JPS6366383B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an austenitic stainless steel boiler tube that has improved creep properties and high temperature corrosion resistance at the same time. In recent years, with the rise in oil prices, the use of coal has been actively promoted, and it is expected that Japan's thermal power generation will shift from the traditional heavy oil-only combustion to coal-fired power generation in the future. The switch from heavy oil to coal is said to make the corrosive environment inside the boiler even more severe, and countermeasures for corrosion in boiler tubes will become a major technical issue in the future. On the other hand, as energy resources are depleted and prices soar, efforts are being made to save energy, and it is also known that increasing temperature is effective in improving the conversion efficiency from thermal energy to electric power. Since such an increase in operating temperature causes more severe corrosion of the boiler tube as described above, it is desired to develop a boiler tube that has better high-temperature corrosion resistance and creep strength than ever before. It is well known that increasing the amount of Cr is effective in improving high-temperature corrosion resistance, but decreasing the stability of austenite may reduce creep strength, or the formation of σ phase may reduce toughness and strength. This causes problems such as In order to prevent this, it is necessary to contain a large amount of Ni commensurate with the amount of Cr, which leads to an increase in cost. The inventors found that Si is extremely effective in high-temperature corrosion resistance, but in order to facilitate the formation of the σ phase, similar to Cr, they decided to increase the amount of Si only in the outer surface layer where corrosion resistance is required. As a result, we succeeded in developing an economical boiler tube that combines these contradictory characteristics. Table 1 shows the range of ingredients used in the present invention.

[Table] The present invention will be explained in detail below. First, the reasons for limiting each component as described above will be described. Although C is necessary to maintain strength, the upper limit was set at 0.20% from the viewpoint of weldability. In other words, austenitic steel is prone to hot cracking during solidification, but C
has a particularly strong negative effect, so the upper limit was set at 0.20%.
Regarding the lower limit, the lower limit was set at 0.03% since it would be difficult to ensure creep rupture strength if it was less than 0.03%. Si is added as a deoxidizer, but it is also an element that improves oxidation resistance and high-temperature corrosion resistance. Therefore, considering weldability and toughness for the internal part, the upper limit was set to 1.0 to keep them within a range that does not damage them.
%. The lower limit was set at 0.3% in order to sufficiently deoxidize and obtain a sound steel ingot. Next, regarding the Si enriched layer on the outer surface, which is the main focus of the present invention, we have defined the Si concentration on the outer surface, the thickness of the Si enriched layer, and the relationship between the two from the viewpoint of corrosion resistance and workability. This is the greatest feature of the present invention. First, regarding the Si concentration on the outer surface, if it exceeds 14%, workability will be significantly inhibited. However, at least on the outer surface, a Si concentration of 4.5% is required.
If it is less than 4.5%, corrosion resistance will be insufficient even with a concentrated layer thickness of 0.4 mm, which will be described later. So at least Si
The upper limit of the outer surface concentration was set at 14% and the lower limit at 4.5%.
In addition, the Si-enriched layer immediately below the outer surface requires a Si content of 4.5% or more in order to be effective in corrosion resistance, so the layer was made to contain 4.5 to 14% Si. The thickness t must be 0.4 mm or less in order to maintain workability and not adversely affect the material quality, but if it is less than 0.04 mm, the outer surface concentration of Si, for example, will decrease.
Even at 14%, the corrosion resistance was still insufficient, so the upper limit was set to 0.4 mm and the lower limit was set to 0.04 mm. Furthermore, corrosion resistance is related to both the Si concentration on the outer surface and the thickness t of the concentrated layer, and when this relationship was determined in the laboratory, sufficient corrosion resistance was ensured when the outer surface Si% x √ was 2.8 or more. I knew what I needed to do. Note that the value in this formula satisfies the above regulations. For example, even if the outer surface Si concentration is 6%, if the concentrated layer thickness t is 0.09 mm, sufficient corrosion resistance will not be exhibited. Therefore, the outer surface Si%
×√ was defined as 2.8 or more. In this case, the present inventors have so far determined that 41% Na ₂ SO ₄ + 8% V ₂ O ₅ +
Synthetic ash with 51% Fe ₂ (SO ₄ ) ₃ was used and the working temperature (650
It has been discovered that corrosion in an actual boiler can be evaluated by heating and melting the test piece at a temperature of ~750℃) and immersing the test piece for 200 hours. For safety reasons, it is desirable that the amount of corrosion is 400 mg/cm ² or less. Mn is a necessary component not only for deoxidizing but also for maintaining strength. The upper limit was set at 3.0% because the effect would be saturated even if added in excess of this, and the lower limit was set at 0.1% in order to deoxidize and obtain a sound steel quality. Cr is an essential element for oxidation resistance and high-temperature corrosion resistance, and is always added to heat-resistant steel, but 13
If the corrosion resistance is less than 25%, the above corrosion resistance is insufficient.
Exceeding this decreases the stability of austenite, weakens its high-temperature strength, and promotes the formation of σ phase, resulting in a decrease in toughness, so the upper limit was set at 25% and the lower limit was set at 13%. Ni increases the stability of austenite and also σ
The effect of suppressing phase formation is remarkable. lower limit to 13
% is a sufficient amount to ensure austenite stability against the lower limit of ferrite-forming elements such as Cr, and the upper limit of 40% is set against the above-mentioned upper limit of ferrite-forming elements. be. Mo and W are elements that significantly increase high-temperature strength through solid solution strengthening and carbide precipitation, so they are added to cope with increases in boiler steam temperature and pressure, but they are expensive and impair oxidation resistance, so the upper limit is The lower limit was set at 0.5% in total, since 0.5% or more has a significant effect on improving creep rupture strength. Ti, Nb, and V are carbide- and nitride-forming elements, respectively, and by finely precipitating them,
Significantly improves creep strength. However, if the amount is less than 0.05% individually or in total, there will be no strengthening effect, and if the amount exceeds 0.5% in total, the precipitates will aggregate and the creep strength will tend to decrease. %. B is an element that has a remarkable effect on increasing grain boundary strength and creep strength.
Less than 0.001% has little effect;
If it exceeds %, hot workability will be inhibited, so the upper limit should be set at 0.01.
%, and the lower limit was set at 0.001%. Next, the effects of the present invention will be described with reference to examples. Table 2 shows the chemical composition of the test steel pipe, the creep rupture time at 700℃ and a stress of 12 Kg/mm ² for the steel pipe with the internal composition, and the 41% Na ₂ SO for the test piece with a two-layer structure with different Si%. ₄ +8%V ₂ O ₅ +51%Fe ₂
( _SO4 ) 200 in molten salt at 700 °C using synthetic ash of ₃
The amount of corrosion after being immersed for hours, the impact value (JIS No. 4 test piece) after 4000 hours at 700°C, and cold bending workability are shown. Among those shown in Table 2, steel pipes D, F, I,
J, K, and N are steel pipes P and Q are steel pipes of the present invention that belong to the invention in Item 1, and the others are comparative steel pipes. Steel pipe A is SUS321HTB, which is used for super heaters in thermal power generation boilers in Japan, and steel pipe B is used for the same purpose in the United States.
Although SUS347HTB has insufficient creep rupture strength and high temperature corrosion resistance when the surface temperature reaches 700℃. In steel pipe C, the Si concentration in the outer layer is outside the lower limit of the range of the present invention, and if the Si content is less than 4.5%, sufficient corrosion resistance cannot be obtained even with a 0.4 mm thick layer. In steel pipe E, the thickness of the Si enriched layer exceeds the upper limit, and cracks occur on the surface during cold bending. Steel pipe G has a Si content that is outside the upper limit, and its toughness deteriorates significantly after long-term aging (embrittlement during use). In steel pipe H, the Si concentration of the outer layer is close to the upper limit of the range of the present invention, but the thickness is outside the lower limit. That is, even though the Si concentration is extremely high at 13.95%, the concentrated layer is too small and the corrosion resistance is insufficient. Steel pipe L has the same composition as steel pipe K, but the value of Si×√ is below the lower limit of the range of the present invention, and its corrosion resistance is insufficient.
The steel pipes M and O contain Ti and Nb which are outside the lower and upper limits of the range of the present invention, respectively, and even if their amounts are outside the appropriate range of the steel pipe of the present invention and are too high or low, sufficient creep rupture strength will not be achieved. I can't get it. In contrast, the steel pipe of the present invention is made of the current SUS321,
It is an excellent steel pipe that has significantly superior creep rupture strength and high-temperature corrosion resistance compared to 347HTB, and has no particular problems with embrittlement or workability during use. In particular, the steel pipes P and Q belonging to the invention in Item 2 are steel pipes made of steel to which B has been added in order to further improve the creep strength. The effect is clear. As detailed above, the steel pipe of the present invention is an excellent steel pipe that can withstand use at higher temperatures than ordinary austenitic stainless steel boiler pipes.

【table】

[Table] (Note) ○: Comparative steel pipe

Claims (1)

[Claims] 1 C0.03 to 0.20%, Si 0.3 to 1.0% in the endoplasmic part,
Further, when the Si concentration at least on the outer surface is 4.5 to 14.0% and the thickness of the Si layer of 4.5 to 14.0% is t, t
=0.04~0.4mm, and further satisfies the relationship of outer surface Si%×√≧2.8, other Mn0.1~3.0%, Cr13~25
%, Ni13~40%, Mo, W, one or two types in total
A high-strength, high-corrosion-resistant boiler characterized by a steel pipe containing 0.5 to 3.0%, one or more of Ti, Nb, and V in a total of 0.05 to 0.5%, with the balance consisting of Fe and substantially impurities. Tube. 2 C0.03-0.20%, Si 0.3-1.0% in the endoplasm,
Further, when the Si concentration at least on the outer surface is 4.5 to 14.0% and the thickness of the Si layer of 4.5 to 14.0% is t, t
=0.04~0.4mm, and further satisfies the relationship of outer surface Si%×√≧2.8, other Mn0.1~3.0%, Cr13~25
%, Ni13~40%, Mo, W, one or two types in total
Contains 0.5-3.0%, one or more of Ti, Nb, and V in total of 0.05-0.5%, B0.001-0.01%, and the balance is Fe.
and a high-strength, high-corrosion-resistant boiler tube characterized by being a steel tube consisting essentially of impurities.