Steel material for line pipes, method for producing the same, and method for producing line pipe

Abstract

A method for producing a steel material for line pipes which has a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the method including heating a steel having a specific composition to a temperature of 1000° C. to 1200° C.; performing hot rolling such that a cumulative rolling reduction ratio in a non-recrystallization temperature range is 60% or more, a cumulative rolling reduction ratio in a temperature range of (a rolling finish temperature +20° C.) or less is 50% or more, and a rolling finish temperature is the Ar.sub.3 transformation point or more and 790° C. or less; and subsequently performing accelerated cooling from a cooling start temperature of the Ar.sub.3 transformation point or more, at a cooling rate of 10° C./s or more, until the temperature of a surface of a steel plate reaches 300° C. to 500° C.

Claims

1. A method for producing a steel material for line pipes, the steel material having a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the method comprising heating a steel having a composition containing, by mass, C: 0.030% to 0.10%, Si: 0.01% to 0.20%, Mn: 1.0% to 2.0%, Nb: 0.005% to 0.050%, Ti: 0.005% to 0.025%, and Al: 0.08% or less, the composition further containing one or more elements selected from, by mass, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, and V: 0.1% or less, wherein a Ceq value represented by Formula (1) is 0.350 or more, a Pcm value represented by Formula (2) is 0.20 or less, and an Ar.sub.3 transformation point represented by Formula (3) is 750° C. or less, with the balance being Fe and inevitable impurities, to a temperature of 1000° C. to 1200° C.; performing hot rolling such that a cumulative rolling reduction ratio in a non-recrystallization temperature range is 60% or more, such that a cumulative rolling reduction ratio in a temperature range of (a rolling finish temperature+20° C.) or less is 50% or more, and such that a rolling finish temperature is the Ar.sub.3 transformation point or more and 790° C. or less; and subsequently performing accelerated cooling from a cooling start temperature of the Ar.sub.3 transformation point or more, at a cooling rate of 10° C./s or more, until the temperature of a surface of a steel plate reaches 300° C. to 500° C., such that a metal microstructure is formed in the steel material that is composed primarily of bainite, wherein an area fraction of polygonal ferrite at a position of ¼ plate thickness is 10% or less, an area fraction of martensite-austenite constituent at the position of ¼ plate thickness is 5% or less, and an average grain size of bainite at a position of ½ plate thickness is 10 μm or less,
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5  (1)
Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10  (2)
Ar.sub.3(° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo  (3) wherein, in Formulae (1) to (3), the symbol of each element represents the content (mass %) of the element and is zero when the composition does not contain the element.

2. A method for producing a line pipe having a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the method comprising cold forming a steel material for line pipes produced by the method according to claim 1 into a steel pipe shape by butting opposed edges of the steel material to one another; joining the butting edges to each other by seam welding; and subsequently performing pipe expansion at an expansion ratio of 1.2% or less to produce a steel pipe.

3. A steel material for line pipes, the steel material having a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the steel material comprising a composition containing, by mass, C: 0.030% to 0.10%, Si: 0.01% to 0.20%, Mn: 1.0% to 2.0%, Nb: 0.005% to 0.050%, Ti: 0.005% to 0.025%, and Al: 0.08% or less, the composition further containing one or more elements selected from, by mass, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, and V: 0.1% or less, wherein a Ceq value represented by Formula (1) is 0.350 or more, a Pcm value represented by Formula (2) is 0.20 or less, and an Ar.sub.3 transformation point represented by Formula (3) is 750° C. or less, with the balance being Fe and inevitable impurities, the steel material further comprising a metal microstructure composed primarily of bainite, wherein an area fraction of polygonal ferrite at a position of ¼ plate thickness is 10% or less, an area fraction of martensite-austenite constituent at the position of ¼ plate thickness is 5% or less, and an average grain size of bainite at a position of ½ plate thickness is 10 μm or less,
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5  (1)
Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10  (2)
Ar.sub.3(° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo  (3) wherein, in Formulae (1) to (3), the symbol of each element represents the content (mass %) of the element and is zero when the composition does not contain the element.

4. A method for producing a line pipe having a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the method comprising cold forming a steel material for line pipes according to claim 3 into a steel pipe shape by butting opposed edges of the steel material to one another; joining the butting edges to each other by seam welding; and subsequently performing pipe expansion at an expansion ratio of 1.2% or less to produce a steel pipe.

Description

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(1) An embodiment of the present invention is described below. When referring to the contents of constituent elements, the symbol “%” refers to “% by mass” unless otherwise specified. 1. Chemical Composition
C: 0.030% to 0.10%

(2) C is an element most effective in increasing the strength of a steel plate produced by accelerated cooling. However, if the C content is less than 0.030%, a sufficiently high strength may not be maintained. On the other hand, if the C content is more than 0.10%, toughness may become degraded. In addition, the formation of MA may be accelerated. This results in a reduction in compressive strength. Accordingly, the C content is limited to 0.030% to 0.10%. Preferable lower limit of C content is 0.040% and preferable upper limit is 0.098%.

(3) Si: 0.01% to 0.20%

(4) Si is contained for deoxidization. However, if the Si content is less than 0.01%, a sufficient deoxidation effect may not be achieved. On the other hand, if the Si content is more than 0.20%, toughness may become degraded. In addition, the formation of MA may be accelerated. This results in a reduction in compressive strength. Accordingly, the Si content is limited to 0.01% to 0.20%. Preferable lower limit of Si content is 0.03% and preferable upper limit is 0.15%.

(5) Mn: 1.0% to 2.0%

(6) Mn: 1.0% to 2.0%. Mn is contained for increasing strength and enhancing toughness. However, if the Mn content is less than 1.0%, the above advantageous effects may not be produced to a sufficient degree. On the other hand, if the Mn content is more than 2.0%, toughness may become degraded. Accordingly, the Mn content is limited to 1.0% to 2.0%. Preferable lower limit of Mn content is 1.5% and preferable upper limit is 1.95%.

(7) Nb: 0.005% to 0.050%

(8) Nb reduces the size of microstructures and thereby enhances toughness. Nb also causes the formation of carbides, which increase strength. However, if the Nb content is less than 0.005%, the above advantageous effects may not be produced to a sufficient degree. On the other hand, if the Nb content is more than 0.050%, the toughness of a weld heat-affected zone may become degraded. Accordingly, the Nb content is limited to 0.005% to 0.050%. Preferable lower limit of Nb content is 0.010% and preferable upper limit is 0.040%.

(9) Ti: 0.005% to 0.025%

(10) Ti suppresses coarsening of austenite grains during heating of slabs by the pinning effect of TiN and thereby enhances toughness. However, if the Ti content is less than 0.005%, the above advantageous effects may not be produced to a sufficient degree. On the other hand, if the Ti content is more than 0.025%, toughness may become degraded. Accordingly, the Ti content is limited to 0.005% to 0.025%. Preferable lower limit of Ti content is 0.008% and preferable upper limit is 0.023%.

(11) Al: 0.08% or Less

(12) Al is contained as a deoxidizing agent. However, if the Al content is more than 0.08%, the cleanliness of steel may become degraded and toughness may become degraded. Accordingly, the Al content is limited to 0.08% or less. The Al content is preferably 0.05% or less.

(13) In accordance with aspects of the present invention, one or more elements selected from Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, and V: 0.1% or less are contained.

(14) Cu: 0.5% or Less

(15) Cu is an element effective in improving toughness and increasing strength. However, if the Cu content is more than 0.5%, the HAZ toughness of a weld zone may become degraded. Accordingly, in the case where Cu is contained, the Cu content is limited to 0.5% or less. The lower limit for the Cu content is not specified. In the case where Cu is contained, the Cu content is preferably 0.01% or more.

(16) Ni: 1.0% or Less

(17) Ni is an element effective in improving toughness and increasing strength. However, if the Ni content is more than 1.0%, the HAZ toughness of a weld zone may become degraded. Accordingly, in the case where Ni is contained, the Ni content is limited to 1.0% or less. The lower limit for the Ni content is not specified. In the case where Ni is contained, the Ni content is preferably 0.01% or more.

(18) Cr: 1.0% or Less

(19) Cr is an element that enhances hardenability and thereby effectively increase strength. However, if the Cr content is more than 1.0%, the HAZ toughness of a weld zone may become degraded. Accordingly, in the case where Cr is contained, the Cr content is limited to 1.0% or less. The lower limit for the Cr content is not specified. In the case where Cr is contained, the Cr content is preferably 0.01% or more.

(20) Mo: 0.5% or Less

(21) Mo is an element effective in improving toughness and increasing strength. However, if the Mo content is more than 0.5%, the HAZ toughness of a weld zone may become degraded. Accordingly, in the case where Mo is contained, the Mo content is limited to 0.5% or less. The lower limit for the Mo content is not specified. In the case where Mo is contained, the Mo content is preferably 0.01% or more.

(22) V: 0.1% or Less

(23) V is an element that forms complex carbides as well as Nb and Ti and is markedly effective in increasing strength by precipitation strengthening. However, if the V content is more than 0.1%, the HAZ toughness of a weld zone may become degraded. Accordingly, in the case where V is contained, the V content is limited to 0.1% or less. The lower limit for the V content is not specified. In the case where V is contained, the V content is preferably 0.01% or more.

(24) In accordance with aspects of the present invention, the Ceq value represented by Formula (1) is 0.350 or more, the Pcm value represented by Formula (2) is 0.20 or less, and the Ar.sub.3 transformation point represented by Formula (3) is 750° C. or less.

(25) Ceq Value: 0.350 or More

(26) The Ceq value is limited to 0.350 or more. The Ceq value is represented by Formula (1) below. The Ceq value has a correlation with the strength of base metal and is used as a measure of strength. If the Ceq value is less than 0.350, a high tensile strength of 570 MPa or more may not be achieved. Accordingly, the Ceq value is limited to 0.350 or more. The Ceq value is preferably 0.360 or more.
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5  (1)

(27) In Formula (1), the symbol of each element represents the content (mass %) of the element and is zero when the composition does not contain the element.

(28) Pcm Value: 0.20 or Less

(29) The Pcm value is limited to 0.20 or less. The Pcm value is represented by Formula (2) below. The Pcm value is used as a measure of weldability; the higher the Pcm value, the lower the toughness of a welded HAZ. The Pcm value needs to be strictly limited particularly in a thick-walled high-strength steel, because the impact of the Pcm value is significant in the thick-walled high-strength steel. Accordingly, the Pcm value is limited to 0.20 or less. The Pcm value is preferably 0.19 or less.
Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10   (2)

(30) In Formula (2), the symbol of each element represents the content (mass %) of the element and is zero when the composition does not contain the element.

(31) Ar.sub.3 Transformation Point: 750° C. or Less

(32) The Ar.sub.3 transformation point is limited to 750° C. or less. Formula (3) below represents the Ar.sub.3 transformation point. The higher the Ar.sub.3 transformation point, the higher the temperature at which ferrite is formed and the more the difficulty in achieving the metal microstructure according to aspects of the present invention. In addition, it becomes more difficult to achieve both intended compressive strength and intended toughness. Accordingly, the composition is controlled such that the Ar.sub.3 transformation point is 750° C. or less.
Ar.sub.3(° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo   (3)

(33) In Formula (3), the symbol of each element represents the content (mass %) of the element and is zero when the composition does not contain the element.

(34) The remaining part of the composition which is other than the above-described constituents, that is, the balance, includes Fe and inevitable impurities. The composition may contain an element other than the above-described elements such that the action and advantageous effects according to aspects of the present invention are not impaired. 2. Metal Microstructure
Composed Primarily of Bainite

(35) The metal microstructure according to aspects of the present invention is composed primarily of bainite in order to limit the reduction in compressive strength due to the Bauschinger effect. The expression “the metal microstructure according to aspects of the present invention is composed primarily of bainite” means that the area fraction of bainite in the entire metal microstructure is 85% or more. For limiting the reduction in compressive strength due to the Bauschinger effect, the metal microstructure is desirably composed only of bainite in order to prevent the dislocation accumulation at the interfaces between different phases and the hard second phase. When the fraction of the balance microstructures other than bainite is 15% or less, they may be acceptable. Note that, the area fraction of bainite is measured at a position of ¼ plate thickness.

(36) Area Fractions of Polygonal Ferrite and Martensite-Austenite Constituent at Position of ¼ Plate Thickness Are 10% or Less and 5% or Less, Respectively

(37) For reducing the Bauschinger effect and achieving a high compressive strength, it is desirable to form a uniform microstructure free of a soft polygonal ferrite phase or a hard martensite-austenite constituent in order to reduce the likelihood of dislocations being locally integrated inside the microstructure during deformation. Accordingly, in addition to forming a microstructure composed primarily of bainite as described above, the area fractions of polygonal ferrite and the martensite-austenite constituent at a position of ¼ plate thickness are limited to 10% or less and 5% or less, respectively. The area fractions of polygonal ferrite and the martensite-austenite constituent may be 0%.

(38) Average Grain Size of Bainite at Position of ½ Plate Thickness Is 10 μm or Less

(39) It is effective to form a fine microstructure for producing a thick-walled steel plate having sufficiently high base metal toughness particularly at a position of ½ plate thickness. The above advantageous effects may be produced by adjusting the grain size of bainite at a position of ½ plate thickness to 10 μm or less. Accordingly, the average grain size of bainite at a position of ½ plate thickness is limited to 10 μm or less.

(40) The metal microstructure according to aspects of the present invention may include any phases other than bainite, polygonal ferrite, or the martensite-austenite constituent as long as it includes the above-described structure. Examples of the other phases include pearlite, cementite, and martensite. The amount of the other phases is preferably minimized; the area fraction of the other phases at a position of ¼ plate thickness is preferably 5% or less. 3. Method for Producing Steel Material for Line Pipes

(41) The method for producing a steel material for line pipes according to aspects of the present invention includes heating a steel slab having the above-described chemical composition, hot rolling the steel slab, and subsequently performing accelerated cooling. The reasons for limiting the production conditions are described below. Hereinafter, the term “temperature” refers to the average temperature of the steel plate (steel material) in the thickness direction, unless otherwise specified. The average temperature of the steel plate (steel material) in the thickness direction is determined on the basis of thickness, surface temperature, cooling conditions, etc. by simulation calculation or the like. For example, the average temperature of the steel plate (steel material) in the thickness direction may be calculated from a temperature distribution in the thickness direction determined by a finite difference method.

(42) Steel Slab Heating Temperature: 1000° C. to 1200° C.

(43) If the steel slab heating temperature is less than 1000° C., NbC does not dissolve sufficiently and, consequently, precipitation strengthening may not be achieved in the subsequent step. On the other hand, if the steel slab heating temperature is more than 1200° C., low-temperature toughness may become degraded. Accordingly, the steel slab heating temperature is limited to 1000° C. to 1200° C. Preferable lower limit of the steel slab heating temperature is 1000° C. and preferable upper limit is 1150° C.

(44) Cumulative Rolling Reduction Ratio in Non-Recrystallization Temperature Range: 60% or More, and Cumulative Rolling Reduction Ratio in Temperature Range of (Rolling Finish Temperature +20° C.) or Less: 50% or More

(45) For achieving high base metal toughness, it is necessary to perform sufficient rolling reduction within the non-recrystallization temperature range in the hot rolling process. However, if the cumulative rolling reduction ratio in the non-recrystallization temperature range is less than 60% or the cumulative rolling reduction in the temperature range of (rolling finish temperature +20° C.) or less is less than 50%, the size of crystal grains may not be reduced to a sufficient degree. Accordingly, the cumulative rolling reduction ratio in the non-recrystallization temperature range is limited to 60% or more, and the cumulative rolling reduction in the temperature range of (rolling finish temperature +20° C.) or less is limited to 50% or more. The cumulative rolling reduction ratio in the non-recrystallization temperature range is preferably 65% or more. The cumulative rolling reduction ratio in the temperature range of (rolling finish temperature +20° C.) or less is preferably 55% or more.

(46) Rolling Finish Temperature: Ar.sub.3 Transformation Point or More and 790° C. or Less

(47) For limiting the reduction in strength due to the Bauschinger effect, it is necessary to form a metal microstructure composed primarily of bainite and suppress the formation of soft microstructures, such as polygonal ferrite. This requires the hot rolling to be performed within the temperature range of the Ar.sub.3 transformation point or more, in which polygonal ferrite does not form. Accordingly, the rolling finish temperature is limited to the Ar.sub.3 transformation point or more. For achieving high base metal toughness, it is necessary to perform the rolling at lower temperatures in the temperature range of the Ar.sub.3 transformation point or more. Accordingly, the upper limit for the rolling finish temperature is set to 790° C. The rolling finish temperature is preferably 780° C. or less.

(48) Cooling Start Temperature: Ar.sub.3 Transformation Point or More

(49) If the cooling start temperature is less than the Ar.sub.3 transformation point, the area fraction of polygonal ferrite at a position of ¼ plate thickness may exceed 10% and a sufficiently high compressive strength may not be achieved due to the Bauschinger effect. Accordingly, the cooling start temperature is limited to the Ar.sub.3 transformation point or more. The cooling start temperature is preferably (the Ar.sub.3 transformation point +10° C.) or more.

(50) As described above, the Ar.sub.3 transformation point can be calculated using Formula (3).
Ar.sub.3(° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo   (3)

(51) In Formula (3), the symbol of each element represents the content (mass %) of the element and is zero when the composition does not contain the element.

(52) Cooling Rate: 10° C./s or More

(53) Accelerated cooling performed at a cooling rate of 10° C./s or more is a process essential for producing a high strength steel plate having high toughness. Performing cooling at a high cooling rate enables strength to be increased due to transformation strengthening. However, if the cooling rate is less than 10° C./s, a sufficiently high strength may not be achieved. Furthermore, diffusion of C may occur during cooling. This results in concentrating of C at non-transformed austenite and an increase in the amount of MA formed. Consequently, compressive strength may be reduced, because the presence of hard second phases, such as MA, accelerates the Bauschinger effect as described above. When the cooling rate is 10° C./s or more, diffusion of C which occurs during the cooling may be suppressed and, consequently, the formation of MA may be reduced. Accordingly, the cooling rate in the accelerated cooling is limited to 10° C./s or more. The cooling rate is preferably 20° C./s or more.

(54) Cooling Stop Temperature: Temperature of Surface of Steel Plate Is 300° C. to 500° C.

(55) Performing rapid cooling until the temperature of the surface of the steel plate reaches 300° C. to 500° C. by the accelerated cooling subsequent to the rolling suppresses the formation of MA and pearlite and enables the formation of a uniform microstructure composed primarily of bainite. However, if the cooling stop temperature is less than 300° C., MA may be formed. This results in a reduction in compressive strength due to the Bauschinger effect and degradation of toughness. When the cooling stop temperature at the surface of the steel plate is set to 300° C. or more, MA becomes decomposed due to recuperation and, consequently, a uniform microstructure may be formed. On the other hand, if the cooling stop temperature is more than 500° C., pearlite may be formed. This makes it not possible to achieve a sufficiently high strength and results in a reduction in compressive strength due to the Bauschinger effect. Accordingly, the cooling stop temperature is determined such that the temperature of the surface of the steel plate is 300° C. to 500° C. Preferable lower limit of the cooling stop temperature is 350° C. and preferable upper limit is 490° C. 4. Method for Producing Line Pipe

(56) In accordance with aspects of the present invention, a steel pipe (line pipe) is produced using a steel plate (steel material) produced by the above-described method. Examples of a method for forming the steel material include a method in which a steel material is formed into the shape of a steel pipe by cold forming, such as a UOE process or press bending (also referred to as “bending press”). In the UOE process, the edges of a steel plate (steel material) in the width direction are subjected to edge preparation and then the edges of the steel plate in the width direction is crimped using a C-press machine. Subsequently, the steel plate is formed into a cylindrical shape such that the edges of the steel plate in the width direction face each other using a U-ing press machine and an O-ing press machine. Then, the edges of the steel plate in the width direction are brought into abutment with and welded to each other. This welding is referred to as “seam welding”. The seam welding is preferably performed using a method including two steps, that is, a tack welding step of holding the cylindrical steel plate, bringing the edges of the steel plate in the width direction into abutment with each other, and performing tack welding; and a final welding step of subjecting the inner and outer surfaces of the seam of the steel plate to welding using a submerged arc welding method. After the seam welding, pipe expansion is performed in order to remove welding residual stress and to improve the roundness of the steel pipe. In the pipe expansion step, the expansion ratio (the ratio of a change in the outer diameter of the pipe which occurs during the pipe expansion to the outer diameter of the pipe before the pipe expansion) is set to 1.2% or less. This is because, if the expansion ratio is excessively high, compressive strength may be significantly reduced due to the Bauschinger effect. The expansion ratio is preferably 1.0% or less. The expansion ratio is preferably 0.4% or more and is more preferably 0.6% or more in order to reduce welding residual stress and enhance the roundness of the steel pipe.

(57) In the press bending, the steel plate is repeatedly subjected to three-point bending to gradually change its shape and, thereby, a steel pipe having a substantially circular cross section is produced. Then, seam welding is performed as in the UOE process described above. Also in the press bending, pipe expansion may be performed after the seam welding.

EXAMPLES

(58) Slabs were manufactured from steels (Steel types A to K) having the chemical compositions described in Table 1 by a continuous casting process. Steel plates (Nos. 1 to 25) having a thickness of 35 to 40 mm were manufactured from the slabs. Steel pipes were manufactured from the steel plates by the UOE process. Seam welding was performed by four-wire submerged arc welding such that one welding path is formed on both of the inner and outer surfaces of the seam. The heat input during the welding was selected from the range of 20 to 80 kJ/cm in accordance with the thickness of the steel plate. Table 2 summarizes the conditions under which the steel plates were produced and the condition under which the steel pipes were produced (expansion ratio).

(59) TABLE-US-00001 TABLE 1 Ar.sub.3 Steel Composition (mass %) Ceq Pcm transformation type C Si Mn Nb Ti Al Cu Ni Cr Mo V value.sup.(1) value.sup.(2) point.sup.(3) Remark A 0.050 0.050 1.85 0.028 0.010 0.030 0.100 0.100 0.300 0.100 0.020 0.456 0.175 727 Invention B 0.043 0.140 1.70 0.020 0.015 0.033 0.200 0.120 0.364 0.142 748 example C 0.095 0.060 1.54 0.025 0.010 0.020 0.020 0.210 0.398 0.189 740 D 0.065 0.070 1.60 0.030 0.011 0.025 0.150 0.150 0.200 0.030 0.398 0.170 748 E 0.060 0.040 1.90 0.025 0.020 0.033 0.020 0.020 0.050 0.389 0.161 734 F 0.050 0.050 1.78 0.018 0.012 0.030 0.300 0.320 0.100 0.408 0.168 721 G 0.028 0.060 1.90 0.028 0.020 0.025 0.005 0.346 0.126 749 Comparative H 0.080 0.050 2.10 0.030 0.013 0.033 0.200 0.250 0.200 0.100 0.520 0.218 688 example I 0.055 0.100 1.55 0.012 0.032 0.200 0.150 0.357 0.153 763 J 0.140 0.150 1.55 0.025 0.011 0.028 0.200 0.200 0.425 0.236 728 K 0.065 0.250 1.80 0.030 0.015 0.025 0.200 0.200 0.100 0.010 0.414 0.184 723 *The underlined values are outside the scope of the present invention. Formula (1): Ceq = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 Formula (2): Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 Formula (3): Ar.sub.3 = 910 − 310C − 80Mn − 20Cu − 15Cr − 55Ni − 80Mo

(60) To determine the tensile properties of the steel pipes produced as described above, a full-thickness test piece in the circumferential direction of the pipe was taken from each of the steel pipes as a test piece for tensile test and the tensile strength of the test piece was measured by a tensile test. In a compression test, a test piece having a diameter of 20 mm and a length of 60 mm was taken from the inner surface-side portion of each of the steel pipes in the circumferential direction of the pipe and the 0.5% compressive proof strength of the test piece was measured as a compressive yield strength.

(61) TABLE-US-00002 TABLE 2 Cumulative rolling Ar.sub.3 reduction ratio Rolling Cooling Cooling transfor- Heating Non-recrystal- Below (rolling finish start stop Expan- mation Thick- temper- lization finish temper- temper- Cooling temper- sion Steel point ness ature temperature temperature + ature ature rate ature ratio No. type (° C.) (mm) (° C.) range (%) 20° C.) (%) (° C.) (° C.) (° C./s) (° C.) (%) Remark 1 A 727 40 1050 75 70 760 755 25 430 0.8 Invention 2 A 727 40 1030 75 55 765 750 20 440 0.8 example 3 A 727 40 1040 75 75 780 770 20 430 0.8 4 A 727 40 1060 75 70 765 755 27 350 0.8 5 A 727 40 1050 75 70 760 750 22 490 0.8 6 B 748 35 1100 80 75 775 770 30 400 0.8 7 C 740 35 1060 75 75 775 765 35 390 1.0 8 D 748 35 1100 75 70 770 760 30 460 1.0 9 E 734 35 1050 75 70 775 765 28 400 1.0 10 F 721 40 1050 75 70 755 745 32 400 0.6 11 A 727 40  950 75 70 770 760 20 450 0.8 Comparative 12 A 727 40 1250 75 70 765 760 25 420 0.8 example 13 A 727 40 1050 55 55 760 750 25 410 0.8 14 A 727 40 1040 75 45 765 760 25 440 0.8 15 A 727 40 1030 75 70 725 720 20 380 0.8 16 A 727 40 1050 75 70 800 790 30 460 0.8 17 F 721 40 1060 75 75 760 750  5 450 0.8 18 F 721 40 1030 75 70 770 760 30 250 0.8 19 F 721 40 1070 75 75 760 750 20 550 0.8 20 F 721 40 1040 75 75 760 750 26 370 1.6 21 G 749 35 1060 75 70 775 770 25 430 0.8 22 H 688 40 1030 75 70 760 750 20 450 1.0 23 I 763 35 1050 80 75 775 760 20 400 0.8 24 J 728 40 1080 75 75 765 755 25 390 1.0 25 K 723 35 1030 75 70 770 760 25 410 1.0 *The underlined values are outside the scope of the present invention.

(62) A DWTT test piece was taken from each of the steel pipes in the circumferential direction of the pipe. Using the DWTT test piece, the temperature at which the percent ductile fracture reached 85% was determined as 85% SATT.

(63) For determining the HAZ toughness of the joint, the temperature at which the percent ductile fracture reached 50% was determined as vTrs. The position of the notch was determined such that the fusion line was located at the center of the notch root of the Charpy test piece and the ratio between the weld metal and the base metal (including welded heat-affected zone) at the notch root was 1:1.

(64) For determining metal microstructure, a sample was taken from the inner surface-side portion of each of the steel pipes at a position of ¼ plate thickness. The sample was etched using nital after polishing, and the metal microstructure was observed using an optical microscope. The area fractions of bainite and polygonal ferrite were calculated by image analysis of 3 photographs captured at a 200-fold magnification. For observing MA, the sample used for measuring the area fractions of bainite and polygonal ferrite was subjected to nital etching and then electrolytic etching (two-step etching). Subsequently, the metal microstructure was observed with a scanning electron microscope (SEM). The area fraction of MA was calculated by image analysis of 3 photographs captured at a 1000-fold magnification. The average grain size of bainite was determined by a linear analysis using a micrograph obtained by taking a sample from the inner surface-side portion of each of the steel pipes at a position of ¼ plate thickness, etching the sample using nital after polishing, and observing the metal microstructure using an optical microscope.

(65) Although the metal microstructures of the steel pipes are determined in Examples, the results may be considered as the metal microstructures of the respective steel plates (steel materials).

(66) Table 3 shows the metal microstructures and mechanical properties measured.

(67) TABLE-US-00003 TABLE 3 Metal microstructure Plate thickness ¼ position Plate thickness Mechanical properties Area Area fraction of ½ position DWTT HAZ Area fraction of martensite- Bainite Tensile Compressive property toughness Steel fraction of polygonal austenite grain size strength strength 85% SATT vTrs No. type bainite (%) ferrite (%) constituent (%) Balance (μm) (MPa) (MPa) (° C.) (° C.) Remark 1 A 93.8 3.5 2.4 θ 7.0 654 515 −25 −37 Invention 2 A 93.0 4.2 2.3 θ 9.0 637 491 −20 −37 example 3 A 98.1 0.0 1.5 θ 9.5 660 523 −18 −37 4 A 95.4 1.8 2.8 — 6.0 678 453 −30 −38 5 A 92.1 3.8 2.2 θ, P 7.5 637 440 −27 −37 6 B 91.3 5.1 3.2 θ 8.0 610 450 −25 −50 7 C 91.1 4.8 4.1 — 6.5 626 452 −35 −25 8 D 88.0 7.5 2.8 θ 7.5 628 461 −30 −27 9 E 96.9 0.0 2.5 θ 7.0 608 451 −20 −40 10 F 95.2 2.2 2.3 θ 7.5 597 471 −25 −35 11 A 97.1 0.0 2.2 θ 6.5 559 454 −30 −38 Comparative 12 A 97.6 0.0 1.9 θ 20.0  772 594 0 −38 example 13 A 95.0 2.3 2.4 θ 18.0  655 494 −5 −37 14 A 96.9 0.0 2.6 θ 18.5  650 503 0 −36 15 A 75.8 20.0  4.2 — 6.0 543 423 −32 −37 16 A 96.8 0.0 2.4 θ 21.0  673 508 0 −37 17 F 88.2 1.2 6.1 θ, P 9.0 586 394 −15 −40 18 F 92.4 0.0 7.6 — 7.0 634 322 −7 −41 19 F 76.5 6.5 2.7 θ, P 9.1 567 339 −17 −41 20 F 96.1 1.5 2.4 — 6.8 604 338 −25 −41 21 G 94.3 4.1 1.0 θ 8.2 560 443 −20 −55 22 H 93.1 0.0 4.8 θ, P 6.2 709 523 −32 −5 23 I 85.6 12.0  2.1 θ 9.5 574 420 −12 −42 24 J 87.0 3.2 7.2 θ, P 6.7 645 438 −23 0 25 K 93.2 0.0 6.8 — 7.2 619 420 −22 −28 *The underlined values are outside the scope of the present invention. *In the above table, “θ” and “P” denote cementite and pearlite, respectively.

(68) In Table 3, all of Nos. 1 to 10 had a tensile strength of 570 MPa or more; a compressive strength of 440 MPa or more; as for DWTT property, a 85% SATT of −10° C. or less; and a HAZ toughness of −20° C. or less. That is, all of Nos. 1 to 10 were evaluated as good.

(69) In contrast, in Nos. 11 to 20, although the composition fell within the scope of the present invention, the production method was outside the scope of the present invention and therefore the intended microstructure was not formed. As a result, Nos. 11 to 20 were evaluated as poor in terms of any of tensile strength, compressive strength, and DWTT property. In Nos. 21 to 25, the chemical composition was outside the scope of the present invention. As a result, Nos. 21 to 25 were evaluated as poor in terms of any of tensile strength, compressive strength, DWTT property, and HAZ toughness.

(70) According to aspects of the present invention, a high-strength steel pipe of API-X70 grade or more which has excellent low-temperature toughness and an excellent DWTT property may be produced. Therefore, the steel pipe according to aspects of the present invention may be used as deep-sea line pipes that require high collapse resistant performance.