The final quality of the product is determined by the quality of the supplied ingot. In general, the so-called continuous casting ingot quality refers to the severity of the ingot defects allowed to obtain qualified products. Its meaning is:
Ingot destruction degree (number, shape, distribution, gas, etc. of inclusions).
Ingot surface defects (cracks, slag inclusions, pores, etc.).
Ingot internal defects (cracks, segregation, inclusions, etc.)
Ingot thoroughness is mainly determined by the process of molten steel entering the former. Since the molten steel is destined to be “cleaner”, it is necessary to work hard on each process before the molten steel enters the former, such as smelting and alloying process control, selection of appropriate refining outside the furnace, tundish metallurgy, protective pouring, etc. The surface defects of the casting are mainly determined by the solidification process of the molten steel in the former. It is related to the shell formation before the crystallizer billet, and the liquid level of the former must control the parameters affecting the surface quality within the value to produce defect-free ingots, which is the premise of hot delivery and direct casting. The internal defects of the billet are mainly determined by the cooling of the billet during the secondary cooling zone and the billet support system. Reasonable secondary cooling water distribution, centering of the support rollers, and prevention of billet bulging are the front distributions to improve the internal quality of the billet.
Therefore, in order to obtain good billet quality, different process technologies can be used in different stages of continuous casting such as ladle, tundish, melter and secondary cooling zone according to the different requirements of steel grades and products to effectively control the billet quality.
What measures can be taken to improve the cleanliness of continuous casting steel?
Cleanliness refers to the number, shape and distribution of non-metallic inclusions in steel. To reduce the non-metallic inclusions in steel to the required level according to the steel grade and product quality, the following five aspects should be taken:
Reducing the [O] content in steel
Preventing the reaction of molten steel with air
Reducing the reaction of molten steel with refractory materials
Reducing the slag from being drawn into the molten steel
Improving the fluidity of molten steel to promote the floating of inclusions in molten steel
From the perspective of process operation, the following measures should be taken:
(1) Slag-free steelmaking: The converter uses slag balls (or slag cones) to prevent a large amount of slag from falling into the ladle.
(2) Ladle refining: Select appropriate refining methods according to the steel grade to uniform temperature, pollutant composition, reduce oxygen content, remove gas inclusions, etc.
(3) Oxidation-free pouring: After the molten steel is refined in the ladle, the total oxygen content in the steel drops from 130ppm to below 20ppm. If the ladle-tundish injection is not protected or poorly protected, the total oxygen content in the molten steel in the tundish will rise to the range of 60-100 ppm, returning to a level close to that before refining outside the furnace, making the refining effect outside the furnace go to waste.
(4) Tundish metallurgy: The use of large capacity in the tundish, adding retaining walls and dams are effective measures to promote the floating of inclusions.
(5) Immersed nozzle + protective slag: The protective slag should be able to fully absorb inclusions. The material, shape and insertion depth of the immersing nozzle should be conducive to the floating separation of inclusions.
The internal quality of the ingot refers to low-multiple structure, component segregation, center segregation, center segregation and cracks.
After the ingot is hot-processed, some defects disappear, some are deformed, and some remain intact, which brings different degrees of dissolution to the product performance. The generation of internal defects in the ingot involves the heat conduction, mass transfer and dangerous effects of the ingot, and the generation is complex. But in general, the internal defects of the ingot are controlled by the solidification process of the ingot in the secondary cooling zone. The measures to improve the internal quality of the ingot are:
(1) Casting structure: Control to expand the equiaxed crystal area in the center of the casting and inhibit the growth of columnar crystals. This can reduce center segregation and center porosity. Therefore, the use of technologies such as low superheat pouring of molten steel and electromagnetic stirring are effective methods to expand the equiaxed crystal area.
(2) Reasonable secondary cooling system: The surface temperature of the casting is evenly distributed in the secondary cooling zone, and the surface temperature at the straightening point is greater than 900℃. Pay attention to straightening without liquid core. For this purpose, computer control of secondary cooling water distribution, air-water spray cooling, etc. is adopted.
(3) Control of the stress and deformation of the casting in the secondary cooling zone: The stress and deformation of the solidified shell in the secondary cooling zone are the root cause of cracks. For this purpose, multi-point bending straightening, accurate arc alignment, roller gap centering, compression casting technology, etc. are adopted.
(4) Control the flow of molten steel in key points: to promote the floating of inclusions and improve their distribution. For example, the former adopts electromagnetic stirring technology and improves the design of immersion nozzles.
What types of continuous casting defects are there?
Continuous casting defects can be divided into the following three categories:
(1) Surface defects: including surface cracks, surface transverse cracks, subcutaneous slag inclusions, subcutaneous pores, surface supplements, etc.;
(2) Internal defects: including middle cracks, subcutaneous cracks, pressure cracks, inclusions, center cracks and eccentric analysis, etc.;
(3) Shape defects: square billet rhombus deformation (de-squareness) and slab bulging.
What are the causes of longitudinal cracks on the surface of continuous casting and how to prevent them?
Longitudinal cracks on the surface of continuous casting billets will affect the quality of rolled products. For example, a longitudinal crack of 300mm long and 2.5mm deep will leave a 1125mm delamination defect on rolling. If the longitudinal crack is serious, it will cause leakage and scrap.
Defects in the formation of longitudinal cracks on the surface of castings: From metallographic examination, it can be seen that MnS is analyzed locally at the cracks, the cracks at the surface extend along the primary dendrites, and the deep layers extend along the austenite grain boundaries. From the low-power inspection of the cross-section of the slab, it can be seen that for steel with C=0.10%~0.16%, the longitudinal cracks originate from the uneven thickness of the solidified shell chill layer, and the thin defects in the chill layer have a crack depth of 0.5~2.5mm. This indicates that the thickness of the primary shell of the surface crack in the meniscus area of the crystallizer is uneven, and the thin tensile force acting on the shell exceeds the high-temperature allowable strength and strain of the steel, resulting in stress concentration at the weak point of the shell, leading to longitudinal cracks, and continuing to expand in the secondary cooling zone after the crystallizer. From a theoretical analysis, the stresses acting on the shell are: thermal stress σf formed by the uneven temperature of the solidified shell; bulging force op generated by the lateral width contraction of the slab under the action of the static pressure of the molten steel; friction force σf generated by the uneven contact between the shell and the copper plate; air gaps are generated, and the wide surface model shrinks and is constrained by the narrow surface, causing the shell to bear bending strain 0b.
Generally speaking, 0t and ob are the largest in the center area of the wide surface of the slab. Experiments show that the incidence of longitudinal cracks in slabs with a width of 7 and a width of 1890mm is twice that of slabs with a width of 1520mm.
The chemical composition of steel, especially the carbon content, has an increasingly greater impact on the longitudinal cracks of continuous casting slabs. A large number of production statistics show that when the C content in steel is 0.12~0.17%, the longitudinal cracking of the continuous casting billet is the most serious. As the pulling speed increases, the longitudinal cracking tends to be safe.
The melting speed of the protective slag is too fast or too slow, making the slag layer too thick or too thin, or the slag viscosity is not appropriate, and the slag thickness between the billet shell and the copper plate is uneven, resulting in uneven melting of the melt melter, causing local increase of ot and ob and promoting the occurrence of longitudinal cracks
The swell and increase of the casting liquid surface of the mold melter, the wider the casting slab, the more serious the cracking tendency. When the melting liquid surface appears greater than 10mm, the probability of longitudinal cracking is 30%; when the insertion depth of the immersion nozzle changes more than 40mm, the probability of longitudinal cracking is 20%. The causes of longitudinal cracking can be summarized as follows:
(1) The interaction between the nozzle and the former produces bias flow scouring centrifugation.
(2) When the C content in steel is 0.12~0.17%, the tendency of longitudinal cracking increases.
(3) Poor melting performance of protective slag, too thick or too thin liquid slag layer leads to uneven thickness of slag film, making the local solidified shell too thin. When the liquid slag layer is <10mm, the longitudinal cracks increase significantly.
(4) Fluctuation of the liquid level in the mold. When the liquid level fluctuates >10mm, the longitudinal cracks occur about 30%. (5) S+P content in steel. When S>0.02% and P>0.017% in steel, the high strength and plasticity are significantly reduced, and the longitudinal crack tendency of steel increases.
In short, the formation of longitudinal cracks is the result of the combined effect of multiple factors. However, the basic conditions for the formation of longitudinal cracks are: uneven thickness of the primary shell, local stress concentration at the weak part of the shell, segregation along the dendrite elements (C, Mn, S, P) and the same part, and the opening and expansion of cracks are always in the place where the segregation is serious. The indicators to prevent longitudinal cracking are:
(1) The nozzle and the crystallizer should be aligned to prevent the steel flow from eroding the billet shell;
(2) The wave surface fluctuation of the melter is stable at ≤±10mm. When the wave surface fluctuation increases from 5mm to +20mm, the longitudinal crack index increases from 0 to 2.0;
(3) Appropriate insertion depth of the immersion nozzle.
(4) Appropriate taper of the former.
(5) The arc of the melter and the upper part of the secondary cooling zone should be accurate.
(6) Appropriate protective slag performance.
(7) Appropriate superheat of molten steel: When the superheat of molten steel increases by 10℃, the high-temperature molten steel flowing in the melter will eat up 2mm of the solidification shell;
What are the causes of transverse cracks on the surface of continuous casting billets and how to prevent them?
Transverse cracks are located at the trough of the vibration mark on the inner arc surface of the billet, which is usually hidden and invisible. After pickling inspection, it was pointed out that the crack depth was 7mm and the width was 0.2mm. The crack was located in the ferrite network area, and the network area happened to be the primary austenite grain boundary. There were AIN or Nb (CN) particles precipitated at the austenite grain boundary. When the particles at the austenite grain boundary are coarse and sparsely distributed, the scrap generated by the transverse cracks of the billet increases. Therefore, controlling the coarsening of the particles precipitated at the austenite grain boundary or controlling the particles (such as AIN, TIN) and MnS) not to be analyzed at the grain boundary can reduce the load on the cracks.
Causes of transverse cracks:
(1) Too deep vibration marks are the origin of transverse cracks.
(2) The increase in the content of A1 and Nb in the steel pushes the particles (AIN) in the grain boundary precipitation, inducing transverse cracks.
(3) Straightening at the brittle temperature of the ingot at 900-700℃.
(4) The secondary cooling is too strong.
Measures to prevent transverse cracks:
(1) The use of high frequency (200-400 times/min) and small amplitude (2-4mm) in the melter is an effective way to reduce the depth of vibration marks.
Vibration marks and transverse cracks coexist, and the depth of transverse cracks and vibration marks should be reduced. The larger the amplitude, the deeper the vibration mark; the longer the negative slip time, the deeper the vibration mark; the lower the vibration frequency, the deeper the vibration mark. The support grains deep in the vibration mark are coarse and the solute elements are enriched. When it becomes a casting, it is easy to produce cracks.
(2) Weak cooling is used in the secondary cooling zone, and the surface temperature of the casting (900℃~1050℃) during straightening is the particle precipitation temperature or multi-→temperature transformation to ignore the low ductility zone
(3) Reduce the content of S, O, and N in the steel, or add T, Zr, and Ca to inhibit the precipitation of CN compounds and clarifiers at the grain boundary, or make the CN compound particles coarser to improve the hot ductility of austenite grains;
(4) Reduce the ripples on the liquid surface of the casting machine and use good protective slag with low surface tension and lubrication performance; (5) Refine the austenite grains. Transverse cracks are often distributed along the coarse austenite grain boundaries under the adjacent layer of the ingot. The austenite grains are refined through secondary cooling to reduce the structure of the cracked grains.
Causes of transverse cracks:
(1) Too deep vibration marks are the origin of transverse cracks.
(2) The increase in the content of A1 and Nb in the steel pushes the particles (AIN) in the grain boundary precipitation to induce transverse cracks.
(3) Straightening at the brittle temperature of the ingot at 900-700C.
(4) The secondary cooling is too strong.
Measures to prevent transverse cracks:
(1) The use of high frequency (200-400 times/min) and small amplitude (2-4mm) in the mold-melting machine is an effective way to reduce the depth of vibration marks; vibration marks and transverse cracks coexist, and to reduce transverse cracks, the depth of vibration marks must be reduced. The larger the amplitude, the deeper the vibration mark; the longer the negative slip, the deeper the vibration mark; the lower the vibration frequency, the deeper the vibration mark. Vibration marks are used to detect coarse grains deep inside, and solute elements are enriched. When the casting is affected by defects, it becomes the origin of cracks.
(2) The second cooling zone adopts weak cooling, and the surface temperature of the casting (900℃~1050℃) during straightening is the particle precipitation temperature or more → temperature transition to ignore the low ductility zone.
(3) Reduce the content of S, O, and N in the steel, or add T, Zr, and Ca to inhibit the analysis of CN compounds and compounds at the grain boundary, or make the CN compound particles coarser to improve the hot ductility of austenite grains;
(4) Reduce the fluctuation of the liquid level of the product maker, and use protective slag with low surface tension and good lubrication performance; (5) Refine the austenite grains. Transverse cracks are often distributed along the coarse austenite grain boundaries under the surface layer of the ingot. The austenite grains are refined through secondary cooling to reduce the cracks.
What are the causes and preventive measures for the longitudinal cracks at the corners of continuous casting billets?
The longitudinal cracks at the corners may be located near the edge of the wide face and the narrow face, some are 10~15mm away from the edge, and some are located exactly on the edge. In severe cases, it will cause steel leakage. The causes are: For square billets: It may be that the thickness of the water seam along the height of the former is uneven, causing the corner of the former to be cold. For slabs, it may be due to: improper support of the narrow face causing the narrow face to bulge. The narrow face has a bulge of 6~12mm along the edge of the corner, resulting in steel leakage. (2) Improper taper. (3) Insufficient cooling water on the narrow face.
Improvement method:
For square billets (1) Control the mold shape to prevent deformation. (2) Appropriate fillet radius. (3) When assembling the mold, keep the thickness of the cooling water seam consistent to ensure uniform cooling. (4) Appropriate amount of cooling water. (5) Align the nozzle with the mold and do not deviate.
What are the causes and preventive measures for the transverse cracks at the corners of continuous casting?
This is a small transverse crack located at the corner of the billet. The possible causes are: (1) The taper of the former is manual. (2) The surface of the former is scratched. (3) The arc of the former outlet and the zero section is not aligned.
Improvement method: adjust the taper of the former, strictly align the arc, and adjust the secondary cooling so that the temperature of the corner of the billet during correction cannot be less than 800℃.
How are the subcutaneous bubbles of the continuous casting billet formed?
Below the casting billet, there are large bubbles with a diameter and length of more than 1 mm and 10 mm respectively, growing in the direction of columnar crystals. These bubbles are called surface bubbles if they are exposed to the outside, and subcutaneous bubbles if they are not exposed. They are smaller than bubbles and are densely packed with small holes and subcutaneous pinholes.
In the heating furnace, the surface bubbles of the billet or the inner surface of the subcutaneous bubbles are oxidized to form a decarburized layer, which cannot be welded after rolling and forms surface defects. Shallowly buried bubbles can be removed by grinding wheels, wind shovels and flame cleaning. Deeply buried bubbles are difficult to find and will cause cracks in the product.
Insufficient deoxidation of molten steel is an important reason for the formation of bubbles. For example, enhanced deoxidation can be used to reduce the oxygen content in steel and make the aluminum content in molten steel reach 0.01~0.015% to eliminate bubbles. In addition, the bubble content in molten steel (especially hydrogen) is also an important reason for the formation of bubbles. The materials added to the molten steel should be dry, the ladle and the intermediate ladle should be baked according to the standard, and attention should be paid to the use of protective pouring for running water. These measures have a significant effect on reducing bubbles.
What is the surface folding defect of continuous casting billet?
There are transverse folding marks on the surface of the casting, and in severe cases, there are transverse cracks. Reasons:
(1) The suspension inside the mold causes solidification marks on the shell. Due to the strong cooling of the mold, the molten steel leaked at the tearing point immediately forms solidification marks on the surface
(2) Improper adjustment of the vibration parameters of the molding machine;
(3) The arc of the mold outlet and the secondary cooling section is poor;
(4) The crystallizer is poorly lubricated, and the shell and copper wall soap.
