What is the meaning of continuous casting billet quality?
The quality of the final product is determined by the quality of the billet supplied. In a broad sense, the so-called continuous casting billet quality refers to the severity of the billet defects allowed to obtain qualified products. Its meaning is:
Billet purity (number, shape, distribution, gas, etc. of inclusions).
Billet surface defects (cracks, slag inclusions, pores, etc.).
Billet internal defects (cracks, segregation, inclusions, etc.)
Billet purity is mainly determined by the treatment process before the molten steel enters the copper mould tube. In other words, to make the molten steel “cleaner”, it is necessary to work hard on each process before the molten steel enters the copper mould tube, such as smelting and alloying process control, selection of appropriate refining outside the furnace, tundish metallurgy, protective pouring, etc. The surface defects of the billet are mainly determined by the solidification process of the molten steel in the crystallizer. It is related to the formation of the crystallizer billet shell, the fluctuation of the crystallizer liquid level, the design of the submerged nozzle, and the performance of the protective slag. It is necessary to control all parameters affecting the surface quality within the target value to produce defect-free ingots, which is the premise of hot delivery and direct rolling. The internal defects of the ingot are mainly determined by the cooling process of the ingot in the secondary cooling zone and the ingot support system. Reasonable secondary cooling water distribution, centering of support rollers, and prevention of ingot bulging are the prerequisites for improving the internal quality of the ingot.
Therefore, in order to obtain good ingot quality, different process technologies can be used in different stages of continuous casting such as ladle, tundish, crystallizer and secondary cooling zone according to the different requirements of steel grades and products to effectively control the ingot quality.
What are the measures to improve the purity of continuous casting steel grades?
Purity refers to the number, shape and distribution of non-metallic inclusions in steel. To reduce the inclusions in steel to the required level according to the steel grade and product quality, the following five aspects should be taken into consideration:
Reduce the [O] content in steel as much as possible
Prevent the interaction between molten steel and air
Reduce the interaction between molten steel and refractory materials
Reduce the slag from being drawn into the molten steel
Improve the fluidity of molten steel and promote the floating of inclusions in molten steel
From the perspective of process operation, the following measures should be taken:
(1) Slag-free steel tapping: The converter uses slag blocking balls (or slag blocking 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 uniformly adjust the temperature, fine-tune the composition, reduce the oxygen content, remove gas inclusions, etc.
(3) Non-oxidizing pouring: After the molten steel is refining in the ladle, the total oxygen content in the steel can be reduced from 130ppm to below 20ppm. If the ladle-tundish injection flow is not protected or poorly protected, the total oxygen content in the molten steel in the tundish will rise to the range of 60~100ppm, returning to a level close to that before refining outside the furnace, making the effect of refining outside the furnace completely lost.
(4) Tundish metallurgy: The tundish adopts a large capacity, and 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 immersed nozzle should be conducive to the floating and separation of inclusions.
What is the heavy skin defect on the surface of the continuous casting billet?
The lateral discontinuity on the surface of the billet and the obvious traces of incomplete welding are called heavy skin. Causes:
(1) The injection of the copper mould tube is suddenly stopped, or the billet drawing is stopped instantly. If the stop time is too long, obvious reconnection will be formed on the surface of the billet;
(2) The molten steel is too viscous, the temperature is too low, the nozzle is blocked, the injection flow is deviated, etc., which may cause heavy skin.
What measures should be taken to improve the internal quality of continuous casting billets?
The internal quality of billets refers to low-multiple structure, component segregation, central porosity, central segregation and cracks.
After hot processing, some defects of the billet may disappear, some may be deformed, and some may remain intact, which may cause different degrees of harm to product performance. The generation of internal defects of the billet involves the effects of solidification heat transfer, mass transfer and stress of the billet, and the generation mechanism is extremely complex. But in general, the internal defects of the billet are controlled by the solidification process of the billet in the secondary cooling zone. The measures to improve the internal quality of the billet are:
Control the billet structure: The first thing is to expand the equiaxed crystal area in the center of the billet and inhibit the growth of columnar crystals. This can reduce central segregation and central porosity. For this purpose, the measures to improve the internal quality of the billet are:
(1) Control the billet structure: The first thing is to expand the equiaxed crystal area in the center of the billet and inhibit the growth of columnar crystals. This can reduce central segregation and central porosity. To this end, the use of low superheated steel pouring, electromagnetic stirring and other technologies are effective ways to expand the equiaxed crystal zone.
(2) Reasonable secondary cooling system: The surface temperature of the billet is evenly distributed in the secondary cooling zone, and the surface temperature at the straightening point is greater than 900℃, and liquid core straightening is avoided as much as possible. For this purpose, computer control of secondary cooling water distribution, air-water spray cooling, etc. is used.
(3) Control of the stress and deformation of the billet 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 used.
(4) Control the flow of molten steel in the liquid phase hole to promote the floating of inclusions and improve their distribution. For example, the crystallizer adopts electromagnetic stirring technology, improves the design of submerged nozzle, etc.
What are the types of continuous casting defects?
Continuous casting defects can be divided into the following three categories:
(1) Surface defects: including surface longitudinal cracks, surface transverse cracks, subcutaneous slag inclusions, subcutaneous pores, surface depressions, etc.;
(2) Internal defects: including middle cracks, subcutaneous cracks, pressure cracks, inclusions, center cracks and segregation, etc.; () Shape defects: square billet rhombus (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 the rolled plate. If the longitudinal crack is serious, it will cause leakage and scrap.
Mechanism of formation of longitudinal cracks on the surface of the casting billet: From metallographic examination, it can be seen that MnS is precipitated locally at the crack, the cracks extend along the primary dendrites at the surface, and extend along the austenite grain boundaries at depth. 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 chill layer of the solidified shell. At the weak point of the chill layer, the crack depth is 0.5~2.5mm. This shows that the thickness of the primary shell of the surface longitudinal crack is uneven in the meniscus area of
The melting speed of 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 thickness of slag flowing into the billet shell and the copper plate is uneven, resulting in uneven heat conduction of the crystallizer, causing ot and ob to increase locally and promote the occurrence of longitudinal cracks.
The fluctuation of the casting liquid level in the crystallizer increases. The wider the casting slab, the more serious the cracking tendency. When the crystallizer liquid level fluctuation is 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 nozzle and the crystallizer are not centered, resulting in biased flow to scour the solidified shell.
(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 liquid level in crystallizer. When the liquid level fluctuates >10mm, the probability of longitudinal cracks is 30%.
(5) S+P content in steel. When S>0.02% and P>0.017% in steel, the high temperature strength and plasticity of steel are significantly reduced, and the tendency of longitudinal cracks increases.
In short, the formation of longitudinal cracks is the result of the combined action 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 segregation is serious. Measures to prevent longitudinal cracks are:
(1) The water inlet and the crystallizer should be aligned to prevent the steel flow from scouring the shell;
(2) The fluctuation of the crystallizer wave surface is stable at ≤±10mm. The wave surface fluctuation increases from 5mm to +20mm, and the longitudinal crack index increases from 0 to 2.0;
(3) Appropriate immersion nozzle insertion depth.
(4) Appropriate crystallizer taper.
(5) The arc of the crystallizer and the upper part of the secondary cooling zone must be accurate.
(6) Appropriate protective slag performance.
(7) Appropriate molten steel superheat: When the molten steel superheat increases by 10℃, the high-temperature molten steel flowing in the crystallizer 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 marks on the inner arc surface of the billet and are usually hidden and invisible. Pickling inspection shows that the crack depth can reach 7mm and the width is 0.2mm. The cracks are located in the ferrite network area, which happens to be the primary austenite grain boundary. AlN or Nb (CN) particles are precipitated at the austenite grain boundary. When the particles at the austenite grain boundary are coarse and sparsely distributed, the scrap generated by transverse cracks in the billet increases. Therefore, controlling the coarsening of particles precipitated at the austenite grain boundary or controlling the particles (such as AlN, TIN) and MnS) from precipitating at the grain boundary can reduce the sensitivity to cracks.
Causes of transverse cracks:
(1) Too deep vibration marks are the origin of transverse cracks.
(2) The increase in the Al and Nb content in the steel promotes the precipitation of particles (AlN) at the grain boundary, inducing transverse cracks.
(3) The billet is straightened at the brittle temperature of 900~700C. (4) The secondary cooling is too strong.
Measures to prevent transverse cracks:
(1) Using high frequency (200-400 times/min) and small amplitude (2~4mm) in the crystallizer 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 time, the deeper the vibration mark; the lower the vibration frequency, the deeper the vibration mark. The dendrites in the deep part of the vibration mark are coarse and rich in solute elements. When the ingot is subjected to stress, it becomes the origin of cracks.
(2) Weak cooling is used in the secondary cooling zone. During straightening, the surface temperature of the ingot (900°C~1050°C) is higher than the particle precipitation temperature or higher than the y→a transformation temperature to avoid 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 C-N compounds and sulfides at the grain boundary, or make the C-N compound particles coarser to improve the hot ductility of austenite grains;
(4) Reduce the fluctuation of the liquid level in the grain setter 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 can be refined by secondary cooling to reduce the sensitivity to 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 where the wide face and the narrow face meet. Some are 10-15mm away from the edge, and some are just on the edge. In severe cases, it may cause steel leakage. The causes are: For square billets: It may be that the thickness of the water seam along the height of the crystallizer is uneven, resulting in poor cooling of the corners of the crystallizer; the crystallizer taper is too small, and the crystallizer corner radius is too small. For slabs, it may be due to: improper support of the narrow face causing bulging of the narrow face. The narrow face has a bulge of 6-12mm accompanied by longitudinal cracks at the corners, resulting in steel leakage. (2) The taper is not appropriate. (3) Insufficient cooling water on the narrow face.
Improvement methods:
(1) Control the geometry of the crystallizer to prevent deformation.
(2) Appropriate corner radius.
(3) When assembling the crystallizer, keep the thickness of the cooling water seam consistent to ensure uniform cooling.
(4) Appropriate amount of cooling water.
(5) Align the water inlet with the crystallizer and do not deviate.
What are the causes and preventive measures of transverse cracks at the corners of continuous casting?
This is a fine transverse crack located at the corners of the ingot. The possible causes are:
(1) The taper of the crystallizer is too large.
(2) The surface of the crystallizer is scratched.
(3) The arc of the crystallizer outlet and the zero section are not aligned.
Improvement method: adjust the taper of the crystallizer, strictly align the arc, and adjust the secondary cooling so that the temperature of the corner of the ingot during straightening cannot be less than 800℃.
How are subcutaneous bubbles in continuous casting formed?
Located below the surface of the ingot, 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, subcutaneous bubbles if they are not exposed, and small holes smaller than bubbles and densely packed are called subcutaneous pinholes.
In the heating furnace, the surface bubbles of the ingot 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. Shallow 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 the main cause of bubbles. If enhanced deoxidation is used to reduce the oxygen content in steel, the aluminum content in molten steel will reach 0.01~0.015%, thereby eliminating bubbles. In addition, the gas content in molten steel (especially hydrogen) is also an important cause of bubble generation. Therefore, all materials added to molten steel should be dried, the ladle and the intermediate ladle should be baked according to the standard, and the injection flow should be protected. 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 billet, and in severe cases, there are transverse cracks. Causes:
(1) The suspension in the crystallizer causes the solidified shell to tear. Due to the strong cooling of the crystallizer, the molten steel leaked at the tear immediately solidifies on the surface to form folding marks
(2) Improper adjustment of copper mould tube vibration parameters;
(3) The arc of the crystallizer outlet and the secondary cooling section is poor;
(4) The crystallizer is poorly lubricated, and the billet shell is bonded to the copper wall.
