JPWO2007138815A1 - Austenitic stainless steel - Google Patents

Abstract

By mass%, C: 0.10% or less, Si: 0.01 to 1.0%, Mn: 0.01 to 2%, Cr: 16 to 18%, Ni: more than 10% and less than 14%, Mo: more than 2.0% to 3.0 % Or less, N: 0.03 to 0.10%, and one or more of V, Nb and Ti are contained in an amount satisfying the following formulas (1) and (2), and the balance is composed of Fe and impurities. Austenitic stainless steels having a certain P of 0.04% or less and S of 0.003% or less have excellent corrosion resistance, particularly intergranular corrosion resistance. 0.0013 ≦ (V / 51) + (Nb / 93) + (Ti / 48) ≦ 0.0025 (1) {(C / 12) + (N / 14)}-{(V / 51) + (Nb /93)+(Ti/48)}≦0.0058 (2) However, the element symbol in the formulas (1) and (2) is the content (% by mass) of the element. It is desirable that Nb is 0.030% or less and Ti is 0.050% or less.

Description

  The present invention relates to an austenitic stainless steel having excellent corrosion resistance, particularly intergranular corrosion resistance, used for structural members such as nuclear power plants and chemical plants.

  SUS316 stainless steel containing Mo is superior in corrosion resistance such as pitting corrosion resistance and overall corrosion resistance compared to SUS304 stainless steel, and is excellent in workability and mechanical properties. ing. However, when welding or high-temperature heating is performed, significant intergranular corrosion may occur in the heat-affected zone due to the welding or high-temperature heating. This phenomenon that causes intergranular corrosion is called sensitization, and as Cr carbide precipitates at the grain boundaries, the surrounding Cr concentration decreases, resulting in the formation of a Cr-deficient layer with insufficient corrosion resistance. is there. Furthermore, depending on the stress state of the material, intergranular stress corrosion cracking occurs.

  Conventionally, as a countermeasure for sensitization, the C content is kept low, or C is fixed in the grains as a Ti or Nb compound to suppress the precipitation of Cr carbide at the grain boundary, thereby suppressing the formation of a Cr-deficient layer. Although approaches have been taken, there are still cases where it is insufficient to prevent intergranular corrosion.

  An austenitic stainless steel to which the C content is suppressed low or V, Nb, Ti or the like is added is disclosed in, for example, the following documents.

  In Patent Document 1 (Japanese Patent Laid-Open No. 55-89458), one or more of Ti, Nb, Ta, Zr and V is added in an amount of 0.1 to 0.1 for the purpose of preventing deterioration of stress corrosion cracking resistance based on N. An austenitic stainless steel for high temperature and low chlorine concentration environment containing 1% is disclosed. This is because, in a steel containing a relatively large amount of N, by forming a nitride with Ti, Nb, Ta, Zr and V, the amount of N dissolved in the matrix of the stainless steel is reduced. This is to prevent stress corrosion cracking based on N. However, only the reduction of the C content is considered for the deterioration of the stress corrosion cracking resistance caused by the sensitization by C.

  Patent Document 2 (Japanese Patent Laid-Open No. 2003-213379) discloses an austenitic stainless steel having excellent corrosion resistance in which precipitation of Cr nitrides at grain boundaries is suppressed by containing Ti or / and Nb. . However, this document does not consider that Ti and / or Nb immobilize not only N but also C in the grains, and that V has the same effect. Also, there is no disclosure of appropriate addition amounts of Ti and Nb depending on the contents of C and N.

  Patent Document 3 (Japanese Patent Laid-Open No. 5-59494) discloses an austenitic stainless steel excellent in irradiation-induced segregation containing at least one of Ti, Zr, Hf, V, Nb and Ta. In this steel, Ti, Zr, Hf, V, Nb, and Ta reduce point defects generated by neutron irradiation, and move Cr from grain boundaries and Ni, Si, P, and S to grain boundaries. However, it is not considered that the above elements have a role of fixing C or N as carbonitrides in the grains. Further, a large amount of Ti, Zr, Hf, V, Nb and Ta is required.

  Patent Document 4 (Japanese Patent Laid-Open No. 2005-23343) discloses austenitic stainless steel containing one or more of V, Nb, Ti and Zr and having a fine surface. In this steel, V, Nb, Ti, and Zr are added to make the crystal grains finer, but the interaction with other elements such as C and N is not considered regarding the amount of addition.

  Patent Document 5 (Japanese Patent Laid-Open No. 57-158359) discloses a corrosion-resistant austenitic stainless steel containing 0.05 to 0.10% of Nb + Ta. This document states that the combined addition of Nb and Ta suppresses grain boundary precipitation of carbides and nitrides, but if Nb is contained in an amount of 0.05% or more, there is a concern about the occurrence of pitting corrosion or scratches. Is done.

JP-A-55-89458 JP2003-213379 JP-A-5-59494 JP 2005-23343 JP-A-57-158359

  An object of the present invention is to provide an austenitic stainless steel having excellent corrosion resistance, particularly intergranular corrosion resistance.

  The basic idea of the present invention is to prevent precipitation of Cr carbonitrides at grain boundaries by precipitating C as intragranular carbonitrides, thereby preventing intergranular corrosion due to sensitization of austenitic stainless steel. In the point.

  In order to suppress the precipitation of Cr-based carbonitrides at grain boundaries in SUS316 stainless steel, C, N, which is the source of carbonitride formation, V, Nb, Ti, which have higher affinity for C and N than Cr It is preferable to fix it as a carbonitride in the grains. From the standard free energy of formation, V, Nb, and Ti form nitrides more easily than carbides.

  When using SUS316 stainless steel as a structural member, N is contained in an amount of about 0.03 to 0.10% to ensure strength. However, when V, Nb, or Ti is added to SUS316 stainless steel to which N is added, nitriding is performed. Form things preferentially. Therefore, in order to suppress intergranular corrosion by immobilizing C as carbide in the grains, in addition to the amount of immobilizing N as nitride, a large amount of V, Nb that can further immobilize C as carbide. It was thought that addition of Ti was necessary.

  On the other hand, when the contents of V, Nb, and Ti increase and the amount of intragranular precipitates (carbonitrides) increases, production defects such as a decrease in pitting resistance and a scratch on the ground may be promoted. Considering this, it is not preferable to add a large amount of V, Nb and Ti.

  Then, when the appropriate content with which the sensitization suppression effect is acquired paying attention to carbonitride formation elements V, Nb, and Ti, the following new knowledge was obtained.

  (a) As a result of confirming the precipitation state of carbonitride when steel material was actually produced, even when V and Nb and Ti were added in amounts to form N and nitride, V, Nb and Ti were both C It was confirmed that they bonded to form a carbonitride. This is because not only the above-mentioned equilibrium theory but also kinetics are added, so not only the binding with N by the equilibrium theory is preferentially performed, but V, Nb, and Ti are also bonded with C as well as N. It is considered a thing.

  (b) Next, as a result of investigating the intergranular corrosion resistance using SUS316 stainless steel having various V, Nb, and Ti contents, an appropriate amount of V, Nb satisfying the following formula (1): In addition to containing Ti, the relationship between the C and N content and the V, Nb, and Ti content to satisfy the following formula (2) is also within a specific range, thereby preventing intergranular corrosion resistance. It was confirmed that excellent stainless steel can be obtained.

0.0013 ≦ (V / 51) + (Nb / 93) + (Ti / 48) ≦ 0.0025 (1)
{(C / 12) + (N / 14)}-{(V / 51) + (Nb / 93) + (Ti / 48)} ≦ 0.0058 (2)
However, the element symbol in the formulas (1) and (2) is the content (% by mass) of the element.

  If the value of (V / 51) + (Nb / 93) + (Ti / 48) in the above formula (1) is smaller than 0.0013, the intergranular corrosion resistance effect cannot be obtained. On the other hand, if the contents of V, Nb, and Ti are excessively increased and the amount of intragranular precipitates is increased, manufacturing defects such as a decrease in pitting resistance and scratches are promoted. Therefore, the upper limit of the value of (V / 51) + (Nb / 93) + (Ti / 48) is set to 0.0025.

  Furthermore, if the value on the left side of equation (2) exceeds 0.0058 due to the relationship between the content of C and N and the content of V, Nb, and Ti, the amount of carbonitride that precipitates at the grain boundary increases and the grain boundary Corrosiveness deteriorates.

  In addition, Nb and Ti have a stronger affinity with C than V, so carbide formation is easy, but intragranular precipitates grow and pitting corrosion resistance deteriorates, so excessive addition of Nb and Ti More preferably, Nb is 0.030% or less and Ti is 0.050% or less.

  The present invention has been made on the basis of the above findings, and the gist thereof is the following austenitic stainless steel.

  (1) By mass%, C: 0.10% or less, Si: 0.01 to 1.0%, Mn: 0.01 to 2%, Cr: 16 to 18%, Ni: more than 10% and less than 14%, Mo: 2.0% Exceeding 3.0% or less, N: 0.03-0.10%, and one or more of V, Nb, and Ti are contained in an amount that satisfies the following formulas (1) and (2), with the balance being Fe and impurities An austenitic stainless steel in which P as an impurity is 0.04% or less and S is 0.003% or less.

0.0013 ≦ (V / 51) + (Nb / 93) + (Ti / 48) ≦ 0.0025 (1)
{(C / 12) + (N / 14)}-{(V / 51) + (Nb / 93) + (Ti / 48)} ≦ 0.0058 (2)
However, the element symbol in the formulas (1) and (2) is the content (% by mass) of the element.

  (2) The austenitic stainless steel according to (1) above, characterized by mass%, Nb being 0.030% or less or Ti being 0.050% or less, or Nb being 0.030% or less and Ti being 0.050% or less.

  The austenitic stainless steel of the present invention is particularly excellent in intergranular corrosion resistance. Therefore, it is very suitable as a member used in an environment where intergranular corrosion is a concern.

  The reason why the chemical composition of the austenitic stainless steel of the present invention is specified will be described below. In addition, "%" regarding content of each component means "mass%".

C: 0.10% or less C is used for the purpose of deoxidizing steel and securing strength. However, in order to prevent the precipitation of carbides from the viewpoint of corrosion resistance, the content should be as low as possible. Therefore, 0.10% was made the upper limit. More preferred is 0.05% or less. However, when the strength as a structural member is ensured with C, the content is preferably 0.01% or more, and more preferably 0.015% or more.

Si: 0.01-1.0%
Si is used for the purpose of deoxidizing steel. In the steel of the present invention, the content is set to 0.01% or more. However, since excessive inclusion of Si promotes the formation of inclusions, the content is desirably as low as possible, and is set to 0.01 to 1.0%.

Mn: 0.01-2%
Mn is an element effective for deoxidizing steel and stabilizing the austenite phase, and its effect can be obtained with a content of 0.01% or more. On the other hand, Mn forms a sulfide with S, and the sulfide becomes a non-metallic inclusion. Moreover, when steel materials are welded, it concentrates preferentially on the surface of a welding part, and reduces the corrosion resistance of steel materials. Therefore, the proper content of Mn is 0.01-2%.

Cr: 16-18%
Cr is an indispensable element for maintaining the corrosion resistance of steel. If it is less than 16%, sufficient corrosion resistance cannot be obtained. In the assumed use environment of the steel of the present invention, a content of up to 18% is sufficient, and beyond this, there is a problem in terms of workability degradation, cost as a practical steel and austenite phase stability. Therefore, the upper limit of the content is 18%. More preferred is 17.5% or less.

Ni: Over 10% and less than 14%
Ni is an important element for stabilizing the austenite phase and maintaining the corrosion resistance. From the viewpoint of corrosion resistance, a content exceeding 10% is necessary. The upper limit of the Ni content has a correlation with the Cr content from the viewpoint of weldability, and is set to less than 14%. A more preferred lower limit is 10.5%, and a more preferred upper limit is 13%.

Mo: 2.0% to 3.0% or less
Mo is effective in stabilizing the passive film, and is an indispensable element for maintaining pitting corrosion resistance and overall corrosion resistance. However, if it precipitates at the grain boundary as an intermetallic compound together with Fe, Ni, Cr, etc., the intergranular corrosion resistance is lowered. Therefore, the range of maintaining the overall corrosion resistance without adversely affecting the intergranular corrosion resistance is more than 2.0% and not more than 3.0%. A more preferred upper limit is 2.5%.

N: 0.03-0.10%
The N content is 0.03% or more to ensure the strength of the steel. However, since N combines with Cr in the steel to form a nitride and lowers the intergranular corrosion resistance, its content is made 0.10% or less. A more preferred lower limit is 0.04%, and a more preferred upper limit is 0.08%.

V, Ti, and Nb: a range that satisfies the above formulas (1) and (2) when at least one kind is satisfied. A content of V, Ti, and Nb is a content that satisfies the above formulas (1) and (2). The reason is as described above. The reason why Nb and Ti are preferably 0.030% or less and 0.050% or less is also as described above.

  In addition to the above components, the balance of the stainless steel of the present invention consists of Fe and impurities. However, impurities P and S must be regulated as follows.

P: 0.04% or less Since the corrosion resistance decreases when the P content increases, the content is preferably as low as possible. Therefore, the upper limit was made 0.04%.

S: 0.003% or less S is an element that forms sulfides that are non-metallic inclusions and that inhibits hot workability. Therefore, the upper limit was made 0.003%.

  Stainless steel having the chemical composition shown in Table 1 was melted, and a 6 mm thick plate was produced by hot forging and hot rolling. This hot-rolled material was cold-rolled to a thickness of 4 mm, held at 1060 ° C. for 15 minutes, and then subjected to a solution treatment that was water-cooled. After that, sensitization treatment was performed by heating at 650 ° C for 2 hours and then air cooling, and the corrosion rate was evaluated by sulfuric acid / ferric sulfate corrosion test (JIS G 0572) which is a typical evaluation method of intergranular corrosion resistance. It was measured. Table 1 also shows the value of “(V / 51) + (Nb / 93) + (Ti / 48)” in equation (1) and the value on the left side of equation (2).

Table 2 shows the intergranular corrosion resistance test results and evaluation results. In the intergranular corrosion resistance test, the variation of the example of the present invention was small at the number of repetitions 2, but the variation of the comparative example was large at the number of repetitions of 2, so four more tests were added and the evaluation was performed at the number of repetitions of 6. did. The reason why the variation is large in the comparative example is that the intergranular corrosion resistance is inferior and degranulation occurs. The evaluation of intergranular corrosion resistance, the case where the corrosion rate is less than in all the plurality of test 3g / m 2 · ^h "○", one with even 3g / m 2 · ^h or more among the plurality of test The case where there existed was made into "x".

  As is apparent from Table 2, the inventive examples Nos. 1 to 5 all exhibit a low corrosion rate and are excellent in intergranular corrosion resistance. On the other hand, No. 6 and No. 7 in the comparative examples are steels whose chemical compositions deviate from the formula (1) or (2) defined in the present invention, and therefore the intergranular corrosion resistance is poor. It was.

According to the present invention, an austenitic stainless steel having excellent intergranular corrosion resistance and excellent pitting corrosion resistance and overall corrosion resistance can be obtained. This stainless steel exhibits an excellent effect as a structural member of a nuclear power plant or a chemical plant.

Claims (2)

In mass%, C: 0.10% or less, Si: 0.01 to 1.0%, Mn: 0.01 to 2%, Cr: 16 to 18%, Ni: more than 10% and less than 14%, Mo: more than 2.0% to 3.0 % Or less, N: 0.03 to 0.10%, and one or more of V, Nb and Ti are contained in an amount satisfying the following formulas (1) and (2), and the balance is composed of Fe and impurities. An austenitic stainless steel in which some P is 0.04% or less and S is 0.003% or less.
0.0013 ≦ (V / 51) + (Nb / 93) + (Ti / 48) ≦ 0.0025 (1)
{(C / 12) + (N / 14)}-{(V / 51) + (Nb / 93) + (Ti / 48)} ≦ 0.0058 (2)
However, the element symbol in the formulas (1) and (2) is the content (% by mass) of the element. 2. The austenitic stainless steel according to claim 1, wherein, in mass%, Nb is 0.030% or less or Ti is 0.050% or less, or Nb is 0.030% or less and Ti is 0.050% or less.