JP3353969B2 - Melting method of high Al ferritic stainless steel - Google Patents

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 a high-Al ferritic stainless steel with improved medium precision in composition.

[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. Rare earth metals, Y, and the like added to the high Al ferritic stainless steel strengthen the oxide film formed on the surface of the steel material and improve the high temperature oxidation resistance of the steel material. In addition, it also has the effect of fixing harmful elements such as S contained in steel. However, if a rare earth metal, Y, or the like is excessively added, hot workability and toughness deteriorate, and it becomes difficult to produce a steel material by hot rolling. Moreover,
As a result of producing a large amount of nonmetallic inclusions in steel by reacting with S and the like, it also causes deterioration of the surface properties. Rare earth metal,
For the addition of Y and the like, an alloy element is introduced from the upper part of the ladle after the VOD is opened, and the molten metal is stirred with Ar to achieve a uniform dispersion of the added element. The prepared molten metal is cast into a slab, an ingot, and the like, and sent to a lower process. In this manufacturing process, in order to improve the high-temperature oxidation resistance while suppressing the adverse effects caused by the rare earth metal, Y, and the like, it is required that the amount of the rare earth metal, Y, and the like be strictly controlled.

[0003]

SUMMARY OF THE INVENTION Rare earth metals, Y, etc.
Oxygen affinity and specific gravity are large, and it is difficult to achieve the target content due to oxidation loss, segregation, etc., even if it is added to the molten metal in an appropriate amount. For example, when the rare earth metal or Y is added to the molten metal after or during the VOD treatment and the components are adjusted while stirring with Ar gas, the yield of the rare earth metal, Y, etc., has a large variation of 18 to 60%. Such a variation exhibits the same tendency when an alkaline earth metal imparting properties similar to those of the rare earth metal and Y is added. In order to obtain the target content, a certain amount of rare earth metal, Y, alkaline earth metal or the like is added in consideration of the yield. However, a steel material containing excessive amounts of rare earth metal, Y, alkaline earth metal and the like can be obtained if the loss such as oxidation loss is small under the condition of addition in a large amount and the amount incorporated into steel is large. As a result, the above-mentioned adverse effects appear. Conversely, if the content does not reach the lower limit of the target content, the effect of improving properties by rare earth metals, Y, alkaline earth metals, and the like is not exhibited.
The present invention has been devised to solve such a problem. When rare earth metals, Y, alkaline earth metals, and the like are added to a molten stainless steel, the concentrations of these added elements in the steel are set to a value that is lower than that of the molten metal. Focusing on the fact that there is a close relationship with time, the aim is to produce high-Al ferritic stainless steels containing rare earth metals, Y, alkaline earth metals, etc. with high elemental and medium precision by controlling the stirring conditions. And

[0004]

SUMMARY OF THE INVENTION In order to attain the object, a smelting method according to the present invention comprises a ferritic stainless steel containing 3 to 6% by weight of Al and one of rare earth metals, Y and alkaline earth metals. Species or two or more species in total of 0.01-0.
When included 2 wt%, rare earth metals to the melt of the stainless steel, the addition of one or more of Y and alkaline earth metals, the melt is stirred by gas bubbling, in accordance with the target amount X _t following The stirring is stopped at a time point t defined by the equation (1). t = log (X ₀ / X _t ) / K _c (1) where t in the formula (1) is one or two or more of a rare earth metal, Y and an alkaline earth metal. X ₀ is the initial concentration (mol%) of the rare earth metal, Y and the alkaline earth metal immediately after the addition, and X _t is the rare earth metal, Y and Alkaline earth metal concentration (mol%) and K
_c indicates an apparent rate constant (/ min).

[0005] The high Al ferritic stainless steel produced according to the present invention is, for example, 3 to 6% by weight of Al and 0.01 to 0.2% by weight of one or more of rare earth metals, Y and alkaline earth metals or Besides two or more, C: 0.03% by weight or less, Si: less than 0.25% by weight, Mn: less than 0.25% by weight, P: 0.03% by weight or less, S: 0.001% by weight N: 0.03% by weight or less and Cr: 15 to 3
0% by weight, and if necessary, Mo: 3% by weight or less. Rate constant K _c of apparent, rare earth metals, determined in advance for stainless steel melt content of the alloy elements excluding Y and alkaline earth metals are definite. Rate constant K _c of apparent is in the range of 0.05 to 0.1 / min, is used to determine the stirring duration t into Equation (1).

[0006]

In the conventional smelting method in which a rare earth metal, Y, an alkaline earth metal, etc. are added, the large variation in the yield of the added elements means that it is difficult to set the addition conditions because the reaction mechanism is unknown. Due to that. The present inventors have studied the reaction rates of rare earth metal, Y, alkaline earth metal and the like to the molten metal on a laboratory scale according to this premise. The slag-metal was melted in a 100 kg induction melting furnace, and one or more of rare earth metals, Y, alkaline earth metals, etc. were added, and the molten metal was sampled periodically. And the reduction rate of rare earth metals, Y, alkaline earth metals, etc. was investigated. For example, the Y concentration in steel (hereinafter, referred to as
[Y] is reduced as shown in FIG. 1 with the lapse of time from the addition of Y. Assuming that [Y] decreases due to the reaction between the added Y and other elements, -log (X
_t / X ₀ ) and the stirring time t were investigated. As a result, in a region as shown in FIG. 2 [Y] is more than 0.05 wt%, between -log (X _t / X ₀₎ and stirred elapsed time _{t, -log (X t / X} 0 ) = T × K _c and found that a linear proportional relationship holds. The coefficient K _c is
This is the slope of the straight line shown in FIG. 2 and is an apparent speed constant.

Experiments were conducted by changing the conditions such as slag composition, atmosphere or atmosphere such as vacuum, and the reaction between the added Y and the molten metal and slag was investigated. As a result, [Y] was changed to MgO in slag.
Oxidation by M
It was inferred that the oxidation reaction was limited by g (g) bubbles. The oxidation of [Y] by MgO in the slag is presumed to be due to the following equation. 2 [Y] +3 (MgO) = (Y ₂ O ₃ ) +3 [Mg]
Or 2 [Y] +3 (MgO) = (Y ₂ O ₃ ) + 3Mg (g) Based on the above findings, the actual VOD operation was analyzed. Y
As shown in FIG. 3 showing the analysis results in relation to the stirring elapsed time t from the addition, the VOD process in which the molten metal was stirred by Ar injection and the state in which the stirring was not performed from the VOD outlet to the casting. A great difference was observed in the rate of decrease of [Y]. That is, in the VOD step in which the molten metal is stirred by Ar blowing, the reaction rate is high, and the tendency due to oxidation of [Y] appears strongly. On the other hand, in the non-stirring process from the VOD outlet to the casting, the decreasing rate of [Y] is very small.

Further, assuming that the process of adding Y from the molten metal is caused by the primary reaction, the respective experimental data were arranged in relation to the elapsed stirring time t and −log (X _t / X ₀ ). The relationship shown in FIG. From FIG.
In the stirring step to OD Denabe, it is seen that the concentration X _t of Y remaining from the initial doping concentration X ₀ and stirred elapsed the time t by the following relationship in the steel are determined. X _t = X ₀ × 10 ^{(−Kc · t) In} the stainless steel melt of the component system targeted by the present invention, the rate constant K _c is in the range of 0.05 to 0.1,
It can be determined in advance for each stainless steel melt to be melted. Therefore, after adding a predetermined amount X ₀ of Y,
The target concentration X is simply adjusted by adjusting the elapsed stirring time t.
Molten steel containing _t of Y can be produced. [Y] after stirring for a predetermined time varies depending on the amount of Y added. However, the relationship between the stirring time elapsed t and -log (X _t / X _0), was substantially constant even initial concentration X ₀ and Y is changed. For example, the change over time of [Y] when the initial density X ₀ is changed is shown in FIG.
Shown in From the results of FIG. 5, regardless of the initial concentration X _0, X _t
= X ₀ × 10 ^{(-Kc · t)} .

[0009] for the other rare earth metals or alkaline earth metals, the initial doping concentration X ₀ and remaining in the steel from stirring the elapsed time t element concentration X _t is estimated by the same method, stirring the elapsed time t liquid steel target concentration X _t is obtained by the adjustment. The stainless steel to which the smelting method of the present invention is applied preferably contains a predetermined content of an alloying element. Hereinafter, each alloy element and its content will be described. C: 0.03% by weight or less In high Al ferritic stainless steel, it is a harmful element that adversely affects 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: less than 0.25% by weight 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: less than 0.25% by weight An alloy element effective for improving hot workability, but having 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 to deteriorate 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 steel. 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 of 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.

Rare earth metal, Y and alkaline earth metal 1
Species or two or more: 0.01 to 0.2% by weight These alloy elements remarkably improve the protective action 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: 3% by weight or less

In the present invention, although it is an optional element that is added as necessary, the addition of Mo significantly improves the high-temperature oxidation resistance and the high-temperature strength. However, excessive addition deteriorates the toughness of the steel and causes the productivity to be deteriorated. Further, the high Al ferritic stainless steel according to the present invention can be improved in toughness by adding Nb, V, Ti or the like for fixing C and / or N. For applications exposed to severe high-temperature atmospheres, Nb, V, T
It is effective to improve the high-temperature strength by adding 0.05% by weight or more of i or the like.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Stainless steel melt 5 having the composition shown in Table 1
6 tons were melted. During melting, VOD is released and 0.1
108 kg of Y, corresponding to 9% by weight, were added. After the addition of Y, the VOD was closed again, Ar gas was blown into the molten metal for 3 minutes, and the molten metal was stirred by gas bubbling. After stirring,
The molten metal was sent to a casting process and cast into a slab. Table 1 also shows the results of component analysis of the obtained slab. At this time, the rate constant K _c is 0.092 / min, and X _t = X ₀ ×
The Y concentration [Y] in the steel calculated according to 10 ^{(−Kc · t)} was 0.10% by weight. This calculated value is the same value as the actually measured value shown in Table 1, and it has been confirmed that the Y content can be adjusted with high component-specific accuracy. Thereafter, by casting, Y decreased by 0.03% by weight, and Y in the product became 0.07% by weight.

[0015]

[Table 1]

[0016]

As described above, in the smelting method of the present invention, the content of rare earth metal, Y, alkaline earth metal and the like contained in steel is adjusted by the duration of stirring. The component adjustment based on the stirring duration is based on the premise that the added amount of rare earth metal, Y, alkaline earth metal, etc. is consumed and reduced by the oxidation reaction, and the Y concentration in the steel in the molten metal stirring stage in the VOD process. Utilizes the fact that is highly proportional to the duration of stirring. Further, since the concentration in steel can be estimated from the initial addition amount and the stirring duration, it is possible to stably obtain a steel material having a target composition at a high yield. In addition, since the target component can be reached only by adjusting the duration of stirring, a high-Al ferritic stainless steel excellent in high-temperature oxidation resistance can be stably manufactured by a very simple operation.

[Brief description of the drawings]

FIG. 1 is a graph showing a time-dependent change in Y concentration in steel in a lab test.

Figure 2 is a graph which organizes the stirring duration results assuming primary reaction of lab tests and -log (X _t / X ₀₎

FIG. 3 is a graph showing the change over time in the Y concentration in steel when applied to an actual VOD operation.

FIG. 4 is a graph in which the results of actual VOD operation are summarized assuming the primary reaction and the duration of stirring and −log (X _t / X ₀ ).

FIG. 5 is a graph showing the effect of the amount of Y added on the relationship between the duration of stirring and the Y concentration in steel.

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C22C 33/04

Claims (3)

(57) [Claims] 1. A ferritic stainless steel containing 3 to 6% by weight of Al and a total of 0.01 to 0.2% by weight of one or more of rare earth metals, Y and alkaline earth metals.
When including the rare earth metal, Y
And adding one or more alkaline earth metals, that the melt is stirred by gas bubbling stops stirred at time t defined by the following formula (1) according to the target content X _t Melting method for high Al ferritic stainless steel. t = log (X ₀ / X _t ) / K _c (1) “However, after adding one or more of rare earth metal, Y and alkaline earth metal, stirring of the molten metal is performed. Continuous time (minutes) X ₀ : Initial concentration of rare earth metal, Y and alkaline earth metal immediately after addition (mol%) X _t : Rare earth metal at time t after the addition,
Y and concentration of alkaline earth element (mol%) K _c : apparent rate constant (/ min)
" 2. The ferritic stainless steel according to claim 1, wherein 3 to 6% by weight of Al and 0.01 to 0.2% by weight.
In addition to one or more of rare earth metals, Y and alkaline earth metals, C: 0.03% by weight or less, Si: 0.25%
% By weight, Mn: less than 0.25% by weight, P: 0.03
Wt% or less, S: 0.001 wt% or less, N: 0.03
% Of Cr and 15 to 30% by weight of Mo and optionally 3% by weight or less of Mo.
Melting method of ferritic stainless steel. 3. A rare earth metal, previously obtained the apparent rate constant K _c for stainless steel melt content is definite alloy elements excluding Y and alkaline earth metals, 0.05
The method melting the high Al ferritic stainless steel to determine the assignment to stir duration t in equation (1) of claim 1, wherein the rate constant K _c in the range of 0.1 / min.