JPS59136464A - Boiler tube - Google Patents

Abstract

PURPOSE:To provide a boiler tube for high temp. steam, formed of steel having a specific composition consisting of C, Si, Mn, Ni, Cr, Mo, Nb, Ta, B, Ti, Cu, N and Co and an all austenite structure. CONSTITUTION:A boiler tube comprises steel which contains, on a wt. basis, 0.02-0.15% C, 0.5-3.5 Si, 2% or less Mn, 20-40% Ni, 20.5-27% Cr, 0.5-3% Mo, 1% or less Nb+Ta, 0.0005-0.005% B and further one or more of 0.5% or less Ti, 0.5% or less Zr, 4% or less Cu, 0.05-0.1% N and 2% or less Co and has substantially an all austenite structure and excellent in high temp. strength and corrosion resistance. This boiler tube can be suitably used as a power plant including coal combustion and generating steam with a temp. of about 600- 650 deg.C.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to boiler tubes, and particularly to boiler tubes involving coal combustion.
The present invention relates to a boiler tube suitable for power generation plants for obtaining steam temperatures of 00C or higher.

[Prior art]

As a conventional boiler tube material for power generation plants, 2
-! Low alloy steels such as -Cr-IMo and 9Cr-1Mo steels are used, and on the high temperature side, austenitic stainless steels such as 5US304, 5U8321 and SUS 347 are used.

However, in recent years, from the point of view of effective use of resources and energy,
The boiler fuel was changed from heavy oil to coal, and the steam temperature was set at 600 C for reasons such as power generation efficiency.

Furthermore, there is a trend toward increasing the temperature and pressure to around 650C. Therefore, the temperature of the steam inside the boiler tube and the temperature of the outer wall of the tube has been lowered by about 30 to 100' compared to conventional methods.
c and increase the pressure to 250-350 KSI f/c
〇 However, conventional austenitic stainless steels, which are generally used in the steam temperature range of around 600C, have extremely low creep rupture strength when the temperature exceeds 650C, and steam oxidation and gas corrosion also increase. do. Therefore, as a material for boiler tubes for steam temperatures of 650tZ' or higher, 8US304,5
US321, 8US347: A--Higher strength than stenitic stainless steel and superior corrosion resistance are required. Such materials include 2i%Cr-32%
Ni-based austenitic steel is considered.

However, in the case of this austenitic steel, the 1000-hour creep rupture strength at a boiler tube outer wall temperature of 720C is about 5.3 Kff / sea bream 2, and at a pressure of 350
There is a problem with using the boiler tube for ATG's 6507R turbine at a pressure higher than the allowable pressure of 5.5 Kp/rIrln.

[Purpose of the invention]

An object of the present invention is to provide a boiler tube that has excellent high-temperature strength and corrosion resistance and can be used in a depot plant with a main steam temperature of 600 to 650C.

[Summary of the invention]

The first aspect of the present invention is that the N amount ratio is C: 0.02 to 0.15%,
s t: o, s~a, s%, Mn: 2% or less, N
i: 20-42%, Cr: 20.5-27%, MO
: 0.5 to 3%, Nb+'l''a: 1% or less, B:
0.0005 to 0.005%, Ti: 0.5% or less, Zr: 0.5% or less, Cu: 4% or less, N: 0.0
It is a boiler tube containing at least one or more of 5 to 0.1% and 2% or less of Co, and has a substantially entirely austenite structure.

The second aspect of the present invention is the weight ratio -? l'C: 0.02~0.
15%, Si: 0.5-3.5%, Mfl: 2%
Below, Ni: 20-42%, Cr: 20.5-27
%, MO: 0, 5-3%, Nb+Ta: 1% or less, B
: 0.0005~0.005%, A,! : 0.
02-0.5%, Ti: 0.5% or less, Zr: 0
．． 5% or less, Cu: 4% or less, N: 0.05 to 0.1%
A boiler tube containing substantially all orstenite structure.

The present invention will be explained in more detail below.

C combines with carbide-forming elements such as MO, Nb, '[:'i, etc. to form carbides and increases high-temperature strength, but 0.15
If the content exceeds 0.15%, workability, ductility, and bath weldability will be significantly reduced, so the content must be kept at 0.15% or less. Particularly preferred is 0.02 to 0.1%.

8i improves corrosion resistance at 0.5% or more, but 3.5%
If it exceeds this, manufacturability and workability will be impaired and a ferrite phase will precipitate, so the content must be 3.5% or less.

Like Si, Mn is an important deoxidizing component, but if it is too large, oxidation resistance decreases, so it is preferably 2.0% or less.

Ni coexists with Cr to improve workability (kM+), keep the austenite structure stable, and increase high-temperature strength. Inventive steel Fi20.5-27% Cr and Mo.

Since it contains ferrite-forming elements such as Nb, Ti, and 3i, in order to obtain a stable austenite phase, Ni must be between 20 and 20%.
42% is required. The more Ni there is, the more stable the structure can be obtained at high temperatures; however, if it is too much, the columnar crystals become coarse and workability deteriorates. Particularly preferable Ni amount is 30 to 35
%.

Or is effective against high temperature corrosion caused by coal combustion gas,
It is also effective against steam oxidation, and needs to be 20.5% or more. However, even if it exceeds 27%, the effects of high-temperature corrosion and steam oxidation remain unchanged, and on the contrary, hot workability is significantly impaired. A particularly desirable Cr content is 21 to 25%.

MO does not adversely affect high-temperature corrosion caused by coal combustion, and is required to be present in an amount of 0.5% or more in order to strengthen the austenite matrix, partially precipitate as carbides, increase high-temperature strength, and strengthen grain boundaries. However, if it exceeds 3% gold, the workability decreases and the precipitation of sigma phase becomes easy. A particularly preferred amount of MO is 1.0 to 2.0%.

Nb and Ta precipitate as carbides and increase the still-temperature strength and ductility. In order to obtain both this ductility and strength, it must be less than 1%.

B is an element that improves creep rupture strength, particularly long-term creep rupture strength, and is also effective in improving workability, corrosion resistance, and weldability as a boiler tube. Here, the effects of B addition will be explained in particular in relation to the boiler tube manufacturing process and usage conditions. Boiler tubes are usually manufactured through melting, agglomeration, blooming, hot extrusion, cold drawing, and solution treatment. Hot extrusion adjusts the grain size, cold drawing adjusts the structure, and solution treatment
The test is carried out under conditions of 0 to 1200C. The boiler tube has an outer diameter of 30 to 80 mm and a wall thickness of 7 to 16 gn, and is installed in the power plant by welding.

That is, boiler tubes are manufactured by hot working and cold working, and the degree of working changes depending on the amount of B added. In Fig. 1, when the amount of B added exceeds 0.0005%, the workability improves, but when the amount of B added exceeds 0.005%, borides are generated, and when the amount of B added exceeds 0.01%, The amount of boride increases and eutectic is also produced. Since borides act as deposits that reduce processability, the amount of B added must be 0.0005 to 0.005% from the perspective of processability.

Also, from the viewpoint of creep rupture strength, B must be present in an amount of 0.0005% or more, and if it exceeds 0.005%, the strength decreases. Furthermore, the addition of B does not reduce weldability.

A4 is a deoxidizing agent for bath water and also strengthens the base material through interaction with MO. Furthermore, it has a high affinity with N,
N bN, T iN, Z r No Cr, N.

It is an important element for improving creep rupture strength because it suppresses the precipitation of harmful nitrides such as BN. Note that since At is a ferrite-forming element, it must be kept at 0.5% or less. Particularly preferred is 0.05 to 0.20%.

Ti and 7. r partially acts as a deoxidizing agent, and is precipitated as a carbide in the matrix to improve high INA ductility, refine grains, and improve strength. However, too much content impairs weldability and causes welding defects, so each content must be 0.5% or less. Especially 0
．． It is preferably 3% or less.

Cu is an austenite-forming element and is effective as a substitute for Ni. However, when added in a large amount, it promotes grain boundary embrittlement at high temperatures and increases hot cracking susceptibility, so the upper limit needs to be 2%. In particular, 2% or less is preferable.

N has the effect of stabilizing the austenite structure, and the appropriate N content in austenitic steel is about 1% of Cr, but the amount of N added is 0.05 to 1%.

CO is an austenite-forming element that improves oxidation resistance as well as high temperature strength and ductility. If the gap is too large, the weldability and weldability will deteriorate, so the gap must be 2% or less.

[Embodiments of the invention]

Table 1 shows the chemical components (% by weight) of the samples used in the experiment. Comparative steel Al (S US 321), A2 (SU8
316) and A3 (A11oy800) have a wall thickness of 8 to 1
A test piece was taken from a 4 m long tube with an outer diameter of 40 to 60 mm. Comparative steel &1 to Jf6.3 and invention steel A1 to A12
A creep rupture test was conducted at 720C using the sample material. All test pieces were tested with a diameter of 6 runs and between 30 parallel sections.

Figure 2 shows the creep rupture strength at 720tr for 1000 hours. The steels A1 to A12 of the present invention exhibit higher strength than the comparative steels, and I6 is the strongest among the comparative steels. (Alloy800
), the steel of the present invention has a strength that is 1.1 to 1.20 times higher.

The second garment is 720 tl? , the results obtained by linear extrapolation of the 10' hour creep rupture strength are shown. 650tZ', 3
The design stress of a 50atg critical pressure boiler tube is 5.5KqtZ2 or higher at 720C for 10' hours, and the steel of the present invention can be used satisfactorily.

Table 2 ■ 1 House [Effects of the Invention] As described above, according to the present invention, it is possible to provide a boiler tube that can satisfy the design conditions of an ultra-supercritical power generation plant with a main steam temperature of 650 C and a pressure of 350 atg.

[Brief explanation of the drawing]

Claims (1)

[Claims] At a weight ratio of 10, C: 0.02 to 0.1 s%t Si
:0, 5~3.5%, Mn: 2% or less, Ni: 20~
42%, Cr: 20.5-27% 9Mo: 0.5-3%
, Nb+Ta: 1% or less, B: 0.0O05 to 0.
005%, further Ti: o, s% or less, Zr: 0.5%
A boiler tube characterized by containing at least one of the following: CLI: 4% or less, N: 0.05 to 0.1%, and co=2% or less, and having a substantially entirely austenite structure. 2. Weight ratio: C: 0.02-0.15%, Sl
:0, 5-3.5%, Mn: 2% or less, Ni: 2o-
42%, Cr: 20.5-27%, Mo: 0
．． 5 to 3% sNb+Ta: 1% or less, B: 0.0
005-0, 005%, A4: 0.02-0.5
%, and further Ti = 0.5% or less, Zr: 0.5% or less,
Contains Cu: 4% or less, N: 0.05 to 0.1%,
A boiler tube characterized in that it has a substantially all-austenitic structure.