JP2017179482A - Electroseamed steel pipe for linepipe and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electroseamed steel pipe for linepipe having excellent SSC resistance even with high strength.SOLUTION: The electroseamed steel pipe for linepipe has a chemical composition containing, by mass%, C:0.01 to 0.1%, Si:0.01 to 0.4%, Mn:0.5 to 2%, P:0.03% or less, S:0.001% or less, Al:0.01 to 0.05%, N:0.003 to 0.008%, Nb:0.01 to 0.05%, Ti:0.005 to 0.02%, Ni:0 to 0.2%, Mo:0 to 0.2% and Ca:0 to 0.0050% and the balance Fe with impurities. It has yield strength of 450 to 600 MPa and tensile strength of 535 to 765 MPa. Maximum Vickers hardness of an inner surface layer is 248 HV or less and smaller than maximum Vickers hardness of an outer surface layer by 5 HV or more.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a steel pipe and a method for manufacturing the same, and more particularly to an electric resistance welded steel pipe for a line pipe and a method for manufacturing the same.

Crude oil and natural gas produced in recent years contain wet hydrogen sulfide (H ₂ S). Since pipelines that transport excavated crude oil and natural gas are exposed to such a sour environment, hydrogen embrittlement due to hydrogen sulfide becomes a problem. Hydrogen embrittlement includes hydrogen sulfide cracking (Sulphide Stress Cracking: hereinafter referred to as SSC) that occurs in steel under static external stress and hydrogen induced cracking (Hydrogen Induced Cracking) that occurs in steel without external stress. HIC). A particularly excellent SSC resistance is required for an electric resistance welded steel pipe for use in a pipeline.

  Furthermore, recent pipelines are required to improve transportation efficiency. If the operation pressure is increased, the transportation efficiency is improved. However, in order to increase the operation pressure, it is necessary to increase the strength of the ERW steel pipe for the line pipe constituting the pipeline. Specifically, an ERW steel pipe having a high strength with a yield strength of 450 to 600 MPa and a tensile strength of 535 to 765 MPa is required.

  Japanese Patent Laying-Open No. 2013-11005 (Patent Document 1) proposes a manufacturing method for improving the sour resistance of a high-strength hot-rolled steel sheet for welded steel pipes for line pipes.

  The high-strength hot-rolled steel sheet for welded steel pipes for line pipes disclosed in Patent Document 1 is mass%, C: 0.01 to 0.07%, Si: 0.40% or less, Mn: 0.5 to 1. 4%, Al: 0.1% or less, Nb: 0.01 to 0.15%, V: 0.1% or less, Ti: 0.03% or less, N: 0.008% or less, and Nb , V and Ti are contained so as to satisfy Nb + V + Ti <0.15 and further satisfy Cm of 0.12 or less, and have a chemical composition comprising the balance Fe and inevitable impurities. Here, Cm = C + Si / 30 + (Mn + Cu) / 30 + Ni / 60 + Mo / 7 + V / 10. It has a structure containing a bainite phase or a bainitic ferrite phase in an area ratio of 95% or more, and the maximum hardness in the plate thickness direction is 220 HV or less. The yield strength is 450 MPa or more. This describes that high strength and excellent sour resistance can be obtained.

JP 2013-11005 A

  However, in Patent Document 1, only HIC resistance is evaluated as sour resistance. Therefore, the high-strength hot-rolled steel sheet for welded pipe for line pipe disclosed in Patent Document 1 may have low SSC resistance.

  An object of the present invention is to provide an ERW steel pipe for a line pipe having excellent SSC resistance even with high strength.

  The electric resistance welded steel pipe for line pipe according to the present embodiment is mass%, C: 0.01 to 0.1%, Si: 0.01 to 0.4%, Mn: 0.5 to 2%, P: 0. 0.03% or less, S: 0.001% or less, Al: 0.01 to 0.05%, N: 0.003 to 0.008%, Nb: 0.01 to 0.05%, Ti: 0.0. 005 to 0.02%, Ni: 0 to 0.2%, Mo: 0 to 0.2%, and Ca: 0 to 0.0050%, the balance having a chemical composition consisting of Fe and impurities . It has a yield strength of 450 to 600 MPa and a tensile strength of 535 to 765 MPa. The maximum Vickers hardness of the inner surface layer is 248 HV or less, and is 5 HV or less smaller than the maximum Vickers hardness of the outer surface layer.

  The ERW steel pipe for line pipes according to the present invention has excellent SSC resistance even at high strength.

FIG. 1 is a part of a cross-sectional view of an electric resistance welded steel pipe. FIG. 2 is a diagram showing a pipe making process of a method for producing an electric resistance steel pipe for line pipe. FIG. 3 is a front view of a tensile test piece used in the tensile test.

  The present inventors investigated and examined the SSC resistance of the high-strength ERW steel pipe for line pipes, and obtained the following knowledge.

  (A) If the C content is 0.01 to 0.1% and strong cooling is performed in the cooling step after hot rolling, high strength can be obtained. However, strong cooling increases the surface hardness and strength, and may cause SSC.

  (B) In order to improve the SSC resistance, it is effective to reduce the tensile strength. Furthermore, SSC resistance is required on the inner surface of the ERW steel pipe. This is because the inner surface of the ERW steel pipe is in contact with a fluid (crude oil or natural gas) containing wet hydrogen sulfide.

  (C) Therefore, in the cooling step after hot rolling, strong cooling is performed on the first surface of the steel plate for line pipe, and weak cooling is performed on the second surface opposite to the first surface. Then, the ERW steel pipe is formed so that the second surface becomes the inner surface of the ERW steel pipe. In this case, since the hardness of the inner surface of the ERW steel pipe is low, the SSC resistance can be improved. On the other hand, since the outer surface of the ERW steel pipe is the first surface that is strongly cooled, the strength of the outer surface can be increased. As a result, it is possible to obtain an ERW steel pipe for line pipes having high strength and excellent SSC resistance.

  The ERW steel pipe for line pipes of the present embodiment completed based on the above knowledge is in mass%, C: 0.01 to 0.1%, Si: 0.01 to 0.4%, Mn: 0.00. 5 to 2%, P: 0.03% or less, S: 0.001% or less, Al: 0.01 to 0.02%, N: 0.003 to 0.008%, Nb: 0.01 to 0 0.05%, Ti: 0.005 to 0.02%, Ni: 0 to 0.2%, Mo: 0 to 0.2%, and Ca: 0 to 0.0050%, with the balance being Fe And a chemical composition consisting of impurities. It has a yield strength of 450 to 600 MPa and a tensile strength of 535 to 765 MPa. The maximum Vickers hardness of the inner surface layer is 248 HV or less, and is 5 HV or less smaller than the maximum Vickers hardness of the outer surface layer.

  The chemical composition may contain one or more selected from the group consisting of Ni: 0.001 to 0.2% and Mo: 0.1 to 0.2%.

  The chemical composition may contain Ca: 0.0005 to 0.0050%.

The method for producing an ERW steel pipe for a line pipe according to the present embodiment includes a step of heating the above-described material at 1100 to 1250 ° C. and then hot rolling at a finish rolling temperature of 780 to 830 ° C. to produce a steel plate, The first surface is cooled at a cooling rate V1 of 5 to 25 ° C./s, and the second surface opposite to the first surface is cooled at a cooling rate V2 that satisfies Equation (1), and the steel plate is 600 ° C. or less. And forming a hot coil by using the hot coil and forming and welding the second surface to be an inner surface to manufacture an electric-welded steel pipe.
V2 ≦ V1−4.09 (1)

  Hereinafter, the ERW steel pipe for line pipes of this embodiment will be described in detail. “%” Regarding an element means mass% unless otherwise specified.

[Chemical composition]
The chemical composition of the hot-rolled steel sheet of this embodiment contains the following elements.

C: 0.01 to 0.1%
Carbon (C) increases the strength of the steel. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, carbides are generated and the toughness and ductility of the steel are reduced. If the C content is too high, the weldability further decreases. Therefore, the C content is 0.01 to 0.1%. The minimum with preferable C content is 0.03%, More preferably, it is 0.04%. The upper limit with preferable C content is 0.08%, More preferably, it is 0.07%.

Si: 0.01 to 0.4%
Silicon (Si) deoxidizes steel. If the Si content is too low, this effect cannot be obtained. On the other hand, if the Si content is too high, the toughness of the steel decreases. Therefore, the Si content is 0.01 to 0.4%. The minimum with preferable Si content is 0.02%, More preferably, it is 0.1%. The upper limit with preferable Si content is 0.35%, More preferably, it is 0.3%.

Mn: 0.5-2%
Manganese (Mn) increases the hardenability of the steel and increases the strength of the steel. If the Mn content is too low, this effect cannot be obtained. On the other hand, if the Mn content is too high, the toughness and SSC resistance of the steel will decrease. Therefore, the Mn content is 0.5-2%. The minimum with preferable Mn content is 0.6%, More preferably, it is 0.8%. The upper limit with preferable Mn content is 1.8%, More preferably, it is 1.5%.

P: 0.03% or less Phosphorus (P) is an impurity. P segregates at the grain boundary and embrittles the grain boundary. Therefore, P decreases the toughness and SSC resistance of steel. Therefore, it is preferable that the P content is small. The P content is 0.03% or less. The upper limit with preferable P content is 0.02%, More preferably, it is 0.01%.

S: 0.001% or less Sulfur (S) is an impurity. S combines with Mn to form a Mn-based sulfide. Mn-based sulfides are easy to dissolve. Therefore, the toughness and SSC resistance of the steel are reduced. Accordingly, the S content is preferably as low as possible. S content is 0.001% or less. The upper limit with preferable S content is 0.0005%, More preferably, it is 0.0002%.

Al: 0.01 to 0.05%
Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect cannot be obtained. On the other hand, if the Al content is too high, the Al nitride becomes coarse and the toughness of the steel decreases. Therefore, the Al content is 0.01 to 0.05%. The minimum with preferable Al content is 0.012%, More preferably, it is 0.013%. The upper limit with preferable Al content is 0.019%, More preferably, it is 0.018%. In addition, Al content in this Embodiment means content of all the Al in steel.

N: 0.003 to 0.008%
Nitrogen (N) increases the strength of the steel by solid solution strengthening. If the N content is too low, this effect cannot be obtained. On the other hand, if the N content is too high, the carbonitride is coarsened and the SSC resistance is lowered. Therefore, the N content is 0.003 to 0.008%. The minimum with preferable N content is 0.004%, More preferably, it is 0.005%. The upper limit with preferable N content is 0.007%, More preferably, it is 0.006%.

Nb: 0.01 to 0.05%
Niobium (Nb) combines with C and N in the steel to form fine Nb carbonitride. Fine Nb carbonitride increases the strength of the steel by dispersion strengthening. If the Ni content is too low, this effect cannot be obtained. On the other hand, if the Nb content is too high, the Nb carbonitride becomes coarse and the SSC resistance of the steel decreases. Furthermore, if the Nb content is too high, the toughness of the welded portion decreases. Therefore, the Nb content is 0.01 to 0.05%. The minimum with preferable Nb content is 0.02%, More preferably, it is 0.03%. The upper limit with preferable Nb content is 0.045%, More preferably, it is 0.04%.

Ti: 0.005-0.02%
Titanium (Ti) combines with N in the steel to form Ti nitride and / or Ti carbonitride. Ti nitrides and / or Ti carbonitrides refine steel grains. If the Ti content is too low, this effect cannot be obtained. On the other hand, if the Ti content is too high, coarse Ti nitrides and / or Ti carbonitrides are formed. Therefore, the SSC resistance of the steel is reduced. Therefore, the Ti content is 0.005 to 0.02%. The minimum with preferable Ti content is 0.007%, More preferably, it is 0.01%. The upper limit with preferable Ti content is 0.018%, More preferably, it is 0.016%.

  The balance of the chemical composition of the linepipe ERW steel pipe according to the present embodiment is composed of Fe and impurities. Here, the impurities are mixed from the ore as a raw material, scrap, or the manufacturing environment when the ERW steel pipe for a line pipe is manufactured industrially, and the electric pipe for the line pipe of this embodiment is used. It means that it is allowed as long as it does not adversely affect the sewn steel pipe.

[Arbitrary elements]
The electric resistance welded steel pipe for line pipe according to the present embodiment may further contain one or more selected from the group consisting of Ni and Mo instead of a part of Fe. All of these elements are optional elements and increase the strength of the steel.

Ni: 0 to 0.2%
Nickel (Ni) is an optional element and may not be contained. When contained, Ni increases the strength of the steel by solid solution strengthening. Ni further increases the toughness of the steel. However, if the Ni content is too high, the weldability of the steel decreases. Therefore, the Ni content is 0 to 0.2%. The minimum with preferable Ni content is 0.001%, More preferably, it is 0.005%. The upper limit with preferable Ni content is 0.18%, More preferably, it is 0.15%.

Mo: 0 to 0.2%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo increases the hardenability of the steel and increases the strength of the steel. Furthermore, since micro segregation of Mo hardly occurs, the generation of SSC due to center segregation is suppressed. However, since Mo is expensive, if Mo is contained excessively, the manufacturing cost increases. Therefore, the Mo content is 0 to 0.2%. The minimum with preferable Mo content is 0.1%, More preferably, it is 0.12%. The upper limit with preferable Mo content is 0.18%, More preferably, it is 0.15%.

  The electric resistance welded steel pipe for line pipe according to the present embodiment may further contain Ca instead of a part of Fe.

Ca: 0% to 0.0050%
Calcium (Ca) is an optional element and may not be contained. When contained, Ca makes the form of MnS, the starting point of SSC, spherical, and suppresses the generation of SSC. Ca further forms CaS and suppresses the generation of MnS. However, if Ca is contained excessively, the effect is saturated and the manufacturing cost increases. Therefore, the Ca content is 0% to 0.0050%. The minimum of preferable Ca content is 0.0005%, More preferably, it is 0.0007%. The upper limit of the preferable Ca content is 0.0045%, and more preferably 0.0040%.

[Hardness of inner surface layer and outer surface layer]
In the ERW steel pipe for line pipes according to this embodiment, the hardness of the inner surface layer is 248 HV or less in terms of Vickers hardness, and is 5 HV or less smaller than the highest Vickers hardness of the outer surface layer.

  The hardness of the inner and outer surface layers is measured by the following method. FIG. 1 is a part of a cross-sectional view of a line pipe ERW steel pipe (that is, a cross-sectional view cut in the radial direction of the line pipe ERW steel pipe). Referring to FIG. 1, the linepipe ERW steel pipe 100 is equally divided into 20 in the thickness t direction. A range from the outer surface 1 of the line pipe ERW steel pipe 100 to t / 5 in the inner surface 2 direction is defined as an outer surface layer OL. A range from the inner surface 2 to t / 5 in the direction of the outer surface 1 is defined as an inner surface layer IL.

  The outer surface layer OL and the inner surface layer IL at a position 180 ° (a position facing away from the welded portion) from the welded portion of the linepipe ERW steel tube are specified by looking at the linepipe ERW steel tube in the axial direction.

  In the specified outer surface layer OL and inner surface layer IL, a Vickers hardness test in accordance with JIS Z2244 (2009) is performed at a depth of 0.1 mm from the outer surface and a depth of 0.1 mm from the inner surface. The test force is 100 gf = 0.98N.

  In the outer surface layer OL and the inner surface layer IL, the maximum value among the plurality of values obtained by the Vickers hardness test is specified, and these are defined as the highest Vickers hardness of the outer surface layer OL and the inner surface layer IL.

  The maximum Vickers hardness of the inner surface layer IL defined by the above method is 248 HV or less. If the maximum Vickers hardness of the inner surface layer IL exceeds 248 HV, the toughness of the steel decreases, and the SSC resistance of the ERW steel pipe for line pipes decreases. The lower limit of the maximum Vickers hardness of the inner surface layer IL is not particularly limited, but if the maximum Vickers hardness of the inner surface layer IL is too low, the strength of the ERW steel pipe for line pipes cannot be sufficiently obtained. Therefore, a preferable lower limit of the maximum Vickers hardness of the inner surface layer IL is 180 HV, and more preferably 185 HV. A preferable upper limit of the maximum Vickers hardness of the inner surface layer IL is 245HV.

  Further, the maximum Vickers hardness of the inner surface layer IL is 5 HV or less smaller than the maximum Vickers hardness of the outer surface layer OL.

  That is, the upper limit of allowable hardness is made different between the inner surface layer and the outer surface layer. The reason for this is as follows. Since the inner surface of the ERW steel pipe for line pipes is exposed to the sour environment as described above, high SSC resistance is required. On the other hand, the outer surface of the ERW steel pipe for line pipes is less exposed to the sour environment than the inner surface. Therefore, the outer surface of the ERW steel pipe for line pipes is required to have high strength rather than SSC resistance. If the maximum Vickers hardness of the inner surface is made 5HV or less smaller than the maximum Vickers hardness of the outer surface, excellent SSC resistance can be obtained on the inner surface while obtaining high strength on the outer surface.

[Yield strength and tensile strength]
The yield strength YS of the ERW steel pipe for line pipe is 450 to 600 MPa, and the tensile strength TS is 535 to 765 MPa. In this case, the strength required for the ERW steel pipe for line pipe can be obtained.

  Yield strength and tensile strength are measured by the following methods. Take a tensile specimen from an ERW steel pipe for line pipe. Specifically, a full thickness tensile test specimen is taken from a position 180 ° from the welded part of the linepipe ERW steel pipe in the axial direction (position facing away from the welded part). . The cross section of the tensile test piece is arcuate, and the longitudinal direction of the tensile test piece is parallel to the rolling direction of the steel pipe. Using a tensile test piece, a tensile test is performed at room temperature in accordance with the API standard 5CT. Based on the test results, the yield strength YS (MPa) and the tensile strength TS (MPa) of the ERW steel pipe for line pipes are obtained.

[Production method]
An example of the manufacturing method of the above-mentioned ERW steel pipe for line pipe will be described. The slab which is a raw material is manufactured using the molten steel which satisfy | fills the chemical composition mentioned above (raw material preparation process). The manufactured slab is heated in a heating furnace (heating process). The heated slab is rolled with a rolling mill to produce a steel plate (rolling process). The manufactured steel sheet is cooled with a water-cooling device (cooling process) and wound into a hot coil (winding process). A hot coil is formed and welded to produce a pipe, and an ERW steel pipe for a line pipe is produced (pipe making process). In this manufacturing process, in the cooling process, the first surface and the second surface of the steel sheet are cooled at different cooling rates. Thereby, the ERW steel pipe for line pipes which has the outer surface layer and inner surface layer from which hardness differs can be manufactured. Hereinafter, each process will be described in detail.

[Material preparation process]
A material having the above chemical composition is prepared. Specifically, molten steel having the above-described chemical composition is manufactured. The raw material is manufactured using molten steel. The slab may be manufactured by a continuous casting method. The ingot may be manufactured using molten steel, and the ingot may be subjected to ingot rolling to manufacture a slab.

[Heating process]
In the heating process, the manufactured material is heated in a heating furnace. The heating temperature of the raw material in a heating furnace is 1100-1250 degreeC. If the heating temperature is too high, the austenite grains become coarse, so that the crystal grains cannot be refined, and the strength of the steel decreases. On the other hand, if the heating temperature is too low, refinement of crystal grains during rolling and precipitation strengthening after rolling cannot be obtained, and the strength of the steel decreases. Accordingly, the heating temperature is 1100 to 1250 ° C.

[Rolling process]
In the rolling process, the heated material is hot-rolled using a roughing mill and a finish rolling mill to form a steel plate. Both the rough rolling mill and the finish rolling mill include a plurality of rolling stands arranged in a row, and each rolling stand includes a roll pair.

  In the rolling process, the surface temperature of the steel plate on the exit side of the final stand of the finish rolling mill is defined as the finish rolling temperature (° C.). The finish rolling temperature (° C.) is 780 to 830 ° C. If finish rolling temperature is less than 780 degreeC, the rolling resistance of a steel plate will increase and productivity will fall. Furthermore, the steel sheet is rolled in a two-phase region of ferrite and austenite. In this case, the microstructure of the steel sheet forms a layered structure, and the mechanical properties deteriorate. On the other hand, if the finish rolling temperature exceeds 830 ° C., the hardness of the steel pipe for line pipe becomes too hard and the strength becomes too high. Accordingly, the finish rolling temperature is 780 to 830 ° C. The rolling reduction in the austenite non-recrystallization temperature region is preferably 70 to 80%. In this case, the unrecrystallized structure is refined.

[Cooling process]
After completion of rolling, the steel sheet is air-cooled as necessary, and then strong cooling and weak cooling are performed using a water cooling device. The material surface temperature at the start of strong cooling and weak cooling (hereinafter referred to as water cooling start temperature) is preferably 740 ° C. or lower. If water cooling start temperature is 740 degrees C or less, since the production | generation of a bainite structure can be suppressed, obtaining high intensity | strength, the further outstanding SSC resistance will be obtained.

  Let the cooling rate of the 1st surface of a steel plate be V1 (degreeC / s), and let the cooling rate of the 2nd surface on the opposite side to the 1st surface be V2 (degrees C / s). The first surface is cooled by setting V1 to 5 to 25 ° C./s.

On the other hand, the cooling rate V2 (° C./s) of the second surface satisfies the following formula (1).
V2 ≦ V1−4.09 (1)

  In short, the cooling rate V2 of the second surface is made slower than the cooling rate V1 of the first surface. If the cooling rate V2 does not satisfy the formula (1), the maximum Vickers hardness of the inner surface layer does not decrease by 5 HV or more of the maximum Vickers hardness of the outer surface layer. In this case, even if high strength is obtained, the SSC resistance of the inner surface is lowered. If the cooling rate V1 is 5 to 25 ° C./s and the cooling rate V2 satisfies the formula (2), the yield strength YS of the ERW steel pipe after the pipe forming process described later is 450 to 600 MPa, and the tensile strength TS. Becomes 535 to 765 MPa. Furthermore, the maximum Vickers hardness of the inner surface is reduced by 5 HV or more, which is the maximum Vickers hardness of the outer surface.

  The cooling rate V2 is not particularly limited, but if the cooling rate V2 is too slow, the strength of the ERW steel pipe for line pipes cannot be sufficiently obtained. Therefore, the preferable lower limit of the cooling rate V2 is 0.5 ° C./s, more preferably 0.8 ° C./s.

[Winding process]
The hot-rolled steel sheet manufactured in the hot rolling process is wound up to form a coil. The surface temperature of the hot-rolled steel sheet at the start of coil winding (hereinafter referred to as winding temperature) is preferably 600 ° C. or lower. If the coiling temperature is too high, the crystal grains become coarse and the strength of the steel decreases. The lower limit of the winding temperature is not particularly limited, but if the winding temperature is too low, the productivity is lowered. Therefore, the minimum with preferable winding temperature is 500 degreeC, More preferably, it is 530 degreeC.

[Pipe making process]
While rewinding the coiled hot-rolled steel sheet, as shown in FIG. 2, a line pipe ERW steel pipe 200 is manufactured with the second surface of the line pipe steel sheet as the inner surface 2 and the first surface as the outer surface 1. Specifically, an open pipe is formed by bending a steel sheet for line pipe using a continuous forming roll. At this time, bending is performed so that the second surface of the steel plate for line pipe becomes the inner surface 2 of the open pipe. Subsequently, the seam portion of the open pipe, that is, both end faces 3 in the longitudinal direction of the steel plate for line pipe are welded by the electric resistance welding method, and the electric resistance welded steel pipe 200 for line pipe is manufactured. In FIG. 2, both end surfaces 3 are welded by an electric seam welding method using a power supply 60 and a welding roll 70.

  By the above manufacturing process, the electric resistance welded steel pipe of the present embodiment can be manufactured. In the ERW steel pipe for line pipe of the present embodiment, the first surface may be the upper surface and the second surface may be the lower surface, or the first surface may be the lower surface and the second surface may be the upper surface.

  Slabs were produced by continuously casting molten steels of Steel A to Steel G shown in Table 1.

  Using a plurality of slabs of steel A to steel G, ERW steel pipes for line pipes having test numbers 1 to 15 shown in Table 2 were manufactured.

  Specifically, each test number slab was heated in a heating furnace. The heating temperature (° C.) was as shown in Table 2. The heated slab was rolled using a plurality of hot rolling mills to produce a steel plate. The rolled steel sheet was air-cooled and then water-cooled. The water cooling start temperature (° C.) of the steel plates of each test number was as shown in Table 2. The cooling rate by water cooling was (° C./sec) as shown in Table 2.

  After manufacturing the steel plate by the above manufacturing process, the steel plate of each test number was wound at the winding temperature (° C.) shown in Table 2 to form a coil. The pipe was manufactured by the above-described method to produce an electric resistance welded steel pipe for line pipe having an outer diameter of 406.4 mm and a wall thickness of 15.9 mm.

[Test method]
[Vickers hardness test]
Based on the above-mentioned Vickers hardness test method, the maximum value (HV) of the hardness of the inner and outer surface layers of the ERW steel pipe for line pipe of each test number was determined.

[Strength test]
Based on the above-described yield strength and tensile strength test methods, the yield strength YS (MPa) and tensile strength TS (MPa) of the electric resistance welded steel pipe for each test number were determined. The size of the tensile test piece was as shown in FIG. 3, the length of the parallel part was 50.8 mm, and the width of the parallel part was 38.1 mm. The numerical value in FIG. 3 shows the dimension (a unit is mm) of the corresponding site | part of a test piece.

[SSC resistance evaluation test]
A round bar specimen was taken from the center of the thickness of the steel sheet for line pipe of each test number. The longitudinal direction of the round bar test piece was parallel to the longitudinal direction of the steel plate. The outer diameter of the parallel part of the round bar test piece was 6.35 mm, and the length of the parallel part was 25.4 mm. According to the NACE (National Association of Corrosion Engineers) TM0177A method, the SSC resistance of each round bar test piece was evaluated by a constant load test. The test bath was room temperature 5% sodium chloride + 0.5% acetic acid aqueous solution saturated with 1 atm hydrogen sulfide gas. Each round bar test piece was loaded with a load stress of 325 MPa and immersed in a test bath for 720 hours. After 720 hours from the immersion, it was confirmed whether each round bar specimen was broken. When no fracture was observed in the round bar test piece, it was judged that the SSC resistance of the steel was high. When breakage was observed in the round bar test piece, it was judged that the SSC resistance of the steel was low.

[Test results]
Table 2 shows the test results. In the “outer surface layer” and “inner surface layer” columns of “Vickers hardness” in Table 2, the maximum hardness values (HV) of the outer surface layer and the inner surface layer of the ERW steel pipe for line pipe are described. In the “internal / external hardness difference” column, the difference between the highest Vickers hardness of the inner surface layer and the highest Vickers hardness of the outer surface layer is described.

The yield strength (MPa) is described in the “YS” column in Table 2. In the “TS” column, the tensile strength (MPa) is described. In the “SSC resistance” column, the SSC resistance evaluation test results are described. “◯” means that no breakage was observed in the round bar test piece and excellent SSC resistance was shown. “X” means that a fracture was observed in the round bar specimen and the SSC resistance was low.

  Referring to Table 2, the chemical compositions and production conditions of Test No. 1 to Test No. 7 were within the scope of the present invention. Furthermore, the yield strength was 450 to 600 MPa, and the tensile strength was 535 to 765 MPa. Further, the maximum Vickers hardness of the inner surface layer of the ERW steel pipe for line pipes was 248 HV or less, which was 5 HV or less smaller than the maximum Vickers hardness of the outer surface layer. Therefore, no fracture was observed in the SSC resistance evaluation test, and Test Nos. 1 to 7 showed excellent SSC resistance.

  On the other hand, in the test number 8, the heating temperature at the time of rolling exceeded 1250 degreeC. Therefore, the crystal grains became coarse, the yield strength YS was less than 450 MPa, and the tensile strength TS was less than 535 MPa, which was low.

  In test number 9, the finish rolling temperature exceeded 830 ° C. Therefore, the hardness of the ERW steel pipe for line pipe became too hard, the yield strength YS exceeded 600 MPa, and the tensile strength exceeded 765 MPa, too high.

  In test number 10, the upper surface cooling rate V1 exceeded 25 ° C./s. Therefore, the hardness of the ERW steel pipe for line pipe became too hard, the yield strength YS exceeded 600 MPa, and the tensile strength exceeded 765 MPa, too high.

  In Test No. 11, the upper surface cooling rate V1 was less than 5 ° C./s. Therefore, the hardness of the ERW steel pipe for line pipes was too low, the yield strength YS was less than 450 MPa, and the tensile strength TS was less than 535 MPa, which was low.

  In test number 12, the cooling rate V2 on the lower surface exceeded the upper limit of formula (1). Therefore, the maximum Vickers hardness of the inner surface layer exceeded 248 HV, the difference from the Vickers maximum hardness of the outer surface layer was less than 5, and SSC was observed. Furthermore, the hardness of the ERW steel pipe for line pipe was too hard, the yield strength YS exceeded 600 MPa, and the tensile strength exceeded 765 MPa, too high.

  In test number 13, the winding temperature exceeded 600 ° C. Therefore, the crystal grains became coarse, the yield strength YS was less than 450 MPa, and the tensile strength TS was less than 535 MPa, which was low.

  In test number 14, the heating temperature during rolling was less than 1100 ° C. Therefore, the yield strength YS was less than 450 MPa, and the tensile strength TS was less than 535 MPa, which was low.

  In test number 15, the finish rolling temperature was less than 780 ° C. Therefore, the hardness of the ERW steel pipe for line pipes was too high, the yield strength YS exceeded 600 MPa, and the tensile strength exceeded 765 MPa, too high.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

1 outer surface 2 inner surface 100 ERW steel pipe for line pipe 200 ERW steel pipe for line pipe

Claims (4)

% By mass
C: 0.01 to 0.1%
Si: 0.01-0.4%
Mn: 0.5-2%
P: 0.03% or less,
S: 0.001% or less,
Al: 0.01-0.05%,
N: 0.003 to 0.008%,
Nb: 0.01-0.05%
Ti: 0.005 to 0.02%,
Ni: 0 to 0.2%,
Mo: 0 to 0.2%, and
Ca: 0 to 0.0050% is contained, the balance has a chemical composition consisting of Fe and impurities,
Having a yield strength of 450-600 MPa, and a tensile strength of 535-765 MPa,
An ERW steel pipe for line pipes having a maximum Vickers hardness of 248 HV or less on the inner surface layer and 5 HV or less smaller than the maximum Vickers hardness of the outer surface layer. An electric resistance welded steel pipe for a line pipe according to claim 1,
The chemical composition is
Ni: 0.001 to 0.2%, and
Mo: ERW steel pipe for line pipes containing at least one selected from the group consisting of 0.1 to 0.2%. An electric-welded steel pipe for a line pipe according to claim 1 or 2,
The chemical composition is
An electric resistance welded steel pipe for line pipes containing Ca: 0.0005 to 0.0050%. After heating the raw material which has a chemical composition of any one of Claims 1-3 at 1100-1250 ° C, hot rolling at a finish rolling temperature of 780-830 ° C, and manufacturing a steel plate,
Cooling the first surface of the steel sheet at a cooling rate V1 of 5 to 25 ° C./s, and cooling the second surface opposite to the first surface at a cooling rate V2 that satisfies Equation (1);
Winding the steel sheet at 600 ° C. or lower to form a hot coil;
A method for producing an electric-welded steel pipe for line pipes, comprising: using the hot coil to form and weld the second surface to be an inner surface and producing an electric-welded steel pipe.
V2 ≦ V1−4.09 (1)

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