JPH06145793A - Method for preventing decarburization of seamless steel tube - Google Patents

Abstract

PURPOSE:To prevent decarburization in the course of seamless steel tube production. CONSTITUTION:In a decarburization preventing method in the course of seamless steel tube production on a mandrel mill system, the surface temp. of the steel tube in a cooling stage after finish rolling is detected, and cooling is done through the region between the ferrite precipitation initiating point of the material to be treated and the ferrite precipitation finishing point at >=5 deg.C/sec cooling rate. By this method, the necessity of carburizing heat treatment can be obviated and the cutting allowance of decarburized part in customer use can be reduced, and as a result, yield can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preventing decarburization from occurring in the manufacturing process of seamless steel pipe.

[0002]

2. Description of the Related Art Steel pipes used for mechanical structures are subjected to heat treatment such as induction hardening, and processing such as cutting and forging. However, if the material is decarburized, cracking during quenching and product quality will occur. Since it affects (wear resistance, fatigue life resistance, etc.), decarburization regulations are established. However, in the manufacturing process of the steel pipe, since the material is carbon steel and the material to be treated is heated by the direct-fired heating furnace, decarburization inevitably occurs in the heating step. Decarburization in hot pipes can be reduced by cold working and atmospheric annealing, but in high carbon steel pipes with a remarkable carbon content exceeding 0.5%, the decarburization in hot pipes is remarkable. In some cases, the decarburization regulations may be exceeded. In this case, since it is customary to remove the portion where decarburization has occurred, the cost of production will increase due to the yield loss due to the removal work of the decarburization layer and the removal allowance of the decarburization layer.

As a method for preventing decarburization of the hot seamless steel pipe, a method of applying a decarburization inhibitor to the billet of the material (Japanese Patent Laid-Open Nos. 54-107410 and 56-43).
770, JP-A-56-72120, etc.), a method of performing carburization in a predetermined temperature range to recover carbon in a decarburized layer (JP-A-3-126858, JP-A-3-1882).
No. 56, etc.), or a heating furnace for hot pipe making, a low air-fuel ratio in a reheating furnace, and a high workability rolling of a stretch reducer.

[0004]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The method of applying a decarburizing inhibitor to a billet such as 7410 publication is not effective because the inner surface decarburization of the steel pipe occurs in the steps after the perforation, and the methods are not disclosed in JP-A-3-126858 and JP-A-3-188256. The method for carburizing heat treatment disclosed in Japanese Patent Publication is not applicable when the carburizing heat treatment not only increases the cost but also makes the carbide spherical due to the gradual cooling of the ferrite precipitation area and the ferrite and pearlite structures are prescribed. Further, when the air-fuel ratio is lowered in the heating furnace and the reheating furnace, O ₂ in the furnace cannot be completely eliminated, and decarburization occurs.
The high workability rolling of the stretch reducer increases the thickness and is not a very effective method for preventing decarburization.

An object of the present invention is to provide a method for preventing decarburization of a seamless steel pipe, which can prevent decarburization in the process of manufacturing a seamless steel pipe without recovering the decarburized layer by carburizing heat treatment.

[0006]

[Means for Solving the Problems] The inventors of the present invention have made extensive studies to achieve the above object. As a result, if the ferrite precipitation region is gradually cooled in a decarburizing atmosphere, decarburization will proceed, and this is exactly the case of cooling the material after the hot pipe making is completed. Therefore, it was clarified that decarburization, especially inner surface decarburization can be prevented by rapidly cooling the ferrite precipitation region in the steel during the cooling of the material after the hot pipe forming, where carbon diffusion is severe, at a predetermined cooling rate. Reached

That is, according to the present invention, in the decarburization preventing method in the manufacturing process of the seamless steel pipe by the mandrel mill system, the steel pipe surface temperature in the cooling process after finish rolling is detected, and the ferrite precipitation from the ferrite precipitation starting point of the material to be treated is detected. The cooling is performed at a cooling rate of 5 ° C./sec or more until the end point.

[0008]

In the present invention, the surface temperature during cooling of the steel pipe after finish rolling of the hot seamless steel pipe is detected, and the temperature from the ferrite precipitation starting point to the ferrite precipitation ending point is 5 ° C /
By cooling at a cooling rate of sec or more, the ferrite precipitation area from the ferrite precipitation start point to the ferrite precipitation end point is rapidly cooled, and the reaction between the atmosphere and carbon in the steel during this period is suppressed and decarburization is prevented. It

In the present invention, the cooling rate from the ferrite precipitation start point to the ferrite precipitation end point is 5 ° C./s.
The reason for setting ec or more is that the effect of preventing decarburization is not sufficient at less than 5 ° C / sec. The cooling method in the present invention may be any means as long as a cooling rate of 5 ° C./sec or more can be secured. In addition, the reason why the rapid cooling is from the ferrite precipitation start point to the ferrite precipitation end point is that at the ferrite precipitation start point or higher, the progress of decarburization is small due to the relationship between the cooling rate and the microstructure in the steel, and the ferrite precipitation end point This is because if the temperature is lower than the point, the diffusion of carbon in the steel becomes slow and decarburization does not proceed.

[0010]

[Example] S45C specified in JIS G 4051
(C: 0.46%, Si: 0.28%, Mn: 0.80
%, P: 0.010%, S: 0.015%) of a billet of mechanical structural steel with a diameter of 187 mm and a length of 3600 mm, as shown in FIG. 1, in a rotary hearth type heating furnace at 1200 ° C.
After soaking for 1 hour at 1100 ~ 1150 ℃ with a punch
Perforated and rolled with an outer diameter of 192 mm, a wall thickness of 21.84 mm,
It was a blank tube having a length of 8320 mm. Then, the blank tube was stretch-rolled by a 7-stand mandrel mill at 1100 to 1150 ° C. to have an outer diameter of 151 mm, a wall thickness of 11.5 mm, and a length of 1.
9270mm seamless steel tube, 1000 ℃ in reheating furnace
After heating for 20 minutes in a stretch reducer 89
Finish rolling was carried out at 0 to 920 ° C. to produce a seamless steel pipe having an outer diameter of 50.8 mm, a wall thickness of 10.75 mm and a length of 71500 mm.

Next, as shown in FIG. 2, the seamless steel pipe 1
After cutting to a length of 13700 mm on the cooling floor 2,
While being transported to the quenching zone 3 in the cooling floor 2 and rotating the seamless steel pipe 1 at a fixed position at a pitch of 60 times / min, the quenching zone 3
Using the radiation thermometer 4 provided on the upper part, while measuring the outer surface temperature of the central part in the longitudinal direction of the pipe, the pressure from one end is 5 kg / c from the ferrite precipitation start point to the ferrite precipitation end point.
As shown in Table 1, m ² water was sprayed into the pipe while changing the amount of water to cool the inner surface of the pipe. A test piece was taken from the obtained seamless steel pipe, the thickness of the decarburized layer on the inner and outer surfaces was measured, and the relationship between the cooling rate from the ferrite precipitation start point to the ferrite precipitation end point and the decarburized layer thickness on the inner and outer surfaces is shown in Table 1. Shown in.

[0012]

[Table 1]

As shown in Table 1, in the cooling process after finish rolling, the cooling rate from the starting point of ferrite precipitation to the ending point of ferrite precipitation was 5 m / sec or more. In case of 4, inner decarburization is almost completely prevented,
The decarburized layer thickness is reduced to 0.01 mm. On the other hand, the test No. in which the cooling rate from the ferrite precipitation start point to the ferrite precipitation end point was less than 5 m / sec. In the case of Nos. 2 to 3, the inner decarburized layer has a thickness of Test No. 5 of 1
It is not significantly reduced to about 0%.

[0014]

As described above, according to the method of the present invention, decarburization in the hot seamless steel pipe manufacturing process, especially decarburization of the inner surface of the seamless steel pipe, can be prevented, and carburizing heat treatment is unnecessary. In addition, the cutting cost of the decarburization part at the customer can be reduced, and the yield can be improved accordingly.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a pipe manufacturing process in an example.

FIG. 2 is an explanatory view of quenching a hot seamless steel pipe after finish rolling in an example.

[Explanation of symbols]

 1 Seamless steel pipe 2 Cooling bed 3 Quenching zone 4 Radiation thermometer

Claims (1)

[Claims] 1. A method for preventing decarburization in a manufacturing process of a seamless steel pipe by a mandrel mill method, which detects a surface temperature of a steel pipe in a cooling process after finish rolling and detects a ferrite precipitation starting point of a material to be processed to a ferrite precipitation ending point. A method for preventing decarburization of a seamless steel pipe, characterized in that the cooling is performed at a cooling rate of 5 ° C./sec or more.