KR100595874B1 - Method for continuously casting of 409 statinless steel - Google Patents

Abstract

The present invention relates to a continuous casting method of 409 stainless steel, the continuous casting method is nitrogen concentration of molten steel, [% N] atT = Tmold <[% N] <[% N] atT = Ttundish, log [% N ] = -19755 / (T + 273) + 7.78 + 0.07 [% Ti] -log [% Ti] + 0.045 [% Cr], where [%] is the concentration of element in molten steel (wt.%) ), T is the molten steel temperature (° C.), Tmold is the molten steel temperature (° C.) in the mold during casting, and Ttundish is the molten steel temperature (° C.) in the tundish during casting. By appropriately adjusting the titanium, nitrogen, total oxygen content and molten steel temperature, it is possible to secure sufficient equiaxed crystallization rate without lowering molten steel temperature and casting speed and without using expensive EMS.

Equiaxed crystal, titanium nitride, titanium oxide, crystal nucleus, surface defect

Description

Method for continuously casting of 409 stainless steel             

1 is a view schematically showing a solidification structure in the thickness direction of a typical playing cast.

Figure 2 is a graph showing the relationship between the appropriate Ti concentration, N concentration and temperature for securing an equiaxed crystal using titanium nitride.

Figure 3 is a graph showing the equiaxed rate of the cast slab according to the AOD and VOD process.

4 is a diagram showing a mechanism for forming titanium nitride in steel;

Figure 5 is a graph showing the product defect rate by inclusion of the cast iron isometric axis and titanium oxide according to the total amount of oxygen in the steel.

Figure 6 is a graph showing the equiaxed crystal index and surface defect index according to the conventional embodiment and the present invention.

<Description of Symbols for Major Parts of Drawings>

1: quenching tissue formation zone

2: columnar solidification tissue formation area

3: equiaxed crystal formation zone

The present invention relates to a continuous casting method of 409 stainless steel, in particular, 409 stainless steel having a solidified structure by increasing the equiaxed crystallization rate using titanium (Ti) oxide and titanium nitride (TiN) without excessive low-temperature casting and low-speed casting It relates to a continuous casting method.

In general, 409 stainless steel is a steel grade containing a large amount of titanium, as shown in Table 1. Titanium fixes carbon (C) in steel in the form of titanium carbide (TiC), and prevents the formation of chromium carbide (Cr ₂₃ C ₆ ) by reacting carbon and chromium in steel, so 409 stainless steel requires good corrosion resistance. Widely used in automobile exhaust pipes.

Chemical composition of 409 stainless steel (wt.%) C Si Mn Cr Ni N Ti 0.02 0.5 0.3 11.3 〈0.3 0.05 to 0.02 0.2 to 0.3

Such stainless steel is produced as slabs by continuous casting after melting scrap in an electric furnace, performing tantalum and deoxidation in steel in argon oxygen decarburization (AOD) or vacuum oxygen decarburization (VOD). The solidification structure in the thickness direction of the cast steel produced by such a continuous casting process is as shown in FIG.

That is, as shown in Figure 1, the surface in contact with the cold mold is formed a chill zone (chill zone 1), which is a fine quench solidification structure of a thin thickness. Since the cooling speed decreases as the depth deepens from the cooling stage, the columnar top region 2 having a dendrite structure is formed, and an equiaxed crystal region 3, which is a fine solidification structure, is formed at the center of the slab. .

If there are too many columnar tablets in the coagulation structure, segregation is encouraged inside the slab, which is left as a surface defect during processing. Therefore, it is required to reduce columnar heads and to increase equiaxed heads.

Normally, the degree of equiaxed crystal is represented by an equiaxed crystal ratio expressed as a percentage of the length having the equiaxed crystal divided by the total slab thickness. On the other hand, in order to increase the equiaxed crystallinity, an EMS (electro magnetic stirring) method of lowering the molten steel temperature to be cast or decreasing the casting speed and increasing the equiaxed crystal by giving an artificial flow to the molten steel with electromagnetic force is recently used.

However, in the case of 409 stainless steel, when the casting steel temperature is lowered, titanium and nitrogen in the steel react to form titanium nitride, and these titanium nitride particles contained in the steel are coalesced with each other in the pre-mold process to form a steel inclusion. Form large titanium nitride clusters. Such inclusions, in later processing, cause fatal surface defects.

In addition, when the casting temperature is low, since 409 stainless steel is a steel grade having a very small liquid-liquid coexistence temperature range, molten steel easily solidifies, causing nozzle clogging, and thus the continuous casting process is stopped. Reducing the casting speed not only decreases the productivity of 409 stainless steel, but also causes nozzle clogging.

In addition, the EMS method increases the production cost of 409 stainless steel because the device itself is very expensive and the operation cost is expensive.

In order to solve the conventional problems as described above, the present invention can improve the productivity of 409 stainless steel as well as prevent the increase in production cost by improving the equiaxed crystal in the cast steel of 409 stainless steel without lowering the molten steel temperature and casting speed The purpose is to provide a continuous casting method.

According to the present invention, in order to achieve the above object, the continuous casting method of 409 stainless steel is the nitrogen concentration of molten steel is formula, [% N] atT = Tmold <[% N] <[% N] atT = Ttundish, log [% N] = -19755 / (T + 273) + 7.78 + 0.07 [% Ti] -log [% Ti] + 0.045 [% Cr], where [%] is the concentration of element in molten steel (wt. %), T is the molten steel temperature (° C.), Tmold is the molten steel temperature (° C.) in the mold during casting, and Ttundish is the molten steel temperature (° C.) in the tundish during casting.

Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

According to the present invention, since titanium nitride acts as a cause of inclusions in the steel and at the same time a strong ferrite forming element, when titanium nitride is uniformly distributed in steel, titanium nitride does not form a cluster and surface defects occur. It can prevent. In addition, since titanium nitride acts as an effective non-uniform coagulation crystal nucleus of 409 stainless steel, equiaxed crystals may be secured by crystal nuclei uniformly distributed in steel.

Therefore, in the present invention, it is important to maintain the production amount of titanium nitride properly, and in order to control the production of such titanium nitride, it is possible to derive the concentrations of titanium and nitrogen and the molten steel temperature conditions, using actual operating performance data. In the graph of FIG. 2, three regions are divided and displayed.

The distinction between these areas is based on the equilibrium relationship between titanium and nitrogen at 1540 ° C, which is the tungsten molten steel temperature of 409 stainless steel, and 1510 ° C, which is the molten steel in the mold. Almost identical.

That is, in the graph of FIG. 2, the region indicated by reference numeral 4 has a very high concentration of titanium and nitrogen, and thus, titanium nitride is formed at a high temperature, and the fine titanium nitride particles formed in the pre-tundish process are attached to each other to form a titanium nitride cluster. Since it forms in the inside, it is an area | region where a large amount of surface defects of a slab generate | occur | produce.

On the other hand, the region indicated by reference numeral 6 is a region in which titanium nitride is very low and titanium nitride is not formed, but the equiaxed crystallization rate of the cast steel is very small, 30% or less. This is because titanium nitride, which contributes to the formation of equiaxed crystals, does not exist in molten steel just before solidification.

In addition, the area indicated by the reference number 5 is not only the surface defects caused by the cluster of titanium nitride, but also the region where the equiaxed crystallization rate of the cast steel is 50% or more. The presence of this region means that the crystallization temperature at which titanium nitride is crystallized from molten steel is very important. In other words, if the crystallization temperature of titanium nitride is maintained between the molten steel temperature in the tundish and the molten steel temperature in the mold, surface defects by preventing cluster formation of titanium nitride are prevented. Immediately before the molten steel solidifies, the titanium nitride is uniformly distributed in the molten steel and acts as a crystal nucleus for promoting solidification of the 409 stainless steel, thereby forming a solidified structure as an equiaxed crystal.

These conditions were applied in the production of 409 stainless steel. However, it can be seen that the equiaxed crystallinity of cast steel produced at the same composition and casting temperature varies greatly depending on the process. That is, as shown in Figure 3, it can be seen that the equiaxed crystallization rate of the cast through the AOD and the equiaxed crystallization rate through the VOD is significantly different.

This difference can be understood by explaining the formation mechanism of titanium nitride unit particles in steel, as shown in FIG.

That is, the titanium nitride does not exist as a pure titanium nitride in the steel, it can be seen that the fine titanium oxide (TiO ₂ ) acting as a crystal nucleus is formed of a titanium nitride present inside.

Therefore, the reason why the equiaxed crystallization rate in the cast steel through VOD is small is that because the oxygen concentration in the molten steel is smaller than that of the AOD treated cast steel due to the characteristics of the VOD treatment, sufficient crystal nucleus, that is, titanium oxide is not formed.

On the other hand, it is understood that titanium nitride is required to secure sufficient equiaxed crystallization, and that titanium oxide, which serves as a crystal nucleus for forming titanium nitride, is required to form titanium nitride effectively.

However, if the oxygen in the molten steel is too large, a large amount of titanium oxide is formed, these oxides are coalesced with each other to form large inclusions, which causes nozzle clogging during the casting process.

Therefore, as in the case of titanium nitride, it is important that the titanium oxide is uniformly distributed in the steel without growing into the inclusions in the steel so that the crystal nuclei of the titanium nitride formation can be derived. It is the concentration of oxygen in the molten steel that controls the formation of such titanium oxide inclusions.

That is, since the Ti concentration in the steel is constant, the variable controlling the formation of titanium oxide in a process is oxygen in molten steel.

Therefore, in the present invention, the experiment was performed while changing the concentration of oxygen in order to derive the optimum oxygen concentration.

After the experiment, the defect index by the titanium oxide cluster and the equiaxed crystallinity of the solidified cast steel were investigated. The results are shown in FIG.

That is, as can be seen in Figure 5, the equiaxed crystallinity increases linearly as the total amount of oxygen increases, but the tendency to increase when the total amount of oxygen becomes 40 ppm or more is slowed.

On the other hand, the defect index by the cluster of titanium oxide according to the total amount of oxygen increases little by little to about 50 ppm, but when the total amount of oxygen is 50 ppm or more, the defect index tends to increase rapidly. Therefore, it can be seen that an appropriate total oxygen concentration capable of securing an equiaxed crystallization rate without generating defects due to titanium oxide clusters is 40 ppm to 50 ppm, which is an area indicated by reference numeral 7.

Hereinafter, an embodiment of the present invention will be described.

<Example 1>

Table 2 below compares the cast quality and equiaxed crystal in case of deviating from the method of the present invention in the results of applying the embodiment of the present invention to the actual production of 409 stainless steel and the conventional embodiment.

Comparative material Invention One 2 3 One Cr% 11.3 11.3 11.3 11.3 TI% 0.2 0.2 0.2 0.2 N% 140 55 85 90 Total oxygen (ppm) 40 45 60 43   Cast quality Titanium oxide cluster Not Occurred Not Occurred Occur Not Occurred Titanium nitride cluster Occur Not Occurred Not Occurred Not Occurred Isometric Rate (%) 90 30 60 70

(T _mold : 1500 ℃, T _tundish : 1540 ℃)

In this case, the Cr and Ti concentrations of the steel are constant.

First, according to the embodiment of the present invention, the nitrogen concentration [% N] of the molten steel during continuous casting is adjusted to the concentration range calculated by the following equation.

[% N] atT = Tmold 〈[% N] 〈[% N] atT = Ttundish

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

Where [%] is the concentration of the element in the molten steel (wt.%), T is the molten steel temperature (° C), Tmold is the molten steel temperature (° C) in the mold during casting, and Ttundish is the molten steel temperature (° C) in the tundish during casting )to be.

First, the appropriate nitrogen concentration calculated from the molten steel temperature in the mold and tundish in the formulas and examples shown in the present invention is 72-127 ppm.

Comparative material 1 is a case where a large amount of titanium nitride cluster is formed on the cast steel because the concentration of nitrogen is too high, Comparative material 2 is a cluster of titanium nitride and titanium oxide was not generated because the concentration of nitrogen is lower than the range of the present invention, but the equiaxed crystallinity This is not the case.

In Comparative Material 3, the equiaxed crystallization rate was secured, but the total amount of oxygen was too high to generate a large amount of clusters of titanium oxide.

On the other hand, in the case of the present invention, the concentrations of nitrogen and total oxygen content both satisfy the scope of the present invention, and show a good result that 70% equiaxed crystallinity is secured without generating clusters of titanium nitride and titanium oxide.

Figure 6 compares the average value of the defect index and equiaxed rate of the surface of the cast steel as a result of applying this method to the front. As a result of applying the present invention, the defects on the surface of the cast steel show the same level as in the case of the prior art, while the slab equiaxed crystal shows a greatly improved result.

Therefore, according to the present invention, in casting of 409 stainless steel, it is possible to secure a sufficient equiaxed crystallization rate without lowering the molten steel temperature and casting speed by using titanium, nitrogen, total oxygen amount and molten steel temperature appropriately and without using expensive EMS. have.

The foregoing merely illustrates preferred embodiments of the invention, and those skilled in the art to which the invention pertains may make modifications and changes to the invention without departing from the spirit and scope of the invention as set forth in the appended claims. Can be.                     

Claims (2)

In the continuous casting method of 409 stainless steel, Nitrogen concentration of molten steel is [% N] atT = Tmold 〈[% N] 〈[% N] atT = Ttundish log [% N] = -19755 / (T + 273) + 7.78 + 0.07 [% Ti] -log [% Ti] + 0.045 [% Cr], Where [%] is the concentration of the element in the molten steel (wt.%), T is the molten steel temperature (° C), Tmold is the molten steel temperature (° C) in the mold during casting, and Ttundish is the molten steel temperature (° C) in the tundish during casting Continuous casting method characterized in that). The continuous casting method according to claim 1, wherein the total oxygen concentration of the cast steel of the continuous casting is adjusted to 40 ppm to 50 ppm.

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