JPH02197550A - High purity heat-resistant steel - Google Patents

Abstract

PURPOSE:To obtain the high purity heat-resistant steel having excellent high temp. creep rupture strength and toughness by specifying the compsn. of a 12 Cr series heat-resistant steel constituted of C, Cr, Mo, V, W, N, Ni, Co and Fe and extremely reducing inevitable impurities. CONSTITUTION:The compsn. contg., by weight, 0.05 to 0.25% C, 9 to 13% Cr, 0.1 to 2.0% Mo, 0.1 to 0.5% V, 0.5 to 2.5% W and 0.03 to 0.10% N, furthermore contg. one or both of 0.5 to 1.5% Ni and 0.5 to 1.5% Co and the balance Fe with inevitable impurities is formed to obtain the high purity heat-resistant steel combining excellent high temp. creep rupture strength and excellent toughness and used for a turbine rotor shaft of an extra super critical pressure steam turbine or the like. In the above steel, if required, one or both of 0.02 to 0.10% Nb and 0.02 to 0.10% Ta can moreover be incorporated. Furthermore, in the above inevitable impurities, <=0.05% Si, <=0.05% Mn, <=0.005% P and <=0.005% S are preferably regulated as allowable contents.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a high-purity heat-resistant steel that has both excellent high-temperature creep rupture strength and excellent toughness and is used for turbine rotor shafts of ultra-supercritical pressure steam turbines and the like.

[Conventional technology]

In recent years, in order to improve the thermal efficiency of large thermal power plants,
Ultra-supercritical pressure steam turbines have been developed to increase the main steam temperature. Materials used for turbine rotor shafts and the like used under ultra-supercritical pressure are required to have both excellent creep rupture strength and toughness at high temperatures.

Various materials have been developed in line with this requirement. For example, 12c, which has excellent creep rupture strength at high temperatures as well as ductility and toughness, is disclosed in JP-A-59-2322:II.
An r-Mo-V-Nb (Ta)-N steel is disclosed.

However, in numerically 12cr heat-resistant steel,
By changing the heat treatment or changing the amount of alloying elements added,
Increasing the high-temperature creep rupture strength conversely impairs the toughness of the material near room temperature, so it is extremely difficult to obtain a 12Cr heat-resistant steel with excellent properties in both high-temperature creep rupture strength and toughness. The material disclosed in Japanese Patent Publication No. 59-232231 cannot be said to be sufficient in this respect, and there is still room for further improvement.

(Problem to be solved by the invention) In the conventional 12Gr heat-resistant steel,
As disclosed in 2231, various properties such as toughness and creep rupture strength at high temperatures have been improved by adding appropriate amounts of alloying elements and applying appropriate heat treatment, but there are already many similar steel types. At the current state of development, it is difficult to expect significant improvements over conventional steels simply by adding appropriate amounts of alloying elements and performing appropriate heat treatment.

The present invention was made in view of the above circumstances, and it is an object of the present invention to provide a new 12 Cr,% heat-resistant steel that has both high temperature creep rupture strength and toughness that are significantly superior to conventional steels.

[Means to solve the problem]

The specific means for achieving the above objective is to first add and blend an appropriate amount of an alloying element that provides the required strength and improves excellent toughness and high-temperature creep strength to a 12Cr-based alloy, and then incorporates Si as a deoxidizer or alloying element. , Mn was not added, and its content was reduced as much as possible along with p and s as inevitable impurities, in order to achieve high purity.

That is, the first invention has C0.05 to 0.25 in weight%.
%, Cr9-13%, Mo 0.1-2.0%, V0
．． 1~0.5%, WO05~2.5%, N0.03~
Contains 0.10%, roughly Ni 0.5-1.5%,
It is a high-purity heat-resistant steel containing one or two types of Co 0.5 to 1.5%, with the balance consisting of Fe and inevitable impurities.

The second invention is ': In addition to the alloy composition of the JS1 invention, Nb 0.02 to 0.10% and Ta 0.02 to 0% by weight are added.
．． It is a high-purity heat-resistant steel containing 10% of one or two types.

The third invention is 5iO105% by weight of the inevitable impurities contained in the high purity heat resistant steel of the first or 71g2 invention.
Below, Mn 0.05% or less, P 0.005% or less,
It is a high-purity heat-resistant steel with an allowable S content of 0.005% or less.

[Effect]

Next, the effects of each element contained in the steel of the present invention and the reasons for limiting each component will be explained.

C stabilizes the austenitic structure by forming a solid solution in iron at high temperatures, promotes martensitic transformation by rapid cooling, and improves strength at high temperatures and room temperature.
Nb, V, M0. It combines with elements such as the stomach to form carbides and improves high-temperature creep rupture strength.

If the content is less than 0.05%, almost no annotation effect is observed. On the other hand, if it is contained in excess of 0.25%,
The content is limited to 0.05 to 0.25% because coarse carbides tend to form and agglomerate, resulting in a decrease in high-temperature creep rupture strength and poor low-temperature toughness.

Cr: 9-13% C is a main component of the heat-resistant steel of the present invention, and is a solid solution in iron, and is an element necessary to improve the strength of the alloy as well as oxidation resistance and corrosion resistance. If the content is less than 9%, sufficient strength and oxidation resistance cannot be obtained, and if the content exceeds 13%, a delta ferrite structure is generated, which improves ductility and toughness at low temperatures and creep rupture strength at high temperatures. Since it reduces the
It was limited to 13%.

Mo: 0.1 to 2.0% Mo is an element that is dissolved in solid solution in the alloy and is necessary to improve the strength at low and high temperatures and to suppress temper embrittlement. If the content is less than 0.1%, the effect will be small, while if it is more than 2%, a delta ferrite structure will be generated and the strength at low and high temperatures will be reduced. It was limited to 2.0%.

V: 0.1-0.5% ■ is an element necessary to improve high-temperature creep strength, and if it is less than 0.1%, its effect is insufficient;
If the content exceeds 0.5%, a delta ferrite structure is generated and the high temperature creep rupture strength decreases, so the content was limited to 0.1 to 0.5%.

W: 0.5 to 2.5% W is an element that is dissolved in the alloy and is necessary to improve the strength at low and high temperatures. When its content is less than 0.5%, its effect is hardly recognized;
If the content exceeds 0.5%, a delta ferrite structure is generated and the strength at low and high temperatures decreases, so the content was limited to 0.5 to 2.5%.

N: 0.03-0.10% N improves strength at high and low temperatures, and
It is an element that improves high-temperature creep rupture strength. When the Km content is 0.03% or more, its effects are noticeable, but when it is added over 0.1%, it becomes difficult to manufacture steel ingots and the hot workability deteriorates. , its content was limited to 0.03-0.1%.

Ni Co: 0.5-1.5% Ni and CO have the effect of stabilizing the austenite structure at high temperatures and suppressing the formation of ferrite, and this effect improves the low-temperature toughness and high-temperature creep rupture strength. Mn also has the same effect as Ni and Co, but Ni
, Co does not provide any great effect, so in the present invention, Ni and C are used instead of Mn.

was added to suppress the formation of ferrite and improve toughness and high-temperature creep rupture strength.

It should be noted that if their content is less than 0.5%, the above effect will not be noticeable, and if it exceeds 1.5%, the creep rupture strength will tend to decrease.
The content of each was limited to 0.5 to 1.5%.

Nb, Ta: 0.02-0.10% Nb and Ta
has the same effect, combining with carbon and nitrogen in the alloy to form carbides, nitrides, and carbonitrides, which are finely precipitated and dispersed in the matrix of the alloy, improving high-temperature creep rupture strength. , an element necessary to prevent coarsening of crystal grains during forging and heat treatment and improve toughness at low temperatures.
It is necessary to contain at least 0.02%.

However, if these elements are contained in an amount exceeding 0.10%, a delta ferrite structure is generated and the amount of carbonitrides becomes too large, resulting in a decrease in toughness. Therefore, the upper limit of the content is set at 0.1%. I made it.

Sf: 0.05% Mn: 0.
05%P: 0.005% or more, S: 0.00
5%Si is usually used as a deoxidizing agent, and the content in that case is usually about 0.30 to 0.50%, and if the Si content is further reduced by vacuum carbon deoxidation method etc. , macro-segregation, especially inverted V-segregation, becomes slight, and the non-uniformity of ductility and toughness within the wall thickness is improved. Moreover, when the Si content is high, the susceptibility to temper embrittlement becomes extremely high, and notch toughness is impaired. Therefore, it is desirable that its content be as low as possible, and in the present invention it is treated as an unavoidable impurity, and its permissible content is limited to an industrially possible 0.05% or less.

Mn is generally required as a deoxidizing and desulfurizing agent during dissolution. However, Mn combines with S to form nonmetallic inclusions, reducing toughness. On the other hand, un has the effect of stabilizing the austenite structure at high temperatures and suppressing the formation of ferrite that is harmful to high temperature creep rupture strength and toughness.

Therefore, in the present invention, the effects of Mn are replaced by Ni and GO, the Mn content is reduced as much as possible industrially, it is treated as an unavoidable impurity, and its permissible content is limited to 0.05% or less.

P is an element that promotes susceptibility to temper embrittlement, and it is desirable to reduce it as much as possible in order to obtain materials with little aging deterioration. Considering the current level of refining technology, the allowable content is set at 0.005. % or less.

Even a small amount of S in large steel ingots causes V-shaped or inverted V-shaped segregation and deteriorates the quality of the steel, so it is desirable to reduce it as much as possible, and as with P, the current refining technology level should be considered. and its allowable content is 0.005%.
Limited to the following.

In addition to the above-mentioned unavoidable impurities, Cu is an impurity element that deteriorates steel quality, and As, Sb, and other impurity elements that promote temper embrittlement are mentioned, but these impurity elements should be reduced as much as possible. is preferred. However, these impurity elements are unavoidably mixed in with the raw materials, and are difficult to remove by refining. Therefore, it largely depends on the careful selection of raw materials, and from the standpoint of steel quality modification, Cu 0.10% or less, As
0.008% or less, Sn 0.010% or less, Sb
It is desirable to suppress it to 0.005% or less.

〔Example〕

The invention steel and comparative steel having the compositions shown in Table 1 were melted in a vacuum melting furnace to produce a 50 kg steel ingot. Then, it was heated to 1200°C and then forged. Test pieces were cut from these forged materials and heat treated to simulate the heat treatment of actual rotor shaft materials (held at 1050°C for 5 hours, then oil cooled → held at 560°C for 5 hours, then furnace cooled → held at 660°C for 24 hours) (furnace cooling) and used as a test material. Table 2 shows the material test results for these test materials. As is clear from Table 2, the steel of the present invention has a longer creep rupture time and a lower fracture surface transition temperature than the comparative steel, which is a conventional material. These indicate that the creep rupture strength and toughness are excellent, respectively.

(Effects of the invention) A new high-purity heat-resistant steel with excellent high-temperature creep rupture strength and toughness has been created by adding ffi alloying elements to 12CR heat-resistant steel and reducing unavoidable impurities as much as possible to achieve high purity. I was able to get it.

Patent applicant: Japan Steel Works, Ltd.

Claims (3)

[Claims] (1) C0.05-0.25% by weight, Cr9-13
%, Mo0.1-2.0%, V0.1-0.5%, W0
．． 5-2.5%, N0.03-0.10%, and further contains 0.5-1.5% Ni, 0.5-1.5% Co.
High-purity heat-resistant steel containing one or two species, with the remainder consisting of Fe and unavoidable impurities. (2) The composition of claim (1) further includes 0.0% by weight of Nb.
A high-purity heat-resistant steel containing one or both of 0.02 to 0.10% and 0.02 to 0.10% of Ta. (3) Among the inevitable impurities, Si0.05% or less, Mn0.05% or less, P0.005% or less, S0.
Claim (1) or (2) where the permissible content is 0.005% or less.
) High-purity heat-resistant steel.