Hot-rolled seamless steel pipe: Hot rolling is relative to cold rolling, cold rolling is rolling below the recrystallization temperature, and hot rolling is rolling above the recrystallization temperature.
Advantages of hot rolled seamless steel pipe:
It can destroy the casting structure of the steel ingot, refine the grains 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 a certain extent; bubbles, cracks, and looseness formed during pouring can also be welded under the action of high temperature and pressure.
Disadvantages of hot-rolled seamless steel pipe:
1. After hot rolling, the non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, and delamination (interlayer) occurs. Delamination greatly deteriorates the properties of the steel in tension through the thickness, and there is a possibility of interlaminar tearing as the weld shrinks. The local strain induced by the shrinkage of the weld often reaches several times the strain at the yield point, which is much larger than the strain caused by the load;
2. Residual stress caused by uneven cooling. Residual stress is the internal self-equilibrium stress without external force. Hot-rolled steel sections of various sections have this kind of residual stress. Generally, the larger the section size of the section steel, the greater the residual stress. Although the residual stress is self-equilibrium, it still has a certain influence on the performance of the steel member under the action of external force. For example, it may have adverse effects on deformation, stability, fatigue resistance, etc
3. For hot-rolled steel products, it is difficult to control the thickness and side width. We are familiar with thermal expansion and contraction because even if the length and thickness of the hot-rolled products meet the standard at the beginning, there will still be a certain negative difference after cooling. The wider the side width and the thicker the thickness of this negative difference, the more obvious. Therefore, for large-sized steel, the side width, thickness, length, angle, and edge of the steel cannot be too precise.
There are two types of seamless steel pipes: hot-rolled and cold-rolled (dial) seamless steel pipes.
Hot-rolled seamless steel pipes are divided into general steel pipes, low and medium-pressure boiler steel pipes, high-pressure boiler steel pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes, geological steel pipes, and other steel pipes.
In addition to general steel pipes, low and medium-pressure boiler steel pipes, high-pressure boiler steel pipes, alloy steel pipes, stainless steel pipes, petroleum cracking pipes, and other steel pipes, cold-rolled (dial) seamless steel pipes also include carbon thin-walled steel pipes, alloy thin-walled steel pipes, Stainless thin-walled steel pipes, special-shaped steel pipes. The outer diameter of the hot-rolled seamless steel pipe is generally greater than 32mm, and the wall thickness is 2.5-75mm. The diameter of cold-rolled seamless steel pipe can reach 6mm, and the wall thickness can reach 0.25mm. The outer diameter of a thin-walled pipe can reach 5mm, and the wall thickness is less than 0.25mm. Cold rolling has higher dimensional accuracy than hot rolling.
Seamless steel pipes for general use: are made of high-quality carbon steel such as 10, 20, 30, 35, 45, 16Mn, 5MnV, and other low-alloy structural steels or 40Cr, 30CrMnSi, 45Mn2, 40MnB and other alloy steels that are hot-rolled or cold-rolled. Seamless steel pipes made of low carbon steel such as 10 and 20 are mainly used for fluid conveying pipelines. Seamless steel pipes made of medium carbon steel such as 45 and 40Cr are used to manufacture mechanical parts, such as stressed parts of automobiles and tractors. Generally, seamless steel pipes are used to ensure strength and flattening tests. Hot-rolled steel pipes are delivered in a hot-rolled or heat-treated state; cold-rolled steel pipes are delivered in a heat-treated state.