JPH0718332A - Method for manufacturing seamless steel pipe - Google Patents

Abstract

(57) [Summary] [Purpose] Reduction of indentation defects and bending defects due to scale. [Structure] 500-600 in a low oxygen atmosphere after hot rolling
A method for producing a seamless steel pipe, which comprises cooling to ℃ and straightening at 450 to 550 ℃ after descaling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength seamless steel pipe having few surface defects.

[0002]

2. Description of the Related Art Generally, a seamless steel pipe is used for billet heating,
It is manufactured by the steps of drilling, rolling, constant diameter and straightening. Since these steps are performed at a high temperature with low deformation resistance, scale is generated by oxidation and the surface quality is deteriorated. Therefore, descaling is performed at appropriate times to obtain a product. For example, it is common to heat the billet, before piercing, rolling, and cooling after the diameter, and descaling before straightening.

[0003]

However, the above-mentioned conventional method of performing straightening after the constant diameter and cooling has the following problems. 1) Since the tube temperature during straightening varies depending on the rolling size,
It is difficult to control the tube temperature during correction to an appropriate value. In addition, 2) the temperature of the steel pipe varies due to the difference in cooling conditions during cooling, and a considerable difference occurs depending on the position of the pipe and between the pipes. The outside temperature also affects this.

As a result, if a portion with a high straightening temperature is formed, scale will be generated after straightening, and the scale will remain on the tube surface. Further, if the straightening temperature is low, the scale generated before straightening is pushed into the tube surface during straightening, causing scale indentation flaws.

3) On the other hand, also in the descaling process, even when the descaling device is installed in front of the straightening machine, if the descaling temperature is high, scale is generated after the descaling. On the other hand, when the descaling temperature is low, the descaling effect becomes insufficient and the scale remains on the tube surface.

Further, in the case of cooling in the air before descaling, the scale produced during the cooling is considerably large and the scale produced thickly causes partial exfoliation, and the scale is newly renewed in that portion. When this occurs and is descaled, surface unevenness is generated in a portion where peeling and regeneration are not performed, which causes a surface defect. It is an object of the present invention to provide a method for manufacturing a seamless steel pipe that can solve the above problems.

[0007]

The method for producing a seamless steel pipe according to the present invention is carried out in a low oxygen atmosphere after hot rolling in a range of 500-60.
It is characterized in that it is cooled to 0 ° C. and, after descaling, is straightened at 450 to 550 ° C.

[0008]

By cooling in a low oxygen atmosphere after rolling and sizing, the scale produced in the atmosphere before rolling, sizing and low oxygen atmosphere grows slowly. In order to create a low oxygen atmosphere, it is necessary to isolate it from the outside air. For example, when an atmosphere furnace is used, the steel pipe is cooled slowly in a controlled state. In this state, the scale does not peel off,
Therefore, a uniform scale is obtained without peeling or new generation.

Further, even when slowly cooled in a low oxygen atmosphere, scale formation is small, descaling afterwards is easy, and when the descaling time is short, the temperature of the steel pipe can be easily controlled. Since the correction can be performed within the target temperature range, the strength is appropriately maintained. Furthermore, it is also possible to correct the temperature after the correction so that the scale does not occur.

[0010]

EXAMPLE The method of the present invention comprises providing a cooling furnace and a descaler whose atmosphere has been adjusted after a constant diameter and straightening a descaled steel pipe with a straightening machine. After the constant diameter, that is, the temperature at the time of charging into the cooling furnace is generally 800 to 900 ° C.

The cooling furnace is a relatively long furnace, which is in a low oxygen atmosphere by burning the fuel at a low excess air ratio of 4% or less, thereby suppressing generation of scale. The steel pipe is cooled while passing through this long furnace, and the steel pipe is extracted at a temperature of 500 to 600 ° C.

FIG. 1 shows a change in temperature and a change in scale amount during the execution of the present invention. In contrast to the case of using the cooling furnace of the present invention (solid line), the case of cooling in the atmosphere (broken line) is shown.

In the case of standing cooling, even if the cooling start time is the same, a difference occurs in the cooling conditions, so that the cooling curve has a considerable width. This width is naturally recognized even after descaling, and the variation in the straightening temperature is as large as 400 to 600 ° C., which may result in low-temperature straightening, and in this case, it causes strength reduction. On the other hand, if the descaling temperature is high, the scale regenerates after the descaling.

On the other hand, when cooling is performed in the cooling furnace, the cooling rate becomes slower and the time required for cooling becomes longer than that in the case of cooling. However, the width of the cooling curve becomes smaller, and the descaling temperature can be 500 to 600 ° C. Therefore, the straightening temperature is also constant (450-550 ° C)
Since it is too high, it is possible to reduce the scale formation again after straightening, and it is also possible to prevent the strength from being lowered due to the Bauschinger effect due to being too low.

In the case of atmospheric cooling, the cooling rate is high, but the scale generation increases because it is cooled in an atmosphere containing a large amount of oxygen, and the srail loss also increases. Further, the scale loss curve has a width because it is not cooled under a constant cooling condition. Further, the temperature may be high even after the descaling, and a new scale is generated.

On the other hand, when cooling is performed in the cooling furnace, the cooling time is long, but since the cooling is carried out in a low oxygen atmosphere, the scale formation is reduced. Further, since the temperature for descaling and correction can be kept constant, scale generation after descaling can be prevented.

When the method of the present invention was applied to the actual operation, it was possible to reduce the rate of occurrence of indentation flaws due to the scale from 0.5% to 0.1%. In addition, the warm correction could reduce the occurrence rate of bending defects from 0.3% to 0.1%.

[0018]

The method of straightening a seamless steel pipe according to the present invention is as described above, and it is possible to greatly reduce the occurrence rate of indentation flaws due to scales and the strength reduction occurrence rate due to variations in straightening temperature as compared with the prior art. .

[Brief description of drawings]

FIG. 1 is an explanatory view showing a relationship between a tube temperature and a scale loss% in a case of cooling (dashed line) and a case of using a cooling furnace of the present invention (solid line).

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Claims (1)

[Claims] 1. After hot rolling, in a low oxygen atmosphere, 500 to
Cool to 600 ° C, 450-550 after descaling
A method for producing a seamless steel pipe, which comprises straightening at ℃.