KR19980027313A - Manufacturing method of type 321 stainless steel to ensure casting stability and to improve product surface quality - Google Patents

Abstract

The present invention provides a method for producing a type 321 stainless steel which effectively prevents the nozzle clogging problem, which is a problem in casting a typical Ti-added stainless steel, and suppresses the formation of TiN to secure casting stability and improve product surface quality. For the purpose of

To this end, in the present invention, the ratio of Ti / C in weight% in molten steel is 8 or more, satisfies the composition of C: 0.025-0.04%, Ti: 0.2% or more, and the concentration of nitrogen is tundish molten steel temperature T (° C). Nozzle clogging during casting by adjusting below the concentration calculated by the formula Log (N%) = -19755 / (T + 273) + 7.78 + 0.07 [% Ti]-log [% Ti] + 0.045 [% Cr] The present invention provides a method for producing Type 321 stainless steel, which prevents the formation of TiN and suppresses the formation of TiN to ensure casting stability and improve product surface quality.

Description

Manufacturing method of Type 321 stainless steel to ensure casting stability and to improve product surface quality

The present invention relates to a shroud nozzle between a ladle and a tundish and a dipping nozzle between a tundish mold by mixing Ti oxide inclusions and molten steel produced in the steel during continuous casting of Type 321 stainless steel containing Ti. This invention relates to a method for manufacturing Type 321 stainless steel which prevents clogging of the submerged entry nozzle and prevents nozzle clogging and improves product surface quality by appropriately adjusting components such as oxygen, titanium, carbon and nitrogen in molten steel.

In general, Ti added stainless steel is in the range of 0.1 to steel grade which requires high corrosion resistance - which is the most important element for ensuring the corrosion resistance of stainless written because secure the steel carbon in TiC form lecture by the addition of Ti up to 0.4% Cr the Cr ₂₃ to combine with the carbon It is added to prevent precipitation in the form of C ₈ . The Ti-containing stainless steel production process is produced by the general stainless steel production process: electric furnace-refinery-bubbling sheet-continuous casting process. In addition, it has to create molten metal to melt the scrap and ferro-alloy in an electric furnace in a refining using oxygen and argon gases to the molten metal, and the polishing of the tantalum, Tallinn and desulfurization, bubbling (Bubbling) chapter moderately temperature and the component in a continuous casting Adjust In particular, Ti addition of Ti addition steel is performed in a bubbling site. Molten steel with controlled temperature and composition produces slab-shaped products by continuous casting. Table 1 shows the composition of type 321 stainless steel.

However, a critical disadvantage in the addition of Ti is that Ti has a very high affinity with oxygen and nitrogen, forming Ti oxide (TiO ₂ ) and Ti nitride (TiN) in molten steel during the steelmaking process. The melting point is very high, causing nozzle clogging during casting and adversely affecting the quality of the product surface. Particularly, when the nozzle is clogged during casting, the shroud nozzle between the ladle and the tundish and the immersion nozzle between the tundish and the mold are simultaneously clogged and severely blocked, the molten steel is not supplied to the tundish and the mold so that the casting itself is impossible. It causes the most deadly situation that fails to produce.

In addition, even if the nozzle clogging is not so severe as to stop casting, casting is possible, but the molten steel supply from the immersion nozzle to the mold becomes unstable, causing the molten steel hot water level in the mold, which is most important for product surface quality, to become unstable. The material itself is cut off and incorporated into the molten steel, resulting in extremely poor surface quality of the product by large inclusions.

As such, clogging of the nozzle during casting has a great influence not only on the productivity side, but also on the surface quality of the product, and thus many research and reduction techniques have been developed. 1 (a) and (b) show the nozzle clogging of a typical Ti-added stainless steel, which shows the appearance of nozzle clogging due to severe clogging of the nozzle. B) is a diagram schematically showing a portion A of (a) in an enlarged manner. As shown in Figure 1 (b), it can be seen that the nozzle clogging cross-section is divided into four layers. That is, it consists of four layers of the original nozzle refractory molten layer 4, the layer 5 which directly caused the nozzle clogging, the layer 6 in which molten steel penetrated into the nozzle, and the dense Ti oxide layer 7. 6) is a mixture of molten steel and nozzle material, the penetration thickness is about 4㎜. The layer 7 is a dense Ti oxide layer, which has been investigated by various researchers, and has a thickness of about 1 mm in which Ti in molten steel is formed by reducing SiO ₂ in the refractory material by the following Reaction (1).

SiO ₂ (nozzle refractory) + Ti (in molten steel) = TiO ₂ + Si ------------ (1)

The layer 5 is a mixture of numerous TiO ₂ particles 8 and molten steel 9 having a size of 10-20 μm and a thickness of 10-20 mm. In the figure, the flow direction of molten steel during casting is indicated by (10). It can be seen that the nozzle clogging was mainly caused by Ti oxide (TiO ₂ ) in the steel.

The conventional method for preventing such a nozzle clogging is, firstly, a method of reducing Ti. That is, the theory is that reducing Ti will reduce the amount of Ti oxide and Ti nitride produced. However, in this case, there is a burden of simultaneously reducing carbon (C). That is, as shown in Table 1, the Ti-added stainless steel is based on the design of adding more than a certain ratio of Ti in accordance with the content of C to ensure corrosion resistance. Usually, the ratio (Ti / C) is about 7-10.

In 321 stainless steel, which is the object of the present invention, Ti / C> 8 is the design basis. Therefore, in order to satisfy the same ratio, carbon should be lowered when Ti is small. Lowering the carbon increases the amount of oxygen blown for decarburization in the refining furnace, which increases the amount of dissolved oxygen in the molten steel. In addition, as the refining time increases, the molten steel temperature rises, and a considerable amount of coolant is added to lower the temperature after the tapping, and the molten steel comes into contact with the atmosphere during the coolant input, and there is a possibility of generating a large amount of inclusions by reoxidation.

The second method is to remove dissolved oxygen by adding a large amount of Al, which has affinity with oxygen than Ti, before the addition of Ti to reduce Ti oxide. While this method rarely forms Ti oxide, a large amount of Al deoxidation product Al ₂ O ₃ is formed on the surface quality which is more harmful than Ti oxide, so in order to make this Al ₂ O ₃ a less harmful CaO-Al ₂ O ₃ . Ca must be added to molten steel. However, in the case of Ca treatment, due to the strong vaporization of Ca itself, it is very difficult to accurately control the concentration. If too little or too much is added, the desired inclusion shape is not obtained, and the surface defects are increased. Reaction of molten steel between molds reacts with Al ₂ O ₃ of the stopper refractory to form CaO-Al ₂ O ₃ , which is a low melting point compound, causing severe stoppage of the stopper. Therefore, molten steel injection control becomes unstable, and the product quality is greatly deteriorated.

SUMMARY OF THE INVENTION The present invention has been made to solve the problems of the prior art described above, and is a type 321 stainless steel that effectively prevents the clogging of the nozzle during continuous casting of stainless steel and suppresses the formation of TiN to ensure casting stability and improve product surface quality. It is an object to provide a manufacturing method.

In order to achieve the above object, the present invention, the ratio of Ti / C in molten steel to 8 or more, by weight%, satisfies the composition of C: 0.025-0.04%, Ti: 0.2% or more, tundish the concentration of nitrogen At the molten steel temperature T (℃), below the concentration calculated by the formula Log (N%) = -19755 / (T + 273) + 7.78 + 0.07 [% Ti]-log [% Ti] + 0.045 [% Cr] The present invention provides a method for producing Type 321 stainless steel that prevents nozzle clogging during casting and suppresses the formation of TiN to ensure casting stability and improve product surface quality.

FIG. 1A is a view showing a nozzle clogging form of Type 321 stainless steel, (B) is an enlarged view of a nozzle clogging layer indicated by A in (A), and

FIG. 2 (A) shows a contact state between an Al ₂ O ₃ substrate and a type 321 stainless steel that simulates a non-metallic inclusion, and FIG. 2 (B) shows a contact state between a TiO ₂ substrate and a type 321 stainless steel.

3 is a graph showing the equilibrium relationship between titanium (Ti) and oxygen (O) in molten steel as a function of temperature in type 321 stainless steel;

Figure 4 (a) is a graph showing the relationship between the carbon content of molten steel in the manufacture of stainless steel and dissolved oxygen (Free Oxygen) just before tapping in the refining furnace, (b) is a graph showing the relationship between the carbon content and the molten steel temperature when tapping Degrees,

5 is a graph showing the equilibrium relationship between titanium (Ti) and nitrogen (N) in molten steel in type 321 stainless steel;

FIG. 6 is a graph showing a comparison of the lead ratio of 321 stainless steel in the method of the present invention and the prior art.

* Description of the symbols for the main parts of the drawings *

DESCRIPTION OF SYMBOLS 1: Nozzle refractory material 2: Nozzle blocking material 4: One nozzle refractory layer

5: Nozzle Clogging Layer 6: Nozzle Content Hard Penetration Layer 7: TiO ₂ Layer

8 TiO ₂ inclusion particles 9: solidified molten steel 10 molten steel flow direction

11 Al ₂ O ₃ substrate 12 TiO ₂ substrate 13 molten steel particles

14,15: contact angle

The invention will now be described in detail with reference to the accompanying drawings, in which preferred embodiments are shown.

Figure 2 (a), (b) is a result of measuring the contact state between the molten steel and the inclusions using a heating microscope (Heating Microscope) in order to find out why Ti oxide is mixed with the molten steel to easily cause nozzle clogging. Indicates. Figure 2 (a) shows the state in which the type 321 stainless steel 13 is melted and contacted on the plate 11 of Al ₂ O ₃ material, Figure 2 (b) is type 321 stainless on the substrate 12 of TiO ₂ material The state where the steel 13 melted and contacted is shown. As can be seen from the figure, in the case of Al ₂ O ₃ , the contact angle 14 is 135 °, which is larger than 100 °, which is the contact angle 15 of TiO ₂ . In other words, it can be seen that the contact with molten steel is larger than TiO ₂ than Al ₂ O ₃ (contact angle is 0 ° in case of full contact, and contact angle is 180 ° when completely non-contact). These results indicate that when the TiO ₂ inclusions are mixed with the molten steel, the mutual contact between them is very good, so that the molten steels between the inclusions do not move easily and are in contact with each other, so that the TiO ₂ inclusions + molten steel are present to prevent the nozzle clogging in FIG. 1. This is because the resulting layer 5 is formed. Thus, the most effective way to prevent nozzle clogging is to reduce TiO ₂ inclusions. The method of reducing TiO ₂ inclusions is to reduce the concentration of Ti or reduce the amount of free oxygen. Therefore, in the present invention, first, the deoxidation equilibrium relationship between Ti and oxygen in the following equation was thermodynamically investigated.

Ti + 2.0 = TiO ₂ ----------- (2)

3 shows the equilibrium relationship between Ti and oxygen as a function of temperature using thermodynamic data. That is, if oxygen is present above the equilibrium oxygen concentration determined at any Ti concentration and temperature, Ti will deoxidize until the equilibrium oxygen concentration is reached, resulting in deoxygenation inclusions of TiO ₂ , on the contrary, oxygen is below equilibrium concentration TiO ₂ inclusions will not be formed.

As shown in FIG. 3, the equilibrium oxygen concentration decreases with decreasing temperature at a constant Ti concentration. In addition, the effect of Ti concentration on the equilibrium oxygen concentration is sensitively decreased as Ti increases when Ti is less than 0.1%, but when Ti concentration is above 0.1%, the equilibrium oxygen concentration is hardly affected by Ti concentration. It can be seen that.

From these results, it can be seen that it is not meaningful to lower the Ti concentration in order to reduce the Ti oxide because the Ti range of type 321 stainless steel is 0.1-0.4%. On the contrary, if the carbon is reduced to reduce the Ti concentration, the load due to decarburization in the refining furnace increases, and the dissolved oxygen and molten steel temperature due to prolonged decarburization increase. Therefore, the inclusion of Ti oxide increases and the reoxidation of molten steel is increased by the use of a large amount of coolant, which is likely to reduce the cleanliness and the nozzle clogging.

In order to reduce the TiO ₂ , the inventors secured the Ti concentration to some extent to reduce the load on decarburization in the refining furnace, and to reduce the dissolved oxygen and molten steel temperature due to the increase of carbon (C) to prevent nozzle clogging. It was pointed out that the improvement of product quality could be satisfied at the same time. First, the present inventors investigated the effect of carbon in molten steel on dissolved oxygen and molten steel temperature for stainless steel to determine the optimum range of carbon concentration in molten steel.

Figure 4 (a) shows the effect of carbon in the molten steel on the dissolved oxygen shows that the dissolved oxygen increases almost linearly as the carbon in the molten steel decreases. Figure 4 (b) shows the effect of the concentration of carbon in the molten steel on the molten steel temperature (just before the tapping of the refining furnace), the overall temperature of the molten steel increases as the concentration of C decreases, especially when the carbon is less than 0.025% Shows a significant increase. That is, at the carbon concentration below this concentration, it is judged that simultaneously with the decarburization reaction, other components Cr, Mn, Si, etc. are simultaneously oxidized, and the molten steel temperature rises rapidly. From the above results, it can be seen that in order to reduce TiO ₂ inclusions in 321 stainless steel, it is advantageous to increase the carbon concentration in molten steel as much as possible. However, there is a limit to increasing the carbon concentration. In other words, if the carbon is high, the hardness of the final product is too high to be used as a product, and it is impossible to secure the minimum temperature necessary for steelmaking and performance after refining in the refining furnace. Therefore, for most 321 steels, the maximum amount of C is 0.04%. From the above results, the present inventors derived that the appropriate carbon content of 321 stainless steel is about 0.025-0.04%.

The temperature of Ti corresponding to the content of C is 0.2-0.32% since the Ti / C ratio should be 8 or higher due to the characteristics of the 321 product. However, when Ti is added in an amount greater than 0.2% of the Ti concentration when the carbon is 0.025% or less, a problem is TiN inclusions formed by the reaction of Ti and nitrogen in molten steel. Since TiN has a very high melting point around 2000 ° C., when TiN particles are formed in molten steel and aggregate and grow, the TiN particles are extremely detrimental to the surface quality of the product, although they do not adversely affect the nozzle clogging than TiO ₂ inclusions. Therefore, it is important to control TiN not to form in molten steel, that is to say, before tundish molten steel. Therefore, the present inventors have thermodynamically investigated the TiN formation reaction of Ti and nitrogen represented by the following formula.

Ti + N = TiN ---------- (3)

Equilibrium relations have been published in several academic sources. In the present invention, after reviewing and calculating these various data, the following equation that best matches the TiN formation result in actual 321 stainless steel was adopted.

log (N%) = -19755 / (T + 273) + 7.78 + 0.07 [% Ti]-log [% Ti] + 0.045 [% Cr] ---- (4)

(T: 。C)

FIG. 5 shows the equilibrium relationship between Ti and nitrogen as a function of temperature for 321 stainless steels using this equation. As can be seen from FIG. 5, unlike the relationship between Ti and oxygen in FIG. 2, the influence of Ti concentration on the equilibrium nitrogen concentration in the Ti concentration range (0.15-0.4%) of the 321 steel is slight. Therefore, it can be seen that reducing the Ti concentration is effective to reduce TiN. Therefore, in order to reduce the concentration of oxygen in the molten steel, carbon and Ti may be added to a certain concentration, but it can be seen that the Ti concentration must ensure that the nitrogen concentration that does not form TiN in the tundish molten steel. In the drawing, the table shows TiN clusters (multiple TiNs having a size of 100 μm or more) in the product. ○ The table shows that the molten steel exhibited good surface quality due to the absence of TiN clusters. The relationship between Ti and N in the tundish is shown. As can be seen from the figure, it can be seen that they are divided on the basis of the equilibrium line of Ti and nitrogen at 1500 ° C tundish temperature. That is, most of TiN clusters exist in the composition of the region 16 that exists above the 1500 ° C equilibrium line, and there is no TiN cluster in the composition that exists in the region 17 below the equilibrium line. It is interpreted that when TiN is formed after tundish, they do not have a chance to grow coherently with each other, so that TiN clusters are not formed and consequently, the product surface quality is also good. Therefore, it can be seen that the concentration of nitrogen should be controlled below the nitrogen concentration calculated from Eq. (4) at the tundish molten steel temperature.

The present inventors verified the effects on the present invention described above through the examples.

Hereinafter, the present invention will be described with reference to the examples.

Example 1

Table 2 compares the manufacturing method of 321 stainless steel by the conventional method and the composition of molten steel and the degree of nozzle clogging and the number of TiN clusters in the state of cast steel. Here, the degree of clogging of the nozzle is obtained by measuring the length of the nozzle clogging layer 5 shown in FIG. 1 (b) after casting. Using an optical microscope, the number of TiN clusters having a size of 100 μm or more was divided by the area of the specimen. As can be seen from Table 2, Comparative materials 1 and 2 lowered Ti and nitrogen so that TiN clusters were not found at all, but the oxygen was high at the time of tapping, and as a result, a large amount of inclusions in the deoxidation product of TiO ₂ was generated, resulting in nozzle clogging during casting. The result was a severe shutdown in the middle of the continuous casting. In addition, the comparative material 3 is a case where the concentration of C and Ti satisfies the scope of the present invention, but the nitrogen concentration is excessively high to generate a myriad of TiN clusters on the cast steel, and a significant amount of nozzle clogging by TiN occurs. On the other hand, the inventive materials 1, 2, 3 produced by the method of the present invention showed good nozzle clogging degree of 10 mm or less, and TiN clusters were also present but not a problem.

Example 2

FIG. 6 is a view showing an increase in the lead ratio by applying the operating method of the present invention to an actual process to prevent nozzle clogging. Here, the lead cast ratio refers to the number of hits (Heat) continuously cast after the start of continuous casting, one heat (heat) in the present invention means a molten steel contained in one ladle and usually one heat is 90 tons of molten steel It contains. As can be seen from the figure, the conventional method shows a soft fuel ratio of about 1.3 due to clogging of the nozzle, while the soft fuel ratio is 3.0, which is twice or more, as a result of applying the operation method according to the present invention, which greatly improves the productivity. Results are shown.

According to the method for producing type 321 stainless steel which ensures the casting stability of the present invention as described above and has good product surface quality, the ratio and composition of carbon (C) and titanium (Ti) in molten steel are properly adjusted and the nitrogen concentration is in a predetermined range. The following adjustment prevents blockage during casting of the shroud nozzle between the ladle and the tundish and the submerged entry nozzle between the tundish mold and prevents the formation of TiN to secure casting stability and to ensure the product surface. An effect of improving quality is obtained, and as a result, an effect of improving productivity is achieved.

Claims (1)

A method for producing Type 321 stainless steel, in which the weight ratio of molten steel is set to 8 or more, the ratio of Ti / C is 8 or more, the composition of C: 0.025-0.04%, Ti: 0.2% or more, and the concentration of nitrogen are tundished. At the molten steel temperature T (℃), below the concentration calculated by the formula Log (N%) = -19755 / (T + 273) + 7.78 + 0.07 [% Ti]-log [% Ti] + 0.045 [% Cr] A method for producing Type 321 stainless steel which prevents nozzle clogging during casting and suppresses the formation of TiN, thereby ensuring casting stability and improving product surface quality by controlling.