JPS60221525A - Manufacture of ductile cast iron pipe with high strength - Google Patents

Abstract

PURPOSE:To manufacture simply a ductile cast iron pipe with high strength by pouring necessary boiling water on the inside of a centrifugally cast ductile cast iron pipe. CONSTITUTION:Molten ductile cast iron contg. 2.2-3.5wt% Si is centrifugally cast. When the temp. of the resulting cast pipe is in the austenite range, the cast pipe is rapidly cooled to 500-550 deg.C without forming pearlite by pouring necessary boiling water on the inside of the pipe while rotating the pipe. The pipe is then air-cooled so as to form a bentonite structure. During the air-cooling, the pipe is rotated or kept stationary.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a method of manufacturing a ductile cast iron pipe using centrifugal force casting, and more specifically, a method of manufacturing a ductile cast iron pipe using centrifugal force casting. This invention relates to a method for manufacturing high-strength ductile cast iron pipes, in which the structure of the cast pipe is made into a bainite structure without using a kudzu austempering furnace, which is normally used to obtain a bainite structure, and without holding the pipe at a constant temperature.

<Prior art and its problems> Normally, in order to make the matrix structure of a ductile cast iron pipe into a bainite structure with excellent toughness, the material must be maintained at a constant temperature of 300 to 400°C from the austenitizing temperature (usually 900 to 950°C). Methods such as rapid cooling and holding in a salt bath furnace are used, but the process is complicated and is less economical.

Therefore, Japanese Patent Application Laid-open No. 161748/1983 proposed a method of utilizing the heat during casting during heat treatment.

This is approximately 90% after centrifugal casting of ductile iron pipes.
Spray a mist of water onto the inner surface of the cast tube from 0°C to rapidly cool it to 350-400°C, then cool it to 350-400°C.
The austempering treatment is carried out by placing the material in a heat-retaining furnace at a constant temperature of ℃.

However, in this method, the temperature of about 900°C is
Rapid cooling of cast pipes to avoid pearlite formation is
It is extremely slow and has a practical problem.

Furthermore, even if the temperature is rapidly cooled to 350 to 400°C, it is difficult to measure the temperature, and furthermore, an insulating furnace must be used to maintain the temperature at a constant temperature of 350 to 400°C, which is disadvantageous from an economic standpoint. be.

<Means for Solving the Problems> The present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to easily and reliably perform the rapid cooling operation from the austenite region, and to change the structure to bainite without requiring a heat insulating furnace. is possible,
The objective is to provide an economically superior method for manufacturing high-strength ductile cast iron pipes, and the technical means thereof is to reduce the silicon content to 2.
When ductile cast iron/8 hot water having a concentration of 2 to 3.5 wt% is centrifugally cast and the temperature of the resulting cast tube is in the austenite range, the formation of pearlite is avoided under the rotation of the cast tube, and the A required amount of boiling water is injected into the inner surface of the casting tube for rapid cooling to .degree. C., followed by air cooling to obtain a haynite structure while the casting tube is rotating or stationary.

Functions and Effects> The ductile cast iron used in the present invention has a St content of 2
．． Since it is specified as 2 to 3.5 wt%, C, C, T.

In the figure, the pearlite formation region can be shifted to the high temperature/long time side, and the formation of pearlite can be reliably prevented when the cast pipe obtained by centrifugal force casting of the molten metal is rapidly cooled from the austenite region. . Also,
During rapid cooling from the austenite region, boiling water is injected into the inner surface of the cast tube as it rotates, so the high-temperature tube does not deform, and the cooling boiling water acts uniformly on the inner surface of the tube, making it more effective. Cooling can be performed in a timely manner. In other words, the boiling water injected into the inner surface of the tube spreads uniformly over the inner peripheral surface of the tube, and when it comes into contact with the tube, it instantly removes the heat of vaporization from the inner surface of the tube, vaporizes, and becomes water vapor, which is ejected toward the inside of the tube and is also affected by the action of centrifugal force. New boiling water is continuously supplied and vaporized instantly,
The tube is rapidly cooled efficiently. Also, boiling water is always at 100°
Since the temperature is maintained at C, the quenching effect remains constant and does not decrease. Furthermore, the tube temperature was lowered from the austenite range by 5
The amount of boiling water for rapid cooling from 00 to 550'C is
It can be easily calculated using the inner surface area and wall thickness of the pipe.
In the present invention, since a predetermined amount of the boiling water is injected into the inner surface of the tube, the tube automatically reaches the predetermined temperature when the boiling water evaporates and disappears, and after that, it is inevitably air-cooled and kept warm. Retained austenite turns into bainite without requiring a furnace, making it possible to strengthen and thin the cast pipe. As described above, the method of the present invention can easily obtain a high-strength ductile cast iron pipe by simply injecting the required amount of boiling water into the inner surface of the centrifugally cast ductile cast iron pipe, and the operation is simple. Furthermore, it is economically superior and has great value as a means of industrial production.

<Example> Next, an example of the present invention will be described in detail with reference to the drawings.

1 and 2 schematically represent a centrifugal casting apparatus used to carry out the present invention. As shown in the figure, a centrifugal force casting mold 1 with bands 2 for stopping hot water attached to both ends and a wet coating 3 applied to its inner surface as usual is rotated at a predetermined speed (Go, 40 to 60 degrees). ), molten ductile cast iron is poured into the inner surface of the mold 1 from the pouring trough 4. Incidentally, one or more inoculations can be optionally applied during the casting.

Here, a typical composition of the ductile cast iron molten metal used in the present invention is shown below. The unit is weight %.

C: 3.2-3.8% S: 0.010% or less Si
:2.2~3.5% Mg: 0.02~0.05%
Mn: 0.6% or less P: o, os% or less The remainder is essentially Fe Among the above components, the Si component range is characteristic, and the reason for this limitation is 2.2
If it is less than 3.5%, cementite may remain and there is little effect of shifting the pearlite generation region to the high temperature and long time side in the C, C, T diagrams.On the other hand, if it exceeds 3.5%, embrittlement due to Si may occur. appears markedly, the tensile strength and elongation decrease, and the impact value also decreases.

After casting the molten metal, when the mold 1 is kept rotating and the casting tube 5 is solidified and red-hot, that is, when the temperature of the casting tube is in the austenite range (usually 900 to 950°C), the inside of the casting tube is heated. Depending on the surface area and wall thickness, the cast tube
Pre-existing boiling water is injected into the inner surface of the tube from the water cooling trough 6 for rapid cooling to 0 to 550°C. At this time, air may be supplied to the inside of the casting tube 5 from an air supply device 7 installed at one end of the mold 1. Note that the method of injecting boiling water is not limited to the above-mentioned pouring, but may also be performed by showering.

The amount of boiling water injected into the casting tube 5 is calculated from the cooling temperature and the inner surface area and wall thickness of the casting tube 5. When the boiling water is vaporized and disappears, the temperature of the casting tube 5 is automatically adjusted to the desired temperature of 500 to 500. 5
Set the temperature to 50℃. After boiling water is injected and vaporized, it is air cooled, but at this time, mold 1
may remain rotating or may be stopped.

If boiling water is used to cool the cast tube 5, it will instantly absorb vaporization heat from the inner surface of the tube and vaporize when it comes into contact with a high-temperature tube, so the tube will be effectively rapidly cooled and the temperature will be lower than that of water in the form of mist at room temperature. The quenching effect is significant, and the formation of pearlite, which causes a decrease in the strength of the cast tube 5, can be suppressed as much as possible, which is preferable.

The cooling situation during this time will be explained using diagrams C, C, and T shown in FIG. 3 as follows. In the same figure, the cooling curve shown by the solid line is the case when the method of the present invention is performed, and A
The point is the austenite region temperature (900℃), A-B
indicates the part quenched with boiling water, B indicates the point at which the boiling water evaporates, and B-C indicates the air-cooled part. In the figure, the region A represents pearlite formation, and the opening represents a bainite formation region. A
Since it is rapidly cooled from point B to point B using boiling water, it passes through the pearlite formation region (it can reach point B without passing through it, and since it is slowly cooled by air cooling from point B, it passes slowly through the bainite formation region during cooling. Bainite can be suitably generated from retained austenite.Here, the reason why point B is set to 500 to 550°C is that point B is 500 to 550°C.
If the temperature exceeds 50°C, there is a risk of passing through the pearlite formation region during air cooling, while if it is below 500°C, there is a possibility of forming lower bainite, and this structure has tensile strength but no tensile or elongation. This is because it is not desirable. still,
As described above, in order to shift the pearlite generation region to the high temperature and long time side, St: 2.2 to 3.5% was specified. Moreover, in the figure, the cooling curve when cooling with mist water is shown by a broken line. In this case, passage through the pearlite generation zone is unavoidable, making it difficult to strengthen the structure.

Next, more specific examples will be described.

Example of manufacturing a high-strength ductile cast iron pipe with an outer diameter of 500 x 2000β x 10t (mm) (1) Manufacturing method■ A centrifugal casting mold with a wet coating on the inner surface was rotated at 40 to 60 degrees with a GNo. Molten ductile cast iron was poured onto the inner surface at a casting temperature of 1310°C.

(wt%) C: 3.4% P: 0.041% Si: 2.6% S: 0.008%Mn:
0.33% Mg: 0.039% balance essentially Fe ■ After casting, the temperature of the casting tube was kept at 9% while the mold was being rotated.
At 00°C, 31.1ffi of boiling water was uniformly sprayed on the inner surface of the tube using a shower. About 30 seconds after spraying, all the boiling water was vaporized, and the tube temperature was about 500°C.

■ After air-cooling the mold to room temperature while rotating, the rotation was stopped and the structure of the tube was examined, and it was found to be a haynite structure containing some austenite.

[Brief explanation of drawings]

1 and 2 are cross-sectional views schematically showing a centrifugal casting apparatus for carrying out the present invention, and FIG. 1 shows a method in which molten ductile cast iron is poured onto the inner surface of a centrifugal casting mold. FIG. 2 is a sectional view showing a state in which boiling water is injected into the inner surface of the casting tube after centrifugal force casting, and FIG. ,T, is a diagram. 1... Mold, 2... Hand, 3... Wet coating, 4... Trough for pouring, 5... Casting pipe, 6... Trough for water cooling. Patent applicant Kubota Iron Works Co., Ltd.

Claims (1)

[Claims] 1. When a molten ductile cast iron with a silicon content of 2.2 to 3.5 wt% is centrifugally cast and the temperature of the resulting cast tube is in the austenite range, pearlite is not formed under the rotation of the cast tube. After cooling by injecting the required amount of boiling water into the inner surface of the casting tube to avoid and rapidly cool it to 500-550℃,
A method for manufacturing a high-strength ductile cast iron pipe, characterized in that the cast pipe is air-cooled to obtain a bainitic structure while the pipe is rotating or stationary.