Technical field
[0001]
The present invention relates to a seamless steel pipe and a manufacturing method thereof.
BACKGROUND
[0002]
Those of the cylindrical among mechanical structural member, conventionally, by performing forging or elongation rolling to bars, or after further machining and subjected to a desired shape, the heat treatment is performed, necessary for mechanical structural members were often mechanical properties is given.
[0003]
　However, in recent years, receives the trend of large size and high strength of the mechanical structure, weight reduction is achieved by replacing the cylindrical machine structural member into a hollow seamless steel pipe. In particular, the steel pipe used in the crane boom, in addition to the size of the crane for skyscrapers, since there is a need such as to work in cold climates, high toughness is required along with high strength. Specifically, recently, as the application to a crane boom having a tensile strength of at least 980 MPa, and has come to be required seamless steel pipe having excellent toughness at a low temperature of -40 ° C..
[0004]
　For high strength and high toughness of seamless steel pipe and a manufacturing method thereof, various techniques have been disclosed.
[0005]
　For example, Patent Document 1, without the addition of expensive alloy steel, by the online thermomechanical processing, a method capable of producing a high strength seamless steel pipe having excellent toughness is disclosed.
[0006]
　Patent Document 2, a tensile strength of at least 950 MPa, a yield strength at least 850 MPa, and the Charpy absorbed energy at -40 ℃ discloses seamless steel pipe and a manufacturing method thereof is not less than 60 J.
[0007]
　Patent Document 3, a tensile strength of at least 950 MPa, yield strength of 850MPa or more, and is not less Charpy absorbed energy at -40 ° C. 60 J or more, seamless steel pipe and a manufacturing method thereof a wall thickness of 30mm than is disclosed ing.
CITATION
Patent Document
[0008]
Patent Document 1: JP 2001-240913 Patent Publication
Patent Document 2: WO 2010/061882 Patent
Patent Document 3: JP 2012-193404 JP
Summary of the Invention
Problems that the Invention is to Solve
[0009]
　High strength seamless steel pipe disclosed in Patent Document 1, the tensile strength is 899MPa at most, strength can not be said to be sufficient for the crane boom.
[0010]
　On the other hand, a seamless steel pipe disclosed in Patent Document 2, the tensile strength has a 950MPa or more and a yield strength 850MPa or more high strength and excellent toughness at low temperatures, moreover, satisfaction with regard properties after welding is a level that can be. Further, seamless steel pipe disclosed in Patent Document 3, in the case of a thick wall thickness is more than 30mm also has a higher tensile strength 950MPa and yield strength 850MPa or more high strength, and also the toughness at low temperatures excellent.
[0011]
　Here, the crane boom applications, the steel pipe, in addition to the high strength and high toughness is also required high weldability. As a guideline for evaluation of weldability, the following [A] Pcm (weld crack susceptibility composition (%)) of the formula is well known.
　Pcm = C + (Si / 30 ) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + 5B ··· [A ]
　However, the element symbol in the [a] type, means in the steel content of each element (mass%), if not contained and zero.
[0012]
　In general, cold cracking is likely to occur in the welded portion is large Pcm. Therefore, Pcm, in actual welding, are often used as an indicator of the management of preheat temperature.
[0013]
　Further, recently, in order to avoid the complexity of welding, preheating omitted or tend to direct as low as possible preheating. Therefore, the crane boom seamless steel products, Pcm rather than merely as a measure of weldability, (specifically, for example, Pcm ≦ 0.30) that the value is equal to or less than a predetermined value, is required as a specification also have arisen if that. In this case, Pcm> 0.30 product even if the welding of the product as were those no practical problem at all if indeed evaluated previously Pcm value proceeding such actual evaluation It would be a rejected with.
[0014]
　Seamless steel pipe disclosed in Patent Document 2 will include Cr and Mo at a high value. Therefore, it is conceivable that not satisfied with severe demand 0.30 or less Pcm.
[0015]
　Further, seamless steel pipe disclosed in Patent Document 3 also, because it contains Cr and Mo at a high value, it is conceivable that not satisfied with severe demand 0.30 or less Pcm. Furthermore, the manufacturing method of the seamless steel pipe, after the low-alloy steel and pipe-in ​​hot, and performs the quenching and tempering at least twice. Therefore, it becomes disadvantageous in terms of productivity, energy costs it is contemplated that increased.
[0016]
　The present invention, a tensile strength of more than 980 MPa, impact value at -40 ℃ with 2mmV notch Charpy test piece (hereinafter, simply referred to as "Charpy impact value at -40 ℃".) Is 75 J / cm 2 or more There, moreover Pcm is 0.30 or less, and an object thereof is to provide a seamless steel pipe and a manufacturing method thereof.
Means for Solving the Problems
[0017]
　The present invention has been made in order to solve the above problems, the gist lies in seamless steel pipe and the manufacturing method thereof described below.
[0018]
　(1) chemical composition, in
　mass%,
　C: 0.10 ~
　0.20%, Si: 0.05 ~ 1.0%, Mn: 0.05
　~ 1.2%, P: 0.025%
　hereinafter, S: 0.005% or
　less, Cu: 0.20% or
　less, N: 0.007% or
　less, Ni: 0.20
　~ 0.50%, Cr: less than 0.30% to 0.50% 　Mo:
　0.30 ~ 0.50%, 　Nb: 0.01 ~ 0.05%, Al: 0.001 ~ 0.10%, B: 0.0005 　~ 0.0020%, Ti: 0.003 ~ 　% 0.050, V: 0.01 ~ 　0.20%, Ca, any one or more of the total of Mg and REM: 0 ~ 0.025%, 　the balance is Fe and impurities, 　the following [a] formula in the value of Pcm represented is 0.30 or less, 　metal structure, in area%, 　tempered martensite: 90% or more, 　the tensile strength Saga 980MPa or 　more, Charpy impact value at -40 ℃ with 2mmV notch test piece 75 J / cm 2 is not less than, 　the seamless steel pipe. 　Pcm = C + (Si / 30 ) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + 5B ··· [A ] 　However, the element symbol in the [a] type, means in the steel content of each element (mass%), if not contained and zero.

[0019]
　(2) A method for producing a seamless steel pipe according to (1),
　by using a steel slab having a chemical composition described in the above (1), the following from [i] to [iv] Step the applied to the order,
　method of producing a seamless steel pipe.
　[i]: After heating the billet to 1200 ~ 1300 ° C., to produce a mother tube by performing the processing 40 to 99% by reduction of area, the hot steel pipe process
　[ii]: the mother tube Ac 1 cooled to a temperature below the point, the cooling step
　[iii]: the cooled hollow shell Ac 3 after heating to point ~ 950 ° C., rapidly cooled, quenching step
　[iv]: heating the quenched hollow shell to 500 ~ 600 ° C. after cooled to room temperature, tempering process
Effect of the invention
[0020]
　According to the present invention, tensile strength excellent in low temperature toughness and has a high strength of at least 980 MPa, and Pcm is possible to obtain a seamless steel pipe excellent in small weldability 0.30.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021]
[1] tempering area ratio of martensite is a tissue photograph of Test No. 1 was less than 95% 90%.
[2] the area ratio of tempered martensite is a tissue photograph of Test No. 3 was less than 90%.
[3] the area ratio of tempered martensite is a tissue photograph of Test No. 31 was 95% or more.
DESCRIPTION OF THE INVENTION
[0022]
　The present inventors, there is the chemical composition based on inexpensive low alloy steels, after pipe-in ​​hot, only by performing quenching and tempering once, a predetermined strength and Charpy impact value secured, moreover, Pcm is overlaid intensive studies on a method for obtaining a seamless steel pipe is 0.30 or less. As a result, we got an important knowledge of the following.
[0023]
　(A) a Pcm terms of weldability in order to control a low value of 0.30 or less may be lowered content of the alloy elements contained in the [A] type described above. However, simply reducing the alloy element content is not sufficient hardened structure inviting reduction in hardenability is obtained. Therefore, even if possible to secure a good weldability can not be compatible with certain strength and toughness.
[0024]
　(B) by mass%, when the content of B is 0.0020%, limiting the content upper limit of Cr and Mo both 0.50% for Pcm reduction, including in combination thereof in the case of the steel also without coarse boron carbon products are formed during tempering, it can ensure good low-temperature toughness. That, B and By including appropriate amounts, relatively inexpensive to increase the hardenability, component of low alloy steels exist that can satisfy both strength and toughness.
[0025]
　(C) On the other hand, only by performing quenching and tempering once, in order to achieve both high strength and high toughness may be austenite grains during 該焼 inserting fine.
[0026]
　The present invention has been made based on the above findings. It will be described in detail below each requirement of the present invention.
[0027]
　(A) Chemical composition
　reasons for limiting the chemical composition of seamless steel pipes and steel strips according to the present invention is as follows. "%" For the content of each element in the following description means "mass%".
[0028]
　C: 0.10 ~ 0.20%
　C is an indispensable element in order to increase the strength. If C content is less than 0.10%, the tensile in relation to other elements strength it may be difficult to obtain a high strength of more than 980 MPa. On the other hand, when the C content exceeds 0.20%, weldability is significantly decreased. Therefore, C content is from 0.10 to 0.20%. C content is preferably at 0.12% or more is preferably not more than 0.18%.
[0029]
　Si: 0.05 ~
　1.0% Si has a deoxidizing effect, also improves the action of the strength and hardenability. To obtain these effects, Si content should be 0.05% or more. However, when the Si content exceeds 1.0%, toughness and weldability is decreased. Therefore, Si content is 0.05 to 1.0%. Si content is preferably at least 0.1%, is preferably 0.6% or less.
[0030]
　Mn: 0.05 ~
　1.2% Mn has a deoxidation, some improvement effect of strength and hardenability. To obtain these effects, it is necessary to include Mn 0.05% or more. However, if the Mn content exceeds 1.2%, toughness decreases. Therefore, Mn content is from 0.05 to 1.2%. Mn content is preferably 0.30% or more is preferably not more than 1.10%.
[0031]
　P: 0.025% or less
　When the P content exceeds 0.025%, it is difficult to secure a predetermined Charpy impact value decrease in toughness becomes remarkable. Therefore, the P content as an impurity is 0.025% or less. P content is preferably at 0.020% or less.
[0032]
　S: 0.005% or less
　When S content exceeds 0.005%, it is difficult to secure a predetermined Charpy impact value decrease in toughness becomes remarkable. Therefore, the S content as an impurity is 0.005% or less. S content is preferably 0.003% or less.
[0033]
　Cu: 0.20% or less
　Cu content can lead exceeds 0.20%, a deterioration of hot workability. Therefore, the Cu content as an impurity is 0.20% or less. Cu content is preferably 0.05% or less.
[0034]
　N: 0.007% or less
　When N content exceeds 0.007% or the coarse nitrides are formed, ensuring the solid solution B becomes difficult, particularly in seamless steel pipe thick, the B is hardenability improving effect and insufficient or not obtained sufficient hardened structure, because the decrease in toughness becomes remarkable, it is difficult to secure a predetermined Charpy impact value. Therefore, the N content as an impurity is 0.007% or less. N content is preferably 0.006% or less.
[0035]
　Ni: 0.20 ~
　0.50% Ni has the effect of improving the hardenability, strength and toughness. To obtain these effects, it is necessary to include Ni 0.20% or more. On the other hand, when the content exceeds 0.50% the Ni, increase alloy cost. Therefore, Ni content is from 0.20 to 0.50%. Ni content is preferably 0.30% or more is preferably 0.40% or less.
[0036]
　Cr: less than 0.30% to 0.50%
　Cr has the effect of improving the hardenability and strength. To obtain these effects, it is necessary to include Cr 0.30%. Meanwhile, in order to ensure good hardenability, together with from 0.0005 to 0.0020 percent of B to be described later, when the low alloy steel containing by combining Cr and Mo, Cr content is 0.50% or more When the, resulting in deterioration of toughness coarse boron carbon product during tempering is formed. Further, weld cracking is likely to occur becomes high Pcm (weld crack susceptibility composition) it is. Therefore, Cr content is less than 0.50% 0.30%. Cr content is preferably 0.40% or more. Further, it is preferable Cr content is less 0.47%, is preferably 0.45% or less.
[0037]
　Mo: 0.30 ~
　0.50% Mo has the effect of improving the hardenability and strength. To obtain these effects, it is necessary to include Mo 0.30%. Meanwhile, in order to ensure good hardenability, together with from 0.0005 to 0.0020 percent of B to be described later, when the low alloy steel containing in combination of Mo and Cr, Mo content is 0.50% exceeds the, resulting in deterioration of toughness coarse boron carbon product during tempering is formed. Further, weld cracking is likely to occur becomes high Pcm (weld crack susceptibility composition) it is. Therefore, Mo content is 0.30 to 0.50%. Mo content is preferably 0.40% or more is preferably not more than 0.45%.
[0038]
　Nb: 0.01 ~
　0.05% Nb combines with C and / or N to form fine precipitates, thereby suppressing the coarsening of austenite grains, has the effect of improving the toughness. To ensure by the effect of the stable, it is necessary to include Nb 0.01% or more. However, the inclusion of Nb in an amount exceeding 0.05%, the amount of precipitates increases, there is a case of rather deteriorate the toughness. Therefore, Nb content is 0.01 to 0.05%. Nb content is preferably 0.02% or more is preferably not more than 0.04%.
[0039]
　Al: 0.001 ~
　0.10% Al is an element having a deoxidizing action. To ensure this effect, it is necessary to include Al of 0.001% or more. On the other hand, in terms of when the content exceeds 0.10% of Al above effect is saturated, the more the occurrence of Chikizu. Therefore, Al content is 0.001 to 0.10%. Al content is preferably at 0.025% or more, preferably not more than 0.055%. Note that the Al content of the present invention refers to the content by acid-soluble Al (so-called "sol.Al").
[0040]
　B: 0.0005 ~ 0.0020%
　B is a seamless steel pipe thick that suppresses Pcm to a low value of 0.30 or less from the viewpoint of weldability, very important to be provided with a sufficient hardened structure a element, it is necessary to contain 0.0005% or more. However, when the B content exceeds 0.0020% and the content upper limit of Cr and Mo is a 0.50% both in the case of them comprising in combination, coarse boron carbon product during tempering it is formed, which may lead to a decrease in toughness. Therefore, B content is 0.0005 to 0.0020%. B content is preferably at least 0.0008%, preferably not more 0.0015% or less.
[0041]
　Ti: 0.003 ~
　0.050% Ti has the effect of precipitated as Ti carbides during tempering, thereby improving the strength. The Ti, to fix the N, also act to ensure the effective solid solution B to exert the hardenability improving effect of B. These effects, Ti content is obtained in 0.003% or more. However, if the content of Ti exceeds 0.050%, coarse Ti carbonitride in a high temperature region such as during solidification forms, and because the amount of precipitation of Ti carbides during tempering becomes excessive, toughness is reduced . Therefore, Ti content is 0.003 to 0.050%. Ti content is preferably 0.005% or more is preferably 0.015% or less.
[0042]
　Further, as described above, to secure the N, it is preferable to satisfy the Ti / N ≧ 48/14.
[0043]
　V: 0.01 ~ 0.20%
　V has the effect of precipitated as V carbides during tempering, thereby improving the strength. This effect, V content is obtained with 0.01% or more. However, when the V content exceeds 0.20%, the precipitation amount of V carbides during tempering becomes excessive, toughness is lowered. In addition, Pcm increases, weld cracking is likely to occur. Therefore, V content is 0.01 to 0.20%. Incidentally, it is preferable V content is 0.04% or more is preferably not more than 0.15%.
[0044]
　Ca, Mg and any one or more of the total REM: 0 ~
　0.025% Ca, Mg and REM improves the form of inclusions by either forming a sulfide reacts with S, toughness It has the effect of improving. Therefore, Ca, may contain any one or more of Mg and REM, if necessary. To get this effect stably, the content of these components is preferably total is 0.0005% or more. On the other hand, when the total content of these components exceeds 0.025%, the inclusions amount is cleanliness of the steel is lowered by increasing, rather toughness is lowered. Therefore, the upper limit of the total content of these elements is 0.025%. Preferably the total content is 0.01% or less, and more preferably 0.005% or less.
[0045]
　The term "REM" in the present invention refers to Sc, Y, and total 17 lanthanoid elements, the "content of REM", REM is the content in the case of one, in the case of two or more thereof It refers to the total content. Further, REM is generally also supplied as mischmetal which is an alloy of a plurality of types of REM. Therefore, it may be contained by adding one or more separate elements, for example, it may be added in the form of misch metal.
[0046]
　Seamless steel pipes and steel strips according to the present invention comprises each of the above-mentioned elements and the balance of Fe and impurities. Here, the "impurities", when the industrial production of steel materials, ores, raw material scraps, a component mixed by various factors of the manufacturing process, the allowable range of the present invention does not adversely affect It is the means something.
[0047]
　Pcm: 0.30
　according to the present invention seamless steel pipe and steel pieces, Pcm is 0.30 or less represented by the following [A] expression.
　Pcm = C + (Si / 30 ) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + 5B ··· [A ]
　However, the element symbol in the [a] type, means in the steel content of each element (mass%), if not contained and zero.
[0048]
　Note that each element of the right side of Pcm because an effect of each enhancing the strength of the steel pipe, Pcm is if too small may not necessary strength can not be obtained. The lower limit of practical Pcm to get a high strength of more than 980MPa tensile strength stably is considered to be about 0.22.
[0049]
　(B) metal structure
　seamless steel pipe according to the present invention, in order to achieve both high strength and high toughness, having a metal structure mainly composed of tempered martensite. Specifically, the area ratio of tempered martensite is 90% or more. No particular limitation is imposed on the rest of the tissues, bainite, it may include one or more selected from ferrite and pearlite.
[0050]
　In the present invention, the metal structure is measured by the following method. First, the seamless steel tube, so that the vertical cross section becomes an observation plane in the rolling direction, collecting the test piece for observation. Then, after polishing the observation surface, performing nital etching. Then, determine the area ratio of tempered martensite structure photograph taken at 500X magnification of the optical microscope.
[0051]
　(C) characteristics
　according to the present invention seamless steel tensile strength (hereinafter, referred to as "TS".) Is not less than 980 MPa. If TS is 980MPa or more, so can be performed stably lighter, as application to enable crane boom corresponding in size of the crane can be used stable enough. The preferred lower limit of the TS of the seamless steel pipe is 1000MPa. In addition, the preferred upper limit of the TS of the seamless steel pipe is 1100MPa. Incidentally, the yield stress of the seamless steel pipe according to the present invention (hereinafter, referred to as "YS".) Is preferably at least 890MPa, and more preferably at least 900 MPa.
[0052]
　Also, Charpy impact value at -40 ℃ of seamless steel pipe according to the present invention is 75 J / cm 2 is not less than. -40 Charpy impact value at ℃ is 75 J / cm 2 if more, even application to the crane boom work in cold climates is performed, it is possible to use stable enough. A preferred lower limit of the Charpy impact value at -40 ℃ of the seamless steel pipe is 125 J / cm 2 is, the higher preferred.
[0053]
　(D) thickness
　for the wall thickness of the seamless steel pipe according to the present invention, not provided particularly limited. However, the thickness is less than 10 mm, it may be impossible to ensure the strength required when used as a machine structural member. On the other hand, when the thickness exceeds 45 mm, bainite occurs easily, it becomes difficult to tissue tempered martensite mainly. Therefore, it is preferable thickness is 10 ~ 45 mm. Is preferably thick is 20mm or more, preferably 40mm or less.
[0054]
　(E) the production method
　seamless steel pipe according to the present invention can be produced by the following method.
[0055]
　Said steel having a chemical composition described in item (A), was melted in a manner similar to a general low alloy steel, and an ingot or slab by casting. Incidentally, by the so-called "round CC" method, it may be cast slab having a circular billet shape for pipe production.
[0056]
　As a next step, the cast ingot or slab is subjected to slabbing, or hot forging. The process, the final hot-manufactured tube (e.g., drilling in hot, the pipe producing by pipe producing or hot extrusion press, by rolling and stretching step) is a step of obtaining a material to be used for. Incidentally, by the "round CC" method, the slab is circular billet shape, it is possible to finish directly using it to seamless steel pipe, it is not always necessary to apply the slabbing or hot forging.
[0057]
　Prepared in slabbing or hot forging the final use in hot steel tube billet which is a raw material and a circular billet shape (hereinafter, "billet" hereinafter.) From the given below [i] the [vi] to step subjected sequentially, a seamless steel pipe of the present invention is produced.
[0058]
　[i]: After heating the billet to 1200 ~ 1300 ° C., to produce a mother tube by performing the processing 40 to 99% by reduction of area, the hot steel pipe process
　the above-described steel piece 1200 ~ 1300 ° C. after heating, to produce a mother tube having a predetermined shape by performing the processing 40 to 99% by reduction of area. When the heating temperature of the steel strip is below 1200 ° C., to the next cross-section reduction rate is increased deformation resistance pipe producing facilities undergo increased load at the time of processing 40 to 99% processing such as scratches or cracks which may cause failure. On the other hand, if the heating temperature of the steel strip exceeds 1300 ° C., which may lead to high temperatures grain boundary cracking or decrease in ductility. Therefore, the hot pipe making process, first, the heating temperature of the steel strip and the 1200 ~ 1300 ° C..
[0059]
　Also the heating temperature of the steel strip is in the range described above, the cross-sectional reduction rate in the hot steel pipe after heating is less than 40%, even after the cooling step described below [ii], quenching process of [iii] in not a fine hardened structure, it may not be possible to provided the desired mechanical properties to the seamless steel pipe. On the other hand, the pipe producing process of more than 99 percent reduction of area, there is a case where expansion or the like of the pipe producing equipment is required. Therefore, the hot pipe making process, and to perform the processing 40 to 99% by reduction of area.
[0060]
　Heating temperature in the step of the [i] refers to the temperature at the surface of the steel strip. Retention time in the above temperature range, depending on the size and shape of the steel strip preferably 60 to 300 minutes. Also, blank tube finishing temperature in the hot steel pipe is preferably set to 850 ~ 950 ° C.. Raw tube finishing temperature above refers to the temperature at the outer surface of the base pipe. In the step [i], a preferable lower limit of the heating temperature is 1230 ° C., also preferred upper limit is 1280 ° C.. Furthermore, 50% preferred lower limit of the reduction of area is also preferable upper limit is 90%.
[0061]
　[ii]: the mother tube Ac 1 is cooled to a temperature below point, cooling step
　mother pipe was finished into a predetermined shape, Ac in order to obtain a quenching process with a fine hardened structure of [iii] 1 below point It is cooled to the temperature. The cooling rate at this time is not particularly limited. Incidentally, after hot steel tube The raw tube, once after cooling to room temperature, reheated may be subjected to a process of the following [iii], after hot pipe making, Ac 1 below point after cooling to a suitable temperature, by heating directly from the temperature may be subjected to a process of the following [iii]. Cooling temperature in the step of the [ii] refers to the temperature at the outer surface of the base pipe.
[0062]
　[iii]: The cooled hollow shell Ac 3 after heating to point ~ 950 ° C., rapidly cooled, quenching step
　in the blank tube was cooled in the process of the [ii], then, Ac 3 temperature points ~ 950 ° C. quenching is subjected to a process for quenching after heating to. Heating temperature Ac 3 is less than point, because austenite is not completed, it may not be possible to include a predetermined mechanical properties seamless steel. On the other hand, if the heating temperature exceeds 950 ° C., in one quenching treatment may fine austenite grains can not be obtained, can not be provided with a predetermined mechanical properties seamless steel. Therefore, the heating temperature during the hardening process Ac 3 to point ~ 950 ° C..
[0063]
　Holding time at the above heating temperature, depending on the size of the mother tube preferably 5 to 30 minutes. Substantially uniform heating is possible, it may be a rapid thermal annealing for a short time using an induction heating. Heating temperature in the step of the [iii] refers to the temperature at the outer surface of the base pipe. The quenching is as long as sufficient quenching structure is obtained, may be used an appropriate method such as water cooling or oil cooling. In the step [iii], the preferred lower limit of the heating temperature is 880 ° C., also preferred upper limit is 920 ° C..
[0064]
　[iv]: After heating the quenched hollow shell to 500 ~ 600 ° C., cooled to room temperature, the tempering step
　in the hardened hollow shell in the process of the [iii], predetermined mechanical properties as seamless steel pipe to provided, after heating to 500 ~ 600 ° C., cooled to room temperature, the tempering process is performed. In the case of the chemical composition described in the item (A), when the heating temperature of the tempering is below 500 ° C., a predetermined strength (TS) is lowered low temperature toughness can be secured, Charpy at -40 ℃ impact value 75 J / cm 2 may be below. On the other hand, if the heating temperature of the tempering is above 600 ° C., (Charpy impact value at -40 ° C.) predetermined low temperature toughness to decrease obtained even strength, that TS can not ensure a high strength of more than 980MPa is there. Therefore, the heating temperature in the tempering process is to 500 ~ 600 ° C..
[0065]
　Holding time at the above heating temperature, depending on the size of the mother tube preferably 30 to 60 minutes. Heating temperature in the step of the [iv] refers to the temperature at the outer surface of the base pipe. The cooling rate during tempering, there is no particular limitation. For this reason, allowed to cool in the air, straightening air cooling, mist cooling, oil cooling, such as water cooling, cooling it to perform in accordance with the equipment. In the step [iv], the preferred lower limit of the heating temperature is 525 ° C., also preferred upper limit is 575 ° C..
[0066]
　Hereinafter, examples, the present invention will be described more specifically, the present invention is not limited to these examples.
Example
[0067]
　(Example 1)
　Steel A ~ K having the chemical compositions shown in Table 1 were melted using a 100kg vacuum melting furnace, to obtain an ingot cast in a mold, then, each ingot was hot-forged, thickness 50 mm, width 120 mm, was processed into a block shape of length 190 mm, and cooled to room temperature. Thus each block obtained by, after heating 30 minutes at 1250 ° C., to simulate the production of seamless steel pipe, as shown in Table 2, the width as reduction of area is 40% or 60% finishing temperature to restrain the is cooled to room temperature after hot rolling to be in the range of 850 ~ 950 ° C., to obtain a sheet having a thickness of 20mm or 30 mm.
[0068]
　Steel A ~ D in Table 1 are steels chemical composition is within the range defined in the present invention, on the other hand, the steel E ~ K is a steel chemical composition out of the range regulated by the present invention. In Table 1, the formula (1) and Ac obtained from equation (2) below 1 point and Ac 3 were shown together points.
　Ac 1 point (° C.) = 723 + 29.1 × Si-10.7 × Mn-16.9 × Ni + 16.9 × Cr · · ·
　(1) Ac 3 point (° C.) = 910-203 × C 0.5 +44 .7 × Si-15.2 × Ni + 31.5 × Mo + 104 × V- (30 × Mn + 11 × Cr + 20 × Cu-700 × P-400 × Al-400 × Ti) ··· (2)
[0069]
[Table 1]

[0070]
　Was subjected to a quenching and tempering under the conditions shown in Table 2 in plate thickness 20mm or 30mm obtained as described above was performed to investigate the following. Incidentally, the quenching was performed all immersed in stirred water bath. All cooling at the time of tempering was allowed to cool in the air.
[0071]
　First, from each plate (Test Nos. 1 to 26), so that the vertical cross section becomes an observation plane in the rolling direction, a test piece for observation were taken, after polishing the observation surface was subjected to nital etching. Then, determine the area ratio of tempered martensite structure photograph taken at 500X magnification of the optical microscope.
[0072]
　An example of a structure photograph in FIG. 1 and 2. Figure 1 is a structural photograph of the test No. 1 area ratio was less than 95% more than 90% tempered martensite, 2, the area ratio of tempered martensite test No. 3 was less than 90% is an organization photograph.
[0073]
　Then, in parallel to the rolling longitudinally from a thickness center of each plate, cut out 10 No. Tensile specimens according to Annex D of JIS Z 2241-2011, and a tensile test at room temperature in air to determine the YS and TS. Further, quenching - parallel from a thickness center portion of the tempered sheet in the rolling width direction, cut out 2mmV notch full size test piece width 10 mm, subjected to Charpy impact test at -40 ° C., to investigate the absorption energy shock to determine the value.
[0074]
　Table 2 also shows the respective findings of the above.
[0075]
[Table 2]

[0076]
　As shown in Table 2, using the steel A-D having the chemical composition defined in the present invention, the test numbers 1, 4, 5, 7 and 9 of the present invention example was prepared by the method defined in the present invention, 11 and 14-16, the TS more than 980 MPa, YS is also clear that also excellent in low temperature toughness and has a high strength of more than 890MPa, also since Pcm low as 0.30 or less, excellent weldability also easily conceived comprise a.
[0077]
　In contrast, in the case of Test No. Comparative Example, or not at least a predetermined mechanical characteristics can be obtained, inferior in weldability.
[0078]
　That is, as shown in Test No. 2,3,6,10,12,13 and 17, even using steel A ~ D having the chemical composition defined in the present invention, the manufacturing conditions defined in the present invention when departing from the condition, TS has not reached the low 980 MPa.
[0079]
　On the other hand, if the chemical composition of the steel used is outside the conditions specified in the present invention, as shown in Test Nos. 18-26, the manufacturing conditions satisfy the requirements of the present invention, regardless not satisfied, at least a predetermined if the mechanical properties are not obtained, inferior in weldability for Pcm is high.
[0080]
　(Example 2)
　was Steels L having the chemical compositions shown in Table 3, the converter - by continuous casting process to cast a rectangular billet. Rectangular billet further round billet outer diameter 191mm by hot forging, and formed into a circular billet of circular billet and the outer diameter 310mm outer diameter 225 mm, and cooled to room temperature.
[0081]
[table 3]

[0082]
　Each round billet of the above, and heated at 1240 ° C., Mannesmann - the mandrel method, so that the finishing temperature is in the range of 850 ~ 950 ° C., to produce various thickness of a seamless steel pipe shown in Table 4, at room temperature It was cooled to. Each seamless steel pipes thus obtained, under the conditions shown in Table 4, is subjected to quenching and tempering, and producing a product steel pipe. Incidentally, all quenching was performed by water quenching. All cooling at the time of tempering was allowed to cool in the air.
[0083]
　Then, for each product steel tube (Test No. 27-38) was determined area ratio of tempered martensite in the same manner as in Example 1. Figure 3 is a structural photograph of the test No. 31 area ratio of tempered martensite was 95% or more.
[0084]
　Then, for each product steel tube, longitudinal one end position or end positions (T end tip side rolling direction, the distal and B end) from No. 12 in Annex E of JIS Z 2241-2011 cut out test pieces, by a tensile test in air at room temperature to determine the YS and TS. Furthermore, for each product steel tube, parallel from the longitudinal one end position or end positions in the rolling longitudinal direction, 2 mm V notch full size test piece width 10 mm (if the product wall thickness of 20mm or 38mm) or width 3. 2mmV notch test piece of 3mm is cut out (if the product thickness is 5.74mm) by each triplet, conducted Charpy impact test at -40 ° C., investigate the average absorbed energy of each three, this the impact value was determined using.
[0085]
　Table 4 also shows the respective findings of the above.
[0086]
[Table 4]

[0087]
　From Table 4, using the steel L having the chemical composition defined in the present invention, the steel pipe of the test numbers 27-38 of the present invention example was prepared by the method defined in the present invention, in any dimension, TS is 980MPa or more, YS is also clear that also excellent in low temperature toughness and has a high strength of more than 890MPa, also since Pcm is 0.30 or less, can be easily envisioned that with excellent weldability.
Industrial Applicability
[0088]
　Seamless steel pipe of the present invention is excellent in low temperature toughness along with the tensile strength has a high strength of at least 980 MPa, and since Pcm and a small 0.30, for machine structural members, is suitable among others for the crane boom . Further, the production method of the present invention, it is possible to obtain the seamless steel pipe at a low cost.

The scope of the claims
[Requested item 1]
　Chemical composition, in
　mass%,
　C: 0.10 ~
　0.20%, Si: 0.05 ~ 1.0%, Mn: 0.05
　~ 1.2%, P: 0.025% or
　less, S 0.005% or
　less, Cu: 0.20% or
　less, N: 0.007% or
　less, Ni: 0.20
　~ 0.50%, Cr: less than 0.30% or more 0.50%,
　Mo: 0
　~
　0.50%
.30,
　B: 0.0005 ~ 0.0020%, Ti: 0.003 ~ 0.050
　%, V: 0.01 ~
　0.20%, Ca, any one or more of the total of Mg and REM: 0 ~ 0.025%,
　the balance is Fe and impurities,
　represented by the following [a] formula the value of Pcm is 0.30 or less,
　metal structure, in area%,
　tempered martensite: 90% or more,
　tensile strength of 9 80MPa or
　more, Charpy impact value at -40 ℃ with 2mmV notch test piece 75 J / cm 2 is not less than,
　the seamless steel pipe.
　Pcm = C + (Si / 30 ) + (Mn / 20) + (Cu / 20) + (Ni / 60) + (Cr / 20) + (Mo / 15) + (V / 10) + 5B ··· [A ]
　However, the element symbol in the [a] type, means in the steel content of each element (mass%), if not contained and zero.
[Requested item 2]
　A method of manufacturing a seamless steel pipe according to claim 1,
　using a steel slab having a chemical composition according to claim 1, performed in the order of steps up to [iv] the following [i],
　the seam method of manufacturing a steel pipe.
　[i]: After heating the billet to 1200 ~ 1300 ° C., to produce a mother tube by performing the processing 40 to 99% by reduction of area, the hot steel pipe process
　[ii]: the mother tube Ac 1 cooled to a temperature below the point, the cooling step
　[iii]: the cooled hollow shell Ac 3 after heating to point ~ 950 ° C., rapidly cooled, quenching step
　[iv]: heating the quenched hollow shell to 500 ~ 600 ° C. after cooled to room temperature, tempering process