JP3473589B2 - ERW steel pipe and manufacturing method thereof - Google Patents

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 manufacturing the same, and more specifically to an electric resistance welded steel pipe having excellent groove-shaped corrosion resistance of an electric resistance welded portion, which is warm finished or hot rolled. The present invention relates to a technique that can be provided at low cost by finishing.

[0002]

As is well known, forged pipes and electric resistance welded steel pipes are usually used as steel pipes for general piping. When this electric resistance welded steel pipe is used in a humid environment, for example, as water pipes or underground pipes, the corrosion rate of the electric resistance welded portion becomes higher than that of the base metal. Selective corrosion ("groove corrosion" or "groove corrosion") Corrosion ”) may occur.

If this groove-like corrosion occurs severely, leakage from the ERW steel pipe occurs, or if the corrosion spreads to the base metal portion, the service life of the ERW steel pipe greatly falls below the design period. Therefore, conventionally, for applications in such a wet environment, forged welded steel pipes (solid pressure welded steel pipes) have been used because of their excellent groove corrosion resistance.

However, in the forged steel pipe, it is difficult to completely remove the surface scale on the abutting end faces during solid-phase joining at high temperature. For this reason, welding defects such as biting of scale into the joint portion occur, and the joint strength of the joint portion is likely to be inferior to that of the base metal portion. Therefore, the forged steel pipe has a problem that the reliability of pipe joint processing such as bending and widening and the surface quality of the pipe joint portion are easily inferior to those of the electric resistance welded steel pipe. Therefore, there has been a demand for an electric resistance welded steel pipe which is excellent not only in groove corrosion resistance but also in workability in pipe joint processing.

Therefore, many inventions have been proposed so far for improving the groove corrosion resistance of ERW steel pipes. For example,
Japanese Patent Publication No. 60-37173 discloses an invention which improves the groove corrosion resistance by limiting the composition of the electric resistance welded steel pipe to a specific value and performing local heat treatment during the pipe making process.

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

Further, Japanese Patent Application Laid-Open No. 9-165648 discloses an invention for improving groove corrosion resistance by melting both edges of an open pipe by laser irradiation and upsetting and welding. There is.

Further, in JP-A-11-131189, a raw 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
There has been proposed an invention for producing an electric resistance welded steel pipe excellent in both toughness and ductility by performing ferrite rolling at a cumulative temperature reduction rate of 20% or more in a rolling temperature region of 400 ° C.) to make ferrite crystal grains fine.

[0009]

However, the annealing time due to the heat treatment in a normal line is 1 to 1 due to process restrictions.
I can only secure about 2 minutes. For this reason,
In the invention disclosed in Japanese Patent No. 37173, it is difficult to uniformize the structure in the vicinity of the welded portion, and it is not possible to suppress the occurrence of groove corrosion.

Further, according to the studies by the present inventors, JP-A-56-123349, JP-A-56-98451, JP-A-53-100120, and JP-A-55-473.
In the invention proposed by Japanese Patent Laid-Open No. 64, further, 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 manage it. Even if the desired corrosion resistance could be secured, the manufacturing cost would increase, and it was difficult to realize it from the economical point of view.

Further, in the invention disclosed in Japanese Patent Application Laid-Open No. 11-131189, an electric resistance welded portion which has been heated to the Ac ₃ transformation point or higher at the time of manufacturing the material steel pipe and the drawing process after the temperature rise to the Ac ₃ transformation point or lower. Since there is a difference in crystal grain size between the portion and the portion that has been received, the groove-shaped corrosion resistance of the electric resistance welded portion is insufficient. Therefore, providing the electric resistance welded steel pipe provided by the present invention for use in a wet environment has been a problem in view of groove-like corrosion resistance.

The present invention has been made in view of the above problems of the prior art, and is an electric resistance welded steel pipe having excellent welding quality, groove corrosion resistance and pipe joint workability. It is an object of the present invention to provide a technique capable of producing a product with high productivity by hot finishing or warm finishing.

[0013]

Means for Solving the Problems As a result of repeated studies to solve the above-mentioned problems, the present inventors have (1) details of the influence of the material composition and structure on the selective corrosion of the electric resistance welded portion. As a result of the study,
When producing electric resistance welded steel pipe by hot-finishing or warm-finishing by performing electric resistance welding, reheating and drawing rolling, the heating temperature of the steel strip is set to the Ac ₃ transformation point or higher depending on the composition of the material, and By setting the end temperature of the squeeze rolling to be the Ar ₃ transformation point or higher or (Ar ₃ transformation point −50 ° C.) or higher and the start temperature of the squeeze rolling to be higher than the termination temperature by 50 ° C. or more, the crystal grains of the electric resistance welded steel pipe can be obtained. A uniform structure having a diameter aspect ratio of 5.0 or less can be obtained, whereby the difference in structure between the base material portion and the electric resistance welded portion can be eliminated, and the groove resistance of the electric resistance welded portion can be eliminated. Corrosion can be remarkably improved. (2) Depending on the composition of the material, the electric resistance welding pipe made of alloy steel for special applications such as corrosive fluid can also be heated at a heating temperature of Ac ₃ Beyond the transformation point, and further draw rolling The end temperature with the Ar ₃ transformation point or more or (Ar ₃ transformation point -50 ° C.) or higher,
By increasing the start temperature of the squeeze rolling by 50 ° C. or more higher than the end temperature thereof, the groove-like corrosion resistance of the electric resistance welded portion can be improved, and (3) these (1) and Unlike the item (2), even if the heating temperature of the strip steel is less than the Ac ₃ transformation point, the steel pipe before the drawing rolling is heated to the Ac ₃ transformation point or more by reheating performed after electric resistance welding, Furthermore, the end temperature of the squeeze rolling is set to an Ar ₃ transformation point or higher, or (Ar ₃ transformation point −50 ° C.) or higher, and the start temperature of the squeeze rolling is set to 50 ° C. or more higher than the end temperature, whereby electric resistance sewing is performed. We have obtained a new and important finding that the groove corrosion resistance of the weld can be improved.

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

The electric resistance welded steel pipe used in the wet environment according to the present invention has, for example, C: 0.001 to 0.5% (hereinafter, "%" unless otherwise specified in the present specification). Means "mass%"), 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 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: having a steel composition containing one or more of 0.010% or less, Cu, Ni, Cr and M
o is 0.005% or more and 5.0% or less in total, and N
It is illustrated that b, V and Ti are 0.005% or more and 3.0% or less in total. It is exemplified that the electric resistance welded steel pipe according to the present invention is warm-finished or hot-finished.

From another point of view, the present invention continuously forms a heated strip steel by, for example, a forming roll to form an open tube, and locally heats an edge portion of the open tube by an induction heating device. When electric resistance welding is performed to form a steel pipe, reheating is further performed to make the temperature distribution in the circumferential direction of the steel pipe substantially uniform, and then draw rolling is performed to produce an electric resistance welded steel pipe. A method for producing an electric resistance welded steel pipe is characterized in that the end temperature is set to the Ar ₃ transformation point of the steel pipe or more and the start temperature of the drawing rolling is set to be 50 ° C. or more higher than the end temperature of the drawing rolling.

In this case, the steel strip has a 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%
Hereinafter, having a steel composition containing N: 0.01% or less and S: 0.03% or less, or C: 0.001 to
0.5%, Si: 0.01 to 3.0%, Mn: 0.01
~ 3.0%, Al: 0.01-2.00%, P: 0.1
% Or less, N: 0.01% or less, S: 0.03% or less, and 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, and contains one or more kinds of Cu. , Ni, C
It is exemplified to have a steel composition in which r and Mo are 0.005% or more and 5.0% or less in total, and Nb, V, and Ti are 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 to 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: Contains 0.03% or less and Cu: 2.0%
Below, 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 strip steel having a steel composition of 5% or more and 5.0% or less and Nb, V, and Ti in total of 0.005% or more and 3.0% or less is formed into an open pipe, and this open pipe is formed. By performing electric resistance welding to form a steel pipe by locally heating the edge part of the pipe with an induction heating device, and then performing reheating to make the temperature distribution in the circumferential direction of the steel pipe substantially uniform and then performing squeeze rolling. When manufacturing an electric resistance welded steel pipe, the end temperature of the squeeze rolling is set to the (Ar ₃ transformation point −50 of the steel pipe.
C) or higher and the starting temperature of the squeeze rolling is set to be 50 ° C. or more higher than the end temperature of the squeeze rolling. In this case, the strip steel C
It is exemplified that the content is 0.001% or more.

In these ERW steel pipe manufacturing methods according to the present invention, it is exemplified that the ERW steel pipe has an aspect ratio of the crystal grains of the main phase structure of 5.0 or less. In these methods of manufacturing an electric resistance welded steel pipe according to the present invention, it is exemplified that the strip steel is heated to the Ac ₃ transformation point or higher, or the steel pipe subjected to the drawing rolling is heated to the Ac ₃ transformation point or higher. It

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

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of an electric resistance welded steel pipe and a method of manufacturing the same according to the present invention will be described below in detail with reference to the accompanying drawings.

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

(Heating Step) The steel strip 2 is a pipe-making material for the electric resistance welded steel pipe 8, and its composition may be appropriately set according to the performance required for the product electric resistance welded steel pipe 8. It is not limited to a particular composition. That is, the composition of the strip steel 2 is equally applied as long as it is a well-known and commonly used composition of this type of electric resistance welded steel 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, further 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, or 2. It has a steel composition containing two or more kinds, 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. It is 0% or less. Hereinafter, the reasons for limiting the composition of the strip steel 2 in this embodiment will be described.

C: 0.001 to 0.5% C increases the strength of steel by containing 0.001% or more, but if the content exceeds 0.5%, the strength increases excessively. Ductility and hot workability are deteriorated, defects are easily generated in the welded joint, the welding condition becomes unstable, and the groove corrosion resistance of the electric resistance welded portion is deteriorated. Therefore, in the present embodiment, the C content is 0.001% or more and 0.
Limited to 5% or less, preferably 0.01% or more and 0.3%
It is below, and more preferably from 0.03% to 0.3%.

Further, when the C content is 0.2% or less, preferably 0.001% or more and 0.2% or less, the finish temperature of the reduction rolling described later is (Ar ₃ transformation point-50 ° C) of the steel pipe.
It is desirable because it can be expanded to the above range and the appropriate range of rolling temperature can be expanded.

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

Mn: 0.01 to 3.0% Mn also increases the strength of the steel by being contained in an amount of more than 0.01%, but if the content exceeds 3.0%, the weldability and In addition to deteriorating the ductility, MnS, which is a non-metallic inclusion, is easily dissolved in the peripheral portion, so that this MnS becomes a starting point of groove corrosion and groove corrosion resistance deteriorates. Therefore,
In this embodiment, the Mn content is 0.01% or more and 3.0%.
Limited to: From the viewpoint of harmonization of strength and elongation, the lower limit of the Mn content is preferably 0.05%, and the upper limit thereof is preferably 2.0%, more preferably 0.20%. It is above 1.6%.

Al: 0.01 to 2.00% Al also acts as a deoxidizing element by containing 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. It is preferably 0.015% or more and 1.0% or less, and more preferably 0.015% or more and 0.03% or less.

P: 0.1% or less P is an unavoidable impurity and segregates at the crystal grain boundaries to deteriorate both toughness and groove-like corrosion resistance, so it is desirable that the content thereof be small. However, since the 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 0.04% or less, more preferably 0.02% or less.

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

N: 0.01% or less N is a strengthening element of steel and an unavoidable impurity. Although the amount of impurities usually contained is about 0.005%, the inclusion of up to 0.01% is acceptable without any particular harm. Therefore, in this 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, and by adding at least one of the elements described below as an optional additional element to this basic component, it is even more excellent. It is possible to obtain an electric resistance welded steel pipe having both groove corrosion resistance and other properties. Therefore, these optional additional elements will be described below.

Cu: 2.0% or less Cu is an element that improves the corrosion resistance of the base material and also 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: 2.0% or less Ni, like Cu, is an element that improves the groove corrosion resistance of the electric resistance welded portion, but even if added in excess of 2.0%, the groove resistance will increase. The effect of improving the corrosiveness is saturated and the cost is increased. Therefore, when Ni is added, its content is preferably limited to 2.0% or less.

Cr: 3.0% or less Cr is an element that improves the strength as well as the corrosion resistance of the base material and the electric resistance welded portion, but if added in excess of 3.0%, the base material Further, the effect of improving the corrosion resistance of the electric resistance welded portion is saturated, which only increases the cost. 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 all elements that improve both strength and groove-like corrosion resistance of the electric resistance welded portion, but the contents of these elements are 2.0% and 1.0%, respectively. , 1.0%, the effect of improving the groove corrosion resistance is saturated and the cost is increased. Therefore, when adding Mo, Nb, and V, the content is Mo: 2.0% or less, Nb, V:
It is desirable that both are limited to 1.0% or less.

Ti: 1.0% or less Ti is a deoxidizing element, and is an element that contributes to both miniaturization of crystal grains and high strength by precipitating as a nitride or a carbonitride. However, if the Ti content exceeds 1.0%, adverse effects such as deterioration of toughness occur. Therefore, when Ti is added, the content is preferably limited to 1.0% or less.

Ca: 0.01% or less Ca is an element that has the effect of controlling the form of inclusions to improve workability and also improves the resistance to groove corrosion of the electric resistance welded portion. However, the Ca content is 0.0
If it exceeds 1%, the cleanliness of the steel decreases and the ductility deteriorates.
Therefore, when Ca is added, its content is 0.0
It is desirable to limit it to 1% or less.

B: 0.010% or less B is an element that contributes to both finer crystal grains and higher strength. However, if the B content exceeds 0.010%, both weldability and toughness deteriorate. Therefore, when B is added, its content is preferably 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
Hereinafter That, Cu, Ni, since the Cr and Mo has the effect of improving the hardenability, to control the crystal grain size and area ratio of ferrite and rest phase of the cooling process is facilitated. In addition to enhancing the hardenability, Cu also has the effect of enhancing corrosion resistance. Therefore, for the above-mentioned purpose, C
One or more u, Ni, Cr and Mo may be contained, but if the total content of these is less than 0.005%, the quenching effect is difficult to obtain, while the total content of these is 5.
If it exceeds 0%, the above effect is saturated and, conversely, ductility decreases. Therefore, when one or more kinds of Cu, Ni, Cr and Mo are added, the total content thereof is 0.
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. The upper limit is more preferably 1.0%, further preferably 0.5%.

On the other hand, Nb, V and Ti have a function of fixing the solid solution C, the solid solution N and the solid solution S contained in the steel as precipitates to render them harmless, and particularly ductility and deep drawability are not so great. It has the effect of increasing the strength of steel without damaging it. Therefore, in order to enhance the deep drawability of steel, Nb, V and T
One or more i may be contained, but the content is 0.0
If it is less than 05%, it is difficult to obtain such an effect. On the other hand, if the total content of these exceeds 3.0%, the above effect is saturated and, conversely, ductility and deep drawability are deteriorated. So Nb,
When adding one or more of V and Ti, it is desirable that the total content thereof be limited to 0.005% or more and 3.0% or less. From the same viewpoint, the lower limit of these contents is more preferably 0.01% in total,
It is more preferably 0.02%. On the other hand, the upper limit of these elements is more preferably 0.5%, and the upper limit of 0.
It is more preferably 2%.

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

The heating of the strip steel 2 causes a temperature difference that is inevitably generated between the welding edge and the base metal portion in the vicinity thereof during electric resistance welding performed later by the forming and induction heating welding device 4. This is done in order to homogenize the structure by suppressing and preventing the formation of heat-affected hardening structures.

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

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

If the heating temperature of the strip steel 2 is lower than the Ac ₃ transformation point, the structure of the strip steel 2 after heating does not become uniform austenite grains, and local phase transformation occurs during the subsequent drawing rolling. Occurs, and the grain size of the final product becomes uneven. Therefore, particularly in the heat-affected zone near the electric resistance welded portion, remarkable nonuniformity of the grain size of the structure is likely to occur. In this case, it is possible to prevent the nonuniformity of the structure by lowering the welding speed, prolonging the time of the reheating treatment to be performed later, and further raising the temperature of the mother tube before the drawing rolling to the Ac ₃ transformation point or higher. Although you can
Productivity decreases.

Therefore, in the present embodiment, the strip steel 2 is 400
It is desirable to heat to a temperature of not lower than 0 ° C., and it is more desirable to heat to a temperature of Ac ₃ transformation point or higher 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 sheet is strongly worked by drawing rolling to be performed later, the crystal grains of the final product will become coarse and sufficient strength will be obtained. In addition, the scale of the surface of the strip steel 2 is likely to occur and the surface quality of the steel pipe after the reduction rolling is deteriorated.

Therefore, in the present embodiment, it is desirable to heat the strip steel 2 in a temperature range of 400 ° C. or higher and 1300 ° C. or lower, and more preferably in a temperature range of Ac ₃ transformation point or higher and 1300 ° C. or lower.

(Molding process) In FIG. 1, the strip steel heating furnace 3
The strip steel 2 heated to the above 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, the arrangement, roll schedule, etc. may be appropriately diverted or remodeled for hot working by using a device such as an ordinary electric resistance welded steel pipe. Is omitted.

(Electrical Resistance Welding Step) This open tube is locally heated to its melting point by the induction heating coil (not shown) provided in the forming and induction heating welding device 4 with its both edge portions as targets.

Induction heating of both edge parts is performed so that the entire cross section of the open pipe is below the strip steel heating temperature and (Curie point −50).
(° C) or higher is desirable. If the strip steel temperature at the time of electric resistance welding is lower than (Curie point −50 ° C.), the efficiency of induction heating for electric resistance welding greatly decreases, so that it is necessary to reduce the welding speed, which impedes productivity. This is because there is a risk of

As these induction heating coils, well-known and conventional ones may be used, and therefore the description of the induction heating coil will be omitted. The open pipe whose both edges are locally heated to the melting point is abutted and welded by a squeeze roll (not shown) provided in the forming and induction heating welding device 4, and becomes a mother pipe for later reduction rolling. It is a steel pipe.

On the inner and outer surfaces of the steel pipe that has been butt-welded by the squeeze roll, molten beads discharged during the butt-welding are generated. Therefore, it is desirable to remove the generated molten bead at this stage.

The method for removing the molten beads is not a specific method. For example, it is exemplified to use a well-known and conventional cutting and removing method. This cutting removal method is a fixed cutting bite method used for ordinary electric resistance welded steel pipes or a rotating bite method to prevent wear of the bite during hot work, and the pressing force of the cutting bite can be adjusted by the built-in hydraulic cylinder. Therefore, a well-known and conventional cutting and removing method such as a method capable of automatically responding to a change in the wall thickness of an electric resistance welded steel pipe can be used.

(Reheating Step, Drawing and Rolling Step) The steel pipe from which the molten bead generated on the inner and outer surfaces has been removed makes the temperature difference between the welded portion and the surrounding base metal part uniform and performs rolling in the later drawing rolling. For the purpose of adjusting the temperature, the tube reheating device 5 heats up or soaks the heat.

The reheating method is not limited to a specific heating means as long as it is a heating means capable of soaking the tube. For example, any one of a gas combustion type continuous heating furnace, an induction heating device for tubes, an electric heating type, or a combination thereof can be used.

Then, the mother tube having the temperature distribution in the circumferential direction of the tube subjected to substantially uniform heat treatment by the tube reheating device 5 is subjected to reduction rolling to a predetermined outer diameter by the reduction rolling machine 6 to be a final product. In the present embodiment, the rolling temperature of the squeeze rolling by the squeeze rolling mill 6 is such that the rolling end temperature is the Ar ₃ transformation point or more, but the C content is 0.2% or less or 0.001% or more 0.2 %, The rolling end temperature is (Ar ₃
The transformation temperature is set to −50 ° C.) or higher, and the start temperature of the drawing rolling is set to be 50 ° C. higher than this end temperature. As a result, the rolling process can be finished 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 less than (Ar ₃ transformation point −50 ° C.) even if it is 0.2% or less, the above-mentioned JP-A-11-
Since the transformation from austenite occurs during the drawing rolling as in the invention disclosed in Japanese Patent No. 131189, the crystal grains are transformed between the structure transformed after being processed in the austenite region and the structure processed after transformation. There is a difference in diameter. Therefore, the structure of the final product is a mixed-grain structure of coarse particles and fine particles, and in a wet environment or a corrosive environment such as underground piping, a local battery occurs due to this difference in structure.
Groove corrosion resistance deteriorates. In particular, since the electric resistance welded portion is once heated to the melting temperature (1400 to 1500 ° C.) at the time of welding, the austenite grains around the welded portion become coarse. For this reason, if this portion passes through the transformation point without undergoing the rolling process, it will become coarse crystal grains, and the difference in structure from the base material portion will become even larger. Therefore, in this embodiment,
The finish temperature of the squeeze rolling is Ar ₃ transformation point or higher, and particularly when the C content is 0.2% or less or 0.001% or more and 0.2% or less, the finish temperature of the squeeze rolling is (Ar ₃ transformation point- 5
0 ° C) or higher.

If the finish temperature of the drawing rolling exceeds 1000 ° C., the grain refining effect of the drawing process decreases, and the grain size of the final product becomes coarse and sufficient strength cannot be obtained. Is preferably 1000 ° C. or lower.

On the other hand, if the starting temperature of the squeeze rolling is not higher than the ending temperature by 50 ° C. or more, when the ending temperature is the Ar ₃ transformation point or higher, or the C content is 0.2% or lower or 0.001 or less. % Or more and 0.2% or less and 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 the periphery thereof. Since the difference in structure from the base metal part is not completely eliminated, there is a difference in 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 corrosion resistance deteriorates. Therefore, in the present embodiment, the finish temperature of the reduction rolling is the Ar ₃ transformation point or higher, particularly when the C content is 0.2% or lower or 0.001% or higher and 0.2% or lower (Ar ₃ transformation point- 50 ° C.) or higher, and the start temperature of the squeeze rolling is set to be 50 ° C. higher than this end temperature.

The upper limit of the temperature difference between the start temperature and the end temperature of the squeeze rolling need not be specified, but the upper limit of the start temperature is preferably 1300 ° C. or less. This is because when the starting temperature exceeds 1300 ° C, the austenite grains become coarse, and the grain size of the final product becomes coarse even if the strong working by drawing is performed, and sufficient strength cannot be obtained, as with the heating of the strip steel. This is because the scale of the surface of the steel pipe is likely to occur and the normality of the surface of the steel pipe after reduction rolling may be deteriorated.

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

The processing rate k in this reduction rolling is exemplified to be 20% or more and 90% or less from the viewpoint of grain refinement due to the introduction of processing strain. The processing constant k is reducing rolling D ₁ the outer diameter of the front of the steel pipe, D ₂ the outer diameter of the final product as well as the thickness and t _1, when the thickness to t _2, K = {1-
t ₂ (D ₂ −t ₂ ) / t ₁ (D ₁ −t ₁ )} × 100
(%).

Unlike the present embodiment, even when the steel pipe or the steel 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 below 0.2% C content or 0.0
In the case of 01% or more and 0.2% or less (Ar ₃ transformation point-5
(0 ° C.) or higher and the start temperature of the drawing rolling higher than the end temperature by 50 ° C. or more, and the heating temperature of the strip steel or the steel pipe before the drawing rolling is set to the Ac ₃ transformation point or more, It is possible to remarkably improve the groove corrosion resistance of the electric resistance welded portion.

(Cutting Step) The steel pipe that has been subjected to the reduction rolling by the reduction rolling machine 6 is cut into a predetermined length by the pipe cutting device 7 to be an electric resistance welded steel pipe 8.

The electric resistance welded steel pipe 8 manufactured in this manner is
After this, it is further transported to a cooling bed (not shown) and allowed to cool until the transformation is completely completed. Then, it may be water-cooled to a temperature (room temperature) suitable for handling.

(The obtained ERW steel pipe 8) In the present embodiment,
In this way, 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 to 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 Cu: 2.0% or less, Ni: 2.0%, if necessary.
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%
Below, Ca: 0.01% or less, and B: 0.005%
Having 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, it is 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
1 to 3.0%, Mn: 0.01 to 3.0%, Al: 0.
01-2.00%, P: 0.1% or less, N: 0.01%
Hereinafter, S: 0.03% or less is contained, and if necessary, Cu: 2.0% or less, Ni: 2.0% or less, C
r: 3.0% or less, Mo: 2.0% or less, Nb: 1.0
% Or less, V: 1.0% or less, Ti: 1.0% or less, C
a: 0.01% or less and B: 0.010% or less, and one or more of them are contained, and Cu, Ni, Cr and Mo are 0.005% or more and 5.0% or less in total,
Nb, V and Ti total 0.005% to 3.0%
Below, it is manufactured by performing warm finishing or hot finishing.

In this 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%.
, The area ratio of the first-phase structure is less than 50%. In this case, however, 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 the area ratio thereof is, for example, an optical microscope or a scanning electron microscope (SE).
Several views (for example, 5 views) of the tissue are confirmed by M), and an image analyzer is used to analyze a tissue photograph in which the main phase tissue is colored in black and the second phase tissue is colored in white.
It may be calculated from the average value of the visual field. In this 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 this 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) 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, 5 fields of view of the tissue are photographed with an optical microscope (manufacturer: Nikon, product number: OPTI PHOTO) or a scanning electron microscope (SEM, manufacturer: JSM, product number 6300), and using this structure photograph, JI
A method based on the cutting method of the ferrite grain size test method for steel specified in S G 0552, which is based on the number of grains of the main phase structure cut by two orthogonal line segments of a certain length in the microscope visual field. By the linear cutting method to calculate, the maximum value of the diameter of the main phase structure crystal grains, the "maximum diameter" showing the minimum value, "minimum diameter" is obtained, these "maximum diameter", "minimum diameter" obtained
The values obtained by multiplying the above by 1.13 are the maximum grain size d ₃ and the minimum grain size d ₄ of the crystal grains. Since the aspect ratio β in the present invention needs to be 5.0 or less in any measurement cross section, in the present embodiment,
Aspect ratio β ₁ in the longitudinal cross section of the steel pipe and aspect ratio β _{2 in the} cross section perpendicular to the longitudinal direction,
I decided to ask.

If this aspect ratio β, that is, in the present embodiment, at least one of the aspect ratios β ₁ and β ₂ is larger than 5.0, a non-uniform mixed grain structure results. Therefore, the aspect ratios β ₁ and β ₂ At least one of them is 5.0
Larger electric resistance welded steel pipe 8 as underground buried pipe,
When used in a moist environment or a corrosive environment, a local battery is generated due to this difference in structure, and groove corrosion is likely to proceed. On the other hand, the electric resistance welded steel pipe 8 of this embodiment has an aspect ratio β
_{Since 1} and β ₂ are both 5.0 or less, a uniform structure is formed, which makes it difficult to form a local battery, and the groove-like corrosion property is significantly improved.

As described above, the electric resistance welded steel pipe 8 of this embodiment is
Since the aspect ratio is 5.0 or less, it is possible to eliminate the structural difference between the base material portion and the electric resistance welded portion of the electric resistance welded steel pipe 8, and the groove-shaped corrosion resistance of the electric resistance welded portion is remarkably high. improves.

Further, the electric resistance welded steel pipe 8 of the present embodiment is superior in the reliability of pipe joint processing such as bending and widening and the surface quality of the pipe joint portion to the conventionally used forged steel pipe. Excellent not only in groove corrosion resistance but also in pipe joint processing.

Therefore, according to this embodiment, an electric resistance welded steel pipe having both excellent welding quality, groove corrosion resistance, and pipe joint workability can be hot-finished or warm-finished.
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, the heating temperature and drawing rolling temperature (drawing rolling starting temperature and drawing rolling ending temperature) of 31 kinds of strip steels 2 having the steel composition shown in Table 1 as raw materials are shown in Table 2. 31 kinds of electric resistance welded steel pipes 8 were manufactured by changing as shown.

That is, the width is 365 mm and the thickness is 3.7 mm.
The steel strip 2 is heated by the steel strip heating furnace 3, formed and electric resistance welded by the forming and induction heating welding device 4 to form a steel pipe having an outer diameter of 114.3 mm, and the steel pipe is heated by the pipe reheating device 5 After being continuously reheated by means of, the roll is stretch-rolled by a 3-roll type stretch reducer (drawing mill 6), and further cut by the pipe cutting device 7 into a predetermined pipe length, thereby having an outer diameter of 48.6 mm and a wall thickness. 3.31 m
The electric resistance welded steel pipe 8 is m.

[0079]

[Table 1]

Then, the groove corrosion resistance and aspect ratio of these 31 types of ERW steel pipes were investigated. The groove corrosion resistance performance was investigated by conducting a neutral salt spray test on the welded portion of the steel pipe. This neutral salt spray test was performed on each of 31 types of electric resistance welded steel pipes, including 100 welded parts.
mm, width 70 mm, and an arc-shaped test piece, 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: The test conditions were 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 this test. FIG. 3 (a) shows the state before the test and FIG. 3 (b) shows the state after the test. Shown respectively. The groove corrosion resistance was evaluated by the index A value shown by the following formula in FIG.

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

On the other hand, the aspect ratio of the electric resistance welded steel pipe 8 is as shown in FIG.
As described above with reference to
-: Nikon, product number: OPTI PHOTO)
Then, using the average value of 5 fields of view of 100 to 200 times,
Index β indicated by the formula₁, Β ₂The value was evaluated. This
Where β₁Is the aspect ratio in the longitudinal direction of the steel pipe,
β₂Indicates 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) in the major axis direction of crystal grains in the cross section of the electric resistance welded steel pipe 8. Means d ₄ means the length (μm) of the crystal grains in the minor axis direction in the cross section of the electric resistance welded steel pipe 8. According to this measuring method, the closer the values of β ₁ and β ₂ are to 1, the more uniform the structure is without 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 indicate 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, the sample No. which satisfies both the drawing rolling temperature and the drawing rolling ending temperature in the range specified in the present invention. 1-Sample No. It can be seen that all of Nos. 21 have no groove corrosion, and have extremely excellent groove corrosion resistance.

On the other hand, sample N whose C content exceeds 0.2%
o. 22-Sample No. 24, sample No. 30, sample N
o. In all Nos. 31, the drawing rolling end temperature was lower than the Ar ₃ transformation point, so that deep groove corrosion occurred.

Sample No. 25 and sample No. Two
No. 6 had a C content of 0.2% or less, but the drawing rolling end temperature was lower than (Ar ₃ transformation point −50 ° C.), so that deep groove corrosion occurred.

Sample No. In No. 27, the starting temperature of the squeeze rolling was not higher than the ending temperature thereof by 50 ° C. or more, so that deep groove-like corrosion occurred. Further, the sample No. 23, sample No. 25 and sample No. In No. 26, since both the drawing rolling start temperature and the drawing rolling end temperature were below the Ar ₃ transformation point, very deep groove corrosion occurred.

In particular, the sample No. In No. 26, the band steel heating temperature was below the Ac ₃ transformation point, and both the drawing rolling start temperature and the drawing rolling end temperature were below the Ar ₃ transformation point, so very deep groove 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 corrosion resistance, and pipe joint workability can be hot-finished or warm-finished. With finishing, it can be manufactured with high productivity.

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

[Brief description of 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 crystal grains in the embodiment.

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 of the example, and FIG.
Shows before the test and FIG. 3B shows after the test.

[Explanation of symbols]

2 band steel 3 band steel heating furnace 4 Forming and induction heating welding equipment 5 tube reheating equipment 6 Drawing and rolling mill 7 Tube cutting device 8 ERW steel pipe

Continuation of the front page (56) Reference JP-A-2002-69584 (JP, A) JP-A-2002-20841 (JP, A) JP-A-2-301540 (JP, A) JP-A-3-267316 (JP, A) ) JP-A-60-15082 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/10

Claims (11)

(57) [Claims] 1. In the longitudinal and circumferential directions of the main phase structure
Electric resistance welded steel pipe for use in a wet environment, characterized in that that is the aspect ratio of the crystal grains of 5.0 or less. 2. C: 0.001 to 0.5% by mass,
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, which has a steel composition containing N: 0.01% or less and S: 0.03% or less. 3. 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, or Contains two or more kinds, and contains 0.005 in total of Cu, Ni, Cr and Mo.
% Or more and 5.0% or less, and Nb, V, and Ti are 0.005% or more and 3.0% or less in total, The electric resistance welded steel pipe according to claim 2. 4. A heated steel strip is formed into an open pipe, and the edge portion of the open pipe is locally heated by an induction heating device to perform electric resistance welding to form a steel pipe,
Further, when the electric resistance welded steel pipe is manufactured by performing re-heating to substantially uniformize the circumferential temperature distribution of the steel pipe and then performing draw rolling, the finish temperature of the draw rolling is set to an Ar ₃ transformation point of the steel pipe or more. At the same time, the starting temperature of the drawing rolling is set to be higher than the ending temperature of the drawing rolling by 50 ° C. or more, and the method for producing an electric resistance welded steel pipe according to claim 1 . 5. The steel strip, in mass%, C: 0.001
~ 0.5%, Si: 0.01 to 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, which has a steel composition containing 1% or less, N: 0.01% or less, and S: 0.03% or less. 6. The steel strip according to% by mass, C: 0.001
~ 0.5%, Si: 0.01 to 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, and 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 is contained, and Cu, Ni,
Cr and Mo are 0.005% or more and 5.0% or less in total, and Nb, V, and Ti have a steel composition which is 0.005% or more and 3.0% or less in total, It was described in Claim 4. Manufacturing method of ERW steel pipe. 7. Mass%, C: 0.2% or less, Si:
0.01-3.0%, Mn: 0.01-3.0%, A
1: 0.01 to 2.00%, P: 0.1% or less, N:
A heated strip steel having a steel composition containing 0.01% or less and S: 0.03% or less is formed into an open tube, and an edge portion of the open tube is locally heated by an induction heating device to generate electricity. After producing a steel pipe by performing sewn welding, further manufacturing the electric resistance welded steel pipe by performing draw rolling after further reheating to make the circumferential temperature distribution of the steel pipe substantially uniform, the end temperature of the draw rolling Is the (Ar ₃ transformation point-
50 ° C.) or more, and the starting temperature of the drawing rolling is set to be 50 ° C. or more higher than the ending temperature of the drawing rolling, the method for producing an electric resistance welded steel pipe used in a wet environment . 8. In mass%, C: 0.2% or less, Si:
0.01-3.0%, Mn: 0.01-3.0%, A
1: 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:
Contains one or more of 0.01% or less and B: 0.010% or less, Cu, Ni, Cr and M
o is 0.005% or more and 5.0% or less in total, and N
b, V and Ti are molded into a heated strip steel having a steel composition of 0.005% or more and 3.0% or less in total, and an edge portion of the open tube is locally heated by an induction heating device. When the electric resistance welded steel pipe is manufactured by performing electric resistance welding to obtain a steel pipe, and further performing reheating to make the temperature distribution in the circumferential direction of the steel pipe substantially uniform and then performing draw rolling to produce the electric resistance welded pipe. The rolling end temperature is set to the (Ar ₃ transformation point-
50 ° C.) or more, and the starting temperature of the drawing rolling is set to be 50 ° C. or more higher than the ending temperature of the drawing rolling, the method for producing an electric resistance welded steel pipe used in a wet environment . 9. The method for producing an electric resistance welded steel pipe according to claim 7, wherein the C content of the strip steel is 0.001% or more. 10. The electric resistance welded steel pipe has a main phase structure in a longitudinal direction.
And the aspect ratio of the crystal grains in the circumferential direction is 5.0 or less, the method for producing an electric resistance welded steel pipe according to any one of claims 7 to 9. 11. The method according to claim 4, wherein the strip steel is heated to an Ac ₃ transformation point or higher, or the steel pipe before the drawing rolling is heated to an Ac ₃ transformation point or higher.
The method for producing an electric resistance welded steel pipe described in any one of 1 to 3 above.