JPH02294452A - Ferritic heat resisting steel excellent in toughness in welded bond zone - Google Patents

Abstract

PURPOSE:To remove the remaining of delta-ferrite and to improve toughness in a welded bond zone by specifying the components of a steel and particularly limiting the relationship among the additive quantities of Mn, Ni, and Cu. CONSTITUTION:A ferritic heat resisting steel has a composition which consists of, by weight, 0.01-0.30% C, 0.02-0.80% Si, 0.20-3.00% Mn, 8.00-13.00% Cr, 0.05-1.00% Ni, 0.005-1.00% Mo, 0.50-3.00% W, 0.05-0.50% V, 0.02-0.12% Nb, 0.0003-0.008% B, 0.10-5.00% Cu, 0.0005-0.10% Zr, 0.01-0.10% N, <=0.050% P, <=0.010% S, <=0.020% O, and the balance Fe with inevitable impurities and in which the condition of Mn%+Ni%+2Cu%<=12 is further satisfied, or further, Ta and/or Hf and Co and/or Ti are incorporated to the above composition and the condition of Mn%+Ni%+Co%+2Cu%<=12 is satisfied. This steel is applicable to a steel for boiler steel tube, etc.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a ferritic heat-resistant steel, and more specifically to a ferritic Cr-containing boiler steel pipe steel used in high temperature and high pressure environments. .

(Prior Art) In recent years, the operating conditions of thermal power generation boilers have become significantly higher in temperature and pressure, and in some cases, temperatures have reached 566°C. It is planned to operate at 310 atmospheres. In the future, 650°C. Conditions of up to 350 atmospheres are assumed, which is an extremely harsh condition for the materials used.

When the operating temperature exceeds 550°C, when selecting the material to be used, from the viewpoint of oxidation resistance and high temperature strength, for example,
18 from ferritic 2'/4Cr I Mo steel
At present, materials such as high-grade austenitic steel such as -8 stainless steel are used which are excessively high in terms of material properties and cost.

Steel materials that fill the gap between 2% Cr I Mo steel and austenitic stainless steel have been sought for the past several decades. Boiler steel pipes with intermediate Cr content, such as 9Cr and 12Cr, are heat-resistant steels developed based on the above background.
Increasing the creep strength deteriorates the weld properties.

Therefore, Boi. This method has the problem of being difficult to put into practical use because it significantly reduces work efficiency during construction and renovation. From this point of view, the emergence of 9Cr and 12Crtg4, which have high creep strength and excellent weld properties, has been eagerly awaited. In addition, the welding process for manufacturing boilers includes welding and post-weld heat treatment.
At+went (hereinafter referred to as PWHT) or a method of performing welding-PWHT after hot working is adopted. Therefore, it goes without saying that the performance required of such boiler steel is excellent weldability, and even after being subjected to this thermal history, the welded part remains intact. It is important that both the base metal maintain sufficient strength and toughness.

From this point of view, new steels with improved weldability and significantly higher creep rupture strength than conventional materials have already been published in JP-A-63-89644 and JP-A-6.
This method is disclosed in Japanese Patent Publication No. 1-231139 and Japanese Patent Application Laid-Open No. 62-297435. These steels are made of conventional heat-resistant steel that has dramatically increased creep strength by incorporating W as a solid solution, but on the other hand,
The addition of W increases the Cr equivalent value, which is higher than that of conventional materials. Therefore, although the base metal has a martensite or tempered martensite single phase structure, the cooling rate at the weld bond increases. The present inventors subsequently discovered that because the ferrite phase just below the melting point (hereinafter referred to as δ ferrite for convenience) remains untransformed in a band shape along the bond, the toughness of the weld bond significantly decreases. This was revealed through detailed research by et al.

Furthermore, it has been found that the untransformed residual δ ferrite does not disappear by PWHT, and it is most effective to completely transform it during cooling after welding.

The present inventors further conducted research and added Cu to conventional steel, and further increased the amount of Mn, Ni, and Cu added to Mn%+N.
i%+2Cu%≦12 When the condition is satisfied, the welded part properties are improved without any loss of the excellent high temperature properties of these steels. In particular, we succeeded in developing a heat-resistant steel with excellent weld bond toughness.

There is almost no precedent for heat-resistant steel in which W is solidified to increase creep strength and Cu is added to improve weld toughness.Cu
Heat-resistant steel containing 0.4 to 1.5% of
Although disclosed in Publication No. 304, this steel has a W of 0.0.
As low as 5 to 0.5%, it is impossible to simultaneously achieve a high creep strength such as that of the steel of the present invention. Cu 1.0
JP-A-60-155649 as heat-resistant steel with addition of less than %
There is a disclosure of the issue. This steel has Mo+W of 0.5 to 2.
5%, and at the same time a certain degree of creep strength can be obtained, but there are no restrictions on N and Nb, which are necessary to improve the toughness and strength of the base metal, and the toughness. Both strength cannot be equal to that of the steel of the present invention.

(Problems to be Solved by the Invention) The present invention solves the above-mentioned conventional drawbacks, namely, the toughness caused by δ ferrite remaining in a band shape at the weld bond part during welding in Cr-containing ferritic heat-resistant steel with high creep strength. This makes it possible to manufacture heat-resistant steel with excellent welding properties by preventing the deterioration of the welded part.
Contains 0 to 5.00%, and also contains Mn, Ni, Co,
Addition amount of Cu is Mn%+Ni%+Co%+2Cu%≦
By limiting the content to satisfy the condition 12, δ
The objective is to provide a heat-resistant steel with no residual ferrite.

(Means for Solving the Problems) The present invention has been made based on the above findings, and its gist is that C: 0. 0 1~0.
30%, Si: 0.0 2~0.8 0%, M
n: 0.20~3.00%, Cr: 8.0 0~
13.00%, Ni: 0.05-1. 0
0%, W:o. so~3. O O%, Mo: 0.00
5-1.00%, V: 0.05-0.50%, Nb: 0
．． 02-0.12%, B: 0.0003-0
．． 0 0 8%, Cu: 0.10-5.00
%, Zr: 0.0 0 0 5~0. Contains 10%, P: 0. 0 5 0% or less, S: 0.01
0% or less, 0:0. 0 2 0% or less, or further (A) Ta: 0.01 to 1.00%, Hf: 0
．． 01 to 1.00% of one or two types and/or (
B) Co: 0.01-1.00%, Ti; 0.01-0
．． Contains 10% of one or two of Mn, N
i. The amount of Co and Cu added is Mn% + Ni% + Co%
It is a ferritic heat-resistant steel with excellent weld bond toughness, which satisfies the condition of ±2Cu%≦12, with the remainder consisting of Fe and unavoidable impurities.

The present invention will be explained in detail below.

(Function) First, the reason for limiting the range of each component in the present invention as described above will be described below.

C is necessary to maintain strength, but if it is less than 0.01%, it is insufficient to ensure strength, and if it is more than 0.30%, the weld heat affected zone will be significantly hardened, causing cold cracking during welding. Narume, set the range to 0. Ol~0.30%.

Si is an element that is important for ensuring oxidation resistance and is necessary as an auxiliary deoxidizer for Zr, but if it is less than 0.02%, it is insufficient, and if it exceeds 0.80%, the creep strength will decrease, so it is not necessary. The content was set at .02 to 0.80%.

Mn is a necessary component not only for deoxidizing but also for maintaining strength. In addition, since it is an austenite stabilizing element,
It has the effect of reducing residual δ ferrite in the weld bond. In order to sufficiently obtain both effects, it is necessary to add 0.20% or more, and if it exceeds 3.00%, the base material may become brittle due to an excessive increase in strength.
It was set at 3.00%.

Cr is an essential element for oxidation resistance, and at the same time combines with C to create M! ! Fine precipitation in the base material matrix in the form of C&, M&C, l'hc (where M represents a metal element), etc. contributes to an increase in creep strength.

From the viewpoint of oxidation resistance, the lower limit is 8.00%, and the upper limit is:
1 3. For the purpose of limiting the Cr equivalent value to a low value in order to ensure the toughness of the weld bond part. It was set as 00%.

W is an element that significantly increases creep strength through solid solution strengthening and precipitation strengthening by precipitation as carbides.
In particular, the long-term creep strength is significantly increased at high temperatures of 550°C or higher. If added in excess of 3.00%, a large amount of carbide will precipitate and significantly reduce the toughness of the base material, so the upper limit was set at 3.00%. Moreover, since the effect of precipitation strengthening is insufficient if it is less than 0.50%, the lower limit is set to 0.50%.

Mo is an element that significantly increases high temperature strength through solid solution strengthening, but if it is less than 0.005%, the effect is insufficient.
More than 1.00% is MO! The upper limit was set at 1.00% because if added at the same time as W, the toughness of the base material may be significantly reduced due to large amounts of C-type carbide precipitated.

Like W, (2) is an element that significantly increases the high-temperature strength of steel, whether dissolved in the matrix or precipitated as a precipitate.

Especially in the case of precipitation, LzsCa, M6C as V4C3
, M! It becomes a precipitation nucleus of C and has a remarkable effect on fine dispersion of precipitates. Less than 0.05% has no effect; 0.50
If the amount exceeds 0.0%, the toughness decreases, so the range of addition is limited to 0.0%.
05 to 0.50%.

Nb increases the high-temperature strength through the precipitation of Nb (CN), and, like (2), promotes fine precipitation as precipitation nuclei of MzsCb, M&C, l'hc, etc. In order to exhibit the effect of addition, the lower limit is set to 0.02%, and if it exceeds 0.12%, the precipitates will aggregate and coarsen, reducing the strength, so the upper limit is set to 0.02%.
It was set at 12%.

B is well known as an element that significantly improves hardenability, but in heat-resistant steel, creep strength is improved by grain boundary strengthening due to fine precipitation of boride at grain boundaries. In order to exhibit the effect of B, the lower limit is set to O. O O O is set at 3%, and the upper limit is set at 0.0% so as not to impair toughness. 0 0
It was set at 8%.

Zr controls the deoxidation equilibrium in steel and suppresses the formation of oxides by significantly lowering oxygen activity. In addition, it has a high affinity for N, suppresses the precipitation of BN due to nitridation of B, and prevents the effect of B addition from being impaired when a large amount of nitrogen is added. o
．． Less than oos% is insufficient for controlling the deoxidation equilibrium;
0. If more than 10% is added, coarse ZrN, Zr
Since C precipitates in large quantities and significantly reduces the toughness of the base material, it is limited to a range of 0.0005 to 0.10%.

N is an element that increases creep strength by solid solution in the matrix or precipitates as nitrides and carbonitrides, but the lower limit is set at 0.01% to ensure creep strength, and to avoid blowholes during casting. In order to obtain a sound steel ingot, the upper limit was set to 0.10%.

Ni is a typical austenite stabilizing element, and -
Added to prevent the formation of δ ferrite in the base material. Therefore, it is also possible to prevent δ ferrite from remaining in the weld bond. Less than 0.05% has little effect;
Addition of more than 1.00% lowers the creep strength, so the addition range was limited to 0.05 to 1.00%.

Cu is an additive element that is the main focus of the present invention, and has the effect of reducing the Cr equivalent value without reducing the creep strength and preventing δ ferrite from remaining even during rapid cooling during welding heat cycles. There is. If the addition is less than 0.10%, the reduction in Cr equivalent value is insufficient, and if the addition exceeds 5.00%, pure Cu will precipitate at the grain boundaries when the steel is exposed to high temperatures for a long time, causing material embrittlement. 0. 1 0-5.0
The range was 0%.

P, S, and O are mixed as impurities in the steel of the present invention, but in order to exert the effects of the present invention, the upper limit values of P and S, and 0 as oxides lower the toughness, must be set at their respective upper limits. 0.050%, o. oto%, 0.020%.

The above are the basic components of the present invention, but in the present invention, in addition to these, (A) Ta: 0.01 to 0.01 to
1.00%, Hf: 0.01 to 1.00%, and/or (B) Co: 0. 0 1 ~
1.00% and Ti: 0.01 to 0.10%.

Ta and Hf are elements that act as auxiliary deoxidizers for Zr at low concentrations, and finely precipitate as carbides at high concentrations, increasing creep strength. All 0.01%
There is no effect if it is less than 1. If added in excess of 0.01 to 1.0%, carbides become coarse and toughness decreases.
00% range.

Co and Ti are elements that precipitate as carbides and improve the high-temperature strength of the base material. 0 each. If the Ti content is less than 0.1%, there is no effect; if the CO content exceeds 1.00%, coarse carbides will precipitate, and if the Ti content exceeds 0.10%, coarse nitrides will precipitate, leading to a decrease in toughness.
Co:0.01-1.00%, Tt:O. O
The range was 1 to 0.10%.

Each of the above-mentioned alloy components may be added individually or in combination.

Among the alloy components mentioned above, elements that are highly effective in improving the toughness of welded bonds, and which may also cause a decrease in creep strength or cause the steel to deteriorate at high temperatures for long periods of time, namely Mn, Nt, Co, and Cu. is not limited only by the upper limit of each individual concentration, but must satisfy the following formula Mn%+Ni%+CO%+2CLI%≦12 based on the research of the present inventors.

The above inequality was determined by the following experiment.

Heat-resistant steel having the components shown in claims (1) to (4) of the present invention was melted using a vacuum induction heating furnace and cast into a 2 ton ingot. A billet was cut into a specified size from the ingot, heated to 1180'C, hot extruded to form a pipe with a diameter of 50.8 mm and a wall thickness of 9.5 m, and baked at 1050°C for 1 hour and 760°C for 1 hour. Semi-
A test specimen was prepared by subjecting it to tempering treatment.

A test piece was prepared by welding threaded joints cut separately from the same ingot to both ends of a vibrator cut to a length of 500+++ m, and subjected to a real pipe creep test using a large-scale high-temperature atmosphere controlled tensile testing machine.

The test conditions were 600°C, and the breaking strength was investigated for up to 100,000 hours, and after the creep curve was taken, the breaking strength at 100,000 hours was evaluated.

FIG. 1 is a diagram in which Mn%+Ni%+Co%+2Cu% is plotted on the horizontal axis and 100,000 hour breaking strength is plotted on the vertical axis.

It can be seen that when the value of Mn%+Ni%+Co%+2Cu% is 12 or less, the creep rupture strength exhibits 16 kg/1 or more, but when it exceeds 12, it rapidly decreases. From the results shown in FIG. 1, it can also be seen that the breaking strength of the steel of the present invention at 6,000,000 hours of CIO is 16 kg/1 or more.

With the result in Figure 1, the inequality is h%+Ni%+Co%+2Cu%≦12 was determined.

Furthermore, since the present invention provides a heat-resistant steel having excellent weld bond toughness and high creep rupture strength, the steel of the present invention can be manufactured by various methods and subjected to heat treatment depending on the purpose of use. However, this does not impede the effects of the present invention in any way.

First, as a melting process, VIM (vacuum induction heating furnace)
, EF (electric furnace), and LD (converter furnace) can be used and are useful. Next, we will introduce ll! as a method of cleaning molten steel using outside-furnace refining equipment. SR (Electro S
lag Remelting). A O D (Ar
gon OxygenDecarbrizaLion)
, V A D (Vacuum Argon De
carbrization)+V O D (Vacu
m Oxygen Decarbrization)+
and LF (Ladle Furnace) and other processes using vacuum degassing or powder blowing refining equipment (e.g. R}l, DH, CAS, etc.) can be used alone or in combination and are suitable. .

Molten steel is cast into a mold and then into a slab or billet using a continuous casting machine to form a steel ingot, which can then be processed into shapes suitable for various manufacturing processes.

The manufacturing process includes processing into round billets or square billets and then processing them into seamless pipes and tubes by hot extrusion or various seamless rolling methods, and hot rolling into thin sheets, cold rolling, and then electric resistance welding. Den IJ! A method of making steel pipes, and TIG,
MIG, SAW, LASER, and EB welding (either singly or in combination) can be used to form welded steel pipes, and each of the above methods can be followed by hot or warm S-welding.
It is also possible to additionally perform R (reduction rolling) or shaping rolling, and it is possible to expand the applicable size range of the steel of the present invention.

The steel of the present invention can be produced not only in the form of steel pipes, but also in the form of thick plates and thin plates, and can be used in the form of various heat-resistant materials, either as hot-rolled or by using plates that have been subjected to the necessary heat treatment. It is possible to do so without affecting the effects of the present invention.

The above-mentioned steel pipes, plates, and heat-resistant members of various shapes can be subjected to various heat treatments depending on their purpose and use.
It is also important for fully exhibiting the effects of the present invention.

Normally, products are made through a normalizing + tempering process, but in addition to this, it is possible and useful to perform quenching, tempering, and normalizing processes either alone or in combination. . It is also possible to repeat each of the above steps multiple times within the range necessary to fully develop the material properties, and this does not affect the effects of the present invention in any way.

The above steps may be appropriately selected and applied to the manufacturing process of the steel of the present invention.

[Example] 1 ton of each steel having a composition according to any one of claims 1 to 4 shown in Tables 1 to 4 was melted using a vacuum induction heating furnace, and cleaned by ESR treatment to reduce impurities. After that, it was cast into a mold, processed into a round billet, and hot extruded to an outer diameter of 60mm. A tube with a wall thickness of 10M was seamlessly rolled to produce a pipe with an outer diameter of 380 mm and a wall thickness of 50 mm. tube. The pipe was normalized twice at 1050°C for 1 hour and then tempered at 760°C for 1 hour.

The creep characteristics are determined in the axial direction 6 of the steel pipe 5, as shown in FIG.
A creep test piece 7 with a diameter of 6 mmφ was cut out parallel to the
The creep rupture strength was evaluated at 600°C for up to 100,000 hours. A creep rupture strength of 16.0 kg/1 was used as the threshold for creep strength evaluation.

The toughness of the weld bond was determined by processing a U-groove at each end of a steel pipe with the same outer diameter and wall thickness, butting a pair of specimens together, and performing 'rtc welding with an appropriate heat input. After annealing for 1 hour at 600°C and aging for 100,000 hours at 600°C, as shown in Fig. 2m in the thickness direction at the position where line 3 crosses center line 1 of the thickness
A Charby impact test piece 4 with a V-notch was taken, and θ
Evaluation was made using the absorbed energy value at °C.

The base metal toughness was measured by taking samples parallel to the axis of the tube in the same way as the creep test specimens, making a 2mm+ V-notch, and measuring the absorbed energy at 0°C.

Both the weld bond part toughness value and the base metal part toughness value are set at 5.0 kgf-m at 0°C as the evaluation threshold.

The creep rupture strength and weld pound toughness after 100,000 hours are shown in Tables 1 to 4. In addition, the bond part toughness investigation results in the table are the average values of 5 Charpy test points at O''C.Also, NIE. means Mn%+N
It is the value of the formula i%+CO%+2Cu% (unit: -t%). For comparison, steel having components that do not fall under any of claims 1 to 4 of the present invention was melted and produced in the same manner. evaluated. The chemical components and evaluation results are shown in Table 5. Figure 4 is C
The effect of u addition on the weld bond toughness is shown.

Cu is 0. It can be seen that the Charby impact value of the bond at 0°C after 600" CIO aging for 10,000 hours increases significantly when the content is 1% or more. Figure 5 shows that the δ ferrite area ratio (from the bond line to It is a diagram showing that the ratio of the area of residual δ ferrite to the total area within 50 μm on the base metal side decreases.With the addition of 0.1% or more of Cu, the area ratio of residual δ ferrite becomes almost 0%. The results shown in Figure 4 are brought about by the effects shown in Figure 5.

FIG. 6 is a diagram showing the influence of Cu addition on the base material toughness after 100,000 hours of aging. It can be seen that when the Cu content is 5.0% or less, the Charpy impact value of the base material at 0°C is high.

No. 161 among the comparative steels shown in Table 5! iii, 16
Because No. 2 steel had insufficient Cu content or no additive, a large amount of δ ferrite remained in the welding pad.
An example of failure to ensure the toughness of the weld bond, No. 163 steel.
Steel No. 164 had too much Cu added, so pure Cu was present at the grain boundaries.
An example of steel No. 165 precipitated and became brittle, and the toughness value at 0°C after aging for 100,000 hours at 600"C of the base metal was low. Although the Cu content of No. 166 steel was appropriate, Mn
%+Ni%+Co%+2Cu% value (NiE in the table) exceeds 12, and the creep rupture strength at 600″ CIO decreases.
An example where the creep strength decreased due to a high W content, and the toughness after aging of the base metal also decreased. An example where No. 168 steel had a low W content and the creep rupture strength at 600°CtO 10,000 hours decreased. Because No. 169 steel had an excessive W content, its creep strength at 600° CIO was high, but the Charpy impact value at O″C of the weld bond and base metal after aging at 600° CIO was low. This is an example of a decrease in

[Effects of the Invention] The present invention provides a Cr-containing ferritic heat-resistant steel that has a high toughness value at the welded bond and also has extremely excellent creep strength, making an extremely significant contribution to the development of industry. .

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the value of the formula Mn% + Ni% + Co% + 2Cu% and creep rupture strength at 600°C for 100,000 hours, and Figure 2 is a schematic diagram showing the procedure for collecting creep test pieces from steel pipe specimens. Figure 3 is a schematic diagram showing the procedure for collecting Charpy impact test pieces from welded bond parts, Figure 4
The figure shows the effect of Cu addition on the toughness of the weld bond, Figure 5 shows the relationship between the Cu content and the area ratio of residual δ ferrite in the weld pound, and Figure 6 shows the relationship between the Co content and the 600
It is a diagram showing the relationship between the Charby impact absorption value and the base material at 0° C. after aging for 10,000 hours. l... Steel pipe plate thickness center line, 2... Weld bond part, 3...
...Welded bond wire, 4...JIS d full size impact test piece. 5... Steel pipe test specimen. specimen. 6...Axial direction, 7...Cree Mn fat't (of2cg Fig. 3 Fig. 4 θ・/ ノ2 B Cu house evaluation! 04/13) ノK (Wt%) Cu joint party

Claims (4)

[Claims] (1) C: 0.01-0.30%, Si: 0.
02-0.80%, Mn: 0.20-3.00%, Cr
:8.00~13.00%, Ni:0.05~1.00
%, Mo: 0.005-1.00%, W: 0.50-3
．． 00%, V: 0.05-0.50%, Nb: 0.02
~0.12%, B:0.0003~0.008%, Cu
:0.10~5.00%, Zr:0.0005~0.1
0%, N: 0.01-0.10%, P: 0.0
50% or less, S: 0.010% or less, O: 0.020%
In addition, the amount of Mn, Ni, and Cu added satisfies the following conditions: Mn%+Ni%+2Cu%≦12, and the remainder is Fe and unavoidable impurities. Ferritic heat-resistant steel. (2) C: 0.01-0.30%, Si: 0.
02-0.80%, Mn: 0.20-3.00%, Cr
:8.00~13.00%, Ni:0.05~1.00
%, Mo: 0.005-1.00%, W: 0.50-3
．． 00%, V: 0.05-0.50%, Nb: 0.02
~0.12%, B:0.0003~0.008%, Cu
:0.10~5.00%, Zr:0.0005~0.1
0%, N: 0.01-0.10%, and Ta:
Contains one or two of 0.01-1.00%, Hf: 0.01-1.00%, P: 0.050% or less, S:
0.010% or less, O: 0.020% or less, and in addition, the amount of Mn, Ni, and Cu added satisfies the following conditions: Mn% + Ni% + 2Cu%≦12, and the remainder consists of Fe and unavoidable impurities. Ferritic heat-resistant steel with excellent weld bond toughness. (3) C: 0.01-0.30%, Si: 0.
02-0.80%, Mn: 0.20-3.00%, Cr
:8.00~13.00%, Ni:0.05~1.00
%, Mo: 0.005-1.00%, W: 0.50-3
．． 00%, V: 0.05-0.50%, Nb: 0.02
~0.12%, B:0.0003~0.008%, Cu
:0.10~5.00%, Zr:0.0005~0.1
0%, N: 0.01 to 0.10%, and further Co:
Contains one or two of 0.01 to 1.00%, Ti: 0.01 to 0.10%, P: 0.050% or less, S:
O: 0.010% or less, O: 0.020% or less, and the addition amount of Mn, Ni, and Cu satisfies the following conditions: Mn% + Ni% + Co% + 2Cu%≦12, and the remainder is Fe and unavoidable impurities. Ferritic heat-resistant steel with excellent weld bond toughness. (4) C: 0.01-0.30%, Si: 0.
02-0.80%, Mn: 0.20-3.00%, Cr
:8.00~13.00%, Ni:0.05~1.00
%, Mo: 0.005-1.00%, W: 0.50-3
．． 00%, V: 0.05-0.50%, Nb: 0.02
~0.12%, B:0.0003~0.008%, Cu
:0.10~5.00%, Zr:0.0005~0.1
0%, N: 0.01-0.10%, and Ta:
0.01 to 1.00%, Hf: 0.01 to 1.00%, or further Co: 0.01
~1.00%, Ti: 0.01~0.10%, P: 0.050% or less, S: 0.01
0% or less, O: 0.020% or less, and Mn
A ferritic heat-resistant steel with excellent weld bond toughness, characterized in that the added amounts of Ni and Cu satisfy the following conditions: Mn%+Ni%+Co%+2Cu%≦12, with the remainder consisting of Fe and unavoidable impurities.