Cold-rolled And Hot-rolled

- Jan 12, 2018 -

Hot rolling:

Continuous casting slab or slab as raw material, heated by walking furnace, high-pressure water after descaling into the roughing mill, roughing material through the first, tail, and then into the finishing mill, the implementation of computer-controlled rolling, After the end of the rolling laminar flow cooling (computer-controlled cooling rate) and coiler winder, a straight hair volume. Straight hair volume head and tail tend to be tongue and fish tail shape, thickness, width accuracy is poor, the edge often exist in the waves, flaps, towers and other defects. The heavy weight, coil diameter of 760mm. (The tube industry like to use.) The straight hair volume by cutting the head, tail cutting, trimming and multi-pass straightening, leveling and other finishing lines, and then re-board or re-roll, that is: Steel, flat rolled steel coil, Slitting belt and other products. If the hot-rolled finishing roll after pickling to remove the oxide coating and oiled into a hot-rolled pickling coil.

Cold rolling:

Hot rolled coil as raw material, pickling to remove the oxide after cold tandem rolling, the finished product is rolling hard roll, due to continuous cold deformation caused by cold hardening so hard roll strength, hardness, toughness index decreased , Stamping performance deterioration, can only be used for simple deformation of the parts. Rolled hard roll can be used as raw material for hot dip galvanizing plant, because hot galvanizing units are equipped with annealing line. Hard rolling weight is generally 6 to 13.5 tons, coil diameter 610mm. The cold-rolled sheet and coil should be annealed by continuous annealing (CAPL unit) or hood furnace to eliminate the cold hardening and rolling stress and reach the standard mechanical properties. Cold-rolled steel surface quality, appearance, dimensional accuracy are better than hot-rolled plate.

The advantages and disadvantages of cold rolling and hot rolling: 

The advantages and disadvantages of hot rolling:

Advantages: the ingot can destroy the casting organization, refine the grain of steel, and eliminate the defects of the microstructure, so that steel tissue dense, mechanical properties are improved. This improvement is mainly reflected in the rolling direction, so that steel to some extent is no longer isotropic body; pouring bubble formation, cracks and loose, but also under high temperature and pressure welded.

Disadvantages:

1. After hot rolling, non-metallic inclusions (mainly sulfides and oxides, as well as silicates) in the steel are pressed into flakes and stratification (interlayer) occurs. Stratification greatly deteriorates the properties of the steel in tension in the thickness direction and may cause interlayer tearing when the weld shrinks. The local strain induced by the shrinkage of the weld often reaches several times the yield point strain and is much larger than the strain caused by the load.

2. Inconsistent cooling caused by residual stress. Residual stress is the internal self-equilibrium stress in the absence of external force. Hot-rolled steel of all kinds of cross-section has such residual stress. The larger the general section size of steel, the greater the residual stress. Although the residual stress is self-balanced, but the performance of steel components in the external force or have a certain impact. Such as deformation, stability, fatigue and other aspects may have adverse effects.

The advantages and disadvantages of cold rolling:

Advantages: forming speed, high yield, and does not damage the coating, can be made into a variety of cross-sectional forms to meet the needs of the use of conditions; cold rolled steel can have a great plastic deformation, thereby increasing the yield of steel point.

Disadvantages:

1. Although there is no thermal plastic compression in the forming process, residual stress still exists in the cross section, which inevitably has an impact on the overall and local buckling characteristics of the steel.

2. Cold-rolled steel profile is generally open cross-section, making the section of the lower free torsional stiffness. In the bending prone to torsion, prone to flexion and torsion buckling pressure, poor torque performance;

3. Cold-formed steel wall thickness is small, in the convergence of the plate has no thickening of the corner, to withstand the localized capacity of the concentrated load is weak.

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