# Patent

## Title
High-strength thick-walled electric resistance welded steel pipe having excellent low-temperature toughness and method of manufacturing the same

## Abstract

A high-strength thick-walled electric resistance welded steel pipe has excellent low-temperature toughness and excellent HIC resistance and a yield strength of 400 MPa or more.
The steel has a chemical composition consisting of C: 0.025% to 0.084%, Si: 0.10% to 0.30%, Mn: 0.70% to 1.80%, controlled amounts of P, S, Al, N, and O, Nb: 0.001% to 0.065%, V: 0.001% to 0.065%, Ti: 0.001% to 0.033%, and Ca: 0.0001% to 0.0035% on a mass percent basis and the remainder being Fe and incidental impurities, and satisfies Pcm of 0.20 or less.

## Background
TECHNICAL FIELD
This disclosure relates to a high-strength thick-walled electric resistance welded steel pipe and more particularly to an improvement in the reliability of an electric resistance welded portion. The term “high-strength”, as used herein, refers to a yield strength YS of 400 MPa or more. The term “thick-walled”, as used herein, refers to a thickness of 16 to 32 mm.
BACKGROUND
Electric resistance welded steel pipes are used to drill or transport oil or natural gas. However, because of their low reliability due to low toughness of electric resistance welded portions, the electric resistance welded steel pipes are only used for applications that do not require severe specifications with respect to low-temperature toughness and resistance to hydrogen induced cracking (hereinafter also referred to as HIC)
To address such a problem, for example, Japanese Examined Patent Application Publication No. 01-58253 discloses a method of manufacturing a high-strength electric resistance welded steel pipe having excellent low-temperature toughness that includes performing electric resistance welding of a steel sheet having a predetermined chemical composition, heating the electric resistance welded portion at a temperature in the range of 790° C. to 1050° C. for at least 5 seconds, and rapidly cooling the electric resistance welded portion at a temperature of 770° C. to 890° C. at a cooling rate of 30° C./s to 150° C./s, thereby forming an electric resistance welded portion having a fine acicular ferrite microstructure. However, the steel sheet used in Japanese Examined Patent Application Publication No. 01-58253 has a small thickness of approximately 8.0 mm. Furthermore, the electric resistance welded portion of the steel pipe thus manufactured has a fracture (appearance) transition temperature of approximately −40° C. at most and, therefore, the low-temperature toughness of the electric resistance welded portion is not significantly improved. Japanese Examined Patent Application Publication No. 03-60888 discloses a method of manufacturing a high-strength electric resistance welded steel pipe having excellent low-temperature toughness that includes performing electric resistance welding of a steel sheet having a predetermined chemical composition, heating the electric resistance welded portion at a temperature of 790° C. to 1050° C. for at least 5 seconds, rapidly cooling the electric resistance welded portion at a temperature of 750° C. to 950° C. at a cooling rate of 30° C./s to 150° C./s, thereby forming an electric resistance welded portion having a fine acicular ferrite microstructure, and heating the electric resistance welded portion at a temperature of 400° C. to 700° C. for 1 minute or less to perform a stress relief heat treatment.
Japanese Examined Patent Application Publication No. 07-42509 discloses a method of manufacturing a high-strength electric resistance welded steel pipe having excellent low-temperature toughness that includes performing electric resistance welding of a steel sheet having a predetermined chemical composition, heating the electric resistance welded portion at a temperature of 850° C. to 1000° C., rapidly cooling the electric resistance welded portion from the Ar3 transformation point or higher to a finish cooling temperature in the range of (Ar1 transformation point−50° C.) to (Ar1 transformation point−100° C.) at a cooling rate of more than 30° C./s to 100° C./s, and performing weak cooling of the electric resistance welded portion.
However, application of the method disclosed in Japanese Examined Patent Application Publication No. 03-60888 to a thick-walled electric resistance welded steel pipe having a thickness of more than 20 mm requires a huge furnace for stress relief (stress removal) heat treatment, thus causing a economical problem. In the application of the method disclosed in Japanese Examined Patent Application Publication No. 07-42509 to a thick-walled electric resistance welded steel pipe having a thickness of more than 20 mm, it is difficult to heat the thick-walled electric resistance welded steel pipe from the outer surface thereof so that the central portion temperature in the direction of thickness of pipe is 850° C. to 1000° C.
It could therefore be helpful to provide a high-strength thick-walled electric resistance welded steel pipe having not only excellent low-temperature toughness but also excellent HIC resistance. The term “excellent low-temperature toughness”, as used herein, means that both the base steel portion and the electric resistance welded portion have a circumferential absorbed energy vE−50 of 150 J or more at a test temperature of −50° C. in a Charpy impact test in accordance with JIS Z 2242. The term “excellent HIC resistance”, as used herein, means that both the base steel portion and the electric resistance welded portion have a crack area ratio CAR of 5% or less after immersion in a NACE Solution A (0.5% CH3COOH+5% NaCl+saturated H2S) specified in NACE TM0284.
SUMMARY
We thus provide:
(1) A high-strength thick-walled electric resistance welded steel pipe having excellent low-temperature toughness and excellent HIC resistance characterized by having; a chemical composition consisting of C: 0.025% to 0.084%, Si: 0.10% to 0.30%, Mn: 0.70% to 1.80%, P: 0.001% to 0.018%, S: 0.0001% to 0.0029%, Al: 0.01% to 0.10%, Nb: 0.001% to 0.065%, V: 0.001% to 0.065%, Ti: 0.001% to 0.033%, Ca: 0.0001% to 0.0035%, N: 0.0050% or less, O: 0.0030% or less on a mass percent basis, and the remainder being Fe and incidental impurities, wherein Pcm defined by the formula (1) is 0.20 or less,

Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B  (1)

wherein C, Si, Mn, Cu, Ni, Cr, Mo, V, and B denote the amounts (mass %) of the corresponding elements,
a microstructure which includes 90% by area or more of quasi-polygonal ferrite having a grain size of 10 μm or less in each of the base steel portion and the electric resistance welded portion of the steel pipe,
a yield strength YS of 400 MPa or more,
and
an absorbed energy vE−50 of 150 J or more at −50° C. in a Charpy impact test.
(2) The high-strength thick-walled electric resistance welded steel pipe according to (1), characterized in that the chemical composition further contains B: 0.0030% or less on a mass percent basis.
(3) The high-strength thick-walled electric resistance welded steel pipe according to (1) or (2), characterized in that the chemical composition further contains at least one selected from Cu: 0.001% to 0.350%, Ni: 0.001% to 0.350%, Mo: 0.001% to 0.350%, and Cr: 0.001% to 0.700% on a mass percent basis.
(4) The high-strength thick-walled electric resistance welded steel pipe according to any one of (1) to (3), characterized in that the total amount of Si, Mn, Al, Ca, and Cr in inclusions having an equivalent circular diameter of 2 μm or more contained in the electric resistance welded portion is 0.0089% or less on a mass percent basis.
(5) A method of manufacturing a high-strength thick-walled electric resistance welded steel pipe having excellent low-temperature toughness and excellent HIC resistance, including a hot-rolling step of producing a hot-rolled steel strip from steel by heating, hot-rolling, cooling, and coiling, and a pipe-forming step of continuously roll-forming the hot-rolled steel strip after the hot-rolling step to form a tubular product having a substantially circular cross section and then butt-welding circumferential ends of the tubular product by electric resistance welding to produce an electric resistance welded steel pipe, characterized in that;
the steel has a chemical composition consisting of C: 0.025% to 0.084%, Si: 0.10% to 0.30%, Mn: 0.70% to 1.80%, P: 0.001% to 0.018%, S: 0.0001% to 0.0029%, Al: 0.01% to 0.10%, Nb: 0.001% to 0.065%, V: 0.001% to 0.065%, Ti: 0.001% to 0.033%, Ca: 0.0001% to 0.0035%, N: 0.0050% or less, O: 0.0030% or less on a mass percent basis, and the remainder being Fe and incidental impurities, wherein Pcm defined by the formula (1) is 0.20 or less,

Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B  (1)

wherein C, Si, Mn, Cu, Ni, Cr, Mo, V, and B denote the amounts (mass %) of the corresponding elements,
the hot-rolling step is performed by heating the steel to a temperature in the range of 1200° C. to 1280° C., maintaining the temperature for 90 min or more, hot-rolling the steel at a hot-rolling reduction of 20% or more in an unrecrystallized austenite region, after the completion of the hot-rolling, cooling the steel to a finish cooling temperature of 630° C. or less at a cooling rate in the range of 7° C./s to 49° C./s, the cooling rate being the average cooling rate at a temperature from 780° C. to 630° C. at the central portion in the thickness direction, and coiling the steel at a coiling temperature of 400° C. or more and less than 600° C.,

the pipe-forming step is followed by a heat treatment that includes heating the electric resistance welded portion of the electric resistance welded steel pipe on a production line such that the electric resistance welded portion has a temperature in the range of 800° C. to 1150° C. over the total wall thickness, then cooling the electric resistance welded portion to a finish cooling temperature of 630° C. or less at a cooling rate in the range of 7° C./s to 49° C./s, the cooling rate being the average cooling rate at a temperature from 780° C. to 630° C. at the central portion in the thickness direction, and then allowing the electric resistance welded portion to air-cool,

and

the base steel portion and the electric resistance welded portion of the electric resistance welded steel pipe have a yield strength YS of 400 MPa or more and an absorbed energy vE−50 of 150 J or more at −50° C. in a Charpy impact test.

(6) The method of manufacturing a high-strength thick-walled electric resistance welded steel pipe according to (5), characterized in that when a tapered groove is formed in the end faces of the hot-rolled steel strip in the width direction by fin pass forming during the roll-forming in the pipe-forming step, the distance between a taper starting position of the tapered groove and a surface that will become a pipe outer surface or a surface that will become a pipe inner surface in the steel strip thickness direction ranges from 2% to 60% of the hot-rolled steel strip thickness.
(7) The method of manufacturing a high-strength thick-walled electric resistance welded steel pipe according to (5) or (6), characterized in that the atmospheric oxygen partial pressure in the electric resistance welding in the pipe-forming step is adjusted at 900/foxy mass ppm or less, wherein the foxy represents the degree of oxidizability of molten steel defined by the following formula (2),

foxy=Mn+10(Si+Cr)+100Al+1000Ca  (2)

wherein Mn, Si, Cr, Al, and Ca denote the amounts (mass %) of the corresponding elements.
(8) The method for manufacturing a high-strength thick-walled electric resistance welded steel pipe according to any one of (5) to (7), characterized in that the chemical composition of the steel further contains B: 0.0030% or less on a mass percent basis.
(9) The method of manufacturing a high-strength thick-walled electric resistance welded steel pipe according to any one of (5) to (8), characterized in that the chemical composition of the steel further contains at least one selected from Cu: 0.001% to 0.350%, Ni: 0.001% to 0.350%, Mo: 0.001% to 0.350%, and Cr: 0.001% to 0.700% on a mass percent basis.
(10) The method of manufacturing a high-strength thick-walled electric-resistance-welded steel pipe according to any one of (5) to (9), characterized in that the cooling in the heat treatment includes installing at least two lines of cooling headers in a conveying direction above the electric resistance welded portion, the cooling headers being coupled to a nozzle through which a rod-like flow of cooling water can be ejected at a water flow rate of 1 m3/m2·min or more, and ejecting the rod-like flow of cooling water through the nozzle at a rate of 1 m/s or more.
(11) The method of manufacturing a high-strength thick-walled electric resistance welded steel pipe according to (10), characterized in that the plurality of cooling headers are configured to independently control the ejection of cooling water.
A high-strength thick-walled electric resistance welded steel pipe that includes a base steel portion and an electric resistance welded portion having excellent low-temperature toughness and excellent HIC resistance can be easily and stably manufactured. Thus, our pipes and methods have an industrially advantageous effect. The high-strength thick-walled electric resistance welded steel pipe includes an electric resistance welded portion having excellent low-temperature toughness and excellent HIC resistance and therefore having improved reliability. Thus, the high-strength thick-walled electric resistance welded steel pipe can be stably used in applications that require excellent low-temperature toughness and excellent HIC resistance.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph of the vE−50 of an electric resistance welded portion as a function of the heating temperature and the cooling rate after heating.
FIG. 2 is a graph of the crack area ratio CAR after the immersion of an electric resistance welded portion in a NACE Solution A as a function of the heating temperature and the cooling rate after heating.
FIG. 3 is a graph of the vE−50 of an electric resistance welded portion and the crack area ratio CAR after the immersion of the electric resistance welded portion in a NACE Solution A as a function of the total amount (mass ppm) of Si, Mn, Al, Ca, and Cr in inclusions having an equivalent circular diameter of 2 μm or more contained in the electric resistance welded portion.



DETAILED DESCRIPTION
We systematically and extensively studied the effects of the microstructure and the oxides (inclusions) on the low-temperature toughness and the HIC resistance of the base steel portion and the electric resistance welded portion of a thick-walled electric resistance welded steel pipe having a thickness of more than 16 mm. We found that an electric resistance welded steel pipe having improved low-temperature toughness and HIC resistance both in the base steel portion and in the electric resistance welded portion, and thus giving particularly improved reliability can be manufactured by controlling the chemical composition of steel used and the hot-rolling conditions within particular ranges in conjunction with a heat treatment method after electric resistance welding.
First, the results of experiments on the improvement in the reliability of an electric resistance welded portion we performed will be described below.
A thick-walled electric resistance welded steel pipe (having an outer diameter of 660.4 mm) was prepared that had a chemical composition of 0.01% to 0.20% C, 0.01% to 1.00% Si, 0.50% to 3.00% Mn, 0.001% to 0.100% Al, 0% to 0.150% Nb, 0% to 0.150% V, 0% to 0.150% Ti, 0% to 0.0050% Ca, and 0.005% to 0.0100% N on a mass percent basis and had a thickness in the range of 16 to 32 mm.
The electric resistance welded portion of the electric resistance welded steel pipe was then subjected to a heat treatment (heat treatment after electric resistance welding) using an induction heating apparatus at different heating temperatures and under different cooling conditions after the heating. Test specimens were sampled from the electric resistance welded portion after the heat treatment and were subjected to an impact test, a HIC test, and a measurement of the amount of inclusions. These test and measurement methods are described below.
(1) Impact Test
V-notch Charpy impact test specimens (having a thickness of 10 mm) were sampled in the circumferential direction from the electric resistance welded portion in accordance with JIS Z 2242 such that the notch coincided with the central portion of the electric resistance-welded portion. The absorbed energy vE−50 of each test specimen was measured in a Charpy impact test at a temperature of −50° C. The number of test specimens in measurement was three.
(2) HIC Test
Immersion test specimens (dimensions: 10 mm in thickness×20 mm in width×160 mm in length) were sampled from the electric resistance welded portion and were immersed in a NACE Solution A (0.5% CH3COOH+5% NaCl+saturated H2S) specified in NACE TM0284 for 96 hours. After the immersion, the crack area ratio CAR of each test specimen was determined using an ultrasonic flaw inspection method.
(3) Measurement of Amount of Inclusions
Sample sheets (dimensions: 2 mm in width×thickness:wall thickness×length:wall thickness) were cut from the center of the electric resistance welded portion and were subjected to electroextraction in a 10% AA electrolyte solution. After electroextraction, inclusions (having an equivalent circular diameter of 2 μm or more) were extracted with a filter mesh having an opening size of 2 μm and were subjected to alkali fusion. The Si, Mn, Al, Ca, and Cr contents were measured in an inductively coupled plasma (ICP) analysis. The total amount of Si, Mn, Al, Ca, and Cr was calculated. The total amount of Si, Mn, Al, Ca, and Cr in the inclusions having an equivalent circular diameter of 2 μm or more was considered to be the amount of inclusions in the electric resistance welded portion.
The results are shown in FIGS. 1 and 2 as a function of the heating temperature and the cooling rate after the heating in the heat treatment. FIG. 1 shows the vE−50, and FIG. 2 shows the CAR. The cooling rate after the heating was the average cooling rate at a temperature from 780° C. to 630° C. at the central portion in the thickness direction. FIGS. 1 and 2 show that when the heating temperature of the electric resistance welded portion ranges from 800° C. to 1150° C. and the cooling rate after the heating ranges from 7° C./s to 49° C./s on average between 780° C. and 630° C., the electric resistance welded portion has excellent low-temperature toughness of vE−50=150 J or more and excellent HIC resistance of CAR=5% or less.
Thus, we found that an electric resistance welded portion subjected to the heat treatment under the conditions as described above after electric resistance welding had excellent low-temperature toughness and excellent HIC resistance.
On the basis of these results, FIG. 3 shows the relationship between vE−50, CAR, and the total amount of Si, Mn, Al, Ca, and Cr in inclusions having an equivalent circular diameter of 2 μm or more.
FIG. 3 shows that when the total amount of Si, Mn, Al, Ca, and Cr in inclusions having an equivalent circular diameter of 2 μm or more is more than 89 mass ppm, the electric resistance welded portion has decreased vE−50, increased CAR, that is, deteriorates low-temperature toughness and deteriorates HIC resistance.
Inclusions having an equivalent circular diameter of 2 μm or more in an electric resistance welded portion were paid attention as inclusions that had an influence on the characteristics of the electric resistance welded portion. We found that when the amount of inclusions having an equivalent circular diameter of 2 μm or more in the electric resistance welded portion exceeded a predetermined value, the electric resistance welded portion had significantly deteriorated low-temperature toughness and deteriorated HIC resistance. The present invention has been accomplished on the basis of these findings and is summarized as described below.
The high-strength thick-walled electric resistance welded steel pipe has a thick wall having a thickness in the range of 16 to 32 mm and includes a base steel portion and an electric resistance welded portion that have a yield strength YS of 400 MPa or more and excellent low-temperature toughness represented by a circumferential absorbed energy vE−50 of 150 J or more at a test temperature of −50° C. in a Charpy impact test. The electric resistance welded steel pipe also has excellent HIC resistance represented by a crack area ratio CAR of 5% or less after the immersion of the electric resistance welded steel pipe in a NACE Solution A for 96 hours.
The reason for limiting the chemical composition of the electric resistance welded steel pipe will be described below. The mass % in the following chemical composition is simply referred to as %.
C: 0.025% to 0.084%
C forms hard phases such as pearlite, quasi-pearlite, cementite, bainite, or martensite, and increases the strength of a steel pipe. C has an influence on the formation of oxides in an electric resistance welded portion through solidifying point depression or CO-forming reaction with atmospheric O2 during electric resistance welding. Such effects require a C content of 0.025% or more. The desired yield strength YS of 400 MPa or more cannot be achieved at a C content of less than 0.025%. A high C content of more than 0.084% results in a hard phase percentage of more than 10% in an electric resistance welded portion and a base steel portion, causing deterioration of low-temperature toughness, that is, an absorbed energy of less than 150 J at −50° C. in a Charpy impact test. This also results in a crack area ratio CAR of more than 5% after immersion in a NACE Solution A specified in NACE TM0284 for 96 hours, causing deterioration of HIC resistance. Thus, the C content is 0.025% to 0.084%, preferably 0.030% to 0.060%.
Si: 0.10% to 0.30%
Si increases the strength of an electric resistance welded steel pipe through solid solution strengthening. Si has higher affinity with O than Fe and can form eutectic oxides having a high viscosity with Mn oxides in an electric resistance welded portion. A Si content of less than 0.10% results in a high concentration of Mn in eutectic oxides and a melting point of the oxides higher than the molten steel temperature, thus often causing the oxides to remain in an electric resistance welded portion. This results in more than 89 mass ppm of Si, Mn, and Al in total in inclusions having a size of 2 μm or more contained in the electric resistance welded portion, causing deterioration of low-temperature toughness and deterioration of HIC resistance. Thus, the Si content is limited to 0.10% or more.
A Si content of more than 0.30% results in a high concentration of Si in eutectic oxides and a melting point of the oxides higher than the molten steel temperature, thus often causing the oxides to remain in an electric resistance welded portion. This results in more than 89 mass ppm of Si, Mn, and Al in total in inclusions having a size of 2 μm or more and an increased absolute amount of oxides, causing deterioration of low-temperature toughness and deterioration of HIC resistance. Thus, the Si content is limited to 0.30% or less. The Si content is preferably 0.15% to 0.25%.
Mn: 0.70% to 1.80%
Mn increases the strength of an electric resistance welded steel pipe through solid solution strengthening and transformation strengthening. Mn has higher affinity with O than Fe and can form eutectic oxides having a high viscosity with Si oxides in an electric resistance welded portion. A Mn content of less than 0.70% results in a high concentration of Si in eutectic oxides and a melting point of the oxides higher than the molten steel temperature, thus often causing the oxides to remain in an electric resistance welded portion. This results in more than 89 mass ppm of Si, Mn, and Al in total in inclusions having a size of 2 μm or more contained in the electric resistance welded portion, causing deterioration of low-temperature toughness and deterioration of HIC resistance. A Mn content of less than 0.70% results in a base steel portion and an electric resistance welded portion formed of coarse polygonal ferrite having a grain size dα of more than 10 μm, thus causing deterioration of low-temperature toughness. Thus, the Mn content is limited to 0.70% or more.
A high Mn content of more than 1.80% results in a high concentration of Mn in eutectic oxides and a melting point of the oxides higher than the molten steel temperature, thus often causing the oxides to remain in an electric resistance welded portion. This results in more than 89 mass ppm of Si, Mn, and Al in total in inclusions having an equivalent circular diameter of 2 μm or more contained in the electric resistance welded portion and an increased absolute amount of oxides. A Mn content of more than 1.80% results also in a hard phase percentage of more than 10% by area in a base steel portion and an electric resistance welded portion. This causes deterioration of low-temperature toughness and deterioration of HIC resistance. Thus, the Mn content is limited to 0.70% to 1.80%. The Mn content is preferably 0.85% to 1.65%.
P: 0.001% to 0.018%
P is an element that contributes to strengthening but deteriorates toughness because of segregation at grain boundaries or other regions. P cosegregates with Mn and deteriorates HIC resistance of a base steel portion and an electric resistance welded portion. Thus, it is desirable to minimize P content, but an extreme reduction in P content increases the refining cost. A P content of more than 0.018% results in remarkably deterioration of low-temperature toughness and deterioration of HIC resistance. Thus, the P content is limited to 0.001% to 0.018%, preferably 0.001% to 0.013%.
S: 0.0001% to 0.0029%
S precipitates as MnS in an electric resistance welded portion and a base steel portion and deteriorates low-temperature toughness and HIC resistance. Thus, it is desirable to minimize S content, but an excessive reduction in S content increases the refining cost. A S content of more than 0.0029% results in remarkably deterioration of low-temperature toughness and low HIC resistance. Thus, the S content is limited to 0.0001% to 0.0029%, preferably 0.0001% to 0.0019%.
Al: 0.01% to 0.10%
Al is an element that acts as a deoxidizing agent in a steel making process. Al precipitates as AlN in austenite, causes suppression of grain growth during heating of austenite, and improves low-temperature toughness. Al has higher affinity with O than Si or Mn and forms oxides as a solid solution in Mn—Si eutectic oxides such as 2MnO—SiO2 (Tephroite). Such an effect requires an Al content of 0.01% or more. An Al content of less than 0.01% results in insufficient deoxidation ability in a steel making process, low cleanliness of the steel, and more than 89 ppm of Si, Mn, and Al in total in inclusions having an equivalent circular diameter of 2 μm or more contained in an electric resistance welded portion. This causes deterioration of low-temperature toughness and deterioration of HIC resistance.
An Al content of more than 0.10% results in a high concentration of Al in eutectic oxides and a melting point of the oxides higher than the molten steel temperature, thus often causing the oxides to remain in an electric resistance welded portion. Thus, the total amount of Si, Mn, and Al in inclusions having an equivalent circular diameter of 2 μm or more contained in an electric resistance welded portion exceeds 89 ppm. This causes deterioration of low-temperature toughness and deterioration of HIC resistance. Thus, the Al content is limited to 0.01% to 0.10%, preferably 0.03% to 0.08%.
Nb: 0.001% to 0.065%
Nb precipitates mainly as carbides and increases the strength of an electric resistance welded steel pipe. Such an effect requires a Nb content of 0.001% or more. A high Nb content of more than 0.065% results in residual large undissolved Nb carbonitrides, causing deterioration of low-temperature toughness and deterioration of HIC resistance. Thus, the Nb content is limited to 0.001% to 0.065%, preferably 0.005% to 0.050%.
V: 0.001% to 0.065%
Like Nb, V precipitates mainly as carbides and increases the strength of an electric resistance welded steel pipe. Such an effect requires a V content of 0.001% or more. A high V content of more than 0.065% results in residual large undissolved V carbonitrides, causing deterioration of low-temperature toughness and deterioration of HIC resistance. Thus, the V content is limited 0.001% to 0.065%, preferably 0.005% to 0.050%.
Ti: 0.001% to 0.033%
Like Nb and V, Ti precipitates mainly as carbides and increases the strength of an electric resistance welded steel pipe. Such an effect requires a Ti content of 0.001% or more. A high Ti content of more than 0.033% results in residual large undissolved Ti carbonitrides, causing deterioration of low-temperature toughness and deterioration of HIC resistance. Thus, the Ti content is limited to 0.001% to 0.033%, preferably 0.005% to 0.020%.
Ca: 0.0001% to 0.0035%
Ca is an element that makes sulfides in the steel spherical and improves low-temperature toughness and HIC resistance in the vicinity of an electric resistance welded portion. Such an effect requires a Ca content of 0.0001% or more. A Ca content of more than 0.0035% results in a high concentration of Ca in oxides because of high affinity of Ca with O. This results in a melting point of the oxides higher than the molten steel temperature, thus often causing the oxides to remain in an electric resistance welded portion. Thus, the total amount of Si, Mn, Al, Ca, and Cr in inclusions having an equivalent circular diameter of 2 μm or more contained in the electric resistance welded portion exceeds 89 mass ppm, and the absolute amount of oxide increases. This causes deterioration of low-temperature toughness and deterioration of HIC resistance. Thus, the Ca content is limited to 0.0001% to 0.0035%, preferably 0.0002% to 0.0028%.
N: 0.0050% or Less
N in an electric resistance welded portion and a base steel portion precipitates as Ti(N, C) or remains as solid solution and deteriorates low-temperature toughness and HIC resistance. It is therefore desirable to minimize N content, but an excessive reduction in N content increases the refining cost. Thus, it is desirable to limit the N content to 0.0001% or more. A N content of more than 0.0050% results in remarkably deterioration of low-temperature toughness and deterioration of HIC resistance. Thus, the N content is limited to 0.0050% or less, preferably 0.0001% to 0.0040%.
O: 0.0030% or Less
O in an electric resistance welded portion and a base steel portion remains as oxide inclusions and deteriorates low-temperature toughness and HIC resistance. Thus, it is desirable to minimize O content. An O content of more than 0.0030% results in remarkably deterioration of low-temperature toughness and deterioration of HIC resistance. Thus, the O content is limited to 0.0030% or less. However, an excessive reduction in O content increases the refining cost. Thus, the O content is preferably 0.0001% or more. The O content is preferably 0.0020% or less.
These elements are basic. In addition to these elements, B: 0.0030% or less and/or at least one selected from Cu: 0.001% to 0.350%, Ni: 0.001% to 0.350%, Mo: 0.001% to 0.350%, and Cr: 0.001% to 0.700% may be contained, if necessary.
B: 0.0030% or Less
B improves quench hardenability and thereby contributes to strengthening of an electric resistance welded steel pipe. The B content is preferably 0.0001% or more to obtain such an effect. However, such an effect levels off at a B content of more than 0.0030% and is not expected to be proportional to the B content beyond this threshold.
Thus, when an electric resistance welded steel pipe contains B, the B content is preferably limited to 0.0030% or less, more preferably 0.0020% or less.
At least one selected from Cu: 0.001% to 0.350%, Ni: 0.001% to 0.350%, Mo: 0.001% to 0.350%, and Cr: 0.001% to 0.700%
Cu, Ni, Mo, and Cr are elements that contribute to strengthening of a base steel portion and an electric resistance welded portion of a thick-walled electric resistance welded steel pipe and controlling of coarse polygonal ferrite. At least one of Cu, Ni, Mo, and Cr may be contained, if necessary.
Cu ensures the desired strengthening and suppresses the formation of coarse polygonal ferrite having a grain size dα of more than 10 μm because of improved quench hardenability of a base steel portion and an electric resistance welded portion of a thick-walled electric resistance welded steel pipe. Cu also improves the HIC resistance of an electric resistance welded steel pipe. The Cu content is preferably 0.001% or more to obtain such effects. However, such effects level off at a Cu content of more than 0.350% and are not expected to be proportional to the Cu content beyond this threshold. Thus, when an electric resistance welded steel pipe contains Cu, the Cu content is preferably limited to 0.001% to 0.350%, more preferably 0.05% to 0.290%.
Like Cu, Ni ensures the desired strengthening and suppresses the formation of coarse polygonal ferrite having a grain size dα of more than 10 μm because of improved quench hardenability of a base steel portion and an electric resistance welded portion of a thick-walled electric resistance welded steel pipe. Ni also improves the HIC resistance of an electric resistance welded steel pipe. The Ni content is preferably 0.001% or more to obtain such effects. Such effects level off at a Ni content of more than 0.350% and are not expected to be proportional to the Ni content beyond this threshold. Thus, when an electric resistance welded steel pipe contains Ni, the Ni content is preferably limited to 0.001% to 0.350%, more preferably 0.05% to 0.290%.
Like Ni and Cu, Mo ensures the desired strengthening and suppresses the formation of coarse polygonal ferrite having a grain size dα of more than 10 μm because of improved quench hardenability of a base steel portion and an electric resistance welded portion of a thick-walled electric resistance welded steel pipe. Mo also improves the HIC resistance of an electric resistance welded steel pipe. The Mo content is preferably 0.001% or more to obtain such effects. Such effects level off at a Mo content of more than 0.350% and are not expected to be proportional to the Mo content beyond this threshold. Thus, when an electric resistance welded steel pipe contains Mo, the Mo content is preferably limited to 0.001% to 0.350%, more preferably 0.05% to 0.290%.
Like Mn, Cr contributes to strengthening of an electric resistance welded steel pipe through transformation strengthening, ensures the desired strengthening, and causes suppression of the formation of coarse polygonal ferrite. The Cr content is preferably 0.001% or more to obtain such effects. Cr has higher affinity with O than Fe and tends to form oxides. A Cr content of more than 0.700% results in a high concentration of Cr in oxides and a melting point of the oxides higher than the molten steel temperature, thus often causing the oxides to remain in an electric resistance welded portion. Thus, the absolute amount of oxides increases, and the total amount of Si, Mn, Al, and Cr in inclusions having an equivalent circular diameter of 2 μm or more contained in the electric resistance welded portion exceeds 89 mass ppm. This causes deterioration of low-temperature toughness and deterioration of HIC resistance. Thus, when an electric resistance welded steel pipe contains Cr, the Cr content is preferably limited to 0.001% to 0.700%, more preferably 0.01% to 0.700%, still more preferably 0.02% to 0.290%.
The amount of these elements are in the range described above, and Pcm defined by formula (1) satisfies 0.20 or less. Among these elements, the amount of element(s) not contained is zero.

Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B  (1)

wherein C, Si, Mn, Cu, Ni, Cr, Mo, V, and B denote the amounts (mass %) of the corresponding elements.
Pcm is a parameter related to microstructure formation after rapidly cooling an electric resistance welded portion. When Pcm is 0.20 or less, the microstructure of the electric resistance welded portion includes 90% by area or more of quasi-polygonal ferrite having a grain size of 10 μm or less. When Pcm is more than 0.20, the microstructure of the electric resistance welded portion includes less than 90% by area or more of quasi-polygonal ferrite. This causes deterioration of low-temperature toughness. The lower limit of Pcm is preferably, but is not limited to, 0.070 or more in order to stably ensure a yield strength YS of 400 MPa or more.
The reason for limiting the microstructure of the high-strength thick-walled electric resistance welded steel pipe will be described below. 90% by area or more of each of the base steel portion and the electric resistance welded portion of the high-strength thick-walled electric resistance welded steel pipe according to the present invention is occupied with quasi-polygonal ferrite having a grain size of 10 μm or less. The term “quasi-polygonal ferrite”, as used herein, refers to “quasi-polygonal ferrite” (αq) described in “Hagane no beinaito shashinsyu-1 (Photographs of bainite in steel-1)” (edited by Kiso kyodo kenkyu kai beinaito chosa kenkyu bukai (Basic joint research society, bainite research study group) of The Iron and Steel Institute of Japan: “Hagane no beinaito shashinsyu-1”, p. 4, issued on Jun. 29, 1992, publisher: The Iron and Steel Institute of Japan). αq is non-regular shape ferrite formed at a lower temperature than polygonal ferrite αp across an austenite grain boundary before transformation. Transformation strain is mostly recovered in αq.
When the area percentage of quasi-polygonal ferrite is less than 90%, the desired strengthening and low-temperature toughness cannot be achieved because of increased amount of coarse polygonal ferrite, or the desired high low-temperature toughness cannot be achieved because of excessive strengthening due to increased amount of bainite. Thus, the area percentage of quasi-polygonal ferrite is limited to 90% or more, preferably 92% or more. When the grain size dα of quasi-polygonal ferrite is increased to more than 10 μm, the desired strengthening and low-temperature toughness cannot be achieved. Thus, the grain size dα of quasi-polygonal ferrite is limited to 10 μm or less. The grain size is measured using an intercept method according to JIS G 0551 (2005).
A second phase other than quasi-polygonal ferrite may be pearlite, quasi-pearlite, cementite, bainite, and/or martensite, which occupies less than 10% by area in total.
In the electric resistance welded portion of the electric resistance welded steel pipe, the total amount of Si, Mn, Al, Ca, and Cr in inclusions having an equivalent circular diameter of 2 μm or more contained in the electric resistance welded portion is preferably 0.0089% or less. Among these elements, the amount of element(s) not contained is considered to be zero in the total amount. The total amount of Si, Mn, Al, Ca, and Cr in inclusions having an equivalent circular diameter of 2 μm or more means the amount of inclusions that have an influence on the characteristics. The amount of inclusions increases with the total amount.
Among the inclusions (oxides) in an electric resistance welded portion, when the total amount of Si, Mn, Al, Ca, and Cr in inclusions having an equivalent circular diameter of 2 μm or more is less than 89 mass ppm, the crack area ratio (CAR) after immersion in a NACE Solution A (0.5% CH3COOH+5% NaCl+saturated H2S) specified in NACE TM0284 is 5% or less, which indicates improved HIC resistance. Furthermore, the Charpy impact absorbed energy vE−50 of the electric resistance welded portion at a test temperature of −50° C. is more than 150 J, which indicates excellent low-temperature toughness. When the total amount of Si, Mn, Al, Ca, and Cr in the inclusions having an equivalent circular diameter of 2 μm or more is more than 89 ppm, this results in deterioration of HIC resistance and deterioration of low-temperature toughness. Thus, the total amount of Si, Mn, Al, Ca, and Cr in the inclusions having an equivalent circular diameter of 2 μm or more is preferably limited to 89 mass ppm or less, more preferably 39 mass ppm or less.
Among the inclusions contained in an electric resistance welded portion, the total amount of Si, Mn, Al, Ca, and Cr in inclusions having an equivalent circular diameter of 2 μm or more is determined as described below.
Sample sheets (dimensions: 2 mm in width×thickness:wall thickness×length:wall thickness) were cut from the center of the electric resistance welded portion of the electric resistance welded steel pipe and were subjected to electroextraction in a 10% AA electrolyte solution. After the electroextraction, inclusions having a size of 2 μm or more were extracted with a filter mesh having an opening size of 2 μm and are subjected to alkali fusion. The Si, Mn, Al, Ca, and Cr contents were measured in an ICP analysis. The total amount of the elements was calculated and considered to be the total amount of Si, Mn, Al, Ca, and Cr in the inclusions having an equivalent circular diameter of 2 μm or more.
The reason for limiting the method of manufacturing an electric resistance welded steel pipe will be described below.
A steel such as a slab having the chemical composition as described above is formed into a hot-rolled steel strip in a hot-rolling step. The hot-rolled steel strip is continuously roll-formed to form a tubular product. The tubular product is subjected to electric resistance welding in a pipe-forming step to produce an electric resistance welded steel pipe. The steel may be manufactured using any method. Preferably, a molten steel having the chemical composition as described above is produced using an ordinary melting method, for example, using a converter and is formed into a steel, such as a slab, using an ordinary casting method such as a continuous casting method.
The steel such as a slab is formed into a hot-rolled steel strip in the hot-rolling step.
In the hot-rolling step, the steel having the chemical composition as described above is heated to a temperature of 1200° C. to 1280° C., is held at the temperature for 90 min or more, and subjected to hot-rolling at a hot-rolling reduction rate (reduction rate) of 20% or more in an unrecrystallized austenite region (unrecrystallization temperature range). After the completion of the hot-rolling, the steel is cooled to a finish cooling temperature of 630° C. or less at a cooling rate of 7° C./s to 49° C./s. The cooling rate is the average cooling rate at a temperature from 780° C. to 630° C. at the central portion in the thickness direction. After cooling, the steel is coiled at a coiling temperature of 400° C. or more and less than 600° C. (400° C. to 599° C.) to form a hot-rolled steel strip.
Heating Temperature: 1200° C. to 1280° C.
The steel heating temperature has an influence on the strength, low-temperature toughness, and HIC resistance of a base steel portion of a steel pipe. At a heating temperature of less than 1200° C., precipitation strengthening elements such as Nb, V, and Ti, are not redissolved and remain as coarse precipitates. Thus, the desired high yield strength YS of 400 MPa or more cannot be achieved. The residual coarse precipitates reduce HIC resistance. A high heating temperature of more than 1280° C. results in coarsening of crystal grains and coarsening of the resulting quasi-polygonal ferrite. Thus, the desired grain size dα of 10 μm or less cannot be satisfied. Coarsening of the microstructure deteriorates low-temperature toughness. Thus, the heating temperature is limited to 1200° C. to 1280° C. The heating holding time is 90 min or more. A heating holding time of less than 90 min results in residual coarse precipitates of undissolved precipitation strengthening elements such as Nb, V, and Ti, at the central portion in the thickness direction, which deteriorate HIC resistance. Thus, the heating holding time is limited to 90 min or more.
The heated steel is subjected to hot-rolling, which includes rough rolling and finish rolling. In the finish rolling, the hot-rolling reduction rate (reduction rate) in an unrecrystallized austenite region (unrecrystallization temperature range) is 20% or more, and the finishing temperature is 780° C. or more.
Hot-Rolling Reduction Rate (Reduction Rate) in an Unrecrystallized Austenite Region (Unrecrystallization Temperature Range): 20% or More
When the hot-rolling reduction rate (reduction rate) in an unrecrystallized austenite region (unrecrystallization temperature range) is less than 20%, the microstructure is coarsened, and the desired low-temperature toughness cannot be achieved. Thus, the hot-rolling reduction rate (reduction rate) in an unrecrystallized austenite region (unrecrystallization temperature range) is limited to 20% or more, preferably 30% or more.
Finishing Temperature: 780° C. or More
The finishing temperature of the finish rolling is preferably 780° C. or more. A finishing temperature of less than 780° C. results in residual rolling strain and deterioration of low-temperature toughness of the hot-rolled steel sheet.
After completion of the hot-rolling, the hot-rolled steel sheet is cooled on a hot-rolling runout table. The steel sheet is cooled to a finish cooling temperature of 630° C. or less at a cooling rate of 7° C./s to 49° C./s. The cooling rate is the average rate at a temperature from 780° C. to 630° C. at the central portion in the thickness direction. The steel sheet is then coiled at a coiling temperature of 400° C. or more and less than 600° C. (400° C. to 599° C.).
Average Cooling Rate from 780° C. to 630° C.: 7° C./s to 49° C./s
When the average cooling rate is less than 7° C./s, coarse polygonal ferrite is formed, and the desired high low-temperature toughness and high strength cannot be achieved. When the average cooling rate is more than 49° C./s, bainite or martensite is formed, and the desired high low-temperature toughness cannot be achieved because of excessively high strength. Thus, the cooling rate from 780° C. to 630° C. ranges from 7° C./s to 49° C./s on average. Preferably, the average cooling rate is 29° C./s or less such that the amount of quasi-polygonal ferrite is 92% or more.
In the cooling after the hot-rolling, it is desirable that except for an uppermost surface layer having a thickness of 0.2 mm, a deviation from the cooling rate at the central portion in the thickness direction is within 5° C./s at the slowest position and within 20° C./s at the fastest position.
The steel sheet is cooled to a temperature of 630° C. or less at the central portion in the thickness direction at such a cooling rate and is then coiled.
Finish Cooling Temperature: 630° C. or Less
When the finish cooling temperature is more than 630° C., the desired fine microstructure cannot be formed, and the desired high strength and high low-temperature toughness cannot be achieved in the base steel portion. Thus, the finish cooling temperature is limited to 630° C. or less, preferably 600° C. to 550° C.
Coiling Temperature: 400° C. or More and Less than 600° C. (400° C. to 599° C.)
When the coiling temperature is 600° C. or more, the microstructure is coarsened and cannot have the desired grain size and the desired area percentage of quasi-polygonal ferrite. When the coiling temperature is less than 400° C., the steel sheet contains a large amount of bainite and has increased strength, deteriorated low-temperature toughness and deteriorated HIC resistance. Thus, the coiling temperature is limited to 400° C. or more and less than 600° C. (400° C. to 599° C.), preferably 550° C. to 450° C.
In the hot-rolled steel strip resulting from the hot-rolling, the cooling, and the coiling, the area percentage of fine quasi-polygonal ferrite having a grain size dα of 10 μm or less is 90% or more, the remainder being pearlite, quasi-pearlite, cementite, bainite, and/or martensite. The hot-rolled steel strip is used as a base steel to manufacture a steel pipe that includes a base steel portion having high strength represented by a yield strength YS of 400 MPa or more, excellent low-temperature toughness represented by a Charpy impact absorbed energy vE−50 of 150 J or more at a test temperature of −50° C., and excellent HIC resistance represented by a crack area ratio CAR of 5% or less after immersion in a NACE Solution A specified in NACE TM0284 for 96 hours.
The hot-rolled steel strip is then cut in a predetermined width and subjected to a pipe-forming step to form an electric resistance welded steel pipe having a predetermined size.
The pipe-forming step may be any generally known process of manufacturing an electric resistance welded pipe under any conditions, provided that the electric resistance welded steel pipe has a predetermined size.
Preferably, an electric resistance welded steel pipe is formed by continuously roll-forming a hot-rolled steel strip to form a tubular product having a substantially circular cross section, butting the circumferential ends of the tubular product, heating the circumferential ends to at least the melting point by high-frequency resistance heating or high-frequency induction heating, pressing the circumferential ends with squeeze rolls, and welding the seam by electric resistance welding.
In the roll-forming, a tapered groove is preferably formed by fin pass forming in the end faces of a hot-rolled steel strip in the width direction. The tapered groove can promote removal of oxides from the electric resistance welded portion and thereby impart excellent low-temperature toughness and excellent HIC resistance to the electric resistance welded portion. The distance in the steel strip thickness direction between a taper starting position of the tapered groove at the ends in the width direction and a surface that will become a pipe outer surface or a surface that will become a pipe inner surface preferably ranges from 2% to 60% of the thickness of the steel strip. This can promote removal of oxides and reduce the total amount of Si, Mn, Al, Ca, and Cr in inclusions having an equivalent circular diameter of 2 μm or more contained in the electric resistance welded portion by approximately 10 mass ppm. A taper outside this range makes it difficult to remove oxides and results in an electric resistance welded portion having deterioration of low-temperature toughness. The shape of the taper is not limited to a straight line and may be a curved line.
The atmosphere in the electric resistance welding in the pipe-forming step is preferably controlled such that the atmospheric oxygen partial pressure is 900/foxy mass ppm or less, wherein the foxy denotes the degree of oxidizability of molten steel defined by formula (2),

foxy=Mn+10(Si+Cr)+100Al+1000Ca  (2)

wherein Mn, Si, Cr, Al, and Ca denote the amounts (mass %) of the corresponding elements.
Lowering the atmospheric oxygen partial pressure in the electric resistance welding can reduce the amount of coarse oxides in the electric resistance welded portion. When the atmospheric oxygen partial pressure in the electric-resistance-welded portion is 900/foxy mass ppm or less, the total amount of Si, Mn, Al, Ca, and Cr in inclusions having an equivalent circular diameter of 2 μm or more can be reduced by approximately 20 mass ppm.
The atmospheric oxygen partial pressure in the electric resistance welded portion may be reduced by sealing the electric resistance welded portion in a box structure and supplying a nonoxidizing gas to the electric resistance welded portion. In this method, however, the supply of the nonoxidizing gas may accompany the ambient atmosphere, and the atmospheric oxygen partial pressure in the electric resistance welded portion may be increased. To avoid this, the nonoxidizing gas is preferably supplied as a laminar flow through a nozzle having a multilayer structure, such as a three-layer structure. The concentration of oxygen in the electric resistance welded portion is preferably measured by bringing a probe of an oxygen meter close to the electric resistance welded portion.
The electric resistance welded portion of the electric resistance welded steel pipe after the pipe-forming step is subjected to heat treatment.
The low-temperature toughness of the electric resistance welded portion depends on the amount of oxides in the electric resistance welded portion and on the microstructure. Therefore, the electric resistance welded portion is further subjected to heat treatment on a production line. In the heat treatment, the electric resistance welded portion is heated to a temperature of 800° C. to 1150° C. over the total thickness and then cooled to a finish cooling temperature of 630° C. or less at a cooling rate of 7° C./s to 49° C./s. The cooling rate is the average cooling rate at a temperature from 780° C. to 630° C. at the central portion in the thickness direction. The electric resistance welded portion is then air-cooled. The heating of the electric resistance welded portion is preferably mainly performed with an induction heating apparatus on a production line in terms of productivity.
Heating Temperature of Heat Treatment: 800° C. to 1150° C.
When the heating temperature is less than 800° C., the microstructure of the electric resistance welded portion is composed of coarse polygonal ferrite, and it is difficult to ensure the desired high strength and high low-temperature toughness. A high heating temperature of more than 1150° C. results in the formation of coarse quasi-polygonal ferrite having a grain size dα of more than 10 μm, causing deterioration of low-temperature toughness. Thus, the heating temperature of the heat treatment is limited to 800° C. to 1150° C., preferably 850° C. to 1100° C.
Average Cooling Rate after Heating in Heat Treatment: 7° C./s to 49° C./s
When the average cooling rate from 780° C. to 630° C. after heating is less than 7° C./s, the microstructure is composed of coarse polygonal ferrite, and it is difficult to ensure the desired high strength and high low-temperature toughness. When the average cooling rate is more than 49° C./s, this promotes the formation of bainite and reduces the area percentage of quasi-polygonal ferrite to less than 90%, thus resulting in increased strength, deteriorated low-temperature toughness and deteriorated HIC resistance. Thus, the average cooling rate after heating is limited to 7° C./s to 49° C./s from 780° C. to 630° C. Preferably, the average cooling rate is 29° C./s or less such that the percentage of quasi-polygonal ferrite is 93% or more.
In the cooling after heating of the electric resistance welded portion, it is desirable that except for an uppermost surface layer having a thickness of 0.2 mm, a deviation from the cooling rate at the central portion in the thickness direction is within 5° C./s at the slowest position and within 20° C./s at the fastest position. This reduces variations in characteristics in the thickness direction.
Finish Cooling Temperature: 630° C. or Less
In the cooling after heating, when the finish cooling temperature is more than 630° C., the desired fine microstructure cannot be formed, and the desired high strength and high low-temperature toughness cannot be achieved in the electric resistance welded portion. Thus, the finish cooling temperature is limited to 630° C. or less, preferably 550° C. to 200° C.
In the electric resistance welded portion subjected to such heat treatment, the area percentage of fine quasi-polygonal ferrite having a grain size dα of 10 μm or less is 90% or more, and the remainder are pearlite, quasi-pearlite, cementite, bainite, and/or martensite. The resulting electric resistance welded steel pipe includes an electric resistance welded portion having high strength represented by a yield strength YS of 400 MPa or more, excellent low-temperature toughness represented by a Charpy impact absorbed energy vE−50 of 150 J or more at a test temperature of −50° C., and excellent HIC resistance represented by a crack area ratio CAR of 5% or less after immersion in a NACE Solution A specified in NACE TM0284 for 96 hours.
To control the cooling after heating in the heat treatment of an electric resistance welded portion of a thick-walled electric resistance welded steel pipe having a thickness of more than 16 mm and ensure an electric resistance welded portion having the desired microstructure, it is necessary to examine the flow rate of ejected cooling water and determine the cooling method and the temperature controlling method.
In heat treatment, at least two lines of cooling headers are arranged in a conveying direction above the electric resistance welded portion. The cooling headers are coupled to a nozzle through which a rod-like flow of cooling water can be ejected at a water flow rate of 1 m3/m2·min or more. The rod-like flow of cooling water is preferably ejected from the nozzle at a rate of 1 m/s or more. The at least two lines of cooling headers are preferably configured to independently control the ejection of cooling water. The cooling rate at the electric resistance welded portion is adjusted to the target cooling rate by measuring the temperature of the electric resistance welded portion on the downstream side in the conveying direction and performing the on-off control of the ejection of cooling water from each of the cooling headers in response to the temperature of the electric resistance welded portion. This can improve temperature control, allows the electric resistance welded portion to be stably cooled to 630° C. or less at a cooling rate of 7° C./s to 49° C./s as desired, the cooling rate being the average cooling rate at a temperature from 780° C. to 630° C. at the central portion in the thickness direction, and allows the desired microstructure to be stably formed.
When the water flow rate is less than 1 m3/m2·min, or less than two lines of nozzles are arranged in the conveying direction of the steel pipe, or the ejection rate of cooling water is less than 1 m/s, the desired cooling rate cannot be achieved because of a boiling film, for example. In addition to the water flow rate to ensure the cooling rate by rapidly removing the boiling film, it is effective to incline the nozzles or install opposed nozzles.
EXAMPLES
Example 1
A steel slab (steel) (thickness: 250 mm) having a chemical composition shown in Table 1 was subjected to a hot-rolling step including heating, finish rolling, cooling after the finish rolling, and coiling under the conditions shown in Table 2 to produce a hot-rolled steel strip having a thickness shown in Table 2. The hot-rolled steel strip was cut in a predetermined width by slitting and continuously roll-formed in an ordinary pipe-forming step to form a tubular product having a substantially circular cross section. An electric resistance welded steel pipe (having an outer diameter of 26 inches (660.4 mm)) was formed in a pipe-forming step that included butting the circumferential ends of the tubular product, heating the circumferential ends to at least the melting point by high-frequency resistance heating, pressing the circumferential ends with squeeze rolls, and performing electric resistance welding. No tapered groove was formed in the ends of the steel strip in the roll-forming. The electric resistance welding was performed in air.
After the pipe-forming step, the electric resistance welded portion was subjected to heat treatment.
In the heat treatment, the electric resistance welded portion was subjected to heating and cooling on the production line under the conditions shown in Table 2. A high-frequency induction heating apparatus on the production line was used in the heating. The cooling after the heating was performed with ten lines of cooling headers that were disposed above the electric resistance welded portion in the conveying direction of the steel pipe and coupled to a nozzle through which a rod-like flow of cooling water could be ejected at a water flow rate of 2 m3/m2·min. The cooling headers could independently perform the on-off control of the ejection of cooling water. A rod-like flow of cooling water could be ejected through the nozzle at a rate of 2 m/s. The cooling rate at the electric resistance welded portion was controlled by measuring the temperature of the electric resistance welded portion on the downstream side in the steel pipe conveying direction and performing the on-off control of the ejection of cooling water from each of the cooling headers in response to the temperature of the electric resistance welded portion.
Test specimens were sampled from the base steel portion and the electric resistance welded portion of the electric resistance welded steel pipe and subjected to a tensile test, an impact test, a HIC test, and a measurement of the amount of inclusions. The test methods were described below.
(A) Tensile Test
JIS 12C test specimens according to JIS Z 2241 were sampled from the base steel portion of the electric resistance welded steel pipe. The tensile direction for the base steel portion was the axial direction of the pipe. JIS 1A test specimens according to JIS Z 2241 were sampled from the electric resistance welded portion of the steel pipe such that the tensile direction was the circumferential direction. The tensile properties (the yield strength YS and the tensile strength TS) were measured in the tensile test.
(B) Impact Test
V-notch Charpy impact test specimens (having a thickness of 10 mm) were sampled in the circumferential direction from the base steel portion and the electric resistance welded portion of the electric resistance welded steel pipe in accordance with JIS Z 2242 such that the notch in the electric resistance welded portion coincided with the central portion of the electric resistance welded portion. The absorbed energy vE−50 of each test specimen was measured in a Charpy impact test at a temperature of −50° C. The number of test specimens in measurement was three.
(C) HIC Test
Immersion test specimens (dimensions: 10 mm in thickness×20 mm in width×160 mm in length) were sampled from the base steel portion and the electric resistance welded portion of the electric resistance welded steel pipe and were immersed in a NACE Solution A (0.5% CH3COOH+5% NaCl+saturated H2S) specified in NACE TM0284 for 96 hours. After the immersion, the crack area ratio CAR of each test specimen was determined using an ultrasonic flaw inspection method.
(D) Measurement of Amount of Inclusions
Sample sheets (dimensions: 2 mm in width×thickness:wall thickness×length:wall thickness) were cut from the center of the electric resistance welded portion of the electric resistance welded steel pipe and were subjected to electroextraction in a 10% AA electrolyte solution. After the electroextraction, inclusions having a size of 2 μm or more were extracted with a filter mesh having an opening size of 2 μm and were subjected to alkali fusion. The Si, Mn, Al, Ca, and Cr contents were measured in an ICP analysis. The total amount of Si, Mn, Al, Ca, and Cr was calculated. The total amount of Si, Mn, Al, Ca, and Cr in the inclusions having an equivalent circular diameter of 2 μm or more was considered to be the amount of coarse inclusions in the electric resistance welded portion. Table 3 shows the results.









TABLE 1








Chemical composition (mass %)























Steel No.
C
Si
Mn
P
S
Al
Nb
V
Ti
Ca
N
O
Others
Pcm
Note





A
0.037
0.16
0.87
0.005
0.0004
0.036
0.045
0.045
0.01
0.002
0.0035
0.0009
-
0.09
Example


B
0.045
0.17
1.01
0.004
0.0006
0.03
0.052
0.052
0.012
0.0019
0.0027
0.0012
Cu: 0.19, Ni: 0.16,
0.13
Example















Mo: 0.19




C
0.046
0.21
1.4 
0.012
0.0005
0.032
0.056
0.062
0.015
0.0025
0.0036
0.0011
Cu: 0.19, Ni: 0.15
0.14
Example


D
0.052
0.21
1.62
0.012
0.0018
0.035
0.062
0.023
0.014
0.0018
0.0033
0.0014
Cu: 0.28, Ni: 0.21,
0.17
Example















Mo: 0.14




E
0.027
0.15
1.53
0.01 
0.0015
0.041
0.047
0.035
0.011
0.0025
0.0039
0.0009
B: 0.0007
0.12
Example


F
0.061
0.22
1.78
0.01 
0.0016
0.037
0.058
0.024
0.016
0.0021
0.0037
0.0011
Cr: 0.003
0.16
Example


G
0.024
0.27
1.44
0.014
0.0012
0.063
0.023
0.032
0.021
0.0002
0.0014
0.0018
-
0.11
Comparative

















example


H
0.085
0.16
1.74
0.011
0.0008
0.026
0.014
0.012
0.016
0.0004
0.0031
0.0012
-
0.18
Comparative

















example


I
0.055
0.05
1.52
0.016
0.0021
0.074
0.054
0.042
0.008
0.0007
0.003
0.0012
-
0.14
Comparative

















example


J
0.042
0.36
1.58
0.005
0.0015
0.038
0.052
0.054
0.018
0.0018
0.0038
0.0044
-
0.14
Comparative

















example


K
0.037
0.19
0.64
0.017
0.002
0.049
0.061
0.055
0.012
0.0021
0.0018
0.0007
-
0.08
Comparative

















example


L
0.036
0.23
1.95
0.012
0.0008
0.021
0.025
0.014
0.017
0.0019
0.0033
0.0015
-
0.14
Comparative

















example


M
0.042
0.27
1.23
0.021
0.0022
0.038
0.045
0.064
0.016
0.0026
0.0008
0.0015
-
0.12
Comparative

















example


N
0.055
0.21
1.05
0.014
0.0032
0.035
0.003
0.015
0.014
0.0024
0.0026
0.0016
-
0.12
Comparative

















example


O
0.071
0.19
1.28
0.018
0.0021
0.003
0.024
0.025
0.01
0.0011
0.0032
0.0011
-
0.14
Comparative

















example


P
0.028
0.2
1.44
0.012
0.0025
0.123
0.024
0.043
0.008
0.0009
0.0039
0.0022
-
0.11
Comparative

















example


Q
0.036
0.2
1.36
0.006
0.0021
0.046
-
0.042
0.002
0.0016
0.0042
0.0021
-
0.11
Comparative

















example


R
0.061
0.24
1.48
0.011
0.0008
0.051
0.075
0.042
0.016
0.0032
0.0036
0.0025
-
0.15
Comparative

















example


S
0.051
0.19
1.25
0.01
0.0004
0.026
0.014
-
0.022
0.0022
0.0039
0.0008
-
0.12
Comparative

















example


T
0.071
0.21
0.87
0.012
0.0006
0.031
0.014
0.072
0.028
0.0034
0.0025
0.0016
-
0.13
Comparative

















example


U
0.061
0.23
0.97
0.008
0.0007
0.041
0.054
0.012
-
0.0015
0.0014
0.0016
-
0.12
Comparative

















example


V
0.064
0.2 
0.95
0.009
0.0005
0.045
0.064
0.002
0.039
0.0008
0.0026
0.0018
-
0.12
Comparative

















example


W
0.043
0.22
1.35
0.018
0.0009
0.024
0.047
0.005
0.023
-
0.0019
0.0006
-
0.12
Comparative

















example


X
0.029
0.2 
1.53
0.009
0.0021
0.042
0.042
0.018
0.031
0.0039
0.0039
0.0024
-
0.11
Comparative

















example


Y
0.034
0.18
1.35
0.004
0.0018
0.064
0.056
0.045
0.024
0.0021
0.0056
0.0016
-
0.11
Comparative

















example


Z
0.036
0.19
1.2
0.008
0.001
0.042
0.052
0.022
0.011
0.0048
0.0025
0.0034
-
0.1 
Comparative

















example


AA
0.072
0.23
1.75
0.008
0.0005
0.044
0.062
0.059
0.016
0.0015
0.0047
0.0023
Cu: 0.25, Ni: 0.25,
0.21
Comparative















Mo: 0.25, Cr: 0.15

example





Underline refers to outside of the scope of the present invention.















TABLE 2








Hot-rolling step























Average cooling







Heating
Heating
Rolling
Finishing
rate after
Finish cooling
Coiling
Steel strip


Steel
Steel
temperature
holding time
reduction
temperature
rolling**
temperature***
temperature
thickness


pipe No.
No.
(° C.)
(min)
rate (%)*
(° C.)
(° C./s)
(° C.)
(° C.)
mm





1
A
1240
100
50
820
15
520
495
24


2
B
1240
100
50
820
15
520
495
24


3
C
1240
100
50
820
15
520
495
24


4
D
1240
100
50
820
15
520
495
24


5
E
1240
100
50
820
15
520
495
24


6
F
1240
100
50
820
15
520
495
24


7
G
1240
100
50
820
15
520
495
24


8
H
1240
100
50
820
15
520
495
24


9
I
1240
100
50
820
15
520
495
24


10
J
1240
100
50
820
15
520
495
24


11
K
1240
100
50
820
15
520
495
24


12
L
1240
100
50
820
15
520
495
24


13
M
1240
100
50
820
15
520
495
24


14
N
1240
100
50
820
15
520
495
24


15
O
1240
100
50
820
15
520
495
24


16
P
1240
100
50
820
15
520
495
24


17
Q
1240
100
50
820
15
520
495
24


18
R
1240
100
50
820
15
520
495
24


19
S
1240
100
50
820
15
520
495
24


20
T
1240
100
50
820
15
520
495
24


21
U
1240
100
50
820
15
520
495
24


22
V
1240
100
50
820
15
520
495
24


23
W
1240
100
50
820
15
520
495
24


24
X
1240
100
50
820
15
520
495
24


25
Y
1240
100
50
820
15
520
495
24


26
Z
1240
100
50
820
15
520
495
24


27
AA
1240
100
50
820
15
520
495
24












Heat treatment












Pipe-forming step

Heating














Electric resistance welding
Steel pipe
temperature
Average



















Atmospheric



dimensions
range of electric
cooling
Finish

















Steel
Roll-forming
oxygen



Wall
Outer
resistance
rate after
cooling


pipe
Formation of
concentration

900/
Nonoxidizing
thickness
diameter
welded portion
heating
temperature


No.
groove****
(ppm)
f oxy
f oxy
gas blowing
(mm)
(mmφ)
(° C.)
(° C./s)
(° C.)





1
—
Air
8.1
112
—
24
660.4
1050
20
300


2
—
Air
7.6
118
—
24
660.4
1050
20
300


3
—
Air
9.2
98
—
24
660.4
1050
20
300


4
—
Air
9
100
—
24
660.4
1050
20
300


5
—
Air
9.6
93
—
24
660.4
1050
20
300


6
—
Air
9.8
92
—
24
660.4
1050
20
300


7
—
Air
10.6
85
—
24
660.4
1050
20
300


8
—
Air
6.3
142
—
24
660.4
1050
20
300


9
—
Air
10.1
89
—
24
660.4
1050
20
300


10
—
Air
10.6
85
—
24
660.4
1050
20
300


11
—
Air
9.5
94
—
24
660.4
1050
20
300


12
—
Air
8.3
109
—
24
660.4
1050
20
300


13
—
Air
10.3
87
—
24
660.4
1050
20
300


14
—
Air
9.1
99
—
24
660.4
1050
20
300


15
—
Air
4.6
197
—
24
660.4
1050
20
300


16
—
Air
16.6
54
—
24
660.4
1050
20
300


17
—
Air
9.6
94
—
24
660.4
1050
20
300


18
—
Air
12.2
74
—
24
660.4
1050
20
300


19
—
Air
8
113
—
24
660.4
1050
20
300


20
—
Air
9.5
95
—
24
660.4
1050
20
300


21
—
Air
8.9
101
—
24
660.4
1050
20
300


22
—
Air
8.3
109
—
24
660.4
1050
20
300


23
—
Air
6
151
—
24
660.4
1050
20
300


24
—
Air
11.6
77
—
24
660.4
1050
20
300


25
—
Air
11.7
77
—
24
660.4
1050
20
300


26
—
Air
11.9
76
—
24
660.4
1050
20
300


27
—
Air
10
90
—
24
660.4
1050
20
300





Underline refers to the outside of the scope of the present invention.


*Unrecrystallization temperature range


**Average cooling rate at the central portion temperatures in the thickness direction from 780° C. to 630° C.


***Central portion temperature in the thickness direction


****Distance between the taper starting position and the top surface/Total thickness × 100%















TABLE 3








Base steel portion
Electric resistance welded portion












Tensile properties

HIC
Structure
















Steel

Structure
Yield
Tensile
Toughness
resistance

QPF
QPF Grain


















pipe
Steel

Percentage
Grain size
strength
strength
vE-50
CAR

Percentage
size


No.
No.
Type*
(% by area)
(μm)
YS (MPa)
TS (MPa)
(J)
(%)
Type*
(% by area)
(μm)





1
A
QPF + B + C
QPF: 96
6.8
489
532
360
0  
QPF + B + C
QPF: 95
8.5


2
B
QPF + B + C
QPF: 94
5.9
496
540
380
0  
QPF + B + C
QPF: 93
6.8


3
C
QPF + B + C
QPF: 94
5.7
497
538
370
0  
QPF + B + C
QPF: 94
6.5


4
D
QPF + B + C
QPF: 93
3.6
603
652
312
3.8
QPF + B + C
QPF: 93
4.2


5
E
QPF + B + C
QPF: 93
2.6
725
790
280
4.5
QPF + B + C
QPF: 93
3.5


6
F
QPF + B + C
QPF: 93
3.1
610
662
292
4.7
QPF + B + C
QPF: 93
4.4


7
G
PF + P
PF: 95
11.5 
390
405
340
1.2
PF + P
PF: 96
12.3 


8
H
QPF + B + C
QPF: 76
4.5
659
717
135
5.9
QPF + B + C
QPF: 76
4.9


9
I
QPF + B + C
QPF: 86
5.2
479
510
165
4.7
QPF + B + C
QPF: 87
5.4


10
J
QPF + B + C
QPF: 91
4.6
512
560
170
4.9
QPF + B + C
QPF: 93
4.9


11
K
PF + P
PF: 94
12.7 
365
388
337
4.8
PF + P
PF: 95
13.2 


12
L
B + M
B: 78
3.8
669
716
125
7.8
B + M
B: 77
4.2


13
M
QPF + B + C
QPF: 90
4.4
489
531
168
5.5
QPF + B + C
QPF: 90
4.7


14
N
QPF + B + C
QPF: 93
4.1
467
509
134
11.4 
QPF + B + C
QPF: 93
4.5


15
O
QPF + B + C
QPF: 91
5.4
501
544
143
6.8
QPF + B + C
QPF: 92
5.2


16
P
QPF + B + C
QPF: 92
4.1
436
476
125
6.9
QPF + B + C
QPF: 93
4.2


17
Q
PF + P
PF: 96
10.5 
370
408
310
4.3
PF + P
PF: 96
11  


18
R
QPF + B + C
QPF: 86
3.7
518
564
135
6.9
QPF + B + C
QPF: 86
4.3


19
S
PF + P
PF: 95
10.2 
380
418
343
0  
PF + P
PF: 95
10.4 


20
T
QPF + B + C
QPF: 88
5.2
520
562
144
6.5
QPF + B + C
QPF: 88
5.8


21
U
PF + P
PF: 93
12  
390
450
280
0  
PF + P
PF: 92
12.7 


22
V
QPF + B + C
QPF: 93
4.7
516
560
110
6.9
QPF + B + C
QPF: 93
5.4


23
W
QPF + B + C
QPF: 96
5.3
490
533
148
5.4
QPF + B + C
QPF: 96
5.8


24
X
QPF + B + C
QPF: 95
4.3
469
519
105
8.9
QPF + B + C
QPF: 97
4.4


25
Y
QPF + B + C
QPF: 92
4.6
490
546
102
5.4
QPF + B + C
QPF: 92
5  


26
Z
QPF + B + C
QPF: 93
4.4
497
546
 97
9.9
QPF + B + C
QPF: 93
4.3


27
AA
B + M
B: 75
3.5
658
728
130
5.7
QPF + B + C
QPF: 75
4.1













Electric resistance welded portion















Total alloy







amount in






Tensile properties
inclusions

HIC
















Steel
Yield
Tensile
having a size
Toughness
resistance




pipe
strength
strength
of 2 μm or
vE-50
CAR




No.
YS (MPa)
TS (MPa)
more (ppm)**
(J)
(%)
Note






 1
486
552
29
370
0  
Example



 2
490
555
24
385
0  
Example



 3
500
570
26
378
0  
Example



 4
593
680
32
315
2.6
Example



 5
715
810
35
282
3.6
Example



 6
603
687
39
300
3.5
Example



 7
367
417
66
326
1.2
Comparative example



 8
623
708
42
128
6.5
Comparative example



 9
444
505
96
104
6.2
Comparative example



10
480
545
128 
 69
8.7
Comparative example



11
342
390
83
168
3.4
Comparative example



12
629
715
82
119
7.6
Comparative example



13
456
518
74
106
6.5
Comparative example



14
444
505
72
 76
23.9 
Comparative example



15
480
545
96
102
7.8
Comparative example



16
410
466
108 
 76
10.4 
Comparative example



17
450
524
68
289
3.2
Comparative example



18
480
545
68
116
7  
Comparative example



19
360
432
64
324
1.2
Comparative example



20
490
557
66
121
6  
Comparative example



21
371
431
66
245
1.1
Comparative example



22
487
553
78
 78
8.9
Comparative example



23
482
525
75
116
7.9
Comparative example



24
450
511
107 
 86
9.8
Comparative example



25
462
525
82
108
5.2
Comparative example



26
469
533
106 
 68
10.4 
Comparative example



27
618
702
64
121
5.9
Comparative example





Underline refers to the outside of the scope of the present invention.


*QPF: quasi-polygonal ferrite, PF: polygonal ferrite, B: bainite, P: perlite, QP: quasi-perlite, C: cementite, M: martensite


**Total amount of Si, Mn, Al, Ca, and Cr in inclusions having an equivalent circular diameter of 2 μm or more (mass ppm)






The base steel portion and the electric resistance welded portion of the electric resistance welded steel pipe according to our examples have a microstructure in which fine quasi-polygonal ferrite having a grain size dα of 10 μm or less occupies 90% by area or more. The base steel portion and the electric resistance welded portion have high strength represented by a yield strength YS of 400 MPa or more, excellent low-temperature toughness represented by a Charpy impact absorbed energy vE−50 of 150 J or more at −50° C., and excellent HIC resistance represented by a crack area ratio CAR of 5% or less after immersion in a NACE Solution A specified in NACE TM0284 for 96 hours. The remainder other than quasi-polygonal ferrite were pearlite, quasi-pearlite, cementite, bainite, and martensite, which occupies less than 10% by area in total.
In the comparative examples, the base steel portion and the electric resistance welded portion do not have a microstructure mainly composed of fine quasi-polygonal ferrite, or the total amount of Si, Mn, Al, Ca, and Cr in the inclusions having an equivalent circular diameter of 2 μm or more contained in the electric resistance welded portion increases to more than 89 mass ppm. Thus, the comparative examples do not have the desired high strength or have deterioration of low-temperature toughness or deterioration of HIC resistance.
In the comparative examples in which any of C, Mn, Nb, V, and Ti is lower than our range (steel pipes Nos. 7, 11, 17, 19, and 21), the base steel portion and the electric resistance welded portion have a softer polygonal ferrite microstructure and a low yield strength YS of less than 400 MPa. In the comparative examples in which any of C, Mn, Nb, V, and Ti is higher than our range (steel pipes Nos. 8, 12, 18, 20, and 22), the base steel portion and the electric resistance welded portion have deterioration of low-temperature toughness and deterioration of HIC resistance. In the comparative examples in which any of Si, Al, Ca, and O is outside our range (steel pipes Nos. 9, 10, 15, 16, 24, and 26), the total amount of Si, Mn, Al, Ca, and Cr in the inclusions having an equivalent circular diameter of 2 μm or more contained in the electric resistance welded portion is more than 89 mass ppm, and the electric resistance welded portion has deterioration of low-temperature toughness and deterioration of HIC resistance. The comparative examples in which any of P, S, Ca, N, and Pcm is outside our range (steel pipes Nos. 13, 14, 23, 24, 25, and 27) have deteriorated low-temperature toughness and deteriorated HIC resistance.
Example 2
Steels (slabs) of steel Nos. A to F shown in Table 1 were used in a hot-rolling step under the conditions shown in Table 4 to produce hot-rolled steel strips having a thickness shown in Table 4. The hot-rolled steel strips were cut in a predetermined width by slitting, were continuously roll-formed, and subjected to electric resistance welding in a pipe-forming step to form electric resistance welded steel pipes having the dimensions shown in Table 4. In the roll-forming of some of the steel pipes, a tapered groove having the dimensions shown in Table 4 was formed in the ends of the steel strip in the width direction. The electric resistance welding was performed in the air except for some steel pipes. In some of the steel pipes, a nonoxidizing gas was blown into the atmosphere of the electric resistance welding. In this case, gas blowing nozzles were arranged in three layers, and the oxygen partial pressure reduced to 45 mass ppm.
The electric resistance welded portions of the electric resistance welded steel pipes were subjected to heat treatment including heating and cooling under the conditions shown in Table 4. An induction heating apparatus on the production line was used in the heating. In the cooling, ten lines of cooling headers were disposed above the electric resistance welded portion in the conveying direction and coupled to a nozzle through which a rod-like flow of cooling water could be ejected at a water flow rate of 2 m3/m2·min. A rod-like flow of cooling water was ejected through the nozzle at a rate of 2 m/s or more. The cooling headers were configured to independently control the ejection of cooling water. The cooling rate at the electric resistance welded portion was controlled by measuring the temperature of the electric resistance welded portion on the downstream side in the conveying direction and performing the on-off control of the ejection of cooling water from each of the cooling headers in response to the temperature of the electric resistance welded portion.
In the same manner as in Example 1, test specimens were sampled from the electric resistance welded steel pipe and subjected to a tensile test, an impact test, a HIC test, and a measurement of the amount of inclusions. The test methods were described in Example 1. Table 5 shows the results.








TABLE 4








Pipe-forming step













Electric



Hot-rolling step

resistance
























Average




welding





Heating


cooling
Finish



Atmospheric


Steel

Heating
holding
Rolling
Finishing
rate after
cooling
Coiling
Steel strip
Roll-forming
oxygen


pipe
Steel
temperature
time
reduction
temperature
rolling**
temperature***
temperature
thickness
Formation of
concentration


No.
No.
(° C.)
(min)
rate (%)*
(° C.)
(° C./s)
(° C.)
(° C.)
mm
groove****
(ppm)





A1
A
1240
120
45
820
17
590
570
25.4
—
Air


A2
A
1290
120
45
820
17
590
570
25.4
—
Air


A3
A
1180
120
45
820
17
590
570
25.4
—
Air


A4
A
1240
 70
45
820
17
590
570
25.4
—
Air


A5
A
1240
120
15
820
17
590
570
25.4
—
Air


A6
B
1250
100
60
820
27
510
490
16
—
Air


A7
B
1250
100
60
820
27
510
490
16
—
Air


A8
B
1250
100
60
820
27
510
490
16
—
Air


A9
B
1250
100
60
820
27
510
490
16
—
Air


A10
B
1250
100
60
820
27
510
490
16
—
Air


A11
C
1220
110
25
820
26
570
550
32
—
Air


A12
C
1220
110
25
820
26
570
550
32
—
Air


A13
C
1220
110
25
820
26
570
550
32
—
Air


A14
C
1220
110
25
820
26
570
550
32
Outer
Air












surface:













10% + inner













surface 10%



A15
C
1220
110
25
820
26
570
550
32
—
Air


A16
D
1270
105
50
820
 6
500
480
22.5
—
Air


A17
D
1270
105
50
820
13
500
480
22.5
—
Air


A18
D
1270
105
50
820
26
500
480
22.5
—
Air


A19
D
1270
105
50
820
45
500
480
22.5
—
Air


A20
D
1270
105
50
820
63
500
480
22.5
—
Air


A21
E
1240
 95
40
820
13
615
595
25.4
—
Air


A22
E
1240
 95
40
820
13
615
595
25.4
—
Air


A23
E
1240
 95
40
820
13
615
595
25.4
—
Air


A24
E
1240
 95
40
820
13
615
595
25.4
—
45


A25
E
1240
 95
40
820
13
615
595
25.4
—
Air


A26
F
1250
110
60
820
19
560
540
20.6
—
Air


A27
F
1250
110
60
820
19
560
540
20.6
—
Air


A28
F
1250
110
60
820
19
560
540
20.6
—
Air


A29
F
1250
110
60
820
19
560
540
20.6
—
Air


A30
F
1250
110
60
820
19
560
540
20.6
—
Air













Heat treatment














Pipe-forming step

Heating





Electric resistance
Steel pipe
temperature
Average




welding
dimensions
range of electric
cooling rate



















Steel


Nonoxidizing
Wall
Outer
resistance
after
Finish cooling




pipe

900/
gas
thickness
diameter
welded portion
heating
temperature




No.
f oxy
f oxy
blowing
(mm)
(mmφ)
(° C.)
(° C./s)
(° C.)
Note






A1
8.1
112
—
25.4
660.4
910~1060
19
300
Example



A2
8.1
112
—
25.4
660.4
910~1060
19
300
Comparative example



A3
8.1
112
—
25.4
660.4
910~1060
19
300
Comparative example



A4
8.1
112
—
25.4
660.4
910~1060
19
300
Comparative example



A5
8.1
112
—
25.4
660.4
910~1060
19
300
Comparative example



A6
7.6
118
—
16
660.4
920~1080
 6
300
Comparative example



A7
7.6
118
—
16
660.4
920~1080
14
300
Example



A8
7.6
118
—
16
660.4
920~1080
25
300
Example



A9
7.6
118
—
16
660.4
920~1080
46
300
Example



A10
7.6
118
—
16
660.4
920~1080
56
300
Comparative example



A11
9.2
98
—
32
508
760~1050
28
300
Comparative example



A12
9.2
98
—
32
508
810~1080
28
300
Example



A13
9.2
98
—
32
508
900~1120
28
300
Example



A14
9.2
98
—
32
508
900~1120
28
300
Example



A15
9.2
98
—
32
508
950~1160
28
300
Comparative example



A16
9
100
—
22.5
609.6
890~1030
17
300
Comparative example



A17
9
100
—
22.5
609.6
890~1030
17
300
Example



A18
9
100
—
22.5
609.6
890~1030
17
300
Example



A19
9
100
—
22.5
609.6
890~1030
17
300
Example



A20
9
100
—
22.5
609.6
890~1030
17
300
Comparative example



A21
9.6
93
—
25.4
457.2
780~1060
12
300
Comparative example



A22
9.6
93
—
25.4
457.2
820~1070
12
300
Example



A23
9.6
93
—
25.4
457.2
910~1110
12
300
Example



A24
9.6
93
Three-layer
25.4
457.2
910~1110
12
300
Example






nozzle









A25
9.6
93
—
25.4
457.2
950~1160
12
300
Comparative example



A26
9.8
92
—
20.6
660.4
900~1090
 6
300
Comparative example



A27
9.8
92
—
20.6
660.4
900~1090
27
300
Example



A28
9.8
92
—
20.6
660.4
900~1090
27
   300*****
Example



A29
9.8
92
—
20.6
660.4
900~1090
35
300
Example



A30
9.8
92
—
20.6
660.4
900~1090
53
300
Comparative example





Underline refers to the outside of the scope of the present invention.


*Unrecrystallization temperature range


**Average cooling rate at the central portion temperatures in the thickness direction from 780° C. to 630° C.


***Central portion temperature in the thickness direction


****Distance between the taper starting position and the top surface/Total thickness × 100%


*****Tempering (450° C. × 1 min)
















TABLE 5








Base steel portion
Electric resistance welded portion















Structure
Tensile properties

Tensile properties
Total alloy




















Steel
Grain
Yield
Tensile
Toughness
HIC
Structure
Yield
Tensile
amount in
Toughness
HIC

























pipe
Steel

Percentage
size
strength
strength
vE-50
resistance

Percentage
Grain size
strength
strength
inclusions
vE-50
resistance



No.
No.
Type*
(% by area)
(μm)
YS (MPa)
TS (MPa)
(J)
CAR (%)
Type*
(% by area)
(μm)
YS (MPa)
TS (MPa)
(ppm)**
(J)
CAR (%)
Note





A1
A
QPF + B + C
QPF: 95
6.6
490
533
370
0  
QPF + B + C
QPF: 95
8.3
486
552
30
369
0  
Example


A2
A
QPF + B + C
QPF: 93
10.8 
483
538
130
2.6
QPF + B + C
QPF: 94
9.6
476
534
47
287
2.4
Comparative



















example


A3
A
QPF + B + C
QPF: 93
7.8
387
437
270
5.6
QPF + B + C
QPF: 94
9.2
389
438
57
356
5.6
Comparative



















example


A4
A
QPF + B + C
QPF: 93
6.8
427
489
287
5.9
QPF + B + C
QPF: 93
8.8
429
489
44
246
5.6
Comparative



















example


A5
A
QPF + B + C
QPF: 92
12.4 
436
492
127
2.5
QPF + B + C
QPF: 92
9.8
435
479
43
190
2.4
Comparative



















example


A6
B
QPF + B + C
QPF: 94
5.4
501
550
375
0  
PF + P
PF: 93
12.4 
387
440
43
147
1.5
Comparative



















example


A7
B
QPF + B + C
QPF: 94
5.4
501
550
375
0  
QPF + B + C
QPF: 94
7.8
488
555
28
379
0  
Example


A8
B
QPF + B + C
QPF: 94
5.4
501
550
375
0  
QPF + B + C
QPF: 93
6.6
498
566
26
387
0  
Example


A9
B
QPF + B + C
QPF: 94
5.4
501
550
375
0  
QPF + B + C
QPF: 93
6.3
512
582
38
256
1.1
Example


A10
B
QPF + B + C
QPF: 94
5.4
501
550
375
0  
B + M
B: 88
5.9
578
657
44
123
6.3
Comparative



















example


A11
C
QPF + B + C
QPF: 93
5.4
489
533
368
0  
PF + P
PF: 94
10.4 
393
442
45
143
1.7
Comparative



















example


A12
C
QPF + B + C
QPF: 93
5.4
489
533
368
0  
QPF + B + C
QPF: 94
5.9
498
560
34
366
0  
Example


A13
C
QPF + B + C
QPF: 93
5.4
489
533
368
0  
QPF + B + C
QPF: 93
6.4
505
576
29
375
0  
Example


A14
C
QPF + B + C
QPF: 93
5.4
489
533
368
0  
QPF + B + C
QPF: 93
6.3
506
569
19
412
0  
Example


A15
C
QPF + B + C
QPF: 93
5.4
489
533
368
0  
QPF + B + C
QPF: 87
12.3 
561
630
45
 87
6.5
Comparative



















example


A16
D
PF + P
PF: 93
10.8 
395
489
125
6.3
QPF + B + C
QPF: 94
5.4
547
615
45
245
4.7
Comparative



















example


A17
D
QPF + B + C
QPF: 93
5.2
589
640
304
3.6
QPF + B + C
QPF: 93
4.9
565
635
36
314
3.2
Example


A18
D
QPF + B + C
QPF: 93
3.7
610
663
310
3.5
QPF + B + C
QPF: 93
4.7
595
682
34
308
2.9
Example


A19
D
QPF + B + C
QPF: 91
3.5
623
677
245
4.8
QPF + B + C
QPF: 93
4.5
594
667
34
285
4.2
Example


A20
D
B + M
B: 78
3.2
656
725
135
5.9
QPF + B + C
QPF: 91
4.2
605
680
42
178
4.8
Comparative



















example


A21
E
QPF + B + C
QPF: 93
3.4
720
780
265
4.9
PF + P
PF: 95
10.6 
389
437
45
115
6.8
Comparative



















example


A22
E
QPF + B + C
QPF: 93
3.4
720
780
265
4.9
QPF + B + C
QPF: 93
3.2
712
800
38
289
3.6
Example


A23
E
QPF + B + C
QPF: 93
3.4
720
780
265
4.9
QPF + B + C
QPF: 93
3.7
706
806
37
268
3.9
Example


A24
E
QPF + B + C
QPF: 93
3.4
720
780
265
4.9
QPF + B + C
QPF: 93
3.7
706
806
17
468
1.2
Example


A25
E
QPF + B + C
QPF: 93
3.4
720
780
265
4.9
QPF + B + C
QPF: 88
12.8 
714
802
44
 89
8.6
Comparative



















example


A26
F
QPF + B + C
QPF: 93
3.3
623
672
285
4.9
PF + P
PF: 93
10.3 
394
489
45
135
6.4
Comparative



















example


A27
F
QPF + B + C
QPF: 93
3.3
623
672
285
4.9
QPF + B + C
QPF: 93
4.6
594
669
38
300
3.7
Example


A28
F
QPF + B + C
QPF: 93
3.3
623
672
285
4.9
QPF + B + C
QPF: 93
4.6
587
661
38
302
3.6
Example


A29
F
QPF + B + C
QPF: 93
3.3
623
672
285
4.9
QPF + B + C
QPF: 91
4.3
605
680
42
245
4.2
Example


A30
F
QPF + B + C
QPF: 93
3.3
623
672
285
4.9
QPF + B + C
QPF: 77
4.1
656
737
42
105
7.4
Comparative



















example





Underline refers to the outside of the scope of the present invention.


*QPF: quasi-polygonal ferrite, PF: polygonal ferrite, B: bainite, P: perlite, QP: quasi-perlite, C: cementite, M: martensite


**Total amount of Si, Mn, Al, Ca, and Cr in inclusions having an equivalent circular diameter of 2 μm or more (mass ppm)






The base steel portion and the electric resistance welded portion in the examples have a microstructure in which fine quasi-polygonal ferrite having a grain size dα of 10 μm or less occupies 90% by area or more. The electric resistance welded steel pipes according to the examples have high strength represented by a yield strength YS of 400 MPa or more, excellent low-temperature toughness represented by a Charpy impact absorbed energy vE−50 of 150 J or more at −50° C., and excellent HIC resistance represented by a crack area ratio CAR of 5% or less after immersion in a NACE Solution A specified in NACE TM0284 for 96 hours. The remainder other than quasi-polygonal ferrite were pearlite, quasi-pearlite, cementite, bainite, and martensite, which occupies less than 10% by area in total.
The comparative examples do not have the desired high strength or have deteriorated low-temperature toughness or deteriorated HIC resistance.
In the comparative example (steel pipe No. A2) in which the heating temperature of the hot-rolling was higher than our range, the comparative example (steel pipe No. A5) in which the rolling reduction in the unrecrystallization temperature range in the hot-rolling was lower than our range, and the comparative example (steel pipe No. A16) in which the cooling rate after the hot-rolling was lower than our range, the base steel portion has a coarse microstructure and has deterioration of low-temperature toughness. In the comparative example (steel pipe No. A3) in which the heating temperature of the steel in the hot-rolling was lower than our range, the comparative example (steel pipe No. A4) in which the heating holding time of the steel in the hot-rolling was lower than our range, and the comparative example (steel pipe No. A20) in which the cooling rate after the hot-rolling was higher than our range, the base steel portion has deterioration of HIC resistance.
In the comparative examples (steel pipes Nos. A11 and A21) in which the heating temperature in the heat treatment was lower than our range and the comparative examples (steel pipes Nos. A6 and A26) in which the cooling rate after the heating in the heat treatment was lower than our range, the electric resistance welded portion has a coarse microstructure and has deterioration of strength and deterioration of low-temperature toughness. In the comparative examples (steel pipes Nos. A15 and A25) in which the heating temperature in the heat treatment was higher than our range and the comparative examples (steel pipes Nos. A10 and A30) in which the cooling rate after the heating in the heat treatment was higher than our range, the electric resistance welded portion has a microstructure that is different from the desired microstructure composed of fine quasi-polygonal ferrite and has deterioration of low-temperature toughness and deterioration of HIC resistance.
In the example (steel pipe No. A14) in which a groove was formed in the ends of the steel strip in the width direction in the electric resistance welding and the example (steel pipe No. A24) in which the atmosphere was controlled in the electric resistance welding, the total amount of Si, Mn, Al, Ca, and Cr in the inclusions having an equivalent circular diameter of 2 μm or more contained in the electric resistance welded portion is as small as 20 mass ppm or less, and the electric resistance welded portion has significantly improved low-temperature toughness with the vE−50 being 400 J or more. The example (steel pipe No. A28) in which the heating and cooling were followed by tempering at 450° C. for 1 min in the heat treatment also has excellent low-temperature toughness and excellent HIC resistance.

## Claims

1. A high-strength thick-walled electric resistance welded steel pipe having excellent low-temperature toughness and excellent HIC resistance comprising:
a base metal composition consisting of, on a mass percent basis,
C: 0.025% to 0.084%, Si: 0.10% to 0.30%,
Mn: 0.70% to 1.80%, P: 0.001% to 0.018%,
S: 0.0001% to 0.0029%, Al: 0.01% to 0.10%,
Nb: 0.001% to 0.065%, V: 0.001% to 0.065%,
Ti: 0.001% to 0.033%, Ca: 0.0001% to 0.0035%,
N: 0.0050% or less, O: 0.0030% or less, and
optionally, one or more selected from the group consisting of B: 0.0030% or less, Cu: 0.001% to 0.350%, Ni: 0.001% to 0.350%, Mo: 0.001% to 0.350%, and Cr: 0.001% to 0.700% and the remainder being Fe and incidental impurities,

wherein Pcm defined by formula (1) is 0.20 or less,


Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B  (1)


wherein C, Si, Mn, Cu, Ni, Cr, Mo, V, and B denote amounts (mass %) of corresponding elements,

a microstructure which includes 90% by area or more of quasi-polygonal ferrite having a grain size of 10 μm or less in each of a base steel portion and an electric resistance welded portion of the steel pipe,
a yield strength YS of 400 MPa or more,
and
an absorbed energy vE−50 of 150 J or more at −50° C. in a Charpy impact test,
wherein the electric resistance welded steel pipe is formed by rounding a steel strip having the base metal composition to form a butt joined seam by electronic resistance welding.

2. The welded steel pipe according to claim 1, wherein the total amount of Si, Mn, Al, Ca, and Cr in inclusions having an equivalent circular diameter of 2 μM or more contained in the electric resistance welded portion is 0.0089% or less on a mass percent basis.

3. A method of manufacturing a high-strength thick-walled electric resistance welded steel pipe having excellent low-temperature toughness and excellent HIC resistance, comprising:
a hot-rolling step of producing a hot-rolled steel strip from steel by heating, hot-rolling, cooling, and coiling,
a pipe-forming step of continuously roll-forming the hot-rolled steel strip after the hot-rolling step to form a tubular product having a substantially circular cross section and
butt-welding circumferential ends of the tubular product by electric resistance welding to produce an electric resistance welded steel pipe, wherein

the steel has a chemical composition consisting of C: 0.025% to 0.084%, Si: 0.10% to 0.30%,
Mn: 0.70% to 1.80%, P: 0.001% to 0.018%,
S: 0.0001% to 0.0029%, Al: 0.01% to 0.10%,
Nb: 0.001% to 0.065%, V: 0.001% to 0.065%,
Ti: 0.001% to 0.033%, Ca: 0.0001% to 0.0035%,
N: 0.0050% or less, O: 0.0030% or less on a mass percent basis, and
optionally, one or more selected from the group consisting of B: 0.0030% or less, Cu: 0.001% to 0.350%, Ni: 0.001% to 0.350%, Mo: 0.001% to 0.350%, and Cr: 0.001% to 0.700% and the remainder being Fe and incidental impurities,
wherein Pcm defined by formula (1) is 0.20 or less,


Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B  (1)


wherein C, Si, Mn, Cu, Ni, Cr, Mo, V, and B denote the amounts (mass %) of the corresponding elements,

the hot-rolling step is performed by heating the steel to a temperature of 1200° C. to 1280° C., maintaining the temperature for 90 min or more, hot-rolling the steel at a hot-rolling reduction of 20% or more in an unrecrystallized austenite region, after completion of the hot-rolling, cooling the steel to a finish cooling temperature of 630° C. or less at a cooling rate of 7° C./s to 49° C./s, the cooling rate being an average cooling rate at a temperature from 780° C. to 630° C. at a central portion in the thickness direction, and coiling the steel at a coiling temperature of 400° C. or more and less than 600° C.,
the pipe-forming step is followed by a heat treatment that includes heating the electric resistance welded portion of the electric resistance welded steel pipe on a production line such that the electric resistance welded portion has a temperature of 800° C. to 1150° C. over the total wall thickness, then cooling the electric resistance welded portion to a finish cooling temperature of 630° C. or less at a cooling rate of 7° C./s to 49° C./s, the cooling rate being the average cooling rate at a temperature from 780° C. to 630° C. at the central portion in the thickness direction, and then allowing the electric resistance welded portion to air-cool,
and
the base steel portion and the electric resistance welded portion of the electric resistance welded steel pipe have a yield strength YS of 400 MPa or more and an absorbed energy vE—50 of 150 J or more at −50° C. in a Charpy impact test.

4. The method according to claim 3, further comprising forming a tapered groove in end faces of the hot-rolled steel strip in the width direction by fin pass forming during the roll-forming in the pipe-forming step, a distance between a taper starting position of the tapered groove and a surface that will become a pipe outer surface or a surface that will become a pipe inner surface in the steel strip thickness direction is 2% to 60% of the hot-rolled steel strip thickness.

5. The method according to claim 4, wherein atmospheric oxygen partial pressure in the electric resistance welding in the pipe-forming step is adjusted at 900/foxy mass ppm or less, and the foxy represents a degree of oxidizability of molten steel defined by formula (2),


foxy=Mn+10(Si+Cr)+100Al+1000Ca  (2)

wherein Mn, Si, Cr, Al, and Ca denote the amounts (mass %) of the corresponding elements.

6. The method according to claim 3, wherein atmospheric oxygen partial pressure in the electric resistance welding in the pipe-forming step is adjusted at 900/foxy mass ppm or less, and the foxy represents a degree of oxidizability of molten steel defined by formula (2),


foxy=Mn+10(Si+Cr)+100Al+1000Ca  (2)

wherein Mn, Si, Cr, Al, and Ca denote the amounts (mass %) of the corresponding elements.

7. The method according to claim 3, wherein the cooling in the heat treatment includes installing at least two lines of cooling headers in a conveying direction above the electric resistance welded portion, the cooling headers being coupled to a nozzle through which a rod-like flow of cooling water can be ejected at a water flow rate of 1 m3/m2·min or more, and ejecting the rod-like flow of cooling water through the nozzle at a rate of 1 m/s or more.

8. The method according to claim 7, wherein the plurality of cooling headers are configured to independently control the ejection of cooling water.