Among the above analytical methods, the spectrophotometric method is a so-called wet chemical analysis and is high in the analytical accuracy, but a long time such as several hours is usually taken in the measurement.
Therefore, this method is not daily used in the analysis of the S concentration at steel-making step, while the two methods capable of analyzing relatively rapidly such as the emission spectrometric method and infrared absorption method are mainly used.
In the emission spectrometric method, however, surface nature such as surface roughness or the like on an analyzing surface exerts on the analytical value, so that a smooth surface having a diameter of about 30 mm is required as an analyzing surface, and a polishing time is required, and hence there is a problem that a time is taken until an analytical result on the S concentration is obtained usually about 15 minutes.
Further, a time is taken until molten steel is taken out and poured into a mold for an analytical sample and cooled and a sample is taken out therefrom. Also, there is a tendency that the emission spectrometric method is poor in the analytical accuracy as compared with the infrared absorption method. To this end, when the S concentration is particularly necessary to be accurately analyzed in the low-sulfur steel or extremely low-sulfur steel, the infrared absorption method is frequently used. In the desulfurization of molten steel at the steel-making step, therefore, there are problems such as failure of S concentration, increase of production cost and so on.
Furthermore, excessive desulfurization apprehending about the failure of S concentration was carried out in the secondary refining of molten steel. The invention is made in view of the above problems retaining in the conventional techniques. That is, it is an object of the first aspect of the invention to propose a method for desulfurizing molten steel which is capable of controlling S concentration of steel in a high accuracy by rapidly and accurately analyzing S concentration of a sample taken out from molten steel after the tapping from a converter, and a method of manufacturing molten steel by using such a desulfurization method.
Also, it is an object of the second aspect of the invention to propose a method for secondarily refining molten steel which is capable of shortening desulfurization time in secondary refining and also reducing an amount of a desulfurizer or the like used by rapidly and accurately analyzing S concentration of a sample taken out during the secondary refining, and a method of manufacturing molten steel by using such a method.
The reason setting the above objects is due to the fact that when the S concentration can be analyzed rapidly and accurately, the analytical results of S concentration in the converter sample or during the secondary refining are reflected in the subsequent desulfurization refining, whereby not only S concentration in molten steel can be controlled accurately to improve on-target ratio of S concentration but also the excessive addition of the desulfurizer and the prolongation of the treating time become needless.
Furthermore, fruitless treating time in the secondary refining can be reduced by accurately grasping the S concentration in molten steel, and hence the productivity can be increased. The inventors have made various studies on a method for rapidly and accurately analyzing S concentration in molten steel after the tapping from a converter for solving the above tasks.
As a result, it has been found out that the above task can be solved by combusting and oxidizing a sample taken out from molten steel after the tapping from a converter under a high frequency induction heating in a pure oxygen atmosphere to convert all S included in the sample into SO 2 for a short time and analyzing a concentration of SO 2 through an ultraviolet fluorescence method, and the first aspect of the invention has been accomplished. The method of analyzing the S concentration in the molten steel desulfurization method according to the first aspect of the invention is preferable to comprise a high frequency induction heating step wherein the sample is combusted and oxidized under the high frequency induction heating in a pure oxygen atmosphere to convert S in molten steel into SO 2 and an analyzing step wherein SO 2 -containing gas produced in the high frequency induction heating step is analyzed through an ultraviolet fluorescence method to quantify S concentration of the sample.
In the molten steel desulfurization method according to the first aspect of the invention, it is preferable that a target S concentration in the desulfurization is not more than 0.
The desulfurizing condition in the molten steel desulfurization method according to the first aspect of the invention is preferable to be at least one of an amount of a desulfurizer charged in the desulfurization and a treating time molten steel stirring time or the like.
In the molten steel desulfurization method according to the first aspect of the invention, it is further preferable that when molten steel after the tapping from the converter is continuously subjected to a secondary refining, S concentration of a sample taken out from molten steel during the secondary refining is analyzed by using the above ultraviolet fluorescence method for determining the subsequent desulfurizing condition based on such S concentration. Also, the first aspect of the invention is a method of manufacturing molten steel by using any one of the aforementioned molten steel desulfurization methods.
Furthermore, the inventors have made various studies on a method for rapidly and accurately analyzing S concentration of molten steel during the secondary refining for solving the above task. As a result, it has been found out that the above task can be solved by combusting and oxidizing a sample taken out from molten steel during the secondary refining under a high frequency induction heating in a pure oxygen atmosphere to convert all S included in the sample into SO 2 for a short time and analyzing such a concentration of SO 2 through an ultraviolet fluorescence method, and the second aspect of the invention has been accomplished.
That is, the second aspect of the invention is a method for secondarily refining molten steel tapped from a converter, wherein S concentration of a sample taken out from molten steel during the refining is analyzed by using an ultraviolet fluorescence method for determining subsequent desulfurizing condition based on an analytical value of the S concentration. The method of analyzing the S concentration in the secondary refining method according to the second aspect of the invention is preferable to comprise a high frequency induction heating step wherein the sample is combusted and oxidized under the high frequency induction heating in a pure oxygen atmosphere to convert S in molten steel into SO 2 and an analyzing step wherein SO 2 -containing gas produced in the high frequency induction heating step is analyzed through an ultraviolet fluorescence method to quantify S concentration of the sample.
In the secondary refining method according to the second aspect of the invention, it is preferable that a target S concentration in the desulfurization is not more than 0.
The desulfurizing condition in the secondary refining method according to the second aspect of the invention is preferable to be at least one of an amount of a desulfurizer charged in the desulfurization and a treating time molten steel stirring time or the like. Also, the second aspect of the invention is a method of manufacturing molten steel by using any one of the aforementioned molten steel desulfurization methods.
According to the first aspect of the invention, the S concentration of molten steel after the tapping from the converter can be analyzed and grasped rapidly and accurately, so that not only the desulfurization of molten steel can be rationalized to improve the on-target ratio of S but also the step disturbance due to the failure of S concentration can be prevented and the increase of the production cost due to the excessive desulfurization can be suppressed, and hence the industrially successful effect is large.
According to the second aspect of the invention, the S concentration in molten steel during the secondary refining can be analyzed and grasped rapidly and accurately, so that not only the desulfurization of molten steel can be rationalized to improve the on-target ratio of S but also the increase of the production cost due to the excessive desulfurization can be suppressed or the productivity can be improved by reducing the desulfurizing time or the step disturbance due to the failure of S concentration can be prevented, and hence the industrially successful effect is very large.
Molten steel after the completion of decarburization blowing in a converter is poured from a tapping port into a ladle by tilting the converter, at where an alloying iron, a deoxidizer and the like are added to molten steel in the ladle.
Thereafter, a converter sample taken out from molten steel in the ladle is analyzed to determine operational conditions in subsequent secondary refining.
For example, an amount of a desulfurizer initially charged in the secondary refining is calculated from S analytical value of the converter sample, target S concentration and molten steel amount.
After the start of desulfurization refining, samples are taken out repeatedly on the way to monitor desulfurization state, and the desulfurizer is additionally charged, if necessary, and the desulfurization refining is completed when S concentration of molten steel reaches to a given target concentration.
Thus, the S concentration in molten steel is properly analyzed in the steel-making step, and the analyzed results are reflected on the operative conditions. However, if the accuracy of S analysis is poor, deficiency and excess of desulfurization becomes large to bring about the failure of S concentration or increase of the production cost due to the unnecessary addition of the desulfurizer.
Table 1 shows an example that either converter sample or ladle sample is outside of S concentration in charging for a product having a standard value of S concentration of not more than 0. The charge No. On the other hand, the charge Nos. The inventors have also considered that it is necessary to determine operative conditions of secondary refining desulfurizing conditions by rapidly and accurately grasping S concentration of molten steel during the secondary refining and conducted examinations on a method for rapidly analyzing S in steel with a high accuracy the second aspect of the invention.
Consequently, it has been found that the above problems can be solved by using an ultraviolet fluorescence method as the method for analyzing the S concentration, and as a result, the invention has been accomplished.
The method for analyzing S concentration with the ultraviolet fluorescence method in the first aspect and second aspect of the invention will be described below. The ultraviolet fluorescence analyzing apparatus 1 comprises a pure oxygen supply means 2 , a high frequency induction heating furnace 3 for combusting and oxidizing a sample 5 taken out from molten steel in a pure oxygen atmosphere supplied from the pure oxygen supply means 2 to convert S included in the sample 5 into SO 2 , a dust filter 4 for removing grit and dust dust from SO 2 -containing gas produced by combustion of the sample 5 , and an ultraviolet fluorescence analyzer 6 for analyzing SO 2 -containing gas after the removal of the dust by ultraviolet fluorescence method to quantify S in the sample.
The pure oxygen supply means 2 comprises a pure oxygen supply source not shown having an oxygen concentration of not less than As the flow controller may be used a well-known flow controller, but it is preferable to use a mass flow controller capable of controlling a mass flow of pure oxygen from a viewpoint of controlling the flow amount of the gas supplied.
In the inside of the high frequency induction heating furnace 3 are arranged a ceramic crucible 31 for dissolving and combusting the sample 5 , and a coil 32 enclosing the ceramic crucible 31 , wherein the coil 32 is connected to an alternating-current source not shown. In the combustion of the sample 5 , it is preferable to use a combustion improver such as tin, tungsten or the like. Because, the sample 5 can be rapidly combusted by charging the sample 5 and the combustion improver into the ceramic crucible 31 and heating them, and hence the analysis of the S concentration can be conducted rapidly.
The dust filter 4 is disposed between the high frequency induction heating furnace 3 and the ultraviolet fluorescence analyzer 6 for removing dusts, which are generated from the sample 5 and the combustion improver, from the SO 2 -containing gas generated in the high frequency induction heating furnace 3 to protect the ultraviolet fluorescence analyzer 6 arranged at subsequent stage.
As the dust filter 4 , it is preferable to use ones having an excellent air permeability made from a material not adsorbing SO 2 such as silica fiber or polytetrafluoroethylene. In the ultraviolet fluorescence analyzer 6 , an ultraviolet ray having, for example, a wavelength of nm is irradiated to the SO 2 -containing gas and then a fluorescence wavelength of nm emitted from SO 2 in turning from an excited state to a ground state is measured for a certain time, and thereafter S amount included in the sample 5 is calculated from an integration value of fluorescence intensity measured with a previously prepared calibration curve.
As the ultraviolet fluorescence analyzer 6 can be used a well-known ultraviolet fluorescence analyzer, particularly an ultraviolet fluorescence analyzer comprising an ultraviolet generating source, a fluorescent cell for irradiating an ultraviolet ray to SO 2 -containing gas and a photomultiplier tube PMT measuring an excitation light. Next, the method of quantitatively analyzing S concentration of the sample 5 taken from molten steel will be described with the use of the ultraviolet fluorescence analyzing apparatus 1.
At first, the sample 5 and the combustion improver are charged into the ceramic crucible Then, pure oxygen is continuously supplied from the pure oxygen supply means 2 to the high frequency induction heating furnace 3 , while an alternating current is applied to the coil 32 to combust oxidize the sample 5 in the pure oxygen atmosphere.
After dusts included in SO 2 -containing gas produced by combustion of the sample 5 is removed by the dust filter 4 , S concentration included in the sample 5 is quantified by measuring SO 2 amount of the SO 2 -containing gas with the ultraviolet fluorescence analyzer 6.
According to the ultraviolet fluorescence analyzing apparatus 1 , the sample 5 can be combusted rapidly and sufficiently with the high frequency induction heating furnace 3 in the pure oxygen atmosphere. In the ultraviolet fluorescence analyzing apparatus 1 , SO 2 amount produced by combustion of the sample 5 is measured by the ultraviolet fluorescence analyzer 6 , so that it is substantially free of the influence of steam included in a gas to be measured or a temperature of such a gas as compared to the conventional infrared absorption method conducting the measurement with an infrared ray detector.
In the ultraviolet fluorescence analyzing apparatus 1 , it is not also required to use a reference gas comparison gas during the measurement as used in the conventional technique. Desulphurisation rate — The sulphide capacity as well as the desulphurization potential describe the maximum ability of slags to absorb sulphur at thermodynamic equilibrium. In practice, there is a ladle treatment time which is required to be adapted as per the production requirement of the steel melting shop.
A rapid refining process in the ladles is desirable. It has been seen that at steelmaking temperatures the reaction rate between the metal and the slag phase is mostly determined by the mass transport. In this case a rapid mass transfer caused by intensive gas stirring leads to an acceleration of the desulphurization rate.
Today the gas stirring treatment of liquid steel in the ladle is a standard operational practice. For the performance of gas stirring treatment, inert gas normally argon gas is injected into liquid steel through porous plugs located in the ladle bottom or using a submerged top lance.
The injected gas ascends in the liquid steel in the form of gas bubbles and is separated on the bath on the surface. Because of the buoyancy force of gas bubbles a circulating flow field of liquid steel is formed in the ladle.
The pattern of the steel flow field is of great importance for the desulphurization process. The flow velocity of liquid steel increases with the increase of the stirring gas rate. Hence, an acceleration of reactions between the steel and the slag phase can be achieved. Fig 1 Influence of lime saturation index on the desulphurization potential The desulphurization rate increases considerably if the gas stirring rate exceeds a critical value.
Theoretical aspects of slag emulsification Liquid steel desulphurization by slag-metal reaction is an exchange reaction between two non-miscible phases, thermodynamically governed by the sulphur partition ratio between the two phases, and kinetically governed by the inter phase exchange area and sulphur transfer driving force.
In several steelmaking processes in which bath stirring is concerned, mixing times depend on the power transferred to the bath in the ladle at the power of 0. The mono-plug bottom blowing stirring which ensure the shortest mixing times for a fixed bath and a fixed gas flow rate supply is achieved with plug eccentrical with respect to the ladle.
A position between quarter and half ladle radius is generally desired. Multiple porous plugs stirring in the ladle is to be set carefully in order to have the relative velocity at the interface slag-metal to favour emulsification. Asymmetrical plug positions proved to be of maximum efficiency in reducing mixing times. With symmetrical plug positions, flow recirculations are induced in the ladle with zones which have counteracting flows destroying their stirring effects.
Lance blowing is beneficial for emulsification, whereas bottom stirring is beneficial for ladle mixing. A suitable combined blowing merges the two desired effects. Studies carried out on the effect of the slag properties on the emulsification phenomenon show that there are critical conditions which are required to be met for steel velocity at the interface with the slag and flow rate of gas blown from the plugs to allow emulsification onset.
These relationships take into account slag physical properties such as viscosity and density. Among the parameters used to define improved conditions for mixing in the ladle and mass transfer at the slag metal interface, of great importance are the ratio between ladle diameter D and the bath height H. Data available in literature on the effect of gas flow rate injection on desulphurization rate show that the most interesting aspect is that an onset gas flow rate is to be found for enhancing significantly desulphurization rate.
Process of desulphurization Primarily, there are two options available for the removal of sulphur from the liquid steel. Slag adjustment with respect to i slag deoxidation, and ii lime saturation Slag homogenization and liquefying Reduction of FeO and MnO Intensive stirring for desulphurization Depending on the metallurgical reactors Vacuum degassing unit, and ladle furnace etc.
For covering the surface of liquid steel to reduce the heat losses. For avoiding reoxidation of liquid steel from atmospheric oxygen For removal of inclusions from the liquid steel. Issues related to synthetic slag practice Synthetic slag practice appears to be simple and not much capital investment is needed.
Desulphurization may vary from one heat to other if slag carry- over from the primary steel melting furnace is not controlled. Oxygen level of liquid steel is required to be same for consistent results. CaO is the main component. It is hygroscopic and leads to the pick-up of hydrogen H2 in the steel. Argon bubbling is done to stir the bath.
Temperature drop due to bubbling of argon can be of the order of 10 deg to 25 deg C for a heat of weight tons to tons. The temperature drop is resulting from radiation heat losses from the surface and heat transfer due to the argon bubbling. Tokovoi, D. Akhmetov, D. Shaburov, V. Ladle desulfurization of steel by solid slag mixtures.
Steel Transl. Download citation. Published : 27 June Issue Date : March Anyone you share the following link with will be able to read this content:. And when adopting containing vanadium-titanium-iron-water smelting electrical steel, owing to containing vanadium titanium in desulfurization slag, its viscosity is high, poor fluidity is difficult for pulling out, and this returns the serious problem of sulphur after causing molten iron to enter converter. Can find out, in the process adopting containing vanadium-titanium-iron-water smelting electrical steel, if do not solve back sulphur problem, will have a strong impact on the quality of electrical steel.
In the prior art, some steel mill has taked to increase desulfurization in RH refining procedure and has processed to reach the object of controlling sulphur content in steel, but this can cause the increase of smelting cost, meanwhile, to rhythm of production, also certainly will bring certain influence.
For this reason, contriver is from considering from cost and rhythm of production, and the mode that proposes to adopt pretreatment desulfurizing, whole process to control back sulphur is smelted electrical steel. In one exemplary embodiment, utilize the process containing vanadium-titanium-iron-water smelting electrical steel to comprise vanadium extraction, half steel desulfurization, converter smelting, LF refining, RH vacuum-treat and continuous casting.
Wherein, what adopt is the molten iron that Panxi Diqu vanadium iron magnetite obtains through blast-furnace smelting containing vanadium-titanium-iron-water, its composition comprises by weight percentage: the Mn of C But the invention is not restricted to this, working method of the present invention is applicable to all kinds of containing vanadium-titanium-iron-water. First, traditional pretreatment desulfurizing operational path blast-melted-desulfurizing iron-vanadium extraction-converter smelting is optimized, adopt the operational path of blast-melted-vanadium extraction-half steel desulfurization-converter smelting, adopt the technology generations of desulfurization after first vanadium extraction for the technique of vanadium extraction after traditional first desulfurization.
In addition, containing vanadium-titanium-iron-water vanadium, titanium content wherein after vanadium extraction is processed, significantly reduce, now, then carry out desulfurization processing, the vanadium in desulfurization slag, titanium content reduce naturally, have solved the problem that desulfurization slag is difficult for removing. Here, in traditional technology, conventionally need to add vermiculite or carburelant to play the effect of insulation, in this application, due to the technique of desulfurization after the first vanadium extraction of employing, molten iron temperature can meet desulfurization needs, and in vermiculite or carburelant, contains a certain amount of sulphur, therefore, do not add in the present invention vermiculite or carburelant.
Before half steel is blended into converter smelting, first smelt the steel of a few stove low sulfur contents so that converter is carried out to prepurging operation, the increasing sulphur that while avoiding smelt other compared with high-sulfur steel grade early stage, residual sulphur causes.
After converter tapping finishes, to the ladle top of the slag, add first modification agent; After RH refining finishes, to the ladle top of the slag, add second batch modification agent. The object that adds modification agent is that ladle slag is carried out to upgrading, improves its sulfur capacity, avoids process to return sulphur. And the object that first adds a collection of modification agent after converter tapping is because once sulphur returns molten steel, what adopt because of it is pre-deoxidizing technology, refining process is to be difficult to take off, unless increased cost, adopt RH desulfurization, thereby just first add first modification agent after converter tapping, to increase ladle slag sulfur capacity, avoid its time sulphur or reduce back sulphur degree, if secondly modification agent adopts disposable adding after RH refining finishes, due to the problem of activity time, it is to the oxygen removal poor effect in slag.
Particularly, when smelting electrical steel, conventionally, the slag system that the subsequent handling after converter tapping adopts is CaO-Al 2 o 3 slag system this CaO-Al 2 o 3 slag system not only can be used for the external refining processes such as LF, RH, VAD and VOD, also can be used as the coverture of casting process tundish molten steel. If CaO content is too low, the optical basicity of slag and sulfur capacity are just low, in slag, sulphur just easily enters molten steel, if CaO too high levels, can cause the mobility of slag bad, make to control the dynamic conditions variation of sulphur, simultaneously also bad to Control and Inclusion Removal, therefore CaO content is controlled in
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