JP2000061577A - Production of two-phase stainless steel forged pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a pipe having large diameter and thickness by casting a hollow ingot with a two-phase stainless steel containing specific contents of Cr, Mo and N, hollow-forging in the hot-state, executing solid-solution heat treatment and machining. SOLUTION: This material is the two-phase stainless steel having >=40% of index calculated with the equation of Cr%+3.3×Mo%+16×N%. The two- phase stainless steel is melted with an Heroult type arc furnace and cast into a mold having a core with a bottom pouring method and formed as the hollow steel ingot by cutting off a runner and a head part. The hollow-state ingot is heated and hollow-forged by inserting a core metal into the hole of the hollow steel ingot and formed as a half-product of a forged pipe. The half-product is formed as the forged pipe by bore-expanding hollow-forging and formed as the forged pipe. The solid-solution heat treatment is executed to the forged pipe and thereafter, the forged pipe is machined to remove the surface. In the forged pipe, σ phase is precipitated at the time of hollow-forging and crack is developed with the heat loss through the core metal, but since the crack is shallow, this crack is easily removed by machining.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for manufacturing a duplex stainless steel forged pipe.

[0002]

2. Description of the Related Art Duplex stainless steel has high corrosion resistance and strength, so that it is highly useful in equipment for seawater such as seawater desalination equipment, chemical plants such as heat exchangers for petroleum refining, and rotating bodies of various centrifuges. Demand is increasing for applications such as strength structures. Conventionally, this duplex stainless steel forged pipe is manufactured by forging a duplex stainless steel ingot, making a hole with a punch or a boring machine, and then hollow forging this.

However, SUS329J1 (C: 0.08
% Or less, Si: 1.00% or less, Mn: 1.50% or less, P: 0.040% or less, S: 0.030% or less, N
i: 3.00 to 6.00%, Cr: 23.00 to 28.
00%, Mo: 1.00 to 3.00%, residual Fe), which is an index indicating the degree of pitting corrosion resistance, that is, Cr + 3.3 ×.
Low-grade duplex stainless steel with Mo% + 16 × N% of about 31.4% is used when manufacturing a forged pipe.
When the wall thickness of semi-finished products is 600 mm or more (diameter is 600 mm or more for round bars), when cooling by drilling holes etc., air cooling (blast cooling) slows down the cooling of the central part, so the σ phase Since precipitation cracking occurs in water and thermal stress cracking occurs when water cooling is performed so that the σ phase does not precipitate, it has not been possible to manufacture a forged pipe having a wall thickness of 600 mm or more during cooling such as during manufacturing. .

Further, SUS329J4L ((ASTMA
182F55, C: 0.03% or less, Si: 1.
00% or less, Mn: 1.50% or less, P: 0.040%
Hereinafter, S: 0.030% or less, Ni: 5.50 to 7.5
0%, Cr: 24.0 to 26.0%, Mo: 2.5
0 to 3.50%, N: 0.08 to 0.30%, residual Fe)
When manufacturing forged pipes, the high-grade duplex stainless steel with the above-mentioned index of 40 or more such as, for example, has a wall thickness of a steel ingot, a semi-finished product or the like of 300 mm or more (a diameter of 300 for a round bar).
If the thickness is 3 mm or more), the above cracks occur when cooling is performed to open a hole, etc., so it was not possible to manufacture a forged pipe having a wall thickness of 300 mm or more during cooling such as during manufacturing. Therefore, duplex stainless steel, especially high-grade duplex stainless steel with the index of 40 or more, could not manufacture a forged pipe having a large diameter, a large wall thickness, or a large diameter and a large wall thickness.

[0005]

DISCLOSURE OF THE INVENTION The present invention relates to a duplex stainless steel having a diameter, a wall thickness or a diameter and a wall thickness larger than those of conventional ones, in particular, a duplex stainless steel having an index of 40 or more, without cracking. It is an object to provide a method for manufacturing a forged pipe.

[0006]

In order to solve the above-mentioned problems, in the method for producing a duplex stainless steel forged pipe produced by hollow forging a steel ingot according to the present invention, a duplex stainless steel as a steel ingot, particularly the above Using a hollow steel ingot (hollow ingot) of duplex stainless steel with an index of 40 or more, a usual method, that is, hollow forging in hot, and then solution heat treatment,
Machining as needed to produce duplex stainless steel forged pipes.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the present invention will be described in detail.
The duplex stainless steel suitable for manufacturing by the method for manufacturing a duplex stainless steel forged pipe of the present invention is the above-mentioned SUS.
In addition to 329J1 and SUS329J4L, SUS32
9J3L, those obtained by adding necessary alloying elements to these, and those known as duplex stainless steels other than the above.

Further, the hollow steel ingot of the duplex stainless steel used in the method for producing a forged duplex stainless steel pipe of the present invention can be produced by a known method for producing a hollow steel ingot, and the present applicant Japanese Patent Application No. 9-16969
No. 2 discloses a method for manufacturing a hollow steel ingot, that is, a cylindrical non-fired refractory made of a non-fired refractory in a mold installed on a surface plate for lower pouring with a space outside a pipe-shaped core metal. It can also be manufactured by a casting method in which a sleeve is installed, and a core in which a refractory granular material is placed between the core metal and the sleeve is installed in a mold in which the molten metal is poured by the down-pouring method. In the hollow forging in the method for producing a duplex stainless steel forged pipe of the present invention, the ingot temperature is heated to 1200 to 1220 ° C,
It can be carried out by a known hollow forging method in which a core metal is put in the hole of the hollow steel ingot to make the length, the outer shape and the diameter of the hole into predetermined dimensions.

[0009]

In the present invention, when manufacturing a forged pipe of duplex stainless steel, a hollow steel ingot having a cooling rate faster than that of a solid steel ingot of the same volume is used. However, since precipitation cracking of the σ phase does not occur during cooling,
A forged pipe can be manufactured by using a steel ingot (hollow steel ingot) having a volume larger than that of the solid steel ingot. Therefore, it is possible to manufacture a forged pipe of duplex stainless steel having a diameter, a wall thickness, or a diameter and a wall thickness which are larger than those of conventional ones. Further, since a hollow steel ingot is used, it is not necessary to make holes, and cracking does not occur during cooling for making holes.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an embodiment of the present invention will be described with reference to FIGS. C: 0.014%, Si: 0.41%, Mn: 0.66
%, P: 0.021%, S: 0.0006%, Cu:
0.69%, Ni: 6.34%, Cr: 25.88%,
SUS329J4L having a composition composition of Mo: 3.27%, N: 0.25%, and residual Fe (the above-mentioned index: 40.7).
(Corresponding to ASTM A182F55) is melted in an Eru arc furnace, and this molten steel is pipe-shaped in the same manufacturing method as the method for manufacturing the hollow steel ingot, that is, in a mold installed on a surface plate for under-pouring. A cylindrical sleeve made of non-fired refractory was installed outside the heat-resistant steel cored bar, and a core containing CaO particles was installed between the cored bar and the sleeve. It was cast in a mold by the down pouring method, and then the runner and the head were cut off to manufacture a hollow steel ingot having the shape and dimensions shown in FIG.

This hollow steel ingot was placed in a heating furnace at 1220 ° C.
After heating, the hollow steel ingot is filled with a core metal having a diameter of 290 mm, and hollow forging is performed to manufacture a semi-finished product of a forged pipe having a shape and size as shown in FIG. A forged pipe having a shape and dimensions as shown in FIG. 3 was manufactured by forging. The forging ratio of this forged pipe was 1.5S. In this forged pipe, cracks occurred in the surface layer and the inner diameter surface due to precipitation of σ phase during hollow forging and heat loss due to the core metal, but the cracks were shallow enough to be removed by machining. .

This forged pipe was subjected to solution heat treatment for cooling with an air blast and then machined to have an outer diameter of 845 m shown in FIG.
m, inner diameter 500 mm, length 1900 mm forged pipe product.

When the forged pipe was visually inspected,
The surface layer, both end surfaces and the inner diameter surface were not cracked at all. Furthermore, when the test pieces were cut out from the above forged pipe product from each part and direction and the tensile properties and hardness were measured, the tensile properties and hardness were A at all parts and directions.
It satisfied the standard of STMA182F55. In other words, the 0.2% proof stress in 584~629N / mm ^2, and the standard of 550 N / mm ² or more, more tensile strength 780
~ 798 N / mm ² , standard 750 N / mm ² or higher, elongation 32-39%, standard 25% or higher, hardness HB241-255, standard HB3
It was less than 02. Also, an ultrasonic flaw inspection was conducted, but there was no defect inside.

Comparative Example C: 0.017%, Si: 0.44%, Mn: 0.65
%, P: 0.020%, S: 0.0005%, Cu:
0.69%, Ni: 6.40%, Cr: 25.79%,
SUS329J4L having a composition of Mo: 3.27%, N: 0.25%, and residual Fe (the above-mentioned index: 40.6).
(Corresponding to ASTM A182F55) was melted in an Eru arc furnace, cast into a mold by the down pouring method, and then the runner and the head were cut off to produce a solid steel ingot with a diameter of 800 mm and a length of 2000 mm. Put the steel ingot in a heating furnace and 1200 ℃
To 450mm by hot forging and length 2000
When a forged cylindrical body of mm was manufactured and cooled by a boring machine to make a hole, the periphery was cracked down and a crack was generated inside, and it became impossible to perform the subsequent processing.

Although the above examples relate to high-grade duplex stainless steels, forged pipes can be manufactured similarly for low-grade duplex stainless steels. Further, the above-mentioned example relates to the production of a large forged pipe that cannot be produced by the method using the conventional solid steel ingot, but the small forging as can be made by the method using the conventional solid steel ingot. Pipes can be manufactured as well.

[0016]

EFFECTS OF THE INVENTION The method for producing a forged duplex stainless steel pipe of the present invention has the following excellent effects due to the above constitution. (1) It is possible to manufacture a forged pipe having a large diameter, a large wall thickness, or a large diameter and a wall thickness, which cannot be manufactured by the conventional method. In particular, high-grade duplex stainless steel can produce only small-diameter forged pipes, so that this effect is great.

(2) Since it is not necessary to make a hole, it is not necessary to cool the steel ingot in order to make a hole, and cracking does not occur at that time. (3) Since it is not necessary to make a hole with a punch or boring machine, the processing time can be shortened. (4) Since a hollow steel ingot is used, there is nothing to be scrapped by drilling, so that the raw material to be melted and the casting amount can be reduced.

[Brief description of drawings]

FIG. 1 is a front view (a) and a side view (b) showing the shape and dimensions of a hollow steel ingot manufactured according to an embodiment of the present invention.
Is.

2 is a front view (a) and a side view (b) showing the shape and dimensions of a semi-finished product of a forged pipe obtained by hollow forging the hollow steel ingot shown in FIG.

FIG. 3 is a front view (a) showing the shape and dimensions of a forged pipe manufactured by hollow forging by expanding the hole in the semi-finished product shown in FIG.
It is a side view (b).

4 is a front view (a) and a side view (b) showing the shape and dimensions of a forged pipe product obtained by finishing the forged pipe shown in FIG. 3 by machining.

Claims (2)

[Claims] 1. A method for producing a duplex stainless steel forged pipe produced by hollow forging a steel ingot, wherein a hollow steel ingot of duplex stainless steel is used as the steel ingot. Manufacturing method. 2. The duplex stainless steel is Cr + 3.3.
An index calculated by the formula xMo% + 16xN% is a duplex stainless steel having 40% or more.
A method for manufacturing the duplex stainless steel forged pipe described.