JP2003105442A - Welded steel tube of bent plate and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve reliability by raising the strength of a weld zone in the longitudinal direction of a welded steel tube of a bent plate which is produced by bending a steel plate into a cylindrical shape and welding the end parts of the resultant tube in the longitudinal direction. SOLUTION: In the welded steel tube of the bent plate which is made by bending the steel plate containing Cr, Mo, Nb, V and N into a cylindrical shape or a semicylindrical shape and welding the end parts of the cylinder in the longitudinal direction, the difference between the minimum hardness in the heat-affected zone in the weld zone in the longitudinal direction and the average hardness of the surrounding base material is smaller than 15 by Vickers hardness and the difference between the maximum hardness of the weld metal and the average hardness of the surrounding base material is smaller than 30 by Vickers hardness. Furthermore, by this manufacturing method of the steel tube, after bending the steel plate and after making the plate into a tubular shape by performing the welding in the longitudinal direction by using a welding material in which the quantities of Cr, Mo, Nb, V and N are contained in the range of the composition specification of the base metal, normalizing and tempering treatments or quenching and tempering treatments are performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plate-bending welded steel pipe in which a steel plate is bent into a cylindrical or semi-cylindrical shape and the ends are welded in the longitudinal direction to form a steel pipe, and particularly to a boiler, a chemical plant, etc. The present invention relates to a plate bent steel pipe suitable for high strength steel used under conditions of high temperature or high pressure and a method for producing the same.

[0002]

2. Description of the Related Art In a power generation boiler, various heat exchangers, etc., a large number of heat transfer tube groups and a header and a pipe for assembling the heat transfer tubes are used under high temperature and high pressure conditions. In recent years, particularly in large-capacity power generation boilers, steam conditions are becoming higher in temperature and pressure in order to improve power generation efficiency, and as a heat transfer pipe material or piping material, high temperature strength is high in place of the conventional 2.25Cr ferrite steel. Many 9Cr-based ferritic steels have come to be used.

This material is based on 9Cr-1Mo steel and contains N
It is a so-called heat-treated steel in which b, V and N are added and the strength is increased as a tempered martensite structure by normalizing / tempering treatment.

A large-diameter pipe material having a wall thickness exceeding 60 to 70 mm in the pipe is manufactured as a seamless steel pipe by hot extrusion or forging. On the other hand, the wall thickness is 50m
A relatively thin large-diameter pipe material having a thickness of about m or less is often manufactured as a plate bending welded steel pipe. The plate bending welded pipe material is a steel pipe formed by bending a steel plate into a cylindrical shape and welding the ends of the cylinder in the longitudinal direction to form a tubular shape.

A general procedure for manufacturing a plate bending welded steel pipe according to the prior art will be described with reference to FIG. First, a steel sheet is bent into a cylindrical shape by pressing or rolling. At that time, depending on the workability and the strength of the steel sheet, the steel sheet is heated to a high temperature to facilitate bending, and hot bending is performed. In the case of the high-strength heat-treated steel described above, it is difficult to process it into a cylindrical shape at room temperature because it has high strength at high temperature and poor workability.
In many cases, it is heated to 00 to 1000 ° C. and hot-bent. At this time, when the heat-treated steel is heated to a high temperature, its structure is transformed and its strength is lowered. Therefore, it is necessary to perform normalizing / tempering treatment after hot bending to recover the structure.

Normally, normalizing is performed at 1,050 ° C. or higher, and tempering is performed by heating at about 780 ° C. Next, the ends are welded and joined in the longitudinal direction from the outer surface side or the inner surface side of the cylinder to form a steel pipe. If the diameter is relatively small, a semi-cylindrical shape may be formed by pressing, and two pieces may be butt welded together to form a steel pipe.

By the way, since tempered martensitic steel is softened by Vickers hardness of about 20 to 40 in the weld heat affected zone (HAZ portion) during welding, the softened region is widened in welding with high heat input. The creep rupture strength of the joint may decrease. Especially in the longitudinal weld of the pipe, the circumferential stress generated by the internal pressure is perpendicular to the weld line,
Since large stress acts on the softened region, it was necessary to pay close attention to keep the heat input small during welding so that the width of the softened region does not become wide.

As a method of recovering the softening of the HAZ, a method of performing the above-mentioned normalizing / tempering treatment after welding can be considered. In this case, the HAZ structure is restored to the state before welding, but with the conventional welding materials, there arises a problem that the creep rupture strength of the weld metal decreases. That is, according to the conventional concept of component design of welding material, it is premised that the weld metal is used after being subjected to stress relief annealing at about 740 ° C. in the solidified and rapidly cooled structure, to prevent weld cracking and to improve strength and structure. C is lower than the base metal for the purpose of adjusting
n is made higher than the base material, and the amounts of other additive elements are also adjusted.

As a result, if normalizing is performed after welding and further tempering is performed at a high temperature of about 780 ° C., the high temperature strength of the weld metal is reduced. Particularly in the case of welding methods with large heat input welding, such as submerged arc welding, where the amount of welding per pass is large, the amounts of C and Nb are reduced to prevent weld cracking, and creep rupture occurs after normalizing and tempering. The strength is greatly reduced.

As described above, when a plate-bending welded steel pipe is manufactured for a tempered high-strength steel by a general conventional method, the plate-bending welded steel pipe has a HAZ at a longitudinal weld portion or a strength of a weld metal. There is a possibility that it will be low, and depending on the welding construction conditions and usage conditions, it was necessary to make the wall thickness thicker than the seamless steel pipe in order to provide a safety margin in terms of design.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, to provide a plate bending welded steel pipe which can be easily manufactured and which has increased strength of a longitudinal weld portion and improved reliability. Especially.

[0012]

In order to solve the above problems, the present invention mainly employs the following configurations. 9
Cr-1Mo-Nb-V and N steels, and finally used as a heat-treated steel that has been strengthened by subjecting it to normalizing / tempering or quenching / tempering which is a heat treatment for refining. A steel sheet used under high temperature conditions is bent into a cylindrical or semi-cylindrical shape without heat treatment for tempering, and then the end portion of the cylindrical or semi-cylindrical steel sheet is longitudinally Cr , Mo, C, Nb, V and N are welded using a welding material having the same composition as the base metal to form a pipe shape,
Subsequently, a normalizing / tempering process or a quenching / tempering process is performed to recover the HAZ portion softened during the welding to a strength equivalent to that of the base metal, and the weld metal after the welding to the base metal. A method for manufacturing plate-bend welded steel pipes having the same strength.

Further, in the method for producing a plate-bending welded steel pipe, the difference between the minimum hardness of the heat-affected zone in the longitudinal weld zone and the average hardness of the surrounding base metal is within 15 Vickers hardness, and the welding is performed. A method for producing a plate bending welded steel pipe in which the difference between the maximum hardness of metal and the average hardness of the surrounding base materials is 30 or less in Vickers hardness.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION A first embodiment of the present invention will be described in detail below with reference to the drawings. First, a basic manufacturing procedure of a plate bending welded steel pipe will be described with reference to FIG. As a raw material, unrolled steel sheet 1 whose strength is suppressed to a low level by being gradually cooled or tempered after rolling is used, and is bent into a cylindrical shape by pressing or rolling. Depending on the workability, it may be worked into a semi-cylindrical shape, or either cold bending or hot bending. After plate bending, longitudinal welding is performed.

The welding method is a narrow groove TIG, MAG or MIG welding method in which the amount of deposition per pass is small and welding cracks are less likely to occur. As for the welding material, in order to make the strength of the weld metal after normalizing and tempering equal to that of the base metal, the Cr, Mo, C, Nb, V and N amounts that greatly affect the strength are included in the base metal component specification range. Use the one that is available. After longitudinal welding, perform the prescribed normalizing / tempering or quenching / tempering.

Next, a concrete construction example will be shown. The steel plate used for the test is 9Cr-1Mo-Nb-VN with a plate thickness of 40 mm.
It is a steel that is only annealed after rolling. The chemical composition is shown in FIG. Bending radius of 900m for this steel plate
Cold bending was performed under the condition of m. Normalizing normal steel
0.2% proof stress at room temperature in the tempered state is 60 kg /
It is around mm2, but here it is about 40 by annealing.
Since it was as low as kg / mm2, it could be easily processed into a semi-cylindrical shape without any problem even in cold bending.

Longitudinal welding (section) 2 was carried out by butt welding of multi-layer welding by the narrow groove TIG welding method using the welding material (1) shown in FIG. FIG. 2 shows the welding groove shape. After the longitudinal welding, normalizing was performed at 1,050 ° C for 1 hour and tempering was performed at 780 ° C for 1 hour.
(This joint is referred to as A.) For comparison, a conventional manufacturing method, that is, two welded joints are manufactured using a conventional welding material (2) on a tempered steel plate, and one welded joint is 740 A general post-weld heat treatment was carried out at a temperature of 1.5 ° C. for 1.5 hours, while the (C) was subjected to normalizing at 1,050 ° C. for 1 hour and tempering at 780 ° C. for 1 hour.

According to the experimental results, when observing the cross-section macrostructure of the longitudinal weld portion (A) of the plate bending welded steel pipe according to the present invention, welding cracks such as cold cracks and hot cracks did not occur in the weld metal. The HAZ part has the same structure as the base metal part due to the normalizing / tempering process after welding, and the macro structure cannot be distinguished.

FIG. 4 shows the hardness distribution in the central portion of the wall thickness. For comparison, the hardness distribution of the welded portion of (B) is also shown, but in the conventional method (B), about 20 clear HAZ portion softening is recognized with respect to the base metal due to Vickers hardness.
In the present invention, no softened region was found in the HAZ portion, and the range of variation in hardness from the position corresponding to the HAZ portion to the base material was within 10 Vickers hardness. Considering a hardness variation range of about 5 for this type of heat-treated steel, it can be considered that the strength is equivalent to that of the base metal if the variation range of Vickers hardness is 15 or less.

Also, the difference in hardness between the weld metal and the base metal is 50 or more in Vickers hardness in the conventional method, whereas it is about 20 in the present invention, which is a value close to the hardness of the base metal. There is. From the viewpoint of suppressing the strength discontinuity of the welded portion as much as possible, the difference in hardness (strength) between the base metal and the weld metal is about 3 in Vickers hardness.
It is desirable to keep it within 0.

That is, the difference between the minimum hardness of the heat-affected zone in the longitudinal weld zone and the average hardness of the surrounding base metal is within 15 in terms of Vickers hardness, and the maximum hardness of the weld metal and the average hardness of the surrounding base metals. The difference in the Vickers hardness is preferably within 30.

Next, the joint test pieces were cut out from these welds and the creep rupture strength was examined. The results are shown in FIGS. 5 and 6. The test piece of the joint (B) of the welded part according to the conventional manufacturing procedure and the test piece of the joint (C) of the welded part which was heat-treated after welding using the conventional welding material were fractured at the HAZ part and the weld metal, respectively. In both cases, the strength is lower than that of the base material.
On the other hand, all the test pieces of the joint (A) according to the present invention were fractured at the base metal portion and had the same strength as the base metal.

As described above, in the plate bending welded steel pipe shown in the present embodiment, the problems of softening of the HAZ portion of the longitudinal weld portion and deterioration of the strength of the weld metal can be solved, and reliability equivalent to that of the seamless steel pipe can be obtained. it can. In the present embodiment, a steel plate that has not been tempered is used as the material, but in the case of a small degree of bending work or hot bending, it is possible to work with a relatively small working capacity. May be used as the material.

Next, a second embodiment according to the present invention will be described below. FIG. 7 shows a manufacturing procedure for a large-diameter pipe to which branch pipes such as a pipe head and a T-tube are attached. Although the basic idea is the same as that of the first embodiment, in the present embodiment, longitudinal welding 2 is carried out, and branch pipes 3 and peripheral welds 4 are also welded with the same welding material and then tempered and tempered. It is something to do. In this case, if the branch pipe is a heat transfer pipe having a small diameter, the amount of deposition at the time of welding is small, and thus the welding method may be ordinary fillet groove TIG, MAG, MIG or covered arc welding.

In this embodiment, a large heating furnace is required at the time of normalizing and tempering depending on the length of the attached branch pipes. The strength of the welded portion can be increased in the portion having the same as in the first embodiment, and the effect of improving reliability is great.

Further, a third embodiment according to the present invention will be described below. FIG. 8 shows a manufacturing procedure of a pipe to be applied to a pipe in which a bending process is performed after the plate-welded steel pipe is manufactured, such as an elbow pipe. The basic idea is the same as in the first embodiment, but in the present embodiment, longitudinal welding is performed, bending is performed as piping, and then normalization and tempering are performed.

Since the strain history or heat history during bending of the pipe is eliminated by normalizing and tempering, the bending may be performed by high-frequency heating regardless of whether it is cold or hot. In this embodiment as well, the strength of the welded portion can be increased similarly to the first embodiment, and by using these embodiments together, welding with improved reliability can be performed on most of the piping system in an actual plant such as a boiler. Plate bending steel pipe can be applied.

As described above, in the present invention, the following operation and function can be obtained by adopting the following configuration.

According to the present invention, Cr, Mo, C, Nb, V is subjected to plate bending using a non-heat treated steel plate as a raw steel plate which is annealed as it is or after rolling.
And N content are included in the range of component specifications of the base material, after performing longitudinal welding by the narrow groove welding method with a small welding amount per pass, the strength is adjusted by normalizing and tempering. It is achieved by reducing the difference in hardness between the base material and the HAZ part.

Since the strength of the raw steel sheet can be suppressed to be low during bending, bending workability is improved. Further, since welding is performed by the narrow groove welding method in which the amount of deposition per pass is small, welding cracks do not occur even if a welding material having the same composition as the base material is used. The structure of the HAZ part softened during welding is restored to the same level as the general base metal by normalizing and tempering, and the weld metal is also normalized and tempered with the same composition as the base metal. The same strength as the material can be obtained.

[0031]

EFFECTS OF THE INVENTION In the plate bending welded steel pipe of the present invention, since bending can be performed while the strength of the steel plate material is kept low, the bending workability of the steel plate is improved, the strength of the welded part is improved, and the quality reliability is improved. To do. Along with this, it is possible to reduce the wall thickness by making the plate bending steel pipe have a design margin equivalent to that of a general seamless steel pipe, which is industrially effective.

Further, since the material steel sheet does not require normalization and tempering treatment after rolling, not only can the manufacturing cost of the steel sheet be reduced, but the range of workability that can be subjected to cold bending can be expanded as compared with the conventional method, and the number of bending steps can be reduced. It has a great effect and, in turn, the construction cost of the plant can be reduced, so that the economical effect is also great.

[Brief description of drawings]

FIG. 1 is a diagram showing a procedure for manufacturing a plate bending welded steel pipe according to the first embodiment of the present invention.

FIG. 2 is a diagram showing a weld groove shape of a longitudinal welded portion according to the first embodiment of the present invention.

FIG. 3 is a diagram showing a chemical composition of a steel plate and a welding material in the first embodiment.

FIG. 4 is a diagram showing a hardness distribution of a longitudinal welded portion in the first embodiment.

FIG. 5 is a diagram showing a result of a creep rupture test in the first embodiment.

FIG. 6 is a diagram showing data relating to a creep rupture test in the first embodiment.

FIG. 7 is a view showing a procedure for manufacturing a plate bending welded steel pipe according to the second embodiment of the present invention.

FIG. 8 is a diagram showing a procedure for manufacturing a plate bending welded steel pipe according to the third embodiment of the present invention.

FIG. 9 is a diagram showing an example of a procedure for manufacturing a plate bending welded steel pipe according to a conventional technique.

[Explanation of symbols]

1 steel plate 2 Longitudinal welding (part) 3 branches 4 circumference welding (part)

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) B23K 9/025 B23K 9/025 B 9/23 9/23 A C21D 9/50 101 C21D 9/50 101A C22C 38/00 302 C22C 38/00 302Z 38/48 38/48 // B23K 101: 06 B23K 101: 06 (72) Inventor Koji Tamura 3-36 Takaracho, Kure-shi, Hiroshima Babcock-Hitachi Co., Ltd. (Reference) 4E001 BB06 CA04 CC03 DF06 DG04 EA05 4E028 CA02 CA03 CB06 CB08 KA04 4E063 AA01 BA09 BB06 KA15 MA02 4E081 AA08 BA19 CA08 CA09 CA11 4K042 AA06 AA24 BA01 CA07 CA08 CA09 CA10 CA13 DA01 DA02

Claims (5)

[Claims] 1. A 9Cr-1Mo-Nb-V and N steel, and a heat-treated steel that has been strengthened by subjecting it to normalizing / tempering or quenching / tempering which is a heat treatment for refining. Steel sheet that is used for a long time under a high temperature condition is bent into a cylindrical or semi-cylindrical shape without heat treatment for tempering, and then the end of the cylindrical or semi-cylindrical steel sheet is bent. C in the longitudinal direction
r, Mo, C, Nb, V and N contents are welded into a pipe shape by using a welding material having a composition similar to that of the base metal, and subsequently, normalizing / tempering treatment or quenching / tempering treatment is performed, A method for manufacturing a plate-bending welded steel pipe, characterized in that the HAZ portion softened during welding is restored to a strength equivalent to that of the base material, and the weld metal after the welding is made to a strength equivalent to that of the base material. 2. A 9Cr-1Mo-Nb-V and N steel, which is a heat-treated steel that has been strengthened by subjecting it to normalizing / tempering or quenching / tempering which is a heat treatment for refining. Steel sheet that is used for a long time under a high temperature condition is bent into a cylindrical or semi-cylindrical shape without heat treatment for tempering, and then the end of the cylindrical or semi-cylindrical steel sheet is bent. C in the longitudinal direction
r, Mo, C, Nb, V, and N are welded into a pipe shape by using a welding material having the same composition as the base metal, and then the same welding material as the welding material is applied to the steel pipe having the pipe shape. The branch pipe is welded, and then normalizing / tempering or quenching / tempering is performed to recover the HAZ portion softened during the welding to a strength equivalent to that of the base metal, and after the welding, A method for producing a plate-bend welded steel pipe, characterized in that the weld metal has the same strength as that of the base material. 3. A 9Cr-1Mo-Nb-V and N steel, which is a heat-treated steel that has been strengthened by subjecting it to normalizing / tempering or quenching / tempering which is a heat treatment for refining. Steel sheet that is used for a long time under a high temperature condition is bent into a cylindrical or semi-cylindrical shape without heat treatment for tempering, and then the end of the cylindrical or semi-cylindrical steel sheet is bent. C in the longitudinal direction
The amount of r, Mo, C, Nb, V and N is welded using a welding material having the same composition as the base metal to form a pipe shape, and then the steel pipe having the pipe shape is bent, and subsequently, Normalizing / tempering or quenching / tempering is performed to recover the HAZ portion softened during the welding to the same strength as the base metal, and the weld metal after the welding has the same strength as the base metal. A method for manufacturing a plate bending welded steel pipe, which comprises: 4. The method for manufacturing a plate bending welded steel pipe according to claim 1, 2, or 3, wherein a difference between a minimum hardness of the HAZ portion and an average hardness of a surrounding base material in a weld portion in the longitudinal direction is Vickers hardness. And the difference between the maximum hardness of the weld metal and the average hardness of the surrounding base materials is 30 or less in terms of Vickers hardness. 5. A plate bending welded steel pipe manufactured by the method for manufacturing a plate bending welded steel pipe according to claim 1, 2, 3 or 4.