Central segregation can affect the service life and quality of cast slabs. This paper takes steel billet as the research object, and first studies the mechanism of central segregation from the theories of solidification crystal bridge, hole pumping, enrichment and solute precipitation. Then, using the cast slab of a steel company as the test object, the influence of the superheat of molten steel was analyzed, and the parameter was controlled at around 20°. By combining the two continuous casting parameters of the casting speed and the secondary cooling specific water volume, the optimal ratio was obtained, and it was determined to increase the secondary cooling specific water volume by 0.04kg/t. Reducing the converter’s casting speed to 0.06m/min is the optimal continuous casting process.
Keywords: casting slab; continuous casting process; superheat of molten steel; casting speed
High carbon hard wire steel is widely used in the field of manufacturing prestressed metal products, such as tire steel wire, construction wire rope, steel wire strand and stress steel wire, etc. Among them, the service life and quality of high carbon hard wire steel are closely related to the drawing strength. The solidification temperature range of high carbon steel is relatively wide, and the mushy area during the solidification process of steel materials is larger, resulting in problems such as central shrinkage cavities or central segregation in the continuous casting billet. Literature research shows that the main cause of pull-out fracture in high carbon steel wire rods during cold working is central shrinkage cavities or central segregation. Therefore, efforts to improve the central shrinkage cavity or central segregation during the formation of continuous cast slabs are of great significance to the quality of high carbon steel wire rods.
Formation mechanism of segregation in the center of the slab
During the solidification process of molten steel, the solute elements in the solid-liquid phase are redistributed, and the solute elements of the unsolidified molten steel are enriched in the rapidly growing columnar intercrystals. As a result, the solute elements in the slab are unevenly distributed, and the contents of sulfur, phosphorus and carbon elements in the center of the slab are significantly higher than in other locations.
Solidified crystal bridge
According to this theory, the columnar crystals of the steel billet in the solidification structure develop relatively quickly. As the high-carbon steel casting process proceeds, the heat conduction of the billet during the solidification process is uneven, causing the columnar crystals to grow at different rates. A bridge is formed in the center of the cast slab, and the upper molten steel cannot replenish the lower molten steel in time, causing the lower molten steel to shrink and form central segregation, porosity or shrinkage holes.
During the solidification process of molten steel, the solute elements between the solid and liquid phases are redistributed. The columnar crystals of the billet in the solidified structure develop rapidly, and the solutes in the unsolidified molten steel are enriched. As the end of the liquid phase cavity and the bulge of the billet shrink and solidify, a large suction force is exerted on the center of the billet. This suction force causes the solute elements enriched in the unsolidified molten steel to be sucked into the center, causing the central elements to segregate.
Enrichment and precipitation of solute elements
In the casting slab during the crystallization process, solute elements move and dissolve in equilibrium at the liquid phase boundary, such as phosphorus, sulfur, manganese, and carbon. The solute precipitated from the columnar crystals enters the unsolidified molten steel. As the cast slab continues to crystallize, elements that are prone to segregation will be enriched at the ends or in the center of the cast slab during solidification, resulting in central segregation.
Experiment and process parameter adjustment
Experimental methods and results
The corresponding samples were intercepted according to the billet experiment. The thickness of the billet was about 90mm according to the billet casting process of a certain steel company. After rough machining, it was acid etched. Allocate low-magnification inspection samples according to different positions, and intercept the positions as shown in Figure 1. The samples after cold acid etching were cut across the width.
Figure 1 Sample interception position
When the superheat of the molten steel is low, the segregation of the slab can be significantly reduced and the probability of the occurrence of equiaxed crystals can be increased. In the production process, if the temperature of the superheat of the molten steel is close to the liquidus temperature, the problem of blocking of the molten steel may occur. Therefore, the superheat of the molten steel should be controlled at about 20° according to the actual production process. For brand-new bags, new slag wire bags, etc., the barbecue system must be strictly implemented to avoid the generation of black bags.
The literature points out that the solidification structure of steel materials is closely related to the carbon element content. The carbon element content has a great influence on the growth ratio of equiaxed crystals and the growth rate of columnar crystals. Therefore, it must play a decisive role in the central segregation of the slab. Research shows that under the same other continuous casting processes, when steel with a carbon content of 0.3%, 0.1% and 0.5% is poured, the center porosity, center segregation index and columnar crystal length of the cast slab increase in sequence. Therefore, in the converter production process, improving the hit rate of carbon elements is the key to controlling the carbon content in steel.
In molten steel, phosphorus and sulfur are elements that are relatively easy to segregate. The distribution and content of these elements in the molten steel will have a certain impact on the center looseness and segregation of the cast slab. Under normal circumstances, clean steel smelting technology is basically used to reduce the sulfur and phosphorus content in the molten steel to improve the purity of the molten steel and in turn prevent the occurrence of segregation.
For the experiment, the author investigated the quality of the cast slab samples in the experiment. It can be seen that the cleanliness of the molten steel is low, the segregation level of the acid corrosion samples is higher than 1.0 (Figure 2), and the hot acid corrosion samples tested at low magnification all have Severe cracks.
Figure 2 Low magnification test sample
In addition, inclusion analysis was conducted on the low-magnification inspection samples in this test, as shown in Table 2. It can be seen that there are more inclusions with a particle size higher than 20 μm, basically about 50%. After analysis, it can be seen that the reason why the cleanliness of this batch of molten steel is not high is mainly due to the problem of back-packing in the middle of the molten steel and the long time of smelting. During the solidification process of molten steel, the inclusions at the dendrite boundaries are enriched, which weakens the grain boundary strength and reduces the resistance of the cast slab. As the continuous casting process proceeds, cracks are prone to occur in the middle area.
The residence time of the molten steel in the crystallizer decreases with the increase of the pulling speed. The removal of the superheat of the molten steel can extend the columnar crystal area and extend the liquid phase cavity, eventually forming a loose center and a solidification bridge. The surface temperature of the cast slab decreases with the increase of cooling intensity in the secondary cooling zone, which increases the temperature gradient of the cross section and easily forms columnar crystals with a larger width. Reducing the cooling intensity of secondary cooling will lead to the expansion of equiaxed grains and the shrinkage of columnar grains. Matching and optimizing the secondary cooling ratio and drawing speed of the sample slab can effectively reduce the occurrence of central segregation.
Figure 3 Crack morphology in triangular zone
During the continuous casting process of the sample slab, the secondary cooling specific water volume was increased by 0.04kg/t, and the converter’s pulling speed was reduced to 0.06m/min. The purpose is to increase the cooling intensity of the molten steel to avoid unwanted reheating and ensure the internal quality of the cast slab. The optimization results show that the center segregation rating of the sample is reduced to 0.5 during low-magnification inspection, and the occurrence of cracks in the center is basically eliminated. The porosity results meet the standard, and only some minor cracks exist in the triangular area, as shown in Figure 3.
According to the theories of solidified crystal bridges, hole pumping, enrichment and solute precipitation, segregation in the center of the slab is caused by a variety of factors. Iron and steel enterprises must go through long-term continuous casting production practice before they can prepare reasonable continuous casting process parameters, improve central segregation, and improve the life and quality of steel materials. Reducing the superheat of molten steel can effectively reduce the occurrence of central segregation and increase the probability of the occurrence of equiaxed crystals in the slab.
In the actual process, it is suitable to control the superheat of molten steel at about 20°, which will play a certain role in improving the occurrence of central segregation. Another key point in improving the central segregation of the slab is the purity of the molten steel. In the actual continuous casting process, the smelting cycle should be shortened as much as possible to avoid the precipitation of phosphorus and sulfur elements from weakening the strength of the columnar crystals. According to this test, increasing the secondary cooling specific water volume by 0.04kg/t and reducing the converter pulling speed to 0.06m/min are the best processes and can effectively avoid the occurrence of central segregation.