JP2002146429A - METHOD FOR PRODUCING AUSTENITIC HIGH Mn STAINLESS STEEL - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method by which the reduction of oxygen in high Mn stainless steel is solved, and further, the composition and form of inclusions can be controlled. SOLUTION: In this method for producing austenitic high Mn stainless steel, Al is added to a molten steel having a composition containing <=0.15% C, <=1% Si, 5 to 10% Mn, <=0.06% P, <=0.03% S, 3.5 to 6.0% Ni, 16 to 19% Cr and <=0.25% N, and the balance Fe to remain Al by 0.008 to 0.03 mass%. The molten steel is refined by using slag in which the ratio of CaO/SiO2 is 1.5 to 2.5 in a laddle refining furnace to control the value of oxygen in the molten steel to <=35 ppm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing austenitic high Mn stainless steel.

[0002]

2. Description of the Related Art Conventionally, in the field of stainless steel making,
Out-of-pile refining technologies such as the D method, RH-OB method, and AOD method have been adopted, and high-performance stainless steels such as ultra-low carbon stainless steel and high-purity stainless steel have been produced. Austenitic stainless steel with excellent workability is used for many functional materials, and high-Mn-based stainless steel has the characteristics of high-strength, corrosion-resistant stainless steel. This type of material requires a high degree of cleanliness, but is difficult to deoxidize because of its high Cr content and low oxygen activity. In particular, high-Mn stainless steel is a steel type that is even more difficult to deoxidize.

As one of the deoxidizing techniques, a method of applying out-of-pile refining to stainless steel is disclosed in Japanese Patent Application Laid-Open No. Hei 10-23759.
No. 8, for example. Further, Japanese Patent Application Laid-Open No. Hei 10-23
No. 7598 discloses that, for a specific austenitic stainless steel, the ratio of Si / Al is regulated and the composition of nonmetallic inclusions is adjusted so that MnO—SiO ₂ becomes a main component, thereby reducing the susceptibility to processing cracks. Technical proposals have been made to do so. This proposal is excellent in that the inclusions are separated during hot and cold working by controlling the composition of the inclusions, thereby reducing the work-induced cracking caused by the inclusions.

[0004]

Although the above-mentioned techniques of refining outside the furnace and controlling the composition of inclusions are advantageous for the production of ultra-low carbon stainless steel and high-purity stainless steel,
Regarding the production of high-Mn stainless steel, there was a problem that it was difficult to control oxygen reduction and deoxidation products. These problems become significant problems in controlling the form of inclusions to render them harmless. An object of the present invention is to provide a high and high Cr content.
An object of the present invention is to provide a method for producing austenitic high Mn stainless steel in which the oxygen content of Mn stainless steel is solved and the composition and morphology of inclusions are controlled.

[0005]

The present inventors have found that austenitic stainless steels have a high Cr content and a low oxygen activity, making it difficult to deoxidize them. In addition, high-Mn stainless steels are more difficult to deoxidize. Consider the problem of being
It has been found that by adding l and refining slag, it is possible to reduce the oxygen content of the steel and control the composition and morphology of inclusions, and have reached the present invention.

That is, in the present invention, C: 0.15% by mass%
Below, Si: 1% or less, Mn: 5-10%, P: 0.06% or less, S:
0.03% or less, Ni: 3.5 to 6.0%, Cr: 16 to 19, N: 0.25%
Hereafter, Al is added to molten steel whose balance is substantially Fe, and Al is adjusted to 0.008 to 0.03% by mass. After deoxidation, the slag with a CaO / SiO ₂ ratio of 1.5 to 2.5 is used in a ladle refining furnace. This is a method for producing austenitic high-Mn stainless steel in which molten steel is refined using the method described above to reduce the oxygen value of molten steel to 35 ppm or less.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An important feature of the present invention is that a high deoxidizer, Al, is added to a high Mn stainless steel having a specific composition in a melting step before ladle refining to 0.008 to 0.03 mass%. After the adjusted deoxidation, refining was performed in a ladle refining furnace using slag with a CaO / SiO ₂ ratio of 1.5 to 2.5. The present inventors increased the amount of Al used before refining to enhance the deoxidizing power, and then removed the generated Al ₂ O ₃ by slag refining to make the molten steel oxygen amount reach an equilibrium value. I found it. Further examination of the refining slag composition, refractory composition, and the like revealed that it is effective to define the slag basicity.

In the present invention, the Al value is important in considering the deoxidation equilibrium. If the Al value is less than 0.008% by mass, the oxygen content of the molten steel with high Mn cannot be sufficiently reduced. Conversely, if it exceeds 0.03% by mass, there is no change in the deoxidized state, and it is not advisable to increase the amount added. Smelting slag composition is Al ₂
It is important to have excellent O ₃ adsorption ability. Slag basicity C
Although the aO / SiO ₂ ratio is specified to be 1.5 to 2.5, if the basicity is less than 1.5, the adsorption of the deoxidized product does not easily progress and the refining takes a long time. Conversely, if it exceeds 2.5, the slag becomes less hardened and hardened, and the refining effect is reduced. Therefore, the optimal value of the slag basicity is calculated as CaO / SiO ₂ =
1.5 to 2.5.

In the present invention, the refined molten steel oxygen value is 3
It was specified to be 5 ppm or less. If the oxygen value exceeds 35 ppm, the inclusion cleanliness will not be good. This is because chain-like inclusions may be generated. In the present invention, in order to further enhance the refining effect, it is effective to effectively advance the contact between the molten steel and the refining slag, and it is effective to apply Ar bottom blowing with a porous plug during ladle refining. . Preferably, it is of a double porous type, and the higher the agitation of the molten steel, the higher the refining effect.

The refractory of the ladle refining furnace is preferably dolomite. The composition of dolomite refractories is MgO: about 6
The higher the MgO component containing about 33% CaO with 3% as the main component, the higher the erosion resistance. When the basicity of the smelting slag is 1.4 or less, the dolomite refractory tends to be easily melted. Therefore, it is appropriate that the slag basicity is 1.5 to 2.5 even in consideration of the erosion resistance of the dolomite refractory. The use of dolomite-based refractories has the characteristic that even if refining proceeds, the slag basicity does not change significantly due to refractory erosion, so that the effect on refining is small. Generally, the refractory of the ladle refining furnace is made of MgO-C brick, but during refining, the C of the brick elutes into the molten steel and the molten steel C% rises by about 0.005 to 0.01% / h. Go. Therefore MgO-C
Refining of low-C stainless steel in a ladle refining furnace equipped with high quality brick is not preferable, and dolomite refractories are appropriate.

By the way, an artificial graphite electrode is generally used as a heating source of a ladle refining furnace. However, in refining of low carbon stainless steel, so-called C pickup in which C% of molten steel is increased due to erosion of a part of the graphite electrode is likely to occur. In order to prevent this, a heating torch instead of the graphite electrode is effective, and a plasma arc using a metal torch as the cleanest heating source may be used.

The range of alloy components (% by mass) contained in the austenitic high Mn stainless steel to which the present invention is applied will be described below. C: 0.15% or less C is a solid solution strengthening element, and when contained in a large amount, the proof stress increases by 0.2% and hardens the steel material. In addition, when the addition amount is large, the work-induced cracking may occur due to an increase in proof stress and hardness, and thus the C content is set to 0.15% or less.

Si: 1% or less Si is a component used for deoxidizing molten steel. However, if a large amount of Si exceeding 1% by mass is contained, the steel material becomes hard. With the hardening of steel materials, work-induced cracking may occur,
The Si content was set to 1% or less. Mn: 5 to 10% Mn is a component effective for ensuring hot workability, but is a component added to save Ni in high Mn stainless steel. Has the effect of increasing the solid solubility of N to harden steel
Although 10% is added, adding more than 10% does not contribute to improvement in oxidation resistance.

P: 0.06% or less P is a component that is easily segregated, and deteriorates the deformability of the material. Therefore, P should be kept low, preferably 0.04
% Or less is good. S: 0.03% or less S is an element that has an adverse effect on hot workability similarly to P, and it is better to keep it low. The upper limit is 0.03%.

Ni: 3.5-6.0% Ni is an austenite-forming element. In high-Mn stainless steel, Mn and N are added to save Ni. Therefore, the addition amount was set to 3.5 to 6.0%. Cr: 16 to 19% This is a basic component necessary for improving corrosion resistance. The effect becomes significant when the Cr content is 16% or more. If contained excessively, the material becomes hard and workability deteriorates, so the upper limit was made 19%. N: 0.25% or less N is a solid solution strengthening element, and when contained in a large amount, hardens the material. In high-Mn stainless steels, Mn and N were added to save Ni. Since the solid solubility of N was higher in Mn than in Ni, the upper limit was set to 0.25% by mass.

[0016]

EXAMPLE Austenitic high Mn stainless steel SUS201 having the composition shown in Table 1 was melted in a 15-ton electric arc furnace, the components were roughly adjusted, Al deoxidation was performed, and then ladle refining was performed. In FIG. 1, the raw material was melted in the electric arc furnace 1, then the molten steel was transferred to the ladle 2, and Ar was blown from the porous brick 6 arranged at the bottom of the ladle refining furnace 3 to perform slag refining in the ladle. . Deoxidation was performed before tapping of the arc furnace, and the Al mass% was adjusted to 0.008 to 0.03 mass%.
Also, the CaO / SiO ₂ ratio of the slag is adjusted by adjusting the basic composition of the slag before tapping from the arc furnace, but the basicity is also adjusted by adding CaO and CaF ₂ in the ladle refining furnace.

[0017]

[Table 1]

FIG. 2 shows the Al deoxidation equilibrium of SUS201 steel and the results of Al and [O] values before and after ladle refining. As shown in FIG. 2, in the comparative example in which the Al value was less than 0.008%, the temperature was almost 1%.
Can be plotted on the line in the case where the activity of a _{Al2 O3} = 1.0 alumina of 873 K, the molten steel oxygen value it can be seen that not be sufficiently reduced. Table 2 shows the Al value before tapping from the arc furnace to 0.00.
The compositions of inclusions in molten steel after refining in the case of 4% aim and in the case of adjusting to 0.01% aim in the present invention are shown.

[0019]

[Table 2]

At the aim of 0.004%, the main composition of the inclusion is Si
O ₂ -MnO, which indicates that Al is consumed as a deoxidizing agent during refining when deoxidation is not sufficiently performed. On the other hand, when Al is aimed at 0.01%, the main composition of the inclusions is SiO ₂ —MnO, but about 23% by mass of Al ₂ O ₃ is contained, and Al ₂ O ₃ generated during refining is adsorbed by slag and molten steel It can be seen that this contributes to the reduction of the oxygen value. As a result, the molten steel oxygen content could be reduced to about 30 ppm. Further, Table 3 shows changes in the slag composition before and after refining under the same conditions as those of the present invention in comparison with before and after refining. No major change was seen in the slag composition,
As the refining progresses, Al ₂ O ₃ gradually increases, indicating that the generated Al ₂ O ₃ is adsorbed on the slag.

[0021]

[Table 3]

[0022]

According to the present invention, the oxygen reduction of austenitic high Mn stainless steel can be remarkably improved,
This is an indispensable technology for practical use of detoxifying inclusions.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a process of the present invention.

FIG. 2 is a diagram showing a relationship between an Al value and an oxygen amount.

[Explanation of symbols]

1. Electric arc furnace, 2. Ladle, 3. Ladle refining furnace, 4. Refractory, 5. Electrode, 6. Porous brick, 7. template

Claims (1)

[Claims] C .: 0.15% or less by mass, Si: 1% or less,
Mn: 5-10%, P: 0.06% or less, S: 0.03% or less, Ni: 3.5
~ 6.0%, Cr: 16 ~ 19, N: 0.25% or less, with the balance being substantially
Al is added to molten steel having a composition of Fe, and Al: 0.008 to 0.03
After deoxidation, the CaO / SiO ₂ ratio was 1.5 in a ladle refining furnace.
Refining molten steel using a slag of ~ 2.5 and reducing molten steel oxygen value to 3
A method for producing an austenitic high-Mn stainless steel, characterized in that the content is 5 ppm or less.