JP5131727B2 - Stainless steel melting method - Google Patents

Description

  The present invention relates to a method for melting stainless steel.

  Stainless steel is widely used as an outer plate material for kitchen products and electrical appliances because of its excellent corrosion resistance and surface properties. Especially when used in these applications, it is processed into various shapes. Is also required to be excellent.

  The production process of stainless steel is versatile and is not necessarily limited to a single process, but generally includes processes such as melting, casting, hot rolling, cold rolling, and heat treatment.

  In the melting process, non-metallic inclusions such as metal sulfides and oxides may be generated in the molten steel. This non-metallic inclusion is brought to the final product with almost no disappearance even if its shape is changed as it goes through the subsequent steps after melting.

  Non-metallic inclusions are classified into A-based inclusions, B-based inclusions, and C-based inclusions when distinguished by their shapes. Here, A, B, and C inclusions are respectively defined in Japanese Industrial Standard G0555-1956. That is, the A-based inclusions are inclusions that are elongated in a continuous state in the processing direction, and the B-based inclusions are those in which the granular inclusions are arranged discontinuously in the processing direction, C-based inclusions are inclusions such as granular oxides that are independently and irregularly dispersed in steel. The Japanese Industrial Standard is hereinafter abbreviated as JIS.

  The A-based inclusions that are elongated in the processing direction are considered to deteriorate the workability of stainless steel compared to the C-based inclusions that are granular and irregularly dispersed (for example, see Patent Document 1). Therefore, for example, in Patent Document 1, in ferritic stainless steel, S and O are regulated to 0.005% by weight or less and 50 ppm or less, respectively, and Ti or a Ti-containing material is added at the time of adjusting molten steel components in the final refining stage. Thus, it is disclosed that the A type inclusion is replaced with the C type inclusion to improve the workability.

  However, in the method disclosed in Patent Document 1, an expensive Ti raw material must be used, and in order to limit S and O to a low level, the flux basic unit increases and the refining time becomes long, resulting in an increase in production efficiency. There is a problem of lowering.

  Moreover, in patent document 1, when Ti is not substantially added, it is shown that a nonmetallic inclusion can be made into a C type inclusion by adding Al. However, in order for Al to effectively act on molten steel, slag generated in the process prior to the final refining stage must be removed, so valuable metals that exist in the form of oxides in the slag. However, there is a problem that it takes time to remove the hair and the workability is deteriorated.

As another prior art for controlling inclusions in order to improve the workability of stainless steel, in a stainless steel refining furnace, the slag composition after reductive refining is regulated, and then an appropriate amount of Al is added to increase the There is a technique of improving the workability of stainless steel by generating inclusions having a melting point and hardness and promoting recrystallization in a solution treatment after hot rolling (see Patent Document 2). The technology disclosed in Patent Document 2 suppresses deformation and refinement of inclusions in processing after casting by generating spinel containing high melting point and hard Al ₂ O ₃ or Al ₂ O _3. In the subsequent solution treatment, the inclusions do not become a barrier for recrystallization, and the coarsening of the crystal grains is promoted.

However, in the technique disclosed in Patent Document 2, the basicity of the slag after reductive refining is set to 1.5 ≦ (CaO) / (SiO 2) ≦ 2.0, and Al ₂ O ₃ in the slag is used. And MgO content must be regulated so that 5 ≦ (Al ₂ O ₃ ) ≦ 15 and 8 ≦ (MgO) ≦ 15, respectively. Accordingly, there is a problem that the adjustment flux unit for adjustment to be within this range is increased, and there is a problem that adjustment to keep the slag composition within the above-mentioned regulation range is complicated and workability is lowered.
Japanese Patent No. 3101411 JP 2000-129402 A

  An object of the present invention is to provide a method for melting stainless steel that is excellent in workability, can suppress loss of valuable metals, and can easily improve workability.

In order to achieve the above-mentioned object, the method for melting stainless steel according to the present invention provides a stainless steel melt obtained by roughly refining and adjusting the components of the stainless steel produced by melting the raw material, and further refining the stainless steel before being used for casting. A method of melting stainless steel including a final refining process,
In the final refining process, decarburization refining by blowing an oxidizing gas into the molten stainless steel in a state where the slag exists without removing the slag generated in the process prior to the final refining process,
After decarburization, metal silicon or silicon alloy is added from above the slag layer to deoxidize,
After deoxidation, linear or rod-shaped metal aluminum or aluminum alloy is charged into the molten stainless steel through the slag layer ,
The amount of metal aluminum or aluminum alloy charged into the molten stainless steel is 0.2 kg or more and 3.0 kg or less in terms of pure aluminum per ton of stainless molten steel,
The non-metallic inclusions in the stainless steel are only C-type inclusions .

In smelting process stainless steel of the invention, when end-polishing, the amount of slag to be subjected to refining with stainless molten steel has preferable be over stainless molten steel per ton 10 kg. Moreover, it is preferable that Cr content of the molten stainless steel in the step of charging metal Al or Al alloy is 10% by weight or more.

  According to the method for melting stainless steel according to the present invention, since the final decarburization refining together with the molten stainless steel without removing the slag generated in the process prior to the final refining process, there is no need for removal and workability. Excellent. In addition, since Si or Si alloy is introduced from the top of the slag layer after decarburization, valuable metal oxides in the slag can be reduced and recovered with Si, and the molten stainless steel can be deoxidized by Si reaching the molten stainless steel. can do. Preferably, the amount of slag used for final refining together with the molten stainless steel is 10 kg or more per ton of molten stainless steel. This makes it possible to recover valuable metals, especially Cr, without waste. Further, since Si is not excessively reduced by deoxidation with Si, adjustment of the Si content of the molten stainless steel becomes easy.

  Further, since the linear or rod-like metal Al or Al alloy is charged into the molten stainless steel through the slag layer after deoxidation, the Al can be easily charged into the molten stainless steel with almost no reaction with the slag. With such a simple operation, Al can be effectively acted on the molten stainless steel so that the non-metallic inclusions in the stainless steel can be made only of C-based inclusions, so that the workability of the stainless steel can be improved. By setting the Cr content in the step of charging the metal Al or Al alloy to 10% by weight or more, a stainless steel satisfying the corrosion resistance and workability can be obtained.

  FIG. 1 shows an example of a schematic manufacturing process for carrying out the method for melting stainless steel according to the present invention.

  In the first step a1, for example, scrap or other raw materials are melted in an electric furnace to produce stainless steel. In order to coarsely refine the stainless hot metal and adjust the components in the converter, for example, the steel is discharged from the electric furnace to the converter in step a2. In step a3, in order to protect the refractories constituting the inner wall and the bottom of the converter, CaO that is a slag material is added.

  In step a4, the stainless steel hot metal is roughly refined in a converter. The rough smelting in the converter is a rough decarburization mainly by oxygen blowing to lower the C concentration from several weight% to less than 1 weight%. By this rough decarburization, the C concentration of the stainless hot metal is lowered to obtain a molten stainless steel. In this rough refining process, the CaO previously charged melts into slag, and Cr and Si in the stainless steel are oxidized and transferred into the slag.

  Here, the amount of slag generated in the converter in the rough refining process will be described. It is assumed that the set value of the amount of Si contained in the stainless hot metal before oxygen blowing is 0.20% by weight, and this Si is entirely oxidized by oxygen blowing and moves into slag.

The amount of SiO ₂ produced by oxidation of Si by oxygen blowing per ton of stainless steel hot metal can be obtained by equation (1). In the following formula (1), the term (60/28) is for calculating the mass increase from Si to SiO ₂ from the molecular weight.
1000 × 0.2 ÷ 100 × (60/28) = 4.29 [kg] (1)

When the slag is designed so that the basicity of the slag: (CaO) / (SiO ₂ ) is 1.5 or more for the purpose of protecting the refractory, the minimum amount of CaO used as the slagging material is obtained by the formula (2). It is done.
5 × 4.29 = 6.44 [kg] (2)

Therefore, the amount of slag produced per ton of stainless steel is 10.7 kg / ton by adding the (SiO ₂ ) amount and (CaO) amount obtained from the above formulas (1) and (2). . In addition, since part of the Cr in the stainless steel molten iron is oxidized through oxygen blowing and moves into the slag, the amount of slag produced actually exceeds 10.7 kg / ton.

  In step a5, Cr, which is the main component of stainless steel, and other components are adjusted. The molten stainless steel, which has been coarsely refined and adjusted in the converter, is further refined in the final refining process before being used for casting. The refining means in the final refining process is not particularly limited, and for example, a vacuum decarburization method or an argon-oxygen decarburization method can be used. Here, the case of final refining by a vacuum decarburization method is illustrated, and the vacuum decarburization method is abbreviated as VOD, and the device used for VOD is abbreviated as VOD device.

  In step a6, the molten stainless steel that has been subjected to rough refining and component adjustment in the converter is tapped into the ladle together with the slag generated in the converter. One feature of the present invention is that the slag produced in the process prior to the final refining process is used for the final refining process together with the molten stainless steel without removing the slag.

  As described above, since slag contains valuable metal, especially Cr, if removed, Cr in the slag is lost wastefully. However, by bringing slag into the processes after the final refining as in the present invention, Cr can be reduced and recovered in the molten stainless steel for effective use as will be described later. In addition, since it is not removed, the time required for removal is not wasted and workability can be improved.

  The amount of slag transferred from the converter to the ladle is preferably the total amount of slag generated by the converter in order to suppress Cr loss as much as possible. Here, the total amount of slag means the total amount that can be substantially transferred to the ladle, except for a slight amount remaining on the furnace wall.

  Specifically, since at least 10.7 kg of slag is produced per ton of molten stainless steel in the converter as described above, substantially 10% / ton or more of slag can be transferred to the pan if substantially 10% / ton is obtained. You can think that you were able to move. Therefore, it is preferable to supply 10 kg / ton or more to the final refining process together with the molten stainless steel as a specific amount of slag. This can suppress the loss of Cr existing as an oxide in the slag.

  The ladle from which the molten stainless steel and slag are extracted is carried into a VOD apparatus, and in step a7, final refining is performed with the VOD apparatus. In the final refining process in the VOD, the inside of the VOD apparatus containing the ladle is made a reduced pressure atmosphere, and decarburization refining is performed by blowing an oxidizing gas, for example, oxygen gas, into the molten stainless steel in a state where the slag generated in the converter exists. This lowers the C concentration in the molten stainless steel to the target C concentration as a stainless steel product.

After this decarburization refining, in step a8, metal Si or Si alloy is introduced from above the slag layer while deoxidizing the inside of the VOD apparatus to deoxidize. Deoxidation with Si is one of the features of the present invention. The introduced Si reduces Cr oxide in the slag layer. The reduced Cr is recovered in the molten stainless steel and effectively used as the main component of the stainless steel. On the other hand, since Si as a deoxidizing material does not reduce SiO ₂ in the slag, the amount of Si to be added at the time of deoxidation is an amount for reducing Cr oxide in the slag, deoxidizing the molten stainless steel. It is only necessary to consider the amount to be added and the amount to be contained in the molten stainless steel as a stainless steel component. That is, since it is not necessary to consider the amount of Si reduced from the slag, the adjustment of the Si content in the stainless steel can be facilitated. In this VOD process, an alloy element may be added as necessary to finely adjust the components.

  After deoxidation, the VOD device is opened to the atmosphere in step a9, and in step a10, a linear or rod-shaped metal Al or Al alloy is charged into the molten stainless steel through the slag layer at atmospheric pressure. The most characteristic feature of the present invention is the insertion of linear or rod-like metal Al or Al alloy into the molten stainless steel. Here, a case where a wire made of a linear Al alloy is inserted will be exemplified.

  FIG. 2 shows a state in which the Al wire 2 is inserted into the molten stainless steel 7 using the wire supply device 1. The wire supply device 1 includes a wire reel 3 in which an Al wire 2 is wound in a coil shape, a wire feeder 4, and a guide pipe 5.

  The wire feeder 4 is a device for feeding the Al wire 2 unwound from the wire reel 3 so as to be loaded into the molten stainless steel 7 in the pan 6. The guide pipe 5 is a pipe provided so as to be positioned between the wire feeder 4 and the ladle 6 when the Al wire 2 is inserted into the molten stainless steel 7, and the Al wire 2 is inserted into the pipe. The Al wire 2 fed by the wire feeder 4 is guided to a position above the slag layer 8. The Al wire 2 is fed by the wire feeder 4, guided to the upper side of the slag layer 8 by the guide pipe 5, and the Al wire 2 is charged into the stainless steel melt 7 through the slag layer 8 by further feeding.

  For example, when lump Al is introduced from above the slag layer 8 formed on the stainless steel molten steel 7, the lump Al reacts with the slag, and Al cannot effectively act on the stainless steel molten steel 7. However, by inserting the Al wire 2 as in the present invention, it is possible to pass through the slag layer 8 with almost no reaction with the slag and easily reach the molten stainless steel 7. As a result, Al contained in the Al wire 2 reacts with the molten stainless steel 7 to replace non-metallic inclusions in the steel from A-based inclusions to C-based inclusions. In this way, the non-metallic inclusions in the stainless steel can be made only of C-based inclusions by a simple operation, so that the workability of the stainless steel can be improved.

The reason why only C-based inclusions are obtained by inserting Al into the molten stainless steel 7 is considered as follows. The charged Al reacts with oxygen in the molten stainless steel to form non-metallic inclusions of spinel structure containing Al ₂ O ₃ and forms A-based inclusions when forming the non-metallic inclusions of the spinel structure It is presumed that since the metal component that is easy to do is taken in, the A-based inclusion is not formed and only the C-based inclusion is formed.

  In the present invention, a preferable amount of the Al wire 2 charged into the molten stainless steel 7 is 0.2 kg or more and 3.0 kg or less in terms of pure Al per ton of stainless steel molten steel. This pure Al content per ton of molten stainless steel is called Al basic unit. In order to sufficiently improve the workability of stainless steel by using only C-based inclusions as the non-metallic inclusions in the stainless steel, the Al basic unit is preferably set to 0.2 kg / ton or more.

On the other hand, the upper limit of the Al basic unit is not particularly limited from the viewpoint of improving workability. However, when a large amount of Al is charged, a part of it reacts with the slag, Al reduces the SiO ₂ in the slag, and the reduced Si is recovered into the stainless molten steel 7 and the Si of the molten stainless steel 7 is recovered. Since the concentration is increased, it becomes difficult to adjust the Si content of the stainless steel.

  FIG. 3 shows the relationship between the Al basic unit and the Si concentration increase of the molten stainless steel. It can be seen from FIG. 3 that the degree of increase in the Si concentration of the molten stainless steel increases as the Al basic unit charged into the molten stainless steel increases. In order to make the Si content of stainless steel easy to hit, setting the allowable value of the Si concentration increase by reduction recovery to 0.2% by weight or less increases the Si concentration by 0.2% by weight in FIG. The required Al basic unit corresponds to 3.0 kg / ton. Therefore, 3.0 kg / ton is set as a preferable upper limit value of the Al basic unit.

  This Al basic unit can be obtained as follows. From the density of the Al wire 2, the concentration of Al contained in the Al wire 2, the diameter of the Al wire 2, the feeding speed and the feeding time, the total charged amount of pure Al can be calculated. The Al basic unit can be obtained by dividing the total charged amount of the pure Al by the amount of the molten stainless steel 7.

  The material used as the Al wire 2 is not particularly limited, but a material having a high Al purity is preferable in order to keep impurities mixed into the molten stainless steel 7 low. For example, Al and Al alloy rods and wires defined in JIS H4040-2006 are preferably used.

  When the Al wire 2 is inserted and the shape control of the nonmetallic inclusions in the molten stainless steel 7 is performed, the melting method of the stainless steel of the present invention is completed, and then the molten stainless steel 7 is sent to the casting process.

  The Cr content of stainless steel to which the melting method of the present invention is suitably used, that is, the Cr content of molten stainless steel in step a10 in which metal Al or Al alloy is charged is preferably 10 wt% or more.

In order to satisfy the corrosion resistance, which is a basic property of stainless steel, it is effective to contain 10% by weight or more of Cr. By applying the melting method of the present invention, the workability is improved, and by containing 10% by weight or more of Cr, a stainless steel having corrosion resistance can be obtained. The upper limit of the Cr content is not particularly limited, but is set to about 30% by weight from the viewpoint of manufacturability and practicality.

  For example, the melting method of the present invention includes 10 Cr, such as martensitic stainless steel, ferritic stainless steel, austenitic stainless steel containing Ni together with Cr, and duplex stainless steel composed of two phases of austenite and ferrite. It can be applied to all stainless steels containing at least% by weight. Among these, the effect can be remarkably exhibited in so-called Cr-based stainless steels such as martensitic stainless steel and ferritic stainless steel.

(Example)
Examples of the present invention will be described below. In this example, the case where the stainless steel product is melted so that the Cr concentration becomes 14% by weight is illustrated. Scrap and other raw materials were melted in an electric furnace to produce stainless steel. In order to change the Al basic unit charged in the subsequent process in various ways, a total of 7 charges of stainless steel was produced. The amount of stainless steel hot metal for one charge is about 75 tons. Table 1 shows the chemical composition range of the 7-charged stainless steel.

Next, the hot metal of each charge was roughly refined in a converter and the components were adjusted. In the converter, before rough refining and component adjustment, the target value of basicity: (CaO) / (SiO ₂ ) was set to 1.8, and CaO as a slagging material was added. Coarse refining in the converter was mainly rough decarburization by oxygen blowing, and the stainless steel molten steel was made by lowering the C concentration of the stainless steel to 0.05 to 0.30% by weight. CaO melted through this rough refining and produced slag together with SiO ₂ produced by oxidation of Si in the stainless steel and Cr oxide produced by oxidation of Cr. In the converter, the components were adjusted so that the Cr concentration was 12 to 17% by weight.

  After rough refining and component adjustment, both the molten stainless steel and the generated slag were discharged from the converter to the ladle without deoxidation. At this time, the amount of slag discharged to the ladle along with the molten stainless steel was calculated from the amount of stainless steel melted and melted in the electric furnace and the total amount of the molten stainless steel and slag discharged to the ladle. It was 10 kg or more per 1 ton of molten stainless steel.

  The ladle containing the molten stainless steel and slag was accommodated in a VOD apparatus, and the inside of the VOD apparatus was made into a reduced pressure atmosphere, and then decarburized and refined by blowing oxygen gas into the molten stainless steel in a state where the slag was present. This decarburization refining was performed until the target C concentration of the stainless steel product was 0.03% by weight or less.

  After decarburization refining, an FeSi alloy was introduced from above the slag layer in a reduced-pressure atmosphere. With the Si in the FeSi alloy, Cr oxide in the slag produced through rough refining in the converter and decarburization refining at the VOD is reduced to recover Cr, which is a valuable metal, in the molten stainless steel. The stainless steel with high oxygen concentration was deoxidized through rough refining and decarburization refining at VOD.

Since valuable metals in the slag are reduced and recovered by this Si deoxidation, the slag composition is changed and the basicity: (CaO) / (SiO ₂ ) is reduced to about 1.4.

Incidentally, it was composition analysis were taken slag from the charge after decarburization refining and Si deoxidation, (MgO) is less than 8 wt%, and (Al 2 _{O 3)} is of less than 5 wt%.

  After deoxidizing Si, the VOD apparatus was opened to the atmosphere, and Al wire was charged into the molten stainless steel through the slag layer using the wire supply apparatus 1 shown in FIG. Of the 7 charges, 1 wire was not charged at all, and the remaining 6 charges were charged with Al wire while changing the Al basic unit within the range of 0.1 to 5.0 kg / ton.

  The Al wire used in this example was a wire of alloy number 1070 defined by JIS H4040-2006 with a diameter of 8.98 mm, and its Al purity was 99.80% by weight. The feeding speed when charging the Al wire into the molten stainless steel was appropriately selected for each charge between 150 and 250 m / min.

  As described above, the adjustment of the Al basic unit is performed by obtaining the total amount of Al pure charged from the density of the Al wire, the diameter of the Al wire, the Al purity of the Al wire, the feeding speed and the feeding time. The value obtained by dividing by the amount of molten steel was set to a predetermined value selected in the range of 0.1 to 5.0 kg / ton.

  After the Al wire was inserted, molten stainless steel was cast and formed into a slab. The slab was hot-rolled and further cold-rolled to a thickness of 2.0 mm. The rolled stainless steel plate was further annealed and pickled to finish the final product. Table 2 shows the chemical composition of the stainless steel plate produced from each charge together with the Al basic unit. The balance other than the components shown in Table 2 of each charge is Fe and inevitable impurities. In addition, the test number of S1-S7 was attached | subjected to each charge for convenience. Hereafter, each charge is called with a test number.

  A test piece for measuring the cleanliness of non-metallic inclusions was cut out from the stainless steel plate produced for each of S1 to S7, and the cross section of the test piece was polished to be used for measurement. The cleanliness was measured based on JIS G0555-1956, and the cleanliness of non-metallic inclusions was measured by classifying each of the A system, the B system, and the C system. The results of measuring the cleanliness for each charge are shown in Table 3 together with the Al basic unit.

  B-type inclusions were not detected in any of S1 to S7. In S1 and S2, both A-type and C-type inclusions were detected, and in S3 to S7, only C-type inclusions were detected. It can be seen that by setting the Al basic unit to 0.2 kg / ton or more, non-metallic inclusions are only C-based inclusions.

  Next, a specimen for a bending test was collected from the stainless steel plate manufactured for each of S1 to S7, and an adhesion bending test was performed according to JIS Z2248-1996. As a result, no cracks or cracks occurred in S3 to S7 where the non-metallic inclusions were only C-based inclusions. On the other hand, in S1 and S2 in which the nonmetallic inclusions include A-based inclusions and C-based inclusions, fine cracks occurred. This shows that the workability of stainless steel can be improved by making the non-metallic inclusions in the stainless steel only C-based inclusions.

  Since the non-metallic inclusions are only C-based inclusions by making the Al basic unit charged into the molten stainless steel 0.2 kg / ton or more, there is a particular problem even if the Al basic unit is large from the viewpoint of workability. There is no. However, if the Al basic unit exceeds 3.0 kg / ton, Si is excessively reduced from the slag and recovered in the molten stainless steel as described above, and it becomes difficult to target the Si content. It is preferable to be 3.0 kg / ton or less.

The example of the schematic manufacturing process for enforcing the melting method of the stainless steel of this invention is shown. The state which inserts the Al wire 2 in the stainless steel molten steel 7 using the wire supply apparatus 1 is shown. The relationship between the Al basic unit and the Si concentration increase of the molten stainless steel is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Wire supply apparatus 2 Al wire 3 Wire reel 4 Wire feeder 5 Guide pipe 6 Ladle 7 Stainless steel molten steel 8 Slag layer

Claims (3)

A method for producing stainless steel comprising a final refining step of refining a stainless steel molten iron produced by melting raw material into a stainless steel by rough refining and component adjustment, and before the stainless steel molten steel is subjected to casting,
In the final refining process, decarburization refining by blowing an oxidizing gas into the molten stainless steel in a state where the slag exists without removing the slag generated in the process prior to the final refining process,
After decarburization, metal silicon or silicon alloy is added from above the slag layer to deoxidize,
After deoxidation, linear or rod-shaped metal aluminum or aluminum alloy is charged into the molten stainless steel through the slag layer ,
The amount of metal aluminum or aluminum alloy charged into the molten stainless steel is 0.2 kg or more and 3.0 kg or less in terms of pure aluminum per ton of stainless molten steel,
A method for melting stainless steel, characterized in that the non-metallic inclusions in the stainless steel are only C-type inclusions .   The method for melting stainless steel according to claim 1, wherein the amount of slag used for refining together with the molten stainless steel is 10 kg or more per ton of molten stainless steel in the final refining. The method for melting stainless steel according to claim 1 or 2 , wherein the chromium content of the molten stainless steel in the step of charging metal aluminum or aluminum alloy is 10 wt% or more.

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