JPH0440103B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD OF THE INVENTION The present invention relates to continuous casting of steel. BACKGROUND TECHNOLOGY During continuous casting of steel, a molten metal level protectant is added to the mold. The role of this molten metal surface protectant is to coat the surface of molten steel and keep it warm by adding it to the mold, and to prevent solidification and oxidation of the molten steel surface. To clean slabs by absorbing non-metallic inclusions floating up from molten steel, and to prevent defects caused by inclusions in products. Uniform inflow lubrication between the mold and the solidifying shell ensures uniform cooling of the mold, thereby preventing surface cracking and restraint breakout of the slab. In order to ensure the above properties, the hot water surface protectant is
The base material is oxides such as SiO ₂ , Al ₂ O ₃ , CaO, Fe ₂ O ₃ , MgO, etc. In addition, oxides, carbonates, or fluorides of alkali metals and alkaline earth metals are used as physical property modifiers. Contains carbon as a melting rate regulator. As the carbon component, powders or granules of coke, carbon black, natural graphite, artificial graphite, coal, etc. are used. The ingredients of common hot water surface protectants include SiO ₂ 20
~45wt%, CaO20~45wt%, _{Al2O3 0.5} ~10wt
%, MgO1~20wt%, Na2O1~20wt%, F ^- ₂ ~
It is composed of 20wt% or less and C10wt% or less, and LiO ₂ , B ₂ O ₃ and other metal oxides, metal fluorides, and metal carbonates are also used as necessary. Analytically, fluorides are expressed as oxides and F ^- , and carbonates are expressed as oxides and C. In terms of physical properties, it has a viscosity of 0.5 to 15 poise at 1300°C and a melting point of 800 to 1200°C. On the other hand, in recent years there has been an increasing demand for cost reduction through energy conservation, resource conservation, and improvements in yield and unit consumption. Therefore, high-speed continuous casting and furthermore,
Direct feed rolling (CC-DR), in which high-temperature slabs produced by high-speed continuous casting are sent directly to the rolling process without cooling and reheating, is increasingly being adopted. In order to carry out this direct rolling, it is necessary to completely eliminate defects in the slabs manufactured by high-speed continuous casting (surface cracks, powdery defects in the slabs due to the inclusion of surface protection agent into the molten steel during the solidification process, etc.). Or, it must be extremely small to the extent that there is no problem even if direct rolling is performed, and product defects caused by defects in the slab must be prevented. However, if the surface protection agent used at conventional casting speeds is used as is for high-speed continuous casting,
The consumption of the surface protective agent decreases, which causes poor lubrication between the mold and the slab, making it more likely that a restraining breakout will occur. As a preventive measure, the viscosity of the surface protectant was lowered to 1300℃ due to high-speed casting.
By lowering the viscosity to 0.5 to 1.5 poise, the consumption of hot water surface protectant was increased and lubricity was maintained. (Japanese Patent Application Laid-open No. 150752/1983). Problems to be Solved by the Invention However, although reducing the viscosity of the molten metal surface protective agent has the effect of preventing restrictive breakout, the molten surface protective agent may flow into the molten steel due to the intense injection flow into the mold due to increased speed. As a result, the number of powder defects in the slab increases dramatically, which dramatically increases the number of defects in the product, creating a new major problem in implementing high-speed casting and direct rolling. In addition, even in medium and low speed casting, in order to further improve the quality of materials with strict quality requirements such as tinplate materials and steel plate materials for automobiles (Ti-Sulc steel), it is necessary to powder the slab by rolling in a surface protectant. We need to eliminate all sexual defects. The present invention completely prevents the occurrence of restraint breakout during high-speed casting, and at the same time suppresses the increase in powdery defects in slabs due to rolling in of the surface protectant, which is a problem in the above-mentioned conventional technology.
The present invention provides a new continuous casting method that not only enables direct rolling but also produces slabs with no powdery defects even during medium, low-speed or high-speed casting, thereby making it possible to obtain products of even higher quality. . Means for Solving the Problems In order to solve the above-mentioned drawbacks during casting, the inventors of the present invention have conducted various experiments and studies, and have discovered that under specific casting conditions there is an excellent region of extremely high viscosity. I was able to do it. The present invention was made based on these findings, and its characteristics are as follows:
Mold long piece taper [(mold top opening thickness - mold bottom opening thickness)/mold long piece height x 100%]
When continuously casting slabs under conditions of 0.1% or less, CaO, SiO ₂ and Al ₂ O ₃ are the main components, and CaO/
SiO ₂ (wt% ratio) is in the range of 0.5 to 1.6, and contains one or more oxides, carbonates, and fluorides of alkali metals, alkaline earth metals, and other metals, and carbon as a melting rate regulator. , surface tension at 1250℃ is 290dyne/cm or more, break point is
Viscosity η below 1000℃ and at 1300℃
A continuous steel casting method characterized by casting a slab using a surface protectant (powder) that satisfies the relationship between (poise) and casting speed V (m/min): 6.0<ηV It is. That is, the present invention newly introduces surface tension and break point as physical property values, specifies the range of the viscosity of the molten metal surface protective agent with respect to the casting speed, and casts the long piece taper of the mold in the range of 0.1% or less. This allows sufficient inflow lubrication between the mold and the slab to prevent restrictive breakout, not only in low-medium speed casting of 1.5 m/min or less, but also in high-speed casting of 1.5 m/min or more, and prevents restrictive breakout. The present invention provides a continuous casting method that can produce slabs with extremely few slab defects and, in the case of high-grade steel, has few inclusions. Function As mentioned above, in the continuous casting method using a surface protection agent, the molten layer of the surface protection agent is drawn into the molten steel by the molten steel flow in the mold, solidifies, and remains in the slab as inclusions, which impairs quality. The purpose is to prevent the deterioration of the lubricity and maintain smooth lubrication ability with the inner wall surface of the mold. Various studies were conducted to prevent this rolling and to ensure lubricity. First, regarding the prevention of entrainment, since the cause of entrainment is the disturbance of the molten steel surface due to the inflow of molten steel during casting, we focused on surface tension as a new physical property of the molten metal surface protectant, and measured the surface tension of commercially available products. . The measurement of surface tension is the first
Figure 2 shows the platinum cylindrical drawing method (Dipping).
Cylinder method). First, about 60g of a hot water surface protectant that had been decarburized for 1 hour in a Matsufuru furnace at 600°C was placed in a graphite crucible and heated to 1400°C.
Melt in a Matsufuru furnace for 10 minutes. This molten sample was placed in a platinum crucible 3, maintained at 1250° C., and inserted into an electric furnace 2 of a measuring device (FIG. 12) that was replaced with Ar. After the temperature stabilizes, the platinum cylinder 4 connected to the upper balance 1 is immersed in the melt and the balance is balanced. Next, weights are added from the platinum cylinder 4 to lower the liquid level until the balance reaches equilibrium. Repeat this operation until the platinum cylinder leaves the liquid surface, and record the maximum weight at the moment it leaves. This measurement is performed six times, and the average value excluding the maximum and minimum values is taken as Wmax (g). The calculation formula is expressed as σ=g·Wman/4πR×V. Here, σ is the surface tension (dyne/
cm), g is the acceleration of gravity (980cm/sec ² ), Wmax
is the maximum pull (gr) exerted on the platinum cylinder, R is the radius of the cylinder (cm), and V is a correction coefficient, which is determined by the cylinder used and the density of the melt. The surface tension measurement results of the conventional products investigated were 240~
It was 290dyne/cm. In addition, the viscosity of the conventional product is
It was 0.6 to 3.0 poise at 1300°C, and no correlation was observed between viscosity and surface tension. Next, a casting test was conducted with different surface tension levels, and the relationship with the quality of the slab was investigated. The results are shown in FIG. The casting conditions were: slab size 1500 x 250 mm, melt composition C: 0.04, Si: 0.02,
Mn: 0.24, P: 0.02, S: 0.01, Al: 0.05 (%),
TD temperature 1560℃, casting speed 1.5 m/min, hot water surface protection agent (main components CaO: 39.0%, Al ₂ O ₃ : 2.0%, CaO:
36.0%, _Na2O : 14.0%, ^F- : 6.0%) viscosity 1300
Standardized at ℃1.6poise. As is clear from Figure 2, the higher the surface tension, the
Although there is a tendency for inclusion defects to decrease, surface tension
A large decrease in inclusions can be seen at 290 dyne/cm. This means that the higher surface tension means that the surface binding force of the molten metal surface protective agent is stronger than the degree of turbulence of the molten metal surface caused by the casting conditions. As a result, under continuous casting,
We have newly discovered that if it is 290 dyne/cm or more, it is outside the critical condition for entrainment. However, although efforts have been made to reduce inclusion defects, conditions for eliminating them entirely have not been found. Therefore, the design was changed to one that sacrificed lubricity (BO generation rate), the viscosity was set to 3.2 poise at 1300°C, and a casting test was conducted again at 1.5 m/min under the above operating conditions. The results are shown in FIG. As is clear from Figure 3, it has been newly discovered that increasing the viscosity reduces the incidence of inclusions, but increasing the surface tension to 290 dyne/cm or higher can completely eliminate inclusion defects in slabs. did. However, by increasing the viscosity, the lubricity decreased and the rate of occurrence of restrictive BO increased, both of which were 1
(0.1 in Figure 2), which did not guarantee sufficient lubricity. Accordingly, the present inventors conducted research and investigation into means for ensuring this lubricity, and as a result, obtained the following knowledge.
That is, it is said that the physical property most closely related to the unit consumption is viscosity, and it has been conventional knowledge that as the casting speed increases, the viscosity of the surface protectant decreases. Generally, the viscosity is
The measured value at 1300℃ is used, which is
The average temperature of the inflow film just below the meniscus is approximately
This is because the temperature is 1300°C and the measured values can be obtained relatively stably. The present inventors melted 200g of various hot water surface protectants after decarburization at 1400℃ for 10 minutes using a rotor rotation method.
Viscosity measurement was carried out after holding at 1300℃, and then 20
The temperature was lowered by ℃ and the viscosity was measured after holding at each temperature to obtain 1ogη-1/T (η is the viscosity poise, T
I created a graph (indicating the absolute temperature). During this measurement, crystals crystallized in the sample due to temperature drop, and the sample ceased to be a Newtonian liquid, and the breakpoint was defined as a temperature 10°C higher than the temperature at which measurement became impossible. Figure 4 shows an example of the measurement. As is clear from Figure 4, it was found that there are hot water surface protective agents that have vastly different break points even if the viscosity at 1300°C is the same. Therefore, we prototyped a sample with a different breakpoint, conducted an actual casting test, and investigated the effect on the breakout occurrence rate (hereinafter referred to as BO occurrence rate). The steel type and casting speed were the same as before, and the viscosity was 3.2 poise at 1300℃. The results are shown in Figure 5. As is clear from Figure 5, there is a large correlation between the BO incidence rate and the breakpoint, and it is possible to reduce the BO incidence rate by lowering the breakpoint, and the breakpoint temperature can be reduced. We have newly discovered that by keeping the temperature below 1000°C, the BO incidence can be completely eliminated. As shown in Figure 6b, this is the conventional hot water surface protection agent 7.
is liquid lubrication 8 only in the meniscus part, and the other lower parts are in a solid state 9, and 1300
Only the viscosity in °C controlled the basic unit of powder, that is, the lubricity. Furthermore, as shown in Figure 6a, the fact that the break point of the molten metal level protectant 10 of the present invention is 1000°C or lower means that the surface of the slab at the lower end of the mold is generally said to be at 1000°C, and for this reason, molten water is also present at the lower end of the mold. The surface protective agent exists as a liquid, and the entire surface of the mold (the entire surface of the cast piece in the mold) has liquid lubrication.
1, indicating an ideal lubrication condition unlike the conventional one. As a result, the incidence of restrictive breakouts was completely eliminated. Furthermore, by regulating a certain characteristic condition of the mold, that is, the mold taper on the long side, the entire surface between the mold inner wall surface and the surface of the solidified shell is lubricated by the liquid level protectant, and if the taper is small, the break point and surface are the same. Even under tension, high-viscosity molten metal surface protection could be used and good slabs were obtained. Based on this knowledge, we conducted a test. In other words, we varied the mold long piece taper and investigated the optimum unit consumption (Kg/Ton Steel), quality, and operability.
At this time, the characteristics of the hot water surface protective agent and the operating conditions were the same as before. The results are shown in Figure 7. As is clear from Figure 7, a large relationship was found between the mold taper of the long piece and the basic unit of the hot water surface protective agent, and this relationship showed that there was a clear difference at the taper of the long piece of 0.1%. I found it. The reason for this is that if it is greater than 0.1%, in the mold,
The bulging of the slab becomes large and the inflow is inhibited, but if it is less than 0.1%, the opening of the mold toward the lower end is large and the amount of inflow increases rapidly. Similarly, if low clay is used under conditions of greater than 0.1%, the basic unit will be at a normal value, but as mentioned above, the amount of entrainment in the slab will be large and the quality will deteriorate. Furthermore, when a high viscosity is used, the quality is good, but the unit consumption is low and casting troubles (surface flaws due to large amount of mold wear) become serious. Even if there is no breakout rate and the quality of the slab is good, if the consumption of surface protection agent is high, the cost of casting the slab will increase and the amount of slag film attached to the slab will increase. This results in poor peeling during rolling of the slab and increases in rolling defects. Furthermore, when the unit consumption decreases, the amount of wear on the inner surface of the mold increases, resulting in increased surface flaws or a very high mold maintenance rate. In other words, an optimal basic unit exists. These relationships are shown in Figures 8 (operating conditions are the same as in Figure 7), 9 and 10. Based on these results, it was found that the optimum basic unit was 0.20 to 0.6Kg/Ton Steel. Next, to find the optimal viscosity range, the surface tension is
290dyne/cm or more at 1250℃, breakpoint
We prototyped a hot water surface protective agent with a viscosity that varied within the range of 1000℃ or less, and set the long piece taper of the mold to 0.1% or less.
The slab quality was checked by casting on an actual machine using various combinations of casting speeds. The results are shown in FIG. The mark ○ indicates a product that passed the overall evaluation (example of the present invention), and the mark x indicates a product that failed the test (comparison product). From Figure 1, for casting speed,
There is an appropriate viscosity range. Here, the long piece taper of the mold used in the present invention is as shown in Fig. 13: Upper end opening thickness B - lower end opening thickness C/long piece height A x 100
It is defined as %. Carbon in the hot water level protectant adjusts the melting rate of the hot water level protectant while being heated and burned depending on the molten steel temperature when added, and the melting rate is influenced by the amount of carbon contained therein. In other words, if the content is high, the melting rate will be slow, and a content suitable for casting operations is required. That is, for the casting speed V, the viscosity η at 1300° C. is designed to be in the range of 6.0<ηV, preferably ηV is 20 or less. This is because if it is too high, the basic unit may decrease. Furthermore, as shown in Fig. 11, not only does the high surface tension in Fig. 2 reduce the slab defect index (incorporation of surface protection agent) due to inclusions, but also the general surface tension lowers the break point. The defect index due to inclusions within the piece was reduced, and as shown in FIG. 11, a remarkable effect could be found with this combination by creating a high surface tension and a low break point. In this way, by making the surface tension high (290 dyne/cm or more) and the break point low (1000°C or less), we were able to significantly reduce the occurrence of inclusions. It was discovered that under these conditions, good slabs without casting defects could be produced. Based on the above results, the reasons for limiting the scope of the present invention can be summarized as follows. The component system of the hot water surface protectant used may be the composition of a commonly used hot water surface protectant, and has the physical properties of a surface tension of 290 dyne/cm or more and a break point of 1000°C or less as claimed in the claims, CaO/SiO ₂ (wt% ratio)
Only 0.5 to 1.6. It is CaO/SiO ₂ <
In the case of 0.5, a large amount of flux components (Na ₂ O, Li ₂ O, F ^-, etc.) are required to adjust the physical properties, especially the viscosity, which increases the amount of erosion of the immersion nozzle and increases the cost, making it impractical. But also CaO/
If SiO ₂ > 1.6, glassy properties are lost during inflow lubrication due to denaturation within the mold (absorption of Al ₂ O ₃ inclusions, reduction of SiO ₂ in the molten metal surface protectant by Al in the steel). , causing recrystallization. The break point is below 1000℃.
This means that it maintains glassiness even at low temperatures, and in terms of ionic crystals, SiO ₂ connections form a linear structure rather than a lumpy group, making it difficult to form crystals. When crystals occur, heat removal from the slab becomes uneven, making it easier for breakouts to occur due to surface defects and poor lubrication, so CaO/SiO ₂ =
The range was 0.5 to 1.6. The reason why the surface tension is set to 290 dyne/cm or more at 1250℃ is because, as mentioned above, if it is 290 dyne/cm or less, the molten layer of the molten metal surface protectant will be easily engulfed by the molten steel flow, causing inclusion defects. Surface tension is 1250
The temperature must be 290dyne/cm or higher at ℃. The reason why the breakpoint is below 1000℃ is that the viscosity is relatively high and the BO generation rate can be completely eliminated as mentioned above, and above 1000℃, there is a solid surface protectant in the mold as mentioned above. It is necessary to keep the temperature below 1000℃, as this will cause poor lubrication in the gap between the mold and the slab, making breakouts more likely to occur. The reason why the viscosity η at 1300℃ is limited to 6.0<ηV with respect to the casting speed V is that when ηV≦6, the long piece taper
Under the condition of 0.1% or less, the viscosity is too low for the casting speed, and the liquid of the slag film in the mold flows freely, resulting in too high a unit consumption, resulting in defects, such as rolling scratches, and problems (increased costs). This is because mold taper is 0.1.
In this case, the occurrence of surface defects increases as the basic unit decreases. When ηV>6.0 and the taper is 0.1 or more, surface flaws increase dramatically as described above. The reason for this is, as mentioned above, the characteristics of the hot water surface protectant, the break point of 1000℃ or less, and the surface tension.
When it is 290 dyne/cm or more, liquid lubrication occurs within the mold and slabs of good quality can be obtained, but by further regulating the mold taper, it is possible to use a surface protectant with a high viscosity. Very good slabs can be obtained without mixing in the surface protection agent. For these reasons, it is necessary to set the mold long piece taper to 0.1% or less, and the viscosity η at 1300°C to be in the range of 6.0<ηV with respect to the casting speed V. Na ₂ O, Li ₂ O, F ⁻ , B ₂ O ₃ , AlF ₃ , for break points or slight variations in surface tension.
It can be adjusted with Na ₃ AlF ₆ , MgF _{2 ,} etc. Regarding the slab width, if it is 600 mm or less, the length of the long pieces is short and it is easy to form a relatively uniform shell.
Surface defects are difficult to occur, and if the width is 600 mm or more, it becomes non-uniform and surface defects are likely to occur. Therefore, when operating according to the present invention, it is preferable that the casting width is 600 mm or more. Examples Table 1 shows the components and physical properties of the hot water surface protectant. Using this hot water surface protective agent, casting was carried out using a continuous slab caster. Casting conditions are mold size 250 x 1600
mm, and the casting speed was 1.0 to 2.0 m/min. The casting results at that time are shown in Table 2. In addition, breakout occurrence index: (number of occurrences/1000
charge), slab defect index: (amount of occurrence / slab m ² ×
100). Examples A, B, C, D, E, and F are examples of the present invention, and the BO generation rate index was 0, and both slab quality and casting operability showed good results. G, H, J, and K are comparative products (conventional products). G
The breakpoint is within the scope of the present invention, but the surface tension and ηV are outside the scope of the present invention, and B.
Although the O generation rate was low, inclusion defects occurred in the slab. H is an example in which the surface tension is within the range of the present invention, but the break point and ηV are outside the range, and B.
The O generation rate was high, and surface cracking of the slab occurred. J is a break point, the surface tension is within the range of the present invention, but ηV is too low, and although the BO generation rate is low, inclusion defects and surface cracks occur. K is a conventional product (because the break point is high, the surface tension is low, it is outside the scope of the present invention, and ηV is too low),
BO occurred and there were many cracks caused by the molten metal surface protection agent being rolled into the slab. A′ and C′ include the newly obtained mold taper conditions, and as a result of operating with a mold taper of 0.11%, which is outside the operation of the present invention, the basic unit was small, the amount of mold wear was large, and the casting amount was Deterioration of surface quality was observed. In addition, D' used the same powder as D, but was cast at a higher speed, and the results were fully satisfactory in the overall evaluation. K,
Both K' were outside the scope of the present invention, and were the results of casting under operating conditions of mold taper of 0.11% and 0.05%, and both were not satisfactory in the overall evaluation.

【table】

【table】

[Table] Effects of the Invention The present invention enables the production of maintenance-free slabs, which was not possible with conventional technology, as continuous steel casting technology advances and the trend is toward higher quality, faster speeds, and maintenance-free casting. It is meant to be.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between casting speed, viscosity, and surface tension, and Figures 2 and 3 are graphs showing the relationship between surface tension, slab surface defect index, and breakout occurrence index, and Figure 4 is a graph showing the relationship between surface tension and slab surface defect index and breakout occurrence index. Graph showing an example of measurement of viscosity and break point, FIG. 5 is a graph showing the relationship between break point and breakout occurrence index, FIG. FIG. 1 is an explanatory diagram of a conventional lubrication model within a mold. The seventh is a graph showing the relationship between the mold long piece taper and the basic unit, and FIG.
It is a graph showing the relationship between the basic unit, the amount of slag adhering to a slab, and the occurrence of rolling defects. FIG. 9 is a graph showing the relationship between the mold long piece taper and the amount of mold inner wear;
Fig. 10 is a graph showing the relationship between casting time and slab surface flaw occurrence, Fig. 11 is a graph showing the relationship between break point and slab defect index, and Fig. 12 is a graph showing the relationship between casting time and slab surface flaw occurrence.
FIG. 1 is a schematic diagram of a surface tension measuring device. Figure 13 shows
FIG. 3 is an explanatory diagram of the definition of a mold long piece taper. 1... Balance, 2... Electric furnace, 3... Platinum crucible, 4...
Platinum cylinder, 5... Pt-Pt/Ph thermocouple, 6... heat shielding plate, 7, 10... hot water surface protective agent, 8, 11... melting part,
9...Solid, 12...Slab, 13...Mold.

Claims (1)

[Claims] 1 Long piece taper of the mold [(Mold upper end opening thickness -
Thickness of the lower end of the mold) / Height of the long piece of the mold x 100%]
When continuously casting slabs under conditions of 0.1% or less, CaO, SiO ₂ and Al ₂ O ₃ are the main components, and CaO/
SiO ₂ (wt% ratio) is in the range of 0.5 to 1.6, and contains one or more oxides, carbonates, and fluorides of alkali metals, alkaline earth metals, and other metals, and carbon as a melting rate regulator. , surface tension at 1250℃ is 290dyne/cm or more, break point is
Viscosity η below 1000℃ and at 1300℃
A continuous steel casting method characterized by casting a slab using a surface protectant (powder) that satisfies the relationship between (poise) and casting speed V (m/min): 6.0<ηV .