JPS63145750A - Low alloy steel for turborotor - Google Patents

Abstract

PURPOSE:To obtain a low alloy steel for a turborotor having increased ductility and toughness, and excellent antitemper brittleness without lowering the strength at elevated temp., by specifying the comps. consisting of C, Si, Mn, Ni, Cr, Mo, V and Fe. CONSTITUTION:The low alloy steel for the turborotor consists of, by weight, 0.10-0.35% C, <=0.05% Si, <=0.30% Mn, <=0.60% Ni, 0.5-3.0% Cr, 0.3-1.5% Mo, 0.1-0.3% V, and further at need, <=0.005% P, <=0.008% As, <=0.010% Sn, the balance Fe with inevitable impurities. Said steel has the excellent ductility, toughness and antitemper brittleness without lowering the strength, particularly creep break strength.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention relates to a low alloy steel for a turbine rotor used in a steam turbine high pressure or intermediate shaft, and particularly relates to a low alloy steel for a turbine rotor with improved tempering embrittlement resistance. This invention relates to low alloy steel for turbine rotors.

(Prior art) NiCrMoV steel, which has been widely used for steam turbine low-pressure rotors, has high susceptibility to temper embrittlement due to the Ni it contains. temperature within the embrittlement temperature range (usually 3
They are used in areas where the steam temperature is below 350°C, or they are manufactured with a reduced amount of impurity elements that cause embrittlement.

(Problem to be solved by the invention) On the other hand, Cr, which is often used in high and medium pressure rotors,
The operating temperature range of MoV tr4 is 350-560℃
Despite the fact that the embrittlement temperature range was exactly the same as the embrittlement temperature range, it was thought that temper embrittlement would not occur, and no countermeasures were taken.

However, according to destructive inspections of this type of rotor material conducted in recent years, cases have been reported in which significant temper embrittlement has occurred even in rotors for high, medium, and high-grade rotors made of low-alloy steel such as CrMoV steel.

The object of the present invention is to provide a low-alloy steel for a turbine rotor that has significantly increased ductility and toughness without reducing the strength required for the turbine rotor, particularly high-temperature strength, and also has excellent temper brittleness.

[Structure of the invention]

(Means and effects for solving the problems) The present invention has two features:
Weight ratio: C0.10-0.35%, Si0.05
% or less, Mn 0.30% or less, Nl 0.60%
Below, cr0.5-3.0%, No. 0.3-1.
5%, V0.1-0.3%, balance Fe and incidental impurities, of which Mn is 0.50% or less and incidental impurities p0. o
os% or less, As 0.008% or less, sb0.0
This is a low alloy steel for turbine rotors with a Sn content of 0.015% or less and a Sn content of 0.010% or less.

The present invention improves the chemical composition of conventionally used CrMoV low alloy steel by reducing the amount of temper embrittlement and at the same time increasing the strength and
This was done as a result of consideration to avoid reducing the creep rupture strength, and the impurity elements P, ^S, and Sb
, Mn and Si are reduced more than the Sn content is reduced, and this structure not only achieves the intended purpose but also improves the creep rupture strength, especially on the low-stress, long-time side.

Next, the reasons for limiting the composition of the low alloy steel for turbine rotors of the present invention will be described.

First, C, Cr, N0. Regarding V, the composition range of the 9th invention steel is also limited in hopes of achieving the same effect as the conventional "0 class low alloy steel". Rumors and mockery of Mengyeki
〒4 out of 9, on the other hand 9 Mn, Si,
.

In particular, Mn has conventionally been added in an amount of around 0.7% for the purpose of suppressing its desulfurization effect and remaining free S. However, by applying out-of-furnace refining, S can be reduced to 0.
．． On the other hand, conventional NiCrMoV steel is embrittled using the step cool method devised in the United States, and the embrittlement is reduced by J-factor.
: (P + 5n) (Si + Mn) A good correlation is observed when organized by 10', and using this, the CrMoV! shown in Table 1 is calculated. When evaluating I, as shown in Figure 1, a good positive correlation was observed as with NiCrMoV m.s P * When As, Sbt and Sn are suppressed to normal contents, the contents of Si and S are extremely low. It has been found that when the material has a low Mn content of 0.30% or less, good resistance to tempering and brittleness is exhibited.

By the way, Mn was thought to be an important element to improve hardenability, especially in large structural materials such as turbine rotors, to obtain uniform strength all the way to the center.
As a result of an experiment simulating a large rotor of about 1.4 mm by keeping n at 0.30% or less, the same strength as the conventional rotor was obtained up to the center, and from the perspective of hardenability, it was low MRI. However, it turned out that there was no problem. FIG. 2 shows how the hardness and particle size in the □ center area change with respect to the Mn content.

In addition, when low Mn is used, the upper shelf impact value is significantly improved when arranged at the same strength as shown in each example in Table 2, and even though the room temperature strength is slightly reduced, the impact value on the low stress side is It was confirmed that creep rupture strength was improved.

Next, Si was conventionally added for deoxidation.

Applying vacuum casting and vacuum carbon deoxidation eliminates the need for this, and also forms non-metallic inclusions that improve toughness and
It is desirable to reduce the content as much as possible without impairing ductility. However, with the current refining technology, *-tt inevitably remains, so 0. OS% or less.

S is also similar to SL, and has the effect of particularly impairing creep ductility. 0.0 less than the scouring level possible with outside furnace scouring
04% or less.

Regarding Ni, it is considered desirable to reduce it because it has the effect of promoting embrittlement like Mn, but 0
．． When it is set to 6Q% or less, tMrt is set to 0.5% or less. Almost no embrittlement is observed due to C and Nl, and a certain amount of C content is required in balance with C to ensure hardenability, so the content is set to 0.60% or less.

Finally, since P, Sn, Sb, and ^S are elements that directly contribute to embrittlement, it is desirable to reduce them as much as possible.

However, other elements except P cannot be removed by scouring. According to experiments conducted by the inventors.

When the Mn content is 0.30% or less, no embrittlement is observed even if these elements are contained in normal amounts, and the Mn content is 0.30% or less.
In the case of 30 to 0.50%, P 0.005% or less,
As 0.008% or less, sb 0. It has been found that no embrittlement is observed when the Sn content is 0.010% or less.

(Examples) The present invention will be more easily understood from the experimental results of Examples and Comparative Examples described below.

Each of the CrMoV steels having the compositions shown in Table 1 was melted in an induction heating furnace in an Ar atmosphere, heated to 1100°C for forged, held at 960°C for 5 hours, and then heated to 120°C/H.
After cooling at 670°C for 10 hours and air cooling, Comparative Examples 1 to 3. Each test piece of Examples 1 to 4 was produced.

The mechanical properties of the test pieces thus obtained are shown in Table 2.

In Table 2, tempering embrittlement is indicated by ΔF' A TT, and this value is F A TT after the above heat treatment.
From the difference between the FATT value and the FATT value after treatment by the step cool method, it can be determined how much embrittlement has occurred due to the increase in the FATT value due to the step cool method.

(Margin below) Step cool υmi is achieved through a series of heat treatments shown in Figure 3.

After being held at each temperature for a specified time, it is cooled in the furnace and finally heated to 315℃.
It is furnace-cooled to a temperature of 100% and then water-cooled and quenched.

Regarding this ΔFATT, all comparative examples and examples are plotted in FIG. 1 in the case of CrMoV steel.

As is clear from the values in Table 2, Examples 1 to 4 according to the present invention have slightly lower tensile strength than the comparative examples, but are excellent in elongation reduction, and furthermore, FATT, Hi part Ja? The impact toughness expressed by the tl@ value is extremely excellent. In addition, in particular, ΔFATT after step cool method was determined in Examples 1 to 4.
It is understood that the low alloy steel of the present invention has excellent resistance to temper embrittlement. Regarding the creep rupture strength, Examples 1 to 4 were approximately equal to or higher than those of the comparative example, and it was found that there were no problems.

〔Effect of the invention〕

Since the present invention is characterized by a low Mn content, 9 According to the present invention, it is possible to obtain a low alloy steel for a turbine rotor that has improved resistance to temper embrittlement without reducing strength. .

[Brief explanation of the drawing]

Figure 1 is a diagram showing the correlation between J-4actor and ΔFATT, Figure 2 is a diagram showing the relationship between Mn content in the center of the rotor and Vickers hardness, and Figure 3 is an outline of the step school method. It is a schematic diagram. Agent Patent Attorney Yudo Nori Chika Hirofumi Mitsumata, Tsuji fo-ctor ((Mn+5j)X (P
+5A) x /Q') Figure 1 Q 0. / 0.2 0.3 0
．． 4 0.5μ M/I Figure 2

Claims (2)

[Claims] (1) Weight ratio: C0.10-0.35%, Si0.0
5% or less, Mn 0.30% or less, Ni 0.60% or less,
Cr0.5-3.0%, Mo0.3-1.5%, V0.
Low alloy steel for turbine rotors consisting of 1 to 0.3%, balance Fe and incidental impurities (2) Weight ratio: C0.10-0.35%, Si0.0
5% or less, Mn 0.50% or less, Ni 0.60% or less,
Cr0.5-3.0%, Mo0.3-1.5%, V0.
1 to 0.3%, P 0.005% or less, As 0.008%
Low alloy steel for turbine rotor consisting of Sb 0.0015% or less, Sn 0.010% or less, balance Fe and incidental impurities.