KR100380726B1 - Method for cooling billet having high carbon and small cross section - Google Patents

Abstract

PURPOSE: A method for cooling billet having high carbon and small cross section is provided to improve internal quality of the billet by reducing central segregation of the billet when continuously casting high carbon steel. CONSTITUTION: In a method for continuously casting billet by injecting high carbon molten steel into a mold having small cross section, the method comprises a step of injecting a pin having a melting temperature lower than molten steel into the mold, measuring position of the pin, and determining a time point when solidification of the billet is completed; a step of determining a time point when an inner temperature distribution gradient of the billet is greater than an outer temperature distribution gradient of the billet as a time point when a surface cooling speed of the billet is higher than an inner cooling speed of the billet by calculating inner temperature distribution and surface temperature distribution of the billet by ordinary heat transfer analysis and comparing gradients for the calculated both temperature distributions; and a step of injecting cooling water from a billet position corresponding to the time point when a surface cooling speed of the billet is higher than an inner cooling speed of the billet to a billet position corresponding to the time point when solidification of the billet is completed in the cooling water flow rate range of 10 to 100 L/min per unit area of the billet.

Description

Cooling method of high carbon small section billet cast

The present invention relates to continuous casting of high carbon steel, and more particularly, to a method for cooling a high carbon small section billet slab capable of improving internal quality due to reduction of central segregation.

In line with the development of the industrial structure, the demand for automobile parts and various mechanical parts is increased, and the high value added of steel products is developed. Many high grade steels, such as bearing steel, tire cord material, piano wire, free cutting steel, and welding bar, have been developed. It is produced, but all steel companies are pursuing the method of playing because of the improvement of error rate and cost reduction.

Generally, in the continuous casting method, as shown in FIG. 1, molten steel 4 received in the tundish 1 is injected into the mold 3 through the immersion nozzle 2, and the injected molten steel 4 is formed of a mold copper plate. The solidification layer 7 is formed while being transported by the roll 5 downward by the cooling water circulated therein, and is made through a process of manufacturing the final cast. However, this control is very important because the center segregation generated during casting production does not disappear in the post-processing products and remains, and causes problems such as disconnection and mechanical property degradation during the final product processing.

Usually, the central segregation that occurs inside the cast steel is caused by the flow of thickened steel by the solute discharge and the solute discharge during solidification, and it is known that the direct causes of this flow are the coagulation shrinkage and the slab bulging. In small section billet, slab bulging is not a big problem, but solidification shrinkage inevitably occurs, so the center segregation is inevitable during continuous casting. In particular, the higher the carbon content, the greater the degree of segregation, so the control of the central segregation is more important in continuous casting of high quality steel.

The central segregation of cast pieces is greatly influenced by the mechanical factors inherent to the machine in addition to metallurgical factors such as solidification structure, chemical composition, molten steel superheat.

That is, during slab casting, the slab bulging effect has a great effect, which causes the movement of solute concentrate, which is a performance equipment factor that causes the slab bulging. There is this. These all generate molten steel flow by rolling down and subsequent bulging to increase central segregation. According to a report investigating the effect of bulging on segregation during solidification of cast steel, it is known that in the case of columnar solidification, segregation worsens even with 0.2 mm bulging. Although the amount of bulging between rolls is known to reduce segregation due to shortening of the roll pitch and the adoption of a split roll, there are problems such as high installation cost and difficulty in management.

In order to reduce such central segregation, many attempts have been made in the past, such as an electronic stirring method, a low pressure method, a low temperature and a low speed casting method.

Hereinafter, a conventional method of controlling center segregation will be described.

First, low-temperature and low-speed casting methods to reduce molten steel superheat and low-speed casting form a granular equiaxed grain structure at the center to suppress segregation at the center.However, low temperature casting prevents operation instability and reduced productivity due to nozzle closure. It has the disadvantage of causing.

Then, the electron agitation method is a method of preventing central segregation by stirring the residual molten steel by an electromagnetic force to uniformly coagulate the solidified structure to suppress the movement or accumulation of the solute-enriched molten steel. The equiaxed coagulation structure is insensitive to the effects of molten steel flow due to bulging and as a result, the segregation of the core can maintain a relatively low level of quality. However, semi-macro segregation dispersed in equiaxed crystal structures other than the central part and the central part exists, and when the size thereof is large, hardened structures are generated on the rolled plate to cause hydrogen organic cracks.

In addition, the low pressure method has recently been recognized as the most effective method for reducing segregation by preventing molten steel movement and reducing central segregation by deteriorating the area where molten steel flow due to solidification shrinkage occurs. .

The above-mentioned low temperature casting or electronic stirring device is quite advantageous in terms of dispersing the center segregation by creating an equiaxed band in the center of the slab, but relatively V (s) segregation and semi-macro segregation are generated, so that the As a countermeasure, the low pressure method is in the spotlight, but the equipment is expensive and its adoption is not generalized.

In addition, there is a segregation control method by cooling. The segregation control method by strong cooling is explained as follows.

First, as shown in FIG. 1, the cooling in the cast during continuous casting includes indirect cooling (8) by cooling the mold copper in the mold (3) and direct cooling by water spray below the lower end of the mold (hereinafter, 'water cooling'). 9) and natural cooling by air (hereinafter referred to as 'air cooling') (10). Generally, the cooling length of water cooling is about 3-8m at the bottom of the mold, and the air cooling method is adopted below. However, central segregation occurs at the solidification point, which corresponds to the air cooling zone of the machine cooling zone. Upon continuous casting, i.e., when the solidification is completed, as shown in Fig. 2, the cooling of the casting center portion is much larger than the cooling rate of the surface portion. During the solidification process, the molten steel was able to maintain a constant temperature due to the transformation of the molten steel from the liquid phase to the solid phase while the solidification progressed, but when the solidification was completed, the temperature dropped sharply as there was no heat compensation due to the latent heat. On the other hand, the surface of the cast steel has a slow cooling rate because the cooling proceeds rapidly due to water cooling in the early stage of solidification, but only by the cooling effect of atmospheric cooling in the air cooling area. Therefore, at the end of the solidification, there is a difference in the shrinkage rate due to the difference between the cooling rate of the cast steel surface and the cooling speed of the center part. Is happening. Therefore, by forced cooling (strong cooling) the surface of the slab at the end of coagulation, the central segregation can be suppressed by reducing the void at the center of the slab causing segregation by suppressing the generation of the central void due to the difference in cooling capacity. .

On the other hand, as a representative example of the segregation control method by such cooling, the applicant has received a Korean patent by suggesting a method of strong cooling at the end of the solidification applied during bloom continuous casting. (Korean Patent No. 95,383)

However, the steel cooling applied during the bloom casting according to the invention has the following problems.

First is the selection of the coagulation completion position. In other words, in order to achieve the purpose of the central segregation control, the selection of the solidification completion position is most important.In the above-mentioned patent invention, the cast slab heat analysis is performed to confirm the solidification completion point, and the lead tracer is mainly used for verification. It was put in the bias. The solid phase rate at which lead can penetrate into molten steel is defined as 0.5 to 0.6, and the solidification completion point is estimated based on this. However, there is no more reliable verification of the exact location of the solidification point because it is difficult to verify the solid phase rate at which lead can penetrate and there is no interpretation.

Secondly, the cast was cooled only in the localized area of the cast from the solidification point. As can be seen in Figure 1, as the cast solids undergo three changes in physical properties. That is, it can be roughly divided into the liquid-liquid coexistence layer 6 in which the liquid phase, the liquid phase and the solid phase coexist, and the solid phase. The solidification completion point has a solid state. At this point, the movement of the residual molten steel is difficult, and even cooling to the surface of the cast steel does not significantly affect the movement of the concentrated molten steel.

Due to these problems, although the trend of segregation improvement is suggested in the effect analysis according to the strong cooling, there is a shortcoming that it is insufficient to confirm the effect of the accurate cooling method because it does not present quantitative data.

Accordingly, the present invention has been proposed to solve the above-mentioned problems, and in the continuous casting of high carbon steel, in particular, in the continuous casting of high carbon small section billet cast steel, derivation of the final solidification site of the cast slab is generated The purpose of the present invention is to provide a cooling method with excellent internal quality by minimizing heat shrinkage due to the difference in cooling rate between the surface of the cast and the surface by cooling the terminal part of the solidification under appropriate cooling conditions.

1 is a configuration diagram of a typical continuous casting machine

Figure 2 is a graph showing the surface and internal temperature distribution of the high-carbon playing billet cast

Figure 3 is a graph comparing the solidified shell thickness measurement value and the calculated value of the high carbon performance billet cast steel at each speed

4 is a graph showing the relationship between the time point when the internal cooling rate of a plurality of high carbon performance billet slats becomes greater than the surface cooling rate and the point of occurrence of V segregation from the surface of the cast slab;

Figure 5 (a) is a graph showing the temperature distribution change of the high carbon billet cast steel prepared at a casting speed of 1.7m / min

FIG. 5 (b) is a graph showing the ratio of the surface and internal cooling rates of the cast steel corresponding to FIG. 5 (a)

6 is a graph showing the relationship between the surface and internal cooling rate ratio of the high-carbon playing billet cast steel and the amount of cooling water injected at the end of the solidification cast

7 is a graph showing changes in the internal and surface temperature distribution of the cast when 100 l / min.m2 of cooling water is injected into the final solidification portion of the high-carbon playing billet cast steel.

8 is a graph showing the degree of carbon segregation in the center portion of the high carbon playing billet slab according to the amount of cooling water at the end of solidification at each circumferential speed

9 is a graph showing the distribution of Mn segregation of the center portion of the high carbon playing billette slab according to the amount of cooling water at the end of solidification at each casting speed

In the present invention for achieving the above object is a method of continuously casting a billet (billet) by injecting a high-carbon molten steel into a mold of a small cross-section,

First, the step of inserting a pin having a lower melting temperature than molten steel in the mold, measuring the position of the pin and determining the time when the solidification of the cast is completed;

Then, the internal temperature distribution and the surface temperature distribution of the slab are calculated by normal electrothermal analysis, and the gradient difference of the calculated temperature distribution is compared to the gradient of the outer temperature distribution of the slab. Setting a larger time point to a time point at which the cooling rate of the surface of the cast steel becomes larger than the internal cooling rate of the billet slab; And

Cooling water in the range of 10-100 l / min per unit area of the slab from the position of the slab corresponding to the time when the cooling speed of the surface of the slab becomes larger than the internal cooling rate of the billet slab to the position of the slab corresponding to the time when the solidification of the slab is completed. It relates to a cooling method of the high-carbon small section billet cast steel spraying.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

First of all, high carbon steel according to the present invention refers to steel containing about 0.6 to 0.75% of carbon suitable for producing small cross-section billets.

Here, the small sectional billet cast iron refers to a billet having a cross-sectional size of about 100 × 100 to 160 × 160 mm, which is sufficient to exhibit a cooling effect according to the present invention by water cooling during continuous casting. . Of course, the target steel grade and billet cross-sectional size of the present invention is not limited to the above-described range, any of the steel billet of the extent that the shrinkage of the surface and the inside of the cast steel can be minimized by the cooling method according to the present invention.

On the other hand, in applying the cooling method according to the present invention, it is necessary to know the appropriate amount of cooling water and the cooling water injection position in order to eliminate the central segregation inside the cast steel.

The cooling water injection position is the position of the cast steel corresponding to the completion of the solidification of the cast from the position of the cast steel corresponding to the time when the internal cooling speed of the cast is greater than the surface cooling speed (hereinafter, also referred to as 'end of solidification' only) )to be.

The solidification completion position is possible by artificially inserting a pin having a lower melting temperature than molten steel for each cast position and measuring the molten position. The pin is inserted to confirm the solidification completion position, similar to the shape of the nail and when the pin is inserted into the cast steel, it is common to use a melting point lower than the temperature of the molten steel so that it can be easily melted in the molten steel. In addition, in general, the pin may be heat-treated to have a hardness value of 40-50 on the HRC basis in order to prevent judgment or bending before insertion into the cast steel.

In addition, the time point at which the internal cooling rate of the cast steel becomes larger than its surface cooling rate can be obtained through electrothermal analysis using the conventional heat transfer equation of the surface and internal temperature distribution of the cast steel. As an example, FIG. 2 is obtained by electrothermal analysis and shows the temperature distribution of the surface and central portion of the slab during continuous billet casting of 160 × 160 mm section size. As can be seen in Figure 2 at the end of the solidification can be seen that the cooling rate of the center of the cast steel is much greater than the cooling rate of the surface of the cast steel.

Figure 3 shows the solidified shell measured by the insertion of the pin according to the casting speed during the billet casting and the result of the electrothermal analysis, the measurement results and the calculation results are in good agreement.

4 shows the relationship between the time of movement of the residual molten steel existing between the column heads due to the negative pressure (due to solidification shrinkage) occurring in the center of the cast steel and the time when the cooling speed inside the cast steel becomes larger than the surface cooling speed. Is a graph.

As can be seen in FIG. 4, V (V) segregation, which is a kind of central segregation in the slab, is generated at the terminal stage of coagulation.

Therefore, cooling rate control of the surface of the cast steel means that it must be applied at the end of the solidification.

5 (a) and 5 (b) show the temperature distribution of the high carbon steel billet slab, and the cooling rate distribution of the slab center portion and the surface portion under the conditions of a casting speed of 1.7 m / min, respectively. 5 (b) can be obtained from the cast temperature distribution as shown in Fig. 5 (a), it is possible to obtain the time point (A in the figure) that the cooling speed of the central portion is greater than the surface portion in the cooling rate distribution. When the solidification completion point A 'is confirmed, the strong cooling section is A-A'.

In this way, for the 160 × 160mn billet slab, the position of the slab at the time of solidification is completed from the position at which the cooling speed of the center of the slab becomes larger than the surface. Is 13.9-16.2m from the mold's tap surface, and it is about 14.8-17.2m when the circumferential speed is 1.8m / min, and about 15.8 ~ 18.2m when the circumferential speed is 1.9m / min.

Then, the amount of cooling water is determined as follows. As mentioned above, in order to minimize the amount of shrinkage in the center of the cast, the surface cooling rate of the slab in the final solidification region must be greater than the cooling rate of the center of the cast steel.

FIG. 6 shows the cooling rate ratios of the surface and the inside of the slab according to the amount of cooling water when forcibly cooling the slab at the end of the solidification obtained in FIG. 5. As can be seen in Figure 6 it can be seen that the cooling water of at least 10ℓ / min.㎡ should be applied. In general, if the recuperation amount is over 100 ° C. after the surface of the cast is cooled, the surface is likely to be cracked. Therefore, the cooling during continuous casting manages the recuperation amount to be less than 100 ° C.

In the case of high carbon steel, phase transformation occurs from austenite to ferrite near 750 ° C. In this case, since the density of the austenite structure is smaller than that of the ferrite structure, expansion occurs due to the density change at the phase transformation temperature. Expansion of the surface of the slab may be contrary to the original intention to increase the amount of shrinkage by forced cooling on the slab surface.

Considering the above two, the maximum amount of cold cooling can be determined. That is, Figure 7 shows the temperature distribution when the amount of 100ℓ / min. ㎡ applied at the end of the solidification, in this case it can be seen that the surface temperature of the cast slab is more than 750 ℃ and the recuperation amount is also about 100 ℃. Therefore, it is preferable that the maximum quantity is determined to be 100 l / min. The quantity corresponds to 3.2 times that of the cooling rate surface of the cast steel surface.

According to this cooling method, the center segregation is suppressed in the high-carbon small-section billet slab to improve the internal quality.

Hereinafter, the present invention will be described in detail through examples.

Example

A molten steel having a carbon content in the range of 0.6-0.75% was manufactured using a continuous casting facility as shown in FIG.

At this time, the casting speed was 1.7-1.9m / min, and a cooling nozzle was installed at the end of the solidification of about 13.9-16.2m from the molten steel bath surface of the mold. Was cooled.

The cooling nozzle is a nozzle capable of cooling the entire four surface portion of the cast steel, such a nozzle is disclosed in Korean Patent No. 95,383.

The internal quality of cast steels under such strong cooling conditions was evaluated as follows. In other words, the integrity of the test piece center was evaluated by examining the carbon concentration and Mn segregation area ratio of the center of the test piece.

The carbon concentration of the slab center portion was sampled with a slab chip having a diameter of 5 mm at the slab center portion for 300 mm slabs in the slab length direction, and the components thereof were subjected to wet analysis. In addition, Mn segregation is analyzed by means of an electron microscope to determine the segregation particles having a value of Mn / Mno> 1.2 (Mn: center concentration, Mno: the total concentration of the cast steel) with respect to the center of the slab, the segregated particles occupy the entire analysis area Comparison was made with area ratios. The method is an analytical method commonly applied in the art.

The carbon segregation in the center of the cast steel according to the casting speed and the amount of cold water at the end of solidification is shown in FIG. 8.

As shown in Fig. 8, in the case of casting speed 1.7m / min, the central carbon concentration ratio (center carbon concentration / slab average concentration) value of the cast steel is reduced according to the increase in the cooling water quantity, and the conventional cooling water is cooled at 100 l / min. The segregation value of ash is greatly reduced from 1.2 to 1.2. However, there was no cooling effect when the casting speed was more than 1.7m / min.

This is because the solidification completed position coincides with the cold-cooled nozzle installation position at the casting speed of 1-7m / min, while the solidification is completed after the cold-cooled nozzle installation position at the higher casting speed, and thus the cooling effect is not shown.

On the other hand, the Mn segregation distribution, as can be seen in Figure 9, at the casting speed of 1.7m / min was reduced to 0.5% when applying the maximum volume at the conventional 1% level, but there was no effect at more casting speed.

Therefore, it was confirmed that the end-solidification cold cooling at a constant circumferential speed for a certain end-solidification nozzle position is an effective means of reducing central segregation.

As described above, the present invention improves the internal quality by suppressing segregation of the center of the slab by minimizing heat shrinkage between the surface and the inside of the slab by cooling the end portion of the slag to a suitable cooling condition during continuous casting of high carbon steel. Sound steel can be obtained, and this cooling method is particularly useful for producing high-carbon small-section billet cast steel.

Claims (5)

In the method of continuously casting billet slab by injecting molten high-carbon steel into a mold of a small section, First, the step of inserting a pin having a lower melting temperature than molten steel in the mold, measuring the position of the pin and determining the time when the solidification of the cast is completed; Then, the internal temperature distribution and the surface temperature distribution of the slab are calculated by normal electrothermal analysis, and the gradient difference between the calculated temperature distributions of both sides is compared, and the gradient of the inner temperature distribution of the cast steel is greater than the gradient of the external temperature distribution. Setting a larger time point to a time point at which the cooling rate of the surface of the cast steel becomes larger than the internal cooling rate of the billet slab; And Cooling water in the range of 10-100 l / min per unit area of the slab from the position of the slab corresponding to the time when the cooling speed of the surface of the slab becomes larger than the internal cooling rate of the billet slab to the position of the slab corresponding to the time when the solidification of the slab is completed. Cooling method of high-carbon small-section billet cast steel, characterized in that the spraying. The method of claim 1, wherein the high carbon steel has a carbon content of 0.6-0.75%. 3. The cooling system of claim 1 or 2, wherein the injection position of the cooling water is in the range of 13.9-16.2 m from the hot water surface of the mold when the casting speed of the billet slab is 1.7 m / min. Way. 3. The cooling system of claim 1 or 2, wherein the injection position of the cooling water is in the range of 14.8-17.2 m from the hot water surface of the mold when the casting speed of the billet slab is 1.8 m / min. Way. 3. The cooling system of claim 1 or 2, wherein the injection position of the cooling water is in the range of 15.8-18.2 m from the hot water surface of the mold when the casting speed of the billet slab is 1.9 m / min. Way.