JP5454132B2 - Surface melting method, surface modified steel slab, processed product - Google Patents

Description

  The present invention relates to a method for melting a surface layer portion of a steel slab to achieve high cleaning, and a method for obtaining a composite steel material by adding a metal element, an alloy, or a dissimilar steel material to the surface layer melting treatment portion to achieve high functionality. The present invention relates to steel slabs obtained by these methods, steel plates obtained by rolling these steel slabs, and processed products such as steel products and multilayer steel plates obtained by processing the steel plates. In the present specification, the steel slab refers to a steel slab in the middle of rolling and a slab after continuous casting.

  Conventionally, a slab composed of an inner layer and an outer layer using two kinds of metals having different components has utility value as a composite material having a composite function, and various manufacturing methods have been proposed. .

  For example, as a continuous casting method for slabs having different slab surface layer and inner layer components, the present inventors have employed a method of simultaneously casting two types of molten steel (Patent Document 1) and a lubricant. A method (Patent Document 2) in which elements are mixed is disclosed.

Furthermore, in order to improve the conventional method for producing these multilayer slabs, the present inventors have melted the surface layer of the slab by induction heating, plasma heating, or both, to obtain a molten steel. A melt reforming method (Patent Document 3) is proposed in which an additive element or an alloy thereof is added to a surface layer portion of a slab.
As a method for melting by plasma heating, a method of forming flat plasma that performs a wide and stable reciprocating motion by preheating the metal to be treated to a temperature higher than the demagnetization temperature is proposed (Patent Document 4). doing.
In addition, the steel slab is obtained by melting the surface layer of the steel slab by one or both of induction heating and plasma heating, and adding an additive element or an alloy thereof to the surface layer portion of the molten steel slab. A surface layer reforming method (Patent Document 5) has been proposed.

JP 63-108947 A Japanese Patent Application Laid-Open No. 07-276019 JP 2005-305532 A JP 2006-124763 A JP 2004-195512 A

  However, the method of simultaneously casting two types of molten steel as disclosed in Patent Document 1 can obtain multi-layer slabs of various combinations, while preparing two types of components at the molten steel stage. In addition, it is necessary to prepare two types of ladles for putting the molten steel, and tundishes and nozzles necessary for pouring the molten steel into the mold. Further, it is not possible to add different elements on one side and the other side of the slab, or to add elements only on one side. In addition, the method of mixing an element in the lubricant as disclosed in Patent Document 2 can reduce the cost, but the addition amount of the element component is stable because it is added through the lubricant. And there is a problem that the amount of addition is limited due to lack of heat source, and as in Patent Document 1, different elements are added on one side and the other side of the slab, or elements are added only on one side. There is a problem that it cannot be added.

  On the other hand, in the method disclosed in Patent Document 3, although the problems of Patent Documents 1 and 2 can be solved, the contents specifically disclosed in Examples in Patent Document 3 are those of the slab by induction heating. A surface layer of 20 mm is melt-processed, and specific conditions for melt-processing a cast piece by plasma heating are not disclosed.

  Then, as what melts a slab by plasma heating, as disclosed in the example of Patent Document 4, the melting depth is about 5 mm. Thus, in comparison with induction heating, in plasma heating, It can be seen that the melting depth is small. When the surface modified layer is about 5 mm, there is a problem that the surface modified layer is also easily detached or removed by a descaling process for removing oxide scale generated in the subsequent rolling step. Further, as disclosed in the examples of Patent Document 5, as disclosed in a method of melting a surface layer of 20 mm of a slab by performing a melting modification process using induction heating and plasma heating in combination. Nothing is disclosed that melts the surface layer of the cast slab exceeding 5 mm only by plasma heating.

  According to induction heating, it is possible to carry out a melting treatment of the slab surface layer of about 20 mm, but since induction heating generates electromagnetic force, distribution of electromagnetic force occurs on the surface of the slab being processed, For this reason, there is a problem that the variation in the depth direction becomes large, whereas the plasma heating has an advantage that the variation in the depth direction can be reduced. However, as described above, plasma heating has a problem that only the surface layer of the cast slab can be melted.

  By the way, if the moving speed of the slab to be melt-treated is extremely slow, it can be carried out even with a melting process of about 20 mm on the surface layer of the slab, but the production efficiency is remarkably lowered. is not.

  The present invention relates to a method for producing a slab having a thicker surface layer and a smaller variation in the depth direction of the molten portion without lowering productivity, and a steel slab obtained by the method. An object of the present invention is to provide a steel plate obtained by rolling a steel slab and a processed product such as a steel product obtained by processing the steel plate.

The gist of the present invention is as follows.
(1) Plasma using DC arc plasma in which the steel slab being conveyed is preheated by at least one of burner heating, induction heating, and plasma heating, and then vibrated by an AC magnetic field of 100 Hz or higher. by heating at 20W / mm ² or more surface heat input density, characterized by melt processing a surface layer portion of 10mm or more from the steel slab surface, the surface melting treatment method of steel slabs.
(2) At least one of a metal element, an alloy, or a dissimilar steel material is added to a surface layer portion melted portion of a steel slab that is melt-processed by the plasma heating, according to (1), A method for surface layer melting treatment of steel slabs.
(3) The metal element is one or more selected from silicon, manganese, nickel, chromium, molybdenum, copper, aluminum, magnesium, titanium, niobium, vanadium, cerium, lanthanum, and neodymium. The method for surface layer melting treatment of a steel slab according to (2).
(4) Elements contained in the alloy or different steel materials include carbon, nitrogen, phosphorus, sulfur, boron, silicon, manganese, nickel, chromium, molybdenum, copper, aluminum, magnesium, titanium, niobium, vanadium, cerium, The method for surface layer melting treatment of a steel slab according to (2) above, wherein the method is one or more selected from lanthanum and neodymium.
(5) The steel slab before performing the melting treatment is a steel slab containing copper, and the metal element is nickel, or the element contained in the alloy or dissimilar steel material is nickel. The method for surface layer melting treatment of a steel slab according to (2) above.
( 6 ) The method for surface layer melting treatment of a steel slab according to any one of (1) to (5) , wherein a waveform of the alternating current that generates the alternating magnetic field is a sine wave.
( 7 ) The method for surface layer melting treatment of a steel slab according to any one of (1) to (5) , wherein a waveform of the alternating current that generates the alternating magnetic field is a rectangular wave.
( 8 ) When the steel slab is melted by the above plasma heating, Ar gas is used as a base gas as a plasma torch gas, and He gas, H ₂ gas, H ₂ O gas, N ₂ gas, NH ₃ gas, CO _The method for surface layer melting treatment of a steel slab according to any one of (1) to ( 7 ), wherein a mixed gas containing at least one selected from _two gases and hydrocarbon gas is used.
( 9 ) The present invention is characterized in that only one side of a steel slab is melted, that both sides are melted under the same conditions, or that one side and the other side are melted under different processing conditions. The method for surface layer melting treatment of a steel slab according to any one of (1) to ( 8 ).
( 10 ) A steel slab obtained by the method according to any one of (1) to ( 9 ), wherein the steel slab has a melt-treated part in a surface layer part having a thickness of 10 mm or more from the surface of the steel slab. A surface layer modified steel slab.
( 11 ) A surface layer-modified product obtained by further cold-rolling the steel slab described in (10) after hot rolling, thick plate rolling, or hot rolling.

  According to the present invention, it is possible to manufacture a slab having a small variation in the depth direction by melting a surface layer portion of 10 mm or more of the steel slab by plasma heating without reducing productivity. Furthermore, the surface layer part of the steel slab is melt-treated to achieve high cleaning, or a metal element, alloy, or dissimilar steel material is added to the surface layer melt-treated part of 10 mm or more to achieve high functionality and composite. Steel material can be obtained. In addition, while maintaining the ductility and toughness of the base material, it is possible to obtain cast slabs with a surface layer modified layer of 10 mm or more, and the surface layer portion stabilizes processed products with excellent corrosion resistance, wear resistance and durability. And can be used for applications that require durability, such as special automobile parts, machine parts, and structural materials.

It is a figure for demonstrating the fusion processing state of the metal material by the plasma at the time of applying this invention, (a) is a front view, (b) has shown the side view. It is a figure for demonstrating the oscillation principle of direct-current plasma in FIG. 1, (a) is the case where the action direction of an alternating current magnetic field is from the left back side to the front right side, and (b) is the action direction of the alternating magnetic field from the right front side. The action direction of each electromagnetic force in the case of the left back side direction is shown. It is a graph which shows the fusion depth with respect to a surface heat input density. It is a graph which shows the slab heat input efficiency with respect to a plasma arc control current frequency.

  The inventors say that when the surface layer modification layer is about 5 mm, the surface layer modification layer is also easily detached or removed by the descaling process for removing the oxide scale generated in the subsequent rolling step. We proceeded with the examination of the problem. As a result, if the surface layer reformed layer is 10 mm or more, the surface layer reformed layer is maintained soundly even after the hot rolling process or further cold rolling process, and coarse inclusions in the base material are removed by the rolling. It has been experimentally found that the defect rate can be made extremely small because it is not pushed into the part and can be processed without troubles such as cracks and wrinkles.

Accordingly, the present inventors have intensively studied the conditions for melting the surface layer portion of 10 mm or more from the surface of the slab by plasma heating without reducing productivity, and the surface heat input density to the slab by plasma heating. When the surface heat input density is 20 W / mm ² or more, it is found that the melting depth increases rapidly, and the slab surface layer can be stably melted with a thickness of 10 mm or more. I found.

  Furthermore, since harmful oxide-based non-metallic inclusions in the melted surface layer are removed, a slab having a highly purified layer is obtained, and a metal element, alloy, or dissimilar steel material is added to the surface melted portion. Since a slab having a highly functionalized layer is obtained, and a product processed using a melt-modified slab can also be obtained, the product application can be greatly expanded. This will be described in detail below. First, the experiment that led to the completion of the present invention will be described.

  The inventors conducted an experiment of melting the surface layer of the slab while moving the slab using plasma heating. As a result, in order not to reduce the productivity, it is preferable to secure a moving speed of the cast slab of at least 0.1 m / min. For this purpose, the melting process is performed only by plasma heating without preheating. It turned out to be difficult in practice. In addition, since the one where the moving speed of slab is large improves productivity, 0.5 m / min or more is preferable, 1.0 m / min or more is more preferable, 1.2 m / min or more is still more preferable.

  Therefore, preheating is performed before the surface layer of the slab is melted using plasma heating. As a method of performing preheating, burner heating, induction heating, or plasma heating can be used, and these may be used in combination. In addition, the preheating temperature is not particularly specified, but the surface temperature of the slab can be about 800 to 900 ° C.

  After this preheating, the surface layer of the slab was melted using plasma heating. This will be specifically described with reference to FIGS. As shown in FIG. 1, the plasma torch 1 forms plasma by direct current plasma between the introduced plasma gas 7 and the metal material 11 by applying a voltage from a direct current power source 8. Two rectangular loop-shaped plasma vibration coils 2 and 2 are arranged opposite to each other in the vicinity of the plasma ejected from the plasma torch 1 toward the metal material 11, and the plasma vibration coils 2 and 2 cause the plasma to flow. Vibrate. The two plasma vibration coils 2 and 2 connected to the AC power source 9 are respectively installed in the width direction of the metal material 11 so as to face each other with the plasma interposed therebetween, and the coil current flows in the metal material width direction.

  The generated plasma has a plasma torch 1 as a cathode and a metal material 11 as an anode. As shown in FIG. 2, an electron flow 13 is emitted from the plasma torch 1 and a current 12 flows from the metal material 11 to the plasma torch 1. . When an alternating magnetic field 14 generated by energizing the two plasma oscillation coils 2 and 2 is applied to the current 12, the front view of FIG. A Lorentz force is generated in the vertical direction in the left and right directions in the side view of FIG. 1B, and the plasma is vibrated in the vibration direction 10 by the electromagnetic force indicated by the reciprocating arrow in FIG. 2 (a) and 2 (b) is an electromagnetic force that vibrates the plasma. As shown in FIG. 2 (a), the acting direction of the AC magnetic field 14 is from the left rear side to the right front side (upper left of the drawing). 2), the electromagnetic force 15 acts on the right side in the figure as shown in FIG. 2A, and the acting direction of the AC magnetic field 14 is on the right front side as shown in FIG. 2B. 2 to the left rear side direction (lower right to upper left direction in the drawing), the electromagnetic force 15 acts on the left side in the drawing as shown in FIG. As a result of the action of the alternating magnetic field 14, the working direction of the electromagnetic force 15 becomes opposite, so that a fan-shaped flat plasma 6 spreading toward the metal material 11 side is formed as shown in FIG. Due to the vibration of the plasma and the movement of the metal material 11 at a constant speed (indicated by the movement direction 5 in FIG. 1A), the metal material 11 is continuously melted in the longitudinal direction with a width along the plasma amplitude, The melting part 3 is generated. In addition, 4 in Fig.1 (a) shows the part which the melting part 3 resolidified.

In this experiment, a thermocouple was embedded in the slab, which is the metal material 11, the temperature of the slab was measured, and the amount of heat input to the slab was calculated.
Specifically, first, two or more thermocouples having different depth positions are arranged in the slab from the surface opposite to the surface on which the plasma heating is performed in the slab, and at least two locations in the depth direction. The temperature is measured and the heat flux from the plasma heating side to the opposite side is determined.

  Next, assuming the amount of heat radiated on the melt-treated surface of the slab with respect to the amount of heat output from the plasma, (the amount of heat output from the plasma) − (the amount of heat radiated on the melt-treated surface of the slab) − ( The amount of heat from the plasma heating side to the opposite surface side) is calculated as the amount of heat input in the slab surface layer, and the temperature distribution inside the slab is obtained.

  The amount of heat input to the slab surface layer is obtained by repeatedly calculating this amount of heat by changing the amount of heat radiated on the melt-treated surface of the slab until it substantially matches the temperature measured by the thermocouple. Therefore, the slab surface heat input density can be determined according to the moving speed of the slab.

Thus, the slab was heated and a melting experiment was performed, and the relationship between the melting depth of the slab and the heat input to the slab surface was investigated and analyzed in detail. As a result, as shown in FIG. 3, when the heat input density to the slab surface (hereinafter sometimes referred to as “heat input density”) is 20 W / mm ² or more, the melting depth is dramatically improved. And found that a melt depth of 10 mm or more can be achieved. Moreover, since the melting treatment was performed by plasma heating, it was also confirmed that a melting depth of 10 mm or more was obtained uniformly.

Although the details of the reason are unknown, it is considered that when the heat input density is 20 W / mm ² or more, the molten steel flow due to thermal convection or Marangoni convection is dramatically increased. When the heat input density is relatively small, a part of the surface layer of the slab is melted, but the surface layer is considered to be in a state where the temperature is higher and heat convection hardly occurs. Moreover, since the heat input density is small, the temperature difference with other surface layer portions is small, and it is considered that Marangoni convection hardly occurs. However, when the heat input density is increased to 20 W / mm ² or more, the temperature difference from other surface layer portions increases, causing Marangoni convection and accompanying heat convection, a large flow is generated. Thus, it is considered that the melting depth is drastically improved at 20 W / mm ² or more.

Moreover, as a result of investigating and analyzing the number density of the oxide-based nonmetallic inclusions in the melting depth portion of 10 mm or more, no coarse inclusion exceeding 10 μm which causes defects is observed at all. It was also found that a slab having a cleansing layer was obtained.
This is presumably because the surface layer of the slab is melted by the plasma heating, so that coarse inclusions existing in the melt-treated portion float to the outermost layer portion.

  By obtaining a slab having a surface reforming portion having a thickness of 10 mm or more obtained in this way, it is possible to stably form a modified layer having a thickness greater than the depth that is oxidized and scaled down in a subsequent heating furnace or the like. In addition, it can be obtained reliably and the applicable products can be dramatically expanded.

  Moreover, in this invention, you may add at least any one of a metal element, an alloy, or a dissimilar steel material to the surface layer part fusion | melting part of the steel slab melt-processed by plasma heating. In this way, by adding at least one of a metal element, an alloy, or a dissimilar steel material, the function of the added element is added, so a slab having a highly functional layer with a thickness of 10 mm or more is provided. Can be obtained. Also in this case, the variation in the depth direction of the surface layer melted portion can be reduced.

  Incidentally, as a method of adding a metal element, an alloy, or a dissimilar steel material to the molten portion of the surface layer portion of the steel slab being melt-processed, for example, it is efficient to add it as a wire to the molten portion. May be added to the partially melted part of the slab, or may be sprayed or blown as a powder on the melted part.

  Examples of the metal element component to be added include silicon, manganese, nickel, chromium, molybdenum, copper, aluminum, magnesium, titanium, niobium, vanadium, cerium, lanthanum, and neodymium.

  Moreover, as an element component contained in the alloy or different steel material to be added, in addition to the above-mentioned metal element components, carbon, nitrogen, phosphorus, sulfur, boron, and a compound with nitrogen or oxygen are included.

Below, the characteristic of the element added is demonstrated.
Carbon: It works to increase the strength of steel. For example, by adding carbon only to the surface layer of an ultra-low carbon steel, it is possible to produce a steel sheet that is excellent in both strength by maintaining the workability with the internal steel and increasing the strength at the surface layer.
Nitrogen: Increases the strength of steel. In addition, various elements and fine nitrides can be formed and used for controlling the material structure.
Silicon, manganese: It has the effect | action which improves a strength especially with respect to an iron-type alloy.
Phosphorus: Adds to ultra-low carbon steel to increase strength.
Sulfur: Has the effect of enhancing the machinability of steel.
Nickel, chromium: has the effect of improving the corrosion resistance of steel. For example, the surface layer can be made of stainless steel by adding nickel and chromium to the surface layer of the low carbon steel. Moreover, since nickel has the effect | action which raises the solid solubility limit to the steel of copper as it mentions later, the surface red hot embrittlement by copper can be suppressed. For this reason, it is used together when adding copper.
Molybdenum: Adding molybdenum in addition to the above nickel and chromium has the effect of further improving the corrosion resistance.
Copper: Workability and strength can be increased at the same time. However, as described above, it is also an element that exerts an adverse effect on the surface red hot brittleness, so it is usually used together with nickel.
Boron: The hardenability of steel can be improved.
Aluminum: Corrosion resistance can be increased by adding to ordinary steel.
Magnesium, titanium, niobium, vanadium, cerium, lanthanum, neodymium: Oxygen, sulfur, carbon, and nitrogen in steel are combined to produce fine oxides and sulfides, reducing the structure of steel materials, and steel pipe materials, etc. When used for the material to be welded, the structure becomes rough in the heat-affected zone of welding and the strength decreases, but this can be suppressed with a fine compound.

  The alloy of the additive element is not particularly limited as long as it is a multi-component alloy of the additive element, but ferromanganese, ferronickel, ferroline, and other iron alloys are usually used.

  Further, regarding the compound of the additive element component and nitrogen, when looking at stainless steel, these structures can be reduced by adding nitrogen, which is an austenite forming element. That is, adding nitrogen such as iron nitride in the form of an alloy has the effect of reducing crystal grains, so that the surface roughness during rolling can be kept uniform and the surface shape of steel can be improved. In addition, the compound of the additive element component and oxygen has the effect of refining the structure by adding oxygen, such as magnesium oxide, in the form of an alloy. Reduction can be prevented. However, in some ferritic stainless steels, there is also a problem that the texture of the slab ends is rough and the surface properties are poor, so it is important to select whether or not to add depending on the intended stainless steel. .

  In addition, when copper is contained in the target steel slab, it is as described above by adding nickel as a metal element, an alloy containing nickel, or a dissimilar steel material to the surface molten portion of the steel slab. Further, the red hot brittleness of the steel surface can be suppressed.

  Steel containing copper is oxidized when exposed to a high-temperature oxidizing atmosphere, and copper precipitates as liquid copper and infiltrates austenite grain boundaries to generate surface cracks. This is surface red hot brittleness. However, by adding nickel, the solid solubility limit of copper in iron can be increased, so that it is difficult to precipitate as liquid copper, and thus surface cracks can be prevented.

  However, until now, there was no method for reliably adding nickel, which is an expensive rare metal, only to the surface layer of the steel slab. Therefore, in order to suppress the red hot brittleness of the steel containing copper, it is necessary to adjust the composition of the molten steel. Since nickel is added at the stage and nickel is dispersed and present throughout the steel slab, nickel is wastedly added to portions other than the surface layer portion of the steel slab that does not require nickel. Therefore, steel slabs containing no copper have been directed.

  For this reason, there is a problem that inexpensive low-grade scrap with a large amount of copper is rarely used, and that the advantage of simultaneously increasing the workability and strength of copper is hardly enjoyed. .

  On the other hand, in the present invention, nickel can be reliably added only to the surface layer melted portion of the steel slab containing copper, so that resources such as inexpensive low scrap having a high copper content are effectively used. In addition, the advantage of simultaneously increasing the workability and strength of copper can be enjoyed.

Next, frequency characteristics of an alternating current that flows through a control coil for generating an alternating magnetic field that controls vibration of a plasma arc when plasma heating is performed will be described.
In the present invention, an AC magnetic field is applied to a DC plasma arc to vibrate the plasma arc. When the frequency is less than 100 Hz, the efficiency of heat input from the plasma arc to the slab is as low as about 30%. Here, the slab heat input efficiency is defined as (slab surface heat input amount) / (heat amount output from plasma arc) × 100 per unit area and per unit time.

  Therefore, the inventors conducted experiments to increase the frequency, and as a result, found that the heat input efficiency of the slab suddenly improved from 100 Hz or higher as shown in the graph of FIG.

  As for this phenomenon, the phenomenon of the oscillating DC plasma arc was photographed with a high-speed camera, and this phenomenon was elucidated by analyzing the image. Specifically, as a result of image analysis, when the frequency of the plasma arc is less than 100 Hz, the plasma arc vibrates relatively slowly, and a phenomenon occurs in which the arc is reflected at the lowest point reaching the slab. . That is, the plasma torch gas that generates a plasma arc collides with the slab and flows upward, thereby reducing heat input.

  On the other hand, when the frequency exceeds 100 Hz, the plasma arc vibration is accelerated, so that the arc can be pinched while maintaining the arc vibration width to some extent, and the phenomenon that the arc is reflected does not occur. all right. For this reason, it has been found that the amount of heat input to the slab is improved because the arc is pinched in a state where it cannot flow upward even when the plasma torch gas collides with the slab.

  Therefore, by using a DC arc plasma oscillated by an AC magnetic field of 100 Hz or more, it becomes possible to achieve a heat input efficiency of 35% or more, and the slab moving speed is increased to improve productivity. Alternatively, it is possible to obtain a slab having a larger melting depth without reducing the productivity, and it is possible to expand the required varieties.

  And it discovered that the waveform of the alternating current sent through the control coil for vibrating this plasma arc was concerned with control of the shape of a fusion | melting part. That is, when the waveform of this alternating current is a sine wave, the interface between the melted part and the non-melted part becomes wavy. In addition, it was confirmed that the wavy maximum and minimum fluctuation widths were about 1 mm or less. However, in the present invention, since the melting depth is ensured to be 10 mm or more, the fluctuation width of about 1 mm or less is sufficiently small, so that it is practical to obtain a clean thick wall or to roll to a thin plate. There is no problem. Therefore, the waveform of the alternating current can be a sine wave. The sine wave is obtained with a general-purpose alternating current, has high versatility, and is effective for cost reduction.

  Further, when the waveform of the alternating current is a rectangular wave, it is possible to smooth the interface between the melting part and the non-melting part by creating a trapezoidal waveform. Since a highly uniform melting depth and a highly uniform component can be obtained, it is suitable for obtaining a high-strength thick wall.

Further, the plasma torch gas will be described. As a plasma torch gas, Ar gas is used as a base gas, and He gas, H ₂ gas, H ₂ O gas, N ₂ gas, NH ₃ gas, CO ₂ gas, and hydrocarbon gas are used alone or in combination of two or more. By doing so, high thermal efficiency and high heat transfer coefficient can be obtained by the polyatomic molecular gas, which is very effective for obtaining high thermal efficiency.

Moreover, since a reducing atmosphere can be created by using these He gas, H ₂ gas, N ₂ gas, NH ₃ gas, and hydrocarbon gas, it is effective when it is desired to suppress surface oxidation. On the other hand, if H ₂ O gas or CO ₂ gas is used, it is possible to obtain a weak oxidation state by controlling the blending ratio with Ar gas, and it is very effective for forming an oxidized non-moving body on the surface. is there. Similarly, the nitriding treatment can be controlled by controlling the mixing ratio of N ₂ gas and NH ₃ gas. Therefore, the gas species to be blended and the blending ratio can be appropriately set according to the purpose.

  Further, in the melting process, either one of the slabs is processed, the both surfaces are processed under the same conditions, or the processing is performed under different processing conditions on one surface and the other surface. By carrying out the method, a slab according to the purpose can be obtained.

  Specifically, by performing the process on only one side of the cast piece, it is possible to provide a necessary function only on one side. For example, in a rail steel or the like, a required alloy can be added by increasing the strength of only one side. In addition, the cost can be reduced by providing a necessary function only on one side.

  Moreover, it becomes possible to acquire the same effect in any surface of steel materials by processing on both surfaces on the same conditions. For example, if you want to provide corrosion resistance of steel on both sides in structural steel, or if you want to improve the cleanliness of both surfaces of steel in automobile steel, etc., or if you want to increase the strength of the surface layer on both sides, This is effective when, for example, steel materials with remarkable surface cracks are desired to be highly functional so that only the surface layer does not break.

  Furthermore, by performing the treatment under different treatment conditions on one surface and the other surface, it is possible to impart different functions to the surface of the steel material. For example, the portion corresponding to the inner surface for transporting the liquid in a steel pipe or the like is effective for improving the corrosion resistance, and the portion corresponding to the outer surface is effective for increasing the strength or improving the weather resistance.

  As described above, the slab obtained by the present invention is subjected to hot rolling, thick plate rolling, or cold rolling after hot rolling to obtain necessary structural steel, automotive steel, steel pipe, rail steel. It becomes possible to manufacture processed products such as.

  Incidentally, the processed products include steel plates such as thin plates and thick plates, rails, shaped steels, steel pipes, etc., but all steel products obtained by processing slabs by a normal steel process are targeted. Also included are semi-finished products such as hot rolled coils.

After cutting the slab after completion of continuous casting, preheating is performed in a range of 800 to 900 ° C. by induction heating, and then a melt reforming process is performed by plasma heating in which 28 plasma torches are arranged in parallel. 2000 mm, thickness 250 mm, length 10 m 0.001% C-0.11% Mn-0.01% Si-0.015% P-0.009% S-0.045% Al-0.005% The surface layer of a continuous cast slab excluding 50 mm both ends of the width of Ti (unit: mass%) is 1.2 m / at a surface heat input density of 24 W / mm ² with a plasma torch controlled at a control current frequency of 110 Hz. Melt processing was performed at a rate of min.

  The obtained slab was subjected to cross-sectional analysis. As a result, the variation in the depth direction was plus or minus 1 mm, and the surface layer highly purified layer thick slab with a maximum particle size of oxide inclusions of 6 μm. By processing this slab, a steel sheet for thin plate having good workability as a whole and having a very small defect rate including the cold rolling and plating processes could be obtained.

After cutting the slab after completion of continuous casting, preheating is performed in the range of 800 to 900 ° C. by induction heating, and then a melt reforming process is performed by plasma heating in which 21 plasma torches are arranged in parallel. 1500mm, thickness 250mm, length 10m 0.04% C-2.0% Mn-1.0% Si-0.007% P-0.004% S-0.045% Al-0.03% The surface layer 12 mm in the continuous cast slab excluding 50 mm both ends of the width of Ti (unit: mass%) is a plasma torch controlled with a control current frequency of 110 Hz, and a surface heat input density of 24 W / mm ² is 1. It melt-processed at a speed | rate of 2 m / min, carbon alloy addition was performed using the carbon wire, and only the carbon component of the surface layer was 0.10 mass%.

  The obtained slab was subjected to a cross-sectional analysis. As a result, the variation in the depth direction was plus or minus 1 mm, and the variation in the element component was within 3%. By processing this slab, it was possible to obtain a steel sheet for a thin plate having a surface layer excellent in fatigue strength, a high ductility inside, and a good workability as a whole.

After cutting the slab after completion of continuous casting, preheating is performed in the range of 800 to 900 ° C. by induction heating, and then a melt reforming process is performed by plasma heating in which 17 plasma torches are arranged in parallel. 0.07% C-0.6% Mn-0.6% Si-0.03% P-0.005% S-16.5% Cr (unit: mass%) of 1200mm, thickness 250mm, length 10m The surface layer of the continuous cast slab excluding the width 50 mm at both ends of the continuous cast slab of 12) is a plasma torch controlled with a control current frequency of 110 Hz, and a surface heat input density of 24 W / mm ² is 1.2 m. At that time, the melting treatment was performed, and nitrogen was added by changing the plasma gas to argon-nitrogen, so that only the nitrogen component of the surface layer was 0.07% by mass.

  The obtained cast slab was subjected to a cross-sectional analysis. As a result, the variation in the depth direction was plus or minus 1 mm, and the variation in the element component was within 5%. By processing this slab, a stainless steel plate for a thin plate having a good surface property at the end could be obtained.

After cutting the slab after completion of continuous casting, preheating is performed in a range of 800 to 900 ° C. by induction heating, and then a melt reforming process is performed by plasma heating in which 28 plasma torches are arranged in parallel. 2000 mm, thickness 250 mm, length 10 m 0.001% C-0.11% Mn-0.01% Si-0.015% P-0.009% S-0.045% Al-0.005% the surface layer 13mm continuous casting slab with the exception of the width end portions 50mm of Ti (units wt%), the frequency of the control current in the controlled plasma torch as 1000 Hz, a line speed at the surface heat input density of 50 W / mm ² Melting was performed at a speed of 1.4 m / min, which was about 1.2 times.

  The obtained slab was subjected to cross-sectional analysis. As a result, the variation in the depth direction was plus or minus 1 mm, and the surface layer highly purified layer thick slab with a maximum particle size of oxide inclusions of 6 μm. By processing this slab, a steel sheet for thin plate having good workability as a whole and having a very small defect rate including the cold rolling and plating processes could be obtained.

After cutting the slab after completion of continuous casting, preheating is performed in a range of 800 to 900 ° C. by induction heating, and then a melt reforming process is performed by plasma heating in which 28 plasma torches are arranged in parallel. 2000 mm, thickness 250 mm, length 10 m 0.001% C-0.11% Mn-0.01% Si-0.015% P-0.009% S-0.045% Al-0.005% Surface heat input of 24 W / mm ² with a plasma torch controlled with a control current frequency of 110 Hz on the surface layer of a continuous cast slab excluding 50 mm at both ends of Ti-0.3% Cu (unit: mass%). A melt treatment was performed at a density of 1.2 m / min. At that time, a nickel alloy was added using a nickel alloy wire, and only the nickel component of the surface layer was made 0.3 mass%.

  The obtained slab was subjected to a cross-sectional analysis. As a result, the variation in the depth direction was plus or minus 1 mm, and the variation in the element component was within 10%. Further, the slab after the treatment had no cracks, and was heated at 1250 ° C. for 2 hours in a heating furnace, and then hot-rolled. The obtained hot-rolled sheet has no surface cracking, and a good hot-rolled sheet can be obtained. Thereafter, a steel sheet for a thin sheet having a very small defect rate including cold rolling and plating processes can be obtained. .

As described above, even in the case of a slab containing Cu, by adding nickel to the surface layer portion, surface cracking could be prevented both after the melting treatment and hot rolling. In addition, since the rare metal and expensive nickel can be added only to the surface layer of the slab that needs to be added, the cost can be reduced.
[Comparative Example 1]

After cutting the slab after completion of continuous casting, preheating is performed in a range of 800 to 900 ° C. by induction heating, and then a melt reforming process is performed by plasma heating in which 28 plasma torches are arranged in parallel. 2000 mm, thickness 250 mm, length 10 m 0.001% C-0.11% Mn-0.01% Si-0.015% P-0.009% S-0.045% Al-0.005% The surface layer of a continuous cast slab excluding 50 mm width ends of Ti (unit: mass%) is a plasma torch controlled at a control current frequency of 60 Hz, and a surface heat input density of 18 W / mm ² is 1.2 m / Melt processing was performed at a rate of min.

  The obtained slab was subjected to a cross-sectional analysis. As a result, the variation in the depth direction was plus or minus 1 mm, and the oxide layer inclusion had a maximum particle size of 6 μm. The thickness of the quality layer was as small as 5 mm.

DESCRIPTION OF SYMBOLS 1 Plasma torch 2 Plasma vibration coil 3 Melting part 4 Resolidification part 5 Movement direction 6 Flat plasma 7 Plasma gas 8 DC power supply 9 AC power supply 10 Vibration direction by electromagnetic force 11 Metal material 12 Current 13 Electron current 14 AC magnetic field 15 Electromagnetic force

Claims (11)

The steel slab being conveyed is preheated by at least one of burner heating, induction heating, and plasma heating, and then by plasma heating using DC arc plasma oscillated by an AC magnetic field of 100 Hz or more , A method for melting the surface layer of a steel slab, comprising melting a surface layer portion of 10 mm or more from the surface of the steel slab at a surface heat input density of 20 W / mm ² or more. 2. The steel slab according to claim 1, wherein at least one of a metal element, an alloy, or a dissimilar steel material is added to a surface layer portion melted portion of the steel slab that is melt-processed by the plasma heating. Surface layer melting method. The metal element is at least one selected from silicon, manganese, nickel, chromium, molybdenum, copper, aluminum, magnesium, titanium, niobium, vanadium, cerium, lanthanum, and neodymium. The method for surface layer melting treatment of the steel slab according to 2. Elements contained in the alloy or different steel materials are carbon, nitrogen, phosphorus, sulfur, boron, silicon, manganese, nickel, chromium, molybdenum, copper, aluminum, magnesium, titanium, niobium, vanadium, cerium, lanthanum, neodymium The method for surface layer melting treatment of a steel slab according to claim 2, wherein the method is one or more selected from the group consisting of: The steel slab before performing the melting treatment is a steel slab containing copper, wherein the metal element is nickel, or the element contained in the alloy or dissimilar steel material is nickel, A method for surface layer melting treatment of a steel slab according to claim 2. The method for melting a surface layer of a steel slab according to any one of claims 1 to 5, wherein the waveform of the alternating current that generates the alternating magnetic field is a sine wave. The method for surface layer melting treatment of a steel slab according to any one of claims 1 to 5, wherein a waveform of the alternating current for generating the alternating magnetic field is a rectangular wave. When the steel slab is melted by the plasma heating, Ar gas is used as a plasma torch gas as a base gas, He gas, H ₂ gas, H ₂ O gas, N ₂ gas, NH ₃ gas, CO ₂ gas, The method for melting a surface layer of a steel slab according to any one of claims 1 to 7 , wherein a mixed gas containing at least one selected from hydrocarbon gas is used. The melting treatment is performed only on one surface of the steel slab, the melting treatment is performed on the both surfaces under the same conditions, or the melting treatment is performed on the one surface and the other surface under different processing conditions. The surface layer fusion processing method of the steel slab as described in any one of 1-8 . A steel slab obtained by the method according to any one of claims 1 to 9 , wherein the steel slab has a melt-treated portion in a surface layer portion having a thickness of 10 mm or more from the surface of the steel slab. Quality steel slab. The steel slab according to claim 1 0, hot rolling, further characterized by being obtained by cold rolling after thick plate rolling, or hot rolling, the surface reforming process product.

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