KR101159927B1 - Method for evaluating Primary Solidified Layer in continuous casting slab - Google Patents

Abstract

The present invention relates to an initial solidification layer appearance method in a mold of a continuous casting slab. The present invention includes an etching step of immersing a slab sample in an etching solution consisting of picric acid, sodium alkylaryl sulfonate, and residual distilled water; A washing step of washing the slab sample with alcohol after etching is finished; And a measuring step of measuring the tissue of the washed slab sample with an optical microscope.

According to the present invention, the initial solidification layer of the slab may be exhibited according to the carbon content of the slab, and thus, the histological analysis of the cause of the slab surface defects may be possible through the initial solidification layer analysis.

Initial solidification layer, etching

Description

Method for evaluating primary solidified layer in continuous casting slab

The present invention relates to an initial solidification layer manifestation method of a continuous casting slab in a mold, and more particularly, to an initial solidification layer manifestation method of a continuous casting slab which can infer the occurrence of surface defects of the slab.

In the steelmaking process, liquid molten steel treated at the desired composition and temperature is cooled while passing through a continuous casting facility and solidified into a solid slab.

In a continuous casting facility, molten steel first forms a solidification layer (coagulation shell) while passing through a water cooling mold, and then passes through a strand to completely solidify through spraying of coolant to become a semi-finished solid slab. And the slab is manufactured in the form of a steel sheet through a rolling process.

Since the molten steel which forms the solidification layer in the mold starts to solidify from the mold wall surface, it is important to determine the state of the initial solidification layer to determine the quality of the steel.

This is because most surface defects are caused by the state of the initial solidification layer, and the initial solidification layer formed on the mold wall is formed by rapidly solidifying the molten steel by contact with the mold wall surface which is cooled by water cooling. This is because they show completely different histological patterns.

For the reasons mentioned above, identification of the initial solidification layer in the mold of the slab is very important for future defect inference or study of solidification tendency.

SUMMARY OF THE INVENTION An object of the present invention is to provide an initial solidification layer appearance method in a mold of a continuous casting slab that can clearly distinguish between a portion initially rapidly solidified in a mold and a tissue produced in a subsequent solidification process.

According to a feature of the present invention for achieving the object as described above, the present invention comprises an etching step of immersing the slab sample in an etching solution consisting of picric acid, sodium alkylaryl sulfonate and distilled water; A washing step of washing the slab sample with alcohol after etching is finished; And a measuring step of measuring the tissue of the washed slab sample with an optical microscope.

The weight% of picric acid is 0.5 to 1.5 wt%, the weight% of sodium alkylaryl sulfonate is 0.05 to 0.15 wt%, and the balance is distilled water.

The said etching liquid is 40-60 degreeC in temperature.

If the carbon content in the slab is less than 0.03wt%, the etching time of the etching step is 10-15 minutes.

When the carbon content in the slab is 0.03 wt% or more and less than 0.08 wt%, the etching time of the etching step is 4-6 minutes.

If the carbon content in the slab is 0.08 wt% or more but less than 0.20 wt%, the etching time of the etching step is 2 to 3 minutes.

When the carbon content in the slab is 0.20 wt% or more, the etching time of the etching step is 1 to 2 minutes.

In the present invention, the development of the etching method enables the initial appearance of the slab solidified layer according to the carbon content of the slab, thus enabling the histological analysis of the cause of the surface defects of the slab through the initial solidified layer structure analysis.

Therefore, there is an effect that is expected to bring great development to the slab solidification research.

Hereinafter, embodiments of the present invention will be described in detail.

The initial solidification layer appearance method in the mold of the continuous casting slab of the present invention is immersed by etching the slab sample in an etching solution consisting of picric acid, sodium alkylarylsulfonic acid and the remaining distilled water, and the etched slab sample is washed with alcohol and then washed. The tissue of one slab sample is observed with an optical microscope.

In order to identify the initial solidification layer in the mold of the slab, it is necessary to expose the slab solidification structure, and the most widely used method to expose the slab solidification structure is to observe the internal structure after etching with nitric acid (HNO ₃ ) etchant. .

However, in the case of using the nitric acid etchant, as shown in FIG. 1, it is difficult to clearly distinguish the initial solidified portion from the mold and the tissue produced during the subsequent solidification process, and only the general cast structure is exhibited. There is this.

Thus, a new etching solution is prepared to show the initial solidification layer in the mold of the continuous casting slab.

Specifically, look at the process, the etched first picric acid _{(C 6 H 3 N 3 O} 7), and then heating the etching solution consisting of an alkyl aryl sulfonic acid sodium, and the balance distilled water to the bath, immersing the slab samples in the heated etching solution.

The weight% of picric acid is 0.5 to 1.5 wt%, and the weight% of sodium alkylaryl sulfonate is 0.05 to 0.15 wt%.

The picric acid reacts slowly with the cast structure, resulting in a lower slab cast structure with low energy at the grain interface.

If the content of picric acid is less than 0.5wt%, the etching time increases, and if it is more than 1.5wt%, excessive etching occurs, so that the rapidly solidified portion in the mold and the tissue formed during the subsequent solidification process are clearly observed. Indistinguishable.

Sodium alkylarylsulfonic acid is a type of surfactant that exhibits an initial solidification layer in the slab. Sodium alkylarylsulfonic acid reacts first with an initial coagulation layer made of microcrystals having higher grain-interface energy than a coarse cast structure, and systematically distinguishes an initial coagulation layer and a later coagulation layer.

Sodium alkyl aryl sulfonate has an insignificant effect when the content is less than 0.05 wt%, and when it is more than 0.15 wt%, a tacky reactant is formed on the etching surface to hinder the etching reaction.

The etching liquid is heated to 40-60 degreeC. Heating should be done using a hot water bath to prevent the risk of fire or explosion. The heating of the etching liquid is for promoting the reaction between the etching liquid and the cast structure.

If the temperature of the etchant is less than 40 ℃ etch reaction is slow, if it exceeds 60 ℃ flow gas is generated and may be a risk due to high temperature steam.

The etching (immersion) time is adjusted according to the carbon content in the slab.

More specifically, when the carbon content in the slab is less than 0.03wt% (ultra low carbon steel), the etching time is 10 to 15 minutes, and when the carbon content in the slab is 0.03wt% or more and less than 0.08wt% (low carbon steel), the etching time is If the carbon content in the slab is 0.08 wt% or more and less than 0.20 wt% (medium carbon steel), the etching time is 4 to 6 minutes. And when the carbon content in slab is 0.20 wt% or more (high carbon steel), etching time shall be 1-2 minutes. At this time, the slab is a steel slab, the maximum carbon content in the slab may be 2.0wt%.

The reason for adjusting the etching time according to the carbon content in the slab is that a difference occurs in the reaction time between the etching solution and the cast structure according to the carbon content in the slab.

The higher the carbon content in the slab, the faster the etching reaction. Therefore, if the etching time is long, excessive etching occurs.

In the case of excessive etching, the picric acid and the tissue reaction are activated so that even when the alkyl aryl sulfonate is added, the initial coagulation layer does not appear, and only the coarse columnar structure appears large. On the other hand, if the etching time is short, the reaction time between the alkylarylsulfonic acid sodium and the initial coagulation layer is not sufficient, so that not only the large cast structure but also the initial coagulation layer structure does not appear.

For this reason, the etching time is adjusted according to the carbon content in the slab.

On the other hand, the slab sample is finished in fine polishing.

Subsequently, the etched slab sample is washed with alcohol to remove deposits on the surface and to dry.

After drying, the analytical surface of the slab sample is observed using an optical microscope. When the cast structure is etched through the etching process as described above, the initial solidification layer and the internal solidification structure of the slab may be divided to evaluate the initial solidification layer.

Table 1 shows the results of applying the initial solidification layer appearance method in the mold of the continuous casting slab according to the etching conditions for the slab sample having a carbon content of 0.05wt%.

At this time, the etchant was maintained at 50 ° C. using a bath apparatus, and the etchant immersion time was 5 minutes.

Note that. In the preparation of the etching solution, distilled water was first maintained at 50 ° C., followed by dissolving picric acid in Table 1, followed by addition of sodium alkylarylsulfonate. Thereafter, the analysis surface of the slab sample processed in the mirror polishing state was immersed in the etching solution and maintained for 5 minutes, and then the etched analysis surface was washed with ethyl alcohol. The washed analysis surface was observed with an optical microscope.

division picric acid
(wt%) Sodium alkylarylsulfonic acid
(wt%) Balance
(wt%) result
(After 5 minutes soaking) Remarks One 0.4 0.10 Distilled water Unable to verify casting organization Comparative example 2 0.5 0.10 Distilled water Initial solidification Inventive Example 3 1.0 0.10 Distilled water Initial solidification Inventive Example 4 1.5 0.10 Distilled water Initial solidification Inventive Example 5 1.6 0.10 Distilled water Initial solidification cannot be confirmed Comparative example 6 1.0 0.04 Distilled water Initial solidification cannot be confirmed Comparative example 7 1.0 0.05 Distilled water Initial solidification Inventive Example 8 1.0 0.10 Distilled water Initial solidification Inventive Example 9 1.0 0.15 Distilled water Initial solidification Comparative example 10 1.0 0.16 Distilled water Initial solidification cannot be confirmed Comparative example

As shown in Table 1, if the content of picric acid is lower or higher than the set range, the initial coagulation layer is impossible to exhibit, and if the content of sodium alkylarylsulfonate is lower or higher than the set range, the initial coagulation layer is not possible.

On the contrary, when the initial solidification layer appearance method in the mold of the continuous casting slab of the present invention was applied, the initial solidification layer appeared.

Table 2 shows the measurement of the initial solidification layer appearance according to the carbon content and the etching time of the slab.

At this time, 50 degreeC etching liquid which consists of 1.0 wt% of picric acid, 0.10 wt% of sodium alkylaryl sulfonates, and remainder distilled water is applied.

division Out of slab
Carbon content (wt%) Etching time
(min) Result _ Initial Coagulation Floor Decision
Remarks One
Ultra low carbon steel
(C <0.03)
9 Neither massive casting structure nor initial solidification layer Comparative example 2 10 Initial solidification Inventive Example 3 15 Initial solidification Inventive Example 4 16 Only large, columnar organizations emerge Comparative example 5
Low carbon steel
0.03≤C <0.08
3 Neither massive casting structure nor initial solidification layer Comparative example 6 4 Initial solidification Inventive Example 7 6 Initial solidification Inventive Example 8 7 Only large, columnar organizations emerge Comparative example 9
Medium carbon steel
0.08≤C <0.20
One Neither massive casting structure nor initial solidification layer Comparative example 10 2 Initial solidification Inventive Example 11 3 Initial solidification Inventive Example 12 4 Only large, columnar organizations emerge Comparative example 13
High carbon steel
C≥0.20
0.5 Neither massive casting structure nor initial solidification layer Comparative example 14 One Initial solidification Inventive Example 15 2 Initial solidification Inventive Example 16 2.5 Only large, columnar organizations emerge Comparative example

As shown in Table 2, the initial coagulation layer did not appear depending on the carbon content of the slab.

Table 3 shows the results of applying the initial solidification layer appearance degree according to the etching solution temperature conditions for the slab sample having a carbon content of 0.05wt% in the slab.

At this time, an etchant consisting of 1.0 wt% of picric acid, 0.10 wt% of sodium alkylarylsulfonate, and the remaining distilled water is applied. The etching time of the slab sample was 5 minutes.

division Etch solution bath temperature
(℃) Result _ Initial Coagulation Floor Decision
(After 5 minutes soaking) Remarks One 30 Neither massive casting structure nor initial solidification layer Comparative example 2 40 Initial solidification Inventive Example 3 50 Initial solidification Inventive Example 4 60 Initial solidification Inventive Example 5 70 Only large, columnar organizations emerge Comparative example 6 80 Only large, columnar organizations emerge Comparative example

As shown in Table 3, the initial solidification layer was exhibited in the range of the bath water temperature of 40 ~ 60 ℃, the reaction time between the etching liquid and the solidification structure is not enough when the bath water temperature of the etching solution is low, the huge casting structure and Neither initial coagulation layer was present.

In addition, when the bath temperature of the etching solution was higher than the set range, only the coarse columnar structure appeared to be huge due to excessive etching. In addition, although not shown in Table 3, a flow gas is generated and a danger due to high temperature steam is caused.

Figure 2 (a) is a structure photograph showing the solidification structure to which the initial solidification layer manifestation method applied in the mold of the continuous casting slab according to the present invention, (b) is an enlarged boundary surface of the initial solidification layer of (a).

As shown in FIG. 2, the initial coagulation layer and the slab internal coagulation tissue are clearly distinguished based on the interface. That is, it is confirmed that the initial coagulation layer has a fine structure and the slab internal coagulation structure has coarse coagulation structure.

Through this, it can be seen that applying the initial solidification layer appearance method in the mold of the continuous casting slab of the present invention it is possible to histological analysis of the cause of the slab surface defects through the initial solidification layer analysis.

Within the scope of the basic technical idea of the present invention, many other modifications are possible to those skilled in the art, and the scope of the present invention should be interpreted based on the appended claims. will be.

1 is a tissue photograph showing the internal structure of the slab after etching with nitric acid (HNO ₃ ) etching solution.

Figure 2 (a) is a structure photograph showing the solidified structure to which the initial solidification layer manifestation method applied in the mold of the continuous casting slab according to the present invention, (b) is an enlarged boundary surface of the initial solidification layer of (a).

Claims (7)

An etching step of immersing the slab sample in an etchant consisting of picric acid, alkylaryl sulfonate, and distilled water to etch the slab sample; A washing step of washing the slab sample with alcohol after etching is finished; And It comprises a measuring step of measuring the tissue of the washed slab sample with an optical microscope, The weight percent of the picric acid is 0.5 ~ 1.5wt%, the weight percent of the alkyl aryl sulfonate is 0.05 ~ 0.15wt% and the remainder is distilled water, the initial solidification layer manifestation method in the mold of the continuous casting slab. delete The method according to claim 1, The etching solution is an initial solidification layer in the mold method of the continuous casting slab, characterized in that the temperature is 40 ~ 60 ℃. The method according to claim 1 or 3, When the carbon content in the slab is less than 0.03wt%, the etching time of the etching step is 10 ~ 15 minutes, characterized in that the initial solidification layer manifestation method in the mold of the continuous casting slab. The method according to claim 1 or 3, When the carbon content in the slab is 0.03wt% or more and less than 0.08wt%, the etching time of the etching step is 4 ~ 6 minutes, characterized in that the initial solidification layer manifestation method in the mold of the continuous casting slab. The method according to claim 1 or 3, When the carbon content in the slab is 0.08wt% or more less than 0.20wt%, the etching time of the etching step is an initial solidification layer manifestation method in the mold of the continuous casting slab, characterized in that 2 to 3 minutes. The method according to claim 1 or 3, When the carbon content in the slab is 0.20wt% or more, the etching time of the etching step is characterized in that the initial solidification layer manifestation method in the mold of the continuous casting slab, characterized in that 1 to 2 minutes.

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