Advantages and disadvantages of hot rolling and cold drawing process of seamless steel tube in rolling

- Dec 31, 2019-

       The rolling process of seamless steel tube is divided into hot rolling and cold drawing. The rolling deformation of metal above recrystallization temperature is called hot rolling, otherwise it is called cold drawing.
        Hot rolled seamless steel tube: first of all, it can eliminate some defects in the cast metal, enter the welding pores, break the coarse grains and change the distribution of inclusions, so as to improve the compactness and mechanical properties of the steel tube. Secondly, the raw material of seamless steel tube is heated, the plasticity is improved, and the deformation force is reduced. Therefore, the deformation can be increased during rolling, which is conducive to improving productivity. At the same time, due to the reduction of deformation resistance, the requirements of rolling equipment can be reduced accordingly, so the equipment cost can be reduced, and the energy consumption of motor can be greatly reduced. The disadvantage of hot rolling is that high temperature rolling will produce oxide scale and reduce metal yield. In order to eliminate oxide scale, a lot of investment should be added in technology and equipment. The surface of hot rolling products is not smooth enough, the size accuracy of products is not accurate enough, and the mechanical properties are not as good as cold rolling.
        Compared with the hot rolling seamless steel tube, the cold rolling seamless steel tube has the advantages of good surface quality, accurate size, good mechanical properties, and can produce thin and small rolled pieces. The disadvantages of cold rolling are high rolling pressure, high rolling strength and precision, high motor power and large energy consumption. Therefore, the raw material of hot rolling is ingot or billet, while the raw material of cold rolling production is the rolled piece after hot rolling.

        Seamless steel pipe is mainly divided into cold-rolled seamless steel pipe and hot-rolled seamless steel pipe according to the production technology, so we will introduce the differences and advantages and disadvantages of the two seamless steel pipes:
Difference in technology
       1. The local buckling of cold-rolled steel section is allowed, which can make full use of the post buckling bearing capacity of the member; while the local buckling of hot-rolled steel section is not allowed.
       2. The causes of the residual stress are different between hot rolled and cold rolled sections, so the distribution of the residual stress 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 torsion rigidity of hot rolled section steel is higher than that of cold rolled section steel, so the torsion resistance of hot rolled section steel is better than that of cold rolled section steel.
Different advantages and disadvantages
        Cold rolled seamless pipe refers to the steel plate or strip processed 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. the cold rolled section is generally open section, which makes the free torsional rigidity of the section lower. 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.
       Compared with the cold rolled seamless pipe, the hot rolled seamless pipe is rolled under the recrystallization temperature, and the hot rolled seamless pipe is rolled above the recrystallization temperature.
        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.
       3. The thickness and edge width of hot rolled steel products are not well controlled. We are familiar with thermal expansion and cold shrinkage. Even if the length and thickness of hot rolling are up to standard at the beginning, there will be a certain negative difference after cooling. The wider the negative difference is, the thicker the thickness is. Therefore, for large-scale steel, the edge width, thickness, length, angle and edge line of steel cannot be too precise.

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