Good surface hardness and wear resistance. During the operation of the forklift, the forklift mast is not only subjected to complex loads, but also has rolling friction between its bearing surfaces and the rollers. In order to ensure that the electric pallet truck
mast has a high service life, Channel steel must have good surface hardness and wear resistance. Production process flow Q420C grade 22 hot-rolled forklift mast channel steel product billet is continuously cast into a rectangular billet of 240mm375mm specification by the steelmaking plant through hot metal pretreatment, microalloying, and LF furnace refining treatment. It is used during continuous casting. Process measures such as low superheat pouring and optimization of the secondary cooling system to improve the internal and surface quality of the billet. In the section steel plant, the billet is heated by a walking heating furnace with automatic control combustion, and then rolled by high-pressure water descaling, BD machine billet rolling, universal finishing mill and other equipment. In the rolling process, recrystallization controlled rolling is used to achieve the purpose of refining the structure and ensuring the product has high strength and good performance. After rolling, the water-cooled line behind the finishing mill is used to achieve rapid cooling of the surface of the rolled piece, ensuring that the product has higher surface hardness and wear resistance. Carbon is an element that significantly reduces the plasticity and toughness of steel, and the higher the carbon content, the worse the welding performance. For this reason, the C content is designed to be controlled within the range of 0.14% 0.20%. Silicon can significantly increase the yield strength and tensile strength of the product, and effectively increase the elastic limit, which helps to improve the performance of the product under overload conditions. At the same time, Si is one of the important elements of product strength and wear resistance. However, when Si is greater than 0.5%, the cold brittle transition temperature will increase and the aging sensitivity of steel will increase. Therefore, the silicon content of the composition design is 0.30% 0.45%. In order to ensure that the product has the requirements of high strength, good toughness and welding performance, the addition of trace niobium, vanadium and other alloy elements in the composition design can not only ensure that the finished product has good physical properties, but also Greatly reduce production costs. The addition of niobium and vanadium as microalloying elements to steel will increase the strength of the steel. This is mainly achieved through two mechanisms of grain refinement and precipitation strengthening, but the effects of these two mechanisms on toughness are different. Niobium microalloying mainly relies on controlled rolling to achieve grain refinement, which not only increases the strength of the product, but also improves its toughness. In the production practice process, it is found that when the niobium microalloying process is carried out, adding a small amount of B can make a small amount of Nb realize grain refinement and precipitation strengthening, and can produce bainite hardening effect together with B. However, further studies have found that only when the N element is less than 0.005%, can the effect of B significantly delay the phase transformation of polygonal ferrite and promote the formation of fine bainite. The strength of vanadium mainly depends on precipitation strengthening, and the component of grain refinement is small, so its contribution to the strength is greater, but it will reduce the toughness of the hot-rolled product. However, when vanadium is added alone, it cannot inhibit the formation of ferrite, but accelerates the formation of pearlite. Utilizing this characteristic of vanadium can improve the performance of the welded area of u200bu200bthe channel steel during the welding process. Because it promotes the formation of ferrite on the austenite grain boundaries in the welding heat-affected zone (HAZ), and promotes the formation of ferrite in the grains, thereby increasing the toughness of the HAZ. However, when vanadium and niobium exist at the same time, it is easy to form a bainite structure, and vanadium is precipitated in the bainite, which will greatly reduce the toughness of the HAZ. Therefore, when the niobium-vanadium composite microalloyed steel is cooled at a faster rate after rolling, it is easy to form a brittle bainite structure, which can increase the strength, but the ductility and plasticity decrease faster. At the same time, when the steel is welded, it is easy to cause the problem of the decrease of HAZ toughness <1>. Composition and physical properties Laiwu Steel used the above production process equipment and strengthening principles to develop Q420C-class 22 hot-rolled forklift mast channel steel products. Randomly select 10 finished products from mass-produced new products for composition and performance statistics. The physical quality of this type of steel produced by Laiwu Steel fully meets the requirements of users, and in some aspects is significantly higher than the requirements. The chemical composition of the steel is well controlled; the finished steel has high strength, low yield ratio, good toughness and surface hardness.
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