By summarizing the experience of the entire steel grit and aluminum production line of Gansu Dongxing Aluminum Co., Ltd. from installation and commissioning to smooth production, the production process of the four-wheel steel grit and aluminum continuous casting and rolling production line is systematically discussed.
Keywords: steel grit aluminum; continuous casting and rolling; production technology
Oxygen can produce many harmful phenomena in molten steel. When molten steel solidifies, oxygen and other elements form non-metallic inclusions, which exist between the grain boundaries of the steel body and destroy the microscopic continuity of the steel. As the temperature decreases, excess oxygen will react with C to form CO gas. Pinholes will be formed along the path that CO gas passes when it is precipitated from the molten steel. The CO gas that has not been discharged will remain in the steel body to form bubbles. At the same time, oxygen can also aggravate the harm of sulfur and make the hot brittleness tendency of steel more serious. Therefore, deoxidation is an important process step in the steelmaking process.
Steel grit aluminum is an important deoxidizer used in steelmaking. Due to its unique physical structure, it can easily penetrate the protective slag layer above the molten steel and reach the bottom of the ladle. Characteristics of complete deoxidation of molten steel: After the steel grit and aluminum enter the molten steel, the internal iron pellets will drive the unmelted aluminum blocks to the bottom of the ladle under the action of gravity, and the molten steel will be completely deoxidized during the descent. It avoids the phenomenon that when the steel core aluminum is added to the molten steel, the steel core and the aluminum block separate, and the aluminum block floats on the surface of the liquid after melting. The deoxidation value of steel grit aluminum per kilogram is 2.81ppm³.
Steel grit aluminum production line structure and process flow
The steel sand and aluminum production line mainly consists of a mixing holding furnace, a chute, a four-wheel casting machine, a steel sand feeding and heating device, a traction device, a front roller shear, a continuous rolling mill, a rear roller shear and a granulator. The production process of steel sand aluminum continuous casting and rolling mainly includes steps such as aluminum liquid adjustment, continuous casting, continuous rolling, granulation, testing, packaging, and weight inspection. The aluminum liquid in the mixing furnace enters the crystallization wheel cavity along the chute and ladle after composition adjustment, refining, slag removal, and temperature adjustment. The molten aluminum forms a continuous strip of cast slab under the action of cooling water, and comes out of the mold cavity as the crystal wheel rotates. After the casting is stable, add steel grit. The heated steel grit and aluminum liquid enter the crystallization wheel together and are dispersed in the aluminum liquid to form steel grit aluminum billet. The casting process is shown in Figure 1.
The billet that reaches the front roller shear along the traction bridge cuts off the waste rod in the front section and then enters the continuous rolling mill. The rolled continuous rod material is cut into semi-finished products with a length of about 3m at the rear roller shear. Then use a granulator to cut the semi-finished products into finished granules with a length of 5cm and bag them by the ton. The specific process is shown in Figure 2.
Figure 1 Four-wheel continuous casting machine
Figure 2 Steel grit aluminum production process
Common process problems and research
Cast billet forming
During the casting process, the billet often breaks. After testing, it was found that the main reasons for this phenomenon are: unreasonable composition of aluminum liquid, irregular casting temperature and unskilled operating technology.
Effect of molten aluminum composition on cast slab
Each trace element in the aluminum liquid has varying degrees of influence on the casting performance of the aluminum liquid. The content of each trace element in the electrolytic primary aluminum used by Gansu Dongxing Aluminum Co., Ltd. to produce steel grit aluminum is shown in Table 1:
Table 1 Chemical composition of aluminum liquid (%)
It can be seen from the table that except for the high content of Si element and Fe element, the content of other elements is low, and the impact on the casting performance is negligible.
The influence of Si element. As the Si content increases, the alloy grains become finer, the metal fluidity is enhanced, and the casting performance becomes better. However, the crystallized Si easily forms hard points. As the Si content increases, the tensile strength increases and the plasticity decreases, which is not conducive to steel grit aluminum production. Generally, the Si content is controlled within 0.1%.
The influence of Fe element. As the Fe content increases, the shrinkage porosity in the alloy will increase. The main reason is that the needle and flake brittle iron phase β-Al5FeSi that splits the alloy matrix is formed in the alloy (as shown in Figure 3). It hinders the aluminum liquid from filling the channels between dendrites, making the mechanical properties of the alloy worse, especially in terms of toughness. As a result, the steel sand aluminum cast billet rod is very brittle and easily broken. The experiment concluded that the maximum Fe content in the aluminum liquid used to produce steel grit aluminum cannot exceed 1.0%.
Figure 3 Acicular intermetallic compounds formed by iron in aluminum alloy
Effect of molten aluminum temperature on cast slab
As one of the three elements of casting, the casting temperature at which the aluminum liquid enters the crystallization wheel is an important factor affecting the molding of the billet. When the temperature of the molten aluminum is too high (higher than 750°C), the temperature of the cast slab will be too high or the solidification will be incomplete or even impossible.
Cast slabs at high temperatures are prone to twisting before entering the rolling mill and sticking to the rolls during the rolling process, which can even jam the rolling mill and cause equipment damage.
When the temperature of the aluminum liquid is low (lower than 710°C), the steel sand entering the aluminum liquid will cause the surrounding liquid aluminum to solidify rapidly. The steel sand cannot be dispersed in the molten aluminum and is concentrated inside the cast slab. The steel sand may even form a concentrated band on the upper surface of the cast slab. During the subsequent rolling and shearing work, a large amount of steel grit fell off, resulting in a substandard steel grit content. In addition, the fluidity of molten aluminum decreases at low temperatures, and the mold cavity formed by the crystallization wheel and steel strip solidifies before it is filled with molten aluminum. This leads to uneven thickness of the cast slab and even the phenomenon that the air in the unfilled area of the mold cavity instantly expands under the high temperature of the molten aluminum and causes a physical explosion, which brings great safety risks to the workers.
When the temperature of the aluminum liquid is too low (lower than 690°C), the steel sand entering the aluminum liquid will cause the aluminum liquid to solidify instantly. Subsequently, the aluminum liquid is hindered by the solidified billet and cannot continue to flow forward in the crystallization wheel and overflows from both sides of the crystallization wheel. . A large amount of steel grit accumulates at the joining position, eventually causing billet breakage.
In summary, the optimal casting temperature of molten aluminum is between 720 and 740°C.
Effect of cooling water on cast slab
The cooling water is located in the inner and outer rings and both sides of the crystal wheel. It is composed of semicircular pipes with water nozzles, which are called internal cooling, external cooling, and left and right cooling. The water pressure of each cooling water pipe can be adjusted individually. In actual production, the slab coming out of the crystallizing wheel will be slightly offset to the side with greater water pressure.
The main reason for this phenomenon is that the aluminum liquid solidifies quickly on the side with greater cooling water pressure, which produces tensile stress on the surrounding aluminum materials. The test found that the cooling water pressure should be maintained between 03 and 0.5MPa. The water nozzles of the inner and outer cooling should be aligned with the center line of the crystallization wheel cavity, and the water pressure of the left and right colds should be relatively consistent.
Effect of operating technology on cast slabs
When the aluminum liquid flow rate is too small and the cavity is not filled, the cast slab will be insufficiently full, uneven in thickness, cold shut, or the cast slab may be full on the outside but hollow on the inside or even broken. It should be noted that when the flow rate of molten aluminum is quickly adjusted from small to large, the air sealed in the cavity will physically explode and the molten aluminum will splash.
Steel grit content is not up to standard
The factors that affect the content of steel grit in steel grit aluminum mainly include the quality of the steel grit and the position of the compression wheel.
Steel grit quality
The quality of steel grit includes the diameter of steel grit and the density of steel grit.
Tests have found that steel grit with a smaller diameter (for example, 1.5mm) cannot enter the depth of the aluminum liquid due to its limited gravity, and is mostly concentrated on the upper surface of the cast slab to form a steel grit concentration belt. In the subsequent processing, “sand falling” was serious, and the steel grit content of the product was low. Due to the excessive gravity of the steel grit with a larger diameter (for example, 4.0mm), most of it settles at the bottom of the cast slab, forming a steel grit concentration belt at the bottom of the cast slab. The steel sand in the cast slab is unevenly distributed and “sand falling” is serious during the rolling and shearing processes, and the product quality is not up to standard. After testing with different types of steel grit, it was finally concluded that the diameter of the steel grit is 2.7mm or 3.0mm.
In addition, steel sand has a certain empty shell rate. When the empty shell rate of steel sand is high, the overall density of steel sand added with aluminum liquid is low. Although the added macro volume has been saturated, the iron content of the product cannot reach the standard of 20% to 25%. This point must be strictly controlled when purchasing steel grit.
Position of the pressure wheel
The location where steel grit is added has a great influence on the steel grit content in the product and the distribution of steel grit in the cast slab. The position of the steel belt compression wheel determines the adding position of steel sand. When the pressing wheel is directly above the crystallizing wheel (as shown in Figure 4), the aluminum liquid at the steel grit addition point is in the upward stage and has limited kinetic energy to move forward. When the added steel grit is within the index range, the aluminum liquid cannot drive the steel grit forward, and steel grit will accumulate, causing the product to fail. When the compaction wheel is at the eleven o’clock direction of the crystallization wheel (as shown in Figure 5), the aluminum liquid at the steel grit addition point is in the downward stage and has sufficient kinetic energy to move forward. It can easily drive the added steel sand forward together and use the kinetic energy of the forward punch to stir the steel sand so that it is evenly distributed in the cast slab.
Steel sand aluminum casting is a type of continuous casting and rolling. It is necessary to start from the three elements of casting and conduct research and control on casting temperature, casting speed, cooling pressure and other physical principles based on the actual situation. Gansu Dongxing Aluminum Co., Ltd. has produced qualified steel grit aluminum products based on the above research results.