JPH034619B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a ferritic stainless steel that has excellent stress corrosion cracking resistance, particularly in a high-temperature, high-pressure pure water environment. There are various types of equipment that use high-temperature pure water as an environment in fields such as the nuclear power industry and the chemical industry, but austenitic stainless steel is most commonly used in such equipment. However, austenitic stainless steel has Cl ^-
In environments containing ions, there is a high risk of intragranular stress corrosion cracking (IGSCC) occurring, and for this reason, especially when trace amounts of Cl ^- ions are included in high-temperature water, it is necessary to take measures to increase the alloying properties, such as increasing the amount of Ni. Needed. Furthermore, depending on the type of equipment, the material may be required to have high thermal conductivity due to thermal efficiency, etc., and the austenitic stainless steel mentioned above is a material with low thermal conductivity. For this reason, there have recently been a number of cases in which ferritic stainless steel is used as an alternative for high-temperature water environments containing trace amounts of Cl ^- ions. This is because ferritic stainless steels are generally not susceptible to chloride stress corrosion cracking and have approximately 50% better thermal conductivity than austenitic stainless steels. However, this ferritic stainless steel actually undergoes heat treatment such as welding during assembly and low-temperature heat treatment (SR treatment) after welding.
There is a strong possibility that stress corrosion cracking (SCC) from the grain boundaries will occur. Also Cr16
% or more, so-called high Cr ferritic stainless steel has a temperature of 475°C, which is unique to ferritic stainless steel.
This is a major disadvantage as it causes brittleness and therefore limits the temperature of use. The present invention does not exhibit 475℃ brittleness, and is not susceptible to SCC even after being subjected to sensitization treatment during welding, low-temperature heat treatment after welding, and long-term continuous use in high-temperature water (approximately 200 to 300℃). The purpose is to provide a ferritic stainless steel that does not produce ferrite. It is generally well known that SCC resistance improves when the amount of C+N is 0.01% or less in ferritic stainless steel.
Reducing the amount to low levels below 0.01% is disadvantageous, resulting in a significant increase in steelmaking costs. For this reason, when good SCC resistance is required, instead of drastically reducing the amount of C+N,
Measures are taken to add stabilizing elements such as C and N. Now, there is a 13% Cr-Ti type ferritic stainless steel that uses Ti as a stabilizing element. It has the advantage of being resistant to chloride SCC, having a low Cr content so that it does not suffer from 475°C embrittlement, and has superior thermal conductivity than austenitic stainless steel. However, this 13%
As a result of detailed experiments by the present inventors regarding the Cr-Ti system, Ti%/(C%+N%)≧15...(A) Addition of Ti in the above range does not increase sensitization to grain boundary SCC during welding. Although it can be avoided, 500℃ after that
It has become clear that IGSCC occurs when SR processing is performed for approximately 2 hours. The mechanism of this IGSCC occurrence is as follows. First of all, Ti is
After welding, N is fixed preferentially and C is not fixed effectively, and as a result, unfixed free C forms Cr carbides and precipitates at grain boundaries during low-temperature heat treatment after welding. This indicates susceptibility to IGSCC. The present inventors have discovered that ferritic stainless steel
Based on the belief that it is necessary to suppress the precipitation of Cr carbides during the low-temperature heat treatment process after welding to completely prevent SCC, we conducted further experiments and studies on the method, and as a result, we found that Nb is optimal for fixing unfixed C, and Ti based on (A) above
At the same time, it has been found that by adding Nb in a range of Nb%/C%≧8, it is possible to prevent the occurrence of SCC after welding→low temperature heat treatment. In other words, the gist of the present invention is that C0.015
% or less, Si1.0% or less, Mn1.0% or less, Cr11~16
%, N0.015% or less, and Ti and Nb are contained within the above range. The rest is made of ferritic stainless steel, which has excellent stress corrosion cracking resistance and consists of Fe and unavoidable impurities. The reasons for limiting each component of the steel in the present invention will be described below. C: An element that significantly increases the SCC susceptibility of ferritic stainless steel in a high-temperature water environment, and the lower the amount, the more preferable it is. C≦0.02% for austenitic stainless steel
However, in the case of ferritic stainless steel, due to the low solid solubility of C, it is realistically impossible to prevent the occurrence of SCC only by reducing C, and stabilizing elements are required. It is necessary to use the addition of Unless the C content is 0.015% or less, the occurrence of SCC will be unavoidable even if Ti and Nb as stabilizing elements are used within the range that satisfies the above () (). N: Like C, it is an element that increases SCC sensitivity, and for the same reason as C, it needs to be kept at 0.015% or less. Cr: An essential component for improving the corrosion resistance of stainless steel. 11% against general corrosion in high temperature pure water environment
It is enough. When the Cr content is 16% or more,
475°C There is a risk that brittleness may occur, which limits the operating temperature, which is undesirable. Si: This is an element necessary as a deoxidizing agent during steel manufacturing, but if it exceeds 1%, it accelerates the precipitation of σ phase etc. and adversely affects workability. Mn: Like Si, it is an essential element as a deoxidizing agent, but it is also an austenite forming element, and if it exceeds 1%, it deteriorates the stability of the ferrite structure. Ti: It has the effect of preventing sensitization during welding and suppressing the occurrence of IGSCC by fixing C and N.
If it is less than 15 (C%+N%), sufficient effects cannot be obtained. On the other hand, addition of more than 0.8% causes a decrease in toughness. Nb: Especially C that remains free from Ti after welding
Fixed, due to low temperature sensitization after
It is effective in preventing the occurrence of IGSCC.
It is necessary to add more than ×C%, but if this exceeds 1%, weldability will be adversely affected. Next, the effectiveness of the present invention will be specifically explained with reference to Examples. A 5 mm thick steel plate made of each component (1) to (11) shown in Table 1 was heated at 800°C for 30 minutes, cooled with water, and then
A process that provides a heat cycle equivalent to welding (TIG welding), such as air cooling at 1350°C for 1 second (cooling from 800°C to 500°C in 7 seconds) (hereinafter referred to as welding equivalent treatment), and an additional 500°C Heat treatment equivalent to SR treatment after welding was performed by heating for 100 minutes and cooling in air. From each steel plate treated above, 75mm length x 10mm width x
A 2 mm thick test piece was taken and the SCC resistance was investigated using a double U bend test. In the test, the above two test pieces were overlapped and bent into a U-shape, which was then further bent for 5 mm.
This was restrained to form a so-called double U-bend test piece, which was immersed in high-temperature water at 240°C containing 5 ppm of Cl ^- ions for 14 days. ) was conducted by measuring the maximum crack depth. The results are shown in Table 2. In the same table, ○ indicates that SCC was not observed, × indicates that SCC occurred, and in the case of ×, the value written in the same column indicates the maximum depth of that SCC.

【table】

【table】

[Table] From the above table, it is clear that the steel of the present invention can maintain excellent SCC resistance without becoming sensitized not only after welding but also after welding and low-temperature heat treatment. Also, from the results of comparative examples, C, N > 0.015% (steel (7),
(10), (11)) or Ti%/(C%+N%)<15(steel
(8)) or Ti%/(C%N%) ≧15 but Nb%/C%
<8 (steel (9)), it can be seen that the SCC resistance deteriorates due to welding and subsequent low-temperature heat treatment. Figure 1 shows 11-16% of each species as shown in Table 1.
SCC resistance of Cr ferritic stainless steel after the above-mentioned welding equivalent treatment (heating at 1350°C for 1 second followed by air cooling)
Figure 2 shows the properties in correspondence with the values of C% and Ti%/(C%+N%).
Ti%/(C%+N%), Nb%/C
It is expressed in correspondence with the value of %. 1st
The figure shows the case where N is 0.015% or less, and Figure 2 shows the case where both C and N are 0.015% or less. In the figure, ○: SCC does not occur, ●: SCC occurs. First, from Figure 1, C must be less than 0.015%.
SCC occurs, but C0.015 or less (and N≦
0.015% or less), Ti%/(C%+N%) is 15
It can be seen that SCC occurs below this level. Furthermore, the second
According to the figure, C, N≦0.015% and Ti%/
Even if (C%+N%)≧15 is satisfied, Nb%/C
If the percentage is less than 8, there will be no problem after welding, but
SCC occurs when low-temperature heat treatment is applied after welding. Figure 3 A shows 0.01% at room temperature after welding equivalent treatment.
C-13.2%Cr-0.009%N-0.55%Ti (Ti%/(C
%+N%)≒29≧15) Momentary sensitization, or stress, of the steel when it is held at each heating temperature of 400℃, 500℃, 600℃, and 700℃ for 1 to 1000 minutes. TTS shows the occurrence of corrosion cracking (SCC) as a curve and a plot, with the heating temperature on the vertical axis and the holding time on the horizontal axis.
(Temperature − Time −
Sensitization (sensitization) curve diagram is shown, and the figure (b) shows 0.01%C-13.1% at room temperature after welding equivalent treatment.
Cr-0.008%N-0.50%Ti-0.011%Nb system (Ti
%/(C%+N%)≒28, Nb%/C%≒11) system ferritic stainless steel similar to the above TTS curve diagram is shown. In the figure, 〇 and ● are the same as in Figure 1, etc. mentioned above. In Figure 3 A, the range is 600 to 500 as shown by the curve and ●.
SCC occurs when held at a heating temperature of ℃ for 30 to 1000 minutes. The cause of this SCC is the precipitation of Cr carbides at grain boundaries, not Cr nitrides. In other words, in the cooling process of a process equivalent to welding, Ti first fixes N preferentially as TiN, and then this
TiN fixes C in the form of Ti(CN),
In this case, N is effectively fixed first, but C is only insufficiently fixed, and C remains unfixed.
remains, and this is what precipitates Cr carbide. However, if Nb is added to this in a ratio of Nb%/C%≧8, the Nb will sufficiently fix C as NbC, and as a result, the SCC occurrence as shown by the curve and ● in Figure 3 A. The area disappears in Figure 3 (b). Example 2 Inventive steel A and comparative steel B having the components shown in Table 3 were melted, forged, and hot rolled to produce a 10 mm thick steel plate. was subjected to solution treatment by heating at 850°C for 30 minutes and then cooling with water. Next, after bringing these steels A and B to room temperature, they were heated in the atmosphere at 475°C, 400°C, 350°C, and 300°C for 1 to 10,000 hours each, and then these steel plates were An impact test piece measuring 55 mm long x 10 mm wide x 5 mm thick with a 2 mm V-notch was prepared from the sample.

[Table] This test piece was heated to 80°C and then subjected to a Charpy impact test. The test temperature was 80℃ because
This is to determine that there is no practical problem if the material exhibits a Charpy impact value of 15 ^kg-m /mm ² or more at 80°C. The results are shown in FIG. As is clear from the figure, in the test at a heating temperature of 350°C, steel B had a shear py impact value of 15 ^kg-m /mm ² after 1000 hours (approximately 41 days), and steel A after 10,000 hours (approximately 416 days). The embrittlement time of Steel A is extremely longer than that of Steel B. In addition, the embrittlement time at a relatively low heating temperature of 280 to 300℃ is quite long, so no tests were conducted, but the embrittlement points of the test results at 300 to 475℃ are connected and expressed as a straight line. Judging from the extended dotted line,
It can be estimated that the embrittlement time at 280°C (the operating temperature in actual equipment) is 2154 x 10 ⁴ hours (2.45 years) for steel B and 3414 x 10 ⁵ hours (38.9 years) for steel A. Combining the various test results above, we found that 13% Cr ferritic stainless steel has excellent SCC resistance.
The properties are C, N≦0.015% and Ti% (C%+N%)
It is clear that this can be achieved by simultaneously satisfying the conditions of ≧15 and Nb%/C%≧8. In other words, the ferritic stainless steel of the present invention has excellent SCC resistance, does not show sensitization to SCC even by welding or low-temperature heat treatment after welding, and has a Cr content of less than 16%, so it can be heated at 475°C.
Since the steel of the present invention does not cause brittleness, it can be applied to nuclear power and chemical-related equipment that is exposed to high-temperature water environments that may or may not contain trace amounts of Cl ^- ions, and can be highly effective in improving their durability. Can be done.

[Brief explanation of the drawing]

Figure 1 shows the SCC resistance of 11 to 16% Cr ferritic stainless steel after welding equivalent treatment.
Figure 2 is a plot diagram showing the SCC resistance of the same steel after welding equivalent treatment + low-temperature heat treatment in relation to the values of Ti%/(C%+N%).
Figure 3 A shows the heating temperature, holding time, and sensitization of ferritic stainless steel that satisfies the compositional conditions of the present invention except that it does not contain Nb. TTS showing the relationship between
Figure 4 shows a plot diagram showing the relationship between heating temperature, holding time, and Charpy impact value for the invention steel and comparative steel. ing.

Claims (1)

[Claims] 1 C 0.015% or less, Si 1.0% or less, Mn 1.0% or less,
Contains 11% to less than 16% Cr, 0.015% or less N, Ti and Nb, 15 (C% + N%)≦Ti≦0.8% 8×C%≦Nb≦1.0% The rest is Fe and Ferritic stainless steel with excellent stress corrosion cracking resistance due to unavoidable impurities.