JP2005334964A - Method for producing seamless steel tube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a seamless steel tube in which the generation of inside flaws can be suppressed. <P>SOLUTION: A columnar steel having an old austenite intergranular structure comprising ferrite or ferrite and pearlite and an old austenite transgranular structure having a Vickers hardness of ≥240 is charged to a heating furnace. The charged columnar steel is heated at a heating furnace temperature T (K) satisfying the inequality (1). After the heating, the columnar steel discharged from the heating furnace is pierced, and is further passed through a mandrel mill and a reducer, so as to be a seamless steel tube: 50≥6.35×10<SP>-16</SP>×(D/2)<SP>2</SP>×T<SP>4</SP>(1); wherein, D is the outer diameter (mm) of the columnar steel. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for manufacturing a seamless steel pipe, and more particularly to a method for manufacturing a seamless steel pipe including a step of heating cylindrical steel such as round cast pieces and round steel pieces in a heating furnace.

  Seamless steel pipes need to reduce not only the outer surface defects but also the inner surface defects. For example, when an inner surface flaw is present in a seamless steel pipe used for an oil well pipe or the like, cracks such as hydrogen sulfide cracking occur starting from the inner surface flaw.

  It is confirmed that the inner surface defects include machine defects generated due to tools in the piercing and rolling process, and defects due to the quality of cylindrical steel such as round cast slabs and round steel slabs that are the material of seamless steel pipes. Has been.

  For example, mechanical punches are generated when a tool such as a plug comes into contact with the column steel while drilling the column steel. On the other hand, wrinkles due to quality occur due to, for example, center segregation, axial cracks, porosity, etc. in the cylindrical steel.

  Several countermeasures against these internal defects have been reported so far. For example, Patent Document 1 below has reported a countermeasure against mechanical defects. In Patent Document 1, a film containing carbide is formed on the plug surface of a piercing and rolling machine to suppress the occurrence of seizure on the plug surface. Therefore, it is possible to prevent inner surface flaws from occurring in the seamless steel pipe.

  Countermeasures against wrinkles due to quality are reported, for example, in Patent Document 2 below. In patent document 2, in the continuous casting of a round slab, the round slab at the end of solidification is reduced to reduce center segregation in the slab. In order to reduce center segregation and the like, it is possible to prevent the occurrence of internal flaws in the seamless steel pipe.

However, even in the case of a seamless steel pipe that has taken measures against these inner surface defects, inner surface defects may still occur. Therefore, further measures are required to suppress the occurrence of inner surface flaws in the seamless steel pipe.
JP-A-9-52105 Japanese Patent No. 3402250

  The objective of this invention is providing the manufacturing method of the seamless steel pipe which can suppress generation | occurrence | production of an internal surface flaw.

Means for Solving the Problems and Effects of the Invention

  The present inventors investigated the generation | occurrence | production condition of the inner surface flaw which generate | occur | produced in the seamless steel pipe, and the shape of the inner surface flaw. As a result, it has been found that different types of inner surface defects may occur from machine defects and defects caused by quality. This inner surface flaw depended on the outer diameter of the column steel. Specifically, the larger the outer diameter of the column steel, the higher the frequency of occurrence of this inner surface flaw.

  As a result of further investigation, it was found that this inner surface flaw occurred due to a crack extending in the radial direction of the cylindrical steel. Specifically, as a result of drilling a cylindrical steel containing cracks extending in the radial direction to form a seamless steel pipe, internal flaws occurred. This crack did not occur in the column steel before charging into the heating furnace, but occurred in the column steel after extraction from the heating furnace.

  From the above results, the present inventors have found that this crack is a thermal stress crack generated during heating of the column steel in the heating furnace.

  In order to examine measures to prevent the occurrence of internal flaws due to thermal stress cracking, a microstructural test was conducted on a plurality of columnar steels where thermal stress cracking occurred first. As shown in FIG. 1, in a cracked columnar steel, a prior austenite grain boundary structure (hereinafter referred to as a grain boundary structure) has low hardness ferrite (FIG. 1A), or ferrite and pearlite (FIG. 1). 1 (B)).

  Furthermore, according to JISZ2244, as a result of measuring the Vickers hardness of the structure in the prior austenite grains (hereinafter referred to as the intragranular structure), the Vickers hardness of the intragranular structure was 240 or more. In short, the hardness of the intragranular structure was higher than the hardness of the grain boundary structure.

  As a result of the above, the present inventors considered that thermal stress cracking occurs in a columnar steel having a non-uniform hardness structure. When a columnar steel having such a structure is heated in a heating furnace, the ferrite in the grain boundary structure extends due to thermal stress, but the intragranular structure with high hardness does not extend. Therefore, it was considered that the structure was deformed unevenly and thermal stress cracking occurred.

  As one of the measures for preventing the occurrence of thermal stress cracking, a method for preventing the occurrence of such a non-uniform structure can be considered. However, to change the structure, the chemical composition of the columnar steel must be adjusted. If the chemical composition is adjusted, the characteristics of the cylindrical steel and the seamless steel pipe are changed. The chemical composition of the cylindrical steel is determined based on the characteristics of the seamless steel pipe corresponding to the final application. For this reason, the degree of freedom in designing the composition of cylindrical steel is small. In other words, it is difficult to design the composition so that the seamless steel pipe has the desired characteristics and the structure of the cylindrical steel is uniform. Therefore, it is not possible to employ a method for preventing the generation of a non-uniform structure by adjusting the chemical composition.

  As another method for preventing the generation of a non-uniform structure, there is a method of adjusting a cooling rate when manufacturing cylindrical steel. For example, when manufacturing a round slab by a continuous casting method, the method of adjusting the cooling rate at the time of solidification and making a structure | tissue uniform can be considered. However, when the outer diameter of the round slab is large, it is difficult to control the temperature inside the round slab by cooling. Therefore, even if the cooling rate is adjusted, there is a high possibility that the tissue becomes non-uniform. Therefore, a method for adjusting the cooling rate cannot be adopted.

  As a result of the above examination, the present inventors reduced the thermal stress generated in the cylindrical steel having a non-uniform structure, not as a non-uniform structure, as a measure for preventing the occurrence of thermal stress cracking. I studied how to do it.

  When columnar steel is heated in a heating furnace, the greater the difference between the surface temperature and the center temperature of the columnar steel, the greater the thermal stress. Since there is a correlation between the heating furnace temperature and the temperature of the cylindrical steel, it is considered that the higher the heating furnace temperature, the greater the difference between the surface temperature and the center temperature, and the greater the thermal stress. Moreover, in order to manufacture seamless steel pipes of various sizes, cylindrical steels having various outer diameters are used. If the same heating temperature is used, the larger the outer diameter of the cylindrical steel, the larger the thermal stress applied to the cylindrical steel. Conceivable. As a result of the above examination, the present inventors considered that the heating furnace temperature and the outer diameter of the cylindrical steel are related to the thermal stress.

  Based on the above view, the present inventors investigated the relationship between the outer diameter of the cylindrical steel, the furnace temperature, and the thermal stress using a cylindrical steel in which the hardness of the grain boundary structure and the hardness of the grain structure are not uniform. did.

  In the investigation, cylindrical steels having outer diameters of 225 mm, 292 mm, 310 mm, and 360 mm were used. The grain boundary structure of all the columnar steels contained ferrite, or ferrite and pearlite, and the Vickers hardness of the grain structure was 240 or more. Each columnar steel was charged into a heating furnace and heated at a predetermined heating furnace temperature. After heating, the cylindrical steel was extracted from the heating furnace and cooled. After cooling, it was investigated whether thermal stress cracking occurred in the cylindrical steel. Specifically, the cooled cylindrical steel was cut in the axial direction, and the presence or absence of thermal stress cracking was examined by visually observing the cut surface.

The survey results are shown in FIG. Cylindrical steels without thermal stress cracking are indicated by ○ in the figure, and cylindrical steels with thermal stress cracking are indicated by x in the figure. As a result of the investigation, the present inventors have found that thermal stress cracking does not occur in the cylindrical steel if the expression (1) is satisfied.
50 ≧ 6.35 × 10 ⁻¹⁶ × (D / 2) ² × T ⁴ (1)
Here, D is the outer diameter (mm) of the cylindrical steel, and T is the furnace temperature (K).

  The present inventors further examined the case where the heating furnace is divided into a plurality of zones and the temperatures of adjacent zones are different. Usually, the heating furnace is divided into a first zone to an Nth zone in order from the charging port to the extraction port from which the column steel is charged to the extraction port from which the column steel is extracted (N is a natural number). For example, the rotary hearth furnace 10 shown in FIG. 3 is divided into four zones (first zone to fourth zone).

The rotary hearth-type heating furnace 10 includes a plurality of burners 11 on a cylindrical outer peripheral wall 15 and an inner peripheral wall 16. The columnar steel 20 is charged from the charging port 12 and heated by the radiant heat of the plurality of burners 11. The donut-shaped movable hearth 14 rotates clockwise by driving a rack gear and a pinion gear (not shown). Therefore, the cylindrical steel 20 moves to the vicinity of the extraction port 13 after being heated for a predetermined time. The cylindrical steel moved to the vicinity of the extraction port is extracted from the extraction port 13. The rotary hearth furnace 10 can set each zone to a different temperature. For example, the heating temperature of the first zone including the charging port 12 can be set low, and the temperature of the fourth zone including the extraction port 13 can be set high. The temperature of each zone is adjusted by a plurality of burners 11. Thus, when there are a plurality of zones having different temperatures in the heating furnace, the present inventors have found that the temperature T ₁ of the first zone satisfies the formula (2) and the temperature difference T _i between adjacent zones. It has been found that if the cylindrical steel is heated so that -T _i-1 (i = 2 to N) satisfies the formula (3), thermal stress cracking does not occur.
50 ≧ 6.35 × 10 ⁻¹⁶ × (D / 2) ² × (T ₁ ) ⁴ (2)
50 ≧ 6.35 × 10 ⁻¹⁶ × (D / 2) ² × (T _i −T _i−1 ) ⁴ (3)

  Based on the above findings, the present inventors have completed the following present invention.

The method for producing a seamless steel pipe according to the present invention includes a step of charging a cylindrical steel having an old austenite grain boundary structure containing ferrite and an old austenite grain structure having a Vickers hardness of 240 or more into a heating furnace, The method includes a step of heating the charged cylindrical steel at a furnace temperature T (K) satisfying 1) and a step of drilling the heated cylindrical steel in the axial direction.
50 ≧ 6.35 × 10 ⁻¹⁶ × (D / 2) ² × T ⁴ (1)
Here, D is the outer diameter (mm) of the columnar steel.

A method for producing a seamless steel pipe according to the present invention includes a columnar steel having an old austenite grain boundary structure containing ferrite and an old austenite grain structure having a Vickers hardness of 240 or more from an inlet into which the columnar steel is charged. The step of charging the heating furnace divided into the first zone to the Nth zone in order from the charging port to the extraction port from which the column steel is extracted, and the temperature T1 of the heating furnace first zone are expressed by the equation (2). And heating the charged cylindrical steel so that the temperature difference Ti-Ti-1 (i = 2 to N) of adjacent zones of the heating furnace satisfies the formula (3), and heating Drilling the cylindrical steel in the axial direction.
50 ≧ 6.35 × 10 ⁻¹⁶ × (D / 2) ² × (T ₁ ) ⁴ (2)
50 ≧ 6.35 × 10 ⁻¹⁶ × (D / 2) ² × (T _i −T _i−1 ) ⁴ (3)
Here, D is the outer diameter (mm) of the cylindrical steel, and i and N are natural numbers.

  Preferably, the prior austenite grain boundary structure further contains pearlite.

  Preferably, the columnar steel is in mass%, C: 0.15 to 0.35%, Si: 0.10 to 0.50%, Mn: 0.20 to 1.50%, Cr: 0.10 1.50%, P: 0.03% or less, S: 0.01% or less, Sol. Al: 0.001 to 0.1%, N: 0.0070% or less, with the balance being Fe and impurities.

  Preferably, the column steel further contains at least one of Mo: 0.05 to 1.00% and B: 0.0005 to 0.0025%.

  Preferably, it contains at least one of Ti: 0.005 to 0.050%, Nb: 0.005 to 0.040%, and V: 0.03 to 0.30%.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

1. Structure The grain boundary structure of the columnar steel used in the method for producing a seamless steel pipe according to the present invention contains ferrite, or ferrite and pearlite. Further, the Vickers hardness of the intragranular structure is 240 or more. The columnar steel having such a structure is manufactured by a combination of a chemical composition described later and a cooling rate after continuous casting. For example, a cylindrical steel having chemical compositions of steel types BT3 and BT4 in Examples described later is manufactured by a continuous casting method and then cooled at a cooling rate of 4 to 10 ° C./min. In this case, first, ferrite, or ferrite and pearlite is precipitated at the prior austenite grain boundaries. After precipitation of ferrite, or ferrite and pearlite, the remaining structure (that is, the intragranular structure) becomes bainite or martensite, and the Vickers hardness of the intragranular structure becomes 240 or more. The column steel may be a round cast piece or a round steel piece. That is, the column steel may be manufactured by rolling a steel ingot or cast slab with a block mill. Moreover, you may manufacture by forging.

  The cylindrical steel having the above structure has, for example, the following chemical composition.

C: 0.15-0.35%
C is an effective element that increases the hardenability of the steel and increases the strength of the steel. In order to maintain the strength required for steel, the lower limit of the C content is 0.15%. On the other hand, excessive addition of C causes burn cracking. Further, excessive addition of C deteriorates the toughness of the steel pipe. Therefore, the upper limit of the C content is set to 0.35%. Preferably, the C content is 0.20 to 0.29%.

Si: 0.10 to 0.50%
Si is an element effective for deoxidation of steel. Furthermore, Si increases the temper softening resistance of steel. In order to achieve these effects, the lower limit of the Si content is 0.10%. On the other hand, excessive addition of Si reduces the hot workability of steel. Therefore, the upper limit of the Si content is 0.50%.

Mn: 0.20 to 1.50%
Mn is an effective element that increases the hardenability of steel and increases the strength and toughness of steel. In order to maintain the strength and toughness required for steel, the lower limit of the Mn content is 0.20%. On the other hand, excessive addition of Mn increases segregation in the thickness direction of the steel and lowers the toughness of the steel. Therefore, the upper limit of the Mn content is 1.50%. Preferably, the Mn content is 0.40 to 1.40%.

P: 0.03% or less P is an impurity and segregates at the grain boundary to lower the toughness of the steel. For this reason, the P content is preferably as low as possible. Therefore, the P content is limited to 0.03% or less. Preferably, the P content is limited to 0.015% or less.

S: 0.01% or less S is an impurity, and forms inclusions such as MnS in steel. This inclusion is stretched by hot rolling to form a needle shape. Since the inclusions in the form of needles cause stress concentration at their ends, the toughness of the steel decreases. Therefore, the S content is limited to 0.01% or less. Preferably, the S content is limited to 0.005% or less.

Cr: 0.10 to 1.50%
Cr increases the hardenability of the steel and increases the strength of the steel. Further, Cr prevents carbon dioxide corrosion of steel. In order to achieve these effects, the lower limit of the Cr content is 0.10%. On the other hand, excessive addition of Cr forms coarse carbides in the steel. Coarse carbides degrade the toughness of the steel. Therefore, the upper limit of the Cr content is 1.50%. Preferably, the Cr content is 0.20 to 1.15%.

sol. Al: 0.001 to 0.1%
Al is an element necessary for deoxidation of steel. In order to exert this effect, sol. The lower limit of the Al content is 0.001%. On the other hand, excessive addition of Al reduces the toughness of the steel. Therefore, sol. The upper limit of the Al content is 0.1%. Preferably, sol. The Al content is 0.010 to 0.060%.

N: 0.0070% or less N is an impurity and forms a nitride by combining with Al in steel, Ti or Nb described later. These nitrides reduce the toughness of the steel. Therefore, the N content is preferably as low as possible. Therefore, the N content is limited to 0.0070% or less.

  The balance is composed of Fe, but impurities may be included due to various factors in the manufacturing process. The impurities here include, for example, 0.08% or less of Ni or 0.06% or less of Cu.

  The cylindrical steel having the structure may further contain one or more of Mo and B in addition to the chemical composition. Hereinafter, each element will be described.

Mo: 0.05-1.00%
Mo increases the hardenability of the steel and increases the strength of the steel. Furthermore, embrittlement of the steel due to P or the like is suppressed. In order to obtain these effects, the lower limit of the Mo content is set to 0.05%. On the other hand, excessive addition of Mo promotes precipitation of coarse carbides. Therefore, the upper limit of the Mo content is 1.0%. Preferably, the Mo content is 0.08 to 0.80%.

B: 0.0005 to 0.0025%
B significantly improves the hardenability of the steel. In order to obtain this effect, the lower limit of the B content is 0.0005%. On the other hand, excessive addition of B causes carbonitride to precipitate at the grain boundaries and lowers the toughness of the steel. Therefore, the upper limit of the B content is 0.0025%. Preferably, the B content is 0.0008 to 0.0018%.

  The columnar steel having the structure may further contain one or more of Ti, Nb, and V in addition to the chemical composition. Hereinafter, each element will be described.

Ti: 0.005 to 0.05%
Ti precipitates as TiN without dissolving N alone. Since TiN prevents coarsening of crystal grains, the strength and toughness of steel are improved. In order to obtain this effect, the lower limit of the Ti content is set to 0.005%. However, excessive addition of Ti precipitates TiC and reduces the toughness of the steel. Therefore, the upper limit of the Ti content is 0.05%. Preferably, the Ti content is 0.008 to 0.030%.

Nb: 0.005 to 0.040%
Nb forms NbN or NbC. NbN and NbC prevent the coarsening of the crystal grains in the high temperature range, so that the strength of the steel is improved. In order to obtain this effect, the lower limit of the Nb content is set to 0.005%. However, excessive addition of Nb increases segregation in the steel. Therefore, the upper limit of Nb content is 0.04%. Preferably, the Nb content is 0.006 to 0.030%.

V: 0.03-0.30%
V forms VC and increases the strength of the steel. However, excessive addition of V reduces the toughness of the steel. Therefore, the content of V is set to 0.03 to 0.30%. Preferably, the content is 0.05 to 0.22%.

2. Manufacturing Method A method for manufacturing a seamless steel pipe according to this embodiment will be described.

  A columnar steel in which the grain boundary structure contains ferrite or ferrite and pearlite and the intragranular structure has a Vickers hardness of 240 or more is charged into a heating furnace. The heating furnace is, for example, a rotary hearth type heating furnace, a walking hearth type heating furnace, a walking beam type heating furnace or the like shown in FIG. These heating furnaces may be divided into a plurality of zones.

  The charged cylindrical steel is heated in a heating furnace. At this time, the temperature of the heating furnace is adjusted based on the outer diameter D (mm) of the column steel. Specifically, when the heating furnace temperature is set to a predetermined temperature T, the cylindrical steel is heated at a temperature T that satisfies Equation (1).

In addition, when the heating furnace is divided into the first zone to the Nth zone in order from the charging port and the temperatures of the adjacent zones are different, the temperature T ₁ is set so that the temperature T ₁ of the first zone satisfies the formula (2). adjust. Further, the temperature difference T _i -T _i-1 is adjusted so that the temperature difference T _i -T _i-1 (i = 2 to N) between adjacent zones satisfies the expression (3). At this time, based on the measurement result of the temperature measuring device provided in each zone of the heating furnace, the temperature of each zone is adjusted by a plurality of burners. The temperature adjustment may be performed automatically or manually.

  The columnar steel is extracted from the heating furnace after heating the columnar steel at a temperature satisfying the formula (1) or the formula (2) and the formula (3). Subsequently, the extracted cylindrical steel is drilled in the axial direction by a drilling machine. Thereafter, it is formed into a seamless steel pipe having a predetermined size by a mandrel mill, a reducer or the like.

A round slab having the chemical composition shown in Table 1 was heated in a heating furnace and then rolled into a seamless steel pipe, and the state of occurrence of inner surface defects was investigated.

  A plurality of round slabs having chemical compositions of steel types BT1 to BT4 shown in Table 1 were produced by a continuous casting method. A plurality of round cast slabs whose steel types are BT1 or BT2 have an outer diameter D of 310 mm. On the other hand, for steel type BT3, a plurality of round cast pieces having an outer diameter D of 225 mm and a plurality of round cast pieces having an outer diameter D of 310 mm were manufactured. For steel type BT4, a plurality of round cast pieces having outer diameters D of 225 mm, 292 mm, 310 mm, and 360 mm were manufactured. In Table 1, Ni and Cu are impurities contained in each columnar steel due to various factors during the manufacturing process.

  A micro sample was taken from a round slab for each steel type, and the structure was observed by a micro structure test. Specifically, for each steel type, three arbitrary round slabs were selected from a plurality of round slabs. A micro sample was taken from the selected round slab. Two micro samples were taken at a position D / 4 in the radial direction from the center of the cross section perpendicular to the axial direction of the round slab. The size of the micro sample was 15 mm × 20 mm. After polishing the surfaces of the collected micro samples, the surfaces were etched with ethanol nitrate. The surface structure of each etched micro sample was observed. In the tissue observation, 15 visual fields were observed for each micro sample at a magnification of 100 times using an optical microscope.

  After performing the microstructure test, a Vickers hardness test was performed on each micro sample based on JISZ2244. Specifically, Vickers hardness was measured for nine intragranular structures in each micro sample. The load for the Vickers hardness test was 1 kg. The average value of the measured Vickers hardness (measured 54 points for each steel type) was defined as the Vickers hardness of each steel type.

  As shown in Table 1, the Vickers hardness of the intragranular structure of steel type BT1 satisfied the specified value of the present invention, but the grain boundary structure was martensite different from the specified value of the present invention. The grain boundary structure of steel type BT2 was ferrite and pearlite specified in the present invention, but the Vickers hardness of the intragranular structure was less than the specified value of the present invention.

  On the other hand, the grain boundary structure of steel type BT3 is ferrite and pearlite specified in the present invention, and the Vickers hardness of the intragranular structure also satisfies the specified value of the present invention. Further, the grain boundary structure of steel type BT4 is ferrite defined in the present invention, and the Vickers hardness of the intragranular structure also satisfies the specified value of the present invention.

Round slabs of steel types BT1 to BT4 having the above structure were heated under the heating conditions shown in Table 2, and the heated round slabs were pierced and rolled to obtain seamless steel pipes having the outer diameters shown in Table 2. Details will be described below.

50 steel types and round billets with outer diameters in each test number in Table 2 were prepared. A round billet of each test number was heated under the heating conditions shown in Table 2 using a rotary hearth-type heating furnace divided into two zones (first and second zones). At this time, for each test number test, the FT (Furnace Temperature) ₁ value of the first zone shown in Equation (4) and the FT ₂ value of the second zone shown in Equation (5) were obtained.
FT ₁ value = 6.35 × 10 ⁻¹⁶ × (D / 2) ² × (T ₁ ) ⁴ (4)
FT ₂ value = 6.35 × 10 ⁻¹⁶ × (D / 2) ² × (T ₂ −T ₁ ) ⁴ (5)

Here, T ₁ is the temperature of the first zone, and T ₂ is the temperature of the second zone.

In short, the FT ₁ value is the right side of Equation (2), and the FT ₂ value is the right side of Equation (3). In the tests of Test Nos. 1 to 5, which are examples of the present invention, both the FT ₁ value of the first zone and the FT ₂ value of the second zone are within the scope of the present invention. On the other hand, in the tests of test numbers 6 to 11 which are comparative examples, the FT ₁ value exceeded the range of the present invention.

  After heating the round slab in a heating furnace, one of the round slabs was extracted from the heating furnace and air-cooled without piercing and rolling. Further, the remaining (49) round slabs were extracted from the heating furnace, pierced by a piercing mill, and then made into seamless steel pipes by a mandrel mill and a reducer.

[Thermal stress cracking investigation]
Of the round slabs of each test number, it was investigated whether or not thermal stress cracking occurred in the round slabs cooled after extraction from the heating furnace. Thermal stress cracks occur along the radial direction of the round slab. Therefore, the round cast slab was cut in the axial direction to investigate whether or not thermal stress cracks occurred on the cut surface. The presence or absence of cracks in the cut surface was visually observed. When thermal stress cracking occurred at one location in the round slab, it was judged that thermal stress cracking occurred. “Yes” in the column of thermal stress cracking in Table 2 indicates that thermal stress cracking occurred, and “None” indicates that thermal stress cracking did not occur.

[Inner surface inspection]
Inner surface flaws were examined on seamless steel pipes manufactured from round slabs of each test number. Specifically, visual observation and ultrasonic flaw detection were carried out on all manufactured seamless steel pipes, and the presence or absence of internal flaws was confirmed. The flaw detection conditions for ultrasonic flaw detection are defined as API2CT SR2 regulations. For seamless steel pipes with inner surface defects, the cause of inner surface defects was further investigated. As described above, the inner surface defects include defects due to thermal stress cracking, mechanical defects, and defects due to the quality of the round slab. In the present embodiment, only the inner surface defects caused by thermal stress cracking are targeted for investigation, and therefore inner surface defects due to other factors (machine defects and defects caused by quality) must be excluded. In this example, the generation factor of the inner surface flaw was estimated from the shape of the inner surface flaw, and the inner surface flaw generated by the thermal stress cracking was specified.

  In each seamless steel pipe of each test number, the number of seamless steel pipes in which internal flaws due to thermal stress cracking occurred was counted. The value obtained by dividing the number of counts by the total number of seamless steel pipes produced was defined as the rate of occurrence of internal flaws.

[Survey results]
In all the tests of Test Nos. 1 to 5 which are examples of the present invention, no thermal stress cracking occurred in the round cast slab. Moreover, the inner surface flaw occurrence rate of the manufactured seamless steel pipe was 0%.

In the tests of test numbers 6 and 7, which are comparative examples, the FT ₁ value exceeded the range of the present invention, but no thermal stress cracking occurred in the round cast slab, and no inner surface flaws occurred in the seamless steel pipe. The steel type BT1 of the round slab used in Test No. 6 was a steel type outside the specified range of the present invention, and the grain boundary structure was martensite. Steel type BT1 has the same grain boundary structure and intragranular structure, and it is considered that thermal stress cracking did not occur because the difference in hardness between the two was small. Similarly, the steel type BT2 of the round slab used in Test No. 7 had a grain boundary structure of ferrite and pearlite, and the hardness of the intragranular structure was relatively low. It is considered that thermal stress cracking did not occur because the hardness of both the intragranular structure and the grain boundary structure was low and the hardness difference between the two was small.

On the other hand, in the tests of Test Nos. 8 to 11, since the FT ₁ value exceeded the range of the present invention, thermal stress cracking occurred in the round cast slab and inner surface flaws also occurred in the seamless steel pipe. The round slabs used in these tests had a structure that satisfied the provisions of the present invention. That is, the hardness of the grain boundary structure and the hardness of the intragranular structure were non-uniform. When a round slab having such a structure was heated, the FT ₁ value exceeded the provisions of the present invention, and thermal stress cracking, which was a cause of internal flaws, occurred.

As described above, in this example, whether the heating temperature of each test number satisfies the expressions (2) and (3) using the FT ₁ value and the FT ₂ value was examined. However, regarding the test numbers in which the temperatures of the first zone and the second zone of the heating furnace are the same, it may be examined whether or not Expression (1) is satisfied. For example, in test number 1 of the example of the present invention, the temperatures of the first and second zones of the heating furnace are both 1523K. In this case, it can be said that the soot generation rate was 0% because the heating temperature (1523K) of Test No. 1 satisfied Expression (1).

  Further, in this example, the steel type BT3 having a structure satisfying the provisions of the present invention contained Mo, Ti, Nb, V, and B. Even if these alloy elements are not contained, the grain boundary of the columnar steel is obtained. The structure contains ferrite, or ferrite and pearlite, and the Vickers hardness of the intragranular structure is 240 or more.

  While the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out 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 thereof.

  The method for producing a seamless steel pipe according to the present invention can be widely applied to a method for producing a seamless steel pipe using a heating furnace, but is particularly applicable to a method for producing a seamless steel pipe used as an oil well pipe.

It is a figure for demonstrating the structure | tissue in the column steel used for this invention. It is a figure which shows the relationship between the outer diameter of cylindrical steel, the temperature in a heating furnace, and a thermal stress. It is sectional drawing which looked at the rotary hearth type heating furnace from the upper surface.

Claims (6)

Charging a columnar steel having a prior austenite grain boundary structure containing ferrite and a prior austenite grain structure having a Vickers hardness of 240 or more into a heating furnace;
Heating the charged cylindrical steel at a furnace temperature T (K) satisfying the formula (1);
And a step of drilling the heated cylindrical steel in the axial direction.
50 ≧ 6.35 × 10 ⁻¹⁶ × (D / 2) ² × T ⁴ (1)
Here, D is the outer diameter (mm) of the cylindrical steel. Columnar steel having a prior austenite grain boundary structure containing ferrite and a prior austenite intragranular structure having a Vickers hardness of 240 or more, from an inlet into which the columnar steel is inserted to an extraction port through which the columnar steel is extracted Charging a heating furnace divided into a first zone to an Nth zone in the order close to the charging port,
The temperature T ₁ of the first zone of the heating furnace satisfies the formula (2), and the temperature difference T _i −T _i-1 (i = 2 to N) between adjacent zones of the heating furnace is the formula (3). And heating the charged cylindrical steel so as to satisfy
And a step of drilling the heated cylindrical steel in the axial direction.
50 ≧ 6.35 × 10 ⁻¹⁶ × (D / 2) ² × (T ₁ ) ⁴ (2)
50 ≧ 6.35 × 10 ⁻¹⁶ × (D / 2) ² × (T _i −T _i−1 ) ⁴ (3)
Here, D is the outer diameter (mm) of the cylindrical steel, and i and N are natural numbers. It is a manufacturing method of the seamless steel pipe according to claim 1 or 2,
The prior austenite grain boundary structure further contains pearlite, and the method for producing a seamless steel pipe. It is a manufacturing method of the seamless steel pipe according to any one of claims 1 to 3,
The columnar steel is, in mass%, C: 0.15 to 0.35%, Si: 0.10 to 0.50%, Mn: 0.20 to 1.50%, P: 0.03% or less, S: 0.01% or less, Cr: 0.10 to 1.50%, Sol. A method for producing a seamless steel pipe, comprising Al: 0.001 to 0.1%, N: 0.0070% or less, with the balance being Fe and impurities. It is a manufacturing method of the seamless steel pipe according to claim 4,
The cylindrical steel further contains at least one of Mo: 0.05 to 1.00% and B: 0.0005 to 0.0025%. It is a manufacturing method of the seamless steel pipe according to claim 4 or 5,
The cylindrical steel further contains one or more of Ti: 0.005 to 0.050%, Nb: 0.005 to 0.040%, and V: 0.03 to 0.30%. A method for producing a seamless steel pipe.

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