JP2002339048A - HIGH Al FERRITE STAINLESS STEEL HAVING EXCELLENT HIGH TEMPERATURE OXIDATION RESISTANCE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide high Al ferritic stainless steel which has improved high temperature oxidation resistance by reducing the concentration of Mg in the steel to thereby prevent the occurrence of fine bubbles on casting and reduce surface defects in the steel. SOLUTION: The high Al ferrite stainless steel has a composition containing, by weight, <=0.03% C, <=0.25% Si, <=0.25% Mn, <=0.03% P, <=0.001% S, <=0.03% N, 15 to 30% Cr, 3 to 6% Al and 0.01 to 0.2% of one or more kinds selected from rare earth metals and Y, and in which the content of Mg is controlled in <=0.015%. The stainless steel may contain <=4% Mo.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high Al ferritic stainless steel exhibiting excellent high temperature oxidation resistance.

[0002]

2. Description of the Related Art High Al ferritic stainless steel is used as a chimney material, an electrothermal material, a catalytic converter base material of an exhaust gas purifying device, etc. because of its excellent high temperature oxidation resistance. Metallic converters using high Al ferritic stainless steel as a base material have significantly higher thermal shock resistance than conventional ceramic converters, and have the advantage that they can be installed closer to the engine. In addition, it is attracting attention as a material suitable for applications such as structural materials exposed to other high-temperature oxidizing atmospheres. The present inventors have also been developing a high Al ferritic stainless steel of this system. For example, Japanese Patent Application Laid-Open No. 4-147945
In this publication, a high Al ferritic stainless steel to which alloy elements such as rare earth metals, Y, and alkaline earth metals are added has been introduced. These alloy elements strengthen the oxide film formed on the surface of the steel material and improve the high-temperature oxidation resistance of the steel material. Further, it also has an action of removing or fixing harmful elements such as S contained in steel.

[0003]

In recent years, the continuous casting method has been applied to the production of stainless steel. However, when producing a slab of high Al ferritic stainless steel by continuous casting, fine tubular bubbles are likely to be generated near the slab surface. The fine tubular bubbles are a phenomenon peculiar to the high Al ferritic stainless steel, and cause surface defects such as surface roughening and cracks in hot rolling and cold rolling of the slab and in a processing step. According to the investigation by the present inventors, the fine bubbles are A
It has a pipe-like form that is clearly different from the pinhole caused by r. The present invention has been devised to solve such a problem, and based on the finding that the cause of the generation of fine tubular bubbles is the Mg concentration in steel, the present invention reduces the Mg concentration in steel and reduces High Al that prevents the generation of tubular bubbles and reduces the surface defects of steel and improves high-temperature oxidation resistance
An object of the present invention is to provide a ferritic stainless steel.

[0004]

The high-Al ferritic stainless steel of the present invention having excellent resistance to high-temperature oxidation has a C content of 0.03% by weight or less and a Si content of 0.1%.
25% by weight or less, Mn: 0.25% by weight or less, P:
0.03% by weight or less, S: 0.001% by weight or less,
N: 0.03% by weight or less, Cr: 15 to 30% by weight
, Al: 3 to 6% by weight, one or more rare earth metals and Y: 0.01 to 0.2% by weight, and the Mg content is regulated to 0.015% by weight or less. . This high A
l Ferritic stainless steel further contains 4% by weight or less of M
o may be included.

[0005] Such high-Al ferritic stainless steels, Al: the molten metal of a high Al ferritic stainless steel containing 3 to 6 wt%, CaO-CaF ₂ -Al ₂ containing CaF ₂ 5-30 wt% After contacting an O ₃ -based slag, one or more of a rare earth element and Y is added, and then continuous casting is performed. Alternatively, a high Al ferritic stainless steel containing at least 3 to 6% by weight of Al in a container lined with an alumina-based refractory when one or more rare earth elements or Y is added in a pre-process of continuous casting. Of molten steel in the steel
A high-Al ferritic stainless steel having a low concentration can be obtained. The alumina-based refractory is used in any form in which the container wall itself of the container or the inner surface of the container is lining.

[0006]

The high Al ferritic stainless steel of the present invention
This is to reduce the Mg concentration in steel to prevent the generation of tubular fine bubbles during casting, to provide good surface properties in a rolled state, and to improve high-temperature oxidation resistance. The present inventors,
The mechanism of the generation of tubular microbubbles in high Al ferritic stainless steel was investigated and studied. Slag-metal having a predetermined composition is melted in an induction melting furnace, and rare earth metals,
After adding Y, alkaline earth metal, etc., the molten metal was sampled periodically. In addition, the molten metal whose components were adjusted was cast, and the generation state of bubbles was observed. Mg concentration in steel [Mg]
And when the experimental results were arranged by the hydrogen concentration [H] in steel,
It has been found that the relationship shown in FIG. 1 is established. That is, when the Mg concentration [Mg] in the steel exceeded 150 ppm, generation of fine tubular bubbles was observed in the obtained slab.
On the other hand, for [Mg] of 150 ppm or less, 3 to 4 ppm
Even when [H] fluctuated in the range, generation of tubular bubbles was not detected. From this, it was inferred that the main cause of the generation of fine tubular bubbles was Mg gas released from the molten metal during solidification.

[0007] In a molten metal of a high Al ferritic stainless steel containing Al and rare earth metals, Y, etc., it is considered that the following reactions (1) to (3) occur with respect to Mg. 2 [Al] +3 (MgO) = (Al 2 O 3) +3 [Mg] ···· (1) 2 [Y] +3 (MgO) = (Y 2 O 3) +3 [Mg] ···· ( 2) 2 [Y] +3 (MgO) = (Y ₂ O ₃ ) + 3Mg (g) (3) The reaction of the formula (1) is confirmed by the increase of [Mg] by the addition of Al. . That is, when Al is added to the molten metal, MgO in the slag is reduced according to the reaction formula (1), and the Mg concentration [Mg] in the steel increases as shown in FIG. When Y is added, [Mg] increases to the saturation solubility according to the reaction formula (2). When Y is further added, Mg gas is generated as shown in equation (3). Mg gas rises through the slag layer and burns when it comes into contact with air. The present inventors collected powder from white smoke generated and analyzed the components. As a result, most of the white smoke powder was Mg
O was confirmed.

[0008] On the other hand, when the rate of reduction of Y added to the molten metal was analyzed, it was found that it could be arranged by the primary reaction. For example, in a molten metal that comes into contact with slag having low viscosity, the Y concentration [Y] in steel sharply decreases. This means that the slag layer
It is presumed that the g-gas rises vigorously and the oxidation reaction of [Y] according to the formula (3) progresses accordingly. As described above, the reaction for decreasing [Y] is limited by the diffusion of elements involved in the reaction in the slag, and Mg is the main limiting factor in the present system. From the above results,
In the state where the high Al ferritic stainless steel is being melted, the Mg concentration [Mg] in the steel has reached an equilibrium, and when the molten metal is cast and solidified as a slab, the Mg in the steel exceeding the saturation limit becomes outside. It is considered that the released traces of Mg are released as fine tubular bubbles and remain in the slab. Therefore, in order to reduce [Mg] which is a main cause of bubbles, the concentration of MgO should be reduced under a constant temperature and pressure, and the activity of Al ₂ O ₃ , Y ₂ O _3, etc. should be increased. It is expected from Equations (1) and (2) that these are effective.

[0009] To increase the activity amount of Al ₂ O _3, Al ₂
It is effective to relatively reduce CaO, which lowers the activity of O ₃ , in other words, to reduce the CaO / Al ₂ O ₃ ratio. However, in the process of producing a high Al molten metal, a large amount of Al migrates from metal to slag, and the CaO / Al ₂ O ₃ ratio in the slag is at a low level of about 0.6. Decreasing the CaO / Al ₂ O ₃ ratio further than this would result in C
aO-Al ₂ O ₃ 1600 in -MgO ternary phase diagram
There is a limit in view of the liquidus at ℃. Increasing the activity of Y ₂ O ₃ or the like is effective, but consumes a large amount of an expensive Y source and causes an increase in manufacturing cost. Therefore, the present inventors have studied means for reducing the solubility of MgO in slag as a method of reducing the concentration of MgO.
Then, was investigated MgO solubility in relation to the composition for various slag, 5-30 wt% of the CaO-CaF ₂ -Al ₂ O ₃ system slag containing CaF ₂ have been used conventionally CaO- It has been found that, compared to slag of the Al ₂ O ₃ system, an effect of significantly reducing the solubility of MgO is exhibited.

Specifically, in a CaO—CaF ₂ —Al ₂ O ₃ slag containing 5% by weight or more of CaF ₂ , CaO—
[Mg] could be reduced to about 0.005% by weight or more as compared with the Al ₂ O ₃ system. [M
g] was remarkable when the CaF ₂ content exceeded 5% by weight. _{However, CaO-CaF 2 -Al 2 O} 3 slag containing large amounts of CaF ₂ of more than 30% by weight, erosion of refractories is severely reduces the life of the furnace material.
The MgO solubility of the slag can be made substantially zero by using an alumina-based refractory. That is, MgO in slag
Is supplied from dolomite refractories. Therefore, when an alumina-based refractory is used at least for lining a container instead of a dolomite-based material, Mg supplied from the refractory is used.
There is no O and MgO in the slag is regulated to a very low concentration level. In this case, the Mg concentration in steel [Mg] is 50p
pm or less, and a slab free of tubular bubbles is obtained.

By the way, Mg is an element which improves high-temperature oxidation resistance like rare earth metals and Y. However, as described above, when the Mg concentration in steel increases, tubular bubbles are easily generated in the slab. When the slab in which the tubular bubbles are generated is rolled, the tubular bubbles deteriorate the surface properties of the steel material, and as a result, the high-temperature oxidation resistance is reduced. Therefore, in the high-Al ferritic stainless steel of the present invention, the upper limit of the Mg content is set to 0.015% by weight from the viewpoint of suppressing the generation of tubular bubbles during casting and obtaining a slab or a hot-rolled steel sheet having good surface properties. Regulated. Further, the stainless steel of the present invention contains an alloy element in a predetermined content other than Mg. Hereinafter, each alloy element and its content will be described.

C: 0.03 wt% or less In high Al ferritic stainless steel, it is a harmful element that has a bad influence on oxidation resistance. As the C content increases, abnormal oxidation tends to occur. Further, it also causes deterioration of the toughness of the slab or the hot coil. Si: 0.25% by weight or less Si is generally considered to be an effective alloy element for high-temperature oxidation resistance. However, in high Al ferritic stainless steel, the high-temperature oxidation resistance is significantly improved by reducing the Si content. The effect of reducing the Si content becomes remarkable in conjunction with the regulation of the Mn content. Mn: 0.25% by weight or less Although it is an alloy element effective for improving hot workability, it has a high A
(1) Ferritic stainless steel adversely affects high-temperature oxidation resistance. Therefore, in order to secure high-temperature oxidation resistance, the Mn content is reduced to less than 0.25% by weight. In addition, toughness is improved by reducing the Mn content.

P: 0.03% by weight or less P is an element that has an adverse effect on high-temperature oxidation resistance properties.
The lower the P content, the better. Further, when the P content is reduced, the toughness of the hot-rolled sheet is also improved. S: 0.001% by weight or less Bonds with rare earth metals, Y, etc., and becomes nonmetallic inclusions, deteriorating the surface properties of the steel material. Further, since rare earth metal, Y, and the like effective for high-temperature oxidation resistance are consumed, when a large amount of S is contained, it is necessary to add a large amount of rare earth metal, Y, etc. in anticipation of loss. In addition, since the reaction with the rare earth metal, Y or the like is not constant, the yield of the added rare earth metal, Y or the like greatly varies. Therefore, in the present invention, the S content is 0.001% by weight.
It is necessary to regulate strictly as follows. N: 0.03% by weight or less N is a harmful element that lowers the toughness of the steel material. In addition, the amount of Al effective for high-temperature oxidation resistance is consumed by the generation of AlN,
It causes abnormal oxidation. Therefore, the N content is
The upper limit is set to 0.03% by weight.

Cr: 15 to 30% by weight Cr is a basic alloying element in the high-Al ferritic stainless steel according to the present invention. When Cr is contained in an amount of 15% by weight or more, a remarkable effect on improvement in high-temperature oxidation resistance is observed. . However, a large amount of Cr exceeding 25% by weight deteriorates the toughness of the slab or the hot coil, resulting in poor manufacturability. Al: 3 to 6% by weight Like Cr, it is an alloy element necessary for ensuring high-temperature oxidation resistance. When used in the form of a foil material or the like, 3% by weight or more of Al is required to suppress the occurrence of abnormal oxidation. However, a large content of Al exceeding 6% by weight deteriorates the toughness of the slab or the hot coil and deteriorates the manufacturability. One or more rare earth metals, Y and alkaline earth metals
Above: 0.01 to 0.2% by weight These alloy elements remarkably improve the protective effect of the oxide film formed on the surface of the steel material and improve the adhesion of the oxide film to the base steel. As a result, the high temperature oxidation resistance of the high Al ferritic stainless steel is improved. Such an effect becomes remarkable when the content is 0.01% by weight or more. However, when it is contained in a large amount exceeding 0.2% by weight, not only the production becomes difficult due to the deterioration of hot workability and toughness, but also the formation of a large amount of inclusions deteriorates the surface properties of the steel material. Become.

Mo: not more than 4% by weight In the present invention, although it is an optional element added as necessary, the addition of Mo significantly improves the high-temperature oxidation resistance of steel and the high-temperature strength. You. However, excessive addition of Mo deteriorates the toughness of the steel and causes the cleaning property to deteriorate, so that the maximum content is 4% by weight.
Furthermore, the high Al ferritic stainless steel of the present invention
And / or the addition of Nb, V, Ti or the like for fixing N can also improve toughness. For applications exposed to a severe high-temperature atmosphere, it is effective to improve the high-temperature strength by adding 0.05% by weight or more of Nb, V, Ti, and the like.

[0016]

EXAMPLE 1 82 tons of molten stainless steel whose components were adjusted as shown in Table 1 were produced. Table 2 shows the slag composition at this time. The VOD was opened to the atmosphere and 410 kg of Y was added, corresponding to 0.2% by weight. Then, the molten metal was stirred for 3 minutes by blowing Ar gas, and then sent to a continuous casting step to perform casting. As shown in Table 1, the slab obtained had a Mg concentration [Mg] of 0.008% by weight, and no tubular bubbles were observed in the slab. By hot rolling this slab, a steel sheet having good surface properties without defects such as rough skin was obtained. The slab grinding yield at this time was 98%, and the flaw removal yield after pickling was 99.5%.

[0017]

[0018]

Example 2 Using a ladle lined with an Al ₂ O ₃ refractory, 82 tons of molten stainless steel whose components were adjusted as shown in Table 3 were produced. Table 4 shows the slag composition at this time. The VOD was released to the atmosphere, Y was added under the same conditions as in Example 1, the melt was stirred for 3 minutes, and then sent to a continuous casting step to perform casting. The composition of the obtained slab,
The results are shown in Table 3. The slab contained 0.08 wt% Y and 0.004 wt% Mg. Further, no tubular bubbles were observed in the slab, and as in Example 1, it was rolled into a steel sheet having good surface properties without defects such as rough skin. The slab grinding yield at this time was 98%, and the flaw removal yield after pickling was 99.7%.

[0020]

[0021]

Comparative Example: 82 tons of molten stainless steel whose components were adjusted as shown in Table 5 were produced. Table 6 shows the slag composition at this time. VOD was released to the atmosphere, and Example 1
After adding Y under the same conditions as above and stirring the molten metal for 3 minutes,
It was sent to a continuous casting process and cast. Table 5 also shows the composition of the obtained slab. The slab contained 0.07 wt% Y and 0.016 wt% Mg. Fine tubular bubbles were generated in the slab. The hot rolled steel sheet produced from this slab had surface defects derived from tubular bubbles. At this time, the slab grinding yield was 96%, and the flaw removal yield after pickling was 94.9%.

[0023]

[0024]

Example 3 82 tons of molten stainless steel adjusted to the components shown in Table 7 were produced. Table 8 shows the slag composition at this time. VOD released to atmosphere, 0.2% by weight
410 kg of Y was added. After stirring for 3 minutes by blowing Ar gas, the mixture was sent to a continuous casting step and cast into a slab. As shown in Table 7, the obtained slab had a Mg concentration [Mg] of 0.007%, and no tubular bubbles were observed in the slab. By hot rolling this slab, a steel sheet having few surface flaws and good surface properties was obtained. The slab grinding yield at this time is 98.
%, And the yield of removing flaws after pickling was 99.5%.

[0026]

[0027]

Example 4: Using a ladle lined with an Al ₂ O ₃ refractory, 82 tons of molten stainless steel adjusted to the components shown in Table 9 were produced. Table 10 shows the slag composition at this time. The VOD was released to the atmosphere, Y was added under the same conditions as in Example 1, the molten steel was stirred for 3 minutes, and then sent to a continuous casting step for casting. The obtained slab had a composition of 0.07% by weight of Y and 0.00 as shown in Table 9.
It contained 4% by weight of Mg. Further, no tubular bubbles were observed in the slab, and the steel sheet was rolled into a steel sheet having few surface flaws and good surface properties as in Example 1. The slab grinding yield at this time is 98%, and the flaw removal yield after pickling is 99.5.
%Met.

[0029]

[0030]

[0031]

As described above, in the present invention, the generation of fine tubular bubbles in a slab during continuous casting is suppressed by reducing the Mg concentration in steel. The obtained slab is rolled into a steel sheet having good surface properties. Due to its good surface properties, it can sufficiently exhibit the high-temperature oxidation resistance inherent in high-Al ferritic stainless steel, and can be used in a wide range of fields as structural materials, various equipment for heating appliances, base materials for metallic converters, etc. A high-temperature steel material used in the present invention is provided.

[Brief description of the drawings]

Fig. 1 Effect of H concentration in steel and Mg concentration in steel on generation of tubular bubbles

Fig. 2 Effect of Al concentration in steel on Mg concentration in steel

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/22 C22C 38/22 (72) Inventor Morihiro Hasegawa 4976 Nomura Minamimachi, Shinnanyo-shi, Yamaguchi Nisshin F-term in Steel Research Laboratories of Steel Corporation (reference) 4E004 MB14 NC02

Claims (2)

[Claims] C: 0.03% by weight or less;
25% by weight or less, Mn: 0.25% by weight or less, P:
0.03% by weight or less, S: 0.001% by weight or less,
N: 0.03% by weight or less, Cr: 15 to 30% by weight
, Al: 3 to 6% by weight, one or more rare earth metals and Y: 0.01 to 0.2% by weight, and the Mg content is regulated to 0.015% by weight or less. High Al ferritic stainless steel with excellent high temperature oxidation resistance. 2. C: 0.03% by weight or less and Si: 0.
25% by weight or less, Mn: 0.25% by weight or less, P:
0.03% by weight or less, S: 0.001% by weight or less,
N: 0.03% by weight or less, Cr: 15 to 30% by weight
And Al: 3 to 6% by weight, Mo: 4% by weight or less, and one or more kinds of rare earth metals and Y: 0.01 to 0.2.
A high-Al ferritic stainless steel with excellent high-temperature oxidation resistance, containing 0.1% by weight and containing Mg at 0.015% by weight or less.