JP2002167649A - Electric resistance welded tube and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technology capable of manufacturing an electric resistance welded tube having excellent welding quality, grooving corrosion resistance and workability of a tube joint with high productivity by the hot or warm finish. SOLUTION: The electric resistance welded tube 8 is manufactured by heating a steel strip 2 by a steel strip heating furnace 3, and performing the forming and electric resistance welding by a forming and induction heating welding equipment 4 to manufacture a steel tube. After the steel tube is re-heated continuously by a tube re-heating device 5, the steel tube is reduced by a reduction mill 6, and further cut to a predetermined tube length by a tube cutter 7 to manufacture the electric resistance welded tube 8. If the finish temperature of the reduction is set to be >=Ar3 point, and, in particular, if C content is <=0.2% or >=0.001% to <=0.2%, the finish temperature of the reduction is set to be >=(Ar3 transformation point -50 deg.C), and the aspect ratio of a main phase structure of the electric resistance welded tube 8 is set to be <=5.0 by setting the starting temperature of the reduction to be higher than the finish temperature by 50 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric resistance welded steel pipe and a method for producing the same, and more particularly, to an electric resistance welded steel pipe having an excellent resistance to groove-like corrosion in an electric resistance welded part, which is subjected to warm finishing or hot working. The present invention relates to a technology that can be provided at a low cost by finishing.

[0002]

2. Description of the Related Art As is well known, forged pipes and ERW steel pipes are generally used as steel pipes for general piping. When this electric resistance welded steel pipe is used in a humid environment, for example, as a water supply pipe or a pipe buried underground, selective corrosion (“groove pitting” or “groove pitting”) in which the corrosion rate of the electric resistance welded portion becomes larger than that of the base metal portion "Corrosion") may occur.

[0003] If the groove-like corrosion is severely generated, leakage from the ERW pipe occurs, or if the corrosion spreads to the base material, the service life of the ERW pipe is greatly reduced from the design life. For this reason, conventionally, for applications in such a wet environment, forged steel pipes (solid-phase pressure welded steel pipes) have been used because of their excellent groove corrosion resistance.

[0004] However, it is difficult to completely remove the surface scale at the butted end faces in the solid-state welding at a high temperature in the forged steel pipe. For this reason, welding defects such as bites of the scale in the joint are generated, and the joint strength of the joint is likely to be inferior to that of the base material. For this reason, the welded steel pipe has a problem in that the reliability of the pipe joint processing such as bending and push-spread and the surface properties of the pipe joint portion are easily inferior to the ERW steel pipe. For this reason, an electric resistance welded steel pipe which is excellent not only in groove corrosion resistance but also in workability of pipe joint processing has been desired.

[0005] In view of the above, a number of inventions have been proposed to improve the groove-like corrosion resistance of ERW steel pipes. For example,
Japanese Patent Publication No. 60-37173 discloses an invention in which the composition of an electric resistance welded steel pipe is limited to a specific value and a local heat treatment is performed during a pipe forming process, thereby improving groove corrosion resistance.

Further, Japanese Patent Application Laid-Open No. 56-123349,
JP-A-56-98451, JP-A-53-100120 and JP-A-55-47364 disclose Ca and C.
An invention has been disclosed in which the groove corrosion resistance is improved by adding an alloy element such as u, Ti, and Cr.

Further, Japanese Patent Application Laid-Open No. 9-165648 discloses an invention in which both edges of an open pipe are melted by laser irradiation, and are set up and welded to improve the groove corrosion resistance. I have.

Further, Japanese Patent Application Laid-Open No. 11-131189 discloses that a material steel pipe having a specific composition is heated to a temperature range of (Ac ₃ transformation point to 400 ° C.), and this (Ac ₃ transformation point to 400 ° C.)
An invention has been proposed in which an ERW steel pipe having fine ferrite grains and excellent toughness and ductility is produced by performing rolling in a rolling temperature range of 400 ° C.) at a cumulative diameter reduction ratio of 20% or more.

[0009]

However, the annealing time for the normal heat treatment in the line is 1 to 3 due to process restrictions.
Only about 2 minutes can be secured. For this reason,
In the invention disclosed in Japanese Patent No. 37173, it is difficult to homogenize the structure near the welded portion, and it is not possible to suppress the occurrence of groove corrosion.

According to the study of the present inventors, JP-A-56-123349, JP-A-56-98451, JP-A-53-100120, and JP-A-55-473.
In the invention proposed in Japanese Patent Application Laid-open No. 64 and Japanese Patent Application Laid-Open No. 9-165648, the corrosion resistance of the obtained electric resistance welded steel pipe is insufficient for the current target, and the composition and the like are carefully adjusted to somehow. Even if the desired corrosion resistance can be ensured, the production cost increases, and it has been difficult to realize this in terms of economy.

Further, in the invention disclosed in Japanese Patent Application Laid-Open No. H11-131189, an electric resistance welded portion heated to a temperature above the Ac ₃ transformation point during the production of a raw steel pipe, and a drawing process is performed after the temperature rise to a temperature below the Ac ₃ transformation point. Since there is a difference in the crystal grain size between the received portion and the received portion, the groove-like corrosion resistance of the ERW welded portion is insufficient. For this reason, providing the electric resistance welded steel pipe provided by the present invention for use in a humid environment was a problem in terms of groove-like corrosion resistance.

The present invention has been made in view of the above-mentioned problems of the prior art, and has been made in consideration of the above-mentioned problems, and is an electric resistance welded steel pipe having excellent welding quality, groove corrosion resistance, and workability of a pipe joint. Is to provide a technology that can be manufactured with high productivity by hot finishing or warm finishing.

[0013]

The inventors of the present invention have conducted various studies to solve the above-mentioned problems. As a result, (1) the influence of the composition of the material and the structure on the selective corrosion of the ERW weld is described in detail. As a result of the study, continuous heating, forming,
Electric resistance welding, were re-heating and reducing rolling, upon the production of more electric resistance welded steel pipe for hot finishing or warm finish, depending on the composition of the material, the heating temperature of the steel strip and Ac ₃ transformation point or higher, further By making the end temperature of the reduction rolling equal to or higher than the Ar ₃ transformation point or (Ar ₃ transformation point −50 ° C.) and increasing the start temperature of the reduction rolling by 50 ° C. or more than the end temperature, the crystal grain of the ERW steel pipe is obtained. A uniform structure having an aspect ratio of the diameter of 5.0 or less can be obtained, whereby the difference in structure between the base material portion and the ERW portion can be eliminated. Corrosion can be remarkably improved. (2) Depending on the composition of the material, the heating temperature of the material is similarly set to Ac ₃ for an ERW steel pipe made of alloy steel for a special use such as a corrosive fluid. Above the transformation point, and then rolling The end temperature with the Ar ₃ transformation point or more or (Ar ₃ transformation point -50 ° C.) or higher,
By setting the starting temperature of the reduction rolling at 50 ° C. or more higher than the end temperature, the groove corrosion resistance of the electric resistance welded portion can be improved, and (3) these items (1) and (2) according to the composition of the material. Unlike the item (2), even if the heating temperature of the steel strip is lower than the Ac ₃ transformation point, the steel pipe before the drawing rolling is heated to the Ac ₃ transformation point or higher by the reheating performed after the electric resistance welding, Furthermore, by making the end temperature of the reduction rolling equal to or higher than the Ar ₃ transformation point or (Ar ₃ transformation point −50 ° C.) or higher, and increasing the start temperature of the reduction rolling by 50 ° C. or more than the end temperature, the electric resistance is reduced. A new and important finding was obtained that the groove corrosion resistance of the welded portion could be improved.

The present inventors have further studied based on these novel findings (1) to (3), and as a result,
The present invention has been completed. The present invention is the electric resistance welded steel pipe characterized in that the aspect ratio of the crystal grains of the main phase structure is 5.0 or less.

The electric resistance welded steel pipe according to the present invention is, for example,
C: 0.001 to 0.5% (hereinafter, "%" means "% by mass" unless otherwise specified in the specification), Si: 0.01 to 3.0%, Mn : 0.01 ~
3.0%, Al: 0.01 to 2.00%, P: 0.1%
Hereinafter, N: 0.01% or less, S: 0.03% or less, and further, if necessary, Cu: 2.0% or less, Ni: 2.0%
Below, Cr: 3.0% or less, Mo: 2.0% or less, N
b: 1.0% or less, V: 1.0% or less, Ti: 1.0%
Or less, Ca: 0.01% or less, and B: 0.010%
It has a steel composition containing one or more of the following, with a total of 0.005% of Cu, Ni, Cr and Mo
It is exemplified that Nb, V, and Ti are 0.005% or more and 3.0% or less in total. The electric resistance welded steel pipe according to the present invention is exemplified by being subjected to warm finishing or hot finishing.

From another viewpoint, the present invention provides a method for continuously forming a heated steel strip by, for example, a forming roll into an open pipe, and locally heating an edge portion of the open pipe by an induction heating device. After performing ERW welding to form a steel pipe, the reheating is further performed to make the circumferential temperature distribution of the steel pipe substantially uniform, and then, the ERW steel pipe is manufactured by performing the reduction rolling. A method for producing an electric resistance welded steel pipe, characterized in that the end temperature is equal to or higher than the Ar ₃ transformation point of the steel pipe and the start temperature of the reduction rolling is 50 ° C. or more higher than the end temperature of the reduction rolling.

In this case, the steel strip has a C content of 0.001-0.
5%, Si: 0.01 to 3.0%, Mn: 0.01 to
3.0%, Al: 0.01 to 2.00%, P: 0.1%
Hereinafter, having a steel composition containing N: 0.01% or less, S: 0.03% or less, or C: 0.001 to
0.5%, Si: 0.01 to 3.0%, Mn: 0.01
To 3.0%, Al: 0.01 to 2.00%, P: 0.1
%, N: 0.01% or less, S: 0.03% or less, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 3.0% or less, Mo: 2. 0% or less, Nb:
1.0% or less, V: 1.0% or less, Ti: 1.0% or less, Ca: 0.01% or less, and B: 0.010% or less. , Ni, C
It is exemplified that r and Mo have a steel composition of 0.005% or more and 5.0% or less in total, and Nb, V and Ti have a steel composition of 0.005% or more and 3.0% or less in total.

In the present invention, C: 0.2% or less, S
i: 0.01 to 3.0%, Mn: 0.01 to 3.0%,
Al: 0.01 to 2.00%, P: 0.1% or less, N:
Steel composition containing 0.01% or less, S: 0.03% or less, or C: 0.2% or less, Si: 0.01-3.
0%, Mn: 0.01 to 3.0%, Al: 0.01 to
2.00%, P: 0.1% or less, N: 0.01% or less,
S: 0.03% or less and Cu: 2.0%
Ni: 2.0% or less, Cr: 3.0% or less, M
o: 2.0% or less, Nb: 1.0% or less, V: 1.0%
Hereinafter, one or more of Ti: 1.0% or less, Ca: 0.01% or less, and B: 0.010% or less are contained, and Cu, Ni, Cr and Mo are 0.00 in total.
A heated steel strip having a steel composition of 5% or more and 5.0% or less and a total of Nb, V and Ti of 0.005% or more and 3.0% or less is formed into an open pipe, and the open pipe is formed. By performing ERW welding to form a steel pipe by locally heating the edge of the pipe with an induction heating device, and further performing reheating to make the circumferential temperature distribution of the steel pipe substantially uniform, and then performing drawing rolling. When producing the ERW pipe, the end temperature of the reduction rolling is set to the (Ar ₃ transformation point −50) of the steel pipe.
C.) or more, and a start temperature of the reduction rolling is set to be 50 ° C. or more higher than an end temperature of the reduction rolling. In this case, C
It is exemplified that the content is 0.001% or more.

In the method for manufacturing an electric resistance welded steel pipe according to the present invention, it is exemplified that the electric resistance welded steel pipe has an aspect ratio of crystal grains having a main phase structure of 5.0 or less. In the method for producing an electric resistance welded steel pipe according to the present invention, it is exemplified that the steel strip is heated to the Ac ₃ transformation point or more, or the steel pipe to be subjected to the reduction rolling is heated to the Ac ₃ transformation point or more. You.

Further, in the method for manufacturing an electric resistance welded steel pipe according to the present invention, after the electric resistance welding is performed and before reheating, the molten beads on the inner and outer surfaces of the steel pipe are removed. Is done.

[0021]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of an electric resistance welded steel pipe according to the present invention.

FIG. 1 is an explanatory view schematically showing one example of a manufacturing process 9 of the electric resistance welded steel pipe 8 in the present embodiment. In the present embodiment, the ERW steel pipe 8 is manufactured by hot finishing or warm finishing by continuously performing heating, forming, ERW welding, reheating, drawing rolling, cutting, and the like on the steel strip 2. You. Hereinafter, the manufacturing process 9 of the present embodiment will be described with reference to FIG.
Will be described.

(Heating Step) The steel strip 2 is a pipe forming material for the ERW pipe 8, and its composition may be appropriately set according to the performance required of the ERW pipe 8 as a product. It is not limited to a particular composition. That is, the composition of the steel strip 2 is equally applicable as long as it is a well-known and commonly used composition for this type of ERW pipe. In the present embodiment, as an example, C:
0.001-0.5%, Si: 0.01-3.0%, M
n: 0.01 to 3.0%, Al: 0.01 to 2.00
%, P: 0.1% or less, N: 0.01% or less, S: 0.
03% or less, and if necessary, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 3.0% or less, Mo:
2.0% or less, Nb: 1.0% or less, V: 1.0% or less, Ti: 1.0% or less, Ca: 0.01% or less, and B: 0.010% or less It has a steel composition containing two or more types, Cu, Ni, Cr and Mo are 0.005% or more and 5.0% or less in total, and Nb, V and Ti are 0.005% or more in total. 0% or less. Hereinafter, the reasons for limiting the composition of the steel strip 2 in the present embodiment will be described.

C: 0.001 to 0.5% When C is contained in an amount of 0.001% or more, the strength of the steel is increased. However, when the content exceeds 0.5%, the strength is excessively increased. As a result, the ductility and hot workability are reduced, and defects are liable to be generated in the welded joint, so that the welding condition is not stable, and the groove-like corrosion resistance of the ERW weld is deteriorated. Therefore, in the present embodiment, the C content is 0.001% or more and 0.1% or more.
5% or less, preferably 0.01% to 0.3%
Or less, more preferably 0.03% or more and 0.3% or less.

Further, when the C content is 0.2% or less, preferably 0.001% or more and 0.2% or less, the end temperature of the reduction rolling described later is set to the (Ar ₃ transformation point −50 ° C.)
This is preferable because the above range can be expanded and the appropriate range of the rolling temperature can be expanded.

Si: 0.01 to 3.0% Si acts as a deoxidizing element when contained at 0.01% or more, but when the content exceeds 3.0%, ductility is deteriorated. Causes adverse effects. Therefore, in the present embodiment, the Si content is limited to 0.01% or more and 3.0% or less. Preferably it is 0.01% or more and 2.0% or less, more preferably 0.10% or more and 0.3% or less.

Mn: 0.01 to 3.0% Mn also increases the strength of steel by containing more than 0.01%, but when the content exceeds 3.0%, the weldability and In addition to deteriorating ductility, the periphery of MnS, which is a non-metallic inclusion, is easily dissolved, so that MnS becomes a starting point of groove-like corrosion, and the groove-like corrosion resistance deteriorates. Therefore,
In the present embodiment, the Mn content is 0.01% or more and 3.0%.
Limited to the following. From the viewpoint of harmonizing strength and elongation, the lower limit of the Mn content is preferably 0.05%, and the upper limit is preferably 2.0%, more preferably 0.20%. Not less than 1.6%.

Al: 0.01 to 2.00% Al also acts as a deoxidizing element when it is contained at 0.01% or more, but when the Al content exceeds 2.00%, the amount of inclusions increases. As a result, the cleanliness of the steel is lowered and the corrosion resistance is lowered. Therefore, in the present embodiment, Al
The content is limited to 0.01% or more and 2.00% or less. Preferably it is 0.015% or more and 1.0% or less, more preferably 0.015% or more and 0.03% or less.

P: 0.1% or less P is an unavoidable impurity, and segregates at crystal grain boundaries to deteriorate both toughness and groove corrosion resistance. Therefore, it is desirable that the content of P is small. However, since an extreme reduction of P involves a corresponding increase in cost, the upper limit of the P content is set to 0.1% in this embodiment. It is preferably at most 0.04%, more preferably at most 0.02%.

S: 0.03% or less S is an unavoidable impurity, and forms a sulfide, thereby deteriorating both the cleanliness and the groove corrosion resistance of steel. Therefore, it is desirable that the content of S is small. . However, since an extreme reduction in S involves a corresponding increase in cost, the upper limit of the S content is preferably set to 0.03%, more preferably 0.015% or less in the present embodiment.

N: 0.01% or less N is a strengthening element of steel and an inevitable impurity. Although the amount normally contained as an impurity is about 0.005%, the content up to 0.01% is acceptable without any particular problem. Therefore, in the present embodiment, the N content is preferably limited to 0.01% or less, and more preferably 0.005% or less.

These elements are the basic components of the electric resistance welded steel pipe according to the present embodiment. By further adding at least one of the following elements to the basic components as an optional additive element, more excellent properties can be obtained. An electric resistance welded steel tube having both groove-like corrosion resistance and other properties can be obtained. Therefore, these optional elements will be described below.

Cu: not more than 2.0% Cu is an element that improves the corrosion resistance of the base material and improves the groove corrosion resistance of the electric resistance welded portion, but the Cu content exceeds 2.0%. And the hot workability is significantly impaired. Therefore, when Cu is added, its content is preferably limited to 2.0% or less. More preferably 1.0
% Or less.

Ni: not more than 2.0% Ni, like Cu, is also an element that improves the grooving corrosion resistance of the electric resistance welded portion. The effect of improving the corrosiveness is saturated, and only the cost is increased. Therefore, when adding Ni, it is desirable to limit the content to 2.0% or less.

Cr: not more than 3.0% Cr is an element that improves the strength and also improves the corrosion resistance of both the base material and the electric resistance welded portion. Further, the effect of improving the corrosion resistance of the electric resistance welded portion is saturated, and only the cost is increased. Therefore, when Cr is added, its content is preferably limited to 3.0% or less.

Mo: 2.0% or less, Nb, V: 1.0%
Hereinafter, Mo, Nb, and V are elements that improve both the strength and the groove corrosion resistance of the electric resistance welded joint, and the contents of these elements are 2.0% and 1.0%, respectively. If it exceeds 1.0%, the effect of improving the groove-like corrosion resistance is saturated, and the cost only increases. Therefore, when Mo, Nb, and V are added, the content is Mo: 2.0% or less, and Nb, V:
In any case, it is desirable to limit it to 1.0% or less.

Ti: 1.0% or less Ti is a deoxidizing element and an element that contributes to both refinement of crystal grains and enhancement of strength by precipitating as nitride or carbonitride. However, when the Ti content exceeds 1.0%, adverse effects such as deterioration of toughness occur. Therefore, when adding Ti, it is desirable to limit the content to 1.0% or less.

Ca: 0.01% or less Ca is an element having an effect of controlling the form of inclusions to improve workability and improving the resistance of the ERW weld to groove corrosion. However, when the Ca content is 0.0
If it exceeds 1%, the cleanliness of the steel decreases, and the ductility deteriorates.
Therefore, when adding Ca, the content is 0.0
It is desirable to limit it to 1% or less.

B: 0.010% or less B is an element contributing to both refinement of crystal grains and enhancement of strength. However, when the B content exceeds 0.010%, both weldability and toughness deteriorate. Therefore, when B is added, its content is desirably limited to 0.010% or less. Cu, Ni, Cr and Mo: 0.005% or more in total
5.0% or less Nb, V and Ti: 0.005% or more and 3.0% in total
In other words, since Cu, Ni, Cr and Mo have an effect of improving hardenability, it is easy to control the crystal grain size and area ratio of ferrite and the remaining phase in the cooling process. In addition to enhancing the hardenability, Cu also has the effect of increasing corrosion resistance. For this reason, C
One or more of u, Ni, Cr and Mo may be contained, but if their total content is less than 0.005%, the quenching effect is difficult to obtain, while their content is 5.
If it exceeds 0%, the above effect is saturated and, at the same time, ductility decreases. Therefore, when one or more of Cu, Ni, Cr and Mo are added, their contents are in total of 0.1.
It is desirable to limit it to 005% or more and 5.0% or less.
From the same viewpoint, the lower limit of these contents is 0.1% in total.
% Is more preferable. Further, the upper limit is more preferably 1.0%, and further preferably 0.5%.

On the other hand, Nb, V and Ti have the effect of fixing solid solution C, solid solution N and solid solution S contained in steel as precipitates to render them harmless. It has the effect of increasing the strength of steel without loss. Therefore, in order to enhance the deep drawability of steel, Nb, V and T
One or more kinds of i may be contained, but the content is 0.0
If the content is less than 05%, it is difficult to obtain such effects. On the other hand, if the content of these components exceeds 3.0% in total, the above effects are saturated and, conversely, the ductility and the deep drawability decrease. Then, Nb,
When one or more of V and Ti are added, their contents are preferably limited to a total of 0.005% or more and 3.0% or less. From the same viewpoint, the lower limits of these contents are more preferably 0.01% in total,
More preferably, it is 0.02%. On the other hand, the upper limit of these elements is more preferably 0.5%.
More preferably, it is 2%.

The composition other than the above of the steel strip 2 is Fe and substantially has a steel composition. In FIG. 1, prior to forming the strip 2, the strip 2 discharged from the uncoiler 1 is charged into a strip heating furnace 3 to heat the strip 2.

The heating of the steel strip 2 minimizes the temperature difference that inevitably occurs between the welding edge and the base material in the vicinity during the electric resistance welding performed later by the forming and induction heating welding device 4. This is done to homogenize the tissue by suppressing and preventing the occurrence of heat affected hardened tissue.

The heating of the steel strip 2 may be performed using the steel strip heating furnace 3 as in this embodiment, but is not limited thereto. For example, heating using an induction coil, Heating using resistance heating by energization may be used.

The heating temperature of the strip 2 by the strip heating furnace 3 is 4
If the temperature is lower than 00 ° C., the deformation resistance of the steel strip 2 at the time of forming becomes large, and roll scratches due to biting out from the corners of the forming roll easily occur on the surface of the steel pipe, which is not desirable.

If the heating temperature of the steel strip 2 is lower than the Ac ₃ transformation point, the structure of the steel strip 2 after heating does not become uniform austenite grains, and local phase transformation occurs during the subsequent drawing rolling. Are generated, and the grain size of the final product becomes non-uniform. Therefore, particularly in the heat-affected zone in the vicinity of the electric resistance welded portion, a remarkable unevenness in the structure grain size is likely to occur. In this case, the non-uniform structure can be prevented by lowering the welding speed, lengthening the time of the reheating treatment performed later, and further raising the temperature of the mother pipe before the reduction rolling to the Ac ₃ transformation point or more. Although you can,
Productivity decreases.

Therefore, in this embodiment, the steel strip 2 is 400
It is desirable to heat to not less than ° C., and it is more desirable to heat to above the Ac ₃ transformation point in order to suppress a decrease in productivity.

On the other hand, if the heating temperature of the steel strip 2 exceeds 1300 ° C., the austenite grains become coarse, and even if the steel sheet is subjected to a strong working by drawing rolling, the crystal grains of the final product become coarse, and sufficient strength can be obtained. At the same time, the scale of the surface of the steel strip 2 is likely to be generated, which causes deterioration of the surface properties of the steel pipe after the rolling.

Therefore, in this embodiment, the heating of the steel strip 2 is preferably performed in a temperature range of 400 ° C. or more and 1300 ° C. or less, and more preferably in a temperature range of the Ac ₃ transformation point or more and 1300 ° C. or less.

(Forming Step) In FIG.
The steel strip 2 heated to the above-mentioned temperature range is continuously formed into an open pipe by a plurality of forming rolls (not shown) provided in the forming and induction heating welding device 4.

Since these forming rolls are well-known and commonly used, their arrangement and roll schedule may be appropriately changed or remodeled to a normal device such as an electric resistance welded steel pipe for hot use. Is omitted.

(Electrically Welding Step) This open pipe is locally heated to the melting point at both edges thereof by an induction heating coil (not shown) provided in the forming and induction heating welding apparatus 4.

In the induction heating of both edge portions, the entire cross section of the open pipe is not more than the steel strip heating temperature and (Curie point −50).
C) or more. If the temperature of the steel strip during ERW is less than (Curie point −50 ° C.), the efficiency of induction heating for ERW is greatly reduced, and it is necessary to reduce the welding speed, which hinders productivity. This is because there is a risk of doing so.

As these induction heating coils may be used well-known common ones, the description of the induction heating coil is omitted. The open pipe whose both edge portions are locally heated to the melting point is subjected to abutment welding by a squeeze roll (not shown) provided in the forming and induction heating welding device 4 to be a mother pipe for the subsequent reduction rolling. It is a steel pipe.

Further, molten beads discharged at the time of the impact welding are generated on the inner and outer surfaces of the steel pipe which has been impact welded by the squeeze roll. For this reason, it is desirable to remove the generated molten beads at this stage.

The method for removing the molten bead is not by a specific means. For example, use of a well-known and commonly used cutting and removing method is exemplified. This cutting and removing method is a fixed cutting bit method used for ordinary ERW steel pipes or a rotating bit method to prevent hot bite wear, and furthermore, the cutting bit pressing force can be adjusted with the built-in hydraulic cylinder. Thus, a known and commonly used cutting and removing method such as a method capable of automatically responding to a change in wall thickness of the ERW steel pipe can be used.

(Reheating Step, Draw Rolling Step) The steel pipe from which the molten beads generated on the inner and outer surfaces have been removed is made uniform in the temperature difference between the welded portion and the surrounding base material, and is subjected to rolling in the subsequent drawing rolling. For the purpose of adjusting the temperature, the tube is reheated by heating or soaking by the tube reheating device 5.

The reheating method is not particularly limited as long as it is a heating means capable of soaking the pipe. For example, a gas-fired continuous heating furnace, an induction heating device for pipes, or an electric heating method, or a combination thereof can be used.

The mother tube whose temperature distribution in the circumferential direction of the tube has been substantially soaked by the tube reheating device 5 is drawn and reduced to a predetermined outer diameter by a drawing rolling machine 6 to obtain a final product. In the present embodiment, the rolling temperature of the reducing rolling by the reducing mill 6 is such that the rolling end temperature is a temperature not lower than the Ar ₃ transformation point, but the C content is 0.2% or less or 0.001% or more and 0.2% or less. %, The rolling end temperature is (Ar ₃
(Transformation point −50 ° C.) or more, and the start temperature of the reduction rolling is set to be 50 ° C. or more higher than the end temperature. As a result, the rolling process can be completed in the austenite region, and the entire cross section of the steel pipe including the welded portion can have a substantially uniform structure.

That is, when the rolling end temperature is lower than the Ar ₃ transformation point, the C content is 0.2% or less or 0.001%.
If the rolling end temperature is lower than (Ar ₃ transformation point −50 ° C.) even if the rolling end temperature is not less than 0.2% or less, the above-mentioned Japanese Patent Application Laid-Open No.
As in the invention disclosed in Japanese Patent No. 131189, the transformation from austenite occurs during the reduction rolling, so that the grain size is changed between the structure transformed after being worked in the austenite region and the structure worked after the transformation. A difference in diameter occurs. For this reason, the structure of the final product becomes a mixed-grain structure of coarse particles and fine particles, and in a humid environment or a corrosive environment as an underground pipe or the like, a local battery is generated due to this structure difference,
The groove corrosion resistance deteriorates. Particularly, the electric resistance welded portion is once heated to a melting temperature (1400 to 1500 ° C.) during welding, so that austenite grains around the welded portion are coarsened. For this reason, if this portion passes through the transformation point without being subjected to rolling, it becomes coarse crystal grains, and the difference in structure from the base material portion is further increased. Therefore, in this embodiment,
The ending temperature of the reduction rolling is equal to or higher than the Ar ₃ transformation point, particularly when the C content is 0.2% or less or 0.001% or more and 0.2% or less, the ending temperature of the reduction rolling is (Ar ₃ transformation point− 5
0 ° C) or more.

On the other hand, if the end temperature of the drawing rolling exceeds 1000 ° C., the effect of grain refinement by drawing is reduced, and the crystal grains of the final product become coarse and sufficient strength cannot be obtained. Is desirably 1000 ° C. or less.

On the other hand, if the start temperature of the drawing rolling is not higher than the end temperature by 50 ° C. or more, the end temperature is not less than the Ar ₃ transformation point, or the C content is 0.2% or less or 0.001% or less. % Or more and 0.2% or less, and even when the end temperature of the reduction rolling is (Ar ₃ transformation point −50 ° C.) or more, the welded portion where the austenite grains are once coarsened by the electric resistance welding and its surroundings Since the difference in structure from the base metal part is not completely eliminated, a difference occurs in the crystal grain size, and the structure of the final product is
Since it has a mixed grain structure of coarse grains and fine grains, the groove-like corrosion resistance is deteriorated. Therefore, in the present embodiment, the end temperature of the reduction rolling is equal to or higher than the Ar ₃ transformation point, and particularly when the C content is 0.2% or less or 0.001% or more and 0.2% or less, (Ar ₃ transformation point− 50 ° C.) or higher, and the start temperature of the reduction rolling is set to be 50 ° C. or higher than the end temperature.

The upper limit of the temperature difference between the starting temperature and the ending temperature of the reduction rolling does not need to be particularly defined, but the upper limit of the starting temperature is desirably 1300 ° C. or less. This is because the austenite grains are coarsened when the starting temperature exceeds 1300 ° C., and the crystal grains of the final product are coarsened even if the deep working is performed by drawing, as in the heating of the steel strip, and the sufficient strength cannot be obtained. This is because the scale of the surface of the steel pipe is likely to be generated, and the normality of the surface of the steel pipe after the reduction rolling may be deteriorated.

Further, the heating temperature of the steel strip before the electric resistance welding was 4
If it is less than 00 ° C. or higher Ac ₃ transformation point, in the reheating step of the steel pipe, it is necessary to set the heating temperature and the Ac ₃ transformation point or more.

The working ratio k in the reduction rolling is, for example, 20% or more and 90% or less from the viewpoint of crystal grain refinement by introducing working strain. The working ratio k is given by K = ｛1−1, where D _{1 is} the outer diameter of the steel pipe before rolling, T ₁ is the wall thickness, D _{2 is} the outer diameter of the final product, and t ₂ is the wall thickness.
t ₂ (D ₂ −t ₂ ) / t ₁ (D ₁ −t ₁ )｝ × 100
(%).

Unlike the present embodiment, even when the steel pipe or the strip 2 is made of an alloy steel for a special use such as a corrosive fluid, the end temperature of the reduction rolling is set to Ar _3.
Above the transformation point, especially when the C content is 0.2% or less or 0.0%
In the case of not less than 01% and not more than 0.2%, (Ar ₃ transformation point −5)
0 ° C.) or higher, and in addition to raising the starting temperature of the drawing rolling by 50 ° C. or more than its end temperature, and by setting the heating temperature of the steel strip or the steel pipe before the drawing rolling to the Ac ₃ transformation point or more, The groove-like corrosion resistance of the ERW weld can be significantly improved.

(Cutting Step) The steel pipe which has been subjected to the reduction rolling by the reduction mill 6 is cut into a predetermined length by a pipe cutting device 7 to form an ERW steel pipe 8.

The ERW steel pipe 8 thus manufactured is
Thereafter, it is conveyed to a cooling bed (not shown) and left to cool until the transformation is completely completed. Then, water cooling may be performed to a temperature (room temperature) suitable for handling.

(Electrically Welded Steel Pipe 8 Obtained) In this embodiment,
Thus, the electric resistance welded steel pipe 8 is manufactured. The electric resistance welded steel pipe 8 according to the present invention has, for example, C: 0.001-0.
5%, Si: 0.01 to 3.0%, Mn: 0.01 to
3.0%, Al: 0.01 to 0.20%, P: 0.1%
Hereinafter, N: 0.01% or less, S: 0.03% or less, and further, if necessary, Cu: 2.0% or less, Ni: 2.0%
Below, Cr: 3.0% or less, Mo: 2.0% or less, N
b: 1.0% or less, V: 1.0% or less, Ti: 1.0%
Or less, Ca: 0.01% or less, and B: 0.005%
It has a steel composition containing one or more of the following, Cu, Ni, Cr and Mo are 5.0% or less in total, and Nb, V and Ti are 3.0% or less in total And manufactured by performing warm finishing or hot finishing.

The electric resistance welded steel pipe 8 according to the present invention has, for example, C: 0.001% or more and 0.2% or less, Si: 0.0% or less.
1-3.0%, Mn: 0.01-3.0%, Al: 0.
01-2.00%, P: 0.1% or less, N: 0.01%
Hereinafter, while containing S: 0.03% or less, Cu: 2.0% or less, Ni: 2.0% or less, C
r: 3.0% or less, Mo: 2.0% or less, Nb: 1.0
%, V: 1.0% or less, Ti: 1.0% or less, C
a: 0.01% or less and B: one or more of 0.010% or less, wherein Cu, Ni, Cr and Mo are 0.005% or more and 5.0% or less in total;
Nb, V and Ti are 0.005% or more and 3.0% in total
The following is manufactured by performing warm finishing or hot finishing.

Further, in the electric resistance welded steel pipe 8, the aspect ratio of the crystal grains of the main phase structure is 5.0 or less. In the present invention, the “main phase structure” means a structure having the largest area ratio, and is not limited to a structure having an area ratio of 50% or more. For example, the area ratio of the first phase structure is 45%, the area ratio of the second phase structure is 30%, and the area ratio of the third phase structure is 25%.
In this case, the area ratio of the first phase structure is less than 50%. In this case, the first phase structure corresponds to the structure having the largest area ratio. Corresponds to.

The main phase structure of the electric resistance welded steel pipe 8 of this embodiment is ferrite or bainite, and its area ratio is determined by, for example, an optical microscope or a scanning electron microscope (SE).
M), several fields of view of the tissue (for example, 5 fields of view) were confirmed, and a tissue photograph in which the main phase tissue was black and the second phase tissue was white was analyzed using an image analyzer, and these 5 photographs were analyzed.
What is necessary is just to obtain from the average value of a visual field. In the present embodiment, the area ratio of the main phase structure is about 40 to 100%.

FIG. 2 is an explanatory diagram showing the aspect ratio of crystal grains in the present embodiment. As shown in FIG. 2, the aspect ratio β of the crystal grains in the present embodiment is the maximum value of the (maximum diameter) / (minimum diameter) value of each crystal grain of the main phase structure. The “maximum diameter” of the crystal grain means the longest diameter d ₃ of the crystal grain, and the “minimum diameter” of the crystal grain means the shortest diameter d ₄ of the crystal grain.
Then, for example, the tissue is photographed in five visual fields using an optical microscope (manufacturer: Nikon, product number: OPTI PHOTO) or a scanning electron microscope (SEM, manufacturer: JSM, product number 6300).
This method is based on the cutting method of the ferrite grain size test method for steel specified in SG0552, and is based on the number of main phase structure grains cut by two orthogonal segments having a fixed length in a microscope visual field. By the calculated linear cutting method, the maximum value of the diameter of the main phase structure crystal grains, the "maximum diameter" indicating the minimum value, "minimum diameter" was determined, and these determined "maximum diameter", "minimum diameter"
Are obtained by multiplying each by 1.13 times to obtain the “maximum diameter d ₃ ” and the “minimum diameter d ₄ ” of the crystal grains. In the present embodiment, the aspect ratio β in the present invention needs to be 5.0 or less in an arbitrary measurement cross section.
The aspect ratio β ₁ in the longitudinal section of the steel pipe, and the aspect ratio β _{2 in the} section perpendicular to the longitudinal direction,
I decided to ask.

If the aspect ratio β, that is, at least one of the aspect ratios β ₁ and β ₂ in this embodiment is larger than 5.0, a non-uniform mixed grain structure results, so that the aspect ratios β ₁ and β ₂ At least one of which is 5.0
ERW steel pipe 8 larger than
When used in a humid environment or a corrosive environment, a local battery is generated due to the difference in structure, and groove-like corrosion is apt to progress. In contrast, the ERW steel pipe 8 of the present embodiment has an aspect ratio β
_Since both ₁ and β ₂ are 5.0 or less, the structure becomes uniform, so that a local battery is hardly generated, and the groove-like corrosion property is remarkably improved.

As described above, the electric resistance welded steel pipe 8 of the present embodiment is
Since the aspect ratio is 5.0 or less, the difference in the structure between the base material portion and the ERW weld of the ERW steel pipe 8 can be eliminated, and the groove-like corrosion resistance of the ERW weld is remarkably improved. improves.

Further, the electric resistance welded steel pipe 8 of the present embodiment is superior in the reliability of the pipe joint processing such as bending and push-out and the surface properties of the pipe joint to the conventionally used forged steel pipe. It is excellent not only in groove corrosion resistance but also in workability of pipe joint processing.

Therefore, according to the present embodiment, an ERW steel pipe having both excellent welding quality, groove corrosion resistance, and workability of a pipe joint can be formed by hot finishing or warm finishing.
It can be manufactured with high productivity.

[0077]

EXAMPLES The present invention will be described more specifically with reference to examples. Using the manufacturing process 9 shown in FIG. 1 and using 31 kinds of strip steels 2 having the steel compositions shown in Table 1 as raw materials, their heating temperature and drawing rolling temperature (drawing rolling start temperature and drawing rolling end temperature) are shown in Table 2. As shown, 31 kinds of ERW steel pipes 8 were manufactured.

That is, a width of 365 mm and a thickness of 3.7 mm
The steel strip 2 is heated by a steel strip heating furnace 3 and formed and subjected to electric resistance welding by a forming and induction heating welding device 4 to obtain a steel pipe having an outer diameter of 114.3 mm. After continuous re-heating, a three-roll type stretch reducer (reducing rolling mill 6) performs reducing rolling, and further cuts the pipe to a predetermined length by a pipe cutting device 7 to obtain an outer diameter of 48.6 mm and a wall thickness. 3.31m
m electric resistance welded steel pipe 8.

[0079]

[Table 1]

Then, the groove corrosion resistance and the aspect ratio of these 31 types of ERW steel pipes were investigated. The groove corrosion resistance was investigated by performing a neutral salt spray test on the welded portion of the steel pipe. In the neutral salt spray test, 100 kinds of welded parts cut out from each of 31 kinds of ERW steel pipes were used.
Test solution: 5 wt% saline, PH: 7.0, supply air pressure: 2.
3 kg / cm, spray amount: 1.8 ml / h, temperature: 35 ° C,
Time: Performed under each test condition for 30 days.

FIG. 3 is a cross-sectional view schematically showing an example of the state of corrosion thinning of the sample before and after the test, FIG. 3 (a) showing the state before the test, and FIG. 3 (b) showing the state after the test. Shown respectively. In FIG. 3, the groove-like corrosion resistance was evaluated by an index A value represented by the following equation.

A = d ₁ / d ₂ ... In this equation, d ₁ is the corrosion depth (m
m), and d ₂ means the corrosion depth (mm) of the base material. “A = 1.0” in the equation indicates that no groove corrosion has occurred. According to this evaluation method, the greater the value of A is closer to 1, the more excellent the groove corrosion resistance is.

On the other hand, the aspect ratio of the electric resistance welded steel pipe 8 is shown in FIG.
As described above with reference to, the optical microscope (manufacturer
ー: Nikon, product number: OPTI PHOTO)
Using the average value of 5 fields of view 100 to 200 times,
The index β shown by the equation₁, Β _TwoThe value was evaluated. This
Where β₁Is the aspect ratio in the longitudinal direction of the steel pipe,
β_TwoIndicates the aspect ratio in a cross section perpendicular to the longitudinal direction.

Β ₁ = d ₃ / d ₄ β ₂ = d ₃ / d ₄ ... In this equation, d ₃ is the length (μm) of the crystal grains in the cross section of the ERW pipe 8 in the major axis direction. D ₄ means the length (μm) in the minor axis direction of the crystal grain in the cross section of the ERW pipe 8. According to this measurement method, the closer the values of β ₁ and β ₂ are to 1, the more uniform the structure has no anisotropy.

The test conditions and test results are summarized in Table 2. Α, β, P, and M in the column of the structure of Table 2 represent ferrite, bainite, pearlite, and martensite, respectively. Table 3 also shows the evaluation criteria in Table 2 and the index A value at that time.

[0086]

[Table 2]

[0087]

[Table 3]

From Tables 1 to 3, it can be seen that Sample No. which satisfies both the reduction rolling temperature and the reduction end temperature within the ranges specified in the present invention. No. 1 to No. 1 No. 21 shows that no groove corrosion occurred at all, indicating that the sample No. 21 has extremely excellent groove corrosion resistance.

On the other hand, the sample N in which C exceeds 0.2%
o. 22 to sample no. 24, sample no. 30, sample N
o. In No. 31, deep groove-shaped corrosion occurred because the end rolling reduction temperature was lower than the Ar ₃ transformation point.

The sample No. 25 and sample no. 2
In No. 6, although the C content was 0.2% or less, deep groove corrosion occurred because the drawing rolling end temperature was lower than (Ar ₃ transformation point −50 ° C.).

Further, the sample No. In No. 27, deep groove corrosion occurred because the start temperature of the reduction rolling was not higher than the end temperature by 50 ° C. or more. Further, the sample No. 23, sample no. 25 and sample no. In No. 26, since both the reduction rolling start temperature and the reduction rolling end temperature were lower than the Ar ₃ transformation point, very deep groove corrosion occurred.

In particular, in particular, in the case of sample no. In No. 26, since the strip steel heating temperature was lower than the Ac ₃ transformation point, and both the drawing rolling start temperature and the drawing rolling end temperature were lower than the Ar ₃ transformation point, very deep groove-like corrosion occurred.

Further, the sample No. 28 and sample no.
In No. 29, the C content was limited to 0.2% or less, so that groove-like corrosion did not occur and was good.

[0094]

As described in detail above, according to the present invention, an electric resistance welded steel pipe having both excellent welding quality, groove-like corrosion resistance and pipe joint workability can be hot-finished or warm-welded. With finishing, it can be manufactured with high productivity.

The significance of the present invention having such effects is extremely remarkable.

[Brief description of the drawings]

FIG. 1 is an explanatory view schematically showing an example of a manufacturing process of an electric resistance welded steel pipe in an embodiment.

FIG. 2 is an explanatory diagram showing an aspect ratio of a crystal grain in the embodiment.

FIG. 3 is a cross-sectional view schematically showing an example of a state of corrosion thinning of a sample before and after a test of an example is completed, and FIG.
3 shows a state before the test, and FIG. 3B shows a state after the test.

[Explanation of symbols]

 2 Strip Steel 3 Strip Steel Heating Furnace 4 Forming and Induction Heating Welding Equipment 5 Pipe Reheating Equipment 6 Drawing Rolling Machine 7 Pipe Cutting Equipment 8 ERW Steel Pipe

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/58 C22C 38/58 (72) Inventor Tsuneaki Nagamichi 4-33 Kitahama, Chuo-ku, Osaka-shi No. Sumitomo Metal Industries, Ltd. F-term (reference) 4K032 AA01 AA02 AA04 AA05 AA08 AA11 AA12 AA14 AA15 AA16 AA17 AA19 AA20 AA21 AA22 AA23 AA24 AA27 AA29 AA31 AA32 AA35 AA36 BA03 CA01 CC03

Claims (11)

[Claims] An aspect ratio of a crystal grain having a main phase structure of 5.
An electric resistance welded steel pipe characterized by being 0 or less. 2. In mass%, C: 0.001 to 0.5%,
Si: 0.01 to 3.0%, Mn: 0.01 to 3.0%
%, Al: 0.01 to 2.00%, P: 0.1% or less,
The electric resistance welded steel pipe according to claim 1, having a steel composition containing N: 0.01% or less and S: 0.03% or less. Further, in mass%, Cu: 2.0% or less, Ni: 2.0% or less, Cr: 3.0% or less, Mo:
2.0% or less, Nb: 1.0% or less, V: 1.0% or less, Ti: 1.0% or less, Ca: 0.01% or less, and B: 0.010% or less It contains two or more kinds, and Cu, Ni, Cr and Mo are 0.005 in total.
The electric resistance welded steel pipe according to claim 2, wherein the Nb, V, and Ti are 0.005% or more and 3.0% or less in total. 4. A heated steel strip is formed into an open pipe, and the edge of the open pipe is locally heated by an induction heating device to perform electric resistance welding to form a steel pipe.
Furthermore, when reheating is performed to make the circumferential temperature distribution of the steel pipe substantially uniform, and then the reduction rolling is performed to manufacture the ERW steel pipe, the end temperature of the reduction rolling is set to the Ar _{3 of the} steel pipe.
A method for producing an electric resistance welded steel pipe, wherein the temperature is not less than the transformation point, and the starting temperature of the reduction rolling is set to be 50 ° C. or more higher than the end temperature of the reduction rolling. 5. The steel strip according to claim 1, wherein C is 0.001% by mass.
-0.5%, Si: 0.01-3.0%, Mn: 0.0
1 to 3.0%, Al: 0.01 to 2.00%, P: 0.
The method for producing an electric resistance welded steel pipe according to claim 4, having a steel composition containing 1% or less, N: 0.01% or less, and S: 0.03% or less. 6. The steel strip according to claim 1, wherein C is 0.001% by mass.
-0.5%, Si: 0.01-3.0%, Mn: 0.0
1 to 3.0%, Al: 0.01 to 2.00%, P: 0.
1% or less, N: 0.01% or less, S: 0.03% or less, Cu: 2.0% or less, Ni: 2.0%
Below, Cr: 3.0% or less, Mo: 2.0% or less, N
b: 1.0% or less, V: 1.0% or less, Ti: 1.0%
Hereinafter, one or more of Ca: 0.01% or less and B: 0.010% or less are contained, and Cu, Ni,
5. The steel composition according to claim 4, wherein Cr and Mo have a steel composition of 0.005% or more and 5.0% or less in total, and Nb, V and Ti have a steel composition of 0.005% or more and 3.0% or less in total. Manufacturing method of ERW steel pipe. 7. In mass%, C: 0.2% or less, Si:
0.01-3.0%, Mn: 0.01-3.0%, A
l: 0.01 to 2.00%, P: 0.1% or less, N:
A heated steel strip having a steel composition containing 0.01% or less and S: 0.03% or less is formed into an open pipe, and the edge of the open pipe is locally heated by an induction heating device to form an electric power. After performing seam welding to form a steel pipe, when reheating is further performed to substantially uniform the circumferential temperature distribution of the steel pipe, and then drawn and rolled to produce an electric resistance welded steel pipe, the end temperature of the drawn rolling Is the (Ar ₃ transformation point −
50 ° C.) or higher, and the start temperature of the reduction rolling is set to be 50 ° C. or higher than the end temperature of the reduction rolling. 8. In mass%, C: 0.2% or less, Si:
0.01-3.0%, Mn: 0.01-3.0%, A
l: 0.01 to 2.00%, P: 0.1% or less, N:
0.01% or less, S: 0.03% or less, Cu: 2.0% or less, Ni: 2.0% or less, Cr:
3.0% or less, Mo: 2.0% or less, Nb: 1.0% or less, V: 1.0% or less, Ti: 1.0% or less, Ca:
One or more of 0.01% or less and B: 0.010% or less, containing Cu, Ni, Cr and M
o is 0.005% or more and 5.0% or less in total;
A heated steel strip having a steel composition in which b, V and Ti are 0.005% or more and 3.0% or less in total is formed into an open pipe, and the edge of the open pipe is locally heated by an induction heating device. After performing ERW welding to form a steel pipe, and then performing re-heating to make the circumferential temperature distribution of the steel pipe substantially uniform, and then performing rolling by rolling, an ERW steel pipe is manufactured. The rolling end temperature is set to the (Ar ₃ transformation point−
50 ° C.) or higher, and the start temperature of the reduction rolling is set to be 50 ° C. or higher than the end temperature of the reduction rolling. 9. The method for producing an electric resistance welded steel pipe according to claim 7, wherein the C content of the steel strip is 0.001% or more. 10. The electric resistance welded steel pipe according to claim 4, wherein an aspect ratio of crystal grains having a main phase structure is 5.0 or less.
The method for producing an electric resistance welded steel pipe according to any one of the above. 11. The steel strip is heated to an Ac ₃ transformation point or higher, or the steel pipe before being subjected to the drawing rolling is heated to an Ac ₃ transformation point or higher.
The method for producing an electric resistance welded steel pipe according to any one of the above.