RU2706257C1 - Seamless steel pipe and method of its production - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, namely to seamless steel pipe used as main pipe for oil and gas transportation. Pipe has the following chemical composition, wt%: C: from 0.02 to 0.15, Si: from 0.05 to 0.5, Mn: from 0.30 to 2.5, P: up to 0.03, S: up to 0.006, O: up to 0.004, Al: 0.01 to 0.10, Ti: 0.001 to 0.010, N: up to 0.007, Cr: 0.05 to 1.0, Mo: not less than 0.02 and less than 0.5, Ni: from 0.03 to 1.0, Cu: from 0.02 to 1.0, V: from 0.020 to 0.20, Ca: from 0.0005 to 0.005, Nb: from 0 to 0.05, the rest is iron and impurities. Main phase of microstructure from surface layer to middle in wall thickness is martensite of tempering or bainite tempering. Size of pre-urethane grains in microstructure according to scale of crystal grains ASTM E112-10 is less than 6.0. Part located between position spaced 1 mm from the inner surface and position spaced 1 mm from the outer surface has a Vickers hardness of 250 Hv or lower and a carbon equivalent of Ceq of not less than 0.430 % and less than 0.500 %.

EFFECT: pipe has yield point of not less than 555 MPa, as well as high resistance to sulphide cracking under stress.

6 cl, 9 dwg, 4 tbl

Description

DESCRIPTION OF THE INVENTION

2420-549421EN / 71

SEAMLESS STEEL PIPE AND METHOD FOR PRODUCING IT

FIELD OF THE INVENTION

[0001] The present invention relates to a seamless steel pipe and to a method for its production, and more particularly to a seamless steel pipe suitable for a main pipe and to a method for its production.

BACKGROUND

[0002] The oil and gas resources of wells located on land and in shallow seas are ending, and it is therefore necessary to develop an increasing number of deposits in the deep seas. In an offshore oil field, oil or gas must be transported from a crane head of an oil or gas well installed on the seabed to a platform above the sea using a production pipeline or riser. A production pipeline is a main pipe laid along the topography of the surface of the earth or the seabed. The riser is a main pipe running from the seabed to the platform (i.e. up).

[0003] The inner side of a steel pipe, which is part of a deep-seated fishing pipeline, is exposed to high internal fluid pressure from deep layers, and when work stops, the main effect is the pressure of sea water at great depths. The steel pipe, which is part of the riser, is additionally influenced by repeated deformations caused by ocean waves. Accordingly, it is desirable that the steel pipes used in such applications have high strength and high toughness. In addition to this, oil and gas wells are developed in an acidic environment that is more aggressive than conditions for conventional wells, such as deep sea depths and low temperatures. Offshore pipelines laid in such aggressive acidic environments are required to have higher strength (i.e. pressure resistance) and toughness compared to conventional pipelines, and in addition to this are required to have resistance to hydrogen corrosion (resistance to HIC) and sulfide resistance stress cracking (resistance to SSC).

[0004] Patent Document 1 discloses a seamless steel pipe with a large wall thickness for a main pipe having high strength and good viscosity, containing C: from 0.03 wt.% To 0.08 wt.%, Si: 0.15 wt. % or less, Mn: from 0.3 wt.% to 2.5 wt.%, Al: from 0.001 wt.% to 0.10 wt.%, Cr: from 0.02 wt.% to 1.0 wt.%, Ni: from 0.02 wt.% to 1.0 wt.%, Mo: from 0.02 wt.% to 1.2 wt.%, Ti: from 0.004 wt.% to 0.010 wt.% , N: from 0.002 wt.% To 0.008 wt.%, And one or more of Ca, Mg and REM: from 0.0002 wt.% To 0.005 wt.%, With the remainder being Fe and impurities, the impurities being in itself P: 0.05 wt.% or less, S: 0.005 wt.% or less e, and having a wall thickness of 30 to 50 mm.

[0005] Patent document 2 discloses a high-strength seamless steel pipe with a large wall thickness, which is produced by quenching and tempering and has a yield strength of more than 450 MPa, for a main pipe with good resistance to an acidic medium, in which the Vickers hardness number HV5, measured in the outermost or innermost position of the pipe with an applied load of 5 kgf (with a force of 49 N) is 250 HV5 or less.

[0006] Patent Document 3 discloses a seamless steel pipe for a main pipe containing C: from 0.02 wt.% To 0.10 wt.%, Si: 0.5 wt.% Or less, Mn: from 0.5 wt.% up to 2.0 wt.%, Al: from 0.01 wt.% to 0.1 wt.%, Ca: 0.005 wt.% or less, and N: 0.007 wt.% or less, and also one or more elements selected from the group consisting of Ti: 0.008 wt.% or less, V: less than 0.06 wt.% and Nb: 0.05 wt.% or less, with the remainder of Fe and impurities, where the content of Ti, V and Nb is less than 0.06 wt.%, the carbon equivalent of Ceq defined by the following equation is 0.38% or more, and the size p carbonitride particles containing one or more elements of Ti, V, Nb and Al are 200 nm or less.

Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15

[0007] Patent Document 4 discloses a seamless steel pipe with a chemical composition C: from 0.02 wt.% To 0.10 wt.%, Si: from 0.05 wt.% To 0.5 wt.%, Mn: 1.0 wt.% to 2.0 wt.%, Mo: 0.5 wt.% to 1.0 wt.%, Cr: 0.1 wt.% to 1.0 wt.%, Al : from 0.01 wt.% to 0.10 wt.%; P: 0.03 wt.% Or less, S: 0.005 wt.% Or less, Ca: from 0.0005 wt.% To 0.005 wt.%, V: from 0.010 wt.% To 0.040 wt.%, And N : from 0.002 wt.% to 0.007 wt.%, as well as one or more elements selected from the group consisting of Ti: from 0.001 wt.% to 0.008 wt.%, and Nb: from 0.02 to 0.05 wt. %, with a residue of Fe and impurities, where the carbon equivalent of Ceq is from 0.50 to 0.58%, containing the specified carbide.

BACKGROUND OF THE INVENTION

PATENT DOCUMENTS

[0008] [Patent Document 1] JP 2010-242222 A

[Patent Document 2] JP 2013-32584 A

[Patent document 3] WO 2011/152240

[Patent Document 4] JP 5516831 B

SUMMARY OF THE INVENTION

[0009] Even when one or more of the above conventional techniques are used, a seamless steel pipe having a strength of X80 or higher in accordance with American Petroleum Institute (API) standards (that is, having a lower yield strength of 555 MPa or higher) may not have good resistance to sulfide stress cracking in a reliable manner.

[0010] In order to improve the strength and toughness of a seamless steel pipe produced by quenching and tempering, the content of alloy elements such as carbon can be increased to increase hardenability. However, if the content of alloy elements such as carbon increases, the strength (i.e. hardness) of the surface of the steel pipe increases. In a seamless steel pipe produced by quenching and tempering, the surface layer cools at high speed during quenching, and can be easily hardened by increasing hardness, while the metal in the wall will have low hardness. This trend may continue after the holidays. In fact, in a seamless steel pipe having a strength of API X80 or higher, the hardness of the surface layer may exceed 250 Hv, which is the highest limit required for acid resistance in accordance with API 5L standards.

[0011] Although the methods of Patent Document 1 are effective in achieving high strength and high toughness, they do not sufficiently consider reducing the hardness of the surface layer, and thus improving the resistance to sulfide stress cracking. Patent Document 2 indicates that the hardness of the surface layer of a steel pipe can be adjusted to 250 HV5 or lower; however, apparently, this requires a special production process. Patent Document 3 contains some considerations regarding resistance to sulfide stress cracking; however, after hot forming, it is necessary to perform direct quenching or continuous quenching, and then reheating and quenching. Patent Document 4 contains some considerations regarding hardness of the surface layer of a steel pipe and resistance to HIC; however, a stage of reheating and hardening is necessary, and also, after hot molding, direct hardening or continuous hardening is used as necessary, which means unnecessary production costs.

[0012] An object of the present invention is to provide a seamless steel pipe that can be produced by a relatively acceptable manufacturing process and provide a yield strength of 555 MPa or higher, as well as good resistance to sulfide stress cracking in a reliable manner.

[0013] A seamless steel pipe in one embodiment of the present invention has a chemical composition, in wt.%, C: from 0.02 wt.% To 0.15 wt.%; Si: from 0.05 wt.% To 0.5 wt.%; Mn: 0.30 wt.% To 2.5 wt.%; P: up to 0.03 wt.%; S: up to 0.006 wt.%; O: up to 0.004 wt.%; Al: from 0.01 wt.% To 0.10 wt.%; Ti: from 0.001 wt.% To 0.010 wt.%; N: up to 0.007 wt.%; Cr: from 0.05 wt.% To 1.0 wt.%; Mo: not less than 0.02 wt.% And less than 0.5 wt.%; Ni: from 0.03 wt.% To 1.0 wt.%; Cu: from 0.02 wt.% To 1.0 wt.%; V: from 0.020 wt.% To 0.20 wt.%; Ca: from 0.0005 wt.% To 0.005 wt.%; and Nb: from 0 to 0.05 wt.%, with a residue of Fe and impurities, where the carbon equivalent Ceq defined by equation (1) below is not less than 0.430% and less than 0.500%, the main phase of the microstructure from the surface layer to the inner part of the wall is tempering martensite or tempered bainite, the size of pre-austenitic grains in the microstructure is less than 6.0 in accordance with the ASTM E112-10 standard crystal grain size scale, the part between the position 1 mm apart from the inner surface and the position distant m is 1 mm from the outer surface has a Vickers hardness of 250 Hv or less, and a yield strength 555 MPa or higher,

Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ... (1),

where the symbol of each element in equation (1) means the content of this element in wt.%.

[0014] A method of manufacturing a seamless steel pipe in one embodiment of the present invention includes: preparing a raw material having a chemical composition, in wt.%, C: from 0.02 wt.% To 0.15 wt.%; Si: from 0.05 wt.% To 0.5 wt.%; Mn: 0.30 wt.% To 2.5 wt.%; P: up to 0.03 wt.%; S: up to 0.006 wt.%; O: up to 0.004 wt.%; Al: from 0.01 wt.% To 0.10 wt.%; Ti: from 0.001 wt.% To 0.010 wt.%; N: up to 0.007 wt.%; Cr: from 0.05 wt.% To 1.0 wt.%; Mo: not less than 0.02 wt.% And less than 0.5 wt.%; Ni: from 0.03 wt.% To 1.0 wt.%; Cu: from 0.02 wt.% To 1.0 wt.%; V: from 0.020 wt.% To 0.20 wt.%; Ca: from 0.0005 wt.% To 0.005 wt.%; and Nb: from 0 to 0.05 wt.%, with the remainder of Fe and impurities; hot processing of this raw material in order to produce a hollow sleeve; hardening of this hollow sleeve by direct hardening or continuous hardening; and leave this hardened hollow sleeve. Between quenching and tempering, no reheating and quenching is performed. The carbon equivalent Ceq defined by equation (3) below is not less than 0.430% and less than 0.500%, the Larson-Miller parameter PL defined by equation (4) below is not less than 18800,

Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ... (3), and

PL = (T + 273) × (20 + log (t)) ... (4).

The symbol of each element in equation (3) means the content of this element in wt.%. In equation (4), T is the tempering temperature, and t is the holding time at this temperature. T is measured in ° C, and t is measured in hours.

[0015] The present invention provides a seamless steel pipe that can be produced by a relatively acceptable manufacturing process and provides a yield strength of 555 MPa or higher, as well as good resistance to sulfide stress cracking in a reliable manner.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] [FIG. 1] FIG. 1 is a block diagram illustrating one example of a production line.

[FIG. 2] FIG. 2 is a flowchart illustrating a process for manufacturing a seamless steel pipe.

[FIG. 3] FIG. 3 shows the change in surface temperature of the workpiece during production as a function of time.

[FIG. 4] FIG. 4 is a graph illustrating the relationship between the Larson-Miller parameter PL and the yield strength YS for steel B.

[FIG. 5] FIG. 5 is a graph illustrating the relationship between the Larson-Miller parameter PL and the yield strength YS for steel A.

[FIG. 6] FIG. 6 is a graph illustrating the relationship between the Larson-Miller parameter PL and the hardness on the outer surface, inside the wall, and on the inner surface for steel B.

[FIG. 7] FIG. 7 is a graph illustrating the relationship between the Larson-Miller parameter PL and the hardness on the outer surface, inside the wall, and on the inner surface for steel A.

[FIG. 8] FIG. 8 is a graph illustrating the relationship between the Larson-Miller parameter PL and the maximum difference in hardness for steel B.

[FIG. 9] FIG. 9 is a graph illustrating the relationship between the Larson-Miller parameter PL and the maximum difference in hardness for steel A.

MODES FOR CARRYING OUT THE INVENTION

[0017] The authors of the present invention performed a study to find a method for producing a seamless steel pipe that provides a yield strength of 555 MPa or higher, as well as good resistance to sulfide stress cracking in a reliable manner. It has been found that limiting the carbon equivalent of steel to a suitable range and reducing the difference between the hardness of the surface layer and the hardness inside the wall of a seamless steel pipe provides a yield strength of 555 MPa or higher and good resistance to sulfide stress cracking in a reliable manner when only direct hardening or continuous hardening is performed after hot forming, and no reheating and quenching is performed.

[0018] During quenching after rolling, the surface layer of the seamless steel pipe cools at high speed and can be easily hardened. In fact, the surface layer of a seamless steel pipe tends to become more durable and can exceed hardness values defined by API 5L standards or DNV-OS-F101 standards. On the other hand, parts located in the middle of the wall thickness of the seamless steel pipe are cooled at a lower speed and cannot be easily hardened, so that they can include non-hardened structures such as ferrite. Thus, there is usually a difference between the hardness of the surface layer and the hardness of the internal parts in the wall, and this trend may persist after tempering under certain tempering conditions. In addition, in a seamless steel pipe with a high carbon equivalent, such as that used in high-strength steel of grade X80 or higher, the difference between the hardness of the surface layer and the hardness of the internal parts in the wall tends to be significant. Such a high hardness of the surface layer can be a problem when good acid resistance is to be achieved reliably.

[0019] If the carbon equivalent is too low, it is difficult to guarantee a certain strength of the seamless steel pipe. If the carbon equivalent is too high, it becomes difficult to reduce the Vickers hardness of the surface layer to 250 Hv or lower using a manufacturing process that does not contain reheating and quenching, in which direct quenching or continuous quenching is the only quenching step. The reason for this is that if quenching after hot forming is direct quenching or continuous quenching, austenitic grains tend to become coarse compared to realizations that reheat and quench, which increase the overall hardenability. With this in mind, the value of Ceq defined by the following equation (1) should be at least 0.430% and less than 0.500%:

Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ... (1),

where the symbol of each element in equation (1) means the content of this element in wt.%.

[0020] In order to reduce the difference between the hardness of the surface layer and the hardness of the internal parts in the wall, it is effective to limit the carbon equivalent as well as the tempering conditions. Thus, if tempering is not performed sufficiently, the decrease in hardness of the surface layer is insufficient, so that some parts may have a Vickers hardness higher than 250 Hv. More specifically, the Larson-Miller parameter PL defined by equation (2) below is 18800 or higher.

PL = (T + 273) × (20 + log (t)) ... (2).

In equation (2), T is the tempering temperature (in ° C), and t is the holding time (in hours) at this temperature.

[0021] The present invention has been made based on the above findings. A seamless steel pipe in one embodiment of the present invention will now be described in detail with reference to the drawings. The same or corresponding parts in the drawings are marked with the same reference characters, and their description will not be repeated.

[0022] [Chemical composition]

The seamless steel pipe in this embodiment has the chemical composition described below. In the following description, “%” for the content of the element means wt.%.

[0023] C: from 0.02 wt.% To 0.15 wt.%

Carbon (C) increases the strength of steel. If the C content is lower than 0.02 wt.%, This effect cannot be sufficiently achieved. If the C content is higher than 0.15 wt.%, The toughness of the steel is reduced. With this in mind, the content of C should be in the range from 0.02 wt.% To 0.15 wt.%. The content of C is preferably higher than 0.02 wt.%, And more preferably 0.04 wt.% Or higher. The content of C is preferably 0.10 wt.% Or lower, and more preferably 0.08 wt.% Or lower.

[0024] Si: from 0.05 wt.% To 0.5 wt.%

Silicon (Si) deoxidizes steel. This effect can be reliably achieved if the Si content is 0.05 wt.% Or higher. However, if the Si content is higher than 0.5 wt.%, The toughness of the steel is reduced. With this in mind, the Si content should be in the range from 0.05 wt.% To 0.50 wt.%. The Si content is preferably higher than 0.05 wt.%, More preferably 0.08 wt.% Or higher, and even more preferably 0.10 wt.% Or higher. The Si content is preferably less than 0.5 wt.%, More preferably 0.25 wt.% Or lower, and even more preferably 0.20 wt.% Or lower.

[0025] Mn: from 0.30 wt.% To 2.5 wt.%

Manganese (Mn) increases the hardenability of steel to increase the strength of steel. These effects cannot be sufficiently achieved if the Mn content is less than 0.30 wt.%. If the Mn content is higher than 2.5 wt.%, Mn segregates in the steel, reducing its toughness. With this in mind, the Mn content should be in the range from 0.30 wt.% To 2.5 wt.%. The Mn content is preferably higher than 0.30 wt.%, More preferably 1.0 wt.% Or higher, and even more preferably 1.3 wt.% Or higher. The Mn content is preferably less than 2.5 wt.%, More preferably 2.0 wt.% Or lower, and even more preferably 1.8 wt.% Or lower.

[0026] P: up to 0.03 wt.%

Phosphorus (P) is an impurity. P reduces the toughness of steel. Thus, lower P contents are preferred. With this in mind, the content of P should be 0.03 wt.% Or lower. The content of P is preferably less than 0.03 wt.%, More preferably 0.015 wt.% Or lower, and even more preferably 0.012 wt.% Or lower.

[0027] S: up to 0.006 wt.%

Sulfur (S) is an impurity. Sulfur combines with Mn to form coarse MnS particles, and thus reduces the toughness of steel and its resistance to HIC. Thus, lower sulfur contents are preferred. With this in mind, the sulfur content should be 0.006 wt.% Or lower. The sulfur content is preferably less than 0.006 wt.%, More preferably 0.003 wt.% Or lower, and even more preferably 0.002 wt.% Or lower.

[0028] O: up to 0.004 wt.%

Oxygen (O) is an impurity. Oxygen forms coarse oxide particles or clusters of oxide particles, thereby reducing the toughness of steel. Thus, lower O contents are preferred. With this in mind, the O content should be 0.004 wt.% Or lower. The content of O is preferably 0.003 wt.% Or lower, and more preferably 0.002 wt.% Or lower.

[0029] Al: from 0.01 wt.% To 0.10 wt.%

Aluminum (Al) combines with N to form small nitride particles, increasing the toughness of steel. This effect cannot be sufficiently achieved if the Al content is less than 0.01 wt.%. If the Al content is higher than 0.10 wt.%, Coarse Al nitride particles are formed which reduce the toughness of the steel. With this in mind, the aluminum content should be in the range from 0.01 wt.% To 0.10 wt.%. The aluminum content is preferably higher than 0.01 wt.%, And more preferably 0.02 wt.% Or higher. The aluminum content is preferably less than 0.10 wt.%, More preferably 0.08 wt.% Or lower, and even more preferably 0.06 wt.% Or lower. As used herein, the term “Al content” means “acid-soluble Al content” (i.e., so-called “soluble Al”).

[0030] Ti: from 0.001 wt.% To 0.010 wt.%

Titanium (Ti) combines with N in steel and forms TiN, suppressing a decrease in the toughness of steel due to dissolved N. In addition, dispersed and released small particles of TiN increase the toughness of steel. These effects cannot be sufficiently achieved if the Ti content is less than 0.001 wt.%. If the Ti content is higher than 0.010 wt.%, Coarse TiN particles or coarse TiC particles are formed, reducing the toughness of the steel. With this in mind, the titanium content should be in the range from 0.001 wt.% To 0.010 wt.%. The titanium content is preferably higher than 0.001 wt.%, And more preferably 0.002 wt.% Or higher. The titanium content is preferably less than 0.010 wt.%, More preferably 0.006 wt.% Or lower, and even more preferably 0.005 wt.% Or lower.

[0031] N: up to 0.007 wt.%

Nitrogen (N) combines with Al and forms small particles of Al nitride, increasing the toughness of steel. However, if the N content is higher than 0.007 wt.%, Dissolved N reduces the toughness of the steel. In addition, if the N content is too high, coarse particles of carbonitride and / or nitride are formed, reducing the toughness of the steel. With this in mind, the nitrogen content should be 0.007 wt.% Or lower. The nitrogen content is preferably less than 0.007 wt.%, More preferably 0.006 wt.% Or lower, and even more preferably 0.005 wt.% Or lower. The nitrogen content is preferably 0.002 wt.% Or higher.

[0032] Cr: from 0.05 wt.% To 1.0 wt.%

Chrome (Cr) increases the hardenability of steel, and also increases the strength of steel. Cr further increases the softening resistance of steel during tempering. These effects cannot be sufficiently achieved if the chromium content is less than 0.05 wt.%. If the chromium content is higher than 1.0 wt.%, The toughness of the steel is reduced. With this in mind, the chromium content should be in the range from 0.05 wt.% To 1.0 wt.%. The chromium content is preferably higher than 0.05 wt.%, And more preferably 0.2 wt.% Or higher. The Cr content is preferably lower than 1.0 wt.%, And more preferably 0.8 wt.% Or lower.

[0033] Mo: not less than 0.02 wt.% And less than 0.5 wt.%

Molybdenum (Mo) improves the strength of steel due to transformational teasing and hardening of the solid solution. This effect cannot be sufficiently achieved if the Mo content is less than 0.02 wt.%. If the Mo content is higher than 0.5 wt.%, The toughness of the steel is reduced. With this in mind, the Mo content should be no less than 0.02 wt.% And lower than 0.5 wt.%. The Mo content is preferably higher than 0.02 wt.%, More preferably 0.05 wt.% Or higher, and even more preferably 0.1 wt.% Or higher. The Mo content is preferably 0.4 wt.% Or lower, and more preferably 0.3 wt.% Or lower.

[0034] Ni: from 0.03 wt.% To 1.0 wt.%

Nickel (Ni) increases the hardenability of steel, and also increases the strength of steel. In addition, Ni has the effect of improving the adhesion of the scale formed on the surface of the steel during the quenching heating step, as well as the effect of reducing the increase in hardness of the surface layer of steel, since the scale reduces the cooling rate on the steel surface during the quenching cooling step. These effects cannot be sufficiently achieved if the nickel content is less than 0.03 wt.%. If the Ni content is higher than 1.0 wt.%, The resistance of the steel to sulfide stress cracking is reduced. With this in mind, the nickel content should be in the range from 0.03 wt.% To 1.0 wt.%. The Ni content is preferably 0.05 mass% or higher, more preferably 0.08 mass% or higher, and even more preferably 0.10 mass% or higher. The nickel content is preferably less than 1.0 wt.%, More preferably 0.7 wt.% Or lower, and even more preferably 0.5 wt.% Or lower.

[0035] Cu: from 0.02 wt.% To 1.0 wt.%

Copper (Cu) increases the hardenability of steel, and also increases the strength of steel. In addition, copper has the effect of improving the adhesion of the scale formed on the surface of the steel during the heating step for hardening, as well as the effect of reducing the increase in hardness of the surface layer of steel, since the scale reduces the cooling rate on the surface of the steel during the cooling step during hardening. These effects cannot be sufficiently achieved if the copper content is less than 0.02 wt.%. If the copper content is higher than 1.0 wt.%, The weldability of the steel is reduced. In addition, if the Cu content is too high, the grain boundary strength of the steel at high temperatures decreases, reducing the hot workability of the steel. With this in mind, the copper content should be in the range from 0.02 wt.% To 1.0 wt.%. The copper content is preferably 0.05 mass% or higher, more preferably 0.08 mass% or higher, and even more preferably 0.10 mass% or higher. The copper content is preferably less than 1.0 wt.%, More preferably 0.7 wt.% Or lower, and even more preferably 0.5 wt.% Or lower.

[0036] V: from 0.020 wt.% To 0.20 wt.%

Vanadium (V) binds to C in steel and forms V carbide, increasing the strength of the steel. In addition, V dissolves in Mo carbide, forming carbide. Carbide containing V is less likely to form coarse particles. These effects cannot be effectively achieved if the V content is less than 0.020 wt.%. If the V content is higher than 0.20 wt.%, Coarse carbide particles are formed. With this in mind, the content of vanadium should be in the range from 0.020 wt.% To 0.20 wt.%. The vanadium content is preferably higher than 0.020 wt.%, And more preferably 0.04 wt.% Or higher. The vanadium content is preferably lower than 0.16 wt.%.

[0037] Ca: from 0.0005 wt.% To 0.005 wt.%

Calcium (Ca) binds to sulfur in steel to form CaS. As CaS is formed, the formation of MnS is suppressed. Thus, Ca increases the strength and resistance of steel to HIC. These effects cannot be sufficiently achieved if the Ca content is less than 0.0005 wt.%. If the Ca content exceeds 0.005 wt.%, The purity of the steel is reduced, reducing the viscosity and resistance of the steel to HIC. Thus, the Ca content should be in the range from 0.0005 to 0.005 wt.%. The Ca content is preferably higher than 0.0005 wt.%, More preferably 0.0008 wt.% Or higher, and even more preferably 0.001 wt.% Or higher. The calcium content is preferably less than 0.005 wt.%, More preferably 0.003 wt.% Or lower, and even more preferably 0.002 wt.% Or lower.

[0038] The remainder of the chemical composition of the seamless steel pipe in this embodiment consists of Fe and impurities. An impurity in this context means an element originating from ore or scrap used as a raw material for steel, or an element that enters steel from the environment and the like. during the manufacturing process.

[0039] In addition, the chemical composition of the seamless steel pipe in this embodiment may contain Nb instead of a certain amount of Fe.

[0040] Nb: from 0 wt.% To 0.05 wt.%

Niobium (Nb) is an additional element. Nb binds to C and / or N in steel and forms small particles of Nb carbide and / or carbonitride, increasing the toughness of the steel. In addition, Nb dissolves in Mo carbide and forms a given carbide, thereby preventing the formation of coarse particles of a given carbide. On the other hand, if the Nb content is higher than 0.05 wt.%, Coarse carbide particles are formed. With this in mind, the niobium content should be in the range from 0 wt.% To 0.05 wt.%. The above effects can be reliably achieved if the Nb content is 0.010 wt.% Or higher. The Nb content is preferably 0.015 mass% or higher, and more preferably 0.020 mass% or higher. The niobium content is preferably 0.040 wt.% Or lower, and more preferably 0.035 wt.% Or lower.

[0041] (Carbon equivalent of Ceq)

In the seamless steel pipe of this embodiment, the carbon equivalent of Ceq defined by equation (1) is not less than 0.430% and less than 0.500%.

Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ... (1),

where the symbol of each element in equation (1) means the content of this element in wt.%.

[0042] If the carbon equivalent of Ceq is lower than 0.430%, it is difficult to guarantee the specified strength of the seamless steel pipe. If the carbon equivalent of Ceq is 0.500 or higher, it becomes difficult to reduce the Vickers hardness of the surface layer to 250 Hv or lower using a manufacturing process in which hardening after hot forming is the only direct hardening or continuous hardening step.

[0043] [Microstructure]

In the microstructure of a seamless steel pipe in this embodiment, the main phase from the surface layer to the parts within the wall is tempering martensite or tempered bainite. The seamless steel pipe in this embodiment does not contain recrystallized ferrite in at least a region deeper than 1 mm from the surface. Recrystallized ferrite extremely reduces the hardness of the surface layer with a thickness of 1 mm of seamless steel pipe.

[0044] The main phase, which is tempering martensite or temper bainite, generally means a microstructure in which the volume fraction of tempering martensite is 50% or higher, a microstructure in which the volume fraction of tempering bainite is 50% or higher, or a microstructure in which the sum the volume fraction of tempering martensite and the volume fraction of tempering bainite is 50% or higher. In other words, the aforementioned phase means a microstructure in which the volume fraction of a structure that is neither tempering martensite nor tempering bainite (e.g. ferrite) is lower than 50%.

[0045] [Crystal grain size number]

In the microstructure of the seamless steel pipe of this embodiment, the pre-austenitic grain size is lower than 6.0 on the ASTM E112-10 standard crystal grain size scale.

[0046] The pre-austenitic grain size number can be measured in accordance with ASTM E112-10 by cutting a test sample from each steel pipe, preferably before tempering and after quenching, so that a cross section perpendicular to the length of the steel pipe (ie, to the direction of pipe formation) formed the observed surface, and pouring this sample into the resin, and then using the Béchet-Bozhar method of etching with a saturated aqueous solution of picric acid to reveal the boundaries of pre-austenitic grains.

[0047] Alternatively, the ASTM number of the size of the pre-austenitic crystalline grain in the tempered steel pipe can be determined using methods such as electron backscattering diffraction (EBSD) based on the crystal orientation ratio. In such cases, the microstructure of the metal of the steel pipe after tempering is observed using EBSD as follows: the sample is obtained from the middle of the wall thickness in the cross section of the tempered seamless steel pipe (that is, in a cross section perpendicular to the axial direction of the seamless steel pipe); the resulting sample is used to perform a crystal orientation analysis using EBSD for an observed area of 500 × 500 μm, and lines are drawn where the boundary of the pre-austenite grain is defined as the grain boundary with a misorientation angle in the range of 15-51 °, and based on the resulting drawing, the size number crystalline grains are calculated in accordance with ASTM E112-10.

[0048] Theoretically, the size of the pre-austenitic grain after quenching and before tempering is the same as the size of the pre-austenitic grain after tempering. The pre-austenitic grain size determined by EBSD after tempering is essentially equal to the value obtained by observing the crystal grains developed by the Béchet-Bozhar method after quenching and before tempering, with an error of approximately ± 0.2 in the grain size number. Thus, the phrase “pre-austenitic grain size used in the present invention is lower than 6.0 according to the ASTM E112-10 standard crystal grain size scale” means that if the crystalline grain size after quenching is unknown, at least the crystalline grain size number, determined by EBSD after tempering not exceeding 5.8 is within the scope of the present invention. In the following description, unless explicitly stated otherwise, the size of the pre-austenitic grain is the value obtained by the Béchet-Bozhar method for the sample after quenching, but before tempering.

[0049] If the pre-austenitic grains are small grains with a crystal grain size number of 6.0 or greater, sufficient hardenability cannot be achieved in a material with a low carbon equivalent Ceq, as in this embodiment. Thus, a predetermined strength cannot be obtained. In addition, it is difficult to produce a microstructure with such fine grains using a manufacturing process in which quenching after hot forming is the only step in direct quenching or continuous quenching. The crystal grain size number of the pre-austenitic grains is preferably 5.5 or lower, and more preferably 5.0 or lower.

[0050] [Vickers hardness and yield strength]

In the seamless steel pipe in this embodiment, the part between the position 1 mm apart from the inner surface and the position 1 mm apart from the outer surface has a Vickers hardness of 250 Hv or lower. More specifically, in a seamless steel pipe in this embodiment, the Vickers hardness measured in accordance with JIS Z 2244 in any position between a position 1 mm apart from the inner surface and a position 1 mm apart from the outer surface is 250 Hv or lower.

[0051] The seamless steel pipe of the present invention has smaller variations in hardness along the wall thickness direction. More specifically, the difference between the Vickers hardness of the part located at a depth of 1 mm from the inner surface and the Vickers hardness of the part located in the middle of the wall thickness, the difference between the Vickers hardness of the part located 1 mm from the outer surface and the hardness The Vickers hardness of the part located in the middle of the wall thickness and the difference between the Vickers hardness of the part located at a depth of 1 mm from the inner surface and the Vickers hardness of the part located at a depth of 1 mm from the outer surface, composition yayut each Hv 25 or less.

[0052] The seamless steel pipe in this embodiment has a yield strength corresponding to grade X80 or higher (that is, 555 MPa or higher) in accordance with API standards.

[0053] A seamless steel pipe in this embodiment can be suitably used, although not limited to, like a seamless steel pipe with a wall thickness of 25-55 mm. More preferably, to rationalize the use of alloys, the wall thickness of the seamless steel pipe is in the range of 25-40 mm.

[0054] [Production Method]

Next, one example of a method for manufacturing a seamless steel pipe in this embodiment will be described. However, the method of manufacturing a seamless steel pipe in this embodiment is not limited to this.

[0055] [Production Line]

FIG. 1 is a block diagram illustrating one example of a production line. In FIG. 1, the production line includes a heating furnace 1, a piercing mill 2, an elongator mill 3, a calibration rolling mill 4, an additional heating furnace 5, a water cooling device 6, and a tempering device 7. Many transport rollers 10 are located between these devices.

[0056] [Production Process]

FIG. 2 is a flowchart illustrating a process for manufacturing a seamless steel pipe in this embodiment. FIG. Figure 3 shows the surface temperature changes of the workpiece (i.e., steel raw material, a hollow sleeve or seamless steel pipe) during manufacture as a function of time. In this graph, A1 means point Ac ₁ when considering a heated workpiece, and means point Ar ₁ when considering a cooled workpiece. In this graph, A3 means the Ac ₃ point when considering the heated workpiece, and means the Ar ₃ point when considering the cooled workpiece.

[0057] As shown in FIG. 1-3, the manufacturing process includes first heating the steel feed using a heating furnace 1 (heating step S1). Steel raw materials may be, for example, a round billet. Steel raw materials can be produced using a continuous casting system, such as a round ingot continuous casting system. Steel raw materials can be produced using hot processing (such as forging or blooming) of an ingot or slab. Next will be described the case of steel raw materials, which is a round billet.

[0058] The hot round billet is hot worked to produce a seamless steel pipe (steps S2 and S3). More specifically, the round billet is rolled on piercing mill 2 in order to produce a hollow sleeve (piercing rolling step S2). Then, the hollow sleeve is rolled on an elongator mill 3 and a calibration rolling mill 4 in order to produce a seamless steel pipe (an extension rolling step and a calibration rolling step S3).

[0059] A seamless steel pipe produced by hot working is heated to a predetermined temperature using an additional heating furnace 5 as necessary (additional heating step S4). The hot rolled seamless steel pipe or the heated seamless steel pipe is tempered using a water cooling device 6 (quenching step S5). In any case, the seamless steel pipe obtained by hot working is quenched without cooling to a temperature below Ar3. The hardened seamless steel pipe is released using the tempering device 7 (tempering step S6).

[0060] Thus, in the above production method, hardening is performed immediately after the hot treatment is completed. More specifically, after hot working, hardening is performed before the seamless steel pipe is left to cool to about room temperature. Heat treatment in which the seamless steel pipe after hot treatment is rapidly cooled before the surface temperature becomes lower than the Ar ₃ point is hereinafter referred to as “direct quenching”, and heat treatment in which the seamless steel pipe after additional heat treatment is additionally heated at a temperature not lower than the Ac ₃ point, and then quickly cooled, hereinafter referred to as “continuous hardening”. The use of direct hardening or continuous hardening makes the microstructure grains more coarse than during heat treatment, in which the pipe is cooled after its production and then rapidly cooled (hereinafter referred to as reheating and hardening). More specifically, the crystalline grain size number after quenching is less than 6.0. This improves the hardenability of the microstructure compared to reheating and quenching, and thus guarantees high strength even when using steel material with a low carbon equivalent Ceq.

[0061] These steps will be described in more detail below.

[0062] [Heating step (S1)]

The round billet is heated in the heating furnace 1. The heating temperature is preferably in the range from 1100 to 1300 ° C. Heating a round billet to this temperature range leads to the dissolution of carbonitride in steel. If the round billet is to be made from a slab or ingot of metal by hot working, it is only necessary that the slab or ingot be heated to a temperature of 1100-1300 ° C, and the temperature to which the round billet is heated by the heating furnace 1 should not be in the range 1100-1300 ° C, because the carbonitride in steel dissolves when the ingot or slab is heated. The heating furnace 1 may be, for example, a walking beam hearth furnace or a rotary furnace.

[0063] [Firmware stage (S2)]

The round billet is removed from the heating furnace 1, and the hot round billet is pierced by rolling on the piercing mill 2 in order to produce a hollow sleeve. Flashing mill 2 includes a lot of skewed rolls and firmware. The firmware is located between the skewed rolls. Preferably, piercing mill 2 is a transverse piercing mill. A cross-type piercing mill is preferred because it can do piercing with a high level of pipe expansion.

[0064] [Extension rolling step and calibration rolling step (S3)]

Then, the hollow sleeve is rolled. More specifically, the hollow sleeve is rolled on an elongator mill 3. The elongator mill 3 includes a plurality of stands arranged in series. The elongator mill 3 may be, for example, a mandrel rolling mill. After that, the hollow sleeve, which has been subjected to extension rolling, is crimped on a calibration rolling mill 4 in order to produce a seamless steel pipe. The calibration rolling mill 4 includes a plurality of stands arranged in series. The calibration rolling mill 4 may be, for example, a calibration pipe rolling mill or a pressure reducing mill. The extension rolling step and the calibration rolling step together may be referred to simply as the rolling step.

[0065] [Stage of additional heating (S4)]

The additional heating step (S4) is performed as necessary. In other words, the production method in this embodiment is not required to include an additional heating step (S4). More specifically, the additional heating step (S4) is performed in such a way that the temperature of the seamless steel pipe is at a predetermined level that is equal to or higher than the Ac3 point, immediately before the water cooling of the quenching stage (S5). If the additional heating step (S4) is not performed, the method shown in FIG. 2, from step S3 proceeds immediately to step S5. If the additional heating step (S4) is not performed, the additional heating furnace 5 shown in FIG. 1 may be excluded.

[0066] If the finish temperature of the rolling step (that is, the surface temperature of the seamless steel pipe immediately after the completion of the rolling step) is lower than 800 ° C, it is preferable to perform an additional heating step (S4). In the additional heating step (S4), the seamless steel pipe is placed in the additional heating furnace 5 and heated. The heating temperature in the additional heating furnace 5 is preferably in the range of 900-1100 ° C. If the holding time is too long, carbonitrides consisting of Ti, Nb, C and N, i.e. (Ti, Nb) and (C, N), can be released and form coarse particles. In an additional heating step, the additional heating furnace 5 may be replaced by an induction heating device.

[0067] [The stage of hardening (S5)]

A seamless steel pipe is cooled by water in a water cooling device 6. The temperature (i.e. surface temperature) of the seamless steel pipe immediately before water cooling is equal to or higher than the Ac ₃ point, and preferably equal to or higher than 800 ° C.

[0068] For water cooling, it is preferable that the cooling rate for the temperature range of the seamless steel pipe from 800 ° C to 500 ° C be equal to or higher than 5 ° C / s (300 ° C / min). This provides a uniform, hardened microstructure. Cooling stops at a temperature that is equal to or lower than the point Ar ₁ . The temperature at which cooling is stopped is preferably 450 ° C. or lower, and cooling can be performed up to room temperature. The quenching step (S5) changes the structure of the matrix, making it mainly composed of martensite or bainite.

[0069] For example, the water cooling aperture 6 used for the hardening step (S5) may have the following structure: the water cooling device 6 includes a plurality of rotating rollers, a laminar water flow device, and a water jet flow device. The rotating rollers are arranged in two rows, and a seamless steel pipe is located between the two rows of rotating rollers. In this case, the rotating rollers in these two rows are in contact with the lower parts of the outer surface of the seamless steel pipe. When the rotating rollers rotate, a seamless steel pipe rotates around its axis. The laminar water flow device is located above the rotating rollers and pours water on top of a seamless steel pipe. In this case, water poured onto a seamless steel pipe forms a laminar flow of water. The jet water flow device is located near the end of a seamless steel pipe mounted on rotating rollers. The jet water stream apparatus emits a jet stream of water from the end of a seamless steel pipe inside this steel pipe. These devices simultaneously cool the external and internal surfaces of a seamless steel pipe. A water cooling device 6 of this design is suitable for accelerated cooling of a seamless steel pipe with a large wall thickness of 25 mm or more.

[0070] The water cooling device 6 may be a device different from the device discussed above, including rotating rollers, a laminar water flow device, and a water jet stream device. The water cooling device 6 may be, for example, a water tank. With such an implementation, a seamless steel pipe is immersed in a tank of water and thus is subjected to accelerated cooling. Alternatively, water cooling device 6 may include only a laminar water flow device. To summarize, the cooling device 6 is not limited to any particular type.

[0071] [Vacation Stage (S6)]

Hardened seamless steel pipe is exposed to tempering. More specifically, the hardened seamless steel pipe is heated to a predetermined tempering temperature, which is lower than the point Ac ₁ , and maintained at this temperature for a predetermined period of time so that the value of the Larson-Miller parameter PL defined by the following equation (2) was 18800 or higher:

PL = (T + 273) × (20 + log (t)) ... (2).

In equation (2), T is the tempering temperature (in ° C), and t is the holding time (in hours) at this temperature. Log (t) is the base 10 logarithm of t.

[0072] If the PL value is lower than 18800, the decrease in surface hardness becomes insufficient, and some parts may have a Vickers hardness in excess of 250 Hv. The value of PL is preferably 18900 or higher.

[0073] If the PL value is too high, ferrite recrystallization occurs in an area spaced 1 mm or deeper from the surface, which can cause a significant decrease in strength, a decrease in acid resistance in the surface layer and bubble formation. The value of PL is preferably 20,000 or lower, and more preferably 19500 or lower.

[0074] The lower limit of the tempering temperature is preferably 600 ° C, more preferably 630 ° C and even more preferably 650 ° C. The upper limit of the tempering temperature is preferably 700 ° C. and more preferably 680 ° C. The lower limit of the exposure time is preferably one hour, more preferably two hours, and even more preferably three hours. The upper limit of the exposure time is preferably six hours, more preferably five hours, and even more preferably four hours.

[0075] The above production process provides a seamless steel pipe with a wall thickness of 25 mm or more, having good strength, toughness and resistance to HIC. The production method described above is particularly suitable for a seamless steel pipe with a wall thickness of 25 mm or more, and can even be used to produce a seamless steel pipe with a wall thickness of 40 mm or more. The upper limit of the wall thickness is not limited to a specific value, but is usually 60 mm or less.

[0076] Seamless steel pipe in one embodiment of the present invention and a method for its production have been described above. This embodiment provides a seamless steel pipe that can be produced by a relatively acceptable manufacturing process and provides a yield strength of 555 MPa or higher, as well as good resistance to sulfide stress cracking in a reliable manner.

Examples

[0077] The present invention will now be described using specific examples. The present invention is not limited to these examples.

[0078] A plurality of seamless steel pipes with various chemical compositions were produced, and their yield strength, tensile strength, surface hardness, and acid resistance were investigated.

[0079] [Research Methods]

Many steels having the chemical compositions shown in Table 1 were smelted and subjected to continuous casting in order to obtain round billets for forming a pipe. Steels A, C, D1, D2 and J in Table 1 are steels in which the chemical composition or Ceq value does not meet the requirements of the present invention.

[0080] [Table 1]

Steel Chemical composition (in wt.%, With the remainder of iron and impurities) Ceq C Si Mn P S Cu Cr Ni Mo Ti V Nb Al Ca O N A 0.059 0.12 1,53 0.005 0,0007 0.20 0.28 0.23 0.10 0.003 0.05 - 0,031 0,0008 0.0015 0.0036 0.429 B 0,061 0.11 1.51 0.008 0,0009 0.20 0.31 0.31 0.25 0.003 0.05 - 0,032 0.0017 0.0018 0.0050 0.468 C 0,070 0.09 1.42 0.011 0,0005 0.41 0.31 0.39 0.35 0.005 0.05 - 0,030 0.0013 0.0017 0.0048 0.502 D1 0,066 0.12 1.46 0.009 0.0010 0.02 0.23 0.08 0.09 0.010 0.05 - 0,037 0.0017 0.0018 0.0039 0.390 D2 0,065 0.12 1.44 0.009 0.0010 0.08 0.26 0.09 0.06 0.007 0.05 - 0,041 0.0016 0.0016 0.0041 0.390 E 0,068 0.11 1.51 0.009 0.0019 0.37 0.28 0.49 0.25 0.004 0.05 - 0,030 0.0012 0.0012 0.0032 0.493 F 0,061 0.11 1.51 0.010 0.0010 0.20 0.20 0.28 0.25 0.004 0.05 - 0,030 0.0014 0.0015 0.0043 0.445 G 0,060 0.12 1,52 0.009 0.0010 0.21 0.21 0.28 0.25 0.005 0.05 0,020 0,034 0.0012 0.0011 0.0050 0.448 I 0,062 0.12 1,52 0.005 0,0008 0.21 0.27 0.23 0.11 0.006 0.05 0,025 0,031 0,0008 0.0015 0.0036 0.431 J 0,061 0.11 1.42 0.011 0.0018 0.36 0.28 0.49 0.51 0.005 0.04 - 0,030 0.0013 0.0013 0.0032 0.520 K 0.058 0.12 1,50 0.008 0.0010 0.20 0.31 0.32 0.26 0.003 0.05 - 0,033 0.0016 0.0017 0.0055 0.467

[0081] The obtained round billets were heated in a heating furnace to a temperature in the range from 1100 to 1300 ° C. After that, these round billets were pierced in a piercing mill to produce hollow shells. After that, the mandrel rolling mill was used for the extension rolling of hollow shells. After that, the calibration tube mill was used to calibrate the hollow shells to produce seamless steel pipes having the outer diameters and wall thicknesses shown in Tables 2 and 3.

[0082] [Table 2]

No. Steel Pipe forming conditions Pre-austenite grain size number after quenching Holiday Terms Mechanical properties Pre-austenitic grain size number after quenching and tempering Ferrite Recrystallization Notes External diameter Wall thickness Languishing temperature Exposure time PL Ys TS Hv 10 kgf (maximum among positions) (mm) (mm) (° C) (min) (MPa) (MPa) Outer surface In the wall Inner surface Difference one A 273.1 25.0 4.3 660 204 19156 518 592 202 198 208 10 4.2 missing Comparative example 2 A 273.1 25.0 4.3 660 219 19185 501 577 212 200 213 thirteen 4.3 missing Comparative example 3 A 273.1 25.0 4,5 665 234 19314 509 580 197 195 162 35 4.4 is present Comparative example 4 A 273.1 25.0 4,5 650 204 18951 524 602 203 203 220 17 4,5 missing Comparative example 5 A 273.1 25.0 4.3 650 219 18979 519 593 202 196 214 eighteen 4.6 missing Comparative example 6 A 273.1 25.0 4.3 650 234 19006 511 585 197 201 215 eighteen 4.1 missing Comparative example 7 A 273.1 25.0 4.3 650 249 19030 506 585 202 200 219 nineteen 4.6 missing Comparative example 8 A 273.1 25.0 4.3 650 264 19054 514 588 200 201 219 nineteen 4,5 missing Comparative example nine A 273.1 25.0 4.3 650 294 19097 497 573 199 194 198 5 4.4 missing Comparative example 10 A 273.1 25.0 4.3 650 205 18953 544 619 218 218 220 2 4.3 missing Comparative example eleven A 273.1 25.0 4.3 630 204 18540 543 622 213 212 248 36 4.3 missing Comparative example 12 A 273.1 25.0 4.3 630 219 18568 541 620 209 210 236 27 4.4 missing Comparative example thirteen A 273.1 25.0 4.3 630 234 18594 531 610 213 208 242 34 4.3 missing Comparative example 14 A 273.1 25.0 4,5 630 249 18618 531 610 206 202 240 38 4.4 missing Comparative example fifteen A 273.1 25.0 4,5 630 264 18641 536 615 211 209 239 thirty 4,5 missing Comparative example sixteen A 273.1 25.0 4.3 630 294 18683 526 602 210 203 238 35 4.3 missing Comparative example 17 A 273.1 25.0 4.3 600 204 17924 531 622 210 213 257 47 4.4 missing Comparative example eighteen B 323.9 25.0 4.3 700 294 20132 503 582 209 193 170 39 4,5 is present Comparative example nineteen B 323.9 25.0 4.3 700 204 19977 575 641 192 193 209 17 4.3 missing An example of the present invention twenty B 323.9 25.0 4.3 690 204 19772 578 646 203 206 211 8 4.4 missing An example of the present invention 21 B 323.9 25.0 4,5 680 204 19566 579 646 212 220 222 10 4,5 missing An example of the present invention 22 B 323.9 25.0 4.3 670 204 19361 597 659 236 220 239 nineteen 4.2 missing An example of the present invention 23 B 323.9 25.0 4.3 665 149 19131 621 688 238 240 239 2 4.3 missing An example of the present invention 24 B 323.9 25.0 4.3 660 204 19156 606 670 225 226 239 14 4.4 missing An example of the present invention 25 B 323.9 25.0 4.3 660 219 19185 601 665 224 230 239 fifteen 4.3 missing An example of the present invention 26 B 323.9 25.0 4.3 665 234 19314 600 664 233 226 235 nine 4.3 missing An example of the present invention 27 B 323.9 25.0 4,5 650 204 18951 631 697 248 244 249 5 4,5 missing An example of the present invention 28 B 323.9 25.0 4,5 650 219 18979 620 683 235 235 248 thirteen 4.7 missing An example of the present invention 29th B 323.9 25.0 4,5 650 234 19006 620 681 235 235 248 thirteen 4,5 missing An example of the present invention thirty B 323.9 25.0 4.3 650 249 19030 617 683 242 226 248 22 4.3 missing An example of the present invention

[0083] [Table 3]

No. Steel Pipe forming conditions Pre-austenite grain size number after quenching Holiday Terms Mechanical properties Pre-austenitic grain size number after quenching and tempering Ferrite Recrystallization Notes External diameter Wall thickness Languishing temperature Exposure time PL Ys TS Hv 10 kgf (maximum among positions) (mm) (mm) (° C) (min) (MPa) (MPa) Outer surface In the wall Inner surface Difference 31 B 323.9 25.0 4,5 650 264 19054 617 679 236 232 247 fifteen 4.7 missing An example of the present invention 32 B 323.9 25.0 4,5 650 294 19097 612 674 227 226 237 eleven 4.3 missing An example of the present invention 33 B 323.9 25.0 4.3 650 315 19125 619 683 236 234 237 3 4.6 missing An example of the present invention 34 B 323.9 25.0 4.3 630 204 18540 649 720 241 241 269 28 4.4 missing Comparative example 35 B 323.9 25.0 4,5 630 219 18568 644 715 251 242 268 26 4.3 missing Comparative example 36 B 323.9 25.0 4.6 630 234 18594 654 725 256 240 266 26 4.7 missing Comparative example 37 B 323.9 25.0 4.3 630 249 18618 627 698 229 244 268 39 4.3 missing Comparative example 38 B 323.9 25.0 4.3 630 264 18641 624 693 228 240 266 38 4.3 missing Comparative example 39 B 323.9 25.0 4.6 630 294 18683 628 697 241 234 260 26 4,5 missing Comparative example 40 B 323.9 25.0 4.3 600 204 17924 639 728 260 253 286 33 4.7 missing Comparative example 41 B 323.9 25.0 4,5 655 105 18786 635 708 245 232 263 31 4.2 missing Comparative example 42 B 323.9 25.0 4,5 650 105 18684 636 707 251 233 264 31 4,5 missing Comparative example 43 C 323.9 25.0 4.6 650 105 18684 636 714 248 244 282 38 4.6 missing Comparative example 44 C 323.9 25.0 4.6 650 185 18911 573 657 232 228 269 41 4,5 missing Comparative example 45 D1 323.9 25,4 4,5 656 117 18849 478 561 178 178 205 27 4,5 missing Comparative example 46 D2 323.9 25,4 4.3 650 130 18770 468 582 179 191 207 28 4.2 missing Comparative example 47 E 406.4 38.1 4.3 600 204 17924 618 701 267 228 222 45 4.1 missing Comparative example 48 E 406.4 38.1 4.6 630 204 18540 603 673 255 220 227 35 4,5 missing Comparative example 49 E 406.4 38.1 4,5 630 234 18594 609 679 254 217 219 37 4.4 missing Comparative example fifty E 406.4 38.1 4.3 630 264 18641 609 674 252 214 228 38 4.2 missing Comparative example 51 E 406.4 38.1 4.3 630 294 18683 609 675 251 223 219 32 4.4 missing Comparative example 52 E 406.4 38.1 4.3 650 204 18951 599 665 237 222 214 23 4.3 missing An example of the present invention 53 E 406.4 38.1 4,5 650 234 19006 575 644 231 212 211 twenty 4,5 missing An example of the present invention 54 E 406.4 38.1 4,5 650 264 19054 575 641 230 209 208 22 4.6 missing An example of the present invention 55 E 406.4 38.1 4,5 650 294 19097 565 641 226 204 208 22 4.4 missing An example of the present invention 56 E 406.4 38.1 4.6 660 204 19156 565 635 228 205 206 23 4.6 missing An example of the present invention 57 E 406.4 38.1 4.3 660 234 19211 559 628 220 198 200 22 4.4 missing An example of the present invention 58 F 323.9 25.0 4,5 650 200 18942 570 643 212 219 224 12 4.7 missing An example of the present invention 59 G 323.9 25.0 5,4 650 200 18942 597 665 236 223 238 fifteen 5.7 missing An example of the present invention 60 I 273.1 25.0 5.5 650 200 18942 575 647 200 203 211 eleven 5.5 missing An example of the present invention 61 J 323.9 25.0 4.7 665 149 19131 680 734 261 252 283 31 4.7 missing Comparative example 62 * K 323.9 25.0 6.8 665 234 19314 543 625 236 208 228 28 6.9 missing Comparative example

* 62: continuous quenching at 950 ° C + reheating at 950 ° C and quenching + tempering

[0084] The seamless steel pipes that were calibrated rolled were heated in an additional heating furnace to 950 ° C, after which quenching was performed using a water cooling device in which the pipes were cooled to room temperature with a cooling rate of 5 ° C / s or higher.

[0085] After hardening, the seamless steel pipes were tempered at the languishing temperatures and holding times shown in Tables 2 and 3. However, during the production of steel No. 62, after the aforementioned hardening was completed, hardening in which the steel was performed before tempering was re-separately heated to 950 ° C and aged for 20 minutes, and then cooled with water.

[0086] The following evaluation tests were carried out on seamless steel pipes produced in the aforementioned manufacturing process.

[0087] [Tests for yield strength and tensile]

The yield strength of each seamless steel pipe was investigated. More specifically, sample No. 12 (with a width of 25 mm and a length of 50 mm) in accordance with JIS Z 2241 was taken so that the longitudinal direction of the sample was parallel to the longitudinal direction of the steel pipe (i.e., direction L). This sample was used for tensile testing in accordance with JIS Z 2241 in an atmosphere at room temperature (25 ° C), and the yield strength (YS) and tensile strength (TS) were determined. The yield strength was determined using the method of full elongation of 0.5%. The defined yield strength (in MPa) and tensile strength (in MPa) are shown in Tables 2 and 3. The YS columns in Tables 2 and 3 show the yield strength, and the TS columns show tensile strength for samples with different numbers.

[0088] [Test of surface hardness]

Four samples were taken from each seamless steel pipe, offset 90 ° from each other along the pipe circumference, and the Vickers hardness test in accordance with JIS Z 2244 was carried out at three arbitrary points on the cross section (i.e., the cross section perpendicular to the central axis) of each sample spaced 1 mm in the direction of the wall thickness from the inner surface. Strength F in Vickers hardness tests was 10 kgf (i.e., 98.07 N). The maximum among the values for the 12 points that were obtained was used as the value of hardness "at a distance of 1 mm from the inner surface".

[0089] Similarly, the Vickers hardness test was carried out at arbitrary three points of each of the four samples of each seamless steel pipe, spaced 1 mm apart in the direction of the wall thickness from the outer surface, and the maximum among the values of 12 points that were obtained was used as the value hardness "at a distance of 1 mm from the outer surface." In addition, the Vickers hardness test was carried out at arbitrary three points of each of the four samples of each seamless steel pipe, located near the middle in the wall thickness, and the maximum among the values of 12 points that were obtained was used as the “inside wall” hardness value.

[0090] For each seamless steel pipe, the hardness values “at a distance of 1 mm from the outer surface”, “at a distance of 1 mm from the inner surface” and “inside the wall” are shown in Tables 2 and 3 in the columns labeled “Outer surface”, “B wall ”and“ Inner surface ”.

[0091] The largest value is between the difference between hardness “at a distance of 1 mm from the outer surface” and hardness “inside the wall”, the difference between the hardness “at a distance of 1 mm from the inner surface” and the hardness “inside the wall”, and the difference between the hardness “ at a distance of 1 mm from the outer surface "and hardness" at a distance of 1 mm from the inner surface "(hereinafter referred to as the" maximum difference in hardness ") is shown in the column" Difference "in Tables 2 and 3.

[0092] [Observation of the microstructure]

A sample containing an inner surface, an outer surface, and a mid-wall thickness was taken from each seamless steel pipe, and the microstructure of these samples was observed. More specifically, each sample was etched with nital to reveal the microstructure, which was then observed using an optical microscope.

[0093] Each seamless steel pipe had a microstructure, the main phase of which was tempering martensite or bainite tempering. However, in some seamless steel pipes in a region spaced 1 mm or deeper from the surface, ferrite recrystallized. The presence of such a fact of recrystallization of ferrite in the region spaced 1 mm or deeper from the surface is shown in the column labeled “Recrystallization of ferrite” in Tables 2 and 3.

[0094] The crystal grain size number of pre-austenitic grains in the microstructure was measured using the following method: first, a sample was cut from each steel pipe so that a cross section perpendicular to the length of the hardened steel pipe (that is, to the pipe forming direction) forms an observable surface, and poured with resin; then, the Béchet-Bozhar method was used, that is, etching with a saturated aqueous solution of picric acid to reveal the boundaries of pre-austenitic grain, which was then observed using an optical microscope (with a magnification of 200x), and the size number of pre-austenitic grain was measured in accordance with ASTM E112-10. Such grain size numbers are shown in the column “Pre-austenitic grain size number after quenching” in Tables 2 and 3.

[0095] Since the size number of pre-austenitic grains after tempering cannot be measured using etching with an aqueous solution of picric acid, it was measured using EBSD. EBSD analysis was performed by cutting the sample so that a cross section perpendicular to the length of the tempered steel pipe formed an observable surface and a mirror polished observable surface followed by electrolytic polishing, after which a region of 500 × 500 μm in the middle of the thickness of the steel pipe was observed. A detector was used for EBSD mounted on a FE-SEM (DigiViewIV manufactured by EDAX). The obtained crystal orientation data were processed using analytical software (OIM Analysis version 6 manufactured by EDAX) to draw lines along the boundaries between the crystalline grains with misorientation angles of 15-51 °, and the obtained lines were used to measure the size of the pre-austenite grain in according to ASTM E112-10. Such grain size numbers are shown in the column “Pre-austenitic grain size number after quenching and tempering” in Tables 2 and 3.

[0096] [Study Results]

As shown in Tables 1-3, seamless steel pipes No. 19-33 and 52-60 had a chemical composition corresponding to the present invention, and had a Ceq carbon equivalent of not lower than 0.430% and lower than 0.500%. In these seamless steel pipes, ferrite did not recrystallize in a region 1 mm or deeper from the surface, a structure having tempering martensite or tempered bainite as the main phase was present from the surface layer to parts inside the wall, and the size number of pre-austenitic grains was lower than 6.0. In addition, these seamless steel pipes had Vickers hardness values “at a distance of 1 mm from the outer surface”, “at a distance of 1 mm from the inner surface” and “inside the wall”, which did not exceed 250 Hv, and had a yield strength of 555 MPa or higher. These seamless steel pipes had a maximum hardness difference of 25 Hv or lower.

[0097] Seamless steel pipes No. 1-17 had a yield strength lower than 555 MPa. The reason for this, apparently, is that the carbon equivalent Ceq of steel A was too low.

[0098] In seamless steel pipe No. 18, recrystallization of ferrite occurred in an area 1 mm or deeper from the surface. Therefore, seamless steel pipe No. 18 had a yield strength lower than 555 MPa. The reason for this, apparently, is that the Larson-Miller parameter PL of seamless steel pipe No. 18 was too high.

[0099] Seamless steel pipes No. 34-42 and 47-51 had a Vickers hardness value “at a distance of 1 mm from the outer surface”, “at a distance of 1 mm from the inner surface” or “inside the wall” above 250 Hv. In addition, seamless steel pipes had a maximum hardness difference greater than 25 Hv. The reason for this, apparently, is that the values of the Larson-Miller parameter PL of seamless steel pipes No. 34-42 and 47-51 were too low.

[0100] Seamless steel pipes No. 43 and 44 had a Vickers hardness "at a distance of 1 mm from the inner surface" higher than 250 Hv. The reason for this, apparently, is that the carbon equivalent Ceq of steel C was too high.

[0101] Seamless steel pipes No. 45 and 46 had yield strengths lower than 555 MPa. The reason for this, apparently, is that the carbon equivalents Ceq of steels D1 and D2 were too low.

[0102] In No. 61 seamless steel pipe, the Vickers hardness was higher than 250 Hv at all measurement points. The reason for this, apparently, is that the carbon equivalent Ceq of steel J was too high.

[0103] Seamless steel pipe No. 62 had a yield strength lower than 555 MPa. The reason for this, apparently, is that both continuous hardening and reheating and hardening were used, which led to too small pre-austenitic grains, reducing hardenability and thus leading to insufficient strength.

[0104] FIG. 4 is a graph showing the relationship between the Larson-Miller parameter PL and the yield strength YS for steel B. As shown in FIG. 4, the yield strength YS decreases with increasing value of the Larson-Miller parameter PL. Steel B provided a yield strength of 555 MPa or more, with the exception of seamless steel pipe No. 18, in which ferrite recrystallized.

[0105] FIG. 5 is a graph showing the relationship between the Larson-Miller parameter PL and the yield strength YS for steel A. Steel A did not provide a yield strength of at least 555 MPa, even though the tempering conditions were controlled. The reason for this, apparently, is that the carbon equivalent Ceq of steel A was too low.

[0106] FIG. 6 is a graph showing the relationship between the Larson-Miller parameter PL and the hardness on the outer surface, inside the wall and on the inner surface for steel B. As shown in FIG. 6, the hardness on the outer surface, inside the wall, and on the inner surface decreases as the value of the Larson-Miller parameter PL increases. As shown in FIG. 6, when the Larson-Miller parameter PL was 18800 or higher, the hardness on the outer surface, inside the wall, and on the inner surface was 250 Hv or lower. On the other hand, when the Larson-Miller parameter PL was less than 18800, the hardness on the outer surface, inside the wall and on the inner surface was higher than 250 Hv.

[0107] FIG. 7 is a graph showing the relationship between the Larson-Miller parameter PL and the hardness on the outer surface, inside the wall and on the inner surface for steel A. For steel A, like steel B, the hardness on the outer surface, inside the wall and on the inner surface decreases as the value of the Larson-Miller parameter PL increases.

[0108] FIG. 8 is a graph showing the relationship between the Larson-Miller parameter PL and the maximum difference in hardness for steel B. As shown in FIG. 8, when the Larson-Miller parameter PL was 18800 or higher, the maximum difference in hardness was not more than 25 Hv. Seamless steel pipe No. 18 had a large maximum difference in hardness, apparently because ferrite recrystallized in an area 1 mm or deeper from the surface.

[0109] FIG. 9 is a graph showing the relationship between the Larson-Miller parameter PL and the maximum difference in hardness for steel A. As shown in FIG. 9, this relationship between the Larson-Miller parameter PL and the maximum difference in hardness for steel A shows similar trends. Seamless steel pipe No. 3 had a large maximum difference in hardness, most likely because ferrite recrystallized in an area 1 mm or deeper from the surface.

[0110] [Assessment of resistance to acidic environment]

An acid resistance rating as described below (i.e., HIC resistance test, four-point bending test) was carried out on some of the rooms' seamless steel pipes.

[0111] [HIC Resistance Test]

A sample containing the inner surface, a sample containing the middle of the wall thickness, and a sample containing the outer surface were taken from each seamless steel pipe. Each of these samples had a thickness of 20 mm, a width (along the circumference) of 20 mm, and a length of 100 mm. The HIC resistance of each sample was evaluated in accordance with the NACE (National Association of Corrosion Engineers) standard ТМ 0284-2011. The test bath in which the samples were immersed contained an aqueous solution of 5% salt + 0.5% acetic acid, saturated with gaseous hydrogen sulfide at a pressure of 1 atm at a temperature of 24 ° C.

[0112] After 96 hours of immersion, an ultrasound inspection (UT) of the samples was performed to determine the location of the largest crack, and the sample was cut at that location. This cross section was perpendicular to the longitudinal direction of the steel pipe, i.e., the thickness of the sample × the width of the sample were measured. The cut specimen was used to determine the fraction of the CLR crack length (= crack length (mm) / specimen width (mm)). The maximum CLR value of the sample taken from each steel pipe was used as the CLR value for this pipe.

[0113] In addition, it was determined whether each sample had bubbles after the test (that is, swollen parts due to cracks near the surface), and the number of such bubbles was counted. The maximum number of bubbles per sample taken from each steel pipe was used as the number of bubbles for a given pipe.

[0114] [Four-Point Bending Test]

A stress of 95% of the actual yield strength (i.e., the yield strength of each seamless steel pipe) was applied to a sample containing the mid-wall thickness of this seamless steel pipe using a four-point bending test tool in accordance with ASTM G39. Samples to which stresses were applied were placed in a test bath. The test bath in which the samples were immersed contained an aqueous solution of 5% salt + 0.5% acetic acid, saturated with gaseous hydrogen sulfide at a pressure of 1 atm at a temperature of 24 ° C. After 720 hours, visually determined whether there are cracks in these samples. If the material did not have cracks, it was believed that this material has good resistance to sulfide stress cracking.

[0115] [Evaluation Results]

The acid resistance results are shown in Table 4.

[Table 4]

No. PL YS (MPa) Hv 10 kgf (maximum for positions) HIC Resistance Test Four-Point Bending Test Number of bubbles Outer surface In the wall Inner surface Difference 10 18953 544 218 218 220 2

0

3 19314 509 197 195 162 35

3 - eighteen 20132 503 209 193 170 39

2 - 23 19131 621 238 240 239 2

0

33 19125 619 236 234 237 3

0

37 18618 627 229 244 268 39 CLR 1% 0 - 40 17924 639 260 253 286 33 CLR 2% 0 - 43 18684 636 248 244 282 38 CLR 2% - - 44 18911 573 232 228 269 41 CLR 2% - - 52 18951 599 237 222 214 23

0 - 57 19211 559 220 198 200 22

0 - 58 18942 570 212 219 224 12

0 - 59 18942 597 236 223 238 fifteen

0 - 60 18942 575 200 203 211 eleven

0 -

» в колонках «Тест стойкости к HIC» и «Испытание на изгиб с четырьмя точками» указывает, что никаких трещин в соответствующем тесте не было. Значок «-» в колонках «Тест стойкости к HIC» и «Испытание на изгиб с четырьмя точками» означает, что соответствующий тест не проводился.[0116] In Table 4, the icon "

”In the columns“ HIC Resistance Test ”and“ Four-Point Bending Test ”indicates that there were no cracks in the corresponding test. A “-” in the “HIC Resistance Test” and “Four-Point Bend Test” columns indicate that the corresponding test has not been performed.

[0117] As shown in Table 4, in seamless steel pipes with a yield strength of 555 MPa or higher and Vickers hardness "at a distance of 1 mm from the outer surface", "at a distance of 1 mm from the inner surface" and "inside the wall" not higher than 250 Hv, no cracks are formed in either the HIC resistance test or the four-point bending test, and good acid resistance is reliably ensured. On the other hand, seamless steel pipes with Vickers hardness “at a distance of 1 mm from the outer surface”, “at a distance of 1 mm from the inner surface” or “inside the wall” higher than 250 Hv provide insufficient resistance to acidic conditions. These results confirm the relationship between Vickers hardness and acid resistance.

[0118] Although embodiments of the present invention have been described, these embodiments are merely examples that can be used in implementing the present invention. Accordingly, the present invention is not limited to the aforementioned embodiments, and the aforementioned embodiments can be modified as appropriate without departing from the gist of the present invention.

Claims (45)

1. Seamless steel pipe having a chemical composition, wt.%: C: 0.02 to 0.15; Si: 0.05 to 0.5; Mn: 0.30 to 2.5; P: up to 0.03; S: up to 0.006; O: up to 0.004; Al: 0.01 to 0.10; Ti: from 0.001 to 0.010; N: up to 0.007; Cr: 0.05 to 1.0; Mo: not less than 0.02 and less than 0.5; Ni: 0.03 to 1.0; Cu: 0.02 to 1.0; V: from 0.020 to 0.20; Ca: 0.0005 to 0.005; and Nb: from 0 to 0.05; the rest is iron and impurities, the value of the carbon equivalent Ceq defined by the following equation (1) is not less than 0.430% and less than 0.500%, the main phase of the microstructure from the surface layer to the middle in the wall thickness is tempering martensite or bainite tempering, the size of pre-austenitic grains in the microstructure is lower than 6.0 on the ASTM E112-10 standard crystal grain size scale, a part between a position 1 mm apart from the inner surface and a position 1 mm apart from the outer surface has a Vickers hardness of 250 Hv or lower, and yield strength is 555 MPa or higher, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ... (1), where the symbol of each element in equation (1) means the content of this element in wt.%. 2. Seamless steel pipe according to claim 1, in which the chemical composition contains, wt.%: Nb: 0.010 to 0.05. 3. Seamless steel pipe according to claim 1, in which the difference between the Vickers hardness of the part located at a depth of 1 mm from the inner surface and the Vickers hardness of the part located in the middle of the wall thickness, the difference between the Vickers hardness of the part located at the depth 1 mm from the outer surface, and the Vickers hardness of the part located in the middle of the wall thickness, and the difference between the Vickers hardness of the part located at a depth of 1 mm from the inner surface, and the Vickers hardness of the part located at a depth of 1 mm from uzhnoy surface each constitute 25 Hv or less. 4. Seamless steel pipe according to claim 2, in which the difference between the Vickers hardness of the part located at a depth of 1 mm from the inner surface and the Vickers hardness of the part located in the middle of the wall thickness, the difference between the Vickers hardness of the part located at the depth 1 mm from the outer surface, and the Vickers hardness of the part located in the middle of the wall thickness, and the difference between the Vickers hardness of the part located at a depth of 1 mm from the inner surface, and the Vickers hardness of the part located at a depth of 1 mm from uzhnoy surface each constitute 25 Hv or less. 5. Seamless steel pipe according to any one of paragraphs. 1-4, in which: seamless steel pipe is produced by quenching and tempering, and the Larson-Miller parameter PL, defined by equation (2) below, is 18800 or higher: PL = (T + 273) × (20 + log (t)) ... (2), where T is the tempering temperature in ° C, and t is the exposure time at this temperature in hours. 6. A method of manufacturing a seamless steel pipe, including: preparation of raw materials having a chemical composition, wt.%: C: from 0.02 to 0.15; Si: 0.05 to 0.5; Mn: 0.30 to 2.5; P: up to 0.03; S: up to 0.006; O: up to 0.004; Al: 0.01 to 0.10; Ti: from 0.001 to 0.010; N: up to 0.007; Cr: 0.05 to 1.0; Mo: not less than 0.02 and less than 0.5; Ni: 0.03 to 1.0; Cu: 0.02 to 1.0; V: from 0.020 to 0.20; Ca: 0.0005 to 0.005; and Nb: from 0 to 0.05, the rest is iron and impurities; hot processing of this raw material by piercing rolling on a piercing mill to obtain a hollow sleeve with subsequent rolling to obtain a seamless pipe billet; hardening of this billet seamless pipe by direct hardening or continuous hardening; and tempering a hardened billet of a seamless pipe to obtain a seamless steel pipe, moreover, between hardening and tempering, reheating and hardening are not performed, the value of the carbon equivalent Ceq defined by the following equation (3) is not less than 0.430% and less than 0.500%, the Larson-Miller parameter PL defined by equation (4) below is not less than 18800, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ... (3), and PL = (T + 273) × (20 + log (t)) ... (4), where the symbol of each element in equation (3) means the content of this element in wt.%, and in equation (4) T is the tempering temperature in ° C, and t is the exposure time at this temperature in hours.

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