Classifications

• • 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
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
• • 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
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • 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
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
• • 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
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • 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 an austenitic stainless steel, having excellent corrosion resistance, in particular, excellent intergranular corrosion resistance, which can be used as structural members for a nuclear power plant, a chemical plant, or the like.
• a marked intergranular corrosion occurs in the heat affected zone which is produced by welding or by high temperature heating. This intergranular corrosion is called “sensitization”, and is caused by the formation of the Cr depleted zone which is poor in corrosion resistance.
• the above-mentioned Cr depleted zone is formed, in the welding or the heating process, by Cr carbide precipitation at the grain boundary and a decrease of the Cr concentration there. Furthermore, depending on the stress condition of the materials, intergranular stress corrosion crackings may also occur.
• Austenitic stainless steels with a lowered C content and/or with a small additional amount of V, Ti, Nb, and so on are, for example, disclosed in the following Patent Documents.
• the Patent Document 1 Japanese Laid-Open Patent Publication No. 55-89458 discloses an austenitic stainless steel for use in an environment of high temperature and low chlorine concentration, containing one or more elements selected from Ti, Nb, Ta, Zr and V in order to prevent a deterioration of resistance to stress corrosion cracking due to N.
• stress corrosion cracking due to N can be prevented by lowering the solute N within the matrix of the stainless steel by forming nitrides with Ti, Nb, Ta, Zr or V
• the deterioration of resistance to stress corrosion cracking caused by the sensitization due to C only the lowering the C content is considered.
• Patent Document 2 Japanese Laid-Open Patent Publication No. 2003-213379 discloses an austenitic stainless steel having excellent corrosion resistance, containing Ti and/or Nb in order to suppress the Cr nitride precipitation on the grain boundaries.
• Ti and/or Nb can fix not only N, but also fix C within the grains. Similar effects of V on the fix of N and C in the grains are also not considered.
• proper amounts for adding of Ti and Nb, which should depend on the contents of N and C, are not disclosed.
• the Patent Document 3 Japanese Laid-Open Patent Publication No. 5-594944 discloses an austenitic stainless steel excellent in suppressing irradiation assisted segregation, which contains one or more elements selected from Ti, Zr, Hf, V, Nb, and Ta. According to the said Patent Document, a point defect formation by neutron irradiation is suppressed by Ti, Zr, Hf, V, Nb, or Ta, leading to the suppression of element migration, that is, Cr from the grain boundaries, and Ni, Si, P and S to the grain boundaries. However, the role of the above-described elements on fixing C and/or N as carbo-nitrides within the grains is not considered. Furthermore, according to the said Patent Document, a large amount of Ti, Zr, Hf, V, Nb, and Ta is necessary to achieve the above-described effects.
• Patent Document 4 Japanese Laid-Open Patent Publication No. 2005-23343 discloses an austenitic stainless steel with a fine grain structure on the surface layer, containing one or more elements selected from V, Nb, Ti and Zr.
• V, Nb, Ti and Zr are added for grain refinement.
• interaction between other elements such as C or N and above-described alloying elements is not considered.
• Patent Document 5 Japanese Laid-Open Patent Publication No. 57-158359 discloses a corrosion resistant austenitic stainless steel, containing Ti+Nb of 0.05 to 0.10%.
• the said Patent Document mentions that grain boundary precipitation of carbides and/or nitrides is suppressed by the composite addition of Nb and Ta.
• Nb is more than 0.5%, pitting corrosions or macro-streak-flaws may occur.
• Patent Document 1 Japanese Laid-Open Patent Publication No. 55-89458,
• Patent Document 2 Japanese Laid-Open Patent Publication No. 2003-213379,
• Patent Document 3 Japanese Laid-Open Patent Publication No. 5-59494,
• Patent Document 4 Japanese Laid-Open Patent Publication No. 2005-23343,
• Patent Document 5 Japanese Laid-Open Patent Publication No. 57-158359.
• the objective of the present invention is to provide an austenitic stainless steel having excellent corrosion resistance, in particular, excellent intergranular corrosion resistance.
• the basic technical idea of the present invention is to prevent intergranular corrosion due to sensitization in austenitic stainless steels, by suppressing the Cr carbo-nitrides precipitation on the grain boundaries by means of fixing C as Cr carbo-nitrides within the grains.
• the carbo-nitride formation elements of C and N should be fixed within the grains as carbo-nitrides with V, Nb and Ti, whose affinities with C and N are much greater than with Cr. It can be seen from the aspect of the standard formation free energy that it is easier for V, Nb and Ti to form a nitride than to form a carbide.
• SUS 316 type stainless steels used for structural members usually, contain 0.03 to 0.10% N, in order to ensure the strength.
• N in order to ensure the strength.
• V, Nb or Ti is added to the SUS 316 type stainless steels with high N content, nitrides of these elements are preferentially formed. Therefore, according to the conventional idea, in order to fix C as carbides within the grains for suppressing the intergranular corrosion, a large amount of V, Nb or Ti which can fix C as carbides should be added in addition to the amount to fix N as nitrides.
• the increase of intragranular precipitates (carbo-nitrides) owing to the excessive contents of V, Nb and/or Ti may cause a deterioration of pitting corrosion resistance or may encourage production defects such as macro-streak-flaws. From these points of view, the excessive addition of V, Nb or Ti is not desirable.
• the present inventors investigated the precipitation behaviors of the carbo-nitrides for practical steel products, and found that, even in the case when just enough of V, Nb and/or Ti for forming nitrides with N were added, the said elements also combined with C, and therefore, carbo-nitrides were also formed.
• This can be ascribed to the kinetic behavior in addition to the above-described equilibrium thermodynamics. That is to say, V, Nb and Ti combine not only with N by equilibrium thermodynamics but also with C.
• each element symbol in the formulas (1) and (2) represents the content (by mass %) of the element concerned.
• Nb and Ti easily form carbides, because of the strong affinity with C, compared to V, and grow into intragranular precipitates, leading to a deterioration in pitting corrosion resistance. Therefore, the excessive addition of Nb and Ti should be avoided and preferable contents are less than 0.030% for Nb and 0.050% for Ti.
• the present invention has been accomplished on the basis of the above-described findings.
• the gists of the present invention are the following austenitic stainless steels.
• An austenitic stainless steel which comprises by mass percent, C: not more than 0.10%, Si: 0.01 to 1.0%, Mn: 0.01 to 2%, Cr: 16 to 18%, Ni: more than 10% to less than 14%, Mo: more than 2.0% to not more than 3.0%, N: 0.03 to 0.10%, one or more elements selected from V, Nb and Ti satisfying the following formulas (1) and (2), with the balance being Fe and impurities, wherein the content of P is not more than 0.04% and the content of S is not more than 0.003% among the impurities.
• each element symbol in the formulas (1) and (2) represents the content (by mass %) of the element concerned.
• An austenitic stainless steel in the present invention has excellent corrosion resistance, in particular, excellent intergranular corrosion resistance. Therefore, it is very suitable to be used as a structural member where an intergranular corrosion may occur.
• the C is used for deoxidation and ensuring the strength of steels.
• the upper limit of the C content is set to 0.10%.
• a more preferable content of C is not more than 0.05%.
• the C content is preferably not less than 0.01%. More preferably, the content of C is not less than 0.015%.
• Si is used for deoxidation of steels.
• the content of Si is set to not less than 0.01%. Since excessive Si content encourages forming inclusions, it is preferable to make the Si content as low as possible. Therefore, the content of Si is set to 0.01 to 1.0%.
• Mn is effective in deoxidizing the steel and stabilizing the austenitic phase. The said effects are obtained if the content of Mn is not less than 0.01%.
• Mn forms sulfides with S, and the said sulfides exist as non-metallic inclusions in the steel.
• Mn preferentially condenses the surface of the welds and therefore brings out a deterioration of corrosion resistance in the steel products. Therefore, the proper Mn content is set to 0.01 to 2%.
• Cr is an indispensable element in order to ensure the corrosion resistance of steels. A sufficient corrosion resistance is not obtained, when the content of Cr is less than 16%. In the present invention, a Cr content of not more than 18% is sufficient. A Cr content exceeding 18%, leads to the deterioration of workability and also increases the cost of steels for practical use. Moreover, it makes difficult to keep the austenitic phase stable. Therefore, the upper limit of the Cr content is set to 18%. The content of Cr is more preferably not more than 17.5%.
• Ni is an important element for the stabilization of the austenitic phase and maintains the corrosion resistance. From the view point of corrosion resistance, the Ni content of more than 10% is necessary. From a view point of weldability, the upper limit of the Ni content has a relationship with the content of Cr, and is set to less than 14%. The lower limit of the Ni content is more preferably 10.5%, and the upper limit thereof is more preferably 13%.
• Mo has an effect for stabilizing the passive film, and is an indispensable element to ensure the pitting corrosion resistance and/or general corrosion resistance.
• Mo precipitates as intermetallic compounds with Fe, Ni, Cr and the like on the grain boundaries, the intergranular corrosion resistance is deteriorated. Therefore, the content of Mo, which ensures the general corrosion resistance without deteriorating the intergranular corrosion resistance, is set to more than 2.0% to less than 3.0%. The more preferable upper limit content of Mo is 2.5%.
• the content of N is set to not less than 0.03%, in order to ensure the strength of steels.
• N forms nitrides with Cr in the steel, and the said nitrides cause a deterioration of intergranular corrosion resistance.
• the content of N is set to not more than 0.10%.
• the lower limit of the N content is more preferably 0.04%, and the upper limit thereof is more preferably 0.08%.
• V, Ti and Nb one or more elements selected from these, within a range satisfying the above-mentioned formulas (1) and (2)
• the contents of V, Ti and Nb are set within the range satisfying the formulas (1) and (2). Also, by the reason described in paragraph [0023], the preferable contents of Nb and Ti are set to not more than 0.030% and not more than 0.050%, respectively.
• the stainless steel according to the present invention comprises the components described above with the balance being Fe and impurities. It is necessary, however, to suppress the contents of P and S among the impurities in the following manner.
• the content of P is preferable as low as possible. Therefore, the upper limit of the P content is set to 0.04%.
• the content of S is preferable as low as possible. Consequently, the upper limit of the S content is set to 0.003%.
• Stainless steels having chemical compositions shown in Table 1 were melted and cast to form ingots.
• the ingots were hot-forged and hot-rolled into 6 mm thick plates, and then the said hot rolled plates with 6 mm thickness were cold rolled into 4 mm thick plates.
• the cold rolled plates were subjected to solution treatment, namely maintained at 1060° C. for 15 minutes and water cooled. Then they were subjected to a sensitizing treatment, that is to say, they were heated at 650° C. for 2 hours and cooled in air. After the said sensitizing treatment, the corrosion rate was measured according to the Method of Ferric Sulfate-Sulfuric Acid Test (JIS G 0572), which is a typical test method for evaluating the intergranular corrosion resistance.
• JIS G 0572 Method of Ferric Sulfate-Sulfuric Acid Test
• the test and evaluation results of intergranular corrosion resistance are shown in Table 2.
• the scatter of the two specimens was negligibly small in the examples of the present invention.
• the comparative examples since the scatter of the two specimens was very large, an additional 4 specimens were tested and so a total of 6 specimens were evaluated.
• the large scatter of the data in the comparative examples originated from falling-off of surface grains during testing due to the poor intergranular corrosion resistance.
• the intergranular corrosion resistance was evaluated by “ ⁇ ”, when corrosion rates of all the specimens were less than 3 g/m 2 h, and was evaluated by “x”, when at least one result of the plural specimens exceeded 3 g/m 2 h.
• an austenitic stainless steel having excellent intergranular corrosion resistance, and so having both excellent pitting corrosion and general corrosion resistance can be provided.
• This stainless steel can show excellent effects, when they are used as structural members for a nuclear power plant, a chemical plant, or the like.

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• Heat Treatment Of Sheet Steel (AREA)
• Heat Treatment Of Steel (AREA)

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• 2007
  • 2007-04-26 CN CNA200780020250XA patent/CN101460643A/zh active Pending
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