1, Purity and quality of the relationship
Purity refers to the number, form and distribution of non-metallic inclusions in steel. Compared with die casting, continuous casting machine process links, pouring time is long, so the inclusions of a wide range of sources, composition is also more complex; inclusions from the crystallizer liquid phase cavity floating more difficult, especially the high drawing speed of small billet inclusions more difficult to exclude. The presence of inclusions destroys the continuity and denseness of the steel matrix. Larger than 50μm large inclusions are often accompanied by cracks, resulting in low times the structure of the continuous casting billet failed, plate delamination, and damage to the surface of cold rolled steel, etc., the steel is very harmful. The size, shape and distribution of inclusions on the quality of steel is also different, if the inclusions are small, spherical, diffuse distribution, the impact on the quality of steel than the concentration of the presence of less; when the inclusions are large, is incidental distribution, although the number of small but also more harmful to product quality.
For example: from deep-drawn steel plate punching crack scrap inspection found that cracks are present at 100 ~ 300μm irregular CaO-Al2O3 and Al2O3 heterometallic large inclusions.
For example, due to the presence of Al2O3 inclusions under the skin of the continuous casting billet, rolled into the surface of the automotive sheet with black line defects, resulting in poor surface coating of the sheet.
There are also used for packaging tin-plated plate, in addition to the requirements of high cold forming performance, the size and number of inclusions also have corresponding requirements. For the thickness of 0.3mm thin steel plate, in 1m2 area, the particle size of less than 50μm inclusions should be less than 5, in order to achieve the scrap rate of 0.05% or less, that is, deep punching 2000 DI cans, flat less than 1 scrap. Thus see how important it is to reduce the number of inclusions in continuous casting billets to improve the quality of deep-drawn sheet steel.
For very fine steel wire (such as 0.01 ~ 0.25mm diameter tire steel wire) and very thin steel plate (such as the thickness of 0.025mm tin-plated plate), the size of the inclusions contained in it is even more stringent requirements.
In addition, the size and number of inclusions on the quality of steel and the impact of the specific surface area of the cast billet. General slab and billet unit length of surface area (S) and volume (V) ratio in 0.2 ~ 0.8. With the development of thin plate and thin strip technology, S / V up to 10 ~ 50, if the same inclusions in steel, thin plate and thin strip steel, it means that inclusions closer to the surface of the cast billet, the greater the impact on the production of thin plate quality. So reduce the inclusions in steel is more important.
2, To improve the purity of the measures
Improve the purity of steel should be in the steel into the crystallizer before, from the process to minimize the pollution of the steel, and maximize the inclusions from the steel excluded. To this end, the following measures should be taken.
a, No slag out of steel: converter should block slag out of steel, electric furnace using eccentric furnace bottom out of steel, to prevent the slag into the steel ladle or steel bucket, tank.
b, According to the needs of the steel grade to choose the appropriate refining treatment to pure steel, improve the morphology of inclusions.
C. Adopt oxidation-free pouring technology. After refining treatment, the oxygen content of the molten steel has been reduced to less than 20×10-6; in the steel ladle → intermediate tank → crystallizer are used to protect pouring; intermediate tank using a double layer of slag covering agent, the steel is isolated from the air to avoid secondary oxidation of the molten steel.
d. Give full play to the role of the intermediate tank metallurgical purifier. The use of blowing Ar stirring, improve the flow of steel, eliminate the dead zone of the intermediate tank; increase the capacity of the intermediate tank and deepen the depth of the melt pool, extend the residence time of the molten steel in the intermediate tank, promote the floating of inclusions, and further purify the molten steel.
f, Continuous casting machine system use high refractoriness, melt loss is small, high-quality refractory materials to reduce the inclusions in the steel.
g, Give full play to the role of the crystallizer molten steel purifier and billet surface quality controller. The selected immersion spout should have a reasonable opening shape and angle, control the movement of the injection stream, to promote the separation of inclusions floating; and complemented by good performance of the protection slag, absorbing dissolved floating inclusions purified molten steel. In addition, the crystallizer can also be fed into the cored alloy wire to achieve micro-alloying in the crystallizer, which not only improves the absorption rate of the alloy, and can accurately control the composition of the molten steel, adjust the solidification structure, improve the morphology of inclusions, which is conducive to improving and purifying the quality of steel.
h. Electromagnetic stirring technology is used to control the movement of the injection stream. Calculations point out that in the static state, greater than 1mm slag particles floating speed of about 100 ~ 200cm / s; and injection flow downward flow speed of 60 ~ 10cm / s; visible crystallizer liquid phase cavity within the injection flow flow strand impact area inclusions floating is difficult; some inclusions are likely to be trapped by the solidification of the dendrite.
In fact, in the cast billet surface below 10 ~ 20cm often higher inclusions. The installation of electromagnetic brakes can inhibit the movement of the injection flow, promote the floating of inclusions and improve the purity of the steel.
Post time: Dec-23-2022