JP2008208425A - Method for reforming surface layer of steel slab, surface-reformed steel slab, and worked product - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for reforming the surface layer of a steel slab, with which variation in the concentration distribution of nickel in the surface layer is reduced, the surface layer of the steel slab comprising copper is securely modified, and occurrence of cracks upon hot working can be prevented, and to provide a modified slab and a worked product. <P>SOLUTION: When the surface layer of a steel slab 1 comprising copper is heated and melted by a plasma jet 4, a wire 6 made of nickel or a nickel alloy fed to the molten pool 8 so as to modify the surface layer of the steel slab 1, and to the melting treatment velocity Vt of the steel slab 1 and the length Lp of the molten pool 8 formed by plasma heating, the feed velocity with which droplets fed from the wire 6 attain Lp/Vt spacing is defined as Vw<SB>0</SB>, the feed velocity at which one droplet falls to the part per 20 mm of treatment length is defined as Vw<SB>2</SB>, and the feed velocity at which droplets are continuously added to the molten pool 8 is defined as Vw<SB>1</SB>, the feed velocity Vw is made to satisfy Vw<SB>2</SB><Vw<1.2×Vw<SB>1</SB>in the case of Vw<SB>0</SB><Vw<SB>2</SB>, and is made to satisfy Vw<SB>0</SB><Vw<1.2×Vw<SB>1</SB>in the case of Vw<SB>0</SB>≥Vw<SB>2</SB>. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for modifying the surface layer of a continuous cast slab of steel containing copper, and a surface-modified steel slab and a processed product obtained by modifying the surface layer by this method.

  When it contains copper, the hot workability of steel is remarkably hindered, and cracks are generated on the surface in a normal production process such as continuous casting, reheating or hot rolling. This cracking of the steel surface is caused by the fact that when the steel is oxidized by exposure to oxygen in the atmosphere during continuous casting, reheating, or hot rolling, copper having a low solubility in the steel and a low melting point is iron oxide and steel. It is considered that this occurs because the liquid copper enters the crystal grain boundaries of the steel and lowers the interface strength.

  On the other hand, it is difficult to remove copper in the steel smelting process, and as a method for preventing this phenomenon, an element that prevents the copper from being mixed in the steel or improves the solubility of the copper in the steel. There is known a method of adding nickel. However, in recent years, due to the increasing demand for a recycling society, scraps containing a large amount of copper have been used as a raw material for steel, and it has become difficult to prevent copper from being mixed into steel. ing. In addition, even if diluted with iron made from ore, copper gradually accumulates, so it is necessary to detoxify copper in steel by adding nickel, but nickel is a rare and expensive element, which increases production costs .

  Therefore, conventionally, the steel slab surface layer containing copper is reliably and inexpensively modified, and the surface layer of the steel slab is subjected to induction heating and plasma heating in order to prevent cracking during hot working. There has been proposed a method of melting either one or both of them and adding nickel or a nickel alloy to the surface layer portion of the molten steel slab (see Patent Document 1). This Patent Document 1 describes using wire or sheet-like nickel or a nickel alloy as a method of adding nickel to the steel slab surface layer melting portion.

JP 2004-195513 A

  However, the technique described in Patent Document 1 described above has the following problems. That is, when modifying the surface layer of a steel slab containing copper by adding nickel, the nickel concentration of the surface layer varies depending on the method and conditions for adding nickel to the molten surface layer of the steel slab and cannot be uniformly modified. There is a problem. Specifically, when nickel or nickel alloy powder is used when nickel is added to the steel slab surface melted part, the powder position changes due to the magnetic field when placed on the surface of the slab. When sprayed on the surface of a piece, it is repelled by a plasma stream or the like, so that the nickel concentration in the surface layer varies. On the other hand, when a nickel or nickel alloy sheet is used, the above-mentioned problem does not occur, but practical application is technically difficult. Further, when wire-like nickel or nickel alloy is used, simply adding a wire may cause the wire to adhere to the surface of the slab or cause variations in the nickel concentration of the surface layer. Further, Patent Document 1 does not discuss the wire supply method and conditions at all. Therefore, in the conventional method, since the surface layer of the steel slab cannot be uniformly modified, there is a problem that the yield decreases in hot working or the like.

  The present invention has been made in view of the above-described problems, and it is possible to reduce the variation in the nickel concentration distribution in the surface layer, to reliably reform the surface layer of the steel slab containing copper, An object of the present invention is to provide a method for modifying the surface layer of a steel slab, a surface-modified steel slab, and a processed product that can prevent cracking during processing.

The method for modifying the surface layer of a steel slab according to the present invention comprises melting a surface layer of a steel slab containing copper by plasma heating, and adding nickel or a nickel alloy to the surface layer portion of the molten steel slab. A method for modifying a surface layer of a piece, in which a molten droplet supplied from a wire corresponds to a melting processing speed Vt (mm / second) of the steel slab and a molten pool length Lp (mm) formed by plasma heating. Lp / Vt (second) interval supply speed is Vw ₀ (g / second), one droplet drops per processing length of 20 mm is Vw ₂ (g / second), and droplets continuously When the supply rate added to the molten pool is Vw ₁ (g / second), when Vw ₀ <Vw ₂ , the supply rate Vw (g / second) is set in the range defined by the following formula (1), and Vw for ₀ ≧ Vw ₂ feed rate Vw (g / sec) of the following formula (2) As the range defined, and supplying a wire made of nickel or nickel alloy.

  The modified slab according to the present invention is characterized by being subjected to the surface modification method described above.

  The processed product according to the present invention is obtained by processing the above-described surface-modified steel slab.

  According to the present invention, since the supply speed Vw of the wire made of nickel or a nickel alloy supplied to the steel slab surface layer melted portion is optimized, the variation in nickel concentration in the surface layer portion is suppressed, and the steel casting containing copper The surface layer of the piece can be reliably modified to prevent cracking during hot working.

  Hereinafter, the best mode for carrying out the present invention will be described in detail. In the method for modifying the surface layer of a steel slab according to the present invention, the surface layer of a steel slab containing copper is melted by plasma heating, and the surface layer portion of the molten steel slab is nickel-coated with a wire made of nickel or a nickel alloy. Add elements. Fig.1 (a) is a perspective view which shows typically one form of the surface modification method of the steel slab of this invention, FIG.1 (b) is the sectional drawing. As shown in FIGS. 1 (a) and 1 (b), in the method for modifying the surface layer of a steel slab according to the present invention, for example, after the surface layer of the steel slab 1 is induction-heated by a high frequency coil 2, the steel slab 1 A plasma jet 4 is blown toward the surface of the steel slab 1 from the plasma torch 3 disposed above the surface, and the surface layer of the steel slab 1 is heated and melted. At that time, the width of the plasma jet 4 is controlled and expanded by the plasma control coil 5 disposed between the steel slab 1 and the plasma torch 3. And the wire 6 which consists of nickel or a nickel alloy is supplied from the water cooling nozzle 7 for wire supply toward the fusion | melting part in the surface layer of the steel slab 1, ie, the fusion | melting pool 8. FIG. At this time, since the wire 6 is heated and melted by the plasma jet 4 or the like, the component of the wire 6 (nickel or nickel alloy) is added to the molten pool 8 in the form of droplets.

Furthermore, in the method for modifying the surface of a steel slab according to the present invention, the length of the molten pool 6 formed by the melting treatment speed Vt (mm / second) of the steel slab 1 and heating by the plasma jet 4 in the above-described steps. The supply rate at which the droplets supplied from the wire 6 are spaced at intervals of Lp / Vt (seconds) with respect to Lp (mm) is Vw ₀ (g / second), and one droplet drops per processing length of 20 mm. When the speed is Vw ₂ (g / second) and the supply speed at which droplets are continuously added to the molten pool 8 is Vw ₁ (g / second), the supply speed of the wire 6 when Vw ₀ <Vw ₂ Vw (g / sec) is defined as the range defined by the following formula (3), and when Vw ₀ ≧ Vw ₂ , the supply speed Vw (g / sec) of the wire 6 is defined as the range defined by the following formula (4) To do.

  The thickness of the surface layer portion of the steel slab 1 to which nickel is added in the method for modifying the surface of the steel slab of the present invention is not particularly limited as long as it is equal to or greater than the thickness of the oxide film. It is a range of about 0.5 to 5 mm from the surface of the slab 1.

  Further, in the steel slab surface modification method of the present invention, the wire 6 is heated and melted by the radiant heat from the plasma jet 4 and the steel slab 1 and the heat transfer from the melting pool 8. Then, a droplet is formed at the tip of the wire 6 and grows. When the weight of the wire 6 surpasses the surface tension, the droplet becomes a droplet and falls to the steel slab surface layer melting portion. At this time, if the wire supply speed Vw is slow, the temperature distribution of the droplets becomes small and the droplets do not drop until they become larger than usual, so the drop frequency decreases. Further, as the supply Vw of the wire 6 becomes faster, it cannot be melted only by the radiant heat from the plasma jet 4 and the steel slab 1, and is further heated by the heat of the molten pool 8. Specifically, the melting time Lp of the steel slab surface defined based on the melting processing speed Vt (mm / sec) of the steel slab 1 and the length Lp (mm) of the molten pool 8 formed by plasma heating. If at least one droplet is not supplied from the wire 6 to the steel slab surface layer melting portion (molten pool 8) per / Vt (seconds), an extremely low nickel concentration region is formed.

Accordingly, when the supply speed Vw ₀ at which the droplets supplied from the wire 6 are spaced at intervals of Lp / Vt (seconds) is equal to or higher than the supply speed Vw ₂ at which one droplet drops per processing length of 20 mm, the wire 6 feed rate Vw of faster than Vw _0. On the other hand, when the supply speed Vw ₀ at which the droplets supplied from the wire 6 are spaced at intervals of Lp / Vt (seconds) is less than the supply speed Vw ₂ at which one droplet drops per processing length of 20 mm, the wire 6 feed rate Vw of faster than Vw _2. The supply speed Vw ₀ at which the droplets supplied from the wire 6 are spaced at intervals of Lp / Vt (seconds) varies depending on the preheating temperature by induction heating of the steel slab 1, the plasma output, and the processing speed.

On the other hand, when the wire supply speed Vw becomes 1.2 times or more of the supply speed Vw ₁ at which the droplets are continuously added to the molten pool 8, the wire 6 is added to the molten pool 8 with insufficient heating. become. As a result, the molten pool 8 is cooled by the cold wire 6 and the cooled portion is solidified, so that the flow diffusion amount of nickel is reduced, and the nickel concentration is increased only in the added portion. Accordingly, the wire feed speed Vw is less than 1.2Vw _1.

  In addition, about the drop frequency of the above-mentioned droplet, for example, using an imaging unit such as a television camera and a CCD camera, the number of frames is 1/30 or more and the shutter speed is about 1/1000. It can be measured by counting the number of droplets from the image. At that time, it is necessary to appropriately adjust the filter or the diaphragm according to the plasma input and the distance between the camera and the video so that the plasma light can be reduced and the droplet can be confirmed.

  As described above, in the method for modifying the surface layer of the steel slab according to the present invention, the supply rate Vw of nickel or nickel alloy added to the steel slab surface layer melting portion is within the appropriate range described above. Variation in density can be suppressed within ± 50%. As a result, the surface layer of the steel slab containing copper can be reliably modified, and cracking during hot working can be prevented. In addition, since the amount of nickel added can be reduced, the manufacturing cost can also be reduced.

  In the method for modifying the surface layer of a steel slab of the present invention, it is desirable to oscillate the wire in a direction orthogonal to the processing direction at a speed 3 to 10 times the processing speed. Thereby, the variation in nickel concentration can be suppressed within ± 20%. At that time, if the oscillation speed is less than 3 times the processing speed, the variation in nickel concentration cannot be reduced to within ± 20%, and if the oscillation speed exceeds 10 times the processing speed, Under the condition of continuously supplying the wire, welding (wire stick) of the wire to the steel slab occurs.

  Further, the steel slab whose surface layer has been modified according to the present invention is suitable as a steel material used in, for example, shipbuilding, automobiles, building materials, bridges, and the like because corrosion resistance and fatigue characteristics are improved by the effect of the additive element.

  Further, in the method shown in FIG. 1, plasma heating is performed after preheating with the high frequency coil 2, but the present invention is not limited to this, and preheating is performed using a heating furnace, a burner, plasma, a laser, or the like. The main heating for melting may be performed by plasma using another heat source.

  Hereinafter, examples of the present invention and comparative examples that are out of the scope of the present invention will be given to specifically explain the effects of the present invention. In this example, SUS310s having a wire diameter of 1.2 mm is used for the wire. Then, the surface layer modification of the steel slab containing copper was performed by changing the wire supply speed Vw. The processing conditions at that time are shown in Table 1 below. In addition, the underline in following Table 1 shows that it is outside the scope of the present invention.

  And the variation of the nickel concentration in a surface layer part was investigated about each slab of an Example and a comparative example. At that time, the nickel concentration is preliminarily ground with a grinder or the like where the melting treatment and alloy addition have been performed, for example, by a metal material discriminator called mobile countback (for example, PMI-MASTAR manufactured by WAS, etc.) Measurements were taken at 10 mm intervals. The results are also shown in Table 1 above.

As shown in Table 1 above, Comparative Example No. 1 and no. In the steel slab of No. 6, since the supply amount of the wire was insufficient, the variation in nickel concentration exceeded 50%. On the other hand, Comparative Example No. 2 and no. In the steel slab of No. 7, since the molten pool was cooled due to excessive supply of wire, the variation in nickel concentration exceeded 50%. Comparative Example No. Since the steel slab of No. 3 had the wire placed in the high-temperature part, excessive supply occurred in the amount of wire necessary for realizing continuous supply, and as a result, the variation in nickel concentration increased significantly. Further, Comparative Example No. In the steel slab No. 4, since the heating of the wire was insufficient, the wire was not melted, and the surface layer of the steel slab could not be modified. Furthermore, Comparative Example No. The steel slab of No. 5 had a wire distribution rate slower than the supply rate Vw _{2 at} which one droplet dropped per treatment length of 20 mm, so the variation in nickel concentration was as large as 100% and the concentration distribution was poor. .

  On the other hand, Example No. which performed the modification process within the scope of the present invention. 8-No. The 12 steel slabs each had a nickel concentration variation of 50% or less, and the variation in the nickel concentration distribution in the surface layer was reduced as compared with the steel slab of the comparative example described above.

(A) is a perspective view which shows typically one form of the surface modification method of the steel slab of this invention, (b) is the sectional drawing.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Steel slab 2 High frequency coil 3 Plasma torch 4 Plasma jet 5 Plasma control coil 6 Wire 7 Water cooling nozzle for wire supply 8 Molten pool Lp Length of molten pool 8

Claims (3)

A method for modifying the surface layer of a steel slab comprising melting a surface layer of a steel slab containing copper by plasma heating, and adding nickel or a nickel alloy to the surface layer portion of the molten steel slab,
Supply of droplets supplied from a wire at intervals of Lp / Vt (seconds) with respect to the melting processing speed Vt (mm / second) of the steel slab and the molten pool length Lp (mm) formed by plasma heating Vw ₀ (g / sec) for the velocity, Vw ₂ (g / sec) for the supply rate at which one droplet drops per treatment length of 20 mm, and Vw for the rate at which the droplet is continuously added to the molten pool. ₁ (g / sec), when Vw ₀ <Vw ₂ , the supply speed Vw (g / sec) is in the range defined by the following formula (A), and when Vw ₀ ≧ Vw ₂ , the supply speed Vw A method for modifying the surface layer of a steel slab, characterized in that a wire made of nickel or a nickel alloy is supplied within a range defined by the following mathematical formula (B).

  A surface-modified steel slab, wherein the surface-modifying method according to claim 1 is applied.   A processed product obtained by processing the surface-modified steel slab according to claim 2.

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