JP4170193B2 - Stainless steel plate with less heat generation and method for producing the same - Google Patents

Description

  The present invention relates to a steel plate in which water-soluble sulfide-based non-metallic inclusions represented by CaS, which is the starting point of a stainless steel plate, are rendered harmless, and a method for producing the same.

  In steels containing Ca as well as ferritic stainless steel plates, glazing starting from water-soluble sulfide-based nonmetallic inclusions becomes a problem. As a method for suppressing wrinkling due to CaS, MnS, or the like, a method that is performed under melting conditions is known.

  For example, Patent Document 1 proposes that the amount of Ca in steel be less than 10 ppm in order to suppress the formation of CaS. In this method, when the composition of the oxide becomes high CaO, CaS of water-soluble sulfide-based nonmetallic inclusions is generated around it and becomes a starting point.

  A method of suppressing precipitation of water-soluble sulfide-based non-metallic inclusions CaS by controlling the composition of oxide-based non-metallic inclusions and the amount of equilibrium S dissolved in the non-metallic inclusions as shown in Patent Document 2. is there. This method has a problem of increasing the raw material cost as well as increasing the load during melting because the composition of non-metallic inclusions is controlled.

  Further, the nitric acid electrolytic treatment used after bright annealing of a stainless steel plate is generally aimed at strengthening the passive film by anodic electrolytic treatment.

  Patent Document 3 proposes a method for improving pitting corrosion resistance by strengthening a passive film by anodic electrolysis in a phosphoric acid solution. That is, unlike the alternating electrolytic treatment performed for the purpose of descaling after cold rolling of the 2B product, the electrolytic treatment after bright annealing is anodic electrolysis. No study has been made on detoxification of non-metallic inclusions by cathodic electrolysis treatment and improvement of cracking.

  On the other hand, there are Patent Documents 4 to 7 as methods for improving the corrosion resistance by cathodic electrolysis, but these are special aqueous solutions (for example, special and expensive chemicals such as chromic acid, phosphoric acid, ammonium molybdate, sodium dichromate). Is used to improve the corrosion resistance of stainless steel and has the disadvantage of high cost.

Japanese Patent Laid-Open No. 06-0000599 JP 2001-107178 A Japanese Patent Application Laid-Open No. 07-216591 JP 07-243094 A Japanese Patent Application Laid-Open No. 08-176881 Japanese Patent Laid-Open No. 09-184096 Japanese Patent Laid-Open No. 01-004494

  In the manufacturing method that suppresses the starting point of the stainless steel plate, the method of modifying the sulfide-based non-metallic inclusion composition as described above is not only the raw material blending method but also the introduction of a deoxidizer in the melting process Very strict process control such as time is required. Depending on the composition of the nonmetallic inclusions produced during deoxidation, sulfides may precipitate and grow in the vicinity of the nonmetallic inclusions in the subsequent heating step, which may be the starting point of initiation. Unlike the conventional methods for suppressing the starting point of the present invention, the present invention does not suppress CaS, but dissolves and removes the water-soluble sulfide-based nonmetallic inclusions that become the starting point in a short time in the final step. The purpose is to provide a method for improving drought.

As a result of various studies to solve the above-mentioned problems, the present inventors have selectively dissolved a water-soluble sulfide-based non-metal that becomes a starting point of initiation by subjecting a stainless steel plate to cathodic electrolysis. Appropriate conditions were found by paying attention to the fact that the surface gloss of the stainless steel plate was not dissolved and the surface gloss was not deteriorated.
(1)
The surface of the stainless steel plate, S concentration of sulfide-based nonmetallic inclusions which are exposed is less than 2%, and rust is less stainless steel plate number of the hemispherical hole is present 1 / cm ² or more.
(2)
Cathodic electrolytic treatment of stainless steel sheet in an aqueous solution of oxidizing acid ,
Cathodic electrolytic treatment conditions are
-Solution: 5 to 25 vol% oxidizing acid aqueous solution-Electrolytic potential: -200 to -600 mV
Electrolysis time: 1 to 15 seconds.
( 3 )
The oxidizing acid described in ( 2 ) is nitric acid.
( 4 )
Following the cathodic electrolysis described in ( 2 ) or ( 3 ), an anodic electrolysis is performed.

  The present invention is capable of dissolving and removing water-soluble sulfide-based non-metallic inclusions present on the surface of a stainless steel plate by cathodic electrolysis treatment, and can greatly suppress the generation of The value is extremely high.

  The present invention is described in detail below. The present inventors conducted various investigations on a method for dissolving and removing sulfide-based nonmetallic inclusions on the surface of the steel sheet by electrolytic pickling. As a result, it has been found that water-soluble sulfide-based nonmetallic inclusions are selectively dissolved and removed when the potential during cathodic electrolysis is within a specific condition in electrolytic treatment.

  It is clear from Patent Document 1 that water-soluble sulfide-based non-metallic inclusions are the starting point of the initiation. Here, the water-soluble sulfide-based non-metallic inclusion is a non-metallic inclusion in which a clear peak of S (sulfur) exists at a position different from O (oxygen) by a composition distribution survey using an X-ray microanalyzer. It is a thing. Further, a salt spray test-JIS-G-0577 (hereinafter referred to as SST) was carried out, and as a result of investigating and analyzing sulfide-based non-metallic inclusions present on the surface of the steel sheet with a SEM (scanning electron microscope), non-metallic inclusions were obtained. It was clarified that when the peak of S generated from is small, it is not the starting point of the starting. As shown in FIG. 1, when the S concentration is less than 2%, it does not occur.

  From FIG. 2, the upper limit was set to an S concentration of sulfide inclusions of less than 2 mass%. Preferably it is 1 mass% or less.

FIG. 2 shows the relationship between the electrolytic potential and the presence / absence of removal of sulfide-based nonmetallic inclusions when a constant potential electrolytic treatment is performed by the indirect energization method. Determination of removal of sulfides, SEM (scanning electron microscope) at 20 field observation magnification of 1000 using a (9020μm ^2/1 field) was observed, non-metallic inclusions and their inclusions present in the observation monitor removed The number of hemispherical holes estimated to have been made was measured. The electrolyte used was a 10% nitric acid aqueous solution. When the potential is −200 mV or less, hemispherical holes are always confirmed. Hemispherical holes were observed as traces removed by cathodic electrolysis. It became clear that the S peak of this hemispherical hole was hardly detected.

  As an electrolytic solution, the aqueous electrolytic solution is preferably an aqueous nitric acid solution, but may be an aqueous solution of an oxidizing acid. As electrolysis conditions, an electrolysis potential is −200 to −600 mV, preferably −300 to −500 mV. The electrolysis time is not economical because the cathodic dissolution treatment for a long time increases the number of hemispherical holes, requires an extension of the electrolytic treatment tank, and involves the construction of a huge facility. In a short time, sulfide-based nonmetallic inclusions are not removed. Therefore, the cathodic electrolysis time was set in the range of 1 to 15 seconds.

FIG. 3 shows the relationship between the number of hemispherical holes and the number of sputtered SSTs (pieces / 100 cm ² ). As the number of hemispherical holes increases, the number of sprinkles decreases. Although it is best that no rusting occurs, it can be used in a general environment as long as it has several levels.

  The selective removal of sulfide-based nonmetallic inclusions is influenced by the hydrogen produced during cathodic electrolysis. Hydrogen produced from the nitric acid aqueous solution by the cathodic electrolysis treatment reacts with sulfur in the sulfide to form hydrogen sulfide gas. Since sulfide-based non-metallic inclusions are formed of oxides and sulfides, the generation of gas from this sulfide is the same as the generation of gas between sulfide-based non-metallic inclusions and the base metal. As a result, sulfide inclusions are floated and removed. FIG. 4 shows an SEM photograph of the surface of the steel sheet that was subjected to cathodic dissolution treatment at a potential of −300 mV for 1 second. The result shows that hemispherical holes exist on the surface of the steel sheet, and only sulfide-based nonmetallic inclusions are floated and removed.

  From this result, the electrolysis potential in the present invention is −200 mV or less, preferably −200 mV to −600 mV, and the electrolysis time is preferably 1 second or more and less than 15 seconds. Cathodic electrolysis treatment of less than −600 mV is not so preferable because it only increases the size of the device and increases the required current, and the improvement effect obtained is small. In addition, an aqueous solution containing an oxidizing acid that promotes hydrogen generation during electrolysis is effective as the electrolytic solution to be used, and it is preferable to use nitric acid that is easily available in the present invention. The concentration of the electrolytic solution was adjusted to 25% by volume or more in order to reduce the number of additions of the solution, so the upper limit was set to 25%. Preferably 10-20 vol% is good.

  In order to increase the effect of the cathodic electrolysis treatment, it is desirable to use an oxidizing acid that easily generates hydrogen during electrolysis. An inexpensive oxidizing acid is nitric acid. FIG. 5 shows the relationship between the concentration of nitric acid during nitric acid electrolysis and the number of occurrences in SST. The numbers in the figure indicate the electrolysis time, and the electrolysis potential was -500 mV. As can be seen from the figure, the number of occurrences decreases with increasing concentration, but this effect is saturated at over 20%. In addition, the effect of electrolysis time is reduced by extending the time, but the effect is saturated after more than 10 seconds.

  Although strengthening of the passive film in the anodic electrolytic treatment is known, the effect can be obtained in a short time by applying it after the cathodic dissolution treatment. That is, the sulfide-based non-metallic inclusions present on the steel sheet surface are removed by cathodic electrolysis. This sulfide-based non-metallic inclusion is the starting point when the passive film is destroyed, but since it does not exist, the passive film can not be destroyed and a stable film can be formed. is there. On the other hand, if the electrolytic potential during anodic electrolysis is too high, the base material dissolves and the surface properties are significantly deteriorated.

  From the above, as a method for selectively removing sulfide-based non-metallic inclusions existing on the surface of the steel sheet and preventing it from becoming a starting point, the cathodic electrolysis treatment is performed, and the anodic electrolysis treatment is performed in the subsequent steps. A method in which the passive film is strengthened and the corrosion resistance is greatly improved is optimal.

After the stainless steel plate (thickness 0.4 mm) shown in Table 1 was brightly annealed, electrolytic treatment was performed by changing the electrolytic potential and the electrolytic time. The electrolytic conditions and results are shown in Table 2. As is apparent from Table 2, when electrolytic treatment is performed within the conditions of the present invention, the number of SSTs generated is stably less than 10/100 cm ^2, and the effect of the cathodic electrolytic treatment is clear.

  Anodic electrolysis was additionally performed on the stainless steel plate described in Example 1 (plate thickness: 0.4 mm). As shown in Table 2, it is clear that if the anodic electrolysis treatment is applied under the conditions of the present invention, the destruction time of the passive film can be stably improved and the passive film can be strengthened stably.

The figure which shows the relationship between S density | concentration in sulfide type nonmetallic inclusion, and the number of generation | occurrence | production of SST. Figure showing the relationship between the electrolytic potential and the presence or absence of hemispherical pores when performing constant potential electrolytic treatment The figure which shows the relationship between the number of hemispherical holes and the number of SST occurrences SEM photograph of steel plate surface subjected to cathodic electrolysis for 1 second by -300mV constant potential electrolysis The figure which shows the relationship between the nitric acid concentration of cathodic electrolysis processing, electrolysis time, and the number of SST generations

Claims (4)

On the surface of the stainless steel plate, the slag is characterized in that the S concentration in the exposed sulfide-based nonmetallic inclusion is less than 2% and the number of hemispherical holes is 1 / cm ² or more. Less stainless steel sheet. Cathodic electrolytic treatment of stainless steel sheet in an aqueous solution of oxidizing acid ,
The cathodic electrolytic treatment conditions are
Electrolytic aqueous solution: 5 to 25 vol% oxidizing acid aqueous solution
Electrolytic potential: -200 to -600 mV
・ Electrolysis time: 1 to 15 seconds
A method for producing stainless steel with less heat generation. 3. A method for producing a stainless steel plate with less heat generation, characterized in that the aqueous solution of oxidizing acid according to claim 2 is an aqueous nitric acid solution. A method for producing a stainless steel plate with less heat generation, characterized by performing anodic electrolysis after the cathodic electrolysis according to claim 2 or 3 .