阅读说明：本技术 低碳低硅钢铝质复合脱氧剂及其制备和使用方法 (Low-carbon low-silicon aluminum composite deoxidizer and preparation and use method thereof ) 是由 周伟 王建 郑之旺 吴国荣 王敏莉 于 2021-09-13 设计创作，主要内容包括：本发明公开了一种低碳低硅钢铝质复合脱氧剂及其制备和使用方法,属于钢铁冶金技术领域。本发明为改善低碳低硅钢种可浇性差、夹杂降级率高的问题,提供了一种低碳低硅钢铝质复合脱氧剂,其包括：金属铝：50～60％；7Al-(2)O-(3)·12CaO预熔渣：10～20％,钝化CaO：10％～20％,Fe：10～20％,其余为微量杂质元素。本发明通过深入研究铝脱氧后钢中夹杂物的行为,提供了一种低碳低硅钢铝质复合脱氧剂,通过在转炉出钢过程加入该脱氧剂,能够有效降低钢中大型夹杂比例、对夹杂物改性,在不延长精炼时间的情况下降低夹杂物总量,改善目前此类钢种生产过程中存在的可浇性差,夹杂降级率高的问题。(The invention discloses a low-carbon low-silicon steel aluminum composite deoxidizer and a preparation and use method thereof, belonging to the technical field of ferrous metallurgy. The invention provides a low-carbon low-silicon aluminum composite deoxidizer for improving the problems of poor castability and high inclusion degradation rate of low-carbon low-silicon steel seeds, which comprises the following components in percentage by weight: metal aluminum: 50-60%; 7Al 2 O 3 12CaO pre-slag: 10-20%, passivating CaO: 10-20%, Fe: 10-20% and the balance of trace impurity elements. The invention provides a low-carbon low-silicon aluminum composite deoxidizer by deeply researching the behavior of inclusions in steel after aluminum deoxidation, and the deoxidizer is added in the tapping process of a converter, so that the proportion of large inclusions in the steel can be effectively reduced, the inclusions are modified, the total amount of the inclusions is reduced under the condition of not prolonging the refining time, and the problems of poor castability and high inclusion degradation rate in the production process of the steel at present are solved.)

1. The low-carbon low-silicon aluminum composite deoxidizer is characterized by comprising the following components in percentage by weight: the composite material comprises the following components in percentage by mass: metal aluminum: 50-60%; 7Al₂O₃12CaO pre-slag: 10-20%, passivating CaO: 10-20%, Fe: 10-20% and the balance of trace impurity elements.

2. The low-carbon low-silicon aluminum composite deoxidizer of claim 1 is characterized in that: the particle size of the composite deoxidizer is 10-35 mm.

3. The low-carbon low-silicon aluminum composite deoxidizer of claim 1 or 2 is characterized in that: the composite deoxidizer is prepared by the following method:

A. taking the components according to the proportion, grinding the components to be less than 3mm, mixing and stirring the dry materials for 8-11 min, and uniformly mixing;

B. preparing a pellet blank by dry-type pellet pressing by using a powerful pellet press, wherein the pressure is 12-14 GPa, and the specification is 20-35 mm;

C. and screening the pellet blank by adopting a sieve with a sieve pore size not smaller than 10mm, and selecting pellets with a particle size not smaller than 10mm as the composite deoxidizer.

4. The use method of the low-carbon low-silicon aluminum composite deoxidizer of any one of claims 1 to 3 is characterized in that: the method comprises the following steps:

A. after the molten steel meets the tapping requirement, adding a compound deoxidizer 2min after tapping according to the oxygen determination result of the converter, and ensuring the bottom blowing effect of a steel ladle during the adding period so that the compound deoxidizer is fully mixed with the molten steel; the adding amount of the composite deoxidizer is 0.83-0.87 kg per 1ppm of oxygen, and 0.8-1.0 kg/nominal capacity of the composite deoxidizer is additionally added;

B. after the deoxidizing agent is added, adding lime, and keeping argon stirring for a steel ladle during adding;

C. after tapping, adding a calcium top slag modifier to the slag surface, and blowing argon at the bottom of the steel ladle in the adding process without blowing turnover.

5. The use method of the low-carbon low-silicon aluminum composite deoxidizer of claim 4 is characterized in that: in the step B, the adding amount of the lime is 2-3 kg/ton of steel tapping.

6. The use method of the low-carbon low-silicon aluminum composite deoxidizer of claim 4 is characterized in that: in the step C, the calcium-based top slag modifier comprises the following components in percentage by mass: CaC₂：30％～50％，Al₂O₃: 10-30%, CaO: 20 to 40 percent of the total weight of the alloy, less than or equal to 0.2 percent of S, less than or equal to 0.1 percent of P, and the balance of inevitable impurities.

7. The use method of the low-carbon low-silicon aluminum composite deoxidizer of any one of claims 4 to 6 is characterized in that: in the step C, the addition amount of the calcium top slag modifier is 1-2 kg/ton of steel tapping.

Technical Field

The invention belongs to the technical field of ferrous metallurgy, and particularly relates to a low-carbon low-silicon aluminum composite deoxidizer and a preparation and use method thereof.

Background

The low-carbon low-silicon steel (such as SPH series) is mainly used for shells of household appliances and instruments, is used as a deep processing material for deep pressing, panel forming and the like, requires higher stamping forming performance, and uses aluminum for deoxidation when smelting the low-carbon low-silicon steel.

The products of the aluminium deoxidation are mainly solid Al₂O₃It mainly involves three aspects: 1) al (Al)₂O₃Nucleation of the particles; 2) al (Al)₂O₃Aggregation and growth of nucleation particles; 3) al (Al)₂O₃Separating and removing the inclusions. Al (Al)₂O₃The quantity, form and size distribution of inclusions in the deoxidized steel have great influence on the cleanliness of molten steel, the blockage of a water gap in the pouring process, the quality of steel and the like, and if the deoxidation is poor, a casting blank can generate subcutaneous air holes and surface defects; excessive deoxidation increases the viscosity of molten steel, deteriorates the fluidity, and produces Al₂O₃High-melting-point inclusions are easy to accumulate and form nodules on the inner wall of a tundish nozzle to block the nozzle, so that smooth casting is influenced. This is particularly prominent in low carbon, low silicon aluminum killed steels, which are characterized by poor castability, poor cleanliness, or a combination of both. Related literature research shows that most domestic manufacturers face common problems in producing the steel grades, and make a great deal of research, and the problems of the Sichang steel vanadium related products exist, such as the hot-rolled pickling SPH series inclusion degradation rate reaching about 4 percent.

In the current process, four main methods for removing alumina inclusions in low-carbon and low-silicon steel are adopted: firstly, blow the argon in to the ladle, strengthen the molten steel stirring, promote the inclusion come-up, but this kind of method needs more consuming time, still needs the white slag operation simultaneously, has the risk of returning the silicon when producing low-silicon steel kind. Secondly, feeding calcium alloy into the molten steel to modify the impurities in the alumina. However, the calcium element is active in property and is easy to gasify at the steelmaking temperature, so that the yield of calcium in the calcium treatment process is not high and is only 10-20 percent; meanwhile, the calcium treatment has higher requirement (less than or equal to 0.008 percent or even lower) on the sulfur content in steel; secondly, the addition amount of calcium is difficult to control, too little calcium is added, a good modification effect cannot be achieved, too much calcium is added, and the calcium can react with sulfur in molten steel to generate CaS inclusion to block a water gap and also react with refractory materials on the wall of a ladle to generate new inclusion; the smelting cost is increased with a high probability and the denaturation effect is not ideal. And thirdly, the vacuum light treatment mode is adopted for production, and the method adopts the vacuum carbon deoxidation mode, so that the total amount of impurities is reduced, the production efficiency can be obviously improved, the production cost is reduced, and the method has obvious advantages. Fourthly, remove through vacuum circulation and mix with it all is that the physical mode gets rid of to mix with first one, and mix with and get rid of efficiency and also be higher than first mode, and effective clearance reaches about 40%, but faces the problem of third mode equally, and when the low carbon aluminum killed steel of production general quality, adopt vacuum mode cost and quality requirement obviously not to match simultaneously.

The low-carbon low-silicon steel produced by the Xichang vanadium steel at present mainly adopts a first mode and a third mode, and when the first mode is adopted, the first mode accounts for more than 80 percent of the subsequent degradation proportion caused by inclusion. But the prior RH production capacity is insufficient, so that the steel grades can not be produced by the mode three.

In addition, CN202010280770.1 discloses a method for controlling inclusions in low-carbon low-silicon steel by using refining slag, which comprises the following components in percentage by weight: 45-60 percent of CaO, 5-10 percent of MgO, 5-10 percent of SiO2, 30-40 percent of Al2O3, less than or equal to 1 percent of FeO and MnO, and the balance of impurities which have no influence on a slag system. Although this patent has solved a series of problems such as the quantity that reduces inclusion in the steel and the proportion of large granule inclusion, nevertheless because its main adoption is refine slagging-off in order to reach the purpose to inclusion control, it is long consuming time, has the risk of returning the silicon simultaneously, and the operation degree of difficulty is great.

CN201910134558.1 discloses a method for producing low-carbon low-silicon steel seeds, which comprises the following steps: 1. controlling the end point of the converter; 2. deoxidizing and alloying; 3. slag blocking operation of the converter; 4. operating an argon station; 5. controlling an LF furnace; 6. and (4) continuous casting operation. The method can optimize the deoxidation process of the low-carbon low-silicon aluminum killed steel, further improve the deoxidation effect, reduce high-melting-point inclusions in the steel, avoid the problem of nozzle nodulation of a continuous casting tundish, and solve the quality problem of bubbles of a continuous casting blank caused by insufficient deoxidation; however, the method adopts the processes of calcium treatment and reducing slag making, so that the cost is high and the production time is long.

CN201811416892.8 discloses a deoxidation method for producing low-carbon low-silicon steel by a small converter, which controls the proportion of aluminum and silicon deoxidizers by adding aluminum deoxidizers and silicon deoxidizers, so that the silicon content in the final steel is reduced from 0.10-0.20% to below 0.10%, the silicon content is reduced, the surface quality of steel is improved, the problem of flocculation at a water gap of a crystallizer of a sizing ladle continuous casting machine is not easily caused, and the purity of molten steel is improved. The patent mainly achieves the purpose of controlling the silicon content in the steel by adjusting the addition type and the addition amount of the deoxidizer, and does not relate to inclusion control.

CN201810706078.3 discloses a process for deoxidizing low-carbon low-silicon steel by using barium as a deoxidizing agent, wherein a silicon-calcium-barium alloy is used as the deoxidizing agent in the traditional deoxidizing process, and the adding amount and the adding method of the barium are the same as those of the deoxidizing agent used in the traditional deoxidizing process. The patent adopts the deoxidation alloy as aluminum and silicon calcium barium, and the deoxidation alloy is used for carrying out calcium treatment on the inclusions to achieve denaturation treatment of the inclusions, so that the problems of poor stability and high cost exist.

Disclosure of Invention

Aiming at the problems in the prior art, the invention develops a deoxidizer and a process by deeply researching the behavior of inclusions in steel after aluminum deoxidation so as to effectively reduce the proportion of large-scale inclusions in the steel, modify the inclusions, reduce the total amount of the inclusions under the condition of not prolonging the refining time and improve the problems of poor castability and high inclusion degradation rate in the production process of the steel at present.

The invention firstly provides a low-carbon low-silicon steel aluminum composite deoxidizer which comprises the following components in percentage by mass: metal aluminum: 50-60%; 7Al₂O₃12CaO pre-slag: 10-20%, passivating CaO: 10-20%, Fe: 10-20% and the balance of trace impurity elements.

Wherein the granularity of the low-carbon low-silicon aluminum composite deoxidizer is 10-35 mm.

The invention also provides a preparation method of the low-carbon low-silicon aluminum composite deoxidizer, which comprises the following steps:

A. taking the components according to the proportion, grinding the components to be less than 3mm, mixing and stirring the dry materials for 8-11 min, and uniformly mixing;

B. preparing a pellet blank by dry-type pellet pressing by using a powerful pellet press, wherein the pressure is 12-14 GPa, and the specification is 20-35 mm;

C. and screening the pellet blank by adopting a sieve with a sieve pore size not smaller than 10mm, and selecting pellets with a particle size not smaller than 10mm as the composite deoxidizer.

The invention relates to Al according to the aluminum deoxidation process₂O₃The inclusion forming mechanism and the change in the subsequent process also provide a use method of the low-carbon low-silicon aluminum compound deoxidizer, which comprises the following steps:

A. after the molten steel meets the tapping requirement, adding a compound deoxidizer 2min after tapping according to the oxygen determination result of the converter, and ensuring the bottom blowing effect of a steel ladle during the adding period so that the compound deoxidizer is fully mixed with the molten steel; the compound deoxidizer is added in an amount of 0.83-0.87 kg per 1ppm of oxygen, and 0.8-1.0 kg per nominal capacity of the compound deoxidizer is additionally added;

B. after the deoxidizing agent is added, adding lime, and keeping argon stirring for a steel ladle during adding;

C. after tapping, adding a calcium top slag modifier to the slag surface, and blowing argon at the bottom of the steel ladle in the adding process without blowing turnover.

In the using method of the low-carbon low-silicon aluminum composite deoxidizer, in the step B, the addition amount of lime is 2-3 kg/ton of steel tapping.

In the using method of the low-carbon low-silicon aluminum composite deoxidizer, in the step C, the calcium top slag modifier comprises the following components in percentage by mass: CaC₂：30％～50％，Al₂O₃: 10-30%, CaO: 20 to 40 percent of the total weight of the alloy, less than or equal to 0.2 percent of S, less than or equal to 0.1 percent of P, and the balance of inevitable impurities.

In the using method of the low-carbon low-silicon aluminum composite deoxidizer, in the step C, the addition amount of the calcium top slag modifier is 1-2 kg/ton of steel tapping.

The invention has the beneficial effects that:

the invention provides a low-carbon low-silicon aluminum composite deoxidizer by deeply researching the behavior of inclusions in steel after aluminum deoxidation, and the deoxidizer is added in the tapping process of a converter, so that the proportion of large inclusions in the steel can be effectively reduced, the inclusions are modified, the total amount of the inclusions is reduced under the condition of not prolonging the refining time, and the problems of poor castability and high inclusion degradation rate in the production process of the steel at present are solved.

Drawings

FIG. 1 shows the low-carbon low-silicon aluminum composite deoxidizer of the invention.

Detailed Description

The invention aims to reduce the quality loss of steel mills by deeply researching the behavior of inclusions in the steel after the aluminum deoxidation, and meanwhile, if the process is successfully implemented, the process can be popularized and applied to other steel types adopting the aluminum deoxidation so as to improve the cleanliness control level.

According to the aluminium deoxidation process Al₂O₃The invention is mainly optimized and designed from the following two aspects:

one is in Al₂O₃In the production process, seed inclusion is artificially added to capture fine Al₂O₃So that the mixture is rapidly gathered to form large-size inclusion floating removal capable of floating upwards:

the motion of the inclusions is found to be random. When a certain inclusion has proper diameter and surface area and collides with other inclusions with different diameters, the inclusions can be adsorbed and become the core of the long inclusion, which is technically called as seed inclusion. When other inclusions move around the seed inclusion, the seed inclusion can adsorb the surrounding inclusion particles under the action of viscous force and fluid power. Once the adsorption is successful, the adhered inclusions form protrusions on the surface of the seed inclusions, thereby increasing the volume and surface area of the seed inclusions and providing more favorable conditions for the next adsorption process. Meanwhile, the adsorption of the inclusions has persistence: the adsorbed inclusions tend to become the next "active" surface to adsorb, and these protrusions eventually grow into tentacles of larger size.

The alumina has strong agglomeration and growth effects, and can trap fine alumina generated in the deoxidation process when added into the molten steel, grow up and be easily floated and removed, and the effect is similar to that of water purification by alum. With Al₂O₃The wetting angle between the inclusion and the molten steel is more than 90 degrees, the inclusion belongs to iron-phobic inclusion, the inclusion and the molten steel can be mutually adhered after collision, and can be quickly sintered into coral community under the action of molten steel static pressure and high temperature, the size reaches more than 100 mu m, even is much larger, and the inclusion and the molten steel can be quickly floated and removed. Therefore, Al₂O₃Is a good choice for seed inclusion.

Secondly, the impurities are denatured in the deoxidation process. According to the research of removing alumina inclusions by calcium treatment in the current industrial production, the core of the calcium treatment is to provide active Ca components in molten steel, Ca entering the steel is combined with O or S in the steel, and formed CaO is combined with Al₂O₃The low melting point of the generated aluminum calcium salt is easy to float and mix. The main reaction is as follows:

[Ca]+[O]＝CaO(s) (1)

CaO(s)+Al₂O₃(s)＝CaO·Al₂O₃(s) (2)

12CaO(s)+7Al₂O₃(s)＝12CaO·7Al₂O₃(l) (3)

therefore, the calcium treatment process is the process of converting the alumina inclusion modification into the aluminum calcium salt. However, the alloy treated by calcium has low yield and unsatisfactory modification effect, so the invention designs CaO-Al aiming at the processes of nucleation, collision modification, growth and floating of common inclusions₂O₃Is pre-melted slag, the slag is CaO and Al₂O₃Has strong holding capacity, and simultaneously, because the surface tension of the alumina inclusion and the molten steel is very large, the repulsive force of the molten steel to the alumina inclusion is relatively large, so that the alumina inclusion can be combined with premelting slag entering the molten steel more easily, and the removal of the alumina inclusion is researched according to the fact that the alumina inclusion is removedAnd (5) effect. The principle is as follows: adding CaO-Al into the molten steel while deoxidizing₂O₃The process is characterized in that pre-melted slag is adopted, after a binary pre-melted phase enters molten steel, alumina inclusions in the steel collide and combine with the aluminum calcium acid salt phases in the molten steel, and are denatured into low-melting-point liquid inclusions, and the low-melting-point liquid inclusions grow and float upwards continuously by taking the low-melting-point liquid inclusions as a core, so that the effect of removing the alumina inclusions is achieved.

Based on the research, the invention provides a low-carbon low-silicon aluminum composite deoxidizer which comprises the following components in percentage by mass: metal aluminum: 50-60%; 7Al₂O₃12CaO pre-slag: 10-20%, passivating CaO: 10-20%, Fe: 10-20% and the balance of trace impurity elements.

In the low-carbon low-silicon aluminum composite deoxidizer, metal aluminum is used as the deoxidizer and is matched with Als components, and the metal aluminum is controlled to be 50-60% so as to ensure the deoxidizing effect; 7Al₂O₃12CaO can rapidly adsorb deoxidation product Al₂O₃Inclusion, simultaneously forming large inclusion floating removal, playing a role of seed inclusion, controlling 7Al for ensuring the effects of adsorbing and modifying the inclusion₂O₃12CaO premelting slag 10-20%; passivating CaO and deoxidation product Al₂O₃The inclusion reaction forms CaO-Al with low melting point₂O₃Impurities are mixed, and 10-20% of the impurities are controlled in order to ensure that the impurities are modified and form large impurities to be quickly removed in an upward floating manner; the Fe increases the density of the composite deoxidizer, avoids the deoxidizer from rapidly floating up in molten steel to reduce the deoxidizing effect, controls the deoxidizer to be 10-20% through tests, and enables the density of the composite deoxidizer to be just proper.

In addition, in order to control the return silicon of the low-carbon low-silicon steel, the refining time is generally not too long, so the invention adopts 7Al₂O₃12CaO premelting and passivating CaO, and controlling the ratio of the two to avoid silicon return.

The particle size of the composite deoxidizer is controlled to be 10-35 mm.

The invention also provides a preparation method of the low-carbon low-silicon aluminum composite deoxidizer, which comprises the following steps:

A. taking the components according to the proportion, grinding the components to be less than 3mm, mixing and stirring the dry materials for 8-11 min, and uniformly mixing;

B. preparing a pellet blank by dry-type pellet pressing by using a powerful pellet press, wherein the pressure is 12-14 GPa, and the specification is 20-35 mm;

C. and screening the pellet blank by adopting a sieve with a sieve pore size not smaller than 10mm, and selecting pellets with a particle size not smaller than 10mm as the composite deoxidizer.

In the step A of the preparation method, a proper amount of binding agent can be added according to the requirement.

Based on the research and according to the characteristics of the low-carbon low-silicon aluminum composite deoxidizer provided by the invention, the invention also provides a use method of the low-carbon low-silicon aluminum composite deoxidizer, which comprises the following steps:

A. after the molten steel meets the tapping requirement, adding a compound deoxidizer 2min after tapping according to the oxygen determination result of the converter, and ensuring the bottom blowing effect of a steel ladle during the adding period so that the compound deoxidizer is fully mixed with the molten steel; the compound deoxidizer is added in an amount of 0.83-0.87 kg per 1ppm of oxygen, and 0.8-1.0 kg per nominal capacity of the compound deoxidizer is additionally added;

B. after the addition of the deoxidizer is finished, adding lime (generally, adding lime as soon as possible after the addition of the deoxidizer is finished), and keeping argon stirring for a steel ladle during the addition;

C. after tapping, adding a calcium top slag modifier to the slag surface, and blowing argon at the bottom of the steel ladle in the adding process without blowing turnover.

The aluminum composite deoxidizer is designed aiming at low-carbon low-silicon steel, and is suitable for various specific low-carbon low-silicon steel in the field. In actual production, the tapping of molten steel is required to be controlled according to the conventional standard of low-carbon low-silicon steel in the field.

In the step B of the method, the addition amount of lime is 2-3 kg/ton steel (steel tapping amount). After the composite deoxidizer is added, lime is generally added immediately, and the effect of adding the lime is to react with the residual deoxidizer Al₂O₃The inclusion reaction forms CaO-Al with low melting point₂O₃And (4) impurity inclusion, namely modifying the impurity inclusion and forming large-scale impurity to quickly float and remove. But the lime has high melting point and insufficient refining timeIf not, it cannot be combined with Al₂O₃The impurities are fully reacted, but the invention aims at the condition that the refining time of low-silicon steel is too long and silicon return easily occurs to cause silicon to exceed the component requirement range, so expensive 7Al needs to be added in the early stage₂O₃12CaO pre-slag component.

In the step C of the method, after tapping is finished, adding 1-2 kg/ton steel (steel tapping amount) of calcium-based top slag modifier to the slag surface to reduce the oxidability of ladle slag and enhance the submerged arc effect; the calcium series top slag modifier comprises the following components in percentage by mass: CaC₂：30％～50％，Al₂O₃: 10-30%, CaO: 20 to 40 percent of the total weight of the alloy, less than or equal to 0.2 percent of S, less than or equal to 0.1 percent of P, and the balance of inevitable impurities.

The present invention is further illustrated by the following examples, which are not intended to limit the scope of the invention.

Example 1

In the embodiment, the composition of the deoxidizer adopted is (mass percent): metal aluminum: 54.6 percent; 7Al₂O₃12CaO pre-slag: 12.3%, passivated CaO: 17.1%, Fe: 15 percent of trace impurity elements; the calcium series top slag modifier comprises the following components in percentage by mass: CaC₂：46％，Al₂O₃: 24%, CaO: 28 percent of S is less than or equal to 0.2 percent, P is less than or equal to 0.1 percent, and the balance is inevitable impurities.

The semisteel after being extracted with water, vanadium and desulfurized contains 3.51 percent of C, 0.040 percent of Mn, 0.063 percent of P, 0.007 percent of S, 0.03 percent of V, trace amounts of Cr, Si and Ti, and the balance of iron and inevitable impurities.

The method comprises the following specific steps:

A. 235 tons of the semi-steel are added into a top-bottom combined blowing converter with the capacity of 220 tons (nominal capacity), and the semi-steel is primarily smelted into molten steel by utilizing the function of oxygen blowing and decarburization of the top-bottom combined blowing converter. When the molten steel is initially smelted to the temperature of 1660 ℃ and the C content is 0.028 wt%, the Mn content is 0.032 wt%, the P content is 0.0081 wt%, the S content is 0.0081 wt%, and the thick slag starts to discharge steel into a ladle;

B. after tapping for 2min, according to the oxygen determination result of 600ppm, 710kg of deoxidizer is added, 500kg of lime (the tapping amount is 218t) is added immediately after the deoxidizer is added, and the whole process is that argon is blown at the bottom of a ladle in large air quantity;

C. 300kg of calcium top slag modifier is added into the steel ladle after tapping is finished, and argon is blown from the bottom of the steel ladle in the adding process, so that no blowing turnover phenomenon exists;

D. the problems of large liquid level fluctuation and poor castability caused by flow change and serious blockage of a water gap are not found in the casting process; the casting blank is sampled and analyzed for T [ O ] (total inclusion index) and inclusion type. The T O result was 12ppm, the average T O content of the cast slab of this type of steel of the prior art (deoxidised with ferro-aluminium, added in an amount of 0.9kg ferro-aluminium per 1ppm oxygen, then an additional 200kg ferro-aluminium) was 15ppm, with a reduction in the inclusion weight of about 20%; meanwhile, most of the inclusions are below 2 mu m (improved to more than 85 percent from about 60 percent of the original process), and the inclusions with the particle size of more than 20 mu m are not found.