KR970004990B1 - Decarburizing method of stainless steel - Google Patents

Abstract

The method of removing carbon from crytocarbon stainless steel is made by injecting 4.5-5.5 Kg/T of fluorite in inert gas under the condition of oxygen in the final step of removing carbon and refluxing for 2-3 minutes, just before the step of desulfurization in a reduction condition, injecting 7.5-10 Kg/T of a deoxidizer as a slag goods use and refluxing for 2-3 minutes.

Description

Decarburization of Ultra Low Carbon Stainless Steel

The present invention relates to a method for decarburizing ultra low carbon stainless steel, and more particularly, to improve decarburization efficiency by securing the fluidity of slag by proper input of fluorite (CaF ₂ ) and deoxidizer (Fe-Si) during blowing at a refining degree. A method for decarburizing ultra low carbon stainless steel.

In general, carbon in stainless steels causes grain boundary corrosion in austenitic systems and greatly reduces corrosion resistance and toughness of welded parts. Therefore, it is necessary to control the carbon in stainless steel to the extreme as long as economic efficiency is secured.

Meanwhile, the refining process for decarburizing stainless steel is divided into a decarburization method using an inert gas such as Ar or nitrogen and a vacuum decarburization method using a vacuum. In particular, the dilution decarburization method is performed by dividing into a step and a reduced degassing step.

The decarburization step of the dilute decarburization method is blown according to the carbon concentration in molten steel by blowing inert gas such as argon (Ar) or nitrogen and oxygen in 4-5 steps in order to effectively remove only carbon in stainless steel containing a large amount of chromium. By properly adjusting the ratio of oxygen and inert gas, carbon is first decarburized while preventing the oxidation of chromium as much as possible.

In the reduced degassing step after the decarburization step, a large amount of decarburizing agent and ferroalloy for component adjustment are introduced for the purpose of degassing the valuable metal oxide, especially chromium, contained in the slag.

In the reduction and degassing step, it is necessary to remove carbon as much as possible while suppressing oxidation of valuable metals such as chromium. In particular, in the final step of decarburization, the carbon concentration in molten steel is very low, thereby increasing the inert gas ratio such as argon to lower the CO partial pressure. Decarburization is performed to suppress oxidation of valuable metals.

Thus, when decarburizing only carbon containing a large amount of stainless steel first, decarburization efficiency and decarburization efficiency include decarburization temperature, CO partial pressure, molten steel stirring power, and slag conditions. The higher the decarburization temperature, the lower the CO partial pressure. The higher the molten steel stirring force and the higher the basicity of the slag, the better the decarburization.

On the other hand, the higher the decarburization temperature, the more carbon is decarburized than chromium, but increasing the furnace temperature increases the erosion of the furnace wall refractory, which leads to a cost increase, and thus it is not possible to increase the temperature above the critical temperature.

In addition, lowering the CO partial pressure means lowering the partial pressure of CO gas generated by reacting carbon and oxygen in molten steel by increasing an inert gas ratio such as oxygen to argon. The higher the inert gas ratio to oxygen, the lower the CO partial pressure is advantageous for decarburization.

However, as the supply oxygen ratio is lowered, the decarburization time becomes relatively long, and therefore, an appropriate inert gas to oxygen ratio should be adjusted in consideration of economical efficiency.

On the other hand, improving the agitation power of the molten steel is limited to the equipment and can improve the agitation power by using the inert gas flow rate to the maximum, but at this time, it can cause not only cost increase but also damage of the furnace wall refractory, so that the proper flow within the equipment specification can be achieved. You need to choose.

The quicklime is added as a coolant in the decarburization stage in consideration of slag formation in the refining furnace. The quicklime is used to adjust the basicity of the furnace in the reduced desulfurization stage after decarburization.

The quicklime added in the decarburization step is not recycled due to the lack of silica (SiO ₂ ) and is present in the solid state, and is recycled by silica (SiO ₂ ) generated by ferrosilicon (Fe-Si) introduced as a deoxidizer in the reduction degassing step. .

Normally, quicklime introduced in the decarburization stage is prevented from being recycled. Especially, if there is slag with fluidity that is refined in the early and middle stages where decarburization is strong, it forms a slag layer on the surface of the molten steel, and the CO generated in the molten steel by the stolen oxygen gas. This is because the decarburization efficiency is lowered by preventing the gas from being ejected from the molten steel.

On the other hand, in the decarburization step, it is known that chromium is first oxidized by the injected oxygen, and chromium oxide and carbon generated at this time react and decarburize.

However, some chromium oxides do not react with carbon and are present in the slag. In addition, the metal drop aggregates around the quicklime of the slag layer in the high carbon region at the initial stage of decarburization.

Since the metal drop contains a lot of carbon, the slag material is reduced during the reduction desulfurization step, causing carbon to be picked up in the molten steel.

The present invention is devised to improve the decarburization effect compared to the conventional decarburization described above, and the chromium oxide contained in the slag by appropriately recycling the slag under the same CO partial pressure, decarburization temperature and molten steel stirring force conditions immediately before the reduction desulfurization step It is an object of the present invention to provide a method for decarburizing ultra low carbon stainless steel that can induce a reaction with carbon in the molten steel and also prevent the pickup of carbon in the slag during reduction and degassing.

The figure is a graph showing the carbon concentration in the molten steel of the XM-7 steel grade at the degree of refinement.

In order to achieve the above object, in the present invention, the fluorite (CaF ₂ ) and the deoxidizer (Fe-Si) are added as an auxiliary agent at the end of the decarburization at the degree of refining (AOD) to promote the goods of the slag, thereby securing the flowability of the slag It provides a decarburization method of ultra-low carbon stainless steel characterized by inducing decarburization by means of removing carbon present in the slag.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

In the decarburization method of the ultra-low carbon stainless steel of the present invention, the fluorspar is refluxed for 2-3 minutes after 4.5-5.5Kg / T in the same inert gas-to-oxygen ratio condition at the final stage of decarburization, and the deoxidizer is used for slag ash immediately before the reduction decarburization step ( Fe-Si) to 7.5-10Kg / T, characterized in that the reflux 2-3 minutes.

In the decarburization step, by injecting inert gas such as argon (Ar) or nitrogen and oxygen as in the prior art, only carbon is first decarburized while controlling the ratio of oxygen and inert gas blown in accordance with the carbon concentration in molten steel to prevent oxidization of chromium as much as possible. . At the end of the process of decarburizing while injecting such inert gas and oxygen, injecting only inert gas such as argon to reflux and decarburizing, injecting fluorite at the completion of the process of decarburizing while injecting inert gas and oxygen as described above. Silicon is introduced into the 7.5-10Kg / T in the process of decarburization while refluxing only the inert gas.

Therefore, the slag fluidity of the slag is ensured to facilitate the reaction between chromium oxide in the slag and the carbon present in the molten steel, thereby promoting decarburization and removing carbon contained in the slag immediately before reduction and degassing, thereby improving decarburization efficiency. Let's do it.

Immediately before reducing deflow, the ratio of silica (SiO ₂ ) in the slag is increased from 4-6% to 15-25% and fluorite is used to ensure the fluidity of the slag so that the metal oxide in the slag reacts easily with the carbon in the molten steel.

In this way, the ferrosilicon is added to remove the carbon contained in the slag immediately before the reflux desulfurization step, and then reduced and degassed in an inert gas atmosphere such as argon as in the prior art.

However, when the amount of ferro-silicon (Fe-Si) is more than 10Kg / T, the maximum amount of silicon may react with oxygen to inhibit decarburization. Therefore, limit the amount to less than a certain level.If less than 7.5Kg / T, slag It is desirable to limit the input to 7.5-10Kg / T because the goods of the is inhibited and there is no decarburization effect.

In addition, it is effective to induce it to react only in the slag layer without directly reacting with molten steel using fine ferro silicon (Fe-Si).

In addition, when the fluorspar is added at less than 4.5Kg / T, the decarburization effect is poor because the reactivity with carbon is poor, and when it is added at more than 5.5Kg / T, the erosion of the furnace wall increases the erosion of the furnace wall refractory. It is uneconomical as it shortens the lifespan and leads to cost increase. Therefore, fluorspar is charged in the range of 4.5-5.5Kg / T.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example)

Example 1

This test was carried out on XM-7, a low carbon and ultra nitrogen nitrogen grade, as a bloom material at a refinement degree (AOD) with a 90 ton top lance, and at the final stage of decarburization, fluorite 5.5Kg / T was used for slag preparation. After refluxing for 2 minutes, and after completion of the decarburization step, 7.5 Kg / T of ferrosilicon (Fe-Si) was added immediately before the decarburization step, and fine decarburization was performed for 3 minutes.

Comparative Example 1

Subsequently, the reduction and dehydration was carried out by the conventional method, and as shown in the drawing, the carbon concentration in the molten steel was remarkably low as compared with the case by the conventional method. At this time, the blowing time was 99-114 minutes, which is usually In the case of similar level.

Example 2

As another example, this test was conducted on ultra low carbon stainless steel containing 0.02% by weight or less of carbon (C), and the carbon steel contained 0.01% by weight of stainless steel by decarburizing for 60 minutes by injecting oxygen, argon or nitrogen. At the time of completion of the decarburization step while incorporating this oxygen, 4.5-55Kg / T of fluorspar was added for the purpose of slag preparation and 2-3 minutes before the reduction degassing step in an argon gas atmosphere where no oxygen was blown. Si) 7.5-10 Kg / T was added to reflux decarburization to obtain a 0.005% by weight of stainless steel, the reflux decarburization time was 10 minutes. Thereafter, reduction and dehydration was performed for 10 minutes in an argon gas atmosphere. Carbon content of the obtained stainless steel was 0.007 weight%.

Comparative Example 2

For comparison, the general stainless steel containing the same carbon as above was decarburized for 70 minutes while blowing oxygen and argon or nitrogen to obtain 0.010% by weight of stainless steel, followed by reduction and dehydration for 10 minutes in an argon gas atmosphere. The carbon content of the obtained stainless steel was 0.014 weight%.

In the case of Example 2 and Comparative Example 2, the total blowing time was the same.

As can be seen from the results of Examples 1 and 2 and Comparative Examples 1 and 2, it can be seen that according to the method of the present invention, the decarburization effect is remarkably improved as compared with the case of manufacturing stainless steel by the conventional method.

In the present embodiment has been described in relation to ultra-low carbon stainless steel containing less than 0.02% by weight of carbon, it can be easily understood that the decarburization effect is improved according to the method of the present invention as compared to the conventional method for all stainless steels in general. .

Therefore, as described above, according to the decarburization method of the ultra low carbon stainless steel according to the present invention, effective decarburization is possible by injecting an appropriate amount of fluorite and ferrosilicon (Fe-Si) in a proper time within the same refining time, and thus ultralow carbon Since the carbon concentration of stainless steel can be significantly lowered, under the same CO partial pressure, decarburization temperature, and molten steel stirring force, slag is recycled just before the desulfurization step to induce a reaction between chromium oxide contained in slag and carbon in molten steel. The effect can be obtained such that picking up of carbon in the slag can be prevented during the discharge.

Claims (1)

In the decarburization method of ultra-low carbon stainless steel comprising the step of decarburizing the ultra-low carbon stainless steel containing 0.02% by weight or less of carbon while blowing an inert gas and oxygen, and subsequently reducing deflowing in an argon gas atmosphere, the decarburization step is inert Decarburization is performed by degassing while blowing gas and oxygen and reflux decarburization in an inert gas atmosphere without oxygen injection, and decarburization is performed by injecting oxygen having a degree of refinement (AOD) of fluorite (CaF ₂ ) 4.5-5.5Kg / T. At the end of the step, ferrosilicon (Fe-Si) 7.5-10Kg / T was added to the reflux decarburization step, which decarburizes without oxygen injection immediately before the reduction and degassing step, to induce slag ashing, thereby decarburizing molten steel by slag. It promotes and effectively removes carbon in the slag to improve the decarburization efficiency at the refinement rate when manufacturing ultra low carbon stainless steel Decarbonization method of ultra-low-carbon stainless steel that.