At present, there are many types of wear-resistant pipes in China, mainly cast steel pipes, cast iron pipes, welded pipes using plate coil welding, bimetal composite pipes, surfacing composite pipes, ceramic composite pipes and other double-layer composite pipes.
Wear-resistant pipelines are widely used for transporting filling materials, mineral powder and tailings in mines with severe wear. Wear-resistant pipelines for powder delivery, slag removal, and ash transportation in coal-fired thermal power plants are also very suitable. Since the conveying medium generally has the characteristics of good hardness, fast flow rate, and large flow rate, and will continue to impact, wear, and corrode the pipe wall for a long time in the transportation process, causing the pipe to fatigue and gradually wear through, so the ideal wear-resistant tubes made of high-impact, high-toughness and high-plastic materials are an important way to reduce abrasive wear.
At present, there are many types of wear-resistant pipes used for the above purposes in China. What distinguishes this type of pipe from other pipes is the ultra-high hardness of the inner wall (greater than and equal to 40HRC), which is also the most important factor in ensuring its wear resistance and service life. There are mainly the following types:
①Cast steel pipes and cast iron pipes generally have poor plasticity and weldability.
②Welded pipes using plate coil welding need to go through coiling and welding, and the process flow is long.
③Double-layer composite pipes such as bimetallic composite pipes, cladding composite pipes, and ceramic composite pipes have good wear resistance on the inner wall and good toughness and weldability on the outer wall. However, the preparation process of these pipes is complicated and the processing cycle is long; the material is thick, and a single pipe is short due to processing technology limitations. When making long-distance pipelines, there are many interfaces, so the overall cost is high.
A method for manufacturing wear-resistant
seamless steel pipes is proposed, which takes into account the good wear resistance of the inner wall and the good plasticity, toughness and weldability of the outer wall. The length of a single steel pipe can reach more than 12 meters, which is especially suitable for long-distance transportation wear-resistant pipes.
Test raw materials and processes of wear-resistant seamless steel pipes
A wear-resistant seamless steel pipe is a seamless steel pipe that can meet the inner wall hardness (greater than and equal to 40HRC, while the outer wall hardness is slightly lower, and has good plasticity, toughness and weldability. Therefore, the selection of chemical composition is particularly important.
C: Increase the hardenability, strength and hardness of the material, but reduce the plasticity and toughness, increase the ductile-brittle transition temperature, and deteriorate the welding performance of the steel pipe. The different carbon content is selected according to the hardness to be achieved by the product, which is between 0.10 and 0.40%.
Si: It is a deoxidizing element in steel and improves the strength of steel in the form of solid solution strengthening. When the Si content is less than 0.10%, the deoxidation effect is poor; when the Si content is high, the toughness is reduced and the welding performance deteriorates. The Si content in this article is controlled to be between 0.15 and 0.40%.
Mn: It is an element that improves the hardenability of steel and plays a role in solid solution strengthening to improve the strength and hardness of steel. However, excessive Mn can easily reduce the plasticity, toughness and weldability of steel. To achieve a match of strength, plasticity and toughness, the Mn content in this article is controlled at 0.50 and 1.50%.
P: Deteriorate the welding performance of steel and reduce the plasticity and toughness of steel. The P content is controlled to be less than and equal to 0.015%.
S: Strip-shaped sulfide inclusions will increase the crack sensitivity of steel during heat treatment and welding. The S content is controlled to be less than and equal to 0.005%.
Cr: Increase the hardenability, strength and toughness of the material. The Cr content is controlled to be less than and equal to 1.00%.
A1: It has functions of nitrogen fixation and deoxygenation. The AIN formed by A1 and N can effectively refine the grains, but if the content is too high, it will damage the toughness of the steel and worsen the castability of molten steel. The Alt content is controlled between 0.015 and 0.050%.
Ti: It mainly plays the role of refining grains and fixing nitrogen. Ti has a good affinity with O, C, and N. It forms tiny particles and is dispersed in the steel. It can play a role in steel rolling, heating and welding. The pinning effect promotes the grain refinement of the steel matrix and welding seam. In B-containing steel, the combination of Ti and N can also reduce the reaction between B and N, giving full play to the role of B in improving hardenability. The content of Ti is controlled between 0.005 and 0.050%.
Nb: It is an element that plays a significant role in grain refinement in rolling and welding processes. During the recrystallization rolling stage, Nb hinders the recovery and recrystallization of deformed austenite through strain-induced precipitation, thereby refining the grains. This provides a basis for the steel pipe to still have a fine structure during hot forming, heat treatment and welding, which is beneficial to improving the toughness of the matrix and welding seam. However, Nb that is too high cannot be dissolved in solid solution due to the limitation of C content and the influence of heating temperature, which cannot play a role and increases the cost. Therefore, the content is controlled to be between 0.005 and 0.050%.
B: It is the most significant element that improves the hardenability of steel. It can play an important role in replacing Si, Mn, Cr, and Mo, thereby ensuring that thick-walled pipes have better welding performance under the premise of hardenability. The B content is controlled to be less than and equal to 0.0050%.
The specifications of the seamless wear-resistant steel pipe are 219×7mm; the length is 12m, and the chemical composition C 0.10 to 0.40%, Si 0.15 to 0.40%, Mn 0.50 to 1.50%, Cr less than and equal to 1.50%, A1 0.015 to 0.050%, Ti 0.005 and 0.050%, and Nb 0.005 to 0.050%.
The smelting raw materials are sequentially subjected to converter smelting or electric furnace smelting, LF refining, RH degassing or VD degassing to produce molten steel that meets the composition requirements; the molten steel is poured into continuous casting billets, and the continuous casting billets are sent at temperatures between 300 to 600°C or put into the slow cold pit for more than 32 hours before exiting the pit. Heat the continuous casting billet to 1150 to 1280°C; keep it warm for 3 to 10 hours, and then take it out; perforate the billet at the temperature between 1100 and 1150°C to make a billet. The billet is rolled at temperatures between 1000 to 1100°C to make a pierced billet. After taking out the mandrel, the billet is finally sized and formed on the sizing unit to make a seamless steel pipe; the cooled seamless steel pipe is heated to 840 to 920°C, and after coming out of the furnace, it is cooled to below 150°C by spraying water on the inner wall. It can then optionally be tempered at temperatures between 150 and 250°C to relieve heat treatment stress.
