EP3418410B1

EP3418410B1 - Seamless steel pipe and manufacturing method of same

Info

Publication number: EP3418410B1
Application number: EP16890483.7A
Authority: EP
Inventors: Keiichi Kondo; Taro Oe; Yuji Arai; Yusuke Sendai; Hiroki KAMITANI
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2016-02-16
Filing date: 2016-02-16
Publication date: 2021-04-07
Anticipated expiration: 2036-02-16
Also published as: CA3013287A1; CN108699644A; AU2016393486A1; AU2016393486B2; BR112018007744A2; BR112018007744B1; RU2706257C1; JPWO2017141341A1; MX2018005240A; JP6112267B1; WO2017141341A1; EP3418410A4; EP3418410A1; CA3013287C; CN108699644B; US20180355451A1

Description

TECHNICAL FIELD

The present invention relates to a seamless steel pipe and a method of manufacturing the same, and, more particularly, to a seamless steel pipe suitable for line pipe and a method of manufacturing the same.

BACKGROUND ART

Oil and gas resources from oil wells located on land and in shallow seas are drying up and, to address this, increasing numbers of offshore oil fields in deep seas are being developed. In an offshore oil field, crude oil or gas must be transported from the pithead of an oil well or gas well installed on the seabed to the platform above the sea using a flow line or a riser. A flow line is a line pipe laid along the topography of the surface of the earth or the seabed. A riser is a line pipe disposed to rise from the seabed toward the platform (i.e. upward).
The inner side of a steel pipe forming part of a flow line laid in the deep sea is subject to a high interior fluid pressure having a pressure from deep strata added thereto and, when the operation is halted, is also affected by seawater pressures of the deep sea. A steel pipe forming part of a riser is further affected by repeated distortions by ocean waves. Accordingly, it is desirable that steel pipes used for such applications have high strength and high toughness. In addition, oil and gas wells are being developed in sour environments, which are harsher than conditions for conventional wells, such as deep seas and cold regions. Offshore pipe lines laid in such harsh sour environments are required to have a higher strength (i.e. pressure resistance) and toughness than conventional ones, and are further required to have hydrogen-induced cracking resistance (HIC resistance) and sulfide stress corrosion cracking resistance (SSC resistance).
Patent Document 1 discloses a seamless steel pipe with a large wall thickness for line pipe having high strength and good toughness, containing C: 0.03 to 0.08 %, Si: 0.15 or less, Mn: 0.3 to 2.5 %, Al: 0.001 to 0.10 %, Cr: 0.02 to 1.0 %, Ni: 0.02 to 1.0 %, Mo: 0.02 to 1.2 %, Ti: 0.004 to 0.010 %, N: 0.002 to 0.008 % and one or more of Ca, Mg and REM:0.0002 to 0.005 % in total, the balance being Fe and impurities, where P in the impurities: 0.05 % or less, S: 0.005 % or less, and the wall thickness is 30 to 50 mm.
Patent Document 2 discloses a high-strength seamless steel pipe with a large wall thickness that is made by quenching and tempering and having a yield strength higher than 450 MPa for line pipe with good sour resistance where the Vickers hardness HV5 measurable at an outermost or innermost position of the pipe with an applied load of 5 kgf (with a force in the test of 49 N) is 250 HV5 or lower.
Patent Document 3 discloses a seamless steel pipe for line pipe containing, in mass %, C: 0.02 to 0.10 %, Si: 0.5 % or less, Mn: 0.5 to 2.0 %, Al: 0.01 to 0.1 %, Ca: 0.005 % or less, and N: 0.007 % or less, and one or more selected from the group consisting of Ti: 0.008 % or less, V: less than 0.06 % and Nb: 0.05 % or less, the balance being Fe and impurities, where the total content of Ti, V and Nb is smaller than 0.06 %, the carbon equivalent Ceq defined by the following equation is 0.38 % or more, and the size of carbonitride particles containing one or more of Ti, V, Nb and Al is 200 nm or less. $Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15$
Patent Document 4 discloses a seamless steel pipe with a chemical composition of, in mass %, C: 0.02 to 0.10 %, Si: 0.05 to 0.5 %, Mn: 1.0 to 2.0 %, Mo: 0.5 to 1.0 %, Cr: 0.1 to 1.0 %, Al: 0.01 to 0.10 %, P: 0.03 % or less, S: 0.005 % or less, Ca: 0.0005 to 0.005 %, V: 0.010 to 0.040 %, and N: 0.002 to 0.007 % and one or more selected from the group consisting of Ti: 0.001 to 0.008 % and Nb: 0.02 to 0.05 %, the balance being Fe and impurities, where the carbon equivalent Ceq is 0.50 to 0.58 %, the pipe containing a specified carbide.
Patent Document 5 discloses a seamless quenched and tempered steel pipe having a wall thickness (WT) higher than or equal to 35 mm and lower than or equal to 80 mm. Embodiments of the steel pipe can comprise C, Mn, Cr, Ni, Mo, Al, Ca, N, Nb, Ti, Zr, and Ta. Further, for some embodiments of the steel pipe wherein, defining a first parameter P1=50×C+Cr+l0×Mo+70×V, the chemical composition can satisfy a first condition P1≦8.0.
Patent Document 6 discloses a seamless steel pipe consisting of: in mass%, C: 0.03 to 0.08%, Si: not more than 0.25%, Mn: 0.3 to 2.0%, P: not more than 0.05%, S: not more than 0.005%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02 to 0.8%, N: 0.002 to 0.008%, Ca: 0.0005 to 0.005%, and Nb: 0.01 to 0.1%, the balance being Fe and impurities, and having a wall thickness of not less than 50 mm. In a cross section perpendicular to an axial direction of the seamless steel pipe, an average crystal grain size of prior austenite grains in a near surface portion is less than 80 µm, the near surface portion being a 500 µm x 500 µm area centered at a position of a depth of 2 mm from a surface, and a difference between the average crystal grain size of the prior austenite grains in the near surface portion and an average crystal grain size of prior austenite grains in a central portion of a wall thickness of the cross section is less than 50 µm, the central portion being a 500 µm x 500 µm area centered at a center position of the wall thickness.

Prior Art Document

Patent Document

[Patent Document 1] JP 2010-242222 A
[Patent Document 2] JP 2013-32584 A
[Patent Document 3] WO 2011/152240
[Patent Document 4] JP 5516831 B
[Patent Document 5] US2014/299235 A1
[Patent Document 6] EP 2 843 072 A1

DISCLOSURE OF THE INVENTION

Even when one or more of the above conventional techniques are used, a seamless steel pipe having a strength of X80 grade or higher as defined by the American Petroleum Institute (API) standards (i.e. a lower limit yield strength of 555 MPa or higher) may not have good SSC resistance in a reliable manner.
To improve the strength and toughness of a seamless steel pipe produced by quenching-and-tempering, the content of alloy elements such as carbon may be increased to increase hardenability. However, if the content of alloy elements such as carbon is increased, the strength (i.e. hardness) of the surface of the steel pipe increases. In a seamless steel pipe produced by quenching-and-tempering, the surface layer is cooled at a high rate during quenching and can easily be hardened, increasing the hardness, while the in-the-wall portions have low hardness. This tendency may remain after tempering. As such, in a seamless steel pipe having a strength of X80 grade or higher, a surface layer hardness may exceed 250 Hv, which is the higer limit required in the sour resistance grade according to the API 5L standards.
Although the techniques of Patent Document 1 are effective in achieving high strength and high toughness, they do not sufficiently consider reducing the hardness of the surface layer or thus improving SSC resistance. Patent Document 2 states that the hardness of the surface layer of a steel pipe can be controlled to be 250 HV5 or lower; however, it appears to require a special manufacturing process. Patent Document 3 provides some considerations about SSC resistance; however, after hot forming, it is necessary to perform direct quenching or in-line quenching and then reheating-and-quenching. Patent Document 4 provides some considerations about the hardness of the surface layer of a steel pipe and HIC resistance; however, a reheating-and-quenching step is necessary and, after hot forming, direct quenching or in-line quenching is used as necessary, which means manufacturing costs that are not very reasonable.
An object of the present invention is to provide a seamless steel pipe that can be manufactured by a relatively reasonable manufacturing process and that provides a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner.
A seamless steel pipe, and a method of manufacturing a seamless steel pipe, according to the present invention are as described in the appended claims.
The present invention provides a seamless steel pipe that can be manufactured by a relatively reasonable manufacturing process and that provides a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] FIG. 1 is a block diagram illustrating an example of a manufacturing line.
[FIG. 2] FIG. 2 is a flow chart illustrating a process for manufacturing the seamless steel pipe.
[FIG. 3] FIG. 3 shows changes in the surface temperature of a workpiece during a manufacture versus time.
[FIG. 4] FIG. 4 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and yield strength YS for steel B.
[FIG. 5] FIG. 5 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and yield strength YS for steel A.
[FIG. 6] FIG. 6 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and hardness at an outer surface, an in-the-wall portion and an inner surface for steel B.
[FIG. 7] FIG. 7 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and hardness at an outer surface, an in-the-wall portion and an inner surface for steel A.
[FIG. 8] FIG. 8 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and maximum difference in hardness for steel B.
[FIG. 9] FIG. 9 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and maximum difference in hardness for steel A.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The present inventors did research to find a method of providing a seamless steel pipe that ensures a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner. They found out that limiting the carbon equivalent of a steel to an appropriate range and reducing the difference between the hardness of the surface layer and the hardness of the in-the-wall portions of the seamless steel pipe ensures a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner, where only direct quenching or in-line quenching is performed after hot forming and no reheating-and-quenching is performed.
During the quenching after rolling, the surface layer of a seamless steel pipe is cooled at high rate and can easily be hardened. As such, the surface layer of the seamless steel pipe tends to be hard and may exceed the values of hardness specified by the API 5L standards or DNV-OS-F101 standards. On the other hand, the portions located in the middle in the wall thickness of the seamless steel pipe is cooled at a lower rate and cannot easily be hardened such that non-quenched structures such as ferrite may be included. Thus, there is typically a difference between the hardness of the surface layer and that of the in-the-wall portions, and this tendency may remain after tempering for certain tempering conditions. Further, in a seamless steel pipe with high carbon equivalent such as those used in high-strength steel with X80 grade or higher, the difference between the hardness of the surface layer and that of the in-the-wall portions tends to be significant. Such a high hardness of the surface layer may be a problem when good sour resistance is to be achieved in a reliable manner.
If the carbon equivalent is too low, it is difficult to ensure a certain strength of a seamless steel pipe. If the carbon equivalent is too high, it is difficult to reduce the Vickers hardness of the surface layer to 250 Hv or lower with a manufacturing process in which reheating-and-quenching is eliminated, direct quenching or in-line quenching being only one step of the quenching. This is because, if the quenching after hot forming is direct quenching or in-line quenching, the austenite grains tend to be coarse compared with implementations where reheating-and-quenching is performed, which increases overall hardenability. In view of this, Ceq as defined by equation (1) below is to be not less than 0.430 % and less than 0.500 %: $Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15$
where the symbol of each element in equation (1) is substituted by the content of this element in mass %.
To reduce the difference between the hardness of the surface layer and that of the in-the-wall portions, it is effective to limit the carbon equivalent and, in addition, the tempering conditions appropriately. That is, if tempering is not sufficiently done, the reduction in the hardness of the surface layer is insufficient such that some portions may have a Vickers hardness higher than 250 Hv. More specifically, the Larson-Miller parameter PL as defined by equation (2) below is 18800 or higher. $PL = (T + 273) \times (20 + \log (t))$
In equation (2), T is a tempering temperature (in °C) and t is a holding time (in hours) for that temperature.
The present invention was made based on the above findings. A seamless steel pipe in one embodiment of the present invention will now be described in detail with reference to the drawings. The same or corresponding portions in the drawings are labeled with the same characters and their description will not be repeated.

[Chemical Composition]

The seamless steel pipe in the present embodiment has the chemical composition described below. In the following description, "%" for the content of an element means mass %.

C: 0.02 to 0.15 %

Carbon (C) increases the strength of the steel. If the C content is lower than 0.02 %, this effect cannot be sufficiently achieved. If the C content is higher than 0.15 %, the toughness of the steel decreases. In view of this, the C content should be in the range of 0.02 to 0.15 %. The C content is preferably higher than 0.02 %, and more preferably 0.04 % or higher. The C content is preferably 0.10 % or lower, and more preferably 0.08 % or lower.

Si: 0.05 to 0.5 %

Silicon (Si) deoxidizes steel. This effect can be clearly achieved if the Si content is 0.05 % or higher. However, if the Si content is higher than 0.5 %, the toughness of the steel decreases. In view of this, the Si content should be in the range of 0.05 to 0.5 %. The Si content is preferably higher than 0.05 %, and more preferably 0.08 % or higher, and still more preferably 0.10 % or higher. The Si content is preferably lower than 0.5 %, and more preferably 0.25 % or lower, and still more preferably 0.20 % or lower.

Mn: 0.30 to 2.5 %

Manganese (Mn) increases the hardenability of steel to increase the strength of the steel. These effects cannot be sufficiently achieved if the Mn content is lower than 0.30 %. If the Mn content is higher than 2.5 %, Mn segregates in the steel, decreasing the toughness of the steel. In view of this, the Mn content should be in the range of 0.30 to 2.5 %. The Mn content is preferably higher than 0.30 %, and more preferably 1.0 % or higher, and still more preferably 1.3 % or higher. The Mn content is preferably lower than 2.5 %, and more preferably 2.0 % or lower, and still more preferably 1.8 % or lower.

P: up to 0.03 %

Phosphorus (P) is an impurity. P decreases the toughness of steel. Thus, lower P contents are preferable. In view of this, the P content should be 0.03 % or lower. The P content is preferably lower than 0.03 %, and more preferably 0.015 % or lower, and still more preferably 0.012 % or lower.

S: up to 0.006 %

Sulphur (S) is an impurity. S bonds with Mn to form coarse MnS particles and thus decreases the toughness and HIC resistance of the steel. Thus, lower S contents are preferable. In view of this, the S content should be 0.006 % or lower. The S content is preferably lower than 0.006 %, and more preferably 0.003 % or lower, and still more preferably 0.002 % or lower.

O: up to 0.004 %

Oxygen (O) is an impurity. O forms coarse oxide particles or clusters of oxide particles, decreasing the toughness of the steel. Thus, lower O contents are preferable. In view of this, the O content should be 0.004 % or lower. The O content is preferably 0.003 % or lower, and more preferably 0.002 % or lower.

Al: 0.01 to 0.10 %

Aluminum (Al) bonds with N to form fine nitride particles, increasing the toughness of the steel. This effect cannot be sufficiently achieved if the Al content is lower than 0.01 %. If the Al content is higher than 0.10 %, coarse Al nitride particles result, decreasing the toughness of the steel. In view of this, the Al content should be in the range of 0.01 to 0.10 %. The Al content is preferably higher than 0.01 %, and more preferably 0.02 % or higher. The Al content is preferably lower than 0.10 %, and more preferably 0.08 % or lower, and still more preferably 0.06 % or lower. As used herein, Al content means the content of acid-soluble Al (i.e. so-called "sol. Al").

Ti: 0.001 to 0.010 %

Titanium (Ti) bonds with N in a steel and forms TiN, suppressing the reduction in the toughness of the steel due to dissolved N. Further, the dispersed and precipitated fine TiN particles increase the toughness of the steel. These effects cannot be sufficiently achieved if the Ti content is lower than 0.001 %. If the Ti content is higher than 0.010 %, coarse TiN particles result or coarse TiC particles are produced, decreasing the toughness of the steel. In view of this, the Ti content should be in the range of 0.001 to 0.010 %. The Ti content is preferably higher than 0.001 % and more preferably 0.002 % or higher. The Ti content is preferably lower than 0.010 %, and more preferably 0.006 % or lower, and still more preferably 0.005 % or lower.

N: up to 0.007 %

Nitrogen (N) bonds with Al and forms fine Al nitride particles, increasing the toughness of the steel. However, if the N content is higher than 0.007 %, dissolved N decreases the toughness of the steel. Further, if the N content is too high, coarse carbonitride and/or nitride particles result, decreasing the toughness of the steel. In view of this, the N content should be 0.007 % or lower. The N content is preferably lower than 0.007 %, and more preferably 0.006 % or lower, and still more preferably 0.005 % or lower. The N content is preferably 0.002 % or higher.

Cr: 0.05 to 1.0 %

Chromium (Cr) increases the hardenability of steel and increases the strength of the steel. Cr further increases the temper softening resistance of the steel. These effects cannot be sufficiently achieved if the Cr content is lower than 0.05 %. If the Cr content is higher than 1.0 %, the toughness of the steel decreases. In view of this, the Cr content should be in the range of 0.05 to 1.0 %. The Cr content is preferably higher than 0.05 %, and more preferably 0.2 % or higher. The Cr content is preferably lower than 1.0 %, and more preferably 0.8 % or lower.

Mo: not less than 0.02 % and less than 0.5 %

Molybdenum (Mo) improves the strength of steel by transformation toughening and solute strengthening. This effect cannot be sufficiently achieved if the Mo content is lower than 0.02 %. If the Mo content is higher than 0.5 %, the toughness of the steel decreases. In view of this, the Mo content should be not lower than 0.02 % and lower than 0.5 %. The Mo content is preferably higher than 0.02 %, and more preferably 0.05 % or higher, and still more preferably 0.1 % or higher. The Mo content is preferably 0.4 % or lower, and more preferably 0.3 % or lower.

Ni: 0.03 to 1.0 %

Nickel (Ni) increases the hardenability of steel and increases the strength of the steel. Further, Ni has the effect of improving the adherence of scales formed on the surface of the steel during the heating step for quenching, and also the effect of reducing the increase in the hardness of the surface layer of the steel since the scales reduce the cooling rate at the surface of the steel during the cooling step for quenching. These effects cannot be sufficiently achieved if the Ni content is lower than 0.03 %. If the Ni content is higher than 1.0 %, the SSC resistance decreases. In view of this, the Ni content should be in the range of 0.03 to 1.0 %. The Ni content is preferably 0.05 % or higher, and more preferably 0.08 % or higher, and still more preferably 0.10 % or higher. The Ni content is preferably lower than 1.0 %, and more preferably 0.7 % or lower, and still more preferably 0.5 % or lower.

Cu: 0.02 to 1.0 %

Copper (Cu) increases the hardenability of steel and increases the strength of the steel. Further, Cu has the effect of improving the adherence of scales formed on the surface of the steel during the heating step for quenching, and also the effect of reducing the increase in the hardness of the surface layer of the steel since the scales reduce the cooling rate at the surface of the steel during the cooling step for quenching. These effects cannot be sufficiently achieved if the Cu content is lower than 0.02 %. If the Cu content is higher than 1.0 %, the weldability of the steel decreases. Further, if the Cu content is too high, the grain boundary strength of the steel at high temperatures decreases, decreasing the hot workability of the steel. In view of this, the Cu content should be in the range of 0.02 to 1.0 %. The Cu content is preferably 0.05 % or higher, and more preferably 0.08 % or higher, and still more preferably 0.10 % or higher. The Cu content is preferably lower than 1.0 %, and more preferably 0.7 % or lower, and still more preferably 0.5 % or lower.

V: 0.020 to 0.20 %

Vanadium (V) bonds with C in a steel and forms a V carbide to increase the strength of the steel. Further, V is dissolved in an Mo carbide to form a carbide. A carbide containing V is less likely to form coarse particles. These effects cannot be effectively achieved if the V content is lower than 0.020 %. If the V content is higher than 0.20 %, coarse carbide particles result. In view of this, the V content should be in the range of 0.020 to 0.20 %. The V content is preferably higher than 0.020 %, and more preferably 0.04 % or higher. The V content is preferably lower than 0.16 %.

Ca: 0.0005 to 0.005 %

Calcium (Ca) bonds with S in steel to form CaS. As CaS is formed, the formation of MnS is suppressed. Thus, Ca increases the toughness and HIC resistance of the steel. These effects cannot be sufficiently achieved if the Ca content is lower than 0.0005 %. If the Ca content is higher than 0.005 %, the cleanliness of the steel decreases, decreasing the toughness and HIC resistance of the steel. Thus, the Ca content should be in the range of 0.0005 to 0.005 %. The Ca content is preferably higher than 0.0005 %, and more preferably 0.0008 % or higher, and still more preferably 0.001 % or higher. The Ca content is preferably lower than 0.005 %, and more preferably 0.003 % or lower, and still more preferably 0.002 % or lower.
The balance of the chemical composition of the seamless steel pipe in the present embodiment is made of Fe and impurities. Impurity in this context means an element originating from ore or scraps used as a raw material of steel or an element that has entered from the environment or the like during the manufacturing process.
Further, the chemical composition of the seamless steel pipe in the present embodiment may contain Nb in lieu of some of Fe.

Nb: 0 to 0.05 %

Niobium (Nb) is an optional element. Nb bonds with C and/or N in steel and forms fine Nb carbide and/or carbonitride particles to increase the toughness of the steel. Further, Nb is dissolved in an Mo carbide and forms a specified carbide, thereby preventing coarse particles of a specified carbide from being produced. On the other hand, if the Nb content is higher than 0.05 %, coarse carbide particles result. In view of this, the Nb content should be in the range of 0 to 0.05 %. The above effects can be clearly achieved if the Nb content is 0.010 % or higher. The Nb content is preferably 0.015 % or higher, and more preferably 0.020 % or higher. The Nb content is preferably 0.040 % or lower, and more preferably 0.035 % or lower.

[Carbon Equivalent Ceq]

In the seamless steel pipe in the present embodiment, a carbon equivalent Ceq as defined by equation (1) is not less than 0.430 % and less than 0.500 %. $Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15$
where the symbol of each element in equation (1) is substituted by the content of this element in mass %.
If the carbon equivalent Ceq is lower than 0.430 %, it is difficult to ensure a certain strength of a seamless steel pipe. If the carbon equivalent Ceq is 0.500 or higher, it is difficult to reduce the Vickers hardness of the surface layer to 250 Hv or lower with a manufacturing process in which the quenching after hot forming is only one step of direct quenching or in-line quenching.

[Microstructure]

In the microstructure of the seamless steel pipe in the present embodiment, the main phase from the surface layer to the in-the-wall portions is tempered martensite or tempered bainite. The seamless steel pipe in the present embodiment contains no recrystallized ferrite at least in a region deeper than a position 1 mm deep relative to the surface. Recrystallized ferrite extremely reduces the hardness of a portion at 1 mm from the surface layer of the seamless steel pipe.
The main phase being tempered martensite or tempered bainite generally means a microstructure in which the volume fraction of tempered martensite is 50 % or higher, a microstructure in which the volume fraction of tempered bainite is 50 % or higher, or a microstructure in which the sum of the volume fraction of tempered martensite and the volume fraction of tempered bainite is 50 % or higher. In other words, the above phrase means a microstructure in which the volume fraction of a structure that is neither tempered martensite nor tempered bainite (for example, ferrite) is lower than 50 %.

[Crystal Grain Size Number]

In the microstructure of the seamless steel pipe of the present embodiment, the size of the prior austenite grains is lower than 6.0 in crystal grain size number, as defined in ASTM E 112-10.
The prior austenite grain size number may be measured in accordance with ASTM E112-10 by cutting out a test specimen from each steel pipe preferably before tempering and after quenching, such that a cross section perpendicular to the length of the steel pipe (i.e. pipe forming direction) forms the observed surface, and imbedding the test specimen into a resin and then using the Bechet-Beaujard method where it is corroded by a picric acid saturated aqueous solution to let prior austenite grain boundaries appear.
Alternatively, the ASTM grain size number of prior austenite crystal grains of the tempered steel pipe may be determined by using methods such as electron beam backward scattering diffraction (EBSD) based on the orientation relationship of crystals. In such cases, the metal microstructure of a steel pipe after tempering is observed by EBSD in the following manner: A sample is obtained from the middle in the wall thickness in a cross section of a tempered seamless steel pipe (i.e. cross section perpendicular to the axial direction of the seamless steel pipe); the obtained sample is used to perform crystal orientation analysis by EBSD for an observed area of 500 × 500 µm², and lines are drawn where a prior austenite grain boundary is defined as the boundary of grains in a misorientation angle in the range of 15 to 51° and, based on the resulting drawing, the crystal grain size number is calculated in accordance with ASTM E112-10.
Theoretically, the prior austenite grain size after quenching and before tempering is the same as the prior austenite grain size after tempering. The prior austenite grain size determined by EBSD after tempering is substantially equal to the value obtained by observing crystal grains that were caused to appear by the Bechet-Beaujard method after quenching and before tempering, with an error of about ±0.2 in grain size number. Thus, "the size of the prior austenite grains is lower than 6.0 in crystal grain size number, as defined in ASTM E112-10" as in the present invention means that, if the crystal grain size after quenching is not known, at least, a crystal grain size number determined by EBSD after tempering being lower than 5.8 is in the scope of the present invention. In the following description, unless specifically stated, prior austenite grain size is a value obtained by the Bechet-Beaujard method for a test specimen after quenching and before tempering.
If the prior austenite grains are fine grains with a crystal grain size number of 6.0 or higher, sufficient hardenability cannot be achieved in a material with a low carbon equivalent Ceq, as in the present embodiment. Thus, a predetermined strength may not be obtained. Further, it is difficult to produce a microstructure with such fine grains with a manufacturing process in which the quenching after hot forming is only one step of direct quenching or in-line quenching. The crystal grain size number of prior austenite grains is preferably 5.5 or lower, and more preferably, 5.0 or lower.

[Vickers Hardness and Yield Strength]

In the seamless steel pipe in the present embodiment, a portion between a position at 1 mm from the inner surface and a position at 1 mm from the outer surface has a Vickers hardness of 250 Hv or lower. More specifically, in the seamless steel pipe in the present embodiment, the Vickers hardness measured in compliance with JIS Z 2244 at any position between a position at 1 mm from the inner surface and a position at 1 mm from the outer surface is 250 Hv or lower.
The seamless steel pipe of the present invention has smaller variations in hardness along the wall thickness direction. More specifically, the difference between the Vickers hardness of a portion at 1 mm from the inner surface and that of a portion in the middle in the wall thickness, the difference between the Vickers hardness of a portion at 1 mm from the outer surface and that of a portion in the middle in the wall thickness, and the difference between the Vickers hardness of a portion at 1 mm from the inner surface and that of a portion at 1 mm from the outer surface is 25 Hv or lower.
The seamless steel pipe in the present embodiment has a yield strength of X80 grade or higher (i.e. 555 MPa or higher) according to the API standards.
The seamless steel pipe in the present embodiment may be suitably used as, although not limited thereto, a seamless steel pipe with a wall thickness of 25 to 55 mm. More preferably, to rationalize the use of alloys, the wall thickness of a seamless steel pipe is in the range of 25 to 40 mm.

[Manufacturing Method]

An example of a method of manufacturing the seamless steel pipe in the present embodiment will be described below. However, the method of manufacturing the seamless steel pipe in the present embodiment is not limited thereto.

[Manufacturing Line]

FIG. 1 is a block diagram illustrating an example of a manufacturing line. Referring to FIG. 1, the manufacturing line includes a heating furnace 1, a piercing machine 2, an elongation rolling mill 3, a sizing rolling mill 4, a supplementary heating furnace 5, a water-cooling apparatus 6, and a tempering apparatus 7. A plurality of transport rollers 10 are disposed between these apparatuses.

[Manufacturing Flow]

FIG. 2 is a flow chart illustrating a process for manufacturing the seamless steel pipe in the present embodiment. FIG. 3 shows changes in the surface temperature of a workpiece (i.e. a steel raw material, hollow shell or seamless steel pipe) during a manufacture versus time. In the graph, A1 indicates the Ac₁ point when considering a workpiece being heated, and indicates the Ar₁ point when considering a workpiece being cooled. Further, in the graph, A3 indicates the Ac₃ point when considering a workpiece being heated, and indicates the Ar₃ point when considering a workpiece being cooled.
As shown in FIGS. 1 to 3, the manufacturing process involves first heating a steel raw material using the heating furnace 1 (heating step: S1). The steel raw material may be a round billet, for example. The steel raw material may be produced by a continuous casting system such as round CC. The steel raw material may be produced by hot working (e.g. forging or blooming) an ingot or slab. A case with a steel raw material that is a round billet will be described below.
The heated round billet is hot-worked to produce a seamless steel pipe (S2 and S3). More specifically, the round billet is piercing-rolled by the piercing machine 2 to produce a hollow shell (piercing-rolling step: S2). Further, the hollow shell is rolled by the elongation rolling mill 3 and sizing rolling mill 4 to produce a seamless steel pipe (elongation rolling step and sizing rolling step S3).
The seamless steel pipe produced by the hot working is heated to a predetermined temperature by the supplementary heating furnace 5 as necessary (supplementary heating step: S4). The seamless steel pipe produced by the hot working or the heated seamless steel pipe is quenched by the water-cooling apparatus 6 (quenching step: S5). In either case, the seamless steel pipe produced by the hot working is quenched without being cooled to lower than Ar₃ temperature. The quenched seamless steel pipe is tempered by the tempering apparatus 7 (tempering step S6).
That is, in the above manufacturing method, quenching is performed promptly after the hot working is finished. More specifically, after hot working, quenching is performed before the temperature of the seamless steel pipe is left to cool to decrease to around room temperature. A heat treatment where a seamless steel pipe after hot working is rapidly cooled before the surface temperature becomes lower than the Ar₃ point will be hereinafter referred to as "direct quenching", and a heat treatment where a seamless steel pipe after hot working is supplementarily heated at a temperature not lower than the Ac₃ point and then rapidly cooled will be hereinafter referred to as "in-line quenching". The use of direct quenching or in-line quenching makes the grains of the microstructure coarser than with a heat treatment in which a pipe is cooled after its production and then rapidly cooled (hereinafter referred to as reheating-and-quenching). More specifically, the crystal grain size number after quenching is smaller than 6.0. This improves the hardenability of a microstructure compared with the reheating-and-quenching, and thus ensures a high strength even when a steel material with a low carbon equivalent Ceq is used.
The steps will be described in more detail below.

[Heating Step (S1)]

Around billet is heated in the heating furnace 1. The heating temperature is preferably in the range of 1100 to 1300 °C. Heating the round billet to this temperature range causes the carbonitride in the steel to dissolve. If a round billet is to be produced from a slab or ingot by hot working, it is only required that the slab or ingot be heated to a temperature of 1100 to 1300 °C, and the temperature to which the round billet is heated by the heating furnace 1 does not have to be in the range of 1100 to 1300 °C, because the carbonitride in the steel dissolves when the ingot or slab is being heated. The heating furnace 1 may be a walking-beam furnace or a rotary furnace, for example.

[Piercing Step (S2)]

The round billet is removed from the heating furnace 1 and the heated round billet is piercing-rolled by the piercing machine 2 to produce a hollow shell. The piercing machine 2 includes a plurality of skewed rolls and a plug. The plug is disposed between the skewed rolls. Preferably, the piercing machine 2 is a cross-type piercer. A cross-type piercer is preferable because it can do piercing at high pipe expansion rate.

[Elongation Rolling Step and Sizing Rolling Step (S3)]

Next, the hollow shell is rolled. More specifically, the hollow shell is elongation-rolled by the elongation rolling mill 3. The elongation rolling mill 3 includes a plurality of roll stands disposed in series. The elongation rolling mill 3 may be a mandrel mill, for example. Subsequently, the hollow shell that has been subjected to elongation rolling is subjected to reduction rolling by the sizing rolling mill 4 to produce a seamless steel pipe. The sizing rolling mill 4 includes a plurality of roll stands disposed in series. The sizing rolling mill 4 may be a sizer or stretch reducer, for example. The elongation rolling step and sizing rolling step together may be referred to simply as rolling step.

[Supplementary Heating Step (S4)]

The supplementary heating step (S4) is performed as necessary. That is, the manufacturing method in the present embodiment need not include the supplementary heating step (S4). More specifically, the supplementary heating step (S4) is performed in such a way that the temperature of the seamless steel pipe is at a predetermined level that is equal to or higher than the Ac₃ point directly before the water cooling of the quenching step (S5). If the supplementary heating step (S4) is not performed, the method in FIG. 2 proceeds from step S3 to step S5. If the supplementary heating step (S4) is not performed, the supplementary heating furnace 5 in FIG. 1 may not be provided.
If the finishing temperature of the rolling step (i.e. surface temperature of the seamless steel pipe directly after the rolling step is finished) is lower than 800 °C, it is preferable to perform the supplementary heating step (S4). At the supplementary heating step (S4), the seamless steel pipe is inserted into the supplementary heating furnace 5 and heated. The heating temperature in the supplementary heating furnace 5 is preferably in the range of 900 to 1100 °C. The soaking time is preferably 30 minutes or less. If the soaking time is too long, carbonitrides made of Ti, Nb, C and N, i.e. (Ti, Nb) and (C, N), may precipitate and form coarse particles. At the supplementary heating step, the supplementary heating furnace 5 may be replaced by an induction heating apparatus.

[Quenching Step (S5)]

The seamless steel pipe is water-cooled in the water-cooling apparatus 6. The temperature (i.e. surface temperature) of the seamless steel pipe directly before water cooling is equal to or higher than 800 °C.
For water cooling, the cooling rate for the temperature range of the seamless steel pipe from 800 °C to 500 °C is equal to or higher than 5 °C/sec (300 °C/min). This provides a uniform quenched microstructure. The temperature at which cooling is stopped is 450 °C or lower, and the cooling may be done down to room temperature. The quenching step (S5) changes the structure of the matrix to a structure mainly composed of martensite or bainite.
For example, the water-cooling aperture 6 used for the quenching step (S5) may have the following construction: The water-cooling apparatus 6 includes a plurality of rotating rollers, laminar water flow device, and a jet water flow device. The rotating rollers are disposed in two rows, and the seamless steel pipe is positioned between the two rows of rotating rollers. At this time, the rotating rollers in the two rows are in contact with bottom portions of the outer surface of the seamless steel pipe. When the rotating rollers rotate, the seamless steel pipe rotates about its axis. The laminar water flow device is located above the rotating rollers and pours water from above the seamless steel pipe. At this time, the water poured toward the seamless steel pipe forms a laminar water flow. The jet water flow device is located near an end of the seamless steel pipe positioned on the rotating rollers. The jet water flow device emits a jet water flow from the end of the seamless steel pipe toward the interior of the steel pipe. The laminar and jet water flow devices cool the outer and inner surfaces of the seamless steel pipe at the same time. A water-cooling device 6 with such a construction is suitable for accelerated cooling for a seamless steel pipe with a large wall thickness of 25 mm or larger.
The water-cooling device 6 may be a device other than the one including rotating rollers, laminar water flow device and jet water flow device discussed above. The water cooling device 6 may be a water tank, for example. In such implementations, the seamless steel pipe is immersed in the water tank and thus subjected to accelerated cooling. Alternatively, the water-cooling device 6 may include a laminar water flow device only. To sum up, the cooling device 6 is not limited to a specific type.

[Tempering Step (S6)]

The quenched seamless steel pipe is subjected to tempering. More specifically, the quenched seamless steel pipe is heated to a predetermined tempering temperature that is lower than the Ac1 point, and is held at this temperature for a predetermined period of time in such a way that the Larson-Millar parameter PL as defined by equation (2) below is 18800 or higher: $PL = (T + 273) \times (20 + \log (t))$
In equation (2), T is a tempering temperature (°C), t is a holding time (in hours) for that temperature. Log (t) is the logarithm of t whose base is 10.
If PL is lower than 18800, the reduction in surface hardness is insufficient and some portions may have a Vickers hardness exceeding 250 Hv. PL is preferably 18900 or higher.
If PL is too high, the recrystallization of ferrite occurred in a region of a depth of 1mm or deeper from the surface, which may cause an extreme reduction in strength, a reduction in the sour resistance in the surface layer and production of blisters. PL is preferably 20000 or lower, and more preferably 19500 or lower.
The lower limit of tempering temperature is preferably 600 °C, and more preferably 630 °C, and still more preferably 650 °C. The upper limit of tempering temperature is preferably 700 °C, and more preferably 680 °C. The lower limit of holding time is preferably one hour, and more preferably two hours, and still more preferably three hours. The upper limit of holding time is preferably six hours, and more preferably five hours, and still more preferably four hours.
The above manufacturing process provides a seamless steel pipe with a wall thickness that is as large as 25 mm or more having good strength, toughness and HIC resistance. The above manufacturing method is particularly suitable for a seamless steel pipe with a wall thickness of 25 mm or larger, and can even be used for a seamless steel pipe with a wall thickness of 40 mm or larger. The upper limit of wall thickness is not limited to a specific value, but is typically 60 mm or lower.
The seamless steel pipe in one embodiment of the present invention and the method of manufacturing the same have been described. The present embodiment provides a seamless steel pipe that can be manufactured by a relatively reasonable manufacturing process and that provides a yield strength of 555 MPa or higher and good SSC resistance in a reliable manner.

Examples

The present invention will be described using specific examples. The present invention is not limited to these examples.
A plurality of seamless steel pipes with various chemical compositions were produced and their yield strength, tensile strength, surface hardness and sour resistance were investigated.

[Investigation Methods]

A plurality of steels having the chemical compositions shown in Table 1 were melt and were subjected to continuous casting to produce round billets for pipe forming. Steels A, C, D1, D2 and J in Table 1 are steels in which the chemical composition or the value of Ceq does not meet the requirements of the present invention.

[Table 1]

TABLE1

Steel	Chemical Composition (in mass %, balance being Fe and impurities)																Ceq
Steel	C	Si	Mn	P	S	Cu	Cr	Ni	Mo	Ti	V	Nb	Al	Ca	O	N	Ceq
A	0.059	0.12	1.53	0.005	0.0007	0.20	0.28	0.23	0.10	0.003	0.05	-	0.031	0.0008	0.0015	0.0036	0.429
B	0.061	0.11	1.51	0.008	0.0009	0.20	0.31	0.31	0.25	0.003	0.05	-	0.032	0.0017	0.0018	0.0050	0.468
C	0.070	0.09	1.42	0.011	0.0005	0.41	0.31	0.39	0.35	0.005	0.05	-	0.030	0.0013	0.0017	0.0048	0.502
D1	0.066	0.12	1.46	0,009	0.0010	0.02	0.23	0.08	0.09	0.010	0.05	-	0.037	0.0017	0.0018	0.0039	0.390
D2	0.065	0.12	1.44	0.009	0.0010	0.08	0.26	0.09	0.06	0.007	0.05	-	0.041	0.0016	0.0016	0.0041	0.390
E	0.068	0.11	1.51	0.009	0.0019	0.37	0.28	0.49	0.25	0.004	0.05	-	0.030	0,0012	0.0012	0.0032	0.493
F	0.061	0.11	1.51	0.010	0.0010	0.20	0.20	0.28	0.25	0.004	0.05	-	0.030	0.0014	0.0015	0.0043	0.445
G	0.060	0.12	1.52	0.009	0.0010	0.21	0.21	0.28	0.25	0.005	0.05	0.020	0.034	0.0012	0.0011	0.0050	0.448
I	0.062	0.12	1.52	0.005	0.0008	0.21	0.27	0.23	0.11	0.006	0.05	0.025	0.031	0.0008	0.0015	0.0036	0.431
J	0.061	0.11	1.42	0.011	0.0018	0.36	0.28	0.49	0.51	0.005	0.04	-	0.030	0.0013	0.0013	0.0032	0.520
K	0.058	0.12	1.50	0.008	0.0010	0.20	0.31	0.32	0.26	0.003	0.05	-	0.033	0.0016	0.0017	0.0055	0.467

The round billets produced were heated by the heating furnace to a temperature in the range of 1100 to 1300 °C. Subsequently, the round billets were piercing-rolled by the piercing machine to produce hollow shells. Subsequently, the mandrel mill was used to elongation-roll the hollow shells. Subsequently, the sizer was used to reduction-roll (i.e. sizing-roll) the hollow shells to produce seamless steel pipes having the outer diameters and wall thicknesses shown in Tables 2 and 3.

[Table 2]

TABLE2

No.	Steel	Pipe-Forming Conditions		AsQ Prior γ grain size No.	Tempering Conditions			Mechanical Properties						AsQ Prior γ grain size No.	Ferrite Recrystallization	Remarks
		Outer Diameter	Wall Thickness		Soaking Temp.	Holding Time	PL	YS	TS	Hv10kgf (maximum among positions)
		(mm)	(mm)		(°C)	(min)	PL	(MPa)	(MPa)	Outer Surface	In Wall	Inner Surface	Difference
1	A	273,1	25,0	4,3	660	204	19156	518	592	202	198	208	10	4,2	absent	comparative ex.
2	A	273,1	25,0	4,3	660	219	19185	501	577	212	200	213	13	4,3	absent	comparative ex.
3	A	273,1	25,0	4,5	665	234	19314	509	580	197	195	162	35	4,4	present	comparative ex.
4	A	273,1	25,0	4,5	650	204	18951	524	602	203	203	220	17	4,5	absent	comparative ex.
5	A	273,1	25,0	4,3	650	219	18979	519	593	202	196	214	18	4,6	absent	comparative ex.
6	A	273,1	25,0	4,3	650	234	19006	511	585	197	201	215	18	4,1	absent	comparative ex.
7	A	273,1	25,0	4,3	650	249	19030	506	585	202	200	219	19	4,6	absent	comparative ex.
8	A	273,1	25,0	4,3	650	264	19054	514	588	200	201	219	19	4,5	absent	comparative ex.
9	A	273,1	25,0	4,3	650	294	19097	497	573	199	194	198	5	4,4	absent	comparative ex.
10	A	273,1	25,0	4,3	650	205	18953	544	619	218	218	220	2	4,3	absent	comparative ex.
11	A	273,1	25,0	4,3	630	204	18540	543	622	213	212	248	36	4,3	absent	comparative ex.
12	A	273,1	25,0	4,3	630	219	18568	541	620	209	210	236	27	4,4	absent	comparative ex.
13	A	273,1	25,0	4,3	630	234	18594	531	610	213	208	242	34	4,3	absent	comparative ex.
14	A	273,1	25,0	4,5	630	249	18618	531	610	206	202	240	38	4,4	absent	comparative ex.
15	A	273,1	25,0	4,5	630	264	18641	536	615	211	209	239	30	4,5	absent	comparative ex.
16	A	273,1	25,0	4,3	630	294	18683	526	602	210	203	238	35	4,3	absent	comparative ex.
17	A	273,1	25,0	4,3	600	204	17924	531	622	210	213	257	47	4,4	absent	comparative ex.
18	B	323,9	25,0	4,3	700	294	20132	503	582	209	193	170	39	4,5	present	comparative ex.
19	B	323,9	25,0	4,3	700	204	19977	575	641	192	193	209	17	4,3	absent	inventive ex.
20	B	323,9	25,0	4,3	690	204	19772	578	646	203	206	211	8	4,4	absent	inventive ex.
21	B	323,9	25,0	4,5	680	204	19566	579	646	212	220	222	10	4,5	absent	inventive ex.
22	B	323,9	25,0	4,3	670	204	19361	597	659	236	220	239	19	4,2	absent	inventive ex.
23	B	323,9	25,0	4,3	665	149	19131	621	688	238	240	239	2	4,3	absent	inventive ex.
24	B	323,9	25,0	4,3	660	204	19156	606	670	225	226	239	14	4,4	absent	inventive ex.
25	B	323,9	25,0	4,3	660	219	19185	601	665	224	230	239	15	4,3	absent	inventive ex.
26	B	323,9	25,0	4,3	665	234	19314	600	664	233	226	235	9	4,3	absent	inventive ex.
27	B	323,9	25,0	4,5	650	204	18951	631	697	248	244	249	5	4,5	absent	inventive ex.
28	B	323,9	25,0	4,5	650	219	18979	620	683	235	235	248	13	4,7	absent	inventive ex.
29	B	323,9	25,0	4,5	650	234	19006	620	681	235	235	248	13	4,5	absent	inventive ex.
30	B	323,9	25,0	4,3	650	249	19030	617	683	242	226	248	22	4,3	absent	inventive ex.

[Table 3]

TABLE3

No.	Steel	Pipe-Forming Conditions		AsQ Prior γ grain size No.	Tempering Conditions			Mechanical Properties						AsQ Prior γ grain size No.	Ferrite Recrystallization	Remarks
		Outer Diameter	Wall Thickness		Soaking Temp.	Holding Time	PL	YS	TS	Hv10kgf (maximum among positions)
		(mm)	(mm)		(°C)	(min)	PL	(MPa)	(MPa)	Outer Surface	In Wall	Inner Surface	Difference
31	B	323,9	25,0	4,5	650	264	19054	617	679	236	232	247	15	4,7	absent	inventive ex.
32	B	323,9	25,0	4,5	650	294	19097	612	674	227	226	237	11	4,3	absent	inventive ex.
33	B	323,9	25,0	4,3	650	315	19125	619	683	236	234	237	3	4,6	absent	inventive ex.
34	B	323,9	25,0	4,3	630	204	18540	649	720	241	241	269	28	4,4	absent	comparative ex.
35	B	323,9	25,0	4,5	630	219	18568	644	715	251	242	268	26	4,3	absent	comparative ex.
36	B	323,9	25,0	4,6	630	234	18594	654	725	256	240	266	26	4,7	absent	comparative ex.
37	B	323,9	25,0	4,3	630	249	18618	627	698	229	244	268	39	4,3	absent	comparative ex.
38	B	323,9	25,0	4,3	630	264	18641	624	693	228	240	266	38	4,3	absent	comparative ex.
39	B	323,9	25, D	4,6	630	294	18683	628	697	241	234	260	26	4,5	absent	comparative ex.
40	B	323,9	25,0	4,3	600	204	17924	639	728	260	253	286	33	4,7	absent	comparative ex.
41	B	323,9	25,0	4,5	655	105	18786	635	708	245	232	263	31	4,2	absent	comparative ex.
42	B	323,9	25,0	4,5	650	105	18684	636	707	251	233	264	31	4,5	absent	comparative ex.
43	C	323,9	25,0	4,6	650	105	18684	636	714	248	244	282	38	4,6	absent	comparative ex.
44	C	323,9	25,0	4,6	650	185	18911	573	657	232	228	269	41	4,5	absent	comparative ex.
45	D1	323,9	25,4	4,5	656	117	18849	478	561	178	178	205	27	4,5	absent	comparative ex.
46	D2	323,9	25,4	4,3	650	130	18770	468	582	179	191	207	28	4,2	absent	comparative ex.
47	E	406,4	38,1	4,3	600	204	17924	618	701	267	228	222	45	4,1	absent	comparative ex.
48	E	406,4	38,1	4,6	630	204	18540	603	673	255	220	227	35	4,5	absent	comparative ex.
49	E	406,4	38,1	4,5	630	234	18594	609	679	254	217	219	37	4,4	absent	comparative ex.
50	E	406,4	38,1	4,3	630	264	18641	609	674	252	214	228	38	4,2	absent	comparative ex.
51	E	406,4	38,1	4,3	630	294	18683	609	675	251	223	219	32	4,4	absent	comparative ex.
52	E	406,4	38,1	4,3	650	204	18951	599	665	237	222	214	23	4,3	absent	inventive ex.
53	E	406,4	38,1	4,5	650	234	19006	575	644	231	212	211	20	4,5	absent	inventive ex.
54	E	406,4	38,1	4,5	650	264	19054	575	641	230	209	208	22	4,6	absent	inventive ex.
55	E	406,4	38,1	4,5	650	294	19097	565	641	226	204	208	22	4,4	absent	inventive ex.
56	E	406,4	38,1	4,6	660	204	19156	565	635	228	205	206	23	4,6	absent	inventive ex.
57	E	406,4	38,1	4,3	660	234	19211	559	628	220	198	200	22	4,4	absent	inventive ex.
58	F	323,9	25,0	4,5	650	200	18942	570	643	212	219	224	12	4,7	absent	inventive ex.
59	G	323,9	25,0	5,4	650	200	18942	597	665	236	223	238	15	5,7	absent	inventive ex.
60	I	273,1	25,0	5,5	650	200	18942	575	647	200	203	211	11	5,5	absent	inventive ex.
61	J	323,9	25,0	4,7	665	149	19131	680	734	261	252	283	31	4,7	absent	comparative ex.
62*	K	323,9	25,0	6,8	665	234	19314	543	625	236	208	228	28	6,9	absent	comparative ex.

*62: in-line quenching at 950°C + reheating at 950°C and quenching + tempering

The seamless steel pipes that had been subjected to sizing rolling were heated by the supplementary heating furnace to 950 °C, and quenching was then performed by the water-cooling apparatus where the pipes were cooled to room temperature at a cooling rate of 5 °C/sec or higher.
After the quenching, the seamless steel pipes were tempered at the soaking temperatures and holding times shown in Tables 2 and 3. However, during the production of the steel of No. 62, after the above quenching was performed, before tempering, quenching was performed where the steel was reheated off-line to 950 °C and soaked for 20 minutes and then water-cooled.
The following evaluation tests were conducted on the seamless steel pipes produced in the above production process.

[Yield Strength and Tensile Strength Tests]

The yield strength of the seamless steel pipe of each number was investigated. More specifically, a No. 12 test specimen (with a width of 25 mm and a gauge length of 50 mm) as specified in JIS Z 2241 was taken out so that the longitudinal direction of the specimen for tensile testing was parallel to the longitudinal direction of the steel pipe (i.e. L direction). The test specimen that had been taken out was used to conduct a tensile test in compliance with JIS Z 2241 in the atmosphere at room temperature (25 °C), and the yield strength (YS) and tensile strength (TS) were determined.
The yield strength was determined using a 0.5 % total elongation method. The determined yield strength (in MPa) and tensile strength (in MPa) are shown in Tables 2 and 3. The columns labeled "YS" in Tables 2 and 3 have yield strength and the columns labeled "TS" have tensile strengths determined for the test specimens of the various test numbers.

[Surface Hardness Test]

Four test specimens were taken from the seamless steel pipe of each number, the specimens being displaced from each other by 90° along the pipe's circumference, and a Vickers hardness test in compliance with JIS Z 2244 was conducted on arbitrary three points on a transverse cross-section (i.e. cross-section perpendicular to the center axis) of each test specimen, the points being at 1 mm inwardly in the wall thickness direction from the inner surface. The force in the Vickers hardness tests, F, was 10 kgf (i.e. 98.07 N). The maximum among the values for the 12 points that had been obtained was used as the value of hardness "at 1 mm from the inner surface".
Similarly, a Vickers hardness test was conducted on arbitrary three points of each of the four test specimens of the seamless steel pipe of each test number, the points being at 1 mm inwardly in the wall thickness direction from the outer surface, and the maximum among the values of the 12 points that had been obtained was used as the value of hardness "at 1 mm from the outer surface". Further, a Vickers hardness test was conducted on arbitrary three points of each of the four test specimens of the seamless steel pipe of each test number, the points being near the middle in the wall thickness, and the maximum among the values of the 12 points that had been obtained was used as the value of hardness "in the wall".
For the seamless steel pipe of each test number, the value of hardness "at 1 mm from the outer surface", the value of hardness "at 1 mm from the inner surface" and the value of hardness "in the wall" are shown in Tables 2 and 3, in the columns labeled "Outer Surface", "In Wall" and "Inner Surface".
The largest value among the difference between the hardness "at 1 mm from the outer surface" and the hardness "in the wall", the difference between the hardness "at 1 mm from the inner surface" and the hardness "in the wall", and the difference between the hardness "at 1 mm from the outer surface" and the hardness "at 1 mm from the inner surface" (hereinafter referred to as "maximum difference in hardness") is shown in the column labeled "Difference" in Tables 2 and 3.

[Observation of Microstructure]

A sample was taken from the seamless steel pipe of each number, the sample containing the inner surface, outer surface and middle in the wall thickness, and the microstructure was observed. More specifically, each sample was etched by a nital etching solution to cause the microstructure to appear, which was observed using optical microscopy.
The seamless steel pipe of each number had a microstructure having a main phase of tempered martensite or tempered bainite. However, in some seamless steel pipes, recrystallization of ferrite had occurred in a region of a depth of 1mm or deeper from the surface. Whether recrystallization of ferrite occurred in a region of a depth of 1mm or deeper from the surface is shown in the column labeled "Ferrite Recrystallization" in Tables 2 and 3.
The crystal grain size number of the prior austenite grains of the microstructure was measured by the following method: First, a test specimen was cut out from each steel pipe such that a cross section perpendicular to the length of the steel pipe as quenched (i.e. pipe forming direction) forms the observed surface, and was imbedded into a resin; then the Bechet-Beaujard method was used where it is corroded by a picric acid saturated aqueous solution to let prior austenite grain boundaries appear, which were observed by optical microscopy (with a magnification of 200 times), and the prior austenite grain size number was measured in accordance with ASTM E112-10. Such grain size numbers are shown in the column "AsQ Prior γ grain size No." in Tables 2 and 3.
Since the grain size number of prior austenite grains after tempering cannot be measured using picric acid saturated aqueous solution corrosion; in view of this, the number was measured with the help of EBSD. EBSD was performed by cutting out a test specimen such that a cross section perpendicular to the length of a tempered steel pipe forms the observed surface, finishing the observed surface by mirror polishing and electrolysis polishing, and an area of 500 × 500 µm² in the middle in the thickness of the steel pipe was observed. A detector for EBSD mounted on an FE-SEM (DigiViewIV from EDAX) was used. Based on the obtained crystal orientation data, analysis software (OIM Analysis ver. 6 from EDAX) was used to draw lines along the boundaries between crystal grains in misorientation angles of 15 to 51 °, and the resulting line drawing was used to measure the prior austenite grain size number in accordance with ASTM E112-10. Such grain size numbers are shown in the column "QT Prior γ Grain Size No." in Tables 2 and 3.

[Results of Investigation]

As shown in Tables 1 to 3, the seamless steel pipes of Nos. 19 to 33 and 52 to 60 had a chemical composition falling in the scope of the present invention and had a carbon equivalent Ceq not lower than 0.430 % and lower than 0.500 %. In these seamless steel pipes, recrystallization of ferrite did not occur in a region of a depth of 1mm or deeper from the surface, and a structure was present having a main phase of tempered martensite or tempered bainite from the surface layer to the in-the-wall portions, and the crystal grain size number of the prior austenite grains was lower than 6.0. Further, these seamless steel pipes had Vickers hardness values "at 1 mm from the outer surface", "at 1 mm from the inner surface" and "in the wall" that were not higher than 250 Hv and had a yield strength of 555 MPa or higher. These seamless steel pipes had a maximum difference in hardness of 25 Hv or lower.
The seamless steel pipes of Nos.1 to 17 had a yield strength lower than 555 MPa. This is presumably because the carbon equivalent Ceq of steel A was too low.
In the seamless steel pipe of No. 18, recrystallization of ferrite occurred in a region of a depth of 1mm or deeper from the surface. Consequently, the seamless steel pipe of No.18 had a yield strength lower than 555 MPa. This is presumably because the Larson-Miller parameter PL of the seamless steel pipe of num No.ber 18 was too high.
The seamless steel pipes of Nos.34 to 42 and 47 to 51 had a Vickers hardness value "at 1 mm from the outer surface", "at 1 mm from the inner surface" or "in the wall" that was higher than 250 Hv. Further, these seamless steel pipes had a maximum difference in hardness higher than 25 Hv. This is presumably because the Larson-Miller parameters PL of the seamless steel pipes of Nos. 34 to 42 and 47 to 51 were too low.
The seamless steel pipes of Nos.43 and 44 had a Vickers hardness "at 1mm from the inner surface" higher than 250 Hv. This is presumably because the carbon equivalent Ceq of steel C was too high.
The seamless steel pipes of Nos.45 and 46 had yield strengths lower than 555 MPa. This is presumably because the carbon equivalents Ceq of steels D 1 and D2 were too low.
In the seamless steel pipe of No.61, the Vickers hardness was higher than 250 Hv at all the measurement points. This is presumably because the carbon equivalent Ceq of steel J was too high.
The seamless steel pipe of No.62 had a yield strength lower than 555 MPa. This is presumably because both in-line quenching and reheating-and-quenching were used, which produced too fine prior austenite grains, reducing hardenability and thus leading to insufficient strength.
FIG. 4 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and yield strength YS for steel B. As shown in FIG. 4, the yield strength YS tended to decrease as the Larson-Miller parameter PL increased. Steel B provided a yield strength of 555 MPa or larger except for the seamless steel pipe of No. 18, in which the recrystallization of ferrite progressed.
FIG. 5 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and yield strength YS for steel A. Steel A did not provide a yield strength not lower than 555 MPa even though tempering conditions were adjusted. This is presumably because the carbon equivalent Ceq of steel A was too low.
FIG. 6 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and hardness at an outer surface, an in-the-wall portion and an inner surface for steel B. As shown in FIG. 6, the hardnesses at the outer surface, in-the-wall portion and inner surface tended to decrease as the Larson-Miller parameter PL increased. As shown in FIG. 6, when the Larson-Miller parameter PL was 18800 or higher, the hardnesses at the outer surface, in-the-wall portion and inner surface were 250 Hv or lower. On the other hand, when the Larson-Miller parameter PL was lower than 18800, the hardness at the outer surface, in-the-wall portion or inner surface was higher than 250 Hv.
FIG. 7 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and hardness at an outer surface, an in-the-wall portion and an inner surface for steel A. In steel A, similar to steel B, the hardnesses at the outer surface, in-the-wall portion and inner surface tended to decrease as the Larson-Miller parameter increased.
FIG. 8 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and maximum difference in hardness for steel B. As shown in FIG. 8, when the Larson-Miller parameter PL was 18800 or higher, the maximum difference in hardness was not more than 25 Hv. The seamless steel pipe of No. 18 had a large maximum difference in hardness presumably because the recrystallization of ferrite progressed in a region of a depth of 1mm or deeper from the surface,.
FIG. 9 is a scatter plot illustrating the relationship between Larson-Miller parameter PL and maximum difference in hardness for steel A. As shown in FIG. 9, the relationship between Larson-Miller parameter PL and maximum difference in hardness in steel A exhibited similar tendencies. The seamless steel pipe of No.3 had a large maximum difference in hardness presumably because the recrystallization of ferrite progressed in a region of a depth of 1mm or deeper from the surface.

[Evaluation of Sour Resistance]

A sour resistance evaluation as described below (i.e. HIC resistance test, four-point bending test) was conducted on the seamless steel pipes of some of the numbers.

[HIC Resistance Test]

From each seamless steel pipe were taken out a test specimen containing the inner surface, a test specimen containing the middle in the wall thickness, and a test specimen containing the outer specimen. Each test specimen had a thickness of 20 mm and a width (along the circumference) of 20 mm, and a length of 100 mm. The HIC resistance of each test specimen was evaluated in accordance with NACE (National Association of Corrosion Engineers) TM 0284-2011. The testing bath in which the test specimens were immersed was a 5 % salt + 0.5 % acetic acid aqueous solution saturated with hydrogen sulfide gas at 1 atm at a temperature of 24 °C.
After 96 hours of immersion, ultrasonic inspection (UT) was conducted on the test specimens after being tested to determine the location of the largest crack, and the specimen was cut at this location. The cross-section at this time was a cross-section of thickness × width of the test specimen, i.e. perpendicular to the longitudinal direction of the steel pipe. The cut test specimen was used to determine the crack-length ratio CLR (=crack length (mm)/width of test specimen (mm)). The maximum value among the CLR values of the test specimen taken from each steel pipe was used as the crack-length ratio CLR for this test number.
Further, it was determined whether each test specimen after being tested had a blister (i.e. a swollen part due to a crack near the surface), and the number of blisters produced on the test specimen was counted. The maximum among the numbers of blisters on the test specimen taken from each steel pipe was used as the number of blisters for this test number.

[Four-Point Bending Test]

A stress of 95 % of the actual yield strength (i.e. yield strength of the seamless steel pipe of each number) was applied to a test specimen containing the middle in the wall thickness of this seamless steel pipe using a four-point bending jig in accordance with ASTM G39. The test specimens to which stresses were applied were placed in a test bath. The test bath was a 5 % salt + 0.5 % acetic acid aqueous solution saturated with hydrogen sulfide gas at 1 atm at a temperature of 24 °C. After 720 hours, it was visually determined whether there was a crack in the test specimens. If a plate material had no crack, it was determined that this material had good SSC resistance.

[Evaluation Results]

The results of sour resistance evaluation were shown in Table 4.

[Table 4]

TABLE4

No.	PL	YS (MPa)	Hv10kgf (maximum for positions)				HIC Resistance Test		Four-Point Bending Test
			Hv10kgf (maximum for positions)					Number of Blisters
			Outer Surface	In Wall	Inner Surface	Difference		Number of Blisters
10	18953	544	218	218	220	2	○	0	○
3	19314	509	197	195	162	35	○	3	-
18	20132	503	209	193	170	39	○	2	-
23	19131	621	238	240	239	2	○	0	○
33	19125	619	236	234	237	3	○	0	○
37	18618	627	229	244	268	39	CLR 1%	0	-
40	17924	639	260	253	286	33	CLR 2%	0	-
43	18684	636	248	244	282	38	CLR 2%	-	-
44	18911	573	232	228	269	41	CLR 2%	-	-
52	18951	599	237	222	214	23	○	0	-
57	19211	559	220	198	200	22	○	0	-
58	18942	570	212	219	224	12	○	0	-
59	18942	597	236	223	238	15	○	0	-
60	18942	575	200	203	211	11	○	0	-

In Table 4, "○" in the columns labeled "HIC Resistance Test" and "Four-Point Bending Test" indicates that there was no crack in the relevant test. "-" in the columns labeled "HIC Resistance Test" and "Four-Point Bending Test" indicates that the relevant test was not conducted.
As shown in Table 4, in the seamless steel pipes with a yield strength of 555 MPa or higher and Vickers hardness values "at 1 mm from the outer surface", "at 1 mm from the inner surface" and "in the wall" not higher than 250 Hv, no crack occurs in both the HIC resistance test and four-point bending test, and a good sour resistance was provided in a reliable manner. On the other hand, the seamless steel pipes with Vickers hardness values "at 1 mm from the outer surface", "at 1 mm from the inner surface" or "in the wall" higher than 250 Hv provided a poor sour resistance. These results prove a relationship between Vickers hardness and sour resistance.
Although embodiments of the present invention have been described, these embodiments are merely examples that may be used to carry out the present invention. Accordingly, the present invention is not limited to the above embodiments and the above embodiments can be modified as appropriate within the scope of the appended claims.

Claims

A seamless steel pipe having a chemical composition of, in mass %,
C: 0.02 to 0.15 %;
Si: 0.05 to 0.5 %;
Mn: 0.30 to 2.5 %;
P: up to 0.03 %;
S: up to 0.006 %;
O: up to 0.004 %;
Al: 0.01 to 0.10 %;
Ti: 0.001 to 0.010 %;
N: up to 0.007 %;
Cr: 0.05 to 1.0 %;
Mo: not less than 0.02 % and less than 0.5 %;
Ni: 0.03 to 1.0 %;
Cu: 0.02 to 1.0 %;
V: 0.020 to 0.20 %;
Ca: 0.0005 to 0.005 %; and
Nb: 0 to 0.05 %,
the balance being Fe and impurities,
where a carbon equivalent Ceq as defined by equation (1) below is not less than 0.430 % and less than 0.500 %,
a main phase of a microstructure from a surface layer to an in-the-wall portion is tempered martensite or tempered bainite,
a size of prior austenite grains in the microstructure is lower than 6.0 in crystal grain size number according to ASTM E112-10,
a portion between a position at 1 mm from an inner surface and a position at 1 mm from an outer surface has a Vickers hardness of 250 Hv or lower, and
a yield strength is 555 MPa or higher, $Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15$
where a symbol of each element in equation (1) is substituted by a content of this element in mass %.
The seamless steel pipe according to claim 1, wherein the chemical composition contains, in mass %:
Nb: 0.010 to 0.05 %.
The seamless steel pipe according to claim 1 or 2, wherein a difference between a Vickers hardness of a portion at 1 mm from the inner surface and that of a portion in a middle in a wall thickness, a difference between a Vickers hardness of a portion at 1 mm from the outer surface and that of a portion in the middle in the wall thickness, and a difference between a Vickers hardness of a portion at 1 mm from the inner surface and that of a portion at 1 mm from the outer surface are each 25 Hv or lower.
The seamless steel pipe according to any one of claims 1 to 3, wherein:
the crystal grain size number of the prior austenite grains is 5.5 or lower.
A method of manufacturing a seamless steel pipe, comprising:
preparing a raw material having a chemical composition of, in mass %, C: 0.02 to 0.15 %; Si: 0.05 to 0.5 %; Mn: 0.30 to 2.5 %; P: up to 0.03 %; S: up to 0.006 %; O: up to 0.004 %; Al: 0.01 to 0.10 %; Ti: 0.001 to 0.010 %; N: up to 0.007 %; Cr: 0.05 to 1.0 %; Mo: not less than 0.02 % and less than 0.5 %; Ni: 0.03 to 1.0 %; Cu: 0.02 to 1.0 %; V: 0.020 to 0.20 %; Ca: 0.0005 to 0.005 %; and Nb: 0 to 0.05 %, the balance being Fe and impurities;

hot working the raw material to produce a hollow shell;

quenching the hollow shell by direct quenching or in-line quenching,
wherein the hollow shell is water cooled from 800°C or higher, a cooling rate for the temperature range of the hollow shell from 800°C to 500°C is equal to or higher than 5°C/sec, and a temperature at which cooling is stopped is 450°C or lower; and
tempering the quenched hollow shell at a tempering temperature that is lower than Ac₁ point,

wherein no reheating-and-quenching is performed between the quenching and tempering,

a carbon equivalent Ceq as defined by equation (3) below is not less than 0.430 % and less than 0.500 %,

a Larson-Miller parameter PL as defined by equation (4) below is not less than 18800, $Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15$
and $PL = (T + 273) \times (20 + \log (t))$

a symbol of each element in equation (3) is substituted by a content of this element in mass %, and in equation (4), T is the tempering temperature, and t is a holding period for this temperature, and T is in °C, and t is in hours.
The method of manufacturing a seamless steel pipe according to claim 5, further comprising supplementary heating of the shell before the quenching.