JP2001011533A - Heat treatment of heat resistant steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the application of high quenching temp. at refining time and a high temp. strength and fine crystal grains by applying a pre-heat treatment at lower temp. than the quenching temp. at the refining time before the refining. SOLUTION: The kind of steel to be the objective is a martensitic high Cr heat resistant steel containing >=8 wt.% Cr together with one or more kinds of Nb, Ta, V, and particularly, super large forged steel product. Such the forged steel product is uniformly heated at 1,100-1,200 deg.C and forged and slowly cooled after forging, and annealed into pearlitic structure. Then, successively, the pre-heat treatment is applied. This treatment comprises cooling to not higher than the Mf point after heating to 1,000-1,050 deg.C and holding the temp., and the cooling contains rapid-cooling, air-cooling and slow-cooling. Thereafter, the refining consisting of quenching and tempering is executed. In the refining quenching, the rapid-cooling is applied after heating to a higher temp. than the pre-treatment temp. and holding the temp. In the rapid-cooling, the air- cooling is contained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method for heat-resistant steel, and more particularly to a method for heat treatment of a large forged high Cr steel for which high-temperature strength is required and it is difficult to obtain fine crystal grains.

[0002]

2. Description of the Related Art Large-sized forged steel products, such as turbine rotors and gas turbine disks, are required to have high-temperature strength. It is known that in order to achieve high-temperature strength, the higher the quenching temperature during refining (the process of “tempering” following “quenching” to control the final properties of the material), the higher it is. However, when the quenching temperature is increased, the crystal grain size tends to increase. When the crystal grain size becomes large, it causes a crack in the product or a deterioration in transmittance at the time of ultrasonic inspection. Therefore, in order to eliminate such a defect, it is necessary to refine the crystal grains of the heat-resistant steel.

[0003] As a heat treatment technique for such a large forged steel product, a specialized technical magazine, "Heat Treatment 18-4" (August 1978
Monthly), pp. 203 to 211, as examples of power generation components, a high pressure turbine shaft (1% Cr-1% Mo-V steel), a low pressure turbine shaft, and a generator shaft (3% Ni-Mo-V, 3% Ni). -Cr-Mo-V steel) is described (hereinafter referred to as "prior art 1"). The prior art 1 is a low Cr steel having a Cr content of 5.5% or less, and the preliminary heat treatment temperature is higher than the tempering temperature.

[0004] Japanese Patent Application Laid-Open No. 8-1766671 discloses a heat treatment of a Cr steel containing V or Nb (hereinafter referred to as "prior art 2"). In the prior art 2, in the heat treatment of “assuming the central part of the high-pressure part”,
The normalizing temperature after the pearlitizing treatment in the pre-heat treatment is set lower than the quenching temperature in the heat treatment. However, this Cr steel is a low Cr steel having a content of 3% or less. Further, the technique disclosed in Japanese Patent Publication No. 61-48567 discloses a technique for heat-treating a coarse acicular ferrite structure material of a low alloy steel large cast and forged steel product to an austenitizing temperature range.
Prior to austenitization, in a ferrite region, heating and holding were performed at a predetermined temperature for a predetermined time, and thereafter, crystal grains at the time of austenitizing heat treatment were refined (hereinafter, referred to as “austenitic”).
"Prior art 3").

[0005] That is, "Prior Art 3" relates to a heat treatment method for a large cast and forged low alloy steel containing V and is 1200 ° C.
Is heated to 550 to 650 ° C. in a ferrite region, maintained at this temperature, and heated at 30 ° C./hr after the heating and holding. By heating to 900 ° C. at a high speed to austenitize, the temperature was kept for 2 hours and then cooled to room temperature, whereby the γ crystal grains were refined. On the other hand, as a high Cr steel having a Cr content of 10% or more, the one described in Japanese Patent Application Laid-Open No. 7-26329 is known (hereinafter referred to as “prior art 4”). Such high Cr steels
It is suitable as a member requiring high-temperature strength such as a turbine rotor.

[0006]

However, the above-mentioned prior arts 1 to 3 relate to a low Cr steel having a Cr content of 5.5% or less, and cannot be applied to a high Cr steel as shown in the above prior art 4. Was something. That is, in the above-mentioned prior arts 1 to 3, the quenching temperature at the time of tempering was relatively low at 970 ° C., and the high-temperature strength required for high Cr steel could not be obtained. Therefore, an object of the present invention is to provide a heat treatment method for a high Cr heat resistant steel that can apply a high quenching temperature at the time of refining, obtain high-temperature strength, and can achieve the refinement of crystal grains. .

[0007]

Means for Solving the Problems The present inventors have proposed Nb,
In the heat treatment of a high Cr heat resistant steel containing at least one or more of Ta and V, it has been found that if a conventional method is used, it is difficult to refine crystal grains, and a heat treatment specific to the steel type is required. That is, the steel type of the prior art has a bainite structure. In the repetition of γ-formation (normalization) from bainite, during γ-formation, new γ grains precipitate around the triple point of the old γ grain boundaries and the grain boundaries. It is presumed that the target steel type of the present invention is martensite, whereas when gamma-forming from martensite, the new γ grains are difficult to refine because the old γ grains are easily grown as they are, while the steel type is martensite. Therefore, as a result of intensive studies, as a method of suppressing the crystal grain growth even at a high tempering quenching temperature, the temperature is sufficiently small at a temperature equal to or higher than the gamma temperature (Ac3) and lower than the tempering quenching temperature before the tempering quenching. By forming crystal grains (former γ grains) and precipitating fine Nb, Ta or V carbonitrides (hereinafter simply referred to as “carbonitrides”) at the grain boundaries,
It has been found that even if the tempering quenching temperature in the next step is high, it is possible to stop the crystal grains from growing slightly due to the pinning effect of the carbonitride.

That is, the feature of the present invention is that
In a heat treatment method for a high Cr heat resistant steel containing at least one of b, Ta, and V, preliminary heat treatment is performed at a temperature lower than the quenching temperature at the time of tempering before tempering. In addition,
“Tempering” in the present invention refers to “quenching” and “tempering”, which are final heat treatments of a product. The preliminary heat treatment,
The heating is carried out at a temperature not lower than the Ac3 transformation point, and thereafter, it is cooled to a temperature not higher than the Mf point. The Ac3 transformation is one of the allotropic transformations of iron, and refers to the change from α-iron to γ-iron during heating, and the Mf point refers to the temperature at which the transformation from austenite to martensite is completed during cooling.

Further, the high Cr steel preferably has a Cr content of 8% by weight or more and has a martensitic structure. In order to refine the crystal grains after tempering, it is necessary to make the crystal grains before tempering and quenching sufficiently small. Because the mass effect is large in ultra-large forged steel, the crystal grains are not sufficiently refined before tempering, so even if tempering is performed in that state, the crystal grains in the final product will be sufficiently fine. Granulation cannot be achieved. In the present invention, before the tempering, the preliminary heat treatment is performed at a temperature lower than the quenching temperature during the tempering, so that the crystal grains can be refined before the tempering. Therefore, the crystal grains are refined, and the refining is performed in a state where fine nitrides effective for pinning are precipitated at the crystal grain boundaries, so that the crystal grains of the final product can be refined.

[0010] Even when the tempering quenching temperature is set to a high temperature, high-temperature strength and crystal grain refinement are achieved at the same time.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION
b, Ta, V
The present invention is applied to a martensitic high Cr heat resistant steel having an r content of 8% by weight or more, and particularly used for an ultra-large forged steel product such as a turbine rotor or a gas turbine disk. Such a large forged steel product has a temperature of 1100 ° C to 120 ° C.
The steel is uniformly heated to 0 ° C., forged, and after being forged, gradually cooled and annealed to a pearlite structure.

Then, the "preliminary heat treatment" of the present invention is continued.
Is applied. In this treatment, the substrate is heated to 1000 ° C. to 1050 ° C. and then cooled to the Mf point or lower. This cooling includes rapid cooling, air cooling, and slow cooling. Thereafter, if necessary, after tempering, machining or the like is performed. After that, tempering of quenching and tempering is performed. The temper hardening is rapidly cooled after heating and holding at a temperature higher than the preliminary heat treatment temperature. This rapid cooling includes air cooling. Thereafter, tempering is performed one or more times at 550 to 700 ° C.

In the steel type of the present invention, carbonitrides are finely precipitated at the grain boundaries before tempering and quenching. Pinning works effectively and growth of crystal grain boundaries is suppressed. The reason for setting the quenching temperature before tempering lower than the quenching temperature during tempering is that crystal grains before tempering quenching must be sufficiently small because crystal grains grow further during tempering quenching. Because there is.

[0014]

EXAMPLES Table 1 shows the chemical composition of a martensitic high Cr heat resistant steel requiring high temperature strength. Steel types A, C, and D are steels according to the present invention, and steel type B is a comparative steel. That is, steel types A, C, and D contain V and Nb, and steel type B does not contain any of V, Nb, and Ta. Since Ta is an element of the same family as Nb, N
The content of b is regarded as the content of Ta.

[0015]

[Table 1]

[0016] Steels A and B having the components shown in Table 1 were heated to 1100 ° C, processed (forged), and then heat-treated under the conditions shown in Table 2.

[0017]

[Table 2]

In Table 2, “Test Nos.” 1 to 5 are comparative examples.
The grain size numbers for steel types A and B when tempering was performed in the first and second stages after quenching at a temperature changed from 00 ° C to 1090 ° C. "Test number" 6-1
0 is the quenching temperature of the preliminary heat treatment of 1000 ° C. to 1090 ° C.
After performing the preliminary heat treatment while changing the tempering quenching temperature to 1
The grain size number was determined for steel types A and B when tempering quenching was performed at a constant temperature of 070 ° C., and then the first and second tempers were performed.

Test Nos. 11 to 13 were carried out by setting the quenching temperature of the pre-heat treatment to a constant value of 1025 ° C., and then changing the temper quenching temperature to 1050 ° C. to 1090 ° C. After the first and second tempering, the grain size numbers of the steel types A and B were determined. FIGS. 1 to 3 show the results of Table 2 above. FIG. 1 shows the relationship between the temper quenching temperature and the crystal grain size in Test Nos. 1 to 5 and 11 to 13 for steel type A. From the results shown in this figure, it can be seen that crystal grains are finer when the preliminary heat treatment is performed (black circles) than when the preliminary heat treatment is not performed (white circles).

FIG. 2 is the same as FIG. 1 for steel type B, and the effect of the preliminary heat treatment is not recognized for steel type B. That is, in the steel type containing no V, Nb and Ta, there is no relation between the presence or absence of the preliminary heat treatment and the refinement of the crystal grains. 1 and 2, it can be seen that under the same conditions, the steel type A has finer crystal grains than the steel type B. FIG. 3 illustrates the results of Test Nos. 6 to 10, and for steel type A, when the quenching temperature of the preliminary heat treatment is lower than the tempering quenching temperature (in the case of 1070 ° C. or less), The effect of micronization is remarkably recognized.
No effect is observed for steel type B.

From the above results, it was found that in the case of steel type A, before the tempering, the preliminary heat treatment was performed at a temperature lower than the quenching temperature at the time of the tempering, whereby the crystal grains were clearly refined.
In addition, the same result as the steel type A was obtained also in the steel types C and D. Note that the present invention is not limited to the above embodiment.

[0022]

According to the present invention, in a high Cr heat resistant steel containing at least one kind of Nb, Ta and V, the crystal grains can be refined without lowering the tempering quenching temperature, so that the high temperature strength can be improved. Can be obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between temper quenching temperature and crystal grain size for steel type A.

FIG. 2 is a graph showing the relationship between temper quenching temperature and grain size for steel type B.

FIG. 3 is a graph showing a relationship between a quenching temperature before tempering and a crystal grain size.

Claims (3)

[Claims] 1. A heat treatment method for a high Cr heat resistant steel containing at least one of Nb, Ta, and V, wherein a preliminary heat treatment is performed at a temperature lower than a quenching temperature at the time of the tempering before the tempering. Heat treatment method for heat resistant steel. 2. The heat treatment method for heat-resistant steel according to claim 1, wherein the preliminary heat treatment is performed by heating and maintaining the temperature at a temperature equal to or higher than the Ac3 transformation point and then cooling the temperature to a temperature equal to or lower than the Mf point. 3. The high Cr steel has a Cr content of 8% by weight.
The heat treatment method for heat-resistant steel according to claim 1 or 2, wherein the heat treatment steel has a martensite structure.