WO1994026947A1

WO1994026947A1 - High-strength austenitic heat-resisting steel with excellent weldability and good high-temperature corrosion resistance

Info

Publication number: WO1994026947A1
Application number: PCT/JP1994/000767
Authority: WO
Inventors: Hiroyuki Mimura; Masao Kikuchi; Satoshi Araki; Hisashi Naoi
Original assignee: Nippon Steel Corporation
Priority date: 1993-05-13
Filing date: 1994-05-12
Publication date: 1994-11-24
Also published as: CA2162704A1; EP0708184A1; JPH06322488A; EP0708184A4

Abstract

A high-strength austenitic heat-resisting steel that has excellent weldability and good high-temperature corrosion resistance and can exhibit excellent performance when used as the material of boilers to be used under the conditions becoming more and more severe. The steel comprises less than 0.02 % (by mass, the same will apply hereinbelow) of carbon, at most 1.5 % of silicon, 0.3-1.5 % of manganese, at most 0.02 % of phosphorus, at most 0.005 % of sulfur, 18-26 % of chromium, 20-40 % of nickel, 0.5-10.0 % of tungsten, 0.05-0.4 % of niobium, 0.01-0.2 % of titanium, 0.003-0.008 % of boron, 0.05-0.3 % of nitrogen, and if necessary at least one member of 0.5-2.0 % of molybdenum and/or 0.001-0.05 % of magnesium, 0.001-0.05 % of calcium and 0.001-0.15 % of rare earth element (REM), and the balance consisting of iron and inevitable impurities.

Description

Description High strength austenitic heat-resistant steel with excellent weldability and good high-temperature corrosion resistance

The present invention is an austenitic heat-resistant material that has extremely good high-temperature strength, has excellent weldability and good high-temperature corrosion resistance, and exhibits excellent performance when applied to a boiler whose operating environment is becoming severer. It concerns steel. Background art

For thermal power plants, ultra-supercritical boilers with high-temperature and high-pressure steam conditions are being planned in terms of improving economic efficiency and suppressing carbon dioxide emissions in recent years. Examples of high-strength materials that can withstand use in such harsh environments include “Iron and Steel”, 70th year, S1409, or “Thermal Nuclear Power”, volume 38, page 75. As shown in, heat-resistant austenitic steels utilizing precipitation strengthening with carbonitrides such as Nb and Ti and solid solution strengthening with Mo have been developed.

However, since these heat-resistant steels contain a large amount of alloying elements, welding cannot be said to be easier than conventional austenitic heat-resistant steels, for example, SUS347H, and welding workability is an issue. Was.

It is a well-known fact that purifying steel, that is, reducing the amount of C together with the amount of P and S, is a powerful means to improve weldability. However, as described above, most heat-resistant steels are strengthened by carbonitrides, and a decrease in the amount of C causes a decrease in high-temperature strength.

On the other hand, the increase in the amount of Mo, which is often added for the purpose of solid solution strengthening of steel, It is known to degrade the corrosion properties.

An object of the present invention is to provide an austenitic heat-resistant steel having good weldability and excellent high-temperature strength and high-temperature corrosion resistance. Disclosure of the invention

The present inventors conducted various experiments on steel to which Mo and W were added to compensate for the decrease in high-temperature strength due to the reduction in the amount of C by solid solution strengthening. And succeeded in producing heat-resistant steel with high-temperature corrosion resistance. That is, the gist of the present invention is as follows.

(1) In mass%,

C less than 0.02%,

S i 1.5% or less,

Mn 0.3 to 1.5%>

P 0.02% or less,

S 0.005% or less,

Cr 18-26%,

N i 20-40%,

W 0.5-1 0 0.0%>

Nb 0.05-0.4%,

T i 0.0 0.2% 0.2%,

B 0.003 to 0.008%,

N 0.05-0.3%

A high-strength austenitic heat-resistant steel with excellent weldability and good high-temperature corrosion resistance, characterized by containing Fe and the balance of Fe and unavoidable impurities.

(2) Further Mo: 0.5 to 2.0%

A high-strength austenitic heat-resistant steel having excellent weldability and excellent high-temperature corrosion resistance as described in (1) above, characterized by containing:

(3) Further

Mg: 0.001 to 0.05%,

C a: 0.000% 0.05%,

Rare earth element (REM): 0.001 to 0.15%

A high-strength austenitic heat-resistant steel having excellent weldability and good hot-corrosion resistance as described in (1) or (2) above, characterized by containing one or more of these. BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 is a graph showing the effect of Mo and W on the high temperature corrosion resistance of 20Cr-25Ni steel.

FIG. 2 is a graph comparing the creep rupture strength of the steel of the present invention and the comparative steel with the loss of hot corrosion.

FIG. 3 is a graph showing the results of ballistic strain tests of SUS 347H and steels in which the main alloying elements other than C are within the scope of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the reason why the range of the alloy element is determined as described above in the present invention will be described.

C:

It is necessary to reduce the C content as much as possible to prevent high temperature cracking and decrease in ductility during welding.However, to ensure good weldability, the upper limit of the C content is set based on the following experiment did. Figure 3 shows the main alloying sources other than C Steel (Cr: 20 Ni: 25 W: 3%) in which the amount of carbon was changed (marked with 騮 in the figure) and S US 347 H ( This corresponds to the comparative steel K in the examples described later.In the figure, the results of the varestraint test for evaluating the weldability of the squares are shown. The test conditions were: specimen thickness: 5 mm, welding method: GTAW, welding voltage: 10 V, welding current: 80 A, welding speed: 80 mm / min, and added strain: 2%. Based on the results of this test, the upper limit of the amount of C to secure good weldability was set at less than 0.02%, aiming at the same level as SUS347H.

S i:

Si is an element that not only is effective as a deoxidizing agent, but also improves oxidation resistance and high-temperature corrosion resistance.However, too much Si reduces creep rupture strength and toughness. . Therefore, the upper limit was set to 1.5%.

Mn:

Mn is an element that has a deoxidizing effect and improves weldability and hot workability. The lower limit of Mn was set to 0.3% in order to sufficiently deoxidize and obtain a sound lump. However, if the amount of Mn is too large, the oxidation resistance deteriorates, so the upper limit was set to 1.5%.

C r:

Cr is an element indispensable for oxidation resistance, steam oxidation resistance, and high temperature corrosion resistance. In order to ensure the same or better characteristics as the conventional austenitic stainless steel, the lower limit of the Cr content was set to 18%, which is the same as the Cr content of the austenitic stainless steel. However, an increase in the Cr content lowers the stability of austenite, weakens the high-temperature strength, promotes the formation of intermetallic compound phases, and lowers the toughness.

N i:

Ni increases the austenite stability and suppresses the formation of the σ phase. It is an essential element. In order to achieve austenite stability against the content of ferrite-forming elements such as Cr, the Ni content must be 20% or more. On the other hand, if the amount of Ni exceeds 40%, it is disadvantageous in terms of price, so the amount of Ni was set to 20 to 40%.

Mo, W:

Both M0 and W are elements that significantly increase the high-temperature strength by solid solution strengthening, etc., but their effects are small when added to less than 0.5%, respectively, and when W exceeds 10%, Laves phase etc. This causes precipitation of intermetallic compounds, and reduces creep rupture ductility. When Mo is added alone, the high-temperature corrosion resistance deteriorates as the amount of Mo increases. On the other hand, experiments have shown that when W is added alone, it does not degrade the high-temperature corrosion resistance, and when it is combined with Mo, the high-temperature corrosion resistance can be improved compared to the steel with only Mo added. Therefore, W must be added, and the range is 0.5 to 10%. If Mo is added in excess of 2.0%, the high-temperature corrosion resistance is particularly deteriorated even when W is added in a complex manner. Therefore, 0.5 to 2.0% is added as necessary.

Nb, T i:

Nb and Ti form fine carbonitrides and significantly improve the long-term cleave rupture strength. However, if the 1 ^ 13 amount is less than (3.05% and the Ti amount is less than 0.01%, the above effect cannot be obtained, so the lower limits of the Nb and Ti amounts are set to 05% and 0.01%, respectively. The above effect is more remarkable as the amount of Nb and Ti that can form a solid solution at the solution heat treatment temperature increases, but when the Nb and Ti are added beyond the solid solubility limit, undissolved carbon-nitride Therefore, the upper limit of the amount of Nb and T i is set to 0.4% and 0.2%, respectively, and the amount of solid solution (Nb + T i) is increased within that range. To do this, Nb and Ti were added in combination. B:

B is an element that improves the cleave rupture strength as a result of increasing the grain boundary strength, but its effect is small when it is less than 0.03%, and when it exceeds 0.08%, the weldability and hot work Since the workability is reduced, the range of the B content is set to 0.003 to 0.008%.

P:

If P is added in a large amount, the weldability is remarkably deteriorated, so the upper limit was made 0.02%.

S:

Since S is deflected to the grain boundary, deteriorating hot workability and promoting grain boundary embrittlement during creep, the upper limit was made 0.005%.

N:

N is an element that significantly improves the creep rupture strength by solid solution strengthening and the formation of nitrides. If N is less than 0.05%, the decrease in strength due to the low C content in order to improve weldability cannot be compensated for, and even if added over 3%, the increase in long-term creep rupture strength will not increase. Less, further reduces toughness. Therefore, the range of the amount of N was set to 0.05 to 0.3%.

Mg, C a, Rare earth elements (REM):

These elements cleanse the steel by deoxidation and desulfurization, and enhance the hot workability. To obtain the effect, it is necessary to add at least one of these elements in an amount of 0.0001% or more. Mg: 0.05%, Ca: 0.05% REM: Addition exceeding 0.15% will adversely affect hot workability. : 0.001 to 0.05%, Ca: 0.001 to 0.05% ^ REM: 0.001 to 0.15%. Example Next, the present invention will be described specifically with reference to examples.

Tables 1 and 2 (continued from Table 1) show the chemical composition and material properties of the test steel. These steels were subjected to a solution treatment at 1250 ° C, a creep rupture test at 700,750 and a high temperature corrosion test at 700 ° C. Regarding the creep rupture strength, the data was arranged by the Lars on-M i11 er method, and the rupture strength at 700 ° C for 100,000 hours was estimated. Hot corrosion test information _{_{_{about, K 2 S0 4: Na 2}}} S0 4: F e 2 (SO 4) 3 = 0 28:. 0. 2: a coal-fired simulated combustion ash 0.5 (weight ratio) After immersing the test steel for 20 Oh, the corrosion loss was measured. Table 2 shows the test results.

Of the steels shown in Tables 1 and 2, A to J are the present invention steels, and K to

U is a comparative steel. Among the comparative steels, Κ is SUS 347 Η equivalent steel that has been commonly used. The steel of the present invention has extremely excellent high-temperature strength and high-temperature corrosion resistance as compared with SUS 347H steel. Among the comparative steels, L to 0 are examples in which the high-temperature strength is low without the addition of Mo and W and the Nb or B content is out of the range of the present invention. In addition, although P to U have relatively high strength at high temperatures, even when Mo is added alone or in combination with W, the amount of Mo is large and the high temperature corrosion resistance is poor.

Fig. 1 shows the effect of Mo and W on the high-temperature corrosion resistance of 20Cr—25Ni steel. When Mo alone is added (indicated by hatching in the figure), the corrosion loss is large. It can be seen that the high-temperature corrosion resistance is improved by adding a 1.5% composite (in the figure). In addition, it can be seen that corrosion loss is not changed by adding W alone (indicated by □ in the figure).

Figure 2 compares the cleave rupture strength and hot corrosion weight loss of the steel of the present invention and the comparative steel.The comparative steel has low high-temperature strength and / or high-temperature corrosion resistance. It can be seen that the steel of the present invention is excellent in both high-temperature strength and high-temperature corrosion resistance.

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£ 9Z-00 / t76df / XOd [LP69ZIP6 OAV Table 2 (continued from Table 1)

Note: * indicates that it is outside the range of components of the present invention. Industrial applicability

According to the present invention, it is possible to realize an austenitic heat-resistant steel with excellent weldability and high-temperature strength and high-temperature corrosion resistance. It can be done.

Claims

The scope of the claims

1. In mass% (hereinafter abbreviated as%),

C-less than 0.02%,

S i 1.5% or less,

Mn 0.3 to 1.5%,

Ρ 0 • 02% or less,

s 0.005% or less,

Cr 18-26%,

N i 20-40%,

W 0 .5-1 0 .0%,

Nb. 0 • 05-0.4%,

T i 0.0 0.2% 0.2%,

B. 0.003-0.008%,

N 0 .05-0.3%

A high-strength austenitic heat-resistant steel with excellent weldability and good high-temperature corrosion resistance, characterized by containing Fe and unavoidable impurities.

2. Further

Mo: 0.5-2. 0%

2. A high-strength austenitic heat-resistant steel having excellent weldability and excellent high-temperature corrosion resistance according to claim 1, characterized by comprising:

3. Further

Mg: 0.001 to 0.05%,

C a: 0.001 to 0.05%,

Rare earth element (REM): 0.000 1-0.15%

Claim 1 or Claim 2, characterized by containing one or more of A high-strength austenitic heat-resistant steel with excellent weldability and good high-temperature corrosion resistance as described in 2.