JP2010227944A - Continuous casting method for steel cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To inexpensively and stably manufacture a cast slab which hardly causes defects due to nonmetallic inclusions such as an alumina cluster on the surface layer thereof, and is clean and has high quality, without deteriorating productivity. <P>SOLUTION: The continuous casting method for the cast slab is directed for continuously casting a cast slab of extra-low carbon steel containing ≤0.003 mass% C. In the continuous casting method, the molten metal flowing rate of the cast slab at the side facing a solidified shell is controlled to be within the range of formula (1): V≥[165,308×(1-K<SB>E</SB><SP>Ti</SP>)×Vs×[Ti]+1,613,307×(1-K<SB>E</SB><SP>S</SP>)×Vs×[S]]<SP>2/3</SP>, with respect to the solidifying speed of a solidified shell of the cast slab and the Ti content and the S content of molten metal components of the cast slab. In formula (1), V is a molten metal flowing rate (m/s) at the side facing a solidified shell, Vs is a solidifying speed (m/s) of a solidified shell in a mold, K<SB>E</SB><SP>Ti</SP>is an effective distribution coefficient (-) of Ti, K<SB>E</SB><SP>S</SP>is an effective distribution coefficient (-) of S, [Ti] is a Ti concentration (mass%) in a molten metal, and [S] is an S concentration (mass%) in a molten metal. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a continuous casting method of a steel slab, and more particularly, to a continuous casting method for casting a steel slab having a small number of alumina clusters captured by a slab surface layer in a mold.

When manufacturing ultra-low carbon steel such as steel sheets for automobiles, the molten steel is deoxidized with Al, so mixing of alumina (Al ₂ O ₃ ) into the molten steel is inevitable at the end of refining. As a result, the alumina in the molten steel is agglomerated in a transport container or a tundish of a continuous casting facility to form an alumina clasler. During continuous casting of molten steel, this alumina cluster flows into the mold together with the molten steel from the tundish, and is trapped by the solidified shell of the slab and becomes a surface defect of the slab. The quality of carbon steel slabs is significantly reduced.

  Therefore, when there is a surface defect in the cast slab, a work for removing the portion where the surface defect exists by welding, a so-called “care work” is performed. However, this maintenance work not only causes an increase in cost due to a decrease in steel yield and an increase in work processing cost, but also causes a problem that the manufacturing process is extended and the efficient production system is hindered.

  Therefore, in order to improve the quality of the slab, non-metallic inclusions such as alumina clusters adhering to the solidified shell can be obtained by applying a flow velocity to the molten steel in front of the solidified shell using an electromagnetic stirring device installed on the back of the mold. Many methods have been proposed for cleaning and thereby reducing non-metallic inclusions such as alumina clusters in the slab surface layer (see, for example, Patent Document 1).

  However, in the method of applying a flow rate to the molten steel in front of the solidified shell using an electromagnetic stirrer, a flow rate higher than necessary may be applied to the molten steel. In such a case, the mold added on the molten steel surface in the mold The entrainment of powder occurs, and on the contrary, the quality of the slab surface layer is deteriorated, and there is a problem that energy is wasted due to an unnecessary increase in electric consumption.

  As other countermeasures, Patent Document 2 and Patent Document 3 capture bubbles in the solidified shell by controlling the surface tension due to the concentration gradient of C, S, N, and O in the molten steel in front of the solidified shell. A method of suppressing casting, that is, a method of continuous casting after adjusting the concentrations of C, S, N, and O in the molten steel in advance so that the surface tension becomes a predetermined value or less has been proposed.

  However, Patent Document 2 and Patent Document 3 do not discuss the trapping of alumina clusters in the solidified shell. In addition, although it is suggested that the trapping of bubbles in the solidified shell depends on the molten steel components, the relationship between the trapping of bubbles and the flow velocity of molten steel at the solidification interface has not been clarified. Cannot be grasped. This is because in the actual mold, simultaneously with the surface tension due to the concentration distribution of C, S, N, O (= capturing force to the solidified shell), the drag due to the molten steel flow velocity also works, This is because when considering capturing non-metallic inclusions, it is necessary to consider the drag due to the molten steel flow velocity.

JP-A-10-180426 JP 2003-205349 A JP 2003-251438 A

  As described above, in spite of the desire to produce a slab, which is a raw material of a steel plate that requires strict quality such as a steel plate for automobiles, at low cost without impairing productivity, the conventional However, there was no effective means, and defects due to alumina clusters occurred in the surface layer portion of the slab, and the defects were inevitably removed by scouring with a scarfer, resulting in an increase in manufacturing cost.

  The present invention has been made in view of the above circumstances. The object of the present invention is to produce a clean, high-quality slab with less defects due to non-metallic inclusions such as alumina clusters in the surface portion of the slab. It is an object to provide a continuous casting method of a steel slab that can be manufactured inexpensively and stably without impairing the above.

  In order to solve the above-mentioned problems, the present inventors have intensively studied and studied. As a result, it produces slabs that are made of steel plate materials that require clean quality, such as automotive steel plates, with few defects due to non-metallic inclusions such as alumina clusters in the surface layer of the slabs. In order to manufacture stably and inexpensively without impairing the properties, the flow rate of the molten steel at the solidification interface is reduced by using an electromagnetic stirrer or using the discharge flow discharged from the discharge hole of the immersion nozzle. Given that the first condition is to clean the alumina cluster, depending on the Ti content and S content of each steel type and the solidification rate in the mold in order to prevent the entrainment of mold powder, etc. It was also found that it was necessary to provide an appropriate molten steel flow rate.

The present invention has been made on the basis of the above knowledge, and the continuous casting method of a steel slab according to the first invention is a continuous casting method of an ultra-low carbon steel slab containing 0.003% by mass or less of C. The molten steel flow velocity at the front of the solidified shell of the slab is within the range of the following formula (1) with respect to the solidification rate of the solidified shell of the slab and the Ti content and S content of the molten steel components. It is characterized by being controlled so as to be cast.
V ≧ [165308 × (1-K _E ^Ti ) × Vs × [Ti] + 1613307 × (1-K _E ^S ) × Vs × [S]] ^2/3 … (1)
However, in (1), V is, the molten steel flow velocity in the solidified shell front (m / s), Vs is the solidification speed (m / s) of the mold in the solidified shell, K _E ^Ti is effective partition coefficient Ti (−), K _E ^S is the effective distribution coefficient (−) of S, [Ti] is the Ti concentration (mass%) in the molten steel, and [S] is the S concentration (mass%) in the molten steel.

  The continuous casting method of the steel slab according to the second invention is the method according to the first invention, wherein the molten steel flow velocity at the front of the solidified shell and the solidification rate of the solidified shell are 300 to 500 mm away from the molten steel surface in the mold in the casting direction. It is characterized by the solidified shell front molten steel flow velocity and solidification velocity in the solidified shell near the position.

  In the continuous casting method of the steel slab according to the third invention, in the first or second invention, the ultra-low carbon steel contains, as a chemical component other than C, Si: 0.05% by mass or less, Mn: 1 0.0 mass% or less, P: 0.05 mass% or less, S: 0.015 mass% or less, Al: 0.010 to 0.075 mass%, Ti: 0.050 mass% or less, with the balance being It consists of Fe and inevitable impurities.

  In the continuous casting method of a steel slab according to a fourth invention, in any one of the first to third inventions, the molten steel flow velocity at the front surface of the solidified shell is controlled by an AC moving magnetic field applying device arranged at the back surface of the mold. It is characterized by this.

  According to the present invention, since the molten steel flow velocity in front of the solidified shell is controlled to an appropriate flow velocity according to the molten steel component and the solidification velocity in the mold, mold powder is not entrapped and is caused by non-metallic inclusions such as alumina clusters. It is possible to produce a clean and high-quality slab with extremely few surface defects at low cost and stably without impairing productivity.

It is a figure which shows the position which measured the inclination angle of the dendrite dendrite of slab.

  Hereinafter, the present invention will be described in detail.

  An ultra-low carbon steel having a C content of 0.003% by mass or less is decarburized and refined under atmospheric pressure in a converter and decarburized and refined under reduced pressure in a vacuum degassing facility such as an RH vacuum degassing apparatus. It is made from hot metal by decarburizing and refining twice ("vacuum decarburizing and refining"). Decarburization refining does not proceed unless the concentration of dissolved oxygen in the molten steel is increased to some extent. Therefore, a large amount of dissolved oxygen (also referred to as “free oxygen”) remains in the molten steel at the end of decarburization refining. Since the cleanliness of steel deteriorates when a large amount of dissolved oxygen remains, in the melting process of ultra-low carbon steel, metal Al is added to the molten steel after vacuum decarburization refining, and the molten steel is deoxidized. It is processed. By this deoxidation treatment, the dissolved oxygen concentration in the molten steel is rapidly lowered, and alumina is formed as a deoxidation product. Note that oxygen in alumina is chemically bonded to Al, and even if alumina is suspended in molten steel, oxygen in alumina is not called dissolved oxygen. The total of oxygen present in molten steel as an oxide such as alumina and dissolved oxygen is referred to as total oxygen (also referred to as “TO”).

  The alumina produced as a deoxidation product agglomerates with the passage of time for the period during which molten steel is transported from the vacuum degassing equipment to the continuous casting equipment and the time it is injected into the mold after being injected into the tundish. To form an alumina cluster. When this alumina cluster is injected into the mold together with molten steel and is captured by the solidified shell of the slab, it becomes a surface defect of the ultra-low carbon steel slab, and the quality of the slab deteriorates.

The present inventors paid attention to the force acting on the nonmetallic inclusions in the molten steel in the front of the solidified shell in examining the conditions for preventing the nonmetallic inclusions such as alumina clusters from being trapped in the solidified shell. And as a result of considering two forces of “force acting in the direction of solidification interface based on the gradient of interfacial tension between non-metallic inclusions and molten steel” and “drag due to molten steel flow velocity”, the above-mentioned problem is solved by the following means: The knowledge that it can be solved was obtained. That is, “the molten steel flow velocity at the front of the solidified shell of the slab is within the range of the following formula (1) with respect to the solidification rate of the solidified shell of the slab and the Ti content and S content of the molten steel components: It is a method of “controlling and casting to be”.
V ≧ [165308 × (1-K _E ^Ti ) × Vs × [Ti] + 1613307 × (1-K _E ^S ) × Vs × [S]] ^2/3 … (1)
However, in the formula (1), V is a molten steel flow velocity (m / s) in front of the solidified shell, Vs is a solidification rate of the solidified shell (m / s), and K _E ^Ti is an effective distribution coefficient of Ti (− ), K _E ^S is the effective distribution coefficient (−) of S, [Ti] is the Ti concentration (mass%) in the molten steel, and [S] is the S concentration (mass%) in the molten steel.

Here, “(1-K _E ^Ti ) × Vs × [Ti]” and “(1-K _E ^S ) × Vs × [S]” in equation (1) are formed on the front of the solidified shell during continuous casting. Is a measure of the attractive force in the direction of the solidified shell due to the interfacial tension gradient that acts on the nonmetallic inclusions that have entered the concentration boundary layer of the solute element (hereinafter also simply referred to as “solute”). It is a parameter that determines the molten steel flow velocity in front of the solidification interface in the mold. In addition, the present invention aims to prevent the capture of alumina clusters in the solidified shell. Therefore, the molten steel flow velocity V and the solidification velocity Vs of the solidified shell in the equation (1) It is preferable to target the solidified shell front molten steel flow velocity and solidification rate in the solidified shell near the position 300 to 500 mm away from the molten steel surface in the mold in the casting direction, where the alumina cluster is likely to be captured.

  Hereinafter, a method for deriving the expression (1) will be described.

Regarding “force acting in the direction of solidification interface based on gradient of interfacial tension between non-metallic inclusion and molten steel”, as shown in the publication: Iron and Steel (80 (1994) p.527), Based on the interfacial tension gradient K in the concentration boundary layer, that is, dσ / dx (σ: interfacial tension between non-metallic inclusion and molten steel, x: distance), the force F that the inclusion receives in the direction of the solidified shell is It is shown by the formula (2).
F =-(8/3) × πR ² K… (2)
Here, F is the force (N) received by the inclusions, π is the circumference, R is the radius (m) of the inclusions, and K is the interfacial tension gradient (N / m ² ).

The interfacial tension gradient K is the product of the change in interfacial tension due to the solute concentration and the component concentration gradient, as shown in the following equation (3).
K = dσ / dx = (dσ / dc) × (dc / dx) (3)
Here, σ is the interfacial tension (N / m) between the nonmetallic inclusion and molten steel, x is the distance (m) from the solidification interface, and dσ / dc is the interfacial tension between the nonmetallic inclusion and molten steel. The change due to the solute concentration (N / m · mass%), dc / dx is the concentration gradient (mass% / m) of the component.

From the solidification theory, the concentration gradient dc / dx of the component under the condition where the molten steel flow velocity exists in the mold is expressed by the following equation (4).
dc / dx = -C ₀ x (1-K _E ) x (V _s / D) x exp [-V _s x (x-δ) / D] ... (4)
Here, C ₀ is the solute concentration (mass%) in the molten steel before casting, V _s is the solidification rate (m / s) in the mold, and D is the diffusion coefficient of the solute in the molten steel (m ² / s). , Δ is the thickness (m) of the concentration boundary layer. K _E is the effective partition coefficient (−) of the solute and is expressed by the following equation (5).
K _E = K ₀ / [K ₀ + (1-K ₀ ) × exp (−Vsδ / D)] (5)
Here, K ₀ is the equilibrium partition coefficient (−) of the solute.

When x = δ is substituted in the equation (4), the concentration gradient (dc / dx) at x = δ is obtained by the following equation (6).
dc / dx = -C ₀ x (1-K _E ) x (V _{s /D)...(6} )
By substituting Eq. (6) into Eq. (3), the interfacial tension gradient K between the non-metallic inclusion and molten steel at x = δ, that is, the force acting immediately after the alumina cluster enters the concentration boundary layer. An interfacial tension gradient K indicating a scale can be obtained by the following equation (7).

K = (dσ / dc) x [-C ₀ x (1-K _E ) x (V _{s /D)]...(7} )
When the formula (7) is substituted into the formula (2), the force acting on the alumina cluster immediately after entering the concentration boundary layer is obtained by the following formula (8).

F =-(8/3) × πR ² × (dσ / dc) × [-C ₀ × (1-K _E ) × (V _s /D)]...(8)
For "drag by molten steel flow speed", publication: As shown in transport phenomena for Materials Engineering (p.45), the drag F _D the ball is subjected in the fluid represented by the following formula (9) It is.
F _D = C _D × (πR ² ) × [(1/2) × ρV ² ] ... (9)
Here, F _D is the drag (N), C _D is the drag coefficient of a sphere is subjected in the fluid (-), [rho is the molten steel density (kg / m ^3).

Resistance coefficient C _D is sought from the Re number (Reynolds number), also calculation formula is different according to the value of Re number. When considering inclusion behavior in the mold, Re number is about 30 to 300, and the in this case, the resistance coefficient C _D is formula Allen represented by the following equation (10) is in good agreement.
C _D = 10 / Re ^1/2 (10)
(10) By substituting expression (9) below, the drag F _D received by the inclusions in the mold is determined by (11) below.

F _D = (10 / Re ^1/2 ) × (πR ² ) × [(1/2) × ρV ² ] ... (11)
In this case, the Re number is obtained by the following equation (12).
Re = 2RρV / μ… (12)
Here, μ is the viscosity (N · s / m ² ) of the molten steel. By substituting this equation (12) into equation (11), the following equation (13) is obtained.

F _D = (5/2 ^1/2 ) × π × (μρ) ^1/2 × (RV) ^3/2 (13)
Thus, the “force acting in the direction of the solidification interface based on the gradient of the interfacial tension between inclusions and molten steel” and “the drag due to the molten steel flow velocity” are as shown in the equations (8) and (13), respectively. . The former “force acting in the direction of the solidification interface based on the gradient of the interfacial tension between inclusions and molten steel” is the force that pulls inclusions toward the solidification interface, while the latter “force due to molten steel flow velocity” solidifies inclusions. It is the force away from the interface. That is, as shown in the following formula (14), if a force larger than the attractive force in the direction of the solidification interface acting on the inclusion is applied in the vertical direction of the attractive force, the inclusion is not trapped and the solidification interface is formed. I will leave.

F <F _D (14)
By substituting the equations (2) and (13) into the equation (14), a conditional equation of the molten steel flow velocity at which inclusions are not trapped at the solidification interface can be derived as the following equation (15).

V ≧ [(8 × 2 ^1/2 / 15) × K × R ^1/2 × (μρ) ^-1/2 ] ^{2 / 3} … (15)
Here, the value of the interfacial tension gradient K is determined for each solute component, and the inclusion receives a force for each interfacial tension gradient of each component. As a result of the study, the elements that have a great influence on the value of the interfacial tension gradient K among the chemical constituent elements of the ultra-low carbon steel are Ti and S, and these elements are only calculated. It was found that there was no problem in considering the trapping of inclusions in the solidified shell even when the value of the interfacial tension gradient K shown in the following equation (16) was used.

K = (dσ / dc _Ti ) × [-C _Ti × (1-K _E ^Ti ) × (Vs / D _Ti )] + (dσ / dc _S ) × [-C _S × (1-K _E ^S ) × (Vs / D _S )]… (16)
Here, dσ / dc _Ti is the change in the interfacial tension between the molten steel and inclusions due to the Ti concentration (N / m · mass%), C _Ti is the Ti concentration (mass%) in the molten steel, and K _E ^Ti is the effective Ti Distribution coefficient (−), D _Ti is the diffusion coefficient of Ti in the molten steel (m ² / s), dσ / dc _S is the change in the interfacial tension between the molten steel and inclusions due to the S concentration (N / m · mass%), C _S is S concentration (mass%) in the molten steel, K _E ^S is the effective distribution coefficient S (-), D _S is the diffusion coefficient of S in the molten steel (m ² / s).

The dσ / dc _Ti and dσ / dc _S shown in the equation (16) are described in the publication: Handbook of Physical Properties of Molten Iron and Hot Metal (Edited by the Japan Iron and Steel Institute), and the diffusion coefficient of each component, molten steel The density ρ and the viscosity μ are shown in a publication “Metal Data Book” (edited by the Japan Institute of Metals) and the like. Further, in the surface quality of the steel plate for automobiles, inclusions having a diameter of about 100 μm are often problematic. Therefore, the inclusion radius R in the equation (15) is set to 50 μm. Further, the concentration boundary layer thickness δ is set to several 10 to 1000 μm, and in this study, if it is set to 100 μm, it agrees well with the experimental result, so it is set to 100 μm. By substituting these values into the equations (15) and (16), the above-described equation (1) is derived.

  By controlling the flow velocity V of the molten steel in front of the solidified shell, that is, in front of the solidified interface, within the range of the formula (1), it is possible to prevent the alumina cluster from being captured by the solidified shell. The range for controlling the molten steel flow velocity V on the front surface of the solidified shell is a range corresponding to the surface layer portion of the slab, and as described above, the front surface of the solidified shell near the position 300 to 500 mm away from the molten steel surface in the mold in the casting direction. It is preferable to use a molten steel flow rate at. Needless to say, the control target may be an upstream side in the casting direction or a range on the lower sulfur side in the casting direction from a position 300 to 500 mm away. Moreover, since the thickness of the solidified shell varies depending on the casting conditions, and the position of the magnetic field application device differs depending on the continuous casting machine, the position for controlling the molten steel flow rate may be changed as appropriate.

  As a method for obtaining the solidification rate Vs in the mold, an isotope labeling method and a method for analyzing a heat conduction system in the mold are well known.

  The isotope labeling method is described in, for example, the publication: Iron and Steel (55 (1969) S108). 198Au is introduced into a mold as a tracer to capture 198Au at the solidification interface, and then autoradiograph. The solidified shell thickness is examined, the solidified shell thickness is compared with the casting speed, the time change of the solidified shell thickness is calculated, and thereby the solidification rate Vs in the mold is obtained. If the solidification rate is examined in advance for each casting condition, the molten steel flow velocity at the front surface of the appropriate solidification interface can be calculated by substituting the examined solidification rate into the equation (1).

  The method of analyzing the heat conduction method in the mold is one-dimensional as specified in the publication: Iron and Steel (58 (1972) S396) and the publication: Iron and Steel (56 (1970) S268). The unsteady heat transfer is solved by a function of casting conditions such as casting speed, and the unknown parameter is determined by comparing the actual operation value with the calculation result, and the time change of the solidified shell thickness in the mold is calculated. This is a method for calculating the coagulation rate. Comparing with the result of the labeling method using the isotope described above, the calculation accuracy is further improved. If the solidification rate is examined in advance for each casting condition, the molten steel flow velocity at the front surface of the appropriate solidification interface can be calculated by substituting the examined solidification rate into the equation (1).

  As a method of controlling the molten steel flow velocity in front of the solidification interface, the size, angle, immersion depth, etc. of the discharge hole of the immersion nozzle for injecting the molten steel in the tundish into the mold are adjusted and discharged from the discharge hole. A method using a discharge flow of molten steel, a method using a magnetic field applied from a magnetic field application device arranged on the back of a mold, and using an electromagnetic force formed by the magnetic field and the molten steel flow can be used. There are two types of magnetic field generators: an alternating current movement application device and a direct current magnetic field (static magnetic field) application device. Even if the casting speed is changed, the molten steel flow velocity in front of the solidified shell can be adjusted arbitrarily. It is preferable to use a magnetic field application device. In particular, it is preferable to control by an alternating-current moving magnetic field applying device arranged over the entire back surface of the long side of the mold.

  An AC moving magnetic field application device is placed across the entire width of the back side of the long side of the mold of the slab continuous casting machine, and the moving direction of the moving magnetic field applied from this AC moving magnetic field application device is set to the opposite magnetic field application device. By setting the direction from the mold short side to the immersion nozzle side, the discharge flow of the molten steel discharged from the immersion nozzle is decelerated, and the molten steel flow velocity in front of the solidification interface is reduced accordingly (referred to as “deceleration magnetic field application”). Conversely, the molten steel discharged from the immersion nozzle is set so that the moving magnetic field applied from the AC moving magnetic field application device is in the direction from the immersion nozzle side to the mold short side with the opposite magnetic field application device. As a result, the molten steel flow velocity in front of the solidification interface is increased (referred to as “acceleration magnetic field application”). Further, the moving magnetic field applied from the AC moving magnetic field applying device arranged on the back side of one long side of the mold is set to the same direction, and the moving magnetic field applied from the AC moving magnetic field applying device opposite to the slab is sandwiched. By moving the moving direction in the opposite direction, the molten steel in the mold is agitated so as to rotate in the horizontal direction, and the molten steel flow velocity in front of the solidification interface is increased accordingly (referred to as “swirl magnetic field application”). To do.

  In this way, the flow velocity of the molten steel at the front of the solidification interface is reduced by applying a magnetic field with three types of magnetic field application patterns appropriately selected according to the casting speed by the AC moving magnetic field application device arranged at the entire back surface of the mold long side. Alternatively, it can be accelerated, and the molten steel flow velocity in front of the solidification interface can be controlled to an arbitrary flow velocity regardless of the casting speed.

  In the case of a direct current magnetic field application device, the magnetic field application device is placed opposite to the back of the long side of the mold of the slab continuous casting machine with the slab interposed therebetween, and the molten steel moves by applying a magnetic field penetrating in the mold thickness direction. Is applied with a braking force to control the molten steel flow velocity in front of the solidification interface. In the case of a DC magnetic field application device, the molten steel is not only decelerated, but the molten steel discharge flow from the immersion nozzle flows so as to bypass the DC magnetic field application device, so depending on the installation position of the DC magnetic field application device, The molten steel flow velocity may also increase.

  However, if the molten steel flow velocity at the front of the solidification interface becomes too high due to excessively strong stirring strength, the molten steel flow on the molten steel surface in the mold will increase accordingly, and the mold powder added on the molten steel surface in the mold will Since entrainment occurs, it is preferable to control the molten steel flow velocity in front of the solidification interface within a range in which entrainment of mold powder does not occur. If the flow rate of the molten steel surface in the mold is 0.48 m / s or less based on known literature (Gakushin 19 Committee, No. 10977), the mold powder will not be entrained. What is necessary is just to control the molten steel flow velocity of the solidification interface front surface so that it may become in the range below 0.48 m / s.

The present invention can be applied to any steel type as long as it is an ultra-low carbon steel having a C content of 0.003% by mass or less. As chemical components, Si: 0.05 mass% or less, Mn: 1.0 mass% or less, P: 0.05 mass% or less, S: 0.015 mass% or less, Al: 0.010 to 0.075 mass %, Ti: 0.050 mass% or less, and the effect is particularly remarkable when the steel is made of Fe and inevitable impurities.
The reason for defining the components will be described below.

  C deteriorates the workability of the thin steel sheet when its content increases. Therefore, the upper limit is set to 0.003 mass% at which excellent elongation and deep drawability can be obtained as IF steel (Interstitial-Free steel) when carbide forming elements such as Ti and Nb are added.

  Si is a solid solution strengthening element, and if the content is large, the workability of the thin steel sheet deteriorates. In consideration of the influence on the surface treatment, 0.05 mass% was made the upper limit.

  Mn is a solid solution strengthening element and increases the strength of steel. However, the present invention assumes mild steel and gives priority to workability. Therefore, the upper limit was set to 1.0 mass%.

  P is a solid solution strengthening element and increases the strength of steel. However, if the content exceeds 0.05% by mass, workability and weldability deteriorate, so the upper limit was made 0.05% by mass.

  S causes cracking during hot rolling and generates A-based inclusions that lower the workability of the thin steel sheet. Therefore, the content thereof needs to be reduced as much as possible. Therefore, in the present invention, the upper limit is set to 0.015% by mass.

  Al functions as a deoxidizing agent, and in order to obtain a deoxidizing effect, it is necessary to contain 0.010% by mass. Moreover, adding more Al than necessary causes an increase in cost. Therefore, in the present invention, the range of Al content is set to 0.010 to 0.075% by mass.

  Ti fixes C, N, and S in the steel as precipitates and improves workability and deep drawability. However, if the content exceeds 0.05% by mass, the steel sheet becomes hard and workability deteriorates. Therefore, in the present invention, the upper limit of the Ti content is set to 0.050% by mass.

  As described above, according to the present invention, the molten steel flow velocity at the front of the solidified shell is controlled to an appropriate flow velocity according to the molten steel components and the solidification velocity in the mold, so that no mold powder is involved, such as alumina clusters. Therefore, it is possible to produce a clean and high quality slab inexpensively and stably without impairing productivity.

  Hereinafter, the test casting result of 8 charges performed with the slab continuous casting machine will be described.

  8 charges (test No. 1-8) of ultra-low carbon steel with a charge of about 200 tons is cast into a slab slab having a thickness of 220 mm and a width of 1160 mm, a casting speed of 1.04 m / min, and a cast amount of molten steel Was cast at 2.1 tons / min. Table 1 shows the chemical composition of the molten steel of each test charge, and the solidification rate calculated from the casting rate and the heat transfer calculation. In the slab continuous casting machine, the molten steel flow rate at the front surface of the solidification interface at a position about 400 mm away from the molten steel surface in the mold in the casting direction satisfies the condition of the above-described formula (1), 1) Casting was performed under conditions that did not satisfy the range of the equation. That is, Table 1 shows the necessary minimum flow velocity obtained from the equation (1). In Test Nos. 1 to 4, the conditions satisfying the range of the equation (1) (examples of the present invention) are used, and the test Nos. 5 to 8 are performed. Then, it was set as the conditions (comparative example) which do not satisfy | fill the range of (1) Formula. In calculating the equation (1), the analytical value of the sample collected from the molten steel at the end of refining in the RH vacuum degassing apparatus was used as the chemical component of the molten steel. The oxygen concentration shown in Table 1 is the dissolved oxygen concentration.

  The molten steel flow velocity at the front surface of the solidification interface was controlled by using an AC moving magnetic field application device disposed across the entire width of the back surface of the mold long side with the slab interposed therebetween. Specifically, when the molten steel flow velocity at the front of the solidification interface in the vicinity of a position about 400 mm away from the molten steel surface in the mold in the casting direction is set to 0.05 m / s, the rotating magnetic field having a magnetic flux density of 0.025 Tesla. In the case where the molten steel flow velocity at the solidification interface front is 0.10 m / s, a swirl magnetic field application with a magnetic flux density of 0.050 Tesla is applied, and the molten steel flow velocity at the solidification interface front is 0.15 m / s. In this case, a swirl magnetic field with a magnetic flux density of 0.075 Tesla was applied, and when a molten steel flow velocity at the front of the solidification interface was 0.20 m / s, a swirl magnetic field with a magnetic flux density of 0.10 Tesla was applied.

  The molten steel flow velocity at the front of the solidification interface was confirmed from the solidification structure of the sample taken from the cast slab. That is, a sample having a full thickness (220 mm) × full width (1160 mm) was taken from the cast slab, and after the sample was mirror-finished, it was corroded with acid to reveal a solidified structure. Measure the tilt angle of dendritic dendrites in the solidified structure at the position, and use the Okano et al. Formula (see publication: Iron and Steel (61 (1975) p.69)) to determine the molten steel flow velocity from the measured tilt angle. Obtained and confirmed from the average value of 6 locations.

Further, after collecting the inclusion investigation sample from the vicinity of the solidification structure investigation sample, and mirror-finishing the collected inclusion investigation sample, the range from the slab surface to a position within 20 mm using an optical microscope In addition to counting the number of alumina clusters present in the sample, the major and minor axes of the alumina clusters were measured to calculate the area of the alumina clusters per unit area in the slab. Further, after the slab was rolled into a thin steel plate, the presence or absence of surface defects in the thin steel plate was also investigated. Table 2 shows the results of investigations on slabs and thin steel sheets. In addition, it has been found from the research by the present inventors that surface defects occur when the area of alumina clusters per unit area exceeds 0.001 mm ² / mm ² in the ultra-low carbon steel for automobiles.

As shown in Table 2, in Test Nos. 1 to 4 which are examples of the present invention, the area of the alumina cluster in the slab is 0.001 mm ² / mm ² or less, and the surface of the rolled steel sheet is also a surface. There were no defects. On the other hand, in test Nos. 5 to 8 which are comparative examples, the area of the alumina cluster in the slab exceeds 0.001 mm ² / mm ² , and surface defects can be confirmed even in the rolled steel sheet. It was.

Claims (4)

C is a continuous casting method of an ultra-low carbon steel slab containing 0.003% by mass or less, wherein the molten steel flow velocity in front of the solidified shell of the slab is the solidification rate of the solidified shell of the slab and the molten steel component A continuous casting method for a steel slab, wherein the casting is controlled so as to be within the range of the following expression (1) with respect to the Ti content and the S content.
V ≧ [165308 × (1- K E Ti) × Vs × [Ti] + 1613307 × (1-K E S) × Vs × [S]] 2/3 ... (1)
Where V is the molten steel flow velocity (m / s) at the front of the solidified shell, Vs is the solidification rate of the solidified shell in the mold (m / s), and K _E ^Ti is the effective distribution coefficient of Ti. (-), K _E ^S is the effective distribution coefficient S (-), [Ti] is, Ti concentration (mass%) in the molten steel, [S] is the S concentration in molten steel (mass%).   The molten steel flow velocity at the front of the solidified shell and the solidification rate of the solidified shell are the solidified shell front molten steel flow velocity and solidification rate in the solidified shell near the position 300 to 500 mm away from the molten steel surface in the mold in the casting direction. The continuous casting method of a steel slab according to claim 1.   The ultra-low carbon steel has, as chemical components other than C, Si: 0.05% by mass or less, Mn: 1.0% by mass or less, P: 0.05% by mass or less, S: 0.015% by mass or less, The steel according to claim 1 or 2, characterized in that Al: 0.010-0.075 mass%, Ti: 0.050 mass% or less, the balance being Fe and inevitable impurities. A continuous casting method for slabs.   The continuous casting of a steel slab according to any one of claims 1 to 3, wherein the molten steel flow velocity at the front surface of the solidified shell is controlled by an AC moving magnetic field applying device arranged at the back surface of the mold. Method.

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