Advantages and disadvantages of hot-rolled seamless steel tubes

　　Generally, the production process of seamless steel pipe can be divided into cold drawing and hot rolling. Hot rolling, as the name implies, has high temperature of rolled piece, so the deformation resistance is small and large deformation can be achieved. Take the rolling of steel plate as an example. Generally, the thickness of continuous casting slab is about 230 mm, and after rough rolling and finishing rolling, the final thickness is 1~20 mm. At the same time, due to the small width thickness ratio of the steel plate and the relatively low requirements for dimensional accuracy, the shape problem is not easy to occur, and the convexity is mainly controlled.

　　Hot rolled seamless steel pipe: hot rolling is relative to cold rolling. Cold rolling is rolling below the recrystallization temperature, while hot rolling is rolling above the recrystallization temperature.

　　advantage:

　　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. Therefore, the steel is no longer an isotropic body to a certain extent; Bubbles, cracks and looseness formed during pouring. It can also be welded under high temperature and pressure.

　　Disadvantages:

　　After hot rolling, non-metallic inclusions (mainly sulfide and oxide, as well as silicate) inside the steel are pressed into thin sheets and delamination (interlayer) occurs. Delamination greatly deteriorates the tensile properties of steel along the thickness direction, and may cause interlaminar tearing when the weld shrinks. The local strain induced by weld shrinkage often reaches several times of the yield point strain, which is much larger than the strain caused by load;

　　Residual stress caused by uneven cooling. Residual stress is the internal self balanced stress without external force. Hot rolled steel sections of various sections have this kind of residual stress. Generally, the larger the section size of steel section, the greater the residual stress. Although the residual stress is self phase equilibrium, it has some influence on the performance of steel members under external forces. For example, it may have adverse effects on deformation, stability and fatigue resistance.

　　The thickness and side width of hot-rolled steel products are not well controlled. We are familiar with thermal expansion and cold contraction. Even if the length and thickness of the hot rolled products meet the standard at the beginning, a definite negative difference will still appear after the final cooling. The wider the width of the negative difference is, the more obvious the thickness is. Therefore, for large-sized steel, the side width, thickness, length, angle and side line of steel cannot be too precise.

　　Hardness Test of Hot Rolled Seamless Pipe

　　The hardness test is to slowly press a hard indenter into the surface of the specimen under specified conditions, and then test the depth or size of the indentation to determine the hardness of the material. Hardness test is the simplest, fastest and easiest method in material mechanical property test. The hardness test is non-destructive, and there is an approximate conversion relationship between the material hardness value and the tensile strength value. The hardness value of the material can be converted into the tensile strength value, which is of great practical significance.

　　Overview of process flow

　　Hot rolling (extrusion of seamless steel tubes): round tube blank → heating → piercing → three roll cross rolling, continuous rolling or extrusion → pipe stripping → sizing (or reducing) → cooling → blank tube → straightening → hydrostatic test (or flaw detection) → marking → warehousing.

　　Cold drawn (rolled) seamless steel pipe: round pipe blank → heating → piercing → heading → annealing → pickling → oiling (copper plating) → multi pass cold drawn (cold rolled) → blank pipe → heat treatment → straightening → hydrostatic test (flaw detection) → marking → warehousing.

　　Seamless steel pipes are divided into hot rolled and cold rolled (rolled) seamless steel pipes.

　　Hot rolled seamless steel pipe is divided into general steel pipe, low and medium pressure boiler steel pipe, high pressure boiler steel pipe, alloy steel pipe, stainless steel pipe, petroleum cracking pipe, geological steel pipe and other steel pipes.

　　In addition to ordinary steel tubes, low and medium pressure boiler steel tubes, high pressure boiler steel tubes, alloy steel tubes, stainless steel tubes, petroleum cracking tubes, and other steel tubes, cold rolled (rolled) seamless steel tubes also include carbon thin-walled steel tubes, alloy thin-walled steel tubes, stainless thin-walled steel tubes, and special-shaped steel tubes. The outer diameter of hot-rolled seamless pipe is generally greater than 32mm, and the wall thickness is 2.5-75mm. The outer diameter of cold-rolled seamless pipe can reach 6mm, the wall thickness can reach 0.25mm, and the outer diameter of thin-walled pipe can reach 5mm, and the wall thickness is less than 0.25mm. Cold rolling has higher dimensional accuracy than hot rolling.