JP2591234B2 - 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 that reason,
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 transformed from austenite, and the so-called attempt to further refine the crystal grains by taking into account its action. "Accelerated cooling technology" has also been developed.

However, even with the technology that combines "controlled rolling" with "accelerated cooling", there is naturally a limit on the "final grain size of austenite before transformation by cooling", and a uniform ultrafine austenitic structure that overcomes this limit is obtained. That was impossible. 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 encountered in these prior arts is that the austenite grains produced by hot working become practically no longer finer when they are refined to a certain degree. Derived from the limitations of technology, even if trying to produce a fine ferrite structure by accelerated cooling from an austenite structure that is not sufficiently refined, a satisfactory uniform ultrafine structure cannot be obtained at all It is a translation.

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 a continuous rolling mill for research (a 24-stand sinking reducer employing a common drive system for the drive) are a means to break down the austenitic structure itself during hot rolling of seamless steel pipes. The result was as follows.

Experiment 1 Using a steel pipe with a diameter of 21.5φ and a wall thickness of 2.5t as a test material, draw-rolling by changing the drawing ratio of the heating temperature (the ratio of the length of the product after drawing to the length of the tube before drawing) An experiment was conducted to measure the inlet temperature just before entering the rolling mill and the outlet temperature immediately after the reducing rolling, and the working heat generated by the reducing 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.

By the way, in the reduction rolling experiment, the inlet temperature was set to six levels (1150
C., 1050.degree. C., 950.degree. C., 850.degree. C., 750.degree. C., and 650.degree. C.) and the draw ratio was changed at four levels (1.5, 2, 3, and 4).

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" ( In each case, the draw ratio is set as a parameter).

The following findings were obtained from this experiment. That is, (a) the generation of processing heat becomes more remarkable as the entry-side temperature decreases, and this tendency appears more remarkably as the drawing ratio of the reduction rolling increases. For example, the temperature rise caused by the processing heat when the inlet temperature is 650 ° C. and the stretching ratio is 2 is approximately 170 ° C., and reaches 270 ° C. when the stretching ratio is 4. When the inlet temperature is 750 ° C. and the stretching ratio is 2, the temperature rise by the processing heat is about 130 ° C., and when the perforation ratio is 4, it is about 210 ° C.

(B) note is the entry side temperature (Material heating temperature), by the choice whether the heating temperature and the draw ratio to the temperature range of not lower than Ac ₁ point of the following temperature range Ac ₁ point, Ac at _one point or more And Ac
A temperature range of below ₃ points to the temperature range of not lower than Ac ₃ point, even Ac ₁
The point is that the reverse transformation from the temperature range below the temperature point to the temperature range above the Ac ₃ point can be realized at once. For example, if the inlet temperature is 680
If the ° C., Ac ₁ from the temperature range of less than Ac ₁ point at a draw ratio of 1.5
From the temperature range below Ac ₁ point to A
c Reverse transformation to ₃ points or more is sufficiently possible, and if the inlet temperature can be set to 780 ° C., at a stretching ratio of 1.5, the temperature range from ₁ point or more to Ac and ₃ points or less from Ac to ₃ points or more from Ac Reverse transformation to the temperature range is sufficiently possible.

In other words, the applied continuous rolling mill for research was composed of up to 24 stands as described above,
In this continuous rolling mill, the reduction in area of the single stand is small, but the continuous rolling is performed at an extremely high speed as in the case of the cotton rolling mill, so that strain is accumulated in the low-temperature side rolling. Therefore, base pipe the Ac ₃ point or less, even if reducing rolling at a high reduction ratio at a low temperature region to say the temperature below Ac ₁ point, A from the temperature range of less than Ac ₁ point
c To the temperature range of ₁ point or more or from the temperature range of ₃ points or less to Ac ₃
It is considered that the temperature can be raised all at once to the temperature range above the temperature point, and further from the temperature range of the temperature of less than Ac ₁ point to the temperature range of more than Ac ₃ point.

Experiment 2 A steel pipe equivalent to SCM430 with a diameter of 21.5φ and a thickness of 2.5t was used as the test material, the draw ratio was fixed at 2.5, the inlet temperature was changed from 1200 ° C to 600 ° C every 50 ° C, and immediately after rolling. The austenitic grain size and the ferrite grain size after cooling were observed and investigated. The other experimental conditions are the same as those in "Experiment 1".

The effect of the inlet temperature on the austenite grain size immediately after drawing rolling and the ferrite grain size after cooling obtained in this experiment is summarized in FIG.

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

(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 close to 15, and the ferrite grain size after cooling. Indicates 15 or more. In addition, the ferrite grain size near 12 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 of Ac to the temperature range of ₃ points or more of Ac, and these reverse transformation processing heat treatments It is sufficiently possible to reduce the ferrite particle size after cooling to 7 μm or less.

In this experiment, since the stretching ratio was unified to 2.5, as a result, the temperature was raised to the temperature range of Ac ₃ points or more by reduction rolling in all temperature ranges, but the stretching ratio was low. It seems that the ferrite particle size after cooling can be sufficiently reduced to 10 μm even by the reverse transformation from the temperature range below the Ac ₁ point to the temperature range above the Ac ₁ point.

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, Ac ₃ transformation point although different, if properly choose the heating temperature and draw ratio from Ac ₁ point below the temperature range to Ac ₁ point or more temperature range, or Ac ₁ or more points And Ac
_From the temperature range of ₃ points or less to the temperature range of ₃ points or more of Ac,
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 point is exceeded to reversely transform to the austenite structure, the processing must be performed to sufficiently complete the reverse transformation. After the process of temperature rise due to heat is completed, it is preferable that the temperature is maintained for a certain period of time at or above the A ₁ transformation point (ie, Ae ₁ point) or A ₃ transformation point (ie, Ae ₃ point) in a perfect equilibrium state.

(C) The ultrafine 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, while “drawing a rolling mill at a draw ratio of 1.5 or more and performing plastic working of“ a thin hollow hollow shell at least partially made of ferrite ”at a low temperature. , whereby the temperature range from Ac ₁ point below the temperature range of not lower than Ac ₁ point by processing heat generated, or Ac at ₁ or more points and to a temperature range of not lower than Ac ₃ point from the temperature range following _three Ac, more Desirably, the temperature is raised from the temperature range of Ac ₁ point or less to the temperature range of Ac ₃ points or more at once, and if necessary, the temperature is raised to the temperature range of Ac ₁ point or more, preferably Ae ₃ points or more. By holding the material, a part or all of the structure composed of the ferrite is reversely transformed into austenite, thereby realizing a uniform ultrafine austenitic structure, and further cooling to obtain an ultrafine structure (ferrite grain size of 10 to 5 μm). Below) Intensity, and has toughness, ductility, and wherein the hot-rolled seamless steel tube having a corrosion resistance to the stable point to allow produced. "

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 is obtained from commercial low carbon steel to pure iron, and not only carbon steel but also 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 draw ratio is less than 1.5, the working strain is small, and the generation of working heat tends to be insufficient.
It is difficult to make the temperature of the workpiece reach the temperature at which the ferrite is transformed back into austenite. In addition, even if the reverse transformation from ferrite to austenite could be achieved, the induced generation by processing of fine austenite grains was insufficient, and the target austenite grain size to be formed was targeted.
It is difficult to make the thickness 15 μm or less. In other words, a uniform fine austenite structure having an average grain size of 15 μm or less can be relatively easily realized only by setting the amount of strain in the plastic working at the time of reverse transformation from ferrite to austenite to 1.5 or more in a draw ratio. However, in order to realize a uniform fine austenitic structure that is stable and uniform on site in consideration of all types of steel, the amount of plastic working strain to be applied at the time of reverse transformation from a ferrite phase to an austenite phase is 2% at a draw 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 increased by the processing heat while performing plastic processing. The reasons are as follows: a) Refinement of ferrite grains by processing in the ferrite region, and b) Work hardening. This is for the purpose of working-induced generation of fine austenite grains from ferrite grains, c) miniaturization by processing austenite grains, and further promoting 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, when the rolled material is kept in a temperature range of _one point or more of Ae or _three or more points of Ae by an induction heating device or the like after the completion of the rolling under the required conditions, the ferrite grains with built-in processing strain become austenite. There is enough time for reverse transformation, and the intended purpose can 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.However, in this example, since it 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 to about 900 ° C. in a reheating furnace (4), and the outer diameter is reduced to a predetermined outer diameter and wall thickness by a stretch reducer (5). It is left to cool on the cooling floor.

As the stretch reducer (5), a three-roll type has recently become widespread, and a method is adopted in which a “diaper shape” is drawn close to a perfect circle by a finishing pass while being squeezed and rolled into a “reverse diaper shape”. Twenty-four stand stretch reducers are the most popular, but 28-stand stretch reducers have come into use. In each case, each stand is generally driven independently, and the roll rotation speed of each stand is set so as to give a tension between stands of at most 85% of the deformation resistance. The thickness can be adjusted.

By the way, when the embodiment of the present invention is carried out, as shown in FIG. 4, the induction heating device (6) is provided immediately after the stretch reducer (5) of the Mannesmann-mandrel mill line.
Was specially arranged.

The following examples were all carried out according to the Mannesmann-mandrel mill process in which the induction heating device (6) was specially arranged.

<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 round slab of 0 ° C, Ae ₃ transformation point: 790 ° C, Ac ₃ transformation point: 795 ° C) as a test material, it is heated to 1250 ° C in a rotary hearth heating furnace and has a cone-type main roll. A 228 mm x 17.5 t holographic piece was formed by drilling as usual under the conditions of an inlet side temperature of 1210 ° C, a roll crossing angle of 7 ° C, and an inclination angle of 12 ° by a piercing rolling mill of the inclined rolling method. The thickness was reduced mainly by elongating and rolling in a mill, and the hollow shell was allowed to cool as a hollow shell of 198φ × 6.5t. Next, this was charged into a reheating furnace maintained at a temperature of 700 ° C., and after keeping heat for 15 minutes, it was drawn and reduced to 76.2φ × 6.0 t by a stretch reducer of 24 stands.

At this time, the piercing ratio in the piercing mill was 3.4, the stretching ratio in the mandrel mill was 3.0, and the stretching ratio in the stretch reducer was 3.0.

At this time, the inlet temperature of the stretch reducer is 68
5 ° C., the outlet side temperature of the following temperature range Ac ₁ point a 870 ° C. A
c The temperature was surely raised to the temperature range of _three or more points, confirming that the reverse transformation from the ferrite phase to the austenite phase occurred sufficiently.

Microscopic observation of the ferrite grains after cooling on the cooling floor of the seamless steel pipe manufactured in this way revealed that an extremely uniform ultrafine grain ferrite structure having a ferrite grain size of 3 μm and a grain size number of 14 or more was achieved as intended. Was.

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 1250 ° C in a rotary hearth heating furnace, and was drilled by a piercing and rolling mill using an inclined rolling method under the conditions of an inlet temperature of 1210 ° C, a roll crossing angle of 7 ° C, and an inclination angle of 12 ° as usual. 186φ ×
27.5t hollow piece, followed by elongation rolling with an 8-stand mandrel mill to reduce the thickness mainly to 158φ × 15t
And allowed to cool. Next, it was charged into a reheating furnace maintained at a temperature of 760 ° C., kept for 15 minutes, and then drawn and reduced to 88.9φ × 15 t by a stretch stand of 22 stands. Immediately after the stretchducer, the temperature was maintained at 850 ° C. for a few seconds by a special induction heating device, and then cooled on a cooling floor.

At this time, the piercing ratio in the piercing mill was 2.0, the stretching ratio in the mandrel mill was 2.0, and the stretching ratio in the stretchducer was 1.9. At this time, the inlet temperature of the stretchducer was 745 ° C, and the outlet side was 745 ° C. The temperature was 840 ° C.

Therefore, by the processing with this stretch transducer, the temperature of the material is surely raised from the temperature range of ₁ point or more Ac to ₃ points of Ac to the temperature range of ₃ points or more of Ac, and from the ferrite + austenite two phase region to the austenite phase. It was confirmed that the reverse transformation had sufficiently occurred.

Microscopic observation of ferrite grains after cooling on a cooling bed of the seamless steel pipe thus manufactured showed
An ultrafine-grained ferrite structure with a size of 4.5 μm and a grain size of around 13 was realized.

Example 3 Example 1 and exactly the same pass schedule by S10C equivalent material (Fe-0.1% C-0.25 % Si-0.45% Mn at Ae ₁ transformation point: 7
20 ° C, Ac ₁ transformation point: 730 ° C, Ae ₃ transformation point: 865 ° C, Ac ₃ transformation point: 875
C), a 76.2φ × 6.0t seamless steel pipe was manufactured as a test material for 225φ round steel slabs, and heat retention was performed for several seconds by a special induction heating device immediately after the stretch reducer to complete the product.

At this time, the stretch reducer inlet temperature is 68
Although it was 5 ° C, S10C generated SCM
Stretch reducer outlet temperature is 820, not as high as 430
° C. And the heat retention temperature of the induction heating device is 83
It was 0 ° C.

Therefore, the material temperature during the rolling in this case is:
The temperature was raised from the temperature range of ₁ point or less to the temperature of ₁ point or more of Ac, but did not reach the ₃ point of Ac (875 ° C.).

For this reason, the ferrite grains of the seamless steel pipe product after cooling following induction heating are not as fine as those of Example 1, but still have a grain size of 6 μm and a grain size number of about 12 in the prior art. In the controlled rolling technique, an ultra-fine ferrite structure of a completely inexperienced level was obtained.

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, it can be applied to a sinking reducer or a sizing mill as well as a stretch reducer in a mandrel mill line or other seamless steel pipe production line. It should be noted that the reduction rolling machine such as a stretch reducer is not limited to a 2-roll, 3-roll or 4-roll type.

<Summary of Effects> As described above, according to the present invention, it is possible to mass-produce seamless steel pipes having a uniform ultra-fine structure, which was impossible in the past, on an industrial scale, and to obtain excellent strength and 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 rolling mill of a seamless steel pipe material. FIG. 2 is a graph showing the relationship between the temperature on the inlet side of the rolling mill of the seamless steel pipe material and the generated processing heat. FIG. 3 is a graph showing the relationship between the inlet side temperature of the reduction rolling mill of the seamless steel pipe material, the austenite grain size immediately after the reduction rolling, and the ferrite grain size after cooling. FIG. 4 is a schematic process diagram of a Mannesmann-mandrel mill process in which an induction heating device is specially provided.

Claims (5)

(57) [Claims] 1. A drawing mill with a draw ratio of 1.5 or more, and a thin hollow hollow shell made of at least a part of ferrite at a temperature of not more than ₁ point of Ac and not less than ₃ points of Ac while utilizing processing heat. A method for producing a seamless steel pipe having an ultrafine structure, comprising a step of drawing and rolling while raising the temperature to a temperature range, once transforming all of the ferrite structure into austenite, and then cooling. 2. A drawing mill having a draw ratio of 1.5 or more, and forming a thin hollow hollow shell at least partially made of ferrite at a temperature of ₁ point or more and Ac ₁
It is characterized in that it includes a step of drawing and rolling while raising the temperature from a temperature range below the Ac point to a temperature range above the Ac ₃ point, and once transforming all of the ferrite composition back to austenite and then cooling. A method of manufacturing a seamless steel pipe having a structure. 3. A drawn material which is drawn and rolled while raising the temperature to a temperature range of ₃ points or more of Ac, and subsequently maintained in a temperature range of ₃ points or more of Ae by a heating device to promote reverse transformation to austenite. 3. A method for producing a seamless steel pipe having an ultrafine structure according to 1 or 2. 4. A drawing mill with a draw ratio of 1.5 or more, forming a thin hollow hollow shell composed of at least a part of ferrite from a temperature range of ₁ point or less of Ac and ₁ point or more of Ac while utilizing processing heat. A process having an ultra-fine structure, characterized by including a step of subjecting a part of the structure composed of ferrite to reverse transformation to austenite once and cooling after elevating the temperature to a temperature range of ₃ points or less. Manufacturing method of steel pipe. 5. A drawn material which is drawn and rolled while raising the temperature to a temperature range of not less than Ac ₁ point and not more than Ac ₃ point, and subsequently heated by a heating device.
The method for producing a seamless steel pipe having an ultrafine structure according to claim 4, wherein the temperature is maintained at a temperature of ₁ point or more and Ae ₃ points or less to promote reverse transformation to austenite.