JP3365348B2 - Rolling method of metal tube - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rolling method for reducing the outer diameter of a metal tube with a constant diameter rolling mill such as a sizer. More specifically, the present invention relates to a rolling method capable of preventing hexagonal tension and biting flaws and finishing metal pipes having different wall thicknesses using the same hole type roll. 2. Description of the Related Art Generally, in a process of manufacturing a seamless metal pipe (hereinafter, also referred to as a metal pipe), a hollow shell pierced by a piercer is reduced in thickness using a stretching rolling machine such as a mandrel mill. Then, it is rolled to a required outer diameter by using a constant diameter rolling machine such as a sizer or a stretch reducer. The sizer and the stretch reducer have substantially the same configuration, and will be described below with the sizer. FIG. 1 is a schematic diagram schematically showing a roll arrangement of a sizer. 2A and 2B are cross-sectional views of a roll stand of a three-roll type sizer. FIG. 2A is a cross-sectional view taken along line AA of FIG. 1, and FIG. 2B is a cross-sectional view taken along line BB of FIG. 1 and 2, reference numeral 1 denotes a roll stand, 2 denotes a hole-shaped roll, 3 denotes a metal tube, and O denotes a pass center. [0004] The sizer, which is a constant diameter rolling mill, is shown in FIG.
As shown in (b), a plurality of roll stands (hereinafter, also referred to simply as a stand, usually referred to as a stand), in which three hole-type rolls are generally incorporated radially around the pass center O at an interval of 120 ° around the pass center O, as shown in FIG. To 15 stands) are connected and arranged as shown in FIG. The adjacent stands are
The holes are rolled and connected around the path center with a phase difference of 60 ° so that the mutual arrangement of the grooved rolls is as shown in FIG. 2 (a) and FIG. 2 (b). The number of stands used during rolling depends on the finished outer diameter of the metal tube. In the last one to several stands of the sizer, an arcuate hole-shaped roll is used for constant-diameter finishing, and in other stands, an elliptical arc-shaped hole-shaped roll is used. As shown in FIGS. 2 (a) and 2 (b), the actual grooved roll has a gap between the rolls, and the flange end F is not at a position of 60 ° with respect to the groove bottom point E. The description will be made on the assumption that the flange end position is located at a point F of 60 °. In rolling a sizer, there is no constraint on the inner surface of the tube. Therefore, in rolling of one stand, the wall thickness increases at the bottom of the groove and the flange, resulting in uneven deformation. Since the material to be rolled is rolled while the angle between the groove bottom and the flange is alternately changed by 60 °, the thickness of the intermediate point between the groove bottom and the flange (meaning the position at 30 ° from the groove bottom or the flange) is Becomes thicker or thinner than the bottom or flange. This is called hexagonal tension or square tension, and is particularly likely to occur in thick-walled pipes. In order to prevent the occurrence of the hexagonal tension, it is necessary to reduce the circumferential wall thickness by rolling the hole so as to have a shape close to a perfect circle and pressing down the flange. However, when a material having a hole shape close to a perfect circle is applied to the rolling of a thin material, biting occurs at a flange portion, and a biting flaw is a problem. [0007] Therefore, it is necessary to hold the roll in accordance with the wall thickness, and the number of rolls to be held increases, causing problems such as an increase in equipment cost and a shortage of roll storage space. Also,
Frequent roll replacement is required depending on the wall thickness, and therefore a great deal of time and effort must be spent, which is also a problem in terms of productivity. Japanese Patent Application Laid-Open No. Hei 7-16616 discloses a technique for reducing the number of rolls and reducing the frequency of roll exchange. Rolling method that adjusts the distance between the path center and the roll axis in accordance with the wall thickness and outer diameter of the pipe, changes the ellipticity and average outer diameter of the hole-shaped roll, and suppresses the inner surface angularity of the pipe Have been. [0009] The method disclosed in the above publication adjusts the distance between the path center and the roll axis by paying attention to the ellipticity of the hole-type roll. It was found that the effect of improving the flaw was insufficient. [0010] An object of the present invention is to solve the above-mentioned deficiencies of the prior art in a size rolling mill such as a sizer and to suppress the occurrence of flaws and hexagonal tension by biting a wide range of products with the same hole-shaped roll. To provide a method of rolling a metal tube that can be rolled. The inventor of the present invention has recognized that the relationship between the hole-shaped roll shape between adjacent stands is important for preventing hexagonal tension and biting flaws. Various rolling experiments were performed, and the following findings were obtained. (A) Hexagonal tension which tends to occur when rolling a thick material and biting flaws which tend to occur when rolling a thin material occur during rolling on an intermediate stand having a higher rolling reduction than the first stand and the final stand. I do. (B) The hexagonal tension is determined by the distance between the pass center of the stand and the flange end of the grooved roll (A) and the distance between the pass center of the immediately preceding stand and the groove bottom of the grooved roll (B).
Is suppressed by reducing the difference (A−B) from
That is, by reducing the difference, the restraint of the material to be rolled at the flange portion is increased, and occurrence of uneven thickness in the circumferential direction is suppressed. (C) The biting flaw is the difference between the distance (A) between the pass center of the stand and the flange end of the grooved roll and the distance (B) between the path center of the immediately preceding stand and the groove bottom of the grooved roll. It is suppressed by increasing AB). In other words, by increasing the difference, the restraint of the material to be rolled at the flange portion is reduced, and biting is prevented. (D) Prevention of hexagonal tension and biting flaws can be achieved by controlling the side relief coefficient of each stand within an appropriate range. The (i) side relief coefficient S _i of the stand is expressed by the following equation. [Equation 2] (E) The side relief coefficient is determined by the ratio (t / D) between the thickness (t) of the product of the metal tube and the outer diameter (D), and is in the range from Smin to Smax expressed by the following equation. By doing so, the occurrence of hexagonal tension and edge flaws can be prevented. [Equation 3] The present invention has been completed based on the above findings, and the gist thereof is as follows. (1) A method for rolling a metal tube using a constant-diameter rolling mill comprising a plurality of stands having three or four hole-shaped rolls in the same plane in one stand, wherein the constant-diameter rolling mill is moved from upstream to downstream. In the first stand, the plurality of intermediate stands, and all the intermediate stands when the final stand is formed, the path center and the hole-shaped roll groove bottom are set according to the thickness (t) and the outer diameter (D) of the product of the metal tube. By adjusting the distance between them, the side relief coefficient S _i of the (i) th stand expressed by the following equation (1) is set to a value not less than Smin and not more than Smax expressed by the following equations (2) and (3). A method for rolling a metal tube, characterized in that it is within the range. [Equation 4] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The constant diameter rolling mill used in the present invention may be a known rolling mill such as a sizer or a stretch reducer, and may be in the same plane as shown in FIGS. 2 (a) and 2 (b). A plurality of stands (usually sizers: 4-1) in which three or four (not shown) hole type rolls are radially assembled around the path center O at intervals of 120 ° and 90 °, respectively.
(5 stands, stretch reducer: 4 to 28 stands) are used. The rolls of the three-roll type or four-roll type constant-diameter rolling mill are arranged so as to change the phase between adjacent stands by 60 ° and 45 °, respectively. FIG. 3 is a schematic cross-sectional view of a main part of the grooved roll, schematically illustrating a distance A between the path center O and the flange end F of the grooved roll and a distance B between the path center O and the groove bottom E of the grooved roll. It is. In the present invention, the distance between the path center and the groove bottom of the hole-shaped roll is determined according to the thickness t of the product and the outer diameter D (FIG. 3B).
Is adjusted so that the side relief coefficient S _i of the stand (i) expressed by the equation (1) is in the range from Smin to Smax expressed by the equations (2) and (3), respectively. FIG. 4 is a graph showing an appropriate range of the side relief coefficient according to the present invention. In the figure, lines 1 and 2 respectively correspond to S in equation (2).
max and Smin in equation (3), and the area surrounded by line 1 and line 2 is the appropriate range of the side relief coefficient. In other words, if the side relief coefficient exceeds Smax, the product shape becomes poor due to the occurrence of hexagonal tension, and if it is less than Smin, biting flaws occur and the quality becomes poor. Therefore, the side relief coefficient is not less than Smin and not more than Smax. Preferably, (Smax + 3 · Smin) / 4 or more, (3 · Smax +
Smin) / 4 or less. Hereinafter, the distance between the pass center and the groove bottom of the hole type roll is also referred to as a groove bottom distance. The groove bottom distance can be adjusted either off-line or on-line by providing an electric or hydraulic pressure-reducing device on the stand, but it is desirable to perform the on-line. Here, the adjustment of the groove bottom distance to set the side relief coefficient to an appropriate range is performed by moving the constant diameter rolling mill from the upstream to the downstream from the first stand, the plurality of intermediate stands, and the rounding. It is sufficient to perform only the hole-shaped rolls of all the intermediate stands when the final stand is used for the above, but it may be performed on the hole-shaped rolls of all the stands including the first stand and the final stand. . Generally, in the rolling of the first stand and the final stand, the rolling reduction is set lower than that of the rolling of the intermediate stand, and there are few problems associated with the occurrence of hexagonal tension and biting flaws. As for the side relief coefficient S ₁ of the first stand, B ₀ when i = 1 in the equation (1) is defined by the radius of the material to be rolled (raw tube) on the first stand entry side. It is a numerical value. Next, the rolling method is specifically described in which two types of metal tubes having an outer diameter (D) of 230 mm and a wall thickness (t) of 8 mm and 45 mm are provided with 11 stands (first stand, intermediate stand). An example in which rolling is performed at nine (the last stand) will be described. In the rolling of a metal tube having a thickness of 8 mm, Smin is -3.25 and Smax is 1.84 according to equations (2) and (3).
It becomes. Similarly, in rolling of a 45 mm thick metal tube, Smi
n is -4.86 and Smax is -0.24. Therefore, when rolling a metal tube having a thickness of 8 mm, each hole shape of the intermediate stand is set so that the side relief coefficient in rolling at the intermediate stand is a value within the range of -3.25 to 1.84. By adjusting the groove bottom distance by raising and lowering the roll, hexagonal tension and biting flaws can be prevented. The groove bottom distance between the first stand and the last stand may be adjusted so that the side relief coefficient in rolling at the first stand and the last stand also has a numerical value within the above range. When rolling a 45 mm thick metal tube,
Similarly, the side relief coefficient in rolling at the intermediate stand may be set to a value within the range of -4.86 to -0.24. According to the above-mentioned method, metal tubes having different thicknesses can be rolled using the same hole-type roll while suppressing hexagonal tension and biting flaws. EXAMPLE A total of 11 stands including a first stand, nine intermediate stands and a final stand, and all the stands have a function of adjusting a groove bottom distance by a hydraulic pressure reduction method.
Using a roll sizer, outer diameter 300φ, wall thickness 7mm, 2
The base tubes of 3 mm and 44 mm were rolled into metal tubes having an outer diameter of 230 mm and wall thicknesses of 8 mm, 25 mm and 45 mm, respectively. Table 1 shows the hole-shaped roll dimensions A and B of each stand when the groove bottom distance is set as a reference. [Table 1] Rolling was performed by adjusting the groove bottom distance to change the side relief coefficient of each stand.
Table 2 shows the set values of the side relief coefficient of each stand. [Table 2] The state of occurrence of biting flaws and hexagonal tension of the product obtained by rolling was investigated. As for the scratches,
The case where no flaw was observed on the surface was evaluated as ◎, the case where it appeared to be thin in a part of the longitudinal direction and there was no problem on the product, and the case where flaw care was required was evaluated as x. For hexagonal tension,
The degree of the occurrence is evaluated by the hexagonal tension ratio P defined by the following formula, and the hexagonal tension ratio P is less than 5.0% ◎, 5.0% or more and less than 10.0% is ○, 10.0% or more Is represented as x. As the overall evaluation, both the bleeding flaw and the hexagonal tension were judged to be acceptable if they were ◎ or ○. Table 3 shows the survey results. [Equation 5] [Table 3] As shown in Table 3, when the wall thickness is 8 mm, the side relief coefficient of the intermediate stand is −2.9 to −2.9.
Test No. 1.0 within the range of 1.0 Examples 3 to 6 of the present invention passed. In particular, when the side relief coefficient is -1.0
-No. In the case of No. 5, the evaluation of both the scratching out and the hexagonal tension was ◎, which was extremely good. On the other hand, Test No. The comparative examples 1 and 2 were defective due to the biting flaws. Comparative Example 7 had a hexagonal tension ratio of 10% or more. In the case of the thickness of 25 mm, the test No. in which the side relief coefficient of the intermediate stand is in the range of -3.9 to -0.1. 9 to 12 of the present invention examples passed. In particular, the test Nos. Having the side relief coefficients of -1.9 to -1.0. 11 is the evaluation of both the scratches and hexagonal tension.
It was extremely good. On the other hand, Test No. The comparative example of No. 8 was defective due to a scratching out, and the test No. 8 was defective. 13, 14
In Comparative Example 2, the hexagonal tension ratio was 10% or more. In the case of a wall thickness of 45 mm, test No. 3 in which the side relief coefficient of the intermediate stand is in the range of -3.9 to -1.0. 16 to 18 examples of the present invention passed. In particular, the test No. with the side relief coefficient of -2.9 to -2.0. 17 is the evaluation of both the scratches and hexagonal tension.
It was extremely good. On the other hand, Test No. The comparative example of No. 15 was defective due to a scratching out and the test no. 19-2
Comparative Example 1 had a hexagonal tension ratio of 10% or more. According to the present invention, a metal pipe having a wide range of thickness can be rolled by using the same hole type roll while suppressing the occurrence of flaws and hexagonal tension. Therefore, the productivity can be improved by reducing the equipment cost by reducing the number of hole-shaped rolls and shortening the roll replacement time.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram schematically showing a roll arrangement of a sizer. 2A and 2B are cross-sectional views of a roll stand of a three-roll type sizer. FIG. 2A is a cross-sectional view taken along line AA of FIG. 1, and FIG. 2B is a cross-sectional view taken along line BB of FIG. FIG. 3 is a schematic diagram for explaining a distance between a path center and a flange end of the hole-shaped roll and a distance between a path center and a groove bottom of the hole-shaped roll in a cross section of a main part of the hole-shaped roll. FIG. 4 is a graph showing an appropriate range of a side relief coefficient according to the present invention. [Description of Signs] 1: Roll stand, 2: Roll type roll, 3: Metal tube,
O: pass center, F: flange end, E: groove bottom point.

Claims (1)

(57) [Claim 1] A method for rolling a metal tube using a constant diameter rolling mill comprising a plurality of stands having three or four hole-shaped rolls in one stand in the same plane. The thickness (t) and outer diameter (D) of the product of the metal tube in all the intermediate stands when the constant diameter rolling mill is a first stand, a plurality of intermediate stands, and a final stand from upstream to downstream. ) To adjust the distance between the pass center and the bottom of the roll groove,
The side relief coefficient S _i of the (i) th stand represented by the following equation (1) is set to be in a range of Smin to Smax represented by the following equations (2) and (3). Rolling method of metal tube. (Equation 1)