JP2002035809A - Method for rolling seamless steel tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for rolling seamless steel tubes by which a thin tube can be manufactured in the mandrel mill system by solving a perforation problem in the mandrel mill rolling, specifically, for example, to provide a method for rolling seamless steel tubes by which thin tubes having the following outside dia. ratio are obtained: the wall thickness/outside dia. ratio is two percent or less in common steel and the wall thickness/outside dia. ratio is four percent or less in martensitic stainless steel. SOLUTION: The oblique angle β of the cone type roll provided by pinching a pass line is 8 deg. to 20 deg. provisionally and the cross angle γ is 5 deg. to 35 deg. and a solid-core billet is perforated into a hollow raw tube and next, the hollow raw tube wherein a core bar is inserted is passed through rolling rolls of plural stands and an average wall thickness degree of processing of the rolling roll fillet portion shall be 0.40 or less so that the seamless steel tubes are rolled by pressing down the tube wall thickness.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for rolling a seamless steel pipe. More specifically, the present invention relates to a method for rolling a seamless steel pipe capable of producing a pipe having a small thickness / outside diameter ratio by a mandrel mill method.

[0002]

2. Description of the Related Art FIG. 1 is an explanatory view showing a method of manufacturing a seamless steel pipe by a mandrel mill method. As shown in the figure, when manufacturing a seamless steel pipe, first, a solid billet 10 is heated to a predetermined temperature in a heating furnace 20. Next, the heated solid billet 10 is pierced and rolled by a piercing mill 30 to form a hollow shell 35. A mandrel 41 is inserted into the hollow shell and stretched and rolled by a mandrel mill 40 to adjust the wall thickness of the pipe. Thereafter, the mandrel is extracted and, if necessary, reheated, then formed into a predetermined outer diameter by a reducing mill 50 such as a sizer or a stretch reducer, and finished to product dimensions.

A piercing mill is provided between a pair of cone-type rolls (also referred to as main rolls), which are provided on the left and right or up and down with a pass line therebetween, a plug serving as an inner surface regulating tool, and the main rolls, with the pass line interposed therebetween. The main roll has a three-dimensional inclination with respect to the pass line, that is, the inclination angle (the angle formed by the axis of the roll with respect to the horizontal or vertical plane of the pass line). ) And an intersection angle (an angle formed by the axis of the roll with respect to the vertical or horizontal plane of the pass line) γ.

In this piercing rolling mill, a plug is pushed into the inner surface of a solid billet, and the solid billet and the hollow shell are pressed against the disk roll surface while the solid billet is reduced between the plug and the main roll. Into a hollow shell.

[0005] A mandrel mill is generally composed of four to eight stands each having a pair of grooved rolling rolls, and each stand is continuously arranged along a pass line. Between the adjacent stands, the rolling rolls are arranged crosswise with the rolling direction shifted by 90 degrees in a plane perpendicular to the pass line. In this mandrel mill, elongation rolling is performed by passing a hollow shell having a mandrel inserted into the inner surface between rolling rolls of each stand.

In the manufacture of a seamless steel pipe by the mandrel mill method, there is a problem that it is difficult to form a thin-walled pipe having a small thickness / outside diameter ratio. Therefore, conventionally, the manufacture of a thin-walled tube has been performed by cold drawing. However, cold drawing has drawbacks of low productivity and high production cost. Therefore, production of a thin tube by a mandrel mill method is desired.

For the production of a thin-walled tube, it is necessary to make the hollow shell thinner by piercing and rolling. In Japanese Patent Publication No. 5-23842, the inclination angle β and the cross angle γ of the cone-shaped rolls supported at both ends opposed to each other with the pass line therebetween are set to 8 ° ≦ β ≦ 20 °,
Perform piercing rolling while maintaining the range of 5 ° ≦ γ ≦ 35 °,
A method for manufacturing a seamless tube for obtaining a hollow shell having a thickness / outer diameter ratio of 6.5% or less is disclosed.

However, in the rolling of a thin-walled tube with a mandrel mill, even if a hollow shell having a wall thickness / outer diameter ratio of 6.5% or less is used, the generation of a hole defect becomes a problem. This perforation defect becomes more remarkable as the wall thickness / outer diameter ratio after mandrel mill rolling is smaller.

[0009] For example, in Japanese Patent Application Laid-Open No. 63-84720, a change in the rolling load of each stand is detected in a mandrel mill, and when the detected value exceeds a reference value, There is disclosed a method of correcting a roll gap in a preceding stand of the stand or correcting a wall thickness of a hollow shell.

[0010]

However, according to the prior art disclosed in the above-mentioned publications, for example, when the material to be rolled is ordinary steel, the thickness / outer diameter ratio is 2 in a mandrel mill system.
% Or less was difficult to roll. Also,
Martensitic stainless steel with a high Cr content has poor hot workability, making it difficult to reduce the thickness of the hollow shell in piercing and rolling compared to ordinary steel. In addition, piercing defects also occur in the subsequent mandrel mill rolling. And the rolling of a seamless steel pipe having a thickness / outer diameter of about 4% or less by a mandrel mill method was difficult.

An object of the present invention is to provide a method of rolling a seamless steel pipe capable of solving the problem of perforation in mandrel mill rolling and producing a thin-walled pipe by a mandrel mill method. Specifically, the object of the present invention is, for example, that the thickness / outer diameter ratio is 2.0% or less for ordinary steel, and the thickness / outer diameter ratio is 4.0% or less for martensitic stainless steel. The present invention provides a method for rolling a seamless steel pipe capable of obtaining a thin-walled pipe.

[0012]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventor has studied the countermeasures for perforated defects and has obtained the following knowledge.

FIG. 2 is an explanatory diagram showing a state of occurrence of a hole defect. As shown in FIG. 1, the perforation defect 1 occurs in a part of the material 3 to be rolled which is not in contact with the grooved roll 2. A compressive stress acts on a portion 5 of the material 3 to be rolled corresponding to the bottom portion 4 of the rolling roll groove, and a tensile stress acts on a portion 7 of the material to be rolled corresponding to the flange portion 6 of the rolling roll. Since the portion 7 is not in contact with the rolling roll or the mandrel, the portion 7 is in a uniaxial tension state. When the tensile stress becomes excessive, necking occurs and the hole 7 extends to a hole. This tensile stress increases as the thickness / outer diameter ratio after mandrel mill rolling decreases. Therefore, in order to prevent this perforation, it is important to reduce the tensile stress at the portion 7.

In order to reduce the tensile stress, it is effective to limit the thickness reduction at the bottom of the roll groove. Hereinafter, the roll roll groove bottom and the roll roll flange are also referred to as the groove bottom and the flange, respectively.

FIG. 3 is a cross-sectional view of the rolling roll 2, the mandrel 8, and the material 3 to be rolled for explaining the thickness reduction at the groove bottom in the i-th stand. In the figure, reference numeral 4 denotes a roll bottom, Mc denotes a roll bottom (hereinafter also referred to as a bottom), and 6 denotes a roll flange. In addition, the groove bottom refers to a portion where the rolling roll diameter is minimum.

The thickness reduction of the groove bottom of the i-th stand is calculated by subtracting the thickness reduction of the groove bottom of the i-th stand from the (i-2) th.
The ratio to the thickness of the groove bottom at the outlet side of the stand (thickness reduction / thickness). As shown in FIG. 3, the thickness t of the material to be rolled 3 changes from the groove bottom Mc toward the flange portion 6 side. Therefore, the thickness reduction of the groove bottom 4 is not constant, but changes from the groove bottom Mc toward the flange 6. The relationship between the wall thickness reduction of a perforated defects investigated in detail, that the average wall thickness reduction degree r _i being defined defect open hole exceeds 0.40 prone by the following formula (1) understood. Note that this average thickness reduction is geometrically determined.

R _i = (t _i−2 −t _i ) / t _i−2 (1) where r _{i is} the average thickness reduction of the i-th stand, and t _i is the groove on the exit side of the i-th stand. Average wall thickness between 0 and 45 degrees around the path center with respect to the bottom, t _i-2 : the angle around the path center with respect to the groove bottom at the exit side of the (i-2) th stand being 0 degree Average wall thickness between to 45 degrees.

When i is 1 or 2, t _i-2
Is the wall thickness of the hollow shell. The present invention has been completed based on the above findings, and the gist is as follows.

(1) A method in which a hollow raw tube having a cored bar inserted therein is passed between rolling rolls of a plurality of stands to reduce the wall thickness and roll it into a seamless steel pipe. The average thickness reduction at the bottom of the roll groove is 0.
A method for rolling a seamless steel pipe, wherein the rolling rate is 40 or less.

(2) A solid billet is pierced into a hollow tube by a cone-shaped roll provided with a pass line interposed therebetween, and then the tube into which a core bar is inserted is inserted between a plurality of stands of a rolling roll. And rolling the pipe into a seamless steel pipe by reducing the wall thickness of the base pipe, wherein the inclination angle β and the intersection angle γ of the cone type roll satisfy the following formula, and the rolling of each stand of the plurality of stands is performed. A method for rolling a seamless steel pipe, wherein the average thickness reduction at the bottom of the roll groove is 0.40 or less.

8 ° ≦ β ≦ 20 °, 5 ° ≦ γ ≦ 35 ° (3) The shell / outer diameter ratio is 6.5% or less, and the material of the seamless steel pipe is ordinary steel, (1) wherein the seamless steel pipe has a thickness / outer diameter ratio of 2.0% or less.
The method for producing a seamless steel pipe according to item or (2).

(4) The material of the seamless steel pipe is C: 0.1
A martensitic stainless steel containing 2 to 0.30% by mass and Cr: 11 to 14% by mass, wherein the seamless steel pipe has a thickness / outer diameter ratio of 4.0% or less. A method for rolling a seamless steel pipe according to the above mode (1) or (2).

[0023]

Embodiments of the present invention will be described below with reference to the accompanying drawings. Hereinafter, in mass%, C:
0.12% or more, 0.30% or less, Cr: 11% or more,
Martensitic stainless steel containing 14% or less is also simply referred to as martensitic stainless steel. Hereinafter, the description of the present embodiment is based on the description of the rolling of a seamless steel pipe made of carbon steel having a thickness / outer diameter ratio of 2.0% or less. Rolling of a martensitic stainless steel seamless steel pipe having a diameter ratio of 4.0% or less is taken as an example.

The apparatus for manufacturing a seamless steel pipe according to the present embodiment includes a piercing mill and a mandrel mill, and forms a solid billet into a hollow shell (hereinafter also referred to as a hollow shell) by the piercing mill. Piercing and rolling by a mandrel mill to form the hollow shell into a seamless steel pipe.

FIG. 4 is a schematic plan view showing a piercing mill according to this embodiment, and FIG. 5 is a schematic side view of the same.
As shown in FIGS. 4 and 5, the piercing mill 30 includes a pair of cone-shaped main rolls 31a supported at both ends, which are inclined and opposed to each other across a pass line XX through which the round billet 10 passes, 31b, a pair of disk rolls 32a and 32b opposed to each other with the pass line being shifted by 90 ° from the main roll with a pass line therebetween, and a plug 34 supported by a mandrel 33 and disposed between the main rolls. Is provided. Note that a guide shoe may be used instead of the disc roll.

As shown in FIG. 4 and FIG.
a, 31b, the extension of which is a solid billet 1
0 is inclined and disposed so as to intersect with a vertical plane (or horizontal plane) including a pass line passing through 0, and in a direction opposite to a horizontal plane (or vertical plane) including the pass line. Are inclined at an equal angle β, and are rotated in the same direction at the same rotational speed. Note that the angle γ is called an intersection angle, and the angle β is called an inclination angle.

In this piercing mill 30, the solid billet 10 fed along the pass line is made up of a main roll 31.
After being bitten between a and 31b, it spirally rotates and moves,
A hole is made in the axial center portion by the plug 34, and piercing and rolling is performed so that the wall thickness is gradually reduced and the hollow shell 35 is formed.

In this piercing rolling, the inclination angle β is not less than 8 ° and not more than 20 °, and the crossing angle γ is not less than 5 ° and 35 °.
The following is assumed. If the inclination angle and the crossing angle are out of these ranges, the piercing and rolling becomes unstable, and a hollow shell with a small thickness / outer diameter ratio supplied to the mandrel mill rolling cannot be obtained.

In the above piercing and rolling, if the thickness / outer diameter ratio of the hollow shell increases, the number of rolling stands becomes 4 to 8
In the base mandrel mill, the rolling reduction per stand is excessive, and a hole defect is likely to occur. Accordingly, the wall thickness / outer diameter ratio of the hollow shell is 6.5% or less when the material of the seamless steel pipe is ordinary steel, and 7.0% or less when the material of the seamless steel pipe is martensite stainless steel. It is desirable to be within the range. By setting the thickness / outer diameter ratio of the hollow shell to the above range, the occurrence of perforation defects is prevented, and the thickness / outer diameter ratio of ordinary steel is 2.0% or less. Stainless steel can be rolled into a seamless steel pipe having a thickness / outer diameter ratio of 4.0% or less.

FIG. 6 is an explanatory view schematically showing a mandrel mill according to the present embodiment. This mandrel mill 4
0 denotes five stands (# 1 to # 5) arranged in tandem.
Stand). FIG. 6 shows a state in which three stands are arranged in tandem for convenience of explanation. The # 1 stand is the first stand, the # 3 stand is the downstream stand, and the # 4 stand and the # 5 stand are omitted.

In the present embodiment, after the mandrel 41 is inserted into the hollow shell 35, which is a hollow material to be rolled, pierced by a piercing mill, elongation rolling is performed by a mandrel mill 40.

Each of the stands is provided with a pair of rolls including grooved rolls 42a and 42b, a pair of rolls including rolls 43a and 43b, and a pair of rolls including rolls 44a and 44b. Each is provided.

The rolling direction of the roll pair consisting of the rolling rolls 42a and 42b and the roll pair consisting of the rolling rolls 44a and 44b is horizontal, while the rolling pair 43a,
The rolling direction of the roll pair 3b is a vertical direction. In this manner, each pair of rolling rolls is arranged so that the rolling direction differs between adjacent stands by 90 °.

In the mandrel mill 40, the thickness of the hollow shell 35 is reduced by the rolling rolls 42a to 44b and the mandrel 41 and elongation rolling is performed, but the bottom of the groove of the rolling roll defined by the formula (1) is performed. When the average thickness reduction of 0.4% exceeds 0.40, a hole defect occurs. By setting the average thickness reduction of each of the rolling rolls 42a to 44b to 0.40 or less, occurrence of perforations can be prevented.

In the present invention, the average thickness reduction at the groove bottom of the rolling roll is specified, but the sectional shape of the groove bottom is not limited. The cross-sectional shape of the groove bottom may be, for example, a single arc shape as shown in FIG. 3 or a plurality of arc shapes.

In the present embodiment, in the mandrel mill, the wall thickness / outer diameter ratio of ordinary steel is 2.0% or less, and the wall thickness / outer diameter ratio of martensitic stainless steel is 4.0% or less. Although an example of rolling into a thin-walled tube has been described, the present invention is not limited to this. However, the present invention relates to a mandrel mill, a thin wall tube made of ordinary steel having a thickness / outer diameter ratio of 2.0% or less, and a martensitic stainless steel having a wall thickness / outer diameter ratio of 4.0% or less. This is a preferred method for producing thin-walled tubes made of stainless steel.

[0037]

EXAMPLE A piercing mill shown in FIGS. 4 and 6 and a mandrel mill composed of five stands were used.
For ordinary steel (C: 0.2% by mass) and 13Cr stainless steel (C: 0.2% by mass, Cr: 13% by mass), the outer diameter is 65 mm, and the thickness / outer diameter is 1.8 to 4. A 0.0% tube was produced. Table 1 shows the drilling conditions and the mandrel mill rolling conditions.

[0038]

[Table 1] Table 2 shows the test results.

[0039]

[Table 2] As shown in Table 2, in ordinary steel, the inclination angle: 12 °, the cross angle: 20 °, the thickness / outer diameter ratio of the hollow shell: 6.4% or less,
Average wall thickness: No. 0.39 or less. 3, 4
No porosity defects occurred, and it became possible to roll thin-walled tubes having a thickness / outer diameter ratio of 2.0% and 1.8%, respectively. Test No. In Nos. 1 and 2, a hole defect occurred.

In the case of 13Cr stainless steel, the inclination angle was 12 °, the crossing angle was 20 °, the thickness / outer diameter ratio of the hollow shell was 6.9%, and the average thickness reduction was 0.38 or less. Test No. In No. 7, no porosity defect was generated, and rolling of a thin-walled tube having a thickness / outer diameter ratio of 4.0% became possible. Test No.
In Nos. 5 and 6, a hole defect occurred.

[0041]

According to the present invention, it is possible to prevent the occurrence of perforation defects and to manufacture a thin-walled pipe by a mandrel mill method. Specifically, the occurrence of perforation defects in a mandrel mill is prevented, and thin pipes with a wall thickness / outer diameter ratio of 2.0% or less can be manufactured from ordinary steel. In addition, it is possible to produce a thin-walled tube having a wall thickness / outer diameter ratio of 4.0% or less in martensitic stainless steel.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a method of manufacturing a seamless steel pipe by a mandrel mill method.

FIG. 2 is an explanatory diagram showing a state of occurrence of a hole defect.

FIG. 3 is a schematic diagram of a cross section of a grooved roll and a mandrel bar for explaining a degree of thickness reduction of a groove bottom in an i-th stand.

FIG. 4 is a schematic plan view showing a piercing mill according to the present embodiment.

FIG. 5 is a schematic side view showing a piercing mill according to the present embodiment.

FIG. 6 is an explanatory view schematically showing a mandrel mill according to the present embodiment.

[Explanation of symbols]

1: hole defect, 2: rolling roll, 3: rolled material, 4:
Roll roll groove bottom (groove bottom) 5, 7: site, 6: roll roll flange (flange), 8: mandrel, Mc: roll roll groove bottom (groove bottom), 10: solid billet, 20: heating Furnace, 30: piercing mill, 31a, 31b: main roll,
32a, 32b: disc roll, 33: mandrel, 3
4: plug, 35: hollow shell, 40: mandrel mill,
41: Mandrel, 42a, 42b, 43a, 43b, 44
a, 44b: rolling roll, 50: reduction rolling mill.

Claims (4)

[Claims] 1. A method of passing a hollow raw tube having a core bar inserted therein between rolling rolls of a plurality of stands to reduce the thickness of the raw tube and roll it into a seamless steel pipe, comprising the steps of: A method for rolling a seamless steel pipe, wherein the average thickness reduction at the bottom of the rolling roll groove of each stand is 0.40 or less. 2. A solid billet is pierced into a hollow tube with a cone-shaped roll opposed to a pass line, and
A method in which the raw tube in which a metal core rod is inserted is passed between rolling rolls of a plurality of stands to reduce the thickness of the raw tube and roll it into a seamless steel pipe, wherein the cone-shaped roll has an inclination angle β.
And the crossing angle γ satisfies the following formula, and the average thickness reduction at the bottom of the rolling roll groove of each of the plurality of stands is 0.4.
A method for rolling a seamless steel pipe, wherein the rolling rate is 0 or less. 8 ° ≦ β ≦ 20 °, 5 ° ≦ γ ≦ 35 ° 3. The seamless tube has a thickness / outer diameter ratio of 6.5% or less, the seamless steel tube is made of ordinary steel, and the seamless tube has a thickness / outer diameter ratio of 2.0%. The method for producing a seamless steel pipe according to claim 1 or 2, wherein: 4. The material of the seamless steel pipe is C: 0.12 to 0.12.
A martensitic stainless steel containing 0.30% by mass and Cr: 11 to 14% by mass.
2. An outer diameter ratio is 4.0% or less.
Or the method of rolling a seamless steel pipe according to item 2.