JP2000319749A - NiCrMoV STEEL AND CAST FORGED STEEL MEMBER USING THE NiCrMoV STEEL - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide NiCrMoV steel (for refining) exhibiting high strength and high toughness even without using carbide forming elements such as Nb and ta, and to provide a cast forged steel member using the NiCrMoV steel. SOLUTION: In NiCrMoV steel for refining, it is mainly formed of a bainitic structure in which the number of acicular cementite of >=1 μm length is controlled to <=0.15 pieces/μm2 on the average in the observed field. This NiCrMoV steel for refining is subjected to quenching in which heating is executed at 800 to 1000 deg.C for at least one time or quenching and normalizing for at least one time, respectively, and tempering in which holding is executed at 500 to 700 deg.C for 3 to 60 hr for at least one time, by which NiCrMoV steel exhibiting high strength and high toughness can be obtd. The NiCrMoV steel obtd. in this way is suitable as the material for a large cast forged steel member.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a tempered NiCrMoV steel exhibiting high strength and toughness by heat treatment, a tempered NiCrMoV steel, and a cast and forged steel member (for example, steam) using the above NiCrMoV steel. Turbine rotor, marine diesel engine crankshaft, etc.)
It is about.

[0002]

2. Description of the Related Art As a rotor material of a steam turbine used for thermal power generation and nuclear power generation, NiCrMoV steel having large tensile strength and good notch toughness is employed. In recent years, the amount of Ni has been increased in order to meet the demand for improvement in strength and toughness accompanying the increase in capacity of a steam turbine. However, since Ni enhances the temper embrittlement susceptibility, a technique for reducing the amount of Ni added by adding Nb, Ta, or the like to refine the γ grain size has been proposed (for example, JP-B-63-38420
JP-A-1-230723, JP-A-6-25689
No. 3). However, since Nb, Ta, and the like form carbides at high temperatures and impair forgeability, NiCrMo can be used without using such carbide-forming elements.
There has been a demand for the development of a technique capable of improving the strength and toughness of V steel.

[0003]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and exhibits high strength and high toughness without using a carbide-forming element such as Nb or Ta. It is an object of the present invention to provide a NiCrMoV steel and a cast and forged steel member using the NiCrMoV steel.

[0004]

The NiCrMoV steel for tempering according to the present invention, which has solved the above-mentioned problems, is characterized in that acicular cementite having a length of 1 μm or more has an average of 0.15 in the observation visual field.
The main object of the present invention is that the bainite structure is not more than 1 piece / μm ^2.
The oV steel is subjected to at least one quenching by heating to 800 to 1000 ° C. or at least one quenching and normalizing, and at least one tempering at 500 to 700 ° C. for 3 to 60 hours. As a result, a NiCrMoV steel exhibiting high strength and high toughness can be obtained. The NiCrMoV steel thus obtained is suitable as a material for large cast and forged steel members.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have assumed that NiCrMn is used on the assumption that carbide forming elements such as Nb and Ta are not used.
Intensive research was conducted on the relationship between the microstructure in the manufacturing process of the oV steel and the obtained characteristics. As a result, it has been found that, at any stage of the heat treatment, if a specific structure described later is obtained, the strength and toughness obtained in the subsequent heat treatment step are significantly increased. The above specific structure has an average density of 0.1 μm or more of needle-like cementite having a length of 1 μm or more observed in an arbitrary cross section.
It is a bainite structure of 15 / μm ² or less. Bainite can be classified into three types according to the cooling conditions, and as shown in Fig. 1, they are called type I, type II and type III, of which type II is called upper bainite and type III is called lower bainite. In the upper bainite, acicular cementite is formed at the lath interface, and in the lower bainite, the acicular cementite is formed inside the lath.

In the present invention, in order to obtain high strength and high toughness, an intermediate material of NiCrMoV steel (NiCrMoV for tempering) is used.
The structure suitable for steel) is a bainite structure having a small amount of large acicular cementite, and the average density of acicular cementite having a length of 1 μm or more in an observation visual field is 0.15 / μm ² , mainly in lower bainite. It satisfies the following conditions.

In obtaining the lower bainite structure according to the present invention, an austenite region (800 to 1300 ° C.)
After training or normalizing (especially 300 to 450
By lowering the cooling rate (between C and C) or reducing the coarse acicular cementite generated when the alloy is kept in the above temperature range, the balance between strength and toughness after tempering (quenching-tempering) can be obtained. It can be significantly improved over NiCrMoV steel.

[0008] If the coarse cementite density is high, the crystal grains obtained after subsequent normalizing or quenching become large,
FATT [fracture appearance transition temperatu
re fracture surface transition temperature] (toughness decreases). Coarse cementite (hereinafter sometimes referred to as coarse θ) is due to the formation of upper bainite (type II). When the upper bainite is formed, the lath structure does not collapse during normalizing or quenching heating, and the austenite generated by reverse transformation is aligned and united and grows, thereby increasing the crystal grain size. In addition, even when the finally obtained crystal grains are the same, the FATT becomes high because the difference in orientation between the finally obtained austenite grains is small, so that the effective grain size is different even if the apparent grain size is the same. It is thought to be.

In order to lower the coarse θ density, the following method may be used. FIG. 2 shows an example of the relationship between the cooling time and temperature from the Ac ₃ point and the obtained structure in the NiCrMoV steel.
Shown in Based on the graph shown in FIG.
Austenite range (800 to 13)
(00 ° C.). Especially 300
A method of controlling the cooling rate between 〜450 ° C. to 1５０50 ° C./hr is recommended. If the temperature exceeds 50 ° C./hr, it is necessary to pay attention to the fact that island martensite increases and toughness is deteriorated. Further, it is not always necessary to cool at a constant cooling rate, and a cooling pattern (for example, after holding at 600 ° C. for several tens of hours and then quenching, from the austenite region to the 250 to 280 ° C. region at a cooling speed that does not transform martensite) (Cooling pattern such as holding at ℃).

In the present invention, the composition of the NiCrMoV steel is not limited, and ordinary NiCrMoV steel may be adopted. According to the ASTM, the Cl of A469 shown in Table 1 is used.
ass6,7,8, A470 class 5,6,7, A471 class
1 to 9 can be exemplified.

[0011]

[Table 1]

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples do not limit the present invention, and any design changes based on the preceding and following gist will be described. Are included within the technical scope of

[0013]

[Example] [Production method of test material] Table 2 by vacuum induction melting
150 kg of an ingot having the composition of
After soaking for 20 hours at 0 ° C, forging and 123
The heating at 0 ° C. was repeated several times, and the forging ratio (ingot diameter / forged member diameter) 3 was processed. Then, it is heated again to 1230 ° C., and the cooling rate of 630 ° C. or less is set to 0.2 to 100 ° C. /
hr and the form of cementite produced was controlled to change the density of acicular cementite.

[Measurement Method of Acicular Cementite] A sample was corroded with a picric acid / nitric acid solution, and then subjected to a scanning electron microscope.
After photographing several fields of view in the area of about 1000 μm ² at
The number of needle-like cementite (needle-like θ) having a length of 1 μm or more was measured, and the average value was defined as the coarse θ-density.

[Measurement of Heat Treatment and Strength and Toughness] Using the above test materials, tempering was performed under heat treatment conditions shown in Table 3 (heat patterns 1 to 4 are shown in FIG. 3). FATT was measured. In Table 3, the plurality of temperatures and times in the columns of normalization, quenching, and tempering are the conditions of the preceding heat treatment process in order from the left, and the temperature and the holding time are measured not for the atmosphere but for the substance measurement. .

The results are shown in Table 3.

[0017]

[Table 2]

[0018]

[Table 3]

When the heat pattern 1 is adopted, in the example of the present invention, the proof stress is more than 710 MPa and the tensile strength is 83
Characteristics exceeding 0 MPa and FATT of -20 ° C or less were obtained. In the heat patterns 2 to 4, the FATT tends to decrease (the toughness improves) as the crystal grain becomes smaller (as the grain size number increases) by repeating normalizing and quenching. It can be seen that when the number exceeds 1 piece / μm ² , FATT becomes high (poor in toughness).

[0020]

Since the present invention is constituted as described above, NiCrMoV exhibiting high strength and high toughness (for heat treatment) without using carbide forming elements such as Nb and Ta.
It has become possible to provide a cast and forged steel member made of steel and the NiCrMoV steel.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing three types of bainite of (a) to (c).

FIG. 2 is a graph showing a relationship between a cooling condition and a structure.

FIG. 3 is an explanatory diagram showing a heat treatment method (heat pattern) employed in an example.

Claims (3)

[Claims] 1. A needle-like cementite having a length of 1 μm or more,
A NiCrMoV steel for refining, which mainly has a bainite structure having an average of 0.15 / μm ² or less in an observation visual field. 2. The NiCrMoV for refining according to claim 1.
At least one quenching performed by heating the steel to 800 to 1000 ° C. or at least one quenching and normalizing, and at least one tempering maintained at 500 to 700 ° C. for 3 to 60 hours. NiCrMoV steel characterized by the following. 3. A cast and forged steel member using the NiCrMoV steel according to claim 2.