JPH07126810A - Ferritic heat resistant steel for fusion reactor excellent in weldability and its production - Google Patents

Abstract

PURPOSE:To produce a heat resistant steel sheet excellent in low inducing activation, weldability, high temp. creep strength and toughness by subjecting an alloy steel slab having a specified compsn. to heating to a specified temp. or above, subjecting it to hot rolling and thereafter executing normalizing and tempering treatment at a specified temp. CONSTITUTION:A low carbon steel having a compsn. contg., by weight, 0.04 to 0.09% C, 0.08 to 0.25% Si, 0.01 to 0.50% Mn, <0.01% P, <0.007% S, 8 to 10% Cr, 1.5 to 2.2% W, 0.15 to 0.25% V, 0.05 to 0.10% Ta, 0.0005 to 0.0030% B and 0.025 to 0.06% N is heated to a temp. >=T1( deg.C) expressed by the formula and is subjected to hot rolling to form into a hot rolled steel sheet. This steel sheet is subjected to heat treatment of normalizing treatment at 930 to 1080 deg.C, tempering treatment at 750 to 800 deg.C or the like, by which the ferritic heat resistant steel excellent in the balance of high temp. creep fracture strength and toughness both in the base metal and weld heat affected zone as well as weldability at the time of welding and in which low activation is simultaneously realized is developed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a fusion reactor first wall which has low induction activation, excellent weldability, and excellent balance of high temperature creep strength and toughness in both the base metal and the weld heat affected zone. The present invention relates to a heat-resistant steel sheet used for, for example, and a manufacturing method thereof.

[0002]

2. Description of the Related Art A nuclear fusion reactor has a track record as an experimental research reactor and has already achieved critical plasma conditions. Currently, SUS316 is used as the first wall material. However, SUS316 cannot be used at the neutron irradiation dose expected in a commercial reactor due to the problem of void swelling. Instead, JPCA steel in which Ti is added to SUS316 and HT-9 steel (12Cr-1Mo-W-V), which is a ferrite type and has less void swelling, have been developed. further,
"Journal of Nuclear Mater
als 133 & 134 (1985) 149-15
As described in “5”, it is economically remarkably advantageous to reduce radioactive contamination at the time of repair during use and at the time of disposal after shutdown. As materials are required, Ni, Cu, M, which are ferritic steels and elements with high induction activation,
Steel with a minimum reduction of o, Nb, etc. has been required.

As the low activation steel, "Alloy De"
velocity for Irradiation
Performance / Semiannual P
Rogless Report For Period
Ending September 30.198
5; U. S. Department of Energ
y, p117-123 ", 9Cr-2W
-Ta-V steel, "Heat-resistant materials 123rd Committee research report Vo
l. 27 No. 1 pp105-117 ”, 9Cr-2W steel, and 8Cr-2W-Ta-V steel as described in Japanese Patent Publication No. 3-61749 have been developed. In addition to solid solution strengthening of W, these steels are intended to have high temperature strength equal to that of HT-9 by dispersion strengthening by Ta carbonitride and V carbonitride.

Regarding the weldability of low activation steel, 8Cr-2
There is known technology only for W-Ta-V steel. C, T
It is described in Japanese Examined Patent Publication No. 3-61749 that welding hot cracking occurs when a, V and W are excessively added. In addition, "Journal of Nuclear Mater
als 179-181 (1991) p693-69
6 ", the characteristics of the 8Cr-2W-Ta-V steel welded joint are reported, but the welding preheating temperature is 200 to 250 ° C and HT.
It is constructed at a preheating temperature as high as 9 steel.

Also, the welding heat affected zone of low activation steel (hereinafter referred to as H
Regarding the high temperature creep strength and toughness in AZ), there is no technology disclosed hitherto.

[0006]

Problems to be Solved by the Invention 9Cr-2W-Ta-
In V steel and 9Cr-2W steel, W is coarse F during holding at high temperature.
The high temperature creep rupture strength is not sufficient because the amount of W that precipitates as e ₂ W and contributes to solid solution strengthening decreases, and V carbonitrides and Ta carbonitrides coarsen in the HAZ and the dispersion strengthening effect decreases. . Further, since the 8Cr-2W-Ta-V steel described in Japanese Patent Publication No. 3-61749 is added with Ni from the viewpoint of ensuring toughness, it cannot be said to be a sufficiently low activation material and the same reason as for the 9Cr-2W steel. Therefore, the high temperature creep rupture strength of HAZ is not sufficient. Furthermore, it is necessary to further reduce the welding preheating temperature to reduce the welding construction cost.

It is an object of the present invention to provide a ferritic heat-resistant steel which has excellent weldability and a good balance between high temperature creep rupture strength and toughness for both the base metal and HAZ, and at the same time has achieved low emission, and a method for producing the same.

[0008]

The present invention has optimized the amounts of C, Si, V, Ta, B and N in order to improve weldability. Further, in order to improve the balance between creep strength and toughness of both the base metal and HAZ, the amounts of C, W, V, Ta, B and N, the rolling heating temperature and the heat treatment conditions were optimized. The gist of the present invention is as follows.

(1) C: 0.04 to 0.09% Si: 0.08 to 0.25% Mn: 0.01 to 0.50% P: 0.01% or less S: 0.0. 007% or less Cr: 8 to 10% W: 1.5 to 2.2% V: 0.15 to 0.25% Ta: 0.05 to 0.10% B: 0.0005 to 0.0030% N : Ferrite heat-resistant steel for a fusion reactor having excellent weldability, characterized in that it contains 0.025 to 0.06% and the balance is Fe and inevitable impurities.

(2) C: 0.04 to 0.09% Si: 0.08 to 0.25% Mn: 0.01 to 0.50% P: 0.01% or less S: 0.0. 007% or less Cr: 8 to 10% W: 1.5 to 2.2% V: 0.15 to 0.25% Ta: 0.05 to 0.10% B: 0.0005 to 0.0030% N : Steel containing 0.025 to 0.06% and the balance Fe and inevitable impurities is represented by the following formula: T ₁ (° C.)

[0011]

[Equation 2]

Nuclear fusion excellent in weldability, which is characterized in that it is heated to a temperature of not less than the above temperature and rolled, and heat-treated under the conditions of normalizing temperature: 930 to 1080 ° C. and tempering temperature: 750 to 800 ° C. Method for producing ferritic heat-resistant steel for furnaces.

[0013]

The present invention will be described in detail below. C acts as a solid solution strengthening element and also produces carbides to improve the high temperature creep strength. It also suppresses the formation of δ ferrite and improves toughness. For that purpose, 0.04% or more is necessary. On the other hand, if it exceeds 0.09%, the weldability deteriorates, so it was limited to 0.04 to 0.09%.

Si is required to be 0.08% or more in order to reduce the viscosity of molten steel and stabilize the castability and weld bead shape. On the other hand, if it exceeds 0.25%, the toughness decreases and HAZ
Since the hardness increases, it was limited to 0.08 to 0.25%. M
It is necessary to add 0.01% or more of n as a deoxidizing agent.
On the other hand, when it exceeds 0.50%, Mn segregation becomes remarkable and the toughness is lowered, so the content is limited to 0.01 to 0.50%.

Since P causes embrittlement at the grain boundary, it is limited to 0.01% or less. It is desirable to be as low as possible. Since S causes deterioration of toughness, it is limited to 0.007% or less. It is desirable to be as low as possible. Cr is an essential element for heat resistant steels that improve corrosion resistance and oxidation resistance at high temperatures. In ferritic steel, the amount of Cr with the smallest neutron irradiation embrittlement is 8
~ 12%. On the other hand, if it exceeds 10%, δ ferrite is formed and the toughness is lowered, so the content is limited to 8 to 10%.

W remarkably improves the high temperature creep rupture strength by solid solution strengthening. On the contrary, if the addition amount is excessive, a coarse intermetallic compound (Fe ₂ W) is formed and the toughness is remarkably reduced, so the content is limited to 1.5 to 2.2%. V: High temperature creep strength due to solid solution strengthening and precipitation strengthening, especially HAZ
Increases high temperature creep strength. The effect becomes remarkable at 0.15% or more, but addition of more than 0.25% leads to a decrease in toughness due to the formation of δ ferrite and a decrease in weldability. Therefore, the effect was limited to 0.15 to 0.25%. .

Ta improves the high temperature creep strength by precipitation strengthening as TaN and TaC. Further, Ta in solid solution improves toughness. For this purpose, 0.05% or more is necessary. On the other hand, if it exceeds 0.10%, the high-temperature creep strength decreases conversely and the weldability deteriorates.
It was limited to 5 to 0.10%. B refines the structure of the HAZ and improves the toughness of the HAZ. It also has the effect of slightly increasing creep strength. On the other hand, excessive addition deteriorates the weldability, so the content was limited to 0.0005 to 0.0030%.

N suppresses the formation of δ ferrite and enhances toughness, and forms fine precipitates such as TaN and VN to enhance high temperature creep strength, particularly HAZ creep strength. For that purpose, 0.025% or more is necessary. On the other hand, 0.06%
If added in excess of 0, the castability and weldability will be deteriorated.
It was limited to 025 to 0.06%. The rolling heating temperature T ₁ is
This is an index discovered to secure the strength and toughness of a high Cr ferritic steel with a low C content in consideration of weldability. Ta, V, C, N when heated to above T ₁ and rolled
Is a sufficient solid solution and carbonitrides are finely precipitated on the dislocations introduced during rolling to improve the high temperature creep strength and toughness.
When the heating temperature is lower than T _{1, the} carbonitrides become relatively coarse and the high temperature creep strength cannot be sufficiently secured. Therefore, the rolling heating temperature is T ₁ (° C) shown in the following formula.

[0019]

[Equation 3]

The temperature is limited to the temperature above. If the normalizing temperature is too low, the creep strength decreases, and if it is too high, the toughness deteriorates. Further, when heated to more than 1080 ° C., the effect of fine carbonitrides formed during rolling disappears at all. If the heating is performed at 1080 ° C. or less, even if the carbonitride re-dissolves, the carbonitride precipitated after rolling when it is re-precipitated inherits the tendency of being fine. Therefore, the normalizing temperature is limited to 930 to 1080 ° C.

If the tempering temperature is lower than 750 ° C., the toughness decreases, and if it exceeds 800 ° C., the short-time strength decreases. Therefore, the tempering temperature is limited to 750 to 800 ° C. With respect to the unavoidable impurities, it is desirable that Mo, Nb, Ni, Co, Ag, and Cu be made as low as possible within a range that practically does not increase the cost.

[0022]

EXAMPLE A steel sheet was prepared by hot rolling an ingot adjusted to the chemical composition shown in Table 1 to a thickness of 25 mm in a 50 kg vacuum melting furnace, air-cooling, normalizing and tempering.
A joint was prepared from a part of the plate by submerged arc welding and subjected to post-weld heat treatment at 740 ° C / 8h. 6φ × 30GL creep test piece and 2 from each of the base metal and joint
A mmV notch Charpy test piece was processed. The creep test piece of the joint was arranged so that the entire HAZ was included in the parallel portion. The Charpy test piece of the joint had a notch in the HAZ portion 2 mm from the fusion line. Creep test is 6
Breaking strength of 10,000 hours at 00 ° C and absorbed energy of 0 ° C for Charpy test were investigated. In addition, the maximum hardness test of HAZ, which is an index of the welding preheating temperature, is also performed.
Weldability was investigated. Generally, the higher the maximum hardness, the HAZ
The cracking susceptibility is high and the preheating temperature must be high.

The results are shown in Table 2. As can be seen from this table, in Examples 1 to 15 of the present invention, both the base metal and HAZ have high creep rupture strength and good toughness. Furthermore, it has a low maximum hardness and excellent weldability. In Comparative Example 16, since C is less than the lower limit, solid solution strengthening and precipitation strengthening are insufficient and the creep strength is low. Further, δ ferrite is precipitated and the toughness is low.

In Comparative Example 17, C is more than the upper limit, so the maximum hardness is increased and the weldability is deteriorated. Comparative Example 18 is Si
Is less than the lower limit, the weld bead shape is not stable and HAZ
Creep strength and HAZ toughness are deteriorated. In Comparative Example 19, since the Si content exceeds the upper limit, the toughness is reduced. Also, the maximum hardness is increasing.

In Comparative Example 20, the toughness is deteriorated because Mn exceeds the upper limit. In Comparative Example 21, P is more than the upper limit, so the toughness is reduced. In Comparative Example 22, the toughness is deteriorated because S exceeds the upper limit. In Comparative Example 23, since Cr exceeds the upper limit, δ ferrite is precipitated and the toughness is reduced. Also, the maximum hardness is increasing.

In Comparative Example 24, since W is less than the lower limit, solid solution strengthening is not sufficient and creep strength is lowered. Comparative example 2
In No. 5, since W exceeds the upper limit, a coarse intermetallic compound precipitates and the toughness decreases. Also, the maximum hardness is increasing. In Comparative Example 26, since V is less than the lower limit, solid solution strengthening and precipitation strengthening are not sufficient, and especially the creep strength of HAZ is lowered.

In Comparative Example 27, since V exceeds the upper limit, δ ferrite is precipitated and the toughness is lowered. Also, the maximum hardness is increasing. In Comparative Example 28, Ta is less than the lower limit, so precipitation strengthening is insufficient and creep strength is reduced. It also has low toughness. In Comparative Example 29, Ta exceeds the upper limit, so the creep strength is reduced. Also, the maximum hardness is increasing.

In Comparative Example 30, B was less than the lower limit, so HAZ
Of the HAZ becomes coarse and the toughness of HAZ is lowered. In Comparative Example 31, B is more than the upper limit, so the maximum hardness is increased.
In Comparative Example 32, since N is less than the lower limit, precipitation strengthening is insufficient and creep strength is reduced. Further, δ ferrite is precipitated and the toughness is lowered.

In Comparative Example 33, the maximum hardness is increased because N exceeds the upper limit. In Comparative Example 34, since the rolling heating temperature is less than the lower limit, the precipitates are coarsened and the creep strength is reduced. In Comparative Example 35, the normalizing temperature is less than the lower limit and the creep strength is lowered. In Comparative Example 36, since the normalizing temperature exceeds the upper limit, the crystal grains are coarsened and the toughness of the base material is lowered.

In Comparative Example 37, the tempering temperature is less than the lower limit, so the toughness of the base material is lowered. In Comparative Example 38, the tempering temperature is higher than the upper limit, so that the short-time tensile strength is 10 as compared with the inventive example.
It decreased by about 0 MPa.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

As described above, according to the ferritic steel for a fusion reactor excellent in high temperature strength of the present invention, for example, the high temperature strength, especially the high temperature creep rupture strength as compared with the conventional fusion reactor steel. Is significantly improved and the weldability is excellent, so that it is extremely effective as a first wall material for a fusion reactor.

Claims (2)

[Claims] 1. C: 0.04 to 0.09% Si: 0.08 to 0.25% Mn: 0.01 to 0.50% P: 0.01% or less S: 0.007 by weight % Or less Cr: 8 to 10% W: 1.5 to 2.2% V: 0.15 to 0.25% Ta: 0.05 to 0.10% B: 0.0005 to 0.0030% N: A ferritic heat-resistant steel for a fusion reactor having excellent weldability, which contains 0.025 to 0.06% and the balance Fe and inevitable impurities. 2. C: 0.04 to 0.09% Si: 0.08 to 0.25% Mn: 0.01 to 0.50% P: 0.01% or less S: 0.007 by weight % Or less Cr: 8 to 10% W: 1.5 to 2.2% V: 0.15 to 0.25% Ta: 0.05 to 0.10% B: 0.0005 to 0.0030% N: A steel containing 0.025 to 0.06% and the balance Fe and unavoidable impurities is represented by the following formula: T ₁ (° C.) Of the ferrite for fusion reactors, which is excellent in weldability, characterized in that it is heated to a temperature equal to or higher than the above temperature and rolled, and heat-treated under the conditions of normalizing temperature: 930 to 1080 ° C and tempering temperature: 750 to 800 ° C. -Based heat-resistant steel manufacturing method.