Reduction of straight crack defect rate at the edge of slab process practice
According to the mechanism of slab side straight crack, measures such as improving the cleanliness of steel, improving the composition of steel and optimizing the casting method have been taken to effectively control the slab side straight crack defects, which greatly reduces the rate of cast slab crack and significantly improves the product quality. This paper discusses how to reduce the defect rate of slab side straight cracking by combining the production practice of continuous casting line of Hand Steel.
Continuous casting slab side straight cracks are surface cracks occurring on the wide side of the slab parallel to the casting direction, which is a major product quality defect problem and seriously affects the surface quality and mechanical processing performance of the slab. This kind of straight edge cracking often occurs in batches and can lead to the scrapping of a large number of products in the batch in serious cases. In the production line of continuous casting of Hand Steel, the quality problem of straight edge cracking of slabs occurred during the production of different specifications of cast slabs. After the analysis of the continuous casting production process, it is believed that the slab side straight crack is mainly related to the uneven cooling of the steel in the crystallizer, resulting in the uneven thickness of the cast billet shell and the cracking of the weak part of the side. After the billet out of the crystallizer, due to uneven cooling and shrinkage of the billet, resulting in the billet straight cracks continue to expand, and eventually form more serious crack defects. According to the mechanism of slab side straight crack, measures such as improving the cleanliness of steel, improving the composition of steel and optimizing the casting method are taken to effectively control the slab side straight crack defects, which greatly reduces the rate of cast billet cracks and significantly improves the product quality. This paper discusses how to reduce the defect rate of slab side straight cracking by combining the production practice of continuous casting line of Hand Steel.
Analysis of the causes of slab edge straight cracks
Slab edge cracking is a surface defect problem that seriously affects the quality and yield of continuous casting products. 2018, a large number of slabs cast in the continuous casting line of Hand Steel had straight edge cracks, which directly led to the scrapping of cast slabs in serious cases and caused large economic losses to the enterprise. Most of the quality problems on the surface of the cast slab are related to crystallizer pollution and steel composition.
The influence of steel composition and cleanliness
(1) The influence of carbon content
The carbon content of the steel has a certain influence on the solidification shrinkage of the steel. When the carbon content of the molten steel in the range of 0.08% to 0.15%, the molten steel cooling solidification will occur and produce a more significant volume shrinkage of the package crystal reaction. At this time, due to the volume contraction of the billet, the crystallizer wall and the billet will form a large gap between. As the heat loss in the middle gap is slower, it leads to the inconsistent and uneven cooling rate between the middle part and the side of the billet surface; the thickness of the primary billet shell in the middle and the side is also different. Under the influence of various forces such as static pressure of steel and thermal stress, cracks will be formed on the side of the billet, and the cracks will gradually increase with the cooling process. According to the statistics of billets with different carbon content, the carbon content is 0.1%-0.13% when it belongs to the crack sensitive area.
(2) Effect of sulfur content
Sulfur in the molten steel increases the thermal embrittlement of the cast billet. Therefore, the higher the sulfur content in the molten steel, the more likely it is that the billet will form surface cracks. Sulfur has very little solubility in the steel, and FeS formed with Fe can form thermally brittle co-crystals in the steel and precipitate at the grain boundaries. The presence of sulfur significantly reduces the stress that the billet shell can withstand, the role of static pressure and thermal stress in the steel billet surface crack formation. Therefore, reduce the sulfur content in the steel is conducive to improving the slab straight cracking.
(3) The impact of steel cleanliness
Silicon, manganese, sulfur and other inclusions in the molten steel will be enriched in the process of crystallization or precipitation of the cast billet, and make the primary cast billet thermal resistance increased. During the cooling process, uneven cooling and deformation can occur in the middle and sides of the billet, resulting in side cracks. In addition, the amount of shrinkage of the inclusions during cooling is different from the rest of the billet. Therefore, the edge part of the inclusions is also the site where cracks occur more often. Sampling analysis shows that silicates, metal oxides, MnS, etc. are the most common inclusions in the molten steel.
The effect of pulling speed change
Due to the change of production rhythm and steel temperature, composition fluctuations, the casting machine pulling speed changes frequently, easy to form a non-stationary casting. The constant change of pulling speed will make the protective slag inflow speed, water mouth side hole injection flow impact intensity, etc., and then lead to non-stationary, uneven heat transfer in all parts of the billet shell; especially the central part of the billet and the side, the heat division difference is more obvious. This leads to the occurrence of straight cracks in the side of the billet. In production practice, it is found that the side straight cracks mostly occur in the open casting furnace. In this regard, it is necessary to avoid frequent fluctuations of the pulling speed within a short period of time by stabilizing the steel temperature and the production rhythm.
Sealing water mouth argon volume is large, although there is enough argon seal, but also lead to large fluctuations in the liquid level of the steel, and destroy the stability of the liquid slag layer, resulting in the uneven inflow of liquid slag leads to the generation of straight cracks on the surface of the cast billet. According to production statistics, the cast billet surface straight crack with the increase in liquid level fluctuations, liquid level fluctuations of more than 3mm, the incidence of cracking significantly higher. Secondly, the slag, slag operation is improper, often picking the slag shell or depth stirring slag surface is also easy to make the protective slag layer is damaged; poor alignment of the water mouth, resulting in the existence of bias flow when the steel is injected into the crystallizer, the deviation of the flow field will then lead to uneven temperature distribution of the steel, which is also an important cause of the cast billet surface straight crack.
Reduce the defect rate of slab side straight cracking process practice
From the above analysis, it can be seen that the large temperature difference between the middle and the side of the molten steel in the crystallizer is the root cause of straight cracking on the slab side, which can be solved by the following measures.
Optimize the steel composition
First of all, the carbon content of the molten steel should be reasonably controlled, and try to avoid the crack-sensitive area where the carbon content is in the range of 0.1%-0.13%. Then it is necessary to start from the previous ladle refining link to strengthen the desulfurization effect, reduce the S content in the molten steel as much as possible, and reduce the thermal embrittlement of the cast billet. In order to improve the cleanliness of the molten steel, ladle down slag automatic detection technology can be used, according to the characteristics of high melting point, high viscosity and light specific gravity of the steel slag. Through the monitoring of the slag quantity and slag time, timely slag retrieval is carried out.
Optimization of crystallizer structure and process parameters
Under the fast-paced, high-drawing speed continuous casting production conditions, the possibility of steel leakage and straight cracking defects on the casting surface increases. During production, it was found that the possibility of side straight cracks was greater for medium carbon steel, low carbon steel and thin slabs. The analysis concluded that the uneven cooling of the cast billet on the wide and narrow sides in the crystallizer is the main reason for the bulging of the cast billet and the straight cracks on the sides. In this regard, the crystallizer and cooling parameters were adjusted. Firstly, the clamping device of the crystallizer was improved to ensure that the crystallizer can be clamped stably during the casting process and the taper does not change; and the change of the crystallizer taper was checked after each stoppage, and the crystallizer was replaced when the taper was less than the allowed value. Second, adjust the crystallizer cooling process parameters. In the original production process, there is no adjustment of the crystallizer wide surface cooling water flow according to the different characteristics of steel grades, resulting in uneven cooling of the crystallizer billet. The technical department optimized the crystallizer cooling process parameters by calculating the heat flow of the crystallizer for different steel grades, combined with the billet cooling and deformation mechanism. The cooling water volume of low-carbon aluminum ballast steel was increased to 3260L/min for wide surface cooling; the cooling water volume of low alloy steel and normal carbon steel was adjusted to 400L/min for narrow surface cooling. thirdly, the production management organization was added to reduce the fluctuation of pulling speed of continuous casting line by taking measures such as constant temperature and speed steel pouring and stable steel supply production. Make the nature of steel grade, pulling speed, section casting more matching.
Optimization of protective slag
The protective slag has the role of lubrication and heat transfer control, and its performance needs to be optimized under fast-paced, high-drawing speed production conditions. Through production studies, it was found that different protective slag properties have different effects on the generation of straight cracks in different sizes of steel grades. For example, A2 slag with an alkalinity of 1.1 is the best for thin gauge slabs; A4 slag with an alkalinity of 0.96 is less effective when applied to normal gauge mild steel. Therefore, for the crystallizer heat transfer faster, liquid level fluctuations in the thin specifications of the slab, it is appropriate to use low viscosity, low melting point and low solidification temperature of the protective slag; and for thicker specifications of the slab, it is more applicable viscosity and melting point of the higher protective slag. In the future production, but also continue to different characteristics of the protective slag and the impact of the product quality of each steel grade, in order to reduce the cast billet surface cracking defects.
Improve the structure of the parameters of the immersion spout
By appropriately reducing the area of the water spout spit hole, eliminating the low-pressure reflux area in the upper part of the two spit holes; at the same time, the use of open hole area and adjust the water spout jet to reduce the speed of steel movement, so that the high-temperature zone of the steel in the crystallizer is significantly shifted upward. Thus improving the effect of slag melting and billet shell lubrication effect.
Through the above technical measures, the product quality of continuous casting billet is effectively improved, and the defects of straight cracks on the billet edge are significantly improved. The product cracking rate decreased by 0.5 percentage points. High quality slab raw material was provided for the subsequent rolling production.
Author: Wang Youxin
Source: China Steel Plate Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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-  Tang Shengbin. Causes of longitudinal cracks in slab edges and measures to solve them [J]. Steelmaking 2008,(1).
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-  Wang Xiaofeng, Lv Ya, Liu Lei. Analysis of the causes of longitudinal cracks on the surface of slab and preventive measures [J]. Metal Materials and Metallurgical Engineering 2013,(02).