JPH02145751A - Cr alloy steel - Google Patents

Abstract

PURPOSE:To improve the durability and thermal conductivity of the title Cr alloy steel by specifying respective contents of C, Si, Mn, Ni, Mo, V, Nb, N, P, and S in a Cr alloy steel. CONSTITUTION:The steel has a composition consisting of, by weight ratio, 0.05-0.20% C, <=0.10% Si, <=0.30% Mn, <=1.0% Ni, 8.0-9.5% Cr, 0.5-1.0% Mo, 0.15-0.30% V, 0.05-0.15% Nb, 0.03-0.08% N, <=0.005% P, <=0.005% S, and the balance Fe with accompanying impurities. Further, 0.20% W is added to the above composition, if necessary. By the above composition, Mn content is reduced, strength is increased, and durability and thermal conductivity are improved. By this method, a Cr alloy steel increased in low-cycle fatigue strength can be provided by the reduction, in particular, of Si, Mn, and S contents.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention is applied to, for example, turbine casings, steam valves, pressure vessels such as fast breeder reactors, etc.

This invention relates to Cr alloy steel that has been improved to further enhance various properties such as creep rupture and embrittlement.

(Prior art) For example, in steam turbine equipment, a reheat regeneration method is adopted under the Rankine cycle to increase output.
The main steam temperature is 53.
8°C, reheat steam temperature 566°C, and steam pressure 246 kg/- are used.

For this reason, in conventional machines, materials are selected to ensure strength that meets the steam conditions; for example, the main steam pipe that sends steam from the boiler to the turbine is made of alloy steel or ICr.
I Mo 4 V alloy steel is used.

However, in this type of field, where the aim is to increase output efficiency in conjunction with the increase in electricity demand, there is a need for the emergence of so-called ultra-supercritical pressure plants, which are even more advanced than the steam conditions used in conventional machines. Research and development of materials that meet these requirements is underway.

(Problem to be solved by the invention) In this trend, this type of field
If the alloy steel were to be used as is for sideways sliding, the wall thickness would inevitably increase significantly compared to conventional machines, and the increased wall thickness would create an undesirable temperature difference between the inside and outside of the vessel. For example, if there is a temperature difference between the inside and outside of the vessel, excessive thermal stress will be generated locally, and even worse, if the operation is started and stopped frequently, thermal fatigue caused by excessive thermal stress will occur. It has a defect that reduces the durability of the material.

Since such defects are largely caused by changes in the metallographic structure of the material, there is a need for a Cr alloy steel with a stable metallographic structure. Furthermore, in the field of fast breeder reactors, there is a demand for a Cr alloy steel with high thermal conductivity to replace the conventional austenitic stainless steel.

This invention was made based on the above-mentioned circumstances, and it is a Cr alloy steel that has improved material durability and thermal conductivity so that it can be applied even if the steam conditions used are higher than before. The object of the present invention is to disclose a high Cr alloy steel.

[Structure of the invention]

(Means for solving the fa problem) The first aspect of the invention related to Cr alloy steel is that, in terms of weight ratio, C0.0
5-0.20%, Si 0.10% or less, Mn 0.3
% or less, Ni 1.0% or less, Cr 8.0-9.5%
, Mo 0.5-1.5%, V0.15-0.30
%, Nb 0.05-0.15%, N0.03-0.0
8%, P 0.005% or less, S 0.005% or less,
This is a Cr alloy steel with a composition consisting of the balance F6 and incidental impurities.

The second invention has a weight ratio of C0.05 to 0.20%, S
i 0.10% or less, Mn 0.3% or less, Ni 1.0
% or less, Cr8.0-9.5%, Mo 0.5-1.5
%, V0.15-0.30%, Nb 0.05-0.1
5%, N0.03-0.08%, balance Fe and incidental impurities, and 0.2-2.0% W by weight.
The structure is such that it adds .

The reason for the quantitative limitation of the above composition is as follows.

C (Carbon): C stabilizes the austenite phase during quenching and further generates carbides to increase the creep-rubber strength, and for this purpose, it is necessary in an amount of 0.05% or more. However, if it exceeds 0.20%, carbides become excessive, and creep-rubber strength decreases. Therefore, C should be 0.05 to 0.20%.

Cr (Chromium): Cr is an element necessary to prevent oxidation in high-temperature ring tombs and to improve creep rubble strength.

For this purpose, it is necessary to add 8.0% or more, but if it exceeds 9.5%, elongation and toughness will decrease. Therefore,
The amount of Cr is 8.0 to 9.5%.

No (molybdenum): MO is an effective element for improving creep rubber strength, and for this purpose, it is necessary to add 0.5% or more. However, if it exceeds 1.5%, delta ferrite is produced and creep roughness strength and toughness are reduced, so the content is set at 0.5 to 1.5%.

■ (Vanadium): ■ is an element effective in improving creep rubber strength. For this purpose, it is necessary to add 0.15% or more, but if it exceeds 0.30%, delta ferrite is likely to be produced, so the content is set at 0.15 to 0.30%.

Nb (on the order of Nb): Nb has the effect of refining crystal grains and increasing ductility and toughness. Furthermore, Nb forms carbides and carbonitrides, finely disperses and precipitates in the matrix, and significantly improves creep properties. In order to obtain these effects, it is necessary to add at least 0.05%, but if it exceeds 0.15%, delta ferrite is produced and carbides are precipitated coarsely, which is not preferable. Therefore, the amount added is 0.0
5 to 0.15%.

N (nitrogen): N is effective in suppressing the formation of ferrite phase,
It is also an element necessary to form Nb carbonitride.

For this purpose, it is necessary to add 0.03% or more, but if it exceeds 0.08%, the elongation and toughness will decrease, so the content should be 0.03 to 0.08%.

Mn (manganese): Regarding Mn, which is a feature of the present invention, it has been used in the past for the purpose of suppressing its desulfurization effect and remaining free S.
．． Approximately 5% was added. However, by applying out-of-furnace refining, it becomes possible to reduce S to around 0.002%, and Mn addition becomes unnecessary. With the reduction of S, non-metallic inclusions of MnS are also extremely reduced, resulting in improved cleanliness, improved elongation and toughness, and as a result, improved low cycle fatigue strength. Therefore, Mn is 0.30
% or less.

By the way, Mn was thought to be an important element to improve hardenability and to obtain uniform strength down to the center in large thick structural materials such as pressure vessels, but when Mn was added to 0.30 % or less, we conducted an experiment simulating a thick material with a thickness of 500 mm, and as a result, the same strength as before was obtained up to the center, and from the viewpoint of hardenability, there was no problem even with low Mn. found. FIG. 1 shows how the hardness and grain size in the center change with respect to the Mn content.

SL (Silicon): Conventionally, Si was added for deoxidation, but by applying vacuum casting and vacuum carbon deoxidation, this need is eliminated and it also prevents the formation of nonmetallic inclusions. However, it is desirable to reduce the content as much as possible because it impairs elongation and toughness and promotes embrittlement over time. However, with the current refining technology, it is inevitable that trace amounts of Since it remains, the content should be 0.10% or less.

P (Phosphorus): P is an element that is directly involved in lowering creep-lubber strength and ductility, and also lowers weldability, so it is desirable to reduce it as much as possible.

However, with the current refining technology, a trace amount inevitably remains, so the content is set to 0.005% or less.

S (sulfur): Like Si, S also has the effect of particularly impairing creep ductility. It is desirable to reduce it as much as possible. however,
With the current refining technology, a trace amount remains unavoidably, so 0. oos% or less.

Ni (nickel): Ni is an austenite-forming element and is effective in stabilizing the austenite phase during quenching and preventing the formation of delta ferrite, but if too much is added, the creep-rubber strength decreases. However, since the Aex transformation point is lowered, the content is set to 1.0% or less.

W (tungsten): Similar to Mo, W has the effect of improving creep rubber strength, and for this purpose it is necessary to add more than 0.2%, but if it exceeds 2.0%, the delta The amount of addition is limited because it produces ferrite, which causes a decrease in elongation and toughness, and a decrease in creep rubber strength.

The content should be 0.2 to 2.0%.

By selecting the elements and determining the weight ratio of each element in this way, quality assurance can be achieved in the field of application.

(Example) An embodiment of the Cr alloy steel according to the present invention will be described in detail.

Table 1 shows the 'Cr alloy steel according to the present invention and the conventional Cr alloy steel.
This is the weight ratio of the composition compared with alloy steel.

(Left below) To produce a Cr alloy steel having the above composition, raw materials were blended in the above weight ratio, melted in a high frequency vacuum melting furnace, and then cast into a mold to obtain an ingot. After the surface of this ingot was shaved off by machining, it was charged into a heavy oil furnace, heated to 1200°C, hammer forged, and forged into a round bar with a diameter of 60 mm.

The thus obtained round bar was heated and held at 1050° C. for 3 hours in an electric furnace, and then air-cooled to room temperature. After that, it was heated and tempered in an electric furnace at 750°C for 8 hours. After this heat treatment, each material was divided into two groups, and one group was machined as it was to prepare test pieces, which were then subjected to a tensile test. Charpy impact test, creep-butture test and low cycle fatigue test were conducted. As for the remaining group.

After heat treatment, it was heated in an electric furnace at 550° C. for 10,000 hours, and then machined to prepare a test piece and subjected to a Charpy impact test. These results are shown in Table 2.

Tensile tests were conducted at room temperature. The creep love test was conducted under two different conditions of temperature and stress, and the love time, elongation at break, and area of area at break are also shown in Table 2. Additionally, Charpy impact tests were conducted at multiple temperatures ranging from -100°C to +100°C, and the fracture surface transition temperature (FAT)
T) was calculated.

From the test results shown in Table 2, it is clear that Examples 1 to 6, which are Cr alloy steels according to the present invention, have significantly superior creep rubber strength compared to Comparative Examples 1 to 4. . This is particularly noticeable in Examples 3 to 5, in which P was reduced and W was added. Also.

At 550°C, i0. FATT after heating for ooo hours
Looking at the change in ΔFATT (FATT of heated material - FATT of non-heated material), Comparative Examples 1 to 4 all had a temperature of 36°C.
In contrast to the above increase, in Examples 1 to 6, the Si
, il is 0.03-0.05%, so FATT
The increase in temperature remains at 5 to 12°C, and aging embrittlement is suppressed to a low level.

Furthermore, at 550°C, over the entire strain range, Δεt is 1.0
%, the number of crack occurrences in the low cycle fatigue test increased in Examples 2 to 5, especially for SL+Mn,
An improvement in low cycle fatigue strength due to the reduction of S was observed.

〔Effect of the invention〕

From the above explanation, in this invention, in order to increase durability or thermal conductivity, Cr alloy steel is used with preferable elements selected and the weight ratio appropriately determined, and its reliability will be further improved when applied to this type of field. We can expect it to increase.

[Brief explanation of the drawing]

The drawing is a graph showing the relationship between Vickers hardness and crystal grain size with respect to the increase in Mn weight ratio. Agent Patent Attorney Noriyuki Chika Yudo Ken Daishimaru

Claims (2)

[Claims] (1) Weight ratio: C0.05-0.20%, Si0.1
0% or less, Mn 0.3% or less, Ni 1.0% or less, Cr
8.0-9.5%, Mo0.5-1.5%, V0.15
~0.30%, Nb0.05~0.15%, N0.03
Cr alloy steel consisting of ~0.08%, P 0.005% or less, S 0.005% or less, balance Fe and incidental impurities. (2) Weight ratio: C0.05-0.20%, Si0.1
0% or less, Mn 0.30% or less, Ni 1.0% or less, C
r8.0-9.5%, No.0.5-1.5%, V0.1
5-0.30%, Nb0.05-0.15%, N0.0
A Cr alloy steel characterized in that 0.2 to 2.0% by weight of W is added to a composition consisting of 3 to 0.08% of Fe, the balance being Fe and incidental impurities.