JPS63290606A - Manufacture of seamless steel pipe - Google Patents

Abstract

PURPOSE:To obtain a seamless steel pipe having a smooth inner surface by specifying a surface roughness of a mandrel bar and specifying a mean stretch coefficient at the time of rolling through a stretch reducer. CONSTITUTION:A billet 2 is heated through a rotary hearth type heating furnace 4, then subjecting to piercing rolling to obtain a hollow blank pipe 8A by a Mannesmann piercing mill 6. Then, a mandrel lever 12 having Rmax 30 mum sur face roughness is inserted into the inner part to make a blank pipe 8B for finish rolling by a mandrel mill 10. After this blank pipe 8B for finish rolling is heated through a reheating furnace 16, then it is made into a finished pipe 20 having >=0.58 a mean stretch coefficient by a stretch reducer 18. In this way, a plug seizure is prevented to obtain a smooth pipe without furrows in its inner surface.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method for manufacturing seamless steel pipes.

[Prior art] Seamless steel pipes are generally manufactured by rolling methods such as the mandrel mill method and plug mill method, or hot extraction methods such as the Eugene Séjourne method and the Erhard-Buschbench method. Mandrel mill rolling is widely used for pipe manufacturing because of its excellent productivity and dimensional accuracy.

In the mandrel mill method, for example, as shown in FIG.
The tube is pierced and rolled into a hollow tube 8A. Alternatively, this hollow tube 8A may be directly manufactured by the continuous casting machine 5 for manufacturing hollow tubes.Since this hollow tube 8A is thick and short, it is thinned and stretched by a mandrel mill 10, which is an elongation rolling machine. be done. The mandrel mill 10 has a hollow tube 8
This is a rolling mill that performs elongation rolling with a mandrel bar 12 inserted in A, and is usually composed of 6 to 8 roll stands, and each roll stand carries two grooved rolls 14.
The rotary axes of the grooved rolls 14 are arranged to be shifted by 80 degrees between adjacent roll stands 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. After the raw pipe 8B for finish rolling is reheated to a predetermined temperature in the reheating furnace 16 as necessary,
Finish rolling is performed by a stretch reducer 18, which is a finish rolling mill. 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 raw material billet, and the outer surface has several stands of perfect circular holes on the final side of the stretch reducer 18. Since the finished tube 20 is shaped using mold rolls, a finished tube 20 with relatively excellent outer diameter dimensional accuracy can be obtained.

Here, the stretch reducer 18 is constituted by a rolling roll group consisting of a plurality of stands, and the rolling rolls of each stand are arranged, for example, in a 30-row arrangement as shown in FIG. 2, and the roll rotation speed ratio of each stand is adjusted. By doing so, the tension applied to the raw tube during rolling is controlled, and its wall thickness can be controlled and used.

[Problems to be Solved by the Invention] By the way, when a material with relatively poor hot deformability, such as low alloy steel or stainless steel, is rolled with a stretch reducer, as shown in FIG. Longitudinal wrinkles, so-called wrinkle defects, occur on the inner surface of the tube. If this wrinkle flaw is sharp and deep, the scale stuck in the wrinkle flaw will remain unremoved even by pickling in the subsequent cold grain pretreatment process, and will remain after the area reduction in the cold drafting process. Not only do they remain as wrinkles on the inner surface and the inner surface is not smooth,
In some cases, plug seizure may cause trouble such as inability to operate. Therefore, for these reasons, conventionally, for products that require smoothness of the inner surface of the pipe after finish rolling, a polishing process must be added to remove wrinkles after rolling, resulting in lower yields and lower costs. This was the cause of the rise.

An object of the present invention is to reduce or prevent wrinkles that occur on the inner surface of a pipe after stretch reducer rolling, and to produce a seamless steel pipe with a smooth inner surface.

[Means for Solving the Problems] The present invention involves stretching and rolling a hollow tube into which a mandrel bar is inserted, using a mandrel mill, finishing after rolling with the mandrel mill, and forming the tube for rolling into a plurality of stands. In a method for manufacturing a seamless steel pipe by reducing and rolling with a stretch reducer, the surface roughness of the mandrel bar used for rolling with a mandrel mill is Rmax 30W layer or less, and the rolling rolls of each stand are used for rolling in the stretch reducer. Stretch coefficient (tensile stress/
The average value of yield stress for all stands is set to 0.58 or more.

[Function] When a foreign material (for example, a non-metallic inclusion layer) exists near the inner surface of the tube during straight 2-chire reducer rolling, in order to prevent the inner layer from breaking or peeling on the inner surface of the tube after rolling,
One idea is to avoid applying tensile stress above a certain level in the longitudinal direction of the pipe. To that end, the Special Publickate Sho 131-908
Publication No. 4 proposes setting the average stray 2,000 coefficient Zm to 0.45 or less.

However, as a result of careful investigation by the inventors,
It has become clear that wrinkles on the inner surface of low-alloy steel, stainless steel, etc. are a completely different phenomenon from the inner surface defects that occur due to the mechanism based on the presence of different materials as described above. Therefore, as a result of repeated research through repeated and more detailed experiments, we found that: ■ Wrinkles on the inner surface of the pipe that occur after stretch reducer rolling are caused by compressive force in the circumferential direction that is generated on the inner surface of the pipe due to drawing deformation during stretch reducer rolling. What happens due to buckling on the material surface? ■Buckling on the material surface due to compressive force in the circumferential direction generated on the inner surface of the tube is caused locally by minute irregularities that originally exist on the inner surface of the raw tube for finish rolling. If the inner surface of the raw pipe for finish rolling is smooth, buckling will be less likely to occur, thus reducing the occurrence of wrinkling. By reducing the compressive strain, buckling becomes less likely to occur, and therefore, by increasing the tension applied in the longitudinal direction of the pipe during stretch reducer rolling, wrinkling defects can be reduced. It has come to this.

By the way, in general, a new or modified mandrel bar is used for rolling 2000 to 5000 times, depending on the roughness of the bar surface and the degree of change in outer diameter due to wear.
The size is reduced by modification and used again for rolling. Immediately after recutting, the bar surface is finished with an R+*ax of about 10 to 20g+s, but after being used for rolling, the surface becomes rough and wears out just before the next recutting. Depending on the case, it can be as low as 100 ru. There are also localized cracks and scratches.

Figure 4 shows the relationship between the surface roughness Rmax of the bar and the roughness Rmax of the inner surface of the tube for finish rolling when the bar is used for rolling.A water-soluble graphite lubricant is applied to the bar surface during mandrel mill rolling. The roughness of the bar surface is not necessarily transferred as is to the inner surface of the raw tube for finishing rolling, and the roughness of the bar surface is higher than the bar surface roughness Rmax. Although the inner surface roughness Rmax of the raw tube for rolling is smaller, as the roughness of the bar surface increases, the roughness of the inner surface of the raw tube for finish rolling also increases.

Stretch reducer rolling was performed under the same conditions using such a raw pipe for rolling, and the roughness Rraax of the inner surface of the raw pipe for finish rolling and the depth of wrinkles on the inner surface of the finished pipe after rolling were measured. As a result, as shown in FIG. 5, the smaller the roughness Rmax of the inner surface of the raw pipe for finish rolling, the smaller the wrinkle flaw depth.

On the other hand, the rolling theory of stretch reducer by Neumann and Hancke (Stahl and Eisen)
75 (1955).

No. 22) is analyzed as follows.

Condition of constant volume, φ sentence + φθ + φr = O (1) where φ sentence: Axial logarithmic strain φθ: Circumferential direction logarithmic strain
σm) = (σ - σm): (σθ - σm) = φr: φ or: φθ (2) where σr: Radial stress σ Biaxial stress σθ: Circumferential stress σm: Average stress [= (στ + σ + σθ) / 3] On the other hand, the yield condition expression of τresca and σ statement - σθ = k f (3) where kf: radial balance approximation expression of deformation resistance thin-walled pipe % expression % (4) where Input: Using the wall thickness and outer diameter ratio of the pipe, the average stress is σm = (σλ + σθ + σr) / 3 = [(2 + in) · σ - (1 + in) · kf] / 3 (5) Here, If we define the stretch coefficient Z as Z = σ statement/kf (8), then φr: φ intersection: φθ (7) = 2 according to equations (2), (5), and (8).
(Enter-1)・Z-(2 people-1): (1-person)・Z+ (1+λ) = (1-person)-Z-(2-person) From equation (7), φr, The φ sentence and φθ change as a function of Z and input as shown in FIG.

It can be seen that the larger the stretch coefficient Z, the smaller the compressive strain φθ in the circumferential direction acting on the inner surface of the tube. The physical meaning of the stretch coefficient is the ratio of the longitudinal stress to the deformation resistance of the material, as is clear from equation (6). In other words, it is a parameter representing how much longitudinal tension is applied during stretch reducer rolling.

Furthermore, φ r/φ θ = K
If we take (8) and solve equation (7) for Z, we get (1-person)-2(enter-1).

Equation (9) is a relational expression that holds true for each stand during stretch reducer rolling, but it is a parameter that can be used for all stands as an easy guide to know how much tension is acting in the tube axis direction during actual operation. The average stretch coefficient Zm throughout is generally expressed by the following equation.

K-(1-person 11)-2 (input m-1) K=φr/φθ Tt (Dt −Tt) Ts: Wall thickness of the raw tube for finish rolling Ds Outer diameter of the raw tube for second finish rolling Tt: After finishing rolling Pipe wall thickness Dt = Pipe wall thickness after finish rolling In the cold drawing process after stretch reducer rolling, as shown in Table 1, if the wrinkle depth is less than about IQpm, there will be a problem up to at least an area reduction of 33%. However, if the wrinkle depth is 13 pm, the reduction rate is 15%.
In this case, the wrinkle flaw is not crimped and remains even after cold drawing, resulting in a defect. Furthermore, if the wrinkle flaw depth is 18 gm, plug seizure occurs during cold drawing, making cold drawing impossible.

On the other hand, using raw tubes for raw rolling that were subjected to mandrel mill rolling using mandrel bars with different surface roughnesses, the wrinkle depth on the inner surface of the finished tube after stretch reducer rolling was determined by changing the average stretch coefficient Zm. The results of measuring the elasticity are shown in Figure 1. The larger the average stretch coefficient Zm and the smaller the surface roughness of the mandrel bar used, the smaller the wrinkle depth. ;The plug is burned out.

Therefore, in order to achieve a wrinkle depth of 10IL11 or less that leaves no wrinkle defects during cold wheat and does not cause plug seizure, a mandrel bar with at least a surface roughness Rmax of 30 pm or less should be used during mandrel mill rolling, and the average It is necessary to perform stretch reducer rolling with a stretch coefficient of 0.58 or more. The average stretch coefficient can theoretically take a value between O-1, but the slip between the roll and the material during stretch reducer rolling,
Or, due to operational problems such as material breakage, it is practically 0.
．． The upper limit is about 65 to 0.70.

[Example] Next, the present invention will be explained by examples.

Using 5US304 as material, 110φX 1301
After heating the billet 2 to 1240°C in a rotary hearth heating furnace 4, it was heated to 110φ×11 in a Mannesmann piercer 6.
．． Puncture-rolled into 8A hollow tube of 25tX 3200mm,
A mandrel bar 12 with a surface roughness of Rmax 28 gm is inserted inside, and a mandrel bar 12 of 80φ x 3.80 mm is milled using a mandrel mill 10.
The raw tube for finish rolling 8B of tX11eO01 was used.

This raw pipe 8B for finish rolling is heated to 860°C in a reheating furnace 16.
After heating, the stretch reducer 18 is used to stretch 27.2φX with an average stretch coefficient of 0.58. OtX 4850
The finished tube 20 was made with 0 sentences. The wrinkle depth on the inner surface of this finished tube was 8 gm. After that, a cold draft tube with a length of 45,001 mm was prepared from this finished tube 20, and heated to 1,080 mm in length in a heat treatment furnace.
After holding for lO min at °C, water cooling and further 9% nitric acid-3 at 48 °C.
% hydrofluoric acid solution for 30 minutes to perform a series of pretreatment steps such as descaling, water washing, and chemical conversion coating treatment, and then cold-drawn to 25.2 Φ x 2.4 t x 6070 mm, which corresponds to an area reduction rate of 26%. A smooth tube without plug seizure and without wrinkles on the inner surface was obtained.

Although the present invention is effective for manufacturing seamless steel pipes made of general ordinary carbon steel, it is particularly suitable for seamless steel pipes made of low-alloy steel, austenitic stainless steel, etc., which have poor deformability in hot conditions. It is suitable for use in the production of

[Effects of the Invention] As described above, according to the present invention, mandrel mill rolling is performed with the surface roughness of the mandrel bar set to Rx 30g1 or less, and the average stretch coefficient is set to 0.58 or more during stretch reducer rolling. By setting this, the depth of wrinkles on the inner surface of the finished pipe can be reduced to, for example, 10 layers or less, preventing the plug from seizing in the subsequent cold drafting process, and also preventing wrinkles on the inner surface of the cold drafting finished pipe. can. In other words, it is possible to reduce or prevent wrinkles that occur on the inner surface of the tube after stretch reducer rolling, and to reduce or omit the load on the polishing process for removing wrinkles after rolling. Smooth seam with no temperature control Figure 1 is a diagram showing the relationship between the average stretch coefficient and the wrinkle depth on the inner surface of the finished tube in stretch reducer rolling when the mandrel bar surface roughness is changed. Figure 2 is a seam made using the mandrel mill method. A schematic diagram showing an example of a steelless pipe manufacturing line, Fig. 3 is a schematic diagram showing the occurrence of wrinkles on the inner surface of the finished pipe, and Fig. 4 shows the relationship between the mandrel bar surface roughness and the inner surface roughness of the raw pipe for finish rolling. Figure 5 is a diagram showing the relationship between the inner surface roughness of the raw tube for finish rolling and the wrinkle depth on the inner surface of the finished tube, and Figure 6 is a line showing the relationship between strain and stretch coefficient in straight 2-chire reducer rolling. It is a diagram.

8A...Hollow tube, 8B...Material pipe for finish rolling, 10... Mandrel mill, 12... Mandrel bar-1 14... hole-shaped roll, 18...Stretch reducer, 20... Finished pipe.

Agent Patent Attorney Osamu Shiokawa Vegetables 1 Diagram Mandrel bar surface roughness Figure 3 Figure 4

Claims (1)

[Claims] (1) A method for manufacturing a seamless steel pipe, in which a hollow tube into which a mandrel bar is inserted is stretched and rolled in a mandrel mill, and the tube for finish rolling after rolling by the mandrel mill is reduced and rolled in a stretch reducer consisting of multiple stands. The surface roughness of the mandrel bar used for rolling with a mandrel mill is Rmax 30 μm or less, and the stretch coefficient (tensile stress/yield stress) given to the raw pipe by the rolling rolls of each stand during rolling in a stretch reducer is A method for producing a seamless steel pipe, the method comprising: setting an average value of 0.58 or more.