What is hot rolled and cold rolled?

There are mainly two kinds of stainless steel plate, hot rolling and cold rolling. The difference between them is very obvious from appearance to application performance. This paper will give a detailed description.

Contents:

20191205074207 51546 - What is hot rolled and cold rolled?

What is hot rolling?

Hot rolling is to take slab (mainly continuous casting slab) as raw material, after heating, it is made into strip steel by roughing mill and finishing mill. The hot steel strip from the last finishing mill is cooled to the set temperature by laminar flow, and then rolled into steel strip by coiler. The cooled steel strip coil is processed into steel plate, flat coil and longitudinal cut steel strip products by different finishing lines (leveling, straightening, cross cutting or longitudinal cutting, inspection, weighing, packaging and marking) according to different needs of users. To put it simply, after heating a billet (that is, the red hot steel on TV), it is refined and rolled several times, then cut edges, and corrected into steel plates, which is called hot rolling.
Hot rolling is to soften the alloy at a temperature higher than the recrystallization temperature, and then press the material into a thin sheet or billet cross section with a press wheel to make the material deform, but the physical properties of the material do not change. 
Hot rolling (forging) state: the state in which the metal material is no longer subject to special heat treatment after hot rolling or hot forging and is delivered directly after cooling. The end temperature of hot rolling (forging) is generally 800-960 ℃, and then it is cooled in air, so the hot rolling (forging) state is equivalent to normalizing treatment. The metal materials delivered in hot rolling (forging) state are covered with a layer of oxide film on the surface, so they have certain corrosion resistance. The storage, transportation and storage requirements are not as strict as the materials delivered in cold drawing (rolling). For example, large, medium-sized section steel and medium thick steel plates can be stored in the open yard or after being covered.

What is cold rolling?

Cold rolling: the hot rolled steel coil is used as raw material, and the oxide skin is removed by pickling. The finished product is hard rolled coil. The cold work hardening caused by continuous cold deformation makes the strength, hardness and toughness plastic index of hard rolled coil increase, so the stamping performance will deteriorate, and it can only be used for simple deformed parts. The hard rolled coil can be used as the raw material of the hot-dip galvanizing plant, because the hot-dip galvanizing units are equipped with annealing lines. The weight of hard rolled coil is generally 6 ~ 13.5 tons, and the hot-rolled pickling coil is continuously rolled under normal temperature. The inner diameter is 610mm.
Product features: because it has not been annealed, its hardness is very high (HRB is greater than 90), and its machining performance is very poor, so it can only carry out simple bending processing with directionality less than 90 degrees (perpendicular to coiling direction).
To put it simply, cold rolling is based on the hot-rolled strip. Generally speaking, it is a process of hot rolling – Pickling – cold rolling.
Cold rolling is made of hot-rolled plate at room temperature. Although in the process of rolling, the plate will also heat up, it is still called cold rolling. As a result of continuous cold rolling after hot rolling, the mechanical properties of cold rolling are relatively poor and the hardness is too high. The mechanical properties can only be restored after annealing. The one without annealing is called hard roll. The hard roll is generally used to make products without bending and stretching, and the lucky two sides or four sides with thickness less than 1.0 are bent.

Cold rolling is a process in which the material that has been hot rolled, pitting and oxidation removed is further rolled with a roller at a temperature lower than the recrystallization temperature of the alloy to make the material recrystallize. After repeated cold pressing recrystallization annealing cold pressing (repeated for 2-3 times), the metal in the material changes in molecular level (recrystallization), and the physical properties of the formed alloy change. Therefore, its surface quality is good, its surface finish is high, its dimension precision is high, and its performance and structure can meet some special requirements, such as electromagnetic performance, deep drawing performance, etc. Cold rolled strip can be used in automobile manufacturing, electrical products, rolling stock, aviation, precision instruments, food cans, etc.

Advantages and disadvantages of hot rolling and cold rolling

Both hot rolling and cold rolling are forming processes of section steel or steel plate, which have a great influence on the structure and properties of steel. The rolling of steel is mainly hot rolling, and cold rolling is only used to produce small section steel and thin plate.
I. Hot rolling
Advantages:

It can destroy the casting structure of the ingot, refine the grain of the steel, and eliminate the defects of the microstructure, so that the steel structure is dense and the mechanical properties are improved. This improvement is mainly reflected in the rolling direction, so that the steel is no longer isotropic to some extent; the bubbles, cracks and looseness formed during pouring 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, as well as silicate) inside the steel are pressed into thin sheets, resulting in delamination (interlayer). Delamination makes the tensile properties of the steel along the thickness direction deteriorate greatly, and it is possible that the interlaminar tear occurs when the weld shrinks. The local strain induced by weld shrinkage is often 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, which exists in all kinds of hot-rolled sections. 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. Such as deformation, stability, anti fatigue and other aspects may have adverse effects.

2. Cold rolling refers to the processing of steel plates or strips into various types of steel by cold drawing, cold bending, cold drawing and other cold processing under normal temperature.
Advantages:

Rapid forming speed, high output, and no damage to the coating, can be made into a variety of cross-section forms to meet the needs of the use conditions; cold rolling can make the steel produce great plastic deformation, thus improving the yield point of the steel.
Disadvantages:

1. Although there is no hot plastic compression in the forming process, there is still residual stress in the section, which will inevitably affect the overall and local buckling characteristics of the steel;
2. Cold rolled section steel is generally open section, which makes the free torsional rigidity of the section low. In bending, it is easy to twist, in compression, it is easy to bend and twist, and its torsional performance is poor;
3. The wall thickness of cold-rolled steel is small, and there is no thickening at the corner where the plate connects, so the ability to bear local concentrated load is weak.
Main differences between hot rolling and cold rolling:

  • 1. The cold-rolled section is allowed to buckle locally, which can make full use of the post buckling bearing capacity of the member; the hot-rolled section is not allowed to buckle locally.
  • 2. The causes of residual stress are different between hot-rolled section and cold-rolled section, so the distribution on the section is also very different. The distribution of residual stress on the cold-formed thin-walled section is curved, while that on the hot-rolled or welded section is thin-film.
  • 3. The free torsional rigidity of hot rolled section steel is higher than that of cold rolled section steel, so the torsional resistance of hot rolled section steel is better than that of cold rolled section steel.

Coil

  • The coil plate belongs to one of the plates in steel, which is actually a long and narrow sheet steel supplied in coils.
  • Roll and flat are almost a cut package.
  • The cold and hard coil is obtained by pickling and cold rolling. It can be said that it is a kind of cold rolled coil.
  • Cold rolled coil (annealed): hot rolled coil is obtained by pickling, cold rolling, bell type annealing, leveling and finishing.

There are three main differences between the two:

  • 1. On the appearance, generally, the cold and hard rolling plate is slightly black.
  • 2. The surface quality, structure, dimensional accuracy and other cold-rolled plates are better than the hard ones.
  • 3. In terms of performance, the cold-rolled coil directly obtained from the cold rolling process hardens during cold rolling, resulting in the increase of yield strength and residual internal stress, which is called cold-rolled coil because of its “hard” appearance.

Yield strength: the cold-rolled coil is larger than the cold-rolled coil (annealed state), which makes the cold-rolled coil (annealed state) more conducive to stamping.
Generally, the default delivery state of cold rolled coil is annealing state.

Hot rolled coil

It takes slab (mainly continuous casting slab) as raw material and is made into strip steel by roughing mill and finishing mill after heating.
The hot strip from the last finishing mill is cooled to the set temperature by laminar flow. The coiler is used to coil the hot strip. The cooled coil is processed by different finishing lines (leveling, straightening, cross cutting or longitudinal cutting, inspection, weighing, packaging and marking) according to different needs of users to become steel plate, flat coil and longitudinal cutting steel strip products.
To put it simply, after heating a billet (that is, the red hot steel on TV), it is refined and rolled several times, then cut edges, and corrected into steel plates, which is called hot rolling.
Because of its high strength, good toughness, easy processing and good weldability, hot rolling steel plate products are widely used in shipbuilding, automobile, bridge, construction, machinery, pressure vessel and other manufacturing industries.
With the development of new technologies for controlling the size accuracy, shape and surface quality of hot rolling as well as the appearance of new products, hot rolling steel plate and strip products have been more and more widely used and become more and more competitive in the market.

Related parameters

Thickness (mm)
Width (mm)
Length (mm)
Inner diameter of coil (mm)
1.5 to 4
800~1600
2000~2500 (or C)
Phi 580
The allowable deviation of the thickness of the steel plate and the steel strip should comply with the requirements of Table 2.
Nominal thickness (mm)
Thickness tolerance (mm)
Width direction after slicing
Thickness deviation (mm)
Rolling direction after slicing
Thickness deviation (mm)
>1.5-2.6
+ 0.10
Less than 0.05
Less than 0.05
>2.6-3.0
+ 0.12
Less than 0.05
Less than 0.05
>3.0-4.0
-0.17 ~ +0.12
Less than 0.05
Less than 0.05
  • The steel plate’s roughness should not exceed 12mm. The unit length of the roughness measurement is 1000mm.
  • The sickle of steel plate and steel strip should not exceed 4mm per 2 meters.
  • The width tolerance of steel plate and steel strip should comply with the requirements of the following table.
Allowable width deviation (mm)
Thickness range (mm)
>2.0-2.8
>2.8-3.5
Trimming
0~+3
0~+5
Untrimming
+20 (target value +10)
  • Steel coil head and tail overflow edge should not be greater than 40mm, the middle overflow edge, tower height should not be greater than 30mm.
  • The weight of each coil should be 5-25 tons.

Steel grade

1. SPHC — the abbreviation of the first S bit steel Steel, P is the abbreviation of Plate, H is the abbreviation of hot Heat, the abbreviation of C commercial Commercial, as a whole, it usually uses hot-rolled steel plate and steel strip.
2. SPHD – represents hot rolled steel plates and strips for stamping.
3. SPHE – for deep drawing hot rolled steel plate and steel strip.
4. SPCC – generally used for cold rolled carbon steel sheet and strip. The third letter C is the abbreviation of cold Cold.To ensure the tensile test, add T at the end of the brand to SPCCT.
5. SPCD – indicating cold rolled carbon steel sheet and strip for stamping.
6. SPCE — cold rolled carbon steel sheet and strip for deep drawing. To ensure non timeliness, add N to SPCEN at the end of the brand.
Cold rolled carbon steel sheet and steel strip quenched and tempered Code: annealing state is A, standard quenching and tempering is S, 1/8 hardness is 8, 1/4 hardness is 4, 1/2 hardness is 2, hardness is 1. The surface machining code is: the matte finish rolling is D, the bright finish rolling is B. For example, SPCC-SD is used for standard quenched and tempered, non glossy finish rolling, and generally cold rolled carbon sheets. Another example is SPCCT-SB, which represents standard quenching and tempering, bright machining, and cold rolled carbon sheets requiring mechanical properties.
7. JIS mechanical structural steel brand designation method is: S+ carbon content + letter code (C, CK), of which carbon content is expressed in the middle value of 100, the letter C: carbon K: carbon carburizing steel. Such as carbon knot plate S20C its carbon content is 0.18-0.23%.

Representation of silicon steel sheets in China

  • (1) cold rolled non oriented silicon steel strip (sheet) expression method: DW+ iron loss value (at 50HZ frequency, sinusoidal peak value of magnetic force is 1.5T of unit weight iron loss value). 100 times + 100 times the thickness. If DW470-50 indicates that the iron loss is 4.7w/kg and the thickness is 0.5mm, the cold rolled non oriented silicon steel is now 50W470.
  • (2) cold rolling oriented silicon steel strip (sheet) expression method: DQ+ iron loss value (at 50HZ frequency, sinusoidal magnetic peak value of 1.7T, unit weight iron loss). 100 times + 100 times the thickness. Sometimes the iron loss value plus G indicates a high magnetic sense.
  • (3) hot rolled silicon steel plate hot rolled silicon steel sheet is expressed by DR. According to silicon content, it is divided into low silicon steel (silicon content less than 2.8%) and Gao Guigang (silicon content >2.8%).
Description: the DR+ iron loss is 100 times the value of the unit weight iron loss when the maximum magnetic flux density of 50HZ is changed by sinusoidal shape and the maximum value of magnetic flux density is 1.5T, and 100 times the thickness value. For example, DR510-50 indicates that the iron loss is 5.1 and the thickness of 0.5mm is hot-rolled silicon steel sheet. The grades of hot-rolled silicon steel sheets for household appliances are represented by JDR+ iron loss value and thickness value. For example, JDR540-50 cold-rolled coils are rolled strips made of hot rolled steel strip and rolled by a coiler at room temperature. Hot rolled coil is a coil of steel rolled by coiler, which is made from slab as raw material and heated to a certain temperature after rough rolling and finishing mill cooling to set temperature.Compared with hot-rolled steel sheet, cold rolled steel sheet has better surface quality and higher dimensional accuracy. Combined with Q345B[1] annealing treatment, its mechanical and technological properties are better than hot-rolled steel plates. It is applied to many areas with high accuracy, especially in household appliance manufacturing and automobile industry. But the thickness of cold rolled sheet is thin, generally less than 4mm. The hot-rolled steel plate can only be used beyond a certain thickness.

Mechanical property

Nominal thickness
Yield strength (Mpa)
Tensile strength (Mpa)
Elongation A50mm%
180 degree bending test
>1.5-2.5
Less than 290
More than 310
More than 38
D=0a
>2.5-3.0
Less than 290
More than 300
More than 38
D=0a
>3.0-4.0
Less than 290
More than 300
More than 40
D=1/2a

Explain:

  • 1. Tensile test and bending test in the table are applicable to longitudinal specimens.
  • 2. Tensile tests were carried out with L0=50 mm, b=25 mm specimens.
  • 3. The sampling plan for bending test shall be carried out according to the No. 3 specimen of JIS Z 220
  • 4. The test method shall be carried out according to JIS Z 2248, and the hardness test shall be carried out according to the provisions of JIS Z 2245.
  • 5. Hardness HRB has a target value of 60 + 8. The actual value is provided in the warranty, but not as acceptance requirements.

Surface quality
The surface of steel plate or steel strip is not allowed to have harmful defects such as inclusions, cracks, rust and so on. No parking spots are allowed. It is allowed to have defects that do not affect the molding performance, such as slight scratches without hand feel, light yellow film after pickling, slight pock marks, scratches, roller marks and indentation, color difference and slight wrinkle without hand feeling.
Because the steel strip has no chance to remove the defect and allow defective delivery, the defective part does not exceed 6% of the total length of the coil.
Oiling refers to the surface of steel plate with rust proof performance of electrostatic antirust oil, oil coating light oil, medium oiled, heavy oiling, the buyer side specified in the contract. In normal transportation and storage, the supplier shall ensure that the product is free from rust within 3 months.

Product classification
Hot rolling steel plate products include steel strip (roll) and steel plate with its shear. The steel strip (roll) can be divided into straight hair roll and finishing roll (divided roll, flat roll and longitudinal cut roll).
Main products
Hot continuous rolling can be divided into ordinary carbon structural steel, low alloy steel and alloy steel according to their different materials and properties. According to the different uses, it can be divided into: cold forming steel, structural steel, automobile structural steel, corrosion-resistant structural steel, mechanical structural steel, welding gas cylinder and pressure vessel steel, pipeline steel, etc.
Production process
The hot-dip galvanizing hot-rolled strip mill adopts the improved senjimir annealing process, and the raw material is hot-rolled pickling strip coil. The production process is as follows:
Hot rolling and pickling plate coil → uncoiling → cutting head and tail → welding → inlet looper → improved senjimir horizontal annealing furnace → hot galvanizing → cooling after plating → zinc layer thickness gauge → finishing and straightening → passivation treatment → inspection platform → electrostatic oiling → coiling → weighing and packaging → warehousing of finished products.
Dezine reason

  • (1) The residual impurities on the surface of the substrate were not removed before immersion in the zinc pot, which affected the adhesion of the zinc layer.
  • (2) The iron oxide layer is attached to the substrate, which proves that the substrate is not completely reduced to the sponge like pure iron required for galvanizing in the annealing furnace.
  • (3) The content of Al in the alloy layer is less, and there is no reasonable Fe 2Al 5 alloy layer which plays a decisive role in the adhesion of zinc layer, or the alloy layer is damaged by the over grown Fe Zn alloy layer.
  • Based on the above analysis, in order to solve the problem of zinc layer falling off, we should improve the product quality of hot-dip galvanized hot-rolled plate by improving the surface cleanliness of the substrate, ensuring the atmosphere control in the annealing furnace, ensuring the complete reduction of the substrate and adjusting the composition temperature of the zinc liquid.

Cold rolled coil

Cold rolled coil is made of hot rolled coil and rolled under recrystallization temperature at room temperature, including plate and coil. Among them, those delivered in sheets are called steel plates, also known as box plates or flat plates; those delivered in rolls with long length are called steel strips, also known as rolled plates.
Applicable brands: Q195, Q215, Q235, Q275; SPCC (Japan); ST12 (Germany)
Symbol:

  • 1. Q – the code of yield point (limit) of ordinary carbon structural steel, which is the case of the first Chinese pinyin letter of “yield”; 195, 215, 235, 255, 275 – the value of their yield point (limit), unit: MPa (n / mm2); because Q235 steel has the best comprehensive mechanical properties in ordinary carbon structural steel, such as strength, plasticity, toughness and weldability, It can meet the general use requirements well, so it has a wide range of applications.
  • 2. S-STEEL, P-plate, c-cold, fourth c-common.
  • 3. St steel, 12 ordinary cold rolled steel sheet, mark: dimensional accuracy dimension steel plate variety standard cold rolled steel sheet: steel number technical condition standard mark example: b-0.57501500-gb708-88; steel plate, Standard No. Q / bqb402, brand No. SPCC, heat treatment state annealing + leveling (s), central processing state of table is pockmarked D, surface quality of FB grade trimming (trimming EC, no trimming) Edge EM) steel plate, thickness 0.5mm, class B precision, width 1000mm, class a precision, length 2000mm, class a precision, unevenness precision PF. A, then the mark is: steel plate ecq / BQB 402-spcc-sd-fb / (0.51000a2000a-pf. A);

Properties of cold rolled coil
Cold rolled sheet has good performance, that is to say, through cold rolling, we can get thinner and higher precision cold rolled strip and steel plate, with high straightness, high surface finish, clean and bright surface of cold rolled sheet, easy to be coated and processed, various varieties and wide use, and also has the characteristics of high stamping ability, aging resistance and low yield point. Therefore, cold rolled sheet has a wide range of uses and main applications At the same time, it is the best material for producing organic coated steel plate.

Chemical composition (Wt%)

Brand name
C
Mn
P
S
Alta
Tib
DC01,SPCC
Less than 0.12
Less than 0.60
Less than 0.045
Less than 0.045
More than 0.020
DC02,SPCD
Less than 0.10
Less than 0.45
Less than 0.035
Less than 0.035
More than 0.020
DC03 , SPCE
Less than 0.08
Less than 0.40
Less than 0.030
Less than 0.030
More than 0.020
DC04 , SPCF
Less than 0.06
Less than 0.35
Less than 0.025
Less than 0.025
More than 0.015

Mechanical property

DC01 , DC02, DC03 , DC04
Brand name
Yield strengthRcL
Mpa
tensile strengthRm
Mpa
elongationA80mm
%
Plastic strain ratio R90
Strain hardening exponent N90
DC01
140~280
270~410
More than 28
DC02
140~240
270~370
More than 34
More than 1.4
DC03
120~210
270~350
More than 38
More than 1.8
More than 0.18
DC04
120~180
270~330
More than 40
More than 2
More than 0.20
SPCC, SPCD , SPCE
Brand name
Yield strengthRelb) MPa
tensile strengthRmMPa
Plastic strain ratioR90e), d)
Strain hardening exponent N90d)
SPCC
130~300
270~410
SPCD
120~240
270~370
More than 1.3
SPCE
120~210
270~350
More than 1.8
More than 0.18

Note:

  • A) Tensile test Take the transverse pattern; the width B is 25mm and the pattern L0 is 50mm.
  • B) When there is no obvious Yield point Rel can be replaced by Rp0.2; when the product thickness is <0.70mm and is equal to 0.50mm, Yield strength The upper limit can increase 20N/mm2, and the thickness can increase by 40 N/mm2 when <0.50mm thickness.
  • E) When the product thickness is >1.50mm and is less than 2.0mm, the value of r90 or R is allowed to decrease by 0.1 (absolute value); when the thickness is >2.0mm, the value of r90 or R is allowed to decrease by 0.2 (absolute value).
  • D) The values of r90 and N90 or R and N are only applicable to products of thickness greater than 0.50mm; r= (r0+ r45+ r90) /4, N= (n0+ n45+ N90) /4.

Common defects of cold rolled coil
Edge crack of cold rolled coil
The enrichment of large inclusions in the edge is one of the main causes of edge cracks. Because the funnel mold is used in the thin slab, the flow, heat transfer unevenness and liquid level fluctuation in the mold are more complex than that in the traditional slab continuous casting. During the casting process, the slag is often rolled, some of the slag remains on the surface of the slab to form surface inclusions, in which the larger inclusions are deposited at the edge of the slab, resulting in the enrichment of large oxide inclusions at the edge.
Pressing defect of iron oxide sheet
The primary oxide scale which is not completely removed is pressed into the strip during the subsequent rolling process, resulting in the surface defects of the strip. The key to the defect is that the secondary descaling water is not opened. The results of defect tracking show that the pressed defects of iron oxide sheet can be eliminated in the subsequent pickling and cold rolling process, and have little effect on the cold rolled sheet.
Surface defects of cold rolled coil
Black line defects on the surface of cold rolled strip after pickling. The surface defects of cold rolled strip mainly come from the defects of hot rolled base material, and the main defects are the inclusions on the surface of hot rolled strip. The possibility of defects in cold rolling production line is very small.
Application field of cold rolled coil
Galvanized steel strip products are mainly used in construction, light industry, automobile, agriculture, animal husbandry, fishery, commerce and other industries. Among them, the construction industry is mainly used to manufacture anti-corrosion industrial and civil building roof panels, roof grilles, etc.; the light industry is mainly used to manufacture home appliance shells, civil chimneys, kitchen appliances, etc.; the automobile industry is mainly used to manufacture corrosion-resistant parts of cars, etc.; the agriculture, animal husbandry and fishery are mainly used to store and transport grain, frozen processing appliances of meat and aquatic products, etc.; the commerce is mainly used as the storage of materials Transportation, packaging, etc.
Cold rolled sheet steel is the abbreviation of common carbon structural steel cold rolled sheet, also known as cold rolled sheet, commonly known as cold sheet, sometimes mistakenly written as cold rolled sheet. The cold plate is a hot-rolled steel strip of common carbon structural steel, which is further cold-rolled into a steel plate with a thickness of less than 4mm. Because the cold plate is rolled at normal temperature without oxide scale, its surface quality is good and its dimension precision is high. In addition to annealing treatment, its mechanical and technological properties are superior to those of hot-rolled steel sheet. In many fields, especially in the field of home appliance manufacturing, it has gradually replaced hot-rolled steel sheet.

Source: China Steel Plate Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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.)

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Summary
what is hot rolled and cold rolled - What is hot rolled and cold rolled?
Article Name
What is hot rolled and cold rolled?
Description
There are mainly two kinds of stainless steel plate, hot rolling and cold rolling. The difference between them is very obvious from appearance to application performance. This paper will give a detailed description.
Author
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www.steeljrv.com
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