JPH0617119A - Method and equipment for heat treatment for hot extruded steel tube - Google Patents

Abstract

PURPOSE:To apply solution heat treatment to an austenitic stainless steel tube directly after hot extrusion. CONSTITUTION:In the heat treatment method for successively subjecting an austenitic stainless steel tube directly after hot extrusion to water cooling down to a temp. in the vicinity of ordinary temp. to obtain a hot extruded steel tube having an austenite structure in solid solution state and free from bending, cooling water constituted so that it is allowed to flow at >=4kg/cm<2> pressure in a direction parallel to a tube axis is kept being supplied from one tube end side of a rolling steel tube 22 the amount >= at least 60% of the cross-sectional area of internal surface of the steel tube. By this method, the solution heat treatment for the steel tube can be done in the state free from bending while obviating the necessity of large amounts of heat energy with inexpensive plant and equipment investment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-extruded steel pipe having an austenite structure immediately after hot-extrusion and having a solution-processed austenite structure and being water-cooled to near room temperature. And a heat treatment method for obtaining the same.

[0002]

2. Description of the Related Art Austenitic stainless steel pipes produced by a hot extrusion method are subjected to solution treatment (hereinafter referred to as water toughness) in order to exert their original corrosion resistance. The conventional method of water toughness is as follows. That is, as shown in FIG. 3, the austenitic stainless steel pipe 22 to be subjected to water toughness is sent in the pipe axial direction by the hearth roller 31, is charged into the heat treatment furnace 32 through the charging port 33, and is extracted through the extraction port 34. The temperature is raised to a predetermined temperature before being extracted. And the heat treatment furnace 3
While passing through the water cooling device 35 provided on the downstream side of 2, the water toughness is completed by cooling to room temperature.

Another method of water toughness is shown in FIG. In this method, after the austenitic stainless steel pipe 22 is heated to a predetermined temperature in a heat treatment furnace 42 configured to convey the austenitic stainless steel pipe 22 in a direction orthogonal to the pipe axis, the heat treatment furnace 4
The water toughness is obtained by passing through a water tank or a cooling zone 45 provided on the downstream side of 2.

FIG. 5 is a graph showing a temperature history when performing water toughness by the method shown in FIG. The outer diameter of the steel pipe is 2
1.7 mm x 2.16 mm thickness, outer diameter 55.0 m
m × wall thickness 4.0 mm, and outer diameter 96.0 mm ×
The temperature histories of three types of steel pipes having different thicknesses from those having a wall thickness of 6.0 mm are shown. Temperature of steel pipe during extraction is 1050 ℃
The temperature at the start of cooling is 9
It is 50 ° C. The cooling rate is 8.2 ° C./sec for the smallest size and 6.6 ° C./sec for the largest size.

[0005]

However, the above-mentioned conventional method for water toughening of an austenitic stainless steel pipe has the following problems. That is, in the method shown in FIG. 3, the portion of the pipe that normally leaves the furnace is cooled sequentially, for example, by applying cooling water from above, so that the rear end of the pipe remains in the furnace. Therefore, it is very difficult to rotate the tube around the axis, and the cooling becomes uneven, so that the tube is bent.

If the inner surface of the pipe is cooled in an attempt to make the cooling uniform, the cooling water will flow into the furnace, so that the inner surface of the pipe cannot be cooled. Of course, it is possible to cool the inner surface of the tube after the whole tube exits from the furnace, but the temperature of the tube at the beginning of cooling depends on the rate at which the tube is extracted from the furnace not so fast. There is a problem that it is different at both ends. The reason why the portion outside the furnace is immediately cooled when the pipe is moved in the longitudinal direction is to keep the temperature of the pipe high at the start of cooling and to secure the corrosion resistance.
It may be possible to increase the extraction temperature in order to increase the temperature of the tube at the start of cooling as much as possible, but there is a problem that the structure becomes coarse.

On the other hand, the method shown in FIG. 4 requires a furnace having a width corresponding to the length of the tube, and in order to ensure productivity, it is necessary to lengthen the furnace length as well, resulting in enormous capital investment. There is a problem that Further, in the case of this method, a method of immersing the pipe in a water tank is generally adopted, but there is a problem that equipment cost and power cost for circulating the cooling water are required. However, even in this method, the cooling is not always performed uniformly, and there is a problem that the bending of the pipe cannot be completely prevented because the cooling of the inner surface of the pipe is particularly delayed.

The present invention has been made in order to solve the above-mentioned problems of the prior art, and an austenitic stainless steel pipe without bending and in a solution heat treated state requires a large amount of heat energy. It is an object of the present invention to provide a heat treatment method and a heat treatment apparatus which can be manufactured at low cost.

[0009]

In the heat treatment method for hot extruded steel pipe according to the present invention, an austenitic stainless steel pipe immediately after hot extrusion is continuously water-cooled to near room temperature to obtain an austenite structure in a solution treatment state. In the heat treatment method for hot extruded steel pipe, from the one end side of the rolling steel pipe,
Cooling water configured to flow at a pressure of kg / cm ² or more and parallel to the pipe axis is used at least 60 times the inner surface cross-sectional area of the steel pipe.
We will continue to supply more than 100%.

Further, in the heat treatment apparatus for hot extruded steel pipe according to the present invention, the transfer chain 2 having the push plate 1 configured so that the steel pipe 22 can be transferred on the skid 21 in the direction orthogonal to the pipe axis while rotating. And a cooling water chamber 3 located on one side of the transport path of the steel pipe 22 for receiving cooling water from the cooling water pipe 23, and a parallel flow provided on the side of the cooling water chamber 3 facing the one end of the steel pipe 22. It is composed of a plurality of cylindrical nozzles 4 having a constant length for forming.

[0011]

In the heat treatment method for the hot extruded steel pipe according to the present invention, the pressure of the cooling water is set to 4 kg / cm ² or more.
This is because, as a result of repeating the experiment, it was found that the complete water toughness effect cannot be expected unless the weight is 4 kg / cm ² or more in the case of a large single heavy material.

Further, the cooling water flowing parallel to the pipe axis while rotating the steel pipe is cooled so that the cooling water occupies 60% or more of the inner surface sectional area of the steel pipe. This is because it was found that a perfect water toughness effect could not be expected, as in the case of cooling water pressure, due to the bending.

The heat treatment apparatus for hot extruded steel pipes according to the present invention is for realizing the above-mentioned heat treatment method for hot extruded steel pipes, and an effective water toughness effect can be expected by inexpensive equipment investment.

[0014]

EXAMPLES The temperature history in the heat treatment method for hot extruded steel pipes according to the present invention cannot be measured with a thermocouple, so the temperature history was estimated by the following method. That is, 0.05%
25Cr-20Ni steel pipe of C (SUS 310 ST
P, outer diameter 96.0 mm, wall thickness 6.0 mm)) by the method shown in FIG. 3 and the method according to the present invention, and a sample for microstructure observation is taken from the steel tube after the heat treatment, 10% oxalic acid etch test method (JIS G
The etched structure was observed based on 0571). The results are shown in Table 1. From these results, it was estimated that the method of the present invention has a higher cooling rate in the temperature range in order to secure the corrosion resistance (intergranular corrosion resistance) than the conventional method.

[0015]

[Table 1]

In the graph of FIG. 1, the filling rate of the cooling water in the pipe is 30
The comparison of the amount of bending per unit length of the steel pipe between the case of 70% and the case of 70% is shown. Although not shown, the average amount of bending per unit length when the filling rate in the pipe is 60% is 12 mm / m, and when 50%, it is 19 mm.
/ M, the amount of bending per unit length of the steel pipe is significantly reduced in the present invention as compared with the case where it is not,
It can be seen that the effect of uniform cooling can be expected.

Next, the heat treatment apparatus for hot extruded steel pipe according to the present invention will be described with reference to FIG. 2A is a side view of the heat treatment apparatus for hot extruded steel pipes according to the present invention, and FIG. 2B is a plan view of the heat treatment apparatus for hot extruded steel pipes according to the present invention. The heat treatment apparatus for hot extruded steel pipe according to the present invention is a skid 21.
A transfer chain 2 having a push plate 1 configured so that the steel pipe 22 can be transferred in a direction orthogonal to the pipe axis while the steel pipe 22 is being rotated.
And the cooling water pipe 23 which is located on one side of the transport path of the steel pipe 22.
A cooling water chamber 3 for receiving the cooling water from the cooling water chamber 3 and a plurality of cylindrical nozzles 4 provided on the side of the cooling water chamber 3 facing the one end of the steel pipe 22 and having a constant length for forming a parallel flow. It is composed. Reference numeral 5 in FIG. 2 is a cover for preventing scattering of cooling water.

Since the heat treatment apparatus for hot extruded steel pipes according to the present invention is configured as described above, the heat treatment method for hot extruded steel pipes according to the present invention described above can be realized with inexpensive equipment investment. it can.

[0019]

According to the present invention, an austenitic stainless steel pipe immediately after hot extrusion is continuously water-cooled to near room temperature to have an austenite structure in a solution treatment state,
In addition, it is possible to obtain steel pipes that do not bend with inexpensive equipment investment.

[Brief description of drawings]

FIG. 1 shows a case where the filling rate of cooling water in a pipe is 30%, and
It is a graph which shows the amount of bending per unit length of a steel pipe when it sets it as 0%.

FIG. 2A is a side view of a heat treatment apparatus for hot extruded steel pipe according to the present invention, and FIG. 2B is a plan view of a heat treatment apparatus for hot extruded steel pipe according to the present invention.

FIG. 3 is an explanatory view showing a conventional heat treatment method for a hot extruded steel pipe.

FIG. 4 is an explanatory view showing another conventional heat treatment method for a hot extruded steel pipe.

FIG. 5 is a graph showing a temperature history when performing water toughness on a hot extruded steel pipe.

[Explanation of symbols]

 1 Push plate 2 Transfer chain 3 Cooling water chamber 4 Cylindrical nozzle 5 Cover

Continuation of front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical display location C21D 6/00 102 A 9269-4K 8/10 D 7412-4K C22C 38/00 302 Z 38/40 // C21D 8/00 E 7412-4K

Claims (2)

[Claims] 1. A heat treatment method for a hot-extruded steel pipe, wherein an austenitic stainless steel pipe immediately after hot-extrusion is continuously water-cooled to near room temperature to obtain an austenite structure in a solution treatment state. 4k from the pipe end
Cooling water configured to flow at a pressure of g / cm ² or more and parallel to the pipe axis should be at least 60% of the inner cross-sectional area of the steel pipe.
A heat treatment method for a hot-extruded steel pipe without bending, which is characterized by continuously supplying the above amount. 2. A transfer chain (2) having a push plate (1) configured so that the steel pipe (22) can be transferred on the skid (21) in a direction orthogonal to the pipe axis while rotating, and the steel pipe (22). ) Cooling water piping (23) located on one side of the transport path
A cooling water chamber (3) for receiving cooling water from the cooling water chamber, and a plurality of cooling water chambers (3) having a certain length for forming a parallel flow provided on the side facing the one end of the steel pipe (22). A heat treatment apparatus for hot extruded steel pipe, characterized in that it is composed of a tubular nozzle (4).