JP3962301B2 - Metal casting method and apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is a metal casting method, in particular, when directly casting small cross-section steel slabs such as billets and blooms without going through a batch rolling process, the solidified structure is made fine equiaxed and the segregated grain size is minimized. The present invention relates to a casting method and a casting apparatus that can set the casting conditions necessary for the conversion to the conditions such as the slab size and the casting speed.
[0002]
[Prior art]
In the past, high-carbon steel wire rods such as steel cords were manufactured through the wire rod rolling process after the bloom slab was once rolled into billets, but directly cast the slab of the size to be used for wire rod rolling in the casting process. If it can be done, the process can be omitted greatly. In recent years, blooms and billets have been directly cast by continuous casting. In order to perform the rolling without trouble by directly sending the cast slab to the rolling process, the center segregation in the cast slab is dispersed as much as possible during casting, in other words, the equiaxed crystallization of the slab solidified structure is achieved. It is necessary to make the individual equiaxed crystals as small as possible, that is, to ultimately minimize the segregated grain size.
For example, according to the knowledge of the present inventors, in a billet of 120 mm to 160 mm square, when wire rod rolling is performed using a billet whose segregation particle size is minimized to 2 mm or less, it can be rolled without breaking. I know that.
[0003]
On the other hand, in order to prevent segregation and porosity at the center of the slab during continuous casting of steel, a stirring flow by electromagnetic stirring is applied to the front of the solidified shell during casting to equiax the solidified structure. Has been done. However, the casting conditions for minimizing the segregation grain size are clear because the equiaxed crystal ratio and grain size of the equiaxed crystal in the slab differ depending on the slab size, casting speed, molten steel superheat degree, or steel type. No specific proposal has been made in the past, and it remained unclear.
[0004]
[Problems to be solved by the invention]
Therefore, the present invention provides a casting necessary for miniaturizing the segregated grain size while achieving fine equiaxed crystallization of the solidified structure when directly casting a small-section slab such as a billet without going through the ingot rolling process. It is an object of the present invention to provide a metal casting method capable of easily setting conditions according to conditions such as slab size and casting speed, and a casting apparatus for carrying out this method.
[0005]
[Means for Solving the Problems]
The gist of the present invention is as follows.
(1) During the continuous casting of metal, the flow rate of the solidified shell front surface is determined by flow analysis, the cooling rate and solidification rate at each thickness is determined by heat transfer solidification analysis, and the solid phase rate washed by the flow is determined. The number of dendritic secondary branches in the cleaning region is obtained, and the number density of secondary branches in the molten metal pool is estimated based on the amount of secondary branches discharged into the residual molten metal pool. A casting method for a metal, characterized in that casting is performed by setting casting conditions such that the segregation particle size represented by using a segregation particle size is less than or equal to a segregation particle size allowable value determined by a slab size.
(2) The casting method according to (1), wherein a constant flow action is imparted to the molten metal in the continuous casting mold by electromagnetic force.
(3) The casting method according to (2), wherein a flow having a component parallel to the front surface of the solidified shell is imparted to the front surface of the solidified shell in two stages with respect to the molten metal in the mold and immediately below the mold.
(4) Below the mold, in the region where the ratio L / V _c between the distance L (m) from the molten metal surface and the casting speed V _c (m / min) is 1 or less, the front surface of the solidified shell is parallel to the front surface of the solidified shell. (1) The metal casting according to (1), wherein the ratio of the flow rate of cooling water sprayed onto the metal surface and the mass of the metal passing through the region is 1 liter / Kg or more. Method.
(5) The metal casting method according to (4), wherein an electromagnetic force having a component parallel to the front surface of the solidified shell in the mold is applied.
(6) The metal casting method according to any one of (1) to (5), wherein the estimated number density of secondary branches is 10 pieces / mm ³ or more.
(7) An apparatus capable of applying a flow having a component parallel to the front surface of the solidified shell to the front surface of the solidified shell in two stages with respect to the molten metal in the mold and immediately below the mold is a distance L (m) from the molten metal surface and a casting speed V _c. ( M / min) ratio L / V _c is set in a region of 1 or less, and the ratio of the flow rate of cooling water sprayed to the metal surface and the mass of metal passing through the region is 1 liter / Kg or more. A metal casting apparatus characterized by being capable of injecting water.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
Details of the present invention will be described below. In the following description, continuous casting of steel is taken as an example, but the present invention is not limited to this, and is effective for other metals such as aluminum and copper. It can be applied.
In continuous casting of steel, Takahashi et al. (Takahashi et al .: Iron and Steel, 61 (1975), 9, 2198) studied the phenomenon in which solute elements concentrated between dendritic trees were washed by flow and washed by flow. solid fraction S _h which is the report that is represented by a flow rate U coagulation speed V by the following equation (1).
S _h = U / (7500V + U) (1)
[0007]
FIG. 1 schematically shows a part of a dendrite of a solidified structure that is generated when the molten steel injected into a casting mold solidifies during continuous casting of a steel billet, (a) is a partial plan view, (B) is a side view of a certain range. In the figure, reference numeral 1 denotes a primary branch of a dendrite that extends from the outer periphery toward the center of the slab in the slab cross section, and 2 denotes a secondary branch of the dendrite derived from the primary branch 1. Based on the idea expressed by the above formula (1), it can be considered that a part of the secondary branch 2 of the dendrite that is forming on the primary branch 1 is melted by the flow at the same time. It is thought that a crystal is formed. At that time, to estimate the secondary branch to the solid phase ratio in S _h to be cleaned is present much, some of which is transferred into the residual molten steel pool, again depending on the temperature distribution of the remaining molten metal If it is determined that the melting or remaining will be determined, the number density n of equiaxed crystal nuclei in the residual molten metal pool can be estimated. Usually, since one equiaxed crystal is formed from one nucleus, if the number density of equiaxed nuclei in the molten metal pool can be estimated, the diameter of the equiaxed crystal in the slab can be estimated.
[0008]
Here, a case where a stirring flow having a constant flow rate is applied in a horizontal section will be described as an example. However, the stirring flow rate and the direction need not be constant, and the direction of the stirring flow may be a vertical direction. This will be specifically described below. First, by solving the continuity formula, the momentum conservation law, and the energy conservation law, the flow velocity U, solidification velocity V, temperature gradient G and cooling rate CR at each thickness, and temperature distribution in the residual molten metal pool are obtained. Ask. Together determine these flow rates U and the from the solidification rate V (1) solid phase rate S _h which is washed with formula at a length ΔL to be cleaned using a temperature gradient G at the time of solidification (2) Ask. Further, the dendrite primary branch interval λ ₁ and the secondary branch interval λ ₂ can be determined from the cooling rate CR by the following equations (3) and (4). Each symbol is also shown in FIG.
ΔL = (T _L −T _S ) / G × S _h (2)
λ ₁ = 767.4 (CR) ^-0.424 (3)
λ ₂ = 191.0 (CR) ^-0.447 (4)
[0009]
Next, based on the dendrite primary branch interval λ ₁ and secondary branch interval λ ₂ obtained above, a certain area ΔS of the front surface of the solidified shell, for example, the height Δz in the casting direction shown in FIG. The following formulas (5) and (6) determine how many dendrite primary branches and how many secondary branches exist in the area ΔS of the horizontal Δx. n ₁ represents the number of dendritic primary branches, and n ₂ represents the number of dendritic secondary branches. Based on these numbers, the amount q of the dendrite secondary branch discharged into the volume ΔV of the front surface of the solidified shell while the target portion (height Δz) is descending at the casting speed V _C is (7) Calculate with the formula.
n ₁ = 4ΔS / πλ ₁ ² (5)
n ₂ = 4ΔL / πλ ₂ (6)
q = fn ₁ n ₂ V _C / ΔzΔS (7)
The above operation is performed at all solidification interfaces, and the nucleus flows into the residual molten steel pool in the amount q, and the temperature in the residual molten steel pool determined earlier is equal to or higher than the liquidus (temperature at which solidification starts). The number density distribution of equiaxed nuclei in the residual molten steel pool is estimated by the following equation (8) by analyzing the distribution of the nuclear number density in the residual molten metal pool, assuming that the nuclei disappear in the region. Can do. Note that N in the equation (8) means that if the temperature in the molten metal pool is equal to or higher than the liquidus temperature, it is a function that makes the number density n in that region zero.
(∂n / ∂t) + U · ∇n = ∇ · D∇n + ∇ · q + N (8)
[0010]
FIG. 2 shows the result of analyzing the relationship between the distance from the meniscus and the number density of nuclei in molten steel with and without applying electromagnetic stirring (EMS). As shown in FIG. 2, it can be seen that the number density of nuclei is greatly increased by applying electromagnetic stirring, and that the value approaches a constant value at the bottom of the pool. In this analysis, 14% (f = 0.14) of the secondary branches in the region to be cleaned is cleaned, and the nucleus disappears if the temperature of the residual molten metal is equal to or higher than the liquidus temperature.
Next, casting was performed under various stirring conditions, and the same analysis was performed under those conditions, and the relationship between the asymptotic value of the nucleus number density at the bottom of the pool and the segregated grain size observed at the center of the slab thickness. investigated. The result is shown in FIG. 3, and it was found that there is a good correlation between the two, and the relationship between the nucleus number density n and the segregated particle diameter D can be expressed by the following equation (9).
D = 2.97n ^-0.19 (9)
[0011]
As described above, by adopting the casting method according to the present invention, it is possible to estimate the segregation particle size in the slab determined from the casting conditions, so the segregation particle size is minimized among the assumed casting conditions. The casting conditions can be selected as appropriate, and casting according to the selected conditions makes it possible to always obtain the best quality slab, which can be directly supplied to the hot rolling process. become.
For example, when casting a 120 mm square billet, if the allowable value of the segregated grain size, which does not cause any trouble in the wire rolling, is finally 2 mm, the number density of equiaxed nuclei can be found by equation (9). Casting conditions (flow rate, cooling rate, solidification rate, etc.) for obtaining the density can be set. In actual casting, the operation should be carried out according to the set casting conditions.
[0012]
In the present invention, as described above, it is desirable to install an electromagnetic stirrer as a means for imparting a flow action to the molten steel in front of the solidified shell, but the stirring action only affects the molten steel in the mold. It is effective not only to increase the number density of equiaxed crystal nuclei, but also to install the electromagnetic stirrer in two stages so that the stirring action can be exerted on the molten steel directly under the mold as well as in the mold. It was confirmed that
[0013]
Here, the position of the electromagnetic stirrer installed immediately below the mold is preferably a region where the cooling rate is fast, that is, a region where the distance between the primary branch and the secondary branch of the dendrite is small. In addition, by providing two electromagnetic stirring devices, the direction of stirring flow can be selected in various ways. By combining the circulation flow in the horizontal section and the circulation flow in the vertical direction, the stirring region can be formed over a wide range of the pool, or the flow in the same direction can be imparted to increase the stirring flow rate.
[0014]
Even if the secondary branch is melted by the flow and transferred to the residual molten metal pool, it melts again and disappears if the temperature of the residual molten metal is high. As a method for preventing this, it is conceivable to lower the temperature of the molten metal originally injected into the mold as much as possible. However, the nozzle for injecting into the mold is clogged, so that the normal injection temperature is usually limited. Thus, it is important how quickly the temperature of the molten metal injected into the mold is reduced. First, if flow is applied to the front surface of the solidified shell, heat transfer at the solid-liquid interface can be promoted. In addition, the temperature of the lower part of the molten metal at the upper part of the pool is reduced by the stirring flow formed in the pool. The temperature of the molten metal can be lowered by two effects that it can be mixed with a low molten metal. Furthermore, the metal mass W passing through the flow rate Q and the area of the coolant flow in the solidified shell front in downward for injecting the specific as well as metal surfaces of the distance L between the casting speed V _C from granted and bath level than the mold It has been confirmed that the ratio of the ratios of the equiaxed nuclei is effective for increasing the number density of equiaxed nuclei. The relationship is shown in FIG.
[0015]
As shown in FIG. 4, the ratio between the distance L from the molten metal surface and the casting speed V _C is 1 or less, and the ratio of the flow rate Q of the cooling water sprayed onto the metal surface and the metal mass W passing through the region is 1 liter / It can be seen that the number density of equiaxed crystal nuclei is increased by setting the weight to kg or more. This is because the temperature of the residual molten metal can be lowered by vigorously cooling the slab surface with good heat transfer at the solid-liquid interface due to the flow of the solidified shell front surface, and remelting of equiaxed crystals can be prevented. . In addition, the number of primary branches and secondary branches of the dendrite can be increased by increasing the cooling rate of the stirring region, and the number density of secondary branches to be cleaned is increased. When L / V _C is in the region of more than 1, the thickness of the solidified shell increases and the thermal resistance increases, which is not preferable because the cooling effect on the surface is reduced. Moreover, the effect can be made higher by installing an electromagnetic stirring apparatus in the area | region which lets water flow on these conditions.
Furthermore, by controlling the magnetic stirring and the flow rate of the cooling water, a state where the number density of equiaxed nuclei was increased was formed, and the relationship with the casting speed was examined. As shown in FIG. If the density can be 10 pieces / mm ³ or more, the same equiaxed crystal ratio can be obtained even if the casting speed is changed, and the segregated particle size can be stably reduced.
[0016]
FIG. 6 shows an example of a suitable continuous casting apparatus for carrying out the casting method of the present invention described above. In the figure, 3 is a mold for continuous casting, 4 is an immersion nozzle for pouring molten steel immersed in the molten steel 5 in the mold 3, 6 is powder to be poured into the molten steel surface, and 7 is drawn while forming a solidified shell 8. It is a slab that is pulled out. In such a casting apparatus, electromagnetic force devices 9a and 9b for applying an electromagnetic force to the molten steel are disposed in the mold 3 and immediately below the mold 3, respectively. By these electromagnetic force devices 9a and 9b, two steps are performed. A stirring flow 10 having a component parallel to the front surface of the solidified shell 8 can be applied to the molten steel. The installation positions of the electromagnetic force devices 9a and 9b are set in a region range in which the ratio of the distance L from the molten metal surface to the casting speed V _C is 1 or less.
Further, in the continuous casting apparatus, the slab first cooled in the mold is secondarily cooled by spraying cooling water in the roll band below the mold. Cooling in a region where the ratio of the distance L from the molten metal surface and the casting speed V _C is 1 or less is the ratio between the flow rate Q of cooling water sprayed on the surface of the slab and the metal mass W passing through the region. By defining it as 1 liter / kg or more, the number density of equiaxed crystal nuclei is increased. The cooling water 11 in FIG. 6 is displayed in such a manner that this point is simply emphasized, and does not mean that the cooling water is injected only at this position.
[0017]
【Example】
[Example 1]
The inventors of the present invention have examined appropriate electromagnetic stirring conditions in casting a 120 mm square billet using a vertical continuous casting machine. As a result, as shown in FIG. 6, the electromagnetic stirrer is installed at two positions in the mold and immediately below the mold (two-stage EMS in FIG. 7), compared with the case where the electromagnetic stirrer is installed only in the mold. Thus, it was confirmed that the number density of equiaxed crystal nuclei (solid line in FIG. 7) can be greatly increased. Therefore, when an electromagnetic stirrer was actually installed based on such conditions and a casting test was carried out, a slab filled with fine equiaxed crystals could be obtained, and the segregated particle size (broken line in FIG. 7) Can also be made 2 mm or less.
[0018]
[Example 2]
Moreover, although the same examination was performed on the conditions of a 160 mm square billet, the stirring conditions were examined using the method described in the present invention. As a result, as shown in FIG. 6, the electromagnetic stirrer is installed in the mold and directly under the mold (two-stage EMS in FIG. 8). It was confirmed that the number density of equiaxed crystal nuclei (solid line in FIG. 8) can be increased. Therefore, when an electromagnetic stirrer was actually installed under such conditions and a casting test was carried out, a slab filled with fine equiaxed crystals could be obtained, and the segregated particle size (broken line in FIG. 8) was 2 mm or less. And was able to.
[0019]
[Example 3]
The same examination was conducted under the condition of a 190 mm square billet. In this case, it was found that the cross-sectional area becomes large, and the region above the liquidus temperature may become excessive in the residual molten metal pool. Therefore, when an electromagnetic stirrer is installed in the mold and the cooling water flow rate in the region where the stirring flow just below the mold is changed, the distance L from the molten metal surface and the casting speed are changed as shown in FIG. The number density of equiaxed nuclei is increased by setting the ratio of the flow rate Q of cooling water sprayed on the metal surface to a metal mass W passing through the region when the V _C ratio is 1 or less. In addition, the effect could be further enhanced by installing electromagnetic stirring in that region. As a result, even in this cross-sectional size, the segregation particle size could be stably controlled to 2 mm or less.
[0020]
Further, the three-size slabs listed above were cast so that the nucleus number density was as high as possible, and the relationship with the casting speed was investigated. The results are shown in FIG. 10. Although the value of equiaxed crystal ratio varies depending on the size, if the number density of nuclei is 12 / mm ³ , almost the same equiaxed crystal even if the casting speed varies for each size. With a slab of a high rate, a segregated particle size of 2 mm or less could be stably formed.
[0021]
【The invention's effect】
According to the above-described method of the present invention, conditions for making the solidified structure fine equiaxed and minimizing the segregated grain size according to casting conditions such as slab size and casting speed can be easily set. Therefore, it is possible to cast a high-quality slab that can be directly used for wire rolling without performing the process, and a significant omission of the process can be achieved.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram schematically showing the state of generation of dendrite in a mold during steel casting, (a) is a partial plan view of the dendrite, and (b) is a side view of the dendrite in a certain range.
FIG. 2 is a diagram showing the difference in the number density of equiaxed crystal nuclei between when a magnetic stirrer is applied and when it is not applied in steel casting.
FIG. 3 is a graph showing the relationship between the number density of nuclei and the segregated grain size at the bottom of the residual molten steel pool.
FIG. 4 is a diagram showing the relationship between the flow rate / weight ratio of cooling water and metal and the number density of equiaxed crystal nuclei when L / V _C is changed.
FIG. 5 is a diagram showing the relationship between the number density of equiaxed crystal nuclei and the equiaxed crystal ratio in relation to the casting speed.
FIG. 6 is an equipment outline diagram showing an example of a casting apparatus according to the present invention suitable for carrying out the method of the present invention.
FIG. 7 is a diagram showing the influence of electromagnetic stirring on the number density of equiaxed nuclei and the segregated particle diameter in an example of the present invention using a 120 mm square billet.
FIG. 8 is a diagram showing the influence of electromagnetic stirring on the number density of equiaxed nuclei and the segregated particle size in an example of the present invention using a 160 mm square billet.
FIG. 9A shows the relationship between the ratio of cooling water flow rate and metal mass and the number density of equiaxed nuclei, and FIG. 9B shows the relationship between the ratio of cooling water flow rate and metal mass and the segregated particle size. FIG.
FIG. 10 is a diagram showing the relationship between the casting speed and the equiaxed crystal ratio in a three-size slab in an example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Primary branch of dendrite 2 Secondary branch of dendrite 3 Mold 4 Immersion nozzle 5 Molten steel 6 Powder for casting 7 Cast piece 8 Solidified shell 9a, 9b Electromagnetic force apparatus 10 Stir flow 11 Cooling water

Claims (7)

  In continuous casting of metal, flow analysis and heat transfer solidification analysis are used to determine the flow velocity, solidification rate, temperature gradient and cooling rate at each thickness, and temperature distribution in the residual molten metal pool, respectively, and flow from these. Obtain the solid fraction to be washed and the number of dendritic secondary branches in the washing region, and the molten metal based on the amount of secondary branches discharged into the residual molten metal pool and the temperature distribution in the residual molten metal pool. Estimate the number density of secondary branches in the pool, and set the casting conditions so that the segregated particle size represented by this number density is less than the segregated particle size allowable value determined by the slab size. A method for casting a metal, characterized in that:   2. The metal casting method according to claim 1, wherein a flow having a component parallel to the front surface of the solidified shell is imparted to the molten metal in the continuous casting mold by electromagnetic force.   The metal casting method according to claim 2, wherein a flow having a component parallel to the front surface of the solidified shell is imparted to the front surface of the solidified shell in two stages with respect to the molten metal in the mold and immediately below the mold. Below the mold, in the region where the ratio L / V _c between the distance L (m) from the molten metal surface and the casting speed V _c (m / min) is 1 or less, a component parallel to the front surface of the solidified shell is formed on the front surface of the solidified shell. 2. The metal casting method according to claim 1, wherein the ratio of the flow rate of the cooling water sprayed onto the metal surface and the mass of the metal passing through the region is 1 liter / Kg or more.   5. The metal casting method according to claim 4, wherein an electromagnetic force having a component parallel to the front surface of the solidified shell in the mold is applied. The metal casting method according to claim 1, wherein the estimated number density of secondary branches is 10 pieces / mm ³ or more. An apparatus capable of imparting a flow having a component parallel to the front surface of the solidified shell to the front surface of the solidified shell in two stages with respect to the molten metal in the mold and immediately below the mold is a distance L (m) from the molten metal surface and a casting speed V _c (m / Min) ratio L / _Vc is set in an area of 1 or less, and cooling water having a ratio of the flow rate of cooling water injected to the metal surface and the mass of metal passing through the area can be injected at 1 liter / Kg or more. A metal casting apparatus characterized by the above.

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