JPH0518644B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] 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 mandrel mill method and plug mill method, or hot extraction methods such as Eugene Sedgyurne method and Erhardt push bench method, but they are manufactured in relatively small diameter size. The mandrel mill rolling method is widely used for manufacturing pipes because of its excellent productivity and dimensional accuracy. In the mandrel mill method, for example, the second
As shown in the figure, after the billet 2 is heated to a predetermined temperature in a rotary hearth heating furnace 4, it is pierced and rolled by a Mannesmann piercer 6 to form a hollow shell 8A. Alternatively, the hollow shell 8A may be directly manufactured by the continuous casting machine 5 for producing hollow shells. 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 by 90 degrees from each other 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 needed, and then finish rolled in 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 billet. Since the tube is shaped by several stands of perfectly circular hole 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,
The rolling rolls on each stand are arranged in a three-roll arrangement as shown in Figure 2, for example, and by adjusting the roll rotation speed ratio of each stand, the tension applied to the raw pipe during rolling can be controlled and the wall thickness can be controlled. It can be controlled. [Problems to be solved by the invention] By the way, materials with relatively poor deformability under hot conditions,
For example, when low-alloy steel, stainless steel, etc. are rolled with a stretch reducer, longitudinal wrinkles called so-called wrinkle defects occur on the inner surface of the tube after rolling, as shown in FIG. If this wrinkle flaw is sharp and deep, the scale stuck in the wrinkle flaw will remain unremoved, for example, even by pickling in the subsequent cold draft pretreatment process, and it will not be removed during the cold drafting process. After surface reduction, wrinkles remain on the inner surface, which not only prevents the inner surface from becoming smooth, but also causes problems such as plug seizure, which may make it impossible to operate. Therefore, for these reasons, conventionally, for products that require smoothness of the inner surface of the tube after finish rolling, a polishing process has to be added to remove wrinkles after rolling, resulting in a decrease in yield and This was a cause of an increase in costs. An object of the present invention is to reduce or prevent wrinkles that occur on the inner surface of a tube after stretch reducer rolling, and to produce a seamless steel tube 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, and stretching the tube for finish rolling after rolling by the mandrel mill into a stretching system consisting of a plurality of stands. In a method for manufacturing seamless steel pipes that is stretched in a stretch reducer, the surface roughness of the mandrel bar used for rolling with a mandrel mill is Rmax 30 μm or less, and the rolling rolls of each stand are flat during rolling in a stretch reducer. Stretch coefficient (tensile stress/
The average value of yield stress for all stands is set to 0.58 or higher. [Function] When a foreign material (for example, a non-metallic inclusion layer) exists near the inner surface of the tube during stretch reducer rolling, the tube length must be One idea is to avoid applying tensile stress above a certain level in the hand direction. To this end, Japanese Patent Publication No. 61-9084 proposes setting the average stretch coefficient Zm to 0.45 or less. However, as a result of careful investigation by the present inventors, wrinkle defects on the inner surface that occur in low-alloy steel, stainless steel, etc. are completely different from the inner surface defects that occur due to the mechanism based on the presence of different materials as described above. It has become clear that this is a phenomenon. Therefore, as a result of repeated research and repeated more detailed experiments, we found that wrinkles on the inner surface of the tube that occur after stretch reduction rolling are caused by narrowing deformation during stretch reduction rolling that occurs in the circumferential direction. Buckling of the material surface due to compressive force occurs. Buckling of the material surface due to compressive force in the circumferential direction that occurs on the inner surface of the tube originally originates from minute irregularities that 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 wrinkles, and the inner surface of the pipe during stretch reducer rolling. If the compressive strain generated in the circumferential direction is reduced, 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. We have come to the knowledge shown in ~. Generally speaking, a new or modified mandrel bar is rolled 2,000 to 5,000 times, depending on the roughness of the bar surface and the degree of change in outer diameter due to wear, after which it is downsized and rolled again. used. The bar surface immediately after modification is Rmax.
It is finished to a thickness of about 10 to 20 μm, but after being used for rolling and just before the next modification, due to surface roughness and wear, the thickness may range from 40 μm to about 100 μm in some cases. There are also localized cracks and scratches. FIG. 4 shows the relationship between the surface roughness Rmax of the bar and the roughness Rmax of the inner surface of the raw tube for finish rolling when the bar is used for rolling. During mandrel mill rolling, a water-soluble graphite lubricant is applied to the bar surface, and relative slippage occurs between the bar and the material during rolling, so the roughness of the bar surface is not necessarily transferred as is to the inner surface of the blank tube for finish rolling. However, the inner surface roughness of the raw pipe for finish rolling is higher than the bar surface roughness Rmax.
Although Rmax 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 finish rolling, and the roughness Rmax 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. However, as shown in Figure 5, the roughness of the inner surface of the raw tube for finish rolling
The smaller Rmax is, the smaller the wrinkle depth is. On the other hand, the rolling theory of the stretch reducer by Neumann and Hancke (Stahl und Eisen 75
(1955), No. 22), it is analyzed as follows. Condition of constant volume, φ + φθ + φr = 0 (1) where φ: Axial logarithmic strain φθ: Circumferential logarithmic strain φr: Radial logarithmic strain From the Levy-Mises stress-strain relationship, (σr-σm): (σ −σm): (σθ−σm) =φr:φ:φθ (2) Here, σr: Radial stress σ: Axial stress σθ: Circumferential stress σm: Average stress [=(σr+σ+σθ)/3] On the other hand, Tresca's yield condition equation and σ−σθ=kf (3) where kf: deformation resistance Approximate balance equation in the radial direction of thin-walled pipe σr=λ・σθ (4) where λ: wall-thickness-outer-diameter ratio of the pipe When used, the average stress becomes σm=(σ+σθ+σr)/3 = [(2+λ)・σ −(1+λ)・kf]/3 (5). Here, if the stretch coefficient Z is defined as Z=σ/kf (6), then φr：φ：φθ=2(λ−1)・Z−(2λ−1 ):
(1-λ)・Z+(1+λ):(1-λ)・Z-(2+
λ) (7). The larger the average stretch coefficient Zm, the smaller the wrinkle depth on the inner surface of the finished tube. However, when R _nax on the mandrel bar surface exceeds 30 μm, even if the average stretch coefficient Zm is increased, its effect is saturated, and there is a limit to the minimum wrinkle depth that can be achieved. Therefore, if you want to achieve a wrinkle depth of 10 μm or less that does not leave any wrinkle defects and does not cause plug seizure during cold drawing, the surface roughness R _nax must be 30 μm.
The following mandrel bars are required to be stretched reducer rolled using mandrel mill rolling and with an average stretch coefficient of 0.58 or more. 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, if we set φr/φθ=K (8) and solve equation (7) for Z, we get Z=K・(2−λ)+(1−2λ)/K・(1−λ)
−2(λ−1)(9) is obtained. 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 as a simple guide to know how much tension is acting in the tube axis direction during actual operation. Average stretch coefficient Zm across all stands
is generally expressed by the following formula. Zm=K・(2−λm)+(1−2λm)/K・(1−λm
)−2(λm−1)(10) K=φr/φθ φr=nTs/Tt φθ=n(Ds−Ts)/(Dt−Tt) λm=1/2・[Ts/(Ds−Ts) +Tt /(Dt-Tt)] Ts: Thickness of the raw tube for finish rolling Ds: Outer diameter of the raw tube for finish rolling Tt: Thickness of the tube after finishing rolling Dt: Thickness of the tube after finishing rolling Now, after the stretch reducer rolling In the cold drawing process, as shown in Table 1, if the wrinkle depth is approximately 10 μm or less, cold drawing can be performed without any problem up to at least an area reduction of 33%, but if the wrinkle depth is 13 μm, At a reduction rate of 15%, wrinkles are not crimped and remain as defects even after cold drawing, and the wrinkles are even deeper.
At 18μm, plug seizure occurs during cold drafting, making cold drafting impossible. On the other hand, using a raw tube for finish rolling that has been subjected to mandrel mill rolling using mandrel bars with different surface roughness, the wrinkle depth on the inner surface of the finished tube after stretching reducer rolling was determined by changing the average stretch coefficient Zm. The results of the measurements are shown in Figure 1. The meaning of equation (7) is as follows. When the stretch coefficient Z becomes 1, φr:φ:φθ=-1:
2:-1, and the tube length is increased (φ
) is the thickness reduction (φr), and the remaining 1/2 is the circumference reduction (φθ). On the other hand, when the stretch coefficient Z becomes 0, when λ=0, φr:φ:φθ=1:1:-2, and 1/2 of the decrease in circumference (φθ) is the increase in tube length (φ), and the remaining 1/2 is the wall thickness increase (φr). Therefore, as the stretch coefficient Z increases, the strain φθ in the circumferential direction acting on the inner surface of the pipe changes from the compression direction to the tension direction.

[Table] ○: Good
△: Remaining wrinkles
×: Plug seizure The average stretch coefficient can theoretically take a value between 0 and 1, but due to operational problems such as slip between the roll and material during stretch reducer rolling or material breakage, Practically speaking, the upper limit is about 0.65 to 0.70. [Example] Next, the present invention will be explained with reference to an example. Using SUS304 as the material, a 110φ×1300 billet 2 was heated to 1240℃ in a rotary hearth heating furnace 4, and then 110φ×
A hollow tube 8A of 11.25t x 3200mm is punch-rolled, a mandrel bar 12 with a surface roughness of Rmax 28μm is inserted inside, and a 90φ x 3.80t tube is machined with a mandrel mill 10.
The raw tube for finish rolling was 8B with a size of 11,600 mm. After heating this raw pipe 8B for finish rolling to 960°C in a reheating furnace 16, a finished pipe 2 of 27.2φ×3.0t×48500 with an average stretch coefficient of 0.59 is heated in a stretch reducer 18.
It was set to 0. The wrinkle depth on the inner surface of this finished tube is 8μm.
It was hot. After that, the length is 4500 mm from this finishing tube 20.
Prepare a cold drag tube and heat it to 1080℃ for 10 minutes in a heat treatment furnace.
After holding for 30 minutes, cooling with water, and then immersing in a 9% nitric acid-3% hydrofluoric acid solution at 48℃ for 30 minutes, performing a series of pretreatment steps such as descaling, washing with water, and chemical conversion coating treatment, the area reduction rate is equivalent to 26%. Although it was cold drawn to a size of 25.2φ x 2.4t x 6070, no plug seizure occurred and a smooth tube with no 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 being Rmax 30 μm or less, and the average stretch coefficient is set to 0.58 or more during stretch reducer rolling. As a result, the depth of wrinkles on the inner surface of the finished tube can be reduced to, for example, 10 μm or less, and it is possible to prevent the plug from seizing in the subsequent cold drawing step, and also prevent wrinkles on the inner surface of the finished cold drawing tube. In other words, the inner surface can reduce or prevent wrinkles that occur on the inner surface of the pipe after stretching reducer rolling, and reduce or omit the load in the polishing process for removing wrinkles after rolling. It is possible to manufacture smooth seamless steel pipes.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the average stretch coefficient in stretch reducer rolling when the mandrel bar surface roughness is changed and the wrinkle depth on the inner surface of the finished tube. Figure 2 is a diagram showing the relationship between the wrinkle depth on the inner surface of the finished tube and the manufacturing of seamless steel tubes using the mandrel mill method. Fig. 3 is a schematic diagram showing an example of the line; Fig. 3 is a schematic diagram showing the state of occurrence of wrinkles on the inner surface of the finished tube;
The figure is a diagram showing the relationship between the mandrel bar surface roughness and the inner surface roughness of the raw tube for finish rolling. Figure 5 is the 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. It is. 8A... Hollow tube, 8B... Finish rolling tube,
10... Mandrel mill, 12... Mandrel bar, 14... Hole roll, 18... Stretch reducer, 20... Finishing tube.

Claims (1)

[Claims] 1. A method for producing 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 was
Rmax is set to 30 μm or less, and the average value of the stretch coefficient (tensile stress/yield stress) given to the raw pipe by the rolls of each stand during rolling in the stretch reducer for all stands is set to 0.58 or more. A method for manufacturing seamless steel pipes.