JPH0454737B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a high-strength ferritic heat-resistant steel, and more specifically to a ferritic Cr-containing boiler steel pipe steel with improved creep properties at high temperatures and excellent weldability and toughness. be. (Conventional technology) In recent years, thermal power boilers have become larger in size, at higher temperatures, and at higher pressures. However, when selecting materials for temperatures exceeding 550°C, ferrite-based boilers are preferred due to their oxidation resistance and high-temperature strength. From 1/4Cr−1Mo steel
Currently, high-grade austenitic steels such as 18-8 stainless steel are being used. However, as materials become more high-grade, such as low-alloy steel, stainless steel, and superalloys, costs rise and boiler construction costs become expensive. A supercritical pressure boiler is used. By the way, steel materials to fill the gap between 2 1/4Cr-1Mo steel and austenitic stainless steel have been sought for the past few decades, but boiler steel pipes with intermediate Cr content, such as 9Cr and 12Cr, have improved weldability by increasing their strength. However, the reality is that it is difficult to put it into practical use because it significantly reduces work efficiency in boiler construction. From this point of view, the emergence of a steel with creep strength that fills the gap between 2 1/4Cr-1Mo steel and austenitic stainless steel has been awaited. In view of these circumstances, the present inventors have developed a new steel type that has improved weldability and has a creep rupture strength that is significantly higher than conventional materials, and has disclosed (a) Japanese Patent Publication No. 56-34628, (b) Japanese Patent Application Publication No. 59-153865,
Alternatively, (c) is proposed in Japanese Patent Application No. 59-68377. Of these, steel (a) is a steel that secures creep rupture strength by adding V and Nb appropriately, and has improved weldability by reducing the amount of C.
Steel (b) is a steel that further improves toughness by limiting Si, and maintains a balance between strength and toughness by determining the correlation between V and Si. Steel (c) is a steel that aims to improve toughness by limiting Si and also improve creep strength by adding B and N and limiting the amount of oxygen. All of these steels (a) to (c) are excellent steels that can withstand long-term use at 600°C. However, it is predicted that boilers will start and stop frequently in response to further rises in steam temperature and fluctuations in power demand, and in this case, in order to reduce thermal stress, the wall thickness will be reduced at any time. Improvement in creep strength is desired. On the other hand, Japanese Patent Publication No. 58-17820 discloses that addition of W is effective in improving creep strength. However, in this steel, no proposal has been made regarding the optimal range of W, and also
No consideration was given to additions. (Problems to be Solved by the Invention) The present inventors have developed W, which has a high melting point and a slow diffusion rate, in order to increase the creep rupture strength at 600°C and at the same time enable its use in a higher temperature range. 1.8
% or more, and that it is also effective to replace part of W with a very small amount of Mo. Based on this, we have developed a steel with significantly superior creep rupture strength. was successfully developed. (Means for Solving the Problems) In order to solve the above problems, the present invention provides the following steel. That is, the steel of the present invention has C0.03 to 0.12% by weight,
Mn0.1~1.5%, Cr8.0~13.0%, W2.0~3.0%,
V0.05~0.30%, Nb0.02~0.12%, N0.02~0.10
%, contains Mo 0.02% or more and less than 0.1%, Si 0.01~
Limited to 0.25%, if necessary, B0.001% or more 0.008%
This is a high-strength ferrite-based boiler steel pipe steel characterized by containing the following: The present invention will be explained in detail below. First, the reason for limiting each component contained in the steel of the present invention will be described.C is necessary to maintain strength, but from the viewpoint of weldability, it is set to 0.12% or less. In other words, this type of steel has very good hardenability in relation to the Cr content, which will be described later.
The weld heat-affected zone hardens significantly, causing cold cracking during welding. Therefore, in order to completely perform welding, preheating to a considerably high temperature is required, and as a result, welding workability is significantly impaired. However, if C is kept at 0.12% or less, the maximum hardness of the weld heat affected zone will decrease and weld cracking can be easily prevented, so the upper limit was set at 0.12%. Regarding the lower limit, since it becomes difficult to ensure creep rupture strength if the C content is less than 0.03%, the lower limit was set at 0.03%. Mn is a necessary component not only for deoxidizing but also for maintaining strength. The upper limit was set at 1.5% because it is undesirable from the viewpoint of toughness if it exceeds this, and the lower limit was set at 0.1% as the minimum amount necessary for deoxidation. Cr is an essential element for oxidation resistance and is always added to heat-resistant steel, increasing high-temperature strength through fine precipitation of M ₂₃ C ₆ and M ₆ C (where M refers to a metal element). However, the lower limit was set at 8.0%, where precipitation hardening was noticeable, and the upper limit was set at 13.0% from the viewpoint of weldability and toughness. W is an element that significantly increases high-temperature strength by solid solution strengthening and suppressing coarsening by dissolving in carbides, and is particularly effective in strengthening for long periods of time at temperatures exceeding 600°C. The effect tends to increase rapidly after reaching 1.8%, but the lower limit was set at 2.0%, where the effect is stably large. Also, if it exceeds 3%, it will impair weldability and oxidation resistance, so the upper limit should be set.
It was set at 3.0%. Like W, V is an element that significantly increases the high-temperature strength of steel, whether dissolved in the base material or precipitated as a precipitate.
Especially in the case of precipitation, V ₄ C ₃ as well as M ₂₃ C ₆ ,
Substitutes a part of M ₆ C and shows a remarkable effect on suppressing coarsening of precipitates. However, at around 600℃, SUS347
Less than 0.05% is insufficient to achieve creep rupture strength that exceeds that of stainless steel, and exceeding 0.30% causes a decrease in strength, so the upper limit is set at 0.30%.
%, with a lower limit of 0.05%. Nb increases high temperature strength through precipitation of Nb (CN), but initial fine dispersed precipitation also follows.
It also contributes to long-term creep strength by finely controlling the precipitation state of M ₂₃ C ₆ , M ₆ C, etc. If the amount is less than 0.02%, it will not be effective, and if it exceeds 0.12%, it will cause coarsening of the agglomeration and reduce the strength.
The upper limit was set to 0.12% and the lower limit was set to 0.02%. Note that the amount of V+Nb is 0.15 from the perspective of creep strength.
% to 0.35% is preferred. N is an element that dissolves in solid solution in the matrix or precipitates as nitrides and carbonitrides and increases creep rupture strength, but if it is less than 0.02%, the strength will drop rapidly, and if it exceeds 0.10%, blowholes will occur during casting. Since this causes problems such as difficulty in forming a sound steel ingot, the upper limit was set at 0.10% and the lower limit was set at 0.02%. On the other hand, Si is an element originally added for deoxidation, but from a material standpoint, it is an element that has an adverse effect on toughness. Therefore, when we investigated the effect on toughness, we found that
It was found that heating embrittlement is reduced when the content is kept below 0.25%. Therefore, the Si content is limited to 0.25% or less. Note that the preferable range is 0.10% or less. Furthermore, the lower limit was set at 0.01% because deoxidation, etc., is insufficient and a product with a healthy internal quality cannot be obtained below this value. Although Mo has the same effect as W and is effective in increasing high-temperature strength, it is less effective in suppressing carbide refinement and coarsening than W. However, in the range of W2.0% or more, the synergistic effect of (W+Mo) increases, so they are added simultaneously. FIG. 1 shows the influence of the Mo content on the creep rupture life when the W content is different, and it can be seen that the strength-improving effect of adding a small amount of Mo is particularly large when the W content is high. However, if the amount is too large, weldability and oxidation resistance will be adversely affected. Therefore, the upper limit of the Mo content is 0.1%, at which point a substantial effect on weldability begins to appear when the W content is high.
less than Further, the lower limit was set at 0.02%, at which the effect on creep rupture life appears when the W content is high. Further, in the present invention, B can be further contained for the purpose of increasing creep strength. B is well known as an element that significantly improves hardenability, but as mentioned above, creep strength is significantly improved by adding a small amount of B. If the amount is less than 0.001%, it has almost no effect, and if it exceeds 0.008%, hot workability and weldability are impaired, so the upper limit was set at 0.008% and the lower limit was set at more than 0.001%. Furthermore, depending on the melting history, impurities may be present in the steel.
Although it may contain 0.3% or less of Ni and Co,
This does not impair the properties of the steel of the present invention. (Example) Next, the effects of the present invention will be described in more detail with reference to Examples. Table 1 shows the chemical composition of the test steel, 650℃, 18Kgf/
The creep rupture time and elongation at break under mm ² conditions, the crack stop temperature in the diagonal Y-shaped weld cracking test that indicates weldability, the impact value after aging at 600°C for 1000 hours, and room temperature tensile properties are shown. Among those shown in Table 1, No. 8, No. 10 to 11, and No. 16 to 19 steels are the steels of the present invention, and the others are comparative steels. Among the comparative steels, No. 1 steel is 2 1/4Cr-1Mo steel, which is usually used as a low-alloy heat-resistant steel.
Steel No. 2 is an alloy steel tube for boiler heat exchangers that has further improved high-temperature corrosion resistance, but has low creep rupture strength. No. 3 is a steel type currently used for superheater tubes and reheater tubes in coal-fired boilers in Germany and Europe, but it has significantly higher C content than the steel of the present invention, making it easier to weld and form. There are some difficulties. No.4～
In steel No. 6, the amount of W is below the lower limit, and sufficient creep rupture strength cannot be ensured. Although the W content of No. 7 and No. 9 steels is within the appropriate range, the Mo content is below the lower limit of the appropriate range, and sufficient creep rupture strength cannot be ensured. No.12 steel is the
The amount of Mo exceeds the upper limit of its suitable range, and weldability is poor. Steel No. 13 has a W content that exceeds the upper limit of its suitable range, and its toughness is significantly reduced after long-term use at high temperatures. Steels No. 14 and No. 15 have C content exceeding the lower limit and upper limit, respectively, and have low creep rupture strength or poor weldability. On the other hand, the steel of the present invention is considerably superior to Comparative Steel Nos. 1 to 3, which are existing ferrite-based boiler steel pipe steels, and can be used at considerably higher temperatures at the same stress level. Furthermore, the toughness is equivalent to that of the existing X20CrMoV121 steel (comparative steel No. 3), and there is virtually no problem at all. In addition, No.
16~18 steel respectively 0.25% Ni and 0.26 as impurities
%Co and 0.15%Ni+0.14%Co, but it is comparable in properties to other invented steels. In addition, the steel of the present invention has a weldability of 2 1/4Cr.
It is similar to -1Mo steel and is extremely easy to use.

【table】

[Table] (Note) ○: Comparative steel (effects of the invention) As described above, the steel of the present invention has achieved an increase in high-temperature strength that can cope with higher temperatures and higher pressures in equipment compared to conventional ferrite-based boiler steel pipe steels. It is a steel with excellent practical properties such as weldability and toughness, making it an extremely valuable contribution to industry.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the influence of Mo content on creep rupture life at 650°C and 18 Kgf/mm ² .

Claims (1)

[Claims] 1. C0.03 to 0.12%, Mn 0.1 to 1.5% by weight,
Cr8.0~13.0%, W2.0~3.0%, V0.05~0.30%,
Nb0.02~0.12%, N0.02~0.10%, Mo0.02% or more
A high-strength ferritic boiler steel pipe steel containing less than 0.1% Si, limited to 0.01-0.25% Si, and the remainder consisting of Fe and unavoidable impurities. 2 C0.03-0.12%, Mn0.1-1.5% by weight,
Cr8.0~13.0%, W2.0~3.0%, V0.05~0.30%,
Nb0.02~0.12%, N0.02~0.10%, B>0.001%
Contains 0.008% or less, Mo 0.02% or more and less than 0.1%,
A high-strength ferritic boiler steel pipe steel, characterized in that Si is limited to 0.01 to 0.25%, and the remainder consists of Fe and unavoidable impurities.