Requirements of high quality molybdenum slab and Study on its processing technology
Molybdenum is a rare metal with high melting point (2620 ± 10 ℃), high elastic modulus (320-360 GPA), small linear expansion coefficient (20-100 ℃, 5.8x10-6-6.2x10-6), high melting point, low steam pressure and low evaporation rate. There are many kinds of molybdenum products, such as molybdenum rod, molybdenum plate, molybdenum electrode, molybdenum crucible, molybdenum shaped parts and so on. In the application of various forms of molybdenum, plate is the most. However, when the plate is applied, it usually needs rolling and various types of forming processing. In the current processing and production of molybdenum wafer, the yield rate is only 40% - 50%. Therefore, improving the quality of molybdenum plate products and processing yield is an urgent problem to be solved.
Problems in molybdenum processing and processing materials
Molybdenum is a hard to deform metal, its production process is very technical, and the "heredity" between processes is also very strong. As long as one process in the production process goes wrong, it will lead to the collapse of the whole production line and cause irreparable losses. Molybdenum slab is the foundation, and its quality directly affects the yield and mechanical properties of processed materials.
The defects such as delamination, peeling and blistering are easy to occur during the processing of molybdenum billet, and the finished product rate is only about 40%.
Requirements of high quality molybdenum slab
According to the requirements of processing technology and material properties, the chemical composition, microstructure and density of high-quality molybdenum slab should be controlled.
The purity of molybdenum slab is an important index to measure the quality of slab, and the purity of slab also affects the processing performance of molybdenum. The plastic brittle transition temperature is very sensitive to the impurity content during molybdenum processing. With the increase of the impurity content in the gap, the processing brittleness increases and the finished product rate is low.
With the increase of O, N, C content, the ductile brittle transition temperature increases, and the brittleness of molybdenum increases, which leads to the reduction of yield during the processing. At the same time, a large number of studies show that the impurity with low melting point is an important factor causing defects in molybdenum sheet. Fe, Ni, Co are the main causes of pitting and white spots. These impurities are easy to volatilize at high temperature to form pits and white spots. The main reason for the delamination and point fracture of molybdenum sheet is that interstitial impurities such as Al, Mg, Si combine with O to form hard deformation oxides with high melting point and high hardness. K、Ca and other impurities are the main cause of bubble and hole.
The purity of molybdenum slab not only seriously affects the yield, but also affects the physical and mechanical properties of the processed products. Therefore, high quality molybdenum slab requires strict control and reduction of impurity element content in molybdenum slab.
Density and microstructure of molybdenum slab
The density and structure of slab directly affect the quality of processed materials and the rate of finished products. When the density of slab is lower than 90% of the theoretical density, the blank is "under fired", and cracking or surface cracking is easy to occur in subsequent processing. Although the crystal position of the blank with low density is small, but there are many pores, it can still be processed after back burning. However, when the billet is "overburned", the slab is very brittle and can not be processed. The microstructure is characterized by coarse grains, flat grain boundaries and inclusions gathering on the grain boundaries. If the structure of the billet is not uniform, such as the surface of the billet is not consistent with the central part, it will lead to inconsistent deformation, peeling, delamination and even cracking.
The purity of high quality molybdenum slab should be above 99.97%, the grain structure is uniform, the grain boundary is fine and clear, the sintering hole is fine and evenly distributed, and has good processing performance.
Test process and method of molybdenum plate rolling
The molybdenum slab of 12mm x 60mm x 142mm was hot rolled by 250 two high rolling mill and heated by molybdenum wire furnace protected by nitrogen. The slab was rolled at 1200 ℃, 1300 ℃ and 1400 ℃ respectively. The slab thickness was rolled from 12mm to 7.5mm, 6mm and 4.5mm by two passes of one fire.
With the increase of deformation, molybdenum sheet tends to fiber structure gradually. Because of the difference of total deformation, the difference of microstructure is also relatively large.
Hot rolling and breaking is the first process in the production process of molybdenum plate. This process not only needs to complete the change of geometry, but also complete the transformation from sintered state to processed state, so as to change the mechanical properties and technological properties of the slab, and lay a good foundation for subsequent processing. In the hot rolling blooming stage, the blooming temperature and pass deformation rate should be strictly controlled.
The hot rolling blooming temperature has obvious influence on the properties of molybdenum plate. When rolling at 1300 ℃, the room temperature tensile properties of molybdenum plate are the highest, and with the increase of rolling temperature, the room temperature tensile properties decrease significantly; the elongation index is higher after rolling at 1300 ℃, and has little change with the decrease of the first rolling temperature, but the elongation decreases greatly with the increase of rolling temperature.
Molybdenum belongs to body centered cubic metal. With the increase of deformation, it tends to form a single structure. Due to the different rolling process, the quality and yield of finished products are different greatly. The results show that the deformation rate of the first pass is too small, and the rolling pressure is not enough to press the molybdenum billet into the range of atomic gravity. The grains are mechanically meshed due to the initial plastic deformation. There is still a large gap between the grains and the bonding strength of the grains is not high. With the increase of the deformation rate, the larger rolling pressure makes more molybdenum atoms in the blank. The range of metal bond attraction makes the bonding strength between grains increase significantly. However, when the deformation rate is too high, the grain interface bonding is relatively dense, and the strength is not increased much. Because of the poor plasticity of the plate, the edge crack of the phase plate is serious, and the defects caused by deformation increase. In addition, the load of the rolling mill is too heavy.
During rolling, heated molybdenum plate contacts with air, which will contaminate molybdenum plate with nitrogen, oxygen and other gases, forming a contamination layer. When nitrogen interacts with molybdenum, MoN and Mo2N are formed when the temperature is lower than 1000 ℃, while mon is formed when the temperature is higher than 1000 ℃. In the process of nitriding, with the increase of nitriding layer thickness, larger internal stress will be produced, and when the stress is relaxed, microcracks will be produced; oxygen and molybdenum will produce different types of oxides with different temperature, which will cause large loss of raw molybdenum, and when there are defects on the surface of molybdenum plate, oxygen will penetrate into the cracks. The higher the heating temperature, the more serious the contamination layer. Due to the higher hardness, lower elongation and uneven distribution of the contamination layer on the surface of molybdenum plate, with the increase of rolling passes and heating times, the contamination layer will be gradually pressed into the surface layer of the substrate. When it peels off, the surface of the whole molybdenum plate will produce pits and pockmarks with different depth, which will seriously affect the quality of molybdenum plate. Therefore, after hot rolling, the slab transferred to warm rolling must be removed by alkali and acid pickling, otherwise it should not be transferred to warm rolling. The total processing rate of warm rolling is an important factor to ensure the continuous refinement of texture structure in cold working, reduce the plastic brittle transition temperature of molybdenum and its alloy, and improve the low-temperature plasticity. It is the key to make the plate suitable for cold working production. The higher the total processing rate, the lower the transition temperature. When the total processing rate is more than 80%, the transition temperature has a sharp downward trend. However, if the processing rate is too high, the rolling passes will be increased, the surface work hardening will be serious, resulting in the increase of non-uniformity, which will lead to peeling, delamination and other defects on the plate surface.
If the total processing rate of warm rolling is too small, the plasticity of molybdenum plate will be reduced and the internal physical properties and mechanical state of molybdenum plate will be uneven; however, if the total processing rate of warm rolling is too large, the work hardening of the plate will be serious and cracks will appear in the plate. It can be seen that the suitable total processing rate of warm rolling should be selected. It can be seen that when the total processing rate of warm rolling is controlled at 66.67%, the product yield is the highest.
(1) The preparation of high-quality molybdenum plate should be strictly controlled from the aspects of chemical composition, density and microstructure of the slab, and the impurity element content in the aluminum slab should be reduced as far as possible to improve the purity of the slab.
(2) The results show that it is appropriate to use 12mm x 60mm x 142mm molybdenum slab and 250 two high rolling mill for hot rolling. It is appropriate to control the blooming temperature at 1300 ℃ and the total deformation of the slab at about 50%.
(3) For the plate rolled by the above process, if the 175 two high rolling mill is continued to be used for rolling, the first warm rolling is 1150 ℃, the second warm rolling is 1050 ℃, and the total processing rate is controlled at 66.67%, the product yield is the highest.
