JPH0472601B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a Mannesmann Piaser piercing and rolling method for seamless martensitic stainless steel pipes used for oil country tubular goods, line pipes, and the like. <Conventional technology> Martensitic seamless stainless steel pipes, represented by SUS410 steel or SUS420 steel, are
It is used for oil country tubular goods, geothermal country tubular goods, line pipes, etc. because it exhibits excellent corrosion resistance in corrosive environments containing CO ₂ . Incidentally, in the perforation process during the manufacture of seamless stainless steel pipes, the inclined rolling method represented by Mannesmann Piaser or the hot extrusion method represented by the Eugene Sejourne method is applied, but martensitic stainless steel Certain types of steel have poor hot workability, and as a result, defects such as cracks and cracks may occur on the outer or inner surface during the piercing-rolling process using a Mannesmann piercer. Therefore, to manufacture seamless steel pipes of these steel types, it is best to use a hot extrusion method typified by the Eugene-Ségiurne process, which is said to be suitable for manufacturing materials with poor hot workability. Normal. <Problems to be Solved by the Invention> By the way, when directly perforating a billet by applying the hot extrusion method, the length of the billet is 5 to 50% of the diameter.
If the thickness is increased by 7 times, the uneven thickness will increase, making it difficult to manufacture long seamless steel pipes. In order to solve this problem, the length of the billet is limited to approximately 15 times the diameter even in the so-called expansion method, in which a guide hole is previously formed in the center of the billet by machining and then expanded by pressing. Furthermore, the typical hot extrusion method, Eugene Sejourne, uses a glass lubricant, so it is necessary to peel off the glass lubricant after extrusion, but this peeling process requires considerable effort and cost. There is. In hot extrusion methods such as the Eugene Sejourne method, it is difficult to increase productivity beyond a certain level because the billet length is restricted, and the yield is inevitably low due to the use of short billets. However, there is a problem in that it does not provide good performance and is disadvantageous in terms of cost. On the other hand, the mandrel mill method and plug mill method can produce long tubes compared to hot extrusion methods such as the Eugene Sejille method.
Therefore, it is known to have various advantages such as high productivity and cost advantage. However, since a Mannesmann piercer is generally used in the drilling process to manufacture the hollow tube, there is the problem of a significant decrease in yield due to defects such as cracks and flaking during piercing and rolling as described above. Therefore, it was considered difficult to apply to some types of martensitic stainless steel. In order to prevent the occurrence of defects during punch-rolling with a Mannesmann piercer, for example, as disclosed in Japanese Patent Laid-Open No. 59-208055, the impurity elements P and S in steel are reduced, and the Methods to improve hot workability have been proposed, but even if such rolled materials are used, if the billet heating conditions or piercing rolling setting conditions are inappropriate, it may be difficult to completely prevent defects from occurring during piercing rolling. It is impossible to avoid this. The present invention was made in view of the above-mentioned circumstances, and it is possible to improve the production of martensitic stainless steel, which has traditionally been difficult to manufacture into grainless steel pipes due to the problem of defects occurring during Mannesmann piercer piercing rolling. Mannesmann Piercer makes it possible to practically apply Mannesmann Piercer piercing rolling to some steel types, thereby making it possible to manufacture this type of seamless martensitic stainless steel pipe with high productivity, low cost, and high yield. The purpose of the present invention is to provide a method for piercing and rolling a sauna. <Means for solving the problems> The present inventors have developed C:
0.12~0.30% (weight%, same below), Cr: 11~14
As a result of intensive investigation and research into the causes of defects such as cracks and flaking on the inner and outer surfaces that occur when piercing and rolling martensitic stainless steel containing After discovering that the presence of ferrite has a significant effect, further research revealed that it was possible to suppress ferrite precipitation during billet heating, and to suppress the precipitation of ferrite due to the rise in material temperature caused by heat generation during drilling and rolling of Mannesmann Pimassa. It was discovered that by suppressing the above-mentioned defects, piercing rolling becomes possible without producing the defects described above, and based on this knowledge, the present invention was accomplished. That is, in the present invention, C: 0.12 to 0.30% by weight,
In piercing rolling of martensitic stainless steel containing 11 to 14% by weight of Cr using a Mannesmann piercer, the temperature is at least 30°C higher than the boundary temperature between the austenite single-layer region and the ferrite-austenite two-phase region on the equilibrium phase diagram. A Mannesmann-Piercer piercing rolling method characterized by heating a billet at a low temperature and setting the strain rate during piercing rolling to an average strain rate ε~ determined by the following formula of 1.3 sec ^{-1 or} less. Note ε~=ln (A ₀ /A) / △T (L _d /L _h ) (sec ^-1 ) Here, A ₀ ; Cross-sectional area of billet (mm ² ) A; Cross-sectional area of hollow shell after rolling ( mm ² ) △T; Time from start to end of piercing rolling (sec) L _d ; Projected contact length in the piercing direction between the rolled material and the roll (mm) L _h ; Length of the hollow tube after rolling (mm) This achieves the above objective. <Function> Detailed experiments and investigations by the present inventors have revealed that defects such as cracks and flaking on the inner and outer surfaces of hollows that occur after piercing and rolling of martensitic stainless steel with a Mannesmann piercer can be prevented during billet heating or with a Mannesmann piercer. It became clear that this was caused by ferrite precipitated by the temperature rise of the rolled material due to heat generation during piercing and rolling.
In other words, when a small amount of ferrite phase precipitates in the austenite phase, the additional shear strain generated during Mannesmann-Piercer piercing and rolling is concentrated in the ferrite phase, which has lower mechanical strength than the base austenite phase, and the ferrite phase and austenite phase are separated. Failure occurs near the boundary with the phase, leading to defects such as cracks and flaking on the inner and outer surfaces. Therefore, in order to prevent the occurrence of such defects, it is necessary to reduce the amount of precipitation of the ferrite phase in the austenite phase of the drawn material during Mannesmann-Piercer piercing rolling, or preferably, to prevent the precipitation of the ferrite phase. It is. Figure 2 shows, as an example, an equilibrium diagram of 13% Cr steel depending on carbon content and temperature. Carbon content is 0.12-0.30%
In the vicinity of 1100°C to 1350°C, which is the piercing and rolling temperature of the general Mannesmann Piaser, the austenite single phase (γ) or the ferrite-austenite two-phase region (α+γ) is predominant. Ferrite may precipitate during heating of the billet in a heating furnace, or may precipitate due to temperature rise associated with heat generation during Mannesmann-Piercer piercing and rolling. Therefore, in order to reduce the amount of ferrite precipitated or to prevent ferrite from precipitating during piercing rolling with Mannesmann Piaser, the heating temperature of the rolled material should be lowered,
In addition, it is necessary to reduce the temperature rise of the rolled material due to processing heat generation during Mannesmann piercer piercing rolling. The processing heat value Q per unit time and unit volume is generally given by the following equation (1). Q・=Kf・ε・・△V ……(1) Here, Kf: Deformation resistance (Kg/cm ² ) ε・: Equivalent strain rate (sec ^-1 ) △V: Unit volume (mm ³ ) This ( As is clear from equation 1), it is possible to reduce the temperature rise of the rolled material due to processing heat generation by reducing the strain rate during piercing rolling. Here, the strain during Mannesmann Piaser piercing rolling,
To handle the strain rate, the average strain ε~ and strain rate ε~ will be defined as follows. ε〜=ln(A _p /A) ……(2) ε〜=ε〜／△τ ……(3) △τ＝△T(L _d /L _h ) ……(4) Here, A ₀ ; Cross-sectional area of billet (mm ² ) A: Cross-sectional area of hollow tube after rolling (mm ² ) △τ: Time during which a minute part of the material to be rolled is actually plastically deformed (sec) △T: Start of piercing rolling Time from to completion (sec) L _d ; Projected contact length in the perforation direction between the rolled material and the roll shown in Figure 3 (mm) L _h ; Length of the hollow tube after rolling (mm) Therefore, The average strain rate ε~ is determined by the following equation (5). ε~=ln( _Ap /A)/△T( _Ld / _Lh )...(5) That is, the average strain rate ε~ is the cross-sectional area A and length _Lh of the hollow tube after piercing and rolling. , the time ΔT from the start to the end of perforation, and the projected contact length L _d between the rolled material and the roll in the perforation direction. Therefore, each steel type having the typical martensitic stainless steel components shown in Table 1 was perforated and rolled using a Mannesmann piercer with a roll gorge outer diameter of 1045 mm at various heating temperatures and average strain rates. Basic

[Table] (Note) Ferrite precipitation temperature; austenite single phase region on equilibrium phase diagram
We investigated the occurrence of defects such as cracks and flakes on the inner and outer surfaces of the boundary temperature tube between the ferrite and ferrite-austenite two-phase regions. The survey results are summarized in Table 2. In addition, the determination results of the defect occurrence status in the table were visually observed, and were expressed as ○: No defect occurred, Δ: Defect occurred only at the rear end, ×: Defect occurred over the entire length. Based on this Table 2, the relationship between the difference between the ferrite precipitation temperature and the heating temperature and the average strain rate ε is shown in FIG. As is clear from Fig. 1, the billet is heated at a temperature that is at least 30°C lower than the ferrite precipitation temperature, that is, the boundary temperature between the austenite single-layer region and the ferrite-austenite two-phase region on the equilibrium phase diagram, and the average strain It can be seen that when the speed ε~ is 1.3 sec ^-1 or less, no defects such as cracks or baldness occur on the inner and outer surfaces of the hollow tube after drilling. Therefore, the billet is heated at a temperature that is at least 30°C lower than the boundary temperature between the austenite single-layer region and the ferrite-austenite two-phase region on the equilibrium diagram, and the average strain rate ε~ is 1.3 sec ^-1.

【table】

[Table] By performing piercing rolling with a Mannesmann piercer under the following regulations, it is possible to prevent defects such as cracks and baldness on the inner and outer surfaces of the hollow tube after piercing. Here, in order to prevent precipitation of the ferrite phase, the lower the billet heating temperature is, the more advantageous it is, but if the temperature drops too much, the deformation resistance increases and the rolling load of the piercer mill increases. this is,
From the viewpoint of tool life, the higher the billet heating temperature is, the better, because not only does it exceed the mill capacity limit, but also the rolling load increases, which shortens the life of tools such as plugs. Therefore, it is realistic to set the billet heating temperature to 1100°C or higher. Furthermore, in order to reduce the temperature rise of the material due to processing heat generation, the smaller ε~ is, the better.
As a means to reduce ε~, it is necessary to reduce the amount of processing by the piercer or to slow down the peripheral speed of the rolls and to lengthen the rolling time. However, if the piercer's machining amount becomes smaller, the load on the mill in the downstream process will increase, so there is a limit, and if the roll circumferential speed is slow and the rolling time is long, the load on tools such as plugs will increase and the tool life will increase. This is a problem because it reduces the Therefore, it is realistic to set ε~ to 0.4 or more. <Examples> Examples of the present invention will be described below. FIG. 4 schematically shows an example of the Mannesmann-Piercer mandrel mill method for rolling martensitic stainless steel seamless pipes. The material billet 2 is heated to a predetermined temperature in the rotary hearth type heating furnace 1, and then punch-rolled by a Mannesmann piercer 6 to form a hollow shell 8A.
Since this hollow tube 8A is thick and short, it is stretched to reduce its thickness by a mandrel mill 10, which is a stretching mill. The mandrel mill 10 is a rolling mill that performs elongation rolling with a mandrel bar 12 inserted into a hollow tube 8A, and usually consists of 6 to 8 roll stands, and each roll stand has two grooved rolls 14. The rotary axes of the grooved rolls 14 are arranged between adjacent roll stands so as to be shifted from each other by 90 degrees in a plane perpendicular to the rolling axis. The hollow blank tube 8A is stretched to a length 2 to 4 times its original length by a mandrel mill 10, and becomes a blank tube 8B for finish rolling. The raw pipe 8B for finish rolling is reheated to a predetermined temperature in a reheating furnace 16 as necessary, and then finish rolled in a finish rolling mill, for example, a stretch reducer 18. The outer diameter of the raw pipe is reduced by up to 75% by the stretch reducer 18 and stretched to more than 40 times the length of the material billet, and the outer surface is further reduced by the stretch reducer 18.
Since the finished tube 20 is shaped by several stands of perfect circular hole rolls on the final side of the tube, a finished tube 20 with relatively excellent outer diameter dimensional accuracy can be obtained. As the martensitic stainless steel that is the material to be rolled, C: 0.19%, Si: 0.45%, Mn: 0.45%,
P; 0.015%, S; 0.0013%, Cr; 12.9%, N;
0.022% SUS420J2 was used. Austenite single layer region and ferrite on the equilibrium phase diagram of this material
The boundary temperature with the austenite two-phase region was 1285°C. A billet with an outer diameter of 207 mmφ and a length of 2000 mm was heated in a rotary hearth type heating furnace 1 at 1250°C for 163 minutes in the furnace, resulting in a billet with an outer diameter of 222 mmφ, a wall thickness of 20 mm, and a length of 5300 mm.
Piercing and rolling was performed using a Mannesmann piercer 6 so that the following results were obtained. The piercer used had a roll gorge diameter of 1045 mm, a gorge interval of 189 mm, a lead of 62 mm, a plug diameter of 156 mmφ, a roll rotation speed of 104 rpm, a roll inclination angle of 9°, and the projected contact length L _d at this time was 505 mm.
Drilling time △T is 9.8sec, △τ is 0.93sec, average strain ε~
was 1.12, and the average strain rate ε~ was 1.2 sec ^-1 . As a result, there were no defects on the inner and outer surfaces of the hollow tube after piercing and rolling. <Effects of the Invention> As explained above, according to the present invention, it is possible to prevent the occurrence of serious defects such as cracks and flaking during punch-rolling with the Mannesmann piercer, thereby improving the yield and further improving the present invention. It is now possible to roll steel types that were previously impossible to roll, and it is possible to manufacture seamless martensitic stainless steel pipes with higher productivity, higher yield, and lower cost compared to hot extrusion methods such as Eugene Sejourne. It is.

[Brief explanation of drawings]

Figure 1 is a characteristic diagram showing the relationship between the difference between ferrite precipitation temperature and heating temperature and average strain rate regarding the presence or absence of defects, and Figure 2 is a characteristic diagram showing the equilibrium state depending on the carbon content and temperature of 13% Cr steel. FIG. 3 is a perspective view schematically showing the piercing and rolling of the Mannesmann Piaser, and FIG. 4 is a perspective view schematically showing an example of the Mannesmann Piaser mandrel mill type production line. 1... Rotary hearth type heating furnace, 2... Material billet, 3... Mannesmann Piaser, 4... Plug bar, 5... Plug, 6... Piaser roll, 8A...
...Hollow tube.

Claims (1)

[Claims] 1. In piercing rolling of martensitic stainless steel containing 0.12 to 0.30% by weight of C and 11 to 14% by weight of Cr using a Mannesmann Piaser, an austenite single phase region and ferrite on an equilibrium phase diagram are formed. - The billet is heated at a temperature at least 30°C lower than the boundary temperature with the austenite two-phase region, and the strain rate during piercing rolling is such that the average strain rate ε~ determined by the following formula is 1.3 sec ^-1 or less A Mannesmann-Piercer piercing rolling method characterized by: Note ε~=ln (A ₀ /A) / △T (L _d /L _h ) (sec ^-1 ) Here, A ₀ ; Cross-sectional area of billet (mm ² ) A; Cross-sectional area of hollow shell after rolling ( mm ² ) △T; Time from start to end of piercing rolling (sec) L _d ; Projected contact length in the piercing direction between the rolled material and the roll (mm) L _h ; Length of the hollow tube after rolling (mm)