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Seamless steel pipes are divided into hot-rolled and cold-rolled (drawn) 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, oil cracking pipe, geological steel pipe and other steel pipes. Cold rolled (drawn) 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 and other steel pipes, as well as carbon thin-walled steel pipe, alloy thin-walled steel pipe, stainless thin-walled steel pipe and special-shaped steel pipe. The outer diameter of the hot-rolled seamless pipe is generally larger than 32mm, the wall thickness is 2.5-75mm, the outer diameter of the cold-rolled seamless pipe can be 6mm, the wall thickness can be 0.25mm, the outer diameter of the thin-walled pipe can be 5mm, the wall thickness is less than 0.25mm, and the dimension accuracy of the cold-rolled seamless pipe is higher than that of the hot-rolled seamless pipe.

In general, seamless steel pipe is made of high-quality carbon Steel 16Mn, 5mnv and other low alloy structural steels such as 10 × 10, 20 × 30, 35 × 45, etc. or 40Cr, 30CrMnSi, 45Mn2, 40MnB and other composite steel by hot rolling or cold rolling. The seamless pipes made of 10 × 20 × and other low carbon steel are mainly used for fluid transmission pipes. Seamless tubes made of medium carbon steel such as 45, 40Cr are used to make mechanical parts, such as the stressed parts of automobiles and tractors. The strength and flattening test of seamless steel pipe shall be ensured. Hot rolled steel pipes shall be delivered in hot rolling or heat treatment state; cold rolled steel pipes shall be delivered in heat treatment state.
Hot rolled seamless steel pipe, as the name implies, the temperature of the rolled piece is high, so the deformation resistance is small, and large deformation can be achieved. Taking the rolling of steel plate as an example, the thickness of continuous casting slab is generally about 230mm, and the final thickness is 1 ~ 20mm after rough rolling and finishing rolling. At the same time, because the width thickness ratio of steel plate is small and the requirement of dimension accuracy is relatively low, the shape problem is not easy to appear, and the control of convexity is the main way. The microstructure and mechanical properties of the strip are generally controlled by controlling rolling and cooling, that is to say, controlling the opening rolling temperature, finishing rolling temperature and rolling temperature.