KR19990051469A - Manufacturing method of stainless steel with improved cleanliness by suppressing titanium nitride production - Google Patents

Abstract

The present invention relates to a method for producing stainless steel having excellent cleanliness by suppressing the production of titanium nitride (TiN) when titanium is added during refining of stainless steel.

In the stainless steel production method of improving cleanliness by suppressing the production of titanium nitride of the present invention, the silicon (Si) concentration at the end of refining in the AOD refining furnace is 0.1% or less and the nitrogen (N) concentration is 0.003 to 0.006% {( C + N) ≤ 160ppm} to step out; to control the titanium (Ti) 0.15 ~ 0.25% in ladle refining; It is a technical idea to control the production of titanium nitride in molten steel by controlling the final nitrogen concentration of 0.0010% or less immediately before solidification during continuous casting.

Description

Manufacturing method of stainless steel with improved cleanliness by suppressing titanium nitride production

The present invention relates to a method for producing stainless steel having excellent cleanliness by suppressing the production of titanium nitride (TiN) when titanium is added during refining of stainless steel.

In general, in the stainless steel used for automobile exhaust pipe, washing machine, etc., titanium is added in the range of 0.1% to 0.5% in molten steel after decarburization in a refining furnace to obtain material properties having excellent high temperature oxidation resistance and corrosion resistance.

Titanium nitride produced by the addition of titanium in the steelmaking process reduces the cleanliness of molten steel as an impurity and grows on the inner wall of the nozzle during continuous casting to reduce the nozzle inner diameter during casting, thereby lowering the continuous casting ratio due to the interruption of casting, thereby decreasing productivity.

In addition, they are not only an important cause of slab (Billet) surface defects during casting, but also the inclusion of inclusion defects having a length of 10 to 1000 mm on the coil surface during hot and cold rolling. This is a direct cause of deterioration in productivity as well as product quality.

Therefore, in order to solve this problem, the focus has been on the development of a technology to suppress the production of titanium nitride by lowering the nitrogen concentration equilibrium with titanium by refining in an Argon Oxygen Decarburization (AOD) refining furnace or a vacuum refining furnace.

In other words, the Nagayama report reported by Aichi Steel Co. in CAMP-ISIJ (Vol.7, 1994,1115) -166 in Japan shows that the furnace-AOD refining furnace-ladle gas vacuum treatment was performed as a stainless steel refining process. Degassing) controls nitrogen to about 500 ppm.

In addition, according to Japan's CAMP-ISIJ-165, in order to increase the denitrification efficiency of high chromium (Cr), aluminum is deoxidized using vacuum pressure in a vacuum decarburization furnace (VOD) and the internal decarburization is promoted to obtain low nitrogen levels. I'm reporting how.

In this way, the denitrification reaction occurring using the vacuum decarburization furnace is shown in Equation (1, 2). Where P _N2 is the partial pressure of nitrogen, a _N is the activity of nitrogen, and K is the equilibrium constant.

[Equation 1]

N ₂ (g) = 1/2 N (ℓ)

[Equation 2]

log K = a _N / P _N2 =-5.18 / T-1.063

ΔG。 = 9916 + 20.17 T

As can be seen from Equation (2), it can be seen that lowering the partial pressure of nitrogen at a constant temperature can lower the nitrogen level in the molten steel.

However, when the titanium-containing ferritic stainless steel refining is performed without an vacuum refining process (VOD or LVD), it is produced in an electric furnace → AOD refining furnace → continuous casting process under atmospheric pressure, that is, in a process of decarburizing and deoxidizing under atmospheric pressure. There is a limit to manufacturing.

Because during the AOD process, nitrogen is drawn into the molten steel from the atmosphere while tapping nitrogen into the low nitrogen region (typically 100 ppm), it is very difficult to produce low nitrogen stainless steel.

Therefore, when titanium is added, nitride is easily generated, which reduces the cleanliness of the steel and causes various defects of the product.

FIG. 1 shows a typical example of a hot-rolled coil surface defect, that is, an inclusion line, during rolling due to titanium nitride mainly present in titanium-added stainless steel (SUS409) steel. The length of the defect is usually distributed in the range of 100 to 1000 mm so that all these coils must be surface polished to cause a huge problem that can be sold to the consumer.

FIG. 2 is an enlarged electron microscope photograph of the surface defect of FIG. 1. As can be seen from the photograph, it shows that a defect is generated due to hexagonal titanium nitride and spherical oxides at the defect site. Titanium nitrides generally have a hexagonal shape.

The steel was SUS409 steel produced through an electric furnace-AOD refining furnace-continuous casting process. At this time, the titanium component was 0.27% by weight, nitrogen was 0.012% by weight, and silicon was 0.5% by weight. The thermodynamic possibility of TiN formation of this steel is as follows.

In general, TiN generation can be expressed as in Equation (3), and when Equation (4) is summarized, it is as in Equation (6). Where K is the equilibrium constant, a _TiN is TiN activity 1, a _Ti is Ti Ti activity f _Ti [% Ti], a _N is N activity f _N [% N], and f _Ti is Ti activity The coefficient, f _N, is the N activity coefficient.

[Equation 3]

Ti + N = TiN

[Equation 4]

log K (= a _TiN / a _Ti a _N ) =-19800 / T + 7.78

[Equation 5]

logf _Ti = ∑e _i ^j [% j]

[Equation 6]

2.06 [% N] + log [% N] = 0.558 [% Ti] -log [% Ti] -19775 / T + 7.769

Therefore, using equation (6), considering the titanium component 0.27%, nitrogen 0.012%, silicon 0.5% and so on, the components in the stainless steel generated in FIG. 1 are calculated, and the theoretical nitrogen concentration equilibrating to 0.26% Ti is about In the case of the steel as 0.0008% (8ppm), since the nitrogen concentration is 0.012% (120ppm), which is much higher than the theoretical nitrogen concentration of 0.0008%, it is considered that nitride formation is easy and the cleanliness deteriorates, resulting in many surface defects.

Therefore, the AOD refining should be performed at a level lower than the nitrogen concentration equilibrating to a given Ti component, but it is limited to reduce the nitrogen concentration in the electric furnace → AOD refining furnace → continuous casting process to prevent various defects caused by nitride. It becomes a problem.

The present invention has been made to solve the problems of the prior art as described above, the production of titanium nitride by controlling the silicon concentration in the AOD refining furnace → AOD refining furnace → ladle refining → continuous casting process in the production of titanium-containing stainless steel The purpose of the present invention is to provide a method for manufacturing stainless steel with improved cleanliness.

1 is a photograph showing typical inclusion defects occurring on a hot rolled coil of titanium-containing stainless steel;

FIG. 2 is an enlarged view of the photograph of FIG. 1;

3 is a view showing the effect of silicon on the nitride distribution in the steel in the low nitrogen region,

4 is a view showing the effect of silicon on the nitride distribution in the steel in the high nitrogen region,

5 is a view showing the effect of the silicon amount on the steel cleanliness in the low nitrogen region.

In order to achieve the above object, the method of manufacturing stainless steel which improves the cleanliness by suppressing the production of titanium nitride of the present invention, the silicon (Si) concentration at the end of refining in the AOD refining furnace is 0.1% or less, the nitrogen (N) concentration is 0.003 It is adjusted to -0.006% {(C + N) ≤ 160ppm}; the titanium is controlled to 0.15 to 0.25% during ladle refining; In the continuous casting, the final nitrogen concentration immediately before solidification is controlled to 0.0010% or less to suppress the formation of titanium nitride in molten steel.

Hereinafter, the present invention will be described in detail with reference to the preferred embodiments.

Table 1 shows the nitride distribution ratio [{nitride number / total large inclusions (<10㎛) number} × 100] and the cleanliness results of the specimens according to the change of silicon, titanium, and nitrogen concentrations by dissolution test under atmospheric pressure as an example. Giving. The smaller the silicon concentration, the smaller the overall nitride distribution ratio, and the superior the cleanliness.

Figure 3 shows the nitride distribution in relation to the titanium concentration in the low nitrogen region, that is, nitrogen ranges from 0.0036 to 0.0081%, when titanium is 0.18% or less, almost no nitride is produced regardless of the silicon concentration. In the case of 0.2% or more, it can be seen that the nitride distribution ratio increases significantly as the silicon concentration increases.

As can be seen from FIG. 4, when the nitrogen region is 0.0082 to 0.015% in the high nitrogen region, that is, the nitride distribution ratio is somewhat correlated with the increase in the silicon concentration even in the titanium range of 0.13 to 0.18%. The relationship is shown, especially in the case of 0.5% or more, the distribution ratio of nitride tends to increase significantly.

That is, even if titanium is 0.24% level and nitrogen is somewhat high (range of 0.0082 ~ 0.015%), silicon is less than 0.1%, which means that the nitride distribution ratio in molten steel is low, so that steel having excellent cleanliness can be manufactured.

No. Ingredient (% by weight) Nitride distribution rate (%) cleanliness C Si Ti Cr N (ppm) One 0.01 0.01 0.15 11.6 36 0 0.01 2 0.015 0.12 0.14 11.7 50 0 0.02 3 0.012 0.53 0.17 11.5 77 0 0.02 4 0.013 0.01 0.21 11.9 38 0 0.01 5 0.011 0.13 0.22 11.5 68 0 0.08 6 0.013 0.55 0.24 11.5 47 20 0.04 7 0.01 0.01 0.36 11.3 52 5 0.12 8 0.012 0.13 0.32 11.5 80 10 0.12 9 0.014 0.48 0.4 11.5 75 30 0.22 10 0.013 0.01 0.13 11.0 110 0 0.02 11 0.01 0.09 0.15 11.1 88 0 0.03 12 0.01 0.49 0.15 11.5 120 10 0.06 13 0.015 0.01 0.19 11.7 130 10 0.03 14 0.014 0.12 0.2 11.2 130 10 0.06 15 0.017 0.5 0.22 11.8 98 60 0.03 16 0.011 0.01 0.35 10.7 92 40 0.12 17 0.013 0.12 0.34 11.5 96 80 0.24 18 0.015 0.54 0.38 12.0 150 90 0.28

5 shows that the cleanliness is superior to that of 0.5% when the silicon concentration is 0.1% or less.

This is because, in general, electric furnace → AOD refining furnace → ladle refining → continuous casting process, and when refining Ti-containing stainless steel, decarburization in AOD, denitrification refining and silicon deoxidation are followed.

In this case, the nitride equilibrium equation and the equilibrium relationship are the same as mentioned in the equations (3, 4, 6), but when the respective interaction coefficients in the equation (5) are substituted, Since e _i ^j (i = Ti, j = Si) = 2.1 is higher than other values, we can reconstruct it considering binomial.

[Equation 7]

2.06 [% N] + log [% N] = 0.558 [% Ti] -log [% Ti] -19775 / T + 7.769-2.1 [% Si]

It can be seen that the smaller the silicon, the higher the nitrogen concentration in equilibrium with titanium, so that the formation of nitride is difficult even if the nitrogen concentration in steel is rather high. For example, when Si is 0.1 and 0.5 at 1600 ° C, the equilibrium concentrations of titanium and nitrogen are shown in Table 2, respectively.

(weight%) Si Ti N Real condition example Whether to generate TiN 0.1 0.180.4 0.00720.0057 Ti; 0.18N; 0.007 Can't create 0.5 0.180.4 0.00100.0008 Ti; 0.18N; 0.012 Can be generated

In other words, when Si is 0.1% and 0.5%, N equilibrating to 0.18% of Ti is 0.0072% and 0.0010%, respectively. If silicon is 0.1%, the equilibrium nitrogen concentration is 0.0071% when the actual nitrogen is 0.0070%. Nitride is difficult to produce due to the higher equilibrium concentration, but silicon is 0.5%, and the actual nitrogen is 0.0070%, which means that the nitride can be easily generated because the nitrogen is much higher than the equilibrium nitrogen concentration of 0.0010%.

In general, the silicon concentration of titanium-containing stainless steel is generally reported to be in the range of 0.4 to 0.6%. Therefore, in the AOD process, the nitrogen concentration equilibrating to the same titanium concentration is low, and nitride is easily generated even in a region where nitrogen is relatively low and 0.0036 to 0.0081% or less, which causes poor cleanliness.

That is, as the silicon increases, the titanium nitride distribution ratio in the molten steel increases as the Ti concentration increases.

However, as can be seen from Table 1, even though titanium concentration is 0.2% or more and nitrogen is 0.0081% to 0.015%, the titanium concentration is less than 0.1 ± 0.05%. It shows great advantage.

As can be seen from FIG. 5, it can be seen that controlling the silicon component in the range of 0.1 ± 0.05% is effective for cleanliness.

Therefore, according to the present invention as described above, by controlling the concentration of silicon (Si) in the AOD refining furnace in the stainless steel manufacturing process to suppress the production of titanium nitride (TiN) to improve the cleanliness of the steel and the generation of defects due to inclusions in the product By preventing the above, useful effects such as high quality stainless steel with excellent cleanliness can be achieved.

Claims (1)

At the end of refining in the AOD refining furnace, the silicon (Si) concentration is adjusted to 0.1% or less, and the nitrogen (N) concentration is adjusted to 0.003 to 0.006% {(C + N) ≤ 160 ppm}; Is controlled to 0.15 to 0.25%; A method of manufacturing stainless steel with improved cleanliness by suppressing the production of titanium nitride in the molten steel by controlling the final nitrogen concentration immediately before solidification during continuous casting to 0.0010% or less.