JPS6115147B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to an austenitic stainless steel that has excellent corrosion resistance and high tensile strength (hereinafter simply referred to as strength) at both high and ordinary temperatures. [Prior art and its problems] Austenitic stainless steel, represented by SUS304, is widely used in chemical plants, power generation plants, etc., but some of these uses include, for example, piping used in light water reactors for nuclear power generation. teeth
There are products in which high-temperature and high-pressure water of about 300°C flows, and at such high temperatures and high pressures, it is necessary to have high strength exceeding a specified value and sufficient corrosion resistance (i.e., the ability to prevent the occurrence of stress corrosion cracking caused by high-temperature, high-pressure water). required to have one. In order to improve the corrosion resistance of austenitic stainless steel, it is necessary to reduce the C content or to improve the corrosion resistance of austenitic stainless steel.
SUS304L, SUS304L, etc. have been proposed for a long time.
Steel types such as SUS321 and SUS347 are known. However, in such steel types, the amount of solid solute C, which is a reinforcing component of the steel, is small, so the strength may be insufficient depending on the application. For example, large-diameter pipes used in power plant piping systems are often manufactured by hot forging, thick plate forming, or welding, but such products have coarse grains due to the small processing ratio. However, it is not possible to expect an increase in strength due to grain refinement as in extruded or drawn products. On the other hand, the large-diameter pipes are required to have excellent corrosion resistance against high-temperature, high-pressure water, steam, etc., and reducing and stabilizing C is essential.
A reduction in will result in a lack of strength. In order to meet the above-mentioned demands, the present invention developed an austenitic stainless steel that is comparable to conventional steel in terms of corrosion resistance and that can ensure sufficient strength even if the processing rate to reach the finished product is small. This is what we provide. [Means for solving the problem] As a means for solving the above problem, the austenitic stainless steel according to the present invention was provided with the following composition. C...0.03% or less Si...1.0% or less Mn...2% or less Cr...17.0 to 19.0% Ni...9.0 to 13.0% Mo...0.2 to less than 0.5% N...0.05 to 0.15% Nb... Exceeding 0.3% and 1.0% Balance: Substantially Fe Here, the balance substantially Fe means that it contains impurities that are inevitably mixed in during the steelmaking process. Typical impurities are P and S, which need to be suppressed to 0.04% or less and 0.03% or less, respectively. In the present invention, the lower the C content, the better, but 0.02
% or less than 0.03% is unavoidable. In addition, in order to ensure corrosion resistance that satisfies the extremely strict specifications of power plant piping, etc.
It is necessary to keep the amount below 0.03%, preferably
It is recommended that the content be less than 0.02%. If such a small amount of C is contained, it is possible to prevent Cr carbide caused by the combination of C and Cr from being precipitated at grain boundaries by the action of Nb, which will be described later. However, since C is a component that generally contributes to improving the strength of steel, reducing the amount of C as described above inevitably leads to a decrease in strength. One of the features of the present invention is that the decrease in strength due to the reduction in the amount of C is compensated for by adding other components. The type and amount of added components for this purpose must be selected with due consideration to the effects on corrosion resistance and other properties. Si is necessary as a deoxidizing agent, but if it exceeds 1%, it causes deterioration of weldability. Mn acts as a deoxidizing agent and at the same time contributes to improving the hot workability of steel. However, since excessive addition causes a decrease in strength, the upper limit of the content is set at 2%. Cr and Ni need to be in a range that makes the steel structure stable as austenite and ensures basic properties such as corrosion resistance. That is, the range is 17.0 to 19.0% for Cr and 9.0 to 13.0% for Ni. If it deviates from this range, the balance of required properties will be lost or the price will increase unnecessarily. Mo and N are added mainly for the purpose of compensating for the decrease in strength due to the reduction in C. In particular, Mo is N
The synergistic effect with this greatly contributes to the improvement of high-temperature strength. This effect becomes noticeable when Mo is 0.2% or more, but when it exceeds 0.5%, the effect cannot be exerted on the rate of increase in Mo content, which is disadvantageous from a cost perspective. Furthermore, Mo in this range also improves corrosion resistance. As mentioned above, N improves the strength of steel at high temperatures due to its synergistic effect with Mo.
It increases not only the high temperature strength but also the strength at room temperature. In the present invention, in order to ensure the strength of the steel in a wide temperature range from high temperature to room temperature, the amount of N added is actively added in excess of the amount contained in normal austenitic steel. A specific content of 0.05% or more is required, but if this amount becomes excessive, the amount of nitrides that combine with Nb (described later) will increase, and the effective Nb that fixes C will decrease. ,
It begins to impair the corrosion resistance of steel. To avoid such negative effects, the N content should be limited to 0.15%. Nb fixes C, prevents grain boundary precipitation of Cr carbides, and improves the stress corrosion cracking resistance of steel. In the present invention, C is suppressed to 0.03% or less, but the unavoidably mixed C combines with Cr and reduces the Cr concentration in areas near grain boundaries, thereby promoting intergranular corrosion, etc. There is a risk of causing To eliminate such C effects, an amount of Nb greater than 0.3% is required. However, if it exceeds 1.0%, it deteriorates the weldability of the steel. Since the addition of Nb is extremely important in the present invention, the present inventors investigated the influence of Nb on intergranular stress corrosion cracking of austenitic stainless steel. The investigation was conducted using the following method. In other words, as shown in Table 4, the steel type has a C content of 0.015%.
We prepared 11 types of Nb containing 0.10% level Nb additive-free and gradually increasing amounts, and used them as test materials by cutting them into plates with a length of 75 mm, width of 10 mm, and thickness of 2 mm. , after sensitizing the plate,
It was based on the autoclave test method. The sensitization treatment at that time involves heating at 1250℃ for 20 minutes and cooling.
This is followed by heating at 650°C for 30 hours. The conditions for the autoclave test were to stack two sensitized plates, curve them, and tighten both ends with stainless steel bolts and nuts to form a U-shaped test piece, which had a dissolved oxygen content of 8 ppm (atmospheric saturation). ), which is immersed in pure water at a temperature of 250°C for 500 hours.
The results of this intergranular stress corrosion cracking investigation were as shown in FIG. As seen in the same figure, Nb is 0.1
It can be seen that even when Nb is added in a small amount of about 0.3%, the cracking depth decreases rapidly, and as the amount of Nb increases, the cracking depth becomes even smaller, and when it exceeds 0.3%, cracking susceptibility is almost not observed. The Nb content was defined as described above based on the results of this investigation. In the present invention, there is another important reason why Nb was specifically selected and added to fix C.
Generally, in order to prevent intergranular corrosion of stainless steel, a means of fixing C by adding Ti is used. However, in the present invention, the addition of Ti eliminated this. The reason for this is that the relationship with N, which is an extremely important additive element in the present invention, was taken into consideration. In other words, Ti has a greater affinity for N than Nb, and if Ti is added, Mo
This is because it combines with N, which was added with the intention of improving high-temperature strength through a synergistic effect with N, and reduces the effect of N, which is actively added in large amounts in the present invention. Moreover, since TiN produced by the combination of N and Ti tends to grow in size, it impairs the cleanliness of the steel and causes an increase in surface flaws on the steel plate. Therefore, in the present invention, Ti was excluded and Nb was added. [Examples] The present invention will be described below based on specific examples. Table 1 shows the chemical composition of the sample materials. After melting and hot rolling these steels, a, b,
c Three types of two-stage heat treatment were performed. The first part of this heat treatment
The purpose of stage heating is to generate in the sample material coarse crystal grains that correspond to the coarse crystal grains of products with a small working ratio, such as forged products and thick plates. In a, the particle size number is approximately 2
Hereinafter, in b, particles of particle size No. 4 to 6 are obtained.
The second stage heating (1050°C x 20 minutes) is a process to keep the amount of solid solution constant. JIS No. 14A tensile test pieces were cut from the material treated as described above, and tensile tests were conducted at room temperature and 300°C. The results are shown in Tables 2 and 3, respectively. Based on the data in Tables 2 and 3 above, N and
Figures 2 and 3 illustrate the influence of changes in the amount of Mo in the coexistence of Mo on the strength of steel.
It is a diagram. In the same figure, for J, K, L, and M steels, N is
It is a low N material of 0.0218~0.0320%, H, D, E,
A, O, and F steels are high N materials with N content of 0.0847 to 1.008%. As is clear from these figures, the strength of the low-N material does not increase even when the Mo content increases, regardless of whether it is at room temperature or at a high temperature of 300°C. On the other hand, when looking at high N materials, the strength at room temperature is
It can be seen that the strength at high temperatures increases as the Mo content increases, whereas it hardly increases even if the Mo content increases. Especially Mo
In the range where the Mo content is less than 0.5%, a significant increase in strength can be seen even when a small amount of 0.2% is added, and the strength increase sharply increases almost in proportion to the increase in Mo content. Although the strength increases even when the amount of Mo is 0.5% or more, the rate of increase tends to slow down, indicating that even if a large amount of expensive Mo is added, a significant effect can be expected. From these results, it is clear that an appropriate amount of N is needed to improve not only the strength at room temperature, but also the strength at high temperatures.
You can learn the importance of using both and Mo together. Moreover, Nb improves the stress corrosion cracking resistance without reducing the high-temperature strength and room-temperature strength.

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〔Effect of the invention〕

As described above, the present invention can ensure high-temperature strength and room-temperature strength even for products in which grain refinement cannot be expected due to a small working ratio. In addition, in the present invention, while reducing C, the effect of N, which acts synergistically with Mo to compensate for the lack of strength due to low C, is not diminished.
The addition of Nb prevents intergranular corrosion and at the same time improves stress corrosion cracking resistance, resulting in a steel comparable to conventional austenitic stainless steel.

[Brief explanation of the drawing]

Figure 1 is a graph showing the heat pattern of the test heat treatment, Figure 2 is a graph showing the results of the room temperature tensile test,
FIG. 3 is a graph showing the high temperature tensile test results, and FIG. 4 is a graph showing the intergranular corrosion cracking investigation results.

Claims (1)

[Claims] 1 C0.03% or less, Si1.0% or less, Mn2% or less,
Corrosion resistant high strength consisting of Cr17.0~19.0%, Ni9.0~13.0%, Mo0.2~less than 0.5%, N0.05~0.15%, Nb over 0.3%~1.0%, and the balance essentially Fe. Austenitic stainless steel.