JP2826260B2 - Manufacturing method of wear-resistant cast steel curved pipe - Google Patents

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 bent pipe by high-frequency bending using a wear-resistant cast steel straight pipe cast by centrifugal force as a raw pipe.

[0002]

2. Description of the Related Art The manufacture of elbows, U-shaped pipes, and other curved pipes by subjecting a straight pipe centrifugally cast to high-frequency bending is performed by bending a steel pipe such as ordinary carbon steel having good workability. It is widely practiced as a law.

[0003] However, when high frequency bending is performed to make a centrifugally cast straight pipe of wear-resistant cast steel into a bent pipe, the backside of the bent pipe is formed.
Cracks occur on the (tension side), and a healthy curved tube cannot be obtained. This is because the wear-resistant cast steel pipe has a metal structure with martensite precipitated, is hard and brittle, and the heating and cooling of the tube in high-frequency bending is rapid heating and cooling, and the bending of the tube is Water cooling or the like for preventing the cross-sectional diameter of the processed portion from becoming elliptical also acts as a factor that induces cracking.

[0004] Therefore, the applicant has previously machined the inner and outer surfaces of the wear-resistant cast steel straight pipe cast by centrifugal force to form a raw tube, and under high-frequency heating, a pipe temperature of 800 to 900 ° C and a processing speed of 0.5 to 0.5. 1m
It has been proposed to produce a wear-resistant cast steel curved pipe without causing defects such as cracks by performing bending under the conditions of m / sec, radius of curvature (R): 2D or more, and cooling: cooling. (Japanese Patent Application No. 62-94086).

[0005]

However, the above-mentioned method for producing a wear-resistant cast steel bent pipe has a problem in that the wear resistance of the inner surface of the pipe, which is a characteristic of the centrifugally cast pipe, is deteriorated. That is, the centrifugally cast tube used as the raw tube has a property that the carbon amount increases due to the centrifugal force acting on the inner surface side at the time of manufacturing, and the hardness becomes higher than other parts, and generally the hardness is higher if the same component is used. It is said that the higher the abrasion resistance, the better.

However, since the inner surface having a high hardness is removed by machining, the abrasion resistance is reduced. In addition, if the cooling during high-frequency bending is allowed to cool, the deformation becomes large and becomes elliptical, and when the bent pipe is used as a part of a transport pipe for transporting dust, earth and sand, there is a problem that the transport efficiency is reduced. Was. The present invention has been made in view of the above, and has made it possible to manufacture a wear-resistant cast steel curved pipe without causing defects such as cracks by a high-frequency bending method using a centrifugally cast straight pipe as a raw pipe. Things.

[0007]

The method for producing a wear-resistant cast steel curved pipe according to the present invention comprises the steps of: centrifugally casting an as-cast, wear-resistant cast steel straight pipe;
c After normalizing at a temperature of ₃ or more than Acm, under high frequency heating, tube temperature: 800-900 ° C, processing speed: 0.5-1mm /
Second, radius of curvature (R): 2D or more, machining cooling: 0.3 ° C / sec ~
Bending is performed under the condition of forced air cooling of 2.7 / sec, and after the bending, quenching and tempering are performed.

[0008]

According to the method of the present invention, a bent pipe having an arbitrary bending angle, for example, an elbow or a U-shaped pipe can be manufactured. Hereinafter, the method of the present invention will be described in detail.

[0009]

EXAMPLE A straight pipe made of wear-resistant cast steel manufactured by centrifugal casting is used as a raw pipe. The typical structure of the tubing is C: 0.25-0.
30%, Si: 0.2-0.5%, Mn: 0.7-1.1%, Cr: 0.
6-0.8%, Ni: 0.4-0.6%, Mo: 0.2-0.4%,
P: 0.03 or less, S: 0.02 or less, and the balance substantially consists of Fe.

Prior to high frequency bending, the centrifugally cast straight pipe is subjected to a normalizing treatment as a pretreatment. The normalizing process is performed to normalize the structure of the steel and disperse the stress generated in the tube during the subsequent bending to prevent stress concentration and the occurrence of cracks due to the stress concentration. The normalizing treatment is achieved by heating to a temperature of 30 to 50 ° C. or higher from the Ac ₃ or Acm transformation point, and then cooling in a quiet atmosphere.

[0011] After the normalizing treatment, bending is performed under heating by a high-frequency heating coil. The processing temperature of the tube in the bending is important. FIG. 1 shows the relationship between the tensile properties and the temperature of the wear-resistant cast steel. The composition of the test material satisfies the representative structure of the tube material. As shown, 800-9
It shows the best ductility in the temperature range of 00 ° C. Therefore,
In the present invention, the bending is performed at 800 to 900 ° C.

Further, in the bending, the processing speed and the processing speed are important factors. Because
Even if other conditions are appropriate, if the processing speed is too high, cracks occur in the pipe, and if it is too low, the pipe diameter of the bent portion becomes elliptical. As a result of detailed experiments, the inventor has found that the optimum processing speed is 0.5 to 1 mm / sec. Therefore, according to the present invention, bending is performed at a processing speed of 0.5 to 1 mm / sec. The processing speed means an intermediate position between the tension side and the compression side, that is, on the pipe axis.

The present invention also provides a curvature radius of a bent portion of a tube.
(R) is twice or more the outer shape (D) of the tubular body, that is, R ≧ 2D
And This is because, when a strong bending process with a smaller radius of curvature (R) is performed, fine cracks are likely to occur on the back side (tensile side) of the tube. The bending angle of the bent portion is arbitrary, and
Character bending is also possible.

The bending is performed from one end to the other end of a predetermined region (a predetermined axial length region in the tube axis direction) to which bending is to be applied, for example, at a position (processing start point) in FIG. ) Is performed along the pipe axis toward the position (processing end point). In an area from the processing start point to the end point, each part is subjected to predetermined processing and is cooled sequentially. The cooling is called processing cooling in this specification.

The cooling rate in the process cooling process is the most characteristic part of the present invention. The present invention provides a processing temperature (800 to 90
(0 ° C.) and air cooling at a cooling rate of 0.3 ° C./sec to 2.7 ° C./sec. Air-cooled pipes (curved pipes) after the bending process avoid the precipitation of martensite, have a relatively ductile mixed structure of ferrite and pearlite, and suppress thermal distortion, etc., during the cooling process. Cracking is effectively avoided.

The pipe body (curved pipe) after working and cooling is then subjected to a quenching and tempering treatment for refining. The quenching treatment is preferably carried out by heating and holding at 930 ° C. ± 10 ° C. and then cooling with water.
This is suitably achieved by heating and holding at 0 ° C. ± 10 ° C., followed by air cooling. By this heat treatment, the bent tube has a homogeneous martensite structure, and has both high hardness and high wear resistance and good mechanical properties.

[0017]

[Table 1]

Table 1 shows the results of a high frequency bending test of a centrifugally cast abrasion resistant cast steel straight pipe. Trial No. 1 and N
No. 2 is an example of bending work that satisfies the conditions specified in the present invention, and No. 3 omits normalizing processing on the raw tube before bending, and performs cooling without air cooling instead of air cooling. 0 by cooling.
No. 4 is an example of cooling at a cooling rate of 2 ° C./second or less.
This is an example in which an as-cast straight pipe is annealed at a temperature equal to or lower than the transformation point, and the working cooling is forcibly air-cooled. In the case of No. 4, cracking occurred as a result even if the processing cooling was forced air cooling.

As shown in Table 1, No. 1 and No. 2 satisfying the conditions of the present invention achieve a predetermined bending without cracking. On the other hand, the normalizing process before bending was omitted, and the working cooling was performed by cooling.
In No. 3, the tube was greatly deformed and became elliptical. The as-cast tube that had been subjected to the annealing treatment had cracks.

FIG. 3 shows the measurement results of the outer diameter, ellipticity, perimeter, and wall thickness of the curved tube obtained in 1. As shown in FIG. 3, the measurement positions are the following seven positions.
As shown in FIG. 4, the TC of "outer diameter" is an outer diameter in a direction passing through the pipe axis and passing through the tension side (T) and the compression side (C), and NN 'is perpendicular to the TC direction passing through the pipe axis. The outer diameter in the direction. “Ellipticity” is (outer diameter NN′−outer diameter TC) / outer diameter (dimension before bending) × 100
(%). As shown in FIG. 5, the curved pipe manufactured by the method of the present invention has good dimensional accuracy in terms of pipe diameter, wall thickness, and ellipticity.

[0021]

[Table 2]

Table 2 shows the results of the mechanical test after the quenching and tempering of the curved tube. Measurement is at position A (back side)
And point B (ventral). From this measurement result, it can be seen that the strength, ductility, etc., are all high, the dispersion is small, and the homogeneity is excellent. FIG. 6 (tensile side), FIG. 7 (central part), and FIG. 8 (compressive side) show the hardness (H
v) The distribution is shown by sampling from the tension, bending center, and compression side at a bending angle of 45 ° shown in FIG. It can be seen that the hardness has been increased by the heat treatment.
The present invention is not limited to the configuration of the above embodiment,
Various modifications are possible within the scope described in the claims.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a tensile characteristic and a temperature of a wear-resistant cast steel.

FIG. 2 is an explanatory diagram of a compression side and a tension side of a curved pipe.

FIG. 3 is an explanatory view showing a test portion of a curved tube.

FIG. 4 is an end view of a curved tube.

FIG. 5 is a graph showing an outer diameter, an ellipticity, and the like of a curved tube.

FIG. 6 is a hardness distribution diagram on the tension side of a curved tube.

FIG. 7 is a hardness distribution diagram at a bending center of a curved tube.

FIG. 8 is a hardness distribution diagram on the compression side of a curved tube.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI F16L 9/02 F16L 9/02

Claims (1)

(57) [Claims] 1. An as-cast , abrasion-resistant cast steel pipe in an as-cast state, which has been centrifugally cast, is subjected to a normalizing treatment at a temperature of Ac ₃ or Acm or more, and then processed at a pipe temperature of 800 to 900 ° C. under high frequency heating. Speed: 0.
5-1 mm / sec, radius of curvature (R): 2D or more, machining cooling: 0.3
A method for producing a wear-resistant cast steel curved pipe, comprising performing bending under the condition of forced air cooling at a rate of ° C / sec to 2.7 / sec, performing quenching and tempering after bending.