Carburized wear-resistant plate 18CrNiMo7-6 is a surface-hardened steel with the advantages of high strength, high toughness and high hardenability. It is widely used in industrial fields such as mining, transportation, locomotive traction, lifting and internal power. A large number of low-magnification inclusions appear in 18CrNiMo7-6 steel produced by a certain factory, which seriously affects product quality and market reputation. For defective parts of the product, low-magnification, high-magnification, and scanning electron microscopy are used for analysis and research.
18CrNiMo7-6 wear-resistant plate production process route: electric furnace smelting → LF refining → VD vacuum → pouring → slow cooling in the slow cooling pit → steel ingot heating → forging → post-forging annealing → non-destructive testing → inspection, inspection → delivery.
The specific production process operations are as follows: when charging, the amount of pig iron or molten iron is greater than 30% of the total amount of steel; during initial refining, P ≤ 0.010% and T > 1580°C, steel is tapped; during the refining process, white foam slag is produced as soon as possible, and white foam is produced. The slag retention time is greater than 20 minutes. When [S] ≤ 0.010% and the temperature ≥ 1650°C, vacuum is applied; in the VD stage, the vacuum pump is turned on step by step, the vacuum degree reaches 67Pa, and is maintained for more than 15 minutes before dissipating; the pouring temperature and pouring speed are strictly controlled during the pouring process, and it is strictly prohibited to pour 1/ After 3 adjust the flow rate.
Observing the distribution of inclusions from the low-magnification test piece, we found that the particles were large and numerous, irregularly arranged, and the locations where inclusions occurred were not fixed. Some appeared at the head of the steel ingot, some at the tail, and some in the middle.
Although the forging process will cause the accumulation of inclusion defects in the as-cast structure, the occurrence of defects in such a large amount is initially confirmed to be a problem in the smelting process, mainly the deoxidation process. From the inclusion defect analysis, it can be seen that in the low-magnification test piece, except for inclusions, there are no other low-magnification defects such as general looseness. The inclusions are distributed in a large range, some are diffusely distributed, and some are aggregated. The edges of the inclusions are The base has sharp corners. High-magnification inspection also proves that the inclusion particles are large in size, have sharp corners and are embedded in the matrix. It can be seen from the scanning electron microscope results that, in addition to the matrix content, the average proportion of oxygen in defects is 54.55%, aluminum is 32.66%, magnesium is 4.01%, calcium is 3.74%, silicon is 1.02%, manganese is 0.82%, and iron is 2.67%. Judging from the composition of the inclusions, the secondary inclusions are mainly composed of oxygen and aluminum, and the ratio of oxygen to aluminum is close to 3:2, so it is determined that the main composition of the inclusions is Al2O3.
In order to reduce inclusion defects, scientific researchers have improved the production process of 18CrNiMo7-6 wear-resistant plates and adopted two processes for production: one is to not add aluminum ingots during the initial steelmaking and tapping process, and to feed aluminum wires to the refining station. ; The other is to add aluminum ingots for deoxidation during the initial steelmaking and tapping process. After reaching the refining station, a small amount of aluminum wire is replenished depending on the aluminum content in the steel.
Process research results show:
(1) The main component of inclusion defects in 18CrNiMo7-6 wear-resistant plate is Al2O3.
(2) The timing of deoxidation has little to do with whether inclusion defects occur.
(3) Under the premise of a certain amount of aluminum per ton of steel, increasing the amount of aluminum used for precipitation deoxidation and reducing the amount of aluminum used for diffusion deoxidation will help eliminate Al2O3 inclusion defects.
(4) Experience shows that no matter what production process is used, aluminum deoxidation should be completed as early as possible in the early stage of refining.
