JPS5948932B2 - Drawing method for high Ni-Cr centrifugally cast steel pipes - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention involves drawing a centrifugally cast raw tube of high Ni-Cr alloy steel to refine its crystal grains and improve its mechanical properties. The purpose is to eliminate troubles such as damage to the raw pipe or drawing jig that often occur during processing, and to easily obtain a healthy recrystallized pipe.

For example, commonly known as Alloy 800 centrifugal casting tube (0.05% C-
20%Cr-32%Ni-Ti, AI, balance Fe) has coarse grains, so it cannot satisfy the mechanical properties as a heat-resistant and corrosion-resistant steel pipe, and has problems such as poor weldability. This is significantly inferior to forged and forged steel pipes, and for this reason, this type of centrifugally cast pipe has traditionally been further drawn, recrystallized its coarse crystals into fine pieces, and improved its room-temperature strength in the same way as forged or forged steel pipes. Although the aim is to increase the strength by increasing the strength of the steel and stabilizing the creep rupture strength, the conventional drawing process has the following problems.

That is, in the prior art, there are two types of cold drawing for centrifugally cast raw pipes: a type using dies and plugs as shown in FIG. 1, and a type using only dies as shown in FIG.

In FIG. 1, 1 is a raw tube, 2 is a die, 3 is a plug, and 4 is a drawing member.

When this type is used, the outer diameter of the tube can be reduced using the die 2, and the inner surface of the tube can be sufficiently forged using the plug 3, but since the tightening caused by the contraction of the inner surface of the tube is strong, there is a large amount of pressure on the plug 3 side. When the load is applied, the plug may break at the A part of the plug 3 as shown in the figure, or the B part of the raw pipe 1 may break.
Problems such as cracks occurring on the inner surface of the tube or damage to the tube itself may occur.

In addition, in the case of using only the die 2 shown in Fig. 2 and not using the plug 3, the load applied to the pipe 1 is only on the die 2 side, so it is easier to pull it out compared to the means shown in Fig. 1. With this method, in the tube 1 after drawing, the outer diameter of the tube becomes smaller but the tube wall thickness increases, and it is insufficient to give a forging effect to the inner surface of the tube. However, it is difficult to sufficiently recrystallize the crystals, and therefore, it is difficult to obtain the desired crystal grain refinement.

In terms of forging effect, the model shown in Figure 1 is more effective, but in the end, the damage to the plug 3 and the tube 1 is caused only by the drawing process, which is insufficient for effective recrystallization. This problem arises because you try to give all the ratios all at once.

In particular, in the case of Alloy 800 centrifugally cast tubes mentioned above, cracks are likely to occur when the raw tube is subjected to such drawing processing, and this is because the crystal grains are coarse, so impurities are concentrated at the grain boundaries. As a result, the grain boundaries become brittle and cannot withstand plastic working by drawing out, resulting in cracks.

The present invention solves the problems described above when drawing centrifugally cast raw pipes of high Ni-Cr alloy steel, and makes it possible to easily obtain soundly recrystallized pipes. , and eliminates the problem of breakage caused by drawing.The feature is that when drawing a centrifugally cast raw pipe made of high Ni-Cr alloy steel, the cross-sectional area of the pipe is first determined by extrusion processing. After reducing the reduction rate to about 5 to 10%, the tube is annealed at about 500 to 900°C to relieve stress, and then subjected to solution treatment at about 1000 to 1200°C to refine the crystal grains. After preventing cracking due to plastic deformation, the material is formed by cold drawing.

The present invention will be described in detail below with reference to the illustrated embodiment. As illustrated in FIG. Extrusion processing is performed through the die 2, and in the present invention, in the extrusion processing, first, the tube cross-sectional area reduction rate is 5 to 1.
Plastic deformation of approximately 0% is performed.

This is because extrusion is more effective than drawing in preventing cracks caused by plastic working of the cast product.

In addition, the reason why the pipe cross-section reduction rate is set at about 5 to 10% is because
If it is less than about 5%, the processing rate, which is a prerequisite for recrystallization by solution treatment to be described later, is too low, and grain refinement is difficult.

On the other hand, if the processing rate exceeds about 10%, equipment such as dies and press molds for processing must be enlarged, which is disadvantageous in terms of cost.

When applying plastic deformation of about 5 to 10% as primary plastic deformation through extrusion processing, the deformation rate is small, so there is no need to upgrade or enlarge dies or molds as extrusion equipment, and the equipment is reduced. It is also advantageous in terms of operation.

Raw pipe 1 subjected to plastic deformation within a certain limit in this way
is subjected to an annealing treatment for stress relief, and the appropriate annealing temperature is about 500 to 900°C.

This is because it has been confirmed that at temperatures below about 500°C, stress relief is insufficient and there is a high risk of cracking in the raw tube during subsequent solution treatment, whereas at temperatures above about 900°C, recrystallization occurs. This is because crystal grains will start to become grainy, and it will be difficult to refine the crystal grains even with the subsequent solution treatment, and this will be economically disadvantageous.

The stress-removed raw tube is further heated to 1000 to 1200℃.
By performing a solution treatment that heats the material to a certain degree and recrystallizing it, the crystal grains are refined to some extent, and the cracking resistance against subsequent plastic deformation is greatly improved.

Here, the reason for limiting the temperature to about 1000 to 1200°C is as follows.
If it is below about 1000℃, recrystallization will be insufficient and coarse crystal grains from casting will remain, and there is a risk that cracks will occur during the drawing process after solution treatment.On the other hand, if it exceeds about 1200℃, recrystallization will occur. This is because crystal grains become coarser and cracks are more likely to occur during drawing.

The raw tube 1 that has been subjected to limited extrusion processing and heat treatment as described above is subjected to drawing processing for the first time, and during this drawing processing, fine recrystallization has already progressed through the above-mentioned treatments. Therefore, the cracking resistance due to processing has been greatly improved, so it is safe to perform drawing processing using only die 2 without a plug as shown in Fig. 2, and the area reduction processing rate can be greatly increased. However, no cracks occur in the raw tube 1, and a tube body with a dense and strong structure that has been sufficiently refined and recrystallized can be obtained.

In addition, even in the drawing process shown in Fig. 2, the processing rate can be greatly increased (processing rate 20 to 30%), so there is no forging effect on the inner side due to not using the plug 3. A healthy drawn pipe with sufficient coverage, improved strength at room temperature, and stable creep rupture strength can be obtained.

Of course, it is also possible to perform the drawing process using the plug 3 shown in FIG. 1 in combination.

According to the present invention, it is thought that two pieces of equipment are required for extrusion processing and drawing processing, but this can be done by adding, for example, a hydraulic press for extrusion to the drawing device shown in Fig. 2, so both equipment can be used. It is not necessary to provide them separately, and there is no disadvantage in terms of cost.

Next, specific examples and comparative examples will be listed and explained.

(2) A molten high Ni-Cr alloy steel having the following chemical composition (wt%) was centrifugally cast to obtain a specimen of a centrifugally cast blank pipe.

Molten metal chemical composition C: 0.07% Ni: 33.21% Si: 0
．． 43% Cr: 21.60% Mn: 0.
62% Cu: 0.04%P: 0,010
%AA: 0.41%S: 0.00
6% Ti: 0.39% balance substantially F
e. The centrifugally cast raw pipe was drawn under the conditions shown in Table 1.

Note that Table 1 also lists the processing conditions.

■ From Table 1, sample material 3.4, which corresponds to the example of the present invention, showed good results with no cracks occurring on the surface after solution treatment and cold drawing processing (processing rate of 20% and heavy processing). was gotten.

On the other hand, in sample material 1 whose reduction rate during extrusion processing was lower than the specified value, the degree of processing was low and recrystallization was insufficient even after the subsequent solution treatment, and cracks occurred during the final cold drawing.

In sample material 2, since the annealing temperature was as low as 400° C., stress relief was insufficient and cracks occurred after the solution treatment.

Sample material 6.7 is unsuitable for recrystallization because the solution treatment temperature is not controlled to a predetermined value (sample material 6 has coarse crystal grains remaining from casting, sample material 7 has recrystallization) In both cases, cracks occurred during cold drawing due to coarsening of crystal grains due to excessive carbonization.

In sample material 5.8, no cracks occurred even after cold drawing, but in sample material 5, the annealing temperature was quite high at 1100°C, and crystal grains coarsened due to the progress of recrystallization. This is thought to have occurred, and subsequent solution treatment may reduce the recrystallization effect, and in severe cases, cracks may occur during cold drawing.

On the other hand, sample material 8 had a high cross-sectional reduction rate of 15% during extrusion processing, which placed a large load on the extrusion equipment, and required careful management of extrusion speed to prevent deformation of the material itself.

As described above, according to the present invention, by combining extrusion processing, drawing processing, and heat treatment means, a healthy forged pipe without cracks can be easily obtained.

In particular, in the present invention, after imparting a reasonable plastic change to the raw pipe 1 through extrusion processing, it is subjected to stress relief annealing treatment and solution heat treatment to effectively advance its fine recrystallization. It is excellent in that a large area reduction rate can be easily and effortlessly applied in the drawing process, and the desired final product can be smoothly obtained without causing any damage to the raw tube. Targeting centrifugally cast steel pipes made of as-cast Ni-Cr stainless steel and other high-Ni-Cr alloy steels with coarse grains, this product is extremely effective in drawing the pipes without causing damage to the jig or the pipe side. It has great utility value.

[Brief explanation of drawings]

FIGS. 1 and 2 are explanatory views of conventional drawing means, and FIG. 3 is an explanatory view of an embodiment of extrusion means according to the method of the present invention. 1...Main pipe, 2...Dice, 3...
...Plug, 4...Gripping member, 5...
...Mold, 6...Press.

Claims (1)

[Claims] 1. When drawing a centrifugally cast raw pipe made of high Ni-Cr alloy steel, first the raw pipe is extruded to reduce the cross-sectional area reduction rate of the pipe to about 5-10%, and then the alloplastic pipe is
It is annealed at about 00℃ for stress relief, then 100℃
A high Ni-
A drawing method for Cr centrifugally cast steel pipes.