JPH04365838A - Ferritic heat resisting steel excellent in hot workability and strength at high temperature - Google Patents

Abstract

PURPOSE:To attain the thinning of equipment and also to prolong the service life of equipment by constituting a heat resisting steel of a composition consisting of specific weight percentages of C, Si, Mn, Cr, Mo, W, V, Nb, B, Ta, Ca, and N and the balance Fe. CONSTITUTION:The ferritic heat resisting steel has a composition consisting of, by weight, 0.03-0.20% C, <=1.0% Si, 0.1-1.5% Mn, 7.0-13.0% Cr, 0.4-2.5% Mo, 0.05-2.5% W, 0.02-0.35% V, 0.01-0.15% Nb, 0.0008-0.0100% B, 0.01-0.10% Ta, 0.0005-0.0150% Ca, 0.005-0.08% N, and the balance Fe with inevitable impurities. Further, either or both of 0.01-0.40% Ti and 0.1-1.0% Ni are incorporated into the above composition. By this method, this ferritic heat resisting steel can be used as a substitute for 18Cr-8Ni type austenitic stainless steel.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is applicable to ■ evaporator tubes, heater tubes,
Materials for supercritical pressure and ultra-supercritical pressure boilers for thermal power generation, such as reheater tubes, main steam piping, and header tubes, ■Materials for chemical industry plants, such as heater tubes and heat exchanger tubes, ■Steam generation The present invention relates to a ferritic heat-resistant steel with excellent hot workability and high-temperature strength, which is suitable for use as materials for fast breeder reactors such as vessel tubes and heater tubes, and for first wall materials of various fusion reactors.

0002

[Prior Art] Conventionally, supercritical pressure boilers have been used as boilers for thermal power generation, but recently, in order to increase thermal efficiency and save fuel, efforts have been made to convert them to ultra-supercritical pressure boilers. There is. This kind of ultra-supercritical pressure boiler is
Due to shutting down operations at night when electricity demand is low,
High-temperature/low-temperature thermal cycle operation has become inevitable, and materials of superior quality are required.

By the way, austenitic stainless steel has conventionally been used as a material for such boilers. However, this steel type has a large coefficient of thermal expansion,
There are problems with thermal fatigue and scale peeling. Particularly, exfoliated scale accumulates at bent portions of steel pipes, leading to localized high temperatures, which can lead to rupture, and this scale can also reach the turbine, causing various problems.

Ferritic heat-resistant steel is also known as the boiler material. Compared to the austenitic stainless steel mentioned above, this heat-resistant steel has the advantages of not only a lower coefficient of thermal expansion, but also reduced stress corrosion cracking and intergranular corrosion at high temperatures, high thermal conductivity, and low cost. It is equipped with For this reason, ferritic heat-resistant steel has conventionally been used as a material for the above-mentioned supercritical pressure and ultra-supercritical pressure boilers, and as a material for various equipment for the chemical industry.

[0005]

[Problems to be Solved by the Invention] However, the above-mentioned conventional ferritic heat-resistant steels, for example, 9Cr-1Mo to 2
Although Mo-based steel has superior oxidation resistance compared to 21/4 Cr-1Mo steel, it has low high-temperature strength, which limits its use.

[0006] Furthermore, the above-mentioned 21/4 Cr-1Mo steel and S
TBA26 steel (9Cr-1Mo steel), STBA27
As a steel with improved high temperature strength of steel (9Cr-2Mo steel), super 9Cr steel, i.e. ASTM A213
T91 steel (9Cr-1Mo-Nb-V steel) is also known. However, although this T91 steel has improved high-temperature strength to some extent compared to conventional steel, it is still insufficient, and there has been a need to develop a material that can cope with further increases in steam temperature.

[0007] In response to such requests, conventionally, 9 to 12C
The high-temperature strength has been improved by adding Nb, V, or W to r-steel.Furthermore, composite steel is being developed by adding appropriate combinations of various elements such as Ti, B, and W. Ferritic steel whose high-temperature strength is further improved by adding additives is disclosed in JP-A-63-65059 and JP-A-6
It is disclosed in Publication No. 3-76854 and the like. However, these ferritic steels required further improvement in high-temperature strength in order to be used in boiler tubes for super-supercritical pressure power generation.

[0008] Furthermore, these steels (9-12Cr steel)
In order to improve high temperature strength, Mo, W, Nb,
Addition of elements such as V causes a decrease in hot deformability, that is, hot workability, and as a result, there is a problem in that manufacturability deteriorates. That is, a decrease in hot deformability has a negative effect on manufacturability, and the effect is particularly large, for example, not only in thick plates and forged steel products, but also in the case of steel pipes that are processed under severe processing conditions. In particular, when seamless steel pipes are manufactured using the Mannesmann method, the 9-12Cr steel is made of Mo, W,
Because of the addition of elements such as Nb and V, hot workability is poor, and as a result, flaws are likely to occur during pipe making, resulting in lower production efficiency and yield, and an increase in manufacturing costs.

As explained above, the above-mentioned 9-12Cr steel has achieved improvement in high-temperature strength, particularly creep rupture strength, but it cannot be said to be sufficiently high, and in addition, it has the problem of poor hot workability. Therefore, the present invention
The purpose of the present invention is to overcome these problems and provide a steel that has both excellent high-temperature strength and hot workability.

0010

[Means for Solving the Problems] As a result of intensive research to achieve the above-mentioned object, the present inventors have found that high-Cr ferritic steel containing W has improved hot workability and high-temperature strength. As a result of our study on the premise that high levels of B and B can be obtained, we found that B,
It has been found that adding Ca and Ta in combination is effective. That is, the basis of the alloy design of the steel of the present invention is that an appropriate amount of Mo,
It was discovered that both hot workability and high-temperature strength were improved by adding B, Ca, and Ta in combination to 9-12Cr steel to which W, V, and Nb were added, and the present invention was completed. It was.

That is, in the present invention, C: 0.03 to 0.
20 wt%, Si: 1.0 wt% or less, Mn: 0
．． 1 to 1.5 wt%, Cr: 7.0 to 13.
0 wt%, Mo: 0.4-2.5 wt%, W: 0.0
5-2.50wt%, V: 0.02-0.35wt
%, Nb: 0.01-0.15wt%, B: 0.0
008-0.0100wt%, Ta: 0.010
~0.100 wt%, Ca: 0.0005~0.01
50 wt%, N: 0.005 to 0.080 wt%, and the balance consists of Fe and unavoidable impurities, and is characterized by excellent hot workability and high-temperature strength, and furthermore, Ti :0.01~0.4
It is a ferritic heat-resistant steel with excellent hot workability and high-temperature strength, containing one or two of 0 wt% or 0.1 to 1.0 wt% Ni.

0012

[Function] As mentioned above, the present invention has been developed as a result of a detailed study of the effects of various alloying elements on improving hot workability and high-temperature strength. It has been found that by adding appropriate amounts of B, Ca and Ta in combination, not only hot workability but also high temperature strength, especially high temperature creep rupture strength, is improved.

[0013] Figure 1 shows 0.10wt% (hereinafter referred to as "%").
) C-0.18%Si-0.41%Mn-11
．． 9%Cr-0.47%Mo-0.09%Nb-0.2
1%V-2.1%W-0.04%N steel 0.0032
Hot deformability of steel containing %B-0.0021%Ca-0.016%Ta (A1 steel) and steel without B, Ca and Ta (A2 steel) with the same composition system ( This is a comparison graph of processability). This hot deformability is obtained by manufacturing the above A1 steel and A2 steel ingots in a high frequency furnace.
This steel ingot is hot rolled into a 20 mm thick steel plate, which is then subjected to the following Greeble test. In this Greeble test, a round bar tensile test piece with a parallel part of 6 mmφ is heated to 1250°C, then lowered to each temperature, and a tensile test is performed at that temperature. It was evaluated. The larger the diameter reduction rate, the better the hot deformability.

[0014] In addition, a creep rupture test was conducted to clarify the high temperature strength level. In this test, the steel plate was further normalized at 1050°C, and then heated to 760°C for 1
A round bar test piece with a parallel portion of 6 mmφ was taken from the treated steel plate after being tempered for a period of time. As can be seen from the results shown in Figure 1, B, Ca and T
It is clear that the diameter reduction rate, that is, the hot workability, of the A1 steel with the composite addition of a is greatly improved compared to the A2 steel without these additions.

Next, FIG. 2 shows 0.11% C- containing Cr.
0.16%Si-0.45%Mn-0.51%Mo-0
．． 08%Nb-0.22%V-1.9%W-0.04
%N steel containing 0.0029% B, 0.0018% Ca, and 0.022% Ta (B1 steel), and steel with the same composition system but without B, Ca, and Ta (B1 steel).
The graph shows the results of evaluating the diameter reduction rate (hot workability) of B2 steel by changing the Cr content using the Greeble test. Although the components varied slightly depending on the sample tested, the values were close to the values listed above. As can be seen from this figure, in the case of B1 steel containing B, Ca, and Ta, the diameter reduction rate is good at 65 to 85%, whereas it is clear that B2 steel, which does not contain these elements, is significantly inferior. It is.

As can be seen from these test results, hot workability, that is, manufacturability, can be improved by adding appropriate amounts of B, Ca, and Ta in a high Cr-Mo-W steel containing appropriate amounts of Nb and V. It was confirmed that there was also a significant improvement. Note that steel types with good workability, such as A1 steel and B1 steel, can be made into pipes using the Mannesmann method without producing any flaws.

Next, the high temperature strength was investigated and the results will be explained. This study is about creep rupture properties. Figure 3 shows 0.10% Cr content.
C-0.15%Si-0.43%Mn-0.49%Mo
-0.09%Nb-0.21%V-2.0%W-0.
04%N steel with 0.0031%B, 0.0019%Ca, and 0.034%Ta added (C1 steel), the Cr content was changed, and 11kg was produced at 650℃.
The creep rupture properties were evaluated when a stress of /mm2 was applied. The procedures for heat treatment and sample collection are the same as in the Greeble test. As shown in this figure, by adding appropriate amounts of B, Ca, and Ta in combination, this steel type had a sufficiently high creep rupture time, that is, creep rupture strength.

[0018] As is clear from the above test results and explanation, hot working is improved by adding appropriate amounts of B, Ca, and Ta in a high Cr-Mo-W steel containing appropriate amounts of Nb, V, etc. It can be seen that not only the properties are greatly improved, but also the creep rupture time, that is, the creep rupture strength is sufficiently high.

The reason why the ferritic heat-resistant steel according to the present invention is limited to the above-mentioned composition will be explained below. C: C is an element that contributes to improving creep rupture strength through the formation of low-temperature transformation products and precipitation of carbides. If it exceeds 0.20%, the hardenability increases significantly and the strength increases, but weldability and workability deteriorate, so the upper limit of the C content was set to 0.20% or less. On the other hand, if it is less than 0.03%, it becomes difficult to ensure high temperature strength, so the lower limit was set at 0.03%. Si: Si is added as a deoxidizer, but if used in a large amount, the toughness of the steel deteriorates, so the upper limit was set at 1.00%. Note that it is desirable that this Si content be low in order to improve high-temperature long-term strength and toughness. Mn: Mn is an element useful as a deoxidizing and desulfurizing agent, and for obtaining a suitable structure with improved strength and hot workability. If the Mn content is less than 0.1%, the above effects will not be achieved, while if it exceeds 1.5%, the hardenability will increase, and although the strength will increase, it will cause deterioration of workability such as bending and toughness. , 0.1 to 1.5%. Cr: Cr is added to improve high-temperature oxidation resistance and high-temperature long-term strength.The high-temperature long-term strength of 650°C or higher is high when Cr: 7.0 to 13.0%, and this When the Cr content exceeds 13.0%, δ ferrite increases and the high temperature long-term strength decreases. On the other hand, 7.
If it is less than 0%, precipitation strengthening due to Cr carbides and solid solution strengthening of Cr cannot be expected, and high-temperature long-term strength decreases, and high-temperature oxidation resistance also decreases. Therefore, the Cr content is 7
．． The content was set at 0 to 13.0%. Mo, W: Mo and W are indispensable elements for heat-resistant steel because both significantly increase high-temperature long-term strength. These two elements not only strengthen the steel by forming a solid solution in the steel, but also precipitate carbides to improve creep strength, but this effect is not achieved when Mo is less than 0.4% and W is less than 0.05%. do not have. On the other hand, Mo exceeding 2.5% or W exceeding 2.5% increases the amount of δ ferrite and lowers the high temperature strength. Moreover, since Mo and W are expensive elements, the cost is high and it is not preferable from an economic point of view, so the upper limit was set at 2.5%. In addition, when Mo is added alone, 600
Since the creep strength at temperatures exceeding .degree. C. is insufficient, it is necessary to add at least 0.05% of W. Nb, V: Nb, V precipitates as carbides or carbonitrides and has the effect of suppressing a decrease in high temperature strength over a long period of time. The solubility product of Nb carbide or Nb carbonitride is smaller than the solubility product of V carbide or V carbonitride, and because it is easy to precipitate, the high-temperature short-time strength is significantly increased. However, when these are added alone, Nb carbide, Nb
Carbonitrides tend to aggregate and coarsen, and it becomes difficult to maintain high-temperature strength for a long time. That is, the combined addition of Nb and V is essential for improving long-term strength. This is because Nb (C, N) precipitated during the manufacturing process,
This is because it suppresses the coarsening of carbides: M23C6 and M6C that precipitate at high temperatures. That is, in addition to the V4C3 carbide, V in a solid solution state is the carbide M23C6,
By diffusing into M6C, coarsening of these carbides is suppressed. In this way, due to the combined addition of Nb and V, these finely precipitated precipitates, as well as M23C6, M6C, which finely precipitates after a long period of time,
Solid solution V improves high-temperature long-term strength. therefore,
If V is used alone, fine carbides M23C6, M6C cannot be obtained and long-term strength cannot be improved. In addition, V,
If Nb is less than 0.02%, the above effects are insufficient. In addition, if too much V or Nb is present, the carbides will become coarser and the creep rupture strength will be lowered, as well as notch toughness and weldability. Therefore, Nb was set to 0.15% or less, and V was set to 0.35% or less. . B: B is a high Cr steel containing appropriate amounts of V and Nb, Mo and W, and additionally appropriate amounts of Ca and Ta.
By adding it in combination with C, it greatly contributes to improving hot workability and increases high-temperature creep strength. This effect cannot be obtained with B alone, and is particularly noticeable when B is added in combination with Ca and Ta. The B content to obtain this effect is
It is necessary to add at least 0.0008% or more; on the other hand, if the addition exceeds 0.01%, the effect will be saturated and the hot workability will deteriorate.
Must be 0.01% or less. Ca, Ta: Ca and Ta can significantly improve hot workability and high-temperature creep rupture strength by adding them together with an appropriate amount of B in high Cr steel containing appropriate amounts of Nb, V, and W. can be improved. As described above, the improvement effect cannot be obtained by adding only Ca and Ta, but is noticeable only by the combined addition of the trinity of Ca, Ta, and B. To obtain this effect, Ca is 0.0005
% or more, and Ta needs to be added in an amount of 0.010% or more. Furthermore, if Ca is added in excess of 0.0150%, and Ta is added in excess of 0.100%, the effect will be saturated and the cost will increase. 0.015
0% or less, and 0.100% or less of Ta is contained. Ni: Ni is added to appropriately control the amount of δ ferrite, maintain high temperature strength, and improve toughness. If the amount is less than 0.10%, there is no effect; on the other hand, if the amount is less than 0.10%,
%, the toughness improving effect tends to be saturated. Moreover, since Ni is an expensive element, the upper limit is set to 1.0.
%. Ti: High-temperature creep rupture strength can be improved by further adding Ti to high-Cr steel containing appropriate amounts of Nb, V, W, and Mo. To obtain this effect, it is necessary to add Ti content of 0.01% or more; on the other hand, if it exceeds 0.4%, the effect will be saturated and the carbides will tend to coarsen. Therefore, it must be kept at 0.4% or less. N: N is an element that forms nitrides or carbonitrides, and furthermore, inherent N is an element that improves high-temperature long-term strength. If the N content is less than 0.005%, there will be no effect, while if it exceeds 0.08%, blowholes will be formed during welding and weldability will be significantly deteriorated.
．． 008%.

[0021]

[Example] Table 1 shows the chemical composition of heat-resistant ferritic steels according to the present invention as well as test results of creep rupture time and diameter reduction rate of these steels, and Table 2 shows the results of comparison steels. shows. The above test results were obtained by manufacturing a steel ingot with each component composition in the table in a high-frequency melting furnace, hot rolling this steel ingot into a 20 mm thick steel plate, and then conducting a Greeble test to determine hot workability. The evaluation was based on the diameter reduction rate (%) at 1100°C. In addition, the creep test was performed by normalizing at 1050°C and tempering at 760°C for 1 hour, and then processing into a 6mmφ round bar tensile test piece and conducting it at 650°C with a stress of 11kg/mm2, and the rupture time. This is what we sought.

[0022] The steels of the present invention indicated by symbols A to R include C, Si, Cr, Mo, Nb, V, W.
, B, Ta, Ca, Ti, and Ni.

[0023] Among the comparative steels, steel A contains Nb, V, W
B steel is a steel in which B is added in addition to Nb, V, and W; C steel is a steel in which B and Ta are added in addition to Nb, V, and W, and D steel is a steel in which B and Ta are added in addition to Nb, V, and W. In addition, B and Ca
E steel is a steel that has Ta and Ca added in addition to Nb, V, and W. F steel is a steel that has Ta, Ca, and B added in addition to Nb, V, and W. This is an example in which the amount of B is less than the amount required for the steel of the present invention. All of the comparative examples are examples of cases in which the components of the present invention are not compatible.

[0024] From the results shown in Tables 1 and 2, first, the steel type according to the present invention has good hot workability of 70% or more as evaluated by the diameter reduction rate at 1100°C by the Greeble test, and It has a creep rupture time of 4000 hours or more and has excellent high temperature strength.

On the other hand, in the case of the comparative steels shown in Table 2, the creep rupture times are shorter and inferior to the above-mentioned steel of the present invention. Moreover, it can be seen that the hot workability evaluated by the diameter reduction rate at 1100° C. by the Greeble test is also inferior to the steel of the present invention.

[0026]

[Table 1]

[0027]

[Table 2]

As described above, the steel of the present invention has significantly better hot workability, which is an important property for manufacturability, than conventional steels, and also has excellent long-term strength at high temperatures. Therefore, it is possible to use the steel as an alternative to the expensive 18Cr-8Ni austenitic stainless steel, and the steel of the present invention is extremely useful as a heat-resistant steel for heat exchange equipment and the like.

As explained above, the ferritic heat-resistant steel according to the present invention has a higher temperature than the conventional ferritic heat-resistant steel.
Hot workability, which affects manufacturing properties, has been significantly improved, and strength at high temperatures, especially above 600℃, has been significantly improved.
This is a steel with significantly improved creep rupture strength (especially creep rupture strength). Therefore, it is suitable for use in super-supercritical pressure boiler materials used in high-temperature, high-pressure environments, steam generator tubes for FBR, heater tubes, etc., and the use of this steel material makes these devices thinner and has a longer lifespan. can be realized.

[Brief explanation of the drawing]

FIG. 1 is a graph comparing the effect of the presence or absence of combined addition of B, Ca, and Ta on the diameter reduction rate determined by the Greeble test.

[Fig. 2] Fig. 2 is a graph comparing the effect of the presence or absence of combined addition of B, Ca, and Ta on the diameter reduction rate determined by the Greeble test for steels with varying amounts of Cr.

[Fig. 3] Fig. 3 is a graph comparing the effect of the presence or absence of combined addition of B, Ca, and Ta on the creep rupture time of steels with varying amounts of Cr.

Claims (2)

[Claims] [Claim 1] C: 0.03 to 0.20 wt%,
Si: 1.0 wt% or less, Mn: 0.1 to 1.5 w
t%, Cr: 7.0 to 13.0wt%, Mo: 0
．． 4 ~ 2.5 wt%, W: 0.05 ~ 2.50
wt%, V: 0.02-0.35wt%, Nb: 0
．． 01-0.15wt%, B: 0.0008-0.01
00wt%, Ta: 0.010 ~ 0.100w
t%, Ca: 0.0005 to 0.0150 wt%, and N: 0.005 to 0.080 wt%,
A ferritic heat-resistant steel with excellent hot workability and high-temperature strength, characterized in that the remainder consists of Fe and unavoidable impurities. [Claim 2] C: 0.03 to 0.20 wt%,
Si: 1.0 wt% or less, Mn: 0.1 to 1.5 w
t%, Cr: 7.0 to 13.0wt%, Mo: 0
．． 4 ~ 2.5 wt%, W: 0.05 ~ 2.50
wt%, V: 0.02-0.35wt%, Nb: 0
．． 01-0.15wt%, B: 0.0008-0.01
00wt%, Ta: 0.010 ~ 0.100w
t%, Ca: 0.0005 to 0.0150 wt%, and N: 0.005 to 0.080 wt%, in addition to T
i:0.01-0.40wt% or Ni:0.1
Contains up to 1.0 wt% of one or two types, with the remainder F
A ferritic heat-resistant steel with excellent hot workability and high-temperature strength, which is characterized by comprising e.g. and inevitable impurities.