Surface Quality Defects and Control Measures of Continuous Casting Slabs

Through the analysis of the causes of different surface defects, this paper puts forward suggestions on surface quality control from the aspects of mold flux performance optimization, crystallizer liquid level control, vibration and secondary cooling.

Key words: continuous casting slab, surface defect, control measures

Foreword

The quality of the surface quality of the continuous casting slab directly determines the quality of the steel surface quality, and it is also the prerequisite for whether the casting slab can be hot-sent and directly rolled, so it not only affects the product quality, but also affects the production efficiency and steel yield . The reasons for the surface defects of slabs are extremely complicated, but mainly due to the control of the solidification process of molten steel in the mold and the influence of secondary cooling. The common surface defects of slabs mainly include surface longitudinal cracks, surface transverse cracks, and corner transverse cracks. , pores, abnormal vibration marks, slag inclusions, etc. In addition, there are secondary solidification, double pouring (joint marks or reconnection), abrasions, subcutaneous slag inclusions and subcutaneous air bubbles, etc., especially subcutaneous slag inclusions and air bubbles cannot be directly inspected And judgment, the harm to the quality of steel is greater. Surface defects are mainly produced during the formation and growth of the molten steel shell in the mold, and are related to casting temperature, billet casting speed, mold flux performance, submerged nozzle structure, mold copper tube inner cavity quality, water gap uniformity, crystallization It is related to factors such as the vibration of the mold and the stability of the liquid level of the crystallizer. This paper analyzes the causes of each defect and proposes control measures.

Causes of surface defects and analysis of measures

The main defects on the surface of the slab are shown in Table 1.

Analysis of causes and preventive measures for longitudinal cracks on the surface of slab

Cause Analysis of Longitudinal Cracks on Slab Surface

The longitudinal cracks on the cast surface are caused by the uneven growth of the primary billet shell. Under the action of the inward contraction force of the billet shell in the area 50-100mm below the meniscus, a depression toward the center occurs at the weak point of the billet shell, and longitudinal cracks occur in severe cases. Cracks, even breakouts. Longitudinal cracks are firstly closely related to steel types, especially peritectic steels with a carbon content of 0.09-0.14% are the most serious. When the steel type is determined, the factors that affect the uniform cooling of the slab in the mold and the thickness of the slab shell will lead to the generation of longitudinal cracks. The influencing factors include the viscosity and heat transfer performance of mold slag, the vibration parameters of the mold and the stability of vibration. , crystallizer installation accuracy and taper, nozzle installation accuracy, molten steel composition (including residual elements Cu and As), superheat, castability, etc. In addition, the tundish stopper blowing Ar is too large and the frequent changes in punching operation and casting speed Poor operations that affect liquid level fluctuations can also lead to longitudinal cracks. If the cooling of the foot roll section is uneven, the centering (or arc alignment) of the casting machine is poor, and the opening of the clamping roll is too large, the slab will bulge, which will further aggravate the generation of longitudinal cracks or internal cracks in the sag of the slab.

Measures to prevent longitudinal cracks on the surface of the slab

The key to preventing longitudinal cracks is to ensure the uniform growth of the shell of the meniscus and to suppress the generation of depressions. The specific measures are to control molten steel Mn/S≥25, preferably greater than 30, S+P+As≤0.075%; improve the cleanliness and castability of molten steel, adopt full protection to prevent secondary oxidation; optimize vibration parameters and crystallizer liquid level Automatic control to reduce liquid level fluctuations and ensure constant casting speed and steady pouring; according to the difference in solidification and shrinkage of steel types, select the appropriate mold slag viscosity, melting point and melting speed to increase and stabilize the liquid slag layer to ensure the stability and stability of the initial solidification. Uniform cooling of the crystallizer; reduce the heat flux density of the crystallizer and the weak cooling of the crystallizer; align the nozzle and prevent the stopper rod from blowing too much Ar and the impact of the “punch rod” operation on the liquid level; choose a reasonable primary cooling and secondary cooling system to ensure Cool evenly.

Causes of surface transverse cracks and analysis of preventive measures

The transverse cracks on the surface all appear at the vibration valleys of the slab, and there are more inner arcs than outer arcs, and AlN precipitation often occurs at the transverse cracks.

Cause analysis of surface transverse cracks

The uneven heat flux of the mold and too deep vibration marks are the origin of transverse cracks. The precipitation of AlN at the grain boundary in the steel, the straightening of the slab in the embrittlement zone at 700-900 °C and the strong secondary cooling aggravate the formation of transverse cracks. . Therefore, the main reason for the occurrence of transverse cracks is that the vibration marks of the slab are too deep. The elements that easily form nitrides such as Al, Nb, Ti, and B in the steel are the internal causes of transverse cracks. The straightening temperature is too low or straightened in the brittle zone. is the external cause of transverse cracks. In addition, mold powder, mold taper, excessive vibration amplitude, etc. can also easily lead to the generation of transverse cracks.

 Measures to prevent surface transverse cracks

On the basis of ensuring the installation accuracy and uniform cooling of the crystallizer water jacket and copper pipes, the main measure to prevent surface cracks is to use low surface tension and good lubricating mold powder to ensure uniform cooling of the mold; use high frequency (200 ~ 400 Vibration times/min) with small amplitude (2-4mm); automatic control of the liquid level is adopted to ensure the stability of the liquid level of the crystallizer; the secondary cooling adopts a uniform weak cooling system to avoid repeated rises in the surface temperature of the slab and increase the temperature entering the tension leveler , so that the surface temperature of the slab during straightening is higher than the particle precipitation temperature or the γ→α transition temperature to avoid the low ductility zone; reduce the content of S, O, and N in the steel, or add Ti, Zr, and Ca to suppress C-N compounds and The sulfide precipitates at the grain boundary or makes the C-N material point thicker to improve the hot ductility of the austenite grain.

Analysis of causes and preventive measures for transverse cracks at corners

Analysis of the causes of transverse cracks at corners

The root cause of the transverse cracks at the corner of the slab is the stress concentration at the trough of the vibration mark. In the case of uneven cooling of the mold, due to the excessive crystallization taper, scratches on the surface of the mold, and the uneven alignment between the outlet of the mold and the zero section Accelerate the formation and expansion of cracks when the tensile stress exceeds the high-temperature allowable strength of the steel, resulting in corner transverse cracks.

 Measures to prevent transverse cracks at corners

First of all, within the allowable range of steel composition, optimize the composition control range and reduce the content of aluminum, sulfur, nitrogen, etc., increase the temperature of the low plasticity zone of the slab, and reduce the crack sensitivity of the slab; optimize the performance of the mold slag, and appropriately increase the crystallization of the mold slag Temperature, reduce viscosity, make mold powder melt and mold cooling more uniform, so as to reduce the depth of vibration marks at the corner and achieve good lubrication performance; choose the appropriate mold cone angle, ensure the quality of the mold copper tube, and strictly align the arc Reduce the resistance of the slab; reduce the secondary cooling strength, optimize the distribution of the secondary cooling section and the layout of the nozzle, so that the temperature of the corner of the slab during straightening can avoid the brittle temperature; improve the stability of the continuous casting equipment, on the basis of stabilizing the molten steel temperature , implement constant pulling speed, stable liquid level pouring.

Analysis of the causes and preventive measures of star-shaped and network-shaped cracks on the surface of the slab

Usually, the surface star cracks and network cracks are difficult to find when the slab is covered with oxide scale, and they appear after technical treatment. They are often small intergranular cracks grouped together, or star-shaped or It is distributed in a net shape, and some are also called cracks, and its depth is about 3mm. When straightening, it may expand into transverse cracks, which are cracked along the grain boundaries.

Cause analysis of star and network cracks on slab surface

The main reason for star cracks is [2]: the high content of harmful elements (P, S, Cu and “five harmful elements”, etc.) leads to the precipitation of low melting point compounds along the grain boundaries, and the presence of grain boundaries on the shell surface (Fe, Mn ) S—O and five harmful elements have poor high-temperature strength. When the friction force exceeds the high-temperature strength of the billet shell, star-shaped cracks are prone to occur. When the chromium plating layer of the mold copper tube falls off due to poor quality or other reasons or the crystallization temperature or crystallization rate of the mold slag film is high (usually caused by high alkalinity), the thermal resistance of the heat transfer from the billet shell to the copper tube Larger, the solidification of the billet shell is slow, and the thickness is thinner. Mold slag has poor lubricating properties, and a large friction force is generated between the inner surfaces of the middle and lower parts of the crystallizer, which is caused by the coarse grains on the surface of the slowly cooled billet shell and the low strength. After the chromium plating layer of the crystallizer is damaged, the friction force causes the copper plate to be partially worn by the steel slab, and the copper shavings adhere to the surface of the slab and penetrate into the grain boundary, resulting in copper cracks at the grain boundary. The cooling system in the molten steel solidification process is not suitable, which leads to the straightening temperature in the low plasticity zone of the steel grade. Therefore, for carbon steel and alloy steel, it is required that the surface temperature of the billet before the straightening point should avoid the “low plasticity zone”. Because at 900°C to 700°C, the phase transformation of γ→α and the precipitation of AlN at the grain boundary occur in the steel, and the inner arc surface of the slab generates tension during straightening, and the larger the specification, the greater the tension , slab star cracks are prone to occur.

 Measures to prevent star cracks

According to the analysis of the causes of star-shaped cracks, the main measures to prevent star-shaped cracks are to control the harmful and residual element content of steel; to maintain the quality of mold copper tubes, especially to prevent the chromium plating layer from falling off and the high-temperature billet surface from absorbing Cu Cause cracks; control the straightening temperature of the billet to prevent straightening in the low plasticity temperature zone.

Analysis of causes and preventive measures for continuous casting slab vibration marks

The vibration marks are caused by the relative movement between the solidified slab shell and the mold caused by the vibration of the mold in the continuous casting process, and the overflow of molten steel at the top of the meniscus. If the vibration marks are too large or abnormal, it will lead to horizontal cracks, horizontal cracks at corners and straightening cracks, etc. If there are many impurities in the continuous casting slab, it will also cause network cracks and even steel penetration. . Therefore, controlling the depth and uniform distribution of vibration marks is very important to the quality of the slab.

Analysis of factors affecting the depth of vibration marks

Vibration parameters such as amplitude, frequency, negative slip time and vibration mode are the key factors affecting the depth of vibration marks; the viscosity, thermal insulation performance, surface performance and consumption of mold flux and the solidification characteristics of steel all have important influences on the depth of vibration marks, especially It is when the carbon content in the steel and the Ni/Cr ratio in the steel have the most prominent influence. When the carbon content in the steel is about 0.1%, Ni/Cr≈0.55, the vibration marks on the surface of the slab are the deepest.

Measures to reduce the depth of vibration marks

Using small amplitude (s), high frequency (f) and reducing negative slip time (tN) can effectively reduce the depth of vibration marks; using non-sinusoidal vibration can reduce the depth of vibration marks, because non-sinusoidal vibration has negative slip The time tN is shorter than that of sinusoidal vibration; the use of mold slag with low slag consumption and high viscosity can make the depth of vibration marks shallower; the use of mold slag with good thermal insulation performance and can increase the radius of meniscus can reduce the depth of vibration marks; improve stainless steel, The superheat of molten steel, especially stainless steel containing titanium and aluminum, is effective in reducing the depth of vibration marks on the steel surface; increasing the temperature difference between the cooling water entering and leaving the crystallizer is also beneficial to reducing the depth of vibration marks.

 Analysis of causes and preventive measures for surface slag inclusions

Cause analysis of surface slag inclusions

The slag inclusion on the surface of the slab is caused by mold slag or scum being involved in the solidification shell due to the fluctuation of the liquid level of the mold. In the case of open casting, it is related to the content of Mn/Si or aluminum, and in the case of protected casting, it is related to the stability of the liquid level. , molten steel flow field, mold slag performance, etc. The main components of slag inclusions are high-temperature oxides, namely FeO, MnO, SiO2 and Al2O3, which are formed by mold flux, and the slag circle is the cause of this oxide accumulation zone.

Measures to prevent surface slag inclusion

The main measures to prevent surface slag inclusion are as follows: ensure the crystallizer liquid level is stable, and the liquid level fluctuation is less than 3mm; the insertion depth of the submerged nozzle should be 125±25mm; the inclination angle of the submerged nozzle outlet should be such that the outlet stream does not stir the meniscus slag The surface is the principle; control the appropriate argon blowing amount of the tundish plug rod to prevent the bubbles from floating up and enhance the agitation of the steel slag interface; suitable mold slag viscosity, Al2O3 content and liquid slag layer thickness; liquid slag layer at 1/4 width of the crystallizer 8~ 12mm can prevent unmelted slag from being involved in the billet shell.

Analysis of the causes of stomata and preventive measures

After pickling, the surface of the slab is randomly distributed with tiny round holes with a diameter of about 1mm and a depth of about 2mm. Its formation mechanism is that the higher gas content dissolved in molten steel drops sharply with the decrease of molten steel temperature, resulting in gas overflow and remaining as pores.

Causes of porosity

The main causes of pores are pinholes caused by CO bubbles caused by poor deoxidation; external gases (air, protective gas); alloys, slagging materials, large and medium-sized covering agents, and crystallizer protection added during the refining process. The slag contains a certain amount of water, and part of the water decomposes into [H], [0] and enters the molten steel; during the continuous casting process, the water cooling system of the casting machine generates water vapor, and due to the insufficient capacity of the exhaust fan, the water vapor will rise along the casting machine , water droplets condense on the lower surface of the upper cover plate of the crystallizer, flow into the crystallizer from the edge of the copper plate of the crystallizer, enter the mold powder, and even part of the water vapor enters and rises from the corner seam of the combined crystallizer, and enters the mold powder. The mold slag is wet and slagging on the meniscus, resulting in continuous casting failure and pores; the water in the refractory material, such as the refractory material such as the tundish, is not dry after baking. In the previous stage of casting (mainly continuous casting furnace) The first few billets or the first furnace), all the water vapor enters the steel to become [H], [0] atoms, the characteristic of the bubbles is that only the first few billets of the first furnace of the pouring time appear bubbles, the more to Later, there are fewer bubbles.

Measures to prevent porosity

According to the analysis of the cause of pores, the corresponding measures are: strengthen deoxidation, for low-silicon and low-aluminum quasi-boiling molten steel, the casting speed can be appropriately increased, the segregation and enrichment of [C] and [O] phase interfaces can be reduced, and pinhole bubbles can be reduced At the same time, the molten steel can quickly enter the lower area of high static pressure, which can effectively inhibit the growth of CO pinhole bubbles; adopt the whole process of protective pouring measures to reduce the amount of external gas; ensure the drying of alloy materials or take measures to ensure that The moisture content of the factory’s covering agent and mold slag should be below 0.5% to prevent moisture; increase the air volume of the secondary cooling fan to prevent the secondary cooling steam from overflowing. The baking temperature and time of the middle bag should be guaranteed.

 Main measures to control the surface defects of billets

Through the analysis of the causes and measures of the main surface defects of the slab, it can be seen that the various surface defects of the slab also influence and correlate with each other. After comprehensive analysis of various measures, the measures for controlling the surface quality of the slab are summarized as follows:

(1) Control the composition, cleanliness and residual elements of molten steel. By controlling the composition of molten steel within a certain range, the plasticity of steel in the low-temperature plastic zone or the temperature range of the low-temperature plastic zone can be increased to prevent straightening cracks; improving the cleanliness and inclusions of the steel can not only improve the fatigue life of the steel, but also prevent it from being caused by Defects such as subcutaneous inclusions and surface cracks caused by continuous casting nozzle nodules and continuous casting mold liquid level instability.

(2) By adopting measures such as full protection pouring, full baking of the tundish, moisture control of mold slag and thermal insulation agent, increasing the fan capacity of the secondary cooling room to prevent steam overflow, etc., avoid continuous casting instability or external water caused by secondary oxidation Steam causes defects such as pores.

(3) Through the use of mold powder suitable for specific steel types, precise control of the liquid level of the crystallizer, and high-frequency and small-amplitude vibration, it is ensured that a stable and uniform primary shell is formed, and subcutaneous inclusions, excessively deep vibration marks or micro-cracks on the surface are prevented. .

(4) Prevent longitudinal cracks, transverse cracks and corner transverse cracks by controlling copper pipes and water seams in the mold and adopting appropriate primary cooling and secondary cooling processes.

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