Technical field
[0001]
　The present invention relates to a process for the preparation of an oil well pipe for low alloy steels and low alloy steels OCTG, and more particularly relates to a method of manufacturing a low alloy steel for oil well pipes having excellent sulfide stress cracking resistance and low alloy steels OCTG.
Background technique
[0002]
　OCTG are used as casing or tubing for oil wells or gas wells. Oil wells and gas wells (hereinafter collectively oil and gas well, simply referred to as "oil wells") by deep wells of high strength oil well pipes is required. Conventionally, 80 ksi grade (a yield stress of 80 ~ 95 ksi, i.e., 551 ~ 654MPa) and, 95 ksi grade (a yield stress of 95 ~ 110 ksi, i.e., 654 ~ 758 MPa) OCTG has been widely used. In recent years, 110ksi grade (yield stress is 110 ~ 125ksi, ie, 758 ~ 862MPa) and oil country tubular goods is beginning to be use of, is considered in the future, increasing the need for further strengthening.
[0003]
　Many deep wells developed recently containing hydrogen sulphide having a corrosive. Therefore, an oil well pipe is not only high strength, resistance to sulfide stress cracking (SSC resistance) is also required.
[0004]
　JP-A-2004-2978 discloses a low alloy steel is disclosed having excellent pitting resistance. JP Kohyo 2013-534563, low-alloy steel is disclosed having a yield strength of at least 963MPa. In Japanese Patent No. 5522322, an oil well steel pipe having a yield strength of at least 758MPa is disclosed. Japanese Patent No. 5333700, a low alloy steel for oil well pipes having a yield strength of at least 862MPa is disclosed. The JP 62-54021 discloses, 75 kgf / mm 2 process for producing a high strength seamless steel pipe having the above yield strength is described. The JP 63-203748 discloses, 78Kgf / mm 2 high strength steel is disclosed having the above yield strength.
Disclosure of the Invention
[0005]
　By tempering at a high temperature, it is known that can improve the SSC resistance of the steel. By tempering at a high temperature, it is it is possible to reduce the density of dislocations the trap sites of hydrogen. On the other hand, when the dislocation density decreases, strength of the steel decreases. It has been attempted to increase the content of alloying elements to improve the temper softening resistance, there is a limit.
[0006]
　SSC is likely to occur as the intensity increases. Be applied techniques disclosed in Patent Documents described above, in the low alloy steel oil well pipe having a yield strength of at least 965MPa, it may not be obtained stably excellent SSC resistance.
[0007]
　An object of the present invention is to provide a method for producing a high strength and excellent SSC resistance and stable low alloy steel for oil well pipes obtained, and low alloy steel oil country tubular goods.
[0008]
　Low alloy steel for oil well pipes according to the present invention, the chemical composition, in mass%, C: 0.65% than 0.45% or less, Si: 0.05 ~ 0.50%, Mn: 0.10 ~ 1 .00%, P: 0.020% or less, S: 0.0020% or less, Cu: 0.1% or less, Cr: 0.40 ~ 1.50%, Ni: 0.1% or less, Mo: 0 .50 ~ 2.50%, Ti: 0.01 % or less, V: 0.05 ~ 0.25%, Nb: 0.005 ~ 0.20%, Al: 0.010 ~ 0.100%, B : 0.0005% or less, Ca: 0 ~ 0.003%, O: 0.01% or less, N: 0.007% or less, and the balance: an Fe and impurities, tissues, and tempered martensite, the volume fraction consists of a residual austenite of less than 2% at a rate, grain size number of prior austenite grains in the tissue is not less than 9.0, 5 Carbonitride inclusions the number density having a particle size of more than 0μm has 10/100 mm 2 or less, yield strength is not less than 965MPa.
[0009]
　Method of manufacturing a low alloy steel oil country tubular goods according to the present invention, the chemical composition, in mass%, C: 0.65% than 0.45% or less, Si: 0.05 ~ 0.50%, Mn: 0. 10 ~ 1.00%, P: 0.020% or less, S: 0.0020% or less, Cu: 0.1% or less, Cr: 0.40 ~ 1.50%, Ni: 0.1% or less, Mo: 0.50 ~ 2.50%, Ti: 0.01% or less, V: 0.05 ~ 0.25%, Nb: 0.005 ~ 0.20%, Al: 0.010 ~ 0.100 %, B: 0.0005% or less, Ca: 0 ~ 0.003%, O: 0.01% or less, N: 0.007% or less, and the balance: a step of preparing a raw material is Fe and impurities, wherein a step of producing a cast material by casting a raw material, a step of producing a blank tube of the cast material to hot working, a step of hardening the blank tube, wherein the sintering And a step of tempering the is the base pipe. In the casting step, the cooling rate of the temperature range of 1500 ~ 1000 ° C. in thickness 1/4 position of the cast material is at 10 ° C. / min or more
[0010]
　According to the present invention, the SSC resistance and excellent high-strength stable low alloy steel for oil well pipes obtained and low alloy steel oil well pipe can be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011]
[Figure 1A] Figure 1A is a diagram for explaining a cluster-like inclusions.
FIG 1B] Figure 1B is a diagram for explaining a cluster-like inclusions.
FIG. 2 is the particle size of the sub-organization, which is the old austenite grain boundary map of the tissue is 2.6 [mu] m.
FIG. 3 is the particle size of the sub-structure is high-angle grain boundary map of the tissue is 2.6 [mu] m.
FIG. 4 is the particle size of the sub-organization, which is the old austenite grain boundary map of the tissue is 4.1 .mu.m.
FIG. 5 is the particle size of the sub-structure is high-angle grain boundary map of the tissue is 4.1 .mu.m.
FIG. 6 is a flow diagram of a method for manufacturing a low alloy steel oil country tubular goods according to an embodiment of the present invention
DESCRIPTION OF THE INVENTION
[0012]
　The present inventors have conducted various studies about the strength and SSC resistance of a low alloy steel for oil well pipes, to obtain a knowledge of the following (a) ~ (e).
[0013]
　(A) in order to get a high strength superior and SSC resistance stably, it is effective to use a large steel with C content. Increasing the C content, thereby improving the hardenability of steel, the amount of carbides precipitated in the steel is increased. Thus, without depending on the dislocation density, it is possible to improve the strength of steel.
[0014]
　(B) in order to better SSC resistance obtained stably, it is important to control the particle size of the carbonitride inclusions. When coarse carbonitrides inclusions in the plastic region formed forwardly propagating crack is present, it cracks occurred in starting point is because it is considered that crack propagation is facilitated.
[0015]
　Specifically, the carbonitride inclusions the number density with a particle size of more than 50 [mu] m, 10 pieces / 100 mm 2 if the following is obtained excellent fracture toughness. More preferably, in addition to the above, the carbonitride inclusions the number density with a particle size of more than 5 [mu] m, 600 pieces / 100 mm 2 to less than. In the present invention, the carbonitride inclusions, JIS G 0555 (2003) B is defined in the "type of inclusions" Annex 1,4.3 Section 2 inclusions and C 2 inclusions the It is intended to refer to.
[0016]
　The particle size of the carbonitride inclusions can be controlled by the cooling rate during the casting of steel. Specifically, the cooling rate of the temperature range of 1500 ~ 1000 ° C. in thickness 1/4 position of the cast material, to 10 ° C. / min or more. If during this period of cooling rate is too small, carbonitride inclusions become coarse. On the other hand, during the cooling rate is too-high, there are cases where cracking occurs in the cast material surface. Therefore, the cooling rate is preferably 50 ° C. / min or less, more preferably to 30 ° C. / min or less.
[0017]
　(C) low alloy steel for oil well pipes is quenched and tempered after pipe-is adjusted to structure mainly a tempered martensite. When the volume fraction of residual austenite is increased, it becomes difficult to get a high strength stably. To get a high strength stably, the volume fraction of residual austenite to less than 2%.
[0018]
　(D) tempered martensite is composed of a plurality of prior austenite grains. As prior austenite grain is fine, excellent SSC resistance can be stably obtained. Specifically, if the grain size number conforming to ASTM E112 of prior austenite grains is 9.0 or more, even if it has a yield strength of at least 965MPa, obtained excellent SSC resistance stable .
[0019]
　(E) in order to obtain a more excellent SSC resistance, in addition to the above, it is preferable to refine the sub tissue within prior austenite grains. Specifically, it is preferable that the equivalent circle diameter of the sub-tissue as defined below 3μm or less.
[0020]
　Each of the old austenite grains is composed of a plurality of packets. Each of the plurality of packets, is composed of a plurality of blocks, each of the plurality of blocks composed of a plurality of class. Packet boundaries, the block boundaries, and among the lath boundaries, the crystal orientation differences of boundaries of 15 ° or more is defined as "high angle grain boundary". In tempered martensite, packet boundaries, the block boundaries, and among the regions defined by the boundaries of lath boundaries, a region surrounded by high angle grain boundaries is defined as a "sub-organization".
[0021]
　Equivalent circle diameter of the sub tissue can be controlled by quenching conditions. Specifically, the quenching start temperature Ac 3 and points above the temperature, quenching stop temperature to 100 ° C. or less. That is, a base tube Ac 3 was heated to a temperature above points, to cool the heated mother pipe to 100 ° C. or less. Furthermore, this time of cooling, a 1 ° C. / sec or higher 15 ° C. / less than a second cooling rate of a temperature range of 100 ° C. from 500 ° C.. This can be a circle equivalent diameter of the sub-organization 3μm or less.
[0022]
　Based on the above findings, the present invention has been completed. Hereinafter, low alloy steel for oil well pipes according to an embodiment of the present invention, and illustrating a method of manufacturing a low alloy steel oil country tubular goods in detail.
[0023]
　[Chemical composition]
　Low alloy steel for oil well pipes according to the present embodiment has a chemical composition described below. In the following description, "%" of the content of the element means mass%.
[0024]
　C: 0.65% exceed 0.45% or less
　carbon (C) is a carbide was precipitated in the steel, increasing the strength of steel. Carbide is, for example, cementite, alloy carbides (Mo carbides, V carbides, Nb carbide, Ti carbide, etc.). Further, the sub-structure was refined, increase the SSC resistance. If the C content is too small, not the effect. On the other hand, when the C content is excessive, it reduces the toughness of the steel, increases the susceptibility to cracking. Therefore, C content is 0.65% or less than 0.45%. The preferable lower limit of C content is 0.47%, more preferably 0.50%, more preferably from 0.55%. The preferable upper limit of C content is 0.62%, more preferably 0.60%.
[0025]
　Si: 0.05 ~ 0.50%
　silicon (Si), the deoxidizing steel. If Si content is too small, the effect can not be obtained. On the other hand, when the Si content is excessive, the SSC resistance is reduced. Therefore, Si content is 0.05 to 0.50%. The lower limit of the preferred Si content is 0.10%, more preferably 0.20%. The upper limit of the preferred Si content is 0.40% and more preferably 0.35%.
[0026]
　Mn: 0.10 ~ 1.00%
　manganese (Mn), the deoxidizing steel. If Mn content is too small, the effect can not be obtained. On the other hand, when the Mn content is excessive, segregated in the grain boundary together with impurity elements such as phosphorus (P) and sulfur (S), the SSC resistance of the steel is lowered. Therefore, Mn content is 0.10 to 1.00%. The lower limit of the preferred Mn content is 0.20%, more preferably 0.28%. The upper limit of the preferred Mn content is 0.80%, more preferably 0.50%.
[0027]
　P: 0.020% or less
　phosphorus (P) is an impurity. P is segregated in the grain boundary to lower the SSC resistance of the steel. Therefore, P content is preferably small. Accordingly, P content is 0.020% or less. Preferred P content is 0.015% or less, still more preferably not more than 0.012%.
[0028]
　S: 0.0020% or less
　of sulfur (S) is an impurity. S is segregated in the grain boundary lowers the SSC resistance of the steel. Therefore, S content is preferably small. Thus, S content is 0.0020%. Preferred S content is at 0.0015% or less, still more preferably 0.0010% or less.
[0029]
　Cr: 0.40 ~ 1.50%
　chromium (Cr) increases the hardenability of steel and increases the strength of steel. On the other hand, when the Cr content is excessive, toughness of the steel is lowered and the SSC resistance of the steel is lowered. Therefore, Cr content is 0.40 to 1.50%. A preferable lower limit of Cr content is 0.45%. The preferable upper limit of the Cr content is 1.30%, more preferably 1.00%.
[0030]
　Mo: 0.50 ~ 2.50%
　molybdenum (Mo) is a carbide is formed and increasing the temper softening resistance. If the Mo content is too small, the effect can not be obtained. On the other hand, when the Mo content is excessive, the effect is saturated. Therefore, Mo content is from 0.50 to 2.50%. A preferable lower limit of Mo content is 0.60%, more preferably 0.65%. The preferable upper limit of the Mo content is 2.0%, more preferably from 1.6%.
[0031]
　V: 0.05 ~ 0.25%
　vanadium (V) is a carbide is formed and increasing resistance to temper softening. If the V content is too small, the effect can not be obtained. On the other hand, when the V content is excessive, toughness of the steel is lowered. Therefore, V content is 0.05 to 0.25%. The preferable lower limit of V content is 0.07%. The preferable upper limit of the V content is 0.15%, more preferably 0.12%.
[0032]
　Ti: 0.01% or less
　titanium (Ti) is an impurity. Ti forms a carbonitride inclusions, to destabilize the SSC resistance of the steel. Therefore, Ti content is preferably small. Therefore, Ti content is 0.01% or less. The upper limit of the preferred Ti content is 0.008%, more preferably 0.006%.
[0033]
　Nb: 0.005 ~ 0.20%
　niobium (Nb) forms carbides, nitrides, or carbonitrides. These precipitates, the steel sub-tissue comminuted by pinning (pinning) effect, enhancing the SSC resistance of the steel. If the Nb content is too small, the effect can not be obtained. On the other hand, when the Nb content is excessive, carbonitride inclusions is excessively formed, to destabilize the SSC resistance of the steel. Therefore, Nb content is from 0.005 to 0.20%. The preferable lower limit of Nb content is 0.010%, more preferably 0.012%. The preferable upper limit of Nb content is 0.10%, more preferably 0.050%.
[0034]
　Al: 0.010 ~ 0.100%
　of aluminum (Al), the deoxidizing steel. If the Al content is too low, insufficient deoxidation of steel, the SSC resistance of the steel is lowered. On the other hand, when the Al content is excessive, oxides are generated, the SSC resistance of the steel is lowered. Therefore, Al content is 0.010 to 0.100%. A preferable lower limit of Al content is 0.015%, more preferably 0.020%. The preferable upper limit of Al content is 0.080%, more preferably 0.050%. The content of the "Al" referred to herein, "acid soluble Al", that means the content of "sol.Al".
[0035]
　B: 0.0005% or less
　Boron (B) is an impurity. B is, M in the grain boundary 23 CB 6 is formed and reduces the SSC resistance of the steel. Therefore, B content is preferably small. Therefore, B content is 0.0005%. The upper limit of the preferred B content is 0.0003%, more preferably 0.0002%.
[0036]
　O: 0.01% 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 small. Therefore, O content is 0.01% or less. Preferred O content is 0.005% or less, more preferably 0.003% or less.
[0037]
　N: 0.007% or less
　nitrogen (N) is an impurity. N forms nitrides, destabilize the SSC resistance of the steel. Therefore, N content is preferably small. Therefore, N content is 0.007% or less. Preferred N content is 0.005% or less, more preferably 0.004% or less.
[0038]
　Cu: 0.1% or less
　Copper (Cu) is an impurity in the present invention. Cu, although to enhance the hardenability of steel has the effect of strengthening the steel, the content exceeds 0.1%, local curing organizations to occur, the cause of non-uniform corrosion of the steel surfaces or it becomes. Therefore, Cu content is 0.1% or less. Preferred Cu content is 0.05% or less, more preferably 0.03% or less.
[0039]
　Ni: 0.1% or less
　of nickel (Ni) is an impurity in the present invention. Ni also, although to enhance the hardenability of steel has the effect of strengthening the steel, the content exceeds 0.1%, SSC resistance is reduced. Therefore, Ni content is 0.1% or less. Preferred Ni content is 0.05% or less, more preferably 0.03% or less.
[0040]
　The remainder of the chemical composition of the low alloy steel for oil well pipes according to the present embodiment is composed of Fe and impurities. Impurities as referred to herein, refers to an element being mixed from the environment, such as ore and scrap, or the manufacturing process is used as a raw material of steel.
[0041]
　[For the selected element]
　low alloy steel for oil well pipes according to the present embodiment, instead of a part of the Fe, may contain Ca.
[0042]
　Ca: 0 ~ 0.003%
　of calcium (Ca) is a selective element. Ca combines with S in steel to form a sulfide, enhance the toughness of the steel by improving the shape of the inclusions. If Ca is contained even a little, the effect of the above can be obtained. On the other hand, when the Ca content is excessive, its effect is saturated. Therefore, Ca content is from 0 to 0.003%. The lower limit of the preferred Ca content is 0.0005%, more preferably 0.0010%. The upper limit of the preferred Ca content is 0.0025%, more preferably 0.0020%.
[0043]
　Organization (Microstructure)]
　tissue of low alloy steel for oil well pipes according to the present embodiment is mainly tempered martensite. Specifically, the matrix phase in the tissue is composed of a tempered martensite, and residual austenite of less than 2% by volume fraction.
[0044]
　Tissues other than the tempered martensite, for example, bainite or the like is mixed, the strength becomes unstable. Further, the residual austenite is to produce a variation in the strength, the volume fraction is preferably lower. The volume fraction of the retained austenite for example, using X-ray diffraction method, is measured as follows. Obtaining a sample comprising a thick central portion of the produced low alloy steel oil country tubular goods. The collected surface of the sample to chemical polishing. To chemical polished surfaces, using CoKα line as the incident X-ray, to implement the X-ray diffraction. (211) plane of the ferrite, (200) plane, and the integrated intensity of the (110) plane, (220) plane of the austenite, (200) plane, from the integral intensities of the (111) plane, the volume fraction of retained austenite determined by quantitative.
[0045]
　The crystal structure of tempered martensite and bainite are the same BCC structure as the ferrite. As described above, the tissue of the low alloy steel for oil well pipes according to the present embodiment is mainly tempered martensite. Therefore, (211) plane of the ferrite, (200) plane, the integrated intensity of the (110) plane, so that measures the tempered martensite.
[0046]
　[Prior austenite grains grain size]
　grain size number of prior austenite grains of the low alloy steel for oil well pipes according to the present embodiment is 9.0 or more. Grain size number of prior austenite grains is measured according to ASTM E112. If the grain size number of prior austenite grains is 9.0 or more, even steel having a yield strength of at least 965MPa, excellent SSC resistance can be obtained. Preferred grain size number of prior austenite grains is greater than 9.0, still more preferably 10.0 or more.
[0047]
　Grain size number of prior austenite grains after quenching may be measured using a tempered before the steel (so-called hardened Mom material), may be measured using a tempered steel. Using either of steel, grain size number of prior austenite grains is not changed.
[0048]
　[Carbonitride inclusions the number density]
　Furthermore, in low alloy steel for oil well pipes according to the present embodiment, carbonitride inclusions the number density with a particle size of more than 50μm is 10/100 mm 2 or less. As already mentioned, the coarse carbonitride inclusions in the plastic region formed forwardly propagating crack is present, it cracks occurred in starting point, the crack propagation is facilitated. Accordingly, coarse inclusions the number density is preferably as low as. The number of carbonitride-type inclusions having a particle size of more than 50μm is 10/100 mm 2 if less, resulting excellent fracture toughness.
[0049]
　Particle size and number density of the inclusions is measured in the following manner. Area including the thickness center in the cross section parallel to the axial direction of the low-alloy steel oil well pipe is 100 mm 2 to take a sample containing the observation region. Mirror polished surface (observation surface) including the observation area. Of the observation plane of the polished samples, inclusions observation region (sulfide inclusions (MnS, etc.), oxide inclusions (Al 2 O 3 , etc.), and carbonitride inclusions) an optical microscope identified by. Specifically, in the observation area, on the basis of the contrast and shape of the optical microscope, oxide inclusions, identifying sulfide inclusions, and carbonitrides inclusions.
[0050]
　Among the inclusions specified, the particle size measured carbonitrides inclusions. The particle size herein means the largest of the straight line ([mu] m) connecting two different points on the interface between the inclusions and the matrix. However, the cluster-like particles determines the particle size is regarded as one of the inclusions. More specifically, as shown in FIGS. 1A and 1B, whether in individual inclusions straight line, the distance d is 40μm or less, when the distance between the centers s are present in 10μm or less, these one One of the regarded as inclusions. Hereinafter referred carbonitride inclusions having a particle size of more than 50μm and coarse inclusions.
[0051]
　In each observation area, counting the total number of coarse inclusions. Then, a total number TN of coarse inclusions in all observation regions. Based on the total number TN obtained from the following equation (A), 100 mm 2 obtains coarse inclusions the number density N per.
　N = TN / total area × 100 · · · in the observation area (A)
[0052]
　More preferably, in addition to the above, the carbonitride inclusions the number density with a particle size of more than 5 [mu] m, 600 pieces / 100 mm 2 to less than. Carbonitride inclusions the number density with a particle size of more than 5μm can be calculated in the same manner as in the case of the carbonitride inclusions the number density with a particle size of more than 50 [mu] m.
[0053]
　[Suborganization circle equivalent diameter]
　The present embodiment according to the low alloy steel for oil well pipes is preferably in the tempered martensite, packets among the blocks and lath boundaries, the crystal orientation difference is surrounded by 15 ° or more boundary equivalent circle diameter of the sub-organization is 3μm or less.
[0054]
　In steel having a high strength of at least 965MPa, SSC resistance is not only the particle size of prior austenite grains, but also on the size of the sub-organization. Grain size number of prior austenite grains is not less than 9.0, further, if the circle equivalent diameter of the sub-organization 3μm or less, in the low alloy steel for oil well pipes having a high strength of at least 965MPa, the excellent SSC resistance stable obtained. Further preferred equivalent circular diameter of the sub-structure is at 2.5μm or less, more preferably 2.0μm or less.
[0055]
　Equivalent circle diameter of the sub-tissue is measured by the following method. In the axial direction perpendicular to the cross-section of low alloy steel oil country tubular goods, taking samples with a viewing surface of 100 [mu] m × 100 [mu] m around the center of the wall thickness. With respect to the observation plane, performing the crystal orientation analysis by electron backscatter diffraction pattern method (EBSP). Then, based on the analysis result, in the observation plane, by drawing a boundary having a crystal misorientation than 15 °, to identify a plurality of sub-organizations. A plurality of specific sub-organization, for example, be carried out by image processing using a computer.
[0056]
　Measuring the circle equivalent diameter of each sub-organization identified. The circle-equivalent diameter is meant the diameter of a circle in the case of converting the area of ​​the sub-structure to a circle of the same area. Measurements of the equivalent circle diameter, for example, be carried out by image processing. The average circle equivalent diameter of each sub-tissue obtained is defined as the equivalent circle diameter of the sub-organization.
[0057]
　2 and 3, the particle size of the sub-organization to illustrate the tissue is 2.6 [mu] m. FIG. 2 is a prior austenite grain boundary map, and FIG. 3 is a large angle grain boundary map. 2 and 3, the grain size number of prior austenite grains is 10.5, C: 0.51%, Si: 0.31%, Mn: 0.47%, P: 0.012%, S : 0.0014%, Cu: 0.02%, Cr: 1.06%, Mo: 0.67%, V: 0.098%, Ti: 0.008%, Nb: 0.012%, Ca: 0.0018%, B: 0.0001%, sol. Al: 0.029%, N: a tissue obtained from 0.0034% of the steel.
[0058]
　4 and 5, the particle size of the sub-organization to illustrate the tissue is 4.1 .mu.m. Figure 4 is a prior austenite grain boundary map, FIG. 5 is a large angle grain boundary map. 4 and 5, a grain size number of prior austenite grains is 11.5, C: 0.26%, Si: 0.19%, Mn: 0.82%, P: 0.013%, S : 0.0008%, Cu: 0.01%, Cr: 0.52%, Mo: 0.70%, V: 0.11%, Ti: 0.018%, Nb: 0.013%, Ca: 0.0001%, B: 0.0001%, sol. Al: 0.040%, N: a tissue obtained from 0.0041% of the steel.
[0059]
　[Manufacturing Method]
　Hereinafter, a method for manufacturing a low alloy steel oil country tubular goods according to an embodiment of the present invention.
[0060]
　Figure 6 is a flow diagram of a method for manufacturing a low alloy steel oil well pipe according to the present embodiment. Method of manufacturing a low alloy steel oil well pipe according to the present embodiment includes a step (step S1) of preparing a raw material, a step of producing a cast material by casting a raw material (step S2), and with a cast material to hot working and step (step S3) to produce a mother tube, and step (step S4) of the intermediate heat treatment a base tube, a step of quenching the intermediate heat-treated raw pipe (step S5), and step (the step of tempering the hardened hollow shell S6) and a.
[0061]
　Preparing a raw material of the above-mentioned chemical composition (Step S1). Specifically, Steels of the above-mentioned chemical composition and refining.
[0062]
　By casting a raw material to cast material (step S2). Casting, for example, continuous casting. Cast material is, for example, a slab or bloom or billet. Continuous casting material may be a round billet is continuously cast.
[0063]
　At this time, the thickness 1/4 position of the cast material, the cooling rate of the temperature range of 1500 ~ 1000 ° C., to 10 ° C. / min or more. If during this period of cooling rate is too small, carbonitride inclusions become coarse. On the other hand, during the cooling rate is too-high, there are cases where cracking occurs in the cast material surface. Therefore, the cooling rate is preferably 50 ° C. / min or less, more preferably to 30 ° C. / min or less. Cooling rate in the thickness 1/4 position can be determined by simulation calculation. In actual production, conversely, to previously obtain a such cooling conditions such that the appropriate cooling rate in advance simulation calculation may be applied that condition. The cooling rate of lower than 1000 ° C. temperature range, as good as any speed.
[0064]
　Note that the thickness 1/4 position from the surface of the cast material, which is at a depth of 1/4 of the thickness of the cast material. For example if the cast material is round billet is continuously cast, the position depth of one-half of the radius from the surface, in the case of rectangular bloom, depth from the surface is a quarter of the long side 1 is the length of the position.
[0065]
　The cast material slabbing or forging to make the shape of the round billet. The round billet to produce a raw tube by hot working (Step S3). With the is continuously cast round billet, it is possible to omit the slabbing or forging process. Hot working example, a Mannesmann tube. Specifically, a round billet was pierced and rolled by the piercing mill, a mandrel mill, reducer, and hot rolled by a sizing mill or the like to base pipe. The processing method between other heat may be prepared mother tube from a round billet.
[0066]
　The mother tube manufactured by hot working may be intermediate heat treatment (step S4). Intermediate heat treatment is an optional step. That is, the intermediate heat treatment may not be performed. By carrying out intermediate heat treatment, it can be finer steel crystal grains (prior austenite grains), SSC resistance is further enhanced.
[0067]
　Intermediate heat treatment is, for example, a quota rise (normalizing). Specifically, base pipe the Ac 3 point or more temperature and, after holding a predetermined time, for example, 850-950, and allowed to cool. Retention time, for example, 15 to 120 minutes. Norma Rise usually after hot working, it is carried out after cooling the mother pipe to room temperature. However, in the present embodiment, after hot working, without cooling to room temperature, the blank tube Ac 3 was maintained at a temperature of above points, it may be allowed to cool.
[0068]
　As an intermediate heat treatment, instead of the norm rise above it may be carried out quenching. The hardening is a separate heat treatment performed to the quenching step S5. That is, when the quenching is carried out as an intermediate heat treatment, quenching is performed multiple times. Quenching, specifically, base pipe the Ac 3 point or more temperature and, after holding a predetermined time, for example 850-950, quenched. In this case, immediately after the hot working, base tube the Ac 3 may be quenched from points above the temperature (hereinafter, this process called "direct quenching").
[0069]
　Intermediate heat treatment is heat treatment at a 2-phase region temperature of ferrite and austenite (hereinafter, referred to as "2-phase region heating") the same effect even. In intermediate heat treatment, at least a portion of the structure of the steel if the transformation to austenite, favorable results can be obtained for the grain refinement. Therefore, the intermediate heat treatment, at least the base tube Ac 1 it is preferable to soaking at a temperature above points.
[0070]
　The intermediate heat-treated raw tube, implementing the quenching (Step S5). When not carrying out the intermediate heat treatment is carried quenching (step S5) against hot working mother pipe which is manufactured by (step S3).
[0071]
　Quenching, quenching start temperature Ac 3 and points above the temperature, quenching stop temperature preferably set to 100 ° C. or less. That is, a base tube Ac 3 was heated to a temperature above points, it is preferable to cool the heated mother pipe to 100 ° C. or less. During this cooling, preferably to 1 ° C. / sec or higher 15 ° C. / less than a second cooling rate of 100 ° C. temperature range from 500 ° C.. This can be a circle equivalent diameter of the sub-organization 3μm or less. In cooling rate is less than 1 ° C. / sec, it becomes difficult to the circle equivalent diameter of the sub-organization 3μm or less. When the cooling rate exceeds 15 ° C. / sec, quench cracking may increase to occur. The lower limit of the cooling rate is preferably 2 ° C. / sec, more preferably 5 ° C. / sec or more.
[0072]
　Quenching are hollow shell tempering (step S6). Specifically, the quenching is hollow shell, Ac 1 soaking tempering temperatures below points. Tempering temperature is adjusted according to the yield strength of the chemical composition and objectives of blank tube. Preferred tempering temperature is lower than 700 ° C. 650 ° C. or higher, preferably soaking time is 15 to 120 minutes. Tempering temperature, Ac 1 is less than points, higher temperatures are preferred.
[0073]
　Above, low alloy steel for oil well pipes according to an embodiment of the present invention, and described a method of manufacturing a low alloy steel for oil well pipes. According to this embodiment, the SSC resistance and excellent high-strength stable low alloy steel for oil well pipes obtained and low alloy steel oil well pipe can be obtained.
Example
[0074]
　The following examples illustrate the present invention more specifically. The present invention is not limited to these examples.
[0075]
　Steel A ~ F having the chemical compositions shown in Table 1 was melted.
[0076]
[Table 1]

[0077]
　From each of steels A ~ F, a round CC (round continuous casting), a plurality prepared round billet outside diameter 310 mm. Alternatively, hot working the bloom obtained by continuous casting process, a plurality prepared round billet outside diameter 310 mm. It was produced raw tube by hot working from Kakumaru billet. Specifically, after heating to 1150 ~ 1200 ° C. The round billet in a heating furnace, performing a piercing by the piercing mill, carried out elongation rolling by a mandrel mill, to implement a constant 径圧 extended by reducer, base pipe It was prepared. By carrying out various heat treatments in the blank tube was produced low alloy steel OCTG numbers 1-44. Low alloy steel oil country tubular goods of each number, external shape 244.48Mm, wall thickness was 13.84Mm. Table 2 shows the processing conditions of low alloy steel oil country tubular goods for each number.
[0078]
[Table 2]

[0079]
　In Table 2, "○" in the column of "casting conditions", the cooling rate of the temperature range of 1500 ~ 1000 ° C. indicates that was 10 ~ 30 ° C. / min. "×" in the column indicates that the cooling rate in the temperature range was less than 10 ° C. / min. "920 ° C. Norma" in the column of "intermediate heat treatment" indicates that it has performed the quota rise of soaking temperature 920 ° C. as an intermediate process. "Inline Q" in the column of "intermediate heat treatment" as intermediate heat treatment, blank pipe temperature after hot working Ar 3 that was carried quenching water cooling after soaking at 920 ° C. from the state does not reach below point show. The column of "intermediate heat treatment", "-" indicates that it did not implement the intermediate heat treatment. "Mist Q" in the column "Method" "quenching conditions" indicates that it has performed the mist cooling as a cooling at the time of quenching. "WQ" in this column indicates that it has performed the water cooling as a cooling at the time of quenching. The column of "Tempering", "-", indicates that it did not implement the tempering. Low alloy steel OCTG number 42, because the cracks during quenching has occurred and has not carried out tempering.
[0080]
　[Tensile Test]
　from the low alloy steel oil country tubular goods of each number, were taken arcuate tensile specimen. Cross section of the arcuate tensile specimen is arcuate, longitudinal arcuate tensile test piece was parallel to the longitudinal direction of the steel pipe. Using an arcuate tensile specimens, API in accordance with the provisions of 5CT of (American Petroleum Institute) standards, and a tensile test at room temperature. Based on the test results, the yield strength YS of each steel tube (MPa), tensile strength TS (MPa), and was determined the yield ratio YR (%).
[0081]
　[DCB Test]
　thickness 9.53 ± 0.05 mm from the low alloy steel oil country tubular goods of each number, width 25.4 ± 0.05 mm, were taken DCB specimen length 101.6 ± 1.59 mm. Using the collected DCB specimen, NACE compliant with (National Association of Corrosion Engineers) TM0177-2005Method D, it was carried out DCB tests. Room temperature 50g / L NaCl + 4g / L CH saturated with hydrogen sulfide gas 0.03atm the test bath 3 was used COONa solution. PH of the test solution was adjusted to pH3.5 with hydrochloric acid. The DCB specimens were immersed for 720 hours in the test bath and conducted DCB tests. Specimen is placed under the opening stress using a wedge to provide the displacement of 0.51mm (+ 0.03 / -0.05mm) to the two arms of the DCB specimens were exposed to a 30-day test solution. After the test was measured crack propagation length a that occurred DCB specimen. From the measured crack propagation length a and wedge opening stress P, a stress intensity factor K based on the equation (B) ISSC was determined (ksi√inch). In the formula (B), h is the height of each arm of the DCB specimen, B is the thickness of the DCB test specimen, Bn is a web thickness of the DCB test specimen. These are defined in the NACE TM0177-2005MethodD.
[0082]
[Number 1]

[0083]
　[Structure Observation]
　Samples were taken from the thick central portion of the low alloy steel OCTG each number was determined the volume fraction of residual austenite by X-ray diffraction method.
[0084]
　[Counting inclusions]
　from each low alloy steel oil country tubular goods, the polishing surface is parallel to the rolling direction were taken inclusions quantitative test strip to include the thickness center of the steel pipe. The collected test specimen was observed at 200 magnification. Those that are made in a cluster-like, as measured at 200 to 1000 times, was determined whether or not the cluster. Carbonitride inclusions the number having a particle size of at least 50 [mu] m, and a carbonitride inclusions the number having a particle size of at least 5 [mu] m, were counted in each of the two field of view. Calculated by determining the number densities the number counted divided by the area of the field of view, toward the number density obtained in 2 field large, and the carbonitride inclusions the number density of each low-alloy steel oil country tubular goods.
[0085]
　[Austenite grain size Test
　low alloy steel oil country tubular goods of each number, surface orthogonal to the axial direction (hereinafter, referred to as observation surface) were taken test piece having a. The observation surface of each specimen was mechanically polished. After polishing, using a picral (Picral) etchant, it was to appear prior austenite grain boundaries in the observation plane. Then, in compliance with ASTM E112, were determined grain size number of prior austenite grains of the observation plane.
[0086]
　[Suborganization circle equivalent diameter measured]
　Samples were taken from the cross-section of low alloy steel oil country tubular goods for each number, and carrying out crystal orientation analysis by EBSP, it was determined circle equivalent diameter of the sub-organization.
[0087]
　The results of each test are shown in Table 3. The low alloy steel OCTG any number also had a tempered martensite, the structure consisting austenite of less than 2% by volume fraction.
[0088]
[table 3]

[0089]
　The yield strength in the column of "YS" in Table 3, the tensile strength in the column of "TS", the yield ratio in the column of "YR", described respectively. The column of "old γ grain number", describing the grain size number of prior austenite grains. Incidentally, in each column of Table 3, "-" indicates that it did not implement the test or measurement.
[0090]
　Number low alloy steel OCTG 1,2,4,10,11,13,19,21,33,35,37 to 39, the yield strength of 140ksi (965MPa) above, the stress intensity factor of more 22ksi√inch the had. Low alloy steel OCTG These numbers carbonitride inclusions the number density with a particle size of more than 50μm is 10/100 mm 2 or less, the carbonitride inclusions the number density having a particle size of at least 5μm 600 / 100Mm 2 was less than.
[0091]
　Low alloy steel OCTG numbers 6 ~ 9, 15 ~ 18, 23 ~ 25, the yield strength was less than 140 ksi. It is believed that this is because the tempering temperature was too high.
[0092]
　Yield strength of low alloy steel OCTG numbers 26-32 was less than 140 ksi. This is probably because the carbon content of the steel E was too low.
[0093]
　Although the yield strength of the low alloy steel OCTG number 3,5,12,14,20,22,34,36,40 was at least 140 ksi, the stress intensity factor was less than 22Ksi√inch. This carbonitride inclusions the number density with a particle size of more than 50μm is 10/100 mm 2 that was higher than, or carbonitride inclusions the number density having a particle size of at least 5μm is 600/100 mm 2 from presumably because it was also high. The coarse carbonitride inclusions the number density was high, in the casting process, presumably because the cooling rate was too small.
[0094]
　Although the yield strength of the low alloy steel OCTG numbers 41, 43, 44 was at least 140 ksi, the stress intensity factor was less than 22Ksi√inch. This is equivalent circle diameter of the sub-structure is considered for greater than 3 [mu] m. The equivalent circle diameter of the sub-organization is larger than 3μm is probably because the quenching conditions were inappropriate. The low alloy steel OCTG number 42 cracking during quenching occurs. This cooling rate during quenching is probably because too large.

claims
[Claim 1]
　Chemical composition, in
　mass%, C: 0.65% than 0.45% or
　less,
　Si: 0.05 ~ 0.50%, Mn: 0.10
　~ 1.00%, P: 0.020%
　hereinafter, S: 0.0020% or
　less, Cu: 0.1% or
　less, Cr: 0.40 ~
　1.50%, Ni: 0.1% or
　less, Mo: 0.50 ~ 2.50%,
　Ti: 0.01% or
　less,
　V:
　0.05 ~ 0.25%, Nb: 0.005 ~ 0.20%, Al: 0.010
　~ 0.100%, B: 0.0005% or
　less, Ca: 0 ~
　0.003%, O: 0.01% or
　less, N: 0.007% or less,
　the balance is Fe and impurities,
　the tissue, and tempered martensite, and residual austenite of less than 2% by volume fraction becomes,
　the grain size number of prior austenite grains in the tissue is not less than 9.0,
　50 [mu] m or more particle Carbonitride inclusions the number density having ten / 100 mm 2 or less,
　yield strength is not less than 965MPa, low alloy steel for oil well pipes.
[Claim 2]
　A low alloy steel for oil well pipes according to claim 1,
　carbonitride inclusions the number density having a particle size of at least 5μm is 600/100 mm 2 or less, low alloy steel for oil well pipes.
[Claim 3]
　A claim 1 or 2 low alloy steel for oil well pipe according to,
　in the tempered martensite, packet, block, and out of the boundaries of lath, sub tissue misorientation is surrounded by the boundaries of more than 15 ° circle equivalent diameter of 3μm or less, low alloy steel for oil well pipes.
[Claim 4]
　Chemical composition, in mass%, C: 0.65% than 0.45% or less, Si: 0.05 ~ 0.50%, Mn: 0.10 ~ 1.00%, P: 0.020% hereinafter, S: 0.0020% or less, Cu: 0.1% or less, Cr: 0.40 ~ 1.50%, Ni: 0.1% or less, Mo: 0.50 ~ 2.50%, Ti: 0.01% or less, V: 0.05 ~ 0.25%, Nb: 0.005 ~ 0.20%, Al: 0.010 ~ 0.100%, B: 0.0005% or less, Ca: 0 ~ 0.003%, O: 0.01% or less, N: 0.007% or less, and the balance: a step of preparing a raw material is Fe and impurities,
　a process for producing a cast material by casting the raw material,
　a step of producing a blank tube of the cast material to hot working,
　a step of hardening the blank tube,
　and a step of tempering the quenched hollow shell,
　before In the casting process, the cooling rate of the temperature range of 1500 ~ 1000 ° C. in thickness 1/4 position of the cast material is 10 ° C. / min or more, a manufacturing method of the low alloy steel oil country tubular goods.
[Claim 5]
　A method of manufacturing a low alloy steel oil country tubular goods according to claim 4,
　in the casting process, the cooling rate of the temperature range of 1500 ~ 1000 ° C. in thickness 1/4 position of the cast material is 30 ° C. / min or less in a method of manufacturing a low alloy steel oil country tubular goods.
[Claim 6]
　A method of manufacturing a low alloy steel oil country tubular goods according to claim 4 or 5,
　wherein the step of quenching,
　the raw tube Ac 3 and heating to a temperature above points,
　100 ° C. the heated raw tube and a step of cooling to below
　the in cooling to step, the cooling rate of the temperature range of 100 ° C. from 500 ° C. is less than 15 ° C. / sec 1 ° C. / sec or more, a manufacturing method of the low alloy steel oil country tubular goods.