JPH0691319A - Seamless steel pipe rolling method with few outer surface defects - Google Patents

Abstract

(57) [Abstract] [Purpose] Effective in reducing the seizure of the guide in the rolling process of the seamless steel pipe by a pair of opposed rolling rolls and restraint guides, and obtaining a seamless steel pipe with less outer surface defects. Providing various means. [Structure] After the material to be rolled leaves the rolls, high-pressure cooling water is sprayed onto the outer surface until it comes into contact with the guides, and only the surface layer is rolled while the temperature is momentarily lowered. The position where the surface of the material to be rolled is rapidly cooled is preferably just before starting contact with the guide in order to limit the quenching layer to the surface layer as much as possible. With respect to the high-pressure jet water, it is desirable that a jet water curtain having a width equal to or smaller than the width of the guide is vertically applied to the surface of the steel pipe, and the temperature within 500 μm of the surface layer is lower by about 50 to 150 ° C. than the temperature inside the material to be rolled. It has effects of facilitating sliding friction at the contact interface between the material to be rolled and the restraint guide, suppressing seizure, peeling of metal and scale, and cooling the restraint guide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the rolling of seamless steel pipe which reduces the generation of surface defects.

[0002]

2. Description of the Related Art Generally, a seamless steel pipe is continuously formed into a hollow shape by heating a solid material to a high temperature and then subjecting it to the main steps of perforating, drawing, mandrel rolling and sizing.

Among these steps, the perforation and stretching steps in the former stage are generally performed by a pair of rolls and a pair of restraining guides which face each other.

In this step, since it is desirable that the deformation resistance of the material to be rolled is as small as possible, the material to be rolled is rolled in a state of being heated to a high temperature as much as possible. At the time of rolling the heated material to be rolled, the roll is constantly sprayed with cooling water, and the temperature rise on the roll surface is suppressed to a low level.
The restraint guide is performed without water injection, and is continuously used without being replaced during continuous rolling of tens to hundreds of steel pipes.

During this process, a strong sliding friction phenomenon occurs between the restraint guide and the material to be rolled, and seizure between the guide and the surface layer of the material to be rolled easily occurs. Moreover, the scale or metal on the surface layer of the rolled material is easily scraped off or stripped off by the guide. When these phenomena occur, surface flaws called wraps, dips, and pits are generated in the rolled steel pipe product. The blemish is caused by the fact that the metal flow in the surface layer is abnormal and the layers overlap, or the material is cracked due to shear deformation, and the jump is caused by the peeled metal or scale debris being incompletely pressed against the surface of the rolled material. It is considered that the pits are generated and the pits are formed when the adhering substances that have jumped in are separated without being pressed.

In any case, in the conventional seamless steel pipe rolling, these rolling defects are chronically occurring to a greater or lesser extent. Defects are required to reduce the yield of seamless steel pipe products and take care to remove defects, which is a major cause of problems such as increased manufacturing costs, complicated logistics, and prolonged manufacturing period. . In addition, when defects occur frequently, there is a problem that the service life of the restraint guide is reduced, and the dimensional accuracy of the steel pipe product tends to deteriorate.

Conventionally, as a measure for reducing the seizure of these guides, JP-A-1-148405 discloses that the sliding surface of the guide has a specific composition, and JP-A-2-2248.
Japanese Patent Laid-Open No. 08-2008 proposes to supply an anti-seizure agent to the guide surface.

However, these measures do not eliminate the cause of the seizure of the guide itself in the process of punching and stretching, so that the result is naturally limited.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to reduce seizure of guides in a rolling process of a seamless steel pipe by a pair of rolling rolls and a constraining guide which are opposed to each other, so that a seamless steel pipe with less outer surface flaws is generated. It is to provide an effective means for obtaining.

[0010]

SUMMARY OF THE INVENTION In the present invention, the contact between the hot rolled material and the guide is the main cause of the flaw generation, and the sliding friction between the guide and the rolled material is used to prevent the flaw. Based on the finding that reducing the coefficient to prevent seizure is an effective means, we have found that a specific method is to quench only the surface layer of the material to be rolled.

That is, according to the present invention, in a rolling process of a seamless steel pipe by a pair of rolling rolls and a restraining guide which face each other, high-pressure cooling water is applied to the outer surface of the rolled material until it comes into contact with the guide after leaving the roll. It is characterized by spraying and rolling only the surface layer while instantaneously lowering the temperature.

A wide range of positions for quenching the surface of the material to be rolled is assumed, but in order to limit the quenching layer to only the surface layer as much as possible, the gap between the roll and the guide is a necessary condition, and the contact with the guide is necessary. Just before the start is desirable. At other positions, the cooling portion becomes too large, and the rolling reaction force in the subsequent rolling increases, so that the ductility deteriorates.

For the same reason, it is preferable to use high-pressure jet water as the cooling method. The pressure and flow rate of water are not particularly limited,
It is desirable to apply a jet water curtain that is less than the width of the guide vertically to the steel pipe surface.

The degree of quenching is 500 μm on the surface layer.
Within is the temperature inside the rolled material (usually about 1000-120
It is preferable that the temperature is lower than 0 ° C. by about 50 to 150 ° C.

[0015]

[Function] By rapidly cooling only the surface of the material to be rolled with high-pressure jet water, sliding friction at the contact interface between the material to be rolled and the restraint guide can be facilitated, and seizure, peeling of metal, and scale can be suppressed. it can.

By spraying the high-pressure cooling water onto the outer surface, it is possible to cool the material to be rolled and at the same time cool the restraint guide.

[0017]

[Example] Carbon steel (0.13% C-0.2% S)
i-1.2% Mn), Cr-Mo low alloy steel, and 1
A seamless steel tube was rolled using 3% Cr steel.

The rolled size of the steel pipe is the finished size and the outer diameter:
The constant conditions were 300 mmφ, wall thickness: 11 mm, and length: 11.5 m. Steel tube rolling is performed in the order of process: punching machine-drawing machine-
Plug mill-tube polisher-reheat furnace-sizer
The method of the present invention was applied to a stretch rolling mill. The number of rolling was 5 for each level.

The test results are shown in Table 1 in comparison with the conventional method.

[0020]

[Table 1] The evaluation of the results was carried out by the average number of defects generated per pipe, but in the case of the method of the present invention, it is clear that the defects on the outer surface of any of the steel types are extremely reduced as compared with the conventional method. . On the other hand, it was confirmed that the service life of the restraint guide was significantly extended.

[0021]

According to the present invention, the occurrence of external surface flaws in a seamless steel pipe product is drastically reduced. This contributes not only to the improvement of the surface quality of the product, but also to the improvement of the yield and the reduction of the defect maintenance cost.

Claims (1)

[Claims] 1. In a rolling process of a seamless steel pipe using a pair of rolling rolls and a constraining guide facing each other, high-pressure cooling water is sprayed on the outer surface of the surface layer until the material to be rolled leaves the rolls and contacts the guide. A method for rolling seamless steel pipe with few outer surface defects, which is characterized by rolling only a single instant while lowering the temperature.