JPS60230965A - Low alloy steel for shaft of turbine generator - Google Patents

Abstract

PURPOSE:To obtain the titled steel which is less expensive than a 3.5% Ni-Mo-V steel and has superior toughness and hardness by using an increased amount of Mn in place of part of Ni in the 3.5% Ni-Mo-V steel having a specified composition, regulating the relation among the amounts of Mn, Ni and Cr, and restricting the amounts of Si, P and Mo. CONSTITUTION:This low alloy steel consists of, by weight, 0.13-0.30% C, <=0.10% Si, 0.60-2.00% Mn, <=0.010% P, 0.40-2.00% Cr, 0.20-2.50% Ni, 0.10- 0.50% Mo, 0.05-0.15% V, 0.005-0.040% Al, 0.0050-0.0150% N (Ni+2Mn+2Cr= 4-8%) and the balance Fe. The steel is the low alloy Mn steel having a reduced Ni content, and the temper embrittlement is prevented by regulating the amounts of Mn, Ni and Cr as mentioned above and restricting the amounts of Si, P and Mo.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a low alloy steel used for a turbine generator shaft.

In general, the turbine generator shaft, i.e., the rotor shaft or turbine shaft of a turbine generator, rotates at a high speed of several thousand revolutions per minute, so it must have excellent toughness and high mechanical strength to prevent accidents during operation. It is required to have strength. Therefore, 3.5% NiM□V steel, which has excellent strength and toughness, has been used for the turbine generator shaft. This 3,5S NiMoV steel has Ni 3.2
5-3.75 To, M.

This steel contains approximately 0.0925% to 0.35%, and 0.09% to 0.15%, with the remainder being essentially Fe.However, this steel has the disadvantage of high manufacturing costs because it contains a large amount of expensive Ni. be. Therefore, there is a strong desire to develop low-alloy steel for turbine generator shafts that has strength and toughness equal to or greater than 3.5% NiMoV steel and is inexpensive.

By the way, as a way to reduce the manufacturing cost of low-alloy steel for turbine generator shafts without reducing its toughness, 3.5% NiCrMoV steel, which is a slightly different steel type from the above-mentioned 3.5 Z NiMoV steel, has been proposed. For example, Japanese Patent Publication No. 50-753 discloses a method of reducing the amount of expensive Ni by incorporating inexpensive Cu into the material, thereby reducing costs.
It is proposed by No. 0. However, in this method, since a large amount of Cu is contained, there is a problem that red heat embrittlement is likely to occur during hot forging. Further, since Cu is not such an inexpensive element, even if Ni is replaced with Cu, the cost reduction effect is small. Therefore, with this proposed method, it was difficult to sufficiently achieve the objective of reducing manufacturing costs without impairing toughness and other properties.

This invention was made against the background of the above-mentioned circumstances, and it is an object of the present invention to provide a low-alloy steel for turbine generator shafts that is sufficiently cheaper than the currently used 3.5% NiMoV steel and has superior toughness. The purpose is to

Expensive N in currently used 3.5% NiMoV steel
As a substitute element for i, it must be very inexpensive and must not deteriorate hardenability, toughness, or other properties. As an element that satisfies such conditions, Mn, which is cheaper than Cu used in the above proposal, is considered to be more promising. However, Mn is known to be an element that significantly increases the susceptibility to tempering embrittlement, and therefore, in conventional low-alloy steel for turbine generator shafts, the Mn content is generally limited to a low level. In other words, for large generator shaft materials with a diameter of 500 fins or more,
Since the cooling strength after tempering is inevitably slow and tempering embrittlement during cooling becomes a problem, Mn is usually 0.60.
% or less. However, the present inventors have conducted various experiments and studies in order to find a method that can suppress the occurrence of temper embrittlement even when the amount of cheap Mn added is significantly increased in order to reduce the expensive Nii as much as possible. As a result, Ni
Even in Mn-based low alloy steel with low Mn, Ni,
The amount of Cr is adjusted within a certain range in relation to each other, as well as Si and P.

By limiting the amount of Mo, tempering embrittlement can be prevented, and it is possible to provide a low-alloy steel suitable for turbine generator shafts that is cheaper than the 3.5% NiMoV steel used to achieve this, and has superior toughness. They discovered this and came up with this invention.

That is, the low alloy steel for turbine generator shaft of this invention is C
0.13~0.30%, SiO, IO%, Mn 0
．． 60~2.00 To, P O, 01096 or more Ts
CrO, 40~2.00 eIbSNi O, 20~
2.50%, M.

0, 10-0.50 slV 0.05-0.15%,
AJo, 005~0.040chi, NO, 0050~0
．． 0150- and Ni + 2Mn +
It is characterized in that 2Cr is adjusted to be within the range of 4 to 8-, and the remainder consists of Fe and inevitable impurities.

Below, the low alloy steel for turbine generator shafts of the present invention will be explained in more detail.

First, the reasons for limiting each component in this invention will be explained.

CO, 13-0.30chi: C is an element effective in improving hardenability and strength.
If O is less than 13%, good hardenability and predetermined strength cannot be ensured. On the other hand, if the C content exceeds 0.30 inches, the toughness will deteriorate significantly, so the C content should be 0.13 to 0.3
It must be within the range of 0%.

SiO, 1O: Si is an element that increases the susceptibility to temper embrittlement, and the steel of this invention tends to have a high susceptibility to temper embrittlement, especially since it contains large amounts of Mn and Cr. It is desirable to keep the content as low as possible. However, if Si is 0.
Since tempering embrittlement does not occur if the Si content is 10% or less, Si is specified as less than 0.10%.

Mn 0.60-2.00%: Mn is an effective element for improving hardenability and toughness, and is also an inexpensive element, and is an extremely important element in this invention. In other words, the steel of this invention is expensive N
The decrease in strength and toughness due to the decrease in the amount of i added is compensated for by increasing the addition of inexpensive Mn, and in order to obtain this effect, it is necessary to add 0.60% or more of Mn. When the amount of Mn added is increased in this way, the susceptibility to tempering embrittlement generally increases, but in the case of the steel of this invention, Mn
n, Cr.

Adjust the amount of Ni in relation to each other, and adjust the amount of P and Si.

By limiting the amount of Mo, it is possible to suppress the susceptibility to tempering embrittlement and ensure toughness even when Mn is added in an amount of 0.60% or more. However, if the Mn content exceeds 2.00%, even the steel of the present invention will not be susceptible to tempering embrittlement, and the toughness will deteriorate. Therefore, the amount of Mn is 0.60 to 2.0
It was set within the range of 0%.

po, oto% or less: Since P is an element that increases the susceptibility to tempering embrittlement, it is desirable that the P content be as low as possible, but 0.0IO'
If it is less than 1, tempering embrittlement will not occur, so o, ot
It was limited to 0% or less.

CrO, 40-2.00 To: Cr is an element that improves hardenability and strength, and is necessary together with Mn to obtain the reinforcing structure in the steel of this invention (F), Cr is less than 0.40% This effect is small. On the other hand, if Cr exceeds 2.00%, the susceptibility to tempering embrittlement increases, so Cr was limited to a range of 0.40 to 2.00%.

Mo　0.10=0.50%: Mo is an element effective in suppressing temper embrittlement.

In order to prevent tempering embrittlement that occurs during slow cooling during tempering in the steel of this invention, Mo 0.10- or more is required. However, the effect of suppressing embrittlement due to burning is 0.
Above 50 S, the addition of Mo in excess of (,0,5(l) is meaningless. Therefore, M.

was limited to a range of 0.10 to 0.501.

VO, 05-0.15%: (1) is an element that improves strength by reducing resistance to temper softening. Although it is necessary to add 0.05% or more to obtain a predetermined strength, if ■ exceeds 0.15%, the toughness deteriorates, so ■ was limited to a range of 0.05 to 0.15 inches.

AA! 0.005~0.040chi, NO,0050~0
．． 015 (1: A/ and N effectively suppress tempering embrittlement by forming A#J in steel and refining crystal grains. For this purpose, Al should be 0.005 or more, N is 0.005
0% or more is required, but if A/ exceeds 0.040% or N exceeds 0.00150 inches, the toughness will deteriorate, so A/ is 0.005 to 0.040% and N is 0. .0
It was limited to a range of 0.050% to 0.0150%.

Ni0120~2.50ch: Ni is an effective element for improving strength and toughness, but Ni0
．． If it is less than 20%, the effect will not be exhibited. However, since Ni is an expensive element, in this invention
Since cheap Mn is used as a substitute for i, Ni is limited to 250% or more and is limited to a range of 0.020 to 250.

Furthermore, in the steel of this invention, Ni (%), 2XMn (%
).

The sum of 2xcr (%), i.e. Ni + 2Mn +
Ni, Mn, so that the value of 2Cr is 4 to 8%
It is necessary to adjust the amount of Cr added in relation to each other. The reason is as follows. That is, N, M
Both n and Cr are elements effective in improving toughness, but if they increase more than necessary, they will cause deterioration of powderiness due to tempering embrittlement. Therefore, the present inventors investigated Ni for toughness.

The combined effect of Mn and Cr should be investigated (, Ni
.

When the fracture surface transition temperature vTrs in the V-noctic Charpy impact test was investigated by varying the amounts of Mn and Cr, the results shown in FIG. 1 were obtained. From Fig. 1, high toughness when the fracture surface transition temperature is about 0°C is found in the shaded area in Fig. 1, that is, when Ni + 2Mn + 2Cr is 4~
It has been found that this is only possible in areas within 8%. N
If i + 2Mn + 2Cr exceeds 8 inches, toughness will deteriorate due to tempering embrittlement, and -10,000Ni + 2Mn +
If 2Cr is less than 4, the effect of improving toughness due to these elements cannot be sufficiently obtained. Therefore, in the low alloy steel of the present invention, the condition is that Ni + 2Mn + 2Cr satisfies the range of 4 to 8%.

In addition to the above-mentioned components, Fe and other unavoidable impurities may be used.

Furthermore, the method for producing the low alloy steel of the present invention may be carried out in accordance with conventionally known methods, such as melting a molten metal with the required components and casting it into a steel ingot.
After forging, it may be annealed, and then quenched and tempered.

EXAMPLES Experimental steel ingots having the components shown in sample numbers 1 to 11 in Table 1 were melted, hot forged into 25mm thick steel plates, annealed, quenched, and tempered, and then subjected to tensile testing and An impact test was conducted. Here, the annealing treatment conditions are 90 o
'cx After heating for 5 hours, cooling was performed at a Z cooling rate of 30°C. The quenching conditions were: 860°C x 5 hr hold, 4 V
Hardening was performed at a cooling rate of min.

Further, the tempering conditions were as follows: after holding at 630°C x 1 Ohr, cooling was performed at a cooling rate of 25'c/hr. These heat treatment conditions correspond to the heat treatment conditions for the center of a generator shaft material having a diameter of about 1000 nm. In addition, each sample 1~ shown in Table 1
Of No. 11, steels A1 to A5 all have compositions within the condition range of this invention, and have a composition of Ni + 2Mn + 2
The Cr value also satisfies the range of 4 to 8%.

On the other hand, the comparison steel of sample number 6 is made of 3.5% NiMoV, steel, M
n, teeth, and Cr content are outside the scope of this invention. In addition, the comparative steel of sample number 7 is made of Si and Mn.

P is out of the range of this invention, and the comparison steel of sample number 8 is MO
is out of the scope of the present invention, and furthermore, the comparative steel of sample number 9 has Mn out of the scope of the present invention. Furthermore, sample number t
In the comparative steels of o and xi, the value of N+2Mn+2Cr is outside the range of the present invention.

Test results for each sample as described above are shown in Table 2. As is clear from Table 2, the current 3,5'ir
For comparison steel of sample number 6, which corresponds to NiMoV steel,
Although the comparative steels of sample numbers 7 to 11 all have approximately the same tensile performance, their impact properties are significantly inferior. In other words, good toughness cannot be obtained with low alloy steel that falls outside the range of conditions specified in the present invention.

On the other hand, an appropriate amount of Mn limits the Si and P contents.

The present invention @ (sample numbers 1 to 5) containing Nl, Cr, and Mo has lower strength and low-temperature toughness than the current 3.5% NiMoV steel (sample number 6), despite having a lower Ni content. It is clear that it is superior to

As is clear from the above examples, the low-alloy steel for turbine generator shafts of the present invention contains 3.5%Ni, which is currently in use.
It is cheaper than MoV steel and has superior strength and toughness, and is useful for shaft materials used in rotor shafts or turbine shafts of turbine generators, especially large diameter shaft materials.

Table 2

[Brief explanation of drawings]

Figure 1 shows the effects of Ni and Mn in steel on impact transition temperature.
- It is a correlation diagram showing the influence of Cri.

Claims (1)

[Claims] CO, 13 to 0.30 inches (weight, same below), Si
0.10% or less, Mn 0.60-2.00%, Po
, otos or less, CrO, 40-2.0O1, Ni0,
20-2.50%, Mo 0.10-0.50 fb
, ■0.05~0.15%, AIo, 005~0.0
40 chi, containing -0,0150 chi to NO,0050,
A low alloy steel for a turbine generator shaft, which is adjusted so that Ni+2Mn+2Cr is within a range of 4 to 8-, and the remainder is Fe and inevitable impurities.