JPH1088274A - High strength heat resistant steel and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a high strength heat resistant steel increased in toughness at ordinary temp., excellent in high temp. strength, and having economical efficiency by specifying a composition consisting of C, Si, Mn, Ni, Cr, Mo, V, N, Al, O, and Fe. SOLUTION: This steel is a high strength heat resistant steel which has a composition consisting of, by weight, 0.15-0.25% C, <=0.2% Si, <=1.5% Mn, <=1.5% Ni, 2.8-4.5% Cr, 0.5-2.O% Mo, 0.10-0.20% V, 0.008-0.025% N, <=0.03% Al, <=0.0030% O, and the balance Fe with inevitable impurities and further containing, if necessary, 0.5-1.5% Cu and/or 0.001-0.007% Ca and can be suitably used for disk, rotor material, etc., for thick and large-diameter gas or steam turbine. This heat resistant steel can be obtained by subjecting an ingot or slab of steel with the above composition to heating up to >=1000 deg.C and to hot forging, finishing the working at >=650 deg.C steel slab central temp., applying cooling, performing reheating one or more times at a temp. in the range between (Ac3 transformation point + 50 deg.C) and 1,100 deg.C, and then subjecting the steel to hardening and further to tempering at <=Ac1 one or more times.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant steel having high room-temperature toughness and excellent high-temperature strength, and a method for producing the same. Particularly, the invention is used for disks and rotors of power generation gas turbines and steam turbines. The present invention relates to an improved heat-resistant steel and a method for producing the same, and can also be used as a heat-resistant steel for general chemical plants.

[0002]

2. Description of the Related Art Conventionally, Cr-containing steel has been used as a steel material in this field with an emphasis on heat resistance such as tensile strength at high temperatures, resistance to creep and rupture, and oxidation resistance. Invention is provided. In particular, in the field targeted by the present invention, heat-resistant steel in which Co: 6% is added to 12% Cr-based steel is widely used, but it contains 6% of Co, which is a scarce resource, so that it is extremely expensive steel. Not get. For this reason, Japanese Patent Application Laid-Open No. 56-13466 discloses a non-Co-based 10 to 12.5% Cr steel in which the economy and toughness are improved.

[0003] In addition, a heat-resistant steel having a reduced Cr content is, for example, a 4-6% Cr steel in consideration of high temperature strength and toughness disclosed in Japanese Patent Application Laid-Open No. 2-20440.
No. 0-245772, a heat-resistant steel containing 1.8 to 2.8% Cr for a high-low pressure integrated steam turbine rotor,
There is a heat-resistant steel containing 0.4 to 1.4% Cr which considers toughness and bendability disclosed in JP-A-1-316441.

[0004]

As described above, conventional disk materials for gas turbines include 12% Cr-6% Co.
Although steel is used, improvements in economy and toughness have been strongly sought. Although there is an intention to improve conventional steels represented by the above-mentioned publications, there are drawbacks in consideration of economy and the like. That is, JP-A-56-1346
No. 6 does not contain expensive Co, and satisfies the target values of strength and toughness, but Cr is 10 to 12.5%, which is the same as the conventional one, and furthermore, firing for securing such strength and toughness is performed. The charging temperature is set as high as 1150 ° C,
As a result, there is a problem that not only the crystal grain size becomes coarse and the toughness is impaired, but also the burden on the heat treatment furnace increases, and it is difficult to remarkably improve the economic efficiency after all. Next, JP-A-2-
In the publication No. 20440, the Cr content is reduced to 4.0 to 6.0%, but as described in the literature described in the publication, high-temperature baking at 1100 ° C. or more is performed to obtain high creep rupture strength. break is required, also in addition to the burden on the heat treatment furnace is large, lower than the level of the target at most 60 kgf / mm ² grade-level intensity from restrictions on components, 12% Cr-6% It cannot be a substitute for Co steel and large high-strength rotor materials. Japanese Patent Application Laid-Open No. 60-245772 discloses a steel containing 1.8 to 2.8% Cr, which has strength, toughness, and heat resistance closest to the level intended for a rotor material, but has low strength or toughness at the center. In order to obtain the desired material, the strength-toughness balance is inappropriate. JP-A 1-3
Japanese Patent No. 16441 discloses 0.4 to 1.4% Cr steel, but the room temperature and high temperature strengths are insufficient with respect to target levels.

The present invention relates to a steel material having unprecedented economic efficiency and tough heat resistance for a disk and a rotor of a thick and large-diameter power generating gas turbine or a steam turbine, which is expected to be increasingly demanded in the future. The method of manufacture is provided. That is, specifically, without containing expensive Co, a room temperature tensile strength of 102 kgf / mm ² or more, a notch tensile strength of 15 kgf / mm ² or more.
8 kgf / mm ² or more, the load stress 53kgf / mm ² at 482 ° C. × 1
An object of the present invention is to provide a high-toughness heat-resistant steel having high-temperature properties in which the amount of plastic strain after holding for 00 hours is 0.15% or less, and a normal temperature impact value of 2.1 kgf · m or more and a transition temperature of 40 ° C. or less.
Further, as a turbine rotor material, a room temperature tensile strength of 8
5 kgf / mm ² or more, yield strength at 400 ° C (or 0.2
% Proof strength) 65kgf / mm ² or more high strength and normal temperature impact value 3.5
It is an object of the present invention to provide a tough heat-resistant steel having a toughness of not less than kgf · m and a transition temperature of not more than 40 ° C., and also having a creep resistance at a high temperature of not less than 400 ° C. and a method for producing the same.

[0006]

Means for Solving the Problems In order to achieve the above object, the present inventors have conducted a detailed study on the chemical components constituting steel and the heat treatment conditions. As a result, the Cr content and the austenitizing temperature are closely related to the strength of the steel through the quenched structure, and 1100 when the Cr content exceeds 9%.
High strength cannot be obtained unless heated to a temperature higher than ℃, in other words, when the tempering conditions are the same, the lower the Cr content is less than 9%, the higher the strength is obtained at a lower austenitizing temperature. I found that. Further, C
When the content of u is 0.5 to 1.5%, high strength can be more easily obtained, and the mass effect of an extremely thick steel product such as a change in strength due to tempering conditions can be reduced. We found an important fact to stabilize the material by granulation. In addition, when Nb is added, Nb carbon / nitride precipitates largely in the segregation zone of the steel ingot, and is not easily decomposed during subsequent forging heating or forging, and as a result, the toughness of the product is significantly reduced. It was found that it was desirable not to contain them.

The present invention has been made based on these new facts, and is based on C, Si, Mn, Cu, Ni, Cr, M
o, V, N, Al as a main component, a martensite-bainite mixed structure is formed in a target steel product by quenching heat treatment, and at the same time, V is formed on the transformation dislocation during quenching transformation.
Is characterized by finely depositing carbon / nitride, and then tempering to control the structure recovery, precipitate precipitation, consistency and amount. In this process, steel components and heat treatment conditions are regulated in order to properly control the crystal structure, grain size, and distribution, amount, and size of precipitates.

That is, the present invention has the following constitution. C: 0.15% to 0.25%, Si ≦ 0.2%, Mn ≦ 1.5%, Ni ≦ 1.5%, Cr: 2.8 to 4.5%, Mo: 0% by weight. 5 to 2.0%, V: 0.10 to 0.20%, N: 0.008 to 0.025%, Al ≦ 0.03%, O ≦ 0.0030%, if necessary, Cu: 0 0.5% to 1.5%, Ca: 0.001% to 0.007%, and the balance consists of Fe and inevitable impurities. The heat-resistant steel is used for a disc or a rotor for a gas turbine or a steam turbine, and is 100 kgf / mm ² or more or 85 kg.
It has a tensile strength at room temperature of f / mm ² or more.

Further, according to the present invention, a steel ingot or a steel slab composed of the above-mentioned steel component is heated to a temperature range of 1000 ° C. or more, and then hot forging is performed.
Finish at a temperature range of 0 ° C. or higher, and after cooling, the Ac ₃ transformation point +
Production of heat-resistant steel with high room temperature toughness and excellent high-temperature strength characterized by quenching after reheating to a temperature of 50 ° C. or more and 1100 ° C. or less once and then quenching at a temperature of Ac ₁ or less. Is the way.

[0010]

Next, the reasons for limiting the components of the present invention will be described. C is an important element for ensuring the strength at room temperature and high temperature. Especially for thick and large diameter steam for power generation
It is necessary to stably obtain a martensite-bainite mixed structure over a wide range of quenching speeds for turbines, gas turbine disks, rotors, and the like. An excessive addition causes carbide agglomeration and coarsening to cause a decrease in high-temperature strength, and also significantly deteriorates toughness, so the upper limit is made 0.25%.

Si is used as a deoxidizing agent in the steel of the present invention because the addition of Al is restricted for the reasons described below.
The upper limit is limited to 0.2% because tempering embrittlement is likely to occur during high temperature use. Like Mn, Mn is used as a deoxidizing agent, and also fixes S to increase the strength of crystal grain boundaries. However, when added in a large amount, tempering embrittlement is promoted, so the upper limit was regulated to 1.5%.

Ni has the effect of facilitating cross-slip and improving toughness. However, if added excessively, it increases the temper brittleness susceptibility. Cr forms a strong oxide film and enhances oxidation resistance, but when added in a large amount, it reduces creep rupture strength and warps at high temperature tempering to decrease the strength, so that the desired strength and toughness are secured. As the most appropriate ranges, the lower limit was 2.8% and the upper limit was 4.5%.

Mo plays an important role in forming carbides or trapping vacancies to prevent dislocation rising motion and maintain strength at high temperatures, but its effect is less than 0.5%. However, if it exceeds 2.0%, it saturates and impairs toughness, so its range is set to 0.5 to 2.0%. V is
Contributes to the strengthening of steel by forming carbides and nitrides. In particular, in the present invention, VN precipitates in the austenite (γ) region due to N added in a larger amount than ordinary steel materials, and the γ grains are refined. At the time of transformation, it precipitates at the transformation interface to increase the γ / α crystal grain conversion ratio, further refine the steel structure, improve strength and toughness, and significantly increase high temperature strength and creep rupture strength. The effect appears sufficiently at 0.05 to 0.20%,
Further addition decreases the toughness.

N is an important element for controlling the above-mentioned effects, and the amount of N is 0.0
080-0.250% is required. Al is used as a deoxidizing agent. However, if solid solution Al remains excessively, compounding with N increases and the amount of precipitation of VN, which is a feature of the steel of the present invention, is reduced. I do. O slightly forms a solid solution in steel, but most of them form oxides with Si, Al, Cr and the like, and significantly impair the toughness. In particular, in the case of a high-strength steel such as the steel of the present invention, there is a high possibility of becoming a starting point of a fatigue crack during use. 0.003 to prevent these phenomena
It is necessary to make it 0% or less.

The above are the basic components of the steel of the present invention, but one or more of Cu and Ca can be contained for the purpose of improving the strength at room temperature and high temperature, the toughness and the creep rupture strength. Cu is effective for improving room temperature and high temperature strength,
It is an important element for the purpose of obtaining particularly high strength of the present invention, and it is effective when achieving both high creep rupture strength and toughness, but the range of addition in consideration of cracking during hot working Was set to 0.5% to 1.5%. Ca has an effect as a deoxidizing agent and an effect of subdividing sulfide (MnS), improving ductility and toughness of a base material, and suppressing anisotropy. However, less than 0.001% has no effect,
If it exceeds 07%, coarse Ca sulfide oxides are generated, and ductility and toughness are reduced.

Next, the manufacturing method of the present invention will be described. The steelmaking furnace for the steel of the present invention may be any of the current melting furnaces such as an electric furnace and a converter, and is not particularly limited. In addition, degassing, ladle refining technology, and remelting and refining such as ESR and VAR can also be applied.

[0017] The steel of the present invention, which has been made into a steel ingot by ingot and ingot or ingot-bulking or continuous casting in this way, is forged, but the reheating temperature at that time is as follows. It is restricted to a temperature range of 1100 to 1300 ° C. In order to maximize the effect of adding V and Mo, which are important for securing high-temperature strength, these elements must be fully dissolved in the heating before hot forging, so the lower limit of the reheating temperature is 1100.
° C, and the upper limit is 1300 from the performance and economy of the heating furnace.
° C.

The end temperature in hot forging is 6 at the center temperature.
50 ° C. or higher. The preferred temperature range is 800 to 105
0 ° C. If the final forging temperature is lower than 650 ° C., the precipitation of VN on the introduced dislocations will increase rapidly, causing a rapid increase in strength and a decrease in hot workability, which may cause cracking. Therefore, when the processing time becomes long and the processing end temperature of the steel slab falls to 650 ° C. or lower, the steel slab is charged again into the heating furnace and reheated.

In the present invention, the quenching and tempering tempering heat treatment is defined as the basic heat treatment. The austenitizing temperature for the quenching treatment is from the Ac ₃ transformation point + 50 ° C to 1100 ° C,
Preferably, the temperature is in the range of 900 to 1050 ° C. in consideration of the heat treatment cost and the balance between normal temperature and high temperature strength and toughness.
Do more than once. In the steel of the present invention, the toughness is good but the strength is insufficient at a low quenching temperature side, and the strength is good at a high temperature side, but the toughness tends to be insufficient. Quenching is water-cooled,
Either oil cooling or forced air cooling may be used.

In the tempering treatment, the tempering temperature and time are selected so that the room-temperature tensile strength attains the target strength and toughness at a temperature of not more than _one point Ac, preferably at 600 to 700 ° C. In particular, when substituting for 12% Cr-6% Co steel, the tempering time is selected within the above temperature range so that the room temperature strength is 102 kgf / mm ² or more. As a result, a new heat-resistant steel characterized by low components, low-temperature short-time heat treatment, and high performance can be manufactured at lower cost than conventional heat-resistant steel.

Thus, according to the present invention, a room temperature tensile strength of 10
2 kgf / mm ² or more, notch tensile strength 158kgf / mm ² or more, plastic strain amount after loading stress 53kgf / mm ² at 482 ° C. × 100 hours retention has high temperature characteristics of 0.15% or less, yet cold shock A high toughness heat-resistant steel having a value of 2.1 kgf · m or more and a transition temperature of 40 ° C. or less can be obtained. Further roomtemperature tensile strength 85 kgf / mm ² or more as a turbine rotor material, 40
Yield strength at 0 ° C (or 0.2% proof stress) 65kgf / mm ²
A tough heat-resistant steel having the above high strength, normal temperature impact value of 3.5 kgf · m or more, transition temperature of 40 ° C. or less, and creep resistance at a high temperature of 400 ° C. or more can be obtained.

[0022]

Examples of the present invention will be described below. Table 1 shows the chemical components of the steel of the present invention and the steel of the comparative example. Tables 3 and 4 show the results (mechanical test characteristics) obtained by the treatment under the heat treatment conditions shown in Table 2.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

[0026]

[Table 4]

From these results, it can be understood that the steels A to J according to the present invention are all heat resistant steels having high room temperature toughness and excellent strength, which are the objects of the present invention, by selecting appropriate heat treatment conditions. . In particular, from Table 3, the load stress 53
kgf / mm plastic strain of ² at 482 ° C. × 100 hours retention has high temperature characteristics of 0.15% or less, tough heat resisting steel is obtained having excellent creep resistance at a high temperature of Table 4 from 550 ° C. You can see that.

On the other hand, the steels of K to O are those whose chemical composition and heat treatment conditions are outside the specified values of the present invention, and all of which have one or both of the strength and toughness outside the target values. The difference due to the effect of the present invention is remarkable.

[0029]

As is clear from the above embodiments,
According to the present invention, the toughness is far superior at the same strength as compared with the steel obtained by the conventional method, and a heat-resistant steel having a desired balance between strength and toughness can be produced at low cost. In particular, it is suitable for disks or rotors for gas turbines and steam turbines, and has an extremely large industrial effect.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI F01D 5/28 F01D 5/28

Claims (5)

[Claims] 1. Weight%, C: 0.15 to 0.25%, Si ≦ 0.2%, Mn ≦ 1.5%, Ni ≦ 1.5%, Cr: 2.8 to 4.5 %, Mo: 0.5 to 2.0%, V: 0.10 to 0.20%, N: 0.008 to 0.025%, Al ≦ 0.03%, O ≦ 0.0030% A heat-resistant steel having high room-temperature toughness and excellent high-temperature strength, the balance being made of Fe and inevitable impurities. 2. The composition of claim 1, further comprising one or more of Cu: 0.5% or more and 1.5% or less, and Ca: 0.001% or more and 0.007% or less. 2. The heat-resistant steel according to claim 1, which has high room temperature toughness and excellent high-temperature strength. 3. A disk for a gas turbine or a disk for a steam turbine and having a tensile strength of 10 at room temperature.
^{3. The method according} to claim 1, wherein the weight is 0 kgf / mm ² or more.
2. A heat-resistant steel having high normal-temperature toughness and excellent high-temperature strength as described in 1. 4. A tensile strength at room temperature of 85 kgf / mm used for a gas turbine rotor or a steam turbine rotor.
^{3. The} heat-resistant steel according to claim 1, wherein the heat-resistant steel has high room temperature toughness and excellent high-temperature strength. 5. A steel ingot or a steel slab comprising the steel component according to claim 1 or 2 is heated to a temperature range of 1000 ° C. or higher, and then hot forging is performed.
Finished in a temperature range of ℃ or more, and cooled, then the Ac ₃ transformation point +5
A heat-resistant steel with high room-temperature toughness and excellent high-temperature strength, characterized in that it is quenched after reheating to a temperature of 0 ° C or more and 1100 ° C or less once or more and tempered at a temperature of Ac ₁ or less. Production method.