The hot-rolled coil passes through a pickling line, where scale breaker machines and hydrochloric acid solutions are used to remove surface scale and oxide film, which cause surface flaws during the final stage of cold rolled steel processing.
Pickled coils are cold rolled in tandem mills to a specified thickness, typically 40~90%, of original material dimensions. Fully automated thickness and shape adjustment is ensured through state of the art process machinery.
The purpose of electrolytic cleaning is to remove lubricant oil and contaminants on the cold rolled steel prior to the annealing process. All traces of surface oil are removed through mechanical and chemical action as the cold rolled coil passes through an alkaline solution bath.
After cold-rolling, the steel is hard and brittle, and its grains are elongated in the rolling direction. To obtain the desired grain structure and improve the mechanical properties, the material is reheated in a furnace and subjected to cycles of rapid heating and cooling. Steel products with extra deep drawing qualities and high tensile strength can be produced via this high roductivity manufacturing method. Two annealing methods are commonly used: batch annealing and continuous annealing.
A final rolling process is performed in order to remove minor surface defects such as stretch marks and to produce a smooth, lustrous surface. Skin Pass results in a further thickness reduction of about 1%.
Finishing and Inspection
During this final stage of manufacturing, cold rolled strips are trimmed to customer specified size. The finished product is subject to final inspection, where thickness, width and surface quality are verified. Samples are sent for laboratory testing of mechanical properties as appropriate for specific end use.
1. Cold-Rolled Steel
|Used for exposed or unexposed parts where bending, shallow drawing, forming and welding are involved||Refrigerator doors
|Class 2||CSP2||Used for fabricating parts which require drawability||Automobile oil pans|
|Class 3||CSP3||Used for deep drawing parts which require more drawability than CSP 2||Roofs, fenders and hoods of automobiles|
|CSP3N||Known for non-aging deep-drawing quality, which guarantees consistent steel properties even after a long period of time|
|CSP3E||Used for automobile parts which require more drawability than CSP 3N||Quarters and springhouses of automobiles|
|CSP3X||Used for automobile parts which require more drawability than CSP 3E||Car side panels
Motorcycle fuel tanks
2. Structural & Hard Quality Cold-Rolled Steel
|Used in structural materials which do not need drawability but need strength
(The minimum value of tensile strength is guaranteed.)
|Structural materials for construction|
|When customer makes a request, strip hardness is guaranteed||Coated pipes, motor core|
3. Cold-Rolled Steel for Porcelain Enameling
|Manufactured with extremely low carbon steel to which titanium and boron are added. Therefore, this steel has excellent drawability and the characteristics that ensure the best adhesion for porcelain enameling and the elimination of fish scales and pin holes.||Washers
Exterior materials for construction
4. High Strength Steel
|Adding hardening elements, such as Nb, Cr and V, to low carbon steel enhances its strength and yield point.It has excellent crack resistance and is used for structural purposes where strength is needed.||Seats, rail levers and parking brakes of automobiles|
|This is high strength steel with solid solution hardening elements, such as phosphate.It is used in automobile panels for its increased strength.||Center floors and brackets of automobiles|
|This is ultra-low carbon steel with special elements added.It has a high elongation rate and is used in deep drawing parts of automobile outer panels.||Outer panels, such as fenders and hoods|
|Low Yield Ratio||*CHSP35EB||In constant temperature, the steel forms a ferrite matrix grain and has a lower yield strength ratio than that of high strength steel.It is used as a strength enhancing material|
|It has a lower yield ratio compared with solid solution hardening steel. Therefore it is used as a strength-enhancing material which requires drawability.By baking after press forming, the yield strength of the steel is heightened and its dent-resistance is improved.The formability is the same as CHSP 35E||Doors and bumpers of automobiles|
5. High Strength Steel
|Classification||Steel Type||DBTT by D/R||Applicability||Remark|
|1.92||1.80||1.80 and under|
|CR||35E||−30||−50||−70||applicable to all parts||Steel without B(Boron)|
|35S||−40||Not measured yet||applicable to all parts||Steel without B|
|40E||−50||−70||−90||applicable to all parts||Steel with B added|
|45E||−20||−50||−60||Prior consultation is necessary||Steel with B added|
Thickness AllowancePOSCO Standards
Width AllowancePOSCO, KS, JIS Standards
|Cutting Method||Width(mm)||POSCO||KS, JIS Allowance(mm)|
Length AllowancePOSCO, KS, JIS Standards
|Re-cutting & Precision Cutting||0~+3||0~+4||0~+6||0~+8||-|
PlanarizationPOSCO, KS, JIS Standards
|Width(mm)/Types||Curvature||Edge deformation||Center deformation|
- 1. In general, numbers inside parentheses are applied to stretcher leveler processed steel.
- 2. Curvature: bending of the entire plate. There are two axes of bending: in the rolling direction and perpendicular to the rolling direction.
- 3. Edge deformation: Bending occurs at the edges in the width axes and the center remains flat.
- 4. Center deformation: Bending occurs in the center and the edges remain flat.
Horizontal BendingPOSCO, KS, JIS Standards
|Width(mm)/Division||Steel Plates(mm)||Steel Rods|
|~630||4||4 per random length of 2,000|
|630~||2||2 per random length of 2,000|
Dull Finish, also called Pear-Skin Finish or Egg-Shell Texture, is a steel surface finish in which designated roughness is produced in its surface. To create these fine roughness, roll surface is grinded and treated with special abrasive blasting method. This surfice finish allows lubricant to adhere to the surface more evenly, thus reducing friction during subsequent machining. Also, paint adheres better and lasts longer. Bright Finish, created by passing the steel through highly polished rollers, produces exquisite surface smoothness and a mirror-like luster. Its highly luminous surface makes such treated steel well suited for decorative use.
|Dull Finish, Ra (µm)||Bright Finish, Ra (µm)|
POSCO protects finished products by spraying rust-preventative oil on the steel surface to stop rust from occurring during shipment and warehousing. Upon ordering, our customers can specify the type and amount of oil applied in accordance with their machining needs. Of course, no-oiled steels are very rust-prone and require extreme care to avoid it. Also, rusting remains a likely occurrence with DOS oiled steels.
|Specification||Code||Amount of oil used on surfaces(mg/m²)|
1. Tension Testing
Tension test is used to ascertain several mechanical properties. A specimen is deformed
usually to fracture with a gradually increasing tensile load that is applied uniaxially along
the long axis of a specimen. Tensile testing is performed to determine the yield strength,
tensile strength, and elongation of steel sheet.
2. Test Piece
All types and sizes of test pieces for tension testing are prescribed in the standards,
such as KS, JIS, and ASTM
Tension test results are used as a basis for determining the workability and formability of
- a. Elongation
- igher elongation means better workability.
- b. Yield Point
- Low yield point means well-shaped end product.
- c. Yield Ratio : (Yield point/tensile strength)
- Low yield ratio means a wide difference between tensile strength and yield point.
Greater difference translates into better workability of sheets.
- d. Modulus of Elasticity
- It represents the "stiffness" of the material, that is, resistance to elastic strain. This manifests itself as the amount of deformation in normal use below the yield strength and the "springiness" of material during formin
- e. n-Value (Modulus of Work Hardening)
- This is measured when the stress- strain curve approximates. Workability is proportionate to the n-value.
- f. r-Value (Modulus of Plastic Deformation)
- In wo / w
In to / t
- wo, w= width before and after testing, respectively - to, t=thickness before and after testing, respectively Surface shrinkage in the direction of thickness is in an inverse relationship with the r-value and the shrinkage in the direction of width is directly proportionate to the r-value. The higher the value, the less susceptible to fracture, hence better drawability.
4. DBTT Test: Ductile to Brittle Transition test
Test Flow : Cup Forming (Blanking, Punching) →Temperature Change → Drop weight test → Inspect for brittle fracture (Transition temperature is the lowest temperature without fracture.)
5. DBTT Test Conditions (No-trimming after Forming Cup)
|Forming Conditions||Blank Dia(mm)||96|
|Punch type||Flat Cup|
|Drop Weight Test||Load(kgf)||4.44|
|Test Piece position||Laid on to the side|
Note)Drawing ratio range (1.7~2.16) : 85mm(1.7)~108mm(2.16)
6. Bending Test
Bending test is performed to determine the ductility of steel sheet. Test specimen for cold-rolled steel sheet is designated in KS B 0801 No.3. In the test, the specimen is bent to a specified angle on a mandrel or a specified radius until fracture. The ductility of the sheet is judged by the cracks on the outside of the bent specimen. In case of cold-rolled sheet the specimen is bent 180 degrees.
Harness of steel sheet is closely related to other properties like strength, wear resistance, and workability. Hardness test is a good indicator of various properties of steel sheet. For measuring the hardness of cold-rolled steel sheet, Rockwell Hardness test is used.
- Rockwell Hardness Test
Material resistance to indentation is a qualitative indication of its strength. Steel ball or diamond is often used as an indenter. With this system, hardness value is determined by the difference in penetration depth from an application of initial minor load onto the indenter followed by a major load.
B-scale uses 1/16(1.588mm) steel ball indenter and major load is 100kg. The B-scale most accurately measures specimens with the thickness of 0.762mm(0.030in.) or over.
F-scale uses 1/16(1.588mm) steel ball indenter and major load is 60kg. It is preferable to use the F-scale for specimen with the thickness under 0.762mm.
8. Drawing Test
The drawability of cold-rolled steel sheet can be measured with various testing methods. Drawing involves many intricate fabrication process, thus it is nearly impossible to determine the drawability of a steel sheet by just applying one testing method. The following methods are most commonly used.
- Erichsen Test
This test measures the drawability of steel sheets. Steel sheets of 0.1mm~2.0mm thickness are used for testing. A test piece is punched down with a 10mm-radius steel ball plunger until the test piece cracks. Value h, measured when fracture occurs, is the Erichsen value. Usually, a larger Erichsen value means better drawability and workability.
- Erichsen Test
- Conical Cup Test
This method has gained popularity in recent years. The specimen is pressed with a flat or round puncher as shown in the diagram. The test value is obtained by measuring the diameter of the cup which the specimen has been drawn into. Because the value closely corresponds to the actual performance of steel sheet fabrication, this test method is widely used by automobile makers.
- Conical Cup Test