KR20040057214A - Casting method of continuously cast billet with good center soundness - Google Patents

Abstract

PURPOSE: A forming method of continuously cast billet with generation of center segregation being reduced is provided to improve generation of center segregation of high class steel billet, enable steel for high class wire rod to be continuously cast into billet and improve actual yield, thereby reducing manufacturing cost. CONSTITUTION: The forming method of continuously cast billet with generation of center segregation(8) being reduced comprises a step of promoting equiaxed solidification as an equiaxed structure ratio of 40% or more with billet thickness ratio by applying electromagnetic force to non-solidified molten steel being cast by electromagnetic stirrer mounted on mold, thereby stirring the molten steel; a step of adjusting the billet so that a position in which solid phase ratio of central part of the billet is 0.7 to 0.8 is positioned in rear of pinch rolls(1) when taking a casting direction(2) as a front side; and a step of strongly reducing the billet to 2-10 mm through the pinch rolls, wherein there are two pinch rolls, and a position in which solid phase ratio of central part of the billet is 0.7 to 0.8 is positioned in rear of the pinch rolls when taking a casting direction as a front side.

Description

Casting method of continuously cast billet with good center soundness

The present invention relates to a technique for reducing center segregation by casting electron stirring and pinch roll reduction during continuous billet casting of steel.

Since the central segregation generated during continuous casting of steel remains in the final product and acts as a defect, it must be actively reduced in the production of high quality steel. Moreover, the smaller the cast steel size, the lower the reduction ratio in the rolling process, so that the central segregation of the cast steel remains in the final product.

The central segregation generating mechanism of the bloom or billet at the time of equiaxed solidification can be represented as shown in FIG. Figure 1 illustrates the process of generating the central segregation divided into eight steps. In the first and second stages, the growth of free equiaxed growth continued following the steady growth, and in the third phase, the growth of equiaxed crystals and central solidification begins. In step 4, the solid state of the liquid coexistence zone is shown, and as the solidification proceeds, the fluidity of the solid solution coexistence zone decreases in step 5, forming a dendritic network. The residual molten steel begins to penetrate between the dendritic meshes, and segregation passages are formed in step 6, and the central segregation path is formed in step 7, and then solidification is completed in step 8. The influence of solidification shrinkage is large as a driving force for generating such central segregation.

As the core segregation reduction technology, methods such as electron stirring, light pressure, secondary cooling super cold cooling and late solidification cooling have been applied or attempted. Electro-stirring technology promotes equiaxed solidification by stirring and applying electromagnetic force to unsolidified molten steel during casting, and reduces central segregation by reducing the flow of segregation materials at the end of solidification. However, in the production of high-grade steel, the billet cast steel, which has a relatively low rolling reduction ratio compared to the bloom cast, requires a sufficient reduction in the center segregation, but electronic stirring technology is not enough.

The low pressure technique reduces segregation by reducing the amount of solidification shrinkage within the central segregation zone where liquid segregation material flows between the solid phases, that is, within the range of the central solid phase 0.3 to 0.7, as much as shrinkage due to liquid / solid transformation in the solid-liquid coexistence zone at the end of solidification. It reduces central segregation by preventing mass flow. This method stops production of existing players for about 2 months and adapts the equipment to reduce the rolling pressure of about 2mm or less per roll down roll. It is mainly applied to bloom players of 200mm or more. The disadvantage is that it has to be expensive.

In the second cooling super-cooling technology, after the billet slab exits the mold, the cooling water is formed by high pressure cooling, and the solidification of the cast steel becomes super cold by 2.5 ~ 3L / kg, which is about 5 times the normal amount. It proceeds to the vicinity of the center part, and the center part is a technology which reduces central segregation and point segregation by solidifying with equiaxed crystal by casting electron stirring. This technology has been applied recently, but the cross section size is 150mm. It is known that the effect is only when it is below, it is easy to generate surface cracks during cold cooling, and there is a problem such as costly to introduce super-cooling cooling equipment.

The end-solidification cold-cooling technology is characterized by the fact that the difference between the temperature gradient on the surface of cast steel and the temperature in the core is so large that tension stress is applied to the core and the shrinkage is concentrated on the core. This method reduces the center segregation by reducing the gradient difference. This method is not currently applied because it is difficult to control the precise cooling section during continuous casting casting operation.

As mentioned above, there are various technologies for reducing the center segregation, but in case of electronic stirring, it is insufficient to reduce the center segregation of high-grade steel billets. A lot of burdens are inevitable, such as the interruption of production for a certain period of time and the cost of introducing the equipment. In the case of the application of ultra-cold technology, there are problems such as the limitation of the billet cross-sectional size, the problem of surface cracking, and the cost of introducing the super-cooling equipment. In addition, the end-solidification cold-cooling technology, such as on-site production under the mass production system, there is a problem in the field utilization, such as no steel mill to be applied because there is a problem to be cooled in the exact cold cooling section every moment during the casting by steel type, casting conditions.

Since the billet cast steel has less rolling reduction during post-process rolling compared to BLUM, the central segregation must be solved when producing high quality steel billet. In order to solve this problem, there are various central segregation reduction technologies, but there are various problems as described above.

The present invention is to solve the above problems, by operating the mold electronic stirring device to the billet player to ensure the equiaxed coefficient of 40% or more in the thickness of the cast steel, the portion of the central solid phase of 0.7 ~ 0.8 is the existing pinch It aims to prevent the generation of high quality steel billet segregation without internal cracking by lowering the cast piece by 2 to 10 mm per pinch roll with the pinch roll so that it is located behind the roll (when the mold direction is viewed forward).

In addition, an object of the present invention is to enable the production of high-quality steel for billet performance while at the same time to reduce the rate of central segregation to improve the error rate.

1 is a view showing a central segregation generating mechanism during equiaxed solidification.

2 is a schematic diagram of a conventional billet player.

3 is a schematic view showing the inner state of the cast steel before and after pinch roll pressing.

Figure 4 is a graph showing the internal crack generation sensitivity according to the casting structure.

5 is a graph showing the center segregation of the billet of the invention and comparative examples.

Figure 6 is a graph showing the wire quality and the final product disconnection rate of the invention example and comparative example.

Explanation of symbols on the main parts of the drawings

1: pinch roll

2: casting direction

3: Movement of segregation molten steel

4: Zero crack start temperature isotherm (ZDT)

5: Residential coagulation section

6: equiaxed solidification section

7: limit of possible occurrence of central segregation (solid state ratio = 0.7 to 0.8)

8: center segregation

As a constituent means for achieving the above object, the present invention is a method for forming a billet playing cast piece, by applying an electromagnetic force to the unsolidified molten steel during casting by stirring with an electronic stirring device mounted on the mold 40% in the thickness ratio of the cast steel Encouraging equiaxed crystal solidification while securing the above equiaxed constant; Adjusting the position of the solid state of the slab center to be 0.7 to 0.8 to be positioned before pressing down by the pinch roll; And a step of lowering the cast steel by 2 to 10 mm through a pinch roll.

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

The present invention aims to develop a technology that can solve the central segregation problem of high-grade steel by using the electronic stirring equipment and the pinch roll in the mold to solve the central segregation problem that occurs during continuous casting of steel billet. Put it.

First, sufficient equiaxed solidification proceeds when the mold stirrer is applied.

Second, internal cracks are less likely to occur even if the strain received inside the slab is sufficiently solidified.

Third, the center segregation generation section can be brought to the existing pinch roll position.

Fourth, by reducing the pinch roll pressure it is possible to reduce the amount of reduction more than 2mm.

Fifth, to reduce the central segregation, a sufficient rolling reduction should be applied. When the columnar solidification is not equiaxed, it is impossible to apply a sufficient rolling reduction due to internal cracks.

From the above fact, by applying the mold electronic stirrer, the equiaxed solidification is progressed and the operation is controlled so that the center segregation section is at the pinch roll position, and the cast steel is pushed down by 2 ~ 10mm per pinch roll with one or two pinch rolls. This eliminates the problem of central segregation of fine steel billets without internal cracking. I tried to solve.

2 shows a schematic diagram of a billet player. The molten steel 15 is injected into the mold to form a coagulation layer from the part in contact with the mold.At this time, the molten steel is stirred by the electron stirring 11 in the mold to reduce the superheat degree and pass through the fine resinous phases of the coagulation interface. Fine crystal pieces are generated to promote equiaxed solidification. The cast piece exiting the mold continues to solidify while passing through the secondary cooling zone 12 and is drawn out at a constant speed by the pinch roll 1. At this time, by controlling the casting speed and the cooling water to bring the position (A) of the central solid phase ratio of 0.7 ~ 0.8 to the back of the pinch roll (1) to increase the pinch roll pressure to reduce the cast piece 2 ~ 10mm.

If the position of the solid phase of 0.7 ~ 0.8 is located in front of the pinch roll (1), the center segregation is already generated, and even if it is pressed down by the pinch roll, there is no effect of reducing the center segregation. In addition, if the rolling reduction exceeds 10mm, wire rods are impossible to apply.

Fig. 3 is a schematic view showing the state of the cast steel before and after pinch roll pressing. Reference numeral 5 denotes columnar solidification section, and reference numeral 6 represents an equiaxed solidification section. 4 denotes an internal crack generation start isotherm. In the case of pinch roll pressing, the portion 7 having a solid phase ratio of 0.7 to 0.8 or less, which is a region capable of generating central segregation, has a certain thickness, but in the case of the pinch roll pressing, the central liquid-liquid coexisting portion 7 including the coagulation shrinkage hole is pressed down. It is shown that the thickness is significantly reduced. In addition, when the equiaxed crystallinity is 40% or more, almost no internal crack is generated even when the pressure is reduced by 2 ~ 10mm since the temperature section of 50 ~ 100C directly below the solidus temperature, which is the origin of internal crack generation, is in the equiaxed structure. As described above, it can be seen that the central segregation 8 is largely generated before the pinch roll pressing is performed.

4 is a graph showing the internal crack generation sensitivity according to the casting structure. As can be seen in Figure 4 is the case of the lower surface of the cast slab solidification structure, it can be seen that the internal crack is much more difficult to occur during the isometric crystal solidification. This is because the equiaxed crystals do not have directivity, and thus internal cracks do not easily occur when tensile stress perpendicular to the columnar growth direction is applied as in the columnar crystals. Therefore, in case of low pressure installation, due to the occurrence of internal crack, the rolling reduction of about 2mm or less and total rolling reduction of 5 ~ 10mm are inevitable due to the occurrence of internal crack, whereas in the case of billet performance, the electronic agitation in the mold is carried out to about 40% or more If you have a fixed rate, one or two pinch rolls can be reduced by 2 ~ 10mm per pinch roll without the occurrence of internal cracks.

EXAMPLE

The billet caster of 160x160 mm ² section was produced under the conditions as shown in Table 1 of high carbon reinforcement for automobile bead wire rod. At least 55% of equiaxed crystals were used when using MEMS, and the pinch roll rolling amount was 0 ~ 6mm and the pinch roll rolling position was 0.7 ~ 0.8 at the center of solid state. Each change was made. The analysis of the central segregation of the cast steel thus tested was performed with a round bar of diameter 5mm and a length of 200mm around the central axis, and then cut into a lathe to collect about 1.5g of chips per 10mm in length to make a total of 20 samples. The sample was analyzed by carbon and sulfur simultaneous analyzer, and the average value of five carbon concentration maximums divided by the steel composition was taken as the central segregation.

Figure 5 shows the center segregation of the invention and comparative examples cast as shown in the graph, the center segregation of the cast steel produced by the present invention technology 1.18 ~ 1.24 as compared to the center segregation of 1.38 ~ 1.45 Significantly reduced. Figure 6 compares the wire rod quality test results of the billet cast steel produced as Inventive Example 1 with Comparative Example 1. The above graphs show the out-of-order error yield, the center segregation of the wire rod and the breakage incidence rate during the final product processing. As can be seen in the graph of FIG. 6, the product quality level of the billet produced by the present technology was 5.4% 0% of the order rate, wire segregation degree 1.4-0.9, and final product disconnection rate of 4.5 (times / ton) and 1.5 (times / ton). As a result, the cost reduction and demand contributed to the improvement of satisfaction.

Table 1 Test Conditions

As described above, according to the present invention, the mold stirrer is operated on the billet player to secure an equiaxed coefficient of 40% or more in the thickness ratio of the slab, and the portion of the central solid phase ratio of 0.7 to 0.8 is located behind the existing pinch roll (the mold direction is By lowering the cast steel by 2 ~ 10mm per pinch roll with pinch rolls, which greatly improves the generation of central segregation of high quality steel billets without the occurrence of internal cracks. The production of wire rod steel can be produced in bilet, and the real error rate can be greatly contributed to cost reduction.

While the invention has been shown and described with respect to particular embodiments, it will be understood that various changes and modifications can be made in the art without departing from the spirit or scope of the invention as set forth in the claims below. It will be appreciated that those skilled in the art can easily know.

Claims (2)

In the method of forming a billet playing cast, Encouraging equiaxed solidification while securing an equiaxed constant of at least 40% by the thickness ratio of the slab by applying an electromagnetic force to the unsolidified molten steel during casting by an electronic stirring device mounted on the mold; Adjusting the position of the solid state of the slab center to be 0.7 to 0.8 to be located behind the pinch roll when the mold direction is viewed forward; And And pressing down the cast iron by 2 to 10 mm through a pinch roll. According to claim 1, wherein the pinch roll is two, the position of the solid state ratio of 0.7 ~ 0.8 of the center portion of the cast steel is formed on the rear of the rear pinch roll when viewed in the mold direction, forming a billet playing cast piece Way.