JP7256371B2 - Steel manufacturing method and tempering equipment - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel manufacturing method and tempering equipment.

Due to the deepening of oil wells and gas wells (hereinafter collectively referred to as "oil wells"), steel materials for oil wells are required to have higher strength. Many deep wells are sour environments containing corrosive hydrogen sulfide. As used herein, a sour environment means an acidified environment containing hydrogen sulfide. Note that the sour environment may contain carbon dioxide. Oil well steel materials used in such a sour environment are required to have not only high strength but also sulfide stress cracking resistance (hereinafter referred to as SSC resistance).

On the other hand, when the strength of the steel material is increased, the SSC resistance of the steel material may decrease. Specifically, increasing the dislocation density in the steel increases the yield strength of the steel. However, dislocations can occlude hydrogen. Therefore, if the dislocation density of the steel material increases, the amount of hydrogen occluded by the steel material may also increase. If the hydrogen concentration in the steel increases as a result of increasing the dislocation density, the SSC resistance of the steel decreases even though high strength is obtained.

Thus, when the strength of the steel material increases, the SSC resistance of the steel material tends to decrease. Therefore, if the strength of the steel material increases locally, the SSC resistance may decrease locally. That is, when a single steel material includes a portion with high strength, the steel material cannot obtain stable SSC resistance. Therefore, in order to obtain stable SSC resistance in steel materials used in a sour environment, it is preferable to reduce variations in strength in the steel materials.

Techniques for reducing variations in strength of steel materials typified by steel materials for oil wells are disclosed in JP-A-2000-219914 (Patent Document 1), JP-A-2013-129879 (Patent Document 2), and International Publication No. 2016/035316. (Patent Document 3) and Japanese Patent Application Laid-Open No. 2017-166059 (Patent Document 4).

The method for producing a seamless steel pipe disclosed in Patent Document 1 comprises C: 0.15-0.35%, Si: 0.1-1.5%, Mn: 0.1-2.5%, P: 0.03% or less, S: 0.005% or less, sol. Al: 0.001 to 0.1% or less, Cr: 0.1 to 1.5%, Mo: 0 to 1.0%, N: 0.0070% or less, V: 0 to 0.15%, B : 0 to 0.0030%, Ti: 0 to A%, where A = 3.4 × N (%), and Nb: 0.005 to 0.012%. , rolling is performed, and a seamless steel pipe is produced by making a pipe under the conditions of a final rolling temperature of 900 to 1100°C, and quenching and tempering are performed while maintaining the temperature in the _Ar3 point or higher. As a result, by adjusting the composition of the steel and the final rolling temperature, the austenite grain size can be adjusted to No. 1 according to the ASTM standard. Patent Document 1 describes that a fine structure of 6 or more is obtained, and the strength variation is reduced.

The seamless steel pipe disclosed in Patent Document 2 contains, in mass%, C: 0.15 to 0.50%, Si: 0.1 to 1.0%, Mn: 0.3 to 1.0%, P : 0.015% or less, S: 0.005% or less, Al: 0.01 to 0.1% or less, N: 0.01% or less, Cr: 0.1 to 1.7%, Mo: 0.01% or less, 40 to 1.1%, V: 0.01 to 0.08%, Nb: 0.01 to 0.08%, Ti: 0.03% or less, B: 0.0005 to 0.003% The inner surface side, which is a region of 2.54 to 3.81 mm from the inner surface of the pipe, has a composition and a structure in which the main phase is a tempered martensite phase and the prior austenite grains have a grain size number of 8.5 or more. According to Patent Document 2, it has a hardness distribution in which 295HV is 10 or less on the outer surface side at the same distance from the outer surface and at the center of the plate thickness and at four positions separated from each other by 90° in the circumferential direction. Are listed.

The thick steel pipe for oil well disclosed in Patent Document 3 has C: 0.40 to 0.65%, Si: 0.05 to 0.50%, and Mn: 0.10 to 1.0% in mass %. , P: 0.020% or less, S: 0.0020% or less, sol. Al: 0.005 to 0.10%, Cr: more than 0.40 to 2.0%, Mo: more than 1.15 to 5.0%, Cu: 0.50% or less, Ni: 0.50% or less , N: 0.007% or less, O: 0.005% or less, V: 0-0.25%, Nb: 0-0.10%, Ti: 0-0.05%, Zr: 0-0. 10%, W: 0 to 1.5%, B: 0 to 0.005%, Ca: 0 to 0.003%, Mg: 0 to 0.003%, and rare earth elements: 0 to 0.003% , and the balance has a chemical composition consisting of Fe and impurities. This thick oil well steel pipe has an equivalent circle diameter of 100 nm or more and contains 2 carbides/100 μm ² or less containing 20% by mass or more of Mo. The thick oil well steel pipe further has a yield strength of 827 MPa or more, and a difference between the maximum yield strength and the minimum yield strength in the thickness direction of 45 MPa or less. As a result, the thick oil well steel pipe has a wall thickness of 40 mm or more, excellent SSC resistance, high strength (827 MPa or more), and little variation in strength in the thickness direction. is described in

The high-strength oil-well steel pipe material disclosed in Patent Document 4 contains, by mass %, C: 0.20-0.50%, Si: 0.05-0.40%, Mn: 0.3-0. 9%, P: 0.015% or less, S: 0.005% or less, Al: 0.005-0.10%, N: 0.001-0.006%, Cr: 0.1-0.8 %, Mo: 0.1-2.0%, V: 0.02-0.3%, Nb: 0.001-0.04%, B: 0.0003-0.0030%, O (oxygen) : 0.0030% or less, and the balance has a composition consisting of Fe and unavoidable impurities, and the Rockwell hardness is the formula (15.6 × [% C] + 29.2 ≤ HRC < 60.5 × [ % C]+31.1). According to this high-strength steel pipe material for oil wells, when the steel pipe satisfies the yield strength of 862 MPa or more and less than 963 MPa, it has excellent SSC resistance and can suppress the change in strength against tempering conditions. It is described in Patent Document 4.

JP-A-2000-219914 JP 2013-129879 A WO2016/035316 JP 2017-166059 A

However, even if the techniques disclosed in Patent Documents 1 to 4 are applied, there are cases where variations in yield strength cannot be reduced in steel materials (for example, steel materials for oil wells). Furthermore, it is preferable that the variation in the yield strength of the steel material can be reduced by techniques other than the techniques disclosed in Patent Documents 1 to 4 above.

SUMMARY OF THE INVENTION An object of the present disclosure is to provide a method for manufacturing a steel material with reduced variation in yield strength, and a tempering facility capable of implementing the manufacturing method.

A steel manufacturing method according to the present disclosure includes a preparation process, a quenching process, and a tempering process. In the preparation step, in mass%, C: 0.20 to 0.50%, Si: 0.05 to 0.50%, Mn: 0.01 to 1.00%, P: 0.030% or less, S : 0.010% or less, Al: 0.005-0.100%, Cr: 0.20-1.20%, Mo: 0.30-1.50%, Ti: 0.001-0.030% , V: 0.01 to 0.50%, Nb: 0.002 to 0.100%, B: 0.0001 to 0.0050%, Cu: 0.01 to 0.50%, Ni: 0.01 ~0.50%, N: 0.0100% or less, O: less than 0.0050%, Ca: 0-0.0100%, Mg: 0-0.0100%, rare earth element: 0-0.050%, Then, an intermediate steel material having a chemical composition containing W: 0 to 2.00% and the balance being Fe and impurities is prepared. In the quenching step, after the preparation step, the intermediate steel material at 850 to 1000°C is cooled at an average cooling rate of 300°C/min or more between 800°C and 500°C. In the tempering process, after the quenching process, the intermediate steel material is heated between 100° C. and 600° C. at an average heating rate of more than 10 to 50° C./sec and held at 650 to 720° C. for 10 to 180 minutes.

A tempering installation for tempering steel according to the present disclosure comprises a rapid heating device and a tempering furnace. The rapid heating device can heat steel at an average heating rate between 100°C and 600°C at more than 10-50°C/sec. The tempering furnace is arranged downstream of the rapid heating device, can be charged with the steel material heated by the rapid heating device, and can perform heat treatment on the charged steel material.

The steel production method according to the present disclosure can produce steel with reduced variation in yield strength. A tempering installation according to the present disclosure is capable of carrying out the method of manufacturing steel according to the present disclosure.

FIG. 1 is a flow chart showing an example of a steel material manufacturing method according to the present embodiment. FIG. 2 is a functional block diagram showing an example of a manufacturing equipment line used for the quenching process and the tempering process according to this embodiment. FIG. 3 is a plan view showing an example of conventional tempering equipment. FIG. 4 is a diagram showing the heat pattern of the intermediate steel material in the tempering process. FIG. 5 is a diagram showing the relationship between the heating rate (° C./second) in the tempering process and the variation (MPa) of the yield strength of the steel material. FIG. 6 is a flowchart showing another example of the method for manufacturing steel according to this embodiment. FIG. 7 is a plan view of tempering equipment according to this embodiment. FIG. 8 is a plan view of tempering equipment according to another example of the present embodiment. FIG. 9 is a diagram showing the heat pattern of the intermediate steel material in another example of the tempering process according to this embodiment.

The present inventors have found that the steel material used in a sour environment, in mass%, C: 0.20 to 0.50%, Si: 0.05 to 0.50%, Mn: 0.01 to 1.00% , P: 0.030% or less, S: 0.010% or less, Al: 0.005 to 0.100%, Cr: 0.20 to 1.20%, Mo: 0.30 to 1.50%, Ti: 0.001-0.030%, V: 0.01-0.50%, Nb: 0.002-0.100%, B: 0.0001-0.0050%, Cu: 0.01- 0.50%, Ni: 0.01 to 0.50%, N: 0.0100% or less, O: less than 0.0050%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Focusing on steel materials having a chemical composition containing rare earth elements: 0 to 0.050% and W: 0 to 2.00%, with the balance being Fe and impurities, investigating methods for reducing variations in strength of steel materials. We also studied and obtained the following findings.

FIG. 1 is a flow chart showing an example of a steel material manufacturing method according to the present embodiment. Referring to FIG. 1, when manufacturing a steel material that has the chemical composition described above and is assumed to be used in a sour environment, the intermediate steel material prepared in the preparation step (S1) is subjected to a quenching step (S2). be implemented. A tempering step (S3) is performed on the intermediate steel material that has been subjected to the quenching step (S2).

In this specification, "quenching" means quenching an intermediate steel material having a point of _A3 or higher. In the present specification, the term "tempering" means reheating the intermediate steel material after quenching at A _c1 point or less, holding it, and then cooling it to room temperature. In this specification, the term "intermediate steel" refers to plate-like steel when the final product is a steel plate, and refers to a blank pipe when the final product is a steel pipe. As used herein, "normal temperature" means a normal temperature in the environment in which steel materials are manufactured, for example, 0 to 40°C.

Here, when the intermediate steel material having the chemical composition described above is subjected to the quenching step (S2), the microstructure becomes mainly martensite and bainite. A tempering step (S3) is performed on the intermediate steel material that has been subjected to the quenching step (S2) to produce a steel material having the desired yield strength and having the chemical composition described above.

In this way, the yield strength of the steel material having the chemical composition described above is determined by the tempering step (S3). That is, it is thought that by more precisely controlling the tempering step (S3) in the steel manufacturing method, the yield strength of the manufactured steel can be more precisely controlled. Therefore, the present inventors paid attention to the tempering step (S3) and studied in detail the manufacturing conditions in the tempering step (S3) of the steel material having the chemical composition described above and variations in yield strength.

FIG. 2 is a functional block diagram showing an example of a manufacturing equipment line used in the quenching step (S2) and the tempering step (S3) according to this embodiment. Referring to FIG. 2, the manufacturing equipment line includes quenching equipment 20 and tempering equipment 10 in order from upstream to downstream. A transport path 15 is arranged between the hardening equipment 20 and the tempering equipment 10 .

FIG. 3 is a plan view showing an example of conventional tempering equipment 10. As shown in FIG. As shown in FIG. 3 , in the conventional tempering equipment 10 , the intermediate steel material M transported by the transport path is charged into the tempering furnace 11 . The inside of the tempering furnace 11 is heated to a tempering temperature and the temperature is maintained. Therefore, the intermediate steel material M charged into the tempering furnace 11 is heated to the tempering temperature and held at the tempering temperature. After that, the intermediate steel material M extracted from the tempering furnace 11 is air-cooled to normal temperature.

FIG. 4 is a diagram showing the heat pattern of the intermediate steel material M in the tempering step (S3). A dashed line in FIG. 4 indicates the thermal history of the intermediate steel material M in the previous tempering step (S3). Referring to FIG. 4, in the previous tempering step (S3), the temperature of the intermediate steel material M changes as follows. An intermediate steel material M at room temperature P (°C) is charged into a tempering furnace 11 heated to a tempering temperature Q (°C). Therefore, the intermediate steel M whose heating is started from time _t1 is heated to the temperature Q (°C) in the tempering furnace 11 by time _t3 . Subsequently, the intermediate steel material M is held at a tempering temperature Q (°C) for a period of time _t3 to _t4 . Thereafter, the intermediate steel material M extracted from the tempering furnace ₁₁ at time t4 is allowed to cool to room temperature P from time _t4 to _t5 .

In contrast to the conventional tempering furnace 11, the inside is filled with atmospheric gas at a desired temperature. In short, in the conventional tempering step (S3), the intermediate steel material M is heated by the atmosphere gas in the tempering furnace 11 to the tempering temperature Q (°C). That is, the time from when the intermediate steel material M is charged into the tempering furnace 11 until the temperature of the intermediate steel material M reaches the tempering temperature Q (° C.) (that is, between times t ₁ and t ₃ ) is long. Therefore, depending on the position in the intermediate steel material M, there is a possibility that the time required to reach the tempering temperature Q (°C) will vary. In particular, when the intermediate steel material M has a size larger than a certain level or has a complicated shape, the time required to reach the tempering temperature Q (°C) tends to vary depending on the position of the intermediate steel material M.

As described above, in the conventional tempering step (S3), depending on the position in the intermediate steel material M, there are parts where the heating time to the tempering temperature Q (°C) is short and parts where the heating time is long. become. As a result, the steel material after the tempering step (S3) may have variations in yield strength at each position.

Therefore, the present inventors have conducted various studies on techniques for reducing the deviation in the time required to reach the tempering temperature Q (°C) depending on the position in the intermediate steel material M. As described above, in the conventional tempering step (S3), it takes a long time for the temperature of the intermediate steel material M to reach the tempering temperature Q (°C). Therefore, depending on the position of the intermediate steel material M, the time required to reach the tempering temperature Q (° C.) tends to vary greatly. Therefore, the present inventors have found that if the speed at which the intermediate steel material M is heated to the tempering temperature Q (°C) is increased, the deviation that occurs in the time until the tempering temperature Q (°C) is reached depending on the position in the intermediate steel material M can be reduced. I thought it might be possible.

On the other hand, as described above, in the previous tempering step (S3), the intermediate steel material M is heated by the atmosphere gas in the tempering furnace 11 to the tempering temperature Q (°C). Therefore, the present inventors presumed that by heating the intermediate steel material M before being charged into the tempering furnace 11 in advance, the speed at which the intermediate steel material M is heated to the tempering temperature Q (° C.) can be increased. I wondered.

Specifically, by rapidly heating the intermediate steel material M before being charged into the tempering furnace 11, the speed at which the intermediate steel material M is heated to the tempering temperature Q (°C) is increased. In this case, depending on the position in the intermediate steel material M, there is a possibility that the deviation occurring in the time until reaching the tempering temperature Q (° C.) can be reduced. As a result, there is a possibility that variations in yield strength of the steel material after the tempering step (S3) can be reduced. Therefore, the present inventors investigated in detail the heating rate in the tempering step (S3) and the variation in the yield strength of the steel material after the tempering step (S3).

FIG. 5 is a diagram showing the relationship between the heating rate (° C./sec) in the tempering step (S3) and the yield strength variation (MPa) of the steel material. FIG. 5 was obtained by the following method. Among the examples to be described later, for steel materials in which the conditions other than the heating rate in the tempering step (S3) satisfy the range of the present embodiment, the heating rate (° C./sec) in the tempering step (S3) and the obtained yield strength and the maximum difference (MPa). The yield strengths of the steel materials shown in FIG. 5 were all 758 to 862 MPa (110 ksi class).

With reference to FIG. 5, it is clear that if the heating rate in the tempering step (S3) exceeds 10° C./sec for the intermediate steel material M having the chemical composition described above, the variation in the yield strength of the steel material can be greatly reduced. became. The present inventors consider this point as follows.

In a steel manufacturing method that is assumed to be used in a sour environment, the tempering temperature Q (°C) is set higher than in other technical fields in order to reduce the dislocation density. Specifically, steel materials intended for use in a sour environment are tempered in a high temperature range of 650 to 720°C. In tempering in such a high temperature range, deviations in yield strength due to slight differences in temperature are more likely to occur than in tempering in a low temperature range. Therefore, it is considered that the yield strength of steel materials that are assumed to be used in a sour environment is likely to vary.

On the other hand, by rapidly heating the intermediate steel material M before being charged into the tempering furnace 11, the time (between _t1 and _t3 ) required to reach the tempering temperature Q (°C) depending on the position in the intermediate steel material M It is possible to reduce the deviation that occurs in As a result, it is considered that the yield strength of the steel material after the tempering step (S3) is less likely to vary.

The steel manufacturing method according to the present embodiment completed based on the above knowledge includes a preparation step, a quenching step, and a tempering step. In the preparation step, in mass%, C: 0.20 to 0.50%, Si: 0.05 to 0.50%, Mn: 0.01 to 1.00%, P: 0.030% or less, S : 0.010% or less, Al: 0.005-0.100%, Cr: 0.20-1.20%, Mo: 0.30-1.50%, Ti: 0.001-0.030% , V: 0.01 to 0.50%, Nb: 0.002 to 0.100%, B: 0.0001 to 0.0050%, Cu: 0.01 to 0.50%, Ni: 0.01 ~0.50%, N: 0.0100% or less, O: less than 0.0050%, Ca: 0-0.0100%, Mg: 0-0.0100%, rare earth element: 0-0.050%, Then, an intermediate steel material having a chemical composition containing W: 0 to 2.00% and the balance being Fe and impurities is prepared. In the quenching step, after the preparation step, the intermediate steel material at 850 to 1000°C is cooled at an average cooling rate of 300°C/min or more between 800°C and 500°C. In the tempering process, after the quenching process, the intermediate steel material is heated between 100° C. and 600° C. at an average heating rate of more than 10 to 50° C./sec and held at 650 to 720° C. for 10 to 180 minutes.

In this specification, the steel material is not particularly limited, but includes, for example, steel pipes, steel plates, and steel bars.

According to the steel material manufacturing method according to the present embodiment, it is possible to manufacture a steel material with reduced variations in yield strength.

The chemical composition is Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, and one or more selected from the group consisting of rare earth elements: 0.001 to 0.050% may contain.

The above chemical composition may contain W: 0.01 to 2.00%.

In the tempering step, the intermediate steel material held at 650 to 720°C for 10 to 180 minutes may be cooled at an average cooling rate of 100°C/min or more between 600°C and 200°C.

The preparation process may include a material preparation process of preparing a material having the chemical composition, and a hot working process of hot working the material to produce an intermediate steel material.

The steel material may be an oil well steel material.

As used herein, oil well steel refers to steel widely used for oil well applications. For example, it may be a steel bar for downhole members, a steel pipe for line pipes, or an oil country tubular good. The shape of the steel material for oil wells is not limited, and may be, for example, a steel bar, a steel plate, a seamless steel pipe, or a welded steel pipe. Oil country tubular goods are steel pipes used for casing and tubing applications, for example.

The steel material may be a seamless steel pipe.

If the steel material according to the present embodiment is a seamless steel pipe, more stable SSC resistance can be obtained as compared with other shapes.

The tempering equipment for tempering the steel material according to the present embodiment includes a rapid heating device and a tempering furnace. The rapid heating device can heat steel at an average heating rate between 100°C and 600°C at more than 10-50°C/sec. The tempering furnace is arranged downstream of the rapid heating device, can be charged with the steel material heated by the rapid heating device, and can perform heat treatment on the charged steel material.

The tempering equipment may comprise a cooling device. The cooling device is arranged downstream of the tempering furnace and can cool the steel material heat-treated by the tempering furnace at an average cooling rate of 100°C/min or more between 600°C and 200°C.

Hereinafter, the steel manufacturing method and tempering equipment according to the present embodiment will be described in detail. "%" for elements means % by weight unless otherwise specified.

[Shape of steel]
The shape of the steel material manufactured by this embodiment is not specifically limited. Steel materials are, for example, steel pipes and steel plates. The steel material may be a solid material (steel bar) or a hollow material (steel pipe). When the steel material is a steel pipe for oil wells, the wall thickness is preferably 9 to 60 mm. The steel material according to this embodiment may be a thick-walled seamless steel pipe.

[Steel manufacturing method flow]
As described above, FIG. 1 is a flow chart showing an example of the steel manufacturing method according to the present embodiment. Referring to FIG. 1, the steel manufacturing method according to the present embodiment includes a preparation step (S1), a quenching step (S2), and a tempering step (S3). Other steps are optional steps.

[Preparation step (S1)]
The preparation step (S1) prepares an intermediate steel material M having the following chemical composition.

[Chemical Composition of Intermediate Steel M]
The chemical composition of the intermediate steel material M according to this embodiment contains the following elements.

C: 0.20-0.50%
Carbon (C) enhances the hardenability of the steel material and enhances the yield strength of the steel material. Further, C promotes spheroidization of carbides during tempering during the manufacturing process and enhances the SSC resistance of the steel material. Dispersed carbides further increase the yield strength of the steel. If the C content is too low, these effects cannot be obtained. On the other hand, if the C content is too high, the toughness of the steel material is lowered and quench cracks are likely to occur. If the C content is too high, coarse carbides are formed in the steel material, and the SSC resistance of the steel material is lowered. Therefore, the C content is 0.20-0.50%. A preferred lower limit for the C content is 0.22%, more preferably 0.24%. A preferable upper limit of the C content is 0.45%, more preferably 0.40%.

Si: 0.05-0.50%
Silicon (Si) deoxidizes steel. If the Si content is too low, this effect cannot be obtained. On the other hand, if the Si content is too high, the SSC resistance of the steel is lowered. Therefore, the Si content is 0.05-0.50%. The lower limit of the Si content is preferably 0.10%, more preferably 0.15%. A preferred upper limit for the Si content is 0.45%, more preferably 0.40%.

Mn: 0.01-1.00%
Manganese (Mn) deoxidizes steel. Mn further enhances the hardenability of the steel material and enhances the yield strength of the steel material. If the Mn content is too low, these effects cannot be obtained. On the other hand, if the Mn content is too high, Mn will segregate at grain boundaries along with impurities such as P and S. In this case, the SSC resistance of the steel is lowered. Therefore, the Mn content is 0.01-1.00%. A preferred lower limit for the Mn content is 0.02%, more preferably 0.03%. A preferable upper limit of the Mn content is 0.90%, more preferably 0.80%.

P: 0.030% or less Phosphorus (P) is an impurity. That is, the P content is over 0%. P segregates at grain boundaries and lowers the SSC resistance of the steel material. Therefore, the P content is 0.030% or less. A preferable upper limit of the P content is 0.025%, more preferably 0.015%. The lower the P content is, the better. However, an extreme reduction in the P content greatly increases manufacturing costs. Therefore, considering industrial production, the preferable lower limit of the P content is 0.0001%, more preferably 0.0003%.

S: 0.010% or less Sulfur (S) is an impurity. That is, the S content is over 0%. S segregates at grain boundaries and lowers the SSC resistance of steel materials. Therefore, the S content is 0.010% or less. A preferable upper limit of the S content is 0.008%, more preferably 0.005%. It is preferable that the S content is as low as possible. However, an extreme reduction in the S content greatly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0003%.

Al: 0.005-0.100%
Aluminum (Al) deoxidizes steel. If the Al content is too low, this effect cannot be obtained, and the SSC resistance of the steel material is lowered. On the other hand, if the Al content is too high, coarse oxide-based inclusions are formed and the SSC resistance of the steel is lowered. Therefore, the Al content is 0.005-0.100%. A preferable lower limit of the Al content is 0.010%, more preferably 0.020%. A preferable upper limit of the Al content is 0.080%, more preferably 0.060%. The "Al" content referred to in this specification means the content of "acid-soluble Al", that is, "sol. Al".

Cr: 0.20-1.20%
Chromium (Cr) enhances the hardenability of the steel material and enhances the yield strength of the steel material. Cr also increases temper softening resistance and enables high temperature tempering. As a result, the SSC resistance of the steel is enhanced. If the Cr content is too low, these effects cannot be obtained. On the other hand, if the Cr content is too high, coarse carbides are formed in the steel material, and the SSC resistance of the steel material is lowered. Therefore, the Cr content is 0.20-1.20%. A preferable lower limit of the Cr content is 0.25%, more preferably 0.30%, and still more preferably 0.40%. A preferable upper limit of the Cr content is 1.15%, more preferably 1.10%.

Mo: 0.30-1.50%
Molybdenum (Mo) enhances the hardenability of steel and enhances the yield strength of steel. Mo also forms fine carbides and increases the temper softening resistance of the steel material. As a result, the SSC resistance of the steel is enhanced. If the Mo content is too low, these effects cannot be obtained. On the other hand, if the Mo content is too high, the above effects are saturated. Therefore, the Mo content is 0.30-1.50%. A preferred lower limit for the Mo content is 0.40%, more preferably 0.50%. A preferred upper limit for the Mo content is 1.40%, more preferably 1.30%.

Ti: 0.001-0.030%
Titanium (Ti) forms nitrides and refines crystal grains by a pinning effect. This increases the yield strength of the steel material. If the Ti content is too low, this effect cannot be obtained. On the other hand, if the Ti content is too high, a large amount of Ti nitrides will be formed and the SSC resistance of the steel material will be lowered. Therefore, the Ti content is 0.001-0.030%. A preferable lower limit of the Ti content is 0.003%, more preferably 0.005%. A preferable upper limit of the Ti content is 0.025%, more preferably 0.020%.

V: 0.01-0.50%
Vanadium (V) combines with C and/or N to form carbides, nitrides or carbonitrides (hereinafter referred to as “carbonitrides, etc.”). Carbonitrides and the like refine the prior austenite grains of the steel due to the pinning effect, and improve the SSC resistance of the steel. V also forms fine carbides during tempering. Fine carbides increase the temper softening resistance of steel and increase the yield strength of steel. If V is too low, these effects will not be obtained. On the other hand, if the V content is too high, the toughness of the steel will decrease. Therefore, the V content is 0.01-0.50%. A preferable lower limit of the V content is 0.02%, more preferably 0.04%, and still more preferably 0.06%. A preferable upper limit of the V content is 0.45%, more preferably 0.40%, and still more preferably 0.35%.

Nb: 0.002-0.100%
Niobium (Nb) combines with C and/or N to form carbonitrides and the like. Carbonitrides and the like refine the prior austenite grains of the steel due to the pinning effect, and improve the SSC resistance of the steel. Nb further combines with C to form fine carbides. As a result, the yield strength of the steel is increased. If the Nb content is too low, these effects cannot be obtained. On the other hand, if the Nb content is too high, carbonitrides and the like are excessively formed, and the SSC resistance of the steel is lowered. Therefore, the Nb content is 0.002-0.100%. A preferable lower limit of the Nb content is 0.005%, more preferably 0.010%, and still more preferably 0.020%. A preferable upper limit of the Nb content is 0.080%, more preferably 0.060%, still more preferably 0.050%.

B: 0.0001 to 0.0050%
Boron (B) forms a solid solution in steel to enhance the hardenability of the steel and increase the strength of the steel. If the B content is too low, this effect cannot be obtained. On the other hand, if the B content is too high, coarse nitrides are formed and the SSC resistance of the steel deteriorates. Therefore, the B content is 0.0001-0.0050%. A preferable lower limit of the B content is 0.0003%, more preferably 0.0007%. A preferable upper limit of the B content is 0.0030%, more preferably 0.0025%, and still more preferably 0.0020%.

Cu: 0.01-0.50%
Copper (Cu) enhances the hardenability of the steel material and enhances the yield strength of the steel material. If the Cu content is too low, this effect cannot be obtained. On the other hand, if the Cu content is too high, the hardenability of the steel becomes too high and the SSC resistance of the steel deteriorates. Therefore, the Cu content is 0.01-0.50%. A preferred lower limit for the Cu content is 0.02%, more preferably 0.05%. A preferred upper limit for the Cu content is 0.35%, more preferably 0.25%.

Ni: 0.01-0.50%
Nickel (Ni) enhances the hardenability of the steel material and enhances the yield strength of the steel material. If the Ni content is too low, this effect cannot be obtained. On the other hand, if the Ni content is too high, it promotes localized corrosion and reduces the SSC resistance of the steel material. Therefore, the Ni content is 0.01-0.50%. A preferable lower limit of the Ni content is 0.02%, more preferably 0.05%. A preferable upper limit of the Ni content is 0.30%, more preferably 0.20%, and still more preferably 0.10%.

N: 0.0100% or less Nitrogen (N) is inevitably contained. That is, the N content is over 0%. N combines with Ti to form fine nitrides and refine crystal grains. On the other hand, if the N content is too high, N forms coarse nitrides, which lowers the SSC resistance of the steel material. Therefore, the N content is 0.0100% or less. A preferable upper limit of the N content is 0.0080%, more preferably 0.0050%. A preferable lower limit of the N content for obtaining the above effect more effectively is 0.0010%, more preferably 0.0020%.

O: less than 0.0050% Oxygen (O) is an impurity. That is, the O content is over 0%. O forms coarse oxides and lowers the corrosion resistance of steel materials. Therefore, the O content is less than 0.0050%. A preferable upper limit of the O content is 0.0040%, more preferably 0.0030%, and still more preferably 0.0020%. It is preferable that the O content is as low as possible. However, the drastic reduction of the O content greatly increases the manufacturing cost. Therefore, considering industrial production, the lower limit of the O content is preferably 0.0001%, more preferably 0.0003%.

The remainder of the chemical composition of the intermediate steel material M according to this embodiment consists of Fe and impurities. Here, the impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when the steel material is industrially manufactured, and are within a range that does not adversely affect the steel material according to the present embodiment. means acceptable.

[Regarding arbitrary elements]
The chemical composition of the intermediate steel material M described above may further contain one or more selected from the group consisting of Ca, Mg and rare earth elements instead of part of Fe. All of these elements are optional elements and enhance the SSC resistance of the steel material.

Ca: 0-0.0100%
Calcium (Ca) is an optional element and may not be contained. That is, the Ca content may be 0%. When contained, Ca refines the sulfides in the steel material and enhances the SSC resistance of the steel material. This effect can be obtained to some extent if even a small amount of Ca is contained. However, if the Ca content is too high, the oxides in the steel material will coarsen and the SSC resistance of the steel material will decrease. Therefore, the Ca content is 0-0.0100%. A preferable lower limit of the Ca content is more than 0%, more preferably 0.0001%, still more preferably 0.0003%, still more preferably 0.0006%, still more preferably 0.0010% is. A preferable upper limit of the Ca content is 0.0080%, more preferably 0.0060%, and still more preferably 0.0040%.

Mg: 0-0.0100%
Magnesium (Mg) is an optional element and may not be contained. That is, the Mg content may be 0%. When contained, Mg detoxifies S in the steel material as sulfides and enhances the SSC resistance of the steel material. This effect can be obtained to some extent if even a small amount of Mg is contained. However, if the Mg content is too high, the oxides in the steel material will coarsen and the SSC resistance of the steel material will decrease. Therefore, the Mg content is 0-0.0100%. The preferred lower limit of the Mg content is more than 0%, more preferably 0.0001%, still more preferably 0.0003%, still more preferably 0.0006%, still more preferably 0.0010% is. A preferable upper limit of the Mg content is 0.0080%, more preferably 0.0060%, still more preferably 0.0040%.

Rare earth element (REM): 0-0.050%
A rare earth element (REM) is an optional element and may not be contained. That is, the REM content may be 0%. When REM is contained, REM refines sulfides in the steel material and enhances the SSC resistance of the steel material. REM further binds with P in the steel material and suppresses the segregation of P at grain boundaries. Therefore, the deterioration of the SSC resistance of the steel due to the segregation of P is suppressed. These effects can be obtained to some extent if even a small amount of REM is contained. However, if the REM content is too high, the oxides become coarse and the SSC resistance of the steel decreases. Therefore, the REM content is 0-0.050%. The preferred lower limit of the REM content is more than 0%, more preferably 0.001%, still more preferably 0.003%, still more preferably 0.006%, still more preferably 0.010% is. A preferable upper limit of the REM content is 0.045%, more preferably 0.040%, and still more preferably 0.035%.

In this specification, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoid (La) with atomic number 57 to atomic number 71. One or more elements selected from the group consisting of lutetium (Lu). Moreover, the REM content in this specification is the total content of these elements.

The chemical composition of the intermediate steel material M described above may further contain W instead of part of Fe.

W: 0-2.00%
Tungsten (W) is an optional element and may not be contained. That is, the W content may be 0%. When included, W forms a protective corrosion film in a hydrogen sulfide environment and inhibits hydrogen penetration. As a result, the SSC resistance of the steel is enhanced. This effect can be obtained to some extent if even a small amount of W is contained. However, if the W content is too high, coarse carbides are formed in the steel material and the SSC resistance of the steel material is lowered. Therefore, the W content is 0-2.00%. A preferable lower limit of the W content is more than 0%, more preferably 0.01%, still more preferably 0.03%, still more preferably 0.05%. A preferable upper limit of the W content is 1.50%, more preferably 1.00%, still more preferably 0.80%, and still more preferably 0.50%.

The manufacturing method of the intermediate steel material M is not particularly limited as long as it has the chemical composition described above. In this specification, the intermediate steel material M is a plate-shaped steel material when the final product is a steel plate, and is a blank pipe when the final product is a steel pipe.

FIG. 6 is a flowchart showing another example of the method for manufacturing steel according to this embodiment. Referring to FIG. 6, the preparation step (S1) of the steel manufacturing method according to the present embodiment includes a step of preparing a material (material preparation step) and a step of hot working the material to manufacture an intermediate steel material M ( hot working step). Hereinafter, the case where the material preparation process and the hot working process are included will be described in detail.

[Material preparation process]
In the material preparation step, the material is manufactured using molten steel having the chemical composition described above. A method for manufacturing the raw material is not particularly limited, and a known method may be used. Specifically, a slab (slab, bloom, or billet) is produced by continuous casting using molten steel. You may manufacture an ingot by an ingot casting method using molten steel. If desired, the slab, bloom or ingot may be bloomed to produce a billet. A raw material (slab, bloom, or billet) is manufactured by the above steps.

[Hot working process]
In the hot working process, the intermediate steel material M is manufactured by hot working the prepared material. If the steel material is a seamless steel pipe, the intermediate steel material M corresponds to a blank pipe. Hereinafter, as an example of a method for manufacturing steel materials, a method for manufacturing a blank pipe will be specifically described in detail.

First, the billet is heated in a heating furnace. Although the heating temperature is not particularly limited, it is, for example, 1100 to 1300.degree. A billet extracted from a heating furnace is subjected to hot working to produce a blank pipe (seamless steel pipe). For example, the Mannesmann process is carried out as hot working to produce a mother tube. In this case, the round billet is pierced and rolled by a piercing machine. In the case of piercing-rolling, the piercing ratio is not particularly limited, but is, for example, 1.0 to 4.0. The pierced-rolled round billet is further hot-rolled by a mandrel mill, a reducer, a sizing mill, or the like to form a mother tube. The cumulative area reduction rate in the hot working process is, for example, 20 to 70%.

A blank pipe may be manufactured from a billet by other hot working methods. For example, in the case of a short thick steel material such as a coupling, a blank pipe may be manufactured by forging such as the Ehrhardt method. A blank pipe is manufactured by the above steps. Although the wall thickness of the blank tube is not particularly limited, it is, for example, 9 to 60 mm.

As described above, when the steel material is a steel plate, the intermediate steel material M means a plate-shaped steel material. That is, in the method for manufacturing steel materials according to the present embodiment, a blank tube may be manufactured as described above, or a plate-shaped steel material may be manufactured by another method. Furthermore, the intermediate steel material M having other shapes may be manufactured by other methods.

The intermediate steel material M produced by hot working may be air-cooled (As-Rolled). The intermediate steel material M produced by hot working may also be directly quenched after hot working without being cooled to room temperature, or quenched after supplementary heating (reheating) after hot working. may However, when performing direct quenching or quenching after supplementary heat, it is preferable to stop cooling during quenching or perform slow cooling for the purpose of suppressing quench cracks.

When quenching is performed directly after hot working, or when quenching is performed after reheating after hot working, stress relief annealing is performed after quenching and before heat treatment in the next step for the purpose of removing residual stress. (SR treatment) is preferably performed.

As described above, the intermediate steel material M is prepared in the preparation step (S1). The intermediate steel material M may be manufactured by the preferred steps described above, the intermediate steel material M manufactured by a third party, or a factory other than a factory where the quenching step (S2) and tempering step (S3) described later are performed. An intermediate steel material M manufactured in another factory or another place of business may be prepared.

[Quenching step (S2)]
In the quenching step (S2), the prepared intermediate steel material M is quenched. As described above, "quenching" in this specification means quenching the intermediate steel material M having a point of _A3 or higher.

A preferred quenching temperature is 850-1000°C. The quenching temperature corresponds to the surface temperature of the intermediate steel material M measured by a thermometer installed on the delivery side of the final hot working device when quenching is performed directly after hot working. The quenching temperature also corresponds to the temperature of the reheating furnace or heat treatment furnace when quenching is performed using a reheating furnace or heat treatment furnace after hot working.

If the quenching temperature is too high, the crystal grains of the prior austenite grains become coarse, and the SSC resistance of the steel material may deteriorate. Therefore, the quenching temperature is 850-1000°C. A more preferable upper limit of the quenching temperature is 950°C.

The quenching method includes, for example, continuously cooling the intermediate steel material M from the quenching start temperature and continuously lowering the temperature of the intermediate steel material M. A method of continuous cooling treatment is not particularly limited, and a known method may be used. The method of continuous cooling treatment includes, for example, a method of cooling the intermediate steel material M by immersing it in a water tank, and a method of accelerating cooling the intermediate steel material M by shower water cooling or mist cooling.

If the cooling rate during quenching is too slow, the microstructure of the intermediate steel material M after quenching will not consist mainly of martensite and bainite. In this case, the SSC resistance of the steel material after tempering may not be obtained. Therefore, in the steel material manufacturing method according to the present embodiment, the intermediate steel material M is rapidly cooled during quenching. Specifically, in the quenching process, the average cooling rate when the temperature of the intermediate steel M during quenching is between 800° C. and 500° C. is defined as the cooling rate during quenching CR _800-500 (° C./min). More specifically, the cooling rate during quenching CR _800-500 is determined from the temperature measured at the surface of the intermediate steel material M to be quenched.

In the quenching step (S2) according to this embodiment, the cooling rate CR _800-500 during quenching is 300° C./min or more. In this case, the microstructure of the intermediate steel material M after quenching is stably mainly composed of martensite and bainite. A more preferable lower limit of the cooling rate CR _800-500 during quenching is 500°C/min. Although the upper limit of the cooling rate CR _800-500 during quenching is not particularly limited, it is, for example, 12000° C./min.

Further, preferably, the blank tube is heated in the austenitic region a plurality of times, and then quenched. In this case, since the austenite grains before quenching are refined, the SSC resistance of the steel is further enhanced. By performing quenching multiple times, heating in the austenite region may be repeated multiple times, and by performing normalizing and quenching, heating in the austenite region may be repeated multiple times.

[Tempering step (S3)]
In the tempering step (S3), tempering is performed on the intermediate steel material M that has undergone the above-described quenching step (S2). As described above, “tempering” in this specification means reheating the intermediate steel material M after quenching at A _c1 point or lower, maintaining the temperature, and then cooling to room temperature.

In the tempering step (S3) according to this embodiment, the tempering temperature Q (°C) is 650-720°C. As described above, steel materials intended for use in a sour environment are tempered in a high-temperature region for the purpose of increasing SSC resistance. As a result, the yield strength at each position of the steel material after the tempering step (S3) tends to vary. Therefore, in the tempering step (S3) according to the present embodiment, the heating rate is increased to near the tempering temperature (650 to 720°C).

[Regarding tempering equipment 10]
The tempering equipment 10 will be described in detail below with reference to the drawings. FIG. 7 is a plan view of the tempering equipment 10 according to this embodiment. With reference to FIG. 7 , tempering equipment 10 according to this embodiment includes a rapid heating device 12 and a tempering furnace 11 . The rapid heating device 12 is arranged upstream of the tempering furnace 11 .

The intermediate steel material M transported by the transport path is rapidly heated by the rapid heating device 12 . The temperature range in which the intermediate steel material M is heated by the rapid heating device 12 includes 100 to 600° C. and is below the _Ac1 point. The intermediate steel material M is heated, for example, from room temperature to a temperature 10° C. lower than the tempering temperature by the rapid heating device 12 .

As described above, in the tempering process according to this embodiment, the tempering temperature is 650-720°C. If the intermediate steel material M is rapidly heated to such a high temperature range, the temperature of the intermediate steel material M may rise above the target temperature. Therefore, in the present embodiment, the temperature range for heating the intermediate steel material M by the rapid heating device 12 is preferably from 100° C. or lower to a temperature 10° C. lower than the tempering temperature. In this case, the intermediate steel material M can be stably heated by the rapid heating device 12 to a temperature in the range of 100 to 600° C. and lower than the tempering temperature.

The heated intermediate steel material M is charged into a tempering furnace 11 maintained at a tempering temperature Q (°C). The intermediate steel material M charged into the tempering furnace 11 is held in the tempering furnace 11 at a tempering temperature Q (650-720° C.). After that, the intermediate steel material M is extracted from the tempering furnace 11 and air-cooled by being exposed to air.

As described above, in the tempering equipment 10 according to the present embodiment, the intermediate steel material M conveyed through the conveying path is heated by the rapid heating device 12 before being charged into the tempering furnace 11 . Here, the rapid heating device 12 is, for example, a high frequency induction heating device. When the rapid heating device 12 is a high-frequency induction heating device, the intermediate steel material M is arranged so as to be surrounded by heating coils. In this case, the intermediate steel material M is rapidly heated by applying an electric current to the heating coil.

When tempered by the tempering equipment 10, the heat pattern of the intermediate steel material M becomes as indicated by the solid line in FIG. Specifically, referring to FIG. 4, in the tempering step (S3) according to the present embodiment, the temperature of the intermediate steel material M is changed from normal temperature P (°C) to around tempering temperature Q (°C) from time _t2 to _t3 . It heats up rapidly during Subsequently, the intermediate steel material M is held at a tempering temperature Q (650-720° C.) for a period of time t ₃ to t ₄ . After that, it is cooled from the tempering temperature Q (°C) to the normal temperature P (°C) during time _t4 to _t5 .

More specifically, the heating rate of the tempering step (S3) according to this embodiment is controlled as follows. In the tempering step (S3), the average heating rate when the temperature of the intermediate steel material M during heating is in the range of 100 to 600° C. is defined as the heating rate during tempering HR _100-600 . As described above, if the heating rate HR _100-600 during tempering is too slow, variations in the temperature of the intermediate steel M tend to occur during heating in the tempering step (S3). As a result, the variation in the yield strength of the steel material after the tempering step (S3) increases. On the other hand, if the temperature rise rate during tempering is too fast, the above effects are saturated. Therefore, the heating rate HR _100-600 during tempering should be more than 10 to 50°C/sec.

A preferable lower limit of the heating rate HR _100-600 during tempering is 11° C./second, more preferably 12° C./second. A more preferable upper limit of the heating rate HR _100-600 during tempering is 45°C/second, more preferably 40°C/second.

As described above, the heated intermediate steel material M is charged into the tempering furnace 11 maintained at the tempering temperature Q (° C.), and held at the tempering temperature Q (650 to 720° C.) in the tempering furnace 11. . That is, in this specification, the holding time (tempering time) at the tempering temperature Q (°C) means the time from when the intermediate steel material M is charged into the tempering furnace 11 until it is extracted. The intermediate steel material M according to this embodiment is held at a tempering temperature Q (° C.) for 10 to 180 minutes in the tempering step (S3). In short, in the tempering step (S3) according to the present embodiment, the time from charging the intermediate steel material M into the tempering furnace 11 to extracting it is 10 to 180 minutes.

In the tempering step (S3), if the tempering time is too short, a microstructure mainly composed of tempered martensite and/or tempered bainite may not be obtained. On the other hand, if the tempering time is too long, the above effects are saturated. Therefore, in this embodiment, the tempering time is 10-180 minutes. A preferred lower limit to the tempering time is 15 minutes. A more preferable upper limit of the tempering time is 120 minutes, more preferably 90 minutes.

Tempering facility 10 may also have other configurations. Specifically, FIG. 8 is a plan view of a tempering facility 10 according to another example of this embodiment. With reference to FIG. 8 , tempering equipment 10 includes a rapid heating device 12 , a tempering furnace 11 and a cooling device 13 . As described above, the rapid heating device 12 is arranged upstream of the tempering furnace 11 . A cooling device 13 is arranged downstream of the tempering furnace 11 .

That is, the tempering equipment 10 according to this embodiment may include a cooling device 13 on the delivery side of the tempering furnace 11 . The cooling device 13 is not particularly limited as long as it can rapidly cool the intermediate steel material M extracted from the tempering furnace 11 . The cooling device 13 may be, for example, a shower cooling device or a cooling device that immerses the steel material in a cooling liquid.

When the tempering equipment 10 according to another example of the present embodiment shown in FIG. 8 is used, the heat pattern of the intermediate steel material M is as follows. FIG. 9 is a diagram showing the heat pattern of the intermediate steel material M in another example of the tempering step (S3) according to this embodiment. Referring to FIG. 9, in the tempering step (S3) according to the present embodiment, heating is rapidly performed from normal temperature P (°C) to tempering temperature Q (°C) during time _t2 to _t3 . Subsequently, it is held at a tempering temperature Q (°C) for a period of time _t3 to _t4 . After that, it is rapidly cooled from the tempering temperature Q (°C) to the normal temperature P (°C) during time _t4 to _t6 .

More specifically, preferably, the cooling rate of the tempering step (S3) according to this embodiment is controlled as follows. In the tempering step (S3), the average cooling rate when the temperature of the intermediate steel material M during cooling is in the range of 600 to 200° C. is defined as the cooling rate during tempering CR _600-200 . As described above, if the cooling rate during tempering CR _600-200 is high, the variation in the temperature of the intermediate steel M during cooling in the tempering step (S3) is likely to be reduced. As a result, variations in yield strength of the steel material after the tempering step (S3) are reduced. Therefore, the cooling rate CR _600-200 during tempering is preferably 100° C./min or more.

A more preferable lower limit of the cooling rate during tempering CR _600-200 is 110°C/min, more preferably 120°C/min. A more preferable upper limit of the cooling rate during tempering CR _600-200 is 400°C/min, more preferably 200°C/min.

The method for manufacturing the steel material according to the present embodiment has been described above. In the manufacturing method described above, the method for manufacturing a seamless steel pipe has been described as an example. However, the steel material according to this embodiment may be a steel plate or other shapes. The method of manufacturing steel sheets and other shapes also includes preparation, quenching, and tempering steps, similar to the manufacturing method described above.

Moreover, the tempering equipment 10 described above has been described using a continuous furnace as an example of the tempering furnace 11 . However, tempering furnace 11 may also be a batch furnace. The tempering furnace 11 is not particularly limited, and a known tempering furnace may be used.

Molten steel having the chemical composition shown in Table 1 was produced.

A billet was produced from the molten steel by a continuous casting method. After heating the manufactured billet to 1250° C., hot rolling was performed to obtain a blank pipe (seamless steel pipe) of each test number having an outer diameter (mm) and a wall thickness (mm) shown in Table 2. manufactured. Table 2 shows the types of steel used for each test number. After hot-rolling, the mother tube of each test number was allowed to cool, and the temperature of the mother tube was normal temperature (25°C).

After standing to cool, the blank tube of each test number was heated and held at the quenching temperature (920° C.) for 20 minutes. Here, the temperature of the furnace in which the reheating was performed was defined as the quenching temperature (°C). After reheating, the blank tube of each test number was water-cooled by water-cooling equipment. The cooling rate in the quenching process of the blank tube of each test number is the quenching temperature (920 ° C), the surface temperature of the blank tube of each test number on the outlet side of the water cooling equipment, and the cooling rate from the entry side to the exit side of the water cooling equipment. sought from time. The cooling rate in the quenching process for each test number obtained was 300° C./min or more. Therefore, the cooling rate CR _800-500 during quenching for each test number was considered to be 300° C./min or more.

Tempering was performed on the blank tube of each test number. The blank tube of each test number was heated from 25° C. to a temperature 10° C. lower than the tempering temperature (° C.) shown in Table 2 at the heating rate (° C./sec) shown in Table 2. The heating rate was controlled using a high frequency heating device as a rapid heating device. The heating rate was obtained from the surface temperature of the blank tube of each test number on the inlet side and outlet side of the rapid heating device and the time from the inlet side to the outlet side of the rapid heating device. Table 2 shows the obtained heating rate HR _100-600 (°C/sec).

The heated mother pipes of each test number were charged into a tempering furnace maintained at the tempering temperature (°C) shown in Table 2. After being held in the tempering furnace for the holding time (minutes) shown in Table 2, the blank tube was extracted from the tempering furnace. The blank tube of each test number extracted from the tempering furnace was cooled to room temperature.

Cooling was performed at the cooling rate (°C/min) shown in Table 2. The cooling rate was controlled using a shower water cooling system. The cooling rate was obtained from the surface temperature of the blank tube of each test number on the entry side and the exit side of the shower water cooling equipment and the time from the entry side to the exit side of the shower water cooling equipment. Table 2 shows the obtained cooling rate CR _600-200 (°C/sec). Steel pipes (seamless steel pipes) of each test number were manufactured by the above steps.

[Evaluation test]
The tensile test described below was performed on the seamless steel pipes of each test number manufactured by the above process.

[Tensile test]
The yield strength was measured for the seamless steel pipes of each test number. Specifically, a tensile test was performed according to ASTM E8 (2013). Nine round-bar tensile test specimens each having a parallel portion diameter of 4 mm and a parallel portion length of 35 mm were prepared from the thickness central portion of the seamless steel pipe of each test number. First, of the seamless steel pipes of each test number, the portion that is first charged into the tempering furnace is the “steel pipe tip”, the portion that is finally charged into the tempering furnace is the “steel pipe end”, and the seam in the longitudinal direction The central portion of the non-steel pipe was defined as the “steel pipe central portion”.

The tip of the seamless steel pipe of each test number was divided into three regions in the circumferential direction. One of the above-described round bar tensile test pieces was produced from each of the divided regions. The axial direction of the round bar tensile test piece was parallel to the axial direction of the seamless steel pipe. Similarly, the end portion and center portion of the seamless steel pipe of each test number were divided into three regions in the circumferential direction, and one of the above-mentioned round bar tensile test pieces was produced from each region.

Using the round bar test piece of each test number, a tensile test was performed at 25° C. in the atmosphere to obtain the yield strength (MPa) of the round bar tensile test piece of each test number. In this example, the stress at 0.2% elongation (0.2% proof stress) obtained in the tensile test was defined as the yield strength of each test number. Table 2 shows the yield strength (MPa) obtained. Furthermore, among the obtained yield strengths (MPa), the difference between the maximum value and the minimum value was determined and defined as the maximum strength difference (MPa). Table 2 shows the maximum strength difference (MPa).

[Test results]
With reference to Tables 1 and 2, the chemical compositions of the seamless steel pipes of test numbers 1 to 24 were appropriate, and the heating rate HR _100-600 during tempering was more than 10 to 50°C/sec. As a result, the maximum difference in yield strength was 60 MPa or less, and the variation in yield strength could be reduced.

The seamless steel pipes of test numbers 11 and 16 had the same chemical composition (steel type A), the same outer diameter and wall thickness, and the heating rate during tempering was HR _100-600 (16.7°C/sec. ) were identical. However, in the seamless steel pipe of test number 16, the cooling rate _CR600-200 during tempering was 100°C/min or more, but in the seamless steel pipe of test number 11, the cooling rate _CR600-200 during tempering was 100°C. /min. As a result, the maximum difference in yield strength of the seamless steel pipe of test number 16 was smaller than that of the seamless steel pipe of test number 11.

The seamless steel pipes of test numbers 14 and 18 had the same chemical composition (steel type C), the same outer diameter and wall thickness, and the heating rate during tempering was HR _100-600 (16.7°C/sec. ) were identical. However, in the seamless steel pipe of test number 18, the cooling rate _CR600-200 during tempering was 100°C/min or more, but in the seamless steel pipe of test number 14, the cooling rate _CR600-200 during tempering was 100°C. /min. As a result, the maximum difference in yield strength of the seamless steel pipe of test number 18 was smaller than that of the seamless steel pipe of test number 14.

On the other hand, in the seamless steel pipes of test numbers 25 to 29, the heating rate HR _100-600 during tempering was 10°C/sec or less. As a result, the maximum difference in yield strength exceeded 60 MPa. That is, the variation in yield strength could not be reduced.

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit of the present invention.

Claims (7)

in % by mass,
C: 0.20 to 0.50%,
Si: 0.05 to 0.50%,
Mn: 0.01 to 1.00%,
P: 0.030% or less,
S: 0.010% or less,
Al: 0.005 to 0.100%,
Cr: 0.20-1.20%,
Mo: 0.30-1.50%,
Ti: 0.001 to 0.030%,
V: 0.01 to 0.50%,
Nb: 0.002 to 0.100%,
B: 0.0001 to 0.0050%,
Cu: 0.01-0.50%,
Ni: 0.01 to 0.50%,
N: 0.0100% or less,
O: less than 0.0050%,
Ca: 0 to 0.0100%,
Mg: 0-0.0100%,
Rare earth element: 0 to 0.050%, and
A preparatory step of preparing an intermediate steel material having a chemical composition containing W: 0 to 2.00%, the balance being Fe and impurities;
After the preparation step, a quenching step of cooling the intermediate steel material at 850 to 1000° C. at an average cooling rate of 300° C./min or more between 800° C. and 500° C.;
After the quenching step, the intermediate steel material is heated at an average heating rate of 10 between 100°C and 600°C.
a tempering step of heating at more than ~50°C/sec and holding at 650-720°C for 10-180 minutes. A method for manufacturing the steel material according to claim 1,
The chemical composition is
Ca: 0.0001 to 0.0100%,
Mg: 0.0001 to 0.0100%, and
Rare earth element: A method for producing a steel material containing one or more selected from the group consisting of 0.001 to 0.050%. A method for manufacturing a steel material according to claim 1 or 2,
The chemical composition is
A method for producing a steel containing W: 0.01 to 2.00%. A method for manufacturing a steel material according to any one of claims 1 to 3,
In the tempering step, the intermediate steel material held at 650 to 720° C. for 10 to 180 minutes is
A method for manufacturing steel, comprising cooling at an average cooling rate of 100°C/min or more between 00°C and 200°C. A method for manufacturing a steel material according to any one of claims 1 to 4,
The preparation step includes a material preparation step of preparing a material having the chemical composition according to any one of claims 1 to 3;
and a hot working step of hot working the raw material to produce the intermediate steel. A method for manufacturing a steel material according to any one of claims 1 to 5,
The steel material manufacturing method, wherein the steel material is oil well steel material. A method for manufacturing a steel material according to any one of claims 1 to 6,
The steel material manufacturing method, wherein the steel material is a seamless steel pipe.

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