JP2000334506A - Manufacture of seamless steel tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the occurrence of flaws at the time of tube making when Mannesmann piercing is mainly applied to diffucult-to-work materials which is inferior in hot workability and taken as being unsuitable to the material for seamless steel tubes or base stocks bad in quality in the center part of which porous cavity or the like is present because they remains as-cast. SOLUTION: In a method for manufacturing the seamless steel tubes by a skew piercing method using billets consisting of the difficult-to-work material or a material which is as-cast, the rolling is executed by keeping a piercing parameter PP positive, which is shown by the next equation, that is, PP =-0.00012BT+(1.6856×10-5×Dr)-(-0.000327×Vr)+0.0255 (Where, BT is the diameter of the billet of the base stock (mm), Dr is the diameter of the main roll for piercing (mm) and Vr is the number of revolution of the main roll for piercing (rpm).). Thus, by keeping the tip draft of a plug at >=95%, the occurrence of the flaws on the inside surface of the tube is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a seamless steel pipe, and more particularly, to a difficult-to-work material or a cast as it has been conventionally unsuitable as a seamless steel pipe due to poor hot workability. Therefore, a method of manufacturing a seamless steel pipe from a difficult-to-machine material, which can suppress generation of flaws at the time of pipe making, mainly when performing a Mannesmann piercing and rolling on a poor quality material having a zaku or the like in the center of the material. Things.

[0002]

2. Description of the Related Art A seamless steel pipe is generally produced by working a cast slab or using a round or square billet-shaped billet made by a continuous casting machine. Extrusion and the like are performed to prepare a hollow shell, and thereafter, the hollow shell is stretched by a rolling machine such as an elongator, a plug mill or a mandrel mill, and the hollow shell thus stretched is finally stretched. It is manufactured by sizing over a sizer or stretch reducer.

[0003] As for the material of the seamless steel pipe, continuous casting into a round billet can be performed relatively easily like ordinary low carbon steel having a small alloying component, and the material having good hot workability is not cast. If the hot workability is poor, such as stainless steel, which tends to cause porosity and segregation in the center of the slab as cast, then cast it into a steel ingot, and then apply large processing to the round steel. Make a piece.

[0004] Main causes of the deterioration of workability are segregation and porosity in the center of the slab. In other words, the porosity generation mechanism that has a particularly large effect is generally such that voids are generated in the slab in the final solidification stage of the continuous cast slab, and where molten steel should be supplied, in the case of high alloy steel, etc., The reason is that the molten steel is not supplied to the gap because the viscosity of the molten steel increases due to a high content of Cr and the like.

For example, the relationship between the amount of Cr and the viscosity is shown in FIG. 4. As is clear from FIG. 4, when the amount of Cr exceeds 0.5%, the viscosity of the molten steel sharply increases, so that the porosity Is more likely to occur.

[0006] As described above, when a material having a possibility of having a defect inside is used as cast, it is subjected to severe processing called Mannesmann drilling at the beginning of the pipe making process. Pipe inner surface flaws occur. For this reason, carbon steels such as steels containing a large amount of alloy components and free-cutting steels containing a relatively large amount of sulfur S which deteriorates the workability, which are difficult-to-process materials, are also used, as well as ordinary steels having a high carbon content and carbon steels.
It has been said that even for a steel type to which r is added, pre-piercing rolling is required to improve the internal quality.

[0007] For example, as a method for producing a seamless pipe material of high Cr steel, a square bloom having a relatively large cross section of 400 × 520 mm is cast by a continuous casting machine or the like, and after heating,
The method is such that a round steel ingot having a small cross section, that is, a round billet is prepared from a rectangular shaped steel ingot by using slab rolling or the like.
Such a pipe manufacturing method in which a cast material is pre-processed into a material for a seamless steel pipe is an increase in the number of processes and heating during pre-processing compared to the case where a steel type that can be used as a material as cast is used. Etc. leads to an increase in cost. Therefore, there has been a demand for a method of producing pipes without adding pre-processing as much as possible.

[0008] In order to solve this problem, many proposals have been made, one for improving the casting method and one for improving the pipe making method.

With respect to the improvement of the casting method, a method of rolling down a continuous cast material during casting and mechanically pressing a defect is often applied to a slab or other steel piece having a rectangular cross section.
The major problems when applying this light reduction process to round billets are the deterioration of the slab shape caused by the reduction rolls and the cracks in the solidification interface that can occur when the reduction amount is increased.

If the round slab is simply rolled down by a pair of flat rolls, the roll-down portion is naturally collapsed and the cross section of the slab is flattened. In addition, cracks are likely to occur due to the generation of tensile stress in the cross section in a direction orthogonal to the reduction direction in the cross section. Also, if the amount of reduction is increased in order to enhance the porosity pressure bonding effect, the shape is further away from a perfect circle. As a result, the rate of occurrence of cracks increases, and when used as a round billet for manufacturing seamless pipes, it is impossible to carry out by rolling the billet, or the biting during drilling becomes unstable. In the end, the internal quality of the slab is inadequate and must be put up with.

In order to solve this problem, for example, Japanese Patent Application Laid-Open No. Hei 7-108358 (hereinafter referred to as Prior Art 1) discloses a casting method using an elliptical mold in which a portion to be reduced is enlarged in advance. .

[0012] However, this method is effective under light pressure, but it causes new defects due to fluctuations in the molten metal surface and powder pull-in caused by non-uniform flow during casting as compared with a perfect circular mold. It is clear that this is the cause, and cannot be a complete solution.

On the other hand, in the pipe making process, there are many cases in which a technique for preventing the occurrence of inner surface flaws caused by Mannesmann cracking during drilling has been studied. For example, JP-A-55-106611
(Hereinafter referred to as Prior Art 2) discloses a technique in which not only the generation limit of the inner surface flaw but also the perforation conditions are specified for biting and uneven thickness.

However, as described in Prior Art 2, the inner surface flaw generation limit depends on the material, temperature,
Due to the dependence on the manufacturing method, the generation of flaws cannot be prevented even if the prior art 2 is applied to a raw material that is continuously cast or a difficult-to-process material.

[0015] Further, there is a technique for suppressing the occurrence of inner surface flaws by changing the inclination angle of the piercing and rolling roll from before to immediately after the biting, and immediately before and after the trailing edge, and between these levels and during steady rolling. It is disclosed in registration number 1811008 (hereinafter referred to as prior art 3). Prior art 3
Applies a reasonable method from the viewpoint of flaw control. That is, as means for suppressing the Mannesmann crack, it is desired to continue punching in a state where the roll inclination angle is high. In that case, however, it is easy to cause biting failure. Things.

However, Prior Art 3 is not universal. That is, according to Prior Art 3, when the material is inferior, the roll must be disposed at a high inclination angle in order to maintain the efficiency immediately after being engaged at a considerably low inclination angle, Significant mechanical loads occur and there is a risk of mechanical failure. In addition, since the flaw itself is generated during the biting at a low inclination angle, the flaw generated at the time of biting and at the end of the tail, that is, at the front and rear ends of the pipe cannot be suppressed. However, it is difficult to produce a product with good internal quality while suppressing a large decrease in production efficiency due to a major failure or the like. In addition, equipment must be designed to withstand a large mechanical load, so that equipment costs are high.

On the other hand, when the workability of the material is poor,
There are many examples of studying the manufacturing method. For example, the registration number 1828639 (hereinafter referred to as Prior Art 4) has a material tip outer diameter when manufacturing a seamless pipe made of free-cutting steel such as sulfur free-cutting steel or lead free-cutting steel by the Mannesmann pipe method. A technique has been disclosed in which taper processing is performed to reduce the roll opening degree or less to suppress inner surface flaws.

Prior art 4 refers to a prior art such as Japanese Patent Application Laid-Open No. 56-89307, which is considered to be a problem because the processing of the material and the maintenance after pipe production increase. However, this method does not exceed the limit in that tape processing is performed, and is not a technology that can reduce the manufacturing cost.

In order to reduce the manufacturing cost, as a technique for producing a pipe while preforming the material as little as possible, for example, Japanese Patent Application Laid-Open No. 1-228603 (hereinafter referred to as Prior Art 5) discloses a specific technique. A technique for limiting the working degree and the heating temperature of a round billet continuously cast to a steel type, here a duplex stainless steel, to a specific value is disclosed. Japanese Patent Application Laid-Open No. 6-106209 (hereinafter referred to as Prior Art 6) discloses a technique for drilling. A piercing and rolling method based on a geometric relationship consisting of a mill and a plug is disclosed.

The latter does not limit the material and at first glance seems to be a fairly effective proposal. However, when the material quality is inferior, as described in “Iron and Steel Journal No. 56, No. 7,“ Techniques for manufacturing seamless steel pipes ”(hereinafter referred to as prior art), etc. Such a proposal is completely powerless because there is a certain reduction ratio that can suppress the occurrence of the problem, and at least the properties of the material must be grasped.
This is because the state of the billet shaft core, which is the source of the inner surface flaw, is completely different depending on the steel type of the material or the manufacturing method, because it undergoes severe processing called Mannesmann drilling.

Moreover, when the material is cast, especially when it is a high alloy steel, the internal quality of the billet deteriorates remarkably, and the inner surface flaw cannot be suppressed very much at the rolling reduction shown in the above-mentioned prior document. is the current situation. Further, even if it is attempted to change the drilling conditions in order to obtain the rolling reduction, there is a problem in that the material may cause a biting failure this time, and as a result, the inner surface flaw cannot be suppressed.

For this reason, even after a tube is formed from a difficult-to-machine material, particularly a difficult-to-machine material such as a high-alloy steel as cast, through substantially the same process as in the prior art, it is necessary to keep the inner surface flaw of the tube. It was extremely difficult. Therefore, there has been a strong need for a technology that solves the "demerit of increased cost", which uses a material which is not subjected to pre-processing such as casting as much as possible and which does not produce internal flaws by drilling without large capital investment. .

Accordingly, it is an object of the present invention to reduce the material cost by using a material having a remarkably deteriorated internal quality such as a round cast slab as cast. The perforation parameters represented by the roll diameter and the number of rotations of the main roll for perforation rolling were maintained positive to realize perforation under light pressure, thereby preventing flaws on the inner surface of the pipe even when performing Mannesmann perforation. A method of manufacturing a steel pipe is provided.

[0024]

According to the first aspect of the present invention,
In a method of manufacturing a seamless steel pipe by an inclined piercing and rolling method using a slab made of a difficult-to-process material or an as-cast material,
The following formula PP = −0.00012BT + (1.6856 × 10 ⁻⁵⁾
× Dr) − (0.000327 × Vr) +0.0255 where BT: diameter of material billet (mm) Dr: diameter of main roll for piercing and rolling Vr: number of rotations of main roll for piercing and rolling The piercing and rolling is performed while maintaining the piercing parameter PP indicated by (rpm) positive, and thus the plug tip reduction rate is maintained at 95% or more, thereby suppressing the occurrence of flaws on the inner surface of the pipe. It is.

According to a second aspect of the present invention, the difficult-to-work material is
It is characterized in that it contains more than 0.5% by weight of Cr (chromium).

According to a third aspect of the present invention, the difficult-to-work material is
Containing more than 0.5% by weight of Cr (chromium),
001% by weight or more of S (sulfur) and Cr
(Chromium), Ni (Nickel) and Mo (Molybdenum) have a total content of more than 1.5% by weight.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The principle of the present invention will be described below.

[0028] In view of the findings of the prior art documents mentioned above,
It has been suggested that in order to pierce and form a raw material having poor workability without flaws, it is necessary to perform piercing under light pressure. In order to realize this, it is necessary to advance the perforated plug toward the material side to create a lightly reduced state. Although this seemingly contradicts, the Mannesmann effect,
In other words, it means that it is possible to produce pipes of any material without causing internal surface flaws by drilling while suppressing the rotary forging effect as much as possible.

The inventors of the present invention have conducted intensive studies and have found that perforation can be performed with a much smaller pressure than in the prior art and that the inner surface flaw can be suppressed. That is,
The following formula PP = −0.00012BT + (1.6856 × 10 ⁻⁵⁾
× Dr) − (0.000327 × Vr) +0.0255 where BT: diameter of material billet (mm) Dr: diameter of main roll for piercing and rolling Vr: number of rotations of main roll for piercing and rolling It has been found that by maintaining the perforation parameter PP represented by (rpm) positive, it is possible to suppress the inner surface flaw of the pipe.

The diameter of the piercing roll is, for example,
When the roll shape used for the Mannesmann perforation is a barrel shape, it is the maximum point, that is, the portion having the largest roll diameter.

There is a limit to the light reduction drilling, that is, the expansion of the reduction ratio of the plug tip defined by the following equation in a normal drilling and rolling state.

Plug tip reduction ratio = roll interval (d) at the plug tip position / billet diameter (D) This relationship is shown in FIG. In FIG. 5, 1 is a billet, 2 is a main roll for piercing and rolling, and 3 is a piercing plug.

According to the above formula, increasing the roll interval at the plug tip is equivalent to increasing the plug tip rolling reduction. The method includes increasing the roll interval,
A method is considered in which the plug is advanced in advance in the billet direction and the plug is pierced. In any case, the contact area from the contact of the billet to the roll to the contact with the plug is reduced, and it is difficult to obtain a thrust from the piercing roll that can overcome the drag of the plug. For this reason, the billet does not advance and drilling cannot be performed. On the other hand, from the results of many experiments, the inventors of the present invention have found that the plug tip reduction rate of as-cast material with deteriorated inner quality and alloy steel called difficult-to-machine material is 95%.
It has been found that if the above can be secured, flaws can be almost completely eliminated.

The present inventors have repeated many studies on how efficiently the material can be drawn into the roll and a thrust that can overcome the drag of the plug can be applied. As a result, it was concluded that it was advantageous to increase the friction coefficient between the roll and the material. As a method of increasing the material pull-in force of the roll, a technique for imparting unevenness to the roll surface has already been disclosed in Japanese Patent Application Laid-Open No. 251305/1990. ,
If the irregular shape is inappropriately large, there is a problem that the surface properties of the product are deteriorated.

On the other hand, with respect to the coefficient of friction between the roll and the material, for example, Japanese Patent Application Laid-Open No. 5-57307 discloses a technique in which a friction increasing agent is supplied between a piercing roll and a material billet. However, this technique is also liable to occur in high alloy steel, etc., resulting in "slip" caused by deterioration of the roll surface.
Is reduced, and the billet advance efficiency accompanying the increase in the number of perforations is compensated. Also, as clearly stated in the examples of the publication, of the application examples, those having a flaw are expressed as material properties, in other words, the inner flaws caused by the material properties. Cannot be suppressed. In any case, an effective rolling method capable of always maintaining a certain coefficient of friction and providing a thrust enough to overcome the effect of the perforated plug has not been confirmed.

Therefore, the inventors of the present invention have focused on the rotation speed of the roll and made an investigation. It has been said that there is a correlation between frictional force and roll rotation speed or rolling speed. However, this correlation is a matter confirmed by ordinary sheet rolling using a roll whose surface has not been subjected to any processing and which is completely perpendicular to the direction of material movement.

On the other hand, piercing rolling is a completely different type of rolling in which, as described above, the surface is sufficiently worked and the rolls are placed on a shaft having a slight angle with the direction of travel of the material. It is. Moreover, there is a perforated plug that prevents the material from advancing. Further, no correlation was found between the friction coefficient and the plug tip reduction rate.

Therefore, as a result of many experiments of the present invention, it was confirmed that there is a clear correlation between the roll rotation speed and the reduction rate of the plug tip as shown in FIG. In FIG. 1, the horizontal axis represents the number of rotations of the roll, and the vertical axis represents the reduction ratio of the plug tip at the above-described biting limit, and plots the presence or absence of occurrence of an inner surface flaw.

In addition, the biting is generally correlated with the roll diameter, and also affects the size of the material. Furthermore, in the case of complicated rolling such that the material is advanced while rotating the material, such as piercing rolling, the billet weight may also be involved, so the piercing conditions include a plug tip reduction rate,
The piercing roll diameter, the material billet diameter, and the number of roll rotations were taken into account. For these, the relationship between the tip reduction rate, which is the limit of bite due to the drag by the plug, and the respective parameters was examined, and as the billet diameter and roll rotation speed increased, and as the roll diameter decreased, the plug tip reduction decreased. The rate was observed to decrease. The present inventors repeated such experiments a number of times, accumulated data and considered these, and as a result, sufficient rolling was performed to suppress the inner surface flaw with respect to the above-mentioned poor-quality material. It has been found that the present invention has a condition that a plug tip reduction rate of 95%, which is a condition, can be ensured.

It is to be noted that even the as-cast material of general carbon steel) can be perforated under light pressure by applying the present invention, and the incidence of internal surface flaws can be reduced to almost zero. In addition, it is recognized that the occurrence of internal surface flaws can be suppressed to the same level as conventional carbon steel, even for difficult-to-process materials such as carbon steel and high alloy steel containing a large amount of sulfur, whose internal quality is easily deteriorated, and for as-cast materials. Was.

FIG. 2 shows the thrust load and the roll load of the piercing roll applied to the piercing plug when the biting failure occurs. Here, it is shown that a load starts to be applied to the piercing plug almost at the same time when the raw material bites into the piercing roll, and the raw material does not bite into the piercing roll even after a lapse of time.

On the other hand, FIG. 3 shows the load at the time of the normal drilling. Here, the roll load becomes sufficiently large, that is, after the contact area becomes sufficiently large, the roll is in contact with the drilling plug. It is shown. Thus, when the present invention is applied, it is recognized that the piercing time is longer than when piercing and rolling is performed at a high speed, but is substantially the same as the normal piercing state as shown in FIG. The plug tip rolling reduction at the biting limit was determined with the maximum when the same situation as that shown in FIG. 3 was shown.

[0043]

The present invention will be described in more detail with reference to examples.

Example 1 The present invention was studied using a hot model drilling machine. For the material billet, 0.4% carbon steel as cast is used, and the material heating temperature is 1250.
° C, the entrance side angle of the main roll for piercing and rolling is 2.5 degrees, the inclination angle of the main roll for piercing and rolling is 9 degrees, the main roll for piercing and rolling is barrel-shaped, and its surface processing is called normal knurling.
A perforation experiment was performed under the condition that the unevenness was processed to a depth of about 5 mm. In addition, regarding the pipe inner surface defect, 10 holes were drilled at each level, and the ratio of the length of the raw pipe at the portion where the pipe inner surface flaw was generated per total pressure extension was evaluated as a defective rate. % If not less than 5% to 15%
Less than △ was 1% or more and less than 5%, and を was not flawed during the test. Table 1 shows the results.

[0045]

[Table 1]

As is evident from Table 1, in the comparative example in which PP, which is outside the scope of the present invention, is a negative condition, the inner surface flaw of the pipe tends to increase as the roll rotation speed increases.
At 00 rpm, the defect rate was 11%, indicating that flaws could not be suppressed. However, in the rolling at 76 rpm under the condition that the PP which is the applicable range of the present invention is positive, perforation capable of suppressing the inner surface flaw to about 3% is possible. Further, in the example in which the roll diameter was changed from 300 to 550, the PP could be made positive, and the defect rate due to perforation under such conditions was 1%, confirming that the applicable range of the present invention was correct. Furthermore, when the billet diameter was reduced and the PP was increased,
The defect rate became 0. This is because the increase in the ratio of the roll diameter to the billet diameter has improved the biting property, and it has become possible to secure a higher plug tip reduction rate.

(Example 2) An experiment was conducted using a 13% Cr directly cast (as-cast) material and a 13% Cr pre-processed material subjected to conventional pre-processing under the same drilling conditions as in Example 1. Was done. Table 2 shows the results.

[0048]

[Table 2]

As is apparent from Table 2, when the pre-processed material of 13% Cr was perforated, the defect rate was slightly less than 10% at 100 rpm where the PP was -0.00778, which is slightly high for industrial production. The results are shown. On the other hand, in the comparative example directly cast, the defect rate was as high as 20% even under the same drilling condition,
Industrial production does not hold.

However, when the present invention is applied, a defect rate of 4% is obtained by drilling at a rotational speed of 76 rpm and a PP of 0.0003 in the pre-processed material, and a pipe can be formed with no scratches at a drilling speed of 69 rpm. In addition, as for the directly cast material, the PP to be perforated at a roll rotation speed of 39 rpm is 0.012.
4 to reduce the defective rate to 1%, and the roll rotation speed to 29.5 rp.
Under the drilling condition where the PP at m is 0.0155, the defect rate is 0
%, And the cost of material pre-processing can be reduced as it is.

Example 3 An experiment was conducted using various steel materials which were pre-processed according to the same drilling conditions as in Example 1. Table 3 shows the results.

[0052]

[Table 3]

As is evident from Table 3, in all the steel types to which the present invention is applied, the defective rate exceeds 5.5%, and it is found that considerable care is required. When the present invention was applied to these, the defective rate was 1 to 5%
It was found that in each case, the inside surface flaw can be significantly reduced.

Pre-processed material of 0.52Cr, 1Cr, 2.25Cr, direct cast material of 2.25Cr, 1Cr-0.7Mo-
0.01S pre-processed material and 0.6Ni-0.65
It was found that even with a direct cast material of Cr-0.3Mo-0.05S, sufficient suppression of internal surface flaws was possible.

[0055]

As described above, according to the present invention, a difficult-to-work material which has been poor in hot workability and is unsuitable as a seamless pipe material or is cast as it is. When piercing and rolling a material of poor quality in which there is, it is possible to optimize the piercing conditions without making significant changes to the equipment, thereby eliminating flaws on the inner surface of the pipe, such as high value-added high alloy steel pipes Useful effects such as cost reduction and productivity improvement can be obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a roll rotation speed and a plug tip reduction rate under a certain perforation condition.

FIG. 2 is a graph showing a relationship between a piercing plug load and a piercing rolling roll load at the time of poor biting.

FIG. 3 is a graph showing a relationship between a piercing plug load and a piercing rolling roll load during normal piercing.

FIG. 4 is a graph showing the relationship between the amount of Cr and the viscosity of molten steel.

FIG. 5 is an explanatory diagram showing a relationship between a roll interval (d) and a billet diameter (D) at a plug tip position.

[Explanation of symbols]

 1: Billet 2: Perforated rolling roll 3: Perforated plug

Continuation of the front page (72) Inventor Shozo Higashi 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Kinomasa 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan F term in the company (reference) 4E024 AA12 BB01 BB02 DD12 DD20

Claims (3)

[Claims] 1. A method of manufacturing a seamless steel pipe by an inclined piercing-rolling method using a slab made of a difficult-to-process material or an as-cast material, wherein PP = −0.00012BT + (1.6856 × 10 ⁻⁵⁾
× Dr) − (0.000327 × Vr) +0.0255 where BT: diameter of material billet (mm) Dr: diameter of main roll for piercing and rolling Vr: number of rotations of main roll for piercing and rolling (Rpm) piercing rolling is performed while maintaining the piercing parameter PP indicated by (rpm) positive, and thus, the plug tip reduction rate is maintained at 95% or more, thereby suppressing the occurrence of tube inner surface flaws. Manufacturing method of seamless steel pipe. 2. The method for producing a seamless steel pipe according to claim 1, wherein the hard-to-process material contains more than 0.5% by weight of Cr (chromium). 3. The hard-to-process material contains more than 0.5% by weight of Cr (chromium), and more than 0.0001% by weight of S.
(Sulfur) and the total content of Cr (chromium), Ni (nickel) and Mo (molybdenum) is 1.
2. The composition according to claim 1, wherein the content is more than 5% by weight.
Manufacturing method of seamless steel pipe.