KR102043003B1 - Continuous casting method for high alloy steel - Google Patents

Abstract

The present invention relates to a continuous casting method of high alloy steel.
Continuous casting method according to one aspect of the present invention comprises the steps of ejecting a cast of a high alloy steel from the mold; Bending the slab discharged from the mold in a horizontal direction; And an unbending step of flattening the bent slab back into a flat shape, wherein the surface temperature of the slab at the point where the unbending is started can be 570 to 670 ° C.

Description

Continuous casting method of high alloy steel {CONTINUOUS CASTING METHOD FOR HIGH ALLOY STEEL}

The present invention relates to a continuous casting method of high alloy steel.

The continuous casting method of steel is a more productive method than the ingot method, and is a method of manufacturing steel recently adopted by most steel mills.

Commercially available continuous casting methods include vertical type, curved type, vertical curved type, and the like. Among them, the continuous casting method of curved type or vertical curved type with less space constraint is most used.

In the vertically curved or curved continuous casting method, in order to overcome the height constraints of the continuous casting machine, after the cast (strand) is discharged from the mold, there is a bending area for bending the cast in the horizontal direction, and the curved cast is again There is an unbending area that flattens to a flat shape.

Bending and unbending are both stresses on the cast steel, and the strongest stress acts on the surface of the cast steel, causing surface defects.

These surface defects are one of the major causes of economic loss by lowering product quality and error rate. In particular, in recent years, the need for higher strength of steel has been getting stronger, and as a result, the ratio of high alloy steel containing a large amount of alloying components is produced by the continuous casting method and is being supplied to the post process. As a sensitive steel grade, problems such as surface cracking may occur during casting.

According to one aspect of the present invention, there is provided a continuous casting method capable of reducing the occurrence of surface cracks as a continuous casting method of high alloy steel.

The problem of the present invention is not limited to the above description. Anyone of ordinary skill in the art will have no difficulty in understanding the additional subject matter of the present invention from the general contents of the present specification.

Continuous casting method according to one aspect of the present invention comprises the steps of ejecting a cast of a high alloy steel from the mold; Bending the slab discharged from the mold in a horizontal direction; And an unbending step of flattening the bent slab back into a flat shape, wherein the surface temperature of the slab at the point where the unbending is started can be 570 to 670 ° C.

In this case, the high alloy steel may have a composition including Mn: 1.5 to 2.5%, Cu: 0.05 to 0.4%, Ni: 0.3 to 0.6%, and Nb: 0.01 to 0.1% by weight.

Moreover, the temperature of the point of 9 mm depth from the surface of a slab can be made into 600-700 degreeC at the point which starts the said unbending.

As described above, the present invention can effectively prevent the occurrence of surface cracks by performing the unbending of the cast by avoiding the characteristic softening section that appears only in the high alloy steel during continuous casting of the high alloy steel.

1 is a photograph (observing a part of a cast was observed) and a conceptual diagram showing the appearance of surface cracks in a high alloy steel cast.
2 is a view showing a test method of a high temperature tensile test to simulate the phenomenon that the surface crack occurs in the player unbending area.
Figure 3 is a graph showing the results of the high temperature tensile test in the manner shown in Figure 2 as a change in the maximum cross-sectional reduction rate with temperature.
4 is a photograph of the cross section of the test piece tested at 550 ° C. with a scanning electron microscope (SEM).
FIG. 5 is an enlarged photograph of the test piece of FIG. 4 in order to analyze a cause of grain boundary fracture at 550 ° C. FIG.
6 is a view conceptually illustrating the phenomenon occurring in the grains of the test piece for each temperature in conjunction with the graph of FIG.
7 is a conceptual diagram illustrating a concept of controlling secondary cooling to control the temperature at the unbending point.
8 is a graph showing the frequency of occurrence of cracks on the surface of the cast steel obtained in one embodiment of the present invention according to Comparative Examples and Inventive Examples.

Hereinafter, the present invention will be described in detail.

High alloy steel targeted in one embodiment of the present invention may have a composition containing, by weight, Mn: 1.5-2.5%, Cu: 0.05-0.4%, Ni: 0.3-0.6%, Nb: 0.01-0.1% have. More preferably, the high alloy steel may have a weight%, Mn: 1.8 to 2.0%, Cu: 0.1 to 0.3%, Ni: 0.35 to 0.55%, Nb: 0.02 to 0.04%.

The alloy steel targeted by the present invention is not necessarily limited thereto, but in addition to the alloying elements described above, C: 0.04 to 0.1%, Si: 0.1 to 0.5%, Cr: 0.05 to 0.3%, Tot-Al: 0.005 to It may further include one or more selected from 0.07%, Sol-Al: 0.005 ~ 0.06%, in addition, it may further include an impurity such as P, S and the like within the tolerance generally included. The remaining components in the alloy steel may be Fe and inevitable impurities. This high alloy steel had a problem that a large amount of cracks on the surface during casting, the present invention is made based on the results of the inventors of the present invention to solve this problem.

In order to suppress surface cracks in the cast, it is necessary to perform bending or unbending in consideration of the cooling process of the cast to avoid the temperature range where the cast is brittle. When bending down slabs, a strong tensile stress acts on the lower surface of the slab, so surface cracks generated during bending occur on the lower surface of the slab. On the contrary, when un bending the slab, the tension is applied on the upper surface of the slab. Because of the stress, surface cracks during unbending are found mainly on the upper surface of the cast.

The photograph included in FIG. 1 shows a part of a cast slab in which a crack has occurred. The surface cracks of the high alloy steel slab targeted in the present invention are mainly found on the upper surface of the slab (slab), as shown in Fig. 1, most of which occur during unbending. In particular, as shown in the upper conceptual diagram of FIG. 1, the surface crack is mainly generated at a point about 8 to 11 mm deep from the outermost surface, not the outermost surface of the slab. It can remain on the surface and act as a defect. Accordingly, the present invention seeks to reduce cracks generated on the surface of cast steels by appropriately controlling the unbending conditions.

The surface cracks of the slabs occur because the stress on the slab surface exceeds the ductility limit of the slabs. Therefore, in order to reduce the surface cracks of the cast steel, it is possible to consider a method of reducing the stress applied to the surface of the cast steel or increasing the ductility limit of the cast steel. However, since the stress applied to the cast steel is not easy to be changed due to the characteristics of the continuous casting operation in which the size and shape of the cast steel are strictly determined, the present invention adopts a method of increasing the ductility of the cast steel at the time of stress application.

In other words, the slab reduction rate, which is one measure of ductility, varies depending on the temperature, but it depends on the composition of the slab, but the section reduction rate shows a minimum value at a high temperature between 800 and 1000 ° C, and then gradually decreases as the temperature decreases. It is increasing. Therefore, if surface cracks occur in the cast during unbending, it is a general recognition that the temperature is lowered more than the temperature at which surface cracks occur to perform unbending.

However, according to the research results of the present inventors, it was found that the cast steel of the high alloy steel which is the object of the present invention does not exhibit the temperature-sectional reduction rate relationship that the ordinary cast steel exhibited, but exhibits unique behavior.

That is, the inventors conducted a test in consideration of both the cooling rate and strain rate in the strand during continuous casting to simulate the phenomenon that the surface crack in the player unbending region. For the experiment, the specimen having the composition shown in Table 1 (unit: wt%) was given a temperature history as shown in FIG. That is, all the specimens were heated at a temperature increase rate of 10 ° C./s to a temperature of 1400 ° C. and maintained for 5 minutes, and then cooled at a cooling rate of 10 ° C./s to a target test temperature in consideration of the cooling rate directly under the mold. . The cooled specimen was held at the target test temperature for 1 minute and then subjected to a tensile test at that temperature to obtain the maximum cross-sectional reduction rate. In FIG. 2, the target test temperature (T) applied to the test is shown together in a table, and the specifications of the test piece used for the test are also shown.

C Si Mn P S Cu Nb Ni Cr Ti Sn 0.06 0.26 1.9 0.0054 0.0010 0.179 0.03 0.45 0.14 0.0134 0.0032

Figure 3 shows the result of the high temperature tensile test of the maximum cross-sectional reduction rate with temperature Shown as a change graph.

As can be seen in the drawing, the cross-sectional reduction behavior at high temperatures of high-alloyed steels is unlikely to be expected, after having the most vulnerable section at about 800 ° C, and then at a temperature of about 600 ° C where the ductility (section reduction) reaches its maximum value. As the falling, the ductility decreases again, and the so-called W-shaped temperature-section reduction rate is shown. Since the high alloy steel targeted by the present invention had a problem that cracks were observed from the upper surface rather than the lower surface, surface cracking during unbending becomes a problem. Therefore, in the present invention, it is important to control the unbending temperature not in the temperature range near 800 ° C. but in the temperature range of 700 ° C. or lower, which is a temperature range related to the unbending of high alloy steel. However, at a temperature lower than the temperature at which the cross-sectional reduction rate is extremely small, the lower the temperature, the higher the cross-sectional reduction rate. Therefore, it was common common knowledge that the cracks could be suppressed on the surface of the cast steel by unbending the cast steel at a temperature lower than 700 ° C. In the present invention, as described above, a completely different result was found.

The present inventors came to the following conclusion as a result of examining the reason. That is, FIG. 4 is a photograph of the cross section of the test piece tested at 550 ° C. using a scanning electron microscope (SEM). As can be seen from the figure, it can be seen that grain boundary fracture occurs. This is quite different from intragranular destruction at higher 600 ° C or lower 500 ° C. In order to analyze the cause of grain boundary fracture at 550 ° C., the specimens were further enlarged and analyzed to form precipitates formed by scanning electron microscopy as shown in FIG. 5. As can be seen from the drawings, it was clearly observed that at 550 ° C., cementite having a size of 1 μm or less was formed at grain boundaries around cracks. 6 shows the results of analyzing such a phenomenon.

Referring to the cross-sectional reduction rate behavior for each temperature band in Figure 6 as follows. First, in the high temperature range 900 ℃, precipitates such as NbC and TiC precipitate at the grain boundaries, causing the grain boundary to embrittle, thus reducing the cross-sectional reduction rate.In 800 ℃, the cross section is formed due to the mechanism of forming a very thin and low-strength ferrite film of about 10 μm at the grain boundaries. The reduction rate is even lower. On the other hand, at 600 ~ 700 ℃, the ferrite film is distributed at the grain boundary, but the thickness of the film becomes thicker than that of 800 ℃, resulting in a better cross-sectional reduction rate. However, when the temperature is lowered to 550 ° C, the cooling rate of the cast steel must be increased to meet the unbending temperature, so that the ferrite phase distribution is not seen at all, the hard phase (bainite) is formed, and the cement boundary is about 1 μm thick. Grains are observed. When cementite is formed at the grain boundary, [C] is depleted around the grain boundary, and various segregation materials are filled into the empty space, which may make the grain boundary more vulnerable. Below 500 ° C, cementite is evenly developed evenly in the grain boundary / mouth area, and the cross-sectional reduction rate is increased compared to 550 ° C. That is, the ductility value shows a recovery behavior.

Therefore, the present inventors have found that the unbending operation in which the cast steel is stressed should be avoided at 550 ° C. However, since the unbending operation is carried out over a certain length of the cast steel, even if the temperature of the starting point of the unbending is different from 550 ℃, the temperature of the cast steel to or near the 550 ℃ during the continuous unbending operation Since it may fall, it is necessary to take this into consideration so that the temperature at the point where the unbending is started is well controlled.

That is, in the present invention, the surface temperature of the cast steel at the point where the unbending is started is controlled to 570 to 670 ° C. At this time, the slab surface temperature means the temperature of the central portion in the width direction of the upper surface of the cast steel. Since the surface defects of the cast steel are not initiated at the outermost surface of the cast steel, they are started at a depth of 9 mm from the surface of the cast steel (usually starting at 8 to 11 mm and selecting 9 mm as the representative position), so that the temperature at this point becomes 550 ° C. It is important not to. In consideration of this, the surface temperature of the point at which the unbending is started is set to 570 ° C or higher, so that the temperature of the 9 mm point directly below the surface of the cast steel until the unbending is substantially finished can be prevented from becoming 550 ° C. More preferably, the surface temperature of the cast steel may be 580 ° C or more, and most preferably 590 ° C or more. In addition, even if the temperature of the point at which the unbending starts is increased, surface cracks may occur at the starting point, so that the surface temperature at the point at which the unbending is initiated may be 670 ° C. or less, preferably 660 ° C. or less. And most preferably, 650 degreeC or less.

According to one embodiment of the present invention, the surface temperature as described above is such that the temperature of the 9mm point directly below the surface where cracks occur (this point also means the center portion in the width direction of the slab as the starting point of the unbending) is 600 to 700 ° C. It can be, Preferably it can be 610-690 degreeC, Most preferably, it can be 620-680 degreeC.

According to one embodiment of the present invention, the temperature of such an unbending point can be controlled by appropriately changing the amount of secondary coolant when injecting the secondary coolant in a manner as shown in FIG. 7 during continuous casting.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it should be noted that the following examples are only intended to illustrate the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

C: 0.06%, Si: 0.26%, Mn: 1.9%, P: 0.0054%, S: 0.0010%, Cu: 0.179%, Nb: 0.03%, Ni: 0.45%, Cr: 0.14%, Ti: 0.0134%, A molten steel having a composition of Sn: 0.0032% was manufactured into a slab (long side: 2200 mm, short side: 400 mm) using a vertical curved continuous casting machine. After exiting the continuous casting machine mold, the strands were bent to have a curved surface after passing through a vertical section, and the surface temperature was controlled by secondary cooling with coolant. During casting, the slab surface temperature at the point where the unbending was initiated was shown in Table 2. The slab was cut after the solidification completion point to the solidification is completed the unbending section, and the frequency of the cracks were found on the surface of the obtained slab was shown in the graph of FIG.

division Comparative Example 1 Comparative Example 2 Inventive Example Temperature at the unbending start point 730 ℃ 550 ℃ 650 ℃

As can be seen from FIG. 8, in the case of the invention example (unbending start point temperature: 650 ° C.) in which the temperature of the point at which the unbending is started is in the range of the present invention, the temperature of the point at which the unbending is started is in the range of the present invention. It can be seen that the incidence of cracks was significantly reduced compared to Comparative Example 1 (unbending start point temperature: 730 ° C.) and Comparative Example 2 (unbending start point temperature: 550 ° C.).

Thus, the advantageous effects of the present invention could be confirmed.

Claims (3)

By weight%, Mn: 1.5-2.5%, Cu: 0.05-0.4%, Ni: 0.3-0.6%, Nb: 0.01-0.1%, and also C: 0.04-0.1%, Si: 0.1-0.5%, Cr: Discharging the cast iron from the mold having a composition composed of at least one selected from 0.05 to 0.3%, Tot-Al: 0.005 to 0.07%, and Sol-Al: 0.005 to 0.06% and the balance of Fe and unavoidable impurities;
Bending the slab discharged from the mold in a horizontal direction; And
Unbending step to flatten the bent cast back to a flat form
In the continuous casting method comprising a,
Cast steel surface temperature of the point which starts the said unbending is 570-670 degreeC,
Continuous casting method which makes the temperature of the point of 9 mm from the cast surface at 600-700 degreeC at the point which starts the said unbending.
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