JP2576254B2 - Manufacturing method of seamless steel pipe with ultrafine structure - Google Patents

Description

The present invention relates to a method for stably producing a seamless steel pipe having a uniform and ultrafine structure on an industrial scale.

<Prior art and its problems> It has been widely known that the properties (eg, low-temperature toughness, ductility, yield strength, corrosion resistance, superplasticity, etc.) of a seamless steel pipe improve as its structure becomes finer. ,
For example, a so-called "controlled rolling technique" in which the rolling conditions during hot rolling are regulated in accordance with the adjustment of the component composition has remarkably developed, and many proposals relating to the technique have been made. Furthermore, recently, the cooling rate after controlled rolling was also adjusted to increase the number of nuclei generated in the ferrite crystal grains to be transformed and refined from austenite, and the so-called attempt was made to further refine the crystal grains by taking the action into account. "Accelerated cooling technology" has also been developed.

However, even with the technology that combines "controlled rolling" with "accelerated cooling", there is naturally a limit to the "final grain size of austenite before transformation by cooling". It was impossible to get. In addition, it was difficult for the "structure after cooling" formed on the basis of this structure to have a limit to the degree of fineness. Because, unless the original austenite grains themselves are refined, it is extremely difficult to refine the martensite grains produced when they are cooled as intended. Is only a fatal problem that only an unexpected "semi-quenched structure composed of ferrite and martensite" can be obtained.

Of course, various proposals have been made regarding grain refinement in addition to controlled rolling and accelerated cooling, but in any case, there is a limit to the refinement of “austenite grains before transformation by cooling”, so the final product is limited. It could not be a technology that breaks down the conventional limits related to the miniaturization and uniformity of semiconductors. In other words, the problem found in these conventional techniques is that the austenitic grains produced by hot working become practically no longer finer to a certain degree when they become finer to a certain degree. The reason is that even if an attempt is made to forcibly produce a fine ferrite structure from an austenite structure that is not sufficiently refined by accelerated cooling, a satisfactory uniform ultrafine structure cannot be obtained at all. It is.

Therefore, unless the austenitic structure itself during hot working of the seamless steel pipe material is made to be an ultra-fine structure by special means, the above-mentioned limit relating to the ultra-fine and uniform structure of the structure at the final product stage is drastically reduced. Was considered impossible to wipe off.

For these reasons, the main object of the present invention is to provide a uniform ultra-fine austenitic structure (average austenite crystal grain size: 15 μm or less) which was impossible in the prior art in the hot working stage of the seamless steel pipe manufacturing process. ) Was found, and based on this, the “ultrafine structure (average ferrite grain size: 1
0 μm or less).

<Means for Solving the Problems> The present inventors have conducted intensive studies from various viewpoints in order to achieve the above object, and have completed the present invention. Here are the results of the two basic experiments that triggered this. These experiments using the “inclined rolling type piercing mill” for research were realized as a means of remarkably refining the austenitic structure itself during hot rolling of seamless steel pipes. There was something like that.

Experiment 1 A 70 mm diameter solid round steel slab was used as a test material, and the piercing and rolling experiment was performed by changing the heating temperature and the drilling ratio (the ratio of the length of the hollow piece after drilling to the length of the solid round slab before drilling). Then, the inlet temperature just before entering the piercing mill and the outlet temperature immediately after piercing rolling were measured, and the processing heat generated by piercing rolling was actually measured. here,
The heating temperature was adjusted so as to obtain a predetermined inlet temperature.

The material of the test material was SCM430 equivalent material (Fe-0.29% C-
0.22% Si-0.64% Mn- 1.08% Cr-0.24% Mo) and is (hereinafter,% represents the component ratio is the weight%), the Ae ₁ point is 725 ° C., Ac ₁ point is 730 ° C., Ae ₃ The point was 790 ° C and the Ac ₃ point was 795 ° C.

In the drilling experiment, the cone-shaped main roll (intersection angle: 15
傾斜, tilt angle: 12 ゜) and the expansion ratio (ratio of the outer diameter of the hollow piece after drilling to the outer diameter of the solid round steel piece before drilling) is 1.
Approximately 05, the inlet temperature to the pier mill was 6 levels (1150 ° C, 1050 ° C, 950 ° C, 850 ° C, 750 ° C and 650 ° C), and the piercing ratio was 5 levels (1.5, 2, 3). , 4 and 5). Here, the reason why the cross angle is given by adopting the cone-shaped main roll is to prevent the occurrence of internal flaws due to the deterioration of the deformability at the time of low-temperature drilling.

Fig. 1 shows the "relationship between the inlet side temperature and the outlet side temperature" obtained by this experiment, and Fig. 2 shows the "relationship between the inlet side temperature and the processing heat". (All are organized using the piercing ratio as a parameter.) The following findings were obtained from this experiment. That is, (a) the generation of the processing heat becomes more remarkable as the entry-side temperature decreases, and this tendency appears more remarkably as the drilling ratio increases. For example, when the inlet temperature is 650 ° C. and the piercing ratio is 2, the temperature rise due to the processing heat is approximately 200 ° C., and when the piercing ratio is 4, it reaches approximately 300 ° C. When the inlet temperature is 750 ° C. and the piercing ratio is 2, the temperature rise by the processing heat is approximately 150 ° C., and when the piercing ratio is 4, it is approximately 225 ° C.

(B) note is the entry side temperature (Material heating temperature), the temperature range of less than Ac ₁ point by selecting whether the heating temperature and the piercing ratio to the temperature range of the Ac ₁ point, Ac at _one point or more And Ac ₃
To a temperature range of not lower than Ac ₃ point from the temperature range point, even a point reverse transformation from a temperature range below Ac ₁ point to at once Ac temperature range above points ₃ can be realized. For example, if the inlet temperature is 680 ° C
If the perforation ratio to the temperature range of not lower than Ac ₁ point from Ac ₁ point below the temperature range at 1.5, Ac ₃ from a temperature range of less than Ac ₁ point in piercing ratio 2
Reverse transformation of the above points is quite possible, also, if possible entry side temperature of 770 ° C., the Ac ₁ point or more piercing ratio 1.5 and A
reverse transformation from c ₃ points below the temperature range to a temperature range of not lower than Ac ₃ point is also sufficiently possible.

Experiment 2 A solid round steel equivalent to SCM430 with a diameter of 70φ was used as the test material, the piercing ratio was fixed at 2.5, and the inlet temperature was 1200 ° C to 50 ° C.
Each time the temperature was changed to 600 ° C., the austenitic grain size immediately after piercing and rolling and the ferrite grain size after cooling were observed and investigated. The other experimental conditions are the same as those in "Experiment 1".

FIG. 3 summarizes the "Effects of the inlet temperature on the austenite grain size immediately after piercing and rolling and the ferrite grain size after cooling" obtained in this experiment.

The following findings were obtained from this experiment. (A) The effect of the temperature on the inlet side of the piercing mill on the austenite grain size immediately after piercing and rolling and the ferrite grain size after cooling is clear, and the crystal grain size becomes significantly smaller as the inlet side temperature is lower.

(B) In particular, the austenite grain size in the case of reverse transformation from the temperature range of less than Ac ₁ point to the temperature range of more than Ac ₃ points at a time is refined to a grain size number near 16 and the ferrite grain size after cooling. Indicates 16 or more. In addition, a ferrite grain size near 13 by the grain size number is also obtained by the reverse transformation from the temperature range of ₁ point or more of Ac and the temperature range of ₃ points or less to the temperature range of ₃ points or more of Ac, and these reverse transformation heat treatments It is sufficiently possible to reduce the ferrite particle size after cooling to 5 μm or less.

In this experiment, since the piercing ratio was unified to 2.5, as a result, the temperature was raised to a temperature range of _three or more Ac by piercing rolling in all temperature ranges. also the ferrite grain diameter after cooling 10μm by reverse transformation from the temperature range below Ac ₁ point of the lower case to the Ac ₁ point or more temperature range appears to be sufficiently possible.

Now, with the above two basic experiments, the present inventors accumulated full-scale research on reverse transformation working heat treatment and came to the following conclusions (A) to (D).

(A) Ac ₁ transformation point of steel species, although Ac ₃ transformation point are different, if properly choose the heating temperature and piercing ratio to a temperature range of not lower than Ac ₁ point from Ac ₁ point below the temperature range, or Ac ₁ or more points And
From the temperature range below Ac ₃ points to the temperature range above Ac ₃ points,
Reverse transformation from the temperature range of ₁ point or less to the temperature range of ₃ points or more of Ac is possible at once, and the ultra-fine austenite structure that could not be obtained by conventional controlled rolling etc. by this reverse transformation processing heat treatment Can be realized.

(B) As described above, when the temperature is raised by the working heat while plastic working is performed on the ferrite structure and the transformation is performed to exceed the transformation point and reversely transforms to the austenite structure, it is necessary to complete the reverse transformation sufficiently. after completion process of temperature rise due to heat, a ₁ transformation point in complete equilibrium (i.e. Ae ₁ point)
Alternatively, it is preferable that the temperature is kept at or above the A ₃ transformation point (ie, Ae ₃ point) for a certain period of time.

(C) The ultra-fine austenite structure obtained in this manner can be obtained by any of various cooling means (for example, cooling, slow cooling, cooling after heat retention, accelerated cooling, quenching, or cooling while applying processing). Even when cooled, it becomes a "uniform and extremely fine isotropic transformed structure" which could not be obtained by the prior art.

(D) In addition, according to the above-mentioned means of reverse transformation working heat treatment, since the material is immersed in the phase transformation of “ferrite → austenite → ferrite”, the use of carbides and nitrides precipitated during plastic working is also intended. For example, it is possible to strengthen the steel without embrittlement.

The present invention has been completed on the basis of the above findings and the like, and describes a method of forming a solid or hollow round steel slab at least partially composed of ferrite on an inclined rolling mill at a piercing ratio or a stretching ratio. while applying plastic working than 1.5, Ac ₃ this time to a temperature range of not lower than Ac ₁ point from Ac ₁ point below the temperature range by the processing heat generated, or Ac at _one point or more and a temperature range below Ac ₃ point
To the temperature range above the temperature, more preferably, the temperature is raised from the temperature range below the Ac ₁ point to the temperature range above the Ac ₃ point at a stroke, and if necessary, following this temperature rise, the Ae ₁ point or more, preferably Ae A part or all of the structure composed of ferrite is reverse-transformed to austenite by maintaining the temperature range of _three or more points, thereby realizing a uniform ultrafine austenite structure, and further cooling the ultrafine structure ( Ferrite grain size
(10 to 5 μm or less) to stably produce a hot-rolled seamless steel pipe having excellent strength, toughness, ductility, corrosion resistance, and the like ”.

The term “ferrite structure” as used herein means a structure composed of a ferrite phase with respect to an austenite phase. Not only an isotropic ferrite structure but also a needle-like ferrite structure, a pearlite structure, a bainite structure, a martensite structure. And any form of ferrite structure containing a ferrite phase as a constituent element, such as a tempered martensite structure.

Further, the round billet material targeted by the present invention does not ask for other constituent components and compositions as long as the steel has a structure at least partially composed of ferrite (that is, a ferrite single structure or a mixed structure containing ferrite). There is no problem with carbon steel or alloy steel. That is, according to the present invention, an ultrafine structure can be obtained from commercial low carbon steel to pure iron, and in addition to carbon steel, various alloy steels, stainless steels and the like are particularly affected by alloy components. Since the microstructure can be remarkably refined without the need, the C content of the target material steel and the composition range of components other than C need not be particularly limited. However, if the C content is too large, huge eutectic cementite or graphite appears, and it tends to be difficult to homogenize and refine the structure. Therefore, it is preferable to use a material having a C content of 1.5% or less. Is good.

 Hereinafter, the present invention will be described in more detail together with its operation.

<Action> In the present invention, the "structure of the round billet material to be applied is" a ferrite single structure "or" a mixed structure containing ferrite "
The reason for this is that, as described above, the present invention has an important requirement that "the reverse transformation from the ferrite phase to the austenite phase occurs while performing plastic working". This is because fine austenite grains, which are unprecedented in the art, are formed, and a uniform and ultrafine transformed structure is developed from the fine austenite grains by subsequent cooling.

It is important that the amount of strain applied by the plastic working at this time is an amount sufficient to cause the following three actions.

The first effect is that very fine austenite crystal grains are generated from the ferrite that has been subjected to the work and hardened by the work.

The second is the effect of the heat generated during processing for raising the temperature of the workpiece to the transformation point at which the ferrite reversely transforms into austenite.

The third function is to work harden the generated fine austenite crystal grains and to generate work-induced transformation to generate finer ferrite grains during the subsequent ferrite generation.

However, in the process of manufacturing a seamless steel pipe, when the strain amount of plastic working is less than 33%, that is, when the piercing ratio or the stretching ratio is 1.5% or less.
If it is less than 1, the processing strain is small and the generation of processing heat tends to be insufficient, making it difficult to reach the temperature of the workpiece to the temperature at which the ferrite is transformed into austenite. Further, for example, even if a reverse transformation from ferrite to austenite can be performed, the induced generation by processing of fine austenite grains becomes insufficient, and it becomes difficult to reduce the generated austenite grain size to a target of 15 μm or less.
That is, a uniform fine austenite structure having an average grain size of 15 μm or less can be relatively easily realized only by setting the strain amount of the plastic working at the time of reverse transformation from ferrite to austenite to 1.5 or more in the piercing ratio or the stretching ratio. However, in order to realize a uniform and fine austenitic structure that is stable and uniform in the field in consideration of all types of steel, the amount of plastic working strain to be applied during the reverse transformation from the ferrite phase to the austenite phase is 2% by the piercing ratio or the stretching ratio. It is desirable to make the above.

Next, regarding the temperature rise temperature of the workpiece, even if the temperature rise temperature is “the temperature range in which the ferrite reversely transforms to austenite (ie, the temperature range of _one or more points of Ac)”, the temperature becomes Ac If it is less than ₃ points, it becomes a two-phase mixed structure of ferrite and austenite, but in the present invention, processing is performed while increasing the temperature, so that as long as the temperature rise temperature is ₁ point or more, less than Ac ₃ points Even in this temperature range, the crystal grains are sufficiently refined by processing and recrystallization. Of course, in order to fully exert the effects of the present invention, it is desirable to raise the temperature to a temperature range of Ac ₃ points or more, but depending on the product such as duplex stainless steel, the dual phase structure of ferrite and austenite is required. Needless to say, it is necessary to keep the heating temperature of such a product in a temperature range lower than the Ac ₃ point.

As described above, when reverse transformation is performed from the ferrite phase to the austenite phase, the temperature is raised by the processing heat while applying plastic working. A) The ferrite grains are refined by processing in the ferrite region, and b) the work hardening occurs. This is for the purpose of working-induced generation of fine austenite grains from ferrite grains, and c) to refine the austenite grains by working, and to promote strain-induced transformation of fine ferrite grains from work-hardened austenite grains. The various actions and effects are condensed into a unique reverse transformation processing heat treatment technology of "raising the temperature by processing heat while processing".

By the way, in the steel type that forms carbide, the carbide in the slab is mechanically crushed and finely dispersed in the process of raising the temperature by the processing heat while processing, but this carbide becomes the core of the reverse transformation from ferrite to austenite. This promotes the formation of an ultrafine inverse transformed austenite structure, so that this phenomenon can be positively utilized.

Further, in the present invention, Ac ₁
It is recommended that the temperature be raised to a temperature range of not less than the temperature of at least ₃ points or at least ₃ points of Ac, and then maintained at a temperature of not less than ₁ point of Ae or not less than ₃ points of Ae. It is a very effective means to realize it.

That is, in the process of manufacturing a seamless steel pipe, since the processing speed is high and the temperature tends to rise rapidly, in reality, there is a time margin for the reverse transformation to austenite to proceed as described above. Is scarce. In this case, the above-mentioned functions and effects aimed at by the present invention cannot be obtained, and the object of the present invention cannot be sufficiently achieved. Therefore, in this case, if the rolled material is kept in a temperature range of _one or more Ae points or _three or more points of Ae by a reheating furnace, an induction heating device, etc. after the completion of the rolling under the required conditions, the ferrite with a built-in processing strain. Allows time for the grains to transform back to austenite,
The intended purpose will be reliably achieved. The holding time at this time varies significantly depending on the rolling conditions and the type of steel, and in the case of high-purity iron, a unit of seconds that can be said to be almost instantaneous is sufficient, but about 10 minutes is required for a high alloy. There is also.

Subsequently, the effects of the present invention will be described more specifically with reference to examples.Because this example is in accordance with the Mannesmann-mandrel mill process, which is the most typical manufacturing process of a seamless steel pipe, first, The outline of the Mannesmann-mandrel mill process will be described.

FIG. 4 is a schematic process diagram of the Mannesmann-mandrel mill process. In a normal process, a solid round slab is heated to a temperature of 1200 to 1250 ° C. in a rotary hearth type heating furnace (1) and is tilted. A hollow thick hollow piece is pierced by a piercing and rolling mill (2) of a rolling system, and then continuously rolled by a mandrel mill (3) with a mandrel bar inserted into the inner surface of the pipe to mainly perform thickness reduction. Next, the hollow shell from which the mandrel bar has been removed is reheated in a reheating furnace (4), and the outer diameter is reduced to a predetermined outer diameter by a stretch reducer (5).

The following examples were all performed according to the Mannesmann-Mandrel Mill process.

<Example> Example 1 SCM430 equivalent material (Fe-0.29% C-0.22% Si-0.64% Mn-
1.08% Cr-0.24% Mo, Ae ₁ transformation point: 725 ° C, Ac ₁ transformation point: 73
Using a 187φ round slab of 0 ° C, Ae ₃ transformation point: 790 ° C, Ac ₃ transformation point: 795 ° C) as a test material, this was heated to 700 ° C in a rotary hearth heating furnace, and the inlet temperature: 675 At ℃, roll crossing angle: 7 °, tilt angle: 1
A hollow piece of 186φ × 27.5t was formed by drilling under the condition of 5 °, and this was drawn and rolled with an eight-stand mandrel mill to reduce the thickness mainly to obtain a 158φ × 15t hollow shell. Then strip the mandrel bar
After charging in a reheating furnace at 870 ° C. and keeping the heat for 15 minutes, it was squeezed and rolled to 88.9 × 15 ton by a stretch reducer and allowed to cool on a cooling floor.

The piercing ratio at this time was 2.0, and the temperature of the hollow piece immediately after piercing and rolling was 870 ° C. Therefore, by this processing, the material jumps over the Ac ₁ point from the temperature range below the Ac ₁ point and rises to the temperature range above the Ac ₃ point at a stroke, and the reverse transformation from the ferrite phase to the austenite phase is sufficient. Met.

The draw ratio by the mandrel mill was 2.0, and the draw ratio by the stretch reducer was 1.9.
It stopped within 50 ° C. Then, the reduction rolling by the stretch reducer was performed under conditions almost similar to normal rolling.

Microscopic observation of the ferrite grains after cooling of the thus manufactured seamless steel pipe revealed that a very uniform ultrafine grain ferrite structure having a grain size of 2 μm and a grain size number of 15 or more was achieved as intended.

Example 2 S50C equivalent material (Fe-0.5% C-0.25% Si-0.75% Mn,
e ₁ transformation point: 720 ° C, Ac ₁ transformation point: 730 ° C, Ae ₃ transformation point: 765 ° C, Ac ₃
(Transformation point: 775 ° C)
This was heated to 760 ° C. in a rotary hearth heating furnace, and pierced at an inlet temperature of 740 ° C. to form a hollow piece of 186φ × 27.5 t. Then, 8 stands were prepared according to the pass schedule according to the case of Example 1. And rolled mainly by a mandrel mill to reduce the wall thickness to form a hollow shell of 158φ × 15t. Next, the mandrel bar was stripped, charged into a reheating furnace at 870 ° C., and kept for 15 minutes, then squeezed and rolled to 88.9 × 15 t by a stretch reducer, and allowed to cool on a cooling floor.

The piercing ratio at this time was 2.0, and the temperature of the hollow piece immediately after piercing and rolling was 860 ° C. Therefore, by this processing, the temperature of the material is surely raised from the temperature range of Ac ₁ point or more to Ac ₃ point or less to the temperature range of Ac ₃ point or more, and the reverse transformation from ferrite and austenite two phase region to austenite phase is sufficient. Met.

Microscopic observation of the ferrite grains after cooling of the thus produced seamless steel pipe revealed that an ultrafine grained ferrite structure with a grain size of 3.5 and a grain size number of around 14 was realized.

Example 3 The S10C equivalent material (Fe-0.1% C-0.25% Si-0.45% Mn, Ae ₁ transformation point: 720 ° C., Ac ₁ transformation point: 730 ° C., Ae _{3) according} to exactly the same pass schedule as in Example 1. (Transformation point: 865 ° C, Ac ₃ transformation point: 875 ° C)
Finished into a seamless steel pipe product.

Since S10C has a low C content and low deformation resistance, the heat generation during drilling was not as high as that of SCM430, and the temperature of the hollow piece immediately after drilling was about 840 ° C. Moreover, the Ac ₃ transformation point of S10C is about 80 ° C. higher than that of SCM430. Therefore, in this case was heated to a temperature above Ac ₁ point from Ac ₁ point below the temperature range, it did not reach the Ac ₃ point (875 ° C.).

For this reason, the ferrite grains of the seamless steel pipe product after drawing and cooling are not as fine as those in Example 1, but still have a grain size of 5 μm and a grain size of 13 μm.
An ultra-fine ferrite microstructure was obtained at a near-inexperienced level by the conventional control rolling technique.

In these embodiments, examples based on the most typical Mannesmann-mandrel mill line as a process for manufacturing small and medium diameter seamless steel pipes have been described. However, the reverse transformation heat treatment method according to the present invention is based on the Mannesmann-plug mill line, PPM
(Press piercing mimill)-Plug mill line, PPM-
Naturally, the present invention can be applied to a rotary elongator and the like as well as an inclined rolling piercing mill in a mandrel mill line and other seamless steel pipe production lines. In addition,
It goes without saying that the piercing rolling mill or the elongating rolling mill of the inclined rolling type may be of any type of two rolls or three rolls.

<Summary of Effects> As described above, according to the present invention, seamless steel pipes having a uniform and ultrafine structure, which were impossible, can be mass-produced on an industrial scale, and excellent strength, toughness, Industrially extremely useful effects can be obtained, such as a stable supply of a hot-rolled seamless steel pipe having ductility and corrosion resistance.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the inlet side temperature and the outlet side temperature of a piercing mill for a seamless steel pipe material. FIG. 2 is a graph showing the relationship between the temperature on the piercing and rolling mill entrance side of the seamless steel pipe material and the generated processing heat. FIG. 3 is a graph showing the relationship between the temperature on the piercing and rolling mill side of a seamless steel pipe material, the austenite grain size in piercing rolling, and the ferrite grain size after cooling. FIG. 4 is a schematic process diagram of the Mannesmann-mandrel mill process.

Claims (5)

(57) [Claims] 1. A piercing ratio or elongation ratio of 1.5 or more in an inclined rolling mill, and a single or hollow round steel slab having at least a part of ferrite structure is processed to ₁ point of Ac or less using processing heat. Piercing or elongation rolling while raising the temperature from the temperature range of Ac to the temperature range of ₃ or more points, and a step of once transforming all of the ferrite structure to austenite, characterized in that it includes a step of having a superfine structure. Manufacturing method of steelless pipe. 2. A piercing ratio or elongation ratio of 1.5 or more in an inclined rolling mill, and at least _one point of Ac using a heat of processing a "solid or hollow round steel slab at least partially composed of ferrite". in and by drilling or elongation rolling while raising the temperature to a temperature range above ₃ points Ac from temperature range following _three Ac, characterized in that it comprises a step of temporarily reverse-transformed into austenite all tissues composed of ferrite, A method for producing a seamless steel pipe having an ultrafine structure. 3. A pierced or stretched material that has been pierced or stretched and rolled while being heated to a temperature range of ₃ or more points of Ac, and then maintained in a temperature range of ₃ or more points of Ae by a heating device to perform reverse transformation to austenite. A method for producing a seamless steel pipe having an ultrafine structure according to claim 1 or 2, which is promoted. 4. A piercing ratio or elongation ratio of 1.5 or more in an inclined rolling mill, and a single or hollow round steel bar having a structure composed of at least a part of ferrite is subjected to processing heat to obtain an Ac of ₁ point or less. Piercing or elongation rolling while raising the temperature from the temperature range of Ac ₁ point or more to the temperature range of Ac ₃ points or less, comprising a step of once transforming part of the ferrite structure to austenite. , A method of manufacturing a seamless steel pipe having an ultrafine structure. 5. A pierced or stretched material which has been pierced or stretched and rolled while being heated to a temperature range of not less than Ac ₁ point and not more than Ac ₃ points,
5. The method for producing a seamless steel pipe having an ultrafine structure according to claim 4, wherein a reverse transformation to austenite is promoted by maintaining the temperature in a temperature range of ₁ point or more and ₃ points or less of Ae by a heating device.