JPH0573504B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial Application Field The present invention is a cast composite steel material in which an iron-coated alloy wire is added to the mold, the surface layer of the slab is made of the same composition as the molten steel in the tundish, and the alloy component is contained only in the core of the slab. Relating to a method of manufacturing. BACKGROUND TECHNOLOGY Composite steel materials, in which the surface layer and core portion of the steel material have different characteristics, are used as high value-added steel materials, and there have recently been reports on their production by continuous casting. For example, "Tetsu to Hagane" 72 (1986), S985
``Continuous casting method for clad steel round billets'',
Alternatively, there is "Continuous casting method for stainless steel hollow round plumes" in "Materials and Processes" 1 (1988), S298, which relates to methods for producing composite steel materials by these casting methods. As a method for manufacturing composite steel materials, a core addition method in which alloying elements are added into a mold is known. For example, Japanese Patent Publication No. 55-14847 describes a method for manufacturing enameled molten steel sheets by adding iron-coated titanium-filled wire to the core, and Japanese Patent Publication No. 62-142053 describes a method for producing molten steel sheets by adding iron-coated sulfur-filled wire to the core. A method for manufacturing sulfur free-cutting steel is disclosed. Problems to be Solved by the Invention In the production of cast composite steel materials by the core addition method, a surface layer of a predetermined thickness is ensured with the same composition as the molten steel in the tundish, and the core part is further alloyed to keep the composition within a predetermined range. By adjusting to
The objective is to obtain a composite steel material in which the surface layer portion and the core portion have different components. The problem with the above method is that, as shown in Figure 1, it is necessary to separate and maintain the pool of molten steel components forming the surface layer and the pool of molten steel components containing additive alloys forming the core within the continuous slab. Therefore, controlling the melting position of iron-coated alloy wire filled with alloy becomes an important technology. The flow of molten steel in the mold is generally caused by the discharge flow from the discharge hole of the immersion nozzle.
The area is roughly divided into the upward circulation flow area (section B) and the area below it (section B in the figure) where the upward circulation flow has subsided. In order to secure the two pools with different molten steel compositions mentioned above, it is important to complete the melting by inserting the iron-coated alloy wire into the B part at a position where the required surface layer thickness can be secured. It is necessary. However, from the above point of view, if a wire that is difficult to melt is used in an attempt to melt excessively below the mold (section C in the figure), the molten steel in the mold solidifies and adheres to the outer periphery of the wire and remains in the slab. Furthermore, even if the wire is melted, there is a problem that uniform mixing and diffusion of the alloy is not achieved and the alloy locally becomes concentrated. In the present invention, in the production of composite steel materials by continuous casting, the method for controlling the melting position of the iron-coated wire is optimized, and the composite material can be stably manufactured using the core addition method. Means for Solving the Problems The present invention solves the above-mentioned problems, and first, means for controlling the melting position of the iron-coated wire will be described below. When the target thickness of the surface layer of the composite material to be manufactured is L _s (m), the wire melting position L (m) from the melt surface in the slab can be calculated from the solidification equation (1).
It can be found using the formula. L=(L _S /K) ² ·V _C (1) Here, K is the solidification coefficient (m/min _1/2 ) and V _C is the casting speed (m/min). On the other hand, if the wire feeding speed is V _W (m/min), it takes T (min) for the wire fed into the slab to reach the melting position L (hereinafter referred to as melting time).
is expressed by equation (2). T=L/V _W ...(2) From equations (1) and (2), equation (3) is obtained. L = (L _S /K) ²・V _C = V _W・T ...(3) In this way, if the target surface layer thickness L _S and casting speed V _C are given, the position where the wire should be melted is L is determined and can be expressed as the product of the wire feeding speed V _W and the wire melting time T. The inventors determined that the thickness t and diameter D of the iron wire portion covering the alloy to which the wire melting time is to be added are
tried to control it. Naturally, while passing through the molten steel, the wire goes through the process of solidification, adhesion, and melting of the molten steel, but the melting time referred to here is the time when the iron coating finally melts and the alloying elements filled inside are discharged. say. In addition, here, we are focusing on the iron-coated part as the melting time because the alloy that is filled into the iron-coated wire is generally filled in the form of powder, so the presence of air spaces between the powders causes heat transfer resistance. big,
This is because the heat transfer from molten steel is slower than in the iron-coated area. As shown in Fig. 2, the inventor's investigation also found that the melting time was measured when the molten steel temperature was kept just above the melting point and the iron coating components of the molten steel and wire were almost the same. The melting time is determined by the wall thickness t and diameter D, and can be approximately expressed by the cross-sectional area of the iron coating as shown in FIG. From the above, in order to clarify the factors that determine the melting time of iron-coated wire and further clarify the relationship between these factors and the melting position L in practical use, we conducted an actual composite material manufacture and calculated the equation as shown in equation (4). Finally, we obtained a formula that serves as an index for controlling the melting of iron-coated wire. L=C3・_ρs・S・_VW ...(4) Here, L is the melting position (m), _ρs is the density of the wire coating (Kg/ ^m3 ), and _VW is the wire feeding speed ( m/min), S is the cross-sectional area of the iron-coated wire (m ² ),
C3 is a coefficient of about 0.3 to 0.6. S is the inner diameter Di (m) and outer diameter D _O (m) of the iron-coated wire
It can be found using the following formula. S=π/4・(D _O ² − _{D i} ² ) ...(5) Equation (4) is an equation based on the idea that the melting position L is controlled by the heat capacity of the wire that supplies it, and the coefficient is Since the coefficient varies somewhat depending on the temperature of the molten steel and mold size, the wire feeding speed may be controlled by selecting the most appropriate coefficient within the above range. Here, the cross-sectional shape of the iron-coated wire can be determined by treating the cross-sectional area S of the iron-coated wire in the same manner according to each shape, whether it is circular as described above, oval, square, or other shapes (4) You may apply Eq. The method for adjusting the melting position L using the iron-coated wire has been outlined above. Next, a method for adjusting and supplying alloying elements to the core portion will be described. The cross-sectional area of the molten steel in the solidified shell of the slab at the wire melting position is A (m ² ), the density of the molten steel is ρ _M (Kg/m ³ ), and the target concentration difference (increase) of alloy addition to the core is ΔC ( %),
The inner diameter of the wire is D _i (m), the packing density of the alloying element in the wire is ρc (Kg/m ³ ), and the addition yield of the alloying element is η
(%), and the number of wires to be melted is N.
The material balance equation (6) is found. A × V _C × ρ _M × ΔC = π/4・D _i ² × ρc × V _W × η × N ... (6) Here, the alloy addition amount Q supplied by the wire
is Q=π/4・D _i ² ×ρc×V _W ×N (7), so the operating factor of the iron-coated wire can be expressed as follows. D _i ² V _W = Q (ρc・π/4・N) ……(8) From the above, the desired amount of alloy Q is added to the core of the iron while satisfying the desired melting position L (surface layer thickness L _S ). The coated wire conditions are obtained by combining the thickness, diameter, feeding speed, etc. of the iron coated wire that satisfies formulas (4) and (8). Using the above method, we have demonstrated the concept of a method for controlling the melting position of wire in the manufacture of composite materials using the core addition method. Using the method described above, it is possible to determine the necessary melting position of the wire and the outline of the wire specifications when manufacturing composite materials, but the wire melting position must be determined by taking into account the flow state of the molten steel in the mold, which is the issue mentioned above. The next point to be made is to define the conditions for determining whether the above is sufficient, and the details are described below. When the wire is melted in part A in the mold shown in FIG. 1, mixing and diffusion of the additive alloy becomes a problem in manufacturing the composite material. Regarding this point, Figure 4 shows the investigation results of the upward circulation flow in the mold through water model tests. As a standard, it is approximately (1 to 2) times the width of the long side of the mold. Therefore, mixing of the additive alloy into part A in the mold,
To prevent diffusion, the melting position L of the wire must be
(m) needs to be the following formula. L>(C1×H+L _N )÷1000...(9) Here, H is the long side width of the mold size (m), L _N is the immersion depth of the nozzle (mm), and C1 is a coefficient of about 1 to 1.5. It is. Furthermore, the present inventors disclose a means for optimizing the flow in the mold by changing the discharge hole of the immersion nozzle, that is, a means for relaxing the shallowest melting position constraint defined by equation (9). (1) Rotating nozzle method The discharge hole of the immersion nozzle is placed against the inner wall of the mold.
This is a method of reducing the area A in FIG. 2 by suppressing the kinetic energy of the discharge flow from reaching the under-mold area by injecting at an angle as shown in FIG. 5a. This method is particularly effective when the mold size is large. (2) Straightening of the immersion nozzle In addition to the conventional discharge hole facing the inner wall of the mold, a new discharge hole is provided below the mold at the bottom of the immersion nozzle as shown in Fig. 5b. It is characterized by supplying the amount of hot water for forming the thickness of the surface layer with as little kinetic energy as possible, and further ensuring the amount of hot water for forming the core at the latter discharge hole and the stirring energy for uniformly dispersing the added alloy. This method is particularly effective when the mold size is small. The above describes the regulations regarding the required shallowest depth for the melting position of iron-coated wire in the production of composite materials, and methods for relaxing the constraints. Next, we will discuss the maximum depth limit from the perspective of the melting position of iron-coated wire. The maximum depth limit is determined from the problem of residual metal adhering to the iron-coated wire, and is the position where the molten steel at the lower end of area B in Figure 1 has lost its heating degree ΔT (=molten steel temperature - melting point) and fluidity. It is defined. This condition is approximately determined by the decrease in the temperature of the molten steel in the mold due to heat removal due to the growth of the solidified shell relative to the heat capacity of the mold. The second term on the left side of equation (10) is an expression that defines the solidification rate in the mold, and the left side expresses the unsolidified rate (%) in the mold. Based on actual hot water tests, etc., we find the conditions under which no metal remains attached to the iron-coated wire, and it is necessary to complete melting when the unsolidified rate of the slab at the molten steel position L of the wire is 40 to 50% or more. be. 1-K・(L/V _C ) _1/2・2(H+B)/(H×B)
＞0.4～0.5...(10) Here, K is the solidification coefficient (mm/min _1/2 ), L is the melting position of the iron-coated wire (m), V _C is the casting speed (m/min), H
is the width of the long side of the mold (mm), B is the thickness of the short side of the mold (mm)
represents. From equation (10), the maximum depth L from the melting point at the melting point is determined by the following equation. L<W _C・((0.5~0.6)×(H×B)/K・2(H+B
)) ² ...(11) To summarize the above, the shallowest depth from the melting surface of the iron-covered wire from the melting point is defined by formula (9), and the critical maximum depth L is determined by formula (11). By specifying C1 as 1 to 1.5, the melting position of iron-coated wire is
C2 is set to 0.5 to 0.6 and defined by equation (12). C1×H×LN/1000<L<V _C・(C2×(H×B)/K×2
(H+B)) ² ...(12) Effects The present invention describes the melting position control of an iron-coated wire filled with an additional alloying element. The second effect is to clarify the control range of the melting position of the wire, and the second effect is to control the melting position within the above control range by using the dimensions and feeding speed of the iron-coated wire as control factors, and to apply the wire to the core as required. The aim is to clarify the method of supplying alloying elements according to the requirements. By controlling the melting position of the iron-coated wire according to the present invention, it is possible to manufacture a good composite material. Examples Examples of continuous casting of sulfur free-cutting steel having a high core sulfur concentration will be described below. 0.06%C, 0.02%Si, 0.50%Mn, 0.025 in converter
Molten steel with a composition system of %P, 0.02%S, and 0.015%Al is melted, and a billet with a cross-sectional size of 162mm x 162mm is produced using a curved continuous machine with a curvature radius of 12m using a 5-hole immersion nozzle (4 horizontal holes). (having one vertical discharge hole) at a casting speed V _C =1.8 to 2.0 m/min. A wire feed guide was installed above the mold, and casting was performed using a wire feeder to continuously feed an iron-coated wire filled with powdered sulfur into the mold from between the mold and the immersion nozzle. A steel grain with a core sulfur concentration of 0.150 to 0.250%S
Two types of sulfur-filled wires are prepared to manufacture B steel main body with 0.050 to 0.100% S, and the outer diameter for A steel type is
6.5mmφ, iron coating thickness 0.9mm, outer diameter 4.5mm for B steel type
φ, iron coating thickness was 1.0 mm. The target thickness of the surface layer of the slab without adding sulfur was 10 to 25 mm for all steel types. Table 1 shows the manufacturing conditions for the above-mentioned sulfur free-cutting steel, the measurement results of the surface layer thickness in a cross-sectional sample taken from the obtained billet slab, the analysis results of the sulfur concentration in the core, and the residual metal content in the core. Observation results regarding the presence or absence of sulfur and the presence or absence of local concentration of sulfur are shown. FIG. 6 shows the melting position of the wire calculated from the surface layer thickness after solidification with respect to the plunge speed of the iron-coated wire. Here, a specific application method of the present invention will be described in detail in association with the above embodiment. Here, as a basic condition, for the slab size,
In order to secure a surface layer thickness of 15 to 20 mm at V _C = 1.8 m/min and solidification coefficient K = 24 mm/min _1/2 , from equation (1), the wire melting position should be set at 0.7 to 1.25 mm from the melt surface in the slab.
The depth must be m. On the other hand, in equation (12), which defines the conditions for separating the two molten steel pools in the slab and the conditions for complete melting of the wire, in order to produce a good composite material, the left side is given when the nozzle immersion depth is 0.2 m. (0.36 to 0.44) m, and the right side is in the range of (1.28 to 1.80) m, so if the melting position of the wire is controlled to the depth of 0.7 to 1.25 m, it is likely that the desired composite material will be obtained. Next, the wire dimensions and wire supply speed that can supply the target concentration to the core portion while controlling the melting position as described above are determined. Now, the core part starts from 0.050%S.
The required amount of sulfur to increase to 0.200%S is
0.42Kg/min and the density of sulfur is 2000Kg/m ³ by combining equation (8) for the mass balance and equations (4) and (5) for controlling the melting position of the iron-coated wire. The outer diameter D _O = 6.5 mmφ, the iron coating thickness 0.9 mm, the wire feeding speed V _W = 10.7 m/min, and the iron coating feeding speed 2.6 kg/min are determined. This feeding rate allows control to the melting position shown in FIG. 6, where the composite material can be stably produced. A similar method can be applied to cases where casting conditions are different.

[Table] Effects of the Invention As explained above, according to the present invention, continuous casting of composite steel materials is possible by adding iron-coated alloy wire in the mold, and manufacturing of composite steel materials that was conventionally cast by the ingot method is now possible. The effects on cost reduction and quality stabilization are extremely large.

[Brief explanation of the drawing]

Figure 1 is a longitudinal cross-sectional view, Figure 2 is a diagram showing the relationship between iron coating thickness and wire melting time, Figure 3 is a diagram showing the relationship between iron coating cross-sectional area and wire melting time, and Figure 4 is a diagram showing the relationship between iron coating thickness and wire melting time. Figure 5 showing the survey results of upward circulation flow.
Figures a and b are explanatory diagrams of means for optimizing the flow within the mold, and Figure 6 is a diagram showing the feeding speed and melting position of the wire. 1... Nozzle, 2... Discharge port, 3... Wire, 4... Surface layer portion, 5... Core layer.

Claims (1)

[Claims] 1. In continuous casting of steel, the molten steel supplied from the tandate forms a surface layer (rim layer) of the mother molten steel, and furthermore, by adding alloy to the unsolidified portion, the molten steel is different from the mother molten steel. In a cast composite steel production method that forms the inside of the component (core layer), the melting position L of the iron-coated wire expressed as the distance from the hot water surface is
It is characterized by controlling the additive alloy to be above the specification on the left side of the equation below so that it does not mix into the surface layer of the slab, and below the specification on the right side of the equation below so that the iron-coated wire does not remain unmelted in the slab. Method for manufacturing cast composite steel materials. L _N +CI×H/1000＜L＜Vc・(C2・(H×B)/K・2
(H+B)) ^2Where , L: Distance from the mold surface to the melting position of the iron-coated alloy wire (m) L _N : Distance from the mold surface to the discharge hole of the immersion nozzle (mm) H: Length of the mold Side width (mm), B: Short side thickness of mold (mm) Vc: Casting speed (m/min), K: Solidification coefficient (mm/
min ^1/2 ) Cl: coefficient (1 to 1.5), C2: coefficient (0.5 to 0.6) 2 As a means of controlling the melting position of the iron-coated alloy wire within the range of claim 1, the melting position L is determined by the following formula. As shown, a method for producing a cast composite steel material is characterized by an approximate control method in which the speed is proportional to the input weight speed of the wire coating. L=C3・ρs・S・V _wwhere , ρs: Density of wire coating (Kg/m ³ ), S: Cross-sectional area of wire coating (m ² ) = π/4・(D _O ² −D _i ² ) D _O : Outer diameter of coated wire (m), D _i : Inner diameter of coated wire (m) V _w : Feeding speed of iron coated wire (m/min), C3: Coefficient (0.3 to 0.6) 3 Claim By adding the following equation as a material balance equation to one of the control conditions to satisfy terms 1 and 2 and obtain the required amount of alloy addition, the wire conditions and charging conditions are finally determined, and the desired core is obtained. A method for manufacturing a cast composite steel material, characterized in that components are arbitrarily selected. D _i ² V _w =Q/N・π/4・ρc Here, D _i : Inner diameter of iron-coated wire (m) N: Number of wires V _w : Feed-in speed of iron-coated wire (m/min) Q: Alloy addition amount (Kg/min) ρc: Alloy packing density (Kg/m ³ ) 4 Suppresses the upward circulation flow of molten steel in the mold, promotes uniform dispersion of the added alloy, and/or improves the rim In order to obtain a composite steel material with a thin layer, the discharge hole of the immersion nozzle is inclined with respect to the inner wall surface of the mold (rotary flow), or the discharge hole is provided at the bottom of the immersion nozzle and the discharge is directed downward (downward flow). The method for manufacturing a cast composite steel material according to claims 1 to 3, characterized in that: