JP7388371B2 - ERW steel pipe and method for manufacturing ERW steel pipe - Google Patents

Description

The present invention relates to an electric resistance welded steel pipe, and particularly to an electric resistance welded steel pipe that has high strength and excellent low-temperature toughness and can be suitably used as a structural member for construction machinery such as a crane lattice and a crane boom. The present invention also relates to a method for manufacturing the electric resistance welded steel pipe.

Steel pipes are used for a variety of purposes, and are also widely used as structural members for construction machinery such as crane lattices and crane booms. The steel pipes used in these applications are required to have high strength and toughness to accommodate larger cranes and use in extremely cold regions. For example, in recent years, steel pipes with a tensile strength (TS) of 780 MPa class are required to have excellent toughness even at a low temperature of -20°C.

For these applications, steel pipes with a wide range of wall thicknesses are used, ranging from relatively thin walled pipes with a wall thickness of about 4 mm to thick walled pipes with a wall thickness of over 10 mm. Until now, seamless steel pipes or ERW steel pipes have been used as thin-walled steel pipes, and seamless steel pipes have been used as thick-walled steel pipes. When producing thick-walled steel pipes, a method has been used to ensure the required strength by performing heat treatment (quenching and tempering) after pipe production.

For example, Patent Document 1 proposes a method for manufacturing a seamless steel pipe with a wall thickness of more than 30 mm, which is used as a mechanical structural member such as a boom of a crane. In the method proposed in Patent Document 1, by performing heat treatment two or more times after pipe making, high strength such as tensile strength of 950 MPa or more and yield strength of 850 MPa or more, and Charpy absorbed energy of 60 J or more at -40°C is achieved. Seamless steel pipes with high toughness can be obtained.

Japanese Patent Application Publication No. 2012-193404

As described above, according to the method proposed in Patent Document 1, a seamless steel pipe with excellent strength and low-temperature toughness can be manufactured. However, seamless steel pipes are more expensive to manufacture than ERW steel pipes. In addition, in the method of Patent Document 1, it is necessary to perform heat treatment two or more times after pipe formation in order to obtain desired characteristics, which further increases manufacturing costs.

In addition, in the method proposed in Patent Document 1, the steel pipe is heated to high temperatures for a long time in the pipe making process and the heat treatment process, so there is a problem that the surface of the steel pipe becomes pocked and the surface texture becomes poor. . If the surface texture is poor, not only the appearance quality (beauty) is impaired, but also the paintability is reduced.

As described above, although seamless steel pipes obtained by conventional techniques have excellent strength and low-temperature toughness, they have problems in that they are expensive to manufacture and tend to have a pocked surface.

In order to solve the above-mentioned problems, it is conceivable to replace relatively thick-walled steel pipes, for which seamless steel pipes have conventionally been used, with electric resistance welded steel pipes that can be manufactured at a relatively low cost. However, in order to obtain the desired strength in ERW steel pipes, it is necessary to increase the amount of added elements such as C, Mn, Si, Ti, and Nb. Welding quality will deteriorate.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an electric resistance welded steel pipe that satisfies the following requirements and can be suitably used for structural members of construction machinery such as crane lattices and crane booms. do.
・It must be an ERW steel pipe that can be manufactured at low cost.
・Both high strength and excellent low-temperature toughness without the need for heat treatment after pipe making.
・Excellent surface quality.
- Excellent weldability.
・It is possible to manufacture relatively thick-walled pipes, which conventionally used seamless steel pipes.

The present inventors conducted studies to solve the above problems, and found that by controlling the component composition and microstructure, a high tensile strength of 780 MPa and an excellent Charpy absorbed energy of 31 J or more at -20°C were achieved. We have discovered that it is possible to obtain ERW steel pipes that have both low-temperature toughness and low-temperature toughness. The above-mentioned ERW steel pipes can be manufactured without heat treatment after pipe production by controlling the composition and manufacturing conditions of the steel material, so in addition to being inexpensive, they also have excellent surface properties. . Furthermore, although it is necessary to increase the amount of alloying elements added to improve strength, strength can be ensured by controlling the content of Si and Mn, which are elements that tend to form oxides, within a specific range. At the same time, deterioration in welding quality can be prevented.

The present invention has been made based on the above-mentioned knowledge, and has the following gist.

1. In mass%,
C: 0.050-0.15%,
Si: 0.001 or more, less than 0.050%,
Mn: 1.5-2.5%,
P: 0.1% or less,
S: 0.01% or less,
Al: 0.01-0.10%,
Ti: 0.05-0.20%,
Nb: 0.01-0.10%, and N: 0.0005-0.01%
, with the remainder consisting of Fe and unavoidable impurities, and having a component composition in which the ratio of Mn content to Si content, Mn/Si, is more than 36,
It has a microstructure consisting of a bainite phase and a ferrite phase with an area ratio of 10% or less,
An electric resistance welded steel pipe having mechanical properties such as a tensile strength of 780 MPa or more and a Charpy absorbed energy of 31 J or more at -20°C.

2. The component composition is in mass%, and further,
Mo: 0.05-0.30%,
Cr: 0.05-0.50%,
Cu: 0.001 to 0.5%,
Ni: 0.001 to 0.5%,
W: 0.001-0.05%,
V: 0.001 to 0.01%,
The electric resistance welded steel pipe according to 1 above, containing at least one selected from the group consisting of Ca: 0.0001 to 0.0050%, and REM: 0.02% or less.

3. In mass%,
C: 0.050-0.15%,
Si: 0.001% or more, less than 0.050%,
Mn: 1.5-2.5%,
P: 0.1% or less,
S: 0.01% or less,
Al: 0.01-0.10%,
Ti: 0.05-0.20%,
Nb: 0.01-0.10%, and N: 0.0005-0.01%
, the balance is Fe and unavoidable impurities, and the ratio Mn/Si of Mn content to Si content is more than 36, heated to a heating temperature of 1100 to 1250 ° C.,
The heated steel material is hot-rolled at a finish rolling finish temperature of 700 to 830°C to form a hot-rolled steel strip,
Water-cooling the hot rolled steel strip at an average cooling rate of 50° C./s or more to a cooling end temperature of 500° C. or less,
Coiling the water-cooled hot rolled steel strip at a coiling temperature of 350 to 500°C,
Roll forming the hot rolled steel strip into a substantially cylindrical open tube;
A method for producing an ERW steel pipe, comprising ERW welding the open pipe to produce an ERW steel pipe.

4. The component composition is in mass%, and further,
Mo: 0.05-0.30%,
Cr: 0.05-0.50%,
Cu: 0.001 to 0.5%,
Ni: 0.001 to 0.5%,
W: 0.001-0.05%,
V: 0.001 to 0.01%,
3. The method for manufacturing an electric resistance welded steel pipe according to 3 above, containing at least one selected from the group consisting of Ca: 0.0001 to 0.0050%, and REM: 0.02% or less.

According to the present invention, it is possible to obtain an electric resistance welded steel pipe that has both high tensile strength of 780 MPa or more and excellent low-temperature toughness such as Charpy absorbed energy of 31 J or more at -20°C. The electric resistance welded steel pipe of the present invention can be manufactured without performing heat treatment after pipe production, so it is not only inexpensive but also has excellent surface properties and weldability. The electric resistance welded steel pipe of the present invention can be suitably used as a structural member of construction machinery, particularly for a crane lattice or a crane boom.

The present invention will be explained in detail below. Note that the present invention is not limited to this embodiment.

[Component composition]
The electric resistance welded steel pipe of the present invention has the above-mentioned composition. Each component included in the component composition will be explained below. In addition, unless otherwise specified, "%" as a unit of content of a component in this specification means "mass %."

C: 0.050-0.15%
C is an element that has the effect of improving strength. In order to ensure the strength required for structural members of construction machinery, the C content is set to 0.050% or more. On the other hand, when the C content exceeds 0.15%, low temperature toughness deteriorates. Therefore, the C content is set to 0.15% or less, preferably 0.12% or less.

Si: 0.001 or more, less than 0.050% Si is an element that acts as a deoxidizing agent and also as a solid solution strengthening element. In order to obtain the above effect, the content must be 0.001% or more. Therefore, the Si content is set to 0.001% or more. On the other hand, since Si is an element that is easily oxidized, if the Si content is excessive, oxides will be produced significantly during electric resistance welding, and welding quality will deteriorate. Therefore, the Si content is made less than 0.050%. Note that as the wall thickness increases, it tends to become more difficult to ensure welding quality. Therefore, especially when the wall thickness is 6 mm or more, the effect of suppressing the Si content to less than 0.050% is particularly significant.

Mn: 1.5-2.5%
Mn is an element that forms a solid solution and contributes to improving the strength of steel. Moreover, since Mn has the effect of improving the hardenability of steel, by appropriately adding Mn, a structure consisting mainly of bainite can be obtained even when the wall thickness is large. In order to ensure the strength required for structural members of construction machinery, the Mn content is set to 1.5% or more, preferably 1.6% or more. On the other hand, if the Mn content exceeds 2.5%, not only the toughness decreases but also the hardness becomes excessively high, making it difficult to form a pipe. Therefore, the Mn content is 2.5% or less, preferably 2.0% or less.

P: 0.1% or less P is an element contained in steel as an impurity, segregates at grain boundaries, etc., and reduces weld cracking resistance and toughness. In order to prevent a decrease in weld cracking resistance and toughness, it is necessary to reduce the P content to 0.1% or less. Therefore, the P content is 0.1% or less, preferably 0.05% or less.

S: 0.01% or less S is an element that exists as sulfide inclusions in steel and reduces hot workability and toughness. In order to prevent a decrease in hot workability and toughness, it is necessary to reduce the S content to 0.01% or less. Therefore, the S content is set to 0.01% or less, preferably 0.005% or less.

Al: 0.01~0.10%
Al acts as a deoxidizing agent, and also combines with N to precipitate as AlN, which has the effect of increasing strength. In order to obtain the above effect, the content must be 0.01% or more. Therefore, the Al content is set to 0.01% or more. On the other hand, if it is contained in a large amount exceeding 0.10%, the amount of oxide inclusions will increase and workability will decrease. Therefore, the Al content is 0.10% or less, preferably 0.05% or less.

Ti: 0.05-0.20%
Ti forms carbides such as TiC. By forming carbides, coarsening of austenite grains during the slab heating stage is suppressed. Therefore, by adding Ti, the strength of steel can be increased by grain refinement and precipitation strengthening. In order to obtain the above effect, the Ti content is set to 0.05% or more. On the other hand, when the Ti content exceeds 0.20%, ductility decreases. Therefore, the Ti content is set to 0.20% or less. More preferably, it is 0.06 to 0.15%.

Nb: 0.01~0.10%
Like Ti, Nb combines with C in steel to form carbides such as NbC. Bainite is strengthened by finely dispersing the generated carbides in bainite, which is the main phase. In order to obtain the above effect, the Nb content should be 0.01% or more. Therefore, the Nb content is set to 0.01% or more. On the other hand, if the Nb content exceeds 0.10%, the effect of addition is saturated and no effect commensurate with the content can be obtained, which is economically disadvantageous. Therefore, the Nb content is 0.10% or less, preferably 0.05% or less.

N: 0.0005-0.01%
N is an element that is inevitably contained as an impurity. N combines with nitride-forming elements in steel and contributes to suppressing coarsening of crystal grains. In order to obtain the above effect, the content must be 0.0005% or more. Therefore, the N content is set to 0.0005% or more. On the other hand, if the N content exceeds 0.01%, the toughness of the weld will decrease. Therefore, the N content is set to 0.01% or less, preferably 0.005% or less.

The component composition in one embodiment of the present invention can be such that the above elements and the remainder are Fe and unavoidable impurities.

Mn/Si: More than 36 In order to ensure the welding quality of electric resistance welded pipes, it is important to appropriately control the ratio of Mn content to Si content, Mn/Si. When electric resistance welding is performed, oxides of Si and Mn are formed on the joint surface, and the melting temperature of the formed oxides of Si and Mn depends on the value of Mn/Si. If the value of Mn/Si is low, the melting temperature of the oxide becomes high, making it difficult for the oxide to be discharged outside the tube when upset, and reducing the joint strength. In order to suppress this, the Mn/Si ratio is set to exceed 36. Note that the ratio Mn/Si here is the mass ratio of Mn and Si.

Moreover, in another embodiment of the present invention, the above-mentioned component composition can further optionally contain at least one selected from the following elements in the amounts described below.

Mo: 0.05-0.30%,
Mo is an element that forms a solid solution and contributes to further improving the strength of steel. When Mo is added, the Mo content is set to 0.05% or more in order to obtain the above effect. On the other hand, when the Mo content exceeds 0.30%, the effect becomes saturated and the cost increases significantly. Therefore, the Mo content is 0.30% or less, preferably 0.2% or less.

Cr: 0.05-0.50%
Cr is an element that forms a solid solution and contributes to further improving the strength of steel. When adding Cr, the Cr content is set to 0.05% or more in order to obtain the above effects. On the other hand, when the Cr content exceeds 0.50%, oxides are likely to be formed, and Cr oxides remain in the electric resistance welding portion, resulting in a decrease in electric resistance welding quality. Therefore, the Cr content is set to 0.50% or less, preferably 0.3% or less.

Cu: 0.001-0.5%
Cu is an element that has the effect of improving corrosion resistance. When adding Cu, in order to obtain the above effect, the Cu content is set to 0.001% or more. On the other hand, since Cu is an expensive alloying element, if the Cu content exceeds 0.5%, material costs will rise. Therefore, the Cu content is 0.5% or less, preferably 0.30% or less.

Ni: 0.001-0.5%
Ni, like Cu, is an element that has the effect of improving corrosion resistance. When adding Ni, in order to obtain the above effects, the Ni content is set to 0.001% or more. On the other hand, since Ni is an expensive alloying element, if the Ni content exceeds 0.5%, material costs will rise. Therefore, the Ni content is 0.5% or less, preferably 0.30% or less.

W: 0.001~0.05%
Like Ti and Nb, W is an element that forms fine carbides and contributes to further increase in strength (hardness). When adding W, the W content is set to 0.001% or more in order to obtain the above effect. On the other hand, if the W content exceeds 0.05%, the effect of addition is saturated and no effect commensurate with the content can be obtained, which is economically disadvantageous. Therefore, the W content is set to 0.05% or less, preferably 0.03% or less.

V:0.001~0.01%
Like N and Ti, V is an element that has the effect of further strengthening bainite by combining with C in steel to form carbides and being finely dispersed in bainite, which is the main phase. When V is added, the V content is set to 0.001% or more in order to obtain the above effect. On the other hand, if the V content exceeds 0.01%, the effect of addition is saturated and no effect commensurate with the content can be obtained, which is economically disadvantageous. Therefore, the V content is set to 0.01% or less, preferably 0.008% or less.

Ca: 0.0001-0.0050%
Ca is an element that has the effect of improving ductility by controlling the morphology of sulfide-based inclusions. When adding Ca, the Ca content is set to 0.0001% or more, preferably 0.0010% or more in order to obtain the above effects. On the other hand, when the Ca content exceeds 0.0050%, the amount of inclusions becomes excessive and the cleanliness of the steel decreases. Therefore, the Ca content is 0.0050% or less, preferably 0.0040% or less.

REM: 0.02% or less REM (rare earth metal), like Ca, is an element that has the effect of controlling the morphology of sulfide-based inclusions into fine, approximately spherical inclusions. However, when the REM content exceeds 0.02%, the amount of inclusions that serve as starting points for fatigue cracks becomes excessive, resulting in a decrease in corrosion resistance and fatigue properties. Therefore, the REM content is set to 0.02% or less, preferably 0.01% or less. On the other hand, the lower limit of the REM content is not particularly limited, but from the viewpoint of enhancing the effect of REM addition, when REM is added, it is preferable that the REM content is 0.001% or more.

[Microstructure]
The electric resistance welded steel pipe of the present invention has a microstructure consisting of a bainite phase and a ferrite phase with an area ratio of 10% or less. By having the above-mentioned microstructure, it becomes possible to have both desired high strength and excellent low-temperature toughness.

Ferrite area ratio: 10% or less When the ferrite area ratio exceeds 10%, desired strength cannot be achieved. Therefore, the area ratio of ferrite is 10% or less, preferably 5% or less. The lower the area ratio of ferrite, the better; therefore, it may be 0%. In other words, the microstructure of the electric resistance welded steel pipe of the present invention may be a bainite single-phase structure consisting only of bainite.

[Mechanical properties]
TS: 780 MPa or more In order to suitably use it as a structural member for construction machinery such as a crane lattice and a crane boom, the tensile strength (TS) is set to 780 MPa or more. On the other hand, the upper limit of the tensile strength is not particularly limited, but if the strength is too high, not only will it be difficult to mold it, but it will also be difficult to ensure low-temperature toughness. Therefore, the tensile strength is preferably 1180 MPa or less.

vE _-20 : 31J or more For use in cold regions, it is required to have excellent low-temperature impact properties. If the Charpy absorbed energy vE _-20 at -20°C, which is an index of low-temperature impact properties, is less than 31 J, there is a significant risk of brittle fracture when used in cold regions. Therefore, vE _-20 is set to 31J or more. On the other hand, the higher vE _-20 is, the better, so the upper limit of vE _-20 is not particularly limited. Here, vE _-20 is a V-notch with a width of 10 mm x height of 5 mm x length of 55 mm, notch angle of 45°, notch depth of 2 mm, and notch bottom radius of 0.25 mm, in accordance with the regulations of JIS Z 2242. It is defined as the absorbed energy measured by performing a Charpy impact test using a test piece at a test temperature of -20°. More specifically, it can be measured by the method described in Examples.

[Production method]
Next, a method for manufacturing an electric resistance welded steel pipe according to an embodiment of the present invention will be described.

In one embodiment of the present invention, a steel material is subjected to heating, hot rolling, water cooling, winding, roll forming, and ERW welding under predetermined conditions in order, thereby producing an ERW that satisfies the above conditions. Steel pipes can be manufactured.

[Steel material]
As the steel material, any steel material having the above-mentioned composition can be used. A steel slab is typically used as the steel material.

The steel material is obtained, for example, by melting molten steel having the above-mentioned composition using a conventional melting method such as a converter, and forming it into a shape such as a slab using a continuous casting method or an ingot-rolling method. be able to.

[heating]
Heating temperature: 1100℃~1250℃
First, the above steel material is heated to a heating temperature of 1100 to 1250°C. In order to precipitate fine precipitates that are effective for strengthening and grain refinement during the manufacturing process, it is necessary to sufficiently dissolve precipitates such as Ti and Nb contained in the steel material. For this purpose, it is necessary to set the heating temperature to 1100° C. or higher. On the other hand, when the heating temperature exceeds 1250° C., the austenite grains become significantly coarsened and the toughness decreases. Therefore, the heating temperature is 1250°C or lower, preferably 1220°C or lower.

[Hot rolling]
Finish rolling finish temperature: 700-830℃
Next, the heated steel material is hot-rolled at a finish rolling finish temperature of 700 to 830°C to form a hot-rolled steel strip. When the finish rolling end temperature is less than 700°C, the hardness of the steel material is high, making hot rolling and subsequent forming difficult, resulting in a decrease in productivity. Therefore, the finish rolling end temperature is set to 700° C. or higher. On the other hand, if the finish rolling end temperature is higher than 830°C, the crystal grains become coarse and the required strength cannot be achieved. In addition, the surface skin deteriorates, impairing the appearance of the product. Therefore, the finish rolling end temperature is 830°C or lower, preferably 810°C or lower.

Note that the above-mentioned hot rolling can include rough rolling and finish rolling. In other words, in the hot rolling, the steel material can be roughly rolled to form a sheet bar, and then the sheet bar can be finish rolled to form a hot rolled steel strip. In the present invention, rough rolling conditions are not particularly limited.

[Water cooling]
Next, the hot rolled steel strip is water cooled prior to winding. The water cooling can be performed by any method without particular limitation. For example, the hot rolled steel strip may be water cooled on a runout table.

Average cooling rate: 50°C/s or more Cooling end temperature: 500°C or less In order to obtain a bainite-based microstructure, the water cooling must be performed at an average cooling rate of 50°C/s or more until the cooling end temperature is 500°C or less. Cooling is important. If the average cooling rate is less than 50° C./s, the ferrite phase increases, resulting in a decrease in strength. Therefore, the average cooling rate is set to 50° C./s or more. Note that the average cooling rate is defined as the average cooling rate from the start of water cooling to the end of water cooling. The upper limit of the average cooling rate is not particularly limited, but if the average cooling rate is excessively high, it will be necessary to significantly increase the threading speed to achieve the desired coiling temperature, making operation difficult. Become. In some cases, this may lead to material instability. Therefore, the average cooling rate is preferably 150° C./s or less.

[Winding]
Winding temperature: 350-500℃
Next, the water-cooled hot-rolled steel strip is wound into a coil at a winding temperature of 350 to 500°C. If the coiling temperature is less than 350°C, the hot rolled steel sheet becomes excessively hard, making subsequent shaping difficult. Therefore, the winding temperature is set to 350°C or higher. On the other hand, if the winding temperature is higher than 500°C, the required strength cannot be obtained. Moreover, if the coiling temperature is higher than 500° C., upper bainite, which is disadvantageous to low-temperature toughness, is likely to be formed. Therefore, the winding temperature is set to 500°C or less.

[Roll forming]
Next, the hot rolled steel strip is roll formed into a substantially cylindrical open tube. The roll forming can be performed according to a conventional method.

[ERW welding]
Next, the ends of the open pipe in the width direction are abutted against each other and are electrically welded to form an electrical resistance welded steel pipe. The ends of the open tube in the width direction can be brought together by any method, but usually, it can be done using a squeeze roll. Further, the electric resistance welding is preferably performed by, for example, high frequency resistance welding or induction heating welding.

Hereinafter, in order to confirm the effects of the present invention, an electric resistance welded steel pipe was manufactured according to the procedure described below, and its characteristics were evaluated.

First, molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (steel material) using a continuous casting method. The slab was sequentially subjected to heating, hot rolling, water cooling, winding, roll forming, and electric resistance welding under the conditions shown in Table 2 to produce an electric resistance welded steel pipe. In the water cooling, the hot rolled steel strip was cooled at the average cooling rate shown in Table 2 to a cooling end temperature of 500° C. or less. The cooling end temperature was adjusted so that the winding temperature was as shown in Table 2.

The outer diameter of the electric resistance welded steel pipe was 114.3 mm, and the wall thickness was as shown in Table 2. The obtained electric resistance welded steel pipe was not subjected to heat treatment such as quenching or tempering. However, for comparison, Comparative Example No. No. 21 ERW steel pipe was subjected to heat treatment consisting of quenching and tempering after pipe making. The quenching was performed by holding at 925° C. for 5 minutes and then water quenching. Moreover, the said tempering was performed by holding at 550 degreeC for 10 minutes.

The resulting electrical resistance welded steel pipe was subjected to structural observation, tensile test, and Charpy impact test according to the following procedures.

(organizational observation)
Ten specimens for microstructural observation (observation of cross section in the tube circumference (C) direction) were collected from the obtained electric resistance welded steel pipe. After polishing the cross section of the test piece, it was corroded using a nital solution to reveal a fine structure. Next, using an optical microscope, the surface of the test piece was imaged in 10 or more fields of view per test piece at a magnification of 200 times to obtain an image of the microstructure. Analyze the images obtained using an image analysis device, identify the types of structures such as bainite and ferrite, and calculate the area ratio of ferrite as the average value of the observation results in all fields of view of 10 test pieces x 10 fields or more. Calculated. The results obtained are shown in Table 2. Note that Comparative Example No. The ERW steel pipe No. 21 had a ferrite area ratio of 0% and a microstructure consisting of 100% tempered martensite.

(Tensile test)
A JIS No. 12 B tensile test piece (gauge length: 50 mm) was taken from the obtained electric resistance welded steel pipe in accordance with the provisions of JIS Z 2241 so that the tensile direction was in the direction of the pipe axis (L). Next, using the tensile test piece, a tensile test was conducted according to the provisions of JIS Z 2241, and 0.2% proof stress (YS), tensile strength (TS), and elongation (El) were determined. The results obtained are shown in Table 2.

(Charpy impact test)
From the obtained ERW steel pipe, a V-notch test piece (width 10 mm x height 5 mm x length 55 mm, notch angle 45° , notch depth 2 mm, notch bottom radius 0.25 mm) was sampled from the tube axis direction. A Charpy impact test was conducted using the test piece at a test temperature of -20°C, and the Charpy absorbed energy vE _-20 at -20°C was measured. The results obtained are shown in Table 2.

(welding quality)
In order to evaluate the welding quality, a 90-degree flattening test was conducted based on the method described in JIS G3445, in which the welded part was turned right sideways and crushed with an upper limit flat plate. In the flattening test, welding quality is considered to be good if the flattening height (distance of the flat tube) at which cracks occur in the welded part is 7/8 x tube outer diameter (D) or less; If it exceeded the tube outer diameter (D), it was determined that the welding quality was poor.

(Surface skin)
The outer and inner surfaces of the obtained electric resistance welded steel pipes were visually inspected, and if rough skin, residual scale, pock-like irregularities, etc. were present on the surface, it was determined that the surface texture was poor.

As can be seen from the results shown in Table 2, ERW steel pipes that meet the conditions of the present invention have high strength and excellent low-temperature toughness, and are also superior to seamless steel pipes that are manufactured by heat treatment after pipe manufacturing. It also had excellent surface properties. It also had excellent weldability. In contrast, ERW steel pipes that did not meet the conditions of the present invention were inferior in at least one of strength, low-temperature impact properties, surface texture, and welding quality. In addition, Comparative Example No. 1 was heat-treated after pipe making. Although No. 21 had excellent strength and low-temperature impact properties, the surface skin became pocked and the beauty was poor.

Claims (4)

In mass%,
C: 0.050-0.15%,
Si: 0.001% or more, less than 0.050%,
Mn: 1.5-2.5%,
P: 0.1% or less,
S: 0.01% or less,
Al: 0.01-0.10%,
Ti: 0.05-0.20%,
Nb: 0.01-0.10%, and N: 0.0005-0.01%
, with the remainder consisting of Fe and unavoidable impurities, and having a component composition in which the ratio of Mn content to Si content, Mn/Si, is more than 36,
It has a microstructure consisting of a bainite phase and a ferrite phase with an area ratio of 10% or less,
An electric resistance welded steel pipe having mechanical properties such as a tensile strength of 780 MPa or more and a Charpy absorbed energy of 31 J or more at -20°C. The component composition is in mass%, and further,
Mo: 0.05-0.30%,
Cr: 0.05-0.50%,
Cu: 0.001 to 0.5%,
Ni: 0.001 to 0.5%,
W: 0.001-0.05%,
V: 0.001 to 0.01%,
The electric resistance welded steel pipe according to claim 1, containing at least one selected from the group consisting of Ca: 0.0001 to 0.0050%, and REM: 0.02% or less. In mass%,
C: 0.050-0.15%,
Si: 0.001% or more, less than 0.050%,
Mn: 1.5-2.5%,
P: 0.1% or less,
S: 0.01% or less,
Al: 0.01-0.10%,
Ti: 0.05-0.20%,
Nb: 0.01-0.10%, and N: 0.0005-0.01%
, the balance is Fe and unavoidable impurities, and the ratio Mn/Si of Mn content to Si content is more than 36, heated to a heating temperature of 1100 to 1250 ° C.,
The heated steel material is hot-rolled at a finish rolling finish temperature of 700 to 830°C to form a hot-rolled steel strip,
Water-cooling the hot rolled steel strip at an average cooling rate of 50° C./s or more to a cooling end temperature of 500° C. or less,
Coiling the water-cooled hot rolled steel strip at a coiling temperature of 350 to 500°C,
Roll forming the hot rolled steel strip into a substantially cylindrical open tube;
A method for manufacturing an ERW steel pipe, comprising ERW welding the open pipe to produce an ERW steel pipe,
The electric resistance welded steel pipe is
has a microstructure consisting of a bainite phase and a ferrite phase with an area ratio of 10% or less, and
A method for manufacturing an electric resistance welded steel pipe having mechanical properties such as a tensile strength of 780 MPa or more and a Charpy absorbed energy of 31 J or more at -20°C . The component composition is in mass%, and further,
Mo: 0.05-0.30%,
Cr: 0.05-0.50%,
Cu: 0.001 to 0.5%,
Ni: 0.001 to 0.5%,
W: 0.001-0.05%,
V: 0.001 to 0.01%,
The method for producing an electric resistance welded steel pipe according to claim 3, containing at least one selected from the group consisting of Ca: 0.0001 to 0.0050%, and REM: 0.02% or less.