[0001]The present invention relates to a seamless steel tube and a manufacturing method thereof, and more particularly, to a suitable seamless steel pipe and a manufacturing method thereof for a line pipe.
Background technique
[0002]Petroleum of oil field is located in the land or shallow water, are becoming depleted gas resources in recent years, the development of deep-sea offshore oil has become active. In offshore oil, oil wells installed in seabed, from wellhead of gas wells, it is necessary to transport crude oil and gas with a flow line and risers up offshore platform. The flow line is laid line pipes along the terrain surface or seafloor. Risers and is a line pipe which is arranged to stand up from the sea floor to the platform direction (ie upwards).
[0003]
　Inside the steel tube constituting the laid Flow lines deep sea, it takes high internal fluid pressure deep strata pressure is applied, also affected by sea water pressure deepwater during shutdowns. Meanwhile, the steel pipe constituting the riser further also affected by repeated distortion due to waves. Therefore, as a steel pipe for use in such applications, and high-toughness steel pipes it has been desired high strength. In more recent years, typified by deep sea and cold regions are being developed oil and gas wells in harsh sour environment than conventional. Such harsh submarine pipelines laid in sour environment is higher than conventional strength (pressure resistance) and toughness are required, further, resistance to hydrogen-induced cracking resistance (HIC resistance) and sulfide stress corrosion cracking sex (SSC resistance) are required.
[0004]
　Patent Document 1, C: 0.03 ~ 0.08%, Si: 0.15% or less, Mn: 0.3 ~ 2.5%, Al: 0.001 ~ 0.10%, Cr: 0 .02 ~ 1.0%, Ni: 0.02 ~ 1.0%, Mo: 0.02 ~ 1.2%, Ti: 0.004 ~ 0.010%, N: 0.002 ~ 0.008 % and Ca, and containing from 0.0002 to 0.005% in total of one or more of Mg and REM, the balance being Fe and impurities, P in the impurities of 0.05% or less, S There is 0.005% or less, and good line thick seamless steel pipe of toughness at high strength is disclosed, wherein the wall thickness of 30 ~ 50 mm.
[0005]
　Patent Document 2, is subjected to quenching and tempering process, yield strength: 450 MPa exceeds a thick high-strength seamless steel pipe having a tube outermost or tube innermost at a load: 5 kgf (test force : measurable Vickers hardness HV5 in 49N) is thick high-strength seamless steel pipe for line pipe superior in sour resistance, characterized in that it 250HV5 less is disclosed.
[0006]
　Patent Document 3, by mass%, C: 0.02 ~ 0.10% , Si: 0.5% or less, Mn: 0.5 ~ 2.0%, Al: 0.01 ~ 0.1% , Ca: 0.005% or less, and, N: containing 0.007% or less, further, Ti: 0.008% or less, V: less than 0.06%, and, Nb: 0.05% or less It made containing one or more members selected from the group, and the balance Fe and impurities, Ti, V, less than 0.06% the content of Nb is in total, carbon is defined by the following formula equivalent Ceq is at 0.38% or more, Ti, V, magnitude of the one or carbonitride containing two or more kinds of Nb and Al is 200nm or less, the line pipe for the seamless steel pipe disclosed It is.
　Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15
[0007]
　Patent Document 4, chemical composition, in mass%, C: 0.02 ~ 0.10%, Si: 0.05 ~ 0.5%, Mn: 1.0 ~ 2.0%, Mo: 0 .5 ~ 1.0%, Cr: 0.1 ~ 1.0%, Al: 0.01 ~ 0.10%, P: 0.03% or less, S: 0.005% or less, Ca: 0. 0005 ~ 0.005%, V: 0.010 ~ 0.040%, and, N: containing 0.002 to 0.007%, further, Ti: 0.001 ~ 0.008% and, Nb : containing one or two kinds selected from the group consisting of 0.02 to 0.05%, the balance is an Fe and impurities; a carbon equivalent Ceq is from 0.50 to 0.58 percent; specific carbide containing; seamless steel pipe, wherein it is disclosed.
CITATION
Patent Document
[0008]
Patent Document 1: JP 2010-242222 Patent Publication
Patent Document 2: JP 2013-32584 JP
Patent Document 3: WO 2011/152240 Patent
Patent Document 4: Japanese Patent No. 5516831
Disclosure of the Invention
[0009]
　Be applied to the prior art described above, the seamless steel pipe having a strength of API (American Petroleum Institute) X80 grade or higher, which is specified in the Standard (or lower yield strength 555MPa), stably excellent SSC resistance obtained there is a case that is not.
[0010]
　Quenching - in order to improve the strength and toughness of seamless steel pipe manufactured by the tempering process is to increase the content of alloying elements such as carbon, it may be increased hardenability. However, increasing the content of alloying elements such as carbon, the strength of the steel pipe surface (hardness) is high. Quenching - seamless steel pipe produced by tempering process, the quenching process, the surface layer is high hardness since the cooling rate is likely to be caused fast baked found in meat hardness decreases. This trend may be after tempering remains. Therefore, in the seamless steel pipe having an X80 grade or higher strength may exceed the upper limit hardness 250Hv the surface hardness required as sour grade API 5L standards.
[0011]
　Patent Document 1 is effective for realizing high strength and high toughness, the improvement of SSC resistance according to the hardness suppressed and this surface layer portion is not necessarily paid consideration. Patent Document 2 is a possible to control the hardness of the steel tube surface portion 250HV5 below, it appears to require a special manufacturing process. In Patent Document 3, although consideration has been made regarding SSC resistance performs direct quenching or inline quenching after hot steel pipe, it is necessary to perform further reheating quenching. In Patent Document 4, as it concerns with respect to hardness and HIC resistance of the steel pipe surface portion has been made, the reheating quenching step was essential, in combination with direct quenching or inline quenching after hot pipe making necessary in the cost rationality of production is not necessarily high.
[0012]
　An object of the present invention is to provide a relatively reasonable there can be manufactured in a manufacturing process, and stably obtained seamless steel pipes and more yield strength and excellent SSC resistance 555MPa.
[0013]
　Seamless steel pipe according to an embodiment of the present invention, the chemical composition, in mass%, C: 0.02 ~ 0.15% , Si: 0.05 ~ 0.5%, Mn: 0.30 ~ 2. 5%, P: 0.03% or less, S: 0.006% or less, O: 0.004% or less, Al: 0.01 ~ 0.10%, Ti: 0.001 ~ 0.010%, N 0.007% or less, Cr: 0.05 ~ 1.0%, Mo: less than 0.02% or more 0.5%, Ni: 0.03 ~ 1.0 %, Cu: 0.02 ~ 1. 0%, V: 0.020 ~ 0.20 %, Ca: 0.0005 ~ 0.005%, Nb: 0 ~ 0.05%, the balance is Fe and impurities, defined by the following formula (1) that the carbon equivalent Ceq is less than 0.500% or more 0.430%, tissue, from the surface layer to meat, the tempered martensite or tempered bainite and the main phase, The size of the prior austenite of the serial tissue is 6.0 less than in grain size number conforming to ASTM E112-10, between the position and the outer surface of 1mm from the inner surface of the position of 1mm, Vickers hardness below 250Hv Yes, the yield strength is greater than or equal to 555MPa.
　Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ... (1)
　The equation (1) element symbol, in mass%, the content of the corresponding element is assigned.
[0014]
　Method of producing a seamless steel pipe according to an embodiment of the present invention, the chemical composition, in mass%, C: 0.02 ~ 0.15% , Si: 0.05 ~ 0.5%, Mn: 0.30 ~ 2.5%, P: 0.03% or less, S: 0.006% or less, O: 0.004% or less, Al: 0.01 ~ 0.10%, Ti: 0.001 ~ 0.010 %, N: 0.007% or less, Cr: 0.05 ~ 1.0%, Mo: 0.02% to less than 0.5%, Ni: 0.03 ~ 1.0 %, Cu: 0.02 ~ 1.0%, V: 0.020 ~ 0.20%, Ca: 0.0005 ~ 0.005%, Nb: 0 ~ 0.05%, remainder: a step of preparing a raw material is Fe and impurities , to tempering the steps of manufacturing a mother tube by hot working the material, and a step of hardening by direct quenching or inline quenching mother pipe, quenching is hollow shell And a step. In between the quenching and tempering is not performed reheating quenching. Carbon equivalent Ceq defined by the following formula (3) is less than 0.500% or more 0.430%, Larson defined by the following formula (4) - is the mirror parameter PL is 18800 or more.
　Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) /
　15 ... (3) PL = (T + 273) × (20 + log (t)) ... (4)
　The formula (3) in each element symbol, in mass%, the content of the corresponding element is assigned. In the formula (4), T is a tempering temperature, t is the holding time at that temperature. Unit of T is ℃, unit of t is the time.
[0015]
　According to the present invention, a relatively reasonable there can be manufactured in a manufacturing process, and stably obtained seamless steel pipes and more yield strength and excellent SSC resistance 555MPa can be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016]
[1] Figure 1 is a block diagram showing an example of a production line.
FIG. 2 is a flowchart showing a manufacturing process of a seamless steel pipe.
FIG. 3 is a diagram showing a change in surface temperature versus time for the workpiece during manufacture.
[4] FIG. 4, for steel B, Larson - it is a scatter diagram plotting the relationship between the mirror parameters PL and the yield strength YS.
FIG. 5 is the steel A, Larson - is a scatter diagram plotting the relationship between the mirror parameters PL and the yield strength YS.
FIG. 6, for steel B, Larson - a mirror parameters PL, an outer surface, in the meat, and is a scatter diagram plotting the relationship between the hardness of the inner surface.
FIG. 7 is for the steel A, Larson - a mirror parameters PL, an outer surface, in the meat, and is a scatter diagram plotting the relationship between the hardness of the inner surface.
FIG. 8, for steel B, Larson - is a scatter diagram plotting the relationship between the mirror parameters PL and the maximum difference in hardness.
[9] FIG. 9, for steels A, Larson - is a scatter diagram plotting the relationship between the mirror parameters PL and the maximum difference in hardness.
DESCRIPTION OF THE INVENTION
[0017]
　The present inventors have found that in the seamless steel pipe to ensure a yield strength of at least 555MPa, and excellent SSC resistance was studied how to stably obtain. As a result, the carbon equivalent of the steel is limited to an appropriate range, and by reducing the difference between the hardness in the hardness and meat surface of a seamless steel pipe, only direct quenching or inline quenching after hot pipe making, without reheating quenching, ensuring the yield strength of more than 555MPa, and excellent SSC resistance was found to be obtained stably.
[0018]
　In quenching after rolling, the surface layer of seamless steel pipe faster cooling rate is likely to contain the burn. Therefore, seamless surface of steel pipe is liable to be hard, in some cases more than the hardness value of the stipulated by API 5L standards and DNV-OS-F101 standard. On the other hand, the wall thickness central part of the seamless steel pipe for the cooling rate is slow, baked hardly enters, there are cases where non-hardened structure such as ferrite is mixed. Thus, usually that hardness difference at the surface and the meat occurs, the trend is depending on the conditions of the tempering may also remain after tempering. Also, at high seamless steel pipe carbon equivalent as applied to X80 grade or higher high strength steel, there is a tendency that the hardness difference between the surface layer and the meat becomes remarkable. The reason why the surface layer hardness is high, a problem in terms of stably obtain a good sour resistance.
[0019]
　The carbon equivalent is too low, it is difficult to ensure the strength of the seamless steel pipe. On the other hand, when the carbon equivalent is too high, the manufacturing process and only one direct quenching or inline hardening without reheating quenching, it becomes difficult to the surface layer of Vickers hardness below 250 Hv. This is because when the direct quenching or inline hardening quenching after hot pipe making, easy to austenite grains coarsened as compared with the case of the reheating quenching, because the overall increases hardenability. Therefore, the Ceq defined by the following formula (1), is less than 0.500% or more 0.430%.
　Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ... (1)
　The equation (1) element symbol, in mass%, the content of the corresponding element is assigned.
[0020]
　In order to reduce the hardness difference between the surface layer and in the meat, in addition to carbon equivalent, it is effective to appropriately limit the tempering conditions. That is, if the tempering is insufficient, the reduction of the surface hardness is insufficient, Vickers hardness may become point occurs greater than 250 Hv. Specifically, Larson defined by equation (2) below - the mirror parameters PL than 18800.
　PL = (T + 273) × (20 + log (t)) ... (2)
　In the formula (2), T is a tempering temperature (° C.), t is the holding time at that temperature (time).
[0021]
　Based on the above findings, the present invention has been completed. Hereinafter, with reference to the drawings, detailed seamless steel pipe according to an embodiment of the present invention. Its description will not be repeated the same reference numerals designate like or corresponding parts in FIG.
[0022]
　[Chemical composition]
　seamless steel pipe according to the present embodiment has a chemical composition described below. In the following description, "%" of the content of the element means mass%.
[0023]
　C: 0.02 ~ 0.15%
　carbon (C) increases the strength of steel. If C content is less than 0.02%, the effect is not sufficiently obtained. On the other hand, if the C content exceeds 0.15%, the toughness of the steel is lowered. Therefore, C content is 0.02 to 0.15%. C content, in terms of the lower limit is preferably higher than 0.02%, more preferably 0.04% or more. C content, in terms of the upper limit is preferably not more than 0.10%, more preferably not more than 0.08%.
[0024]
　Si: 0.05 ~ 0.5%
　silicon (Si), the deoxidizing steel. If Si content is less than 0.05%, the effect can be obtained remarkably. However, when the Si content exceeds 0.5%, the toughness of the steel is lowered. Therefore, Si content is 0.05 to 0.5%. Si content is in terms of the lower limit is preferably higher than 0.05%, more preferably 0.08% or more, further preferably 0.10% or more. Si content is in terms of the upper limit is preferably less than 0.5%, more preferably 0.25% or less, still more preferably not more than 0.20%.
[0025]
　Mn: 0.30 ~ 2.5%
　manganese (Mn) increases the hardenability of steel and increases the strength of steel. If Mn content is less than 0.30%, the effect is not sufficiently obtained. On the other hand, when the Mn content exceeds 2.5%, Mn segregates in steel, the toughness is lowered. Therefore, Mn content is 0.30 to 2.5%. Mn content, in terms of lower limit is preferably higher than 0.30%, more preferably 1.0% or more, more preferably 1.3% or more. Mn content, in terms of the upper limit is preferably less than 2.5%, more preferably not more than 2.0%, more preferably not more than 1.8%.
[0026]
　P: 0.03% or less
　phosphorus (P) is an impurity. P lowers the toughness of the steel. Accordingly, P content is preferably as small as possible. Therefore, P content is limited to 0.03% or less. P content is preferably less than 0.03%, more preferably 0.015% or less, still more preferably not more than 0.012%.
[0027]
　S: 0.006% or less
　sulfur (S) is an impurity. S combines with Mn to form coarse MnS, lowers the toughness and HIC resistance of the steel. Thus, S content is preferably as small as possible. Therefore, S content is limited to 0.006% or less. S content is preferably less than 0.006% or, more preferably, not more than 0.003%, more preferably not more than 0.002%.
[0028]
　O: 0.004% or less
　oxygen (O) is an impurity. O is coarse oxides, or to form a cluster of oxides lowers the toughness of the steel. Therefore, O content is preferably as small as possible. Therefore, O content is limited to 0.004% or less. Preferred O content is 0.003% or less, more preferably 0.002% or less.
[0029]
　Al: 0.01 ~ 0.10%
　of aluminum (Al) is combined with N to form fine nitrides, improve the toughness of the steel. The Al content is less than 0.01%, the effect is not sufficiently obtained. On the other hand, Al content is higher than 0.10%, Al nitrides are coarsened, the toughness of the steel is lowered. Therefore, Al content is from 0.01 to 0.10%. Al content is in terms of the lower limit is preferably higher than 0.01%, more preferably 0.02% or more. Al content is in terms of the upper limit is preferably less than 0.10%, more preferably 0.08% or less, still more preferably not more than 0.06%. Al content in the present specification means the content of acid-soluble Al (so-called Sol. Al).
[0030]
　Ti: 0.001 ~ 0.010%
　titanium (Ti) combines with N in steel to form a TiN, suppressing a decrease in the toughness of the steel by solid solution the N. Further, the fine TiN dispersed precipitate enhances the toughness of steel. The Ti content is less than 0.001%, the effect is not sufficiently obtained. On the other hand, if the Ti content is higher than 0.010%, TiN is or coarse, coarse TiC is generated, the toughness of the steel is lowered. Therefore, Ti content is 0.001 to 0.010%. Ti content, in terms of the lower limit is preferably higher than 0.001%, more preferably 0.002% or more. Ti content, in terms of the upper limit is preferably less than 0.010%, more preferably 0.006% or less, further preferably 0.005% or less.
[0031]
　N: 0.007% or less
　Nitrogen (N) combines with Al to form fine Al nitride, increasing the toughness of steel. However, if is higher than 0.007% N content, solid solution of N decreases the toughness of the steel. Further if the N content is too high, carbonitrides and / or nitrides are coarsened, the toughness of the steel is lowered. Therefore, N content is 0.007% or less. N content, in terms of the upper limit is preferably less than 0.007% or, more preferably 0.006% or less, further preferably 0.005% or less. N content, in terms of the lower limit is preferably 0.002% or more.
[0032]
　Cr: 0.05 ~ 1.0%
　chromium (Cr) increases the hardenability of steel and increases the strength of steel. Cr further enhance the temper softening resistance of the steel. The Cr content is less than 0.05%, the effect is not sufficiently obtained. On the other hand, when the Cr content exceeds 1.0%, the toughness of the steel is lowered. Therefore, Cr content is from 0.05 to 1.0%. Cr content in terms of the lower limit is preferably higher than 0.05%, more preferably 0.2% or more. Cr content in terms of the upper limit is preferably less than 1.0%, more preferably 0.8% or less.
[0033]
　Mo: less than 0.02% to 0.5%
　molybdenum (Mo) improves the strength of steel by the solid solution strengthening and transformation strengthening. The Mo content is less than 0.02%, the effect is not sufficiently obtained. On the other hand, when the Mo content is 0.5% or more, the toughness of the steel is lowered. Therefore, Mo content is 0.5% or more and less than 0.02%. Mo content is in terms of the lower limit is preferably higher than 0.02%, more preferably 0.05% or more, further more preferably 0.1% or more. Mo content is in terms of the upper limit is preferably 0.4% or less, more preferably 0.3% or less.
[0034]
　Ni: 0.03 ~ 1.0%
　nickel (Ni) increases the hardenability of steel and increases the strength of steel. Further, Ni is a heating step for hardening, has the effect of improving the adhesion of the scale formed on the surface of the steel, the scale results suppress the cooling rate of the steel surface in the cooling stage of quenching, the steel surface effect of suppressing increase in the hardness of the parts also. If Ni content is less than 0.03%, the effect is not sufficiently obtained. On the other hand, Ni content is higher than 1.0%, SSC resistance is reduced. Therefore, Ni content is 0.03 to 1.0%. Ni content, in terms of lower limit, preferably at least 0.05%, even more preferably 0.08% or more, further preferably 0.10% or more. Ni content, in terms of the upper limit is preferably less than 1.0%, more preferably 0.7% or below, further more preferably 0.5% or less.
[0035]
　Cu: 0.02 ~ 1.0%
　copper (Cu) increases the hardenability of steel and increases the strength of steel. Further, Cu is heating step for hardening, has the effect of improving the adhesion of the scale formed on the surface of the steel, the scale results suppress the cooling rate of the steel surface in the cooling stage of quenching, the steel surface effect of suppressing increase in the hardness of the parts also. If Cu content is less than 0.02%, the effect is not sufficiently obtained. On the other hand, Cu content is higher than 1.0%, weldability of the steel is lowered. Further if the Cu content is too high, decrease the grain boundary strength of the steel at high temperatures, decreases the hot workability of the steel. Therefore, Cu content is 0.02 to 1.0%. Cu content in terms of the lower limit is preferably 0.05% or more, more preferably 0.08% or more, further preferably 0.10% or more. Cu content in terms of the upper limit is preferably less than 1.0%, more preferably 0.7% or below, further more preferably 0.5% or less.
[0036]
　V: 0.020 ~ 0.20%
　vanadium (V) is a V carbide formed by combining with C in steel to increase the strength of steel. V further form carbides and solid solution in Mo carbides. By including V, carbides hardly coarsened. V content is less than 0.020% or above effects are not effectively obtained. On the other hand, V content is higher than 0.20%, carbides are coarsened. Therefore, V content is from 0.020 to 0.20%. V content, in terms of the lower limit is preferably higher than 0.020% or, more preferably 0.04% or more. V content, in terms of the upper limit is preferably less than 0.16%.
[0037]
　Ca: 0.0005 ~ 0.005%
　of calcium (Ca) forms CaS combines with S in steel. The formation of CaS, formation of MnS is suppressed. Therefore, Ca enhances the toughness and HIC resistance of the steel. The Ca content is less than 0.0005%, the effect is not sufficiently obtained. On the other hand, Ca content is higher than 0.005%, reduces the cleanliness of the steel, which lowers the toughness and the HIC resistance of the steel. Therefore, Ca content is 0.0005 to 0.005%. Ca content, in terms of lower limit is preferably higher than 0.0005%, more preferably 0.0008% or more, further preferably 0.001% or more. Ca content, in terms of the upper limit is preferably less than 0.005%, more preferably 0.003% or less, more preferably 0.002% or less.
[0038]
　The remainder of the chemical composition of a seamless steel tube according to the present embodiment is Fe and impurities. Impurities as referred to herein, refers to an element incorporated from the environment, such as ores and scrap or manufacturing process, which is used as a raw material of steel.
[0039]
　The chemical composition of a seamless steel tube according to the present embodiment further includes, in place of part of Fe, may contain Nb.
[0040]
　Nb: 0 ~ 0.05%
　niobium (Nb) is a selective element. Nb combines with C and / or N in the steel to form fine Nb carbide, enhance the toughness of the steel. Nb further includes a solid solution in Mo carbides to form specific carbides, inhibits the coarsening of certain carbides. On the other hand, Nb content is higher than 0.05%, carbides and / or carbonitrides become coarse. Therefore, Nb content is 0 to 0.05%. If the Nb content is 0.010% or more, the effect can be obtained remarkably. Nb content is in terms of the lower limit is preferably 0.015% or more, further preferably 0.020% or more. Nb content is in terms of the upper limit is preferably not 0.040% or less, still more preferably not more than 0.035%.
[0041]
　[Carbon equivalent Ceq]
　seamless steel pipe according to the present embodiment is a carbon equivalent Ceq defined by equation (1) is less than 0.430% or more 0.500%.
　Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (1)
　The formula (1) each element symbol in the content of the corresponding element (mass%) is substituted.
[0042]
　The carbon equivalent Ceq is less than 0.430%, it is difficult to ensure the strength of the seamless steel pipe. On the other hand, in the carbon equivalent Ceq is 0.500 or more, the manufacturing process of quenching after hot steel tube only once the direct quenching or inline quenching, it becomes difficult to the surface layer of Vickers hardness below 250Hv .
[0043]
　Organization
　embodiment according seam steel pipe tissue from the surface layer to meat, the tempered martensite or tempered bainite as a main phase. Seamless steel pipe according to the present embodiment, the 1mm or more deep region from at least the surface does not include a re-crystallized ferrite. Recrystallized ferrite, extremely reducing the hardness of the position of 1mm from the surface of the seamless steel pipes.
[0044]
　Incidentally, the tempered martensite or tempered bainite as the main phase generally includes a volume fraction of tempered martensite is 50% or more of the tissue, the volume ratio of the tempered bainite 50% or more of the tissue, or volume fraction of tempered martensite the sum of the volume fraction of tempered bainite is meant 50% or more tissues. In other words, the volume of the tissue nor without tempered bainite in tempered martensite (e.g., ferrite) is meant less than 50% tissue.
[0045]
　[Grain size number]
　seam steel pipe tissue according to the present embodiment, the size of prior austenite grains is 6.0 less than the crystal grain size number defined in ASTM E112-10.
[0046]
　Prior austenite grain size, so that a cross section perpendicular to the length direction (pipe-direction) of the steel pipe becomes the test surface, preferably, after quenching, cut out test pieces from the steel pipes before tempering embedded in a resin, by Bechet-Beaujard method to corrode in picric acid saturated aqueous solution by causing to appear prior austenite grain boundary, it can be measured prior austenite grain size number according to ASTM E112-10.
[0047]
　Incidentally, with respect to the steel pipe after tempering, using methods such as electron backscatter diffraction (EBSD), it is also possible to determine the ASTM grain size number of prior austenite grains from orientation relationship of a crystal. In this case, by EBSD metallographic of the steel pipe after tempering, it is measured as follows. Taking samples from the thickness center of the cross section of seamless steel pipe after tempering (seamless steel pipe axial direction and the cross section perpendicular). Collected 500 × 500 [mu] m using a sample 2 by EBSD observation range make crystal orientation analysis, the boundaries of grains with each other Misorientation Angle is in the range of 15 ~ 51 ° is defined as the prior austenite grain boundary, to line drawing , based on the drawing view, in conformity with ASTM E112-10 determine grain size number.
[0048]
　Prior austenite grain size of the theoretically after prior austenite grain size and tempering before tempering after hardening are the same. Prior austenite grain diameter determined by EBSD method after tempering is consistent with almost error of about ± 0.2 as the grain size number, the result of observation of the crystal grains to appear by Bechet-Beaujard method before tempering after quenching . Therefore, the present invention "prior austenite grain size is, ASTM E112-10 and is a less than 6.0 grain size number defined." And, in case grain size after quenching is unknown, at least, the crystal grain size number determined by the EBSD method under the state after the tempering means that if it is less than 5.8 in the range of the present invention. Hereinafter, unless otherwise mentioned, the prior austenite grain size is described assuming numerical observed in Bechet-Beaujard method for samples before tempering after quenching.
[0049]
　When prior austenite grains is 6.0 or more fine grains in the grain size number, the material having a low carbon equivalent Ceq as in the present embodiment, sufficient hardenability can not be obtained. Therefore, there is a case where a predetermined strength can not be obtained. In the manufacturing process of quenching after hot steel tube only once the direct quenching or inline quenching, it is difficult to such a fine organization. Grain size number of prior austenite grains is preferably 5.5 or less, more preferably 5.0 or less.
[0050]
　[Vickers hardness and yield strength]
　seams according to the present embodiment steel pipe, between the position and the outer surface of 1mm from the inner surface of the position of 1mm, Vickers hardness is less than 250 Hv. More particularly, a seamless steel pipe according to the present embodiment, in any position between the position of 1mm from the position and the outer surface of 1mm from the interior surface, the Vickers hardness is measured according to JIS Z 2244 is less than or equal to 250Hv is there.
[0051]
　Seamless steel pipe according to the present invention, the difference in the hardness of thickness direction is small. Specifically, the difference Vickers hardness between the inner surface of the position and the wall thickness center of 1mm, the difference Vickers hardness between the outer surface of the position and the wall thickness center of 1mm, and the inner surface 1mm of difference Vickers hardness between a position and an outer surface 1mm position and are both at 25Hv or less.
[0052]
　Seamless steel pipe according to the present embodiment has a yield strength of X80 grade or higher as specified in API standards (over 555MPa).
[0053]
　Seamless steel pipe according to the present embodiment is not limited thereto, the wall thickness can be suitably used as a seamless steel pipe 25 ~ 55 mm. The thickness of the seamless steel pipe, from the viewpoint of alloy streamlined, more preferably from 25 ~ 40 mm.
[0054]
　[Manufacturing Method]
　Hereinafter, an example of a method of producing a seamless steel pipe according to the present embodiment. However, method of producing a seamless steel pipe according to the present embodiment is not limited thereto.
[0055]
　[Production Line]
　FIG 1 is a block diagram showing an example of a production line. Referring to FIG. 1, the production line includes a heating furnace 1, a drilling machine 2, the elongation rolling machine 3, a constant径圧rolling mill 4, and Honetsuro 5, a water cooling device 6, and a tempering device 7 provided. Between the respective devices, a plurality of conveying rollers 10 are disposed.
[0056]
　[Production Flow
　Figure 2 is a flowchart showing a manufacturing process of a seamless steel tube according to the present embodiment. Figure 3 is a diagram showing changes in the surface temperature on the time of the workpiece in the manufacturing (steel material, base pipe and seamless steel pipe). Here, reference numeral A1, when the workpiece is heated Ac 1 shows the point, when the workpiece is cooled Ar 1 illustrates a point. In the drawings, A3, when the workpiece is heated Ac 3 shows the points, when the workpieces are cooled Ar 3 shows the points.
[0057]
　As shown in FIGS. 1 to 3, in the manufacturing process First, heating the steel material in the heating furnace 1 (heating step: S1). Steel material, for example, a round billet. Steel material may be produced by continuous casting apparatus, such as a round CC. Further, the steel material may be produced an ingot or slab hot working (forging or slabbing etc.) to. The following describes a case steel material is round billet.
[0058]
　The heated round billet to machining to seamless steel pipe hot (S2 and S3). Specifically, the round billet piercing to the drilling machine 2 to blank pipe (piercing rolling step: S2). Further, the mother tube was rolled by a drawing rolling mill 3 and the constant 径圧 rolling mill 4, to seamless steel tube (elongation rolling step and shaped rolling step S3).
[0059]
　Hot seamless steel pipe manufactured by the processing, if necessary, heated to a predetermined temperature by Honetsuro 5 (supplementary heating step: S4). Hot seam steel pipes manufactured by machining, or the heated seamless steel pipe, quenching by water cooling device 6 (quenching step: S5). In either case, a seamless steel pipe produced by hot working, Ar 3 is hardened without being cooled to below points. Quenching seam steel pipe, tempering by tempering device 7 (tempering step S6).
[0060]
　That is, in the above-described manufacturing method, after the pipe producing a seamless steel pipe, rapidly implementing quenching. More specifically, after hot working, prior to reduction to near room temperature the temperature of the seamless steel pipe by cooling, carrying out quenching. Here, the surface temperature of the seamless steel pipe after hot working Ar 3 heat treatment of quenching before the below points called "direct quenching", the seamless steel pipe after hot working Ac 3 above points Temperature in the heat treatment of quenching from the heated complement referred to as the "in-line quenching". According to direct quenching or inline quenching, once cooled after pipe-heat treatment for subsequent rapid cooling (hereinafter, referred to as a reheating quenching.) Compared to the tissue becomes coarse. Specifically, the grain size number after quenching is less than 6.0. Therefore, as compared with the case of reheating quenching improves hardenability of tissue, even when the carbon equivalent Ceq is used less steel, it is possible to ensure high strength.
[0061]
　It will be described in detail for each step.
[0062]
　[Heating Step (S1)]
　heating the round billet in a heating furnace 1. Preferred heating temperature is 1100 ℃ ~ 1300 ℃. By heating the round billet at this temperature range, dissolution of the carbonitride in the steel. If the slab or ingot to produce a round billet by hot working, may be a heating temperature of the slab or ingot 1100 ~ 1300 ° C., the heating temperature of the round billet in the heating furnace 1 may not be 1100 ~ 1300 ° C. . When the ingot and slab are heated, because carbonitrides in the steel is dissolved. Furnace 1 is, for example, a walking beam furnace or a rotary furnace.
[0063]
　[Piercing process (S2)]
　extracts the round billet from the heating furnace 1, a heated round billet is pierced and rolled by the piercing mill 2, the blank tube. Drilling machine 2 comprises a plurality of inclined rolls and a plug. Plug is disposed between the inclined rolls. Preferably, the drilling machine 2 is a cross-type piercing mill. With cross-type piercing mill, it is possible perforation with high expansion ratio preferred.
[0064]
　[Elongation rolling step and the constant-radius rolling step (S3)]
　Next, rolled base tube. Specifically, the elongation rolling by elongation rolling machine 3 base tube. Elongation rolling machine 3 includes a plurality of roll stands arranged in series. Elongation rolling machine 3 is, for example, a mandrel mill. Subsequently, the stretched rolled base tube and rolled diaphragm by a constant径圧rolling mill 4, to produce a seamless steel pipe. Constant径圧rolling mill 4 includes a plurality of roll stands arranged directly. Constant径圧caster 4, for example, a sizer, a stretch reducer or the like. Incidentally, collectively elongation rolling step and shaped rolling may be simply referred to as the rolling process.
[0065]
　[Supplementary heating step (S4)]
　supplementary heating step (S4) is performed if necessary. That is, the manufacturing method according to the present embodiment may not include the supplementary heating step (S4). Specifically, supplementary heating step (S4), in water cooled immediately before the quenching step (S5), the temperature of the seamless steel pipe is Ac 3 is carried to a predetermined temperature above points. If not carried out supplementary heating step (S4), in FIG. 2, the process proceeds from step S3 to step S5. If not carried out supplementary heating step (S4), in FIG. 1, Honetsuro 5 may not be arranged.
[0066]
　If the finishing temperature of the rolling process (the surface temperature of the seamless steel pipe after rolling step is completed) is less than 800 ° C., it is preferable to carry out the supplementary heating step (S4). In supplementary heating step (S4), and heated by inserting the seamless steel pipe to Honetsu furnace 5. Preferred heating temperature in Honetsuro 5 is 900 ~ 1100 ° C.. Preferred soaking time is less than 30 minutes. When the soaking time is too long, because Ti, Nb, carbonitrides consisting of C and N (Ti, Nb) (C, N) are precipitated, there is a possibility that coarse. In the supplementary heating step, instead of Honetsuro 5, it may be used an induction heating device.
[0067]
　[Quenching process (S5)]
　to water cooling the seamless steel pipe by water cooling device 6. Water cooling immediately before the seam steel pipe temperature (surface temperature), Ac 3 not less than points, preferably 800 ° C. or higher.
[0068]
　Water cooling, the cooling rate between the seam temperature of steel pipe is 800 ° C. ~ 500 ° C., it is preferable that the 5 ° C. / sec (300 ° C. / min) or more. Thereby, a uniform quenched structure can be obtained. Cooling stop temperature, Ar 1 is below points. Preferred cooling stop temperature is 450 ° C. or less, may be cooled to room temperature. The quenching process (S5), the tissue of the mother phase (matrix) will structure mainly martensite or bainite.
[0069]
　Configuration of the water-cooling unit 6 used in the quenching step (S5) is, for example, as follows. Water cooling device 6 comprises a plurality of rotary rollers, a laminar flow device, a water jet device. A plurality of rollers are arranged in two rows, seamless steel pipe is disposed between the plurality of rotary rollers arranged in two rows. In this case, the rotation roller two rows each of which contacts the outer surface lower portion of the seamless steel pipe. When the rotary roller rotates, seamless steel pipe is rotated about the axis. Laminar water flow device is positioned above the rotating rollers, pour water from above seamless pipes. At this time, water is poured into a seamless steel pipe forms a laminar sheet-like stream of water. Water jet device is arranged at the end near the disposed rollers seamless steel. Water jet device injects water jet toward the steel tube inside the edge of the seamless steel pipes. The laminar flow device and water jet device, an outer surface and an inner surface of the seamless steel pipe is cooled at the same time. Such a configuration of the water cooling device 6, particularly suitable for accelerated cooling of the seamless steel pipe thick having a thickness of more than 25 mm.
[0070]
　Water cooling device 6, rotating rollers described above, it may be a device other than laminar flow device and water jet device. Water-cooling device 6, for example, may be a water tank. In this case, seamless steel pipe is immersed in the water tank is accelerated cooling. Water-cooling device 6 may also be only the laminar flow equipment. In short, the type of the cooling device 6 is not limited.
[0071]
　[Tempering process (S6)]
　with respect to hardened the seamless steel pipe to perform the tempering. Specifically, the hardened seam steel pipe, Ac 1 and heated to a predetermined tempering temperature below points, for a predetermined period of time at that temperature. At this time, Larson defined by the following formula (2) - Miller parameter PL is set to be more than 18800.
　PL = (T + 273) × (20 + log (t)) ... (2)
　In the formula (2), T is a tempering temperature (° C.), t is the holding time at that temperature (in hours). log (t) is the logarithm of t to the base 10.
[0072]
　The PL is less than 18800, the reduction of the surface hardness is insufficient, there is a case where locations Vickers hardness of more than 250Hv occurs. PL is preferably 18900 or more.
[0073]
　On the other hand, when the PL is too high, recrystallization of ferrite occurs by 1mm or more deep region from the surface, an extreme decrease in strength, decrease in the surface layer of the sour resistance, may cause the occurrence of blisters. PL is more preferably preferably 20,000 or less, and 19500 or less.
[0074]
　The lower limit of the tempering temperature is preferably 600 ° C., more preferably from 630 ° C., more preferably from 650 ° C.. The upper limit of the tempering temperature is preferably 700 ° C., more preferably still preferably 680 ° C.. The lower limit of the retention time is preferably 1 hour, more preferably from 2 hours, more preferably 3 hours. The upper limit of the holding time is preferably 6 hours, more preferably from 5 hours, more preferably from 4 hours.
[0075]
　Above by the manufacturing process, even a seamless steel pipe having a wall thickness of more than 25 mm, it is possible to obtain excellent strength, toughness and HIC resistance. Manufacturing method described above is particularly suitable for a seamless steel pipe having a wall thickness of more than 25 mm, it can also be applied to a seamless steel pipe having a wall thickness of more than 40 mm. The upper limit of thickness is not particularly limited, but usually, it is 60mm or less.
[0076]
　Above, seamless steel pipe according to an embodiment of the present invention, and a manufacturing method described. According to this embodiment, a relatively reasonable there can be manufactured in a manufacturing process, and stably obtained seamless steel pipes and more yield strength and excellent SSC resistance 555MPa can be obtained.
Example
[0077]
　The following examples illustrate the present invention more specifically. The present invention is not limited to these examples.
[0078]
　To produce a plurality of seamless steel pipe having various chemical compositions, yield strength, tensile strength, surface hardness, and investigated the sour resistance.
[0079]
　[Methodology]
　was melted a plurality of steel having the chemical compositions shown in Table 1, were prepared round billet for pipe production by continuous casting. Table 1 Steel A, C, D1, D2, and J, the value of the chemical composition or Ceq is steel does not satisfy the requirements of the present invention.
[0080]
[Table 1]

[0081]
　And heated to 1100 ~ 1300 ° C. By heating furnace Kakumaru billet produced. Followed by the raw tube by piercing and rolling by drilling machine Kakumaru billet. Followed by elongation rolling each base tube by a mandrel mill. Subsequently, rolling diaphragm each blank pipe (constant-radius rolling) by sizer to produce a seamless steel pipe having an outer diameter and thickness shown in Table 2 and Table 3.
[0082]
[Table 2]

[0083]
[table 3]

[0084]
　The shaped rolled seamless steel pipe, after heating to 950 ° C. by the auxiliary heat furnace, the water cooling apparatus, was carried quenching to cool to room temperature at 5 ° C. / sec or more cooling rate.
[0085]
　After quenching was performed tempered at soaking temperature and the holding time shown in Table 2 and Table 3 for each seamless steel pipes. However, No. After carrying out the hardening for 62, before tempering, after soaking 20 minutes and then re-heated to 950 ° C. offline, it was quenched to water ordinance.
[0086]
　Against seamless steel pipe manufactured by the above manufacturing steps were carried out the following evaluation tests.
[0087]
　[Yield strength and tensile strength test]
　investigated the yield strength of each number of seamless steel pipe. Specifically, 12 test piece No. (width 25 mm, gauge length 50 mm) defined from seamless steel tube in JIS Z 2241, and the longitudinal direction (L direction) in the longitudinal direction steel tensile strength test piece in parallel with It was composed as collected. Using harvested specimens, a tensile test according to JIS Z 2241, was performed in air at room temperature (25 ° C.), it was determined yield strength (YS) and tensile strength (TS). Yield strength was determined by 0.5% total elongation method. The resulting yield strength (MPa) and tensile strength (MPa) shown in Table 2 and Table 3. Table "YS" of 2 and in Table 3 shows the yield strength obtained by the test pieces of each test number, "TS" indicates tensile strength.
[0088]
　[Surface Hardness Test]
　For seamless steel pipes of each number, a total of four specimens were taken for each circumferentially 90 °, in cross-section of each specimen (the central axis in a cross section perpendicular), the thickness from the inner surface in any three points 1mm inward direction was conducted Vickers hardness test according to JIS Z 2244. Test force F of the Vickers hardness test was 10kgf (98.07N). The maximum value of the obtained values of the 12 points were the hardness of "1mm position from the inner surface".
[0089]
　Similarly, the Vickers hardness test was performed at three arbitrary points of 1mm inward from the outer surface in the thickness direction of the four specimens of the seamless steel pipe of each test number, the maximum value among the values ​​of 12 points obtained were the hardness of the "1mm position from the outer surface". Further, the Vickers hardness test was performed at three arbitrary points in the vicinity of the wall center thickness of the four specimens of the seamless steel pipe of each test number, the maximum value among the obtained values ​​of the 12 points, "in the meat and the hardness of the ".
[0090]
　Hardness of "1mm position from the outer surface" of the seamless steel pipe of each test number, the hardness of the "1mm position from the inner surface", and each hardness of the "meat", "exterior surface" in Table 2 and Table 3, " in the meat ", shown in the column of the inner surface".
[0091]
　The difference hardness of the hardness and "meat" of the "1mm position from the outer surface", and hardness of the difference in hardness between the hardness of the "1mm position from the inner surface" and "meat" and "1mm position from the outer surface" of the difference between the hardness of the "1mm position from the inner surface", the largest value (hereinafter, referred to as "maximum hardness difference") are shown in the column of "difference" in Table 2 and Table 3.
[0092]
　[Structure Observation]
　inner surface of seamless steel pipes of each number, an outer surface, and the samples were taken, including the wall thickness center, was measured tissue. Specifically, each sample was corroded by nital corrosion solution to appear the microstructure was observed by an optical microscope.
[0093]
　Seamless steel pipes of each number are all had a structure that the main phase of tempered martensite or tempered bainite. However, in some of seamless steel pipe, re-crystallization of ferrite in 1mm or more deep region from the surface has occurred. The presence or absence of recrystallization of the ferrite in the surface 1mm above the deep region, shown in the column "ferrite recrystallization" of Table 2 and Table 3.
[0094]
　Grain size number of prior austenite grains tissues were measured by the following method. First, as a cross section perpendicular is the test surface in the length direction of the steel pipe at the time as quenched (manufactured tube direction), embedded in resin A test piece was cut out from each steel pipe, corroded with a picric acid saturated aqueous Bechet to appear prior austenite grain boundaries by -Beaujard method, was observed with an optical microscope (200 times), it was measured prior austenite grain size number according to ASTM E112-10. It shows this grain size number in the column 'AsQ old γ grain size No. "in Table 2 and Table 3.
[0095]
　Further, the grain size number of prior austenite grains after tempering, can not be measured by picric acid saturated aqueous corrosion was measured by incorporated EBSD. EBSD is cut out specimens as vertical section becomes the test surface in the length direction of the steel pipe after tempering, finishing the test surface at mirror polishing and electrolytic polishing, 500 × thick central portion of the steel pipe 500 [mu] m 2 was performed on the region. Incidentally, using detector EBSD mounted on FE-SEM and (EDAX Inc. model DigiViewIV). From the obtained crystal orientation data, using analysis software (EDAX Inc. OIM Analysis ver), was drawn in a line boundary of the crystal grains corresponding to Misorientation Angle 15 ~ 51 °, with a line drawing view , it was measured prior austenite grain size number according to ASTM E112-10. It shows this grain size number in the column "QT old γ grain size No." in Table 2 and Table 3.
[0096]
　Survey Results
　As shown in Tables 1 to 3, seamless steel pipe No. 19-33, and 52-60 is in the range of chemical composition the present invention, 0 the carbon equivalent Ceq is more than 0.430% It was less than .500%. These seamless steel pipe, the recrystallization of the ferrite in a deep from the surface 1mm or more regions does not occur, the surface layer to meat, have a tissue that the main phase of tempered martensite or tempered bainite, austenite grains grain size number of is less than 6.0. These seamless steel pipe further "1mm position from the outer surface," "1mm position from the inner surface", and in any of the "meat" also Vickers hardness is not more than 250 Hv, have a yield strength of at least 555MPa It was. These seamless steel pipe, the maximum difference in hardness was less than 25 HV.
[0097]
　Seamless steel pipe of the numbers 1 to 17, the yield strength was less than 555MPa. This is probably because the carbon equivalent Ceq of the steel A was too low.
[0098]
　Seamless steel pipe No. 18, recrystallization of ferrite in 1mm or more deep region from the surface occurs. Therefore, seamless steel pipe No. 18, the yield strength was less than 555MPa. This Larson seamless steel pipe No. 18 - probably because Miller parameter PL is too high.
[0099]
　Seamless steel pipe No. 34-42, and 47-51 are "1mm position from the outer surface", or Vickers hardness of "1mm position from the inner surface" and "meat" is higher than 250 Hv. These seamless steel pipe, the maximum difference in hardness was higher than 25 HV. This number 34-42, and 47 seam to 51 of steel pipe Larson - Miller parameter PL is considered to be because too low.
[0100]
　Seamless steel pipe of the number 43 and 44, the Vickers hardness of the "1mm position from the inner surface" was higher than 250Hv. This is probably because the carbon equivalent Ceq of the steel C was too high.
[0101]
　Seamless steel pipe of the number 45 and 46, the yield strength was less than 555MPa. This is probably because the carbon equivalent Ceq of the steel D1 and steel D2 is too low.
[0102]
　Seamless steel pipe of the number 61, the Vickers hardness was higher than 250Hv at all measurement points. This is probably because the carbon equivalent Ceq of the steel J was too high.
[0103]
　Seamless steel pipe No. 62, the yield strength was less than 555MPa. This is too old austenite grains fine due to a combination of the in-line quenching and reheating quenching, the hardenability is considered that become insufficient strength by which low.
[0104]
　Figure 4, for steel B, Larson - is a scatter diagram plotting the relationship between the mirror parameters PL and the yield strength YS. As shown in FIG. 4, the yield strength YS is Larson - showed lower tendency as Miller parameter PL increases. In steel B, and except for the seamless steel pipe No. 18 has advanced recrystallization of ferrite, a yield strength of at least 555MPa was obtained.
[0105]
　Figure 5, for steel A, Larson - is a scatter diagram plotting the relationship between the mirror parameters PL and the yield strength YS. In steel A, it is adjusted quenching conditions, did not give a yield strength of at least 555MPa. This is probably because the carbon equivalent Ceq of the steel A was too low.
[0106]
　6, the steel B, Larson - a mirror parameters PL, an outer surface, in the meat, and is a scatter diagram plotting the relationship between the hardness of the inner surface. As shown in FIG. 6, the outer surface, in the meat, and none of the hardness of the inner surface, Larson - showed lower tendency as Miller parameter PL increases. As shown in FIG. 6, Larson - the mirror parameter PL is 18800 or more, the outer surface, in the meat, and both the hardness of the inner surface could be to 250Hv or less. On the other hand, Larson - the Miller parameter PL is less than 18800, an outer surface, in the meat, and any of the hard inner surface is higher than 250 Hv.
[0107]
　7, the steel A, Larson - a mirror parameters PL, an outer surface, in the meat, and is a scatter diagram plotting the relationship between the hardness of the inner surface. In the case of steel A, as in the case of steel B, the outer surface, in the meat, and none of the hardness of the inner surface, Larson - showed lower tendency as Miller parameter PL increases.
[0108]
　8, the steel B, Larson - is a scatter diagram plotting the relationship between the mirror parameters PL and the maximum difference in hardness. As shown in FIG. 8, Larson - Miller parameter PL is the 18800 or more, the maximum hardness difference is equal to or less than 25 HV. Note that seamless steel pipe No. 18, since the progress in recrystallization ferrite 1mm or more deep region from the surface, the maximum hardness difference is considered increased.
[0109]
　9, the steel A, Larson - is a scatter diagram plotting the relationship between the mirror parameters PL and the maximum difference in hardness. As shown in FIG. 9, Larson - the relationship between the mirror parameters PL and the maximum difference in hardness showed a similar tendency also in steel A. Seamless steel pipe No. 3, since the progress in recrystallization ferrite 1mm or more deep region from the surface, the maximum hardness difference is considered increased.
[0110]
　[Sour resistance Evaluation
　for several seamless steel pipes of each number, sour resistance evaluation (HIC resistance test, four-point bending test) of the following were performed.
[0111]
　[HIC resistance test]
　from the seamless steel pipes, the test piece including the inner surface, the test strip comprising a thick central, were taken respectively specimens containing the outer surfaces. The thickness of each specimen is 20 mm, the width is (circumferential direction) 20 mm, it was 100mm in length. According NACE (National Association of Corrosion Engineers) TM0284-2011, was evaluated HIC resistance of each specimen. Test bath of immersing the test piece was 5% salt + 0.5% acetic acid aqueous solution of temperature 24 ° C. saturated with hydrogen sulfide gas 1 atm.
[0112]
　After immersion for 96 hours after the lapse to identify the maximum crack locations to implement the ultrasonic testing (UT) to the test piece after the test were cut that site. Section at this time was the cross-section of thickness × width of test piece (circumferential direction). With a cutting specimens to determine the crack length ratio CLR (= crack length (mm) / width of the specimen of (mm)). Of CLR of each test piece was taken from each steel pipe, the highest value was defined as the crack length ratio CLR of the test numbers.
[0113]
　Further, to confirm the presence or absence of blisters of the test piece after the test (blisters by cracks in the vicinity of the surface), and counts the number of blisters that occurred in the test piece. Among blister number in each test piece taken from each steel pipe, the largest value was defined as the blister the number of the test numbers.
[0114]
　[4-point bending test]
　to the test piece including the thickness center of each seamless steel pipe, using a 4-point bending jig, in compliance with ASTM G39, the actual yield strength (yield strength of seamless steel pipes of each number) 95% of the stress of the load. The specimen stress is loaded and placed in the test chamber. Test bath was 5% salt + 0.5% acetic acid aqueous solution of temperature 24 ° C. saturated with hydrogen sulfide gas 1 atm. After a lapse of 720 hours, whether or not cracks in the test specimen has occurred was visually observed. If cracking has not occurred, it was evaluated and its sheet material is excellent in SSC resistance.
[0115]
　[Evaluation Result]
　Table 4 shows the results of the sour resistance evaluation.
[Table 4]

[0116]
　In Table 4, "○" in the column of "HIC resistance test" and "4-point bending test" shows that crack in the test did not occur. In the column of "HIC resistance test" and "4-point bending test", "-" indicates that it did not implement the test.
[0117]
　As shown in Table 4, the yield strength is at least 555MPa, and "1mm position from the outer surface," "1mm position from the inner surface", and a seamless steel Vickers hardness is below 250Hv in any of the "meat" is HIC resistance test, no cracking occurred in any of the four-point bending test, good sour resistance is stably obtained. On the other hand, "1mm position from the outer surface" high seamless steel than the Vickers hardness of 250Hv in either "1mm position from the inner surface" and "meat" is sour resistance was FuKaoru. The results, the relationship between Vickers hardness and sour resistance is corroborated.
[0118]
　Having described the embodiments of the present invention, the above-described embodiment is merely an example for implementing the present invention. Accordingly, the present invention is not limited to the embodiments described above, it can be implemented by modifying the above-described embodiments without departing from the scope and spirit thereof as appropriate.

claims

Chemical composition, in
　mass%, C:
　0.02 ~
　0.15%, Si: 0.05 ~ 0.5%, Mn: 0.30 ~
　2.5%, P: 0.03% or
　less, S 0.006% or
　less, O: 0.004% or
　less,
　Al: 0.01 ~ 0.10%, Ti: 0.001 ~
　0.010%, N: 0.007% or less,
　Cr: 0.05
　1.0% ~, Mo: 0.02% or more and less than%
　0.5,
　Ni: 0.03 ~ 1.0%,
　Cu: 0.02 ~ 1.0%, V: 0.020 ~ 0.20
　%, Ca: 0.0005
　~ 0.005%, Nb: 0 ~ 0.05%,
　the balance is Fe and impurities,
　0 carbon equivalent Ceq is more than 0.430%, which is defined by the following formula (1). less than 500%,
　tissue, from the surface layer to meat, tempered martensite or tempered bainite and the main phase,
　the former of the tissue austenite The size of the bets grains is 6.0 less than the crystal grain size number conforming to ASTM E112-10,
　Between the position and the outer surface of 1mm from the inner surface of the position of 1mm, and Vickers hardness is 250Hv or less,
　the yield strength is not less than 555MPa, a seamless steel pipe.
　Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ... (1)
　The element symbol in the formula (1), by mass%, the content of the corresponding element is assigned.
[Requested item 2]
　A seamless steel pipe according to claim 1,
　wherein the chemical composition, by
　mass%, Nb: 0.010 ~ 0.05%,
　containing, seamless steel pipes.
[Requested item 3]
　A seamless steel pipe according to claim 1 or 2,
　Vickers between the difference from the outer surface of 1mm position and wall thickness center position of the Vickers hardness between the position and the wall thickness center of 1mm from the inner surface difference in hardness, and the difference Vickers hardness between a position and the outer surface of 1mm from the inner surface 1mm position and is in any 25Hv or less, a seamless steel pipe.
[Requested item 4]
　A seamless steel pipe according to any one of claims 1 to 3,
　are produced is hardened and tempered,
　Larson defined by the following formula (2) - is a mirror parameter PL is 18800 or more, Mu seams steel pipe.
　PL = (T + 273) × (20 + log (t)) ... (2)
　In Formula (2), T is a tempering temperature, t is the holding time at that temperature. Unit of T is ℃, unit of t is the time.
[Requested item 5]
　Chemical composition, in mass%, C: 0.02 ~ 0.15% , Si: 0.05 ~ 0.5%, Mn: 0.30 ~ 2.5%, P: 0.03% or less, S 0.006% or less, O: 0.004% or less, Al: 0.01 ~ 0.10%, Ti: 0.001 ~ 0.010%, N: 0.007% or less, Cr: 0.05 ~ 1.0%, Mo: 0.02% to less than 0.5%, Ni: 0.03 ~ 1.0 %, Cu: 0.02 ~ 1.0%, V: 0.020 ~ 0.20 %, Ca: 0.0005 ~ 0.005% , Nb: 0 ~ 0.05%, remainder: a step of preparing a raw material is Fe and impurities,
　a process for manufacturing a mother tube of the material to hot working When,
　a step of quenching by direct quenching or inline quenching said raw tube,
　and a step of tempering the hardened are hollow shell,
　the quenching and tempering In, without performing reheating quenching,
　the carbon equivalent Ceq defined by the following formula (3) is less than 0.500% or more 0.430%,
　Larson defined by the following formula (4) - Miller parameter PL There is 18800 or more, the production method of a seamless steel pipe.
　Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) /
　15 ... (3) PL = (T + 273) × (20 + log (t)) ... (4)
　The element symbol in the formula (3), in mass% , the content of the corresponding element is assigned. In the formula (4), T is a tempering temperature, t is the holding time at that temperature. Unit of T is ℃, unit of t is the time.