The content of nitrogen and hydrogen in steel smelted by various processes is presented in table. 3.16.
HYDROGEN. The oxygen-converter method of steel production has prerequisites that ensure a lower hydrogen content in steel compared to the open-hearth process. During the smelting process, the hydrogen content changes from the initial content in the metal charge, mainly in cast iron (3-7 cm3/100 g) to the values indicated in the table. These values are less than the critical values at which the harmful effect of hydrogen on the quality of the cast metal begins to appear. This is very important for critical steels.
The main sources of hydrogen entering the converter bath are cast iron and steel scrap. A significant amount of scrap contributes in the form of rust. A lot of moisture can come from stale, partially hydrated lime.
The hydrogen content in the metal depends little on the moisture content in the source materials if they are not introduced into the converter at the end of the purge. The main part of hydrogen during the smelting process is removed with the gas phase. In addition, very mobile hydrogen is intensively washed out of the metal by CO bubbles.
The main reason for the low hydrogen content in oxygen-converter steel is the low moisture and hydrogen content in the gas phase above the metal. Unlike the atmosphere of the working space of an open-hearth furnace, where fuel is burned to form H2O and H2, the content of these gases in the converter cavity does not exceed 1%.
The process of hydrogen dissolution can be expressed by the following inequality:
It follows from this that the hydrogen content in steel is higher, the higher its partial pressure in the converter gases.
The transition of hydrogen into steel from water vapor is described by the equation:
(H20) = 2[H] + [O].
The equilibrium constant of this reaction is
The hydrogen content in steel increases with increasing partial pressure of water vapor in the converter gases, which is mainly determined by the humidity of the blast, and decreases with increasing oxidation of the steel.
The main source of hydrogen in the converter process is oxygen used for purge. Technical oxygen contains 8-10 g/m3 of moisture. Entering the reaction zone, water vapor dissociates and releases hydrogen to the metal. According to V.I. Yavoisky, the equilibrium concentration of [H] under these conditions can reach 10-13 cm3/100 g. The actual hydrogen content during the blowing process is much lower, which is associated with the degassing effect of boiling the bath.
In the practice of converter production, cases of very high hydrogen content in the metal are known. This is usually due to the use of a leaking purge lance and the entry of cooling water into the reaction zone.
It must be remembered that during the production and deoxidation of steel, the hydrogen content in it increases due to its entry from ferroalloys and carburizing additives.
The dynamics of changes in hydrogen during metal purging in the converter are shown in Fig. 3.54.
The initial level of hydrogen content in the metal is determined by its content in the charge. During the first period of purging (4-6 minutes), there is an increase in the concentration of hydrogen in the metal, which is associated with its entry from the rust of scrap metal, hydrate moisture of lime and from the converter atmosphere; The partial pressure of water vapor in the atmosphere during this period is quite high, since the rate of carbon oxidation is low. The significant instability of hydrogen concentration values obtained in different melts is explained by different amounts of hydrogen coming from different sources; after 4-6 minutes, the carbon content in the metal decreases, which is associated with an increase in the rate of decarbonization and the leaching effect of CO bubbles. At the end of blowing, the hydrogen content in the metal increases, which is explained by a decrease in the rate of carbon oxidation.
The final hydrogen content in the converter bath largely depends on the moment of introducing hydrogen-containing additives - the closer this moment is to the end of the purge, the higher the hydrogen concentration.
A radical means of reducing the hydrogen content in steel is evacuation, during which the degree of hydrogen removal is 50-70% or more.
NITROGEN. Due to its inherent features (low partial pressure of nitrogen in oxygen blast, high rate of carbon oxidation, absence of air leaks into the converter cavity), the oxygen-converter process is well suited for producing metal with a low nitrogen content at the outlet.
The nitrogen content in the oxygen-converter metal is of greatest interest in the production of low-carbon steels intended for cold plastic deformation. The influence of nitrogen on the ductility of steel and its tendency to age during service and, in particular, at low temperatures completely disappears when its concentration in steel does not exceed 0.001-0.0005%. Taking into account the decisive role of nitrogen in the quality indicators of steel, this issue should be discussed in more detail.
Nitrogen dissolves well in liquid iron - at 1600°C 0.044% N dissolves - and very limitedly in solid iron. In iron at room temperatures, the solubility of nitrogen becomes significantly less than its actual content. However, unlike hydrogen, nitrogen does not release from the steel upon cooling, forming a supersaturated solution. The loss of nitrogen from a supersaturated solution, which is possible during thermomechanical treatment of metal, leads to a decrease in the ductility of steel and is called aging.
The nitrogen concentration in the metal that is in equilibrium with the gas phase is determined by Sieverts’ law:
The proportionality constant is a function of the composition of the metal bath and the temperature. As the temperature increases, the Kn values increase, which leads to an increase in the solubility of nitrogen in the metal.
For the oxygen-converter process, the partial pressure of nitrogen in the blast Pn is of particular importance. This is due to the fact that the temperature of the reaction zone can reach 2500°C.
Since the metal in the reaction zone is in contact with the oxygen stream and contains virtually no carbon, the solubility of nitrogen in it will correspond to its solubility in pure iron. Calculations by V.I. Yavoisky showed that the maximum solubility of nitrogen at an oxygen purity of 97% (PN2 = 0.18 at g and T = 2200°C) is 0.0256%.
The metal, saturated with nitrogen in the reaction zone, is transferred to a volume of metal removed from it. Naturally, the nitrogen concentration in the entire volume of the bath will be many times lower, not only due to the higher content of bath impurities and lower metal temperature, but also due to a sharp decrease in the partial pressure of nitrogen at a relatively high rate of carbon oxidation and a high concentration of carbon-containing gases in the exhaust gases. However, the above indicates the advisability of reducing the temperature of the reaction zone, in particular by injecting dust-like materials.
Factors determining the nitrogen content in the converter bath.
The main sources of gases entering converter steel include:
- charge materials;
- atmosphere of the melting unit;
- technical oxygen;
- ferroalloys and additives introduced into metal;
- the atmosphere surrounding the liquid metal during its release and casting, etc.
Charge materials and ferroalloys. Below are data on the gas content of various materials.
The data in Table 3.17 indicates that the most important sources of nitrogen entering converter steel are primarily charge materials (cast iron, scrap, ferroalloys, etc.). For the oxygen-converter process, the main component of the charge is cast iron, so it is obvious that the share of nitrogen introduced by cast iron must be significant. The nitrogen content in cast iron from different plants varies between 0.003-0.014%. The different levels of nitrogen in pig iron of a number of plants are explained by the specific production conditions at these enterprises. In general, up to 75% of the total amount is added to the converter bath with cast iron.
Blast melting mode. One of the factors determining the nitrogen content in steel is the degree of purity of the oxygen blast. As studies conducted under industrial conditions at NLMK and the Krivorozhstal metallurgical plant have shown, a low nitrogen content (about 0.002%) can only be obtained by blowing the metal with high-purity oxygen (more than 99.7%). A decrease in the purity of oxygen blast to 99.2-99.5% leads not only to an increase in the nitrogen content, but also to an increase in the spread of this value from heat to heat. The relationship between oxygen purity η and nitrogen content in low-carbon steel before its release from the converter is characterized by the following data presented in Table 3.18.
Blowing the metal with oxygen of about 98% purity leads to an increase in the nitrogen content in the metal to 0.0063-0.0090%. The change in nitrogen content in the metal depending on the degree of purity of the blast is due to the fact that gas with a very low partial pressure of nitrogen is blown into the metal. In this case, the higher the degree of rarefaction (oxygen purity) and the intensity of mixing the bath with released carbon monoxide, the higher the removal of nitrogen from the metal (Fig. 3.55).
As can be seen from the figure, in region I, after 5-6 minutes of blowing, the process of denitrogenation of the melt becomes more intense than the transition of nitrogen into the metal, and this process develops up to a certain nitrogen content in the metal (0.003-0.004%) and depends on its partial pressure in the reaction zone and on the rate of decarbonization during this period. When blowing with oxygen of reduced purity (92-99% O2), the nitrogen content in the metal reaches a minimum at about the 12th minute, after which it begins to increase. In the last third of the blowdown period, stabilization of the nitrogen content is observed, which indicates the relative equality of the flows of nitrogen input and removal.
However, if optimal smelting conditions are not observed, an increase in the nitrogen concentration in the metal is observed, even with high oxygen purity. One of the factors determining the nitrogen content in the metal is the intensity of oxygen supply. It has been established that when blowing with high-purity oxygen, increasing the blowing intensity helps to reduce the nitrogen content in steel.
The significant level and limits of fluctuations in the nitrogen content in the metal are quite understandable if we take into account the large amount introduced by oxygen blast compared to other sources (Table 3.19).
In the process of conducting research on 130 converters, V.I. Yavoisky and his colleagues found that the nitrogen content in the metal after blowing is determined by the purity of the oxygen blast and the carbon content:
Another important factor in the blast regime that affects the nitrogen content in the metal is the position of the tuyere above the metal surface.
Nitrogen from the atmosphere can pass into steel as a result of its ejection by a stream of oxygen. Ejection is impossible when blowing in the “flooded” jet mode. However, at the end of blowing, with increasing tuyere height, the nitrogen content increases due to its suction through the neck (Fig. 3.56).
To prevent this process, it is proposed to blow argon into the tap hole at the end of blowing and add limestone, mill scale, and iron ore to the converter, which makes it possible to stabilize the nitrogen content in the metal after blowing at the level of 0.002%.
Blowers. It is especially necessary to dwell on the role of post-blowing in terms of increasing nitrogen in steel. Particularly unpleasant consequences occur with two or more blowings (Fig. 3.57).
Additional blowing inevitably leads to an increase in nitrogen in the metal for the following reasons:
1 - during additional blowing, the atmosphere in the converter is renewed;
2 - air suction increases, since additional blowing is carried out in the “open jet” mode.
It should be noted that post-blowing carried out at low carbon content is especially dangerous (Fig. 3.57).
Slag regime. An important technological factor that determines the final nitrogen content in the metal is the slag mode of converter smelting, first of all, the state of the slag, its quantity, which is associated with the protective effect of the slag layer. In the optimal slag melting mode, the slag during the blowing process is in a foamed state and the metal is isolated from contact with the atmosphere. If slag coagulation is observed during the blowing process, then metal with a high nitrogen content is obtained before tapping the melt. It has been experimentally established that saturation of the metal with nitrogen in the converter begins after 60% of the purging time (Fig. 3.58), and therefore, in order to obtain low nitrogen concentrations, it is necessary to maintain the slag in a foamed state in the last 50% of the purging time.
Saturation of steel with nitrogen during tapping and casting. Nitrogen uptake during tapping, deoxidation, and casting has a significant impact on the nitrogen content of the finished metal, and there are many factors that influence the process of nitrogen uptake during tapping. Thus, it has been established that the absorption of nitrogen by the metal increases with a decrease in the diameter of the tap hole and an increase in its length, with an increase in the thickness of the slag cover, and decreases with a decrease in the height of the metal fall and the oxygen content in the liquid steel. It also significantly depends on the duration of release and the geometry of the jet, i.e. determined by the size of the contact surface of the metal with atmospheric air during the draining process.
The saturation of the metal with nitrogen is also facilitated by the introduction of deoxidizers and alloying agents into the steel-pouring ladle during its filling.
Chemical composition of the metal. All elements, according to the degree of their influence on the solubility of nitrogen in iron, can be divided into three groups:
1. Elements that form fairly stable nitrides. When doped with these elements, the solubility of nitrogen in iron increases. Elements of this group include Cr, V, B, Mn, Si, Al, Ti, Nb, Zr and rare earth metals. The activity coefficient of nitrogen in iron alloys with these elements decreases.
2. Elements - analogues of iron, which practically do not affect the solubility of nitrogen: Ni, Co.
3. Elements that reduce the solubility of nitrogen - C, P.
In general, it can be stated that the oxygen-converter process is unique in terms of obtaining low nitrogen in the metal after purging. This process can easily achieve 0.002% or less if the following conditions are met:
1. High purity of oxygen (99.7-99.8%).
2. Use of cast iron with low nitrogen content. In this sense, the use of cast iron treated with lime and natural gas is promising, where low concentrations of sulfur (0.003 - 0.005%) and nitrogen (less than 0.003%) are simultaneously achieved. When converting such cast iron, steel with 0.0013% nitrogen is obtained.
3. Evacuation of steel with low sulfur content.
4. Complete elimination of additional blowing when producing low-nitrogen steels.
5. Reducing the temperature of the reaction zone by introducing lime, ore, sinter, etc.
6. Additive to the ladle during smelting of gasifying additives (limestone, ore, etc.) in an amount of up to 2 kg/t, which act as both boiling intensifiers and active gas-forming agents.
7. Prevention of metal saturation with nitrogen during tapping, out-of-furnace processing and casting, since evacuation of steel for the purpose of denitrogenation is ineffective.
OXYGEN. Providing specified oxygen contents for different types of steel is achieved by an appropriate deoxidation regime. With a certain deoxidation technology, the higher the oxygen content in the metal at the time of casting, the higher its content at the time of deoxidation.
In addition, the contamination of the finished metal with oxide inclusions (deoxidation products), the behavior of the metal during casting and the structure of the ingot (blank), as well as the concentration of oxygen remaining in the solid solution, depend on the oxygen content in liquid steel; the latter contributes to the aging of steel, increases its fragility, increases electrical resistance and negatively affects magnetic properties.
In liquid steel, oxygen can exist in an active, unbound form and in the form of oxide inclusions. Oxygen is currently the only element whose activity can be determined by direct measurement - by measuring the E.M.F. in a high temperature concentration galvanic cell.
The transition of oxygen from blast to metal occurs in two stages; At the first stage, oxidation occurs predominantly of iron:
on the second - partial dissolution of oxygen in the metal:
Naturally, the oxygen content in the metal of the reaction zone is higher than in the entire volume of the bath. The metal of the reaction zone is essentially an oxygen donor to the entire bath.
Along with the process of transition of oxygen from slag to metal, there are processes of oxidation of other impurities, as a result of which oxygen is removed from the slag and metal, and the oxygen content at each moment of melting is determined by the rate of these two mutually opposite processes. The change in oxygen content during purging is shown in Fig. 3.59 (according to R.V. Starov).
At the very beginning of purging, with a low rate of carbon oxidation and poor mixing of the bath, the oxygen content slightly increases. With the onset of intense oxidation of carbon, the oxygen content decreases, and the spread of values also decreases, st. A decrease in the oxidation rate and carbon content leads to a sharp increase in both absolute values and the spread of oxygen content values.
The invention relates to ferrous metallurgy, in particular to the production of stainless steel. The method involves supplying nitrogen to the metal through a device installed in the lining of the ladle bottom. Before supplying nitrogen, the sulfur content in the metal is determined and saturation with nitrogen is carried out taking into account its content. The maximum nitrogen consumption is determined depending on the sulfur content in the metal. Nitrogen consumption must be within the range of at least 0.5 of the maximum value and not more than the maximum value of nitrogen consumption, determined by the ratio. The metal temperature is maintained within 1520°-1640°C. It is possible to add nitrided ferroalloys at the finishing stage. The use of the invention makes it possible to increase the degree of assimilation of gaseous nitrogen by the metal while reducing its consumption. 2 salary files, 3 tables.
The invention relates to the field of metallurgy, namely to the production of stainless steel mainly in out-of-furnace metal processing installations, for example ladle-furnace units, evacuation and vacuum-oxygen decarbonization (refining) units.
There is a known method of alloying steel with nitrogen using nitrided ferroalloys. Nitrided ferroalloys are added to the furnace or ladle during the release of metal from the furnace (Sviyazhin A.G. “Alloying steel with nitrogen.” Bulletin of scientific and technical information “Ferrous Metallurgy”, issue 6 (1094), 1990, p. 23).
The disadvantages of this method when used in after-furnace treatment plants are unstable and low nitrogen absorption. In the case of subsequent vacuum treatment, there is an almost complete loss of nitrogen introduced with ferroalloys. Feeding nitrided ferroalloys into 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.
There is a known method for nitriding liquid steel in a ladle, which includes blowing the melt with gaseous nitrogen through a submersible lance and supplying nitrogen from above in jets to the surface of the bubbling zone under the protection cone (RF Patent No. 2009209, IPC S21S 7/072, 03/15/94).
The process is characterized by high nitrogen consumption - 125-250 l/t·min and is accompanied by metal seething in the upper part of the ladle. This leads to rapid wear of the ladle lining in the slag zone.
The closest to the claimed method in technical essence is the method of alloying stainless steel with nitrogen, which includes saturating the metal with gaseous nitrogen at a flow rate of 17-36 l/t·min (Rimkevich V.S., Butskiy E.V., Kurasov V.I., Sazhin I.V. ., Savchenko S.G. "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 nitrogen absorption, not exceeding 32%. A significant portion of the supplied gas is released into the atmosphere. The process is accompanied by intense mixing of metal and slag, which leads to increased erosion of the lining in the slag zone of the ladle and accelerated cooling of the steel. When alloying steel with gaseous nitrogen in after-furnace treatment plants without heating means, the latter can lead to overcooling of the metal and failure to achieve the required nitrogen content.
The problem solved by the invention is to eliminate all identified shortcomings, namely to reduce erosion of the ladle lining, reduce nitrogen consumption, reduce heat losses by optimizing the nitriding process.
This task is achieved by the fact that in the method of alloying stainless steel with nitrogen, which includes saturating the metal with gaseous nitrogen by supplying nitrogen to the metal, before supplying 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 consumption of at least 0.5 and no more than the maximum nitrogen consumption, which is determined by the ratio:
Q=12-29.9[S]+16.9[S],
29.9; 16.9 - empirical coefficients;
It is advisable to saturate the metal with nitrogen gas 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 nitrogen purging, to supply nitrided ferroalloys at the finishing stage.
It has been experimentally established that the dissolution of nitrogen in the metal from the gaseous phase is influenced by the content of sulfur as a surface-active element, which blocks the dissolution of nitrogen in the metal and, as a consequence, affects the rate of saturation of stainless steel with nitrogen. As the flow rate of supplied nitrogen gas increases, the rate of saturation of steel with nitrogen increases.
When the maximum rate of saturation of steel with nitrogen is exceeded, the release of excess (undissolved) amount of nitrogen from the metal begins, accompanied by turbulent movement of 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 consequence, a decrease in the degree of assimilation, as well as to an intensification of the movement (mixing) of metal and slag in the ladle and an increase in wear of the ladle lining. This increases the cooling rate of the metal.
The proposed method makes it possible 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 consumption of nitrogen at which the maximum rate of saturation of the metal with nitrogen is achieved, which means that a high degree of nitrogen assimilation by the metal is ensured. When the maximum consumption of gaseous nitrogen, determined by the ratio, is exceeded, the 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 (washout) of its lining, as well as increased cooling of the metal and excessive consumption of nitrogen. At the same time, the degree of nitrogen absorption decreases, since part of the nitrogen is removed into the atmosphere. When the consumption of gaseous nitrogen is less than 0.5 determined by the ratio, the alloying process is delayed, which leads to overcooling of the metal, while the specified 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 nitrogen gas at different values of sulfur content in stainless steel was determined by changing its consumption. The results of the experiments are given 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 beginning of softening of the refractory lining of the ladle. At a temperature of less than 1520°C, supercooling of the metal occurs, since this temperature is in the temperature range for casting stainless steel.
At temperatures above 1640°C, intensification of erosion of the ladle lining begins, caused by the movement of metal and slag occurring under the influence of nitrogen blowing, because the indicated temperature corresponds to the beginning of softening of ladle refractories under load used for out-of-furnace processing.
To obtain a nitrogen content of more than 0.1% in stainless steel, it is not enough to alloy the steel with nitrogen gas only due to the duration of the process, overcooling of the metal and the need for significant heating of the metal, which negatively affects the durability of the ladle lining. The introduction of nitrided ferroalloys at the steel finishing stage makes it possible to achieve specified values of nitrogen content in steel without increasing the duration of the process. Thus, the technical result is an increase in the degree of assimilation of gaseous nitrogen by the metal while reducing nitrogen consumption, washing out the ladle lining and reducing heat losses.
The method is carried out as follows.
After releasing the (semi-finished) stainless steel into the ladle, the necessary steel processing operations (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 consumption per ton of steel is determined using the given ratio and the metal is saturated with gaseous nitrogen by supplying nitrogen through a device installed in the bottom of the ladle, taking into account the sulfur content to the specified nitrogen values in of stainless steel. Nitrogen consumption is varied within the specified limits by at least 0.5 of the calculated value of the maximum nitrogen consumption and not exceeding the calculated value of the maximum nitrogen consumption. If necessary, the metal is heated to the optimum temperature of steel saturation with nitrogen gas, which is 1520-1640°C.
At a given nitrogen content in steel of more than 0.1%, at the metal finishing stage, together with nitrogen purging, nitrided ferroalloys are added according to calculation.
Examples of method implementation.
Example 1. At a vacuum-oxygen refining installation in a ladle with a capacity of 30 tons, corrosion-resistant steel 04Х18Н10 was alloyed with gaseous nitrogen supplied to the ladle with metal through a bottom tuyere (plug) with slotted holes 0.2 mm wide.
After releasing the intermediate product into a ladle lined with periclase-chromite refractories, vacuum-oxygen treatment and deoxidation of the metal and slag were carried out under vacuum conditions. Immediately before blowing the metal with nitrogen, the sulfur content in the metal was determined. The maximum consumption of gaseous nitrogen was calculated using the above ratio, taking into account the sulfur content in the steel, and the steel was alloyed with nitrogen with nitrogen consumption in the limiting values determined by the specified ratio. The process of saturating steel with nitrogen was carried out at a temperature of 1520-1640°C. The degree of erosion of the ladle lining was determined by the content of magnesium oxide in the slag. The degree of gas assimilation by the metal was determined by the ratio of the assimilated nitrogen to the total consumption.
The parameters of the melts of metal alloying with nitrogen gas and the results in comparison with the prototype are given in Table 2.
When the maximum consumption of gaseous nitrogen, determined by the ratio, is exceeded, the degree of its assimilation decreases and the erosion of the ladle lining increases, as evidenced by an increase in magnesium oxide in the slag (option 6, table 2).
When nitrogen consumption is less than 0.5 of the maximum nitrogen consumption (option 8 of Table 2), the specified 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 ladle lining increases, 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 a ladle-furnace unit alloying of SV-10Kh16N25AM6 stainless steel was carried out, with a nitrogen content of 0.1-0.2%.
The metal (semi-product) was smelted in an electric arc furnace and released into a ladle lined with periclase-carbon refractories. Before saturating the metal with nitrogen gas, a sample of the metal was taken to determine its chemical composition, incl. sulfur content. The maximum nitrogen consumption was set according to the given ratio, taking into account the sulfur content in the steel. The metal temperature was maintained at 1520-1640°C.
The supply of nitrogen gas through the purge device was turned on after installing the ladle on the transport cart (steel carrier). In the process of saturating the metal with gaseous nitrogen at the finishing stage, the nitrogen content in stainless steel was determined and, according to calculations, nitrided ferroalloy-nitrided chromium with a nitrogen content of 8% was supplied to the ladle with the metal. The results of metal alloying melts using the claimed method and the prototype method are given in Table 3.
When the maximum consumption of gaseous nitrogen, determined by the ratio (option 5 of Table 3), is exceeded, the degree of nitrogen assimilation is reduced by 10%, and the content of magnesium oxide in the slag increases by 30%.
When nitrogen consumption is less than 0.5 of the maximum nitrogen consumption (option 7 of Table 3), the specified nitrogen content in the steel is not achieved.
When the upper temperature limit (1640°C) of steel saturation with nitrogen was exceeded, the intensity of erosion of the ladle lining increased (option 4, Table 3). When the lower limit of the temperature range decreased (1520°C), solidified metal remained on the bottom in the amount of 580 kg (losses) (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 nitrogen assimilation by the metal by 1.9-2.5 times, while the erosion of the ladle lining is reduced by 1.5-2 times, nitrogen consumption is reduced by 1.5-3.8 times compared with prototype. Heat losses and melting duration are reduced.
1. A method of alloying stainless steel with nitrogen, including saturating the metal with nitrogen gas by supplying nitrogen to the metal, characterized in that before supplying 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 consumption of at least 0.5 and not more than the maximum nitrogen consumption, which is determined by the ratio
Q=12-29.9[S]+16.9[S] 2 ,
where Q is the maximum nitrogen consumption, l/t·min;
12 - nitrogen consumption corresponding to the maximum rate of saturation of steel with nitrogen, l/t·min;
29.9; 16.9 - empirical coefficients;
2. The method according to claim 1, characterized in that the metal is saturated with nitrogen gas in the temperature range 1520-1640°C.
3. The method according to claim 1, characterized in that to obtain a nitrogen content in steel of more than 0.1%, together with nitrogen purging, nitrided ferroalloys are supplied at the finishing stage.
Atomic nitrogen and dissolves, And forms chemical compounds in steel.
Increased solubility have dissociated nitrogen.
Educated chemical compounds - nitrides. In steels, nitrogen forms nitrides with both iron and most impurities.
With iron, nitrogen produces two types of nitrides (chemical compounds): Fe 4 N contains 5.88% N 2, Fe 2 N - 11.1% N 2. Ionic type nitrides are obtained by the interaction of metals with nitrogen at temperatures of 700-1200 °C. Nitrides are formed in plasma in arc, high-frequency and ultra-high-frequency plasmatrons. In the latter case, nitrides are formed as ultrafine powders with a particle size of 10-100 nm.
Nitrogen also forms nitrides with steel alloying elements, often significantly more stable than iron nitrides. They are particularly resistant to high temperatures silicon and titanium nitrides.
For welding, Fe 4 N is of greater importance. Ultrafine refractory nitrides with particle size 10-100 nm solidify in the weld pool faster than iron, therefore, with an increased cooling rate of the metal, iron nitrides may not have time to precipitate from the ά-Fe solution, and the latter will be oversaturated nitrogen.
Welding heating introduces deviations from the equilibrium state of N solubility in Fe. The total amount of nitrogen dissolved in the metal due to heating of the metal can be enlarged.
The solubility of nitrogen in iron depends significantly on temperature(Fig. 113). As the temperature rises, the solubility of nitrogen increases, undergoing abrupt changes at the moments of polymorphic transformations of iron and during its transition from solid to liquid. Abrupt changes in solubility lead to the formation of gas bubbles.
Research process of saturating metal with nitrogen showed that it is possible these are the ways it proceeds:
1) dissociated nitrogen directly dissolves in liquid metal drops. Upon subsequent cooling of the metal at appropriate temperatures iron nitrides are formed;
2) dissociated nitrogen forms at high temperatures persistent nitrides, which, dissolving in drops of liquid metal, saturate it with nitrogen.
3) dissociated nitrogen forms in high temperature region nitric oxide NO, which dissolves in droplets. At metal temperatures below 1000 °C, nitrogen oxide precipitates from the solid solution and dissociates; in this case, atomic nitrogen forms iron nitrides, and oxygen forms oxides.
So: during welding, nitrogen simultaneously dissolves in the metal and forms chemical compounds (iron nitrides) and ultimately saturates the iron with nitrogen and its chemical compounds.
Being in a metal in one state or another, nitrogen greatly affects its properties. From Fig. 114 it is clear that with An increase in nitrogen content increases the strength and fluidity limits of the metal.
At the same time the plastic properties and, especially sharply, the impact strength of steel decrease. Along with this, the tendency of the metal to aging appears, the tendency to cold brittleness and blue brittleness increases, the ability to harden increases, and the
magnetic permeability, the electrical resistance of the metal increases.
Thus, in the general case, nitrogen is an undesirable impurity in the weld metal, especially when such metal is exposed to a dynamic load.
However in welding conditions of high-alloy steels of the austenitic class, nitrogen increases the stability of austenite and acts as an alloying additive that can replace a certain amount of nickel.
Nitrogen is usually introduced into steel in the form of nitrided ferroalloys (ferrochrome, ferromanganese) containing from 1.5 to 7.0% nitrogen.Maximum nitrogen absorption is about 0.3%. Attempts to obtain steel with a higher nitrogen content led to castings being affected by gas bubbles. In the authors' studies, nitrogen was introduced in the form of manganese nitrogen. The chemical composition of the experimental melts and the amount of gases in the metal are given in Table. 1.
The degree of nitrogen absorption by the metal at a low nitrogen content was about 70% of the input. With an increase in the amount of introduced nitrogen, the degree of assimilation decreases to 55% (m3) and lower.
Alloying with nitrogen gives a slight increase in the yield strength of steel and a decrease in the values of relative elongation and contraction (Table 2).
The nature of the microstructure of steel with an increased amount of nitrogen remained almost unchanged.
Impact strength, determined on standard notched samples, at all test temperatures is lower than for samples with a normal amount of nitrogen (Fig. 1).
The wear resistance of steel increases by approximately 10-15% with increasing nitrogen content. The loss of metal during the test cycle (70 thousand revolutions with rolling friction with 10% slip, P = 70 kg) is P1 - 1.04 g for melt samples; swimming trunks M1 - 0.81 g and swimming trunks.
When some elements are introduced into steel, for example vanadium, chromium, etc., the solubility of nitrogen in iron increases to a greater extent than when manganese is introduced.
Thanks to this, when introduced into steel, in addition to manganese nitride, chromium nitride, containing 9.0% nitrogen, it was possible to retain 57 cm3/100 g of nitrogen in the metal. The degree of “assimilation” of nitrogen by steel is 36%. The surface of the cast samples was not smooth, although the entire volume of metal did not yet have gas shells.
The chemical composition of this group of samples is given in Table. 3.
The simultaneous presence of chromium and nitrogen in manganese steel has a beneficial effect on the mechanical properties (Table 4) and microstructure. The austenite grain size decreases to No. 4-6.
As follows from the table. 4, nitrogen and chromium in manganese steel very significantly increase the yield strength and tensile strength, without reducing the ductility of the steel.
The impact strength at all test temperatures is at the same level as for samples alloyed with nitrogen alone (Fig. 2).
The wear resistance of steel when alloyed together with nitrogen and chromium increased by approximately 15% compared to ordinary manganese steel tested under the same conditions, i.e., it turned out to be no higher than when alloying steel with nitrogen alone.
Based on the results of laboratory studies, two melts were carried out at one of the plants and experimental cores were cast from manganese steel with nitrogen and chromium additives. The chemical composition of the cores is given in table. 5.
Previously, the metal of smelting No. 1 was deoxidized in the furnace to a content of manganese oxide and iron oxide of 8.1 and 2.0%, respectively. Nitrogen in an amount of 0.043% was introduced into the steel in the form of manganese nitride after precipitation deoxidation of the metal in a ladle with aluminum.
The nitrogen content of the finished core, determined by vacuum melting, was 0.033%. Consequently, nitrogen absorption was approximately 70%. The actual nitrogen content of the steel was probably higher, since analysis carried out by conventional methods always gives somewhat underestimated results due to the volatility of manganese and its ability to combine chemically with gases released from the sample.
The metal of smelting No. 2 in the furnace was alloyed with ferrochrome in addition to manganese before release. After deoxidation in a ladle with aluminum, manganese nitride containing 6.0% nitrogen was added to the steel. From each experimental melt, four cores of the P50 type 1/11 were cast and crosspieces were assembled, which were put into operation on various railways.
On the West Siberian Railway, systematic monitoring of cores was established, and data from control measurements are annually submitted to the Central Research Institute. There, a control crosspiece with a mass-produced high-manganese steel core was simultaneously monitored, the operating conditions of which were similar to the conditions of the experimental crosspieces.
Numerous operational observations have shown that the core made of high-manganese steel wears out most intensively in a section with a width of 20 mm.
Research has shown that the wear pattern of experimental crosspieces has not changed compared to standard-production crosspieces (the most intense wear was also observed in the first period of operation), but the wear rate turned out to be lower and the service life of the cores increased.
The mass-production control crosspiece went out of service after passing 152.9 million m of moving load through it. This wear resistance is slightly above average. Two experimental crosspieces were removed from the track after 134.2 and 216.8 million tons of cargo were passed through them. The first one was removed not due to wear, but due to a crack in the guardrail. The remaining two crosspieces, one of which passed more than 200 million tons of cargo, are in operation, and monitoring of them continues.
The results of the work showed that experimental cores made of manganese austenitic steel containing an increased amount of nitrogen can pass a larger amount of moving load up to the maximum permissible wear compared to cores made of G13L steel of standard composition.
25.11.2019
Every modern person sooner or later has to decide where to put a computer desk. We evaluate the free space in the apartment and go ahead - select a model...
25.11.2019
The question of where to place carpets in an apartment is no less important than the ability to choose the right carpet. This article will tell you how to do this....
25.11.2019
In every industry where liquid or viscous products are produced: in the pharmaceutical industry, in the cosmetics industry, in the food and chemical sectors - everywhere...
25.11.2019
Today, mirror heating is a new option that allows you to keep the mirror surface clean from hot steam after taking water procedures. Thanks to...
25.11.2019
A barcode is a graphic symbol depicting alternating stripes of black and white or other geometric shapes. It is applied as part of the marking...
25.11.2019
Many owners of country residential estates who want to create the most comfortable atmosphere in their home,...
25.11.2019
In both amateur and professional construction, profile pipes are in great demand. With their help, they are built to withstand heavy loads...
24.11.2019
Safety shoes are part of a worker’s equipment designed to protect feet from cold, high temperatures, chemicals, mechanical damage, electricity, etc...
The development of new branches of technology, as well as the intensification of existing processes of physical and chemical technology for the production of materials and products require a sharp increase in the quality of metal, the level of service characteristics and reliability of products.
Considering the increasing shortage of the most important alloying elements (nickel, chromium, cobalt, tungsten, molybdenum, etc.), leading steel producers believe that the main direction for increasing the mechanical and physical properties of steel and reducing the weight of structures will be the transition to ultra-pure carbon and low-alloy steels or steels , alloyed with non-deficient elements with more efficient use of the possibilities of controlling the structure and properties of steels through microadditives and temperature and deformation treatment.
One of the promising elements for alloying and microalloying steel is nitrogen. This is an accessible and completely scarce material. Nitrogen, with its widespread availability and low cost, is a strong austenite-forming element and is effectively used in the production of economically alloyed steels for various purposes.
Low-alloy nitride-hardened steels typically contain 0.010 to 0.040% nitrogen, while high-alloy metals may have nitrogen concentrations in excess of 1%.
For alloying with nitrogen, any material that contains nitrogen in sufficient quantities and is capable of dissolving in liquid metal can be used. Due to their low cost and simplicity, nitrogen alloying methods are known that are based on blowing the melt with nitrogen gas.
Therefore, alloying steel with nitrogen to obtain stable austenite and its strengthening is currently becoming increasingly widespread. However, alloying steel with nitrogen presents some difficulties, since in order to evaluate the behavior of nitrogen at various stages of the steelmaking process, it is necessary to have reliable data on solubility, dissolution rate and conditions of interaction of nitrogen with other components of the melt.
The relevance of the problems lies in determining the possibility of predicting the solubility of nitrogen in metal melts depending on their chemical composition, temperature, partial pressure of nitrogen in the gas phase, as well as the need to know the kinetic characteristics of the process as a function of the conditions of the smelting process and after-furnace processing of steel.
A promising method is the alloying of steel with nitrogen gas during its out-of-furnace processing in a ladle. The method is simple and economical and allows you to accurately predict the nitrogen content in the metal.
But the hydrodynamics of the liquid phase have a great influence on the assimilation of nitrogen by steel. In this regard, research is required in laboratory and industrial conditions to determine the necessary conditions and parameters of purging.
Nitrogen in the form of impurities in steels causes non-trivial and even extraordinary changes in their properties.
In particular, this statement refers to the unusual combination of yield strength and fracture toughness. The first study of the mechanical properties of nitrogen steels was probably by Andrew |1|, who obtained Fe-N samples and discovered an increase in the yield strength caused by the introduction of nitrogen and the effect of nitrogen austenitization. Fresher and Kubisch |2| were the first to discover that with increasing nitrogen content, an increase in the yield strength of austenitic steels is accompanied by the expected decrease in strength.
In fact, this fact meant that nitrogen steels represent a new promising class of structural materials. More recently, several studies have also shown that nitrogen in steels can improve fatigue life, low- and high-temperature strength, mechanical hardening, and wear resistance.
Currently, high-strength transition-class chromium-nickel steels (09Х15Н8У, 07ХХ16Н6, 10Х15Н4АМЗ, 08Х15Н5Д2Т, etc.) are used in industry. Their disadvantage is that they contain scarce nickel.
New high-strength nickel-free steels of this class have been developed: 10Kh14AG6, 10Kh14AG6F, 10Kh14AG6MF, 10Kh14AG6D2’M, etc. (A.S. USSR No. 771180, 789626, 996505). They open up a promising direction in the creation of transition-class steels. There is no information about the use of these steels in foreign and domestic practice.
The microstructure of the developed steels is low-carbon lath martensite and metastable austenite, which transforms into martensite under loading. Depending on the specific operating conditions, due to alloying and processing, the amount and degree of stability of austenite changes and, accordingly, the level of mechanical and service properties is regulated. After heat treatment, including quenching (normalization) from 1000 °C and tempering at 200 °C, new steels have a good combination of mechanical properties. A higher level of strength while maintaining good ductility to impact strength is achieved after stepwise hardening with exposure in the range of 100-400 “C (Table 4). Comparative tests for resistance to shock-cyclic loading, simulating the operating conditions of the plates of ring valves of compressors, showed that steel 10Kh14AG6MF has a 1.5-2 times higher level of this characteristic than the well-known chromium-nickel steel 09Kh15N8Yu.
Pilot tests of plates of ring valves of high-pressure compressors - 320/320, operating on the compression of a nitrogen-hydrogen mixture during the production of ammonia at the Slantsekhim Production Association, showed that the durability of plates made of new steel 10Kh14AG6MF is 1.1-1.2 times higher than from steel 10X15N4AMZ (VNS-5), and 1.8 times higher than that of 40X13.
Nickel-free transition-class steels in some cases can successfully replace more expensive nickel-containing steels 111].
Reference: the transition class of metals includes alloys that form both austenitic and martensitic phases.
In industry, the most widely used austenitic steel is 12Х18Н9Т steel. Unfortunately, it is used not only in cases where the failure of parts is due to corrosion, but also when the cause of destruction is cavitation and wear.