JPS62170461A - Heat resistant steel - Google Patents

Abstract

PURPOSE:To provide high creep rupture strength and high toughness at room temp. to a Cr-Ni-Mo-V-Nb-N steel by adding a very small amount of a rare earth element or Ca to the steel. CONSTITUTION:The composition of a steel is composed of, by weight, 0.05-0.25% C, <=0.3% Si, <=1.5% Mn, 1.0-2.5% Ni, 8-13% Cr, 1.0-2.5% Mo, 0.05-0.35% V, 0.02-0.20% Nb and/or Ta, 0.01-0.10% N, at least one between <=0.3% rare earth element and <=0.01% Ca and the balance Fe with inevitable impurities, and a total tempered martensite structure is practically formed as the structure of the steel. Thus, a heat resistant steel having >=50kg/mm<2> creep rupture strength after the lapse of 10<5>hr at 450 deg.C and >4kg-m absorption energy in a V notch Charpy impact test at 25 deg.C can be obtd. The heat resistant steel is suitable for use as a material for a high-speed rotating body such as a gap turbine disk.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention has high creep rupture strength at 450°C to 500°C and high toughness at room temperature.

The present invention relates to a heat-resistant steel suitable for rotating bodies used at temperatures below 500°C.

[Background of the invention]

The rotating bodies (wheels, rotor shafts, blades) of gas turbines and steam turbines have 12Cr-2,5Ni-1,8M o -0, 35V at temperatures below 450°C.
Steel (M 152 steel) becomes 12Cr-I at 500℃ or higher.
Mo-IW-0,25V steel (Crucible 422
steel) is widely used and has achieved good results.

Recently, gas turbines have been trending toward higher temperatures and higher speeds, and the conditions for using high-temperature components have become stricter. Gas turbine wheel materials are required to be high-strength, high-toughness, heat-resistant steels with a creep rupture strength of 50 kg/+nm'' or higher at 450° C. for 105 hours and a room temperature V notch Charpy impact value of 3 kg-m or higher.

Conventionally, lcrlMoo, 25V steel has been widely used for gas turbine wheels and steam turbine rotors.
This 450℃, 105h creep rupture strength is 29~
39 kg/mm", which is insufficient in high-temperature strength as a high-temperature/high-speed gas turbine wheel material. Also, even with the existing 12Cr heat-resistant steel mentioned above.

12Cr-IMo-IW-0,25V steel lacks creep rupture strength and toughness, and 12Cr-2,5Ni-
1,8Mo-0,35Vm has sufficient toughness but insufficient creep rupture strength.

[Purpose of the invention]

The object of the present invention is to solve the conventional I Cr -I M o -0
, 25V steel than low alloy steel and 12Cr alloy steel.

The object of the present invention is to provide a material having high creep rupture strength at 450°C to 500°C and high toughness at room temperature.

[Summary of the Invention] The high-strength, high-toughness steel that has a creep rupture strength of 50 kg/a++++" or more at 450° C. and 10"h creep rupture strength of 50 kg/a++++" or more and a room temperature V notched pea shock absorption energy of 4 kg-m or more, which is the object of the present invention, is 12Cr-Ni- Mo-V-Nb-N
It has been experimentally shown that this can be achieved by containing 0.3% or less of rare earth elements or 0.01% or less of calcium in steel.

The heat treatment of the steel of the present invention involves first uniformly heating the steel to a temperature sufficient to transform into complete austenite, a minimum of 1000°C and a maximum of 1100°C, and then 100°C to obtain a martensitic structure.
/h or more, then heated and held at a temperature of 450°C to 600°C for primary tempering, then 550°C to
Second tempering is performed by heating and maintaining at a temperature of 650°C.

Next, the required values of the present invention will be described. Saturation failure is one of the causes of failure of high-speed rotating bodies. Many researchers believe that materials with high V-notch Charpy impact absorption energy are superior in resistance to saturation fracture. Therefore, a V-notch pea impact test was used to evaluate the safety of the material of the present invention against saturation fracture. The most important factor for high-temperature rotating bodies is creep rupture strength, and the design allowable stress is often determined by the 100,000-hour creep rupture strength. Therefore, the high-temperature strength of the present invention material was evaluated by evaluating its creep rupture strength for 100,000 hours at 450"C, which is the metal temperature of high-temperature and high-speed gas turbine wheel materials.
Creep rupture strength was determined by the commonly used Larson-Miller method.

The reason for limiting the range of components of the material of the present invention will be explained.

C is at least 0.05% to obtain high tensile strength and yield strength.
is necessary. However, if the content is too large, the metal structure becomes unstable when exposed to high temperatures for a long period of time, reducing the creep rupture strength, so it must be kept at 0.25% or less. Most preferred is 0.09-0.13%.

Si is added as a deoxidizing and desulfurizing agent during steel melting, and even a small amount is effective. Si is a δ ferrite-forming element, and addition of a large amount of δ reduces fatigue and toughness.
Since it causes the formation of ferrite, it must be kept at 0.3% or less. but.

According to carbon vacuum deoxidation method, electroslag melting method, etc., it is not necessary to add Si. Most preferably, Si is 0.07% or less.

Adding a large amount of Mn lowers high temperature strength, so add 1.5%.
Must be as follows. Particularly preferred is 0.5 to 0.9%.

Cr increases corrosion resistance and high-temperature strength, but when added in an amount of 13% or more, it causes the formation of a δ-ferrite structure.

If it is less than 8%, corrosion resistance and high temperature strength are insufficient.
It was determined to be ~13%. Particularly preferred is 10.5 to 11.5%.

Mo increases the creep rupture strength through solid solution and precipitation strengthening effects, and at the same time has the effect of preventing pore formation.

If it is less than 1.0%, the creep rupture strength improving effect is insufficient, and if it is more than 2.5%, it will cause the formation of δ ferrite, so it was limited to 1.0 to 2.5%.

Particularly preferred is 1.45 to 1.75%.

■ and Nb precipitate carbides and nitrides to increase high temperature strength and have the effect of improving toughness #VO005% and Nb
If it is less than 0.02%, the effect is insufficient, and VoO,
35% and 0.20% or more of Nb causes the formation of δ ferrite and the creep rupture strength tends to decrease. Especially Vo, 15-0.20%, Nb0.04
~0.08% is preferred.

Ni increases toughness and has the effect of preventing the formation of δ ferrite, but if it is less than 1.0%, the effect is not sufficient.
A content of 2.5% or more lowers creep rupture strength.6 Particularly preferred is 1.4 to 1.6%.

Like Nb, Ta has the effect of improving creep rupture strength and toughness. If it is less than 0.02%, the effect is insufficient, and if it is more than 0.20%, it causes the formation of δ ferrite.

In general, there is a contradictory phenomenon in which increasing toughness lowers creep rupture strength, but the present inventors added appropriate amounts of rare earth elements and/or calcium to 12Cr alloy steel to create a substantially fully tempered martensitic structure. It has been found that creep rupture strength and toughness can be simultaneously improved.

The present invention was obtained based on such findings, and includes at least one of rare earth elements of 0.3% by weight or less and calcium of 0.01% by weight or less, and substantially entirely tempered marten. It is a 12Cr alloy steel with excellent toughness and high-temperature strength consisting of a site structure.

In particular, by adding rare earth elements or calcium to the above-mentioned alloy steel, a heat-resistant steel with excellent creep rupture strength and toughness required as a gas turbine disk material can be obtained.

When 0.3% by weight or less of a rare earth element and 0.01% by weight or less of calcium are added to the 12Gr alloy steel as described above, the toughness and creep rupture strength are increased. As the rare earth element content increases, toughness and creep rupture strength improve, but the effect is saturated even if it exceeds 0.3% by weight, so it is necessary to keep it below 0.3% by weight from the economic point of view. Calcium also has the same effect as rare earth elements, but the effect is saturated even if the amount of calcium added exceeds 0.01% by weight. Furthermore, in the present invention, toughness and creep rupture strength are also improved by the combined addition of rare earth elements and calcium.

[Embodiments of the invention]

10 kg of each tissue sample shown in Table 1 was dissolved, and t
It was heated to 20°C and forged to be used as an experimental material. This material was subjected to heat treatment as shown in Table 1. Creep rupture, tensile and V-notch Charpy impact test specimens were collected from this material and tested.

In Table 1, fragrances 2 to 5 are steels of the present invention.

Flavors 1, 6 and 7 are comparative steels. Comparative Steel 6 is a material used in gas turbine disks and steam turbine blades. Comparison [7] is Crucible 422 steel, which is most widely used as a 12Cr heat-resistant steel. To the perfumers 2 and 3, 0.3% of Mitshu metal (a rare earth element alloy whose main component is La-Ce) was added before casting.

Perfume 4 is an example in which Ca is added, and Perfume 5 is an example in which Ca and rare earth elements are added in combination.

Table 2 shows the mechanical test results for these samples.

First, looking at the results of the currently used materials (Fragrance 6 and 7), Flavor 6 has high impact absorption energy but low creep rupture strength. Fuka 7 has both impact absorption energy and creep rupture strength,
The strength required for high-temperature and high-speed gas turbine disk materials (
450℃, 105h strength) 50 kg/mm2. Room temperature shock absorption energy > 3 kg-I m)
lower. Comparative material No. 1, which was previously developed by the inventors, satisfies the strength required as a high-temperature/high-speed gas turbine disk material, but has low impact absorption energy strength.

In contrast, the present invention materials 2 to 5 were heated at 450 to 500°C;
10sh ri IJ-j rupture? It was confirmed that it has a high absorbed energy and fully satisfies the strength and toughness required as a material for high-temperature and high-speed gas turbines.

〔Effect of the invention〕

The 450-500°C creep rupture strength and impact absorption energy of the steel of the present invention are extremely excellent.

Claims (1)

[Claims] 1. Weight ratio of 0.05 to 0.25% C, 0.3% or less Si, 1.5% or less Mn, 1.0 to 2.5% Ni
, 8-13% Cr, 1.0-2.5% Mo, 0.0
5-0.35% V, 0.02% 0.20% Nb, T
either or both of a, 0.01 to 0.10% N, at least one of rare earth elements 0.3% or less and Ca 0.01% or less, the balance consisting of Fe and inevitable impurities, 4
50℃, 10^5h creep rupture strength is 50kg/mm
A heat-resistant steel having a V-notch mechanical impact absorption energy exceeding 4 kg-m at 25°C.