JPH04371551A - High strength ferritic heat resisting steel - Google Patents

Abstract

PURPOSE:To produce a high strength ferritic heat resisting steel excellent in high temp. strength and toughness at ordinary temp. by preparing a steel having a specific composition in which respective contents of Mo, W, Co, and B are specified. CONSTITUTION:A steel having a composition consisting of, by weight, 0.05-<0.15% C, <=0.20% Si, 0.05-1.50% Mn, 8.00-13.00% Cr, 0.01-1.50% Ni, >0.50-1.50% Mo, >1.00-4.O0% W, 0.05-0.40% V, 0.02-0.15% Nb, 1.00-5.00% Co, 0.002-0.050% Al, 0.0010-0.0300% B, 0.01-0.11% N, and the balance Fe with inevitable impurities is prepared. By this method, the high strength ferritic heat resisting steel having superior strength at high temp. and sufficient toughness at ordinary temp. can be obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength ferritic heat-resistant steel with excellent high-temperature creep properties and room-temperature toughness, and more particularly to steel for boiler pipes.

0002

[Prior Art] Recently, in order to improve thermal efficiency, steam conditions in thermal power generation have been increased in temperature and pressure, from the current 538°C/246kgf/cm2 to 593°C/3cm2.
16kgf/cm2, even 650℃/350kg
We are trying to raise the pressure to the so-called ultra-supercritical pressure condition of f/cm2. With this trend, when selecting materials for boiler tubes, etc., the currently used 2-1/4 Cr-Mo steel will no longer be applicable due to its oxidation resistance and high-temperature strength. On the other hand, application of 18-8 austenitic heat-resistant steel is considered, but there are problems such as increased cost. Therefore, it is desired to develop a high-strength, high-toughness ferritic heat-resistant steel that lies between these two.

On the other hand, 9Cr-
High chromium ferritic heat-resistant steels such as 1Mo steel and 9Cr-2Mo steel have also been used, but these cannot be applied because they lack creep rupture strength under the above steam conditions. In addition, as other related technology, Japanese Patent Application Laid-Open No. 62-29
No. 7435, JP-A-62-297436, JP-A-63
There are those described in various publications such as No.-89644.

0004

[Problem to be solved by the invention] In view of the above circumstances, the object of the present invention is to provide a ferritic heat-resistant steel having high strength and high toughness that can be used as a material for ultra-supercritical pressure boilers, etc. It is said that

[0005]

[Means for Solving the Problems] In order to achieve the above-mentioned objects, the present invention aims to optimize alloy components, optimize the amounts of Mo and W added, and at the same time actively utilize Co and B. This provides a high-strength ferritic heat-resistant steel with excellent high-temperature strength and room-temperature toughness. That is, the gist of the present invention is as follows.

[0006] In weight%, C: 0.05 to 0.15% Si: 0.20 or less Mn: 0.05 to 1.50% Cr: 8.00 to 13.00% Ni: 0.01 to 1 .50% Mo: More than 0.50% to 1.50% W: More than 1.00% to 4.00% V: 0.05 to 0.40% Nb: 0.02 to 0.15% Co: 1 Contains .00~5.00% Al: 0.002~0.050% B: 0.0010~0.0300% N: 0.01~0.11%, with the balance consisting of Fe and inevitable impurities. , a high-strength ferritic heat-resistant steel with excellent high-temperature strength and sufficient room-temperature toughness.

[0007]

[Operation] The reasons for limiting each component of the present invention will be explained below. C mainly precipitates as MC (M refers to an alloying element, the same applies hereinafter) and M23C6 type carbides, and has a large influence on strength and toughness. If it is less than 0.05%, the amount of precipitation is small and it is insufficient for strengthening, and if it is more than 0.15%, toughness decreases and coarsening of carbide agglomeration is promoted, reducing creep rupture strength at high temperatures and long periods of time. So,
Limited to less than 0.05% to 0.15%.

[0008] Although Si has little effect on strength, it deteriorates toughness, so it is limited to 0.20% or less. Mn is δ
Suppresses the formation of ferrite and achieves a minimum of 0.05 in terms of phase balance
%, but if it exceeds 1.50%, the high temperature strength will decrease, so the upper limit was set at 1.50%.

Cr is an essential element for ensuring high-temperature oxidation resistance, and also has the effect of precipitating M23C6 type carbides. If it is less than 8.00%, oxidation resistance at high temperatures will be insufficient and high temperature strength will also decrease. On the other hand, 13.00%
If the content exceeds 8.00% to 13.00%, it becomes difficult to suppress δ ferrite and the strength and toughness are impaired. Ni is an austenite-forming element, has the effect of suppressing δ ferrite, and has a beneficial effect on toughness. A minimum content of 0.01% is required, but if it exceeds 1.50%, the precipitates will aggregate and become coarse, so it is set at 0.01 to 1.50%.

Mo provides solid solution strengthening and at the same time M2
Stabilizes 3C6 and improves high temperature strength. 0.50
% or less, the effect is small, and if it exceeds 1.50%, it promotes the formation of δ ferrite, as well as the precipitation and agglomeration coarsening of M6 C and Laves phases. . W has the effects of solid solution strengthening and fine precipitation of M23C6, and at the same time suppresses the agglomeration and coarsening of carbides and significantly improves the creep rupture strength at high temperatures and long periods of time. A minimum content of more than 1.00% is required, but if it exceeds 4.00%, δ ferrite and coarse Laves phase tend to form, reducing high temperature strength and toughness.
It was set at 4.00%.

[0011] V precipitates as fine carbonitrides and functions to increase high-temperature strength. If it is less than 0.05%, the effect is insufficient, and if it exceeds 0.40%, it not only causes coarsening of V (C, N), but also reduces the amount of C that can be precipitated as M23C6, and conversely reduces high-temperature strength. 0.05~
Limited to 0.40%. Nb precipitates as carbonitride,
Effective for increasing strength. A minimum amount of 0.02% is required, but if it exceeds 0.15%, it will not be completely dissolved in the matrix at the normalizing temperature and a sufficient reinforcing effect will not be obtained. Limited to 15%.

[0012]N is one of the important elements that precipitates nitrides or carbonitrides and increases high-temperature strength. Minimum 0.01
% is necessary, but if it exceeds 0.11%, it not only causes coarsening of the nitride and a decrease in toughness, but also makes it difficult to manufacture, so it is limited to 0.01 to 0.11%. Co
The active use of is one of the major features of the present invention. C
o is an austenite-forming element, which suppresses the formation of δ ferrite, stabilizes precipitates, and increases high-temperature strength. If it is less than 1.00%, the effect is small, and 5.
If it exceeds 00%, the cost will be high and embrittlement will easily occur, so it is limited to 1.00 to 5.00%.

[0013] Al is used as a deoxidizing agent, but its residual amount has a large effect on crystal grain size and mechanical properties. 0.
Less than 0.002% is insufficient for deoxidation, and 0.050%
If the temperature exceeds 0.002, the creep rupture strength will decrease.
Limited to 0.050%. B is grain boundary strengthening effect and M2
3(C,B)6 etc., which has a precipitation strengthening effect,
It has the effect of improving high temperature strength. If it is less than 0.0010%, the effect is insufficient, and if it exceeds 0.0300%, a coarse B-containing phase is formed and embrittlement occurs.
It is limited to 0.0300%.

[0014]

[Example] Steel of the present invention (No.
．． 1 to 7) and comparative steels (No. 8 to 11) were melted into ingots of 20 kg each in a vacuum induction melting furnace, hot rolled into a 15 mm thick plate, and then sintered at 1100°C for 60 minutes. and then tempered at 780°C for 60 minutes.
0℃, 20kgf/mm2 and 650℃, 15kgf/mm2
A creep rupture test was conducted under two conditions of 6 mm2.
After aging at 00°C for 3000 hours, a Charpy impact test was conducted at 0°C. The results are shown in Table 2. As is clear from Table 2, the creep rupture time of the steel of the present invention is 2 to 3 times longer than that of the comparative steel under all conditions, and
The Charpy absorbed energy after aging at 0° C. for 3000 hours is equal to or higher than that of the comparative steel, and it is considered that it can be used at higher temperatures than conventional steel.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

As described above, the present invention has made it possible to supply a ferritic heat-resistant steel having excellent creep rupture strength and good toughness. These steels can be applied to many fields such as ultra-supercritical thermal power generation and nuclear power generation, making an extremely large contribution to industry.

Claims (1)

[Claims] Claim 1: C: less than 0.05 to 0.15% Si: less than 0.20 Mn: 0.05 to 1.50% Cr: 8.00 to 13.00% Ni: 0.01 ~1.50% Mo: More than 0.50% ~ 1.50% W: More than 1.00% ~ 4.00% V: 0.05 ~ 0.40% Nb: 0.02 ~ 0.15% Co : 1.00-5.00% Al: 0.002-0.050% B: 0.0010-0.0300% N: 0.01-0.11%, the balance being Fe and inevitable impurities A high-strength ferritic heat-resistant steel with excellent high-temperature strength and sufficient room-temperature toughness.