JP2017160520A - Austenitic stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic stainless steel excellent in stress corrosion crack resistance and good in cold forgeability.SOLUTION: An austenitic stainless steel is constituted by 0.06 mass% or less of C, 1.0 mass% or less of Si, 0.5 mass% to 2.5 mass% of Mn, 11.0 mass% to 13.0 mass% of Ni, 15.0 mass% to 17.0 mass% of Cr, 3.0 mass% to 4.0 mass% of Cu, 0.15 mass% or less of Mo and 0.05 mass% or less of N and the balance Fe with inevitable impurities. A value of Mdrepresented by Md=551-462(C+N)-9.2Si-8.1Mn-29(Ni+Cu)-13.7Cr-18.5Mo is -150 or less. Further a value of SFE represented by SFE=2.2 Ni+6Cu-1.1Cr-13Si-1.2Mn+32 is 50 or more.SELECTED DRAWING: None

Description

  The present invention relates to an austenitic stainless steel excellent in stress corrosion cracking resistance.

  Conventionally, austenitic stainless steels such as SUS304 and SUS316 have been used for cold forged pipe joints used in water heaters and the like.

  Since pipe joints manufactured by such cold forging have high residual stress, stress corrosion cracking is likely to occur when exposed to a highly corrosive chloride environment. For this reason, the cold forged pipe joint requires regular maintenance and parts replacement.

  Therefore, as a cold forged pipe joint material, as shown in Patent Documents 1 to 4, austenitic stainless steel having excellent stress corrosion cracking resistance is known. Among them, Patent Documents 2 to 4 attempt to improve stress corrosion cracking resistance by containing a large amount of Si and Cu.

Japanese Patent No. 4757561 Japanese Patent No. 4823534 Japanese Patent No. 5500600 JP-A-8-269641

  However, as described above, austenitic stainless steel containing a large amount of Si has high plastic deformation resistance, and formability by cold forging is insufficient.

  Therefore, an austenitic stainless steel having excellent stress corrosion cracking resistance and good cold forgeability has been demanded.

  This invention is made | formed in view of such a point, and it aims at providing the austenitic stainless steel which is excellent in stress corrosion cracking resistance, and has favorable cold forgeability.

The austenitic stainless steel described in claim 1 has C: 0.06 mass% or less, Si: 1.0 mass% or less, Mn: 0.5 mass% or more and 2.5 mass% or less, Ni: 11. 0 mass% or more and 13.0 mass% or less, Cr: 15.0 mass% or more and 17.0 mass% or less, Cu: 3.0 mass% or more and 4.0 mass% or less, Mo: 0.15 mass% or less, and N: 0.05% by mass or less, with the balance being composed of Fe and inevitable impurities, Md ₃₀ = 551-462 (C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr- the value of _{Md 30} is in -150 or less indicated by 18.5Mo, the value of SFE = 2.2Ni + 6Cu-1.1Cr- 13Si-1.2Mn + 32 is a stacking fault difficulty index indicated by SFE is intended 50 or more .

  The austenitic stainless steel according to claim 2 is the austenitic stainless steel according to claim 1, wherein Al: 0.002 mass% to 0.1 mass%, Mg: 0.0003 mass% to 0.01 % By mass or less, Ca: 0.0003% by mass to 0.01% by mass, B: 0.0005% by mass to 0.01% by mass, Nb: 0.01% by mass to 1.0% by mass, Ti : 0.01% by mass or more and 0.5% by mass or less, V: 0.01% by mass or more and 1.0% by mass or less, and Zr: 0.01% by mass or more and 1.0% by mass or less. It contains.

According to the present invention, in the alloy composition defined in the predetermined range, the value of Md ₃₀ is −150 or less, and the value of SFE is 50 or more. Therefore, the stress corrosion cracking resistance can be improved, and cold Forgeability can be improved.

It is a block diagram for demonstrating the test piece of a stress corrosion cracking test. It is a photograph of the test piece surface after the stress corrosion cracking test in a present Example. It is a photograph of the test piece surface after the stress corrosion cracking test in a comparative example.

  Hereinafter, the configuration of an embodiment of the present invention will be described in detail.

  The austenitic stainless steel according to one embodiment of the present invention is used as a material such as a cold forging joint such as a pipe joint of a hot water supply apparatus.

  This austenitic stainless steel has 0.06 mass% or less of C (carbon), 1.0 mass% or less of Si (silicon), 0.5 mass% or more and 2.5 mass% or less of Mn (manganese), 11 0.0 mass% to 13.0 mass% Ni (nickel), 15.0 mass% to 17.0 mass% Cr (chromium), 3.0 mass% to 4.0 mass% Cu ( Copper), 0.15 mass% or less of Mo (molybdenum), and 0.05 mass% or less of N (nitrogen), with the balance being composed of Fe and inevitable impurities.

  Also, 0.002 mass% to 0.1 mass% Al (aluminum), 0.0003 mass% to 0.01 mass% Mg (magnesium), 0.0003 mass% to 0.01 mass% Ca (calcium), 0.0005 mass% to 0.01 mass% B (boron), 0.01 mass% to 1.0 mass% Nb (niobium), 0.01 mass% to 0.00 mass%. At least one of 5% by mass or less of Ti (titanium), 0.01% by mass or more and 1.0% by mass or less of V (vanadium) and 0.01% by mass or more and 1.0% by mass or less of Zr (zirconium). You may contain seeds as needed.

  C and N are elements inevitably contained in the steel. When the contents of C and N are reduced, the steel becomes soft and the workability is improved, and the amount of carbides and nitrides is reduced, and the corrosion resistance of the base material is improved. However, since C and N are strong austenite generating elements, the C content is set to 0.06% by mass or less and the N content is set to 0.05% by mass or less so as not to inhibit the corrosion resistance of the base material. And

  Si is effective against stress corrosion cracking in a chloride environment, and when it is contained in an amount of 0.2% by mass or more, its effect is easily exhibited. On the other hand, if the Si content exceeds 1.0% by mass, the steel may harden and the workability may be impaired. Therefore, the Si content is 1.0% by mass or less (excluding no addition), and preferably 0.2% by mass or more and 1.0% by mass or less.

  Mn is a useful austenite-forming element that is cheaper than Ni and can substitute for the function of Ni, and it is necessary to contain Mn in an amount of 0.5% by mass or more in order to exert its action. On the other hand, when the content of Mn is excessively larger than 2.5% by mass, work hardening accompanying plastic deformation is remarkably increased, so that cold forgeability is lowered. In addition, environmental conservation problems are likely to occur in the steelmaking process. Therefore, the content of Mn is set to 0.5% by mass or more and 2.5% by mass or less.

  Ni is an austenite-forming element and has an effect of suppressing the progress of corrosion with respect to corrosion resistance. Therefore, from the viewpoint of maintaining the austenite structure, improving workability, and improving stress corrosion cracking resistance by controlling the SFE (stacking defect difficulty index), the Ni content is 11.0% by mass or more and 13. 0 mass% or less.

  Cr is an indispensable element for imparting corrosion resistance in stainless steel, and it is necessary to contain 15.0% by mass or more from the viewpoint of corrosion resistance. On the other hand, if the content of Cr is more than 17.0% by mass, the manufacturability and workability may be impaired. Therefore, the Cr content is 15.0% by mass or more and 17.0% by mass or less.

  Cu is an element that suppresses work hardening caused by the formation of work-induced martensite and contributes to softening of austenitic stainless steel. Cu is a very effective element for increasing SFE, and greatly contributes to improvement of stress corrosion cracking resistance by suppressing generation of stacking faults. In order to obtain these effects sufficiently, it is necessary to contain 3.0% by mass or more of Cu. On the other hand, if the Cu content is more than 4.0% by mass, hot workability in production may be hindered. Therefore, the Cu content is set to 3.0% by mass or more and 4.0% by mass or less.

  Mo is an element that improves the corrosion resistance, but if it is excessively contained in an amount of more than 0.15% by mass, cold forgeability may be reduced. Moreover, it is necessary to consider the manufacturing cost and manufacturability associated with the addition. Therefore, the Mo content is 0.15% by mass or less.

  Al, Mg and Ca are effective for deoxidizing steel, and may be added as necessary. Moreover, in order to exhibit the effect | action, it is necessary to contain 0.002 mass% or more of Al, 0.0003 mass% or more of Mg, or 0.0003 mass% or more of Ca. On the other hand, when it contains more than 0.1% by mass of Al, more than 0.01% by mass of Mg, or more than 0.01% by mass of Ca, the deoxidation action is saturated, and conversely coarse oxides (inclusions) May occur and cold forgeability and stress corrosion cracking resistance may deteriorate. Therefore, when any of Al, Mg, and Ca is contained, the Al content is 0.002 mass% to 0.1 mass%, and the Mg content is 0.0003 mass% to 0.01 mass%. The content of Ca is 0.0003 mass% or more and 0.01 mass% or less. Further, more preferable Al content is 0.005 mass% or more and 0.06 mass% or less, and more preferable Mg content is 0.001 mass% or more and 0.006 mass% or less, and more preferable Ca content. Is 0.001 mass% or more and 0.005 mass% or less.

  B is an element effective for improving the hot forgeability, and may be added as necessary. Moreover, in order to exhibit the effect | action stably, it is necessary to contain B 0.0005 mass% or more. On the other hand, when the content of B is more than 0.01% by mass, boride is generated, and cold forgeability and stress corrosion cracking resistance may be deteriorated. Therefore, when B is contained, the content of B is set to 0.0005 mass% or more and 0.01 mass% or less. Further, the more preferable content of B is 0.002 mass% or more and 0.006 mass% or less.

  Nb, Ti, V, and Zr are effective in suppressing the formation of Cr carbonitride and improving the corrosion resistance, and may be added as necessary. Moreover, in order to exhibit the effect, it is necessary to contain Nb 0.01% by mass or more, Ti 0.01% by mass or more, V 0.01% by mass or more, or Zr 0.01% by mass or more. is there. On the other hand, when the content of Nb is more than 1.0% by mass, Ti is more than 0.5% by mass, V is more than 1.0% by mass, or Zr is more than 1.0% by mass, the above-described action is saturated. On the contrary, coarse precipitates may be generated, which may deteriorate the corrosion resistance and stress corrosion cracking resistance. Therefore, when Nb, Ti, V and Zr are contained, the Nb content is 0.01% by mass or more and 1.0% by mass or less, and the Ti content is 0.01% by mass or more and 0% by mass. The V content is 0.01% by mass or more and 1.0% by mass or less, and the Zr content is 0.01% by mass or more and 1.0% by mass or less. A more preferable Nb content is 0.05% by mass or more and 0.6% by mass or less, and a more preferable Ti content is 0.05% by mass or more and 0.3% by mass or less, and a more preferable V content. Is 0.1 mass% or more and 0.6 mass% or less, and more preferable Zr content is 0.05 mass% or more and 0.6 mass% or less.

  Here, in austenitic stainless steel, a sliding surface is formed on the steel surface during plastic deformation by tension, and a new surface on which a passive film is not formed is exposed. Further, when the new surface is exposed to a chloride corrosive environment, corrosion is likely to occur, and the corroded portion becomes a base point for stress corrosion cracking.

  The higher the value of SFE (stacking defect difficulty index) expressed by the formula SFE = 2.2Ni + 6Cu-1.1Cr-13Si-1.2Mn + 32, the narrower the width of the sliding step during plastic deformation, The new surface exposed to the corrosive environment is reduced, and the occurrence of stress corrosion cracking can be suppressed. Further, from the viewpoint of cold forgeability, the lower the SFE value, the more work hardening becomes and the workability decreases. Therefore, the alloy composition is adjusted so that the SFE value represented by the above formula is 50 or more from the viewpoint of stress corrosion cracking resistance due to the formation of the new surface of the sliding step and the workability due to work hardening.

In addition, since austenitic stainless steel having an unstable austenite phase undergoes martensitic transformation when subjected to processing, there is a concern of embrittlement due to hydrogen generated in a corrosive environment. When _{the 30} value _Md given by the equation of Md 30 = 551-462 (C + N ) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo exceeds -150, the cold forging When performed, a very large strain is locally generated, so that a martensite phase may be generated and the austenite phase may become unstable. Therefore, the alloy composition is adjusted so that the value of Md ₃₀ is −150 or less.

  Next, effects and the like of the one embodiment will be described.

According to the austenitic stainless steels, at a defined alloy composition in a predetermined _{range, Md 30 = 551-462 (C +} N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo Since the austenite phase can be stabilized by adjusting the content of each element so that the value of Md ₃₀ represented by the formula is −150 or less, martensitic transformation can be prevented. Further, in the alloy composition defined in a predetermined range, the content of each element is adjusted so that the SFE value represented by the formula of SFE = 2.2Ni + 6Cu-1.1Cr-13Si-1.2Mn + 32 is 50 or more. As a result, the width of the sliding step at the time of plastic deformation can be made fine so that a new surface exposed to the corrosive environment can be reduced, and a decrease in workability due to work hardening can be suppressed. Therefore, the stress corrosion cracking resistance can be improved and the cold forgeability can be improved.

  The austenitic stainless steel is less prone to stress corrosion cracking even in a chloride environment. For example, stress corrosion cracking is less likely to occur even when used as a pipe joint material in a hot water supply device, etc., so the frequency of maintenance can be reduced. .

  Moreover, since the moldability at the time of processing into a pipe joint etc. by cold forging is favorable, manufacturing cost can be reduced.

  Stainless steel having the chemical components shown in Table 1 was melted, and a hot-rolled sheet having a thickness of 4 mm was prepared by hot rolling.

  Then, it cold-rolled to plate thickness 1mm, finish-annealed at 1000-1150 degreeC, and pickled, and it was set as the test material.

  About each test material, the stress corrosion cracking test was implemented based on JISG0576.

  That is, in the stress corrosion cracking test, the test solution was a boiling 42% magnesium chloride solution, and the test piece was a U-shaped piece.

  In the stress corrosion cracking test, the test piece was immersed in a test solution, and the test piece was taken out after a lapse of a certain time (24 hours of immersion time). As shown in FIG. The crack density (pieces / mm) was measured based on the number of cracks in the cross direction with respect to the longitudinal direction (L direction) of the test piece. The crack density was measured based on the number of cracks intersecting with an arbitrary line drawn in the longitudinal direction of the test piece in the observation field of the test piece.

  And considering the maintenance frequency of the pipe joint, if the crack density after immersion for 24 hours is less than 10 pieces / mm, the stress corrosion cracking resistance is evaluated as good (◯), and the crack density is 10 pieces / mm. If it was above, it was evaluated that the stress corrosion cracking resistance was insufficient (x).

Table 1 shows the evaluation results of the chemical composition, SFE value, Md ₃₀ value, and stress corrosion cracking resistance (crack density) of each steel type. FIG. 2 shows cracks observed on the shoulder surface of the U-shaped test piece after the stress corrosion cracking test in steel type A of the present example, and FIG. 3 shows the stress in steel type AA of the comparative example. The crack observed on the shoulder surface of the U-shaped test piece after the corrosion cracking test is shown.

As shown in Table 1, within the specified alloy composition range, the steel types A to T according to the present example having an Md ₃₀ value of −150 or less and an SFE value of 50 or more have a crack density of 10 / It was less than mm, and the stress corrosion cracking resistance was good.

Further, steel type AB as a comparative example having Md ₃₀ of −150 or less but SFE of less than 50 had a crack density of less than 10 pieces / mm and good stress corrosion cracking resistance. In addition, although SFE is less than 50, it is considered that the stress corrosion cracking resistance is good because of the effect of adding a large amount of Si.

On the other hand, steel grade AA as a comparative example having an Md ₃₀ value higher than −150 and an SFE value lower than 50 within the specified alloy composition range has a crack density of 10 pieces / mm or more, and stress corrosion resistance. The crackability was insufficient.

  Further, when Al, Mg, Ca, B, Nb, Ti, V, and Zr are added, the steel types AJ, AK, AL, AM, AN, as comparative examples in which the content of these elements is outside the specified range AO, AP, and AQ had a crack density of 10 pieces / mm or more, and the stress corrosion cracking resistance was insufficient.

  Next, a cold forgeability test was performed on the steel types (steel types A to T which are the present examples and steel types AB and AR which are comparative examples) in which the result of the stress corrosion cracking test was good.

  In the cold forgeability test, first, an ingot of the above steel type was hot rolled into a hot rolled sheet having a thickness of 10 mm, and the hot rolled sheet was held at 1000 to 1150 ° C. for 5 minutes, and then cooled with water.

  The hot-rolled sheet after water cooling was finished into a cylindrical test piece having a diameter of 6 mm and a height of 10 mm so that the rolling direction coincided with the longitudinal direction of the test piece, and the cylinder was installed.

  And until the longitudinal direction of the test piece becomes 5 mm (compression ratio 50%), the presence or absence of appearance cracks when the test piece is deformed at a stroke speed of 10 mm / second at room temperature is visually confirmed, and cold forgeability is achieved. Evaluated. The evaluation results of cold forgeability are shown in Table 1 above.

In the range of the prescribed alloy composition, the steel types A to T as examples of this example having an Md ₃₀ value of −150 or less and an SFE value of 50 or more are not visually observed for cracking, and are cold forgeable. Was good.

  On the other hand, steel types AB and AR as comparative examples having good stress corrosion cracking resistance had appearance cracks that could be visually confirmed, and cold forgeability was insufficient.

  In steel type AB, although the value of SFE is less than 50, the stress corrosion cracking resistance can be improved by adding a large amount of Si. On the other hand, SFE is less than 50 and the workability is increased by adding Si excessively. Is thought to have declined.

  In steel type AR, since the value of SFE was 50 or more, the stress corrosion cracking resistance could be improved, but it was considered that the workability was lowered due to the excessive addition of Si.

  INDUSTRIAL APPLICABILITY The present invention can be used as a material for a cold forged pipe joint such as a hot water supply device.

Claims (2)

C: 0.06 mass% or less, Si: 1.0 mass% or less, Mn: 0.5 mass% or more and 2.5 mass% or less, Ni: 11.0 mass% or more and 13.0 mass% or less, Cr: 15.0% by mass or more and 17.0% by mass or less, Cu: 3.0% by mass or more and 4.0% by mass or less, Mo: 0.15% by mass or less and N: 0.05% by mass or less, and the balance Is composed of Fe and inevitable impurities,
Md ₃₀ = 551-462 (C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo has a value of Md ₃₀ of −150 or less,
An austenitic stainless steel characterized in that the value of SFE, which is a stacking fault difficulty index indicated by SFE = 2.2Ni + 6Cu-1.1Cr-13Si-1.2Mn + 32, is 50 or more. Al: 0.002 mass% to 0.1 mass%, Mg: 0.0003 mass% to 0.01 mass%, Ca: 0.0003 mass% to 0.01 mass%, B: 0.0005 1 mass% or more and 0.01 mass% or less, Nb: 0.01 mass% or more and 1.0 mass% or less, Ti: 0.01 mass% or more and 0.5 mass% or less, V: 0.01 mass% or more The austenitic stainless steel according to claim 1, comprising at least one of 0% by mass or less and Zr: 0.01% by mass or more and 1.0% by mass or less.