JPH0260749B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to high-chromium steel for high temperature use, particularly Nb-added high chromium steel. For heater tubes, superheater tubes, reheater tubes, or heat exchanger tubes for general boilers, petrochemical industry, or nuclear power industry, as well as blade materials for steam turbines or gas turbines, Currently, for example, 0.2C−0.5Ni−12Cr−1Mo−
0.3V, 0.1C−12Cr−1Mo−1W−0.2~0.3V,
9-12% Cr heat-resistant steel such as 0.05C-9Cr-2Mo-0.3V is used. However, in conventional high chromium steel, C
There were many system inclusions, and the cleanliness was not satisfactory. Therefore, when used at high temperatures exceeding 600°C, sufficient high-temperature properties, particularly creep properties, have not always been obtained. By the way, in order to reduce the amount of oxygen in steel and the amount of inclusions, it is necessary to increase the amount of Sol.Al, as is already well known. However, on the other hand, this Sol.Al reduces the high temperature strength. Because there is a tendency to
The addition amount range of Al is extremely narrow, so Sol.
The method of improving cleanliness by increasing Al has not been adopted for high chromium steels so far. On the other hand, it has been a common method to add Nb to increase the high-temperature strength of 9-12% Cr steel.
In either case, Nb is added in a fairly large amount of 0.15 to 0.35% in order to obtain the required high temperature strength. For example, French EM12 steel (0.08C−9Cr−2Mo−
0.3V−0.4Nb) France Sokil T1 (0.20C−11Cr−
0.75Mo−0.5Ni−0.3V−0.4Nb) are typical high Nb-containing steels. Therefore, in order to completely dissolve a large amount of Nb, it is necessary to raise the solid solution temperature, and the undissolved Nb
If NbC carbide remains, it becomes inclusion-like.
This results in a decrease in toughness or ductility. On the other hand, in the case of JP-A-53-140217, Nb0.02~0.15
% is disclosed, but there is no mention of the cleanliness and amount of Sol.Al of the obtained steel, and the obtained creep strength is also 20Kg/mm at 600℃. At 650℃ and 12Kg/mm, it is only 575hr at most. In order to develop a high-chromium steel with high cleanliness and excellent high-temperature strength, the inventors of the present invention discovered that Sol, which had been suppressed in the past because it would harm high-temperature strength, was discovered. By proactively increasing the amount of Al to 0.01-0.07%, we improved cleanliness and prevented harmful defects, and obtained the knowledge that the decrease in high-temperature strength due to Sol.Al could be compensated for by adding Nb. Furthermore, Nb addition of 0.015% or more and less than 0.05% is sufficient to contribute to high-temperature strength, and as the cleanliness of steel improves, even with less than 0.05% addition, high-temperature strength is greater than that seen in conventional steels. The present invention was completed by discovering that this effect can be enhanced synergistically. Therefore, the gist of the present invention is C:
0.04-0.25%, Si: 1.0% or less, Mn: 0.45-1.0%,
Cr: 8~13%, Mo: 0.5~3.0%, V: 0.2~0.35
%, Sol.Al: 0.01~0.07%, Nb: 0.015% or more,
It is a high-chromium steel for high temperature use consisting of Fe with less than 0.05% and optionally Ni: 1.0% or less and/or W: 0.4 to 1.2%, the balance containing unavoidable impurities, Sol.
The aim is to maximize the synergistic effect of increasing the amount of Al added and optimizing the amount of Nb added, and at the same time optimize the composition of other alloy components, thereby synergistically improving the high-temperature properties. . Thus, according to the present invention, the amount of Sol.Al is reduced to 0.01
By significantly increasing the Nb content to ~0.07%, the cleanliness of the resulting steel was significantly improved.
When the amount of Nb added is 0.015% or more and less than 0.05%, at least sufficient high-temperature strength is ensured, and furthermore, the decrease in toughness and ductility that occurs when the amount of Nb added is increased can be prevented. The reasons for limiting the steel composition in the present invention will be explained in detail below. Al: In order to ensure high-temperature strength of steel, the amount of Sol.Al has traditionally been limited to less than 0.01% in high chromium steel for boilers, for example. For example, it was thought that increasing Sol.Al would shorten the rupture time of creep characteristics. But Sol.Al 0.01%
If it is limited to less than 100%, the deoxidation of the steel will be insufficient, and oxide-based inclusions will increase, leading to deterioration of the steel quality. Sol.
When Al is less than 0.01%, the amount of oxygen increases and the variation in the amount of oxygen increases. On the other hand, the amount of Sol.Al is 0.01% or more, preferably
When it increases to 0.02% or more, the amount of oxygen decreases, variation also decreases, and the steel quality becomes stable. However, even if the Sol.Al content increases by 0.015% or more, the oxygen content in the steel does not decrease as markedly as when Sol.Al is 0.01 to 0.015% or less, and if Sol.Al is further increased beyond 0.07% Even if it is added, the effect from the point of view of inclusions will be small. Thus, according to the present invention, 9-12% Cr
Less than 0.01% Sol.Al is insufficient for cleaning steel, while Sol.Al of 0.015% or more causes almost no change in the amount of oxygen in the steel, and even if Sol.Al exceeds 0.07%, Sol.Al If it exists, the breaking strength will be greatly reduced, so the upper limit was limited to 0.07% or less. The relationship between the Sol.Al content and the oxygen content in steel is explained in the first section using 12% Cr steel as an example.
This is shown graphically in the figure. Nb: According to the present invention, the creep rupture strength of conventional steel can be greatly improved by adding a small amount of Nb. In particular, even if the Sol.Al amount is increased to 0.07%,
High temperature strength is higher than conventional steel that does not contain Sol.Al. Therefore, by adding a small amount of Nb as in the present invention, it is possible to widen the allowable addition range of Sol.Al, and sufficient deoxidation and reduction of inclusions can be achieved. On the other hand, a sufficient amount of Sol.
For steel with increased cleanliness by adding Al,
Creep rupture strength increases from the addition of 0.015% Nb
It increases significantly up to the addition of less than 0.05, but after that
Even if 0.05% or more is added, the breaking strength does not increase very much. On the contrary, if it exceeds 0.05%, it is necessary to increase the solution temperature, which increases cost and reduces toughness. Therefore, in the present invention, the amount of Nb added is set to 0.015% or more and less than 0.05%. C: 0.04% or more of C is required to form carbides with other alloying elements and obtain the required strength, but if added in excess of 0.25%, weldability will be impaired, so in the present invention, C is not used. Limit 0.04-0.25%. Si: Si is an element necessary as a deoxidizer and to obtain a proper structure, but if added in too large a quantity, it will impair toughness and have an unfavorable effect on creep strength, so it should be kept at 1.0% or less. do. Mn: Mn is an effective element for improving hot workability and optimizing the structure at the same time.If it is less than 0.45%, sufficient effects cannot be obtained, and if it is added in excess of 1.0%, If the amount added is too large, a hard and brittle phase will be formed, so the amount of Mn added should be 0.45~
It shall be 1.0%. Cr: Cr is the most important component for obtaining the required high-temperature properties in the steel of the present invention, and in view of the purpose of the present invention, it is limited to 8 to 13% Cr. Mo: Mo is an element that increases creep strength because it dissolves in ferrite and strengthens the matrix.
If it is less than 0.5%, it has no effect, while if it exceeds 3.0%, it is unfavorable in terms of oxidation resistance and toughness. V: V precipitates as a carbide and increases high-temperature strength, but for this purpose it is necessary to add 0.2% or more, but on the other hand, even if it is added in excess of 0.35%, the effect will be saturated, and rather In the present invention, it is 0.2 to 0.35% because weldability is impaired.
Limit to V. Ni: Ni is added as necessary, but is an effective element for increasing toughness. However, if the amount added is too large, the hardness increases significantly and cold cracking during welding becomes more likely to occur. In addition, since creep strength on the long-term side decreases, the amount added is set to 1.0% or less. W: Higher creep strength is obtained by adding W, which is a desired element. W
In the case of addition, less than 0.4% W has no effect;
On the other hand, a sufficient effect can be obtained with a content of 1.2% or less, and adding more than that only increases the cost. Oxygen: The amount of oxygen in steel determines the amount of inclusions, that is, the cleanliness of steel, but in the present invention, Sol.Al is set to 0.01
Since it is present in a large amount of ~0.07%, the amount of oxygen at that time is 0.010% or less. That is, in a preferred embodiment of the present invention, the amount of oxygen in the steel is 0.010% or less, and the cleanliness is 0.2% or less. Next, the present invention will be explained in more detail with reference to Examples. Examples Various steels whose compositions are shown in Table 1 were melted in a heating furnace with a capacity of 50 kg, and the resulting ingots were forged at 1150 to 950°C to obtain round bars with a diameter of 18 mm. This round bar was then heated to 1050°C for 1 hour, air cooled, and further heated to 750°C.
℃ for 1 hour and air cooled. For steel type JKO, in addition to such heat treatment, in order to fully dissolve NbC carbide,
After further heating to 1150℃ for 1 hour, air cooling, then 750℃
℃ for 1 hour and air cooled.

[Table] Creep rupture tests and impact tests were conducted on the various test steels obtained. FIG. 2 graphically shows the results of a creep test for time to rupture at 600° C. and under a load of 15.5 Kg/mm ² . The test results are summarized in terms of Nb content and Al addition amount. In the figure, the solid line graph is for the Al content level of 0%, and the dashed-dotted line graph is for the Al content level of 0.03%.
level, and the dotted line graph is Al
The content is at the 0.05% level. Each symbol is the first
Indicates the steel symbol in the table. The horizontal dotted line at 700 hours indicates the rupture time of the highest level of conventional steel in a similar experiment. From FIG. 2, it can be seen that at any level of Al content, the rupture time is improved by adding Nb compared to conventional steel. On the other hand, as before
When adding a large amount of Nb (0.15 to 0.35%),
No difference was observed due to the difference in the amount of Al added, and the effect also tends to be saturated. Table 2 summarizes the test results regarding creep rupture strength and impact value for each sample steel. Comparing steels A, B, and C, the impact value decreases due to an increase in Nb, and excessive addition of Nb is not preferable. The impact values of steels D, E, F and steels G, H, and I, which are steels A, B, and C with a small amount of Al added, are high, indicating that Al is effective in improving toughness. Steels containing a large amount of Al (0.1%), such as O steel, have good impact values, but high temperature strength is very low.
Excessive addition of should be avoided. In particular, what is recognized from the results in Table 2 is that Al
Even with the addition of Nb, the high-temperature strength does not decrease due to the coexistence of Nb; on the contrary, the creep rupture strength is greater than that of the conventional material, and the impact value or creep strength exceeds that of the material without the addition of Al.

【table】

【table】 [Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between Sol.Al content and oxygen content in 12% Cr steel, and Figure 2 is a graph showing the relationship between Sol.Al content and oxygen content in steel.
It is a graph showing experimental data regarding the effect of the amount of Sol.Al and the amount of Nb added on the creep rupture time.

Claims (1)

[Claims] 1 C: 0.04-0.25%, Si: 1.0% or less, Mn: 0.45-1.0%, Cr: 8-13%, Mo: 0.5-3.0%, V: 0.2-0.35%, Sol. Al: 0.01~0.07%, Nb: 0.015% or more, 0.05
High chromium steel for high temperature use, consisting of less than % Fe, the balance containing unavoidable impurities. 2. The high-chromium steel for high temperature use according to claim 1, having a Sol.Al content of more than 0.02% and up to 0.07%. 3 C: 0.04-0.25%, Si: 1.0% or less, Mn: 0.45-1.0%, Cr: 8-13%, Ni: 1.0% or less, Mo: 0.5-3.0%, V: 0.2-0.35%, Sol. High-chromium steel for high temperature use, consisting of Al: 0.01 to 0.07%, Nb: 0.015% or more but less than 0.05%, and the balance Fe containing unavoidable impurities. 4. The high-chromium steel for high temperature use according to claim 3, having a Sol.Al content of more than 0.02% and up to 0.07%. 5 C: 0.04-0.25%, Si: 1.0% or less Mn: 0.45-1.0%, Cr: 8-13%, Ni: 1.0% or less, Mo: 0.5-3.0%, V: 0.2-0.35%, W: 0.4 ~1.2%, Sol.Al: 0.01~0.07%, Nb: 0.015% or more, 0.05
High chromium steel for high temperature use, consisting of less than % Fe, the balance containing unavoidable impurities. 6. The high-chromium steel for high temperature use according to claim 5, having a Sol.Al content of more than 0.02% and up to 0.07%.