JP3771899B2 - Method for surface modification of steel slab containing copper, modified slab and processed product - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a surface layer modification method for a continuous cast slab of steel containing copper, and a modified slab and a processed product obtained using the method.
[0002]
[Prior art]
When steel contains copper, hot workability is remarkably hindered, and cracks occur on the surface in continuous casting, reheating, and hot rolling, which are ordinary steel manufacturing processes. This is because when steel is oxidized by being exposed to oxygen in the atmosphere during continuous casting or reheating and hot rolling, the solubility of copper in the steel is low and the melting point of copper is low. It is disclosed in Non-Patent Document 1 that it exists in a liquid state between steels, and further, this liquid copper penetrates into the grain boundaries of the steel and leads to cracking in order to reduce the interface strength.
In addition, tin promotes the phenomenon of cracking by copper by lowering the solubility of copper in steel, so tin alone is unlikely to be a problem, but it is not a patent when present together with copper. It is disclosed in Document 2.
[0003]
In order to prevent this phenomenon, copper is an element that cannot be removed in the steel refining process, so it is an element that prevents copper from being mixed into the steel or increases the solubility of copper in steel. A method of adding nickel is known.
However, in the current recycling society where a large amount of scrap containing a large amount of copper is used, copper is inevitably mixed, and even when diluted with iron made from ore, it gradually accumulates, gradually adding nickel, and harmless. There is a growing need for However, on the other hand, nickel is a rare and expensive element, and has a problem that the manufacturing cost increases.
From the above, the inventors, when continuously cast, by a method of casting two types of molten steel simultaneously (see Patent Document 1), a method of mixing nickel into the lubricant (see Patent Document 2), etc., It discloses a method of processing at low cost by adding nickel only to the slab surface layer.
[0004]
[Non-Patent Document 1]
`` Material Transaction '' Vol. 43, No. 3, (2002) PP.292-300
[Non-Patent Document 2]
"Feramu" Vol.7, (2002) No.4, P18-22
[Patent Document 1]
JP 63-108947 A [Patent Document 2]
Japanese Patent Application Laid-Open No. 07-26109
[Problems to be solved by the invention]
However, in the former method of Patent Document 1, the manufacturing cost is high because two types of molten steel are prepared, and in the latter method of Patent Document, the portion that later becomes the inside of the slab is added before solidifying yet. Therefore, it was difficult to add only to the surface layer, and it was difficult to stabilize the addition thickness and satisfy the lower limit of the components.
An object of the present invention is to reliably and inexpensively modify the slab surface layer of steel containing copper and prevent the occurrence of cracks during hot working.
[0006]
[Means for Solving the Problems]
The present invention
(1) The surface layer of a steel slab containing copper is melted by one or both of induction heating and plasma heating, and a pure metal or alloy containing nickel is added to the surface layer part of the molten steel slab. A method for modifying the surface layer of a steel slab, which is characterized.
(2) The method for modifying the surface layer of a steel slab according to (1), wherein a pure metal or alloy containing nickel in an amount satisfying the following formula is added to a surface layer portion of the molten steel slab.
[% Cu] + 8 × [% Sn] <α × [% Ni]
[% Cu]> 0
[% Sn] ≧ 0
α> 0
Here, [% Cu], [% Sn], and [% Ni] indicate mass% of the content of each element in the steel.
(3) At least when induction heating is used, the direction of the magnetic field generated by induction heating is generated so as to be in the slab casting direction, and the molten portion is held by electromagnetic force (1) or (2 The method for modifying the surface layer of a steel slab as described in 1).
(4) When using at least plasma heating, pure metal or alloy containing nickel is supplied into the plasma, heated or melted, and then supplied to the molten portion of steel (1) to ( 3) The method for modifying the surface layer of a steel slab according to any one of the above.
(5) When both induction heating and plasma heating are used, the additive pure metal or alloy containing nickel is preheated together with the slab surface layer by induction heating, and then the additive pure metal or alloy is melt-alloyed by plasma heating. The method for modifying the surface layer of a steel slab according to (1) or (2), wherein:
(6) A surface-modified steel slab obtained by the method according to any one of (1) to (5).
(7) A processed product obtained by processing the surface-modified steel cast according to (6).
[0007]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, as a method of depositing nickel, which is an element for increasing the solubility of copper in steel, on the slab surface of steel containing copper, copper is used by using either or both of induction heating and plasma heating. It has been found that the slab surface layer can be reliably and inexpensively modified by melting the slab surface layer of the contained steel and adding a pure metal or alloy containing nickel thereto. This will be described in detail below.
[0008]
For the steel slab containing copper, which is the subject of the present invention, the thickness of the surface layer portion may be equal to or greater than the thickness of the oxide film and is not particularly specified, but is usually 0.5 to 5 mm. Degree. This is because there are not many oxide films less than 0.5 mm, and in the case of more than 5 mm, addition is more than necessary with respect to the thickness of the oxide film. In many cases, the slab is manufactured by continuous casting.
As a method for melting the slab surface layer of steel containing copper, any of induction heating alone, plasma heating alone, or a combination of induction heating and plasma heating may be used.
[0009]
In the case of performing induction heating alone, a pure metal or alloy containing nickel can be added to the slab surface layer melting portion in the form of a wire or a sheet. Specifically, the slab obtained by a continuous casting machine is transported to a melting treatment site after cutting, heated by an induction coil arranged so as to circulate around the slab, and wire or A pure metal or alloy containing nickel is added and welded using a sheet. The induction coil creates a stable melted part surface by pressing the melted part toward the inner surface by electromagnetic force in the slab surface melted part, and then cooled again and solidifies.
[0010]
According to this method, the melting part temperature of the slab surface layer is set to the liquidus temperature (iron contains other components, so the melting state is not determined at one temperature such as the melting point, and the temperature at which solidification starts is the solidus temperature. , The temperature at which it becomes all liquid is called slightly higher than the liquidus temperature), the solidification structure can be reduced by rapidly cooling and solidifying after addition, and the size of the crystal grains can be reduced by reducing the size of the crystal grains. There is an advantage that the adverse effect of strength reduction caused by entering the grain boundary of steel can be suppressed.
[0011]
In addition, when a pure metal or alloy containing nickel is added and welded to the molten portion of the slab surface layer, it is preferable to prevent oxidation of the slab, so that the induction coil has an inert gas atmosphere (for example, argon, nitrogen) in the chamber. Etc.), and in order to prevent the oxidation more reliably, it is preferable to melt and modify the inert gas atmosphere to contain about 2% by volume of hydrogen.
Here, the principle of generation of electromagnetic force by the induction coil is shown in FIG. Due to the interaction between the magnetic field generated by the induction coil and the current induced in the slab, which is a conductor, an electromagnetic force acts on the melted portion. This electromagnetic force has an action of compressing the melted portion called pinch force, and contributes to stabilization of the surface of the melted portion.
[0012]
Next, the case where the method of the present invention is applied when the end of the continuous casting machine, that is, the slab after continuous casting is moving horizontally will be described. In this case, in the vertical bending type continuous casting machine having a general curved mold or a vertical portion over several m below the mold, the slab generally has a horizontal portion after being bent back and before being gas cut. Place in this part. The induction coil is arranged around the slab. In the case of this method, as shown in FIGS. 6 and 7, the upper part of the slab is easily treated because dripping due to gravity does not occur even if the surface part of the slab is melted by heating with an induction coil. Further, the melted portion at the bottom of the slab is held by the action of electromagnetic force based on the principle shown in FIG. 3, and it is possible to prevent the surface shape after dripping or re-solidification due to gravity. This processing can be performed one side at a time by reversing 180 degrees using a slab reversing device after gas cutting. The slab end can be processed by reversing ± 90 degrees using the slab reversing device.
[0013]
Further, the case where the method of the present invention is applied in the continuous casting machine, that is, when the slab is moving vertically will be described. In the continuous casting machine, the slab out of the mold is heated by the induction coil in the secondary cooling zone. The induction coil is held by the electromagnetic force of the melting part, and the molten part is cooled again and solidified after passing through the lower induction coil without dropping by gravity. Here, the slab melted part is more likely to drop by the action of gravity on the lower side of the melted part, so that the induction coil is installed near the slab rather than the upper coil, as shown in FIG. It is preferable because the lower side of the melting part can generate a stronger electromagnetic force and prevent the melting part from leaking.
[0014]
In addition, when plasma heating alone is performed, pure metal or an alloy containing nickel is supplied into the plasma, and is supplied to the molten portion of the steel so that it can be added to the slab surface layer melting portion. Since the plasma is generally axisymmetric, in order to continuously treat the surface of the slab, the plasma torch is scanned in the width direction of the slab, or the plasma disclosed in Japanese Patent Application Laid-Open No. 54-1421545 is used. A flat reciprocating method using electromagnetic force in the slab width direction can be used.
[0015]
The slab obtained by the continuous casting machine is transported to a melting treatment site after cutting, and a pure metal or nickel containing nickel is supplied to the plasma, thereby melting the slab surface layer with the plasma and pure nickel containing nickel. It is welded by supplying a metal or alloy and supplying it to the molten part of steel. Then it is cooled again and solidifies.
[0016]
Also in this method, the melting part temperature of the slab surface layer is set to the liquidus temperature (iron contains other components, so the melting state is not determined at one temperature like the melting point, and the temperature at which solidification starts is the solidus temperature, All liquid temperatures are called liquidus temperatures), and the solidification structure can be reduced by rapidly cooling and solidifying after addition, and by reducing the size of the crystal grains, There is an advantage that it is possible to suppress the adverse effect of strength reduction caused by entering the grain boundaries of steel.
In addition, when a pure metal or alloy containing nickel is added and welded to the molten portion of the slab surface layer, it is preferable to prevent oxidation of the slab, and therefore the gas atmosphere in the chamber is preferably the same as described above.
[0017]
Next, the case where the method of the present invention is applied when the end of the continuous casting machine, that is, the slab after continuous casting is moving horizontally will be described. In this case, in the vertical bending type continuous casting machine having a general curved mold or a vertical portion over several m below the mold, the slab generally has a horizontal portion after being bent back and before being gas cut. Place in this part. In the case of this method, the melting part cannot be held by the force of electromagnetic force as in the case of using an induction coil. Therefore, only the upper surface side is processed first by horizontal processing, and the slab is inverted 180 degrees after gas cutting. It is necessary to take measures such as processing. The slab end can be processed by reversing ± 90 degrees.
[0018]
Further, a case where induction heating and plasma heating are performed in combination will be described with reference to FIGS.
The slab 5 that has been continuously cast by the continuous casting machine 1 is cut, transported to a melting treatment field, heated by the induction coil 3 as shown in FIG. 1, and further supplied with pure metal or alloy containing nickel to the plasma 2. By doing so, a pure metal or alloy containing nickel is added to the slab surface layer melting portion and welded. The induction coil 3 forms a stable molten surface by pressing the molten portion toward the inner surface by electromagnetic force at the slab surface melting portion 8 by electromagnetic force, and then is cooled again and solidified.
[0019]
Even when the above method is used in combination, the heat load on the slab surface layer can be made smaller than in the prior art. Therefore, the melting state is not determined at one temperature, such as the melting point, the temperature at which solidification begins is called the solidus temperature, and the temperature at which it becomes all liquid is called the liquidus temperature). By rapidly cooling and solidifying after the addition, the solidified structure can be reduced, and by reducing the size of the crystal grains, there is an advantage that the adverse effect of strength reduction caused by the penetration of copper into the crystal grain boundaries of steel can be suppressed.
In addition, when a pure metal or alloy containing nickel is added and welded to the molten portion of the slab surface layer, it is preferable to prevent oxidation of the slab, and therefore the gas atmosphere in the chamber is preferably the same as described above. Furthermore, the electromagnetic force due to the induction coil acts as described above.
[0020]
Next, the case where the method of the present invention is applied when the end of the continuous casting machine, that is, the slab after continuous casting is moving horizontally will be described. In this case, in the vertical bending type continuous casting machine having a general curved mold or a vertical portion over several m below the mold, the slab generally has a horizontal portion after being bent back and before being gas cut. Place in this part. The induction coil is arranged so as to go around the slab, and the plasma is also arranged at the upper and lower side ends. In the case of this method, as shown in FIG. 2, the upper part of the slab is easily treated because dripping due to gravity does not occur even if the surface part of the slab is melted by heating with an induction coil. Further, the melted portion at the bottom of the slab is held by the action of electromagnetic force based on the principle shown in FIG. 3, and it is possible to prevent the surface shape after dripping or re-solidification due to gravity. This processing can be performed one side at a time by reversing 180 degrees using a slab reversing device after gas cutting. The slab end can be processed by reversing ± 90 degrees using the slab reversing device.
[0021]
Further, the case where the method of the present invention is applied in the continuous casting machine, that is, when the slab is moving vertically will be described. In the continuous casting machine, the slab out of the mold is heated by the induction coil in the secondary cooling zone, and further pure metal or alloy containing nickel is supplied to the plasma 2 so that the surface of the slab is melted with nickel. A pure metal or alloy containing is added and welded. The induction coil is held by the electromagnetic force of the melting part, and the molten part is cooled again and solidified after passing through the lower induction coil without dropping by gravity. Here, the slab melted part is more likely to drop by the action of gravity on the lower side of the melted part, so that the induction coil is installed near the slab rather than the upper coil, as shown in FIG. It is preferable because the lower side of the melting part can generate a stronger electromagnetic force and prevent the melting part from leaking.
[0022]
Next, it is preferable to add a pure metal or alloy containing nickel in an amount satisfying the following formula to the surface layer portion of the molten steel slab.
[% Cu] + 8 × [% Sn] <α × [% Ni]
[% Cu]> 0
[% Sn] ≧ 0
α> 0
Here, [% Cu], [% Sn], and [% Ni] indicate mass% of the content of each element in the steel.
α may be set to a suitable value so that the frequency of occurrence of cracks and the degree of cracking caused by casting conditions, heating and rolling conditions, etc. are lower than the control index. Further, the upper limit value of α is not particularly specified, and a suitable value may be set in consideration of cost. From such points, α is usually about 0.5 to 1.0.
[0023]
Moreover, as another form in the case of using both induction heating and plasma heating, an added pure metal or alloy containing nickel added to the slab surface layer part in the form of a wire or a sheet together with the slab surface layer by induction heating is used. A method of preheating, and then melting the added pure metal or alloy by plasma heating may be used.
In this method, induction heating is simply used as a preheating function, and the added pure metal or alloy containing nickel is melt-alloyed by subsequent plasma heating. In general, the shape of nickel-containing metal is powder for heating and melting with plasma. However, this method has the advantage that it can be carried out regardless of the shape of the added pure metal or alloy.
[0024]
The surface layer modified steel slab obtained by the above method is in a liquid state between the iron oxide and the steel when the steel is oxidized by exposure to oxygen in the atmosphere during continuous casting or reheating and hot rolling. Further, this liquid copper penetrates into the grain boundaries of the steel, and it is possible to prevent cracks caused by lowering the interface strength.
Therefore, the processed product obtained by processing the above-mentioned surface layer-modified steel slab can be obtained without cracking. Processed products include steel plates such as thin plates and thick plates, shaped steels, steel pipes, etc., but all steel products obtained by processing slabs in a normal steel process are targeted. Also included are semi-finished products such as hot rolled coils.
[0025]
【Example】
(Example 1)
The melt modification treatment was performed after the slab cut using a method in which induction heating and plasma heating as shown in FIGS. A nickel alloy was used for nickel addition and was supplied to plasma.
The surface layer 3 mm of a continuous cast slab having a width of 1500 mm, a thickness of 250 mm, and a length of 10 m containing 0.25% by mass of copper and 0.02% by mass of tin was melted on both sides, and the end of the slab was also melted. Nickel was added at 0.5% by mass.
The slab had already been cracked before it was embrittled by copper, but was repaired by the melting process. The modified slab was placed in a heating furnace and held at 1200 ° C. for 2 hours, and then subjected to hot rolling. The obtained rolled material was in a good state with no cracks usually observed.
[0026]
(Example 2)
About the same slab as Example 1, when the melt processing and nickel addition were performed on the continuous casting machine end on the same conditions using the method shown to FIG. 1, 2, the same favorable state as Example 1 was obtained.
[0027]
(Example 3)
Using a method in which induction heating and plasma heating as shown in FIGS. 4 and 5 are used together in a state where the slab is vertical, using an apparatus for melt reforming treatment in a continuous casting machine, containing 0.3% by mass of copper The surface layer of 3 mm of a continuous cast slab having a width of 2000 mm and a thickness of 250 mm was melted on both sides, and the end of the slab was also melted, and 0.3 mass% nickel was added.
The slab observed at the machine end had no cracks and was placed in a heating furnace and maintained at 1250 ° C. for 2 hours, and then hot rolled. The obtained rolled material contained copper but was in a good state with no cracks usually seen without adding nickel.
[0028]
(Example 4)
For the same slab as in Example 1, using the method of performing induction heating as shown in FIGS. 6 and 7 with the slab in a horizontal state, the slab is melted under the same conditions at the end of the continuous casting machine, and the slab is made of wire. When nickel was added to the melted surface layer, the same good state as in Example 1 was obtained.
[0029]
【The invention's effect】
As described above, by using the method of the present invention, cracking during continuous casting and hot working of steel containing copper can be reliably prevented at a low cost, and the degree of superheat of molten steel during surface modification is kept low. Since the re-solidification speed can be increased, the solidification structure of the steel and the grain size of the crystal can be reduced, and it becomes possible to obtain a steel of good quality.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing an example of the configuration of a method of the present invention (after slab cutting or treatment at the end of a continuous casting machine).
2 shows a configuration of the method of the present invention and is a detailed view of a processing portion of FIG.
FIG. 3 is an explanatory diagram of the principle of the method of the present invention.
FIG. 4 is a schematic cross-sectional view showing another example of the configuration of the method of the present invention (treatment in a continuous casting machine).
FIG. 5 is an explanatory diagram of the configuration of the method of the present invention (process in the continuous casting machine) and the balance of force (detailed processing portion of FIG. 4).
FIG. 6 is a schematic cross-sectional view showing a configuration example of the method of the present invention using only induction heating (processing after cutting a slab or at the end of a continuous casting machine).
7 is a configuration of the method of the present invention using only induction heating (detailed processing portion of FIG. 6).
[Explanation of symbols]
1: Continuous casting machine 2: Plasma having a function of melting and adding elements 3: Electromagnetic induction coil 4: Support roll 5: Cast slab 6: Container for creating an inert gas atmosphere 7: Solidification part 8: Molten part 9: Unsolidified part 10: Continuous drawing or moving direction 11: Coil current 12: AC current 13: Time 14: Electrical conductor 15: Magnetic field 16: Unmelted heating part 17: Nickel-enriched part 18: Electromagnetic force Direction and strength 19: Gravity (static iron pressure) direction and strength 20: Wire-like material containing additive elements

Claims (7)

The surface layer of the steel slab containing copper is melted by one or both of induction heating and plasma heating, and a pure metal or alloy containing nickel is added to the surface layer portion of the molten steel slab. A method for improving the surface layer of steel slabs. The method for reforming the surface layer of a steel slab according to claim 1, wherein a pure metal or alloy containing nickel in an amount satisfying the following formula is added to the surface layer portion of the molten steel slab.
[% Cu] + 8 × [% Sn] <α × [% Ni]
[% Cu]> 0
[% Sn] ≧ 0
α> 0
Here, [% Cu], [% Sn], and [% Ni] indicate mass% of the content of each element in the steel. The steel according to claim 1 or 2, wherein, at least when induction heating is used, the direction of the magnetic field generated by induction heating is generated so as to be in the casting direction of the slab, and the molten portion is held by electromagnetic force. A method for modifying the surface layer of a slab. At least in the case of using plasma heating, pure metal or alloy containing nickel is supplied into the plasma, heated or melted, and then supplied to the molten portion of steel. A method for modifying a surface layer of a steel slab as described. When both induction heating and plasma heating are used, the additive pure metal or alloy containing nickel is preheated together with the slab surface layer by induction heating, and then the additive pure metal or alloy is melt-alloyed by plasma heating. The method for modifying the surface layer of a steel slab according to claim 1 or 2. A surface-modified steel slab obtained by the method according to claim 1. A processed product obtained by processing the surface-modified steel slab according to claim 6.

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