JPH11106860A - Ferritic heat resistant steel excellent in creep characteristic in heat-affected zone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the segregation of Nb and W in a steel by allowing it to have a compsn. composed of specified amounts of C, N, Mn, Ni, Cr, Mo, W, V and Nb, in which the amounts of Mo and W are also regulated to specified ratios, and the balance Fe with inevitable impurities. SOLUTION: A ferritic heat resistant steel having a compsn. contg., by mass, 0.05 to 0.15% C, <=0.015% (including 0%) N, 0.05 to 0.5% Mn, <=0.5% (including 0%) Ni, 7.0 to 10.0% Cr, <=0.7% (including 0%) Mo, 1.0 to 2.5% W, 0.1 to 0.6%. V and 0.01 to 0.05% Nb, in which the amounts of Mo and W simultaneously satisfy 0.7% h(%Mo)+0.5×(%W)<=2.0 and (%W)/(%Mo)>=3.5, and the balance Fe with inevitable impurity elements is prepd. Furthermore, >=0.01% B and 0.1% Si may be added thereto according to necessary. In this way, the ferritic heat resistant steel having proof stress, tensile strength and absorbed energy at a room temp. and at about 550 deg.C can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant ferritic steel having high creep strength and improved creep characteristics of a heat-affected zone of a weld, and particularly to a reactor for a chemical industry and a pressure vessel for petroleum refining. The present invention relates to a ferritic heat-resistant steel used for a high temperature member such as a reactor and a rotor.

[0002]

2. Description of the Related Art Currently, 2.25 mass% (hereinafter, "mass%" is referred to as "%") of Cr-1% Mo steel and 3% Cr- 1%
Mo steel is mainly used, but its upper limit temperature is about 500 ° C. In order to increase operating temperature and pressure,
There is a demand for strength characteristics at even higher temperatures. 50
As a heat-resistant steel usable at 0 ° C. or higher, 9% Cr-based steel has begun to be practically used for steam turbines and boiler tubes. As the 9% Cr heat-resistant steel, US A
A387 Fr. 91 steel (9% Cr-1% M
o-0.2% V-0.1% Nb steel) is known.

Further, the demand for high-temperature strength has increased, and in order to improve the creep strength, a Cr-based ferrite heat-resistant steel to which various alloys have been added using 9% Cr-based ferritic heat-resistant steel as a basic steel has been developed. For example, improvement of creep strength by addition of W (see JP-A-58-17820), improvement of creep strength by addition of Mo and W (JP-A-62-60845, Japanese Patent Publication No. 3-6090)
No. ₅ , Japanese Patent Laid-Open No. 6-65690), and improvement in creep strength by dispersion strengthening of Ta ₂ O ₅ (see Japanese Patent Application Laid-Open No. 6-65690) has been proposed.

[0004] In particular, the ferritic heat-resistant steel disclosed in Japanese Patent Publication No. 3-60905 is a steel having improved weldability as well as creep strength. In this ferritic heat-resistant steel, the addition of C is reduced to improve weldability, and to improve the creep strength, a composite addition of Mo, W, and Nb is performed. Additions are being made.

[0005]

However, the ferritic heat-resistant steel disclosed in Japanese Patent Publication No. 3-60905 has excellent weldability along with the creep strength of the base metal, but does not consider the creep strength of the heat affected zone. In some cases, the creep strength of the heat affected zone is reduced. Normally, the material properties of the heat affected zone are lower than those of the base metal, so that the performance of a high-temperature member having a weld is often affected by the creep strength of the heat affected zone. For high temperature members, it is important to improve the creep strength of the heat affected zone. Further, Japanese Patent Application Laid-Open Nos. 58-17820 and 6-65690 do not consider the creep strength of the heat affected zone, and similarly, the creep strength of the heat affected zone may decrease. is there.

In addition, large steel ingots having a slow cooling rate during solidification are used.
In the case of manufacturing, the above-mentioned Japanese Patent Publication No. 3-60905
In some cases, segregation of Nb or the like may occur.
In the case of Japanese Patent Application Laid-Open No. 8-17820, a large atomic weight W
When the amount of addition is large, weight segregation of W is likely to occur,
In addition, in the case of JP-A-6-65690, Ta _Two
O_FiveMay be difficult to uniformly disperse.

The present invention has been made in view of the above-mentioned problems, and has excellent creep strength of a heat affected zone at a temperature of 500 ° C.
It is an object of the present invention to provide a ferritic heat-resistant steel having strength, toughness and creep strength (base material) from the conventional 9% Cr-based steel (A387 Fr.91 steel based on US ASTM) at the above temperature. is there. Still another object of the present invention is to provide a heat-resistant ferritic steel capable of preventing Nb segregation and W weight segregation during casting of a large ingot.

[0008]

Means for Solving the Problems The inventors of the present invention have been concerned with improving the creep strength (weld heat affected zone and base metal) of heat-resistant ferritic steel and Nb and W for increasing the creep strength when casting large ingots. We conducted intensive research on prevention of segregation of Nb and W. As a result, not only the creep strength of the base metal but also the creep strength of the welded heat-affected zone can be improved by optimizing the amounts and ratios of Mo and W added and further reducing the amount of N added. Further, the present inventors have found that by defining the upper limits of Nb and W, it is possible to prevent segregation of these elements during casting, and have completed the present invention.

That is, the Mo equivalent “(% Mo) +0.5
× (% W) ”in the range of 0.75 to 2.0%, and further, by setting the (% W) / (% Mo) ratio to 3.5, preferably 4.0 or more. During the process, a small amount of Fe ₂ W (intermetallic compound) precipitates in the matrix of the heat-resistant ferritic steel, and this Fe ₂ W maintains the creep strength of the weld heat-affected zone and the base material of the heat-resistant ferritic steel,
It has been found that it contributes to improvement. Further, when the addition amount of N exceeds 0.015%, a new finding is obtained that the crystal grain size of the welded heat-affected zone becomes finer and the creep strength of the welded heat-affected zone is reduced. It has been found that the creep strength of the heat affected zone can be improved by setting the content to 015% or less. Further, the Nb content is 0.06% or less,
It has been found that by setting the W content to 2.5% or less, Nb segregation in a large ingot and weight segregation of W can be suppressed, and cracking during subsequent forging can be prevented.

In the present invention, C: 0.05 to 0.15 by mass% (hereinafter, indicated by%).
%, N: 0.015% or less (including 0%), Mn: 0.
0.5 to 0.5%, Ni: 0.5% or less (including 0%),
Cr: 7.0 to 10.0%, Mo: 0.7% or less (0%
), W: 1.0-2.5%, V: 0.1-0.6.
%, Nb: 0.01 to 0.05%, and Mo
And the amount of W is 0.7% ≦ (% Mo) + 0.5 × (% W)
≦ 2.0% and (% W) / (% Mo) ≧ 3.5 at the same time, with the balance being Fe and unavoidable impurity elements. The reasons for limiting each component are shown below.

(A) C: 0.05 to 0.15% In the present invention, the C content is reduced as much as possible in order to improve the weldability. In order to have a martensitic structure, C is 0.0
5% or more is required. On the other hand, if the C content exceeds 0.15%, the hardness of the heat-affected zone of the weld increases, causing cracks and deteriorating weldability. Therefore, the C content is set to 0.05 to 0.15%.

(B) N: 0.015% or less As in the case of C, the amount of N is reduced as much as possible in order to improve the weldability. However, N is dissolved in a matrix, nitrided or carbonitrided. As the weld heat affected zone and the base material creep strength (hereinafter, unless otherwise specified, simply
This is called "creep strength". ),
It is preferable to add N by 0.005% or more. However, when the N content exceeds 0.015%, the crystal grain size of the weld heat affected zone becomes fine, and the creep strength of the weld heat affected zone decreases. As a result, the N amount is set to 0.015% or less.

(C) Mo equivalent “(% Mo) + 0.5 ×
(% W) ": 0.75 to 2.0% W and Mo are elements that significantly increase high-temperature strength by solid solution strengthening. To obtain sufficient creep strength by solid solution strengthening, Mo equivalent: (% Mo) ) + 0.5 × (% W) needs to be 0.75% or more. On the other hand, when the Mo equivalent exceeds 2%, the amount of the intermetallic compound is increased and the toughness is reduced, and also has an adverse effect on oxidation resistance. Therefore, the Mo equivalent is 0.75 to 2%.

(D) (% W) / (% Mo) ratio: 3.5 or more By setting the (% W) / (% Mo) ratio to 3.5 or more, the creep strength is remarkably improved. This is because, by increasing the proportion of W, a small amount of Fe ₂
This is because the precipitation of W (intermetallic compound) maintains and improves the creep strength.

(E) W: 1 to 2.5% W is an element having a large atomic weight. If a large amount of W is added, weight segregation during casting tends to occur. . Also, to improve the creep strength,
The amount of W must be 1% or more.

(F) Mo: 0.71% or less (including 0%) (d) Mo equivalent specification and (e) (% W) / (% M)
o) From the regulation of the ratio, the amount of Mo is 0.71% or less,
The amount of Mo may be 0.4% or less, furthermore, only W may be added and Mo may not be added.

(G) Nb: 0.01-0.05% Nb is an element that precipitates as carbides and nitrides and controls the dispersion state of carbonitrides that precipitate later, thereby significantly increasing creep strength. . If the Nb content is less than 0.01%, this effect is small, and sufficient creep strength cannot be obtained. On the other hand, when Nb is added in excess of 0.05%, particularly in the case of large steel ingots, C, N and N added to the steel ingot are added.
Carbon nitride and the like due to b generate Nb segregation, and cracks are likely to occur during subsequent forging. Also, in the present invention, since the amounts of C and N are set low from the viewpoint of welding resistance, even if Nb is added in an amount exceeding 0.06%, NbC or NbN is not added during the solution treatment for quenching. Cannot be dissolved sufficiently, and further improvement in creep strength cannot be expected.
For this reason, the Nb content is set to 0.01 to 0.05%.

(H) Cr: 7 to 10% Cr is an important element for securing oxidation resistance and corrosion resistance. However, if the Cr content exceeds 10%, delta ferrite precipitates and high-temperature characteristics deteriorate. I do. If the Cr content is less than 7%, the creep strength, particularly the creep strength of the weld heat affected zone, cannot be sufficiently obtained.

(I) Ni: 0.5% or less (including 0%) Ni is necessary to prevent the delta ferrite phase and to improve the toughness at room temperature. Insufficient strength can be obtained.

(M) Mn: 0.05 to 0.5% Mn is an element necessary for ensuring deoxidation and hardenability, and Mn needs to be 0.05% or more. On the other hand, Mn
If the amount exceeds 0.5%, the creep strength is reduced.

(V) V: 0.1-0.6% V is dissolved in a matrix to increase the high-temperature strength,
Precipitates as carbides and nitrides to improve high-temperature strength. However, if the V content is less than 0.1%, a sufficient effect cannot be obtained.
% There is no difference in the effect of addition. Therefore, the V amount is 0.1
To 0.6%.

A second aspect of the present invention is characterized in that the composition of the first aspect of the present invention contains 0.01% or less of B. A small amount of B is further added to the ferritic heat-resistant steel according to claim 1 (preferably, B is added to 0.1%).
001% or more and 0.01% or less).
It can improve the hardenability of ferritic heat-resistant steel and improve the creep strength. If B is added in excess of 0.01%, the hot workability is significantly reduced, so the upper limit of the B content was made 0.01%.

[0023] Further, the invention according to claim 3 provides the invention according to claim 1.
Alternatively, the composition of the invention described in Item 2 contains 0.1% by mass or less of Si. The addition of Si (preferably, 0.04% or more and 0.1% or less of Si) to the ferritic heat-resistant steel according to claim 1 or 2 makes it possible to remove the ferritic heat-resistant steel during casting. Perform acid and improve oxidation resistance of heat resistant ferritic steel. In order to improve deoxidation and oxidation resistance during casting, Si
The amount needs to be 0.04% or more. On the other hand, when the amount of Si is 0.1
%, The intermetallic compound (Laves phase) is increased in the heat-resistant ferritic steel to lower the toughness.

According to a fourth aspect of the present invention, there is provided a ferritic heat-resistant steel excellent in creep characteristics of a weld heat-affected zone according to the first, second or third aspect of the invention. It is characterized by being used for a high-temperature member. The effect is exhibited by using the ferritic heat-resistant steel of the present invention for a large-sized high-temperature member such as a reactor or a rotor. That is, the ferritic heat-resistant steel of the present invention has a high creep strength not only in the base material but also in the weld heat-affected zone.
It improves the high-temperature characteristics of high-temperature components that require welding, such as rotors, and provides stable and excellent performance. Furthermore, when the present invention is used for large-sized high-temperature members, cracks during forging of the ingot can be prevented, and these large-sized high-temperature members can be easily manufactured.

[0025]

EXAMPLES Next, the effects of the present invention will be described more specifically with reference to examples. Example 1 investigated the effect of alloying elements on the creep strength of the weld heat affected zone and the base metal, and Example 2 investigated the effect of Nb on the segregation of the steel ingot during solidification.
And W were investigated.

Example 1 A 20 kg steel ingot was cast in a vacuum induction melting furnace to have the chemical components shown in Table 1. By observing the internal structure of these ingots, NbC and N
The presence or absence of bN and the weight segregation of W were investigated. 1 (A387 Fr.91 of ASTM standard)
No.) and No. Nb segregation was observed in 20 (Nb: 0.08%) steel ingot. However, for other ingots, Nb
And the weight segregation of W were not recognized.

[0027]

[Table 1]

Next, these ingots are heated to 1200 ° C.
A round bar having a diameter of 20 mm was formed by forging. Further, after heating these round bars at 1050 ° C. for 1 hour, air-cooling normalizing was performed, and then tempering was performed at 720-760 ° C. for 3 hours so that the hardness (Hv) became 220. Was. Hereinafter, these materials are referred to as base materials. Further, in order to reproduce the heat affected zone by welding, these samples were further heated to 1000 ° C., which is an austenitic structure, and simulated heat treatment for welding was performed. Hereinafter, these materials are referred to as HAZ reproduction materials.

Next, at room temperature and 550 ° C.
, And a Charpy impact test at -18 ° C. The results are shown in Table 2. In addition, No. 6
In the sample (Cr: 10%), the above test was not performed because delta ferrite was observed as a result of observation of the structure of the base material.

[0030]

[Table 2]

As shown in Table 2, each sample had N
o. No. 1 (ASTM A387 Fr.91 steel) was confirmed to have proof stress, tensile strength, and absorbed energy at room temperature and 550 ° C. which were at least equal to those of ASTM A387 Fr.91 steel. further,
All of the samples of the present example were Nos. It exhibited a proof stress, tensile strength and absorbed energy higher than 1, confirming that the strength and toughness suitable for a pressure vessel for petroleum refining were sufficiently satisfied.

Next, each base material was heated at 600.degree.
A creep test at 7.4 kg / mm ² was performed. on the other hand,
For HAZ reproduction material, crystal grain size measurement (JIS G
0551 along with austenitic grain size test)
A creep test was performed at 14.0 kg / mm ² at ° C.
Table 3 shows the results. No. of conventional material 1 (AST
MA 4387 Fr. From the results of the creep test of (91 steel), if the rupture time in the creep test of the base material exceeds 342 hours and the rupture time in the creep test of the HAZ reproduction material exceeds 890 hours, this conventional material is used. Level that can be applied to existing oil refining pressure vessels.

[0033]

[Table 3]

In the test materials of the examples of the present invention, the creep rupture times of the base metal and the HAZ reproduction material were all 1000 hours or more. As a result, the test material according to the example of the present invention has ASTM A4387 Fr. It was confirmed that both the base metal and the HAZ reproduction material had better creep strength than the steel No. 91 (No. 1).

Next, the effect of alloying elements on the results of the creep test will be further described. In this example, No. 1 in the component range of the present invention was used. 3 test materials (0.
1% C-7% Cr-0.35% Mo-1.3% W-0.
2% V-0.01% N steel, Mo equivalent: 1%, W / Mo
A test was conducted on the effect of alloying elements based on a ratio of about 3.8).

Regarding the effect of the amount of Cr, Explanation will be made from the results of 2 to 7. No. with a Cr content of 6% or less. HAZ of 2
The creep strength of the reproduction material is low, while the Cr content is about 11%
No. In the test material of No. 7, delta ferrite which deteriorates high temperature characteristics was precipitated. Cr content of the present invention: No. 2 in the range of 6 to 10%. Nos. 2 to 6 show excellent creep properties. The creep rupture time of the HAZ reproduction material of No. 4 (Cr: 8%) was 2000 hours or more, and it was found that the weld heat affected zone had excellent creep strength.

Regarding the influence of the amount of N, 3 and 7-
Explanation will be made from the result of No. 9. As the amount of N increases, the creep rupture time of the base material and the HAZ reproduction material tends to be shortened. The creep rupture time of the test material No. 9 was significantly reduced. Further, it was found that the crystal grain size of the HAZ reproduction material of the test material whose creep strength was reduced had a JIS grain size number of 10 or more when the N content exceeded 0.015%, and the crystal grains were fine.

Regarding the effect of the W / Mo ratio, Explanation will be made from the results of Nos. 3 and 10 to 13. As the W / Mo ratio increases, the creep rupture time of the base material and the HAZ reproduction material tends to increase. In particular, when the W / Mo ratio is 3.5 or more,
o. In No. 3, the creep rupture time of the HAZ reproduction material was 1359
No. and the W / Mo ratio was 4.0 or more. No. 13 was found to exhibit excellent creep properties, with a creep rupture time of 1729 hours or more. In addition, No. In No. 11, it is considered that the creep characteristics were deteriorated because the W amount was less than 0.7%.

Next, the Mo equivalent “(% Mo) + 0.5 ×
No. (% W) ". 3 and No. 3 14
Explanation will be made from the results of Nos. To 17. When the Mo equivalent is less than 0.7%. No. 15 has a low creep strength of the HAZ reproduction material, whereas the Mo equivalent of No. 15 exceeds 2.0%. In Table 17, Table 3
As shown in FIG. 1 (ASTM A387F
r. 91 steel). The addition amount of Mo and W is 0.7% ≦ (% Mo) +0.
5 × (% W) ≦ 2.0%, (% W) / (% Mo) ≧
No. 3.5 prepared in a range that simultaneously satisfies 3.5. 15 and No. In No. 16, good characteristics were obtained.

Regarding the influence of the amount of Nb, 3 and N
o. Explanation will be made from the results of Nos. 18 to 20. The creep rupture time of the base metal and the HAZ reproduction material tends to increase with an increase in the Nb content. 19
The creep rupture time and Nb amount of No. 20
No significant difference was found with the creep rupture time of the sample. Further, when the Nb amount exceeds 0.05%, the Nb amount is 0.1%.
No. 08%. In No. 20, Nb segregation was observed in the steel ingot as described above.

Regarding the influence of the amount of V, 21 and no.
Explanation will be made from the result of No. 22. Similar to Nb, the creep rupture time of the base metal and the HAZ reproduction material tends to increase as the V amount increases. No. 21 having a creep rupture time and a V amount of 0.8%. No significant difference from the creep rupture time of No. 22 was observed.

Regarding the influence of the B amount, 3 and No. 2
Explanation will be made from the result of No. 3. No. Nos. 3 and B were added.
In comparison with No. 23, the addition of B increases the creep rupture time of the base metal and the HAZ reproduction material, thereby improving the creep strength.

Example 2 Further, in order to investigate the influence of Nb and W on segregation of the steel ingot during solidification, Table 4 was used.
A molten metal having the chemical composition shown in Fig. 3 is melted, and the cooling rate during solidification is 5
The molten metal was cast in a mold heated at a rate of ° C./min to produce an ingot. Next, the internal structures of these ingots were observed, and the presence / absence of primary NbC and NbN and the weight segregation of W were investigated. Table 4 shows the results.

[0044]

[Table 4]

As shown in Table 4, the amount of Nb added was 0.0
No. exceeding 5% Nb with Nb carbonitride
No segregation was observed. On the other hand, when the addition amount of W exceeded 2.5%, C and No. Weight segregation of W was found in the D ingot. From this result, the maximum amount of Nb added was 0.05
% And the maximum amount of W added to 2.5%, it was found that a large steel ingot without segregation could be produced.

As explained above, the creep strength of the heat affected zone of the ferritic heat-resistant steel of the present invention is set to 9
% Cr based steel (ASTM A387 Fr.91 steel)
It has been found that it can be remarkably improved. Furthermore, the ferritic heat-resistant steel of the present invention exhibits higher proof stress, tensile strength and excellent impact properties at -18 ° C at normal temperature and 550 ° C, and higher creep strength of the base material than this 9% Cr-based steel. It became clear. Further, the heat-resistant ferritic steel of the present invention,
It has been found that segregation of Nb and W during casting of large ingots can be prevented.

The ferritic heat-resistant steel having such excellent properties can be used in a reactor such as a pressure vessel for petroleum refining and a high-temperature member such as a rotor, which require welding, to obtain a stable and excellent high-temperature material. Demonstrates excellent performance.
In addition, segregation during casting of large ingots can be prevented,
It is effective for application to large high-temperature members.

[0048]

As described above, the heat-resistant ferritic steel of the present invention can be obtained by optimizing the addition amount and the addition ratio of Mo and W, and by controlling the addition amount of N to 0.015% or less. It is possible to improve not only the creep strength of the base material but also the creep strength of the heat affected zone. Further, even if the amount of alloying elements is reduced as compared with the conventional heat-resistant steel, ASTM A387 Fr. 9
It has proof stress, tensile strength and absorbed energy at room temperature and 550 ° C., which are superior to steel No. 1. Further, there is also an effect that the weldability can be improved by reducing the amount of the alloy element.

Further, by defining the upper limits of Nb and W, segregation of Nb carbonitride and the like and weight segregation of W are prevented.
In particular, since segregation of these elements can be effectively suppressed at the time of casting of a large ingot, there is also an effect of preventing forging cracks at the time of forging of a large ingot.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Takanari Okuda 1-5-5 Takatsukadai, Nishi-ku, Kobe-shi, Hyogo Kobe Steel, Ltd. Kobe Research Institute

Claims (4)

[Claims] C: 0.05 to 0.15% by mass%,
N: 0.015% or less (including 0%), Mn: 0.05
0.5% or less, Ni: 0.5% or less (including 0%), C
r: 7.0 to 10.0%, Mo: 0.7% or less (including 0%), W: 1.0 to 2.5%, V: 0.1 to 0.6
%, Nb: 0.01 to 0.05%, and the amount of Mo and W is 0.7% ≦ (% Mo) +0.5
× (% W) ≦ 2.0%, (% W) / (% Mo) ≧ 3.
A ferritic heat-resistant steel that satisfies 5 at the same time, with the balance being Fe and inevitable impurity elements. 2. The heat resistant ferritic steel according to claim 1, further comprising B in an amount of 0.01% by mass or less. 3. The heat-resistant ferritic steel according to claim 1, further comprising 0.1% by mass or less of Si. 4. A high temperature member such as a reactor or a rotor for use in a large temperature member.
A ferritic heat-resistant steel with excellent creep characteristics in the weld heat affected zone described.