CN1299855C - Method for producing ultra low carbon steel slab - Google Patents

Abstract

An ultra-low carbon steel slab having a carbon content of about 0.01 mass percent or less is produced by casting at a casting speed of more than about 2.0 m/min using a mold provided with a casting space having a short side length D of about 150 to about 240 mm and an immersion nozzle provided with discharge spouts each having a lateral width d, the ratio D/d being in the range of from about 1.5 to about 3.0. Accordingly, a ultra-low carbon steel slab can be obtained having superior surface quality without performing slab conditioning such as scarfing.

Description

Method for producing ultra-low carbon steel plate

technical field

The present invention relates to a method for the production of continuously cast ultra-low carbon steel sheets, in particular to the production of steel sheets specifically for the manufacture of mobile phone casings and similar parts with high surface quality.

Background technique

The steel plate used to manufacture such as mobile phone casings is formed into complex shapes by deep drawing and/or deformed, and must have good forgeability. So-called "ultra-low carbon steels" are used, the carbon content of which is reduced to the lowest possible level. The carbon percentage of ultra-low carbon steel is generally 0.01 or less by mass. Among the above-mentioned ultra-low carbon steel sheets, cold-rolled thin steel sheets used for manufacturing mobile phone casings have particularly good appearance and colorability.

In the finishing rolling process of producing ultra-low carbon steel, it is necessary to undergo oxidation with oxygen to remove the carbon content in molten steel. Therefore, a deoxidation process of removing oxygen dissolved in molten steel in the oxidation process is also employed, and deoxidizers such as aluminum, magnesium, and titanium are used. In the deoxidation process, the oxygen dissolved in the molten steel reacts with the deoxidizer to form alumina, magnesia and titanium oxide, and the resulting reactants remain in the molten steel in the form of non-metallic inclusions.

When the plate is hot-rolled and/or cold-rolled into a thin steel plate, if the above-mentioned non-metallic inclusions exist near the surface of the plate, unfavorable defects such as cracks and/or shrinkage cavities will be formed on the surface of the thin steel plate.

In the continuous casting process, argon and casting powder are added to the surface of molten steel to prevent the immersion nozzle that supplies molten steel from the funnel to the mold from clogging. The argon trapped in the molten steel remains in the molten steel only in the form of bubbles, or combines with the reaction product of the above-mentioned deoxidation process (hereinafter referred to as "deoxidation reaction product") to form bubbles remaining in the molten steel . Both of the above situations may produce surface defects. In addition, the added casting powder remains in the molten steel, which also produces surface defects similar to those formed by deoxidation reaction products.

In the past, the ordinary plates of continuous casting were not subjected to surface treatment, that is, they were hot-rolled to produce cold-rolled thin steel plates. However, for the sheet material for the production of mobile phone casings, the surface layer of about 1 to 4 mm thick is partially removed, for example, surface defect cleaning is performed to remove inclusions of deoxidation reaction products, air bubbles, casting flux, and surface after hot rolling. Defective analogues are then hot-rolled and cold-rolled.

The removal of surface defects of the sheet material as described above reduces the yield of the sheet material as a raw material, and in addition, disadvantageously delays the process. Therefore, many attempts have been made to prevent the occurrence of surface defects in the sheet material which are the cause of the defects on the surface of the above-mentioned thin steel sheet in the process of producing the sheet material in the continuous casting equipment.

The basic ideas of these attempts are mainly based on the following points (1) to (6):

(1) The thickness of the plate increases, and then the cross-sectional area increases. Since the width of the plate is limited during rolling, the casting speed (m/min) is reduced. Therefore, the residence time of the molten steel in the mold is increased without lowering the productivity, so the time is increased for impurities such as deoxidation reaction products, casting powder, air bubbles, etc. to move from the inside of the molten steel of the mold to the surface.

(2) The continuous casting equipment used for casting has a vertical part, so that the deoxidation reaction products, casting powder, air bubbles, etc. can be better moved from the inside of the mold molten steel to the surface and separated.

(3) Transverse flow is generated by electromagnetic force near the trough plate to prevent impurities in molten steel from being trapped by the solidification shell (washing effect).

(4) Properly control the viscosity of casting powder to reduce the possibility of casting powder being involved in molten steel.

(5) Appropriately control the vibration (vertical vibration) of the casting mold during continuous casting to reduce the generation of solidification shell edges and corners in the casting mold (a phenomenon in which the solidification shell is inclined to the molten steel side due to vibration), and then reduce the deoxidation reaction products, The amount of casting powder, air bubbles, etc. that are trapped in the corners.

(6) Control the molten steel flow by electromagnetic agitation or applying electromagnetic stagnation to the molten steel entering the mold from the immersion nozzle, so as to prevent the molten steel mixed with the deoxidation reaction product from entering the bottom of the mold.

For example, in a technique disclosed in Unexamined Japanese Patent Publication No. 5-76993, molten steel having a carbon content of less than 0.01% by weight is cast on continuous casting equipment, the longitudinal portion of which is 20 m or more, and the casting speed 1.0m/min or more, with a production rate of 4 ton/min or more to produce plates with a thickness of more than 200mm and a width of more than 900mm, and the viscosity of the powder is set to 1.0 poise or more, and the speed of the inert gas flow into the nozzle is set 1L/min or more, electromagnetic agitation acts in the horizontal direction in the molten steel area from the channel plate to a depth of 1.5m at a speed of 15 to 40cm/sec. This technique is primarily based on paragraphs (1), (2), (3), (4) and (6) above.

In addition, in a technique disclosed in Unexamined Japanese Patent Publication No. 7-155902, the vibration of the casting mold is appropriately controlled to suppress the generation of angular portions where inclusions are easily trapped. Formed during the initial solidification stage. This technique is mainly based on the content of paragraph (5) above.

However, there are still some problems with the above techniques.

As disclosed in Unexamined Japanese Patent Publication No. 5-76993, when the cross-sectional area of the plate is increased, especially the thickness is increased, and the casting speed exceeds 1.5m/min, the near surface of the plate is caused by inclusions or the like The number of defects did not decrease as expected. The reason is that although the molten steel flow rate vm of the channel-shaped plate part is controlled at the optimum value by the transverse electromagnetic force, the production capacity increases after the thickness of the plate increases, and the casting speed (Vc) and plate width are the same when the cross-sectional area does not increase. In the case of (W), the injection velocity Vi of the submerged nozzle increases. Therefore, although the change of the average value of the molten steel flow velocity vm is small, the casting flux is more involved in the molten steel after its change amount increases. It can be seen that the cleanliness of the surface of the plate is not only determined by the flow rate of molten steel near the channel plate.

In addition, the influence of the molten steel flow ejected from the immersion nozzle is significant, and the growth of the shell on the short side of the mold is delayed to some extent. The reason is that in the plate continuous casting equipment, when the molten steel is injected into the mold, the so-called "double nozzle" provides the molten steel to be evenly distributed along the width direction of the casting space of the mold. The short side length D (corresponding to the thickness of the plate) is small, and the flow rate of molten steel changes in the direction of the thickness of the plate. Therefore, molten steel at a high flow rate impinges unevenly on the solidified shell portion along the short side, with the result that the growth of the solidified shell portion is delayed. In addition, the fluctuation of the flow rate of molten steel in the thickness direction of the plate also causes the fluctuation of the flow rate of molten steel near the above-mentioned channel plate to a certain extent.

Next, in the technique disclosed in Unexamined Japanese Patent Publication No. 7-155902, in order to improve the surface quality of the plate, the negative velocity slab time T determined by the casting speed, mold amplitude and vibration frequency is adjusted by The mold vibration state, especially reducing the mold vibration amplitude and increasing the mold vibration frequency, is controlled within a specific range, and the following problems arise at this time.

When the casting speed of ultra-low carbon steel is higher than 2.0m/min, and the vibration frequency of the mold is higher than 185 times/min, the abnormal phenomenon of sudden and large changes in the surface plane of molten steel will occur, although this phenomenon is not frequently observed . As a result, casting flux may be entrained in molten steel or trapped by the solidification shell, causing surface defects in the cast thin steel plate. Therefore, surface defects due to casting flux often occur when the casting speed exceeds 2.0 m/min. Thus, products having a high-quality surface cannot be stably produced.

As mentioned above, when ultra-low carbon steel sheets are used to manufacture mobile phone casings or similar parts, casting at a high speed higher than 2.0m/min, without cleaning the surface defects of the sheet, current technology cannot stably Produce high-quality panels.

It would therefore be very beneficial to provide a continuous casting method for the production of ultra-low carbon sheets which can stably produce sheets with high surface quality at casting speeds even higher than 2.0 m/min without surface defect removal of the sheets of.

Contents of the invention

The invention provides a method for producing ultra-low carbon steel plates, comprising:

Provided is a continuous casting apparatus comprising a casting mold provided with a casting space having a shorter side length D of about 150 to about 240 mm, and an immersion nozzle provided with at least one nozzle opening of a transverse width d, wherein the D/d ratio is between about 1.5 and about 240 mm. In the range of about 3.0;

Introducing molten steel into the mold through immersion nozzles; and

Cast molten steel at a speed higher than 2.0 mm/min in a continuous casting device to produce an ultra-low carbon steel plate with a carbon mass percentage of 0.01% or lower.

Preferably, the sheet continuous casting method may further include vibrating the casting mold at a frequency of 185 times/min or less. The possibility of anomalous phenomena with sudden and large changes in the surface level of the molten steel is reduced. Therefore, when the mold is vibrated at a frequency of 185 times/min or less, the resonance between the molten steel surface and vibration of the mold is reduced, so the amount of defects caused by the flux is reduced.

The casting speed may be about 2.4 m/min or faster. The corner depth becomes about 0.7 mm or less, that is, the depth of impurities trapped does not exceed the corner depth at a casting speed of about 2.4 m/min or faster. Therefore the casting speed is preferably set at 2.4 m/min or faster.

As the above-mentioned immersion nozzle, a cylindrical nozzle (so-called "straight nozzle") or a double nozzle nozzle is generally used. The front end of the double nozzle nozzle is closed, and two approximately circular nozzles are installed on the two short sides facing the mold. of the spout.

When product quality is considered in combination with sheet thickness, submerged nozzle durability and required flow rate, the D/d ratio of the length D of the short side to the lateral width d of the submerged nozzle orifice is preferably about 2.1 to about 2.9.

The above-mentioned ultra-low carbon steel sheet is preferably used as a raw material for cold-rolled thin steel sheets for producing mobile phone casings.

The above continuous casting method for plates preferably further includes applying electromagnetic force stagnation to the molten steel in the casting space of the mold. The following paragraphs (A) to (C) serve as the preferred method of applying electromagnetic force stagnation:

(A) Electromagnetic force stagnation is generated by using a static magnetic field that runs through the thickness of the mold and covers approximately the entire mold, wherein an upper magnetic field generating device and a lower magnetic field generating device are used. The upper magnetic field generating device is installed on the upper part of the casting mold including the molten steel surface plane, and the lower magnetic field generating device is installed below the upper magnetic field generating device. The immersion nozzle is installed between the upper and lower magnetic field generating devices, and the immersion depth is set between about 200 to about 350 mm.

(B) Using the upper magnetic field generating device installed on the upper part of the mold including the surface plane of the molten steel, the static magnetic field and the AC magnetic field are superimposedly applied to the entire mold through the thickness of the mold to generate electromagnetic hysteresis. The immersion nozzle is installed at the lower end of the magnetic field generating device, and the immersion depth is set between about 200 to about 350 mm.

(C) Electromagnetic hysteresis is generated by superimposing a static magnetic field and an alternating magnetic field throughout the mold thickness using an upper magnetic field generating device, and applying a lower magnetic field generating device through the thickness of the mold A static magnetic field is applied throughout the mold. The upper magnetic field generating device is installed on the upper part of the casting mold including the molten steel surface plane, and the lower magnetic field generating device is installed below the upper magnetic field generating device. The immersion nozzle is installed between the upper and lower magnetic field generating devices, and the immersion depth is set between about 200 to about 350 mm.

Description of drawings

Figure 1 shows the relationship between casting speed and bevel depth in accordance with aspects of the present invention;

Figure 2 shows the relationship between the depth h of the retentate from the surface of the plate and the amount of retentate according to aspects of the present invention, obtained at different casting speeds;

Figure 3 shows the relationship between the distance L of the retentate from the channel plate and the amount of retentate according to aspects of the present invention, obtained at different casting speeds;

Figure 4 shows the effect of sheet thickness and casting speed on the amount of crowning produced on the short sides in accordance with aspects of the present invention;

Fig. 5 represents according to aspects of the present invention, the impact of plate thickness on the proportion of product surface defects;

Figure 6 shows the influence of casting speed on the proportion of product surface defects according to aspects of the present invention;

7A to 7C are schematic diagrams of suitable molds for use in accordance with aspects of the present invention, and continuous casting molds with magnetic field generating means, respectively;

8 is a schematic diagram of an example of using an AC oscillating magnetic field according to aspects of the present invention; and

Fig. 9 is a schematic diagram of an example of using an AC traveling wave magnetic field according to aspects of the present invention.

Detailed ways

We have disclosed that the casting speed is properly controlled, the short side length D of the continuous casting mold casting space, the ratio D/d of the short side length D to the transverse width d of the submerged nozzle nozzle, and when necessary, the vibration frequency of the mold is properly controlled, or in The effective use of electromagnetic hysteresis in molten steel flow can better produce ultra-low carbon content plates.

According to aspects of the present invention, the steel is a so-called "ultra-low carbon steel" with a carbon mass percentage of about 0.01% or less. Components other than carbon are not particularly limited. The first choice, however, is a steel that can be deep-drawn to produce cell phone casings and similar components. An advantage of the present invention is that the produced steel has few defects caused by inclusions, and the inclusions hardly exist in the region from the surface of the plate to a certain depth, which does not need to be stripped off in the subsequent process. Many of the advantages of the present invention can thus be obtained in ultra-low carbon steels in which non-metallic inclusions such as alumina are easily formed as deoxidation reaction products during the finishing process.

Typical components in ultra-low carbon steel (excluding the component carbon) are illustrated by the following examples: silicon with a mass percentage of about 0.01% to about 0.04%, manganese with a mass percentage of about 0.08% to about 0.20%, and a mass percentage of 0.008 to about 0.020 percent phosphorus, about 0.003 to about 0.008 percent by mass sulfur, about 0.015 to about 0.060 percent by mass aluminum, about 0.03 to about 0.080 percent by mass Titanium, about 0.002% to about 0.017% by mass niobium, and 0 to about 0.0007% by mass boron.

The continuous casting equipment according to the present invention is a continuous casting equipment for producing steel plates, and can be arbitrarily selected from longitudinal continuous casting equipment, longitudinal deflection continuous casting equipment and bending continuous casting equipment. But among the above-mentioned equipment, the longitudinal deflection continuous casting equipment is particularly advantageous in terms of productivity and product quality.

The casting mold is a so-called "sheet continuous casting mold". Its short sides have a length of about 150 to about 240 mm. The length of the long side of the casting mold is not particularly limited, and is preferably approximately equal to the length of common cold-rolled steel sheets (especially cold-rolled steel sheets for mobile phones), such as about 900 to 2,200 mm. The length of the short side corresponds to the thickness of the board when the board is produced, and the length of the long side corresponds to the width of the board.

The height of the longitudinal mold is not particularly limited. However, since the formed solidified shell has a certain thickness, even when the casting speed exceeds 2.0 m/min, the cast thin steel plate does not bulge through the mold, so the height is preferably set at about 800 to about 1000 mm.

A submerged spout acts as a nozzle to inject molten steel from the funnel into the cavity of the mold. The material of the submerged nozzle can be a common material such as alumina-graphite. But the material is not limited to this.

In addition, regarding the shape of the immersion nozzle, it may be a generally mentioned cylindrical nozzle (so-called "straight nozzle"), or a double-spout nozzle whose front end is closed and placed on two sides facing the mold. Two approximately circular spouts are installed on the short side. The cross-sectional shape of the spout can be circular, square, or rectangular (longer in the transverse direction, or longer in the vertical direction), and there is no special limitation, as long as the maximum width d of the spout meets the type of the situation of the present invention, it can be used.

Also, the casting speed was set at 2.0 m/min or more for the following reasons. The casting speed is preferably set at 2.4 m/min or faster.

When the electromagnetic force is used to stagnate the molten steel in the casting space of the mold of the continuous casting equipment, as a preferred method, for example, Unexamined Japanese Patent Publication No. 2-284750 mentions that along the long side width, the A static magnetic field is applied in the casting mold, or as mentioned in Unexamined Japanese Patent Publication No. 57-17356, a static magnetic field is applied only at the position where molten steel is ejected. The main ideas of JP No.2-284750 and JP No.57-17356 can be combined as a reference here.

According to the present invention, when the short side length (sheet thickness) of the casting space of the mold is set at about 150 to about 240 mm, and the casting speed is set at more than 2.0 m/min, some different phenomena will occur in casting. Next, some new findings concerning the above-mentioned phenomenon will be explained. Inclusions, air bubbles and the like are hereinafter referred to as "impurities".

(1) Reduce the area where impurities are trapped

The formation of the initial solidified shell of the channel plate portion, the so-called "corner", is significantly suppressed when the casting speed Vc is set at 2.0 m/min or more, preferably 2.4 m/min or faster. We believe that the reason is that from any fixed depth below the surface of the molten steel, the thickness of the solidified shell formed will decrease with the increase of the casting speed Vc. Due to the influence of the static pressure of the molten steel, the force acting on the side of the mold is greater than that of the corners tilting towards the molten steel. , the latter is caused by the thermal contraction of the solidified shell and depends on its thickness. In addition, when the thickness of the plate decreases, the absolute value of the shrinkage of the shell in the thickness direction "plate thickness × temperature difference × thermal expansion coefficient" decreases, and the inclination of the shell to the side of the molten steel will be further restrained, and finally the effect of restraining the inclination of the edges and corners will be reduced. more obvious.

Figure 1 shows the effect of casting speed on bevel depth. When the casting speed is higher than about 2.0 m/min and the short side length (sheet thickness) of the casting space of the mold is about 240 mm or less, the corner depth becomes 1 mm or less. Also, at a casting speed of about 2.4 m/min or faster, the corner depth becomes 0.7 mm or less.

(2) Inhibit the absorption of impurities

Along with the solidification process, due to the segregation of dissolved substances concentrated on the solidification shell interface, a surface tension gradient is generated, and the force generated by this gradient makes it easy for impurities to be absorbed or trapped at the solidification shell interface. Attempts were therefore made to reduce the concentration of dissolved elemental sulfur or titanium, which had a marked effect on improving the ability to absorb or retain impurities. But in some cases, changing the composition can have a downside, such as increased cost when reducing sulfur, and reduced quality when reducing titanium.

According to the present invention, increasing the casting velocity Vc reduces the force to absorb or trap impurities at the interface of the solidified shell. That is, when the casting speed Vc is high, such as above 2.0m/min, because the amount of solidification and segregation in the channel plate portion is reduced, the surface tension gradient for attracting impurities is also reduced. Therefore, the amount of impurities absorbed or trapped on the side of the solidified shell is also reduced.

(3) Reduction in the thickness of trapped impurities

Fig. 2 shows the relationship between the interception depth h from the surface of the plate to trap impurities and the amount of trapped impurities on the surface of the plate. In addition, Figure 3 shows the relationship between the amount of trapped impurities and the distance L from the channel plate (surface of molten steel), which is obtained by converting the interception depth h from the surface of the plate:

h=k(L/Vc) ^1/2

In this equation, Vc represents the casting speed, and the solidification constant k is 20 mm·min ^-1/2 .

It can be seen from Figures 2 and 3 that impurities are trapped by the shell in a region from the surface of the molten steel to a depth of 20 mm. In addition, the retention depth decreases when the casting speed increases, and when the casting speed Vc is 2.0 m/min or more, the retention depth from the surface of the plate is 1 mm or less.

When the interception depth is 1mm or less, although the impurities are trapped by the shell, in the subsequent process of producing products through the hot rolling process and cold rolling process, these impurities are peeled off and removed together with the oxide scale on the surface of the cast steel sheet. clear. Defect-free products can thus be obtained without sheet metal processing. In addition, at a casting speed of 2.4 m/min or more, the corner thickness becomes 0.7 mm or less, and the retention depth h does not exceed the corner thickness. Therefore, the casting speed is preferably set at 2.4m/min or above.

(4) Reduction in the possibility of impurity retention

Impurities in the region from the surface of molten steel to a depth of 20mm are easily trapped by the solidification shell, and when the casting speed increases, the residence time of the solidification shell in this area also decreases. Therefore, even with the same amount of impurities in molten steel, the possibility of impurities being trapped in the solidification shell is reduced. For example, when Vc is 3.0m/min, the probability of impurity entrapment is reduced to half that when Vc is 1.5m/min.

(5) The optimal mold vibration frequency to prevent sudden changes in the surface plane of the molten steel

When casting at a speed of Vc higher than about 2.0 m/min, a doming phenomenon occurs, although not very noticeable, because the thickness of the solidified shell in the mold is further reduced. Bumping is the phenomenon of the solidified shell being pushed towards the sides of the mold under the influence of hydrostatic pressure. In the heaving phenomenon, when the shell temperature is high and the type of steel is low carbon steel, or a steel with less shell strength compared to other types of steel, the speed of heaving (being pushed towards the mold) becomes faster than The mold vibration speed is high. The mold usually has a cone to compensate for the volume shrinkage caused by solidification shrinkage and/or heat shrinkage. When the mold is vibrated vertically, the solidified shell rises by the amount _δb as the mold descends. Conversely, as the mold rises, the mold pushes against the housing with a thrust δ _p approximately equal to δ _b and bulges it. After a simple calculation, it can be seen that the change of the surface plane of the molten steel caused by the volume change is small, less than 1mm. However, when the above phenomenon is repeated, the vibration of the surface plane of the molten steel and the vibration of the mold resonate with each other. An abnormal phenomenon of sudden and large-scale changes in the surface level of the molten steel then occurs in a few cases. It is not easy to detect this phenomenon on the molten steel surface with an eddy current type planar sensor because the anomaly occurs at the edge of the mold. However, we first discovered this phenomenon while studying the time-dependent distortion of a vibration signature of a cast steel sheet. Especially, when the casting speed is above 2.0 m/min, and the vibration frequency of the mold is relatively high, for example above about 185 times/min, the above abnormal phenomenon is relatively easy to be observed. As a result, casting flux may be entrained in the molten steel and may be trapped in the solidified shell, which in turn creates defects on the surface of the cast sheet. Accordingly, when the casting speed is above about 2.0 m/min, the surface defects caused by the casting flux suddenly increase. Ultimately reducing surface defects can become difficult.

However, from the relationship between the mold vibration frequency and the ratio of flux-related defects to the total defects, that is, the ratio as an index indicating the occurrence rate of sudden abnormal phenomena, it can be found that when the mold vibration frequency is set to about 185 times/min or less , Even if the casting speed Vc is 2.0m/min or more, the above-mentioned abnormal phenomenon can be effectively prevented.

In addition, the setting of the lower limit of the vibration frequency of the mold can consider reducing the area for trapping impurities, so as not to increase the depth of corners, and also consider preventing the restraint fracture caused by the reduction of lubricant performance (mold flux consumption) in the mold. For example, the negative velocity strand time is preferably about 0.02 seconds or more, and the negative velocity strand length is preferably about 0.1 mm or greater. Negative velocity slab time is a characteristic value defining the vibration condition of the casting mold, and represents a period of time during which the casting mold descends faster than the casting thin steel plate. The negative-speed slab length indicates the maximum distance that the casting mold passes through the drawn cast thin steel plate between the casting mold and the cast thin steel plate within the negative-speed slab time. Assuming that the vibration waveform of the mold is a sinusoidal waveform, πSf/Vc>1 is satisfied, where S represents the amplitude of the mold, f represents the frequency of the mold, and Vc represents the casting speed. For example, when Vc is equal to 2.0m/min and S is equal to 9mm, the lower limit of casting frequency f is 71cpm (times/min), and when S is equal to 5mm, the lower limit is 127cpm (times/min). The vibration waveform of the casting mold is not necessarily limited to a sinusoidal waveform. Also, considering the particularity and controllability of the vibration condition of the continuous casting equipment, the lower limit of the frequency and the waveform can be properly determined.

(6) Prevent the short side from bulging (the reason for setting the upper limit of the length of the short side of the casting mold continuous casting space)

Although the submerged nozzle used satisfies the D/d ratio of the short side length (sheet thickness) D of the mold casting space to the lateral width d of the submerged nozzle nozzle, when the short side length is too large, the casting speed Vc exceeding 2.0m/min will occur question. In particular, bulges on the short sides can lead to shape defects and/or breaks in the sheet. On the contrary, when the length of the short side is small and the casting speed Vc is high, the bulge caused by the static pressure of the molten steel through the short side of the mold plate will be reduced, so that the risk of fracture is small.

However, as shown in Figure 4, when the length of the short side (that is, the thickness of the plate) exceeds 240mm, although the casting speed is 2.4m/min, due to the increase in the thickness of the plate, the speed of the molten steel sprayed from the nozzle nozzle increases, and the electromagnetic stagnation The induced second flow rate is also increased. It is not easy to suppress the delay in the growth of the short-side shells. Swelling of the short side at the bottom end of the mold becomes noticeable, increasing the risk of fracture (swelling amount 10 mm or more).

In addition, when the length of the short side (that is, the thickness of the plate) exceeds 240 mm, based on the same reason above, the fluctuation of the surface plane of the molten steel is aggravated by the backflow and secondary flow of the molten steel jet flowing out from the short side of the solidified shell, and it is easy to cast the casting aid. Flux is entrained and trapped. In addition, due to the increase in plate thickness, molten steel is prone to stagnation at the channel plate, especially at the immersion nozzle. As a result, as shown in Fig. 5, the number of sheet surface defects and product defects increased.

(7) The reason for setting the lower limit of the length of the short side of the casting mold continuous casting space

The short side length (thickness of the plate material) of the mold continuous casting space is preferably not less than about 150 mm for the following reasons.

From the viewpoint of the controllability of the molten steel surface plane, the above effect (1) cannot be obtained when the cross-sectional area of the plate is reduced too much. The reason for this is that when changing the cast size, fluctuations in the surface plane of the molten steel increase compared to when plates with larger cross-sectional areas are produced. Also, since the molten steel produces ripples, the generation ratio of corners having a depth of 1 mm or more increases. In addition, the casting flux is easily involved and trapped due to the fluctuation of the molten steel surface (see Figure 5). In addition, the outer diameter of an ordinary immersion nozzle is determined by the total number of wall thicknesses (about 20mm or more) determined in consideration of durability, and the determination of its inner diameter (about 70mm to about 130mm) is to ensure from 5.4ton/ Min (150mm thick, 2200mm wide, speed Vc is 2.1m/min or faster) to 14.5ton/min (240mm thick, 2200mm wide, speed Vc is 3.5m/min or faster) throughput. In this case, when the short side length D (sheet thickness) is too small, the distance between the outer wall of the nozzle and the long side of the solidified shell is too small (less than 20mm), and the flow between them becomes uneven, which will cause longitudinal cracks. In extreme cases, the solidified shell comes into contact with the nozzle and becomes stuck, causing a fracture. Therefore, the length of the short side D (thickness of the plate) should not be less than 150mm (inner diameter 70mm + thickness of the entire outer wall 40mm (20×2) + distance between the outer wall of the nozzle and the long side of the solidified shell 40mm (20×2)).

In addition, there is no special limitation on the length of the long side of the mold casting space (the width of the plate), which can be equal to the width of ordinary cold-rolled steel sheets (especially cold-rolled steel sheets used for mobile phones). The length is preferably about 900 to 2200 mm.

The mold longitudinal height is not particularly limited. However, the height is preferably set at about 800 to 1000mm, because the solidified shell must be formed in a certain thickness so that the cast steel sheet does not bulge when it passes through the mold, even when the casting speed is above 2.0m/min.

(8) Optimization of the D/d ratio of the length D of the short side of the casting space of the mold to the lateral width d of the spout.

When the molten steel is decelerated, the molten steel sprayed from the submerged nozzle nozzle extends in the width direction until it hits the short side shell. However, the degree of deceleration and dispersion of the jet velocity impinging on the short side shell depends on the sheet width W, the casting velocity Vc and the D/d ratio. When the spout width d of the submerged nozzle is too small (D/d is too large) relative to the length of the short side of the mold casting space (the width of the plate), when D, Vc and W increase, the width of the molten steel impacting the short side shell area at high speed and The ratio of sheet thickness (short side width) decreases. So the growth of the solidified shell becomes uneven and easily disturbed. Also, fractures may occur when the thickness of the solidified shell increases too much. On the other hand, when the nozzle opening width d of the submerged nozzle is too large (D/d is too small) relative to the length of the short side of the mold casting space (slab width) D, when D, Vc and W increase, because the molten steel jet flow is impacting The long side of the solidified shell is impacted before the short side of the solidified shell, so the growth of the long side of the solidified shell is disturbed, resulting in the generation of transverse cracks and/or oblique cracks. In addition, fracture occurs in some cases when the thickness of the solidified shell is too small. It is difficult to observe the effect of sheet width in both cases.

In addition, the molten steel rises after impacting the short side of the solidified shell, and then flows along the surface of the molten steel on the long side. When the D/d ratio exceeds the optimum range due to the speed change in the thickness direction of the plate, the flow velocity change near the grooved plate will also be affected. To a certain extent, the amount of mold flux involved will also increase.

The maximum lateral width d of the orifice determined by securing a throughput from about 5.4 ton/min to about 14.5 ton/min is preferably equal to or smaller than the inner diameter (70 to 130 mm) of the submerged nozzle in consideration of wear resistance. Thus, the optimal short side length (thickness of the plate) D (150 to 240mm) of the mold casting space and the lateral width d (70 to 130mm) of the spout can determine the D/d ratio. When casting for a long time up to 300 minutes or longer, the overall outer wall thickness is preferably set to 25 mm x 2 = 50 mm or more. In addition, the distance between the mold and the nozzle is preferably set to 40mm or more to ensure more stable quality. That is, the required thickness except the inner diameter is 50+40×2=130mm. On the other hand, when casting for a short time, the thickness of the entire outer wall can be set to 20 mm x 2 = 40 mm, and the distance between the mold and the nozzle can be set to about 20 mm. That is, the required thickness except the inner diameter is 40+20×2=80mm.

In Table 1 the results of the investigation on the effect of the D/d ratio on the product quality are shown. The D/d ratio is preferably set between 1.5 and 3.0. However, the D/d ratio is more preferably set between about 2.1 and about 2.9 when considering the optimum sheet thickness, durability of the immersion nozzle, and desired flow rate.

Table 1 serial number Plate thickness D(mm) Sheet width W(mm) Casting speed Vc(m/min) Immersion nozzle spout width d(mm) D/d Mold amplitude S (total amplitude) (mm) Mold vibration frequency f (times/minute) tn ^* (s) Electromagnetic hysteresis Number of cracks on the surface of the plate (≥5mm) (/m ² ) Surface defect rate of cold-rolled steel sheet (%) break Remark 1 220 1100-1800 2.4 60 3.67 7 160 0.098 Type 1 65 2.1 Break BO on short side comparative example 2 220 1100-1800 2.4 70 3.14 7 160 0.098 Type 1 twenty three 0 no comparative example 3 220 1100-1800 2.4 75 2.93 7 160 0.098 Type 1 0 0 no Example 4 220 1100-1800 2.4 80 2.75 7 160 0.098 Type 1 0 0 no Example 5 220 1100-1800 2.4 130 1.69 7 160 0.098 Type 1 5 0 no Example 6 235 1100-1800 2.4 88 2.67 7 160 0.098 Type 2 0 0 no Example 7 235 1100-1800 2.4 100 2.35 7 160 0.098 Type 2 0 0 no Example 8 235 1100-1800 2.4 120 1.96 7 160 0.098 Type 2 1 0 no Example 9 235 1100-1800 2.4 160 1.47 7 160 0.098 Type 2 ≥100 23.5 Break BO on the long side comparative example

Type 1: EMBR Type 2: EMLS tn ^* =60/f-tp tp=60/(πSf)×acos(-1000Vc/πSf) COMP.EX: Comparative example

(9) Electromagnetic force stagnation

When the casting speed Vc is about 2.4 m/min or more, or the throughput is about 7 ton/min or more, although D/d is optimized, a slight increase in product defect rate can be observed.

As mentioned above, it is preferable to use electromagnetic force to stagnate the flow. Under the effect of stagnation, the operation can be made more stable and the quality can be improved.

For the method using electromagnetic force stagnation, the techniques disclosed in the above-mentioned Unexamined Japanese Patent Publication No. 2-284750 and JP No. 57-17356 can be employed.

A continuous casting mold with a magnetic field generating device suitable for use in the present invention is schematically shown in FIGS. 7A to 7C.

FIG. 7A shows that the magnetic field generating device 1 is installed on the upper part of the casting mold including the surface plane of molten steel and at a predetermined distance below it to apply a static magnetic field in two stages. Fig. 7B shows that the magnetic field generating device 2 is installed only on the upper part of the casting mold including the surface plane of the molten steel, and applies the superimposed magnetic field of the static magnetic field and the alternating magnetic field. 7C shows that the magnetic field generating device 2 is installed on the upper part of the casting mold including the molten steel surface plane to apply the superimposed magnetic field of the static and AC magnetic fields, and the magnetic field generating device 1 is installed at a predetermined distance below the magnetic field generating device 2 to apply the static magnetic field.

Among the various magnetic field generating devices described above, when used as a device for generating a static magnetic field, the magnetic flux density value of the DC magnetic field is preferably set to about 1000 to about 7000 Gauss. This value can be applied to both cases, when two units are installed at the upper and lower ends and when only one unit is installed at the lower end.

There are two kinds of AC magnetic fields, namely AC oscillating magnetic field and AC traveling wave magnetic field, and both are preferably used in the present invention.

Fig. 8 shows a magnetic field in which alternating currents whose phases are almost opposite to each other act on adjacent coils in an alternating current oscillating magnetic field, or alternating currents of the same phase act on a magnetic field in coils whose coil directions are opposite to each other, so that insertion in adjacent coils produces magnetic field. When the AC oscillating magnetic field and the DC magnetic field are superimposed on each other, the local flow of molten steel will be reduced. In the figure, the mark 3 represents the DC coil, the mark 4 represents the AC coil, the mark 5 represents the casting mold, and the mark 6 represents the molten steel (the slanted part represents the area with a slow flow rate).

In addition, the AC traveling wave magnetic field is a magnetic field in which an alternating current reversed 360°/N acts on adjacent N coils. As shown in Fig. 9, in general use, N=3 (phase difference 120°) and high frequency can be obtained. As mentioned above, when the AC oscillating magnetic field and the DC magnetic field are superimposed on each other, the local flow of molten steel will be reduced.

When using a magnetic field generator capable of generating an AC magnetic field as described above, the magnetic flux density of the AC magnetic field is preferably set to about 100 to about 1000 Gauss, and the frequency of the oscillating magnetic field is preferably set to about 1 to about 10 Hz.

In addition, when a device capable of producing a static magnetic field superimposed with an AC magnetic field is used, the magnetic flux density value of the DC magnetic field is preferably set to about 1000 to about 7000 Gauss, and the magnetic flux density value of the AC magnetic field is preferably set to about 100 to about 7000 gauss. 1000 Gauss.

As mentioned above, during continuous casting, the electromagnetic force of the magnetic field generator is used to form a stagnation of molten steel. The new discovery of the continuous casting process in the mold as described above will be explained below in conjunction with the reasons for limiting the production conditions of the present invention.

(10) Nozzle immersion depth (distance from the surface of molten steel to the upper end of the nozzle)

The circulating state of molten steel in the mold changes according to the depth of nozzle immersion. In particular, when the continuous casting speed is high, the flow rate from the submerged nozzle is faster, so its submersion depth should be optimized. When the immersion depth is too small, the flow velocity on the surface of molten steel is too high. Therefore, flux is easily involved. On the other hand, when the immersion depth is too large, the flow velocity on the molten steel surface is too slow, and the washing effect at the interface of the solidified shell is weakened. Thus air bubbles and inclusions are easily trapped.

Therefore, based on the above considerations, the optimization of the nozzle immersion depth is studied, and it is found that the range of the nozzle immersion depth is set at about 200 mm to about 350 mm.

In addition, for the material of the immersion nozzle as described above, it is preferable to use a general material such as alumina-graphite. But the material is not limited to this.

For the immersion nozzle as described above, generally a cylindrical nozzle or a double nozzle nozzle is used, the front end of which is closed and two approximately circular nozzles are installed on both short sides facing the mold. The cross-sectional shape of the spout can be circular, square, or rectangular (longer in the transverse direction, or longer in the longitudinal direction), and there is no particular limitation, as long as the maximum width d satisfies the type of the spout shape described below in the present invention.

As already stated, when flux is entrained or inclusions are floating in molten steel, paragraphs (5), (6) and (8) above can also minimize the entrainment of casting flux, and above paragraph ( Paragraphs 2) and (4) can prevent impurities from being trapped in the solidification shell, and even when impurities are trapped, the depth from the surface of molten steel will become shallower due to the above paragraphs (1) and (3) to avoid defects . Therefore, in the production process, especially in the plate heating process, the peeling and removal of impurities on the surface of the plate will become easier.

Therefore, from the above paragraphs (6), (7), (8), and (9), the above effects can be stably achieved, and high productivity can be obtained.

Example 1

Continuous casting was carried out with the continuous casting equipment prepared under the conditions in Table 2A and 2B, wherein the continuous casting space of the mold has different short side lengths, 110mm (for continuous casting equipment test), 200, 215, 220, 235, and 260mm (for longitudinal bending production of continuous casting equipment) and other plate types with different thicknesses, and different plate widths such as 400mm (for continuous casting equipment production) and 900 to 2200mm (for continuous casting equipment longitudinal bending production). In this process, the height of the casting mold is 900mm (continuous casting equipment test) and 700mm (continuous casting equipment test), the immersion nozzle is a double nozzle nozzle made of alumina-graphite material, its wall thickness is 25mm, and the nozzle shape It is square (the thickness of the plate is 220mm or less at this time) or circular (the thickness of the plate is more than 220mm at this time), the downward spray angle is a constant value of 20°, and the nozzle immersion depth (the distance from the surface of the molten steel to the upper end of the nozzle) is set Set at 200 to 250mm. The casting flux material used has a solidification temperature of 1000, a viscosity of 0.05 to 0.2 Pa.s (0.5 to 2.0 poise) at 1300, and a basicity (CaO/SiO ₂ ) of 1.0. In addition, the degree of superheat of the molten steel in the funnel is set at 10 to 30. There is also the composition of molten steel, which has the composition of ultra-low carbon steel, the mass percentage of carbon is 0.0005% to 0.0090%, the mass percentage of silicon is less than 0.05%, the mass percentage of manganese is less than 0.50%, and the mass percentage of phosphorus Less than 0.035%, the mass percentage of sulfur is less than 0.020%, the mass percentage of aluminum is 0.005% to 0.060%, the mass percentage of titanium is less than 0.080%, the mass percentage of niobium is less than 0.050%, and the mass percentage of boron is less than 0.0030 %. In addition, the mold vibration waveform is a sinusoidal waveform.

The maximum short-side bulge, the maximum depth of corners, the maximum defect on the surface of the plate and whether there is fracture were tested for different types of plates produced. The results are shown in Table 3, and the maximum short-side swelling amount is preferably 10 mm or less, more preferably 5 mm or less. The maximum corner depth is preferably 1 mm or less, more preferably 0.7 mm or less.

In addition, the measurement results of the surface defect ratio of the cold-rolled steel sheet are also listed in Table 3 (thickness of the steel sheet is 0.8mm). The cold-rolled steel sheet is obtained by the following steps: Heating at 1200° C. for 2 to 2.5 hours, followed by hot rolling, cold rolling and finally annealing according to conventional methods.

In addition, the effects of casting speed on surface defects of plates and cold-rolled steel sheets are studied and summarized. The result is shown in Figure 6.

In the subsequent process, the surface of the plate is ground to 1mm, and polished with #1000 sandpaper, and then corroded with a mixture of hydrochloric acid and hydrogen peroxide. Larger) (piece/m ² ), the number of alumina clusters (diameter 500 μm or more) and molten slag (including foundry flux, diameter 0.5 mm or more).

In addition, the surface defect ratio of the cold-rolled steel sheet is a ratio, which is the number of defects on a percentage basis, such as the ratio of scratches and chips caused by casting to the total defects. The number of defects is measured on the top and bottom surfaces of cold-rolled steel sheets at intervals of 1000 m.

When at least one fracture occurred in casting under each condition, occurrence of fracture was "Yes".

In addition, electromagnetic force stagnation "type 1" refers to the static magnetic field generating device (EMBR) applied throughout the mold near the bottom end of the mold, and electromagnetic force stagnation "type 2" refers to the application at the submerged nozzle nozzle The static magnetic field generating device (EMLS) in the entire mold, "Type 1" and "Type 2" adopted are based on the technologies disclosed in Unexamined Japanese Patent Publication No. 2-284750 and JP No. 57-17356, respectively.

The negative velocity slab time tn is a characteristic value defining the vibration condition of the casting mold, which means a period of time during which the casting mold descends faster than the casting thin steel plate. As can be seen from Table 3 and Figure 6, when casting sheets according to the present invention, even at relatively high casting speeds, such as exceeding about 2.0 m/min, the surface defects of the sheets are relatively light, with almost no cold-rolled thin spots detected. There are few defects on the surface of the steel plate, even if there are defects.

As can be seen from the foregoing examples, according to the present invention, it is best to optimize process conditions to reach the following conditions:

(1) The relative thrust generated by the static pressure of molten steel on the mold wall increases, acting on the shell solidified near the surface of the molten steel in the mold,

(2) Absorption of inclusions, slag, flux, and air bubbles at the interface of the solidified shell is suppressed, and the possibility of trapping impurities is reduced, and

(3) The depth of impurities trapped in the solidification shell is reduced as much as possible.

Thus, high productivity and stable operation can be obtained even at casting at high speeds, such as higher than about 2.0 m/min, and high-quality sheets of cold-rolled steel sheets for manufacturing mobile phone casings can be provided without surface treatment.

Example 2

Molten steel (approximately 300 tons) melted in the converter and treated with RH is continuously cast into plates by continuous casting equipment, wherein the continuous casting equipment is equipped with the magnetic field generating devices in Figures 7A to 7C, and the composition of the molten steel is: Carbon mass 100 0.0015% by mass, 0.02% by mass silicon, 0.08% by mass manganese, 0.015% by mass phosphorus, 0.004% by mass sulfur, 0.04% by mass aluminum, and 100% by mass titanium The content of iron is 0.04%, and the rest is iron and other unavoidable impurities. The production conditions in this example are listed in Table 2. The immersion nozzle is a square-shaped double-spout nozzle with a downward spray angle of 15°.

Segregation and non-metallic inclusions on the surface of the sheet were then measured as well as surface defects caused by casting flux after cold rolling. The measurement results are listed in Table 3.

Surface segregation was inferred from the visual inspection of the number of segregations per 1 ^m2 after the plate grinding and etching process. In addition, non-metallic inclusions are extracted from the surface of the cast steel sheet at a depth of one quarter below the surface by particle extraction. Then measure the weight of the inclusions. Also, the coil surface defects formed by cold rolling were visually inspected, sampled and analyzed. The number of defects caused by flux can be obtained. For comparison purposes, the surface segregation caused by flux, the amount of inclusions, and the number of defects were simplified into indices, and the worst results obtained in all conditions were taken as an index of 10. Each outcome is a ratio of the worst outcome on the basis that they satisfy the linear relationship assumption.

As can be seen in Table 3, according to the present invention, when the casting speed, the short side length D of the continuous casting space of the mold, the nozzle immersion depth, and the ratio D/d of the short side length D to the transverse width d of the submerged nozzle spout are appropriately simultaneously Controlling and adopting appropriate electromagnetic hysteresis to the molten steel in the mold will reduce the amount of surface segregation, the amount of non-metallic inclusions and the amount of defects caused by casting aids.

When the strength of the oscillating magnetic field is too high, the amount of flux involved in the surface of molten steel increases, resulting in a decrease in surface quality. In addition, when the frequency is too high, the surface plane of the molten steel cannot keep up with the magnetic field, and the washing effect on the solidified shell is weakened, which will lead to an increase in the number of bubbles and inclusion defects.

Table 2A serial number Short side length (sheet thickness) D (mm) Sheet width W(mm) Casting speed Vc(m/min) Throughput of molten steel (ton/min) Spout lateral width d(mm) D/d Mold amplitude S (total amplitude) (mm) Mold vibration frequency f (times/minute) Tn ^* (s) minimum value maximum value minimum value maximum value 1 220 900 1950 1.0 1.6 3.4 80 2.75 6 120 0.177 2 220 900 1950 1.5 2.3 5.1 80 2.75 6 130 0.134 3 220 900 1950 1.8 2.3 6.1 80 2.75 6 150 0.112 4 220 900 1950 2.0 3.1 6.7 80 2.75 6 185 0.099 5 220 900 1950 2.1 3.3 7.1 80 2.75 5 170 0.075 6 220 900 1950 2.2 3.4 7.4 80 2.75 5 180 0.072 7 220 1200 1950 1.5 3.1 5.1 80 2.75 9 190 0.129 8 220 1200 1950 1.8 3.7 6.1 80 2.75 9 190 0.124 9 220 1200 1950 2.0 4.1 6.7 80 2.75 9 190 0.120 10 220 1200 2200 2.3 4.8 8.7 80 2.75 9 160 0.124 11 220 1200 2200 2.3 4.3 8.7 80 2.75 9 185 0.115 12 220 1200 1840 2.3 4.8 7.3 80 2.75 9 195 0.112 13 220 1200 1500 2.3 4.8 6.0 80 2.75 9 205 0.108 14 220 900 1950 2.1 3.3 7.1 80 2.75 6 160 0.096 15 220 900 1950 2.2 3.4 7.4 80 2.75 7 160 0.107 16 220 900 1950 2.3 3.6 7.7 80 2.75 7 160 0.102 17 220 900 2200 2.5 3.9 9.5 80 2.75 6 160 0.071 18 220 900 2200 2.7 4.2 10.3 80 2.75 8 160 0.100 19 220 900 2000 3.0 4.7 10.4 80 2.75 9 160 0.101 20 220 900 1950 3.5 5.4 11.8 80 2.75 9 160 0.086 twenty one 110 400 400 2.5 0.9 0.9 30 3.67 6 160 0.071 twenty two 200 900 1950 2.5 3.5 7.7 70 2.86 6 160 0.071 twenty three 215 900 1950 2.5 3.8 8.2 88 2.44 6 160 0.071 twenty four 235 900 1950 2.5 4.2 9.0 88 2.67 6 160 0.071 25 250 900 1950 2.5 4.4 9.6 88 2.84 6 160 0.071 26 260 900 1950 2.5 4.6 9.9 88 2.95 6 160 0.071 27 220 1200 1950 2.5 5.2 8.4 80 2.75 6 160 0.071 28 235 1200 1950 2.5 5.5 9.0 88 2.67 7 160 0.093 29 235 1200 1950 1.5 3.3 5.4 88 2.67 7 185 0.123 30 235 1200 1950 2.1 4.6 7.6 88 2.67 6 180 0.096 31 235 1200 2200 2.5 5.5 10.1 130 1.81 6 185 0.080 32 220 900 2200 2.5 3.9 9.5 80 2.75 6 185 0.080 33 220 900 2200 2.5 3.9 9.5 80 2.75 6 185 0.080 34 220 900 2200 2.5 3.9 9.5 80 2.75 6 185 0.080 35 220 900 2200 2.5 3.9 9.5 80 2.75 6 185 0.080 36 220 900 1950 2.1 3.3 7.1 80 2.75 6 160 0.096 37 220 900 2000 3.0 4.7 10.4 80 2.75 9 160 0.101

Type 1: Oscillating magnetic field, Type 2: Transforming magnetic field

Table 2B serial number Immersion nozzle depth (mm) AC magnetic field type Upper AC magnetic field (Gauss) Upper DC magnetic field (Gauss) Lower end DC magnetic field (Gauss) 1 280 none 0 0 0 2 280 none 0 0 0 3 280 none 0 0 0 4 280 none 0 0 0 5 280 none 0 0 0 6 280 none 0 0 0 7 280 Type 1 1000 1000 0 8 280 Type 1 700 1000 0 9 280 Type 1 500 1000 0 10 280 Type 1 300 1000 0 11 280 Type 1 300 1000 0 12 280 Type 1 300 1000 0 13 280 Type 1 300 1000 0 14 280 Type 1 300 1000 0 15 280 Type 1 300 1000 0 16 280 Type 1 300 1000 0 17 280 Type 1 0 1000 1500 18 280 Type 1 0 1500 2000 19 280 Type 1 0 2000 2500 20 280 Type 1 0 2500 3000 twenty one 280 Type 1 0 0 0 twenty two 280 Type 1 200 1000 0 twenty three 280 Type 1 200 1000 0 twenty four 280 Type 1 200 1000 0 25 280 Type 1 200 1000 0 26 280 Type 1 200 1000 0 27 280 none 0 0 0 28 280 none 0 0 0 29 280 Type 2 600 0 0 30 280 Type 2 600 1000 0 31 280 Type 2 600 1000 0 32 180 Type 1 200 1000 0 33 200 Type 1 200 1000 0 34 350 Type 1 200 1000 0 35 370 Type 1 200 1000 0 36 280 Type 1 300 1000 1500 37 280 Type 1 300 1000 1500

table 3 serial number Maximum short side bulge (mm) Max corner depth(mm) The maximum number of surface defects of the plate (/m ² ) Surface defect rate (%) break The ratio of powder defects to total defects (%) Remark 1 0 3.5 3.10 no 49 Comparative example 1 2 1 2.7 185 2.35 no twenty four Comparative example 2 3 1 2.6 120 1.23 no 20 Comparative example 3 4 2 1.5 90 0.30 no 36 Comparative example 4 5 2 1.1 55 0.15 no 0 Example 1 6 1 0.7 45 0.05 no 3 Example 2 7 1 3.0 3.10 no 33 Comparative example 5 8 1 2.9 1.54 no 20 Comparative example 6 9 2 2.2 0.50 no 16 Comparative example 7 10 4 0.8 0 no 0 Example 3 11 4 0.9 0.11 no 5 Example 4 12 3 1.3 2.6 no 74 Comparative example 8 13 3 1.3 4.1 no 85 Comparative example 9 14 2 1.0 50 0 no 0 Example 5 15 3 0.6 30 0 no 0 Example 6 16 3 0.5 20 0 no 0 Example 7 17 3 0.2 10 0 no 0 Example 8 18 5 0.2 3 0 no 0 Example 9 19 5 0.1 3 0 no 0 Example 10 20 6 0.2 5 0 no 0 Example 11 twenty one 1 1.4 70 no Comparative example 10 twenty two 1 0.1 15 0.02 no 0 Example 12 twenty three 2 0.2 11 0 no 0 Example 13 twenty four 5 0.3 13 0 no 0 Example 14 25 10 0.8 25 0.3 no 4 Comparative example 11 26 15 1.1 60 0.4 no 60 Comparative example 12 27 9 0.7 0.03 yes 15 Example 15 28 9 0.6 0.05 no twenty one Example 16 29 0 2.5 5.90 no 37 Comparative example 13 30 1 0.8 0 no 0 Example 17 31 2 0.4 0 no 0 Example 18 32 2 0.4 0.05 no 33 Example 19 33 2 0.4 0 no 0 Example 20 34 2 0.4 0 no 0 Example 21 35 2 0.6 1.5 no 67 Comparative example 14 36 2 1.0 20 0 no 0 Example 22 37 3 0.5 12 0 no 0 Example 23

^* Blank: Not measured COM.EX.: Comparative example

Claims (24)

1. A method for producing an ultra-low carbon steel plate, comprising: Provided is a continuous casting plant comprising a casting mold having a casting space with a short side length D of 150 to 240 mm, and a submerged nozzle having at least one spout of transverse width d, wherein the D/d ratio is in the range of 1.5 to 3.0; Introducing molten steel into the mold through immersion nozzles; and Cast molten steel at a casting speed higher than 2.0mm/min in continuous casting equipment to produce ultra-low carbon steel sheets with a carbon mass percentage of 0.01% or lower. 2. The method according to claim 1, further comprising vibrating the casting mold at a frequency of 185 times/min or less. 3. The method according to claim 1, the casting speed is 2.4 m/min or faster. 4. The method according to claim 1, wherein the submerged nozzle is a dual jet nozzle. 5. The method according to claim 1, the D/d ratio being 2.1 to 2.9. 6. according to the method for claim 1, ultra-low carbon steel plate is as the raw material of the cold-rolled thin steel plate of production mobile phone shell. 7. The method according to claim 1, further comprising using electromagnetic force to stagnate the molten steel in the casting space of the mold. 8. The method according to claim 7, wherein the static magnetic field that runs through the thickness of the casting mold and substantially covers the entire casting mold is used to generate electromagnetic force hysteresis, including an upper magnetic field generating device and a lower magnetic field generating device, The upper magnetic field generating device is installed on the upper part of the casting mold including the molten steel surface plane, and the lower magnetic field generating device is installed below the upper magnetic field generating device. as well as The immersion nozzle is installed between the upper and lower magnetic field generating devices, and the immersion depth is set between 200 and 350mm. 9. The method according to claim 7, wherein adopting the superposition of static magnetic field and alternating magnetic field in the whole casting mold through the thickness of the casting mold to produce electromagnetic force hysteresis, and its magnetic field generating device is installed on the casting mold top comprising the molten steel surface plane, and The immersion nozzle is installed at the lower end of the magnetic field generating device, and the immersion depth is set between 200 and 350mm. 10. The method according to claim 7, wherein adopting the superposition of the static magnetic field and the AC magnetic field in the whole casting mold through the thickness of the casting mold to produce electromagnetic force hysteresis, using the upper magnetic field generating device, in addition, using the lower magnetic field through the thickness of the casting mold The generating device generates a static magnetic field in the mold, The upper magnetic field generating device is installed on the upper part of the casting mold including the molten steel surface plane, the lower magnetic field generating device is installed below the upper magnetic field generating device, and The immersion nozzle is installed between the upper and lower magnetic field generating devices, and the immersion depth is set between 200 and 350mm. 11. The method according to claim 1, wherein the molten steel comprises 0.01% by mass or less of carbon, 0.01% to 0.04% by mass of silicon, 0.08% to 0.20% by mass of manganese, and 0.008% to 0.020% phosphorus, 0.003% to 0.008% by mass of sulfur, 0.015% to 0.060% by mass of aluminum, 0.03% to 0.080% by mass of titanium, 0.002% by mass to 0.017% niobium, and 0 to 0.0007% boron by mass, and the rest is iron and unavoidable impurities. 12. The method according to claim 11, wherein the molten steel comprises 0.0005% to 0.0090% by mass of carbon. 13. A method of producing an ultra-low carbon steel sheet, comprising: a casting mold of a casting space having an immersion nozzle having at least one spout of transverse width d, wherein the D/d ratio is in the range of 1.5 to 3.0; and Cast molten steel at a speed higher than 2.0 mm/min in a continuous casting device to produce an ultra-low carbon steel plate with a carbon mass percentage of 0.01% or lower. 14. The method of claim 13, further comprising vibrating the mold at a frequency of 185 times/min or less. 15. The method according to claim 13, the casting speed being 2.4 m/min or faster. 16. The method of claim 13, wherein the submerged nozzle is a dual jet nozzle. 17. The method according to claim 13, the D/d ratio being 2.1 to 2.9. 18. The method according to claim 13, the ultra-low carbon steel sheet is used as a raw material for cold-rolled thin steel sheets for producing mobile phone shells. 19. The method according to claim 13, further comprising using electromagnetic force to stagnate the molten steel in the casting space of the mold. 20. The method according to claim 19, wherein the static magnetic field that runs through the thickness of the casting mold and substantially covers the entire casting mold is used to generate electromagnetic force hysteresis, including an upper magnetic field generating device and a lower magnetic field generating device, The upper magnetic field generating device is installed on the upper part of the casting mold including the molten steel surface plane, and the lower magnetic field generating device is installed below the upper magnetic field generating device. as well as The immersion nozzle is installed between the upper and lower magnetic field generating devices, and the immersion depth is set between 200 and 350mm. 21. The method according to claim 19, wherein the superposition of a static magnetic field and an alternating magnetic field in the entire casting mold through the thickness of the casting mold is used to generate electromagnetic hysteresis, and the magnetic field generating device is on the upper part of the casting mold including the surface plane of the molten steel, and The immersion nozzle is installed at the lower end of the magnetic field generating device, and the immersion depth is set between 200 and 350mm. 22. The method according to claim 19, wherein the superposition of the static magnetic field and the AC magnetic field in the entire mold through the thickness of the casting mold is used to generate electromagnetic stagnation, and the upper magnetic field generating device is used. In addition, the electromagnetic field generating device is used to pass through the casting mold. A static magnetic field is generated in the mold throughout the thickness, The upper magnetic field generating device is installed on the upper part of the casting mold including the molten steel surface plane, the lower magnetic field generating device is installed below the upper magnetic field generating device, and The immersion nozzle is installed between the upper and lower magnetic field generating devices, and the immersion depth is set between 200 and 350mm. 23. The method according to claim 13, wherein the molten steel comprises 0.01% by mass or less of carbon, 0.01% to 0.04% by mass of silicon, 0.08% to 0.20% by mass of manganese, and 0.008% to 0.020% phosphorus, 0.003% to 0.008% by mass of sulfur, 0.015% to 0.060% by mass of aluminum, 0.03% to 0.080% by mass of titanium, 0.002% by mass to 0.017% niobium, and 0 to 0.0007% boron by mass, and the rest is iron and unavoidable impurities. 24. The method according to claim 23, wherein the molten steel comprises 0.0005% to 0.0090% by mass of carbon.

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