JPS5864356A - Stainless steel for high temperature use with superior strength at high temperature, stress corrosion cracking resistance and weldability - Google Patents

Abstract

PURPOSE:To enhance the strength at high temp., stress corrosion cracking resistance and weldability by adding prescribed amounts of C, Si, Mn, Cr and Ni and by carrying out heat treatment at a prescribed temp. to form a structure composed of a certain range of ferrite and austenite. CONSTITUTION:A steel consisting of <=0.2% C, <=3% Si, <=3% Mn, 20-30% Cr, 6-12% Ni and the balance Fe is refined, cast, heated at 1,130-1,250 deg.C, and quenched to form a structure composed of 5-30% by area of ferrite and austenite. Thus, the strength at high temp., stress corrosion cracking resistance and weldability are enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-temperature stainless steel having excellent properties such as high-temperature strength, stress corrosion cracking resistance, and weldability.

Piping materials for heavy oil desulfurization equipment are used under high temperatures and pressures of over 400°C and pressures of 200 kg/c and It, and when shut down, they are exposed to severe stress as they come into contact with corrosive substances such as polythionic acid. Corrosion cracking (hereinafter referred to as
8CC) conditions. Therefore, the piping material must have high high temperature strength and excellent SCC resistance. Of course, as a structural material, it is necessary to have good weldability.

Conventionally, JIS8U8304 was used as a pipe material for such uses.
(8 (!813), 8U8347 or 5U8321
Stainless steel such as is used. however,
None of these conventional materials satisfy all of the above-mentioned characteristics; for example, 8 (!813.5trs30
4 does not have sufficient strength and sea resistance, 5US347 has problems with weldability and SCC resistance of welded parts, and 5US347 has problems with weldability and SCC resistance of welded parts.
321 has the drawbacks of poor SCC resistance of welded parts and insufficient strength. This is why there is a demand for the development of new materials with excellent high-temperature strength, SCC resistance, and weldability. In addition, in order to meet the above-mentioned conditions of use at high temperatures and high pressures, it is desired that a high degree of toughness be maintained even during long-term use at high temperatures.

The present invention is a new high-temperature stainless steel base that meets the above requirements.
Mn8. Q% or less, or more than 30%
% or less, Ni 6 to 12%, balance Fe, and unavoidable impurities.The metal structure of L is a two-phase structure of ferrite with an area ratio of 5 to 30% and the balance austenite, and it is heated at a temperature of 1130 to 1250%. The point is that it has been subjected to solution treatment, which involves heating and holding it at ℃ and then rapidly cooling it.

Hereinafter, the stainless steel of the present invention will be explained in detail.

The reason for limiting the chemical components in the present invention is as follows.

C: 0.20 C or less C is an effective element for improving strength, and in order to obtain high high temperature strength in high temperature applications of around 400'C, which is the aim of the present invention, it is possible to increase the content. is preferred. However, on the other hand, it is accompanied by a decrease in SCC resistance, general corrosion resistance, and weldability, and when the thickness exceeds about 0.20 inch, the SCC resistance decreases significantly, and the toughness deteriorates significantly after long-term use. .

Therefore, the upper limit is set at 0.2o%. However, SCC resistance
As described later, the strength also has a relationship with the amount of ferrite in the steel structure, and if the above C amount (0.20 ts or less), good 8cc resistance can be achieved by the regulation of the amount of ferrite according to the present invention. can be secured. In addition, if emphasis is placed on 8cc resistance, the lower the C content, the better, and it is preferably about O,OS% or less, especially about 0.03% or less, but in order to ensure practical strength in high-temperature applications, about It is preferable to set the lower limit to 0.054.

Sl: B, 0@ or less, Mn: a, ol or less S1 and M
Although n does not particularly characterize the alloy of the present invention, it is usually added at least about 0.2 h each for the purpose of deoxidizing during alloy melting (for Mn, desulfurization in addition to deoxidizing) and ensuring castability. It will be done.

However, there is almost no increase in these effects even if the thickness exceeds 8.0, and especially 8i, it promotes a decrease in toughness after long-term use, which is not preferable. Therefore, the upper limit is set at 3.0% for each.

cr: more than 20% and less than 30% Or not only ensures the oxidation resistance necessary for high-temperature use, but also forms the required amount of ferrite in the austenite/ferrite structure as a ferrite former in relation to the amount of Ni described below. It is necessary for In particular, in the present invention, the upper limit of the C content is set to a relatively high value in consideration of high-temperature strength as described above, so in order to compensate for the tendency for the SCC resistance to decrease due to C, it is necessary to Also a high level C! rii is desired. From this point on, let's assume that the total amount exceeds 20 cm. However, if the amount is too large, the toughness will deteriorate and an excessive amount of ferrite will be produced, so the upper limit is set at 30 inches.

Ni: 6 to 12% Ni is an element necessary for improving general corrosion resistance and forming a ferrite/austenite structure as an austenite former for the above-mentioned Or, which is a ferrite former. The content needs to be at least 6 to ensure corrosion resistance and balance with the Cr content. However, Ni is an expensive element, and it is uneconomical to use a large amount.If the Ni content exceeds 12%, the amount of austenite will increase excessively, making it impossible to obtain the necessary amount of ferrite, so use 12%. Upper limit.

It is desirable that P, B, and other unavoidably mixed impurities be as low as bone ash, but they may be present within the range normally allowed for this type of steel.

For example, P is about 0.04 inch or less, and 8 is about 0.04 inch or less. If it is o4chi or less, there is no particular problem.

The alloy of the present invention has a two-phase structure of austenite/ferrite, and the amount of ferrite is required to be 5 to 30 ts in terms of area ratio. The ferrite phase contributes to improving SCC resistance and weldability. Lower C content at the expense of high-temperature strength,
For example, if the content is limited to 0.05% or less, it is possible to maintain SCC resistance and weldability at a certain level without the aid of the ferrite phase, but in the present invention, the above-mentioned In order to ensure high-temperature strength, the upper limit of C content is set relatively high.In order to prevent the deterioration of 8CC resistance and weldability due to the C content, the lower limit of ferrite content is set to 5 CH. be. However, even with such effective ferrite, when the thickness exceeds 30 mm, the toughness decreases significantly, especially during sensitization treatment or long-term use at high temperatures.

Therefore, the upper limit is set at 30. Of course, controlling the amount of ferrite in this two-phase structure from 5 to 30 is achieved by appropriately balancing the contents of each of the above elements, especially Cr and Ni, within the specified range. It is done well.

The alloy of the present invention needs to be subjected to a heat treatment in which it is heated and maintained in a high temperature range and then rapidly cooled (water cooling may be sufficient).

The heating temperature for the heat treatment must be at least 1130°C. If this were to be subjected to solution treatment at a temperature of about 1100°C ± 25°C, which is applied to ordinary stainless steel, the purpose would not be achieved, or the toughness after long-term use at high temperatures would become extremely poor. However, as mentioned above, by heating and holding at 1130°C or higher and then rapidly cooling, the functions of each alloying element are exhibited for the first time, and the desired material properties are achieved without causing the above-mentioned deterioration of toughness. can be obtained.・Comparing the metallographic structures, the results are as shown in Figures 1 to 5 (magnification: 200x, corrosion: 7-electrolytic etching in 10% oxalic acid). Figure 1 shows
As-cast structure (as-cast), Figure 2 shows heating temperature of 1100℃
, Figure 3 is 1130℃, Figure 4 is 1200℃, Figure 5 is 1200℃.
The figure shows the structure obtained by heating at 125'0°C and cooling with water after holding the temperature for 3 hours. That is, the ferrite phase, which is network-like in the as-cast structure (FIG. 1), is divided by the heat treatment and changes into a rounded shape as the heating temperature increases.

The degree of roundness is not sufficient at the heating temperature (Fig. 2) corresponding to conventional solution treatment. The alloy of the present invention requires that the divided ferrite phase be rounded to the extent shown in FIGS. 3 to 5, and therefore the heating temperature must be at least 1130 DEG C. However, if the heating temperature exceeds 1250℃, there is no improvement in the effect despite the high temperature, which is disadvantageous in terms of thermoeconomics, or it should be
The amount of ferrite that was in the range of 30 inches changes greatly, and the object of the present invention cannot be achieved. Therefore, the upper limit is set at 1250°C.

Next, the stainless steel of the present invention will be specifically explained with reference to examples.0Example: Alloys with various chemical compositions were melted and centrifugally cast tubes (outer diameter 200 mm x wall thickness 20 mm x length 40.00 mm) were made. Cast. Table 1 shows the composition and amount of ferrite of each sample tube material. Test samples 1 to 4 have the chemical composition and amount of ferrite specified in the present invention, and test samples 1'&xll~
16 is a comparison material. Among the comparative materials, N[Lll is JI
S 80. It is a material equivalent to S13.

Table 1 Test pieces were collected from each of the above-mentioned test tube materials, and test pieces 1 to 4 and 12 to 16 were heated to 1150°C according to the present invention.
A heat treatment was performed by holding the sample at 1° C. (holding time: 3 hours) and then cooling it with water. %11 (equivalent to JIS 5C8ia) was subjected to the usual solid solution treatment (1100° C. x 3 hours, water cooling) performed on this alloy.

For each test piece, high-temperature strength was measured by a 450°C high-temperature tensile test, and SCC resistance was evaluated. The results are shown in Table 2.

However, SCC resistance evaluation was performed by intergranular corrosion test (
SCC resistance corresponds to intergranular corrosion resistance and can be evaluated based on its quality.) In addition, the intergranular corrosion test is
The test piece was previously subjected to a sensitization treatment in which the test piece was heated and held at 650° C. for 2 hours and then slowly cooled, and then processed in accordance with the provisions of ASTM A262E method. In the table, "○" indicates that there was no abnormality in the bending test, and "x" indicates that cracks occurred.

Furthermore, Table 3 shows the influence of heat treatment temperature on toughness for sample materials 1 to 4 and fjhl (equivalent to SC813). In both cases, the heating holding time of the heat treatment was 3 Hr,
Rapid cooling after heating was performed by water cooling. The values in the table are 400℃
After aging by heating and holding for 1000 hours, the temperature θ
Impact value (k!!”/cJ) measured in °C
Yes (sample, with 2m+nV notch).

Table 2 Table 3 Impact Value (Yum/cI4) As shown in Table 2 above, the alloy of the present invention has no problem with SCC resistance and has excellent high temperature strength. On the other hand,
Comparative materials N1111 to 16 were 1. Both high temperature strength and SCC resistance cannot be satisfied. That is, t'lh
l'1 (equivalent to 80813) has low high-temperature strength and low SCC resistance due to the small amount of ferrite. N[L1
No. 2 has problems in both high-temperature strength and SCC resistance because the Cr content and ferrite content are low. No. 13 has a relatively high C content, so the strength is good, but the 800 resistance is poor.

In N14, the amount of Ni does not greatly exceed the upper limit specified in the present invention, but the amount of Cr is large, so the amount of ferrite is excessive, and although the strength is high, the SCC resistance is poor. Furthermore, according to a separate test, it is inferior in toughness. ll! In l15, since the amount of N1 is less than the lower limit, the amount of ferrite is excessive, the 8CC resistance is poor, and the toughness is also poor. N [
L16 has good S resistance even though the Ni content exceeds the upper limit.
CC properties are good. However, the high temperature strength is low and is inferior to the alloy of the present invention.

Furthermore, as is clear from Table 3 above, the alloys of the present invention exhibit good toughness when subjected to a prescribed heat treatment.

In other words, the alloy of the present invention cannot be expected to improve its toughness if it is subjected to conventional solid solution treatment in which it is heated and held at a temperature of about 1100°C.
It can be seen that a high degree of toughness can only be obtained through heat treatment at a high temperature of 1130 to 1250°C. Moreover, even when used for long periods of time under high temperature and high pressure, it still maintains sufficient toughness, making it an excellent industrial material.

As mentioned above, the chemical composition of the alloy of the present invention was determined taking weldability into consideration, so there was no problem with weldability, and in a separate weldability test, it was found to be equivalent to or better than conventional materials. We were able to confirm good results.

As described above, the high-temperature stainless steel according to the present invention has excellent high-temperature strength, SCC resistance, and weldability.
In addition, it does not lose sufficient toughness even after long-term use at high temperatures, making it extremely suitable as a material for various equipment that is used at high temperatures and high pressures and requires 8CC resistance, including piping materials for heavy oil desulfurization equipment. be.

[Brief explanation of drawings]

Figures 1 to 5 are photographs (substitute for drawings) showing the metallographic structure of the alloy.
Magnification: 200). Patent applicant Kubota Iron Works Co., Ltd. Agent Patent attorney Arata Miyazaki

Claims (1)

[Claims] (1) 00.20 S or less, 8i9.9% or less, M
n9. Noodles have a two-phase structure of ferrite and austenite of 5 to 30% (area ratio), consisting of Qchi or less, Cr exceeding 29%, 30chi or less, N16 to 12chi, the balance Fe and unavoidable impurities. Skin 1130-1250℃
A high-temperature stainless steel with excellent high-temperature strength, stress corrosion resistance, weldability, etc., which is characterized by having undergone heat treatment of heating holding and rapid cooling.