JPS60199563A - Ingot making method of high-chromium steel containing nitrogen - Google Patents

Abstract

PURPOSE:To prevent segregation of a giant carbide near the central hole of a steel ingot by melting a high-chromium steel contg. >=1 kinds of Nb and Ta and adding a reference value of N to the molten steel subjected to a vacuum degassing treatment then maintaining a nitrogen atmosphere in a casting mold, pouring the molten metal into the mold and casting the ingot. CONSTITUTION:A high-chromium steel contg. >=1 kinds of Nb and Ta is melted and a prescribed reference value of N is added to the molten steel subjected to a vacuum degassing treatment. A nitrogen atmosphere is then maintained in the mold and a molten steel is poured into the mold to cast an ingot. The ratio (H/D) between the height (H) and diameter (D) in the mold space is made 0.5-1. The chromium equiv. expressed by the equation for the high-chromium steel contg. N is made 5-7.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an ingot-forming method for high chromium steel containing at least one of niobium or tantalum and nitrogen. This invention relates to a high chromium steel ingot formation method suitable for preventing precipitation and obtaining a sound rotor shaft material.

[Background of the invention]

Conventionally, martensitic 120r steel has been widely used as a rotor shaft material.

[12Cr7] for rotor shaft materials, alloy steels containing niobium or tantalum have been developed as forged products that have suitable physical properties at high temperatures and room temperatures and can withstand use at high temperatures.

This alloy steel containing niobium and tantalum has a chromium equivalent of 10 or less and is said to be substantially free of δ ferrite.

However, 12 with niobium or tankles
With Cr steel, when manufacturing rotor shafts etc. by forging from large steel ingots of about 10 to 150 tons, δ ferrite segregates near the center of the forged product and is elongated during forging, so the forging direction There is a problem in that the mechanical properties, especially the impact value, in the direction perpendicular to the surface are significantly deteriorated.

Generally, 0.03 niobium and tantalum are added to high chromium steel.
It is already widely known that addition of ~0.12% improves ductility. However, as mentioned above, in large steel ingots, niobium eutectic carbides segregate near the center,
It has the disadvantage of significantly reduced ductility.

[Purpose of the invention]

An object of the present invention is to perform true-wall degassing treatment of the molten metal in a large forged product of simple chromium steel containing nitrogen and at least one of niobium and tantalum, and to contain nitrogen to a target value, thereby producing δ-ferrite. At the same time as preventing the appearance of niobium eutectic carbides, it reduces the segregation of niobium eutectic carbides,
The object of the present invention is to provide a method for forming a high chromium steel containing nitrogen that improves the ductility in the center of the steel ingot in a direction perpendicular to the forging direction.

[Summary of the invention]

The present invention is a method of melting chromium steel containing at least one of niobium and tantalum, adding a predetermined standard value of nitrogen to the molten steel that has been subjected to vacuum degassing treatment, and then casting between the walls of a mold. In particular, the mold is held in a nitrogen atmosphere and molten steel is injected into the mold space.

The chromium-resistant steel targeted by the present invention is niobium.

This material for rotor shafts contains at least one of tantalum and is highly clean, having been degassed to remove oxygen and hydrogen gases that are harmful to physical properties at high and room temperatures.

As this type of chromium steel, 1965-4137
As described in the publication, it is disclosed that when the chromium equivalent is 10 or less, excellent high-temperature properties are exhibited without substantially containing δ-ferrite.

The present inventors manufactured a large forged product weighing approximately 10 to 150 tons by melting, ingotting, and forging such high chromium steel containing at least one of niobium and tantalum. When examining the structure of the steel, we found that a large amount of δ-ferrite was segregated near the center, and that NbC eutectic compounds were segregated around the δ-ferrite, significantly reducing ductility. . Noting that the segregation of δ-ferrite is closely related to the chromium equivalent of chromium steel, it was found that the formation of δ-ferrite can be suppressed by reducing the chromium equivalent to 7.0 or less. On the other hand, if the chromium equivalent is too low, the mechanical properties, especially the yield strength, will be insufficient, so it has become clear that the chromium equivalent needs to be 5.0 or more.

When manufacturing 6 chrome steel for rotor shafts, the molten steel is adjusted to a specified composition in a furnace, etc., and then degassed in a vacuum degassing furnace to improve the cleanliness of the steel. It is necessary to do well. In the vacuum degassing furnace B, oxygen and hydrogen in the molten steel, which are harmful to the temperature characteristics, are removed, and useful nitrogen is also degassed at the same time. Therefore, even if the required amount of nitrogen can be secured before vacuum casting, the actual nitrogen content in the steel ingot after vacuum casting is reduced, making it impossible to secure the necessary ROM equivalent. Therefore, δ-ferrite is easily generated, and eutectic compounds such as NbC are segregated, causing a significant deterioration of ductility near the center.

On the other hand, oxygen and hydrogen in steel cause defects such as white spots and pitting, so vacuum degassing treatment is essential. Therefore, the molten metal adjusted to the specified composition is transferred to a ladle refining furnace and refined, then degassed to remove harmful hydrogen and oxygen, and then nitrogen is added to make up for the deficiency. By injecting the molten metal into the mold space in a nitrogen atmosphere, a certain amount of nitrogen can be contained in the lump.

In addition, when the molten metal hardens in the mold nitride circle, the eutectic carbide of niobium tends to gather in the center and become trapped in gaps such as scratches. These voids such as scratches can be reduced by removing harmful gases such as hydrogen and oxygen from the molten steel, and the ratio of the height (H) and diameter (D> of the mold space (H)
/D) can also be reduced by setting it to 0.5 to 1.0.

[Embodiments of the invention]

When producing steel ingots for casting, conventionally, in order to remove harmful gases such as hydrogen and oxygen contained in the molten metal, hot water that has been heated in a -air furnace is poured into a ladle, and then heated as it is to remove harmful gases such as hydrogen and oxygen contained in the molten metal. The so-called Makabe casting method was used, in which ingots were poured into a cast iron mold (ingot case) placed inside a Makabe tank.

Steel A1 in the first column shows the chemical composition in the ladle at that time, and Boi 2 shows the chemical composition in the center of the steel ingot after the bath water was cast into Makabe. Harmful hydrogen and r due to Makabe casting
It can be seen that while the R element decreases, the shoulder nitrogen also decreases, and as a result, the chromium equivalent J also increases.

Table 1 On the other hand, after receiving the bath water refined in the electric furnace into the ladle refining furnace and removing the hydrogen, barium, and other gases contained in the molten metal, the amount of water that has decreased during the treatment is The center of the lump was cast in a cast iron model 98 placed in a tank with a reduced pressure of about 50 + amHg of the nitrogen surrounding atmosphere. In this case, the chemical composition of A4 in Table 2 was obtained. That is, according to the present example,
Bulk production can be done without reducing the nitrogen content. As a result, there is little change in chromium equivalent, and the appearance of δ-ferrite can be prevented.

Table 2 □ Note) Cr current = Cr + 6Si + 4Mo + 11V + 5Nb
-40C-2Mn-4Ni-3ON-120- (%) When the molten metal of Boil 1 in Table 1 is cast into a mold with H/D = 1.6, both δ-ferrite and niobium are present in the center of the steel ingot. Since crystalline carbides appear and both are stretched in the forging direction during the forging process, the mechanical properties in the lateral direction of the center of the rotor shaft material manufactured using the steel ingot, as shown in 2 of Table 3, are , elongation and reduction of area are extremely low. When the molten steel of No. 3 in Table 2 is cast into a mold with H/D 0.98,
Since δ-ferrite and niobium eutectic carbide were not present in the center of the steel ingot, there was an effect of improving mechanical properties as shown in No. 4 of Table 8143.

FIG. 1 shows the amount of δ-ferrite when the cooling rate during solidification of molten metal with complementary chromium equivalents was varied. It was found that when the chromium equivalent was 7 or less, no δ-ferrite remained after forging and diffusion heating.

The ti442 diagram shows the relationship between the adhesive am equivalent and the yield strength when quenching and tempering is applied. In addition, the quenching and tempering is quenching at a cooling rate of 1050C to IUOC/)lr, and is 565CX20h furnace cooling and 650UX501 furnace cooling tempering. It was found that when the chromium value is less than 5, the yield strength is insufficient, so 5 or more is required.

Therefore, the optimum chromium equivalent is 5 to 7.

The ratio (H/D) of the height H of the steel ingot body to the diameter or distance across flats (Ll) at a position of approximately i of the steel ingot body is 0.
．． When a steel ingot consisting of 5 to 1.0 is produced, the generated niobium eutectic carbide forms near the radial center of the mold until the steel ingot solidifies, and the eutectic carbide floats to the top of the lump. This has the effect of progressing solidification while moving to the feeder section of the steel ingot.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, it is possible to ingot high chromium steel containing at least one of niobium and tantalum, which has excellent ductility, without degassing nitrogen, which improves ductility. Has a remarkable effect.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between chromium equivalent and delta ferrite content after forging and diffusion heating, and FIG. 2 is a diagram showing the relationship between chromium equivalent and yield strength of 4Cr steel. Agent Patent Attorney Tatsuyuki Unuma $ 1 Prison chromium equivalent! $ 2 Diagram Chromium equivalent

Claims (1)

[Claims] 1. High chromium steel containing at least one of niobium and tantalum is melted, a predetermined standard value of nitrogen is added to the molten steel that has been subjected to vacuum degassing treatment, and then cast in a locking room space. A method for ingot-forming high chromium steel containing nitrogen, characterized in that the mold is placed in a nitrogen atmosphere and molten steel is injected. 2. In claim 1, the ratio of the height (II) of the mold gap to the diameter (D) (l/D) is 0.5 to 1.0.
A method for forming high chromium steel containing nitrogen, characterized by: 3. In claim 1, the high chromium steel containing nitrogen has a chromium equivalent of 5.0 to 7.0 as expressed by the following formula (■).
A high chromium-〇 agglomeration method containing nitrogen, characterized by the following. Chromium equivalent (%) = C-+68++4M, +11V+5
Nb 40C2M-4N+ -aON-12WI