-rolled plate is not pickled, it is similar to the surface of many ordinary steel plates on the market. The rusted surface is red, and the non rusted surface is purple black (scale).
The advantages of cold rolled sheet and hot rolled sheet are as follows:

  • (1) The thickness difference of cold rolled strip is not more than 0.01 ~ 0.03mm.
  • (2) The size is thinner, and the thinnest strip can be rolled by 0.001mm; The thinnest hot rolling can reach 0.78mm.
  • (3) The surface quality is better, cold rolled steel plate can even produce mirror surface; However, the surface of hot rolled plate has some defects such as scale and pitting.
  • (4) Cold rolled sheet can adjust its properties such as tensile strength and process properties such as stamping properties according to user requirements. Cold rolling and hot rolling are two different rolling technologies. As the name suggests, cold rolling is to tie the steel at room temperature, which has high hardness. Hot rolling is the rolling of steel at high temperature.

Hot rolled plate has low hardness, easy processing and good ductility.
Cold rolled sheet has high hardness and relatively difficult processing, but it is not easy to deform and has high strength.
Hot rolled plate has relatively low strength, poor surface quality (oxidation, low finish), but good plasticity, generally medium and heavy plate, cold rolled plate: high strength, high hardness, high surface finish, generally thin plate, which can be used as stamping plate.
The mechanical properties of hot rolled steel plate are far less than that of cold working and forging, but it has better toughness and ductility.
Cold rolled steel plate has a certain degree of work hardening, low toughness, but can achieve a better yield strength ratio, which is used for cold bending spring and other parts. At the same time, because the yield point is close to the tensile strength, so there is no foreseeing danger in the use process, and accidents are easy to occur when the load exceeds the allowable load.
By definition, ingots or billets are difficult to deform and process at room temperature. Generally, they are heated to 1100-1250 ℃ for rolling. This rolling process is called hot rolling. Most steel is rolled by hot rolling. However, due to the formation of scale on the surface of steel at high temperature, the surface of hot-rolled steel is rough and the size fluctuates greatly. Therefore, the steel with smooth surface, accurate size and good mechanical properties is required to be produced by cold rolling with hot-rolled semi-finished products or finished products as raw materials.
Rolling at room temperature is generally understood as cold rolling. From the point of view of metallurgy, the boundary between cold rolling and hot rolling should be distinguished by recrystallization temperature. That is, rolling below recrystallization temperature is cold rolling, and rolling above recrystallization temperature is hot rolling. The recrystallization temperature of steel is 450 ~ 600 ℃.
Hot rolling, as the name suggests, the temperature of the rolled piece is high, so the deformation resistance is small, and large deformation can be achieved. Taking the rolling of steel plate as an example, the thickness of continuous casting slab is about 230mm, and the final thickness is 1 ~ 20mm after rough rolling and finishing rolling. At the same time, because the width thickness ratio of steel plate is small, the requirement of dimensional accuracy is relatively low, so the shape problem is not easy to appear, and the crown is mainly controlled. The microstructure and mechanical properties of the strip are controlled by controlling the start rolling temperature, finish rolling temperature and curling temperature.
In cold rolling, there is no heating process before rolling. However, due to the small thickness of the strip, it is easy to have shape problems. Moreover, it is finished product after cold rolling. Therefore, in order to control the dimensional accuracy and surface quality of strip steel, many complicated processes are adopted.
The cold rolling production line is long, the equipment is many, the craft is complex. With the improvement of users’ requirements for strip size accuracy, shape and surface quality, the control model, L1 and L2 system and shape control means of cold rolling mill are more than those of hot rolling mill. Moreover, the temperature of roll and strip is one of the important control indexes.
Cold rolled products and hot-rolled products are the difference between the previous process and the next process. Hot rolled products are the raw materials of cold-rolled products. In cold rolling, pickling treated hot-rolled steel coils are rolled on a roller mill. Rolling is cold forming, mainly rolling thick hot rolled plates into thin cold-rolled plates, Usually, for example, 0.3-0.7mm cold rolled coil can be produced by rolling 3.0mm hot rolled plate on the machine. The main principle is to use extrusion principle to force deformation.

The difference between Q235 hot rolled steel plate and cold rolled steel plate

Hot rolled plate has low hardness, easy processing and good ductility.
Cold rolled sheet has high hardness and relatively difficult processing, but it is not easy to deform and has high strength.
The strength of hot rolled plate is relatively low, and the surface quality is almost (with low oxidation finish), but it is of good plasticity. Generally, it is medium and thick plate. Cold rolled plate: high strength, high hardness and high surface finish. Generally, it is a sheet, which can be used as stamping plate.
The mechanical properties of hot rolled steel plate are far less than that of cold working and forging, but it has better toughness and ductility.
Cold rolled steel plate has a certain degree of work hardening, low toughness, but can achieve a better yield strength ratio, which is used for cold bending spring and other parts. At the same time, because the yield point is close to the tensile strength, so there is no foreseeing danger in the use process, and accidents are easy to occur when the load exceeds the allowable load.
Hot rolling is to make steel plate with a relatively thin point at a higher temperature; Cold rolling is the rolling of steel plate under normal temperature.
Generally, it is hot rolling first and then cold rolling. When the steel plate is thick, it can only be hot rolled and rolled into thin plate before cold rolling. Hot rolled steel plates are divided into two types: thick plate (thickness is more than 4mm) and thin plate (thickness is 0.35-4mm); Only one kind of sheet (thickness 0.2-4mm) is used for cold rolled steel plate.
The end temperature of hot rolling is 800-900 ℃, and then it is cooled in air, so the hot rolling state is equivalent to normalizing treatment. The metal materials delivered in hot rolling state have a certain corrosion resistance because of the surface covered with an oxide film. The storage, transportation and storage requirements are not as strict as those delivered by cold drawing (rolling). For example, large and medium-sized steel and medium thickness steel plates can be stored in open-air freight yard or covered with tarpaulin.
Compared with the hot rolling state, the metal material in cold rolling state has high dimensional precision, good surface quality, low surface roughness and high mechanical properties. However, it is easy to suffer corrosion or rust, and its packaging, storage and transportation are required to be kept in the warehouse, and the temperature and humidity control in the warehouse should be paid attention.

How to distinguish steel plate material is Q235 and Q345?

Q235 and Q345 are generally invisible. The color difference is not related to the material of steel, but the cooling method of steel after rolling out. Generally, the surface is red after natural cooling. If the method of quenching is used, the surface forms a dense oxide layer, then black will be displayed.
Q345 steel is used in general strength design, because Q345 is higher than Q235 steel, saving steel, 15% – 20% compared with 235. It is better to use Q235 in the design of stability control. The price difference is 3% – 8%.
As for identification, there are several statements:
A:

  • 1. The factory can use the method of trial welding to roughly distinguish the two materials. For example, a small round steel is welded with E43 electrode on two steel plates, and then shear force is applied. According to the failure situation, the materials of two kinds of steel plates can be roughly distinguished.
  • 2. The factory can also use grinding wheel to roughly distinguish between two materials. When Q235 steel is polished with grinding wheel, sparks splashed out are round particles with dark color. And Q345 spark is bifurcated, bright color.
  • 3. There are also two kinds of steel that can be roughly distinguished according to the color difference of the shear surface of two steels. Generally, the color of Q345’s cut-off is white.

B:
1. According to the color of steel plate, Q235 and Q345 can be distinguished: Q235 is green and Q345 is red (this is only for steel just entering the site for a long time and can not be distinguished).
2. The most distinguishable material test is chemical analysis. The carbon content of Q235 and Q345 is different, and the chemical content is different (This is a way to be sure.).
3. The distinction between Q235 and Q345 is made by welding: butt connect two plates of unknown material and weld with common welding rod. If one side of steel plate with cracks appears, it is proved to be Q345( This is practical experience).

Why is there any difference between hot rolled steel and cold rolled steel?

Hot rolling and cold rolling are all the processes of forming steel plates or profiles, which have a great influence on the structure and properties of steel.
The rolling of steel is mainly hot rolling, and cold rolling is usually used to produce small size steel and thin plate.
The cold rolling of common steel:

  • Wire: diameter 5.5-40 mm, coil, all hot-rolled. After cold drawing, it belongs to cold drawing material.
  • Round bar: except for the bright materials with precise size, they are generally hot rolled and forged (there are forging marks on the surface).
  • Strip steel: hot rolled and cold rolled, cold rolled materials are generally thin.
  • Steel plate: cold rolled plate is generally thin, such as automobile plate; There are many hot rolled medium and thick plates, which have similar thickness as cold rolling, and have obvious differences in appearance.
  • Angle bar: it is hot rolled.
  • Steel pipe: both hot and cold drawn by welding.
  • Channel bar and H-section bar: hot rolled.
  • Steel bar: hot rolled material.

Hot-rolling

By definition, ingots or billets are difficult to deform and process at room temperature. Generally, they are heated to 1100-1250 ℃ for rolling. This rolling process is called hot rolling.
The end temperature of hot rolling is 800-900 ℃, and then it is cooled in air, so the hot rolling state is equivalent to normalizing treatment.
Most steel is rolled by hot rolling. The steel delivered in hot rolling state has a layer of oxide sheet on the surface due to high temperature, so it has certain corrosion resistance and can be stored in the open air.
However, the oxide sheet also makes the surface of hot rolled steel rough and the size fluctuates greatly. Therefore, it is required that the steel with smooth surface, accurate size and good mechanical properties shall be cold rolled with semi-finished or finished products as raw materials.
Advantage:
The forming speed is fast, the production is high, and the coating is not damaged. It can be made into various cross-section forms to meet the needs of the use conditions; Cold rolling can make the steel produce a great plastic deformation, thus improving the yield point of the steel.
Disadvantages:

  • 1. Although the forming process has not been subjected to hot plastic compression, there is still residual stress in the section, which will inevitably affect the overall and local buckling characteristics of steel;
  • 2. Cold rolled section steel is generally of open section, which makes the free torsional stiffness of the section lower. The torsional behavior is poor, while the bending is easy to occur, and the bending and torsional buckling is easy to occur when the pressure is pressed;
  • 3. The cold rolled formed steel has a small wall thickness, and it is not thickened at the corner of the joint of the plate, and the ability to bear local concentrated load is weak.

Cold rolling

Cold rolling refers to the rolling method of pressing steel with the pressure of the roll and changing the shape of the steel at room temperature. Although the process also causes the steel plate to rise, it is still called cold rolling. In particular, the hot rolled steel coil is used as raw material for cold rolling, and the finished product is hard rolling coil after the oxide scale is removed by pickling.
Generally cold rolled steel, such as galvanized and color steel, must be annealed, so the plasticity and elongation are also good, and are widely used in automobile, home appliances, hardware and other industries. The surface of cold rolled plate has certain smoothness, and the hand touch is relatively smooth, mainly the work of acid washing. The surface finish of hot rolled plate can not meet the requirements, so the hot-rolled steel strip needs cold rolling, and the thinnest thickness of hot-rolled steel strip is generally 1.0mm, and the cold rolling can reach 0.1mm. Hot rolling is the rolling above the crystallization temperature point, and cold rolling is the rolling under the crystallization temperature point.
The change of steel shape by cold rolling belongs to continuous cold deformation. The cold hardening caused by this process makes the strength, hardness and toughness index of the rolled hard roll decrease.
For terminal use, cold rolling makes stamping performance worse, and the product is suitable for simple deformation parts.
Advantage:
It can destroy the casting structure of ingot, refine the grain of steel, and eliminate the defects of microstructure, so that the steel structure is dense and the mechanical properties can be improved. The improvement is mainly reflected in the rolling direction, so that the steel is no longer isotropic to some extent; Bubbles, cracks and looseness formed during casting can also be welded under the action of high temperature and pressure.
Disadvantages:

  • 1. After hot rolling, the non-metallic inclusions (mainly sulfide and oxide, and silicate) inside the steel are pressed into thin sheets, and delamination occurs. Delamination makes the tensile properties of steel in the direction of thickness deteriorate greatly, and it is possible to produce interlayer tearing when the weld is contracted. The local strain induced by weld shrinkage often reaches several times of the yield point strain, which is much larger than that caused by load;
  • 2. Residual stress caused by uneven cooling. The residual stress is the internal self-equilibrium stress without external force. The hot rolled section steel has such residual stress. The larger the section size of general section, the greater the residual stress. Although the residual stress is self-equilibrium, it has a certain influence on the performance of steel members under external force. For example, it may have adverse effects on deformation, stability and fatigue resistance.

Conclusion:
The temperature of the rolling process is the main factor of cold rolling and hot rolling “Cold” is normal temperature and “hot” is high temperature.

From the point of view of metallization, the boundary between cold rolling and hot rolling should be distinguished by recrystallization temperature. That is, rolling below recrystallization temperature is cold rolling, and rolling above recrystallization temperature is hot rolling. The recrystallization temperature of steel is 450 ~ 600 ℃.

Source: Network Arrangement – Stainless Steel Plate Supplier: www.epowermetals.com

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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