JP4506441B2 - Seamless steel pipe manufacturing method - Google Patents

Description

  The present invention relates to a method for producing a seamless steel pipe by a Mannesmann-mandrel mill method.

  Mannesmann-mandrel mill method for the production of seamless steel pipes was achieved by forming round steel pieces heated in a rotary hearth furnace into a hollow shell (shell) using a drilling machine, and applying a lubricant to the surface. A mandrel bar is inserted into the shell in a skewer shape, and is drawn and rolled to a predetermined size by a mandrel mill. After drawing and rolling, the tube from which the mandrel bar was drawn was cut with a hot saw at the bad end of the tube, and then the tube reheated in the reheating furnace was subjected to descaling with high-pressure water on the outer surface, The product dimensions are subject to outer diameter reduction and slight wall thickness reduction. Thereafter, the pipe is cooled in a cooling bed, cut into a required length by a cold saw, subjected to heat treatment, and sent to a finishing line.

  In the production of seamless steel pipes by the Mannesmann-mandrel mill method, the mandrel bar is inserted into a shell heated to a temperature of 1000 to 1100 ° C. and placed in a state where it is easy to be seized. Further, the mandrel bar is pulled out of the shell by a burst ripper after drawing and rolling. However, when the lubricity is poor, the material and the mandrel bar are seized and the bar cannot be pulled out, thereby impairing workability. In addition, the shape and thickness of the tube after drawing and rolling are affected by the friction between the mandrel bar and the material, as well as the roll rotation speed and roll hole type. Therefore, it is necessary to apply a hot rolling lubricant excellent in hot lubrication performance to the mandrel bar surface. As such a lubricant, a water-soluble lubricant mainly composed of graphite (hereinafter referred to as a graphite-based lubricant) which is inexpensive and has very excellent lubricating performance is most often used.

  However, when stretch rolling is performed by inserting a mandrel bar coated with such a graphite-based lubricant, a portion having a high carbon concentration (carburized layer) is generated on the inner surface side of the tube during stretch rolling. Subsequent reheating, finish rolling, and solution heat treatment after rolling diffuse the carbon in the carburized layer, which is the high carbon concentration region, and the carbon concentration becomes low, but a portion with a high carbon concentration still remains.

  Such a carburized layer causes intergranular corrosion and reduces the corrosion resistance of the product. In order to ensure the corrosion resistance of the product having the carburized layer, a process of removing the carburized portion on the inner surface of the product by polishing or the like is required. This process significantly increases costs and decreases productivity. Further, in the case of a product having a small diameter, there may be a case where it is impossible to insert a grinding wheel for polishing into the pipe because the inner diameter is small.

  In order to solve such problems, a non-graphite lubricant as shown in Patent Document 1 has been developed. This non-graphite lubricant can be applied not only to typical stainless steel, which is a problem of the occurrence of carburized layers, but also to ordinary steel, but it increases costs. Therefore, in a facility that also manufactures ordinary steel pipes, a conventional graphite-based lubricant is separately used when producing ordinary steel pipes. However, if a graphite lubricant is used for rolling ordinary steel, the graphite lubricant will adhere to the mandrel bar and the mandrel bar conveying line, so when rolling stainless steel using the same mandrel mill, non-graphite lubricant Even if the agent is used, the graphite that has already adhered is transferred to the bar, resulting in a carburized layer on the inner surface of the stainless steel tube. Therefore, the merit of applying a special non-graphite lubricant only for stainless steel pipes is expected to be small.

  As countermeasures, as shown in Patent Documents 2 and 3, ordinary steel is rolled using a conventional graphite-based lubricant, the mandrel bar and the mandrel bar conveying line are washed with water before rolling the stainless steel, and thereafter It has been proposed to perform rolling by applying a non-graphite lubricant. However, as shown in Patent Document 2, a problem may occur when a water-soluble graphite-based lubricant is used as a lubricant for ordinary steel. That is, in order to prevent the lubricant from being washed off during the rolling of the ordinary steel, it is necessary to perform ON / OFF control of the roll cooling water, but the rolling of the ordinary steel often has a fast pitch, The time zone in which the roll cooling water is turned off becomes relatively long, and the cooling of the roll becomes insufficient, and it becomes easy to induce wear and damage of the roll.

On the other hand, as shown in Patent Document 3, it is possible to control the amount of graphite adhering to the mandrel bar surface to 100 mg / m ² or less, but it takes a lot of man-hours for cleaning, and it seems to be poor in reality.
Japanese Unexamined Patent Publication No. 64-16894 JP 2002-28705 A JP 2000-24706 A

  The present invention provides a more inexpensive manufacturing method that can effectively suppress the occurrence of a carburized layer on the inner surface of a pipe, regardless of the type of lubricant used, even when a seamless steel pipe is manufactured by the Mannesmann-Mandrel Mill method. There is to do.

  The present inventors have investigated in detail the occurrence of carburized layers on the inner surface of a pipe when using a graphite-based or non-graphite-based lubricant mixedly and manufacturing a seamless steel pipe by the Mannesmann-Mandrel mill method. As a result, the following useful findings were obtained.

  (1) Conventionally, it has been considered that the amount of carburization generated on the inner surface of the pipe correlates with the amount of carbon adhering to the mandrel bar surface before rolling. However, the inventors have a strong carburization amount generated on the inner surface of the tube, not only the amount of carbon adhering to the mandrel bar surface, but also the stretching ratio of mandrel mill rolling, and the time until stripping after completion of mandrel mill rolling. We found that there is a correlation.

  (2) This is based on the new finding that the amount of carburization generated on the inner surface of the tube has a correlation with the amount of carbon adhering to the inner surface of the tube after mandrel burst ripping. Since the carbon adhering to the inner surface of the tube is carburized in a subsequent process such as heat treatment, not only the amount of carbon adhering to the surface of the mandrel bar, but also the stretch ratio of the mandrel mill rolling and the stroke after the mandrel mill rolling is completed. The time until ripping is thought to affect the amount of carburization.

  (3) Specifically, the carbon adhering to the mandrel bar surface is transferred from the bar surface to the pipe inner surface between mandrel mill rolling and mandrel burst ripping, but the stretch ratio of mandrel mill rolling is large. It has been found that the amount of transfer to the inner surface of the tube tends to increase as the time from the end of mandrel mill rolling to stripping increases.

  (4) The amount of carbon in (1) refers to a value obtained by converting the total amount of carbon contained in graphite and organic binder contained in the lubricant and carbon contained as a contaminant. In other words, it was also found that graphite and organic binder must be handled in the same manner in terms of carbon content.

Based on the above knowledge, one of the methods to prevent carburization is not only focusing on reducing the amount of carbon adhering to the mandrel bar surface before rolling more than necessary, but also after stripping the mandrel bar. It was concluded that it was important to find conditions that would reduce the amount of carbon adhering to the inner surface of the tube. As a result of diligent examination of the conditions, E: Stretch ratio, T: Time from stripping after mandrel mill rolling to stripping (seconds), C: Carbon amount adhering to the bar surface (g / m ² ),
(0.015 × E + 0.0002 × T−0.02) × C ≦ 2
Preferably
(0.015 × E + 0.0002 × T−0.02) × C ≦ 1
It has been found that the generation of the carburized layer can be suppressed if the above is satisfied.

  Here, the present invention performs piercing and rolling on a steel piece, inserts a mandrel bar into the obtained hollow shell, performs stretching and rolling with a mandrel mill, and performs rolling that satisfies the following conditions in the stretching and rolling. This is a method for producing a seamless steel pipe.

(0.015 × E + 0.0002 × T−0.02) × C ≦ 2
However, E: Stretch ratio, T: Time (seconds) until stripping after completion of mandrel mill rolling,
C: Carbon amount adhering to the mandrel bar surface (g / m ² )
Here, “the amount of carbon adhering to the surface of the mandrel bar” means that the carbon adhering to a predetermined bar surface position (as described later, for example, a suitable range of 50 mm × 100 mm) is scraped off, It can be obtained by measuring the amount, and in actual operation, it can be measured by introducing equipment for automatically scraping off carbon adhering to a predetermined bar surface position.

  Therefore, from another aspect, the present invention is a method for manufacturing a seamless steel pipe in which the amount of carbon adhering to a mandrel bar is obtained and then rolling is performed based on the amount of carbon satisfying the above-described relational expression.

  According to the present invention, even when a stainless steel seamless steel pipe is manufactured by the Mannesmann-mandrel mill system on the same line as ordinary steel, carburized layer generation on the inner surface of the pipe can be suppressed without particularly washing the mandrel bar with water.

  The steel types with the composition shown in Table 1 were coated with 5 types of graphite or non-graphite lubricants shown in Table 2 on a mandrel bar and dried by 100 μm, and then pierced and rolled by a punching machine (P / M) with the setup shown in Table 3. This was stretch-rolled with a mandrel mill (M / M), and then finish-rolled with a stretch reducer (SR / M).

The mandrel bar was passed through the same process as the actual work process, and the 50 mm × 100 mm lubricant was peeled off from the three locations immediately before the stretch rolling, and the amount of carbon contained therein was analyzed. It was confirmed that ² contaminated carbons were attached as pollutants. Therefore, the amount of carbon adhering to the bar surface was the sum of the carbon contained in the lubricant described in Table 2 and the contaminated carbon.

  The time from the end of mandrel mill rolling to stripping was 2 seconds when burst ripping was performed in the rolling line, and 20 seconds when burst ripping was performed in another line.

  The carbon concentration was obtained by collecting a total of twelve samples from three circumferential locations in the longitudinal direction of each product tube obtained, grinding and removing the oxidized scale on the inner surface, and the carbon concentration on the material surface. Was measured using an emission spectroscopic analyzer. The evaluation was performed using the maximum value among 12 samples.

  The results are shown in Table 4.

  As is apparent from this result, when pipes are produced under the conditions of the present invention examples (No. 1 to 28), the occurrence of carburized layers exceeding the upper limit value (C = 0.03%) specified in the JIS standard does not occur. Not happening. However, among these, those with the conditions of No. 11, 12, 15, 16, 20, 22, 23, 24, 26, 27, 28 are out of the more desirable conditions, so C = 0.02% or more Carburized layers are generated. On the other hand, under the conditions of the comparative examples (Nos. 29 to 40), the occurrence of a carburized layer exceeding the upper limit specified in the JIS standard occurs.

Claims (1)

The steel piece is subjected to piercing and rolling, a mandrel bar is inserted into the obtained hollow shell, and the mandrel mill is stretched and rolled, and in the stretching and rolling, the rolling satisfying the following conditions is performed. Steel pipe manufacturing method.
(0.015 × E + 0.0002 × T−0.02) × C ≦ 2
However, E: Stretch ratio, T: Time (seconds) until stripping after completion of mandrel mill rolling,
C: Carbon amount adhering to the mandrel bar surface (g / m ² )