JPH09194998A - Welded steel tube and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a welded steel tube reduced in the amount of oxygen in a weld zone and having mechanical properties, in the weld zone, comparable to those in a base material part while maintaining the advantage of a resistance welding method enabling high efficiency production. SOLUTION: A steel plate, which has a composition containing, by weight, 0.01-0.5% C and 0.5-2% Mn and further containing, each including the case of 0%, <=1.0% Si, <=0.2% Al, <=0.01% N, <=0.2% Ti, <=0.2% Zr, <=0.2% Nb, <=0.2% V, <=1.5% Ni, <=1.5% Cu, <=3.0% Cr, <=1.5% Mo, <=1.5% W, <=0.005% B, <=0.01% Ca, <=0.01% Mg, <=0.1% REM, <=0.02% P, and <=0.03% S, is formed into tubular body. Then, the edges, abutting each other, of the tubular body are melted by laser beam irradiation and subjected to upset welding, by which the welded steel tube in which the amount of oxygen in a weld zone is regulated to <=200ppm is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welded steel pipe used for pipelines, machine structural members, various plants and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Steel pipes are used in a wide variety of applications such as pipelines, machine structural members, heat exchangers, automobile parts, mechanical structural members, and various plants. The seamless steel pipes and welded steel pipes manufactured by the method are used. Among them, electric resistance welding, in which a steel strip is continuously formed into a tubular shape to form a tubular body (open pipe), and its opposite edges are heated by high frequency heating or resistance heating and welded. Welded steel pipes have been produced and used in large quantities from the past because of their high dimensional accuracy, high efficiency in production compared to other pipe production methods, and relatively low cost.

However, when the electric resistance welded steel pipe is used in a field in which high material characteristics and high reliability are required, the material sometimes becomes a problem. The reason why the material of the electric resistance welded steel pipe is inferior to other welded steel pipes is as follows. 1) Since the heat input to the electric resistance welded portion is smaller than that to the arc welding, the cooling speed of the electric resistance welded portion and its vicinity is high during the electric resistance welding. Therefore, the electric resistance welded portion and its vicinity are easily hardened, and the quality thereof is easily deteriorated. 2) The welded portion welded by the electric resistance welding method is in a state that it can be said to be intermediate between ordinary fusion welding such as arc welding and pressure welding, and no clear molten pool is formed at the jointed portion.
On the other hand, a considerable amount of oxides are formed by oxidation during welding, but these oxides are hard to be discharged from the inside of the steel,
As a result of the weld having a high oxygen content, the mechanical properties deteriorate.

Of the above reasons, 1) can be dealt with by performing a heat treatment. For example, the material of the hardened welded portion can be improved by subjecting the electric resistance welded pipe to normalizing on-line or performing heat treatment such as quenching and tempering, for example, as disclosed in JP-A-62-1.
Japanese Patent No. 51509 discloses a technique of performing heat treatment for quenching and tempering on a hardened weld portion.

[0005]

However, among the above reasons, in order to solve the problem 2) caused by oxidation, an additional measure is required. For example, a steel strip is upset to a tubular body during welding. Therefore, it has been attempted to discharge inclusions and the like to the outside as much as possible. However, as a result, a metal flow in the vicinity of the weld rises, so to speak, a cross section of steel during rolling appears on the inner and outer surfaces of the pipe. The steel pipe having such a metal flow has poor fatigue strength,
There is a concern that durability may become a problem when internal pressure is applied. Of course, it is also known that inclusions and the like are not completely discharged even after upsetting, and a considerable amount of inclusions remain, and as a result, the amount of oxygen in the steel increases.

Measures for solving the above problems have been studied, and heat input is controlled, or JP-A-63-24 is used.
As disclosed in Japanese Patent No. 1116, there is known a method of shielding the electric resistance welded portion with a non-oxidizing gas and performing electric resistance welding to reduce the amount of oxygen and the amount of inclusions in the electric resistance welded portion. However, in reality, there is a limit to the improvement by controlling the heat input, and a shield device that has excellent shielding properties and can withstand continuous operation has not been developed.

For these reasons, it has been conventionally impossible to manufacture an electric resistance welded steel pipe in which mechanical properties such as strength and toughness of the welded portion are comparable to those of the base metal portion. As mentioned above,
The mechanical properties such as strength and toughness of the welded part of the electric resistance welded steel pipe manufactured by the conventional technology are inferior to those of the base metal part, which is a major cause of limiting the applications of the electric resistance welded steel pipe. Was becoming.

Therefore, the object of the present invention is to solve the above-mentioned problems and to maintain the advantages of the electric resistance welding method capable of producing with high efficiency, while the oxygen content of the welded portion is small and the mechanical characteristics thereof are the base metal. It is intended to provide a welded steel pipe and a manufacturing method thereof that are comparable to those of the above-mentioned parts.

[0009]

The inventors of the present invention have the following reasons that the mechanical characteristics of the electric resistance welded part are inferior to those of the base metal part: 1) the amount of oxygen and the amount of inclusions in the welded part; The present invention has been completed as a result of repeated studies by finding out that the heat history and 3) the metal flow in the vicinity of the welded portion are particularly affected by 1) and 2).

The basis of the present invention is that a steel strip having an appropriate chemical composition is formed into a tubular body by a multi-stage roll, the abutted edges of the obtained tubular body are preheated at an appropriate temperature, and then, A welded steel pipe is obtained by irradiating a preheated edge portion with a laser beam to melt it, and upset and weld it.

Therefore, unlike the case of the conventional method for producing electric resistance welded steel pipe, the welded portion has a melt-solidified structure and melting of the steel occurs, so that most of the oxides produced by oxidation are melted. Emitted from steel As a result, the oxygen content of the welded part is significantly reduced compared to the welded part of the conventional electric resistance welded steel pipe, and the mechanical properties such as strength of the welded part are reduced to the minimum extent compared with the base metal part. Can be Of course, if the composition of the steel is not appropriate, the mechanical properties of the weld will not be sufficient even if a laser beam is irradiated under optimum conditions to produce a welded steel pipe.

The invention described in claim 1 is characterized by the following. % By weight, C: 0.01-0.5% and Mn: 0.5-2
%, Further, Si: 1.0% or less, Al: 0.2% or less, N: 0.01% or less, Ti: 0.2% or less, Zr: 0.2% or less, Nb: 0.2% or less, V: 0.2% or less, Ni: 1.5% or less, Cu: 1.5% or less, Cr: 3.0% or less, Mo: 1.5% or less, W: 1. 5% or less, B: 0.005% or less, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.1% or less, P: 0.05% or less, and S: 0. A welded steel pipe made of a steel containing 03% or less (including 0), and having an oxygen content of 200 ppm or less at the welded portion.

The invention described in claim 2 is characterized by the following. (A) a step of preparing a steel sheet having the chemical composition as set forth in claim 1, (b) a step of forming the steel sheet into a tubular body, and (c) an abutted edge portion of the tubular body,
A method for manufacturing a welded steel pipe, which comprises the steps of melting by irradiation of a laser beam, upsetting and welding.

The invention described in claim 3 is characterized by the following. (A) a step of preparing a steel sheet having the chemical composition described in claim 1, (b) a step of forming the steel sheet into a tubular body,
(C) preheating the butted edges of the tubular body, and (d) melting the preheated edges of the tubular body by laser beam irradiation and upsetting and welding. Manufacturing method of welded steel pipe.

The invention described in claim 4 is characterized by the following. (A) a step of preparing a steel sheet having the chemical composition described in claim 1, (b) a step of forming the steel sheet into a tubular body,
(C) The abutted edge portions of the tubular body are
Preheating to a temperature in the range of ˜1300 ° C., and
(D) a step in which the preheated edge portion of the tubular body is melted by laser beam irradiation and is upset and welded; and (e) the welded portion is Ac ₃ + 20 ° C.
A method for producing a welded steel pipe, comprising the steps of heating to a temperature range of up to 1000 ° C and then cooling.

The invention described in claim 5 is characterized by the following. (A) a step of preparing a steel sheet having the chemical composition described in claim 1, (b) a step of forming the steel sheet into a tubular body,
(C) The abutted edge portions of the tubular body are
Preheating to a temperature in the range of ˜1300 ° C., (d) melting and upsetting and welding the preheated edge of the tubular body by laser beam irradiation, and (e) the welded welding. Part is Ac ₃ + 20 ° C to 10
Heating to a temperature in the range of 00 ° C., and (f)
Then, the temperature in the range of 100 ° C. to A _C1 (however, 30
A method for manufacturing a welded steel pipe, which comprises a tempering step (except 0 to 400 ° C.).

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic explanatory view showing an example of pipe manufacturing equipment for carrying out the present invention, and FIG.
FIG. As shown in the drawings, the contact tip 2 and the top roll 4 are arranged along the moving direction of the tubular body 1.
The side roll 5, the first high-frequency heating device 6, the water cooling zone 7, and the second high-frequency heating device 8 are arranged in this order.

The steel strip is formed into a tubular body 1 by a multi-stage roll (not shown), and then the butted edges of the tubular body 1 are preheated by a contact tip 2 or a high frequency induction heating device. Then, the laser beam 3 is applied to the preheated edge portion of the tubular body 1 to melt it and upset and weld it. Next, in the first high-frequency heating device 6, the water-cooling zone 7 and the second high-frequency heating device 8, normal heat treatment or post-heat treatment such as quenching and tempering is performed in accordance with the purpose of use of the steel pipe.

The basis of the present invention is to irradiate a laser beam to melt and weld the abutted edges of the tubular body 1, discharge the oxide produced during welding to the outside of the steel, and to discharge the oxygen in the weld. Make the amount 200 ppm or less. As a result, oxides, which make up the majority of the oxygen content of the steel, are discharged to the outside of the steel pipe.

The oxygen content of fully deoxidized steel is usually 10
It does not exceed 0 ppm. On the other hand, the oxygen content of the welded portion of the electric resistance welded steel pipe manufactured by the conventional method is 2
The range is more than 00 ppm to 300 ppm.

FIG. 3 shows a welded steel pipe having an inner diameter of 76.3 mmφ and a thickness of 7.5 mm, which is prepared by using steel strips having the compositional compositions of No. 1 to No. 5 in Table 1 described later, at a preheating temperature of 700 ° C. A comparison was made between the oxygen content of the welded portion produced by the method of the present invention under the conditions of laser output of 5 kw and welding speed of 3 m / min, and the oxygen content of the electric resistance welded portion produced by the conventional electric resistance welding method. FIG. As is apparent from FIG. 3, the oxygen content in the welded portion of the welded steel pipe of the present invention is about 150 ppm,
The amount of oxygen in the weld of conventional ERW welded steel pipe is about 250ppm
Met.

The oxygen content of the weld is influenced by the preheating condition. That is, when the preheating temperature is increased, the load of laser irradiation on welding is reduced and the manufacturing efficiency is improved. On the other hand, since the amount of oxides generated during preheating increases,
As a result, the amount of oxide increases and the amount of oxygen also increases, deteriorating the mechanical properties. Note that the amount of inclusions, the amount of oxides, and the amount of oxygen have a substantially proportional relationship.

On the other hand, when the preheating temperature is lowered, the load of laser irradiation on welding becomes large and the manufacturing efficiency decreases,
Alternatively, a large-capacity laser irradiation equipment is required, resulting in high cost. On the other hand, as a result of reducing the amount of oxides generated during preheating, the amount of inclusions decreases, the amount of oxygen decreases, and the mechanical properties improve.

In the present invention, the welded portion is a general term including a portion having a different microstructure (metal flow, different etching sensitivity, etc.) from the base material portion, including a welded portion by laser beam irradiation. is there.

FIG. 4 is a diagram showing the influence of the preheating temperature on the toughness. Using the same welded steel pipe as described with reference to Fig. 3 above, the steel pipe subjected to quenching and tempering treatment (quenching temperature 950 ° C, tempering temperature 650 ° C, cooling rate 50 ° C / sec) had its preheating temperature and fracture surface in Charpy test. The relationship with the transition temperature ( _v Trs) was investigated. As is clear from FIG. 4, as the preheating temperature increases, the oxygen content increases, _v Trs increases, and the toughness decreases.

_V Tr below -50 ° C, which is a measure of high toughness
In order to secure s, the preheating temperature must be 1000 ° C. or lower, and further, in order to secure _v Trs of −70 ° C. or lower, the preheating temperature must be 750 ° C. or lower. In addition, the amount of oxygen when the preheating temperature is 1000 ° C. is 150 to 1
The amount of oxygen is 90 ppm, and the oxygen amount is about 120 to 150 ppm when the preheating temperature is 700 ° C.

Next, the reason why the chemical composition of the welded steel pipe of the present invention is limited as described above will be explained. C has the effect of increasing the strength of the steel. However, if the C content is less than 0.01 wt.%, It is difficult to secure the strength. On the other hand, if the C content exceeds 0.5 wt.%,
Hot workability, weldability and toughness deteriorate. Therefore, C
The content should be limited to the range of 0.01-0.5 wt.%.

Mn also has a function of increasing the strength of steel. However, if the Mn content is less than 0.05 wt.%, Sufficient strength cannot be obtained. On the other hand, when the Mn content exceeds 2 wt.%, The weldability and toughness deteriorate. Therefore, Mn
The content should be limited within the range of 0.05-2 wt.%.

Si is a deoxidizing element, but if its content exceeds 1.0 wt.%, Adverse effects such as deterioration of weldability and toughness occur. Therefore, the upper limit of the Si content is 1.
0 wt.% When Si is added as a deoxidizing agent, it is preferably added in an amount of 0.05 wt.% Or more.

Al is also a deoxidizing element, but if its content exceeds 0.2 wt.%, Adverse effects such as deterioration of toughness occur. Therefore, the upper limit of the Al content is 0.2 wt.%. In addition, when adding Al as a deoxidizing agent,
Addition of 0005 wt.% Or more is preferable.

N is an element mixed into the steel as an impurity from the atmosphere during melting, and its content is 0.01 wt.%.
If it exceeds, weldability and toughness are adversely affected. Therefore, the upper limit of the N content is set to 0.01 wt.%. Note that N
Has the effect of increasing the strength of steel as with C, and the effect is recognized by the inclusion of about 0.001 wt.%. Therefore, when utilizing the strengthening action of N, 0.0
Addition of more than 01 wt.% Is preferable.

Ti, Zr, Nb and V have the effects of increasing the strength of the steel and forming carbides and nitrides to refine the austenite grains and improve the strength and toughness. Therefore, at least one element selected from the above group is contained if necessary. However,
If the content exceeds 0.2 wt.%, The toughness deteriorates. Therefore, when at least one element selected from the group consisting of Ti, Zr, Nb and V is contained, the upper limit value is set to 0.2 wt.%. The content is 0.005
If less than wt.%, the effect is small. Therefore, when it is contained, it is preferably 0.005 wt.% or more.

Cr, Cu, Ni, Mo, W and B are
It has the effect of increasing the strength of steel. Therefore, if necessary, at least one element selected from the above group is contained. However, when the Cr content exceeds 3 wt.%, The weldability and toughness deteriorate, and the Cu content is 1.5 w.
If it exceeds t.%, the hot workability, weldability and toughness deteriorate, and if the contents of Ni, Mo and W exceed 1.5 wt.%, the effect of addition is saturated, and the B content is 0. .005
If it exceeds wt.%, the toughness of the welded part deteriorates. Therefore, the upper limit of each element in the case of containing at least one element selected from the above group is 3 wt.% For Cr and 1.5 for Cu.
wt.%, each of Ni, Mo and W is 1.5 wt.%, B is 0.0
It is set to 05 wt.%. In addition, preferable lower limit values for each element to exert its effect are Cr, Cu, Ni, and M.
Each of o and W is 0.03 wt.% and B is 0.0001 wt.%.

Ca, Mg and REM combine with S to granulate inclusions, improve the corrosion resistance in various corrosive environments through morphology control, and increase the toughness value. Therefore, at least one element selected from the above group is contained if necessary. However, if the Ca and Mg contents exceed 0.01 wt.% And the REM content exceeds 0.1 wt.%, The toughness of the steel deteriorates. Therefore, the upper limit of each element in the case of containing at least one element selected from the above group is Ca
And Mg are 0.01 wt.% And REM is 0.1 wt.%. The preferable lower limit value for each element to exert its effect is 0.0005 wt.%.

The inevitable impurities P and S are usually contained in steel, that is, P is 0.05 wt.% Or less, and S is 0.
Less than 03 wt.% Is acceptable. However, P and S
Is harmful to the toughness, so that P is preferably 0.02 wt.% Or less and S is 0.01 wt.% Or less when the toughness is highly required. In particular, if the S content is limited to 0.005 wt.% Or less, much more excellent toughness can be obtained.

Next, a method for manufacturing the welded steel pipe of the present invention will be described. Using the equipment shown in FIGS. 1 and 2,
The butted edges of the tubular body 1 are preheated by the contact tip 2 or high frequency induction heating. The preferable preheating temperature of the edge portion is in the range of 600 to 1300 ° C. If the preheating temperature is less than 600 ° C., the preheating is insufficient and the burden on the laser beam irradiation becomes excessive. On the other hand, when the preheating temperature is higher than 1300 ° C., the load on the laser beam irradiation is light, but the oxidation amount of the steel increases and the oxide amount increases, resulting in an increase in the oxygen amount.
The preheating temperature is preferably limited to 1000 ° C. or lower in order to keep the oxygen content in the welded portion at 200 ppm or lower.

The butted edges of the tubular body 1, which have been preheated as described above, are irradiated with the laser beam 3 to melt and weld them. The shape of the edge portion abutted by the top roll 4 at the time of welding is formed into an I shape, and the side roll 5 upsets. As shown in FIG. 1, the plurality of top rolls 4 are
It is arranged so that the tubular body 1 is held obliquely from above and the laser beam 3 passes between the top rolls 4, and the welded portion is heated and melted by the laser beam 3 passing between the top rolls 4. Note that preheating is not an essential condition, and electric resistance welding can be performed only by irradiation with a laser beam.

In this way, since the fusion welding is performed by the laser beam 3, most of the oxide formed at the edge portion during the preheating is discharged to the outside of the fusion pool, and a welding portion having a small amount of oxygen is formed. It It goes without saying that the amount of oxides formed during preheating is significantly smaller than that in the conventional method for producing electric resistance welded steel pipes, which is heated to a temperature just below the melting point.

When manufacturing a conventional electric resistance welded steel pipe,
Although it was necessary to apply a strong upset in order to discharge a large amount of oxides generated from the welded portion, the method of the present invention requires an extremely low upset amount as compared with the conventional method. This means that not only the deterioration of mechanical properties due to the rising of the metal flow can be reduced, but also burrs generated during upset can be removed easily.

If necessary, post-heat treatment such as normalizing, quenching and tempering is applied to the welded portion of the steel pipe after welding. Of course, such post heat treatment may not be necessary depending on the composition of the steel and the purpose of use.

When the normalizing process is performed, for example, in the first high-frequency heating device 6, Ac ₃ + 20 ° C. to 1000 ° C.
After heating in the temperature range of ℃, it is cooled. In the case of local heating near the welded part, heat is absorbed by the non-heated part of the steel pipe,
Special cooling may not be necessary, but air cooling may be performed if necessary. Of course, the heat treatment may be applied to the entire pipe without being limited to the vicinity of the welded portion. In this case, since there is no boundary with the normalizing portion, a steel pipe of a more uniform material can be obtained. The lower limit of the heating temperature is to increase the heat treatment effect,
Ac ₃ + 20 ° C., and its upper limit is 1000 ° C. at which austenite crystal grains are not coarsened.

Even when the quenching and tempering process is performed, first, the first high frequency heating device 6 is used for Ac ₃ + 20 ° C. to 1000 ° C.
In the cooling zone 7, quenching is performed by water cooling, mist cooling, or the like. The cooling rate at this time was 30 ° C./sec. With the above, the structure should be fully baked. The cooling stop temperature needs to be 400 ° C. or lower, and the cooling is performed with a sufficient amount of water or a sufficient cooling zone.

After quenching, tempering performed in the second high-frequency heating device 8 is performed in the temperature range of 100 ° C. to AC ₁ .
If the tempering temperature is less than 100 ° C, the tempering effect cannot be sufficiently obtained. Note that tempering at a temperature in the range of 300 to 400 ° C. causes embrittlement, so the temperature in the above range is excluded. The upper limit temperature for tempering is Ac ₁ , but a rough guideline is 760 ° C.

[0044]

EXAMPLES Next, the present invention will be described based on examples together with comparative examples. Steels having chemical composition within the scope of the present invention shown in Tables 1 and 2 (hereinafter referred to as steel of the present invention) and steels having chemical composition outside the scope of the present invention shown in Table 3 (hereinafter referred to as comparative steels) ) Was melted.

[0045]

[Table 1]

[0046]

[Table 2]

[0047]

[Table 3]

Inventive Steel No. 1 shown in Table 1 1 to No. 5,
No. 12, No. 17, steel No. 17 of the present invention shown in Table 2. 2
1 to No. No. 28 was used, welding was carried out under the conditions shown in Table 4 by the method of the present invention, and then quenching and tempering treatments were carried out under the conditions shown in Table 5, thus giving an inner diameter of 76.3 mm.
A φ and 7.5 mm thick welded steel pipe was manufactured. The production efficiency was equal to or higher than that of the conventional electric resistance welding method.

[0049]

[Table 4]

[0050]

[Table 5]

Inventive Steel No. 1 to No. This consists of 5
QT treatment (quenching) for welded steel pipes manufactured by the bright method
Welding at a temperature of 950 ℃ and tempering temperature of 650 ℃
Part of _VT_rsFigure 5 shows the effect of cooling rate on
QT treatment (quenching temperature 950 ° C, cold
Of the welded part when subjected to a rejection speed of 50 ° C / sec)_VT_rsTo
FIG. 6 shows the effect of tempering temperature. 5 and 6?
As is clear from the above, when the cooling rate is 30 ° C / sec or more,
Sufficiently low when tempering temperature is over 500 ℃_V
T_rswas gotten.

Inventive Steel No. 1 to No. No. 5, using the method of the present invention and a conventional welded steel pipe produced by the conventional electric resistance welding method without laser beam irradiation, when performing QT treatment (tempering temperature 650 ° C., cooling rate 50 ° C./sec) 7 shows the _V T _rs of each part in relation to the quenching temperature. As is clear from FIG. 7, in the case of the manufacturing method of the present invention, the _V T with the quenching temperature of 1000 ° C. or lower
_rs is sufficiently low, but _V above when manufactured by the conventional method
T _rs was high even if the composition of the steel was within the scope of the present invention.

Inventive Steel No. 21-No. No. 26, the welded steel pipe produced by the method of the present invention and the conventional electric resistance welding method not irradiating a laser beam was subjected to a normalizing treatment, and the _V T _rs of the welded portion was compared with the heating temperature. The relationship is shown in FIG. From FIG. 8, in the case of the method of the present invention, _V T _rs is low when the normalizing temperature is 1000 ° C. or lower. The normalizing temperature is 1000 ℃
The _V T _rs increases exceeds, but the difference between the present invention method and the conventional method is the same as normalizing temperature is 1000 ° C. or less.

Next, according to the invention steel No. 9-No. 11, N
o. 13-No. 16, No. 18-No. 20, N
o. 29, no. 30 and comparative steel No. 31-N
o. 38 was melted in the laboratory by vacuum melting to obtain 50 kg.
It was cast into an ingot.

Next, these ingots were heated at 1200 ° C.
And then hot-rolled to a plate thickness of 50 mm to form a steel plate,
Air cooled. 50 x 150 x 400 mm from steel plate after air cooling
Cut out the plate, heating temperature 1200 ℃, rolling end temperature 8
It was rolled at 20 ° C. to a thickness of 12 mm. Immediately after the end of rolling, the cooling rate was about 10 ° C./sec. At 550
After cooling to ℃, it was charged into an electric furnace preheated to 550 ℃ and cooled. These steps simulate the conditions for manufacturing a steel strip by hot rolling.

From a steel sheet cooled to room temperature, 6 × 35 ×
A 1000 mm test piece was cut out and welded using an electric resistance welding simulator. As shown in the schematic perspective view of FIG. 9, the electric resistance welding simulator device feeds two test steel strips 9 by guide rolls 10 and supplies the high frequency current supplied from the contact tip 2 to the edges of the steel strips 9 facing each other. After preheating with, the squeeze roll 11 is pressure-welded, so that the carbon dioxide gas laser beam 3 is irradiated to the edge joint portion while focusing on the edge portion of the steel strip 9 which is abutted from vertically above. There is.

The welding conditions are as follows, and the heat treatments for quenching and tempering were carried out under the conditions shown in Table 5. The amount of oxygen in the weld was 140 to 150 ppm. Preheating temperature: 700 ° C., welding speed: 10 m / min. Input power from contact tip: 200 kw, laser output: 5 kw, beam diameter at focus position: 0.5 mm.

A Charpy impact test piece of 5 mmt was sampled from the welded portion of the welded steel pipe manufactured by the above-mentioned method.
_V T _rs was _calculated . Using the steels of the present invention shown in Tables 1 and 2 and the comparative steels shown in Table 3, manufacturability and laser irradiation weldability of steel strips when a welded steel pipe is produced by the method of the present invention, and the above electric resistance welding The toughness ( _V T _rs ) of the welded portion of the welded steel pipe welded using a welding simulator was investigated, and evaluated by the following, and shown in Table 6.

Good manufacturability: When cracks do not occur during hot rolling Manufacturability: When cracks occur during hot rolling but the next step (welding) can be performed by flaw removal, Poorness: If cracks are large during hot rolling and cannot be removed, good weldability: If no oxides, etc. are observed in the laser irradiation weld, weldability is possible: Oxides, etc. in laser irradiation weld However, if welding is possible, weldability is poor: laser irradiation welds have a large amount of oxides, etc., and welding is virtually impossible. Good toughness: _V T _{rs of} weld is -30 ° C. In the following cases, poor toughness: When _{V Trs of the} welded portion exceeds −30 ° C.

[0060]

[Table 6]

As is clear from Table 6, Comparative Steel No. 3
1, No. 33, no. 34 and No. 34. For No. 38, the manufacturability was poor and a large crack occurred during hot rolling, so welding was stopped. Comparative Steel No. 32, No. 35-No.
In No. 37, the laser irradiation weldability was poor in all cases, and the _{V Trs of the} welded portion was -30 ° C. or more and was poor. From this, it can be seen that the component composition has a great influence on _V T _rs .

On the other hand, according to the invention steel No. 1 to No.
5, no. 9-No. No. 27 had good steel strip manufacturability and laser irradiation weldability. The steel No. of the present invention.
28-No. No. 30 had fine cracks and flaws at the end of the steel plate, but welding could be performed by removing the flaws.

The steel of the present invention was used for the production by the method of the present invention both when it was manufactured into an electric resistance welded steel pipe at the factory and when the test molten material was welded using an electric resistance welding simulator. In case of _V T _rs , the preheating temperature is 7
In the case of 00 ° C., the temperature was −30 ° C. or less, which showed extremely good toughness. The toughness of the steel having a low S content and a relatively small amount of strengthening elements was further excellent.

Good toughness was also obtained when normalizing treatment was performed after welding. When the normalizing treatment is applied, _V
Although T _rs is high, the weld zone of the steel of the present invention is
_{V Trs} was -20 ° C or lower even when the preheating temperature was 900 ° C.

The steel No. of the present invention shown in Table 1 was used. 11-N
o. 12, No. 14-No. 15, No. 17 and No. 17 Since No. 19 contains a predetermined amount of Ca and / or Mg, HIC (hydrogen induced cracking) resistance and S resistance
It also had SCC properties (sulfide stress corrosion cracking).

[0066]

As described above, according to the present invention, while maintaining the advantages of the electric resistance welding method that can be manufactured with high efficiency, the amount of oxygen in the welded portion is small and the mechanical characteristics thereof are the base metal. The welded steel pipes are comparable to those of other parts and are most suitable for machine structures, various plants, line pipes, etc. When used in various corrosive environments, it goes without saying that a high-strength / high-toughness steel pipe having corrosion resistance can be obtained by selecting the component composition as necessary.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing an example of pipe manufacturing equipment for carrying out the present invention.

FIG. 2 is a view taken along the line AA of FIG. 1;

FIG. 3 is a diagram showing a comparison of oxygen concentrations in welded portions of electric resistance welded steel pipes manufactured by the method of the present invention and the conventional method.

FIG. 4 is a diagram showing a relationship between a preheating temperature of a welded steel pipe manufactured by the method of the present invention and _V T _rs .

FIG. 5 is a diagram showing the relationship between the cooling rate during QT processing and the _V T _{rs of the} welded portion in the method of the present invention.

FIG. 6 is a diagram showing the relationship between the tempering temperature during QT processing and the _V T _{rs of the} welded portion in the method of the present invention.

FIG. 7 is a diagram showing the relationship between the quenching temperature and _V T _rs during the QT treatment of the welded steel pipe produced by the method of the present invention using the present invention steel and the comparative steel.

FIG. 8 is a diagram showing a relationship between a normalizing temperature and a _V T _rs of a welded part when a normalizing treatment is performed on a welded steel pipe manufactured by the method of the present invention and the conventional method.

FIG. 9 is a schematic perspective view showing an electric resistance welding simulator device.

[Explanation of symbols]

 1 Tubular Body 2 Contact Tip 3 Laser Beam 4 Top Roll 5 Side Roll 6 First High Frequency Heating Device 7 Water Cooling Zone 8 Second High Frequency Heating Device 9 Steel Strip 10 Guide Roll 11 Squeeze Roll

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location C21D 9/50 101 C21D 9/50 101A C22C 38/04 C22C 38/04 38/58 38/58 ( 72) Inventor Masaki Omura 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (5)

[Claims] 1. By weight%, C: 0.01 to 0.5% and Mn: 0.5 to 2
%, Further, Si: 1.0% or less, Al: 0.2% or less, N: 0.01% or less, Ti: 0.2% or less, Zr: 0.2% or less, Nb: 0.2% or less, V: 0.2% or less, Ni: 1.5% or less, Cu: 1.5% or less, Cr: 3.0% or less, Mo: 1.5% or less, W: 1. 5% or less, B: 0.005% or less, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.1% or less, P: 0.05% or less, and S: 0. A welded steel pipe, which is made of steel containing 03% or less (including 0), and has an oxygen content of 200 ppm or less in a welded portion. 2. A method for manufacturing a welded steel pipe, comprising the following steps. (A) a step of preparing a steel sheet having the chemical composition described in claim 1; (b) a step of forming the steel sheet into a tubular body; and (c) a laser beam for abutting edge portions of the tubular body. The process of melting by irradiation with a beam and upsetting and welding. 3. A method for manufacturing a welded steel pipe, comprising the following steps. (A) a step of preparing a steel sheet having the chemical composition described in claim 1, (b) a step of forming the steel sheet into a tubular body, (c) a step of preheating the abutted edge portions of the tubular body, And (d) a step in which the preheated edge portion of the tubular body is melted by irradiation with a laser beam and is upset and welded. 4. A method for manufacturing a welded steel pipe, comprising the following steps. (A) a step of preparing a steel sheet having the chemical composition of claim 1; (b) a step of forming the steel sheet into a tubular body;
Preheating to a temperature in the range of ˜1300 ° C., (d) melting and upsetting and welding the preheated edge portion of the tubular body by irradiation with a laser beam, and (e) the welding Parts are heated to a temperature range of Ac ₃ + 20 ° C. to 1000 ° C. and then cooled. 5. A method for manufacturing a welded steel pipe, comprising the following steps. (A) a step of preparing a steel sheet having the chemical composition of claim 1; (b) a step of forming the steel sheet into a tubular body;
Preheating to a temperature in the range of ˜1300 ° C., (d) melting and upsetting and welding the preheated edge portion of the tubular body by laser beam irradiation, and (e) the welding portion. , Ac ₃ + 20 ° C. to 1000 ° C., and (f) tempering at a temperature in the range of 100 ° C. to AC ₁ (excluding 300 to 400 ° C.).