JP4548715B2 - Method for melting surface layer of metal material - Google Patents

Description

  The present invention relates to a method for melting a surface layer of a metal material, and in particular, a technique for improving the surface roughness of the metal material by melting and resolidifying the surface of the metal material, or by adding other solute components during melting. The present invention relates to a technique for modifying a metal material surface layer or a technique for producing a multilayer metal material.

In general, the following techniques are known as techniques for melting and processing a surface layer of a metal material and related techniques.
For example, the surface layer of a continuous cast slab of steel is melted by one or both of plasma heating and induction heating, and the surface layer is reformed by adding other solute components to the melted surface layer portion, thereby reducing the cost. A technique for obtaining a composite steel material is disclosed in (Patent Document 1). In (Patent Document 2), a steel casting in which an induction heating coil is arranged so as to go around a steel slab, a conductive segment container is installed inside the coil, and an element addition device is provided at an appropriate position. A technique has been proposed in which a surface layer is reformed by a single surface layer reforming apparatus while the molten metal surface is stably held by a pinch force.
Among the above, (Patent Document 1) related to the present invention will be described with reference to the drawings. FIG. 6A shows a melting treatment of the slab S moving in the horizontal arrow direction 15 by using induction heating and plasma heating together. The case where is performed is shown. That is, while providing the induction coil 9 so as to go around the slab S, the plasma torch T is disposed between the induction heating coils, and a predetermined portion of the slab is heated and melted by Joule heat by the coil and gas heat by the plasma, The slab surface layer is modified. The plasma 10 from the plasma torch T has a solute component (for example, Ni, Cr, C, Si, Mn, P, S, Mo, Cu, Al, REM, or an alloy thereof) added to the molten surface layer portion 14. After the molten surface layer portion 14 is supplied and solidified, the layer 12 enriched with additive components is formed on the slab surface layer portion. 11 is a magnetic flux, 13 is a Joule heating part.
FIG. 6B shows an example in which the melting process is performed using induction heating and plasma heating together when the slab S moves in the vertical direction. An induction coil 9 is provided so as to go around the slab S, and a plasma torch T is disposed between the induction heating coils, and if necessary, added to the plasma 10 from the plasma torch T to the molten surface layer portion 14. The supply of an appropriate solute component is the same as in the case of the horizontal movement described above. In this example, the electromagnetic force is maintained on the molten surface layer portion 14, but in the vertical movement, the lower side of the molten surface layer portion 14 tends to drop due to the action of gravity, so the lower coil of the induction coil 9 is moved from the upper side. A stronger electromagnetic force acts on the lower side of the molten surface layer portion 14 in the vicinity of the slab.

Furthermore, (Non-Patent Document 1) shows a technique for applying a magnetic field perpendicular to the weld, but electromagnetic force generated by the interference between the magnetic field and current is used for stirring the molten pool, There is no intention to improve the surface properties of the melted part. FIG. 7 shows an outline of this technique, in which a molten pool is formed by a plasma torch 18 arranged vertically with respect to a sample 19, and a magnetic field is applied to the pool by a Helmholtz coil 20 in the vertical direction. 21 is a Helmholtz coil power supply, and 22 is a plasma torch power supply.
JP 2004-195512 A JP 2004-195514 A M. Malirowski. Brodnicka, G. den Ouden and WJP Vink, Welding Research Supplement, Feb., (1990), 52.

As described above, in the conventional patent documents, there are various techniques relating to a technique for modifying a metal material surface layer by adding other solute components during melting, a technique for producing a multilayer metal material, and a technique for maintaining the surface of a molten part. In particular, no specific proposal has been made for improving the surface roughness of the metal material by suppressing the undulation of the molten pool surface. In addition, even in (Non-patent Document 1), the electromagnetic field is applied for the purpose of stirring the melted part, and there is no mention of wave suppression.
As a result of repeated experiments on the melting treatment of the surface of a material such as a continuous cast slab, the inventors have conducted convection, thermal convection, electromagnetic force induction due to a difference in surface tension due to temperature distribution. Since the free surface is disturbed by surface tension waves and gravity waves due to the flow generated in the molten pool due to flow, plasma gas flow, etc., the undulating phenomenon on the surface of the molten pool cannot be avoided and the molten surface layer The shape of the part, especially the surface roughness, is affected by the ripple phenomenon of the molten pool before solidification, and it was found that the surface roughness can be improved by effectively suppressing this ripple, and the present invention was completed. It is a thing.
The present invention has been made in view of the above points, and by improving the surface roughness of the metal material by finding the optimum condition for suppressing the undulation of the shallow surface layer portion of the surface melt pool of the metal material. In addition, it is an object of the present invention to provide a surface layer melting treatment method that can improve the surface layer of a metal material and obtain a high-quality multilayer metal material.

The surface layer melting treatment method according to the present invention for solving the above-described problems satisfies the following equation for an AC electromagnetic field having a component parallel to the surface of the molten pool when forming a molten pool by melting the metal material surface layer It is characterized by suppressing the undulation of the molten pool by applying to the molten pool under conditions.
d / δ ≧ 1 where δ = 1 / √ (πμσf)
However, d: Melt pool depth [m], f: Frequency of electromagnetic field [Hz], π: Circumference ratio,
μ; 4π × 10 ⁻⁷ , σ; electrical conductivity at the melting temperature of the metal material [1 / Ωm]
δ: Skin depth [m]
In the above melting treatment method, it is preferable to use either or both of the heat of the gas in the plasma state and the Joule heat of the alternating electromagnetic field when the metal material surface layer is melted.

  The surface melting process of the metal material according to the present invention effectively suppresses the undulation phenomenon in the shallow surface layer of the molten pool, thereby improving the surface roughness of the metal material and improving the surface layer. Thus, a high-quality multilayer metal material can be obtained.

Hereinafter, embodiments of the present invention will be described. Before that, the principle of the present invention will be described with reference to the drawings.
The molten metal 1 causing the undulation shown in FIG. 1 is schematically represented by taking out only the molten portion. When an alternating magnetic field 2 having a horizontal component parallel to the molten pool surface 5 of the molten metal 1 is applied, an induced current 3 perpendicular to the magnetic field is induced in the molten pool. In order to apply the desired AC magnetic field 2, an appropriate induction coil is installed above the molten pool. Due to the interaction between the induced current 3 and the magnetic field 2, an electromagnetic force 4 is generated vertically downward. When the molten pool surface 5 has a wave as shown in the figure, the electromagnetic force acts strongly at the convex portion and the electromagnetic force is weakened at the concave portion, so that the action of suppressing the ripple of the molten pool surface 5 is expressed. . The degree of penetration of the electromagnetic field into the molten metal pool is governed by a parameter called magnetic skin depth (referred to as skin depth in the present invention). This is expressed by the following equation.
δ = √ {2 / (μσω)}
Here, δ is skin depth, and the absolute value of the magnetic field or induced current is attenuated to 1 / e (e is the base of natural logarithm, 2.71828...) In the metal as compared with the value on the metal surface. Shows its depth. In the molten metal, μ is almost the same value as the vacuum and is 4π × 10 ⁻⁷ , which is called magnetic permeability. σ is electric conductivity and is determined by the type of metal. In the case of steel, it is 0.7 × 10 ⁶ [S / m]. The molten nickel is 1.2 × 10 ⁶ [S / m]. Since ω is an angular frequency and the frequency of the electromagnetic field is f, it is expressed as ω = 2πf, so the above equation is expressed as δ = 1 / √ (πμσf)
It can be seen that the greater the frequency, the smaller the skin depth δ.

The inventors conducted an experiment on the relationship between the skin depth δ and the molten pool depth d in an experimental facility using liquid gallium as shown in FIG. It has been found that the higher the is, the greater the effect of suppressing undulations, and in particular, when the skin depth is below the depth of the molten pool, the suppression effect becomes significant. In FIG. 2, a nonmagnetic stainless steel container 8 having a depth d is filled with molten metal 1 (in this case, liquid gallium), and a wave is generated on a molten pool surface 5 by a mechanical vibrator 6 and an alternating magnetic field 2 is applied. The laser displacement meter 7 measures the attenuation of the wave when it is applied. The horizontal axis of FIG. 3 is the pool depth / skin depth (d / δ), and the vertical axis is the decay time of the molten pool surface wave normalized with reference to the decay time when no magnetic field is applied. This shows that the ripple suppression effect is large.
As shown in FIG. 3, when the skin depth indicating the penetration depth of the magnetic field into the molten metal is about twice the pool depth of the molten metal (d / δ = 0.5), the effect of suppressing surface undulations is obtained. When the depth is about ½, and the depth is the same as the skin depth (d / δ = 1), the suppression effect becomes remarkable at about 1/10. Qualitatively, it is understood that when the skin depth is deep, the electromagnetic force in the pool is almost the same in all places, and the electromagnetic force is the same in the undulating convex part and the concave part. Conversely, when the skin depth is reduced by increasing the frequency, the electromagnetic force concentrates near the surface of the molten metal, and the distance between the electromagnetic coil providing the magnetic field and the convex part, and between the electromagnetic coil and the concave part. It is understood that the difference between the distances affects the action of the electromagnetic force, and is strongly pressed by the convex part and weakly pressed by the concave part, and the effect of suppressing surface undulations is strengthened.

From the above, in the present invention, the range in which the magnetic field acts is limited only to the surface portion of the molten metal, and only the surface is pushed by an electromagnetic field to effectively suppress the ripple phenomenon and improve the surface shape. Therefore, as described above, it is necessary to apply an electromagnetic field so as to satisfy the condition of d / δ ≧ 1 (where δ = 1 / √ (πμσf)).
As for the relationship between the direction of the wave and the direction of the magnetic field, the same effect can be obtained in any direction as long as the magnetic field has a component parallel to the surface of the molten pool, whether parallel or orthogonal as shown in FIG. Is expressed.

As described above, the present invention aims to suppress the ripple when the depth d of the molten metal pool is shallow. This shallow pool depth refers to a depth within the range affected by the surface wave, and specifically It can be said that the depth is equal to or smaller than the smaller size of the molten metal pool.
Regarding the suppression of undulations on the surface of a deep pool compared to the width or length of a molten pool of metal, for example, technical literature (M. Garnier R. Moreau: J. Fluid mech. (1983), 127, pp365-377). There are many studies, but none of them is a technology to suppress the ripple phenomenon at the same depth of skin depth and molten pool.

  In the present invention, when the metal material surface layer is melted, the combined use of induction heating and plasma heating shown in FIG. 6 can be used. Of course, the induction heating alone type may be adopted, or depending on the mode of the molten pool. Plasma heating alone is also possible. Further, the present invention can be applied not only to the melting process of the entire surface of the metal material surface but also to the partial melting process of the surface layer of the metal material. For example, when the melting process is performed by plasma heating, an AC magnetic field is applied to the plasma, and the plasma is vibrated in a direction intersecting with the movement (longitudinal) direction of the metal material to form a melt pool having a constant width. On the other hand, in the case of induction heating alone, for example, an induction heating coil is installed so as to face a metal material, thereby generating Joule heat in the metal material to perform a predetermined melting process, and for the induction heating coil. By adjusting the frequency, it is possible to appropriately control the skin depth. When the metal material is a continuous cast slab, the slab is horizontal or vertical depending on whether the slab is located at the end of the continuous caster or in the continuous caster. Above the material, vertical metal material will be placed on the side of the material.

In the present invention, the means for forming a molten metal pool on the surface of the metal material is not limited to one or both of plasma heating and induction heating, and laser heating or burner heating can be employed depending on circumstances. As described above, it is desirable to use either or both of gas heat and Joule heat of an AC electromagnetic field in a plasma state which is efficient and advantageous in terms of operation.
Furthermore, in the present invention, an example in which the surface roughness of the metal material is improved by suppressing the ripple of the molten pool has been described. However, the present invention is not limited to this, and other solutes can be applied by applying an electromagnetic field for suppressing the ripple. Even when the components are added to the molten pool, stable supply of solute components and uniform melting can be expected, and good modification of the surface layer is achieved, which in turn enables the production of high quality multilayer materials. .

300A and 400A DC plasma is vibrated by a vibration coil that generates an AC magnetic field of 50 Hz and 20 × 10 ⁻⁴ T, and is set to become a flattened plasma with a width of 100 mm. A horizontal sample cut from the aluminum killed steel continuous cast slab was melt-treated on the upper surface of the slab at a speed of 0.3 m / min in the longitudinal direction. The depth of the molten pool was about 3 mm and about 4 mm at 300 A and 400 A, respectively, and it was confirmed in advance by examining the cross section after processing. An AC magnetic field is applied to the part melted by the plasma under four conditions of 5, 10, 20, and 50 kHz using a high-frequency induction coil installed at an upper position facing the slab, and the roughness of the slab surface after treatment Was measured. The result is shown in FIG. 4. As shown in FIG. 4, the roughness (Ra) becomes 1/10, and the roughness is sufficiently improved when the pool depth / skin depth is 1 or more for both 3 mm and 4 mm depths. I found out.

  An induction coil was installed above a horizontal nickel plate sample having a width of 150 mm and a thickness of 50 mm, and the upper surface of the sample was melted at a speed of 0.5 m / min in the longitudinal direction. The current passed through the coil was adjusted for each frequency described later so that the depth of the molten pool was 5 mm. The frequency of the coil current was melted under five conditions of 1, 5, 10, 20, and 50 kHz, and the roughness after the surface treatment was measured. The result is shown in FIG. 5. As in the case of the plasma melting of Example 1, as a condition for the roughness (Ra) to be 1/10, the pool depth / skin depth is 1 or more and the roughness is sufficiently high. It was found that it improved.

It is a schematic diagram for demonstrating the principle of this invention method. It is explanatory drawing which shows the outline | summary of the experimental apparatus for calculating | requiring the relationship between a skin depth and a molten pool depth. It is a figure which shows the relationship between the skin depth / melting pool depth obtained by the experimental apparatus of FIG. 2, and the decay time of a molten pool surface wave. It is a figure which shows the result of Example 1 of this invention. It is a figure which shows the result of Example 2 of this invention. (A) is a cross-sectional schematic diagram which shows the specific example of the melting process of the slab which moves horizontally, and (b) respectively shows the specific example of the melting process of the slab which moves vertically. It is explanatory drawing which shows an example of the conventional melting process.

Explanation of symbols

S slab T plasma torch 1 molten metal 2 AC magnetic field 3 induction current 4 electromagnetic force 5 surface of molten pool 6 mechanical vibrator 7 laser displacement meter 8 nonmagnetic stainless steel container 9 induction coil 10 plasma 11 magnetic flux 12 solute component enriched layer 13 Joule heating unit 14 Melting surface layer 15 Slab movement 16 Electromagnetic force 17 Gravity 18 Plasma torch 19 Sample 20 Helmholtz coil 21 Power supply for Helmholtz coil 22 Power supply for plasma torch

Claims (2)

Upon the metallic material surface to melt processing to form a molten pool, to suppress waving of the molten pool by applying an alternating electromagnetic field having a surface component parallel of the molten pool to the molten pool under the conditions satisfying the following formula A method for surface layer melting treatment of a metal material.
d / δ ≧ 1 where δ = 1 / √ (πμσf)
However, d: Melt pool depth [m], f: Frequency of electromagnetic field [Hz], π: Circumference ratio,
μ; 4π × 10 ⁻⁷ , σ; electrical conductivity at the melting temperature of the metal material [1 / Ωm]
δ: Skin depth [m]   2. The method for melting a surface layer of a metal material according to claim 1, wherein one or both of the heat of a gas in a plasma state and the Joule heat of an AC electromagnetic field is used when the surface layer of the metal material is melted.

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