JPH08333657A - Heat resistant cast steel and its production - Google Patents

Abstract

PURPOSE: To produce a heat resistant cast steel excellent in ductility-toughness and high temp. strength, particularly, in high temp. creep fracture strength and suitable for the ultrahigh critical pressure turbine casing of a thermal power plant or the like and to provide a method for producing the same. CONSTITUTION: A heat resistant cast steel having a compsn. contg. 0.05 to 0.15% C, 0.10 to 1.50% Mn, <=1.0% Ni, 9.0 to 13.0% Cr, 0.5 to 1.5% Mo, 0.1 to 0.3% V, 0.005 to 0.10% N, 0.1 to 5.0% W, 0.1 to 5.0% Co and 0.001 to 0.022% B and, if desired, contg. one or more kinds among 0.01 to 0.2% Nb, 0.01 to 0.2% Ta and <=0.1% Ti is refined by out-of-furnace refining and is thereafter subjected to casting production by a sand mold. This cast steel is subjected to annealing, normalizing and tempering heat treatment at prescribed temps. It is excellent in high temp. strength, ductility-toughness and high temp. Creep rupture strength, its use in a thermal power plant in which temp. and pressure are made higher is made possible, and the high efficiency and high outputting of a power plant are made possible.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to heat-resistant cast steel used for turbine casings, valves and the like in thermal power plants and a method for manufacturing the same.

[0002]

2. Description of the Related Art In recent years, in large-scale thermal power plants, development of turbines used at ultra-supercritical pressure has been advanced in order to increase output. As materials for turbine casings, flanges, valves, etc. used under such ultra-supercritical pressure, cast steel products for steam turbines are used, and these cast steel products have high temperature characteristics so that they can withstand harsh operating environments. Not only is it excellent in toughness, but it is also required to have high toughness and little deterioration over time. Conventionally, from such a viewpoint, as a cast steel product used for the above-mentioned application, 12Cr
-Mo-V-Nb-N cast steel and 12Cr-Mo-V-Nb
-N-W cast steel or the like is used.

[0003]

However, since the creep rupture strength of the conventional heat-resistant cast steel is not sufficient with the high temperature and high pressure of the steam temperature, the creep rupture strength is higher, and the ductility is good and the temperature is high. Development of 12Cr heat-resistant cast steel which is also excellent in strength is desired. The present invention has been made in view of the above circumstances, and a novel 12 having excellent ductility and high temperature strength, and particularly excellent high temperature creep rupture strength.
It is an object of the present invention to provide a Cr-based heat-resistant cast steel and a method for producing the same.

[0004]

In order to solve the above problems, the heat-resistant cast steel of the first invention of the present invention is
C: 0.05 to 0.15%, Mn: 0.10 to 1.
50%, Ni: 1.0% or less, Cr: 9.0 to 13.0
%, Mo: 0.5 to 1.5%, V: 0.1 to 0.3%,
N: 0.005 to 0.10%, W: 0.1 to 5.0%,
Co: 0.1-5.0%, B: 0.001-0.022
%, With the balance being Fe and unavoidable impurities. The heat-resistant cast steel according to the second aspect of the invention has the composition of the first aspect of the present invention, further containing Nb: 0.01 to 0.2 by weight.
%, Ta: 0.01 to 0.2%, and Ti: 0.1% or less. The third invention is
An alloy raw material having the target composition of the heat-resistant cast steel according to any one of the first invention and the second invention is melted in an electric furnace, refined by outside furnace refining, and then cast into a sand mold. . 4th invention is 1000-after the said cast molding.
It is characterized in that it is annealed at 1150 ° C., standardized by heating to 1000 to 1200 ° C. and forced cooling, then tempered at 500 to 700 ° C., and secondly tempered at 700 to 780 ° C. To do.

[0005]

The function of the present invention will be described below together with the reasons for limiting each component. C: 0.05 to 0.15% C combines with a carbide-forming element to form a carbide and improves the high temperature strength, but if it is less than 0.05%, the strength is insufficient, while 0.15% %, The carbides coarsen and the high temperature properties deteriorate, so the range is 0.05-
It was set to 0.15%. For the same reason, the lower limit is 0.0
It is desirable that the upper limit is 9% and the upper limit is 0.13%.

Mn: 0.10 to 1.50% Mn is an element used as a deoxidizing agent together with Si, and it is necessary to contain 0.10% or more to obtain a sufficient deoxidizing effect. However, if the content exceeds 1.50%, the toughness is impaired, so the content is limited to 0.10 to 1.50%. For the same reason, the lower limit is 0.45% and the upper limit is 0.70%.
Is desirable. Ni: 1.0% or less Ni is an element that improves hardenability and also improves FATT and toughness. It is desirable to contain 0.25% or more, but if it exceeds 1.0%, high temperature creep occurs. Since the strength decreases, the allowable range is set to 1.0% or less. In addition, desirably, 0.30 to 0.70% is contained. On the other hand, Ni is not added mainly for the purpose of improving the high temperature characteristics. In this case, considering that it is inevitably mixed from the raw material, less than 0.25% is allowed as an impurity.

Cr: 9.0 to 13.0% Cr is a basic alloy component that enhances hardenability and high temperature strength in this steel type, and is required to be 9.0% or more.
If the content exceeds 0%, δ-ferrite crystallizes to deteriorate high temperature properties and notch toughness, so the upper limit is 1
It was set to 3.0%. For the same reason, the lower limit is preferably 9.5 and the upper limit is 10.5%. Mo: 0.5 to 1.5% Mo is required to be 0.5% or more in order to enhance the temper softening resistance and improve the high temperature strength, but even if it is contained in excess of 1.5%, No further effect can be expected, and harmful δ-ferrite is generated to lower the creep rupture strength, so the content is limited to 0.5 to 1.5%. For the same reason, it is desirable to set the lower limit to 0.65% and the upper limit to 0.95%.

V: 0.1 to 0.3% V has the effect of forming stable carbides and improving the creep strength, but if it is less than 0.1%, it has no effect, while V is 0.3%. If it is contained in excess, the ductility decreases, so the content is limited to 0.1 to 0.3%. For the same reason, it is desirable to set the lower limit to 0.15% and the upper limit to 0.25%. N: 0.005 to 0.10% N not only strengthens the matrix, but also coexists with Mo and effectively acts to improve the creep strength. Its content is 0.
If it is less than 005%, the effect is not recognized, and it is 0.1
Since blowholes are generated when the content exceeds 0%, the content thereof is set to 0.005 to 0.10%. For the same reason, the lower limit is 0.01% and the upper limit is 0.06%.
Is desirable.

W: 0.1 to 5.0% W is contained in order to improve high temperature strength.
If it is less than 1%, the effect is not obtained, while if it exceeds 5.0%, the segregation tendency increases and the ductility and toughness decrease, so the content is limited to 0.1 to 5.0%. For the same reason, it is desirable to set the lower limit to 1.5% and the upper limit to 3.5%. Co: 0.1-5.0% Co is contained in order to improve the impact properties by suppressing the precipitation of δ ferrite, and to improve the creep rupture strength. However, if less than 0.1%, there is no effect, and if more than 5.0% is added, the effect is saturated, so the content was limited to 0.1% to 5.0%. For the same reason, it is desirable to set the lower limit to 1.5% and the upper limit to 3.5%.

B: 0.001 to 0.022% B contains a small amount to increase hardenability, improve toughness, and suppress precipitation and agglomeration of carbides at grain boundaries and grains.
Contributes to high temperature creep strength. However, its content is
If it is less than 0.001%, the above effect is insufficient. On the other hand, if it exceeds 0.022%, the high temperature creep ductility is remarkably deteriorated, and the weldability is further deteriorated. Therefore, the content is limited to 0.001 to 0.022%. For the same reason, it is desirable to set the lower limit to 0.002% and the upper limit to 0.015%. Furthermore, 0.003 to 0.007% is more desirable.

Nb: 0.01 to 0.2% Nb forms fine carbonitrides and improves high temperature strength, so Nb is contained as a selective component. However, if the content is less than 0.01%, there is no effect, and if it exceeds 0.2%, carbonitrides increase and the ductility decreases, so the range is set to 0.01 to 0.2%. did. For the same reason, it is desirable to set the lower limit to 0.03% and the upper limit to 0.12%. Ta: 0.01 to 0.2% Ta is contained as a selective component because it precipitates fine carbides and improves high temperature strength. However, if the content is less than 0.01%, there is no effect, and if it exceeds 0.2%, the carbides increase and the ductility decreases, so the range was made 0.01 to 0.2%. For the same reason, it is desirable to set the lower limit to 0.03% and the upper limit to 0.12%. Ti: 0.1% or less Ti is one of the deoxidizers, and forms a carbide or a nitride to improve the high temperature characteristics, so that it is contained as a selective component. However, if the content exceeds 0.1%, a large amount of inclusions are generated and the ductility is reduced, so the upper limit is set to 0.
It was set to 1%. For the same reason, it is desirable to set the upper limit to 0.05%.

Others Si is unavoidably contained because it is used as a deoxidizer. However, if its content is reduced, macro segregation,
In particular, the reverse V segregation becomes slight, and the non-uniformity of ductility and notch toughness inside the wall thickness is improved. Further, if the Si content is high, the temper embrittlement susceptibility becomes extremely high, and the notch toughness is impaired. Therefore, it is desirable that the Si content is low. However, since it is an element used as a deoxidizing agent and its upper limit is extremely low, the manufacturing margin is small and it is not practical, so less than 0.20% is allowed as an impurity.

Since the casing or the like targeted in the heat-resistant cast steel of the present invention has a large casting weight of about 10 to 150 tons (the product weight is 5 to 50 tons), it is necessary to produce a cast steel having good internal quality. Advanced steelmaking and casting techniques are required. The heat-resistant cast steel according to the present invention is obtained by melting an alloy material in an electric furnace, refining it in a furnace outside the furnace, sufficiently performing degassing, and using a sand mold to positively and directionally solidify by casting. A sound cast steel with few casting defects can be manufactured, and a material suitable for the above-mentioned large-sized casing and the like can be obtained.

In addition, the heat-resistant cast steel that has been cast-formed is 100
By annealing at 0 to 1150 ° C., heating to 1000 to 1200 ° C., forced cooling, normalizing at 500 to 700 ° C., and subsequently performing second stage tempering at 700 to 780 ° C. High creep rupture strength can be secured. The annealing and normalizing temperature must be 1000 ° C. or higher in order to form a solid solution of carbonitride and decompose δ-ferrite, but if it is too high, coarsening of crystal grains and retransformation into δ-ferrite occur. Since it occurs, the upper limit temperature was set to 1150 ° C or 1200 ° C. Further, by performing the tempering twice, the retained austenite is completely decomposed, a uniform martensite structure is obtained, and further carbonitrides are finely precipitated to improve the creep rupture strength.

The steel of the present invention may be structurally welded, if necessary.
Welding such as repair welding can be performed. For example, after the series of heat treatments described above, welding is performed, and then 650 ° C to 76 ° C.
Perform stress relief annealing at 0 ° C. The welding can be performed during the above series of steps, that is, after annealing and before normalization, and thereafter, according to the above steps, normalization, tempering, and second-stage tempering are performed. Is done. In this case, the stress relief annealing described above is unnecessary. Further, in this step (including a welding step in the middle of heat treatment), it is also possible to further perform welding after the second-stage tempering, and after the welding, the above-mentioned stress is applied. Perform removal annealing. As described above, when the welding process is included in the middle of the heat treatment process, the normalization and tempering similar to the above are performed on the structural welded part and the repaired welded part, so that high creep also occurs at the welded part. The breaking strength and good toughness can be secured.

[0016]

【Example】

(Example 1) Alloys having the compositions shown in Tables 1 and 2 (Examples and Comparative Examples) were melted in a vacuum melting furnace and casted in a sand mold. After subjecting these test materials to a predetermined heat treatment, their mechanical properties and weldability were evaluated, and the results are shown in Table 3. The weldability was evaluated by the flat plate 1 shown in FIG. 1 (280 mm long × 100 mm wide ×
(Thickness of 30 mm) is manufactured, and 3 passes are welded to the plate surface with a predetermined welding rod, and then 5
The cross section was examined for cracks. This result is B
Fig. 2 shows a summary of the relationship with the content. As is clear from Table 3 and FIG. 2, it was revealed that the material of the present invention is excellent in creep rupture strength and weldability.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

[Table 3]

(Embodiment 2) An alloy raw material for which the heat-resistant cast steel of the present invention is achieved as a target is melted in an electric furnace, and after composition adjustment by outside-furnace refining, degassing and the like are performed, and casting is performed by a sand mold, and casting is performed. A model casing having a weight of 20 tons (product weight: about 9 tons) was manufactured. This cast steel was held at 1070 ° C. for 20 hours, then annealed for furnace cooling, held at 1070 ° C. for 10 hours, then normalized for forced cooling, and further subjected to first-stage tempering at 570 ° C. for 8 hours.
Air-cooled after holding for a time, and then 740 as the second stage tempering
After holding at 16 ° C. for 16 hours, the furnace was cooled. When the mechanical properties of this model casing were evaluated, the creep rupture strength was 60.
12 kgf / mm ² at 10 ° C. for 10 ⁵ hours, 10 at 625 ° C.
^It was possible to secure 9 kgf / mm ² and FATT 60 ° C. in ⁵ hours.

[0021]

As described above, according to the heat-resistant cast steel of the present invention, C: 0.05 to 0.15% and Mn:% by weight.
0.10 to 1.50%, Ni: 1.0% or less, Cr:
9.0-13.0%, Mo: 0.5-1.5%, V:
0.1-0.3%, N: 0.005-0.10%, W:
0.1-5.0%, Co: 0.1-5.0%, B: 0.
001-0.022%, and if desired, N
b: 0.01 to 0.2%, Ta: 0.01 to 0.2%,
Ti: 0.1% or less of at least one is contained, and the balance is composed of Fe and unavoidable impurities, so that it has excellent ductility and high-temperature strength, particularly improved creep rupture strength, and excellent weldability. Due to this characteristic, it can be used in a thermal power plant where the temperature and pressure are higher, and it is possible to contribute to higher efficiency and higher output in the power plant.

When the above ingot is manufactured, the alloy raw material is melted in an electric furnace, refined by out-of-furnace refining, and then cast into a sand mold to form a cast steel with few casting defects and good internal quality. Can be manufactured, and a material suitable for a large casing or the like can be provided.

The heat-resistant cast steel cast by the above process is annealed at 1000 to 1150 ° C., and 1000 to 1
Normalize by heating to 200 ° C and forced cooling, then 50
High creep rupture strength can be secured if tempering is performed at 0 to 700 ° C and then second stage tempering is performed at 700 to 780 ° C.
The toughness can also be improved.

[Brief description of drawings]

FIG. 1 is a schematic view showing a cutting position in a welding evaluation test piece.

FIG. 2 is a graph showing the relationship between the B content and the weld cracking rate.

[Explanation of symbols]

 1 Flat plate 2 Weld beads

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoichi Tsuda 2-4 Suehiro-cho, Tsurumi-ku, Yokohama-shi, Kanagawa Toshiba Heavy Electric Engineering Laboratory Co., Ltd.

Claims (4)

[Claims] 1. C: 0.05 to 0.15% by weight,
Mn: 0.10 to 1.50%, Ni: 1.0% or less, C
r: 9.0 to 13.0%, Mo: 0.5 to 1.5%,
V: 0.1 to 0.3%, N: 0.005 to 0.10%,
W: 0.1-5.0%, Co: 0.1-5.0%, B:
Heat-resistant cast steel containing 0.001-0.022% with the balance Fe and unavoidable impurities 2. The composition according to claim 1, further comprising% by weight.
, Nb: 0.01 to 0.2%, Ta: 0.01 to 0.
Heat-resistant cast steel containing 1% or more of 2% and Ti: 0.1% or less 3. An alloy raw material having the target composition of the heat-resistant cast steel according to claim 1 or 2 is melted in an electric furnace, refined by outside furnace refining, and then cast into a sand mold. Of heat-resistant cast steel characterized by 4. After the cast molding according to claim 3, 100
Annealing at 0 to 1150 ° C., normalizing by heating to 1000 to 1200 ° C. and forced cooling, then tempering at 500 to 700 ° C., and then second stage tempering at 700 to 780 ° C. Characteristic heat-resistant cast steel manufacturing method