RU2333259C1 - Doping technique of noncorrosive steel by nitrogen - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention concerns ferrous metallurgy field. Particularly, it concerns noncorrosive steel making. Method includes nitrogen feeding into metal through the facility, mounted in brickwork of ladle bottom. Before nitrogen feeding it is defined sulphur content in metal and saturation by nitrogen is implemented subject to its content. Maximal nitrogen consumption is defined subject to sulphur content in metal. Nitrogen consumption have to be in the range not less 0.5 of maximal value and no more maximal value of nitrogen consumption, defined by ratio. Metal temperature is kept up in the range of 1520°-1640°C. It is probable addition of nitride ferroalloys at a finishing stage.

EFFECT: increasing of metal recovery degree of gaseous nitrogen while reducing of its consumption.

3 cl, 3 tbl, 2 ex

Description

The invention relates to the field of metallurgy, and in particular to the production of stainless steel primarily in out-of-furnace metal processing plants, for example, “ladle-furnace” units, vacuum and oxygen-oxygen decarburization (refining) plants.

A known method of alloying steel with nitrogen using nitrided ferroalloys. Nitrided ferroalloys are seated in a furnace or in a ladle during the release of metal from the furnace (Sviyazhin A.G. “Alloying with nitrogen”. Bulletin of scientific and technical information “Ferrous metallurgy”, issue 6 (1094), 1990, p.23).

The disadvantages of the method when it is used in plants for secondary processing are unstable and low absorption of nitrogen. In the case of subsequent vacuum treatment, almost complete loss of nitrogen introduced with ferroalloys occurs. The supply of nitrided ferroalloys to a ladle with metal leads to the melting and dissolution of ferroalloys in the surface layer of steel and the removal of most of the nitrogen into the atmosphere.

A known method of nitriding liquid steel in a ladle, including blowing the melt with gaseous nitrogen through an immersion lance and supplying nitrogen from above with jets to the surface of the bubble zone under the protection cone (RF Patent No. 2009209, IPC С21С 7/072, 03/15/94).

The process is characterized by a high nitrogen flow rate - 125-250 l / t · min and is accompanied by metal drilling in the upper part of the bucket. This leads to rapid wear of the lining of the bucket in the slag zone.

Closest to the claimed method by technical essence is a method of alloying stainless steel with nitrogen, including saturation of the metal with gaseous nitrogen with a flow rate of 17-36 l / t · min (Rimkevich BC, Butsky EV, Kurasov V.I., Sazhin I.V. ., Savchenko SG "On the possibility of alloying metal with nitrogen from the gas phase", Electrometallurgy, No. 2, 2000, pp. 14-16 - prototype).

The disadvantage of this method is the high consumption and low degree of assimilation of nitrogen, not exceeding 32%. A significant portion of the supplied gas is vented to the atmosphere. The process is accompanied by intensive mixing of metal and slag, which leads to increased erosion of the lining in the slag zone of the ladle and accelerate the cooling of steel. When steel is alloyed with gaseous nitrogen in out-of-furnace treatment plants without heating means, the latter can lead to overcooling of the metal and failure to achieve the required nitrogen content.

The task of the invention is to eliminate all the identified shortcomings, namely, reducing erosion of the lining of the bucket, reducing nitrogen consumption, reducing heat loss by optimizing the nitriding process.

This task is achieved by the fact that in the method of alloying stainless steel with nitrogen, including saturation of the metal with nitrogen gas by supplying nitrogen to the metal, the sulfur content in the metal is determined before nitrogen is supplied and its saturation with nitrogen is carried out taking into account the sulfur content with a nitrogen flow rate of at least 0.5 and no more than the maximum value of nitrogen flow, which is determined by the ratio:

Q = 12-29.9 [S] +16.9 [S],

where Q is the maximum nitrogen flow, l / t · min;

12 - nitrogen flow rate corresponding to the maximum rate of steel saturation with nitrogen, l / t · min;

29.9; 16.9- empirical coefficients;

[S] - sulfur content in the metal, wt.%;

It is advisable to saturate the metal with gaseous nitrogen in the temperature range 1520-1640 ° C.

In addition, it is advisable to obtain a nitrogen content in steel of more than 0.1%, together with a nitrogen purge, to supply nitrided ferroalloys at the finishing stage.

It was experimentally established that the dissolution of nitrogen in a metal from the gaseous phase is influenced by the sulfur content as a surface-active element that blocks the dissolution of nitrogen in the metal and, as a consequence, affects the rate of saturation of stainless steel with nitrogen. With an increase in the flow rate of nitrogen gas supplied, the rate of steel saturation with nitrogen increases.

If the maximum rate of steel saturation with nitrogen is exceeded, the release of excess (undissolved) amounts of nitrogen from the metal begins, accompanied by turbulent movement of the metal and slag in the ladle. Therefore, a further increase in nitrogen consumption does not affect the rate of saturation of the metal with nitrogen, but leads to its removal into the atmosphere and, as a result, to a reduction in the degree of assimilation, as well as to the intensification of the movement (mixing) of metal and slag in the ladle and increased wear of the lining of the ladle. This increases the cooling rate of the metal.

The proposed method allows to take into account the influence of the sulfur content in the smelted metal on the rate of saturation of the metal with nitrogen and to determine according to the given ratio the maximum nitrogen flow rate at which the maximum rate of saturation of the metal with nitrogen is achieved, which means that a high degree of assimilation of nitrogen by the metal is ensured. If the maximum flow rate of gaseous nitrogen, determined by the ratio, is exceeded, an excess amount of nitrogen is not absorbed by the metal and is removed from the ladle, causing turbulent movement of metal and slag in the ladle and destruction (erosion) of its lining, as well as increased cooling of the metal and excessive consumption of nitrogen. In this case, the degree of assimilation of nitrogen is reduced, as part of the nitrogen is removed into the atmosphere. When the flow rate of nitrogen gas is less than 0.5, determined by the ratio, the doping process is delayed, which leads to hypothermia of the metal, while the set value of the nitrogen content in the finished metal is not achieved.

The relationship between the sulfur content in the metal, the maximum rate of saturation of stainless steel with nitrogen and nitrogen consumption has been experimentally confirmed.

The maximum rate of metal saturation with gaseous nitrogen at various values of the sulfur content in stainless steel was determined by a change in its flow rate. The experimental results are shown in table 1.

It is advisable to carry out the nitriding process at a temperature of 1520-1640 ° C, not exceeding the values of the onset of softening of the refractory lining of the bucket. At a temperature of less than 1520 ° C, metal supercooling occurs, since this temperature is in the temperature range of stainless steel casting.

At temperatures above 1640 ° C, the erosion of the lining of the bucket begins, caused by the movement of metal and slag, which occur under the influence of nitrogen purge, because the indicated temperature corresponds to the beginning of softening of the refractories of the bucket under load, used for out-of-furnace treatment.

To obtain a nitrogen content of more than 0.1% in stainless steel, alloying the steel with gaseous nitrogen alone is not sufficient due to the length of the process, metal overcooling and the need for significant metal heating, which negatively affects the durability of the lining of the bucket. The introduction of nitrided ferroalloys at the stage of finishing the steel allows one to achieve the specified values of the nitrogen content in the steel without increasing the duration of the process. Thus, the technical result is to increase the degree of assimilation of gaseous nitrogen by the metal while reducing nitrogen consumption, erosion of the lining of the bucket and reducing heat loss.

The method is as follows.

After the release (intermediate) of stainless steel into the bucket, the necessary operations for processing steel (deoxidation, refining, etc.) are performed. Immediately before purging the steel with nitrogen, a sample is taken for the sulfur content in the metal, then the maximum nitrogen flow rate per ton of steel is determined from the given ratio and the metal is saturated with gaseous nitrogen by supplying nitrogen through a device installed in the bottom of the bucket, taking into account the sulfur content to the specified nitrogen values in of stainless steel. The nitrogen flow rate varies within the specified limits of at least 0.5 of the calculated value of the maximum nitrogen flow rate and not exceeding the calculated value of the maximum nitrogen flow rate. If necessary, the metal is heated to the optimum temperature of saturation of the steel with gaseous nitrogen, comprising 1520-1640 ° C.

For a given nitrogen content in steel of more than 0.1%, at the stage of metal refinement, together with nitrogen purging, nitrided ferroalloys are added according to calculation.

Examples of the method.

Example 1. On the installation of vacuum-oxygen refining in a ladle with a capacity of 30 tons, alloying of corrosion-resistant steel 04X18H10 with nitrogen gas was carried out, fed into the ladle with metal through a bottom tuyere (plug) with slotted holes 0.2 mm wide.

After the semi-product was discharged into a ladle with a lining of periclase-chromite refractories, vacuum-oxygen treatment and deoxidation of the metal and slag were performed under vacuum. Immediately prior to purging the metal with nitrogen, the sulfur content of the metal was determined. The maximum consumption of gaseous nitrogen was calculated from the above ratio, taking into account the sulfur content in the steel, and alloying the steel with nitrogen with a nitrogen flow rate in the limiting values determined by the specified ratio. The process of steel saturation with nitrogen was carried out at a temperature of 1520-1640 ° C. The degree of erosion of the lining of the bucket was determined by the content of magnesium oxide in the slag. The degree of assimilation of gas by the metal was determined by the ratio of assimilated nitrogen to the total consumption.

The parameters of the melting alloying metal with gaseous nitrogen and the results compared with the prototype are shown in table.2.

When exceeding the maximum flow rate of gaseous nitrogen, determined by the ratio, the degree of assimilation decreases and the erosion of the lining of the ladle increases, as evidenced by an increase in magnesium oxide in the slag (option 6, table 2).

When the nitrogen flow rate is less than 0.5 of the maximum nitrogen flow rate (option 8 of Table 2), the predetermined nitrogen content in the steel is not achieved when the metal is supercooled.

When the metal temperature is less than 1520 ° C, the specified nitrogen content in the metal is not achieved (option 5 of Table 2). At temperatures above 1640 ° C, erosion of the lining of the ladle is enhanced, as evidenced by an increase in the amount of magnesium oxide in the slag (option 4, table 2). The optimal options are 1, 2, 7 of Table 2.

Example 2. On the unit "ladle-furnace" alloying of stainless steel SV-10X16H25AM6, with a nitrogen content of 0.1-0.2%.

The metal (intermediate) was smelted in an electric arc furnace and released into a ladle with a lining of periclase-carbon refractories. Before saturation of the metal with gaseous nitrogen, a metal sample was taken to determine its chemical composition, including sulfur content. The maximum nitrogen flow rate was set according to the given ratio, taking into account the sulfur content in steel. The temperature of the metal was maintained at 1520-1640 ° C.

Turning on the supply of gaseous nitrogen through the purge device was carried out after installing the bucket on a transport trolley (steel truck). In the process of saturation of the metal with gaseous nitrogen, the nitrogen content in stainless steel was determined at the finishing stage, and, as calculated, nitrided ferroalloy-nitrided chromium with a nitrogen content of 8% was fed into the ladle with metal. The results of melting alloying metal of the claimed method and the prototype method are given in table.3.

When exceeding the maximum flow rate of gaseous nitrogen, determined by the ratio (option 5 of Table 3), the degree of assimilation of nitrogen decreases by 10%, and the content of magnesium oxide in the slag increases by 30%.

When the nitrogen flow rate is less than 0.5 of the maximum nitrogen flow rate (option 7 of Table 3), the specified nitrogen content in the steel is not achieved.

When exceeding the upper temperature limit (1640 ° С) of nitrogen saturation of the steel, the erosion intensity of the ladle lining increased (option 4, Table 3). With a decrease in the lower limit of the temperature range (1520 ° C), hardened metal in the amount of 580 kg remained on the bottom (loss) (option 8 of Table 3). The optimal options are 1, 3, 6 of Table 3.

The proposed method of alloying stainless steel with gaseous nitrogen increases the degree of assimilation of nitrogen by metal by 1.9-2.5 times, while erosion of the lining of the bucket is reduced by 1.5-2 times, nitrogen consumption by 1.5-3.8 times compared with prototype. Heat losses and melting time are reduced.

Table 1 The results of experiments to establish the maximum rate of saturation of steel with nitrogen, depending on the sulfur content Top speed Nitrogen consumption Sulfur content nitrogen saturation answering wt.% % / min maximum speed saturation, l / (t · min) 0.015 0.00115 11.6 0,120 0,00078 8.7 0.310 0,00031 4'4

Claims (3)

1. The method of alloying stainless steel with nitrogen, including saturation of the metal with gaseous nitrogen by supplying nitrogen to the metal, characterized in that before the supply of nitrogen, the sulfur content in the metal is determined and its saturation with nitrogen is carried out taking into account the sulfur content with a nitrogen flow rate of at least 0.5 and not more than the maximum nitrogen flow rate, which is determined by the ratio Q = 12-29.9 [S] +16.9 [S] ² , where Q is the maximum nitrogen flow, l / t · min; 12 - nitrogen flow rate corresponding to the maximum rate of steel saturation with nitrogen, l / t · min; 29.9; 16.9 - empirical coefficients; [S] - sulfur content in the metal, wt.%. 2. The method according to claim 1, characterized in that the metal is saturated with gaseous nitrogen in the temperature range 1520-1640 ° C. 3. The method according to claim 1, characterized in that in order to obtain a nitrogen content in the steel of more than 0.1%, together with a nitrogen purge, nitrided ferroalloys are supplied at the finishing stage.