JP2000212631A - Production of high nitrogen steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method, with which a high nitrogen steel containing a large quantity of nitrogen can be produced without such fear as to development of blow hole defect. SOLUTION: A refining apparatus, with which atmosphere can be controlled and a pressurized operation can be executed, is used and refining of molten steel is executed by regulating the nitrogen content in the atmosphere so as to be higher than that in the air or the operational pressure higher than the atmospheric pressure or making the nitrogen partial pressure high in the atmosphere with the above both methods. Then, the nitrogen with dissolvable concn. in the molten steel at the solidifying time of the molten steel, is added to the molten steel, and casting is executed under condition of keeping the high nitrogen partial pressure in the atmosphere to obtain a steel ingot. Nitrogen can be added by charging a nitride alloy together with the above method.

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 high-nitrogen steel, which makes it possible to produce a steel material having a high nitrogen content and no blow defects without using a nitride alloy or with the aid of a nitride alloy. To

[0002]

2. Description of the Related Art For example, attempts have been made to improve the corrosion resistance of austenitic or martensitic stainless steel by increasing the nitrogen content of the steel. If the nitrogen content of steel could be increased without causing blow defects, technological advances in this field could be expected.

[0003] In general, the equilibrium nitrogen concentration observed between an atmosphere containing nitrogen and a molten alloy steel is expressed by the following equation called "Gakushin equation" log [N] in relation to the chemical composition of the alloy. _eq = -518 / T-1.063 + 0.046 [Cr] -0.0002
8 [Cr] ² + 0.02 [Mn]-0.007 [Ni]-0.048 [S
i] +0.12 [O] -0.13 [C] +0.011 [Mo] -0.059
[P] -0.007 [S] where log [N] _eq : equilibrium nitrogen concentration at 1 atm (% by weight) []: amount of each element dissolved in molten steel (% by weight) T: temperature of molten steel (K) It is said that it can be obtained by.

[0004] Ordinary atmospheric pressure dissolution cannot contain nitrogen at concentrations exceeding the equilibrium concentration determined by the above equation. Even if the nitrogen is dissolved to the equilibrium concentration, the solubility decreases during solidification, and an amount of nitrogen corresponding to the difference from the saturated dissolved amount is released, thereby causing blow defects. In actual operation, the dissolution of nitrogen from the atmosphere is slow, and it takes a long time to reach equilibrium. Therefore, nitrogen is added by adding a nitride alloy such as ferrochrome nitrogen to molten steel. The above-described problem of the occurrence of the blow defect similarly occurs in that case.

On the other hand, regarding the relationship between the amount of dissolved nitrogen under atmospheric pressure and the amount of dissolved nitrogen under a pressure P, the following equation called "Sieverts'law" has been known for a long time, [N] _Patm = KP ^1/2 = [N] _{1 atm} × P ^1/2 where [N] _Patm : Atmospheric nitrogen partial pressure Nitrogen solubility at P atmospheric pressure (wt%) P: Atmospheric nitrogen partial pressure (atmospheric pressure) K: Coefficient For example, if the nitrogen partial pressure of the atmosphere is quadrupled, twice as much nitrogen is dissolved.

By increasing the nitrogen partial pressure of the atmosphere, the inventors not only increase the concentration of equilibrium nitrogen dissolved in the liquid phase, ie, molten steel, but also increase the amount of nitrogen remaining in the solid phase, ie, the steel ingot during solidification. Have also been found to increase.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to make use of the above-mentioned new knowledge obtained by the present inventors to produce high-nitrogen steel containing a large amount of nitrogen without causing blow defects. It is to provide a method that can be performed.

[0008]

The method for producing high-nitrogen steel of the present invention uses a refining apparatus capable of controlling the atmosphere and operating under pressure to reduce the nitrogen content of the atmosphere to the atmosphere. Or the operating pressure is higher than the atmospheric pressure, or both, to increase the nitrogen partial pressure of the atmosphere to refine the molten steel to increase the concentration that can be dissolved in the steel during the solidification of the steel. Nitrogen having a concentration not exceeding is added to molten steel, and casting is performed while maintaining a high partial pressure of nitrogen in the atmosphere to obtain a steel ingot.

[0009] The amount of dissolved nitrogen in steel is
As the above Gakushin formula shows, it increases with increasing temperature and decreases with decreasing temperature. On the other hand, in the solid phase, when the temperature is lower, the solubility of nitrogen is higher, so the refining of the molten steel is completed and casting is performed.In the process of changing in the direction of decreasing the temperature exclusively, the solubility of nitrogen changes from the liquid phase. It takes a minimum value at the temperature that transitions to the solid phase. That is, the solubility of nitrogen follows a V-shaped curve with respect to temperature.

According to the present invention, the level of solubility of nitrogen is
By realizing consistently enhanced conditions from refining to casting, the V-bottom, which is the minimum value of nitrogen solubility described above, is raised to a higher point. Since the nitrogen concentration in the molten steel is a concentration that does not exceed the solubility at the temperature at which solidification starts,
In the process of cooling the molten steel, it can be transferred to the solid phase without hitting the V-shaped curve. In this way, it is possible to obtain a steel ingot while avoiding the occurrence of blow.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION
In addition to the addition of nitrogen from the atmosphere to the molten steel, the addition of nitrogen by the addition of a nitride alloy, which has been performed in the production of conventional high-nitrogen steel, can be used together. Since the nitrided alloy has a high specific gravity, it sinks into the molten steel and dissolves therein to give nitrogen, so that the nitrogen content of the molten steel can quickly reach a desired level.

Although the present invention is applicable to a wide range of low and high alloy steels, it is particularly effective that
This is an application to the manufacture of the austenitic or martensitic stainless steel described above, in which the nitrogen content is increased and the corrosion resistance is improved.

In the practice of the present invention, any equipment can be used as long as it can control the atmosphere and can continuously perform melting and casting under pressure. The separately proposed device is useful. The device is airtightly connected with a pressure-resistant induction melting furnace that can be tilted to pour out the molten metal and a pressure-resistant ingot case that receives the molten metal and casts it into a steel ingot. With the induction melting furnace standing upright and the ingot case lying down, melting is first performed under atmospheric pressure to prepare a molten metal, and then nitrogen is added under pressure to obtain the desired structure. After the induction melting furnace is tilted by 90 to 100 degrees after the nitrogen concentration is set to the above, the molten metal inside is moved to an upright ingot case. At the upper part of the furnace, a chamber for pressure operation is detachably installed.

[0014]

EXAMPLE An example was carried out using an apparatus in which the above-described induction melting furnace and an ingot case were integrally connected. In the furnace, a crucible and a coil surrounding the crucible are arranged in a stainless steel closed container, the maximum melting amount is 50 kg, and pressurization up to 5 atm is possible.

[Example 1] By dissolving under atmospheric pressure, SU
S304 stainless steel was melted. The chamber was set to a nitrogen gas atmosphere, pressurized to 4 atm, and held for 60 minutes. Sampling was performed every 15 minutes, and the nitrogen concentration in the molten steel was measured to obtain a graph shown in FIG. After 60 minutes, the nitrogen concentration in the molten steel had reached 0.43%, which is close to the equilibrium nitrogen concentration of 0.44% (calculated value) at a nitrogen partial pressure of 4 atm.

The molten steel was cast, the ingot was cut, and the inside was examined. As a result, no blow defect was found.

[Example 2] SUS304 and SUS3
16 stainless steel was melted. However, Cr and Mn
The content of is considered to be 1 to
2% lower. The chamber was set, a nitrogen gas atmosphere was set, and F-CrN (Cr70
%, N7%) and M-MnN (Mn90%, N6%)
And the nitrogen amount was increased to a level close to the amount of nitrogen added when the equilibrium concentration was approached by maintaining the pressure at 4 atm for a long time. Next, casting was performed while maintaining the nitrogen pressure.

The nitrogen content of the obtained ingot was as follows, and was intact without blow defects.

Possible nitrogen addition amount under atmospheric pressure Target nitrogen amount Actual nitrogen amount SUS304 0.22% 0.44% 0.38% SUS316 0.21% 0.42% 0.41%

[0020]

According to the present invention, when a high-nitrogen steel is manufactured according to the present invention, a steel ingot having a high nitrogen content, which has been conventionally difficult to manufacture, can be easily manufactured without the risk of occurrence of blow defects. As a result, the corrosion resistance of austenitic or martensitic stainless steel is particularly improved.

[Brief description of the drawings]

FIG. 1 shows data of an embodiment of the present invention,
When nitrogen is added from the atmosphere to 04 stainless steel,
4 is a graph showing an increase in nitrogen concentration in molten steel over time.

Claims (4)

[Claims] 1. A smelter capable of controlling the atmosphere and capable of operating under pressure to increase the nitrogen content in the atmosphere above that in the atmosphere or to raise the operating pressure above atmospheric pressure. Or refining the molten steel by increasing the nitrogen partial pressure of the atmosphere by both, and adding a concentration of nitrogen to the molten steel that does not exceed the concentration that can be dissolved in the steel during solidification of the steel,
A method for producing high-nitrogen steel, characterized in that a steel ingot is obtained by performing casting while keeping the nitrogen partial pressure of the atmosphere high. 2. The method for producing high-nitrogen steel according to claim 1, wherein the addition of nitrogen from the atmosphere to the molten steel is performed in addition to the addition of nitrogen by introducing a nitrided alloy. 3. The method according to claim 1, wherein the high-nitrogen steel is austenitic or martensitic stainless steel. 4. The method for producing high-nitrogen steel according to claim 1, wherein the apparatus is implemented by using an apparatus in which an ingot case and an ingot case that can be tilted for pouring out molten metal are airtightly connected. .