EP4050118A1

EP4050118A1 - Steel plate for pressure vessel having excellent resistance to high-temperature post-welding heat treatment, and method for manufacturing same

Info

Publication number: EP4050118A1
Application number: EP20878789.5A
Authority: EP
Inventors: Soon-Taik Hong
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2019-10-22
Filing date: 2020-10-20
Publication date: 2022-08-31
Also published as: CN114585760B; CN114585760A; WO2021080291A1; US20240102137A1; JP7398559B2; KR102280641B1; JP2022553704A; EP4050118A4; KR20210047698A

Abstract

The purpose of the present invention is to provide a steel plate for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment, in which deterioration of strength and toughness is minimized even when a long-term post welding heat treatment (PWHT) is applied at high temperature, and a method for manufacturing same.

Description

Technical Field

The present disclosure relates to a steel material used for boilers, pressure vessels, and the like, in power plants and chemical plants, and more particularly, to a steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment and a method for manufacturing the same.

Background Art

Recently, a demand for materials for medium and high-temperature pressure vessels of about 350 to 600°C, such as boilers, pressure vessels, and the like, in power plants and chemical plants, continues to increase.
In addition, a supply of steel materials having high resistance to high-temperature tempering heat treatment, or the like, is required due to advancements of equipment, longer lifespans, and thickening of the steel materials.
Meanwhile, in the case of welding the steel material in addition to thickening of steel material, post-welding heat treatment (PWHT) will be performed to remove stress generated during welding for the purpose of preventing deformation of the structure after welding and stabilizing the shape and dimensions thereof. Since the post-welding heat treatment process is performed over a long-period of time, and the steel sheet subjected to this process has a problem in that tensile strength of the steel sheet decreases due to coarsening of the structure.
That is, after a long-period of PWHT, strength and toughness are simultaneously deteriorated due to softening of a matrix and grain boundaries, grain growth, and coarsening of carbides.
In order to solve this problem, the following technique has been proposed.
According to Patent Document 1, a method, in which tempering heat treatment pattern is applied to a thick steel sheet containing an appropriate amount of C, Si, Mn, Cr, Mo, Ni, Cu, or the like, that is, a low-temperature heat treatment (low-temperature tempering) after a high-temperature heat treatment (high-temperature tempering) is performed to utilize a precipitation strengthening effect generated by low-temperature tempering to reduce strength due to a decrease in dislocation density during high-temperature tempering, is presented. However, even if this method is applied, there is a problem in that resistance is greatly deteriorated due to the long-term PWHT.
Therefore, it is required to develop a steel material that can be used appropriately in a medium-high temperature environment while minimizing the deterioration of physical properties even after the long-term PWHT.
(Patent Document 1) Republic of Korea Patent Publication No. 2012-0073448

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide a steel plate for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment with minimal deterioration in strength and toughness even when a long-term post welding heat treatment (PWHT) is applied at a high temperature, and a method for manufacturing the same.
The subject of the present invention is not limited to the above. The subject of the present invention will be understood from the overall content of the present specification, and those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional subject of the present invention.

Solution to Problem

According to an aspect of the present disclosure, a steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment, includes by weight %, carbon (C): 0.10 to 0.16%, silicon (Si): 0.20 to 0.35%, manganese (Mn): 0.4 to 0.6%, chromium (Cr): 6.5 to 7.5%, molybdenum (Mo): 0.7 ~0.9%, aluminum (Al): 0.005 to 0.05%, phosphorus (P): 0.015% or less, sulfur (S): 0.020% or less, niobium (Nb): 0.002 to 0.025%, vanadium (V): 0.25 to 0.35 %, and a balance of Fe and other unavoidable impurities, wherein a microstructure is a complex structure having tempered martensite and tempered bainite.
According to another aspect of the present disclosure, a method for manufacturing a steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment, the method includes operations of: preparing a steel slab including, by weight %, carbon (C): 0.10 to 0.16%, silicon (Si): 0.20 to 0.35%, manganese (Mn): 0.4 to 0.6%, chromium (Cr): 6.5 to 7.5%, molybdenum (Mo): 0.7 ~0.9%, aluminum (Al): 0.005 to 0.05%, phosphorus (P): 0.015% or less, sulfur (S): 0.020% or less, niobium (Nb): 0.002 to 0.025%, vanadium (V): 0.25 to 0.35 %, and a balance of Fe and other inevitable impurities; heating the steel slab in a temperature range of 1050 to 1250°C; hot-rolling the heated steel slab in a temperature range of 800 to 1000°C to manufacture a hot-rolled steel sheet; performing heat-treatment for maintaining the hot-rolled steel sheet in a temperature range of 1000 to 1050°C for {(1.3×t)+(10 to 30)} minutes (where, t means a thickness of the steel material (mm)); cooling the heat-treated hot-rolled steel sheet at a cooling rate of 1 to 30°C/s; and performing tempering heat-treatment for maintaining the cooled hot-rolled steel sheet in a temperature range 800 to 825°C for {(1.6×t)+(10 to 30)} minutes.

Advantageous Effects of Invention

According to an aspect of the present disclosure, it is possible to provide a steel material for a pressure vessel in which strength and toughness are not deteriorated even after high-temperature heat treatment, particularly, even when a long-term post welding heat treatment (PWHT) is performed at a high temperature.
In particular, the steel material for a pressure vessel of the present invention has an effect of being suitably applicable as a material for a pressure vessel for medium and high-temperature.

Best Mode for Invention

In manufacturing a steel material for a pressure vessel used as structural steel in an environment such as a power plant, plant industry, or the like, the present inventors have studied in detail a method for greatly improving resistance to deterioration of strength and toughness even after performing post-welding heat treatment (PWHT) for an extended period of time to minimize residual stress generated by welding, thereby resulting in completion of the present disclosure.
In particular, by optimizing an amount of a specific element in an alloy composition, the present invention is technically characterized in that it provides a steel material having excellent resistance to deterioration of strength and toughness even when tempering heat treatment and a long-term post welding heat treatment (PWHT) at a high temperature are performed.
Hereinafter, the present invention will be described in detail.
According to an aspect of the present disclosure, a steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment, may include, by weight %, carbon (C): 0.10 to 0.16%, silicon (Si): 0.20 to 0.35%, manganese (Mn): 0.4 to 0.6%, chromium (Cr): 6.5 to 7.5%, molybdenum (Mo): 0.7 ~0.9%, aluminum (Al): 0.005 to 0.05%, phosphorus (P): 0.015% or less, sulfur (S): 0.020% or less, niobium (Nb): 0.002 to 0.025%, and vanadium (V): 0.25 to 0.35 %.
Hereinafter, a reason for limiting an alloy composition of the steel material for a pressure vessel provided in the present invention as above will be described in detail.
Meanwhile, unless otherwise specified in the present invention, the content of each element is based on a weight, and a ratio of a microstructure is based on an area.

Carbon (C): 0.10 to 0.16%

Carbon (C) is a beneficial element for improving the strength of steel. When a content of C is less than 0.10%, strength of a matrix itself is lowered. On the other hand, when the content of C exceeds 0.16%, the strength is excessively increased, such that there is a concern that toughness may be deteriorated.
Accordingly, the content of C may be 0.10 to 0.16%.

Silicon (Si): 0.20 to 0.35%

Silicon (Si) is an element effective for deoxidation and solid solution strengthening, and is an element accompanied by an increase in an impact transition temperature. In order to achieve target strength, a content of Si may be 0.20% or more, but when the content of Si exceeds 0.35%, there is a problem in that weldability is lowered and impact toughness is deteriorated.
Accordingly, the content of Si may be 0.20 to 0.35%.

Manganese (Mn): 0.4 to 0.6%

Manganese (Mn) combines with sulfur (S) in steel to form MnS, which is a stretched non-metallic inclusion, thereby inhibiting room temperature elongation and low-temperature toughness, and thus a content of Mn may be limited to 0.6% or less. However, when the content of Mn is less than 0.4%, it is not preferable because it becomes difficult to secure an appropriate level of strength.
Accordingly, the content of Mn may be 0.40 to 0.6%.

Chromium (Cr):6.5 to 7.5%

In the present disclosure, chromium (Cr) is advantageous in obtaining an effect of enabling heat treatment (tempering, PWHT) at high temperatures and an effect of increasing strength, and for this purpose, Cr may be added in a content of 6.5% or more. From this, the steel material of the present disclosure may ensure excellent resistance to high-temperature heat treatment.
However, Cr is an expensive element, and when the content of Cr exceeds 7.5%, manufacturing costs are greatly increased.
Accordingly, the content of Cr may be 6.5 to 7.5%.

Molybdenum (Mo): 0.7 to 0.9%

Molybdenum (Mo) is not only an effective element for increasing high-temperature strength like Cr, but is also advantageous in preventing cracks caused by sulfides. In order to sufficiently obtain the above-described effect, Mo may be included in a content of 0.7% or more, but when the content of Mo exceeds 0.9%, an increase in manufacturing costs is caused.
Accordingly, the content of Mo may be 0.7 to 0.9%.

Aluminum (Al): 0.005 to 0.05%

Aluminum (Al) is a strong deoxidizer in a steelmaking process together with the Si. In order to sufficiently obtain a deoxidation effect, the Al may be included in a content of 0.005% or more, but when the content of Al exceeds 0.05%, the deoxidation effect is saturated, but there is a problem in that manufacturing costs are greatly increased.
Accordingly, the content of Al may be 0.005 to 0.05%

Phosphorus (P): 0.015% or less

Phosphorus (P) is an element increasing temper embrittlement susceptibility while inhibiting low-temperature toughness of steel, and an amount of P is preferably controlled to be as low as possible. However, a process for lowering the content of P is difficult, and there is a concern that production costs may increase due to an additional process.
In consideration thereof, P can be limited to 0.015% or less, and the present disclosure reveals that even if P is contained at a maximum of 0.015%, there is no difficulty in securing intended physical properties.

Sulfur (S): 0.020% or less

Sulfur (S) is also an element reducing low-temperature toughness of steel, and since it inhibits the toughness of steel by forming MnS inclusions in the steel, it is preferable to control an amount of S to be as low as possible. However, a process for lowering the content of S is difficult, and there is a concern that production costs may increase due to an additional process.
In consideration thereof, the S may be limited to 0.020% or less, and the present disclosure reveals that even if the S is contained at a maximum of 0.020%, there is no difficulty in securing intended physical properties.

Niobium (Nb): 0.002 to 0.025%

Niobium (Nb) is an effective element in preventing softening of matrix structures by forming fine carbides or nitrides in steel. In order to sufficiently obtain such an effect, Nb may be contained in a content of 0.002% or more, but as an expensive element, when the content of Nb exceeds 0.025%, there is a concern that manufacturing costs may be greatly increased.
Accordingly, the content of Nb may be 0.002 to 0.025%.

Vanadium (V): 0.25 to 0.35%

Vanadium (V) is an element that can easily form fine carbides or nitrides in steel like Nb. In order to sufficiently obtain this effect, the V may be contained in a content of 0.25% or more, but since it is also an expensive element, the content of V may be limited to 0.35% or less in consideration thereof.
Accordingly, the content of V may be 0.25 to 0.35%.
A remainder of the present disclosure may be iron (Fe). However, in a general manufacturing process, inevitable impurities may be inevitably added from raw materials or an ambient environment, and thus, impurities may not be excluded. A person skilled in the art of a general manufacturing process may be aware of the impurities, and thus, the descriptions of the impurities may not be provided in the present disclosure.
The steel material of the present disclosure having the above-described alloy composition may include a mixed structure of tempered martensite and tempered bainite as a microstructure, and may include the above-described mixed structure over an entire thickness regardless of the thickness of the steel material.
It is preferable that the tempered martensite phase in the mixed structure has an area fraction of 40% or more. If a fraction on the tempered martensite phase is less than 40%, target strength cannot be sufficiently secured. More advantageously, the tempered martensite phase may include an area fraction of 40 to 90%.
Hereinafter, a method for manufacturing a steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment according to another aspect of the present invention will be described in detail.
The steel material for a pressure vessel according to the present invention can be manufactured through processes of [heating - hot rolling - heat treatment - cooling - tempering heat treatment] of a steel slab satisfying the alloy composition system proposed in the present invention. Hereinafter, each process will be described in detail.

[Heating steel slab]

After preparing a steel slab satisfying the above-described alloy composition system, the steel slab may be heated in a temperature range of 1050 to 1250°C.
When the heating temperature is less than 1050°C, it is difficult to dissolve solute atoms. On the other hand, when the temperature exceeds 1250°C, a size of austenite grains is too coarse, resulting in inhibiting physical properties of the steel.

[Hot-rolling]

The steel slab heated according to the above may be hot-rolled to be manufactured as a hot-rolled steel sheet. The hot rolling is preferably performed at a reduction ratio of 2.5 to 30% per pass in a temperature range of 800 to 1000°C.
When a temperature during the hot rolling is less than 800°C, there is a problem in that a rolling load increases. On the other hand, when the temperature exceeds 1000°C, there is a problem in that the grains become coarse.
In addition, when the rolling reduction per pass during the hot rolling is less than 2.5%, there is a problem in that rolling productivity is lowered and a manufacturing costs increase. On the other hand, when the reduction ratio per pass exceeds 30%, a load is generated on a rolling mill and there is a risk of having a fatally adverse effect on a facility.
It should be noted that the hot-rolled steel sheet obtained by completing the hot rolling may be cooled to room temperature through air cooling, and subsequent processes can be performed thereafter.

[Heat treatment]

The hot-rolled steel sheet manufactured as described above may be heat-treated by being heated to a specific temperature range.
Specifically, heat treatment for maintaining the hot-rolled steel sheet for {(1.3 × t) + (10 to 30)} minutes (here, t means a thickness of the steel material (mm)) in a temperature range of 1000 to 1050°C is preferably performed.
When a temperature during the heat treatment is less than 1000°C, it becomes difficult to re-dissolve solid-solute elements, making it difficult to secure a target strength. On the other hand, when the temperature exceeds 1050°C, growth of crystal grains occur, resulting in reduced low-temperature toughness.
When a maintaining time during heat treatment in the above temperature range is less than {(1.3×t)+10} minutes, homogenization of the microstructure is difficult. On the other hand, when the time exceeds {(1.3×t)+30} minutes, productivity is inhibited, which is not preferable.

[Cooling]

The hot-rolled steel sheet heat-treated as described above may be cooled at a cooling rate of 1 to 30°C/s, and in this case, cooling may be performed to room temperature. Here, it is noted that the cooling rate is based on a center in the thickness direction of the hot-rolled steel sheet (e.g., 1/2t (t: thickness (mm)) point).
When a cooling rate during the cooling is less than 1°C/s, there is a fear that coarse ferrite grains are generated during cooling. On the other hand, when the cooling rate exceeds 30°C/s, there is a problem in that economic efficiency is lowered due to excessive cooling facilities.

[Tempering heat treatment]

The hot-rolled steel sheet cooled according to the above may be tempered, and specifically, a tempering heat treatment process for maintaining the steel sheet for {(1.6×t)+(10 to 30)} minutes in a temperature range of 800 to 825°C may be performed.
When a temperature during the tempering heat treatment is less than 800 °C, precipitation of fine precipitates becomes difficult, and it is difficult to secure a target strength. On the other hand, when the temperature exceeds 825°C, there is a problem in that growth of precipitates occurs, resulting in deterioration of strength and low-temperature toughness.
When a maintaining time during the tempering heat treatment in the above temperature range is less than {(1.6×t)+10} minutes, it is difficult to homogenize a microstructure. On the time, when the time exceeds {(1.6×t)+30} minutes, it is not preferable because productivity is reduced.
The steel material for a pressure vessel of the present invention manufactured through the above-described process requires a post-welding heat treatment (PWHT) process for the purpose of removing residual stress due to a welding process added during manufacture of the pressure vessel.
In general, deterioration of strength and toughness occurs after long-time PWHT heat treatment, and the steel material manufactured by the present invention has an advantage that welding can be performed without significant deterioration in strength and toughness even if heat treatment is performed for up to 50 hours in a temperature range of 760 to 780°C, which is higher than the normal PWHT temperature.
In particular, the steel material of the present invention has an effect of having excellent strength and toughness with tensile strength of 600 MPa or more and a Charpy impact energy value of 100 J or more at -30°C even after PWHT for up to 50 hours at high temperature.
Hereinafter, the present disclosure will be described in more detail through examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present disclosure. The scope of the present disclosure may be determined by matters described in the claims and matters able to be reasonably inferred therefrom.

Mode for Invention

(Example)

Hereinafter, after preparing a steel slab having an alloy composition shown in Table 1 below, the steel slab was heated at 1150°C for 300 minutes, and then hot-rolled at 800 to 1000°C at a rolling reduction ratio of 5 to 20% per pass to prepare a hot-rolled steel sheet.
Thereafter, the hot-rolled steel sheet was air-cooled to room temperature, and then heat treatment was performed to heat and maintain the hot-rolled steel sheet at 1050°C again. In this case, the heat treatment was maintained for 150 to 280 minutes depending on a thickness of the hot-rolled steel sheet. Thereafter, water cooling was performed to room temperature at a cooling rate of 2.5 to 15°C/s based on a central portion in the thickness direction of the hot-rolled steel sheet. Then, tempering heat treatment and PWHT heat treatment were performed under the conditions shown in Table 2 below.
A tensile test was performed on the hot-rolled steel sheet having undergone all the above-described processes to measure yield strength (YS), tensile strength (TS) and elongation (El). In addition, a Charpy impact test was performed to derive an impact energy value.

The tensile test was performed in accordance with ASTM standards A20 and A370 & E8, and the impact test was performed at -30°C with a Charpy impact test on a specimen 5 having a V notch. Each result is shown in Table 3 below.

[Table 1]

Steel type	Alloy composition (by weight%)
Steel type	C	Mn	Si	P	S	Al	Cr	Mo	Cu	Ni	Nb	V
Inventive Steel A	0.15	0.55	0.25	0.005	0.001	0.028	6.85	0.82	0	0	0.018	0.28
Inventive Steel A					1
Inventive Steel B	0.14	0.52	0.28	0.006	0.001	0.031	7.15	0.80	0	0	0.012	0.30
Inventive Steel B					3
Inventive Steel C	0.13	0.58	0.30	0.008	0.001	0.030	7.25	0.86	0	0	0.020	0.32
Inventive Steel C					5
Comparativ e Steel A	0.14	0.55	0.38	0.008	0.001	0.033	2.28	0.90	0.15	0.15	0	0
Comparativ e Steel A					2

[Table 2]

Steel type	Thickness	Process condition				Classificat
	(mm)	Tempering Temperature (°C)	Tempering time(min)	PWHT Temperature (°C)	PWHT time(Hr)	ion
Inventive Steel A	100	810	180	770	20	Inventive Example 1
	150	810	260	770	35	Inventive Example 2
	200	810	340	770	50	Inventive Example 3
Inventive Steel B	100	810	180	770	20	Inventive Example 4
	150	810	260	770	35	Inventive Example 5
	200	810	340	770	50	Inventive Example 6
Inventive Steel C	100	810	180	770	20	Inventive Example 7
	150	810	260	770	35	Inventive Example 8
	200	810	340	770	50	Inventive Example 9
Comparative Steel A	100	810	180	770	20	Comparative Example 1
	150	810	260	770	35	Comparative Example 2
	200	810	340	770	50	Comparative Example 3

[Table 3]

Classificati on	Microstructure	Mechanical property
Classificati on	T-M(area %)	YS(MPa)	TS(MPa)	El(%)	CVN@-30°C(J)
Inventive Example 1	55	518	672	31	317
Inventive Example 2	53	512	662	32	320
Inventive Example 3	50	510	656	32	319
Inventive Example 4	57	527	685	33	315
Inventive Example 5	55	519	669	32	309
Inventive Example 6	52	513	657	33	311
Inventive Example 7	56	521	674	35	328
Inventive Example 8	53	517	668	33	321
Inventive Example 9	52	515	659	33	313
Comparative Example 1	12	341	451	30	75
Comparative Example 2	10	335	432	32	65
Comparative Example 3	7	324	419	33	58

(In Table 3, T-M means a tempered martensite phase, and a remainder of the structures except for the tempered martensite phase was a tempered bainite (T-B) phase.)
As shown in Tables 1 to 3, Inventive Examples 1 to 9, which satisfy all alloy compositions and manufacturing conditions proposed in the present invention, may have tensile strength of 600 MPa or more and a Charpy impact energy value at -30°C to 100J or more even if the post-welding heat treatment time reaches a maximum of 50 hours.
On the other hand, it can be seen that Comparative Examples 1 to 3, in which the alloy composition deviates from the present invention, has strength of about 150 MPa and a low-temperature toughness of about 200 J, deteriorated from those of the Inventive Examples after a long-term PWHT.
As described above, the steel material obtained by the alloy composition system and manufacturing conditions according to the present invention has excellent resistance thereto even though not only high-temperature tempering heat treatment but also high-temperature post-welding heat treatment (PWHT) for an extended period of time were performed, the steel material has a suitable effect as a steel material for medium and high-temperature pressure vessels.
While example embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present disclosure as defined by the appended claims.

Claims

A steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment, comprising, by weight %,
carbon (C): 0.10 to 0.16%, silicon (Si): 0.20 to 0.35%, manganese (Mn): 0.4 to 0.6%, chromium (Cr): 6.5 to 7.5%, molybdenum (Mo): 0.7 ~0.9%, aluminum (Al): 0.005 to 0.05%, phosphorus (P): 0.015% or less, sulfur (S): 0.020% or less, niobium (Nb): 0.002 to 0.025%, vanadium (V): 0.25 to 0.35 %, and a balance of Fe and other unavoidable impurities,

wherein a microstructure is a complex structure having tempered martensite and tempered bainite.
The steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment of claim 1, wherein the tempered martensite has an area fraction of 40% or more.
The steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment of claim 1, wherein the steel material has tensile strength of 600 MPa or more, and a Charpy impact energy value of 100 J or more, after high-temperature post-welding heat treatment (PWHT).
A method for manufacturing a steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment, the method comprising operations of:
preparing a steel slab including, by weight %, carbon (C): 0.10 to 0.16%, silicon (Si): 0.20 to 0.35%, manganese (Mn): 0.4 to 0.6%, chromium (Cr): 6.5 to 7.5%, molybdenum (Mo): 0.7 ~0.9%, aluminum (Al): 0.005 to 0.05%, phosphorus (P): 0.015% or less, sulfur (S): 0.020% or less, niobium (Nb): 0.002 to 0.025%, vanadium (V): 0.25 to 0.35 %, and a balance of Fe and other unavoidable impurities;

heating the steel slab in a temperature range of 1050 to 1250°C;

hot rolling the heated steel slab in a temperature range of 800 to 1000°C to manufacture a hot-rolled steel sheet;

performing heat treatment for maintaining the hot-rolled steel sheet in a temperature range of 1000 to 1050°C for {(1.3×t)+(10 to 30)} minutes (where, t is a thickness of the steel material (mm));

cooling the heat-treated hot-rolled steel sheet at a cooling rate of 1 to 30°C/s; and

performing tempering heat-treatment for maintaining the cooled hot-rolled steel sheet in a temperature range of 800 to 825°C for {(1.6×t)+(10 to 30)} minutes.
The method of manufacturing a steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment of claim 4, wherein the hot rolling is performed at a reduction ratio of 2.5 to 30% per pass.
The method of manufacturing a steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment of claim 4, further comprising:
an operation of air cooling to room temperature after the hot rolling.
The method of manufacturing a steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment of claim 4, further comprising: an operation of post-welding heat treatment in a temperature range of 760 to 780°C for up to 50 hours after the tempering heat-treatment.