WO2013018629A1 - 高Siオーステナイト系ステンレス鋼の製造方法 - Google Patents
高Siオーステナイト系ステンレス鋼の製造方法 Download PDFInfo
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- WO2013018629A1 WO2013018629A1 PCT/JP2012/068906 JP2012068906W WO2013018629A1 WO 2013018629 A1 WO2013018629 A1 WO 2013018629A1 JP 2012068906 W JP2012068906 W JP 2012068906W WO 2013018629 A1 WO2013018629 A1 WO 2013018629A1
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
Definitions
- the present invention relates to a method for producing a high-Si austenitic stainless steel suitable for use in a high-temperature, high-concentration nitric acid environment.
- Stainless steel forms a stable passive film in nitric acid and exhibits excellent corrosion resistance.
- high-temperature, high-concentration nitric acid for example, temperature: 80 to 90 ° C. and concentration: 90% by mass, is extremely oxidizable and causes general passive corrosion in general stainless steel. Due to the overpassive corrosion, the overall corrosion proceeds with the elution of Cr 2 O 3 forming a passive film.
- Patent Documents 1 and 2 As a material having corrosion resistance in this kind of environment, there is a high Si austenitic stainless steel disclosed by Patent Documents 1 and 2. These high Si austenitic stainless steels exhibit excellent nitric acid corrosion resistance due to the formation of a silicate (SiO 2 ) film in the overpassive region.
- SiO 2 silicate
- Patent Document 3 discloses that the chemical composition is Al: 0.05% or less (in this specification, “%” relating to the chemical composition means “mass%” unless otherwise specified), O : Limiting to 0.003% or less, and eliminating the formed intermetallic compound by performing long-term soaking and / or soaking at 1100 to 1250 ° C. and then hot rolling, It is disclosed to improve hot workability. Inclusions are limited by the total amount, not by type.
- Patent Document 4 discloses that the amount of sol.Al is defined in order to prevent the formation of oxides that impair the work flow corrosion resistance. However, the inclusions generated in the molten steel are not examined. There is no description about the corrosion resistance deterioration resulting from it. In general, since the amount of inclusions such as Al 2 O 3 and the amount of sol.Al are not directly related to each other, the problem caused by inclusions cannot be sufficiently prevented only by specifying the amount of sol.Al.
- Patent Document 5 discloses that since inclusions become a starting point of corrosion, the inclusions are finely dispersed to improve corrosion resistance. However, only the fine dispersion of MnS is achieved by the restriction of S and the restriction of hot rolling conditions, and nothing is disclosed about alumina inclusions.
- Patent Document 6 discloses an invention for preventing pitting corrosion by controlling the composition of inclusions to agglomerate the cluster shape and make it water-insoluble. However, such inclusions inhibit the formation of a silicate film necessary for ensuring corrosion resistance at high temperature and high concentration nitrification.
- An object of the present invention is to provide an austenitic stainless steel that stabilizes the acid resistance of high-Si austenitic stainless steel and has good corrosion resistance.
- inclusions B 1 inclusions described later
- the passive film of Cr 2 O 3 due to overpassive corrosion These inclusions are exposed on the steel surface by elution and elution of the base material.
- the inclusions thus exposed are very large compared to the thickness (several tens of nanometers) of the silicate film, with the size of one particle being several microns or more. Since the affinity between these inclusions and SiO 2 is small, a sufficient silicate film is not formed not only on the surface of the inclusion but also on the interface thereof. Therefore, a gap is inevitably formed between the inclusion and the silicate film, and a situation such as local crevice corrosion occurs, and excessive corrosion proceeds.
- JIS G 0555 (2003) Annex 1 “Microscopic test method for non-metallic inclusions by point calculation” (hereinafter simply referred to as the method described in JIS G 0555) defines the microscopic test method for non-metallic inclusions in steel.
- the types of inclusions are A-type inclusions (subdivided into sulfide A 1 series, silicate A 2 series such as SiO 2 ) that are viscously deformed by processing such as hot rolling, processing direction without B type inclusions granular aligned discontinuously without the population (subdivided into the oxide B 1 system and carbonitride-based B 2 system such as alumina), and viscous deformation to not C-type inclusions such as CaO dispersed regularly are classified.
- B 1 -based inclusions such as alumina are produced by oxidation of Al, but have a high melting point, and therefore exist in a solid state without melting during molten steel refining treatment. These particles are adsorbed and agglomerated with each other when the particles collide during the molten steel treatment, and grow in clusters. Since the individual particles do not have ductility at room temperature or in the hot rolling temperature zone, they remain as small lumps and discontinuously exist as lumps having a size of 1 to several microns in the hot-rolled steel sheet. As a result, the above-described problem occurs.
- Carbon nitrides such as Nb, Ti, and Zr are classified as B 2 inclusions, but these dissolve in high-temperature and high-concentration nitric acid solutions, and thus the above-described problems do not occur.
- C-based inclusions such as CaO are generated by the addition of Ca such as Ca treatment.
- This has a relatively low melting point and melts in the molten steel refining treatment temperature zone by eutectic reaction with other oxides.
- particles collide with each other during the molten steel treatment they exist as liquids, so they grow as the particles become larger, and the size of one particle becomes several microns or more.
- These particles are solidified and exist as solids at a temperature equal to or lower than the hot rolling temperature zone, but are present in the rolled steel sheet as aggregated particles because they do not have ductility.
- the CaO inclusions exposed on the surface layer are dissolved in a high-temperature, high-concentration nitric acid solution, the above-described problem does not occur.
- the A-based inclusions such as SiO 2 have a relatively low melting point like the C-based inclusions, they grow to a size of several microns or more by colliding in a liquid state during the molten steel treatment.
- the A-based inclusion has ductility, it is stretched together with the base material in hot rolling or cold rolling, and is stretched to a thickness of 1 micron or less depending on the rolling ratio.
- a 2 inclusions of an extended inclusions act itself as an alternative to passive layer, improving the nitrate corrosion resistance.
- SiO 2 since it has an affinity for a silicate film formed from eluted Si, even if it is exposed on the surface of steel, the formation of the silicate film is not inhibited.
- the main inclusion that affects the corrosion resistance in high-temperature and high-concentration nitric acid is a B 1 -based inclusion such as alumina, so that the amount must be regulated.
- SiO 2 which is an A 2 -based inclusion is effective for improving the nitric acid corrosion resistance as long as it is within a certain limit. Therefore, it is preferably contained in a high Si austenitic stainless steel.
- C 0.04% or less, Si: 2.5 to 7.0%, Mn: 10% or less, P: 0.03% or less, S: 0.03% or less, N: 0.035 %, Sol.Al: 0.03% or less, Cr: 7-20%, Ni: 10-22%, and if necessary, one or more of Nb, Ti, Ta, Zr
- the total amount of B 1 inclusions measured by the method described in JIS G 0555 is 0.5 when the total content is 0.05 to 0.7% and the balance is Fe and impurities. It is an austenitic stainless steel characterized by being not more than 03 area%.
- the austenitic stainless steel according to the present invention preferably contains SiO 2 that is an A 2 inclusion measured by the method described in JIS G 0555 in an amount of 0.06% or less.
- the high Si austenitic stainless steel according to the present invention exhibits stabilized acid resistance and is excellent in corrosion resistance in a high temperature and high concentration nitric acid environment. Therefore, although this stainless steel is suitable as a constituent material of a nitric acid production plant, it can also be used for other applications requiring acid resistance.
- FIG. 1 is a graph showing an example of the relationship between B 1 inclusions and corrosion rate.
- C is an element that increases the strength of steel, but it causes the formation of Cr carbide at the grain boundary in the heat-affected zone of the weld and causes sensitization (increased susceptibility to intergranular corrosion). It is. Therefore, the C content is set to 0.04% or less.
- the C content is preferably 0.03% or less, and more preferably 0.02% or less.
- Si is contained in an amount of 2.5% to 7% in order to enhance the corrosion resistance in concentrated nitric acid.
- the Si content is set to 2.5% or more.
- the upper limit of the Si content is 7%.
- the lower limit of the Si content is preferably 2.7%, more preferably 2.8%. Further, the upper limit of the Si content is preferably 6.8%, and more preferably 6.6%.
- Mn is an austenite phase stabilizing element and also acts as a deoxidizer, so it is contained in an amount of 10% or less. If the Mn content exceeds 10%, the corrosion resistance is lowered, hot cracking during welding, and further the workability is lowered.
- the Mn content is preferably 5% or less, and more preferably 2% or less. In order to reliably obtain the above effect of Mn, the Mn content is preferably 0.5% or more, and more preferably 1.0% or more.
- P and S Both elements are elements that are particularly harmful to hot workability with respect to corrosion resistance, weldability, and S. The lower the content, the better. Appear in Therefore, the P content is 0.03% or less, and the S content is 0.03% or less.
- N 0.035% or less
- N has a high affinity with Nb, Ti, Ta, and Zr, and inhibits the fixation of C by these elements. Therefore, N is preferably as low as possible. If the N content exceeds 0.035%, its harmfulness becomes remarkable. Therefore, the N content is set to 0.035% or less.
- the N content is preferably 0.020% or less, and more preferably 0.015% or less.
- Al is used as a deoxidizing agent and a reducing agent for slag, but is also included in the alloy, so it is mixed when the alloy is added. Al reacts with dissolved oxygen in the molten steel to produce Al 2 O 3 . In addition, Al 2 O 3 is also generated when Al reduces SiO 2 inclusions in molten steel and oxides in slag.
- the Al 2 O 3 inclusion exposed on the surface layer is insoluble in water, prevents formation of a silicate film necessary for exhibiting corrosion resistance in nitric acid, and causes crevice corrosion. In addition to this, it causes nozzle clogging during casting, appearance defects, cracks and starting points of cracks and corrosion. Therefore, in the present invention, the amount of B 1 -based inclusions whose main component is Al 2 O 3 inclusions is regulated to a certain level or less. Therefore, the sol.Al content is set to 0.03% or less. The sol.Al content is preferably 0.02% or less. Reduction of the Al content can be realized by using an alloy having a low Al content.
- Cr 7-20%
- Cr is a basic element for ensuring the corrosion resistance of stainless steel, and is 7 to 20%. If the Cr content is less than 7%, sufficient corrosion resistance cannot be obtained. On the other hand, if the Cr content is excessive, a co-existence of Si and Nb results in a two-phase structure in which a large amount of ferrite is precipitated, resulting in deterioration of workability and impact resistance. Therefore, the upper limit of Cr content is 20%. To do.
- the lower limit of the Cr content is preferably 10%, and more preferably 15%.
- Ni is a stabilizing element of the austenite phase and has the effect of increasing the zero ductility temperature, so it is contained in an amount of 10 to 22%. If the Ni content is less than 10%, it is not sufficient to obtain an austenite single phase. Excessive addition of Ni only increases the cost, and a sufficient austenite single phase is obtained at 22% or less.
- the upper limit of the Ni content is preferably 18%, and more preferably 14%.
- the lower limit of the Ni content is preferably 11%, and more preferably 12%.
- Nb, Ti, Ta, and Zr are all effective in fixing C and suppressing deterioration in corrosion resistance due to sensitization, and are particularly effective in suppressing sensitization of the heat affected zone. It is an optional element that may be contained accordingly. In order to suppress sensitization, it is effective that the total content of one or more of these is 0.05% or more. If the total content of one or more of these exceeds 0.7%, workability and corrosion resistance are deteriorated. Therefore, when 1 type or 2 types or more selected from Nb, Ti, Ta, and Zr are contained, the total content is set to 0.05% to 0.7%. The lower limit of this total content is preferably 0.3%.
- the balance other than the above elements is Fe and impurities.
- the amount of inclusions is the amount measured according to the method described in JIS G 0555. In addition, the amount (%) of inclusions is area%. In the measurement, 60 visual fields are measured according to the method defined in the above standard, and the average value is taken as the amount of inclusions.
- Total amount of B 1 inclusions 0.03% or less
- most of the B 1 inclusions are alumina (Al 2 O 3 ) in view of the chemical composition.
- the Al 2 O 3 inclusions exposed on the surface of the steel material are insoluble in water, prevent formation of a silicate film that exhibits corrosion resistance in nitric acid, and cause crevice corrosion.
- Al 2 O 3 inclusions in the molten steel cause nozzle clogging and hinder the casting operation.
- the inclusions remaining in the slab become wrinkles due to rolling and are not only visually bad, but also become a starting point of cracking during processing and use, so a step of removing wrinkles is necessary. Therefore, in order to improve these, the amount of B 1 inclusions is set to 0.03% or less. This amount is preferably 0.025% or less.
- a 2 -based inclusion SiO 2 content 0.06% or less
- a 2 inclusions such as SiO 2 have a relatively low melting point like C inclusions, and thus grow to a size of several microns or more during the molten steel treatment.
- it since it has ductility, it is stretched together with the base material in hot rolling or cold rolling, and depending on the rolling ratio, it is stretched to a thickness of 1 micron or less.
- a 2 inclusions such as SiO 2 present in the steel sheet are very thin and serve as a substitute for a passive film.
- the SiO 2 content of the A 2 inclusions exceeds 0.06%, as with the B 1 inclusions, an adverse effect on processing occurs.
- the presence of SiO 2 which is an A 2 inclusion, in an amount of 0.06% or less sufficiently secures resistance to nitric acid corrosion. Therefore, the inclusion is contained in an amount of 0.06% or less. It is preferable to contain.
- the inclusion content is more preferably 0.001% or more and 0.06% or less.
- a method for identifying SiO 2 which is an A 2 inclusion is visual determination.
- the sulfide inclusions that are A 1 inclusions are light in color, whereas the SiO 2 inclusions are dark black, so that the SiO 2 inclusions can be identified visually.
- a composite oxide or mixed oxide may be formed with SiO 2 , CaO, or the like.
- These mixed oxides are not significantly different from C-based inclusions mainly composed of CaO or the like, and are difficult to distinguish without elemental analysis.
- the crystal structure of this oxide is unknown, it dissolves in a high-temperature, high-concentration nitric acid solution, leaving only SiO 2 .
- the inclusions have a size of 10 ⁇ m or more, and cavities are formed in a high-temperature, high-concentration nitric acid solution, and crevice corrosion proceeds to deteriorate the corrosion resistance.
- Al 2 O 3 in the molten steel is generated by adding Al in the presence of dissolved oxygen as shown in the formula (1).
- scraps and alloys are melted in an electric furnace, but raw materials are selected carefully and ones with as low an Al concentration as possible are used. Be careful not to mix Al in scrap.
- decarburization is first performed in an AOD (argon oxygen decarburization) furnace and then in a VOD (vacuumm oxygen decarburization) furnace.
- AOD argon oxygen decarburization
- VOD vacuum oxygen decarburization
- oxygen in the molten steel is used to remove C from the system as CO gas.
- the oxidation of chromium proceeds at the same time, but decarburization is performed while mixing the argon gas and lowering the partial pressure of the CO gas while suppressing the oxidation of chromium.
- chromium oxidizes and migrates into the slag as Cr 2 O 3 . Since chromium is an expensive element, it is reduced to molten steel using a reducing agent after the treatment is completed. In general, reduction is performed using Al or Fe—Si alloy as a reducing agent. However, in the case of the present invention, in order to suppress the formation of alumina inclusions that deteriorate the corrosion resistance in high-temperature high-concentration nitric acid, it is necessary to limit the input of Al. Therefore, in the AOD, reduction is performed using only the Fe—Si alloy without using Al.
- Fe—Si alloy used here an alloy having as low an Al as possible is used.
- An inexpensive Fe-Si alloy that is normally used contains about 1% of Al used in the alloy manufacturing process.
- the cost of the Fe—Si alloy is approximately doubled, but an expensive low Al alloy having an Al content of approximately 0.1%. Fe—Si alloy is used.
- the reduced slag contains alumina.
- alumina in the slag is reduced in the subsequent steps and contained as Al in the steel, and that this Al reduces SiO 2 inclusions and the like to produce Al 2 O 3 inclusions.
- the alumina in the slag is physically excluded from the system by carefully carrying out the removal after completion of the reduction with AOD.
- the generated slag is removed until the surface of the bullion is exposed to about 70%, leaving about 30% of the slag. This is to prevent yield deterioration due to loss of bullion discharged out of the system along with removal.
- Thorough removal is performed until the face appears on the surface.
- COD in the molten steel is removed out of the system as CO gas using oxygen gas to further remove C out of the system by VOD.
- the system is evacuated and decarburized while reducing the partial pressure of CO gas by reducing the pressure and suppressing oxidation of chromium.
- an Fe-Si alloy is introduced for the purpose of adding Si to a predetermined value. .
- the final components and the molten steel temperature are adjusted in the ladle.
- a low Al Fe—Si alloy is also introduced here.
- a small amount of alumina remaining in the slag was reduced by the Fe—Si alloy and dissolved as Al in the steel, and then this Al was reoxidized by reducing inclusions such as SiO 2 and slag.
- Al 2 O 3 inclusions are generated.
- cutting with a snorkel is performed to prevent the fed Fe—Si alloy from directly touching the slag.
- the Si concentration in the Fe—Si alloy is 10 times higher, and the reducibility by Si is high.
- the reduced Al is reoxidized by slag and inclusions, and harmful Al 2 O 3 inclusions are generated. Therefore, to prevent this reoxidation, it is effective to avoid direct contact with the slag when the Fe—Si alloy is charged.
- sol.Al 0.03% or less, and the total of B 1 -based inclusions is reduced to 0.03% or less, both of which have not existed so far.
- a high-Si austenitic stainless steel according to the present invention that exhibits stable acid resistance and good corrosion resistance in nitric acid.
- a 200 mm thick slab is manufactured from molten steel having the composition shown in Table 1 by electric furnace-AOD-VOD-ladder refining-continuous casting, the slab is cut into a predetermined size, and hot rolled to a 6 mm thick A board was used.
- the main production conditions at that time are as shown in Table 1.
- the test steels 1 to 12 thus produced were subjected to a corrosion test after removing the scale from the surface by pickling.
- the corrosion test was conducted by immersing in 98% concentrated nitric acid at 60 ° C. for 700 hours.
- the corrosion rate calculated from the mass of the test piece before and after immersion is shown in Table 1 together with the amounts of B 1 and A 2 inclusions of the test steel obtained by the method described above.
- the amount of SiO 2 inclusion was measured by the visual method described above.
- FIG. 1 is a graph showing an example of the relationship between B 1 inclusions and corrosion rate.
- the test steels 5, 6, 7, and 12 are not plotted.
- Sample steels 1 to 3 which are inventive examples were good, with corrosion rates of less than 0.1 g / m 2 ⁇ hr.
- the test steel 4 has a high corrosion rate because the sol.Al content exceeded the upper limit and the B 1 inclusions also exceeded the upper limit due to the use of an ordinary FeSi alloy.
- the Cr content is outside the lower limit of the present invention, and the corrosion rate is remarkably high.
- the Si content is outside the lower limit of the present invention. Even if a normal FeSi alloy is used, there is little pick-up of Al, but the corrosion rate is remarkably high because of low Si.
- the test steel 7 has a high corrosion rate because the N content is outside the upper limit.
- Sample steel 8 is an example in which the stripping after AOD is insufficient.
- the alumina in the slag is partially reduced in the next step, and the sol.Al content in the molten steel is picked up.
- the B 1 inclusions also deviated from the upper limit value, so the corrosion rate was high.
- the test steel 9 did not use a snorkel when adjusting the final components in the ladle refining, the alumina in the slag was reduced by the high concentration Si in the fed FeSi alloy, and the sol.Al content in the molten steel was the upper limit. It was out of value. As a result, the B 1 inclusions also deviated from the upper limit value, and the corrosion rate was high.
- test steel 11 Since the test steel 11 had a high casting speed, the floating separation of inclusions was insufficient, and the B 1 inclusions deviated from the upper limit of the present invention. Therefore, the corrosion rate is large.
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Abstract
Description
[化学組成]
[C:0.04%以下]
Cは,鋼の強度を高める元素ではあるが、溶接部の熱影響部において粒界にCr炭化物を形成させ、鋭敏化(粒界腐食の感受性増大)の原因となるなど、耐食性を劣化させる元素である。したがって、C含有量は0.04%以下とする。C含有量は、好ましくは0.03%以下であり、さらに好ましくは0.02%以下である。
Siは、濃硝酸中での耐食性を高めるために2.5%以上7%以下含有させる。硝酸中での耐食性を確保するシリケート皮膜を形成するために、Si含有量は2.5%以上とする。一方、Siを過剰に含有させるとステンレス鋼のゼロ延性温度が低下し、熱間圧延を困難にして操業阻害を生じるとともに、コストアップになるだけでなく、溶接性の低下も招く。したがって、Si含有量の上限は7%とする。Si含有量の下限は、好ましくは2.7%であり、さらに好ましくは2.8%である。また、Si含有量の上限は、好ましくは6.8%であり、さらに好ましくは6.6%である。
Mnは、オーステナイト相安定化元素であり、脱酸剤としても作用するので、10%以下の量で含有させる。Mn含有量が10%を超えると耐食性の低下、溶接時の高温割れ、さらには加工性の低下を招く。Mn含有量は、5%以下であることが好ましく、2%以下であることがさらに好ましい。Mnの上記の効果を確実に得るためには、Mn含有量は0.5%以上であることが好ましく、1.0%以上であることがさらに好ましい。
P、S:両元素とも、耐食性、溶接性、Sについては特に熱間加工性に有害な元素であり、その含有量は低いほど良く、いずれも0.03%を超えるとその有害性は顕著に現れる。そこで、Pが含有量は0.03%以下、S含有量は0.03%以下とする。
Nは、Nb、Ti、Ta、Zrとの親和力が高く、これらの元素によるCの固定を阻害するので、できるだけ低い方が好ましい。N含有量が0.035%を超えるとその有害性が顕著に現れる。そこで、N含有量は0.035%以下とする。N含有量は、0.020%以下であることが好ましく、0.015%以下であることがさらに好ましい。
Alは、脱酸剤やスラグの還元剤として用いられるが、それ以外に、合金中に含まれているために、合金添加の際に混入する。Alは、溶鋼中の溶存酸素と反応してAl2O3を生成する。それ以外に、溶鋼中のSiO2介在物やスラグ中の酸化物をAlが還元することでもAl2O3が生成する。
Crは、ステンレス鋼の耐食性を確保するための基本元素であり、7~20%とする。Cr含有量が7%未満では十分な耐食性を得られない。一方、Cr含有量が過剰となると、SiとNbの共存により多量のフェライトが析出した二相組織となって、加工性、耐衝撃性の低下を招くので、Cr含有量の上限は20%とする。Cr含有量の下限は、10%であることが好ましく、15%であることがさらに好ましい。
Niは、オーステナイト相の安定化元素であり、ゼロ延性温度を高める効果もあるので、10~22%の量で含有させる。Ni含有量が10%未満では、オーステナイト単相とするには不十分である。Niの過剰添加は、コストアップを招くだけであり、22%以下で十分にオーステナイト単相となる。Ni含有量の上限は18%であることが好ましく、14%であることがさらに好ましい。Ni含有量の下限は11%であることが好ましく、12%であることがさらに好ましい。
Nb、Ti、Ta、Zrは、いずれも、Cを固定して鋭敏化による耐食性の低下を抑制する効果があり、特に溶接熱影響部の鋭敏化抑制にも有効な元素であるため、必要に応じて含有させてもよい任意元素である。鋭敏化抑制のためには、これら1種または複数の合計含有量が0.05%以上であることが有効である。また、これら1種または複数の合計含有量が0.7%を超えると、加工性、耐食性を劣化させる。したがって、Nb、Ti、Ta、Zrから選んだ1種または2種以上を含有させる場合、その合計含有量を0.05%以上0.7%以下とする。この合計含有量の下限は好ましくは0.3%である。
[介在物]
本発明において介在物の量はいずれもJIS G 0555に記載の方法に従って測定した量である。また、介在物の量(%)はいずれも面積%である。測定は、上記規格に規定された方法に従い、60視野測定して、その平均値を介在物量とする。
本発明に係る高Siオーステナイト系ステンレス鋼の場合、化学組成からみて、B1系介在物はほとんどがアルミナ(Al2O3)である。鋼材の表層に露出したAl2O3介在物は、非水溶性であり、硝酸中で耐食性を発揮するシリケート皮膜の生成を阻み、すきま腐食の原因となる。それ以外にも、溶鋼中のAl2O3系介在物は、ノズル閉塞を引き起こし、鋳込み作業の阻害の原因となる。また、鋳片中に残存した介在物は、圧延により疵となり、見た目に悪いだけでなく、加工中や使用中に割れの起点となるために、疵除去の工程が必要となってくる。したがって、これらを改善するためには、B1系介在物の量を0.03%以下とする。この量は好ましくは0.025%以下である。
上述したように、SiO2のようなA2系介在物は、C系介在物と同様に比較的融点が低いため、溶鋼処理中に数ミクロン以上の大きさに成長する。しかし、伸延性を有しているために、熱間圧延または冷間圧延では母材とともに延ばされ、圧延比にもよるが、1ミクロン以下の厚さに延ばされる。また、鋼板中に存在するSiO2のようなA2系介在物は、非常に薄く、不働態皮膜の代替として働く。しかし、A2系介在物のSiO2が0.06%を超えて存在すると、B1系介在物と同様、加工への悪影響が生じる。
本発明に係る高Siオーステナイト系ステンレス鋼を確実に製造することができる方法を次に説明する。ただし、上記の化学組成および介在物により特定される本発明に係るステンレス鋼を製造することができる限り、他の製造方法を採用することも可能である。
また、Alに比べて酸化性の弱い元素から生成された酸化物の介在物が存在する状態でAlを投入した場合、(2)式に示すようにAlがそれを還元してAl2O3が生成する。
高Si鋼の場合、大量にSiを投入することにより溶鋼中にSiO2介在物が大量に生成する。そこへAlを投入した場合、(2)式に示すAlによる還元反応が起こり、(3)式に示す反応が起こる。
そこで、高Si鋼では、大量にSiを投入した後、SiO2を鋼中に残存させ、Al量を規定することにより、上記(3)式の反応によるAl2O3介在物の生成を抑える。しかし、この手法でAl2O3介在物の生成をある程度抑えることは可能であるが、求められる耐食性を得るには不十分である。従って、Al量の制限に加え、Al2O3介在物量も制限する必要があり、そのために介在物浮上分離処理を行うことが必要となる。
表1に示す組成の溶鋼を、電気炉-AOD-VOD-取鍋精錬-連続鋳造により200mm厚のスラブを製造し、その鋳片を所定の大きさに切断し、熱間圧延により6mm厚の板とした。その際の主な製造条件は、表1に示すとおりである。こうして製造された供試鋼1~12について、表面を酸洗によりスケールを除去した後、腐食試験に供した。
供試鋼6は、Si含有量が本発明の下限値を外れている。通常のFeSi合金を使用してもAlのピックアップは少ないが、低Siであるがゆえに腐食速度が著しく大きい。
供試鋼8は、AOD後の除滓が不十分な例である。スラグ中のアルミナが次工程にて一部還元され、溶鋼中のsol.Al含有量がピックアップした結果、本発明の上限値を外れ。それに伴って、B1系介在物も上限値を外れたため、腐食速度が大きい。
Claims (3)
- 質量%で、C:0.04%以下、Si:2.5~7.0%、Mn:10%以下、P:0.03%以下、S:0.03%以下、N:0.035%以下、sol.Al:0.03%以下、Cr:7~20%、Ni:10~22%、Nb、Ti、Ta、Zrの1種または2種以上:合計で0~0.7%、残部:Feおよび不純物の化学組成を有し、かつJIS G 0555(2003)付属書1「点算法による非金属介在物の顕微鏡試験方法」に記載の方法で測定したB1系介在物の合計量が0.03面積%以下であることを特徴とするオーステナイト系ステンレス鋼。
- 前記化学組成が、質量%で、Nb、Ti、Ta、Zrの1種または2種以上を合計で0.05~0.7%含有する請求項1に記載のオーステナイト系ステンレス鋼。
- JIS G 0555(2003)付属書1「点算法による非金属介在物の顕微鏡試験方法」に記載の方法で測定したA2系介在物であるSiO2を0.06%以下有する、請求項1または2に記載のオーステナイト系ステンレス鋼。
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JP2012550245A JP5212581B1 (ja) | 2011-07-29 | 2012-07-26 | 高Siオーステナイト系ステンレス鋼の製造方法 |
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JP2016079481A (ja) * | 2014-10-20 | 2016-05-16 | 新日鐵住金株式会社 | 複合非金属介在物を含有する高Siオーステナイト系ステンレス鋼 |
JPWO2016052639A1 (ja) * | 2014-10-01 | 2017-06-15 | 新日鐵住金株式会社 | ステンレス鋼材 |
WO2017195372A1 (ja) * | 2016-05-13 | 2017-11-16 | 新日鐵住金株式会社 | オーステナイト系ステンレス鋼 |
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