JPS6315060B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a continuous casting method for aluminum killed steel. Continuous casting of steel has developed rapidly in recent years to achieve energy savings and process savings. However, in order to establish a rational steel manufacturing technology that is consistent with continuous casting and continuous rolling, and to manufacture slabs of even higher quality, it is necessary to establish an even better continuous casting technology. In particular, we will develop a casting method that reduces Al ₂ O ₃ inclusions in aluminum-killed steel, which is widely used in automobile steel plates, etc., and will not cause sliver defects or cracks during deep drawing even when rolled without maintenance after casting. This has become increasingly necessary in recent years from the standpoint of ensuring high-quality products and reducing costs through process-saving steps. In other words, in the steelmaking process, aluminum killed steel is
In order to remove excess oxygen in steel, it refers to steel that uses materials containing Al, such as Al, Fe-Al alloy, Al-Si alloy, Al-Mg alloy, etc. In this aluminum killed steel, naturally Al ₂ O ₃ -based oxides, i.e.
Al ₂ O ₃ based inclusions are formed. Inclusions in continuous casting molds include those that are not removed before entering the mold and flow into the mold, and those that are newly formed when molten steel comes into contact with oxygen when it flows into the mold. . The inclusions that have flowed into the mold flow along with the flow of molten steel.
Then, some of the inclusions in the molten steel float to the surface of the molten steel due to the difference in specific gravity between the molten steel and the inclusions, or due to bubbling of an acid-preventing shielding gas when the molten steel flows into the mold. A portion of the floating inclusions reacts with the molten flux in the mold, is absorbed, and is discharged. However, even if they float to the surface of the molten steel, some of them do not react with the molten flux and are once again caught up in the flow of the molten steel. When the steel solidifies, the inclusions within it are naturally fixed. When observing the distribution of inclusions fixed in this steel, many of them are unevenly distributed on the surface layer of the slab. This causes sliver defects when the slab is rolled or cracks when deep drawing is performed. Therefore, in order to cast a good slab of aluminium-killed steel, it is necessary to promote the floating of Al ₂ O ₃ inclusions and to remove the floated inclusions to prevent them from being drawn into the molten steel again. be. As mentioned above, the floating behavior of inclusions in the mold is influenced by the flow of molten steel, bubbling of shielding gas, and the like. Therefore, to promote the floating of inclusions, it is necessary to optimize the flow of molten steel and the flow of shielding gas. Furthermore, in order to remove more floating inclusions, it is necessary to optimize the flux composition. Furthermore, in order to obtain a good slab in continuous casting, flux flows between the mold and the slab, and is required to have appropriate performance as a lubricant between the mold and the slab and as a heat transfer medium. For this purpose, the flux must flow uniformly between the mold and the slab. Therefore, in order to ensure uniform inflow, it is necessary to ensure an appropriate viscosity of the flux depending on the casting conditions, especially the slab drawing speed. Further, as mentioned above, flux is required to have the ability to absorb and discharge inclusions from molten steel, and when it reacts with these inclusions, changes in viscosity, etc. are expected. Since this viscosity change significantly affects the inflow behavior, it is necessary to reduce the viscosity change and suppress the amount of change within an allowable amount. The present invention involves conducting detailed research experiments on the floating behavior of Al ₂ O ₃ inclusions in aluminum killed steel in a mold and the behavior of the floating inclusions reacting with molten flux and being ejected, and quantitatively analyzing these behaviors. As a result of this research, the gist is that in continuous casting of aluminum-killed steel, inert gas is applied at a rate of 20/min to 130/min per mold unit cross-sectional area (m ² ).
/min into the molten steel, and the center of the molten steel discharge port of the immersion nozzle is 120 mm to 220 mm from the molten steel surface.
The flux added in the mold is regulated to a basicity B of 1.3 or more, which is determined by equation (1).
30 Al ₂ O ₃ inclusions per mold unit cross-sectional area (m ² )
The viscosity ηx (Poise) of the in-mold molten flux discharged from the molten steel and absorbing the discharged Al ₂ O ₃ inclusions is determined by equation (2), and it is determined by the slab drawing speed. A continuous casting method for aluminum killed steel is characterized in that casting is controlled within the range determined by equation (3) according to the following conditions. B=1.53・CaO+1.51・MgO+1.37・_K2O +1.94・_Na2O +1
.53・CaF ₂ +3.55・Li ₂ O+3.27・NaF／1.48・SiO ₂ +0.36
・TiO ₂ +0.10・Al ₂ O ₃ ...(1) However, each component is weight (%) η _x = (0.55 + 0.34 x 1.12 ^x ) / (0.55 + 0.34 x 1.12
^x0 )η(2) However, η: 1300℃ of flux before Al ₂ O ₃ absorption
Viscosity (Poise) x ₀ : Al ₂ O ₃ originally contained in the flux
(%) x: In the molten flux that absorbed Al ₂ O ₃ inclusions
Al ₂ O ₃ (%) 1.0/V≦η _x ≦4.0/V (3) where V: drawing speed of slab (m/min) The present invention will be described in detail below. First of all, the shielding gas mentioned here is Ar・_N2 or
This refers to inert gases such as He, which have traditionally been used as shielding gases. This amount of injection is usually considered as a function of the molten steel injection speed or the cross-sectional area of the immersion nozzle, etc., since it is used as an antioxidant in the immersion nozzle when molten steel is injected into the mold. Inert gas was regulated by the amount of gas blown into the mold (/min) per unit cross-sectional area (m ² ) of the mold. This is because gas injection is a bubbling phenomenon in the molten steel in the mold.
From the viewpoint that it greatly contributes to the floating of Al ₂ O ₃ inclusions, the amount of gas blown per cross-sectional area is regulated, which is considered to represent the frequency of bubbling within the mold. Note that this blown amount is the amount that passed through the immersion nozzle, and does not include the amount that was flowed for shielding around the joint surface of the immersion nozzle and was discharged as is without flowing into the immersion nozzle. The lower limit was set at 20/min per ^1m2 .
This is because if the speed is less than 20/min, the discharge of Al ₂ O ₃ inclusions will be insufficient and the occurrence of sliver defects cannot be prevented.
On the other hand, if the blowing rate exceeds 130/min, the Al ₂ O ₃ inclusions can be sufficiently discharged, but excessive bubbling occurs due to the excessive gas blowing rate, causing slag entrainment, which is the cause of sliver defects. must be avoided. For this reason, the upper limit of the blowing rate was set at 130/min. As shown in the examples below, Al ₂ O ₃ can be discharged even when inert gas is not blown in, but in this case, the molten steel is oxidized while passing through the immersion nozzle, and as a result, Al 2 O 3 is discharged. As a result, many Al ₂ O ₃ inclusions are rolled into the mold, which increases the occurrence of sliver defects. Next, the reason why the molten steel is immersed so that the center of the molten steel discharge port of the immersion nozzle is located at a position of 120 mm to 220 mm from the molten steel surface is as follows. First, when the immersion depth is less than 120 mm, the amount of Al ₂ O ₃ inclusions discharged is large, but slag entrainment and sliver defects occur, which are thought to be due to fluctuations in the molten steel level.
On the other hand, when the immersion depth exceeds 220 mm, the surface temperature of the molten steel tends to become too low, and the amount of Al ₂ O ₃ inclusions discharged is small, causing surface cracks in the slab, slag entrainment, sliver defects, etc. For the above reasons, the immersion depth of the immersion nozzle was defined as 120 mm to 220 mm. Next, we will discuss the regulation of basicity of flux. The higher the basicity of the flux added into the mold, the greater the amount of Al ₂ O ₃ absorbed by the flux. In this case, the basicity of the flux in the present invention is expressed by the following formula (1), and the basicity B is
Requires 1.3 or higher. If B is less than 1.3, cracks will occur on the surface of the slab and it cannot be put to practical use. In equation (1) for B, first find the ion-oxygen attraction force I of each component such as SiO ₂ and CaO, then subtract it from the I of each component using 1.56 as the neutral point, and use this value to calculate the mole% of each component. In order to convert to weight%, calculate the quotient of the molecular weight of each component, and to simplify the coefficient, 100
The numerator is the sum of the coefficient components with negative values, the denominator is the sum of the coefficient components with positive values, and the quotient is B. For example, the ion-oxygen attraction I of SiO ₂ is 2.45, subtracting 1.56 from this gives 0.89, dividing this by the molecular weight of SiO ₂ 60, 0.0148, multiplying this by 100 to get the coefficient of SiO ₂ , and since it takes a positive value, the denominator And so. The coefficient was calculated for fluoride using the same concept. B=1.53・CaO+1.51MgO+1.37・_K2O +1.94・_Na2O +1.5
3・CaF ₂ +3.55・Li ₂ O+3.27・NaF／1.48・SiO ₂ +0.36・
TiO ₂ +0.10・Al ₂ O ₃ ...(1) However, each component is by weight (%). As described above, the composition of the flux used in the present invention is not particularly limited as long as B is 1.3 or more. Furthermore, the form of the flux is not particularly limited, but may include mixing raw materials,
Most preferably, this mixture is melted and pulverized, a melt modifier and a granulating agent are added thereto, and the mixture is granulated and dried. In this case, as raw materials, limestone, wollastonite, silica sand, soda ash, lithium carbonate, fluorite, sodium fluoride, alumina, magnesia, etc. are used in an appropriate combination. Desirable component ranges include SiO ₂ 20-45%;
_{Al2O3 0} ~15%, CaO20~45%, MgO0~20%,
_CaF2 +NaF5~30%, _Na2O + _K2O + _Li2O5 ~29
%, C0.4-7%, FeO, MnO, TiO ₂ , etc. as necessary or as impurities in the raw materials used.
It is also possible to contain BaO and the like in a total amount of about 0.5 to 10%. In this case, as will be described later, it is desirable to keep the amount of Al ₂ O ₃ originally contained in the flux as low as possible. Next, we set the amount of Al ₂ O ₃ inclusions discharged from molten steel to 30 g/min or more per mold unit cross-sectional area (m ² ) because if the discharge is less than 30 g/min, there are many inclusions remaining on the surface layer of the slab. This is because defects such as sliver flaws occur during rolling or cracks occur during deep drawing. Note that this emission amount can be determined as follows. That is, the flux added into the mold, flowed between the mold and the slab, and discharged from the bottom end of the mold was sampled, and the amount of Al ₂ O ₃ contained in this was measured. Determine the amount of emissions by subtracting the amount of ₂ O ₃ and multiplying it by the amount of flux emitted per hour, and then calculate the amount of flux emitted per hour.
By dividing the amount of Al ₂ O ₃ by the internal cross-sectional area of the mold, the amount of discharge per unit cross-sectional area of the mold can be determined, and it is also possible to correct the difference due to the difference in mold size. . In this way, the amount of Al ₂ O ₃ inclusions discharged from molten steel can be controlled by the amount of shielding gas injected, the immersion depth of the immersion nozzle, and the basicity of the flux used, and the combination of optimal conditions for each of these can be controlled. In combination, maximum Al ₂ O ₃ inclusions can be removed. However, conventional prior art, for example, JP-A-52-
As shown in No. 26318, etc., it is necessary to further regulate the viscosity of the molten flux according to the slab drawing speed. For this reason, the molten flux first becomes Al ₂ O ₃
We investigated how the viscosity changes when inclusions are absorbed. The results are shown in FIG.

[Table] This figure shows that for the fluxes having the basic composition shown in Table 1, Al _{2 O 3 was added so that the Al 2 O 3 content} in the flux was 5, 10, 15, 20, and 25%, respectively.
The viscosity of the sample at 1300℃ was measured by adding different levels of Al ₂ O 3, and the viscosity change index for each Al ₂ O ₃ level is shown based on the viscosity when Al 2 O ₃ is 0%. be. ○ mark is flux, ◇ mark is flux, △ mark is flux. The viscosity of any flux increases rapidly when the amount of Al ₂ O ₃ exceeds 15%. As a result of regression analysis of this change, equation (2') was obtained. y=0.55+0.34×1.12 ^x ...(2') Here, y is the amount of change, x is Al ₂ O ₃ in the flux
content (wt%). The values calculated using equation (2') are shown in Figure 1 with ●. It can be seen that the values match well with the measured values. Using this equation (2′), the flux is
The viscosity η _x when Al ₂ O ₃ inclusions are absorbed can be determined by equation (2). η _x = (0.55 + 0.34 x 1.12 ^x ) / (0.55 + 0.34 x 1.
12 ^x0 ) η(2) where η is the flux of 1300℃ before Al ₂ O ₃ absorption
_{viscosity ( Poise ) at _ _} It is. As is clear from this relationship, it is desirable to limit the Al ₂ O ₃ originally contained in the flux to as low a level as possible. Furthermore, there is a clear relationship between the slab drawing speed and the viscosity η _x required to obtain a good slab. That is, according to the findings of the present inventors, the viscosity for obtaining a good slab is within the range shown by equation (3). This relationship was obtained by determining the appropriate viscosity range by performing casting under different casting conditions of viscosity η _x . The casting at this time is Si0.01~0.03%, Mn0.1~0.2
%, Al0.004~0.007%, C0.04~0.11% aluminum killed steel, mold size thickness 210mm, width 950~2100
mm, the slab drawing speed ranges from 0.8 to 1.8 m/min, and the flux has a viscosity of 0.4, 0.6, 1.1 at 1300℃,
At 1.6, 2.1, 3.2, 3.8 Poise, the basicity B at that time is 2.71, 2.42, 1.99, 1.82, 1.71, 1.45, 1.36, respectively.
The amount of Ar injection for shielding is 40 ~
80/min・m ² , immersion depth of immersion nozzle is 160~
This was done within a range of 180mm. Figure 2 shows the number of defects such as cracks and slag entrainment occurring per 1 ^m2 of slab surface, with the horizontal axis representing the drawing speed of the slab and the vertical axis representing the viscosity η _x determined by equation (2).
If the number is less than 7, it is marked with ○, and if it is more than 7, it is marked with △. From the same figure, it is preferable to use a low-viscosity powder when the casting speed is increased, but if the casting speed is decreased excessively, the surface defects of the slab will increase. It turns out that it is necessary to cast it with 1.0/V≦η _x ≦4.0/V …(3) Here, V is the slab drawing speed (m/min), and η _x is the time when the molten flux absorbs Al ₂ O ₃ inclusions (2)
This is the viscosity (Poise) determined by the formula. In equation (3), if the viscosity is less than 1.0/V, excessive flux will flow between the mold and the slab, and local excess flow will occur, causing cracks due to insufficient flow of molten flux. In addition, slag entrainment occurs due to insufficient molten flux spool layer due to excessive inflow. On the other hand, when the viscosity exceeds 4.0/V, the fluidity of the molten flux deteriorates, and the molten flux does not flow uniformly between the mold and the slab, causing cracks. Also, if this inflow failure occurs, the renewal of the molten flux will be delayed, so the amount of flux in the molten flux will increase.
As the amount of Al ₂ O ₃ increases, the fluidity is further deteriorated, and the ability of the molten flux to absorb Al ₂ O ₃ inclusions decreases, and the entrainment of Al ₂ O ₃ inclusions in the slab increases. The effects of the present invention will be illustrated in more detail with reference to Examples below. Example: Sample steel: C0.06%, Si0.02%, Mn0.15%,
Used aluminum killed steel with Al0.007%. The mold sizes were 210 mm and 245 mm thick, 850 to 2100 mm wide, and the slab drawing speed was 1.0 to 1.8 m/min. Ar was used as the shielding gas, and the rate was 0 to 160/min per unit cross-sectional area (m ² ) in the mold. The immersion nozzle had an inverted Y-shaped molten steel discharge port, and the immersion depth was such that the distance from the center of the nozzle discharge port to the surface of the molten steel was 100 to 250 mm. The change in the molten steel surface position was controlled to within ±5 mm. The tested flux was manufactured by mixing the raw materials, melting and pulverizing the mixture, and adding a melting rate regulator and a granulating agent to the mixture and granulating it. The raw materials used were limestone, silica sand, soda ash, lithium carbonate, fluorite, sodium fluoride, alumina, and magnesia, and the melting rate regulators were coke fine powder and carbon black. The composition, basicity, and viscosity of the fluxes are shown in Table 2, and seven types of fluxes A to G were prepared.

【table】

【table】

【table】

[Table] Table 3 shows the results of continuous casting under these conditions. In Examples No. 1 to No. 7, the amount of shielding gas was varied. Figure 3 shows shielding Ar amount (/min・m ² ) and
Amount of Al ₂ O ₃ inclusions discharged from molten steel (g/
min・m ² ). As can be seen from the figure, the amount of Al ₂ O ₃ inclusions discharged increases as the amount of Ar injection increases, except in the case without Ar injection. Example No.
The phenomenon in which the amount of Al ₂ O ₃ inclusions is increased without Ar injection in step 1 is because the oxidation of molten steel is large when it passes through the immersion nozzle, and the amount of Al ₂ O ₃ inclusions entering the mold is large. This is thought to be because it has grown larger. As a result, despite the large amount of Al ₂ O ₃ inclusions discharged,
There were many occurrences of inclusions in the slab and sliver defects. In Example No. 2, when the Ar injection rate was 18/min·m ² , the occurrence of sliver defects was observed when the discharge rate of Al ₂ O ₃ inclusions was 25 g/min·m ² . Example No. 3, No. 4,
When the Ar injection rate of No. 5 is 20, 70, and 120/min・^m2 , the Al ₂ O ₃ inclusion discharge amount is 35, 230, and 460 g/min・
A good slab with less occurrence of sliver defects was obtained at ^m2 . Example No. 6, No. 7 Ar injection amount 140,
At 160/min・m2 ^, the emission amount of Al ₂ O ₃ inclusions is 520,
Although the amount was 600 g/min・m ² , slag rolling-in sliver defects were observed. This is thought to be because the amount of Ar injection was too large, which caused the hub ring phenomenon to become too large, resulting in large ripples on the surface of the molten steel, which caused slag entrainment, and this slag entrainment caused sliver defects. In Examples No. 4 and No. 8 to No. 11, cases were investigated in which the immersion depth of the immersion nozzle was varied. FIG. 4 shows the relationship between the nozzle immersion depth and the amount of Al ₂ O ₃ inclusions discharged. As can be seen from the figure, the deeper the immersion depth, the smaller the amount of Al ₂ O ₃ inclusions discharged. At an immersion depth of 110 mm in Example No. 8, the amount of Al ₂ O ₃ inclusions discharged was as large as 550 g/min·m ² , but slag entrainment sliver defects occurred, which was thought to be due to fluctuations in the molten steel level. At the immersion depths of 130, 170, and 210 mm in Examples No. 9, No. 4, and No. 10, the Al ₂ O ₃ emissions were 280 mm,
The slab quality was also good at 230 and 160g/min・^m2 . At the immersion depth of 230 mm in Example No. 11, the temperature of the molten steel surface layer tends to become too low, and the amount of Al ₂ O ₃ inclusions discharged is also small at 20 g/min・m ² , causing surface cracks in the slab and slag entrainment. , sliver defects were observed. In Examples No. 4 and No. 12 to No. 17, the influence of the basicity of the flux added into the mold was compared. Figure 5 shows the relationship between flux basicity and the amount of Al ₂ O ₃ inclusions discharged. As is clear from the figure, the higher the basicity of the flux, the greater the amount of Al ₂ O ₃ inclusions discharged. The basicity of the flux of Example No. 12 is 1.27, and the amount of Al ₂ O ₃ inclusions discharged is 20 g/
Sliver defects were observed at min·m ² . Example
For No. 13, which had a flux basicity of 1.37, the amount of Al ₂ O ₃ inclusions discharged was 40 g/min·m ² , but cracks on the surface of the slab were observed, which was considered to be due to inappropriate flux viscosity. For Example No. 4 and No. 14, No. 15, and No. 16 whose flux basicity is 1.78, 1.42, 2.01, and 2.49, respectively, the discharge amount of Al ₂ O ₃ inclusions is 230, 50,
The slab quality was also good at 330 and 450g/min・^m2 . Example No. 17 has a flux basicity of 2.69 and is Al ₂ O ₃
The amount of inclusions discharged was high at 510g/min・^m2 , but slag entrainment was observed. This is considered to be because flux G shown in Table 2 had a lower viscosity than fluxes A to F, and as a result, the flow rate of the flux became excessive. Next, Examples No. 3, No. 4, No. 5 and No. 13 to No. 26
The relationship between slab drawing speed and flux viscosity was compared. Figure 6 summarizes the results. In the figure, the horizontal axis is the slab withdrawal speed, and the vertical axis is the viscosity, which is η _x determined by measuring the amount of Al ₂ O ₃ absorbed by the flux and using equation (2). ● marks indicate slabs with cracks on the surface of the slabs, slag entrainment, and sliver defects; ○ marks indicate slabs with no defects and good quality. As is clear from the figure, the viscosity required to obtain a good slab is within the range shown by equation (3). In Examples No. 13, No. 18, No. 22, and No. 23 where η _x exceeds 4.0/V, and in Examples No. 17, No. 21, and No. 26 where η _x is less than 1.0/V, Cracks on the slab surface, slag entrainment, and sliver defects were observed. The effects of the present invention have been described above based on the examples. However, the present invention provides improvements in the amount of shielding gas blown into continuous casting of steel, the immersion depth of the immersion nozzle, the basicity of the flux, the viscosity, etc., which were not previously clear. By quantitatively understanding individual conditions and investigating the effects of their combinations, quantitatively understanding the effects of these conditions on the emission of Al ₂ O ₃ inclusions from aluminum killed steel, and by combining these conditions, It has made it possible to provide a completely new continuous casting method for aluminium-killed steel that has not been found in the past, and its contribution to the development of industry is extremely significant.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relative change in viscosity due to an increase in the amount of Al ₂ O ₃ in the flux, and Figure 2 is a diagram showing the region where good slabs can be obtained in the relationship between slab drawing speed and viscosity η _x . , Fig. 3 is a diagram showing the relationship between the amount of shielding Ar gas blown in and the amount of Al ₂ O ₃ inclusions discharged for the example, and Fig. 4 is a diagram showing the relationship between the immersion depth of the immersion nozzle and the Al ₂ O ₃ inclusions for the example. Figure 5 is a diagram showing the relationship between the basicity B of flux and the discharge amount of Al 2 O ₃ inclusions for Examples, and Figure 6 is a diagram showing the relationship between the flux basicity B and the discharge amount of Al ₂ O 3 inclusions for Examples. FIG. 2 is a diagram showing the relationship between viscosity η _x and

Claims (1)

[Claims] 1. In continuous casting of aluminum-killed steel, inert gas is applied at 20/min or more per mold unit cross-sectional area (m ² ).
Blow into the molten steel at a rate of 130/min, and ensure that the center of the molten steel outlet of the immersion nozzle is 120 mm or more from the molten steel surface.
Immerse within a range of 220mm to add flux inside the mold.
The basicity B determined by formula (1) is controlled to 1.3 or more, and the Al ₂ O ₃ inclusions are reduced to 30% per unit cross-sectional area (m ² ) of the mold.
_{The viscosity η} A continuous casting method for aluminum killed steel, which is characterized by controlling the casting speed to fall within the range determined by equation (3). B= 1.53・CaO+1.51・MgO+1.37・_K2O +1.94・_Na2O +1.53
・CaF ₂ +3.55・Li ₂ O+3.27・NaF／1.48・SiO ₂ +0.36・T
iO ₂ +0.10・Al ₂ O ₃ ...(1) However, each component is weight% η _x = (0.55 + 0.34 x 1.12 ^x ) / (0.55 + 0.34 x 1.12
^x0 )η(2) However, η: 1300℃ of flux before Al ₂ O ₃ absorption
Viscosity (Poise) x ₀ : _{Al 2} O ₃ originally contained in _the flux (%)
Al ₂ O ₃ (%) 1.0/V≦η _x ≦4.0/V (3) However, V: Slab drawing speed (m/min)