JP2009133001A - Austenitic stainless steel having excellent hydrogen embrittlement resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive austenitic stainless steel applicable to a member used in an environment where a hydrogen gas is used. <P>SOLUTION: The austenitic stainless steel having excellent hydrogen embrittlement resistance has a composition comprising, by mass, 0.03 to 0.10% C, 0.20 to 1.00% Si, 0.50 to 2.00% Mn, ≤0.050% P, ≤0.030% S, 7.0 to 12.0% Ni, 16.0 to 20.0% Cr, ≤1.0% Mo, ≤0.10% N and ≤0.01% P, and also comprising one or two kinds selected from 0.1 to 1.0% Ti and 0.2 to 1.0% Nb, and the balance Fe with inevitable impurities, and in which one or more kinds selected from Ti carbide, Ti carbonitride and Nb carbonitride of ≥1 μm in 1 mm<SP>2</SP>cross-sectional area are present by ≥20 pieces. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an inexpensive austenitic stainless steel that does not easily embrittle even in an environment where hydrogen gas is used, and particularly relates to valves, pipes, joints, compressors, and accumulators used in an environment where hydrogen gas is used. Further, the present invention relates to an inexpensive austenitic stainless steel applied to members such as measuring instruments.

  In recent years, technological development of hydrogen energy centered on fuel cell vehicles has been promoted, and metal materials used in high-pressure hydrogen gas atmosphere such as liner materials and high-pressure hydrogen gas piping for high-pressure hydrogen fuel tanks for fuel cell vehicles Research and development is in progress. The materials used for the parts exposed to these high-pressure hydrogen gases are strongly required to be industrially easy-to-use materials as well as low hydrogen embrittlement sensitivity.

  From the above viewpoint, austenitic stainless steel such as SUS304 as a corrosion resistant material has been proposed, but in SUS304, embrittlement after using hydrogen gas is remarkable, and SUS316L stainless steel is proposed as an applicable material. ing. However, this steel is expensive due to the large amount of Ni or Mo alloy added. Moreover, since it is low C, there exists a problem that intensity | strength also becomes low.

Therefore, as disclosed in, for example, Japanese Patent Application Laid-Open No. 2007-126688 (Patent Document 1), an austenitic stainless steel Mn stainless steel that maintains resistance to hydrogen embrittlement superior to that of SUS316 and is applicable to a low-temperature hydrogen environment is provided. Proposed. In addition, International Publication WO 2004-83477 (Patent Document 2) includes, in mass%, C: 0.04% or less, Si: 1.0% or less, Mn: 7-30%, Cr: 15-22%. Ni: 5 to 20%, V: 0.001 to 1.0%, N: 0.20 to 0.50%, and Al: 0.10% or less, the balance being Fe and impurities, P is 0.030% or less, S is 0.005% or less, Ti, Zr and Hf are each 0.01% or less, and the contents of Cr, Mn and N are 2.5Cr + 3.4Mn ≦ 300N High-strength stainless steel having excellent mechanical properties and corrosion resistance under a high-pressure hydrogen gas environment has been proposed.
JP 2007-126688 A International Publication WO2004-83477

  All of those disclosed in Patent Documents 1 and 2 described above are austenitic stainless steel Mn stainless steels that use Mn to maintain hydrogen embrittlement resistance exceeding that of SUS316, and are adapted to low temperature hydrogen environments. However, both of them are high in Mn and N, are inferior in hot manufacturability, and have a significantly higher strength level than SUS316L, making it difficult to cut and weldability. There is an inferior problem.

In order to solve the above-mentioned problems, the inventors have intensively developed, and as a result, Ti or Nb, which is an element forming carbonitride, is added alone or in combination to an inexpensive material based on SUS304. In addition, the presence of 20 or more carbonitrides with a cross-sectional area of 1 mm ² of 1 μm or more prevents elongation and reduction of drawing in a tensile test under a hydrogen gas environment, and the addition of Ni and Mo amounts that are more expensive than SUS316L It is an object of the present invention to provide an austenitic stainless steel with a small amount and an excellent resistance to hydrogen embrittlement.

The gist of the invention is that
(1) In mass%, C: 0.03-0.10%, Si: 0.20-1.00%, Mn: 0.50-2.00%, P: 0.050% or less, S: 0.030% or less, Ni: 7.0 to 12.0%, Cr: 16.0 to 20.0%, Mo: 1.0% or less, N: 0.10% or less, O: 0.01% And containing one or two of Ti: 0.1 to 1.0% and Nb: 0.2 to 1.0%, the balance being Fe and inevitable impurities, and ( Ti + Nb) / (C + N): 3 or more, and 20 or more of 1 type or 2 types or more of Ti carbide, Ti carbonitride or Nb carbonitride having a cross-sectional area of 1 mm ² or more. Austenitic stainless steel with excellent hydrogen embrittlement resistance.
(2) Ni or Cr described in (1) above is Ni: 8.0 to 10.0% or Cr: 18.0 to 20.0%, respectively, hydrogen embrittlement resistance Austenitic stainless steel with excellent properties.

  As described above, according to the present invention, the austenitic stainless steel, which is less expensive than the currently recommended SUS316L, which is unlikely to be brittle even in an environment where hydrogen gas is used, particularly used in an environment where hydrogen gas is used. It provides an inexpensive austenitic stainless steel that is applied to members such as valves, pipes, joints, compressors, accumulators, and measuring instruments.

Hereinafter, the reasons for limiting the component composition according to the present invention will be described.
C: 0.03-0.10%
C is an austenite generating element and is an element necessary for improving the strength, and is useful for forming carbonitrides. However, if it is less than 0.03%, the effect is not sufficient, and if it exceeds 0.10% Since the hot workability is lowered, the range is set to 0.03 to 0.10%.

Si: 0.20 to 1.00%
Si is an element useful as a deoxidizing element, and lowers the melting point of the oxide to soften it. However, if it is less than 0.20%, it is insufficient for lowering the melting point of the oxide, and if it exceeds 1.00%, ferrite is generated and hot workability deteriorates, so the range is 0.20 to 1%. 0.000%.

Mn: 0.50 to 2.00%
Mn is an element useful as a deoxidizing element and a sulfide-forming element. However, if it is less than 0.50%, the Cr concentration in the sulfide increases and the machinability deteriorates. Moreover, since corrosion resistance will deteriorate when it exceeds 2.00%, the range was 0.50 to 2.00%.

P: 0.050% or less, S: 0.030% or less P and S are elements that adversely affect the toughness of steel. Accordingly, it is preferable that the amount be as small as possible. However, if the content is 0.050% or less and 0.030% or less, significant deterioration in the properties of the steel of the present invention is not recognized.

Ni: 7.0 to 12.0%
Ni is an element that stabilizes the austenite phase and improves the toughness of the steel. It is also a γ-generating element. However, if it is less than 7.0%, these effects are not sufficient, and if it exceeds 12.0%, the effect is saturated and the cost is high, so the range is 7.0 to 12.0%. did. Preferably it is set to 8.0 to 10.0%.

Cr: 16.0 to 20.0%
Cr is a ferrite-forming element and is an element that imparts corrosion resistance. However, the effect is not sufficient if it is less than 16.0%. If it exceeds 20.0%, the δ ferrite phase increases and the hot workability deteriorates, so the range was made 16.0 to 20.0%. Preferably it is 18.0 to 20.0%.

Mo: 1.0% or less Mo has a great effect on improving the corrosion resistance of stainless steel. However, less than 0.1% is insufficient to ensure corrosion resistance. On the other hand, if it exceeds 1.0%, the hot workability deteriorates and the δ ferrite phase increases, so the range is made 1.0% or less, preferably 0.1-1.0%. More preferably, it is 0.1 to 0.5%.

Ti: 0.1 to 1.0%
Ti forms carbides or carbonitrides, fixes C in steel, disperses a large number of carbides or carbonitrides in steel, and prevents trapped hydrogen from entering, thereby preventing deterioration of ductility. However, if it is less than 0.1%, the effect cannot be obtained, and if it exceeds 1.0%, the hot workability deteriorates, so the range was made 0.1 to 1.0%.

Nb: 0.2-1.0%
Nb forms carbonitride, fixes C in the steel, disperses a large number of carbonitrides in the steel, and prevents trapped hydrogen from entering, thereby preventing deterioration of ductility. However, if it is less than 0.2%, the effect cannot be obtained, and if it exceeds 1.0%, the hot workability deteriorates, so the range was made 0.2 to 1.0%.

N: 0.10% or less, O: 0.01% or less N is an element effective from the viewpoint of strength. When the N content is high, the strength is increased, but the workability such as cutting and welding and the hot workability are significantly affected. Therefore, the upper limit is regulated to 0.10%. Further, O significantly affects hot workability in a material in which δ ferrite coexists. If it exceeds 0.10%, the hot ductility is remarkably lowered, and a sound hot-worked product cannot be obtained. Therefore, the upper limit of the O content is regulated to 0.01%.

(Ti + Nb) / (C + N): 3 or more (Ti + Nb) / (C + N) is an index necessary for producing Ti carbide, Ti or Nb carbonitride. If it is smaller than 3, the number of carbides and nitrides that become trapped hydrogen trap sites becomes insufficient, and it is not possible to prevent a decrease in ductility.

The presence of 20 or more of one or more of Ti carbide, Ti carbonitride, or Nb carbonitride having a cross-sectional area of 1 mm ² or more, resulting in a decrease in elongation and drawing in a tensile test after hydrogen charging. To prevent. Moreover, compared with SUS316L, there exists an effect which can also suppress addition of expensive Ni and Mo amount. However, fine objects that cannot be observed at the microscopic level are ineffective and require a size of 1 μm or more. Further, a large number of dispersions of Ti carbide, Ti carbonitride, or Nb carbonitride are required, and if less than 20, the effect is insufficient. Note that these Ti carbides, Ti carbonitrides, or Nb carbonitrides have the same effect, and therefore, it is only necessary to ensure the total number without distinguishing the types. Therefore, the range was set to 20 or more of one or more of Ti carbide, Ti carbonitride, or Nb carbonitride having a cross-sectional area of 1 mm ² of 1 μm or more.

Hereinafter, the present invention will be specifically described with reference to examples.
Samples made of an alloy having the chemical composition shown in Table 1 were melted in 100 kg units in a vacuum induction melting furnace, the resulting steel ingot was heated to 1250 ° C., forged to a diameter of 20 mm, and 1050 ° C. for 20 minutes. Various samples were prepared by a solution heat treatment by post-water cooling. Using the test piece as a microstructure, carbonitrides in the L surface middle periphery were observed. Moreover, the parallel part diameter 6mm and # 600 finish were used as a tensile test piece, and the low strain rate tensile test was done for the stroke rate of 1.0 mm / min.

As a test method, the number of carbonitrides is measured by taking optical micrographs at arbitrary three locations, averaging the numbers, calculating the area of the used photo to 1 mm ² , and multiplying the number by the ratio. In the tensile test, Ni wire is electrically welded to the end of the tensile test piece, and the hydrogen intrusion is blocked by the resin coating at the other part than the parallel part. This test piece is immersed in 0.01N sulfuric acid + 0.5 g / l ammonium thiocyanate solution, and charged with hydrogen at 68 A / mm ² by cathode charging method (30 ° C.). A tensile test is performed immediately after hydrogen charging (room temperature, atmospheric pressure, stroke speed 1.0 mm / min). Changes in elongation and drawing after the tensile test were evaluated by the ratio of presence or absence of hydrogen charge.

As described above, the hydrogen charging conditions are as follows: test solution: sulfuric acid (0.01 N) + ammonium thiocyanate (0.5 g / l), temperature: 30 ° C., current density: 68 A / m ² , time: 24 hr To do. As the rate of increase / decrease after hydrogen charging, a tensile test using a tensile specimen with a parallel part diameter of 6 mm was performed at a low strain rate (stroke speed: 1.0 mm / min), and (test value after hydrogen charging) / (hydrogen The ratio of the measured value before charging) was used. The tensile test was conducted in the atmosphere, but the test was completed within 30 minutes after the hydrogen charge. Considering the variation in the tensile test results, it was judged that hydrogen embrittlement was not observed for 0.95 or more. Also, the price was evaluated as x for those that were expensive relative to SUS316L (No. 16).

表１に示すように、Ｎｏ．１〜１４は本発明例であり、Ｎｏ．１５〜２３は比較例である。

As shown in Table 1, no. 1 to 14 are examples of the present invention. 15-23 are comparative examples.

  FIG. 1 is a diagram showing the relationship between (Ti + Nb) / (C + N) and the number of Ti carbides or carbonitrides of Ti and Nb. As shown in this figure, when (Ti + Nb) / (C + N) is 3 or more and the total of one or more of Ti carbide, Ti carbonitride or Nb carbonitride is 20 or more The present invention, for example, the present invention No. 8 is an example. FIG. 2 is a graph showing the relationship between the number of Ti carbides or carbonitrides of Ti and Nb and the increase / decrease ratio after hydrogen charging. As shown in this figure, after hydrogen charge for elongation, drawing and tensile strength when the total of one or more of Ti carbide, Ti carbonitride or Nb carbonitride is 20 or more. It can be seen that the increase / decrease ratio of 1.00 shows a value of 1.00.

  As shown in Table 1, Comparative Example No. No. 15 has a low content of Ti and Nb and a low value of (Ti + Nb) / (C + N). Further, since the number of Ti and Nb carbonitrides is small, the increase / decrease ratio after hydrogen charging is small, and hydrogen embrittlement occurs. Comparative Example No. 16 is equivalent to SUS316L, the content of Ti and Nb is low, and the value of (Ti + Nb) / (C + N) is low, but the increase / decrease ratio after hydrogen charge exceeds 0.95, and hydrogen embrittlement does not occur . In addition, the contents of Ni and Mo are high, and the price base is used. Comparative Example No. No. 17 has a low Ti and Nb content and a low value of (Ti + Nb) / (C + N). Similar to 16, the increase / decrease ratio after hydrogen charge exceeds 0.95 and hydrogen embrittlement has not occurred, but the price increases due to the high content of Ni and Cr.

  Comparative Example No. No. 18 has a high Mo content, a low Ti and Nb content, and a low (Ti + Nb) / (C + N) value. Further, since the number of Ti and Nb carbonitrides was small, hydrogen embrittlement after hydrogen charging occurred. Comparative Example No. No. 19 had a low C and Nb content and a small number of carbonitrides of Ti and Nb, resulting in hydrogen embrittlement after hydrogen charging. Comparative Example No. No. 20 has a low Ti or Nb content and a low value of (Ti + Nb) / (C + N). Further, since the number of Ti and Nb carbonitrides was small, hydrogen embrittlement after hydrogen charging occurred.

  Comparative Example No. No. 21 had a low content of C and Ti, and a small number of carbonitrides of Ti and Nb, resulting in hydrogen embrittlement after hydrogen charging. Comparative Example No. No. 22 has a low Ti content and a low value of (Ti + Nb) / (C + N). Further, since the number of Ti and Nb carbonitrides was small, hydrogen embrittlement after hydrogen charging occurred. Comparative Example No. No. 23 has a low content of Ti and Nb and a low value of (Ti + Nb) / (C + N). Further, since the number of Ti and Nb carbonitrides was small, hydrogen embrittlement after hydrogen charging occurred. On the other hand, No. which is an example of the present invention. Since 1-14 satisfy | fills all conditions, it turns out that the hydrogen embrittlement after hydrogen charge was not seen at all.

As described above, by adding Ti or Nb, which is a carbonitride-forming element, to steel corresponding to SUS304 alone or in combination, 20 or more carbonitrides having a cross-sectional area of 1 mm ^{2 and} having a diameter of 1 μm or more are present in 20 hydrogen charges. This is extremely industrially advantageous because it can prevent elongation and reduction of drawing in a later tensile test, can reduce the amount of expensive Ni and Mo, and can provide inexpensive austenitic stainless steel.

It is a figure which shows the relationship between (Ti + Nb) / (C + N) and the number of carbonitrides of Ti carbide | carbonized_material or Ti and Nb. It is a figure which shows the relationship between the number of carbonized nitride of Ti carbide | carbonized_material or Ti, Nb, and the increase / decrease ratio after hydrogen charge.

Claims (2)

% By mass
C: 0.03-0.10%,
Si: 0.20 to 1.00%,
Mn: 0.50 to 2.00%,
P: 0.050% or less,
S: 0.030% or less,
Ni: 7.0 to 12.0%,
Cr: 16.0 to 20.0%,
Mo: 1.0% or less,
N: 0.10% or less,
O: 0.01% or less, and
Ti: 0.1 to 1.0%,
Nb: 0.2-1.0%
1 or 2 of the following, comprising the balance Fe and inevitable impurities, and (Ti + Nb) / (C + N): 3 or more, and 1 μm or more of Ti carbide, Ti carbonitride or Nb at a cross-sectional area of 1 mm ² An austenitic stainless steel having excellent hydrogen embrittlement resistance, wherein 20 or more of one or more of carbonitrides are present. Ni or Cr according to claim 1 is excellent in hydrogen embrittlement resistance, characterized by being Ni: 8.0 to 10.0% or Cr: 18.0 to 20.0%, respectively. Austenitic stainless steel.

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