SE459184B - PROCEDURES FOR PREPARING CHROME STAINLESS STEEL WITH LOW PHOSPHORUS CONTENT - Google Patents

Description

The invention has been made for the purpose of developing a new process for the production of chromium-containing steel, in which case even phosphorus-containing materials which could not be used in previously known processes can be used as starting materials with good results. To this end, the inventors have carefully studied and tested how to advantageously refine a chromium-containing molten pig iron to low P-grade molten steel in commercial scale plants using dephosphorizing fluxes for use in dephosphorizing chromium-molten molten pig iron. including a flux proposed by the applicant in Japanese Patent Laid-Open No. 56-S910, published January 22, 1981, which corresponds to U.S. Pat. patent 4,290,803, inventor S.

Maruhashi et al, granted September 22, 1981. As a result, a new refining process for the production of chromium steel with a low P content of chromium-containing molten pig iron with a high P content has now been developed.

According to the present invention, a process for the production of low P chromium-containing steel in a vessel equipped with a lance adapted for top blowing of oxygen and having at least one double nozzle of double tube construction adapted for bottom blowing of oxidizing and non-oxidizing gases only or in combination, starting from a chromium-containing molten pig iron containing not more than 4,5% by weight of C, at least 0,010% by weight of P and at least 3,0% by weight of Cr, the following steps: A. addition of a solid carbon source to the chromium-molten pig iron in the vessel in an amount sufficient to give a chromium-containing molten pig iron having a carbon content of at least 4.5% by weight at the end of step D below, B. stirring the contents of the vessel after or during step A by bottom blowing into it by a non-oxidizing gas, C. blowing oxygen to the molten pig iron to produce a chromium-containing molten pig iron with a temperature ranging between 1500 ° C and 700 ° C containing not more than 0.15% by weight of Si and not less than 4.5% by weight of C, axis D. substantially removing a slag formed in the previous step from the vessel, E. adding a dephosphorizing flux to the molten pig iron in the vessel, F. stirring of the contents of the vessel after or during step E by bottom blowing into it of a non-oxidizing gas to facilitate efficient contact of the molten pig iron with a formed slag, while maintaining the concentration of iron oxides in the slag at a level of at least 1 0% by weight, G. substantially removing the slag from the vessel; Optionally, an additional blowing, such as bottom blowing or top and bottom blowing of oxygen to the molten pig iron in the vessel for decarburization, is also performed, with bottom blowing into it by a non-oxidizing gas.

Preferably, at least the last part of step F is carried out while blowing oxygen onto the contents of the vessel in order to thereby keep the concentration of iron oxides in the slag at not less than 1.0% by weight.

If the degree of dephosphorization produced by a cycle of steps E, F and G is unsatisfactory, the cycle can be repeated lying down until a desired level of P is reached, with an intermediate step, between the successive cycles, including raising the temperature of the molten pig iron to a temperature varying between about 1500 DEG C. and about 1600 DEG C. by blowing oxygen with or without the addition of an oxidizable exothermic material to the molten pig iron, with control of the carbon content of the molten pig iron to not less than δ. percent and the silicon content of the molten pig iron to not more than 0.15% by weight.

As the dephosphorizing flux, preferably a flux set forth in Japanese Laid-Open Patent Specification No. 56-5910 corresponding to U.S. Pat. patent 4,290,803. This flux comprises 30 to 80% by weight of at least one compound selected from fluorides and chlorides of alkaline earth metals, 0.4 to 30% by weight of at least one compound selected from lithium oxide and lithium carbonate, 5 to 50% by weight of at least one compound selected from iron oxides and nickel oxide, and 0 to 40% by weight of at least one compound selected from oxides and carbonates of alkaline earth metals.

The process according to the invention is carried out using a refining vessel equipped with a lance adapted for top blowing of oxygen and with at least one nozzle (tuyere) of double tube construction adapted for bottom blowing of oxidizing and non-oxidizing gases alone or in combination, known as a AOD vessel or a top and bottom blown converter. The nozzle comprises concentric inner and outer tubes designed so that oxygen or a mixture of oxygen and a non-oxidizing gas can be blown through the inner tube, while a cooling gas can be blown through the outer tube for an oxygen blowing operation. It is also possible to use such a nozzle for agitation purposes only by blowing through it a non-oxidizing gas, such as argon, nitrogen, gaseous hydrocarbon and water vapor alone or in combination without blowing oxygen.

The method according to the invention comprises three stages. The first stage comprises steps A, B, C and D, while the second stage comprises steps E, F and G, the third stage comprises step H. The first stage is to produce a chromium-containing molten pig iron with a high C- and low Si content with a sufficiently high temperature, which can be suitably dephosphorized in the subsequent stage. The second stage, 459 184 which can be repeated, is to effectively dephosphorize the chromium-containing molten pig iron with high C and low Si content to a desired low P level. The third stage is to decarburize the chromium-containing molten pig iron with a high C and low Si content, which has been dephosphorized in the second stage. The decarburization process is carried out in the third stage and is a known technique per se. According to the invention, the series of these three stages can be efficiently carried out using an AOD vessel.

The first stage is to recarburize and remove silicon from a chromium-containing molten pig iron made of phosphorus-containing materials, and at the same time to maintain or raise the temperature of the molten pig iron at resp. to a sufficiently high temperature. For effective dephosphorization of a chromium-containing molten pig iron with a dephosphorizing flux, the treatment can preferably be carried out with chromium-containing pig iron with a high C and low Si content containing at least 4.5% by weight of C and not more than 0.2% by weight, preferably no more. than 0.5% by weight Si, and the temperature of the molten pig iron must be kept above a predetermined level. The first stage is to realize these conditions. In a case where high carbon ferrochromium is used as the main Cr source for producing a chromium-containing molten pig iron, the carbon content of the obtained molten pig iron is normally lower than the above-mentioned C level, and the silicon content of the pig iron is considerably above the above-mentioned The content unless other special treatment is used. In the first stage of the process, to such a chromium-containing molten pig iron having a low C- and high Si content is added a solid carbon source (for example, powder or particles of coke) in an amount sufficient to give a chromium-containing molten pig iron having a carbon content of at least 4.5% by weight at the end of the first stage; after or during the addition of the carbon source, the contents of the vessel are stirred by bottom blowing therein of a non-oxidizing gas (for example argon or nitrogen), and oxygen blowing (top and / or bottom) is carried out during the bottom blowing of the non-oxidizing gas until the silicon content in the molten pig iron is lowered to a level of not more than 459,184 0.15% by weight. The carbon source can be added from the top of the vessel, but it can also be added from the bottom of the vessel by carrying it with the bottom-blown non-oxidizing gas. Carburization of the molten pig iron is endothermic, but the oxygen blowing which causes the removal of silicon raises the temperature of the molten pig iron. The lower the temperature, the longer the silicon removal should preferably proceed towards the decarburization, and consequently the above-mentioned three conditions which are essential for the slag dephosphorization, i.e. re-carbonization, removal of silicon and temperature increase, are achieved in the first stage. The more vigorous the agitation with the aid of bottom-blown gas, the better the re-carbonation proceeds. In a case where the initial Si content is relatively high and thus a considerable amount of silicon must be removed, it is necessary to control the basicity of a formed slag. This can be achieved, for example, by adding lime. The first stage is completed by removing the slag formed from the vessel, and a further step of raising the temperature primarily by oxidation of carbon can be performed if required. At the end of the first stage, the temperature of the molten pig iron should be in the range from about 1000 ° C to about 170 ° C. if an aphosphorizing flux is added to the molten pig iron at a temperature within this range, the temperature of the molten pig iron decreases more or less depending on the amount of flux added. Nevertheless, the high temperature (approximately 1500 ° C - 1500 ° C) of the molten pig iron at the end of the first stage ensures a high temperature required for efficient slag dephosphorization in the second stage.

In the second stage of the process, a dephosphorizing flux is added to the molten pig iron treated in the first stage; after or during the addition of the dephosphorizing flux, the contents of the vessel are stirred through. bottom blowing therein of a non-oxidizing gas to promote efficient contact of the molten pig iron with a formed slag, 459 184 and the slag is removed from the vessel. The flux can be added from the top of the vessel, or from the bottom of the vessel by entraining it with the bottom blown gas. The required amount of flux can be added at once, but batch or continuous addition is preferred. The stronger the agitation caused by the bottom-blown gas, the better the dephosphorization proceeds. Preferably, the bottom-blown non-oxidizing gas is blown at a flow rate of at least 10 Nm 3 / h. tons. Fluorite, industrial grade lithium carbonate, scale and lime were used as materials for the flux. It is preferred to use materials containing the least possible amounts of SiO2, Al2O3 and MgO to avoid unwanted contaminants of these formed slag. To achieve considerable dephosphorization, at least 30 kg of the flux per tonne of molten pig iron is required. However, the use of the flux in excess of about 80 kg / ton of pig iron should normally be avoided as otherwise unacceptable heat absorption is caused. During the course of the dephosphorization treatment, C in the molten pig iron actively reduces iron oxides in the slag due to the strong agitation, and thus the concentration of iron oxide in the slag decreases rapidly. If this concentration of iron oxides in the slag decreases to a level below about 1.0%, re-phosphorization is obtained. The dephosphorization treatment should therefore preferably be stopped before the concentration of iron oxides in the slag has decreased to a level of about 1, 0§ or lower.

If the bottom blowing of a non-oxidizing gas for stirring the chromium-containing molten pig iron with the added dephosphorizing flux is stopped within a relatively short period of time to avoid the aforementioned reforforation problem due to the reduction of the iron oxide concentration in the slag, P-halt. One way to maintain the iron oxide concentration in the slag during a relatively long dephosphorization time is to use a flux with increased concentration of iron oxides. However, an increase in the content of iron oxide in the flux has a detrimental effect on the fluidity of the slag, and thus the use of a flux with an iron oxide concentration in excess of 50% by weight must be avoided. Another way is to add an additional source of iron oxides, such as embers, to the molten pig iron that is blown. A further way is to top-blow oxygen against the material being treated in the vessel during at least a part of the dephosphorization step, i.e. step F to thereby maintain the concentration of iron oxides in the slag at not less than 1.0% by weight. We prefer the second and third way. A second stage comprising oxygen peak blowing is referred to herein as improved second stage. In the improved second stage, oxygen, as an auxiliary oxidizing agent, can be effectively added to the slag to compensate for the lack of oxygen to be supplied by the iron oxides for dephosphorization. Thus, without suffering from the problem of re-phosphorization due to lack of oxygen to be supplied by the iron oxides in the slag, the dephosphorization treatment can be carried out to a desired low P content.

If a desired low P content is not obtained at the end of the second stage or improves the second stage, this stage can be repeated or performed cyclically until the desired low P content is reached, with an intermediate step, between two successive cycles, of raising the temperature of the molten pig iron by bottom and / or top blowing of oxygen with or without the addition of a carbon source, silicon source or other similar oxidizable exothermic material to the molten pig iron. When the second stage or improved second stage is repeated, a temperature loss of the molten pig iron is inevitable. However, such a temperature loss can be conveniently and easily compensated for by using the known characteristics of the vessel used. To raise the temperature of the molten pig iron by oxygen blowing, C and Si in the molten pig iron can be used. If required, externally added materials may also be used for this purpose, including, for example, a carbon source such as those useful in the first stage, a silicon source such as FeSi, and 459 184 Al. When the heat of oxidation of materials other than C is used, a basicity adjusting agent such as CaO may need to be added. In all cases, this temperature-raising step is carried out to obtain a chromium-containing molten pig iron having a temperature in the range between about 1500 ° C and 1600 ° C, and containing at least 4.5% by weight of C and not more than 0.15% by weight of Si. After blowing, any slag formed can be removed if required. One of the advantages of the process according to the invention is that the second stage can be repeated, which enables dephosphorization of the chromium-containing molten pig iron to a desired low P content without significant inconvenience due to oxidation loss of chromium.

When the first and second stages have been completed, a chromium-containing molten pig iron with a low P, low Si and high C content is obtained in the vessel, which can be further processed in the same vessel by a so-called _AOD process for the production of stainless steel with low P content, i.e. it can be subjected to a coarse-cooling refining in the same vessel (by top- or bottom-blowing of oxygen) and further a reducing final decarburization in the same vessel with bottom-blowing of an inert gas or a mixture of an inert gas and oxygen. It should be noted, however, that the above-mentioned chromium-containing pig iron with low P, low Si and high C content from the dephosphorization step or the chromium-containing pig iron with low P, low Si and low C content from the coarse decarburization in the same vessels can also be decarburized by a known process; for example, the VOD and RH-OB processes, in another vessel.

With the method according to the invention, an additional effect is obtained that desulphurisation also proceeds. In addition, a reduced content of N in the steel is also achieved with the method according to the invention.

Example 1 An AOD vessel with three nozzles (seals) at the bottom adapted for blowing oxygen and a non-oxidizing gas and also provided with a lance adapted for top blowing of oxygen was loaded with 30 tons of chromium-containing molten pig iron with the composition listed in Table 1 (listed in Table 1 as "stage 1, before treatment"). To the molten pig iron was added 1.2 tons of particulate coke and 1 ton of lime, and the first stage treatment was carried out by bottom blowing of 500 Nm 3 / h. argon and 150 Nm3 / h total oxygen from the seals and top blowing of 600 Nm3 / h oxygen for 20 minutes The composition and temperature of the molten pig iron at the end of the first stage are shown in Table 1.

To the molten pig iron treated in this way were added 150 kg of lithium carbonate, 300 kg of lime, 900 kg of fluorite and 750 kg of scale, and the second stage treatment was carried out by bottom blowing 700 Nm3 / h argon through the seals for 8 minutes.

The composition and temperature of the molten pig iron at the end of the second stage are shown in Table 1.

The molten pig iron treated in this way was subjected to decarburization, reductive refining and component adjustment according to a conventional AOD process whereby a low P stainless steel having a composition given in Table 1 (such as "stage 3, after treatment ") was obtained.

Table 1 Metal components (wt%) 41 lcqx C Si P Ni Cr Before 3.63 1.47 0.020 8.90 18.45 14S0 ° C lza stage After 5.32 0.03 0.021 8.81 18.37 1600 ° C Before - - ~ - - - 2nd stage After 5.14 0.02 0.009 8.35 18.16 1450 ° C Before ~ - - - - - 3rd stage After 0.061 0.57 0.011 8.40 18 , 32 459 184 ll Exemgel 2 An AOD vessel with three seals (bottom nozzles) at the bottom adapted for blowing oxygen and a non-oxidizing gas and also equipped with a lance adapted for top blowing of oxygen was loaded with 30 tons of chromium-containing molten pig iron with a composition listed in Table 2 (designated in Table 2 "stage 1, before treatment"). To the molten pig iron was added 1.2 tons of particulate coke and 1 ton of lime, and the first stage treatment was carried out by bottom blowing 500 Nm3 / h argon and 1500 Nm3 / h oxygen from the seals for 20 minutes. The composition and temperature of the molten pig iron at the end of the first stage are shown in Table 2.

To the molten pig iron treated in this way were added 150 kg of lithium carbonate, 300 kg of lime, 900 kg of fluorite and 500 kg of scale, and the second stage treatment was carried out by bottom blowing 700 Nm3 / h argon through the seals and top blowing 600 Nm3 / h oxygen through the lance for 8 minutes.

The composition and temperature of the molten pig iron at the end of the second stage are shown in Table 2.

The molten pig iron treated in this way was subjected to decarburization, reductive refining and component adjustment according to a conventional AOD process whereby a low P ~ stainless steel having a composition given in Table 2 (such as "stage 3, after treatment ") was obtained. 459 184 Example gel 3 The procedure of Example 2 was repeated except that the second stage of Example 2 was repeated twice and that between the first cycle of the second stage and the second 12 Table 2 Metal components (% by weight) TÉWL c 'si P Ni cr Before 3.41 1.53 0.022 8.93 18.33 14s0 ° c 1st Smdietgfter 5.40 0.06 0.022 8.82 18.23 1000 ° c Before - - "'' '' 2nd Stagegfter 5 .17 0.02 0.010 8.87 17.34 1000 ° c Before - "" "" 3rd stage after 0.063 0.58 0.012 8.43 111.20 - the cycle of the second stage the temperature of the molten pig iron was raised to 1570 ° C by adding 1.0 tonne of lime, 0.3 tonne ferric silicon and 0.5 tonne particulate coke to the molten pig iron, and top blowing 600 Nma / h oxygen through the lance with bottom blowing of 500 Nm3 / argon and 1500 Nm3 / h oxygen through the seals.

The results are shown in Table 3. 459 184 13 Table 3 Meta 1 lcomponents (weight t-%) Temp.

C Si P Ni Cr Before 3.57 1.47 0.023 8.85 18.21 14s0 ° c After 5.53 0.10 0.022 8.80 18.18 1600 ° C 1st stage Before '' '' 2: a stage After 5.45 0.03 _0.01l 8.79 18.01 .1450 ° C _. - O Tanperatul? Fore - - _ _ _ 14> 0 C increase After 5.30 0.07 0.012 s, 7s' 1s, o0 1sv0 ° c Before - - ~ - - - 2nd stage After 5.20 0.02 0.006 S, 79 17.83 14S0 ° C Before - - - - - ~ 3rd stage After 0.066 0.53 0.008 8.54 18.34 - Example gel 4 A stainless steel with a low P content (SUS 304) was prepared using the diagram given below: Description of Removal 1st de- Terrpera- material down -> of silicon, ---> phosphorus- -> turhöj- - high P-content recalcification x 2 nd de- Tarxpera- 3rd de- Conventional -ä phosphori- -> turhöj- k »phosphori- -> refining --- sering ning sering Strand casting An ADD vessel equipped with a lance for top blowing of oxygen and with three seals with concentric double pipe construction for bottom blowing of nitrogen and oxygen was loaded with about 30 tons of molten pig iron containing in weight percent 3.7% C, 1.5% Si, 0.025% P, 0.60% S, 9.0% Ni and l8.0% Cr . To the molten pig iron was added at a temperature of about 1450 ° C 1.6 l tons of particulate coke and 1.5 tons of lime. During bottom blowing, a small amount of nitrogen was added. After the addition of the coke was started, bottom blowing of nitrogen in an amount of 17 Nm 3 / h was started. tons and: bottom blowing of oxygen in an amount of S0 Nm3 / h. tons, and S minutes later, top blowing of oxygen was started in an amount of 22 Nm3 / h. tons. After about 20 minutes from the start of the blowing of nitrogen and oxygen, all blowing was stopped and the slag was removed. The molten pig iron in the vessel contained S, 3% C and 0.12% Si and had a temperature of about 1600 ° C.

To the molten pig iron treated in this way was added 6 kg / ton of industrial grade lithium carbonate, 29 kg / ton of fluorite and 15 kg / ton of scale. The contents of the vessel were treated by bottom blowing of 28 Nm3 / h. tons of nitrogen and top blowing of 22 Nm3 / h. tons of oxygen for 8 minutes during which 5 kg / ton of embers were introduced into the vessel by bringing this with bottom-blown nitrogen. At the end of the period, the blowing was stopped and the slag was removed from the vessel. The molten pig iron in the vessel contained 0.11% P ccn ned a temperature of about 145o ° c.

The dephosphorization treatment was repeated two more times. Before each of the 2nd and 3rd dephosphorization stages, the temperature of the molten pig iron was raised to about 1000 ° C by adding thereto 20 kg / ton of lime, 20 kg / ton of particulate coke and 7 kg / ton. ferro-silicon, top blowing of 22 Nm3 / h. tons of oxygen with bottom blowing of l7 Nm / h. tons of nitrogen and 50 Nm3 / h. tons of oxygen, and removal of slag from the vessel.

The molten pig iron treated in this way was refined using a conventional AOD process. The final product (SUS 304 stainless steel) contained 60 ppm P.

At each dephosphorization stage, the degree of dephosphorization achieved was in the range of about 50% to 60%, and the oxidation loss of chromium was about 0.3% by weight.

Claims (5)

459 184 15 PATENT REQUIREMENTS Process for the production of low P chromium-containing steel in a vessel equipped with a lance adapted for top blowing of oxygen and with at least one bottom nozzle (tight) of double pipe construction adapted for bottom blowing of oxidizing and non-oxidizing gases only or in combination, comprising bottom blowing or top and bottom blowing of oxygen to a chromium-containing pig iron melt in the vessel for decarburization during bottom blowing of a non-oxidizing gas, characterized in that before the charring the chromium-containing pig iron is produced from the vessel, a chromium-containing molten pig iron containing not more than 4.5% by weight of C, at least 0.010% by weight of P and at least 3.0% by weight of Cr, the process comprising the steps of: A. adding a solid carbon source to the chromium-containing molten pig iron in the vessel in an amount which is sufficient to give a chromium-containing molten pig iron with a carbon content of at least 4% by weight at the end of step D below, B. stirring g of ° the contents of the vessel by bottom blowing therein of a non-oxidizing gas, C. blowing oxygen to the molten pig iron to produce a chromium-containing molten pig iron having a temperature ranging between 150 ° C and 1700 ° C containing no more than 0.15% by weight Si and at least 4.5% by weight C, D. substantial or substantial removal from the vessel of a slag formed in the preceding steps, E. addition of a dephosphorating flux to the molten pig iron in the vessel, F stirring the contents of the vessel by bottom blowing therein a non-oxidizing gas to facilitate efficient contact of the molten pig iron with a formed slag, while maintaining the concentration of iron oxides in the slag at a level of at least 1.0% by weight, and G. substantially or substantially removing the slag from the vessel. 459 184 lbs 2. A method according to claim 1, characterized in that a cycle of steps E, F and G is repeated until a desired P content is reached, with an intermediate step between successive cycles comprising raising the temperature of the molten pig iron to a temperature in the range 1500-1700 ° C oxygen, while controlling the carbon content of it to not less than 4.5% by weight and the silicon content of the molten pig iron to not more than 0.15% by weight. by blowing the molten pig iron 3. A method according to claim 2, characterized in that at least one intermediate step comprising raising the temperature of the molten pig iron is carried out with molten pig iron, to which an oxidizable exothermic material has been added. A process for the production of low P chromium-containing steel according to any one of the preceding claims, characterized in that at least the last part of step F is carried out with top blowing of oxygen against the contents of the vessel to thereby maintain the concentration of iron oxides in the slag at a level of at least 1.0% by weight. 5. A process for producing low P chromium-containing steel according to any one of the preceding claims, characterized in that the dephosphorizing flux used comprises 30-80% by weight of at least one compound selected from fluorides and alkaline earth metal chlorides. 0.4 to 30% by weight of at least one compound selected from lithium oxide and lithium carbonate, 5-50% by weight of at least one compound selected from iron oxides and nickel oxide and 0-40% by weight of at least one compound selected from oxides and carbonates of alkaline earth metals .